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In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust. [ABSTRACT FROM AUTHOR]

The article discusses a study which examined the causes of the formation of normal faults at the Leh basin in the Northwest Himalaya. Topics covered results which showed that gravity tectonics is the cause of normal faults in the basin, the difference between the normal faults mapped within the intermontane basins with those that are tectonically driven, and the distribution of lithology, location of major Himalayan faults and the major fluvial streams.

[Display omitted] • Surin-Mastgarh Anticline (SMA) is a Polyharmonic Anticlinorium with Flexural slip. • SMA has suffered both N-S and E-W compressional tectonic stress. • It is a Type-II superimposed folding as per Ramsay's classification. • A Blind Thrust is sandwiched between the SMA and Medlicott-Wadia Thrust. • Duplex-Thrust Complex lying on MHT are potential Hydrocarbon reservoir. The Surin-Mastgarh Anticline (SMA) at the frontal Siwalik range of the NW Himalaya possesses great interest for the evolutionary processes and hydrocarbon prospect. Most of the previous models have been proposed from geological data, which alone cannot picturise the SMA in the subsurface. This work aims to delineate subsurface characteristics, geometry of SMA, and stratigraphic divisions from seismic imaging along and across the eastern flank of SMA to shed light on inter-tectonics with the surrounding Main Himalayan Thrust (MHT) and Medlicott-Wadia Thrust (MWT), and evaluation of hydrocarbon resources. The seismic attributes including interval velocity have been computed, and seismic amplitudes have been traced for better comprehension of the objectives. It is inferred that the SMA is a polyharmonic anticlinorium with flexural slip consisting of stratigraphic layers: Lower Siwalik, Middle Siwalik and Upper Siwalik. It shows that during the formation of anticlinorium, flexural flow occurred within the upper and lower layers of the anticline. A blind thrust overran the Northern Limb of SMA, which was later overrun by the MWT. The MHT has no direct relation with the SMA, and is terminated in the south. It is also elucidated that this anticline experienced both major N-S compressional and minor E-W neo-tectonic stress regimes, which led to Type-II superimposed folding as per Ramsay's classification. The presence of Late Proterozoic Limestone and/or marine Pre-Siwalik Tertiary formations, which are organic-rich, below the MHT could generate hydrocarbons. This can be trapped by the overlying Duplex-Thrust Complex, and thus providing hydrocarbon prospects in this study area. [ABSTRACT FROM AUTHOR]

This study presents hazard estimation of Kashmir Basin, NW Himalaya using regional ground motion relations, representing one of the most seismically active region in the Himalayan belt. Fault-level seismic recurrence parameters are determined from an updated earthquake catalogue spanning from 25 to 2018 AD for all possible seismic sources. The estimated hazard maps are presented for three ground motion parameters (PGA, short and long period spectral acceleration) for 50, 100, 500 and 2500 years return periods. Moreover, uniform hazard response spectrums and hazard curves are presented for all ten districts of the basin. The southern section of the basin consisting districts of Budgam, Shopian, Pulwama and Kulgam show higher hazard levels due to presence of numerous seismogenic structures in close vicinity. Our results highlight that the imposed seismic hazard in Kashmir basin is highly underestimated which need to be redressed by modifying the current provisional design standards. [ABSTRACT FROM AUTHOR]

The Himalayan foreland basin is characterised by three principal sedimentary successions. These are the Subathu Group (Palaeocene-Eocene), the Murree or Dharamsala Group (Oligocene-Miocene) and the Siwalik Group (Miocene-Pleistocene). The basin was linked in the east to the Bay of Bengal through the Bengal Basin and its western part was connected to the Arabian Sea through Rajasthan and Kachchh, Gujarat. The Himalayan foothills have long been known for yielding rich mammalian fossil occurrences and significant localities in the Indian Siwaliks include Kalagarh, Ramnagar, Haritalyangar, Chandigarh and Jammu regions. The Bugti Basin and Zinda Pir area of Sulaiman Range of Pakistan were also part of the foreland basin, as Lower and Middle Siwalik fossils are known from the Vihowa and Litra formations, respectively. The Marwat Formation of Bhittani and Marwat Ranges, Pakistan have also yielded Pinjor-age vertebrate fossils. The Middle and Upper Siwalik-age fossils are also known from the Irrawaddy Formation of Myanmar, where older Pegu beds have also yielded scanty Lower Siwalik-age fossils. This faunal succession in the Irrawaddy Valley may have geologically and biogeographically belonged to the Himalayan foreland basin in the past. Additional mammalian fossils having an affinity with the Lower and Middle Siwaliks are known from the Khari Nadi Formation of Kachchh. The Bokabil Formation of Tripura also yielded scanty Lower to Middle Siwalik fossils. A younger Siwalik late Miocene assemblage is known from Piram Island of Gujarat. These areas are also part of the foreland basin. Most recently, a late Miocene suid species was discovered and reported from near Baripada, Odisha. The paper discusses the respective faunal lists of the various successions mentioned above. Finally, the paper discusses the implications of the foreland faunas. The faunas of Lower and Middle Siwaliks of foreland basin have links with Europe, Turkey, Africa and Central Asia and throw light on the uplift of the Himalaya. The post-Siwalik faunas of central and Peninsular India and Indo-Gangetic Plains have also yielded remnant Siwalik taxa, including Stegodon insignis, Elephas hysudricus and Potamochoerus theobaldi. The Kurnool Cave Fauna and Recent Fauna of India have also generic resemblance with the foreland faunas. [ABSTRACT FROM AUTHOR]

The central Himalaya has attracted much attention due to its potential to host a great earthquake – an inference based on the history of earthquakes and the geodetic slip estimates. The largest and earliest historically documented earthquake in the central Himalaya occurred in 1803 (M ∼ 7.7) near Uttarkashi on its western part, much before the great event of 1934 (Mw 8.2, located farther to the east) - both of which were felt severely in the Gangetic Plain south of the mountain front. The seismotectonic model by Seeber and Armbruster (1981) predicts that large/great earthquakes originate on the Main Himalayan Thrust (MHT) and their ruptures propagate southward to the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8) Gorkha (Nepal) earthquake, however, was southeast-directed with a southwest-directed slip, and its Mw 7.3 aftershock on May 12 also showed a similar slip direction. Post-2015, we revisited the 1991 Uttarkashi (Mw 6.7) and the 1999 Chamoli (Mw 6.5) earthquakes in India, both of which had originated on the MHT flat, but show SSW-directed slips, as reported in this study. Ruptures of both these earthquakes fell short of reaching the MFT, just as the historically documented 1803 earthquake, but their shaking effects were felt in the southern plains. These observations compel us to conclude that the varying rupture modes exhibited by the Himalayan earthquakes are controlled primarily by the geometry of the flat-ramp-flat system. [ABSTRACT FROM AUTHOR]

A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side. Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts. [ABSTRACT FROM AUTHOR]

The region to the east of Palghat gap is of low elevation and nearly circular in shape. It forms a part of the Kaveri river basin. The predominantly gneissic terrain is surrounded by the charnockitic hill ranges, prominent among which are Nilgiris and Biligirirangan to the north; and Anaimalai and Kodaikanal to the south. The charnockite massifs have a steep slope facing the circular feature and a gentler slope in the opposite direction. Fractures/faults/shear zones are noticed in many parts. The Bouguer anomaly in the gneissic terrain is elliptical in shape and positive, relative to the surrounding elevated region. The magnetic contours are also elliptical and the magnetic basement is deeper by about one km compared to regions in the periphery. The shallow seismic velocity picture from Chennimalai to Palani indicates a graben structure. The velocity structure also depicts a 4-5 km Moho upwarp near Chennimalai. Junction between the gneissic and charnockitic terrain and even beyond, is marked by the presence of pseudotachylites and breccia. Field and petrographic studies indicate presence of suevite, Planar Deformation Features (PDF), Planar Fractures, diaplectic glass of quartz and plagioclase and spherical inclusion in suevite. These evidences taken together point to an extra-terrestrial impact which created a crater of approximately 120 km in diameter. Several lines of indirect evidences point to Neoproterozoic age for the impact. Kaveri Crater is the fourth largest crater on the surface of the Earth; other larger craters being Vredefort (160 km), Chicxulub (150 km) and Sudbury (130 km). [ABSTRACT FROM AUTHOR]

