Wide-angle seismic reflection studies of the Elberton granite and Carolina Terrane, southern Appalachians, northeast Georgia
Khalifa, Mohamed Omran
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The first part of the study uses wide-angle seismic reflections to investigate the tectonic relationship between the Elberton granite and Appalachian thrusting and to image the base of the granite. Field experiments were designed to take advantage of the increase in reflection coefficients near the critical angle to image relatively subtle contrasts in the acoustic impedance between the Elberton and underlying gneisses. Shot gathers show strong first arrivals and high amplitude events arriving shortly after the direct P wave that are interpreted as post-critical reflections from a “layered” complex at depths of 3 to 4 (possibly 2-4) km. Possible interpretations of the layering include migmatitic rocks, mafic cumulates, or a mylonitic shear zone at the base of the granite. The migrated sections show southeast-dipping features that span a depth of 5-10 km. The transition from horizontal to dipping reflections beneath the Elberton granite supports the hypothesis that the granite is a tabular body. A high-amplitude, multicyclic event at ~ 11 km is interpreted as the master decollement. The second part of the study involves migration of wide-angle data collected along the axis of the East Coast gravity high within the Carolina Terrane. The study compares the reflection response of the crust to COCORP-style vertical profiling and wide-angle profiling. The two data sets differ in dominant frequency and therefore are sensitive to different scales of layering. Depths of some migrated events match the depths of prominent reflection packages previously interpreted as the master decollement and Modoc zone. The migrated wide-angle sections also show prominent sub-horizontal events at 30 km that do not appear in the COCORP sections. If basement rocks here do indeed represent rifted continental crust as proposed by earlier workers, then the reflectors could be mafic sills. In this study, very highamplitude events interpreted as near-critical reflections from the Moho appear at projected vertical two-way times that are slightly greater (11.6-11.9 s) than two-way times for Moho reflections in the COCORP sections (11 s). Overall, the elevated reflection coefficients at wide angles have proved quite useful for imaging subtle constrasts in acoustic impedance otherwise missed by near-vertical recording.