Lithosphere rheology and active tectonics in Mongolia: relations between earthquake source parameters, gravity and GPS measurements

ABSTRACT

In this paper, we use observations of earthquake source parameters and gravity to investigate the mechanical properties and the active faulting of the lithosphere in Mongolia. Well-determined earthquake centroid depths, including 10 from inversions of P and SH waveforms that are presented here for the first time, show that the seismogenic thickness  (T_s)  within Mongolia itself is less than 20 km. However, to both the east, in parts of the Lake Baikal rift system, and the west, adjacent to the Junggar basin and Kazakhstan platform, the seismogenic thickness is considerably greater, and includes essentially the whole crust. From the admittance between the free-air gravity and the topography, and also from profiles across a flexural foreland basin, we determine the effective elastic thickness  (T_e)  in central Mongolia to be <10 km, though it may be a little greater (<20 km) adjacent to the Gobi-Altay range in the south. Further west, adjacent to the Kazakhstan platform, the same techniques show that  T_e > 30 km . In both Mongolia and its surroundings,  T_e  is comparable with  T_s  and, where it is well determined,  T_e < T_s . Nowhere do the data require that  T_e > T_s . These data are consistent with the view that the strength of the continental lithosphere resides in its seismogenic part, which in Mongolia is the upper crust, but to both the east and west appears to be the whole crust. The earthquake source parameters also allow us to ask how the active faulting in Mongolia accommodates the velocity field revealed by GPS measurements. It is likely that the entire Mongolian Altay range in the west rotates counter-clockwise relative to stable Asia, and is responsible for the distributed E–W left-lateral shear seen further east in central Mongolia. The admittance observations show no evidence at the present day for convective mantle support, or a 'hotspot', responsible for the elevated region of the Hangay dome in central Mongolia, even though the geochemical data from nodules in late Cenozoic basalts and seismic tomography studies suggest elevated temperatures at shallow depths (<125 km) and probably thinned lithosphere.