Variation in fracture patterns in damage zones related to strike-slip faults interfering with pre-existing fractures in sandstone (Calcione area, southern Tuscany, Italy)

摘要

This paper deals with the effects of pre-existing fractures on the development of damage zones related to meter and decameter strike-slip faults, which affected Late Oligocene-Early Miocene sandstone in the Calcione area, southern Tuscany (Italy). Strike-slip faults dissected rock masses locally affected by pervasive, pre-existing fractures related to previous deformational events. Two different domains were identified: rock masses affected and unaffected (or relatively unaffected) by pre-existing fractures. Strike-slip faults affecting the two domains made it possible to compare the different configurations of fracture patterns related to isolated or interacting fault segments. New data is provided on scaling relations between: i) fault length and the length of related fractures; ii) mean step length and width for overstepping fault segments; iii) angular values at the intersection of the fault plane and related fractures. Scaling relations obeyed a positive power law for faults dissecting sandstone unaffected by pre-existing fractures. On the contrary, no scaling relations were recognized for faults dissecting sandstone affected by pre-existing fractures. The development of strike-slip faults on rock masses affected by pre-existing fractures can simulate splays apparently related to faulting. Three main points were categorized for detecting deceptive cases. There is a real chance of a deceptive fracture pattern if: i) the angular values at the intersection of the fault and its fractures (mode I opening fracture) exceed 50° in the wall damage zone; ii) the scaling relation between fault length and maximum fracture length does not fit a power law; iii) fractures acted as shear fractures. The resulting framework provides additional data for understanding the development and architecture of strike-slip faults.