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Exceptionally well-exposed comendite coulees crop out in the northern part of San Pietro Island (SW Sardinia, Italy). Large-scale ductile and brittle structures are well visible, even from the aerial photographs, and can be related to the lava emplacement and cooling. Several generations of folds, foliations, thrusts, and fractures are recognized and their overprinting relationships, both in space and time, unravel the synemplacement deformative history of these highly viscous lava flows. We suggest that superposition of folding and thrusting is the result of a progressive, time-transgressive deformation of the coulees related to the continuous extrusion of magma from the vent. Unlike deformative structures observed in other lava flows, where the surface ridges are interpreted as having been derived from the folding of an upper, high-viscosity layer of the coulee, the field data clearly show that folding developed over the whole thickness of the coulee. Ductile deformation progresses toward the frontal area, with first-generation, quasi-cylindrical F 1 upright folds progressively overturning and warping to form F 2 , convex downflow folds. Deformation along the margins of the coulees results in a prominent shear foliation, which develops along the primary foliation planes. Brittle structures like thrusts only develop in the frontal zone of the lava flows, where the largest deformation is associated with a large stiffening of the flowing lava. We suggest that the San Pietro Island comendite lava flows represent an end member of the possible styles of deformation commonly shown by silicic lava flows.

Most lavas are viscous laminar flows, and the roles played by different physical, chemical, and geometric parameters on the mechanism of flow emplacement are relatively well known. However, the kinematics of lavas and the style of flow deformation are still poorly known. Strain indicators of lavas include stretched vesicles, enclaves, crystal and Anisotropy of magnetic susceptibility fabrics, and folds. Enclaves may show different shapes: subcircular to ellipsoidal enclaves, rolling-like structures, boudin-like structures, banding-like structures, and folds. Here, we discuss the constraints that must be satisfied for the use of enclaves as strain indicators, and review the analytical techniques that can be used to determine the finite pure shear α and simple shear strain γ. We review three lava flows cropping on the Porri Volcano (Salina Island, southern Tyrrhenian Sea, Italy) as case study. In these lavas, enclaves of different shapes have been selected at different places (vent, intermediate, and front zone). The structural analysis of the enclaves allows us to determine the ratio between α and γ. The kinematic vorticity number, Wk , which is a measure of noncoaxiality, ranges between 0.1 and 0.7 in the vent zone, and increases up to 0.9–1 at the front. The prevailing pure shear deformation at the vent is due to the lateral extension of the flow, whereas the simple shear increments are acquired during the lava flow motion owing to gravity. Lava flows are extensional to hyperbolic flows at the vent and intermediate zones, whereas they may be considered as ideal simple shear flows near the front zone.

The anisotropy of magnetic susceptibility (AMS) is a powerful technique that can be used to explore in detail the mineral fabric of many types of rocks. In particular, it is well suited to determine mineral fabric of massive, otherwise featureless rocks, like the internal parts of many lava flows and dikes. Although the AMS technique relies on the use of an external magnetic field for its measurement, the methodology and general assumptions behind the AMS technique are more akin to other traditional petrofabric techniques than they are to paleomagnetic works. Furthermore, like most other mineral fabrics, the AMS is mainly acquired at a stage when flow-related deformation promotes a mineral array within the liquid (albeit viscous and probably with a yield strength) lava. The effort required to obtain three-dimensional information of such a mineral array using AMS, however, is less than the effort required by other more traditional methods of fabric analysis. It must be noted that due to differences in the shape of various minerals it is possible to find some differences between magnetic and optically determined mineral fabrics. When attention is given to the systematic variations of the AMS within a lava flow or dike, however, the AMS method allows us to infer aspects of lava (magma) emplacement that are not easy to study through other traditional petrofabric techniques, like mapping the regions of a lava flow that experienced late shearing during emplacement. Detailed knowledge of such deformation partition within a lava flow can be used in turn as a useful criterion to determine whether the flow grew by endogenous or exogenous processes. Also, in some circumstances it may be possible to delineate zones that experienced the same deformation regime (i.e., pure or simple shear) using the information provided by the AMS method alone, or in combination with other petrofabric techniques, therefore allowing us to obtain a detailed record of the internal deformation of one flow unit. For these reasons, the AMS method is an efficient means for collecting a large number of observations, which, in turn, are required to fully characterize lava flow fabric and kinematics.