Northern Ladakh Research Articles

Rudists (Bivalvia: Hippuritida) from the Lower Cretaceous Khalsi Formation, Ladakh, India, and their palaeogeographic implications

Rudist fauna composed of the Radiolitidae and the Polyconitidae is reported from the Aptian–Albian Khalsi Formation, southern Ladakh, India. The rudists are identified as Auroradiolites gilgitensis, A. biconvexus, Magallanesia rutogensis, and Shajia tibetica. Among them, A. biconvexus and M. rutogensis are described first time in Ladakh. Additionally, two radiolitid specimens reported from the Saltoro Formation of the SSZ in northern Ladakh by Upadhyay (2014), are assigned to A. biconvexus, and one single specimen from Saltoro Formation which was assigned to Horiopleura sp. by Upadhyay (2014), is questionably transferred to S. tibetica in this study. The occurrence of A. gilgitensis in the early Aptian of Ladakh provides evidence for the eastern dispersal pattern of A. gilgitensis during early Aptian. The finding of M. rutogensis in Ladakh fills the geographical gap between the first plagioptychid, early Turonian Plagioptychus haueri in the Mediterranean and the previous westernmost record of M. rutogensis in Lhasa Block, and further strengthens the hypothesis that Plagioptychidae was evolved from Magallanesia or related form. The association of A. biconvexus, A. gilgitensis, and M. canaliculata support the hypothesis of southwest Asian- Pacific province for the palaeobiogeographic linkage of Auroradiolites and Horiopleura haydeni–Praecaprotina–Magallanesia. This study provides additional evidence in favour of the relatively close geographical connection and tectonic equivalence between the Ladakh and Lhasa blocks during the mid-Cretaceous. Meanwhile, it highlights the need for detailed biostratigraphic studies in other microcontinents along the northern margin of the East Tethys, which may provide critical information for their tectonic and palaeogeographic evolution.

Evolution and palaeogeographical dispersion of the radiolitid rudist genus Auroradiolites (Bivalvia, Hippuritida) with descriptions of new material from Tibet and archived specimens from Afghanistan

AbstractDiagnostic characters of the recently established new genus of radiolitid rudist Auroradiolites include an entirely compact outer shell layer, a distinctly convex upper (left) valve and a robust myocardinal apparatus surrounding a strongly internally projected ligamentary infolding. Until now, A. biconvexus (previously considered to be exclusively late Albian in age) has been reported only from the Langshan Formation, which crops out along the northern portion of the Lhasa block, Tibet. Here, it is recorded for the first time from the Sangzugang Formation of the Xigaze Forearc Basin, situated on the southern margin of the Lhasa block. A. biconvexus differs from the south‐west Asian type species A. gilgitensis (late Aptian to Albian) by its relatively larger size, more strongly domed left valve and distinct radial undulations of the outer shell layer. The characters of A. gilgitensis are further clarified from archived material from central Afghanistan, also newly identified and described herein. All Auroradiolites records to date are revised. The recognition of examples of A. biconvexus from upper Aptian strata increases both the stratigraphical and geographical ranges of the species, indicating that it had already branched off from A. gilgitensis in the late Aptian. Some new radiolitid specimens that combine both compact and cellular calcitic outer shell layer structures are also described from the Langshan Formation, but a number of internal differences from Auroradiolites cast doubt on their constituting a sister group to the latter and we assign them to Eoradiolites cf. hedini. Rather, the evolution of Auroradiolites directly from the ancestral radiolitid genus Agriopleura is favoured on the grounds of parsimony. So far, the genus Auroradiolites has been recorded from Iran, central and eastern Afghanistan, the type locality of Yasin in north‐western Pakistan, southern and northern Ladakh, the Lhasa block, as well as Hokkaido in northern Japan. During the late Aptian to Albian interval, all of these localities were associated with terranes and blocks that were limited to the north‐eastern margin of Tethys and the western Pacific margin, making Auroradiolites an indicator of a SW Asian to Pacific faunal province.

