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The tectonic and topographic history of the Himalaya-Tibet orogenic system remains controversial, with several competing models that predict different exhumation histories. Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate asymmetric exhumation of Mount Everest consistent with a scenario in which the southern edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. Northward plateau retreat was caused by erosional incision during the Pliocene. Our results suggest that the South Tibetan Detachment was a localized structure and that no coupling between precipitation and erosion is required for Miocene exhumation of Greater Himalayan Sequence rocks on Mount Everest. [ABSTRACT FROM AUTHOR]

The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, midcrustal layer "flowing" southward relative to the upper and lower crust during late Oligocene-Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Himalaya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak "active channel" to a strong "channel plug" at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and latestage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene-early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillimanite-grade decompression melting, as commonly recorded elsewhere in the Himalaya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]--the proposed channel) had a greater viscosity than in other Himalayan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny. [ABSTRACT FROM AUTHOR]

Although the Himalayan range is classically presented as cylindrical along strike, segmentation of the range in terms of structure, topography, precipitation, and erosion patterns is becoming widely recognized. The potential climatic or tectonic controls on these lateral variations remain controversial. Thermokinematic models predict that the geometry of the main Himalayan detachment controls the kinematics, exhumation, and topography of the orogen: where the detachment includes a major crustal ramp, the topography shows a steep gradient that focuses orographic precipitation and exhumation, whereas the topography is gentler and exhumation less focused above a flatter detachment. We test this prediction by comparing the patterns of river incision (specific stream power) and long-term exhumation (from apatite fission track thermochronology) in central Nepal, where a major crustal ramp has been imaged by geophysical methods, with new exploratory data from the remote Karnali River transect in western Nepal, where a ramp is predicted to be absent or minor. Our results show that both exhumation rates and river incision capacity are significantly higher and focused on the crustal ramp in central Nepal, whereas they are lower and the pattern is more diffuse in western Nepal. These differences support a model in which lateral variations in topography and exhumation are controlled by variations in the geometry of the detachment, and imply that along-strike climatic variations in the Himalaya respond to tectonics rather than driving it. [ABSTRACT FROM AUTHOR]

40Ar/39Ar ages from detrital muscovites have been analyzed from six modern rivers in central and western Nepal; the size of the drainage basins associated with these samples ranges from a few square kilometers to >40,000 km2. These data, when combined with previously published ages of detrital muscovites from other modern rivers in the region, suggest that a good correspondence between normalized age and normalized topography (the comparison of t* and z*) is rare, due to either nonuniform rates of passage through the ~400 °C isotherm or subsequent faulting in the drainage area. The closure temperature of Ar in muscovite is perhaps too high to make meaningful comparisons to modern topography in tectonic analysis of active orogens. The distribution of 40Ar/39Ar ages from detrital muscovites from the Karnali basin in western Nepal is much older than that for the Narayani basin in central Nepal. The Karnali muscovites, when combined with previously published muscovites from the Siwalik Group in western Nepal and zircon fission track ages from modern and ancient samples from the region, suggest a thermal history for western Nepal consistent with vigorous tectonics (and attendant erosion) before the middle Miocene but a significant diminution in the rate of erosion since ca. 10 Ma. 40Ar/39Ar ages of detrital muscovites from the Narayani basin in central Nepal suggest a markedly different history with an acceleration of the rate of erosion since ca. 10 Ma and reactivation of major faults; this is consistent with the abundant bedrock data from the Narayani basin. The strong difference in the erosional history of the adjacent Karnali and Narayani basins, as evidenced by the 40Ar/39Ar ages from detrital muscovites, is not likely to have been due to variations in climate, but rather due to strain partitioning within the Himalaya during and after the Miocene. [ABSTRACT FROM AUTHOR]

