Rheological partitioning during multiple reactivation of the Palaeozoic Brevard Fault Zone, Southern Appalachians, USA
Published:January 01, 2001
Robert D. Hatcher, Jr., 2001. "Rheological partitioning during multiple reactivation of the Palaeozoic Brevard Fault Zone, Southern Appalachians, USA", The Nature and Tectonic Significance of Fault Zone Weakening, R. E. Holdsworth, R. A. Strachan, J. F. Magloughlin, R. J. Knipe
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The Brevard Fault Zone is a linear, NE-trending, gently to moderately SE-dipping fault zone traceable some 750 km from Alabama to Virginia in the crystalline southern Appalachians. It ranges from 1 to 3 km wide and contains a mappable lithostratigraphy. The Brevard Fault Zone has been interpreted as a thrust, strike-slip fault (both dextral and sinistral), a suture and terrane boundary, and a fundamental crustal tectonic boundary. Deformation was partitioned in space and time, and motion was both strike-slip (dextral) and dip-slip (thrust). Early strike-slip and thrust movement was coupled to map-scale structures in the deep Inner Piedmont, late Palaeozoic dextral motion was confined to a zone of 1-3 km width, and the latest reactivation consisted of brittle thrusting confined to a zone of 100 m width. The fault zone is cut by undeformed NW-trending Mesozoic dolerite dykes. The Brevard Fault Zone is characterized by the presence of a prominent retrograde (chlorite-muscovite stable) S–C fabric that indicates dextral motion. This fabric is related to late Palaeozoic (Alleghanian) dextral reactivation of the fault zone, with an unknown displacement at a time when huge volumes of fluid were fluxed through the zone. The deformation overprints an earlier (Acadian) high-tempera-ture (garnet–staurolite–kyanite) fabric that also yields a dextral motion sense, and involved a component of thrusting. This mid-Palaeozoic deformation was coupled with west-directed, near-metamorphic peak thrusting and flow from the deep Inner Piedmont (to the east) that was buttressed against the primordial Brevard Fault Zone so that the motion became SW directed, and plastic flow became constricted in this narrow 1-3 km zone. Both of these plastic deformations were overprinted by late Alleghanian NW-directed dip-slip brittle deformation confined to the NW side of the Brevard Fault Zone. This last deformation involved at least 10 km of displacement and was related to reactivation of this block of crust as part of the late Alleghanian, NW-directed Blue Ridge-Pied-mont megathrust sheet, and formed out of sequence with respect to the megathrust sheet. The Brevard Fault Zone was clearly a zone of crustal weakness that had a suitable mech-anical stratigraphy that imparted sufficient anisotropy to localize the initial Acadian fault(s). Early Alleghanian fluid fluxing weakened the already strongly anisotropic fault zone and probably focused ductile reactivation at a shallower crustal depth during the early Alleghanian event. Late Alleghanian reactivation occurred even shallower as an almost discrete boundary in the brittle regime. This is one of the few faults in the Appalachians to have undergone deformational partitioning to permit multiple reactivation during Palaeozoic time.
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The Nature and Tectonic Significance of Fault Zone Weakening
Many faults appears to form persistent zones of weakness that fundamentally influence the distribution, arichitecture and movement patterns of crustal-scale deformation and associated processes in both continental and oceanic regions. They act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards.
This book brings together papers by an international group of Earth Scientists to discuss a broad range of topics centred upon the controls of fault weakening and the role of such faults during lithosphere deformation.
The book will be of interests to both academic and industrial Earth Scientists with an interest in geodynamics, structure at all scales, tectonics and the migration of petroleum and water.