The recognition of reactivation during continental deformation

Abstract

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

Localisation of ductile and brittle shear zones along the Szentlőrinc-1 well in the Mecsekalja Zone using quartz microstructural and well-log data

“…The generations of previous cohesionless fractures, grain refinement processes, general reaction softening, geometric and fabric softening and thermal perturbations in faults and shear zones lead to long-term weakening. Other processes, such as shear heating, increase in pore fluid pressure, transient fine-grained reaction products, transformational plasticity, variations in pore fluid chemistry and fluid-assisted diffusive mass transfer may cause transient weakening in faults and shear zones (Holdsworth et al 1997).…”

“…The early faults and shear zones in the continental crust operate as softened planes, so when the crust is experienced new stress field, those areas perform as preferred regions for reactivation (White et al 1986). Holdsworth et al (1997) defined the reactivation as the emplacement of geologically separable displacement episodes (intervals [1 Ma) along the primarily formed structures. In the continental crust, the structural reactivation is an essential procedure of the deformation.…”

“…In the continental crust, the structural reactivation is an essential procedure of the deformation. The long-lived weakening zones in the continental crust recurrently reactivate, and thus, the subsequent crustal strains may cause the formation of new regions of displacements (Hills 1956;Prucha 1992;Holdsworth et al 1997). The stress in the continental crust is the particular cause of the reactivation, but it is also a function of the orientation of the fault-zone with respect to the imposed stress field, their capability to accommodate the resultant strains and the presence of rocks that form along faults (fault gouges, cataclasites, mylonites) (White et al 1986).…”

The onset of the North Atlantic Igneous Province in a rifting perspective

“…Increased fluid pressure further weakens all affected rock assemblages in the lithosphere (Hirth & Kohlstedt, 1996;Jackson et al 2008). Upper mantle heterogeneity and the presence of old shear zones in the lithosphere may play a prominent role in incipient rifting both by enhancing partial melting and by reactivation of old shear zones (Holdsworth, Butler & Roberts, 1997;Ryan & Dewey, 1997;Kohlstedt, Evans & Mackwell, 1995).…”

Protracted deformation during cooling of the Paleoproterozoic arc system as constrained by 40Ar/39Ar ages of muscovite from brittle faults: the Transamazonan Bacajá Terrane, Brazil

“…These anisotropies are locations that enhance tectonic reactivation. The Bacajá domain contains the first two of the four types of criteria used by Holdsworth et al (1997) to identify reactivation features (structural, geochronological and stratigraphic).…”