Spatial variation of crustal tectonic stress in the southern Calabrian Arc.

Calabrian Arc is the area characterized by the greatest strain rate in the whole Italian peninsula (Palano, 2015). The crustal tectonic stress of this area is highly variable and the cause lies in the interaction between large scale geodynamic processes with the regional and local tectonic ones. We collected a dataset of 447 crustal stress indicators, published data and original ones, with the aim of displaying the trajectories of the minimum horizontal stress throughout the study area. The catalogue comprise two datasets. The former collects published and unpublished information relative to Quaternary outcrop scale and regional scale structural features. We also carried out dedicated field surveys in order to collect new data focusing on the areas lacking information. We measured about 1,400 meso-scale structural features at 39 different sites. All the collected information have been inverted with numerical techniques to obtain the associated stress tensors. The second dataset consists 387 focal solutions all coming from published papers. The information provided by the different authors has been homogenised by calculating the directions of T, P and B axes. From the stress indicators we considered the direction of the least principal stress s3with plunge <20°. The catalogue was initially elaborated as unique, then it has been subdivided in order to explore separately the stress acting in the shallower and in the lower crust. Data were interpolated on a 0.1° stepped grid using the algorithm and software proposed by Carafa et al.(2015). At this stage the indicators have been considered equal and no weight was assigned during the interpolation. In the shallower layer (0-10 km)the pattern of the Sh min clearly shows a general trend roughly E–W to ESE–WNW oriented; in general Sh min is homogeneous and varies with an angular span of about 50°. The average direction is N116°. In the deeper layer (10-35 km) the Sh min trend shows higher variability. The general ESE–WNW trend is slightly recognizable and marks only relatively small sub-positions. In particular, Sh min is NE-SW in the southeastern and southwestern parts and roughly NNW-SSE in the central eastern part of the study area. In several nodes of the grid, the interpolated Sh min orientation from the shallow dataset is almost orthogonal to the deeper one. Considering the stress indicators in the whole crust, the Sh min trajectories agree with the first-order tectonic stress field being ESE-WNW and Sh min is always at high angle with respect to the major normal faults. This distribution confirms the existence of a dominating geodynamic process active (at least) since late Quaternary. Conversely, discriminating the Sh min at shallow and deep crustal levels some differences arise. In the shallow crust, the Sh min trajectories are generally more coherent with the regional picture but in the deeper crust they show a higher variablility. Such second-order deflections of the stress field are rather concentrated below 10 km depth and this seems not justified by the role of the major faults (De Guidi et al., 2013). Probably other causes have to be find in the deeper crustal levels where several Vp anomalies have been highlighted by seismic crustal tomography (Presti et al., 2013; Palano et al; 2015). The approach followed in this study represents a new tool to investigate the tectonic stress and its continue spatial variation across the southern Calabrian Arc and it also allows a better comparison with geodetic or tomographic information.