This network of faults show that the Himalayan deformation reaches further [about 40 kilometres further south] than we previously thought. A fault is a fracture or zone of fractures between two blocks of rock. The biggest faults mark the boundary between two plates. Strike-slip faults are usually vertical, while normal and reverse faults are often at an angle to the surface of the Earth. Sources: the Hindu. Previously unknown faults at the foot of the Himalaya discovered Context : An oil and gas exploration company has helped geologists discover a series of faults at the foot of the Himalaya.
This fault system lies in the southeastern region of Nepal and has the potential to cause earthquakes in the densely populated country. National Science Foundation to rush to Nepal the dozen seismometers that Stanford owns to become part of a network of 45 seismometers in the ground.
The deployment was complicated by the difficulty of traveling in high-altitude regions with poor infrastructure damaged further by ongoing earthquake and monsoon-induced landslides. Speed was essential because the rate of occurrence of aftershocks rapidly decreases after the main earthquake, and the existing network of aftershock-measuring devices, known as seismic stations, was very limited. Without data on the aftershocks, including their locations and magnitudes, it would not have been possible to develop this more detailed understanding of the fault, according to Klemperer.
The Stanford seismometers were kept in place for one year after the main Gorkha earthquake, and were visited twice in that period to download data and service the instruments. The detailed locations of over 8, small earthquakes recorded by the temporary array of devices allowed researchers to recognize that what had previously been thought of as a single earthquake fault actually consists of multiple earthquake faults.
Now the researchers want to find out whether the same duplex structure that controlled the location and rupture of the April magnitude 7. This story was adapted from a press release issued by UC Riverside. Stanford geophysicist Simon Klemperer discusses how the Gorkha earthquake that shook Kathmandu in central Nepal gave researchers new information about where, why and how earthquakes occur.
These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges such as the Himalayas and the Rocky Mountains. The Himalayan mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate which began 50 million years ago and continues today. Other names: thrust fault, reverse-slip fault or compressional fault]. Examples: Rocky Mountains, Himalayas.
In a strike-slip fault, the movement of blocks along a fault is horizontal. These faults penetrate from the basement up through the Quaternary Upper Siwalik units. They suggest that this thrust fault may be the early stages of the formation of a range-front salient. The configuration of cross faults bounding a thrust segment is consistent with the idea that cross faults may play a role in limiting lateral thrust propagation. Based on the seismic imaging data, Duvall et al.
Figure 2. Simplified geologic map of eastern Nepal, India, and western Bhutan. The red dashed lines are the approximate locations of the Benkar, Kosi, Gish, and Dhubri-Chungthang cross faults. The solid yellow lines are the bounds of the Benkar fault zone that have been mapped in the Greater Himalayan Sequence Seifert, Sikkim M w 6.
Map compiled by Bibek Giri]. The availability of high-resolution topographic data and satellite imagery visually illuminates the lateral heterogeneities in the southern mountain front of the Himalaya.
The dun valleys of the Sub-Himalayan zone are enclosed valleys often bound by fault-related folds of the Siwalik Group. These valleys also mark diachronous and contrasting geomorphic expressions of deformation along the range Kimura, In the northwestern Indian Himalaya, there have been several efforts to quantify amounts of shortening and shortening rates in the Sub-Himalayan zone Powers et al. Dubey et al. They conclude that this variability may actually be a result of different approaches to cross section balancing.
Dubey conducted analog modeling of an oblique ramp in a convergent setting and makes a connection to the geometry of reentrants in the northwestern Indian Himalaya. In the region of the Indian Himalaya where the Yamuna and Ganga rivers emerge, the range front is visibly offset. Sahoo et al. Further mapping in that area, which overlies the Delhi-Hardwar basement ridge, has confirmed the presence of cross faults at the recess boundaries Sahoo et al.
Geologic mapping in deformed molasse and foreland sediment, known as the Siwaliks, of western Nepal demonstrated the need for an orthogonal transfer zone where the structural nature of the range front thrust systems changed abruptly along strike Mugnier et al.