Spread over countries including Pakistan, India, Nepal, Bhutan, and China, the Himalayan mountain chain, the most spectacular result of the Indo-Eurasian plate collision, is a locus of destructive earthquakes. Past earthquakes from this region have impacted vast swathes of frontal Himalaya and stretches of alluvial plains south of the range front. Risk from future earthquakes has increased, considering the burgeoning population and an ever-expanding built-environment in the region. While considerable ambiguities exist on the locations, ruptures, and sizes of the earthquakes during the first half of the last millennium (1255, 1344, and 1505 CE), those during the latter half (1803 and 1833 CE) are quite well-documented, all reported from the central Himalayan segment comprising eastern Nepal, Kumaun, and Garhwal. While dormancy prevailed in the central segment in the intervening period, the Himalayan arc elsewhere witnessed three large/great earthquakes in the last century, namely, 1905 Kangra (Mw 7.8), 1934 Bihar-Nepal (Mw 8.2), and 1950 Upper Assam (Mw 8.6), the last one being the largest intra-continental earthquake in the recorded history. The April 25, Gorkha (Nepal) earthquake (Mw 7.8) located in the central seismic gap terminated the period of low-level seismic productivity that followed the 1950 event. The Himalayan arc and its contiguous regions are now being investigated using various tools including paleoseismology. Quite understandably, the interpretations and tectonic models do not always converge, given the diverse quality and type of data. The data from variously sized modern-day earthquakes sourced in the diverse structural settings of the Himalaya provide better constraints on the source properties of the Himalayan earthquakes. The source complexity and diversity is a striking point of interest for the post-1950 events, and at least some of them seem to deviate from the generally accepted model of great ruptures on a shallow dipping detachment. A prime example of such anomalies is the 2005 Kashmir earthquake (Mw 7.6) sourced on an out-of-sequence thrust. The sources of the 1991 Uttarkashi (Mw 6.8), 1999 Chamoli (Mw 6.6), and the 2015 Gorkha (Nepal) (Mw 7.8) earthquakes - all in the central Himalayan segment - are attributed to the ramp-flat on the down-dip part of the Main Himalayan Thrust (MHT). In contrast, the 2011 Sikkim (Mw 6.9) and the 2016 Imphal (Mw 6.7) earthquakes are intraplate events sourced within the subducting slab. Source models of modern-day earthquakes (1991–2016) discussed in this paper bring out this plurality of mechanisms expected of a complex subduction-cum-collisional boundary. This review presents the status of current seismotectonic understanding of the Himalaya from the analyses of significant earthquakes, from the past as well as recent events with a focus on the central Himalayan segment. [ABSTRACT FROM AUTHOR]

Terraces in the Himalayas are important geomorphic archives which preserve the signature of tectonics and past environment. Terraces in eastern part of the Himalayan fold and thrust belt have not received much attention of the geologists. A geochemical approach using luminescence dating has been applied to understand the evolution of paired and deformed terraces between major thrust boundaries of the eastern Himalayas, on the either banks of the Tista River in Darjeeling-Sikkim-Tibet wedge. These terraces are located at the confluence of Tum Thang khola and the Tista River. Three levels of terraces are present in general and also in the study area. The terrace T3 was formed during last interglacial period and the T2 terrace during last glacial maximum (LGM) and in the humid phases after LGM. The top section of T2 terrace (∼2.5 m thick) was formed in the transition phase (arid to humid) after Younger Dryas event. The region has experienced several deformational events, (i) one after 45 ka which raised the T3 terrace to its present level, (ii) another one after 11.9 ka which raised the T2 to its present level and this event is also associated with the shifting of the Tum Thang khola, and (iii) the region is still tectonically active as shown by the warping of the T1 and T0 surfaces, which are of recent origin. These terraces have complex input of sediments from Higher Himalayan Crystalline (HHC) rocks and from locally present Lingtse granites. [ABSTRACT FROM AUTHOR]

The basement in Upper Assam foreland basin exhibits a typical flexure pattern. An elongated continuous subsurface basement high has developed between Himalaya and Naga-Patkai ranges. A few prominent basement lows have developed adjacent to the foothill regions. It is noticed that the general topographic slope near the foothill regions is not only controlled by aggradation process, but also influenced by the flexured basement of the region. The basement lows have influenced the overlying topography significantly. Rivers flowing over those regions have shown unidirectional lateral migration. North bank tributaries like Subansiri, Jiadhal and Dikrang have been affected by the Subansiri basement low. Coseismic subsidence of sediments over Subansiri basement low had resulted subsidence of the North Lakhimpur-Ranga Nadi region in 1950 Assam earthquake. Some south bank tributaries like Disang and Dikhow have been affected by Nazira basement low. Topographic elevation along the Subansiri river is lower than that part of the Brahmaputra located south of Majuli. This typical topographic setting of the region makes the mouth of the Subansiri river and Majuli region highly susceptible to erosion. [ABSTRACT FROM AUTHOR]

The tectonic and topographic history of the Himalaya-Tibet orogenic system remains controversial, with several competing models that predict different exhumation histories. Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate asymmetric exhumation of Mount Everest consistent with a scenario in which the southern edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. Northward plateau retreat was caused by erosional incision during the Pliocene. Our results suggest that the South Tibetan Detachment was a localized structure and that no coupling between precipitation and erosion is required for Miocene exhumation of Greater Himalayan Sequence rocks on Mount Everest. [ABSTRACT FROM AUTHOR]

The leucogranites in the Higher Himalayan Sequence (HHS) provide a probe to elucidate the crustal melting of continental collisional orogen. An integrated geochemical and geochronological study of the Yadong leucogranites, southern Himalaya, shows that these rocks have relatively high SiO 2 contents of 69.77 to 75.32 wt.% and alumina saturation index (A/CNK) of 1.09–1.40, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and depleted HFSE (Zr, Hf, Nb and Ta). LA–ICP–MS U–Pb zircon dating of ten samples yields crystallization ages ranging from 21.0 to 11.7 Ma. The zircons have variable ε Hf ( t ) values of − 26.3 to − 3.5 and corresponding Hf two-stage model ages of 2.77–1.33 Ga. The present study reveals that the muscovite–biotite leucogranites (2ML) have higher TiO 2 , MgO, CaO, Sr, Ba and Zr contents, lower Rb/Sr ratios than the tourmaline–muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681–784 °C) of the 2ML are 20–80 °C higher than those (663–705 °C) of the TML. Combining with previous results, we propose that the TML were derived from the muscovite-dehydration melting, whereas the 2ML dominantly resulted from the biotite-dehydration melting during the prograde metamorphism of the pelitic and felsic granulites of the HHS. Therefore, the Himalayan leucogranites were probably formed during the subduction of the Indian crust following the India and Asia collision. [ABSTRACT FROM AUTHOR]

The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, midcrustal layer "flowing" southward relative to the upper and lower crust during late Oligocene-Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Himalaya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak "active channel" to a strong "channel plug" at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and latestage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene-early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillimanite-grade decompression melting, as commonly recorded elsewhere in the Himalaya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]--the proposed channel) had a greater viscosity than in other Himalayan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny. [ABSTRACT FROM AUTHOR]