Early Cretaceous subvolcanic calc-alkaline granitoid magmatism in the Nubra-Shyok valley of the Shyok Suture Zone, Ladakh Himalaya, India: Evidence from geochemistry and U–Pb SHRIMP zircon geochronology

The lithounits constituting the Ladakh Himalaya are exposed along the Indus and Shyok Sutures Zones of northwest Himalaya. The Shyok Suture Zone (SSZ) in northern Ladakh represents a highly tectonized zone of a back-arc basin, which is mainly composed of volcano-sedimentary formations (Shyok and Khardung Formations) intimately, associated with intrusive granitoids. In the Nubra-Shyok valley of the SSZ calc-alkaline granitoids of batholithic dimension are exposed in the Tirit region, referred herein as the Tirit granitoids, which are intrusive evidently into the Shyok volcanic (rhyolite) rocks belonging to the Shyok Formation. In this valley the northern margins of granitoids of the Ladakh batholith can also be found intrusive into the metasediments (shale/slate) and metavolcanics of the Shyok Formation. The compositions and crystallization pressures (~66 to 91MPa) of amphiboles in the intrusive Tirit granitoid corroborate a calc-alkaline nature and solidification of Tirit granitoid melt at subvolcanic level equivalent to a minimum of 2.5km to a maximum of 3.5km thick overburden of Shyok volcanics. U–Pb SHRIMP zircons from the Tirit granitoids have yielded mean crystallization ages of 109.4±1.1Ma and 105.30±0.80Ma, which strengthen the idea of Early Cretaceous subduction beneath the Karakoram terrain. Inherited older zircon cores (278–393Ma, 476–519–713–952Ma and 1933Ma) suggest a contribution from heterogeneous Palaeozoic and Proterozoic sources in the generation of the Tirit granitoids similar to those observed elsewhere in the Karakoram-Kohistan region. A mean crystallization age (105.30±0.80) of zircons in the Tirit granitoid hosting xenoliths of porphyritic volcanics places a minimum eruption age of ca. 105Ma for the Shyok volcanics. The Ladakh granitoid, Tirit granitoids and porphyritic volcanic xenolith belong to a calc-alkaline series. A mean crystallization age (67.32±0.66Ma) for zircon in the Ladakh granitoid implies that the Shyok metasediments (shale/slate) were deposited and metamorphosed prior to ca. 68Ma. Present and earlier determined ages on Tirit granitoids revealed that the subduction-related calc-alkaline magmatism in the Nubra-Shyok valley of the SSZ prevailed between 110 and 68Ma, and hence a minimum age of Early Cretaceous can be suggested for development of the SSZ.

Palaeogeographic Significance of 'Yasin-Type' Rudist and Orbitolinid Fauna of the Shyok Suture Zone, Saltoro Hills, Northern Ladakh, India

Along the Shyok Suture Zone in northern India, a ∼200 m thick limestone succession within the larger Saltoro Formation has been identified as a carbonate platform margin with build-ups. This limestone succession is overlying volcanic rocks of island arc affinity, most probably a volcanic ridge. The partly recrystallized reefal limestone contains abundant rudists, echinoid spines, gastropods, algae and a rich orbitolinid assemblage. This faunal assemblage reflects a shallowwater tropical environment and also shows a close affinity with that recorded from the Yasin Group sediments exposed along the Northern Suture Zone in Kohistan, northwestern Pakistan. The rudist fauna from both sites resembles the Horiopleura haydeni/Eoradiolites gilgitensis association from the Yasin Group, and even the very distant classic latest Aptian-Early Albian rudist fauna of the Santander area in NW Spain. The presence of Late Aptian Horiopleura, Radiolitidae and different forms of Orbitolina and other microfaunal assemblages in the Saltoro reefal limestone dates the underlying volcanic edifice as middle Cretaceous or older. Rudists, nerineids, corals and foraminifers of Early-Middle Cretaceous age are widely distributed as reefal frameworks all along the tropical and subtropical Euro-African-Asiatic regions of the northern and southern margin of the Tethys. However, rudist build-ups may also occur far from the continental margins, associated with volcanic edifices like in the Caribbean, Sicily, and seamounts in the central Pacific region or, in our cases, volcanic arcs within the Tethys.

