Sedimentological and Geochemical Characteristics of Turbidites Related to Earthquake Activity in the Sea of Marmara
Abstract
The Sea of Marmara (SoM) is a tectonically active basin being located on a major continental transform fault boundary between the Eurasian and Anatolian plates. It consists of three transtensional major subbasins in excess of -1250 m and smaller ones with -100 to -200 m forming the E-W elongated gulfs and bays. The major subbasins have steep slopes, especially in the north, with slope angles greater than 18°. The sedimentary infill sequence in the deep basins consists of about 75% turbidite-homogenite units (THU) and 25% hemiplagic sediments, deposited at sedimentation rates of 1 to 3 m/ka. Deposition of most of the THU has been triggered by seismo-tectonic activity that constitute a serious geohazard in the densely populated coastal areas. Identification and dating of the THUs are therefore important in the repeat-time determination of the earthquakes on different fault segments, and thus, for the probabilistic earthquake risk assessment in the region. We studied the sedimentological, physical and chemical characteristics of THUs in several cores recovered from different Marmara basins, and identified the record of the devastating (Mw=7.4) 1999 Izmit earthquake, using digital X-Ray Radiography, XRF Core Scanner, MSCL, stable isotope and grain-size analyses. The units were dated using AMS C-14 and radionuclide methods. THUs are characterized by a relatively thin (commonly mm to several cm thick) sand-silt unit at the base and thick (commonly several tens of cm) homogeneous mud at the top. Digital X-ray radiography indicates that the THUs have multiple sand-silt laminae in the basal unit showing bidirectional foresets and a sharp and often erosional basal contact. These features indicates deposition by a single turbidity current reflecting or deflecting from the opposite slopes. The XRF Core Scanner analysis indicates two specific geochemical anomalies associated with the turbiditites sampled over the active faults: Ca enrichment in the basal coarse part, and Mn depletion within the THU unit. Both of these anomalies can be explained by the anaerobic methane oxidation (AMO) at or near the seafloor. The high upward methane flux towards the seafloor during the crustal deformation and fault rupture and the associated AMO lead to suboxic-anoxic bottom-water conditions in the confined Marmara basins and the formation of the black sulphide patches and carbonate crusts along the active segments of the Main Marmara Fault. This conclusion is supported by several lines of evidence, including O- and C-isotope data on carbonates, oceanographic surveys in the Izmit Gulf before and after the 1999 earthquake, and the Nautile submersible observations in 2007. The sedimentary records of earthquakes identified and dated in the cores can be confidently matched with the historical earthquake records.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.U51A0021C
- Keywords:
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- 1051 Sedimentary geochemistry;
- 3004 Gas and hydrate systems;
- 3022 Marine sediments: processes and transport;
- 7250 Transform faults