New paleomagnetic results from basaltic drill cores of the Nauru Basin, Western Pacific: Tectonic and magnetostratigraphic implications

Available online at www.sciencedirect.com

Earth and Planetary Science Letters 267 (2008) 175 – 187 www.elsevier.com/locate/epsl

New paleomagnetic results from basaltic drill cores of the Nauru Basin, Western Pacific: Tectonic and magnetostratigraphic implications Maodu Yan a,⁎, Xixi Zhao a , Peter Riisager b a b

CSIDE/IGPP and Department of Earth and Planetary Sciences, UC Santa Cruz, CA 95064, USA Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København K, Denmark

Received 16 April 2007; received in revised form 20 November 2007; accepted 21 November 2007 Available online 8 December 2007 Editor: R.W. Carlson

Abstract The voluminous volcanic eruptions in the Nauru Basin, Western Pacific, have long been regarded as important research targets for tectonic history of the Pacific Plate and for the widespread Cretaceous volcanic activity in the Western Pacific. The Nauru Basin volcanic rocks were recovered at Site 462 by Deep Sea Drilling Project (DSDP) Legs 61 and 89, where more than 600 m of lavas and sills were drilled, thereby making it the deepest penetration into crust of Cretaceous age in the Pacific Ocean. For paleomagnetism, this section represents a unique possibility for averaging out secular variation to obtain a reliable paleolatitude estimate. However, previous paleomagnetic studies have only been subjected to alternating field (AF) demagnetization on several core samples, thus, unable to provide comprehensive understanding on the paleolatitude of the basin. The work reported here aims to determine the Cretaceous paleomagnetic paleolatitude for the Pacific Plate and define the magnetostratigraphy for the basaltic sections drilled in the Nauru Basin. A total of 391 basaltic rock samples were carefully re-sampled from DSDP Sites 462 and 462A. Stepwise thermal and AF demagnetizations have isolated characteristic components in the majority of the samples. The most important findings from this study include: (1) Two normal and one reversed polarity intervals are identified in Site 462, and six normal and six reversed polarity intervals are found in Site 462A, although possible erroneous markings of the opposite azimuth for some reversed polarity cores during the DSDP coring cannot be completely ruled out. (2) Based on previous radiometric ages, the magnetostratigraphic correlations with the Geomagnetic Polarity Time Scale (GPTS) indicate that the lower-basaltic flow unit in Site 462A began to erupt at least before 130 Ma. No correlation is available for the upper-sill unit. (3) Paleosecular variation for the lower-flow unit has been sufficiently averaged out; whereas bias may exist for that of the upper-sill unit; (4) The calculated mean inclination of ∼ −50° for the lower-flow unit yields a paleolatitude of 30.8°S for the Nauru Basin at the time of emplacement. This value is well to the north of suggested location in plate reconstruction models, suggesting that there has been a significant amount of apparent polar wander of the Nauru Basin and Pacific plate since 130 Ma. In addition, the paleolatitude for the Nauru Basin is ∼ 7° further south and the basin's age is more than 10 my older than those of the Ontong Java Plateau (OJP), which suggest that the volcanic eruptions of the lower flows in the Nauru Basin are unlikely related to the emplacement of the Ontong Java Plateau. © 2007 Elsevier B.V. All rights reserved. Keywords: paleolatitude; magnetostratigraphy; Nauru Basin; Western Pacific

1. Introduction The most voluminous large igneous province, the Ontong Java Plateau in the Western Pacific, erupted in the MidCretaceous (120 Ma) (Neal et al., 1997). The massive Ontong ⁎ Corresponding author. E-mail address: [email protected] (M. Yan). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.11.047

Java volcanism is considered as the result of a hot mantle plume. However, traditional mantle plume models fail to explain all observables and other mechanisms have been called upon (Ingle and Coffin, 2004). Many workers have even suggested that the maximum extent of Ontong Java related volcanism may go well beyond the plateau proper. The early Cretaceous (Aptian) lava flows in the Nauru, Pigeffeta, and East Mariana basins appear likely to be related closely to the plateau, and the Manihiki

176

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

Fig. 1. Bathymetry map of the Nauru Basin and the Ontong Java Plateau (OJP) in the western Pacific, showing the location of Sites 462 and 462A (colored stars) (Modified from Riisager et al. (2003a). The two sites are located at 7.2°N, 165°E. Black dots and stars are previous ODP-DSDP sites drilled at the OJP.

Plateau to the southeast is coeval with Ontong Java and may have been constructed from the same broad source (e.g., Tarduno et al., 1991; Castillo et al., 1994 Neal et al., 1997; Castillo, 2004; Tejada et al., 2004). It has been suggested that eruptions at these volcanic centers have greatly contributed to the warmer climate, higher sea level, and mass extinctions in Cretaceous (Coffin and Eldholm, 1994; Tarduno et al., 1998; Eldholm and Coffin, 2000). Knowledge of the origin and evolution of these Cretaceous intraplate volcanic events are crucial to further our understanding of mantle circulation, the emplacement of large igneous provinces, and environmental change in the past. Magnetostratigraphy is a very useful tool to test the temporal relationship between Ontong Java volcanism and volcanism in other Early Cretaceous West Pacific volcanic centers. Another motivation for the present study is ongoing discussions concerning the Pacific Apparent Polar Wander Path (APWP), which plays a central role for our understanding of hotspot fixity and true polar wander (Tarduno and Smirno, 2001, 2002; Camps et al., 2002; Torsvik et al., 2002). Although

the Pacific Plate is the largest plate on Earth, the APWP for the Pacific Plate is still poorly defined, especially during Cretaceous and Jurassic, due to difficulties in obtaining azimuthally oriented samples, relative scarcity of basalt cores, and/or lack of reliable paleomagnetic results and age constraints (e.g., Cottrell and Tarduno, 2000; Sager and Koppers, 2000; Riisager et al., 2003a; Sager, 2006). Detailed paleomagnetic studies of the Cretaceous basalts in the Western and Central Pacific are able to increase our knowledge of their paleolatitudes, origins and evolutions, and help better constrain the Cretaceous Pacific APWP. The 600+ m of Cretaceous basalts and sills recovered at Site 462 by DSDP Legs 61 and 89 represent the deepest penetration into the basaltic crust of Cretaceous age in the Pacific Ocean and thus provides a potentially idea opportunity for paleomagnetic study because secular variation may be averaged out (although as discussed later, this is a parameter we must investigate because the thickness of a lava sequence does not always correlate linearly with time). The available Ar–Ar ages for Site

Fig. 2. Plots of downhole variation in ChRM inclinations and observed polarities for Leg 61 Sites 462, 462A, and Leg 89 Site 462A (Supplementary Table 1). Polarity columns were derived based on Supplementary Table 1. Possible correlation for the upper sills between Sites 462 and 462A was made based on the thickness of the sills and the lithology units. To calculate the mean inclinations, all the positive inclinations were inversed into negative. The open circles show inclination values for individual samples, and larger open squares show the mean inclination for each inclination group of basaltic samples, with error bars denoting α95 (95% confidence). Gray dots are transitional and excursion directions, which are eliminated from mean inclination calculations. Black circles filled with pattern are volcaniclastic directions, which are also included in mean inclination calculations. Dashed lines represent possible correlations.

