INTEGRATING DIVERSE DATA SETS FROM THE LAURENTIAN MARGIN AND ACCRETED TERRANES IN THE NORTHERN APPALACHIANS AND WESTERN CALEDONIDES

Appalachian-Caledonide orogen development has been traditionally illustrated using cross-sections showing terrane accretion and collision over time. This approach is valuable but involves implicit assumptions: subduction was initiated at passive continental margins; convergence was mainly orthogonal; terranes had ribbon-like geometry parallel to continental margins; and present-day orogen geometry is a valid "end point" for reconstructions. Post-Pangea tectonic evolution provides little support for these assumptions.

Several under-utilized data sets, primarily from sedimentary rocks, are relevant to the reconstruction of the orogen: (1) Over 350 detrital zircon data sets may be compiled and compared using new display techniques in geographical information systems and plate reconstruction software, to show proximity of terranes to major continental blocks that are best candidates for sedimentary provenance; (2) Paleomagnetic information including declinations, so as to evaluate systematic vertical-axis rotations as well as latitude changes; (3) Legacy biostratigraphic data using calibrated timescales to place sedimentary units relative to isotopically dated igneous units; (4) Estimates of late Paleozoic and Mesozoic plate motions, to restore a valid mid-Devonian geometry from which to build back in time.

These data sets can inform kinematic models for the orogen in several ways. The arrival of terranes at the Laurentian margin is marked by an influx of late Mesoproterozoic "Grenville" grains, indicating that terranes attributed to Ganderia and associated Gondwana-derived arcs arrived at different times from Ordovician to Devonian. The Taconian Orogeny is explained as the result of diachronous arc-continent collision that involved both Laurentia-derived and Gondwana-derived units, followed by subduction-polarity reversal at the Laurentian margin. Salinian deformation resulted from subduction-accretion at this margin, over a period of time from Late Ordovician to nearly the end of the Silurian. Acadian deformation resulted from sinistral and convergent motions at an Early Devonian along-margin boundary that may have varied from transpressional in New England to ideal strike slip in Britain and Ireland.