Tectonometamorphic Discontinuities Research Articles

Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis

The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity.

Mapping tectono-metamorphic discontinuities in orogenic belts: implications for mid-crust exhumation in NW Himalaya

A newly identified high-temperature shear zone, the Badrinath shear zone (BSZ), is described within the metamorphic core of the Himalayan belt along the Alaknanda valley, Garhwal Himalaya (NW India). A multidisciplinary approach comprising fieldwork, microstructural analyses, petrology and in situ monazite geochronology was addressed to this high-strain zone to better understand its role in the Greater Himalayan Sequence (GHS) evolution along the study area. The Badrinath mylonite displays top-to-the-south thrust-sense of shear and affects sillimanite-bearing gneiss showing evidence of partial melting. The data integration allowed us to reconstruct the following model for BSZ development: the pre-mylonitic stage took place during prograde metamorphic path reaching conditions of 700–720 °C and 10 kbar during the time interval of 34 and 23 Ma, with incipient partial melting, followed by nearly-isothermal decompression triggered by the shear activity between 23 and 19 Ma, with exhumation rate of ±0.3 cm yr−1. Moreover, the rocks from the lower part of GHS, in the BSZ footwall, experienced metamorphic conditions of c. 660–700 °C and c. 10–11 kbar, the partial melting and exhumation took place ~3 Ma after the BSZ. Such findings led us to correlate the BSZ with the High Himalayan Discontinuity (HHD) of Central Himalaya. The BSZ is the first reported HHD branch in NW Himalaya (Garhwal). It corroborates the regional extent of the HHD accomplishing an important role during the GHS exhumation. In the Alaknanda valley, the BSZ activity accommodated deformation during early exhumation stages, 3 Ma before the MCTz and STDS coeval motion took place. Our observations point to a shift in time and place of the deformation responsible to trigger the GHS exhumation.

The Greater Himalayan Thrust Belt: Insight Into the Assembly of the Exhumed Himalayan Metamorphic Core, Modi Khola Valley, Central Nepal

AbstractStrike‐parallel tectonometamorphic discontinuities within the Himalayan metamorphic core are typically interpreted to reflect thrust‐sense movement. However, there is disagreement on the nature and sense of movement across one such structure in central Nepal. Using an integrated approach, this study characterizes multiple structural breaks in the Modi Khola region. Thermobarometric calculations combined with petrochronological investigation show that rocks across the Sinuwa thrust record similar histories with prograde garnet growth ca. 35 Ma and peak pressures of ~11.0 kbar, anatexis at ca. 28 Ma, and followed by cooling and exhumation between ca. 24 and 15 Ma. Rocks below the structurally lower Bhanuwa fault record similar garnet growth at ca. 35 Ma and pressures of ~11.5 kbar but experienced melting and retrogression after ca. 21 Ma. Finally, rocks in the footwall of the Main Central thrust, the structurally lowest break investigated, record prograde metamorphism ca. 17–13 Ma with peak pressures of ~7.0 kbar. This down‐structural migration of prograde metamorphism, anatexis, and subsequent cooling and exhumation of the footwall is consistent with models of progressive underplating and in‐sequence thrusting. When paired with published cooling ages across the Bhanuwa fault, results from this study are consistent with normal‐sense reactivation of the structure during middle‐late Miocene time. This new data set shows that the final assembly of the Himalayan metamorphic core is a result of progressive deformation and juxtaposition of multiple thrust sheets through a combination of in‐sequence thrusting, out‐of‐sequence thrusting, and normal faulting. We refer to this as the Greater Himalayan thrust belt.

