Mantle Deformation in the Eastern Himalaya, Burmese Arc and Adjoining Regions

Abstract

This study presents new results of shear wave splitting at 56 broadband stations installed in the Eastern Himalaya, Burmese Arc, and adjoining regions. The fast polarization directions (FPDs) within the Himalaya, Burmese Arc and the foredeep are parallel to the strike of the orogens, suggesting a coherently deformed lithospheric mantle under compression. The FPDs follow the Main Boundary Thrust and the Main Central Thrust in the central segment of the Arunachal Himalaya. Smaller delay times (∼0.7 s) can be best explained by multiple layers of anisotropy, where combined effects of the anisotropic fabric of a coherently deformed lithosphere of the Arunachal Himalaya and plate motion related strain are playing a role. Null measurements in the Siang window may be due to the Indian plate interaction with Eurasia and Burma plates, causing different layers of anisotropy. Another possibility is the coincidence of source polarization direction with the fast axis. In the Bengal Basin, the Nulls could be due to annihilation of frozen anisotropy in the downwelling Indian lithosphere and the APM‐related strain, or a highly heterogeneous mantle affected by the Kereguelen plume magmatism, at ∼116 Ma.

Upper Mantle Anisotropy beneath the Western Segment, NW Indian Himalaya, Using Shear Wave Splitting

“…A recent study in the Eastern Himalaya also found that the FPAs are mostly oriented along the arc curvature, lending credence to the hypothesis that lithospheric deformation is the primary source of anisotropy [26]. Further west in the Sikkim Himalaya, strike parallel FPA orientation follows [19].…”

“…Further, due to the higher convergence rate, the amount of shortening is found to be more for the eastern Himalaya [49]. This might cause the FPAs in Arunachal Himalaya to orient along the strike of the orogen, suggesting that the coherently deformed mantle is under compression [26]. The amount of shortening decreases towards west.…”

“…Numerous shear wave splitting studies have been performed for the Indo-Eurasian collision zone, which infer a complex deformation pattern beneath the region (Figure 1) [10][11][12][13][14][15][16][17][18][19]. The existing data sets show that seismic anisotropy is different in the diverse tectonic units like the Indian subcontinent, Northeast Himalaya and its foothills, Western Himalaya, and Southern Tibet [14,[18][19][20][21][22][23][24][25][26]. Shear wave splitting studies for the Indian subcontinent are interpreted in terms of the absolute plate motion-(APM-) related strain in the mantle due to the simple progressive shear at the base of the lithosphere [14,23,24,27].…”

“…In the NE Himalaya and its foothills, the FPAs are mostly oriented along the NNE-WSW direction and/or are parallel to the strike of the orogens ( [18,25,26]). In the Northern and Central India, the FPAs are mostly oriented in accordance with the plate motion direction of the Indian plate.…”

Toroidal Mantle Flow Induced by Slab Subduction and Rollback Beneath the Eastern Himalayan Syntaxis and Adjacent Areas

“…On the Indian plate, while some early studies (Barruol & Hoffmann, ; Chen & Özalaybey, ; Sandvol et al, ) suggest a lack of observable splitting based on measurements at a few stations, significant anisotropy is detected by most of the more recent investigations (Figure ; Hazarika et al, ; Heintz et al, ; Saikia et al, , ; Singh et al, , ). Using data from 56 broadband seismic stations in the eastern Himalayan and Burmese arc region, Saikia et al () report weak orogen‐parallel anisotropy. Weak or the absence of anisotropy reported at stations on the Indian plate is attributed to multiple layers of anisotropy with nearly orthogonal fast orientations.…”