Slip vectors and fault mechanics in the Makran Accretionary Wedge, southwest Pakistan

Abstract

The Makran is a broadly east–west trending arcuate accretionary wedge currently forming where the oceanic floor of the Gulf of Oman is being subducted north‐northeastward beneath the south Asian margin. Two traverses across the Makran Coast Ranges, extending about 80 km inland from the coastline (which lies about 100 km north of the wedge front), show that the structure of this region is dominated by large back‐rotated south directed thrust faults and associated folds. Related structures include duplexes, fault gouge containing Reidel shears, and melange along fault zones. These structures are cut by later north directed backthrusts, out‐of‐sequence south directed thrusts, and apparently conjugate sets of NNE trending sinistral and NW trending dextral strike‐slip faults. Slip vectors were determined from slickenlines, fiber lineations, gouge fabrics, and Riedel shears. Vectors from early thrusts and bedding‐parallel slip surfaces, corrected for tectonic rotation, form a broad south directed concentration lying between the normal to the regional strike (varying from 160° to 180°) and the present plate convergence vector (200°). Locally, the concentration is bimodal. This pattern may represent the interaction between plate forces and body forces in the accretionary wedge. The late thrusts have widely variable slip vectors and stair‐step off the early thrusts. They overlap in orientation and slip vector with gently dipping sinistral strike‐slip faults, suggesting that they also stair‐step in plan off the latter. The steep strike‐slip faults represent the final tectonic stage, involving minor N‐S shortening and longitudinal extension. These three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew.