Investigation of the inner structure of La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, Spain)

La Crosa de Sant Dalmai volcano is the largest volcanic edifice of the Catalan Volcanic Zone (NE Spain). It is a very well preserved maar-type structure, 1.5 km in diameter, excavated at the contact between a hard substrate and a soft substrate formed by Palaeozoic granites and Pliocene and Quaternary gravels, respectively. In order to infer the uppermost inner structure of La Crosa de Sant Dalmai maar and to characterise its main geological and tectonic constraints, we have performed a multiparametric geophysical study including gravimetry, magnetometry, self-potential, and electrical resistivity tomography. The results obtained together with a field geology revision and additional geological data from two drill cores, provide a detailed picture of the post-eruptive maar infill sequence as well as of the uppermost part of the maar-diatreme structure. This information helps in understanding the origin and subsequent evolution of the volcano, which included an alternation of phreatomagmatic and Strombolian phases. Geophysical data show that the last Strombolian phase, which culminated with the formation of a scoria cone inside the maar, was associated with a NW–SE trending regional fault. The little erosion and degradation of the original tephra ring suggest a much younger age of La Crosa de Sant Dalmai maar than was previously stated.

► Geophysical methods used to obtain 3D geological information of La Crosa maar
► We characterise the post-eruptive deposits that infill the maar crater.
► A NW–SE regional fault was identified inside the basement.
► The NW–SE regional fault exerts a control on the location of the Strombolian cone.