Rubielos de la Cérida impact structure

The c. 80 km x 40 km sized Rubielos de la Cérida impact structure (impact basin, resp.) is located in northeast Spain north of Teruel and is considered

a companion of the Azuara impact structure and part of a large multiple impact event, the Azuara impact event, that from stratigraphic considerations and paleontological dating happened in the Upper Eocene or Oligocene (roughly 30 - 40 Mill. years ago). The name is attributed to the village of Rubielos de la Cérida located in the central uplift of the most northerly part of the structure.

Originally the Rubielos de la Cérida structure was limited to and described as a 40 km-diameter circular impact crater with a prominent central uplift but is now considered to comprise a whole crater chain forming an impact basin accompanied by a central uplift chain. In the eastern part, the morphology of the original basin is in parts buried beneath post-impact Upper Tertiary sediments.

Conventionally, the impact basin is considered to represent tectonic structures, the Jiloca-Calatayud graben and the Alfambra-Teruel graben. Since a tectonic graben is typically characterized by tensional features, the common interpretation meets basic difficulties with the structurally dominant compressive style that is almost everywhere to be observed and easily explained by an impact origin.

Impact features

The impact nature is documented by impact melt rocks, suevites, abundant polymictic and monomictic breccias, breccia dikes (dike breccias), extensive megabreccias, impact ejecta, dislocated megablocks, peculiar geological structures in the rim and central uplift zones, and shock metamorphism. Shock effects comprise melt glass, diaplectic glass from quartz and feldspar, planar deformation features (PDFs) in quartz and feldspar and moderate shock effects like intense kink banding and multiple sets of planar fractures (PFs) in quartz, multiple sets of kink bands in mica, and intense micro-twinning in calcite. Moderately developed shatter cones add to the impact signature.

Controversy

As a companion to the Azuara impact structure and like many other impact structures in the world, such as Nördlinger Ries, the Vredefort crater, or the Sudbury Basin, the origin of the Rubielos de la Cérida structure has been debated, and Spanish geologists still remain opposed to an impact origin. In their opinion and with regard to the Spanish impact event, the shock effects are tectonic features, the impact ejecta are Quaternary alluvial fans or Tertiary conglomerates and impact breccias and dike breccias are interpreted as karst features and soil formations.

The opposition against the impact origin for Rubielos de la Cérida (and Azuara) has been supported by a misleading analysis and paper (Langenhorst & Deutsch 1996) denying the occurrence of shock metamorphism in Azuara rocks, which has provoked considerable dissent.

Despite the clear impact evidence for Rubielos de la Cérida underlined by the occurrence of strong shock metamorphism, the crater (like Azuara) is not listed in the Canadian Impact Data Base thus highlighting the controversy about this remarkable impact event.

Rubielos de la Cérida is however listed as a confirmed impact structure in other data bases, e.g. Moilanen, J. and the Expert Database on Earth Impact Structures (EDEIS).