Upper mantle seismic shear discontinuities of the Pacific

Abstract

[1] Shear wave impedance discontinuities are inventoried for eight paths connecting circum-Pacific earthquakes with seismic stations on Hawaii and Oahu. In addition to the transition zone discontinuities, we observe a consistent impedance decrease at a depth of $80 km that marks the transition from the fast seismic lid to the low-velocity zone. The interval over which this transition occurs is less than 30 km. The requisite impedance decrease, ascribed almost entirely to diminished velocities in the low-velocity zone, exceeds predictions for peridotite chemistries of appropriate lithospheric age. In all cases, the transition is better matched by lithosphere of roughly half of the true age. Four paths show clear evidence of the X discontinuity, an impedance increase of 3-8% near 300 km depth. The only viable explanations would require extensive eclogite or basalt-depleted mantle in the upper mantle of the western Pacific. The impedance contrast of the 410-km discontinuity, which depends on the modal fraction of mantle olivine, does not signal the presence of either, although a mixture of the two remains a possibility. We find no evidence of a low-velocity layer atop of the 410-km discontinuity. If present, it is either consistently thin ( 15 km), highly variable in thickness (topography in excess of 60 km) or has little impedance contrast ( 2-3%) with overlying mantle. The apparent absence of melt atop the 410-km discontinuity, an intermittent and weak 520-km discontinuity, and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central Pacific.

Thermo-chemical and thermo-physical properties of the high-pressure phase anhydrous B (Mg14Si5O24): An ab-initio all-electron investigation

“…) have discounted the orthorhombic to monoclinc transition of pyroxene as a potential candidate for the 300 km discontinuity based on seismological grounds. However, the argument of Bagley and Revenaugh (2008) against the suggestion of Ganguly and Frost (2006) is completely flawed as they misinterpreted the phase diagram and concluded that the reaction Fo + Per = Anh-B could only be accessed by a subduction zone geotherm and hence is not a viable reaction for the explanation of the X-discontinuity, while on the contrary a normal mantle adiabat crosses the reaction boundary.…”

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

“…These seismic studies determined that the X possesses a relatively low positive shear impedance contrast (3-5 %) but is seismically sharp, with the gradient constrained to be <5 km across the discontinuities (Revenaugh and Jordan 1991;Zheng et al 2007;Bagley and Revenaugh 2008). The X is primarily observed beneath the regions of subduction (Zhang and Lay 1993), continents (Wajeman 1988), and a few hot spots (Courtier et al 2007;Bagley et al 2009).…”

“…The primary focus of this chapter, the X-discontinuities, or more simply the X, is a set of intermittently observed interfaces with seismic detections at 250-330 km depth beneath the continents and oceanic regions, spanning a wide variety of seismic probes, including PP precursors (Wajeman 1988), ScS reverberations (Revenaugh and Jordan 1991;Williams and Revenaugh 2005;Courtier et al 2007;Bagley and Revenaugh 2008), receiver functions (Eagar et al 2010;Shen et al 2014), and SS precursors (Zhang and Lay 1993;Deuss and Woodhouse 2002;Schmerr et al 2013). These seismic studies determined that the X possesses a relatively low positive shear impedance contrast (3-5 %) but is seismically sharp, with the gradient constrained to be <5 km across the discontinuities (Revenaugh and Jordan 1991;Zheng et al 2007;Bagley and Revenaugh 2008).…”

Geophysical and petrological constraints on ocean plate dynamics

“…Observations from the western Pacific and the Philippine Sea, over a range of seafloor ages (~40-130Ma), further identify a sharp LAB with the suggestion that the underlying asthenosphere contains some degree of partial melt [Kawakatsu et al, 2009]. Another observed seismic feature is an abrupt drop in seismic velocity (the so-called G discontinuity) at depths of ~70-80 km independent of plate age, which is too sharp to be thermal in origin [e.g., Bagley and Revenaugh, 2008], and has been interpreted variously in terms of partial melt, decreased grain size, a change in anisotropy, and/or the presence of a hydrous asthenosphere. A self-consistent quantitative explanation for the observed LAB structure has yet to be determined, leaving the relationship between the G discontinuity and the LAB unclear.…”

“…However, such analyses have generally used data sets that span ocean basins and thus do not well constrain critical, more subtle features that are diagnostic of composition and evolutionary processes. Constraints on the depth dependence of shearvelocity structure in the oceanic lithosphere and across the LAB come primarily from regional surface-and body-wave studies [Nishimura and Forsyth, 1989;Gaherty et al, 1996;Gu et al, 2005;Tan and Helmberger, 2007], studies of mantle reflectivity from old Pacific lithosphere [Revenaugh and Jordan, 1991;Collins et al, 2002;Bagley and Revenaugh, 2008], receiver function studies [Rychert and Shearer, 2009;Kawakatsu et al, 2009], as well as from analysis of seismic phase SS precursors [Schmerr, 2012]. These studies showed that the transition from the lithosphere to the asthenosphere is complex with multiple seismic discontinuities observed within the transitional region at various oceanic plate ages.…”