Mud volcanic episodicity : subduction zone water budget, long-term monitoring and laboratory case studies

Abstract

Mud volcanism is a widespreaded phenomenon that has been reported both on land and on the seafloor. Despite their worldwide occurrence, more than 60% of mud volcanoes are distributed in proximity of subduction zones. In these tectonically active areas, the presence of overpressured fluids and sediments released through the accretionary prism fuels the formation of such piercement structures. Hence, the mud volcanoes act as windows to depth, making available a wealth of material that otherwise could not be sampled by scientific drilling due to technical limitations. The nature of mud volcano ejecta is various since a lot of different processes interact during the ascent to the seafloor. Nevertheless, studies on mud volcanoic products contributed to elucidate the dynamics of the so-called subduction factory shedding some light on the fluid cycling in subduction zones. At the same time, in order to understand the role of mud volcanoes as fluid expulsion structures, their formation as well as evolution has to be constrained. The main mechanisms, which have been hold responsible for that, are either diapiric ascent of a buoyant body of fluidized mud or hydrofracturing in areas of structural weekness, setting the path to mud intrusions. Once they break trough the seafloor, mud volcanoes are subject to numerous internal and external triggers, which hamper or enhance their activity. Earthquakes, as well as episodic inflation and deflation of an underlying mud reservoir might play a major, concomitant role in the release of accumulated pressure at depth. This role as pressure release valves for deeper sediments makes mud volcanoes complex, dynamic environments governed from a multitude of different processes. The aim of this dissertation is to enhance the understanding of mud volcanism by summarizing the current knowledge on the matter with new insights from case studies, i) explaining the role of mud volcanoes in the Nankai subduction zone offshore Japan, ii) focussing on factors that govern mud volcanic activity at the Athina MV, in the Eastern Mediterranean and finally iii) answering the open questions regarding origin and evolution of these sedimentary structures through laboratory analog experiments. Thirteen mud volcanoes have been reported in the Kumano Basin, landwards of the Nankai Trough. Their role in the water budget of the Nankai subduction zone have been evaluated creating a model of water circulation in the upper and lower plates using constraints from IODP holes of the NanTroSEIZE project, in situ geochemical and heat flow data and water isotopes measurements. Clay minerals have been identified as an important origin of water release in the region whereas mud volcanoes are deemed only minimally responsible for water expulsion, which seems to be diffusively emitted through the whole basin. Admittedly, paroxysmal phases in mud volcanoes activity could not be taken into account in this study. However this aspect of mud volcanism have been tackled with the deployment of a multiparametric observatory on top of the Athina MV, south of Turkey. More than two years of pore pressure, tilt and temperature data revealed an unprecedented connection (in the submarine environment) between variations of those parameters and earthquake occurrence. Such external triggers of mud volcano activity have been linked to magnitude/distance ratio, ground motion and local factors like geology and tectonic structures. Moreover, signals not related to external triggers gave hints on the internal dynamics of the Athina system. Finally, to assess the existing concepts of formation and development of mud volcanoes, a set of analogue experiments on mud breccia samples have been conducted. The unicity of these tests lies in the material used to model hydraulic failure: sediment samples from three different submarine mud volcanoes (Japan, Costa Rica and Mediterranean Sea). The experiments, which showed the characteristics of the plumbing system and the surficial structures forming before and after hydraulic failure of the samples, confirmed hydrofracturing as formation mechanism responsible for mud volcano formation and demonstrated how episodicity in and around mud volcanoes can be the result of inflation and deflation of a mud reservoir at depth.