Himalayan Orogen Research Articles

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

Petrogenesis of the Himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U-Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8±0.1Ma, 15.1±0.1Ma, and 12.0±0.1Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), low zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating that they were derived from the HHCS. The characteristics of relatively high Na2O and Rb contents, high Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, and Sr contents indicate that both the ca. 18Ma Lhozag tourmaline leucogranites and the ca. 15Ma Lhozag two-mica granites were derived from fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12Ma Khula Kangri two-mica granites imply that these granites are derived from fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO, and CaO contents, higher Rb concentrations and Rb/Sr ratios could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent melting to fluid-present melting, implying that there were P-T-XH2O variations in the deep crust. The tectonic evolution would give rise to variation of P-T-XH2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behavior of the Lhozag leucogranites, which implies the tectonic evolution from earlier N–S extension to later E–W extension in the eastern Himalaya at ca. 12Ma.

Leucogranite geochronological constraints on the termination of the South Tibetan Detachment in eastern Himalaya

The South Tibetan Detachment System (STDS) developed between the very-low-grade Tethyan Himalayan Sequence (THS) and the high-grade metamorphic Greater Himalayan Sequence (GHS) over an along-strike distance exceeding 2000km within the Himalayan orogen. Motion along the STDS has been associated with the exhumation of the GHS and the uplift of the Himalayan Mountains. Geological field mapping and U–Th–Pb geochronological analyses were performed to trace and date the STDS in the Yadong region of southern Tibet, where it is a top-to-the-north shear zone juxtaposing the Yadong klippe of the THS with the high-grade GHS. This shear zone is intruded by multiple generations of leucogranites emplaced mainly at ca. 23Ma, ca. 20–18Ma and ca. 17–16Ma. The undeformed textures of the leucogranites and their cross-cutting relationships with the shear zone indicate that ductile motion along the STDS terminated by ca. 20Ma. This is largely consistent with the deformation history of the STDS in central and western Himalaya. The termination of shearing along the STDS during the Early Miocene occurred well before the rapid cooling of the GHS during the Middle Miocene, suggesting that exhumation and decompression of the GHS was still ongoing after the cessation of motion along the STDS.

Migmatization and intrusion of “S‐type” granites in the trans‐Himalayan Ladakh Magmatic Arc of north India and their bearing on Indo‐Eurasian collisional tectonics

The Ladakh Magmatic Arc (LMA) of trans‐Himalaya was formed due to subduction of the Tethyan Ocean beneath the Eurasian continental plate. This was followed by the Indo‐Eurasian continental collision that gave rise to the Himalayan orogen. In this work, we present results from U–Pb geochronology of zircon of 2 samples from a previously unknown migmatite body within the LMA and one two‐mica granite body that lies adjacent to this migmatite and intrudes the LMA. One sample of mesosome from this migmatite gives a crystallization age of 66.9 ± 1.5 Ma (n = 11 zircon) along with a younger population of zircon (n = 24) giving an age of 62.1 ± 1.3 Ma. This sample also shows the presence of a younger cluster of zircon (n = 7) giving a concordant age of 50.0 ± 2.9 Ma. The other sample of one leucosome contains a cluster of inherited zircon (n = 5) giving a concordant age of 72.86 ± 0.83 Ma. However, the majority of the population (n = 10) gives a concordant crystallization age of 55.31 ± 1.5 Ma. Limited Hf isotopic analysis of the leucosome shows positive εHft values ranging from 7 to 15, comparable to that of the LMA. High and positive εHft values and absence of any Palaeozoic xenocrystic zircon help us infer that the Indian continental crust was not involved in the partial melting and leucosome generation process. Therefore, this event of migmatization predates Indo‐Eurasian collision. By comparing Th/U ratio versus age for both the mesosome and leucosome and their respective age spectra, we infer that this part of the LMA underwent pronounced partial melting between 55 and 60 Ma and generated leucosome till ~50 Ma. The two‐mica granite contains muscovite, indicating its crustal origin. It gives 2 closely spaced age spectra of 38.1 ± 1.1 Ma (n = 7) and 34.31 ± 0.48 Ma (n = 4). This two‐mica granite also contains xenocrysts of Palaeozoic ages. By integrating our studies with earlier works, we infer that this migmatization of the LMA indicates its collision with the Indian plate, prior to Indo‐Eurasian continental accretion, at 55–50 Ma. The two‐mica granite contains xenocrystic material from both the LMA and the Indian continental crust and indicates S‐type granite magmatism related to India–Eurasia continental collision at 35–40 Ma.

