Himalayan Fold-thrust Belt Research Articles

Potential source zones for Himalayan earthquakes: constraints from spatial–temporal clusters

The Himalayan fold-thrust belt has been visited by many disastrous earthquakes (magnitude > 6) time and again. This active collisional orogen bordering Indian subcontinent in the north remains a potential seismic threat of similar magnitude in the adjoining countries like India, Pakistan, Nepal, Bhutan and China. Though earthquake forecasting is riddled with all conjectures and still not a proven presumption, identifying likely source zones of such disastrous earthquakes would be an important contribution to seismic hazard assessment. In this study, we have worked out spatio-temporal clustering of earthquakes (Mb ≥ 4.5; 1964–2006) in the Himalayas. ‘Point density’ spatial statistics has helped in detecting 22 spatial seismicity clusters. Earthquake catalog is then treated with a moving time-distance window technique (inter-event time 35 days and distance 100 ± 20 km) to bring out temporal clusters by recognizing several foreshock-main shock-aftershock (FMA) sequences. A total of 53 such temporal sequences identified in the process are confined within the 22 spatial clusters. Though each of these spatio-temporal clusters deserves in-depth analysis, we short-listed only eight such clusters that are dissected by active tectonic discontinuities like MBT/MCT for detail study. Spatio-temporal clusters have been used to constrain the potential source zones. These eight well-defined spatio-temporal clusters demonstrate recurrent moderate to large earthquakes. We assumed that the length of these clusters are indicating the possible maximum rupture lengths and thus empirically estimated the maximum possible magnitudes of eight clusters that can be generated from them (from west to east) as 8.0, 8.3, 8.2, 8.3, 8.2, 8.4, 8.0 and 7.7. Based on comparative study of the eight cluster zones contemplating with their temporal recurrences, historical seismic records, presence of intersecting faults and estimated magnitudes, we have guessed the possibility that Kangra, East Nepal, Garhwal and Kumaun–West Nepal clusters, in decreasing order of earthquake threat, are potential source zones for large earthquakes (≥7.7 M) in future.

Basin formation in the High Himalaya by arc-parallel extension and tectonic damming: Zhada basin, southwestern Tibet

[1] The late Miocene–Pleistocene Zhada basin in southwestern Tibet provides a record of subsidence and basin formation within an active collisional thrust belt. The >800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of Tethyan strata that were previously shortened in the Himalayan fold-thrust belt. Modal sandstone petrographic data, conglomerate clast count data, and detrital zircon U-Pb age spectra indicate a transition from detritus dominated by a distal, northern source to a local, southern source. This transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin-centric. At the same time the depositional environment in the Zhada basin changed from a large, braided river to a closed-basin lake. Sedimentation in the Zhada basin was synchronous with displacement on the Qusum and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively. These observations indicate that accommodation for Zhada basin fill was produced by a combination of tectonic subsidence and damming, as midcrustal rocks were evacuated from beneath the Zhada basin in response to arc-parallel slip on crustal-scale detachment faults.

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N. Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.

Statistical analysis on yearly seismic moment release data to demarcate the source zone for an impending earthquake in the Himalaya

Tectonism in the Himalayan fold-thrust belt had generated great earthquakes in the past and will spawn more in the future. Sequential cumulative moment release data of macroearthquakes (Mb ≥ 4.5) over the years 1964-2006 in four zones of the Himalaya was analysed by nonparametric RUD method. The Z values of RUD analysis had neither rejected nor supported the null hypothesis of randomness. However, the Hurst analysis and plot, a statistical procedure to identify clustering of low and high values in a time series, brought out a pattern for earthquake prognostication. The pattern was a negative sloping segment representing a sluggish moment release over years, followed by a positive sloping segment indicating a sudden high moment release with occurrence of medium/large size earthquake(s). In recent past, such a negative sloping has been found in Zones B (1992-2006) and D (1998-2006), indicating an impending moderate/mega earthquake event in near future.

Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system

Preliminary mapping and stratigraphic correlation of Lesser Himalayan rock in eastern Bhutan using field characteristics, U–Pb detrital zircon dating, and ɛNd geochemistry define the first-order stratigraphic architecture of the Indian passive margin sequence in the eastern Himalaya. We use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern Himalayan fold-thrust belt in Bhutan. We propose that Lesser Himalayan rock in Bhutan can be divided into lower Lesser Himalayan rocks with a Paleoproterozoic detrital zircon signal, and upper Lesser Himalayan rocks with detrital zircon signals of ∼ 1000–500 Ma. The ∼ 500 Ma detrital grains are from rocks in the frontal portions of the fold-thrust belt north of the Main Boundary thrust as well as directly in the footwall of the Main Central thrust in the hinterland. Our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross-section through the Kuru Chu valley in eastern Bhutan which provides the first image of the geometry and amount of shortening through Bhutan. The Main Frontal thrust tilts a 6 km of thick section of Neogene foreland basin deposits. These deposits are separated from folded and faulted upper Lesser Himalayan rocks by the Main Boundary thrust. North of the Main Boundary thrust, we propose two duplex systems. The southernmost duplex contains 9 repeated sections of upper Lesser Himalayan units, including the Permian Gondwana Sequence and the Cambrian (or younger) Baxa Group. The northernmost duplex is located in the footwall of the Main Central thrust, and is comprised of two repeated sections of the Proterozoic Shumar and Daling Formations. The southern boundary of this northern duplex is the Shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the Ramgarh thrust in Nepal and India. We propose that the development of the duplex systems passively folded overlying Tethyan and Greater Himalayan rocks. Minimum shortening for this part of the Himalayan fold-thrust belt is 359 km, all of which has occurred from 22 Ma to present defining a long-term shortening rate of 16 mm/yr.

Paleostrain stratigraphic analysis of calcite twins across the Cambrian–Ordovician unconformity in the Tethyan Himalaya, Spiti and Zanskar valley regions, India

Calcite strain analyses were conducted on low-grade Cambrian and Carboniferous limestone samples collected above and below the Cambrian–Ordovician unconformity in the Spiti and Zanskar valley regions of the NW Himalaya in order to compare strain patterns in rocks that bracket an enigmatic early Paleozoic tectonic episode. All samples record a layer-parallel shortening strain at a high angle to folds and faults in the Tethyan Himalayan fold-thrust belt. In the Carboniferous samples, we relate these layer-parallel strains to the onset of Cenozoic deformation within the Tethyan Himalayan fold-thrust belt. The Cambrian sample from the Spiti area contains a layer-parallel shortening strain even though the Cambrian–Ordovician unconformity is angular. This suggests that the twinning strains in the Cambrian sample may have formed at the onset of early Paleozoic folding and subsequent erosion, and that early phases of Cenozoic shortening were coaxial to early Paleozoic shortening. The maximum shortening axis in the Carboniferous samples, which is probably parallel to the early thrust transport direction in the Tethyan Himalayan fold-thrust belt, is parallel to the NE movement of India with respect to Eurasia in the Middle Eocene, suggesting that it might closely correspond to the India/Eurasian slip direction during this time period.

Exhumation of Greater Himalayan rock along the Main Central Thrust in Nepal: implications for channel flow

Abstract South-vergent channel flow from beneath the Tibetan Plateau may have played an important role in forming the Himalaya. The possibility that Greater Himalayan rocks currently exposed in the Himalayan Fold-Thrust Belt flowed at mid-crustal depths before being exhumed is intriguing, and may suggest a natural link between orogenic processes operating under the Tibetan Plateau and in the fold-thrust belt. Conceptual and numeric models for the Himalayan-Tibetan Orogen currently reported in the literature do an admirable job of replicating many of the observable primary geological features and relationships. However, detailed observations from Greater Himalayan rocks exposed in the fold-thrust belt’s external klippen, and from Lesser Himalayan rocks in the proximal footwall of the Main Central Thrust, suggest that since Early Miocene time, it may be more appropriate to model the evolution of the fold-thrust belt using the critical taper paradigm. This does not exclude the possibility that channel flow and linked extrusion of Greater Himalayan rocks may have occurred, but it places important boundaries on a permissible time frame during which these processes may have operated.

Clustering of Earthquake Events in the Himalaya - Its Relevance to Regional Tectonic Set-up

The earthquake events of Himalaya of magnitude ≥5.0 from the time window 1905–2000 are statistically analysed. The inter-event time between earthquakes shows Hurst phenomena of temporal clustering which are spatially located in five distinct domains along the Himalayan fold-thrust belt. Out of these, two domains, one around Uttaranchal-Nepal border and the other around Nepal-Sikkim border reveal maximum number of temporal clusters and thus considered as seismically most potential zones of the Himalaya. Both these zones are located at the interface of the orthogonally disposed major tectonic discontinuities of the Peninsular Shield and Himalayan fold-thrust belt. Such zones are geologically most favourable locales for strain accumulation during later-tectonic movement. Statistical analysis points towards a probability of recurrence of seismic events in near future in these two zones. However, validity of such statistical results can be ascertained by detailed geological and geophysical modelling of the terrain.

The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes

Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium isotopic data from the Himalayan fold-thrust belt in Nepal ( n=35) reveal that the Lesser Himalayan zone consistently has a more negative ϵ Nd(0) value than the Greater and Tibetan Himalayan zones. Our data show the average ϵ Nd(0) value in the Lesser Himalayan zone is −21.5, whereas the Greater and Tibetan Himalayan zones have an average ϵ Nd(0) value of −16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative ϵ Nd(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits ( n=34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The ϵ Nd( T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones. At ∼10 Ma in Khutia Khola and ∼11 Ma in Surai Khola, a shift in ϵ Nd( T) values from −16 to −18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the ϵ Nd( T) values remain close to −16 throughout Miocene time because there has been less erosional unroofing in this region.

Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal

Research Article| March 01, 2000 Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal T. P. Ojha; T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar R. F. Butler; R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar P. G. DeCelles; P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar D. Richards; D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar B. N. Upreti B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Search for other works by this author on: GSW Google Scholar Author and Article Information T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Publisher: Geological Society of America Received: 13 Aug 1998 Revision Received: 18 Mar 1999 Accepted: 26 Mar 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (3): 424–434. https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Article history Received: 13 Aug 1998 Revision Received: 18 Mar 1999 Accepted: 26 Mar 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. P. Ojha, R. F. Butler, J. Quade, P. G. DeCelles, D. Richards, B. N. Upreti; Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal. GSA Bulletin 2000;; 112 (3): 424–434. doi: https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The middle Miocene–Pliocene Siwalik Group was deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the Siwalik Group in central and western Nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies. Sandstones and paleosols of the Siwalik Group, however, generally display highly erratic paleomagnetic behavior during thermal demagnetization. On the basis of these observations, siltstones from a well-exposed, 2423-m-thick section of the Siwalik Group in Khutia Khola, far western Nepal, were sampled for magnetic polarity stratigraphy. The Siwalik Group is composed of informal lower, middle, and upper members. Correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik boundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of sediment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increasing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vertical axis 16.6° counterclockwise with respect to the Indian subcontinent. Measurements of δ13C in paleosol carbonate indicate the predominance of C3 plants until 7.65 Ma, and the clear presence of C4 plants higher in the undated portion of the section. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal

Research Article| March 01, 2000 Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal T. P. Ojha; T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar R. F. Butler; R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar P. G. DeCelles; P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar D. Richards; D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar B. N. Upreti B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (3): 424–434. https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Article history received: 13 Aug 1998 rev-recd: 18 Mar 1999 accepted: 26 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. P. Ojha, R. F. Butler, J. Quade, P. G. DeCelles, D. Richards, B. N. Upreti; Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal. GSA Bulletin 2000;; 112 (3): 424–434. doi: https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The middle Miocene–Pliocene Siwalik Group was deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the Siwalik Group in central and western Nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies. Sandstones and paleosols of the Siwalik Group, however, generally display highly erratic paleomagnetic behavior during thermal demagnetization. On the basis of these observations, siltstones from a well-exposed, 2423-m-thick section of the Siwalik Group in Khutia Khola, far western Nepal, were sampled for magnetic polarity stratigraphy. The Siwalik Group is composed of informal lower, middle, and upper members. Correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik boundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of sediment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increasing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vertical axis 16.6° counterclockwise with respect to the Indian subcontinent. Measurements of δ13C in paleosol carbonate indicate the predominance of C3 plants until 7.65 Ma, and the clear presence of C4 plants higher in the undated portion of the section. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India

The latest Paleocene–middle Eocene Subathu Formation and the Oligocene–Miocene Dagshai and Kasauli Formations of the Indian foreland basin record the early evolution of the Himalayan fold-thrust belt. Sandstone petrography of the Subathu Formation shows a predominantly recycled sedimentary source, with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the Dagshai Formation. Sandstones in the Dagshai and Kasauli Formations consist predominantly of metapelitic detritus. The metamorphic grade of metapelitic lithic grains increases with time, from dominantly very low grade at the base of the Dagshai Formation to dominantly low grade in the Kasauli sandstones. Mudstone geochemistry documents the presence of a mafic-ultramafic source during the time of deposition of the Subathu Formation that becomes significantly less important by the time of deposition of the Dagshai Formation. Compositions of Subathu Formation detrital spinels show they were either derived from both mid-ocean ridge basalt–type and arc-type ophiolites or from an ophiolite of composite origin, and that southerly derivation from the Deccan Trap continental flood basalts is unlikely. Kasauli Formation garnet compositions suggest derivation from medium-grade metamorphic rocks to amphibolite facies. Subathu Formation composition reflects provenance influence from the Indus suture zone during latest Paleocene–middle Eocene time, indicating initiation of continent-continent collision and development of the foreland basin by this time. Suture-zone influence was drastically reduced by the time of deposition of the Dagshai Formation, when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the Himalayan foreland basin. The first appearance of Himalayan metamorphic detritus occurs in the Dagshai Formation at the close of Oligocene time, whereas the Kasauli Formation records erosion to deeper metamorphic levels during earliest Miocene time. The occurrence of garnet and higher grade metamorphic lithic grains during early Neogene time is coincident with the timing of displacement along the Main Central thrust and South Tibetan detachment zone. Composition of early foreland basin sediments from Pakistan (Balakot, Murree and Kamlial Formations) to Nepal (Bhainskati and Dumri Formations) and Bangladesh (Kopili, Barail Formations; Surma Group) indicates diachronous arrival of ophiolitic to low-grade metamorphic detritus derived respectively from the Indus Suture and early Himalayan thrust sheets in the north. This is consistent with progressively later closure of Neotethys along the suture, from latest Paleocene time in the west to Eocene time or even later in the east.

Thrust tectonics and evolution of domes and the syntaxis in eastern Himalaya, India

The low grade pre-Tertiary metamorphic nappe rocks, underlying widely exposed high grade crystalline rocks in eastern Himalaya, override the narrow frontal belt of Neogene Siwalik molasse sediments along the Main Boundary Thrust (MBT). Marine Eocene or early Miocene sediments are only exposed as narrow thrust slivers over wide lateral extent close to and beneath the MBT, and also within the northern parts of the Neogene sediments. The slivers of pre-Tertiary rocks within the Neogene belt have two modes of occurrence, more frequently they occur as klippen over the Neogene sequence and floored by the MBT, or as basement wedge, which at times preserve unconformable relation with the Neogene cover. Domal window structures in eastern Himalaya, north of frontal belt, expose low grade pre-Tertiary metasediments occurring beneath the crystalline nappe. The largest Siang window located close to eastern syntaxis of the Himalaya, is distinctive and exposes a duplex arch of late Palaeocene-Eocene sediments and the Abor volcanic rocks. The MBT represents the folded roof-thrust of the window. The overlying Himalayan pre-Tertiary nappe rocks are passively folded. The Siang and other windows from eastern Himalaya, located at the external fringes of the Himalayan nappe sheets, have many structural similarities. These are inferred to have evolved in a similar way and during the process of emplacement of the crystalline and low grade pre-Tertiary Himalayan nappes over the Tertiary foreland basin. During the early-mid Eocene period, contemporaneous to terminal collision at the northern margin of the Himalaya, there was continental flood basalt and other volcanic extrusion in the foreland basin located at the marginal parts of the Himalayan fold-thrust belt. The Eocene foreland rocks at present are largely tectonically concealed. Thrust movements involving the pre-Tertiary Himalayan nappes, the subjacent Tertiary sub-thrust rocks and their architecture influenced the structure of the Himalayan nappes at their external fringes and often-formed duplexes and windows.&#x0D; The Siang window was possibly produced by the interaction between the NE projecting indenture of the Indian continent that acted as an oblique crustal ramp over which the Himalayan and Trans-Himalayan nappes were propagated. Several nappes in the eastern limb of the dome are greatly attenuated in width and are overridden by the frontally advanced Trans-Himalayan granitoid nappe. Convergence of tectonic movements at the eastern syntaxis produced imbricate thrusts and duplex arch in the sub-thrust Palaeogene rocks, which breached the MBT and passively folded overlying Himalayan nappes. The western limb of the complimentary synform, west of the Siang window, is obliquely truncated by a major N-S trending dextral tear-fault, which has also climbed from the floor-thrust of the Siang window and has breached the MBT as well as the Main Central Thrust (MCT). It resembles the imbricate thrusts within the Palaeogene rocks from the Siang window. In other windows, the sub-thrust Tertiary rocks appear to have played more passive role and may not have been tectonically arched up.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

Geochemical characteristics of the Abor volcanic rocks, NE Himalaya, India: nature and early Eocene magmatism

The Abor volcanic rocks. exposed in the Siang window, NE Himalaya. occur as dismembered thrust-hound packages interbanded with fossiliferous shelf sediments of Upper Palaeocenc to Middle Eocene age. Their age is broadly contemporaneous with the age of collision between the Indian and Tibetan continents. They were. therefore, erupted in a convergent setting through channel ways in the leading edge of the Indian continent. The volcanic rocks occur beneath the Lesser Himalayan thrust sheets and were transported with them during their southward propagation. The Abor volcanic rocks were erupted in an early Palaeogene shallow basin located within marginal parts of the Himalayan fold-thrust belt south of the collision zone. They may thus represent foreland basin magmatism. The abundances of some major and large-ion-lithophile elements have been affected by alteration of the samples. Ti and some other trace elements remain unaffected however, and their abundances indicate that the volcanic rocks form a chemically coherent group of tholciites and alkalinc basalts. The tholeiitic and alkaline basalts reflect different degrees of melting, and low-pressure fractional crystallization involving olivine and plagioclase has played a significant role in the evolution of the basalts. On the basis of ratios of strongly incompatible elements, the two basalt types appear to have been generated from sources having similar character. This source is inferred to he enriched sub-continental mantle.

Crustal configuration of NW Himalaya based on modeling of gravity data

DOI = 10.3126/hjs.v5i7.1244 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.39