Nappe Research Articles

Rift thermal inheritance in the SW Alps (France): insights from RSCM thermometry and 1D thermal numerical modelling

Abstract. Conceptual models of orogenic accretionary prisms assume that peak temperatures (Tmax⁡) increase towards the internal domains as crustal rocks are accreted from the lower to the upper plate. However, the recognition of pre-orogenic heating events in mountain belts questions the magnitude of thermal overprint during nappe stacking. Using Raman spectroscopy on carbonaceous material (RSCM) to calculate Tmax⁡, we have investigated the thermal record of Lower Jurassic to Eocene strata exposed along six stratigraphic sections at the front of the Digne Nappe (SW Alps), from the Devoluy Massif to the Castellane Arc. Our results highlight two groups of depth-dependent temperatures: (1) a regionally extensive and constant Tmax⁡ up to 300–330 ∘C measured in the Jurassic succession and (2) regionally variable lower temperatures (<150 ∘C) recorded either in the Upper Mesozoic or the syn-orogenic sequence. Modelling shows that the highest palaeotemperatures were achieved during the Early Cretaceous (∼ 130 Ma), associated with the Valaisan–Vocontian rifting, while the lowest Tmax⁡ reflect post-rift thermal relaxation in the Alpine foreland basin. This study provides a striking new example where mid-crustal palaeotemperatures measured in sediments accreted from the downgoing plate are inherited. An estimated peak thermal gradient of 80–90 ∘C km−1 requires a crustal thickness of 8–10 km during the Early Cretaceous, hence placing constraints for tectonic reconstruction of rift domains and geophysical interpretation of current crustal thickness in the SW Alps. These results call for the careful interpretation of palaeothermal data when they are used to identify past collisional thermal events. Where details of basin evolution are lacking, high-temperature records may be misinterpreted as syn-orogenic, which can in turn lead to an overestimation of both orogenic thickening and horizontal displacement in mountain belts.

The metabasites from the Texel Unit (Austroalpine nappe stack): markers of Cretaceous intracontinental subduction and subsequent collision

Abstract Eclogites in the Texel Unit (Austroalpine nappe stack; Eastern Alps, South Tyrol, Italy) represent the westernmost out-crops of the E-W striking Eoalpine High-Pressure Belt (EHB). East of the Tauern Window, the EHB forms part of a Cretaceous intracontinental south-dipping subduction/collision zone. The impetus of this contribution is to extend the geothermo-barometric data of the eclogites from the EHB using conventional geothermobarometry and multi-equilibrium calculations as well as Zr-in-rutile/titanite geothermometry and to put the P-T results of eclogites and amphibolites from the Texel Unit into the geodynamic framework. The investigated samples of this study are from the Spronser- and the Saltaus valleys (S-Tyrol) in the Austroalpine Texel Unit. The Texel Unit is composed mainly of paragneisses with minor intercalations of micaschists, orthogneisses, amphibolites and subordinately eclogites. The amphibolites from the Spronser valley contain the mineral assemblage amphibole + plagioclase + garnet + clinozoisite/epidote + quartz + titanite ± ilmenite ± rutile ± apatite ± calcite. Chemical zoning in plagioclase and amphibole shows two main growth stages: an older P-dominated stage (e.g. albite and barroisite cores) and a younger amphibolite-facies stage. The core of the amphiboles shows barroisite composition, the rim can be chemically classified as hornblende, edenite, tschermakite and pargasite. Geothermobarometric calculations with multi-equilibrium geothermobarometry (THERMOCALC v.3.21) yield temperatures of 600–654°C and pressures of 0.98–1.17 GPa for the same samples. The eclogites from the Saltaus valley contain the peak mineral assemblage omphacite + amphibole + garnet + clinozoisite/epidote + muscovite + quartz + titanite ± ilmenite ± rutile. Thermobarometric calculations, were performed in the system CaO-FeO-MgO-Na2O-Al2O3-FeO-SiO2-H2O with the assemblage clinopyroxene + garnet + amphibole + clinozoisite/epidote + muscovite + quartz ± H2O. The calculations involved an H2O-absent invariant point (mode-1 calculation) as well as two types of average P-T mode-2 calculations. The obtained average P-T H2O-absent mode 1 P-T conditions are 1.89 ± 0.18 GPa and 578 ± 60°C. Using the average P-T mode-2 two types of calculations were done: (1) calculations without amphiboles but with H2O present, which yield mean P-T conditions of 1.95 ± 0.28 GPa and 601 ± 55°C and (2) calculations with amphiboles but without H2O, which yield mean P-T conditions of 1.95 ± 0.26 GPa and 666 ± 77°C. Calculations using THERMOCALC v.3.33 yield similar results with slightly higher pressures of 0.3 GPa. Based on the present geothermobarometric data in conjunction with available mineral ages the eclogites represent the Eoalpine intracontinental subduction stage whereas the amphibolites reflect the subsequent, P-accentuated stage of decompression associated with the subsequent Eoalpine collisional stage.

