RIFT-TO-DRIFT IN THE GULF OF MEXICO: EVIDENCE FOR SYN-RIFT VOLCANSIM AND SEAFLOOR-SPREADING FROM MARINE SEISMIC REFRACTION DATA

Continental rifting and seafloor-spreading between North America and the Yucatán Block during the Jurassic to early Cretaceous formed the Gulf of Mexico (GoM). The lack of deeply-penetrating geophysical data in this small ocean basin limited early reconstructions of the timing, location, and nature of the rift-to-drift transition. To better understand the deep structure of this economically important basin, we acquired four marine seismic refraction profiles in the northern GoM from the shelf to deep water as part of the 2010 Gulf of Mexico Basin Opening project (GUMBO). We use travel times from long-offset reflections and refractions to image compressional seismic velocities in the sediments, crystalline crust, and upper mantle using an iterative tomographic inversion. GUMBO Line 3 extends from offshore Alabama through the De Soto Canyon towards the central GoM. We interpret velocities >5.0 km/s in the sediment layer landward of the Florida Escarpment as a Lower Cretaceous carbonate platform. Crystalline crust with velocities between 5.5-7.5 km/s thins significantly from 23 km to 7 km across a narrow necking zone. A deep, localized region of anomalously high seismic velocities (>7.5 km/s) at the base of crystalline crust exceeds those of continental lower crust in the eastern US. We interpret this section of GUMBO 3 to represent mafic under-plating and/or infiltration of asthenospheric melts, common at volcanic rifted margins. The seaward end of GUMBO 3 has seismic velocities consistent with mafic ocean crust produced by normal seafloor-spreading (6.0-7.5 km/s). GUMBO Line 2 extends from offshore Louisiana southward across the Sigsbee Escarpment. We find a massive sediment package with substantial lateral heterogeneities, which we attribute to salt tectonics. GUMBO 2 crust shows variations in thickness of 3-10 km with seismic velocities between 6.0-8.0 km/s. We interpret the majority of GUMBO 2 as oceanic crust formed by slow to ultraslow seafloor-spreading, with a volcanic rift margin closer to the present-day coastline than most prior reconstructions. We invoke a ridge jump to explain ocean crust asymmetry between North America and the Yucatán peninsula. We further suggest that the effects of heat and asthenospheric melt were more impactful, and the rift-to-drift transition more immediate, in the eastern GoM.