The Nidar Ophiolite and its surrounding units in the Indus Suture Zone (NW Himalaya, India): new field data and interpretations

The Nidar Ophiolite is located between the North Himalayan nappes and the Indus Suture Zone in NW Himalaya in eastern Ladakh (India). Based mainly on geochemical argument, this ophiolite is classically interpreted as a relic of an intra-oceanic arc (Mahéo et al. 2000; Mahéo et al. 2004), which developed at around 140 Ma, prior to the collision between the Indian and Eurasian plates (Ahmad et al. 2008). From top to bottom, this ophiolite is composed of various sedimentary rocks (radiolarites, polygenic conglomerates and carbonates), volcanic rocks (pillow lavas, basaltic to andesitic in composition), gabbros (Fe- and layered gabbros, pegmatites and minor troctolites), serpentinites, dunites, pyroxenites and peridotites (mainly harzburgites). The Nidar Ophiolite underwent an anchizonal metamorphism with preservation of primaries structures (layering) and volcanic textures (pillow lavas). This study is mainly focused on new field observations across the ophiolite and the surrounding units. A new detailed geologic map of the ophiolite between the Nidar village and Kyun Tso area is presented. The upper part of the ophiolitic complex is an alternation of volcanic and sedimentary rocks (500- 1000 m thick) and the lower part consists of large outcrops of gabbros (3000m thick). These mafic rocks are separated from the serpentinized ultramafic rocks by a 200m thick ophiolitic breccia and continental Indus Molasse slices. The Nidar Ophiolite is made up of the classical rock type succession (ultramafites, gabbros, pillow basalts, radiolarites), but the internal structure is far more complex than previously suggested. New field data (geologic and structural maps, lithologic sections, etc.) coupled with new geochemical analysis will help to constrain the geodynamic context and deformation history. Ahmad, T., T. Tanaka, H.K. Sachan, Y. Asahara, R. Islam, et P.P. Khanna. 2008. « Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone ». Tectonophysics 451 (1–4): 206‑ 24. Mahéo, Gweltaz, Hervé Bertrand, Stéphane Guillot, Georges Mascle, Arnaud Pêcher, Christian Picard, et Julia De Sigoyer. 2000. « Témoins d’un arc immature téthysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde) ». Comptes Rendus de l’Académie des Sciences - Series IIA - Earth and Planetary Science 330 (4): 289‑ 95. Mahéo, Gweltaz, Hervé Bertrand, Stéphane Guillot, Igor M. Villa, Francine Keller, et Paul Capiez. 2004. « The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys ». Chemical Geology 203 (3–4): 273‑ 303

Geology of the eastern part of the Tso Morari nappe, the Nidar Ophiolite and the surrounding tectonic units (NW Himalaya, India)

The Tso Morari nappe is well known for having suffered ultrahigh-pressure metamorphism in the northwest Himalaya. Its pressure-temperature evolution has been widely studied and debated since the end of the 1990s. However, the lithostratigraphy, structures and metamorphism of the tectonic units around the Tso Morari nappe are poorly known, especially in the eastern part of the nappe. Here, a detailed geologic map based on recent field work is presented. The survey area is the eastern part of the ultrahigh-pressure Tso Morari nappe, the surrounding tectonic units and the Nidar Ophiolite. This map improves the knowledge of the geology and geometry of the North Indian continental plate and the ocean-derived units south of the Eurasian continent in eastern Ladakh. Knowledge of the present-day geometry will help to reconstruct the initial setting in the kinematic models of the formation and exhumation of the Tso Morari nappe in relation to its surrounding tectonic units

Ophiolites in the North Himalayan nappes and Indus Suture Zone in Eastern Ladakh (NW Himalaya, India)

Ophiolites are fragments of ancient oceanic lithosphere, preserved in orogenic belts in a context of plate convergence. They are generated at mid-ocean ridges, in a supra-subduction zone or volcanic arc. Commonly, several magmatic events are recorded, as shown, for instance, in the Oman Ophiolite (Goodenough et al. 2014). The Ophiolitic rocks of Eastern Ladakh are subdivided in two main groups, based on the geodynamic setting during their formation: the supra-subduction zone ophiolite and the ophiolitic “mélanges”, corresonding both to the Indus Suture Zone. Recent detailed studies North-East of the Tso Moriri area revealed a large diversity of ophiolitic rocks and associated sediments. We identified three distinct tectonic units containing ophiolites: The Nidar Ophiolite, the Drakkarpo nappe and the Karzok-Ribil nappe. The Nidar supra-subduction zone Ophiolite represents a complete ophiolitic sequence, from mantle to sediments, which underwent a low greenschist facies metamorphism. This ophiolitic sequence was thrusted towards the South. They record a first magmatic event in a mid-ocean ridge setting, and a second one in a supra-subduction zone at around 130 Ma. The Drakkarpo nappe is a “mélange” unit composed of thick polygenic conglomerates and volcano-sedimentary rocks, mainly composed of tuffs and augite-basalts (OIB), serpentinites, pillow lavas and gabbros. This unit is interpreted as being a part of an accretionary wedge containing slices of oceanic islands arc. This nappe marks the Indus Suture Zone. The Karzok-Ribil nappe is a newly defined tectonic unit involved in the North Himalayan nappe stack. It can be followed at the top of the Tetraogal nappe and around the Tso Morari dome. The Karzok-Ribil nappe is composed of segments of ophiolitic sequence (serpentinites, gabbros, pillow lavas), radiolarites, polygenic conglomerates, agglomeratic slates from the indian margin, augite-basalts (OIB) and limestones. It is interpreted as being originally a seamount, located close to the Indian passive margin in a ocean-continent transition zone. The new lithostratigraphy and structural analyses of the Eastern Ladakh ophiolites and their associated sediments allow us to better constrain the formation and emplacement mechanisms of these tectonic units. It defines or precises the paleogeography and geometry of the north Indian passive margin, prior to the Himalayan collision. REFERENCES Goodenough, Kathryn M., Robert J. Thomas, Michael T. Styles, David I. Schofield, and Christopher J. MacLeod. 2014. “Records of Ocean Growth and Destruction in the Oman–UAE Ophiolite.” Elements 10 (2): 109–114