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

462 range from 110 to 144 Ma (e.g., Naughtom quoted by Moberly and Jenkyns, 1981; Ozima et al., 1981; Takigami et al., 1986). However, revision of the stratigraphic age strongly

177

indicates that the formation should be Aptian (Moberly et al., 1986), the same as for the Ontong Java Plateau. Previous paleomagnetic studies of the Nauru Basin only performed

178

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

sparse sampling and alternating field (AF) demagnetization on the volcanic rocks (Steiner, 1981a; Shipboard Scientific Party, 1986). These studies produced two rather different paleolatitudes for rocks in the Nauru Basin (∼20°S and ∼ 31°S, respectively, see Steiner, 1981a; Shipboard Scientific Party, 1986). The lower paleolatitude value (∼ 20°S) was favored and used to argue for an Ontong Java origin of the Nauru Basin (e.g., Steiner, 1981a; Larson et al., 1981). However, due to dearth of sampling of these flows, paleomagnetic field secular variation cannot be ruled out from these data (Steiner, 1981a; Larson et al., 1981). Hence, paleolatitude of the Nauru Basin in the Cretaceous is still poorly constrained. To test the hypothesis that the Nauru Basin was part of the OJP and to obtain a reliable paleolatitude for the basin, a denser paleomagnetic sampling with more detailed analysis is needed. In this paper we present new paleomagnetic results from such a study, which involves rock-magnetic and magnetostratigraphic studies and paleosecular variation analysis on rocks from DSDP Sites 462 and 462A. These new data is combined with previously existing paleomagnetic results, to provide a precisely

constrained paleolatitude, and magnetostratigraphy for the Cretaceous Nauru Basin. 2. Geological setting and sampling The Nauru Basin extends from about 12°N to 3°S along longitude 165°E and lies at depths greater than 4000 m (Fig. 1) (Larson and Schlanger, 1981). It is bounded by the Marshall– Gilbert Seamount chain to the northeast and the Ontong Java Plateau to the southwest. Seismic reflection data indicate that some 1,750,000 km2 of the Nauru Basin is flooded by intraplate basalts (Larson and Schlanger, 1981; Saunders, 1986; Shipley et al., 1993). The total volume of the Early Cretaceous igneous complex is estimated to be 3.3 × 106 km2 (Mochizuki et al., 2005). The exact timing of the igneous eruptions is not precisely known. The eruptions have been suggested to possibly be: (i) Late Jurassic oceanic basement (Schlanger and Jenkyns, 1976; Larson et al., 1981), or (ii) due to post-ridge-crest igneous activity (Moberly and Jenkyns, 1981), or (iii) originated from Cretaceous mantle plume (e.g., Schlanger and Moberly, 1986;

Fig. 3. Magnetostratigraphic results versus lithostratigraphic position for Site 462A in the Nauru Basin. a–d are the four possible magnetostratigraphic correlations with GPTS of Gradstein et al. (2004). GPTS, reference geomagnetic polarity time scale.

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

Castillo et al., 1991; Mahoney et al., 2003). Better age control is crucial to test these speculations. The Nauru Basin is divided into the northern and southern sub-basins. During the two DSDP drill cruises (Legs 61 and 89), two holes (Leg 61 Hole 462 at 7°14.25′N, 165°01.83′E, Legs 61 and 89 Hole 462A at 7°14.50′N, 165°01.90′E) were drilled in the northern Nauru Basin (Fig. 1) (Larson and Schlanger, 1981; Moberly et al., 1986), which are located in the oldest and deepest part of the basin between magnetic anomalies M26 and M27 (∼155 Ma) (Larson and Schlanger, 1981). More than 600 m of Cretaceous basalts and sills were recovered (Fig. 2, igneous group columns). The northern Nauru Basin is characterized by distinct seismic layering in the shallow crustal section that correlate to interbedded sills and sediments sampled in the upper 160 m of basement at Leg 61 Site 462 (Shipboard Scientific Party, 1986) (Fig. 2). Seismic investigations during cruises (Leg 61 and Leg 89) indicate no apparent tectonic tilting occurred at the Site (Larson et al., 1981; Moberly et al., 1986; Shipley et al., 1993). The 1063 m section cored at Leg 61 Hole 462A consists of upper 450 m of sedimentary units, and a lower 500 m of igneous rock, with an intervening 113 m of volcaniclastic sediments (Fig. 2). This igneous section was lithologically subdivided into an upper unit consisting basalt sills with intercalated volcanogenic sediments (Cores 15 to 41) (upper-sill unit), and a lower unit consisting of basalt sheet flows of variable thickness (Cores 44 to 89) (Larson et al., 1981) (Fig. 2) (lower-flow unit). An additional 140.5 m of cores (Cores 93-109) was obtained from the lower unit at Leg 89 Site 462A (Fig. 2), which is the deep continuum of the Leg 61 Site 462A (Moberly et al., 1986). We obtained a total of 391 basaltic core samples for this study, which include 36 azimuthally unoriented paleomagnetic core samples from Leg 61 Site 462 961-462), 239 samples from Leg 61 Site 462A (61-462A, including 9 volcaniclastic sedimentary samples), and 116 from Leg 89 Site 462A (89-462A, Supplementary Table 1). All these 2.5-cm cylindrical samples were drilled from the core sections that contained long pieces (so that there is no chance for them to freely rotate within the core liner), using a water-cooled nonmagnetic drill bit attached to the standard drill press. In addition, the uphole direction was carefully recorded on the sample by means of an orientation arrow before removal from the core section in all cases. These cores were also carefully selected so that they do not duplicate previous studied

179

cores in order to maximize the distribution of paleomagnetic samples from these sites. 3. Laboratory methods and results 3.1. Demagnetization procedures and magnetostratigraphic results Paleomagnetic analyses were carried out in the paleomagnetic laboratory at the University of California Santa Cruz. Most of the samples were subjected to detailed thermal demagnetizations in 10–100 °C temperature steps from room temperature to 585 °C or until the intensity was less than 5% of the natural remanent magnetization (NRM) (i.e., Supplementary Fig. 1a, c, e, g). Some samples were also demagnetized using stepwise AF treatment in increments of 1–5 mT to 50–60 mT (i.e., Supplementary Fig. 1b, d, f, h). All experiments were carried out in a magnetic shielded room. Thermal and AF demagnetization results are consistent in that all samples show small viscous magnetizations (Supplementary Fig. 1). After the removal of the low field viscous magnetizations, most samples show univectorial decay to the origin (e.g., Supplementary Fig. 1a, b, d, e, f); only few samples show unstable directions (Supplementary Fig. 1g, h). Based on the demagnetization data, the characteristic remanent magnetization (ChRM) directions were calculated by using principal component analysis (Kirschvink, 1980). Only those ChRM directions obtained based on at least four demagnetization steps and their maximum angular deviation (MAD) values less than 15° were used for further analysis. A total of 1 out of 36 samples for Leg 61 Site 462, 28 out of 239 for Leg 61 Site 462A, and 10 out of 116 for Leg 89 Site 462A were rejected. Representative samples of accepted and rejected samples are shown in Supplementary Fig. 1. All the obtained ChRM directions, as well as pre-existing paleomagnetic data (Steiner, 1981a; Shipboard Scientific Party, 1986), are listed in Supplementary Table 1. The obtained inclinations vary largely, and contain many high values of ∼70– 80°, as well as low values of ∼0–20° (both positive and negative). Paleomagnetic plate reconstructions based on neighboring OJP data demonstrate a southern hemisphere origin for the drill sites (i.e., Riisager et al., 2003a, 2004; Hall and Riisager, 2007), the negative inclinations thus should denote normal polarity and positive inclinations should designate reversed polarity. Three

Fig. 4. Typical thermomagnetic curves for representative samples from Leg 61 Site 462, Leg 61 Site 462A, and Leg 89 Site 462A. The directions of red arrows indicate heating curves, whereas the green arrows represent cooling curves. Sample names, i.e. 61-462A-29R1W13-15, represent the sample from Leg 61, Site 462, Core 29R, Section 1 from the working half, and interval 13–15 cm.

