The impact of strike-slip, transtensional and transpressional fault zones on volcanoes. Part 1: Scaled experiments

The activity of a regional strike-slip fault can affect or channel magma migration, can deform a volcano and can destabilise the edifice flanks. The aim of this study is to determine the location, strike, dip and slip of structures that develop in a stable or gravitationally spreading volcanic cone located in the vicinity of a fault with a strike-slip component. This problem is addressed with brittle and brittle-ductile analogue models. The one hundred and twenty three models were deformed by pure strike-slip, transtensional or transpressional fault displacements. The deformation was organized around an uplift in transpressional and strike-slip experiments and around a subsiding area in transtensional experiments. Most displacements are accommodated by a curved fault called Sigmoid-I structure, which is a steep transpressional to transtensional fault. This fault projects the regional fault into the cone and delimits a summit graben that is parallel to the main horizontal stress. The systematic measurements of faults strike and slip in the experiments indicate that extension along the faults in the cone increases with the extensional component of the regional fault and the thickness of the substratum ductile layer. The distribution of the fastest horizontal movements of the analogue cone flanks, which vary depending on the regional fault characteristics and on the composition of the substratum, correspond to the distribution of instabilities in nature. Natural examples of volcanoes sited in strike-slip contexts are described and interpreted in the light of the analogue results in the second article