NOMENCLATURE

There is currently no general agreement concerning the meaning of such terms as "rifting" and "continental breakup." The following definitions are used here:

  1. Ocean–continent transition zone. This refers to crust located between well-defined continental crust (as indicated mainly by seismic refraction studies) and "normal" oceanic crust with an assumed well-layered stratigraphy.
  2. Continental rifting. This refers generally to the complete sequence of events involving crustal extension, beginning with initial faulting of preexisting continental crust and ending with final separation of subcontinental mantle lithosphere to form new oceanic crust. Continental breakup refers in general to the disintegration of continental crust but is only meaningful if the nature of the rifted lithosphere is specified (e.g., upper crust or mantle lithosphere).
  3. Break up of continental crust. This refers to the complete separation of the brittle upper crust, which may be associated with the exhumation of subcontinental mantle lithosphere to form new seafloor. Fragments of brittle continental crust may persist above the subcontinental mantle lithosphere and remain stranded within the ocean–continent transition zone as "extensional allochthons."
  4. Break up of subcontinental mantle lithosphere. This refers to the final and complete separation of the previously exhumed subcontinental mantle lithosphere, which is replaced by juvenile asthenosphere as "normal" seafloor spreading begins.
  5. "Normal" seafloor spreading. This is regarded as oceanic lithosphere with a well-defined layered stratigraphy, as inferred mainly from the study of certain ophiolites (e.g., Troodos and Vourinos ophiolites) (Anonymous, 1972). However, many ophiolites are now known to be of suprasubduction zone type and their stratigraphies are unlikely to be representative of most oceanic lithosphere formed at mid-ocean ridges (e.g., Robertson, 2002). The stratigraphy of the early ("normal") oceanic lithosphere that was formed by seafloor spreading adjacent to the Newfoundland rifted margin remains unknown, as it has not been drilled. This "normal oceanic crust" is assumed to differ from the earlier crust that was formed by mantle exhumation, as sampled at Site 1277.
  6. Timescale. The timescale used here is that of Gradstein et al. (2004).

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