Paleomagnetism of Igneous Rocks from the Shatsky Rise: Implications for Paleolatitude and Oceanic Plateau Volcanism

Downloaded from specialpapers.gsapubs.org on April 20, 2015

The Geological Society of America Special Paper 511 2015

Paleomagnetism of igneous rocks from the Shatsky Rise: Implications for paleolatitude and oceanic plateau volcanism

William W. Sager* Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA

Margaret Pueringer† Department of Geology & Geophysics, Texas A&M University, College Station, Texas 77843, USA

Claire Carvallo† Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Universités–Université Pierre-et-Marie-Curie (UPMC, Université Paris 06), Unités Mixtes de Recherche Centre National de la Recherche Scientiﬁ que (UMR CNRS) 7590, Muséum National d’Histoire Naturelle, Institut de Recherche (IRD) UMR 206, 4 Place Jussieu, F-75005 Paris, France

Masahiro Ooga† Department of Environmental System Science, Doshisha University, 1-3 Tatara Miyakodani, Kyo-Tanabe City, Kyoto 610-0321, Japan

Bernard Housen† Geology Department, Western Washington University, Bellingham, Washington 98225, USA

Masako Tominaga† Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA

ABSTRACT

The eruptive history of the Shatsky Rise, a large oceanic plateau in the northwestern Pacifi c Ocean, is poorly understood. Although it has been concluded that the Shatsky Rise volcanic edifi ces erupted rapidly, there are few solid chronological data to support this conclusion. Similarly, the Shatsky Rise is thought to have formed near the equator, but paleolatitude data from the plateau are few, making it diffi cult to assess its plate tectonic drift with time. To understand the formation history of this oceanic plateau, paleomagnetic measurements were conducted on a total of 362 basaltic lava samples cored from the Shatsky Rise at 4 sites (U1346, U1347, U1349, and U1350) during Integrated Ocean Drilling Program Expedition 324. Examining

*Now at Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA; [email protected]. †[email protected]; [email protected]; [email protected] (Ooga); [email protected]; [email protected]

Sager, W.W., Pueringer, M., Carvallo, C., Ooga, M., Housen, B., and Tominaga, M., 2015, Paleomagnetism of igneous rocks from the Shatsky Rise: Implications for paleolatitude and oceanic plateau volcanism, in Neal, C.R., Sager, W.W., Sano, T., and Erba, E., eds., The Origin, Evolution, and Environmental Impact of Oceanic Large Igneous Provinces: Geological Society of America Special Paper 511, p. 147–171, doi:10.1130/2015.2511(08). For permission to copy, contact [email protected]. © 2015 The Geological Society of America. All rights reserved.

147 Downloaded from specialpapers.gsapubs.org on April 20, 2015

148 Sager et al.

changes in paleomagnetic inclinations, we gain a better understanding of eruptive rates by comparison of observed shifts in inclination with expected paleosecular variation. At three sites (U1346, U1347, and U1349) little change in paleomagnetic directions was observed, implying that the cored sections were mostly erupted rapidly over periods of <~100–200 yr. Only Site U1350 displayed directional changes consistent with signifi cant paleosecular variation, implying emplacement over a period of ~1000 yr. The paleomagnetic data are consistent with the idea that the Shatsky Rise igneous sections were mostly emplaced rapidly, but there were some time gaps and some fl ank locations built up more slowly. Because paleosecular variation was inadequately sampled at all the Shatsky Rise sites, paleolatitudes have large uncertainties, and because of the equatorial location, magnetic polarity is also uncertain. All sites yield low paleolatitudes and indicate that the Shatsky Rise stayed near the equator during its formation. Given that the locus of magmatism moved northward relative to the Pacifi c plate while staying near the equator, the Pacifi c plate must have drifted southward relative to the spin axis during the emplacement of the plateau.

INTRODUCTION sive eruptions above the head of a nascent mantle plume, a thermal diapir that has risen from deep in the mantle to the base of Oceanic plateaus are massive volcanic edifi ces that represent the lithosphere (Coffi n and Eldholm, 1994). This model pre- extraordinary fl ux of magma from mantle sources (Duncan and dicts a massive initial eruption, as may have occurred with the Richards, 1991; Coffi n and Eldholm, 1994). These large igneous Tamu massif, and a waning of volcanism with time, as implied provinces give insight into mantle dynamics and secular changes. by the smaller edifi ce sizes in the northeast part of the plateau The formation and evolution of oceanic plateaus are not well (Sager, 2005). An alternative hypothesis is that plateau vol- known because many are in remote areas beneath the sea, and canism can occur in regions of plate extension, such as mid- their geologic evolution has been diffi cult to determine. Many ocean ridges, caused by decompression melting of fertile upper oceanic plateaus were formed during the Cretaceous Normal Superchron, making it diffi cult to relate their formation to the tectonics of nearby active spreading ridges (Heller et al., 1996). 40° 100 km M11 The Shatsky Rise is unique among the largest plateaus because N M115 it formed during a time of magnetic reversals, thus potentially allowing the evolution of seafl oor spreading to be inferred from Shirshov Massif M13 M10 M11 magnetic lineations. M12 The Shatsky Rise, located in the northwest Pacifi c Ocean, is M13 U1346 4 one of the largest oceanic plateaus, with an area of 4.8 × 105 km2 M15 4 M16 and a total volume of ~4.3 × 106 km3 (Sager et al., 1999). Mag- M14 Ori Massif U1349 U1350 M11 netic lineations around the Shatsky Rise (Fig. 1) indicate that it

formed during the Late Jurassic to Early Cretaceous near a triple M16 M16 M17 M16 junction (Sager et al., 1988, 1999; Nakanishi et al., 1999). It is 35° M17 M17 M11 4 composed of three large mountains, the Tamu, Ori, and Shirshov M15 M18 massifs, and a low ridge, Papanin ridge, extending to the north. U1348 M15 M19 Shatsky Rise ediﬁ ces decrease in both volume and age from M16 M20 5 southwest to northeast along the plateau (Sager et al., 1999). U1347 M14 M18 Plateau formation began with the eruption of the Tamu massif, 12131213 M17 M15 3 the southernmost of the three, and the largest and oldest volcano 4 Tamu Massif M21 (Sager et al., 2013). From an analysis of the positive magnetic 5 anomaly over the Tamu massif, it was argued (Sager and Han, M21 M18 30° 1993) that this large mountain formed rapidly during a single 155°E 160° 165° reversed polarity epoch. The other massifs and ridge formed sub- sequently along the path of the triple junction (Sager et al., 1999; Figure 1. Map showing the location of Integrated Ocean Drilling Pro- Nakanishi et al., 1999; Sager, 2005). gram Expedition 324 sites (U1346-U1350) and Ocean Drilling Pro- gram Site 1213. Bathymetry contours on the Shatsky Rise (shaded The Shatsky Rise has characteristics that ﬁ t two compet- gray) are shown at 500 m intervals. Gray lines show magnetic linea- ing hypotheses for oceanic plateau formation. One, the plume tions and fracture zones from Nakanishi et al. (1999). Filled circles head hypothesis, posits that oceanic plateaus form from mas- show the sites analyzed in this study. Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 149

mantle (Foulger, 2007). This explanation also could apply to well-known rates for the past ~5 m.y. (Johnson et al., 2008; Kent the Shatsky Rise, since it formed along the track of a triple junc- et al., 2010), with changes of ~20°–30° in magnetic inclination tion (Sager, 2005). To gain a better understanding of the volca- over periods of hundreds to thousands of years. Thus, changes nism and formation mechanism of the Shatsky Rise, Integrated in magnetization direction can be interpreted to refl ect the tim- Ocean Drilling Program (IODP) Expedition 324 cored at fi ve ing of lava fl ows. The timing of individual lava fl ows is poorly sites on the plateau to obtain volcanic material. Site U1346 was known because the most precise radiometric dates in basalts of cored on the Shirshov massif, Sites U1347 and U1348 on the Mesozoic age have an uncertainty of ~0.5 m.y., so it is diffi - Tamu massif, and Sites U1349 and U1350 on the Ori massif cult to distinguish the age variation of successive ancient fl ows (Figs. 1 and 2; Sager et al., 2010). Prior coring reached volca- drilled at a single site. Because of the relatively rapid changes in nic basement at only one location, Site 1213 on the south fl ank magnetic inclination caused by PSV, paleomagnetic measure- of the Tamu massif, where a 47 m igneous section was cored ments can be used to infer the relative timing of the individual (Shipboard Scientifi c Party, 2002). lava fl ows. The Shatsky Rise is an anomalously large volcanic Basaltic fl ows were recovered at all sites except Site U1348, construct with massive lava fl ows, thus our working hypothesis where only volcaniclastics were recovered (Sager et al., 2010, is that the cored igneous sections were emplaced rapidly. If that 2011a, 2011b). The igneous rocks are vesicular aphyric and is true, measured paleoinclinations should show limited varia- phyric basalts with mid-oceanic ridge basalt–like chemistry and tion caused by PSV. Our motivation for examining the paleo- isotopic signatures (Sager et al., 2010). These rocks are classi- latitude is that the Late Jurassic to Early Cretaceous location of fi ed as pillow basalts and massive fl ow basalts, the latter as much the Pacifi c plate at the time the Shatsky Rise formed is poorly as ~23 m thick. At Site 1213, three massive fl ows were cored known (Sager, 2006) and thus new data are important for under- (Fig. 2) (Koppers et al., 2010). An average age of 144.6 ± 0.6 Ma standing Pacifi c tectonic history. was calculated using 40Ar/39Ar radiometric dates from the lower two fl ows (Mahoney et al., 2005). Preliminary 40Ar/39Ar dates Prior Paleomagnetic Studies of the Shatsky Rise from Expedition 324 research indicate a similar age for the lower Site U1347 section (143–145 Ma below ~240 mbsf [meters A number of paleomagnetic studies have been conducted below seafl oor]), but there may be a gap of several million years on samples of Cenozoic and Late Cretaceous sediments from between the lower and upper (<220 mbsf) massive fl ows (Heaton the Shatsky Rise sediment cap (e.g., Bleil, 1985; Sager et al., and Koppers, 2014; Geldmacher et al., 2014). Preliminary dating 1993; Evans et al., 2005). Because these studies are not helpful also indicates an age that is ~10 m.y. younger for Site U1350 for understanding the formation of the igneous pedestal of the (ca. 133 Ma) (Heaton and Koppers, 2014) on the Ori massif, Shatsky Rise, we do not discuss them here. Two studies from consistent with the expected age progression from surrounding Site 1213 are informative for our study; in one, 52 samples from magnetic lineations. the 3 massive fl ows were measured using alternating fi eld (AF) Pillow lavas and massive fl ows both occur at most sites, but demagnetization and thermal (TH) demagnetization to isolate appear to show a transition from southwest to northeast on the the primary magnetization (Tominaga et al., 2005). The study Shatsky Rise. Thick massive fl ows are found only at sites on showed that many samples from the Site 1213 massive fl ows pro- the Tamu massif, where they make up a large proportion of the duced a data set of paleoinclinations with relatively high scatter. core. At Site 1213, the entire cored section consists of 3 massive Tominaga et al. (2005) surmised that the coarse-grained massive fl ows, and at Site U1347, the massive fl ows make up ~67% of fl ows contain multidomain (MD) magnetic grains that are sus- the cored section (Sager et al., 2010). At Site U1347 the massive ceptible to acquiring spurious secondary magnetizations, such as fl ows are concentrated in 2 sections where they make up ~100% a drill-string overprint (Audunsson and Levi, 1989). No statisti- of the rock (Fig. 2) (Sager et al., 2011b). Massive fl ows were cal difference was detected between the mean inclinations of the found at some of the other sites, but they are fewer and thinner three igneous units, suggesting that the emplacement occurred (Sager et al., 2010). within a short time period. The mean inclination for the site is Paleomagnetic studies indicate that the Shatsky Rise formed −9.3° +27.5°/–41.8° (Here and elsewhere, we report 95% con- near the equator (e.g., Tominaga et al., 2005; Sager et al., 2005; fi dence limits, which are asymmetric for azimuthally unoriented Sager, 2006), but Pacifi c paleomagnetic data are sparse and the samples; see Cox and Gordon, 1984) and this was interpreted as Pacifi c apparent polar wander path is poorly defi ned, especially recording eruption 4.7° north of the equator during a reversed for the Late Jurassic and Early Cretaceous (Sager, 2006). Here polarity period, possibly chron M19 on the geomagnetic polar- we examine the paleomagnetism of samples from lava fl ows ity time scale (Tominaga et al., 2005). Paleomagnetic data were cored from the Shatsky Rise to gain an appreciation of the timing also measured from Early Cretaceous sedimentary rocks depos- of the eruptions and the paleolatitude for the Shatsky Rise at the ited directly on the igneous basement at Site 1213 (Sager et al., time it formed. The basic premise of the eruption timing study is 2005). Data from 22 samples gave a mean inclination of 9.1° that changes in magnetization inclination of cored samples ver- +13.6°/–14.2°, which was also interpreted as a reversed magne- sus depth provide a record of the changes in the magnetic fi eld tization, but acquired in the Southern Hemisphere. In addition, caused by paleosecular variation (PSV). PSV occurs at relatively paleomagnetic samples were measured from an ~100 m section Downloaded from specialpapers.gsapubs.org on April 20, 2015

