Arnaud Pecher

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effectoftheevolvingtopographycanexplainafractionofthescatterobservedinthedatabut not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumationarelikely.Ithasbeenarguedthatthesteepphysiographictransitionatthefootof the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

Crustal reworking at Nanga Parbat, Pakistan: Metamorphic consequences of thermal‐mechanical coupling facilitated by erosion

Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.

A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet

On the South Karakorum margin, Neogene high-temperature–medium-pressure (HT–MP) gneisses define an east–west trending thermal anomaly. These rocks have been heated from 600 to 750°C during a slight pressure drop from 0.7 to 0.5 GPa. Their retrogressive path cross-cuts the relaxed geotherm of tectonically thickened crust. Such a P–T evolution occurs only if an advective source of heat is involved. Involvement of an advective heat source is also implied by the occurrence of Neogene granitoids and lamprophyres within the HT–MP gneiss area. These rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies. We interpret these geochemical characteristics to be the result of melting of metasomatized Asian lithospheric mantle. The Nd and Sr isotopic compositions of the South Karakorum Neogene magmatic rocks (ϵNd=−12 to −7 and 87Sr/86Sr=0.705–0.725) further suggest they could have originated from mixing between Asian variously metasomatized mantle and Precambrian crust. By contrast, the origin of the youngest magmatic rocks (<10 Myr), here exemplified by the Hemasil syenite and associated lamprophyres, requires involvement of a depleted mantle. The combined ϵHf–ϵNd signature of these rocks (ϵHf=+10.4–+11.5 and ϵNd=+3.4–+4.3) suggests that the source of the Hemasil syenite could have been depleted mantle contaminated by oceanic sediments, likely during the earlier subduction of the Tethyan ocean. Neogene magmatic rocks with the same geochemical characteristics and evolution as those of South Karakorum have previously been described in South Tibet. Based on their location and the geochemical evolution of their source region, we here propose that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab breakoff of the subducting Indian continental margin starting at about 25 Ma. This model is supported by available geophysical data from South Karakorum and South Tibet.

Heat sources for Tertiary metamorphism and anatexis in the Annapurna-Manaslu Region central Nepal

Following the collision of India with southern Tibet, crustal rocks of the leading edge of India (1) underwent regional metamorphism to upper amphibolite grade, (2) melted locally to produce anatectic granitoids, and (3) were sheared and thrust onto lower grade rock along the Main Central Thrust, yielding an inverted metamorphic sequence. This sequence is exemplified in the Annapurna-Manaslu region. We use simple physical calculations to examine the heat sources involved in the different phases of metamorphism. The regional metamorphism apparently is due to the burial of the northern edge of India beneath the accretionary prism along the southern edge of Tibet. The observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation, during the 10–30 m.y. before slip on the Main Central Thrust began, of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mW m−2. The release of water from the footwall of the Main Central Thrust apparently facilitated melting of the overlying crust in the second phase. Such melting could have occurred in the first million years or so of thrusting, if warm (550–650°C) crust in the footwall contained the necessary water. If melting did not occur in the earliest stages of slip on the Main Central Thrust, dissipative heating, with shear stresses of 10 to 100 MPa, is required for temperatures near the Main Central Thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material. The thickness (6–8 km) of the zone of inverted isograds associated with the fault, if undisrupted and due solely to thermal diffusion, implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30–40 km was 4–8 m.y. The apparent inverted temperature gradients (about 10–25°C/km) in this zone can be understood as the combined result of two processes. Diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10–20°C/km. The peak temperatures in such a zone, however, would not have exceeded about 350°C without an additional source of heat. Dissipative heating at shear stresses of about 100 MPa can account for peak temperatures in excess of 600°C during slip on the fault and would have contributed as much as 13°C/km to the inverse gradient. Although inversion of metamorphic isograds could have occurred as a result of deformation within the Main Central Thrust zone, the high temperatures during slip on this zone still require dissipative heating, unless the duration of slip exceeded 25 m.y.

Strain partitioning along the Himalayan arc and the Nanga Parbat antiform

Shortening along the Himalayan arc of continental convergence is approximately in the radial direction. If the underthrusting foot-wall block (India) is not deformed, the hanging-wall block (Tibet) needs to stretch along the arc, as suggested by radial grabens in southern Tibet. In contrast, the Nanga Parbat‐Haramosh massif and the western Himalayan syntaxis are part of a 250-km-long antiform that strikes in the radial direction (northeast) and verges northwest. The Nanga Parbat antiform is the structural and topographic expression of arc-parallel shortening that compensates for arc-parallel extension in southern Tibet. This shortening is predicted to be as high as 12 mm/yr.

