[1] Multichannel 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.Citation: Van Ark, E. M., R.

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1-2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is ϳ50-200 m deep and 1-8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

[1] We use multichannel seismic reflection data to image the upper crustal structure of 0-620 ka crust along the southern Juan de Fuca Ridge. The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow (Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment to 2.3 km at the northern end near the site of the 1980s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250-300 m thick at the ridge axis along the Cleft segment and 300-350 m thick along the axis of the Vance segment. However, off-axis layer 2A is similar in both segments (500-600 m), indicating $90% and $60% off-axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3-4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.

No abstract