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Recent movements along the Main Boundary Thrust of the Himalayas: Normal faulting in an over-critical thrust wedge?
Abstract
The Main Boundary Thrust (MBT) is one of the major Himalayan thrusts occurring during the Cainozoic, and it is presently incorporated within the Himalayan thrust wedge (Lesser and Outer Himalayas) displaced above the Indian lithosphere. Nonetheless the MBT shows recent normal displacement along most of its length. We suggest that the orientation of the major principal stress within the Himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back-tilted thrusts, the normal component of displacement may act along these faults. Steep north-dipping segments of the MBT therefore show a normal component of displacement if a geometrical definition is used, but they are faults in a compressional regime where the major principal stress axis has deviated from the horizontal. Micro-structural data recorded along the Surkhet-Ghorahi segment of the MBT are consistent with a strong deviation of the state of stress. The presence of such peculiar normal faulting along the MBT is used to calibrate the mechanical characteristics of the belt considered as a Coulomb wedge. The following characteristics are suggested: (a) very poor strength contrast between basal decollement and rocks in the wedge body, (b) a high pore fluid pressure ratio (probably close to 0.8?0.9) and a higher fluid pressure ratio (close to 1.0) along the active normal faults if a high internal friction angle (close to the Byerlee value) is considered. The strong deviation in principal stress direction may have recently increased, due to a taper of the Himalayan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the Himalayan range.
... Geodetic measurements at Jogindernagar in Himachal Pradesh and Dakpathar in the Yamuna valley show evidences of recent to sub-recent activity along the MBT (Ansari et al., 1976;Arur & Rajal, 1981). Landforms related to the normal faulting have been reported from the MBT zone and its thrust splays from the MBT zone of the Kumaun and Nepal Himalaya (Kothyari et al., 2010;Luirei et al., 2021;Mugnier et al., 1994;Valdiya et al., 1992). Structural control on the rivers in the MBT zone is described, revealing the strikeslip movement along the MBT as the bedrocks have a high angle (>70 ) of dip (Valdiya, 1980a). ...
... The MBT is a regional tectonic boundary characterized by imbricating thrusts, where sudden elevational gain in altitude is observed across this thrust (Medlicott, 1864;Valdiya, 1984Valdiya, , 1998. Recent tectonic activities along the MBT are reported from various segments of the T A B L E 1 Generalized stratigraphic succession of the area (Shanker et al., 1993;Tiwari et al., 2013) Himalayan arc (Luirei & Bhakuni, 2008;Mugnier et al., 1994;Valdiya, 1992). ...
... The alternating sedimentary layers are devoid of sediments of river-borne origin which suggests that the lacustrine materials were deposited in a sag pond that originate from tectonic Normal faulting is also reported from other segments, where the movement has truncated fan terrace producing a fault scarp almost 2.5 km-long and 37-m-high at Logar in Gaula River (Kothyari et al., 2010;Valdiya, 1992). Mugnier et al. (1994) the Kumaun Himalaya (Valdiya, 1986;Valdiya et al., 1984). Steeply tilted colluvial deposit resting over the Lesser Himalayan rocks in the hanging wall that dips towards the slope is also evidences of Recent tectonic activity along the MBT is also evident from the tilted colluvial deposits in the hanging wall that are inclined towards the slope. ...
... Geodetic measurements at Jogindernagar in Himachal Pradesh and Dakpathar in the Yamuna valley show evidences of recent to sub-recent activity along the MBT (Ansari et al., 1976;Arur & Rajal, 1981). Landforms related to the normal faulting have been reported from the MBT zone and its thrust splays from the MBT zone of the Kumaun and Nepal Himalaya (Kothyari et al., 2010;Luirei et al., 2021;Mugnier et al., 1994;Valdiya et al., 1992). Structural control on the rivers in the MBT zone is described, revealing the strikeslip movement along the MBT as the bedrocks have a high angle (>70 ) of dip (Valdiya, 1980a). ...
