TERTIARY VOLCANISM IN NORTHWEST OF SAVEH, IRAN, AND ITS GEOTECTONIC SIGNIFICANCE

The study area is part of the Cenozoic Urumieh-Dokhtar continental arc of central Iran. The Saveh area is also centered in the widest segment (>150 km) of the magmatic arc. The oldest rock unit is a sequence of volcanosedimentary deposits that is labeled as unit E3. It is topped with some ignimbrites (E4). Unit E5c is a red conglomerate and is overlain by sandstones, siltstones, shales and nummulitic limestones (E51), and finally a sequence of multicolor tuffs (E52) completes the Lower-Middle Eocene package. Stratigraphically above unit E52, is a thick and widespread alternation of tuffs-ignimbrites (E6) and mafic flows (E6ba1, E6ba2) that could be of Upper Eocene age. The Paleogene volcanic complex ends with olivine basalt flows (EOb) that are given the age of Upper Eocene to Oligocene. Younger rock units comprise Oligocene-Miocene Lower Red, Qom, and Upper Red Formations. This area, in its larger extent, illustrates the greatest abrupt thickness variations observed in the marine carbonates of Qom Formation. One of the most interesting features in terms of magma genesis in this region is the bimodal composition of the units E6, E6ba1and E6ba2 in which, alternation of felsic and mafic flows occurs with high frequency. In addition, field observations such as the abundance of pillow-like mafic enclaves in ignimbrites attest to extensive mixing of the two magma types represented in units E6, E6ba1and E6ba2. Since bimodal volcanism has universally been considered as key evidence for continental extension, we infer that an intra-arc spreading could have given way to eruptions of E6 ignimbrites and E6ba1, E6ba2 basalts. The subsequent eruption of olivine basalts (unit EOb) signifies the more advanced stage of the rifting that resulted in the formation of the Neogene sedimentary basins in this area. Deposition of Qom Formation carbonates in a horst and graben like basin seems as the only way of explaining why it shows such abrupt changes in facies and thickness in the general Saveh area. The intra-arc continental extension also justifies why the Urumieh-Dokhtar zone is so wider in this area. Our intra-arc spreading expalnation reconciles the continental rift model, favored by some authors, with the well-documented subduction scenario for the formation of the Urumieh-Dokhtar zone.