Session 43: Palaeoclimate, palaeoceanography and tectonics of the Mediterranean region

Processes of mud volcanism on the Mediterranean Ridge

Achim Kopf1

(kopf@perm.geologie.uni-freiburg.de)

Alastair H.F. Robertson2 (Alastair.Robertson@glg.ed.ac.uk)

1 Geologisches Institut, Albert-Ludwigs-Universitaet Freiburg, Albertstrasse 23B, 79104 Freiburg, Germany

2 University of Edinburgh, Department of Geology and Geophysics, West Mains Road, Edinburgh EH9 3JW, UK

Drilling the Milano and Napoli mud domes on the Mediterranean Ridge during ODP Leg 160 (Sites 970/971) provided insights into processes of mud volcanism. Eruption of largely mud debris flows began prior 1.5 Mabp, based on biostratigraphic evidence. Both mud volcanoes evolved through a distinct cycle. Early extrusion built up cones of clast-rich debris flows and turbidites, overlain by pelagic sediments, and then followed by successive eruptions of matrix-supported debris flows containing fewer clasts. Both layering and gradational changes are indicated by core observations and formation microscanner data. Upbuilding was accompanied by subsidence to form peripheral moats that still exist in the case of Napoli. The location of the mud volcanoes near the rear part of the Mediterranean Ridge accretionary wedge is critically dependent on collision with the Cyreniaca Peninsula of North Africa to the south. This collision resulted in backthrusting of the evaporite-bearing accretionary wedge against a rigid backstop of Cretan continental crust to the north. This backthrusting allowed egress of fluid- and gas-rich muds (presumably overpressured) from depth, possibly from within the decollement zone. Many of the clasts and some of the matrix material, however, are more likely to have originated from the overlying accretionary wedge as no evidence for diagenetic alteration or pervasive deformation or metamorphism was observed. Physical plucking and/or hydraulic fracturing by rising overpressured muds, followed by upward migration and seafloor extrusion of multiple debris flows is the most probable mechanism of mud dome genesis. Combined results from permeability and shear strength tests, grain size analyses, study of sedimentary textures, and estimates of the viscosity of mud volcanic deposits provide further clues to eruptive processes. Together with side-scan sonar images a quantitative estimate of the mud efflux related to tectonic shortening can be made.