The Late Cretaceous-Cenozoic history of western Baluchistan Pakistan—the northern margin of the Makran subduction complex

Abstract

Summary The study area lies some 400 km N of the Makran coast, and extends westwards for some 450 km from the Chaman Fault, a major sinistral transform fracture zone which marks the suture between the Eurasian and Indian plates. It lies to the north of the Makran ranges of Tertiary flysch, and flanks the southern margin of a stable, aseismic area known as the Dashi-i-Margo block. The Makran region as a whole has been interpreted as an accretionary prism of Late Cretaceous to Holocene age, resulting from the northward subduction of oceanic lithosphere. The geological record of the study area is consistent with such an interpretation, for the period Late Cretaceous-Palaeocene at least. Rapid northward subduction during the Late Cretaceous gave rise to profuse andesitic volcanism on the sub-parallel, ENE-trending arc massifs of the Chagai Hills and Ras Koh geanticlines. The waning of this volcanism during the Maestrichtian coincided with the accumulation and deformation of flysch sediments in a trench which extended from Mirjawa to the south of Ras Koh; also with thick forearc sedimentation in the Dalbandin Trough, between the geanticlines. In late Palaeocene times, deformed flysch—interpreted as a subduction complex—was elevated to form a structural high in the Mirjawa range. In the western Ras Koh, the flysch, along with ultramafic slices, became emplaced against the arc massif to form a new landmass. This emplacement may have been a result of a late Palaeocene collision between the northward-migrating Indian continental plate and the continental lithosphere of the Dasht-i-Margo block, the Ras Koh flysch belt taking the brunt of the initial impact. The principal forearc faults have regular, arcuate traces which, in the south-west, parallel the structural grain of the Mirjawa flysch belt. Movement on the faults influenced sedimentation in the forearc, particularly during the Eocene and Miocene. There is evidence of tensional transform movement in Eocene times. The forearc faults were especially active during the Pliocene. Movement across them during this period was compressional and associated with major folding, particularly in the Dalbandin Trough. The trend of this folding may imply dextral transform displacement within the trough.