180

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

Fig. 5. a), Day et al. (1977)-type diagrams for basement samples from DSDP Site 462 (Table 1). Ms is saturation magnetization, Mr is saturation remanent magnetization, Hc is coercivity, and Hcr is remanent coercive force. The plot is usually divided into regions: single-domain (SD) for Mr/Ms N0.5 and Hcr/Hc b1.5, multidomain (MD) for Mr/Ms b0.05 and Hcr/Hc N4, and pseudo-single-domain in between (PSD). All samples fall within pseudo single domain. b), Diagrams of room temperature hysteresis loops for representative rock samples for the three Holes. Vertical axis is normalized magnetization with slope corrected.

polarity intervals (two normal and one reversed) are observed at Leg 61 Site 462. A total of six normal and six reversed polarity intervals recorded in Site 462A, marked as N1–6, R1–6, respectively (Fig. 3). If a single sample indicates a different polarity from that of adjacent samples (e.g., sample at 1078.25 m (94R2W14-16) in Fig. 3) we have retained it in the inclination column. However, such samples are marked as gray rectangles in the “observed polarity” column, and have not been used to define polarity intervals. During thermal demagnetization experiments, an interesting feature has been observed in four basaltic samples in Sites 61462 and 61-462A (61-462: 67R1W15-17 and 68R3W59-61; 61-462A: 29R1W13-15 and 29R7W79-81). Three magnetic components can be identified from these four basaltic samples. One component has low unblocking temerature with negative inclinations. One has intermediate to high unblocking temperature with negative inclinations. The third component has higher unblocking temperature with positive inclination and decays to the origin (i.e., Supplementary Fig. 1c). The intermediate-high temperature component is antiparallel to the low and high temperature components. This apparent reversal of remanence is not observed during any AF demagnetization experiments. This behavior has been reported previously from basalts of the OJP and other ocean drilling project samples and has sparked much discussion and concern (Mayer and Tarduno, 1993; Riisager et al., 2003a; Doubrovine and Tarduno, 2004a,b, 2006). Whether these potential self-reversals were due to ionic reordering or alteration of titanomaghemite during laboratory heating is often not clear (Doubrovine and Tarduno, 2006). For this reason, the four samples were excluded from further paleomagnetic analysis in this study. 3.2. Rock-magnetic characterization Temperature dependence of initial magnetic susceptibility was measured in argon atmosphere using a KLY-2 susceptibility meter equipped with a furnace. Hysteresis measurements were performed using a Princeton Measurements Vibrating Sample Magnetometer equipped with a cryostat at UC Santa Cruz.

Representative plots of magnetic susceptibility as function of temperature are shown in Fig. 4. Three distinctive groups of samples can be recognized from Site 462A. One group displays low Curie temperature around 150–350 °C (i.e., Fig. 4, 61-462A21R1W51-53, also see Steiner, 1981b), typical of Ti-rich titanomagnetite or hydrothermally oxidized titanomaghemite (Dunlop and Özdemir, 1997). The second group has multiple Table 1 List of rock magnetic properties of Site 462 Ms (µAm2)

Hcr (mT)

Leg 61 Site 462 60r1w69 3.365 63r3w134 2.004 67r1w15 1.390 68r1w36 0.603

9.274 7.782 6.609 7.438

12.880 7.589 18.170 10.900

Leg 61 Site 462A 18r1w98 1.997 21r1w51 1.449 21r1w94 1.726 23r2w30 1.272 29r1w13 1.097 29r1w99 0.857 29r4w79 0.844 39r5w46 2.069 42r2w69 2.401 47r1w59 2.632 47r2w14 1.374 47r3w36 3.280 48r1w115 1.244 50r3w135 1.030 53r2w95 1.712 64r5w73 1.121

7.509 7.947 7.712 8.216 7.219 7.483 7.432 8.046 8.157 7.783 7.662 8.016 6.768 7.632 7.632 8.259

Leg 89 Site 462A 97r1w142 1.227 100r2w1 0.695 103r1w101 1.319 103r1w15c 1.565 103r1w24 1.170 104r1w37 1.839 104r1w8 1.195

7.544 8.581 8.075 7.971 8.564 7.942 8.197

Samples

Mrs (µAm2)

Other details are in Figs. 4 and 5.

Hc (mT)

Mrs/Ms

Hcr/Hc

9.301 4.995 9.834 3.701

0.363 0.258 0.210 0.081

1.385 1.519 1.848 2.945

9.089 8.654 7.750 5.927 13.360 10.660 10.430 8.887 31.800 18.890 10.510 23.390 10.820 8.010 10.270 9.157

5.681 3.395 4.485 3.223 5.707 4.170 4.126 5.862 14.850 11.990 5.516 15.360 5.797 3.848 5.811 4.346

0.266 0.182 0.224 0.155 0.152 0.114 0.114 0.257 0.294 0.338 0.179 0.409 0.184 0.135 0.224 0.136

1.600 2.549 1.728 1.839 2.341 2.556 2.528 1.516 2.141 1.575 1.905 1.523 1.866 2.082 1.767 2.107

10.210 7.253 10.960 14.020 10.470 17.070 13.370

5.117 2.591 5.275 7.073 4.779 8.705 5.844

0.163 0.081 0.163 0.196 0.137 0.232 0.146

1.995 2.799 2.078 1.982 2.191 1.961 2.288

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

181

1969). Hence, it is important to ensure that paleosecular variation has been averaged out so that the site mean inclination represents a time-averaged geocentric axial dipole field. This point is particular important for flood basalt provinces with high volcanic eruption rates (e.g. Riisager et al., 2003b). To average out paleosecular variation, the sampled lava flows must be time independent and must cover a certain time span of some thousands of years (e.g., McFadden et al., 1988, 1991). A common way to define independent lava units is to use the cooling/lithology units defined by the shipboard scientific party (Shipboard Scientific Party, 1986), and to calculate the mean inclination for each unit. The values from adjacent units are compared, and considered independent if their means differ significantly at the 95% confidence level (Crow et al., 1965; Kono, 1980). If they do not pass this criterion, the means are combined (Tarduno and Cottrell, 1997; Tarduno et al., 2002). A more conservative method is first combine the observed cooling units into lava units using criteria based on physical volcanology and geochemical data (Tarduno et al., 2003); such methodology was not developed until ODP Leg 197, and therefore we do not have the data available to apply this approach. Because two successive flow units may not be separated by a significant time interval and therefore may record the same geomagnetic field, and because also the identified individual lava flows and cooling units may not be straightforward enough to be defined only based on visual inspections of the cores due to the limited recovery (Riisager et al., 2003a; Sager, 2006), inclination groups are also often defined mainly based on the paleomagnetic inclination data (e.g., Kono, 1980; Riisager et al., 2003a; Antretter et al., 2004), followed by the criterion mentioned above (Crow et al., 1965; Kono, 1980). Hence, paleomagnetic data-based inclination groups are also defined for comparison (Supplementary Table 1). Volcaniclastic samples are included in the mean inclination calculation, as their inclinations usually do not show significant difference from those above and/or below.

magnetic phases with a low Curie temperature phase around 330– 380 °C and a high Curie temperature phase around 500–550 °C (i.e., Fig. 4, 61-462A-29R1W13-15). The magnetic carriers for the former and latter phases are most likely titanomaghemites and titanomagnetites, respectively. The third group is characterized by a single ferromagnetic phase with Curie temperatures between 480° and 580 °C, compatible with that of Ti-poor titanomagnetites, or magnetites (i.e., Fig. 4, 89-462A-100R2W102-104). Directions carried by the last two groups are generally stable; whereas those carried by the first type behavior are generally unreliable. Zhao et al. (2002, 2004, 2006) also found same types of magnetic behaviors in other ODP and DSDP oceanic basalts. They suggested that samples with single phase of Ti-poor titanomagnetite are mostly likely good paleomagnetic recorders and probably have observed original and stable magnetic remanences. Magnetic hysteresis parameters are displayed in Fig. 5 and summarized in Table 1. The grain sizes of most samples analyzed in this study fall into pseudo single domain (PSD) range (Fig. 5a), probably indicating a mixture of multidomain (MD) and single domain (SD) grains. The narrow-shaped hysteresis loops (Fig. 5b) for all samples suggest more MD-like hysteresis parameters along the MD-SD trend (Gee and Kent, 1999). Room temperature magnetic susceptibility was also measured for most samples (Supplementary Table 1). In general, it shows a trend of increasing susceptibility downhole, and remains relatively high values that are constant below 1000 m below sea floor (mbsf) (Supplementary Fig. 2). This variation trend is similar to previous studies for the Hole 462 and is consistent with down-hole-ward increasing trend in grain size (Steiner, 1981a; Shipboard Scientific Party, 1986). 3.3. Paleosecular variation and inclination groups Lavas that occur in sequence usually do not provided uniformly distributed readings of the past magnetic field (Cox,

