Abstract In two separate areas of western and central Elba Island (Italy), Late Miocene granite porphyries are found as shallow-level intrusions inside a stack of nappes rich in physical discontinuities. Detailed mapping of intrusive rocks, along with their relations with country rocks, show that outcrops from western and central Elba Island expose the same rock types, with matching intrusive sequence, petrography and geochemical features. Structural and geological data indicate that these layers were originally part of a single sequence that was split by eastward-directed décollement and tilting. The two juxtaposed portions of the original sequence allow the restoration of a 5-km thick sequence, made up of nine main intrusive layers, building three Christmas-tree laccoliths nested into each other to support a structural dome. During their construction, the role of the neutral buoyancy level was of minor significance with respect to the role played by the relatively thin overburden and/or the large availability of magma traps inside the intruded crustal section. Emplacement of the Monte Capanne pluton into the base of the domal structure likely caused oversteepening and initiated decapitation of the complex, with gravity sliding of the upper half off the top.

Abstract Magmatic activity in the western part of Elba Island at the north end of the Tyrrhenian Sea lasted approximately 1.5 Ma during the Late Miocene, building a complex of nested Christmas-tree laccoliths, a 10 km-diameter pluton (Monte Capanne) and, finally, the steeply-dipping Orano dyke swarm (ODS). This igneous activity occurred as an extensional regime and followed the wake of eastward-migrating compression of the Apennine front. The ODS consists of hybridized mantle-derived magmas, constituting about 200 dykes totalling a length of approximately 90 km. These dykes intruded the northwestern part of the pluton (NW of the Pomonte–Procchio geomorphic lineament) and its contact aureole, as well as several kilometres of sedimentary rock above. The ODS intruded near the close of pluton crystallization, above a source region marked by a positive magnetic anomaly located NW of the Pomonte–Procchio lineament. Dyke orientations are dominated by a major system trending N78E, with dykes concentrated in belts that locally produced up to 15% extension; between these belts, a minor system of Orano dykes dominates with N38W and N22E trends. ODS emplacement patterns preserve the strain that resulted in exploitation of Riedel fractures in a NE–SW dextral shear zone; local internal zones of sinistral shear account for one set of the minor system. This shearing occurred between offset segments of the Elba Ridge in the western Elba transfer zone, where strain concentrated magma flow to build the western Elba magmatic complex. This zone developed as a result of different extension rates that produced north-trending Neogene–Quaternary sedimentary basins north and south of the zone. Such basins are connected regionally by NE-trending lineaments previously active during the formation and destruction of the Tethys Ocean. All the magmatic centres in the northern Tyrrhenian–Tuscan area are distributed along such lineaments and developed as a wave moving northeastward across the region, suggesting that magmatism was focused by transfer zone development as back-arc extension migrated in that direction and reactivated former faults.

Abstract Regional considerations on the tectonic regime during the emplacement of the Early Devonian magmatic units in the Sierra de San Luis are inferred from combined field, petrographic and AMS observations. Devonian granitoids of the Sierra de San Luis, in central Argentina, constitute elliptical composite batholiths and make up the most voluminous magmatism that appears in the Sierra. Detailed fabric studies have been carried out on the La Totora batholith (33°09′S, 65°42′W), which complement previous studies on two of the largest plutons in the Sierra de San Luis: The Renca and Las Chacras-Potrerillos batholiths. The studies comprised systematic field surveys, petrographic observations and anisotropy of magnetic susceptibility (AMS) measurements. Microstructural studies indicate that the batholith rocks are mainly characterized by magmatic microstructures with limited sub-magmatic to high-temperature solid-state deformation. All three batholiths possess concentric foliation patterns. The average magnetic foliation patterns in the studied plutons agree well with the macroscopic fabrics measured in the field indicating that the AMS-data can be used to study the orientation of fabric elements. Bulk susceptibility indicates a predominance of ferromagnetic contributions, although some paramagnetic sub-units are also present. Most foliations and lineations reflect magmatic flow and their attitude is linked to the interference between regional deformation and batholith inflation, i.e. fabrics may be due to regional strain in combination with the internal dynamics of the magma bodies. Rock fabrics are mainly described by oblate magmetic fabric ellipsoids. Magnetic lineations generally show a NNE-SSW trend that is interpreted to be controlled by the opening transtensional pull-apart structures during batholith inflation. It turns out that the Devonian batholiths intruded the basement syn-kinematically with respect to the Achalian deformational cycle.