Seismicity and Anisotropic Imaging Reveal an Active Detachment Beneath the Northern Alaska Range Foothills
AbstractNorth of the Denali Fault, the collision between the Yakutat block with North America is accommodated by a fold-thrust belt that gives rise to the northern Alaska Range foothills. At the western end of the belt, the Kantishna Hills anticline hosts prominent microseismicity and surface deformation, together interpreted as active folding of the Kantishna Hills anticline above a midcrustal detachment. Here, we test for such a detachment by using anisotropy-aware receiver functions to image fabric contrasts within the crust and comparing the depths of such contrasts to seismicity statistics. Seismic stations near the crest of the Kantishna Hills anticline and near its southern flank show a single strong contrast in dipping fabric at depths of 12 and 13 km, near where the microseismicity clusters at depth and consistent with a detachment plane beneath the fold. A minimum in b-value at 10-13 km depth is consistent with seismicity on the detachment, compatible with the imaged anisotropic contrast, while off-fault seismicity is shallower, deeper, and limited to smaller magnitudes. South-dipping imbricate thrusts in schist characterize the northern Alaska Range foothills structure and support our interpretation of the observed anisotropy as reflecting SSW-SSE-dipping foliation above a detachment at ~10-13 km depth that may exploit existing crustal weaknesses along more subtle fabric contrasts observed in the seismically quiescent region north of the actively deforming belt.