The average crustal density is 2810 kg m−3. This sharp crustal thickness gradient agrees with the presence of a crustal tectonic buttress guiding block motion west and south towards the subduction zone. In interior Alaska, the crustal thickness changes abruptly across the Denali fault, from 34–36 to the north to above 30 km to the south. Offshore, 28–30-km-thick crust is predicted near the Bearing slope break and 36–38 km in the northern Chukchi Shelf. The obtained crustal model shows northwest-directed long wavelength thickening (32–36 km), with additional localized trends of thicker crust in the Brooks Range (40 km) and in the Alaska and St Elias ranges (50 km). We also perform 3-D forward modelling and inversion of Bouguer anomalies to analyse density heterogeneities at the crustal level. This study presents for the first time an integrated image of the crust and lithospheric mantle of Alaska and its adjacent western shelves of the Chukchi and Bering seas based on joint modelling of potential field data constrained by thermal analysis and seismic data. That and other evidence suggest the presence of a wedge of ductile crust that partially decouples the subducting mantle lithosphere from the upper crust in the area near the suture with the Yakutat microplate. West of the syntaxis, faults inferred from inversion of the GPS data are above the megathrust inferred from seismic imaging. GPS velocity vectors show a large rotation across the syntaxis at Mount St. The top of the plow is near the Seward Glacier, where previous studies showed near world-record exhumation rates. The east side is inferred from geologic mapping and slip models of the 1899 Yakutat Bay earthquake sequence. The west side of the plow is the eastern limit of the Aleutian megathrust constructed from the union of constraints from STEEP seismic results and slip models of the 1979 St. We call this model the middlebuster model because the geometry is similar to a two-sided plow with that name. We constructed a 4D kinematic model of crustal deformation in the vicinity of Mount St. Kinematic reconstructions using that anchor suggest the modern Fairweather fault is likely inherited from motion of the margin in the 6-10 Ma period. We model the eastern edge of the subducting lithosphere using the southern tip of the Yakutat microplate as an anchor. The model is defined by three surfaces: (1) a top of the subducting lithosphere surface, (2) Moho surfaces, and (3) a base of subducting lithosphere surface. Elias Erosion/tectonics Project (STEEP), other studies, and seismicity data to build a comprehensive, threedimensional model of the lithosphere of the subduction corner in southern Alaska. Elias Mountains is obtained via a duplex system that produces a vertically growing antiformal stack of sedimentary and metasedimentary material. We argue that crustal thickening and topography at 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 present several geologic models for crustal thickening beneath the Chugach-St. 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. Moho depths are consistent with Airy isostasy beneath the orogen. 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. We carry out an interface inversion of ~19,000 PmP reflection picks from 50 receivers to constrain a 2-D Moho interface model. Average velocities at the surface are slower near the coast (3.8 km/s) than under the highest topography (4.7 km/s).
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. We use a tomographic inversion of ~61,000 first-arrival picks from 64 receivers to create a 3-D velocity model. 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.
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