Upper mantle structure under the Zagros collision zone; insights from 3D teleseismic P-wave tomography.

We map the lateral variations of P-wave velocity beneath the Zagros collision zone in Iran down to 800 km depth by regional travel time tomography. We invert 32,293 relative travel time residuals from five temporary seismic networks across the collision zone in NW and central Zagros and northern Iran, and a large number of permanent stations distributed over the Iranian Plateau. A salient feature of our model is a ~225 km thick lithosphere beneath the Zagros, almost twice as thick as in the rest of Iran. Slab detachment at a depth range of 250–300 km from the base of the subducted continental Arabia is clearly distinguishable in central Zagros. In NW Zagros detachment appears to be at an earlier stage. Beneath the central Iran Plateau, we observe shallow low-velocity regions down to 200 km depth, and smaller patches with significantly higher velocity in the 200–400 km depth range. This pattern might suggest a post-collisional foundering of the mantle lithosphere in the upper plate. The subducted slab seems to penetrate into the lower mantle in a segmented fashion, with no evidence for a stagnant slab above the 660 km interface. We map the lateral variations of P-wave velocity beneath the Zagros collision zone in Iran down to 800 km depth by regional travel time tomography. Our tomographic study documents variations of lithospheric thickness between the Zagros and central Iran and a post-collisional slab detachment around 250 km depth. If we assume that the detachment occurred right beneath the base of the subducted Arabian continental margin, then the maximum thickness of the continental root must be no >225 km. This would imply a doubling of the Zagros lithosphere as a result of collision. The absence of high-velocity anomalies far north from the MZT at the lithospheric depth range indicates that the lithosphere thickening has not propagated into the interior of central Iran. The high-velocity anomalies at 200–300 km depth beneath central Iran suggest the presence of fragments of delaminated mantle lithosphere. They also suggest post-collisional slab break-off in central Zagros that seems to be at an earlier stage in NW Zagros compared to central Zagros. From the shallow depth of the top of the detached slab, we infer that the break-off is recent (5–10 Ma), in good agreement with geological records. Effective detachment of an oceanic slab from continental lithosphere is often followed by the upwelling of the asthenosphere through slab windows, eduction, slab retreat, and eventually volcanism and exhumation of ultra-high-pressure rocks, which are not currently observed in central Iran. On the other hand, lack of deep seismicity, especially deep extensional events in the Zagros, indicates that the subducted slab does not exert gravitational pull on the base of the continental plate to reconcile these apparently contradictory observations, we propose that slab break-off in NW Zagros is very young. [Display omitted] • Upper mantle structure of the Zagros collision zone via teleseismic tomography. • Evidence for a thicker lithosphere beneath the Zagros versus thinner lithosphere in the surrounding regions. • Slab detachment near the base of the thickened Zagros lithosphere. • Seismological evidence for mantle lithospheric delamination beneath central Iran. [ABSTRACT FROM AUTHOR]