South Tibetan Detachment Research Articles

Geochronology of granulitized eclogite from the Ama Drime Massif: Implications for the tectonic evolution of the South Tibetan Himalaya

The Ama Drime Massif (ADM) is an elongate north‐south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(‐Th)‐Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite‐facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite‐grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High‐grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high‐temperature metamorphism, decompression, and exhumation of the ADM postdates mid‐Miocene south directed midcrustal extrusion and is kinematically linked to orogen‐parallel extension.

Empirical constraints on extrusion mechanisms from the upper margin of an exhumed high-grade orogenic core, Sutlej valley, NW India

The Early–Middle Miocene exhumation of the crystalline core of the Himalaya is a relatively well-understood process compared to the preceding phase of burial and prograde metamorphism in the Eocene–Oligocene. Highly deformed rocks of the Greater Himalayan Sequence (GHS) dominate the crystalline core, and feature a strong metamorphic and structural overprint related to the younger exhumation. The Tethyan Sedimentary Series was tectonically separated from the underlying GHS during the Miocene by the South Tibetan Detachment, and records a protracted and complex history of Cenozoic deformation. Unfortunately these typically low-grade or unmetamorphosed rocks generally yield little quantitative pressure–temperature–time information to accompany this deformation history. In parts of the western Himalaya, however, the basal unit of the Tethyan Sedimentary Series (the Haimanta Group) includes pelites metamorphosed to amphibolite facies. This presents a unique opportunity to explore the tectono-thermal evolution of crystalline rocks which record the early history of the orogen. Pressure–temperature–time–deformation ( P–T–t–d) paths modelled for two Haimanta Group pelitic rocks reveal three distinct stages of metamorphism: (1) prograde Barrovian metamorphism to 610–620 °C at c. 7–8 kbars, with garnet growing over an early tectonic fabric (S 1); (2) initial decompression during heating to 640–660 °C at c. 6–7 kbars, with development of a pervasive crenulation cleavage (S 2) and staurolite and kyanite porphyroblast growth; (3) further exhumation during cooling, with minor retrograde metamorphism and modification of the pervasive S 2 fabric. Monazite growth ages constrain the timing of initial garnet growth (> 34 Ma), the start of D 2 and maximum burial (c. 30 Ma), and the termination of garnet growth (c. 28 Ma). Muscovite Ar/Ar ages indicate cooling through c. 300 °C at c. 13 Ma, from which we derive an initial exhumation rate of c. 1.3 mm year − 1 for the Haimanta Group. The underlying GHS was exhumed at a rate of 2.2 to 3 mm year − 1 during this time. The difference in exhumation rate between these two units is considered to reflect Early Miocene displacement on the intervening South Tibetan Detachment. Slower exhumation (c. 0.6 mm year − 1 ) of both units after c. 13 Ma followed the cessation of major displacement on this structure, after which time the Haimanta Group and the GHS were exhumed as one relatively coherent tectonic block.

Nd isotopic compositions of the Tethyan Himalayan Sequence in southeastern Tibet

The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread metamorphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sampled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a ɛ Nd-T DM plot. The first is defined by the ɛ Nd(210 Ma) values of −3.45 to −7.34 and T DM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with ɛ Nd(130 Ma) values from −15.24 to −16.61 and T DM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has ɛ Nd(130 Ma) values of −10.89 to −16.32 and Nd model ages (T DM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.

Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO2

Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water‐rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N‐S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 gas range from −5.5 to +3.8‰ and −13.1 to −0.3‰ versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near‐surface effervescence of CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 range from −2.2 to +2.1‰ (versus AIR) and are explained by mixtures of air‐derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near‐surface biogenic processes. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

Structures, kinematics, thermochronology and tectonic evolution of the Ramba gneiss dome in the northern Himalaya

