HR: 17:45h
AN: T14B-08    [Abstracts]
TI: Continental-Arc Geotherms, How Much do we Understand Their Shape?
AU: * Depine, G V
EM: gvd2@cornell.edu
AF: Department of Earth and Atmospheric Sciences Cornell University, Snee Hall, Ithaca, NY 14853, United States
AU: Andronicos, C L
EM: ca98@cornell.edu
AF: Department of Earth and Atmospheric Sciences Cornell University, Snee Hall, Ithaca, NY 14853, United States
AU: Phipps - Morgan, J
EM: jp369@cornell.edu
AF: Department of Earth and Atmospheric Sciences Cornell University, Snee Hall, Ithaca, NY 14853, United States
AB: The distribution of temperature with depth in orogenic belts controls deformation, metamorphism and plutonism. Investigating the shape of geotherms in mountain belts as well as the major parameters that modify them is very important in order to understand the tectonics and geodynamics of orogens. Common observations in orogenic belts are: 1) the presence of rocks that show a wide range of pressures but similar high temperatures; 2) the appearance of rocks which crossed the andalusite-sillimanite transition, which indicate a very hot geothermal gradient in the upper middle crust; 3) the abundance of plutonic rocks and migmatites at all levels of the crust, which indicate that melting processes and pervasive migration of melt play a major roll in shaping the geotherm; 4) the predominance of granites over more mafic compositions for the plutonic bodies, indicating partial melting of the crust is widespread. In this work we modeled the shape of geotherms to explain the above observations in orogens using a 1-D advection-diffusion-melting equation. The initial condition is a conductive geotherm for the crust and asthenospheric temperatures below the Moho. Melt is produced when the temperature of the rocks reaches their solidus temperature. We use amphibolite as the dominant composition for the lower/middle crust, based on studies which show amphibolite as the likely source rock for calc-alkaline plutons. Advection is allowed only within the crust by moving melts generated by anatexis after they reach a threshold of 10 vol.%. The models do not account for changes in crustal thickness due to erosion, thickening or extension. The modeled geotherms represent a continuous thermal evolution of an orogen with constant crustal thickness and heat input from the mantle. Different models evaluate the effect of the heat input into the deep crust on the geotherms. By doing this we found that when the Moho heat flux is 120 mW/m² or higher, the quasi-steady-state is obtained after 35 Ma. When the Moho heat flux is less than 120 mW/m² as is common for mountain belts, the steady-state geotherm is not obtained in geologically realistic time spans. By introducing the process of melt focusing through the modeled profile, a hot geotherm that fits the field observations in orogens can be obtained. Melt focusing is the process of lateral migration of melt from adjacent regions at the base of the crust which then rises through the column. If the melt is focused by a factor of two, a quasi-steady state is predicted to occur after 25.5 Ma for a mantle heat flux of 47.5 mW/m². The modeled melt-enhanced quasi-steady-state geotherm has very steep geothermal gradients of more than 50 ° C/km in the upper crust consistent with the generation of andalusite-sillimanite metamorphic terranes. The middle and lower crust are nearly-isothermal because the temperature is buffered by the amphibolite solidus curve. This thermal profile for the crust produces conditions that would favor orogenic instability, and may be a necessary precondition for late orogenic extensional collapse. For terranes where collapse occurs, exhumation would follow the pressure peak and be nearly-isothermal because the entire middle and lower crust is hot. Thus exceptionally fast exhumation rates are not necessary for nearly-isothermal decompression. For terranes that do not collapse, isobaric heating and cooling paths would be followed, with long lived slow cooling.
DE: 1020 Composition of the continental crust
DE: 1037 Magma genesis and partial melting (3619)
DE: 3652 Pressure-temperature-time paths
DE: 8110 Continental tectonics: general (0905)
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2007 Fall Meeting