Abstract. Fault and fracture planes (FFPs) affecting multilayer sequences can be
significantly refracted at layer–layer interfaces due to the different
mechanical properties of the contiguous layers, such as shear strength,
friction coefficient and grain size. Detailed studies of different but
coexisting and broadly coeval failure modes (tensile, hybrid and shear)
within multilayers deformed in extensional settings have led to infer
relatively low confinement and differential stress as the boundary stress
conditions at which FFP refraction occurs. Although indeed widely recognized
and studied in extensional settings, the details of FFP nucleation,
propagation and refraction through multilayers remain not completely
understood, partly because of the common lack of geological structures
documenting the incipient and intermediate stages of deformation. Here, we
present a study on strongly refracted strike-slip FFPs within the mechanically
layered turbidites of the Marnoso Arenacea Formation (MAF) of the Italian
northern Apennines. The MAF is characterized by the alternation of sandstone
(strong) and carbonate mudstone (weak) layers. The studied refracted FFPs
formed at the front of the regional-scale NE-verging Palazzuolo anticline and
post-date almost any other observed structure except for a set of late
extensional faults. The studied faults document coexisting shear and hybrid
(tensile–shear) failure modes and, at odds with existing models, we suggest
that they initially nucleated as shear fractures (mode III) within the weak
layers and, only at a later stage, propagated as dilatant fractures (modes
I–II) within the strong layers. The tensile fractures within the strong
layers invariably contain blocky calcite infills, which are, on the other
hand, almost completely absent along the shear fracture planes deforming the
weak layers. Paleostress analysis suggests that the refracted FFPs formed in a
NNE–SSW compressional stress field and excludes the possibility that their
present geometric attitude results from the rotation through time of faults
with an initial different orientation. The relative slip and dilation
potential of the observed structures was derived by slip and dilation
tendency analysis. Mesoscopic analysis of preserved structures from the
incipient and intermediate stages of development and evolution of the
refracted FFPs allowed us to propose an evolutionary scheme wherein
(a) nucleation of refracted FFPs occurs within weak layers; (b) refraction is
primarily controlled by grain size and clay mineral content and variations
thereof at layer–layer interfaces but also within individual layers;
(c) propagation within strong layers occurs primarily by fluid-assisted
development ahead of the FFP tip of a “process zone” defined by a network
of hybrid and tensile fractures; (d) the process zone causes the progressive
weakening and fragmentation of the affected rock volume to eventually allow
the FFPs to propagate through the strong layers; (e) enhanced suitable
conditions for the development of tensile and hybrid fractures can be also
achieved thanks to the important role played by pressured fluids.
Scaling of Quasi-Brittle Fracture and the Fractal Question