We study the Makran subduction zone, along the southern coasts of Iran and Pakistan, to gain insights into the kinematics and dynamics of accretionary prism deformation. By combining techniques from seismology, geodesy and geomorphology, we are able to put constraints on the shape of the subduction interface and the style of strain across the prism. We also address the long-standing tectonic problem of how the right-lateral shear taken up by strike-slip faulting in the Sistan Suture Zone in eastern Iran is accommodated at the zone's southern end. We find that the subduction interface in the western Makran may be locked, accumulating elastic strain, and move in megathrust earthquakes. Such earthquakes, and associated tsunamis, present a significant hazard to populations around the Arabian Sea.

The Makran Subduction Zone (Figure 1), which extends ∼1,000 km from the Strait of Hormuz in the west to Sonmiani Bay near Karachi in the east, results from the ongoing northward subduction of the Arabian Sea Plate beneath southeastern Iran and southwestern Pakistan. It is characterized by a shallow, gently-dipping subduction interface (e.g., Kopp et al., 2000;Smith et al., 2012), a high sedimentation rate (e.g., Ellouz-Zimmermann, Deville, et al., 2007;McCall, 1997), an extremely broad accretionary prism which is largely sub-aerial and no bathymetric trench. While great historic earthquakes (e.g., Byrne et al., 1992) accompanied by tsunamis (e.g., Hoffmann et al., 2013) have occurred on the Makran Subduction Zone, in general, seismicity is low, especially in the western Makran (e.g., Engdahl et al., 2006). A number of studies have focused on the structure of the offshore Makran accretionary prism (e.g., Kopp et al., 2000;Smith et al., 2012) and teleseismic tomographic studies show a high-velocity, steep northward-dipping feature in the upper mantle beneath southern Iran and Pakistan (e.g., Van der Meer et al., 2018). However, although the Makran Subduction Zone is the primary seismological

The Iranian Plateau, a broad zone of continental deformation, has formed as a result of the Arabia-Eurasia collision which initiated about 25 million years ago (e.g., Agard et al., 2011;Hatzfeld & Molnar, 2010). It is the site of one of the youngest collision zones in the world and the study of its structure can provide clues to understanding the younger stages of continental collision and plateau growth. The evolution of the Plateau is intimately associated with the opening and closing of the Paleo-Tethys and Neo-Tethys Oceans. Following the closure of the Paleo-Tethys in the Triassic (e.g., Berberian, 1981;Stöcklin, 1974;Şengör et al., 1984), the landmass of the Iranian Plateau began to form with the coalescing of island arcs and dispersed continental fragments which had rifted from the northern margin of Gondwana during the Permian. These fragments accreted to the Turan Platform, the southern margin of Eurasia at that time (e.g., Besse et al., 1998;McElhinny et al., 1981). The initiation of subduction of the Neo-Tethys Ocean is a subject of debate as to whether it occurred in the Late Triassic or Late Jurassic (e.g.,

SUMMARY We inverted 3555 regional and teleseismic P-wave relative time residuals to resolve deep velocity structure beneath the NW part of the Zagros collision zone. The data were gathered by 46 seismic stations installed along a ∼520-km-long seismic profile crossing the Zagros Mountains, Central Iran and the western Alborz Mountains. The obtained tomogram reveals a high velocity lithospheric root beneath the Zagros Mountains and a low velocity wedge in the frontal edge of the Arabian Plate beneath the suture that might be interpreted as beginning of delamination of lower part of the Arabian mantle lithosphere from its upper part. A significant deep (depth >350 km) high velocity feature is observed in the lower part of the upper mantle to the north of the Zagros suture and beneath Central Iran. We interpret this feature as the remains of oceanic slab of the Neotethys lying in the lower portion of the upper mantle and the transition zone.

The convective circulation generated within the Earth's mantle by buoyancy forces of thermal and compositional origin is intimately controlled by the rheology of the rocks that compose it. These can deform either by the diffusion of point defects (diffusion creep, with a linear relationship between strain rate and stress) or by the movement of intracrystalline dislocations (nonlinear dislocation creep). However, there is still no reliable map showing where in the mantle each of these mechanisms is dominant, and so it is important to identify regions where the operative mechanism can be inferred directly from surface geophysical observations. Here we identify a new observable quantity--the rate of downstream decay of the anomalous seafloor topography (swell) produced by a mantle plume--which depends only on the value of the exponent in the strain rate versus stress relationship that defines the difference between diffusion and dislocation creep. Comparison of the Hawaiian swell topography with the predictions of a simple fluid mechanical model shows that the swell shape is poorly explained by diffusion creep, and requires a dislocation creep rheology. The rheology predicted by the model is reasonably consistent with laboratory deformation data for both olivine and clinopyroxene, suggesting that the source of Hawaiian lavas could contain either or both of these components.