Middle Siwalik Research Articles

Detrital thermochronology and sediment petrology of the middle Siwaliks along the Muksar Khola section in eastern Nepal

Sediment derived from erosion of the Himalaya during the Miocene–Pliocene was deposited in the Himalayan foreland basin and is now exposed in the Siwalik Formation between the Main Boundary Thrust and Main Frontal Thrust. These sediments hold important information on orogenic exhumation during this time. In this preliminary study we sampled the middle Siwalik Formation in eastern Nepal along the Muksar Khola section for thermochronologic and sediment petrologic analysis. Detrital zircon fission-track thermochronology shows that the age signal of the middle Siwalik Formation of eastern Nepal has some similarities but also differences with published data for western-central Nepal. The Muksar Khola section samples show two static peaks at about 14 and >140 Ma, which is similar to the Siwalik Formation of western-central Nepal. In contrast, a signal of consistently fast exhumation was not observed in the upper middle Siwaliks in eastern Nepal. This could be related to a position of the Main Central Thrust further south, to overall less erosional exhumation of Higher Himalayan rocks, or to a reduced exposure of the underlying Lesser Himalayan units in comparison with western-central Nepal. Based on our apatite fission-track data we propose that above the section dated by Ojha et al. (2009) with magnetostratigraphy about 1000 m of overlying upper Siwalik Formation exist.

Palynofossils from the Middle Siwalik Sediments of Nepal with Remarks on Age and Palaeoenvironment

Palynofossils from the Middle Siwalik Sediments of Nepal with Remarks on Age and Palaeoenvironment

Neotectonic study along mountain front of northeast Himalaya, Arunachal Pradesh, India

To study neotectonics, the structural and morphotectonic aspects are studied along a part of mountain front region of Northeast Himalaya, Arunachal Pradesh, India. Unpaired river terraces are recognized near north of transverse Burai River exit, which is cut by an oblique fault. Across this fault, fluvial terraces are located at heights of 22.7 and 3 m, respectively, on the left and right banks. A water gap is formed along the river channel where the uplifted Middle Siwalik sandstone beds dipping 43° towards ENE direction, thrust over the Quaternary deposit consisting of boulders, cobbles, pebbles and sandy matrix. This river channel incised the bedrock across the intraformational Ramghat Thrust along which the rocks of the Middle Siwalik Formation thrust over the Upper Siwalik Formation. Recent reactivated fault activity is suggested north of the Himalayan Frontal Thrust that forms the youngest deforming front of the Himalaya. The uplifting along the stream channel is noticed extended for a distance of ~130 m and as a result the alluvial river channel became a bedrock river. The relative displacement of rocks is variable along the length of strike–slip faults developed later within the Ramghat Thrust zone. Longitudinal and Channel gradient profiles of Burai River exhibit knick points and increase in river gradient along the tapering ends of the profiles. The study suggests active out-of-sequence neotectonically active thrusting along the mountain front. Neotectonics combined with climatic factor during the Holocene times presents a virgin landscape environment for studying tectonic geomorphology.

Boselaphines (Artiodactyla, Ruminantia, Bovidae) from the Middle Siwaliks of Hasnot, Pakistan

In this paper, boselaphine material from several localities in the area of the Hasnot Pakistan, is described, identified, and discussed. Four species that belong to three different genera of the tribe Boselaphini have been found: Selenoportax vexillarius, S. lydekkeri, Pachyportax latidens and Eotragus sp. Eotragus sp. is reported for the first time from the Hasnot and consequently from other Upper Middle Siwalik sediments of Pakistan and equivalent strata of the world, extending the range of the genus from the Lower to the Middle Siwaliks. Reviewing the Siwaliks’ Selenoportax species, S. dhokpathanensis Akhtar and S. tatrotensis Akhtar are synonymized with S. lydekkeri and S. vexillarius, respectively.

