Abstract
Abstract. The Sea of Marmara, in northwestern Turkey, is a transition zone where the dextral North Anatolian Fault zone (NAFZ) propagates westward from the Anatolian Plate to the Aegean Sea Plate. The area is of interest in the context of seismic hazard of Istanbul, a metropolitan area with about 15 million inhabitants. Geophysical observations indicate that the crust is heterogeneous beneath the Marmara basin, but a detailed characterization of the crustal heterogeneities is still missing. To assess if and how crustal heterogeneities are related to the NAFZ segmentation below the Sea of Marmara, we develop new crustal-scale 3-D density models which integrate geological and seismological data and that are additionally constrained by 3-D gravity modeling. For the latter, we use two different gravity datasets including global satellite data and local marine gravity observation. Considering the two different datasets and the general non-uniqueness in potential field modeling, we suggest three possible “end-member” solutions that are all consistent with the observed gravity field and illustrate the spectrum of possible solutions. These models indicate that the observed gravitational anomalies originate from significant density heterogeneities within the crust. Two layers of sediments, one syn-kinematic and one pre-kinematic with respect to the Sea of Marmara formation are underlain by a heterogeneous crystalline crust. A felsic upper crystalline crust (average density of 2720 kg m−3) and an intermediate to mafic lower crystalline crust (average density of 2890 kg m−3) appear to be cross-cut by two large, dome-shaped mafic high-density bodies (density of 2890 to 3150 kg m−3) of considerable thickness above a rather uniform lithospheric mantle (3300 kg m−3). The spatial correlation between two major bends of the main Marmara fault and the location of the high-density bodies suggests that the distribution of lithological heterogeneities within the crust controls the rheological behavior along the NAFZ and, consequently, maybe influences fault segmentation and thus the seismic hazard assessment in the region.
Highlights
The Sea of Marmara in NW Turkey is an extensional basin associated with a right-stepping jog in the orientation of the North Anatolian Fault zone (NAFZ; Fig. 1), a westwardpropagating right-lateral strike-slip fault that constitutes the plate boundary between the Anatolian and the Eurasian plates (Fig. 1a; McKenzie, 1972; Sengör et al, 2005)
The results show that an average density of 2890 kg m−3, equivalent to the value assigned for the lower crust average density, would fit the gravity response of Model III to the ImprovedTOPEX dataset best (Fig. 8g)
3-D crustal density configurations are presented for the Sea of Marmara that integrate available seismological observations and are consistent with observed gravity
Summary
The Sea of Marmara in NW Turkey is an extensional basin associated with a right-stepping jog in the orientation of the North Anatolian Fault zone (NAFZ; Fig. 1), a westwardpropagating right-lateral strike-slip fault that constitutes the plate boundary between the Anatolian and the Eurasian plates (Fig. 1a; McKenzie, 1972; Sengör et al, 2005). E. Gholamrezaie et al.: 3-D crustal density model of the Sea of Marmara of approximately 250 years (Fig. 1b and c); this section experienced the last earthquake in 1509 and 1766, suggesting that the fault is mature and that the potential for a large seismic event is regarded as high (Ambraseys, 2002; Barka et al, 2002; Parsons, 2004; Janssen et al, 2009; Murru et al, 2016; Bohnhoff et al, 2013, 2016a, b, 2017b). A key question is if this 150 km long seismic gap will rupture in the future in one event or in several separate events due to segmentation of the MMF, an issue that will depend a lot on the stress evolution along the strike among other forcing factors In this regard, three-dimensional (3-D) geological models are the fundament of geomechanical models, and the distribution of density is of key importance, as density controls body forces. Gravity models can help to assess the density distribution at greater depths where borehole observations and/or seismic surveys have limitations
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