Asthenospheric imprints on the lithosphere in Central Mongolia and Southern Siberia from a joint inversion of gravity and seismology (MOBAL experiment)

Abstract

International audienceWe present a joint inversion of gravity and teleseismic data to enlighten the lithospheric structures of the Baikal–Mongolia region, an area characterized by high topographic contrasts, sporadic Cenozoic volcanism, extension and large transcurrent faulting in the vicinity of the Baikal Rift, Central Asia. The study uses a 1000 km long seismic transect that cross-cuts the main tectonic structures from north to south (namely, the Siberian platform, Tunka basin, Hangay Dome and Gobi-Altai belt). The Siberian platform depicts a high-velocity lithosphere down to about 150 km. We evidence strong velocity contrasts within the crust below the Hangay Dome and the Tunka depression, interpreted as a thickened crust. A low-velocity/density region is located at various depths below the Hangay Dome. Thanks to the dense spatial coverage of gravity data, we are able to define the 3-D geometry of this particular low-velocity/density anomaly. The Hangay Dome anomalous body extends from 60 to 225 km depth at his largest point and slightly thins to no more than 40 km at its easternmost end. A deep low-velocity zone (below 150 km) with only a weak negative density contrast is observed on the eastern part of the transect. We propose that the late Cenozoic uplift of the whole Mongolian Plateau and associated rifting, magmatism, high heat flow and lithospheric thinning are not externally driven by the India–Asia collision, but results from the interaction of two mantle plumes with the overlying lithosphere. Conversely, recent faulting may be the major expression of the India–Asia collision in this region, as well as N–S striking rift basins that connect and interact with major strike-slip faults, such as the Bolnai fault. The comparison between time residuals of this experiment and the Baikal Rift seismic data shows an undeniable disproportion for the Hangay anomaly. The Hangay time residuals are far more positive than through the Baikal, arguing for an asthenospheric signature that is not seen beneath the Baikal Rift. Furthermore, the southern edge of the thick Siberian cratonic lithosphere may favour the rise of a sublithospheric mantle flow at the contact with the Baikal–Mongolia lithosphere

East Asia: Seismotectonics, magmatism and mantle dynamics

“…The result shows a prominent low-V anomaly under the Baikal rift zone extending down to 660 km depth and high-V anomalies in the lithosphere under the Siberian Craton. The result is generally consistent with the lithospheric and asthenospheric features revealed by other geophysical studies in this region (e.g., Achauer and Masson, 2002;Gao et al, 2003;Liu and Gao, 2006;Tiberi et al, 2008). The low-V anomaly is interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal rift zone and the formation of Cenozoic volcanism in Baikal.…”

Studying deep sources of volcanism using multiscale seismic tomography

“…It was concluded that if these plumes had existed, they would have been robustly resolved by tomography. It should be added that studying the mantle beneath the volcanic areas in Mongolia using the teleseismic tomography scheme of Tiberi et al (2008) did not reveal any feature which could be unambiguously interpreted as a plume. Therefore, we propose that volcanism in this region is caused by a general large-scale heating of the mantle rather than by small-scale separate plumes.…”

A new perspective on evolution of the Baikal Rift

“…The presence of low-velocity mantle material under the Baikal Basin was discovered in the late 1970's (Logatchev and Zorin, 1987 and references therein) and confirmed by later studies, including teleseismic tomography, which provided better constraints on the position of the plume (Gao et al, 1994(Gao et al, , 2003Zorin et al, 2003;Zorin and Turutanov, 2005;Tiberi et al, 2008). According to the teleseismic data, asthenosphere in the southwestern Baikal region reaches the crustal base in a wedge-shaped upwarp thickening away from the Siberian craton (Tiberi et al, 2008;Mordvinova et al, 2010). The width of the upwarp, in an WeE direction, is estimated to be about 400e500 km from 3D teleseismic tomography and gravity data Tiberi et al, 2008), as well as from the distribution of volcanism (Rasskazov, 1993).…”

“…According to the teleseismic data, asthenosphere in the southwestern Baikal region reaches the crustal base in a wedge-shaped upwarp thickening away from the Siberian craton (Tiberi et al, 2008;Mordvinova et al, 2010). The width of the upwarp, in an WeE direction, is estimated to be about 400e500 km from 3D teleseismic tomography and gravity data Tiberi et al, 2008), as well as from the distribution of volcanism (Rasskazov, 1993). The asthenospheric upwarp maintains increasing NW extension by creating gravity instability .…”