Himalayan Orogenesis Research Articles

Petrogenesis of the Ramba leucogranite in the Tethyan Himalaya and constraints on the channel flow model

Abstract The Himalayan leucogranites are critical for understanding the geodynamic process of Himalayan orogenesis. In this article, we present petrological, geochronological and geochemical results of the Ramba leucogranites that crop out in the center of the Ramba dome within the Tethyan Himalaya. U–(Th)–Pb dating of zircon, monazite and xenotime revealed three episodes of leucogranitic magmatism in the Ramba dome at ca. 44 Ma, ca. 28 Ma and ca. 8 Ma, respectively. The ca. 44 Ma and ca. 28 Ma magmatisms are both represented by strongly deformed porphyritic two-mica granite gneiss dykes that intruded into the margin of the dome. These granite gneisses have high CaO and Sr contents, relatively high Na/K, Sr/Y and low Rb/Sr ratios, and much lower initial 87Sr/86Sr ratios but higher eNd(t) and eHf(t) values than the ancient Indian crustal material. These results indicate that the granite gneisses were not derived from metasedimentary rocks of the High Himalayan Sequence but most likely were derived from partial melting of mafic rocks at a thickened crustal condition. The ca. 8 Ma leucogranites consist of a two-mica granite pluton in the core of the dome and garnet-bearing granite dykes in the margin of the dome. Major and trace element compositions indicate that these Miocene leucogranites are typical S-type granites and were most likely derived from muscovite and/or biotite dehydration melting of metapelites. The relatively lower TiO2, TFeO, MgO and CaO contents, higher Rb concentration and Rb/Sr ratio, and more strongly negative Eu anomaly of the garnet-bearing granite indicate that it was more evolved than the coeval two-mica granite. The non-CHARAC (CHarge-And-Radius-Controlled) trace element behavior and the tetrad effect of the REE distribution of the garnet-bearing granite further suggest that it is a highly fractionated granite. Besides, our study detected some Asian-like materials in the Ramba leucogranites, such as Mesozoic-aged inherited zircons and the relatively depleted Sr–Nd–Hf isotopic compositions of some leucogranite samples. These features of leucogranites in the Tethyan Himalaya have been previously considered as direct geological evidence for the channel flow model. However, detailed geochronological and geochemical studies of the country rocks suggest that these Asian-like materials are most likely sourced from the country rock rather than the Asian lithosphere, thus they may not be used as solid evidence for the channel flow model.

Relations des structures anatomiques neuro-vasculaires avec les voies d’abords arthroscopiques postérieures et trans-septales dans les différents degrés de flexion du genou

The Paleogene succession of the Himalayan foreland basin is immensely important as it preserves evidence of India-Asia collision and related records of the Himalayan orogenesis. In this paper, the depositional regime of the Paleogene succession of the Himalayan foreland basin and variations in composition of the hinterland at different stages of the basin developments are presented. The Paleogene succession of the western Himalayan foreland basin developed in two stages, i.e. syn-collisional stage and post-collisional stage. At the onset, chert breccia containing fragments derived from the hanging walls of faults and reworked bauxite developed as a result of erosion of the forebulge. The overlying early Eocene succession possibly deposited in a coastal system, where carbonates represent barriers and shales represent lagoons. Up-section, the middle Eocene marl beds likely deposited on a tidal flat. The late Eocene/Oligocene basal Murree beds, containing tidal bundles, indicate that a mixed or semi-diurnal tidal system deposited the sediments and the sedimentation took place in a tide-dominated estuary. In the higher-up, the succession likely deposited in a river-dominated estuary or in meandering rivers. In the beginning of the basin evolution, the sediments were derived from the Precambrian basement or from the metasediments/volcanic rocks possessing terrains of the south. The early and middle Eocene (54.7–41.3 Ma) succession of the embryonic foreland possibly developed from the sediments derived from the Trans-Himalayan schists and phyllites and Indus ophiolite of the north during syn-collisional stage. The detrital minerals especially the lithic fragments and the heavy minerals suggest the provenance for the late Eocene/Oligocene sequences to be from the recycled orogenic belt of the Higher Himalaya, Tethyan Himalaya and the Indus-suture zone from the north during post-collisional stage. This is also supported by the paleocurrent measurements those suggest main flows directed towards southeast, south and east with minor variations. This implies that the river system stabilized later than 41 Ma and the Higher Himalaya attained sufficient height around this time. The chemical composition of the sandstones and mudstones occurring in the early foreland basin sequences are intermediate between the active and passive continental margins and/or same as the passive continental margins. The sedimentary succession of this basin has sustained a temperature of about 200 °C and undergone a burial depth of about 6 km.

