P-T-t-D paths of Everest Series schist, Nepal (original) (raw)
Related papers
Journal of the Geological Society, 2003
This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (.620 8C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 AE 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene-early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite-cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 158. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12-18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest-Lhotse-Nuptse massif contains a massive ballooning sill of garnet þ muscovite þ tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U-Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and eastwest extension in south Tibet (,18 Ma).
New samples collected from a transect across the summit limestone of Mount Everest (Qomolangma Formation) show that multiple distinct deformational events are discretely partitioned across this formation. Samples from the highest exposures of the Qomolangma Formation (Everest summit) preserve a well-developed mylonitic foliation and microstructures consistent with deformation temperatures of ≥250 °C. Thermochronologic and microstructural results indicate these fabrics were ingrained during initial contractile phases of Himalayan oro-genesis, when crustal thickening was accommodated by folding and thrusting of the Tethyan Sedimentary Sequence. In contrast, samples from near the base of the Qomolangma Formation (South Summit) preserve extensional shear deformation, indicate metasomatism at temperatures of ~500 °C, and contain a synkinematic secondary mineral assemblage of muscovite + chlorite + biotite + tourmaline + rutile. Shear fabrics preserved in South Summit samples are associated with activity on the Qomolangma detachment, while the crystallization of secondary phases was the result of reactions between the limestone protolith and a volatile, boron-rich fluid that infiltrated the base of the Qomolangma Formation, resulting in metasomatism. The 40 Ar/ 39 Ar dating of synkinematic muscovite indicates the secondary assemblage crystallized at ca. 28 Ma and that shear fabrics were ingrained at ≥18 Ma. This paper presents the first evidence that Everest's summit limestone records multiple phases of deformation associated with discrete stages in Himalayan orogenesis, and that the structurally highest strand of the South Tibetan detachment on Everest was initially active as a distributed shear zone before it manifested as a discrete brittle detachment at the base of the Qomolangma Formation.
Journal of Metamorphic Geology, 2011
This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U-Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of 340°C. Temperatures increase down-section, reaching 650°C at the base of the shear zone, defining an apparent metamorphic field gradient of 310°C km )1 across the entire structure. U-Th-Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.
2011
This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U-Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of 340°C. Temperatures increase down-section, reaching 650°C at the base of the shear zone, defining an apparent metamorphic field gradient of 310°C km )1 across the entire structure. U-Th-Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.
Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)
Journal of Asian Earth Sciences, 1999
In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, aecting the S1 highgrade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are aected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is aected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E±W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The ®rst stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55±0.65 GPa) and high T (750±7708C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The ®rst event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.
The core of the Greater Himalayan Sequence in the Mugu-Karnali area (Western Nepal) is affected by a thick shear zone with development of nearly 4 km of mylonites (Mangri shear zone). It is a contractional shear zone showing a top-to-the-SW and WSW sense of shear. The shear zone developed during the decompres-sion, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamor-phism deformed under the kyanite stability field. P–T conditions indicate that the footwall experienced higher pressure (1.0–0.9 GPa) than the hanging wall (0.7 GPa) and similar temperatures (675°–700 °C). U–Pb in-situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 Ma. Samples from the lower part of the Greater Himalayan Sequence underwent similar ductile shearing at ~17–13 Ma. These ages and the associated P–T–t paths revealed that peak metamorphic conditions were reached ~ 5–7 Ma later in the footwall of the shear zone with respect to the hanging-wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself. Mangri Shear Zone, with the other recently documented tectonic and metamorphic discontinuities within the Greater Himalayan Sequence, point to the occurrence of a regional tectonic feature, the High Himalayan Discontinuity, running for more than 500 km along the strike of the Central Himalayas. It was responsible of the exhumation of the upper part of the Greater Himalayan Sequence starting from 28 Ma, well before the activation of the Main Central Thrust and the South Tibetan Detachment. Our data point out that exhumation of the Greater Himalayan Sequence was partitioned in space and time and different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one.
Earth and Planetary Science Letters, 2017
North-dipping, low-angle normal faults of the South Tibetan detachment system (STDS) are tectonically important features of the Himalayan-Tibetan orogenic system. The STDS is best exposed in the N-S-trending Rongbuk Valley in southern Tibet, where the primary strand of the systemthe Qomolangma detachmentcan be traced down dip from the summit of Everest for a distance of over 30 km. The metamorphic discontinuity across this detachment implies a large net displacement, with previous studies suggesting >200 km of slip. Here we refine those estimates through thermal-kinematic modeling of new (U-Th)/He and 40 Ar/ 39 Ar data from deformed footwall leucogranites. While previous studies focused on the early ductile history of deformation along the detachment, our data provide new insights regarding the brittle-ductile to brittle slip history. Thermal modeling results generated with the program QTQt indicate rapid, monotonic cooling from muscovite 40 Ar/ 39 Ar closure (ca. 15.4-14.4 Ma at ca. 490˚C) to zircon (U-Th)/He closure (ca. 14.3-11.0 Ma at ca. 200˚C), followed by slower cooling to apatite (U-Th)/He closure at ca. 9-8 Ma (at ca. 70˚C). Although previous work has suggested that ductile slip on the detachment lasted only until ca. 15.6 Ma, thermal-kinematic modeling of our new data suggests that rapid (ca. 3-4 km/Ma) tectonic exhumation by brittleductile to brittle fault slip continued to at least ca. 13.0 Ma. Much lower modeled exhumation rates (≤0.5 km/Ma) after ca.13 Ma are interpreted to reflect erosional denudation rather than tectonic exhumation. Projection of fault-related exhumation rates backward through time suggests total slip of ca. 61 to 289 km on the Qomolangma
Thermal structure and exhumation history of the Lesser Himalaya in central Nepal
Tectonics, 2004
1] The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last $20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°-550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°-50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3-4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m 2 /yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.
1] A high-temperature shear zone, Toijem shear zone, with a top-to-the-SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°-700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite-bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33 Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS) and Main Central Thrust (MCT) zones. As a consequence, western Nepal represents a key area where the channel-flow-driven mechanism of exhumation, supposed to be active from Bhutan to central-eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation. Citation: Carosi, R., C. Montomoli, D. Rubatto, and D. Visonà (2010), Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal), Tectonics, 29, TC4029,
Journal of Geology, 2010
In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the Ama Drime and Mount Everest massifs. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan Series (GHS) until the Middle Miocene. Field-and laboratorybased structural data presented here indicate that early melt-present deformation in the footwall of the STDS accommodated large-scale flow of a low-viscosity middle crust. Decompression-related anatexis and emplacement of leucogranites in the structurally highest positions of the GHS (∼16-18 Ma) mark the final stages of south-directed extrusion within a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions, from ultramylonitic leucogranite (∼400Њ-500ЊC; 51%-67% pure shear) to mylonitic marble (1300ЊC; 41%-55% pure shear) to moderately foliated marble ∼200Њ-300ЊC (46%-59% pure shear). Previous investigations demonstrated that the cessation of movement on the STDS at ∼16-13 Ma was followed by anatexis in the lower portion of the middle crust (∼12-13 Ma; Ama Drime Massif). The Ama Drime and Nyö nno Ri detachments (∼8-12 Ma; Dinggyê graben) exhumed rocks from the deepest structural position in the central Himalaya during orogen-parallel extension. Brittle faulting dissected the upper crust (!4 Ma; Tingri-Kung Co graben) in a setting that was kinematically linked to extension in the interior of the Tibetan plateau.