During Leg 180 we collected core and borehole data from a transect of sites drilled in a region of rapid extension (Moresby rift and Moresby Seamount) and its less extended northern margin (Woodlark Rise) that is at the point of continental breakup adjacent to the Woodlark spreading center. Postcruise analysis of these data and parallel investigations provide new insights into the nature of the continental crust being rifted, the style of synrift (and prerift) sedimentation, the mechanisms of rifting (including shallow-angle normal faulting and lower crustal flow), and the extent of the deep biosphere.
The upper crust of the Papuan region (onshore and offshore) is composed of a variety of basement types (dominantly ophiolitic normal mid-ocean-ridge basalt [N-MORB] and enriched mid-ocean-ridge basalt [E-MORB], but also island arc rocks) and ages (late Maastrichtian, Paleocene, and middle Eocene). The Site 1117 gabbro crystallized in the late Maastrichtian (66 Ma) and, together with the thick E-MORB dolerite sill complex at Sites 1109 and 1118 and probably the metadolerite at Site 1114, cooled into the Paleocene (59-54 Ma), and all were partially altered through the early Oligocene. Archean continental components exist in the lower crust.
This amalgam of oceanic, arc, and continental basement terranes was the site of early Miocene to Holocene arc magmatism related to southward subduction at the Trobriand Trough. A thick forearc basin developed in the Miocene, which filled to sea level in the late Miocene from paleowater depths >500 m. Volcaniclastic turbidites indicate source terranes of calc-alkaline extrusives, dominantly basaltic, with lesser rhyolitic and, rarely, alkalic rocks.
A regional unconformity at 8.4 Ma marks the onset of rifting and provides a paleo-sea level surface for tracking the subsequent subsidence of the northern margin sites from paralic to shelf to the present bathyal water depths. The northward Miocene drainage was reversed as the margin thinned and subsided southward toward the active rifts. Moresby Seamount was not a topographic high in the late Miocene. It was part of a wide graben system that included the basin to the south as well as what is now Moresby rift. All the master faults that currently bound these basins were formed early in the rift history, and at least one of them initiated at shallow dips.
Clastics, largely deposited by turbidity currents with higher energy environments in the rift basins than on the margins, were derived from calc-alkaline volcanics plus pelitic metamorphic rocks and lesser ultramafic rocks. The turbidite bed thicknesses fit a power-law model, most likely controlled by earthquakes. High-K volcanic ash layers and muds rich in tephra intersected at Sites 1109 and 1115 at <2.3 Ma record explosive eruptions from rhyolitic volcanoes of the D'Entrecasteaux Islands, both peralkaline (Dawson Strait) and calc-alkaline (Moresby Strait).
Movement in the Pleistocene on the normal faults that bound Moresby Seamount uplifted the footwall seamount, lowered the hanging-wall rift basins, and substantially changed the paleogeography and depositional pathways. Terrigenous sediments ceased to onlap the northern margin during the Pleistocene (~1.2 Ma), and since that time sedimentation in Moresby rift, other than pelagics, has been limited to talus derived from fault scarps plus sediments eroded from the northern margin.
Within the ~100-m-thick Moresby normal fault zone at Site 1117 (gouge, mylonite, breccia, and undeformed quartz gabbro), mylonites and calcite twins indicate depths and temperatures greater than those indicated by other lines of evidence. For example, the gabbros were not thermally reset from their 66-Ma ages and must have stayed at shallow and cool levels in the crust. Dynamic shear heating may explain this paradox.
The Moresby normal fault dips ~30°N, consistent with the frictional properties expected for mature faults with well-developed gouge zones. The propensity for failure at the shallow dips observed is overdetermined; there is evidence for both
Such shallow-angle normal faults may be a common feature of strain localization in the transition from rifting to spreading.
The up to 3 km of subsidence of the cold (30 mW/m2) northern margin, with little attendant brittle deformation, requires substantial crustal thinning by lower crustal flow. This ductile extension is demonstrably not activated by temperature but presumably by fluids and occurs synrift, not just postbreakup.
Organic carbon and pyrolysis gas chromatographic data indicate that there is no significant source rock potential at Site 1108, although sufficient organic matter (0.6 ± 0.5 wt%) is present for microbial processes to generate the limited gas present within the recovered cores. The hydrocarbons encountered appear to be indigenous and not a migrated product or contaminant, suggesting that Site 1108 can be revisited safely in order to penetrate and characterize the in situ properties of the Moresby normal fault at depth.
Culturable anaerobic bacteria and realistic rates of anaerobic bacterial activity (sulfate reduction, methanogenesis, and thymidine incorporation) are present in the deepest samples from the subseafloor biosphere analyzed to date for microbial populations (842 meters below seafloor [mbsf]; Site 1118) and activities (800 mbsf; Site 1115).
1Taylor, B., and Huchon, P., 2002. Active continental extension in the western Woodlark Basin: a synthesis of Leg 180 results. 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/synth/synth.htm>. [Cited YYYY-MM-DD]
2Department of Geology and Geophysics, SOEST, University of Hawaii, 1680 East-West Road, Honolulu HI 96822-2285, USA. taylorb@hawaii.edu
3Géosciences Azur, Observatoire océanologique de Villefranche s/mer, Université Pierre et Marie Curie, BP 48, 06235 Villefranche s/mer, France.
Initial
receipt: 18 July 2001
Acceptance: 7 January 2002
Web publication: 3 May 2002
Ms 180SR-150