In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Recent identification of tectono-metamorphic discontinuities within the Himalayan metamorphic core has challenged previous understanding of mountain-building processes/models during continental collision. However, their exact position, spatial extent, and temporal evolution still remain in debate. Monazite petrochronology and thermobarometry have been applied to metapelites of the Gyirong-Syabrubensi-Langtang transect in central Nepal. Peak metamorphic temperatures and pressures were determined by conventional thermobarometry and Zr-in Rutile thermometry. P-T conditions increase structurally upward from 540–580 °C, 8–10 kbar at LHS to 640–710 °C, 8–12 kbar at lower GHC, and 680–720 °C, 6–9 kbar at upper GHC. Petrochronology studies indicate prograde metamorphism in the LHS, partial melting of 20–18 Ma in the lower GHC and prolonged partial melting at 29–23 Ma in the upper GHC, demonstrating a diachronism across these units. The Rasuwagadhi Shear Zone (RSZ), is marked by a jump in T/depth and peak metamorphic timing from lower GHC (<25 °C/km, ~19 Ma) to upper GHC (>30 °C/km, ~29 Ma), and can be correlated with the regional High Himalayan Discontinuity recently identified within the GHC in other Himalayan transects. This shear zone acted almost synchronously with the South Tibetan Detachment around 24–18 Ma and was relayed by movement along the Main Central Thrust after 18 Ma, presenting south-ward in-sequence thrust propagation style. Therefore, a hybrid model, In-sequence Buoyancy Extrusion that involves a combination of underplating and buoyancy mechanisms is proposed to explain the overall exhumation of HMC, and perhaps is applicable to metamorphic core in similar large and hot collisional orogen.

Contrasting P-T-t paths reveal a metamorphic discontinuity in the New Quebec Orogen: Insights into Paleoproterozoic orogenic processes

The large scale Paleoproterozoic Trans-Hudson Orogen exposed in Canada is recognized to record one of the first complete Wilson cycle. Although it is considered as a prototype of modern accretionary orogen, the strain pattern, thermal state, and architecture in its wedge are still poorly constrained. Part of it, the accretionary New Quebec Orogen (NQO, 1.82–1.77 Ga) resulted from oblique collision between the Superior Craton lower plate and a composite upper plate including the Kuujjuaq Domain (KD) and the Core Zone, and exposes supracrustal sequences of the Rachel-Laporte Zone (RLZ), a continental forearc basin that evolved in a collisional foreland basin upon collision. We performed an integrated analysis, using petrography, phase equilibria modelling and Lu-Hf garnet, and U-Pb zircon, monazite and rutile chronology on supracrustal rocks in the central segment of the New Quebec Orogen. On the one hand, the RLZ experienced a tight clockwise metamorphic evolution, with a peak at 650 °C/0.7 GPa. Prograde garnet and monazite growth is estimated at 1804 ± 8 Ma and 1796 ± 4 Ma respectively. On the other hand, the metamorphic peak for restitic granulites from the KD is estimated at 790 °C/0.72 GPa and follows isothermal decompression. Geochronology yields ages of 1836 ± 5 Ma for prograde garnet growth, and 1807 ± 4 Ma for crystallization of zircon from anatectic melt, which was followed by rapid cooling recorded by rutile at 1798 ± 16 Ma. We propose that the Lac Turcotte Fault that separates both domains has acted as a tectonometamorphic discontinuity that limits contrasting P-T-t paths. We suggest sequential burial of supracrustal sequences along an apparently continuous Barrovian sequence, with progressive recycling of inner zones in the foreland basin and subsequent involvement of the latter in the orogenic wedge.

Structural evolution, metamorphism and melting in the Greater Himalayan Sequence in central-western Nepal

AbstractJoining geological mapping, structural analysis, petrology and geochronology allowed the internal architecture of the Greater Himalayan Sequence (GHS) to be unraveled. Several top-to-the-south/SW tectonic–metamorphic discontinuities developed at the regional scale, dividing it into three main units exhumed progressively from the upper to the lower one, starting from c. 40 Ma and lasting for several million years. The activity of shear zones has been constrained and linked to the pressure–temperature–time–deformation (P–T–t–D) evolution of the deformed rocks by the use of petrochronology. Hanging wall and footwall rocks of the shear zones recorded maximum P–T conditions at different times. Above the Main Central Thrust, a cryptic tectonometamorphic discontinuity (the High Himalayan Discontinuity (HHD)) has been recognized in Central-Eastern Himalaya.The older shear zone, that was active at c. 41–28 Ma, triggered the earlier exhumation of the uppermost GHS and allowed the migration of melt, which was produced at peak metamorphic conditions and subsequently produced in abundance at the time of the activation of the HHD. Production of melt continued at low pressure, with nearly isobaric heating leading to the genesis and emplacement of andalusite- and cordierite-bearing granites.The timing of the activation of the shear zones from deeper to upper structural levels fits with an in-sequence shearing tectonic model for the exhumation of the GHS, further affected by out-of-sequence thrusts.

