active faults, seismicity and stresses in an internal ...hera.ugr.es/doi/15781823.pdf · active...

www.elsevier.com/locate/tecto

Tectonophysics 396

Active faults, seismicity and stresses in an internal boundary

of a tectonic arc (Campo de Dalıas and Nıjar,

southeastern Betic Cordilleras, Spain)

Carlos Marın-Lechadoa, Jesus Galindo-Zaldıvarb,*,

Luis Roberto Rodrıguez-Fernandeza, Inmaculada Serranoc, Antonio Pedrerab

aInstituto Geologico y Minero de Espana, Rıo Rosas 23, 28003 Madrid, SpainbDepartamento de Geodinamica, Universidad de Granada, 18071 Granada, SpaincInstituto Andaluz de Geofısica, Universidad de Granada, 18071 Granada, Spain

Received 9 June 2004; received in revised form 4 October 2004; accepted 2 November 2004

Abstract

The Betic-Rif Cordilleras, formed by the interaction of NW–SE convergence between the Eurasian and African plates and

the westward motion of their Internal Zones, provide a good example of an active tectonic arc. The Campo de Dalıas and

Campo de Nıjar constitute outcropping sectors of Neogene and Quaternary rocks located in the southeastern border of the Betic

Cordilleras and allow us to study the recent deformations developed in the internal border of this tectonic arc.

The main active faults with related seismicity, representing a moderate seismic hazard, associated to the southeastern Betic

Cordilleras boundary, include high-angle NW–SE-oriented normal faults that affect, at least, the upper part of the crust, a main

detachment located at 10 km depth, and probably another detachment at 20 km as well. Seismite structures, recent fault scarps

with associated colluvial wedges that deform the drainage network and the alignment of the coastline, indicate that the high-

angle faults have been active at least since the Quaternary.

Paleostresses determined from microfault analysis in Quaternary deposits generally show an ENE–WSW trend of extension.

Present-day earthquake focal mechanisms include normal, strike-slip and reverse faulting. Normal and strike-slip focal

mechanisms generally indicate ENE–WSW extension, and strike-slip and reverse focal mechanisms are related to NNW–SSE

compression.

The maximum horizontal compression has a consistently NNW–SSE trend. The deep activity of detachments and reverse

faults determines the NNW–SSE crustal shortening related to the Eurasian–African plate convergence. At surface, however, the

predominance of normal faults is probably produced by the increase in the relative weight of the vertical stress axis, which in

turn may be related to relief uplift and subsequent horizontal spreading. The internal mountain front boundary of the Betic

Cordilleras developed through the activity of a set of structures that is more complex than a typical external mountain front,

0040-1951/$ - s

doi:10.1016/j.tec

* Correspon

E-mail addr

(L.R. Rodrıguez

(2005) 81–96

ee front matter D 2004 Elsevier B.V. All rights reserved.

to.2004.11.001

ding author. Tel.: +34 958 24 3349; fax: +34 958 24 8527.

esses: [email protected] (C. Marın-Lechado)8 [email protected] (J. Galindo-Zaldıvar)8 [email protected]

-Fernandez)8 [email protected] (I. Serrano).

C. Marın-Lechado et al. / Tectonophysics 396 (2005) 81–9682

probably as a consequence of a vertical variable stress field that acted on previously deformed rocks belonging to the Internal

Zone of the cordilleras.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Internal mountain front; Active faulting; Seismicity; Betic Cordilleras

1. Introduction

The active structures related to mountain fronts

have been generally studied in the external boundaries

of cordilleras, where they deform sedimentary rocks

and limit the foreland basins as in the Alps (Ricci-

Lucchi, 1986; Linzer et al., 2002) or the Rif

Cordilleras (Bargach et al., 2004). In these sectors,

the main active deformations are compressive struc-

tures such as reverse faults and folds that may be

associated to blind faults (Keller and Pinter, 1996;

Burbank and Anderson, 2001). In most cordilleras,

deformation propagates towards the mountain front,

leading to piggy-back sequences. This setting con-

trasts their internal zones, where extension occurs and,

in some cases, back-arc basins develop (e.g. Aegean

sea, Doutsos and Kokkalas, 2001).

There are also curved cordilleras or tectonic arcs,

characteristic of the alpine Mediterranean chains,

where mountain fronts are formed in the internal

borders. These mountain fronts, less studied up to

date, usually develop upon previously deformed

rocks; as a consequence, the features of the active

structures may be different than in the external fronts.

The previous discontinuities, with variable trends,

generate complex kinematics for the fault system due

to reactivation processes (Bott, 1959).

An example of internal mountain front associated

with an arched orogen can be studied in the Betic and

Rif Cordilleras. The latter are separated by the

Alboran Sea, which constitutes an internal basin

(Fig. 1A). The Alboran Sea has decreased in size

since the Tortonian because of the propagation of

deformation related to the African–Eurasian conver-

gence. Previous studies carried out in the SE Betic

Cordilleras (Fourniguet, 1976; Baena and Ewert,

1983; Goy and Zazo, 1986; Rodrıguez-Fernandez

and Martın-Penela, 1993; Montenat and D’Estevou,

1995; Martınez-Dıaz, 2000; Marın-Lechado et al.,

2003) indicate the presence of several faults with

recent activity, although there is no consensus on the

kinematics and their significance in the regional

context. The geodetical study of Gimenez et al.

(2000) evidences the presence of active tectonic

structures, mainly NW–SE normal faults, that deform

this region with rates that reach up to 1.5 mm/year

near Almerıa. The seismic activity in the Betic-Rif

cordilleras is widely distributed, but the southeastern

boundary of the Betic Cordilleras shows more intense

seismicity, as was evidenced by Mezcua and Marti-

nez-Solares (1983) in the Spanish Earthquake Cata-

logue. The recorded seismicity, including a seismic

series in 1994–1995, indicates a higher seismic hazard

for this region.

The aim of this contribution is to study the

active mountain front located in the boundary

between the SE Betic Cordilleras and the Alboran

Sea. The main features of active faults along this

internal mountain front were studied together with

seismological and geological data in order to

propose a tectonic model that would explain the

great variability of observed structures. In addition,

this paper aims to determine the location and

features of major active faults with the associated

seismic hazard. Paleostress will also be determined

in order to improve our knowledge of the recent

evolution of the mountain front.

