HR: 14:10h
AN: T32D-03    [PDF]
TI: Window into the middle crust: A deeply exhumed Proterozoic orogenic plateau in the western U.S.
AU: * Shaw, C A
EM: shawca@uwec.edu
AF: University of Wisconsin, Eau Claire, 105 Garfield Ave., Eau Claire, WI 54702 United States
AU: Karlstrom, K E
EM: kek1@unm.edu
AF: University of New Mexico, 200 Yale Blvd. NE, Albuquerque, NM 87131 United States
AB: A widespread 1.45-1.35 Ga tectonic episode involving A-type granitic magmatism, regional metamorphism, and widespread deformation profoundly modified Proterozoic continental lithosphere throughout the southwestern United States. We suggest that this intercontinental tectonism is best understood in the context of a broad orogenic plateau that formed inboard of an Andean-type convergent margin that stretched more than 5000 km from the Baltic craton to southern California and perhaps into Australia. An extensive 40Ar/39Ar thermochronologic study (more than 250 new dates) in the southern Rocky Mountains suggests that the current level of exposure in basement rocks broadly coincides with a ca. 1.4 Ga mid-crustal magma-rich layer where regional heating enhanced by advection in the lower crust gave way to purely convective heat transfer in the upper crust. Thus, 1.4 Ga tectonism in the southwestern United States may represent a deeply exhumed example of magmatic modification of the mid-crustal brittle-plastic rheologic transition. Similar processes may occur in mid-crustal magma layers that are inferred beneath modern intracontinental orogenic plateaus such as Tibet and the Andean Altiplano-Puna plateau An orogenic plateau model for ~1.40 Ga tectonism is also supported by: widespread NW directed contraction and crustal thickening during syntectonic pluton emplacement, widespread elevated temperatures at middle crustal depths with higher temperatures towards an inferred plate margin to the south, slow cooling/lack of rapid unroofing following 1.45-1.35 Ga metamorphism, and the necessity to provide the gravitational potential to drive post- 1.4 Ga erosional removal of ~ 10 km of crust over tens of millions of years to produce the Great Unconformity. Geophysical evidence for a high-velocity lower crust is consistent with a ~1.4 Ga basaltic underplate related to the production of A-type magmas that carried heat into the middle crust. If our model is correct, the southwestern U.S. may be a deeply exhumed analog for modern intracontinental orogenic plateaus that could provide insights into mid-crustal processes of intracontinental deformation.
DE: 1035 Geochronology
DE: 8099 General or miscellaneous
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting