No abstract

[1] The nappe-structured belt of Calabria constitutes the eastward termination of the southern branch of the Alpine Mediterranean belt that delimits the northern edge of the Africa plate. Contrasting hypotheses for the origin and tectonic significance of the north Calabrian nappe edifice have been proposed, and kinematic data from north Calabria have been used to support different interpretations of the Alps-Apennines linkage and the polarity of the Tethyan subduction in the Apennine region. We reconstruct the architecture of the north Calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40 Ar/ 39 Ar geochronology. Results from this study indicate that north Calabria consists of a Tertiary nappe stack, resulting from superimposed top-to-thewest extensional shearing (late Oligocene to middle Miocene in age) onto a previously structured top-tothe-east compressional belt (Eocene to Oligocene in age). This study also documents that the top-to-thewest extensional tectonics was achieved by means of regionally sized extensional detachment fault systems, stretching apart and translating as allochthonous fragments the previously accreted units. Thinning operated by top-to-the-west extensional detachment tectonics also resulted in the direct juxtaposition of non-Alpine or slightly Alpine metamorphosed units (upper plate complex) onto the previously exhumed deep-seated portions of the orogenic wedge, metamorphosed under blueschist facies metamorphic conditions (lower plate complex). These findings support a new tectonic scenario for the orogenic history of north Calabria, which may be adequately framed within the Tertiary Apennine-Tyrrhenian system evolution.

This study describes the structural setting, petrogenesis, and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the Tertiary ophiolitic suture zone of the Sabzevar Range, NE central Iran. These intrusive bodies show tabular geometries with solid-state fabrics documenting syntectonic emplacement during crustal shortening. In the total alkalis-silica (TAS) diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite fields. They show characteristic low MgO (0.15–0.60 wt%) and Ni (<20 ppm), high Sr contents, a negligible Eu anomaly, and extremely fractionated rare earth element (REEs), with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. They also show zircon Hf isotope compositions compatible with a mid-ocean-ridge basalt (MORB)–type oceanic crust. Inverse and forward thermobarometry constrains conditions of magma crystallization in the upper-pressure field of the amphibolite facies (ca. 1.2–1.5 GPa and 750 °C). Integrated U-Pb zircon and 40Ar/39Ar white mica and amphibole geochronology constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Genesis of the Sabzevar magmatic suite is interpreted in terms of prograde, wet amphibolite melting during oceanic subduction, within a pressure-temperature range between a plagioclase-out and a hornblende-out boundary. Magma differentiation and high-pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the Sabzevar structural zone. Implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the Neotethyan subduction during the Mesozoic–Tertiary time span

[1] We report on the paleomagnetism of 34 sites from lower Oligocene-middle Miocene sediments exposed in the Tertiary Piedmont Basin (TPB, northern Italy). The TPB is formed by a thick ($4000 m) and virtually undeformed sedimentary succession unconformably lying upon Alpine nappes decapitated by extensional exhumation, which in turn are tectonically stacked over the Adriatic foreland. Paleomagnetic directions from 23 (mostly Oligocene) sites were chronologically framed using new biostratigraphic evidence from calcareous nannoplankton. Our data, along with published paleomagnetic results, show that the TPB rotated $50°counterclockwise with respect to Africa in Aquitanian-Serravallian times. The rotation was likely driven by underneath nappe stacking and was synchronous with (further) bending of the Alpine chain. Both the rotation magnitude and its timing are similar to those documented for the Corsica-Sardinia microplate. Therefore the formation of the western Alpine arc (or at least part of its present-day curvature) occurred during the rollback of the Apenninic slab and related back-arc spreading of the Liguro-Provençal Basin and drift of the Corsica-Sardinia block. This suggests a common dynamics driving both the Alpine and the Apennine slab motions. Paleomagnetic data also document that the Adriatic plate has undergone no paleomagnetic rotation since mid-late Miocene times. Anisotropy of magnetic susceptibility data suggests that the TPB, an enigmatic basin arising from a controversial tectonic setting, formed in an extensional regime characterized by a stretching direction approximately orthogonal to the main trend of the underlying chain.

Terra Nova, 22, 26–34, 2010 Abstract The ophiolitic mélange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia–Eurasia convergence history. Occurrence of km‐scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole‐dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(‐Th)–Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short‐lived back‐arc oceanic basins accompanied the long‐lasting history of the Neotethyan subduction in the region.