The Ramba gneiss dome, one of the north Himalayan gneiss domes, is composed of three tectono-lithologic units separated by an upper and a lower detachment fault. Low-grade metamorphic Tethyan Himalayan sedimentary sequence formed the upper unit above the brittle upper detachment fault. Mylonitic gneiss and a leucogranite pluton made up the lower unit beneath the ductile lower detachment fault. Mylonitic middle-grade garnet-, staurolite- and andalusite-schist constituted the middle unit between the two faults, which may be that the basal part of the upper unit experienced detachment shear. The Ramba dome underwent three episodes of deformation in its tectonic evolution. The first episode was a top-down-to-north-northwest sliding possibly related to the activity of the south Tibetan detachment system (STDS). The second episode was the dominant deformation related to a east–west extension, which resulted in a unique top-down-to-east kinematics and the major tectonic features of the dome. The third episode was a collapse sliding toward the outsides of the dome. The Ramba gneiss dome is possibly a result of the east–west extension and magmatic diapir. The lower detachment fault is probably the main detachment fault separating the sedimentary sequence from the crystalline basement during the east–west extension in the dominant deformation episode. The diapir of the leucogranite pluton formed the doming shape of the Ramba gneiss dome. This pluton intruded in the core of the dome in a late stage of the dominant deformation, and its Ar–Ar cooling ages are about 6 Myr. This indicates that the dominant deformation of the dome happened at the same time of the east–west extension represented by the north–south trending rifts throughout the northern Himalaya and southern Tibet. Therefore, the formation of the Ramba gneiss dome should be related to this east–west extension.

Relationship between the Higher Himalayan Crystalline and Tethyan Sediments in the Kali Gandaki area, western Central Nepal: South Tibetan Detachment revisited

During our recent fieldwork in Kali Gandaki valley in westerncentral Nepal, we observed a definite intrusive relationship between the augen gneiss of the Higher Himalayan Crystalline Sequence (HHCS, Formation III of Le Fort 1975) and the base of the Tethyan Sedimentary Sequence (TSS). Hagen (1968) also reported beautiful outcrops of the similar relationship from Namun range east of the Annapurna Range. This means that the observed contact between the HHCS and TSS is the intrusive, exemplified by the intrusion of the augen gneiss unit between the remaining part of HHCS and the overlying TSS. Between the TSS and underlying HHCS, a northdipping normal fault system has been reported from various parts of the Himalayan Orogen from Kashmir to Bhutan through Nepal; this fault system has been identified as the South Tibetan Detachment System (STDS) and been considered to form one of the major fault systems dividing principal geologic units in the Himalayan Orogen (e.g., Burchfiel and Royden 1985). In the Kali Gandaki area, the STDS was identified as the Annapurna Detachment by Brown and Nazarchuk (1993) and studied in detail by Godin (1999). Our small observation above conforms neither to these studies, nor even to other earlier studies, which mentioned a tectonic contact between the two (e.g., Bodenhausen and Egeler 1971, Garzanti and Frette 1991). We also observed that there is a good lithological similarity between the lower part of the TSS and the metasedimentary gneisses of the HHCS, the metasedimentary gneisses forming the lower part of the HHCS and being separated from the TSS by the intrusive augen gneiss body as mentioned above. Worth noting is that the metasedimentary gneisses are quite consistent throughout HHCS, their lithology changing gradually downwards from calcareous to psamopelitic, maintaining structural conformity throughout as pointed out in earlier studies (e.g., Gansser 1964, Stocklin 1980). Many small reverse folds and related small structures with southerly vergence were observed, while normal sense faults and folds were rarely observed in the lower part of TSS. Based on these lines of evidences, we suspect a continuous development of the Annapurna Detachment in the Kali Gandaki area. We also consider a possibility that the metasedimentary gneisses of the HHCS are actually the lower equivalent or a lower formation of the TSS intervened by the intrusive body of the augen gneiss. This view is in conformity with the classical observation by Le Fort (1975), and is supported by recent geochronologic data suggesting that the age of the HHCS could be in the range of 800–480 Ma (e. g., DeCelles et al. 2000). Our observations also support in part a recent proposal by Gehrels et al. (2003) who proposed Cambro-Ordovician thrusting tectonics at the base of the TSS, and pointing out a possibility of repetition of the lower formations of the TSS into the underlying HHCS. If the above are to be admitted, we may have to consider the redefinition of the Tethyan Sedimentary Sequence so that it could include all metasediments of HHCS, with the stratigraphic and structural base lying just above the MCT. This consideration leads us to re-examine the role of the STDS in the Himalayan Orogen.