СТРОЕНИЕ КЫЛЛАХСКОЙ ЗОНЫ (ЮЖНЫЙ СЕКТОР ВЕРХОЯНСКОГО СКЛАДЧАТО-НАДВИГОВОГО ПОЯСА) ПО МАТЕРИАЛАМ СЕЙСМИЧЕСКОГО ПРОФИЛИРОВАНИЯ

The structure of the Kyllakh thrust zone of the Verkhoyansk fold and thrust belt has been studied by seismic profiling. The construction of geoseismic sections refined the thickness of individual sedimentary complexes and the total thickness of the sedimentary cover, the structure of thrust sheets, and the position of detachments. Also, geoseismic sections were used to estimate the amount of the minimum slip of allochthons and to calculate compression coefficients for individual parts of the Kyllakh zone. The seismic and geological data were compared. A brief description is given of the sequence of geodynamic events that resulted in thrust folding in the region.

Geology, Structure, Metamorphism and Deformation of Botechaur-Chaurjhari Area of Western Nepal

The Lesser Himalayan unit of the Nepal Himalaya, especially the western and central parts comprise numerous metamorphic crystalline thrust sheets occupying the cores of large synclinorium. Despite having complex geological settings, some areas in western parts of Nepal lack detailed geological mapping with credible information. This study aims to explore one least explored section; with emphasis on lithostratigraphy, structural setting, deformation, metamorphism, and magmatism. Detailed geological mapping along the Botechaur-Chaurjhari section revealed three tectonostratigraphic units separated by the Main Boundary Thrust (MBT) and the Mahabharat Thrust (MT); the Siwaliks (Middle Siwalik), autochthonous units of the Lesser Himalaya (Dandagaon Phyllite, Nourpul Formation and Dhading Dolomite) and the Lesser Himalayan Crystalline units (Kalagaon Formation and Chaurjhari Formation) from south to north respectively. From the microstructural analysis and measurements of shear sense indicators, the MT shows a top-to-the-south sense of shear indicated by asymmetric boudins, parasitic folds, and thrusting indications in blasto-mylonitic augen gneiss and asymmetric porphyroblast of garnet and the shearing in the garnet. The area also displays an inversion of metamorphism, supporting the development and existence of thrust. The metamorphic grade ranges from chlorite grade to kyanite grade having metamorphic facies assemblage of zeolite to amphibolite facies. Five phases of deformation (D1 to D5) are prevalent within the area, two of which (D1 and D2) belong to the pre-Himalayan (pre-Tertiary) orogeny and the rest of the deformation syn-to post-Himalayan.

