White Paper Title: 
Observations of the northern, “magma-starved”, segment of the East Pacific Rise ISS

A key assertion about mid-ocean ridges is that the morphological segmentation (depth, cross-sectional shape, distance from ridge offsets) is linked to the supply of magma to the ridge axis.   Spreading rate exerts the strongest control on that supply.   It is generally presumed that crustal thickness, in turn, may be correlated with spreading rate and magma supply.  This is observed with the transition from ulta-slow to slow and intermediate spreading rates, and on slow-spreading rifted ridges (e.g. Dunn et al., 2005).  For intermediate to fast-ridges, the role of morphological segementation is well established, as is an apparent link to magma supply (e.g. Macdonald and Fox, 1988).   However, despite these correlations, these ridges   exhibit apparent striking uniformity in crustal structure and thickness, even when the morphological features change.

The 8o-11oN’  ISS on the East Pacific Rise (EPR) spans a full range of ridge morphologies, and several significant discontinuities.  From 8oN to 10oN, between the Clipperton and Sequiros Transforms, the first order segment of the ridge is broad in cross-section and shallow, and it includes a substantial axial magma chamber reflection, one that is nearly continuous, except at the second order boundary (OSC) at 9oN.  Between 10o and 11oN, and immediately north of the Clipperton, the ridge is relatively deep and has a triangular cross section, evidence for a magma starved segment, although the ridge shoals and broadens to the north.  This view is reinforced by the absence of a magma chamber reflection for 70 km north of the Clipperton.  We would expect a thin crust under this segment. and thus, it became the type-locality for a “magma-starved” segment on a fast-spreading ridge.  Consistent with this model was the identification of more evolved basalts north of Clipperton, compared to those to south. However, unlike magma-depleted segments of ridges spreading at intermediate to slow rates, there is no large rift valley.

Almost all of the research connected with 8o-11oN  ISS has been conducted on the magmaticaly robust segment south of the Clipperton Transform, with particular focus on the axis at 9o50’N eruption site.  These efforts have resulted in exciting observations extending over 20 years.   To a lesser extent workers have conducted research on other portions of segment between the Clipperton and Sequiros Transforms.  Far less work has been devoted to  north of the Clipperton (10o -11oN).

Subsequent research has revealed several surprises for this latter segment.  Barth and  (1986) noted that the crust appeared unusually thick near  the Clipperton, and pointed out that the presence of Ridge-Transform Intersection highs where the northern segment intersected the transform, perhaps evidence of excess magma on the axis.  Based on a seismic refraction survey in 1994, Begnaud and others (1997) noted a low velocity zone in the lower crust beneath the axis.  The seismic velocities associated with this zone were not low enough to indicate a large magma chamber, but may have suggested a partial melt zone.    In 2003, Alvin dives revealed that relatively large volcanic eruptions had occurred in the months immediately preceding the dives (McClain  et al., 2004).   The lavas were high in Titanium and Iron (FeTi) basalts.  The observation of a volcanic eruption suggests that eruptions do not require a significant (with an AMC reflection) magma chamber.  Alternatively it may reveal that the magma chamber has changed since the 1985 reflection survey, or that reflection survey failed to cover the chamber.  In any event, appears that the EPR north of Clipperton is not magma-starved, but instead is magma-deficient. It is not known if the more evolved magmas originate at the segment apex at 11oN and flowed southward, or if they evolved at the latitude of the eruption.   Regardless, it appears that the crust that results from this magma-depleted segment of the ridge does not appear to be different in structure or thickness that the crust generated on the magmatically robust segment south of Clipperton.

Barth, G. A. & Mutter, J. C., 1996,  Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise, J.  Geophy. Res.101, 17951–17975.

M. Begnaud, J. McClain, G. Barth, J. Orcutt, and A. Harding,1997,Velocity structure from forward modeling of the eastern ridge-transform intersection area of the Clipperton Fracture Zone, East Pacific Rise, J. Geophys. Res., 102, B4, doi:10.1029/96JB03393.

Dunn, R. A., V. Lekić, R. S. Detrick, and D. R. Toomey, 2005,  Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N): Evidence for focused melt supply and lower crustal dike injection, J. Geophys. Res., 110, B09101, doi:10.1029/2004JB003473.

Macdonald, K. C., and P. J. Fox, 1988. The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions, Earth and Planetary Science Letters 88, 119-131.

McClain, J.S. R.A. Zierenberg, J.R. Voight, K.L.Von Damm, K.L. Rubin, 2004,A Recent Volcanic Eruption on a "Magma Starved" Segment of the East Pacific Rise ISS, "10o44'N, EOS, Trans. AGU, 85(47), Fall Meet. Suppl. Abstract B13A-0177.