Table 2 Data of mean inclinations, secular variation parameters, paleolatitdues and paleocolatitudes Hole

N/n

Mean Inc

α95

k

S

Su

Sl

Paleolatitude (o)

Paleocolatitude (o)

Mean

Min

Max

Inclination groups defined by inclinations 61-462 10/62 61-462A-Upper-sill unit 18/144 Sill mean (Leg 61) 28/193 Flow mean (61 + 89-462A) 39/327 61-462⁎ 10/36 61-462A-Upper-sill unit⁎ 18/67 Sill Mean (Leg 61) 28/103 Lower Mean (61 + 89-462A)⁎ 39/249

− 39.5 − 47.8 − 44.9 −49.4 − 39.6 − 49.6 − 46.0 − 50.6

6.3 4.6 2.8 3.7 6.5 4.4 3.8 3.6

29.7 28.8 25.3 20.3 27.7 31.5 25.6 20.4

14.9 15.1 15.8 18.0 15.4 14.4 16.0 18.0

21.0 19.5 19.3 21.3 21.8 18.6 19.6 21.2

11.5 12.1 13.4 15.6 11.9 11.6 13.6 15.5

− 22.4 − 28.9 − 26.5 −30.3 − 22.5 − 30.4 − 27.4 − 31.3

− 18.1 − 25.2 − 23.6 −27.1 − 18.1 − 26.7 − 24.4 − 28.2

−27.2 −33.0 −29.6 −33.7 −27.5 −34.5 −30.6 −34.7

112.4 + 4.8/− 4.3 118.9 + 4.1/− 3.7 116.5 + 3.1/− 2.9 120.3 + 3.4/−3.2 112.5 + 5.0/− 4.4 120.4 + 4.1/− 3.7 117.4 + 3.2/− 3.0 121.3 + 3.4/− 3.1

Inclination groups defined by lithology groups 61-462 7/62 61-462A-Upper-sill unit 13/144 Sill mean (Leg 61) 20/193 Flow mean (61 + 89-462A) 27/327

− 37.3 − 44.8 − 42.2 −50.0

8.8 5.3 4.5 4.6

24.0 28.7 26.5 20.7

16.6 15.1 15.7 17.8

25.3 20.4 19.9 21.8

12.4 12.0 13.0 15.0

− 20.9 − 26.4 − 24.4 −30.8

− 15.2 − 22.4 − 21.1 −26.9

−27.5 −30.9 −27.9 −35.1

110.9 + 6.6/− 5.7 116.4 + 4.5/− 4.0 114.4 + 3.6/− 3.3 120.8 + 4.3/−3.9

α95 is the statistical parameters associated with the mean directions; k is the precision parameter; S is the angular standard deviation (Su for upper limit, Sl for lower limit). The final mean inclination of the Nauru Basin and other associated numbers are highlighted in bold (the mean of the lower-flow unit). Co-Lat, colatitude; STDEV, standard deviation. N, number of units; n, number of samples.

182

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

Although undoubtedly valid paleofield vectors, the transitional magnetic directions in Leg 89 Site 462A and excursions were excluded from further mean inclination, paleosecular variation, and paleolatitude analyses. We treat sample's direction as excursional if it has an inclination that deviates more than 30° from the group mean. These excluded samples have been indicated in the “inclination” column (gray circles) of Fig. 2, but not included in the “inclination group” column (i.e., samples at 604.11 mbsf depth at Leg 61 Site 462, 1013.15 mbsf at Leg 61 Site 462A, see Supplementary Table 1). We have used both lithology units and paleomagnetic inclination data to define the inclination groups. A total of seven (by lithology units) or ten (by paleomagnetic data) inclination groups are identified for Leg 61 Site 462 (Supplementary Table 1, Fig. 2). In Site 462A, we have identified forty inclination groups by lithologic method and fifty seven groups by paleomagnetic inclinations. The upper-sill unit of Site 462A

contains inclination groups 1–13 (lithology units) or paleomagnetic groups 1–18, whereas the lower-flow unit encompasses lithologic units 14–40 or paleomagnetic groups 19–57. The positive inclinations are inversed to negative inclinations to calculate the mean. The mean inclination for each group is then calculated based on the statistical procedures of McFadden and Reid (1982) using Lisa Tauxe's pmag software package (Tauxe, 1998) followed by the inclination criterion mentioned above, and is shown in Supplementary Table 1 and Fig. 2. The calculated mean inclination is ∼ 38.4° (mean of − 37.3 ± 8.8° and − 39.5 ± 6.3°, from lithology-based and paleomagnetic data-based, respectively) for Leg 61 Site 462, ∼−46.3° (mean of −44.8 ± 5.3° and −47.8 ± 4.6°) for the upper-sill unit of Leg 61 Site 462A, ∼−43.5° (mean of −42.2 ± 4.5° and −44.9 ± 2.8°) for the mean of the upper-sill unit, and ∼−49.7 (mean of −50.0 ± 4.6° and −49.4 ± 3.7°) for the lower-flow unit of Site 462A (combined from both Legs 61 and 89) (Table 2).

Fig. 6. Plot of the dispersions of the mean directions from Table 2 with data taken from a global data bank of McFadden et al. (1991) for the time intervals of 80–110 Ma and 110–195 Ma.

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

183

Fig. 7. Mean paleocolatitudes and corresponding 95% confidence interval (gray area) for the upper-sill and lower-flow units from Site 462. The paleomagnetic poles of the Pacific plate are based on a combination of different data sources, such as seamount anomaly modeling, basaltic cores, sediment rocks and magnetic lineation skewness (open squares; i.e., Sager and Pringle, 1988; Petronotis et al., 1992; Tarduno and Sager, 1995; Cottrell and Tarduno, 2003). The obtained mean paleocolatitude for the lower-flow unit of Site 462A is indistinct at the 95% confidence level from the M0-9 APWP poles (Petronotis et al., 1992), but statistically distinct from the paleocolatitude of the 120 Ma Ontong Java Plateau (Riisager et al., 2003a).

Following the work on paleosecular variation of Cox (1969) and McFadden et al. (1991), we calculate the angular standard deviation (ASD) (S in Table 2) of the inclination groups transformed into pole space following McFadden et al. (1991). We then compare with the dispersion of virtual geomagnetic poles from global igneous rocks at two windows of 80–110 Ma and 110– 195 Ma (McFadden et al., 1991) (Fig. 6). The obtained angular dispersions are indistinguishable from the predicted virtual geomagnetic pole scatter at the 110–195 Ma window (McFadden et al., 1991) for both the upper-sill unit and the lowerflow unit, but differ at the 80–110 Ma window (Fig. 6). 3.4. Paleolatitude Seismic investigations during cruises (Leg 61 and Leg 89) indicate no apparent tectonic tilting occurred in the drill sites (Larson et al., 1981; Moberly et al., 1986; Shipley et al., 1993). Thus, no tectonic correction is needed for these inclinations of the two holes, which yield a mean paleolatitude of − 26.5° for the upper-sill unit, and − 30.3° for the lower-flow unit, based on groups defined by inclination data, and −25.5° for the upper-sill unit and − 30.9° for the lower-flow unit, based on groups defined by lithology units (Table 2). The corresponding mean paleocolatitudes and their confidence limits of the above mentioned mean inclinations are listed in Table 2. The mean colatitudes of the upper-sill and lower-flow units are shown on Fig. 7.

