Fault-propagation folding in the Cotiella extensional fault system (Pyrenees)

The Cotiella Upper Cretaceous basins in the central southern Pyrenees host an outstandingly exposed extensional fault system and related folds in the southern Pyrenees. These structures developed by the extensional gravitational collapse of the postrift carbonate platform which was deposited above a Triassic salt layer at the rift margins. Extensional deformation was coeval with salt expulsion and formation of salt structures. The Cotiella extensional basins developed in the eastern part of the Bay of Biscay-Pyrenean Atlantic arm. Subsequent Pyrenean contractional deformation preserved the main extensional features, and as result, these basins provides an almost unique exposed analogue for gravity-driven extensional folds developed on passive margins. Among the Cotiella extensional structures, the Armeña rollover involves more than 3 km of middle Coniacian to lower Santonian wedge-shaped syn-kinematic sediments which are overlying middle Albian to lower Coniacian pre-kinematic strata detached above an Upper Triassic salt layer. Although the rollover was partially inverted during the Pyrenean orogeny, its first order extensional architecture and the associated syn-extensional fracture pattern are well preserved. This communication will focus on an integrated macro- and mesostructural analysis of the relationships between folding and fracturing in the Armeña fold.

Both the hanging wall and foot wall of the NE-dipping Armeña fault exposes two sets of bed-perpendicular joints which are oriented about parallel (longitudinal) and perpendicular (transverse) to the fault. The frequency of longitudinal joints increases towards the fault, being progressive in the central portions of the rollover and transitional in its lateral termination, where two panels of highly and poorly fractured strata are divided by a very narrow area. These panels coincide with the SW- and NE-dipping limbs of the hanging wall syncline of the rollover.

Our research points out that fracture density varies in the different structural positions of the fold. First, widespread longitudinal jointing occurred as part of the regional extensional framework, affecting the entire rock volume with a rather constant density. Second, laterally decreasing fault-propagation rate caused the development of a hanging wall syncline, preferentially located at the lateral termination of the rollover. There, folding of strata above the slowly propagating fault’s tip was ensured by development of further longitudinal joints that underwent shearing during the later stage of fault-propagation. Understanding of fracture patterns and distribution in extensional rollovers may provide new insights to better understand fluid flow and hydrocarbon accumulation in traps developed in similar settings.