Karakoram Shear Zone Research Articles

Geological Evaluation of EIGEN-6C4 and GOCE Derived Gravity Models in and around Karakoram Shear Zone, Leh, India

ABSTRACT New generation high resolution gravity models derived from space-borne gravity data, integrated with land based surveys, have enabled understanding of regional gravity field over regions, which are till date considered to be inaccessible for land surveys, especially in mountainous terrains. In this study, we evaluate two high resolution gravity models EIGEN-6C4 and GO_CONS_GCF_2_TIM_R5 in order to understand its usability in identification of meso-scale regional geological features and lithological boundaries around the Karakoram shear zone, in Leh, India. The EIGEN-6C4 is a “hybrid” model integrating data from space-borne sensors and terrestrial data, whereas GO_CONS_GCF_2_TIM_R5 is a model derived from the latest space-borne GOCE sensor. Bouguer gravity anomaly has been derived for both the models and compared. It is seen that, the GOCE derived model pertains to the regional gravity field of the region and compares well with the regional derivative of the EIGEN-6C4 model. Further, the EIGEN-6C4 has been analyzed using horizontal derivatives (dx, dy), analytical signal (ANS) and tilt derivative (TDR) techniques. These, derived maps are then overlain on published geological map of the area to understand the correlation between sub-surface geology vis a vis gravitational signal. The major and distinct geological signatures as derived from the various derivative maps correlate well with the existing geological map. The source boundaries derived from the TDR map agrees reasonably well with the lithological boundaries. Further, the anomaly and derivative maps from EIGEN-6C4 indicates towards a possible continuation of the Shyok suture zone in the region. Therefore, for the given spatial extent of the area under consideration, the GOCE derived model represents the regional field, whereas the EIGEN-6C4 data and derivatives are of sufficient resolution for understanding the geological variability in and around the Karakoram shear zone.

Himalayan leucogranites and migmatites: nature, timing and duration of anatexis

AbstractWidespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence (GHS), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T– conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c. 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas. The relationship between granites and movement on the South Tibetan Detachment (STD) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD. In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS, or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD, it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.

Kinematic Vorticity Analysis along the Karakoram Shear Zone, Pangong Mountains, Karakoram: Implications for the India–Asia Tectonics

Abstract: The Karakoram Shear Zone (KSZ) is a northwest-southeast trending dextral ductile shear zone that has mylonitized the Tangste and Darbuk granitoids of the southern margin of the Asian plate. Kinematic vorticity (Wk) has been estimated in 6 mylonitized Tangste granite samples, using Porphyroclast Hyperbolic Distribution (PHD) and Shear Band (SB) Analyses methods on well-developed quartz and feldspar porphyroclasts, and synthetic and antithetic shear bands respectively to visualize the overall deformation of the KSZ. The PHD and SB analyses yield Wk values ranging from Wk=0.29 to 0.43 and 0.45 to 0.93, respectively, thus indicating distinct pure and simple shear dominant regimes during different stages of the evolution of the KSZ. Strain has essentially been pure shear when southern edge of the Asian plate was initially juxtaposed against the Indian plate around 70 Ma, and changed to simple shear, possibly during the reactivation of this shear zone during 21-13 Ma to produce the shear bands.

Aseismic Layer Detected in the Mid-Crust of Nubra-Siachen Region, India Using Local Seismic Data

The local seismicity observed by seismic network in siachen-nubra region during January 2010-December 2012 shows that the middle part of the crust (17 - 40 km) is aseismic. This aseismic layer (17 - 40 km) is sandwiched between two seismically active layers and depicts a good spatial correlation with the observations of low resistivity reported from magnetotelluric studies for the same region. The local seismicity shows a trend along the Karakoram fault and clustering of events in Shyok Suture zone and Karakoram shear zone. The moment magnitude of these events lies between 1.3 and 4.3. Most of these events have been originated in upper crust.

Age and origin of granites in the Karakoram shear zone and Greater Himalaya Sequence, NW India

