Melt-Enabled Lithospheric Delamination in the Western US

Abstract:

Signs of convective downwelling in non-subduction environments have been increasingly prevalent in recent studies. From seismic images of the “Isabella Anomaly” beneath the southern Sierra Nevada, to studies on the origin of the Columbia River flood basalts, to long standing speculation on the state of the lithosphere beneath the Colorado Plateau, the lithospheric delamination hypothesis potentially accounts for a substantial amount of otherwise anomalous topographic uplift and magmatism in western North America and elsewhere.

Efforts to model numerically the underlying processes behind such events have also been gaining popularity. Typically, such models have relied on drastically weakened lithosphere or highly non-linear rheologies at the boundaries of dense regions to allow the propagation of delamination in reasonable timescales. However, these low viscosities are rarely justified by a systematic investigation of mantle rheology.

Based on suspected cases of delamination, the process seems to be commonly associated with localized magmatism. For instance, the Columbia River flood basalts, the renewed magmatism in the southern Sierras, and the radially converging magmatism propagating though the Colorado Plateau, all match well temporally with suspected delamination cases. Geochemical signatures are consistent with delamination in all of these cases.

Because of the spatial and temporal correlations between delamination and magmatism, we are interested in investigating the rheological role that melt might play in the initiation and propagation of delamination. Moderate melt fraction in the asthenosphere is a potential source for the low viscosity necessary to appropriately predict timescales of these events. To that end, we are working on tracking melt generation and migration though geodynamic models using the Eulerian finite element code, ASPECT. Preliminary results predict melt being generated during delamination events but the magnitudes and rheological implications remain unclear.