150 Sager et al.

North South Shirshov Ori Tamu Massif Massif Massif U1346 U1350 U1349 U1347 1213

130

140 440 150 450 160 460 170 470 180 480 190 490 200 500 210

220

230 Depth (mbsf) 240

250

260

270

280

290

300

310

320

chert & ooze/chalk limestone pillow basalt

clastic sediment sandstone basalt intercalating sediment

volcaniclastics massive basalt

Figure 2. Graphic lithology sections for Integrated Ocean Drilling Program Expedition 324 sites used in this study. Ocean Drilling Program Site 1213 is also shown (modiﬁ ed from Sager et al., 2010); mbsf—meters below seaﬂ oor. Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 151 of basalt fl ows cored from abyssal seafl oor at Site 1179, located ings (Sager et al., 2010) and are combined here with our shore- slightly north of the Shatsky Rise (Sager and Horner-Johnson, based results (Tables 1–4). 2004). The 122 measured samples describe 13 independent magnetic units that indicate a paleolatitude of 1.9°N ± 6.8°. Among Onshore Measurements the important fi ndings of these studies is the fact that the Shatsky Rise formed near the equator. Site U1349 A downhole geomagnetic study was reported from wire- Samples from Site U1349 were measured at Doshisha Uni- line logging data collected during Expedition 324 within the versity in Japan. Cubic 8 cm3 samples were cut into multiple Site U1347 borehole (Tominaga et al., 2012). A change in the specimens and demagnetized using both AF and TH demagne- sign of the vertical component of the magnetic fi eld in the lower tization, so data from the two different methods are available for part of the section was interpreted as a shift of paleoinclina- most samples. The AF samples were demagnetized with a 2G tion in surrounding lava fl ows from negative to positive in the Enterprise 755R magnetometer in steps of 5 mT from 5 to 50 mT upcore direction. This change was interpreted as the result of and steps of 20 mT up to 100 mT. Thermally demagnetized sam- the Tamu massif moving from the Southern Hemisphere (nega- ples were demagnetized with a TDS-1 (Natsuhara Giken) oven in tive inclination) to the Northern Hemisphere (positive inclina- steps of 25–30 °C up to 700 °C. The resulting inclinations from tion) (Tominaga et al., 2012). each sample set were usually similar, so in our analysis, each Several allied paleomagnetic studies have been conducted of these 62 paleocolatitudes is the average of the primary mag- using Expedition 324 samples. Three reports give magnetic prop- netizations determined from both AF and TH demagnetization erties data from Expedition 324 sites (Carvallo and Camps, 2013; of subsamples (Table 3). Of the remaining three samples treated Pueringer et al., 2013; Carvallo, 2014); another describes mag- using only a single demagnetization type, two were demagne- netic properties as background for a study of paleofi eld intensity tized using AF and the other was TH demagnetized. (Carvallo et al., 2013). These studies provide important background data to help us understand the magnetic recording process Sites U1346, U1347, and U1350 of Expedition 324 samples. A brief outline of the methods used Measurements of samples from Sites U1346, U1347, and is given in the next section and cogent results are presented in the U1350 were made in a Lodestar Magnetics shielded room at section that follows. the Pacifi c Northwest Paleomagnetism Laboratory at Western Washington University (WWU; Pueringer et al., 2013). The sam- METHODS ples were measured for magnetic susceptibility on an AGICO KLY3-S magnetic susceptibility bridge and then the NRM was Because of the international team of paleomagnetic sci- measured on an AGICO JR6 dual speed spinner magnetometer. entists working on Expedition 324, samples were measured in Half of the samples were demagnetized using TH demagnetiza- several different laboratories. Although measurements were tion and the other half using AF demagnetization. This division made from all Expedition 324 sites, this division led to slight was maintained because it was unclear after the shipboard study differences in equipment, handling, and types of measurements. which method produced better results. For samples being TH Although the data are therefore not as homogeneous as would be demagnetized, pilot batches were heated and cooled in an argon expected from a single laboratory, suffi cient data exist to under- atmosphere (so that oxidation of the samples would be mini- stand the magnetic properties of the Shatsky Rise basalt samples. mized) and a regular atmosphere, using an ASC model TD48 or an ASC model TD48-SC thermal demagnetizing oven. No sig- Expedition 324 Shipboard Measurements nifi cant difference in the results from these two techniques were detected, so the rest of the samples were TH demagnetized in air During Expedition 324, samples from every site were mea- at steps of 40 °C from 80° to 500 °C and steps of 20–25 °C above sured for paleomagnetic vector data, using both AF and TH 500 °C. The AF demagnetized samples were demagnetized with demagnetization (Sager et al., 2010). To monitor changes in mag- a D-Tech D2000 AF demagnetizer at steps of 5 mT from 5 mT netic mineralogy, bulk susceptibility measurements were made to 60–120 mT. Hysteresis loops from 4 samples from each site between steps using a Kappa Bridge KLY 4S (Geofyzika Brno). were conducted with a vibrating sample magnetometer (Princ- AF demagnetization was performed with a D-Tech demagnetizer eton Measurements Corporation MicroMag model 3900). in 2–5 mT steps up to 30 mT and 10–20 mT steps from 30 to Because there was concern that high scatter in shipboard 120 mT. TH demagnetization was performed with a Schoenstedt measurements of Site U1347 massive fl ows resulted from MD thermal demagnetizer (model TSD-1) in steps of 25–50 °C up to magnetic grain behavior, all samples from the massive fl ows 500–600 °C. After each step for both AF and TH demagnetized measured at WWU were subjected to a low-temperature (77 K) samples, the natural remanent magnetization (NRM) was mea- treatment. In a magnetically shielded room the samples were sured with a Molspin Minispin magnetometer or on a 2G cryo- immersed in liquid nitrogen for 20 min then warmed to air tem- genic magnetometer (Sager et al., 2010). The results from Sites perature and measured. This low-temperature procedure was per- U1346, U1347, and U1350 were reported in the IODP proceed- formed before the samples were demagnetized, and the process Downloaded from specialpapers.gsapubs.org on April 20, 2015

152 Sager et al.

TABLE 1. SITE U1346 PALEOMAGNETIC DATA Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC 6R-1, 83–85 139.73 19 1 AF 25–60 −24.0 8 2.8 102.6 6R-1, 111–113 140.01 20 1 TH 300–475 −26.4 6 1.7 103.9 7R-1, 53.5–55.5 142.24 22 1 TH 300–475 −20.5 6 2.4 100.6 7R-1, 92–94 142.62 23 1 TH 300–425 −9.8 4 2.3 94.9 7R-2, 39–41 143.50 23 1 AF 10–60 −19.0 8 0.8 99.8 7R-3, 49–51 145.10 24 1 TH 310–450 −18.9 4 2.4 99.7 7R-4, 12–14 145.59 26 1 TH 300–425 −25.7 4 1.5 103.5 7R-4, 53–55 146.00 26 1 AF 20–50 27.6 7 1.2 75.4* 8R-1, 136–138 149.96 31 1 TH 280–450 −24.0 5 2.5 102.6 8R-2, 11–13 150.13 31 1 TH 200–525 −18.9 9 3.0 99.7 9R-1, 104.5–106.5 155.88 32 1 TH 220–450 −21.9 6 6.1 101.4 9R-2, 95–97 155.78 32 1 TH 300–450 −13.4 5 3.3 96.8 10R-1, 21.5–23.5 158.42 34 1 AF 10–100 −20.8 11 3.6 100.8 10R-1, 40–42 158.60 34 1 AF 10–50 −26.9 9 1.0 104.0 10R-2, 75–77 158.74 35 1 TH 310–440 −26.5 4 6.4 103.8 11R-1, 35–37 163.35 36 1 TH 300–450 −17.7 6 6.2 99.1 11R-1, 97–99 163.97 37 1 AF 15–55 −16.1 9 1.8 98.2 13R-1, 5–7 172.55 37 1 AF 25–50 −5.9 6 1.7 93.0* 13R-1, 76–78 173.26 38 1 TH 300–500 −21.5 8 3.0 101.1 13R-2, 38–40 174.15 39 1 AF 10–100 −15.7 11 1.6 98.0 13R-2, 59–61 174.36 40 1 AF 20–55 −29.1 10 1.3 105.6* 14R-1, 14–16 177.54 40 2 TH 300–500 −22.3 7 1.6 101.6 14R-1, 136–138 178.76 45 2 TH 400–540 −26.2 7 4.3 104.0 14R-2, 32–34 179.16 46 2 TH 100–525 −24.1 11 1.7 102.6 14R-2, 121–123 180.05 47 2 TH 100–475 −24.3 9 2.2 102.7 15R-1, 6–8 182.26 50 2 AF 10–80 −17.2 10 0.7 98.8 16R-1, 78–80 187.78 52 2 AF 25–55 −23.1 7 1.7 102.0 16R-2, 35–37 188.31 54 2 TH 310–450 −20.5 4 0.9 100.6 16R-2, 89–91 188.85 56 2 TH 300–525 −31.8 9 3.4 107.2 Note: U1346—Integrated Ocean Drilling Program Expedition 324 site. Core/sect/int—core number, section, and interval in section (full sample designation is 324-U1346-core-section-interval); mbsf—meters below seafloor; Flow—flow number described by shipboard scientists (Sager et al., 2010); Group—statistically distinct inclination group (see Table 5); Treat—demagnetization treatment: AF—alternating field demagnetization (mT); TH—thermal demagnetization (°C); Steps—demagnetization steps used for principle component calculation; Inc—inclination of primary magnetization; N—number of demagnetization steps used to calculate mean; MAD—maximum angular deviation; VGC—virtual geomagnetic colatitude (calculated assuming normal polarity). *Indicates sample rejected for average paleocolatitude calculations.

of immersion, warming, and measuring was repeated until there (as reported in Carvallo and Camps, 2013; Carvallo et al., 2013), was little change in magnetic intensity after exposure to the liquid and with a vibrating sample magnetometer (Carvallo, 2014). nitrogen. The rationale of this treatment is that low- temperature Low-temperature measurements were made with a Quantum treatment helps remove the magnetic remanence of MD mag- Design Magnetic Properties measurement system. Susceptibility netic minerals (Dunlop and Özdemir, 2007). versus temperature measurements were made by cooling or heating samples in a CS-L cryostat and CS-3 furnace in argon atmo- Magnetic Properties Measurements sphere and measuring with a KLY-3 Kappabridge instrument. Measurements were made on samples from Sites U1346, SEM photographs were made with a SEM/FRG Zeiss Ultra 55, U1347, and U1350 using the facilities at the Université Pierre observing rock powders mounted on carbon stubs. et Marie Curie and Université de Montpellier II to understand the magnetic properties of Expedition 324 igneous samples (Car- Principle Component Analysis vallo and Camps, 2013; Carvallo et al., 2013; Carvallo, 2014). All characteristic magnetization directions were calculated The objective of most measurements was to fi nd the best samples from demagnetization data using principle component analysis for paleofi eld intensity determinations (Carvallo et al., 2013). (PCA) (Kirschvink, 1980). For each sample, a series of high- The measurements included magnetic hysteresis, fi rst-order temperature or high AF fi eld measurements that displayed uni- reversal curve (FORC) diagrams, low-temperature magnetiza- vectorial decay toward the origin of an orthogonal vector plot tion, susceptibility versus temperature curves, and scanning elec- (Zijderveld plot) was assumed to represent the primary magneti- tron microscope (SEM) photographs. Hysteresis and FORC mea- zation and used for the PCA determination. Because some sam- surements were made with an alternating gradient magnetometer ples displayed short sections of partial self-reversal at medium Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 153

