Thermochronology of the Main Central Thrust system, Nepal
Models that implicate flow of semi-viscous midcrustal rocks south towards the front of the Himalayan orogen, 'channel flow', are able to account for many geologic observations in the Himalaya. However, alternativemodels of collision, particularly 'thrust-wedge taper', demonstrate that the observed geology could have formed in the absence of a low-viscosity midcrustal layer. Whether channel flow and thrust-wedge taper models are mutually exclusive or whether they represent a continuum of deformation in time and/or space within a single collisionalsystem is of crucial importance for explaining the evolution of the Himalayan orogen and, by extension, for understanding the evolution of Earth’s many continent-continent collision zones. A key difference in these contrasting models is the location, nature and relationships of thrust faults within the Himalayan foreland; specifically, the spatial and kinematic association of the Main Central Thrust and the Ramargh Thrust. In the channel flow model of the Himalaya, exhumed midcrustal rocks are predicted to act as a pervasively-deformed tectonic package bounded at its base by a single major fault; the Ramgarh and Main Central thrust are synthetic and coeval components of that diffuse structure. In contrast, thrust-wedge taper models interpret the Main Central thrust and Ramgarh thrust to be kinematically distinct, and formed in sequence within a foreland propagating fold and thrust belt.
Funding:
•NSF-EAR-1119380 to J. Cottle: 07/01/11 - 06/30/14 “How Does the Mid-crust Accommodate Deformation in Large, Hot Collisional Orogens? Insight from the Himalaya-Tibet System” NSF Abstract
Publications:
-Larson, K.P., Cottle, J.M., Godin, L. 2011. Geochronologic record of the upper Greater Himalayan sequence, Manaslu-Himal Chuli Himalaya, west-central Nepal. GSA Lithosphere. Larson_etal_2011_Lithosphere.
Collaborators:
Dr Kyle Larson (University of Saskatchewan)
Dr. Dawn Kellett (Canadian Geological Survey)
Dr. Santa Man Rai (Tribhuvan University, Kathmandu)
Dr. Malay Mukul (Indian Institute of Technology, Mumbai)
Funding:
•NSF-EAR-1119380 to J. Cottle: 07/01/11 - 06/30/14 “How Does the Mid-crust Accommodate Deformation in Large, Hot Collisional Orogens? Insight from the Himalaya-Tibet System” NSF Abstract
Publications:
-Larson, K.P., Cottle, J.M., Godin, L. 2011. Geochronologic record of the upper Greater Himalayan sequence, Manaslu-Himal Chuli Himalaya, west-central Nepal. GSA Lithosphere. Larson_etal_2011_Lithosphere.
Collaborators:
Dr Kyle Larson (University of Saskatchewan)
Dr. Dawn Kellett (Canadian Geological Survey)
Dr. Santa Man Rai (Tribhuvan University, Kathmandu)
Dr. Malay Mukul (Indian Institute of Technology, Mumbai)
Orogen Parallel Extension in the Himalaya
The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that protrudes70 kilometers north from the crest of the south Tibetan Himalaya and offsets the position of the South Tibetan Detachment system (STDS). A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the Mid- to Late-Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma. and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland.
The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite faciesmetamorphism at 750°C and 0.7-0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with syn-kinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by post-kinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated dataset indicates that high-temperature metamorphism, decompression and exhumation of the ADM post-dates mid-Miocene south-directed mid-crustal extrusion and is kinematically linked to orogen-parallel crustal-scale extension.
The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite faciesmetamorphism at 750°C and 0.7-0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with syn-kinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by post-kinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated dataset indicates that high-temperature metamorphism, decompression and exhumation of the ADM post-dates mid-Miocene south-directed mid-crustal extrusion and is kinematically linked to orogen-parallel crustal-scale extension.
Leo Pargil Dome, NW India & Ama Drime Complex, South Tibet
The Himalaya provide an exceptional opportunity to investigate the evolution of mid-crustal rocks in an active collisional system. Along the collisional front, the major structures that bound these mid-crustal rocks, the Main Central thrust belowand South Tibetan detachment above, are offset or reactivated by young metamorphic domes like the Leo Pargil, the focus of this study. Such domes provide an important opportunity to study the generation of partial melt, the potential feedback between partial melt and exhumation, the affects of shear zone nucleation, and the rheology of the crust during dome formation. The high-grade rocks of the Leo Pargil dome have been exhumed during extension and dome formation within an active convergent setting along orogen-parallel thrust faults and low angle detachment faults that were active until the middle Miocene. In collaboration with US and international academic organizations this project will employ a field-and lab-based interdisciplinary investigation to explore the evolution of these rocks using 1) geologic mapping and detailed microstructural analysis, 2) isotope geochronology (U/Pb, Ar/Ar, [U+Th]/He), and 3) determination of metamorphic P-T-t paths. Results from Leo Pargil will be contrasted with that from other domes along the Himalayan front to generate a high-resolution model for the timing and kinematics of orogen-parallel mid-crustal flow and exhumation along the southern margin of the Tibetan plateau since the middle Miocene.