Large thrusting and late faulting shape the Aiguilles Rouges crystalline massif (Western Alps), structural implications

Although the Helvetic nappes of the Western Alps have been widely studied, their style, amount, and timing of deformation during the Alpine orogeny remain a subject of controversy. The discussion partly arises from various interpretations of the external crystalline massifs geometry and of the cover/basement relationships. Here, we apply structural analysis based on new field data, microtectonics, and 3D geological modeling on the Aiguilles Rouges and Mont-Blanc massifs, to better quantify their tectonic evolution and their relationship with the nappe stacking. Our results show large thrusting of the Helvetic Mesozoic sedimentary cover above the Aiguilles Rouges basement towards the WNW. The basal contact of the Helvetic nappes, which we call the Helvetic Basal thrust (HBT), often corresponds to a décollement level localized above the Triassic unconformity, but that locally also affects the basement. We suggest that most of the subalpine ranges are klippes above the HBT, and estimate the shortening of the thrust from previously published cross-sections to be between 13 and 32 km. According to published ages, we argue that the HBT was active in the Oligo-Miocene, between ∼30 and ∼14 Ma. After ∼14 Ma and until ∼6 Ma, the HBT is offset by late steep out-of-sequence faults, including back-thrusts and the Mont-Blanc shear zone that was previously a ramp of the HBT. The transition of the major thrust system from the HBT rooting above (east) of the Mont-Blanc to the Alpine sole thrust rooting below (west) of the Aiguilles Rouges occurred between 18 and 14 Ma.

Vývoj metamorfních fluid pozdního stádia regionální metamorfózy v keprnické klenbě silezika

Neo-Proterozoic rocks (Brunovistulian Unit) are dominant in the studied area of Silesicum, which have complex nappe, thick-skinned and thrust sheet geological structures. The Keprník Dome (Keprník nappe) represents the easternmost part of crystalline complexes of the Silesian domain and is dominated by various types of orthogneiss, gneiss, mica schist and phyllite, with a small occurrence of calc-silicate rocks. These rocks are affected by pre-Variscan and Variscan metamorphism, dominantly staurolite-sillimanite and garnet zone and late Variscan to Cenozoic brittle deformation. Hydrothermal Variscan quartz veins and surrounding rocks (biotite-bearing paragneiss and orthogneiss) were studied in the Keprník dome geological unit to confront the type of fluids enclosed in fluid inclusions and the intensity of metamorphism that affected the host rocks. The study of fluid inclusions in quartz veins was supplemented by Raman spectrometry and microscopic study of rocks, EMPA and mineral thermometry. Temperatures 616 to 643 °C obtained by the Ti-in-biotite method for biotite paragneisses are compatible with the peak metamorphic conditions estimated by previous authors. Thermometer for orthogneiss provided temperatures between 688 to 699 °C using the Ti-in-biotite thermometer. The lower temperatures (483 to 529 °C) obtained using magnetite-ilmenite thermometer document later retrogression. Three types of fluids were found in the quartz of the Variscan hydrothermal veins based on the phases present, respectively components: I. L+V, H2O–NaCl ± K+ ± Mg2+ ± Ca2+, II. L+V+S±S2, H2O–NaCl–CaCl2 ± K+ ± Mg2+, III. L1+L2+V, H2O–NaCl–CO2 ± CH4 ± K+ ± Mg2+ ± Ca2+ (L – liquid, V – vapour, S – solid). Type II inclusions contain aqueous fluids with high salinities, which would correspond to the post-Variscan systems found (Slobodník et al. 2010a, 2020) in the Silesicum area. On the other hand, they have high Th temperatures, reaching values of over 300 °C. Only results from type I could be used to calculate isochores and derive P-T conditions for the formation of Variscan veins, because in type III often occur fluid inclusion decrepitation and the impossibility of measuring total homogenization. Types I and III according to the structural position of fluid inclusion (uncertain or primary and pseudosecondary inclusions), microthermometric data and composition, they are considered to be metamorphic types of fluids generated by Variscan metamorphism. The evaluation of isochores of the first type of fluids using a lithostatic thermobaric gradient (50 °C/270 bar/km) indicates the entrapment of fluids, or formation of inclusions under conditions of 190–300 °C and 90–160 MPa. This type of fluid was very likely captured at a later stage of metamorphism, after the crystallization of magnetite and ilmenite, and represents a retrograde stage of metamorphism.

