A-Introduction: salient-recess transitions

1. Transpressive shear zones in large scale salient-recess transitions. A case study from the Gibraltar Arc, Southern Spain. Balanyá J.C., Expósito I., Jiménez-Bonilla A., Barcos L., Díaz-Azpiroz, M. / Universidad Pablo de Olavide, Cra. de Utrera km1, 41013 Sevilla, Spain Surveyed area in this work C-The salient-recess transition zone It corresponds to an E-W dextral transpressive zone (The Torcal Shear Zone, TSZ).The TSZ is a 70 km long and 4-5 km wide band, made up by units belonging to the external thrust and fold belt. The southern wall of the TSZ bounds the internal metamorphic zone. Deformation at TSZ is mainly Upper Miocene to Recent (Figs. C-1 and C-3). Thrusts and folds oblique to the shear walls coexist with nearly orthogonal normal faults accommodating moderate extension (Fig. C-1). Kinematics within the TSZ shows that it is the result of a triclinic dextral transpression, in which strain was highly partitioned into simple and pure shear dominated domains (Fig. C-2). Theoretical models of triclinic transpression (Díaz-Azpiroz et al., 2012) applied to the TSZ kinematic data indicate the far field vector was approximately WNW-ESE. Jura WGA Orocline Arc Area of data in Fig. B-1 Piedmont Glacier Primary Arc A-1. Tectonic traits of the Gibraltar Arc. Structural trend-line pattern depicts a major salient west of A-A´points (the WGA salient). To the NE, a wide recess zone is identified between de WGA salient and the so-called Prebetic Arc. Redrawn from Balanyá et al. (2007). B-2. Stretching and shortening in the Ronda Basin sector. Left: normal faults (n=51) and associated-NE-SW extension (47 slickenlines). Right: axial traces and reverse faults (n= 55). Data from Jiménez-Bonilla et al. (2011 and 2012). C-1. Central sector of the transpressive Torcal Shear Zone (TSZ). Structural map showing therelationships among folds and reverse faults, normal faults, and dextral strike-slip faults. Compiled from Barcos et al. (2011 and 2012). A-2. The Jura Arc compared with the WGA salient (redrawn from Crespo-Blanc et al., 2012). Kinematics of two arcs have been refered as a “Primary arc” (Jura, Hindle and Burkhard, 1999) and to a “Piedmont Glacier” type (WGA, Balanyá et al., 2007). A-Introduction: salient-recess transitions Structural trend-line patterns in orogenic arcs commonly depict alternating convex (salients) and concave (recesses) to the foreland segments. Studies carried out on a variety of natural cases together with analogue modelling show that the factors controlling the geometric features of these curved segments are very different in nature: variations in stress trajectories, predeformational thickness, indenter shape, tectonic transport direction patterns, and the strength of detachment levels (Macedo and Marshack, 1999). Lateral zones of salients and salient-recess transitions often coincide with relative narrow bands where strike-slip motion is dominant. The Gibraltar Arc, 1200 km long, is a tigth curved orogen that closes the Mediterranean Alpine system at its western end. Within it, trace patterns form second order curved segments of 100-200 km in their length cord. The hinge zone of the Arc, called the Western Gibraltar Arc (WGA), defines itself a major salient that ends at the apex of two recessing zones located in the Betic (northern branch) and Rif (southern branch) chains. Northern half of the WGA salient zone Salient-recess transition zone (TSZ) Western Gibraltar Arc shortening-stretching directions Central sector of the TSZ (Figs. C-1 and C-2) Ronda Basin Sector (Figs. B-2 and B-3) Structural trend B-1. Shortening (blue dots) and stretching (red dots) vs structural trend. WGA data base from Balanyá et al. (2012). New data from the