Defining the Himalayan Main Central Thrust in Nepal

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1–3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2–3 km thick, bounded by an upper thrust, MCT-2 (= Vaikrita thrust), and a lower thrust, MCT-1 (= Munsiari thrust). Some workers define an ‘upper Lesser Himalaya’ thrust sheet that shows similar P – T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (<5 Ma) Th–Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (early–middle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogen-parallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms that exhumed trans-Himalayan antiformal structures during orogen-parallel extension along the southern margin of the Tibetan Plateau.

Forward modeling the kinematic sequence of the central Himalayan thrust belt, western Nepal

The Himalayan thrust belt is often cited as an example of a thrust system that propagated from hinterland to foreland; however, this kinematic sequence is not well documented, and the process of formation of the thrust belt has not been well supported. This study uses forward modeling and timing data to reveal a detailed view of the evolution of the central Himalayan thrust belt from the footwall of the South Tibetan detachment system southward to the Main Frontal thrust. By using a reasonable configuration of undeformed stratigraphy, the surface deformation in western Nepal can be dynamically reproduced, confirming that the cross sections from which the undeformed sections were derived are viable and propagated from hinterland to foreland. In addition, this study yields detailed step-by-step reconstructions of three cross sections and is the first of its kind in any thrust belt system. These detailed views are useful for understanding and bracketing erosion data, the basin sediments, and geodynamic models. Modeling shortening estimates are between 495 and 733 km from the Main Frontal thrust to the South Tibetan detachment system, and are within the range predicted for shortening in western Nepal obtained from balanced cross sections (485–743 km). Thus, the Himalayan thrust belt in western Nepal is essentially a forward-propagating thrust belt from hinterland to foreland, with minor out-of-sequence (

Contact metamorphism in the Malashan dome, North Himalayan gneiss domes, southern Tibet: an example of shallow extensional tectonics in the Tethys Himalaya

AbstractCombined petrographic, structural and geochronological study of the Malashan dome, one of the North Himalayan gneiss domes, reveals that it is cored by a Miocene granite, the Malashan granite, that intruded into the Jurassic sedimentary rocks of Tethys Himalaya. Two other granites in the area are referred to as the Paiku and Cuobu granites. New zircon SHRIMP U‐Pb and muscovite and biotite 40Ar‐39Ar dating show that the Paiku granite was emplaced during 22.2–16.2 Ma (average 19.3 ± 3.9 Ma) and cooled rapidly to 350–400 °C at around 15.9 Ma. Whole‐rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the High Himalayan Crystalline Sequence. Abundant calcareous metasedimentary rocks and minor garnet‐staurolite‐biotite‐muscovite ± andalusite schists record contact metamorphism by three granites that intruded intermittently into the Jurassic sediments between 18.5 and 15.3 Ma. Two stages of widespread penetrative ductile deformation, D1 and D2, can be defined. Microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield P–T conditions of 4.8 ± 0.8 kbar at 550 ± 50 °C during a non‐deformational stage between D1 and D2, and 3.1–4.1 kbar at 530–575 °C during syn‐ to post‐D2. The pressure estimates for the syn‐ to post‐D2 growth of andalusite suggest relatively shallow (depth of ∼15.2 km) extensional ductile deformation that took place within a shear zone of the South Tibetan Detachment System. Close temporal association between intrusion of the Malashan granite and onset of D2 suggests extension may have been triggered by the intrusion of the Malashan granite.

Structural insights into the early stages of exhumation along an orogen-scale detachment: The South Tibetan Detachment System, Dzakaa Chu section, Eastern Himalaya

Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20 Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision.

Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U‐Th]/He analyses

New apatite and zircon fission track and (U‐Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7 zircon fission track (ZFT), and 7 apatite (U‐Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie ∼2 km above the adjacent valleys, and a transect of &gt;20 valley bottom samples spans from the Lesser Himalaya across the Greater Himalaya and into the Tethyan strata. As a consequence, these data provide one of the more comprehensive low‐temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFT ages range from &gt;3.8 to 0 Ma, ZFT ages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the south central zone of the field area) yielded significantly older ZFT and AFT ages of ∼2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFT and AFT ages from a vertical transect along this ridge indicate exhumation rates of ∼1.5 km Myr−1 (r2 &gt; 0.7) from ∼2 to 0.6–0.8 Ma, whereas AHe ages indicate a faster exhumation rate of ∼2.6 km Myr−1 (r2 = 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr−1 (all with low r2 values of &lt;0.6) for the time period from ∼1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long‐term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than in the Greater Himalaya and may be a response to a drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data are further evaluated with thermokinematic modeling in the companion paper by Whipp et al.

