G. M. Purdy

Segmentation and morphotectonic variations along a slow-spreading center: The Mid-Atlantic Ridge (24°00′ N– 30°40′ N)

Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00′ N and 30°40′ N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points.

Bathymetry of the mid-atlantic ridge, 24°-31°N: A map series

This paper presents a series of eleven maps of the bathymetry of a 900 km long section of the crestal region of the Mid-Atlantic Ridge. Along with a twelfth key map, this series defines the morphology of fifteen discrete spreading segments and shows convincingly that no transform faults exist between the Kane and Atlantis fracture zones. The publication of these multi beam bathymetry data with a contour interval of 50 m and at a scale of 30 inches per degree of longitude is intended to allow easy access by a broad community of marine earth scientists to this unique and powerful data set.

An Ocean Bottom Hydrophone instrument for seismic refraction experiments in the deep ocean

Tests of a new Ocean Bottom Hydrophone (obh) instrument have recently been completed at Woods Hole Oceanographic Institution. This instrument is designed to float ∼3 m above the seafloor at depths of up to 6100 m for periods of up to 10 days and continuously records the output of a single hydrophone on a four-channel 0.064 cm/s (1/40 in./s) analog magnetic tape recorder. This instrument has an acoustic transponder and release system and is designed primarily for multiple deployments as a fixed ocean bottom receiver for seismic refraction work.

A deep towed explosive source for seismic experiments on the ocean floor

A new seismic source for carrying out high resolution measurements of deep ocean crustal structure has been constructed and successfully used in a number of ocean bottom refraction experiments on the Mid Atlantic Ridge near 23° N. The source is towed within 100 m of the ocean floor on a conventional 0.68″ coaxial cable and is capable of firing, upon command from the research vessel, up to 48 individual 2.3 kg explosive charges. The explosive used was commercially available Penta-Erythritol-Tetra Nitrate (PETN) that was activated by 14.9 gm m−1 Primacord and DuPont E-97 electrical detonators. For safety reasons each detonator was fitted with a pressure switch that maintained a short until the source was at depth in excess of approximately 300 m. In addition, a mechanical protector isolated the detonator from the main charge and was only removed by the physical release of the explosive from the source package. These and other safety precautions resulted in several misfires but three experiments were successfully completed during the summer of 1985 at source depths of 3000–4000 m.

Reflection profiling in the deep ocean using a near bottom hydrophone

A 20 km long high resolution seismic reflection profile was carried out approximately 300 km southwest of Bermuda. The data were collected using a small airgun sound source and a single hydrophone receiver towed ∼100 m above the seafloor at a depth of ∼5400 m. Comparisons with nearby conventional seismic reflection profiles show the considerable improvement of resolving power provided by this method, particularly of the basement morphology beneath the 700 m thick sediment column. The data were recorded digitally and selected data examples show the enhancement provided by filtering, stacking, source deconvolution and corrections for hydrophone motion. The precise picture of basement topography that results from this data is compared with deep tow bathymetry profiles of the present day mid-Atlantic Ridge flanks, and is seen to be remarkably similar.

A determination of the seismic velocity structure of sediments using both sources and receiver near the ocean floor

A trial experiment proves the power and practicality of using both sources and receivers near the ocean floor to make precise measurements of deep (∼6000 m) ocean sediment velocity structure. A digitally recording ocean bottom hydrophone receiver operating at a sampling rate of 1800 Hz recorded clear arrivals with bubble pulse frequencies of ∼500 Hz from 41b. explosive charges detonated at depths of 5500m along a 4 km long wide angle reflection profile. It is shown that corrections for changes in source depth may be computed without approximation and without prior knowledge of the velocity structure. The experiment was located at longitude 56° W in the trough of the Kane Fracture Zone. The velocity structure of the 1 km thick sedimentary section reveals a 310 m thickness of 3 km s−1 material overlying igneous basement.

Seismic reflection profiling with a deep-towed vertical hydrophone array

A seismic reflection profiling system consisting of a 264 m long, deep-towed, 15-element, end-fire, vertical array and a 40 cubic inch airgun was successfully used to profile a sediment pond in the trough of the inactive segment of the Kane Fracture Zone close to its intersection with the Mid-Atlantic Ridge at 24° N. The increased signal to noise ratio achieved with the array demonstrates that it is a useful tool for detailed seismic profiling in areas of rough topography in the deep ocean.

A simple ocean bottom hydrophone with 200 megabyte data capacity

Funding provided by the National Science Foundation under Grant Nos. OCE-9019918 and OCE-8917628.

Source signature measurements of underwater explosives at very high ambient pressures

The interpretation of seismic refraction experiments carried out on the deep-ocean floor has been hampered by lack of knowledge of the source signatures of explosives at these high ambient pressures. In order to fully exploit the data from such experiments, the source signatures of four explosives detonated at deep water depths have been measured. The explosives were commercially available Pentolite and three U.S. Navy explosives: PBXN-103, PBXN-111, and PBXW-127. A total of 75 ∼10 lb (4.54 kg) charges were detonated at depths of 2350–4593 m, and the source signatures were recorded at offset ranges of 160–230 m. For all four explosives, the period of the first oscillation follows the theoretically established relationship T=K*M1/3(Z+10)−5/6, where T is the period (s), M is the charge mass (kg), and Z is the detonation depth (m). The optimum value of K for Pentolite (2.07±0.01 s m5/6 kg−1) is identical to the value previously reported from the analysis of explosives detonated at depths of less than a few h...