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Author(s): Daniel Lizarralde (corresponding author) [1]; Gary J. Axen [2]; Hillary E. Brown [3]; John M. Fletcher [4]; Antonio González-Fernández [4]; Alistair J. Harding [5]; W. Steven Holbrook [3]; Graham [...]

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3-4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to Keywords: continental margin, seafloor spreading, extension tectonics, continental breakup.

Many parts of the world's oceans contain layers with contrasting temperature and salinity that produce fine-scale thermohaline structure (1, 2), such as thermohaline staircases (3), which occur in tropical and [...], We present acoustic images of oceanic thermohaline structure created from marine seismic reflection profiles across the major oceanographic front between the Labrador Current and the North Atlantic Current. The images show that distinct water masses can be mapped, and their internal structure imaged, using low-frequency acoustic reflections from sound speed contrasts at interfaces across which temperature changes. The warm/cold front is characterized by east-dipping reflections generated by thermohaline intrusions in the uppermost 1000 meters of the ocean. Our results imply that marine seismic reflection techniques can provide excellent spatial resolution of important oceanic phenomena, including thermohaline intrusions, internal waves, and eddies.

New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermodynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, un-faulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration. Keywords: methane hydrate, methane flux, gas dynamics, seismic reflection, waveform inversion.

We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.

Marine sediments can contain large quantities of natural gas (usually methane) in the form of gas hydrate, a solid compound binding water and gas molecules, which is stable at the [...], Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.

The relative motion between the North American and Pacific plates occurs over a wide zone in the San Francisco Bay Area (1), inconsistent with a single vertical plate boundary. Indeed, [...], Results from the San Francisco Bay area seismic imaging experiment (BASIX) reveal the presence of a prominent lower crustal reflector at a depth of 15 kilometers beneath San Francisco and San Pablo bays. Velocity analyses indicate that this reflector marks the base of Franciscan assemblage rocks and the top of a mafic lower crust. Because this compositional contrast would imply a strong rheological contrast, this interface may correspond to a lower crustal detachment surface. If so, it may represent a subhofizontal segment of the North America and Pacific plate boundary proposed by earlier thermomechanical and geological models.

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedge reaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWTT (two-way traveltime) dip southeastward at low angles ( -15[degrees]). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the U.S. Atlantic margin may be the top of the Jurassic magmatic-underplating layer, the northwest-dipping reflection truncates the southeast-dipping 7.0 s TWTT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the cast and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeast-dipping 7.0 s reflection probably represent the Taconic suture.

We compare seismic refraction and reflection results along the PASSCAL/COCORP 40 [degrees] N transect in the northern Basin and Range of Nevada in order to determine the origin of the prominent reflections from the deep crystalline crust. Reflection data along the transect show a thick zone of discontinuous, subhorizontal reflections, beginning at 4-6 s two-way traveltime (10-20 km depth) and ending at 9-11 s (27-35 km). Two independently derived velocity models, based on refraction data, are largerly similar and agree on many important aspects of the reflectivity-velocity relation. Both models show that the top of the reflective zone lies 3-8 km above a prominent mid-crustal velocity discontinuity, which is interpreted to separate bulk silicic from bulk dioritic-gabbroic crust; in most places, the silicic mid-crust is more strongly reflective than the mafic lower crust. This pattern is expected in areas where ductile shearing is the mechanism responsible for the reflectively. One of the velocity models, however, suggests that, in places, the strongest reflectively spans both the middle (6.1-6.3 km/s) and lower (6.6 km/s) crust; this pattern suggest that the combined influence of ductile strain fabrics and mafic intrusions gives rise to crustal reflections. Both models show that the lowermost crust and crust/mantle transition are highly reflective, also suggesting the presence of mafic and/or ultramafic intrusions. Thus the observed reflection patterns suggest that ductile shearing and the intrusion of mantle-derived magma--both of which are likely to have accompanied the extreme Cenozoic extension--are important factors in generating deep crustal reflections.

Analysis of the earth's crust of the Marquesas Islands confirms the presence of crustal underplating above an oceanic hotspot that is characterized by plumes of hot material arising from the Earth's mantle. Underplating involves the addition of magmatic material to the base of the crust. The hotspot of the Marquesas Island that lies on the Pacific plate has similar magnetic thickness as that of its counterpart on the Hawaiian Islands, lying on the same plate. The high-velocity material of the hotspot predominantly consists of pre-existing mantle.