River profiles along the Himalayan arc as indicators of active tectonics (original) (raw)
Related papers
Normalized Steepness Index along the Himalayan Arc as a proxy for Indian plate segmentation
Authorea (Authorea), 2022
The Indian plate underthrusting the Himalaya is considered to be segmented along the collision belt arc and seismic images of the Indian mantle lithosphere (IML) suggest along-arc variations in the angle of underthrusting and its northern limit beneath Tibet. The pre-existing transverse tectonic structures of the Indian plate mapped in the Ganga foreland basin have been related to these segmentation boundaries. These segmentations imply changes in mechanical properties of adjoining blocks which should manifest in the form of spatial variations in topography build-up. We have analysed a geomorphic index, normalized channel steepness (ksn), along the Himalayan arc using the ALOS elevation dataset to test whether there is any correlation between the and these segmentation boundaries. Our results bring out spatial variability in the along the arc. Based on these results, the arc can be segmented into five blocks, similar to the ones delineated based on correlation between the width of the Ganga foreland basin and the disposition of major Himalayan thrusts from the foothills. Thus, the can be used as a proxy to demarcate different tectonic blocks along the Himalayan arc. Further, we have found a good correlation between the basin width and the northern limit of the IML for all block except the Uttarakhand block. We infer that transverse crustal heterogeneities in this block due to the continuation of different litho-units of the Aravalli-Delhi Fold Belt could be a plausible cause for this anti-correlation.
The northwestern termination of the Himalayan Mountain Front: Active tectonics from microearthquakes
Journal of Geophysical Research, 1978
The seismicity of the northwestern Himalayan syntaxial bend and the geologically complex area to the west, between the Hazara thrust system (HTS) and the higher mountains of Indus-Kohistan, is examined in a wider tectonic context by using data from about 1,800 microearthquakes. The microearthquake data were obtained from a telemetered seismic network in northern Pakistan centered at Tarbela dam on the Indus River and were collected during an 11-month period prior to impounding of the Tarbela reservoir. The observed seismicity indicates that a branch of the main boundary thrust (MBT) traverses the region as a straight northwesterly extension of the Murree thrust, the mapped section of the MBT southeast of the syntaxial bend along the Kashmir Himalayas. Seismic release on this extension of the MBT, here named the Indus-Kohistan seismic zone (IKSZ), is highest in the upper 25 km of the crust and correlates with a pronounced topographic step. Deeper activity on the IKSZ extends to a depth of 70 km. Seismicity in the lower crust defines a second lineation 100 km southwest of the IKSZ and parallel to it. The syntaxial bend and the eastern HTS, microseismically virtually inactive at present but associated with recent and historical microseismicity, overprint the two northwesterly seismic lineaments. The two tectonic regimes may be simultaneously active at different depths separated by an incompetent layer. A set of steeply dipping faults, either parallel or perpendicular to the IKSZ, is active in the region between the HTS and the. IKSZ. Seventeen composite fault plane solutions show a predominant pattern of either reverse or strike slip faulting with the inferred slip vectors oriented such that north-south compressional stresses are relieved. Such a stress field is compatible with the north-south convergence between the Indian and EUrasia n plates inferred from plate tectonics. Bird, 1970; Molnar and Tapponnier, 1975]. In relation to Eurasia the Indian plate is presently moving north at a rate of about 3.7 cm/yr near the longitude of Tarbela and at rates exceeding 4 cm/yr near the eastern termination of the Himalayas [Minster et al., 1974]. Probably a major portion of this convergence is taken up by concentrated thrusting along the Himalayan front, while the remaining convergent movement is probably distributed over a wide area including the Pamirs, Tien Shan, and the Tibetan plateau. Some convergence occur,, Copyright ¸ 1978 by the American Geophysical Union. throughout central Asia apparently by pushing large blocks sideward along major strike slip faults [Molnar and Tapponnier, 1975]. Prior to 30-40 m.y.B.P. the convergence between the Indian and Eurasian plates occurred at higher rates exceeding 10 cm/yr when Tethyan oceanic lithosphere was attached to the Indian plate and was subducted beneath an island arc structure of the Andean type along the Indus suture line. The rather linear and elongated eraplacements of welldeveloped ophiolite suites along the Indus suture line [Gansser, 1964, p. 253] approximately coincide with the upper-Indus and Tsangpo river valleys (Figure 1). The ophiolites are interpreted as the remnants of the oceanic crust of the Tethys Ocean trapped during the final stages of island arc subduction and the beginning of collision between the Indian and Eurasian continental blocks. This transition from oceanic subduction to continental collision tectonics occurred at about late Eocene to early Oligocene times. Some mafic rocks can be followed around and behind the two syntaxial bends near the eastern and western terminations of the .Himalayas. Along the western Indian plate margin, mafic rocks can be traced southward along the Baluchistan and Afghanistan border region into the Kirthar range [Abu Bakr, 1964]. These mafic rocks, usually emplaced into Mesozoic sedimentary formations, have not yet been studied sufficiently to clearly identify them as ophiolite suites. The Himalayan front appears to have migrated southward in time. Thrusting probably occurred first along the Indus suture at the time of initial collision (45 m.y. ago). Some time later the Indus suture became inactive, and from about 25 m.y. ago the convergent movement was taken up mostly on the main central thrust (MCT) [Mattauer, 1975]. Probably 10 m.y. ago the movement shifted to a new front farther south, the main boundary thrust (MBT) [Le Fort, 1975]. The typical cross section of the Himalayan front is usually divided into three main zones. The sub-Himalayas consist of postcollisional, upper Tertiary clastic sediments (Siwaliks) folded and faulted but not metamorphosed. This belt forms Paper number 7B0415.
