EXHUMATION OF THE MIDDLE CRUST WITHIN A CONTINENTAL-SCALE STRIKE-SLIP FAULT SYSTEM: INSIGHTS FROM THE CLEARWATER METAMORPHIC CORE COMPLEX, IDAHO

Several Eocene metamorphic core complexes (MCCs) in the northern U.S. Rocky Mountains have been kinematically and temporally linked to dextral movement along a regionally extensive set of strike-slip faults called the Lewis and Clark fault zone (LCFZ). The Clearwater complex (CMCC) in northern Idaho is unique from other MCCs in this region in that it lies in a releasing bend in the LCFZ and thus allows for the study of mid-crustal exhumation processes within continental-scale strike-slip fault zones. Structural and ⁴⁰Ar/³⁹Ar thermochronological data presented here provide constraints on the exhumation history of the CMCC. Previous workers have divided the CMCC into two lithologically and structurally distinct zones: an internal and external zone, both of which contain rocks that yield Eocene ⁴⁰Ar/³⁹Ar cooling ages. However, the evolution of the CMCC has remained cryptic due to a scarcity of structural and thermochronological data. In particular, a set of previously unknown E-verging normal faults termed the Surveyors Ridge fault zone in the NE part of the external zone was identified. Footwall rocks in this area preserve E-directed ductile kinematic indicators and yield ⁴⁰Ar/³⁹Ar mica cooling ages of ~44 Ma; rocks in the hanging wall of the easternmost of these faults yield muscovite cooling ages >70 Ma, and thus delineates the eastern boundary of Eocene exhumation in the CMCC.

The results of this study show that the Clearwater complex can be subdivided into at least four structural blocks, each with distinct exhumation histories. The internal zone and western and eastern external zones were exhumed from 48-41 Ma along E- and W-verging brittle-ductile normal faults. The Crescendo Peak block (CPB) lies between the internal and external zones and yields ⁴⁰Ar/³⁹Ar mica cooling ages of 58-54 Ma. The upper greenschist facies metamorphic grade of these rocks differs significantly from the upper amphibolite facies rocks of the internal and external zones and suggests that the CPB was not buried as deeply as other parts of the CMCC. The CPB may therefore represent the remnants of a strike-slip duplex that formed in the upper crust during the initial stages of dextral movement in the LCFZ. These results suggest that the complex exhumation history of the Clearwater complex may be due to inheritance of pre-existing structural features.