The crustal-scale Karakoram shear zone structurally distinguishes the western Himalaya from—and provides an opportunity to compare to— the central and eastern portions of the orogen. To evaluate the tectonic evolution of the western Himalaya, this paper presents granite U/Th-Pb ages and zircon Hf isotopic signatures along the two major structures in northern India: the Karakoram shear zone and the Zanskar shear zone, the westernmost limb of the South Tibetan detachment system. Leucogranites in Zanskar crystallized 27-20 Ma and exhibit Precambrian to Paleozoic inheritance and predominantly negative e Hf (t) values typical of the Greater Himalayan Sequence. Karakoram shear zone leucogran- ites have igneous crystallization ages over a prolonged period from 22 Ma to <13 Ma, contain Late Cretaceous through Paleocene inherited cores, and have e Hf (t) values from +1 to +9. These inherited ages and mostly positive e Hf (t) values compare closely to the adjacent Ladakh batholith, but low e Hf (t) values along the Karakoram shear zone suggest an input of older crustal material from the proximal Karakoram terrane or subducted Indian crust. The Zanskar Greater Himalayan Sequence contains two suites of Paleozoic granites: (1) Pan-African Cambrian- Ordovician granites at the cores of gneiss domes and (2) Mississippian-Permian granites related to magmatism associated with the Panjal Traps. Monazite ages record peak through retrograde metamorphic conditions from 27.3 ± 1.2 Ma to 17.2 ± 0.9 Ma concurrent with anatectic leucogranite crystallization. Cenozoic partial melting in the Greater Himalayan Sequence occurred contemporaneously across the Himalayan orogen, but lower degrees of partial melting and ubiquitous doming distinguish the westernmost Greater Himalayan Sequence in Zanskar.

Hornblende Chemistry in Meta- and Diatexites and its Retention in the Source of Leucogranites: an Example from the Karakoram Shear Zone, NW India

A suite of calc-alkaline arc rocks in the Pangong Metamorphic Complex in the Karakoram Shear Zone, Ladakh, NW India, was migmatized at upper amphibolite facies under water-fluxed conditions. Meta- and diatexites contain abundant hornblende in both leucosome and melanosome. These form idiomorphic, poikilitic megacrysts in the leucosome and in melanosome rims around leucosomes. Hornblende chemistry, particularly its rare earth element (REE) pattern, is highly variable in protoliths and neosomes, and in most cases the REE contents do not reflect hornblende partition coefficients reported in the literature. These variations are the result of the complex crystallization histories of the migmatites. REE variations in whole-rock diatexites and leucogranites demonstrate the role of hornblende in controlling the REE budget. Formation of heavy REE (HREE)-enriched hornblende and its accumulation in the source results in leucogranitic magmas with high Lan/Ybn and Sr/Y values typical of adakites. We show that leucogranitic magmas, depleted in HREE, can be generated by water-fluxed partial melting of arc-related rocks in the mid-crust without requiring garnet retention in the source.

The dike swarm of the Karakoram shear zone, Ladakh, NW India: Linking granite source to batholith

Debate regarding migration of granitic magmas arises from the fact that the physical link between migmatites and plutons and batholiths is rarely documented. Within the Karakoram shear zone, Ladakh, NW India, the synkinematic transfer of magma can be traced from the anatectic source region to the Karakoram Batholith through a complex dike swarm. Melting is due to water fluxing at upper-amphibolite facies, producing Miocene hornblende and garnet–two mica leucogranites. The anatectic zone is characterized by synmagmatic folding and shearing of migmatites, and by pervasive, irregular magma migration paths forming injection complexes. These irregular networks are linked to dike networks that are interpreted to represent a magma transfer zone characterized by a range of styles, such as (1) dike swarms parallel to the shear plane, (2) dikes following the two major foliation planes in the shear fabric, (3) magma sheets in conjugate pairs of ductile fractures, or (4) chaotic dike complexes. Network geometries are controlled by the regional stress field, strain distribution, preexisting anisotropies, and rheological contrasts. Dikes give rise to anastomosing systems with rare crosscutting relationships, and with intersections oriented parallel to the dominant mineral stretching lineation. Dikes feed a number of sills, stocks, plutons, and, ultimately, the leucogranitic Karakoram Batholith. We propose that magma developed an interlinked, continuous dike swarm from source to sink by interacting with deformation and preexisting anisotropies. Unlike theoretical and laboratory predictions, dikes are typically oriented at high angles to the maximum shortening axis, and unlike expectations, water-fluxed melting of the crust produced mobile magma that migrated to form large granitic bodies.

Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram Shear Zone in NW India

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross-cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine-grained material; (iii) compositional zoning of early formed plagioclase and K-feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti-rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine-grained quartz. Mapping of the fine-grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S-C fabric defined by pre-existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro-scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.

