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Determining long-term slip rates in the Northern Zagros Fold-and-Thrust Belt (Kurdistan Region of Iraq) from tectonic geomorphology, structural modeling and OSL-dating

Subject Area Palaeontology
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 393274947
 
Final Report Year 2021

Final Report Abstract

The main results of the project can be summarized as below: • We were able to constrain structural style for this part of the Zagros with consideration to the principals of the balanced cross-sections and kinematic models of the fault related folding. The results indicated that the basement thrusting is responsible for the structural relief across the MFF that was determined at 2.4 to 3 km and averages at c. 2.6 km from the High Folded Zone to the Foothill Zone. The structural evolution is achieved by a multi-stage deformation. It started with thin-skinned deformation by fault-related folding of sedimentary cover above the Triassic basal detachment. Then basement thrusting below MFF was involved in the deformation along with the continuous deformation in the cover. Total displacement of 6.5 km on the basement thrust that dips about 25° NE is sufficient to build the structural step of c. 2.4 km. Up-dip, the main thrust splits into two splay thrusts. The structurally higher splay accommodated c. 4.2 km of slip and amplified frontal anticlines to form the Mountain Front Flexure. The lower and younger splay accommodated c. 2.3 km of slip, which was mainly transferred into the Triassic basal detachment of the Foothill Zone. A regional balanced crosssection was constructed across the belt. The shortening amount calculated from this regional cross-section for this part of the Zagros Belt is c. 24.8 km. The shortening within the topographically low part of the belt to the SW of the 1250 m regional contour suggests that the Late Miocene to Quaternary shortening rate calculated from the cross-section was much lower than the present-day geodetically derived convergence rate. Therefore, a significant amount of shortening during Late Miocene-Quaternary must have been accommodated somewhere within the Turkish-Iranian Plateau further to the NE. • Numerous uplifted river terraces along the banks of the Greater Zab River in the NW Zagros Fold-Thrust Belt were mapped and sampled. Their uplift is interpreted to have formed due to a combination of ongoing fault-related folding and regional uplift during Late Pleistocene- Holocene, and subsequent erosion by the river. Tilted river terraces on the flanks of anticline indicated ongoing folding. A basement-rooted thrust below the Zagros MFF accommodates 1.46±0.60 mm a-1 and more external basement fault to the SW accommodates less than 0.41±0.16 mm a-1 based on the relative uplift rates of terraces in their hanging walls. The horizontal slip rate related to detachment folding within the Foothill Zone is 0.40±0.10 mm a - 1 for the Safin Anticline and 1.24±0.36 mm a-1 for the Sarta Anticline. It can be inferred from this distribution of the river terraces, regional topography, seismic record, and crustal thickness that the basement thrusting and thickening of the crust within the Foothill Zone are restricted to its NE parts. In the SW parts of the zone, the deformation is limited mainly to folding and thrusting of the sedimentary cover above the Triassic basal detachment and is associated with slip on basement thrusts to the NE, including the basement thrust below the MFF. • The landscape maturity of several anticlines in the area was assessed from analyses of geomorphic indices. These analyses allowed us to differentiate between variably degraded landforms in the frontal anticlines of the Zagros Mountain Front Flexure. There is a significant difference in the onset of uplift of these anticlines. Our interpretation is that Akre Anticline was the first one to uplift, followed by Perat Anticline and Harir Anticline. The results of a landscape evolution model show that it would take the Harir Anticline about 80–100 and 160– 200 kyr to reach the maturity level of the Perat and Akre anticlines, respectively, based on the comparison of the geomorphic indices of the model output to those of the present-day topography of these anticlines, and by assuming erosion under constant conditions and constant rock uplift rates. Since the factors controlling geomorphology (lithology, structural setting, and climate) are similar for all three anticlines, and under the assumption of constant growth and erosion conditions, it can be inferred that uplift of the Akre and Perat anticlines started 160–200 and 80–100 kyr, respectively, before that of the Harir Anticline. To sum up, our studies show that geomorphic signals of distributed ongoing deformation of the faultrelated folds recorded by alluvial terraces, when combined with a careful examination of the structural setting, alongside with kinematic fault modeling techniques, can reveal processes acting at depth. This allows determining the ongoing structural uplift at rates of only a few millimeters per year. It is possible to interpret the uplift rates in terms of displacement rates on the relevant faults. These methods can be applied elsewhere in the Zagros Fold-Thrust Belt and may hence be the key to determine more robust long-term slip rate estimates. These can then be compared with GPS-derived, shorter term slip rate measurements. Discrepancies in estimates obtained via these two approaches might thus allow identifying locked fault strands that pose an increased seismic hazard. Additionally, estimating relative age differences in variously degraded anticlines using geomorphic indices, landscape maturity, and landscape evolution models can be applied to many other anticlines along the Zagros MFF and could eventually lead to developing a model for the temporal evolution along the Zagros Belt.

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