Publication Date:
2022-05-25
Description:
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B12104, doi:10.1029/2005JB003630.
Description:
We use multichannel seismic reflection data to image the upper crustal structure of 0-620
ka crust along the southern Juan de Fuca Ridge (JdFR). 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 1980’s eruptive event. Along the Vance segment, the AMC also
deepens from south to north, from 2.4 km 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.
Description:
This study was supported by the National Science Foundation grants OCE-0002551 to
Woods Hole Oceanographic Institution, OCE-0002488 to Lamont-Doherty Earth
Observatory, and OCE-0002600 to Scripps Institution of Oceanography.
Keywords:
Mid-ocean ridge
;
Juan de Fuca
;
Crustal structure
Repository Name:
Woods Hole Open Access Server
Type:
Article
Format:
application/pdf
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