Although the Himalayan range is classically presented as cylindrical along strike, segmentation of the range in terms of structure, topography, precipitation, and erosion patterns is becoming widely recognized. The potential climatic or tectonic controls on these lateral variations remain controversial. Thermokinematic models predict that the geometry of the main Himalayan detachment controls the kinematics, exhumation, and topography of the orogen: where the detachment includes a major crustal ramp, the topography shows a steep gradient that focuses orographic precipitation and exhumation, whereas the topography is gentler and exhumation less focused above a flatter detachment. We test this prediction by comparing the patterns of river incision (specific stream power) and long-term exhumation (from apatite fission track thermochronology) in central Nepal, where a major crustal ramp has been imaged by geophysical methods, with new exploratory data from the remote Karnali River transect in western Nepal, where a ramp is predicted to be absent or minor. Our results show that both exhumation rates and river incision capacity are significantly higher and focused on the crustal ramp in central Nepal, whereas they are lower and the pattern is more diffuse in western Nepal. These differences support a model in which lateral variations in topography and exhumation are controlled by variations in the geometry of the detachment, and imply that along-strike climatic variations in the Himalaya respond to tectonics rather than driving it. [ABSTRACT FROM AUTHOR]

Himalayan mountain building has been dominantly explained by two types of models: extrusion and duplexing. To elucidate possible roles of these mechanisms during emplacement of the Himalayan crystalline core, we investigate an area speculated to contain the southern leading edge of the crystalline core: the northeastern margin of the Dadeldhura klippe, western Nepal. We found an ~ 700 m thick, primarily top-to-the-north shear zone within the klippe; we term this as the Tila shear zone. The shear zone occurs within a right-way-up metamorphic field gradient, and separates footwall gneiss from hanging wall schist. Similarly, deformation temperatures estimated from quartz and feldspar microstructures and quartz c -axis fabrics indicate a right-way-up thermal gradient of ~ 77–189 °C/km. U–Pb zircon dating of post-kinematic leucogranite dikes suggests that ductile shearing along the Tila shear zone occurred prior to ~ 17–14 Ma. We correlate the Tila shear zone to the South Tibet detachment (STD) on the basis of consistent structural fabrics (shear sense), lithologies, metamorphism, and deformation timing. This interpretation, in combination with regional constraints, indicates southwards-increasing proximity of the STD (Tila shear zone) and the Main Central thrust (MCT). These two shear zones are separated by ~ 3 km of structural section in the northern portion of our study area, and become close to within ~ 1 km of separation, in the southern portion. Interpolation suggests that the STD (Tila shear zone) and MCT merge 15 ± 10 km southwest of our study area. The increasing-to-south proximity and potential merger of the two shear zones suggest that the STD formed as a backthrust from the MCT. This interpretation contrasts with the long-standing normal fault interpretation of the STD. Because the STD and MCT bound the Himalayan crystalline core, these findings document crystalline core emplacement at depth via tectonic wedging. This kinematic evolution is consistent with duplexing, but not extrusion to the surface. [ABSTRACT FROM AUTHOR]

Mature orogenic systems built by continent-continent collision feature orogenic plateaus flanked by accretionary wedges. Thermal-mechanical models of these systems predict the development of a thermally weakened orogenic infrastructure that is capable of lateral flow toward the orogenic foreland. Such flow, if it occurs, strongly influences the evolutionary pathway of a wedge. Although the architecture of a wedge features numerous large-displacement faults, three are preeminent in mature orogens: one that marks the base of the wedge and two others that mark the base and top, respectively, of the weakened infrastructure. These structures represent major decoupling horizons separating domains with distinctive deformational and thermal histories. Reviews of the geology of orogenic wedges in two mature orogenic systems-the Cenozoic Himalaya and the Paleozoic East Greenland Caledonides-show how this simple conceptual model provides a valuable context for studies of how collisional orogenic systems develop and how they interact with the surrounding lithosphere. [ABSTRACT FROM AUTHOR]

40Ar/39Ar ages from detrital muscovites have been analyzed from six modern rivers in central and western Nepal; the size of the drainage basins associated with these samples ranges from a few square kilometers to >40,000 km2. These data, when combined with previously published ages of detrital muscovites from other modern rivers in the region, suggest that a good correspondence between normalized age and normalized topography (the comparison of t* and z*) is rare, due to either nonuniform rates of passage through the ~400 °C isotherm or subsequent faulting in the drainage area. The closure temperature of Ar in muscovite is perhaps too high to make meaningful comparisons to modern topography in tectonic analysis of active orogens. The distribution of 40Ar/39Ar ages from detrital muscovites from the Karnali basin in western Nepal is much older than that for the Narayani basin in central Nepal. The Karnali muscovites, when combined with previously published muscovites from the Siwalik Group in western Nepal and zircon fission track ages from modern and ancient samples from the region, suggest a thermal history for western Nepal consistent with vigorous tectonics (and attendant erosion) before the middle Miocene but a significant diminution in the rate of erosion since ca. 10 Ma. 40Ar/39Ar ages of detrital muscovites from the Narayani basin in central Nepal suggest a markedly different history with an acceleration of the rate of erosion since ca. 10 Ma and reactivation of major faults; this is consistent with the abundant bedrock data from the Narayani basin. The strong difference in the erosional history of the adjacent Karnali and Narayani basins, as evidenced by the 40Ar/39Ar ages from detrital muscovites, is not likely to have been due to variations in climate, but rather due to strain partitioning within the Himalaya during and after the Miocene. [ABSTRACT FROM AUTHOR]

The carbon pool stored in soil carbonate is comparable to the soil organic carbon. Therefore, secondary calcite precipitation in supersaturated catchment could be an important, yet poorly constrained, carbon sink within the modern global carbon cycle. The chemical analysis of some dissolved species transported by rivers, such as elevated Sr/Ca and Mg/Ca ratios but also heavy stable Ca isotopic compositions, witness the formation of secondary calcite in rivers draining arid regions. However, in areas affected by active tectonics and rapid physical erosion, co-variations in the fluvial Sr/Ca and Mg/Ca ratios could also be related to incongruent carbonate weathering processes. Here, we present a model to assess the roles played by incongruent carbonate dissolution and secondary calcite precipitation in modern weathering processes. We tested and applied the model to rivers draining the Himalayan-Tibetan region. The results suggest that regional aridity in the drainage basin promotes carbon sequestration as secondary carbonate but that for a given runoff, incongruent dissolution of carbonate possibly related to rapid physical erosion amplifies such sequestration. The isotopic compositions (C/C and O/O) of detrital carbonate transported by the main rivers in South and South-East Tibet imply that around 1% of the suspended material transported by those rivers corresponds to secondary carbonate and can represent between 5% and 15% of the alkalinity flux. Most of these alkalinity transported as particulate material is, nevertheless related to the weathering of carbonate lithologies and is also subjected to dissolution prior its final storage in sedimentary basins. However, on glacial-interglacial timescale this will amplify the significant role of mountain weathering on climatic variations. [ABSTRACT FROM AUTHOR]

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 (NEIC-catalogue, 1973–2013) and GPS measurements (published), collectively substantiate our geomorphic interpretations. [ABSTRACT FROM AUTHOR]

Moment Tensor solutions of 104 earthquakes which were observed at local distances in Lower Siang region of Arunachal Himalaya have been estimated using ISOLA code. The magnitude range of analyzed earthquakes lies between 1.8 ( M w ) and 5.3 ( M w ). Out of 104 earthquakes, only 32 events are having good data. The signal noise ratio of these 32 events is greater than 6 with magnitude greater than 2.5. All possible sources of non-DC such as, noise present in the data, depth of the source, low azimuthal coverage of the event and non-DC as a part of real earthquake source process are examined. The study reveals that non-DC is highly dependent on the source depth as comparatively shallow events shows high CLVD%. The CLVD is also highly affected by the noise present in the data. Another factor is the magnitude of the event. The high magnitude event shows quite high DC%. So the source mechanisms of high magnitude events are double couple (DC). [ABSTRACT FROM AUTHOR]

