Feedbacks between plate structure, deformation, and magmatism: Insights from magma migration and modification of the LAB around the Colorado Plateau, western US
Abstract
Within plate-interiors strain rates are lower than at plate boundaries, and heterogeneities within the plate may strongly control plate deformation. In this study we focus on the effects of heterogeneous plate thickness on deformation and patterns of magmatism. We show that the lithosphere-asthenosphere boundary (LAB) may undergo modification and evolution by the interplay between deformation, magma-migration and infiltration. Using the Colorado Plateau as a natural laboratory, we investigate the effect of the flow of asthenosphere below convex basal protrusions in the lithosphere. Within regions of thicker lithosphere, asthenospheric shear sets up dynamic pressure gradients that influence the motion of partial melt at the LAB. We assume a Newtonian fluid with variable, temperature-dependent viscosity structure (viscosity contrasts between the lithosphere and the asthenosphere ranging between a factor of 10 and 1.e7). Melt-migration is modeled using a simple D'Arcy flow approximation, where the separation of melt from the solid is driven by buoyancy and by dynamic pressure gradients. We find that dynamic pressures (linearly proportional to asthenosphere viscosity) within the protruding lithosphere display a pronounced asymmetry along the flow direction. Asymmetry in dynamic pressures causes an asymmetry in melt migration: melt is driven into the upwind side of the protrusion and away from the downwind side. At the Colorado Plateau, this asymmetry manifests itself in a clear difference between the SE margin (upwind side relative to the direction of uppermost mantle flow beneath North America), where we observe the highest rates of magmatic encroachment, and the NE margin (downwind side) where there is a distinct lack of magmatic encroachment. The predicted rates of magmatic encroachment and migration from our melt-migration calculations are consistent with observations from the ages and distribution of igneous rocks surrounding the Colorado Plateau. Asymmetric magma-infiltration into the base of lithospheric protrusions modifies the physical and chemical state of the lithosphere, essentially rejuvenating it and causing a migration of the LAB. Our calculations suggest that physical properties are likely to vary significantly across the evolving LAB, depending on the degree to which melt is segregated and the length-scales over which porosity varies. Moving beyond the D'Arcy approximation, we also explore the effect of coupling between flow of melt and solid deformation at the LAB, via a porosity-dependent viscosity. We find that melt-infiltration and LAB evolution is a strong function of initial porosity-gradients, with fundamentally different behavior when porosity varies at the compaction length scale vs. over longer spatial scales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T53D..06R
- Keywords:
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- 8103 TECTONOPHYSICS / Continental cratons;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8145 TECTONOPHYSICS / Physics of magma and magma bodies