Estimation of 3D Moho topography and vertical tectonic stress in the eastern Himalayan syntaxis using gravity data

Abstract

Eastern Himalayan Syntaxis (EHS) holds the unique significance and one of the least studied regions in the eastern Himalaya. In this study, Moho topographic undulation map of the study region and the vertical tectonic stress caused by isostatic adjustment are obtained by using the Bouguer gravity anomaly (BGA), topographic and isostatic anomaly data from the WGM2012 model. The source depth and cutoff wavenumber estimated from spectral analysis are found to be ≈ 46 km and 0.012 km⁻¹, respectively. The BGA is then filtered using a low-pass filter with 83 km wavelength to obtain the regional anomaly corresponding to Moho topography. The resulting regional anomaly map is inverted using the Parker–Oldenberg method to obtain a gravity Moho. The gravity Moho is found to be varied from 36 to 56 km. The isostatic Moho depth is computed using the Airy model. The resulting gravity Moho is in good agreement with previous seismological studies in the region. Using the resulting gravity and isostatic Moho, an isostatic compensation map is derived, which shows all three states of isostatic compensation in the region. The state of isostatic compensation obtained in our study corroborates well with the isostatic anomaly map. In addition, we estimated the vertical tectonic stress caused by lithospheric load in the study region. In the Southern Tibet detachment, Namcha Barwa Antiform (NBA), and Lohit plutonic complex (LPC), the vertical stress is negative and reaches the maximum value of 80 MPa. The central zone of the study region (EHS) features tensional and compressional stresses that vary from − 20 to 20 MPa. In the southern EHS, the Assam valley shows a significant increase in vertical compressional stress. In the Assam valley, the compressional vertical stress varies up to 60 MPa. Due to under-compensation, the mountains in the NBA and LPC subside downward, causing tensional negative stress, while the Assam valley has the highest compressional stress due to topographic uplift, which accounts for the surface mass lost during fluvial erosion.