Exhumation history of eastern Ladakh revealed by ⁴⁰ Ar/ ³⁹ Ar and fission-track ages: the Indus River–Tso Morari transect, NW Himalaya

Abstract

Fission-track and 40 Ar/ 39 Ar ages place time constraints on the exhumation of the North Himalayan nappe stack, the Indus Suture Zone and Molasse, and the Transhimalayan Batholith in eastern Ladakh (NW India). Results from this and previous studies on a north-south transect passing near Tso Morari Lake suggest that the SW-directed North Himalayan nappe stack (comprising the Mata, Tetraogal and Tso Morari nappes) was emplaced and metamorphosed by c. 50-45 Ma, and exhumed to moderately shallow depths (c. 10 km) by c. 45-40 Ma. From the mid-Eocene to the present, exhumation continued at a steady and slow rate except for the root zone of the Tso Morari nappe, which cooled faster than the rest of the nappe stack. Rapid cooling occurred at c. 20 Ma and is linked to brittle deformation along the normal Ribil-Zildat Fault concomitant with extrusion of the Crystalline nappe in the south. Data from the Indus Molasse suggest that sediments were still being deposited during the Miocene.

Doming in compressional orogenic settings: New geochronological constraints from the NW Himalaya

“…As cited above, structural relationships reveal that the NE directed crustal thickening phase was followed by contraction related to the southwestward emplacement of the Nyimaling-Tsarap nappe. Geochronological data reveal that the latter ranged from the middle Eocene to the late Oligocene [Vance and Harris, 1999;Walker et al, 1999;Schlup et al, 2003]; therefore the NE directed Shikar Beh nappe emplacement must be older than middle Eocene. Our monazite ages therefore cannot correspond to the peak metamorphic conditions recorded in the migmatitic zone of the Miyar Valley section.…”

“…This event was likely coeval with the SW directed nappe tectonics (e.g., Nyimaling-Tsarap nappe) responsible for the middle Eocene low-grade metamorphism in the Tethyan Himalaya constrained by 40 Ar/ 39 Ar white mica ages (circa 45 -42 Ma [Bonhomme and Garzanti, 1991;Wiesmayr and Grasemann, 1999;Schlup et al, 2003]). This interpretation is consistent with the observations by Patel et al [1993] that the extensional ZSZ reactivates a former thrust.…”

High-pressure Tethyan Himalaya rocks along the India-Asia suture zone in southern Tibet

“…Exhumation and concomitant retrogression occurred between 46.5-31 Ma at Kaghan Valley (Wilke et al, 2010) and 47-34 Ma at Tso Morari (de Sigoyer et al, 2000;Schlup et al, 2003).…”

Rapid early Miocene exhumation of the Ladakh batholith, western Himalaya

“…U-Pb crystallisation ages from the main Batholith range from 65 to 49 Ma , suggesting that magmatism was contemporaneous with the Gangdese Batholith (Scharer et al, 1984) and that magmatic activity ceased with the collision of India and Eurasia. Apatite fission track (AFT) ages of the Ladakh Batholith from Kargil to Chumatang range from 28 to 4.8 Ma (Choubey, 1987;Sorkhabi et al, 1994;Clift et al, 2002;Schlup et al, 2003). The young Pliocene ages are related to minor faulting (Choubey, 1987), however, there remains a spread in single AFT ages from 18 to 28 Ma (Sorkhabi et al, 1994;Clift et al, 2002).…”

“…This region has a vertical extent of just over 1.3 km, from 3921 to 5245 m.a.s.l., over a horizontal distance of 10 km. The profile is between sites of previous studies (Choubey et al, 1987;Sorkhabi et al, 1994;Clift et al, 2002;Schlup et al, 2003). Only sample KHARS 5245 from the top of the profile was analysed using all techniques.…”