IMPACT MELT ROCKS, SHOCK METAMORPHISM, AND STRUCTURAL FEATURES IN THE RUBIELOS DE LA CÉRIDA STRUCTURE, SPAIN: EVIDENCE OF A COMPANION TO THE AZUARA IMPACT STRUCTURE
Kord Ernstson 1, Ferran Claudin 2, Ulrich Schüssler 3, Francisco Anguita 4, and Till Ernstson 5
1 Fakultät für Geowissenschaften, Universität Würzburg, firstname.lastname@example.org; 2IES Giola, email@example.com; 3 Institut für Mineralogie, Universität Würzburg, firstname.lastname@example.org; 4 Departamento de Petrología y Geoquímica, Universidad Complutense, Madrid, email@example.com; 5 firstname.lastname@example.org
About 50 km south of the Azuara impact structure (35 - 40 km diameter) [1, 2], the Rubielos de la Cérida structure is defined by a circular to elliptical basin with a diameter of roughly 40 km, a circular central uplift with a diameter of about 15 km, and a geometrically associated drainage pattern. The stratigraphic sequence in the uplift comprises Muschelkalk in the core up to Creataceous at the flanks, with a stratigraphic uplift  of at least 500 m. Besides soft argillaceous Keuper layers and few Cretaceous and Keuper sandstones, carbonate rocks, mainly limestones, dominate. The most significant feature in the central uplift is the enormous compressive signature including continuous megabrecciation up to chaotic criss-cross layering nearly everywhere. A superfault  is exposed in the uplift where the adjacent carbonate rocks have changed to a white mass probably as the result of decarbonation or/and melting. Allochthonous conglomerates in the uplift are composed of heavily plastically deformed limestone pebbles, cobbles and boulders. Apart from the general megabrecciation, all kinds of monomict and polymict breccias and breccia dikes occur. Like in the Azuara impact structure, a polymict basal breccia  is widely exposed in contact with the uplifted Mesozoic layers. In the rocks emerging from the Upper Tertiary and Quaternary in the basin, quarries and road construction expose large allochthonous megablocks and enable insight into drastic and voluminous brecciation of the limestones (gries ), frequently crushed and ground down to display mortar texture. At the rim of the (semi-circular) depression, the rocks continue to being heavily deformed with varying intensity, frequently showing megabrecciation with large rotated blocks and breccia dikes sharply cutting well-bedded limestones. At the rim near Ollala, a section of inverted stratigraphy (Palaeozoic on top of Muschelkalk overlaying Keuper) shows extreme deformation and fracturing including an extended Palaeozoic - Muschelkalk interface continuously mirror-polished.
Strong evidence for an impact origin of the Rubielos de la Cérida structure is given by the find of compact melt rocks within the structure between the central uplift and the northern rim. The melt rocks (silicate, carbonate, and phosphate melt) occur as blocks of variable size intermixed in a polymict megabreccia (Figs.1, 2). A petrographic description of these melts is given in .
More evidence of impact signature in rocks from the Rubielos de la Cérida structure is given by the occurrence of shock metamorphism. We observe heavily disintegrated feldspars with strong mechanical twinning and multiple sets of PDFs, crossing sets of isotropic lamellae in twinned feldspars, diaplectic quartz and feldspar together with multiple sets of PDFs. Kinkbanding in mica from silicate Creataceous rocks and strong microtwinning in calcite are frequently observed. As a macroscopic shock feature, shatter cones can be found in Palaeozoic siltstones near Olalla. A peculiar shock phenomenon in Triassic Buntsandstein conglomerates surrounding the Rubielos de la Cérida structure is described in [7, 8]. An extended blanket of diamictites with strongly plastically deformed components, similar to the Pelarda Fm. ejecta of the Azuara impact structure , surrounds the Rubielos de la Cérida structure and is spectacularly exposed at the Puerto Mínguez . Since a syn-tectonic sedimentation  and a fluvial deposition can be excluded, these deposits are best explained as ejecta from the Rubielos de la Cérida structure.
Conclusions. - Based on current knowledge of impact cratering and impact structures [e.g.,12, 13, 14], we conclude from the observations and features summarized above that Rubielos de la Cérida is an impact structure. This established, the neighbourhood to the Azuara structure and the stratigraphic age (Lower to Mid-Tertiary) of both structures makes a synchronous impact of a paired projectile very probable hence constituting the largest presently known terrestrial doublet impact structure.
Note: Color images of Rubielos de la Cérida outcrops, rock samples, and photomicrographs (comments included) may be consulted in the web: <http://www.impact-structures.com> or <http://www.estructuras-de-impacto.impact-structures.com>
References: -  Ernstson, K. et al., 1985, Earth Planet.Sci.Let., 74, 361-370.  Ernstson, K. and Fiebag, J., 1992, Geol. Rundschau, 81, 403-427.  Grieve, R.A.F. et al., 1981, Multi-ring Basins, Proc.Lunar Planet.Sci., 12A, 37-57.  Spray, J.G., 1997, Geology, 25, 579-582.  Hüttner, R., 1969, Geologica Bavarica, 61, 142-200.  Schüssler, U. et al., this volume.  Ernstson, K. et al., 2001, Geology, v. 29, 11-14.  Ernstson, K. et al., this volume. . Ernstson, K. and Claudin, F., 1990. N.Jb.Geol.Paläont.Mh., 1990, 581-599.  Claudin, F. et al., this volume.  Casas, A.M. et al. (2000) Geodinamica Acta , 1 , 1-17.  Melosh, H.J., 1989, Impact cratering. A geologic process, Oxford University Press.  Dressler, B. O. et al.(eds.), 1994, Large meteorite impacts and planetary evolution, Geol.Soc. America Special Paper 29.  Grieve, R. A. F., 1987, Ann.Rev.Earth Planet.Sci., 15, 245–270.
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