The extensional South Tibetan fault system (STFS) crops out for more than 2000 km along the length of the Himalayan range crest. Interpretations of the signifi cance of this structure in Himalayan tectonics vary. While many regard it as rooted northward beneath Tibet, with offset estimates ranging as high as >200 km, others regard it as a relatively minor, rootless structure. Such inconsistent interpretations are possible because, except for the Everest region, where glacial erosion has exposed a cross section of the STFS, there are few direct constraints on the amount of slip on this feature. Arguments for very large displacements have been based on indirect lines of evidence (e.g., geothermobarometric data) that are multiply interpretable. However, new fi eld work and remote sensing data reported here directly support the petrologic arguments for large displacements, at least in the eastern Himalaya. Near the western boundary of Bhutan, the trace of the STFS is apparently offset ~70 km across a NNE-SSW--trending graben, the Yadong cross structure. This offset has been attributed by previous authors to left-lateral slip along the Jomolhari fault system, which bounds the eastern edge of the graben. In fact, the basal structure of the STFS is not offset, but forms a regionally continuous low-angle detachment surface. The outcrop pattern of the structure, kinematic indicators in fault-related tectonites, and the lack of matchable correlative hanging wall and footwall units require a minimum displacement of ~65 km for the detachment. Such displacements are comparable with estimates for major contractional structures in the eastern Himalaya, such as the Main Central thrust system, and confirm the importance of STFS extension in the evolution of the orogenic system. [ABSTRACT FROM AUTHOR]

Pressure-temperature-time (P-T-t) conditions of metamorphism have been determined in the Annapurna region of central Nepal that place new constraints on the structural and tectonic evolution of the Himalayan orogenic wedge. Peak P-T conditions increase structurally upward: ~525 °C and 8 kbar in the Lesser Himalayan sequence, 650 °C and 12 kbar at the base of the Greater Hima layan sequence across the Main Central thrust, 750 °C and 12 kbar in the middle of the Greater Himalayan sequence, and 775 °C and 13 kbar near the top of the Greater Hima layan sequence. Metamorphic monazite ages in the Greater Hima layan sequence also increase structurally upward: 16--21 Ma for sub solidus growth at the base of the Greater Himalayan sequence to ~25 Ma for peak-T metamorphism and anatexis near the top of the Greater Hima layan sequence. These ages are several million years older than at equivalent structural levels at Langtang, ~200 km to the east. The P-T-t data recommend reinterpretation of the Bhanuwa fault within the Greater Hima layan sequence as a thrust, and the presence of a different thrust structurally above the Bhanuwa thrust, here named the Sinuwa thrust. The new data are consistent with progressive stacking of tectonic slices, with calculated overthrust rates that are consistent with some (but not all) models that presume ~2 cm/yr convergence across the Himalaya since 25 Ma. Despite differences in absolute ages, similarities among the chemical systematics of monazite, peak P-T conditions, and overthrust rates calculated for Annapurna when compared to Langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers, although the timing and duration of movement on discrete thrust surfaces may differ. [ABSTRACT FROM AUTHOR]

The article discusses accurate measures of geometry and crustal shortening of the Himalayan fold-thrust belt in Bhutan. The researchers showed that there is an eastward continuation of Lesser Himalayan (LH) duplexing identified in northwest India, Nepal and Sikkim. They also presented balanced cross-sections which allow estimation of the minimum shortening accommodated by the LH and Subhimalayan zones to 164-267 kilometers.

Isotopic data from the Gurla Mandhata metamorphic core complex provide insight on the character of rocks exposed within it and permit possible correlations of these rocks to those exposed in the Himalayan thrust belt. Whole-rock Sr and Nd isotopic analyses show that rock units in the metamorphic core have isotopic signatures that correlate to those of Greater and Lesser Himalayan rocks exposed in the foreland. These data are the first to document Lesser Himalayan rocks north of the Higher Himalayan physiographic zone. When combined with previously published structural reconstructions of the Himalayan orogen, these data reveal a crustal architecture that requires significant uplift of Lesser Himalayan rocks in the hinterland with respect to their position in the foreland, a greater amount of underthrusting of the Lesser Himalayan rocks, and more shortening in the hinterland within the Tethyan Himalaya physiographic zone than currently estimated. [ABSTRACT FROM AUTHOR]

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0–1.8 mm/yr) than inside of it in the east (0.55–0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region's recent tectonic evolution. [ABSTRACT FROM AUTHOR]

