FOLDED SURFACES AND EMPLACEMENT-RELATED DEFORMATION IN CRYSTAL-RICH COMENDITE LAVA FLOWS FROM SARDINIA (ITALY)

Exceptionally well-exposed comendite exogenous lava domes crop out in the northern part of San Pietro Island (SW-Sardinia). Here ductile and brittle structures have been related to the emplacement and cooling of lava domes and coulees. The lava is a peralkaline rhyolite (SiO2=75-76 wt%, Total Alkalis=8.2-9.2 wt%, Agpaitic Index=1.0-1.1) rich in mm-sized phenocrysts of Qtz and K-Fld. Phenocrysts content reach up to 50 vol%, giving an high viscosity rate. The main feature of lava flows are strongly curved ridges (wavelength reach up to 300 m), concave toward the vent, whose look like ropy structures. Detailed field mapping and structural analysis highlight that lava domes and coulees undergone a complex deformation history and ropes are first generation folds (F1) refolded by a second generations folds (F2). F1 folds are, when undeformed, cylindrical upright horizontal folds, with their axial plane sub-orthogonal to the flow direction. F2 folds are generally large-scale reclined folds, with axial plane roughly parallel to the flow direction and hinge line dipping toward the vent. The surface folded by F1 folds is a primary layering of mm- and sub mm-scale levels with different phenocryst content. We suggest that superposed folding is the result of a progressive deformation related to the progressive extrusion of lava from the vent. The progressive deformation is highlighted by the decrease of interlimb angle of F2 folds with distance from the vent zone, as well as by the decrease of wavelength of F1 folds and the increase of overturning of axial plane of F1 folds towards the flow direction. Other structural elements have been mapped. Several sets of cleavage affect the whole body of the coulees, whereas the sides of the lava flow suffered an high strain and are characterised by strongly asymmetric small-scale folds and by the occurrence of tension gashes, tectonic breccias and tectoglyphs which indicate the sense of shear of lava flow. Thrusts are clearly exposed only in the frontal portions of the coulees where sub-horizontal shortening reach a maximum, and high viscosity rate cause brittle behaviour of the lava flow. Overprinting relationship, both in space and time, between folds, cleavages, thrusts and joints, allow us to unravel deformation history of lava flow emplacement and several "phases" of deformation can be recognised.