Kinematic Variation Within the Fars Arc, Eastern Zagros, and the Development of Fold‐and‐Thrust Belt Curvature

Abstract

We analyze deformation of the Fars Arc in the eastern Zagros, Iran, including earthquake slip vectors, GPS velocities, paleomagnetism data, and fold orientations, to understand how this fold‐and‐thrust belt works and so better understand the generic issue of fold‐and‐thrust belt curvature. The Fars Arc is curved, convex southward. GPS‐derived rotation rates are ≤0.5° Myr−1: Rotation is clockwise west of 53°E and counterclockwise to the east. These rotation senses are opposite to previous predictions of passive “bookshelf” models for strike‐slip faults during north‐south convergence. West of 53°E, average GPS vectors, thrust earthquake slip vectors, strain axes derived from GPS data, and orthogonal directions to fold trends are all aligned, toward ~218°. East of this meridian, the average GPS vector is toward 208°, but the averages of the other data sets are distinctly different, all toward ~190°. We propose that fault blocks in eastern Fars, each ~20–40 km long, rotate predominantly counterclockwise, whereas in western Fars, the regional clockwise rotation takes place mainly on the array of active right‐lateral faults in this area. Thus, localized block faulting and rotations accumulate to produce the overall strain and regional curvature. Active folds of different orientations in eastern Fars intersect to produce domal interference patterns, without involving separate deformation phases, indicating that fold interference patterns should not be interpreted in terms of changing stress orientations unless there is clear evidence. Fars Arc curvature is best explained by deformation being restricted at tectonic boundaries at its eastern and western margins, with significant gravitational spreading.

Late Miocene Deformation Kinematics Along the NW Zagros Fold‐Thrust Belt, Kurdistan Region of Iraq: Constraints From Apatite (U‐Th)/He Thermochronometry and Balanced Cross Sections

“…Others have proposed thick‐skinned deformation to account for high‐magnitude surface uplift, but not necessarily in an out‐of‐sequence pattern (Mouthereau, Tensi, et al, 2007; Sherkati & Letouzey, 2004). Based on earthquake slip vector scatter and crosscutting folds in the outer eastern Fars segment, a recent study (Edey et al, 2020) suggests that potentially both thin‐skinned and thick‐skinned deformation have been taking place since an early stage with no necessity for switching from a thin‐skinned to a thick‐skinned style.…”

The 2019–2020 Khalili (Iran) Earthquake Sequence—Anthropogenic Seismicity in the Zagros Simply Folded Belt?

“…The sedimentary cover of the Fars arc is detached from the underlying basement by a layer of Ediacaran-early Cambrian Hormuz salt, which also surfaces in numerous diapirs (e.g., Barnhart & Lohman, 2012;Edey et al, 2020;Jahani et al, 2009Jahani et al, , 2017. Estimates of the depth of this interface vary from as little as ∼6-8 km (e.g., Sherkati et al, 2005) to as great as ∼14-20 km (Jahani et al, 2009(Jahani et al, , 2017).…”

“…GPS measurements indicate 10 mm/yr of NNE‐directed convergence across the central Fars arc (e.g., Tatar et al., 2004). This shortening is manifested at the surface in symmetric, range‐parallel folds with amplitudes of up to a few kilometers and wavelengths of ∼10–20 km (e.g., Edey et al., 2020), and at depth in frequent earthquakes on steeply dipping (30°–60°), blind reverse faults (Berberian, 1995; Nissen et al., 2011; Talebian & Jackson, 2004). There are no known examples of coseismic surface rupture in the Fars arc, and the mechanical relationship between buried faults and surface folds remains a matter of debate.…”

The 2019-2020 Khalili (Iran) earthquake sequence - anthropogenic seismicity in the Zagros Simply Folded Belt?

“…GPS measurements indicate 10 mm/yr of NNE-directed convergence across the central Fars arc (e.g., Tatar et al, 2004). This shortening is manifested at the surface in symmetric, range-parallel folds with amplitudes of up to a few kilometers and wavelengths of ∼10-20 km (e.g., Edey et al, 2020), and at depth in frequent earthquakes on steeply dipping (30°-60°), blind reverse faults (Berberian, 1995;Nissen et al, 2011;Talebian & Jackson, 2004). There are no known examples of coseismic surface rupture in the Fars arc, and the mechanical relationship between buried faults and surface folds remains a matter of debate.…”