Salt tectonics in contractional fold belts, the kuqa foreland basin and thrust belt case (tarim basin, china)
- Autores: Oriol Pla Casacuberta
- Directores de la Tesis: Eduard Roca (dir. tes.), Esther Izquierdo Llavall (codir. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2019
- Idioma: español
- Materias:
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- Resumen
The Kuqa fold-and-thrust belt, in the southern foreland of the central Tian Shan Range (NW China) was contractionally deformed during Late Mesozoic and Cenozoic times as recorded by well-preserved syntectonic continental sequences. In addition, its structural evolution was strongly controlled by synorogenic salt (Eocene-Oligocene in age) and presalt décollements with varying spatial distribution. In this scenario, we present a set of six balanced cross-sections, three of them are subsequentially restored across the Kuqa fold-and-thrust belt that provides a new interpretation of the structure beneath the evaporites, in which Paleozoic and Mesozoic strata are deformed by a thrust stack involving (i) a thin-skinned thrust system detached on Triassic-Jurassic coal/mudstone units, and (ii) an ensemble of south-directed basement thrusts. The latter formed from the inversion of Mesozoic extensional faults such as those preserved both in the Tarim Basin and beneath the frontal part of the Kuqa fold-and-thrust belt. Regional cross sections also show a total shortening (from Oligocene to the present-day) ranging from 30.5 km to the West to 24 km to the East (29 km in the central part) that was mostly accommodated from late Miocene to Pleistocene times. Total shortening in the Kuqa fold-and-thrust system decreases progressively eastwards, accordingly to the eastwards shortening decrease in the Tian Shan Range. The regional restorations depict a three-stage evolution for the Kuqa fold-and-thrust belt: i) minor Mesozoic extension; ii) an early compressional stage (Late Cretaceous to early Miocene) with low shortening and syntectonic sedimentary rates; and iii) a later compressional stage (late Pliocene-Pleistocene) characterized by a greater and progressively increasing shortening rate and rapid deposition.
To gain further insights about the influence of synkinematic sedimentation rate, décollement rheology, and the interaction between décollements on the deformation style in foreland areas of fold-and-thrust belts we present an experimental study including four 3D sandbox models inspired by the Kuqa fold-and-thrust belt. These experiments contain two décollements with different areal extents: a weak synorogenic salt layer; and a deeper, preorogenic and frictionless décollement (i.e., coal or mudstone). They show along strike variations of thickness and lithology which affect their rheology and mechanical behavior. The experimental results show that increasing synkinematic sedimentary rate: (i) generates a progressive change from distributed to localized deformation and, (ii) delays the development of frontal contractional structures detached on the salt, favoring the formation and reactivation of more hinterland thrusts and backthrusts. With respect to the rheology, our study reveals that as the viscosity of the prekinematic décollement increases: (i) the deformation propagates more slowly towards the foreland and, (ii) the underlying thrust stack becomes broader and lower and has a gentler thrust taper angle. The rheology of the prekinematic décollement defines the distribution and geometry of the structures detached on it that in turn influence the development of overlying, salt-detached structures. Subsalt structures can: (i) determine the areal extent of the salt and therefore of any fold-and-thrust system detached on it and, (ii) hamper or even prevent the progressive foreland propagation of deformation above the salt. In addition, internal deformation of the salt highlights that there is a balance between salt flow related to the thrust emplacement and salt evacuation below the piggy-back basins.
The integration of surface data, seismic interpretation, and analogue modelling permitted us to a better understanding of the geometry and kinematic evolution of the Kuqa fold-and-thrust belt, as well as provided valuable insights on contractional deformation in the outer parts of fold-and-thrust belts including syntectonic sediments and multiple décollements.