2017_Transverse tectonic structural elements across Himalayan mountain front.pdf
Structural and morphotectonic signatures in conjunction with the geomorphic indices are synthesised to trace the role of transverse tectonic features in shaping the landforms developed along the frontal part of the eastern Arunachal sub-Himalaya. Mountain front sinuosity (S mf) index values close to one are indicative of the active nature of the mountain front all along the eastern Arunachal Himalaya, which can be directly attributed to the regional uplift along the Himalayan Frontal Thrust (HFT). However, the mountain front is significantly sinusoidal around junctions between HFT/MBT (Main Boundary Thrust) and active transverse faults. The high values of stream length gradient (SL) and stream steepness (K s) indices together with field evidence of fault scarps, offset of terraces, and deflection of streams are markers of neotectonic uplift along the thrusts and transverse faults. This reactivation of transverse faults has given rise to extensional basins leading to widening of the river courses, providing favourable sites for deposition of recent sediments. Tectonic interactions of these transverse faults with the Himalayan longitudinal thrusts (MBT/HFT) have segmented the mountain front marked with varying sinuos-ity. The net result is that a variety of tectonic landforms recognized along the mountain front can be tracked to the complex interactions among the transverse and longitudinal tectonic elements. Some distinctive examples are: in the eastern extremity of NE Himalaya across the Dibang River valley, the NW-SE trending mountain front is attenuated by the active Mishmi Thrust that has thrust the Mishmi crystalline complex directly over the alluvium of the Brahmaputra plains. The junction of the folded HFT and Mishmi Thrust shows a zone of brecciated and pulverized rocks along which transverse axial planar fracture cleavages exhibit neotectonic activities in a transverse fault zone coinciding with the Dibang River course. Similarly, the transverse faults cut the mountain front along the Sesseri, Siluk, Siku, Siang, Mingo, Sileng, Dikari, and Simen rivers. At some such junctions, landforms associated with the active right-lateral strike-slip faults are superposed over the earlier landforms formed by transverse normal faults. In addition to linear transverse features, we see evidence that the fold-thrust belt of the frontal part of the Arunachal Himalaya has also been affected by the neotectonically active NW-SE trending major fold known as the Siang antiform that again is aligned transverse to the mountain front. The folding of the HFT and MBT along this antiform has reshaped the landscape developed between its two western and eastern limbs running N-S and NW-SE, respectively. The transverse faults are parallel to the already reported deep-seated transverse seismogenic strike-slip fault. Therefore, a single take home message is that any true manifestation of the neotectonics and seismic hazard assessment in the Himalayan region must take into account the role of transverse tectonics.
Geology, Ecology, and Landscapes
The drainage pattern and the morphology of the piedmont zone of the Himalayas are clear indicators of the active orogenic belt of most recent origin formed by the collision of Indian and Eurasian plate. The foothills of the Himalayas in West Bengal are zones of active tectonics drained by the rivers belonging to the Brahmaputra system. The present study is conducted for the left bank tributaries and sub-tributaries of Tista which bear the imprint of active tectonics of the region as they lie in the zone of Himalayan Frontal Fault, the most active thrust belt of Himalayas. Data and subsequent field experiences which showed that the region is constantly acted upon by recent diastrophism. Various tectonic indices were calculated to evaluate the evidence of tectonism. This include hypsometric integral (HI), fractal dimension (FD), basin asymmetry factor (AF), basin shape index (Bs), stream-length gradient (SL), mountain front sinuosity (Smf) and valley-floor width to valley height ratio (Vf). The results of these indices are used to prepare an index of active tectonics with three classes which is represented in a map. Then the field evidences of deformed landscape are matched with the areas showing high tectonic index values.