Mimicking Alpine thrusts by orogenic passive deformation of synsedimentary normal faults: a record of Jurassic extension of the European margin (Mont Fort nappe, Middle Penninic, Western Alps)

The Mont Fort nappe is the upper tectonic subdivision of the former Grand St-Bernard (GSB) nappe (Escher 1988, Escher et al. 1997). Subsequent research has confirmed its tectonic and stratigraphic independence from the rest of the GSB nappe (e.g. Gouffon 1993). It consists of a Paleozoic basement overlain by a thin Mesozoic sedimentary cover, comprised of very thin, discontinuous Triassic quartzite and dolomite, a thin limestone layer, and for the greater part of breccias of variable thickness mostly made of dolomitic elements (“reconstituted Triassic”). These breccias are overlain by a more massive limestone topped by a thinner upper breccia rich in elements of quartzite. Called by Escher (1988) the Evolène series, this sequence presents remarkable similarities with the stratigraphic column of the Breccia nappe in the Prealps which typically belongs to the Prepiemontese paleogeographic domain (Lemoine 1961, Escher 1988). This similarity strongly suggests an earliest Jurassic age for the lower limestone, a late Early to Middle Jurassic age for the main mass of breccias (Lower Breccia), and a Late Jurassic to earliest Cretacous age for the upper limestone and the Upper Breccia. We will not discuss here the status of the Cretaceous calcschists (Série Rousse, Marthaler 1984) that overlie the Evolène series and are the subject of research in progress. These ages are generally accepted. However, several authors proposed a different tectonic model where the Evolène series would be allochthonous over the Mont Fort basement (e.g. Sartori & Marthaler 1994, Marthaler et al. 2008). Their contact would be an Alpine thrust. The main argument would be that this contact surface often cuts with an angular discordance the stratigraphy of the Evolène series. Consequently, this series would not be the original sedimentary cover of the Mont Fort basement but would belong to a distinct tectonic unit that these authors identified with the Cima Bianca nappe (classically defined as a slice of Late Paleozoic to Mesozoic sediments separating the ophiolitic Zermatt-Saas and Tsaté nappes; Vannay & Allemann 1990, Steck et al. 2015). This proposition has been widely accepted (e.g. Tectonic map of Switzerland 2005). Our observations don’t support this proposition. They rather confirm the tectonic reconstruction of Escher (1988) and suggest that the Evolène series is indeed the autochthonous cover of the Mont Fort basement. The main points are: - At many places the contact is concordant and shows a good preservation of the basal levels of the Mesozoic sequence, without any hint of tectonic disturbance or anomalous rock deformation. - The discordance observed at other places, which can put the Jurassic breccia in contact with all older formations, can be conveniently explained by synsedimentary normal paleofaults. By means of strain theory (e.g. Ramsay 1967) it is easy to demonstrate that passive deformation of such faults during Alpine compression can mimic thrusts. - The sedimentary characteristics of the Evolène series, typicall of the Prepiemontese domain, are very different from those of the Cima Bianca unit, much poorer in breccias and whose relatively thick Triassic formations of quartzite and carbonates show a Briançonnais affinity. Conclusion : The Evolène series is the sedimentary cover of the Mont Fort nappe. During Jurassic times it has been strongly affected by synsedimentary normal faulting, generating stratigraphic gaps, synsedimentary anomalous contacts and internal discordances. These structures provide a spectacular record of the extension of the European margin during the opening of the Alpine Tethys. Then Alpine compression passively deformed these faults in such a way that they mimic Alpine thrusts. REFERENCES Escher, A. 1988: Structure de la nappe du Grand Saint-Bernard entre le val de Bagnes et les Mischabel. Service hydrologique et géologique national, Berne. Escher, A., Hunziker, J., Marthaler, M., Masson, H., Sartori, M., & Steck, A. 1997: Geologic framework and structural evolution of the western Swiss-Italian Alps. In O. A. Pfiffner et al. (Eds.): Deep structure of the Swiss Alps: results of NRP 20, 205–221. Birkhaüser, Basel. Gouffon, Y. 1993: Géologie de la «nappe» du Grand St-Bernard entre la Doire Baltée et la frontière suisse (Vallée d’Aoste, Italie). Mém. Geol. (Lausanne) 12. Marthaler, M., Sartori, M., Escher, A. & Meisser, N. 2008. Feuille 1307 Vissoie. Atlas géologique Suisse 1:25’000, Carte et Notice explicative 122. Sartori, M. & Marthaler, M. 1994: Exemples de relations socle-couverture dans les nappes penniques du Val d’Hérens. C.-r. Excursion Soc. Géol. Suisse et Soc. Suisse Minér. Pétr., Schweiz. Miner. Petr. Mitt. 74, 503-509