Deep-tow Profiles Research Articles

Ground-Truthing 11- to 12-kHz side-scan sonar imagery in the Norwegian-Greenland Sea: Part II: Probable diapirs on the Bear Island fan slide valley margins and the Vøring Plateau

SeaMARC II (11- to 12-kHz) side-scan sonar revealed hundreds of small strong-backscatter spots, tens to 500 m in diameter, along the lips of the Bear Island fan slide valley. New bathymetry, deep-tow side-scan, deep-tow profiles, heatflow, and gravity cores were collected for ground-truth. These mounds are probably mud diapirs (or mud-built mounds) typically 10–75 m high, formed by glacial sediment mobilized by Late Pleistocene slide events. The mounds are arranged along NNE trending lines, suggesting control by intrasedimentary faults ca. 0.5–1 km apart. Diapirs examined on the Voring Plateau exhibit WNW structural control. No heatflow anomaly was found in four stations on or next to diapirs in either area.

Sediment accumulation rates from Deep Tow profiler records and DSDP Leg 70 cores over the Galapagos spreading centre

Abstract Variations in pelagic input rates over abyssal hill areas cannot be inferred easily from sedimentation rate variations between cores because of redistribution by downslope gravity processes and bottom currents. Some spatial averaging or adjustment for sediment transport is required. Relatively accurate mean sedimentation rates may be obtained, however, by regressing sediment thickness with distance from a mid-ocean ridge spreading centre, where the regression averages out the variations due to local redistribution. This is shown using sediment profiler records from the Galapagos spreading centre (SC) at 1° N in the eastern Pacific. Physical property data from Deep Sea Drilling Project (DSDP) Leg 70 are used to correct sediment thicknesses for compaction and to convert sedimentation rates to mass accumulation rates (MARs), which are c . 2 g cm −2 ka −1 in this region. These high MARs are due to enhanced equatorial productivity of pelagic organisms, which is also reflected in a c . 6% higher MAR for the ridge flank closest to the equator, corresponding to a rate of change of MAR with latitude of c . 16% per degree. The equatorial high productivity zone in the Panama Basin lies further south than in the central Pacific; peak sedimentation along the Ocean Drilling Program (ODP) Leg 138 transect (110° W) occurs near 1° N so has nearly zero gradient at this latitude, but has comparable gradient to the Galapagos SC north of 2° N. The zone of peak enhancement at 86° W in the Panama Basin may therefore be 1° or more further south than at 110° W. Some further sedimentation characteristics of the Galapagos spreading centre are also described, such as a scaling of thickness variability and the possibility of dating sea floor using sediment thickness.

Fault scarp statistics at the Galapagos spreading centre from Deep Tow data

Much of the relief of the abyssal hills covering the ocean basins is believed to originate from faulting of oceanic crust at mid-ocean ridges. The timescale over which faults grow is controversial, however, with some authors arguing that faults continue to grow in places for 0.5 m.y. or more based on increasing relief of fault scarps with distance from ridge axes. We examine Deep Tow profiler records of the Galapagos Spreading Centre, in which basement reflections allow scarp relief to be measured beneath the sediment cover, and find that relief does not increase but decreases systematically to 40 km off-axis (1.5 Ma seafloor). Since reversal of fault offsets is unlikely in this tectonic setting, we interpret this result as indicating that variations in fault statistics could reflect temporal variations in the tectonic or volcanic state of the ridge crest, not necessarily progressive fault growth with age as previously assumed. Resolving the issue of fault longevity will therefore require independent data on the timing of fault growth and distribution of present growth activity. We suggest some possible alternative indicators of fault longevity and discuss more generally the implications of volcanic flows to studies of faulting at ridges.

Creep in pelagic sediments and potential for morphologic dating of marine fault scarps

Cores from the flanks of the Galapagos spreading centre show that the top 25 cm of pelagic sediments is highly bioturbated. Kenyan and Turcotte [1985] have proposed that bioturbation may cause down‐slope creep of the sediment surface and bioturbation may therefore smooth sediment topography in a similar way to hillslope creep on land. Assuming this produces movement consistent with the diffusion transport model, sediment topography across scarp crests, digitised from Deep Tow profiler records, is modelled with a simple analytic solution to the diffusion equation and with a mean diffusion constant k of ∼0.007 m²/yr. This small value suggests creep rates are modest, for example, 2 cm/yr for a 30° slope, and is an upper bound since sediment avalanching and bottom currents probably also remove sediment from scarp crests. Although requiring further work, these results suggest that morphologic dating methods may be applicable to marine fault scarps and could be useful in relatively sediment‐free areas, such as mid‐ocean ridges, where it is commonly not possible to use datable offset seismic horizons.

Diffusion transport model for pelagic sediments on the Mid‐Atlantic Ridge

The diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations, which are commonly found between Deep Sea Drilling Project and Ocean Drilling Program sites. If appropriate, the model and stratigraphy could be used, for example, to constrain the amount and timing of fault block rotation. Deep Tow profiler records from the French‐American Mid‐Ocean Undersea Study area of the Mid‐Atlantic Ridge are examined to determine whether they are consistent with a diffusion model. Sedimentary contacts with basement show that a variety of processes may be operating, some of which are inconsistent with the diffusion model. For example, there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities. More than half of the contacts, however, show onlapping, which is qualitatively consistent with the diffusion model. Sediments generally fill low areas of the topography and have smooth surfaces, as expected from a diffusion model. Where the fluxes of sediment entering a basin are constant, the diffusion model predicts that the sediment surface should evolve to a parabola (the steady state solution). Some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m. The slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins (steep surfaces are produced by low diffusivity or high fluxes). This ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity, Kapp, for eacri basin. Flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap, so that the lateral flux equals the width of a basin's pelagic catchment area times the area's mean sedimentation rate S. Using this method, median Kapp is 0.04–0.11 m2 yr−1 (assuming S = 10–30 m m.y.−1). Variations in Kapp and the assumptions behind the diffusion model are discussed, and alternative causes of sediment surface tilts are considered, in particular differential compaction, which is modelled using a simple porosity‐depth function.

