12-3 Middle Eocene to Oligocene exhumation of the Tso Morari ultrahigh pressure nappe from mid-crustal depths (Ladakh, northwestern Indian Himalaya): insights from multi-technique thermochronometry

Session: The Geodynamic Evolution of the Himalaya: From Mountain Building to Modern Seismicity and Climate Change

Presenting Author:

Adelie Ionescu

Authors:

Ionescu, Adelie¹, Long, Sean Patrick², Kohn, Matthew J.³, Thomson, Stuart N.⁴, Konrad, Kevin⁵, Heizler, Matthew T.⁶, Thompson, Jayna⁷

(1) Washington State University, School of the Environment, Pullman, WA, USA, (2) Washington State University, Pullman, WA, USA, (3) Boise State University, Geosciences, Boise, ID, USA, (4) University of Arizona, Dept of Geosciences, Tucson, AZ, USA, (5) Nevada Isotope Geochronology Laboratory, University of Las Vegas, Las Vegas, NV, USA, (6) New Mexico Bureau of GeologyNew Mexico, Socorro, NM, USA, (7) Boise State University, Boise, ID, USA,

Abstract:

Documenting the processes that drive high-magnitude exhumation is crucial for investigating the dynamics of orogenesis. Ultrahigh-pressure (UHP) nappes can provide insight into exhumation from upper mantle depths; however, they can experience complex exhumation paths, which can make it difficult to deconvolve the relative importance of exhumation processes including buoyancy, continental underthrusting, and fold-thrust shortening. To investigate this problem in the NW Indian Himalaya, we integrate new and published thermochronometry from the Tso Morari UHP nappe, overlying nappes to the southwest, and the Indus suture zone to the northeast, to constrain time-temperature (t-T) paths. Previous work shows that the Tso Morari nappe experienced UHP metamorphism at ~90-100 km depths at ~55-49 Ma, and was rapidly exhumed to ~26-44 km depths by ~48-44 Ma, where it underwent amphibolite-facies metamorphism at ~600-700 °C. Our t-T paths show that the nappe then cooled rapidly to ~300–100 °C by ~38–33 Ma (34–50 °C/Myr), followed by slower cooling (2–8 °C/Myr) to the present. Overlying nappes cooled from ~600–425 °C at ~45–43 Ma to ~200–125 °C by ~33–30 Ma (21–43 °C/Myr), followed by slower cooling (3–5 °C/Myr), and Indus suture zone units cooled from ~550–425 °C at ~45–43 Ma to ~335–225 °C at ~37–32 Ma (18–33 °C/Myr), followed by slower cooling (3–10 °C/Myr). Our t-T results, when integrated with published depth reconstructions, show that: 1) the Tso Morari nappe was rapidly exhumed from ~44-26 km to ~18 km depth between ~48-43 Ma (1.6-5.2 mm/yr), which was likely accomplished by distributed, normal-sense ductile shearing; 2) After ~43 Ma, the entire region cooled as a coherent package, and was exhumed from ~18 km to ~10-3 km depth by ~38-30 Ma (0.6-3.0 mm/yr). We interpret that this was the result of uplift generated by underthrusting of thick Indian middle-lower crust, as dated by prograde metamorphism of Greater Himalayan rocks between ~42-30 Ma; 3) The entire region was slowly exhumed from ~10-3 km depth between ~38-30 Ma and the present (0.1-0.3 mm/yr). We interpret that this represents a transition to passive southwestward transport as the deformation front migrated foreland-ward during emplacement of Greater Himalayan rocks and construction of the Lesser Himalayan-Subhimalayan thrust belt over the past ~25 Ma.

 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8504

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