Mantle-derived arc related mafic enclaves and host orthogneiss from the Shyok Suture Zone of NE Ladakh, India: An evidence of magma-mixing

The Shyok valley in the northern Ladakh is a linear NW-SE trending suture zone (Shyok Suture Zone) and separates two distinct terranes, the southern continental margin of Eurasian Plate to north, and Ladakh magmatic arc to south. The crustal growth in the Shyok Suture Zone (SSZ) can be explained by two stage melting process involving the generation of orthogneiss (OG) with mafic enclaves (ME) (pre-collision), and temporal evolution towards increasingly more evolved felsic magmas during reworking processes (post-collision). In this paper, the geochemistry of various ME and host OG that fall within the Shyok‐Darbuk corridor of the suture zone are discussed. The field, petrographic and geochemical data suggest that the ME and host OG are genetically related. The ME has formed by low degree of partial melting of a garnet-bearing source or from the partial melting of a basaltic source with a clinopyroxene‐garnet eclogite as residue. The LILE and LREE enrichment of ME have been influenced by mantle‐magma interaction during the passage of the hydrous primary arc-magma through the overlying lithosphere. The likely interpretation for the genetic linkage of ME and host OG is explained through the mixing of coeval mafic (enclave) and felsic melts, resulting in hybrid magma. Besides, several geochemical lines of evidence suggest that fractional crystallization of the resultant hybrid magma had played a complementary role in the evolution of these rocks.

Early Cretaceous radiolarians from the Spongtang massif, Ladakh, NW India: implications for Neo-Tethyan evolution

Abstract: The discovery of two Early Cretaceous (mid-Valanginian–mid-Aptian range) radiolarian faunal assemblages from ribbon-bedded cherts collected near Photoskar in northern Ladakh, NW India, provides the first robust biostratigraphic age constraints associated with the Spongtang massif. This klippe of relict Neotethyan suprasubduction-zone ophiolitic rocks and related arc volcanic rocks crops out 30 km south of the Indus suture in Ladakh. The radiolarian assemblages, the age assignment of which lies between published radiometric ages, provide new constraints on the evolution of this intra-oceanic island arc system. Critically, from them it can be inferred that the system was appreciably long-lived (Jurassic–Cretaceous) and more continuous than is commonly considered.

Mineral chemistry and K–Ar ages of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh Himalaya, India

Petrographic analyses of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh area revealed that the rocks consist of diorite, granodiorite, granite and leucogranite. Pyroxene and biotite chemistries by EPMA indicate that most of the diorite, granodiorite and granite are calc-alkaline and formed in an arc-related tectonic setting. These calc-alkaline plutons yield K–Ar biotite ages from 49.3 ± 1.1 to 51.2 ± 1.1 for Ladakh batholith in Khardung La and from 60.8 ± 1.3 to 65.8 ± 1.4 Ma for granites and diorites in Hundar area. The K–Ar ages of the Hundar igneous complex are within the age range of 46–70 Ma of the Ladakh batholith near Leh town and the neighbor sites, indicating the Hundar igneous complex as a part of the Ladakh arc. Their ages are also coincident with the Khardung volcanics (67.4 and 60.5 Ma). The calc-alkaline granites from Panamik of Karakoram block yield K–Ar biotite ages of 95.7 ± 2.1 and 96.7 ± 2.1 Ma, suggesting that the granites are member of the plutons (90–120 Ma) occurring in northern Pakistan, Tangtse in Ladakh and western Tibet that emplaced during subduction of the Neo-Tethyan oceanic plate under the Karakoram block. The leucogranite and granodiorite in the Panamik region yield K–Ar biotite and muscovite ages ranging from 9.18 ± 0.21 to 9.45 ± 0.21 Ma. The leucogranites are the S-type related to post-collision tectonic setting and are considered to have emplaced in relating to activation of the Karakoram fault.

