Deborah E. Eason

Ka‘ena Volcano—A precursor volcano of the island of O‘ahu, Hawai‘i

Ka‘ena and Wai‘alu Ridges form prominent submarine ridges NW of the island of O‘ahu, Hawai‘i. We evaluate whether or not either one of these ridges represents a submarine extension of Wai‘anae Volcano on O‘ahu using new bottom observations, geophysical surveys, and geochemical data acquired on new samples from the region. Wai‘alu Ridge has the morphology of a submarine rift zone but is too shallow for its distance from the O‘ahu shoreline; Ka‘ena Ridge also is unusually shallow and is surmounted by two topographic shields. Ka‘ena and Wai‘alu Ridges have similar magmatic and volcanic evolutionary histories, beginning ca. 5 Ma with a submarine, shield phase of volcanism that produced high-SiO 2 , low-FeO* tholeiites with higher 208 Pb/ 204 Pb than in the adjacent Wai‘anae Volcano. Late-shield volcanism included transitional and alkalic rock types, with lower SiO 2 and enrichment in incompatible elements, especially P 2 O 5 , Nb, Zr, Ti, and light rare earth elements. The transition from shield to late-shield stage occurred as the edifice was beginning to emerge from the sea. Geological observations and K/Ar ages indicate that Ka‘ena emerged above sea level ca. 3.5 Ma, reaching a maximum height of ∼4000 m above the abyssal ocean floor and 1000 m above sea level. Relatively weak gravity anomalies, topographic lineaments, and the orientation of dike complexes indicate a volcanic structure that is independent of Wai‘anae Volcano. Thus, volcanic structure, geochemistry, and age all indicate a precursor volcano to the island of O‘ahu, which we call Ka‘ena Volcano. After emergence, Ka‘ena Volcano tilted ∼2° to the south. We estimate a total volume of 20–27 × 10 3 km 3 for Ka‘ena Volcano, taking into account overlapping geometry of concurrently active volcanoes. Sample compositions from the Ka‘ena landslide deposit are entirely consistent with derivation from Ka‘ena, whereas most samples from the Wai‘anae slump are likely derived from Wai‘anae Volcano. Uniformly oriented dikes in the Wai‘anae NW rift zone likely reflect buttressing by a preexisting Ka‘ena Volcano. Unusual isotopic compositions of some Wai‘anae samples, including unique hydrous silicic lavas, probably reflect interaction with underlying Ka‘ena crust. A newly recognized lava flow field on the southern flank of Ka‘ena Ridge extends the previously known distribution of secondary volcanism in the Kaua‘i Channel. Putative submarine volcanic activity in the region in 1956 cannot have built a large edifice and is unlikely to have produced pumice that was found on O‘ahu shores. This eruptive activity therefore remains unconfirmed.

Three‐Dimensional Seismic Structure of the Mid‐Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area

To test models of tectonic, magmatic, and hydrothermal processes along slow-spreading mid-ocean ridges, we analyzed seismic refraction data from the Mid-Atlantic Ridge INtegrated Experiments at Rainbow (MARINER) seismic and geophysical mapping experiment. Centered at the Rainbow area of the Mid-Atlantic Ridge (36°14’N), this study examines a section of ridge with volcanically active segments and a relatively amagmatic ridge offset that hosts the ultramafic Rainbow massif and its high-temperature hydrothermal vent field. Tomographic images of the crust and upper mantle show segment-scale variations in crustal structure, thickness, and the crust-mantle transition, which forms a vertical gradient rather than a sharp boundary. There is little definitive evidence for large regions of sustained high temperatures andmelt in the lower crust or upper mantle along the ridge axes, suggesting that melts rising from the mantle intrude as small intermittent magma bodies at crustal and subcrustal levels. The images reveal large rotated crustal blocks, which extend to mantle depths in some places, corresponding to off-axis normal fault locations. Low velocities cap the Rainbow massif, suggesting an extensive near-surface alteration zone due to low-temperature fluid-rock reactions. Within the interior of the massif, seismic images suggest a mixture of peridotite and gabbroic intrusions, with little serpentinization. Here diffuse microearthquake activity indicates a brittle deformation regime supporting a broad network of cracks. Beneath the Rainbow hydrothermal vent field, fluid circulation is largely driven by the heat of small cooling melt bodies intruded into the base of the massif and channeled by the crack network and shallow faults.