P-to-S Receiver Function Constraints on Crustal Structure and Rift Magmatism in the Southeastern United States

Abstract:

The superposition of Mesozoic continental extension on convergent structure of the Appalachian mountain belt in the southeastern United States provides an opportunity to study the possible influence of reactivation tectonics on the break-up of Pangea. Major offset rift basins are located inboard of the continental margin beneath the Atlantic Coastal Plain (South Georgia, Riddleville), and Central Atlantic Magmatic Province (CAMP) mafic dike swarms intrude rift strata and accreted terranes of the southern Appalachians. New P-to-S receiver function H-k stacking results from the 85-station SESAME array and EarthScope Transportable array provide additional constraints on Moho topography and average crustal Vp/Vs across the region. Previous crustal thickness estimates show that Moho depth gradually increases across strike of the Appalachians from ~37 km beneath the Carolina terrane to a maximum of ~55 km across the Blue Ridge Mountains. New crustal thickness estimates of 46-49 km in the southwest portion of the Inner Piedmont in Georgia also indicate significant along-strike complexity of Moho topography. Low Vp/Vs ratios (1.69-1.74) across a large region (~15,000 km²) spanning the Inner Piedmont and Carolina terrane indicate the crust has a relatively felsic composition, implying that bulk crustal modification by CAMP magmatism was volumetrically minor. Higher Vp/Vs ratios (1.75-1.80) across the Inner Piedmont, Blue Ridge, and Valley & Ridge correlate with regional crustal thickening (40-55 km), likely reflecting an increase in crustal root density unrelated to rift magmatism. Across the array, intra-crustal P-to-S conversions at 0.8-1.5 seconds (6-13 km) corresponding with the depth of the Appalachian décollement provide supporting evidence for the southeastward continuation of the detachment beneath the Carolina terrane and new constraints on the origin of reflectivity along the fault zone. This low-angle discontinuity between the Appalachian orogenic wedge and Grenville lower crust may have localized deformation and basin development within an upper crustal break-away during Mesozoic extension. Additional constraints on crustal thickness across the Atlantic Coastal Plain using the SsPmp phase will allow us to evaluate the extent of crustal thinning beneath rift basins in southern Georgia.