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- Title
X‐Discontinuity Beneath the Indian Shield—Evidence for Remnant Tethyan Oceanic Lithosphere in the Mantle.
- Authors
Srinu, Uppala; Kumar, Prakash; Haldar, C.; Kumar, M. Ravi; Srinagesh, D.; Illa, Bhaskar
- Abstract
Beneath the continents and island arcs, a seismic discontinuity is often detected around 300 km depth, referred to as the X‐discontinuity. Various mineralogical and petrological mechanisms have been put forth to explain its occurrence. Because of the large depth variability, it is challenging to explain its origin invoking a single mineralogical phase transition. In order to investigate this discontinuity beneath India, we analyzed 10,216 P wave receiver functions at seismological stations deployed on the Indian shield and the Himalayas. We detect the X‐discontinuity as a sporadic and thin feature, in the depth range of 246–335 km, with a sharp shear velocity jump of 2.5%–3.6%. It neither bears a clear tectonic affinity nor has any correlation with the transition zone discontinuities. Interpreting its origin due to a single mineralogical change warrants a large spatial variation in the mantle temperature. Therefore, we suggest that the observed widespread X‐discontinuity beneath the Indian shield owes its origin to two mechanisms, that is, Orthoenstatite to high pressure Clinoenstatite transformation which shifts to lower pressures (∼2 GPa) due to the presence of water (0.13 wt% H2O) in MgSiO3 and coesite‐stishovite transition occurring at 8–11 GPa due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during lower Eocene. The identification of such a discontinuity could allow tracking of subducted material within the upper mantle providing a measure of mantle geochemical heterogeneity. Plain Language Summary: In addition to 410 and 660‐km global mantle discontinuities, a number of seismological studies reveal an intriguing phase in the uppermost mantle, termed as X‐discontinuity. A large number of P‐to‐s wave receiver functions have clearly shown such X‐discontinuity below Indian shield at a depth range of 246–335 km. We found that this discontinuity is sporadic, thin, and sharp seismic shear wave velocity jump of 2.5–3.6%. Its large variability in depth below the Indian shield suggests that its origin cannot be explained by single petrological transformation. Therefore, we suggest that the main cause of its occurrence is Oen to HPCen transformation and secondly, coesite‐stishovite transition occurring due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during the lower Eocene. Hence, the closing of the Tethys ocean not only generated the mighty Himalayas and Tibet but also, contributed to the mantle geochemical heterogeneity. Key Points: P‐receiver functions have been analyzed using the seismological data from Indian plate to investigate the X‐discontinuityIt has been observed wide‐spread in Indian mantle at a depth of 246–335 km and originates from Orthoenstatite‐high pressure Clinoenstatite and coesite‐stishovite transitionsIt is suggested that the observed X‐discontinuity is thin body derived from the remnant of subducted Tethys oceanic lithosphere
- Subjects
ISLAND arcs; PHASE transitions; P-waves (Seismology); LITHOSPHERE; MINERALOGICAL research
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 8, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB021890