Flow perturbation folds are frequently associated with crustal scale shear zones (Alsop and Holsworth, 2007). Such folds are generated as a consequence of variable flow within high shear strain zones and can evolve to form both eye folds (Ramsay, 1962) and increasingly stretched sheat folds characterized by intense hinge line curvilinearity. Field survey performed in the area of Montalto (Aspromonte Massif, southern Calabria) and the careful recording of fold and fabric geometries allowed to recognize eye-fold geometry in quartz-feldspar mylonitic leucogneisses and interbedded paragneisses deformed under greenschist facies conditions (Ortolano et al., 2005; Cirrincione et al., 2009; Fazio et al., 2010). Axial planar surfaces of minor S- and Z-type folds and stretching lineation delineate a top-to-thenortheast sense of shear in the present day geograpic coordinates. Vorticity analysis performed on thin sections based on the aspect ratio of rigid clasts (feldspars) within ultramylonite layers (quartz-micaceous matrix) suggests a sub-simple shear for such mylonites with an average value of the vorticity number Wk=0.65. Microstructural analysis also revealed the occurrence of similar eye-folds at thin section scale. A shear strain rate of 4.32×10-12 (1/s) was also inferred by means of recrystallized quartz grainsize paleopiezometry. The pattern of folding occurring in the investigated shear zone may be linked to the same kinematic regime of deformation and could reflect progressive strain of the same event rather than multiple deformational events.
|Titolo:||Flow perturbation folding developed under sub-simple shear regime within a crustal scale shear zone (Calabria)|
|Data di pubblicazione:||2015|
|Appare nelle tipologie:||1.5 Abstract in rivista|