The Eastern Lesser Himalayan fold-thrust belt is punctuated by a row of orogen-transverse domal tectonic windows. To evaluate their origin, a variety of thrust-stack models have been proposed, assuming that the crustal shortening occurred dominantly by brittle deformations. However, the Rangit Window (RW) in the Darjeeling-Sikkim Himalaya (DSH) shows unequivocal structural imprints of ductile deformations of multiple episodes. Based on new structural maps, coupled with outcrop-scale field observations, we recognize at least four major episodes of folding in the litho-tectonic units of DSH. The last episode has produced regionally orogen-transverse upright folds (F4), the interference of which with the third-generation (F3) orogen-parallel folds has shaped the large-scale structural patterns in DSH. We propose a new genetic model for the RW, invoking the mechanics of superposed buckling in the mechanically stratified litho-tectonic systems. We substantiate this superposed buckling model with results obtained from analogue experiments. The model explains contrasting F3-F4 interferences in the Lesser Himalayan Sequence (LHS). The lower-order (terrain-scale) folds have undergone superposed buckling in Mode 1, producing large-scale domes and basins, whereas the RW occurs as a relatively higher-order dome nested in the first-order Tista Dome. The Gondwana and the Proterozoic rocks within the RW underwent superposed buckling in Modes 3 and 4, leading to Type 2 fold interferences, as evident from their structural patterns. [ABSTRACT FROM AUTHOR]

In a recent paper Chakrabarti Goswami et al., 2013 discussed the geomorphology of Matiali fan and the terraces formed along the rivers traversing this fan, in the Jalpaiguri District of West Bengal, eastern Himalaya. The area of their study is tectonically active with a couple of blind thrust traversing the Quaternary fan deposits. The authors have proposed that the tectonic movements along blind thrust faults of the area, were the main control of formation and present disposition of the terraces. In proposing this, the authors have completely ignored the question of aggradation of the terraces, sediment:water ratio in the alluvial system and role of well-known climatic fluctuations during the time span over which these geomorphic features formed in eastern Himalaya. They have reported fault propagation folds deforming the fan sediments, but did not document any deformation or any other features within the younger terraces that could substantiate their claim of tectonics as the main agent for the development of the terraces. In this comment we point out the inconsistencies in the tectonogeomorphic model proposed by them and suggest that the climate induced aggradation played a major role in the formation of the terraces in the river valleys in this area. [ABSTRACT FROM AUTHOR]

Detailed analysis of intensity for ten damaging historical earthquakes in the central arcuate belt between the Himachal and Darjeeling Himalayas was carried out in the backdrop of isoseismal eccentricity, source depth and Indian plate obliquity. Results indicate that the elongated axes of the isoseismals and strike of ruptures for shallow earthquakes are almost parallel with strike of the Himalayan arc, and clearly conformable with the obliquity. An empirical power law relationship between eccentricity and focal depth established under the present study illustrates that the deeper events are more influenced by the bending of the penetrating Indian lithosphere, whereas the shallower events are principally controlled by the obliquity. A positive correlation between eccentricities and obliquity obviously supports this inference. The present study further reveals that the constant decrease in Indian plate obliquity from Himachal to Nepal-Bihar Himalaya is well compatible with the graben structures and horizontal shearing along this arcuate segment. [ABSTRACT FROM AUTHOR]

Interest in hydrocarbon exploration from the Lesser Himalayan Sequence (LHS) has recently been revived amongst the petroleum geoscientists. Understanding the paleostress regime and the deformation mechanism are the two important steps to understand the structural geology of any (petroliferous) terrains. Arc-parallel shear is an integral deformation process in orogeny. The scale of such deformation features can range from micro-mm up to regional-scale. Unlike arc-orogen-perpendicular shear, different driving forces can produce arc-parallel shears. We review these mechanisms/theories from several orogens including the Himalaya, and compile 44 locations worldwide with arc-parallel shear. Due to continuous crustal shortening by the India-Eurasia collision, the squeezed rock mass at the plate interface has been building the Himalayan mountain chain. In addition, the rock mass also escapes laterally along the orogenic trend. Tectonic stress-field governs this mass flow. Field study and microstructural analysis in the northwest LHS (India) reveals arc-parallel brittle and ductile shear movement. Y- and P- brittle shear planes, and the S- and C- ductile shear planes reveal the following shears documented on the ~ NW-SE trending natural rock selections: (i) top-to-NW up, (ii) top-to-SE up, (iii) top-to-NW down, and (iv) top-to-SE down. Our paleostress analysis indicates top-to-SE down and top-to-NW down shears occurred due to stretching along ~131°–311° (D ext), whereas top-to-SE up and top-to-NW up shear fabric originated due to shortening along ~133.5°–313.5° (D compr). Previous authors considered that the arc-parallel extension generated ~15-5 Ma due to vertical thinning of the Himalaya. The NE-trending Delhi-Haridwar Ridge below the LHS plausibly acted as a barrier to the flowing mass, and piled up the rock mass in the form of NW-SE/arc-parallel compression. The NW-SE compression can be correlated with the D 3 of Hintersberger et al. (2011) during ~ 4–7 Ma. • Paleostress analysis from Lesser Himalaya in Garhwal section. • Global review of occurrence and mechanisms of arc-parallel shears. • Himalayan mechanism in the study area and applied aspects. [ABSTRACT FROM AUTHOR]

The stratigraphic and tectonic setting in the northwest part of Himalayan belt is complex and thrusted due to the collision of Indian plate and the Eurasian plate. During the past, the Himalaya is divided into four parts; these are Outer Himalaya, Lesser Himalaya, Greater or Higher Himalaya and Tethys Himalaya. The appearance of basement rocks played a significant role in the Himalayan periphery for stratigraphic, structural and tectonic movement. The deformation pattern of the crustal rocks causing the rise of basement rocks which constitutes an integral part of crustal configuration during the evolutionary stages of the Himalaya. In this study, an attempt has been made to estimate the basement depth configuration using spectral analysis and Euler deconvolution technique of gravity data in the northwest Himalaya region. The elevation increases continuously from 500 m to 5100 m in SW to NE direction, however, Bouguer gravity anomaly decreases continuously from −130 mGal to −390 mGal in SW to NE direction due to the isostaic adjustment. Gravity anomaly is very low near Harsil, Badrinath and Joshimath area and observed higher elevation due to the deep rooted basement. However, there are extrusion of crystalline basement in and around the Badrinath area due to the resettlement of geologic process which are overlaid to the top surface of the sedimentary layers. Euler deconvolution technique has been applied to detect the direct basement depth and results show a good correlation with the average depth of the spectral analysis and other works carried by different authors. Three gravity profiles are selected in appropriate places orienting SW-NE direction with a profile length of 160 km, 150 km and 140 km respectively in the study area for calculating the average depth of the basement rock. The average basement depth calculated is around 11.27 km using the spectral analysis technique and results are well correlated with the results of various workers. Euler deconvolution studies along the three selected profiles have been interpreted. It has been observed that there are more number of cluster points falling between depth ranges of 10 to 15 km, dipping in SW to NE direction. Euler's study shows deep rooted connection near Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), Bearing Thrust (BT) and Vaikrita Thrust (VT) locations as per profile study. Based on these studies three geological models have been prepared along the profiles showing different tectonic resettlement and depth of crystalline basement. Crystalline rocks exposed at the surface may be due to uplift along the shear in the MCT zone by kinetic flow basically, Munsiayari Thrust (MT) and VT in the of NW-Himalaya region. [ABSTRACT FROM AUTHOR]

