Defining the Iapetan Rifted Margin of Laurentia

English Abstract:

The first interpretations of the Iapetan rifted margin of Laurentia explained curved salients and recesses of the Paleozoic Appalachian-Ouachita thrust belt as inherited from the shapes of embayments and promontories, respectively, of the late Proterozoic–Cambrian rifted margin. Contrasting interpretations attributed the shape of the rifted margin (1) to transform offsets of the rift or (2) to three-armed radial-rift triple junctions. Neither initially considered palinspastic location of the rifted margin, which is complicated because parts of the rifted margin are translated in Appalachian allochthons, whereas other Appalachian-Ouachita allochthons carried off-shelf sedimentary rocks onto the passive-margin shelf, leaving the rift margin in the footwall. Retrodeformed balanced cross sections (from outcrop geology, drill data, and seismic reflection profiles) support quantitative reconstruction of palinspastic locations of external basement massifs, synrift sedimentary and igneous rocks, passive-margin shelf carbonates and shelf-edge facies, and the rifted margin. The palinspastic reconstructions, gravity and velocity models, distribution of synrift sedimentary and igneous rocks, passive-margin thermal subsidence, and load-driven subsidence of synorogenic foreland basins contribute to distinction between upper-plate and lower-plate rift margins and transform margins of continental crust. Ages of synrift igneous rocks indicate two phases of extension (~775–650 and ~615–530 Ma), the significance of which, as well as implications for rifting processes, remains to be resolved. A few segments of the margin (e.g., the New York promontory) require more subsurface data for confident restoration. Documentation of extent and provenance of accreted exotic (generally Gondwanan) continental terranes will constrain the extent of native Laurentian crust. Expression of the rift and transform margins within the mantle lithoshpere may be resolved by EarthScope velocity data.