Lithospheric structure beneath the Zagros collision zone resolved by non-linear teleseismic tomography

Abstract

S U M M A R YThe upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.

New constraints on lithospheric thickness of the Iranian plateau using converted waves

“…However, this thickness can not be considered as a reliable factor to control the high elevations of Zagros, which are also not underlain by the thickest crust (~ 50 km). We suggest in good agreement with Shomali et al (2011) that the high elevation in the ZFTB is supported by hot and shallow asthenosphere rather than a thickened crust and lithosphere. The same scenario was previously reported beneath the high Alborz mountain (Sodoudi et al, 2009).…”

“…High resolution tomography images provided by Shomali et al (2011) revealed a thick continental lithosphere (more than 200 km) beneath the Arabian shield (ZFTB) and no (or very thin) lithospheric mantle under the Central Iran. Furthermore, they indicated that the low-velocity anomaly beneath UDMA continues with less intensity underneath the ZFTB to depths of about 200 km.…”

“…Bird (1978) used finite element modeling and showed that the slab detachment can cause a gap at depth between the lithospheres of Arabia and Eurasia under the MZT. The shallow LAB observed beneath the northernmost UDMA and Central Iran is most likely originated by active upwelling of the asthenospheric material into shallower mantle due to slab break-off and sinking into the mantle (Molinaro et al, 2005a;Shad Manaman et al, 2010;Shomali et al, 2011). Another explanation for the observed LAB thickness estimates points to lithospheric delamination underneath the SSZ (Shad Manaman et al, 2011).…”

“…McKenzie and Priestley (2007) estimated a thick lithosphere of about 260 km beneath Iran using low-resolution surface waves and suggested that the process that generated the thickened crust beneath Iranian plateau has also resulted in thick lithosphere that extends beneath the whole plateau. Teleseismic P wave tomography and Rayleigh wave dispersion measurements (Kaviani et al 2007) as well as teleseismic tomography based on S phase inversion (Keshvari et al 2011) revealed a low velocity zone at ~80 km beneath Central Iran and UDMA. Furthermore, a combined study including gravity, geoid, and topography data by Molinaro et al (2005) showed that the lithospheric thickness decreases from 200 km beneath the Arabian platform to about 100 km beneath the UDMA.…”

Lithospheric mantle heterogeneities beneath the Zagros Mountains and the Iranian Plateau: a petrological-geophysical study

“…Tomographic cross-sections presented by Alinaghi et al (2007) show northward-dipping high-velocity mantle anomalies beneath Central Iran, which can be interpreted as remnants of the subducted Neotethys oceanic lithosphere, as was later noted by Paul et al (2010). Shomali et al (2011) investigated the upper-mantle structure of the Zagros Mountains in southwest Iran, using traveltime teleseismic tomography. The results show a thick (more than 200 km) continental lithosphere in the Arabian Platform, while very thin (or no) lithospheric mantle is seen in Central Iran.…”