... The MBT is a regional tectonic boundary characterized by imbricating thrusts, where sudden elevational gain in altitude is observed across this thrust (Medlicott, 1864;Valdiya, 1984Valdiya, , 1998. Recent tectonic activities along the MBT are reported from various segments of the T A B L E 1 Generalized stratigraphic succession of the area (Shanker et al., 1993;Tiwari et al., 2013) Himalayan arc (Luirei & Bhakuni, 2008;Mugnier et al., 1994;Valdiya, 1992). ...
... Evidence of recent tectonic activity along the MBT is the thrust- Normal faulting is also reported from other segments, where the movement has truncated fan terrace producing a fault scarp almost 2.5 km-long and 37-m-high at Logar in Gaula River (Kothyari et al., 2010;Valdiya, 1992). Mugnier et al. (1994) the Kumaun Himalaya (Valdiya, 1986;Valdiya et al., 1984). Steeply tilted colluvial deposit resting over the Lesser Himalayan rocks in the hanging wall that dips towards the slope is also evidences of Recent tectonic activity along the MBT is also evident from the tilted colluvial deposits in the hanging wall that are inclined towards the slope. ...
In the Garhwal Himalaya, the steeply inclined Main Boundary Thrust (MBT) is characterized by landforms that indicate towards tectonic activity in the recent past. Linear active fault trace, sub‐recent fault scarps, strath terrace, uplifted terraces, deflected drainage, and triangular facet cones are some of the morphotectonic evidences of recent tectonic activity along the MBT. The MBT is defined by active fault trace in the form of linear depression and shows a prominent slope break across it. Thrusting of the Lesser Himalayan bedrocks over the colluvial deposits and back tilting of the colluvial deposits are also observed in the MBT zone. The formation of the sag pond and the deposition of the Quaternary lacustrine sediments followed by the deformation of the lacustrine deposits indicate phases of tectonic activities along the MBT. The later phase of deformation is represented by the folded lacustrine and colluvial deposits, where the amplitude of the fold measures about 3 m‐high and the deformed section measures about 23 m in length. The ongoing deformation patterns of the MBT zone have also been monitored with the help of Synthetic Aperture Radar‐based time series analysis. We used Sentinel‐1C dataset in ascending direction with polarization of the VV+ VH acquired between December 2, 2017 to September 7, 2021 to identify phase changes caused by ongoing active deformation. The Persistent Scatterer Interferometry (PSI) result based on Sentinel‐1A suggests that the Dehradun–Mussoorie and adjoining region has undergone an average deformation of 5.5 to −11 mm/year and the ground displacement of 1.5 to −3.8 cm. The time series analysis reveals that the Mussoorie hills in the MBT zone have higher rate of deformation.
... The slip along the MBT apparently ceased when the deformation propagated to the main frontal thrust, about 2 Myr (Mugnier et al. 2004;van der Beek et al. 2006) or 4-5 Myr (McQuarrie et al. 2019 ago. However, there have been recent movements close to the boundary between the Siwaliks and the outer LHS: normal and strike-slip faulting as well as thrust faulting (Nakata 1989;Yeats and Lillie 1991;Mugnier et al. 1994;Thiede et al. 2017). Variations in several parameters (decrease in the basal slope or increase in the topographical slope or of the pore fluid pressure) could have transformed a stable Himalayan Coulomb wedge into an over-critical one (Mugnier et al. 1994) causing internal deformation of the wedge. ...
... However, there have been recent movements close to the boundary between the Siwaliks and the outer LHS: normal and strike-slip faulting as well as thrust faulting (Nakata 1989;Yeats and Lillie 1991;Mugnier et al. 1994;Thiede et al. 2017). Variations in several parameters (decrease in the basal slope or increase in the topographical slope or of the pore fluid pressure) could have transformed a stable Himalayan Coulomb wedge into an over-critical one (Mugnier et al. 1994) causing internal deformation of the wedge. Alternatively, the MBT-parallel normal faults in the outer LHS were interpreted as extensional deformation within the hanging wall of a megathrust system following a major earthquake (Riesner et al. 2021). ...
