HR: 17:00h
AN: T32F-05    [PDF]
TI: The Role of "Dripping" Mantle Lithosphere in the Formation and Exhumation of UHP Metamorphic Crustal Rocks
AU: Brandon, M T
EM: mark.brandon@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, CT 06520-8109 United States
AU: * Pysklywec, R N
EM: russ@geology.utoronto.ca
AF: Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: Ultrahigh-pressure metamorphism requires a rapid burial of crustal rocks to significant depth and subsequent exhumation while maintaining relatively cool deep lithospheric temperatures. The process has commonly been explained in terms of plate subduction, with the continental crust subducted to great depths and then returned within a subduction channel or as an extruding slice. Using numerical models of the thermomechanical evolution of the crust-mantle system, we investigate an alternative mechanism to account for the genesis of UHP crustal rock during continental collision. The numerical experiments show that in the early stages of collision, packages of lower crustal material are entrained to depth into a thickened mantle lithosphere root beneath the young model orogen. Subsequently, the mantle lithosphere root "drips" into the mantle as a Rayleigh-Taylor-type gravitational instability. This results in partial uplift of the buried crust, as the buoyant material separates from the descending root and rises. Late stage exhumation of the crust to the surface occurs more slowly, related to exhumational processes operating within the still active collisional orogen. The P-T evolution of the buried crust is tracked in the experiments and we find its history is consistent with those observed for UHP metamorphism. We propose this model of burial and uplift of crustal material encased in an unstable mantle lithosphere root during young, cold continental plate collision as an alternative to traditional subduction models of UHP crustal burial. The mantle drip model is consistent with a number of geological observables of UHP rocks in continental collision zones and may have several favourable features over a subduction channel/extruding slice models.
DE: 3210 Modeling
DE: 8015 Local crustal structure
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting