Metamorphic response to collision in the
Central Himalayan Orogen
Ben Goscombe1,3*, David Gray2, David A. Foster3.
1Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.
2School of Earth Sciences, University of Tasmania, Hobart Tasmania. Australia.
3Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.
*Corresponding author. Email: ben.goscombe@gmail.com. http://www.terraneanalysis.com.au
Published as Focus Review in Gondwana Research 2018, volume 57, 191-265.
Closure of the Neotethys Ocean and high-angle continent-continent collision between India and Asia after about 55 Ma resulted in low-angle subduction of the Indian plate below the Tibetan Plateau and by ~30 Ma established an arcuate 2,300 km long, shallow north-dipping metamorphic fold-thrust belt in the foreland. This Himalayan Metamorphic Front quickly established an Upper-Plate / Lower-Plate paired metamorphic architecture centred on the median High Himal Thrust that largely controlled subsequent evolution. The Upper-Plate is a thick slab of high-T/moderate-P high-grade migmatized metamorphic rocks, whereas the Lower-Plate is an inverted series of moderate-T/high-P schists in two crustal wedges, the broad Main Central Thrust Zone and at lowest structural levels the Footwall, below the Basal Main Central Thrust. Spatial and temporal patterns of metamorphic response in the evolving Himalayan Metamorphic Front has been characterized in a large-scale integrated structural-metamorphic study based on 8 profiles across eastern Nepal. Metamorphic response at all structural levels is established using a large dataset (n~160) of internally consistent quantitative PT determinations, petrology of metapelite samples, semi-quantitative P-T paths, metamorphic mapping and metamorphic field gradients. These results are integrated with previously published metamorphic studies, structural profiles and metamorphic chronology. From these datasets the architecture and evolution of the Himalayan Metamorphic Front is constrained by rock kinematics, metamorphic field gradients showing discontinuities, and diachronous metamorphism with contrasting P-T evolutions at different structural levels. Each of the three panels constituting the Himalayan Metamorphic Front: Upper-Plate, Main Central Thrust Zone and Footwall, experienced distinctly different tectono-metamorphic histories. Crustal processes operating during metamorphism and exhumation differ between the Upper- and Lower-Plates. The Upper-Plate experienced long-lived metamorphism starting from at least 28-38 Ma and tracking low DP/DT clockwise P-T paths that culminated at ~19-27 Ma in peak high-grade condition with 27-31 ºC/km thermal regimes. Protracted high-grade conditions produced significant partial melt, which facilitated gravity driven southward extrusion involving internal ductile flow processes. Southward extrusion of the Upper-Plate was accommodated by coeval reverse movement on the High Himal Thrust and top down to the north, normal reactivation of the South Tibet Detachment System between ~22-10 Ma. Transport of this thick slab to the south resulted in further prograde burial of the Lower-Plate below, culminating in peak metamorphism at the highest pressures attained in the Lower-Plate rocks. The Lower-Plate consists of at least two deeply buried (7.5-9.4 kb) crustal wedges, both of which experienced steep DP/DT hairpin clockwise P-T paths with isothermal decompression. The Main Central Thrust Zone experienced 20-26 ºC/km metamorphism at ~12-22 Ma and was exhumed along the Basal Main Central Thrust after ~11 Ma. Whereas, loading by the Main Central Thrust Zone gave rise to further prograde burial of the Footwall, which experienced 16-21 ºC/km metamorphism at ~4-10 Ma and was exhumed along the Main Boundary Thrust at ~3-9 Ma. In contrast to the Upper-Plate, both crustal wedges were exhumed by a process of out-wedging, which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls. This three-stage foreland propagating lateral exhumation history resulted in telescoping of the Himalayan Metamorphic Front in concert with peak metamorphic events at different structural levels during main phase orogenesis, and is not the result of superimposed retrograde reactivation of the belt.
Goscombe, B. and Gray, D., 2014.
Metamorphic evolution of Gondwana 4. Post-breakup collisional margins: Metamorphic response to collision in the Himalayan Orogen and obduction at the east Arabian margin. Geoscience Australia Record 2014/XX (in review).