2. Geological setting

The studied area is located in the boundary

between the southeastern Betic Cordilleras and the

northern Alboran Sea Basin. These mountain chains,

together with the Rif Cordilleras, constitute the

westernmost end of the Mediterranean Alpine belt,

resulting mainly from the convergence between the

African and European plates during late Mesozoic and

Cenozoic times, including NW–SE convergence since

the late Miocene (De Mets et al., 1990).

Fig. 1. (A) Geological setting of the Betic and Rif Cordilleras. (B) Geological map of Campo de Dalıas and Campo de Nıjar (SE Betic Cordilleras).

C.Marın

-Lech

adoet

al./Tecto

nophysics

396(2005)81–96

83

Table 1

Historic seismicity in Almerıa region

Date Intensity MSK Locality

09/11/1518 IX Vera

22/09/1522 IX Southern Adra

19/04/1550 VII Almerıa

31/12/1658 VIII Almerıa

13/01/1804 VIII Dalıas

21/01/1804 VIII Adra

25/08/1804 IX Southwestern Adra

28/01/1872 VII Motril


Traditionally, the Betic Cordilleras have been

divided into three tectonic domains: the External

Zones, the Internal Zones and the Campo de Gibraltar

Units (Fig. 1A). The studied area is located within the

Internal zones, which correspond approximately to the

Alboran Domain (Balanya and Garcıa-Duenas, 1987).

The Alboran Sea, located south of the studied area,

has been interpreted as a Mediterranean-style back-arc

basin (Horvath and Berckhemer, 1982).

In Campo de Dalıas and Campo de Nıjar, located

in the Internal Zones of the Cordilleras, both volcanic

rocks and the north Alboran Basin Neogene and

Quaternary sedimentary rocks have emerged in recent

times. The Sierra de Gador, formed by Alpujarride

phyllites and marbles, and the Sierra Alhamilla, which

also includes Nevado-Filabride schists and quartzites

(Fig. 1B), constitute the basement of these basins that

continue towards the Alboran Sea below the Neogene

and Quaternary sediments. The Campo de Nıjar is

limited to the southeast by the Cabo de Gata volcanic

complex, mainly composed by Tortonian volcanic

rocks (Bellon et al., 1983). The Carboneras Fault is

the major brittle structure in the study area and is

thought to represent the on-shore segment of the

major sinistral transcurrent Trans-Alboran shear zone

(Montenat et al., 1987; De Larouziere et al., 1988).

The sedimentary sequence deposited on the meta-

morphic complexes includes intercalations of volcanic

rocks and begins with Middle Miocene marls located at

small outcrops. Most of the sedimentary infill has been

deposited since the Upper Miocene and includes

Tortonian–Messinian calcarenites, marls, limestones

and gypsum cropping-out at the southern boundaries of

the mountain ranges (Perconig, 1976; Addicott et al.,

1979; Baena and Ewert, 1983). The stratigraphic

sequence continues with Pliocene siliciclastic sedi-

ments with well represented calcarenite levels and

quaternary detritic sediments of continental and marine

depositional systems (Fourniguet, 1976; Goy and

Zazo, 1986; Aguirre, 1998). While important active

alluvial fans develop near the main reliefs, uplifted

Pleistocene marine terraces are located along the coast.
12/06/1894 VII Nacimiento
25/05/1901 VII Motril

16/06/1910 VII Adra

16/06/1910 VIII Adra

11/08/1913 VII Albunol

Maximum intensity (MSK) upper than VI from 1500 up to 1994

(from Lopez-Marinas, 1977).

3. Seismicity

The southeastern Betic Cordilleras is characterized

by continuous shallow seismic activity of low to

moderate magnitude and less frequent large earth-

quakes. Historical and archaeological evidence indi-

cates that over the past 2000 years the study area has

been affected by at least 50 destructive earthquakes,

which demonstrate that its seismic hazard is certainly

significant. Major earthquakes and their associated

damage have been often reported since Islamic times.

Arab chroniclers describe in detail main shocks,

surface breaks and related damage distribution in the

southeastern Iberian Peninsula as early as in the ninth

century (Sanchez, 1917). However, it is from the 15th

century onward when we have more precise descrip-

tions and historical data to shed more light on this

seismic region (Table 1). For example, the earthquake

of September 22, 1522 is extensively described by

historical researchers who document the severity of

the damage on the southeastern Iberian Peninsula.

These descriptions point to an epicentre with an

estimated maximum intensity IX (MSK) (Vincent,

1974), showing that the earthquake shook the North-

ern African continent and was one of the most

dangerous earthquakes ever identified in the history

of this area. Later on, in August 1804, researchers

extensively describe a long period of seismic activity

that affected Adra and its surrounding regions. Lopez-

Marinas (1977), using historical sources defined the

epicentral area as an estimated maximum intensity IX

(MSK). According to Vidal (1986), the earthquake of

June 16, 1910 near Adra reached intensity VIII

(MSK). Reported details on this earthquake reflect

C. Marın-Lechado et al. / Tectonophysics 396 (2005) 81–96 85

the severity of the damage in a wide macroseismic

area, from the middle of Spain to Morocco. The

existence of numerous historical descriptions, the

available archaeoseismic evidence and the daily

seismic activity recorded reveal the relatively high

seismic hazard of the southeastern Iberian Peninsula.

Fig. 2. (A) Historical seismicity (upper right picture) and the most recent se

Seismic Network from 1992 to 2003. (B) NNW–SSE cross section located

the active fault surfaces.

The most intensive earthquake activity recorded

recently in this region occurred from December 1994

to January 1995, when a seismic series including a

Mb=4.9 and Mb=5.0 mainshocks took place near Adra

(maximum intensity VII, MSK). Over the following 3

months, 350 events (Mdz1.5) were recorded in the

ismic activity with Mdz1.5 (792 events) recorded by the Andalusian

in the part A of the figure. The black arrows indicate the position of

Fig. 3. Enlarged area of Fig. 4(A) with epicentre locations near

Balanegra Fault. Grey lines point to interpreted epicentre earthquake

lineations.


area, most of their hypocentres ranging from 0 to 12

km in depth. This series and the most recent seismic

activity (792 events) recorded by the Andalusian

Seismic Network (RSA ), from 1992 to 2003

(Mdz1.5), are included in Fig. 2. We exclude the

events shallower than 2 km, which may have been

produced by other sources.

