Iberian Domain Research Articles

Palaeozoic Palaeogeography in Hercynian and Appalachian Belts by Sm-Nd Isotopic Studies of Shales

The Sm-Nd isotopic studies of shales give important informations about the crustal growth and the provenance of the sediments. Because shales integrate the diversity of eroded rocks and because the Sm-Nd isotopic system is left undisturbed by surface processes, the Nd isotopic initial composition reflects an average of the different mechanically eroded sources. This composition reflects a balance between juvenile inputs from the mantle by volcanism and reworked inputs from the continental crust. However, the Sm-Nd isotopic signal is a biased average in which pre-existing sediments and young mountains appear to be over represented. The Nd isotopic initial composition can be expressed by depleted mantle model ages (TDM). Assuming that similar TDM values can characterize a common tectonic domain, comparisons between data from several areas can give new palaeogeographical constraints. First, we test this assumption for the Hercynian belt and we expand it to the Appalachian belt in eastern Canada. Our data from the Hercynian belt concern Sm-Nd isotopic data on 46 shales from the Montagne Noire (southern French Massif Central). The sedimentary record, which comprises one of the most complete Palaeozoic sequences in Europe, was sampled from Early Cambrian to Early Jurassic (560 to 180 Ma). This sequence is interrupted by two hiatuses. The first is situated in Middle Ordovician (490 to 450 Ma) and the second during Carboniferous (330 to 300 Ma). We analysed volcanogenic materials and lavas in order to identify possible source materials. During the studied time span, most of the TDM's are around 1.6 Ga. We observe some rapid TDM changes toward older ages at 520 and 270 Ma and toward younger ages at 300 Ma. These variations are related to different sedimentary settings and to the volcanic event (300 Ma) detected in a continental basin. Our results indicate that in any case, younger TDM do not reflect large Palaeozoic juvenile inputs related to orogenic processes in the south of the French Massif Central. We compare our data with others published data from Brittany (Medio North Armorican Domain) and from Spain (Central Iberian Domain and Cantabrian Domain). The similar TI~M calculated for these areas from Early Ordovician to Early Carboniferous (490 to 330 Ma) suggest that these three areas belonged to the same palaeogeographic entity at least for this time span. This contradicts models that propose there is an oceanic suture between Brittany and Spain and between the two Spanish zones. Conversely, our observations are in agreement with palaeontological data indicating that Brittany and Spain were adjoining during this time span. On the other hand, because the Montagne Noire has younger TI~M from 490 to 400 Ma compare to Brittany and Spain, we propose that this area probably undergone a different tectonic evolution during this time span. The Appalachian belt was created by several accretion events of exotic terranes to the North American shield (Laurentia) during the Palaeozoic. The origin, the timing and the link between the accreted terranes are a matter to debate. In order to give new constraints, we analysed shales from the Dunnage zone, which represent eroded products from the Laurentia, and shales from accreted terranes: the Avalon and Meguma zones. For the Dunnage zone, we observe a rapid TDM shift toward younger ages near 440 Ma. This result is interpreted as an evidence for the accretion of Avalon to the Laurentia around that time.

Albian-Cenomanian and Campanian-Maastrichtian biostratigraphy of southeast Spain

The Upper Cretaceous in southeast Spain, which crops out at the southeastern end of the Iberian Range and on the easternmost border of the Betic Range, contains rich and biostratigraphically significant macrofaunas. The upper Albian is recognized by its inoceramid and ammonite content in the Internal Prebetic, the base of the Cenomanian on the basis of ammonites, and the lower Cenomanian by inoceramids, ammonites, brachiopods and echinoids. The upper Campanian has rudists in the shallow marine facies of the Iberian Domain and inoceramids in the External Prebetic, while in the Internal Prebetic it contains inoceramids, ammonites and echinoids. The lower Maastrichtian is characterized in the Internal Prebetic by inoceramids, ammonites and echinoids and in the Intermediate Units by inoceramids, brachiopods and echinoids. All taxa recorded from the Albian-Cenomanian and Campanian-Maastrichtian intervals are plotted against a reference section for each of the palaeogeographic domains distinguished.