The tectonics of the Himalayan Orogen has been the focus of enormous attention over the last +30 years because it is both an active convergent orogen and the quintessential example of high-angle collisional orogen. As a result taking a special place in influencing the understanding of collisional orogenesis and is often the template used for ground-truthing thermo-mechanical modelling of collisional orogens. The Semail Ophiolite on the East Arabian Margin is the type-example for obduction and only pristine, un-reworked obducted margin worldwide. The East Arabian Margin preserves metamorphic parageneses from a range of early tectonic settings such as subduction, obduction and spreading ridge interactions that are typically obliterated and lost during collisional orogenesis. Consequently, datasets from both the Himalayan Orogen and East Arabian Margin cover the full range of metamorphic response, representing all tectonic episodes, during a collisional orogenic cycle. These two regions of interest, together typify collisions between crustal fragments subsequent to breakup of the Gondwana Supercontinent.
The Himalayan Metamorphic Front consists of two basinal sequences deposited on the north Indian Plate passive margin: the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic-Cambrian Greater Himalayan Sequence. The currently entrenched paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has been called the Main Central Thrust, and that this acted as the fundamental structure that controlled the metamorphic architecture of the Himalayan Metamorphic Front. Geological mapping of East Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define the crustal architecture. In East 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 East 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 into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and basal 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 approximately with the Himalayan Unconformity and has been incorrectly named the "Main Central Thrust", above the originally defined basal Main Central Thrust (or Ramgarh Thrust).
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There is geochronological evidence for an early metamorphic event of ~28-38 Ma, recognised throughout the entire length of the Himalayan Orogen, except in the fareast at Namche Barwa. This M1 or Eohimalayan metamorphic event is restricted to the Upper-Plate high-grade gneissic rocks and gneissic dome windows exposed in the Tibetan Zone. M1 metamorphism is recognised only by inclusion assemblages and U-Pb age determinations, typically from relict zircon and monazite inclusions preserved within M2 garnet porphyroblasts. Because of extensive recrystallization during M2 metamorphism, no matrix mineral parageneses have been correlated with the M1 event. The M1 event is interpreted to represent the first peak of metamorphism attained after crustal thickening during collision at ~40-55 Ma. Crustal thickening was followed by conductive heating and radiogenic heat production during a lag of ~12-17 Ma before the peak of M1 metamorphism was attained. The Eohimalayan metamorphic front was centred on the Eohimalayan Range formed during collision, and running parallel to and north of the current Himalayan Range and metamorphic front. The precursor of the South Tibet Detachment System is interpreted to have been the Eohimalayan Thrust at this time, leading to burial in the now Upper-Plate at lower structural levels. The age of the Eohimalayan event is consistent throughout the whole Himalayan Orogen with monazite and zircon inclusion age determinations ranging 28-38 Ma. Rare low-P hercynitic spinel, sillimanite, biotite, quartz and ilmenite inclusion parageneses within garnet porphyroblasts in metapelites from the Upper-Plate in East Nepal are interpreted as relict M1 assemblages. These parageneses formed at approximately 750-800 ºC and 4.0-6.0 kb, at lower pressures than matrix M2 parageneses at 700-750 ºC and 6.4-7.9 kb. The high pressures and decompressive clockwise P-T paths during M2 metamorphism are incompatible with M1 conditions. M1 must constitute a separate early metamorphic event and not prograde conditions during the M2 event. The low-P inclusion parageneses indicate that M1 Eohimalayan metamorphism was in an orogen supporting lower crustal thicknesses than during the peak of M2. Consequently, it can be inferred that further burial and crustal thickening continued as the orogen evolved from M1, culminating in peak metamorphism and maximum crustal thickness at M2.
