Jean-Claude Vannay

Miocene to Holocene exhumation of metamorphic crustal wedges in the NW Himalaya: Evidence for tectonic extrusion coupled to fluvial erosion

The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high-grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland-directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para- and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present-day regional seismicity, and by hydrothermal circulation linked to elevated near-surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans-Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal-scale tectono-thermal reworking and efficient erosion along major river systems.

Himalayan inverted metamorphism and syn-convergence extension as a consequence of a general shear extrusion

Two paradoxical geological features of the Himalaya are the syn-convergence extension and the inverted metamorphic isograds observed in the crystalline core zone of this orogen. This High Himalayan Crystalline Sequence corresponds to an up to 40 km thick sequence of amphibolite to granulite facies gneiss, bounded by the Main Central Thrust at the base, and by the extensional faults of the South Tibetan Detachment System at the top. Geochronological and structural data demonstrate that coeval movements along both the Main Central Thrust and South Tibetan Detachment System during Early to Middle Miocene times were related to a tectonically controlled exhumation of these high-grade metamorphic rocks. The High Himalayan Crystalline Sequence systematically shows an inverted metamorphic zonation, generally characterized by a gradual superposition of garnet, staurolite, kyanite, sillimanite + muscovite and sillimanite + K-feldspar isograds, from the base to the top of the unit. Recent kinematic flow analyses of these metamorphic rocks demonstrate the coexistence of both simple shear and pure shear during the ductile deformation. The simple shear component of such a general non-coaxial flow could explain a rotation of isograds, eventually resulting in an inversion. The pure shear component of the flow implies a thinning of the metamorphic sequence that must be balanced by a perpendicular stretching of the unit parallel to its boundaries. Inasmuch as seismic data show that both the Main Central Thrust and South Tibetan Detachment System converge at depth, a thinning of the wedge-shaped High Himalayan Crystalline Sequence should induce a ductile extrusion of these high-grade rocks toward the surface. Rapid extension at the top of the sequence could thus be the consequence of a general shear extrusion of this unit relative to its hanging wall. Moreover, this extensional movement should decrease with depth to become zero where the boundaries of the unit meet, accounting for the paradoxical convergence of the South Tibetan Detachment System toward the Main Central Thrust. Furthermore, a general flow combining simple shear and pure shear can reconcile inverted isograds with the lack of inverted pressure field gradient across the High Himalayan Crystalline Sequence, despite an intense non-coaxial deformation. In good agreement with the seismic, kinematic and P – T – t constraints on the Himalayan tectono-thermal evolution, general shear extrusion provides a consistent model accounting for both inverted isograds and rapid extension in a compressional orogenic setting.

Quantitative kinematic flow analysis from the Main Central Thrust Zone (NW-Himalaya, India): implications for a decelerating strain path and the extrusion of orogenic wedges

Quantitative kinematic indicators from the Main Central Thrust Zone (MCTZ) in the NW-Himalaya have been used to characterize the type of flow during deformation. Different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation, forming associated fringe folds. These record the late stage brittle–ductile flow and reveal that a strong pure shear component of deformation occurred throughout the MCTZ. To characterize earlier deformation increments, fabrics from highly deformed quartz ribbons were analyzed. Well-developed shape- and lattice-preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the MCTZ. Investigations of the c-axes patterns reveal a strong asymmetry at the top of the MCTZ, whereas the samples from the base of the MCTZ show almost perfectly symmetrical Type I crossed girdles. Deformation within the MCTZ probably started close to simple shear flow at higher temperatures, which progressively became a more general shear during cooling, and ended in a pure shear dominated flow during the final stages of brittle–ductile deformation (i.e. a decelerating strain path). Using the Higher Himalaya Crystalline as an example, a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested.

Synorogenic extension: Quantitative constraints on the age and displacement of the Zanskar shear zone (northwest Himalaya)

Structural, thermobarometric, and geochronological data place limits on the age and tectonic displacement along the Zanskar shear zone, a major north-dipping synorogenic extensional structure separating the high-grade metamorphic sequence of the High Himalayan Crystalline Sequence from the overlying low-grade sedimentary rocks of the Tethyan Himalaya. A complete Barrovian metamorphic succession, from kyanite to biotite zone mineral assemblages, occurs within the 1-km-thick Zanskar shear zone. Thermobarometric data indicate a difference in equilibration depths of 12 ± 3 km between the lower kyanite zone and the garnet zone, which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the Zanskar shear zone. For the present-day dip of the structure (20°), a simple geometrical model shows that a net slip of 35 ± 9 km is required to regroup these samples to the same structural level. Because the kyanite to garnet zone rocks represent only part of the Zanskar shear zone, and because its original dip may have been less than the present-day dip, these estimates for the finite displacement represent minimum values. Field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the Zanskar shear zone occurred as a continuous process starting at the Barrovian metamorphic peak and lasting throughout the subsequent extension-induced exhumation. Geochronological dating of various leucogranitic plutons and dikes in the Zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19.8 Ma and that extension most likely initiated shortly before 22.2 Ma.

Sense and non-sense of shear in flanking structures

Abstract The deformation of marker horizons across slip surfaces can lead to the development of several types of flanking structures . The development of such structures was investigated using a numerical model to simulate the flow around a slip surface in a viscous medium. The modelled structures can be classified on the basis of three criteria: (1) the extensional or contractional offset of markers, (2) the co- or counter-shearing sense along the slip surfaces and (3) the normal or reverse drag of markers relative to the shear sense along the slip surfaces. As a function of the kinematic vorticity of the flow and the initial orientation of the slip surface, with respect to the shear zone boundaries, three main types of structures can develop: (1) shear bands developed along co-shearing extensional slip surfaces; (2) a-type flanking folds developed along counter-shearing slip surfaces; and (3) s-type flanking folds developed along co-shearing contractional slip surfaces. All these structures can occur with either normal or reverse drag effects. Shear bands and a-type flanking folds recording opposite shear senses can be geometrically very similar and consequently lead to kinematic misinterpretations. However, correctly identified flanking structures can provide quantitative information about the kinematic vorticity of the flow.

Tectonic evolution of the High Himalaya in Upper Lahul (NW Himalaya, India)

The Upper Lahul region in the NW Himalaya is located in the transition zone between the High Himalayan Crystalline (HHC) to the SW and the Tethyan Zone sedimentary series to the NE. The tectonic evolution of these domains during the Himalayan Orogeny is the consequence of a succession of five deformation events. An early Dl phase corresponds to synmetamorphic, NE verging folding. This deformation created the Tandi Syncline, which consists of Permian to Jurassic Tethyan metasediments cropping out in the core of a large-scale synformal fold within the HHC paragneiss. This tectonic event is interpreted as related to a NE directed nappe stacking (Shikar Beh Nappe), probably during the late Eocene to the early Oligocene. A subsequent D2a phase caused SW verging folding in the HHC. This deformation is interpreted as contemporaneous with late Oligocene to early Miocene SW directed thrusting along the Main Central Thrust. In the Tethyan Zone, a D2b phase is marked by a decollement thrust, a system of reverse faults, and gentle folds, associated with SW directed tectonic movements. This deformation is related to an imbricate structure, characteristic of a shallow structural level, and developed in the frontal part of a nappe affecting the Tethyan Zone units of SE Zanskar (Nyimaling-Tsarap Nappe). A later D3 phase generated the Chandra Dextral Shear Zone (CDSZ), a large-scale, ductile, dextral strike-slip shear zone, located in the transition zone between the HHC and the Tethyan Himalaya. The CDSZ most likely represents a part of a system of early Miocene extensional and/or dextral, strike-slip shear zones observed at the HHC-Tethyan Zone contact along the entire Himalaya. A final D4 phase induced large-scale doming and NE verging back folding.

FLOW CONTROLLED INVERTED METAMORPHISM IN SHEAR ZONES

Abstract Numerous processes and models have been developed in order to explain inverted metamorphism in shear zones. This article demonstrates, by means of continuum mechanics, that the orientation of a sample profile across a shear zone before and after deformation can be used to quantify the conditions under which inverted metamorphism is likely to have developed. A thrusting shear zone deforming under ideal simple shear will develop an inverted metamorphic zonation if the angle between the thrust and isotherms is greater than 5–10°, even at only moderate strains. If the flow is general shear, this angle is considerably larger (i.e. >60° for a kinematic vorticity number of W k =0.5). Additionally, the stretch of the sample profile across the shear zone, which is not influenced by the flow type, bears important implications for the interpretation of petrological, geochronological and structural data. Although this kinematic model is clearly an end-member model, neglecting dissipative processes, it can easily account for the structural and petrological data of the inverted metamorphism observed throughout the Higher Himalayan Crystalline wedge in the Sutlej Valley (India).

The Carboniferous Baralacha La basaltic dykes (Upper Lahul, Ladakh) : remnants of an early rifting event along the Indian northern plate

The Lower Carboniferous Baralacha La basaltic dykes were emplaced along transtensional faults. The basalts exhibit tholeiitic and alkaline affinities. The tholeiites are TiO2-poor, moderately enriched in light rare earth (LREE), and display Nb and Ta negative and Th positive anomalies. The alkali basalts, compared to the tholeiites, have higher TiO2, rare earth and highly incompatible trace element contents and greater LREE enrichments. The Nd and Pb isotope compositions of the Baralacha La basalts suggest that they derive from the partial melting of an enriched OIB mantle source, characterized by a HIMU component, and contaminated by the lower continental crust. The Baralacha La dyke swarm represent the remnants of an early rifting event on the northern Indian passive margin.