Late Cenozoic shortening in the west-central Alborz Mountains, northern Iran, by combined conjugate strike-slip and thin-skinned deformation

The west-central Alborz Mountains of northern Iran have deformed in response to the Arabia-Eurasia collision since ca. 12 Ma and have accommodated 53 +/- 3 km of shortening by a combination of range-parallel, conjugate strike-slip faulting and range-normal thrusting. By our interpretation, similar to 17 km of shortening across the Alborz is accommodated by westward relative motion of a crustal wedge bounded by conjugate dextral and sinistral strike-slip fault systems. The Nusha, Barir, and Tang-e-Galu fault zones strike west-northwest, constrain the north side of the wedge, and, prior to ca. 5 Ma, accumulated a total of similar to 25 km of dextral slip. The south side of the wedge is bounded by the active sinistral reverse Mosha and Taleghan faults, which merge northwest of Tehran and have a total slip estimate of 30-35 km. A restored cross section across the range indicates a minimum of 36 +/- 2 km of fold-and-thrust-related, range-normal shortening. Combined, wedge motion, thrusting, and folding yield a net shortening of 53 +/- 3 km across the range, which is within the error of the shortening estimate predicted by assuming that the present-day shortening rate (5 +/- 2 mm/yr) has been constant since ca. 12 Ma (similar to 60 km of predicted shortening). A second restored cross section farther west, which includes the wedge, gives a total shortening of 15-18 km and a long-term shortening rate of 1.25-1.5 mm/yr (constant shortening rate since ca. 12 Ma). These strong along-strike finite-strain and long-term strain-rate gradients are important for our understanding of how long-term strain rates compare with instantaneous strain rates derived from global positioning system (GPS) data, and should be considered when planning mountain belt-scale GPS surveys. Finally, a 60-km-long right-hand bend in the Mosha-Taleghan fault system has driven the development of a transpressional duplex south of the fault. The southern boundary of the duplex is the active Farahzad-Karaj-North Tehran thrust system. The kinematic development of this strike-slip duplex has implications for seismic hazard assessment in the heavily populated Karaj and Tehran areas.