The RSA belongs to the Andalusian Geophysics

Institute of the University of Granada, and was

established in the nineties. It has five short-period

seismic sites near the study area and eight sites

westward. Soon after the mainshock of January 1995,

three short-period sites were deployed to record at

Table 2

Earthquake focal mechanism in the SE Betic Cordilleras (Stich and Mora

Id Date P-axis

Az/plunge

T-axis

Az/plunge

Moment

magnitude Mw

1 97/07/02 354/21 085/04 4.4

2 00/05/27 347/18 077/00 3.6

3 97/08/07 344/08 077/20 3.6

4 94/01/04 177/49 068/15 4.9

5 94/01/04 352/19 112/55 4.9

6 93/12/23 178/47 054/28 4.8

7 98/10/16 147/01 56/58 3.6

8 02/02/04 262/78 077/12 4.7

9 98/04/06 335/11 237/33 3.9

10 99/06/14 145/26 251/29 3.7

short epicentral distances (for events in the study

area). The spatial distribution of seismicity defines an

approximately NNW–SSE elongated region limited to

the North and South by the two mainshocks (Fig. 2).

A detailed observation of the epicentre distribution

allows us to outline approximately 5-km long

lineations with a variable N–S to NW–SE trend

(Fig. 3). These lineations would be parallel to some

of the faults drawn from field data. The NNW–SSE

cross section (Fig. 2) shows that the cutoff depth of

the earthquakes of the Adra series dips 308 northwards(S1, Fig. 2), connecting with the cutoff depth of this

region at 10 km depth (S2, Fig. 2), where a certain

concentration of the seismicity may be related to a

detachment. At 20 km depth (S3, Fig. 2), there is a

less intense band of seismicity that may represent

another deeper detachment fault. In addition, a north-

dipping broad band is identified between 30 and 40

km depth (S4, Fig. 2).

Due to the low-to-medium magnitude of the

events and the small number of seismic sites in the

region before the eighties, their focal mechanisms

were difficult to calculate. In recent times, the

enlarged and improved seismic networks allowed us

to determine the focal mechanisms of earthquakes

with magnitudes over 3.5 with sufficient accuracy

(Rueda et al., 1996; Thio et al., 1999;Stich and

Morales, 2001, 2003). Most focal mechanism sol-

utions shown in this paper (Table 2) are taken from

Stich and Morales (2001) and Stich (2003), applying

time domain moment tensor inversion of waveforms

to these shallow earthquakes. For each event, a

moment tensor inversion and a double-couple grid

search has been performed. For simple faulting

les, 2001; Stich, 2003)

Depth (km) Location lat./long. Reference

10 36.367/�3.255 IAG

16 36.362/�3.131 IAG

16 36.452/�3.238 IAG

2 36.340/�2.490 Stich and Morales, 2001



20 36.949/�2.643 Stich, 2003

10 37.091/�2.547 Stich, 2003

8 37.012/�1.792 Stich, 2003

8 37.338/�2.174 Stich, 2003


events, the grid search is valuable in revealing

potential ambiguities of the solutions and in assessing

confidence limits of fault plane parameters or

principal axes orientation (Stich, 2003). Fault-plane

solutions calculated for most of the earthquakes (sites

1, 2, 3, 9 and 10; Fig. 4) are characterized by strike-

slip faulting, while a few solutions involve normal

faulting (site 8; Fig. 4) or oblique-slip faulting (sites

4 and 6; Fig. 4). Only two of the solutions

correspond to reverse faulting (sites 5 and 7; Fig.

4). The azimuths of the P-axis mainly trend NNW–

SSE and plunge near 08 (except the solution of event

8, where P-axis is near subvertical).

It is possible to determine the orientation of stress

axes using data from solutions of focal mechanisms

(Angelier and Melcher, 1977; Angelier, 1984) and the

ratio between principal stress values (Gephart and

Forsyth, 1984; Gephart, 1990). Main stress axes are

Fig. 4. (A) Earthquake focal mechanism solutions of SE Betic Cordilleras

inside the stereoplots indicate the P-axis. (B) Stress determination from the

and Melcher, 1977; Angelier, 1984).

determined by right-dihedra diagrams (Angelier and

Melcher, 1977; Angelier, 1984), which show the

percentage of compression or extension dihedra of

each orientation in stereographic projections. This

technique was applied to our data set to reveal high

levels of consistency (100% of compression and

tension) with a clear NNW–SSE trending r1 and a

WSW–ENE trending r3 (Fig. 4).

The regional stress directions and the axial ratio

that fit best with the available focal mechanisms are

determined by a grid search of stress ellipsoids

under the assumption of uniform stress in the source

region (Gephart and Forsyth, 1984 and Gephart,

1990). An stress ellipsoid characterized by r1:

03,175 (plunge, trend), r3: 31,267, R=(r2�r3)/

(r1�r3)=0.4 was obtained from our data set. Main

axis orientations are close to those obtained by right-

dihedra diagrams.

obtained by Stich and Morales (2001) and Stich (2003). Black dots

analysis of the focal mechanisms by right-dihedra method (Angelier


4. Recent fault activity

The major fault in the study area is the Carboneras

Fault Zone (CFZ), which is located on the SE side of

the Campo de Nıjar. The CFZ has been described as a

sinistral strike-slip fault, striking approximately NE–

SW (Bousquet and Montenat, 1974). About 30 km of

sinistral slip has taken place since the Tortonian

(Montenat and D’Estevou, 1995) due to NW–SE to

Fig. 5. Field examples of different structures in the Campo de Dalıas and

sedimentary clastic wedge. (B) Fault scarp developed in piedmont and a

Holocene continental sediments south to the El Ejido town. (D) Tensional

with an E–W trend in lower Pliocene calcarenites south of Sierra Alhami

N–S shortening (Montenat and D’Estevou, 1995;

Huibregtse et al., 1998). During the Quaternary, the

CFZ is characterized by vertical uplift rather than

strike-slip movement (Bell et al., 1997).

Other smaller faults widely represented in the

studied area are normal faults with a mean NW–SE

orientation affecting up to Quaternary sediments (Fig.

