HR: 16:45h
AN: T32F-04    [PDF]
TI: Arrest of Ultrahigh-Pressure Terranes and the Evolution of the Continental Crust
AU: * Hacker, B R
EM: hacker@geology.ucsb.edu
AF: Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Walsh, E O
EM: walshe@union.edu
AF: Department of Geology, Union College, Schenectady, NY 12308 United States
AB: Ultrahigh-pressure terranes can, in principle, be generated in any setting where continental crust is forced to depths greater than ~100 km: i) during ophiolite emplacement onto continental margins (e.g., Oman, Timor), ii) in intracontinental convergence zones (e.g., the Hindu Kush), and iii) in continent-continent collisions (e.g., the Himalaya). The most favorable setting might be subduction of a cold microcontinent beneath a thick continental margin arc with a serpentinized mantle wedge. The infancy of field studies means, however, that the mechanisms by which most ancient ultrahigh-pressure terranes were created are unknown even to this gross level of detail. Several mechanisms for the exhumation of ultrahigh-pressure rocks have also been suggested: i) slab breakoff and wedge extrusion, ii) reversal of slab motion, iii) flow in a low-viscosity channel, and iv) diapirism. None of these models explicitly accounts for a curious feature of the three largest, well-studied ultrahigh-pressure terranes: each has a post-ultrahigh pressure metamorphic overprint of 0.8 to 1.2 GPa that almost destroyed the ultrahigh-pressure record. Why? Our combined structural, petrological, and geochronological study of the 220 km x 150 km high- to ultrahigh-pressure terrane of Norway leads us to the conclusion that early buoyancy-driven(?) exhumation through the mantle was abruptly arrested at the Moho. There, at the base of the crust, the formerly ultrahigh-pressure mass underwent vertical flattening, outward flow along the Moho, and nearly complete recrystallization. This highly modified body was subsequently exhumed through the crust, perhaps as a result of the crustal overthickening produced by its arrest at the Moho. This Moho-arrest model for the exhumation of ultrahigh-pressure terranes has significant implications for the geodynamics and geochemistry of the crust. For example, if arrested ultrahigh-pressure terranes are never exhumed through the crust and {\it remain at the Moho}, they replace the lower crust with variably depleted once-upper crustal lithologies. A simple calculation based on probable rates of crustal subduction at active orogenic belts suggests that this mechanism could have processed the entire lower continental crust of Earth.
DE: 1020 Composition of the crust
DE: 3660 Metamorphic petrology
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting