Moho depth and crustal thinning in the Marmara Sea region from gravity data inversion

Abstract

The free‐air gravity in the Marmara Sea reveals that the low density of sedimentary basins is partly compensated in the lower crust. We compiled geophysical upper crust studies to determine the sediment basin geometries in and around the Marmara Sea and corrected the gravity signal from this upper crust geology with the Parker method. Then, assuming long wavelength anomalies in the residual gravity signal is caused by variations in the Moho topography, we inverted the residual to build the Moho topography. The result shows that the Moho is uplifted on an area greater than the Marmara Sea with a maximum crust thinning beneath the basins where the Moho is at about 25 km, 5 km above the reference depth. We then evaluated the Neogene extension by comparing the surface covered by our 3‐D thinned model with the surface covered by an unthinned model with same crustal volume. Comparing this surface with areal extension rate from GPS data, we found a good compatibility indicating that the extension rate averaged over the Sea of Marmara area probably remained close to its present‐day value during major changes of tectonic regime, as the incursion of the North Anatolian Fault system during the Pliocene leads to the establishment of the dominantly strike‐slip present‐day system. We also show that crustal extension is distributed over a wider domain in the lower crust than in the upper crust, and that this may be accounted for by a relatively minor component of lower crustal ductile flow.

A statistical approach to relationships between fluid emissions and faults: The Sea of Marmara case

“…For instance, the transpressional active fault system affecting the Western High (Imren et al, 2001) and the transtensional system determining the present-day subsidence of the central basin (Grall et al, 2012) could be interpreted as flower structures that formed within the sedimentary cover. The sedimentary cover is here 5 to 7 km thick, comprising syn-kinematic basin fill as well as, probably, Eocene-Oligocene sediments (Bayrakci et al, 2013;Kende et al, 2017). The deformation zones we observed at the seafloor around the MMF, and which appear to influence the distribution of fluid emissions, are typically narrower.…”

Crustal Composition and Moho Variations of the Central and Eastern United States: Improving Resolution and Geologic Interpretation of EarthScope USArray Seismic Images Using Gravity

“…As shown in Figure 6, the minimum RMS yields Δρ Moho~1 10 kg/m 3 with a low correlation coefficient (0.03) between the modeled Moho gravity anomaly and the residual gravity. This value is less than the already-low Moho density contrasts estimated by Schmandt et al (2015) and Ma and Lowry (2017) and much less than the 400 kg/m 3 typically estimated for continental crust (Christensen & Mooney, 1995;Kende et al, 2017). To estimate the crustal and mantle dρ dκc and dρ dκm , we use the RMS of the final residual for a range of dρ dκc and dρ dκm .…”

Modelling tectonic deformation along the North-Anatolian Fault in the Sea of Marmara

“…Dextral shear was imposed onto the mobile plate by translating it at a constant displacement rate of 2 cm/h, for a total displacement of 7 cm. The scaling factor of the models was 2 × 10 −6 (1 cm per 5 km): the 15 km-thick upper crust (Kende et al, 2017) was reproduced by a 1.5 cm-thick sand pack, while 0.2 cm of silicone represented the ductile lower crust. Serial cross sections with 0.5 cm spacing were cut at the end of the experiments after wetting the models with tap water and waiting 24 h to ensure complete imbibition.…”

“…A specific feature of most experiments that simulate strikeslip faulting is localization of the master shear zone at a sharp boundary between nondeformable and mobile basal plates. In our model, we used a 2 mm-thick silicone layer, scaled to the relative thickness of the viscous lower crust as suggested by Kende et al (2017). The thickness and viscosity of this layer controls how efficiently basal displacement is transferred to the shallower crust.…”