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M2 metamorphism is restricted to the Upper-Plate and High Himal Thrust, and evident throughout the entire length of the Himalayan Orogen. M2 metamorphism gave the main phase, gneissic and migmatitic regional metamorphic parageneses of upper-amphibolite to granulite facies grade. Maximum peak M2 metamorphic conditions are 700-780 ºC, 6.4-7.9 kb and 27-31 ºC/km in East Nepal. Maximum pressure conditions attained during prograde M2 metamorphism are approximately 8.0-10.5 kb, indicating moderate burial. The Upper-Plate experienced open clockwise P-T paths during M2, with decompression through the peak of metamorphism followed by near isobaric cooling. Peak M2 metamorphism occurred between 19-27 Ma in all sectors of the Himalayan Orogen. Leucogranite melts were generated in the Upper-Plate during M2 and range in age 17-27 Ma throughout the Himalayan Orogen. Melting occurred in response to decompression through the peak of M2 metamorphism. M2 matrix parageneses represent the culmination of a protracted continuum of high-grade metamorphic conditions in the Upper-Plate starting with M1 Eohimalayan metamorphism at 28-38 Ma. Together, M1 and M2 high-grade metamorphic conditions persisted for ~18 Ma and document the protracted period of progressive crustal thickening and heat production. M2 metamorphism of the Upper-Plate was accompanied by extrusion of the Upper-Plate (Extrusion I), starting at peak M2 and continuing throughout late-M2 between 18-24 Ma. Extrusion was accommodated by reverse shear on the High Himal Thrust at the base of the Upper-Plate and initiation of extension in the South Tibet Detachment System at the hanging wall. Direct and indirect age constraints indicate reverse shear in the High Himal Thrust occurred at 19.5-23.9 Ma and possibly 25.5±4.5 Ma in all parts of the orogen, over-lapping with peak M2 metamorphism and extrusion. Extensional reactivation of the South Tibet Detachment System was initiated in late-M2 accompanying extrusion of the Upper-Plate, at 18-24 Ma. Protracted extensional shear continued throughout the M3 period to ~10 Ma. This long-active extensional shear system accommodated two exhumation events at lower structural levels: Extrusion I of the Upper-Plate and Outwedge II of the Main Central Thrust Zone. Metamorphic parageneses in the South Tibet Detachment System reflect this long history of extension and exhumation, ranging from middle-amphibolite to sub-greenschist brittle conditions.
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M3 constitutes the main phase of the Himalayan Metamorphic Cycle in most of the Lower-Plate. M3 metamorphism gave main phase regional parageneses within gneissic domes in the Tibetan Zone, the South Tibet Detachment System, Main Central Thrust Zone and Main Central Thrust and reworking parageneses such as shearbands and foliation seams in the Upper-Plate and High Himal Thrust. M3 metamorphism accompanies burial, peak metamorphism and exhumation of the Main Central Thrust Zone (Outwedge II) in the Lower-Plate. The broad Main Central Thrust Zone is a crustal wedge in the Lower-Plate, bound at lower structural levels by the basal Main Central Thrust (Ramgarh Thrust) and at higher structural levels by the High Himal Thrust at the hanging wall. M3 regional metamorphism gave rise to the middle- to upper-amphibolite facies main phase matrix parageneses with an inverted Barrovian metamorphic field gradient. Lowest grades are at ~580-590 ºC and pooled peak metamorphic conditions in East Nepal range from 613-646 ºC, 8.4-9.1 kb and 19.7-21.0 ºC/km at the base to 680-695 ºC, 7.5-7.9 kb and 24.8-26.3 ºC/km at the top of the Main Central Thrust Zone. Maximum pressures were attained prior to the peak-T of metamorphism, and range 8.3-11.4 kb across 520 to 640 ºC. P-T paths in the Main Central Thrust Zone are tight to open clockwise paths with loading prograde trajectories followed by decompressive heating to peak metamorphic conditions, which were terminated by isothermal decompression.
Prograde metamorphism and burial of the Main Central Thrust Zone occurred in response to further loading by the Upper-Plate, during M2 to late-M2 extrusion (Extrusion I) between 18-24 Ma. Peak M3 metamorphic age determinations indicate a diachronous multi-stage metamorphic history in the Main Central Thrust Zone. Ranging semi-systematically from 18-22 Ma at highest structural levels, through 16-12 Ma, to 8.9 Ma immediately above the basal Main Central Thrust (Ramgarh Thrust). Peak metamorphic conditions coincide with initiation of isothermal decompression during extensional telescoping and exhumation of the Main Central Thrust Zone (Outwedge II), which terminated peak metamorphism. Exhumation of the Main Central Thrust Zone by out-wedging occurred from 18 Ma to at least 9 Ma, after peak metamorphism during isothermal decompression. Out-wedging was accommodated by extensional reactivation of the main foliation at the top of the Main Central Thrust Zone and High Himal Thrust and shearbands in the basal Upper-Plate. Out-wedging was also accommodated by ongoing reverse movement in the basal Main Central Thrust (Ramgarh Thrust), dated at 8.5-19.8 Ma throughout the Himalayan Orogen. Progression in peak metamorphic ages and thus also post-peak exhumation, indicate internal telescoping of the Main Central Thrust Zone, the lowest structural levels being exhumed the last. Exhumation of the Main Central Thrust Zone (Outwedge II) in turn further loaded the Footwall, which was eventually followed by M4 metamorphism in the Footwall at 6-9 Ma.