Timing of fluvial terrace formation and concomitant travertine deposition in the upper Sutlej River (Tirthapuri, southwestern Tibet) and paleoclimatic implications

Travertines are carbonates precipitated from hydrothermal springs and are relatively common on the Tibetan plateau and occur along tectonically active faults. The Karakoram fault system is an active strike-slip fault that extends from the Pamir into southwestern Tibet, where it controls the course of the upper Sutlej River and the occurrence of several hydrothermal springs, including the Tirthapuri hot springs. Multiple fluvial terraces that are partly capped by travertine are preserved in the Tirthapuri area. Four main fluvial terrace levels (labelled as T1 to T4 with increasing height above river) were identified and several meter-thick travertine platforms occur on the current river level as well as the T2 and T3 terraces. Sedimentological and petrographic observations suggest that the travertine platforms were deposited on active floodplains of the paleo- and modern Sutlej River, and preserved from fluvial erosion because travertine precipitation was immediately followed by vertical river-bed incision and thus terrace abandonment. Results of 230Th/U in combination with luminescence dating show that the deposition of travertine platform and river incision that led to the formation of T3 terrace (∼93 m above the Sutlej) took place at ca. 127.5 ka. The development of terrace T2 and overlying travertine platform (∼28 m above the Sutlej) occurred between ca. 10.0 and 8.8 ka. Fluvial incision has arrived at the modern level at least ca. 0.2 ka ago. Both the travertine deposition and major river incision are likely triggered by the intensified Indian summer monsoon and are linked to phases of maximum monsoon strength. During strong monsoon phases, a large quantity of moisture is transported into southwestern Tibet, activating hot springs and thus travertine precipitation, facilitating fluvial incision and stripping off sediments from the regional hill-slopes. At least over the last glacial cycle we suggest that the Tirthapuri travertine and associated fluvial incision are sensitive indicators of (peak) monsoonal activity and can thus provide valuable insights into past climate change and climate-driven landscape evolution on the southwestern Tibetan Plateau. Comparison of our findings with published data further suggests that monsoon-controlled fluvial aggradation and incision during the early Holocene is synchronous in southwestern Tibet and the adjacent sector of the Himalayan orogen (north-western Sub-to High Himalaya).

Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India: Implications for petrogenesis

The Naga Hills Ophiolite (NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite, diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx. Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features. The basalts are characterized by 44.1–45.6 wt.% of SiO2 with 28–38 of Mg#, and the gabbros by 38.7–43.7 wt.% of SiO2, and 26–79 of Mg#. The ultramafic rocks are characterized by 37.4–52.2 wt.% of SiO2, and 80–88 of Mg#. In multi-element diagrams (spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr, Nb, Sr and a gentle negative anomaly of P. However, the rare earth element (REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE (LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite (IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340–4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97–1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources (fertile and refractory).

Structural features and proto-type basin reconstructions of the Bay of Bengal Basin: A remnant ocean basin model

Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin (BOBB) provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to establish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India (ECMI) with graben-horst and to the east by the Sunda convergence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flexure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.

Do along-strike tectonic variations in the Nepal Himalaya reflect different stages in the accretion cycle? Insights from numerical modeling

Whereas the large-scale morphology and dynamics of orogenic wedges are well explained by critical-taper theory, many questions remain unanswered regarding the details of how deformation is accommodated internally. Here, we investigate the dynamics of a collisional orogenic wedge bounded by an over-thickened continental plateau, using two-dimensional thermo-mechanical numerical models. These models, applied to the Himalayan orogen and compared with reference cross-sections, lead us to propose a new hypothesis to explain along-strike variations in tectonic style, topography and exhumation patterns observed along the Himalayan range by a combination of two mechanisms. First, numerical models produce a cycle of crustal ramp formation and advection toward the rear of the wedge. The asynchronous evolution of this cycle along different segments of the range may account for the well-documented lateral variations in the geometry of the Main Himalayan Thrust (MHT) and for the existence of a well-defined topographic transition in some segments of the range. Second, the models suggest that the formation of duplexes leading to the isolation of klippen along the range front may be controlled by rheological contrasts between the Tibetan plateau and/or the Greater Himalayan Sequence and the colliding Indian plate.

