Lithospheric Density Structure and Effective Elastic Thickness Beneath Himalaya and Tibetan Plateau: Inference From the Integrated Analysis of Gravity, Geoid, and Topographic Data Incorporating Seismic Constraints

Abstract

Investigation of deep crustal and lithospheric structures is essential to understand the nature of geodynamical processes beneath the Himalaya and Tibetan plateau of the India-Eurasia collision zone. Our density cross sections across the Himalaya-Eurasia collision zone using integrated 2-D modeling of gravity, topography, and geoid data incorporating constraints from seismic information supports the above contention. Analysis of gravity, geoid, and elevation data over the interior of the Tibetan plateau predicts complete isostatic compensation, whereas margins of the plateau, having large topographic gradients, show lack of isostatic compensation as the Airy Moho differs from flexural Moho and seismic Moho beneath the Himalaya. Our 2-D modeled lithospheric cross sections show thick crust (~75 km) and thick lithosphere (~240 km) beneath the Himalayas and southern Tibetan plateau and relatively thin crust (~60 km) and thin lithosphere (~140 km) beneath the northern Tibetan plateau. Therefore, depth of lithosphereasthenosphere boundary (LAB) mimics the Moho relief. Thinner crust and thin lithosphere under northern Tibetan plateau suggest the importance of the mantle isostasy where the temperature is anomalously high. This corroborates with the presence of recent potassic volcanism, inefficient Sn propagation, east and southeast oriented global positioning system displacements, and large shear wave splitting anisotropy (>2 s). Excellent correlation between effective elastic thickness and lithospheric thickness predicts hot and deformable lithosphere in the northern Tibet and underthrusting of cold Indian mantle beneath the Himalayas.

Constrained Gravity Inversion With Adaptive Inversion Grid Refinement in Spherical Coordinates and Its Application to Mantle Structure Beneath Tibetan Plateau

“…Beneath the northern Tibet, the Ravikumar et al. (2020)'s LAB becomes shallower, where a low‐density anomaly is also seen in our model beneath the JRS in spite of a ∼150 km difference in the horizontal location. Further north, both our model and Ravikumar et al.…”

“…Further north of the Himalaya, Ravikumar et al. (2020)'s lithosphere becomes thicker, whereas thinning of lithosphere indicated by the low‐density anomaly beneath the YZS is shown in our model. This thinning is more obvious in the LITHO1.0 model.…”

“…Third, the difference in methodology also causes the inconsistency between our model and Ravikumar et al. (2020)'s model. By means of existing knowledge and the trial‐and‐error modeling strategy, Ravikumar et al.…”

“…They used Bouguer gravity anomalies that are mainly sensitive to crustal structures, while we apply crustal corrections to obtain gravity anomalies that are mainly due to mantle sources. Third, the difference in methodology also causes the inconsistency between our model and Ravikumar et al (2020)'s model. By means of existing knowledge and the trial-and-error modeling strategy, Ravikumar et al (2020) could model clear layered features in the mantle lithosphere, despite the inherent poor depth resolution of gravity data.…”

“…In addition, Ravikumar et al. (2020)'s model shows lithosphere thinning beneath a vast area of the central Tibetan Plateau. By contrast, in LITHO1.0 model, the lithosphere thinning occurs in both the northern and southern parts of the Tibetan Plateau, which is in a rough agreement with the low‐density anomalies in the southern and northern Tibet in our model.…”