José Manuel Romão

Transient inversion during the opening stageof the Wilson cycle “Sardic phase” in the Iberian Variscides –Stratigraphic and tectonic record

The Variscides of Iberia have a bilateral symmetry with east vergence in the eastern branch and west vergence in the western, on both sides of a Centro-Iberian Zone (CIZ), with predominant steep axial planes. All the structures curve around the Ibero-Armorican Arc (IAA). Unconformities in the sedimentary sequences of Cambrian to Early Ordovician age were ascribed to “Sardic phase” by correlation with similar tectonosedimentary events in Sardinia. Recent studies showed diachronism between these events in Sardinia and Iberia but migration of major geodynamic regime in time may be due to regional variation of major events at plate tectonic scale. We studied in detail two critical areas in the CIZ, the Marão anticline in the NE and the Amêndoa-Carvoeiro synform in the SW. Two unconformities can be put in evidence, as elsewhere in CIZ. A stronger lower unconformity of a Volcano-Sedimentary Complex of Lower Arenig (and Tremadocian?) age on top of a Cambrian clastic sequence with flysch characteristics; and a milder upper unconformity of Armorican Quartzite of Arenig age on both the Volcano-Sedimentary Complex and the Cambrian sequences. The lithostratigraphy of the studied areas is described and correlated with other areas in Iberia. The Volcano-Sedimentary Complex and coeval magmatic bodies with bimodal composition are briefly described. The Sardic event corresponds to folds with steep axial planes at high angles to Variscan structures that produce the penetrative cleavage that cut across the unconformity surfaces. Sardic thrusts are also present and can be explained by thin-skinned compressive tectonics. Sardic folds and thrusts suggest a brief period of transient inversion between a major extensional regime from Cambrian to Devonian. The obliquity of Sardic structures to Variscan compression suggests a component of transpression during the Sardic tectonic event, corresponding to a tectonically enhanced unconformity near the Cambro-Ordovician boundary. The transient Sardic inversion is interpreted in terms of a break-up unconformity related to the migration of an intracratonic rift; in the Ordovician this rift moves into the SW of Ossa Morena Zone (OMZ) and since then become the SW Iberia suture during the Variscan Wilson cycle. This migration induced transient compression and dextral strike-slip in the major boundary between CIZ and OMZ due to presence of incipient primary curvature in this segment of IAA.

Reply to comment by M. Francisco Pereira et al. on ''Geodynamic evolution of the SW Europe Variscides''

] The interpretation reported by Ribeiro et al. [2007]favors the presence of a Cadomian basement within the SWEurope Variscides, eventually including older relics,considering as well the role of this basement in theevolution of the Variscan cycle; this particular issue wasdeveloped in a subsequent paper [Ribeiro et al., 2009]. Thepossibility of Grenville inliers is not definitely ruled out bythe arguments of Pereira et al. [2009]. Indeed, a few (nearlyconcordant) U/Pb zircon ages of 0.94 to 1.2 Ga are reportedby Linnemann et al. [2008]; several other U/Pb zircongeochronological data from Na¨gler et al. [1995], de la Rosaet al. [2002], Sola´etal.[2008], and Cordani et al. [2006](upper intercept date for sample 43B-5.1 assuming lead lossduring Variscan metamorphism at 360 Ma), although notdefinitive, also suggest the inheritance of Grenville ages inOssa-Morena Zone (OMZ) rocks. Furthermore, detritalzircons in Me´rtola Formation (South Portuguese Zone)proximal greywackes(includinghigh-grademetamorphiclith-oclastssimilar to those rocks found at the OMZ E´vora Massif)reveal a frequency age distribution peak at 0.94 Ga (R. Jorge,personal communication, 2009), consistent with derivationfrom inliers of a Greenville orogen, which is well datedbetween 1.2 and 0.95 Ga in the North American craton.Therefore, the possibility of pre-Cadomian cycles remainsdebatable (as signed by a question mark in the text of Ribeiroet al. [2007]), as well as the provenance of detrital zircons andtheconnectionsbetweenIberiaandWestAfrica,AmazoniaandLaurentia cratons prior to the assemblage of Pannotia Super-continent. Further investigations are needed to solve thisproblem within an acceptable range of uncertainty.[