Evolution of the Paleogene succession of the western Himalayan foreland basin

The Paleogene succession of the Himalayan foreland basin is immensely important as it preserves evidence of India-Asia collision and related records of the Himalayan orogenesis. In this paper, the depositional regime of the Paleogene succession of the Himalayan foreland basin and variations in composition of the hinterland at different stages of the basin developments are presented. The Paleogene succession of the western Himalayan foreland basin developed in two stages, i.e. syn-collisional stage and post-collisional stage. At the onset, chert breccia containing fragments derived from the hanging walls of faults and reworked bauxite developed as a result of erosion of the forebulge. The overlying early Eocene succession possibly deposited in a coastal system, where carbonates represent barriers and shales represent lagoons. Up-section, the middle Eocene marl beds likely deposited on a tidal flat. The late Eocene/Oligocene basal Murree beds, containing tidal bundles, indicate that a mixed or semi-diurnal tidal system deposited the sediments and the sedimentation took place in a tide-dominated estuary. In the higher-up, the succession likely deposited in a river-dominated estuary or in meandering rivers. In the beginning of the basin evolution, the sediments were derived from the Precambrian basement or from the metasediments/volcanic rocks possessing terrains of the south. The early and middle Eocene (54.7–41.3 Ma) succession of the embryonic foreland possibly developed from the sediments derived from the Trans-Himalayan schists and phyllites and Indus ophiolite of the north during syn-collisional stage. The detrital minerals especially the lithic fragments and the heavy minerals suggest the provenance for the late Eocene/Oligocene sequences to be from the recycled orogenic belt of the Higher Himalaya, Tethyan Himalaya and the Indus-suture zone from the north during post-collisional stage. This is also supported by the paleocurrent measurements those suggest main flows directed towards southeast, south and east with minor variations. This implies that the river system stabilized later than 41 Ma and the Higher Himalaya attained sufficient height around this time. The chemical composition of the sandstones and mudstones occurring in the early foreland basin sequences are intermediate between the active and passive continental margins and/or same as the passive continental margins. The sedimentary succession of this basin has sustained a temperature of about 200 °C and undergone a burial depth of about 6 km.

The timing of strike-slip shear along the Ranong and Khlong Marui faults, Thailand

[1] The timing of shear along many important strike-slip faults in Southeast Asia, such as the Ailao Shan-Red River, Mae Ping and Three Pagodas faults, is poorly understood. We present 40Ar/39Ar, U-Pb SHRIMP and microstructural data from the Ranong and Khlong Marui faults of Thailand to show that they experienced a major period of ductile dextral shear during the middle Eocene (48–40 Ma, centered on 44 Ma) which followed two phases of dextral shear along the Ranong Fault, before the Late Cretaceous (>81 Ma) and between the late Paleocene and early Eocene (59–49 Ma). Many of the sheared rocks were part of a pre-kinematic crystalline basement complex, which partially melted and was intruded by Late Cretaceous (81–71 Ma) and early Eocene (48 Ma) tin-bearing granites. Middle Eocene dextral shear at temperatures of ∼300–500°C formed extensive mylonite belts through these rocks and was synchronous with granitoid vein emplacement. Dextral shear along the Ranong and Khlong Marui faults occurred at the same time as sinistral shear along the Mae Ping and Three Pagodas faults of northern Thailand, a result of India-Burma coupling in advance of India-Asia collision. In the late Eocene (<37 Ma) the Ranong and Khlong Marui faults were reactivated as curved sinistral branches of the Mae Ping and Three Pagodas faults, which were accommodating lateral extrusion during India-Asia collision and Himalayan orogenesis.

