North Indian Margin Research Articles

Detrital zircon, monazite and tourmaline reveal the magmatic and metamorphic history of the Himalayan orogen from Archean to present

The litho-tectonic structure of the Himalaya is the result of multiple orogenic cycles. However, its pre-Cenozoic history is not well understood, which limits our ability to constrain the stratigraphy, structure, and tectonism in the Himalayan orogen. This issue can be potentially addressed by detrital mineral geochronology and isotopic studies, which can be used to reconstruct the magmatic and metamorphic records from past to present. In this study, we collected three river sand samples from the central Himalaya, which originated from the interior of Ama Drime Massif (ADM), Greater Himalayan Sequence (GHS) outside of the ADM, and the boundary between the ADM and GHS, respectively, and then conducted a geochronological study of detrital zircon and monazite, and obtained detrital zircon Hf and tourmaline B isotope data. Detrital zircon of the river sand from the ADM only records the 1.9–1.7 Ga magmatic event, while age groups of 3.5–2.4, 1.1–0.7, and 0.6–0.4 Ga are observed in the GHS outside the ADM. Detrital tourmaline from the GHS has heavy B isotopic signatures with δ11B = −15‰ to −7‰, similar to the values for GHS leucogranites. Detrital tourmaline from the ADM has δ11B = −20‰ to −13‰, similar to the Lesser Himalayan Sequence (LHS). Integrating these results with previous metamorphic and thermochronological works, we interpret that the ADM represents deeply buried LHS rocks beneath the GHS, which underwent a significant uplift assisted by the detachments on its two sides during the Miocene. Furthermore, zircon Hf isotopic variations indicate that the north Indian margin was an active convergent boundary during the Paleoproterozoic, Neoproterozoic, and Paleozoic, in relation to supercontinent amalgamation. Detrital monazite records two major thermal events in the Cenozoic era. A large proportion of detrital monazite yield ages from 25 to 20 Ma, indicating that the collision entered a mature stage at this time, during which thickened Himalayan crust formed and widespread anatexis occurred. A tectonic style transition in the Himalayan hinterland from shortening that thickened the crust to extension that thinned it occurred during the middle to late Miocene (15–10 Ma) in the Himalaya.

Trace fossils, depositional context, and paleogeography of the upper Tal Group (upper lower Cambrian), Lesser Himalaya, India: a Gondwanan succession with no affinities to the Avalonia microcontinent – discussion of paper by Singh et al. (2019)

Terminal Ediacaran–late early Cambrian deposition, faunas and passive margin evolution of the north Indian margin are recorded in the Nigali Dhar syncline succession. Restudy of the upper Tal Group (upper lower Cambrian Koti Dhaman Formation, KDF) ichnofauna from the Khud-Drabil section reduces it to 18 confidently named forms. The lower KDF (Lower Quartzite Member) Cruziana-Rusophycus assemblage is in subtidal (not intertidal) sandsheet facies. The overlying black Shale Member (SM) records trans-East Gondwanan deepening, not intertidal facies, in the Palaeolenus Zone. The SM, with low diversity Planolites-Palaeophycus assemblage, is overlain by subtidal (not intertidal) sandsheet facies of the middle KDF (Arkosic Sandstone Member, ASM) with shallow burrowers and furrowers (Gordia marina assemblage, new; Cruziana ichnofacies). KDF faunas with Cruziana and Rusophycus are similar to coeval, shallow marine associations elsewhere in Gondwana and NW Laurentia. Interpretation of a second KDF section 20 km from Khud-Drabil has confused an understanding of Lesser Himalaya geologic evolution as it claims Ordovician Cruziana species in the ASM and an angular SM–ASM unconformity caused by the Cambrian–Ordovician boundary Kurgiakh orogeny. However, upper lower Cambrian microfaunas occur in and above the ASM, while the angular SM–ASM unconformity is consistent with submarine sliding. KDF-type ichnofaunas do not show a tropical location of Avalonia, which has the distinctive lithofacies and biotas of a high-latitude continent unrelated to Gondwana.

