School of Geosciences, Monash University, Melbourne, Australia

Magmatic and sub-solidus fabrics in intrusive rocks are frequently used to infer the relative timing of deformation with respect to magma emplacement and cooling. Here, we describe the relationships between strain and fabric development in leucogranite sheets (pegmatite, aplite) emplaced into shear zones that localized post-thermal peak deformation in the contact aureole of an upper crustal pluton (<0.2 GPa) on the Island of Elba, Italy. The leucogranite sheets present igneous, mylonitic, and cataclastic fabrics. Detailed meso- and microscopic structural analysis suggests that the dykes emplaced in the shear zones behaved as competent, rigid bodies during mylonitic deformation of the host rocks. Thermal modelling indicates that emplacement and cooling of the sheets occurred very rapidly (a few days to years) compared to typical tectonic strain rates and strain accumulation timescales in the host rocks. Such a fast cooling does not allow melt or magma-induced thermal softening in the host rocks during deformation. The development of mylonitic and cataclastic fabrics in the dykes was controlled by the localized activation of fluid-controlled reaction softening mechanisms (mylonitic fabric) and embrittlement during cooling in sites of high-strain (cataclastic fabric). We show that a broad spectrum of fabrics can form in igneous sheet intrusions emplaced at the same time and crustal level. The coexistence of isotropic (non-foliated igneous) versus anisotropic (mylonitic and cataclastic) fabrics in igneous sheet intrusions should therefore be evaluated in terms of tectonic strain rates, cooling rates, thermal state of the host, distribution of heterogeneous strain, and activation of strain softening mechanisms. Our observations highlight that the concepts of pre-, syn-, late- and post-tectonic fabrics in intrusive igneous rocks should be used with caution when interpreting relative timing relationships between deformation and magmatism.

The Zuccale Fault, central-eastern Elba Island, has been regarded since the 1990s as a low-angle normal fault that records Neogene crustal extension in the inner (Tyrrhenian side) portion of the northern Apennines. The flat-lying attitude of the fault zone and the strong excision of thick nappes were the main reasons for this interpretation. Previous structural and petrographic studies have focused primarily on the fault rocks themselves without map-scale investigation of the structural setting and deformation structures in the hanging wall and footwall blocks. Furthermore, despite the complex history proposed for the Zuccale Fault, the timing of deformation has not yet been constrained by radiometric age data. We present the findings of recent geological studies on eastern Elba Island that provide significant new insight on the nature and tectonic significance of the Zuccale Fault. We document in detail the architecture of breccias and cataclasites that comprise the Zuccale Fault. Our new observations are consistent with a purely brittle deformation zone that crosscuts older early-middle and late Miocene regional and local tectonic structures. The activity on the fault postdates emplacement of the late Miocene Porto Azzurro pluton, and it displaces a previously formed nappe stack ~6km eastward without any footwall exhumation or hanging wall block rotation. These new data raise questions about the development of misoriented faults in the upper crust.

Detailed structural analysis of tourmaline-rich veins hosted in the contact aureole of the ∼6 Ma Porto Azzurro granite in southeastern Elba Island, northern Tyrrhenian Sea is presented. Using geometric features of the veins, the physical conditions at the time of vein formation are estimated, namely the stress ratio (Φ = (σ2 − σ3)/(σ1 − σ3)), driving stress ratio (R′ = (Pf − σ3)/(σ1 − σ3)) and fluid overpressure (ΔPo = Pf − σ3). Two vein sets (A veins and B veins) have been recognized based on orientation and thickness distributions and infilling material. Analysis of vein pole distributions indicates Φ = 0.57 and R′ = 0.24 for the A veins and Φ = 0.58 and R′ = 0.47 for the B veins, and fluid pressures less than the intermediate stress magnitude. Analysis of geometric features of the veins gives estimated fluid overpressures of between ∼16 MPa (A veins) and ∼32 MPa (B veins). We propose a model for the tectonic environment of vein development, in which formation of secondary permeability in the deforming thermal aureole of the Porto Azzurro pluton was controlled by ongoing development of fracture systems in the hinge zone of a regional NNW–SSE trending fold that favored transport and localization of hydrothermal fluids.