Variations in sediment physical properties and permeability of mud-volcano deposits from Napoli dome and adjacent mud volcanoes

Abstract

Active mud volcanoes on the Mediterranean Ridge accretionary prism were sammpled during Bannock Cruises 88 and 89 and Ocean Drilling Program Leg 160. Permeability tests on undisturbed whole-round samples from Napoli dome (Site 971) using a back-pressured system at effective stresses that ranged from 200 to 700 kPa revealed low hydraulic conductivities of the clayrich, undisturbed 'mud breccias,' which ranged from 5 x 10-8 to 5 x 10-9 mm/s. In general, conductivities of sediments from the footwall, flank, and crest of Napoli dome dropped to half their value when the load was incrementally increased from 500 up to 700 kPa. Pore volume, initially 50%-55%, was reduced to around 25% after multiple incremental loading between the permeability tests. Experiments on remolded mud breccia from Napoli dome (Site 971) and adjacent mud volcanoes using a geotechnical shear box as well as a Vane apparatus revealed peak shear strengths of 200-400 kPa. Plasticity indices also vary significantly for the mud-volcano deposits (i.e., from 25% to >40%), presumably because of variations in the composition and amount of the clay fraction. The friction angles obtained with shear tests indicate that montmorillonites (φ' peak = 10°-15°) and fine grained carbonates (φ' peak 27°-30°) are the two dominant mineral phases in the matrix. By comparison, hemipelagic sediments (calcareous ooze) show plasticity indices of <20% and shear strengths of >600 kPa with friction angles around 30°. The driving force of the Mediterranean Ridge mud volcanism is believed to relate to regional tectonic compression resulting from collision of the African and Eurasian plates. The properties of the evaporitic overburden, as well as the degree of overpressuring of mud at some depth, are thought to primarily control the activity. Phases of hemipelagic 'background' sedimentation, as indicated by interfingering patterns with mud breccia, are accompanied by a build-up of pressure at depth until the subsurficial, clay-rich muds are forced to extrude again. Low permeabilities, together with the grain-size variation (and indirectly viscosity) of the mud breccias, are considered to have a secondary effect on the episodicity in mud-volcanic activity on the Mediterranean Ridge, resulting from blocking vents.