Central Himalayan crystallines as the primary source for the sandstone–mudstone suites of the Siwalik Group: New geochemical evidence

Geochemistry of the Sub-Himalayan foreland basin Siwalik sediments has been used for interpreting the nature of the source rocks. This study has shown that the compositional changes are a function of stratigraphic height, demonstrated by the upward increase of P 2O 5, Na 2O, CaO, MgO and SiO 2 content from Lower to the Upper Siwalik rocks. On the other hand, K 2O, Fe 2O 3, TiO 2 and Al 2O 3 show decrease with the increasing stratigraphic height. These trends are a clear reflection of time-controlled changes in the source lithology. Ratios such as Eu/Eu*, (La/Lu) cn, La/Sc, Th/Sc, La/Co, and Cr/Th suggest a prominent felsic source area for the Siwalik sediments. Chondrite-normalized REE pattern with LREE enrichment and moderately flat HREE pattern with sharp negative Eu anomaly are attributed to a felsic source. Contrary to the existing belief, this study has ruled out any contribution from the mafic sources and highlighted the compositional similarities of Siwalik sediments with the crustal proxies like PAAS, NASC and UCC. The geochemical data point to a significant role played by the Precambrian and early Paleozoic granitic rocks of the Himalayan tectogene in shaping the composition of the foreland sediments. The variable CIA values and marked depletion in Na, Mg and Ca exhibited by the Lower, Middle and Upper Siwalik sediments reflect variable climatic zones and variations in the rate of tectonic uplift of the source area. Our results demonstrate that in the Lower Siwalik and part of the Middle Siwalik, Higher Himalayan Crystalline sequence (HHCS) was the primary source area with minor contributions by the meta-sedimentary succession of the Lesser Himalaya. Later, during the deposition of the upper part of the Middle Siwalik and Upper Siwalik, the source terrain switched positions. These two prominent source terrains supplied sediments in steadily changing proportion through time.

Description of Selenoportax vexillarius Molars from Dhok Pathan Village (Middle Siwaliks), Pakistan

Four molars of Selenoportax vexillarius are collected from village Dhok Pathan of the Middle Siwaliks. Out of four, two belong to upper dentition and the rest of two belong to lower dentition. The enamel plications vary owing to the ecological conditions and this feature can not be used for the comparison. The median basal pillars and median ribs are well developed in the molars and the comparative study is made with the type specimens of the species. The third molar of the species is broken anteriorly.

Dorcatherium majus, a Study of Upper Dentition from the Lower and Middle Siwaliks of Pakistan

Dorcatherium majus, a Study of Upper Dentition from the Lower and Middle Siwaliks of Pakistan

Cuticular studies on the fossil leaves from Churia (Siwalik) sediments of Arjun Khola Sequence, western Nepal

Two fossil leaf impressions along with their cuticle, collected from Middle Siwalik (Upper Miocene) sediments of Arjun Khola Sequence, about 3 km north-west of Lamhi in Deokhuri District of western Nepal, have been identified on the basis of their morphological and cuticular features. They resemble closely with the modern taxa. Sterculia coccinea Jack. and Diospyros toposia Ham. of the Sterculiaceae and Ebenaceae respectively. The present distribution of the modern equivalent taxa of the fossils and the presence of fungal spores in their cuticle collectively indicate the prevalence of warm humid climate in the area during Upper Miocene.

Evolution of fluvial system in Siwalik Group of Chatara-Barahakshetra area, east Nepal Himalaya

Four facies associations (i.e., FA1 to FA4) are recognised in the Siwalik molasse of the Chatara-Barahakshetra area in east Nepal. They are classified based on the proportion of sandstone and mudstone, individual bed thickness, and bedform types. The facies associations are intimately related to the lithostratigraphic units of the Siwalik Group. The lower and upper members of the Lower Siwaliks (FA1- and FA2-facies associations, respectively) are inferred to be the products of fine-grained meandering and flood flow-dominated meandering fluvial systems, respectively. Similarly, the lower and middle members or the Middle Siwaliks (FA3- and FA4-facies associations, respectively) are interpreted as the deposits of sandy meandering and sandy braided fluvial systems, respectively.

A multistorey sandstone complex in the Himalayan Foreland Basin, NW Himalaya, India

Ten parallel stratigraphic sections (1500–1800 m thick) spread over an area of >400 km 2 in Dehra Dun sub-basin (DSB) of the Himalayan Foreland Belt (HFB) were studied to understand the anatomy of one of the largest (900–1200 m thick) fluviatile Multistorey Sandstone Complexes (MSC) of the world using fluvial geometry, compositional data and magnetic fabrics over a magnetostratigraphically controlled master section. The multistorey sandstone complex, between 10–5 Ma representing the Middle Siwalik sub-Group, comprises of grey, medium- to fine-grained lithic arenites to lithic greywacke and records tectonic and/or climatic episodes. Three main facies associations are recognised: sandstone–mudstone, sandstone, and conglomerate-sandstone that represent fluvial fan deposit. Palaeocurrent data show radial palaeoflow pattern with major palaeodrainage towards the southern quadrant. The magnetic fabric studies suggest three major tectonic pulses. The first pulse at ∼8.7 Ma resulted in the development of major depocenter for the MSC, the second pulse at ∼7.65 Ma enhanced the sedimentation and progradation, while the third pulse at ∼6.5 Ma records overlapping earlier fluvial fan by another coarse grained piedmont alluvial fan. Thrust movement in the northern fold belt, basement lineaments and rate of basin subsidence controlled the lateral and vertical facies distribution and palaeodrainage. The sedimentation pattern of the multistorey complex is characterised by mainly sheet flood deposits of laterally avulsing unconfined braided rivers and resembles to the modern megafan sedimentation in the Ganga Basin to the south.

