SUBDUCTION INITIATION ALONG A PASSIVE CONTINENTAL MARGIN: LESSONS FROM THE NEOTETHYAN MESOZOIC ARC IN THE SANANDAJ-SIRJAN ZONE, IRAN

Subduction is the major driver for plate movement, but how it initiates is not well understood. The Atlantic Ocean, despite sections of oceanic lithosphere older than 170 Ma, is largely devoid of subduction zones. Key questions therefore include: what was the age of the oceanic lithosphere in earlier oceans where subduction began, how do subduction nucleate in space and time, and at what rates might initiation propagate parallel to a trench? Answering those requires well-constrained timing of both oceanic opening and subduction initiation. Such information now exists for the continental margin along the Mesozoic Sanandaj-Sirjan arc of Iran, which is well preserved and thus ideal for investigating the timing of subduction nucleation, using arc magmas as proxies, and assuming the lag between initiation and magmatism is brief (a few Myr) in comparison to the total timespan required to develop a continuous arc (a few tens of Myr). Crystallization ages of arc granitoids from the SSZ reveal that N-dipping subduction initiated in the early Jurassic, or slightly before that (depending on the lag). A key observation is that the appearance of arc magmatism was highly localized along the plate boundary at this time, only spreading to the entire arc some 30 to 40 Myr later. The oldest ages are for the plutons at Samen (187 Ma) and Siahkuh (185 Ma) which define two inception centers roughly 1000 km apart. The dataset also suggests a distinct NW younging of the granitoids, which appear to indicate consistent NW migration for subduction initiation from the inception centers through most of Jurassic time, at trench-parallel propagation rates of order 1 cm/yr. For arc granitoids of NW Iran, there are no known Jurassic ages yet, and Cretaceous ages are dominant. Neotethyan subduction appears to have been fully developed across-the-board from mid Cretaceous time onward, when the arc also started shifting farther inland. These data thus contrast with the well known hypothesis that the northern continental margin of the Neotethys had fully transformed into a trench by the beginning of Jurassic time (~200 Ma). They also suggest that, despite the protracted history of initiation, the development of a continuous arc along most of the northern margin of the Neotethys took at least 60 Myr less time than the modern Atlantic, which has yet to produce any such arc.