Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence, Annapurna Himalaya, Central Nepal: Insights from titanite petrochronology

Abstract

The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite 40Ar/39Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.

Protracted Shearing at Midcrustal Conditions During Large‐Scale Thrusting in the Scandinavian Caledonides

“…Additionally, titanite was stable during deformation without any evidence of dissolution‐precipitation processes (e.g., lobate or peninsular edges, mineral inclusions marking transient porosity; Putnis, 2015), and EBSD data show limited or no evidence of deformation by dislocation creep. As these two deformation mechanisms could affect titanite age dating results, their absence, determined from microstructural observations, further supports our interpretation (Papapavlou et al, 2017; Walters & Kohn, 2017).…”

Modeling High‐Resolution Pressure‐Temperature Paths Across the Himalayan Main Central Thrust (Central Nepal): Implications for the Dynamics of Collision

“…The conventional T data and QuiG P for estimated for GHC sample MA45 (Table ) more closely match the peak conditions suggested by the trajectories for hanging wall rocks. The South Tibetan Detachment System or other internal structures recently proposed to be present in the GHC (e.g., Ambrose et al, ; Hodges et al, ; Iaccarino et al, ; Imayama et al, ; Walters & Kohn, ) would only disturb the thermal structure of the hanging wall and have no significant influence on the P‐T paths taken by the footwall samples. These structures, if present along this transect, do not influence the P‐T conditions of GHC sample MA24, which remains at isothermal during its growth period.…”

20 years of geological mapping of the metamorphic core across Central and Eastern Himalayas

“…In the Marshyangdi valley (between the Mt. Manalsu and the Annapurna Range) two strands of the HHD have been recently recognized: un upper one located in the uppermost GHS (Walters and Kohn, 2017) and a lower one, in the mid part of the GHS, at the base of the sillimanite-in isograd (Carosi et al, 2017).…”

“…According to Walters and Kohn (2017) the uppermost shear zone could be connected to the HHD by a lateral ramp or non-continual thrust planes (Corrie and Kohn, 2011). Ambrose et al (2015) and Larson et al (2015) determining different monazite ages of the exhumation of the rocks of the GHS, identified and mapped out-of-sequence-thrusts (OOST) corresponding to the High Himalayan Thrust of Goscombe et al (2006) in Eastern Nepal.…”