Detection of tectonometamorphic discontinuities within the Himalayan orogen: Structural and petrological constraints from the Rasuwa district, central Nepal Himalaya

A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side.Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts.

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region, East Central Nepal

ABSTRACTThe recent identification of multiple strike‐parallel discontinuities within the exhumed Himalayan metamorphic core has helped revise the understanding of convergence accommodation processes within the former mid‐crust exposed in the Himalaya. Whilst the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their identification and characterization has become important for potential correlations across regions, and for constraining the kinematic framework of the mid‐crust. The result of new phase equilibria modelling, trace element analysis and high‐precision Lu–Hf garnet dating of the metapelites from the Likhu Khola region in east central Nepal, combined with the previously published monazite petrochronology data confirms the presence of one of such cryptic thrust‐sense tectonometamorphic discontinuities within the lower portion of the exhumed metamorphic core and provides new constraints on the P–T estimates for that region. The location of the discontinuity is marked by an abrupt change in the nature of P–T–t paths of the rocks across it. The rocks in the footwall are characterized by a prograde burial P–T path with peak metamorphic conditions of ~660°C and ~9.5 kbar likely in the mid‐to‐late Miocene, which are overlain by the hanging wall rocks, that preserve retrograde P–T paths with P–T conditions of &gt;700°C and ~7 kbar in the early Miocene. The occurrence of this thrust‐sense structure that separates rock units with unique metamorphic histories is compatible with orogenic models that identify a spatial and temporal transition from early midcrustal deformation and metamorphism in the deeper hinterland to later deformation and metamorphism towards the shallower foreland of the orogen. Moreover, these observations are comparable with those made across other discontinuities at similar structural levels along the Himalaya, confirming their importance as important orogen‐scale structures.

Spatial and temporal evolution of tectonometamorphic discontinuities in the central Himalaya: Constraints from P–T paths and geochronology

Recent studies evoke dispute whether the Greater Himalayan Crystalline Complex (GHC) was exhumed during more than one phase. This contribution investigates the timing of ductile shear along the South Tibetan Detachment (STD), Main Central Thrust (MCT), and hidden discontinuities within the GHC of the central Himalaya. New data from the Nyalam transect, southern Tibet, suggest that ductile shear along the STD was active from ~27–25Ma to ~17–15Ma, whereas ductile shear along the MCT was active from ~19–16Ma to ~10–8Ma. Pseudosection modeling results and published metamorphic ages indicate a P–T–t path discontinuity within the GHC and the upper GHC reached higher peak metamorphic temperatures (ΔT 100°C), similar peak pressures and older metamorphic ages (~5–10Myr), in favor of the presence of the Nyalam Discontinuity. Summarized results from literature suggest that timing of ductile shear along the STD was ~5–10Myr earlier than that along the MCT across the Nepal, Sikkim and Bhutan Himalayas. In addition, the GHC in Nepal can be separated into two portions by a regional-scale (>800km) in-sequence ductile shear zone — the High Himalayan Discontinuity. This shear zone was active coevally with the STD during 25–16Ma, but ~5–10Myr earlier than the MCT. The High Himalayan Discontinuity could possibly extend to the Sikkim Himalayas, but ends in Bhutan. The identified new architecture suggests that both in-sequence and out-of-sequence tectonometamorphic discontinuities are important components that formed the duplexing structure of the GHC in the central and eastern Himalaya, thus calling for more sophisticated numerical simulation than the current models such as the channel flow and critical taper.

Dominant simple-shear deformation during peak metamorphism for the lower portion of the Greater Himalayan Sequence in West Nepal: New implications for hybrid channel flow-type mechanisms in the Dolpo region

I conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid-crustal rocks exposed in the Dolpo region (West Nepal). My results indicate that the Main Central Thrust is located ∼5 km structurally below the previous mapped locations. Deformation temperature increasing up structural section from ∼450 °C to ∼650 °C and overlap with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at “close” to peak metamorphic conditions. I interpreted the telescoping and the inversion of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm = 0.88–1) pervasively distributed through the lower portion of the GHS. My results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectonometamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth.

