Low-temperature thermochronological record of exhumation of theBitterroot metamorphic core complex, northern Cordilleran Orogen

Abstract

The Bitterroot metamorphic core complex is an exhumed, mid-crustal,plutonic-metamorphic complex that formed during crustal thickening andsubsequent extension in the hinterland of the North AmericanCordilleran Orogen, in the northern Idaho batholith region. Extensionwas accommodated mainly on the Bitterroot mylonite zone, a500-1500-m-thick shear zone that deforms granitic intrusive rocks asyoung as 53-52 Ma, as well as older high-grade metamorphic rocks andplutons. Exhumation of the core complex, in Eocene time, is marked inthe shear zone by the transition from amphibolite-faciesmylonitization, to greenschist-facies mylonitization, chloriticbrecciation, to brittle faulting that progressed from shallowercrustal levels in the west to deeper crustal levels in the east fromca. 53 -30 Ma based on U-Pb, Ar-Ar, and fission-track data. Apatiteand zircon fission-track data record the lower-temperature part of theexhumation history and help define when the shear zone becameinactive, as well as the transition from rapid, core complex-styleextension to slower basin-and-range-style extension. They indicatethat the western part of the complex was exhumed to within 1-2 km ofthe surface by approximately 48-45 Ma, while the eastern part of thecomplex was still at amphibolite-facies conditions and that theeastern part of the complex was not exhumed below approximately 60degrees C until after 30 Ma. Younger apatite fission-track ages <or =26 Ma on the eastern range front of the Bitterroot Mountainssuggest that the present topographic expression of the mylonite frontwas due to Miocene high-angle faulting and widening of the BitterrootValley.

42 samples from this reference