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