The Himalayan range is underlain by a basal detachment which is exposed at the surface along the southern foothills, plunges northwards for at least a couple hundreds of kilometers, and extends laterally for more than 2,000 km along the orogen’s length. Because of its size and capacity to generate earthquakes up to magnitude 9, the Himalayan basal detachment is a megathrust akin to the megathrusts in subduction zones. In orogens, contractional deformation propagates from the core of the mountain belt towards its peripheral foreland. Such in-sequence deformation is manifested by the successive activation of thrust faults and shear zones from the hinterland towards the foreland (Boyer and Elliott 1982; Dahlstrom 1970). In this respect, the Himalayas are similar to other orogenic belts. The main shear zones and faults currently exposed at the surface were formed over the last 30 million years, from the oldest structure in the North, in the interior of the orogen, towards the currently active fault exposed along the southern foothills and carrying the entire orogen over the Indus–Ganges–Brahmaputra alluvial planes (see Volume 2). These structures were active at different depths under different pressure and temperature conditions, ranging from granulite metamorphic grade to near-surface conditions. Accurate interpretations of these structures, therefore, entail the knowledge of their deformation conditions. Consequently, the Himalayas offer a prime natural laboratory to investigate the development and the interaction of structures operating at different crustal levels. Of particular societal interest is the understanding of the interactions between deep aseismic movements and near-surface seismic slip-generating earthquakes.
... The union territory Jammu and Kashmir lies on the west of Main Frontal Thrust (MFT), which is 2000 km in length having convergence 2 rate of cm/yr. From north to south Main Karakoram Thrust (MKT), Main Mantle Thrust (MMT) and Main Boundary Thrust (MBT) flank the Himalayas from west to east but Main Boundary Thrust (MBT) is inactive in Jammu and Kashmir region (Mugnier et al. 1994;De and Kayal 2003;Basharat et al. 2021). The Main Boundary Thrust (MBT) underlies the Pir Panjal range and is known as the Pir Panjal Thrust in this region (Fig. 2). ...
Due to the potential seismicity of the region, transportation infrastructure projects in Jammu and Kashmir require resilient design. In the present study, structural integrity and possible seismic hazard are taken into account while modelling seismic loss of tunnelling projects in the Himalayas. For the predicted hazard and vulnerability functions, the impact of seismic exposure, tunnel lining aging, and construction quality are evaluated. The Pir Panjal tunnel is also assessed for seismic risk and structural damage in post-seismic situations while taking source-to-distance effects into account. The proposed vulnerability parameters will be helpful for monitoring, logistical operations, and post-disaster route functionality. For disaster prevention cells in any nation, decision-making and risk assessment are the two key activities that can profit from the findings of this study.
... The Siwalik formation records critical information regarding Himalaya erosional history, paleoclimate, transitions in paleobotany, and exhumation rates (e.g., Quade et al., 1989Quade et al., , 1995Acharyya, 1994;Sanyal & Sinha, 2010;Najman et al., 2017;Ghosh et al., 2018;Khan et al., 2019). In some locations, the MBT has nearby active steep faults that show normal or strike-slip senses of motion as they accommodate a critical taper (Mugnier et al., 1994;Patra & Saha, 2019). The MFT cuts Siwalik strata in places and is often manifested as growing anticlines (Yeats et al., 1992;Powers et al., 1998;Srivastava et al., 2018). ...
The Himalayan orogen exposes assemblages from low-grade Indian shelf sediments of the Tethyan Formation to eclogite and ultra-high-pressure rocks from the suture zone between the Indian craton and Asian subcontinent. Barrovian-grade pelites in the Himalayan core comprise the Greater Himalayan Crystallines and Lesser Himalayan formations. These units are separated by the Main Central Thrust (MCT), which accommodated a significant amount of convergence. We describe and apply isopleth thermobarometry and high-resolution pressure-temperature (P-T) path modeling to decipher the metamorphic history of garnet-bearing rocks collected across the MCT in central Nepal. Results are compared to with previous reports of conventional rim P-T conditions and P-T paths that used Gibb's method on the same data and assemblages. Predictions of the paths on garnet zoning are also presented for the high-resolution P-T path modeling and Gibb's method using the program TheriaG. Although the approaches yield different absolute conditions and P-T path shapes, all are consistent with the development of the MCT shear zone due to the imbrication of distinct rock packages. Greater Himalayan Crystalline garnets experienced higher grade conditions, making extracting its P-T conditions and paths challenging. Lesser Himalayan garnets appear to behave as closed systems and are ideally suited for thermodynamic approaches.