TABLE 2. SITE U1347 PALEOMAGNETIC DATA Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC 12R-1, 20.5–22.5 159.90 4 1 AF 12–100 −2.3 10 4.9 91.2 12R-1, 69–71 160.29 4 1 AF 12–100 −14.2 10 1.2 97.2 12R-1, 78–80 160.38 4 1 AF 25–100 −1.4 7 3.7 90.7 12R-1, 104–106 160.64 4 1 AF 25–60 4.4 7 2.9 87.8 12R-2, 2–4 160.96 4 1 AF 20–65 3.8 10 2.9 88.1 12R-2, 9–11 161.03 4 1 AF 10–100 7.9 11 3.3 86.0 12R-2, 42–44 161.36 4 1 TH 400–600 34.0 10 4.7 71.4* 12R-2, 81–83 161.75 4 1 TH 480–550 4.6 6 4.9 87.7 13R-1, 7–9 167.07 4 2 TH 310–520 46.6 7 7.5 62.1* 13R-1, 24–26 167.24 4 2 TH 500–600 26.1 5 5.8 76.2 13R-2, 14–16 168.63 4 2 AF 20–60 15.5 8 3.5 82.1 13R-3, 23–25 170.05 4 2 TH 360–520 18.5 6 6.8 80.5 13R-4, 2–4 170.96 4 2 AF 15–120 23.0 11 2.5 78.0 13R-6, 13–15 172.95 4 2 TH 500–600 37.5 5 8.8 69.0* 13R-6, 132–136 174.14 4 2 AF 40–110 17.5 9 3.2 81.0 14R-1 51–53 177.11 5 2 AF 30–65 14.0 8 5.1 82.9 14R-1, 145–146 178.05 5 2 TH 350–600 23.6 4 0.5 77.7 14R-2, 110–112 179.17 5 2 TH 310–520 13.8 7 1.4 83.0* 15R-1, 13–15 186.33 5 2 TH 500–600 26.7 5 5.5 75.9 15R-2, 15–17 187.62 7 2 AF 20–80 23.6 10 1.2 77.6 16R-1, 21–23 195.01 9 2 TH 360–520 35.4 6 4.2 70.4* 16R-1, 89–91 195.69 9 2 TH 500–600 39.1 5 7.3 67.9* 16R-2, 14–16 196.44 9 2 AF 30–70 18.3 7 4.2 80.6 16R-5, 107–109 200.65 9 2 AF 20–100 19.3 8 1.4 80.0 16R-5, 116–118 200.74 9 2 AF 25–60 13.1 7 3.3 83.4 17R-2, 54–56 206.37 11 3 TH 425–600 15.6 8 4.5 82.1 17R-2 94–96 206.77 11 3 TH 310–520 15.4 7 3.4 82.2 17R-3, 86–88 208.15 11 3 AF 20–70 18.1 9 1.4 80.7 17R-3, 107–109 208.36 11 3 TH 500–600 16.0 5 4.7 81.8 18R-1, 44–46 214.44 11 3 AF 10–100 10.2 11 4.3 84.9 18R-3, 75–77 217.34 11 3 AF 10–100 12.9 11 1.1 83.5 18R-3, 143–145 218.01 13 3 TH 280–450 37.3 5 4.6 69.1* 18R-4, 54–56 218.58 14 3 AF 35–120 −2.1 11 1.4 91.1 18R-5, 66–68 220.12 17 3 TH 280–450 10.9 5 2.3 84.5 18R-6, 34–36 221.24 19 3 AF 30–100 12.6 10 2.2 83.6 19R-1, 32–34 223.92 20 3 AF 15–80 20.8 8 9.0 79.2 19R-1, 117–119 224.77 22 3 TH 280–450 10.9 6 4.1 84.5 19R-2, 100–102 225.88 26 3 AF 25–120 4.3 13 1.8 87.8 19R-3, 14–16 226.44 26 3 TH 280–450 11.8 5 3.0 84.0 19R-4, 8–10 227.70 29 3 TH 375–600 22.3 10 8.3 78.4 19R-4, 20–22 227.82 29 3 AF 35–100 1.2 9 1.3 89.4 20R-1, 22–24 233.42 30 3 AF 25–100 8.8 11 2.7 85.6 20R-2, 44–46 235.08 33 3 TH 280–450 25.9 5 2.1 76.4 20R-3, 5–7 235.90 33 3 AF 25–100 5.8 11 3.0 87.1 20R-3, 46–48 236.31 34 3 AF 25–100 3.8 7 3.9 88.1* 20R-3, 118–120 237.03 34 3 TH 200–600 46.3 14 6.1 62.4* 21R-3, 72–74 245.21 37 4 TH 300–500 5.9 9 5.9 87.0 21R-3, 14–16 247.39 37 4 AF 10–100 24.8 11 0.8 76.9 21R-4, 12–14 246.11 37 4 TH 310–450 33.2 4 2.6 71.9* 21R-4, 121–123 253.61 40 4 AF 12–60 24.3 8 1.2 77.3 21R-5, 65–67 247.90 41 4 AF 30–50 15.3 5 9.2 82.2 22R-1, 51–53 252.91 41 4 TH 280–450 37.1 5 4.5 69.3* 22R-2, 74–76 254.60 42 4 AF 15–45 14.8 7 1.0 82.5 22R-4, 12–14 256.84 42 4 TH 150–600 43.8 10 5.6 64.4 22R-4, 135–137 258.06 42 4 AF 10–80 24.2 10 0.9 77.3 22R-5, 9–11 258.19 42 4 AF 25–60 23.8 8 6.7 77.6 22R-5, 61–63 258.71 44 4 TH 500–550 12.6 5 9.5 83.6 22R-5, 86–88 258.96 46 4 TH 200–600 9.7 13 6.2 85.1 23R-2, 76–78 262.98 48 4 TH 425–600 15.9 8 7.0 81.9 23R-2, 134–136 263.56 48 4 TH 220–450 15.9 5 2.8 81.9 23R-3, 9–11 263.72 49 4 TH 200–600 29.3 13 3.0 74.3 23R-3, 34–36 265.38 49 4 AF 20–120 13.0 9 3.5 83.4 23R-3, 62–64 264.26 50 4 AF 30–80 3.9 11 1.7 88.1 23R-4, 103–105 266.07 53 4 TH 280–450 13.2 5 5.1 83.3 23R-6, 96–98 267.79 53 4 TH 450–600 18.7 7 5.7 80.4 24R-3, 61–63 274.70 53 4 AF 10–100 8.0 11 1.6 86.0 24R-3, 136–138 275.45 56 5 AF 40–75 0.7 8 3.0 89.6 24R-5, 41–43 277.38 57 5 AF 20–140 16.4 9 2.5 81.6 24R-5, 49–51 277.46 57 5 TH 280–450 20.6 5 6.8 79.4 24R-6, 30–32 278.45 60 5 AF 15–100 13.6 9 1.7 83.1 (Continued) Downloaded from specialpapers.gsapubs.org on April 20, 2015

154 Sager et al.

TABLE 2. SITE U1347 PALEOMAGNETIC DATA (Continued) Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC 24R-6, 69–71 278.84 60 5 AF 30–55 18.4 6 7.0 80.6 24R-6, 90–92 279.05 60 5 TH 400–525 26.2 4 4.0 76.2 24R-7, 45–47 279.81 61 5 TH 450–600 −7.1 7 0.7 93.6 24R-8, 24–26 280.64 62 5 TH 400–600 30.2 9 5.2 73.8* 24R-8, 127–129 281.67 65 5 AF 25–60 7.7 8 3.0 86.1 25R-1, 44–46 281.64 65 5 AF 15–100 13.9 9 2.9 82.9 25R-1, 84–86 282.04 65 5 AF 45–80 2.2 8 7.4 88.9 25R-2, 81–83 283.31 67 5 TH 350–575 43.2 10 8.6 64.8* 25R-3, 65–67 284.53 68 5 AF 20–80 6.2 7 4.3 86.9 25R-4, 102–104 286.40 69 5 TH 450–600 35.3 9 3.9 70.5* 25R-4, 125–127 286.63 70 5 AF 25–75 5.2 11 1.3 87.4 25R-5, 59–61 287.34 71 5 TH 300–600 48.9 13 4.6 60.2* 26R-1, 103–105 291.73 79 5 TH 310–450 9.3 4 3.3 85.3 26R-2 29–31 294.74 81 5 AF 25–65 −2.6 9 3.1 92.0 27R-2, 23–25 294.74 81 5 AF 20–80 14.1 7 5.1 82.8 27R-2, 78–80 295.29 81 5 AF 20–65 7.1 10 3.4 86.4 27R-2 8–10 295.69 81 5 TH 360–450 1.4 3 7.9 89.3 27R-3, 65–67 296.24 81 5 AF 30–50 −3.2 5 14.2 94.0 27R-5, 4–6 297.90 81 5 AF 25–65 −1.0 9 3.2 90.5 27R-6, 123–125 300.38 81 5 TH 150–325 55.6 5 7.2 53.9* 28R-2, 28–30 301.92 81 5 AF 20–45 20.9 6 7.7 79.2 28R-5, 16–18 305.61 81 5 AF 20–50 26.5 5 9.5 76.0* 29R-3, 68–70 312.89 81 5 AF 15–40 64.4 6 3.5 42.5* 29R-4, 57–59 314.22 81 5 TH 150–325 63.4 5 4.1 45.0* 29R-4, 142–144 315.07 82 5 TH 350–600 30.7 11 2.6 73.5* 29R-5, 86–88 316.01 82 5 AF 35–55 5.3 5 7.4 87.3 Note: U1347—Integrated Ocean Drilling Program Expedition 324 site. Core/sect/int—core number, section, and interval in section (full sample designation is 324-U1346-core-section-interval); mbsf—meters below seafl oor; Flow—fl ow number described by shipboard scientists (Sager et al., 2010); Group—statistically distinct inclination group (see Table 5); Treat—demagnetization treatment: AF—alternating fi eld demagnetization (mT); TH—thermal demagnetization (°C); Steps—demagnetization steps used for principle component calculation; Inc—inclination of primary magnetization; N—number of demagnetization step inclinations used to calculate mean; MAD—maximum angular deviation; VGC—virtual geomagnetic colatitude (calculated assuming normal polarity). *Indicates sample rejected for average paleocolatitude calculations.

temperature levels (250–350 °C) during TH demagnetization, tions. Of the results from Site U1347, ~24% were rejected by this only the measurements at higher temperatures were used for criterion; ~2% of the samples from Site U1350 were eliminated the PCA calculations. For most samples, we obtained >4 high for the same reason. However, no samples from Site U1346 and demagnetization measurements that could be used to defi ne the U1349 were rejected. PCA solution (Pueringer et al., 2013; Tables 1–4). PCA directions can be determined either constraining the Paleoinclination (Paleocolatitude) Analysis vector to pass through the origin of the orthogonal vector plot Azimuthally unoriented samples give a biased mean when (anchored) or with the solution unconstrained by the origin inclinations are averaged (Briden and Ward, 1966), thus requir- (unanchored). Preference between these two methods is not uni- ing the use of methodology designed to mitigate this bias. Several versal, but the two analyses give nearly identical results for well- similar techniques have been devised (Briden and Ward, 1966; behaved samples. For consistency with prior results, we retained Kono, 1980a, 1980b; Cox and Gordon, 1984; McFadden and the anchored PCA solutions determined onboard the ship. How- Reid, 1982; Arason and Levi, 2010) and they give similar results, ever, for measurements made at WWU and Doshisha University, especially in samples magnetized near the equator where the bias PCA fi ts were calculated in the unanchored mode. Although the is small. We used a routine devised by Cox and Gordon (1984) results are similar for most samples, we preferred solutions for because it is easily applied to the protocol of comparing lava fl ow unanchored results with the new data because one less assump- data used in this study. In addition, this routine has a mechanism tion is made. No signifi cant difference was observed between to estimate uncertainty of the mean direction in the absence of data produced by the two variations of PCA calculation, justify- adequate sampling of PSV. Because this method may be unfamil- ing the mixing of results. iar, a brief description is warranted. An indication of measurement repeatability is given by the Cox and Gordon (1984) argued that inclination data should maximum angular deviation (MAD), an angle calculated from be averaged using paleocolatitude (the complement angle to PCA solution eigenvalues (Kirschvink, 1980). Demagnetization paleolatitude, λ, which is related to inclination, I, by tanI = 2tanλ) data that produce large MAD values exhibit large directional because the latter has a more symmetric distribution. Thus, the scatter, so it is suspected that the average may be unreliable. In mean paleocolatitude for a magnetic unit is determined by sim- our analysis, we used MAD = 10° as an arbitrary upper limit and ple averaging of sample colatitude values. A correction is applied did not use samples with larger values for mean direction calcula- to convert the observed mean paleocolatitude to an estimate of Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 155

