HR: 1340h
AN: T43C-1342    [Abstracts]
TI: Intra-Lithospheric Tectonics: Iceberg Model for Stabilization of Archean Cratons
AU: Percival, J A
EM: joperciv@nrcan.gc.ca
AF: Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 Canada
AU: * Pysklywec, R N
EM: russ@geology.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: Thick buoyant mantle lithosphere roots appear to have developed as part of Archean cratons contemporaneously with formation of the crust. However, the genesis of post-tectonic, crustally derived granites requires appreciable late heating, remelting, and fractionation events that imply significant input of heat into the crust and removal of residual mafic-ultramafic lower crustal material. It has been postulated that the heating and removal may be explained by Rayleigh-Taylor (RT)-type instability of portions of the lithosphere or full mantle lithosphere delamination. We suggest that these processes based on modern analogues were less likely in Archean cratons owing to the low density of mantle lithosphere and its apparent durability since craton formation. Rather, we introduce an `iceberg model' of lithosphere keel inversion, in which mafic lower crust enters the eclogite field upon post-orogenic cooling, triggering rollover of a lithosphere cell that is lighter than asthenosphere but internally gravitationally unstable. A series of parameterized numerical experiments were conducted to quantify the dynamical evolution of the crust and mantle in a model Archean setting. The models demonstrate that intra-lithospheric rollover may occur where sufficient eclogitic crust overlies relatively buoyant mantle lithosphere. This is assisted by traction along the base of the lithosphere which may be caused either by underlying subduction, or viscous drag by mantle flow across the deep lithospheric root. The lithospheric rollover leads to a rapid pulse of elevated temperature in the lower crust when $\sim$1350$^{\circ}$C basal lithosphere/asthenosphere is juxtaposed with the new Moho during overturn. The dense eclogitic crust is removed from the lithosphere after inversion of the lithospheric root and descent into the mantle. These events are consistent with the observed enigmatic episodes of late, deep-crustal metamorphism, melting, and widespread granite blooms in some Archean cratons. We show that viscous RT instability or ``dripping'' of eclogite yields little thermal impact on the lower crust since the upward return flow of hot lithosphere material tends to be considerably more distributed and diffuse.
DE: 9619 Precambrian
DE: 8015 Local crustal structure
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting