The deep structure and reactivation of the Kyrgyz Tien Shan: Modelling the past to better constrain the present

Abstract

In the Tien Shan belt, Cenozoic to active deformation is guided by structures and rheological contrasts partly inherited from the Paleozoic Variscan orogeny and reactivated by the India-Asia collision. Cenozoic deformation net estimates are constrained by lithospheric scale geological cross-sections, which rely on reconstructions of the Paleozoic Central Asian Orogenic Belt (CAOB). However, several geodynamic scenarii have been proposed for its formation including a south or a north dipping subduction, which are still debated due to lack of constraints. Here, we designed numerical experiments to test different hypotheses about the initial geometry and the effective rheology of the paleo-sutures and especially the one located between the Tien Shan and the Tarim basin, where most Cenozoic deformation localizes. The different geometries of Paleozoic structures are used as input variables in the thermo-mechanical models, which are then run forward in time. After a finite amount of shortening, the structures that develop self-consistently out of the proposed heterogeneities are then compared to the current Cenozoic finite and active deformations. We find that a crustal south-dipping suture zone lying on a resistant lithospheric mantle best explains the localization of the deformation, the current geometry of the structures and the Moho depth variations. Using the model results, we propose a mechanically consistent depth-interpolated crustal cross-section of the Kyrgyz Tien Shan, which incorporates both geological and geophysical data.