Himalayan Metamorphic Core Research Articles

Influence of reactivated basement structures on evolving orogens: Along-strike diachronous Himalayan metamorphism in far west Nepal

Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.

Structural and metamorphic architecture of the Zanskar Himalaya, Suru Valley region, NW India: Implications for the evolution of the Himalayan metamorphic core

Structural and metamorphic architecture of the Zanskar Himalaya, Suru Valley region, NW India: Implications for the evolution of the Himalayan metamorphic core

Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis

The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity.

Constraining the evolution of shear zones in the Himalayan mid crust in Central–Western Nepal: implications for the tectonic evolution of the Himalayan metamorphic core

AbstractStructural analysis, petrochronology and metamorphic petrology enable identification and bracketing of the timing of a newly mapped high-temperature ductile shear zone (Jagat Shear Zone (JSZ)) in the Himalayan metamorphic core in Central-Western Nepal. In situ U-Th-Pb monazite petrochronology constrains the timing of top-to-the-S/SW shearing between 28–27 Ma and 17 Ma. Burial and prograde metamorphisms in footwall rocks were linked to thrust-sense movement along the JSZ, while the hanging wall rocks were retrogressed and exhumed. The identification and age of the JSZ (as part of a regional system of shear zones: the High Himalayan Discontinuity (HHD)) coupled with the localization and timing of activity of the Main Central Thrust (MCT) (i) fills a gap in tracing the HHD along orogenic strike, (ii) supports the identification of the position and timing of the long-debated MCT and (iii) helps to place the boundaries of the Himalayan metamorphic core and its internal architecture. Thus, our study is a significant step towards a precise identification of the burial, assembly and exhumation mechanisms of the Himalayan metamorphic core.

Tectonometamorphic evolution of the Himalayan metamorphic core in the Makalu–Arun region, eastern Nepal

The metamorphic core of the Himalayan orogen, once viewed as a structurally contiguous unit, is now known to be divided into multiple packages by a series of structural discontinuities. Although these structures have been identified based on distinct metamorphic and kinematic histories of the bounding packages, the number of structures identified across the exhumed, former midcrustal core varies throughout the Himalaya. The scarcity of field evidence for these structures not only impedes their identification, but also hinders our understanding of their role in the development of the Himalaya. This study characterizes the metamorphic and geochronological history of garnet-, sillimanite- and kyanite-bearing gneisses in Makalu–Arun region, Nepal. Phase equilibria modelling coupled with monazite U/Th–Pb petrochronology delineates four rock packages with distinct metamorphic histories separated by at least three thrust-sense structures. The earliest thrust activity and subsequent in-sequence thrusting lasted from c. 23 to 14 Ma, with initiation of late out-of-sequence thrusting after c. 14 Ma. These results are consistent with foreland thrust migration and juxtaposition compatible with models that incorporate underplating, metamorphism and exhumation of midcrustal rocks during orogenesis. Moreover, the demonstrated complex evolution of the metamorphic core in Makalu region is inconsistent with models that incorporate singular, orogenic-wide structures in the Himalaya. Supplementary material : Supplementary tables and figures are available at https://doi.org/10.6084/m9.figshare.c.6420244

A comparison of 87Rb/87Sr and 40Ar/39Ar dates: Evaluating the problem of excess 40Ar in Himalayan mica

The problem of excess 40Ar (ArE) affecting 40Ar/39Ar dates, particularly in biotite, is a long-standing issue in Himalayan geology. The development of in situ Rb-Sr dating presents a convenient alternative method to extract geochronological data from the same material and avoids potential ArE complications. Herein we present a comparison between previously published 40Ar/39Ar data, including many interpreted to reflect ArE, and new in situ Rb-Sr analyses for the same specimens. This work demonstrates that Rb-Sr mica dates from across the exhumed Himalayan metamorphic core typically overlap 40Ar/39Ar dates from the same rocks that are unaffected by ArE. Where 40Ar/39Ar dates have been interpreted to reflect ArE, the Rb-Sr method invariably yields dates that are younger than the ArE date and consistent with other mineral chronometer systems. Finally, while further refinement is required, we propose a technique to extract spot ages for high Rb/low Sr biotite analyses, by correcting for common 87Sr based on the present known ratio of 87Sr/88Sr, removing the need to assume an initial 87Sr/86Sr.