Geochemistry of Rewalsar Lake sediment, Lesser Himalaya, India: implications for source-area weathering, provenance and tectonic setting

Major, trace and rare earth element (REE) chemistry of fresh-water lake sediments, stituated in the middle Siwalik Formation (Pliocene), have been investigated. The log (Fe2O3/K2O) vs log (SiO2/Al2O3) plot of sediments discriminate the rock types of the catchment which are remarkably similar to those observed in thinsections. The Chemical Index of Alteration (CIA) between 75.2 to 78.1 and Chemical Index of Weathering (CIW) between 79.5 to 89.8 are higher than Post-Archean Australian Shale (PAAS), indicating moderate to high weathering at source area. The PAAS normalized REE patterns are similar and differentiate the major rock types on the basis of abundance of REEs in accordance to clay minerals, grain size, with the highest concentration in the finest grain sediments. It also indicates poor mixing of sediments. The consistency of REEs in sediments with those of rock types illustrate almost unaltered, unmodified transport of these from the provenance to the detritus deposited in the basin and also shows that the REEs are enriched in phyllosilicates. The major elements, Fe2O3, TiO2, MgO, K2O show strong positive correlation with Al2O3 and follow the trend of the latter, and REEs also indicate same characteristics, hence phyllosilicates are likely to be the promising host for REEs. The negative correlation of La, Yb and Y with Zr and positive correlation with REEs and Al2O3 do not indicate Zr control on REE distribution in the present case. The petrographic character of minerals in thinsection of rocks as well as of lake sediments and low Na2O/K2O ratio in the latter show short distance transport, textural immaturity and sorting of grains. The higher Zr/Th and Zr/Yb ratios in coarser sediments than in fine grain detritus are supporting evidence of sedimentary sorting. Chondrite normalized REE patterns and negative Eu anomaly of the lake sediments are similar to that of PAAS indicating that they may have been originally derived from differentiated silicic (felsic) and/or recycled sedimentary source and deposited in foreland basin. However, petrographic character, REEs and high field strength elements estimated on sediments, tends to support metamorphic source area for middle Siwalik detritus and is consistent to the observations made on rocks exposed in the catchment area, attesting to the rise of Himalaya in phases, exposing fresh rocks for erosion/weathering, lying northward in Lesser Himalayan domain. Plot of sediment chemistry of lakes on tectonic delineation diagrams show that Rewalsar detritus derived from middle Siwalik rocks are from passive margin setting. The discriminant function plots of sediment samples also indicate that they are derived from craton interior or a quartzose sedimentary orogenic terrain and deposited in a passive margin setting. The chemistry of Rewalsar sediments is thus a proxy for the middle Siwalik rocks deposited in the foreland basin and throw light on provenance, source area weathering, and tectonic setting of the source rock.

Clay-mineral distribution patterns in late Neogene fluvial sediments of the Subathu sub-basin, central sector of Himalayan foreland basin: implications for provenance and climate

