Paper No. 90-4
Presentation Time: 8:50 AM
MEGATHRUST SPLAY FAULTS IN PRINCE WILLIAM SOUND, ALASKA: INTERPLAY BETWEEN UNDERPLATING, EXHUMATION, EXTENSION, AND RHEOLOGY
Megathrust splay faults were first identified by George Plafker in Alaska after the 1964 earthquake. They have since been recognized as a common feature of accretionary prisms and of being capable of generating tsunamis during earthquakes. Here we provide new evidence from Alaska that Plafker’s megathrust splay faults have been conduits of focused exhumation for the last 5 Ma. In most of central Prince William Sound, published and new low-T thermochronology data indicate little to no permanent rock uplift (<0.1-0.3 mm/yr) over tens of thousands of earthquake cycles. However, in southern Prince William Sound on Montague Island, apatite-He ages are as young as 1.1 Ma indicating focused and rapid rock uplift with rates as high as 0.7 mm/yr. Montague Island lies in the hanging wall of the Patton Bay megathrust splay fault system, which ruptured during the 1964 M9.2 earthquake. Our new studies show rapid exhumation in the last 2-3 Ma in a pattern similar to the uplift in the 1964 earthquake. The region of older exhumation correlates with rocks that are older (~55-65 Ma) and metamorphosed and constitute a mechanically strong backstop. The region of rapid exhumation consists of much younger (<35 Ma), colder rocks, which we infer are mechanically weak. The regions of rapid and slow exhumation, and younger and older rocks, are separated by the newly identified Montague Strait fault. New high-resolution bathymetry, seismic reflection profiles, and a M4.8 2012 earthquake show this is a 75-km-long normal fault. There are numerous smaller active normal(?) faults in the region between the Montague Strait fault and the splay faults. This hanging wall extension developed between the rapidly uplifting sliver of younger and weaker rocks on Montague Island from the essentially fixed region to the north. Deep seismic reflection profiles show the splay faults root into the subduction megathrust where there is probable underplating. Thus the exhumation and extension are driven by underplating along the megathrust décollement, and results in thickening the overriding plate where there is a change in rheology at the Montague Strait fault. A comparison with other megathrust splay faults around the world shows they have significant variability in their characteristics, and the conditions for their formation are not particularly unique.