... The Himalayan collision zone hosts major active normal fault systems that strike approximately perpendicular to the trend of the orogeny [7][8][9][10][11][12][13][14][15][16][17]. Earthquake moment tensor analysis indicates the existence of normal faults and other extensional features in different parts of the Himalaya [6,[18][19][20][21][22], and the existence of these extensional structures within the greater compressional regime has led to the complex distribution of stress and strain which controls the resulting active deformations of the MHT [23]. This variation in the mechanism of deformation over the MHT at different depth levels resulted in bimodal focal depth distribution of microseismicity along the Himalayan arc [24,25]. ...
The optimum 1D velocity model is calculated for the Kinnaur sector of the NW Himalaya utilizing the arrival time information of the local earthquakes (137 no.) recorded with 12 broadband seismic network within the azimuthal gap of ≤180°. This optimum 1D velocity model is a five-layer model and ranges from the surface to 90 km in the shallow mantle. P velocity varies from 5.5 km/s to 8.6 km/s in the crust and upper mantle, and S-wave velocity varies between 3.2 km/s and 4.9 km/s for the same range. When we relocated the earthquakes with the Joint Hypocenter Determination program incorporating the optimum 1D velocity model, it resulted in a lower RMS residual error of 0.23 s for the hypocenter locations compared to initial hypo71 locations. A total of 1274 P and 1272 S arrival times were utilized to compute station delays. We observed positive variations in P-station delays from -0.19 s below the PULG station to 0.11 s below the SRHN station. Similarly, for S-station delays, we observed negative delays at each individual site from -0.65 s at LOSR station to -0.16 s at the SRHN station. This large variation in P- and S-station delays corresponds to the 3D nature of the subsurface below the Kinnaur Himalaya. The relocated seismicity is clustered along the STD fault at sub-Moho and Moho depths ranging between 40 km and 80 km. The seismicity distribution aligned across the strike of the STD and along the strike of the Kaurik-Chango fault (KCF) can be attributed to the cross-fault interactions of the KCF and the STD fault in the area. We also observed bimodal depth distribution of seismicity in the Higher and Tethys Himalayas. The occurrence of earthquakes down to a depth of ~0-40 km and 50-80 km in the study area can be interpreted in terms of stress contribution from interseismic stress loading associated with the India-Eurasia collision tectonics. The presence of hypocentres in the shallow mantle at 120 km depth highlights the strength of the mantle, which seems to be deforming in a brittle manner below the region. The computed focal mechanisms exhibit generally the flexing of the Indian plate below the Lesser Himalaya with shear parallel to the strike of the MCT and extension orthogonal to it. This study shows deformation over the entire crust and shallow upper mantle levels, with differential stress conditions. Thus, we can consider the crust and the shallow upper mantle down to depths of 120 km to be seismogenic in nature and is capable of producing the microseismicity beneath the Kinnaur Himalaya.
... The ILH is considered the proximal part of the Indian passive margin, whereas more distal part is represented by the Neoproterozoic to the early Cambrian weakly metamorphosed quartzites, limestones and turbidites constituting the Outer Lesser Himalaya (Ahmad et al., 2000;Brookfield, 1993;Célérier et al., 2009;Colleps et al., 2018;Hughes et al., 2005;Jiang et al., 2007;Oliver et al., 1995;Richards et al., 2005). The Outer Lesser Himalaya (OLH) is thrust southwards along the Main Boundary Thrust (MBT) over shallow marine to continental sedimentary series of the Subathu-Kangra foreland basin, part of the so-called Sub-Himalaya, (Huyghe et al., 2001;Mugnier et al., 1994;Valdiya, 1980). The southern boundary of the Sub-Himalaya is delimited by the youngest major Himalayan thrust, the Main Frontal Thrust (MFT) that places the Sub-Himalaya over the recent river deposits of the Gangetic Plains (Mukhopadhyay and Mishra, 2004;Schelling and Arita, 1991;Srivastava and Mitra, 1994) The Subathu-Kangra basin is the focus of our study and is described in more details below. ...