Anatomy of composition and nature of plate convergence: Insights for alternative thoughts for terminal India-Eurasia collision

The pattern and timing of collision between India and Eurasia have long been a major concern of the international community. However, no consensus has been reached hitherto. To explore and resolve the disagreements in the Himalayan study, in this paper we begin with the methodology and basic principles for the anatomy of composition and nature of convergent margins, then followed by an effort to conduct a similar anatomy for the India-Eurasia collision. One of the most common patterns of plate convergence involves a passive continental margin, an active continental margin and intra-oceanic basins together with accreted terranes in between. The ultimate configuration and location of the terminal suture zone are controlled by the basal surface of the accretionary wedge, which may show fairly complex morphology with Z-shape and fluctuant geometry. One plausible method to determine the terminal suture zone is to dissect the compositions and structures of active continental margins. It requires a focus on various tectonic elements belonging to the upper plate, such as accretionary wedges, high-pressure (HP)-ultra-high-pressure (UHP) metamorphic rocks, Barrovian-type metamorphic rocks and basement nappes, together with superimposed forearc basins. Such geological records can define the extreme limits and the intervening surface separating active margin from the passive one, thus offering a general sketch for the surface trace of the terminal suture zone often with a cryptic feature. Furthermore, the occurrence of the cryptic suture zone in depth may be constrained by geophysical data, which, in combination with outcrop studies of HP-UHP metamorphic rocks, enables us to outline the terminal suture zone. The southern part of the Himalayan orogen records complicated temporal and spatial features, which are hard to be fully explained by the classic “two-plate-one-ocean” template, therefore re-anatomy of the compositions and nature for this region is necessitated. Taking advantage of the methodology and basic principles of plate convergence anatomy and synthesizing previous studies together with our recent research, we may gain new insights into the evolution of the Himalayan orogeny. (1) The Yarlung-Zangbo ophiolite is composed of multiple tectonic units rather than a single terminal suture zone, and a group of different tectonic units were juxtaposed against each other in the backstop of the Gangdese forearc. (2) The Tethyan Himalayan Sequence (THS) contains melanges with typical block-in-matrix structures, uniform southwards paleocurrents and age spectra of detrital zircons typical of Eurasia continent. All of these facts indicate that the THS belonged to Eurasia plate before the terminal collision, emplaced in the forearc of the Gangdese arc. (3) The Greater Himalayan Crystalline Complex (GHC) and Lesser Himalayan Sequence (LHS) comprise complex components including eclogites emplaced into the GHC and the upper part of the LHS. Judging from the fact that HP-UHP metamorphic rocks are exhumed and emplaced in the upper plate, the GHC and the upper part of the LHS where eclogite occur should be assigned to the upper plate, lying above the terminal subduction zone surface. It is the very surface along which the continuous subduction of the India subcontinent occurred, therefore acting as the terminal, cryptic suture. From the suture further southward, the bulk rock associations of the LHS and Sub-Himalayan Sequence (Siwalik) show little affinity of melange, probably belonging to the foreland system of the India plate. By the anatomy of tectonic features of all the tectonic units in the Himalayan orogen as well as the ages of the subduction-accretion related deformation, we conclude that the terminal India-Eurasia collision occurred after 14 Ma, the timing of the metamorphism of the eclogites emplaced into the upper plate. The development of rifts stretching in N-S direction in Tibet and tectonic events with the transition from sinistral to dextral movements in shear zones, such as the Ailaoshan fault in East Tibet, can coordinately reflect the scale and geodynamic influence of the India-Eurasia convergence zone. By conducting a detailed anatomy of the southern Himalayas, we propose a new model for the final collision-accretion of the Himalayan orogeny. Our study indicates that the anatomy of structures, composition, and tectonic nature is the key to a better understanding of orogenic belts, which may apply to all the orogenic belts around the world. We also point out that several important issues regarding the detailed anatomy of the structures, compositions and tectonic nature of the Himalayan orogeny in future.