4. Discussions 4.1. Magnetostratigraphic correlations and age of the basalts One of the most interesting problems of the Nauru Basin basalts is their age. DSDP Site 462 lies on the magnetic anomaly between M26 and M27, which corresponds to about 155 Ma (Larson et al., 1981). But the ages of the basalts were determined to be Aptian (∼ 110 Ma) from the fossils in the interbedded sediments (Larson et al., 1981; Moberly et al., 1986), and vary quite differently from radiometric dating. Radiometric dating for the upper unit sills yield ages of 110 ± 3 Ma for Sample 61-462A-32R1, 46–49 cm, 120 Ma for Sample 61-462-60R1, 65–69 cm (Ar/Ar, Ozima et al., 1981), 127 Ma for Sample 61-462A-32R1, 31–33 cm, (K/Ar, Naughtom quoted by Moberly and Jenkyns, 1981); whereas ages for the lower-flow unit display 127 ± 9 Ma for a sample from 61-462A-50 (Hart quoted by Castillo et al., 1986), 129.7 ± 4.6 Ma for Sample 89-462A-109R1, 106-108 cm (Ar/Ar, Takigami et al., 1986), 131 Ma for Sample 61-462A-50R3, 130–134 cm (Ozima et al., 1981), 134 ± 12 Ma for Sample 61462A-69R10, 50–53 cm, and 144 ± 5 Ma for Sample 61-462A50R3, 44–52 cm (K/Ar, Naughtom quoted by Moberly and Jenkyns, 1981). Given the low potassium content of these particular rocks and the comments of the analysts (Moberly and Jenkyns, 1981; Ozima et al., 1981; Takigami et al., 1986), it is difficult to tell which age of the above is more reliable or more

184

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

precise. The exact timing of the igneous eruptions obviously is still not precisely known. Magnetostratigraphic study for the lower-flow unit of Site 462A reveals 6 normal and 6 reversed polarities (N1–N6, R1– R6, Fig. 3). Because there is no other anchor age derived, based on the five radiometric age constraints of the low flows mentioned above, we can positively consider the upper long normal magnetozone (N1) to be equal to or older than the Cretaceous long normal superchron. Four more conservative correlations for the 12 normal-reversed polarity intervals with Geomagnetic Polarity Time Scale (GPTS) of Gradstein et al. (2004) can be made. These are: 1) Given the radiometric age of ∼ 129.7 Ma at bottom of Leg 89 (Takigami et al., 1986), the observed polarity N1 is best correlated to the Cretaceous long normal superchron, N3 and N4 are correlated to M1n, R1 and R2 to M0r, and R4 to M1r. Slight uncertainties exist in correlations of N5 and R5 to the GPTS. N5 is likely correlated to M3n, R5 is plausibly to M3r. No correlation is available for N2 and R3. N6 composes of only several samples with very shallow inclinations. It is not clear whether N6 represents a normal polarity interval or just an excursion. Hence, a question mark is made beside N6 (Fig. 3). The magnetostratigraphic ages for the basalts of Leg 89 range from ∼124 Ma to ∼130 Ma. Based on these magnetostratigraphic ages, the mean emplacement rate is then calculated, with value of ∼27.9 m/Ma (∼0.038 km3/yr for an area of 1.75 × 106 km2 (Shipley et al., 1993). Assuming a same emplacement rate for the entire lower unit of flows, then the upper limit age for the lower-flow unit at Leg 61 Site 462A (Core 44, at depth ∼729 mbsf, see Supplementary Table 1) should be ∼112.3 Ma. The mean age for the lower-flow unit then would be ∼121 Ma. But Peter Michael (2006, personal communication) suggested that basalts from Leg 61 Site 462A within coring interval between core sections 47R2 through 69R1 may have erupted in a very short time period, such as within a few years or decades or even less, the true upper limit and mean ages of the lower-flow unit may be older. 2) Considering a radiometric age of ∼ 131 Ma for the flow unit at Leg 61 Site 462A Core 50 (Ozima et al., 1981), N1 can be plausibly placed to M6n, R1 and R2 to M6r, N3–N4 to M7n, R4 to M7r, and N5 to M8n, R5 to M8r. No correlation is available for N2 and R3. These correlations yield magnetostratigraphic ages of ∼ 131–134 Ma for the lower-flow unit. 3) If one takes the radiometric age of ∼134 Ma (Naughtom quoted by Moberly and Jenkyns, 1981) for Sample 462A-69R10, 50–53 cm, then N1 is plausibly correlated to M10n, R1 and R2 to M10r, N3 and N4 to M11-1n, indicating magnetostratigraphic ages of ∼134–138 Ma for the low-flow unit. 4) If radiometric age of ∼144 Ma is correct for basalts Sample 61462A-50R3, 44–52 cm (K/Ar, Naughtom quoted by Moberly and Jenkyns, 1981), then N1–N4 and R1–R4 can plausibly be correlated to M19–M20 as in Fig. 3d, exhibiting magnetostratigraphic ages of ∼144–148 Ma for the lower-flow unit. Age of 127 Ma at Leg 61 Site 462A Core 50 is not used for magnetostratigraphic correlations, as the polarity interval

around 127 Ma is a reversed one in GPTS (Gradstein et al., 2004), whereas our observed polarity interval is normal (N1). No obvious correlation seems available. Although it is not clear which one of the above has better correlations, we prefer the first choice (a in Fig. 3), as age of 129.7 Ma (Takigami et al., 1986) is the only Ar–Ar date for cores from Leg 89, and the long normal interval N1 is more likely to be the Cretaceous long normal superchron. The true eruptions in the basin may have been much more complicated than these recorded in the observed magnetostratigraphic sequences. New radiometric age dating is needed for better age constraints on the volcanic eruptions in the basin. At any rate, based on the four possible correlation schemes, we can consider that the eruptions of the Nauru Basin flows began at least before 130 Ma. This magnetostratigraphically derived age is somehow more than 10 m.y. older than these of the OJP, which so far is know as erupted at ∼ 120 Ma (e.g., Tarduno et al., 1991; Parkinson and Schaefer, 2001; Chambers et al., 2002; Mahoney et al., 2002; Shipboard Scientific Party, 2002). Although we selected core samples carefully, one may still argue that only several continued positive inclinations are not enough to regard as a reversed polarity (i.e., R1, R2, R3), as it may represent an erroneous marking of the opposite azimuth during DSDP coring. In other words, the reversed polarity intervals R1–3 may be caused by misorientation during coring. Thus, N1–4, and R1–3 could potentially all belong to one normal polarity interval. R4 and R5 are likely to be true polarity intervals, as the two intervals are defined by samples that span more than 15 m long core sections, and contain more than 10 core samples. Hence, based on the above four correlation schemes, the early eruption of the Nauru basalts should have occurred before 125 Ma, which is still at least 5 Ma earlier than the eruption of the OJP. All these do not appear to support the hypothesis that the volcanic eruptions in the Nauru Basin were related to the emplacement of the OJP. Of course, one may argue that the OJP may have also erupted before 120 Ma, as the DSDP/ODP cruises have only drilled the youngest part of the OJP. Thus far there is no evidence to indicate an early eruption of the OJP before 120 Ma. Most igneous units of sills in Site 462 are normally magnetized. The geology of the Site 462 suggests eruptions of these sills may have occurred during mixed polarity intervals that predate and post-date the Cretaceous long normal interval, i.e., during Campanian (∼75 Ma) (Moberly and Jenkyns, 1981; Larson et al., 1981; Schlanger and Premoli-Silva, 1981; Schlanger and Moberly, 1986). It has been suggested that it is unlikely the eruptions occurred only in the Cretaceous long normal interval, but by mixed magnetic polarity (Moberly and Jenkyns, 1981; Larson et al., 1981). However, as no precise age constraint is available for the upper-sill unit, no magnetostratigraphic correlation is available with the GPTS (Gradstein et al., 2004). The exact timing of the eruptions of the upper unit sills is still not clear. 4.2. Mean inclinations and paleosecular variation As mentioned in Section 3.1, the ChRM directions contain many high values of ∼ 70–80°. It is uncommon to have so many

