Shyam Chand

India–East Antarctica conjugate margins: rift-shear tectonic setting inferred from gravity and bathymetry data

Abstract The Eastern Continental Margin of India (ECMI) has evolved as a consequence of breakup of India from East Antarctica during the Early Cretaceous (ca. 130 Ma). The conjugate margin of ECMI in East Antarctica is represented by the margin extending from Gunneris Ridge in the west to about 95°E in the east. To understand the isostatic compensation mechanism operating beneath these conjugate margins, we have examined the cross spectral correlation between gravity and bathymetry along 21 profiles across the ECMI and 16 profiles across the conjugate East Antarctica Margin using both ship and satellite-derived gravity data. The ECMI is considered as a composite of two segments, one north of 16°N extending beyond 20°N, which is based on its rifted margin character, and the other, south of 16°N extending up to Sri Lanka, which has a transform-rift character. Similarly, the conjugate margin of East Antarctica is also considered to be a composite of two segments, west and east of the central bulge at 50–55°E. Admittance analysis and comparison with various isostatic models suggest a flexural plate model with an elastic thickness of 10–25 km for the northern segment of ECMI and its conjugate segment which is the east Enderby land Margin, comparable to results obtained from the eastern North American Margin. For the southern segment of ECMI, low elastic plate thickness of less than 5 km or a local compensation is obtained with matching results for the west Enderby land Margin. These, in turn, appear comparable to the low Te values inferred for the Ghana transform margin of North Africa and Grand Banks Margin of eastern Canada, thereby indicating that the southern segment of ECMI and its conjugate in East Antarctica have developed as a consequence of shearing rather than rifting in the early stages of continental separation.

Role of tectonic stress in seepage evolution along the gas hydrate‐charged Vestnesa Ridge, Fram Strait

Methane expulsion from the world ocean floor is a broadly observed phenomenon known to be episodic. Yet the processes that modulate seepage remain elusive. In the Arctic offshore west Svalbard, for instance, seepage at 200–400 m water depth may be explained by ocean temperature-controlled gas hydrate instabilities at the shelf break, but additional processes are required to explain seepage in permanently cold waters at depths >1000 m. We discuss the influence of tectonic stress on seepage evolution along the ~100 km long hydrate-bearing Vestnesa Ridge in Fram Strait. High-resolution P-Cable 3-D seismic data revealed fine-scale (>10 m width) near-vertical faults and fractures controlling seepage distribution. Gas chimneys record multiple seepage events coinciding with glacial intensification and active faulting. The faults document the influence of nearby tectonic stress fields in seepage evolution along this deepwater gas hydrate system for at least the last ~2.7 Ma.

Identification of Weak Layers and Their Role for the Stability of Slopes at Finneidfjord, Northern Norway

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer in the fjord-marine sediments before developing retrogressively across the shoreline. The integration of results from sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicates that the slide-prone layer is a regional bed likely sourced from clay-slide activity in the catchment of the fjord. The sediments in this regional layer are softer and more sensitive than the typical bioturbated, fjord-marine deposits, which explains their role in slope instability. In addition, biogenic gas in the stratified event bed may further affect its geotechnical properties. Similar, fine-grained, stratified beds with comparable origin and properties occur in other Norwegian fjords. They are presumably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements.

Rifting between India and Madagascar – mechanism and isostasy

Abstract The nature of isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM) and their conjugate nature are examined. The first part of the study deals with the estimation of effective elastic thickness (Te) of the lithosphere for the two margins through cross-spectral analysis of gravity and bathymetry data. The results bring out comparable Te values of 8–15 km for the WCMI and 10–13 km for the ECMM, despite the WCMI having been traversed along strike by a hotspot trace. We have also compared topographic profiles across the two margins extending inland, to examine the rift-flank uplift experienced by the two landmasses abutting the margins, which bear a striking similarity. These results establish the symmetrical rifting mechanism resulting in the formation of the two margins, and indicate that despite the presence of a hotspot trace along one margin (i.e. the WCMI), the two margins have comparable isostatic compensation mechanisms with low Te values. In the second part of the study we have examined the possible mechanism for the rift-flank uplifts using a process-oriented approach of backstripping the sediments along a traverse across the WCMI and reconstructing the original rift flank topography by flexurally backstacking the amount of eroded material on the present-day topography of the Western Ghats. The backstack load configuration is taken from the amount of erosion that took place over the Western Ghats, obtained from AFTA (Apatite Fission Track Analysis) studies. The reconstructed rift time topography and bathymetry are used to get the Moho configuration, which was used to calculate the gravity anomaly, and is compared with the observed anomaly to derive a possible evolution model for the region. We have tried to examine the observed gravity anomaly through two different processes. In the first process, a Te=15 km for backstripping and backstacking, and a compensated underplating for the topography give a best fit model to the observed gravity. In the second process, a necking model with a deep level of necking (20 km) with Te=15 km and without any underplating explains the topography as well as the observed gravity. The nature and amount of the Western Ghat topography and cratonic age (Archaen) of the pre-rifted crust of western India and eastern Madagascar are more in favor of the rifting mechanism (necking model) rather than due to the passage of the Reunion hotspot (underplating model). This is evidenced by the observed increase in topography away from the influence of the Reunion hotspot trace as well as the presence of a similar topography in Madagascar.