This transfer zone marks the western boundary of several dun structures in the Sub-Himalayan zone and has been referred to as the West Dang transfer zone, a possible east-dipping lateral ramp Mugnier et al. In Central Nepal, in the region of the Chitwan Dun, the range front topography is also irregular.
Divyadarshini and Singh mapped six strands of the MFT at this site, some of which remain distinct for short distances, and others of which merge with each other. Farther east, in eastern Nepal and Sikkim, two other cross faults have been recognized at salient-recess boundaries along the range front Figure 2 , the Kosi fault in eastern Nepal along the Indian border, and the Gish fault in Sikkim Mukul, ; Srivastava et al.
The Kosi fault was recently identified by Mukul et al. This structure aligns with the western boundary of the Munger-Saharsa basement ridge structure in the foreland.
Though it has not yet been mapped to the north, its projection may align with the Pumqu Xianza rift of the hinterland Yin and Taylor, Microseismicity patterns in the area outline a concentration of events that follows the strike of the Kosi fault suggesting that the structure is active Pandey et al. The strike of the fault also aligns with the western edge of the Taplejung window Figure 2 that offsets the MCT in an apparent sinistral sense Upreti et al. Following its recognition, continued research has included geomorphic, structural, and geodetic analysis of the region along, and adjacent to this fault Srivastava et al.
The structural and geomorphic research has focused on the range front of the Himalaya and has documented very different structural styles on either side of the Gish fault, with the Ramgarh Thrust structurally between the MCT and the MBT marking the range front in the Gorubathan recess Matin and Mukul, and a series of blind thrusts toward the foreland, whereas the Dharan salient has multiple exposed thrusts south of the Ramgarh Thrust.
The Ramgarh Thrust is displaced in a sinistral sense across the Gish fault. Deformation style of the Munsiari thrust sheet structurally between the Ramgarh Thrust and the MCT differs across the Gish fault and has fold features that are affected by the Gish fault Matin and Mukul, Mukul has traced this fault across the MCT, though the structure has not been mapped in detail in the area of the MCT or further north.
The Gish fault aligns with the Kishanganj fault on the eastern edge of the Munger-Saharsa ridge of the Indian basement Figure 1. This region has had a number of strike-slip seismic events Ni and Barazangi, ; Paul et al. Along-strike variations have been recognized in the Lesser Himalayan zone from topographic data, seismic data, and from cooling history data, suggesting segmentation in tectonic processes Harvey et al.
Hodges et al. PT2 defined by these authors, is an elevation transition from the highest peaks of the Himalaya, typically consisting of the Greater Himalayan Sequence units, to the region of lower elevations to the south in the Lesser Himalaya. In western Nepal, between the longitudes of They interpret the PT2S south and the PT2N north as locations that transition to areas of faster rock uplift and they further document these transitions with the locations of knick points in river channels.
Harvey et al. They, too, concluded that there is a likely structural change in the ramp geometry of the MHT, possibly involving a lateral ramp crossing the strike of the range.
There is further evidence for lateral changes in the MHT in western Nepal that comes from the differences in peak metamorphic temperatures obtained by the Karnali and Jajarkot klippen as well as the timing of metamorphism Soucy La Roche and Godin, Soucy La Roche and Godin interpret these T-t differences to represent a difference in the depth to the MHT of about 13 km and suggest that this exhumation difference reflects segmentation going back to at least the Oligocene.
They further interpret re-activation of the Lucknow fault on the west side of the Faizabad ridge to have created a tear fault in the overlying units, thus offsetting the MHT. The Lesser Himalayan zone is typically characterized along the length of the range as a duplex structure of metasedimentary units from the Kumaon region of India, across Nepal and Sikkim, and into Bhutan Srivastava and Mitra, ; DeCelles et al.
Structural cross sections from each of these regions reveal significant variations in duplex geometry e. While some of these variations may be differences in interpretation, there are clear differences in thickness of the duplex systems, fold geometry, numbers of horses, and shortening estimates in each section Hauck et al.