The Himalayan Collisional Orogeny: A Metamorphic Perspective

AbstractThis paper introduces how crustal thickening controls the growth of the Himalaya by summarizing the P‐T‐t evolution of the Himalayan metamorphic core. The Himalayan orogeny was divided into three stages. Stage 60–40 Ma: The Himalayan crust thickened to ∼40 km through Barrovian‐type metamorphism (15–25 °C/km), and the Himalaya rose from <0 to ∼1000 m. Stage 40–16 Ma: The crust gradually thickened to 60–70 km, resulting in abundant high‐grade metamorphism and anatexis (peak‐P, 15–25 °C/km; peak‐T, >30 °C/km). The three sub‐sheets in the Himalayan metamorphic core extruded southward sequentially through imbricate thrusts of the Eo‐Himalayan thrust, High Himalayan thrust, and Main Central thrust, and the Himalaya rose to ≥5,000 m. Stage 16–0 Ma: the mountain roots underwent localized delamination, causing asthenospheric upwelling and overprinting of the lower crust by ultra‐high‐temperature metamorphism (30–50 °C/km), and the Himalaya reached the present elevation of ∼6,000 m. Underplating and imbricate thrusting dominated the Himalaya' growth and topographic rise, conforming to the critical taper wedge model. Localized delamination of mountain roots facilitated further topographic rise. Future Himalayan metamorphic studies should focus on extreme metamorphism and major collisional events, contact metamorphism and rare metal mineralization, metamorphic decarbonation and the carbon cycle in collisional belts.

The rheology and deformation of the South Tibetan Detachment System as exposed at Zherger La, east-central Himalaya: Implications for exhumation of the Himalayan metamorphic core

Deformation processes and rheological changes within the South Tibetan Detachment System (STDS) played a key role in accommodating exhumation of the Greater Himalayan Sequence (GHS). We present a study for the STDS in Zherger La, Yadong region, east-central Himalaya, to better quantify the spatial and temporal variations of the shear zone's internal deformation and rheology properties. Quartz microstructures and crystallographic preferred orientations, together with quantitative estimates of deformation temperatures, differential stresses, strain rates and viscosities are deduced from the ductile segment of the detachment zone. The mylonites that comprise the exposed shear zone preserve a range of deformation conditions that are interpreted in terms of progressive exhumation. Strain localization within the STDS is indicated by the microstructural evolution from penetrative deformation of constitutive minerals to localized deformation into quartz, which exhibit interconnected networks and high strain zones. Deformation temperatures decrease from 510 ± 40 to 405 ± 50 °C and differential stresses increase from 24 to 100 MPa structurally upwards, as indicated by two-feldspar thermometry and quartz paleopiezometry, respectively. Strain rates derived from quartz dislocation creep flow law are on the order of 10−12-10−13 s−1 and are broadly consistent with macrostructurally-derived strain rates of 1.8–5.1 × 10−13 s−1, which are calculated from previously determined exhumation rates and shear zone width. The estimated differential stresses and strain rates are further utilized to determine the variation of viscosities across the STDS. The viscosities within the STDS range at 1019–1020 Pa s. In conjunction with the published numerical modelling and analog modeling studies, the estimated viscosity data are compatible with the channel flow model for exhumation.

Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya

Abstract Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre-Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