1B), generally oblique to the elongation of the sierras,

and more abundant towards the SW lateral termina-

Campo de Nıjar. (A) Half graben structure filled with a Holocene

lluvial sediments in the Sierra de Gador. (C) Seismite structure in

joint developed in Quaternary alluvial sediments. (E) Open anticline

lla.


tions of antiforms. The Quaternary faults that develop

in the Sierra de Gador generate visible fault scarps and

deform Holocene alluvial sediments giving rise to

clastic wedges (Fig. 5A). These faults are responsible

for high slopes and sharp topography in the western

part of Sierra de Gador and may deform previous low-

angle faults like the Castala Fault (Galindo-Zaldıvar et

al., 2003) that crop-out in this border of the Sierra.

The recent activity of these faults is pointed out by the

development of half graben structures filled by

Holocene sedimentary clastic wedges (Fig. 5A). Half

grabens are up to several hundred meters wide, and

open generally towards the NE, although opposite

senses of opening are also very frequent. In addition,

synsedimentary faults with E–W to NE–SW trend and

reverse regime have been observed in Pliocene

sediments (Fig. 1B).

Most of the Quaternary faults have straight or

slightly curved traces at cartographic and outcrop

scale. Length is also variable, 2 km in average, but

attain in some cases as much as 8 km (for example:

Balanegra Fault, Loma del Viento Fault and Cuevas

de los Ubedas Fault). Some of these faults are

segmented, like the Loma del Viento Fault. The fault

dips are generally high, reaching in some cases up to

708 southwestwards and even approaching 908. Themaximum vertical slips are generally of tens of

meters, although in the most important faults they

can reach up to a hundred of meters (e.g. Loma del

Viento Fault and Balanegra Fault). The fault planes

are generally striated, showing slickensides that

enable kinematic interpretation. The displacement of

subhorizontal bedding and the topographic scarps also

help to interpret the sense of fault movements. The

Quaternary faults are normal, featuring oblique slip

with dextral or sinistral components, depending on

their orientation, and compatible with a single stress

ellipsoid with WSW–ENE extension (Marın-Lechado

et al., 2003). The most recent faults may be

considered faulted joints, developed from previous

hybrid joints formed in Pliocene times (Marın-

Lechado et al., 2003).

The seismicity in this area is also evidenced by the

presence of liquefaction processes in recent sediments

that can give rise to seismites. In the Campo de Dalıas,

south of the town of El Ejido, these structures can be

recognised in continental sediments of the distal parts

of Holocene alluvial fans. The size of these structures

varies from 0.2 to 1 m in wavelength and 0.1 to 0.5 in

height. The load cast morphology is irregular, sagging

or drop type (Alfaro et al., 1997) (Fig. 5C). The

seismites are made up of cemented clayey silt over-

lying fine sands. In Campo de Nıjar a small seismite

develops in Pliocene sediments near the Cueva de los

Ubedas Fault. Other seismites are located in the

southwest end of Carboneras fault in Pleistocene

sands and clays of continental character. Their size

varies from 0.4 to 1 m in wavelength and 0.2 to 0.5 in

height and the morphology is dome-like with an

irregular load cast.

In addition to these structures, the southern part of

the Nıjar Basin holds evidence of recent landslides

that may have developed with very low slope (around

158), possibly induced by the seismic activity of the

region. These landslides deform the sliced block

considerably and produce a complex array of reverse

faults, strike-slip faults and folds.

5. Other active or recent structures

5.1. Joints

Field data from 14 widely spread sites in Pliocene

and Quaternary rocks of the Campo de Dalıas and

Nıjar area indicate the presence of different sets of

subvertical joints (Fig. 5D). The size of the fractures is

highly variable, ranging from decimetric to hecto-

metric, the latter being recognisable in aerial photo-

graphs. The geometry of fractures allows us to define

their character and put forth some evidence regarding

paleostresses.

The Pliocene and Quaternary rocks are affected

generally by NNW–SSE striking joints distributed in

one or two sets. According to the architecture of the

joint system (Hancock, 1985), these joints may be

interpreted as tensional with one joint set or two

orthogonal joint sets (sites 4, 6, 9 and 10 of Fig. 6),

hybrid with two joint sets forming an angle between

08 and 608 (sites 1, 2, 3, 5 and 12 of Fig. 6) or shear

characterized by two joint sets with angle about 608(sites 7 and 8 of Fig. 6). While in the Campo de Dalıas

predominate the hybrid joints, in the Campo the Nıjar

the shear and tensional joints are most frequent. Late

Pleistocene–Holocene sediments seal the hybrid joints

of some outcrops. In the northern Campo de Dalıas,


these sediments are affected by a single tensional joint

set with N1208E strike (site 4; Fig. 6). In the Campo

de Nıjar region, the Quaternary sediments also are

locally deformed by a system of conjugate hybrid

joints.

5.2. Folds

Large open folds with associated uplift determine

the present relief in the central Cordilleras (Weijer-

mars et al., 1985; Johnson, 1997; Galindo-Zaldıvar et

al., 2003). The Sierra de Gador and Sierra Alhamilla,

among others, are roughly E–W elongated high

mountain ranges, corresponding to antiforms where

metamorphic rocks crop out. The Quaternary folding

and present-day activity is less evident from field

observation, but the generalized regional uplift of

Quaternary marine terraces throughout the region

(Zazo et al., 2003) may be due to the development

of these folds (Figs. 1B and 5E).

Fig. 7. (A) Aerial photograph showing a fault affecting Holocene

alluvial fan located in Fig. 2. (B) Sketch of drainage network pattern

influenced by the fault.

6. Tectonic geomorphology

There are several landforms in the study area that

show evidences of neotectonic activity. Most of the

Quaternary faults exhibit scarps near fault surfaces

that are affected by erosion and can reach several tens

of meters. However, this morphology is more frequent

and evident in Campo de Dalıas than in Campo de

Nıjar. The vertical scarps have average heights of

about 1–2 m, although they can reach 15 m, for

instance in the segmented Loma del Viento Fault (Fig.

7). The Balanegra Fault zone is formed by several

parallel faults with associated scarps of 1–10 m

producing an staircase morphology. Some faults affect

the drainage network. Fig. 7 shows a normal fault

with a right-lateral component of slip that deflects the

drainage network and the channels flow parallel to the

fault. The uplift of the southwestern block contributes

to the development of offset and beheaded streams.