Neogene Kinematic Evolution and Exhumation of the NW India Himalaya: Zircon Geo‐ and Thermochronometric Insights From the Fold‐Thrust Belt and Foreland Basin

AbstractThe kinematic and exhumational evolution of the Lesser Himalaya (LH) remains a topic of debate. In NW India, the stratigraphically diverse LH is separated into the inner LH (iLH) of late Paleo‐Mesoproterozoic rocks and the outer LH (oLH) of Cryogenian to Cambrian rocks. Contradictory models regarding the age and structural affinity of the Tons thrust—a prominent structure bounding the oLH and iLH—are grounded in conflicting positions of the oLH prior to Himalayan orogenesis. This study presents new zircon (U‐Th)/He and U‐Pb ages from the thrust belt and foreland basin of NW India that refine the kinematic and exhumational evolution of the LH. Combined cooling ages and foreland provenance data support emplacement and unroofing of the oLH via southward in‐sequence propagation of the Tons thrust by middle Miocene time. This requires that, before India–Asia collision, the oLH was positioned as the southernmost succession of Neoproterozoic–Cambrian strata along the north Indian margin. This is further supported by detrital zircon U‐Pb ages from Cretaceous–Paleogene strata (Singtali Formation) unconformably overlying the oLH, which yield diagnostic Cretaceous detrital zircons correlative with coeval strata in the frontal Himalaya of Nepal. A pulse of rapid exhumation along the Tons thrust front at ~16 Ma was followed by east‐to‐west development of a midcrustal ramp at ~12 Ma which facilitated diachronous iLH duplexing. This duplexing shifted the locus of maximum exhumation northward, eroding away Main Central Thrust hanging wall rocks until the iLH breached the surface at ~9–11 Ma near Nepal and by ~3–7 Ma within the Kullu‐Rampur window.

Severest crisis overlooked-Worst disruption of terrestrial environments postdates the Permian-Triassic mass extinction.

Generally Early Triassic floras are believed to be depauperate, suffering from protracted recovery following the Permian–Triassic extinction event. Here we present palynological data of an expanded East Greenland section documenting recovered floras in the basal Triassic (Griesbachian) and a subsequent fundamental floral turnover, postdating the Permian–Triassic boundary extinction by about 500 kyrs. This event is marked by a swap in dominating floral elements, changing from gymnosperm pollen-dominated associations in the Griesbachian to lycopsid spore-dominated assemblages in the Dienerian. This turnover coincides with an extreme δ13Corg negative shift revealing a severe environmental crisis, probably induced by volcanic outbursts of the Siberian Traps, accompanied by a climatic turnover, changing from cool and dry in the Griesbachian to hot and humid in the Dienerian. Estimates of sedimentation rates suggest that this environmental alteration took place within some 1000 years. Similar, coeval changes documented on the North Indian Margin (Pakistan) and the Bowen Basin (Australia) indicate the global extent of this crisis. Our results evidence the first profound disruption of the recovery of terrestrial environments about 500kyrs after the Permian–Triassic extinction event. It was followed by another crisis, about 1myrs later thus, the Early Triassic can be characterised as a time of successive environmental crises.

Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)

