NEW EVIDENCE OF SEAFLOOR SPREADING MAGNETIC ANOMALIES IN THE EASTERN GULF OF MEXICO: IMPLICATIONS FOR OPENING OF THE GULF

The opening history of the Gulf of Mexico is poorly constrained because the age and nature of the crust in the deep Gulf are unknown. We have identified lineated magnetic anomalies in the eastern Gulf on profiles collected during the Woods Hole R/V Farnella FRNL85-2 cruise. Forward modeling shows that these anomalies correlate with chrons M21R to M10, indicating creation of new seafloor in the eastern Gulf of Mexico between 149-134 Ma at an average half-spreading rate of 3.2 cm/yr. The oldest anomalies terminate against stretched continental crust of the western Florida shelf on the east and the Yucatan shelf on the west, and are taken to indicate the onset of seafloor spreading in the eastern Gulf. The youngest anomalies define the location of an extinct spreading ridge between Yucatan and Florida. The seismic, gravity, and magnetic characteristics of the crust in the area are similar to rifted margins where subcontinental lithosphere was exhumed during slow amagmatic rifting. Paleogeographic restoration of the lineated magnetic anomaly pattern suggests a 4-phase model for opening of the Gulf. During phase 1 (Early Permian-Late Triassic), Yucatan and associated tectonic blocks that now comprise eastern Mexico were translated eastward from the Pacific realm into positions near the modern western Gulf. During phase 2 (Late Triassic-ca. 160 Ma) Yucatan and the South Florida block were translated southeastward relative to North America, rotating 6.7° counterclockwise about a pole located at 34°N, 74°W. This resulted in ca. 430 km of southeastward extension on the North American coastal plain, 120 km of southward extension on the northern Yucatan shelf, and displacement of the South Florida Block from a pre-rift position on the northwest Florida shelf to its modern position. During phase 3 (ca. 160-149 Ma), Yucatan rotated counterclockwise 46° relative to North America about a pole located at 27.6°N, 84.0°W. Phase 3 may have coincided with seafloor spreading in the central and western Gulf, but predated seafloor spreading in the eastern Gulf. During phase 4 (149-134 Ma), Yucatan moved southwestward relative to North America, rotating counterclockwise 2.2° about a pole located at 17.6°N, 74.2°W and completing opening of the Gulf.