Some of these transitions in style may be gradual, but it has also been noted that duplex or underplating geometry can change across cross faults, tear faults, oblique or lateral ramps Dubey et al.
Much of the discussion in the literature pertaining to lateral variations in the Greater Himalaya has focused on differences in cooling histories, exhumation rates, and topographic profiles, in some cases extrapolated from the Lesser Himalaya e. Further discussions have also included variations in the role of climate on erosion and the differences in the presence or absence of discontinuities within the Greater Himalayan zone Carosi et al. Perhaps one of the most obvious examples of lateral variation is in the Greater Himalayan klippen and Lesser Himalayan windows as seen in map pattern Figure 1 ; Gansser, ; Searle et al.
These map pattern variations are largely attributed to variations in the geometries of the Lesser Himalayan duplexing and variations in the presence, absence, or position of a ramp in the MHT Hodges, ; Robinson et al.
In the northwest Himalaya, there are a number of changes that occur in Greater Himalayan cooling ages and inferred exhumation rates between the Sutlej River valley and the Zanskar region Eugster et al. Eugster et al. Their two southeasternmost transects show younger ages in the Greater Himalayan section and therefore suggest a more recent and more rapid exhumation than the Dhauladhar section to the northwest.
They also note that there is a change in the topography toward the northwest with an elimination of the PT2 topographic change. A number of factors have been suggested as causes for changes in exhumation history in the northwestern Himalaya such as an increase in the obliquity of convergence Thakur et al.
The changes in obliquity of convergence or rainfall would likely produce broad areas of change whereas changes in sediment thickness or basement fault reactivation could create the more abrupt changes in deformation style, topography, or exhumation that are observed Eugster et al. Thakur et al. This ramp has been referred to as the Mandi ramp Yin, and it coincides with the Ropar-Manali lineament described in Thakur et al.
Low temperature thermochronology coupled with kinematic modeling in central Nepal supports the presence of a ramp in the MHT as is imaged in seismic data Robert et al. Robert et al. These authors also suggest that the topographic differences that result from variations in MHT geometry further impact the location of higher precipitation and therefore higher erosion rates.
Thermochronology data and kinematic modeling of an east and west transect in Bhutan Coutand et al. Topographic variations all along the range have also been characterized by river channel steepness k sn in the Greater Himalaya with high values of k sn relating to strain accumulation Cannon et al. The variability of k sn displays a segmentation along the length of the range. Cannon et al.
In eastern Nepal, a fault was recently recognized in the Greater Himalaya that could be related to the segmentation process. The Benkar fault zone was first recognized in the Dudh Kosi valley north of the village of Lukla Hubbard et al.
The deformation is brittle-ductile with much of the slip having occurred on sillimanite-rich layers. Kinematics are fairly consistently right-lateral, normal on a SE-dipping plane. Mapping has not been completed to the north of the Everest basecamp area or to the south of Lukla.
To the south there is topographic evidence for a continuation of the Benkar fault down to the Gangetic plain. This topographic feature aligns with the Motihari-Everest transverse fault suggested by lineament mapping from Satellite images Dasgupta et al.
Timing of displacement along the Benkar fault is unknown, though it is younger than the leucogranites. Figure 3. Photograph showing strand of Benkar Fault zone. The view is looking north at the west ridge of Taboche see location in Figure 4 , a peak in the mapped portion of the Benkar Fault zone, in the Khumbu region of Nepal. Red arrows show a zone of shearing to the right east of the leucogranite exposure.
This zone has apparent normal displacement within the NE-striking, Benkar Fault zone that has overall dextral, normal sense of shear photo by Mary Hubbard. Figure 4. Simplified geologic map of the northern Benkar fault zone. The dotted lines outline the region of non-penetrative, NE-striking shear fabric of the Benkar Fault zone. Kinematics on this shearing are dextral, normal. Historic earthquake data shows the episodic and spatially restricted nature of major thrust fault rupture along the length of the Himalaya Ambraseys and Douglas, ; Bilham, , Wesnousky et al.
