HR: 10:35h
AN: V32A-02 INVITED    [Abstracts]
TI: Continental Crust and the Convective Thinning Mechanism
AU: * Houseman, G A
EM: greg@earth.leeds.ac.uk
AF: School of Earth and Environment University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, United Kingdom
AB: Continental lithosphere is potentially unstable to gravitational instability triggered by convergent orogeny. Such events occur quickly on a geological timescale and are limited in the areal extent of lithosphere affected. The volume of lithospheric mantle that may be removed by one such event is illustrated at one extreme by the Tibetan Plateau or the Basin and Range province, and at the other extreme by the convergence associated with the Alpine Fault in New Zealand and the San Andreas Fault in southern California. Miocene convergence between Africa and Europe has resulted in a complex evolution which has almost certainly involved instability of continental lithospheric mantle in basins such as the Pannonian and Alboran Sea. The replacement of continental lithosphere by hot asthenosphere may result in lithospheric extension occurring even in the middle of a region affected by a convergent stress field. In parts of the Mediterranean the extension has naturally progressed to sea- floor spreading. Seismic tomography provides evidence that the removal of lithospheric mantle from beneath the Mediterranean and from beneath Tibet has occurred rapidly enough that unequilibrated, seismically fast, material remains now in the transition zone between 410 and 670 km depth beneath those regions. The extent to which continental crust is involved in such convective thinning events is questionable. The large density contrast between typical continental crust and mantle clearly would prevent unaltered crust being involved in anything like a Rayleigh-Taylor instability of the mantle lithosphere. If the transition to eclogite facies occurs, however, then the lower crust could well participate and in fact might provide an important contribution to the gravitational energy that drives a convective thinning event. Estimates of the volumes of continental crust that might be involved in continental convective thinning events are relatively small. For example, the present crustal volume of the Tibetan Plateau is consistent with what is expected from mass conservation calculations of the continental collision. It seems unlikely that more than 10% of that volume is unaccounted for. Thus it also seems unlikely that removal of eclogitised continental crust by the mantle drip mechanism is significant relative to the volume of oceanic crust consumed in subduction zones.
DE: 8110 Continental tectonics: general (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8122 Dynamics: gravity and tectonics
DE: 8125 Evolution of the Earth (0325)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting