EARTHQUAKES ON THE KAZERUN LINE IN THE ZAGROS MOUNTAINS OF IRAN - STRIKE-SLIP FAULTING WITHIN A FOLD-AND-THRUST BELT

The Kazerun Line is a transverse valley of about 200 km long that obliquely crosses the regular anticlines of the Zagros fold belt in SW Iran. At its northern end it is a clear fault which can be mapped on the surface. Anticline axes die out or bend towards this valley but do not cross it. Six moderate-sized earthquakes that occurred close the the Kazerun Line, and within a 25 km area involved right-lateral strike-slip motion parallel to the strike of the valley. They indicate that the Kazerun Line is the surface expression of a buried strike-slip fault. Slip vectors in these strike-slip earthquakes are different from those of neighbouring reverse-fault earthquakes, suggesting that the Kazerun Line accommodates some of the shortening between Arabia and central Iran by an elongation of the Zagros mountains parallel to strike. The centroid depths and the source dimensions of these earthquakes, combined with the lack of seismogenic surface faulting in the Zagros, suggest to us that all these earthquakes involve faulting in the metamorphic basement beneath the sedimentary cover. The sedimentary cover is almost certainly decoupled from the basement by several thick evaporite horizons. The seismicity of the Kazerun Line thus demonstrates how lateral interruptions to the regularity of a fold belt can arise from faulting in the basement, and not just from lateral ramps between the thrust sheets that deform the sedimentary cover.