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

Abstract

[1] In the central and southeastern parts of the Himalayas, the High Himalayan Crystalline (HHC) high-grade rocks are mainly exhumed in the frontal part of the range, as a consequence of a tectonic exhumation controlled by combined thrusting along the Main Central Thrust (MCT) and extension along the South Tibetan Detachment System (STDS). In the NW Himalaya, however, the hanging wall of the MCT in the frontal part of the range consists mainly of lowto medium-grade metasediments (Chamba zone), whereas most of the amphibolite facies to migmatitic gneisses of the HHC of Zanskar are exposed in a more internal part of the orogen as a large-scale dome structure referred to as the Gianbul dome. This Gianbul dome is cored by migmatitic paragneisses formed at peak conditions of 800°C and 8 kbar. This migmatitic core is symmetrically surrounded by rocks of the sillimanite, kyanite ± staurolite, garnet, biotite, and chlorite mineral zones. The structural data from the Miyar-Gianbul Valley section reveal that the Gianbul dome is bounded by two major converging thrust zones, the Miyar Thrust Zone and the Zanskar Thrust Zone, which were reactivated as ductile zones of extension referred to as the Khanjar Shear Zone (KSZ) and the Zanskar Shear Zone (ZSZ), respectively. Geochronological dating of monazites from various migmatites and leucogranite in the core of the Gianbul dome indicates ages between 26.6 and 19.8 Ma. These results likely reflect a high-temperature stage of the exhumation history of the HHC of Zanskar and consequently constrain the onset of extension along both the ZSZ and the KSZ to start shortly before 26.6 Ma. Several recent models interpret that ductile extrusion of the high-grade, low-viscosity migmatites of HHC reflects combined extension along the ZSZ and thrusting along the MCT. Hence our new data constrain the onset of the thrusting along the MCT to start shortly before 26.6 Ma. Citation: Robyr, M., B. R.Hacker, and J. M. Mattinson (2006), Doming in compressional orogenic settings: New geochronological constraints from the NW Himalaya, Tectonics, 25, TC2007,

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

“…The formation and extrusion of the High Himalayan nappe occurred between 26 and 19 Ma (Frank et al, 1973(Frank et al, , 1977Kumar et al, 1995;Searle et al, 1999;Dèzes et al, 1999;Walker et al, 1999Walker et al, , 2001Robyr et al, 2002Robyr et al, , 2006. Subsequent exhumation of the High Himalayan nappe is constrained only by 40 Ar-39 Ar mica and zircon-apatite fission track studies of the Mandi granite in the frontal part of the High Himalayan nappe (Lal et al, 1999;Walia et al, 2008).…”

“…Metamorphic grade ranges from the prehnite-pumpellyite zone in the Baralacha La-upper Chandra region ( Fig. 1; Steck et al, 1993;Vannay, 1993;Steck, 2003;Epard and Steck, 2004) to the sillimanite + K-feldspar + melt zone in the High Himalayan Crystalline dome south of Zanskar (Honegger, 1983;Honegger et al, 1982;Searle et al, 1992, 1999; Robyr et al, 2002Robyr et al, , 2006. The Zanskar shear zone was intruded during its formation by the 22.2 ± 0.2 Ma Gumburanjun leucogranite (Dèzes et al, 1999).…”

“…Older monazite ages of 26.6 and 23 Ma were determined in the migmatites of the Gianbul dome below the south-plunging Khanjar Shear zone in the upper Miyar valley (KSZ on Fig. 9; Robyr et al, 2006). These ages may represent the timing of the temperature peak in the High Himalayan Crystalline.…”

“…1; Gansser, 1964). The work follows on detailed geological, structural and petrographic studies of Frank et al (1973Frank et al ( , 1977Frank et al ( , 1995, Fuchs (1986), Thöni (1977), Thakur (1983a,b) and geologists from the University of Lausanne (Spring, 1993;Spring et al, 1993a,b;Vannay, 1993;Stutz and Steck, 1986;Steck et al, 1993Steck et al, , 1998Steck et al, , 1999Epard et al, 1995;Vannay and Steck, 1995;Dèzes et al, 1999;Wyss et al, 1999;Girard and Bussy, 1999;Girard, 2001;Girard et al, , 2001Robyr et al, 2002Robyr et al, , 2006Steck, 2003;Steck, 2004, 2008). While the tectonic history of events in this region relies heavily on structural observations, precise isotopic dating is incomplete.…”

“…While the tectonic history of events in this region relies heavily on structural observations, precise isotopic dating is incomplete. Some geochronological data are available from specific areas within the Lahul and Beas valley (Table 1; Frank et al, 1977;Bonhomme and Garzanti, 1991;Spring et al, 1993a;Dèzes et al, 1999;Lal et al, 1999;Walker et al, 1999;Jain et al, 2000;Robyr et al, 2002Robyr et al, , 2006Vannay et al, 2004;Walia et al, 2008), and from the Tso Morari dome near the Indus suture (Fig. 1;De Sigoyer et al, 2000Schlup et al, 2003).…”