Constructing the upper crust of the Mid-Atlantic Ridge

SMITH AND JOURNAL GEOPHYSICAL CANN: MID-ATLANTIC RESEARCH, RIDGE VOL.UPPER 104, PAGES CRUST 23,579-25,399, NOVEMBER 10, 1999

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Constructing the upper crust of the Mid-Atlantic Ridge: A reinterpretation based on the Puna Ridge, Kilauea Volcano Deborah K. Smith Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Johnson R. Cann Department of Earth Sciences, University of Leeds, Leeds, England

Abstract. The volcanic morphology of a number of segments of the slow spreading Mid-Atlantic Ridge (MAR) have been reinterpreted based on our understanding of dike emplacement, dike propagation, and eruption at the East Rift Zone of Kilauea Volcano, Hawaii and its submarine extension, the Puna Ridge. The styles of volcanic eruption at the submarine Puna Ridge are remarkably similar to those of the axial volcanic ridges (AVRs) constructed on the median valley floor of the MAR. We use this observation to relate volcanic processes occurring at Kilauea Volcano to the MAR. We now consider that volcanic features (e.g., seamounts and lava terraces) built on the flanks of the AVRs are secondary features that are fed from lava tubes or channels, not primary features fed directly from an underlying dike. We examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there. In addition, deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate. The volumes of individual volcanic terraces and seamounts on the Puna Ridge and at the MAR are large (0.1-1 km3) and similar to the volumes of lava flows that are broadly distributed at the subaerial East Rift Zone of Kilauea. This striking difference in the volcanic morphology on a scale of 1-2 km (producing terraces and seamounts underwater and low-relief flows on land) must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment. We suggest that at the MAR a crustal magma reservoir, most likely located beneath shallow, flat sections of the segment, provides magma to the rift axis through dikes that propagate laterally tens of kilometers. The zone of dike intrusion, at least in the neighborhood of the magma body, is likely narrower than the width resurfaced by flows, yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes. At segment ends the zone of dike intrusion is likely to be wider, giving a resulting structure with a more gradual transition from lavas to dikes.

1. Introduction The processes that create new ocean crust leave their mark on the morphology of the ocean floor. The challenge is to interpret the observed morphology to recover the nature of these crust-forming processes. Interpretation of the morphology of the median valley floor of the Mid-Atlantic Ridge (MAR) has given rise to a variety of models of upper crustal construction [e.g., Ballard and van Andel, 1977; Ramberg and van Andel, 1977; Atwater, 1979; Sempere et al., 1993; Smith and Cann, 1992]. These models differ principally in the distribution of eruptive vent features within the median valley floor and the length of the lava flows emerging from these vents, and they result in a range of inferred structures for the upper crust. Here we reexamine the problem of interpreting the morphology of the floor of the median valley, using existing and new survey data from the MAR and also by making comparisons of the morphology with that at Kilauea Volcano’s East Rift Zone (ERZ) and its submarine extension, the Puna Ridge. In particular, we find that the submarine volcanic morphology of the summit and flanks of the Puna Ridge on a scale of several hundreds of meters to kilometers is very similar to that on the floor of the median valley of the MAR. From this comparison, we reinterpret the morphological character of the MAR to give a new model for the construction of layer 2 (dikes and lavas) at a slow spreading ridge. The results of this new approach are very different from our previous models for building the crust at the MAR [e.g., Smith and Cann, 1992] in which each of the multitude of small seamounts scattered across the median valley floor is considered to be fed from an individual, unique magma body (Figure 1a). This type of model, with a wide (up to 15 km) zone of dike emplacement and primarily short lava flows (15 km down to the deep ocean floor. This distance far exceeds either the width of the zone of active fissure eruptions on the subaerial ERZ (1.5-3 km, see above) or the half width of the magnetic source beneath the axis of the Puna Ridge (5-6 km), which presumably represents the total width of the zone of dikes intruded over the lifetime of the ridge. This also supports the argument that volcanic products (terraces and circular volcanoes) on the flanks of the ridge must be fed by lava transported from sites of eruption at or near the summit.

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Flows erupted from submarine rift zones must be able to travel at least 15 km down slopes of 160-240 m/km and must produce the characteristic styles of eruption that we see on the flanks of the Puna Ridge. How does this happen? We consider that flows become channelized in tubes that transport the lava down the slope, just as observed on the subaerial ERZ.

−1000

−1500

Water depth (m)

−2000

Northwestern flank

Southeastern flank

−2500

−3000

−3500

−4000

−4500

−5000

−5500 −15

−10

−5

0

5

10

15

Distance from the axis of the Puna Ridge (km)

Figure 4. Cross-axis profiles along the Puna Ridge. Each profile represents an average over 5 km along the ridge. Note that on the southeastern side of the Puna Ridge (see Figure 3a) the flank slope is more or less constant along the length of the ridge until a water depth of ~3500 m. (The northern flank is slightly more complicated as it comes close to the submarine Hilo ridge.) The constant slope implies that the southeastern flank is resurfaced regularly as the ridge grows and that flows must be transported regularly 10-15 km from the ridge summit.

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Table 1. Rift Zone Characteristics

Rift Zone

MAR

Puna Ridge

Subaerial ERZ

0-100

51; 95*

23

Lateral slopes, m/km

150-250

160-240

50

Lava terrace volumes, km3