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.

Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.

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<p>The Main Marmara Fault (MMF) in NW Turkey south of Istanbul is a segment of the North Anatolian Fault Zone (NAFZ) that constitutes a right-lateral continental transform fault.  Several well-documented strong (M7+) earthquakes indicate that the MMF poses a great risk to the Istanbul metropolitan region. A 150 km long stretch of the MMF has not ruptured since 1766 and the recurrence time of 250 yrs for M7+ events derived from historical records indicate that the fault is overdue. We introduce a new project addressing how the rheological configuration of the lithosphere in concert with active fluid dynamics within the crust and mantle influence the present-day deformation along the MMF in the Marmara Sea region. We test the following hypotheses: (1) the seismic gap is related to the mechanical segmentation along the MMF which originates from the rheological configuration of the crust and lithosphere; (2) variations in deformation mechanisms with depth in response to variations in temperature and (fluid) pressure exert a first-order control on the mode of seismic activity along the MMF, and, (3) stress and strain concentrations due to strength and structural variability along the MMF can be used as an indicator for potential nucleation areas of expected earthquakes. To assess what mechanisms control the deformation along the MMF, we use data from the ICDP GONAF observatory (International Continental Drilling Programme – Geophysical Observatory at the North Anatolian Fault) and a combined work flow of data integration and process modelling to derive a quantitative description of the physical state of the MMF and its surrounding crust and upper mantle. Seismic and strain observations from the ICDP-GONAF site are integrated with regional observations on active seismicity, on the present-day deformation field at the surface, on the deep structure (crust and upper mantle) and on the present-day stress and thermal fields. This will be complemented by numerical forward simulations of coupled thermo-hydraulic-mechanical processes based on the observation-derived 3D models to evaluate the key controlling factors for the present-day mechanical configuration of the MMF and to contribute to a physics-based seismic hazard assessment.</p>