TECTONIC THICKENING OF THE WESTERN MAKRAN ACCRETIONARY WEDGE, SOUTHWEST PAKISTAN

Exposed folds of the southern Makran formed above a detachment that separated mid-Miocene and younger strata from oceanic crust and older strata of the subducted Arabian Plate (AP). The deformed section consists of Panjgur Formation (Pj) turbidite sands, an areally restricted mixed sand-mudstone transitional unit (Branguli facies), and the overlying Parkini Formation (Pk) basin to slope mudstones, an aggregate thickness of ~6km. Offshore the base-Pj detachment is ~8km below msl. Onshore the Pj-Pk occurs in outcrop; restorations from surface data indicate a depth to detachment as shallow as 2km on the crests of anticlinoria. Outcrops of Pj are expressed as large relatively undeformed slabs with locally overturned frontal folds. Wavelength of exposed Pj thrust sheets and frontal folds is generally 6km or less, with a few high-strain zones of very closely-spaced thrusts. In contrast, Pk preserved in synclines has been locally detached from the underlying Pj and tightly folded, often with chaotic trends. The base Pj detachment and overlying structures are folded into broad anticlinoria with a wavelength of ~40km, and amplitudes as great as 12km. The deformed Pj-Pk terrane extends 190km to the north of the coast, with only occasional outcrops of pre-Pj strata. The overall structure of the Makran is interpreted as a two-layered thrust system. Offshore the mid-Miocene through Recent section is detached from the underlying sedimentary section, and thrust-faulted and folded into short wavelength structures. When the top of the oceanic crust reaches a depth of ~12km, near the present shoreline, the Mesozoic(?) through Oligocene section is detached to form duplexes that fold pre-existing structures. To the north, these duplexes increase progressively in amplitude, uplifting the deformed Pj-Pk to form the broad outcrop belt that extends north to the southern margin of the Hamun-I-Mashkel Depression, and filling the space between the topographic surface and the top of the downgoing AP. The two thrusting events appear to be both kinematically and temporally discrete. Abrupt along-strike steps in the deep duplex system are marked by cross-strike discontinuities like the Shadi River line, which defines the NE end of the deep Kulanch Syncline.