Crustal structure and deep reflector properties: Wide angle shear and compressional wave studies of the midcrustal surrency bright spot beneath southeastern Georgia

Abstract

Three‐component (P, SH, and SV) expanding spread profiles (ESP), common‐midpoint profiles, and sparse three‐dimensional P wave data were collected over an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), in southeastern Georgia. Shear wave reflections from the SBS at 10.9 s (16 km depth), and possibly from the lower crust at 18.3 s (29 km depth), were recorded but required substantial source effort (stacking) and were too weak for reliable reflectivity measurements. Reflections on the ESPs delineate a 1.5‐km‐thick Atlantic Coastal Plain section whose seismic properties (Vp=2.53 km/s, Vs=1.51 km/s, Vp/Vs= 1.67) are consistent with quartz‐rich sandstones and siltstones, sitting atop a 15‐km‐thick upper crust (Vp=6.38 km/s, Vs=3.25 km/s, Vp/Vs=1.96), which in turn overlies a 15 km‐thick lower crust of slower material (Vp=6.02 km/s, Vs≈3.26 km/s, Vp/Vs≈1.84). The velocity inversion may result from underthrusting of upper crustal rocks during suturing of Florida to North America. Amplitude‐versus‐offset analyses, combined with an earlier reflection polarity test and waveform modeling, indicate that the SBS originates from a thin (∼80–120 m), high‐impedance layer, most likely a mafic dike or tectonically emplaced ultramafic body.