Tectonic stacking of back-arc formations in the Thelichi section (Indus valley) of the Kohistan arc, northern Pakistan

Metamorphosed volcanic and sedimentary rocks of the Jaglot Group are exposed along the west bank of the Indus River near Thelichi. The structural bottom unit, the Thelichi Formation, is composed of metavolcaniclastic, metavolcanic, metapelitic, and metacalcareous rocks. Bedding planes of the Thelichi Formation trend E–W or NW–SE and dip steeply to the N. The middle unit, Gashu-Confluence Volcanics, is composed of metavolcaniclastic, metavolcanic, and metacalcareous rocks. Bedding planes trend NW–SE and dip moderately to the N. The top unit, the Gilgit Formation, is composed of interlayered metapsammitic and metapelitic rocks. Graded bedding, cross-bedding, and pillow structures are preserved in these metamorphic rocks of the Jaglot Group. Those indicate clastic sedimentary and volcanic origins. There is no major repetition of layers due to folding (so-called “the Jaglot syncline”) as is evidenced by the consistent northward younging of the beds. The three lithological units constitute a north-dipping tectonic stack. The tectonic stack was provably caused by the northward subduction of the back-arc basin under the Asian margin and subsequent collision between the Asia and the Kohistan (the closure of back-arc basin).

Research highlights► This paper describes the Jaglot Group in the Indus valley section near Thelichi. ► Three lithological units of the Group form a north-dipping tectonic stack. ► Volcanic and sedimentary layers of the units consistently young toward north. ► This is incompatible with the accepted presence of the Jaglot syncline there. ► Northward subduction of a back-arc basin fits the fact of tectonic stack structure.