Abstract: The Panxi region of SW China makes up most of the western margin of the Yangtze craton. Its structural pattern, defined by NS-trending deep faults, is superimposed on a zone of late Proterozoic crustal extension. The Panxi region seems to have evolved as a continental rift in the Permian, and was subsequently subjected to compression, caused mainly by the Himalayan collision. This induced the uplift of its axial and western parts, generating a horst-type structure within the former rift graben. At ca. 260Ma, mantle plume-related magmatism in the Panxi region generated the Emeishan flood basalts together with ultramafic and silicic volcanic rocks and with numerous intrusive bodies (peridotites, pyroxenites, layered gabbros, syenites, granites). A geochemical distinction between high-Ti and low-Ti Emeishan basalts can be made, but just with the significance of compositional end-members since the published data define a continuous variation of the entire compositional range. Similar compositional variation occurs in the ultramafic lavas (picrites and komatiites) intercalated in the sequence of the Emeishan basalts, which are considered to be the products of undifferentiated primary magmas. Based on the geochemical data, the primary magmas could have been generated from distinct source materials (mantle plume, asthenosphere, lithospheric mantle), from compositionally heterogeneous mantle plume source or from a common source but with various degrees of mantle melting and crustal contamination. Considering the picritic composition of the primary magmas, a deep-seated differentiation of large volume of magma is needed to produce the extrusion of the relatively evolved Emeishan basalts. World class Fe–Ti–V oxide ore deposits are associated with the layered gabbros, while the ultramafic intrusions can host small Ni–Cu and PGE sulfide deposits. The intrusions containing Fe–Ti–V oxide deposits show remarkably continuous layering and probably derived from mafic magmas differentiated in relatively large chambers. The ultramafic intrusions hosting Ni–Cu(–PGE) sulfide ores most likely formed from sulfide saturated magma moving through conduits. [Copyright &y& Elsevier]

Biostratigraphic analysis is an essential element for understanding global tectonics and the evolution of life on Earth. Quantitative analysis of sedimentary sequences provides the precise age constraints on timing of significant events in Earth's history. This paper presents results from quantitative stratigraphic analysis of Upper Cretaceous Tethyan Himalayan sequences. This analysis resulted in a new composite stratigraphic section for the Cretaceous strata of Tibet (TIBETKCS). The eight Upper Cretaceous sections were analyzed in this study and 12 planktonic foraminifera zones were recognized based on available data. Quantitative measurements were made using a Graphic Correlation with Graphcor 3.0 software and correlated to the world standard Cretaceous Composite Section (MIDKCS). The sections were also examined using Constrained Optimization software by CONOP9. Level Penalty was applied as the rule to measure misfit among automatically correlated sections. The new TIBETKCS correlates well with planktonic foraminifera ages from previous work in southern Tibet. A fitting equation of y=−0.19 x+305 with a correlation coefficient of 0.94 was obtained from this work The ages of the first and last appearances of 64 planktonic foraminifera can be calculated with this equation with ± 0.3 Ma precision. This level of precision is approximately 10 times higher than age determinations with traditional methods. Two extinction events were resolved within this analysis at ∼93.5 Ma and ∼85.5 Ma corresponding to the Ocean Anoxic Events at Cenomanian-Turonian and Coniacian-Santonian boundaries respectively. [ABSTRACT FROM AUTHOR]

Abstract: In the last decade growing interest has emerged in quantifying the spatial and temporal variations in mountain building. Until recently, insufficient data have been available to attempt such a task at the scale of large orogens such as the Himalaya. The Himalaya accommodates ongoing convergence between India and Eurasia and is a focal point for studying orogen evolution and hypothesized interactions between tectonics and climate. Here we integrate 1126 published bedrock mineral cooling ages with a transient 1D Monte-Carlo thermal–kinematic erosion model to quantify the denudation histories along ~2700km of the Himalaya. The model free parameter is a temporally variable denudation rate from 50Ma to present. Thermophysical material properties and boundary conditions were tuned to individual study areas. Monte-Carlo simulations were conducted to identify the range of denudation histories that can reproduce the observed cooling ages. Results indicate large temporal and spatial variations in denudation and these are resolvable across different tectonic units of the Himalaya. More specifically, across >1000km of the southern Greater Himalaya denudation rates were highest (~1.5–3mm/yr) between ~10 and 2Ma and lower (0.5–2.6mm/yr) over the last 2My. These differences are best determined in the NW-Himalaya. In contrast to this, across the ~2500km length of the northern Greater Himalaya denudation rates vary over length scales of ~300–1700km. Slower denudation (<1mm/yr) occurred between 10 and 4Ma followed by a large increase (1.2–2.6mm/yr) in the last ~4Ma. We find that only the southern Greater Himalayan Sequence clearly supports a continuous co-evolution of tectonics, climate and denudation. Results from the higher elevation northern Greater Himalaya suggest either tectonic driven variations in denudation due to a ramp-flat geometry in the main décollement and/or recent glacially enhanced denudation. [Copyright &y& Elsevier]

Abstract: The tectonics of seismically active Sikkim Himalaya, as inferred by numerous seismological studies, is distinct from the conventional thrust tectonics proposed for the Himalayan collision belt. Here, focal mechanisms of several moderate magnitude earthquakes and composite fault plane solutions of microearthquakes have revealed strike–slip motion along faults transverse to the northward convergence direction of the Indian plate. In the present study, we analyze broadband magnetotelluric data of 12 sites located along an approximately N–S profile cutting across major geological sub-domains of Sikkim to test whether magnetotelluric strikes also support such transverse tectonic nature of the region. We have performed strike analysis of the data by two decomposition approaches as well as by phase tensor method. The study has revealed local variations in the strike directions within the region consistent with the geological and tectonic setup and the presence of transverse tectonic features in the region of Main Central Thrust Zone (MCTZ) where major axis of phase ellipses align in NNW–SSE to NW–SE direction. This trend coincides with the one obtained by microseismic data recorded after the September 18, 2011 earthquake (Mw 6.9). Magnetotelluric strike analysis thus supports the presence of NNW-to-NW trending transverse tectonic zone in MCTZ. [Copyright &y& Elsevier]

Two contemporary earthquakes originating in the central Himalayan arc and its foredeep (Sikkim earthquake of 18.09.2011, M 6.9, h: 10-60 (?) km and Bihar-Nepal earthquake of 20.08.1988, Mw 6.8, h: 57 km) are commonly associated with transverse lineaments/faults traversing the region. Such lineaments/faults form active seismic blocks defining promontories for the advancing Indian Craton. These actually produce conjugate shear faulting pattern suggestive of pervasive crustal interplay deep inside the mountains. Focal mechanism solutions allow inferring that large part of the current convergence across the central Himalayan arc is accommodated by lateral slip. Similar slip also continues unabated in the densely populated foredeep for distances up to several tens of kilometers south of the Main Boundary Thrust (MBT). [ABSTRACT FROM AUTHOR]

Abstract: To investigate subsurface structure and seismogenic layers, 3D velocity inversion was carried out in the source zone of 1905 Kangra earthquake (M8.0) in the northwestern Himalaya. P-wave and S-wave phase data of 159 earthquakes recorded by a network of 21 stations were used for this purpose. Inverted velocity tomograms up to a depth range of 18km show significant variations of 14% in Vp and Vs and 6% in the Vp/Vs across the major tectonic zones in the region. Synthesis of seismicity pattern, velocity structure, distinctive focal mechanisms coupled with nature of stress distribution allows mapping of three different source regions that control regional seismotectonics. Accumulating strains are partly consumed by sliding of Chamba Nappe to the southwest through reverse-fault movements along Chamba/Panjal/Main Boundary Thrusts. This coupled with normal-fault type displacements along Chenab Normal Fault in the north account for low magnitude widespread seismicity in upper 8–10km of the crust. At intermediate depths from 8 to 15km, adjusting to residual compressive stresses, the detachment or lower end of the MBT slips to produce thrust dominated seismicity. Nucleation of secondary stresses in local NE–SW oriented structure interacts in complex manner with regional stresses to generate normal type earthquakes below the plane of detachment and therefore three seismic regimes at different depths produce intense seismicity in a block of 30×30km2 centered NE to the epicenter of Kangra earthquake. [Copyright &y& Elsevier]

