IDENTIFICATION OF LARGE-SCALE MELT INJECTION DIKES IN THE MEDIAL UPLIFT OF THE ROOSEVELT IMPACT STRUCTURE, WEST-CENTRAL GEORGIA, USA

We have documented blocks of shocked quartzite in charnockitic rocks as evidence of an impact event approximately 800 Ma in west-central Georgia (Harris et al., LPSC, 2021). The quartzite clasts contain grains that exhibit multiple planes of decorated planar deformation features (PDFs) dominated by ω {1013} and π {1012} orientations. These shocked xenoliths, along with shocked quartz and feldspar xenocrysts, and recently discovered clasts of shocked Grenvillian schists and gneisses, provide convincing evidence that the charnockitic rocks are impact melt breccias representing the remains of a large structure on the order of 150 x 100 km. The dimensions are similar to Hale Crater on Mars which likewise exhibits an elongate medial ridge rather than a central peak.

Despite the identification of shatter cones, shatter cleavages, and multiple-striated surfaces in the quartzites that compose the ridge, there has been resistance to the idea that it was created by uplift associated with the impact event rather than later Appalachian compression. When the totality of the impact deformation evidence is put into context with the fact that the Pine Mountain Terrane, in which the structure is located, lies beneath the Appalachian detachment, the orogenic models become strained. However, we appreciate the desire for overwhelming observational evidence in the face of overturning a century of thought about the region. A recent demonstration of the power of citizen science has provided that evidence.

A citizen in the NE section of the structure posted photos to a rock identification website. While viewers debated between hematite and man-made slag, we asked to see the rocks in-place. Our investigation showed the rocks to be the eroded surface of a pseudotachylitic breccia dike approximately 30 m wide, striking NE-SW, offset slightly north of the medial line of the quartzite ridge. Quartzite clasts exhibit evidence of partial melting or dissolution, micro-comminution, and feather features (FFs) indicative of impact-generated shearing (Poelchau and Kenkmann, JGR-SE, 2011). Isolated quartz grains contain PDFs. The original melt matrix has been replaced by iron hydroxides but retains some primary structure. We suggest that the dike represents a combination of shearing and injection of melt. Subsequently we have identified three more similarly oriented outcrops spanning 40 km downrange.