THERMOCHRONOLOGIC INSIGHTS INTO SLIP PARTITIONING BETWEEN THE EASTERN DENALI AND TOTSCHUNDA FAULTS OF SOUTHERN ALASKA IN RESPONSE TO LATE MIOCENE PACIFIC PLATE MOTION CHANGE

The Totschunda and Eastern Denali Faults of southern Alaska provide long-lived (Cretaceous to present) weaknesses to directly investigate how changes in plate boundary processes effect slip distribution between pre-existing fault strands. We constrain exhumation along the southern and northern extremes of the Totschunda Fault using new geochronology (zircon U-Pb) and thermochronology (zircon and apatite (U-Th)/He and apatite fission track) data. Our Totschunda Fault data indicate an increase in rapid exhumation at ~ 6 Ma that continues until the present and we infer strike-slip motion increased from ~2 mm/yr to the modern rate of ~14 mm/yr at this time. The Pacific Plate convergence angle changed ~18°N at ca. 6 Ma resulting in an incoming vector aligned with a more favorably aligned Totschunda Fault. This relative plate motion change and the progressive alignment of the Totschunda Fault with the Fairweather Transform as the fault was translated along the Denali Fault led to rejuvenation of the Totschunda Fault and general abandonment of strike-slip motion on the Eastern Denali Fault. We corroborate this interpretation with 15 apatite U-Th/He ages from the Totschunda – Denali Fault intersection that document an abrupt shift of exhumation from the Eastern Denali Fault to the Totschunda fault around roughly 6 Ma. U-Pb ages and thermochronology data were also determined from detrital sub-volcanic and volcanic cobbles collected from Miocene fault-bounded sedimentary basin adjacent to the Totschunda Fault. These data support our Late Miocene thermochronologic exhumation inference along the Totschunda fault. We conclude that the Late Miocene Pacific plate motion change caused fault system evolution and reorganization towards mechanical efficiency around the circum-Pacific margin. Furthermore, we suggest strike-slip fault intersections once formed can persist for 10’s of millions of years, while slip distribution between the principal splays varies with time.