Elliptical mud volcano caldera as stress indicator in an active compressional setting (Nirano, Pede-Apennine margin, northern Italy)

The relations between mud volcanism and active tectonic strain were investigated at the Nirano mud volcano field (NMVF), near the active Pede-Apennine thrust front of the Northern Apennines thrust-and-fold belt (Italy). Active fluid release occurs through numerous vents within an elliptical depression developed over the crest of a thrust-related anticline. This depression is interpreted as a caldera collapse-like structure that may have developed in response to the deflation of a shallow mud chamber triggered by eruption and sediment fluid evacuation events. Like many volcanic calderas, the NMVF caldera is elongated nearly parallel to the direction of least horizontal compressive stress [S.sub.h], and is thus deduced to reflect the regional tectonic stress axes. It is concluded that mud volcano calderas exhibit mechanical similarities to the igneous analogs, and that the methods used in this first test could be applied to other calderas imaged by seismic data and in the field. Keywords: mud volcanism, elliptical caldera depression, active thrusting. Northern Apennines.