September 2006 LIP of the Month

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Corresponds to event #19 in LIP record database.

The Nauru Basin

Kimihiro Mochizuki
Earthquake Research Institute, University of Tokyo
1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
kimi@eri.u-tokyo.ac.jp

Millard F. Coffin
Ocean Research Institute, University of Tokyo
1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan
mcoffin@ori.u-tokyo.ac.jp

Olav Eldholm
Dept of Earth Science, University of Bergen
Allegt. 41, Bergen N-5007, Norway

Asahiko Taira
Center for Deep Earth Exploration , Japan Agency for Marine-Earth Science and Technology
3173-25 Showa-machi, Yokosuka, Kanagawa 237-0061, Japan

Extracted from Mochizuki et al. (2005) Download pdf here.

The Nauru Basin in the western Pacific was created by seafloor spreading in late Jurassic and early Cretaceous time, as estimated from observed magnetic lineations, and was subsequently affected by Early Cretaceous flood volcanism approximately contemporaneous with emplacement of the nearby ~120 Ma Ontong Java Plateau (OJP), as revealed by geochronological and geochemical investigations of core samples from the only drill hole, DSDP Site 462 in the oldest part of the basin (~155 Ma). Although the hole penetrated 1200 m into the crust, no basalt samples of Jurassic age were obtained. To investigate the structure and stratigraphy of the basin, we completed a coincident multichannel seismic reflection (MCS) and refraction (sonobuoy) transect from the center of the Nauru Basin to the eastern flank of the OJP in 1998 aboard R/V Hakuho Maru.  Along the transect, we identify different styles of flood basalt volcanism. We estimate the volume of the thick igneous complex from discrepancies between actual seafloor depths and those predicted by an age-depth model, which commonly exceed 1000 m.  We selected an appropriate age-depth model by comparing observed seafloor depths at drill holes in the Mesozoic Pigafetta and East Mariana basins where Cretaceous igneous complexes are thin or absent. Jurassic basalt was recovered from one of the holes in the Pigafetta Basin. From an empirical density-velocity relation, we calculated a basalt loading factor of 780 m/s for subsidence of the Late Jurassic-Early Cretaceous oceanic crust.  We then applied the factor to estimate the thickness of the younger Early Cretaceous igneous complex, using previous seismic reflection (Charcot 2), drilling (DSDP Site 462), and wide-angle seismic data from DSDP Site 462 as controls.  We estimate that the igneous complex is thickest, ~5500 m, in the center of the basin (~147 Ma oceanic crust), is ~2200 m thick in the oldest (~155 Ma) northern end of the basin, and ~4300 m thick in the youngest (~135 Ma) southern end of the basin. We estimate the total volume of the Early Cretaceous igneous complex in the Nauru Basin to b 3.3 x 106 km3.

References:

Gradstein, F.M., D.P. Agterberg, J.G. Ogg, J. Hardenbol, P. van Veen, J. Thierry, and Z. Huang (1994) A Mesozoic time scale. J. Geophys. Res. 99: 24051-24074

Mochizuki, K., M. F. Coffin, O. Eldholm, and A. Taira (2005), Massive Early Cretaceous volcanic activity in the Nauru Basin related to emplacement of the Ontong Java Plateau, Geochem. Geophys. Geosyst., 6, Q10003,  doi:10.1029/2004GC000867, 19 p..

Nakanishi, M., and E.L. Winterer, (1998). Tectonic history of the Pacific-Farallon-Phoenix triple junction from Late Jurassic to Early Cretaceous: An abandoned Mesozoic spreading system in the Central Pacific Basin. J. Geophys. Res 103: 12453-12468.


Figure 1: Topography map of the western Pacific. Dark yellow lines are magnetic anomaly lineations (Nakanishi and Winterer, 1998).  Thick light yellow lines indicate magnetic anomaly M19, which encompasses the Jurassic-Cretaceous boundary (Gradstein et al., 1994).  Thick dotted light yellow lines depict M29 (~156 Ma), a part of which coincides with the northern boundary of the Nauru Basin.  Red lines are 1998 R/V Hakuho Maru cruise KH98-1 MCS profiles.  The north-south thick black line is the 1987 N/O Charcot Mesopac 1  MCS profile 2.  Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites 462, 800, 801, and 802 are shown by closed red filled circles.  The dotted square indicates the area shown in Fig. 2.  EMB: East Mairana Basin.  MGSC: Marshall-Gilbert Seamount Chain.  NB: Nauru Basin.  OJP: Ontong-Java Plateau.  PB: Pigafetta Basin.


Figure 2: Tectonic setting of the Nauru Basin.  Red lines and filled pink circles are 1998 R/V Hakuho Maru cruise KH98-1 MCS profiles and sonobuoy locations, respectively. The north-south black line is the 1987 N/O Charcot Mesopac 1  MCS profile 2. DSDP Site 462 is indicated by a filled red circle near the northern end of the Charcot 2 profile.  Mesozoic magnetic anomaly lineations are shown by yellow lines (Nakanishi and Winterer, 1998).  Bathymetric contour interval is 200 m.  Dotted rectangles indicate partitioning for volume calculations of the igneous complex (see full paper, Mochizuki et al. 2005).


Figure 3: The entire Charcot 2 MCS profile, (top) and its interpretation (bottom) (see Fig. 2 for location).  Sediment thickness varies from 0.3 to 0.5 s TWT.  The seafloor is shallowest in the center of the basin where its age is 147 Ma (magnetic lineation anomaly M21).  The basin is divided into northern and southern basins near its center on the basis of reflection character:  Prominent reflections from basaltic sills and flows are apparent in the southern basin, but are absent in the northern basin.  Thickness of the Cretaceous igneous complex is estimated from the difference between observed and estimated seafloor depths.  Thickness of expected sedimentary layers between the bottom of Cretaceous complex and the top of oceanic crust may change according to the age of the crust; thicker sediment likely accumulated in the older northern part of the basin prior to the relatively instantaneous eruption of Cretaceous volcanic rock throughout the basin.