Evolution and chronology of the Pangong Metamorphic Complex adjacent to the Karakoram Fault, Ladakh: constraints from thermobarometry, metamorphic modelling and U–Pb geochronology

Abstract: Sillimanite- and staurolite-grade metamorphic rocks exhumed along the Pangong fault, the NE branch of the right-lateral Karakoram strike-slip fault in northern Ladakh, NW India, show multiple episodes of metamorphism and fabric development. Debate has centred on whether these metamorphic rocks were formed as a result of shear heating during strike-slip faulting, or whether they are exhumed earlier metamorphic rocks unrelated to movement on the Karakoram fault. Here we constrain the burial and exhumation history of the Pangong Metamorphic Complex combining the pressure–temperature evolution with accessory phase geochronology. Sillimanite-grade metamorphism in graphitic pelites was superseded by the preserved P – T conditions of a Bt + Ms + St + Grt + Qtz + Fsp assemblage at 585–605 °C and 6.05–7.25 kbar, equivalent to c . 20–25 km of burial. Laser ablation monazite U–Pb geochronology reveals that sillimanite-grade metamorphism occurred at 108.0 ± 0.6 Ma in rocks immediately adjacent to the Pangong strand of the Karakoram fault, implying that most metamorphic rocks along the Karakoram fault were not formed by shear heating during Miocene strike-slip faulting. This age correlates closely with the ages of the Hunza granite–granodiorite batholith, and the K2 orthogneiss in northern Pakistan, and confirms that some high-grade metamorphism occurred before collision and accretion of the Kohistan arc and the Indian plate to Asia; protracted high-grade metamorphism, and accompanying crustal thickening lasted at least 100 Ma along the South Asian plate margin. Our P – T and geochronology results also demonstrate the continuity of Cretaceous metamorphism across the Karakoram fault.

Age constraints on ductile deformation and long-term slip rates along the Karakoram fault zone, Ladakh

The dextral Karakoram fault zone, that bounds the southwestern margin of Tibet, is widely regarded as one of the major strike-slip faults that accommodates the eastward extrusion of the thickened crust of Tibet. In northern Ladakh, the Karakoram fault bounds the Pangong transpressional zone, with the active fault trace following the eastern margin of the range. Exhumation within this zone reveals mylonites that have absorbed much of the deformation along the fault. Two sets of dikes are evident within the main fault strand that bounds the transpressional zone. Whilst one dike set is concordant with the dominant foliation and shows mylonitic fabric, the youngest dike set are less deformed and cross-cut the shear zone fabric. Using U–Pb ID-TIMS data from five shear zone samples deformed by differing degrees, we infer that fault initiation occurred between 15.68±0.52 and 13.73±0.28 Ma. Since absolute offset is difficult to determine we couple these age constraints with suggested minimum and maximum offsets of the Baltoro-type granites. The offset range of 40–150 km reveals that the Karakoram fault has a long-term average slip-rate in the range 2.7–10.2 mm/year, the lower rate being compatible with GPS, InSAR and cosmogenic data for the fault. The small range of offset and low slip-rates do not support rapid, large-scale extrusion of Tibet along lithospheric-scale faults and strengthens the argument that Tibet does not behave in a rigid, plate-like, manner.

Earthquake-induced soft-sediment deformation in the lower Shyok river valley, northern Ladakh, India

Soft-sediment deformation structures occur in the ∼150 m thick Pliocene–Quaternary fluvio-lacustrine sediments exposed around the Khalsar and Tirit areas in the lower Shyok river valley, northern Ladakh and eastern Karakoram, India. These structures vary in morphology and pattern and occur at different stratigraphic horizons. They satisfy the criteria for attributing them to seismic events and it is proposed that these structures were formed as a result of earthquake-induced liquefaction. The deformation structures are thus interpreted to represent pre-historic seismic events which occurred up to ∼5 Ma ago and were associated with tectonic activity along the Karakoram fault.

Reply to the comments on “Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone on northern Ladakh (NW Himalaya)” by Weinberg and Dunlap

Reply to the comments on “Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone on northern Ladakh (NW Himalaya)” by Weinberg and Dunlap

Comment on “Middle Cretaceous back-arc formation and arc evolution along the Asia margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)” by Rolland and Pêcher

Comment on “Middle Cretaceous back-arc formation and arc evolution along the Asia margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)” by Rolland and Pêcher

Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites: Comment and Reply

[De Sigoyer et al. (2000)][1], studying Tso Morari eclogites in northern Ladakh (northwest Himalaya), claim to have “dated eclogite formation and therefore the timing of the onset of India—Asian contact in the northwest Himalaya at 55 ± 6 Ma.” Their ages (Lu-Hf 55 ± 12 Ma; Sm-Nd 55 ± 7 Ma)

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

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.

