Eastern Nepal Project

Himalayan Metamorphism: Latest findings 2016. [1]

The Himalayan Orogen is the quintessential collisional orogen, important for both field based studies and template for modelling of collisional orogens. The Himalayan project started in 1992 and has involved 4 field seasons. An almost entirely privately funded project; also with two Adelaide University small grants with Martin Hand (Adelaide University) and support from Prof. David Gray [2] (Melbourne University). Field work has concentrated on traverses along most major rivers in the eastern Nepal region. Outcomes are a preliminary metamorphic study in 2000 and new crustal architecture for the Himalayan Orogen in 2006 that best illustrates the Integrated Terrane Analysis Method, and has recieved the International Association for Gondwana Research best paper award in 2006. Detailed metamorphic studies are to follow.

Below are links to summaries of different aspects of the Himalayan project:

• Crustal architecture of east Nepa [3]l. [3]

• The Integrated Terrane Analysis Method [4]

• Metamorphic architecture of east Nepal [5]

(1) Structural and metamorphic architecture of the east Nepal Himalayas.

Ben Goscombe¹, David Gray², Martin Hand¹

¹School of Earth and Environmental Sciences, Adelaide University, 5005, S.A.
²School of Earth Sciences, Melbourne University, Parkville 3010, Victoria.

See Gondwana Research 10, 232-255. 2006.

The whole of east Nepal between Mount Everest and Kangchenjunga has been mapped and nine detailed structural and metamorphic profiles across the Himalayan Metamorphic Front documented. This data, for the first time, accurately constrains the structural and metamorphic architecture of the east Nepal Himalayas and these results contrast with the current paradigm for the architecture of the Himalayas in general. The Himalayan Metamorphic front is comprised of three discrete structural-metamorphic-chronometric terranes: (1) At the base a low-grade Lower-Plate of >2080m thickness. (2) At the top a high-grade Upper-Plate of >4040-4740m thickness. (3) Wedged between is an inverted Barrovian series in what is called the Main Central Thrust Zone (MCTZ) that ranges 350-4050m in thickness.

The main, crustal-scale structure controlling metamorphism is not the Main Central Thrust at the base of the Main Central Thrust Zone, as asserted for elsewhere in the literature, but is the High Himal Thrust at the base of the over-riding Upper-Plate. The Upper-Plate is composed of poly-deformed high-grade gneisses that constitute part of the Neoproterozoic Greater Himalayan Sequences. Upper-Plate rocks are of granulite grade and metapelites have matrix assemblages of garnet-cordierite-sillimanite-K-feldspar. Peak metamorphism occurred at 20-22 Ma, at conditions of 837±59 ºC and 6.7±1.0 kb, defining an average thermal gradient of 36 ºC/km. This metamorphic terrane shows a discrete metamorphic break from the upper part of the underlying Main Central Thrust Zone, amounting to DT=+187 ºC and DP=–2.1 kb. Peak metamorphic garnets are compositionally flat, which is typical of the homogenisation experienced at high metamorphic grades. The Upper-Plate P-T path has not been confidently established. Apparently contradictory petrological evidence is preserved in these rocks; indicating both decompression (rare cordierite growth after garnet) and low-P hercynite-sillimanite-quartz prograde parageneses, suggesting a complex prograde P-T history that was ultimately terminated by near-isobaric cooling. The base of the Upper-plate is a 200-400 m thick high strain mylonite zone called the High Himal Thrust. Deformation fabrics in the High Himal Thrust show a complex evolution, the latest movement episode produced a pervasive foliation with sillimanite-biotite±garnet±gedrite assemblages that formed at 674±33 ºC and 5.7±1.1 kb. These assemblages are identical to those developed in shearbands that over-print peak metamorphic parageneses in both the basal Upper-Plate and upper part of the underlying MCTZ, indicating that the latest movement along the High Himal Thrust, juxtaposed these metamorphic terranes during their post-peak evolution. The High Himal Thrust is the only discrete, crustal-scale, high-strain zone in the Himalayan Metamorphic Front, and is the main structure controlling the metamorphic architecture.

The MCTZ is comprised of Neoproterozoic to Ordovician metasediments of the Greater Himalayan Sequences and consitutes the inverted Barrovian metamorphic series of the Himalayan Metamorphic Front. Clockwise P-T paths were experienced, with peak metamorphic conditions ranging from 550 ºC to 650 ºC at 8.8±1.1 kb, defining average thermal gradients of 18-21 ºC/km. Garnets from the MCTZ show two-stage growth, recording evidence for two metamorphic peaks, possibly correlating with metamorphic events of 20-22 Ma in the overlying Upper-Plate and 6-13 Ma in the underlying Lower-Plate. There is no discrete, high-strain structural break between the MCTZ and the Palaeoproterozoic Lesser Himalayan Sequences of the Lower-Plate. Strain is equally and pervasively partitioned by schistosic fabric development throughout the MCTZ and Lower-Plate. Furthermore, metamorphic grade varies continuously throughout the entire metamorphic front below the High Himalayan Thrust with no significant metamorphic discontinuity between the MCTZ and the Lower-Plate. Lower-Plate rocks experienced a single metamorphic cycle at 6-13 Ma and peak metamorphic conditions of 561±12 ºC at 8.5±1.9 kb, with average thermal gradient of 19 ºC/km. What has been called the Main Central Thrust in the past is an unconformity between Palaeoproterozoic and Neoproterozic sedimentary sequences and was not a discrete high-strain zone during metamorphism.

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(2) Determining crustal architecture using integrated terrane analysis: example from the eastern Nepal Himalayas

Ben Goscombe^a, David Gray^b

^aContinental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, Adelaide. South Australia. 5005. Australia.
^bSchool of Earth Sciences, University of Melbourne, Melbourne. Victoria. 3010. Australia.

