Elsevier

Tectonophysics

Volumes 548–549, 15 June 2012, Pages 1-21
Tectonophysics

Thrust–wrench interference between major active faults in the Gulf of Cadiz (Africa–Eurasia plate boundary, offshore SW Iberia): Tectonic implications from coupled analog and numerical modeling

https://doi.org/10.1016/j.tecto.2012.04.013Get rights and content

Abstract

Analog and numerical modeling experiments were carried out to investigate the tectonic interference between intersecting major active strike–slip and thrust faults in the Gulf of Cadiz (Africa–Eurasia plate boundary, offshore SW Iberia). The obtained results show that newly mapped tectonic features located in the fault intersection area (corner zone) consist mostly in oblique (dextral-reverse) faults that accommodate significant strain partitioning. Modeling of this corner-zone faults show that they have endured some degree of rotation, displaying successive evolving geometries and kinematics. Numerical modeling results further show that an interbedded shallow soft layer, accounting for a regional (Late Miocene) gravitational “Chaotic” unit, could explain the mild bathymetric expression of the fault pattern in the corner-zone. Moreover, a recognized depth discrepancy, between the (upper crust) interference fault-pattern and the (lithospheric mantle) seismicity, is interpreted as a manifestation of similar thrust–wrench tectonic interference at different lithospheric depths. Accordingly, an intermediate lower crust–upper mantle aseismic (i.e. softened) depth-domain could be explained by pervasive alteration/serpentinization, prompted by fluid percolation through fault-related fractures associated with the newly revealed corner zone fault-network. Overall obtained results reinforce the relevance of a thrust–wrench multi-rupture seismic scenario as the main cause for the moderate seismicity (Mw < 6.0) in the study area.

Highlights

► New map of an intersection zone between two major faults in the Gulf of Cadiz area. ► Coupled analog and numerical modeling of fault interference in the mapped area. ► Recognition of a thrust–wrench interference (“corner zone”) tectonic pattern. ► Recognition of a multi-rupture scenario, relevant for seismic-related hazards. ► Decoupled manifestation of similar tectonic interference at different depths.

Introduction

The Gulf of Cadiz (Fig. 1A) has long been considered a key domain to unravel the complex tectonics of the Eurasia–Africa plate boundary (e.g. Duarte et al., 2011, Gràcia et al., 2003a, Gràcia et al., 2003b, Gutscher et al., 2002, Gutscher et al., 2009a, Gutscher et al., 2009b, Rosas et al., 2009, Sallarès et al., 2011, Sartori et al., 1994, Terrinha et al., 2003, Terrinha et al., 2009, Tortella et al., 1997, Zitellini et al., 2001, Zitellini et al., 2004, Zitellini et al., 2009). It corresponds to a specific segment of this boundary characterized by the interplay between the Iberia and Nubia subplates, connecting the (Atlantic) transform Gloria Fault, to the West, with the dextral transpressive Rif–Tell shear zone (Morel and Meghraoui, 1996) to the East of Straits of Gibraltar, and across the Betic–Rif orogenic arc. In the Gulf of Cadiz domain, the Iberia–Nubia plate boundary has been considered of a diffuse nature (e.g. Medialdea et al., 2004, Sartori et al., 1994). Accordingly, present day WNW–ESE convergence between both plates at a ~ 4–5 mm/yr rate, (e.g. Fernandes et al., 2007, Nocquet and Calais, 2004, Serpelloni et al., 2007, Stich et al., 2006, gray line in the inset of Fig. 1B) is here accommodated by a considerable number of widespread and differently orientated active tectonic structures, mostly consisting of strike–slip and thrust faults (Fig. 1B). During the last decade, the acquisition and interpretation of geophysical data (e.g. reflection/refraction seismics and multi-beam swath bathymetry) led to the progressive discovery of several new morphotectonic features, resulting in the continuous improvement of the Gulf of Cadiz tectonic map (see Fig. 1B, Bartolome et al., 2012, Duarte et al., 2009, Duarte et al., 2010, Gràcia et al., 2003a, Gràcia et al., 2003b, Rosas et al., 2009, Terrinha et al., 2003, Terrinha et al., 2009, Zitellini et al., 2004, Zitellini et al., 2009). Recently, based on a new wide-angle refraction seismic (WAS) profile (Fig. 1B), Sallarès et al. (2011) provided new insight on the nature of the crust across different morphotectonic domains in the central part of the Gulf of Cadiz, and proposed the location of the lithospheric continent–ocean boundary (COB) at a distance of approximately 100 km from the Southern Iberian coast line (see COB in Fig. 1B).