Rainfall variability in the Himalayan orogen and its relevance to erosion processes

AbstractRainfall is an important driver of erosion processes. The mean rainfall rate is often used to account for the erosive impact of a particular climate. However, for some erosion processes, erosion rate is a nonlinear function of rainfall, e.g., due to a threshold for erosion. When this is the case, it is important to take into account the full distribution of rainfall, instead of just the mean. In light of this, we have characterized the variability of daily rainfall over the Himalayan orogen using high spatial and temporal resolution rainfall data sets. We find significant variations in rainfall variability over the Himalayan orogen, with increasing rainfall variability to the west and north of the orogen. By taking into account variability of rainfall in addition to mean rainfall rate, we find a pattern of rainfall that, from a geomorphological perspective, is significantly different from mean rainfall rate alone. Using these findings, we argue that short‐term rainfall variability may help explain observed short and long‐term erosion rates in the Himalayan orogen.

Late Triassic sedimentary records in the northern Tethyan Himalaya: Tectonic link with Greater India

The Upper Triassic flysch sediments (Nieru Formation and Langjiexue Group) exposed in the Eastern Tethyan Himalayan Sequence are crucial for unraveling the controversial paleogeography and paleotectonics of the Himalayan orogen. This work reports new detrital zircon U-Pb ages and whole-rock geochemical data for clastic rocks from flysch strata in the Shannan area. The mineral modal composition data suggest that these units were mainly sourced from recycled orogen provenances. The chemical compositions of the sandstones in the strata are similar to the chemical composition of upper continental crust. These rocks have relatively low Chemical Index of Alteration values (with an average of 62) and Index of Compositional Variability values (0.69), indicating that they experienced weak weathering and were mainly derived from a mature source. The geochemical compositions of the Upper Triassic strata are similar to those of graywackes from continental island arcs and are indicative of an acidic-intermediate igneous source. Furthermore, hornblende and feldspar experienced decomposition in the provenance, and the sediment became enriched in zircon and monazite during sediment transport. The detrital zircons in the strata feature two main age peaks at 225–275 Ma and 500–600 Ma, nearly continuous Paleoproterozoic to Neoproterozoic ages, and a broad inconspicuous cluster in the Tonian–Stenian (800–1200 Ma). The detrital zircons from the Upper Triassic sandstones in the study area lack peaks at 300–325 Ma (characteristic of the Lhasa block) and 1150–1200 Ma (characteristic of the Lhasa and West Australia blocks). Therefore, neither the Lhasa block nor the West Australia blocks likely acted as the main provenance of the Upper Triassic strata. Newly discovered Permian–Triassic basalt and mafic dikes in the Himalayas could have provided the 225–275 Ma detrital zircons. Therefore, Indian and Himalayan units were the main provenances of the flysch strata. The Tethyan Himalaya was part of the northern passive margin and was not an exotic terrane separated from India during the Permian to Early Cretaceous. This evidence suggests that the Neo-Tethyan ocean opened prior to the Late Triassic and that the Upper Triassic deposits were derived from continental crustal fragments adjacent to the northern passive continental margin of Greater India.

The Himalaya in 3D: Slab dynamics controlled mountain building and monsoon intensification

Tectonic models for the Oligocene–Miocene development of the Himalaya mountain range are largely focused on crustal-scale processes, and developed along orogen-perpendicular cross sections. Such models assume uniformity along the length of the Himalaya, but significant along-strike tectonic variations occur, highlighting a need for three-dimensional evolutionary models of Himalayan orogenesis. Here we show a strong temporal correlation of southward motion of the Indian slab relative to the overriding Himalayan orogen, lateral migration of slab detachment, and subsequent dynamic rebound with major changes in Himalayan metamorphism, deformation, and exhumation. Slab detachment was also coeval with South Asian monsoon intensification, which leads us to hypothesize their genetic link. We further propose that anchoring of the Indian continental subducted lithosphere from 30 to 25 Ma steepened the dip of the Himalayan sole thrust, resulting in crustal shortening deep within the Himalayan orogenic wedge. During the subsequent ∼13 m.y., slab detachment propagated inward from both Himalayan syntaxes. Resultant dynamic rebound terminated deep crustal shortening and caused a rapid rise of the mountain range. The increased orography intensified the South Asian monsoon. Decreased compressive forces in response to slab detachment may explain an observed ∼25% decrease in the India-Eurasia convergence rate. The asymmetric curvature of the arc, i.e., broadly open, but tighter to the east, suggests faster slab detachment migration from the west than from the east. Published Lu-Hf garnet dates for eclogite facies metamorphism in the east-central Himalaya as old as ca. 38–34 Ma may offer a test that the new model fails, because the model predicts that such metamorphism would be restricted to middle Miocene time. Alternatively, these dates may provide a case study to test suspicions that Lu-Hf garnet dates can exceed actual ages.

