ACTIVE TECTONICS OF THE SUSITNA RIVER BASIN, ALASKA - INTRAPLATE DEFORMATION DRIVEN BY MICROPLATE COLLISION AND SUBDUCTION

The Susitna River (SR) basin a) lies above a transition from subduction of the Pacific plate to accretion and partial subduction of the Yakutat microplate, b) forms the diffuse structural W boundary of the S Alaska tectonic block, and c) lies between the Alaska Range and the Cook Inlet forearc basin. As such it is a critical area for understanding tectonics of intraplate deformation and Alaska's seismic hazards. Extensive glaciation requires careful searching to unmask geomorphic features associated with active deformation. We document three active structures. 1)The W Castle Mountain fault (CMF) creates a vegetation lineament across the basin along a scarp created by small amounts of local uplift. Slip is transpressional with a previously unknown lateral slip rate and direction. We located and measured an offset channel to establish right lateral slip of ~3 mm/yr. We propose a characteristic earthquake of ~7.1 Mw and rupture length ~62 km. The E segment of the CMF has no Holocene scarp, and two historical earthquakes <5.7 Mw. Does this change in seismicity reflect the underlying transition from accretion to subduction? 2)The Pass Creek fault is delineated by a 6.5 m scarp that disrupts drainage patterns and creates a vegetation lineament across the NW margin of the SR basin. The fault is normal, down to the N with a possible small component of left-lateral slip. It cuts a glacial moraine with a C14 age of 1340±60 yr BP; the last significant rupture had >2 m of uplift. The fault lies above a rapid increase in the subduction angle of the Yakutat microplate. Does bending of the plate transfer normal stress into the overlying continental crust? 3)The Beluga Mtn. anticline is located on the W margin of the SR basin. As the Skwentna River crosses the anticline, sinuosity increases, and width and braiding decrease – indicating active uplift. An escarpment on the E flank of the anticline may be a thrust fault scarp; verification requires field checking. The anticline is one of several anticlines on the W margin of the basin that may be cored by active thrusts. These anticlines lie NE of the last volcano of the Aleutian arc and within the transition from terrane accretion to classic subduction. Are the anticlines linked to accretion, to subduction, or to uplift of the Alaska Range? Mechanical and chronologic models are being used to explore answers to the posed questions.