The seismicity that has been recorded in the Gulf of Cadiz corresponds to a general scenario of moderate magnitude at shallow to intermediate depths (e.g. Borges et al., 2001, Buforn et al., 1995, Buforn et al., 2004, Engdahl et al., 1998, Fukao, 1973, Grimison and Chen, 1986, Stich et al., 2005), in which, though, a direct correlation between earthquake location and known major tectonic structures is not straightforward. Large magnitude instrumental and historical events also occurred, such as the 28/02/1969 earthquake (Ms = 7.9), and the highly destructive 1755 Great Lisbon Earthquake (estimated magnitude between 8.5 and 8.8, e.g. Abe, 1979, Johnston, 1996, Solares and Arroyo, 2004), and associated tsunami (e.g. Baptista and Miranda, 2009, Baptista et al., 1998a, Baptista et al., 1998b, Terrinha et al., 2003, Zitellini et al., 2001). The recurrence interval of these great events (Mw > 8.0) has been estimated in 1800 years based on turbidite paleoseismology approach (Gràcia et al., 2010). In addition, a network of broadband OBS deployed in the area during a year, confirmed greater depths for low-magnitude local earthquakes (40–60 km, Geissler et al., 2010), highlighting the relevance of seismogenic mantle rheology and deep lithospheric structures, complementary to the known (i.e. mapped) shallower crustal faults. The seismic and tsunami hazard posed by the high-magnitude earthquakes continue to trigger the interest, and the need, to search for their seismogenic sources, and hence to better understand the tectonic evolution of the region.

In the study area, a new morphotectonic pattern is revealed in the zone of intersection (corner zone) between a main regional thrust, the so called Horseshoe Thrust Fault (Horseshoe Fault of Gràcia et al., 2003b), and a major dextral strike–slip fault, the SWIM 1 Fault (Zitellini et al., 2009), crossing each other and making an angle of ~ 120°/60° (HTF and SWIM 1in Figs. 1B, Fig. 2, Fig. 3). The new corner zone tectonic structures, and their correspondent geometry and kinematics are here interpreted as resulting from regional, active, thrust–wrench tectonic interference. Based on the previously established regional seismostratigraphy, and on newly interpreted seismotectonic data, this assumption is tested through coupled analog, and 3D finite-element numerical modeling. Both modeling approaches assume (brittle) upper-crust mechanical interference. Obtained results are further compared with the natural example, and ensuing implications for the overall tectonic evolution of the Gulf of Cadiz are discussed.

Section snippets

Regional tectonic setting

In the Gulf of Cadiz tectonic map of Fig. 1B three main sets of major faults can be recognized: 1) The thrust front that bounds the so called Gulf of Cadiz Accretionary Wedge (GCAW, in the eastern half of the study area, Fig. 1B); 2) a set of NE–SW striking thrust-faults, preferably located to the west of the Horseshoe Valley (e.g. Horseshoe Thrust Fault, Marquês de Pombal Fault, Gorringe Fault, and Tagus Abyssal Plain Thrust); and 3) a set of WNW–ESE dextral strike–slip faults, corresponding

Analog modeling

Analog modeling experiments of separated wrench systems (e.g. Dooley and McClay, 1997, Le Guerroue and Cobbold, 2006, Mandl et al., 1977, McClay and Bonora, 2001, Richard et al., 1991, Schopfer and Steyrer, 2001) and thrust systems (e.g. Agarwal and Agrawal, 2002, Bonnet et al., 2007, Ellis et al., 2004, Gutscher et al., 1998a, Gutscher et al., 1998b, Lallemand et al., 1994, Lohrmann et al., 2003, Malavieille, 1984, Malavieille, 2010, McClay et al., 2004, Mulugeta, 1988, Zhou et al., 2007) are

Numerical modeling

Taking into account the same general rheological, geometrical and kinematical constraints described above for the SWIM 1–HTF tectonic system, wrench–thrust mechanical interference was simulated in a three-dimensional plate model using the ABAQUS/Standard software (ABAQUS, Inc. 2009). The main goal was to gain some quantitative insight regarding stress and strain distribution in the thrust–wrench corner zone, which could not be achieved by analog modeling alone. In accordance, the present

Discussion

Both analog and numerical modeling results clearly show the importance of a corner effect expressed by the deformation pattern that is expected to develop in a brittle medium due to the mechanical interference between a dextral strike–slip fault and a thrust, intersecting each other at an angle of 120°/60°. Analog modeling provides a characterization of the resultant basic structural pattern, revealing its essential geometry and kinematics. Numerical modeling provides insights on the way stress

Conclusions

The following main conclusions are drawn:

  • a)

    The newly discovered tectonic pattern in the area of intersection (corner zone) between the SWIM 1 and the Horseshoe faults formed as the result of tectonic interference between active strike–slip and thrust faulting, respectively.

