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

Alastair H.F. Robertson1

(Alastair.Robertson@glg.ed.ac.uk)

Achim Kopf2

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

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

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

Petrographic studies of clasts and matrix within debris flows making up the Milano and Napoli mud volcanoes drilled south of Crete (ODP Sites 970 and 971) provide insights into the palaeo-tectonic setting of the Eastern Mediterranean (Sea of Crete area) during Upper Oligocene-Upper Miocene. Lithified clasts are mainly quartzose sandstones (litharenites), siltstones, calcarenites (both shallow-and deep-water) and pelagic carbonates. Biostratigraphic evidence indicates mainly Middle Miocene ages for fossiliferous clasts, with older microfossils being reworked. Textural evidence (e.g. grading, sorting, grain alignment) suggests the clastic lithologies are turbidites. Sources of the sandstones were mainly plutonic igneous (plutonic quartz, exsolved feldspar, microcline, augite) and metamorphic (polycrystalline quartz, schist, muscovite, foliated biotite). Well-rounded quartz grains are seen as aeolian derived. Provenance was ultimately from the Precambrian basement of North Africa. Contrasting, texturally immature lithic sandstones include serpentinite, basalt, radiolarian chert and sphene, of probable ophiolitic origin, material that is inferred to have been derived from the higher thrust sheets of Crete or adjacent orogenic areas, prior to Plio-Pleistocene erosion and extensional downfaulting of the South Aegean. Shallow-water-derived carbonates were redeposited as calciturbidites, derived from both margins. Contrasting provenances are also indicated by X-ray diffraction of the matrix. Abundant kaolinite has a presumed Nile source, while illite and chlorite are seen as being mainly Eurasian derived. The matrix of the mud debris flows includes numerous small clasts of unfossiliferous claystone and shard-like fragments, showing pseudo-lamination, micro-shearing and cross-cutting veinlets, features that are taken as evidence of high strain and high fluid pressure during matrix formation. Evidence of rare calcite-filled veins and breakage of quartz grains in serpentinite-bearing sandstone clasts are also indicative of some deformation prior to mud volcanism, possibly related to formation of an early accretionary wedge. However, there is no evidence within the clasts of pervasive tectonic deformation (e.g. cleavage, stylolites, or flattening of competent grains), which places constraints on the mode of mud volcanism. 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. Session 43 - Mediterranean