Drew R. Eddy

Deep crustal structure in the eastern Gulf of Mexico

We use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along Gulf of Mexico Basin Opening Line 4, a profile extending from the northwestern Florida peninsula across the Florida Escarpment to the central Gulf of Mexico. Moderately thinned continental crust with a Moho depth of 32–33 km, average sediment thickness of 6 km, and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore Florida. Thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6–7 km; mean seismic velocities of the crust in this region increase from 6.55 km/s to 6.95 km/s from northeast to southwest and are evidence for increased magmatic input as rifting developed. Oceanic crust with an average thickness of 5.6–5.7 km is observed over a distance of 175 km on the southwest end of the profile, with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile. Anomalously high upper oceanic crust velocities of 6.0–6.7 km/s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement. Integrating well, seismic reflection and our seismic refraction data allow us to estimate a full-spreading rate of 2.2 cm/yr for seafloor spreading along the profile; this indicates that oceanic crust was emplaced at a slow-spreading center.

Deep crustal structure of the northeastern Gulf of Mexico: Implications for rift evolution and seafloor spreading

We image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean-bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends ~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0–4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23–10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn-rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full-spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor-spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

Continental rifting and sediment infill in the northwestern Gulf of Mexico

The opening of the Gulf of Mexico was an important Mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting. A new seismic velocity profile based on seismic refraction data in the northwestern Gulf of Mexico offshore Texas, where the basin started opening in the Early Jurassic, shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope. The structure of the thinned crust is consistent with large-scale extensional faulting and moderate amounts of synrift magmatism before continental breakup. These new seismic constraints do not indicate the presence of a volcanic margin along the Texas coast, as has sometimes been proposed based on magnetic data. The Laurentian continental lithosphere of central Texas may have been too thick at the onset of rifting (>100 km) to let magmatic diking control the extension. In contrast, the continental lithosphere of the northeastern Gulf of Mexico may have been thinner, such that magma-assisted rifting formed a volcanic margin there later in the Jurassic.

Structure and origin of the rifted margin of the northern Gulf of Mexico

The wide continental margin of southern Louisiana borders Paleozoic terranes that accreted to Laurentia before Jurassic rifting formed the Gulf of Mexico. It is unclear whether continental rifting here involved widespread or localized crustal extension, or how seafloor spreading in the Gulf of Mexico started. To improve our understanding of this rifting episode, we gathered marine seismic-refraction data along a 396-km-long transect from the continental shelf 50 km off the western Louisiana coast to the central ocean basin as part of the Gulf of Mexico Basin Opening (GUMBO) program. Using travel-time tomography, we imaged the compressional seismic-velocity structure from the shallow sediments to the uppermost mantle. In our geophysical model, the crust tapers in thickness from ~11 km near the Louisiana coast to ~8 km in the deep water of the central Gulf of Mexico. The compressional seismic velocity increases from 5.7 to 5.9 km/s in the shallow basement to 6.8–7.2 km/s above the Moho. The thickness and average wave speed of crust beneath the modern Louisiana coast and continental shelf suggest the presence of uniformly stretched continental crust that was intruded by mantle-derived melts during extension before continental breakup. South of the Sigsbee Escarpment, the crust is thinner with a higher seismic velocity, which is more consistent with thick oceanic crust. A comparison of our seismic-velocity model with coincident seismic-reflection data indicates that the voluminous Louann Salt was likely deposited on rifted continental crust shortly before the onset of seafloor spreading in the Gulf of Mexico.