TABLE 3. SITE U1349 PALEOMAGNETIC DATA Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC VGC ave 7R-1, 102–104 165.12 4 1 AF 40–80 –7.4 5 1.6 89.6 7R-1, 102–104 165.12 4 1 TH 525–600 7.9 4 5.6 83.1 86.4 7R-2, 86–88 166.33 6 1 AF 50–100 –6.8 4 4.0 90.3 7R-2, 86–88 166.33 6 1 TH 225–600 2.9 16 6.2 87.8 89.1 7R-3, 46–48 167.40 8 1 AF 50–100 –4.6 4 0.7 91.7 7R-3, 46–48 167.40 8 1 TH 500–600 –6.4 5 14.0 93.4 92.6 8R-1, 77–79 169.67 12 1 AF 10–100 –8.6 12 3.3 92.4 8R-1, 77–79 169.67 12 1 TH 375–600 –9.7 10 5.7 94.9 93.7 9R-2, 90–92 175.99 16 1 AF 5–100 –4.7 13 2.1 92.4 9R-2, 90–92 175.99 16 1 TH 450–600 0.1 7 7.0 90.0 91.2 9R-3, 67–69 177.05 16 1 AF 5–100 –3.5 13 0.8 91.8 9R-3, 67–69 177.05 16 1 TH 275–600 2.4 14 2.5 88.8 90.3 10R-1, 113–115 184.43 17 1 AF 5–100 –4.2 13 0.4 92.1 10R-1, 113–115 184.43 17 1 TH 275–600 –0.2 14 3.5 90.1 91.1 10R-3, 36–38 186.23 17 1 AF 5–100 –5.7 13 1.0 92.9 10R-3, 36–38 186.23 17 1 TH 300–600 –9.5 13 3.6 94.8 93.9 10R-4, 111–113 188.49 17 1 AF 10–100 –5.3 12 0.4 92.7 10R-4, 111–113 188.49 17 1 TH 225–600 –5.8 16 2.5 92.9 92.8 10R-5, 80–82 189.55 19 1 AF 10–100 –5.1 12 0.5 92.6 10R-5, 80–82 189.55 19 1 TH 225–700 –4.7 20 2.3 92.4 92.5 11R-1, 67–69 193.57 20 1 AF 10–100 –5.5 12 0.5 92.8 11R-1, 67–69 193.57 20 1 TH 225–700 –7.2 20 2.3 93.6 93.2 11R-2, 132–134 195.71 21 1 AF 5–100 8.2 13 2.0 85.8 11R-2, 132–134 195.71 21 1 TH 375–700 0.8 14 3.5 89.6 87.7 11R-3, 85–87 196.71 22 1 AF 10–100 –5.2 12 0.8 92.6 11R-3, 85–87 196.71 22 1 TH 425–700 –7.5 12 2.0 93.8 93.2 11R-4, 20.5–22.5 197.35 22 1 AF 5–100 –3.4 13 0.4 91.7 11R-4, 20.5–22.5 197.35 22 1 TH 500–700 –7.8 9 1.0 93.9 92.8 11R-5, 66.5–68.5 199.13 22 1 AF 10–100 –0.3 12 1.1 90.2 11R-5, 66.5–68.5 199.13 22 1 TH 225–700 –3.9 20 1.9 92.0 91.1 11R-6, 51.5–53.5 200.43 22 1 AF 10–100 –4.0 12 1.3 92.0 11R-6, 51.5–53.5 200.43 22 1 TH 500–700 1.9 9 2.4 89.0 90.5 12R-1, 81–83 203.31 23 1 AF 10–100 0.6 12 1.4 90.3 12R-1, 81–83 203.31 23 1 TH 500–700 4.2 9 2.3 87.9 89.1 12R-2, 27–29 204.21 24 1 AF 10–100 –28.1 12 1.2 104.9 12R-2, 27–29 204.21 24 1 TH 475–700 –27.4 10 1.6 104.5 104.7* 12R-3, 88–90 206.25 24 1 AF 25–100 –3.5 9 0.9 91.8 12R-3, 88–90 206.25 24 1 TH 400–600 4.0 9 4.7 88.0 89.9 12R-4, 33.5–35.5 206.99 24 1 AF 25–100 –8.3 9 4.5 94.2 94.2 13R-3, 29–31 215.25 28 2 AF 5–100 –16.3 13 1.9 98.3 13R-3, 29–31 215.25 28 2 TH 400–700 –17.8 13 3.0 99.1 98.7 13R-4, 69–71 217.11 30 2 AF 5–100 –7.1 13 2.2 93.6 93.6 13R-4, 117–119 217.59 31 2 AF 5–100 –13.5 13 1.7 96.8 13R-4, 117–119 217.59 31 2 TH 225–700 –15.6 20 2.4 97.9 97.4 14R-2, 17–19 223.24 32 2 AF 5–100 –1.0 13 1.0 90.5 14R-2, 17–19 223.24 32 2 TH 225–600 –1.5 16 3.6 90.8 90.7 14R-3, 75.5–77.5 224.65 32 2 AF 10–100 –12.6 12 2.1 96.4 96.4 14R-4, 4.5–6.5 225.44 32 2 AF 5–100 –35.8 13 1.6 109.8 14R-4, 4.5–6.5 225.44 32 2 TH 300–600 –36.5 13 5.0 110.3 110.1* 14R-4, 115.5–117.5 226.55 32 2 AF 10–100 –12.3 12 2.2 96.2 14R-4, 115.5–117.5 226.55 32 2 TH 225–600 –22.1 16 3.4 101.5 98.9 14R-5, 72.5–74.5 227.46 32 2 AF 10–100 –2.0 12 2.1 91.0 14R-5, 72.5–74.5 227.46 32 2 TH 275–600 –4.6 14 3.9 92.3 91.7 14R-6, 77.5–79.5 228.93 32 2 AF Oct–80 –1.6 11 1.6 90.8 14R-6, 77.5–79.5 228.93 32 2 TH 225–600 –1.0 16 2.5 90.5 90.7 15R-1, 61–63 231.91 32 2 AF Oct–80 –3.5 11 1.4 91.8 15R-1, 61–63 231.91 32 2 TH 225–600 –3.5 16 2.2 91.8 91.8 15R-5, 40.5–42.5 236.86 32 2 AF 5–100 –28.9 13 1.0 105.4 15R-5, 40.5–42.5 236.86 32 2 TH 225–600 –29.4 16 2.4 105.7 105.6 16R-1, 103–105 241.93 32 2 AF May–80 –0.7 12 2.4 90.4 16R-1, 103–105 241.93 32 2 TH 225–600 –7.3 16 3.1 93.7 92.1 16R-3, 21.5–23.5 243.75 32 2 AF 5–100 –37.2 13 2.2 110.8 16R-3, 21.5–23.5 243.75 32 2 TH 225–600 –23.9 16 2.9 102.5 106.7 Note: U1349—Integrated Ocean Drilling Program Expedition 324 site. Core/sect/int—core number, section, and interval in section (full sample designation is 324-U1346-core-section-interval); mbsf—meters below seafloor; Flow—flow number described by shipboard scientists (Sager et al., 2010); Group—statistically distinct inclination group (see Table 5); Treat—demagnetization treatment: AF—alternating field demagnetization (mT), TH—thermal demagnetization (°C); Steps—demagnetization steps used for principle component calculation; Inc—inclination of primary magnetization; N—number of demagnetization step inclinations used to calculate mean; MAD—maximum angular deviation; VGC—virtual geomagnetic colatitude (calculated assuming normal polarity); VGC ave—average of VGC data for a single sample. *Indicates sample rejected for average paleocolatitude calculations. Downloaded from specialpapers.gsapubs.org on April 20, 2015

156 Sager et al.

TABLE 4. SITE U1350 PALEOMAGNETIC DATA Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC 7R-1, 53–55 153.13 2 1 TH 150–550 –9.5 14 2.6 87.4 7R-1, 104–106 153.64 2 1 TH 280–530 –1.1 7 2.8 87.2 7R-2, 21–23 154.25 2 1 AF 15–40 5.8 6 3.0 87.0 8R-1, 63–65 155.93 4 1 TH 400–530 –2.0 5 2.8 87.2 8R-2, 4–6 156.80 6 1 AF 25–55 –6.7 7 1.7 88.3 8R-3, 4–6 158.11 7 1 AF 25–45 9.1 5 1.9 88.1 8R-3, 85–87 158.92 7 1 AF 7–100 –1.3 12 1.6 88.4 9R-1, 58–60 162.88 9 1 TH 220–500 –8.0 8 2.8 87.2 9R-2, 69–71 164.04 9 1 TH 150–550 10.0 14 6.3 83.7 9R-3, 51–53 164.93 9 1 AF 15–60 –4.4 10 1.6 88.4 9R-4, 89–91 166.62 10 1 AF 10–100 –2.8 11 0.9 89.1 9R-5, 9–11 167.07 10 1 TH 220–500 –0.3 8 3.3 86.7 9R-6, 67–69 168.96 10 1 AF 20–60 0.5 9 8.3 81.7 10R-2, 38–40 173.73 12 1 AF 15–50 2.5 8 1.0 89.0 11R-1, 51–53 182.01 12 1 AF 10–80 0.5 10 0.6 89.4 11R-1, 128–130 182.78 13 1 TH 440–530 –3.6 4 1.8 88.2 11R-2, 14–16 183.16 14 1 AF 20–45 –2.4 6 2.6 87.4 11R-2, 77–79 183.79 14 1 TH 150–550 7.6 14 6.2 83.8 12R-1, 8–10 191.18 14 2 AF 25–50 18.3 6 2.7 87.3 12R-1, 50–52 191.60 15 2 AF 10–80 27.0 11 1.1 88.9 13R-1, 50–52 196.40 16 2 TH 150–550 43.0 14 4.9 65.0* 13R-1, 100–102 196.90 16 2 TH 310–500 –19.1 6 5.6 100.1* 13R-2, 55–57 197.88 17 3 AF 30–50 7.3 5 3.9 86.1 14R-1, 52–54 201.22 20 3 AF 35–65 2.5 7 2.0 88.0 14R-2, 9–11 201.96 20 3 AF 10–80 –1.0 10 1.3 88.7 14R-2, 21–23 202.08 20 3 TH 440–550 –1.2 7 1.0 89.0 15R-1, 53–55 206.03 25 3 AF 25–50 –3.3 6 2.7 87.3 15R-1, 101–103 206.51 25 3 TH 280–440 37.8 5 6.1 68.6* 16R-1, 35–37 210.65 27 4 TH 300–575 –2.3 12 2.0 88.0 16R-2, 5–7 211.07 27 4 TH 310–530 –6.0 7 1.9 88.1 16R-2, 72–74 211.74 27 4 AF 15–100 –3.5 10 1.8 88.2 16R-3, 105–107 213.01 28 4 AF 25–55 –2.5 7 1.7 88.3 17R-1, 138–140 221.28 30 5 TH 360–530 10.2 6 1.3 88.7 17R-2, 34–36 221.74 30 5 TH 150–575 18.3 15 3.3 86.7 17R-2, 80–82 222.2 30 5 AF 25–45 0.4 5 1.7 88.3 17R-3, 72–74 223.52 34 5 AF 10–80 3.0 11 0.7 89.3 18R-1, 72–74 230.22 36 6 TH 280–440 24.9 5 4.1 85.9 18R-1, 97–99 230.47 36 6 TH 150–575 14.8 51 3.4 86.6 18R-2, 50–52 231.44 37 6 AF 7–60 14.5 11 0.9 89.1 18R-2, 73–75 231.67 37 6 AF 30–55 11.9 6 2.2 87.8 18R-3, 12–14 232.56 37 6 TH 400–500 60.6 4 8.0 62.1* 18R-3, 60–62 233.04 37 6 TH 200–575 29.0 14 1.9 88.1 19R-1, 68–70 239.78 38 7 AF 10–60 6.3 10 0.7 89.3 19R-1, 119–121 240.29 39 7 AF 30–55 6.3 6 2.8 87.2 19R-2, 125–127 241.78 44 7 TH 280–480 16.0 6 3.4 86.6 19R-3, 37–39 242.33 44 7 AF 25–50 –8.1 6 2.4 87.6 20R-1, 31–33 249.01 46 7 TH 350–375 13.4 10 4.5 83.2* 20R-1, 65–67 249.35 46 8 AF 25–50 –8.0 6 2.0 88.0 20R-1, 95–97 249.65 47 8 TH 480–550 –11.8 6 6.3 83.7 20R-2, 67–69 250.70 49 8 TH 440–575 –5.6 6 3.3 86.7 20R-2, 132–134 251.35 50 8 AF 25–50 –6.3 6 2.5 87.5 20R-3, 4–6 251.44 50 8 AF 7–60 –11.0 11 1.4 88.6 21R-1, 23–25 258.53 51 9 TH 360–550 1.6 8 4.0 86.0 21R-1, 35–37 258.65 51 9 TH 325–575 25.1 11 3.0 76.8* 21R-1, 137–139 259.67 52 9 AF 30–50 5.0 5 1.1 88.9 21R-2, 67–69 260.47 52 9 AF 7–80 7.9 12 1.1 88.9 21R-2, 135–137 261.15 53 10 TH 440–575 16.7 6 1.2 88.8 21R-3, 48–50 261.71 53 10 TH 250–575 18.6 13 3.5 86.5 21R-3, 64–66 261.87 54 10 AF 25–45 14.1 5 1.6 88.4 22R-3, 113–115 271.30 54 10 TH 150–525 22.8 13 3.3 78.1* 22R-1, 67–69 268.47 57 11 TH 360–550 3.5 7 5.9 84.1 22R-2, 68–70 269.54 59 11 TH 360–550 1.9 7 3.2 86.8 22R-2, 97–99 269.84 59 11 AF 12–100 –0.3 11 1.9 88.1 22R-4, 87–89 272.43 65 11 TH 360–520 4.1 6 1.6 88.4 22R-5, 71–73 273.69 69 11 AF 15–55 –0.5 9 1.6 88.4 22R-5, 135–137 274.33 70 11 TH 360–540 –11.8 8 1.2 95.9* 23R-1, 22–24 277.62 71 11 AF 25–60 2.4 8 2.8 87.2 23R-1, 78–80 278.18 72 11 AF 15–80 0.4 9 2.1 87.9 23R-2, 49–51 279.31 74 11 TH 425–575 3.6 7 3.8 86.2 23R-3, 125–127 281.43 78 12 AF 25–50 –5.1 6 1.7 88.3 23R-4, 16–18 281.84 79 12 TH 360–520 –9.3 6 1.4 88.6 23R-4, 92–94 282.60 81 12 AF 10–100 –2.5 12 3.3 86.7 (Continued) Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 157

