HR: 15:20h
AN: U53A-06    [Abstracts]
TI: Slab pull, slab weakening and their influence on great earthquakes, deep earthquakes and surface deformation
AU: * Lithgow-Bertelloni, C R
EM: crlb@umich.edu
AF: Dept. of Geological Sciences, 1100 N. University Avenue University of Michigan, Ann Arbor, MI 48109-1005 United States
AU: Conrad, C P
EM: conrad@jhu.edu
AF: Dept. of Earth and Planetary Sciences, 3400 North Charles Street Johns Hopkins University, Baltimore, MD 21218 United States
AU: Bilek, S L
EM: sbilek@nmt.edu
AF: Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801 United States
AB: Viscous flow in the mantle, by coupling to Earth's lithosphere, ultimately determines plate motions, lithospheric stresses, and patterns of seismicity. To constrain the mantle's influence on these varied observables, we synthesize recent predictions of plate motions and lithospheric stresses determined from self-consistent models of mantle flow. We use these models of mantle flow, together with global models of lithospheric heterogeneity, to quantify the origin of the lithospheric stress field. These models suggest that the mantle signal is the strongest component of observed lithospheric stresses, but with large geographic variability. Strong lateral heterogeneity in mantle viscosity or in the rheology of the lithosphere may explain most of these geographic variations. We find in fact that strong cratonic regions couple directly to mantle flow. By accounting for the effects of viscous flow induced by slab buoyancy (lower mantle slab suction) and the slab directly transmitting its excess weight to the plate (upper mantle slab pull), we can entirely explain the difference in speed between subducting and non-subducting plates. The strength of slab pull has increased during the Cenozoic, resulting in an increase in the relative speed of oceanic plates. If the degree of pull and suction varies across subduction zones, we can explain, to first order, the observed variations in seismic moment release for large underthrusting earthquakes: at seismically uncoupled subduction zones, slabs are typically attached to plates and transmit nearly their entire upper mantle weight to the plate directly. At seismically coupled subduction zones that produce great earthquakes, however, slabs are nearly completely detached from their subducting plates. This suggests that slabs subducting in a compressional environment experience stress-induced weakening that prevents the effective transmission of the slab pull force. Indeed, we find strong correlations between strong plate-slab attachment and the presence or absence of back-arc basins. If we further compare moment release at depth with the degree of plate-slab attachment, we find positive correlations with the seismic moment released from intermediate and deep earthquakes. This implies that shallow slab weakening that occurs at trenches where compressive stresses (and great earthquakes) dominate, not only detaches slabs from plates, but is also maintained as the slab descends, discouraging deep seismicity. Our integrated view suggests that self- consistent models of mantle flow, plate driving forces and lithospheric stresses can lead to a greater understanding of interior dynamics and surface deformation.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8155 Plate motions: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Union [U]
MN: Fall Meeting 2005