SPATIAL AND TEMPORAL COMPLEXITIES OF THE EASTERN LAURENTIAN MARGIN: NEW INSIGHT FROM OLD ROCKS OF THE APPALACHIAN BLUE RIDGE AND WESTERN PIEDMONT

Salients and recesses were related to plate tectonics in the past 40 years and include rift axes, volcanic centers, triple junctions, transform faults, hinge zones, plume tracks, two-stage rifts, and upper- and lower- plates. Neoproterozoic-Cambrian rocks contain abundant unconformities and span at least 225 my. They were metamorphosed, deformed, and transported during the late Paleozoic Alleghanian orogeny and constitute slices exposed in an area ~700 km long and 25-75 km wide. Palinspastic restoration and modern geochronology define a complex evolution of the eastern margin.

Final amalgamation of Rodinia locally was 950-900 Ma. Extension at 571-555 Ma led to break-up but was not related to 780-675 Ma extension. Bi-modal volcanic rocks (~780-750), mafic dike swarms (~758 Ma), and terrestrial sediments were deposited locally in 3 pulses that ended with lacustrine glaciogenic deposits in restricted basins. Granitoid plutons were emplaced in extended crust ~751-675 Ma. Subaqueous glaciogenic, turbidite, and rhythmite deposits overlie the plutons. Some restricted basins are fault bounded, contain syndepositional faults, and some are cut by transverse normal faults.

Bi-modal volcanic rocks (~571-555 Ma) and associated dike swarm reflect the terminal rift, yet extension was active in the craton at ~544-520 Ma. Subaerial basalt transitions across a hinge zone across strike into a thick basinal deposit that is overlain by clastic rocks that fine upward to mud. These are overlain by rise-slope deposits that contain mafic volcanic rocks and possible felsic subaqueous tuffs. Basalt flows taper out along strike across a transition zone where aerially extensive thin basalt flows are in coarse fluvial rocks. Fluvial to deep water deposits in a large basin are bounded by a major transverse fault. This basin is comprised of restricted units along and across strike that show evidence of extension but contain no volcanic rocks.

Neoproterozoic rocks were unconformably overlain by Cambrian clastic fluvial to basinal rocks on the subsiding margin. Age populations of detrital zircon correspond to depositional settings and exposed basement prior to the carbonate bank. Eastern Blue Ridge and western Piedmont rocks locally transition along and across strike and reflect an irregular margin telescoped during orogenesis.