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

steep paleosecular variations. Some studies suggested that these steep inclinations may have been induced during the coring (e.g., Steiner, 1981a; Shipboard Scientific Party, 1986). However, as no obvious evidence indicates that such steep inclinations are drilling induced overprints, these steep inclinations may represent some secular variations. They were included in the mean inclination calculation and used for magnetostratigraphic correlations. The ChRM directions also contain some low inclination values (0–20°). Significant fluctuations in inclination is suggestive of secular variation—perusal of an inclination map shows that variations of ±25° are plausible (e.g., Garland, 1982). The directions apparently in the polarity transition zones were excluded in the mean inclination calculation. As mentioned, we have used two methods to define inclination groups, one based on lithology units, and another on inclination data themselves. Although the number of the inclination groups is different, the obtained mean inclinations between the two methods are statistically indistinguishable from each other (Table 2, Supplementary Table 1). The obtained dispersions of the paleosecular variations between the two methods do not show significant differences as well (Table 2).The dispersions for the upper-sill and lower-flow units by both methods are indistinguishable from the predicted virtual geomagnetic pole scatters of 110–195 Ma (Fig. 6), but are different from that of 80–110 Ma. Given magnetostratigraphic ages of N 125 Ma for the lower-flow unit, the mean inclination of ∼ − 50° well represents the time-averaged geomagnetic field of the flows. For the upper-sill units, the age of their eruptions were poorly constrained, and even well separated sills that statistically different may record the same time interval. Although the dispersion of the inclination of the upper-sill unit overlaps with that predicted by global lava flows of 110–195 Ma, whether the sills average out the secular variation is unclear. 4.3. Paleolatitudes Previous paleomagnetic studies for Site 462 revealed two groups of inclinations. One group of inclinations recovered by Leg 61 Site 462 (− 38.0°) and by Leg 89 (36°) yields a paleolatitude of ∼ 20°S (− 21.3°S (Steiner, 1981a), and 19.9 °S (Shipboard Scientific Party, 1986), respectively). Another group of inclinations recovered by Leg 61 Site 462A (− 51°) yields a paleolatitude of ∼ 31.7°S (Steiner, 1981a). Our paleomagnetic study demonstrates a mean inclination of −38.5° (mean of −37.3° and −39.5° of the two methods, Table 2) for Leg 61 Site 462 and −49.7° (mean of −50.0° and −49.4°) for Leg 61 Site 462A, corresponding to paleolatitudes of 21.7°S and 30.5°S, respectively. The two obtained paleolatitudes are consistent with previous results (Steiner, 1981a; Shipboard Scientific Party, 1986). However, the mean inclination for Leg 89 Site 462A (−41.8 and −48.5°) show much higher value than previous study (− 36°) (Shipboard Scientific Party, 1986), yielding a further south paleolatitude (∼26.7°S). We believe that the reasons for this discrepancy are that the number of samples for previous studies is small, and the inclination groups are too limited to average out the paleosecular variations, and

185

many paleomagnetic directions used in previous studies may be transitional. Combining our paleomagnetic inclinations with lithologic units and magnetostratigraphic ages, we can now begin to constrain the paleolatitude of the Nauru Basin. First, the recovered igneous rocks are lithologically divided into the upper-sill unit and the lower-lava flow unit. The two units are believed to represent at least two igneous events (Moberly and Jenkyns, 1981; Steiner, 1981a; Larson et al., 1981; Schlanger and Moberly, 1986). Therefore, the paleolatitudes for the basin should be calculated separately. A mean inclination of ∼− 50° for the lower-flow unit yields a paleolatitude of 30.8°S for the Nauru Basin before 130 Ma, which likely represents the true paleolatitude of the basin, as paleosecular variation has been averaged out; whereas a mean inclination of ∼ −43° for the upper-sill unit corresponds to a paleolatitude of 25°S for the basin, which may not represent a true paleolatitude of the basin, as the inclination dispersion may not be averaged out. The obtained paleolatitude of the lower-flow unit for the Nauru Basin is about 7° further south than that of the OJP, which is ∼ 23.8°S (Riisager et al., 2003a); whereas the present Nauru Basin lies on the east-northeast of the OJP, and the two have similar latitudes. Thus, the ∼ 7° further south in paleolatitude and more than 10 my earlier in first eruption for the Nauru Basin suggest that the volcanic activity of the lower flows in the Nauru Basin was not likely related to the emplacement of the adjacent OJP. This is further supported by geochemical evidences that the Nauru basalts were compositionally distinct from OJP basalts (Castillo et al., 1991). The corresponding paleocolatitudes for the lower-flow and upper-sill units and their 95% confidence interval are calculated and drawn on Fig. 7, together with the Pacific paleomagnetic poles. The Pacific poles are based on combined paleomagnetic data sources, such as seamount anomaly modeling, basaltic cores, sediment rocks and magnetic lineation skewness (i.e., Sager and Pringle, 1988; Petronotis et al., 1992; Tarduno and Sager, 1995; Tarduno and Cottrell, 1997; Tarduno et al., 2003; Cottrell and Tarduno, 2003). There is no significant difference between the obtained mean paleocolatitude for the lower-flow unit and M0-9 APWP pole (Petronotis et al., 1992). It may further suggest that the correlation a in Fig. 3 is most likely, which has mean age of ∼ 121 Ma. This paleocolatitude is statistically distinct from the 120 Ma Ontong Java Plateau (Riisager et al., 2003a), further suggesting a non-Ontong-Java origin of the Nauru lower-flow unit. Based on the obtained Mid-Cretaceous Nauru paleolatitude, it appears that there may have been more significant apparent polar wander during Mid-Cretaceous for the Pacific than original suggested in the plate reconstruction model (Fig. 7), in which the Pacific apparent polar wander path is believed to be standstill during the Early to Mid-Cretaceous (Cottrell and Tarduno, 2003). 5. Conclusions Basaltic samples from DSDP Sites 462 and 462A (Legs 61 and 89) in the Nauru Basin were resampled for a detailed rock

186

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187

magnetic, magnetostratigraphic, and paleolatitude study. Rock magnetic studies for these samples demonstrate three distinctive groups of magnetic behaviors, such as a single magnetic phase with low Curie temperatures at ∼ 150–350 °C, a single ferromagnetic phase with high Curie temperatures at ∼ 480– 580 °C, and multiple magnetic phases with intermediate Curie temperatures at 330–380 °C and high Curie temperatures at 500–550 °C. Stable ChRM directions are only carried by latter two groups, whereas the unstable directions are mostly displayed in the first group. The obtained ChRM directions for the upper-sill unit most denote a positive polarity interval, whereas those from the lower-flow unit reveal 6 normal and 6 reversed polarity intervals, though some reversed polarity intervals were plausibly due to inverted cores (i.e., R1, R3). The paleomagnetic results from the lower-flow unit with previous radiometric ages yield four possible magnetostratigraphic correlations, which demonstrate that the voluminous eruptions in the Nauru Basin occurred at least before 130 Ma. No specific age is derived from the upper-sill unit. The obtained mean inclination is ∼ − 42.2° for the uppersill unit and ∼ 50° for the lower-flow unit, yielding a paleolatitude of 24.4°S and 30.8°S for the basin, respectively. The paleosecular variation for the lower-flow unit seems to have been averaged out; whereas it is not clear whether the paleosecular variation for the upper-sill unit has been averaged out due to the existence of bias of inclination groups. The obtained paleolatitude for the lower-flow unit of the Nauru Basin is at least 7° further south and the age is more than 10 my older than those of the Ontong Java Plateau (OJP), which suggest that the volcanic eruptions of the lower flows in the Nauru Basin are unlikely related to the emplacement of the Ontong Java Plateau. Acknowledgements This research used samples provided by the Deep Sea Drilling Program. We thank our colleagues and collaborators whose help made this research possible. Jerry Bode and Qingsong Liu are thanked for extensive help with sampling and laboratory work, and Rob Coe is thanked for helpful discussions. We thank John Tarduno and an anonymous journal reviewer together with Editor Richard Carlson for their insightful and helpful comments and suggestions that helped to improve this manuscript. Financial support for this research was provided by grants from the U.S. Science Support Program of the Joint Oceanographic Institution, Inc., and NSF grants EAR 0207389, EAR 0310309 and OCE 0327431. Funding was also provided by the Center for the Study of Imaging and Dynamics of the Earth, Institute of Geophysics and Planetary Physics at the University of California Santa Cruz, contribution number 493. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2007.11.047.

