CENOZOIC REACTIVATION OF CRETACEOUS TERRANE ACCRETIONARY STRUCTURES IN THE EASTERN ALASKA RANGE, ALASKA

Oligocene-Present distributed oblique convergence in southern Alaska has reactivated structures inherited from Cretaceous docking of the Wrangellia terrane. Geophysical and regional geologic investigations suggest that structures juxtaposing Wrangellia and the inboard Mesozoic suture zone are key components of the diffuse Cenozoic plate boundary. Though multiple deformation events have been recognized at this terrane boundary, the timing of slip and mechanics of thrust-strike slip fault interaction during reactivation remain enigmatic.

We use geologic mapping, structural analysis, (U-Th)/He and ⁴⁰Ar/³⁹Ar cooling ages, and U/Pb detrital zircon maximum depositional ages (MDA) to unravel the polyphase slip history at the Wrangellia-suture zone boundary in the eastern Alaska Range. The Broxson Gulch fault is the northern margin of Wrangellia in the eastern Alaska Range, which formed as a S-vergent thrust reactivation of a previously high angle structure (Talkeetna fault). The hanging wall of the Broxson Gulch fault contains an inverted metamorphic belt with at least two distinct protolith ages: the Maclaren schist (≤90 Ma MDA) and Clearwater phyllite (≤155 Ma MDA). The Maclaren schist is thrust southward over the Clearwater phyllite by the Valdez Creek fault, a brittle overprint of the Cretaceous Valdez Creek ductile shear zone preserved ~60 km along strike to the SW. In the footwall of the Broxson Gulch fault, imbricate thrusts related to southward propagation of the Broxson Gulch thrust system offset Paleozoic-Triassic Wrangellia strata and Cenozoic fluvial gravels. Bedrock cooling ages show rapid Neogene erosion in the area, likely due to growth of the thrust system. Based on our new data, we favor a structural model wherein Cenozoic shortening nucleated on faults within the Alaska Range suture zone and subsequently propagated southward into Wrangellia. Our model predicts at least 20 km of post-Eocene shortening. Based on neotectonic evidence for coseismic linkage between the dextral Denali fault and splay thrusts in the eastern Alaska Range, we propose that reactivation of Cretaceous collisional structures is a long-term record of coseismic communication between the Broxson Gulch thrust system and Denali fault, which accommodate oblique convergence as part of the southern Alaska plate boundary.