Foreland basins associated with collisional mountain belts undergo postdepositional deformation in response to compressive stress exerted from the evolving orogen. The low thermal regime of such processes requires sensitive methods to unravel the structural evolution of such regions. In this study, we conducted apatite fission track (AFT) and (UTh)/He thermochronology on the sedimentary succession of the Subathu basin in the Himalayan foreland of the Himachal Pradesh region in India, an area preserving the sedimentary record of the India-Asia collision since the late Paleocene-early Eocene. The results show that the Subathu basin exposes a fossilised apatite partial annealing zone (APAZ) depicted by incompletely reset AFT ages and shortened track lengths in the Paleocene-middle Miocene pre-Siwalik units and unreset ages in the Neogene Siwalik Group. We associate the partial annealing of the pre-Siwalik rocks with thickening due to internal imbrication and nappe stacking triggered by shearing along the Main Boundary Thrust and its splays since the middle Miocene. Thermal modelling of the Siwalik rocks projects a rapid heating up to approximately 90–100 °C reached between 4 and 1 Ma. Following the thermal peak, the Siwalik rocks cooled rapidly, as indicated by 2.6–1.2 Ma AHe ages. We associate this recent cooling event with SW-directed thrusting along the Main Frontal Thrust, which placed the foreland basin over the Gangetic Plains. Our results are similar to those obtained for the foreland basin in Nepal, indicating that the movement along the youngest major discontinuity of the Himalaya is synchronous along the orogen.
The evolution of Quaternary landforms in the Champawat area of outer Kumaun Lesser Himalaya is reconstructed from the paleolake sections and the geometry of the landscape. Conjugate sets of normal faults are observed in the bedrocks and Quaternary deposits at Banlek. The most prominent normal faults are those striking NE-SW and NW-SE and most of them are high dipping faults while some are vertical faults. Sediments akin to lacustrine deposits are observed at four sites; while the contact between the bedrock and the lake sediments is observed at only one site that too in the peripheral sides. The exposed sediments thicknesses vary from 8.30 m to 4.8 m and consist of an alternation of black carbonaceous mud and sandy horizons indicating different depositional regimes. The total thickness of the lake appears to be more than 75 m as deduced from elevational differences between the exposed sites. From Site 1 two OSL samples were analyzed collected from 2.5 m and 8 m from the surface and give OSL age of 16 ka and 17 ka, respectively; while a sample collected from Site 4 gives an OSL age of 13 ka. A total of 3653 lineaments were mapped encompassing 2,277.8 sq km from the eastern Kumaun Himalaya and maximum of the lineaments trend ENE-WSW or almost E-W (∼ 46.5%), the major principal stress is assumed to be in NE-SW direction. Lake, mature topography and gentle stream gradient in the Champawat area are the result of highly weathered bedrocks and normal faults.
The fault propagation sequence in an accretionary prism is studied by the use of a numerical model. The marginal stability state of such a wedge is controlled by geometrical parameters and mechanical parameters. For each shortening increment applied on the back-stop of the model, it is determined which part of the wedge has to be thickened by displacement along ramps. Several numerical experiments are realized to study the effects of geometry and scale, for mechanical parameters ranging around a reference example. Very external structures are formed at the beginning of the deformation if the wedge slides easily on its basal decollement, and a backward propagation sequence occurs to tectonically thicken the accretionary prism and fill the piggy-back basin. When the decollement shows lateral variation in dip, the prism may show a forward propagation sequence where it is thin, and a backward sequence where it is thicker. Transfer faults develop to accomodate these lateral variations in thrust sequences. When the prism reaches a steady state, 1) new imbricates are accreted at the front, 2) out-off sequence reactivations occur backward. -from English summary
Short-cut geometries occur during structural inversions when a normal fault is created at the hanging-wall of a thrust reactivated in its deeper part and abandoned in its shallower part, or when a thrust fault is created at the footwall of a partly reactivated normal fault. The isotropic and anisotropic Mohr Coulomb Anderson theory is used here to determine the potential of any part of a fault to be reactivated by calculating the difference between the deviator stress necessary to induce reactivation and the deviator stress necessary to induce fracturing. -from Authors
Active faults in the Nepal Himalayas are identified by means of interpretation of vertical aerial photographs. They are mainly distributed along the major tectonic lines as older geological faults and are classified into four groups, the Main Central Active Fault system, the active faults in the Lower Himalayas, the Main Boundary Active Fault system and active faults along the Himalayan Front Fault. The mode of active faulting is closely related to the strikes of the faults. Along the NW-SE and NE-SW trending faults, lateral displacement with northward drop is prevailing, and right-lateral movement along the former and left-lateral movement along the latter is a rule in the sense of displacements. On the other hand, dip-slip faulting is observed mainly along the E-W trending faults belonging to the Main Boundary Active Fault system. However, apparent displacement along the faults is mostly of northward drop. It is considered that active faulting along the major tectonic lines except the Himalayan Front Fault does not favor the upheaval of the Himalayan ranges during the late Quaternary period.
