International Journal of Earth Sciences

Avalonian rocks in northern mainland Nova Scotia are characterized by voluminous 640–600 Ma calc-alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc. However, after the cessation of arc activity, repeated episodes of felsic magmatism between ca. 580 Ma and 350 Ma are dominated by A-type geochemical characteristics. Sm–Nd isotopic data, combined with zircon saturation temperature estimates, indicate that these magmas were formed by high temperature (800–1050 °C) melting of the same anhydrous crustal source. Regional tectonic considerations indicate that A-type felsic magmatism was produced (1) at 580 Ma in a San Andreas-type strike slip setting, (2) at 495 Ma as Avalonia rifted off Gondwana, (3) at 465 and 455 in an ensialic island arc environment and (4) at 360–350 Ma during post-collisional, intra-continental strike-slip activity as Avalonia was translated dextrally along the Laurentian margin. These results attest to the importance of crustal source, rather than tectonic setting, in the generation of these A-type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane. They also suggest that the shallow crustal rocks in northern mainland Nova Scotia were not significantly detached from their lower crustal source between ca. 620 Ma and 350 Ma, a time interval that includes the separation of Avalonia from Gondwana, its drift and accretion to Laurentia as well as post-accretionary strike-slip displacement.

Here, we report on the Quaternary Volsci Volcanic Field (VVF, central Italy). In light of new 40Ar/39Ar geochronological data and compositional characterization of juvenile eruptive products, we refine the history of VVF activity, and outline the implications on the pre-eruptive magma system and the continental subduction processes involved. Different from the nearby volcanic districts of the Roman and Campanian Provinces, the VVF was characterized by small-volume (0.01–0.1 km3) eruptions from a network of monogenetic centers (mostly tuff rings and scoria cones, with subordinate lava occurrences), clustered along high-angle faults of lithospheric depth. Leucite-bearing, high-K (HKS) magmas (for which we report for the first time the phlogopite phenocryst compositions) mostly fed the early phase of activity (∼761–539 ka), then primitive, plagioclase-bearing (KS) magmas appeared during the climactic phase (∼424–349 ka), partially overlapping with HKS ones, and then prevailed during the late phase of activity (∼300–231 ka). The fast ascent of primitive magma batches is typical of a tectonically controlled volcanic field, where the very low magma flux is a passive byproduct of regional tectonic strain. We suggest that the dominant compressive stress field acting at depth was accompanied by an extensional regime in the upper crust, associated with the gravity spreading of the Apennine chain, allowing the fast ascent of magma from the mantle source with limited stationing in shallow reservoirs.

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphiné twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

The continental redbeds from the Internal Domains of the central-western Mediterranean Chains have an important role in the palaeogeographic and palaeotectonic reconstructions of the Alpine circum-Mediterranean orogen evolution since these redbeds mark the Triassic-Jurassic rift-valley stage of Tethyan rifting. The composition and the sedimentary evolution of the Middle Triassic to Lowermost Jurassic continental redbeds of the San Marco d’Alunzio Unit (Peloritani Mountains, Southern Italy), based on mineralogical and chemical analyses, suggests that the studied mudrock sediments share common features with continental redbeds that constitute the Internal Domains of the Alpine Mediterranean Chains. Phyllosilicates are the main components in the mudrocks. The 10 Å-minerals (illite and micas), the I–S mixed layers, and kaolinite are the most abundant phyllosilicates. The amount of illitic layers in I–S mixed layers coupled with the illite crystallinity values (IC) are typical of high degree of diagenesis, corresponding to a lithostatic/tectonic loading of about 4–5 km. The mineralogical assemblage coupled with the A-CN-K plot suggest post-depositional K-enrichments. Palaeoweathering proxies (PIA and CIW) record intense weathering at the source area. Further, the studied sediments are affected by reworking and recycling processes and, as consequence, it is likely these proxies monitor cumulative effect of weathering. The climate in the early Jurassic favoured recycling and weathering occurred under hot, episodically humid climate with a prolonged dry season. The source-area is the low-grade Paleozoic metasedimentary basement. Mafic supply is minor but not negligible as suggested by provenance proxies.

