cenozoic tectonic evolution of qaidam basin and its ...yin/05-publications/papers/109... ·...

ABSTRACT

Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, fi gures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic refl ection profi les from the southern Qilian Shan-Nan Shan thrust belt and north-ern Qaidam basin. Our geologic observa-tions provide new constraints on the timing and magnitude of Cenozoic crustal thicken-ing in northern Tibet. Specifi cally, our work shows that the southernmost part of the Qil-ian Shan-Nan Shan thrust belt and contrac-tional structures along the northern margin of Qaidam basin were initiated in the Paleo-cene-early Eocene (65–50 Ma), during or immediately after the onset of the Indo-Asian collision. This fi nding implies that stress was transferred rapidly through Tibetan

lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust sys-tem in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and tem-porally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin forma-tion and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65–50 Ma) and was associated with deposi-tion of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in north-ern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central axis of the basin. This basin confi guration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin

edges against basin-bounding thrusts. Resto-ration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomo-geneous, varying from ~20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the confl icting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current ele-vation (~3000–3500 m) and crustal thickness (45–50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continen-tal crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region.

Keywords: Tibetan plateau, Qaidam basin, Qilian Shan, Nan Shan, thrust tectonics.

For permission to copy, contact [email protected]© 2008 Geological Society of America

813

GSA Bulletin; July/August 2008; v.120; no. 7/8; p. 813–846; doi:10.1130.B26180.1; 16 fi gures; 1 table.

Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The southern Qilian Shan-Nan Shan thrust belt and northern

Qaidam basin

An YinStructural Geology Group, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, ChinaPermanent address: Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA

Yu-Qi DangLi-Cun WangWu-Ming JiangSu-Ping ZhouQinghai Oilfi eld Company, Dunhuang 736202, Gansu Province, People’s Republic of China

Xuan-Hua ChenInstitute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, People’s Republic of China

George E. GehrelsDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721-0071

Michael W. McRivetteDepartment of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA

E-mail: [email protected]

Yin et al.

814 Geological Society of America Bulletin, July/August 2008

INTRODUCTION

Understanding the evolution of the Tibetan plateau has two fundamental implications in Earth sciences. First, its growth mechanism pro-vides a key constraint on the dynamics of large-scale continental deformation (e.g., England and Houseman, 1986; Tapponnier et al., 1982, 2001). Second, its growth history provides an observa-tional basis for quantifying feedback processes between lithospheric deformation, atmospheric circulation, and biological evolution (Harrison et al., 1992; Molnar et al., 1993). Despite these important implications, no consensus has been reached on the plateau-forming mechanisms. Hypotheses vary from uniform lithospheric shortening, thrust imbrications in the upper crust and mantle lithosphere associated with fl ow in the lower crust, lower-crustal injection and lateral fl ow, continental subduction, convec-tive removal of mantle lithosphere, to large-scale underthrusting (e.g., Zhao and Morgan, 1985, 1987; England and Houseman, 1986; Dewey et al., 1988; England and Houseman, 1989; Burg et al., 1994; Royden, 1996; Clark and Royden, 2000; Owens and Zandt, 1997; Tapponnier et al., 2001; DeCelles et al., 2002). Similarly, there is little agreement on the formation history of the Tibetan plateau. In one view, the plateau has grown progressively from south to north and the northward-marching deformation front did not reach the present Qilian Shan-Nan Shan region of northern Tibet until 15–5 Ma (e.g., Molnar and Tapponnier, 1975; Burchfi el and Royden, 1991; Métivier et al., 1998). In a contrasting view, deformation induced by the Indo-Asian collision may have occurred across the present extent of the Tibetan plateau at ca. 50 Ma during or soon after the onset of the Indo-Asian colli-sion (Burg et al., 1994; Yin et al., 2002).

The competing views on the formation his-tory of the Tibetan plateau have different impli-cations for the plateau-growth mechanisms. For those who advocate a progressive northward growth of Tibet, the plateau may have been enlarged either in a continuous manner (Eng-land and Houseman, 1986; also see Burchfi el and Royden, 1991) or in discrete fashion via a stepwise jumping of the northern plateau margin (Meyer et al., 1998; Métivier et al., 1998). The continuous-growth model implies the continen-tal lithosphere to behave as a viscous fl uid, while the stepwise-jump model requires continen-tal lithosphere to deform as a strain-softening material capable of localizing and maintaining motion on continental-scale faults for tens of millions of years. Coeval initiation of deforma-tion in present northern Tibet and the onset of the Indo-Asian collision (Yin et al., 2002) implies that the preexisting weakness of the Tibetan pla-

teau and buckling instability of a strong Tibetan lithosphere may have played an important role in controlling the spatial and temporal evolution of the plateau (e.g., Burg et al., 1994; Kong et al., 1997; Yin and Harrison, 2000).

Qaidam basin is the largest active inter-mountain basin inside Tibet (Fig. 1) and fi gures importantly in the debate on the development of the Tibetan plateau. In the classic treatise on the Chinese sedimentary basins, Bally et al. (1986) propose the basin to have formed in the core of a large synclinorium between two thrust belts across the Qilian Shan in the north and the Eastern Kunlun Shan in the south. Expanding on this view, Yin et al. (2002) propose that Qai-dam basin initiated in the middle Eocene and formed as a common foreland basin between a southwest-directed thrust system in the north (Qilian Shan-Nan Shan thrust belt) and a north-east-directed thrust system in the south (Qimen Tagh-Eastern Kunlun thrust belt) (Fig. 1A). Yin et al. (2002) also considered the western basin boundary to have been progressively closed by southwest motion of the Altyn Tagh Range along the Altyn Tagh fault, causing Qaidam to become an internally drained basin since the early Oligocene (i.e., the sliding door model). Burchfi el et al. (1989) postulate a basement-involved thrust belt across the basin, which is linked with the Qilian Shan-Nan Shan thrust belt by a northeast-directed detachment fault in the middle crust. In contrast to the above view, Meyer et al. (1998) and Métivier et al. (1998) suggest that the Qaidam basin may have been trapped in the middle Miocene to Pliocene as a result of northward jumping of the deforma-tion front from the Eastern Kunlun Range to the Qilian Shan-Nan Shan region across the basin. More recently, Wang et al. (2006) postulate the presence of an Oligocene-Pliocene longitudi-nal river sourced in the Pamirs region with its eastern termination migrating progressively eastward across Qaidam basin during eastward extrusion of the northern Tibetan plateau.

To test the various hypotheses on the tectonic development of Qaidam basin, we conducted detailed fi eld mapping in the southern Qilian Shan and Nan Shan thrust belt and performed a systematic analysis of a dense network of seis-mic-refl ection profi les and drill-hole data across northern Qaidam basin. The results presented in this paper represent the fi rst of a three-part series on the Cenozoic evolution of Qaidam basin: the second part deals with the evolution of the southern margin of Qaidam basin and the Qimen Tagh-Eastern Kunlun thrust belt (Yin et al., 2007), while the third part deals with the Cenozoic history of the whole Qaidam basin (Yin et al., 2007). The observations presented in this paper have three important implications

for the formation history and growth mecha-nism of the Tibetan plateau. First, they suggest that Cenozoic contractional deformation in northern Tibet started in the Paleocene and early Eocene (65–50 Ma) during or immediately after the onset of the Indo-Asian collision. Second, the magnitude of Cenozoic crustal shortening strain across the region is suffi cient to explain its current elevation and crustal thickness with-out invoking lower crustal fl ow and a thermal event in the mantle. Third, the sedimentary architecture of Qaidam basin, characterized by its depocenters persistently located along the basin axis, was created by the development of crustal-scale triangle zones tapering from the basin margins toward the basin interior (also see Yin et al. [2007] for the structural evolution of the southern margin of Qaidam basin).

GEOLOGY OF THE QAIDAM BASIN

With an average elevation of ~2800 m, Qai-dam basin is triangular in map view, bounded by the Altyn Tagh fault in the northwest, the Qil-ian Shan-Nan Shan thrust belt in the northeast, and the Eastern Kunlun transpressional system (including the Qimen Tagh-Eastern Kunlun thrust belt and the Kunlun fault) in the south (Fig. 1A). Although the crustal thickness of Qaidam basin is ~10–25 km thinner than its surrounding moun-tain ranges at ~45–50 km (Zhu and Helmberger, 1998; Zhao et al., 2006; Li, S.L., et al., 2006; Li, Y.H., et al., 2006), its effective elastic thickness (Te) is ~70 km, which is signifi cantly greater than the values of 10–30 km for the rest of the Tibetan plateau (Braitenberg et al., 2003).

Cenozoic deformation across Qaidam basin has mostly been investigated by paleomagnetic studies and interpretations of subsurface data (seismic profi les and drill-hole data thanks to 50 years of petroleum exploration across the region) (Sun and Sun, 1959; Huang et al., 1996; Dang et al., 2003). From the studies on Creta-ceous and Cenozoic samples along the Altyn Tagh fault and northern Qaidam basin, Dupont-Nivet et al. (2002) and Sun et al. (2006) con-cluded that no Cenozoic rotation has occurred for the whole Qaidam block. In contrast, paleo-magnetic studies of Jurassic to Neogene strata in northern and southwestern Qaidam led Chen et al. (2002) and Halim et al. (2003) to infer that Qaidam basin has rotated clockwise 16–20°. Although the above confl icting paleomagnetic results could be reconciled by inhomogeneous Cenozoic deformation across the basin, because the samples of each study came from different parts of the basin, no systematic structural stud-ies have been conducted to test this possibility. Bally et al. (1986) used seismic data to show the presence of middle Eocene growth strata against

North Qaidam Basin

Geological Society of America Bulletin, July/August 2008 815

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Yin et al.


the Altyn Tagh fault along the western edge of Qaidam basin and interpreted this observation as indicating the Altyn Tagh fault to have ini-tiated at this time. Using both drill-hole and seismic data in southwestern Qaidam, Song and Wang (1993) infer a major south-dipping thrust along the southern margin of Qaidam basin. Huang et al. (1996) and Zhou et al. (2006) pro-vide the most complete structural synthesis of the whole Qaidam basin with emphasis on high-angle thrusting that requires minimal horizontal shortening across the basin (<10 km).

Qaidam basin preserves a complete record of Cenozoic sedimentation and has been the focus of numerous studies (Bally et al., 1986; Wang and Coward, 1990; Song and Wang, 1993; Huang et al., 1996; Zhang, 1997; Métivier et al., 1998; Xia et al., 2001; Yin et al., 2002; Dang et al., 2003; Sobel et al., 2003; Sun et al., 2005; Rieser et al., 2005, 2006a, 2006b; Zhou et al., 2006). For example, sedimentological development of the basin has been established by analyzing thickness distribution (Huang et al., 1996), paleocurrent directions (Hanson, 1998), lithofacies patterns (e.g., Zhang, 1997), sandstone petrology (Rieser et al., 2005), and 40Ar/39Ar ages of detrital micas (Rieser et al., 2006a, 2006b). These studies suggest that the Cenozoic Qaidam basin has expanded progres-sively eastward since the initiation of the basin in the Paleocene-early Eocene, with its depo-centers consistently located along the axis of the basin. These observations have led Wang et al. (2006) to propose the presence of a paleo-longi-tudinal river fl owing eastward along the axis of Qaidam basin.

Cenozoic stratigraphic division and age assignments across Qaidam basin are based on outcrop geology and its correlation with sub-surface data (seismic refl ection profi les and drill-hole data), terrestrial fossils (i.e., spores, ostracods, and pollen), basin-wide stratigraphic correlation via a dense network of seismic refl ection profi les, magnetostratigraphic studies, and fi ssion-track and 40Ar/39Ar dating of detrital micas (Huo, 1990; Qinghai Bureau of Geology and Mineral Resources [BGMR], 1991; Yang et al., 1992; Song and Wang, 1993; Huang et al., 1996; Xia et al., 2001; Nansheng, 2002; Sun et al., 2005; Rieser et al., 2006a, 2006b). Major Cenozoic stratigraphic units in Qaidam basin include (Table 1): the Paleocene to early Eocene Lulehe Formation (E1+2; 65–49 Ma) (Yang, 1988; Huo, 1990; Yang et al., 1992; Rieser et al., 2006a, 2006b), the middle and late Eocene Lower Xiagancaigou Formation (E3–1; 49–37 Ma) (Yang et al., 1992; Sun et al., 2005), the early Oligocene Upper Xiagancaigou For-mation (E3–2; 37–28.5 Ma) (Sun et al., 1999), the late Oligocene Shanggancaigou Formation

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(N1; 28.5–23.8 Ma) (Sun et al., 1999), the early to middle Miocene Xiayoushashan Formation (N2–1; 23.5–11.2 Ma) (Sun et al., 1999), the late Miocene Shangyoushashan Formation (N2–2; 11.2–5.3 Ma) (Sun et al., 1999), the Pliocene Shizigou Formation (N2–3; 5.3–1.8 Ma) (Sun et al., 1999), the Pleistocene Qigequan Forma-tion (Q1; 1.8–0.01 Ma) (Sun et al., 1999), and the Holocene Dabuxun Yanqiao Formation (Q2) (Yang et al., 1997). Our age division closely fol-lows that of Rieser et al. (2006a, 2006b) and Sun et al. (2005).