The fate of calcareous pelites in collisional orogens

AbstractAlthough calcareous pelites are important constituents of sediments involved in orogenic processes, their prograde metamorphism is significantly less studied than that of pure pelites with negligible amounts of modal calcite. This paper presents mineral equilibria modelling in the system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2, with the aim of constraining the prograde evolution of calcareous pelites in collisional orogenic settings. A suite of model bulk‐rock compositions is used to investigate the influence of different proportions of calcite in the protolith on (a) the equilibrium assemblages at different pressure, temperature and fluid composition (P–T–X(CO2)) conditions; (b) the melt fertility and (c) the fluid evolution and the main decarbonation reactions occurring during prograde metamorphism of calcareous pelites. In spite of being purely theoretical, the reliability of the modelling is tested by comparing the predicted assemblages with those observed in a wide set of natural samples from the Himalayan metamorphic core. Comparison between the predicted and the modelled assemblages demonstrates that even a small amount of calcite in the calcareous pelitic protoliths has a strong influence on the final mineral assemblages and compositions, with potential effects on their melt productivity. Specifically, it appears that up to ~800°C, the melt productivity of calcic metapelites remains low, and melt production occurs gradationally because it is mostly controlled by continuous biotite dehydration melting reactions, rather than by muscovite breakdown. Moreover, the study demonstrates that calcareous pelites could be non‐negligible CO2‐source rocks in orogenic settings, and that in such contexts, an internal buffered behaviour is likely for most of them.

The Greater Himalayan Thrust Belt: Insight Into the Assembly of the Exhumed Himalayan Metamorphic Core, Modi Khola Valley, Central Nepal

AbstractStrike‐parallel tectonometamorphic discontinuities within the Himalayan metamorphic core are typically interpreted to reflect thrust‐sense movement. However, there is disagreement on the nature and sense of movement across one such structure in central Nepal. Using an integrated approach, this study characterizes multiple structural breaks in the Modi Khola region. Thermobarometric calculations combined with petrochronological investigation show that rocks across the Sinuwa thrust record similar histories with prograde garnet growth ca. 35 Ma and peak pressures of ~11.0 kbar, anatexis at ca. 28 Ma, and followed by cooling and exhumation between ca. 24 and 15 Ma. Rocks below the structurally lower Bhanuwa fault record similar garnet growth at ca. 35 Ma and pressures of ~11.5 kbar but experienced melting and retrogression after ca. 21 Ma. Finally, rocks in the footwall of the Main Central thrust, the structurally lowest break investigated, record prograde metamorphism ca. 17–13 Ma with peak pressures of ~7.0 kbar. This down‐structural migration of prograde metamorphism, anatexis, and subsequent cooling and exhumation of the footwall is consistent with models of progressive underplating and in‐sequence thrusting. When paired with published cooling ages across the Bhanuwa fault, results from this study are consistent with normal‐sense reactivation of the structure during middle‐late Miocene time. This new data set shows that the final assembly of the Himalayan metamorphic core is a result of progressive deformation and juxtaposition of multiple thrust sheets through a combination of in‐sequence thrusting, out‐of‐sequence thrusting, and normal faulting. We refer to this as the Greater Himalayan thrust belt.

Prolonged Partial Melting of Garnet Amphibolite from the Eastern Himalayan Syntaxis: Implications for the Tectonic Evolution of Large Hot Orogens

AbstractThe Himalayan orogen, which formed due to collision of the Indian and Asian continents during the Early Tertiary, is a prime example of a large, hot collisional orogen. Despite decades of study, the duration of partial melting of migmatitic rocks exposed in the Himalayan orogenic core remains highly controversial. As such, we have performed detailed petrological and geochronological analyses of garnet amphibolite from the eastern Himalayan syntaxis in order to reveal the thermal conditions, tectonometamorphic mechanisms, and timing and duration of anatexis in metabasic rocks that form a major component of the thickened lower crust of the eastern Himalayan orogen. Phase equilibrium modeling and geothermobarometry show that the studied sample underwent high‐pressure granulite‐facies metamorphism and partial melting at 15−17 kbar and 805−840°C. Dehydration melting of amphibole during prograde metamorphism generated up to 20 vol. % partial melt with a granitic composition, which thus represents a potential source for Himalayan syn‐ to post‐orogenic crustal‐derived granites. Zircon U‐Pb geochronology shows that the garnet amphibolite witnessed a long‐lived anatectic and melt crystallization process lasting more than 30 Myr. Prolonged anatexis from ca. 40 to ca. 20 Ma predates initiation of the extrusion of Himalayan metamorphic core by up to 15 Myr, indicating that the thick and weak crust of the Himalayan‐Tibetan orogen must have remained stationary for this length of time, despite the ongoing continental collision. This study thus provides new insight into the tectonic evolution of hot orogens.

