John D. Mudie

Fast sea-floor spreading on the Chile Ridge

Abstract The orientation and rate of spreading of the Chile Ridge is not well known owing to the paucity and poor orientation of the geophysical observations defining the ridge system. A recent profile crossing the Chile Ridge at 40°S, 92°W suggests that the half-spreading rate may be as high as 41 mm-yr −1 from 0 to 5 myBP and 56 mm-yr −1 from 5 to 9 myBP as opposed to previous estimates of 20 to 30 mm-yr −1 . In contrast to the rugged bathymetric relief which has been previously reported for the Chile Ridge, the new profile shows a 500 km-long section of “smooth” crust on the eastern flank. We interpret most of the rugged relief which has been observed as being due to numerous fracture zones offsetting the ridge and trending 080° ± 10°. Based on a skewness study of the magnetic anomalies, the ridge crest segments have been determined to be oriented along 350° ± 7°. This makes the ridge axis normal to the fracture zones and parallel to the final strike of the Galapagos Rise.

Magnetic Anomalies in the Northern and Central Gulf of California

Magnetic anomalies found in the northern and central Gulf of California do not resemble typical sea-floor spreading anomalies, but are consistent with the existence of spreading centers in this region. The magnetic anomalies appear to delineate the extension of the San Jacinto fault into the Gulf of California and some of the transform faults that connect the inferred spreading centers within the gulf.

A near-bottom survey of lineated abyssal hills in the equatorial pacific

A 250 km2 area of abyssal hills in the vicinity of 14°N, 126°W (between the Clarion and Clipperton Fracture Zones in the Equatorial Pacific) was surveyed in detail using an instrument package towed close to the deep sea floor, the MPL Deep Tow device. Both topography and near bottom magnetic field are lineated perpendicular to the major fracture zones. Except for a few localized depressions, the sediment surface is generally smooth and of low relief with maximum elevation differences of 200 m and slopes of six degrees. Several small graben-like troughs and depressions were observed, most of them near the crest of one abyssal hill. The largest trough is two kilometers long, 250 m wide and 50 m deep with steep sides (>30°). These troughs are tentatively interpreted as the result of tensional separation at the tops of the hills caused by down-slope creep and consolidation of the pelagic sediments.

Detailed geophysical studies on the northern Hawaiian Arch using a deeply towed instrument package

Abstract A survey of a 20 km by 20 km area near the crest of the Hawaiian Arch (24°25′N 157°40′W) has been conducted with near-bottom instruments and a sea-floor acoustic navigation system having positional accuracy on the order of tens of meters. The southern portion of the area is dominated by part of a 100 km long eastwest trending ridge similar to other such ridges that characterize the bathymetry of the northern arch. Narrow-beam, near-bottom echo soundings show the ridge and other bathymetric highs to be flanked by long narrow grabens on the order of 500 m wide and 50–60 m deep. Other such measurements characterize much of the bathymetry of the area as rolling and lobate. A shape similar to a subareal lava dome was mapped on the northern flank of the ridge. The ridge was formed by a combination of volcanic and tectonic processes that uplifted and tilted several hundred meters of the pre-existing sediments. Interpretation of conventional reflection profiles and near-bottom, side-looking sonar data suggests volcanic material has been extruded on the sediment covered ocean floor and intruded into the sedimentary column. Near-bottom magnetic data over an intrusive feature near the center of the area reveals large anomalies up to 1,000 γ in amplitude. Bottom photographs and piston cores show volcanic glass in the form of small (1–5 cm) fragments, substantiating other evidence of recent volcanic activity.

Direct Mapping of the Sea Floor Using Side-Scanning Sonar and Transponder Navigation

The side-scanning sonar, precision bathymetry, and acoustic transponder navigation capabilities of the Marine Physical Laboratory deep-tow device were used to produce a bathymetric chart, a pictographic map of side-scanning sonar data, and a photographic mosaic of the side-scanning sonar records corrected for lateral distortion for a 5 × 6-km section of the La Jolla deep-sea fan. The precise navigation provided by the acoustic transponder system made possible a detailed description of the meanders of the La Jolla Fan-Valley and recognition of subtle depressions and lineaments on the open fan.

Simulation studies of the response of deeply towed vehicle to various towing ship maneuvers

Abstract The behavior of a long cable towed at slow speeds through the ocean depends in a complex fashion on the path followed by the towing ship relative to the water. A cable simulation program was used to characterize the response of the cable by using idealized towing ship maneuvers as input to the program. The response of the cable was noted and it was found that the behavior of the cable was strongly dependent on the fundamental period of the towing vessel maneuvers. Sinusoidal deviations of the towing ship from a straight towing track resulted in delayed and reduced excursions of the towed vehicle from the tract; the estimated response ratio varied from 0·002 to 0·800, depending both on the period of the deviations (periods ranged from 5·5 to 4·0 hr) and on the towing depth (2 or 6 km). The ships speed was 3 km/hr. The time lag between ship motion and vehicle response was approximately 0·5 hr for the shallow case and 1·3 hr for the deep case. Simulations runs of a low dragk (faired) cable showed that the behavior of the vehicle when towed at a depth of 6 km was similar to that obtained with a conventional cable at 2 km depth. The response of the towed vehicle to a right-angle turn of the towing ship was investigated and a generalized model of the response developed. The effects of a controllable side force on the towed vehicle were also simulated and it was noted that a deviation (2-hr period) of the towed vehicle from a straight-line track could be reduced from 40 to 2 m by impressing a side force on the vehicle with an average magnitude of 150 newtons (30 lb).

Computer aided piloting of a deeply towed vehicle

Abstract An instrumented vehicle is towed at the end of a 5 km long cable, gathering data about the deep sea floor and near-bottom water column. Although bottom-moored acoustic transponders are used to determine the vehicle and ship positions precisely and in real time, predicting the ship manoeuvers required to bring the vehicle over an area of interest on the sea floor is far from trivial for the ship driver. Computer software has been developed which recommends courses for the ship to steer so that the vehicle will pass near a desired target. In trials at sea, the computer steered the vehicle 80, 40 and 85 m from pre-selected targets. Analysis of the causes of the misses suggests future developments which may reduce the miss distance, provide information on current structure of the water column, and reduce the level of skill and attention required of the vehicle pilot.