A SEISMOTECTONIC REVIEW OF THE IMPACT OF THE MAKRAN SUBDUCTION ZONE ON THE HELMAND-SISTAN BLOCK

The Makran subduction zone (MSZ) located along the coasts of eastern Iran and western Pakistan is approximately 900 km in length. The MSZ accommodates ~30 to 40 km/Myr of convergence between the subducting Arabian and overriding Eurasian plates. Subduction of the Arabian oceanic slab has formed an elaborate accretionary prism that is emplaced as far inland as southern Afghanistan. The MSZ seismicity and tomography indicate a tear in the Arabian oceanic slab that divides it into a shallower eastern and a steeper western section. The slab has a shallow dip beneath the volcanic arc and bends into a near vertical slab that is foundering beneath the Helmand-Sistan (H-S) continental Block in Afghanistan. The chemistries and effusion rates of the 5 volcanoes in the arc reflect the expiration of volcanism as one moves eastward along the Makran arc. To the west, the MSZ connects with the right-lateral Minab fault system that bounds the eastern Zagros fold and thrust belt as well as with the right-lateral Sistan Suture that merges into the Makran's accretionary prism. To the east, the MSZ is bounded by the transpressional left-lateral strike-slip Ornach-Nal and Chaman fault zones, which forms an unstable triple junction with relative plate convergence of India with Eurasia and the Himalayan orogeny. We propose a trap-door-style seismotectonic model of the H-S Block, given that it is bounded by the left-lateral Chaman Fault and the right-lateral Sistan Suture system, and that the MSZ is retreating slowly southward between these two subparallel, strike-slip plate boundary faults. The Makran slab is foundering ~150 to 200 km beneath the H-S Block and encourages subsidence of the Block in the SW corner of the H-S Basin, evidenced by the circuitous path of the Helmand River, which is fault and fold(?) controlled. Using satellite images and DEMs, we mapped several fault and fracture zones in the H-S Basin. The youngest ~100-km-long Holocene fault zone we named the Zabol Fault is located in a hamun ~15 km west of Zabol, Iran. Another prominent Quaternary fault we named the Helmand Fault offsets Pliocene basin fill and has caused repeated avulsions of the Helmand River. Previous workers have inferred the H-S Block is not internally deforming, yet our recent mapping has identified several structures within the Block, some of which are apparently active.