Journal of Structural Geology 28 (2006) 602–620 www.elsevier.com/locate/jsg Lateral interaction between metamorphic core complexes and less-extended, tilt-block domains: the Alpujarras strike-slip transfer fault zone (Betics, SE Spain) Jose´ Miguel Martı´nez-Martı´nez * Instituto Andaluz de Ciencias de la Tierra and Departamento de Geodina´mica (C.S.I.C.-Universidad de Granada), Avda. Fuentenueva s/n, 18071 Granada, Spain Received 10 May 2005; received in revised form 13 December 2005; accepted 28 January 2006 Available online 13 March 2006 Abstract The Alpujarras area in southeastern Spain exhibits one of the scant documented examples of extension related strike-slip faults bordering core complexes in the world. Faults of the Alpujarras fault zone define a regional-scale complex ENE-striking transfer zone that marks the boundary among the Sierra Nevada elongated dome, a highly-extended, constricted core-complex, and a less-extended domain formed by large-scale tilt- blocks. Detailed mapping and structural analysis show that the Alpujarras fault zone is an integral part of the WSW-directed normal fault systems that thinned the Betic hinterland during the middle Miocene to Recent time. Fault patterns and palaeostress analysis both indicate that dextral movement along the strike-slip faults was induced by a local stress field with a sub-horizontal E–W- to ESE–WNW-trending maximum principal stress axis, which is synchronous with the regional stress field driving the normal fault systems. Palaeostress analysis also indicates subsequent variations in the stress field with a sub-horizontal NW–SE- to N–S-trending maximum principal stress axis, thus producing both the tectonic inversion of the northern fault of the Alpujarras system and shortening of the unloaded extensional detachment footwall. A simplified kinematic model for the tectonic evolution of the Alpujarras area from the middle Miocene to Recent emphasizes the kinematic coupling of normal faults and strike-slip transfer zones in the extensional process. q 2006 Elsevier Ltd. All rights reserved. Keywords: Transfer fault zones; Segmented normal fault systems; Metamorphic core-complexes; Tilt-block domains; Palaeostress analysis; Betics 1. Introduction or oblique structures that were denominated transfer zones, also known as accommodation zones, relay zones and segment The segmentation in normal fault systems is well boundaries (Morley et al., 1990; Gawthorpe and Hurst, 1993; documented within extensional tectonic provinces (Davis and Faulds and Varga, 1998). Burchfiel, 1973; Stewart, 1980; Bally, 1981; Gibbs, 1984; The term ‘transfer zone’ was previously introduced by Rosendahl, 1987; Faulds et al., 1990; Wernicke, 1992). All Dahlstrom (1970) to give a name to cross structures normal faults must terminate both up and down dip and along accommodating differential shortening in thrust terrains. strike. Segmented fault traces are seen over a large range of Transfer zones are structures that serve as a linkage between scales (e.g. Peacock, 2003). Regions near fault tip lines and growing isolated faults. Faults that grow while interacting with between adjacent en e´chelon segments are often zones of other developing faults but not physically intersecting with concentrated strain associated with the progressive loss of slip them are said to be ‘soft linked’. Relay ramps are an example of on individual faults and the transfer of displacement between soft linkage, in which the slip transference from one to another fault segments in order to conserve the regional extensional fault can essentially be accommodated by ductile strain. Faults strain (Morley et al., 1990; Nicol et al., 2002). Accommodation with connecting fault surfaces are described as ‘hard linkages’. of extension between individual faults gives rise to transverse Transfer faults are an example of hard linkage which requires that there is actual physical linkage of fault surfaces allowing slip to be transferred (Walsh and Watterson, 1991; McClay and * Tel: C34 958 249504; fax: C34 958 248527. E-mail address: [email protected]. Khalil, 1998). At the regional scale, transfer faults generally are complex transfer fault zones where both hard linkages and 0191-8141/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. soft linkages can occur together. doi:10.1016/j.jsg.2006.01.012 J.M. Martı´nez-Martı´nez / Journal of Structural Geology 28 (2006) 602–620 603 Transfer faults, originally described in extensional basins 2. Tectonic framework (see Gibbs, 1984), are one type of transverse structure within extended terrains that link spatially separated loci of extension. The Betics in southern Spain form the northern branch of Faults laterally adjoin extended continental blocks showing the peri-Albora´n orogenic system (western Mediterranean) similar or opposite structural asymmetry and/or contrasting (Martı´nez-Martı´nez and Azan˜o´n, 1997), which also includes values of upper-crustal extension (Davis and Burchfiel, 1973; the Rif and Tell mountains in North Africa. The Betics and Rif Burchfiel et al., 1989; Duebendorfer and Black, 