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M4 metamorphism is restricted to the Footwall and accompanies the final component of burial, peak metamorphism and exhumation of these rocks in the latest phase of the Himalayan Metamorphic Cycle. The Footwall is a crustal wedge in the Lower-Plate, bound at lower structural levels by the Main Boundary Thrust and at higher structural levels by the basal Main Central Thrust (Ramgarh Thrust) at the hanging wall. Footwall schists experienced only one metamorphic event evidenced by single-stage, syn-kinematic prograde garnet growth during lower- to middle-amphibolite facies conditions. Peak M4 metamorphic conditions range 538-585 ºC, 8.0-9.4 kb and 16.4-20.9 ºC/km, and occurred at 4.0-10.0 Ma. P-T paths are steep tight clockwise paths, with near isothermal loading to maximum-P conditions near peak metamorphism, which was terminated by isothermal decompression. Loading by the Main Central Thrust Zone during Outwedge II at 9.0-18.0 Ma, gave rise to shear fabrics in the Footwall dated at 8.0-11.0 Ma, accompanying final component of burial during prograde metamorphism. Post-peak isothermal decompression occurred at 2.0-6.0 Ma in isostatic response to loading, resulting in exhumation of the Footwall (Outwedge III) and further extensional telescoping of the metamorphic front. Extensional telescoping was accommodated by extensional reactivation of the main foliation within the Main Central Thrust Zone above the Footwall, and ongoing reverse movements in the Main Boundary Thrust from 9.0 to 3.0 Ma and Main Frontal Thrust from 3.3 to 0.0 Ma. At the same time as M4 metamorphism and out-wedging of the Footwall, E-W extension produced N-S rift grabens in the Tibetan Plateau at 2.5-10.5 Ma. The M5 metamorphic event of 1-11 Ma age is restricted to the Nanga Parbat Massif, producing moderate-P granulites and leucogranite and accompanies very rapid exhumation of these rocks in the Pliocene.
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Extensional telescoping of the Himalayan Metamorphic Front occurred by a sequential history of foreland propagating and foreland-vergent lateral exhumation of crustal wedges, a process called "out-wedging". This dynamical process has been constrained by identifying metamorphic discontinuities and integration with metamorphic evolutions, geochronology and kinematic data. The first exhumation event was extrusion of a thick slab of high-T, high T/depth gneisses of the Upper-Plate between the basal High Himal Thrust and South Tibet Detachment System at the top of the slab (Extrusion I). Transport of this thick slab to the south resulted in further prograde burial of the Main Central Thrust Zone below between 24 and 18 Ma. Peak metamorphism of the Main Central Thrust Zone occurred later at 12-19 Ma and coincides with the initiation of isothermal decompression during extensional telescoping and exhumation of the Main Central Thrust Zone (Outwedge II). Exhumation of the Main Central Thrust Zone by out-wedging occurred between 9-18 Ma. Out-wedging was accommodated by extensional reactivation of the foliation at the top of the Main Central Thrust Zone and within the High Himal Thrust, and ongoing reverse movement in the basal Main Central Thrust, dated at 8.5-19.8 Ma throughout the Himalayan Orogen. Loading below Outwedge II gave rise to further prograde burial of the Footwall (Outwedge III) and eventual peak metamorphism at 6-9 Ma. Isothermal decompression and exhumation of Outwedge III at 2-6 Ma, was accommodated at the base by the Main Boundary Thrust and at higher structural levels by extensional reactivation of the main foliation immediately above the basal Main Central Thrust within the lower Main Central Thrust Zone. This foreland propagating sequence of out-wedging may be the result of thrust stacking giving rise to a southward migration of the isostatic instability generated by crustal over-thickening. The southward transport of Extrusion I, further loaded and deeply buried the rocks in Outwedge II. This over-loading resulted in relatively fast isostatic rebound of Outwedge II, which in an overall convergent system was facilitated by upthrusting along the basal Main Central Thrust and accommodated by top-down to the north extensional reactivation in the vicinity of the High Himal Thrust. Followed in turn by another cycle of over-loading, isostatic instability and exhumation response in the underlying Footwall (Outwedge III).