Structural complexity resulting from pervasive ductile deformation in the Karakoram Shear Zone, Ladakh, NW India

The NW‐SE trending, ∼800 km long, dextral Karakoram Shear Zone bounds southwest Tibet. We investigate an area immediately NW of the Pangong Lake where midcrustal rocks sections of the shear zone are exposed. Here, the dominant shear zone is characterized by dextral mylonitic rocks sheared at amphibolite facies and partly retrogressed to greenschist facies. These rocks define a kilometric, SE plunging inclined‐to‐recumbent fold in the footwall of a north directed, oblique dextral thrust that exhumed anatectic rocks of the Pangong Range. Argon cooling ages reveal that the Pangong Range cooled from ∼700°C to 300°C between ∼18–15 and ∼10 Ma, contemporaneously with overthrusting. Toward the SE, closer to the Pangong Lake, this kilometric fold is rotated from its NW–SE trend to an E–W trend, and the sense of shear changes to sinistral, with a broadly north directed thrusting component. The documented structures contradict predictions of normal movement in this area derived from considerations of rigid block behavior of the crust and support the interpretation that the system records dextral transpression. Thrusting and exhumation of the deeper and hot rocks exposed in the Pangong Range, folding of the thrust footwall, and the development of a sinistral E–W trending shear zone are all compatible with transpression caused by a northward push by a rigid indenter such as the Ladakh Batholith.

Microstructures of mylonites along the Karakoram Shear Zone, Tangste Valley, Pangong Mountains, Karakoram

The Karakoram Shear Zone is a northwest-southeast trending dextral ductile shear zone, which has affected the granitic and granodioritic bodies of the southern Asian Plate margin in three distinct episodes. The ductile shearing of the granitic bodies at Tangste and Darbuk has resulted in the development of mylonites with mylonitic foliation and stretching lineation. More intense deformation is noted in the Tangste granite grading up to orthomylonite, as compared to the Darbuk granite. Kinematic indicators include S-C foliation, synthetic C′ and C″ antithetic shear bands, Type A s-mantled porphyroclasts, oblique quartz foliation, micro-shears with bookshelf gliding, mineral fishes including Group 2 mica fishes, and Type 1 and 2a pull-apart microstructures, and exhibit strong dextral sense of ductile shearing towards southeast. The textural features of the minerals, especially that of quartz and feldspar, indicate temperature of mylonitisation ranging between 300 and 500°C in the upper greenschist facies, and appear to have been evolved during exhumation as a consequence of oblique strike-slip movements along the Karakoram shear zone.

Hybridization of granitic magmas in the source: The origin of the Karakoram Batholith, Ladakh, NW India

Many magmatic bodies have a hybrid isotopic signature suggesting that somewhere during genesis, transport and emplacement, magmas assimilated other rocks or mixed with other magmas. Where and how hybridization takes place is seldom documented. Here, we investigate a magmatic system in the Eastern Karakoram, Ladakh, NW India, comprising an anatectic zone, and a network of sheets, stocks and plutons exposed in the Pangong Metamorphic Complex within the Karakoram Shear Zone, as well as the Karakoram Batholith. These granitic rocks have an isotopic signature indicative of a mixture between mantle and crustal sources. In the anatectic region, calc-alkaline granitoids and their meta-sedimentary country rocks underwent water-fluxed partial melting at upper amphibolite facies between 20 and 14 Ma ago. Anatexis gave rise to leucosomes and intrusive rocks that have a range in composition from leucotonalite to leucogranite. Those related to the partial melting of calc-alkaline rocks contain hornblende, whereas those related to Bt–psammites contain two micas ± garnet. Leucosomes rooting in different source rocks merge with each other and homogenize as they link up to form a hierarchy of magma channels, feeding into stocks, plutons and ultimately into the Karakoram Batholith. This interpretation is supported by Sr and Nd isotopes. Initial 87Sr/ 86Sr and ε Nd values are distinct for each of the magma protoliths in the anatectic zone and for the magmatic products. Calc-alkaline granitoids have initial 87Sr/ 86Sr = 0.7042 to 0.7077 and ε Nd = + 0.6 to + 2.4, indicative of a slightly depleted mantle source region. This is in contrast to the meta-sedimentary rocks that yield initial 87Sr/ 86Sr = 0.7115 to 0.7161 and ε Nd = − 10.0 to − 9.6, suggesting a stronger crustal component. Leucogranitic rocks, including a variety of leucosomes in the anatectic zone and samples from the Karakoram Batholith, yield intermediate values of initial 87Sr/ 86Sr = 0.7076 to 0.7121 and ε Nd = − 3.6 to − 7.1 that can be modelled by mixing of the two source rocks. The hybrid signature of leucosomes and their similarity to intrusive leucogranites indicate that magma hybridization must have taken place within the source region as a result of the confluence of magmas to form the escape channels. We conclude that the voluminous leucogranites of the Miocene Karakoram Batholith result from water-fluxed intracrustal melting of sources with crustal and mantle signatures, and that mixing occurred within the source.