An Early Pliocene thermal disturbance of the main central thrust, central Nepal: Implications for Himalayan tectonics

Since the beginning of the collision between India and Asia at about 50 Ma, the convergence in the Himalaya has largely been taken up along major thrust zones. In this study, samples of the Lesser Himalaya Formations, up to 10 km below the Main Central Thrust (MCT), and the Greater Himalaya Sequence, up to 12 km above the MCT, have been analyzed by the 40Ar/39Ar and U-Pb methods to investigate the thermal history of the MCT. The ages can be summarized as follows: (1) The youngest ages from muscovites (3.1 Ma), biotites (3.4 Ma), and hornblendes (4.1 Ma) all come from within 1 km of the MCT, (2) there is a marked asymmetry of ages between the footwall and the hanging wall of the MCT; the maximum mica age in the hanging wall (Greater Himalaya Sequence) is 13 Ma, whereas a muscovite 5 km below the MCT, in the footwall, has an age spectrum with a gradient from 400 to 1400 Ma, (3) five K-feldspars from −6.2 to 11.9 km above the MCT all give minimum ages in the range 3.0–6.4 Ma, and (4) a 206Pb/238U age on a brannerite from the Greater Himalayan Sequence is 4.8 Ma. Structural and petrologic observations preclude the possibility that this age-distance distribution reflects faulting within the Greater Himalaya Sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses. Infiltration of hot fluids through the MCT zone appears to be the best hypothesis to explain these data. Simple numerical simulations, which account for heat transfer by advection within the fluid flow zone and by conduction outside it, indicate that the observed age distribution could have been produced by infiltration of hot fluids through the MCT zone at circa 5 to 4 Ma within the following range of conditions: the fluids heated rocks to temperatures in the range 470 to 510°C for less than 1 million years in a region narrower than the entire MCT zone. The temperature of the thermal disturbance inferred from the 40Ar/39Ar data is consistent with petrologic data from albitic alteration assemblages in the Greater Himalaya Sequence and the U-Pb age of the brannerite is similar to the youngest 40Ar/39Ar hornblende age. This scenario, hydrothermal heating culminating at about 4 Ma, is similar to the model of Le Fort (1981) for the generation of the High Himalayan leucogranites in the Late Oligocene - Early Miocene by dehydration of the footwall rocks of an active thrust. In the younger instance, we interpret that fluids derived from the footwall rocks of the Main Boundary Thrust migrated upward through the Lesser Himalaya Formations and were subsequently channeled along the MCT, producing the thermal disturbance.

Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)

Abstract The Shyok Suture Zone of the Ladakh palaeovolcanic arc is made of Cretaceous volcanosedimentary formations intruded by granodioritic plutons. Field observations show a tectonic stacking of litho-units from different volcanic arc and back-arc environments. In the western part (Skardu area), the Shyok Suture Zone can be subdivided into two groups: (1) The Northern Group, which consists of olistolith basaltic blocks and tuffs. The basalts are LREE depleted with a LILE enrichment and a slight Nb depletion suggesting a back-arc basin origin. (2) The Southern Group, which consists predominantly of andesites that show LREE enrichment, a flat HREE pattern, strong Nb–Ta depletion, and LILE enrichment. The volcanic rocks of the Southern Group appear to have island-arc tholeiite (IAT) to calc-alkaline affinities. In the eastern part of the suture zone (Nubra–Shyok area), Albian to Cenomanian age silicoclastic sediments predominate. These sediments correspond to a large detrital platform built on the northern part of the Ladakh Arc. At the top, these sediments interlayer with basaltic to andesitic lavas. These lavas appear to be more differentiated and calc-alkaline in nature than the Skardu Southern Group lavas, but show similar volcanic arc affinities. No evidence of a back-arc basin was found in this area. Our data from these two areas shows a northwest–southeast evolution, from back-arc to arc formations in northern Ladakh. Opening of this back-arc basin occurred on the northwestern side of the Ladakh Arc. This back-arc was progressively closing eastward. The arc itself was more mature in the east, resembling the southern Tibetan continental arc, and was characterised by more continental sedimentation. Subsequent Himalayan tectonometamorphic evolution, closure of the back-arc basin and deformation along the Shyok Suture, reflects an early asymmetrical geometry along the Asian margin and Kohistan–Ladakh Arc.

