Thursday, April 16, 2015 AGS Luncheon
"Focused Exhumation Along Megathrust Splay Faults in Prince William Sound, Alaska"
Noon Luncheon 11:30-1:00 pm
Megathrust splay faults are a common feature of accretionary prisms and can be important for generating tsunamis during some subduction zone earthquakes. Here we provide new evidence from Alaska that megathrust splay faults have been conduits for focused exhumation in the last 5 Ma. In most of central Prince William Sound, published and new low-temperature thermochronology data indicate little to no permanent rock uplift over tens of thousands of earthquake cycles. However, in southern Prince William Sound on Montague Island, apatite (U-Th)/He ages are as young as 1.1 Ma indicating focused and rapid rock uplift. Montague Island lies in the hanging wall of the Patton Bay megathrust splay fault system, which ruptured during the 1964 M9.2 earthquake and produced ~9 m of vertical uplift. Recent geochronology and thermochronology studies show rapid exhumation within the last 5 Ma in a pattern similar to the coseismic uplift in the 1964 earthquake, demonstrating that splay fault slip is a long term (3-5 my) phenomena. The region of slower exhumation correlates with rocks that are older and metamorphosed and constitute a mechanically strong backstop. The region of rapid exhumation consists of much younger and weakly metamorphosed rocks, which we infer are mechanically weak. The region of rapid exhumation is separated from the region of slow exhumation by the newly identified Montague Strait Fault. New sparker high-resolution bathymetry, seismic reflection profiles, and a 2012 Mw4.8 earthquake show this feature as a 75-km-long high-angle active normal fault. There are numerous smaller active normal(?) faults in the region between the Montague Strait Fault and the splay faults. We interpret this hanging wall extension as developing 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 in the hanging wall are likely driven by underplating along the megathrust décollement, thickening in the overriding plate and a change in rheology at the Montague Strait Fault to form a structural backstop. 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.
Peter J. Haeussler
Peter J. Haeussler is a Research Geologist at the U.S. Geological Survey in Anchorage, Alaska, where he has lived and worked for the last 24 years. His current research focuses on understanding active tectonic processes in southern Alaska, with studies on the frequency of earthquakes, the location and rate-of-movement of active faults, and mountain building. Other research efforts relate to submarine-landslides and their role in tsunami generation, as well as framework geology for energy and mineral resource assessments. He is author or co-author on more than 100 scientific publications. He completed his B.S. in Geology from Michigan State University, and his Ph.D at the University of California Santa Cruz. Haeussler is the Alaska Coordinator for the USGS’ Earthquake Hazards Program. He has also served on the Muncipality of Anchorage’s Geotechnical Advisory Commission, the State of Alaska’s Geologic Mapping Advisory Board, and is frequently asked to serve on NSF and other agency panels and committees related to active tectonics in Alaska.