The late Neogene (6–0.5 Ma) fluvial succession of the Subathu sub-basin, a part of the Himalayan foreland basin, comprises a 2.4-km-thick pile of conglomerate, grey and buff sandstone, and mudstone, representing Middle and Upper Siwalik subgroup. This basin is filled mainly by major trunk and piedmont drainage, which are nearly perpendicular to each other. The clay-mineral assemblages of this sedimentary succession have illite (7–82%), smectite (0–90%), chlorite (2–23%) and kaolinite (1–13%). The grey sandstones have moderate to abundant smectite (23–90%), whereas the buff sandstones have abundant illite (66–79%) and low to absent smectite (0–14%). The mudstones that dominates the succession (>50%) have clay-mineral assemblages similar to grey and buff sandstones, or intermediate proportion. The temporal distribution of clay minerals in mudstones shows occasional intense zigzag pattern with either smectite (3–81%) or illite (15–82%) abundance. The smectite-rich grey sandstones and mudstones are deposited by trunk drainage, and the illite-rich buff sandstones and mudstones are deposited by piedmont drainage. The intense zigzag distribution pattern of clay minerals in mudstone indicates interfingering of floods from trunk and piedmont drainages. The interfingering was severe, ranging between 4.8 and 3.36 Ma and between 2.60 and 1.77 Ma, related to tectonic activity. The association of smectite (>36%) bearing mudstones and piedmont source-derived buff sandstone and conglomerate towards the upper part of the section (above 1.77 Ma) suggests either floodwater of trunk drainage over spill on the fringe of piedmont alluvial fan or derivation from smectite bearing Middle Siwalik rocks, exposed due to the activity of an intra-foreland thrust (IFT) in the piedmont zone. The occurrence of smectite and its variable proportion with time suggests its probable derivation not only from the sparsely exposed basic rock in the catchment area but also from siliceous and metamorphic rocks under favourable climatic conditions between 6 and 0.5 Ma.

Sandstone body architecture and stratigraphic trends in the Middle Siwalik Succession of the Jammu area, India

Sedimentological analysis has been carried out on the coarsening upward Middle Siwalik Succession of the Jammu area, India. Major sandstone bodies are composed of several storeys, and are tens of metres thick, continuing along strike for tens of kilometers. These sandstone bodies are mostly multistoried, representing several channel fill events; few thin sandstone bodies are single channel fill events. The detailed facies architecture of three of these sandstone bodies is discussed.Vertical sediment variation over a 1450m thick succession was studied to document the changes. The 550m thick upper part of the succession has a greater proportion and thicker sandbodies than the underlying 900m of the succession. This is accompanied by coarser grain size and poorer representation of fine grained deposits in the upper part of the succession, as compared to lower part. In general, palaeocurrent trends show a gradual change from SSW in lower part to SSE in upper part, which may be due to a change in direction of slope. The probable explanations for these up-section variations are tectonic and climatic change.

Fossil woods from Upper Tertiary sediments of Jammu region (Jammu & Kashmir) North-West India and their significance

Petrified dicotyledonous woods are reported for the first time from the Middle Siwalik sediments of Jammu region (Jammu & Kashmir) in the northwestern part of India. The fossil woods resembling modem woods of Dipterocarpus Gaertn.f. (Dipterocarpaceae), Bischofia Blume (Euphorbiaceae), Cassia Linn. and Kingiodendron (Roxb.) Harms (Leguminosae) have been described in detail. Based on the distribution of comparable extant genera and species, prevalence of humid climate and existence of a mixed lowland tropical forest, comprising moist deciduous to evergreen elements, in the area have been deduced. Evidently Bischofia, Dipterocarpus and Kingiodendron existed in the Jammu region during the Late Tertiary time and subsequently shrunk in their distribution due to climatic change. They no longer grow in the area as they require more humid conditions. The occurrence of Dipterocarpus is particularly noteworthy as it extends the limit of Dipterocarpaceae in the geological past as far west as Jammu region in the north-west India. The arboreal C3 forms show that the landscape was well forested.

Lithofacies and palaeosol analysis of the Middle and Upper Siwalik Groups (Plio–Pleistocene), Haripur-Kolar section, Himachal Pradesh, India

The Middle–Upper Siwalik Groups (Plio–Pleistocene) are exposed at Haripur-Kolar, Himachal Pradesh, India. The succession is 2800-m thick and has been subdivided into Unit M1 of the Middle Siwalik and four units U1–U4 of the Upper Siwalik Group, on the basis of facies associations, and type and degree of development of palaeosols. The available magnetostratigraphic ages for bases of Units U1, U3 and U4 are 5.5, 2.6 and 1.77 Ma, respectively. The top of the section has been dated as 19 ka. Lithofacies association and palaeocurrent analysis indicates that the Middle and Upper Siwalik Groups were formed mainly by a basin transverse fluvial system. Two types of river systems, which differ in their size, can be documented in Unit-M1, Unit U1 and Unit-U2: one trunk river system similar to the modern Kosi and the other smaller river system, which formed tributaries to the former. The large rivers were mainly braided in nature. The Unit U3 and lower part of Unit U4 were deposited in the piedmont depositional system mainly by small braided streams and the upper part of the Unit U4 was deposited during a period of arid climate by sediment gravity-flows. Integration of fluvial lithofacies and pedofacies helps to identify two fluvial depositional systems from the modern Indo-Gangetic Plains. The Lowland System involved deposition on alluvial megafans and interfan areas, which resulted in sand-rich and mud-rich sequences with weekly developed soils. The Upland System allowed large tracts to act as high ground for thousands of years, thereby giving rise to sandstone poor intervals with moderately to strongly developed soils. Occurrence of moderately to strongly developed soils was controlled by uplifting and tilting of large tectonic blocks, without any relation to distance from the main channels. Rate of subsidence apparently controlled the occurrence of Lowland and Upland systems. Deposition of the Unit M1, Unit U1 and Unit U2 took place under Upland and Lowland systems, very similar to those identified from the modern Indo-Gangetic Plains. The warm and humid climate between 5.3 and 2.6 Ma led to the formation of red Alfisols with calcrete nodules at places. Slightly cooler and drier climate starting at about 2.6 Ma and approximately coinciding with the onset of global-scale glaciation, produced poorly developed yellow soil with common development of nodular calcretic horizon and calcitc material disseminated in the groundmass. At ca. 0.9 Ma, a probable significant change to still drier and cooler climate produced typical sediment gravity-flows in the piedmont system, that continued until at least up to 19 ka.

Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal

SUMMARY We use two gravity profiles that we measured across Central Nepal, in conjunction with existing data, to constrain the mechanical behaviour and the petrological structure of the lithosphere in the Himalayan collision zone. The data show (1) overcompensation of the foreland and undercompensation of the Higher Himalaya, as expected from the flexural support of the range; (2) a steep gravity gradient of the order of 1.3 mgal km x1 beneath the Higher Himalaya, suggesting a locally steeper Moho; and (3) a 10 km wide hinge in southern Tibet. We compare these data with a 2-D mechanical model in which the Indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland. The model assumes brittle Coulomb failure and nonlinear ductile flow that depends on local temperature, which is computed from a steadystate thermal model. The computed Moho fits seismological constraints and is consistent with the main trends in the observed Bouguer anomaly. It predicts an equivalent elastic thickness of 40‐50 km in the foreland. The flexural rigidity decreases northwards due to thermal and flexural weakening, resulting in a steeper Moho dip beneath the high range. Residuals at short wavelengths (over distances of 20‐30 km) are interpreted in terms of (1) sediment compaction in the foreland (Dr=150 kg m x3 between the Lower and Middle Siwaliks); (2) the contact between the Tertiary molasse and the meta-sediments of the Lesser Himalaya at the MBT (Dr=220 kg m x3 ); and (3) the Palung granite intrusion in the Lesser Himalaya (Dr=80 kg m x3 ). Finally, if petrological transformations expected from the local (P, T) are assumed, a gravity signature of the order of 250 mgal is predicted north of the Lesser Himalaya, essentially due to eclogitization of the lower crust, which is inconsistent with the gravity data. We conclude that eclogitization of the Indian crust does not take place as expected from a steady-state local equilibrium assumption. We show, however, that eclogitization might actually occur beneath southern Tibet, where it could explain the hinge observed in the gravity data. We suspect that these eclogites are subducted with the Indian lithosphere.

New specimen of Conohyus indicus (Lydekker, 1884) (Mammalia: Suidae) from the base of the Late Miocene, Jammu, India

Conohyus indicus is a poorly known tetraconodont suid from the summit of the Lower Siwaliks and the base of the Middle Siwaliks of the Indian Subcontinent. Even though it was first recorded well over a century ago, only 13 specimens have since been described in the literature, consisting of isolated teeth and incomplete mandible and maxilla fragments. We here describe another fragmentary mandible from the Ramnagar Member (uppermost Middle Miocene to basal Late Miocene) of the Siwalik Group, which contains the left canine lacking its tip, the alveolus of P/1 and the left P/2 – P/3 and right P/3 - M/1. The new specimen, albeit incomplete,provides interesting information about the anterior parts of the jaw and throws light on the systematic position of the species as well as the recently described species Conohyus thailandicus from Thailand.

Biostratigraphy and geology of the neogene Siwalik group of the Surai Khola and Rato Khola areas in Nepal

An almost complete sequence of Neogene molasse sediments of the Siwalik Group in the Surai Khola (‘khola’ means small river in Nepali) area in Nepal, which was divided into five lithological units, includes the upper part of the Lower Siwaliks, the Middle and the Upper Siwaliks. It discloses a tectonically uncomplicated southern belt (i.e. the Surai Khola sequence) and a tectonically much disturbed northern belt. The Upper Siwaliks at Surai and Rato Khola yield a rich vertebrate fauna, contained within thick bodies of multistoried sandstones. The data from the pollen and plant fossils indicate vegetational changes from tropical evergreen forests in the Lower and Middle Siwaliks to dry, deciduous forests and grasslands by the beginning of the Upper Siwaliks. Biostratigraphically the Upper Siwalik sediments can be compared with the Hexaprotodon sivalensis and the Elephas planifrons Interval Zones of Pakistan and with the Tatrot and Pinjor formations of Northwest India. Palaeomagnetic dating of the Surai Khola sequence indicate a time scale from 13 to ca. 1 million years.

