The lithospheric density structure below the western Himalayan syntaxis: tectonic implications

Abstract

The differences in the tectonic regimes of the western part of the India -Asia collision zone (western Himalayan syntaxis), consisting of the Ladakh-Karakoram, Hindu Kush and Pamir terrains, are examined through the analyses and modelling of gravity anomalies, computed from global gravity model and terrestrial data. Long-wavelength (.450 km) anomalies are related to isostatic compensation and correspond to topographic variations. The short-wavelength gravity highs are primarily due to upthrust blocks and are mostly linked to crustal seismicity. The intermediate depth-focused earthquake region of Hindu Kush-Pamir is characterized by a gravity low probably caused by crustal underthrusting. The deep density structure of the crust and upper mantle constructed from the modelling of gravity anomalies with constraints from seismological information suggests crustal underthrusting of the Indian and the Asian plates in the Hindu Kush-Pamir section unlike that in the Ladakh-Karakoram region, where underthrust Indian lithosphere underplates beneath the Asian plate. The density model supports a hypothesis of slab breakoff of Indian and Eurasian plates in the Ladakh-Karakoram segment, and of the Indian plate in the Hindu Kush-Pamir region, whereas the Eurasian plate drastically underthrusts deeper (c. 200 km), causing deep seismicity in the Hindu Kush-Pamir section.

Kinematics and dynamics of the Pamir, Central Asia: Quantifying surface deformation and force balance in an intracontinental subduction zone

“…For the slab model, we use the mantle densities from the first model but approximate the presence of the slab by replacing the calculated mantle density with an estimated slab density in columns that contain earthquakes per Sippl et al [] (Figure ). We assume that the slab occupies the entire column (1° × 1° horizontal dimensions) and is composed of 20% lower crustal material ( ρ = 3150 kg m −3 ) and 80% mantle lithosphere ( ρ = 3300 kg m −3 ), which gives an effective slab density of 3270 kg m −3 [ Sippl et al , ; Tiwari et al , ]. Therefore, in the slab model, all columns are isostatically compensated except those containing the slab.…”

“…Journal of Geophysical Research: Solid Earth 10.1002/2017JB014177 lower crustal material (ρ = 3150 kg m À3 ) and 80% mantle lithosphere (ρ = 3300 kg m À3 ), which gives an effective slab density of 3270 kg m À3 [Sippl et al, 2013a;Tiwari et al, 2015]. Therefore, in the slab model, all columns are isostatically compensated except those containing the slab.…”

Kinematics and Dynamics of the Pamir, Central Asia: Quantifying the Roles of Continental Subduction in Force Balance

“…Gravity data point to anomalously low densities (interpreted as crustal material) at 50–100 km depth in the vicinity of the slab (Tiwari et al, , ). However, including low slab densities in our GPE integral invariably results in a worse model fit (Table ).…”

“…For Crust1.0 models, both crustal and mantle densities are from Crust1.0. For each of the three compensation styles, we test two cases: (1) “slab,” in which mantle lithosphere portions of columns that include intermediate‐depth earthquakes (Sippl, Schurr, Yuan, et al, ) are assigned an effective slab density of 3,270 kg/m 3 (Sippl, Schurr, Tympel, et al, ; Tiwari et al, ), and (2) “no slab,” in which mantle lithosphere is unmodified. In the compensated, slab model, the crust is compensated everywhere except in grid areas that contain the slab.…”

Tectonics of the Himalaya: an introduction

“…In their review, the authors mention that the 2008 Kashmir earthquake indicates out-of-sequence thrusting. Density modelling by Tiwari et al (2014) for the western Himalaya indicates local slab break-off in the deeply and steeply subducted Eurasian and Indian plates. In their study area in Ladakh-Karakoram-HindukushPamir, the authors decipher at a few locations gravity highs as upthrust blocks and gravity lows as downthrust units.…”