HR: 0800h
AN: T41A-1287    [Abstracts]
TI: Seismicity, Structure, and Rheology of the Lithosphere in the Lake Baikal Region
AU: * Emmerson, B
EM: emmerson@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom
AU: Jackson, J
EM: jackson@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom
AU: McKenzie, D
EM: mckenzie@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom
AU: Priestley, K
EM: keith@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom
AB: Our investigation of rheological properties of the lithosphere in the lake Baikal region combines observations of seismicity, gravity, topography, and thermal and velocity structures. We use teleseismic waveform modelling and published relocations of microearthquakes to examine the seismogenic thickness (T_s), and conclude that there is no convincing evidence for a seismogenic mantle in the Baikal region. Using the admittance between free air gravity and topography, we estimate the effective elastic thickness (T_e) in the region to be between 5--20~km. Nowhere do the data require that T_e>T_s, allowing the simple interpretation that the long term strength of the lithosphere resides in its seismogenic layer, and that the mantle is relatively weak. A weak mantle in the Baikal region can be explained by its high temperature, which we estimate by combining local geotherm estimates with the regional upper mantle velocity structure, obtained from fundamental and higher-mode surface waves. Geotherms are fitted to pressure and temperature estimates from mantle nodules at four sites, both within and outside the Siberian shield. In order to constrain the temperatures at the Moho, we estimated crustal thicknesses using teleseismic receiver functions. Moho temperatures exceed ~550°C beneath the Siberian shield and are higher in the more recently deformed mountain belts to the south. Based on a recent reassessment of oceanic geotherms and seismicity, it seems likely, therefore, that the mantle in the Baikal region is too hot to be a source of long term strength. This is consistent with the suggestion that the distribution of mantle seismicity in both the oceans and the continents is dependent on temperature alone. Finally, we note that results from S-wave tomography studies, combined with the observed locations of rift-related earthquakes, lead us to suspect that the frequently published position of the edge to the Siberian shield at the surface provides a poor description of that same boundary at depth.
DE: 7205 Continental crust (1219)
DE: 7215 Earthquake source observations (1240)
DE: 8138 Lithospheric flexure
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005