Abstract: Many orogens, including the Central Andes and Himalayas, are characterized by wide areas that have undergone upper crustal shortening and surface uplift. The behaviour of the deep lithosphere is poorly constrained, and in some mountain belts, lower crust and mantle lithosphere appear to have been removed through delamination during orogen development. Thermal–mechanical numerical models demonstrate that as crust thickens during shortening, the lowermost crust may undergo metamorphic eclogitization, which increases its density. Even a small density increase (7% or more) causes shortening to localize above the eclogitic crustal root, promoting the development of thick lithosphere in this area which is then prone to gravitational removal. Complete removal of orogen mantle lithosphere occurs if the eclogitized lower crust is weak enough to allow full detachment of negatively buoyant mantle lithosphere; this can occur even if the lower crust is less dense than the mantle. The onset of delamination may be determined by the hydration state of the lower crust, as the presence of water promotes eclogitization and significantly reduces rock strength. Two distinct styles of delamination are observed: (1) retreating delamination in which weak mantle lithosphere rolls back and peels away from the crust, producing a contemporaneous migration of crustal thickening, surface uplift and magmatism, and (2) stationary delamination in which strong lithosphere separates from the weak lower crust and slides into the deep mantle at a stationary detachment point, followed by widespread crustal deformation and magmatism. [Copyright &y& Elsevier]

Abstract: The Himalayan Frontal Thrust (HFT) forms the southernmost active tectonic mountain front of the Himalaya. To understand the ongoing tectonics further, paleoseismological study has been carried out in the vicinity of the HFT system along the Himalayan Front near Kala Amb, India. The trench excavation survey conducted across an explicit surface exposure of the HFT exhibits two distinct faults considered to be associated with the reactivation of the HFT where the Middle Siwalik rocks (Late Miocene) have repeatedly thrust over the Late Pleistocene and Holocene sediments. Presence of large-sized coseismically induced sand-injection feature and its disposition recognized in the trench also suggest occurrence of large magnitude earthquakes in this region. An uplifted and upwarped strath terrace, 3 to 5m thick alluvium, resting over the 15m high Middle Siwaliks, abruptly truncated by the HFT indicates its latest activity. Optically Stimulated Luminescence dating techniques were employed to constrain the chronology of events. The long term slip rate of the abandoned terraces due to the activity of the HFT is estimated to be 3.4mm/yr or greater since Late Holocene. The paleoseismological investigations have provided unambiguous evidences of at least two large magnitude earthquakes occurred in this region where an earthquake with 12m or larger surface displacement and magnitude 7.5 or greater hit this region in the period between 29.3ka and 17ka in the Late Pleistocene and another great earthquake recurred with 20–22m or more surface displacement and magnitude of 7.7 or greater between 5.8ka and 2ka in the Holocene. The present study is the first time report of multiple large magnitude paleoearthquakes in the northwestern part of the Frontal Himalaya during Late Pleistocene and Holocene. The repeated reactivation of HFT substantiates high seismic potential of the Frontal Himalaya and calls for more extensive study of paleoearthquakes of this vastly populous mountainous region. [Copyright &y& Elsevier]

We present here seismological evidence for an active fault beneath the Tehri dam based on mapping of the earthquake distribution in the Kumaon-Garhwal Himalaya using 40 broadband digital seismographs operated over a period of 39 months during 2005- 2008. The earthquake distribution shows two welldefined bands: (i) following the surface trace of the Munsiari Thrust and (ii) a significant EW trend in the Lower Himalaya following the Tons Thrust Fault in the west and the Ramgarh Thrust in the east. The Tehri dam is in close proximity of the south-dipping Tons Thrust Fault. We also recorded over 20 earthquakes (M 1.6-2.8) in 20 km radius of the dam site. Most of these earthquakes have a thrust mechanism. The tectonic loading on this active Tons Fault due to local seismicity coupled with the reservoir loading and unloading may generate earthquake(s) and cause additional seismic risk in this critically stressed region. [ABSTRACT FROM AUTHOR]

The extensional South Tibetan fault system (STFS) crops out for more than 2000 km along the length of the Himalayan range crest. Interpretations of the signifi cance of this structure in Himalayan tectonics vary. While many regard it as rooted northward beneath Tibet, with offset estimates ranging as high as >200 km, others regard it as a relatively minor, rootless structure. Such inconsistent interpretations are possible because, except for the Everest region, where glacial erosion has exposed a cross section of the STFS, there are few direct constraints on the amount of slip on this feature. Arguments for very large displacements have been based on indirect lines of evidence (e.g., geothermobarometric data) that are multiply interpretable. However, new fi eld work and remote sensing data reported here directly support the petrologic arguments for large displacements, at least in the eastern Himalaya. Near the western boundary of Bhutan, the trace of the STFS is apparently offset ~70 km across a NNE-SSW--trending graben, the Yadong cross structure. This offset has been attributed by previous authors to left-lateral slip along the Jomolhari fault system, which bounds the eastern edge of the graben. In fact, the basal structure of the STFS is not offset, but forms a regionally continuous low-angle detachment surface. The outcrop pattern of the structure, kinematic indicators in fault-related tectonites, and the lack of matchable correlative hanging wall and footwall units require a minimum displacement of ~65 km for the detachment. Such displacements are comparable with estimates for major contractional structures in the eastern Himalaya, such as the Main Central thrust system, and confirm the importance of STFS extension in the evolution of the orogenic system. [ABSTRACT FROM AUTHOR]

Abstract: The alluviation and incision history of Late Quaternary terrace staircases of Yamuna River and its tributaries in the Sub Himalaya, between the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), were studied to understand interlinkages between tectonics and climate in their genesis. We documented five levels of terraces (T1 to T5), cutting across numerous tectonic plains, ranging in elevation from 65–80, 40–44, 28–30, 8–12 and 3–4m from the present day river bed, deposited by both glacier-fed perennial and piedmont-fed ephemeral streams. Temporal and spatial distribution of these terraces, coupled with absolute ages based on optically stimulated luminescence (OSL) chronology documents five distinct fluvial accretions separated by incision events spanning between Marine Isotope Stage (MIS) 3 and MIS 1. Two major accretion phases occur in the Late Pleistocene (>37 to 24ka and >15 to 11ka) and three minor phases in the mid- to late Holocene (7–4ka; 3–2ka; and <2ka). The Phase-I aggradation is thicker and occurs as alluvial fill in a pre-existing valley, whereas younger accretion sequences are cut-in-fill and rest over remnant deposits. The periods of terrace formation are coeval with the Indian Summer Monsoon (ISM) oscillations, however abrupt termination of T1 to T3 terraces in the Himalayan mountain front reflects tectonic upheaval along HFT after their development. This study outlines the climatic control on the terrace genesis in the tectonically active Himalaya on the scale of 103–104 years. [Copyright &y& Elsevier]

The Garhwal Himalaya region with moderate to high seismicity has considerable seismic risk potential arising from complex tectonic evolutionary processes. We assess first approximations of site responses across the region using the empirical horizontal to vertical (H/V) spectral amplitude ratio method of Nakamura (1989). The data comprises recorded ambient seismic noise and local earthquakes using a network of digital broadband seismographs from June 2005 to June 2008. The site response is calculated as the H/V amplitude spectra at each site following the hypothesis of Nakamura. These spectra are used in estimating sitespecific fundamental resonance frequencies and peak amplitudes. The stability of the H/V ratio is studied using standard deviations and 90% confidence intervals. Our results show that seasonal variations in the ambient noise data do not affect frequency estimates. The 'peak' corresponding to fundamental resonance frequency, is prominently present in both ambient noise and earthquake datasets. Observed H/V ratios correspond to the effects of velocity contrast and characteristic sub-surface topographic effect around a certain frequency range. Estimated fundamental resonance frequencies are in the range 2-5 Hz for the studied sites. Our results provide valuable inputs for further studies on site characterization and damage mitigation. [ABSTRACT FROM AUTHOR]

