NEOPROTEROZOIC TO CAMBRIAN RIFT ARCHITECTURE OF EASTERN LAURENTIA: A PROLONGED RECORD OF EXTENSION BEFORE AND AFTER DEVELOPMENT OF THE IAPETUS OCEAN

Mesoproterozoic to Cambrian rocks of the Appalachian Blue Ridge and Middle Cambrian grabens buried beneath the Appalachian basin define the rift architecture of the Laurentian margin. The distribution of the restored, allochthonous Blue Ridge rocks constitute structural blocks where extension developed from south to north. In the southern block, the Ocoee Supergroup was deposited in an extensional basin between about 920 and 700 Ma. To the northeast in the Grandfather Mountain-Mount Rogers block, crustal extension resulted in restricted basins with magmatism and sedimentation from 765-730 Ma. Basalt flows in the Unicoi may represent magmatism from ~570-555 Ma. Farther northeast in the Catoctin block, linear plutons were emplaced within northeast-trending extensional fault zones between 735-680 Ma. Sedimentary and volcanic rocks in the Catoctin block reflect a highland with significant topographic relief in the west to a subsiding basin in the east. Basal sedimentary deposits post-date locally preserved volcanic rocks (719-708 Ma). Northwest-trending, down-to-the-southwest, syn-depositional normal faults locally truncate the Neoproterozoic deposits which thicken across the faults. A rift margin hinge zone in the axial part of the Blue Ridge restricted deep water deposits in the east. Deposition of marble at the top of the clastic section was followed by extensional dike swarms and bimodal volcanism from ~570-555 Ma. A regional disconformity underlies the Early Cambrian clastic sedimentary rocks. Extension continued into the Early Cambrian as there are local syn-depositional normal faults. Regional clastic sedimentation, the extensional Short Hill-South Mountain fault in the Catoctin block, and development of the extensional Rome trough to the west, occurred in the Middle Cambrian after establishment of the passive margin carbonate platform. Thus, extension within Laurentia lasted more than 265 m.y., and bracketed development of the Iapetus Ocean.