Moho interface beneath Yakutat terrane, southern Alaska

Abstract

We use air gun shots recorded by ocean-bottom and land seismometers to constrain a 3-D velocity structure and a Moho interface depth for the Yakutat terrane in southern Alaska. We use a tomographic inversion of ∼61,000 first-arrival picks from 64 receivers to create a 3-D velocity model. The average velocity-depth function north of the coastline has velocities at the surface of ∼4-5 km/s, increasing to velocities of >6 km/s at ∼12-13 km below the surface. Average velocities at the surface are slower near the coast (3.8 km/s) than under the highest topography (4.7 km/s). We carry out an interface inversion of ∼19,000 PmP reflection picks from 50 receivers to constrain a 2-D Moho interface model. The average Moho depth of the Yakutat terrane in the marine portion of our study area is ∼30 km, but depth rapidly increases onshore to 40-45 km beneath the high topography of the Chugach-St. Elias Mountains. Moho depths are consistent with Airy isostasy beneath the orogen. Our data set supports a continuous Moho at the base of the Yakutat terrane, with no evidence for Pacific oceanic crust underthrusting the Yakutat terrane. We present several geologic models for crustal thickening beneath the Chugach-St. Elias orogen that are consistent with the seismic observations and surface geology and compare these models with geologic mapping in the region. We argue that crustal thickening and topography at the Chugach-St. Elias Mountains is obtained via a duplex system that produces a vertically growing antiformal stack of sedimentary and metasedimentary material. Key Points Moho depth increases by 10-15 km beneath Chugach-St. Elias orogen Several geologic models are consistent with seismic observations Favored model is sedimentary and metasedimentary antiformal stack © 2013. American Geophysical Union. All Rights Reserved.