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Changes in timing and style of syn-collisional exhumation across the Himalaya: Evidence from western Bhutan

English title	Changes in timing and style of syn-collisional exhumation across the Himalaya: Evidence from western Bhutan
Applicant	Regis Daniele
Number	142167
Funding scheme	Fellowships for prospective researchers
Research institution	Department of Earth Sciences The Open University
Institution of higher education	Institution abroad - IACH
Main discipline	Geology
Start/End	01.08.2012 - 31.07.2013

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All Disciplines (4)

Discipline

Geology

Mineralogy

Geochronology

Geochemistry

Keywords (5)

Jomolhari Massif; Himalayan Orogen; GHS high-grade metamorphic core; “Petro”chronology and analytical geochemistry; Exhumation mechanisms

Lay Summary (English)

Lead

Lay summary
The world’s greatest mountain belts form when tectonic plates collide. Our understanding of crustal deformation during collisions is underpinned by geochronologic, petrologic and thermobarometric studies of rocks that have been deformed and transformed. Despite increasingly precise data, the processes driving the transportation, deformation and transformation of rocks at different temporal and spatial scales are still poorly understood; these are important for understanding the chemistry and physics of crustal recycling and crust-mantle interactions. The Himalayan orogen is the archetypical example of continental collision, and exhumed metamorphic rocks within the orogen thus provide samples for investigating how continental crust becomes transported, deformed and transformed. The overall aim of this project is to investigate temporal and spatial changes in exhumation rates and mechanisms of some of the most deeply buried rocks from the central Himalaya towards the eastern termination in order to determine the changing behaviour of deeply buried crust in continental collision zones. The metamorphic core (forming the Greater Himalayan Sequence or GHS) of the central Himalaya forms a semi-continuous unit that has exhumed southwards during the Miocene between the overlying normal-sense South Tibetan Detachment (STD) and the underlying Main Central Thrust (MCT). The prevailing, but not universally accepted exhumation mechanism, “channel flow”, suggests that partially molten mid-orogenic crust flowed, and was exhumed, southwards as a result of topographic pressure forcing due to the elevation of the Tibetan Plateau, and focussed erosion at the Himalayan front. This model is based on data from the GHS in Nepal, where peak amphibolite-facies metamorphic conditions (650-750°C and 0.8-1.3 GPa) and peak metamorphic monazite ages (16 - >30 Ma) both increase upwards through the unit. In contrast, the GHS in Bhutan appears to consist of multiple terranes with major metamorphic breaks accommodated by tectonic structures. Recent work by scientists at the Open University and co-workers in Canada has led to the identification of a new unit eclogite and granulite-bearing unit in NW Bhutan with a distinct and unique tectonic history compared with the central Himalayan GHS. A major out-of-sequence thrust, the Kakthang Thrust, accommodates late-orogenic extrusion of this terrane over older amphibolite-facies rocks. Furthermore, recent preliminary chronological data from the Jomolhari massif suggest that it forms a third unit with distinctive metamorphic history within the Bhutan GHS. The formation and exhumation history of these three disparate units is not explained by the “channel flow” hypothesis. Importantly, these rocks include high pressure assemblages which challenge currently accepted models for exhumation within collision zones. The precise determination of the petrological and geochronological history of the poorly mapped and sampled Jomolhari Massif, and detailed comparison between it and other two units in the Bhutan GHS is important for determining spatial and temporal changes in exhumation rate and mechanism from the central to the eastern Himalaya. This project therefore aims to define the detailed pressure-temperature-time evolution, structural history and protolith affinity of the Jomolhari Massif and to explore the significance of the data in terms of constraining exhumation mechanisms of deeply buried crustal rocks within collision zones. The dataset obtained during this project will not only help constrain models of evolution of the Himalayan orogen in particular but also help to constrain the behaviour of deeply buried crust in continental collision zones in general. This project will be based at the Open University where pioneering methods for exploring exhumation processes of high pressure rocks in general and NW Bhutan in particular are being developed. The combination of well-equipped laboratories, experienced staff and my skills in petrology, geochemistry and geochronology that I have successfully applied to western Alpine tectonic problems during my PhD project, make the OU the perfect host institute for the proposed research program.

Lead

Lay summary

The world’s greatest mountain belts form when tectonic plates collide. Our understanding of crustal deformation during collisions is underpinned by geochronologic, petrologic and thermobarometric studies of rocks that have been deformed and transformed. Despite increasingly precise data, the processes driving the transportation, deformation and transformation of rocks at different temporal and spatial scales are still poorly understood; these are important for understanding the chemistry and physics of crustal recycling and crust-mantle interactions. The Himalayan orogen is the archetypical example of continental collision, and exhumed metamorphic rocks within the orogen thus provide samples for investigating how continental crust becomes transported, deformed and transformed. The overall aim of this project is to investigate temporal and spatial changes in exhumation rates and mechanisms of some of the most deeply buried rocks from the central Himalaya towards the eastern termination in order to determine the changing behaviour of deeply buried crust in continental collision zones.

The metamorphic core (forming the Greater Himalayan Sequence or GHS) of the central Himalaya forms a semi-continuous unit that has exhumed southwards during the Miocene between the overlying normal-sense South Tibetan Detachment (STD) and the underlying Main Central Thrust (MCT). The prevailing, but not universally accepted exhumation mechanism, “channel flow”, suggests that partially molten mid-orogenic crust flowed, and was exhumed, southwards as a result of topographic pressure forcing due to the elevation of the Tibetan Plateau, and focussed erosion at the Himalayan front. This model is based on data from the GHS in Nepal, where peak amphibolite-facies metamorphic conditions (650-750°C and 0.8-1.3 GPa) and peak metamorphic monazite ages (16 - >30 Ma) both increase upwards through the unit.

