Shear wave splitting along a nascent plate boundary: the North Anatolian Fault Zone

Abstract

S U M M A R YThe North Anatolian Fault Zone (NAFZ) is a transform structure that constitutes the boundary between the Anatolian Plate to the south and the Eurasia Plate to the north. We analysed the properties of the upper-mantle strain field and mantle anisotropy in the vicinity of NAFZ via splitting of SKS and SKKS phases. We used data from the North Anatolian Fault (NAF) passive seismic experiment. This is the first study that analyses the upper-mantle anisotropy in this region and our results indicate that the observed upper-mantle strain field is uniform underneath the array with consistent NE-SW polarization directions for fast split waves. The measured lag times between the arrivals of the fast and slow split waves varies from 0.5 to 1.6 s for the study area. Smaller lag times are obtained consistently in the eastern part of the array. However, we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary (NAFZ).The uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy. The regional tectonic framework favours a SW direction of asthenospheric flow due to the forces acting on the upper-mantle exerted by the slab-roll-back taking place along the Aegean and the Cyprean Subduction Zones.

Strain Rate Analysis on the Çankiri-Bingöl Segment of the North Anatolian Fault in Turkey

“…This variation in direction for maximum compression and extension is also in agreement with the structural features of the Anatolian crust (Biryol et al, 2010).…”

“…The maximum shear strain rate is less in Region-1 according to the other four regions, but it has significant values (max 0.28 µstrain/yr) in Çankırı basin. Biryol et al (2010) suggest that comparison of fast polarization directions with plate motion directions requires selection of a reference frame that will yield true absolute plate velocities. There exist multiple reference frames for plate motions, based on different assumptions, and each of these has different motion directions and speeds.…”

Crust-mantle mechanical coupling in Eastern Mediterranean and Eastern Turkey

“…In eastern Turkey the mantle lithosphere is largely missing (47, 48) because of delamination and slab break-off around 11 Mya. The anisotropies are, therefore, thought to be asthenospheric and deep-seated there and in the eastern part of the NAFZ (35). The reason for the large delay times is probably the thickness of the anisotropic layer (low-viscosity asthenosphere) and increased shear on the vertical plane created by the interaction between the large-scale asthenospheric flow and the local flow field generated by the sinking slab underneath eastern Turkey (49-51).…”

“…The recorded delay time between these two components is a function of the severity of the anisotopy and the thickness of the anisotropic layer. The shear wave anisotropy studies that I use here are representative down to a depth of around 200 km or more in eastern Turkey (34) and the eastern part of NAFZ (35) being possibly shallower farther west (36). P n waves, on the other hand, propagate typically below the Moho discontinuity along approximately horizontal rays apart from source and receiver legs.…”

“…Therefore, it is more meaningful to compare the P n anisotropy directions with geodetically measured strain rates rather than velocities [see (40) for a study in which P n anisotropy is used to measure the width of mantle deformation across a Continental Transform]. In my analysis, I use the SKS fast-splitting directions from (34)(35)(36)41) for the Aegean, central and eastern parts of the NAF, eastern Turkey, and Dead Sea Transform, respectively (see Fig. 2).…”