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We analyze episodic (1995–2010) and permanent GNSS data quantifying interseismic velocities in the Himalaya of Nepal so as to constrain spatial variations in coupling along the Main Himalayan Thrust. To estimate this coupling, we model the MHT with the help of cross sections allowing us to constrain its changes in geometry from far-western Nepal to central and eastern Nepal. We determine that the upper flat of the MHT is nearly totally locked, the crustal ramp is partially locked, and that free slip is localized only along the lower flat north of the Himalaya. This location of increased coupling probably corresponds to the brittle/ductile change in rheology along the MHT and explains the location of very large (greater than the Gorkha 2015 event) earthquake epicenters north of the crustal ramp. Including the simulation of the Western Nepal Fault System in the continuation of the Karakorum fault does not improve simulation of the interseismic velocity field. The 25 April Gorkha earthquake nucleated in a highly coupled part of the upper flat of the MHT and propagated eastward along a less coupled part of the MHT.

[1] Predominant stretching structures in the Greater Himalayan Crystalline Complex (GHC) trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the GHC between the South Tibet Detachment (STD) and the Main Central Thrust. However, our field investigations in southern Tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen-parallel stretching lineation, which separates the Tethyan Himalayan Sequence and the underlying GHC. The shear zones are well preserved in the upper part of the GHC, south to and structurally lower than the STD. Field criteria, microstructures, and quartz fabrics indicate top-to-the-east shearing in the Yadong shear zone (eastern GHC), coexistence of top-to-the-east and top-to-the-west shearing in the Nyalam shear zone (central GHC), but top-to-the-west shearing in the Pulan shear zone (western GHC). Characteristic microstructures and slip systems of quartz in the high-grade GHC rocks resulted from the lateral flow under upper amphibolite (up to 650–700 °C) to greenschist facies conditions. U-Pb ages of metamorphic zircon rims by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses yield 28–26 Ma for the initiation of the Yadong and Nyalam shear zones and 22–15 Ma for the activation of the Pulan shear zone. In addition, 40Ar/39Ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13–11 Ma on the Yadong shear zone, which is coeval with the activation of the STD. Combined with previous studies, we propose that initiation of orogen-parallel extension marks the transition from burial/crustal thickening to exhumation of the GHC. Due to lateral crustal thickness gradients in a thickened crust, orogen-parallel gravitational collapse occurred within the convergent Himalayan orogen in the late Oligocene-Miocene. This tectonic denudation triggered and enhanced partial melting and ductile extrusion of the GHC in the Miocene.

The maximal density of a measurable subset of $${{\mathbb {R}}}^n$$ avoiding Euclidean distance 1 is unknown except in the trivial case of dimension 1. In this paper, we consider the case of a distance associated to a polytope that tiles space, where it is likely that the sets avoiding distance 1 are of maximal density $$2^{-n}$$ , as conjectured by Bachoc and Robins. We prove that this is true for $$n=2$$ , and for the Voronoi regions of the lattices $$A_n$$ , $$n\ge 2$$ .

The Stak massif, northern Pakistan, is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the Indian continent in the northwest Himalaya. Although this unit was extensively retrogressed during the Himalayan collision, records of the high-pressure (HP) event as well as a continuous pressure-temperature (P-T) path were assessed from a single thin section using a new multiequilibrium method. This method uses microprobe X-ray compositional maps of garnet and omphacitic pyroxene followed by calculations of ∼200,000 P-T estimates using appropriate thermobarometers. The Stak eclogite underwent prograde metamorphism, increasing from 650 °C and 2.4 GPa to the peak conditions of 750 °C and 2.5 GPa, then retrogressed to 700–650 °C and 1.6–0.9 GPa under amphibolite-facies conditions. The estimated peak metamorphic conditions and P-T path are similar to those of the Kaghan and Tso Morari high- to ultrahigh-pressure (HP-UHP) massifs. We propose that these three massifs define a large HP to UHP province in the northwest Himalaya, comparable to the Dabie-Sulu province in China and the Western Gneiss Region in Norway.

The metamorphic evolution of the rocks near the Main Central Thrust in the Annapurna-Manaslu region of central Nepal is examined. In this region, all three types of metamorphic features can be observed: regional metamorphism, anatectic granitoids, and inverted metamorphic isograds. In this work, each phase of metamorphism is treated separately to estimate the heat sources required for each process. This approach makes it possible to identify the important parameters for each process, to draw preliminary conclusions about the heat sources required for each of these phases, and to determine which parameters need to be measured more precisely in order to constrain these heat sources.

