Investigation of gas and gas hydrate accumulations along the continental margin of the Danube Delta (Romania and Bulgaria offshore) using seismic reflection data

In2012, a comprehensive study of the Danube River’s submarine channelscontinental slope was conducted, employing multi-beam bathymetry and over 2300km of high-resolution two-dimensional seismic reflection data. Theinvestigation aimed to delve into the area's morphology, potential for gashydrate presence, and the correlation between stratigraphic units and gas hydrates.Three distinct zones, revealed Bottom Simulating Reflectors (BSRs) indicatingthe base of gas hydrate accumulations in the seismic data. These BSR areasexhibited Type-1 reflections, characterized by continuous cuts across layers.Notably, five discrete levels of BSRs were detected, suggesting a consistentgas composition across them. The multiple BSR formations are attributed tohigher sedimentation rates relative to gas hydrate dissolution rates. Mass transportdeposits (MTDs) within the gas hydrate stability zone (6 in total) wereidentified; their highly consolidated nature could account for the absence ofgas hydrates within them. Additionally, one MTD displayed elevated heat flowmeasurements, indicating a higher geothermal gradient, likely due to itsrelatively high thermal conductivity. This disparity in thermal propertiesexplains the deeper-than-expected BSR in this specific region, as it forms at alower temperature equilibrium level due to efficient heat conduction.Keywords:Danube Channel-Levee System, Multiple BSR, MTD, Geothermal Gradient.</p

Post-500 ka and Holocene activity on distributed faults of the North Anatolian Fault system along the southern shelf of Marmara Sea, Turkey in Tectonophysics.

Geodetic monitoring and patterns of seismicity indicate that the Northern Branch of the North Anatolian Fault(NAF) absorbs the majority of the relative motion between the Eurasia and Anatolia plates along the northernMarmara Sea. Nonetheless, historical seismicity documents that the Central Branch of NAF is also hazardous,with earthquakes diffusely occurring in the southern Marmara Sea. In order to better assess the seismic hazardsfacing large cities along the southern coast, we recently collected geophysical data across the adjacent shelf.These data include closely spaced high-resolution multichannel seismic profiles, sparker seismic profiles, CHIRPsub-bottom profiles, and multibeam bathymetric data. The stratigraphic and structural analyses of this newdataset and prior datasets highlight the geometry of three long faults and many shorter, discontinuous faults. Thethree longer faults are interpreted as primarily strike-slip fault zones. All the mapped faults are Late Quaternaryactive and, in fact, many fault segments are Holocene active. Smaller discontinuous faults are present in Gemlikand Erdek bays, some of them clearly active during Holocene time. This pattern of Late Quaternary active faultscan account for the well-documented dispersed seismicity on the southern shelf. Based on the lengths of thevarious fault segments, we estimate that earthquakes with moment magnitude as high as 7.4 may occur along thesouthern shelf of the Marmara Sea. Therefore, the system of distributed faults that constitutes the Central Branchof the NAF in that area represents a significant seismic hazard for the southern coastal cities.</p