Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

Abstract

[1] The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last $20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°-550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°-50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3-4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m 2 /yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.

K-Ar Dating of White Mica from the Lesser Himalaya, Tansen-Pokhara Section, Central Nepal: Implications for the Timing of Metamorphism

“…622). This trend, i.e., ages getting older southward, is quite similar to the trend shown by the 40 Ar/ 39 Ar Ms ages as a function of distance along a N018ºE section in central Nepal (Bollinger et al 2004) (Fig. 4).…”

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…Estimates for the displacement along the Main Central thrust are between 140 and 200 km [59], the last 30 km of which occurred in the past 3 Ma according to [66]. These authors, as well as [67,68] also suggest that the inverse metamorphic zonation observed at the Main Central thrust is the result of basal accretion of slices in the footwall of the thrust in the course of the Miocene-Pliocene phase of its activity.…”

Pattern of Temperature Structures in Himalayan Subduction Zone: A Review

“…Hossain & Rezaul Similar observation from other active mountain belts has been accomplished at different subduction zones in the world like the Aleutian-Alaska subduction zone (Abers et al 2005), Costa Rica-Nicaragua subduction zone (Peacock et al 2004), Kamchatka subduction zone (Portnyagin and Manea 2008), Cocos plate under the Caribbean plate at the Nicoya Peninsula, Costa Rica (Harris and Wang 2002), Central Andean subduction zone (Springer and Föster 1998) but in our study area this type of approximation is not available, except some of related studies (e.g., Bollinger et al 2004;Bilham et al 1997a;Bilham 2008;Sridevi 2004;Catherine 2004). So, these observations facilitate to provide sufficient interest to review the temperature structures in Himalayan subduction zone.…”