Structural and Geophysical Characterization of the Western Termination of the South Pyrenean Triangle Zone

The South Pyrenean triangle zone represents the southernmost front of the Pyrenees at its central portion deforming the Upper Eocene-Miocene Ebro Basin deposits. Two main structures characterize its western termination, the Barbastro anticline and the San Roman backthrust, which detached on the Barbastro Formation (and lateral equivalents), an Upper Eocene-Lower Oligocene syntectonic evaporite-rich formation that acted as a multidetachment unit. Northward, the south directed Pyrenean thrust unit (i.e., Gavarnie-Sierras thrust sheet) detached along the Middle-Upper Triassic evaporitic rocks to finally ramp up and glide along the same Upper Eocene-Lower Oligocene multidetachment unit. A multidisciplinary approach allowed constructing a detailed structural and stratigraphic model of the study area. The workflow consisted of (1) constraining the geometry and structural architecture based on surface geology, interpretation of seismic lines (>900 km), and wells and (2) obtaining the 3-D density distribution of the multidetachment unit using gravity stochastic inversion (more than 7,000 gravity stations and 1,500 density data). The geometry of the sole thrust of the Gavarnie-Sierras thrust sheet was controlled by the distribution of the evaporite-rich units of the Barbastro Fm. Weak detachments promoted thrust salient formation and thrust flat geometries. The western termination of the South Pyrenean triangle zone is defined as a westward transition from a ramp-dominated and multiple triangle zone to a detachment-dominated one. Its geometry, kinematics, and location were controlled by the heterogeneous lithology of the Barbastro Fm. and its basal, halite-based detachment southern pinch-out.