A palaeogeographic reconstruction of the Dir Group: evidence for magmatic arc migration within Kohistan, N. Pakistan

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Abstract
The Late Palaeocene Dir Group records the resumption of volcanic activity along the southern margin of Eurasia after a prolonged period of uplift and erosion. The group forms an integral part of the Kohistan Batholith and is readily divisible into two distinctly contrasting volcanic successions. The Baraul Banda Slate Formation comprises 2700 m of fore-arc sandstones and siltstones. The basin formed during the collapse of the Kohistan continental margin and was filled initially by subaerial debris and mass-flow deposits. Subsidence was rapid such that >900f the sedimentary record consists of thin-bedded sheet turbidites. These maintain a remarkable lateral continuity and indicate deposition in a restricted deep-water environment. Rare interbedded limestones have yielded a Thanetian (60.2–54.9 Ma) marine fauna of Miscellanea miscella and Actinosiphon tibeticus . In direct contrast the Utror Volcanic Formation comprises 3000 m of volcaniclastic sedimentary material, lava flows and ignimbrites. Eruptive style was predominantly explosive with fragmental material dominating the volcanic record. Lavas range in composition from 53% to 79% SiO 2 with rhyolite the predominant rock type. A basaltic andesite has an 40 Ar- 39 Ar age of 55 ± 2 Ma. Although the volcanic stratigraphy is complex preliminary investigations suggest accumulation in a predominantly subaerial ring-plain or flanking facies distal to the main focus of volcanic activity. Sedimentary thickness and the abundance of silicic lavas and pyroclastic flows imply extensional tectonic control even though the arc developed at a convergent margin. Juxtaposition of the two facies along the Dir Thrust excludes any record of shore-line or coastal processes from the volcanic stratigraphy. The inferred original geographical separation of the two environments coupled with their relationships to isotopically dated plutonic suites reveals that between Late Cretaceous and Early Eocene times the locus of magmatic activity within the Kohistan arc migrated progressively northward. The Shamran volcanic rocks although originally correlated with the island-arc Chalt Volcanic Group are now identified on the basis of a 58 ± 1 Ma ( 40 Ar- 39 Ar) age as a northern continuation of the Late Palaeocene volcanic arc.