FLUID SEEPAGE ALONG THE NORTH ANATOLIAN FAULT, IN THE SEA OF MARMARA, IN RELATION WITH TECTONICS AND SEDIMENTARY ENVIRONMENTS

Along the submerged section of the North Anatolian fault system within the Sea of Marmara, the Main Marmara Fault (MMF) is a case study on coupled fluid and deformation processes. Indeed, numerous sites of fluid venting occur in association with the active deformation at this major transcurrent plate boundary. Recent surveys combining visual observations, acoustic sounding, sampling, and long term instrument deployments permitted us to relate fluid outflow with geomorphologic and tectonic features.

Since the Izmit earthquake in 1999, a wide range of marine datasets were acquired in the Sea of Marmara. Multibeam bathymetric data image seafloor traces of active faults, as well as significant mass wasting processes affecting the steep slopes of the Sea of Marmara. Seafloor deformation and fluid emissions were observed with ROV during the Marmarscarps cruise (2005), with manned submersible during the Marnaut cruise (2007), and recently with AUV during Marmesonet cruise (2009). Various structural contexts were surveyed: strike-slip localized on a single linear fault (e.g. Western High), releasing and compressive jogs on the main strike-slip fault (e.g. Kumburgaz basin and Central High), fault segments with combined strike-slip and normal slip (N Cinarcik scarp), en-echelon normal fault system (S Cinarcik basin), and a basin edge with minor transpressive deformation (NW Tekirdag). Manifestations of fluid seepage are diverse and range from highly focused brackish water outflow emitted from authigenic carbonate chimneys to more extensive and diffuse fluid seepage areas. Fluids are mainly from relatively shallow basin consolidation and gases are mainly of biogenic origin, however several fluid emission sites expel fluids originating from deep within the sedimentary basin and include thermogenic gas, oil, and brines, and possibly, mantle He.

Mapping of seep distribution indicates that fluid emissions are primarily associated with deep-rooted active faults. In particular, gas emissions are found in Cinarcik Basin above a buried transtensional shear zone, which displayed aftershock activity at its eastern end after the Izmit earthquake. In most areas where it is observed, the main strike-slip fault trace presents reflectivity anomalies indicative of fluid outflows. However, secondary extensional (normal faults), compressional structures (anticline axes), and, in some occurrences, Riedel shears also influence the distribution of seepage sites. Gas emissions are observed on NE-SW trending anticlinal ridges a km or more away from where the main fault trace crosses the ridge. Some of the emission sites are not obviously correlated with active faults. In particular, gas emissions are generally present at the base of the slopes along the edges of the basins, but these do not systematically correspond to an active fault trace. Furthermore, observations also suggest that the sedimentary environment plays a role in providing pathways for fluid expulsion, mainly in the case of diffuse seepage and water outflow. Fluid emission sites are observed in close relationship with mass wasting deposits (turbidites, debris flow) or at the toe of destabilized slopes. In the NE Cinarcik basin, fluid seepage has been observed at the base of a scree slope with meter-sized boulders. Avalanche debris and coarse sandy turbidites provide high permeability conduits to drain fluid from the basin towards the active fault scarp.