DETAILED MAPPING OF THE LOWER MOUNT ROGERS FORMATION, SW VIRGINIA: EVIDENCE FOR CRUSTAL INSTABILITY PRECEEDING BREAKUP OF RODINIA

The Mount Rogers Formation (MRF), in the Blue Ridge province of SW VA, records aborted rifting of the Laurentian portion of Rodinia, ~760 million years ago. Successful rifting and opening of Iapetus occurred some 200 million years later, evident from volcanic deposits that outcrop from central Virginia to Maritime Canada. The MRF, however, contains volcanic and clastic sedimentary rocks related to the earlier event. The upper MRF constitutes an eruptive center dominated by voluminous rhyolite lavas and ash-flow sheets, dated at ~760 Ma, whereas the lower MRF contains bimodal volcanic rocks alternating with coarse alluvial fan deposits, which rest unconformably on the 1.1-1.2 Ga Grenville basement. Although the lower MRF holds vital evidence for the initial crustal instability, it has not been previously described in detail. The lower MRF is dominated by a complex assemblage of polymict conglomerate, arkosic sandstone, greenstone, and rhyolite, now metamorphosed to chlorite grade. Recently, RU faculty and students, with support from EDMAP, have undertaken a mapping program with the following goals: 1) differentiate the stratigraphy of the lower MRF, and map the major structures that control distribution of the different lithologies; 2) reconstruct the geometry of the ancient alluvial fan and debris-flow deposits; and 3) describe the nature of the nonconformity (faulted or original?) between the lower MRF and the crystalline basement rocks in the map area. Our mapping thus far indicates that the complex present-day map pattern is controlled by Neoproterozoic syndepositional basin-bounding faults, modified by Paleozoic (Alleghanian?) compressional structures, including tight folds and top-to-NW high strain zones. The foliation typically dips SE, and the upper MRF appears to dip below the lower MRF, suggesting that major portions of the sequence are overturned. Down-dip mineral lineations (e.g., stretched phenocrysts in rhyolite), and stretched clasts in conglomerate consistently trend S40E-S50E, consistent with top-to-NW fault movement. The contact between lower MRF lithologies and the underlying crystalline basement is typically a zone of Intense shearing and mylonitization, indicating that the nonconformable contact was subsequently faulted.