In summary the Himalayan Metamorphic Front evolved through a foreland propagating sequence of reverse movements on crustal structures, younging from north to south. Starting with thrusting during collision at the Indus-Tsangpo Suture Zone at 40-55 Ma, proto-South Tibet Detachment System or Eohimalayan Thrust at 28-38 Ma, High Himal Thrust possibly from 25.5 Ma and predominantly at 19.5-23.9 Ma, penetrative fabrics and thrusts in the Main Central Thrust Zone from 18-24 Ma and possibly continuing to 8.5 Ma, basal Main Central Thrust (Ramgarh Thrust) at 8.5-19.8 Ma, Main Boundary Thrust and fabrics in the Footwall at 4-10 Ma and finally Main Frontal Thrust at 1.7-3.3 Ma. Crustal over-thickening in the Himalayan thrust stack resulted in gravitational instability and isostatic exhumation. Exhumation occurred by a foreland propagating sequence of foreland-vergent crustal sheets or wedges in discrete events: from Extrusion I of the Upper-Plate at ~18-24 Ma, Outwedge II of the Main Central Thrust Zone at ~9-18 Ma and Outwedge III of the Footwall at ~2-6 Ma. These extrusion and out-wedging events resulted in extensional telescoping of the metamorphic front, which was accommodated by a foreland propagating sequence of extensional reactivation in the hanging walls of crustal wedges concomitant with the sequence of thrusting in footwalls. Starting with extensional reactivation in the South Tibet Detachment System from 18-24 Ma and continuing to ~10 Ma, High Himal Thrust and vicinity probably at ~9-18 Ma and Main Central Thrust and lower Main Central Thrust Zone probably at ~2-6 Ma. The thermal peak in each crustal zone was attained after additional burial to maximum depths by the over-riding crustal wedge in the hanging wall. This resulted in further burial of already hot deeply burial rocks, and the thermal peak was attained quickly and generally terminated when the crustal wedge was inturn exhumed by out-wedging. Consequently, the peak of metamorphism also propagates towards the foreland with time, bracketed in time between over-thrusting of the hanging wall and exhumation along the footwall. Starting with peak metamorphism in the Eohimalayan Zone at 28-38 Ma, Upper-Plate at 19-27 Ma, Main Central Thrust Zone from 18-20 Ma at the top of the zone to 8.9 Ma at the base and finally in the Footwall at 2.9-8.0 Ma.
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A series of metamorphic field gradients across the metamorphic front and a summary gradient along the orogen, show marked variation in peak metamorphic pressures and T/depth ratios in different parts of the Upper-Plate. This variation suggests a model of highly partitioned extrusion ("lobes") with faster rates of lateral material transport in different parts of the orogen. The background thermal regime or T/depth ratio in the Upper-Plate is typically 18-23 ºC. Significantly higher T/depth ratios are recorded in the Zanskar profile and Beas profile in NW India, the whole East Nepal region between Makalu and Tamor profiles and Sikkim profile immediately to the east. These high-grade lobes are confirmed by metamorphic mapping of different parts of the Himalayas. These four regions experienced high peak metamorphic temperatures into the granulite facies, at relatively low pressures, and are interpreted to be lobes that experienced higher extrusion rates.