STRUCTURAL GEOLOGY OF NORTH QAIDAM THRUST SYSTEM

Surface Geology

The 500-km-long North Qaidam thrust sys-tem is located in the southernmost part of the Cenozoic Qilian Shan-Nan Shan thrust belt (Fig. 1B). It consists of the following major structures: (1) the northeast-directed Saishit-eng thrust zone that includes the northeast-directed Dasaibei thrust (DS) in the north and the northeast-directed Xiaosaibei thrust (XS) in the south; (2) the southwest-directed Sainan thrust zone (SN) that lies south of the Saishiteng thrust zone and dies out laterally into northwest-trending anticlines; (3) the southwest-directed Qaidam Shan thrust zone (QD) terminating in the west in the Xiaosaibei-thrust hanging wall and in the east in an anticline; (4) the L-shaped and southwest-directed Western and Eastern Luliang thrusts (WL and EL) that merge sepa-rately northward with the overriding Qaidam Shan thrust zone; (5) the southwest-directed Xitie Shan thrust zone (XT) that lies south of the Qaidam Shan thrust zone and dies out lat-erally into folds; (6) the southwest-directed Olongbulak thrust zone (OL) that terminates in the footwall of the Qaidam Shan thrust zone in the west and the Dalang anticline (DLA) in the east; (7) the northeast-directed Aimunik thrust zone (AM) that lies directly south of the Olong-bulak thrust zone and dies out eastward into the

Changshan anticline (CSA); and (8) the Lenghu anticlinorium consisting of a series of folds and lying directly south of the Saishiteng and Sainan thrust zones. Before describing each structure in the North Qaidam thrust system, we briefl y sum-marize the pre-Cenozoic deformation history of the North Qaidam region.

The Cenozoic Qilian Shan-Nan Shan thrust belt was a locus of intense early Paleozoic deformation induced by a series of arc-arc and arc-continent collisional events (Li et al., 1978; Yin and Nie, 1996; Yin and Harrison, 2000; Yang et al., 2001; Gehrels et al., 2003a, 2003b). The area also experienced widespread Mesozoic extension as expressed by the development of Jurassic and Cretaceous extensional basins (e.g., Huo and Tan, 1995; Vincent and Allen, 1999; Xia et al., 2001; Dang et al., 2003; Horton et al., 2004; cf. Ritts and Biffi , 2000), Jurassic and Cretaceous detachment faulting that had induced rapid cooling in the detachment footwalls and growth strata in the hanging-wall half grabens (Chen et al., 2003), and Jurassic basaltic erup-tions (Guo et al., 1999). The southernmost part of the thrust belt against Qaidam basin, where this study was conducted, exposes Precambrian gneisses, early Paleozoic ultrahigh-pressure metamorphic rocks, and metamorphosed Ordo-vician-Silurian arc sequences (e.g., Yang et al., 2001; Zhang et al., 2005; Song et al., 2005; Yin et al., 2008a). The metamorphic rocks are overlain unconformably by unmetamorphosed Devonian to Cenozoic marine and continental strata (Liu, 1988; Qinghai BGMR, 1991; Yang et al., 2001). Pre-Devonian structures are mainly isoclinal folds associated with development of ductile fabrics such as cleavage, gneissic folia-tion, and ductile shear zones (Yang et al., 2001; Yin et al., 2008a) (Figs. 2 and 6). Although early Paleozoic ductile structures can be easily differentiated from Cenozoic brittle structures, contractional deformation in unmetamorphosed Devonian to Cretaceous rocks and Cenozoic strata shares a similar style. For this reason, our mapping focuses mainly on areas exposing Cenozoic strata.

Accurate construction of geologic cross sec-tions requires the portrayed style of deforma-tion to be consistent with that observed in the fi eld. As shown below, map patterns, direct fi eld observations, and structures imaged in seismic refl ection profi les all indicate that folds across northern Qaidam basin exhibit rather broad and round, fold hinge-zone geometry. To simulate this folding style, we fi rst establish the general structural framework using the dip-domain method to honor all the measured bed-ding attitudes at surface (Suppe, 1983; Suppe and Medwedeff, 1990). We then use a round fold-hinge geometry to smooth the artifi cially created kinks so that the fold style in the cross sections matches those observed in the fi eld and seismic refl ection profi les. This hybrid method produces broader fold-hinge zones and smaller fold amplitudes than those created solely by the kink-bend method and matches well the drill-hole data that constrain the depth distribution of fold limbs and fold hinge zones.

The geometric and kinematic plausibility of each cross section (including those constructed from seismic profi les) was verifi ed by a step-by-step palinspastic restoration. The restoration was conducted by removing sequentially the effect of deformation and associated sedimentation from the youngest to oldest deformation event. Strain compatibiltiy of neighboring restored panels sharing a common geologic contact was achieved by bedding-parallel shear after all the beds in the panels were laid down horizontally. The success of this simple procedure in restor-ing all the sections suggests that inter-bedding fl exural slip is the main folding mechanism to have accommodated Cenozoic deformation.

The restored sections allow us to estimate the total amount of shortening and the associated shortening strain by comparing the present and restored lengths of the cross sections. As in all the restored sections, different beds may have differ-ent amounts of shortening as induced by (1) the presence of bedding-parallel faults, (2) develop-ment of growth strata with the younger synkine-matic units only recording a fraction of the total Cenozoic strain, and (3) erosion causing partial removal of some stratigraphic units in the sec-tions. To avoid the above complications, we chose our reference unit with the following two criteria: (1) it has the most complete bed length in the section, and (2) it was deposited immedi-ately prior to the onset of Cenozoic deformation. These criteria led us to use mostly the Jurassic-Cretaceous beds as the reference horizon to cal-culate horizontal shortening.

Saishiteng Thrust ZoneThe Saishiteng thrust zone consists of the

northeast-directed Dasaibei and Xiaosaibei

TABLE 1. MESOZOIC AND CENOZOIC STRATIGRAPHY OF QAIDAM BASIN

egA emit cigoloeG lobmyS seman tinU(Ma)

Dabuxun Yanqiao Formation Q2 Holocene 0.01 Ma–present Qigequan Formation Q1 Pleistocene 1.8–0.01 Shizigou Formation N2-3 Pliocene 5.3–1.8 Shangyoushashan Formation N2-2 late Miocene 11.2–5.3 Xiayoushashan Formation N2-1 early and middle Miocene 23.8–11.2 Shangganchaigou Formation N1 late Oligocene 28.5–23.8 Upper Xiaganchaigou Formation E3-2 early Oligocene 37–28.5 Lower Xiaganchaigou Formation E3-1 middle Eocene–late Eocene 49–37 Lulehe Formation E1+2 Paleocene–Early Eocene >54.8–49 (Jurassic strata, locally overlain by Cretaceous beds)

Jr Jurassic-Cretaceous 206–65

Yin et al.


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gen

thru

st

Xia

oshu

gan

Lak

e

Shug

an

Lak

e

gn2

gn2

gn2

gn2

gr2

gr2

gr1

gr2

Pz1

Pz1

Pz1

gr2

gr2

Pz1

gn2

Pz2-

Tr

Pz1

Pz1

gn2

Kek

esal

th

rust

Jr

Wut

umei

ren

gr1

gr2

Pz2-

Tr

Pz1

Pz1

N2

Tuo

shu

Lak

eKer

ilek

Lak

e

Jr N2

Q1

Pz2-

Tr

N1

Q1

Q1

N2

N2

N2

Q1

Q1

Q1

Q1

Q1

N2

Dab

uxun

L

ake

Q1

N1

N1

N2

E

Jr

E

EN

2

Jr

N2

N1

E

E

K

gn1

gn1

gn1

Pz1

Q1 N

2N

1E

N2

N2

Q1

N2

N2

Jr

Xia

ocai

dan

Lak

e

Dac

aida

n L

ake

N2

Don

gtai

jinai

eL

ake

Xita

ijina

ieL

ake

Mah

aiL

ake

Ren

nie

Lak

e

Jrum

gr1

Pz1

Pz1

Pz2-

Tr

Alty

n Ta

gh

Faul

t

Ar

ahh

gggnnn222

Lap

eiqu

ande

tach

men

t

gr2

N1

EE

N1 E

N2

E

E

EE

9493

JrJNN

krQQQQQQQNNJrJ2222222TTTTTTTTT

96

Fig

. 10

Fig

. 3

Fig

. 5

Fig

. 7 Fig

. 8

Fig

. 4E

Jr

Xitie Shan

Aim

unik

e

S

han

Dan

ghe

Nan

Sha

n

EE

E

Q2

Q2 Q

2

Q2

Q2

Q2

N2

Q2

Q1

N1 E K J Pz1

gn2

gn1

gr2

Hol

ocen

e al

luvi

al a

nd la

cust

rine

dep

osits

,

Plei

stoc

ene

allu

vial

and

lacu

stri

ne d

epos

its

Plio

cene

allu

vial

and

lacu

stri

ne d

epos

its

Mio

cene

allu

vial

and

lacu

stri

ne d

epos

its

Pale

ogen

e al

luvi

al a

nd la

cust

rine

dep

osits

Cre

tace

ous

allu

vial

and

lacu

stri

ne d

epos

its

Jura

ssic

con

glom

erat

e an

d sh

ale

N1

NNN111

EEQ

1Q

Lk

Fig

3F

PPPPPzzzz11

2222NNNNN

thru

stgg

Jgn

2g

9737

00′

3800

′

3900

′

Lat

e Pa

leoz

oic

to T

rias

sic

stra

ta

Ear

ly P

aleo

zoic

str

ata

(arc

and

fly

sch)

Qua

rtzo

-fel

dspa

thic

and

ort

hogn

eiss

es

Ultr

a-hi

gh p

ress

ure

met

amor

phic

roc

ks

Perm

ian

gran

itoid

s

Cam

bria

n-D

evon

ian

gran

itoid

s

KK

Pz2-

Tr

Olo

ngbu

lak

Shan

Fig

. 4G

E

Q1

gr1

A

B

Fig

. 4F

Lul

iang

de

tchm

ent

Lul

iang

de

tchm

ent

DS

XS

SN

WL

EL

XTAM

OL

Qai

dam

th

rust

gn2

QD

Len

ghu-

Lin

gjia

nfa

ult z

one

Fig

. 4D

Fig

s. 4

A-C

Fig

. 6

Fig

. 9

Fig

. 11

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

Fig. 16

95

Lenghu Anticlinoriu

m

N

Fig

ure

2. G

eolo

gic

map

of

the

sout

hern

Qili

an S

han-

Nan

Sha

n th

rust

bel

t, m

odifi

ed f

rom

Liu

(19

88)

base

d on

our

ow

n ob

serv

atio

ns. L

ine

A–B

is lo

cati

on o

f a

regi

onal

geo

logi

c cr

oss

sect

ion

show

n in

Fig

ure

16. L

ocat

ions

of d

etai

led

geol

ogic

map

s: G

aoqu

an a

rea

(Fig

. 3),

Lul

ehu

area

(Fig

. 5),

wes

tern

Lul

iang

Sha

n ar

ea (F

ig. 6

), G

aqiu

are

a (F

ig. 7

), X

iao-

qaid

am a

rea

(Fig

. 8),

Aim

unik

-Olo

ngbu

lak

sect

ion

(A–O

cro

ss s

ecti

on)

(Fig

. 4G

), L

engh

u-4

area

(F

ig. 1

0), L

engh

u-6

area

(F

ig. 1

0) a

re a

lso

show

n. L

ocat

ions

of

fi eld

ske

tche

s in

F

igur

e 4

are

also

indi

cate

d. N

umbe

rs in

par

enth

eses

den

ote

loca

tion

s of

inte

rpre

ted

seis

mic

refl

ect

ion

profi

les

(Fig

. 13)

. Loc

atio

n of

Fig

. 12

coin

cide

s w

ith

line

(1).