On the other hand, the Carboneras fault does not show

evidences for large lateral offset of the Quaternary

Fig. 6. Rose diagrams of joints affecting Pliocene and Quaternary sediments from several measurement sites in the Campo de Dalıas and Nıjar

The number of data (n) and the age of sediments in each site is indicated: Plio., Pliocene; Quat., Quaternary. The grey arrows indicate the trend

of maximum extension direction. Stereoplots of Quaternary fault planes and striae for each region are also included.

marine terraces, at least during the last 100 ka (Bell et

al., 1997).

The coastline of the Campo de Dalıas features

some straight segments several kilometres long and a

NW–SE trend that coincides with some of the

principal faults such us the Balanegra Fault and Punta

Entinas Fault. These straight segments evidence the

recent activity of the faults. Another segment between

Cabo de Gata and El Alquian shows the same

.


orientation, but no fault is visible upon field observa-

tion. The Carboneras Fault, with a very rectilinear

front, does not displace the present coastline.

The incision of a drainage network in Quaternary

alluvial fans and glacis is very pronounced in the

entire region. The alluvial fans develop high slopes in

the half-proximal section and the related deposits may

reach up to 160 m, as deduced from borehole data.

Usually, the main channel is strongly incised in the

head zone. Near the study area, in the northeastern

boundary of the Sierra de Gador, the drainage network

has incision rates between 0.3 and 0.7 m/ka, similar to

the regional uplift rates in the area (Garcıa et al.,

2003).

Pleistocene marine terraces develop in the Almerıa

littoral, providing information about recent uplift of

the region. Near the Loma del Viento Fault, 16 marine

terraces have been described forming a staircase

profile that goes up to 82 m above actual sea level

(Zazo et al., 2003). The uplift rate in the upthrow

block of the Loma del Viento over the last 130 ka is of

0.046 m/ka (Zazo et al., 2003), but uplift varies

throughout the region because of interaction with

development of the folds and faults.

7. Paleostress analysis

Tensional and hybrid joint sets that affect up to

Pleistocene marine terraces indicate a WSW–ENE

trend of extension and NNW–SSE compression. In

addition, fault orientations provide very complete

information about the deviatory stress ellipsoid and

Table 3

Paleostress ellipsoids determined from the search grid method (Galindo-Z

Nıjar

No. Age N N tot r1

1 Quaternary 5 5 144/81

2 Plio–Quaternary 14 17 172/78

3 Pliocene 14 17 073/70

4 Pliocene 11 13 223/71

5 Pliocene 8 9 334/88

6 Middle Messinian 10 13 196/69

7 Early Messinian 15 37 062/68

8 Triassic 5 15 225/75

Sites 1–5 own data in the Campo de Dalıas. Sites 7–8 are obtained from

measurements that fit the calculated tensor. N tot, total number of measurem

R, axial ratio, (r2�r3)/(r1�r3).

allow the determination of the most recent stresses.

The paleostress determination from faults let us

establish the main stress axis direction and the axial

ratio R=(r2�r3)/(r1�r3). Among the various meth-

ods proposed to study microfaults, we used the

Search Grid method (Galindo-Zaldıvar and Gonza-

lez-Lodeiro, 1988) for calculating paleostress ellip-

soids since it permits the determination of

overprinted stages on the basis of fault striae

orientations with known and unknown senses of

motion.

Five sites in the area were studied (Table 3) for the

determination of recent paleostress. We mainly

considered those outcrops where the deformation

affects up to Plio–Quaternary sediments for the

determination of recent paleostress. These sites have

a low amount of data and the results are not strongly

constrained, like in site 1 where only five faults have

been measured. Anyway, calculated stress ellipsoids

have similar features in all the sites. The ellipsoids

determined by Stapel et al. (1996) and Huibregtse et

al. (1998) in the northern boundary of Campo de Nıjar

using Angelier et al. (1982) method were also taken

into account.

The results obtained in the Campo de Dalıas (Table

3) indicate stress ellipsoids characterised by a near-

subvertical r1 axis and a near-subhorizontal, WSW–

ENE r3 trending axis (Fig. 8). The axial ratios of

these ellipsoids are generally low (between 0.04 and

0.29), indicating a prolate shape. Although most stress

ellipsoids show WSW–ENE trending r3 axis, in some

of them (e.g. sites 3 and 5), this orientation correspond

to r2 due to permutation related with low axial ratios.

aldıvar and Gonzalez-Lodeiro, 1988) in the Campo de Dalıas and

r2 r3 R Source

297/8 28/4 0.10 Own data

352/12 82/0 0.26 Own data

242/20 334/3 0.04 Own data

334/10 076/16 0.10 Own data

64/0 154/2 0.29 Own data

103/01 13/21 0.12 Stapel et al., 1996

310/09 216/20 0.58 Stapel et al., 1996

108/13 016/08 0.63 Stapel et al., 1996

Stapel et al. (1996) in the northern Campo de Nıjar. N, number of

ents. r2, r3, r1, trend and plunge of the main axes of stress ellipsoid.

Fig. 8. Stereoplots of stress axes obtained from palaeostress analysis. Equal area projection, lower hemisphere. The right upper number indicates

the site. The grey arrows indicate the trend of extension axes.


At site 3, the NW–SE orientation of r3 has practically

no significance due the close values of r2 and r3

(axial ratio of 0.04). In general, the low axial ratio of

these stress ellipsoids would facilitate local switching

between r2 and r3. The trend of extension in the north

boundary of the Campo de Nıjar obtained by Stapel et

al. (1996) is more variable, with intermediate axial

ratios according to the results obtained by Huibregtse

et al. (1998).

In the Campo de Dalıas, the evidence of synsedi-

mentary faulting helps to establish the age of

paleostresses. Most of the NW–SE striking faults

affect up to Quaternary deposits and lead to develop-

ment of half graben structures, like the Loma del

Viento, Matagorda or Balanegra faults, all indicating

NE–SW extension. In the northern Campo de Nıjar,

Stapel et al. (1996) conclude that this extension took

place during the Neogene–Quaternary, but the ori-

entation and timing of the extensional movements is

poorly constrained. Nevertheless, the similarity of the

stress ellipsoids in both regions may indicate that the

extensional stress ellipsoids predominate in the most

upper crust during the Quaternary.