In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Recent identification of tectono-metamorphic discontinuities within the Himalayan metamorphic core has challenged previous understanding of mountain-building processes/models during continental collision. However, their exact position, spatial extent, and temporal evolution still remain in debate. Monazite petrochronology and thermobarometry have been applied to metapelites of the Gyirong-Syabrubensi-Langtang transect in central Nepal. Peak metamorphic temperatures and pressures were determined by conventional thermobarometry and Zr-in Rutile thermometry. P-T conditions increase structurally upward from 540–580 °C, 8–10 kbar at LHS to 640–710 °C, 8–12 kbar at lower GHC, and 680–720 °C, 6–9 kbar at upper GHC. Petrochronology studies indicate prograde metamorphism in the LHS, partial melting of 20–18 Ma in the lower GHC and prolonged partial melting at 29–23 Ma in the upper GHC, demonstrating a diachronism across these units. The Rasuwagadhi Shear Zone (RSZ), is marked by a jump in T/depth and peak metamorphic timing from lower GHC (<25 °C/km, ~19 Ma) to upper GHC (>30 °C/km, ~29 Ma), and can be correlated with the regional High Himalayan Discontinuity recently identified within the GHC in other Himalayan transects. This shear zone acted almost synchronously with the South Tibetan Detachment around 24–18 Ma and was relayed by movement along the Main Central Thrust after 18 Ma, presenting south-ward in-sequence thrust propagation style. Therefore, a hybrid model, In-sequence Buoyancy Extrusion that involves a combination of underplating and buoyancy mechanisms is proposed to explain the overall exhumation of HMC, and perhaps is applicable to metamorphic core in similar large and hot collisional orogen.

WITHDRAWN: In-sequence buoyancy extrusion of the Himalayan metamorphic core, central Nepal: Constraints from monazite petrochronology and thermobarometry

WITHDRAWN: In-sequence buoyancy extrusion of the Himalayan metamorphic core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Muscovite dehydration melting: Reaction mechanisms, microstructures, and implications for anatexis

AbstractDehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour‐absent) crust, muscovite reacts with quartz to produce K‐feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K‐feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K‐feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K‐feldspar from K‐feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K‐feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K‐feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K‐feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K‐feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic‐tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K‐feldspar trap fine‐grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K‐feldspar grains overgrow and trap fabric‐aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K‐feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K‐feldspar in the Himalayan metamorphic core requires vapour‐absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.

Major shear zone within the Greater Himalayan Sequence and sequential evolution of the metamorphic core in Sikkim, India

The Greater Himalayan Sequence (GHS), bounded by the South Tibetan detachment system, and the Main Central thrust (MCT), constitutes majority of the Himalayan Metamorphic Core. Recent studies identified several shear zones within the GHS, which played significant role in its evolution. Here, we report the presence of a major shear zone, the Chungthang-Thambi thrust (CTT), which is situated within the GHS in Sikkim, eastern India. The CTT exhibits “zone-type” geometry, with mylonite core flanked by protomylonite zones, and its thickness varies from ∼1km to ∼5.6km along the transport direction. New 40Ar/39Ar mica ages, and apatite fission track dates from this study, when integrated with published UThPb monazite, and LuHf garnet ages indicate that the CTT, the MCT, and the Munsiari thrust (MT), at base of the crystalline Lesser Himalayan Sequence (LHCS), developed in-sequence between ∼27Ma and ∼12Ma, and controlled the sequential evolution of the metamorphic core (GHS+LHCS) in Sikkim. Our imbricate model suggests that the upper-middle GHS was initially underthrusted below the Tethyan unit, and underwent prograde metamorphism from ∼37Ma to ∼28Ma. Thrusting on the CTT exhumed the middle GHS, and underthrusted the lower GHS between ∼27Ma and ∼22.5Ma. The MCT exhumed the lower GHS between ∼22.5Ma and ∼12.1Ma, and underthrusted the LHCS, which was later exhumed by the MT between ∼12Ma and ∼10Ma. The thrust sheets exhumed to near-surface conditions between ∼2.6 and ∼0.5Ma, during growth of the underlying Lesser Himalayan Duplex. The MCT showed protracted activity till ∼7.2Ma, and leading edge of the CTT was later reactivated at ∼13.5Ma, and at ∼1.8Ma. The CTT is equivalent to the High Himalayan Discontinuity, and forms part of a regional structure that, spans ∼800km arc-length of the Himalaya from Sikkim to west-central Nepal.

