Metamorphism, Melting, and Extension: Age Constraints from the High Himalayan Slab of Southeast Zanskar and Northwest Lahaul

Walker, James A.; Martin, Mark W.; Bowring, Samuel A.; Searle, Michael P.; Waters, D. J.; Hodges, Kip

doi:10.1086/314360

Cited by 158 publications

(164 citation statements)

References 41 publications

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“…These results support the view that the magma migrated upward with limited, if any, chemical or thermal modifications. There is field evidence that the HHL magmas moved upward through narrow dykes (Le Fort, 1981;Scaillet et al, 1996;Walker et al, 1999), some of which connect to the base of the pluton, such as in the Garwhal Himalaya (Searle et al, 1993;Scaillet et al, 1995a). Harris (1998), together with the P-T conditions inferred for the top and bottom contact aureoles of Manaslu (Guillot et al, 1995a).…”

Section: Conditions During Magma Intrusionmentioning

confidence: 99%

“…(2) The HHL have been alternatively interpreted as plutons emplaced in situ close to their level of production (Visona & Lombardo, 2002), and as intrusive bodies in cold upper crust detached from their source, with vertical transport distances of $8-10 km (Copeland et al, 1990;Guillot et al, 1995b;Scaillet et al, 1996;Walker et al, 1999). The level of pluton intrusion has a major impact on its subsequent thermal evolution (Davidson et al, 1992), and our modelling results aim to distinguish between these hypotheses.…”

Section: Introductionmentioning

confidence: 96%

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Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

Annen¹,

Scaillet²,

Sparks³

2005

Journal of Petrology

102

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The emplacement of the Manaslu leucogranite body (Nepal, Himalaya) has been modelled as the accretion of successive sills. The leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection, and by a thin (<100 m) upper thermal aureole. These characteristics were used to constrain the maximum magma emplacement rate. Models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period. Additionally, the hypothesis of a tectonic top contact, with unroofing limiting heat transfer during magma emplacement, was evaluated. In this latter case, the upper limit for the emplacement rate was estimated at 3Á4 mm/year (or 1Á5 Myr for 5 km of granite). Geological and thermobarometric data, however, argue against a major role of fault activity in magma cooling during the leucogranite emplacement. The best model in agreement with available geochronological data suggests an emplacement rate of 1 mm/year for a relatively shallow level of emplacement (granite top at 10 km), uninterrupted by a long repose period. The thermal aureole temperature and thickness, and the isotopic heterogeneities within the leucogranite, can be explained by the accretion of 20-60 m thick sills intruded every 20 000-60 000 years over a period of 5 Myr. Under such conditions, the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites.

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Section: Conditions During Magma Intrusionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

Annen¹,

Scaillet²,

Sparks³

2005

Journal of Petrology

102

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“…Several authors have suggested that metamorphic isograds were affected by crustal-scale post-peak metamorphic folding and shearing (Searle & Rex 1989;Hubbard 1996;Walker et al 1999;Stephenson et al 2000Stephenson et al , 2001. At least in the NW Himalaya, it can be demonstrated that isograds were inverted by south-vergent folding and associated simple shear along the MCT zone at the base of the extruding layer.…”

Section: Greater Himalayan Sequencementioning

confidence: 99%

“…In the western Himalaya, several studies combining structural mapping with thermobarometry show that metamorphic isograds were folded after peak metamorphism, resulting in a right-way-up metamorphic sequence along the top of the GHS beneath the Zanskar shear zone (ZSZ), and an inverted metamorphic sequence along the MCT zone at the base of the slab (Searle & Rex 1989;Searle et al 1992Searle et al , 1999bWalker et al 1999;Dezes et al 1999;Stephenson et al 2000Stephenson et al , 2001Robyr et al 2002;Vannay et al 2004). Peak sillimanite + K-feldspar-grade metamorphism was concomitant with partial melting at c. 22-20 Ma (Noble & Searle 1995).…”

Section: Himalayan Channel Flow Modelmentioning

confidence: 99%

“…The footwall rocks were extruded out to the south beneath the fault, whilst the hanging wall rocks remained static. Using pressure-depth constraints from footwall gneisses and leucogranites, Walker et al (1999) estimated at least 50 km of southward extrusion of GHS rocks in Zanskar, and Searle et al (2002,2003) estimated at least 100 km of southward motion may have occurred in the footwall GHS rocks along the Rongbuk valley, north of Everest. This supports the channel flow model of ductile extrusion of the middle crust.…”