  • b)

    Modeling results show that in the corner zone domain a preferred concentration of stress and strain occurs (corner effect); the latter being mainly accommodated by reverse oblique faulting, with faults exhibiting different

Acknowledgments

Experiments were performed in the Analogue Modeling Lab of Instituto Dom Luiz (IDL), a research Associate Laboratory funded by FCT. ALMOND — Multiscale modelling of deformation in the Gulf of Cadiz (PTDC/CTE-GIN/71862/2006). TOPOEUROPE/0001/2007-TOPOMED (Plate re-organization in the western Mediterranean: lithospheric causes and topographic consequences). Support by Landmark Graphics Corporation via the Landmark University Grant Program is acknowledged. The authors also acknowledge the support

References (95)

  • G. Dufréchou et al.

    Analogue models of second-order faults genetically linked to a circular strike–slip system

    Journal of Structural Geology

    (2011)
  • S. Ellis et al.

    Comparisons between analogue and numerical models of thrust wedge development

    Journal of Structural Geology

    (2004)
  • Y. Fukao

    Thrust faulting at a lithospheric plate boundary Portugal earthquake of 1969

    Earth and Planetary Science Letters

    (1973)
  • A. González et al.

    Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz

    Tectonophysics

    (1998)
  • E. Gràcia et al.

    Holocene earthquake record offshore Portugal (SW Iberia): testing turbidite paleoseismology in a slow-convergence margin

    Quaternary Science Reviews

    (2010)
  • M.A. Gutscher et al.

    Material transfer in accretionary wedges from analysis of a systematic series of analog experiments

    Journal of Structural Geology

    (1998)
  • M.A. Gutscher et al.

    Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: observations from multi-beam bathymetry and seismic profiling

    Marine and Petroleum Geology

    (2009)
  • M.A. Gutscher et al.

    Deep structure, recent deformation and analog modelling of the Gulf of Cadiz accretionary wedge: implications for the 1755 Lisbon earthquake

    Tectonophysics

    (2009)
  • C. Hensen et al.

    Sources of mud volcano fluids in the Gulf of Cadiz — indications for hydrothermally altered fluids

    Geochimica et Cosmochimica Acta

    (2007)
  • L. Iribarren et al.

    The structure of the Atlantic–Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia–Africa plate boundary)

    Marine Geology

    (2007)
  • H. Koyi et al.

    Tectonic thickening of hanging-wall units over a ramp

    Journal of Structural Geology

    (2007)
  • E. Le Guerroue et al.

    Influence of erosion and sedimentation on strike–slip fault systems: insights from analogue models

    Journal of Structural Geology

    (2006)
  • J. Lohrmann et al.

    The impact of analogue material properties on the geometry, kinematics, and dynamics of convergent sand wedges

    Journal of Structural Geology

    (2003)
  • B. Maillot et al.

    Thrust dip and refraction in fault-bend folds: analogue models and theoretical predictions

    Journal of Structural Geology

    (2006)
  • K.R. McClay et al.

    3D evolution of fold and thrust belts formed by oblique convergence

    Marine and Petroleum Geology

    (2004)
  • T. Medialdea et al.

    Structure and evolution of the “Olistostrome” complex of the Gibraltar Arc in the Gulf of Cadiz (eastern Central Atlantic): evidence from two long seismic cross-sections

    Marine Geology

    (2004)
  • G. Mulugeta

    Modelling the geometry of Coulomb thrust wedges

    Journal of Structural Geology

    (1988)
  • M.C. Neves et al.

    Flexure and seismicity across the ocean–continent transition in the Gulf of Cadiz

    Journal of Geodynamics

    (2009)
  • K. Persson et al.

    Analogue models of orogenic wedges controlled by erosion

    Tectonophysics

    (2002)
  • P. Richard et al.