North Qaidam Basin


thrusts (Figs. 1B and 2). Along most of their traces the thrusts place Precambrian gneisses and early Paleozoic metagraywacke over Qua-ternary deposits (Fig. 2). The two faults merge to the east and west with their common western end linking with the left-slip Altyn Tagh fault (Fig. 1). The Dasaibei thrust is active, exhibiting prominent fault scarps and active foreland sedi-mentation in the footwall directly to the north (i.e., Sugan Lake; Fig. 2). As shown in seismic profi les (Figs. 13A and 13B) and our kinematic reconstructions (Fig. 15A), the thrust zone ini-tiated in the Paleocene and early Eocene and acted as a passive roof thrust of a large south-

west-directed thrust duplex system; its devel-opment also produced an overlying southwest-thickening, growth-strata sequence (Figs. 13A and 13B).

Sainan Thrust ZoneThe southwest-directed Sainan thrust zone is

active and bounds an active foreland depocenter in its footwall to the south (Mahai Lake; Fig. 2). The thrust zone dies out to the east and west into folds involving Jurassic to Neogene strata. The western Sainan thrust zone in the Gaoquan area consists of six major northeast-dipping thrusts (Figs. 3A and 3B). The northernmost Gao-

quan thrust (fault-1) places Devonian strata (D) over Lower Jurassic strata (Jr1). Directly to the south, the Lower Jurassic strata are thrust over Middle Jurassic strata (Jr2) on fault-2, which in turn are thrust over the Paleocene to early Eocene Lulehe Formation (E1+2) on fault-3 (Fig. 3) (Table 1). The southern part of the sec-tion exposes three thrusts with down-dip stria-tions (Fig. 4A): fault-4 places unit E3 over unit N1 with a complex structure in its hanging wall (Fig. 4B); fault-5 is an intra-formational thrust zone in unit E3 and its motion produced a fault-bend fold in the hanging wall (Fig. 4C); fault-6 (the Sainan thrust) juxtaposes middle Eocene to

C Palinspastically restored section

fault-1 = Gaoquan Thrust

fault-6 = Sainan thrust

fault-2fault-3

fault-4fault-52 km

Jr

E1+2

N1-N2

N2-3

Jr

E1+2

N1-N2

N2-3

Jr

E1+2

N1-N2

DQ

A B

N2

N1

43

84

57 40

22 7339

72

82

85

N2-3

E3

N1

N2

E3E1+2

0 1.0 2.0 km

2800

3000

N1

N2

D

E1+2

Q

Jr2Jr1

94°15′

37°25′37°25′

94°15′

78

45

15

7

E3

Sainan Thrust

A

BGaoquan Thrust

Fig. 4BFig. 4C

68

66

37

43

Q

Fig. 4A

3000

2000

1000

0

1000

N2-3

N2

E1+2N1

E3

E3

E1+2

J1J2

J1 J2

QD

meters

3000

2000

1000

0

1000

metersSainan thrust

A B

2000 2000

Present section length = 5.8 km

Restored section length > ~15.6 km

Shortening = 9.8 km

Total shortening strain is 63%

J1

J2

E1+2

E3

fault-1 = Gaoquan thrust

N1

N2pre-Jr

Out-of-syncline thrust

2 km

Position of passive roof thrust duplex in Fig. 4Afault-6

fault-5

fault-4

fault-3

fault-2

Figure 3. (A) Geologic map of the Gaoquan area; see Figure 2 for location. Locations of fi eld photos in Figures 4A and 4B are also indicated. (B) Geologic cross section across Gaoquan area. See Figure 4A for location. See Table 1 for the defi nition of map units. Additional map symbols: Jr1—lower Jurassic, Jr2—middle Jurassic, D—Devonian. (C) Restored geologic cross section. (D) Kinematic reconstruction of the cross sec-tion shown in (A). (Continued on following page.)

Yin et al.


Gaoquan thrustSainan thrust

Kinematic reconstruction

Stage 1DJ1

J2

D

D

E1+2E3

N1N2Q

E3E1+2 J1

J2

J1

J2

Stage 2

D

D

E1+2J2

E3N1N2

Q

Q

J1Stage 3

Stage 4

D

D

E1+2E3

N1N2

J1

J2

D

D

E1+2

J1

J2

E3N1N2

Q

J1

J1

J2

E3

E3

E1+2

E1+2J2

Out-of-syncline thrust

J2

J1

Stage 5

Final section length = 5.8 km Original section length = 15.6 km

Magnitude of shortening = 9.8 km

D

J2

2 km

Total shortening strain = (final length - original length)/original length = 63%

Stage 6

D

E1+2

J1J2

E3

N1

N2Q

E3E3

E3

E1+2

E1+2

J1J2

J1

N2

2 m

South

2 m

South

down-dip striations

Tip of folded, south-tapering thrust wedge

a shale marker bed

A

B

C

Figure 3 (continued).

Figure 4. Field photos, sketches, and schematic cross sections; see Figure 2 for locations. (A) Down-dip stria-tions on a thrust in Eocene red beds, Gaoquan area; see Figure 3A for location. (B) View northeast at south-west-directed minor thrusts truncated by a northeast-directed thrust in the middle Eocene to early Oligocene Xiagancaigou Formation (E3–1 + E3–2; see Table 1 for defi nition). Note complex distribution of a shale marker bed dismembered by two generation of thrust-ing with opposite sense of slip. (C) A fault-bend fold in the Xiagancaigou Formation (E3). See Figure 3A for location. (Continued on following page.)

North Qaidam Basin


E1+2

J

J

J

5 m

SW

SWPaleozoic Luliang Shan

Detachment fault

gn(UHP)

Or(mb)

eclogite

N2-2N2-1

N1

E3

E1+2200 m

overturned

bedding:

S0 = N54W42NE

Xitie Shan thrust:

fault = N35W45NE

upright

isoclinal

foldshallower

strata dip

N2-2

Jr1

Jr2-3Jr2-3Qt

Jr1

Jr2-3Jr1Jr2-3

Qt

Pt

Pt

200 m

Jr1

SW

Jr2-3Jr2-3

klippe

out-of-sequence thrust

D

E

FFigure 4 (continued). (D) Structure of the Qaidam Shan thrust zone along Lucaogou Valley, northern Qaidam Shan, constructed from a collage of photos. The structurally highest thrust carries Precambrian gneiss (Pt) above and truncates Jurassic strata below. Within Jurassic units, internal thrusts repeat Lower (Jr1) and Middle (Jr2) Jurassic formations. Note that the frontal thrust of the imbricate system remains active, causing uplift of a prominent Quaternary terrace (Qt) ~250 m above the active stream. (E) Complex thrusts and folds in the core of an anticline constructed from a fi eld photo. This style of deformation is common in anticlines cored by Jurassic strata. (F) Field sketch of a detachment fold in the footwall of the Xitie Shan thrust. Note that Cenozoic strata are overturned directly below the thrust. The Luliang Shan detachment fault is a top-north, low-angle normal fault exhuming ultrahigh-pressure metamorphic rocks. At this locality, it is overturned and preserves top-north-shear indicators. The southern fold limb exhibits gentler dip, which could be induced either by hinge-zone thickening during folding or by the development of growth strata during a progressive increase in fold amplitude. (Continued on following page.)

Yin et al.


early Oligocene Upper and Lower Xiagancaigou Formations (E3) over Quaternary deposits (Q) (Figs. 3A and 3B). Palinspastic reconstruction of the cross section in Figure 3B indicates that the restored section was at least 15.6 km long, and had absorbed >63% of horizontal shorten-ing (Figs. 3C and 3D). The minimum shortening estimate comes from the fact that the Devonian unit could lie in any position lower than that shown in Figure 3C, which would increase the estimated total shortening and the associated shortening strain.

The Sainan thrust zone extends eastward to the Lulehe area (Fig. 5), where it splits into two thrusts terminating into folds in the east. The ter-mination folds overlap the trace of the Western Luliang thrust along strike to the south, which dies out westward into a Jurassic anticline. The core of the anticline is complexly deformed by top-north and top-south thrusts within Jurassic shale and sandstone directly below relatively little deformed unit E1+2 (Fig. 4D). North of the Sainan thrust zone is the Qaidam Shan thrust zone that juxtaposes Precambrian gneisses over Quaternary strata. Palinspastic restoration of the cross section in Figure 5B suggests that the sec-tion has absorbed at least 31 km of horizontal shortening, which yields a minimum shortening strain of 63% (Fig. 5C). Similar to the Gaoquan cross section in Figure 3B, there is no match-ing unit across the Qaidam Shan thrust, and the position of its hanging wall could be placed any-where below that shown in the restored section in Figure 5C that would increase the shortening estimates. Our restoration requires the Qaidam

Shan thrust to have at least 23 km of fault slip in the area shown in Figure 8A.

Qaidam Shan Thrust ZoneThis is the longest (~200 km) structure in the

North Qaidam thrust system and has the most prominent topographic expression, with its hanging-wall elevation reaching ~5700 m and a structural relief locally exceeding 2 km across the fault (Figs. 1 and 2). The fault zone is active, as indicated by numerous fault scarps cutting Quaternary alluvial fans throughout its trace. The fault zone dies out in the hanging wall of the Xiaosaibei thrust in the west and in the footwall of the Olongbulak thrust in the east (Figs. 1 and 2). The Qaidam Shan thrust zone is also linked geometrically with three major thrust zones to the south: the Sainan, Western Luliang, and Eastern Luliang thrusts (Figs. 1B and 2). The Qaidam Shan thrust zone consists of imbricate thrusts placing Pre-cambrian gneiss over Jurassic strata and Juras-sic strata over Quaternary deposits (Fig. 4E). Active thrusting along the Qaidam Shan thrust zone has raised Quaternary strath terraces in the hanging wall ~150 m above the active streams (Fig. 4E).

Luliang Shan Thrust ZoneThe Luliang Shan thrust zone consists of

the Western and Eastern Luliang thrusts, both exhibiting L-shaped geometry in map view with the northwest-striking thrust segments linking with the east-northeast-striking lateral ramp segments (Figs. 1B and 2). The lateral ramps

are active, as expressed by fault scarps along the fault traces and the development of the active depocenters directly against the faults (i.e., the Daqaidam Lake against the western Luliang lateral ramp and Xiaoqaidam Lake against the Eastern Luliang lateral ramp). Folds in the Mesozoic and Cenozoic strata and synforms in the Luliang Shan high-grade gneisses share the same axes (Fig. 6), a relationship suggest-ing that the crystalline basement did not behave as rigid blocks. Palinspastic reconstruction of a cross section through the area indicates >34-km horizontal shortening and a shortening strain of >55%. Minimum slip on the Qaidam Shan thrust is 11 km (Fig. 6C).

Structures south of the Luliang Shan thrust zone are mapped in the Gaqiu area (Fig. 7A), where several south-directed thrusts are cut by younger north-trending, left-slip faults and northeast-trending, right-slip faults (Fig. 7A). Although both thrusts and strike-slip faults accommodate northeast-southwest shorten-ing, they refl ect a temporal change in the state of stress. The intermediate principal stress axis was horizontal during early thrusting and changed later to a vertical orientation during strike-slip faulting. Palinspastic reconstruc-tion of a cross section through the area requires >22-km horizontal shortening and a shortening strain of >38% (Fig. 7C).

Xitie Shan Thrust ZoneThe Xitie Shan thrust is a northeast-dipping,

basement-involved structure that dies out into folds laterally toward the Luliang Shan to the

N

Pliocene-Quaternary growth strata along northern edge of Qaidam basin

N46W52SWN84W38SWN52W15SW Olongbulak ShanAimunike Shan

D

C

C

C

J

N2-1

N2-2

N2-3 Qal

D

D

N2-2

N2-1

Qal

CD

Qal

Qal

N2-2gn

gn

gn

gn

Qal Qal

0

5

10

3

km

0

5

10

3

km

K

C

1050

km

gn

gn

C

J

N2-1

D

CD

gn

gn

K

C

1050

km

gn

gn

Geology in the shaded area is highly speculative

G

Figure 4 (continued). (G) Schematic cross section across the Aimunik Shan and Olongbulak Shan, shown as A–O cross section in Figure 2. Cenozoic map units are defi ned in Table 1. Additional units: gn—gneiss, D—Devonian, C—Carboniferous, J—Jurassic.

North Qaidam Basin


Q

Q

Q

N2-

2

N2-

1

N2-

3

N2-

2

N2-

1E

3E

1+2

E1+

2

E1+

2

E3

E3

N1

75

85

gn

Q

E3

E3

N2-

2N

2-1

N1

E3

E3

E1+

2

N2-

1

N2-

2J

JE1+

2

60

75 84

30

3072

65

J

gn7070

6550

706520

N1

57

8724

2556

54

E3

E1+

2

55 6677

N1N

2-1

Q

3000

3200

3200

3200

3400

3600

02

4km

Lule He

94°4

5′

K

50

60

Wes

tern

Lul

iang

thru

stN

1

Qai

dam

Sha

n th

rust

A

B

Fig.

