Delineation of the 85°E ridge and its structure in the Mahanadi Offshore Basin,Eastern Continental Margin of India (ECMI), from seismic reflection imaging

The passive Eastern Continental Margin of India (ECMI) evolved during the break up of India and East Antarctica in the Early Cretaceous. The 85°E ridge is a prominent linear aseismic feature extending from the Afanasy Nikitin Seamounts northward to the Mahanadi basin along the ECMI. Earlier workers have interpreted the ridge to be a prominent hot spot trail. In the absence of conclusive data, the extension of the ridge towards its northern extremity below the thick Bengal Fan sediments was a matter of postulation. In the present study, interpretation of high resolution 2-D reflection data from the Mahanadi Offshore Basin, located in the northern part of the ridge, unequivocally indicates continuation of the ridge across the continent–ocean boundary into the slope and shelf tracts of the ECMI. Its morphology and internal architecture suggest a volcanic plume related origin that can be correlated with the activity of the Kerguelen hot spot in the nascent Indian Ocean. In the continental region, the plume related volcanic activity appears to have obliterated all seismic features typical of continental crust. The deeper oceanic crust, over which the hot spot plume erupted, shows the presence of linear NS aligned basement highs, corresponding with the ridge, underlain by a depressed Moho discontinuity. In the deep oceanic basin, the ridge influences the sediment dispersal pattern from the Early Cretaceous (?)/early part of Late Cretaceous times till the end of Oligocene, which is an important aspect for understanding the hydrocarbon potential of the basin.     

Double Relics in the Outskirts of A3376: Accretion Flows Meet Merger Shocks?

The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts. We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al. 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large-scale accretion flows on the outgoing merger shocks as reported in the recent simulations.     

Geomorphic effectiveness of extraordinary floods on three large rivers of the Indian Peninsula

The efficacy of extreme events is directly linked to the flood power and the total energy expended. The geomorphic effectiveness of floods is evaluated in terms of the distribution of stream power per unit boundary area (ω) over time, for three very large floods of the 20th Century in the Indian Peninsula. These floods stand out as outliers when compared with the peak floods per unit drainage area recorded elsewhere in the world. We used flood hydrographs and at-a-station hydraulic geometry equations, computed for the same gauging site or a nearby site, to construct approximately stream-power curves and to estimate the total energy expended by each flood. Critical unit stream power values necessary to entrain cobbles and boulders were estimated on the basis of empirical relationships for coarse sediment transport developed by Williams [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] in order to determine the geomorphological effectiveness of the floods. The estimates indicate that the minimum power per unit area values for all three floods were sufficiently high, and stream energy was above the threshold of boulder movement (90 W m^− 2) for several tens of hours. The peak unit stream power values and the total energy expended during each flood were in the range of 290–325 W m^− 2 and 65–160 × 10⁶ J respectively. The average and peak flood powers were found to be higher or comparable to those estimated for extreme palaeo or modern floods on low-gradient, alluvial rivers.     

Is flooding in South Asia getting worse and more frequent?

South Asia is drained by some of the most flood‐prone rivers in the world. Flooding during the monsoon season is the most recurring, widespread and disastrous natural hazard in South Asia that results in enormous social, economic and environment consequences every year. Several massive floods have occurred in the recent decades causing huge economic losses and human suffering. On average, the total damage is close to USD 1 billion annually. To answer the question whether flooding in South Asia is getting worse and more frequent, all available data were considered: the annual peak discharge data for major rivers, post‐1985 information on floods from the global archive of large floods and palaeoflood records from nine Indian rivers. According to the global archive data, 372 large and 55 extreme flood events have occurred since 1985. Although there is no significant trend, all types of data point to clustering of large floods. Palaeoflood records show that modern floods (post‐1950) have higher flood levels than the late Holocene floods. Notwithstanding the limitations of data, there is enough evidence to conclude that (1) incidences of flood‐generating extreme rainfall event are rising and (2) human interventions have made the recent floods more destructive.     

Implications of new $$^{40}\hbox {Ar}/^{39}\hbox {Ar}$$ age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin,Dharwar Craton,India

The Kaladgi Basin on the northern edge of the Dharwar craton has characters diverse from the other epicratonic Purana basins of Peninsular India. Sedimentological studies in the basin have established the presence of three cycles of flooding separated by an event of intra-basinal deformation accompanied by low grade incipient metamorphism. The overall structural configuration of the basin indicates its development by supracrustal extension accompanied by shearing in a trans-tensional regime during the Mesoproterozoic. This was followed by sagging that yielded Neoproterozoic sedimentation in a successor nested basin. \(^{40}\)Ar/\(^{39}\)Ar dating of an intrusive mafic dyke along the axial plane of a fold has yielded a plateau age of \(1154{\pm }4\,\hbox {Ma}\). This helps constraint the age of the various events during the evolution of this basin.     

Colluvial deposits in northwest Deccan,India: their significance in the interpretation of Late Quaternary history

The Deccan Trap region exhibits an erosional landscape over a relatively ancient and stable Deccan shield. The Quaternary history of the area has been reconstructed on the basis of evidence from alluvial deposits occurring along the major rivers. However, recent investigations have revealed that evidence for geo-environmental change during the Quaternary Period is also contained in the colluvial deposits that occur in the foothill zones. The colluvial deposits, ranging in thickness from 1 to 10 m, invariably occupy gently inclined pediment slopes. The sediments are presently deeply dissected by gullies, and the process of colluviation has almost ceased. These deposits are best preserved in the semi-arid parts of the region. Detailed textural, geochemical and stratigraphical studies at four different sites reveal similar input processes, the slight variations being attributed to local environmental factors. Scanning electron microscopy studies of some grains indicate marginal contribution of aeolian processes at the time of deposition. Mesolithic artefacts and a few U/Th dates indicate that the colluviation took place during the Late Quaternary. The properties of the deposits suggest relatively high energy conditions as well as a remarkable variability in the intensity of hillslope processes. These properties are indicative of semi-arid conditions during which the regolith was stripped from devegetated hillslopes and was deposited on the pediments. A variety of evidence indicates that the period of colluviation coincided with arid conditions during the Last Glacial Maximum. The geomorphological and archaeological evidence also indicates that incision by gully systems was initiated during the early Holocene humid phase. The environmental conditions deduced for the study area are similar to those reported for other parts of the intertropical zone. © 1997 John Wiley & Sons, Ltd.     

