DISCONTINUITIES AND CONNECTIONS IN PIEDMONT AND BLUE RIDGE TERRANES FROM MARYLAND TO VIRGINIA

Rocks of the ancestral rift-shelf-slope-rise of the eastern Laurentian continental margin comprise tectonic belts in MD and VA whose N-S connection is concealed by the early Mesozoic Culpeper Basin (CB). In both places, slope-rise rocks of the western Piedmont were juxtaposed above rift-shelf-slope rocks of the Blue Ridge, but rock types, faults, and ages and conditions of metamorphism change along strike as a function of margin geometry and subsequent tectonic assemblage.

Weakly metamorphosed, ENE-trending Late Neoproterozoic to early Ordovician shallow shelf- to slope rocks of the Westminster terrane sensu stricto (WT) record ancient islands of basalt (subaerial and pillows) rimmed by carbonate and clastic rocks. The WT is east of the Blue Ridge (BR) in MD, and pinches out to the southwest under the CB, reflecting a Precambrian margin that was oblique to the NE tectonic strike of later orogenies. BR cover rocks south of the CB represent a Neoproterozoic to Cambrian shelf-slope underlain by Neoproterozoic subaqueous rift basalts. Piedmont rocks of the slope-rise to the east are similar along strike: phyllites (Marburg Formation and Mine Run Complex) comprise the western part of the Potomac terrane (PT), while the eastern PT contains a continuous ultramafic-bearing melange zone cut by an Ordovician pluton. The western boundary of the melange zone is the Pleasant Grove (MD) and Byrd Mill (VA) faults, which connect under the CB. East of the zone are Ordovician arc plutons (MD) and the Ordovician Chopawamsic volcanic arc (VA).

Tectonothermal histories of PT rocks across the region have both differences and commonalities. Early Taconian 475 Ma high-grade partial melting in MD was followed by protracted lower-grade, westward-propagating events from 430-340 Ma, whereas PT rocks in central VA record only a single event at ~450 Ma before the late Paleozoic. Cooling ages of BR rocks in both MD and VA range from 400-340 Ma and indicate thrusting of western PT rocks above BR rocks along two faults, Martic (MD) and Mountain Run (VA), that are not connected along strike. PT rocks across the region shared a strong Alleghanian deformation as late as ~300 Ma. Thus, regional differences in initial margin geometry and synchrony and diachroneity of tectonic events are revealed by comparing Piedmont and Blue Ridge terranes in MD and central VA.