TABLE 4. SITE U1350 PALEOMAGNETIC DATA (Continued) Depth Inc MAD Core/sect/int (mbsf) Flow Group Treat Steps (°) N (°) VGC 23R-5, 6–8 283.13 81 12 AF 40–65 –3.6 6 3.4 86.6 24R-1, 42–44 287.42 84 12 TH 400–520 –10.0 5 1.2 88.8 24R-1, 109–111 288.09 85 12 TH 150–550 –4.1 14 1.7 88.3 24R-2, 137–139 289.85 89 12 AF 30–50 –7.5 5 1.9 88.1 24R-3, 31–33 290.18 90 12 AF 12–80 3.8 10 1.0 88.1* 25R-1, 24–26 296.94 92 13 TH 325–550 –1.2 10 2.1 87.9 25R-1, 102–104 297.72 94 13 AF 25–50 –3.5 6 3.1 86.9 25R-1, 112–114 297.82 94 13 TH 440–540 –14.2 6 3.6 86.4 25R-2, 60–62 298.80 100 13 AF 25–45 –6.3 5 1.6 88.4 25R-2, 119–121 299.39 101 13 AF 12–80 –12.0 10 1.1 88.9 25R-3, 83–85 300.53 107 13 TH 360–520 –0.4 6 1.5 88.5 25R-4, 71–73 301.75 114 13 TH 150–550 4.9 14 2.6 87.4 25R-5, 57–59 302.88 119 13 AF 10–80 –7.5 11 0.8 89.2 25R-5, 92–93 303.24 120 13 AF 15–45 –3.1 7 1.8 88.2 25R-6, 31–33 303.95 124 13 TH 360–520 1.1 6 1.4 88.6 25R-7, 13–15 304.92 130 13 TH 150–550 –1.0 14 1.6 88.4 25R-7, 91–93 305.69 133 13 AF 20–45 –1.8 6 1.3 88.7 25R-8, 13–15 306.31 138 13 TH 400–520 –4.8 5 2.4 87.6 26R-1, 28–30 306.48 141 13 TH 360–520 –13.2 6 3.0 87.0 26R-1, 78–80 306.98 142 13 AF 10–80 –3.7 10 1.2 88.8 26R-2, 40–42 308.07 145 13 AF 20–45 0 6 2.2 87.8 26R-4, 27–29 310.54 156 14 TH 150–550 4.7 14 1.9 88.1 26R-6, 12–14 313.11 170 14 AF 25–45 –1.5 5 1.4 88.6 26R-7, 13–15 314.35 176 14 AF 10–45 –0.8 8 1.2 88.8 26R-8, 11–13 315.40 181 14 TH 150–550 4.2 14 3.3 86.7 26R-8, 43–45 315.72 183 14 AF 15–45 –2.2 7 0.7 89.3 Note: U1350—Integrated Ocean Drilling Program Expedition 324 site. Core/sect/int—core number, section, and interval in section (full sample designation is 324-U1346-core-section-interval); mbsf—meters below seafl oor; Flow—fl ow number described by shipboard scientists (Sager et al., 2010); Group—statistically distinct inclination group (see Table 5); Treat—demagnetization treatment: AF—alternating fi eld demagnetization (mT), TH—thermal demagnetization (°C); Steps—demagnetization steps used for principle component calculation; Inc—inclination of primary magnetization; N—number of demagnetization step inclinations used to calculate mean; MAD—maximum angular deviation; VGC—virtual geomagnetic colatitude (calculated assuming normal polarity). *Indicates sample rejected for average paleocolatitude calculations.

the true value using a lookup table derived from numerical inte- analysis of those holes. Because the correction for inclination gration of equations for the expected paleocolatitude (Cox and bias varies with latitude, uncertainty estimates become asym- Gordon, 1984). For the Shatsky Rise samples, these corrections metric when the inclination bias correction is applied. Thus 95% were only 0.0°–0.1° because of the low paleolatitude. Cox and confi dence limits given in this study have asymmetric values for Gordon (1984) also considered the propagation of uncertainties lower and upper bounds. to determine confi dence intervals for averaged paleocolatitudes. At the heart of the inclination analysis is the distinction of Although the observed scatter of colatitude values refl ects the independent magnetic units. If the paleocolatitude averages of measurement uncertainty, if PSV is not properly sampled, the two lava fl ows are nearly the same, it is suspected that they were standard error of the mean is likely too small. Thus, Cox and Gor- erupted close together in time. If the averages are greatly differ- don (1984) substituted a model value of the colatitude variance ent, they may have erupted after a signifi cant time interval. It is caused by PSV (Harrison, 1980) when the observed and model appropriate to combine the two units erupted close in time, but values of this variance are signifi cantly different. This may seem treat the units with greatly different means as independent sam- counterintuitive (i.e., to substitute a model value for the observed ples of the paleofi eld direction. To determine whether the means scatter), but this method gives more realistic estimates of uncer- of two paleocolatitude groups are statistically distinct (i.e., their tainty, especially if the number of independent magnetic units means are not equal with 95% confi dence), we used the Z sta- 2 2 1/2 is small. The overall uncertainty estimate for a mean site paleo- tistic of Kono (1980a, 1980b), where Z = (I2-I1)/(s2 /n1 – s1 /n1) colatitude combines the variance from this model along with an and I1, I2 are the two inclination means, s1 and s2 are the standard estimate of the between-unit standard error (i.e., measurement deviations of the two groups, and n1 and n2 are the numbers of uncertainty) and an estimate of systematic error caused by bore- samples in the two groups. If Z > 1.96, the two group means hole tilt (Cox and Gordon, 1984). Most other methods do not treat are distinct. In our analysis, we compared the means of adjacent the systematic error caused by borehole tilt, and this quantity is lava fl ows, treating them as distinct units when appropriate and not always measured. Cox and Gordon (1984) assumed a value combining data from adjacent units if not. At the beginning, we of 2° in the absence of borehole tilt data, and we followed this grouped data by the lava fl ow units defi ned by Expedition 324 suggestion when no such data were available. Wireline logging shipboard scientists (Sager et al., 2010; Tables 1–4). Especially data from Expedition 324 defi ned the tilt of two holes (U1347, in the sections of thin pillow fl ows, it was common to have a 2.8°; U1349, 1.2°), so these measured values were used for the low number of samples in a given fl ow, so we also looked for Downloaded from specialpapers.gsapubs.org on April 20, 2015

158 Sager et al.

groupings of similar paleocolatitudes to indicate where magnetic oxides of different compositions. Consistent with this observa- unit boundaries occur. Often the average of a group of paleoco- tion, at all sites, electron dispersive spectra measurements made latitudes contains data from several adjacent lava fl ows. with the electron microscope show a wide variety in titanium Even though two magnetic units are statistically distinct, content for the magnetic grains. they may not be independent in the strictest sense. Cox and Gor- Low- and high-temperature heating and cooling measure- don (1984) recommended a conservative approach when deter- ments indicate a variety of behaviors, from reversible to highly mining a site average, combining adjacent magnetic units if the irreversible. Curie temperatures of magnetic grains range from colatitude difference is not ~10°. Often adjacent units may be low (~160 °C) to high (550–575 °C), but the most common values statistically distinct, but are only separated by a few degrees. are 400–500 °C. This variability suggests that the samples con- Because this approach yields a more conservative estimate of tain a complex mixture of magnetic grains and phases. Changes uncertainty, we applied it in this study. in magnetization with temperature are consistent with titano- When paleocolatitude data were plotted, it was clear that magnetite with varying titanium content and variable oxidation some values clustered but others appeared to be outliers. We states (Carvallo and Camps, 2013; Carvallo et al., 2013). Suscep- applied several criteria to determine which values are unreliable tibility and temperature measurements for Sites U1347, U1349, and did not use these outliers for the calculation of averages. All and U1350 show some samples with two different Curie tem- samples with MAD > 10° were rejected because these samples peratures, indicating two magnetic phases. After heating, often have low internal consistency in demagnetization steps. We also only the higher temperature component survived, implying that considered a sample to be an outlier if its colatitude is displaced the lower Curie temperature component (titanium rich) inverted from the group mean more than two standard deviations. After to a higher Curie temperature (low titanium) mineral (Carvallo this fi ltering, it was noted that the distribution of samples around and Camps, 2013). Similarly, it was also noted that some basalt the mean in some groups (all at Site U1347) showed a tail of low fl ow samples showed an increase of magnetic susceptibility at colatitude values (corresponding to higher inclinations). Because higher TH demagnetization steps (Sager et al., 2010), a behav- most samples were affected by a steep, downward overprint ior implying the inversion of titanomagnetite to magnetite dur- (probably from the drill string), it is likely that in these samples ing heating (Özdemir and O’Reilly, 1982). This transformation the overprint was incompletely removed. In response, we also did not appear to change magnetization direction, so the newly rejected samples whose magnetization vector did not stop shal- formed magnetite appears to inherit the magnetization direction lowing during the demagnetization process. There were a num- of the original magnetic minerals or the new magnetite has no net ber of samples from Site U1347 in which the differences at high direction (Carvallo et al., 2013; Pueringer et al., 2013). demagnetization steps were small, but the vector was nonetheless Magnetic grain sizes determined from hysteresis and FORC shallower at each successive step. Almost all of these samples measurements indicate that most samples have grains that gave low paleocolatitude values and their removal resulted in a respond as single domain (SD) or pseudosingle domain (PSD) distribution that was more symmetric around the mean value. types (Pueringer et al., 2013; Carvallo, 2014), consistent with thin-section observations of small and broken iron oxide grains RESULTS (Carvallo and Camps, 2013; Carvallo et al., 2013). Hysteresis measurements of samples from Sites U1346, U1347, and U1350 Magnetic Properties are mostly in the SD-PSD range (Fig. 3), implying grain types that usually record the paleomagnetic fi eld reliably (e.g., Butler, Basalt samples from Expedition 324 cores show magnetic 1992). Most samples from Sites U1346 and U1350 plot along properties indicating that they contain a diverse assemblage of an SD-MD mixing curve (Dunlop, 2002a, 2002b) at values of mainly titanomagnetite and titanomaghemite grains of varying 20%–60% MD grains (Fig. 3). Site U1347 results show the most size and behavior. There were indications from thin sections variation, plotting along the same mixing curve from 0%–80% (Sager et al., 2010) and magnetic properties that some samples MD (Fig. 3). In particular, most of the samples that exhibit more from Site U1349 contain hematite. Despite this diversity, most MD behavior are from the massive fl ows, whereas those on the sample properties are consistent with those of seafl oor basalts SD end of the distribution are from the fi ne-grained pillow fl ows. that reliably record the paleomagnetic fi eld. Moreover, immersion of samples in liquid nitrogen (a procedure Magnetic grains observed by SEM in thin section range to demagnetize MD grains) showed little effect on samples from from 1 to 2 µm to several tens of microns across, but in samples Sites U1346 and U1350 and the pillow fl ows of Site U1347. Only from Sites U1347 and U1350 the larger grains have been subdi- some samples from the massive fl ows of Site U1347 displayed vided by alteration and breakage into micron-scale grains (Car- signifi cant changes in magnetization with this treatment (Puer- vallo and Camps, 2013). Samples from Sites U1347 and U1350 inger et al., 2013). These results are consistent with most Expedi- contain skeletal grains typical of alteration, whereas Site U1349 tion 324 samples having SD-PSD behavior, but the massive fl ow samples contain mostly unbroken and little altered grains. Sam- samples of Site U1347 may exhibit MD behavior that can include ples from Sites U1347 and U1350 had iron oxide grains with having an unstable magnetization prone to acquiring an overprint oxy- exsolution patterns (Carvallo and Camps, 2013), implying (e.g., Butler, 1992). Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 159