Stress and palaeostress in the Betic-Rif cordilleras (Miocene to the present)

Palaeostress orientation in the Betic-Rif Cordilleras can be determined for the Miocene to the Quaternary by analysis of brittle microstructures (faults, joints, striated pebbles and stylolites). In the central and northern South Iberian Domain, σ 1 was subhorizontal and trended NW-SE, from the Early Miocene to the present. In the northern Alborán Domain, from the Burdigalian to the Serravallian, palaeostress ellipsoids were oblate, with σ 3 subhorizontal and trending ENE-WSW. In the southern part of the South Iberian Domain and in the northern part of the Alborán Domain (Betic Cordillera) from the Tortonian to the Quaternary, palaeostress ellipsoids were variable: in the Granada region, they were prolate with σ 1 subvertical; but in the Almería region they were triaxial with σ 2 subvertical and σ 3 subhorizontal and trending NE-SW to E-W. In the Rif, the Alborán and African-Maghrebian domains were affected by palaeostresses with σ 1 subhorizontal, varying from NE-SW in Tortonian times to N-S in the Plio-Quaternary. A study of earthquake focal mechanisms by the right-dihedra method indicates that the present-day general stress field around the Betic-Rif cordilleras is compressional, with σ 1 subhorizontal and trending NW-SE. In the Betic-Rif Cordilleras, this general stress field is modified and the present-day orientations of σ 1, σ 2 and σ 3 are nearly the same as those of the palaeostresses in the Pliocene and Quaternary. The distribution and evolution of stresses in the Betic-Rif cordilleras, from the Miocene to the present, can be correlated with the main Neogene deformations: extensional structures in the central and northern Alborán Domain and the southern part of the South Iberian Domain, and compressional structures in the South Iberian and African-Maghrebian domains and in the southern part of the Alborán Domain.

Découverte de brachiopodes du Sinémurien dans la Sierra Harana (Subbétique Interne, Cordilleres Bétiques, prov. de Grenade, Espagne). Implications stratigraphiques

Two Lower Sinemurian brachiopod species from the NW european province [ Calcirhynchia calcaria Buckman and Calcirhynchia calcicosta (Quenstedt)] have been found for the first time in the lower dolomitic beds of the Lower lurassic carbonate ensemble of the Sierra Harana unit (Internal Subbetic). Their presence allow to detail the evolution of the installation of the carbonate platform in the southern iberian domains during the Lower Lias transgression.

Late cenozoic identation/escape tectonics in the eastern Betic Cordilleras and its consequences on the Iberian foreland

The structuration of the southeastern quarter of the Iberian Peninsula (eastern Betics and Iberian foreland), since Messinian time, was the result of a series of complex deformational events, as related to a major NNE-SSW sinistral shear zone disrupting the southeasternmost part of the Betic cordilleras (Trans-Alboran-Palomares shear zone; TAPSZ). Deformation along this shear zone was characterized by two differentiated and sequential scenarios: 1) periods of stress-build-up associated to momentary slip-obstruction along the TAPSZ, involving NW-directed indentation of the southeastern Iberian domain, and 2) periods of stress-release resulting from the lateral escape of wedge-shaped blocks, thus allowing full-scale strike-slip displacements along the TAPSZ. These events imposed compressional stress fields on the Iberian foreland, with a series of consequences such as alkaline volcanism in Calatrava and Cofrentes, protorift zones, changes in previous structural trends, localized uplift/doming processes, and changes in the sedimentary records of sorne basins.

Brachiopodes articulés de la coupe de l'Ile de Rosan (Crozon, Finistère). Formation des tufs et calcaires de Rosan (Caradoc-Ashgill)

Paleontological study of the “Ile de Rosan” section reveals two different fossiliferous levels in the Tufs et calcaires de Rosan Formation. 15 brachiopod genera and 16 species are described including one new genus and six new species ( Aberia havliceki nov. gen. nov. sp., Dolerorthis aberensis nov. sp., Ptychopleurella villasi nov. sp., Mcewanella barroisi nov. sp., Bicuspina armoricana nov. sp., Cliftonia rosanensis nov. sp.). The lower level (n o 5) at the basis of the section is upper Caradoc (Onnian); the level 16 at the top is Cautleyan-Rawtheyan in age. This fauna is typically Mediterranean with some elements from the Anglo-Baltic Province. There are great affinities between these faunas and those from the Eastern Iberian Chains. In the Armorican Massif, but also in the whole Iberian domain, the first limestones show great diversification of the brachiopod faunas, which had been poor in genera and species until then.

Le Cénomanien supérieur-Turonien dans la zone centrale de la chaîne ibérique (Espagne): Une étape sédimentaire très particulière dans l'évolution de la plate-forme crétacée

The Upper Cenomanian-Turonian of the Central area of the Iberian Range constitutes a single sedimentary stage bounded at its bottom and top by stratigraphic disconformities. The sedimentary stage is very different from those other stages of the Cretaceous of this region because: it is the most important transgressive event as it leaves the only pelagic deposits of the Cretaceous of this region; it is the quickest transression as the pelagic deposits even rest over continental facies; it means the most important paleogeographic change of the Cretaceous as it is the first time in which the Atlantic sea floods over the whole Iberian domain; the progradation of the platform occurs during the regression of this stage and it is the only one known in the stratigraphic record of the Iberian Range. This sedimentary evolution is explained by a fastsea level rise to the limit of the Middle Cenomanian to the Upper Cenomanian followed by a sea level decrease during the Turonian.