Need for re-apprehension of basin tectono-depositional set-up during initial stage of Himalayan orogeny from pedogenic evidences

The collision and suturing of Indian and Eurasian plates about 50Ma ago closed the Tethys Ocean, initiated Himalayan orogen and a foreland basin developed towards its south. The last phase of marine sedimentation in Indian sub-continent during Paleocene-Eocene is represented by Subathu Formation overlain by continental Dagshai sediments and nature of this transition, named as ‘Passage beds’ (sensu Bhatia and Mathur, 1965), is controversial. Further, it is poorly understood both in terms of depositional environment and age. Surprisingly, these beds show evidences of pedogenesis but no such palaeopedological studies have been reported so far. The degree and nature of pedogenesis is widely used in solving diverse geological problems in complex geological settings. Therefore, pedogenesis of these beds exposed in the Shimla hills of Subathu sub-basin has been worked out. Morphological and micromorphological studies divide them into older green/greenish-grey pedofacies (GP) and younger red/reddish-brown pedofacies (RP) units. Pedons are up to tens of centimeters thick. Structure largely comprises sub-angular to sub-rounded blocky peds. A suite of pedogenic features include burrows, root traces, infillings, mottling, cutans, segregations, and nodules which vary in dominance from GP to RP unit. Greenish-grey pedons are characterized by mottles, Fe-Mn oxide segregations whereas reddish-brown pedons by root traces commonly filled with carbonates, presence of carbonate segregations and cutans. Detailed field and laboratory studies at various local and regional exposures can reveal the nature and extent of pedogenesis which will be significant to enact depositional environment as well as stratigraphic position, of Passage beds co-relatable at different localities throughout length of the Himalaya from Pakistan to Nepal because of their distinctive lithology and stratigraphic position. Such studies bear global significance in demonstrating the use of palaeosols as a geologic archive in complex geological settings.

Investigation of geological, geotechnical and geophysical properties of Kiratli (Bayburt, NE Turkey) travertine

There are good quality natural stones having different color and texture especially in the Turkey located in Alpine–Himalayan Orogenic Belt. However, there are limited natural stone reserves in NE Turkey. Because Kiratli travertine site is the second largest area with a reserve of 250,000 m3 in the region, this travertine site was chosen as a pilot site for this study. Geological, geophysical and geotechnical investigations were carried out in the study and laboratory tests were conducted. The discontinuity properties of travertine mass were investigated using Ground Penetrating Radar (GPR) method to guide for suitable operating techniques in the natural stone quarry. According to the tests results uniaxial compressive strength values are 25 and 51 MPa, indirect tensile strength values are 3.98–9.18 MPa, P-wave velocities for dry condition are 3717 and 4154 m/s, respectively, for parallel and perpendicular core samples to the deposition direction. The uniaxial compressive strength of the samples was also determined after cycling tests such as wetting–drying and freezing–thawing tests. The travertine samples have less than 10% deterioration after these tests. On the investigated area, the GPR data were acquired with 100 MHz antenna using GPR technique being one of geophysical methods on 7 lines parallel each other in the study. The data were processed utilizing with ReflexW software. It was concluded that the reflection boundaries observed on obtained radargrams originated in discontinuities and weathered parts in dominant rock. It has been suggested that productive raw materials could not be got during the extraction of material travertine in the orientation of quarry when it comes to discontinuities and weathered parts in dominant rock. In these parts, quality travertine material can not be obtained.

Infiltration of meteoric water in the South Tibetan Detachment (Mount Everest, Himalaya): When and why?

