crustal dynamics in northern chile

7/28/2019 Crustal Dynamics in Northern Chile

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Tectonophysics 321 ( 2000) 297325www.elsevier.com/locate/tecto

Crustal dynamics and active fault mechanics during

subduction erosion. Application of frictional wedge

analysis on to the North Chilean Forearc

J. Adam *, 1, C.-D. Reuther

Geologisch-Palaontologisches Institut und Museum, Universitat Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany

Received 23 August 1999; accepted for publication 13 February 2000

Abstract

The forearc region of the non-accreting South American Plate margin in northern Chile is characterised by

subduction erosion and regional uplift. Neotectonic deformation structures reflect simultaneous extensional and

contractional fault kinematics. In the outer forearc, where the brittle crust directly overlies the subducting Nazca

Plate, the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as

seen in neotectonic surface structures and seismic data. In the inner forearc, surface structures indicate a compressional

stress regime also affecting the western rim of the magmatic arc. This stress regime is limited to a brittle crustal wedge

segment which overlies the ductile part of the inner forearc lithosphere. The shape of the two brittle forearc wedges

at the leading edge of the South American Plate is controlled by exogenetic surface processes, internal deformation

processes, contemporaneous basal tectonic erosion and underplating. Mechanical parameter sets controlling andreflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated

and applied to general frictional wedge models. The states of stress within the crustal wedges are controlled by spatial

variations of the basal mechanical parameters in the down-dip direction of the forearc wedge base. The new models

illustrate the fundamental kinematics and dynamic processes of Coulomb-type basal tectonic erosion and mass

transfer modes along active non-accretive convergent margins. The frictional wedge models explain the dynamics of

the simultaneous and contrary deformation processes affecting the forearc crust at the North Chilean convergent

margin shaped by active basal tectonic erosion. 2000 Elsevier Science B.V. All rights reserved.

Keywords: basal tectonic erosion; forearc wedge dynamics; frictional wedge analysis; mass transfer; neotectonics; non-accretive margin

1. Introduction et al., 1977; Huene and Lallemand, 1990; Huene

and Scholl, 1991). Subduction erosion of the conti-nental crust at the leading edge of the SouthThe North Chilean segment of the SouthAmerican Plate was first considered by RutlandAmerican Plate boundary is a non-accretive(1971) to explain the present position of the extinctmargin characterised by subduction erosion ( KulmMesozoic magmatic arc along the Chilean coast.Previous magmatic arcs from the Jurassic and

* Corresponding author. Tel.: +49-331-288-1317;Cretaceous have been situated in the present fore-

fax: +49-331-288-1370.arc setting since Oligocene times. The complexE-mail address: [email protected] (J. Adam)configuration of the active margin and structure1 Present address: GeoForschungsZentrum Potsdam,

Telegrafenberg, D-14473 Potsdam, Germany. of the forearc lithosphere are consequences of

0040-1951/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 0 - 1 9 5 1 ( 0 0 ) 0 0 0 7 4 - 3


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298 J. Adam, C.-D. Reuther /Tectonophysics 321 (2000) 297325

about 200 km eastward migration of the magmatic (Delouis et al., 1996). The inner forearc domain

is compressional (Jolley et al., 1990; Buddin et al.,arc and the related forearc/backarc system since

Jurassic times (Scheuber and Reutter, 1992; 1993). Each geodynamic model is faced with the

problem of the contrasting and contemporaneousScheuber et al., 1994). Thus, neotectonic faults

and crustal discontinuities are strongly influenced fault kinematics. According to Armijo and Thiele(1990 ), the formation of extensional surface struc-by pre-existing structures.

The present forearc is subdivided into an inner tures in the outer forearc region, e.g. on the

Mejillones Peninsula is related to the Coastaland an outer domain separated by the NS-trend-

ing Atacama fault system. Neotectonic surface and Escarpment (Fig. 1). They interpret this asymmet-

ric west-dipping normal fault system of crustaloffshore structures are extensional in the outer

forearc (Buddin et al., 1993; Reichert and CINCA scale as a secondary effect of a dip variation at

the subduction interface. This bend within theStudy Group, 1996). However, within the lower

part of the outer forearc lithosphere, seismological subducting plate, however, was not confirmed by

later geophysical data (Comte et al., 1992).investigations indicate compressional deformation

Fig. 1. Main structural features and upper crustal stresses deduced from neotectonic faults in the North Chilean forearc region

between 22S and 24S (location of Rio Salado area=RS, Talabre Thrust=TT, Quebrada Diabolo=QD).


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299J. Adam, C.-D. Reuther /Tectonophysics 321 (2000) 297325

Wdowinski et al. ( 1989) and Wdowinski and upper plate probable causes of frontal erosion

are (a) the subduction of an oceanic plate with anOConnell (1991) attributed the extension in the

forearc to the shear forces acting along the base intense topography, e.g. fault scarps, horst-and-

graben structures, seamounts and ridges, and (b)of the lithospheric wedge and derive the deforma-

tion processes within the forearc from a flow model the fundamental reconfiguration of the margin dueto a variation of accretionary processes throughon a plate-tectonic scale. In this model, Wdowinski

and OConnell (1991) assume that the apex of the time. An inferred mechanism for basal erosion is

the upward migration of water into fractures alongasthenospheric wedge and the edge of the litho-

sphere are at the same horizontal position, which the base of the upper plate (Marauchi and Ludwig,

1980; Lallemand et al., 1994). Becoming overpres-they identify as the trench. But their model does

not separate the brittle from the ductile part of sured, the fluids induce hydrofracturing that soft-

ens the base of the upper plate producing claststhe forearc crust and does not cover the region

between the trench and the tip of the asthenosph- and slivers forming a melange with subducting

sediments. Along the North Chilean non-accretiveeric wedge, which is the main focus of our investi-

gations on a more regional scale. margin, subduction erosion directly affects the

framework rock at the toe of the upper plate. TheTo investigate the mechanics of basal tectonicerosion and recent deformation processes between strong trench retreat ( 200 km since the Jurassic;

Scheuber et al., 1994) implies that sediment accre-the non-accreting North Chilean margin and the

magmatic arc, we extended the critical taper analy- tion and accretionary wedge formation played no

or only a minor role in the configuration of thissis on to framework forearc-wedge blocks

(Reuther and Adam, 1996, 1997, 1998). We apply segment of the plate margin.

the critical taper analysis (e.g., Coulomb-wedge

modelling), originally developed to analyse the Crustal wedge dynamics

mechanics of accretionary wedges and fold-and-

thrust belts, on to the brittle continental crust of In general crustal wedges (accretionary wedges,

orogenic wedges and fold-and-thrust wedges)an entire non-accretive forearc prism. The

frictional wedge models demonstrate how spatial develop in convergent settings by compressional

deformation of rock material until they exceed thevariations of the basal mechanical parameters indown-dip direction of the wedge base control the shear strength along a basal detachment. To exceed

the basal shear strength and to initiate basal dis-contrasting crustal stresses and neotectonic defor-

mation processes. We develop a general model that placement, the wedge has to attain a critical taper

(Chapple, 1978; Davis and Suppe, 1980; Davisexplains the mechanics and mass transfer modes

of high-friction basal subduction erosion along a et al., 1983; Dahlen, 1984; Dahlen et al., 1984;

Dahlen and Suppe, 1988). In the critically taperednon-accretive active margin.

wedge the basal shear stresses are balanced by

wedge internal stresses on the verge of shear fail-

ure. The crustal stresses are generated by gravita-2. General concepts

tional forces caused by the topographic gradient,

plate-tectonic forces resulting from frictional resis-Subduction erosiontance at the subduction interface and/or from

gravitational forces generated by orogenic features.Subduction erosion has been recognised in vari-

ous regions along the convergent margins of the The shape of a crustal wedge is controlled by

various factors such as frontal or basal accretionPacific (e.g., Scholl et al., 1977, 1980; Karig et al.,

1983; Huene et al., 1985; Huene and Culotta, 1989; of rock material, internal deformation, sedimenta-

tion, surface erosion and tectonic erosion. ALallemand and Le Pichon, 1987). Huene and

Lallemand (1990) distinguish between frontal sub- change of one or more of these factors generates

internal deformation of the wedge caused byduction erosion at the toe of the upper plate and

basal subduction erosion along the base of the internal stress release to regain or to maintain


