Mud volcanoes: Indicators of stress orientation and tectonic controls

This study examines the use of specific mud volcano features (i.e., elongated calderas, aligned vents and elongated volcanoes) as potential indicators of tectonic stress orientation. The stress indicator principles, widely recognised for magmatic systems, have been discussed and applied to mud volcano settings such as in the Northern Apennines and the Azerbaijan Greater Caucasus, as well as in other instances where the analysis was fully based on a remote sensing study. The results of these applications are promising, the obtained maximum horizontal stress (SH) directions generally showing a good correlation with those determined in the upper crust by classical methods (i.e., earthquake focal mechanism solutions, well bore breakouts). Therefore, stress information from mud volcanoes could be used as a proxy for stress orientation (1) where stress data is lacking, (2) where settings are inaccessible (i.e., underwater or the surface of planets), or simply (3) as supplementary stress indicators. This study also pays special attention to structural elements that may control fluid expulsion at various length scales, and pathways that should have spawned the mud volcanoes and controlled their paroxysmal events and eruptions. Different types of sub-planar brittle elements have been found to focus fluid flow rising up-through fold cores, where the vertical zonation of stresses may take part in this process by creating distinctive feeder fracture/fault sets. On a regional scale, mud volcanoes in active fold-and-thrust belts may occur over wider areas, such as the prolific mud volcanism in Azerbaijan, or may cluster along discrete structures like the steep Pede-Apennine thrust in the Northern Apennines, where the generation of overpressures is expected to establish a positive feedback loop allowing for fault movement and mud volcanism.