Channel flow and the Himalayan–Tibetan orogen: a critical review

The movement of a low-viscosity crustal layer in response to topographic loading provides a potential mechanism for (1) eastward flow of the Asian lower crust causing the peripheral growth of the Tibetan Plateau and (2) southward flow of the Indian middle crust to be extruded along the Himalayan topographic front. Thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface. Isotopic constraints on the timing of fault movement, anatexis and thermobarometric evolution of the exhumed garnet- to sillimanite-grade metasedimentary rocks support mid-crustal channel flow during the Early to Mid-Miocene. Exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface, defined by the South Tibetan Detachment System, and strain softening along the base, bounded by the Main Central Thrust. Neotectonic extrusion, bounded by brittle Quaternary faults south of the Main Central Thrust, is positively correlated with the spatial distribution of precipitation across a north–south transect, suggesting climate–tectonic linkage over a million-year time scale. A proposed orogen-wide eastward increase in extrusion rate over 20 Ma reflects current precipitation patterns but climate–tectonic linkage over this time scale remains equivocal.

The Rigid Grain Net (RGN): An alternative method for estimating mean kinematic vorticity number ( Wm)

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number ( W m) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating W m. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used W m plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating W m, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.

The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen

The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence, have a specific structural correlation with the Main Central thrust and South Tibet detachment in the central Himalaya. There, the Main Central thrust places the Greater Himalayan Crystalline complex over the Lesser Himalayan Sequence, and the South Tibet detachment places the Tethyan Himalayan Sequence over the Greater Himalayan Crystallines. Although this division has formed the basis for all Himalayan tectonic models, it fails to explain aspects of the geology of the western Himalaya where the Main Central thrust places the Tethyan Himalayan Sequence directly above the Lesser Himalayan Sequence. Our mapping in NW India shows that this relationship results from southward merging of the Main Central thrust and South Tibet detachment. This finding, in conjunction with observed alternating shear senses on the South Tibet detachment, is inconsistent with the wedge-extrusion and erosion-induced channel-flow models (both require only top-to-the-N motion on the South Tibet detachment) but is consistent with a tectonic-wedging model.

A structural transect in the Lower Dolpo: Insights on the tectonic evolution of Western Nepal

We present the results of a structural transect in Lower Dolpo, cross-cutting the upper part of the Lesser Himalaya (LH), the Higher Himalayan Crystallines (HHC) and the lower part of the Tibetan Sedimentary Sequence (TSS). The MCT zone affects the upper part of the LH as well as the lower part of the HHC and shows a later brittle reactivation. Mean vorticity in the MCT points to non-coaxial deformation. These data, together with available kinematic data along the belt, on the South Tibetan Detachment System (STDS) and in the core of the HHC, point to increasing simple shear toward the tectonic boundaries. A top-to-the-SW high-temperature shear zone (Toijem Shear Zone) is recognized in the middle part of the HHC at the boundary between Units 1 and 2. It developed during the earlier stages of exhumation of the HHC, enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills. Its development could be explained by a change in the velocity profile during the extrusion of the HHC, triggered by first order changes in rock types of the tectonic unit. The STDS is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the HHC to greenschist facies marbles in the lower part of the TSS. The development of a pervasive foliation towards the bottom of the TSS indicates increasing strain, related to top down-to-the-NE tectonic transport. A Low P metamorphic event, marked by the growth of post-D1 biotite porphyroblasts at the base of the TSS, is related to the conductive heating from the underlying HHC.

