GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.2 181 instrumental seismic record of the area includes only some Mw >3 earthquakes, a few of them concentrated around the fault system (Fig. 1b), and the historical catalogue of the area is devoid of remarkable events too. Despite this, the Ubierna-Ventaniella Fault System is included in the Quaternary Active Fault Database of Iberia (García-Mayordomo et al. , 2012), where it is quoted as “debated”. Geological evidence of recent activity for this fault occurs indeed, including: (1) the Ventaniella fault offsets Lower Pleistocene alluvial fan and it is sealed by Upper Pleistocene deposits (Nozal Martín, F., Gracia Prieto, 1990); (2) a weak seismic activity is associated with the Ventaniella Fault (López-Fernández et al. , 2018); (3) in its western portion, the Ubierna fault offsets and folds a Pliocene erosional surface (Gracia Prieto et al. , 1990); (4) at the eastern contractional termination of the Ubierna Fault, i.e. at the Rojas fold system, geomorphic features that could be considered indicative of active folding occur (Tavani et al. , 2011). In detail, tilted Lower to Middle Miocene strata unconformably overlying folded Upper Cretaceous carbonates occur along the Rojas fold system, together with fluvial captures, a tiled fluvial meander, and a tilted alluvial fan. All these structures suggest a late Miocene - or earlier - tectonic activity along a fault system offering rupture areas exceeding 10 3 km 2 , hence having a remarkable seismogenic potential. In this work we analyse the structures indicative of a possible recent activity along the Rojas fold system, with the aim of better defining the timing Fig. 1 - Geological framework of the study area. (A) Elevation map of the northern Iberian Peninsula with the main structural features. (B) Geological map of the Cantabrian Domain and western Pyrenean Domain, with Mw>3 earthquakes indicated.