Magmatism & lithospheric destruction along the Colorado Plateau margin
Abstract
The process of cratonic lithosphere deformation remains mysterious. The Colorado Plateau (CP), including its underlying lithosphere, has persisted for over a 1Ga, while in parts of the adjacent Basin and Range (B&R) Cenozoic extension has thinned the lithosphere by half. Today, extensional processes are focused in the transition zone between these two regions, which is defined by a region of volcanism and active faulting. We combine seismic tomography models from EarthScope data with melt thermobarometry from mafic scoria erupted in three volcanic fields since 100ka to investigate lithospheric deformation in this actively rifting area. Our sample locations lie along the western and southwestern margin of the CP and record different evolutionary stages in the process of lithosphere destruction via melt infiltration. For each volcanic area studied we use seismic profiles of shear wave velocities (Vs) with pressures and temperatures of mantle-melt equilibrium calculated using the Si and Mg thermobarometer (Lee et al 2009). The thermobarometric results depend highly on the water and Fe3+ content of the melts, which were constrained separately for each volcanic field. Magmatic water contents have been determined by ion-microprobe measurements of olivine hosted melt inclusions. Fe/ΣFe+3 ratios were estimated for each volcanic field via LA-ICP-MS analysis of V in olivine and whole rock compositions (Canil 2002). In the northernmost volcanic field, Black Rock (BRVF), Utah, melts are hot (consistent with mantle potential temperature (Tp) >1400°C), dry (≤1 wt% H2O), and have equilibrated at shallow depths (<70 km), within the seismic lid. Shear velocities in this lid, however, are anomalously slow (4.1 km/s), and the mantle beneath (Vs ~ 3.9 km/s), is the slowest in the B&R, coincident with the highest and most focused extension rates (Wasatch Fault Zone). Together, these observations support high mantle temperatures, inefficient melt extraction, and a weak lithosphere due to melt corrosion, which has focused strain. Further south, in Snow Canyon Volcanic Field, Vs in the low-velocity zone and seismic lid are faster than at BRVF and magmas record equilibration depths that coincide with the seismically defined lithosphere-asthenosphere boundary (~70km). In this location both observations are consistent with a stronger lithosphere, which may allow rapid melt ascent by diking from the LAB to the surface, and limited melt-infiltration. Lavas from the most southern volcanic field, the Grand Canyon, record the deepest and largest range of melt equilibration depths (49-139km), the highest water contents (up to 3wt%) and have the most variable geochemistry (Nb/La ranging from asthenospheric to lithospheric values). Several vents contain abundant mantle xenoliths. Mantle Vs here is consistently higher (>4.1 km/s) than all the regions to the north. Here, lower mantle Tp, a stronger lithosphere, and wetter melts appears to enable more efficient melt extraction from all depths. Our results indicate that the destruction of the CP lithosphere occurs via a spectrum of processes along its margin, ranging from diking in the south to extensive melt infiltration and corrosion in the north. The spatial distribution of mantle heat and water content may exert a primary influence on the mode of lithosphere destruction, surface strain and volcanism.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T43C2688B
- Keywords:
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- 1000 GEOCHEMISTRY;
- 8100 TECTONOPHYSICS;
- 8400 VOLCANOLOGY