Tectonic setting of Cretaceous basins on the NE Tibetan Plateau: insights from the Jungong basin

ABSTRACTQuantifying the Cenozoic growth of high topography in the Indo‐Asian collision zone remains challenging, due in part to significant shortening that occurred within Eurasia before collision. A growing body of evidence suggests that regions far removed from the suture zone experienced deformation before and during the early phases of Himalayan orogenesis. In the present‐day north‐eastern Tibetan Plateau, widespread deposits of Cretaceous sediment attest to significant basin formation; however, the tectonic setting of these basins remains enigmatic. We present a study of a regionally extensive network of sedimentary basins that are spatially associated with a system of SE‐vergent thrust faults and are now exposed in the high ranges of the north‐eastern corner of the Tibetan Plateau. We focus on a particularly well‐exposed basin, located ∼20 km north of the Kunlun fault in the Anyemaqen Shan. The basin is filled by ∼900 m of alluvial sediments that become finer‐grained away from the basin‐bounding fault. Additionally, beds in the proximal footwall of the basin‐bounding fault exhibit progressive, up‐section shallowing and several intraformational unconformities which can be traced into correlative conformities in the distal part of the basin. The observations show sediment accumulated in the basin during fault motion. Regional constraints on the timing of sediment deposition are provided by both fossil assemblages from the Early Cretaceous, and by K–Ar dating of volcanic rocks that floor and cross‐cut sedimentary fill. We argue that during the Cretaceous, the interior NE Tibetan Plateau experienced NW–SE contractional deformation similar to that documented throughout the Qinling–Dabie orogen to the east. The Songpan‐Ganzi terrane apparently marked the southern limit of this deformation, such that it may have been a relatively rigid block in the Tibetan lithosphere, separating regions experiencing deformation north of the convergent Tethyan margin from regions deforming inboard of the east Asian margin.

Distinction of Gem Spinels from the Himalayan Mountain Belt

Gem spinel deposits in Myanmar, Vietnam and Tajikistan have their formation in association with Himalayan orogenesis. Gem-quality orange, pink, red and purple spinels from deposits at Mogok (Myanmar), Luc Yen (Vietnam), and Kuh-i-Lal (Tajikistan) have been investigated by 'standard' gemological testing and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Microscopic examination yielded apatite and calcite inclusions together with octahedral negative crystals to be most frequently present in Mogok spinels. The presence of dislocation systems and titanite inclusions are indicative inclusion features for spinels from Luc Yen. Trace elements such as Ti, Fe, Ni, Zn, Zr and Sn differ slightly in spinels from the sources investigated. A distinction of spinels from these deposits is therefore possible by trace element chemistry. This is especially helpful for gem spinels, which often show few inclusions or completely lack inclusion features.

Zircon U-Pb Age and Geochemistry of Granitoids Within Jinla Pb-Zn-Ag Poly-metallic Ore Field Across China and Myanmar Border