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stability. The mode of stress release and strain forces resulting from plateau build-up act on thebounding pro-wedge and retro-wedge systemsaccommodation is governed by the depth-depen-(Willett et al., 1993). The pro-wedge is alsodent rheology within the wedge. Upper crustalaffected by plate-tectonic stresses transmitted fromwedge systems like accretionary prisms and fold-

the subducting Nazca Plate on to the overridingand-thrust belts are formed under brittle condi-South American Plate due to high frictional resis-tions and exhibit a plastic or Coulomb-type rheol-tance between the plates (Fig. 2) . For northernogy. On a regional scale, dynamic processes ofChile, between 18S and 24S, estimated platelarge, brittle crustal wedges can be modelledcoupling extends to depths of about 50 kmfollowing the critical taper theory for non-cohesive(Tichelaar and Ruff, 1991).wedges (Dahlen, 1984). Cohesion can be neglected,

Lithospheric shear zones decouple the uppersince it is insignificant compared with the increas-crust from the lower lithosphere affected by ductileing gravitational stresses with depth and the tec-deformation processes and are marked by antonic stresses that affect the wedge within aabrupt decrease of crustal shear strength.convergent setting.Probably, in the inner forearc segment, a shearzone segment ramps into the upper crust andforms an intracrustal detachment. (ICD; Fig. 2).3. Forearc wedge formation and plate-tectonicAdditionally, translithospheric shear zones like thesettingMain Andean Thrust (MAT ), provide crustalthickening and lithospheric stacking of crustal and

To explain the origin of the deformation pro-upper mantle segments. On the cratonic side, the

cesses within the brittle part of the forearc, weMAT ramps into the upper crust and forms an

briefly discuss the lithospheric structure and theintracrustal detachment beneath the Subandean

forces acting on the North-Central Andean sub-belt. Thus, internal lithospheric stacking of lower

duction system. According to the mechanicalcrust segments leads to the initial formation of

model of doubly vergent compressional orogensthe external fold-and-thrust belt (Roeder and

by Willett et al. (1993), the North Chilean PlateChamberlain, 1995).

margin can be divided into three large lithospheric

The plateau-induced gravitational forcesunits ( Fig. 2) : pro-wedge, orogenic plateau andincrease the contraction in the frictional-based

retro-wedge. As proposed in the model ofbounding wedges and favour west-verging thrust

Wdowinski and OConnell (1991), the compres- formation in the pro-wedge and east-vergingsion in the overriding plate arises from shear thrusts in the retro-wedge. This is confirmed bytraction acting on the base of the lithosphere field observations of west-verging neotectonictoward the asthenospheric wedge tip from both forethrusts in the inner forearc and by east-vergingdirections. Between the trench and the astheno- main thrusts in the Eastern Cordillera and thespheric wedge tip, the lithosphere is sheared by Subandean fold-and-thrust belt.the subducting Nazca Plate and between the

asthenospheric wedge tip and the Brazilian shield,

the base of the lithosphere is sheared by basal drag 4. Forearc wedge configuration

resulting from asthenospheric flow within the con-tinental mantle. Based on field observations and geophysical

The thickened crust of the orogenic plateau of data, we correlate the contrasting deformationthe North-Central Andes, formed by horizontal processes within the framework rock of the North-shortening of the thermally softened lithosphere, Chilean forearc region with the mechanical behavi-magmatic addition, lithospheric thinning, upper our of two distinct frictional forearc wedges inmantle hydration and tectonic underplating different dynamic states of stress an outer(Allmendinger et al., 1997), is in balance with the forearc wedge and an inner forearc wedge.far-field plate-tectonic compressional forces The forearc wedges are bounded at the top by

the present-day topographic slope. The brittle outer(Froidevaux and Isacks, 1984). Gravitational


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Fig.2.Schematicmechanicalmodelandlithosphericwedgeconfigurationo

ftheNorth-CentralAndeansubduction

systemfromtheNascaPlatetotheBra

zilianshield

(adoptedafterthemechanicalmod

elofdoublyvergentcompressionaloro

genesbyWillettetal.(1993)andthelithosphericviscousflowmodelforthe

Andesfrom

WdowinskiandOConnell(1991).

Themodelillustratestheinteractionof

plate-tectonicstressesresultingfromactivesubductionandgravitationallyindu

cedtectonic

stressesbythethickenedorogenic

lithosphere.Stresstransmissionwithinthepro-wedge,orogenicplateauandre

tro-wedgesystemisindicatedbywhite

arrows,the

dottedlinesrefertomaximumstresstrajectorieswithintheuppercrust.Thedashedsegmentofthesubductionzo

necorrespondstotheinactivesubductioninterface

betweentheoceanicandcontinentalcrust.Duringsubductionerosionthe

activesubductionfaultlieswithinthe

upperplate.Dashedboldlines=lithosphericshear

zones;ICD=intracrustaldetachment;MAT=MainAndeanThrustafterRoederandChamberlain(1995);S=singu

larity;blackarrow=activeunderplating;horizontal

hatchedarea=ductileuppercrust;verticalhatchedarea=rheologicalbuffer.


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forearc wedge is bounded along its base by the The inner forearc wedge shows compressional

neotectonic deformations and extends from thesubduction interface; the base of the brittle inner

forearc wedge is marked by an intracrustal disconti- Atacama fault zone to the western margin of the

active magmatic arc. We analyse the deformationnuity acting as intracrustal detachment (Figs. 2 and

3). The outer forearc wedge ends at the Atacama structures observed at the surface for the brittlesubwedge overlying a midcrustal discontinuity atFault system and is backstopped by the inner

forearc wedge. The brittle, inner forearc wedge ends depths between 15 km and 25 km ( Fig. 3) repre-

sented by a low-velocity zone in interpreted seismicat the thermally weakened magmatic arc, which

forms a rheological buffer acting like an interverte- sections between Tocopilla and Chuquicamata/

Calama (Schmitz, 1993; Wigger et al., 1994). Asbral disk between the forearc wedge and the oro-

genic plateau. This rheological buffer transmits shown by thermal modelling (Springer, 1999) and

crustal seismicity (Comte et al., 1992) for thisgravitationally induced compressional stresses on

to the brittle, inner forearc wedge, which are gener- upper crustal segment of the inner forearc litho-

sphere brittle behavior can be expected. The crustalated by the high-elevated Altiplano/Puna plateau

(Fig. 2). wedge overlaying the intracrustal detachment is

not aff

ected by the upward deflection of the iso-In general, high subduction speeds of cold oce-anic plates cause a significant downward shift of therms in the thickened crust of the orogenic

plateau due to the presence of an astenosphericthe isotherms in the forearc segment of the overrid-

ing upper plate (Honda and Uyeda, 1983). Due wedge ( Wdowinski and Bock, 1994a,b; Springer

1999). Thus, for the outer forearc wedge and theto the high convergence rate of the Nazca Plate

(about 9 cm/year; Minster and Jordan, 1978; upper crustal inner forearc wedge segment, the

critical taper analysis following the CoulombDeMets et al., 1990) this downward shift of the

isotherms is observable in the outer forearc wedge Navier failure law for brittle deformation

(Paterson, 1978) or frictional sliding (Byerlee,segment of northern Chile as confirmed by surface

heat-flow density data and 2D thermal modelling 1978) can be applied.

To investigate the fundamental processes of(Springer, 1999) . Therefore in the outer forearc

wedge, thermally driven recrystallisation (power basal tectonic erosion at non-accreting active mar-

gins, in this study, the MejillonesAtacama sectionlaw creep rheology) is not to be expected until adepth of 3040 km which additionally is confirmed is chosen as an example for the application of the

2D critical taper approach. Geological (Scheuberby the presence of interplate and intraplate crustal

seismicity (Comte et al., 1992). and Reutter, 1992), seismological (Comte et al.,

Fig. 3. Forearc wedge configuration of the North-Chilean trench-arc system and states of active crustal stresses obtained from

neotectonic and seismological data. White arrows=active compressional and extensional crustal stresses; vertical hatched=under-

thrusting of erosional debris in deformation or melange zone; oblique hatched=underplating of underthrusted material in basal

duplex zone. LCD=lower crustal detachment, ICD=intracrustal detachment.


