Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan

Abstract

The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc.In Baltistan, this suture has been reactivated by the late 'break-back' Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low-P (2-4 kbar, c. 500" C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent southverging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8-9-kbar rocks over 5-6-kbar rocks). There is little variation in temperature with structural level (550-625" C). These t\ko different terranes have been juxtaposed after metamorphism by the late south-directed MKT.

Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events

“…played by diffusion and advection of heat during an orogenic event. Concerning the relative extent of the two major metamorphic events (M1 and M2), the data presented here complement the pre-existing thermobarometric estimates provided by studies on the metapelitic lithologies of the kyanite-staurolite and sillimanite bearing metapelites of the KMC (Bertrand et al 1988;Searle et al 1989;Allen and Chamberlain 1991;Lemennicier et al 1996;Rolland et al 2001). …”

“…Calc-silicate rocks are also found throughout the KMC metamorphic pile and are more frequent than pelites, especially in the lower-grade part. Previous studies (Bertrand et al 1988;Searle et al 1989;Allen et al 1991;Lemennicier et al 1996) were mainly concerned with the metamorphism of pelites, so the metamorphic evolution of the lower-grade part remains poorly known. In the calc-silicate lithologies, Na 2 O and TiO 2 contents are low, and the sequence of crystallisation can be described in the K-Ca-(Mg/ Fe)-AlSi-H 2 O-CO 2 system (KCMAS-HC).…”

“…The SE Karakorum margin ( Fig. 1), or Karakorum Metamorphic Complex (KMC), presents a 20 km thick crustal section, in which the metamorphic conditions of the pelitic series have already been determined on a regional scale, with early Himalayan Barrovian metamorphism culminating at 0.8-1.0 GPa-650-700°C between 55 and 23 Ma, and late high temperature (HT) metamorphism culminating at 0.5-0.6 GPa-750-800°C between 6 and 7 Ma (Bertrand et al 1988;Hanson 1989;Searle et al 1989;Allen and Chamberlain 1991;Lemennicier et al 1996;Rolland et al 2001). Calc-silicate rocks are also found throughout the KMC metamorphic pile and are more frequent than pelites, especially in the lower-grade part.…”

Tectono‐metamorphic evolution of the Karakorum Metamorphic complex (Dassu–Askole area, NE Pakistan): exhumation of mid‐crustal HT–MPgneisses in a convergent context

“…(1988), Searle et al . (1989), Allen & Chamberlain (1991) and Lemennicier et al . (1996), have established a broad zone of young (Miocene to Pliocene) medium‐pressure high temperature (MP‐HT) gneisses in Southern Karakorum (Fig.…”

Cenozoic unroofing history of the Ladakh Batholith, western Himalaya, constrained by thermochronology and numerical modelling

“…2). In the Baltistan region of Pakistan, the Main Karakoram Thrust runs N608W along the Shyok Valley and has thrust the Karakoram terrane onto the Ladakh terrane in the late Miocene (Allen & Chamberlain 1991). The Main Karakoram Thrust has also been traced as the reactivation of the Shyok suture along a steep to vertical brittle fault west of Ladakh (Pêcher & Le Fort 1999).…”