A line-length balanced palinspastic reconstruction across the Himachal Himalaya is presented, highlighting different phases of Himalayan tectonic development: Eocene shortening of the north Indian margin, Early–Middle Miocene emplacement of the crystalline core, and subsequent growth of the range by underplating. The total preserved shortening is 518 km (72%). The reconstruction demonstrates geometric feasibility of crystalline core emplacement via tectonic wedging, i.e., between a south-directed thrust (the Main Central thrust) and a north-directed backthrust (the South Tibet detachment). Crystalline core exposure between these faults occurs ca. 5 Ma in the reconstruction; initial exposure of these crystalline rocks ca. 11 Ma probably occurred in the hinterland within core complexes accommodating east-west extension. After Early–Middle Miocene crystalline core emplacement, ongoing orogenic growth is dominated by underplating processes. Out-of-sequence faulting accomplishes <5% of this shortening; frontal accretion of foreland rocks occurs, but the resulting imbricate fan is largely eroded away. Over the past 3–5 m.y., an antiformal stack of mid-crustal horses and a hinterland-dipping duplex of upper crustal horses develop simultaneously. Minimum total shortening across the western Himalaya (from undeformed foreland to the India-Asia suture) is estimated by adding the new results and results from prior work to the north; an estimate of ∼703–773 km of assessed shortening is calculated. However, because large portions of the regional deformation remain unassessed, estimates of ∼900–1100 km may more accurately reflect the minimum preserved shortening here. This range is comparable to the ∼1350 km of shortening estimated by plate circuit reconstructions for this region. Apparent mismatch between geologic and plate circuit shortening estimates has recently instigated the new Greater India Basin hypothesis for two Cenozoic collisions along Asia’s southern margin, but the new results suggest that this mismatch may not exist.

U-Pb zircon geochronology of major lithologic units in the eastern Himalaya: Implications for the origin and assembly of Himalayan rocks

Models for the origin and deformation of Himalayan rocks are dependent upon geometric and age relationships between major units. We present field mapping and U-Pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern Himalaya, revealing that: (1) the South Tibet detachment along the Bhutan-China border is a top-to-the-north ductile shear zone; (2) Late Triassic and Early Cretaceous sedimentary samples from the northern Indian margin show a similar age range of detrital zircons from ca. 3500 Ma to ca. 200 Ma, but the Late Triassic rocks are distinguished by a significant age cluster between ca. 280 and ca. 220 Ma and a well-defined age peak at ca. 570 Ma, (3) an augen gneiss in the South Tibet detachment shear zone in southeast Tibet has a Cambrian–Ordovician crystallization age, (4) Main Central thrust hanging-wall paragneiss and footwall quartzites from the far western Arunachal Himalaya share similar provenance and Late Proterozoic maximum depositional ages, and (5) Main Central thrust footwall metagraywacke from the central western Arunachal Himalaya has a Paleoproterozoic maximum depositional age, indicated by a single prominent age peak of ca. 1780 Ma. Recent work in the eastern Himalaya demonstrates that in the early-middle Miocene, the Himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south. A proposed subsequent (middle Miocene) brittle low-angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here. The ca. 280–220 Ma detrital zircons of the Late Triassic strata are derived from an arc developed along the northern margin of the Lhasa terrane. Detritus from this arc was deposited on the northern margin of India during India-Lhasa rifting. Along-strike heterogeneity in Main Central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western Arunachal. Nonetheless, the Late Proterozoic rocks in the Main Central thrust hanging wall and footwall in far western Arunachal can be correlated to each other, and to previously analyzed rocks in the South Tibet detachment hanging wall to the west and in the Indian craton to the south. These findings are synthesized in a reconstruction showing Late Triassic India-Lhasa rifting and Cenozoic eastern Himalayan construction via in situ thrusting of basement and cover sequences along the north Indian margin.

Nature of magmatism and sedimentation at a Columbia active margin: Insights from combined U–Pb and Lu–Hf isotope data of detrital zircons from NW India

Detrital zircons from a Palaeoproterozoic quartzite, deposited between 1.85 and 1.82Ga in the northern Aravalli orogen of NW India, show a distinctive age peak of ca. 1.85Ga and variable, but largely subchondritic εHf1.85 Ga between −1.3 and −21.0 corresponding to hafnium model ages of 2.5 to 3.6Ga. These data indicate an important period of reworking of ancient (Eo- to Neoarchaean), strongly heterogeneous continental crust at this time. Prevalence of ca. 1.85Ga subduction-related granitoids, almost identical U–Pb age spectra and εHft of detrital zircons in ca. 1.85Ga metasedimentary rocks in the Aravalli orogen and the inner Lesser Himalaya indicate similar sediment provenances and thus a geological connection between these two terranes during late Palaeoproterozoic. All together, the data constrain a rapid succession of sedimentation, metamorphism and subduction-related magmatic activity and support the interpretation of an active geodynamic realm along the entire north Indian margin at ca. 1.85Ga. Comparison of detrital zircon data in conjunction with published paleomagnetic data from north India and other crustal blocks of the Columbia supercontinent, additionally, suggest a close affinity of north India with Madagascar, the Cathaysia block of South China and South Korea during Columbia times.

