Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

Abstract

<jats:p>Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can actively control crustal deformation on a scale of several hundred kilometers.</jats:p>

Multiphase Topographic and Thermal Histories of the Wallowa and Elkhorn Mountains, Blue Mountains Province, Oregon, USA

“…The Miocene to present increase in relief in the Wallowa Mountains independent of the Elkhorn Mountains, is consistent with the hypothesis derived from upper-mantle seismic tomography, that the Wallowas were isostatically uplifted due to the development of a lithospheric instability or delamination event in the Miocene (Darold & Humphreys, 2013;Hales et al, 2005). Regional crustal anisotropy further suggests crustal deformation in Northeastern Oregon is linked to upper mantle density variation (Castellanos et al, 2020), consistent with recent lithosphere-scale deformation. Nonetheless, our data set cannot validate whether the Wallowa Mountains were uplifted due to these processes, with any mechanism resulting in local post-Miocene uplift being acceptable.…”

Deformation Microstructures of Phyllite in Gunsan, Korea, and Implications for Seismic Anisotropy in Continental Crust

“…Seismic anisotropy originating in the interior of the earth provides important information for understanding tectonic processes, deep earth structures and geodynamics [1][2][3][4][5][6][7][8]. In continental crust, strong seismic anisotropies have been observed in large-scale tectonic structures such as mountain belts in orogenic systems [9][10][11], strike-slip faults or shear zones near plate boundaries [3,4], and the overriding upper crust in subduction zones [12,13].…”

Evidence of Mantle‐Based Deformation Across the Western United States

“…In the PNW and Rocky Mountains, crustal azimuthal anisotropy is roughly perpendicular to central seismically fast upper mantle structures (Castellanos et al., 2020, Figure 3a). The anisotropy is attributed to crustal flow that is approximately perpendicular to the location of the seismically fast mantle.…”

“…However, if loading occurs more rapidly than crustal flow can supply the compensating crust, a surface deflection will occur, such as the 80 Ma creation of basins in the mechanism. With continued crustal flow and Moho adjustment, isostasy may be ultimately maintained without a surface expression and could be topographically invisible (Crosswhite & Humphreys, 2003; Castellanos et al., 2020). This mechanism differs from tectonically driven changes in crustal thickness, in which crustal thickness variations are associated with (and compensated by) topographic variations.…”