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1992), and GPS data (Klotz et al., 1999) indicate carbonates and calcareous sandstones (Ferraris

and Di Biase, 1978). The entire succession of thethat in this segment of the Chile trench, oblique

convergence (Minster and Jordan, 1978) is actually onshore outer forearc is cut by neotectonic W-

and E-dipping normal faults and large NS-trend-not significantly affecting the active state of stress

and deformation processes in the overriding fore- ing tensile fissures (Armijo and Thiele, 1990;Buddin et al., 1993). This fault geometry indicatesarc crust.

a subhorizontal s3

axis (minimum principal stress)

in an EW direction (Figs. 1 and 3). Quaternary

coastal uplift of the Mejillones Peninsula is indi-5. Wedge geometry and regional geological setting

cated by the existence of high-level regressive ter-

races and shorelines (Armijo and Thiele, 1990). InThe outer forearc wedge extends over an

average distance of 120 km from the trench axis the same region, at depths between 20 and 30 km

beneath the coastal area, earthquake focal mecha-in the west to the Atacama fault zone in the east

(Figs. 1 and 3). The outer forearc wedge is limited nisms confirm thrust faulting (Delouis et al., 1996)

indicating a subhorizontal s1

axis (maximum prin-at the base by the subduction zone with an average

dip ofb=10 towards the East according to seismic cipal stress) slightly inclined to the West (Fig. 3).The inner forearc wedge extends over a distancedata of Wigger et al. (1994). Bathymetric data of

Schweller et al. (1981) at latitudes 22.21S and of 210 km and is situated between the Atacama

Fault and the western margin of the active mag-25.01S demonstrate inner trench slope inclinations

of about a=7 characterising the toe of the outer matic arc (Figs. 1 and 3). Based on linear regres-

sion analyses of topographic data across theforearc wedge. After a pronounced change within

the offshore topography about 50 km landwards Precordillera and the Salar de Atacama basin, the

average surface slope is a=1. The base of theof the trench, the surface continues onshore into

the western slope of the Coastal Cordillera with a brittle inner forearc wedge (ICD in Fig. 2,

intracrustal discontinuity and ICD in Fig. 3) dipsmore gentle inclination ofa=3.5W characterising

the internal segment of the outer forearc wedge. 7 to the east.

As in the outer forearc wedge the crustal struc-Neotectonic and active surface structures

between the Chile trench and the Atacama fault tures of the inner forearc wedge was also createdby the Mid-Cretaceous and Late Cretaceouszone reflect extensional processes. In the frontal

part of the outer forearc wedge, marine seismic Paleogene arc systems. The wedge consists of lower

Cretaceous volcanic rocks, mid-Cretaceous grano-investigations suggest extensive mass movements

and block sliding along the steep, inner trench diorites, uplifted Precambrian to Lower Palaeozoic

metamorphic rocks, Carboniferous to Permo-slope down into the trench (Reichert and CINCA

Study Group, 1996). This slump material is triassic magmatic and sedimentary rocks, lower to

mid-Jurassic carbonates and Cretaceous to recenttrapped in horst-and-graben structures and is car-

ried down into the subduction zone. marine/continental sediments with intercalated

evaporitic layers (Reutter et al., 1988; ScheuberThe outer forearc wedge consists entirely of

framework rock and includes palaeozoic igneous, and Reutter, 1992; Scheuber et al., 1994) providing

potential detachment horizons. The western rimmetamorphic and folded sedimentary rocks of thePre-Andean basement which is well exposed in the of the active magmatic arc is dominated by Upper

Miocene to Pleistocene ignimbrites and NeogeneCoastal Cordillera and on the Mejillones

Peninsula. The major part of the Coastal to recent andesites and dacites (De Silva, 1989).

The inner forearc domain and the westernCordillera consists of gabbros, granodiorites, mafic

to felsic dikes, tuffs and lavas associated with a margin of the active magmatic arc are character-

ised by compressional deformations. Buddin et al.JurassicEarly Cretaceous magmatic arc system

(Scheuber and Reutter, 1992; Scheuber et al., (1993) describe thick-skinned thrust systems in the

Pre-Cordillera with localised thin-skinned tectonics1994). Further forearc rocks are Lower Cretaceous

and Cenozoic continental clastic rocks, marine controlled by detachment horizons along evapo-


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(a)

(b)

Fig. 4. (a) Fault escarpments on the southern Mejillones peninsula caused by post-Miocene and post-Pliocene extension of the outer

forearc. (b) Post-late Miocene west-verging folds and thrust faults in the Rio Salado Valley (RS in Fig. 1). (c) East-verging Talabre

backthrust at the western margin of the magmatic arc with Tumisa volcano in the background. This NS-trending thrust fault

displaces the 8-m-thick Talabre ignimbrite (2.17 Ma) and the underlying Atana ignimbrite (4.09 Ma) of 35 m exposed thickness (TT

in Fig. 1). (d) Low-angle west-verging and east-verging thrusts within the upper Miocene Sifon ignimbrite, Quebrada de Diabolo,

western margin of Salar de Atacama (QD in Fig. 1).


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(c)

(d)

Fig. 4. (continued)


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ritic layers. Structure and seismic stratigraphy of Dahlen (1984). The critical taper models describe

the dynamic relations between the wedge geometrythe Salar de Atacama basin were examined by

Macellari et al. (1991). They explain the Atacama and the acting stresses within the wedges (Fig. 5).

The wedge shape is controlled by the balancebasin as a late Cretaceous halfgraben structure,

which underwent an Eocene to Oligocene compres- of various geological processes, e.g. exogeneticsurface processes, internal deformation, basal sub-sional inversion. Later compression is recognised

in the Rio Salado Valley where west-verging duction erosion and underplating. Moreover, the

critical taper will be influenced by variation offaulted anticlines in upper Miocene strata are

exposed ( Fig. 4a, loc. Fig. 1). Numerous extinct internal mechanical conditions of the Coulomb

wedge. In the critically tapered state, the wedgefault systems and crustal discontinuities within the

forearc crust reflect the complex deformation his- deformation is balanced between internal shear

failure and basal frictional sliding (Coulombtory of the forearc region since Jurassic times and

have potential for reactivation under the active rheology).

Within a critical wedge, an equilibrium betweenstress regime.

The active compressional stage of the inner three main elements exists (Fig. 5):

(1) Basal traction of the compressive wedge.forearc is expressed in east-verging and west-verg-ing thrusts affecting Pliocene and Quaternary (2) Stresses acting at the rear end of the wedge.

(3) Wedge geometry.deposits at the western margin of the Salar de

Atacama (Jolley et al., 1990). Compressional The internal and basal tectonic stresses of the

forearc wedges result from the internal sheardeformation also affects the western margin of the

active magmatic arc. Beneath the Tumisa volcano, strength of the wedge material, the frictional cou-

pling of the convergent plates and the gravitation-in the Quebrada Soncor the 2.03 Ma (0.35) old

Talabre ignimbrite ( K-Ar age obtained from ally induced crustal stresses of the thickened

continental crust. Thus, the three model elementsmultiple determinations; De Silva, 1989) is

thrusted to the east (Fig. 4b, loc. Fig. 1); near are determined by the regional geodynamic and

plate-tectonic setting (Fig. 2):Socaire within the Pliocene Tucucaro ignimbrite

(3.2 Ma, K-Ar biotite age, Ramrez and Gardeweg, (1) Basal traction of the compressive wedge.

Regarding the brittle forearc crust we have to1982) east-verging thrusts and pop-up structuresare exposed. Impressive low-angle west- and east- consider different basal conditions for the inner

and outer forearc wedges. Basal traction of theverging thrusts are exposed within the upper

Miocene Sifon ignimbrite at the western margin outer forearc wedge results from the shear resis-

tance along the subduction interface. Basal tractionof the Salar de Atacama in the Quebrada de

Diabolo area ( Fig. 4c, loc. Fig. 1). In the literature, of the inner forearc wedge results from the shear

resistance along the intracrustal detachmentthe east-verging compressional faults are consid-

ered as forethrusts and the west-verging faults as (intracrustal detachment; Fig. 5).

(2) Stresses at the rear end of the forearc wedgebackthrusts. No conducive explanations for the

thrust faults in the inner forearc or for the recent system are caused by gravitationally induced

stresses of the orogenic plateau. Within the brittlecontradictory deformation processes in the outer

forearc have been proposed until now. crust, these stresses are transmitted by the WesternCordillera/magmatic arc (rheological buffer;

Figs. 2 and 3).

(3 ) Wedge geometry. The critical taper equa-6. Application of critical taper analysis to the

erosive North-Chilean forearc tions (see the Appendix) describe the theoretical

critical taper for crustal wedges (a+b)crit

required

for basal wedge transport without internal wedgeTo explain the mechanics, the active state of

stress and the dynamic evolution of the described deformation. The comparison of this required criti-

cal taper value with the actual wedge geometryforearc-wedge system, we apply the critical taper

theory for non-cohesive Coulomb wedges by (Fig. 6) characterises the active dynamic state of


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Fig. 5. Schematic cross-section of a critical non-cohesive crustal forearc wedge in an active subduction system (brittle part of the

lithosphere), illustrating the internal state of stress, governed by three main elements: (1) basal traction of the compressive wedge;

(2) stresses at the rear end of the wedge; and (3) wedge geometry. Additionally the required geometric and mechanical parameters

for Critical Taper modelling are shown (modified after Dahlen, 1984).

the forearc wedge controlling internal stresses and segments (Fig. 8ad). The pole has the usefulproperty by which stresses determined by thethe kinematics of active deformation structures

(Adam, 1996). second intersection with the Mohr circle of any

line passing through the pole act on a planeAdditionally to the mathematical solution

(Fig. 7ad, Table 1) of Dahlen (1984), we show oriented parallel to that line in physical space.