4d

02

4km

Pres

ent s

ectio

n le

ngth

= 1

8 km

Res

tore

d se

ctio

n le

ngth

= 4

9 km

Tota

l sho

rten

ing

= 31

km

Tota

l sho

rten

ing

stra

in =

63%

Hig

hly

defo

rmed

antic

line

core

(see

Fig

. 4D

)4 3 2 1 0 -1 -2 -3 -4km

gn

N2-

3/Q

N2-

2

N2-

1

N1

E3

E1+

2

J

E1+

2

E3

N1

E1+

2

N2-

1N

2-2

N2-

3

QE

3

J-K

4 3 2 1 0 -1 -2 -3 -4km

AB

Qai

dam

Sha

n th

rust

N2-

3N

2-3

B

C

94°4

5′

38°0

5′

E1+

2 -

E3

N1

- N

2-2

N2-

3/Q

J-K

E1+

2 -

E3N

1 -

N2-

2

J-K

gn

4 km

Min

imum

leng

th o

f th

e or

igna

l sec

tion

= 29

km

Min

imum

leng

th o

f th

e or

igna

l sec

tion

afte

r re

stor

ing

slip

on

Qai

dam

Sha

n th

rust

= 4

9 km

Shad

ed a

rea:

Str

uctu

res

extr

apol

ated

fro

m

surf

ace

geol

ogy;

see

Fig

. 14E

for

dee

per

Cen

ozoi

c st

ruct

ures

.

N2-

3/Q

N2-

3/Q

35°0

5′A

Fig

ure

5. (A

) Det

aile

d ge

olog

ic m

ap o

f the

Lul

ehe

area

. (B

) Geo

logi

c cr

oss

sect

ion:

gn—

gnei

sses

; see

Tab

le 1

for

defi n

itio

n of

oth

er m

ap u

nits

. (C

) Res

tore

d se

ctio

n.

Yin et al.


Palin

spas

tical

ly r

esto

red

sect

ion

C

O-S

(a/b

)gn

(UH

P)

J

Pale

ozoi

c L

ulia

ng S

han

deta

chm

ent

Cen

ozoi

c W

este

rn L

ulia

ng th

rust

Cen

ozoi

c Q

aida

m S

han

thru

st z

one

O-S

(a/b

)

Or(

gr)

O-S

(a/b

)

O-S

(a/b

)

JJ

O-S

(a/b

)

J

E3

E1+

2E

1+2

E1+

2

gn(U

HP)

gn(U

HP)

10 k

m

Ori

gina

l (re

stor

ed)

leng

th o

f th

e se

ctio

n =

62 k

mFi

nal l

engt

h of

the

sect

ion

= 28

km

Shor

teni

ng =

34

kmSh

orte

ning

str

ain

= 55

%

Pale

ozoi

c L

ulia

ng S

han

deta

chm

ent

E1+

2

Res

tore

d se

ctio

n le

ngth

= 6

2 km

Or-

S

Or(

gr)

Or-

S

Or-

S

Qal

N75

°E

P(g

r)

Lu

lian

g S

han

det

ach

men

t

Lu

lian

g d

etac

hm

ent

Yu

ka s

hea

r zo

ne

Wes

tern

Lu

lian

g t

hru

st

gn

(UH

P)

Or-

S

gn

(UH

P)

Lu

lian

g S

han

det

ach

men

t

JJ

fold

ed q

uart

z ve

ins

um

um

um

Yu

ka s

hea

r zo

ne

E1+

2E

3

E1+

2

B

B6 5 4 3 2 1 0 654321 7 8 9km

6 5 4 3 2 1 0 654321 7 8 9kmA

50

72

02

46

810

km

323045

62

65

50

66

35

52

75

55

40

48

45

6868

6275

6067

45

50

45

41

76

Qal

Riv

erY

uka

22

32

605361

5155

5546

31

57 2

5363

50

55

95°0

0′

95°0

0′

95°1

0′

95°1

0′

38°0

0′38

°00′

Or-

S

Or-

S

Or-

S

um

A

Qal

Yuk

a

Yuk

a sh

ear

zone

BO

r(gr

)

gn(U

HP)

P(gr

)

gn(U

HP)

E1+

2

E1+

2

E1+

2

E3

N1

J

Lu

lian

g d

etac

hm

ent

Wes

tern

L

ulia

ng

th

rust

52

50

67

Or-

S

gn(U

HP)

eclo

gite

bl

ocks

Qal

Qal

A Fig

ure

6. (

A) D

etai

led

geol

ogic

map

of

the

wes

tern

Lul

iang

Sha

n ar

ea;

see

Fig

ure

2 fo

r lo

cati

on a

nd T

able

1 f

or d

efi n

itio

n of

map

uni

ts. A

ddit

iona

l un

its:

Or-

S—O

rdov

icia

n m

etas

edim

enta

ry a

nd m

etab

asit

e, g

n(U

HP

)—ul

trah

igh-

pres

sure

met

amor

phic

roc

ks; u

m—

ultr

amafi

c r

ocks

, P(g

r)—

Per

mia

n gr

anit

e, O

r(gr

)—O

rdov

icia

n gr

anit

e. (B

) Geo

logi

c cr

oss

sect

ion.

(C) R

esto

red

sect

ion.

North Qaidam Basin


N2-

1N

2-2

N2-

3

E1+

2KE3

N1-

1N

1-2

E1+

2KE3

N1-

1

E1+

2KE3

N1-

1N

1-2

N2-

3

4 km

N2-

2

E1+

2KE3

N2-

1

N1-

1N

1-2

E1+

2KE3

Q

Unc

onfo

rmity

alo

ng w

hich

N2-

1 an

d N

2-2

are

mis

sing

CPa

linsp

astic

ally

res

tore

d se

ctio

nM

inim

um s

lip o

n th

e fa

ult i

s re

stor

eddu

e to

han

ging

-wal

l cut

off b

eing

ero

ded

away

5 3 1 –1 –3km

5 3 1 –1 –3km

Q

N1-

1E

3

N1-

1

E3

E1+

2K

N1-

1N

1-2

N2-

1 N1-

2N

2-1

N1-

2

N2-

2E

1+2

N2-

3

Q2

E3

E1+

2 K

E1+

2K

–5 –7 –9 –11

–5 –7 –9 –11

E3

N1-

1

E3

K

E1+

2K

E1+

2

K

E3

N1-

1

N1-

2

Q2

02

4

km

AB

Pres

ent s

ectio

n le

ngth

= 3

5.5

kmR

esto

red

sect

ion

leng

th >

57.5

km

Shor

teni

ng >

22

kmTo

tal s

hort

enin

g >

38 %

N2-

3

Unc

onfo

rmity

alo

ng w

hich

N2-

1 an

d N

2-2

are

mis

sing

B

N1-

1

N1-

2

E3

N1-

1N

1-1

E1+

2

KN

1-1

N2-

1

N2-

1

N2-

3

N2-

2

N2-

1

Q2

E3

N1-

1

N1-

2

N2-

1 N2-

1

Q

N1-

1

N1-

1N

1-2

N1-

1N

1-2

N2-

2

N2-

1

3120

2960

2990

Q1

N1-

1

N1-

2N

2-1

N2-

1

N1-

1

N1-

2

N1-

1N

1-2

02

4

km94

°45′

37°4

0′

37°4

0′

94°4

5′

592064

4

20

8

152316

13

83030

20

15

85

2939

40

1531

12

11

9

N1-

1

A

B

late

ral t

hrus

t ra

mps

Q

Q

Q

Q

QQ

QQ

N2-

3

N2-

3

N2-

3

N2-

3

N2-

3

A Fig

ure

7. (

A) D

etai

led

geol

ogic

map

of

the

Gaq

iu a

rea;

see

Fig

ure

2 fo

r lo

cati

on. (

B) G

eolo

gic

cros

s se

ctio

n. T

otal

sho

rten

ing

exce

eds

38%

. See

Tab

le 1

for

defi

nit

ion

of C

enoz

oic

unit

. K—

Cre

tace

ous.

Yin et al.


4 km

C

K

E3

N1 - N2-2

N2-2 - N2-3

E1+2K

E3E1+2

N2-2 - N2-3

N2-1N2-3

Palinspastically restored section Present section length = 26 kmRestored section length = 33.5 kmShortening = 6.5 kmTotal shortening strain = 23%

A B

5

3

1

–1

–3

–5

5

3

1

–1

–3

–5

N2-3

N2-2 N1

N2-2

km

N1

N2-1

N1

N2-1

N1

E3

E1+2E3

E3

E1+2E3

km

K

K

N2-1 is a mappable growth strata unit

Xitieshan thrust

4 km

B

N2-3

95°30′95°15′

Q

E1+2E3

N1

3000

32003200

3400

3400

KE3

E3

E1+2E1+2

N2-2

N2-1

N1

N2-2

gn(UHP)MF

N2-1

E1+2

N2-3

E3 37°30′

37°30′

0 2 4

2526

22 30 22

14

13

17

14

14

714

8

26

9 2345

39 1314

70

307242

40

2444

32

34 5041

78

47

74

32

37

77

4810

17 2554

8882

60

51

46

63

32

1013 14

17 10

957

10

13

16 23 Xitie Shan thrust

80

Bedding-parallel shear zone

6050

6241 80

52

E1+

4

50

Bedding-parallel shear zone

B

A

Xiaoqaidam Lake

OverturnedLuliang Shan

detachment

Fig. 4e

42

78

80

A

N2-1Q

N2-3

Figure 8. (A) Detailed geologic map of the Xiaoqaidam area. Bedding parallel shear zones are observed in units E1+2 and N2–2. gn(UHP)—ultrahigh-pressure metamorphic rocks. (B) Geologic cross section. N2–1 unit displays southward thickening, which is interpreted as growth strata associated with anticlinal folding above the Xitie Shan thrust (i.e., Xitie Shan thrust was active during deposition of unit N2–1). (C) Restored section.

North Qaidam Basin


west and Aimunik Shan to the east (Fig. 2). Along the northern limb of the western termina-tion anticline, bedding-parallel shears are widely developed in units E1+2 and N2–2, across which bed dips vary signifi cantly (Fig. 8A). Along the southern limb, growth strata are well developed as indicated by progressive south-westward thickening of unit N2–1 (Fig. 8B). This relationship requires the Xitie Shan thrust to be active during the early to middle Miocene and is broadly consistent with the seismic data that contractional deformation in the same area was initiated during early Oligocene deposition (Fig. 13H). A tight, upright anticline is devel-oped in the footwall of the Xitie Shan thrust. It has nearly vertical limbs at the fold core and progressively shallower limbs away from the fold hinge zone (Fig. 4F). The lack of intrafor-mational deformation and the presence of pro-gressive thickening of individual beds observed in the fi eld led us to conclude that the shallowing of the fold limbs was produced by synchronous folding (Fig. 4F). Palinspastic reconstruction of the cross section in Figure 8B requires 6.5-km horizontal shortening and a shortening strain of ~23% (Fig. 8C).

Aimunik and Olongbulak ThrustsAlthough Paleogene strata are preserved

in the Xitie Shan area, they are missing in the Aimunik Shan directly to the east (Figs. 2, 4G, and 8). There, only Neogene strata unconform-ably over Devonian-Carboniferous and Jurassic strata are present. The eastward overlapping of Neogene strata over pre-Cenozoic rocks from the Xitie Shan to the Aimunik Shan is consistent with the observation that Qaidam basin has been expanding eastward during the Neogene (Huang et al., 1996; Wang et al., 2006). A narrow, north-west-trending Neogene basin is located between the north-dipping Olongbulak thrust in the north and the south-dipping Aimunik thrust in the south. This basin could be part of a much larger Qaidam basin and was later incorporated into the southern Qilian Shan-Nan Shan thrust belt during the development of the Aimunik thrust. The Aimunik thrust dies out to the west into a fold complex in the Xitie Shan and to the east into the Changshan anticline (Figs. 1B and 2). The Olongbulak thrust dies out to the west into a fold complex linked with the Qaidam Shan thrust zone and terminates to the east at the Dalang anticline (Figs. 1 and 2).