Jinla Pb-Zn-Ag poly-metallic ore field across China and Myanma, is tectonically located in the south segment of the Mengtong–Gengma–Ximeng passive continental margin of Proterozoic era-Palaeozoic era, and the eastern margin of the Baoshan–Shantai Massif. Mineralization related granitoids within the ore field include Laochang amphibole admellite, Menglinshan biotite admellite and Nanla K-feldspar granite-porphyry. This paper systematically presents the characteristics of the above granites on their major elements, REE and trace elements (including ore-forming elements) and their isotopic ages determined by zircon SHRIMP dating. The intrinsic connection and genesis of the granites, and the relationships between granites and Pb-Zn-Ag mineralization are discussed. It comes to the conclusions as follows: (1) Among the three types of granites in Jinla ore field, the U-Pb zircon ages for both Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite are (45.41±1) Ma, and Nanla K-feldspar granite porphyry, with a zircon U-Pb age of (43.41±0.78) Ma, formed at the late stage of a syn-collisional mineralization process of Himalayan orogenesis. The magma may have been resulted from partial melting in the crust due to crust and mantle interaction and therefore, is dominated by crustal source materials mixed with some mantle components. Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite contrain significant amount of mantle source materials, whereas Nanla K-feldspar granite porphyry is dominated by crustal materials mixed with minor mantle component. (2) Major and trace elements (including REE and some trace elements (Zr/Hf, Nb/Ta, Rb/Sr, Rb/Ba, K/Rb, (Rb/Yb) N, Sr*, K* and Zr*) suggest that the granitic intrusions in the research area may have formed in locally extensional tectonic environment. Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellite may have formed at deep crust during the early stage of magma differentiation. Nanla K-feldspar granite porphyry, on the other hand, may have formed at shallow crust during the late stage of magma differentiation. These three granite intrusions are the products of various stages of magma differentiation during magmatic-tectonic evolution process. (3) Our study on the ore-forming elements in these intrusions suggests that in comparison with granites in China, although Laochang porphyraceous amphibole admellite and Menglinshan porphyraceous biotite admellites are relatively rich in some ore-forming minerals, Nanla K-feldspar granite porphyry has the highest ore-forming potential because, comparing with the other two intrusions, it not only contains more ore-forming minerals, such as W, Cu, Bi, Sb, Mo, Sn, Ag, Pb, Au, etc., the granite is also much richer in F, B, As and other mineralizing elements.

Evidence for Early (> 44 Ma) Himalayan Crustal Thickening, Tethyan Himalaya, southeastern Tibet

Comprehensive understanding of Himalayan orogenesis is limited in part by a poor knowledge of the Eohimalayan episode; a phase of tectonic activity predating the better understood Neohimalayan crustal thickening associated with coaxial deformation along the Main Central thrust. The Cambo–Ordovician to Tertiary metasedimentary sequences outcropping within the Tethyan Himalaya are the structurally highest units of the Himalayan Fold and Thrust Belt; they and are inferred to have been the first to have accreted to Asia. A compilation of data from transects along the length of the main Himalayan arc shows that the Tethyan sequences have experienced at least five deformation events, although the timing of these episodes is poorly constrained. Emplacement of a series of undeformed granitoid bodies following the second of these events, which accounts for the majority of crustal thickening in the Tethyan Himalayan units, is constrained by U–Pb zircon dating to be older than 44.1 ± 1.2 Ma. Thus, significant crustal thickening had occurred along the length of the proto-Himalayan arc by the mid-Eocene, or within 10 to 20 myr of the initiation of Himalayan orogenesis.

Marble-hosted ruby deposits from Central and Southeast Asia: Towards a new genetic model

Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and South East Asia. These deposits are located in the Himalayan mountain belt which developed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to granitoid intrusions and are contained in platform carbonates series that underwent high-grade metamorphism. All occurrences are located close to major tectonic features formed during Himalayan orogenesis, directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar–Ar dating of micas syngenetic with ruby and U–Pb dating of zircon included in ruby gives evidence that these deposits formed during Himalayan orogenesis, and the ages document the extensional tectonics that were active, from Afghanistan to Vietnam, between the Oligocene and the Pliocene. The petrography shows that ruby-bearing marbles formed in the amphibolite facies ( T = 610 to 790 °C and P ~ 6 kbar). A fluid inclusion study defines the conditions of gem ruby formation during the retrograde metamorphic path (620 < T < 670 °C and 2.6 < P < 3.3 kbar) for the deposits of Jegdalek, Hunza and northern Vietnam. Whole rock analyses of non-ruby-bearing marbles indicate that they contain enough aluminum and chromiferous elements to produce all the ruby crystals that they contain. In addition, (C, O)-isotopic analyses of carbonates from the marbles lead to the conclusion that the marbles acted as a metamorphic closed fluid system that were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in marbles reveals that it is of organic origin and that it exchanged C-isotopes with the carbonates during metamorphism. Moreover, the O-isotopic composition of ruby was buffered by metamorphic CO 2 released during devolatilisation of marble and the H-isotopic composition of mica is consistent with a metamorphic origin for water in equilibrium with the micas. The (C, O, H)-isotopic compositions of minerals associated with marble-hosted ruby are all in agreement with the hypothesis, drawn from the unusual chemistry of CO 2–H 2S–COS–S 8–AlO(OH)-bearing fluids contained in fluid inclusions, that gem ruby formed at P ~ 3 kbar and 620 < T < 670 °C, during thermal reduction of evaporite by organic matter, at high temperature-medium pressure metamorphism of platform carbonates during the Tertiary India–Asia collision. The carbonates were enriched in Al- and chromiferous-bearing detrital minerals, such as clay minerals that were deposited on the platform with the carbonates, and in organic matter. Ruby formed during the retrograde metamorphic path, mainly by destabilization of muscovite or spinel. The metamorphic fluid system was rich in CO 2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron released by molten salts (NaCl, KCl, CaSO 4). Evaporites are key to explaining the formation of these deposits. Molten salts mobilized in situ Al and metal transition elements contained in marbles, leading to crystallization of ruby.

Degassing of metamorphic carbon dioxide from the Nepal Himalaya

Geothermal activity is common at the foot of the Higher Himalaya near the Main Central Thrust (MCT), Nepal Himalaya. We have sampled hot springs along a 150 km stretch of the Himalayan front and find that they carry large fluxes of CO2 derived from metamorphic reactions. Hot spring fluids are saturated with CO2, have [DIC] from 1.3 to &gt;100 mmol kg−1 and have δ13CDIC values from −13‰ to +13‰(PDB). Analysis of CO2 released by decrepitation of fluid inclusions from syn‐ and postdeformational quartz veins indicate that crustal fluids had δ13C from −15‰ to +2‰(PDB), consistent with production of CO2 from both thermal decomposition of organic matter and decarbonation at depth. Modeling of the hot spring fluid compositions indicates that they are strongly degassed. Combining our degassing calculations with estimates of the fraction of hydrothermal alkalinity in local rivers shows that the metamorphic degassing flux of CO2 in the 32,000 km2 Narayani basin of the central Himalaya is &gt;1.3 × 1010 mol a−1, exceeding the calculated consumption of CO2 by chemical weathering for the Narayani River basin by a factor of four. Our study implies that the net impact of Himalayan orogenesis on the carbonate‐silicate geochemical cycle is not large‐scale drawdown of CO2 because the weathering sink is substantially offset or even exceeded by the metamorphic source.

U–Pb SHRIMP zircon geochronology and T– t– d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/ 238U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D 1), predominately top-to-N ductile shearing and crustal extension (D 2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D 3), and continued N–S contraction, E–W extension and doming (D 4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.

ANATOMICAL REVISION OF THE GENUSMERYCOPOTAMUS(ARTIODACTYLA; ANTHRACOTHERIIDAE): ITS SIGNIFICANCE FOR LATE MIOCENE MAMMAL DISPERSAL IN ASIA