Reconciling Himalayan midcrustal discontinuities: The Main Central thrust system

The occurrence of thrust-sense tectonometamorphic discontinuities within the exhumed Himalayan metamorphic core can be explained as part of the Main Central thrust system. This imbricate thrust structure, which significantly thickened the orogenic midcrustal core, comprises a series of thrust-sense faults that all merge into a single detachment. The existence of these various structures, and their potential for complex overprinting along the main detachment, may help explain the contention surrounding the definition, mapping, and interpretation of the Main Central thrust. The unique evolution of specific segments of the Main Central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position, and structural level of exposure of the mid-crust. This helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt.

Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal

Integrated pseudosection modeling and monazite petrochronology of paragneiss from the Kanchenjunga region of northeastern Nepal reveal the presence of cryptic tectonometamorphic discontinuities within the Himalayan metamorphic core. These new data outline a series of thrust-sense structures that juxtapose rocks that generally record a protracted history of early Eocene to latest Oligocene–early Miocene (ca. 41–23 Ma) prograde metamorphism and lateral extrusion against others that typically record short prograde (between 2 and 7 m.y.) and retrograde (between 3 and 6 m.y.) histories variably spanning the middle Oligocene to the middle Miocene (ca. 31–12 Ma). Retrograde metamorphism in the hanging wall of the thrust faults is typically coeval with prograde metamorphism in the footwall, indicating that overthrusting/underthrusting accommodated crustal shortening and drove metamorphic processes. The structures and juxtaposed panels were cut by early Miocene (ca. 20–18 Ma) out-of-sequence thrusting coincident with the previously mapped High Himal thrust. The resulting kinematic model for the evolution of the Himalayan metamorphic core in the Kanchenjunga area demonstrates that the Himalayan metamorphic core was dominated by underplating from at least the Oligocene through to present, and that the internal structure of the exhumed metamorphic core is significantly more complex than has been documented previously.

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.

Metamorphism and geochronology of the exhumed Himalayan midcrust, Likhu Khola region, east-central Nepal: Recognition of a tectonometamorphic discontinuity

The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist- to upper-amphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland- to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

A P–T–t–D discontinuity in east-central Nepal: Implications for the evolution of the Himalayan mid-crust

Rock specimens collected from lower, middle, and upper portions of the exhumed Himalayan mid-crust in the upper Tama Kosi region of east-central Nepal yield pressure–temperature–time–deformation (P–T–t–D) paths that demonstrate the presence of a structural, metamorphic, and geochronologic discontinuity. A P–T–t–D path from below the discontinuity defines a loading path with deformation at staurolite-grade metamorphic conditions occurring at ~10–8Ma. P–T–t–D paths from above the discontinuity, in contrast, record an earlier and protracted metamorphic history that includes decompressional heating. Monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be >21Ma with decompression-related garnet breakdown initiating at c.19Ma and likely continuing until c.15Ma. These contrasting P–T–t–D histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the Himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath Tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen. These results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen.

Tectonometamorphic discontinuities within the Greater Himalayan Sequence in Western Nepal (Central Himalaya): Insights on the exhumation of crystalline rocks

Abstract The core of the Greater Himalayan Sequence in the Mugu-Karnali area (Western Nepal) is affected by a thick shear zone with development of nearly 4 km of mylonites (Mangri shear zone). It is a contractional shear zone showing a top-to-the-SW and WSW sense of shear. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. P–T conditions indicate that the footwall experienced higher pressure (1.0–0.9 GPa) than the hanging wall (0.7 GPa) and similar temperatures (675°–700 °C). U–Pb in-situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 Ma. Samples from the lower part of the Greater Himalayan Sequence underwent similar ductile shearing at ~ 17–13 Ma. These ages and the associated P–T–t paths revealed that peak metamorphic conditions were reached ~ 5–7 Ma later in the footwall of the shear zone with respect to the hanging-wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself. Mangri Shear Zone, with the other recently documented tectonic and metamorphic discontinuities within the Greater Himalayan Sequence, point to the occurrence of a regional tectonic feature, the High Himalayan Discontinuity, running for more than 500 km along the strike of the Central Himalayas. It was responsible of the exhumation of the upper part of the Greater Himalayan Sequence starting from 28 Ma, well before the activation of the Main Central Thrust and the South Tibetan Detachment. Our data point out that exhumation of the Greater Himalayan Sequence was partitioned in space and time and different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one.