Lenghu AnticlinoriumThe anticlinorium is truncated by the Xiaosai-

bei thrust in the west and terminates at the West-ern Luliang thrust in the east. The fold is arcuate in map view, with a rectilinear northwest seg-ment and a curvilinear southeast segment. The Lenghu-Lingjian thrust zone (Figs. 9, 10, and 11) is part of a passive roof thrust zone above a southwest-directed thrust wedge. The relation-ship between the Lenghu anticlinorium and the Western Luliang thrust is similar to that between the Luliang Shan thrust zone and the Qaidam Shan thrust zone, both having the lateral ramps to merge with an overriding thrust (Fig. 2). A key difference between them is that the Lenghu-Lingjian lateral ramp is blind below an anticline while the Luliang lateral ramps are all exposed at the surface. Deformation associated with the fold above the Lenghu-Lingjian thrust has caused the Western Luliang thrust to be over-turned with a local southward dip (Fig. 9).

We mapped surface structures of the Lenghu anticlinorium in two places (Fig. 2). In the Len-ghu-4 area (Fig. 10), the south-dipping Lenghu-Lingjian thrust zone places Miocene strata over Pliocene strata. Palinspastically restored section

South

gn(UHP) E1+2

Locally overturned Yuka thrust

T1

T2

T3

Three levelsof terraces

B

Yuka River

Maxian thrust is a blind structure below an anticline, which exhibits prominent fold scarps in the field.

10 km

55

70

E1+2E3

N

gn

Western Luliang Thrust

Active lateral ramp cutting alluvial fans

Active W. Luliang thrust is locally folded

A

Figure 9. (A) A schematic geologic map showing the relationship between the anticline below the blind Maxian thrust and the Western Luliang Shan thrust. (B) Locally overturned Western Luliang thrust and uplifted strath terraces at the intersection of the anti-cline and the Western Luliang thrust. gn(UHP)—ultrahigh -pressure metamorphic rocks.

Yin et al.


N1

N2

N2-3

N1

N2

N2-3

Q

Palinspastically restored sectionC500 km

B

N1

N2

N2-3Q

N2

N1

2800

2400

2000

1600

meters

2800

2400

2000

1600

meters

Final section length = 3 kmOriginal section length >4.6 kmShortening >1.6 kmTotal shortening strain >35%

0 200 400 600 800

meter

A B

N2-3

Q

N2

N1

N2

21

19

2119

25

Lenghu

0 200 400 600 800

meters

2800

2900

93°10′ 38°40′8

36

3

Lenghuthrust zone

A

B

N2-3

N2-3

A

Figure 10. (A) Detailed geologic map of the Lenghu-4 area. (B) Geologic cross section. See Table 1 for defi nition of Cenozoic units. (C) Restored section.

North Qaidam Basin


requires >1.6-km horizontal shortening and a minimum strain of ~35% (Fig. 10C). In the Len-ghu-6 area (Fig. 11), the south-dipping Lenghu-Lingjian thrust zone is developed in the Neogene strata, with a tight, southwest-verging anticline in its footwall of the northeast-directed thrust. This relationship leads us to interpret that thrust-ing postdates the footwall fold. Bedding-parallel shear zones are common in Neogene units. Pal-inspastic restoration of a cross section across the area requires 16-km horizontal shortening and a shortening strain of ~39% (Fig. 11C).

Subsurface Geology of Northern Qaidam Basin

Although the laterally overlapping relation-ships among thrusts and their termination folds in the North Qaidam thrust system suggest their coeval development and kinematic linkage, the geometric relationships in map view alone do not permit the establishment of the timing and sequence of deformation across the thrust system. In addition, surface mapping does not provide a complete picture of the upper crustal structures in northern Qaidam basin. To resolve these problems we have acquired and interpreted more than 100 individual seismic refl ection pro-fi les across northern Qaidam basin. Our seismic interpretations were also assisted and verifi ed by drill-hole data. Below we select eight sections to illustrate the style, magnitude, and timing of Cenozoic deformation (Fig. 2). In order to con-struct true-scale geologic cross sections, we fi rst interpret seismic profi les in the time domains, which were then converted to depth sections by assigning known depth-dependent velocity distributions. An example of interpreted time sections is shown in Figure 12. All interpreted seismic profi les use a reference elevation of 2750 m (the lowest point of Qaidam basin) as the zero-km depth.

Seismic Profi le (1) (Fig. 13A)This section exhibits a growth-strata relation-

ship for the whole Cenozoic sequence, which thickens to the southwest toward the basin axis. The section also displays nested southwest-tapering, thrust-duplex wedges (triangle zones, a la Price, 1986). At the surface, the northeast-directed roof thrust zone correlates with the southwest-dipping Xiaosaibei thrust, whereas the south-directed imbricate zone below cor-relates with the northeast-dipping Sainan thrust zone. The oldest unit in the growth strata is the Paleocene to early Eocene Lulehe Formation (E1+2), suggesting the initiation of the triangle zone occurred at this time. The northward-taper-ing, growth-strata sequence (GS-1, Fig. 13A) is subsequently folded by a syncline, which was

in turn associated with a younger sequence of growth strata with an age range from the late Miocene to Pliocene (units N2–2 and N2–3 in GS-2, Fig. 13A). Palinspastic reconstruction of the section requires 26-km horizontal shortening with a shortening strain of ~34% (Fig. 13A).

Seismic Profi le (2) (Fig. 13B)This section has a similar structural style to

that in section (1), exhibiting a large triangle zone and an overlying growth-strata sequence. Again, as in Figure 13A, the northeast-directed passive-roof fault correlates with the Xiaosaibei thrust and the southwest-directed imbricate zone below correlates with the Sainan thrust zone. The northeast-tapering, growth-strata sequence is folded by two later synclines, each generating its own growth strata in the fold cores. The north-ern late syncline started to develop during depo-sition of unit E3-1 (GS-1, Fig. 13B), while the southern late syncline started to develop during deposition of unit N1 (GS-2, Fig. 13B). These temporal relationships again indicate southward propagation of shortening deformation across northern Qaidam basin. This interpreted defor-mation pattern is consistent with the observation that backthrusts from the southern part of the section cut and offset the thrusts in the northern part of the section (Fig. 13B). Palinspastic recon-struction of the section requires 19-km horizon-tal shortening and a shortening strain of ~36%.

Seismic Profi le (3) (Fig. 13C)This section shows a thrust-wedge duplex

(triangle-zone structure) in the north and two widely spaced, southwest-directed thrusts in the south. The fl oor thrust of the duplex is part of the Sainan thrust zone that is also imaged seis-mically in profi les (1) and (2) (Figs. 13A and 13B). The roof thrust is passive, linking with the northeast-directed Xiaosaibei thrust zone. As in sections (1) and (2), the entire Ceno-zoic sequence thickens southwestward directly above the thrust duplex, indicating its develop-ment started during the deposition of unit E1+2 and lasted until at least the late Oligocene dur-ing deposition of unit N-1. The southern thrusts were developed after the initiation of the tri-angle zone to the north, as their related folds deformed the southwest-thickening growth-strata sequence between unit E1+2 and N2–1. The younger southwest-directed fault-1 started to develop during deposition of unit N2–2 in the late to middle Miocene (Fig. 13C), again sug-gesting the deformation front migrated south-ward toward the basin interior. A small, south-dipping normal fault is present in the Jurassic strata, producing a small half graben. Palinspas-tic reconstruction of the section requires 26 km of shortening and a shortening strain of 29%.

Seismic Profi le (4) (Fig. 13D)This section crosses the Sainan thrust zone

and consists of geometrically linked bivergent thrust wedges stacking on top of one another. The northeast-directed thrusts in the profi le are parts of the passive-roof fault zone as imaged in profi les (1) to (3) (Figs. 13A–13C), but with a signifi cantly decreased magnitude of slip, only on the order of hundreds of meters versus greater than a few kilometers of slip as in the other sec-tions. Southwestward thickening of Cenozoic strata remains evident in the section and can be divided into two growth-strata sequences. The northern sequence (GS-1) includes units E1+2 to N2–2, whereas the southern sequence con-tains units E3–2 to N2–2, with unit E3–2 (early Oligocene) having the most prominent varia-tion in thickness. We interpret deposition of the GS-1 during motion on fault-2 and deposition of the GS-2 during motion on fault-1 in the Sainan thrust zone. The growth-strata relation-ships again suggest that the deformation front has migrated southwestward across northern Qaidam basin. Palinspastic reconstruction of the section requires 10-km horizontal shortening and a shortening strain of ~21%.

Seismic Profi le (5) (Fig. 13E)This section crosses the north-dipping Sainan

thrust zone and the lateral ramp segment of the south-dipping Lenghu-Lingjian fault zone. The two faults bound a blind pop-up structure below the Lenghu anticlinorium. The hanging wall of the Lenghu-Lingjian thrust also forms a north-east-tapering thrust wedge that is bounded by a top-southwest thrust lying mostly within unit N2–1. The Paleogene sequence in this section is thinner than that in sections (1) to (4). A syn-clinal trough is developed between the Sainan and Lenghu-Lingjian thrusts, with unit E3–1 as a prominent growth-strata sequence in their common footwall. This relationship indicates that motion on the Lenghu-Lingjian and Sainan thrusts started in the middle to late Eocene dur-ing deposition of unit E3–1. This age is slightly younger than the onset age of Cenozoic defor-mation in sections (1) to (4) in the Paleocene to early Eocene during deposition of unit E1+2. The synclinal folding is active as indicated by the presence of Holocene growth strata in its core. The section also shows that units E3–2 (early Oligocene) to N1 (late Oligocene) have constant thickness, whereas units N2–1 (early to middle Miocene) to Q (Quaternary) exhibit growth-strata geometry across the anticlinorium on its limbs. The latter relationship suggests that motion on the Lenghu-Lingjian fault zone ceased in the early Eocene but was later reac-tivated in the early Miocene, with a dormant period of ~25 m.y.!

Yin et al.


Res

tore

d se

ctio

n le

ngth

= 4

1 km

Fina

l sec

tion

leng

th =

25

kmTo

tal s

hort

enin

g =

16 k

mTo

tal s

hort

enin

g st

rain

= 3

9%

C

4 km

Q N2-

3

N2-

2

N2-

1

E3

N1-

2

E1-

2

N1-

1N

1-1

N1-

1

N2-

1

N1-

2

Res

tore

d se

ctio

n le

ngth

= 4

1 km

E3

Len

ghu

thru

st z

one

Palin

spas

tical

ly r

esto

red

sect

ion

Q N2-

3

N2-

2

N2-

1

N2-

1

N1-

2 N1-

2E

3

N1-

2

E1-

2N

1-1

02

4km

2 0 2 4 6 8 10 12 14km2 0 2 4 6 8 10 12 14km

AB

N1-

1

Len

ghu

thru

st z

one

B

N1-

1

N1-

1

A

B

N1-

1

N1-

2

N1-

1

N2-

2

QN

2-3

38°2

0′

N2-

1

93°2

0′

93°3

0′

38°2

0′

38°4

0′

02

4km

N1-

2N

1-1

80

11

71 7164

bedd

ing-

para

llel

shea

r zo

ne

37

3619

2120

19

8020

80

18

78

38°4

0′

Leng

huth

rust

zone

A

BA

Fig

ure

11. (

A) D

etai

led

geol

ogic

map

of t

he L

engh

u-6

area

. (B

) Geo

logi

c cr

oss

sect

ion.

See

Tab

le fo

r de

fi nit

ion

of C

enoz

oic

unit

s. (C

) Res

tore

d se

ctio

n.

North Qaidam Basin


Several north-dipping, normal-separation faults are present in the Jurassic strata or bound-ing basins containing Jurassic strata, with slip varying from a few hundreds of meters to ~2 km. Half grabens are developed in the hanging walls of the normal-separation faults. A major gliding horizon is present in the Jurassic strata, along which a thrust duplex is developed. Palinspas-tic reconstruction of the section requires 46-km horizontal shortening and a shortening strain of ~31%. Because section (5) passes through our mapped area shown in Figure 5, we compare the overlapping segments of the fi eld-based cross section (Fig. 5B) and seismically based cross section (Fig. 13E), respectively. It is clear that the fi eld-based cross section can only capture the structural geometry in the uppermost few kilometers of the section. It is also clear that the steep bed dips do not penetrate deep in the crustal section, because fold limbs in the deeper part of the seismic section have much shallower dips than those measured at the surface.