Already Dahlen (1984) illustrates in a series ofthe graphical solution of the critical wedge problem

with Mohr stress circles presented by Lehner hypothetical wedge models, that variations in style

of deformation at active convergent margins are(1986). The Mohr circles ( Fig. 8ad.) give a graph-

ical presentation of the wedge mechanics and fault the consequence of the degree of frictional coupling

between the overriding and subducting plates.kinematics of the brittle forearc wedges.

Due to the self-similar wedge geometry, the Small variations of basal friction can easily account

for different deformation patterns. The successionapplied mechanical boundary conditions controlthe depth-independent orientation of the active of deformation processes from low to high friction

in the hypothetical wedge consists of extension bydeformation structures. Therefore the limiting

states of stress (effective normal stress sn and shear normal faulting, sediment subduction without

accretion, accretion and imbricate thrusting andstress t) can be obtained for any point (snz, t

z) at

depth z within the wedge segments. Using the pole subduction erosion.

Long-term basal erosion only occurs in highconstruction method (Terzaghi, 1943; Crans and

Mandl, 1980) the Mohr circles provide the actual frictional wedges on the verge of their existence limit

(no contrast between basal and internal sheararrangement and the sense of movement for the

sets of active faults within the forearc wedge strength; strength ratio x=1, Eq. 7 in the Appendix).


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Fig. 6. Schematic cross-section of the North-Chilean forearc-wedge system illustrating the relations between the present wedge

geometry and basal mass transfer modes within the distinct forearc segments. Numbers in wedge segments refer to wedge-specific

data sets in Table 1. ICD, intracrustal detachment separating the brittle inner forearc wedge from the lower crust; LCD, newly formed

lower crustal detachment in the outer forearc wedge; dashed bold lines, inactive or future faults (vertical exaggeration of factor 2).

Even high frictional wedges with minor contrast stable wedge geometries (a+b) is reduced to a line(as shown in Fig. 7a). Therefore, for a particularbetween internal and basal shear strength (x1) shiftsas the dip of the direction of maximum principlethe wedge beyond the existence limit, basal wedge

stress s1

relative to the wedge base, steepens simulta-transport is stopped and a new detachment must

neously. The actual orientation of the maximum be formed up in the wedge for subduction toprinciple stress s1controls the position of the sets of

continue. The forethrust-related set of slip linesslip lines (potential and active shear planes) in the

paralleling the blocked wedge base will be pre-wedge. With the steepening s

1direction the set of ferred for any new detachment horizon. The pre-

slip lines with trenchward mass transport (e.g. cise location of the wedge internal detachment willforethrusts) rotates to a more wedge base parallel be governed by any pre-existing structures anddirection, whereas the set of slip lines with arcward weakness zones (Dahlen, 1984). The wedge mate-mass transport (e.g. backthrusts) steepens (Fig. 8a). rial in the footwall of the newly formed detachment

If the contrast in shear strength disappears (x= is fixed to the subducting oceanic crust and willbe basally eroded and transported arcward.1) within the stability diagram the former field for


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7. Database and global parameters accretive systems, we model the difference between

internal and basal shear strength in crustal wedges

by variation of the internal and basal pore fluidOur model parameters for northern Chile are

presented in Table 1. The geometrical data, geolo- pressure ratios (llb

) and similar coefficients of

internal and basal friction (m=mb=0.7) reflectinggical data and seismological data are given by theregional geology and structures. The geodynamic a non-accretive forearc system made of prestruct-

ured framework rock.and mechanical parameters differ significantly

from the values in sedimentary accretionary com- The ratio (x) and the related internal and basal

pore fluid pressure ratios (l, lb) are summarisedplexes as the outer forearc wedge consists entirely

of framework rock. Up to now all published values as the wedge-specific parameter set in Table 1. The

applied values for the internal pore fluid pressureare considering submerged accretionary wedges

(Lallemand et al., 1994). ratio vary from l=0.42 (subaerial wedge with

hydrostatic pore fluid pressure), l=0.6 (sub-Invariant physical properties of our wedge

models are summarised in the global parameter merged wedge) to l=0.8 (overpressured wedge).

The basal pore fluid pressure ratio lb

will beset. For the coefficient of internal friction of the

framework rock, we apply m=0.7 (angle of internal adjusted to model diff

erent strength ratios of theparticular forearc wedges. The resulting strengthfriction w=35) characterising a brittle continental

crust in frictional equilibrium on pre-existing faults ratios describe different states of the wedge base

from locked (x=1.0) to weak (x1, the basal traction tb

exceeds the internal ing deformation processes within one distinct time

interval. In our Coulomb-wedge model, contrast-shear strength t and a continuous detachment at

the wedge base cannot develop, the wedge cannot ing states of internal stress within the wedge are

controlled by spatial variations of the basalexist. Therefore, for an active crustal wedge it is

an important assumption that the internal wedge mechanical parameters in the down-dip direction

of the forearc wedge base despite temporalis stronger than its base, which is expressed by the

following inequality: mb

(1lb

) m (1l). To variations.

Continuous erosional mass transfer sincecustomise critical taper calculations it is usual to

fix either the coefficients of internal/basal friction Jurassic times is more easily achieved by a steady-

state process with spatial variation of wedge(m, mb) or basal/internal pore fluid pressure ratios(l, l

b). dynamics than by multiple temporal variations,

requiring a change of geodynamic or externalBecause accretionary wedges and thrust belts

are made of heterogeneous sedimentary sequences factors. Because this region was not affected by

significant variations of geodynamic factors (e.g.with varying mechanical properties, normally the

wedge strength will be modelled by the coefficients rate and direction of convergence; Scheuber et al.,

1994) over the last 10 m.y. and arid climatic condi-of internal and basal friction (m>mb

) and fixed

basal and internal pore fluid pressure ratios tions prevailed at least since the Late Quaternary,

the present wedge geometry reflects the dynami-(l=lb=constant) (Dahlen and Suppe, 1988;

Roeder, 1992; Adam, 1996). In contrast to these cally stable state and the mechanical properties


14/29


Table1

Summaryofthegeometric/geodynamic

database,global/wedge-specificparametersetsandresultsofcriticaltaperanalysisconcerningthethreemodelledf

orearcwedge

segmentsa

Outerforearcwedge

Innerforearcwedge

Toe

Internalouterforearcwedge

Geometry

datase

t

Surfacelengthnormaltotrench

50km

70km

210km

Topographicslope(a)

7

3.5

1.0

Basaldip(b)

10(subductioninterface)

10(subductioninterface)

7(intracrustaldiscontinuityICD)

Actualtaper(a+b)

17

13.5

8

Geo

logica

ldatase

t

Stressregimenearsurfacededuced

[Z:thrusts,seismicdata

Z

[:normalfaultsandfissures,

[Z:fore-andbackthrusts,folds

fromneotectonicstructures

Z[:locallyatdeformationfront

C

oastalScarp,submarineslumps

Recentgeodynamicprocesses

Basaltectonicerosion

C

rustalthickening,upliftofCoastalRange

Crustalthickening

Seismolog

ical

datase

t

Stressregimededucedfromfault

Aseismic

Z

[:surface(Antofagastaquake96)[Z:

planesolution

th

rustfaultingat2030kmdepth

Seismologicalcharacterofwedge

Aseismicsubductionfault;entire

Strongseismiccouplingofsubductionfault;

Seismicactivity;epicentrestracewedge

base(SW-seismologicalwindow)

wedgebaseaboveSW

in

ternalwedgebaseinSW

baseaboveICD

Globalparameterse

t

Coefficientofint./basalfriction(m=mb)

0.7(w=w

b=35)

0.7(w=w

b=35)

0.7(w=w

b=35)

Densityofframeworkrock(d

litho)

2600kg/m3

2600kg/m3

2600kg/m3

Densityofporefluid

1030kg/m3

1030kg/m3

1030kg/m3

Wedgespec

ificparameterse

t

Hypothet

ical

Internalporefluidpressureratio(l)

0.83(calculatedbytapergeometry)

0.77

0.6(submerged)

+

7.9.

