Structural analysis south of the Malakand and adjoining areas, northern Pakistan

Abstract

Two major faults, the Kishora thrust and a back thrust, dominate the structure of the study area. The Kishora thrust divide the study area into two tectonic terranes, the Indus melanges including Dargai ultramafic complex, and the Indian shelf terrane, whereas the back thrust brings higher grade almandine zone rocks in contact with low grade chlorite zone rocks near Bar Bazdara and Zormandai village. An important aspect of this study is the relationship of the structures of Swat with the Peshawar basin. The structures and stratigraphy in Swat extends into the Peshawar basin.

Four deformation phases (D₁, D₂, D₃, D₄) are recorded in the Indian shelf rocks from north to south, each characterized by distinct fabrics and folding. In Domains 1 and 2, D₁ formed S₁. The earliest recognizable fabric S₁ occurs as relict intrafolial in the S2 fabric, pressure shadows and porphyroblasts. No macroscopic folding event related to S₁ fabric has been recognized. During D₂, F₂ folds (large and small scale) were formed. F₁ and F₂ are coaxial and coplanar with isoclinal, recumbent axial surfaces and fold axes that plunge gently toward the NNW are associated with movement' on the Kishora thrust. The dominant S₂ foliation developed following the inter kinematic phase. S₂ is associated with transposition of S1 and rotation of porphyroblasts. S₂ is defined by calcite, muscovite, plagioclase, quartz, and garnet. The post S₂ phase is dominated by fractures and brittle shearing in the schists and annealing recrystallization in the granite gneiss and marbles. During D₃ crenulation of S₂, garnet growth, and the formation of the S₃ occurred. F₃folds are upright and open and plunge gently, but variably towards north or less commonly the south or southeast. F₄ is related to the change of forces from westward to southward in the lower Swat when Kohistan was thrust southward to its present position. The superposition of the E-W trending F₄ folds on the generally N-S trending F₃ folds resulted in a dome and basin structure near Mora Kandao.

In the south, Domain 3 preserves the last two deformation phases. During D₃ deformation S₁ developed. During D₄, S₁ was transposed and also involved with the growth of the magnetite porphyroblasts. F₃ and F₄ folds are best preserved. The largescale F₄ folds are mainly E-W, but at Rustam and Budal they are deflected into a broad arc near the Ambela complex. This is related to the back thrust in that the Ambela complex acts as buttress that deflects the later folds towards north.

Matrix schist records three deformation phases. During the D₂ deformation a transposed foliation produced. During D₃ the early S₁ was folded: During D₄ the S₂ was folded to produce local S₃ space crenulation cleavage.

The Dargai ultramafic complex has been emplaced along the Kishora thrust with the ophiolitic melange.