Palynological evidence for vegetation development and climatic change in the Sub-Himalayan Zone (Neogene, Central Nepal)

A palynological study on Miocene–Pleistocene sediments exposed at Surai Khola in central Nepal yields new information on vegetation and climate change over the last ∼11.5 Ma. Our results show that during the late Middle Miocene to early Late Miocene (∼11.5–∼8 Ma, Lower Siwalik) the Himalayan foothills and the Gangetic floodplain of Nepal were mainly forested with subtropical to temperate broad-leafed (b.l.) taxa (e.g. Quercus, Lithocarpus/ Castanopsis, Alnus), and tropical forest taxa respectively. Grasses were present at the time but were not abundant. Between the early to late Late Miocene (∼8–∼6.5 Ma; lower Middle Siwalik) grassland replaced subtropical and temperate b.l. forest. This change may be related to disturbance of the vegetation on the slopes due to uplift, perhaps enhanced by intensification of the monsoon. Between late Late Miocene to Pliocene/Early Pleistocene? (∼6.5–<2 Ma; Middle–Upper Siwalik) the grassland vegetation became well established and the influence of the subtropical climax vegetation is minor, with some occasional revivals. An increase in peak discharge in the fluvial system (also related to monsoonal intensification?) is recognized by an increase of the aquatic taxon Potamogeton and better preservation of all specimens. Enhanced seasonal flooding in the system during the Pliocene (∼5.5–∼3.5 Ma; top Middle Siwalik) may have produced local lacustrine conditions on the overbanks, evidenced by an abundance of algae ( Spirogyra) and pteridophytes in the pollen assemblages. Evidence of climatic cooling between ∼6.5–∼5 Ma ago is indicated by the onset of steppe taxa, decrease of tropical forest taxa and disappearance of certain Dipterocarpaceae. Pollen assemblages from the Surai Khola section suggest complex vegetation changes, of which the shift to C 4 grass dominance was only one. Global climatic cooling, the intensification of the monsoon, and the Late Miocene uplift may all have contributed to the phytogeography in the Sub-Himalayan Zone.

The geometry and kinematics of the Main Boundary Thrust and related neotectonics in the Darjiling Himalayan fold-and-thrust belt, West Bengal, India

The Main Boundary Thrust (MBT) is well-exposed in a section of the Tista River valley in the Darjiling Himalayan fold-and-thrust belt. The fault trace trends E–W across the N–S flowing Tista and thrusts Gondwana (Permian) age sandstones over Lower Siwalik (Miocene) age sandstones. The MBT has been folded into a fault propagation synform–antiform pair by tectonic activity along a younger, South Kalijhora Thrust (SKT) in its footwall that transports Lower Siwalik rocks over Middle Siwalik sandstones; the SKT cuts through the fault propagation structure preserving the MBT fault–propagation fold pair in its hanging wall. The motion along the SKT rotated the bedding and the limbs of the folded MBT in its hanging wall. Footwall imbrication in front of the SKT further rotated the folded MBT. Footwall imbrication continued to about 30 km south of the study area in the Middle Siwalik and younger section. The region has been subsequently reactivated as evident from uplifted gravel beds and strath terraces along the folded trace of the MBT fault zone and also in the footwall of the SKT. The reactivation resulted in the formation of connective splays off the existing faults, probably forming horses of a connecting splay duplex in the sub-surface within the Siwalik section. The motion along these splays passively folded overlying beds, and formed uplifted gravel beds and strath terraces along the local drainage. The rotations resulting from footwall imbrication and reactivated splays probably caused the southern limb of the fault propagation synform to become overturned. The duplexing also raised the river bed and arrested the flow of the River Tista causing the river to ‘pond’ and deposit its load, resulting in the formation of a bar upstream of the MBT trace. The tectonic reactivation of the area points to taper-building activities in a subcritical wedge probably in response to monsoon-induced excessive erosion in the middle of the Darjiling–Sikkim–Tibet Himalayan thrust wedge.