Palynology of the Lower Triassic succession of Tulong, South Tibet — Evidence for early recovery of gymnosperms

The thermally altered but otherwise well preserved Early Triassic palynomorphs from the Tulong area (South Tibet) allowed for qualitative and quantitative analyses and the differentiation of two distinct palynofloras, a Smithian lycopod-dominated flora and a Spathian gymnosperm-dominated flora. This suggests that gymnosperm vegetation recovered about 2Ma after the end-Permian event on the subtropical North Indian Margin. Together with previously described successions from Pakistan, Australia, and Norway the new palynofloral record suggests an earlier establishment of diverse, gymnosperm-rich plant communities after the end-Permian event than previously proposed. The observed change in the ratio of the two major plant groups (pteridophytes/gymnosperms) in these two assemblages supports the global extent of a major climate change near the Smithian–Spathian boundary. The observed palynofloral patterns are closely related to the changes in the carbon isotope record, which suggests that the floral recovery dynamics were linked to environmental conditions.

Climatic oscillations at the onset of the Mesozoic inferred from palynological records from the North Indian Margin

The beginning of the Mesozoic, the Early Triassic, is characterized by several ecological perturbations following the end-Permian mass extinction. They are reflected in multiple C-isotope excursions coupled with climatic changes. Here we present palynological data from two accurately dated sections from the North Indian Margin (Pakistan and South Tibet). The climate of the Early Triassic was controlled by persistent monsoon circulation. The spore/pollen ratios, used as a proxy for humidity changes, indicate several significant climatic changes coinciding with C-isotope excursions. Comparison with published climate model simulations reveals that the climatic shifts were induced by orbital forcing and probably represent eccentricity cycles. Humidity peaks indicate an insolation forced shift of the intertropical convergence zone towards the North Indian Margin. Comparison with palynological data from Norway and other proxies reveal that the profound climatic change from humid to drier climate across the Smithian–Spathian boundary represents a global event, which affected southern and northern mid-latitudes and coincided with major ammonoid and conodont extinction events. This implies that increased greenhouse gas concentrations owing to recurring volcanic pulses increased the climate system sensitivity, resulting in climatic changes in distant parts of the world. Our data strongly support a link between C-isotope excursions, climatic changes and biotic responses.

Provenance of the Greater Himalayan sequence: Evidence from mafic granulites and amphibolites in NW Bhutan

Mafic granulites and amphibolites in the Masang Kang area of NW Bhutan Himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history. Bulk-rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics. U–Pb zircon SIMS data suggest an age of 1742 ± 39 Ma for mafic magmatism. The age-corrected ε Nd (1742) values of the rocks are highly variable, ranging from high positive (+ 8.4) to negative (− 3.3). The positive value suggests a primitive magma source, similar to that of rift-related tholeiites. We suggest that the rocks of the Masang Kang suite were produced during a major late Paleoproterozoic thermal event that caused the mobilization and enrichment of the sub-continental lithospheric mantle beneath the north Indian margin. The geochemical signature of these rift-related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub-lithospheric mantle may have been derived. Though their occurrence is rare, Paleoproterozoic igneous rocks within the Greater Himalayan Sequence (GHS), in addition to sources identified throughout the LHS, may have contributed to the detrital zircon population that form the 1.7–1.9 Ga peak in the age spectra of the Lesser Himalayan Sequence (LHS). In addition, the coeval Paleoproterozoic magmatism in both LHS and GHS suggests that the two lithotectonic units may have belonged to the same continental plate at that time period.

Subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India

Detailed geological field mapping has allowed the restoration of two full stratigraphic sections through the highly deformed Mesozoic and Early Tertiary fold and thrust belt of the north Indian continental margin. The two sections, which are representative of a proximal and a distal facies on the margin, have been backstripped using standard techniques. Profiles of the tectonic subsidence and uplift through the pre-collisional history of the margin have been constructed and compared with the predictions of simple thermal and mechanical models. The pre-collisional history can be explained by a thermal model with an initial age of rifting of 270 Ma and a stretching factor, β , of c . 1.2. This model accounts for the general exponential decrease in the backstripped tectonic subsidence. The model fails, however, to completely explain the subsidence and uplift history of the margin since the late Cretaceous. The history during this time is characterized by uplift at the most proximal location and an increase in subsidence at the most distal location. We attribute these differential vertical movements to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model is one in which a 70 km wide wedge-shaped load, tapering from 10 to 0 km thick, is emplaced on rifted lithosphere with an elastic thickness, T e , of 5–10 km. These results, which are in accord with the late Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the T e structure of extended continental lithosphere 120–150 Ma after a rifting event.

Photang thrust sheet: an accretionary complex structurally below the Spontang ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India

The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.

Rift-related sedimentation and volcanism of the north-Indian margin inferred from a Permian–Triassic exotic block at Lamayuru, Indus suture zone (Ladakh Himalaya) and regional comparisons

Stratigraphical, petrographical and geochemical work on an important limestone exotic within the Indus suture zone, near Lamayuru, reveals the following history of events: (i) Late Permian deposition on a shallow-water carbonate platform open to the Neotethys Ocean; (ii) Late Permian–Early Triassic break-up, tilting, fissuring and subsidence; (iii) Early Triassic mantling by ammonite-bearing, pink pelagic carbonate and chlorite-rich volcaniclastic sediment; (iv) Deposition of Fe, Mn- and/or Mn-rich oxide-sediments, as crusts and fissure fills. The metal-rich oxides were precipitated from hydrothermal solutions related to nearby volcanism; (v) Late Triassic? eruption of pillowed and massive basalts of within-plate type on an irregular seafloor. Taking account of evidence from similar exotic units associated with Upper Cretaceous melange beneath the Spontang Ophiolite further south in the Zanskar Mountains (e.g. Photoskar unit), the Lamayuru exotic is interpreted as part of a small carbonate platform formed somewhere between the Zanskar/Lamayuru passive continental margin and a Permian–Triassic Neotethyan spreading axis to the north. The carbonates probably capped a volcanic seamount that later collapsed and was covered by pelagic sediments and basic extrusives. The Lamayuru exotic was later detached from its igneous basement and tectonically emplaced, probably in the latest Cretaceous, and was also influenced by Early Tertiary suturing of Neotethys, and mid–late Tertiary backthrusting.

The geodynamic evolution of the South-Tethyan, margin in Zanskar, NW-Himalaya, as revealed by the Spongtang ophiolitic melanges

— The composite nappe of the Spongtang ophiolite is thrust over Mesozoic and Cenozoic sediments of the North-Indian margin and shelf. The ophiolitic peridotiles tectonic-ally overlie a sedimentary melange, the detailed stratigraphy of which reveals the evolution of the South-Tethyan margin from its opening to its closure. The matrix of the melange is dated as Upper Campanian to Lower Eocene. Volcano-sedimentary olistoliths indicate carbonate platform sedimentation associated with alkaline lavas in the Permian, followed by more pelagic sedimentation of Upper Triassic and mid- Cretaceous age. They derived from external platforms near to the Indian shelf, but separated from it. Composite olistoliths of mid to late Cretaceous age, containing Permian elements, are found in the upper Cretaceous to lower Eocene wildflysch. Primary contacts of melange upon serpentinites indicates that the deeply eroded oceanic crust served partly as a substratum for the melange.