P-wave azimuthal seismic anisotropy across the Zagros

The Zagros Fold and Thrust Belt (ZFTB), a part of the Alpine-Himalayan mountain chain, is an orogenic response to the ongoing northward convergence of the Arabian plate towards the Iranian micro continent. This young and active deforming belt located in western and southwestern Iran is a remarkable place to study the processes occurring in convergence zones during early stages of continent collisions. From northeast to southwest, the tectonic units of the Zagros collision zone consist of 1) the Uromieh-Dokhtar Magmatic Arc (UDMA), 2) the Sanandaj-Sirjan Zone (SSZ), and 3) the ZFTB. Main Zagros Thrust (MZT), a suture between the Iranian and Arabian plates separates the ZFTB and SSZ units. Physical properties in an anisotropic media, in contrast to those in an isotropic media, depend on direction; that is, they vary as a function of orientation. Seismic anisotropy occurs when seismic waves propagate faster in one direction than another. The presence of seismic anisotropy in the upper mantle normally depends on the lattice-preferred orientation (LPO) of mineral crystals. Asthenospheric convection flow beneath continents and olivine mineral LPO are the main reasons for anisotropy in this part of mantle. Olivine crystals, as a dominant mineral in the upper mantle, tend to align with the mantle convection. Models obtained for Earth anisotropy show that anisotropy has an axis of cylindrical symmetry. Anisotropy with a horizontal axis of symmetry is called Horizontal Transverse Isotropy (HTI). In such a medium, there is no anisotropy in directions perpendicular to the symmetry axis. In this paper, azimuthal anisotropy was studied in the upper mantle beneath a profile across the Zagros (Zagros profile) to a depth of 460 km using teleseismic P-wave relative residuals. Fifty-six teleseismic earthquakes were selected with epicentral distances between 30 and 90 and with magnitudes greater than 5.5. The data were corrected for the effect of crustal structure before inversion. Using P-residuals (residual spheres), attempts were first made to group 66 seismic stations along the Zagros profile, based on the directional dependence of the data. The stations were divided into seven groups, and rose diagrams were constructed for these data confirmed the result of residual spheres. It is necessary to note that when two rays propagate in opposite directions along the same ray path, it is expected that they have similar relative travel times. Thus, subtracting 180 from back-azimuths between 180 and 360, they are mapped in back-azimuths between 0 and 180 and conduced to data augmentation. The relative residuals obtained were plotted related to back-azimuths beneath each station and then a 4-degree polynomial curve was fit to the data from tt = P0 + P1θ + P2 θ2 + P3 θ3 + P4 θ4, where is the arrival-time relative residuals (s), is the back-azimuth (degree), and P0, P1, P2, P3 and P4 are the curve coefficients for the 4-degree polynomial curve. The fast velocity direction is analogous to the minimum of relative residuals in the curve and vice versa; the maximum of the relative residuals is correlated with the slow axis of anisotropy. The results indicate that the orogen-parallel fast velocity direction (NW-SE) in the upper mantle beneath Central Iran and the UDMA change to orogen-normal (NE-SW) beneath ZFTB and SSZ.