The Garmsar salt nappe and seasonal inversions of surrounding faults imaged by SAR interferometry, Northern Iran

Abstract

AbstractThe allochthonous Tertiary salt of the Garmsar salt nappe extruded from where the most southerly point of the Alborz Mountain front is offset by the Zirab–Garmsar strike-slip fault. We used eleven descending Advanced Synthetic Aperture Radar (SAR) images, acquired by the European Space Agency's ENVISAT satellite from 2003 to 2006, to map surface displacement over 23 increments ranging in time from 30 to 2 months. A 30 month SAR interferogram of the area shows that the regional folds and faults are active south of the mountain front, but are dampened by the allochthonous salt that otherwise appears to be merely degrading at rates that vary with the season. Interferograms for shorter epochs display different patterns of fault blocks in the country rocks that rose and fell with the seasons. By relating surface displacements mapped in these interferograms to the contemporaneous seismic record, we find that seismic faults reactivate repeatedly while their kinematics may have inverted on remarkably short timescales. Seismic disturbances propagate very slowly and faults are longer than expected for earthquakes with ML<3.5, indicating that the regional strains are more aseismic than anticipated by earlier studies.