Descriptive text to the Geological map of Denmark, 1:50 000, Møn 1511 I, 1511 IV and 1512 II

Descriptive text to the Geological map of Denmark, 1:50 000, Møn 1511 I, 1511 IV and 1512 II

Bhumichula plateau: A remnant high-elevation low-relief surface in the Himalayan thrust belt of western Nepal

The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya.

Using geophysics to follow and model the Precambrian basement terranes beneath the Caledonian nappes in Finnmark, northern Norway: A case study

The bedrock geology of Finnmark, northern Norway, consists primarily of rock complexes dating back to the Precambrian and early Mid Palaeozoic eras and comprises (1) a mid-crustal lithospheric basement which is characterised by granitoid and greenstone complexes dating from Neoarchean to late Paleoproterozoic in age; (2) the Caledonide Orogen, characterised by a series of nappes and thrust sheets. Finnmark forms part of the northern margin of the Fennoscandian Shield, which extends across northern Finland into northwestern Russia. Precambrian rock complexes are also found as thrust slices in Caledonian nappes at several places in Finnmark and Troms, as well as in tectonic windows.Recently acquired high-resolution aeromagnetic data have provided spectacular and confirmatory evidence for the continuation of the diverse Precambrian greenstone belts and granulite terranes beneath the magnetically transparent Caledonian nappes.This study aims at analysing this subsurface continuation of the terranes using potential field data and estimating the depths of the main magnetic sources beneath the nappes. In this study, several geophysical methods are used, including 3D Euler deconvolution and 2D Werner deconvolution. We have focused specifically on the depth-to-basement beneath nappes and purposefully excluded anomalies associated with shallow structures, such as dykes and faults. Different methods have produced similar results and indicate an appreciable increase in the depth of the Precambrian basement rocks from south to north. In the south, Precambrian basement rocks are either exposed on the surface southeast of the Caledonian front or reappear within tectonic windows. A prominent positive magnetic anomaly on the Porsanger Peninsula is thought to represent an extension of the anomaly ascribed to the greenstone belt of the Repparfjord Tectonic Window. A 3D model of this has also been provided. According to estimates, the depths-to-basement below nappes close to the Caledonian front range from less than 500 to 2000 m, whilst in the northern part of the study area the depths generally reach 3000 to 4000 m, and are locally estimated to range between 5000 m and 6000 m.

A major Miocene deepwater mud canopy system: The North Sabah–Pagasa Wedge, northwestern Borneo

Abstract Three-dimensional seismic reflection data, well data, and analogues from areas with extensive shale tectonics indicate that the enigmatic deepwater “shale nappe or thrust sheet” region of northern offshore Sabah, Malaysia, now referred to as the North Sabah–Pagasa Wedge (NSPW), is actually a region of major mobile shale activity characterized by mini-basins and mud pipes, chambers, and volcanoes. A short burst of extensive mud volcano activity produced a submarine mud canopy complex composed of ~50 mud volcano centers (each probably composed of multiple mud volcanoes) that cover individual areas of between 4 and 80 km2. The total area of dense mud canopy development is ~1900 km2. During the middle Miocene, the post-collisional NSPW was composed predominantly of overpressured shales that were loaded by as much as 4 km thickness of clastics in a series of mini-basins. Following mini-basin development, there was a very important phase of mud volcanism, which built extensive mud canopies (coalesced mud flows) and vent complexes. The mud canopies affected deposition of the overlying and interfingering deposits, including late middle to early late Miocene deepwater turbidite sandstones, which are reservoirs in some fields (e.g., Rotan field). The presence of the extensive mud volcanoes indicates very large volumes of gas had to be generated within the NSPW to drive the mud volcanism. The Sabah example is only the second mud canopy system to be described in the literature and is the largest and most complex.