    Experiments on simultaneous faulting and folding above a basement wrench fault

    Tectonophysics

    (1991)
  • F.M. Rosas et al.

    Morphotectonic characterization of major bathymetric lineaments in Gulf of Cadiz (Africa–Iberia plate boundary): insights from analogue modelling experiments

    Marine Geology

    (2009)
  • V. Sallarès et al.

    Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin)

    Earth and Planetary Science Letters

    (2011)
  • D. Stich et al.

    Kinematics of the Iberia–Maghreb plate contact from seismic moment tensors and GPS observations

    Tectonophysics

    (2006)
  • P. Terrinha et al.

    Tsunamigenic–seismogenic structures, neotectonics, sedimentary processes and slope instability on the southwest Portuguese Margin

    Marine Geology

    (2003)
  • P. Terrinha et al.

    Morphotectonics and strain partitioning at the Iberia–Africa plate boundary from multibeam and seismic reflection data

    Marine Geology

    (2009)
  • L. Torelli et al.

    The giant chaotic body in the Atlantic Ocean off Gibraltar: new results from a deep seismic reflection survey

    Marine and Petroleum Geology

    (1997)
  • G. Viola et al.

    Analogue modelling of reverse fault reactivation in strike–slip and transpressive regimes: application to the Giudicarie fault system, Italian eastern Alps

    Journal of Structural Geology

    (2004)
  • G. Viola et al.

    Offshore mud volcanoes and onland faulting in southwestern Africa: neotectonic implications and constraints on the regional stress field

    Earth and Planetary Science Letters

    (2005)
  • R. Weijermars et al.

    Rheological and tectonic modeling of salt provinces

    Tectonophysics

    (1993)
  • J. Zhou et al.

    Shortening of analogue models with contractive substrata: insights into the origin of purely landward-vergent thrusting wedge along the Cascadia subduction zone and the deformation evolution of Himalayan–Tibetan orogen

    Earth and Planetary Science Letters

    (2007)
  • N. Zitellini et al.

    The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar

    Earth and Planetary Science Letters

    (2009)
  • K. Abe

    Size of great earthquakes of 1837–1974 inferred from tsunami data

    Journal of Geophysical Research

    (1979)
  • E. Banda et al.

    Iberia Atlantic Margin Group investigates deep structure of ocean margins

    Eos Transactions, American Geophysical Union

    (1995)
  • M.A. Baptista et al.

    Revision of the Portuguese catalog of tsunamis

    Natural Hazards and Earth System Sciences

    (2009)
  • R. Bartolome et al.

    Evidence for active strike–slip faulting along the Eurasia–Africa convergence zone: implications for seismic hazards on the SW Iberian Margin

    Geology

    (2012)
  • C. Bayarsayhan

    1957 Gobi–Altay, Mongolia, earthquake as a prototype for southern California's most devastating earthquake

    Geology

    (1996)
  • C. Bonnet et al.

    Interactions between tectonics, erosion, and sedimentation during the recent evolution of the Alpine orogen: analogue modeling insights

    Tectonics

    (2007)
  • Cited by (41)

    • Structure and kinematics of active faulting in the Hope-Kelly and Alpine Fault intersection zone, South Island, New Zealand

      2021, Tectonophysics
      Citation Excerpt :

      In Alaska, the dextral strike-slip Denali and Totschunda faults are near vertical and are geometrically and kinematically linked with a direct connection of the two faults, that allows earthquake rupture to propagate directly across the intersection (Haeussler et al., 2004; Schwartz et al., 2012). In the Gulf of Cadiz, at the intersection between active strike-slip and thrust faults, slip transfer is accomplished via a complex process zone where strike-slip faults and thrust faults merge both geometrically and kinematically, accommodating a common horizontal slip vector on dextral, oblique, and thrust faults with strikes ranging by up to 120° (Rosas et al., 2012; Rosas et al., 2015). Previous regional scale (1:250,000) mapping indicates that the junction between the Alpine fault in the west and the central Hope fault to the east is defined by the Taramakau section of the Hope fault, the Kelly fault, and associated subsidiary faults in a horsetail-like structure (Fig. 1 and 2A) (Freund, 1971; Nathan et al., 2002; Berryman et al., 2003; Langridge and Berryman, 2005).

    View all citing articles on Scopus
    1

    Now at Monash University, VIC 3800, Melbourne, Australia.

    View full text