SUPERCONTINENT BREAKUP, DIACHRONOUS RIFTING, AND DISPERSAL OF MICROCONTINENTS: THE LAURENTIAN MARGIN OF IAPETUS

The final stages of continental breakup of Rodinia framed the promontories and embayments of the Iapetan margin of Laurentia. Resolution of timing of rifting and breakup rests on integration of age data from (1) synrift volcanic and plutonic rocks, bracketing the time from initial brittle extension to sea-floor spreading; (2) synrift sediment and a post-rift unconformity, bracketing the time from brittle extension to post-rift thermal subsidence; (3) post-rift thermal subsidence at conjugate margins, distinguishing upper and lower plates in low-angle-detachment simple-shear extension; and (4) shelf, shelf-edge, slope, and rise assemblages, confirming the time of initiation of a passive margin. These data document early rifting at ~750 Ma, pervasive continental breakup at ~565 Ma, and late-stage rifting of microcontinents as late as ~530 Ma. Laurentian microcontinents ultimately either were accreted back to the parent continental margin or were dispersed to exotic sites.

Post-rift transition along the Laurentian Blue Ridge margin (New York promontory to Alabama promontory) at ~543 Ma constrains the time of breakup, as well as the minimum age of rifting of microcontinents, now “re-accreted” Appalachian internal basement massifs (e.g., Pine Mountain, Sauratown Mountains, Baltimore domes). The age of a sandstone-carbonate (now quartzite-marble) passive-margin cover on the microcontinents is unconstrained and might represent either ~750 or ~565 Ma rifting and breakup. In contrast, the passive-margin cover of the Argentine Precordillera indicates rifting from the Ouachita embayment of Laurentia during Early Cambrian time (~530 Ma), followed by dispersal and accretion to western Gondwana. Early to Middle Cambrian graben-filling sediment of the Mississippi Valley-Rough Creek-Rome fault system of eastern Laurentia is coeval with rifting of the Precordillera from the Ouachita embayment, and the fault system outlines a potential microcontinent that failed to separate from Laurentia.