9-3 Testing the Role of Strike-Slip Versus Normal-Slip Faulting Along a Major Terrane Boundary in the North Carolina Eastern Piedmont

Session: Advances in Geologic Mapping, Databases, and Dissemination: Student Posters

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Abstract:

The contact between the greenschist-facies easternmost Carolina and amphibolite-facies Falls Lake terranes is a major fault zone in the North Carolina eastern Piedmont. Past mapping indicates that this structure separating Neoproterozoic suprastructural and infrastructural rocks is a segment of the ductile dextral Nutbush Creek fault zone. This strand of the Eastern Piedmont fault system in the southern Appalachian orogen formed during late Paleozoic Alleghanian orogenesis and Pangean amalgamation between Laurentia and Gondwana. While prior map data commonly show planar fabric orientations compatible with regional strike-slip fault trends, linear fabric elements depict dip-slip orientations consistent with normal-slip rather than strike-slip faulting. Constraining the slip history of this terrane boundary is key to understanding regional deformation history. 

To evaluate fault kinematics, we conducted lithological mapping and collected fabric element measurements and oriented rock samples along eight traverses across the fault trace in the Bayleaf and Creedmoor 7.5-minute quadrangles in north Raleigh. The easternmost Carolina terrane is composed of metamorphosed intermediate to felsic and locally ultramafic, arc-related plutonic rocks of the Beaverdam complex. The Falls Lake terrane contains layered biotite muscovite ± garnet schist that locally includes pebble-sized metaplutonic clasts. Protomylonitic to ultramylonic and phyllonitic rocks and bull quartz outcrops to boulder fields mark the terrane boundary. 

We created a detailed geologic map and fabric stereograms across the fault trace. Stereograms highlight a dip-slip mineral stretching lineation in west-dipping foliations. We then analyzed samples oriented parallel and perpendicular to lineation to identify mesoscale to microscale structures including planar S-C elements, porphyroclast and porphyroblast rotations, and lithologic and foliation offsets to determine slip sense. Kinematic indicators reveal that ductile normal-slip was the main displacement mechanism in contrast to dextral slip along the Nutbush Creek fault zone. Only one sample displays an oblique sense of strike-slip motion. Common occurrences of bull quartz, silica cataclasite, and epidosite reflect a brittle overprint. 

These results are compatible with this structure being a ductile-brittle normal fault similar to the Jonesboro and Fishing Creek faults that define the eastern margin of the Deep River Triassic rift basin to the west of our study area. We propose that this terrane boundary be renamed the Upper Barton Creek normal fault that is related to Paleozoic-Mesozoic rifting of Pangea rather than Nutbush Creek strike-slip faulting during Alleghanian orogenesis. 

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