Abstract: The Sikkim Himalaya in the eastern sector of the Himalayan collision belt shows some distinct features, such as the occurrence of the Main Central Thrust as a several-km-wide zone (MCTZ) of intense ductile deformation and its deviation from the general east–west strike of the Himalayan thrusts, suggesting that the tectonics of this region could be different from the regional Himalayan tectonics. We have carried out a MT survey across MCTZ to understand the tectonics of the region. The broadband MT data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90° for frequencies smaller than 1Hz whereas other sites further north of this zone show normal behavior. Such anomalous phase behavior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors. We have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high-grade crystalline gneisses at the northern boundary of MCTZ. Among various models analyzed, a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data. We, thus, infer that the presence of a localized strongly anisotropic block at the northern boundary of MCTZ could be a viable explanation for the observed anomalous phase behavior. [Copyright &y& Elsevier]

From the end of the Eocene through the Pliocene, the Alpine-Himalayan Belt underwent collisional shortening induced by convergence of the Gondwana plates with the Eurasian Plate and varied in orientation from the north-northwestern to the northeastern directions. The collisional shortening was expressed in folding, thrusting of continental crustal tectonic sheets over one another, and closure of the residual basins of Neotethys and its backarc seas; it resulted in local thickening of the crust and its isostatic uplifting. As a rule, the uplifts were not higher than ∼1.5 km. In other words, before the Pliocene, the growth of local mountain edifices was caused by collisional shortening of the belt. Isostatic uplifting of the thickened crust was continued in the Pliocene and Quaternary even more intensely than before, but the general rise of the mountain systems was superposed on this process. The rise substantially exceeded in amplitude the contribution of the uplift caused by shortening and did not depend on the preceding Cenozoic history of either territory. Not only the mountain ridges but also most adjacent basins were involved in rising, which eventually led to the contemporary mountain topography of the belt. The spread of the hot and fluidenriched asthenosphere of the closed Tethys beneath the orogenic belt could have been a cause of such additional rising. The uplift was an isostatic reaction to decompaction of the lithospheric mantle partly replaced with asthenosphere and of the lower crust subject to retrograde metamorphism under the effect of cooled asthenospheric fluids. The deep transformations are also probably responsible for deepening of some basins in the Pliocene-Quaternary and more contrasting transverse segmentation of the belt. [ABSTRACT FROM AUTHOR]

Active faults of the densely forest covered northwestern part of the Ganga Plain have been identified and mapped on the basis of geomorphic signatures as observed in the Digital Terrain Models, remote sensing data and field. The structural limit between the Ganga Plain and Himalaya in the north is defined by the Himalayan Frontal thrust (HFT), which is a direct consequence of the compression resulting from collision of Indian and Eurasian plates and present day principal displacement zone between them. This HFT is offset by dip-slip, oblique-slip and strike-slip faults, extending transversely through the Ganga Basin. Analysis of these faults in conjunction with the available geological and subsurface geophysical data reveals that the development of some of these transverse faults is localized along basement highs of the basin. These faults tear asunder the Himalayan thrust sheets and may have developed during the collision either because of a rheological contrast between the basement highs and adjacent sedimentary fill, or a lateral change in cover thickness across the basement highs, or due to the reactivation of pre-existing basement faults. The continuous pressing and prodding by these basement highs have caused bending of the strata, leading to conspicuous bending of the mountain belt. One of the tear faults has formed as a result of the along-strike propagation of an oblique ramp of the Main Boundary thrust (MBT), along which the Lesser Himalayan succession is thrust over the Siwalik. In addition to these tear faults, some transverse faults of limited lateral extent, cutting through the mountain belt and basin, may have formed during the last event of deformation along the HFT zone. [ABSTRACT FROM AUTHOR]

SUMMARY A molasse sequence comprising 1.5-km-thick sediments of the Lower and Middle Siwaliks Group in the sub-Himalayan North Belt along the Tinau Khola River in west-central Nepal was studied for the low-field magnetic susceptibility and anisotropy using single core specimens of siltstone/sandstone from 127 stratigraphic levels. The magnetic fabric comprises primary sedimentary-compactional and secondary tectonic components. It is characterized by (i) predominantly oblate magnetic susceptibility anisotropy ellipsoids, (ii) low anisotropy ( P′ mostly <1.1) and low magnetic susceptibility (< ca. 10−7 m3 kg−1) contributed mainly by paramagnetic and diamagnetic minerals and (iii) WNW-ESE magnetic lineations subparallel to the fold axes/bedding strikes/thrust front, and hence normal to the direction of palaeotectonic compression. Thermal demagnetization of single core specimens from 103 levels across 1120 m of the lower part revealed a characteristic remanence of the high unblocking temperature (> 600 °C) in hematite. Remanence ratios derived from demagnetization data allowed the first-order estimation of remanence contributions from magnetic minerals (goethite, maghemite, magnetite and hematite), and discrimination of rockmagnetic zones correlatable with distinct lithofacies, which will facilitate objective mapping. We correlated a magnetic polarity sequence, constructed from normal and reverse polarity directions from 77 levels that passed the reversal test and represented primary remanences, with the standard geomagnetic polarity timescale to constrain the depositional age between ca. 13.2 Ma (base of Chron C5AAN, 13.015-13.183 Ma) and the middle of Chron C5n.2n (9.987-11.040 Ma). We calculated the sediment accumulation rate for polarity zones from the chronologically better constrained part below Chron C5n.2n (i.e. below 11.040 Ma) to be 25-61 cm kyr−1 (average, 39 cm kyr−1), which is consistent with the value of 32-50 cm kyr−1 reported from Siwaik sections in Nepal. The notable increase in accumulation rate after 12.1 Ma probably reflects the peak of the earlier phase of uplift and/or unroofing of the Himalayan source region followed by rapid accumulation in the foredeep, and a link to monsoon initiation/intensification. Compared to the expected remanence from the latest APWP for the Indian Plate 10-13 Ma, the best-defined mean (351.8°/20.9°) is rotated anticlockwise by 9.2° and records an inclination shallowing of 25°. Constraining the base of the Tinau Khola north section to 13.2 Ma (i.e. older than the Tinau Khola south by 1.7 Myr) should open up new horizons for multidisciplinary and multiproxy research targeting geotectonic/climatic/environmental palaeoreconsructions of Himalaya-wide events. [ABSTRACT FROM AUTHOR]

The northwest terminus of the Himalayan range forms a syntaxis limited by two strike-slip faults, the Chaman Fault to the west, and the Karakorum Fault to the east. In between these faults, smaller-scale syntaxes are observed, as the Nanga Parbat and the Kashmir syntaxes. The fold axial traces of these syntaxes trend nearly north–south, revealing a component of east–west shortening in the geodynamic context of the north–south convergence between India and Asia. We constructed a sandbox with two edges oblique to the convergence to test the influence of the NW Himalayan syntaxis large-scale structural geometry on the construction of a wedge and on its evolution. We used sand and microbeads as standard analogues for brittle crustal layers approximately 30 km thick. We show that the obliquity of the edges can generate an east–west shortening in a north–south convergence. The amount of east–west shortening generated by the oblique edges of the box is up to 40% of the amount of the north–south convergence. This shortening is accommodated by oblique motion on transpressive faults parallel to the edges. The amount of strike-slip motion is up to 60% of the convergence vector. We conclude that the obliquity of the edges is sufficient to generate an east–west shortening in a north–south convergence, but not sufficient to generate a north–south axial-trending fold. One experiment, with a free southern boundary, allowed the development of pure strike-slip faults within the sand layer, conjugated to pure shortening on one north–south fold. The east–west component of the oblique motion generated by the obliquity of the faults bordering the NW Himalayan syntaxis could be absorbed by a north–south fold as the Nanga Parbat if the motion on the border faults is pure strike-slip. [ABSTRACT FROM AUTHOR]

