LITHOSPHERIC STRUCTURE OF SOUTHEASTERN NORTH AMERICA FROM GEOPHYSICAL AND SURFACE GEOLOGIC DATA

Two complete Wilson cycles are recorded in eastern North America crust-lithosphere. Final phase of supercontinent Rodinia assembly was collision of eastern Laurentia with pre-Gondwanan continents during the ~1Ga Grenville event. Breakup of Rodinia involved separation of Laurentia from West Gondwana and opening of Iapetus ocean 570-535 Ma. Subduction in Iapetus, 470-455 Ma, accreted the Taconian volcanic arc system to SE Laurentia. An ocean remained off eastern Laurentia that closed obliquely in the middle Paleozoic (Neoacadian orogeny) with zippered collision of Carolina superterrane with Laurentia. ~330 Ma thermal activity marked the beginning of the Alleghanian orogeny and construction of Pangaea. Alleghanian collision was oblique north-to-south, producing dextral strike-slip blocks, followed by late Pennsylvanian-Permian head-on collision producing the Blue Ridge Piedmont megathrust sheet and foreland fold-thrust belt. Aeromagnetic (AM) and gravity (G) maps of Alabama and nearby states reveal blocks and boundaries from both Wilson cycles; AM data record upper crustal structure down to the Curie isotherm (20-25 km and G data reveal upper crustal and deeper structures. The combination of AM and G data provides much more crustal information than AM or G data alone. Broad deep-crustal anomalies of the continental interior, possibly including the Grenville front, occur in NW AL. The southern part of the NY-AL lineament is the eastern boundary of a buried crustal block having NE-trending AM highs and G lows. High-frequency AM anomalies superposed on broad, low-amplitude anomalies result from magnetic rocks exposed in the Piedmont NW and SE of the Brevard fault zone, and outline Appalachian terranes and structures, some requiring new field investigation. All NE-SW Appalachian and Grenvillian trends, and the subsurface Brunswick (Charleston) terrane (AM high) are truncated by E-W-trending broad, mostly low amplitude AM anomalies that define the Alleghanian suture, now covered by Triassic-Jurassic strata, in a failed rift. To the south, broader, higher amplitude AM anomalies probably represent Suwannee terrane Gondwanan crust.