Linking deep-Earth processes, basin stratigraphy, and topographic build-up during the Neogene Zagors orogeny in the Kurdistan region of Iraq

Plate tectonics can lead to construction of mountain belts, whereas surface processes destruct the orogenic masses and redistribute the surface load. These processes can be modulated by climate through variation in air temperature and the magnitude-frequency distribution of precipitation. In the northwestern Zagros orogenic belt the driving force for hinterland uplift has been baffling. The key concern is whether uplift is due to upper crustal shortening and related crustal thickening (local uplift) or to deep lithospheric processes (regional dynamic uplift) such as slab breakoff and/or to lithospheric mantle delamination. The stratigraphic record is sensitive to geodynamic processes, yet distinguishing the tectonic signatures from the climate-induced signatures is necessary. The goal of this research is to test these competing mechanisms of orogenesis through field-based evaluations of shifts in foreland basin stratigraphy, provenance, detrital geochemistry, and climate change through time as well as flexural modeling for the northwestern Zagros orogenic belt. In the Kurdistan region of Iraq, the northwestern Zagros orogenic belt is characterized by a well preserved ~4 km thick stratigraphic column of the Neogene synorogenic predominantly clastic continental deposits that coarsen and thicken upwards: The Fatha (middle Miocene), Injana (late Miocene), Mukdadiya (latest Miocene), and the Bai-Hasan (Pleistocene) Formations. These units, in addition to sandstone beds, include thick poorly consolidated mudstone packages that in some places reach ~100 m. Preliminary results show that the frequency and thickness of sandstone-filled channels increases upsection, leading to an amalgamation of sandstone packages towards the top. This thickening-upward trend was additionally associated with an increase in the grain size. These patterns of stratigraphy dynamics hint to a progradation of the depositional systems, driven either by an increase in the sediment flux relative to the subsidence rate, or by a propagation of the orogen front towards the foreland basin. Sm-Nd analysis on the fine material packages revealed a crustal origin (εNd-) comparable to the Arabian shield, with an older crustal age upsection. Weathering proxy data such as chemical index alteration (CIA) and K₂O/Al₂O₃ ratio yield evidence for a weathering intensity that increases upsection. X-Ray diffraction data from the clay-size materials (<2-μm) show contents of smectite, illite, kaolinite and Fe-rich chlorite, with an increasing abundance of smectite minerals upsection. These mineral assemblages demonstrate a semi- arid/humid climate likely with an increasing seasonality through time, which could possibly have resulted in an increasing sediment flux. Furthermore, basic flexural modeling for the northwestern Zagors orogenic belt indicates that the present-day Zagros topography, and thus topographic load alone, cannot explain the observed basin depth. Overall, these evidences suggest that exhumation of the source terranes was enhanced by increased weathering, yet a geodynamic process could have been the main driver for controlling the formation of accommodation space and uplift of the mountain belt.