Resumen de Combined analytical and anlogue modelling of strongly partitioned transpression: the Torcal shear zone (External Betics)
Transpressive high-strain zones are common features in the lateral parts of orogenic arcs where oblique convergence usually occurs. The strain within these zones is highly partitioned when they are located in the upper crust. In order to better understand the meaning of strain partitioning modes and strain localization within transpression shear zones, this thesis provides an exhaustive structural study of a natural case, based on surface structural data combined with analytical and analogue modelling. Thus, a protocol to test a general triclinic transpression model was applied to the so-called Torcal Shear Zone, a highly partitioned brittle-ductile shear zone located in the northern branch of the Gibraltar Arc. In addition, the theoretically obtained far-field parameters together with field data have provided the boundary conditions as well as the main parameters for designing the analogue models. It is noteworthy to mention that this work offers the first analogue model of oblique convergence that simulates triclinic inclined transpression. The combination of analytical and analogue models sheds some light on the deformation processes within the upper crustal transpression zones and, to a broader extend, high strain zones in general. Within the Gibraltar Arc, the Western Gibraltar Arc (WGA) is a protruded salient, 200 km in cord length, closely coinciding with the apex zone of the major arc. The TSZ is located in the External Zones (Subbetics) at the northeast termination of the WGA. The TSZ is a 70 km long and 4-5 km wide brittle-ductile deformation band that forms a W-E topographic alignment along which the kinematic data show an overall dextral transpression. Within the TSZ, strain is highly partitioned at multiples scales into mainly shortening, extensional and strike-slip structures. Regarding their structural associations, three distinctive zones can be defined within the TSZ: a) the central sector which comprises in turn two distinctive structural domains, the Valle de Abdalajís and the Torcal de Antequera massifs (VAM and TAM, respectively), b) the western end of the TSZ which coincides with the Teba-Peñarrubia massif, and c) the eastern end which corresponds to the Cabras- Camorolos sector. All these sectors show evidences of overall dextral transpression during the Late Miocene to the Quaternary. The application of a general analytical triclinic transpression model with oblique extrusion has been made by means of a standard procedure. This procedure comprises five consecutive steps, although some of them could be reordered, or even skipped, depending on the case. The results obtained for the central sector of the TSZ have permitted to estimate the orientation of the far-field velocity vector ((Fd) ¿) responsible for the TSZ bulk deformation. A single N99ºE-N109ºE trending horizontal velocity vector (V ¿) could explain the kinematics of the entire TSZ central sector. Additionally, our results suggest that the spatial distribution of the main structures observed in the central sector reflects different modes of strain partitioning and strain localization, which indicates that each massif accommodated a specific bulk strain. The partitioned flow in the TAM produces larger transpression obliquity and Wk values, with respect to the Valle de Abdalajís massif. The TAM shows a more complex strain partitioning situation with two narrow outer domains that accommodate simple shear- dominated dextral transpression, and a wider inner domain deformed by triclinic pure-shear-dominated transpression. In contrast, the VAM displays a typical discrete partitioning situation, with a discrete, simple shear, dextral strike-slip fault zone at the southern boundary; whereas the rest of the massif is affected by distributed deformation, whose bulk kinematics would approximate slightly triclinic (pseudo-monoclinic), pure-shear-dominated transpression. Such flow partitioning between these massifs appears to be mainly controlled by (1) slight, along strike variations in the orientation of the Alboran Domain ¿ Subbetic unit boundary, (2) lateral differences in the thickness of the weak Triassic layer and (3) the rheological contrast related to the contact between limestones and marly limestones. Analogue modelling carried out to reproduce the structural pattern of the Torcal de Antequera massif was made by simulating triclinic transpression with two inclined parallel backstops in an oblique convergence setting. The oblique convergence angles used in experiments (15º and 30º) were chosen from analytical results to simulate the two differentiated structural domains within the TAM. Results support that: a) the structural style depends on the thickness variation in the ductile layer; b) planar boundaries favour the early nucleation of boundaries-parallel deformation zones where mainly strike-slip, simple shear deformation concentrates; c) different modes of strain partitioning and strain localization between the two different domains are related to different values of the oblique convergence angle: in 15º model strain partitioning is simple and strain localization is high, whereas in the pure-shear-dominated case (30º model) strain partitioning is strong (forming oblique-to-the walls thrusts and parallel-to-the walls strike-slip faults) and deformation spreads over a much wider area. The results concerning the structure and kinematics of the natural case show that the TSZ induces the near vertical extrusion of the Subbetic units and the dextral deflection of the regional structural trend-line pattern. This indicates the TSZ represents the main structure that shapes the northern end of the Western Gibraltar Arc salient. The structure and kinematics of the TSZ central sector are compatible with a single, Late Miocene to Quaternary, transpressional event. This event would be related to a unique velocity vector, whose orientation ranges from N99ºE to N109ºE. The different structural patterns exhibited by the two lateral domains make them comparable to splay-dominated (western end) and thrust-dominated (eastern end) strike-slip fault tips, where the most of the TSZ strike-slip displacement may be accommodated. This underlines the importance of the westward motion of the Alboran Domain relative to the external wedge and fits well with the radial outward thrusting pattern identified in the Western Gibraltar Arc. The identification of a variety of strain partitioning modes at multiple scales is revealed as an essential question to be taken into account in any kinematic model of the Gibraltar Arc system.
