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Regional Acoustic Basement

Acoustic basement was penetrated at five sites during Leg 194 drilling (Sites 1193, 1194, 1195, 1197, and 1198). Prior to Leg 194 drilling, no direct information existed concerning the basement composition of the Marion Plateau.

Leg 133 drilling on the Queensland Plateau recovered altered and deformed metasedimentary and metavolcanic rocks that comprised acoustic basement (Davies, McKenzie, Palmer-Julson, et al., 1991). These metasedimentary rocks consisted of detrital quartzofeldspathic assemblages and igneous rock fragments. In addition, low-grade quartz-muscovite-feldspar schists were also recovered. Recovered igneous rocks were determined to be altered granodioritic to tonalitic in composition and dominated by feldspars and quartz (Feary et al., 1993). Those assemblages were considered a straightforward continuation of the onshore Hodgkinson Formation, predominantly Devonian in age, consisting of thick, monotonous cleaved graywacke, siltstone, shale, and slate successions that in turn were cut by late Paleozoic-early Mesozoic dike swarms.

Prior to Leg 194, the basement of the Marion Plateau was considered to be similar to that of the Queensland Plateau. Planation of the basement supposedly occurred during subaerial exposure in the Mesozoic and Paleogene. Site survey multichannel seismic reflection data suggest that the top of the basement of the Marion Plateau is a topographic surface with tens to hundreds of meters of relief (Fig. F3) and often a major erosional unconformity. In particular, a north-south-trending, eastward-facing ramp exists toward the eastern edge of the plateau. The ramp has a maximum relief of 225 m and a dip of ~1°. Initiation of the MP1 (and perhaps earlier) reef/platform systems was focused along the eastern edge of this ramp (Pigram, 1993). Given the progression of the shallow-water early reef systems to the present water depths of the drilling sites (304-420 m), it is clear that postrift thermal subsidence remains the controlling factor on the long-term accommodation. The postrift subsidence of the region occurs even though unambiguous rift structures (e.g., high-angle normal faults, rotated fault blocks, synrift wedge packages) are not observed across the plateau.

Basement rocks recovered during Leg 194 differ greatly from those drilled on the Queensland Plateau. Rather than metasedimentary rocks, highly altered volcanic flows and volcaniclastics were recovered (Figs. F16, F17, F18, F19). The lack of deformation suggests that these volcanics may have been emplaced during the Late Cretaceous-Paleocene rifting of northeastern Australia from the Papuan Plateau and the Lord Howe Rise.

Site 1194 was drilled on the flank of a topographic basement high (Fig. F16). Recovered volcanic rocks consist of altered amygdaloidal olivine basalts, in which the olivines are large phenocrysts within a fine matrix of pyroxenes and plagioclase lathes (Fig. F16). Many of the olivines have been altered to iddingsite and opaques, probably magnetite. Occasional veins are filled with feldspars that are zoned from fine to coarse, whereas the vugs are infilled with zeolites, most likely natrolite.

Site 1197 was located in a similar setting to Site 1194 (Fig. F17). Recovered basal units of this site are polymictite volcanic breccias (tuffs) deposited in a nonmarine paleoenvironment. At the base of the breccias is an olivine basalt (Fig. F17) showing similar characteristics as the basalts from Site 1194. Glass shards are also present and vugs are filled with feldpars.

Site 1193, a volcanic flow, is highly altered even compared with Sites 1194 and 1197 (Fig. F18). Vug infilling is natrolite, characterized by fine radiating fibers (Fig. F18). Crystal outlines are well represented under normal light.

The recovered basement rocks at Site 1198, although again highly altered, are different from the basalts at the other sites, being composed principally of plagioclase (albite?) lathes. Quartz is observed to infill veins (Fig. F19).

The paleomagnetic response of the basement is characterized by both relatively high intensity and consistent normal and reversed inclinations (Figs. F16, F17, F18, F19). In general, the inclination polarity is almost exactly 180° apart, implying the existence of a clean, primary thermal remanent magnetization. In the upper part of the recovered basement section, the inclination has a greater variation, implying that a magnetic overprint exists. This leads to the possibility that the inclinations, when the corresponding paleomagnetic pole is compared with the Australian apparent polar wander path, may provide age estimates for both the emplacement of the basalts and the timing of low-temperature alteration.

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