Delores M. Robinson

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.

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 (

Reconstructing the Greater Indian margin: A balanced cross section in central Nepal focusing on the Lesser Himalayan duplex

Across much of the Himalaya, rocks in the Lesser Himalayan duplex are covered by roof thrusts of other Paleoproterozoic Lesser Himalayan rocks or Greater Himalayan rocks. However, in central Nepal, between the Main Central thrust and the Main Boundary thrust, Lesser Himalayan rocks are exposed in structurally complex relationships within the Lesser Himalayan duplex. We present two balanced cross sections with different stratigraphy involved in the duplex, one with the basal Kuncha Formation and one without this unit, to test stratigraphic assumptions. Both cross sections have roof thrust sheets consisting of the Main Central thrust, Ramgarh-Munsiari thrust, and Trishuli thrust folded over the hinterland dipping Lesser Himalayan duplex dissected by late faults. Cross section 1 has a shortening estimate from the Main Central thrust to the Main Boundary thrust, including motion on the Main Central thrust, of 359 km or 78%. Cross section 2 has a shortening estimate of 349 km or 76% over the same region. Because the amount and percentage of shortening are not significantly different between the two cross sections, the different stratigraphic assumptions did not change the shortening results. This similarity suggests that many of the choices made when constructing a cross section may be less important than researchers previously thought.

Pulsed deformation and variable slip rates within the central Himalayan thrust belt

Forward modeling reconstructions and data derived from the Himalayan thrust belt and the foreland basin of far western Nepal tie the erosional unroofi ng and associated deposition to the kinematics and age of fault motion. We reproduce the deformation identifi ed at the surface through a forward-propagating, linked fold-and-thrust belt‐foreland basin system. This approach permits estimates of the magnitude of erosion at each time step and the extent, depth, and age of the associated foreland basin. The model reconstructions reveal that the units that supplied the sediment to the foreland basin changed through time: 25‐13 Ma, erosion of the Tethyan Himalaya; ca. 12 Ma, fi rst exposure of the Greater Himalaya; ca. 11 Ma, fi rst exposure of the Lesser Himalaya. In our model, exposure of Greater Himalaya and Lesser Himalaya rock is associated with the formation of a thrust ramp that cuts through 7 km of footwall Lesser Himalaya stratigraphy and translates >7 km of Lesser Himalaya rock over the ramp, forming a Lesser Himalaya duplex. An increase in structural relief focuses erosion over the region of the ramp and facilitates exposure of Greater Himalaya and Proterozoic Lesser Himalaya rocks. As the Lesser Himalaya ramp propagates southward, more Lesser Himalaya thrust sheets are incorporated into the Lesser Himalaya duplex. Although uniquely dating thrust events is challenging, these model reconstructions allow us to associate time steps with an age of deposition or exhumation. What emerges is a tempo of deformation that varies with time, marked by periods of rapid shortening during propagation of the Main Central thrust, Ramgarh thrust, and middle stages of the development of the Lesser Himalaya duplex (~25‐30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalaya duplex development are marked by periods of slow shortening (~13‐14 mm/yr). Although longterm and modern (geodetic) rates of deformation agree at ~20 mm/yr, rates of shortening through time have varied from 4 to 33 mm/yr.

Evidence for a far‐traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 + 7/ − 10 Ma and ~570 Ma, with metamorphic zircon growth at 23–29 Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475 + 7/ − 3 and ~660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile “thrust sheets,” present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.

Tectonics of the Himalaya: an introduction

SOUMYAJIT MUKHERJEE1*, RODOLFO CAROSI2, PETER VAN DER BEEK3, BARUN KUMAR MUKHERJEE4 & DELORES M. ROBINSON5 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, Maharashtra, India Dipartimento di Scienze della Terra, v. Valperga Caluso, 35 10125 Torino, Italy Universite Grenoble Alpes, Institut des Sciences de la Terre, 38041 Grenoble, France Petrology and Geochemistry Group, Wadia Institute of Himalayan Geology, 33 GMS Road, Dehra Dun 248001, India Department of Geological Sciences, University of Alabama, 201 7th Ave, Room 2003 Bevill Building, Tuscaloosa, AL 35487, USA

Structural, geochronological and geochemical evidence for two distinct thrust sheets in the ‘Main Central thrust zone’, the Main Central thrust and Ramgarh–Munsiari thrust: implications for upper crustal shortening in central Nepal

