Mantle dynamics beneath the Pacific Northwest and the generation of voluminous back‐arc volcanism

Abstract

[1] The Pacific Northwest (PNW) has a complex tectonic history and over the past $17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at $20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

“…This indicates that the parental basaltic magmas to this system might be changing through time, toward hotter basaltic magmas and greater degrees of partial melting of the mantle, a changing asthenospheric source composition or interaction with a more depleted mantle lithosphere [cf., Carlson and Hart, 1987]. Modeling of thermal anomalies using trench rollback and extension in the backarc also indicates that mantle temperatures should increase over time [Long et al, 2012]. In our model, slab rollback, back-arc extension, and the progressive steepening of the slab to the north results in enhanced counterflow ( Figure 7) and a plume in not a necessary heat source.…”

“…Other workers using three-dimensional strain modeling [Druken et al, 2011;Long et al, 2012] have recently shown that the downdip only motion (Figure 7) on the Cascadia slab is insufficient to create the mantle wedge flow observed under Oregon and thus a combination of trench rollback, slab steepening, and back-arc extension is required to produce the seismic anisotropy observed. Downdip motion alone only produces weak corner flow and little upwelling from the mantle.…”

“…[48] Many subduction zones are dominated by trench parallel flow (toroidal edge flow) or show heterogeneous flow vectors [e.g., Long and Silver, 2008] but the flow vectors under the HLP are nearly unidirectional (Figure 6d) [Long et al, 2012]. Other workers using three-dimensional strain modeling [Druken et al, 2011;Long et al, 2012] have recently shown that the downdip only motion (Figure 7) on the Cascadia slab is insufficient to create the mantle wedge flow observed under Oregon and thus a combination of trench rollback, slab steepening, and back-arc extension is required to produce the seismic anisotropy observed.…”

“…[49] Geodynamical models from Long et al [2012] require modest back-arc extension to draw material from depth to give the thermal modeling results. But, they do not separate trench rollback induced anisotropy from that related to slab steepening.…”

The Yellowstone “hot spot” track results from migrating basin-range extension

“…The ~ 300 x 600-km, roughly oval flood basalt does not reflect the geometry of the magma source, but the topography of the land at the time of eruption (Christiansen et al, 2002); 4. The geochemistry of the CRB corresponds to shallow adiabatic decompression melting of mantle lithosphere, and the depths and temperatures of melting correspond to ~100 km and normal mantle temperatures near the base of the crust (e.g., Long et al, 2012). Mountain and other main CRB eruptive fissures, and 400 km south of the largest eruptive centers (Camp, 2013;Pierce et al, 2002).…”