X‐Discontinuity Beneath the Indian Shield—Evidence for Remnant Tethyan Oceanic Lithosphere in the Mantle

Abstract

Mantle layering greatly influences the dynamics of the Earth's interior, which is responsible for its evolution. Over the last few decades, the search for upper mantle discontinuities using various seismological techniques has gained a lot of momentum. There is a general agreement that in addition to the ubiquitous 410-and 660-km discontinuities (Shearer & Masters, 1992), there exists a weak seismic discontinuity called the X-discontinuity (Revenaugh & Jordan, 1991), in the depth range of 250-350 km, sometimes referred to as the 300 km discontinuity. The X-discontinuity shows a weak, sharp, positive (velocity increasing downward) impedance contrast and is only intermittently observed, unlike the global transition zone discontinuities. Because of its sporadic nature, the origin of X-discontinuity is controversial. However, all the proposed mechanisms are related to the chemical and thermal properties of the mantle, hence the presence of X-discontinuity has implications on mantle dynamics and geochemical signature of the mantle.Seismologically, it has been mostly detected in active subduction and hotspot regions. Figure 1 and Table S1 (Supporting Information) summarize the global disposition of X-discontinuities identified by numerous researchers along with their probing tools and plausible interpretations. Figure 1 shows that the depth of the X-phase below North America is between 290 and 320 km (

INSA National Report on Seismological Research in India: 2019 – 2022

“…In recent years, Uttarakhand Himalaya has been a focus of seismological studies. A setup of 76 three-component broadband stations (in network and profile modes) and 19 strong motion stations have been operating in this region since 2017 (Srinagesh et al, 2019). This network provides high-quality digital waveforms of thousands of local, regional, and teleseismic earthquakes.…”

On the Detection of Upper Mantle Discontinuities with Radon-Transformed Ps Receiver Functions (CRISP-RF)

“…Our final example targets the detection of the X-discontinuity, which is a deeper upper mantle discontinuity marked by a sharp velocity increase and is generally located at the depth range of 230 to 350 km (Pugh et al, 2021;Schmerr, 2015;Srinu et al, 2021). In this data example, we use teleseismic data from a permanent GSN station (IU.AFI) located at Samoa near the convergent boundary between the Pacific and Australian Plates.…”

“…In this case, the CRISP-RF workflow can be beneficial to improving robust detection of discontinuities by serving as a denoiser and aiding in sparse signal recovery (Figure 9). We point out that in most recent applications of timedomain slowness slant stack in Ps-RF imaging, the linear moveout is assumed instead of the parabolic equations used in our implementation (Guan & Niu, 2017;Pugh et al, 2021Pugh et al, , 2023Srinu et al, 2021). To the best of our understanding, our implementation of the sparse Radon transform with the mixed time-frequency iterative solvers, using a suite of modern compressive sensing algorithms, is the most complete treatment of this problem for global upper mantle imaging with Ps-RFs.…”

“…Global seismic imaging has produced maps of upper mantle layering that have important implications for mantle thermo-chemical heterogeneity, rheology, and dynamics (Deuss, 2009;Fischer et al, 2020;Karato et al, 2015;Karato & Park, 2018;Schmerr, 2015;Shearer, 2000;Tharimena et al, 2017). A few examples include the detection of a ubiquitous middlelithosphere discontinuity (MLD) (Abt et al, 2010;Hopper & Fischer, 2018;Krueger et al, 2021), the global lithosphere-asthenosphere system (Kind et al, 2020;Liu & Shearer, 2021;Mancinelli et al, 2017;Rychert et al, 2005), the Lehmann discontinuity (Deuss & Woodhouse, 2004;Karato, 1992), and the X-discontinuity (Pugh et al, 2021(Pugh et al, , 2023Schmerr, 2015;Srinu et al, 2021). Each of these layers can be explained by invoking some combination of partial-melting, phase-changes, chemical stratification, variable anisotropy, and elasticallyaccommodated grain-boundary sliding (Beghein et al, 2014;Karato et al, 2015;Olugboji et al, 2013;Rader et al, 2015;Rychert et al, 2020;Schmerr, 2015;Selway et al, 2015).…”

Elasticity of Anhydrous Phase B Under Mantle Conditions: Implications for the Deep X‐Discontinuity in the Subduction Zones

“…Seismological observations have identified discontinuities known as the X‐discontinuity at a depth range of 250–350 km beneath hotspots, stable continents, and near subduction zones (Bagley & Revenaugh, 2008; Deuss & Woodhouse, 2002; Revenaugh & Jordan, 1991; Schmerr, 2015; Srinu et al., 2021). The X‐discontinuity is characterized by 2%–8% impedance contrasts and indistinguishable Clapeyron Slopes (Deuss & Woodhouse, 2004; Schmerr, 2015).…”