Broadband array observations of the 300 km seismic discontinuity

Schmerr, N. C.; Kelly, Byron Matthew; Thorne, M. S.

doi:10.1002/grl.50257

Cited by 41 publications

(59 citation statements)

References 37 publications

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“…The locations and elastic contrasts of precursor observations beneath the Pacific of the X-discontinuity can be found in Fig. 3.6 and are compared to earlier results (Deuss and Woodhouse 2002;Schmerr et al 2013). …”

Section: Detections Of the X-discontinuitymentioning

confidence: 95%

“…3.6 Detections of the X-discontinuity by the SS precursors. The symbols are scaled to the size of the shear impedance contrast at the interface; (Deuss and Woodhouse 2002) (crosses), (Schmerr et al 2013) (circles) and this work (triangles). The locations of hot spots (gold stars) (Courtillot et al 2003), plate boundaries (red lines) (Bird 2003), and continents indicate tectonic and volcanic features.…”

Section: % 2% 4% 3%mentioning

confidence: 99%

“…SS precursor studies have established the 410-and 660-km discontinuities as global features, though using relatively long-period energy (periods > 25 s), providing maps with lateral resolution on the order of several thousands of kilometers (e.g., Flanagan and Shearer 1998b;Gu et al 1998). The precursors have also been used to study the X-discontinuity (Deuss and Woodhouse 2002;Deuss 2009;Schmerr et al 2013), although the X precursory arrival is much lower amplitude than the precursors from the 410-and 660-km discontinuities.…”

Section: Seismic Imaging Of Mantle Discontinuitiesmentioning

confidence: 99%

“…The impedance contrast is estimated at the observed discontinuity by measuring the relative amplitude of the stacked SS to the precursory stack and corrected for geometric spreading, attenuation, stack defocusing, and other systematic effects using a synthetic seismograms generated for PREM and stacked alongside the dataset. The resulting measurements are translated into discontinuity depth and impedance contrast as well as the associated uncertainties provided by the bootstrap resampling; prior modeling efforts of 10-15 s period waves by Schmerr et al (2013) indicated that the sharpness of the discontinuity must have a thickness <20 km to resolve the precursor from discontinuities near 300 km depth. The locations and elastic contrasts of precursor observations beneath the Pacific of the X-discontinuity can be found in Fig.…”

Section: Detections Of the X-discontinuitymentioning

confidence: 99%

“…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).…”

Section: Introductionmentioning

confidence: 99%

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Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Schmerr

2015

The Earth's Heterogeneous Mantle

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We use underside reflections of S wave seismic energy arriving as precursors to the seismic phase SS to image the depth and impedance contrast present across mantle discontinuities in the depth range of 230-380 km beneath the Pacific basin. A number of past studies have identified seismic discontinuities at these depths, known as the X-discontinuities, ascribing the interfaces to a variety of mineral physical mechanisms, including the coesite to stishovite phase transition, the formation of hydrous Phase A, and/or the reorganization of orthopyroxene into a C2/c monoclinic structure. Thus, the presence of the X-discontinuity (abbreviated here as the X) may be indicative of the nature of mantle heterogeneity. This study finds discontinuities associated with the X Pacific-wide, with SS precursory reflections present beneath the subduction, hot spots, and ridges. Where detected, the X is at an average depth of 293 ± 65 km and the precursor amplitudes indicate a mean shear impedance contrast of 2.3 ± 1.6 %. We model mantle heterogeneity by comparing the depth and the impedance contrasts at the X with predictions for seismic structure from a mineral physics model in which the mantle is considered to be a mechanically mixed bulk assemblage of subducted basalt and harzburgite. In this model, the average mantle composition of the Pacific is fit by a mixture of ~20 % basalt and 80 % harzburgite, roughly consistent with the bulk chemistry of mantle peridotite (18 % basalt, 82 % harzburgite). In some regions beneath the hot spots and subduction, there is evidence for a bulk chemistry enriched in basalt (basalt fraction ~30-35 %), lending evidence to the hypothesis that the mantle is laterally heterogeneous and that dynamics are stirring an enriched component, perhaps from the deep or shallow Earth, into the upper mantle.

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Section: Detections Of the X-discontinuitymentioning

confidence: 95%

Section: % 2% 4% 3%mentioning

confidence: 99%

Section: Seismic Imaging Of Mantle Discontinuitiesmentioning

confidence: 99%

Section: Detections Of the X-discontinuitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Schmerr

2015

The Earth's Heterogeneous Mantle

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A ubiquitous low‐velocity layer at the base of the mantle transition zone

Shen

Yuan

2014

Geophysical Research Letters

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Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of~600 km, indicating a low-velocity layer at the base of the mantle transition zone. The slant-slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X-discontinuity at 300 km depth is also observed in our global stacks, which can be explained by the coesite-stishovite phase transformation.

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The mantle transition zone beneath West Antarctica: Seismic evidence for hydration and thermal upwellings

Emry

Nyblade

Julià

et al. 2015

Geochem Geophys Geosyst

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Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high-quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than 0.24 and 0.48 Hz, respectively. Single-station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 6 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 6 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

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Broadband array observations of the 300 km seismic discontinuity

Cited by 41 publications

References 37 publications

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

A ubiquitous low‐velocity layer at the base of the mantle transition zone

The mantle transition zone beneath West Antarctica: Seismic evidence for hydration and thermal upwellings

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