Crustal architecture of the Himalayan metamorphic front in eastern Nepal

The Himalayan Metamorphic Front consists of two basinal sequences deposited on the Indian passive margin, the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic–Cambrian Greater Himalayan Sequence. The current paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has been called the Main Central Thrust, and that this acted as the fundamental structure that controlled the architecture of the Himalayan Metamorphic Front. Geological mapping of eastern Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define the crustal architecture. In eastern Nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone. In recognition of this, we introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front. The fundamental structure that controls orogen architecture in eastern Nepal occurs at higher structural levels within the Greater Himalayan Sequence and we suggest the name; High Himal Thrust. This 100–400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition, and separates the Upper-Plate from the Lower-Plate in the Himalayan Metamorphic Front. The high- T/moderate- P metamorphism at ∼ 20–24 Ma in the Upper-Plate reflects extrusion of material between the High Himal Thrust and the South Tibet Detachment System at the top of the section. The Lower-Plate is a broad schistose zone of inverted, diachronous moderate- T/high- P metamorphic rocks formed between ∼ 18 and 6 Ma. The High Himal Thrust is laterally continuous into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and Main Central Thrust (as originally defined). To the west in central Nepal, the Upper-Plate/Lower-Plate boundary has been placed at lower structural levels, coinciding with the Himalayan Unconformity and has been named the Main Central Thrust, above the originally defined Main Central Thrust (or Ramgarh Thrust).

Structure and geochronology of the southern Xainza-Dinggye rift and its relationship to the south Tibetan detachment system

The Xainza-Dinggye rift is one of several north-south trending rifts in central and southern Tibet created by Cenozoic east-west extension during Indo-Asian convergence. The southern part of the rift cuts through the Tethyan and High Himalayas. In the Tethyan Himalaya, this rift consists of an early domal structure and a late normal fault developed during the progressive deformation. The dome is cored by leucogranitic plutons that intruded during extension. Muscovite 40Ar/ 39Ar ages of the mylonitic leucogranite indicate that extension in the Tethyan Himalaya began at ∼8 Ma or before. In the High Himalaya, the rift is controlled by a normal fault dipping to the southeast. This fault has a structural constitution similar to a detachment fault. Its lower block is made up of mylonitic High Himalayan gneiss, intruded by early mylonitic leucogranite sills and late less-deformed biotite-bearing leucogranite dikes. Mica 40Ar/ 39Ar ages of these leucogranites and the retrograded metamorphosed gneiss of the lower block range from ∼13 to ∼10 Ma. In the study area, the south Tibetan detachment system (STDS) is a ductile shear zone composed of mylonitic leucogranite that is intruded by less-deformed leucogranite and overlain by low grade metamorphic rocks. Mica 40Ar/ 39Ar ages of leucogranites in the shear zone and schist from the detachment hanging wall indicate a protracted deformation history of the STDS from ∼19 to ∼13 Ma. The Xainza-Dinggye rift is younger than the STDS because it offsets the STDS; this north-south trending rift belongs to a different tectonic system from the east-west striking STDS, and may be caused by geological process related to India–Asia convergence. This temporal and spatial relationship of the STDS to the rift may indicate an important change in tectonic regime at ∼13 Ma in the building of the plateau.

Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models

We present a new geologic map of western Nepal and three balanced regional cross sections in the Himalayan thrust belt. The minimum shortening between the South Tibetan detachment and the Main Frontal thrust is 485–743 km and suggests that total Himalayan shortening may exceed 900 km. All rocks involved in the thrust belt are of upper crustal affinity, implying that a comparable length of Indian lower crust and mantle lithosphere was subducted beneath Tibet. Major structural features are the Subhimalayan thrust system, Lesser Himalayan imbricate zone, Dadeldhura thrust sheet, Lesser Himalayan duplex, Ramgarh thrust sheet, Main Central thrust sheet, and a north-dipping normal-sense shear zone, possibly related to the South Tibetan detachment. These structures are continuous along the entire Nepalese portion of the Himalayan thrust belt. New 40 Ar/ 39 Ar ages from the Ramgarh thrust zone, Greater Himalayan rocks, and the lower part of the Tethyan sequence support a kinematic model in which major thrust systems in Nepal propagated southward from early Miocene time onward. The geometry and kinematic history of the thrust belt in western Nepal are not compatible with recent models for southward ductile extrusion of Greater Himalayan rocks in a mid-crustal channel. Instead, the thrust belt in western Nepal behaved like a typical forward propagating thrust system, involving unmetamorphosed, brittlely deformed rocks in its frontal part and ductilely deformed, higher-grade metamorphic rocks in its hinterland region. Although our results do not support published versions of the channel flow model, they provide additional geological and geo-chronological data that will assist future attempts to develop geodynamic models for the Himalayan-Tibetan orogenic system.

40Ar/ 39Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes

Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite 40Ar/ 39Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at ∼ 14.6 Ma. The Kampa Granite yields the youngest biotite 40Ar/ 39Ar ages of ∼ 13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.