Characterising the extent of volcanism at the Galapagos Spreading Centre using Deep Tow sediment profiler records

Volcanism at the Galapagos Spreading Centre consists of sheet flows and other distributed extrusives. The surficial volcanics in the axial region should therefore be generally young over a finite area and those on the ridge flanks should be younger, on average, than would be predicted from simple seafloor spreading. A method for determining an upper bound on the mean age anomaly of the flanks, which is interpreted as equivalent to the mean extent of volcanism, is demonstrated using sediment thicknesses digitised from Deep Tow profiler records of the Galapagos Spreading Centre. As found previously, the sediments thicken linearly with distance from the ridge, reflecting simple seafloor spreading up to ∼ 1 Ma. Deviations from the linear trend are due to sediment redistribution (short-wavelength fluctuations) and the age distribution of extrusives, in particular due to large volcanic flows revealed by regions of uniform thickness. The full width of the extrusion zone is obtained by regressing sediment thickness on across-axis distance for both flanks and observing the separation of the two regression lines where they cross zero thickness. Since the method is quantitative and provides an average value for the length of the profile, it is more representative than estimating the width from morphological observations of the spreading axis. Due to scattering from the basement, which obscures sediment in depressions, sediment thickness is generally under-estimated and the measured axial width is an upper bound (10–12 km for the Galapagos Spreading Centre). The mean sediment thickness bias is calculated from high-resolution topography data of lava flows and is ∼ 1.5 m, suggesting that the true mean width of the extrusion zone is 8–10 km. Using this corrected value, the surface of the crust is younger than predicted by seafloor spreading by ∼ 120–150 kyr, on average. This large value implies that the flanks of the Galapagos Spreading Centre are covered with a series of large overlapping volcanic flows.

Hydrothermal plumes, hot springs, and conductive heat flow in the Southern Trough of Guaymas Basin

A 9 km length of one of the axial rift valleys of a Gulf of California spreading center was mapped by Deep Tow and examined on ten “Alvin” dives. The Deep Tow CTD towed 10–100 m above the seafloor sensed 20 thermal plumes with temperature anomalies of up to +0.39°C. Most plumes overlie deposits of hydrothermal minerals (mainly anhydrite and sulfides) that form columns and mounds rising up to 25 m above the muddy rift floor. About 130 such sites were mapped by side-scan sonar, and about 20 of them were photographed and sampled with “Alvin”. Two of the sites that had been raised tectonically above the trough floor have only inactive residual mounds, but all the others examined by submersible had active hydrothermal vents. Fluids discharge diffusely through the porous deposits, as relatively low-temperature (< 100°C) springs in the surrounding mud, and at high velocity and temperature (270–314°C) through chimneys built on the mounds. Chimneys have orifices 2–20 cm in diameter and are commonly capped with hollow horizontal sheets of anhydrite and pyrrhotite. Conductive heat flow measured by multipenetration probe and from “Alvin” was exceptionally high (> 2 W/m 2) adjacent to the thermal springs. The patterns of hydrothermal deposits and measured heat flow, interpreted in light of structural information from the Deep Tow profiler, suggest that shallow sills, some extending laterally from more massive hill-forming intrusions, act as cap rocks for intense, shallow hydrothermal circulations above a magma chamber. Hot discharges from this system occur mainly through tectonic fractures in the sills and around their peripheries; recharge probably occurs on a regional scale, from cooler parts of the rift valley. Some warm discharges, inactive barite deposits, and nonlinear thermal gradients probably mark continuing and past cooling of underlying shallow intrusions, by both recharging and temporary closed-system circulations.

Holocene sedimentation history of the major fan valleys of Monterey fan

There are three major fan valleys on upper Monterey fan. Deep-tow geophysical profiles and 40 sediment cores provide the basis for evaluation of the sedimentation histories of these valleys. Monterey fan valley leads from Monterey canyon to a major suprafan and is bounded by levees that crest more than 400 m above the valley floor. The valley passes through a large z-bend or meander. Monterey East fan valley joins Monterey fan valley at the meander at about 150 m above the valley floor, and marks an earlier position of the lower Monterey fan valley. Ascension valley, a hanging contributary to the Monterey fan valley, appears to have once been the shoreward head of the lower part of the present Monterey fan valley. The relief of Monterey fan valley appears from deep-tow profiles to be erosional. The valley is floored with sand. Holocene turbidity currents do not overtop the levees 400 m above the valley floor, but do at times overflow and transport sand into Monterey East valley, producing a sandy floor. An 1100 m by 300 m dune field was observed on side scan sonar in Monterey East valley. Ascension fan valley was floored with sand during glacial intervals of lowered sea level, then was cut off from its sand source as sea level rose. A narrow (500 m), erosional, meandering channel was incised into the flat valley floor; the relief features otherwise appear depositional, with a hummocky topography perhaps produced in the manner of a braided riverbed. The sand is mantled by about 6 m of probable Holocene mud. Hummocky relief on the back side of the northwestern levees of both Ascension and Monterey valleys is characteristic of many turbidite valleys in the northeast Pacific. The hummocky topography is produced by dune-like features that migrate toward levee crests during growth.