Seismic Profi le (6) (Fig. 13F)This section crosses a southwest-directed,

imbricate thrust zone, with fault-1 at the high-est structural level. In the fault-1 hanging wall unit, E1+2 is signifi cantly thicker in the core of a syncline that is unconformably overlain by unit E3–1. The syncline and unit E3–1 are both cut by fault-1. Across fault 1, unit E1+2 is thin-ner in its footwall than that in its hanging wall,

while unit E3–1 is thicker in the footwall and that in the hanging wall. The above relationships suggest that fault-1 was initiated after deposi-tion of unit E1+2 and during deposition of unit E3–1. In the footwall of fault-1, units E1+2 and E3–1 exhibit prominent thinning in the hang-ing walls of fault-2 and fault-3, suggesting that the faults were active during their deposition in the Paleocene and Eocene. The above rela-tionships suggest two sequential deformation events. First, a broad syncline was developed during deposition of unit E1+2 associated with motion on fault-2 and fault-3 below. Fault-1 was developed later during deposition of unit E3–1 as an out-of-sequence thrust. Continu-ous motion on fault-1 had created a northeast-tapering growth-strata sequence during deposi-tion of unit N2–2 in the southernmost part of the section (Fig. 13F). Palinspastic reconstruc-tion of the section requires 29-km shortening and a shortening strain of ~27%.

Seismic Profi les (7) and (8) (Figs. 13G and 13H)

Sections (7) and (8) record a southwest-ward younging deformation history along a north-south transect. Section (7) represents the southern transect, while section (8) represents the northern transect. Major structures in sec-tion (7) include a northeast-directed, fault-bend fold system and a south-directed, roof fault zone on top of the fault-bend fold to the north. Two

detachment horizons are developed in the Juras-sic and early Oligocene units (Jr and E3–2), along which fault-bend folds and imbricate thrusts are branched off. Only unit Q exhibits a growth-strata relationship, suggesting structural development in the Quaternary. Palinspastic reconstruction of section (7) (Fig. 13G) requires 50-km horizontal shortening and a shortening strain of ~58%.

In contrast to section (7), the initiation age of deformation in section (8) is signifi cantly older, and occurred in the early Oligocene as illustrated by thickness variation of unit E3–2 overlying a thrust wedge (Fig. 13H). Due to the presence of bedding-parallel faults in the Jurassic unit, our palinspastic reconstruction indicates that Juras-sic beds have been shortened for ~21 km (42% strain), while the top of unit E1+2 has been shortened for ~40 km (53% strain).

Although the style of deformation in section (8) in the central segment of the North Qaidam thrust system resembles that in sections (1) to (3) in the western segment of the thrust system, the ages of the two structures are signifi cantly different: the western triangle zone was initiated in the Paleocene-early Eocene, while the east-ern triangle zone was initiated in the early Oli-gocene. This age difference suggests eastward younging of initiation of deformation along the northern margin of Qaidam basin, which was coeval with the southward propagation of defor-mation across the same region.

GS

-1

GS

-2

Xiaosaibei thrust zone

Figure 12. Interpreted seismic profi le (1) in time domain. T0 to T6 are regionally correlative refl ectors; T0 at the top of the Shizigou Formation, T1 at the top of the Shangyoushashan Formation, T2 at the top of the Xiayousha-shan Formation, T3 at the top of the Shangganchaigou Formation, TR at the top of the Lulehe Formation, and T6 at the top of the Jurassic-Cretaceous strata. GS-1 and GS-2 are two growth-strata sequences associated with the development of the triangle zone and later synclinal folding. TWTT—two-way travel time.

Yin et al.


0 4 8 km

0

4

8

12

16

km

E3-1 E3-2N1

N2-1N2-2N2-3

Q

E1+2

pre-Jr

Jr

pre-Jr

pre-Jr

0

4

8

12

16

km

Present section length = 34 km Restored section length = 60 kmTotal shortening = 26 kmTotal shortening strain = 34%

GS-

1 GS-

2

Xiaosaibei thrust zone(passive roof thrust of SW-directed thrust duplex)

Sainan thrust zone

Section (1)

Tip of SW-taperingthrust wedge

E3-1

E3-2N1 N2-1 N2-2 N2-3 Q

E1+2

Jr

Sainan thrust zone

8 km

Restored section length = 60 km

Palinspastically restored section

NE A

0

4

8

12

16

km

0

4

8

12

16

km

E3-1E3-2N1

Jr

N2-1N2-2 N2-3Q

E1+2pre-Jr

0 4 8 km

Present section length = 34 kmRestored section length = 53 kmTotal shortening = 19 km Shortening strain = 36%

GS-

2 GS-1


Sainan thrust zone

Section (2)

pre-Jr

Jr

Jr

E3-1E3-2N1N2-1N2-2

QN2-3

E1+2

Palinspastically restored section

NE

8 km

Sainan thrust zone

Restored section length = 53 km

B

Figure 13. Interpreted seismic refl ection profi les; see Figure 2 for locations. (A) Upper fi gure, geologic cross section inter-preted from seismic profi le (1). Lower fi gure, restored cross sec-tion. (B) Upper fi gure, geologic cross section interpreted from seismic profi le (2). Lower fi gure, restored section. (Continued on following page.)

North Qaidam Basin


E3-

1E

3-2

N1

JrN2-

1N

2-2

N2-

3Q E

1+2

pre-

Jr

0 4 8 12 16km

0 4 8 12 16km

Pres

ent s

ectio

n le

ngth

= 6

5 km

Res

tore

d se

ctio

n le

ngth

= 9

1 km

Tota

l sho

rten

ing

= 26

km

st

rain

= 2

9 %

04

8km

Sain

an

thru

st z

one

Xia

osai

bei

thru

st z

one

Len

ghu

antic

linor

ium

Pass

ive

roof

fau

ltSW

-dir

ecte

d du

plex

Sec

tio

n (

3)

Tip

of

SW-t

aper

ing

wed

geT

ip o

f SW

-tap

erin

g w

edge

E3-

1E

3-2

N1

Jr

N2-

1N

2-2

N2-

3Q

E1+

2

pre-

Jr

Gro

wth

str

ata

rela

ted

to d

uple

x de

velo

pmen

tG

row

th s

trat

a re

late

d to

mot

ion

on f

ault-

1

E3-

1E

3-2

N1

Jr

N2-

1 E1+

2Jr

E1+

2E

3-1

E3-

2

Pal

insp

asti

cally

res

tore

d s

ecti

on

8km

Res

tore

d se

ctio

n le

ngth

= 9

1 km

NE

faul

t-1

faul

t-2

C

Fig

ure

13 (c

ontin

ued)

. (C

) Upp

er fi

gure

, geo

logi

c cr

oss

sect

ion

inte

rpre

ted

from

sei

smic

pro

fi le

(3).

Low

er fi

gure

, res

tore

d se

ctio

n.

Yin et al.


NE

E1+

2E

3-1

E3-

2N

1N

2-1

E1+

2E

3-1

04

8km

0 4 8 12km0 4 8 12km

Pres

ent s

ectio

n le

ngth

= 3

8 km

R

esto

red

sect

ion

leng

th =

48

kmTo

tal s

hort

enin

g =

10 k

mTo

tal s

hort

enin

g st

rain

= 2

1%

Sect

ion

(4)

Sain

an th

rust

zon

e

Xia

osai

bei

thru

st z

one

N2-

2

Tip

of

the

SW-t

aper

ing

wed

ge

Lin

ked

bive

rgen

t thr

ust w

edge

s.

The

sha

red

faul

t is

roof

thru

st to

th

e lo

wer

wed

ge b

ut f

loor

thru

st

to th

e up

per

wed

ge

Tip

of

the

NE

-tap

erin

gw

edge

Palin

spas

tical

ly r

esto

red

sect

ion

Res

tore

d be

d le

ngth

for

E1+

2 to

p =

48 k

m

Res

tore

d be

d le

ngth

for

E1+

2 ba

se =

46

km

8 km

E1+

2

E3-

2N

1N

2-1

N2-

2

E3-

1Jr

pre-

Jr

E1+

2E

3-2N

1N

2-1

E3-

1E

1+2

faul

t-1

GS-

1G

S-2

faul

t-2

N2-

2

Jr

pre-

Jr

D

Fig

ure

13 (

cont

inue

d).

(D) U

pper

fi g

ure,

geo

logi

c cr

oss

sect

ion

inte

r-pr

eted

from

sei

smic

pro

fi le

(4).

Low

er fi

gure

, res

tore

d se

ctio

n. (C

ontin

-ue

d on

follo

win

g pa

ge.)

North Qaidam Basin


Pres

ent s

ectio

n le

ngth

= 6

5 km

R

esto

red

sect

ion

leng

th =

103

km

Tota

l sho

rten

ing

= 29

km

Tota

l sho

rten

ing

stra

in =

27%

NE

JrN1

N2-

1

E1+

2E

3-1

0 4 8

12 16km

0 4 8 12 16km

JrJr

Jr

pre-

Jr

pre-

Jr

pre-

Jr

04

8km

E1+

2E

3-1

E3-

2

N2-

2

Sect

ion

(6)

E3-

2N

1

faul

t-1

Jr

Jr

N2-

1

E1+

2E

3-1

E3-

2N

1

faul

t-2

faul

t-2

faul

t-4

faul

t-5

faul

t-6

JrE

1+2

E3-

1E

3-2

N1

N2-

2N

2-1

Res

tore

d se

ctio

n le

ngth

= 1

03 k

m

(usi

ng th

e ba

se o

f Jr

)

F

Palin

spas

tical

ly r

esto

red

sect

ion

faul

t-6

faul

t-1

faul

t-2

faul

t-3

faul

t-4

faul

t-5

Fig

ure

13 (

cont

inue

d).

(E)

Upp

er fi

gur

e, g

eolo

gic

cros

s se

ctio

n in

terp

rete

d fr

om s

eism

ic p

rofi l

e (5

). H

W—

Han

ging

Wal

l; F

W—

Foo

twal

l. L

ower

fi g

ure,

res

tore

d se

ctio

n.

(F)

Upp

er g

eolo

gic

cros

s se

ctio

n in

terp

rete

d fr

om s

eism

ic p

rofi l

e (6

). L

ower

fi gu

re, r

esto

red

sect

ion.

(C

ontin

ued

on f

ollo

win

g pa

ge.)

Res

tore

d se

ctio

n le

ngth

= 1

49 k

m

Pass

ive

roof

fau

lt of

N

E-v

ergi

ng w

edge

Lin

ked

bive

rgen

t thr

ust w

edge

s

8 1240km

8 1240km

04

8km

Pres

ent s

ectio

n le

ngth

= 1

03 k

m

Res

tore

d se

ctio

n le

ngth

= 1

49 k

mTo

tal s

hort

enin

g =

46 k

mTo

tal s

hort

enin

g st

rain

= 3

1%

Jr

pre-

JrE

1+2

E3-

1E

3-2

N2-

3

N1

N2-

1

N2-

2

Q

Jr

E1+

2E

3-1

E3-

2L

engh

u-L

ingj

ian

thru

st z

one

Sect

ion

(5)

Sain

an th

rust

Len

ghu

antic

linor

ium

pop-

up s

truc

ture

w

ithin

wed

geTip

of

NE

-tap

erin

g w

edge

Tip

of

NE

-ver

ging

w

edge

Palin

spas

tical

ly r

esto

red

sect

ion

8 km

N2-

3Q

N2-

3

HW

thin

ning

and

FW

thic

keni

ng

of E

1+2

assp

coat

ed w

ith th

rust

ing

Fig.

5B

Len

ghu-

Lin

gjia

n th

rust

zon

e

Sain

an th

rust

Jr

N2-

3N

2-2

Q

JrE

1+2

E3-

1E

3-2

N1

N2-

3N

2-2

QN

2-3

N2-

1Q

N2-

1N

2-2

JrE

1+2

E3-

1E

3-2

N1

N2-

3 E1+

2E

3-1

E3-

2

N1

N2-

1N

2-2

NE

E

N2-

1N

2-1

Yin et al.


NE

E3-

1E

3-2

N1

JrN2-

1N

2-2

N2-

3

Q E1+

2

0 4 8

12 16km

04

8km

0 4 8 12 16km

E3-

1

JrE1+

2Jr

E1+

2

JrE

1+2

Jr

E1+

2E

3-2

E3-

1

Pres

ent s

ectio

n le

ngth

= 3

6 km

Res

tore

d se

ctio

n le

ngth

= 8

6 km

Tota

l sho

rten

ing

= 50

km

Tota

l Sho

rten

ing

stra

in =

58%

Sect

ion

(7)

pre-

Jr

E3-

1

E3-

1E

3-2

N1

Jr

N2-

1N

2-2

N2-

3Q

E1+

2

JrJr JrE1+

2

E1+

2E

3-1

Ext

rapo

late

d be

d le

ngth

to p

rodu

ce r

equi

red

foot

wal

l ram

pE

xtra

pola

ted

bed

leng

th to

pro

duce

req

uire

d fo

otw

all r

amp

Res

tore

d se

ctio

n le

ngth

= 8

6 km

8 km

Palin

spat

ical

ly r

esto

red

sect

ion

G

Fig

ure

13 (c

ontin

ued)

. (G

) Upp

er fi

gure

, geo

logi

c cr

oss

sect

ion

inte

rpre

ted

from

sei

smic

pro

fi le

(7).

