Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

Abstract

The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non‐coaxial shear with a top‐to‐the‐south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non‐coaxial shear with a top‐to‐the‐north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post‐date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust.

The geology of the Tama Kosi and Rolwaling valley region, East-Central Nepal

“…Similar culminations in Nepal and Tibet have been interpreted to refl ect outof-sequence rebuilding of the orogenic wedge after the early to middle Miocene southward extrusion of the mid-crust (Godin et al, 2006b;Larson et al, 2010b). In this scenario, the Tama Kosi window would have been developed to facilitate the foreland migration of deformation from the Main Central thrust to the Main Boundary thrust after the metamorphic core on May 9, 2012 geosphere.gsapubs.org Downloaded from was extruded (e.g., Larson et al, 2010b).…”

“…Similar culminations in Nepal and Tibet have been interpreted to refl ect outof-sequence rebuilding of the orogenic wedge after the early to middle Miocene southward extrusion of the mid-crust (Godin et al, 2006b;Larson et al, 2010b). In this scenario, the Tama Kosi window would have been developed to facilitate the foreland migration of deformation from the Main Central thrust to the Main Boundary thrust after the metamorphic core on May 9, 2012 geosphere.gsapubs.org Downloaded from was extruded (e.g., Larson et al, 2010b). Alternatively, the Tama Kosi window may be due to tectonic inversion of local, structurally controlled sedimentary thickness variations (Long et al, 2011), lateral ramp structures (Johnson, 1994), or localized increased erosion along river valley bottoms (Montgomery and Stolar, 2006).…”

Large normal-sense displacement on the South Tibetan fault system in the eastern Himalaya

“…Remote sensing studies such as this have been applied previously to map granite and gneiss domes of the Himalaya (Watts et al, 2005;Larson et al, 2010;Bertoldi et al, 2011), but have not thus far been used to map the position of the STFS.…”

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“…Eocene-Oligocene crustal thickening within the Tethyan Himalaya (Godin et al, 1999;Godin et al, 2001;Lee et al, 2000;Aoya et al, 2005;Kellett and Godin, 2009;Aikman et al, 2008) is widely thought to have induced prograde metamorphism and anatectic melting within Greater Himalayan rocks (Godin et al, 2006;Aoya et al, 2005;Lee and Whitehouse, 2007;Larson et al, 2010). Our interpretation of Middle Eocene to Early Oligocene crustal thickening of the Gangdese arc implies that the orogenic wedge at this time extended from the Tethyan Himalaya in the south, northward across the India-Asia suture zone to the Gangdese batholith.…”

“…These processes compete to maintain a self-similar wedge geometry described by the taper angle (e.g., Dahlen, 1990). The fi rst-order structural elements in the central Himalaya have recently been integrated by Larson et al (2010) into a conceptual model describing orogenic wedge develop-ment characterized by two cycles of accumulation (increase in taper angle) and dissipation (decrease in taper angle) of gravitational potential energy (Fig. 9).…”