Diverse Deformation Mechanisms and Lithologic Controls in an Active Orogenic Wedge: Structural Geology and Thermochronometry of the Eastern Greater Caucasus

AbstractOrogenic wedges are common at convergent plate margins and deform internally to maintain a self‐similar geometry during growth. New structural mapping and thermochronometry data illustrate that the eastern Greater Caucasus mountain range of western Asia undergoes deformation via distinct mechanisms that correspond with contrasting lithologies of two sedimentary rock packages within the orogen. The orogen interior comprises a package of Mesozoic thin‐bedded (<10 cm) sandstones and shales. These strata are deformed throughout by short‐wavelength (<1 km) folds that are not fault‐bend or fault‐propagation folds. In contrast, a coeval package of thick‐bedded (up to 5 m) volcaniclastic sandstone and carbonate, known as the Vandam Zone, has been accreted and is deformed via imbrication of coherent thrust sheets forming fault‐related folds of 5–10 km wavelength. Structural reconstructions and thermochronometric data indicate that the Vandam Zone package was accreted between ca. 13 and 3 Ma. Following Vandam Zone accretion, thermal modeling of thermochronometric data indicates rapid exhumation (∼0.3–1 mm/yr) in the wedge interior beginning between ca. 6 and 3 Ma, and a novel thermochronometric paleo‐rotation analysis suggests out‐of‐sequence folding of wedge‐interior strata after ca. 3 Ma. Field relationships suggest that the Vandam Zone underwent syn‐convergent extension following accretion. Together, the data record spatially and temporally variable deformation, dependent on both the mechanical properties of deforming lithologies and perturbations such as accretion of material from the down‐going to the overriding plate. The diverse modes of deformation are consistent with the maintenance of critical taper.

Burial and thermal history of the eastern transform boundary of the central western carpathians based on 1D basin modeling

This paper represents the first data on the thermal and burial history of Mesozoic and Cenozoic sedimentary rocks in the Maruszyna IG-1 deep borehole from the transpressional shear zone of the Western Carpathians. Representative, organic-rich mudstone and claystone samples were selected to determine thermal maturity using mean random vitrinite reflectance (Ro) and thermal alteration index (TAI) data. The reconstruction of the thermal history of fold-and-thrust units was carried out using 1D modeling. This study uniquely distinguishes various thermal maturity levels in the deep profile determined by strata repetition, different absorption of clay and carbonate mineral settings and structural thermal changes in humic particles and palynomorphs. Reflectance of autochthonous vitrinite varies from 0.77 to 1.95% indicating thermally mature to overmature organic matter. Different subsidence-uplift models were tested during modeling in order to find a plausible scenario of tectonic development and heat flow history. The thermal maturity of the individual thrust sheets was modeled separately allowing their individual thermal histories to be investigated before and after the main and final thrusting phase. The best-calibrated scenario assumes a low paleo heat flow ranging from 45 to 54 mW.m-2, affecting the eastern part of the Western Carpathians. The maximum temperature which lead to maturation of the organic matter seems to be related to its successive burial during Cretaceous due to compressional deformation. Individual thrust sheets reached thermal maturity prior to the final thrusting phase. The overthrust was accompanied by 3000–4000 m burial/erosion resulting in different thermal pathways during wedge growth.

Deep electrical structure of the northern UAE: An investigation along the Dibba zone and the foreland basin using magnetotellurics

Recent geophysical studies are focused on imaging the present-day architecture of the subsurface structures in the northern United Arab Emirates. Previous geophysical data have not fully investigated the deep structures in the middle and lower crust. The foreland basin flanking the UAE-Oman mountains to its west has been investigated by oil companies, mainly using seismic techniques. These studies have been vital in providing a constraint on the subsurface structure of the northern Emirates. This paper reports the results of using broad-band magnetotelluric (BBMT) data acquired for the first time in the UAE to map deep electrical crustal structures. The study aims at determining the geometry of the Semail Ophiolite, the underlying thrust sheets, and the basement structure beneath the allochthonous units and the foreland basin. The MT data were collected from 15 stations along an E-W profile across the Semail Ophiolite and the Dibba zone in the east and the foreland basin in the west. The MT data achieve greater crustal penetration, revealing electrical structures within the upper, mid, and lower crust. 2D and 3D inversion of the data map the high resistivity Ophiolite structure and the associated conductive Hwasina-Haybi thrust sheets in the mountain ranges and in the Dibba zone. In the foreland basin, the Quaternary and Tertiary sediments in the Pabdeh basin are characterized by low resistivity. Furthermore, the MT models detect a high resistivity zone related to the basement structure of the northern UAE in the middle and lower crust. The Ophiolite near the Dibba zone has >7 km thickness as shown in the 2D MT model, and the Hawasina-Haybi sheets have a maximum thickness of ⁓10 km beneath the Ophiolite. The allochthonous units indicate an oceanward dipping as similarly reported in the previous geophysical and geologic studies.