For further info on our Ama Drime & Leo Pargil Dome
see Dr. Micah Jessup’s webpage
Funding:
•NSF-EAR-0911416 to M.J Jessup & J. Cottle: 07/01/09 - 06/30/12. “Collaborative Research: Orogen-parallel mid-crustal flow and exhumation of domes along the southern margin of the Tibetan plateau”
Publications:
•Langille, J.M., Jessup M.J., Cottle J.M., Lederer, G., Ahmad, T. Accepted. Timing of metamorphism, melting, and exhumation of the Leo Pargil dome, NW India. Journal of Metamorphic Geology.
•Jessup, M.J., & Cottle, J.M. 2010. Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet. Journal of Geology
•Langille, J.M., Jessup, M.J., Cottle, J.M., Newell, D.L., & Seward, G. 2010. Kinematic evolution of the Ama Drime detachment: Insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet-Nepal. Journal of Structural Geology.
•Cottle, J.M., Jessup, M.J., Newell, D.L., Horstwood, M.S.A., Searle, M.P., Parrish, R.R., Waters, D.J., and Searle, M.P. 2009a: Geochronology of granulitized eclogite from the Ama Drime Massif: Implications for the tectonic evolution of the South Tibetan Himalaya, Tectonics, 28, TC1002, doi:10.1029/2008TC002256.
•Jessup, M.J., Newell, D.L., Cottle, J.M., Berger, A.L, & Spotila, J.A 2008. Orogen-parallel extension and exhumation enhanced by denudation in the Trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal. Geology, 36, 7 587-590.
•Newell, D.L., Jessup, M.J., Cottle, J.M., Hilton, D.R., Fischer, T. & Sharp, Z. 2008. Geochemistry of mineral springs of the southern Tibetan Plateau, Mount Everest region; a geochemical window into three structural levels. Geochem. Geophys. Geosyst., doi:10.1029/2008GC002021,
Collaborators:
Dr. Micah Jessup (University Tennessee, Knoxville)
Dr. Dennis Newell (Utah State)
For further info on our Ama Drime & Leo Pargil Dome
see Dr. Micah Jessup’s webpage
Funding:
•NSF-EAR-0911416 to M.J Jessup & J. Cottle: 07/01/09 - 06/30/12. “Collaborative Research: Orogen-parallel mid-crustal flow and exhumation of domes along the southern margin of the Tibetan plateau”
Publications:
•Langille, J.M., Jessup M.J., Cottle J.M., Lederer, G., Ahmad, T. Accepted. Timing of metamorphism, melting, and exhumation of the Leo Pargil dome, NW India. Journal of Metamorphic Geology.
•Jessup, M.J., & Cottle, J.M. 2010. Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet. Journal of Geology
•Langille, J.M., Jessup, M.J., Cottle, J.M., Newell, D.L., & Seward, G. 2010. Kinematic evolution of the Ama Drime detachment: Insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet-Nepal. Journal of Structural Geology.
•Cottle, J.M., Jessup, M.J., Newell, D.L., Horstwood, M.S.A., Searle, M.P., Parrish, R.R., Waters, D.J., and Searle, M.P. 2009a: Geochronology of granulitized eclogite from the Ama Drime Massif: Implications for the tectonic evolution of the South Tibetan Himalaya, Tectonics, 28, TC1002, doi:10.1029/2008TC002256.
•Jessup, M.J., Newell, D.L., Cottle, J.M., Berger, A.L, & Spotila, J.A 2008. Orogen-parallel extension and exhumation enhanced by denudation in the Trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal. Geology, 36, 7 587-590.
•Newell, D.L., Jessup, M.J., Cottle, J.M., Hilton, D.R., Fischer, T. & Sharp, Z. 2008. Geochemistry of mineral springs of the southern Tibetan Plateau, Mount Everest region; a geochemical window into three structural levels. Geochem. Geophys. Geosyst., doi:10.1029/2008GC002021,
Collaborators:
Dr. Micah Jessup (University Tennessee, Knoxville)
Dr. Dennis Newell (Utah State)
Evolution of the South Tibetan Detachment System
Current geodynamic models that attempt to describe the exhumation of the Himalayan mid-crust invoke a Channel flow mechanism in which a channel of low viscosity material (the Greater Himalaya Series (GHS)) is extruded from mid-crustal depths southward from beneathTibet. Critical to testing Channel Flow models and understanding the exhumation of the GHS is a detailed knowledge of the bounding structures of this system. This study focused on the tectonic feature defining the top of the slab - the South Tibetan Detachment System (STDS). Field, petrological, thermobarometric and geochronological data from the Dzakaa Chu section of the STDS suggests this system progressed from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(-Th)-Pb dating of post-kinematic leucogranites indicate that, in the lower part of the shear zone, mylonitic fabric development occurred prior to c.20 Ma. By integrating structural and geochronological evidence it is argued that the Dzakaa Chu STDS represents a deeper structural position than elsewhere exposed in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km-wide shear zone without discrete brittle detachments. These findings are an important step toward understanding the development of low-angle detachment fault systems active during continental collision.
