THE ALBORZ AND NW URUMIEH-DOKHTAR MAGMATIC BELTS, IRAN: RIFTED PARTS OF A SINGLE ANCESTRAL ARC

The double subduction-zone model of Cretaceous-Paleogene magmatic belts of the Alborz Mountains (ALB, northern Iran) and central Iran (CI) has serious problems: strong similarities in stratigraphy, structure and paleomagnetic latitudes between ALB and CI, and lack of a suture, preclude an oceanic gap between them. We propose that these two arc segments of the ALB and central Iran-NW Sanandaj-Sirjan zone initiated as one arc that was rifted apart. The ALB contains voluminous calcalkaline Eocene distal pyroclastic strata (>3000 m thick Karaj Fm) deposited in a deep marine basin, but have only minor volcanic sources. Two major rhyodacitic to andesitic, crystal-vitric tuff units are underlain, separated, and overlain by marine shale units in the type area. The basin must have been directly connected to its source arc platform because pre-Eocene clasts are absent. We infer sources to the south because the Karaj Fm thins northward and no sources are known farther north. In contrast, many proximal Eocene ignimbrites and caldera complexes exist in CI, with two major ignimbrite sequences, for which northward wind transport has been documented. These two ignimbrites are separated by sediments and lavas. We interpret the ignimbrites as the proximal facies of the two Karaj tuff units, which were deposited in a volcaniclastic turbidite apron in a continental marginal sea north of the Neotethyan subduction zone arc. The subduction zone and arc migrated southward in middle Tertiary time (slab rollback?), rifting the NW Urumieh-Dokhtar volcanic belt (which remained active in the Oligo-Miocene) and the Eocene ignimbrites and caldera complexes of CI away from the Karaj basin. Subsequent effusive volcanism in ALB was alkaline, consistent with a switch to extension. Subsidence of CI during and after the rifting event is recorded by Oligo-Miocene Lower Red, Qom and Upper Red Fms. This was followed by compression of CI and by rapid uplift of ALB due to closure of the NeoTethys. Recognition of ALB and CI magmatic belts as complementary parts of the same volcanic arc system has been hindered by the extensional event, and by syncollisional structural and topographic inversion and bending of the ALB between surrounding rigid blocks.