Isotropic and Anisotropic P Wave Velocity Structures of the Crust and Uppermost Mantle Beneath Turkey

Abstract

Compressional and extensional tectonics following northward plate convergences since the Miocene have formed the major surface features in Turkey, such as faulting and orogeny. Despite increasing efforts in last few decades aiming to elucidate the current architecture of the crust and mantle beneath Turkey, several issues regarding the depth extent of the deformation zones, crust-mantle interaction (e.g., coupling and decoupling) in relation to the deformation, and stress transmission in the lithosphere remain elusive. Here we present high-resolution 3-D P wave isotropic and azimuthal anisotropic velocity models of the crust and uppermost mantle beneath Turkey by inverting 204,531 P wave arrival times of 8,103 local crustal earthquakes. Our results reveal low-velocity anomalies or velocity contrasts down to the uppermost mantle along the North and East Anatolian Fault Zones. The fast velocity directions (FVDs) of azimuthal anisotropy in the lower crust and uppermost mantle are parallel to the regional maximum extensional directions in western Turkey, and the FVDs in the crust and uppermost mantle are parallel to the surface structures in southeastern Turkey. These results indicate that vertically coherent deformation between the crust and uppermost mantle occurs in western and southeastern Turkey. However, in central northern Turkey, the FVDs in the uppermost mantle are oblique to both the FVDs in the lower crust and the maximum shear directions derived from GPS measurements, suggesting that the crust and lithospheric mantle are decoupled there. Owing to increasing efforts in passive seismic monitoring in Turkey, the number of temporary and permanent seismic stations has increased dramatically in the past decade, which has greatly improved the station coverage and so the spatial resolution of various seismic imaging results (e.g.,

The Influence of the North Anatolian Fault and a Fragmenting Slab Architecture on Upper Mantle Seismic Anisotropy in the Eastern Mediterranean

“…Given the evidence for a thinned lithosphere, the apparent lack of influence of either the NAF or EAF on upper mantle anisotropy, and the discrepancy between plate motion and anisotropic fast directions, it has been suggested that the Anatolian crust and mantle are mechanically decoupled (Mutlu & Karabulut, 2011;L. Jolivet et al, 2013;Wang et al, 2020). On the other hand, low-velocity upper mantle structures beneath the NAF have been imaged in tomographic studies and interpreted as signatures of a plate-scale shear zone (Fichtner et al, 2013;Papaleo et al, 2017Papaleo et al, , 2018, favoring a hypothesis of coherent crust and lithospheric mantle deformation.…”

“…At the NAF, in addition to the lack of fault‐parallel observations in the aforementioned XKS splitting studies (Figure 2a), fault‐oblique uppermost mantle anisotropic fast directions have been observed in anisotropic Pn tomography (Mutlu & Karabulut, 2011), as has a sharp change in fast directions across the Moho in anisotropic P ‐wave tomography (Wang et al., 2020). Given the evidence for a thinned lithosphere, the apparent lack of influence of either the NAF or EAF on upper mantle anisotropy, and the discrepancy between plate motion and anisotropic fast directions, it has been suggested that the Anatolian crust and mantle are mechanically decoupled (L. Jolivet et al., 2013; Mutlu & Karabulut, 2011; Wang et al., 2020). On the other hand, low‐velocity upper mantle structures beneath the NAF have been imaged in tomographic studies and interpreted as signatures of a plate‐scale shear zone (Fichtner et al., 2013; Papaleo et al., 2017, 2018), favoring a hypothesis of coherent crust and lithospheric mantle deformation.…”

“…A hypothesis of relatively low-strain olivine LPO along the NAF provides a good explanation for observations of  . Such a hypothesis obviates the need for a decoupling between crust and mantle that Wang et al (2020) suggest is required to explain fault-oblique  E in the uppermost mantle in their anisotropic P-wave tomography model. However, given that the NAF formed in the scars of suture zones, we also cannot preclude the possibility that some impression remains on LPO fabrics from strain localization prior to the development of the NASZ, although the lack of apparent effect of other suture zones on splitting parameters in Anatolia suggests that this is likely to be minor.…”

“…In the Aegean and far‐western Anatolia, XKS splitting investigations (Figure 2a: Evangelidis et al., 2011; Hatzfeld et al., 2001; Paul et al., 2014; Schmid et al., 2004), anisotropic Pn tomography (Mutlu & Karabulut, 2011), anisotropic P ‐wave tomography (Wang et al., 2020; Wei et al., 2019) and anisotropic surface wave tomography (Endrun et al., 2011) have shown fast directions that parallel ongoing crustal stretching directions and mineral lineations in exhumed shear zones (L. Jolivet et al., 2009), consistent with interpretations that deformation is vertically coherent throughout the crust and mantle lithosphere (L. Jolivet et al., 2009, 2013; Kreemer et al., 2004). This is in contrast with the rest of Anatolia, where early shear‐wave splitting investigations indicated consistently NE/SW‐oriented , despite the complex surface strain field (Biryol et al., 2010; Sandvol et al., 2003).…”

New Insights Into Crustal Properties of Anatolia and Its Surroundings Inferred From P‐Coda Autocorrelation Inversions

“… Tectonic sketch map of the study region (adapted from Wang et al., 2020). The red lines indicate major faults and trenches while suture zones are represented by thick black lines based on Okay and Tüysüz (1999).…”