Structure of the Muruntau gold ore region in the Kyzyl-Kum desert (Central Asia)

The giant gold deposit Muruntau in most works belongs to the “orogenic type”. However, in the description of the geology of the deposit, there is usually no information about the stages (deformation, metamorphism) and the role of regional processes in the formation of host and mineralized rocks of the deposit. Our fieldwork provides unique geological evidence and previously unpublished data that are not reported in any other publications to date. This article presents the results of a detailed structural study of the terrane hosting the Muruntau gold deposit on the example of four sites and the results of deep drilling of two wells. The conducted studies confirm that the lowest Muruntau megaterrane consists of multi-stage metamorphosed Neoproterozoic–Early Paleozoic sedimentary-volcanic rocks and Late Paleozoic carbonate sediments overlain by Late Carboniferous flysch and olistostrome in the top. Two main stages (D1 and D2) of rock deformation in the lower part of the megaterrane associated with the Early Paleozoic accretion and two subsequent stages (D3 and D4) associated with the Late Paleozoic accretion and orogeny are described. Thrust processes D2 and D3 caused the formation of recumbent folds of different orientation and vergence. The normal antiform (D4) extending W–E is an orogenic fold. The Muruntau gold deposit is located in a regional synmetamorphic thrust zone called the Main Muruntau Thrust (MMT), which formed during the Early Paleozoic D2 stage. The MMT is responsible for the formation of secondary rock anisotropy along the cleavage of the D2 recumbent folds and conformal lens-like body of mineralized rocks and gently sloping veins with an area of 7.0 × 2.6 km. The internal structure of the deposit retains fragments of deformed gold quartz veins into recumbent folds. These structural data make it possible to link low-grade gold mineralized zones with Early Paleozoic deformations. We suggest that rock anisotropy within the MMT controlled mineralization twice: in the Early Paleozoic as a synmetamorphic flat high-permeable zone and in the Late Paleozoic as a relatively low-permeable zone for post-magmatic fluids.

Dating fluid infiltration and deformation in the subducted ultramafic oceanic lithosphere by perovskite geochronology

Carbonated ultramafic rocks (i.e., ophicarbonates) form by peridotite serpentinization and carbonation at the seafloor. Their metamorphic evolution is linked to that of serpentinites, hence dating deformation and metamorphism in ophicarbonates will add constraints to the P-T-t history of the subducted ultramafic lithosphere. A potential, yet underexplored, suitable mineral for geochronology in ophicarbonates is perovskite (CaTiO3). We present trace element and geochronological data (LA-ICPMS and ID-TIMS) from perovskite in ophicarbonate rocks and calcite veins from two metamorphic terranes, Lanzo Massif and Val Malenco (Alps). Metamorphic perovskite in ophicarbonates is characterized by enrichment in heavy REE, and strong negative anomalies in Zr and Hf, as well as Sr and Pb, which are attributed to coexistence of a Zr-rich phase and mobilization of these elements by fluids, respectively. Other trace elements broadly reflect the composition of the protolith and allow the distinction of perovskite from mafic and ultramafic rocks.In the Lanzo Massif, the 49.6 ± 1.0 Ma age of perovskite in shear bands constrains H2-mediated carbonate reduction and abiotic methane and graphite formation at near peak metamorphic conditions during deformation. Perovskite from Val Malenco is found in calcite veins cutting across serpentinite shear foliation and its age of 48.9 ± 0.5 Ma coincides with a major thrusting and extension event that is distinct in age from the initial nappe stacking. The Val Malenco perovskite age can be reproduced by LA-ICPMS and ID-TIMS and is proposed as a suitable secondary reference material for metamorphic, low U perovskite. These results show that metamorphic perovskite geochronology yields geologically meaningful UPb ages and is a viable tool to date deformation and fluid flow in the subducted oceanic lithosphere.