International audience; The Himalayan range is one of the best documented continent-collisional belts and provides a natural laboratory for studying subduction processes. High-pressure and ultrahigh-pressure rocks with origins in a variety of protoliths occur in various settings: accretionary wedge, oceanic subduction zone, subducted continental margin and continental collisional zone. Ages and locations of these high-pressure and ultrahigh-pressure rocks along the Himalayan belt allow us to evaluate the evolution of this major convergent zone. (1) Cretaceous (80–100 Ma) blueschists and possibly amphibolites in the Indus Tsangpo Suture zone represent an accretionary wedge developed during the northward subduction of the Tethys Ocean beneath the Asian margin. Their exhumation occurred during the subduction of the Tethys prior to the collision between the Indian and Asian continents. (2) Eclogitic rocks with unknown age are reported at one location in the Indus Tsangpo Suture zone, east of the Nanga Parbat syntaxis. They may represent subducted Tethyan oceanic lithosphere. (3) Ultrahigh-pressure rocks on both sides of the western syntaxis (Kaghan and Tso Morari massifs) formed during the early stage of subduction/exhumation of the Indian northern margin at the time of the Paleocene–Eocene boundary. (4) Granulitized eclogites in the Lesser Himalaya Sequence in southern Tibet formed during the Paleogene underthrusting of the Indian margin beneath southern Tibet, and were exhumed in the Miocene. These metamorphic rocks provide important constraints on the geometry and evolution of the India–Asia convergent zone during the closure of the Tethys Ocean. The timing of the ultrahigh-pressure metamorphism in the Tso Morari massif indicates that the initial contact between the Indian and Asian continents likely occurred in the western syntaxis at 57±1 Ma. West of the western syntaxis, the Higher Himalayan Crystallines were thinned. Rocks equivalent to the Lesser Himalayan Sequence are present north of the Main Central Thrust. Moreover, the pressure metamorphism in the Kaghan massif in the western part of the syntaxis took place later, 7 m.y. after the metamorphism in the eastern part, suggesting that the geometry of the initial contact between the Indian and Asian continents was not linear. The northern edge of the Indian continent in the western part was 300 to 350 km farther south than the area east of the Nanga Parbat syntaxis. Such “en baionnette” geometry is probably produced by north-trending transform faults that initially formed during the Late Paleozoic to Cretaceous Gondwana rifting. Farther east in the southern Tibet, the collision occurred before 50.6±0.2 Ma. Finally, high-pressure to ultrahigh-pressure rocks in the western Himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data.

Graphs with circular symmetry, called webs, are relevant w.r.t. describing the stable set polytopes of two larger graph classes, quasi-line graphs [6,10] and claw-free graphs [5,6]. Providing a decent linear description of the stable set polytopes of claw-free graphs is a long-standing problem [7]. However, even the problem of finding all facets of stable set polytopes of webs is open. So far, it is only known that stable set polytopes of webs with clique number ≤ 3 have rank facets only [3,12] while there are examples with clique number > 4 having non-rank facets [8,10]. The aim of the present paper is to treat the remaining case with clique number = 4: we provide an infinite sequence of such webs whose stable set polytopes admit non-rank facets.

The Nangimali ruby deposit in the southern part of the Nanga Parbat Himalaya, has been investigated through field work, geochemistry, stable and radiogenic isotopes. It outcrops in the Shontar valley in a large north-vergent syncline consisting of high-grade metamorphic gneisses capped by a metasedimentary series dominated by marbles and amphibolites. The ore-body is stratiform. Ruby is found within 0.1–2 cm thick shear-veinlets and gash veins cutting dolomitic marbles and carbonate-bearing bands. The marbles of the Nangimali Formation display restricted ranges in δ18O (from 23.6 to 27.6‰ relative to SMOW) and in δ13C (from −1.9 to 2.6‰ relative to PDB). Fluid infiltration along the shear-zone in the marble has no effect on the isotopic signatures of the carbonates. Fluids are metamorphic and CO2 is derived from the decarbonation of marbles. Mass-balance and geochemical analyses suggest that the mobilisation by the fluids of aluminium and chromium in the marbles is sufficient to enable the formation of ruby in the shear-zone. Rubies have been indirectly dated using a stepwise 40Ar–39Ar laser heating technique on syngenetic phlogopites. The Miocene age records a Neogene cooling in the South of the Nanga Parbat massif and a minimum formation age for ruby of 16 Ma.

