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

Abstract

S U M M A R YIn 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.

Investigation of Lithospheric Structure in Mongolia: Insights From Insar Observations and Modelling

“…Using free-air gravity profile across the Valley of Lakes in the Gobi-Altai to find the minimum misfit in the Gobi-Altai region, the result shows the effective elastic thickness is 10 km. The spectral analysis of admittance presents an average effective elastic thickness of < 10 km over the broad region (Bayasgalan et al, 2005). The solid gravity analysis confirms that Te is comparable to Ts in Gobi-Altai region.…”

“…After searching for the various seismogentic thickness under the viscoelastic relaxation condition, the best fitting model support the seismogenic thickness as 10 km. Comparing the seismogenic thickness (10 km) with the effective elastic thickness (10 km) obtained from Bayasgalan et al (2005), the lithospheric thickness in the Gobi-Altai region is consistent with the perspective of seismic and gravity.…”

“…Bayasgalan et al (2005) also constrain the effective elastic thickness (Te) in central Mongolia is < 10 km, and infer the Gobi-Altai range may be a little greater but less than 20 km. Using free-air gravity profile across the Valley of Lakes in the Gobi-Altai to find the minimum misfit in the Gobi-Altai region, the result shows the effective elastic thickness is 10 km.…”

“…Most of continental earthquakes occurred in the lower crust at depths less than 20 km, which can be defined as seismogenic crust (Wright et al, 2013). Bayasgalan et al (2005) determine 10 earthquake centroid depths by investigating P and SH waveforms, and claim that the seismogenic thickness (Ts) within Mongolia is less than 20 km. This conclusion is consistent with this study.…”

Effects of a low‐viscosity lower crust on topography and gravity at convergent mountain belts during gravitational instability of mantle lithosphere

“…More important, a compilation of estimates of admittance from a variety of regions ( Figure 9) shows observed values of admittance to be consistently positive [e.g., Bayasgalan et al, 2005;D'Agostino and McKenzie, 1999;Komut et al, 2012;Fielding and McKenzie, 2012;McKenzie and Bowin, 1976;McKenzie and Fairhead, 1997;Tiley et al, 2003;Watts, 1978]. We are not aware of a single study reporting negative values for the Earth.…”