SD U1346 SD U1347 0.5 SD 0.5 SD pillows 10 10 upper massive 0.4 0.4 lower massive 20 20

30 30 PSD PSD 0.3 0.3 40 s

s 40 /M /M 50

50 rs rs M M 0.2 60 0.2 60 70 70 80 80 0.1 0.1 90 90

MD+SPM MD+SPM 0.0 0.0 1 234 1234H /H Hcr/Hc cr c

SD U1350 0.5 SD

0.4 20 Figure 3. Plots of magnetic hysteresis parameters, Mrs/Ms versus H /H (Day plot), for samples from Integrated Ocean Drilling Pro- 30 cr c PSD gram Expedition Sites U1346, U1347, and U1350. Data from Site 0.3 40 U1347 are labeled with different symbols for samples from the pil- s low units, upper massive ﬂ ows, and lower massive ﬂ ows. Dashed

/M 50 lines divide the plot into regions characteristic of single domain rs (SD), pseudosingle domain (PSD), and multidomain (MD) grains M 0.2 60 (Day et al., 1977). Gray zone shows range of modeled hysteresis 70 parameters for a mixture of SD and MD grains (Dunlop, 2002a, 2002b) with dots and numbers indicating percentage of MD grains 80 in the mixture. SPM—superparamagnetic. 0.1 90

MD+SPM 0.0 1234 Hcr/Hc

Colatitude (Inclination) Data all sites displayed a downward-directed, drill-string overprint (e.g., Audunsson and Levi, 1989), but this bias was typically Despite the variety of magnetic properties, most samples removed by demagnetization at 10–15 mT for AF-treated yielded consistent demagnetization results with univectorial samples and 150–250 °C for TH-demagnetized samples (Figs. decay at higher steps (Figs. 4–8), implying that these samples 4–8). As shown in Figure 6, for some Site U1347 samples it reliably record a primary magnetization corresponding to the was impossible to remove the drill-string overprint, presum- paleomagnetic fi eld direction. This is especially true for Sites ably because these samples have larger magnetic grain sizes U1346, U1349, and U1350, for which there were few data that are less stable, so they were more completely remagne- rejected as unreliable or outliers. Data from Site U1347 dis- tized by exposure to the drill-string magnetic fi eld. Both AF and played more inconsistency, particularly in the massive fl ow TH demagnetization gave consistent results for most samples, units, requiring more scrutiny and analysis. Most samples at although results were commonly inconsistent for the massive Downloaded from specialpapers.gsapubs.org on April 20, 2015

160 Sager et al.

U1346A-11R-1, 97-99 cm N U1346A-16R-2, 35-37 cm ABN,Up ABN, Up E 500 mA/m 1000 mA/m

NRM

W E W E vertical E horizontal vertical S C 1.0 horizontal 0.8 C 1.0 S,Down 0.6 0.8 0.4 0.6 0.2 0.4 0.0 0.2 0102030405060 AF (mT) 0.0 S, Down 0 100 200 300 400 500 T (°C) Figure 4. Example AF (alternating ﬁ eld) and thermal demagnetization results from Integrated Ocean Drilling Program Expedition Site U1346. (A) An orthogonal vector (Zijderveld) plot. (B) A section of an equal angle projection of magnetization vector end points. NRM— natural remanent magnetization. (C) A plot of magnetization intensity versus demagnetization step. On right, sample U1346A-16R-2, 35–37 cm, shows a small segment of partial self-reversal in the demagnetization path at ~200–250 °C.

fl ow samples of Site U1347. The lowermost massive fl ow at Sites U1346, U1347, and U1350 showed a small segment of Site U1347 was particularly troublesome and many samples self-reversal behavior at ~350–350 °C during TH demagneti- gave poor results (Table 2). zation (Fig. 4; Pueringer et al., 2013). This type of behavior is AF demagnetized samples often displayed low (<10 mT) not uncommon in seafl oor basaltic fl ows (e.g., Doubrovine and median destructive fi eld (MDF) values, although some samples Tarduno, 2004, 2005). The reversed directions were removed had higher MDF. Despite the low MDF, most AF-demagnetized by higher temperature demagnetization and we were able to use samples were characterized by univectorial decay once the over- these higher temperature steps to determine the characteristic print was removed (Pueringer et al., 2013). Some samples from magnetization direction.

U1347A-12R-2, 81-82 cm U1347A-15R-2, 15-17 cm N N ABN,Up ABN,Up 2 A/m

W E W E

vertical NRM horizontal NRM vertical E horizontal W

2 A/m S,Down C 1.0 S,Down C 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0 100 200 300 400 500 600 020406080 T (°C) AF (mT) Figure 5. Example AF (alternating ﬁ eld) and thermal demagnetization results for pillow lava samples from Integrated Ocean Drilling Program Expedition Site U1347. NRM—natural remanent magnetization. Plot conventions as in Figure 3. Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 161

NRM U1347A-27R-5, 4-6 cm N U1347A-28R-1, 8-10 cm W AB ABN,Up N,Up 3 A/m W E 2 A/m W E vertical vertical horizontal horizontal C 1.0 W 0.8 NRM S 1.0 0.6 C0.8 0.4 0.6 0.2 S,Down 0.0 0.4 0 100 200 300 400 500 S,Down 0.2 T (°C) 0.0 0 10203040506070 AF (mT)

Figure 6. Example AF (alternating fi eld) and thermal demagnetization results for massive lava fl ow samples from Integrated Ocean Drilling Program Expedition Site U1347. NRM—natural remanent magnetization. Sample U1347A-27R-5, 4-6 cm produced a principle component analysis vector solution that is interpreted as reliable. Sample U1347A-28R-1, 8-10 cm displays a steep overprint that was not removed by demagnetization, which is common for coarse-grained massive lava fl ow samples. Plot conventions as in Figure 3.

Site 1346 sis showed that only two groups are statistically distinct and the Sample measurements from Site U1346 display a narrow difference in average colatitude is small, 1.8° (Fig. 9; Table 5). range of small negative inclination values (Table 1). Because This small difference implies little change in the magnetic fi eld we do not initially know the magnetic polarity, we assume that direction between fl ow groups, so it is unlikely that signifi cant the measured samples record normal magnetic polarity. This time passed between these eruptions. By the rationale described results in colatitude values ranging from ~95° to 105° for Site in Cox and Gordon (1984), we combine the two groups for a U1346 (Fig. 9). No change in the sign of the paleoinclinations is single average value. The average colatitude for the site is 100.3° observed, so all of the samples record the same polarity. Analy- with a 95% confi dence of +20.7°/–20.1° (Table 5).

U1349A-9R-2, 90-92 cm E U1349A-11R-3, 85-87 cm E ABW,Up ABW,Up

NRM S N S N

500 mA/m 200 mA/m NRM

vertical vertical horizontal horizontal

S S 1.0 C 1.0 0.8 C0.8 0.6 0.6 0.4 E,Down 0.4 0.2 0.2 0.0 E,Down 0 20 40 60 80 100 0.0 0 100 300 500 AF (mT) T (°C)

Figure 7. Example AF (alternating ﬁ eld) and thermal demagnetization results for samples from Integrated Ocean Drilling Program Expe- dition Site U1349. NRM—natural remanent magnetization. Plot conventions as in Figure 3. The small magnetization remaining at high (>575 °C) thermal demagnetization steps implies that the magnetization is partly carried by hematite, but it appears to be a minor component that does not carry a different direction. Downloaded from specialpapers.gsapubs.org on April 20, 2015

162 Sager et al.

N U1350A-24R-2, 137-139 cm U1350A-16R-2, 5-7 cm N AB ABN,Up N,Up NRM 1 A/m 1 A/m vertical horizontal NRM W E E 1.0 E S,Down C 0.8 W E 1.0 vertical C 0.6 horizontal 0.8 0.4 S,Down 0.6 0.2 0.4 0.0 0.2 0 100 200 300 400 500 600 T (°C) 0.0 0102030405060 AF (mT)

Figure 8. Example AF (alternating ﬁ eld) and thermal demagnetization plots for samples from Integrated Ocean Drilling Program Expedition Site U1350. NRM—natural remanent magnetization. Plot conventions as in Figure 3.

