characteristic of mid-plate Cretaceous seamounts. The channels act as conduits for sediment and talus debris. The bathymetry shown for the other seamounts (Figs. 7-12) is modified from Chase and Menard (1973) by changing the water depths from fathoms to meters (Chase, Seekins, and Young, U.S. Geological Survey, unpublished data, 1988). Airgun and 3.5 kHz Lines This section provides a brief description of each of the 37 seismic lines collected on F10-89-CP (Table 3; Figs. 13-' 9). The 156 km long Line 1 crosses Ruwituntun and Look Seamounts from the southeast to the northwest (Figs. 13-17). The shallowest water depth over the summit of Ruwituntun Seamount is 1215 m. Hie rugged summit platform of Ruwituntun Seamount consists of a series to basement knolls and basins filled or partly filled with sediment. Craters can be distinguished on the volcanic pinnacles to the southeast. Sediment thickness varies from 104 m in the summit-edge basins to 32 m for interior basins. The sediment drape is thicker on the southeast sides of volcanic pinnacles. The summit platform may tilt slightly to the southeast. 1 he upper to lower flank slope angles increase from 11° to 28° for the southeast flank and from 13° to 17° for the northwest flank. The pass between the two seamounts is rugged, showing volcanic pinnacles, sediment fill, slump deposits, and talus. The summit of Look Seamount shallows to 1000 m water depth aid consists of mostly small volcanic pinnacles draped by a thin layer of sediment. Sediment thickness varies from 0 m to 40 m on the summit and slumps and talus occur on the lower slopes. Slope angles range between 1 \ and 14°, although the lower northwest slope is 33°. The 33 km long Line 2 crosses the lower north-northwest flank of Look Seamount (Figs. 18, 19). The line shows mostly talus debris and slump structures, with abyssal sediment covering the lowermost slope at the northeast end of the line. Hie 46 km long Line 3 crosses Look Seamount from north to south (Figs. 20, 21). The summit shaUows to 999 m water depth, consistent with the line 1 crossing. The summit topography is irregular with small sediment-filled basins. Maximum sediment thickness is 32 m. The sedimentary section is nearly transparent witp few internal reflectors. Pelagic sediment at least several hundred meters thick laps up on the lower south flank. The water depth to abyssal sediment is about 400 m deeper on the north flank than on the south flank. Lines 4 through 9 total 241 km and cross North and South Laanmqjanjan Seamounts (Figs. 22-32), which are elongated in a northsouth direction. Hie summit of South Laanmqjanjan is about 7 km westnorthwest of its position on the Chase and Menard (1973) base map. Little or no sediment occurs on the summit or upper flanks of these generally very rugged seamounts (see Line 6; Figs. 26, 27). The shallowest water depth, 1090 m, was recorded at the end of line 9 (Pigs. 31, 32), which ended at the summit of North Laanmojanjan. Upper and lower slope angles vary from 9 to 17° and from 9 to 25°, respectively. The w...
We summarize the ages and thicknesses of the volcanics and shallow-water carbonate platform sequences recovered by drilling atop guyots on Leg 144, along with results of material previously dredged or drilled in the same areas. To model the subsidence histories of the guyots, we estimate the thermal reset ages of the lithosphere and amounts of initial emergence of the volcanos above sea level as follows: Site 871 (Limalok), 8 Ma thermal reset age of the lithosphere and 171 m of initial emergence of the volcano above sea level; Site 872 (Lo-En), unknown; Sites 873-877 (Wodejebato), 21 Ma and 236 m; Site 878 (MIT), 19 Ma and 0 m; Sites 879-880 (Takuyo-Daisan), 22 Ma and 138 m. We then compare observed subsidence of the guyots, as measured by the ages and thicknesses of their carbonate platform sediments, to the Parsons and Sclater (1977) model subsidence curve for oceanic crust formed by seafloor spreading. Limalok, Wodejebato, and Takuyo-Daisan guyots all subsided more slowly than oceanic lithosphere formed by seafloor spreading of the same thermal age. This suggests that the Cretaceous lithosphere cooled more slowly beneath these guyots than in a ridge crest subsidence model, and/or that some dynamic uplift was contributed from the mantle below. By contrast, MIT Guyot subsided in agreement with the Parsons and Sclater (1977) model, suggesting a more normal underlying thermodynamic structure. The lithosphere beneath Limalok, Wodejebato, and MIT guyots lost significant amounts of thermal age (17-83 Ma) during volcanic construction of these guyots, while the lithosphere beneath Takuyo-Daisan Guyot was essentially unaffected (2 Ma age loss). The small thermal age loss to the lithosphere beneath Takuyo-Daisan Guyot during volcanic construction seems incompatible with its subsequent subsidence history, which is slower than model ridge crest subsidence of the same thermal age range. The first observation implies that the underlying thermodynamic structure was unaffected during volcanic construction, but the second observation implies subsequently anomalous thermodynamic conditions. Perhaps the volcano formed over lithosphere and mantle that was unaltered by that volcanic event and then drifted over an anomalous upwelling during the subsequent period of subsidence. Although possible, this explanation appears somewhat contrived. A paleomagnetic polar wander path for the Pacific Plate is used to reconstruct the paleolatitude histories of these guyots and to define the age and paleolatitude ranges of "crisis zones" for each guyot. These crisis zones are defined by the ages and paleolatitudes of the youngest carbonate platform sediments on each guyot and coincide with the times of death of the ecological communities that built the carbonate platforms. These data all can be encompassed either within a crisis paleolatitude range of 0°-10°S, suggesting a paleoequatorial cause for the death of some of the carbonate platforms, or within a crisis age range of 100-110 Ma, suggesting a cause of death of some of the carbonate platfor...
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