GEOLOGICAL EVIDENCE FOR MUD DIAPIRISM ON THE MEDITERRANEAN RIDGE ACCRETIONARY COMPLEX

A mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the Mediterranean Ridge. This breccia is composed of subrounded clasts supported by a clay-rich matrix and ranges in age from Cretaceous to early Miocene. Sedimentological analysis allows identification of a "primitive" intrusive and a "reworked" mud breccia, the latter being formed by mud extrusion and gravitational reworking of the former. Homogeneous mud, sorted by grain size, which occurs at the top of the diapirs has been interpreted as a lake deposit. Based on composition and carbon isotopic ratios, gases present within the diapiric material suggest a partial thermogenic origin, and gas escape structures have been identified in the mud breccia and host sediments. Carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation. Their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep-sea lithification in the Mediterranean Sea. The presence of a marker bed composed of Mn nodules and bacterial colonies, the mud lake deposits showing lower carbonate content than primitive mud breccia, and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures. Evidence of active mud diapirism can be found from about 300,000 yrs B.P. to the present.