Le Roux-Mallouf et al. The spatial arrangement of rupture events supports the possibility of segment boundaries that may limit the lateral rupture propagation Hubbard et al.
Aftershock data from the Gorkha earthquake Karplus et al. Imaging the third dimension of this data on the western side also led Mendoza et al. This structural transition to a duplex geometry also occurs along a NE trend. Seismic activity also shows strong lateral variations with a highly active eastern section and low activity to the west of the rupture area. In several areas of the Himalaya, microseismic events align along linear, NE-striking zones Rajaure et al.
Hoste-Colomer et al. Figure 5. Map of aftershock locations from the Nepal earthquakes. Epicenters of the two major earthquakes are shown with stars. Colored dots represent the locations of aftershocks.
The solid red lines denote the bounds of the mapped portion of the Benkar Fault Seifert, and the dotted red lines are the projected traces from satellite imagery of the Benkar Fault and the Gaurishankar lineament Dasgupta et al.
Note the abrupt termination of the zone of aftershocks on the east side that is sub-parallel to both the Gaurishankar lineament and the Benkar Fault. In , Sikkim experienced an Mw 6. Using a moment tensor inversion technique, Paul et al. Aftershocks occurred to the SE of the main shock at depths from 12 to 50 km. These results suggest that much of the deformation was occurring within the subducting Indian plate beneath the MHT.
There is evidence from additional earthquake data that the Sikkim event occurred on the Dhubri-Chungthang fault zone that continues southeastward to the western edge of the Shillong Plateau Diehl et al.
In recent years, abundant geodetic data along the Himalaya has led to studies of interseismic coupling Ader et al. Ader et al. Marechal et al. In addition, protracted dynamic triggering in the Central Himalaya indicates that slow slip may play a role in interseismic deformation, stress loading and its lateral variation Mendoza et al. Dal Zilio et al. These segment boundaries also bound the regions of large earthquake rupture in the last millennium, though Mugnier et al.
Geologic and geophysical data collected over the past century clearly shows that while much of the Himalaya can be characterized by a continuous series of range-parallel thrust faults, there are also important lateral variations in the architecture of the range.
A number of these variations can be tied to specific transverse or cross structures, leading to segmentation of the range. Important questions that come from this recognition of segmentation include the more academic question of what has caused the segmentation and the more applied question of how does the segmentation impact seismicity in terms of fault rupture area and size of earthquake events.
We recognize these questions may not be mutually exclusive and the true answers to these questions will require continued data collection, both in the field and in the laboratory, and from multiple disciplines across different space- and time-scales.
To understand possible causes of segmentation, it is useful to look at other collisional mountain belts that also display features of segmentation including cross structures.
In some cases, the cross structures are identified as tear faults or lateral ramps Appalachians and the Papuan Fold and Thrust belt and in other cases there are transverse extensional structures Alps. In the Himalayan example, one of the primary explanations for segmentation has been variation in the geometry of the MHT and possibly related variations in duplex geometry in the Lesser Himalaya e. Geophysical data from the Gorkha earthquake in Nepal supports these explanations.
Mugnier and Huyghe note the role of lateral variations in sediment thickness of the Ganga basin in partitioning the basin. These lateral variations are largely controlled by basement structures on the Indian craton. Based on similar orientations and adjacent locations of Indian basement faults and transverse faults in the Lesser Himalaya, Valdiya proposed that basement faults influenced the development and position of cross faults in the Himalaya.
Godin et al. It remains unclear whether structures that are more than 50 km below the Tibetan upper crust could have influenced the location of the graben structures we see today. Future mapping of cross-faults withing the Himalaya may ultimately help us to understand these possible connections. The NE-striking basement structures are imaged in the foreland on seismic profiles Duvall et al. Earthquake data from events within the Himalaya show strike-slip kinematics and occur down to depths of 50—60 km supporting the idea that the Indian plate basement faults continue to be active under the major Himalayan thrust faults Paul et al.
This earthquake data implies that at least some of the cross faults or segment boundaries in the Himalaya are tied to the basement structures and are not just tear faults in the hanging wall of the thrusts.
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