THERMOCHRONOLOGIC CONSTRAINTS ON MEGATHRUST SPLAY FAULTING IN THE TRANSITION FROM STRIKE-SLIP TO CONVERGENCE IN THE SOUTHERN PRINCE WILLIAM SOUND, ALASKA

Deformation related to the transition from strike-slip to convergent slip during flat-slab subduction of the Yakutat microplate has resulted in regions of focused rock uplift and exhumation. In the St. Elias and Chugach Mountains, faulting related to transpressional processes and bending of fault systems causes rapid exhumation. Underplating below the syntaxial bend farther west in the Chugach Mountains and northern Prince William Sound (PWS) causes focused, but less rapid, exhumation. Farther south in the PWS, plate boundary deformation transitions from strike-slip to nearly full convergence in the Montague Island (MI) and Hinchinbrook Island (HI) region, which is ~20 km above the megathrust between the Yakutat microplate and overriding North American Plate. Montague and Hinchinbrook Islands are narrow, elongate, and steep, with a structural grain formed by several megathrust fault splays, some of which slipped during the 1964 M9.2 earthquake.

We present 32 new apatite (U-Th)/He and 25 new apatite fission-track ages from the MI and HI region. Most AHe ages are <5 Ma, with some as young as 1.0 Ma. AHe ages are youngest at the SW end of MI, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults during the 1964 earthquake. AFT ages range from ~5 Ma to ~20 Ma and are also younger at the SW end of MI. Exhumation rates based on AHe ages, computed assuming a constant geothermal gradient of 20 deg C/km, are as high as ~4 mm/yr at the SW end of MI and decrease to the NE. Similarly, exhumation rates from AFT ages are as high as 1.3 mm/yr to the SW and decrease to the NE. These ages and exhumation rates indicate that the MI and HI region is a narrow zone of intense deformation probably related to duplex thrusting along one or more megathrust fault splays. We interpret the rates of rock uplift and exhumation to have increased in the last ~5 My, especially at the SW end of the island system and farthest from the region dominated by strike-slip and transpressional deformation to the NE. The narrow band of deformation along these islands may be the westward extension of the St. Elias – Bagley fault systems and may represent the northwestern edge of a broader swath of plate boundary deformation between the MI/HI region and the Kayak Island fault zone.