International audience; New geological, geochemical and geochronological data from the Southern Karakoram (NE Pakistan) indicate the presence of several unexpectedly old and well preserved units along the Asian margin: (1) a Precambrian basement, displaying a minimum amphibole Ar-Ar age of 651 Ma; (2) a thick Cambro-Ordovician platform-type sedimentary unit overlying the Precambrian basement. These series are dated by graptolite and crinoid faunas, and are confirmed by concordant 87Sr/ 86Sr and 13C "ages" of the marbles; (3) a dismembered ophiolitic series formed by slices of metagabbros and metabasalts separated by ultramafic lenses (the Masherbrum Greenstone Complex). The occurrence of such Cambro-Ordovician series overlying a Precambrian basement in south-eastern Karakoram similar to the south-western Karakoram shows that the Karakoram constitutes a continuous tectonic block. The petrology and geochemistry of the Masherbrum Greenstone Complex (mineral chemistry, major and trace element and Sr-Nd isotopic data) are indicative of a supra-subductive environment. The presence of LREE-enriched calc-alkaline rocks 〚(La/Yb) N = 4.4-5.6; (Nb/La) N = 0.2-0.3; ɛNd 565 = 5.1-7.1〛 and LREE-depleted tholeiitic rocks 〚(La/Yb) N = 0.5-1.3; (Nb/La) N = 0.6-0.9; ɛNd 565 = 5.6-7.8〛 are consistent with arc and back-arc settings, respectively. A high-Mg andesitic dolerite and an OIB-type metabasalt, with lower ɛNd ratios (ɛNd 565 = 0.5 and 4.5) are in accordance with source heterogeneity beneath the arc. The Masherbrum Greenstone Complex, along with other Cambro-Ordovician central-eastern volcanic series give evidence of a tectonic situation governed by micro-plate convergent-divergent systems with occurrence of arc - back-arc settings during the Lower Palaeozoic, comparable to that of the current SW Pacific area.

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 (

The South Karakorum margin, east of the Himalayan syntaxis, consist of an E–W elongated zone of young (10–3 Ma) high-grade metamorphic rocks (M2) and related migmatitic domes. This late tectono-metamorphic event post-dates the Palaeogene (55–37 Ma) phase of thickening of the belt featured by NW–SE structures and associated M1 amphibolite facies metamorphism (0.7 GPa, 700 °C). This M2 metamorphism is characterised by low-pressure, high-temperature conditions coeval with migmatite formation in response to a thermal increase of c. 150 °C compared to M1, culminating at a temperature of c. 770 °C and a pressure of 0.5–0.6 GPa. Rapid exhumation of migmatitic domes, at a rate of 5 mm yr−1, was accommodated by vertical extrusion, in the core of E–W crustal-scale folds. These crustal-scale folds formed in response to N–S syn-collisional shortening and were enhanced by thermal weakening of the migmatised continental crust. M2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity, firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid-crustal level. Such relationships between a mantle-related magmatism and a high-temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted Indian slab since 20 Ma.

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.

