REVISED EXTENT OF THE SUWANNEE TERRANE: IMPLICATIONS FOR THE LAURENTIA-GONDWANA BOUNDARY IN THE SOUTHEASTERN U.S

Recent analysis of subsurface (seismic reflection and well) data beneath the Coastal Plain of the southeastern U.S. serves to substantially expand the regional extent of crust of Gondwanan origin. Interpretation of offshore seismic reflection and well data documents a continuous, 4-6 km-thick, >1,200 km-long, NE-SW-striking subcrop of Suwannee basin strata at least as far north as Cape Lookout, off the North Carolina coast (Boote and Knapp, 2016). Onshore well data document that terranes of Peri-Gondwanan origin are separated by a narrow (5-10 km) zone from the northern known extent of these Suwannee basin strata. In turn, Boote et al (2018) documented that in Florida and Alabama, these Suwannee basin strata lie unconformably above a Neoproterozoic continental margin arc complex (Osceola arc), inferred to have formed above a preserved subduction/continental collision zone (Brunswick suture zone) marked by the locus of the arcuate Brunswick magnetic anomaly. Both the Brunswick suture zone and Osceola arc can be traced along strike for over ~1,000 km, from the offshore continental shelf to southern Alabama, where they appear to be sharply truncated across a narrow (<10 km) NE-SW-striking zone against rocks of known Laurentian origin in the buried Valley and Ridge Province of the Southern Appalachians. Comparison of these subsurface constraints for the northern and western extent of Gondwanan crust with the aeromagnetic map of the southeastern U.S. suggests this northwestern edge of Gondwana coincides closely with the Carolina-Mississippi fault of Higgins and Zietz (1983), which these authors interpreted as the Suwannee suture zone.

Based on these observations, the boundary between crust of Gondwanan origin and that of known Laurentian/Peri-Gondwanan origin appears to be characterized as a narrow (~10 km), linear boundary >1,500 km in length from the offshore Atlantic slope to southeastern Mississippi. Pre-Mesozoic upper-crustal rocks are exclusively of Gondwanan affinity south of this boundary, and are of known Laurentian or Peri-Gondwanan affinity north of this boundary. While these observations do not preclude an overthrust origin, the linear trace and close juxtaposition of basement rocks across the Carolina-Mississippi fault implies a strike-slip origin for the Gondwanan-Laurentian boundary.