Nature of the lithosphere across the Variscan orogen of SW Iberia: Dense wide‐angle seismic reflection data

Abstract

Two wide‐angle seismic transects have been acquired across the SW Iberian Massif. They crossed three major geological zones (South Portuguese Zone, Ossa‐Morena Zone, and Central Iberian Zone), with their tectonic contacts and the Pyrite Belt being of greatest interest. A total of 690 digital seismic recording instruments (650 Texans and 40 Reftek 3 component units) from the IRIS‐PASSCAL Instrument Pool were used. The transects (A and B) are each approximately 300 km long and consist of 3 and 6 shot points, respectively, with an approximately 60‐km shot point interval. The charge sizes range from 1000 kg at the edges to 500 kg at the center. These recently acquired experiments were designed to provide velocity constraints on the lithosphere and to complement the previously acquired normal incidence seismic profile IBERSEIS. Both data sets are part of the SW Iberia project, which was developed within the EUROPROBE program and designed to address fundamental questions about the nature and dynamics of the Variscan lithosphere. The acquisition parameters provide closely spaced wide‐angle seismic images of the lithosphere beneath SW Iberia. In transect A, the station spacing was on average 400 m, while along transect B, the receiver spacing was approximately 150 m. Because of this close trace spacing, the lateral continuity of the seismic arrivals is greatly improved. Frequency analysis revealed that the recorded events feature relatively low frequencies (6–25 Hz). After processing, the shot records show high‐amplitude and well‐defined arrivals. The interpreted PmP arrival, located at approximately 11 s (normal incidence traveltime), is characterized by high amplitude and relatively low frequency (6–12 Hz). A well‐defined Pn arrival appears at offsets beyond 120 km. At far offsets greater than 180 km, an upper mantle reflection is observed. Furthermore, within the upper crust, the shots records feature a relatively high‐velocity arrival, located at 4–5‐s normal incidence traveltime. The analysis of this arrival indicates that it probably corresponds to the top of the Iberian Reflective Body identified in the IBERSEIS deep seismic profile. The velocity models obtained by forward modeling show a complex crust, especially in the middle crust. The velocity models are the most detailed ones that have been produced in the area and contain a large amount of new features that are relevant to the understanding of the composition of the crust and upper mantle beneath the zone. The velocity depth functions derived from the velocity models have higher middle crustal velocities than the average in other continental areas. A comparison between laboratory seismic velocity measurements and the velocities of the models was carried out in order to estimate the crustal and the upper mantle composition. Results indicate that the high middle crust velocities correspond to rocks of a mafic composition. The combined data set reveals new aspects related to the lithospheric evolution of this transpressive orogen and allows us to attempt an interpretative cross section of the upper lithosphere in SW Iberia.