Growth and Deformation of the Ladakh Batholith, Northwest Himalayas: Implications for Timing of Continental Collision and Origin of Calc-Alkaline Batholiths.

The calc-alkaline Ladakh batholith (NW Himalayas) was dated to constrain the timing of continental collision and subsequent deformation. Batholith growth ended when collision disrupted subduction of the Tethyan oceanic lithosphere, and thus the youngest magmatic pulse indirectly dates the collision. Both U-Pb ages on zircons from three samples of the Ladakh batholith and K-Ar from one subvolcanic dike sample were determined. Magmatic activity near Leh (the capital of Ladakh) occurred between 70 and 50 Ma, with the last major magmatic pulse crystallizing at ca. 49.8+/-0.8 Ma (2sigma). This was followed by rapid and generalized cooling to lower greenschist facies temperatures within a few million years, and minor dike intrusion took place at 46+/-1 Ma. Field observations, the lack of inherited prebatholith zircons, and other isotopic evidence suggest that the batholith is mantle derived with negligible crustal influence, that it evolved through input of fresh magma from the mantle and remelting of previously emplaced mantle magmatic rocks. The sedmimentary record indicates that collision in NW Himalaya occurred around 52-50 Ma. If this is so, the magmatic system driven by subduction of Tethys ended immediately on collision. The thermal history of one sample from within the Thanglasgo Shear Zone (TSZ) was determined by Ar-Ar method to constrain timing of batholith internal deformation. This is a wide dextral shear zone within the batholith, parallel to the dextral, N 30 degrees W-striking crustal-scale Karakoram Fault. Internal deformation of the batholith, taken up partly by this shear zone, has caused it to deviate from it regional WNW-ESE trend to parallel the Karakoram Fault. Microstructures and cooling history of a sample from the TSZ indicate that shearing took place before 22 Ma, implying that (1) the history of dextral shearing on NW-striking planes in northern Ladakh started at least 7 m.yr. before the <15 Ma Karakoram Fault, (2) shearing was responsible for deviation of the regional trend of the Ladakh batholith, and (3) dextral shearing occured within a zone apporximately 100 km wide that includes the Ladakh batholith and portions of the younger Karakoram batholith.

New field, structural and geochronological data from the Shyok and Nubra valleys, northern Ladakh: linking Kohistan to Tibet

Abstract The Nubra-Shyok confluence in northern Ladakh is a key area for understanding the tectonic evolution of NW Himalaya and provides the basis for linking the geology of Pakistan to that of Tibet. The geology of the confluence area has been the subject of much speculation centred mainly on the existence of ophiolites and their regional significance. These ophiolites are thought to represent the eastward extension of the Shyok Suture Zone (SSZ), which separates the Dras island arc from the southern margin of Eurasia, and which was overprinted by movement along the Khalsar Thrust (often thought to represent the eastern continuation of the Main Karakoram Thrust). The geology of the area is relatively complex and the little information available has hampered regional geological correlations. The Khalsar Thrust (KT) and the dextral Karakoram Fault (KF), two regional tectonic features of NW Himalaya, merge at the confluence defining a triple point and three blocks: the Ladakh block to the south, the Saltoro block to the northwest, and the Karakoram block to the northeast. Close to the triple point, the KF changes strike and movement direction. Movement vector analysis of the triple point indicates that the KT and the two parts of the KF could have moved contemporaneously, and allows prediction of the movement vectors across the faults. The KT and KF shear preferentially volcano-sedimentary rocks of the Shyok and Nubra formations, respectively. Contrary to previous interpretations, these sheared rocks do not represent disrupted ophiolites. Regional tectonic reconstructions, however, require suturing between the Ladakh block and Eurasia and the strike of the SSZ in Baltistan suggests that the suture zone might crop out north of the KT, either along the southern slopes of the Saltoro Range or further north along the Saltoro valley. In the few outcrops of the Saltoro block we were able to visit, we found no evidence of ophiolitic rocks. Instead we found outcrops of the calc-alkaline Tirit batholith. Although our observations do not confirm the presence of the suture-related rocks in the southern Saltoro block, this possibility cannot be ruled out. Zircons from a sample of Tirit granite (U-Pb ion-microprobe age) yielded an age centred at 68 ± 1 Ma. The similar range of modal composition and age of the Tirit and Ladakh batholiths suggest that they are part of the same magmatic event. This result and a number of other observations indicate that the post-75 Ma geology of the Ladakh and Saltoro blocks is similar. Thus, if there is a suture zone in the southern Saltoro block, suturing must have occurred before 75 Ma, as concluded by others along the same tectonic boundary to the west in Pakistan. The KF represents a much younger terrane boundary, juxtaposing rocks of the Ladakh and Saltoro blocks to those of the Karakoram terrane. Rocks related to suturing of continents were not found along the KF. Karakoram leucogranites cropping out in the southern part of Karakoram terrane yielded a U-Pb zircon age centred at 15.0 ± 0.4 Ma (2σ). Because these leucogranites were not found south of the KF, this fault must have initiated after leucogranite intrusion and must therefore be younger than 15 Ma old. At the confluence the KF cuts across the regional rock sequence than can be followed from Kohistan into Baltistan and into the confluence area. Movement on the fault displaces the sequence by approximately 150 km to southeastern Tibet where the regional rock sequence can be regained.

