In summer 2018, the U.S. Geological Survey partnered with the U.S Department of Energy and the Bureau of Ocean Energy Management to conduct the Mid-Atlantic Resources Imaging Experiment (MATRIX) as part of the U.S. Geological Survey Gas Hydrates Project. The field program objectives were to acquire high-resolution 2-dimensional multichannel seismic-reflection and split-beam echosounder data along the U.S Atlantic margin between North Carolina and New Jersey to determine the distribution of methane gas hydrates in below-sea floor sediments and investigate potential connections between gas hydrate dynamics and sea floor methane seepage. MATRIX field work was carried out between August 8 and August 28, 2018 on the research vessel Hugh R. Sharp and resulted in acquisition of more than 2,000 track-line kilometers of multichannel seismic-reflection and colocated split-beam echosounder data, along with wide-angle seismic reflection and refraction data from 63 expendable sonobuoy deployments.

Modern multichannel seismic (MCS) surveys on the U.S. Atlantic margin continental slope and rise have imaged multiple bottom simulating reflections (BSRs) at several locations from the Chesapeake Drift on the north to the Carolina Rise and the inner Blake Ridge on the south. The existence of multiple BSRs highlights the dynamic nature of the deepwater marine gas hydrate system as it responds to and records changes in geologic, oceanographic, and climate processes, most likely since the Last Glacial Maximum (LGM). On the north, the double BSR within the well-known "whale" hydrate prospect over the deepwater part of Hudson Canyon lies ~370 km south of the Massachusetts coast in ~3000 m of water. At this site, the multiple BSRs were previously postulated to represent two distinct bases of gas hydrate stability (BGHS) for a gas mixture containing methane and a higher-order hydrocarbon, even though evidence for a non-methane hydrocarbon is scant on this part of the margin. To the south, five or possibly more generations of BSRs have been newly recognized in recently acquired MCS data on the inner Blake Ridge, evoking comparisons to multiple BSRs analyzed on the Hikurangi Margin. We combine published constraints on sea level rise, glacio-isostatic rebound, and the paleo-position of the Gulf Stream with additional information about inferred bottom water temperatures, major mass transport events, and sedimentation and erosion patterns to develop inputs for numerical models that track the migration of the BGHS in response to complex, but realistic, patterns of geologic and oceanographic change and climate forcing at several key locations. In the whale hydrate prospect, increased sea level should have resulted in BSRs migrating to measurably greater depths. However, the inferred contemporary BSR is shallower than the preserved paleo-BSR, implying that thermal equilibration in response to sedimentation (and increased post-LGM bottom water temperature?) has dominated gas hydrate dynamics. The numerical models also track expected variations in relative BSR strength based on solubility changes that increase or decrease vapor phase gas present below BSRs and constrain the nature and duration of events that are most likely to produce paleo-BSRs preserved in the sedimentary record.

Since the discovery of >570 methane flares on the northern U.S. Atlantic margin between Cape Hatteras and Georges Bank in the last decade, the acquisition of thousands of kilometers of additional water column imaging data has provided greater coverage at water depths between the outer continental shelf and the lower continental slope. The additional high-resolution data reveal >1400 gas flares, but the removal of probable duplicates from the combined database of new flares and those recognized in 2014 yields ~1139 unique sites. Most of these sites occur in clusters of 5 or more seeps, leaving about 275 unique locations (including 47 clusters) for seepage along the margin. As a function of depth, seep distribution is heavily skewed toward the upper continental slope at water depths shallower than 400 m on the southern New England margin ~550 m in the Mid-Atlantic Bight, with additional seeps clustered at ~1100 m and just deeper than ~1400 m in both sectors. Despite little ongoing tectonic deformation or active faulting on this passive margin, a variety of processes driven from below the seafloor (e.g., migration of fluids along faults or through permeable strata, seepage above diapirs or other pre-existing structures) and from above (e.g., erosion, sapping, unroofing) contribute to the development of seeps in different settings along the margin. In addition, the prevalence of seeps on promontories overlooking shelf-breaking canyons may be directly related to the three-dimensional nature of the hydrate stability zone in these locations. As a function of depth, the parts of the slope at the contemporary landward limit of gas hydrate stability are devoid of seeps, and the upper slope zones with the most concentrated seepage were not within the gas hydrate stability zone even during the Last Glacial Maximum. Thus, if the large number of upper slope seeps is at least partially sourced in gas hydrate degradation, the gas emitted at these seeps must have migrated there from greater depths on the continental slope.

In the modern era, marine heat flow surveys are relatively rare, yet information about the thermal state of ocean basins continues to have relevance for models of rifted margin evolution and for assessing hydrocarbon maturation and the suitability of sites for long-term seabed CO2 storage. The thermal state of the U.S. Atlantic margin (USAM) has long been more poorly characterized than that of some other margins. For example, there are few marine heat flow measurements, and temperatures in the USAM-s shallow sedimentary section have only been directly measured in a few sparsely distributed ODP boreholes. Kilometers below the seafloor, fluid inclusion studies by the Bureau of Ocean Energy Management on samples retrieved decades ago in COST wells have constrained thermal maturation, but only at specific stratigraphic levels in a few locations. For this study, we create a thermal map of the USAM from the southern New England margin to the Blake Ridge in the area overlying the transition from rifted continental crust to some of the oldest oceanic crust in the North Atlantic basin and including some offshore Triassic-Jurassic rift-related features. We use a new location-dependent velocity model formulated by combining data from numerous long-offset R/V Langseth seismic surveys to determine the depths of bottom simulating reflections (BSRs) for over one million picks from legacy USGS and commercial data, academic seismic surveys, and the 2018 USGS MATRIX lines. Adopting observed bottom water temperatures as the seafloor boundary condition for geothermal gradients, we determine local and regional sediment thermal gradients and heat flow, plus associated uncertainties, by incorporating various gas hydrate stability constraints. When combined with recent USGS margin-wide analyses of sediment thicknesses over basement, the thermal analyses yield reduced (basement) heat flow near the rift-to-drift crustal transition and insights into the impact of post-rift (Quaternary and Tertiary) processes on regional thermal structure.