The data presented suggest that uplift of the Outer Himalaya in this region is continuing even today through sudden co-seismic elevation changes during large thrust earthquakes and secular aseismic uplift during intervals between such earthquakes. The observed co-seismic ground elevation changes during the Kangra earthquake are interpreted so as to simulate the cross-sectional shape of the buried active thrust fault responsible for this continuing episodic as well as secular uplift of the Outer Himalaya. This fault is assumed to be the surface of detachment between the Himalayan rocks above and the Indian shield rocks below. It is concluded that over most of its extent in Dehra Dun region, the detachment surface has gentle dip to the northeast but a few interspersed, northeast dipping steeper ramps are not ruled out. The depth of the detachment is estimated to be between 0 and 3 km beneath the SW limit and about 10 km beneath the NE limit of the Outer Himalaya in the Dehra Dun region. -from Authors
Active faults and their relevant tectonic features are discussed in order to clarify the neotectonics of the Himalayan collision. Active faults have been discovered by means of interpretation of topographic maps and aerial photographs, as well as by field studies. The frontal zone of the Himalaya is most active at present. Active faulting has been taking place along the Main Boundary Fault (MBF), the Himalayan Front Fault (HFF), and their associated faults. These active faults generally form a north-dipping imbricated thrust zone. Consistent uplift of the Lesser Himalaya has continued along these faults during late Quaternary time. Cumulative uplift by active faulting is estimated to be as much as 1,500 m at an average rate of 3 to 4 m/1,000 yr during the last 400,000 to 500,000 yr. However, regional disparity in the sense of vertical displacement is seen in the Nepal Himalaya, where apparent slip along the active fault traces on the MBF is down to the north, commonly manifested as north-facing reverse scarplets on pressure ridges. Strike-slip displacements are also observed along several discontinuous faults trending northeast and northwest on the HFF. In the central Bhutan Himalaya, northward downthrow has accumulated to form an uphill-facing fault escarpment along the HFF. Active faults within the Himalayan Range mainly reactivate major faults such as the Main Central Thrust (MCT) and the Ban Gad fault. They extend northwest-southeast, oblique to the Himalayan Front, and are characterized by right-lateral displacement with northward downthrow. No active north-over-south thrusting is recognized along sinuous traces of the MCT in the Nepal Himalaya. Under the present stress field, the mode of active faulting is closely related to fault strike. Along northwest-southeast- and northeast-southwest-trending faults, lateral displacement with northward drop prevails, and right-lateral slip along the former faults and left-lateral slip along the latter is the rule. Conversely, dip-slip faulting is observed mainly along the east-west-trending faults on the MBF and HFF. Active faulting does not contribute to the uplift of the Himalaya except along the Himalayan Front. The active faults in the Himalayan Range are arranged right-stepping echelon; they extend to form a large right-lateral fault system with the Karakorum fault, which is conjugate with the Altyn Tagh Fault. Active faulting along these faults causes eastward drift, resulting in normal faulting in the eastern Himalaya and southern part of the Tibetan Plateau.