Origin of mud breccia from the Mediterranean Ridge accretionary complex based on evidence of the maturity of organic matter and related petrographic and regional tectonic evidence

摘要

Drilling the Milano mud dome situated in theOlimpi field on the northern flank of the Mediterranean Ridgeaccretionary complex has documented episodic eruptiveactivity over the last 1 to >1.5 million years. Mud extrusion isrelated to backthrust faulting due to subduction of the AfricaPlate beneath a relatively rigid backstop of Cretan continentalcrust. The mud dome is mainly composed of mud breccia withup to 65 percent of polymictic clasts embedded in a clayey matrix.Previously published results from petrography and newmaturity data of both clasts and mud matrix presented hereallow us to distinguish between two different areas ofprovenance: (1) A northern province, characterised by atrimodal maturity distribution of organic material, was foundfor the majority of the mud breccia matrix samples studied.Similarly, some of the clasts studied show the three vitrinite-reflectance populations. These are carbonate and sandstoneclasts, inferred to have been eroded from the upper nappes ofthe Cretan thrust stack after exhumation in the late Miocene, aswell as mudstone clasts. Clay mineralogy of these clastscoincides with earlier results on seafloor samples of a northernprovenance, recovered in the Hellenic Trench. (2) A southernprovenance is proposed for a second group of clasts with abimodal maturity distribution, not showing the secondpopulation (0.64-0.82 percent Rm). These clasts are litharenites,calcilutites and calciturbidites of a proposed southernprovenance, i.e. North Africa.The presence of huminite in all clast and matrix samplesstudied (0.26-0.45 percent Rm) is indicative of a shallow burialhistory. Estimates regarding the depth of mobilisation of themud using the regional thermal gradient indicate burial notdeeper than 2 km below the seafloor. Organoclastic matrixcomponents mirror the northern clast provenance so that, priorto subduction and subsequent extrusion, deposition of the mudin the Northeastern Mediterranean can be inferred. We proposea model in which sediment from the north was trapped withinthe forerunner of the Pliny trench; this sediment was depositedabove the backstop of the subduction complex, which was lateroverridden by the accretionary prism as a result of incipientcollision; density inversion forces then favoured uprise andextrusion of underconsolidated sediments as mud volcanoes.The mud matrix was probably largely derived from aMessinian succession, although alternatives remain possible.