Structural characteristics of the Late Proterozoic (post-Grenville) continental margin of the Laurentian craton

Abstract

Distribution and orientation data of 2676 segments of 2119 Late Proterozoic to Early Cambrian felsic and mafic dikes were compared with other Late Proterozoic to Early Cambrian features associated with two stages of Iapetan extension, including: (1) the trend of the Robertson River rift zone; (2) the distribution of granitoid plutons; (3) the distribution of volcanic fields; (4) trends of previously recognized surface and subsurface extensional faults; (5) kinematic indicators from previous studies; and (6) some Paleozoic foliation data. This comparison suggests the following: Felsic dikes in northern Virginia are spatially related to the N25°E-trending rift zone filled with Robertson River granitoids and have a principal relict trend of N25°E. Mafic dikes, identified as Catoctin dikes in the same area, have a principal relict orientation of N35°E but exhibit a wider range of orientations than the dikes associated with the rift — many locally bimodal or trimodal. Other mafic dikes in the same area have a N25°E principal trend. Previous studies indicate chemical similarity between these dikes and Catoctin dikes. These other mafic dikes could be either associated with the earlier stage of extension, when the Robertson River rift zone developed but are, coincidently, from chemically similar magmas as the Catoctin volcanic rocks, or they could be associated with the younger stage of Catoctin volcanism but intruded along fractures developed during the earlier stage. Many amphibolite dikes in the eastern part of the Blue Ridge where Paleozoic metamorphic grade was higher (commonly garnet) have preferred orientations parallel to regional Paleozoic foliation trends and generally lack diverse secondary or minor trends. These amphibolites are interpreted to have been partially transposed (≈ 10°-20° rotation) during Paleozoic orogenesis, but they could reflect original trends that coincidentally are parallel to younger foliation. Overall, preferred relict orientations for all dikes suggest Late Proterozoic-Cambrian extension was primarily along an axis oriented ≈ N55°-85°W — nearly perpendicular to documented Late Proterozoic subsurface faults in the southern Appalachians and to the reconstructed Laurentian margin during Iapetan extension. Data from uppermost Late Proterozoic (post-Grenville) to Early Cambrian units of the Blue Ridge, when plotted in possible pre-Paleozoic palinspastic locations, suggest that several centers of felsic and mafic volcanism/plutonism developed along the southeastern portion of Laurentia during the initial stage of rifting (≈ 760–650 Ma) associated with Iapetus. In the northern Virginia Blue Ridge, both granitoids of the Robertson River rift zone and associated dikes were emplaced subparallel to the carton margin, derived in previous studies, but oblique to the trend of the nonmarine/marine hinge line previously recognized from Late Proterozoic sedimentation patterns. During the subsequent stage of rifting (≈ 570–600 Ma), when an extensive mafic volcanic field (Catoctin Formation) occupied the northern Virginia Blue Ridge, the associated dike trend was subparallel to the hinge zone. The ten degree difference in dike orientations between the early and late stages of Iapetan rifting may reflect a similar shift in the local stress-field orientation with time during the Iapetan extensional event.