STRUCTURAL LINKS BETWEEN THE CAPPS GLACIER−CASTLE MOUNTAIN AND BORDER RANGES FAULT SYSTEMS AND THEIR ROLE IN THE MIDDLE EOCENE DISMEMBERMENT OF THE LATE PALEOCENE−EARLY EOCENE ALASKA FOREARC BASIN

Cenozoic stratigraphy of the actively subsiding Cook Inlet forearc basin continues to the northeast into the Matanuska Valley, where post-Paleogene northwest shortening has exposed late Paleocene−early Eocene forearc strata and near trench intrusive rocks. In the upper Cook Inlet and Matanuska Valley, the northeast-trending Capps Glacier (CGF) and Castle Mountain (CMF) dextral faults broadly define the structural boundary between Cenozoic arc and forearc basin domains. Trenchward, the Border Ranges fault system (BRFS) separates basin lithologies from the accretionary prism. Pavlis and Roeske (2007) speculated a component of ≤1,100 km of dextral strike slip on the BRFS was transferred across the Matanuska Valley onto the CMF at a nearly 90° southward bend in the BRFS during waning stages of slip, resulting in ~130 km of dextral separation of the north−northeast-trending Jurassic forearc complex. We build on the work of Terry Pavlis and his collaborators to strengthen links between these fault systems using geologic mapping, fault kinematic analysis, and stratigraphic maximum depositional ages. Our results demonstrate a right step between the CGF and CMF formed a pull-apart basin at their overlap from ca. 47.0 to 38.1 Ma (West Foreland Fm.) that was coeval with right-slip along the Glacier Creek strand of the BRFS at ca. 47.8 to 42.9 Ma. In the CMF and BRFS stepover, minor transtensional faults locally cut ca. 56.7 to 54.4 Ma Matanuska Valley forearc strata (Chickaloon Fm.) and produce nearly identical strain axes to fault-dissected West Foreland strata at the CGF and CMF step-over. Apparent conjugate strike-slip faults and dikes that cut underlying Late Cretaceous Matanuska Fm. produce similar northeast maximum elongation axes to transtensional faults up section. Complementary stratigraphic work suggests that forearc deposition continued parallel to the offset Jurassic arc an additional 50 km northeast of the CMF until ca. 52 Ma, suggesting the CGF−CMF does not constitute the structural basin boundary as commonly viewed. An implication of these findings is that linking of the CGF−CMF−BRFS dismembered the late Paleocene−early Eocene forearc. During the middle Eocene, the system decoupled forearc subsidence northeast of the CGF−CMF from the active system to the southwest in Cook Inlet at the abrupt bend in the BRFS, perhaps in response to development of the Alaska orocline.