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Aeromagnetic Map of the Gulf of Mexico Constrains Late Jurassic Anticlockwise Rotational Seafloor Spreading

Abstract

An aeromagnetic reduced-to-pole total magnetic intensity map of the Gulf of Mexico (GoM) constrains positions of primary fracture zones and probable paleo-spreading ridge fabric in the deep GoM that we interpret as having formed by CCW rotational seafloor spreading. The map also shows two prominent rectilinear magnetic anomalies just basinward of salt and at the limits of the spreading fabric along the NW Florida and Yucatan margins, segmented by apparent minor faults suggestive of seafloor spreading geometry. The two outer anomalies have geometric similarity that we believe indicates a common genesis, denoting now-separated isochrons in GoM opening history. Reconstructing the anomaly pair is straightforward and requires a 32 degree CW rotation of Yucatán against North America (going back in time), with subsequent separation direction in the eastern GoM being NE-SW. The reconstruction nearly juxtaposes areas of Callovian-E Oxfordian autochthonous salt in the N and S GoM, so we suggest a Late Oxfordian age for the reconstruction (Smackover time). The reconstruction also post-dates crustal rifting in the northern and southern passive margins, as the base of salt is regionally planar, is rarely significantly faulted, and overlies filled rift grabens. Each of the anomalies occurs at the basement “step-up” from the outer marginal trough to the ocean crust. The trough correlates with the gravity low rimming the eastern and southern deep GoM, and may be floored by exhumed mantle outboard of rifted continental crust. We suggest the anomaly pair may be caused by one or combinations of an intrusive edge effect between oceanic crust and serpentinized exhumed mantle, metsomatized mantle (magnetite), or basement geometry (step-up). The Late Oxfordian reconstruction probably must nearly depict the onset of seafloor spreading, the kinematics of which probably began slightly earlier as mantle was exhumed from beneath the already-rifted continental crust. The finite rotation pole for the reconstruction does not satisfy younger, more central ridge segments and fracture zone trends, suggesting that incremental stage poles operated or migrated during GoM opening. The magnetic patterns suggest SSE migration of the Yucatan/NA rotation pole from the SE GoM to the Isle of Youth Cuba. A Tithonian reconstruction for the Gulf of Mexico is also proposed, and we judge from regional Geology that the GoM was completely open by earliest Cretaceous time.