Low

er fi

gure

, res

tore

d se

ctio

n. (C

ontin

ued

on fo

llow

ing

page

.)

North Qaidam Basin


(1)

Res

ults

of

line

bala

ncin

g of

the

base

of

Jura

ssic

uni

t:Pr

esen

t sec

tion

leng

th =

28

kmR

esto

red

line

leng

th =

49

kmTo

tal s

hort

enin

g =

21 k

mTo

tal s

hort

enin

g st

rain

= 4

2%

0 4 8

12 16km

0 4 8 12 16km

04

8km

E3-

1

E3-

2N

1

Jr

N2-

1

N2-

2

N2-

3

Q

E1+

2 pre-

Jr

post

-fol

ding

thru

st(1

)(2

)(3

)(4

)

(5)

(6)

(7)

(8)

(9)

(10) (1

1)(1

2)

(13)

(14)

(2)

Res

ults

of

line

bala

nce

of

the

top

of u

nit E

1+2:

Pres

ent s

ectio

n le

ngth

= 2

8 km

Res

tore

d lin

e le

ngth

= 6

8 km

Tota

l sho

rten

ing

= 40

km

.To

tal s

hort

enin

g st

rain

= 5

3%

Sect

ion

(8)

E3-

1E

3-2

N1

Jr

N2-

1N

2-2

N2-

3

Q

E1+

2

Ext

ropo

late

E1+

2 to

p by

th

e le

ngth

of

HW

ram

p

Res

tore

d lin

e le

ngth

of

Jr b

ase

= 49

km

Res

tore

d lin

e le

ngth

of

E1+

2 to

p =

68

km

8 km

Palin

spas

tical

ly r

esto

red

cros

s se

ctio

n(1

)(2

)(3

)

(4)

(5)

(6)

(13)

(12)

(11)

(10)

(9)

(8)

(7)

(14)

(6)

(8)

N1

E3-

2

E3-

1

Thi

nnin

g of

E3-

2 ov

er a

ntif

orm

al d

uple

x

NE

App

roxi

mat

e ov

erla

p re

gion

with

Fig

. 8B

H

Fig

ure

13 (c

ontin

ued)

. (H

) Upp

er fi

gure

, geo

logi

c cr

oss

sect

ion

inte

rpre

ted

from

sei

smic

pro

fi le

(8).

Low

er fi

gure

, res

tore

d se

ctio

n. H

W—

Han

ging

Wal

l.

Yin et al.


Because sections (7) and (8) cross the western part of our mapped area shown in Figure 8, we compare the overlapping segments of the cross sections constructed based on surface geology and seismic interpretations, respectively. In gen-eral, the style of deformation in the two cross sections is quite similar. However, the fi eld-based cross section could not detect the complexly deformed thrust duplexes with multiple levels of bedding-parallel detachment horizons. Because of the presence of the bedding-parallel faults, the estimated shortening strain from surface geol-ogy (~23%) is much less than those estimated from seismic profi les (~53%–58%). We also note that fold amplitudes are much smaller and fold hinge zones are much broader in the seismic profi le than those extrapolated from the surface geology guided by the dip-domain method.

KINEMATIC RECONSTRUCTION OF THE NORTH QAIDAM THRUST SYSTEM

Map-View Reconstruction

The growth-strata relationships in northern Qaidam basin indicate that Cenozoic deforma-tion started fi rst in the west against the left-slip Altyn Tagh fault and has subsequently propa-gated southward and eastward, respectively. Cenozoic deformation was initiated in the Paleo-cene and early Eocene in seismic sections (1) to (4) (Figs. 13A–13D), in the middle to late Eocene in seismic section (5) (Fig. 13E), and in the early Oligocene in section (8) (Fig. 13H). These observations yield an eastward-lengthening rate of ~8 km/m.y. for the North Qaidam thrust sys-tem. The southward propagation of deformation is best exemplifi ed by the formation of the Len-ghu anticlinorium currently lying within Qaidam basin. In contrast to southward propagation of deformation, out-of-sequence thrusting is docu-mented in northern Qaidam basin as observed in seismic section (6) (Fig. 13F). In addition, some thrusts could have an extended dormant period (>20 m.y.) between two active periods (e.g., the Lenghu-Lingjian fault in Fig. 13E). These rela-tionships suggest complex spatial and temporal evolution of contractional deformation across northern Qaidam basin.

Based on the above relationships, we recon-struct the development of the North Qaidam thrust system and associated sedimentation in Figure 14. To simplify the regional geology, we only consider major Cenozoic thrust zones dis-cussed above (Figs. 2 and 14A). In the Paleocene and early Eocene, the Dasaibei thrust, Xiaosai-bei thrust, and Qaidam Shan thrust zone started to develop (Fig. 14B). This was associated with the development of a southwest-tapering thrust triangle zone along the northern margin of Qai-

dam basin and sedimentation of a southwest-ward thickening, growth-strata sequence across northern Qaidam basin.

The age of the Qaidam Shan thrust zone can-not be constrained from our own study. How-ever, apatite fi ssion-track work of Jolivet et al. (2001) provides the constraint, which shows that Luliang Shan gneiss in the footwall of the Qai-dam Shan thrust experienced a reheating event at ca. 50 Ma (early to middle Eocene). These authors attributed the Eocene thermal event to deep burial either related to deposition of a thick Cenozoic sequence or thrusting of a thick hang-ing-wall section from above.

Farther to the southeast, the Sainan and West-ern Luliang Shan thrusts were initiated in the middle to late Eocene that were associated with deposition of unit E3–1 (Fig. 14C). This infer-ence is based on the growth-strata and crosscut-ting relationships in the footwall of the Sainan thrust zone (Fig. 13E), although some of the thrusts in the Sainan thrust duplex to the north-east could have initiated earlier in the Paleocene and early Eocene. In the early Oligocene, the Eastern Luliang and Xitie Shan thrusts started to develop, during which unit E3–2 was depos-ited (Fig. 14D). This age inference is based on the growth-strata relationship in seismic profi le (8) (Fig. 13H).

Between the late Oligocene and middle Mio-cene, deformation was mainly expressed by the development of the Lenghu anticlinorium as a result of southward propagation of deforma-tion across northern Qaidam basin (Fig. 14E). From the late Miocene to Pliocene, the Aimunik-Olongbulak thrust system started to develop (Fig. 14F), producing growth strata along the southern fl ank of the Aimunik Shan (Fig. 4G). Southeastward expansion of Qaidam basin at this time is indicated by the unconformable relation-ship between the Neogene and Paleogene strata in the Xitie Shan in the west and the Neogene and pre-Cenozoic strata in the Aimunik Shan in the east (cf. Figs. 4G and 8). The eastward basin expansion at this time is also indicated by a rapid increase in sedimentation rate over the whole Qaidam basin (Huang et al., 1996; Métivier et al., 1998). Also during the late Miocene and Pliocene, contractional deformation propagated southward into the basin interior and created blind thrusts and related folds (Fig. 14F).

In the above reconstructions (Fig. 14), we hypothesize the presence of a series of south-fl owing rivers carrying sediments from the Qil-ian Shan to the interior of Qaidam basin. The sediments were deposited in depocenters along the axis of the basin, which were deepened rela-tively to its surrounding mountain ranges due to the progressive development of the triangle zone along the basin margin. Also, the eastward

expansion of the North Qaidam thrust system was caused by sequential initiation of south-fl owing river systems from the west to the east. This sce-nario explains both the eastward propagation of thrust deformation and the eastward expan-sion of Qaidam basin. We also envision that the south-fl owing rivers were terminated into large playa, which is consistent with the fi ne-grained lacustrine deposition in central Qaidam basin since the early Oligocene (Huang et al., 1996; Zhang, 1997). The presence of the Oligocene to present lacustrine deposits in central Qaidam is inconsistent with the existence of a large longitu-dinal river as suggested by Wang et al. (2006).

Cross-Section Reconstruction

To better illustrate the structural evolution of the thrust triangle zone in northern Qaidam basin, we kinematically reconstructed seis-mic profi les (1) and (8) (Figs. 15A and 15B). For the development of profi le (1), the triangle zone began to develop in the Paleocene to late Eocene (stage 2 in Fig. 15A). Its development produced a southwest-thickening, growth-strata sequence above a passive-roof duplex (stage 3 in Fig. 15A). This event was followed by continuous shortening of the duplex system from the late Oligocene to Pliocene, produc-ing growth strata above (stage 4 in Fig. 15A). Finally, the older southwest-thickening, growth-strata sequence is folded. The later folding event created a younger Pliocene-Quaternary growth-strata sequence (stage 5 in Fig. 15A).

The kinematic reconstruction of seismic pro-fi le (8) (Fig. 13H) is shown in Figure 15B. The fault numbers in the reconstruction correspond to those shown in Figure 13H. From the growth-strata relationships, we interpret that the initiation of Cenozoic deformation occurred in the early Oligocene, which is signifi cantly younger than the onset age of deformation in section (1) at the western end of the North Qaidam thrust sys-tem. The earliest Cenozoic deformation for sec-tion (8) was expressed by the development of a southwest-directed thrust duplex associated with deposition of an early Oligocene sequence above (stage 2 in Fig. 15A). This duplex system contin-ued to develop in the Miocene (stages 2 and 3 in Fig. 15A) and was associated with the develop-ment of a break-back thrust (stage 4 in Fig. 15A). The fi nal deformation was expressed by motion on two post-folding thrusts at the northern and southern ends of the section in the Pliocene and Quaternary (stages 5–7 in Fig. 15B).

DISCUSSION

In map view, our study suggests that many thrusts in the southern Qilian Shan-Nan Shan

North Qaidam Basin


Daqaidam

50 km

Qaidam thrust

W. Luliang thrust

Altyn Tagh fault system

Sainan thrust

E3-1

Dasaibei thrust

Xiaosaibei thrust

CMiddle to late Eocene (development of Sainan and western Luliang thrusts)

Daqaidam

50 km

Qaidam thrust


E1+2

Dasaibei thrustXiaosaibei thrust

BPaleocene to early Eocene (development of Saibei, Lenghu, and Qaidam thrusts)

Daqaidam

50 kmDasaibei thrust

Qaidam thrust

E. Luliang thrust

W. Luliang thrust

Xitie Shan thrust

Aimunike thrust

Olongbulak

thrust


Xiaosaibei thrust

Sainan thrust

APresent configuration of North Qaidam thrust system

Figure 14. Tectonic evolution of the North Qaidam thrust system and depositional history of northern Qaidam basin. (A) Present tectonic con-fi guration of the North Qaidam thrust belt. (B) Paleocene-early Eocene tectonic confi guration of North Qaidam. (C) Middle to late Eocene tec-tonic confi guration of North Qaidam. (D) Early Oligocene tectonic confi guration of North Qaidam. (E) Late Oligocene to middle Miocene tec-tonic confi guration of North Qaidam. (F) Late Miocene to Pliocene tectonic confi guration of North Qaidam. (Continued on following page.)

Yin et al.


Daqaidam

50 km

Qaidam thrust

E. Luliang thrust

W. Luliang thrust

Xitie Shan thrust

Aimunike thrust

Olongbulak

thrust


Sainan thrust

N2-3

Lenghu anticlinorium


FLate Miocene to Pliocene (development of Aimunik-Olongbulak thrust system and folds in central Qaidam)

N1 and N2-1

Daqaidam

50 km

Qaidam thrust

E. Luliang thrust

W. Luliang thrust

Xitie Shan thrust


Sainan thrustLenghu anticlinorium


Future drainage system

ELate Oligocene to middle Miocene (development of Lenghu anticlinorium)

Daqaidam

50 km

Qaidam thrust

E. Luliang thrust

W. Luliang thrust


Sainan thrust

E3-2

Xitie Shan thrust


Future drainage systemEartly Oligocene (development of Eastern Luliang thrust and Xitie Shan thrust)

D

Figure 14 (continued)

North Qaidam Basin


Stage 1: Jurassic

pre-Jr

Jr

Stage 2: Paleocene to late Eocene (E1+2 to E3-1)

pre-Jr

JrE1+2

E3-1

Stage 3: Early Oligocene (E3-2)

E3-2

pre-Jr

JrE1+2E3-1

Stage 4: Late Oligocene to Miocene (N1-N2-2)

N1-N2-2

pre-Jr

JrE1+2

E3-2

E3-1

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Section shown in Fig. 13A

A

Figure 15. (A) Kinematic reconstruction of structural section shown in Figure 13A. See text for detailed description. (Continued on following page.)

