Salvatore Iaccarino

Tectonometamorphic discontinuities in the Greater Himalayan Sequence: a local or a regional feature?

Abstract The Greater Himalayan Sequence (GHS) is one of the major tectonic units of the Himalaya running for more than 2400 km along-strike. It has been considered as a coherent tectonic unit bound by the South Tibetan Detachment (STD) and the Main Central Thrust (MCT). However, thrusts within it have been recognized in several places and have been mainly interpreted as out-of-sequence thrusts being active after the main phase of exhumation of the crystalline unit after the MCT activated. Recent integrated studies allow the recognition of several ductile shear zones in the core of the GHS, with top-to-the-SW-sense of shear (Higher Himalayan Discontinuity (HHD)). U–Th–Pb in situ monazite ages provide ages older than the MCT. Data on pressure and temperature evolution testify that these shear zones affected the tectonometamorphic evolution of the belt and different pressure and temperature conditions were recorded in the hanging wall and footwall of the HHD. The correlation of the WNW–ESE-trending HHD with other discontinuities recognized in the GHS led to the proposal that it is a tectonic feature running for several hundred kilometres, documented at the regional scale dividing the GHS in two different portions.

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal *

Abstract Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event. The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser ablation−inductively coupled plasma−mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kinematics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonic-metamorphic discontinuity in the Greater Himalayan Sequence (High Himalayan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

Geology and tectono-metamorphic evolution of the Himalayan metamorphic core: Insights from the Mugu Karnali transect, Western Nepal (Central Himalaya)

New structural and tectono-metamorphic data are presented from a geological transect along the Mugu Karnali valley, in Western Nepal (Central Himalaya), where an almost continuous cross section from the Lesser Himalaya Sequence to the Everest Series through the medium-high-grade Greater Himalayan Sequence (GHS) is exposed. Detailed meso- and micro-structural analyses were carried out along the transect. Pressure (P) -temperature (T) conditions and P-T-deformation paths for samples from different structural units were derived by calculating pseudosections in the MnNKCFMASHT system. Systematic increase of P-T conditions, from ~ 0.75 GPa–560 °C up to ≥1.0 GPa–750°C have been detected starting from the garnet zone up to the K-feldspar + aluminosilicate zone. Our investigation reveals how these units are characterized by different P-T evolutions and well-developed tectonic boundaries. Integrating our meso- and micro-structural data with those of metamorphism and geochronology, a diachronism in deformation and metamorphism can be highlighted along the transect, where different crustal slices were underthrust, metamorphosed and exhumed at different times. The GHS is not a single tectonic unit, but it is composed of (at least) three different crustal slices, in agreement with a model of in-sequence shearing by accretion of material from the Indian plate, where coeval activity of basal thrusting at the bottom with normal shearing at the top of the GHS is not strictly required for its exhumation. This article is protected by copyright. All rights reserved.

Tectonic activity along the inner margin of the South Tibetan Detachment constrained by syntectonic leucogranite emplacement in Western Bhutan

In Western Bhutan Himalayas leucogranite dykes emplaced in sub-vertical hybrid fractures that cut across the high-grade rocks of the upper Greater Himalayan Sequence just below to the South Tibetan Detachment. The granitic dykes dip to the north often showing a mylonitic deformation with a top-down to-the-N sense of shear. The high-angle fractures are interpreted to be related to the evolution of the South Tibetan Detachment toward a brittle regime of deformation. U-Pb monazite ages constrain the leucogranite emplacement at 13.9±06 Ma implying that brittle-ductile deformation of the South Tibetan Detachment was active at that time. NNE-SSW to nearly E-W trending large scale antiforms and synforms mapped in NW Bhutan affected the Greater Himalayan Sequence and South Tibetan Detachment only after 14 Ma.

Tectono-metamorphic evolution of the Tethyan Sedimentary Sequence (Himalayas, SE Tibet)

The Tethyan Sedimentary Sequence, one of the major tectonic units of the Himalayan belt cropping out in the inner portion of the chain, has been investigated in SE Tibet to unravel its tectonic and metamorphic evolution.The Tethyan Sedimentary Sequence recorded at least three phases of ductile defomation, all of them associated to the development of folds and related axial plane foliations. A prominent D1 deformation is progressively overprinted by a D2 deformation approaching the Yarlung Tsangpo suture zone to the North. Structural analysis allowed to recognise two first-order different structural domains: a southern domain in which D1 is the prominent deformation and a northern domain in which the D2 overprint predominates up to transpose D1 deformation. F2 folds show a regional backward vergence (northward) with respect to the southward verging F1 folds. Finite strain data show an increase of D2-related strain moving to the North. It is worth to note that new P-T-d data on polydeformed chloritoid schists point out an increase of both temperature and pressure from D1 to D2 deformation indicating prograde burial during D1-D2 phases and support that F2 folds developed in a compressive tectonic framework during crustal thickening in the time span of 35-25 Ma. The integration of our new deformation and P-T data with available literature data will help to deconvolve the long lasted history of this tectonic unit, far away to be well understood.

Structural setting, kinematics and metamorphism in a km-scale shear zone in the Inner Nappes of Sardinia (Italy)

In continental collisional orogenic belts, the hinterland-foreland transition plays a crucial role in the deformation styles and in the exhumation modes of the middle crustal rocks. The Barbagia Thrust (BT), a regional scale shear zone in the Variscan belt of central Sardinia, represents this transition. The BT, separating nappes withdifferent deformation styles, is still poorly characterized.We present new data of the BT and of the nearby tectonic units, using a multidisciplinary approach. We characterized in detail both meso- and micro-structural features, as well as, the metamorphic conditions with the aid of illite and chlorite “crystallinity” of rocks from the footwall, hanging wall and high-strain zone of the BT. Moreover, combining different palaeopiezometer-wet quartzite flowlaw pairs we characterized the rheological parameters (i.e., flow stress and strain rate) present during the BT activity. Three main deformation phases were recognized. After a D1 contractional deformation, a D2 related to the BT movement was associated to the development of a nearly 100 m thick high-strain zone withthe development of a mylonitic foliation, Sm, at middle crustal conditions, near the brittle-ductile” transition. Metamorphic constrains obtained from the footwall, the hanging wall and the high-strain zone supported an epizonal metamorphism with no unambiguous trend related to the strain intensity.Integrating our new metamorphic, deformation and rheological data, with previous ones in the geological literature helps to better unravel the long-lasting history of the nappe emplacement at hinterland-foreland transition in the Sardinian Variscan Belt.