The cretaceous Ladakh arc of NW himalaya—slab melting and melt–mantle interaction during fast northward drift of Indian Plate

The Kohistan–Ladakh Terrane in the NW Himalaya is a remnant of a Cretaceous arc sequence obducted onto the Indian margin. This paper presents a geochemical study (major and trace elements and Sr, Nd, Pb isotopes) of the Mid-Cretaceous lavas of the Ladakh side of the arc sequence, which were erupted in response to northward subduction of Neo-Tethys oceanic crust. Lavas from the western Ladakh in Pakistan can be divided into three groups which, from north to south, are: (1) the Northern Group of back-arc tholeiites [0.5<(La/Yb)N<1.4; 0.3<(Nb/La)N<1.4; 4<eNd<8; 38.66<208Pb/204Pb<38.80], (2) the Southern Group of arc tholeiites [1.8<(La/Yb)N<3.9; 0.1<(Nb/La)N<0.6; 5<eNd<6; 38.40<208Pb/204Pb<38.66], and (3) the Katzarah Formation of tholeiitic Nb-rich lavas [3.4<(La/Yb)N<9.8; 1.4<(Nb/La)N<2.1; 3<eNd<5], including radiogenic Pb lavas [39.31<208Pb/204Pb<39.51] and less radiogenic lavas [38.31<208Pb/204Pb<38.55]. Magmas from the eastern Ladakh in India show a simple series of more evolved arc volcanics from basalts to rhyolites [basalts and basaltic andesites: 2.5<(La/Yb)N<5.7; 0.4<(Nb/La)N<0.5; 1.8<eNd<5.5; 38.70<208Pb/204Pb<38.80]. Isotope and trace element data of western Ladakh lavas are compatible with high-degree melting (14–21%) of a fertile MORB-mantle source. An adakitic lava [(La/Yb)N=55.8; (Nb/La)N=0.3; eNd=1.7; 208Pb/204Pb=39.00] and a Mg-poor Nb-rich basalt [(La/Yb)N=4.6; (Nb/La)N=1.3; eNd=−2; 208Pb/204Pb=39.07] are spatially associated with the tholeiitic arc lavas. Isotope compositions of all the lavas, and in particular the radiogenic Nb-rich and adakitic lavas suggest three-component mixing between depleted mantle similar to the Indian MORB mantle, and enriched components similar to the volcanogenic or pelagic sediments. The geochemical diversity of magma types is attributed to contribution of melts from the subducted crust and associated sediments, and their subsequent interaction with the mantle. Such melt–mantle interactions can also be inferred from relicts of sub-arc mantle found in Indian Ladakh. These results lead to a geodynamic reconstruction of the Kohistan–Ladakh arc as a single entity in the Mid-Cretaceous, emplaced south of the Asian margin. Slab melting imply subduction of young oceanic crust, as already proposed for the Oman ophiolite farther west. The fast northward drift of the Indian Plate could have triggered wide-scale inversion of the divergent tectonic regime responsible for the opening of the Neo-Tethys Ocean. Our results suggest breaking of the young oceanic crust initiated at the ridge rather than at passive plate boundaries.

New constraints on the age of the Manaslu leucogranite: Evidence for episodic tectonic denudation in the central Himalayas

The Manaslu leucogranite of central Nepal transacts one segment of the South Tibetan detachment system, a major extensional feature that helped to moderate large topographic gradients in the Himalayan orogen in middle Tertiary time. 40 Ar/ 39 Ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22-23 Ma, providing a minimum age for formation of the detachment system in the Manaslu region that is at least 2 m.y. older than previous estimates from other parts of the orogen. 40 Ar/ 39 Ar mica ages from the aureole, similar to previously published mica ages from the upper part of the leucogranite, indicate an episode of rapid cooling at ∼19-16 Ma. We attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten-sional faults, and we suggest that gravitational collapse of the orogenic front occurred episodically over the late Oligocene-early Miocene interval in the Manaslu area.

The contact between the Higher Himalaya Crystallines and the Tibetan Sedimentary Series: Miocene large‐scale dextral shearing

Space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the Himalayan belt (east and central Nepal, Garhwal, Zanskar). From one area to the other the evolution is very similar: (1) At the base of the pile, in the Main Central Thrust (MCT) shear zone, the stretching lineation, penetrative and regularly oriented N0°E to N30°E, indicates the MCT transport direction, very constant all along the belt, from the Eohimalayan main metamorphic development up to the late-metamorphic movements. (2) At the top of the pile, at the contact between the crystalline unit and its sedimentary cover, gravity-driven structures are confirmed (north-vergent folds, ductile normal faulting). However, there are numerous local indications of late Miocene (syn- to late emplacement of the leucogranitic plutons) dextral shearing. (3) In between, across the medium part of the pile, the stretching lineation shows a conspicuous progressive regional clockwise rotation, clearly indicated by the strain trajectories mapped in Nepal and Garhwal. Points 1 and 2 show that the crystalline-sedimentary boundary, despite its apparent structural and metamorphic continuity, is not only a normal fault but also an important dextral shear zone, which has acted since the upper Miocene as the main southern limit of the eastward extruding Tibetan block.