Abstract: New fossil remains of the anthracothere genusMerycopotamusFalconer and Cautley, are described. Most of them were discovered by the Harvard University and Geological Survey of Pakistan joint research project (Y‐GSP) in the well‐dated Middle and Late Miocene deposits of the Potwar Plateau in northern Pakistan. This new material led us to revise the systematics of the genus with the validation of three species,M. nanusFalconer (M. pusillusLydekker),M. dissimilisFalconer and Cautley, andM. medioximusLihoreauet al., and allowed us to determine precisely their chronological distributions in a continuous Neogene sequence. Other specimens reported from the late Miocene deposits of the Khorat Plateau in north‐east Thailand by the Department of Mineral Resources are the first remains ofMerycopotamusto have been discovered in that region and are attributed toM. medioximus. These discoveries indicate a wider geographical distribution of the genus in the early Late Miocene. Anatomical investigations highlight the evolution ofMerycopotamusthrough the Miocene towards more amphibious habits. Palaeobiogeographical and palaeoecological information forMerycopotamusstress the role of the Himalayan orogenesis as a dispersal barrier and the impact of a major global regression event on the evolution of Indian Subcontinent faunas from the Middle Miocene to the Late Pliocene.

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.

The Punjab foreland basin of Pakistan: a reinterpretation of zircon fission‐track data in the light of Miocene hinterland dynamics

AbstractSedimentary basins represent an archive of tectonic events of the hinterland source regions. By determining the variation in sediment lagtime over time, events can be distinguished which may no longer be available as the source has been eroded. In regions characterized by rapid exhumation this is most often the case but the erosion products form a record of these events. Detrital zircon fission‐track ages from sediments of the Siwalik basin, Pakistan, originally presented by Cerveny et al. (New Perspectives in Basin Analysis, Springer‐Verlag, New York, 1988, p. 43), have been reinvestigated and reinterpreted using a revised methodological approach. Detrital age populations were determined from different stratigraphic levels and were correlated through time in order to assess the change in lag time over the stratigraphic section. This information was combined with the many new ages from the hinterland to further interpret events in the source region. The new investigation suggests that steady‐state evolution has not always existed. An overall trend of exhumation increasing by 0.1 mm Myr−1 (from 0.9 to 2.65 mm yr−1) from 18 Ma to the present is evident with a major exception of a net pulse between 11.7 and 10.9 Ma associated with an increase in sedimentation increasingly rich in hornblende. Earlier studies suggested that at this time the source of the sediments was the presently outcropping Kohistan Arc. We are able to demonstrate that this cannot be so but was rather the rapidly exhuming Nanga‐Parbat Haramosh syntaxis (&gt; 2 mm yr−1) coevally with transpressional displacement along the Main Karakorum Thrust, whereby the overlying Kohistan Arc sequences were removed. Furthermore, comparison of our detrital thermochronological data set with another one from the same basin and one from another foreland basin to the east, in NW India suggest that the Himalayan orogenesis was probably not synchronous for the late Early–Middle Miocene. Overall, regions that undergoes today's rapid uplift may be useless to reconstruct earlier phases of exhumation as the levels that may have yielded such info were eroded and deposited into the adjacent basin(s). Such scenario is reproducible in most orogens as in the Himalaya in NW Pakistan stressing the high potential of detrital thermochronological studies to trace hinterland dynamics.Terra Nova, 18, 248–256, 2006

Ar–Ar and U–Pb ages of marble-hosted ruby deposits from central and southeast Asia

To date the formation of ruby deposits and link it to the regional metamorphism associated with Tertiary Himalayan orogenesis, 40Ar–39Ar stepwise heating experiments were performed on single grains of phlogopite syngenetic with ruby, and zircon inclusions in ruby and spinel were dated with the U–Pb method by ion-probe. The Ar–Ar ages of phlogopites associated with ruby are Oligocene (24.7 ± 0.3 Ma) at Jegdalek in Afghanistan; Miocene at Mogok in Myanmar (18.7 ± 0.2 to 17.1 ± 0.2 Ma), at Hunza in Pakistan (10.8 ± 0.3 to 5.4 ± 0.3 Ma), and Chumar in Nepal (5.6 ± 0.4 Ma); and Pliocene (4.6 ± 0.1 Ma) at Ruyil in Nepal. In Vietnam, a zircon included in a ruby from the Quy Chau deposit yielded a 238U–206Pb age of 53.8 ± 4.6 Ma, whereas in the Red River shear zone, ruby formed at around 40–36 Ma during ductile deformation under peak metamorphic conditions. The ages obtained in this study are in agreement with those previously published for the ruby-bearing metamorphic belts and document extensional tectonics that were active from Afghanistan to Vietnam between the Oligocene and the Pliocene. Ruby-bearing marbles define a high-quality gem belt linked to the high-temperature metamorphism of the Himalayan fold belt that developed during the Tertiary collision of the Indian plate with Asia.