Subduction and collision related magmatism in the Shyok Suture and eastern Karakoram

The Shyok Suture is represented by distinct sets of volcano-plutonic rock assemblages. The high-Mg tholeiitic basalt and calc-alkaline andesites of the Shyok Volcanics have a subduction zone chemical signature. The REE data on tholeiitic basalt suggest a chemical affinity between primitive N-MORB to E-MORB. The calc-alkaline andesites, however, resembles to transitional nature of basalt between E-MORB to OIB. The geochemical data and regional tectonic setting suggest a close similarity between the Shyok Volcanics of northern Ladakh and the Chalt Volcanics of Kohistan.&#x0D; The mildly deformed trondhjemite-tonalite-granodiorite of the Tirit Granitoids are composite plutons located south of the Shyok Suture melange. These granitoids are subalkaline, I-type and represented by volcanic arc chemical signatures. The regional tectonic setting, the nature of occurrence and the composition of Tirit Granitoids are similar to the plutonic suites of northern Kohistan (Gindai, Matum Das and Nomal plutons).&#x0D; The eastern Karakoram Batholith is dominated by quartz monzonite-tonalite-granodiorite and granite. The subalkaline to calc-alkaline Karakoram Batholith is constituted by both I-and S-type granitoids with volcanic arc and syn-collision chemical signatures. REE data suggest that the I-type granitoids of eastern Karakoram are calc-alkaline magmatism of a subduction zone environment. In contrast, most of the S-type granitoids are crust-derived peraluminous granitoids. New Rb/Sr isotopic whole rock age data indicates that an S-type intrusive phase was active in the eastern Karakoram region during 83±9 Ma. The syn-collision nature of these granitoids are similar to those of north Sost pluton and Karambar pluton of northern Kohistan.&#x0D; This indicates that the collision between Kohistan-Ladakh arc and Karakoram block was active during 83±9 Ma.

Tectonic and magmatic evolution of the eastern Karakoram, India

The Shyok suture zone separates the Ladakh terrane to the SW from the Karakoram terrane to the NE. Six tectonic units have been distinguished. From south to north these are; 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram terrane including the Karakoram batholith. Albian—Aptian Orbitolina-bearing lime-stones and turbidites of the Saltoro formation tectonically overlie high-Mg-tholeiites similar to the tectonically overlying Shyok volcanites. The high-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro molasse is an apron-like, moderately folded association of redgreen shales and sandstones that are interbedded with ~ 50 m porphyritic andesite. Desiccation cracks and rain-drop imprints indicate deposition in a subaerial fluvial environment. Rudist fragments from a polygenic conglomerate of the Saltoro molasse document a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok volcanites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemite-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I-type and were emplaced in a volcanic arc setting. The subalkaline to calcalkaline granitoids of the Karakoram batholith are I-and S-type granitoid. The I-type granitoids represent a typical calcalkaline magmatism of a subduction zone environment whereas the S-type granitoids are crustderived, anatectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladakh are equivalent to the successions exposed along the Northern suture in Kohistan. It is likely that the o istan and Ladakh blocks evolved as one single tectonic domain uring the Cretaceous-Palaeogene. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture (50–60 Ma) together with tectonic activity along the Nanga Parbataramosh divided Kohistan and Ladakh into two arealy distinct magmatic arc terranes. The activity and a dextral offset along the Karakoram fault (Holocene-Recent) disrupted the original tectonic relationships. © 1999 Éditions scientifiques et médicales Elsevier SAS