8. Discussion and conclusions

The Campo de Dalıas and Campo de Nıjar are an

excellent region for analysing the active deformations

related to the internal mountain front of an orogenic

arc, due to the outcropping of Pliocene and Quater-

nary rocks that record recent deformations.

The geologic field observations reveal several

characteristics that indicate a recent activity of most

of the faults. The most relevant onshore faults,

including the Loma del Viento, Balanegra and Punta

Entinas faults, have a normal slip component and

determine the relief features. The fault activity

produces rectilinear scarps, sometimes with staircase

morphology, that also influence the drainage network

of the area. In addition, the generalised drainage

network incision and raised marine terraces may be a

consequence of regional uplift related to the open E–

W folds. The topographic relief of the nearest

mountains develops largely as a result of fluvial

denudation driven by regional uplift. The develop-

ment of clastic wedges with asymmetric filling of

Holocene sediments and the fault scarps cutting to

Holocene sediments are indicative of this activity.

However, the Carboneras Fault Zone, widely

described as a sinistral strike-slip structure since the

Tortonian (Montenat and D’Estevou, 1995) as a

consequence of NW–SE to N–S shortening (Montenat

and D’Estevou, 1995; Huibregtse et al., 1998), has

very scarce evidence of recent strike-slip activity (Bell

et al., 1997). The normal component in the nearest

Quaternary faults, the very low seismic activity and

the lack of evidence of displacement of the coastline

point to the absence of recent strike-slip activity for

the Carboneras Fault Zone. So, most of the faults


active in the uppermost crust have normal slip

component.

The presence of seismites in Pliocene and Quater-

nary sediments of the Campo de Dalıas and Nıjar and

the landslides developed on low slopes in the

southern sector of the Campo de Nıjar are additional

evidences of paleoearthquakes. Ambraseys (1988)

indicates that the minimum magnitude of the earth-

quakes in order to develop liquefaction structures is

M=5 and that liquefaction structures increase largely

with magnitude, being very common when magni-

tudes are above 5.5. The relationship between surface

rupture length versus seismic magnitude (Wells and

Coppersmith, 1994) allow an estimation of the

occurrence of maximum magnitude in the range of

5–5.5, considering that the maximum fault lengths

cropping-out in the region are of 8 km. However, the

projection of hypocenters on a vertical section

evidences the presence of a basal detachment at 10

km depth, which due to its larger surface, may

produce earthquakes of higher magnitude. All these

data suggest that the region has undergone moderate

seismic activity that continues up to present, related

with the development of the mountain front, where

the slip on the outcropping faults may be transferred

to crustal detachments.

The offshore area SW of Campo de Dalıas shows a

higher level of seismic activity (mainly concentrated

in the first 10 km of depth) than other areas of the

cordillera. The shallow seismicity shows NW–SE-

oriented lineations of epicenters that in some cases

coincide with the main Quaternary faults observed in

Campo de Dalıas (Balanegra Fault and Punta Entinas

Fault) (Fig. 3). An analysis of the distribution of

seismic data suggests the presence of a low-angle fault

that intersects the sea bottom south of the Campo de

Dalıas, dipping 308 northwards, and connecting with

the detachment at a depth of 10 km. In the context of

NNW–SSE regional shortening, this low-angle fault

may represent a main reverse fault related to the

frontal part of the mountain front. Its activity is

possibly related to the occurrence of reverse earth-

quake focal mechanisms (5, Fig. 4). The main brittle

deformations and folds are located in the upper block

of the crustal detachment and the low-angle north-

dipping fault. This distribution of seismicity may also

be a consequence of slightly high temperature

gradients in the source region, located near the

Alboran Sea, where high heat flow has been measured

(Basov et al., 1994). A deeper detachment located at

20 km depth (S3, Fig. 2) is less defined. In addition, a

north dipping band of seismicity at 30–40 km depth

(S4, Fig. 2) is also identified, although the hypo-

centers shows more dispersion and the available

geophysical and geological data do not allow to

determine the features of this structure located in the

mantle.

Recent and present-day stresses share many fea-

tures. Reverse low angle faults, hybrid joints and folds

localized in Pliocene sediments are compatible with a

paleostress ellipsoid characterized by a main WSW–

ENE extension and a perpendicular NNW–SSE

compression. During the Holocene, the paleostress

analysis from microfaults and joint sets indicates a

WSW–ENE to radial extension. The earthquake focal

mechanisms in the studied area show different

regimes (strike-slip, normal and reverse solutions).

However, the right-dihedra diagram indicates that a

single stress ellipsoid is compatible with all of the

earthquake focal mechanisms, characterized by a

subhorizontal NNW–SSE compression and a WSW–

ENE extension, with a probable triaxial shape, as

indicated by the Geparth method (Gephart and

Forsyth, 1984 and Gephart, 1990). The paleostress

ellipsoids obtained from shallow faulted Quaternary

sediments, however, show a subvertical r1 and a

subhorizontal WSW–ENE to radial oriented r3. This

variability of stresses may be the consequence of a

relative change of the vertical stresses from depth to

surface. In the deeper part of the crust, the horizontal

NNW–SSE compressive stresses that allow the

development of reverse and strike-slip faults may

predominate, while in the uppermost part of the crust

the relative value of the vertical stress increases and

normal faulting develops. The orientation of main

axes of horizontal stress ellipse remains nearly

constant, in the crust overall, with an ENE–WSW

minimum axis. If the horizontal stress values remain

nearly constant, the relative change of vertical stress

from depth to surface should be the opposite to the

described one: it should decrease as the lithostatic

load does. However, a decrease of horizontal stress

values is necessary, probably induced by horizontal

spreading concomitant to relief uplift.

The active structures described in this internal

mountain front of the Betic Cordilleras and the


standard external mountain fronts include common

features, such as the presence of reverse faults in

depth, dipping towards the mountain front, that join

a basal detachment. However, the studied internal

mountain front is more complex, as it includes a

large variety of linked active extensional and

compressional structures related with the same

horizontal stress ellipse, but which vary greatly in

depth. The upper crust has undergone mainly

normal faulting, with local strike-slip regimes, while

at depth reverse and strike slip faults predominate.

