HR: 0800h
AN: T11D-0418    [Abstracts]
TI: Extensional Acceleration of Diapiric Tectonics
AU: * Bailey, R C
EM: bailey@physics.utoronto.ca
AF: Geology and Physics Depts., University of Toronto, 60 St. George St., Toronto, ON M5S 1A7 Canada
AU: Pearse, J
EM: pearse@physics.utoronto.ca
AF: Physics Dept., 60 St. George St., Toronto, ON M5S 1A7 Canada
AB: A decade ago, Glazner pointed out that density anomalies such as mafic plutons in reheated continental crust will drive vertical tectonics by ductile remobilization and subsidence as a result of buoyancy forces, and that this can be a relatively rapid process. We have examined the surface expression of this process using finite element modeling, and shown in previous work that a basin develops at the surface, succeeded by exhumation into a dome if crustal reheating is strong enough. The two-dimensional finite-element code models the coupled deformation and heat flow in a density-stratified crust (from 2.5 at the surface to about 2.9 at the Moho) with thermally activated viscoelasticity (with viscosity ranging from 10¹⁹ Pa-s after reheating in the middle of the crustal ductile channel at 25 km depth, to 10²⁶ Pa-s at the Earth's surface). The most extreme plausible case investigated used mafic plutons (density 3.0) comparable with the Keeweenawan volcanics of the mid-continent gravity high under the Michigan Basin, embedded with centres between 8 and 15 km depth, and a reheating event comparable with that under the Basin and Range province today (about 100 mW/m²). In this case, subsidence of up to 7 km was followed by exhumation of up to 10 km, leaving a dome. Precise numbers depend on the embedding depth of the pluton and the amount of erosion assumed. Deeper embedding leads to earlier remobilization but reduced vertical displacements at the surface. Anything less than full erosion leads to surface topography but smaller vertical displacements. Basin and dome widths are determined by the flexural parameter of the thermally thinned elastic layer of the crust, and are of order 60 km in the models here. In the absence of regional deviatoric stresses, the subsidence phase lasts of the order of 25 Ma, and the exhumation phase about 10 Ma. However, with extensional stresses present, producing extensional strain rates of the order of 10^-15 s^-1, the modelled timescales for both the subsidence and exhumation are up to an order of magnitude faster. We report on the time evolution of displacements, stresses, topography and temperature for a number of cases illustrating this behaviour.
DE: 8003 Diapir and diapirism
DE: 8020 Mechanics, theory, and modeling
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: Fall Meeting 2005