A LATE PLIOCENE TECTONIC SWITCH FROM TRANSPRESSION TO TRANSTENSION IN THE HARAZ SECTOR OF CENTRAL ALBORZ: IMPLICATIONS FOR THE ORIGIN OF DAMAVAND VOLCANO

Damavand volcano (5670 m above sea level) is a large composite cone of trachyandesites overlying the active fold-thrust belt of the Alborz Mountains in northern Iran. Geophysical, geochemical and geochronological data do not support alliance with active subduction and rather point to a local mantle plume source that initiated in the late Pliocene when the local tectonic regime switched from transpression to transtension, attributable to a change in regional shear from dextral to sinistral. Morphotectonic evidence for a pull-apart system under the volcano is being documented and includes: 1- topographically low-standing bedrock -forming an irregular sink under Damavand- compared to the much higher position of the bedrock in the surrounding ridges, and 2- disruption of the structural and topographical grain of the region by previously unrecognized high angle, small throw, normal faults exposed in Haraz River canyon walls to the east and in Lar Plain to the west of the volcano. Some of these faults correlate with marked steps in topography that lower the bedrock towards the basin under Damavand. The sequence of upper crustal deformation in the Damavand sector of central Alborz since central Iran felt the Arabia-Eurasia collision in the Miocene comprise three distinct phases: 1- regional compression acting in the Miocene that shaped the E-W trending mega-scale folds and N-dipping reverse faults, 2- dextral transpression that superimposed NNE-SSW trending meso-scale folds and shear zones onto the regional folds of phase 1 until Pliocene time, and 3- late Pliocene and younger sinistral transtension that overprinted earlier phases via a normal fault system that dips towards the volcano. Tectonized zones formed during the transpressional phase provided the zones of weakness necessary for Damavand magma ascent during the ensuing transtensional phase. The discussed transtensional system might correspond with a covered diverging step over or releasing bend between the Baijan fault (latest earthquake activity in March 1983) to the northeast and Sefidab fault to the southwest of Damavand.