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.

Provenance of Eocene foreland basin sediments, Nepal: Constraints to the timing and diachroneity of early Himalayan orogenesis

In contrast to Eocene Himalayan foreland basin sedimentary deposits in India and Pakistan, coeval sedimentary rocks of the Bhainskati Formation in Nepal contain scant petrographic evidence of orogenic input. Such data have been used as evidence to promote models of diachroneity of India-Asia collision. In this paper we document orogenic input into the Eocene foreland basin rocks of Nepal, from fission-track analyses of detrital zircons. Our data provide evidence that significantly reduces the possible duration of any diachroneity of collision, and brings the interpretation of the sedimentary record into better agreement with ages of early thrusting and metamorphism in the orogen. We also use our detrital fission-track data to bolster previous age determinations of the overlying Dumri Formation, confirming the basin-wide occurrence of a major unconformity. Comparison of our data set with that from coeval along-strike rocks in India suggests that there is no evidence of diachroneity in early stages of the orogen's exhumation.

Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal: implications for timing and diachroneity of initial Himalayan orogenesis

The onset of mountain building in the western part of the Himalayan orogenic belt has been documented in the synorogenic stratigraphic record of northern Pakistan and India as Early to Middle Eocene (∼52 Ma). Eocene strata in the Tethyan portion of the central part of the Himalayan orogenic belt consist of shallow marine carbonate rocks that lack evidence for initial Himalayan orogenesis, thus leaving open the possibility that the onset of orogeny was significantly diachronous along strike. We report U–Pb ages of detrital zircons and Nd-isotopic and trace element data from associated mudrocks in Cretaceous–Paleocene(?), Eocene, and lower Miocene strata of the southern Lesser Himalayan zone of central Nepal. The Cretaceous–Paleocene(?) Amile Formation is dominated by zircons with Archean–Early Proterozoic U–Pb ages. An abrupt influx of Cambrian–Ordovician and Middle to Late Proterozoic zircons marks the transition into the Eocene Bhainskati Formation, and indicates the onset of erosion of Tethyan rocks in the nascent Himalayan thrust belt. An increased proportion of Late Proterozoic zircons in fluvial litharenites of the lower Miocene Dumri Formation signals initial erosion of Greater Himalayan protoliths. The Nd-isotopic and trace element data support the unroofing history documented by the U–Pb zircon ages. The fact that middle Eocene strata in Nepal were derived from the Himalayan thrust belt reduces the maximum time lag between the onset of orogenesis in the northwest and central Himalaya to no more than ∼2 My.

Break Up of Australia-India-Madagascar Block, Opening of the Indian Ocean and Continental Accretion in Southeast Asia With Special Reference to the Characteristics of the Peri-Indian Collision Zones

