Insights from Lithosphere- to Crustal-scale Fault Zones
Published:January 01, 2001
2001. "Insights from Lithosphere- to Crustal-scale Fault Zones", 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 Salmon River suture zone, western Idaho, is a fundamental lithospheric boundary between the North American craton and the accreted terranes of the Cordilleran margin. The initial juxtaposition along this north–south-oriented structure occurred during Early Cretaceous time. This zone was potentially reactivated twice by subsequent tectonism, once during Cretaceous time and once during Miocene time. The Late Cretaceous western Idaho shear zone formed along the Salmon River suture zone, as denoted by a sharp gradient in the isotopic signature of the granitoids that intruded the lithospheric boundary zone. The reconstructed Late Cretaceous orientation of the western Idaho shear zone contains subvertical fabrics (lineation, foliation). The same boundary also acted as a locus for subsequent Miocene Basin and Range extensional deformation. Domino-style normal faulting and deep (2100 m) basin formation accommodated the motion between the extending accreted terranes to the west and the unextended Idaho batholith to the east. Whereas either the mantle boundary or a crustal-scale structuring controls the regional extent of the extensionally reactivated zone, locally crustal basement faults and lithological contacts control the orientation and precise location of faults that accommodate reactivation. The multiple reactivation of the Salmon River suture zone is critical for several reasons. The Early Cretaceous suture zone apparently created a fundamental lithospheric flaw, which was reactivated after terrane accretion. Whether this zone was a fracture or a shear zone, the fabric in the mantle lithosphere was apparently not ‘healed’ during orogenesis. Thus, juxtaposition of mantle lithosphere, which is inferred to occur by faulting in the uppermost mantle, acts as a weakness during later tectonism. Second, the paucity of strike-slip plate boundaries in the geological record makes sense in the context of reactivation. The vertical, lithospheric-scale nature of these structures makes them particularly susceptible to lithospheric-scale reactivation during both transcurrent and/or extensional deformation. These reactivations both overprint the earlier deformation and modify the original geometry. Steeply dipping fabrics, rather than vertical fabrics, may be the general signature of major, ancient strike-slip faults.
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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.