14-9 Early Mississippian emplacement of the Blue Ridge thrust sheet

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Authors:

Abstract:

The composite Blue Ridge thrust sheet (BRTS) extends from Virginia to Alabama and thrusts crystalline terranes of the Blue Ridge and Piedmont over unmetamorphosed Paleozoic sedimentary rocks of the Valley and Ridge.  Recent geologic mapping in the Wytheville 30x60-minute quadrangle, VA-NC-TN, combined with kinematic, petrographic, and geochronologic studies, altogether advance our understanding of the geologic framework of the Blue Ridge, the conditions and timing of emplacement of the BRTS, early stages of the Alleghanian orogeny, and possible connections with inherited structures across the Virginia Promontory and Tennessee embayment. 

The Cat Face and Stone Mountain faults frame the east side of the Mountain City window and bound the BRTS. These faults are characterized by a zone of mylonite up to 200 m wide. In the west, the Mountain City window is framed by the Iron Mountain fault, a northwest-dipping brittle fault that reemerges to the west as the Holston Mountain fault, placing the BRTS over Paleozoic rocks of the Valley and Ridge.  Published restored cross sections suggest ~50% shortening across the Valley and Ridge, and displacement estimates for the BRTS are upwards of 350–400 km to the NW.  Muscovite Ar/Ar growth ages from mylonite of the Cat Face fault document slip around ~340 Ma and the initial emplacement of the BRTS.  Within the BRTS, the Fries fault zone is a 2–11 km wide anastomosing shear zone that records muscovite Ar/Ar cooling ages of 347–335 Ma. The Gossan Lead fault juxtaposes the amphibolite facies eastern Blue Ridge rocks against Mesoproterozoic rocks of the western BRTS.  Amphibole and muscovite ages from the eastern Blue Ridge are ~341 Ma and ~337 Ma, respectively, and require rapid cooling compared to the western BRTS. We suggest that the Catface fault, and the Fries fault zone were roughly contemporaneous, although the Fries fault zone was a deeper structure because of the intensity of mylonitization and wider shear zones.  Collectively, these structures document the initial emplacement of the BRTS at ~340 Ma during the earliest Alleghanian; this early event may be recorded as deformation, sedimentation and paleoseismicity in the foreland.

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