AbstractThe South Tibetan Detachment (STD) in the Himalayan orogen juxtaposes low‐grade Tethyan Himalayan sequence sedimentary rocks over high‐grade metamorphic rocks of the Himalayan crystalline core. We document infiltration of meteoric fluids into the STD footwall at ~17–15 Ma, when recrystallized hydrous minerals equilibrated with low‐δD (meteoric) water. Synkinematic biotite collected over 200 m of structural section in the STD mylonitic footwall (Rongbuk Valley, near Mount Everest) record high‐temperature isotopic exchange with D‐depleted water (δDwater = −150 ± 5‰) that infiltrated the ductile segment of the detachment most likely during mylonitic deformation, although later isotopic exchange cannot be definitively excluded. These minerals also reveal a uniform pattern of middle Miocene (15 Ma) 40Ar/39Ar plateau ages. The presence of low‐δD meteoric water in the STD mylonitic footwall is further supported by hornblende and chlorite with very low δD values of −183‰ and −162‰, respectively. The δD values in the STD footwall suggest that surface‐derived fluids were channeled down to the brittle‐ductile transition. Migration of fluids from the Earth's surface to the active mylonitic detachment footwall may have been achieved by fluid flow along steep normal faults that developed during synconvergent extension of the upper Tethyan Himalayan plate. High heat flow helped sustain buoyancy‐driven fluid convection over the timescale of detachment tectonics. Low δD values in synkinematic fluids are indicative of precipitation‐derived fluids sourced at high elevation and document that the ground surface above this section of the STD had already attained similar‐to‐modern topographic elevations in the middle Miocene.

Miocene orbicular diorite in east-central Himalaya: Anatexis, melt mixing, and fractional crystallization of the Greater Himalayan Sequence

The Greater Himalayan Sequence and leucogranite forming the core of the Himalayan orogen provide an excellent natural laboratory in which to study processes related to crustal melting, granitoid formation, and the tectonic evolution of mountain belts. In contrast to most previous studies, which considered the Himalaya-aged granitoids as leucogranites, here we report a Miocene orbicular diorite from the Greater Himalayan Sequence in the east-central Himalaya. The diorite consists of ellipsoidal orbicules in a diorite matrix. The orbicules have a diorite core with or without garnet-sillimanite-biotite schist enclaves, an inner shell of tangentially oriented biotite laths, and an outer shell of radial or plumose plagioclase crystals. The diorite is aluminous and calcic, shows a fractionated rare earth element pattern with a strongly positive Eu anomaly, and has elevated Sr concentration. The schist enclaves underwent high-temperature metamorphism and partial melting under conditions of 9.2–12 kbar and 765–900 °C, followed by a retrograde pressure-temperature path of decompression and cooling. The inherited magmatic cores, metamorphic mantles, and magmatic rims of zircon from the diorite yield a protolith age of ca. 494 Ma, metamorphic ages ranging from ca. 26 Ma to 22 Ma, and melt crystallization ages of ca. 18–14 Ma. The inherited magmatic cores of zircon show variable but mostly negative e Hf ( t ) values, whereas the metamorphic mantles and the magmatic rims of zircon yield variable and lower e Hf ( t ) values. Our study shows that the orbicular diorite is a plagioclase + biotite + cordierite cumulate rock that formed by the fractional crystallization of peraluminous melt, which was generated by mixing of melts derived from the partial melting of both early Paleozoic granitoids and old pelitic rocks. The orbicular diorite and the trapped metapelitic schist have a prolonged high-temperature metamorphic, anatectic, and crystallization history that was initiated at ca. 26 Ma and lasted until ca. 14 Ma. The formation of orbicular structures was probably related to decompression during the ascent of anatectic melt, and subsequent rapid cooling. Melt mixing was also a key factor that aided in the formation of the orbicular structure of the diorite.

Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

The Trans-Hudson orogen of North America is a circa 1,800 million year old, middle Palaeoproterozoic continental collisional belt. The orogen may represent an ancient analogue to the Himalayan orogen, which began forming 50 million years ago and remains active today. Both mountain belts exhibit similar length scales of deformation and timescales of magmatism and metamorphism. A notable divergence in this correlation has been the absence of high-pressure, low-temperature metamorphic rocks in the Trans-Hudson compared with the Himalaya. It has been debated whether this absence reflects a secular tectonic change, with the requisite cool thermal gradients precluded by warmer ambient mantle temperatures during the Palaeoproterozoic, or a lack of preservation. Here we identify eclogite rocks within the Trans-Hudson orogen. These rocks, which typically form at high pressures and cool temperatures during subduction, fill the gap in the comparative geologic record between the Trans-Hudson and Himalayan orogens. Through the application of phase equilibria modelling and in situ U–Pb monazite dating we show that the pressure–temperature conditions and relative timing of eclogite-facies metamorphism are comparable in both orogenies. The results imply that modern-day plate tectonic processes featuring deep continental subduction occurred at least 1,830 million years ago. This study highlights that the global metamorphic rock record (particularly in older terrains) is skewed by overprinting and erosion. The timing of onset of modern-style plate tectonics on Earth is unclear. Identification of eclogite rocks—typically formed during subduction—in the Trans-Hudson orogen implies modern-style tectonics may have been active 1,830 million years ago.

