LARGE ZONES OF DAMAGE AND FLUID FLOW IN CONTINENTAL STRIKE-SLIP FAULTS – THE POST LATE CRETACEOUS HISTORY OF THE LAKE CLARK FAULT IN SOUTH-CENTRAL ALASKA

The northern Cordillera is characterized by networks of orogen-parallel, steeply-dipping strike-slip faults geomorphologically expressed as “strike” valleys. The detailed structure, fault dimensions, relations to fluid flow, and the rock record of ancient seismicity are poorly documented in these valleys. The Lake Clark fault (LCF) occupies a NE-trending curvilinear valley ~200 km long and up to a few km wide. New mapping along the LCF shows weak, possibly magmatic flow foliated granitoids to the NW, juxtaposed against granitoids with dextral-reverse fault-parallel penetrative ductile deformation fabrics to the SE. Magma evolution and emplacement appears coupled with deformation along the LCF, where median zircon U-Pb ages are ~7 Ma older on the SE side of the fault, consistent with a component of Late Cretaceous-early Paleogene NW-directed reverse slip.

An extensive brittle regime damage zone at least 1.2 km in width and ~90 km in length overprints all lithologies and ductile fabrics. Damage is asymmetric along the fault trace and consists of striated, hematite-coated, meter-long slip surfaces at several surfaces per cubic meter. Weak argillic alteration and laumontite-filled fault veins host reverse, strike-slip, and normal syn-kinematic small strains.

Although no fault cores were observed, fault-related illite in central exposures were dated at ca. 22.8 to 33.0 Ma using encapsulated ⁴⁰Ar/³⁹Ar. Apatite (U-Th)/He age-elevation data indicates Oligocene to Miocene exhumation at ~ 35 m/m.y., accelerating to ~ 250 m/m.y. at ~10 Ma, without appreciable thermochronometric differences on either side of the fault. Illite geochronology overlaps with apatite thermochronology and the thermal stability of laumontite fault veins, effectively bracketing the age of coupled thermomechanical, fluid flow, and fault kinematic processes.

Our data suggests the localization and evolution of the LCF was influenced by Cretaceous pluton assembly processes producing initial plate margin-parallel ductile fabric anisotropy, moderate exhumation, extensive brittle regime deformation, fluid flow, and hydrothermal alteration. The fault zone accommodated both transpressional and transtensional components of deformation and guided landscape evolution within a complex accretionary plate boundary.