A geological cross-section north of Karakorum, from Yarkand to K2

Franco Rolfo, Chiara Groppo, and Maurizio Gaetani

This paper discuss the geologic setting of selected outcrops along the magnificent trekking route leading to K2 northern base camp in Xinjiang (SW China). From the Yarkand valley to the north, to the Shaksgam valley and K2 base camp through the Surukwat valley and the Aghil Pass to the south, different geologic units are separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude and are: Bazar Dara Slates, Surukwat Complex, Aghil Dara Granodiorite, Shaksgam Sedimentary Belt, Karakorum Fault, Whakan Slates, North Karakorum Sedimentary Belt, K2 Batholith.

The Bazar Dara Slates are best preserved along the Yarkand valley as an unmetamorphosed to very low-grade metasedimentary sequence of sandstones, siltstones and slates, steeply dipping towards SSE and locally intruded by undeformed granitic bodies and dykes. South of a major subvertical fault striking WSW-ENE, the Surukwat Complex is a composite sequence of thrust sheets trending WNW-ESE and steeply dipping SSW in which, safe for the occurrence of few non-metamorphic slivers, a general increase of metamorphic grade is evident southward from lower to higher structural levels. Protoliths vary from carbonate rocks, greywacke, conglomerate, diorite and marl to pelite. Petrologic investigation of metapelite at the top of the sequence constrains the metamorphic peak at moderately high pressure and medium temperature (ca. 600°C and 0.9 GPa). The contact between the Surukwat Complex and the weakly deformed Aghil Dara Granodiorite is marked by a cataclastic zone and by the presence of granitic dykes probably related to the intrusion of the granodiorite. A major tectonic contact to the south gives way to the Shaksgam Sedimentary Belt, non-metamorphic though pervasively folded and faulted towards the Karakorum Fault. The Shaksgam Sedimentary Belt spans from the Lower Permian to Jurassic, as proved by fossils and it is sealed by the Urdok Conglomerate, possibly linked to the first deformation stage of the Karakorum Range in the Cretaceous. To the SW of the Karakorum Fault and north of the K2 Batholith, highly deformed and feebly metamorphosed slices of the Wakhan Slates and of the Northern Sedimentary Belt of the Karakorum are preserved.

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