Ronda Basin are incorporated (squares). C-2. Kinematic model of the Torcal de Antequera massif. CSD, simple shear dominated domain; CS, pure shear dominated domain. Redrawn from Barcos et al. (2011). NE sector of the TSZ (Fig. C-3) B-The northern part of the WGA salient zone In this sector, where is located the intermontane Ronda Basin, NE-SW trending shortening structures (average tectonic transport direction NW-SE) were coeval during the Neogene with normal faults accommodating arc-parallel extension (Figs. B-1, B-2, and B-3). The Ronda Basin formation during the Upper Miocene and its later evolution is the result of this mode of strain partitioning.The Upper Miocene marine infill of the Ronda Basin was coeval to the starting activity at the TSZ. Late evolution stages of the Ronda Basin sector correspond to deformative and erosive events. Deformation has been active up to the Quaternary. • D-Conclusions • Kinematic data of the WGA salient indicate that radial shortening directions were coeval with arc-parallel extension. In its northern part, lateral ending of the salient correspond to an E-W transpressive shear zone (the Torcal Shear Zone, TSZ) that induces a dextral deflection of the regional structural trend of about 50º. • Strain partitioning modes characterizing the WGA salient greatly controlled the inception and later evolution of Intermontane basins. The Ronda Basin results from the combination of NE-SW stretching and NW-SE shortening that finally isolated it from the former foreland marine basin. • The salient-recess transition coincides with the Torcal Shear Zone (TSZ). The TSZ trends oblique to regional transport direction, it dextral transpressive regime being coherent with the WGA-related kinematic frame. The approximately WNW-ESE far-field velocity vector of the TSZ (Díaz-Azpiroz et al., 2012) underline the importance of the westward movement of the Internal Zones relative to the external wedge. • Nor the WGA salient neither the TSZ can be explained by the solely N-S to NW-SE Neogene Europe-Africa plate convergence. A westward migrating hinterland domain is suggested to explain the complete system of orogenic curves. Undifferentiated Subbetic units References Balanyá, J.C., Crespo-Blanc, A., Díaz-Azpiroz, M., Expósito, I., Luján, M., (2007). Tectonics, doi: 10.1029.2005TC001932. Balanyá, J.C., Crespo-Blanc, A., Díaz-Azpiroz, M., Expósito, I.,Torcal F., Pérez-Peña V., Booth-Rea G. (2012). Geologica Acta, doi: 101344/105.000001771. Barcos L., Díaz-Azpiroz M., Balanyá J.C., Expósito I. (2011). Geogaceta, 50-1,31-34. Barcos L., Expósito I., Balanyá J.C., Díaz-Azpiroz (2012). Geo-Temas, 13, 507-601. Crespo-Blanc A., Balanyá J.C., Expósito I., Luján M., Suades E. (2012). J. Geol. Soc. London, doi: 10.1144/0016-76492011-115. Díaz-Azpiroz M., Barcos L., Balanyá J.C., Fernández C., Expósito I., Czeck D. (2012). 2012 GSA Annual Meeting. Expósito I., Balanyá J.C., Crespo-Blanc A., Díaz-Azpiroz M., Luján M. (2012). Tectonophysics, 576-577, 86-98. Hindle D. and Burkhard M. (1999). J. Struct. Geol., 21, 1089-1101. Jiménez-Bonilla A., Balanyá J.C., Expósito I., Díaz-Azpiroz M. (2011). Geogaceta, 50-1, 13-26. Jiménez-Bonilla A., Balanyá J.C., Díaz-Azpiroz M., Expósito I. (2012). Geo-Temas, 13, 499-503. Macedo J.M. and Marshack S. (1999). GSA Bulletin, 111, 1808-1822. B-3. Lower to Middle Miocene shortening structures in the WGA salient. Note the NE-SW trend and their subortogonal tectonic transport direction. Dominant vergence is to the NW. Redrawn from Expósito et al. (2012). C-3. Map of earthquakes focal solutions in the NE sector of the TSZ. Strike-slip earthquakes are dominant, probably corresponding to WNW-ESE dextral faults, according to surface geology. Data base from Balanyá et al. (2012).