Gondwana Research, 2012
Subduction of bathymetric features, such as ridges, seamounts, fractures etc., on the subducting plate influences the arc morphology and earthquake ruptures. We analyse their effect on the development of the arcuate shape of the Himalayan arc and on the ruptures of great and major Himalayan earthquakes. Besides the two most prominent ridges in the Indian Ocean, namely the Chagos-Laccadive-Deccan ridge and the 90°E ridge, which are assumed to extend up to the Himalayan arc, at least three major subsurface ridges have been mapped on the underthrusting Indian plate under the Indo-Gangetic plains. It appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the Himalayan arc. The interaction and subduction of the other subsurface ridges probably influenced the Himalayan arc morphology by causing a localised cusp in the frontal topography. Also, these ridges probably acted as barriers to the ruptures of the major and great Himalayan earthquakes.
The effects of neotectonic activity on geomorphic features have been studied in a large alluvial fan in the foothills area of the eastern Himalayas. The interfluve area between the rivers Mal and Murti is an alluvial fan composed of Quaternary sediments characterized by clay, sand, pebble, and boulder beds. Most of the river valleys in this area show well developed terraces. There are four major terrace surfaces, named as T1, T2, T3 and T4 according to increasing height from the river bed. Two EeW scarps named as Matiali and Chalsa scarps that cut across the fan represent traces of the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) respectively. There are two other NNWeSSE and NNE eSSW lineaments which have partially guided the courses of the Neora and Murti rivers. These are interpreted as conjugate sets of normal faults transverse to the orogenic trend. The EeW scarps are the manifestation of the frontal limbs of the ramp anticlines over two blind thrusts. Fault propagation folding has affected the fan surface. Recurrent movements on the thrusts and consequent downcutting of the rivers have led to the formation of the raised terraces on the banks of these rivers. The terraces are formed by cut-and-fill process. Later transverse normal faulting has given rise to a horst of the NeoraeMal interfluve block.
Direct evidences of faulting along the Himalayan mountain front are very rare. The most conspicuous indicators for the presence of a structural discontinuity and an active tectonic boundary are manifested in the geomorphology. In most of the cases, a topographic break defines the outermost Himalayan thrust fault i.e. Himalayan Frontal Thrust (HFT). However, the nature of this tectono-geomorohic boundary tends to vary significantly along the strike of the mountain belt. In some sectors, it is marked by the presence of elongated intermontane valleys popularly known as 'duns', whereas other sectors are conspicuous by their absence. The Nahan sector of the Western Indian Himalaya is one such area that is characterized by the absence of intermontane valley. Here, in this article we emphasize that the absence of intermontane valleys in the Nahan sector gives rise to a distinctive geomorphic expression of the active tectonic boundary. In the Nahan sector, active slip along the underlying thrust faults and splays has resulted in the accumulation of high relief spread over a relatively smaller area. This led to the absence of the duns in this area. Also, the topographic boundary closely corresponds to a number of drainage anomalies establishing the active tectonic nature of the HFT. The drainage anomalies mainly include the deflection of streams, widening of channels and increase in their sinuosity as they approach the mountain front. The increase in the sinuosity of the channels is also marked by degradation activity and presence of broad flat terrace surface that confirm to the tectonic uplift in this area.
Tectonic evolution of Kashmir basin in northwest Himalayas
Geomorphology, 2015
Geomorphology has long been recognised as a key to evaluate the interplay between tectonics and landscape geometry in the regions of active deformation. We use geomorphic signatures at varied spatial scales interpreted from SRTM-DEM/Landsat-ETM data, supplemented with field observations to review the tectonic evolution of Kashmir basin in northwest Himalayas. Geomorphic evidence is persuasive of a credible NNW-SSE trending dextral strike-slip structure (central Kashmir fault-CKF), with the strike length of ~165 km, stretched centrally over the NNW-SSE length of the Kashmir basin. As a result of the strike-slip motion and subsequent erosion, significant deformation has taken place along the CKF. In addition, broad geomorphic architecture of the basin reveals typical pull-apart characteristics. Hence, we deduce that the Kashmir basin has evolved as a pull-apart Quaternary sediment depression owing to the deformation along the central Kashmir fault. The spatial distribution pattern of seismic events (NEICcatalogue, 1973-2013) and GPS measurements (published), collectively substantiate our geomorphic interpretations.