The role of western Mediterranea tectonic evolution in the geometry of a karstic domain in the Betic Cordilleras (Sierra Gorda, Spain): importance ofa tardy extensional regime

Located in the central part of the Betic Cordilleras, the large carbonate S ierra Gorda Massif provides an example of a west-Mediterranean karstic aqui fer. In spite of a complex polyphased tectonic history, the fracturing pres ents, from aerial views and at outcrop scale a quite organised geometry. Fo ur fracture directions are found over the massif: N000-010, N050-070, N090- 100 and N140-170. The statistical and geostatistical approach allows the ch aracteristics (lengths, orientations) and the spatial structure for each fr acture set to be determined. The N000-010 and N140-170 sets are grouped in packers whereas the two other sets are grouped in bands. The microtectonic study describes the evolution of the massif in the geodynamic context of th is part of Mediterranea, distinguishing three recent stages of brittle tect onic activity in the gu massif: a WNW-ESE Middle Miocene compression, then a NNW-SSE to NW-SE compression with a poorly wrenching regime, and finally a probably pre-Quaternary N-S radial distension. This last stage is essenti al for the karstification of the massif and groundwater circulation. From t he combined analysis of fracture network geometry and palaeostresses a mult iple porosity model in agreement with hydrological observations made inside the massif can be proposed: in particular, the hectometric N090-100 land N 050-070) fractures which are essential for the network connectivity, and ha ve a major drainage role at aquifer scale, while the N000-010 and N140-170 ones have a more local drainage role. This extensive tardy regime, which is for the first time described separately from the internal zones of the Cor dilleras, must be considered as a significant phenomenon on a regional scal e, and henceforth integrated in future geodynamic schemes of this part of M editerranea. (C) Elsevier, Paris.