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Late Quaternary Tectonics and Basin Inversion: a case from the Lower Narmada valley, India part of Vignettes:Vignette Collection
The Indian Plate is currently moving northeast at 5 cm/yr (2 in/yr), while the Eurasian Plate is moving northeast at only 2 cm/yr (0.8 in/yr). This is causing the Eurasian Plate to deform, and the Indian Plate to compress leading to tectonic activity along major fault zones. In tectonically active areas sedimentary basins undergo phases of both crustal extension and contraction leading to basin inversion and hence display features typical of subsidence and uplift. Geomorphic attributes and deformation in late Quaternary sediments are the indicators of active tectonic activity in any sedimentary basin. The geomorphic evolution in such reactivated basins is primarily due to complex interaction between sedimentation processes and tectonics. The peninsular India has been undergoing high compressive stresses due to the sea-floor spreading in the Indian Ocean and locking up of the Indian plate with the Eurasian plate to the north. Much of these N-S directed stresses have been accommodated by the underthrusting of the Indian plate below the Eurasian plate. A part of these compressive stresses are accumulated along the Narmada-Son Fault (NSF), a major E-W trending crustal discontinuity in the central part of the Indian plate (Fig. 1). The Narmada River, largest in peninsular India, flows along this seismotectonically active NSF. A significant feature of the Lower Narmada valley is the deposition of a huge thickness of Tertiary and Quaternary sediments in a fault controlled basin. To the south of the NSF, the Tertiary rocks and the basaltic flows of the Deccan Trap Formation occur on the surface (Fig.1A), while to the north they lie in the subsurface and are overlain by Quaternary sediments having a maximum thickness of 800 m (Fig. 1B). The NSF is a normal fault in the subsurface and becomes markedly reverse near the surface (Fig. 1B). Reactivation of the fault in Late Cretaceous led to the formation of a depositional basin in which marine sediments were deposited. The NSF remained tectonically active since then with continuous subsidence of the northern block which accommodated 6-7 km thick Cenozoic sediments. The total displacement along the NSF exceeds 1 km within the Cenozoic section. However, the movements along this fault have not been punctuated by phases of structural and tectonic inversion. Historical and instrumental records indicate that the compressive stresses still continue to accumulate along the NSF due to continued northward movement of the Indian Plate. The geomorphic testimony to the active tectonics in the lower Narmada valley includes the young mountain-front scarps delimiting the basaltic uplands and marking the Narmada-Son Fault, the youthful channel morphology of the Narmada and other rivers as evidenced by consistent presence of incised vertical cliffs, entrenched meanders, extensive and deep ravines, uplifted Holocene terraces, anomalous slope variations of alluvial plain surface (Fig. 2), especially to the south of the Narmada River, and remarkable correlatibilty of the drainage with structural features in the low Tertiary uplands. The present landscape of the lower Narmada valley is characterized by four distinct geomorphic surfaces (Fig. 2) termed as the alluvial plain surface, the ravine/gullied surface, the early Holocene fan sufaces and the mid-late Holocene valley fill terrace surface. These distinct surfaces evolved during the late Pleistocene-Holocene primarily due to vertical tectonic movements along the NSF in a compressive environment. The almost flat but gently sloping alluvial plain occupies a major part of the area and is extremely dissected in the vicinity of the river valley and exhibits gullies as deep as 20-30 m. The sediment succession of these surfaces comprises marine basal clay overlain by sediments deposited in an alluvial fan and the alluvial plain environment (Fig. 3). The early Holocene fan surface is a gravelly surface comprising a series of alluvial fans deposited along the mountain front scarps of the NSF and is bounded by NW-SE trending fault passing through the river Narmada on its eastern side and by a NNW-SSE trending fault on its western side (Fig. 4). The sediments of this surface comprise alluvial fan gravels formed by the coalescing of small fans deposited along the NSF. The wide flat topped terrace surface is 5-10m high occupying a deeply incised fluvial valley, show no evidence of ravine erosion and abut against the abandoned cliffs. The sediments of the valley fill terrace surface comprise two lithofacies: tidal estuarine facies in the lower reaches and the fluvial sandy facies in the upper reaches. The tidal estuarine facies is dominated by tidal carbonaceous mud with intervening sands showing parallel lamination. These terraces consist mainly of horizontally stratified fluvial silty sands. Lateral accretion surfaces are completely absent indicating aggradation of the incised valley through vertical accretion when the lower reaches of the river were undergoing tidal estuarine sedimentation. However, the change from fluvial to tidal facies is not sharply defined and appears to be transitional. The tectonically induced deformation structures such as intraformational folds and flexures, slump structures, joints (some showing small scale offsets), large vertical fractures affecting entire cliff sections are present in the late Pleistocene surfaces (Fig. 5). The elevation of these surfaces along the river and the NNW-directed tilting of the sediments, observed in areas south of Narmada, provide additional evidence for differential uplift along the NSF. However, the mid-late Holocene sediments do not show any observable offset along the NSF suggesting that the offsetting of the alluvial plain surface took place at a prior date. The morphostratigraphy of the basin indicates two major phases of tectonic activity along the NSF. The first phase occurred during the late Pleistocene, when slow synsedimentary subsidence of the basin took place along the NSF and allowed uninterrupted sedimentation except for brief periods of pedogenesis of basal clays and the overbank sediments. Synsedimentary subsidence of the basin in a compressive tectonic setting is evidenced by the impeded alluvial fan sedimentation, the thick overbank sediments and the associated sediment deformation. The second phase that occurred during the early Holocene is marked by basin inversion due to differential uplift along the NSF. The tectonic activity during the early Holocene formed extensive ravines and a deeply incised fluvial valley followed by another phase during the late Holocene to Recent uplifted the mid-late Holocene sediments forming terraces. The early Holocene tectonic activity recorded in the lower Narmada valley, possibly, has wider ramifications when viewed in the larger perspective of the Indian plate. This suggests a renewed phase of extreme compression of the Indian plate, which led to tectonic inversion along the NSF in the lower Narmada valley. Significant increase in compressive stresses accumulating on an intracrustal fault like the NSF can transform a previously subsiding basin into an uplifting one. The NSF has been characterized by a compressive stress regime throughout the Quaternary and variations in the degree of compression relative to the rates of plate movement are responsible for the late Pleistocene subsidence and the Holocene tectonic inversion in the lower Narmada valley.