The geology of the Karakoram range, Pakistan: the new 1:100,000 geological map of Central-Western Karakoram

Andrea Zanchi(*) & Maurizio Gaetani(**)
(*) Dipartimento di Scienze e Geologiche e Geotecnologie, Università degli studi di Milano-Bicocca, Piazza della Scienza, 4 - 20126 Milano, Italy; andrea.zanchi@unimib.it (**) Dipartimento di Scienze della Terra «A. Desio», Università degli studi di Milano, Via Mangiagalli, 34 - 20133 Milano, Italy.

A new geological map of the central-western part of the Karakoram belt (Northern Areas and North West Frontier Province, Pakistan) is presented with its explanatory notes. The map is printed at a 1:100,000 scale, summarizing original field surveys performed at a 1:25,000 scale, which result from the first systematic reconnaissance of the area. This work represents the synthesis of several years of exploration studies and is mainly based based on original stratigraphic and structural field analyses focused on one of the less known orogenic belts of Central Asia. Original field surveys have been integrated within a GIS using georeferenced Russian topographic maps and grey-tone panchromatic SPOT images. The study area is located along the border between Pakistan and Afghanistan, extending from the top of the Chapursan Valley of the Hunza region to the Yarkhun Valley from the Karambar Pass to Gazin and to the upper part of the Rich Gol, which belong to Chitral. Three major tectonic units are exposed in the study area. From north to south they are: the East Hindu Kush-Wakhan, the Tirich Boundary Zone and the Karakoram Terrane. The first and the last units consist of Gondwana-related terranes showing a Precambrian to earliest Paleozoic basement covered by Paleozoic to Mesozoic sedimentary successions which record their Late Paleozoic rifting from Gondwana, their drifting, and successive accretion to the Eurasian margin. They both show some similarities with the S-Parmir ranges, exposed to the north of the Afghan Wakhan. The Tirich Boundary Zone is a complex assemblage of high grade metabasites and gneiss with small remnants of sub-continental peridotites, which separate East Hindu Kush from the Karakoram. Its emplacement has been related to the possible opening of a basin between the two blocks at the end of the Paleozoic, followed by its deformation during the collision of Kara koram with East Hindu Kush, dating to the end of Triassic or beginning of the Jurassic. Detailed mapping has been carried out in the Karakoram belt, especially along its northern portion, which consists of a complex stack of tectono-stratigraphic units, showing peculiar stratigraphic and structural features. These units were progressively deformed and thrusted during the collision with the Kohistan Paleo-Arc and with India which occurred between the end of the Cretaceous and Paleogene. These collisions were also followed by continuous crustal thickening and by left-lateral shearing, which was especially active along the western margin of the mapped area. Our map also includes parts of the Karakoram Batholith, mainly Cretaceous in age, and of the Darkot-Gazin Metasedimentary Belt, which is exposed to the south of the main intrusive bodies and consists of Permo-Triassic metasediments. A new geological map of the central-western part of the Karakoram belt (Northern Areas and North West Frontier Province, Pakistan) is presented with its explanatory notes. The map is printed at a 1:100,000 scale, summarizing original field surveys performed at a 1:25,000 scale, which result from the first systematic reconnaissance of the area. This work represents the synthesis of several years of exploration studies and is mainly based based on original stratigraphic and structural field analyses focused on one of the less known orogenic belts of Central Asia. Original field surveys have been integrated within a GIS using georeferenced Russian topographic maps and grey-tone panchromatic SPOT images. The study area is located along the border between Pakistan and Afghanistan, extending from the top of the Chapursan Valley of the Hunza region to the Yarkhun Valley from the Karambar Pass to Gazin and to the upper part of the Rich Gol, which belong to Chitral. Three major tectonic units are exposed in the study area. From north to south they are: the East Hindu Kush-Wakhan, the Tirich Boundary Zone and the Karakoram Terrane. The first and the last units consist of Gondwana-related terranes showing a Precambrian to earliest Paleozoic basement covered by Paleozoic to Mesozoic sedimentary successions which record their Late Paleozoic rifting from Gondwana, their drifting, and successive accretion to the Eurasian margin. They both show some similarities with the S-Parmir ranges, exposed to the north of the Afghan Wakhan. The Tirich Boundary Zone is a complex assemblage of high grade metabasites and gneiss with small remnants of sub-continental peridotites, which separate East Hindu Kush from the Karakoram. Its emplacement has been related to the possible opening of a basin between the two blocks at the end of the Paleozoic, followed by its deformation during the collision of Kara koram with East Hindu Kush, dating to the end of Triassic or beginning of the Jurassic. Detailed mapping has been carried out in the Karakoram belt, especially along its northern portion, which consists of a complex stack of tectono-stratigraphic units, showing peculiar stratigraphic and structural features. These units were progressively deformed and thrusted during the collision with the Kohistan Paleo-Arc and with India which occurred between the end of the Cretaceous and Paleogene. These collisions were also followed by continuous crustal thickening and by left-lateral shearing, which was especially active along the western margin of the mapped area. Our map also includes parts of the Karakoram Batholith, mainly Cretaceous in age, and of the Darkot-Gazin Metasedimentary Belt, which is exposed to the south of the main intrusive bodies and consists of Permo-Triassic metasediments.

Karakoram, Hindu Kush, Pakistan, Cartography, Geodynamics, Tectonics, Stratigraphy, Collisional belt.