Julia de Sigoyer

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

Multichronometric studies of the low-temperature eclogitic Tso Morari unit (Ladakh, India) place timing constraints on the early evolution of the northwest Himalayan belt. Several isotopic systems have been used to date the eclogitization and the exhumation of the Tso Morari unit: Lu-Hf, Sm-Nd, Rb-Sr, and Ar-Ar. A ca. 55 Ma age for the eclogitization has been obtained by Lu-Hf on garnet, omphacite, and whole rock from mafic eclogite and by Sm-Nd on garnet, glaucophane, and whole rock from high-pressure metapelites. These results agree with a previously reported U-Pb age on allanite, and together these ages constrain the subduction of the Indian continental margin at the Paleocene-Eocene boundary. During exhumation, the Tso Morari rocks underwent thermal relaxation at about 9 ± 3 kbar, characterized by partial recrystallization under amphibolite facies conditions ca. 47 Ma, as dated by Sm-Nd on garnet, calcic amphibole, and whole rock from metabasalt, Rb-Sr on phengite, apatite, and whole rock, and Ar-Ar on medium-Si phengite from metapelites. Ar-Ar analyses of biotite and low-Si muscovite from metapelites, which recrystallized at <5 kbar toward the end of the exhumation, show that the Tso Morari unit was at upper crustal levels ca. 30 Ma. These results indicate variable exhumation rates for the Tso Morari unit, beginning with rapid exhumation while the Indian margin subduction was still active, and later proceeding at a slower pace during the crustal thickening associated with the Himalayan collision.

Reconstructing the total shortening history of the NW Himalaya

The onset of India-Asia contact can be dated with both biostratigraphic analysis of syn-collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction-collision stages. These data, together with paleomagnetic data show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intra-oceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo-Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo-Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto-Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50-45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm.yr-1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening vs amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.

Evidence of hydration of the mantle wedge and its role in the exhumation of eclogites

Serpentinite samples from the Indus suture zone, representing a shallower part of a paleo-subduction zone, show lowgrade metamorphic recrystallization (chrysotile+magnetite ˛ magnesite ˛ talc). They are cumulates of melts formed in the uppermost mantle or the base of the Nidar intra-oceanic arc. Serpentinite samples associated with the Tso Morari eclogitic unit, representing the more deeply subducted portion of a paleo-subduction zone, exhibit high-grade metamorphic recrystallization (antigorite+magnetite ˛ forsterite ˛ talc) and the trace element chemistry of these samples suggests a strongly depleted mantle wedge origin. Nd concentrations and ONd values show that fluids responsible for hydration of the mantle wedge were derived from subducting clastic sediments overlying Tethyan oceanic crust. The exhumation of eclogites requires a mechanically weak zone at the interface between the subducting plate and the mantle wedge. We suggest that serpentinites associated with the Tso Morari eclogites acted as a lubricant for the exhumation of the eclogitic unit. Geophysical data suggest common occurrences of hydrated ultramafic rocks about 10 km thick along the interface between the mantle wedge and the subducting plate. We propose that such a low-viscosity zone played an important role for the exhumation of eclogitic rocks. fl 2001 Elsevier Science B.V. All rights reserved.

Mantle wedge serpentinization and exhumation of eclogites: Insights from eastern Ladakh, northwest Himalaya

In eastern Ladakh, northwest Himalaya, serpentinite layers occur in close association with eclogites. The occurrence of metamorphic olivine and talc in serpentinites suggests that the serpentinization and eclogitization took place under similar conditions (600 °C, 20 kbar). The serpentinites and eclogites show similar deformation, including the direction of normal shearing. The highly refractory nature of the serpentinite protolith, as shown by the composition of bulk rocks and chromite and the concentrations of Re and platinum group elements, indicates their derivation from mantle wedge. We propose that the serpentinites formed by hydration of the mantle wedge as a result of dewatering of the subducted slab. The serpentinites then facilitated exhumation of the subducted rocks by acting as a lubricant. At shallow depths, sediments are generally considered to be the lubricant for the exhumation, but serpentinites may commonly take over this role at greater depths. Under sediment-poor conditions, serpentinites may contribute to the exhumation even at shallower depths. This may explain the close spatial association of serpentinites and partially hydrated peridotites with many well-known high-pressure to ultrahigh-pressure metamorphic belts worldwide.

