TRANSPRESSION AND TRANSTENSION ALONG THE LAKE CLARK FAULT IN SOUTH-CENTRAL ALASKA: NEW VIEWS ON ITS FORMATION, CHARACTERISTICS, DAMAGE, KINEMATICS, EXHUMATION, AND EVOLUTION

The poorly exposed Lake Clark fault (LCF) is expressed as a northeast-striking curvilinear valley ~200 km long and up to a few km wide. Previous studies based on geologic mapping and geophysical data indicated 12 to 26 km of dextral separation and up to 1 km of SE-directed reverse separation since the Late Eocene.

New data from 20 sites indicates an extensive zone of brittle damage a few tens of meters to at least 1.2 km in width. Damage is asymmetrically distributed along the mapped trace and is particularly well developed where the LCF cuts Cretaceous granitoid plutons. Damage consists of striated, hematite coated, meter-scale slip surfaces associated with argillic alteration. The surfaces reflect small strains at intensities of several surfaces per m. Analyses of 140 surfaces indicates a mean strike and dip of 053/83 and a mean shortening axis plunge and trend of 10/140. However, slip varies from site to site with components of oblique reverse, sinistral, dextral, and normal slip on small surfaces to map-scale faults.

Along part of the LCF trace, non-foliated granodiorite to the NW is juxtaposed against foliated quartz diorite to granodiorite that locally contain mafic screens to the SE. Penetrative ductile fabrics in the plutonic rocks to the SE are parallel to the fault trace. Zircon U-Pb ages are ~74-69 Ma to the NW and ~85-76 Ma to the SE with geochemically distinct fractionation trends on either side of the fault. U-Pb ages and ductile deformation are consistent with a component of NW directed reverse slip on a SE-dipping fault, but it is unclear if the plutons on either side are different or if they represent different stages of the same magmatic system. Thermochronometric data indicates post-Oligocene exhumation rates of >200 m/m.y. and local Late Miocene to Pliocene acceleration in rock cooling. Cooling ages do not vary across the fault.

Integration of our new data suggests the localization and evolution of the LCF was influenced by Cretaceous magmatic arc assembly processes that produced initial arc-parallel rock fabric anisotropy, moderate exhumation rates, and distributed brittle deformation. The resulting fault zone has small separation, a long trace length, and locally substantial width. It accommodated both transpressional and transtensional components of deformation within a complex accretionary plate boundary.