Two types of island-arc occur in the North American Cordillera during the Permian-Triassic times. The first type is exposed in the eastern Klamath and Blue Mountains (fig. 1). Its stratigraphy is continuous from Permian to Triassic, and is composed of arc-tholeiites with minor calc-alkaline lavas. This suite shows high epsilon Nd (sub (T)) values similar to the range of intra-oceanic island-arc [Lapierre et al., 1987; Brouxel et al., 1987, 1988; Charvet et al., 1990; Lapierre et al., 1990, 1994]. In contrast, the second type, exposed in northern Sierra Nevada and central-western Nevada (Black Dyke) (fig. 1), is characterized by an early Permian calc-alkaline suite, with positive to negative epsilon Nd (sub (T)) values. Its basement is inferred to present continental affinities [Rouer et Lapierre, 1989; Rouer et al., 1989; Blein et al., 1996, 2000]. In western Nevada, volcanic rocks of early Triassic age are present in few localities: (1) the Triassic Koipato Group in central Nevada (fig. 1); (2) the Pablo Formation in the Shoshone mountains and the Paradise Range (figs. 1 and 2); and (3) the Garfield Flat formation in the Excelsior mountains (figs. 1 and 2). Silberling [1959] has subdivided the Pablo formation into three members: clastic, limestone, and greenstone (fig. 3). The clastic member consists of andesites, interbedded with volcaniclastic turbidites. The contact between the clastic and the limestone members is gradational and interlensing. The limestones are locally bioclastic with shell fragments, indicating a shallow-water deposition. They yielded a reworked late Permian fauna which suggests a late Permian or younger age. The clastic and limestone members could represent the recurrent rapid deposition in a shallow marine basin of volcanic flows, reworked material from a nearby terrane of volcanic, granitic, and sedimentary rocks. The greenstone member is composed of andesites, volcanic breccias and tuffs. The middle Triassic Granstville formation rests conformably on the Pablo formation. Both formations are affected by Mesozoic polyphase deformations [Oldow, 1985]. The Permian and/or Triassic Garfield Flat formation is composed of ignimbrites and pyroclastic breccia interlayered with conglomerates, sandstones, calcareous and red pelites (fig. 4). The Jurassic-Triassic Gabbs-Sunrise formation rests unconformably on the Garfield Flat formation. Both formations are affected by Mesozoic polyphase deformations [Oldow, 1985]. In the Pablo formation, lavas are shoshonitic basalts and calc-alkaline andesites, while calc-alkaline andesites and rhyolites predominate in the Garfield Flat formation. Basalts and andesites exhibit enriched LREE patterns (fig. 6) with slight negative anomalies in TiO 2 , Nb and Ta typical of subducted-related magmas in the primitive mantle-normalized spidergrams (fig. 7). The lavas show epsilon Sr (sub (T)) and epsilon Nd (sub (T)) values which range between -0.4 to +19.6, and -1.4 to +0.8 respectively (fig. 8). Most of the samples are displaced from the mantle array toward higher epsilon Sr (sub (T)) values, due to the alteration. The epsilon Nd (sub (T)) values, close to the Bulk Earth composition, record an interaction between material from a juvenile pole (mantle or young crust) and from an old crust. The Pablo and Garfield Flat formations differ from the Permian Black Dyke formation. This latter is characterized by calc-alkaline basalts and mafic andesites enriched in LREE, and a mantle source contaminated by subducted sediments or arc-basement [Blein et al., 2000]. The Pablo and Garfield Flat formations show many similarities with the Koipato Group. In central Nevada, the Koipato Group is a sequence of andesites, dacites and rhyolites interbedded with tuffs and volcaniclastic sediments. It rests with a marked angular unconformity on folded Upper Paleozoic oceanic rocks [Silberling and Roberts, 1962]. Fission-track dating on zircon [McKee and Burke, 1972] indicate an age of 225+ or -30 Ma for the Koipato Group. Ammonites, near the top, are considered to be upper early Triassic [Silberling, 1973]. The Pablo and Garfield Flat lavas share in common with the Koipato Group: (1) late Permian to middle Triassic ages; (2) abundant andesites and rhyolites with minor basalts, associated with felsic pyroclastic breccias; (3) LILE and LREE enrichement; (4) low epsilon Nd (sub (T)) values suggesting a juvenile source with slight contamination by a crustal component; (5) La/Nb ratios close to the lower limit of orogenic andesites [Gill, 1981]; and (6) high Nb/Zr ratios suggesting a generation far from a subduction zone [Thieblemont and Tegyey, 1994]. This Triassic high-K calc-alkaline to shoshonitic magmatism is enriched in K, Rb, Th, Nb and Ta relative to the calc-alkaline Black Dyke lavas, and is mainly juvenile judging from Nd isotopic ratios. The source may correspond either to a juvenile crust composed of high-K andesites [Roberts and Clemens, 1993], which could be the Black Dyke lavas, or to phlogopite-K-richterite enriched lithospheric mantle. In both cases, the generation of the high-K calc-alkaline magmatism needs the former existence of an important subduction phase to generate its source. The lavas of the Pablo and Garfield Flat formations are similar to calc-alkaline and shoshonitic lavas emitted in post-collisional setting. Post-collisional arc/continent magmatism is varied from intermediate to felsic, calc-alkaline to shoshonitic, low to high-K and meta-aluminous to hyper-aluminous. The studied lavas may be compared to the arc/passive margin collision of Papua-New Guinea, where a post-collisional magmatism characterized by high-K basalts, andesites and shoshonites [McKenzie, 1976]. In Nevada, this post-collisional event develops after the accretion of the Permian Black Dyke island-arc (Type 2), and before the accretion of the intra-oceanic Permo-Triassic arc (Type 1).

The metamorphic rocks preserved in the Pangong Range, within the Karakoram Fault zone (Western Tibet) yield granulitic and amphibolitic assemblages. The petrography and thermobarometry of the granulites show that they were equilibrated at high temperature (T>800 °C, P∼5.5 kbar), and were subsequently retromorphosed in the amphibolite facies (T∼700–750 °C, P∼4–5 kbar), and in the greenschist facies. These very high temperature conditions suggest that the Karakoram Fault is a lithosphere-scale shear zone.

Microfractures appearing in thin section as fluid inclusion trails in quartz crystals were studied in four core samples of Soultz-sous-Forets granite. Their orientations in four series of three mutually perpendicular thin sections were estimated using a previously described apparent dip method and a new method involving measurements of strike and apparent dips. Three samples display three microfracture sets and one sample displays two sets. In all samples, one set is nearly vertical and strikes N–S. In two samples, one and two sets are nearly vertical and strike E–W. In two samples, two sets strike NW–SE: one is vertical, the other dips gently to the NE (or SW). Comparing microfracture and mesofractures sets in the same cores shows that (1) the N–S microfracture set is always dominant at both scales and (2) all other microfracture sets have no mesoscopic counterpart. The N–S microfracture sets could have been created during E–W extension of earliest Cenozoic age (Rhine Graben rifting). Differences between the two scales are explained by a σ1/σ2 switching which occurred at the crystal scale and generated mutually perpendicular cracks.