The Mirandola anticline represents a buried fault-propagation fold which has been growing during Quaternary due to the seismogenic activity of a blind segment belonging to the broader Ferrara Arc. The last reactivation occurred during the May 2012 Emilia sequence. In correspondence with this structure, the thickness of the marine and continental deposits of the Po Plain foredeep is particularly reduced. In order to better define the shallow geometry of this tectonic structure, and hence its recent activity, we investigated in a depth range which is intermediate between the surficial morphological observations and seismic profiles information. In particular, we carried out numerous passive seismic measurements (single-station microtremor) for obtaining the horizontal-to-vertical spectral ratio. The results of a combined analysis of the peak frequency and its amplitude nicely fit the available geological information, suggesting that this low-cost geophysical technique could be successfully applied in other sectors of wide morphologically flat alluvial plains to investigate blind and completely buried potential seismogenic structures.

The Mbondo-Ngazi Tina area belongs to the eastern part of the Adamawa-Yadé Domain (AYD), which corresponds to the central domain of the Central African Fold Belt (CAFB) in Cameroon. This area comprises three groups of granitoids including granites, syenites, and diorites, which are chemically similar to quartz monzonites and nepheline syenites, and monzodiorites, respectively. These granitoids are metaluminous and I-type rocks, with the dioritic rocks being ferroan and high-K calc-alkaline, while others are magnesian and shoshonitic rocks. The Ti-in-zircon thermometer suggests crystallization temperatures of 678–811 °C and 658–768 °C for monzonites and syenites, respectively. Their zircon trace element distribution and patterns coupled to the whole-rock geochemical features indicate that the Mbondo-Ngazi Tina granitoids were most likely originated from the mixture between crust- and mantle-derived melts, with high crustal portion for the quartz monzonites and nepheline syenites, and strong involvement of mantle component for the monzodiorites. The Ce/Ce*, Eu/Eu*, Th/U, and Zr/Hf of zircons suggest that the crystallization of magma melts took place under variable oxidizing and oxygen fugacity conditions at temperature of 678–768 °C. LA-ICP-MS U–Pb zircon analyses yield emplacement age of 576.4 ± 1.9 Ma and 585.9 ± 2.1 Ma for the quartz monzonites and nepheline syenites, respectively. These ages suggest that these rocks are post-tectonic granitoids emplaced during relaxation stages after the collision between the Amazonian-West African cratons and the São Francisco-Congo and Saharan cratons. The post-collisional extensional regime is related to the development of large shear zones across the Pan-African/Brasiliano belts, which played a role for the ascent and emplacement of post-tectonic granitoids in response to lithospheric delamination. This explains that such granitic plutons are widespread in the CAFB and adjacent Brasiliano belt.

Perpendicular or high-angle relationship of lineations is common in shear zones where synchronous fabric transposition and shearing occur. This paper documents a polyphase early-Variscan structural record exposing the transition from orogenic superstructure to infrastructure along a major detachment shear zone at the western margin of the Teplá–Barrandian domain, Bohemian Massif. On the scale of tens of kilometers this region shows major but continuous reorientation of S2/S3 intersection lineation from its NNE–SSW trends in the superstructure to the ESE–WNW trends in the infrastructure. A simple kinematic numerical model, which allows deformation parameters such as fabric attractor, deformation symmetry, and proportion of simple shear, to be varied, was formulated to explain the rotation of fabrics observed in the field. In line with the results of our kinematic modelling, these rotations are interpreted as a result of progressive transposition of the originally subvertical S2 foliation into the gently SE-dipping S3 marked by stretching at high angle to the incipient S2–S3 intersection. The mineral–fabric relations suggest that the S2 was associated with crustal thickening and is well preserved in the superstructure, while S3 was associated with exhumation of deeper parts of the infrastructure. The kinematic model confirmed the operation of major detachment shear zone and allowed a set of the most probable deformation parameters leading to successful simulation of the fabric geometries to be identified. In addition, the successful model parameters were used to restore the vertical structure of the studied region prior to exhumation on the basis of available pressure–temperature data. We believe that unlike the commonly used balanced cross-section restoration, our restoration technique can be successfully applied in orogen interiors if the mineral–fabric relations are well-constrained and high-quality pressure–temperature estimates exist.