Lithospheric discontinuity structure in Alaska, thickness variations determined bySpreceiver functions

Abstract

We present the first broad-scale image of lithospheric thickness across the major tectonic domains of Alaska based on S wave receiver functions and joint interpretation with the potential field, seismic velocity, and heat flow measurements. Thus, we provide context for the distribution of strain throughout the Alaskan orocline. In the north, below the Brooks Range, a 130 km thick lithosphere is resolved, consistent with the presence of strong lithosphere that deflects strain to the south into central and southern Alaska. In southern Alaska beneath the Chugach and St. Elias Mountains, multiple interfaces are present, and we interpret a thinner (80-90 km) North American lithosphere above a deeper interface that represents the base of the Yakutat microplate, thereby extending it to the area below the Wrangell Volcanic Field and St. Elias Mountains. Immediately north of the E-W striking Denali Fault, shallow negative conversions (80 km) denote thin lithosphere in the greater back-arc region where heat flow is observed to be high. Thin lithosphere in eastern Alaska and adjacent Yukon Territory coincides with the occurrence of inboard crustal seismicity and may be indicative of transmitted compression caused by the collision of the Yakutat microplate. Relatively thin lithosphere (<90 km) south of the Arctic Alaska domain that is deforming throughout the Alaskan orocline may result from lithospheric thinning associated with guided deformation. Expansion of this model using the upcoming Transportable Array will be critical to establish lateral continuity (or lack thereof) of lithospheric structure and directly discriminate between existing regional deformation models.

Lithospheric Structure Across the Alaskan Cordillera From the Joint Inversion of Surface Waves and Receiver Functions

“…From south to north, our tomographic images show crustal velocities (<4.2 km/s) at deeper depths under the Wrangellia Composite Terrane south of the Denali fault that rapidly shallow under interior Alaska, gradually deepen under the Brooks Range before shallowing again under the North Slope (Figures c, c, and ). This crustal thickness pattern has been observed previously in 2‐D cross sections (e.g., Fuis et al, ) and a series of 1‐D crustal thickness estimates (e.g., Miller et al, ; O'Driscoll & Miller, ). Although we do not invert for a specific Moho model parameter, the addition of the receiver function data in the inversion constrains discontinuities better than surface wave inversions alone (Ward et al, , supporting information).…”

Seismic Imaging of the Alaska Subduction Zone: Implications for Slab Geometry and Volcanism

“…Figures and S19 show cross sections through the velocity structure of the DVG (Figure a) and the Aleutian island arc near Mount Spurr (Figure b). These indicate the slab and continental LAB locations, which are inferred from velocity discontinuities and agree with LAB picks from O'Driscoll and Miller () where the models overlap (Figure S21).…”

“…The receiver function studies of O'Driscoll and Miller () and Bauer et al () have attempted to constrain the subduction geometry beneath Alaska by imaging velocity discontinuity structures. The S ‐ P receiver function model of O'Driscoll and Miller () does not resolve moderate‐to‐steeply dipping structures but reveals a flat Yakutat LAB at ~100‐km depth beneath south central Alaska and hints at the existence of a shallow slab beneath the WVF. The plane wave migration technique employed by Bauer et al () imaged dipping features and Yakutat crust ~80 km below the WVF; the extent of the slab north of the volcanoes was unconstrained, however.…”

“…Much of the Alaskan crust comprises rocks associated with the northern Cordilleran Orogen, which represents westward growth of the North American continent by accretion of terranes to the margin of Laurentia since the Late Triassic (Nelson & Colpron, ; Plafker & Berg, ). This process continues today with the convergence and partial subduction of the Yakutat terrane with the southern margin of Alaska, at the northeastern corner of the Pacific plate (e.g., Eberhart‐Phillips et al, ; O'Driscoll & Miller, ; Plafker & Berg, ). This unique geometry chronicles a transition from subduction of the Pacific plate beneath North America in the west into right‐lateral strike‐slip motion in the east.…”

Aseismic Deep Slab and Mantle Flow Beneath Alaska: Insight From Anisotropic Tomography

“…In addition, significant high‐ Vp anomalies are imaged under south central Alaska where intense mountain building is taking place due to the shallow subduction of the Yakutat slab (Figures a, d, and e; Eberhart‐Phillips et al, ; Ferris et al, ; Haynie & Jadamec, ; Jadamec et al, ; Koons et al, ; Mazzotti & Hyndman, ). In addition, a high‐ Vp anomaly is revealed beneath the western Brooks Range and Nulato Hills west of 150°W (Figures d–i), perhaps relevant to a thick lithosphere there (O'Driscoll & Miller, ).…”