Yin et al.


E3-

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North Qaidam Basin


thrust belt and northern Qaidam basin exhibit L-shaped geometry, with the northwest-striking thrusts terminating in the west against the Altyn Tagh fault and in the east along east-striking, left-slip lateral ramps (Fig. 2). Because the lat-eral ramps are subparallel to the left-slip Altyn Tagh fault, their development may result from distributed left-slip deformation that transfers motion from the Altyn Tagh fault to the left-slip ramps via the linking thrusts. Such a mode of distributed left-slip deformation has not been recognized previously across northern Tibet.

In cross-section view, our interpreted seismic refl ection profi les reveal complex structural associations in northern Qaidam basin that have accommodated a signifi cant amount of crustal shortening (20% to >60%). The crustal short-ening has been partitioned vertically through multi-story thrust wedges and duplex systems detached from weak stratigraphic horizons. This mode of deformation was anticipated by insightful analog experiments performed by Burg et al. (1994), which simulated possible crustal thickening processes during the devel-opment of the Tibetan plateau. Specifi cally, their experiments predict the development of bivergent thrust wedges that are common in the North Qaidam thrust system. Additionally, their experiments also predict nearly synchro-nous deformation across Tibet at the initial stage of the Indo-Asian collision, which is consistent with our observations.

Our geologic observations from northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt also have important impli-cations for how the Qaidam basin has devel-oped and its relationship to the overall growth history of the Tibetan plateau. First, the discov-ery of a large triangle zone above a passive-roof duplex along the northern edge of Qaidam basin (Fig. 13) explains the observation that Ceno-zoic depocenters have been located consistently within rather than along the edge of the basin (Huang et al., 1996; Wang et al., 2006). This fi nding in a broad sense supports the early pro-posal by Bally et al. (1986) that Qaidam basin was created by the development of a large synclinorium rather than in a classic foreland basin setting (e.g., Jordan, 1981). The cause of the synclinorium was probably not induced by buckling under simple horizontal compression alone as envisioned by Burg et al. (1994). The fold development and its lateral expansion must have been accomplished by the development of multiple duplex systems at deeper crustal levels along the margins of Qaidam basin (also see Yin et al., 2008a). A well-documented example of a large synclinorium produced by duplex development is the folded Lewis thrust sheet in the southern Canadian Rocky Mountains and

western Montana of the United States (Price, 1981, 1986; Yin and Kelty, 1991).

Our study along the northern margin of the Qaidam basin cannot rule out the proposal that the basin originated from a trapping event due to a sudden northward jump of the northern plateau margin, because we do not have a solid constraint on the timing of structural develop-ment along the southern margin of Qaidam basin from this study (but see Yin et al., 2008b for discussion of this issue). However, if the trapping event did happen as the result of a jump of deformation front, it could not have developed in the Miocene-Pliocene as inferred by Meyer et al. (1998) and Métivier et al. (1998); such an age is too young to be consis-tent with the Paleocene and early Eocene ini-tiation of the southern Qilian Shan-Nan Shan thrust belt as revealed by this study.

Our structural and sedimentological obser-vations from northern Qaidam basin are con-sistent with the early suggestion that the basin has expanded eastward in the Cenozoic (Huang et al., 1996). Wang et al. (2006) suggested that the eastward expansion was induced by lateral extrusion of the northern Tibetan plateau and the development of an east-fl owing longitudinal river system (the paleo-Kunlun River) along the axis of Qaidam basin. Specifi cally, their work in the southwestern part of Qaidam basin indi-cates that the eastward migration of the Qaidam depocenters between ca. 31 and 4 Ma was asso-ciated with thrusting and folding of the Ceno-zoic strata. This inference is broadly consistent with our observations across northern Qaidam basin, where eastward expansion of sedimenta-tion corresponds temporally and spatially with nearby contractional structures. However, the presence of Oligocene to present lacustrine deposits in central Qaidam basin (Huang et al., 1996; Zhang, 1997) is not consistent with the existence of a paleo-longitudinal river along the basin axis as inferred by Wang et al. (2006).

The early Paleogene initiation of crustal shortening in northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt is inconsistent with the prediction of the thin-vis-cous-sheet model, at least in its original form, which assumes uniform material properties across the whole Tibetan plateau (England and Houseman, 1986). This model predicts progres-sive and continuous northward enlargement of the Tibetan plateau and implies crustal shorten-ing in the Qilian Shan-Nan Shan region to have started in the last stage, rather than at the onset, of the Indo-Asian collision. A revised thin-vis-cous-sheet model considering heterogeneous mechanical strength, preexisting weakness, and preexisting topography in Asian lithosphere prior to the Indo-Asian collision better explains

the complex Cenozoic deformation history across Tibet and Asia in general (e.g., Neil and Houseman, 1997; Kong et al., 1997). Paleocene-early Eocene initiation of contractional defor-mation in northern Qaidam basin is consistent with other studies that indicate Eocene initia-tion of deformation along the present northern margin of the Tibetan plateau (Yin et al., 2002; Dupont-Nivet et al., 2004; Horton et al., 2004). Together, these studies require rapid and effi -cient stress transfer from the convergent front of the Indo-Asian collision zone in the south to the interior of the Asian continent >1000 km in the north during the early stage of the continental collision (e.g., Burg et al., 1994).

Although it has long been recognized that the Cenozoic contractional structures in the Qilian Shan-Nan Shan thrust belt are geometrically and kinematically linked with the Altyn Tagh fault (Bally et al., 1986; Burchfi el et al., 1989; Tapponnier et al., 1990), their temporal rela-tionships are only locally known and mostly along the Altyn Tagh fault zone (e.g., Bally et al., 1986; Wang, 1997; Hanson, 1998; Yin et al., 2002; Wang et al., 2006). The results of our study provide new insight into this issue. In the previous studies, several authors have inferred that individual rows of northwest-trending thrust systems in the Qilian Shan-Nan Shan thrust belt were initiated simultaneously along strike, with the thrust front migrating progres-sively to the north (e.g., Wang, 1997; Meyer et al., 1998; Yin et al., 2002). The observations from this study suggest that it may take tens of millions of years for individual rows of thrust systems to propagate laterally from the western end to the eastern end of the Qilian Shan-Nan Shan thrust belt. Our observations across north-ern Qaidam basin also imply that the growth of the Tibetan plateau was closely related to motion on the Altyn Tagh fault; that is, slip on the fault has driven crustal shortening across northern Tibet (Burchfi el et al., 1989; Tappon-nier et al., 1990; Meyer et al., 1998).

Determining the magnitude of Cenozoic shortening is a key to evaluating whether lower-crustal fl ow or thermal events in the mantle were responsible for the uplift of the Tibetan plateau (Clark and Royden, 2000; Molnar et al., 1993). Among the thirteen structural sections we examined, fi ve record shortening strain >50%, fi ve record strain between 30% and 39%, and three record strain between 20% and 29%. If the entire crust has been deformed by pure-shear shortening in the Cenozoic, our estimated short-ening strain on average is more than suffi cient to explain the present elevation (3000–3500 m) and crustal thickness (45–50 km) of the north-ern Qaidam region, because thickening a crust from 35 km to 45–50 km requires a shortening

Yin et al.


strain of 29%–42%. If Cenozoic deformation in the upper and lower crust was decoupled by a detachment (Burchfi el et al., 1989), then the overall Cenozoic shortening strain may be partitioned vertically in a more complex way. For example, it is possible that our observed upper crustal shortening represents pure-shear contraction above the inferred mid-crustal detachment; meanwhile the lower crust of Qai-dam basin has undergone a combined pure- and simple-shear deformation (i.e., general shear) accommodating both crustal thickening and northeastward subduction of the Qaidam mantle lithosphere below the Qilian Shan-Nan Shan thrust belt (Fig. 16). Testing this more complex scenario requires the knowledge of lithospheric-scale structures across the region, which is clearly beyond the scope of this study.

As mentioned above, the magnitude of Ceno-zoic rotation varies spatially across Qaidam basin (Dupont-Nivet et al., 2002; Chen et al., 2002; Halim et al., 2003; Sun et al., 2006). Our observed inhomogeneous strain distribution across northern Qaidam basin, rapid lateral ter-mination of contractional structures from thrusts to folds, and the presence of lateral ramps and

younger strike-slip faults could all help explain these confl icting results. It will be a fruitful research area in the future to quantitatively cor-relate the timing and magnitude of local Ceno-zoic rotation with the evolution of individual structures in cross-section and map views.

Although it was not the main emphasis of this study, seismic profi les have also provided new insights into the Jurassic and Cretaceous tectonic setting across the Qaidam region. The Qaidam region in the Jurassic has been regarded as a single, large foreland basin in front of a north-directed thrust belt in the Eastern Kunlun Shan to the south (Ritts and Biffi , 2000). The single-basin conceptual model was used as a guide to constrain the magnitude of offset across the Altyn Tagh fault by matching possible lake shorelines (Ritts and Biffi , 2000). As indicated by surface mapping and the analysis of seismic data, the central and northern Tibetan plateau (including the Qaidam basin) in the Jurassic and Cretaceous had experienced several phases of extension (Huo and Tan, 1995; Huang et al., 1996; Sobel, 1999; Vincent and Allen, 1999; Kapp et al., 2000, 2003; Xia et al., 2001; Chen et al., 2003; Horton et al., 2004). Specifi cally,

the Qaidam region in Jurassic time consists of several isolated basins (Huang et al., 1996; Xia et al., 2001), which would make the correlation of lake shorelines highly nonunique across the Cenozoic Altyn Tagh fault (i.e., slip across the Altyn Tagh fault could be signifi cantly larger or smaller than that estimated by Ritts and Biffi , 2000). This uncertainty should be kept in mind in any tectonic reconstruction of the Tibetan pla-teau in the future.

CONCLUSIONS

New structural observations from the south-ern Qilian Shan-Nan Shan thrust belt and north-ern Qaidam basin allow us to evaluate the timing and magnitude of Cenozoic deformation. In this study, we show that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contrac-tional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65–50 Ma) during or immedi-ately after the onset of the Indo-Asian collision. This fi nding implies that stress was transferred rapidly and effectively across Tibetan litho-sphere to its northern margin over a distance of

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+ = Simple shear Pure shear General shear

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Figure 16. Schematic regional geologic cross section across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam; see Figure 2 for location. The geology of northern Qaidam basin is based on interpretation of seismic refl ection profi les. Lower crust in northern Qaidam basin may have undergone general shear deformation, accommodating both vertical crustal thickening and top-southwest simple shear.

North Qaidam Basin


>1000 km from the Indo-Asian convergent front to the south. The North Qaidam thrust system is dominated by southwest-directed thrusts with a southwest-tapering thrust wedge (i.e., the triangle zone). The recognition of the tri-angle zone and its longevity in northern Qaidam basin explain a long puzzling observation that Cenozoic depocenters have been located consis-tently along the central axis of the basin, which is opposite to the prediction of a classic fore-land-basin model requiring the thickest part of foreland sediments to lie along the basin edges directly against the bounding thrust belt. Restor-ing balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that the Cenozoic short-ening strain is highly inhomogeneous, varying from ~20% to >60%. However, the averaged shortening strain across northern Qaidam basin is suffi cient to explain the Cenozoic shortening strain of ~20% to >60%, which also implies that no lower-crustal injection or thermal event in the mantle are needed to explain the current elevation (~3000–3500 m) and crustal thickness (45–50 km) of the region. The spatial variable strain also helps explain the confl icting paleo-magnetic results indicating different magnitudes of Cenozoic rotation across Qaidam basin.

ACKNOWLEDGMENTS

We thank J.-P. Burg and Ray Price for their excel-lent reviews. Ray Price’s insightful comments on thrust geometry and cross-section balancing led to signifi cant revision of the original text. We also thank Associate Editor Laurent Godin and Science Editor Brendan Murphy for their constructive suggestions. Their comments have helped improve greatly the original manuscript. An Yin’s research in Tibet was supported by the U.S. National Science Foundation. Research on this project would not be possible with-out the support of the Qinghai Oilfi eld Company of PetroChina and the China University of Geosciences (Beijing). The research was mostly conducted while the fi rst author was a summer visiting professor at the China University of Geosciences, which was sponsored by the Chinese Ministry of Education via a Changjiang Visiting Professor Fellowship.

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