Seismic and physical properties of upper mantle xenoliths from the southwestern United States
Abstract
Petrofabrics, compressional and shear-wave properties of upper mantle xenoliths from southwestern US were investigated to achieve several goals: (1) to obtain in situ upper mantle seismic properties; (2) to provide control in calibrating the interpretation of seismic and teleseismic data; and (3) to study upper mantle anisotropy variations with respect to tectonic settings. Results of this study show that upper mantle seismic velocity and anisotropy are related to tectonics. Xenoliths from tectonically active regions exhibit strong rock fabrics and highly anisotropic, whereas those from stable region exhibit weak fabrics and less anisotropic. Comparisons between tectonics also indicate that at shallower depths (less than 50 km) the mode of upper mantle deformation varies, often greatly, between tectonic settings, but is more uniform at greater depths. Other features investigated in this study are xenolith physical features. Many upper mantle xenoliths are bounded by multiple facets or joints with planar surfaces, which indicate the presence of brittle fracturing in the upper mantle. This phenomena was originally thought to be the product of upper mantle deformation related to regional tectonics. Measurement of joint preferred orientations and comparisons with regional stress field show that these facets have little correlation with tectonics. The most probable mechanism for the formation of these facets is jointing facilitated by hydraulic fracturing. Partial melting in the upper mantle can produce highly pressurized magma fluids, which reduce the effects of lithostatic stress. This will weaken the overall rock strength to enable the initiation of new joints and the propagation of the existing ones.
Degree
Ph.D.
Advisors
Christensen, Purdue University.
Subject Area
Geophysics|Geology
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