Treb.Mus. Geol. Barcelona, ll: 5-65(2002) The mid-Tertiary Azuara and Rubielosde la C6rida paired impact structures (Spain)* Kord ERNSTSON**. FernandoCLAUDIN*i:*. Ulrich SCHUSSLER:r**:Fand Klaudia HRADIL*1.:k* RESUMEN ERNSTSON,K., CLAUDIN, F., SCHUSSLER,U. and HRADIL, K. Las estructuras de impacto doble del terciario medio de Azuara y Rubielosde la Cörida (Espana). El presentearticulo se centra en las estructurasde impacto de Azuara y Rubielos de la C6rida,que con diämetrosde aproximadamente35-40 Km fueron generadasen un objetivo puramentesedimentario. Ambas constituyenla estructuraterrestrc de impacto doble de mayor tamafloconocida hasta el momento.A panir de los datos estratigi4ficosy paleontoldgicos,su edadmr4s probable es Eocenosup. u Oligoceno. La cartografia geoldgica realizada ha pemitido localizar abundantesevidencias de impactoe inclusoefectos del impactoen depdsitosaut6ctonos distantes. La evidencia de impacto mäs importante para ambasestructuras viene dada por la presencia de un intensometamorfismo de choque,incluyendo fundido y vidrio diapldctico,msgos de deformacidn planar (PDFS), diferentes tipos de rocas de fundido de impacto (foma- das a paflir de fundido silicatado, de fundido carbonatado y fundido de carbonato- fosfato)y brechassueviticas. Particulas de vidrio carbonosoamorfo sitasen un com- ponentesölido de C-O puedenestar relacionadas con fullerenos,y haberseformado a partir del enfriamiento de un fundido procedente o bien de carbdn intercamente chocadoo bien de calizasintensamente chocadas. Pensamos cue el imDactotuvo una influencia considerableen el terciario medio de estaregiön del Sistima Ibörico, y sugedmosque aquellosmodelos en los que no se ha tenido en cuentaeste evento peculiary de ampliarepercusi6n necesitan una revisi6nconsiderable. Palabras claye: Estructura de impacto de Azuara, estructura de impacto de rubielosde la C6rida, CadenaIbdrica (Espafla),metamorfismo de choque,rocas de fundido de impacto,brechas de impacto,eyecta, Terciario. 'N Dr. FranciscoAnguita, AssociateProfessor (Planetary Geology), Universidad Complutensede Madrid (Spain), retereeof this work, sent us the following remarks: "In my opinion, the present paper means a quantic jump towards the confirmation of Azuara and Rubielos de la Cdrida structuresas a doublet impact crater Specifically, the analysesperformed at cen timeter to microscopic scales,reveal a body of evidence difficult to contradict. Any future researchon this areawill have to discussnecessarily the interpretationsnow offered by Emstson ?/ a/. Lacking this discussion. any hypothesis defending that Azuara and Rubielos structuresare the result ol "nonnal" sedimentaryand tectonic processeswill be utterly untenable." ** Fakultät für Geowissenschaftender Universität Würzburg, Pleicherwall l, D 9'70'70Würzburg, Germany.www-impact-structures,com *** IES Giola, Llinars del VallEs.Barcelona-08450, Spain- www.impact-strucnres.com **** Institut für Mineralogie der Universität Wüzburg, Am Hubland, D 97074 Würzburg, Germany. ABSTRACT We report on the Azuara impact structure and its Rubielos de la Cerida compa- nion cratei, which establish the largest terrestdal doublet imPact structue presently - known. Both structures have diameters of roughly 35 40 km and they have been formed in a purely sedimeltary target. From strati$aphic considerations anJ palaeontologicdating. an Upper'Eoceneor Oligoceneage.is very probable Geolägicalmapp-ing has eitablishää abundänt geologic impact evidencein the form Jmof;omictic änO"polymicticbreccias and brecciadikes, megabreccias,. dislocated megablocks,remartabie structuralfeatures, extensive impact ejectaand impact sig- natires even in distant autochthonousdeposits. The most striking.impactevidence for both süucturesis given by snong shoik rnetamorphism,including^melt and dia- oiecricslass, planar äeformätion fÄatures lPDFst. differentkinds of impactmelt iocLr (#orn färmer silicate melt. carbonatemelt, carbonate-phosphatemelt) and iuevite breccias.Glassy amorphous carbon particles in a solid C-O com^poundmay be related with fullerenesand may originaie from a quenchedmelt of extemely shockedcoal or from extremely shockedlimestones. It is assumedthat the impact had considerable influence on the Mid-Teniary regional geology of the Iberian System,and we suggestthat respectivegeologic models-which have.so far not con- sideredthis oeculiarand far-reachingevent. needconslderable revlslon' Key words: Azuara impact structure, Rubielos de la C6rida impact structure' Iberian chain (Spain)' shocl metamorphism; impact melt rocks, impact breccias' ej€cta, Tertiary. INTRODUCTION Emstsone/ al. ( 1985)published the hypothesisof theimpact origin for theAzua- ra structuresome 5b km.söuthof ZarJr;goza(41'10'N,0'55'W - Fig l) in 1985'The lzuara impact