Geochemistry of Tuting metavolcanic rocks is being reported for the first time. Narrow slivers of mafic volcanic rocks, as those at Tuting, also occur in close association with slivers of more complete sections of ophiolites at the Tsangpo river section upstream of Tuting and skirt round the Namche Barwa antiform. These detached slivers of the mafic volcanic rocks and the ophiolites represent the easternmost components of the Yarlung Tsangpo Ophiolite, and also define the arcuate shape of the Eastern Himalayan syntaxis. The metavolcanic rocks exposed at the apex of the Siang river dome at Tuting (Tsangpo River named Siang down stream of Tuting) is the only exposure of such rocks from the Himalayan syntaxial area in India. The Tuting metavolcanic rocks correspond to andesite and basaltic andesite as per TAS diagram. The mobility of major elements possibly has affected their classification. As per Zr/TiO - Nb/Y diagram of Winchester and Floyd (1977), proposed for classification of altered igneous rocks, the Tuting samples mainly correspond to 'sub-akaline basalt' and one sample plot as 'andesite/basalt'. These have a flat chrondrite-normalised REE pattern. MORB-normalized multi-elemental plot shows enrichment in large ion lithophile (LIL) and the light rare earth elements (LREE), and depletion in several high field strength elements (HFSE). Based on these trace element patterns and a few discrimination plots, the Tuting metavolcanic rocks are inferred to have generated in supra-subduction zone environment in an intraoceanic arc, back arc setting, or in a mid-ocean ridge process that resembles the Chile Ridge spreading centre. [ABSTRACT FROM AUTHOR]

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite 40Ar/39Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ∼300°C at ∼11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ∼7 million years, documenting the cooling of the GHS to ∼110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ∼55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ∼7 Ma, characterized by a fast cooling rate of ∼2 mm per year. [ABSTRACT FROM AUTHOR]

SUMMARY An active oroclinal bending is discovered in the Shan-Thai Block to the south of the eastern Himalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic techniques, the Middle Cretaceous red beds of the Nanxin Formation were sampled at the Zhengwan (22.8°N, 100.9°E) and Dadugang (22.4°N, 101.0°E) localities in the southern Simao Basin. Most of the studied samples revealed the presence of characteristic remnant magnetization with unblocking temperatures around 680 °C. A primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test. Tilt-corrected mean directions calculated for Zhengwan and Dadugang localities are characterized by large easterly deflected declination; D= 51.8°, I= 47.9°, ks= 45.0, α95= 6.9°, N= 11 and D= 64.1°, I= 48.1°, ks= 36.0, α95= 7.3°, N= 12, respectively. Steep inclination values at both these localities with respect to those expected are in the range previously reported from the Shan-Thai Block, confirming their southward displacement by 6.2°± 1.7° as part of the Shan-Thai Block. Combination of the present data (two localities) with those previously reported from Simao Basin (seven localities) reveals a positive palaeomagnetic oroclinal test, indicating that the present-day arc-like geometry of the Simao Basin was formed by oroclinal bending. Comparison with recent GPS and structural data suggest that formation of the Simao curvature started after the early Pliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribed to southwestward movement of the crustal material across the Ailao Shan-Red River Fault (around the eastern Himalaya syntaxis), which was formed by westward movement of the decollement with progressive eastward deepening of the Lanping-Simao Basin. Decoupling between the upper and the middle-lower crusts is a requisite condition for the arc formation on the continent. [ABSTRACT FROM AUTHOR]

In the early morning hours of 23 July 2007 at 04 : 32 : 13.5 (IST), a moderate (ML = 4.9, Mw = 5.0) earthquake occurred in the Garhwal Himalaya near Kharsali, about 50 km northwest of the epicentre of the strong (M 6.4) Uttarkashi earthquake of 20 October 1991. Sourced in the upper crust (15 km focal depth), this earthquake was followed by a series of shallow-focused aftershocks of magnitudes ≤ 3.4. Data from the local broadband seismic network were used for understanding the source characterization. The fault plane solutions of the main shock and the biggest aftershock (M 3.4) are almost similar, suggesting that the deformation occurred by reverse faulting with a significant strike-slip component. For the main shock, the trend/plunge of the major and minor principal axes P/T was 166°/1° and 258°/67° respectively. In these solutions one fault plane is dipping to the NE, and the strike of the fault plane coincides with the major tectonic faults as well as the longest axis of isoseismals. The seismic moment, source radius and stress drop of the main shock are estimated to be 4.15 × 1016 Nm, 1660 m and 4.15 MPa respectively. [ABSTRACT FROM AUTHOR]

Carbonate and calcareous-quartzite of Miri-Buxa Group in the Siang Window of Eastern Himalaya intruded by mafic rocks of gabbroic affinity. These intrusive rocks are low-Ti tholeiites (Ti/Y = 379−478; Nb/La = 0.99−1.88) and characterized by enriched LILE-LREE, depleted in HFSE with minor REE fractionation [(La/Yb) = 2.72−3.35)]. Geochemical behaviour of the incompatible trace elements with the rare earth elements abundances indicates their cogenetic nature and their emplacement in a continental rift tectonic environment. The liquidus olivine temperature of these mafic rocks ranges from 1262°C to 1380°C showing a gentle decrease of [Mg] with a steep increase of [Fe]. These charters thus imply that the rocks are either related to the extent of common source or fractionational crystallization of plagioclase and clinopyroxene from a single batch parental magma. Petrogenetic modeling of [Mg]-[Fe] and REE indicates that these mafic intrusives probably derived from a mantle source similar to komatiitic composition at moderate to high degree (8%-20%) of partial melting. [ABSTRACT FROM AUTHOR]

This article focuses on two regional-scale synclines of the Chamba Thrust Sheet, northwest Himalaya. These synclines were explained by two different tectonic events. Earlier studies ascribe the NE vergent Tandi syncline and the SW vergent Chamba syncline to the older NE-directed nappe stacking and the younger SW-directed Himalayan deformation events, respectively. The new field data and structural analysis reveal that these synclines define the flanks of a large-scale asymmetric box fold referred to here as the Hadsar-Chobia box fold and to a common phase of deformation. The box fold in the Chamba Thrust Sheet has developed over a ductile-brittle substrate referred to here as the Chamba Thrust. This thrust has translated a portion of the Tethys Himalaya over both the Higher Himalayan Crystallines and the Lesser Himalaya. Structural analysis, particularly the parallelism between the trends of hinge lines of the Hadsar-Chobia box fold and the strike of the regional thrust planes indicate that the folding and translation may have occurred simultaneously during the D deformation episode. [ABSTRACT FROM AUTHOR]

Abstract: The upper bound magnitude (ω) or maximum magnitude in 28 seismogenic source zones in the Hindukush-Pamir Himalaya and the adjacent regions have been computed with the help of a complete and homogeneous earthquake catalogue during the period 1900–2010 to estimate the earthquake hazards in the region. The Gumbel''s third asymptotic distribution (GIII) of extreme value method is used to estimate this parameter. In this study, a comparison of maximum magnitude obtained by GIII distribution is carried out with Kijko–Sellevoll method. It is observed that the maximum earthquake magnitudes estimated by Kijko–Sellevoll and GIII methods are comparable to each other and the average of differences of their values is only 0.13. The results also estimate the most probable earthquake magnitude that can be expected in next 100years (M 100 ) in all 28 seismogenic source zones. An effort is made to make regression relations between ω and maximum magnitude estimated by Kijko–Sellevoll method (M max (KS)) and ω and maximum observed magnitude (M max obs ). The estimated ω values exceeded the value of 7.00 in 15 and 8.00 in 5 of the 28 seismogenic source zones. The geographical distribution of ω and M 100 in 28 seismogenic source zones of the study region is visualized to analyze the localized seismicity parameters. It is observed that earthquake hazard level varies spatially from one zone to another, which suggests that examined region have high crustal heterogeneity and seismotectonic complexity. [Copyright &y& Elsevier]