Contemplations on the Role of Continental Tectonics in Shaping the Post-Pliocene Seabed Morphology off Chilka Lake, Orissa, India

摘要

Shallow seismic reflection data along several coast perpendicular traverses off Chilka Lake in Orissa, has brought out the dispositions of the seabed and sub-seabed reflectors along with a few important morphological upwarps extending over 115 km with a relief of 5-11 m and structural breaks. These breaks are mostly aligned in NE-SW direction. Free-air (FA) gravity anomaly of the offshore and onshore region between the Mahanadi and Godavari basins clearly brings out a NE-trending gravity high or crustal thinning along the Eastern Continental Margin of India (ECMI). This is in sharp contrast to the gravity anomaly as well as shoreline geometry down south where the ECMI suddenly turns N-S along the 80° E meridian. Superimposed on the gravity high referred above, a profound gravity low has been identified in the shelf area off Chilka Lake. Fortuitously, this gravity low zone is found to be an approximately 6-km-wide channel cut, in the shelf area, ahead of the shelf-break, from the seismic reflection survey with a vertical relief of 400 m from the seafloor. The actual relief must be much larger in depth, a part of which is presently filled up by the sediments. Obviously presence of such a NE-SW-trending morphotectonic unit in this part of the shelf is a very significant feature in the shelf morphology which is absent down south.Considering the tensional regime which created the Mahanadi and the Godavari grabens in the Permo-Carboniferous-Triassic period one can visualize a net southeasterly push of the ECMI between these two grabens which is evident from the NE alignment of the coast in this region. The presence of the aseismic 85°E ridge with its deep-seated root in the oceanic lithosphere of the Bay of Bengal hinders this southeasterly push and creates a compressional regime in this part of ECMI. This compression is evident from the upwelling of the seabed morphology and associated faults, mapped in the region over a distance of 115 km. The width of this upwarp is varying from 3.5 km to 8.5 km, with a vertical relief of 5 - 11 m. Presence of this upwarp in the seafloor which is associated with a reverse fault towards the seaward side, with identical downthrow of 8 m, conclusively proves a compressional regime since the post-Cretaceous period, following the emplacement of the 85°E ridge. Presence of a SW-NE-trending canyon in the seafloor is an evidence of strike-slip faulting in the direction, which is also endorsed from the seismo-tectonic study in the area. Thus the morphotectonic features mapped in the continental shelf of Orissa are ideal evidence of a transpressional regime. Furthermore under the joint action of the westward back push of the 85°E ridge and the obliquely converging southeasterly push of the landmass, a resultant northeasterly force is a reality.