Exhumation of the ultrahigh‐pressure Tso Morari unit in eastern Ladakh (NW Himalaya): A case study

Exhumation processes of the ultra-high pressure (UHP) Tso Morari dome (NW-Himalaya) are investigated using structural, petrological and geochronological data. The UHP Tso Morari unit is bounded by the low-grade metamorphic Indus Suture Zone to the NE and Mata unit to the SW. Three deformation phases (D1, D2 and D3) are observed. Only D3 is common to the UHP unit and the surrounding units. In the UHP unit, the first deformation phase (D1) produced upright folds, under eclogitic conditions (> 20 kbar; 580 ± 60 °C). D1 is overprinted by D2 structures related to a NW-SE trending open anticline. This phase is characterized by blueschist mineral associations, and corresponds to the quasi-isothermal decompression from a depth of 90 km (eclogitic conditions) up to 30-40 km. The final exhumation phase of the Tso Morari unit is dominated by tectonic denudation and erosion (D3), associated with a slight temperature increase. Radiochronological analyses indicate that the UHP exhumation process began during the Eocene. Exhumation was fast during D1-D2 and slowed down through D3 in Oligocene time. The change in the deformation style from D1-D2 to D3 in the Tso Morari unit coincides with changes in the exhumation rates and in the metamorphic conditions. These changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge, to the vertical uplift of the Tso Morari unit across the upper crust.

Sm–Nd disequilibrium in high-pressure, low-temperature Himalayan and Alpine rocks

In order to decipher the causes of Sm–Nd isotopic disequilibrium in high-pressure, low-temperature rocks, Sm–Nd isotopic analyses were carried out on minerals from four Himalayan (Tso Morari unit) and four Alpine (Dora-Maira, Monte Viso, Sesia Lanzo) eclogitic rocks of different lithologies and different intensities of post-eclogitic metamorphism. In most of these

Témoins d'un arc immature téthysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde)

Abstract The ophiolites of the South Ladakh are evidence of the Neo-Tethys obduction onto the Indian continental margin, during Late Cretaceous times. The mafic rocks are cogenetic and were extracted from a N-MORB like depleted source slightly metasomatized above a subduction zone. Thermobarometry on ultramafic rocks confirms this geodynamic setting. Considering their position in the Ladakh-Zanskar area and their geochemical signatures, these ophiolites could correspond to an immature arc rather than a back-arc basin. Moreover, they are related to a subduction zone located south of the one related to the Dras arc and the Ladakh Batholith. The model deduced from this study brings new constraints on the thermo-mechanical evolution of the Indian margin.

The Longriqu fault zone, eastern Tibetan Plateau: Segmentation and Holocene behavior

The dextral Longriba fault system (LFS), ~300 km long and constituting of two fault zones, has recently been recognized as an important structure of the eastern Tibetan plateau (Sichuan province), as it accommodates a significant amount of the deformation induced by the ongoing Indo-Asian collision. Although previous paleoseismological investigations highlighted its high seismogenic potential, no systematic quantification of the dextral displacements along the fault system has been undertaken so far. As such information is essential to appraise fault behavior, we propose here a first detailed analysis of the segmentation of the Longriqu fault, the northern fault zone of the LFS, and an offset inventory of morphological features along the fault, using high-resolution Pleiades satellite images. We identify six major segments forming a mature fault zone. Offsets inventory suggests a characteristic coseismic displacement of ~4 m. Two alluvial fans, with minimum ages of 6.7 and 13.2 ka, respectively displaced by 23 ± 7 m and 40 ± 5 m, give an estimate of the maximal horizontal slip rate on the Longriqu fault of 3.2 ± 1.1 mm yr A1. As a result, a minimum ~1340 year time interval between earthquakes is expected.

Differential Exhumation Across the Longriba Fault System: Implications for the Eastern Tibetan Plateau

The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U-Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10-15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25-35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the regions deformation, and in both above-mentioned deformation models.

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite-facies mica 40 Ar/ 39 Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.