Basement geometry and cover-basement relationships in the central-western Jaca basin (southern Pyrenees): along-strike variations and role of structural inheritance

Pre- or syn-orogenic décollements in the external domains of fold-and-thrust belts enhance the decoupling between basement and cover folds and thrusts. In this structural setting, deciphering the degree of basement involvement and the kinematic relationship between thin- and thick-skinned structures can be challenging. This work addresses the study of basement/cover deformation in the central-western Jaca basin, in the southern Pyrenees. The Jaca basin represents the early South Pyrenean foreland basin that was latter deformed, piggy-back thrusted, and incorporated into the South Pyrenean fold-and-thrust belt. Its formation and deformation was coeval with the sedimentation of a thick syn-orogenic sequence consisting of early-middle Eocene turbidites (Hecho Group), late Eocene marls (Arguis Fm.) and late Eocene-Oligocene (Campodarbe Fm.) and Miocene continental units (Uncastillo Fm.). At surface, these syn-orogenic units are affected by a series of NE-SW and E-W-trending folds and thrusts that display frequent along-strike relays and geometrical changes. Debate exists on the geometry, timing and role of basement structures underlying these emerging thrusts and folds.

To shed some light in these discussed geometrical aspects, we carried out a combined structural and gravimetric study covering the central-western part of the Jaca basin. Four serial, seismic-based cross sections have been constructed (from East to West, the Hecho, Ansó, Roncal and Salazar cross-sections). In the cross-sections area, gravimetric data have been acquired along a homogeneous and dense grid, the spacing between gravity sites being ~ 1km.

Conversely to previous interpretations, seismic profiles depict a Paleozoic basement that is significantly faulted and involved in the deep structure of the Jaca Basin. Basement units are affected by numerous south-directed thrusts that partly result from the reactivation of inherited Permian-Triassic extensional faults. They provoke a significant basement uplift from East (Hecho cross-section) to West, with the shallowest basement being identified underneath the central part of the study area (Illón and Leyre surface thrusts). South of this basement uplift (Guarga syncline), the base of the Meso-Cenozoic sequence deepens drastically in the footwall of a main basement-involved structure that accommodates a significant shortening and connects with cover units through a long thrust flat along Middle-upper Triassic evaporites. Gravity data, although influenced by the non-coaxial crustal structure in the region, are consistent with the basement geometry deduced from seismic profiles. The Bouguer anomaly shows a prominent minimum along the Axial Zone and the eastern Jaca basin, a relative positive anomaly marking the central basement uplift (Illón-Leyre thrusts, central-western Jaca basin) and a relative minimum to the south of it, corresponding to the area where the denser basement units reach their deepest position. Both gravity and seismic data reveal a degree of cover/basement coupling which is greater than proposed in previous studies. Basement structures are not cylindrical along the study area, resulting in along-strike changes in outcropping folds and thrusts.