Abstract
AbstractWe 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.
Highlights
This paper addresses the kinematics of the fold‐and‐thrust belt in the Fars Arc region of the eastern Zagros, and the mechanism for its curvature
Rotation rates derived from the GPS data (Figure 8) are ≤0.5° Myr−1, which is lower than likely long‐term rates, derived from paleomagnetic studies (Aubourg et al, 2008) of ~1–2° Myr−1 (Figure 3)
The active, GPS‐derived rotations are of the opposite sense to previous predictions based on the presence of right‐lateral faults in western Fars (Hessami et al, 2001; Talebian & Jackson, 2004) and the interpretation of left‐lateral faults in eastern Fars (Koyi et al, 2016) but consistent with the Fars Arc becoming more arcuate over time (Figure 11)
Summary
This paper addresses the kinematics of the fold‐and‐thrust belt in the Fars Arc region of the eastern Zagros, and the mechanism for its curvature. The Fars Arc shares this property with numerous active and inactive fold‐and‐thrust belts worldwide, such as the Himalaya and the Suleiman Range in Pakistan, but the origins of such curvature have long been debated and are still contentious (e.g., Macedo & Marshak, 1999; Copley, 2012). We analyze earthquake slip vectors, fold patterns, published GPS data, and available paleomagnetic data from the Zagros fold‐and‐thrust belt, with the aim of understanding the deformation of the eastern part of the region, the Fars Arc. The rationale is that combination and comparison of these data sets give insights into tectonic processes that cannot be achieved in ancient, inactive orogens where such approaches are not available. Active regions where there are mismatches between the GPS and earthquake slip vectors strongly imply some form of strain partitioning and/or vertical axis rotations have taken place
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