A GEOPHYSICAL TRANSECT ACROSS THE ALASKA RANGE: RELATIONSHIP BETWEEN CRUSTAL STRUCTURE AND THE NOVEMBER, 2002, M=7.9 DENALI EARTHQUAKE

A suite of geophysical information, including gravity and magnetic, magnetotelluric, and deep-crustal seismic-reflection data as well as outcrop geology and earthquake seismology are used to investigate the crustal structure of the Alaska Range and the November 3, 2002, M-7.9 earthquake that ruptured the right-slip Denali Fault in south-central Alaska. This fault developed during the Cenozoic along a Cretaceous suture zone that separates mainly Paleozoic continental rocks to the north from Mesozoic oceanic rocks to the south. The oceanic rocks include a large Mesozoic flysch basin that is truncated along the Denali Fault. A rock body having low electrical resistivity underlies the surface trace of the Denali Fault at depths greater than about 10 km. Seismicity from 1975 to 2002 nucleated above the middle of the low-resistivity body. Although aftershocks of the Denali earthquake had a similar distribution, they also tended to cluster along the boundaries of the low-resistivity body. The low electrical resistivity might result from electrical conduction by carbon films developed in the Mesozoic flysch deposits truncated along the strike-slip Denali Fault. However, an alternative we prefer is that the interpretation that the low resistivity results from deep fluids, as suggested by the occurrence of the Denali bright spot, a strong and flat seismic reflection. Deep fluids might have weakened the middle and lower crust and helped localize seismicity. Seismically reflective features within the Alaska Range orogen are aseismic under today’s stress regime, and faults that ruptured during the Denali earthquake are non-reflective, most likely because the faults dip steeply. An antiform in the schistocity of metamorphic rocks at the surface is underlain by features that return a sigmoidal reflection band that flattens downward into the lower crust. The coincident location between antiform and sigmoidal reflector suggests that the deep crust may include a crustal-scale duplex structure that likely dates from the mid-Cretaceous collision between terranes separated by the Denali Fault.