Stephen A. Swift

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Gas venting and late Quaternary sedimentation in the Persian (Arabian) Gulf

Abstract High resolution 3.5 kHz echo sounding profiles and piston cores were used to reconstruct the microtopography and late Quaternary depositional history of the Persian Gulf. Perversive throughout the seafloor of the Gulf is an extensive network of pockmarks formed by seepages of thermogenic gas. These gas seeps and bottom water exiting the Gulf via the Strait of Hormuz are the most significant processes controlling present-day sedimentation in the region. Erosion by these seeps has been so intense in the Baiban Shelf in the Strait of Hormuz as to create a “hoodoo” like terrain on the outer shelf. The surfical geology of the Gulf documents a short lived transgression 29,400 to 22,800 years ago during the Wisconsin regression which began 125,000 years ago, the Wisconsin regressive maxima when sea level dropped to −120/−130 m about 21,000/20,000 years ago and the climate was dry and eolian and paralic sedimentation characterized the Gulf, the Holocene transgression 18,000 to 12,000 years ago when the climate was more humid than during the climax of the Wisconsin regression, a dry phase 12,000 to 9000 years ago when the Persian Gulf was a site of eolian and carbonate deposition, and the present sediment cycle during the last 9000 years under a more humid regime. It was during the present cycle that southeast trending marl lobes were deposited off Iran, Arabia acquired its hyper-arid climate about 3000 years ago and the Gulf attained its present configuration about 1000 years ago as a result of the construction of the Tigris Euphrates Delta at its head and tectonism and aggradation along its Arabian and Iranian flanks.

Catastrophic meltwater discharge down the Hudson Valley: A potential trigger for the Intra-Allerød cold period

Glacial freshwater discharge to the Atlantic Ocean during de- glaciation may have inhibited oceanic thermohaline circulation, and is often postulated to have driven climatic fluctuations. Yet attributing meltwater-discharge events to particular climate oscil- lations is problematic, because the location, timing, and amount of meltwater discharge are often poorly constrained. We present ev- idence from the Hudson Valley and the northeastern U.S. conti- nental margin that establishes the timing of the catastrophic drain- ing of Glacial Lake Iroquois, which breached the moraine dam at the Narrows in New York City, eroded glacial lake sediments in the Hudson Valley, and deposited large sediment lobes on the New York and New Jersey continental shelf ca. 13,350 yr B.P. Excess 14 C in Cariaco Basin sediments indicates a slowing in thermohaline circulation and heat transport to the North Atlantic at that time, and both marine and terrestrial paleoclimate proxy records around the North Atlantic show a short-lived (,400 yr) cold event (Intra- Allerod cold period) that began ca. 13,350 yr B.P. The meltwater discharge out the Hudson Valley may have played an important role in triggering the Intra-Allerod cold period by diminishing thermohaline circulation.

Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poissons ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large-scale, well-oriented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poissons ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows. Low Poissons ratio at 850–1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large-aspect ratio cracks are required to produce such low values of Poissons ratio. The cracks were likely created by hydraulic fracturing when hot dikes encountered low-temperature seawater.

Close-up stereo photographs of abyssal bedforms on the Nova Scotian continental rise

Abstract Close-up stereo photographs of the seafloor were taken at five stations on the Nova Scotian rise. Stereo viewing reveals abundant relief with heights in the range 1–50 mm which are not apparent in oblique monoscopic pictures. Objects with dimensions of 0.1 mm can be resolved, and areas up to 1.0 cm 2 can be viewed. Photogrammetric contour mapping at one millimeter intervals of longitudinal ripples gives in-situ dimensions of 12 cm height, 90 cm full width, and 410 cm 2 cross-sectional area. A field of ripples spaced at 5 m with a sediment density of 0.7 g cm −3 contains 5.7 kg dry sediment per m 2 . Seafloor roughness on the scale of centimeters in abyssal, high-energy boundary layers reflects both recent sedimentary and biological processes. The spatial density of eight classes of bed relief varies on scales of tens of kilometers downslope and is consistent with downslope trends in the occurrence of larger bedforms, texture of surface sediments, and bottom boundary layer conditions. Most relief is biogenic, directly or indirectly, and most is streamlined.

Modeling seafloor geoacoustic interaction with a numerical scattering chamber

A numerical scattering chamber (NSC) has been developed to compute backscatter functions for geologically realistic seafloor models. In the NSC, solutions are computed to the elastic (or anelastic) wave equation by the finite‐difference method. This has the following advantages: (a) It includes all rigidity effects in the bottom including body and interface waves. (b) It can be applied to pulse beams at low grazing angles. (c) Both forward scatter and backscatter are included. (d) Multiple interactions between scatterers are included. (e) Arbitrary, range‐dependent topography and volume heterogeneity can be treated simultaneously. (f) Problems are scaled to wavelengths and periods so that the results are applicable to a wide range of frequencies. (g) The method considers scattering from structures with length scales on the order of acoustic wavelengths. The process is discussed for two examples: a single facet on a flat, homogeneous seafloor and a canonically rough, homogeneous seafloor. Representing the ...

