Short Notes

H. W. MENARD S. M. SMITH T. E. CHASE

GUYOTS IN THE SOUTHWESTERN PACIFIC BASIN

Abstract: Four guyots in a northwest trending province characterized by northwest trending lines group of guyots, 2000 km east of New Zealand, of volcanoes has greater extent to the southwest have reliefs of 4200-4500 m and shelf break depths than previously known and comes into contact of 700, 880, 900, and 920 m. Blocky and tabular with the sub-continental Tonga-Kertnadec and cobbles of olivine basalt and detrital rock were Fiji structures. dredged from one of the guyots. The structural

A group of guyots in the Southwestern land in connection with the exploration of Pacific Basin is of unusual interest because it is Antarctica. The group was first noted by the far from other existing and ancient islands and Hydrographic Office. One guyot was surveyed yet shows the northwesterly trend and basaltic and dredged in 1961 during the Monsoon Ex- lithology typical of the whole Pacific Basin. pedition of the Scripps Institution of Oceanog- The group was discovered by various ships of raphy of the University of California. the U. S. Navy sailing to and from New Zea- Four guyots have been discovered along a

no1

50" Figure 1. Known distribution of seamounts and volcanic islands in southwestern Pacific. Widely spaced spot sound- ings not shown. Pronounced northwesterly trend of ocean basin volcanoes is evident. Geological Society of America Bulletin, v. 75, p. 145-148, 2 figs., February 1964 145

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/2/145/3427579/i0016-7606-75-2-145.pdf by guest on 27 September 2021 146 MENARD AND OTHERS—GUYOTS IN SOUTHWESTERN PACIFIC BASIN

line about 1100 km long (Fig. 1). They have Valerie Guyot, named after the wife of one remarkably accordant flat tops with shelf breaks of the expedition leaders, is 50 km wide and at depths of about 700 m, 880 m, 900 m, and probably about 100 km long, although neither 920 m. Each is a rather isolated volcano with end was surveyed (Fig. 2). It appears to be local relief of 4200-4500 m, which is greater asymmetrical with steep eastern and southern than that of most drowned ancient islands in sides. The smooth and almost flat top is 9 by 16 the Pacific. km, slopes gently from a depth of 776 m at

HORIZONTAL SCALE IN KILOMETERS

SKIP'S TRACK

Figure 2. Bathymetry of Valerie Guyot, Southwestern Pacific Basin. Ship's track is that of ARGO on Monsoon Expedition. Original discovery soundings made during Operation Highjump cannot be located accurately relative to survey and are not shown. Contour interval = 250 m

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/2/145/3427579/i0016-7606-75-2-145.pdf by guest on 27 September 2021 SHORT NOTES 147

the center, and is limited by shelf breaks at dorite (1-5 mm) and olivine (1-2 mm). Lab- depths of 863, 955, 977, 1004, 1096, and 1097 radorite microlites, olivine, opaque devitrifying m in different places. Differences in depth may glass, and small amounts of pyroxene make up merely reflect uniform truncation of a slightly the groundmass. Several 2-cm pieces of detri- irregular cone. Relief of the smooth top from tial rock composed of angular to subrounded center to shelf break is about 220 m, which is grains of weathered basalt (1 mm) cemented by far too great to have resulted from even the carbonate were also present. most prolonged marine planation without a This group of guyots is about 2200 km south- change in depth. This guyot, like many others west of the nearest oceanic islands, the Austral in the Pacific, was once an island surrounded group, but the two trends are very close to by a gently sloping shelf. Very slow subsidence parallel. Thus the Pacific structural province enabled waves to cut into the island until characterized by northwesterly trending lines nothing remained except a continuous shelf of large basaltic volcanoes is far more extensive sloping out from what had been the center. than previously known. Another group of A single dredge haul (Monsoon VII-9) was large submarine volcanoes, not known to be made across the shelf break and brought up 100 guyots, is centered at about 32°S., 173°W. (Fig. kg of blocky and tabular cobbles of olivine 1). The province butts into, or is interrupted basalt. The curved tabular pieces have a uni- by, the Tonga-Kermadec and Fiji structures. form aphanitic texture and are highly weath- Previously, considering the distribution only ered, suggesting rapid chilling and long surface of existing islands, it appeared that the province exposure. The blocky cobbles have less surface did not intersect this sub-continental salient weathering, and some show rounding. Their tex- into the Pacific Basin. ture is porphyritic with phenocrysts of labra-

DEPT. EARTH SCIENCES, UNIVERSITY OF CALIFORNIA, SAN DIEGO, LA JOLLA, CALIF. (MENARD, SMITH); U. S. BUREAU OF COMMERCIAL FISHERIES, BIOLOGICAL LABORATORY, SAN DIEGO, CALIF. (CHASE) MANUSCRIPT RECEIVED BY THE SOCIETY, JUNE 17, 1963 CONTRIBUTION FROM THE SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIVERSITY OF CALIFORNIA, SAN DIEGO, NEW SERIES

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/2/145/3427579/i0016-7606-75-2-145.pdf by guest on 27 September 2021