The Central Asia collision zone: numerical modelling of the lithospheric structure and the present-day kinematics

Abstract

The Central Asia region is dominated by the Zagros orogen in the western sector and the Himalaya-Tibetan orogen in the eastern sector, which resulted from the subduction of the Tethys oceanic lithosphere towards the NNE and the subsequent collision of the Arabia and India plates with the Eurasia plate during the Cenozoic. The collisions produced tectonic escapes toward lateral regions (in Anatolia and south-eastern Tibet), oblique convergence in the Zagros fold-and-thrust belt, the formation of the Makran subduction zone and shortening in Himalaya, Karakorum and Tibetan Plateau. Different mountain belts also developed far into the continent interiors, e.g. Caucasus, Alborz, Kopet Dagh, Pamir and Tian Shan.

The lithosphere structure plays an important role in controlling the surface deformation and its propagation inside the continent. The compositional and strength heterogeneities within the lithosphere directly affect the tectonic behaviour of the region and, hence, the evolution of the orogenic systems. This Thesis focalizes on the characterization of the lithospheric structure of the Zagros and the Himalayan-Tibetan orogens and on the role of the lithospheric structure and rheology in the accommodation of the deformation related to the Arabia and India convergence against Eurasia.

The lithospheric structure of the Zagros and the Himalaya-Tibetan orogens has been characterized from the thermal, compositional and seismological viewpoint using an integrated geophysical-petrological modelling approach. The models make compatible seismic, density and thermal modelling findings, and allow quantifying the effect of mineral physics on previous results from integrated thermal models. The results obtained in the Zagros orogen reveal that the transition from the Arabian to the Eurasian lithosphere is characterized by a thinning of the lithospheric mantle extending from the suture zone beneath the Zagros range to the Alborz in the North and the Central Iran. The lithospheric mantle composition is compatible with a Proterozoic peridotitic mantle-type beneath the Arabian Platform, the Mesopotamian Foreland Basin and the accreted terrains of the Eurasia plate, and with a more depleted Phanerozoic harzburgitic mantle-type below the frontal parts of the Zagros range. In the Himalaya-Tibetan orogen, the results suggest that the present-day lithospheric mantle structure is laterally-varying within the Tibetan Plateau in the east-west direction. The lithospheric mantle is thicker and more buoyant in the western sector than in the north-eastern sector. The lherzolitic mantle-type is the dominant mantle composition, but it changes to a more fertile composition beneath the Tarim Basin, to a Fe-Mg-rich mantle beneath Tian Shan, Junggar and Altai regions, and to highly MgO-depleted mantle in the north-eastern Tibetan Plateau.

The results on the present-day lithospheric structure of the Zagros and the Himalaya-Tibetan orogens have been combined with the present-day kinematics, geodetic observations and stress data to characterize the current deformation patterns in the Central Asia region related to the tectonic convergence of the Arabia and India plates with Eurasia. The thin-sheet approach allowed investigating the effect of the lithospheric structure, rheology, boundary conditions, and friction coefficient on the predicted velocity and stress fields. The models reproduce the main directions of the velocities in Central Asia by only imposing the convergence of Arabia and India plates respect to the fix Eurasia, and varying the rheology parameters. The models simulate the observed kinematics including the counter-clockwise rotation of Arabia and Iran triggering the westward escape of Anatolia, and the eastward extrusion of the northern Tibetan Plateau structural domains. Besides the large scale, the models offer a coherent result in regions with little or no data coverage, as in the case of the Arabia-India inter-collision zone, over large areas of Pakistan and entire Afghanistan.

The study has been supported by the project ATIZA (CGL2009-09662-BTE), and the FPI grant associated to.

Asia Central está dominada por dos importantes orógenos, el orógeno del Zagros y el sistema Himalaya-Tibet, resultantes de de la colisión de las placas Arábiga e India con el margen meridional de la placa Eurasiática.

Esta Tesis se focaliza en: 1) la caracterización del manto litosférico a través de un metódo de modelización geofísico-petrológico integrado y 2) el estudio del efecto de la estructura litosférica y de la reología en la deformación neotectónica relacionada con la convergencia de Arabia y de India respecto a Eurasia utilizando una metodología basada en la aproximación de lámina delgada (thin-sheet).

En el caso del orógeno del Zagros, los resultados revelan que el manto litosférico se adelgaza debajo de Irán Central, del Alborz y parcialmente debajo de la cordillera del Zagros. En el caso del sistema Himalaya-Tibet, los resultados indican una litosfera engrosada en el sector occidental, debajo de la cordillera Himalaya, Meseta del Tibet, Kunlun Shan y Tian Shan, y un adelgazamiento debajo de las cuencas de Tarim y de Junggar. En el sector oriental los resultados confirman que la Meseta del Tibet está suportada por una litosfera más adelgazada y caliente en el norte que en el sur. Ha sido necesario introducir variaciones laterales de composición mantélica, relacionadas con procesos del manto litosférico superior, en todos los perfiles modelados evidenciando la presencia de diferentes dominios litosféricos.

El estudio de la deformación neotectónica ha revelado el rol clave de la reología en la reproducción del campo de esfuerzos y de velocidades en Asia Central, sugiriendo una litosfera menos rígida en la Meseta del Tibet que en la meseta de Irán. En conjunto, la deformación es más rápida en la zona de colisión India-Eurasia que en la zona de colisión Arabia-Eurasia. Finalmente, la presencia de un manto adelgazado en el noreste del Tibet y la consecuente disminución de viscosidad debida al aumento de temperatura explicarían la presencia de fallas extensionales en la Meseta del Tibet y reconciliarían el modelo con los datos de flujo de calor elevado y bajas velocidades sísmicas registrados en la región.

Esta tesis ha sido financiada por el proyecto ATIZA (CGL2009-09662-BTE) y la beca FPI asociada.