ON THE PROVENANCE OF SEWARD-MALASPINA GLACIER COBBLES AND THE EXTENT OF RAPID, DEEP EXHUMATION AT THE YAKUTAT PLATE CORNER, SE ALASKA

Our study investigates the spatial development of rapid and deep exhumation at the Yakutat plate corner in the St. Elias Mountains, southeast Alaska and southwest Yukon. There, the Yakutat Terrane collides obliquely with the North American Plate to form the Chugach-St. Elias orogen. We focus on the plate corner region, where tectonics transition from transpressional to convergence style. This results in complex structures and the localization of strain. For the past ~5 myr mountain building processes coevolved with glacial erosion. The extensive glaciation made the orogen a prime example to study climate-tectonics interactions but it also presents a challenge to rock sampling. Opposed to sampling bedrock from high, ice-free elevations, a detrital sampling approach recently revealed an area of rapid and deep exhumation in the wider plate corner region around the northern tip of the transfrom plate boundary.

We present new zircon U/Pb data from 22 cobbles sampled at the lobe of the Malaspina piedmont glacier, which drains the high Seward Ice Field. A previous study reported zircon (U-Th)/He and petrographic data on >50 cobbles from the Malaspina Glacier outwash to trace cobbles that yielded young (<5 Ma) cooling ages back to their origin within the Seward-Malaspina Glacier catchment. However, lithology is not a sufficient provenance tool and, therefore, we complement the study by using U/Pb dating of cobbles with known zircon (U-Th)/He ages and lithology.

The catchment comprises 4 different terranes (Wrangellia-Alexander Terrane, Chugach Terrane, Prince William Terrane, and Yakutat Terrane) that have characteristic U/Pb crystallization ages. The cobbles yield ages between 30 and 277 Ma and support the conclusion that a larger area of the catchment is affected by recent exhumation.

Together with zircon fission-track ages from the northernmost tip of the Fairweather Fault to the east of the Seward-Malaspina Glacier catchment, we can constrain the extent of the area of rapid and deep exhumation and deduce possible structures to accommodate the exhumation. Furthermore, we demonstrate how valuable the detrital sampling approach is and that in combination with lithology and U/Pb dating the provenance within the catchment can be determined.