Site 1347 Site 1349 Sample paleoinclinations from Site U1347 samples are Measured paleoinclinations for Site U1349 are mostly small predominantly positive (Table 2), giving group mean colati- negative values (Table 3), implying magnetization in the South- tudes ranging from 79.7° to 89.8° (normal polarity assumed). ern Hemisphere if the polarity is normal. Statistical analysis indi- Individual sample data from Site U1347 show more scatter cates that Site U1349 has two distinct groups, which give mean than the other sites (Fig. 10) and careful analysis was required colatitudes at 91.3° and 96.2°, implying a paleolatitude near the to distinguish outliers (see Methods discussion). Many outli- equator (Fig. 11; Table 5). The difference between the group ers gave low values of colatitude (high inclination) and were mean colatitudes is only 4.9°. This difference is small, so both recognized as not having had the steep, drill-string over- groups were combined for a single average for the section. The print completely removed. Five statistically distinct mag- small difference between the two groups implies that a short time netic groups were recognized within the cored section (Fig. separated them. The mean colatitude for the site is 93.7° with a 10). Differences between successive group mean colatitudes 95% confi dence interval of +20.4°/–20.3° (Table 5). range from 2.7° to 10.1°. The large change between groups 1 and 2 implies that these two groups were erupted after Site 1350 a signifi cant time gap, but the amount of paleocolatitude Site U1350 is unusual among Expedition 324 sites because change is small for groups 2–5 (2.7°–4.9°), implying that the the cored section displays both positive and negative inclinations sampled paleofi eld did not undergo signifi cant secular varia- in nearly equal amounts (Table 4). To interpret the section, an tion between these groups. Indicating otherwise, preliminary assumption about magnetic polarity must be made. It is unlikely radiometric dates (Heaton and Koppers, 2014; Geldmacher et that the section records numerous magnetic reversals; instead, the al., 2014) show a likely gap of at least ~3 m.y. between the pattern of paleocolatitudes (Fig. 12) appears much like published lower massive fl ows (>260 mbsf) and the upper massive fl ows secular variation curves, especially between 170 and 270 mbsf, (<220 mbsf). Thus, despite magnetic directions being similar where mean colatitude values change systematically. Thus, we between groups 3 and 4 (Fig. 10), there is likely a large age assume that changes in the sign of the paleoinclination result discontinuity. This illustrates that signifi cant time can pass from secular variation near the equator. After statistical analysis, between successive lava fl ows with similar paleoinclinations Site U1350 was divided into 14 statistically distinct groups (Fig. owing to the fact that PSV is quasi-cyclical. For calculating the 12; Table 5). Differences between group means are larger than at site mean paleocolatitude, we assume 3 independent magnetic other sites, ranging from 0.9° to 12.0°, with a median difference of groups: group 1, groups 2 and 3, and groups 4 and 5 (Table 5). 6.0°. Changes are small between colatitudes for groups 3 and 4 as These three groups yield an average colatitude of 84.8° with a well as 11–14, so we combined those groups. Our fi nal grouping 95% confi dence of +13.0°/–13.2° (Table 5). has 10 independent groups, from which the average colatitude is Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 163

U1346 Colatitude (°) sive lava fl ow eruptions, it is likely that they will display different 50 60 70 80 90 100 110 120 130 paleoinclinations. Jurassic geomagnetic fi eld behavior is poorly known, so in this discussion we must assume that Jurassic PSV was similar to the recent record. As shown by Holocene deep- 140 sea and lake sediment cores (which can produce high-resolution records), recent PSV displays multiple shifts of paleoinclination during the Holocene (e.g., Creer, 1981; Turner and Thompson, 1981; Gogorza et al., 2011), with swings of up to 15°–30° in 1 k.y. (Creer, 1981; Daly and Le Goff, 1996; Lund et al., 2005). Some records suggest that inclination changes can be very rapid, 150 e.g., 6° over a 50 yr span (Gogorza et al., 2011). In the past 5 k.y., sediment records often show 4–6 swings >10° (Creer, 1981). Inclinations in sediment records can remain stable for a time, but it is rare to fi nd periods >500–1000 yr in which the fi eld 1 changes <5° (Lund et al., 2005). It is diffi cult to estimate the exact timing represented by a 160 particular change in inclination between lava fl ows. In part this is because we do not have a record of Jurassic PSV and do not know the eruption rate of the Shatsky Rise lava fl ows. Estimat- ing the time gap is also diffi cult because the rate of PSV is variable, so a given time span may produce a change in paleoinclination that can be large or small. Moreover, because PSV is

Depth (mbsf) 170 quasi-periodic, oscillating around an average for any given site, even a long time period between paleomagnetic samples may not produce a large change in the paleomagnetic direction. To gain an idea of the time span implied by a given change in paleoinclination, we used a global model of the Holocene magnetic fi eld (Cals10k, covering the past 10 k.y.; Korte et al., 2011) to 180 estimate PSV. A PSV record was generated for an equatorial site (because the Shatsky Rise was near the equator) and a Monte Carlo routine was used to select two paleoinclination values with Colatitude 2 a given age interval (Fig. 13). With 1000 trials for each assumed Mean colatitude age interval (from 25 to 1000 yr), the angles that encompassed 50% and 95% of the estimates were calculated. As expected, 190 Standard deviation larger age intervals are refl ected by larger angular separations on average. The angle encompassing 95% of the estimates climbs rapidly for intervals <~800 yr (Fig. 13). For a 100 yr period, the Figure 9. Colatitude versus depth (meters below seafl oor, mbsf) 95% angle is ~4°, for 200 yr, it is ~7°, and for 300 yr, it is ~10°. for Integrated Ocean Drilling Program Expedition Site U1346 This angle becomes slightly smaller at intervals >800 yr, likely with statistically distinct fl ow groups labeled in numerical order. Heavy line segments show colatitude group means (Table 5) and because of the limited angular deviation of PSV and the limited the shaded areas show the standard deviation for each mean co- time span of the PSV model. latitude. The dashed gray line represents 90° colatitude (equator). In general, the difference in mean paleoinclination between magnetic groups at the Shatsky Rise sites is small. Of all the Shatsky Rise igneous sites, Site 1213 shows the least variation. calculated as 86.7° with a 95% confi dence interval of +7.2/–7.3 Although the individual sample data show considerable scatter, (Table 5). In this case, the confi dence limits are much smaller than none of the fl ows in this 47 m section is statistically distinct, so those of the other sites because of the 10 independent groups. they appear to record the same paleoinclination. Sites U1346 and U1349 both have only two distinct groups with colatitude aver- DISCUSSION ages 1.8° and 4.9° apart, respectively (inclination averages are 3.2° and 9.3° apart). The PSV model implies that the interval Timing of Lava Flow Emplacement between lava fl ows is likely <100 yr for Site U1346 and <200 yr for Site U1349. Based on changes in paleocolatitude only, we Geomagnetic fi eld directions are constantly changing with would have divided Site U1347 into two groups (group 1 and time owing to PSV. If many years have passed between succes- groups 2–5), but radiometric age data imply that there is likely Downloaded from specialpapers.gsapubs.org on April 20, 2015

164 Sager et al.

TABLE 5. MAGNETIC GROUP AVERAGE COLATITUDES AND SITE AVERAGE COLATITUDES Depth Mean colatitude Colatitude error Inc. Colatitude 95% IODP site Group (mbsf) (°) (°) (°) N confidence U1346 1 139.73–174.36 100.6 2.6 –20.5 18 U1346 2 177.54–188.85 102.4 2.5 –23.7 8 Average 100.3 10.0 1 +20.7°/–20.1° U1347 1 159.9–161.75 89.8 3.7 0.4 7 U1347 2 167.07–200.74 79.7 2.5 19.9 12 U1347 3 206.37–237.03 83.7 3.8 12.5 18 U1347 4 245.21–274.70 81.0 5.7 17.6 17 U1347 5 275.45–316.01 85.9 4.9 8.2 21 Average 84.8 6.5 10.3 3 +13.0°/–13.2° U1349 1 165.12–206.99 91.3 2.2 –3.0 19 U1349 2 215.25–243.75 96.2 5.5 –12.3 12 Average 93.7 10.0 –7.4 1 +20.4°/–20.3° U1350 1 153.13–183.79 90.2 2.8 –0.4 18 U1350 2 191.18–191.6 78.2 3.5 22.7 2 U1350 3 197.88–206.03 89.6 2.1 0.8 5 U1350 4 210.65–213.01 91.8 0.9 –3.6 4 U1350 5 221.28–223.52 85.9 4.1 8.2 4 U1350 6 230.22–233.04 80.2 4.3 19.1 5 U1350 7 239.78–242.33 87.4 5.0 5.2 4 U1350 8 249.35–251.44 94.3 1.4 –8.6 5 U1350 9 258.53–260.47 87.6 1.6 4.8 3 U1350 10 261.15–261.87 81.6 1.2 16.4 3 U1350 11 268.47–279.31 89.1 0.9 1.8 8 U1350 12 281.43–289.85 93.0 1.5 –6.0 7 U1350 13 296.94–308.07 92.1 2.7 –4.2 16 U1350 14 310.54–315.72 89.6 1.7 0.8 5 Average 86.7 3.6 6.6 10 +7.2°/–7.3° Note: IODP—Integrated Ocean Drilling Program; Group—colatitude group; mbsf—meters below seafloor; Mean colatitude of group assumes normal polarity; Colatitude error—symmetric standard error for mean colatitude; Inc.—inclination corresponding to mean colatitude (assuming normal polarity); N—number of samples used to determine mean. a time gap between groups 2–3 and 4–5 (Heaton and Koppers, the full range of inclination variation. Supporting this interpreta- 2014). Between groups 1 and 2 the colatitude shifts by 10.1° tion, the progression of paleocolatitudes in groups 2–10 displays (paleoinclination by 19.5°). The PSV model does not show an serial correlation. Paleocolatitude trends progressively higher in inclination change >17° in our 1000 yr analysis (Fig. 13), imply- groups 2–4, lower in groups 5–6, higher in groups 7–8, and lower ing that the interval between these groups is at least this large. in groups 9–10. Although we cannot say with certainty that there Given that radiometric age data show signifi cant time gaps, the is no jump in time that skips over one or more PSV cycles, the age difference between groups 1 and 2 may be larger. Groups 2–5 U1350 curve displays no abrupt jump in the colatitude of these all have similar averages, with only 2.7°–4.9° of colatitude (5.1°– groups that would imply a large time gap. 9.4° of inclination) separating their means. Given the possible We conclude that the Shatsky Rise lava fl ow sections mostly age gap between the upper and lower massive fl ows, this unifor- built rapidly, with sections of ~50–70 m thickness accumulat- mity may be an artifact of the quasi-periodic nature of PSV. The ing in periods of ~100–200 yr. This is similar to fi ndings from Holocene PSV model implies that the small changes between the Deccan Traps large igneous province, where single eruptive groups 2–3 (4.0°) and 4–5 (4.9°) likely represent <200 yr. events can be 40–180 m thick with very little change in paleo- Site U1350 is unlike the other sites. Both the top ~40 m and inclination (Chenet et al., 2008). Two Shatsky Rise sections bottom ~50 m of the section show consistent colatitudes, imply- indicate greater lapses of time in slightly different situations: ing rapid lava accumulation, but the middle section, between ~190 periods of accumulation that were slow but relatively continu- and 270 m, shows large systematic variations with the appearance ous, and long time gaps at the end of volcano formation. Site of PSV. Differences between magnetic groups in this middle sec- U1350 demonstrates the fi rst situation with a section of ~80 m in tion are typically 6°–7° in colatitude (12°–13° inclination) with which several PSV cycles occur sandwiched between two rapidly an overall colatitude range of ~15° (78.2°–94.3°) and inclination emplaced sections. Site U1350 thus implies pulses of greater and range of ~31° (−8.6° to 22.7°). Even with the observed variation, lesser lava output. Similar observations are made on Hawaiian the between-group colatitude standard error is only ~4.9°, which volcanoes where fl ank eruptions often occur in pulses, some- is less than the expected 9.0° from a PSV model (Harrison, 1980; times with slow accumulation in between (Sharp et al., 1996; from Cox and Gordon, 1984). These observations imply that the Stolper et al., 2009). The second situation occurred at the sum- time spanned by these lava fl ows is ~103 yr, but probably not mit of the Tamu massif, where radiometric dating indicates that ~104–105 yr, which would average PSV completely and record the fi nal eruptions were emplaced after a long interval between Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 165

U1347A Colatitude (°) U1349A Colatitude (°) 10 20 30 40 50 60 70 80 90 100 60 70 80 90 100 110 120 130 140 150 155 160 1 Colatitude 165 Rejected colatitude Mean colatitude 175 170 Standard deviation 2 185

195 180 1 205

215 190

3 225

235 200

245 Depth (mbsf) Depth (mbsf) Depth 255 210 4 Colatitude 265 Rejected colatitude M ean colatitude 220 275 Standard deviation

285 2 295 230 5

305

315 240

325

Figure 10. Colatitude versus depth (meters below seaﬂ oor, mbsf) data 250 for Integrated Ocean Drilling Program Expedition Site U1347. Gray diamonds are colatitudes that were considered outliers (and not used Figure 11. Colatitude versus depth (meters below seaﬂ oor, mbsf) for calculation of the group means; see text for discussion). Plot con- data for Integrated Ocean Drilling Program Expedition Site U1349. ventions as in Figure 9. Plot conventions as in Figures 9 and 10. Downloaded from specialpapers.gsapubs.org on April 20, 2015

166 Sager et al.

30 20 U1350A Colatitude (°) 10 50 60 70 80 90 100 110 120 130 140 0 150 -10

Inclination (°) -20 -30 10000 8000 6000 4000 2000 0 160 Years (BP) 18 1 170 16

14 95% 180 12

190 8 2 Angle (°) 6 50% 200 3 4 2 210 4 0 0 200 400 600 800 1000 Interval between Flows (yr) 220 5 Figure 13. Secular variation and differences in inclination with time (BP—before present). (Top) Secular variation predicted by the Cals10k 230 model (Korte et al., 2011) over the past 10,000 yr at an equatorial site. 6 (Bottom) Differences in inclination between two times separated by a time gap using a Monte Carlo sampling of Cals10k inclinations. The 240 curve labeled 95% is the angle that encompasses 95% of the samples;

Depth (mbsf) 7 the curve labeled 50% is the angle encompassing 50% of the samples.