:0.hydrostatic)

Basalporefluidpressureratio(l

b)

0.83(overpressured)(l=lb)

0.8

0.7

0.6

0.7

Strengthratiobaseinternal/

(x)

1.0(nodiscretewedgebase)

0.68(strong)

0.54(intermediate

)

0.49(intermediate)

0.35

(weak)

Cri

tical

tapermodelresu

lts

Basaleros

ion

Subduc

tion

Underplat

ing

Taper

buildup

Detac

hed

Criticaltaper(a+b) c

rit

17(a

stable=7.0)

13.5(a

crit=3.5)

12(a

crit=2)

10.4(a

crit=3.4)

8(acrit=1)

Angleofs1-direction/wedgebase(y

b)

27.5

16.9

13.2

11.8

8.3

Forethrust-relateds.p.rampangle(d

b)

0(wedgeinternals.p.

d

base)

10.6

14.3

15.7

19.2

Backthrust-relateds.p.rampangle(d b)

55

44.4

40.7

39.3

35.6

Dynamicstateofactualwedge

Onvergeofexistencelimit,stable

Critical

Critical

Subcritical

Critical

a

SW,seismogenicwindow;ICD,intracrustaldetachment;s.p.,shearplane;[

Z

compressionalstressregime;Z[extensionalstressregime.


15/29


and active dynamic processes of basal tectonic circle for the compressional state of stress shows

a gently inclined subhorizontal maximum stresserosion.

This concept is supported by the pronounced direction (s1c

inclined by 13 to the wedge base)

and results in a detached wedge base with an activesegmentation of the outer forearc wedge geometry.

The wedge surface has a significant slope break in subduction interface on top of the oceanic crust.Contemporaneous steeper dipping thrust faultsthe offshore area (50 km east of the trench axis)

separating a toe segment from the internal outer allow basal accretion and underplating of tectoni-

cally eroded forearc wedge material. Thrust fault-forearc wedge (Figs. 3 and 6). This topographic

feature cannot be explained by a geometrical varia- ing indicated by focal mechanisms of shallow

earthquakes (depth30 km, Delouis et al., 1996)tion along the subduction interface (wedge base)

or by a change within the crustal structure of the supports this dynamic model. This basal accretion

of crustal material results in crustal thickening ofupper plate. Thus, it implies different dynamic

behaviours within the toe segment and the internal the rear part of the outer forearc wedge and is a

potential mechanism for the ongoing uplift of thesegment of the outer forearc wedge.

coastal area.

In contrast to this compressive basal accretion8.1. Critical taper model for the internal segment ofthe outer forearc wedge mechanism, neotectonic and active surface struc-

tures in the outer forearc show trench parallel

extension. The normal faults are dynamically inter-The model for the internal segment of the outer

forearc wedge is used to limit the possible dynamic preted as a result of the extensional collapse of the

slightly overcritical wedge due to continuous thick-states (basal erosion to overcritical wedge exten-

sion) for a crustal forearc wedge characterised by ening by underplating. This is shown in our model

by contemporaneous extensional and compres-a present taper of (a+b)topo=13,5. The model

parameters are summarised in Table 1. The wedge- sional limiting states of stress within the internal

segment of the outer forearc. Near-surface exten-specific parameter set (parameter set in Table 1)

describes a partly submerged crustal forearc wedge sion with normal faulting would be favoured by

overpressured crustal detachments. Finding thesewith a high frictional wedge base. Due to an

internal pore fluid pressure ratio l=0.6 charac- detachments is a task for further geophysicalinvestigations.terising a partly submerged wedge, a high frictional

base with an intermediate strength ratio (x#0.5) For a more detailed discussion and evaluation

of the possible dynamic states of the partly sub-can be modelled by a significantly increased basal

pore fluid pressure ratio (lb=0.7). This critical merged high frictional outer forearc wedge, the

stability field diagram for the applied wedgetaper model results in a stable to slightly overcriti-

cal wedge geometry for the internal segment of the specific parameter set is plotted in Fig. 7c. The

critical to slightly overcritical internal segment ofouter forearc wedge with (a+b)crit=12. This

stable to slightly overcritical taper will be obtained the outer forearc wedge will plot inside the stability

field (open circle, Fig. 7c) near the lower boundaryby active underplating at the wedge base (see

Fig. 6, set ). Contemporaneous wedge internal (critical minimum taper).

This position near the lower minimum taperextensional deformation may occur to readjust therequired minimum critical taper. This resulting boundary reflects the steady-state dynamic equilib-

rium between mass transfer and internal deforma-wedge dynamic reflects adequately the active geo-

dynamic setting of the internal outer forearc wedge tion processes in accretionary complexes and

forearc wedges, dominantly shaped by frontal andsegment.

Additionally, the critical taper results are graph- basal accretive mass transfer modes. The exten-

sional surface deformation is the expression ofically presented in a Mohr diagram (Fig. 8c),

illustrating the general state of stress and active regional near-surface stress release within an

overcritical but stable forearc wedge to maintainfault kinematics within the internal segment of the

outer forearc wedge at depth z (sz, t

z). The stress the required critical minimum taper. Pervasive


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17/29


extensional deformation of the entire forearc crust data set in Table 1) the required critical mini-

mum taper (a+b)crit=13.5 is significantly lowercan be excluded because it only occurs in maximum

tapered critical wedges (upper boundary of sta- than the present taper of (a+b)topo=17.

Therefore, the toe segment must represent thebility field). The theoretical maximum critical taper

of the modelled forearc wedge segment with a maximum taper on the verge of the existence limitdue to continuous basal tectonic erosion. In thisbasal dip b=10 is attained with a topographic

slope a=22. This topographic slope is near the case (continuous subduction erosion driven only

by Coulomb wedge mechanics and steady-stateangle of repose of the forearc crust (amax=25)

and may not be realised within the forearc setting dynamic equilibrium of the outer forearc wedge

geometry) the toe geometry reflects the mechanicaleven by a significant increase of basal friction.

Similar arguments are reasonable to exclude conditions of basal tectonic erosion along the

North-Chilean convergent margin. With decreas-active tectonic erosion at the base of the internal

outer forearc wedge as source for the thrust-related ing shear strength contrast, the stability field will

be reduced to a line. For a particular set ofearthquakes near the wedge base. In terms of

Coulomb wedge mechanics for the present forearc mechanical parameters and known basal dip b

only a single topographic slope a exists to establishwedge geometry (a+b=13.5) nearly lithostaticinternal and basal pore fluid pressure ratios must the maximum taper for a wedge suffering basal

erosion. Under these particular conditions for thebe assumed to enable basal tectonic erosion of the

continental forearc crust (see also next paragraph). known taper (a+b) of the toe segment, it is

possible to evaluate the mechanical parameters

controlling active basal erosion by a structural8.2. Critical taper model for the toe segment of the

outer forearc wedge approach.

In the particular case of forearc wedges shaped

by active Coulomb-type basal erosion, the requiredThe taper increase from (a+b)=13.5 to 17

in the toe segment cannot be correlated with a strength ratio x=1 is controlled by the relation of

the pore fluid pressure ratios (l=lb=l

erosion)changing crustal composition and structure of the

upper plate or with a geometrical variation along which now is the only unknown parameter of the

critical taper solution. In the stability field diagramthe forearc wedge base. Thus, the segmentationmust be the expression of the different dynamic the parameter l

erosionfixes the vertical position of

the stability line and can be adjusted by iteration.states within the toe segment and the internal

segment of the outer forearc wedge system. But, Applying this procedure to the toe segment (a=

7, b=10, data set in Table 1), the stabilityeven for a submerged toe segment with strong

basal traction (for example x=0.68 in Fig. 7b, line fits the required toe geometry at

Fig. 7. Stability field diagrams for the North-Chilean outer forearc-wedge system (surface slope a versus basal dip b). (a) Toe segment

of the outer forearc wedge at the verge of the existence limit without discrete wedge base and shaped by basal erosion. Increasing

basal traction leads to a reduction of the stability field resulting from the converging upper and lower limits. During basal erosionthe stability field is minimised to a line. In the state of basal erosion only one stable wedge configuration exists for any possible dip

of the subduction zone. The open circle corresponds to the actual geometry of the toe segment of the outer forearc wedge. (b)

Transitional segment of the outer forearc wedge during underthrusting of eroded toe material. In this case the outer forearc wedge

reflects the minimum critical taper. The open circle corresponds to the actual geometry of the internal segment of the outer forearc

wedge. (c) Outer forearc wedge segment modelled as partly submerged wedge with high frictional wedge base. The open circle

corresponds to the actual geometry of the internal segment of the outer forearc wedge and plots within the stability field near the

minimum taper boundary characterising a stable wedge geometry with a critical to overcritical taper due to active basal underplating

accompanied by surficial extension. (d) Required pore fluid pressure ratio depending on the variation of the angle of internal friction

resulting in a stable wedge geometry for the present toe segment under basal erosion. The graph demonstrates that the variation of

the angle of internal/basal friction over a wide range of suitable values in continental crust is a minor influence on the required

internal/basal pore fluid pressure ratio for basal erosion.


