Open AccessA strong seismic velocity contrast in the shallow mantle across the Zagros collision zone (Iran)
Author(s) -
Kaviani A.,
Paul A.,
Bourova E.,
Hatzfeld D.,
Pedersen H.,
Mokhtari M.
Publication year - 2007
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2007.03535.x
Subject(s) - geology , seismology , mantle (geology) , volcano , transition zone , geophysics
SUMMARY Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P ‐wave traveltime residuals along a dense 620‐km long temporary seismic profile across the Zagros to investigate its upper‐mantle structure. The S ‐wave model determined from Rayleigh wave dispersion in the Zagros fold and thrust belt has high velocities from 4.5 ± 0.2 km s −1 below the Moho to 4.9 ± 0.25 km s −1 at 200 km depth, which are comparable to a shield‐like structure. Beneath the suture region from the Main Zagros Thrust (MZT) to the Urumieh‐Dokhtar volcanic arc, S ‐wave velocities are lower than beneath the Zagros in the top 50 km of the upper mantle, with a minimum of 4.4 ± 0.2 km s −1 at 80 km depth. From 150 km and deeper, S velocities are as high as beneath the Zagros. We suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and/or higher fluid content beneath the northern half of the profile, but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone. Teleseismic P traveltime relative residuals display a long‐wavelength variation along the transect, with a difference of 1.1 s between negative residuals in the Zagros Simple Folded Belt and positive residuals in Central Iran. This difference backprojects into a 6–7 per cent lateral variation of P ‐wave velocity in the shallow upper mantle, with higher V P beneath Zagros and lower V P beneath Central Iran. The main short wavelength variation of the residual is located in the suture region, with late P arrivals in the region of the MZT and early arrivals in the Sanandaj‐Sirjan zone (SSZ). Using synthetic models of V P perturbations, we show that the high velocities of the Arabian platform have to extend laterally at least to the SSZ to fit the observed P delays. This model also predicts Rayleigh wave phase velocities, which are within the error bars of the observed dispersion. It supports the model of crustal‐scale overthrusting at the MZT.