structurewas included in the Earth Impact Database,which was flrst "rr"-tf"aly researchersof the GeologicalSurvey of Canadain the sameyear' The CraterInveniory wasrecently updated and lists 163proven terestrial impact structures (http:':-brifitty,//www.unb.calpassc/Imp-alDalabage/ildgx.htmD' th" ""* f"-"rmation of the Azuara structureby impl9t-Jvaj -b3^sed mailly;n th;'occurrenceof shockmetamorphism- (French and Short, 1968; Stöffler' l972iand others)in polymictic(mixed) breccias. Later, the casefor an impactorigin *ur1tt"ttgth"n"d by geöphysicalmeasurements (Ernstson et al',1987;'Emstson and Fie6ae, tö9, ant tiy'*re aätectionof extendedimpact ejectadeposits (E- rnstson and Claudin,1990.1. Detailed mapping. extensive structural investiSations. photogeolo-g1 -a analyses(Bäle;1988; Fiebag,198& Gwosdek, 1988; König' 1988; f"ttog.uphi" 'iiaasmaier, Linnewebär,^1988; 1988; Hunolistein-Bunjevac,1989; Müller' 1989; Katschorek,'1990; Mayer, 1991;Müller and Ernstson,1990; Emstson and Fiebag' i99Z; g-.irott, 1994;ireld, pers comm.)provided a hostof impaclrelatedfindings and ;bservations.Despite the clear shockmetamorphism, the impact evidenceis questionedby someregional geologists (A]Ultell et a1.,1993;Aurell, !994i Cort's et ät.. ZOOZI,wlo stili claiman endogeneticorigin for mostof the impactfeatures' The idea that the impact event not only produced the Azuara structure but also the Rubielos de la C6rida structure, a twin crater of comparablesize about 50 km to the south (Fig.l), was frst suggestedby Emstsonet al. (1994) on the basisof the occunenceof snongly deformed Buntsandstein conglomerates peripheral to the Azuara struchtre (Emstson€, dl., 2001a).Since 1994,field work, petrographicand geochemicalanalyses have shown(llradll et a1.,200I:'Claudin et aI.,2001; Emstsoner al., 2001b,c) that this Rubielos de la C6rida structure bea$ all evidence of an impact strucnre. Stratigraphic considerations suggestRubielos de la Cdrida to be a companion to Azuara and thus we have establishedthat a large doublet impact s[ucture exists in east-centralSpain. Here, we report on the current stateof investigationsof this doublet impact struc- ture. Most of the data for the Azuara structurehave beenpublished previously or can be found in the many diploma thesesreferred to above.Therefore, this report widely summarizesthe comDarativeresults ofboth the Rubielos de la C6rida and the Azuara structures,with some emphasisfocused on new results for Rubielos de la C6rida and on the specialsituation of a paired impact. Fig. L Location map for the Azuara and Rubielos de la Cdrida impact structures.CAL = Calomocha, CAM = Caminreal. CAR = Cadfiena.MUN = Muniesa. Fig 1. Mapa de situacidn de las estructüas de impacto de Azuara y Rubielos de la C6rida. CAL = Calamocha,CAM = Caminreal, CAR = Carifrena,MUN = Muniesa THE AZUARA IMPACT STRUCTURE Topographic features and target rocks Compared with other large impact structures (e.g., Manicouag:n, Ries' ClearwatärWest), the Azuara s-tructurädoes not show a particularly striking-mor- pholosv. Located at the margin betweenthe Alpidic fold belt of the Iberian Chains äna tnä feniary Ebro Basin. the structureis characterizedby Mesozoiclayers emer- ging from the Ebro Basin Tertiary sedimentsand forming a ring-like pattern(Fig'2)' thJ dia-eter of the ring is about 30 km, while the total diameterof the structureis estimatedbetween 35 and 40 km. The imDactaffected both the Palaeozoicand the Mesozoicof fte EastemIberian "rama is (the aragonesa")Chain. The Lower Tertiary at the ,time of the impact assumedto haväbeen thick (1000- 2000m or evenmore) and predominanfly uncon- solidated molassesediments overlaying the Palaeozoicand Mesozoic stable core (Emstsonand Fiebag, 1992). This sedimentarypile has a thicknessof rnorethan 10 i- (C-tr, 1983;Carls and Monninger, 1974) (Fig.3)' which showsthat despitethe laree diameter,the Azuam structurewas imprinted on a purely sedimentarytarget (see Gri-eveand Robertson, 1979; Grieve, 1982), if we neglecta few igneousintercalations of very small dimensions.The dominantly soft target and the advancederosion may be ths causeof the weak morphologicalsignature of the structure. Impact-geologic features Breccias As hasbeen stated previously (Emstson et al., 1985;Emstson and Fiebag' 1992), polymictic (mixed) anä monomictic (monolithologic) breccias are very abundanl ihr6ughouttie Azuara structureand havebeen mapped in detail mostly on I : 10'000 and 1": 20.000scales (Bärle, 1988; Fiebag, 1988; Gwosdek, 1988; König' 1988; Linneweber, 1988; Waasmaier, 1988;Hunoltstein-Bunjevac, 1989; Müller. 1989; Katschorek1990; Mayer,1991; Feld, pers. comm.). Fig. 2. Morphological signature of the Azuara structue taken from the digital mapof Spain,I : 250.000.The image has beenperformed by Manuel Cabedo. Fig.