Quantifying the rise of the Himalaya orogen and implications for the South Asian monsoon

Abstract We reconstruct the rise of a segment of the southern flank of the Himalaya-Tibet orogen, to the south of the Lhasa terrane, using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern Tibet (Liuqu and Qiabulin) and four previously known floras from the Himalaya foreland basin. U-Pb dating of zircons constrains the Liuqu flora to the latest Paleocene (ca. 56 Ma) and the Qiabulin flora to the earliest Miocene (21–19 Ma). The proto-Himalaya grew slowly against a high (∼4 km) proto–Tibetan Plateau from ∼1 km in the late Paleocene to ∼2.3 km at the beginning of the Miocene, and achieved at least ∼5.5 km by ca. 15 Ma. Contrasting precipitation patterns between the Himalaya-Tibet edifice and the Himalaya foreland basin for the past ∼56 m.y. show progressive drying across southern Tibet, seemingly linked to the uplift of the Himalaya orogen.

The Tethyan Himalayan detrital record shows that India–Asia terminal collision occurred by 54 Ma in the Western Himalaya

The Himalayan orogen is a type example of continent–continent collision. Knowledge of the timing of India–Asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation. Sedimentary rocks on the Indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of Asian-derived material deposited on the Indian plate. However, in the Himalaya, for this approach to be applied successfully, it is necessary to be able to distinguish between Asian detritus and detritus from oceanic island arcs that may have collided with India prior to India–Asia collision. Zircons from the Indian plate, Asian plate and Kohistan–Ladakh Island arc can be distinguished based on their U–Pb ages combined with Hf signatures. We undertook a provenance study of the youngest detrital sedimentary rocks of the Tethyan Himalaya of the Indian plate, in the Western Himalaya. We show that zircons of Asian affinity were deposited on the Indian plate at 54 Ma. We thus constrain terminal India–Asia collision, when both sutures north and south of the Kohistan–Ladakh Island arc were closed, to have occurred in the Western Himalaya by 54 Ma.

Orogen‐parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna‐Dhaulagiri Himalaya, central Nepal

AbstractThe metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna‐Dhaulagiri region, central Nepal, recorded orogen‐parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field‐based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low‐angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right‐lateral orogen‐parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen‐parallel melt migration and development of melt‐depleted regions between km3 scale leucogranite culminations at ~50–100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen‐perpendicular convergence on thrust faults and orogen‐parallel extension on normal and strike‐slip faults. In the Annapurna‐Dhaulagiri Himalaya, orogen‐parallel stretching lineations are interpreted as a record of transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen‐wide transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel extension during the mid‐Miocene. The 3‐D nature of oblate strain and orogen‐parallel stretching cannot be replicated by 2‐D numerical simulations of the Himalayan orogen.

Neoproterozoic granitoids in the eastern Himalayan orogen and their tectonic implications

The Precambrian component and evolutionary history of the Greater Himalayan Sequence (GHS), forming the core of the Himalayan orogen, remain a subject of debate. Here, we report new geochronological and petrochemical data of the Neoproterozoic granitoids that occur as the orthogneisses of the GHS in Cona area, the eastern Himalaya orogen. The zircon U–Pb dating results reveal that these granitoids have crystallization ages of Neoproterozoic of 817–838Ma, and metamorphic ages of ca. 31Ma. The petrochemical data show that the granitoids are medium to high-K, peraluminous granodiorite and granite, display geochemical affinities with volcanic arc granitoids. The zircon Hf isotopic compositions show that the granitoids were probably derived from the partial melting of Paleoproterozoic crustal materials. These data, combined with previous results, indicate that an extensive Neoproterozoic magmatic belt may have existed in the Himalayan orogen, and that the Neoproterozoic granitoids resulted from the Andean-type orogeny that formed along the northwestern margin of Rodinia supercontinent, and experienced the Cenozoic high-grade metamorphism during the Himalayan orogeny.