A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran

The Zagros mountains of SW Iran are one of the most seismically active intra-continental fold-and-thrust belts on Earth, and an important element in the active tectonics of the Middle East. Surface faulting associated with earthquakes is extremely rare, and so most information about the active faulting comes from earthquakes. We use long-period teleseismic P and SH body waves to determine the orientation and depth of faulting in 16 new earthquakes, and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the Zagros. We use this information to investigate the style and distribution of active faulting in the Zagros, and how it contributes to the N–S shortening of the Arabia–Eurasia collision. When the data are ranked in quality and carefully evaluated, simple patterns are seen that are not apparent when routine catalogue data are taken at face value. An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation (‘partitioning’) of the orthogonal strike-slip and shortening components on separate parallel fault systems. By contrast, in the SE, overall convergence is orthogonal to the regional strike and achieved purely by thrusting. In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt. The overall configuration is similar to that seen in other curved shortening belts, such as the Himalaya and the Java–Sumatra trench. All the Zagros earthquakes we have been able to check have centroids shallower than ∼20 km and are confined to the upper crust. Many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover, which is active even beneath areas of known shallow structural decollement such as the Dezful embayment. The dominant style of shortening is high-angle reverse faulting with dips >30° though some lower-angle thrusting occurs in places. Active thrust and reverse faulting is relatively confined to the lower topography on the SW edge of the belt today, and only strike-slip faulting affects the higher topography. Profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the Simple Folded Belt, but has progressively migrated SW over the last 5 Ma. There is no evidence for a seismically active structural decollement, such as a low-angle thrust, beneath the Zagros, nor is there any seismic evidence for active subduction, either beneath the Zagros or beneath central Iran. Instead the Arabian margin seems to have shortened by distributed thickening of the basement. Only in the syntaxis of the Oman Line, at the SE end of the Zagros, is there any evidence for a low-angle thrust of regional extent. Here, earthquakes continue 50 km north of the Zagros Thrust Line (the geological suture between the Arabian margin and central Iran) reaching depths of ∼30 km, and may represent thrusting of Arabian basement beneath central Iran to this extent.