18/29


Fig. 8. Mohr diagrams, illustrating the general state of stress and active fault kinematics within the outer forearc wedge at depth z

(snz, t

z). (a) Compressional limiting state of stress within the maximum tapered stable toe segment of the outer forearc wedge on

the verge of the existence limit due to maximum basal friction and basal tectonic erosion ( a=23.6, a=7, b=10). (b) Compressional

limiting state of stress within the minimum critical tapered transitional segment of the outer forearc wedge characterised by continuous

subduction (underthrusting) of erosional debris from the base of the leading toe segment (a=9.1, a=3.5, b=10). (c) Extensional

and compressional limiting states of stress within the critical to overcritical internal segment of the outer forearc wedge characterised

by active basal underplating and near-surface extensional adjustment by normal faulting (a=3.0, a=3.5, b=10). (d) General

state of stress and active fault kinematics within the subcritical inner forearc wedge at depth z.


19/29


lerosion

=0.83 (Fig. 7a). This fluid pressure ratios Because in this special case the stability field is

reduced to a line, any minor variation of thefor active Coulomb-type basal erosion character-

ises an overpressured toe segment (see Fig. 6 ) and boundary conditions shifts the wedge geometry

into an unstable extensional or compressional lim-is in a similar range to characteristic l values,

measured directly in submarine accretionary com- iting state of stress (see Fig. 7a). Therefore, atemporary unstable wedge geometry may be indi-plexes (average value for l=0.88, Lallemand

et al., 1994). cated by extensional or compressional deformation

structures. Minor thrust faults, normal faults andFurthermore, we evaluate the sensitivity of this

procedure for variations of the angle of internal slump structures in the frontal part of the toe

segment, determined from reflection seismic dataand basal friction (in our model w=wb=35). For

the present taper geometry we calculate lerosion

for (Reichert and CINCA Study Group, 1996), indi-

cate this locally unstable taper. The extensionalw values ranging from 20 w=wb50. The

results are graphically shown in Fig. 7d. The dia- limiting state of stress and active normal faults

(extensional adjustment) are also illustrated ingram shows that the determination of lerosion

is

almost independent from the angle of Fig. 8a.

internal/basal friction. In the limits of geologicalapplicable values (w#30 for accretionary com- 8.3. Critical taper model for the transient segment

of the outer forearc wedgeplexes to w#40 continental crust without pre-

existing fractures; Byerlee, 1978), no significant

fluctuation (lerosion

=0.830.02) can be observed. To allow a gradual variation of wedge dynamics

from simultaneous basal erosion to underplating,The compressional state of stress and active

fault kinematics within the toe segment are graphi- a transitional critically tapered wedge segment is

required which performs the mass transfer of ero-cally presented in a Mohr diagram (Fig. 8a). The

present taper of the submerged toe segment sional debris by basal transport and underthrusting

in arcward direction. With increasing depth, the(a+b=17) is illustrated as a shaded prism. The

active basal subduction erosion results from similar dynamic behaviour of the outer forearc wedge is

controlled by the gradual decrease of basal cou-mechanical properties in the toe segment and along

its base (w=wb[l=l

b). The compressional limit- pling ( lowering ofx, for example by strain harden-

ing or dewatering processes; Moore and Byrne,ing state of stress within the toe segment is charac-

terised by a relatively steep inclined subhorizontal 1987) .

Oceanic crust and erosive material togethermaximum stress direction (s1c

inclined 27.5 to

the wedge base) and the forethrust-related slip form the subducting footwall system of the transi-

tional outer forearc wedge segment. Thus, beneathlines rotate parallel to the wedge base, e.g. the

subduction interface on top of the oceanic crust. the transitional wedge segment, the active subduc-

tion interface of the upper plate/lower plate systemIn this state of stress an additional minimum

increase of the shear strength ratio (x>1 if is located in the hanging wall of the subduction

channel with erosional debris. Beneath the adjacentlb>0.83) starts Coulomb-type basal tectonic ero-

sion. Because basal wedge transport on top of the wedge segment in the coastal area, the basal

mechanical variation in the down-dip directionNasca Plate is stopped, a new detachment will beformed in the toe segment to act as a new active triggers progressive footwall collapse. This process

shifts the active subduction interface down intosubduction interface. The forethrust-related set of

slip lines will be preferred and the material in the the underthrusted material or finally on top of the

oceanic crust. For this reason, the erosional debrisfootwall of the newly formed detachment will be

basally eroded and transported arcward with the partly re-enters the internal outer forearc wedge

by basal underplating.subducting oceanic plate in a high strain deforma-

tion zone or melange zone. The process is compa- In northern Chile, the pronounced topographic

break 50 km east of the trench axis is the surficialrable with the subduction channel model for

sediment subduction (Cloos and Shreve, 1988a,b). indication of this transition between the toe seg-


20/29


ment and the transitional outer forearc wedge support the minimum tapered, stable wedge geom-

etry, to detach the transitional segment and tosegments. The maximum tapered, erosive toe seg-

ment builds up the steep lower and middle trench stop basal tectonic erosion. The entire wedge-

specific parameter set for the transitional segmentslope with atoe=7, whereas the upper trench slope

with an average slope atrans=3.5 characterises the is summarised in Table 1 (parameter set ). It isremarkable that the small reduction of the internalminimum tapered, stable transitional segment (see

Fig. 6). The arcward limit of the transitional seg- pore fluid pressure ratio results in a significant

drop of the strength ratio from x=1 to x=0.68,ment is uncertain, because the minor dynamic

variation from stable-underthrusting to stable- characterising a high-frictional forearc wedge with

strong basal coupling.underplating causes no significant variation in the

topographic slope. The state of stress and active fault kinematics

within the transitional segment are shown in theThe initial conditions for the frictional wedge

model of the transitional outer forearc wedge Mohr diagram of Fig. 8b. The compressional limit-

ing states of stress within the minimum taperedsegment can be summarised as follows:

(1 ) The model parameters of the transitional transitional segment is characterised by ongoing

basal wedge transport. The compressional state ofsegment have to be obtained by a reasonablevariation of the wedge-specific data set of the stress shows a gently inclined, subhorizontal maxi-

mum stress direction (s1c

inclined 16.9 to theerosional toe segment (data set in Table 1),

reflecting the gradual transition of the mechanical wedge base) and results in a detached wedge base

with an active subduction interface located in theconditions in the outer forearc wedge.

(2) The present taper (a+b)trans=13.5 charac- hanging wall of the underthrusted erosional debris.

Steeper dipping thrust faults are able to attain orterises a critical-shaped, stable wedge geometry of

the transitional segment, which is required for readjust the critical wedge geometry by minor

internal deformation. In the associated stabilitybasal wedge transport and active underthrusting

of eroded forearc material. field diagram the transitional segment plots at the

lower minimum-taper boundary (open circle,The arcward transition zone from active basal

tectonic erosion to underthrusting is the conse- Fig. 7b) reflecting the typically stable geometry of

Coulomb wedges in a pro-wedge setting.quence of the decreasing strength ratio (x


21/29


coastal area up to 17 near the trench axis) shiftingaround an intermediate, basic taper of (a+b)=13.5 (indicated by the transitional segment,in Fig. 9).

The increased basal coupling in a trenchwarddirection leads to a reduction of the stability field.As a consequence, the lower minimum-taperboundary ascends and the wedge taper increasesby internal shortening and mass transfer to adjustto the required stable minimum taper (indicatedby the grey arrow in Fig. 9 ). Finally, if the stabilityfield is reduced to a line, the maximum taper isadjusted and basal tectonic erosion begins (toesegment in Fig. 9).

Towards the arc, the stability field is enlargedas a result of decreasing basal coupling. Thislowers the minimum taper boundary. Thus, thewedge taper will be narrowed through extensionaldeformation and surficial mass transfer until thenew minimum tapered state is reached (indicatedFig. 9. Variation within the stability field and derived geody-by the white arrow in Fig. 9). In our model, thenamic behavior of the outer forearc wedge (surface slope a

versus basal dip b) controlled by the changing mechanical con- minimum taper for the internal outer forearcditions within the toe segment ( basal erosion) and the wedge ( in Fig. 9) is not yet adjusted becauseinternal outer forearc wedge ( underthrusting and the present, slightly overcritical taper ( in Fig. 9)underplating and extensional adjustment). Index numbers of

is controlled by continuous basal underplating.the three dynamic states refer to the wedge-specific parameter

Therefore, an important result is that highsets in Table 1.frictional forearc wedges are capable of building

up a long-term, dynamically stable wedge geometrysegment (circle in Fig. 9) is comparable withover a wide range from intermediate to strong

the long-term, static-stable state of non-erosivebasal coupling. Non-accretive outer forearc

outer forearc wedges at non-accretive margins. wedges, characterised by a very strong base, seemThese non-erosive outer forearc wedges seem to to be extremely sensitive to minor variations ofbe controlled by uniform basal mechanical condi- the wedge-specific mechanical parameters. Smalltions and the lack of notable exogenetic mass mechanical modifications can initiate basal tec-transfer and significant internal deformation. tonic erosion and, consequently, shift an active

Otherwise, if a mechanical variation along the margin from a static-stable geodynamic state withbase of a non-erosive outer forearc wedge develops, minor deformation into a contrasting dynamicallythe former static-stable state will be transformed stable geodynamic state with strong deformationinto a dynamic-stable state. Now the interaction and significant orogenic mass transfer.

of tectonic deformation and mass transfer establisha dynamically controlled equilibrium of wedge-

stabilising and destabilising processes. This dynam- 9. Critical taper model for the inner forearc wedgeically controlled state is indicated by a pronounced

geometric segmentation and by a complex, active In contrast to the mainly submerged outerstress regime, as shown by observations of the forearc wedge system, which is situated entirelyerosive North-Chilean outer forearc. upon the subducting oceanic Nasca Plate, the

In the North-Chilean outer forearc wedge subaerial inner forearc wedge is mechanical embed-system this dynamic equilibrium provides stable ded in the overriding South American Plate, proba-

bly mechanical decoupled from the ductile crusttaper variations (a+b) about 5 (from 12 in the


22/29


through a slightly arcward dipping intracrustal The divergence of more than 2 between the

present and required topographic slope indicatesdiscontinuity (Fig. 3).