Tectonic and magmatic evolution of the eastern Karakoram, India

The Shyok suture zone separates the Ladakh terrane to the SW from the Karakoram terrane to the NE. Six tectonic units have been distinguished. From south to north these are: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram terrane including the Karakoram batholith. Albian–Aptian Orbitolina-bearing limestones and turbidites of the Saltoro formation tectonically lay over high-Mg-tholeiites similar to the tectonically overlie Shyok volcanites. The high-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro `molasse' is an apron-like, moderately folded association of red-green shales and sandstones that are interbedded with ~ 50 m porphyritic andesite. Desiccation cracks and rain-drop imprints indicate deposition in a subaerial fluvial environment. Rudist fragments from a polygenic conglomerate of the Saltoro `molasse' document a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok volcanites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemite-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I-type and were emplaced in a volcanic arc setting. The subalkaline to calcalkaline granitoids of the Karakoram batholith are I-and S-type granitoid. The I-type granitoids represent a typical calcalkaline magmatism of a subduction zone environment whereas the S-type granitoids are crust-derived, anatectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladakh are equivalent to the successions exposed along the Northern suture in Kohistan. It is likely that the Kohistan and Ladakh blocks evolved as one single tectonic domain during the Cretaceous–Palaeogene. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture (50–60 Ma) together with tectonic activity along the Nanga Parbat-Haramosh divided Kohistan and Ladakh into two arealy distinct magmatic arc terranes. The activity and a dextral offset along the Karakoram fault (Holocene–Recent) disrupted the original tectonic relationships.

Transpressional tectonics along the Karakoram fault zone, northern Ladakh: constraints on Tibetan extrusion

Abstract The Tibetan plateau north of the Himalaya has approximately double normal crustal thickness (60–75 km) and has been homogeneously shortened since the India-Asia collision at 60–50 Ma ago, yet, with minimal erosion rates, has almost no middle or deep crustal rocks exposed at the surface. In the Karakoram range, west of Tibet, early Tertiary crustal thickening and regional metamorphism resulted in Miocene crustal melting producing the Baltoro monzogranite-leucogranite batholith. Late Tertiary transpression along the western margin of the Tibetan plateau, caused by the continued northward penetration of India into Asia, led to exhumation of migmatites in the Pangong range and Baltoro-type granites along the Karakoram fault. Detailed studies of the Karakoram fault zone in northern Ladakh, India, show that Baltoro-type two-mica ± garnet leucogranites, intruded 21–18.0 Ma ago, have been offset a maximum of 150 km right-laterally. Average slip rates since 18.0 ± 0.6 Ma (2σ) are 8.3 mm/a. 40 Ar/ 39 Ar mica cooling ages are 11.3 Ma on both sides of the main (southwestern) strand of the fault, suggesting that most of the exhumation of the Pangong migmatites and leucogranites must have occurred between 18.0 and 11.3 Ma. During this time, at slip rates of 8.3 mm/a the rocks would have moved horizontally right-laterally for c. 56 km and been exhumed by c. 20 km vertically during transpression, using the measured 20° plunge of lineations. The high exhumation rate (3.0 mm/a) and amount of erosion (20 km) inferred between 18.0 and 11.3 Ma may also reflect the partitioning between an early transpressional strain associated with crustal thickening and exhumation of the Pangong deep crustal migmatites and leucogranites, and a later dominantly dextral strike-slip phase of fault motion along the central part of the Karakoram fault from c. 11 to 0 Ma. This timing may also coincide with the initiation of the N-S aligned normal faults and E-W extension in southern Tibet. We suggest that the relatively minor dextral offset (150 km) and the young age of initiation on this bounding fault do not support the model of large-scale extrusion of Tibetan crust, but they suggest instead that deformation of Tibet was taken up predominantly by crustal thickening.