Alicé S. Davis

Ferromanganese crusts from Necker Ridge, Horizon Guyot and S.P. Lee Guyot: Geological considerations

Abstract Necker Ridge, Horizon Guyot and S.P. Lee Guyot in the Central Pacific were sampled, seismically surveyed, and photographed by bottom cameras in order to better understand the distribution, origin, and evolution of ferromanganese crusts. Necker Ridge is over 600 km long with a rugged crest, pods of sediment to 146 m thick, slopes that average 12° to 20°, and debris aprons that cover some of the lower flanks. Substrate lithologies are mostly hyaloclastite, volcaniclastic breccia, and minor alkalic basalt. Horizon Guyot, 300 km long and 75 km wide, is capped by at least 160 m of sediment, which buries stepped terraces. Substrate lithologies are similar to those on Necker Ridge, although previous workers sampled much tholeiitic basalt on Horizon. S.P. Lee Guyot, 125 km long and 80 km wide, is capped by at least 300 m of sediment, and contains talus aprons along its lower flanks. Ferromanganese-encrusted rocks were recovered in every dredge and are thickest on Necker Ridge. Crust thicknesses average about 2.5, 1.5, and 0.8 cm for Necker, Horizon, and S.P. Lee, respectively. Crusts range from smooth or porous surfaces to knobby and botryoidal. The entire crust is laminated, however, two distinct layers commonly exist, separated by a paper-thin layer of phosphorite. The dominant mineral of all crusts is vernadite (δ-MnO 2 ), while quartz, feldspar, apatite, and, in three rocks todorokite, are minor phases. Quartz and feldspar decrease with decreasing latitude of occurrence, and is suggested to be related to eolian input. On the average, apatite also increases within the crusts with decreasing latitude of occurrence, which may be related to high biological productivity in the zone of equatorial upwelling. Phosphorite substrates are more abundant on Necker Ridge and S.P. Lee Guyot than they are on Horizon Guyot. Seamount ferromanganese nodules are distinct from abyssal nodules in their chemistry and internal structure.

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0N25&N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northernedificesisasoldastheunderlying oceanic crust (>20 Ma), indicating that it originatedataspreadingcenter.Otherseamountlavaagesaremuchyoungerthanthe oceanic crust on which they reside, ranging from 16.8 6 0.3 to <7 Ma for some of the northern seamounts to 270 616 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers orfaultsandfracturezoneswhichareabundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Someoftheepisodesofvolcanismappearto havebeencontemporaneouswithvolcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tappingvariablyenriched,heterogeneousmantle, which occurred sporadically from early Miocene to late Pleistocene.

Eocene basalts from the Yakutat terrane: Evidence for the origin of an accreting terrane in southern Alaska

Basalts from the Yakutat terrane, a composite oceanic and continental tectonostratigraphic terrane, are chemically diverse large ion lithophile element (LILE)–depleted and LILE-enriched tholeiites, interpreted as normal mid-ocean ridge and oceanic island basalt, that originated on seamounts near the Kula-Farallon spreading center during the early to middle Eocene. Coeval and geochemically similar basalts, occurring sporadically in a linear belt from southern Vancouver Island to the southern Oregon Coast Range, are correlative with the Yakutat terrane basalts. Both basaltic sequences were accreted to the continental margin during subduction of the Kula-Farallon Ridge and Kula plate about 48 Ma. The Yakutat basalts were emplaced along the coast of Washington or British Columbia. The Yakutat terrane probably originated during the Neogene as a composite terrane when it was sliced off the continental margin and started to move northward along the Queen Charlotte-Fairweather transform fault toward its present location in southern Alaska.

Volcanic Rocks from Rocas Alijos

The region offshore from southern and Baja California has an abundance of volcanic edifices. Most of the edifices are submarine seamounts or guyots but a few project above sea level. Most are small, conical volcanoes that are round or elliptical in plan view. Others are larger and more complex in shape, and some form elongated ridges (Fig. 1). Unlike many of the distinctive linear island chains of the central and southern Pacific, volcanic edifices offshore from the Californias may be isolated, clustered, or aligned in short chains. Short chains may show a NW trend compatible with direction of plate motion or they may show a variable NE to E trend that is at a high angle to the direction of plate motion.

Marine Sediment from Rocas Alijos

During the 1990 Cordell Expedition to Rocas Alijos, samples of marine sediment were collected from a variety of subtidal sites. This material provides insight into the erosional and depositional processes at Rocas Alijos, as well as information about the dynamics of the hydrologic environment.