The life cycle of a foreland basin: Insights from the Alborz Mountains (Arabia-Eurasia collision zone)

Flexural foreland basins represent first-order geological archives that preserve the record of orogenic processes. These detrital archives provide critical insights into tectonic, climatic and environmental evolution as well as variations in source lithologies, isostasy, eustasy and dynamic lithospheric processes. In this study, we combine published geological data (e.g., facies analysis, magnetostratigraphy, sediment provenance, and carbon and oxygen isotopic records from authigenic minerals) with new zircon U-Pb geochronologic and zircon and apatite (U-Th)/He thermochronologic data from the southern foreland basin of the Alborz Mountains in northern Iran. This orogen, resulting from the Neogene Arabia-Eurasia collision zone, experienced topographic growth and exhumation since ~20 Ma and foreland deposition of a thick pile (> 7 km) of continental red beds.

The foreland-basin fill consists of three major systematic coarsening-upward cycles that each exhibits fine-grained siliciclastic strata at the base with high sediment accumulation rates and younger detrital zircon U-Pb and (U-Th)/He ages (mostly Eocene), followed by coarse-grained sedimentary strata with decreased sediment accumulation rates and older detrital zircon U-Pb and (U-Th)/He ages (> 100 Ma).

The base of each cycle is interpreted as a pulse of enhanced subsidence driven by tectonic loading associated with the growth of new thrust sheets experiencing erosional unroofing. The top of each cycle is interpreted to reflect wanning tectonic subsidence in response to local intra-basinal uplift (cycle 1) as documented by the sedimentary stratal pattern; uplift of the proximal foreland (cycle 3) as suggested by the age distribution of the detrital zircon U-Pb ages, the shift from fluvial to alluvial fan deposits and recycled apatite (U-Th)/He ages from the deepest exhumed red beds. Within these systematic trends, the top of cycle 2 represents an exception as it appears to record an enhanced phase of sediment supply triggered by wetter climatic conditions as documented by oxygen isotope data from paleosols samples.

Overall, our multidisciplinary approach provides a comprehensive overview of the history of a collisional foreland basin, from the forcing mechanisms controlling its stratigraphic architecture and sedimentary composition to its final incorporation into the orogenic wedge associated with basin uplift and erosion.