Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core

Leger, Remington M.; Webb, A. Alexander G.; Henry, Darrell J.; Craig, John A.; Dubey, Prashant K.

doi:10.1002/tect.20020

Cited by 26 publications

(15 citation statements)

References 71 publications

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“…The highest-metamorphic-grade rocks occur in the core of the Phojal fold (600-700 °C, ~6 kbar; Epard et al, 1995;Leger et al, 2013), below the Deo Tibba Ordovician granite (650-700 °C, ~8 kbar; Wyss, 2000), and in the deeply incised Chandra valley (Ky, St, Hbl, Sil, indicating upper amphibolite facies;Epard et al, 1995;our observations). Here, pegmatite and aplite dikes and veins suggest melting in the latest Eocene and Oligocene (Wyss, 2000;Stübner et al, 2014).…”

Section: Geologic Backgroundmentioning

confidence: 51%

Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

et al. 2017

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Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as "too old" on account of "excess Ar. " We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for "too-old" biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.

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Section: Geologic Backgroundmentioning

confidence: 51%

Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

et al. 2017

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“…At the western and northern limits of our study area, LHS rocks are overlain by Tethyan Himalayan Sequence and Greater Himalayan Crystalline complex rocks along the Main Central thrust. Tethyan Himalayan Sequence rocks here are psammitic and pelitic Neoproterozoic metasedimentary rocks, intruded by early Paleozoic granitoids, metamorphosed at upper greenschist‐ to amphibolite‐facies conditions [e.g., Epard et al ., ; Vannay and Grasemann , ; Wiesmayr and Grasemann , ; Leger et al ., ]. The Greater Himalayan Crystalline (GHC) is dominated by similar protoliths that have been metamorphosed at amphibolite‐to‐granulitefacies conditions [e.g., Frank et al ., ; Vannay and Grasemann , ; Manickavasagam et al ., ].…”

Section: Geology Of the Northwest Indian Himalayamentioning

confidence: 99%

Extrusion vs. duplexing models of Himalayan mountain building 1: Discovery of the Pabbar thrust confirms duplex-dominated growth of the northwestern Indian Himalaya since Mid-Miocene

Webb

2015

Tectonics

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Ongoing Himalayan growth is generally thought to be dominated by duplexing and/or extrusion processes. These models may be tested by reconstructing Himalayan fold-thrust belt growth since the middle Miocene. However, our knowledge of basic structural geometry remains too fragmentary to resolve the issue, even in areas with rich stratigraphic diversity such as the northwestern Indian Himalaya. In this region, a primary outstanding question involves the uncertain relationship of the Berinag thrust and the Tons thrust, structures with displacements of >80 km and >40 km, respectively. The uncertain geometry and kinematics allow for the complete range of duplexing or extrusion processes for the integrated kinematic history since the middle Miocene. To address this issue, field mapping and kinematic analysis were performed to reconstruct the deformation of the Lesser Himalayan Sequence in the northwest Indian Himalaya. Our results reveal a new discovery: a~450 m thick top-to-southwest shear zone, termed the Pabbar thrust. The Pabbar thrust placed the Outer Lesser Himalayan Sequence (the Tons thrust hanging wall) directly on the Berinag Group (the Berinag thrust hanging wall). This discovery requires that the Berinag thrust and Tons thrust are, in fact, the same structure, and discrete duplexing processes dominated growth of the northwest Indian Himalaya for the past 10-15 million years. Along-strike extension of these kinematics and corresponding geometries is consistent with the observed orogenic framework and resolves a stratigraphic continuity problem across the India-west Nepal border, where prior work suggests that structures are continuous but stratigraphy does not match.

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“…The geology of the Himachal Himalaya is generally consistent with the stack of four largely north-dipping, fault-bound units described here. A key exception is that from east to west, the immediate Main Central thrust hanging wall changes from an inverted metamorphic sequence consistent with the GHC to a right-way-up metamorphic sequence consistent with the THS (e.g., Thakur, 1998;DiPietro and Pogue, 2004;Leger et al, 2013). Yin (2006) speculated that this transition may result from the merging of the South Tibet detachment and Main Central thrust at the leading edge of the GHC.…”

Section: Geology Of the Himachal Himalayamentioning

confidence: 99%

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Webb

2013

Geosphere

109

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A line-length balanced palinspastic reconstruction across the Himachal Himalaya is presented, highlighting different phases of Himalayan tectonic development: Eocene shortening of the north Indian margin, Early-Middle Miocene emplacement of the crystalline core, and subsequent growth of the range by underplating. The total preserved shortening is 518 km (72%). The reconstruction demonstrates geometric feasibility of crystalline core emplacement via tectonic wedging, i.e., between a southdirected thrust (the Main Central thrust) and a north-directed backthrust (the South Tibet detachment). Crystalline core exposure between these faults occurs ca. 5 Ma in the reconstruction; initial exposure of these crystalline rocks ca. 11 Ma probably occurred in the hinterland within core complexes accommodating east-west extension. After Early-Middle Miocene crystalline core emplacement, ongoing orogenic growth is dominated by underplating processes. Outof-sequence faulting accomplishes <5% of this shortening; frontal accretion of foreland rocks occurs, but the resulting imbricate fan is largely eroded away. Over the past 3-5 m.y., an antiformal stack of mid-crustal horses and a hinterland-dipping duplex of upper crustal horses develop simultaneously. Minimum total shortening across the western Himalaya (from undeformed foreland to the India-Asia suture) is estimated by adding the new results and results from prior work to the north; an estimate of ~703-773 km of assessed shortening is calculated. However, because large portions of the regional deformation remain unassessed, estimates of ~900-1100 km may more accurately refl ect the minimum preserved shortening here. This range is comparable to the ~1350 km of shortening estimated by plate circuit reconstructions for this region. Apparent mismatch between geologic and plate circuit shortening estimates has recently instigated the new Greater India Basin hypothesis for two Cenozoic collisions along Asia's southern margin, but the new results suggest that this mismatch may not exist.

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Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core

Cited by 26 publications

References 71 publications

Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

Extrusion vs. duplexing models of Himalayan mountain building 1: Discovery of the Pabbar thrust confirms duplex-dominated growth of the northwestern Indian Himalaya since Mid-Miocene

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Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core

Cited by 26 publications

References 71 publications

Anomalously old biotite40Ar/39Ar ages in the NW Himalaya

Anomalously old biotite40Ar/39Ar ages in the NW Himalaya

Extrusion vs. duplexing models of Himalayan mountain building 1: Discovery of the Pabbar thrust confirms duplex-dominated growth of the northwestern Indian Himalaya since Mid-Miocene

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Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya

Anomalously old biotite⁴⁰Ar/³⁹Ar ages in the NW Himalaya