THE GREAT ALASKA ESCAPE ROOM: NEW CONSTRAINTS ON AN OLD ESCAPE TECTONICS IDEA

Although tectonic escape is commonly evoked to accommodate the great Alaskan terrane robbery, extension and associated magmatism in southwestern Alaska, and Holocene slip budget and geodetic modeling, there are sparse geologic data to support this scenario in southern Alaska. However, given 1000’s of kms of Cretaceous to Cenozoic northward and westward Cordilleran orogenic movement, some amount of escape across proto and Aleutian-Bower trenches is required. Late Pleistocene-Holocene slip rate gradient and geodetic block modeling indicate at least 90% of recent Denali Fault slip is transferred into shortening. Seismic imaging of crustal thickness, long-term slip gradients, and near-fault exhumation provide more evidence for shortening and crustal thickening along cryptic structures that have siphoned off a portion of Denali slip since at least ca. 45 Ma. Despite long-term transfer of strike-slip slip into shortening along the eastern and central sections of the Denali Fault, the western Denali Fault has experienced ~135 km of right-lateral slip since ca. 85 Ma, providing a potential slip budget to facilitate escape.

Detrital zircon U-Pb age spectra, biostratigraphic information, and sedimentological features support a new correlation across the Denali Fault between Cenomanian-Turonian clastic strata in the Windy terrane north of the Denali Fault and the Kahiltna Old Man petrofacies in the western Alaska Range south of the fault. Dissection of these shallow marine strata implies up to ~360 km of right lateral separation since ca. 90 Ma along the central-western Denali Fault. We also link the McGonagall plutonic and correlative volcanic rocks with age-equivalent igneous rocks near the Post River. These magmatic bodies intruded the dissected Mystic terrane and have similar geochemistry, providing another piercing point along the Denali Fault. These offset magmatic bodies amount to ~180 km of western Denali Fault displacement since ca. 43 Ma. The new and existing offset markers along the Denali Fault support a projected slip gradient ratio since 52 Ma that is broadly similar to Holocene rates. Two newly identified pluton pairs in the Ahklun Mountains may provide additional constraints and allow for 70 km of escape at ~1 mm/yr, similar to geodetic predictions and in alignment with offshore Denali Fault features.