The trends of compression and extension are

slightly oblique to the mountain front, since the

latter was probably controlled by the orientation of

previously inherited structures, from earlier stages

of the tectonic arc development. The deformed

rocks are mainly heterogeneous metamorphic rocks

of the Internal Zones, which may behave very

differently than the low deformed rocks of the

External Zones. Another difference is that the

activity on the basal detachment extends beyond

the mountain front, towards the internal basin,

which is also affected by deformation. The basin

size is progressively reduced by the activity of these

structures. Finally, in the case of the Betic

Cordilleras, the activity of the studied internal

mountain front is substantially greater than the

external front of the Cordillera. All these consid-

erations would mean that internal and external

mountain fronts of arched orogens have different

features and types of evolution.

Acknowledgements

This study was supported by a PhD grant to the first

author from the IGME (Instituto Geologico y Minero

de Espana) and CICYT project BTE2003-01699.

References

Addicott, W.O., Snavely, P.D., Poore, R.Z., Bukry, D., 1979. La

secuencia neogena marina de los Campos de Dalıas y de Nıjar

(Almerıa). Estudios Geologicos 35, 609–631.

Aguirre, J., 1998. El plioceno del SE de la penınsula Iberica

(provincia de Almerıa). Sıntesis estratigrafica, sedimentaria,

bioestratigrafica y paleogeografica. Revista de la Sociedad

Geologica de Espana 11, 297–315.

Alfaro, P., Moretti, M., Soria, J., 1997. Soft-sediment deformation

structures induced by earthquakes (seismites) in Pliocene

lacustrine deposits (Guadix-Baza Basin Central Betic Cordil-

lera). Eclogae Geologicae Helveticae 90, 531–540.

Ambraseys, N., 1988. Engineering seismology. Earthquake Engi-

neering and Structural Dynamics 17, 1–105.

Angelier, J., 1984. Tectonic analysis of fault slip data sets. Journal

of Geophysical Research 89, 5835–5848.

Angelier, J., Melcher, P., 1977. Sur une methode graphique de

recherche des contraintes principales egalement utilisable en

tectonique et en seismologie: la methode des diedres droits.

Bulletin de la Societe Geologique de France 19, 1309–1318.

Angelier, J., Tarantola, A., Valette, B., Manousis, S., 1982.

Inversion of field data in fault tectonics to obtain the regional

stress: I. Single phase fault populations: a new method of

computing the stress tensor. Geophysical Journal of the Royal

Astronomical Society G9, 607–621.

Baena, J., Ewert, K., 1983. Mapa y memoria explicativa de la Hoja

1.058 (Roquetas de Mar). IGME Map 1058, scale 1:50,000.

Balanya, J.C., Garcıa-Duenas, V., 1987. Les directions structurales

dans le Domaine d’Alboran de part et d’autre du Detroit de

Gibraltar. Comptes Rendus de l’Academie des Sciences de Paris

304, 929–933.

Bargach, K., Ruano, P., Chabli, A., Galindo-Zaldıvar, J., Chalouan,

A., Jabaloy, A., Akil, M., Ahmamou, M., Sanz de Galdeano, C.,

Benmakhlouf, M., 2004. Recent tectonic deformations and

stresses in the frontal part of the Rif Cordillera and the SaRsBasin (Fes and Rabat regions, Morocco). Pure and Applied

Geophysics 161, 521–540.

Basov, I.A., Bogdanov, N.A., Comas, M.C., Polyak, B.G.,

Fernandez, M., Khain, V.Y., Khutorskoy, M.D., Agapova,

G.V., Alonso, B., Mazurova, I.S., Soto, J.I., 1994. Recent

sediments, heat flow and gravity field in the Alboran Sea

(Western Mediterranean). Comptes Rendus de l’Academie des

Sciences de Moscow 338, 229–234.

Bell, J.W., Amelung, F., Geoffrey, C., King, C.P., 1997. Preliminary

late quaternary slip history of the Carboneras fault, southeastern

Spain. Journal of Geodynamics 24, 51–66.

Bellon, H., Bordet, P., Montenat, C., 1983. Chronologie du

magmatisme neogene des Cordilleres Betiques (Espagne

Meridionel). Bulletin de la Societe Geologique de France 25,

205–218.

Bott, M.H.P., 1959. The mechanics of oblique faulting. Geological

Magazine 2, 109–117.

Bousquet, J.C., Montenat, C., 1974. Presence de decrochements

nord-est sud-ouest plioquaternaires, dans les Cordilleres

betiques orientales (Espagne). Extension et signification gen-

erale. Comptes Rendus de l’Academie des Sciences de Paris

278, 2617–2620.

Burbank, D.W., Anderson, R.S., 2001. Tectonic Geomorphology.

Blackwell Science, Berlin, pp. 274.

De Larouziere, F.D., Bolze, J., Bordet, P., Hernandez, J., Montenat,

C., Ott D3Estevou, P., 1988. The Betic segment of the

lithospheric Trans-Alboran shear zone during the late Miocene.

Tectonophysics 152, 41–52.

De Mets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current

plate motions. Geophysical Journal International 101, 425–478.


Doutsos, T., Kokkalas, S., 2001. Stress and deformation patterns in

the Aegean region. Journal of Structural Geology 23, 455–472.

Fourniguet, J., 1976. Quaternaire marin et neotectonique sur la cote

andalouse meridionale (Espagne). Comptes Rendus de l’Aca-

demie des Sciences de Paris 282, 1849–1852.

Galindo-Zaldıvar, J., Gonzalez-Lodeiro, F., 1988. Faulting phase

differentiation by means of computer search on grid pattern.

Annales Tectonicae 2, 90–97.

Galindo-Zaldıvar, J., Gil, A.J., Borque, M.J., Gonzalez-Lodeiro, F.,

Jabaloy, A., Marın-Lechado, C., Ruano, P., Sanz de Galdeano,

C., 2003. Active faulting in the internal zones of the central Betic

Cordilleras (SE Spain). Journal of Geodynamics 36, 239–250.

Garcıa, A.F., Zhu, Z., Ku, T.L., Sanz de Galdeano, C., Chadwick,

O.A., Chacon-Montero, J., 2003. Tectonically driven landscape

development within the eastern Alpujarran Corridor Betic

Cordillera, SE Spain (Almeria). Geomorphology 50, 83–110.

Gephart, J.W., 1990. FSMI: a fortran program for inverting fault/

slickenside and earthquake focal mechanism data to obtain the

regional stress tensor. Computer &Geosciences 16 (7), 953–989.