Publications:
•Cottle, J.M., Waters, D.J., Riley, D., Beyssac, O., Jessup, M.J. Accepted. Metamorphic history of the South Tibetan Detachment System, Mt. Everest Region, Revealed by RSCM Thermometry and Phase Equilibria Modeling. Journal of Metamorphic Geology.
•Cottle, J.M., Jessup, M.J., Newell, D.L., Searle, M.P., Law, R.D. Horstwood, M.S.A. 2007. Structural insights into the early stages of exhumation along an orogen-scale detachment: the South Tibetan Detachment System, Dzakaa Chu section, Eastern Himalaya. Journal of Structural Geology, 29(11), p.1781-1797 doi:10.1016/j.jsg.2007.08.007
•Cottle, J.M., Searle, M.P., Horstwood, M.S.A., and Waters, D.J. 2009b: Timing of Mid-crustal metamorphism, melting and deformation in the Mt. Everest region of southern Tibet revealed by U(-Th)-Pb geochronology. Journal of Geology vol. 117, no. 6 doi: 10.1086/605994
•Jessup, M.J., & Cottle, J.M. 2010. Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet. Journal of Geology
•Kellett, D.A., Grujic, D., Warren, C., Cottle, J.M., Jamieson, R.,& Tenzin, T. 2010. Metamorphic history of a syn-convergent orogen-parallel detachment: The South Tibetan detachment system, Bhutan Himalaya. Journal of Metamorphic Geology. doi:10.1111/j.1525-1314.2010.00893.x
•Jessup, M.J., Cottle, J.M., Searle, M.P., Law, R.D., Tracy, R.J., Newell, D.L. & Waters, D.J. 2008. P-T-t-D paths of the Everest Series schist, Nepal. Journal of Metamorphic Geology. doi: 10.1111/j.1525-1314.2008.00784.x
Collaborators:
Dr. Micah Jessup (University Tennessee, Knoxville)
Dr. Dennis Newell (LANL)
Prof. Mike Searle (University of Oxford)
Prof. Rick Law (Virginia Tech)
Dr. Dave Waters (University of Oxford)
Dr. Olivier Beyssac (Ecole Normale Supérieure, Paris)
Note: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation
Publications:
•Cottle, J.M., Waters, D.J., Riley, D., Beyssac, O., Jessup, M.J. Accepted. Metamorphic history of the South Tibetan Detachment System, Mt. Everest Region, Revealed by RSCM Thermometry and Phase Equilibria Modeling. Journal of Metamorphic Geology.
•Cottle, J.M., Jessup, M.J., Newell, D.L., Searle, M.P., Law, R.D. Horstwood, M.S.A. 2007. Structural insights into the early stages of exhumation along an orogen-scale detachment: the South Tibetan Detachment System, Dzakaa Chu section, Eastern Himalaya. Journal of Structural Geology, 29(11), p.1781-1797 doi:10.1016/j.jsg.2007.08.007
•Cottle, J.M., Searle, M.P., Horstwood, M.S.A., and Waters, D.J. 2009b: Timing of Mid-crustal metamorphism, melting and deformation in the Mt. Everest region of southern Tibet revealed by U(-Th)-Pb geochronology. Journal of Geology vol. 117, no. 6 doi: 10.1086/605994
•Jessup, M.J., & Cottle, J.M. 2010. Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet. Journal of Geology
•Kellett, D.A., Grujic, D., Warren, C., Cottle, J.M., Jamieson, R.,& Tenzin, T. 2010. Metamorphic history of a syn-convergent orogen-parallel detachment: The South Tibetan detachment system, Bhutan Himalaya. Journal of Metamorphic Geology. doi:10.1111/j.1525-1314.2010.00893.x
•Jessup, M.J., Cottle, J.M., Searle, M.P., Law, R.D., Tracy, R.J., Newell, D.L. & Waters, D.J. 2008. P-T-t-D paths of the Everest Series schist, Nepal. Journal of Metamorphic Geology. doi: 10.1111/j.1525-1314.2008.00784.x
Collaborators:
Dr. Micah Jessup (University Tennessee, Knoxville)
Dr. Dennis Newell (LANL)
Prof. Mike Searle (University of Oxford)
Prof. Rick Law (Virginia Tech)
Dr. Dave Waters (University of Oxford)
Dr. Olivier Beyssac (Ecole Normale Supérieure, Paris)
Note: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation