Implications of middle Eocene epizonal plutonism for the unroofing history of the Bitterroot metamorphic core complex, Idaho-Montana

Abstract

Petrologic and geochronologic limits on the progression of east-directed extensional unroofing of the Bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation. U-Pb and 4 0 Ar/ 3 9 Ar data from epizonal plutons in the northwestern and southwestern Bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels (1-5 km) by ca. 49 Ma, while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression. U-Pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of Belt Supergroup metasedimentary rocks in the western footwall no later than 56 Ma and emplacement of the adjacent epizonal Lolo Hot Springs batholith at ca. 49 Ma in the northwestern Bitterroot Range. Tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks, suggesting that the initial stages of unroofing may have been accommodated by a planar, moderately east-dipping (30°-50°) shear zone. 4 0 Ar/ 3 9 Ar cooling data show that cooling in the western footwall was synchronous with unroofing. In contrast, we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage. Exposure of a west-dipping mylonitic shear zone along the western limb of the Bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex. If this shear zone can be correlated to the Bitterroot mylonite zone to the east, then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges.