129-8 Re-evaluating the Role of Strike-Slip Versus Normal-Slip Faulting Between the Carolina and Falls Lake Terranes in the Eastern Piedmont of North Carolina

Abstract:

In the North Carolina eastern Piedmont, understanding the kinematics of a major fault zone that separates the suprastructural, greenschist-facies easternmost Carolina terrane from the infrastructural, amphibolite-facies Falls Lake terrane is key to reconstructing the structural evolution of the region. This fault zone was previously considered to be a segment of the ductile dextral Nutbush Creek fault zone, a strand of the Eastern Piedmont fault system formed during late Paleozoic Alleghanian orogenesis. Prior mapping depicts planar fabric orientations compatible with regional strike-slip fault trends. However, linear fabric orientations are orthogonal to strike trends and down-dip plunges are inconsistent with strike-slip faulting.

To re-evaluate fault kinematics, we conducted eight traverses across the fault zone using traditional and digital mapping techniques and collected oriented rock samples and fabric element measurements. The easternmost Carolina terrane contains metamorphosed intermediate to felsic and locally ultramafic arc-related plutonic rocks. The Falls Lake terrane contains layered biotite muscovite ± garnet schist locally meta-ultramafic pods. We created a detailed geologic map and fabric element stereograms across the terrane boundary. The stereograms highlight a dip-slip mineral stretching lineation in west-dipping foliations. We then produced thin sections 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 consistently display ductile normal slip in contrast to dextral slip along the Nutbush Creek fault both north and south of the study area. No evidence for strike-slip kinematics was observed. In addition, occurrences of bull quartz, silicified cataclasite, and epidosite reflect a late brittle overprint.

Based on our lithologic and structural analysis, fault strand slip is consistent with ductile-brittle normal faulting. This is similar to the nearby Jonesboro and Fishing Creek faults that define the eastern margin of the Deep River Triassic rift basin. We propose that the fault zone and terrane boundary in this study, previously considered part of the Nutbush Creek fault, be renamed the Upper Barton Creek normal fault and we suggest that it is associated with Paleozoic-Mesozoic rifting of Pangea as opposed to strike-slip faulting during Alleghanian orogenesis. The Falls Lake terrane may be an uplifted portion of the Carolina terrane in the footwall of the Upper Barton Creek fault due to crustal extension.

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