During Ocean Drilling Program Leg 180, a north-south transect of sites was drilled in the western Woodlark Basin, a region of active continental extension. This transect is perpendicular to the Moresby detachment system. Moresby Seamount is characterized by the presence of a north-northeast-dipping detachment fault zone on its northern flank. Site 1114 is located near the top of Moresby Seamount in the footwall fault block of the detachment zone. This site is characterized by the presence of an antithetical southwest-dipping normal fault zone putting into contact synrift sediment and highly altered metadolerite.
The Formation MicroScanner (FMS) logging tool provides oriented microresistivity images of the borehole wall. In Hole 1114A, FMS data were recorded from 105 to 297 meters below seafloor (mbsf) through the Pliocene sediment and the sediment/basement contact. The structural analysis of FMS images yields the dips and dip directions of beds and fractures and their density distribution vs. depth.
Three fracture zones (FZs) were determined from FMS images analysis: FZ1 (105-140 mbsf), FZ2 (180-215 mbsf), and FZ3 (270-285 mbsf just above the sediment/basement contact). The fracture zones are characterized by a wide range of fracture dips (10°-80°) and dip directions, but a majority of fractures dip northward and north-northwestward. Within the upper section of FZ2 between 180 and 200 mbsf, fractures clearly dip northward. In this zone, beds are also very well organized with a northwest dip direction. At 200 mbsf, there is a sharp change of bedding dip direction from northwest above to southwest below. Otherwise, the bedding dip direction is mainly northwest with a west to north distribution. These observations indicate a differential rotation of strata within fault-bounded compartments.
The sediment/basement contact consists of a several-meters-thick tectonic breccia. The contact occurs along a sharp surface dipping at 60° to the southwest.
The presence of strike-slip and oblique faults observed in cores and of reverse faults observed in both cores and FMS images are indicative of an oblique extensional system on Moresby Seamount.
1Louvel, V., Le Gall, B., Célérier, B., Gardien, V., and Huchon, P., 2002. Structural analysis of the footwall fault block of the Moresby detachment (Woodlark Rift Basin) from borehole images. In Huchon, P., Taylor, B., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 180 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/180_SR/165/165.htm>. [Cited YYYY-MM-DD]
2Laboratoire de Mesures en Forage, ODP-NEB, 13545 Aix-en-Provence Cedex 4, France. Present address: 12, Impasse Jean Giono, 13830 Roquefort-La Bedoule, France. VeroniqueLouvel@aol.com
3Institut Universitaire Européen de la Mer, CNRS UMR 6538, 29280 Plouzané, France.
4ISTEEM, CNRS-Université de Montpellier II, cc MSE, 34095 Montpellier Cedex 5, France.
5Université Lyon I, UMR 5570, 69622 Villeurbanne Cedex, France.
6Géosciences Azur, Observatoire de Villefranche, BP 48, 06235 Villefranche-sur-mer, France.
Initial receipt:
19 December 2000
Acceptance: 16
May 2002
Web publication: 6 August 2002
Ms 180SR-165