Detecting syn-orogenic extension and sediment provenance of the Cilento wedge top basin (southern Apennines, Italy): Mineralogy and geochemistry of fine-grained sediments and petrography of dispersed organic matter

Lawsonite composition and zoning as tracers of subduction processes: A global review

A revision of late Palaeozoic tectonics recorded in Tuscany, Calabria and Corsica is here presented. We propose that, in Tuscany, upper Carboniferous-Permian shallow-marine to continental sedimentary basins, characterized by unconformities and abrupt changes in sedimentary facies, coal-measures, red fanglomerate deposits and felsic magmatism, may be related with a transtensional setting where upper-crustal splay faults are linked with a mid-crustal shear zone. The remnants of the latter can be found in the deep-well logs of Pontremoli and Larderello-Travale in northern and southern Tuscany respectively. In Calabria (Sila, Serre and Aspromonte), a continuous pre-Mesozoic crustal section is exposed, where the lower-crustal portion mainly includes granulites and migmatitic paragneisses, together with subordinate marbles and metabasites. The mid-crustal section, up to 13 km-thick, includes granitoids, tonalitic to granitic in composition, emplaced between 306 and 295 Ma. They were progressively deformed during retrograde extensional shearing, with a final magmatic activity, between 295 ± 1 and 277 ± 1 Ma, when shallower dykes were emplaced in a transtensional regime. The section is completed by an upper crustal portion, mainly formed by a Palaeozoic sedimentary succession deformed as a low-grade fold and thrust belt, and locally overlaying medium-grade paragneiss units. As a whole, these features are reminiscent of the nappe zone domains of the Sardinia Variscan Orogen. In Corsica, besides the well-known effusive and intrusive Permian magmatism of the “Autochthonous” domain, the Alpine Santa Lucia Nappe exposes a kilometer-scale portion of the Permian lower to mid-crust, exhibiting many similarities to the Ivrea Zone. The distinct Mafic and Granitic complexes characterizing this crustal domain are juxtaposed through an oblique-slip shear zone named Santa Lucia Shear Zone. Structural and petrological data witness the interaction between magmatism, metamorphism and retrograde shearing during Permian, in a temperature range of c. 800–400 °C. We frame the outlined paleotectonic domains within a regional-scale, strain–partitioned, tectonic setting controlled by a first-order transcurrent/transtensional fault network that includes a westernmost fault (Santa Lucia Fault) and an easternmost one (East Tuscan Fault), with intervening crustal domains affected by extensional to transtensional deformation. As a whole, our revision allows new suggestions for a better understanding of the tectonic framework and evolution of the Central Mediterranean during the late Palaeozoic.

Late Palaeozoic tectonics in Central Mediterranean: a reappraisal

Abstract We conducted a detailed study on the microstructures and petro-fabrics of massive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NW China. The lattice preferred orientation (LPO) of glaucophane and lawsonite in foliated lawsonite blueschist (LBS) is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid-body rotation, respectively. The LPO of glaucophane is mainly characterized by the [001] axis aligning parallel to lineation and the [100] axis and (110) pole plunging perpendicular to foliation. In contrast, the LPO of lawsonite features the maximum [010] axis concentrated close to lineation and the [001] axis strongly clustered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the finite-volume method (FVM). The mineral LPOs are much stronger in foliated LBS than in massive LBS. In addition, a kinematic vorticity analysis suggests that both pure shear dominant (Wm = 0.18–0.26) and simple shear dominant (Wm = 0.86–0.93) deformation regimes are present in foliated LBS. The [001] axis and (010) pole of glaucophane, and the [100] and [010] axes of lawsonite, tend to distribute in a foliation–parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more lineation–parallel concentrations of a [001] axis of glaucophane and a [010] axis of lawsonite. Because the whole-rock seismic anisotropies in foliated LBS are significantly higher than those in massive LBS and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite, the delay time of fast S-wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle (> 45–60°) is expected to range from 0.03 to 0.09 s (lower bound for massive LBS) and from 0.1 to 0.3 s (upper bound for foliated epidote blueschist).

Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China

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Handling Editor: Donna Whitney Abstract The Neotethyan oceanic Diamante-Terranova unit (DIATU; southern Apennines– Calabria–Peloritani Terrane system) includes basic rocks that during the Cenozoic were subducted and metamorphosed to lawsonite-blueschist facies conditions. Petrological and structural observations (both at the mesoand micro-scale) show that lawsonite growth was continuous during three distinctive ductile deformation stages (D1–D3). These likely occurred close to the metamorphic peak, estimated at 350–390°C and 0.9–1.1 GPa, producing an equilibrium assemblage made of blue Na-amphibole, lawsonite, chlorite and pumpellyite. Locally, pods dominated by quartz and epidote (plus chlorite, calcite and green Ca-amphibole) developed at similar conditions (350–370°C, 0.8–0.9 GPa). Post-peak evolution during the final exhumation of the DIATU along the subduction channel, also consisted of three deformation stages, defined by folding (D4) and normal faulting (D5) and finally by strike-slip faulting (D6), affecting both the blueschist unit and the unconformably overlying Tortonian conglomerates. Vorticity analysis on syn-tectonic lawsonite crystals indicates that severe flattening occurred during the D2 stage, with a significant secondary non-coaxial strain component along the W–E plane. This is associated with an eastward tectonic vergence, consistent with the subsequent D3 and D4 folding stages characterized by a dominant ENE tectonic transport. It is suggested that exhumation started from the D2 stage and continued during D3 at similar HP/ LT metamorphic conditions. The widespread occurrence of unreacted lawsonite crystals suggests that exhumation was very fast and supports the idea that concurrent ductile deformation might play a role in its preservation.

Petrogenesis and deformation history of the lawsonite-bearing blueschist facies metabasalts of the Diamante-Terranova oceanic unit (southern Italy)

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Petrogenesis and deformation history of the lawsonite-bearing blueschist facies metabasalts of the Diamante-Terranova oceanic unit (southern Italy)