It is interpreted that the hot rocks of the Upper-Plate continually re-equilibrated as they extruded southward and upward into shallower crustal levels. Regions experiencing high extrusion rates were able to maintain higher temperatures up into shallower crustal levels. Consequently, when re-equilibration and re-crystallization of the matrix assemblage was finally blocked, mineral parageneses recorded lower pressures and thus higher T/depth ratios compared to slower extruding domains. This extrusion process was unique to the Upper-Plate and different to the out-wedging process experienced during extensional telescoping of the Lower-Plate. This is because the high metamorphic grades in the Upper-Plate ensured that peak metamorphic conditions persisted over a significant period of time (27 to 18 Ma), as advection outpaced conductive cooling, and this is illustrated by the isobaric cooling trajectories. By comparison, there is relatively little along orogen variation in T/depth ratio in the inverted Barrovian series metamorphics of the Main Central Thrust Zone and the Footwall. In the Lower-Plate, peak metamorphism was terminated by very rapid exhumation during out-wedging and this is illustrated by the isothermal decompression trajectories. In this scenario conduction is effectively decoupled from advection and peak metamorphic parageneses accurately record the maximum PT conditions experienced, with little re-equilibration possible during the fast advective termination of metamorphism.
One (among many) plausible explanation for heterogeneous extrusion rates in the Upper-Plate is variation in the large-scale crustal architecture, which ultimately depends on many causal features, such as variation in basin thickness and crustal geometries in the impinging Indian Plate passive margin. Along-orogen variation in the stratigraphic and structural level of the Upper-Plate/Lower-Plate boundary indicates that the fundamental architecture of the Himalayan Metamorphic Front does vary significantly along the length of the Himalayan Orogen. The Upper-Plate/Lower-Plate boundary occurs at the High Himal Thrust at high structural levels within the Greater Himalayan Sequence in East Nepal, Sikkim and Bhutan regions. In contrast, the Upper-Plate/Lower-Plate boundary is located at the Main Central Thrust at lower structural levels, in the vicinity of the unconformity between the Greater- and Lesser Himalayan Sequences, in the central and western Nepal region. This is illustrated by variation in thickness of Greater Himalayan Sequence that is contained within the Main Central Thrust Zone below the Upper-Plate/Lower-Plate boundary, which increases from 1000-2000 m in central Nepal to 4300-6000 m in East Nepal, Sikkim and Bhutan. This rapid change in crustal architecture occurs over a distance of <100 km and coincides with the transition into the East Nepal to Bhutan extrusional lobe identified by metamorphic field gradients. The large dimensions of this extrusional lobe (~400 km wide) suggest the ultimate cause is rooted in systematic change in large-scale first-order variables such as erosion rates (and rainfall) at the orogenic front, crustal strength and thickness, distribution of heat-producing radiogenic elements, basin thickness and original crustal plate geometry.
Unlike the Upper-Plate, T and T/depth do not vary significantly along the length of the Lower-Plate. However, pressures vary significantly, reflecting variation in the crustal depth exhumed during extensional telescoping of the Lower-Plate. Regions that show relatively high-pressures in the Lower-Plate are; the Langtang profile, the entire East Nepal region and both east and west Syntaxes (Nanga Parbat and Namche Barwa). These same regions also show the largest pressure differential between the two crustal wedges in the Lower-Plate: Outwedge II (Main Central Thrust Zone) and Outwedge III (Footwall). The Footwall was exhumed from deeper levels and records pressures 2.8-4.0 kb greater than the Main Central Thrust Zone in East Nepal and 1.5-1.7 kb in Sikkim-Bhutan. These pressure drops into the crustal wedge at higher structural level, confirms extensional telescoping of the Lower-Plate and extensional reactivation of the basal Main Central Thrust during late-stage out-wedging of the Footwall. This pattern of variation in depth of crust exhumed both by the Lower-Plate as a whole and by the Footwall in particular, indicate that the rate of out-wedging was greatest in the East Nepal to Bhutan region. Lower-Plate exhumation occurred by rapid out-wedging and extensional telescoping of the metamorphic front in isostatic response to over-loading and crustal over-thickening. This East Nepal to Bhutan region also coincides with the region of highest extrusion rate in the Upper-Plate. Consequently, the cause of relatively high rates of Upper-Plate extrusion and Lower-Plate out-wedging are unlikely to be independent. It is probable that a number of features coincided in this region to bring about high rates of Upper-Plate extrusion: such as high rainfall and erosion, thick basin sequences and original crustal plate geometry. Once initiated, fast extrusion of the particularly thick Upper-Plate in this region, would have precipitated both deeper burial and consequent rapid out-wedging of the Lower-Plate immediately below.
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