Physiography and Architecture of Marshall Islands Guyots Drilled during Leg 144: Geophysical Constraints on Platform Development

Abstract

Drilling during Ocean Drilling Program Leg 144 provided much needed stratigraphic control on guyots in the Marshall Islands. Information from geophysical surveys conducted before drilling and the ages and lithologies assigned to the sediments sampled during Leg 144 make it possible to compare the submerged platforms of Limalok, Lo-En, and Wodejebato with each other and with various islands and atolls in the Hawaiian and French Polynesian chains. The gross physiography and stratigraphy of the platforms drilled during Leg 144 are similar in many respects to modern islands and atolls in French Polynesia: perimeter ridges bound lagoon sediments, transition zones of clay and altered volcaniclastic sediments separate the shallow-water platform carbonates from the underlying volcanic flows, and basement highs rise toward the center of the platforms. Ridges extending from the flanks of these guyots conform in number to rift zones observed on Hawaiian volcanoes. Edifices appear to remain unstable after moving away from the hotspot swell on the basis of fault blocks perched along the flanks of the guyots.The apparent absence of an extensive carbonate platform on Lo-En, the 100-m-thick sequence of Late Cretaceous shallowwater carbonates on Wodejebato, and the 230-m-thick sequence of Paleocene to middle Eocene platform carbonates on Limalok relate to differing episodes of volcanism, uplift, and subsidence affecting platform development. I analyze three tectonic components to explain these differences in platform development: plate flexure, thermal subsidence of the plate, and edifice size. Flexure modeling on the Anewetak-Lo-En pair suggests that Lo-En lay slightly on the moat side of the flexural arch formed during the construction of Anewetak. The model results are consistent with the negligible difference between the depths to the top of the volcanic substrate of these two guyots and the slight tilting of the Lo-En summit plateau toward Anewetak. The stresses and subsidence associated with flexure of the plate may explain the high-backscatter, cone-and lobe-shaped features cropping out along the flat summit plateau of Lo-En and the absence of an appreciable accumulation of shallow-water carbonate sediments. Age controls on Pikinni-Wodejebato and Mili-Limalok are poor, but flexure modeling on these two pairs of edifices suggests that Pikinni and Wodejebato were probably constructed at approximately the same time, and that construction of a volcano to the south of Limalok may explain a southward fanning of seismic basement reflectors. Neither flexure nor thermal subsidence of the plate appears to be responsible for the demise of the carbonate platforms on top of Wodejebato and Limalok guyots, but uplift or subsidence caused by plate loading may have influenced stratal patterns within the carbonate sediments. Clearly, the use of submerged carbonate platforms drilled during Leg 144 as "dipsticks" for recording the history of Cretaceous sea-level fluctuations requires a better understanding of the sequence and timing ...

Three-dimensional flexure modelling of seamounts near the Ogasawara Fracture Zone in the western Pacific

“…Calculated Te values of the study area are unusually low when comparing to other Te values in the cooling plate model (Watts et al 1980). Many researchers suggested that the lithosphere of the northwestern Pacific shows unusually thin elastic thickness <15 km (Smith et al 1989; Smith 1990; Wolfe & McNutt 1991; Bergersen 1995). Smith et al (1989) suggested the possibility of thermal rejuvenation of the lithosphere rather than the possibility of on‐ridge volcanism.…”

Island Edifice Failures and Associated Tsunami Hazards

“…The remaining 94 (out of 99) atolls display polygonal shapes with large concave 'bites' (suggesting landslide scars) and linear margins (fault scars). Subsequent studies of Pacifc atolls and guyots using side-scan sonar indicate that linear margins appear to mark the head walls of ancient landslides (SMOOT, 1986;KEATING et al, 1991;KEATING, 1994a;BERGERSEN, 1995;SMOOT and KING, 1993). The side-scan sonar images of the margins of volcanic edifices show that many cusps in the coastline mark scars of ancient landslides.…”

“…Both the physical properties of the carbonate cap and the unstable nature of the volcanic foundations may be contributing factors in the failure process. Side-scan sonar images of guyots and atolls in the Pacific often show extensive turbidites draping the flanks of the seamount, and originating below the carbonate cap (e.g., BERGERSEN, 1995). Based upon observations of this nature, it appears likely that the carbonates of many atolls may be undermined by large-scale submarine landslides in the mechanical weak rock of the upper portions of the volcanic edifice, often called the transition zone.…”

Distribution characteristics of seamount cobalt-rich ferromanganese crusts and the determination of the size of areas for exploration and exploitation

“…These data were compared with the seismic profile and Ocean Drilling Project (ODP) data (Bergersen, 1995;Janet and Isabella, 1995;Abrams et al, 1992). The revealed sub-bottom structure can be divided into two layers, layer I and layer II from upper to lower (Fig.…”