Lithospheric Structure of Central Zagros from Seismological Tomography

Category Tectonic & Seismotectonic
Group GSI.IR
Location 4th internetional Conference on Seismology
Holding Date 11 March 2008
     As part of the French-Iranian project set up to study the dynamics of the intracontinental collision zone of Iran, we installed a temporary seismological network across the Zagros for 4.5 months in 2000-2001. This experiment aimed at investigating the lithospheric structure of the mountain belt. The network included 65 broadband, medium-band and short-period stations located along a 600-km long line with an average spacing of ~10 km. The profile ran from the coast of the Persian Gulf to the stable block of Central Iran across the towns of Shiraz and Yazd.
A migrated depth cross-section computed from radial receiver functions displays clear P-to-S conversions at the Moho beneath most of the profile. The average Moho depth is 45 to 50 km between the coast and the Shiraz region. The crust thickens rather abruptly beneath the regions of high elevations NE of Shiraz, the suture zone of the MZT (Main Zagros Thrust) and the Sanandaj-Sirjan (SS) metamorphic zone. The maximum. crustal thickness is ~65 km beneath the SS zone, 50 km NE of the surface trace of the MZT. The region of over-thickened crust is rather narrow (150-200 km) and shifted to the NE with respect to the areas of highest elevations, the strongest negative Bouguer anomaly and the MZT. NE of the volcanic belt, the crust of the block of Central Iran is 40-to-60 km thick, displaying a possible 20-km Moho step beneath the surface exposure of the UrumiehDokhtar magmatic assemblage (UDMA). Two patches of Ps converted energy can be seen below the Moho in the northern half of the transect that cannot be attributed to multiple reflections. As a whole, the radial and transverse receiver functions display rather complicated waveforms along most of the profile, indicative of a very complex structure including a crust-mantle boundary with strongly-dipping segments and diffracted energy both at crustal and mantle level.
We also present preliminary results of the inversion of teleseismic P residual travel times. The maximum amplitude of the variations of time residuals along the profile is as large as 1.5 s with both long (faster arrivals in the SW than in the NE) and short-scale variations (in the MZT region). These teleseismic residuals were inverted for variations of P wave velocity with the ACH technique. The crustallayer exhibits rather strong lateral variations ofVp with lower velocities under the MZT and the UDMA, and faster velocities. under the SS zone. In the mantle, a clear difference appears between the faster P wave velocities of the Arabian craton and the relatively lower velocities of the mantle of Central Iran.

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