Along-strike segmentation and basin evolution in curved fold-and-thrust belts: the study case of the Northern Gibraltar Arc

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Balanya Roure, Juan Carlos
Expósito Ramos, Inmaculada 
Díaz Azpiroz, Manuel 
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Understanding of the geometry and kinematics of fold-and-thrust belts (FTBs) has been greatly improved along the last decades. However, some questions still remain little addressed, mainly those related to the structural variations along curved FTBs. In this respect, this thesis focuses on (1) the strain partitioning modes that operate during the arc-protrusion, and (2) the role of these modes on the topographic and structural segmentation of FTBs. For this purpose, this thesis characterizes the structural associations accommodating strain partitioning in selected segments of the Gibraltar Arc. This study also explores related aspects such as the role of arc-parallel stretching in intermontane basins localization and the influence of the basement mechanical and geometrical properties on eventual lateral variations of the arcuate mountain front. To tackle these questions, both structural and geomorphologic analyses together with seismic interpretations have been done in four field areas, which coincide with structural and topographic discontinuities along-strike the Betic chain (northern branch of the Gibraltar Arc). Additionally, analogue models of piedmonts and models that use a protruding backstop have been performed in order to delve into the strain partitioning modes in arcuate orogens. The Betic FTB is deformed in a thin-skin tectonic style and detached within the Triassic evaporitic-rich substrate. Although in the western-central Betics the main tectonic stacking is lower to middle Miocene in age, this thesis is mostly centered on the post-Serravallian structures that control the current landscape. Both structural data and seismic interpretations show that post-Serravallian shortening was accommodated by arc-parallel, kilometric-scale, upright folds and related reverse faults as well as by tightening of older structures. In the central Betics, across-strike changes of the structural style and the topographic envelope (¿) coincide with the pinch-out of Triassic evaporites and with a change in the basement dip (ß). To explain these features, a tectonic model is proposed consisting in the forelandward propagation of the FTB deformation until Langhian, when it stagnated because of changes in basal friction and dip angles. This stagnation provoked a rapid increase in the topography (¿), thus leading to a subsequent orogen-frontal collapse. The weak behavior of the olistostromic unit produced by this frontal collapse enabled the NW propagation of the FTB during the post-Serravallian deformation. The shortening of the Betic FTB has been coeval with arc-parallel stretching, associated with a conspicuous along-strike relief segmentation. The structural data collected from four selected field areas within the western and central Betics show that these relief discontinuities correspond with normal or strike-slip dominated fault zones. The outcropping normal fault zones, oriented perpendicular to the orogenic grain, accommodate minimum arc-parallel stretching of ca. 15% and accumulate up to 1 km of vertical throw. These normal fault systems often host post-Serravallian depocenters on their hanging walls, being the Ronda intermontane basin the best example in the studied areas. This basin is characterized by an asymmetric graben that is defined by NW-SE (arc-perpendicular) normal fault zones at its SW and NE boundaries. These normal faults provoked the basin inception, as an embayment of the Guadalquivir foreland basin, during the Tortonian to the Messinian. After that, the uplift of the NW boundary of this basin, due to the activity of the ENE-WSW Algodonales-Badolatosa dextral transpressive band, together with regional uplift led to the Ronda basin emersion and isolation respect to its parental foreland basin. Another relief discontinuity, the Ubrique normal fault zone, generates the main structural and topographic drop of the westernmost Betic chain. In this zone, arc-parallel stretching was accommodated in the middle Miocene by the Colmenar Fault that probably favoured the later localization of the Ubrique Normal Fault Zone, which is still active. Structural superposition criteria and geomorphic analyses point out the current activity of the structures that accommodate the prevailing modes of strain partitioning that have also operated during the lower to middle Miocene arc protrusion, thus suggesting that the Gibraltar Arc is still active. As a result of the neotectonic arc-parallel stretching, the development of normal and strike-slip fault zones determines the localization of major along-strike relief discontinuities. In this thesis, analogue models of progressive arcs, both piedmonts and that ones that used a protruding backstop, have been carried out in order to explore the relationship between strain partitioning modes (in particular arc-parallel stretching) and the arc curvature. The distinctive feature of the piedmont models set up is the presence of a gate through which the analogue material is forced to flow. The transfer of material towards the piedmont lobe (outwards the gate) was achieved by transfer faults together with normal fault systems behind the gate that propagated towards the rear of the model. In this inner domain, the converging displacement pattern behind the gate produced shortening, which was accommodated by opposite sense rotation of blocks. Outwards the gate, the development of piedmonts resulted in a divergent particle displacement. Concerning to the second type of the performed analogue models, the set up permits the degree of protrusion of the backstop to increase during the deformation. The resulting curved fold-and-thrust belts show a) arc-parallel thrusts; b) oblique-conjugate, strike-slip faults; and c) arc-perpendicular normal faults. Strike-slip and normal faults, which accommodated arc-parallel stretching, produced the individualization of blocks that rotated up to 40º clockwise and anticlockwise, in the left and right arc limbs, respectively. The passive rotation of some of the structures led to the change of their kinematic pattern. The application of the oroclinal test to these experiments sheds light about the structural evolution of natural cases of progressive arcs. Moreover, analogue models that use a protruding backstop share strong similarities with the Gibraltar Arc and other Mediterranean orogens in terms of kinematic patterns, including size and rotation amount of blocks resulting from the arc segmentation.
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