THE CHALLENGES OF INVESTIGATING POTENTIAL IMPACT STRUCTURES IN ANCIENT TECTONIC TERRANES: A CASE STUDY FROM WOODBURY, GEORGIA, USA

With advances in mobility and remote sensing increasing our ability to investigate astroblemes in the most secluded locations, the remaining frontier of crater studies is how to recognize and analyze ancient structures that have been deformed extensively by tectonics. Sudbury, Beaverhead, and Sante Fe are among a handful of established Precambrian structures shown to have retained signatures of impact metamorphism despite protracted periods of post-impact tectonism. The remains of many additional structures likely exist in tectonic terranes. It is important to find them, not only in pursuit of the impact record, but also to check the accuracy of current structural and petrologic models in those terranes. For instance, how different might a metamorphic history be written if it were determined that the highest grade rocks in a region were pools of ultra-high temperature impact melt rather than granulite?

In seeking to expose this part of the impact record, it is important for us to be open to the possibility that traditional signatures may need to be evaluated through complex P-T-t paths; yet we must equally guard against creating ad hoc criteria. Likewise it is imperative that we not unravel fifty years of impact petrology to buttress endogenic hypotheses against revision, succumbing to ad hocproposals that one fault is the sole exception that created shock metamorphism in the absence of an impact (especially when endogenic and impact models may not be mutually exclusive).

As a case study of the methods, difficulties, and successes (?) in addressing this problem, we present the culmination of a decade of work on Woodbury, a probable Neoproterozoic impact structure in the Pine Mountain Terrane of west-central Georgia. We present new data showing that some “explosion cones” found in megaclasts of peraluminous granite within a 1.1 Ga charnockite (itself containing relict shocked quartz and feldspar) are ancient features and have characteristics consistent with shatter cones. And we show that that the granite clasts do not contain evidence of high-grade metamorphism assumed by structural geologists for the entire region. We propose that Alleghanian docking of a continental fragment containing a buried impact structure can best explain the combined observations of workers with terrestrial and extraterrestrial biases.