Linear belts of Gondwana basins developed in the Indian continent since Late Palaeozoic along favoured sites of Precambrian weak zones like cratonic sutures and reactivated mobile belts. The Tibetan and Sibumasu - West Yunnan continental blocks, that were located adjacent to proto-Himalayan part of the Indian continent, rifted and drifted from the northern margin of the East Gondwanic Indo-Australian continent, during Late Palaeozoic, when the said northern margin was under glacial or cool climatic condition and rift-drift tectonic setting. The Indo-Burma-Andaman (IBA), Sikule, Lolotoi blocks were also rifted and drifted from the same northern margin during Late Jurassic. This was followed by the break-up of the Australia-India-Madagascar continental block during the Cretaceous. The activity was associated with hot spot related volcanism and opening up of the Indian Ocean. The Late Cretaceous and Tertiary phases of opening of the Arabian Sea succeeded the Early Cretaceous phase of opening of the Bay of Bengal, part of the Indian Ocean. The Palaeo- and Neo-Tethyan sutures in Tibet, Yunnan, Laos, Thailand and Vietnam reveal the complex opening and closing history of the Tethys. The IBA block rotated clockwise from its initial E-W orientation because of 90°E and adjacent dextral transcurrent fault movements caused due to faster northward movement of the Indian plate relative to that of Australia. The India-Tibet terminal collision during Early-Middle Eocene initiated Himalayan orogenesis and contemporaneously there was foreland basin development that was accompanied with sporadic but laterally extensive continental-flood-basalt (CFB) type and related volcanism. The Paleogene rocks of the Himalayan foreland basin are involved in tectonism and are mostly concealed under older rocks. The Mesozoic-Early Eocene ophiolite terrane on IBA does not represent the eastern suture of the Indian plate but occurs as klippe on IBA, caused due to oblique collision between Sibumasu and IBA during Late Oligocene. Post-collisional indentation of Y-shaped Indian continent into the Asian collage produced Himalayan syntaxes, clockwise rotation of the Sibumasu block which was then sutured to the Tibetan and SE Asian blocks, and tectonic extrusion of the Indochina block along the Ailao Shan Red River (ASRR) shear zone. Highly potassic magmatic rocks were emplaced during Late Palaeogene at the oroclinally flexed marginal parts of the South China continental lithosphere. These magmatic bodies were dislocated by the ASRR left lateral shear zone soon afterwards. Petrogenetic and tectonic processes that generated the Eocene CFB volcanics at the Himalayan foreland basin may have also produced Late Palaeogene magmatism from outer parts of the Namche-Barwa Syntaxis. Their site-specific location and time sequence suggest them to be genetically related to the India-Asia collision process and Indian continent's indentation-induced syntaxial buckling. Deep mantle-reaching fractures were apparently produced during India-Asia terminal collision at the strongly flexed leading brittle edge of the Indian continental lithosphere, and possibly later in time at the outer oroclinally bent marginal parts of the rigid South China continental lithosphere, generating typical magma. The subduction zone that developed along the western margin of IBA due to oblique convergence between the IBA and the Indian plate is still active. The northern end of IBA ultimately collided with the NE prolongation of the Indian continent and was accreted to it during Mio-Pliocene. The Shillong massif was uplifted and overthrust over the Bengal Basin located over its passive margin to the south, whereas, the Eocene distal shelf sediments of IBA were overthrust over the Tertiary shelf of the Indian continent.

Geological structure of the southern part of the Nanga Parbat massif, Pakistan Himalaya, and its tectonic implications

Abstract The Nanga Parbat massif lies in the core of the major north-south trending, broadly upright antiform that marks the NW syntaxis of the Himalayan arc. However, this antiformal structure is not evident in the trend of foliation and banding within the central and southern parts of the massif. Reconnaissance field studies in this region (Astor, Rama and Rupal areas) have delineated an important shear zone with top-to-the-south overthrust kinematics. This Rupal Shear Zone carries the migmatitic core of the massif onto nonmigmatitic metasediments locally termed the Tarshing Group. The shear zone traces north into a broad high strain zone of steep foliation with gently plunging mineral elongation lineations with no consistent sense of shear. A tentative model is proposed whereby top-to-the-south overshear in the Rupal area passes northwards into a steep belt of apparently constrictional N-S elongation. This type of large-scale transpression may record the early growth of the syntaxis. However, relating these structures to Himalayan orogenesis and the amplification of the NW syntaxis is problematic. The Nanga Parbat massif displays a long and complex history of polyphase deformation, metamorphism and magmatism, as might be expected of a terrane derived from the basement of the Indian sub-continent. Although at least the later part of the constrictional steep belt developed with syn-kinematic leucogranite intrusions (&lt; 10 Ma), the old age limit on the Rupal Shear Zone remains unconstrained.