Evidence for active normal faulting on 5.9 Ma crust near Hole 504B on the southern flank of the Costa Rica rift

Single-channel and multichannel seismic reflection data show evidence for recent movement of faults on 5.9 Ma crust near Ocean Drilling Program (ODP) Hole 504B on the southern flank of the Costa Rica rift, >200 km from the ridge axis. These faults, which are associated with N85°W-trending, ridge-parallel basement escarpments, can be traced upward into the thick, overlying sedimentary section that blankets volcanic crust in this area. The offset of sedimentary horizons indicates the style and history of faulting. It consistently shows the downdropped side to the north, signifying inward-facing normal faults or grabenlike structures indicative of crustal extension perpendicular to the ridge axis. Although most of the movement on these faults appears to have occurred in young crust near the ridge axis, many of these faults have a long history of activity, with cumulative displacements of several tens of metres occurring over the past several million years. These results are inconsistent with inferences from borehole stress measurements made in Hole 504B that the crust in this area is in horizontal compression or with previous assumptions that crustal extension at mid-ocean ridges is limited to within 10–20 km of the spreading axis. Although the broad zone of crustal extension on the south flank of the Costa Rica rift could reflect anomalous stresses within the Nazca plate, several independent lines of evidence suggest that the active “tectonic zone” of crustal extension and normal faulting at mid-ocean ridges may be significantly wider than previously suspected.

Oceanic basement structure, sediment thickness, and heat flow near Hole 504B

A new seismic reflection survey around Hole 504B, the deepest borehole in ocean crust, reveals active faulting, possible volcanic centers, and a lateral change in the relationship of heat flow and basement structure near the borehole. Migration of single channel and multichannel seismic profiles collected in a 25 by 25 km grid with a 1 km line spacing significantly improved the resolution of basement structure and sediment thickness. West of Hole 504B, heat flow is high above east-west lineated basement ridges, whereas heat flow to the east is normal above ridges and high above two buried basement knolls. The difference is probably due to lateral variations in sediment thickness. Small, buried basement knolls are common and may have been point sources for lava flows. Hole 504B lies in a flat-floored basin that slopes gently upward to the west. A recently active fault 1.1–1.2 km south of Hole 504B is indicated by sediment reflector discontinuities that extend up to the seafloor. The fault strikes east-west and crosses a buried volcanic knoll where Holes 678B and 896A were drilled. Regionally, basement relief north of Hole 504B is 100 to 150 m lower than to the south, which we attribute to an increased spreading rate obtained from dating published local magnetic anomaly patterns with a recent timescale. We find at least five graben structures resembling failed rifts which may have formed in response to asymmetric spreading or to the change in tectonic stress accompanying the spreading rate change. South facing scarps on basement ridges are as common as north facing scarps. Sediment thickness is highly correlated to basement depth due to preferential deposition in topographic lows when the crust was 1–2 Ma old and to later winnowing.

Evidence from Hole 504B for the origin of dipping events in oceanic crustal reflection profiles as out-of-plane scattering from basement topography

Dipping reflectors in oceanic crustal seismic reflection profiles have been attributed to either faults cutting through the crustal section or magmatic layering in the mid- to lower crust. Using closely spaced (

The seafloor borehole array seismic system (SEABASS) and VLF ambient noise

The Seafloor Borehole Array Seismic System (SEABASS) has been developed to measure the pressure and threedimensional particle velocity of the VLF sound field (2–50 Hz) below the seafloor in the deep ocean. The system consists of four three-component borehole seismometers (with an optional hydrophone). a borehole digitizing unit, and a seafloor control and recording package. The system can be deployed using a wireline re-entry capability from a conventional research vessel in Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) boreholes. Data from below the seafloor are acquired either onboard the research vessel via coaxial tether or remotely on the seafloor in a self-contained package. If necessary the data module from the seafloor package can be released independently and recovered on the surface. This paper describes the engineering specifications of SEABASS, the tests that were carried out, and preliminary results from an actual deep sea deployment. VLF ambient noise levels beneath the seafloor acquired on the Low Frequency Acoustic-Seismic Experiment (LFASE) are within 20 dB of levels from previous seafloor borehole seismic experiments and from land borehole measurements. The ambient noise observed on LFASE decreases by up to 12 dB in the upper 100 m of the seafloor in a sedimentary environment.