Sub-surface characteristics of pop-up tectonics through field gravity and magnetic modelling: An example of the Shillong Plateau, NE India

The Shillong Plateau (SP) in NE India is one of the most debatable proterozoic basement in the world due to its complex tectonics. Though it had some collisional histories during the formation of the Gondwanaland associated with the Indo-Australo-Antarctic suture, but the signature of crustal and moho depth models gives a different idea about the modified crust under the SP. Despite its peak elevation at around 2000 m, the moho depth observed from the seismic tomography and satellite gravity data under the SP is not more than 34 km, which is remarkably smaller than the surrounding Bengal basin (⁓44 km) and the Brahmaputra basin (⁓44 km). We have tried to solve the problem related to the moho variation, taking into account the field gravity and magnetic anomaly. The major trends in the gravity anomaly predominant along EW direction conforming to the trends of regional geological structures across most of the SP. As our study area concentrates along an NS profile across two different litho units restricted to the central part of the plateau. The corrected field magnetic anomaly across the study area has a little variation between 0 to -500 nT, although some change in anomaly pattern can be seen along the northern side of the SP reaching towards -3500 nT. Moreover, the southern side of the plateau has very little magnetic anomaly variation. The bouguer gravity anomaly varies from ⁓ -70 mGal at the northern boundary to ⁓ +10 mGal with a steep gradient found across the southern side. The gradual change over to positive anomaly under SP, strong -ve anomaly under the Brahmaputra basin to the north and moderate negative anomaly under the Bengal basin towards the south suggested an uplifted moho under SP, which is demonstrated by the 2D gravity modelling. Closely spaced bouguer anomaly contours along the southern part and EW trending magnetic anomaly along the northern part of the SP, indicating two boundary faults viz. Oldham fault/ Brahmaputra valley fault in the North and Dauki fault in the south, dipping towards each other supported the SP for the formation of the pop-up tectonics.