In contrast, the GHS in Bhutan appears to consist of multiple terranes with major metamorphic breaks accommodated by tectonic structures. Recent work by scientists at the Open University and co-workers in Canada has led to the identification of a new unit eclogite and granulite-bearing unit in NW Bhutan with a distinct and unique tectonic history compared with the central Himalayan GHS. A major out-of-sequence thrust, the Kakthang Thrust, accommodates late-orogenic extrusion of this terrane over older amphibolite-facies rocks. Furthermore, recent preliminary chronological data from the Jomolhari massif suggest that it forms a third unit with distinctive metamorphic history within the Bhutan GHS.

The formation and exhumation history of these three disparate units is not explained by the “channel flow” hypothesis. Importantly, these rocks include high pressure assemblages which challenge currently accepted models for exhumation within collision zones. The precise determination of the petrological and geochronological history of the poorly mapped and sampled Jomolhari Massif, and detailed comparison between it and other two units in the Bhutan GHS is important for determining spatial and temporal changes in exhumation rate and mechanism from the central to the eastern Himalaya.

This project therefore aims to define the detailed pressure-temperature-time evolution, structural history and protolith affinity of the Jomolhari Massif and to explore the significance of the data in terms of constraining exhumation mechanisms of deeply buried crustal rocks within collision zones. The dataset obtained during this project will not only help constrain models of evolution of the Himalayan orogen in particular but also help to constrain the behaviour of deeply buried crust in continental collision zones in general.

This project will be based at the Open University where pioneering methods for exploring exhumation processes of high pressure rocks in general and NW Bhutan in particular are being developed. The combination of well-equipped laboratories, experienced staff and my skills in petrology, geochemistry and geochronology that I have successfully applied to western Alpine tectonic problems during my PhD project, make the OU the perfect host institute for the proposed research program.

Direct link to Lay Summary

Last update: 21.02.2013

Responsible applicant and co-applicants

Name	Institute

Regis Daniele

Department of Earth Sciences The Open University

Collaboration

Group / person	Country

	Types of collaboration

The University of Texas at San Antonio

United States of America (North America)

- Publication

NERC Isotope Geosciences Laboratory

Great Britain and Northern Ireland (Europe)

- Publication
- Research Infrastructure

Scientific events

Self-organised

Title	Date	Place

UK Himalaya-Tibet Meeting

06.03.2013

The Open University

Abstract

The Himalayan orogen is the archetypical example of continental collision, and exhumed metamorphic rocks within the orogen thus provide samples for investigating how continental crust becomes transported, deformed and transformed. The overall aim of this project is to investigate temporal and spatial changes in exhumation rates and mechanisms of some of the most deeply buried rocks from the central Himalaya towards the eastern termination in order to determine the changing behaviour of deeply buried crust in continental collision zones. The metamorphic core (forming the Greater Himalayan Sequence or GHS) of the central Himalaya forms a semi-continuous unit that has exhumed southwards during the Miocene between the overlying normal-sense South Tibetan Detachment (STD) and the underlying Main Central Thrust (MCT). The prevailing, but not universally accepted exhumation mechanism, “channel flow”, suggests that partially molten mid-orogenic crust flowed, and was exhumed, southwards as a result of topographic pressure forcing due to the elevation of the Tibetan Plateau, and focussed erosion at the Himalayan front. This model is based on data from the GHS in Nepal, where peak amphibolite-facies metamorphic conditions (650-750°C and 0.8-1.3 GPa) and peak metamorphic monazite ages (16 - >30 Ma) both increase upwards through the unit. In contrast, the GHS in Bhutan appears to consist of multiple terranes with major metamorphic breaks accommodated by tectonic structures. Recent work by scientists at the Open University and co-workers in Canada has led to the identification of a new unit eclogite and granulite-bearing unit in NW Bhutan with a distinct and unique tectonic history compared with the central Himalayan GHS. A major out-of-sequence thrust, the Kakthang Thrust, accommodates late-orogenic extrusion of this terrane over older amphibolite-facies rocks. Furthermore, recent preliminary chronological data from the Jomolhari massif suggest that it forms a third unit with distinctive metamorphic history within the Bhutan GHS. The formation and exhumation history of these three disparate units is not explained by the “channel flow” hypothesis. Importantly, these rocks include high pressure assemblages which challenge currently accepted models for exhumation within collision zones. The precise determination of the petrological and geochronological history of the poorly mapped and sampled Jomolhari Massif, and detailed comparison between it and other two units in the Bhutan GHS is important for determining spatial and temporal changes in exhumation rate and mechanism from the central to the eastern Himalaya. This project therefore aims to define the detailed pressure-temperature-time evolution, structural history and protolith affinity of the Jomolhari Massif and to explore the significance of the data in terms of constraining exhumation mechanisms of deeply buried crustal rocks within collision zones. The dataset obtained during this project will not only help constrain models of evolution of the Himalayan orogen in particular but also help to constrain the behaviour of deeply buried crust in continental collision zones in general. This project will be based at the Open University where pioneering methods for exploring exhumation processes of high pressure rocks in general and NW Bhutan in particular are being developed. The combination of well-equipped laboratories, experienced staff and my skills in petrology, geochemistry and geochronology that I have successfully applied to western Alpine tectonic problems during my PhD project, make the OU the perfect host institute for the proposed research program.