Autonomous geodynamics of the Pamir–Tien Shan junction zone from seismology data

Abstract

The geodynamics of the Tajik Depression, the junction zone of the Pamirs and Tien Shan, is typically considered in the context of plate tectonic concept, which implies intense subhorizontal compression of the zone resulting from the subduction of the Indian and Eurasian lithospheric plates. This convergence has been reliably confirmed by the GPS measurements. However, the joint analysis of the geological structure, seismicity, and geodimeter measurements conducted during a few years at the Garm geodynamical testing site of the Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, demonstrates a widening of the Tajik Depression instead of its shortening, as should be expected from the subhorizontal compression predominant in the present-day stress-state of this region. This conclusion, together with the data from the other regions, suggests that, along with the plate tectonic mechanisms, there are also other, local, autonomous drivers that contribute to the tectogenesis of this region. Besides, the probable existence of these autonomous sources within the Tajik Depression directly follows from the seismology data. Among them is the crustal spreading within the depression suggested by the seismotectonic displacements in the focal mechanisms of the earthquakes. These displacements are directed in different azimuths off the axial’s most subsided part of the depression at a depth of 20–30 km. Above this region the distribution of seismotectonic deformations (STD) is chaotic. This pattern of deformation is barely accounted for by a simple model of subhorizontal compression of the Earth’s crust in the region. In our opinion, these features of the seismotectonic deformation in the crust within the studied part of the Tajik Depression is probably associated with the gain in the volume of the rocks due to the inflow of the additional material, which is supplied from the bottom crust or upper mantle by the deep fluids. This increase in the rock volume produces the state of horizontal compression (or, more exactly, outward all-round pressure). We suggest considering the subvertical pillar-like object, identified as a cluster of the earthquake hypocenters in the northern part of the region with chaotic STD, as one of the probable channels of fluid supply. This structure’ plane is isometric and has a diameter of about 20 km. It stretches downwards to a depth of 40 km, where the seismicity ceases. The suggested scenario of generation of the outward all-round pressure (autonomous geodynamics) can be considered as an independent driver of tectogenesis along with the traditional mechanisms relating the emergence of the stressstate of subhorizontal compression to the plate-tectonic convergence of the Eurasian and Indian lithospheric plates.