Precollision tilt of crustal blocks in rifted island arcs: Structural evidence from the Kohistan Arc

[ 1] We describe results from field and microstructural investigations in the unusually large Chilas plutonic complex, which was emplaced within the Mesozoic Kohistan island arc ( northern Pakistan). Magmatic and deformational fabrics of the Chilas gabbronorite are concordant with magmatic fabrics of associated gabbros, diorites, and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly SSW dipping flow structures over the northeastern part of the complex, which turn to SW-NE striking, subvertical attitudes to the west. Evidence for multiple intrusions includes concordant and discordant intrusive and/or mylonitic contacts between gabbronorites and associated rocks. Owing to structural and kinematic consistency, we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain (and/or strain rate) produced from drag of magma against its margin. We discuss this structural pattern and conclude that the Chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 Ma. In addition, several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body. The latest pegmatite set is composed of variably striking, vertical extensional veins that show no relationship to the magmatic fabric. We argue that the original attitude of these veins, along with that of the enclosing Chilas Complex, has been mostly preserved. We hypothesize that the northward dip of the southern side of the Kohistan is inherited from late Cretaceous rotation of crustal blocks in the hanging wall of the listric faults.