Field and Petrographic Characteristics of Photang Thrust Sheet of Zanskar Tethys Himalaya, Ladakh, India

The Photang thrust sheet is a distinct complex of rocks that lies under the Spongtang ophiolite, showing almost complete ophiolite sequences. The pre-collision tectonic setting, emplacement age, mechanism of association of diverse litho-units within Spongtang klippe, and their regional correlation is still debatable and partially known. The controversy exists as to whether the ophiolite was obducted onto the northern passive margin of the Indian plate during the Eocene or the late Cretaceous. Moreover, the fundamental question emerges whether the Photang thrust sheet is a part of the independent Spong arc, has been tectonically derived from the Ophiolitic Melange Zone in the Dras island arc, or is a representative of the accreted seamounts of the Indus suture zone. This paper presents the application of fundamentals of field geology and petrography to understand the pre-collision tectonic setting of the Photang thrust sheet, Spongtang ophiolite, and associated rock sequences in the Photoksar region of the Zanskar Himalaya. Keywords: Photang thrust sheet, Spongtang ophiolite, Tectonic setting, Obduction, Ophiolitic Melange Zone, Indus suture zone, Himalaya

Accretion of the NW Himalayan foreland pre‐dates Late Cenozoic climate change

AbstractConceptual models demonstrate that climate can control the structural evolution of mountain belts and that Pliocene–Quaternary global climate change modulated Himalayan wedge behaviour. A key test of this hypothesis is the timing of foreland accretion. If foreland accretion pre‐dates climate change, then climate is not the primary driver of mountain building. New apatite (U–Th)/He data from NW Himalaya foreland thrust sheets suggest that accretion had occurred by the latest 4 Ma, prior to the Pliocene–Quaternary global climate change. Climate cannot, therefore, be invoked as the principal driver of Himalayan tectonics in the late Cenozoic.Statement of significanceThis paper presents 66 new, single grain apatite (U–Th)/He (AHe) dates from the foreland basin that constrain NW Himalayan evolution through the late Cenozoic. We show that accretion of the NW sub‐Himalayan foreland was underway by 4 Ma at the latest, several million years earlier than previous estimates, and was followed by distributed, wedge‐internal deformation by 2 Ma. These and other published data present a consistent pattern and timing of deformation observed over nearly 600 km of the NW sub‐Himalaya. Our new constraints on orogen evolution provide key insights into the relative importance of climate and tectonics in driving orogenic systems.

Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

AbstractShortening estimates from regional balanced cross‐sections remain lower than the predicted shortening from paleomagnetism and area‐balancing methods in the Himalaya. To reconcile this shortening deficit, we evaluate the shortening contribution from the often‐ignored penetrative strain of quartz grains and integrate these results with two published regional balanced cross‐sections from Sikkim Himalaya. Penetrative strain progressively decreases from the internal to the external thrusts with changing deformation conditions. Strain increases toward the basal thrust zone within each thrust indicating strain localization. Strain ellipsoids are folded along with the thrusts indicating the strain initiated before thrusting and folding. The layer‐normal shortening (LNS) strain dominates the internal thrusts. A combination of LNS and layer‐parallel shortening (LPS) strain exist in the low‐strained external thrust sheets. Approximately 78–91 km of shortening is recorded as penetrative strain in the Lesser‐ and Sub‐Himalayan thrusts. Lateral structural variations in the thrusts are reflected in the thrust‐sheet scale shortening and in the strain ellipsoid orientations, but not in the grain‐scale shortening. Shortening is partitioned differently across scales; the duplex records the highest thrust‐sheet scale shortening, while the roof thrust toward the hinterland records the highest grain‐scale shortening. Due to the dominance of LNS strain, removing strain from thrusts will collectively decrease ∼54–59 km of base lengths, thereby reducing the total minimum shortening of the Sikkim Himalaya by ∼13%. Therefore, the penetrative strain does not contribute to the shortening deficit in the Sikkim Himalaya.