The present taper of the inner forearc wedge a subcritical state far from the minimum taper

geometry. To shift the subaerial inner forearc(a+b=8) is significantly smaller than in the outer

forearc wedge system. The inner forearc wedge is wedge into a critical state a dramatical decrease ofthe strength ratio from a high frictional, intermedi-modelled as a crustal forearc wedge with a high

frictional wedge base, characterised by an interme- ate wedge base with x#0.5 to a low frictional,

weak wedge base with x#0.35 is required (wedge-diate strength ratio (x#0.5). The wedge-specific

parameter set is summarised in Table 1 (set ). specific parameter set for a hypothetical

detached inner forearc wedge in Table 1).Due to subaerial wedge conditions, a hydrostatic

internal pore fluid pressure ratio l=0.42 is Therefore, the results of the frictional wedge

model for the inner forearc wedge should beassumed for the model. In this case, the intermedi-

ate strength ratio (x#0.5) of the high frictional considered under more general aspects of lith-

ospheric modelling. Some aspects and open ques-base is modelled by a slightly increased basal pore

fluid pressure ratio (lb=0.6). This critical taper tions for future modelling of inner forearc

dynamics are:model results in an unstable subcritical wedgegeometry for the inner forearc wedge with (1) Is the upper crustal, brittle deformation

mechanically decoupled from ductile deformation(a+b)crit=10.4. The present average topographic

slope a=1.0 is significantly smaller than the criti- in the lower lithosphere of the inner forearc, so

that frictional wedge model results may interpretedcal topographic slope acrit=3.4. In this subcritical

state the inner forearc wedge is basally attached quantitatively?

(2) Is the wedge base thermally weakened, soand has to build up a critically tapered, stable

wedge geometry by active compression and that a low frictional model is more suitable to

describe the dynamics of the inner forearc?internal shortening by thrusting and folding (see

Fig. 6, set ). (3) Is it more appropriate to use numerical

models with composite, thermally controlled rheol-The state of stress at depth z (snz,t

z) and the

active fault kinematics within the subcritically ogies (brittle-ductile) for analysing the dynamics

of the inner forearc wedge?tapered inner forearc wedge are shown in theMohr diagram of Fig. 8d. The modelled compres-

sional limiting state of stress within this narrow

wedge is characterised by a subhorizontal maxi- 10. Active fault mechanics and mass transfer in the

North-Chilean forearc systemmum stress axis (s1c

inclined 11.8 to the wedge

base). The subcritical inner forearc wedge is under

active compression and internal deformation pro- The regional correlation between the active state

of stress and the active deformation structurescesses favour active forethrusts and backthrusts

(Fig. 8d) expressed by neotectonic west- and east- between the Chilean trench and the Western

Cordillera are summarised in Fig. 10. Similarverging thrusts. A small-scale field structure

reflecting this state of stress for conjugate thrust mechanical properties within the toe segment and

along its base prohibit a discrete wedge base andfaults is shown in Fig. 4d. Active thrust faultingin the inner forearc wedge is characterised by out- control Coulomb-type basal tectonic erosion. This

results in the formation of a basal melange zoneof-sequence thrusts, favoured by numerous evapo-

ritic layers within the rock succession and previous below a wedge-internal detachment in the outer

forearc crust (LCD in Fig. 10). Within a subduc-structures which will be reactivated when fitting

the actual fault geometry and mechanical condi- tion channel (Fig. 10, ), the erosional debris is

transported arcward with the subducting Nazcations. The result of the applied parameter set ,

which indicate a subcritical wedge increasing its Plate beneath the transitional segment of the outer

forearc wedge (Fig. 10, ).taper by internal thrusting and folding, explains

adequately the neotectonic field observations. Basal accretion of parts of the erosional debris


23/29


Fig. 10. Schematic dynamic cross-section summarising the kinematics of the main active tectonic structures and the active states of

stress at the North-Chilean forearc (numbers refer to explanation in Section 10).

is controlled by the decreasing basal coupling in Pliocene/Pleistocene west-verging forethrusts and

east-verging backthrusts (Fig. 10, ) in the innerthe down-dip direction of the wedge base.

Underplating of the downcarried erosional debris forearc and the western rim of the recent magmatic

arc reflect compressional internal deformation cor-forms crustal stacks at the internal base of theouter forearc-wedge (Fig. 10, ). In the onshore responding to a subcritical crustal wedge.

outer forearc of northern Chile, this mechanism

controls the regional uplift. Trench parallel near

surface extension by normal faults are interpreted 11. General implications of Coulomb-wedge

modelling for mass transfer modes at erosiveas extensional collapse of the slightly overcritical

wedge ( Fig. 10, ). The upper crustal rocks are margins

detached along large listric normal faults in the

hanging wall of overpressured horizons and slide In terms of Coulomb wedge mechanics frontal

and basal tectonic erosion occur only under partic-into the trench (Fig. 10, ). Synchronous post-


24/29


ular conditions. Generally, frontal erosion pro- ing), intensely investigated at accretive margins

(Gutscher et al., 1998a,b) and in analogue sandboxcesses are caused by the short-term modificationexperiments (Kukowski et al., 1994).of the wedge shape due to subducting topographic

asperities of the oceanic crust ( horst-and-graben

structures, seamounts or aseismic ridges, Huene12. Structural approach for evaluation of theand Lallemand, 1990).mechanical conditions for Coulomb-type basalOur models describe the processes and bound-erosion at non-accretive marginsary conditions of continuous, long-term basal tec-

tonic erosion at non-accretive margins completelyIn the state of basal tectonic erosion of thein terms of Coulomb wedge mechanics. Regarding

forearc wedges, only a particular wedge geometryparticular mechanical conditions (strong basal cou-establishes a dynamic-stable state during tectonicpling, x=1) Coulomb wedges will suffer significantdestruction and erosive mass transfer reflecting itsbasal erosion over geological time scales, indepen-mechanical conditions. Because in the erosionaldently from external effects as frontal subductionstate the (a+b) stability field of the wedge isof asperities.

reduced to a stability line, only one correspondingThe mechanical conditions for Coulomb-type topographic slope a exists for any basal dip b.basal erosion, particularly along the wedge base,Thus, the present geometry of erosional wedgecannot be maintained over an extended region insegments is the key to evaluate the significantthe down-dip direction of the outer forearc wedge.mechanical parameters controlling basal tectonic

For this reason, basal tectonic erosion as dominanterosion. The application of this procedure on to

mass transfer mode should occur in the near-the erosive toe segment of the North-Chilean outer

trench segment of high-frictional outer forearcforearc wedge shows, for a wide range of crustal

wedges, characterised by a typical maximum-taperproperties, that Coulomb-type basal tectonic ero-

wedge geometry as observed in northern Chile.sion indicates a remarkable overpressure of the

Consequently, outer forearc segments at non-basal and internal pore fluid pressure ratios

accretive margins, which are shaped through long-(l

erosion#0.8). It can be demonstrated that the

term basal tectonic erosion, have to adjust a typical oversteepend maximum taper geometry up tomass transfer pattern in arcward direction. This

