Tectonostratigraphy of the Mérida Massif reveals a new suture zone exposure in SW Iberia

Dividing a crystalline basement into tectonostratigraphic units, along with the recognition of the nature of their boundaries (primary vs. tectonic), are essential steps to identify major tectonic slices involved in orogeny. The Neoproterozoic and Paleozoic rocks of the Mérida Massif (SW Iberia) have been grouped into five tectonostratigraphic units according to their structural position, continental or oceanic crust affinity, and equivalent tectonometamorphic evolution. Each unit is separated from the rest ones by either crustal-scale thrusts and/or extensional detachments. The lowermost unit (Magdalena Gneisses; lower plate) has continental crust affinity, and rest below a variably strained and metamorphosed mafic-ultramafic ensemble, referred to as the Mérida Ophiolite (suture zone). The Neoproterozoic Montemolín Formation of the Serie Negra Group constitutes a unit with continental crust affinity (Upper Schist-Metagranitoid Unit; upper plate) located on top of the Mérida Ophiolite. A carbonate-rich succession (Carija Unit) occupies the uppermost structural position. Structural and isotopic data suggest that the suture zone depicted by the Mérida Ophiolite and the tectonic piling and main foliation of the Neoproterozoic and Cambrian units were formed during the Cadomian Orogeny. Superimposed shortening during the late Paleozoic formed a train of upright to NE-verging folds and thrusts that affected the Cadomian suture zone and juxtaposed it onto Ordovician strata (fifth tectonostratigraphic unit) during the Variscan Orogeny. Cenozoic contraction during the Alpine Orogeny formed SW-directed thrusts in an intraplate setting. The Mérida Ophiolite represents a new Cadomian suture zone exposure of the Iberian Massif, but its root zone is yet to be identified. This suture zone exposure seems to share a far-travelled nature with other Cadomian and Variscan suture zone exposures in Iberia, making the latter a piece of continental lithosphere built at the expense of allochthonous terranes transferred inland from peri-Gondwana onto mainland Gondwana, both during the Neoproterozoic-Cambrian and the Devonian-Carboniferous.

Precise U–Pb zircon ages from Alderney, Channel Islands: growing evidence for discrete Neoproterozoic magmatic episodes in northern Cadomia

The northernmost exposures of rocks formed during the Late Neoproterozoic Cadomian orogeny in the Channel Islands–northern France region occur on Alderney. The island mainly comprises foliated quartz diorite, once considered to be 2 Ga, pre-Cadomian basement, and an undeformed basic to intermediate plutonic complex. A precise age of 610±2 Ma, based on U–Pb analyses of single and small groups of zircons, for the foliated Fort Tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a Cadomian magmatic event. The age is virtually identical to ages from similar, foliated syntectonic quartz diorite bodies on the islands of Guernsey and Sark and at La Hague (north Normandy), indicating that this magmatic and deformational event was regional in extent. Discordant zircon xenocrysts define an upper intercept age of c. 2 Ga indicating the presence of Palaeoproterozoic basement at depth. Single zircons from the undeformed Bibette Head granodiorite give a precise U–Pb age of 572±1 Ma. This age is closely similar to that for the emplacement of the Northern Igneous Complex of Guernsey. The emerging data indicate that Cadomian magmatism in the northern Channel Islands region was not a protracted continuum, but occurred during two distinct, short-lived events separated by c. 30–40 my.

Proterozoic and Cambrian successions in Upper Silesia: an Avalonian terrane in southern Poland

All Cambrian series and several Cambrian biozones have been recognized using acritarch biochronology within the siliciclastic successions underlying Upper Silesia in southern Poland. The entire Cambrian succession is around 580 m thick and contains rare Lower Cambrian trilobites of the Acado-Baltic faunal province. Acritarch associations are taxonomically comparable to those recorded in Baltica, Laurentia and Gondwana, but their closest taxonomic affinity is with Iberia. The Cambrian succession accumulated in a shallow shelf environment and is almost flat-lying, unmetamorphosed, uncleaved and in normal stratigraphic order. It underlies paraconformably Lower Devonian deposits and overlies unconformably steeply dipping metasediments of undetermined Precambrian age. Tectonic deformation and metamorphism to greenschist grade in these Precambrian strata must have occurred in the Proterozoic, and are attributed to the Cadomian orogeny because similar Cadomian basement complexes occur in the adjoining Brno Massif and in the East Avalonian and Armorican terranes. Upper Silesia appears to be a stable crustal block bordered by deep faults whose sedimentary cover has not been affected by tectonic deformation other than faulting. Based on the recognition of Cadomian age basement, the distribution of trilobites and acritarchs and the tectonostratigraphic relationships to adjacent areas, the Upper Silesia terrane is interpreted to be a distal segment of East Avalonia that in Cambrian times faced Iberia. An extension of the Tornquist Suture from the Intra-Sudetic Fault is seen in the Kraków-Myszków Fault Zone at the margin of Upper Silesia. The Intra-Sudetic Fault zone and the Kraków-Myszków Fault Zone contain Early Palaeozoic rocks deformed during the Caledonian orogeny, and mark the boundary between the Caledonian accretionary belt and areas unaffected by this orogeny.

Neoproterozoic shear zone tectonics within the Icartian basement of Guernsey and Sark, Channel Islands

The Channel Islands of Guernsey and Sark are amongst the few localities within the Neoproterozoic, Cadomian orogenic belt where Palaeoproterozoic basement is exposed. Basement units collectively referred to as ‘Icartian’ comprise orthogneisses (e.g. the c. 2000 Ma Icart granite gneiss), metasediments and amphibolites. On Guernsey, the protolith to the Icart granite gneiss was intruded into metasediments already deformed during a D1 deformation event. Both were variably deformed during a D2 event within an approximately north—south trending, steeply dipping, oblique dextral shear zone. On Sark, metasediments and amphibolites carry a D1 fabric (not necessarily correlatable with that on Guernsey) which was deformed during D2 to form recumbent—gently inclined folds and tectonic fabrics which locally carry top-to-the-south kinematic indicators. Early Cadomian quartz diorites which intruded the Icartian basement carry variably developed magmatic and moderate temperature (400–550 °C) LS solid-state fabrics which are similarly oriented to D2 basement structures. This indicates that the quartz diorites were emplaced and deformed during a protracted D2 event. Published 40Ar/39Ar amphibole cooling ages of c. 600 Ma from Guernsey and Sark quartz diorites and a host Icartian amphibolite are consistent with reworking of basement units during the Cadomian Orogeny. The contrasting kinematic pattern during Cadomian deformation implies that a flat-lying shear zone on Sark may have accommodated oblique dextral strike-slip movement on Guernsey with an overall southerly direction of tectonic transport. Deformation most likely occurred in the hanging-wall of a major south-dipping Neoproterozoic subduction zone.