Gephart, J.W., Forsyth, D.W., 1984. An improved method for

determining the regional stress tensor using earthquake focal

mechanism data: application to the San Fernando earthquake

sequence. Journal of Geophysical Research 89, 9305–9320.

Gimenez, J., Surinach, E., Goula, X., 2000. Quantification of

vertical movements in the eastern Betics (Spain) by comparing

levelling data. Tectonophysics 317, 237–258.

Goy, J.L., Zazo, C., 1986. Synthesis of the Quaternary in the

Almerıa littoral neotectonic activity and its morphologic

features, western Betics Spain. Tectonophysics 130, 259–270.

Horvath, F., Berckhemer, H., 1982. Mediterranean backarc basins.

In: Berckhemer, H., Hsu, K. (Eds.), Alpine–Mediterranean,

Geodynamics, vol. 7, pp. 141–173.

Huibregtse, P., Alebeek, H.V., Zaal, M., Biermann, C., 1998.

Paleostress analysis of the northern Nıjar and southern Vera

basins: constraints for the Neogene displacement history of

major strike-slip faults in the Betic Cordilleras, SE Spain.


Johnson, C., 1997. Resolving denudational histories in orogenic

belts with apatite fission-track thermochronology and structural

data: an example from southern Spain. Geology 25, 623–626.

Keller, E.A., Pinter, N., 1996. Active Tectonics: Earthquakes, Uplift

and Landscape. Prentice Hall.

Linzer, H.G., Decker, K., Peresson, H., Dell, M.R., Frisch, W.,

2002. Balancing lateral orogenic float of the Eastern Alps.


Lopez-Marinas, J.M., 1977. Estudio del terremoto de 22 Septiembre

1522 en Almerıa. Publicacion Hidroelectrica Espanola. 16 pp.

Marın-Lechado, C., Galindo-Zaldıvar, J., Rodrıguez-Fernandez,

L.R., 2003. Joints, faults and paleostress evolution in the

Campo de Dalıas (Betic Cordilleras, SE Spain). Comptes

Rendus de l’Academie des Sciences 335, 255–264.

Martınez-Dıaz, J.J., 2000. Actividad neotectonica en el sureste de

Almerıa y su incidencia en la morfotectonica de la zona

(Cordilleras Beticas). Revista de la Sociedad Geologica de

Espana 13, 417–429.

Mezcua, J., Martinez-Solares, J.M., 1983. Earthquake catalogs

published recently by the National Geographic Institute.

Comunicaciones y Ponencias-Instituto Geografico Nacional 1,

65–67.

Montenat, V., D’Estevou, O., 1995. Late Neogene basins evolving

in the Eastern Betic transcurrent fault zone: an illustrated

review. In: Friend, P.F., Dabrio, C.J. (Eds.), Tertiary Basins of

Spain, pp. 372–386.

Montenat, C., D’Estevou, O., Masse, P., 1987. Tectonic-sedimen-

tary characters of the Betic Neogene Basins evolving in a crustal

transcurrent shear zone (SE Spain). Bulletin Centres Recherche

Exploration Prod. Elf-Aquitaine 11, 1–22.

Perconig, E., 1976. Estudio de algunos cortes del Neogeno de Sierra

de Gador y del Campo de Dalıas (Almerıa Espana). Revista

Espanola de Micropaleontologia 8, 141–178.

Ricci-Lucchi, F., 1986. The Oligocene to recent foreland basins of

the Northern Apennines. In: Allen, P.A., Homewood, P. (Eds.),

Special Publication of the Int. Ass. Sed., vol. 8, pp. 105–139.

Rodrıguez-Fernandez, J., Martın-Penela, J., 1993. Neogene evolu-

tion of the Campo de Dalıas and surrounding offshore areas

(Northeastern Alboran Sea). Geodinamica Acta 6, 255–270.

Rueda, J., Mezcua, J., Sanchez-Ramos, M., 1996. La serie sısmica de

Adra (Almerıa) de 1993–1994 y sus principales consecuencias

sismotectonicas. Avances en Geofısica y Geodesia 1, 91–98.

Sanchez, M., 1917. Datos referentes a la sismicidad de la porcion

oriental de la Penınsula Iberica. Memorias de la Real Academia

de Ciencias y Artes de Barcelona XIII (N8), 179–188.

Stapel, G., Moeys, R., Biermann, C., 1996. Neogene evolution of

the Sorbas basin (SE Spain) determined by paleostress analysis.


Stich, A.G., 2003. Inversion del tensor momento sısmico para

terremotos de la region Iberomagrebı. PhD thesis, University of

Granada, Granada.

Stich, A.G., Morales, J., 2001. The relative locations of multiplets in the

vicinity of the Western Almerıa (southern Spain) earthquake series

of 1993–1994. Geophysical Journal International 146, 801–812.

Thio, H.K., Song, X., Saikia, C.K., Helmberger, D.V., Woods, B.B.,

1999. Seismic source and structure estimation in the western

Mediterranean using a sparse broadband network. Journal of

Geophysical Research 104, 845–861.

Vidal, F., 1986. Sismotectonica de la region Beticas-Mar de

Alboran, PhD thesis, University of Granada, Granada.

Vincent, B., 1974. Les tremblements de terre dans la province

d’Almeria (XV–XIX siecle), Paris. Annales 3, 571–586.

Weijermars, R., Roep, T.B., Van den Eeckhout, B., Postma, G.,

Kleverlaan, K., 1985. Uplift history of a Betic fold nappe

inferred from Neogene–Quaternary sedimentation and tectonics

(in the Sierra Alhamilla and Almerıa, Sorbas and Tabernas

Basins of the Betic Cordilleras SE Spain). Geologie en

Mijnbouw 64, 397–411.

Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships

among magnitude, rupture length, rupture width, and surface

displacement. Bulletin of the Seismological Society of America

84, 974–1002.

Zazo, C., Goy, J.L., Dabrio, C.J., Bajardı, T., Hillarie-Marcel, C.,

Ghaleb, B., Gonzalez-Delgado, J.A., Soler, V., 2003. Pleisto-

cene raised marine terraces of the Spanish Mediterranean and

Atlantic coast: records of coastal uplift, sea-level highstands and

climate changes. Marine Geology 194, 103–133.

active faults, seismicity and stresses in an internal ...hera.ugr.es/doi/15781823.pdf · active...

Documents