The Tertiary continental strata of the Himalayan foreland basin are subdivided into two groups, but the meaning of this subdivision was previously unclear. From the analysis of drill holes, seismic lines, dated sections, outcrops, and balanced cross sections, we find that the southward migration rate of the depositional pinch-out of the younger group is 19 ± 5 mm/yr and equals the Himalayan shortening rate. This equality shows that the flexural foreland basin development is mainly controlled by the motion of the thrust load. The long-term pinch-out migration rate was slower for the older synorogenic group. Erosion locally occurred at the end of its deposition, due to tectonic reactivation of lineaments of the Indian shield. We suggest that this change in the basin development is linked to the detachment of the subducted Indian lithosphere that decreased the slab pull and increased the mean compressive stress within the Indian plate, whereas the plate motion remained constant. The most important implication of our work is that the associated isostatic rebound could have increased the Himalayan elevation prior to 15 Ma. [ABSTRACT FROM AUTHOR]

Reconstructs the tectonic evolution of the Himalayas and the paleogeography of Tertiary foreland basin sedimentary rocks in north India. Evidence of a diachronous arrival of ophiolitic to low-grade metamorphic detritus from the Indus Suture and the early Himalayan thrust sheets in the north; Consistency with the closure of Neotethys along the suture.

Studies the isotopic constraints on the structural relationships between the Lesser Himalayan Series and High Himalayan Crystalline Series from the Garhwal Himalaya thrust system in India. Evidence for out-of-sequence thrusting between the structures; Establishment of the value of isotopic data for lithostratigraphic correlations within orogenic belts.

Analyzes the formation of the Main Boundary thrust in the western Himalaya during the middle-late Miocene. Sandstone stratigraphy; Depositional patterns across the Himalayan foreland; Age and provenance of a middle Siwalik conglomerate in northwestern India; Apatite uplift ages from northwest Pakistan; Displacement of the Main Boundary thrust.

Recent work on Himalayan tectonics indicates that prior to the Cenozoic collision of India and Asia, an enigmatic Cambrian--Ordovician event may have strongly influenced the regional geology of the Himalaya. Stratigraphic and sedimentological analyses of well-preserved Cambrian deposits are critical for understanding the nature of this early tectonic event and its influence on the later tectonic evolution of the Himalaya. The Parahio Formation, defined herein, of the Parahio Valley, Spiti region, in the Tethyan Himalaya of India, is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya. This formation consists of >1350 m of dominantly siliciclastic deltaic deposits. The formation ranges from uppermost Lower Cambrian (Lungwangmiaoan Stage) to middle Middle Cambrian (Hsuchuangian Stage), representing a time span of ∼5-10 m.y. It contains numerous medium-scale shoaling cycles that range from storm-influenced offshore deposits to thick trough cross-bedded fluvial facies. Many thin carbonate beds with abundant trilobite fossils directly overlie the fluvial facies and represent transgressive systems tract deposits. The cycles are interpreted to have resulted from delta-lobe switching, based on a lack of systematic stratigraphic changes in cycle or facies thicknesses. This paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep-sea flysch to shallow-marine tidalites. In addition, our paleoenvironmental analysis and paleocurrent data suggest that the uppermost Lower to Middle Cambrian deposits of the Lesser and Tethyan Himalaya are parts of the same ancient northward-prograding, fluvial-deltaic depositional system of the paleo-Tethys margin of India. An angular unconformity with overlying Ordovician conglomeratic rocks has considerable local relief, with meter-scale scours and a valley fill >100 m thick. The scours have northeast-southwest orientations, which parallel both the paleocurrents in the underlying Parahio Formation as well as published paleocurrent readings from the coarse red beds of the overlying Ordovician strata. The Cambrian-Ordovician unconformity is of regional extent, and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in Spiti is 15 m.y. Our sedimentological analysis and associated paleocurrent data from the Parahio Formation, along with additional data from units both above and below the unconformity, indicate that published models portraying foreland basin development at this time with southward-directed thrusting are problematic. An alternate possibility--that uplift took place south of the Tethyan Himalaya--is also problematic, because no published stratigraphic or structural evidence exists for such an uplift to the south for either the Greater or the Lesser Himalaya lithotectonic zones. [ABSTRACT FROM AUTHOR]