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

AbstractNew phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.

Campaign-style U-Pb titanite petrochronology: Along-strike variations in timing of metamorphism in the Himalayan metamorphic core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core (HMC), in order to document and investigate the causes of along-strike variations in its Oligocene–Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya. This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ∼30 Ma to 12 Ma, recording (re-)crystallization between 625 °C and 815 °C. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen. Disparities in the duration and timing of metamorphism within the HMC are best explained by along-strike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes. These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger, thicker crystalline core in the eastern Himalaya.

Contrasting accessory mineral behavior in minimum-temperature melts: Empirical constraints from the Himalayan metamorphic core

Medium-grained leucogranite in the Tama Kosi region of the Nepalese Himalayan Metamorphic Core yields a relatively narrow range of monazite 208Pb/232Th dates with a dominant population at ~21.0 Ma inferred to represent crystallization of an early plutonic phase. In contrast, the pegmatitic portion of the same intrusive complex, that cross-cuts the medium-grained leucogranite, contains zircon, monazite and xenotime that each display near-identical age spectra, recording semi-continuous (re-)crystallization from 27.5 Ma to 21.0 Ma, followed by a ~2 m.y. hiatus then further (re-)crystallization between 19.4 and 18.6 Ma. The “gap” in pegmatite dates corresponds well to the crystallization age of the older leucogranite, whereas the end of accessory phase growth in the pegmatite coincides with the onset of regional-scale cooling. Detailed textural, trace element and thermochronologic data indicate that the range of zircon, monazite and xenotime dates recorded in the pegmatite reflect inherited components that underwent semi-continuous (re-)crystallization during metamorphism and/or anatexis in the source region(s), whereas dates younger than the hiatus indicate accessory phase recrystallization, related to both fluid influx and a concomitant increase in temperature. In contrast, the lack of an inherited component(s) in the medium-grained leucogranite phase is inferred to be a result of complete dissolution during partial melting. A model is proposed in which influx of heat and H2O-rich fluids associated with early leucogranite emplacement temporarily delayed zircon and monazite and xenotime crystallization, respectively. These data highlight the importance of measuring spatially resolved dates, trace elements and textural patterns from multiple accessory minerals combined with model constraints to better understand the often-complex crystallization history of anatectic melts in collisional orogens.

Influence of inherited Indian basement faults on the evolution of the Himalayan Orogen

Abstract Indian basement faults, which bound three orogen-perpendicular palaeotopographic ridges of Precambrian Indian basement south of the Himalaya, extend to the base of the Indian lithosphere and to the northern extent of the Indian lithosphere underneath Tibet. In the eastern Himalaya, the active orogen-perpendicular Yadong–Gulu graben is aligned with an earthquake-generating strike-slip fault in the high Himalaya. We argue that the graben results from crustal necking during reactivation of the underplated basement fault. In the central Himalaya, along-strike diachronous deformation and metamorphism within the Himalayan metamorphic core, as well as lateral ramps in the foreland thrust belt, spatially correspond to the Lucknow and Pokhara lineaments that bound the subsurface Faizabad Ridge in the Indian basement. Analogue centrifuge modelling confirms that offset along such deep-seated basement faults can affect the location, orientation and type of structures developed at various stages of orogenesis and suggests that it is mechanically feasible for strain to propagate through a melt-weakened mid-crust. We suggest that inherited Indian basement faults affect the ramp-flat geometry of the basal Main Himalayan Thrust, partition the Himalayan range into distinct zones, localize east–west extension resulting in the Tibetan graben and, ultimately, contribute to lateral variability in tectonic evolution along the orogen's strike.