Multiple movements recorded in a crustal weakness zone in NE Iberia. The Vallès-Penedès Fault revisited

Structural analysis and Anisotropy of Magnetic Susceptibility are here applied to the fault zone (several hundreds of meters thick) associated with an E-W striking, 20 km-long segment of the Vallès-Penedès Fault (Catalan Coastal Ranges). The results obtained indicate a complex structure, mainly resulting from the imprint of Paleogene compressional events on a fabric inherited from the Variscan orogeny, and its Late-Variscan and Mesozoic extensional history. The prevailing foliation on the fault gouge and microbreccia is oblique (striking NW-SE) to the main E-W trace of the fault. S-C structures affecting the foliation show both sinistral and dextral shear components on NE-SW and NW-SE planes, respectively, consistent with horizontal striations on quartz rods and planar bodies. Pure reverse and normal dip-slip shear components are also inferred from S-C structures at the meso- and micro-scale, indicating a NW to N (reverse), and S-SSE (normal) transport directions. The magnetic foliation averages the orientation of the observed foliation at the outcrop scale and the magnetic lineation shows two maxima. The first maximum has an overall horizontal plunge, parallel to the strike-slip transport direction inferred from S-C structures. The second maximum is parallel to the dip direction of foliation planes and also parallelizes the reverse transport direction and occasionally the intersection lineation between S and C planes, thus revealing the strong heterogeneity and partitioning of deformation. Cataclasis and pressure-solution are the main deformation mechanisms, in agreement with clay composition and low paleotemperatures determined for the Paleogene. The presented results are consistent with Paleogene transpression occurred through strain partitioning between the main thrust separating the Catalan Coastal Range from the Ebro basin (reverse slip) and the Vallès-Penedès fault zone (sinistral strike-slip), with an important influence of dextral shear associated to the secondary, WNW-ESE, segment of the fault (either an oblique ramp or resulting from the re-activation of the Llobregat Fault). A subsequent, Neogene extensional imprint slightly modified the compressional fabrics in the fault zone.