固体地球物理学术报告通知

报告题目:

1.Lateral viscosity variations in the mantle and dynamic topography in global mantle flow

2.Slab avalanche induced surface tectonic-Implications on the Cenozoic evolution of Southeast Asia

1.      Although the surface topography is mainly induced by density heterogeneity within the lithosphere, dynamic topography also plays an important role in the continental sedimentation and erosion history, gravitational fields, and global and regional sea levels. However, the amplitude of dynamic topography is still under debate. Lateral viscosity variations (LVV) in the mantle influence geodynamic processes and their surface expressions, including dynamic topography and gravitational fields. However, LVV is still poorly constrained. We first invert for depth- and temperature-dependent and tectonically regionalized mantle viscosity. The inversions suggest that long-wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile; the pre-Cambrian lithosphere viscosity is slightly (∼ one order of magnitude) higher than that of oceanic and Phanerozoic lithosphere; plate margins are substantially weaker than plate interiors; and viscosity has only a weak apparent, dependence on temperature, suggesting either a balancing between factors or a smoothing of actual higher amplitude, but short wavelength, LVVs. We then investigate the amplitude of dynamic topography and the relationship between dynamic topography and free air anomaly. Both recent compiled high-accuracy residual topography and the predicted dynamic topography yield ∼1 km amplitude long-wavelength dynamic topography, inconsistent with recent studies suggesting amplitudes of ∼100 to ∼500 m. Such studies use a constant, positive admittance (transfer function between topography and gravity), in contrast to the evidence which shows that the earth has a spatially and wavelength-dependent admittance, with large, negative admittance between ∼4000 and ∼10,000 km wavelengths.

2.The continental core of Southeast Asia, Sundaland is tectonically quiet at present, as evidenced by the low seismicity and strain rate within. However, Sundaland suffered violent deformation in the past, as revealed by the high surface heat flux and low seismic velocity in the uppermost mantle. We use geodynamic models of mantle convection combined with plate reconstructions to investigate the dynamic evolution of Sundaland. Models suggest that a slab stagnates within the transition zone beneath Southeast Asia before the Miocene. The stagnant slab penetrated through the 660 km mantle discontinuity during the early Miocene and formed a slab avalanche event. This avalanche induced large-scale marine inundation, regional compression, and basin inversion across southern Sundaland.

We performed self-consistent mantle convection models to investigate the influences of slab avalanche on surface tectonics in general circumstances. Model results suggest that during the slab avalanche, trench motion changes from trench retreat to trench advance, consistent with plate reconstructions of the Sunda trench. Basins above the avalanched slab are inversed and subsided while basins on the far field are under continuous extension, consistent with the recorded basin history in Sundaland.