Comment on: “Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya?” by Mary L. Leech, Earth and Planetary Science Letters 276 (2008) 314–322

Leech [Leech, M.L., 2008, Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya? Earth Planet. Sci. Lett., 276, 314–322.] proposed that (1) Himalayan granites are significantly more abundant east of the Karakoram fault termination (around Mount Kailas, in SW Tibet) than west of it in the Zanskar–Kumaon region, that (2) the fault may have created a barrier to southward flow of mid-crustal channel flow, and that (3) the fault acted as a vertical conduit for these melts. These inferences are based upon new U–Pb SHRIMP data from the Leo Pargil dome, NW India, and the analysis of published U–Pb ages from additional Himalayan domes. Here we point out the flaws in all these hypotheses and suggest a much closer comparison of granites along the Karakoram shear zone to the widespread Miocene crustal melt granites of the Baltoro Karakoram range in North Pakistan. Field relationships combined with U–(Th)–Pb dating of granites and metamorphic rocks clearly shows that the leucogranites exhumed along the Karakoram fault are related to regional metamorphic and melting events along the Baltoro Karakoram range of the Asian plate and not to Indian plate Himalayan leucogranites at all. We discuss individually the points raised.

Magma migration, folding, and disaggregation of migmatites in the Karakoram Shear Zone, Ladakh, NW India

Effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. We investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, NW India, in which leucosome distribution is inferred to record the permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to the fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding were destroyed. A number of structures developed during this process such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and fl ow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.

Tectonics of the southern Asian Plate margin along the Karakoram Shear Zone: Constraints from field observations and U–Pb SHRIMP ages

New geological observations, recent published data and U–Pb SHRIMP zircon dating from the Karakoram Mountains along the Nubra and Shyok Rivers reveal that the initial subduction of the Tethyan oceanic lithosphere took place ~ 110 Ma beneath the Paleozoic–Mesozoic platform of the southern edge of the Asian Plate. This has produced the I-type plutons within the Karakoram Batholith Complex, well before the juxtaposition of the Asian Plate along the Karakoram Shear Zone. Within this shear zone, U–Pb zircon crystallisation ages of ~ 75 Ma from mylonitised granitoids and 68 Ma from undeformed Tirit granodiorite constrain the timing of suturing of the Karakoram terrain with the Trans-Himalaya between 75 and 68 Ma. Post-shearing leucogranite was episodically generated within frontal migmatised Karakoram Metamorphic Belt and emplaced between 20 and 13 Ma within the shear zone. Presence of a low resistivity zone as a possible indication of mid-crustal partial molten crust underneath the Higher Himalaya–Ladakh–Karakoram terrains manifests the impingement of the Indian Plate along the Main Himalayan Thrust at depth. Physical continuity of the Baltoro granite belt into the Karakoram Batholith is established as well as the continuity of the Shyok suture as the Shiquanhe Suture Zone in western Tibet through the Chushul–Dungti sector. The Karakoram Shear Zone, therefore, displays a complex geological history of movements since ~ 75 Ma and plays a very significant role in the overall India–Asia convergence, rather than merely being a strike-slip fault for eastward extrusion of a segment of Asia in Tibet.

Magma ponding in the Karakoram shear zone, Ladakh, NW India

Granitic melt migration and pluton emplacement are commonly closely associated with transcurrent shear zones. The processes that link granites to shear zones are not yet fully understood. The dextraltranspressive Karakoram shear zone in Ladakh, NW India, exposes anatectic rocks where synkinematic melt migration and ponding at kilometer scale were controlled by competency contrasts. Metasedimentary rocks and a dominantly granodioritic calcalkaline intrusion underwent fl uid-present partial melting at upper-amphibolite facies to produce leucogranite sheets and irregular intrusive masses dated at 21–14 Ma. Leucogranitic magmas ponded in the lowpressure strain shadow of the competent grano dioritic calc-alkaline pluton, giving rise to (a) migmatitic rocks that are pervaded by irregular leucogranite intrusions at a scale of meters or tens of meters, and (b) the growth of the Tangtse pluton, a kilometer-scale sheeted complex. Thus, magmas accumulated during shearing and anatexis in a low-pressure strain shadow within the Karakoram shear zone. This magma provided a readily available magma source that could have been tapped to feed larger plutons at shallower levels by modifi cations in the pressure distribution accompanying changes in shear zone geometry and kinematics. We conclude that shear zones tapping anatectic regions act as magma pumps, creating and destroying magma traps at depth as they evolve, and leading to incremental magma addition to upper-crustal plutons.