INVESTIGATING RIFT BASINS AND UNDERLYING CRUST BENEATH THE SOUTHEASTERN ATLANTIC COASTAL PLAIN USING TELESEISMIC PHASES RECORDED BY THE SESAME BROADBAND ARRAY

Constraining P-wave velocities and ratios between P-wave and S-wave velocities for the thick column of slow unconsolidated Cenozoic sediments, poorly consolidated Cretaceous sedimentary rocks, and more consolidated Triassic rift basin rocks of the southeastern Atlantic coastal plain is critical for imaging deeper underlying crystalline crust and understanding how extension and mafic magmatism during the Mesozoic rifting of the Atlantic margin influenced the lower crust.

Preliminary processing for various individual earthquake events recorded on the N-S and NE trending transects of the SESAME (Southeastern Suture of the Appalachian Margin Experiment) broadband seismometer array reveals very strong reflections at depths corresponding to the Triassic rift basins, lower crust, and Moho.

Clear intra-basement reflections dipping to the northwest on the D line from a 578.4 km-deep magnitude 6.7 Russian event on October 1, 2013 correspond to a notable mid-crustal feature at 16 km depth. Gently south dipping reflections interpreted as the base of the coastal plain sediments from 1-2 km depth up to the surface are observed on both the E and W lines. A deeper concentration of reflections across the E and W line are interpreted as possible lower crustal layering at 9 s TWT with a ray parameter of 0.044 s/km.

A clear grouping of reflectors related to sedimentary features are observed on the E and W line from a 122.9 km-deep magnitude 5.8 South American event from October 8, 2013. Shallow dipping reflectors interpreted as the base of the unconsolidated sedimentary cover are seen from the surface down to 1-2 km depth again. Reflections with a basin-like geometry directly beneath the base of the sedimentary cover on the E line at 4 s TWT with a ray parameter of 0.065 s/km are interpreted as the base of a Triassic rift basin. Deeper reflections interpreted as the Moho are seen around 30-34 s TWT depth on the E and W lines.

Better constraints on the depth and extent of the basement, rift basins, and mafic intrusions beneath the coastal plain will help resolve whether reactivation of the Suwanee-Wiggins suture zone focused crustal thinning and mafic magmatism along the suture’s trend or if rift-related crustal thinning and magmatic underplating is more localized and laterally offset from the suture’s trend.