Insights in the exhumation history of the NWZagros frombedrock and detrital apatite fission-track analysis: evidence for a long-lived orogeny

Abstract

We present the ¢rst ¢ssion-track (FT) thermochronology results for theNWZagrosBelt (SWIran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NWZagros foreland basin: the Palaeocene^early Eocene Amiran^Kashkan succession and theMiocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj-Sirjan Zone. Only apatite ¢ssion-track (AFT) data have been successfully obtained, including 26 ages and11track-length distributions. Five families of AFTages have been documented from analyses of in situ bedrock and detrital samples: pre-middle Jurassic at 171 and 225Ma, early^late Cretaceous at 91Ma, Maastrichtian at 66Ma,middle^late Eocene at 38Ma and Oligocene^early Miocene at 22Ma.The most widespread middle^late Eocene cooling phase, around 38Ma, is documented by a predominant grain-age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFTages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the lateCretaceous oceanic obduction event, during the middle and late Eocene and during the earlyMiocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major £exural foreland episodes, the middle^late Eocene phase mostly produced a long-lasting slow- or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to theNE along the Sanandaj-Sirjan domain and its Gaveh Rud fore-arc basin. As evidenced in this study, the Zagros orogeny was long-lived and multiepisodic, implying that the timing of accretion of the di¡erent tectonic domains that form the Zagros Mountains requires cautious interpretation.