Robert S. Detrick

Crustal thickness and structure along three contrasting spreading segments of the Mid‐Atlantic Ridge, 33.5°–35°N

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick crustal root (excess thickness of 2–4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

Seismic structure of the Endeavour Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity

[1]xa0Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9–1.4 s (∼2.1–3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4–1.2 km wide and appears to dip 8–36° to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 ± 0.09 s (380 ± 120 m) below the seafloor on the along-axis line and 0.38 ± 0.09 s (500 ± 110 m) on the cross-axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9–2.4 s below the seafloor. By projecting hypocenters of well-located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal-related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.

Skew of mantle upwelling beneath the East Pacific Rise governs segmentation

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10° anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

Extrusive thickness variability at the East Pacific Rise, 9°–10°N: Constraints from seismic techniques

We calculate synthetic shot gathers and their corresponding common depth point (CDP) profiles over plausible East Pacific Rise (EPR) shallow velocity structures, based on the structures obtained from high-resolution on-bottom seismic refraction experiments. We then use these results to analyze the variability in layer 2A thickness at the EPR 9°–10°N region, as measured by CDP, wide-aperture profile (WAP), on-bottom seismic refraction experiments, and conventional air gun refraction data. The synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in CDP and wide-angle data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may originate from a gradient within layer 2A if the upper crust contains more than one high-gradient region (our step model). This implies that independent estimates of layer 2A velocity structure are needed to properly interpret CDP and wide-angle data. We also determine that the travel time to the layer 2A reflector, for identical velocity structure, can vary by as much as 50 ms (about 125 m) for differing experimental geometries. This can explain the discrepancy in two-way travel time to the layer 2A reflector imaged on zero-age CDP and WAP lines. The depths to a shallow reflector calculated from CDP and wide-angle data in the 9°–10°N region of the EPR generally correlate with estimated layer 2A thicknesses from on-bottom refraction profiles and conventional air gun refraction lines, which suggests that the upper crustal structure in this area is similar to the gradient model. WAP and conventional air gun refraction data indicate that there is a 100–200 m decrease in off-axis layer 2A thickness at 9°35′N on the EPR, the present-day location of a deviation in axial linearity (deval). There is no bathymetric expression of the 50% decrease in layer 2A thickness. Layer 2A can be interpreted to consist of the extrusive section and transition zone, with the layer 2A/2B boundary corresponding to the top of the sheeted dikes. We suggest that buoyancy forces associated with the axial-magma chamber (AMC) are supporting the extrusive layer and sheeted dikes at the neovolcanic zone. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens, resulting in significantly less subsidence of the seafloor. We speculate that the 50% decrease in dike subsidence and extrusive thickness at the 9°35′N deval is due to a local reduction in magma supply within the axial magma chamber. The off-axis pattern of layer 2A thickness suggests that the 9°35′N deval has persisted for 175,000–275,000 years.

An acoustically linked moored‐buoy ocean observatory

A team from Woods Hole Oceanographic Institution (WHOI; Woods Hole, Mass.) recently developed and successfully deployed a buoy-based ocean observatory that uses acoustic communication to retrieve data from sensors in the water column and on the seafloor out to ranges of about three kilometers from the buoy (Figure 1). n nEach buoy is equipped with an Iridium satellite link that can transmit more than one megabyte of data per day to shore. The near-real-time data provided by this system, and two-way communication that enables control of sensors from shore, affords new opportunities for observing episodic events—such as earthquakes, volcanic eruptions, and phytoplankton blooms—and changing ocean conditions over seasons and years, or during times of the year when measurements from shipboard platforms are simply not possible.