Uplift along the western margin of the Deccan Basalt Province: Is there any geomorphometric evidence?

In line with the passive margin landscape evolutionary model in vogue, sustained erosion and long-distance retreat of the Western Ghat escarpment are widely considered to be the results of erosionally-driven isostatic uplift since Tertiary by many workers. Others have postulated or adduced evidence for strong neotectonic activity in the Ghat region. An obvious question in this regard is whether there is any geomorphometric evidence in support of this widespread view? In order to test the hypothesis of ongoing post-rift fexural uplift or neotectonic activity in the western Deccan Basalt Province (DBP), geomorphometric analysis was carried out and commonly used geomorphic indices of active tectonics (GAT) were derived for 30 selected river basins on both sides of the Western Ghat. SRTM-DEM data and ArcGIS were used to derive the indices. Tectonic geomorphic analysis based on five proxy indicators suggests that the differences in GAT indices, both along strike and across the Western Ghat, are statistically insignificant. The index values are nowhere close to the GAT values typically associated with drainage basins affected by active tectonics and deformation. Mapping of the indices reveals lack of discernable trends. The adduced results indicate that the western DBP belongs to the class of relatively low tectonic activity.     

Channel form and processes of the flood-dominated Narmada River,India

The 1300 km long Narmada River flows along a structural lineament, alternating between constricting rocky gorges and rapids, and meandering wide alluvial reaches. Channel forms and processes were studied in a 120 km long section of an alluvial reach. Channel size, shape and bedforms in the Narmada River are related to very large floods which have occurred three times in this century. During such floods the entire 400 m wide channel is utilized and 10–15 m high cliffs on both sides operate as riverbanks. Normally, even the high flows of the south-western monsoon are insufficient to fill the whole channel, and hence their effects are limited to building of discontinuous floodplains between the cliffs and modifying bedforms and bars. A channel-in-channel topography is thus created. The very large floods are also responsible for erosion of the rocky stretches and building of point bars. The river meanders, but its movement is restricted because of (1) rocky gorges and scablands operating as anchor points at intervals, and (2) the presence of high alluvial cliffs which are topped on extremely rare occasions. In spite of being located in a tectonically active zone in a monsoon setting, it is the exceptional high-magnitude floods at irregular intervals which control the form and behaviour of the Narmada River.     

Temporal variations in the specific stream power and total energy expenditure of a monsoonal river: The Tapi River, India

In this study, an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the Tapi River, in central India. Continuous daily discharge data (1978–1990), hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power (ω) for the Savkheda gauging site in the lower Tapi Basin. The total amount of energy generated by all the monsoon flows was estimated by integrating the area under the ω-graph derived for the monsoon season.The analyses of the 13-year daily discharge data reveal that the average and maximum ω values range from 4–20 W m^{− 2}, and 22–964 W m^{− 2} respectively. Specific stream power duration curve derived for the site shows that for 25% of the time the power per unit area is > 10 W m^{− 2}. Furthermore, unit stream power was found to be above the Williams' [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] threshold of pebble-movement (1.5 W m^{− 2}), cobble-movement (16 W m^{− 2}) and boulder-movement (90 W m^{− 2}) for 71%, 15% and 2% of the time, respectively. Computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 MJ (deficit monsoon years) to 256 MJ (excess monsoon and/or flood years). Large floods have one-third share in the total monsoon energy expenditure. In the absence of appropriate data on the yearwise geomorphic effects, the geomorphic work was evaluated in terms of the total suspended sediment load transported. The total monsoon sediment load is strongly related to the total monsoon energy. The results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons.The present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season, except during the deficient monsoon years, and there is little doubt that large-magnitude floods are effective agents of geomorphic change in monsoonal rivers.     

Effectiveness of monsoon floods on the Tapi River, India: role of channel geometry and hydrologic regime

Geomorphic effects of floods are a function of several controlling factors, such as magnitude, frequency, rate of sediment movement, flood power, duration of effective flows, sequence of events and the channel geometry. In this paper, these measures of effectiveness have been evaluated for the monsoon-dominated, flood-controlled and incised Tapi River, India by defining four flow categories: low flows, moderate flows, floods and large floods. Ratios between effectiveness parameters of moderate flows on one hand and the floods, large floods and maximum floods on the other, were computed to understand the relative importance of moderate and large flows. In addition to this, stream-power graphs for large floods were constructed, and the changes in channel form were analyzed by using multi-date cross-sections. The results of the study indicate that the morphological characteristics of the bedrock as well as the alluvial channels of the monsoonal and incised Tapi River are maintained by large-magnitude, but low frequency floods that occur at long intervals. Because the channel is incised the effectiveness of large flows is accentuated. The incised channel form enhances the role of large floods by reducing the width–depth ratio, and by increasing the velocity as well as the energy per unit area. The low and moderate flows are superior to high-magnitude flows, only in terms of suspended sediment transport and frequency of occurrence. Another conclusion is that the suspended sediment carried by flows may not be the most appropriate criterion for measuring the geomorphic effectiveness of flows, particularly for monsoonal rivers.