(a+b)=17 is controlled by basal tectonic erosion,mass transfer pattern is governed by three different

because these taper values cannot be establishedmodes consisting of: (1 ) basal erosion (toe seg- by tectonic deformation and mass transfer forment); (2) subduction/underthrusting (transitional geologically reasonable mechanical parameter setssegment); and (3) underplating of erosional mate- of the continental forearc crust.rial (internal segment). Each of the three contrast-

ing mass transfer modes generates characteristic

deformation structures and wedge tapers. As 13. Global correlation with other present-dayshown by the Coulomb-wedge analysis of the erosive marginsNorth-Chilean outer forearc system, the transition

between the diff

erent dynamics states and their In the summarised stability field diagram formass transfer modes is controlled by a small active accretionary wedges and non-accretive mar-modification of the shear strength contrast between gins (Fig. 11) we have correlated our results,the crustal wedge and its base, which is probably deduced from the North-Chilean outer forearcdepth-dependent. wedge, with the wedge geometries of various pre-

This dynamical succession of mass transfer sent-day active margins. Stability fields are plottedmodes (erosion, underthrusting and underplating), for typical sedimentary accretionary wedges withas observed along the erosive North-Chilean low frictional base (Lallemand et al., 1994),margin, is similar to typical high frictional wedge crustal forearc wedges with high frictional wedge

base (a,b) and finally, the stability line forsystems (accretion, underthrusting and underplat-


25/29


Fig. 11. Stability field diagram for active subduction-related accretionary wedges and crustal forearc wedges with mean tapers of 30

transects across active continental margins (modified after Lallemand et al., 1994). Stability fields are plotted for typical sedimentary

accretionary wedges with low frictional base (mean friction angles and pore fluid pressure ratios from measurements and structural

considerations from Lallemand et al., 1994), (a+b) crustal forearc wedges with high-frictional wedge base, and stability line

for forearc wedges on the verge of the existence limit shaped by Coulomb-type basal tectonic erosion (mean friction angles and pore

fluid pressure ratios for derived and for calculated by the present wedge geometry of the North-Chilean outer forearc system,

hatched corridor indicates vertical shift of the erosional stability line by minor variation of the porefluid pressure ratio). Data set for

mean tapers of active margins from Lallemand et al., 1994 (table 2, p. 12 039). (i ) accretive; B1 South Barbados, B2 Barbados,

Mq Martinique, Gu Guadeloupe, Bb Barbuda, Hi Hikurangi, Na Nankai, Or Oregon, CA Central Aleutian; (ii)

intermediate: Mn Manila, Su Sumba, SK South Kermadec, Ka Kashima, J1 Japan 37, J2 Japan 3940, J3 Japan

4010, J4 Japan 4040, SK southern Kurile, Pe Peru, NH New Hebrides; (iii) non-accretive: NB New Britain, T1 Tonga

19, T2 Tonga 20, T3 Tonga 23, Os Osbourn, NK North Kermadec, NC2 northern Chile. NC1-Toe toe segment,

NC1-OFW internal segment of outer forearc wedge (this paper).

forearc wedges shaped by Coulomb-type basal lated by the present wedge geometry of the North-

Chilean outer forearc system (NC1-Toe andtectonic erosion. The mean friction angles and

pore fluid pressure ratios for the stability fields NC1-OFW in Fig. 11). In addition, the mean

tapers of 28 transects across active continentalare derived and for the stability line are calcu-


26/29


margins are plotted (compiled in Lallemand gins. Therefore, Coulomb-type basal erosion of

outer forearc wedges seems to be a fundamentalet al., 1994).

The geometric data for the typical accretionary process at non-accretive margins. Future detailed

analyses and global correlation of these erosiveand intermediate accretionary wedges are clustered

near the minimum tapered boundary of the associ- margins should improve our model. Furthermore,it is essential to study the deformation processesated stability fields. Therefore, these wedge geome-

tries are reflecting the dynamically stable state of and tectonic mass transfer during Coulomb-type

basal erosion by numerical and analogue-sandboxaccretionary and crustal wedges, which develop in

a pro-wedge setting at active margins. modelling.

But the most important result is given by the

stability line for erosive wedges, which correlates

with the geometric properties of numerous non-

accretive margins all over the world. The stabilityAcknowledgements

line, deduced by the wedge geometry of the erosive

toe segment in northern Chile, shows a strongField work for this study was carried out within

correlation with the wedge geometries of various the project TP A1, Sonderforschungsbereich 267present-day erosive margins. The data from theDeformationsprozesse in den Anden, Freie

active margins of Tonga (19S) and New BritainUniversitat Berlin, Technische Universitat Berlin

plot on to the erosive stability line, the dataand GeoForschungsZentrum Potsdam, financially

from the active margins of North Kermadec andsupported by the Deutsche Forschungsgemein-

Osbourn in the corridor (hatched area in Fig. 11)schaft, Bonn. We thank S. Lallemand and S.

close to the erosive stability line. This is a strongWdowinski for critical and constructive comments

indication for Coulomb-type basal erosion as theon an earlier version of this paper.

principal deformation processes at these non-accre-

tive margins and for the validity of our general

concept of Coulomb-type basal erosion and the

deduced structural approach for evaluating the

mechanical conditions for Coulomb-type basal Appendix: Numerical procedures: critical taper cal-

culations for non-cohesive Coulomb wedgeserosion at non-accretive margins.

In the present study, the computations for the

critical taper of non-cohesive wedges follow the14. Conclusions

classical approach and numerical procedures of

Dahlen (1984). The mathematical results areLong-term, Coulomb-type basal tectonic ero-

sion occurs under clearly determined mechanical shown in Table 1 (critical taper model results) and

are graphically illustrated by Mohr circleconditions and generates a dynamic-stable wedge

geometry and wedge segmentation with character- presentations.

In Appendix 1 all procedures for the calcula-istic deformation and mass transfer patterns.

Consequently, it is possible to get new insights tions are summarised. The computation and pre-sentation of the procedures are generated by theinto the dynamics of present day erosive outer

forearc systems from structural considerations. As software program MC (trademark of

Mathsoft, Inc.), licensed to C.-D. Reuther). Theshown for northern Chile, the analysis of the

diagnostic features (e.g. the wedge geometry and software workflow requires a prior definition of

units and quantities. Therefore, the equations aresegmentation) by frictional wedge modelling gives

extensive information about the internal and basal shown with an example data set and the order of

presentation is determined by software require-mechanical properties of the outer forearc wedge

which is additionally confirmed by a global geo- ments and differs from the order given in the

referenced literature.metric comparison with other non-accretive mar-


27/29


Appendix 1

Units: MPa)106 Pa; kPa)103 Pa; Grad)p/180 rad

Example data set: internal outer forearc wedge

(data set 3, submerged, intermediate base)

Global parameter set: dw)1030 kg m3

; dlith)2600 kg m3

;w)35 Grad, m)tan (w); m=0.7; w

b=35 Grad; m

b)tan(w

b); m

b=0.7

Wedge specific parameter set: l=0.6 (submerged); lb=0.7

(intermediate base)

Critical topographic slope: a)2 Grad

Numerical procedures for critical taper calculations for non-cohesive Coulomb wedges (Dahlen, 1994):

wb)arctan ( m

b) w

b=27.7 Grad

[1 ] mb)m

bA1l

b

1l BEffective coefficient of basal

mb)tan (w

b) m

b=0.53traction (Dahlen, 1994, eq. 13)

a=3 Grad Modified slope angle[2 ] a)arctan GC

1(dw

/dlith

)

1l D tan (a)H (Dahlen, 1994; eq. 10)Y0=

1.1 Grad Angle between maximum stress direction[3 ] Y0)1

2arcsin

Csin (a

)

sin (w)D1

2a and wedge surface (Dahlen, 1994; eq. 9)

Yb=13.2 Grad Angle between maximum stress direction

[4 ] Yb)1

2arcsin C

sin (wb )

sin (w) D12 wb and wedge base (Dahlen, 1994; eq. 19)[5 ] b)Y

bY

0a b=10 Grad Required dip of wedge base to obtain the

critical taper for given topographic slope

[6 ] a+b=12 Grad Critical taper

[7 ] x)(1+m2)1/2 sin (2Yb

) x=0.54 Ratio of basal to internal shear strength

[8 ] db)1

2arctan (1/m)Y

bdb=14.3 Grad Ramp angle of forethrust-related wedge

internal shear planes

[9 ] db)1

2arctan (1/m)+Y

bdb=40.7 Grad Ramp angle of backthrust-related wedge

internal shear planes

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Cloos, M., Shreve, R.L., 1988a. Subduction-channel model ofAdam, J., 1996. Kinematik und Dynamik des neogenen Falten-

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Cloos, M., Shreve, R.L., 1988b. Subduction-channel model ofAllmendinger, R.W., Jordan, T.E., Kay, S.M., 1997. The evolu-

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and subduction erosion at convergent plate margins: PartAnnu. Rev. Earth Planet. Sci. 25, 139174.II. Implications and discussion. Pure Appl. Geophys. 128,

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forearcs. Nature 304 (5922), 148151.and-thrust belts and accretionary wedges: Cohesive Cou-

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