1992; Faulds are linked across the Strait of Gibraltar to form an extremely and Varga, 1998). Transfer faults can be described as strike- arcuate orogen commonly known as the Gibraltar arc, which slip faults if they strike parallel to the direction of extension, or lies between the converging African and Iberian plates as oblique-slip faults if they strike oblique to the direction of (Balanya´ and Garcı´a-Duen˜as, 1988; Platt et al., 2003) (inset extension. in Fig. 1). Convergence results in a broad zone of distributed Strike-slip transfer faults are dynamically different than deformation and seismicity in the region, resulting in a diffuse strike-slip faults in transcurrent tectonic regimes. The most plate boundary. Kinematic reconstructions reveal continuous fundamental difference is that transcurrent faults strike N–S convergence between Africa and Europe from the late obliquely to the principal stress axes, whereas transfer faults Cretaceous to the upper Miocene (9 Ma), then changing to commonly parallel the extension direction. In addition, the NW–SE until the Present (Dewey et al., 1989; Srivastava et al., sense and magnitude of slip on transfer faults can vary 1990; Mazzoli and Helman, 1994). Nevertheless, convergence significantly along strike due to intimate coupling between in the outermost western Mediterranean was considerably systemsofnormalorreversefaults(Gibbs, 1990; lower than in other more eastward Mediterranean regions (e.g. Duebendorfer et al., 1998; Faulds and Varga, 1998). Finally, Dewey et al., 1989). in transcurrent faults the sense of slip is coherent with the The arcuate orogen results from the collision between the offset that increases in time whereas the sense of slip in Albora´n domain, which moved some 250–300 km westwards transfer faults is contrary to the apparent offset of the relative to Iberia and Africa during the Miocene, and the related normal faults that do not vary in time (Gibbs, 1990). Mesozoic–Cenozoic South-Iberian and Maghrebian continen- The Alpujarras fault zone of southeastern Spain strikes tal margins partially coeval with the obliteration of the Flysch WSW–ENE and has been recognized as a major structural Trough domain (Balanya´ and Garcı´a-Duen˜as, 1988; Platt et al., element of the Betics for nearly 25 years. It was considered 2003; Chalouan and Michard, 2004). Collision produced as a transcurrent fault zone developed during middle-to- tectonic inversion of both continental margins, causing upper Miocene in a stress field with a sub-horizontal development of a fold-and-thrust belt (Platt et al., 1995; WNW–ESE-trending maximum compressive axis (Bernini Crespo-Blanc and Campos, 2001) while the hinterland was et al., 1983; Sanz de Galdeano et al., 1985). Several simultaneously extended (Garcı´a-Duen˜as and Martı´nez-Martı´- observations suggest that another interpretation on the nez, 1988; Galindo-Zaldı´var et al., 1989; Platt and Vissers, origin and nature of this fault zone is possible. (1) The 1989; Garcı´a-Duen˜as et al., 1992; Jabaloy et al., 1993; Vissers spatial and temporal coexistence of the fault zone with et al., 1995; Gonza´lez-Lodeiro et al., 1996; Lonergan and WSW-directed normal fault systems. (2) Despite the large White, 1997; Martı´nez-Martı´nez and Azan˜o´n, 1997). scale of the fault zone (more than 60 km length) the amount The Miocene deformation history of the Albora´n domain is of slip cannot be determined with certainty (see Sanz de dominated by extreme crustal extension. Two episodes of Galdeano et al., 1985; Sanz de Galdeano, 1996). (3) The nearly orthogonal extension in which normal fault systems Alpujarras fault zone is a major structural boundary between developed with directions of extension varying from a NNW– crustal blocks showing different values and modes of SSE system, orthogonal to the belt axis, in the late extension; the highly-extended Sierra Nevada elongated Burdigalian–Langhian, to a Serravallian WSW-directed oro- dome (Martı´nez-Martı´nez et al., 2002) and the Sierra de gen-parallel one have been documented (Garcı´a-Duen˜as et al., la Contraviesa-Sierra de Ga´dor tilt-block domain (Garcı´a- 1992; Crespo-Blanc et al., 1994; Martı´nez-Martı´nez and Duen˜as et al., 1992) with significantly lower values of Azan˜o´n, 1997). The superposition of these two systems extension (Figs. 1 and 2). resulted in a chocolate tablet boudinage mega-structure (e.g. In this paper, I present a detailed structural analysis of the Crespo-Blanc, 1995). Alpujarras fault zone and associated structures focusing on the Inhomogeneous finite extension produces contrasting fault terminations, provide an alternative explanation for their extended crustal domains, with boundaries that are not well development and discuss the role they played in accommodat- outlined, showing significant differences in the mode and rate ing regional extension. It is the first time that the Alpujarras of extension. Extension in the Albora´n basin involves the fault zone is interpreted as a transfer fault zone linking normal whole crust, which has been reduced to 15 km thickness (Torne´ fault systems.