References Antretter, M., Riisager, P., Hall, S., Zhao, X., Steinberger, B., 2004. Modelled Paleolatitudes for the Lousville Hotspot and the Ontong Java Plateau, vol. 229. Geological Society of London, pp. 21–30 (Special Publication). Camps, P., Prévot, M., Daignières, M., Machetel, P., 2002. Comment on “Stability of the Earth with respect to the spin axis for the last 130 million years" by J.A Tarduno and A.Y. Smirnov. Earth Planet. Sci. Lett. 198, 529–532. Castillo, P.R., 2004. Geochemistry of Cretaceous volcaniclastic sediments in the Nauru and East Mariana Basins provides insights into the mantle sources of giant oceanic plateaus. In: Fitton, J.G., et al. (Ed.), Origin and Evolution of the Ontong Java Plateau. Geol. Soc. Spec. Publ., vol. 229, pp. 353–368. Castillo, P., Batiza, R., Stern, R.J., 1986. Petrology and geochemistry of Nauru Basin igneous complex: large-volume, off-ridge eruptions of morb-like basalt during the Cretaceous, Leg 89. In: Moberly, R., Schlanger, S.O., et al. (Eds.), Init. Repts. DSDP, vol. 89. U.S. Govt. Printing Office, Washington, pp. 555–576. Castillo, P.R., Carlson, R.W., Batiza, R., 1991. Origin of Nauru Basin igneous complex: Sr, Nd and Pb isotope and REE constraints. Earth Planet. Sci. Lett. 103 (1–4), 200–213. Castillo, P.R., Pringle, M.S., Carlson, R.W., 1994. East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume? Earth Planet. Sci. Lett. 123 (1–3), 139–154. Chambers, L.M., Pringle, M.S., Fitton, J.G., 2002. Age and duration of magmatism on the Ontong Java Plateau: 40Ar/39Ar results from ODP Leg 192. EOS Trans. AGU 83, Fall Meet. Suppl. Abstract V71B-1271. Coffin, M.F., Eldholm, O., 1994. Large igneous provinces: crustal structure, dimensions, and external consequences. Rev. Geophys. 32, 1–36. Cottrell, R.D., Tarduno, J.A., 2000. Late Cretaceous true polar wander: not so fast. Science 288, 2283a. Cottrell, R.D., Tarduno, J.A., 2003. A Late Cretaceous pole for the Pacific plate: implications for apparent and true polar wander and the drift of hotspots. Tectonophysics 362, 321–333. Cox, A., 1969. Confidence limits for the precision parameter U. Geophys. J. R. Astron. Soc. 18, 545–549. Crow, E.L., Davis, F.A., Maxfield, M.W., 1965. Statistics Manual. New York (Dover), pp. 53–54. Day, R., Fuller, M., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain-size and compositional dependence. Phys. Earth Planet Inter. 13, 260–266. Doubrovine, P.V., Tarduno, J.A., 2004a. Self-reversed magnetization carried by titanomaghemite in oceanic basalts. Earth Planet. Sci. Lett. 222 (3–4), 959–969. Doubrovine, P.V., Tarduno, J.A., 2004b. Late Cretaceous paleolatitude of the Hawaiian Hot Spot: new paleomagnetic data from Detroit Seamount (ODP Site 883). Geochem. Geophys. Geosyst. 5 (11). doi:10.1029/2004GC000745 issn: 1525–2027. Doubrovine, P.V., Tarduno, J.A., 2006. Alteration and self-reversal in oceanic basalts. J. Geophys. Res. 111, B12S30. doi:10.1029/2006JB004468. Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism, Fundamentals and Frontiers. Cambridge Univ. Press, Cambridge, p. 573. Eldholm, O., Coffin, M.F., 2000. Large igneous provinces and plate tectonics. In: Richards, M.A., Gordon, R.G., van der Hilst, R.D. (Eds.), The History and Dynamics of Global Plate Motions. Am. Geophys. Union, Geophys. Monogr., vol. 121, pp. 309–326. Garland, G.D., 1982. Introduction to Geophysics. Sauders (Philadelphia). Gee, J., Kent, D.V., 1999. Calibration of magnetic granulometric trends in oceanic basalts. Earth Planet. Sci. Lett. 170 (4), 377–390. Gradstein, F., Ogg, J., Smith, A., 2004. A Geological Timescale. Cambridge University Press, pp. 344–383. Hall, S., Riisager, P., 2007. Palaeomagnetic palaeolatitudes of the Ontong Java Plateau from 120 to 55 Ma: implications for the apparent polar wander path of the Pacific Plate. Geophys. J. Int 169 (2), 455–470. Ingle, S., Coffin, M.F., 2004. Impact origin for the greater Ontong Java Plateau? Earth Planet. Sci. Lett. 218 (1–2), 123–134. Kirschvink, J.L., 1980. The least squares line and plane and the analysis of paleomagnetic data. Geophys. J. R. Astron. Soc. 62, 699–718.

M. Yan et al. / Earth and Planetary Science Letters 267 (2008) 175–187 Kono, M., 1980. Paleomagnetism of DSDP Leg 55 basalts and implications for the tectonics of the Pacific plate. Init. Repts. DSDP, vol. 55, pp. 737–752. Larson, R.L., Schlanger, S.O., 1981. Geological evolution of the Nauru Basin, and regional implications. Initial Rep. Deep Sea Drill. Proj., vol. 61, pp. 841–862. Larson, R.L., Schlanger, S.O., et al., 1981. Init. Repts. DSDP, vol. 61. Mahoney, J.J., Fitton, J.G., Wallace, P.J., et al. (Eds.), 2002. Proc. ODP Init. Reports, vol. 192. Mahoney, J.J., Fitton, J.G., Wallace, P.J., 2003. The Ontong Java plateau: Insights from Recent ODP Drilling. IUGG, Sapporo, Japan, p. A.211 (Abstract). Mayer, H., Tarduno, J.A., 1993. Paleomagnetic investigation of the igneous sequence, Site 807, Ontong Java Plateau, and a discussion of Pacific true polar wander. Proc. Ocean Drill. Program Sci. Results 130, 51–59. McFadden, P.L., Reid, A., 1982. Analysis of paleomagnetic inclination data. Geophys. J. R. Astron. Soc. 69, 307–319. McFadden, P.L., Merrill, R.T., McElinny, M.W., 1988. Dipole/Quadrupole family modeling of paleosecular variation. J. Geophys. Res. 93 (B10), 11,583–11,588. McFadden, P.L., Merrill, R.T., McElhinny, M.W., Lee, S.-h., 1991. Reversals of the Earth's magnetic field and temporal variations of the dynamo family. J. Geophys. Res. 96, 3923–3933. Moberly, R., Jenkyns, H.C., 1981. Cretaceous volcanogenic sediments of the Nauru Basin, Deep Sea Drilling Project Leg 61. In: Larson, R.L., Schlanger, S.O., et al. (Eds.), Init. Repts. DSDP, vol. 61. U.S. Govt. Printing Office, Washington, pp. 533–548. Moberly, R., Schlanger, S.O., et al., 1986. Init. Repts. DSDP, vol. 89. U.S. Govt. Printing Office, Washington. Mochizuki, K., Coffin, M.F., Eldholm, O., Taira, A., 2005. Massive Early Cretaceous volcanic activity in the Nauru Basin related to emplacement of the Ontong Java Plateau. Geochem. Geophys. Geosyst. 6 (10), Q10003. doi:10.1029/2004GC000867. Neal, C.R., Mahoney, J.J., Kroenke, L.W., Duncan, R.A., Petterson, M.G., 1997. The Ontong Java Plateau. In: Mahoney, J.J., Coffin, M.F. (Eds.), Large Igneous Provinces. AGU Monograph, vol. 100, pp. 183–216. Ozima, M., Saito, K., Takigami, Y., 1981. 40Ar–39Ar geochronological studies on rocks drilled at Holes 462 and 462A, Deep Sea Drilling Project Leg 61. Init. Repts. DSDP, vol. 61, pp. 701–703. Parkinson, I.J., Schaefer, B.F., 2001. ODP Leg 192 Shipboard Scientific Party, A lower mantle origin for the world's biggest LIP? A high precision Os isotope isochron from Ontong Java Plateau basalts drilled on ODP Leg 192. EOS Trans. AGU 82, Fall Meet. Suppl. Abstract V51C-1030. Petronotis, K.E., Gordon, R.G., Acton, G.D., 1992. Determining palaeomagnetic poles and anomalous skewness from marine magnetic anomaly skewness data from a single plate. Geophys. J. Int. 109, 209–224. Riisager, P., Hall, S., Antretter, M., Zhao, X., 2003a. Paleomagnetic paleolatitude of Early Cretaceous Ontong Java Plateau basalts: Implications for Pacific apparent and true polar wander. Earth Planet. Sci. Lett. 208, 235–252. Riisager, J., Riisager, P., Pedersen, A.K., 2003b. Paleomagnetism of large igneous provinces: case-study from West Greenland, North Atlantic igneous province. Earth Planet. Sci. Lett. 214 (3–4), 409–425. Riisager, P., Hall, S., Antretter, M., Zhao, X., 2004. Early Cretaceous Pacific palaeomagnetic pole from Ontong Java plateau basement rocks. In: Fitton, et al. (Ed.), Origin and Evolution of the Ontong Java Plateau, 229. Geological Society of London, pp. 31–44 (Special Publication). Sager, W.W., 2006. Cretaceous paleomagnetic apparent polar wander path for the Pacific plate calculated from Deep Sea Drilling Project and Ocean Drilling Program basalt cores. Phys. Earth Planet. Inter. 156, 329–349. Sager, W.W., Pringle, M.S., 1988. Mid-Cretaceous to early tertiary apparent polar wander path of the Pacific plate. J. Geophys. Res. 93, 11753–11771. Sager, W.W., Koppers, A.A.P., 2000. Late Cretaceous Polar Wander of the Pacific plate: evidence of a rapid true polar wander event. Science 287, 455–459. Saunders, A.D., 1986. Geochemistry of basalts from the Nauru Basin, Deep Sea Drilling Project Legs 61 and 89: implications for the origin of oceanic flood basalts. Init. Rep. DSDP, vol. 89, pp. 499–518. Schlanger, S.O., Jenkyns, H.C., 1976. Cretaceous Oceanic anoxic events: causes and consequences. Geol. En Mijnbouw, vol. 55, pp. 179–184. Schlanger, S.O., Premoli-Silva, I., 1981. Tectonic, volcanic and paleogeographic implications of redeposited reef faunas of Late Cretaceous and

