ROLES OF TRANSFORM FAULTS ALONG THE LAURENTIAN RIFTED MARGIN

At the crustal level of brittle extensional structures, transform faults serve two distinct functions: to offset rift segments and to bound domains of opposite dips of low-angle detachments. Transform faults in continental embayments are associated with anomalously great crustal subsidence recorded in exceptionally thick sedimentary accumulations in both synrift and post-rift synorogenic foreland-basin settings. Along the Iapetan rifted margin of Laurentia, the New Jersey transform fault offsets rift segments to define the New York promontory and Pennsylvania embayment, and the Alabama-Oklahoma transform fault offsets rift segments between the Alabama promontory and Ouachita embayment. The magnitude of rift offset at the Alabama-Oklahoma transform is a well-defined 800 km. The magnitude of rift offset at the New Jersey transform is somewhat uncertain, because alternative possible interpretations of palinspastic restoration of the Berkshire massif and the Reading Prong basement structures leave the palinspastic location of the rifted margin along the New York promontory somewhat uncertain. The thick, synrift Peters Creek Formation extends along the New Jersey transform into the Pennsylvania embayment, which also localizes the maximum thicknesses of synorogenic deposits in the Taconic (Martinsburg) and Acadian (Catskill) foreland basins. In the corner of the Ouachita embayment, where the Alabama-Oklahoma transform intersects the Ouachita rift margin, an exceptional negative gravity anomaly suggests an extraordinary thickness of sedimentary strata, possibly including a very thick synrift accumulation. The same location in the Ouachita embayment is the site of maximum thickness of Carboniferous synorogenic strata in the Arkoma basin in the Ouachita foreland. Localized synrift and synorogenic subsidence along transform faults in continental embayments suggests anomalous lithospheric weakness along transforms, consistent with the interpretation that large-scale transforms are lithosphere-penetrating zones of distributed shear.