Abstract Two orogen-scale thrusts structurally underneath Greater Himalayan (GH) rocks characterize the structural architecture of Himalaya in central Nepal. The Main Central thrust (MCT) is at the base of the GH with the Lesser Himalayan (LH) Robang Formation in the footwall, which is the hanging wall of the Ramgarh–Munsiari thrust (RMT). At Kodari-Tatopani and Malekhu, U–Pb detrital zircon age populations from the RMT sheet yield a maximum depositional age of c. 1838 and c. 1871 Ma. U–Pb analyses of igneous zircons from the RMT sheet yield a crystallization age of c. 1750 Ma at both Galchhi and Kodari-Tatopani. The ϵNd(0) values of pelitic rocks from the RMT sheet at Kodari-Tatopani range from c. −23 to −25; whereas, GH rocks have values from c. −12 to −18. These data indicate that the RMT sheet carries the Palaeoproterozoic LH rock and the MCT carries the GH rock. At Kodari-Tatopani, the thrust previously mapped as the MCT is interpreted to be the RMT. Positively identifying the RMT sheet in all three locations yields a more accurate kinematic evolution and confirms its orogenic-scale presence in central Nepal. Supplementary material: U–Pb geochronological analyses are available at http://www.geolsoc.org.uk/SUP18775

Redefining the tectonostratigraphic and structural architecture of the Almora klippe and the Ramgarh–Munsiari thrust sheet in NW India

Abstract We integrate U–Pb zircon geochronology and ϵNd(0) values with field mapping to determine which tectonostratigraphic units are represented to the north, south and within the Almora klippe in Kumaun, NW India. Rock in the Almora klippe and north of the Main Central thrust (MCT) have Neoproterozoic (c. 900 Ma) detrital zircon ages, coupled with similar ϵNd(0) (−7.6 to −11.8) values, suggesting that these two units are the same tectonostratigraphic unit, and that the Almora klippe is the southern continuation of the MCT or another thrust in the Greater Himalayan thrust system. However, north of the Almora klippe, detrital zircon age populations establish the presence of Palaeoproterozoic rock instead of the previous interpretation of Neoproterozoic rocks. These Palaeoproterozoic Lesser Himalayan (LH) rocks are carried by the Ramgarh–Munsiari thrust (RMT) dipping south and folded underneath the klippe. South of the klippe, the RMT carries both the less metamorphosed Palaeoproterozoic and Neoproterozoic LH rocks, in disagreement with the idea that only Neoproterozoic–Cambrian LH rocks are present south of Almora klippe. These results suggest that previous cross-sections in Kumaun are incorrect and must be re-evaluated. Supplementary material: U–Pb zircon geochronological data table is available at http://www.geolsoc.org.uk/SUP18777.

Zircon U‐Pb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin

Throughout the Himalayan thrust belt, klippen of questionable tectonostratigraphic affinity occur atop Lesser Himalayan rocks. Integrated U-Pb ages, Hf isotopic, and whole rock trace element data establish that the Askot klippe, in northwest India, is composed of Paleoproterozoic lower Lesser Himalayan rocks, not Greater Himalayan rocks, as previously interpreted. The Askot klippe consists of 1857 ± 19Ma granite-granodiorite gneiss, coeval 1878 ± 19Ma felsic volcanic rock, and circa 1800Ma Berinag quartzite, representing a small vestige of a Paleoproterozoic (circa 1850Ma) continental arc, formed on northern margin of the north Indian cratonic block. Detrital zircon from Berinag quartzite shows εHf1850Ma values between 9.6 and 1.1 (an average of 4.5) and overlaps with εHf1850Ma values of the Askot klippe granite-granodiorite gneiss ( 5.5 to 1.2, with an average of 2.7) and other Paleoproterozoic arc-related Lesser Himalayan granite gneisses ( 4.8 to 2.2, with an average of 4.0). These overlapping data suggest a proximal arc source for the metasedimentary rocks. Subchondritic εHf1850 Ma values ( 5.5 to 1.2) of granite-granodiorite gneiss indicate existence of a preexisting older crust that underwent crustal reworking at circa 1850Ma. A wide range of εHf1850Ma values in detrital zircon ( 15.0 to 1.1) suggests that a heterogeneous crustal source supplied detritus to the northernmargin of India. These data, as well as the presence of a volcanogenic massive sulphide deposit within the Askot klippe, are consistent with a circa 1800Ma intra-arc extensional environment.

Tectonics of the Himalaya

The Himalayan mountain belt, which developed during the India–Asia collision starting about 55 Ma ago, is a dramatically active orogen and it is regarded as the classic collisional orogen. It is characterized by an impressively continuous 2500 km of tectonic units, thrusts and normal faults, as well as large volumes of high-grade metamorphic rocks and granites exposed at the surface. This constitutes an invaluable field laboratory, where amazing crustal sections can be observed directly in very deep gorges. It is possible to unravel the tectonic and metamorphic evolution of litho-units, to observe the mechanisms of exhumation of deep-seated rocks and the propagation of the deformation. Himalayan tectonics has been the target of many studies from numerous international researchers over the years. In the last 15 years there has been an explosion of data and theories from both geological and geophysical perspectives. This book presents the results of integrated multidisciplinary studies, including geology, petrology, magmatism, geochemistry, geochronology and geophysics, of the structures and processes affecting the continental lithosphere. These processes and their spatial and temporal evolution have major consequences on the geometry and kinematics of the India–Eurasia collision zone.