Deformation conditions and 40Ar/39Ar dating of thrusting recorded by clay minerals: An example of the Lakora thrust (west-central Pyrenees)

Newly-formed clay minerals in fault zones can provide crucial information about conditions, mechanisms, and timing of deformation at low-grade metamorphism. The present study focuses on the major Lakora thrust in the west-central Pyrenees, that exposes an assemblage of Paleozoic-Upper Cretaceous rocks of the North Pyrenean Zone in the hanging wall and Campanian-Maastrichtian turbidites of the Axial Zone cover in the footwall. Detailed structural, petrological, mineralogical, and chemical investigations were performed on fault rock samples from the Lakora thrust footwall to unravel the deformation conditions related to thrusting. In addition, an attempt to date the thrust activity was made through direct 40Ar/39Ar step-heating on different fractions of muscovite/illite. The structural and petrographic observations suggest a brittle-ductile deformation in two stages along the fault zone: a first stage of bedding-parallel extension and bedding-perpendicular shortening that is marked by boudinage associated with a bedding-parallel cleavage (S0–S1) and a second stage of oblique shearing as marked by reverse faults, folds and oblique cleavage (S2). Pressure-solution and dissolution-recrystallization in the presence of fluids are the main deformation mechanisms that enhanced the neoformation of chlorite and illite associated with cleavage development during the two stages of deformation. The crystallinity, polytypism, and chemistry of illite show that the synkinematic clay minerals with the S2-stage formed under upper anchizone conditions (∼250–300 °C). Such thermal conditions are higher than those expected from the inferred maximal burial (∼6 km), thus arguing for a possible circulation of hot fluids along the thrust footwall. 40Ar/39Ar step-heating results confirm that the newly-formed illite is more abundant in the finer fractions of the most deformed sample, with a total gas age of c. 61.7 ± 0.5 Ma in the finest <0.2 μm fraction. This age is older than the early-mid Eocene age expected from structural constraints for the emplacement of the Lakora thrust sheet due to the mixing of newly-formed illites with small amounts of inherited micas and/or contamination by excess argon carried by fluids.

Active tectonics of the Thoubal‐Chandel Thrust, Manipur Hills, Indo‐Myanmar Range, northeast India: Insights into deformation pattern of adjacent thrust sheets in a growing orogen

The NE–SW trending and SE dipping Thoubal‐Chandel Thrust (TCT) in eastern Manipur Hills of the Indo‐Myanmar Range (IMR) places the Thoubal‐Chandel thrust sheet over the Churachandpur‐Mao thrust sheet. Despite being seismically active, the active tectonic aspects of the major structural discontinuities within the IMR, including TCT, are not yet studied in detail. The present study aims to examine the active tectonics of the TCT and other associated faults, with particular focus on deformation of the Churachandpur‐Mao, and Thoubal‐Chandel thrust sheets. Association and disposition of landforms, and statistical analyses of Mountain‐front Sinuosity Index (Smf), Stream Length‐Gradient Index (SL), Hypsometric Integral (HI), Drainage Basin Asymmetry Factor, and Transverse Topography Symmetry Factor (T) reveal that both of these adjacently lying thrust sheets are actively uplifting due to oblique‐slip movement on their basal thrusts, but with spatially variable rates. In general, higher SL (mode = 219) and HI (mode = 0.527) in large parts suggest faster uplift of the Thoubal‐Chandel thrust sheet than the Churachandpur‐Moa thrust sheet, which has comparatively lower SL (Mode = 140) and HI (mode = 0.416). However, differential along‐strike uplift rates have caused tilting of both the thrust sheets. Faster uplift of the northern part of Thoubal‐Chandel thrust sheet has caused its overall southward down‐tilting, whereas faster uplift of the southern part of Churachandpur‐Mao thrust sheet has caused its overall northward down‐tilting. The study brings out that the deformation pattern of even the adjacent thrust sheets of growing orogens could be at stark variance albeit the same stress regime.