North 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.

The thermal structure of orogenies is subject to wide debate and remains largely uncertain, especially in the lower crust. Eclogite created due to the subduction of the cold Indian crust under Tibet has been widely accepted as the cause of the ‘Moho doublet’, a pair of positive lower crustal wave speed increases in the Tibetan lower crust. Previous studies have used the northward extent of the Moho doublet as a structural marker for the northern extent of the Indian plate. Here, we offer an alternative interpretation: as the Indian eclogite further subducts, the cold eclogitized crust warms into granulite facies and the northern extent of the Moho doublet records the eclogite-granulite transition. This interpretation also implies that the northward extent of the Moho doublet map acts as a seismic thermometer. We calibrate this thermometer using thermodynamic modeling of two endmember average lower crustal compositions. Sensitivity tests to show receiver functions can observe an eclogite doublet >=10 km in thickness in the lower crust. We map the northern extent of the Moho doublet by compiling and calculating receiver functions for all available seismic stations and compare our results with other thermal indicators for the Tibetan lower crust. The presence of a Moho doublet >200 km north of the Bangong–Nujiang suture suggests that Indian crust has subducted further than previously hypothesized. Along-strike variation in the extent of regional cold lower crustal temperatures (<800±100°C at 65–70 km) indicate significant along-strike thermal and tectonic variability.