Hole 1114A (RCB):
9°47.613´S, 151°34.504´E; 406.5 mbsl
0352.80 mbsf cored; 43.78 m recovered (12%)
Site 1114 is located just north of the crest of Moresby Seamount where seismic reflection data indicate that the basement beneath a south-southwest-dipping normal fault is shallowest. The primary objective was to determine the internal structure and composition of Moresby Seamount, particularly the nature of the basement (rock type, pressure-temperature history, structural fabric, and deformation history). A second objective was to determine the sedimentology, biostratigraphy, and vertical motion history of the ~300 m of local sedimentary cover, which may correspond to the uplifted and partly eroded synrift sequence.
In Hole 1114A, we drilled ~286 m of PliocenePleistocene sediments separated by a 6-m-thick tectonic breccia from a metadiabase that forms the basement. The diabase was metamorphosed under low-grade greenschist facies conditions before its upper part, which contained a chilled margin, was reactivated by normal faulting, leading to its unroofing.
The sediments consist of rift-related, mostly volcaniclastic, turbidites deposited in middle bathyal water depths (5002000 m). The benthic foraminiferal assemblage is characteristic of a suboxic environment of deposition, which may correspond either to a basinal situation with restricted circulation or to the oceanographic oxygen minimum situated at middle to upper bathyal water depths. From bottom to top, the deposition changed from mostly coarse-grained sandstones, about 100 m thick, to a finer grained intercalation of sandstones, siltstones, and claystones. The mineralogy of the sandstones suggests that most of the turbidites were derived from calc-alkaline extrusive rocks, but minerals and clasts from ultramafics and metamorphics (serpentinite and calcschists) are also present, as well as ubiquitous bioclasts.
Only the uppermost sediments are Pleistocene. The indurated sediments below 35 cm in Core 1114A-2R are all late and middle Pliocene (from >1.67 Ma to 3.093.25 Ma) and were deposited at rates of at least 176 m/m.y. The offset of the porosity vs. depth curve suggests that from 220 to >400 m of the Pleistocene section has been removed, which we tentatively relate to the uplift of Moresby Seamount in the footwall of bounding normal fault systems.
The deformation in the recovered sedimentary rocks preferentially occurs in the fine-grained strata, where it is expressed as a scaly fabric with numerous striated surfaces. It increases in intensity approaching the tectonic breccia. The bedding dips in the upper ~110 m range from 0% to 30%, then increase to 25%60% below that. The FMS data reveal that (1) bedding dip directions are dominantly northwest, but range from north to west; (2) the basement/sediment faulted contact outlined by the breccia dips ~60% toward the southwest, slightly oblique to the main normal fault that offsets basement by >2 km; and (3) in contrast, faults within the overlying sedimentary rocks mostly dip to the north, but a few dip to the south just above the breccia. The sense of motion on faults within the sedimentary rocks is dominantly normal, but reverse and oblique slip faults also exist. These observations suggest that a component of left-lateral motion exists on the bounding, south-southwest-dipping normal fault, in agreement with regional evidence for north-south extension.
The brecciated fault contact is also marked by an abrupt increase in the degree of hydrothermal alteration. The alteration in the breccia and metadiabase is characterized by massive clay and calcite veins that crosscut numerous quartz and epidote veins. In the breccia, the latter veins are restricted to the clasts. The hydrothermal alteration tends to decrease downward in the metadiabase, suggesting that fluids were channeled into the tectonic breccia.
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