Section: Std As a Passive Roof Fault To The Extruding Channelmentioning

confidence: 99%

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Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust

Searle

Law

Jessup

2006

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115

157

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Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal-south Tibet. The catalyst for the early kyanite _ sillimanite metamorphism (650-680~ 7-8 kbar, 32-30 Ma) was crustal thickening and regional Barrovian metamorphism. Later sillimanite + cordierite metamorphism (600-680~ 4-5 kbar, 23-17 Ma) is attributed to increased heat input and partial melting of the crust. Crustal melting occurred at relatively shallow depths (15-19 km, 4-5 kbar) in the crust. The presence of highly radiogenic Proterozoic black shales (Haimanta-Cheka Groups) at this unique stratigraphic horizon promoted melting due to the high concentration of heat-producing elements, particularly U-bearing minerals. It is suggested that crustal melting triggered channel flow and ductile extrusion of the GHS, and that when the leucogranites cooled rapidly at 17-16 Ma the flow ended, as deformation propagated southward into the Lesser Himalaya. Kinematic indicators record a dominant southvergent simple shear component across the Greater Himalaya. An important component of pure shear is also recorded in flattening and boudinage fabrics within the STD zone, and compressed metamorphic isograds along both the STD and MCT shear zones. These kinematic factors suggest that the ductile GHS channel was subjected to subvertical thinning during southward extrusion. However, dating of the shear zones along the top and base of the channel shows that the deformation propagated outward with time over the period 20-16 Ma, expanding the extruding channel. The last brittle faulting episode occurred along the southern (structurally lower) limits of the MCT shear zone and the northem (structurally higher) limits of the STD normal fault zone. Late-stage breakback thrusting occurred along the MCT and at the back of the orogenic wedge in the Tethyan zone. Our model shows that the Himalayan-south Tibetan crust is rheologically layered, and has several major low-angle detachments that separate layers of crust and upper mantle, each deforming in different ways, at different times.The Tibetan plateau (Fig. 1), covering an area of >5 • 106 km, is an arid plateau with low relief and low erosion rates, and forms the largest area of high elevation (average elevation of 5023 m) and thick crust (65-80 km) on the planet (Fielding

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Synchronous N-S and E-W extension at the Tibet-to-Himalaya transition in NW Bhutan

et al. 2015

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Despite~50 Myr of continuous continent-continent collision, contractional structures in the Himalayan-Tibetan orogen are today limited to the northern and southern margins of the system, while extension dominates much of the interior. On the Tibetan Plateau, Cenozoic E-W extension has been accommodated by strike-slip faults and extensional grabens, while N-S extension at the Tibet-to-Himalaya transition has been accommodated by the South Tibetan fault system (STFS). The genetic relationship between N-S and E-W extension is disputed, although age constraints indicate temporal overlap of at least 7 Myr. In NW Bhutan the two intersect where the STFS basal detachment is cut by the Yadong cross structure (YCS), an extensional half graben that provides a rare opportunity to constrain relative timings. We report U-Pb zircon dates from four STFS footwall leucogranites consistent with episodic magmatism during the middle-late Miocene and in situ U(-Th)-Pb monazite and xenotime dates from three metasedimentary rocks ranging from late Oligocene to middle Miocene. We suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal STFS detachment initiated as a ductile structure in the middle-late Miocene. Late-stage granitic intrusions may reflect footwall melting during extensional exhumation along the STFS, but post-metamorphic and post-intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites. Some of the oldest YCS-related fabrics are found in a deformed 14 Ma leucogranite, implying middle Miocene ductile deformation. This observation, along with evidence for subsequent brittle YCS deformation, suggests that N-S and E-W extensional structures in the area had protracted and overlapping deformation histories.

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Metamorphism, Melting, and Extension: Age Constraints from the High Himalayan Slab of Southeast Zanskar and Northwest Lahaul

Cited by 158 publications

References 41 publications

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust

Synchronous N-S and E-W extension at the Tibet-to-Himalaya transition in NW Bhutan

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