Structural geology of crystal-rich, silicic lava flows: A case study from San Pietro Island (Sardinia, Italy)
Raffaello Cioni, Antonio Funedda, 2005. "Structural geology of crystal-rich, silicic lava flows: A case study from San Pietro Island (Sardinia, Italy)", Kinematics and dynamics of lava flows, Michael Manga, Guido Ventura
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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 F1 upright folds progressively overturning and warping to form F2, 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.