Seismic evidence for orthopyroxene enrichment in the continental lithosphere

Abstract

We assess the ability of predicted seismic velocities (Vp, compressional wave; Vs, shear wave; and Vp/Vs) to identify regions in the upper mantle that are enriched in orthopyroxene relative to normal melt-depleted peridotite compositions. Orthopyroxene enrichment has been found in mantle xenoliths from a number of locations, including the Colorado Plateau and the Kaapvaal craton. We fi nd that the Vp/Vs ratio is very sensitive to orthopyroxene concentration, but it is not sensitive to depletion level. We compare these predicted velocities and Vp/Vs ratios to the high Vs, low Vp/Vs anomaly found above the central Chile-Argentina fl at slab. Within error, the predicted velocities of some of the orthopyroxene-enriched xenoliths match the observed velocities from the Chile-Argentina upper mantle. Because the anomaly above the central Chile-Argentina fl at slab does not conform to known terrane boundaries but does align with the downgoing Juan Fernandez ridge track, we suggest that the orthopyroxene enrichment in this area may be related to fl at slab processes, which would have implications for our understanding of lithospheric silica enrichment in the western U.S. and elsewhere.

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

“…The Vp/Vs ratio ranges 1.7 to 1.77, with an average of 1.73. This value is higher than the unusually low Vp/Vs ratio (1.65) reported by Hacker and Abers (2012), but it is well matched with the result (1.71 to 1.76) of that reported by Wagner et al (2008), which was calculated using the compositions of opxenriched xenoliths from both southern Africa and the Colorado Plateau. Elastic stiffness (Cij) of Vogit-Reuss-Hill average for olivine, orthopyroxene, and clinopyroxene was calculated using the LPO and density of mineral.…”

A data-adaptive, multiscale approach of finite-frequency, traveltime tomography with special reference toPandSwave data from central Tibet

“…This result is consistent with the isopycnic hypothesis [e.g., Jordan, 1979Jordan, , 1988 which predicts that extraction of basalt in the Archean would have left the mantle lithosphere to be most depleted near its top. However, some recent studies questioned if the relationship proposed by Lee [2003], an extrapolation based on data measured at standard temperature and pressure, may be too strong [Faul and Jackson, 2005;Wagner et al, 2008]. So a precise, quantitative relationship awaits more measurements under high pressures and temperatures.…”

Subduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite

“…Recent improvements in the ability to resolve seismic velocities in subduction zones have led to the discovery of anomalously low V P /V S (equivalent to low Poisson's ratio, and used here interchangeably) domains within several subduction‐zone mantle wedges (Figure 1 and pink boxes in Figure 2). Specifically, the V P /V S ratio for a subduction mantle wedge is typically 1.76–1.82 [ Zhang et al , 2004; Syracuse et al , 2008; Tsuji et al , 2008], and yet V P /V S as low as 1.65–1.72 has been reported for depths of 70–100 km above the flat‐slab portion of the Nazca plate at 31°S [ Wagner et al , 2005, 2006, 2008], and V P /V S as low as 1.65 ± 0.03 [ Rossi et al , 2006] or 1.68–1.72 [ Eberhart‐Phillips et al , 2006] has been reported for depths of 60–90 km above the eastern end of the Alaskan slab. Similarly low V P /V S has been suggested for the Kurile back‐arc, using a method with less spatial resolution [ Zheng and Lay , 2006].…”