Timescales of methane seepage on the Norwegian margin following collapse of the Scandinavian Ice Sheet

Gas hydrates stored on continental shelves are susceptible to dissociation triggered by environmental changes. Knowledge of the timescales of gas hydrate dissociation and subsequent methane release are critical in understanding the impact of marine gas hydrates on the ocean–atmosphere system. Here we report a methane efflux chronology from five sites, at depths of 220–400 m, in the southwest Barents and Norwegian seas where grounded ice sheets led to thickening of the gas hydrate stability zone during the last glaciation. The onset of methane release was coincident with deglaciation-induced pressure release and thinning of the hydrate stability zone. Methane efflux continued for 7–10 kyr, tracking hydrate stability changes controlled by relative sea-level rise, bottom water warming and fluid pathway evolution in response to changing stress fields. The protracted nature of seafloor methane emissions probably attenuated the impact of hydrate dissociation on the climate system.

Gravity anomalies, sediment loading and lithospheric flexure associated with the Krishna–Godavari basin, eastern continental margin of India

Abstract Gravity anomalies constrained by seismic data have been used to examine the long-term mechanical properties of the lithosphere beneath the Krishna–Godavari basin, a deltaic sedimentary basin on the eastern continental margin of India. The gravity effect of the rift crust, sedimentary loading and erosional unloading have been computed and the sum effect of these, when compared with the observed gravity anomalies, gives a best fitting Te of 30 km at the margin. The results suggest that the lithosphere, which was weak during the early rifting-stage, has gained strength during sedimentation. The onshore dipping doublet gravity anomaly at the margin reflects a weak rifted continental lithosphere abutting a strong oceanic lithosphere. Comparison with the results of similar investigations for the Bengal basin in the north and Cauvery basin in the south suggests increasing strength for the lithosphere from north to south.

Shallow Landslides and Their Dynamics in Coastal and Deepwater Environments, Norway

In this manuscript, we present the first results of integrated slope stability studies to investigate smaller-scale mass movement processes in different physiographic settings of Norway. These include coastal areas (Sorfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vesteralen) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.

Investigations of Slides at the Upper Continental Slope Off Vesterålen, North Norway

Multibeam bathymetry, high-resolution seismic profiles and sediment cores were collected from the upper continental slope outside Andoya, the northernmost island in Vesteralen, northern Norway (69°N). Eight small slides are identified at water depths between 500 and 800 m. These are linked to a larger slide related to the development of the Andoya Canyon by high resolution seismic. Slope angles adjacent to the headwalls of the small slides are 3–4° while slide deposits have accumulated where slope angles are 2–3°. The slides occurred in parallel-stratified glacial marine sediments, and three seismic horizons are interpreted. One of these horizons coincides with failure planes in four of the slides. A 12 m long core terminates above this level, but penetrates another horizon representing a slip plane in two of the slides. The core comprises silty clay with varying content of ice rafted debris. The upper slope shallower than 450 m appears stable although it may be up to 8° steep. Tension cracks up to 2 m deep on the lower slopes may suggest that deformation is active near the canyon systems.

Subsidence and isostasy along a sheared margin—Cauvery Basin, Eastern Continental Margin of India

We have applied a process-oriented method to a seismo-geological section across Cauvery Basin, a sedimentary basin along the southern part of the Eastern Continental Margin of India (ECMI) to examine the isostatic mechanism operating beneath the margin. The evolution of the ECMI appears to be within a shear-rift tectonic setting, mainly driven by the difference in the relative motions of India and East Antarctica and the subsequent direction of spreading and the Cauvery Basin seems to have formed in this pull-apart tectonic setting. Shearing along the southern ECMI is modeled with gravity anomalies and the resultant crustal configuration, through various models of isostasy. Backstripping analysis suggests a very low Te of 3 km beneath the basin. The shelf edge gravity anomaly reflects a strong contrast across a wrench fault parallel to the coast inferred from gravity forward modeling, denoting coast-parallel transform motion in the initial stages of continental separation prior to rotation and spreading.

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

Source: doi: 10.1190/INT-2015-0023.1 Reuse is subject to SEG terms of use and conditions Society of Exploration Geophysicists