Moho interface beneath Yakutat terrane, southern Alaska

Abstract

[1] 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.

Extending Alaska’s plate boundary: Tectonic tremor generated by Yakutat subduction

“…More recent active source data do not show the aforementioned layering, and this has been interpreted as evidence for west-to-east crustal thickening from ~15 to ~30 km (Worthington et al, 2012). The tomographic and interface inversion shows a continuous Moho below, with no evidence for large-scale underthrusting of Pacific oceanic crust (Christeson et al, 2013), though these results are well updip of the tremor region. Receiver function analysis also corroborates the presence of thickened oceanic crust (Kim et al, 2014).…”

“…A second option is that the Yakutat terrane simply represents a thick oceanic plateau (Worthington et al, 2012;Christeson et al, 2013) extending the Pacific plate. More recent active source data do not show the aforementioned layering, and this has been interpreted as evidence for west-to-east crustal thickening from ~15 to ~30 km (Worthington et al, 2012).…”

Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography

“…They identified an abrupt deepening step in the Moho from offshore to onshore, with Moho depths as great as 46 km. Future surface wave tomographic inversions should use an updated Moho surface with the latest published data, such as Christeson et al [].…”

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…Modern seismic imaging below south-central Alaska show the presence of thickened (11-22 km thick) oceanic crust as far north as the central Alaska Range (Ferris et al, 2003;EberhartPhillips et al, 2006;Gulick et al, 2007;Christeson et al, 2010;Worthington et al, 2012) and the presence of a subducting slab below the modern WA (Eberhart-Phillips et al, 2006;Bauer et al, 2014). These data indicate that subduction of the leading edge of the Yakutat microplate began was active at ~35-30 Ma, subduction of a slab steep enough to cause arc magmatism has been active since ~30 Ma in the WA, and continental collision of the thicker parts of the Yakutat microplate began by ~15-12 Ma, (Christeson et al, 2010;Worthington et al, 2012;Falkowski et al, 2014Falkowski et al, , 2016.…”