187

Tertiary age, from the Nauru Basin and the Line Islands. initial Reports Deep Sea Drilling Project, vol. 61, pp. 817–827. Schlanger, S.O., Moberly, R., 1986. Sedimentary and volcanic history: East Mariana Basin and Nauru Basin. lnit. Rep. DSDP, vol. 89, pp. 653–678. Shipboard Scientific Party, 1986. Site 462. Initial Rep. Deep. Sea Drill. Proj., vol. 89. Shipboard Scientific Party, 2002. Proceedings of the Ocean Drilling Program, Initial Reports, vol. 192. Shipley, T.H., Abrams, L.J., Lancelot, Y., Larson, R.L., 1993. Late Jurassic–Early Cretaceous oceanic crust and Early Cretaceous volcanic sequences of the Nauru Basin, western Pacific. In: Pringle, M., et al. (Ed.), Mesozoic Pacific. Geophys. Monogr. Ser., vol. 77. AGU, Washington, D. C., pp. 77–101. Steiner, M.B., 1981a. Paleomagnetism of the igneous complex, Site 462. In: Larson, R.L., Schlanger, S.O., et al. (Eds.), Init. Repts. DSDP, vol. 61. U.S. Govt. Printing Office, Washington, pp. 717–729. Steiner, M.B., 1981b. Magnetic and mineralogical investigations of opaque minerals: preliminary results, Leg 61. In: Larson, R.L., Schlanger, S.O., et al. (Eds.), Init. Repts. DSDP, vol. 61. U.S. Govt. Printing Office, Washington, pp. 731–742. Takigami, Y., Amari, S., Ozima, M., Moberly, R., 1986. 40Ar/39Ar geochronological studies of basalts from Hole 462A, Nauru Basin, Deep Sea Drilling, Project Leg 89. Initial Rep. Deep Sea Drill. Proj., vol. 89, pp. 519–521. Tarduno, J.A., Sager, W.W., 1995. Polar standstill of the mid-Cretaceous Pacific Plate and its geodynamic implications. Science 269, 956–959. Tarduno, J.A., Cottrell, R.D., 1997. Paleomagnetic evidence for motion of the Hawaiian hotspot during formation of the Emperor Seamounts. Earth Planet. Sci. Lett. 153, 171–180. Tarduno, J.A., Smirno, X.V., 2001. Stability of the Earth with respect to the spin axis for the last 130 million years. Earth Planet. Sci. Lett. 184 (2), 549–553. Tarduno, J.A., Smirno, X.V., 2002. Response to comment on “Stability of the Earth with respect to the spin axis for the last 130 Million Years" by P. Camps, M. Prévot, M. Daignières and P. Machetel. Earth Planet. Sci. Lett. 198 (3–4), 533–539. Tarduno, J.A., Sliter, W.V., Kroenke, L., Leckie, M., Mayer, H., Mahoney, J.J., Musgrave, R., Storey, M., Winterer, E.L., 1991. Rapid formation of Ontong Java Plateau by Aptian mantle plume volcanism. Science 254, 399–403. Tarduno, J.A., Brinkman, D.B., Renne, P.R., Cottrell, R.D., Scher, H., Castillo, P., 1998. Evidence for extreme climatic warmth from the Late Cretaceous Arctic Vertebrates. Science 282, 2241–2244. Tarduno, J.A., Duncan, R.A., Scholl, D.W., et al., 2002. Proceedings of the Ocean Drilling Program, Initial Reports, vol. 197. Tarduno, J.A., Duncan, R.A., Scholl, D.W., Cottrell, R.D., Steinberger, B., Thordarson, T., Kerr, B.C., Neal, C.R., Frey, F.A., Torii, M., Carvallo, C., 2003. The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle. Science 301, 1064–1069. Tauxe, L., 1998. Paleomagnetic Principles and Practice. Kluwer Academic. 299 pp. Tejada, M.L.G., Mahoney, J.J., Castillo, P.R., Ingle, S.P., Sheth, H.C., Weis, D., 2004. Pin-pricking the elephant: Evidence on the origin of the Ontong Java Plateau from Pb–Sr–Hf–Nd isotopic characteristics of ODP Leg 192 basalts. In: Fitton, J.G., et al. (Ed.), Origin and Evolution of the Ontong Java Plateau. Geol. Soc. Spec. Publ., vol. 229, pp. 133–150. Torsvik, T.H., Van der Voo, R., Redfield, T.F., 2002. Relative hotspot motions versus True Polar Wander. Earth Planet. Sci. Lett. 202 (2), 185–200. Zhao, X., Antretter, M., Solheid, P., Inokuchi, H., 2002. Identifying magnetic carriers from rock magnetic characterization of Leg 183 basement cores. Proc. Ocean Drill. Program Sci. Results 183 [CD-ROM]. Available from: Texas A&M University, College Station, TX 77845-9547, USA. Zhao, X., Antretter, M., Riisager, P., Hall, S., 2004. Rock magnetic results from Ocean Drilling Program Leg 192: implications for Ontong Java Plateau emplacement and tectonics of the Pacific. In: Fitton, J.G., Mahoney, J.J., Wallace, P.J., Saunders, A.D. (Eds.), Origin and Evolution of the Ontong Java Plateau, vol. 229. Geological Society, London, pp. 45–61 (Special Publication). Zhao, X., Riisager, P., Antretter, M., Carlut, J., Lippert, P., Liu, Q., Galbrun, B., Hall, S., Delius, H., Kanamatsu, T., 2006. Unraveling the magnetic carriers of igneous cores from the Atlantic, Pacific, and the southern Indian oceans with rock magnetic characterization. Phys. Earth Planet. Inter. 156, 294–328.