A STUDY OF 2 MIDCRUSTAL BRIGHT SPOTS FROM SOUTHEAST GEORGIA (USA)

Two high-amplitude reflections, or 'bright spots', occur on COCORP (Consortium for Continental Reflection Profiling) line Georgia 16 at a record time of 5.9 s, corresponding to nearly 16 km depth. They are significant because bright spots often mark great contrasts in subsurface physical properties. The first is the 'Surrency Bright Spot' (SBS), an exceptionally bright antiformal reflection, and the second is the 'Reedy Creek Reflection' (RCR), which lies 5 km to the south and resembles the south half of a diffraction. The RCR is also imaged on a crossline, Georgia 18, as a relatively symmetric antiformal reflection. These reflections coincide with the Brunswick magnetic anomaly, thought to mark the late Palaeozoic Alleghanian suture. The SBS and the RCR are reflections from bodies that lie within the plane of Georgia 16. Migration and 2-D seismic modelling show the aniformal SBS is from a synformal reflector with a buried focus while the RCR is from a relatively small body. The inferred geometry of the SBS reflector, with its flat portion corresponding to the deepest part of the reflector, argues that it does not represent fluids. These results, combined with recent evidence that the SBS is a reflection from a high-velocity thin layer, suggest that the SBS and the RCR might be reflections from ultramafic slices emplaced within the Alleghanian suture zone during collision or from mafic sheets intruded into the suture zone during subsequent rifting. The brightness of the SBS is attributed to a large impedance contrast, constructive interference, good data quality, and a relatively large reflector size with respect to the Fresnel zone and neighbouring reflectors.