See Gondwana Research 10, 232-255, 2006.

Being the quintessential recent example of a collisional orogen, the Himalayan Orogen has received considerable interest and is the template for thermo-mechanical modelling of continental collision. Underpinning tectonic interpretations of the Himalayas and thermal-mechanical modelling of collision, are assumptions about across-orogen crustal architecture and the extent to which these architectures can be extended laterally along the orogen. The seemingly simple task of determining crust architecture and identifying the key determinate structures is in practice problematic in the recent Himalayan example and significantly more so in deeply eroded old orogenic belts without boundary conditions. Evolving crustal architectures of orogens can be determined by a method of integrated terrane analysis that utilizes fully integrating fundamental datasets. Structural, metamorphic and stratigraphic datasets cannot be used in isolation, but must be fully integrated and linked temporally by direct dating of metamorphic and deformation parageneses. These variables are spatially linked by field gradients across the orogenic front and constrain possible first-order crustal architectures and evolutions of the orogen. There is a feed back loop between modelling studies and ground truthing by field-based geological studies. Variables such as gross fabric geometry, transport, bulk strain, pressure, temperature, average thermal gradient and the age of peak metamorphic parageneses determined in real orogens can be used to test against outcomes of models and validate assumptions.

The Himalayan Metamorphic Front example consists of two basinal sequences deposited on the Indian passive margin, the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic-Cambrian Greater Himalayan Sequence. The current paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has come to be called the Main Central Thrust, and that this acted as the fundamental structure that controlled the architecture of the Himalayan Metamorphic Front. The Main Central Thrust or Ramgarh Thrust was originally defined at lower structural levels than this unconformity. Geological mapping of eastern Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define an alternative crustal architecture. In eastern Nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone. In recognition of this, we introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front. The fundamental structure that controls orogen architecture in eastern Nepal, occurs at higher structural levels within the Greater Himalayan Sequence and we suggest the name; High Himal Thrust. This 100-400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition, and separates the Upper-Plate from the Lower-Plate in the Himalayan Metamorphic Front. The high-T/moderate-P metamorphism at ~20-24 Ma in the Upper-Plate, reflects extrusion of material between the High Himal Thrust and the South Tibet Detachment System at the top of the section. The Lower-Plate is a broad schistose zone of inverted, diachronous moderate-T/high-P metamorphic rocks formed between ~18-6 Ma. The High Himal Thrust is laterally continuous to the east into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and Main Central Thrust (as originally defined). To the west in central Nepal, the Upper-Plate/Lower-Plate boundary has been placed at lower structural levels, coinciding with the Himalayan Unconformity and is referred to in the literature as the Main Central Thrust; though occurs above the originally defined Main Central Thrust (or Ramgarh Thrust).

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(3) Contrasting P-T paths in the Eastern Himalaya, Nepal: Inverted isograds in a Paired Metamorphic Mountain Belt.

Ben Goscombe & Martin Hand

Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia.

See Journal of Petrology 41, 1673-1719, 2000.

Petrology and phase equilibria of rocks from two profiles in Eastern Nepal from the Lesser Himalayan Sequences, across the Main Central Thrust Zone and into the Greater Himalayan Sequences, reveal a Paired Metamorphic Mountain Belt (PMMB) comprised of two thrust-bound metamorphic terranes of contrasting metamorphic style. At the higher structural level, the Greater Himalayan Sequences, experienced high-T/moderate-P metamorphism, with an anti-clockwise P-T path. Low-P inclusion assemblages of quartz+hercynitic spinel+sillimanite have been overgrown by peak metamorphic garnet+cordierite+sillimanite assemblages that equilibrated at 837±59 ºC and 6.7±1.0 kb. Matrix minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re-equilibrated samples preserve evidence of cooling to 600±45 ºC at 5.7±1.1 kb. Below the Main Central Thrust, the Lesser Himalayan Sequences is a continuous (though inverted) Barrovian sequence of high-P/moderate-T metamorphic rocks. Metamorphic zones upwards from the lowest structural levels in the south are;
(A) albite+chlorite+muscovite±biotite
(B) albite+chlorite+muscovite+biotite+garnet
(C) albite+muscovite+biotite+garnet±chlorite
(D) oligoclase+muscovite+biotite+garnet±kyanite
(E) oligoclase+muscovite+biotite+garnet+staurolite+kyanite
(F) bytownite+biotite+garnet+K-feldspar+kyanite±muscovite
(G) bytownite+biotite+garnet+K-feldspar+sillimanite+melt±kyanite.
The Lesser Himalayan Sequences show evidence for a clockwise P-T path. Peak-P conditions from mineral cores, average 10.0±1.2 kb and 557±39 ∞C and peak-metamorphic conditions from rims, average 8.8±1.1 kb and 609±42 ºC in Zones (D) to (F). Matrix assemblages are overprinted by decompression reaction textures and in Zones (F) and (G), progress into the sillimanite field. Both terranes were brought into juxtaposition during formation of sillimanite-biotite±gedrite foliation seams (S3) formed at conditions of 674±33 ºC and 5.7±1.1 kb. The contrasting average geothermal gradients and P-T paths of these two metamorphic terranes suggests they make up a PMMB. The upper-plate position of the Greater Himalayan Sequences produced an anti-clockwise P-T path, with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane. In contrast, the lower-plate Lesser Himalayan Sequences were deeply buried, metamorphosed in a clockwise P-T path and display inverted isograds due to progressive ductile overthrusting of the hot Greater Himalayan Sequences during prograde metamorphism.
Keywords: Thermobarometry; P-T paths; Himalaya; Metamorphism; Inverted Isograds; Paired metamorphic belts.

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