250 8

260 9 large outpourings of lava. This occurrence may be analogous to 10 the late-stage eruptions of large Hawaiian volcanoes, which can occur millions of years after the formation of the main shield 270 11 (e.g., Clague et al., 1989).

Paleolatitudes and Magnetic Polarity 280 12 All Shatsky Rise paleolatitude data place the plateau near 290 the equator at the time of formation. Interpretation of these data is difﬁ cult because of the large uncertainty of the mean paleocolatitudes (±13°–20° for Sites U1346, U1347, and U1349) resulting 300 Colatitude 13 from the small number of independent magnetic units recovered Rejected colatitude at all sites but U1350. Low paleolatitudes complicate the interpre- 310 Mean colatitude tation of magnetic polarity and hemisphere. At low latitude sites, Standard deviation 14 a change in the sign of paleoinclination can occur either because of a magnetic polarity reversal or because of normal PSV. Here 320 we propose plausible interpretations of paleolatitude and magnetic polarity for Shatsky Rise drill sites, but these interpretations Figure 12. Colatitude versus depth (meters below seaﬂ oor, mbsf) data must be treated with caution resulting from this ambiguity. for Integrated Ocean Drilling Program Expedition Site U1350. Plot The two sites drilled on the Tamu massif (1213 and U1347) conventions as in Figures 9 and 10. yielded paleoinclination data with different signs: Site 1213 data have mostly negative paleoinclinations (Tominaga et al., 2005), whereas Site U1347 data are mostly positive (Table 1). Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 167

One interpretation of this difference is that the two sites display the Tamu massif drifted across the equator. This interpretation is different magnetic polarities. However, an alternative is that based on an observed change in sign of the vertical component of the Shatsky Rise drifted across the equator, and one site was the anomalous magnetic fi eld at ~260 mbsf. If this interpretation emplaced in the Southern Hemisphere (negative inclination for is correct, we should see a change in sign of the paleoinclinations normal polarity) and the other in the Northern Hemisphere (posi- in cores from Site U1347. Samples from the lower massive fl ows tive inclination for normal polarity). Moreover, another possibil- were the least reliable and showed the greatest scatter of all Expe- ity is that the change in paleoinclination sign occurred by PSV. dition 324 samples; nevertheless, Figure 10 shows that there are The change of paleoinclination average from −9.3° (Site 1213) apparently reliable measurements from these fl ows, and there is to +10.3° (Site U1347) is within the plausible range of PSV if no change in sign at the bottom of this section. Moreover, radio- the site were at the equator. Without magnetic declination data or metric ages from the lower part of the section are ca. 144–142 Ma some way to determine magnetic polarity, we cannot be certain (Heaton and Koppers, 2014), so there is no evidence of a large which one of these possibilities is correct. age gap at ~260 mbsf consistent with drifting across the equator. Magnetic wireline logs were interpreted to give both mag- The interpretation that the upper section is normally polarized netic polarity and hemisphere of magnetization for Site U1347 in in the Northern Hemisphere is plausible and predicts a normal Tominaga et al. (2012), and it was concluded that the lower part polarity colatitude arc that matches closely to the 145 Ma paleo- of the Site U1347 section (below ~260 mbsf) is normally polar- magnetic pole (Fig. 14). Although it is possible that the Site 1213 ized but formed in the Southern Hemisphere and the upper part data were magnetized with the same polarity, but have opposite of the section is also normally polarized but formed in the North- signs from Site U1347 data owing to PSV, the Site 1213 data give ern Hemisphere after a gap of millions of years, during which a nearly identical paleocolatitude arc to that from Site U1347 if

30 30 U1346270° 60° U1347270° 60° 39 39

80 80 49 49 61 61 60° 68 60° 68 R N

145 145 30° 92 92 112 30° 112 R 300° 300° 123 123

N 330° 0° 330° 0°

30 30 U1349270° 60° 270°U1350 60° 39 39

80 80 49 49 61 61 60° 68 60° 68

R 145 N 145 92 92 112 30° 112 30°

300° 300° N 123 R 123

330° 0° 330° 0°

Figure 14. Mean colatitude arcs for Shatsky Rise sites compared to Pacifi c plate paleomagnetic poles. Pacifi c plate paleomagnetic poles (Sager, 2006; Beaman et al., 2007) are shown as small, fi lled circles. Gray ellipses show 95% confi dence regions for the poles. Numbers in italics are mean ages for the poles (in Ma). Thick arcs labeled N and R show the mean colatitude for each Shatsky Rise site assuming normal and reversed polarity, respectively. Downloaded from specialpapers.gsapubs.org on April 20, 2015

168 Sager et al. it is assumed that the Site 1213 section records reversed polarity Shatsky Rise paleolatitudes are low, it is likely that the plateau (Tominaga et al., 2005). Thus, the most plausible interpretation stayed near the equator during its formation. Although plate of the paleomagnetic data is that the Tamu massif formed slightly rotation can affect paleolatitude, the apparent polar wander path north (~3°–5°) of the equator and is made up of lavas of both indicates that little rotation has occurred (Sager, 2006, 2007), normal and reversed magnetic polarity. This fi nding invalidates so the Shatsky Rise paleolatitude data are mostly a refl ection of the conclusion from a magnetic anomaly study of the Tamu mas- northward and southward drift. sif that the edifi ce formed within a single reversed polarity period Paleolatitudes for both the Tamu and Ori massifs are simi- (Sager and Han, 1993). This fi nding is unsurprising given that the lar, but the Ori massif paleocolatitude arcs imply paleomagnetic magnetic anomalies of volcanic edifi ces average the magnetiza- poles at lower latitudes, consistent with the progression of the tion of a volcano, which may contain complexities (e.g., Gee and apparent polar wander path during the Late Jurassic and Early Nakanishi, 1995). Moreover, preliminary radiometric dates also Cretaceous (Fig. 14). How does this apparent shift in paleo- show that the Site U1347 section likely spans at least several mil- magnetic poles occur when the paleolatitudes show no apparent lion years (Heaton and Koppers, 2014; Geldmacher et al., 2014) change? While the site of eruptions stayed near the equator, as and are thus unlikely to record a uniform magnetic polarity dur- implied by the low paleolatitudes, the plateau eruptions moved ing a period containing magnetic reversals. northward relative to the Pacifi c plate from the Tamu to Ori to No information tells us the magnetic polarity for the other Shirshov massifs. Thus, the Pacifi c plate moved southward while cored sections from Expedition 324. Magnetic wireline logs were the locus of eruptions stayed nearly fi xed in the paleomagnetic collected at Site U1349, but the igneous section was short and reference frame. It is interesting that this fi nding is also consistent partly affected by the magnetic fi eld of the drill string. In the rest with the ~2° paleolatitude of Ocean Drilling Program Site 1179, of the section, the inclinations are anomalously high, implying located on magnetic anomaly M8 (133 Ma) slightly north of the that the borehole magnetic anomaly does not reliably record the Shirshov massif (Sager and Horner-Johnson, 2004). This result paleoinclination (which should be near zero). Even though the confi rms prior research indicating that the Pacifi c plate moved magnetic polarity of the Ori massif sites is unknown, both Sites southward during this time period (Larson and Lowrie, 1975; U1349 and U1350 give colatitudes near 90°, so they constrain Larson and Sager, 1992; Larson et al., 1992). the paleomagnetic pole to a narrow zone that crosses the Pacifi c apparent polar wander path near its farthest extent from the spin CONCLUSIONS axis (Fig. 14). Although samples from Site U1349 record the same paleo- 1. Samples from Sites U1346, U1347, and U1349 do not inclination sign and are therefore of the same magnetic polarity, display substantial colatitude variation between most magnetic interpretation of the Site U1350 section is more diffi cult because groups, implying that the cored sections were erupted during a it contains both positive and negative paleoinclinations. The pro- short period of time, i.e., mostly <100–200 yr. Of these sections, gression of group mean paleocolatitudes at Site U1350 is con- only one shows paleomagnetic evidence of a signifi cant time gap. sistent with PSV, which can cause changes in paleoinclination The uppermost magnetic group at Site U1347 suggests a larger sign when the site is near the equator. Although it is possible that time interval, >1000 yr. The Site U1350 section is the only one more than one polarity period is recorded at Site U1350, pre- that displays signifi cant and systematic variation between fl ow liminary radiometric dates do not show a clear age progression groups that could be interpreted as PSV. This fi nding implies within the section (Heaton and Koppers, 2014), suggesting that emplacement over a period of ~1000 yr. However, the between- a signifi cant time span is not present and this explanation is less group standard error for Site U1350 is smaller than modeled likely. Data from Site U1346 also record one magnetic polarity, PSV, implying that the total eruptive period was too small to but with the large uncertainty in mean paleocolatitude, we cannot completely average PSV and is thus <104–105 yr. know which polarity is recorded, nor can we tell the paleoposi- 2. Site U1347 paleoinclinations are similar to published val- tion of the Shirshov massif except to say that it was also formed ues from nearby Site 1213 but with different signs. We interpret near the equator. that a magnetic polarity reversal occurred between lava emplace- Mean paleocolatitudes for Sites U1347, U1349, and ment at the two sites on the Tamu massif. This contradicts pub- U1350 are all ≤5° from the paleoequator. The Site U1346 aver- lished fi ndings that the Tamu massif formed during a single polar- age is ~10° from the paleoequator, but large uncertainty in the ity period. It is also possible, but less likely, that the different signs mean implies that this difference is probably not signifi cant. are a result of the Tamu massif drifting across the equator, indicat- Paleomagnetic data from Site 1213 gave similar results, with ing a substantial time between the eruptions of these two sections. both basalt and sediment paleolatitudes of 4.6°–4.7° (Tominaga 3. Using the published magnetic wireline log interpretation et al., 2005; Sager et al., 2005). No matter what polarity and that Site U1347 records normal polarity, a comparison of the nor- hemisphere are assumed for the sites in this study, the data are mal polarity paleocolatitude arc for this site (and the reversed consistent with the known Pacifi c apparent polar wander path polarity arc for Site 1213) with the Pacifi c apparent polar wander (Fig. 14) and indicate a near-equatorial paleoposition for the path shows good agreement with the coeval (145 Ma) paleomag- Shatsky Rise at the time of its formation. Because all of the netic pole. Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 169

4. All sites give paleolatitudes near the equator, with values Tertiary boundary using paleomagnetic secular variation: Results from a ≤5° for Sites U1347, U1349, and U1350. Together with an ~2° 1200-m-thick section in the Mahabaleshwar escarpment: Journal of Geo- physical Research, v. 113, B04101, doi:10.1029/2006JB004635. paleolatitude for nearby Site 1179, these data indicate that the Clague, D.A., Dalrymple, G.B., Wright, T.L., Klein, F.W., Koyanagi, R.Y., Shatsky Rise stayed near the equator during its entire forma- Decker, R.W., and Thomas, D.M., 1989, The Hawaiian-Emperor Chain, tion. Given that the locus of volcanism moved northward rela- in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The Eastern Pacifi c and Hawaii: Boulder, Colorado, Geological Society of America, tive to the Pacifi c plate, we conclude that the Pacifi c plate moved The Geology of North America, v. N, p. 187–237. southward during Shatsky Rise formation, consistent with plate Coffi n, M.F., and Eldholm, O., 1994, Large igneous provinces: Crustal struc- motion inferred from the apparent polar wander path. ture, dimensions, and external consequences: Reviews of Geophysics, v. 32, p. 1–36, doi:10.1029/93RG02508. Cox, A., and Gordon, R.G., 1984, Paleolatitudes determined from paleomag- ACKNOWLEDGMENTS netic data from vertical cores: Reviews of Geophysics, v. 22, p. 47–72, doi:10.1029/RG022i001p00047. Creer, K.M., 1981, Long-period geomagnetic secular variations since 12,000 yr Funding for this study came partly from a grant awarded by the B.P: Nature, v. 292, p. 208–212, doi:10.1038/292208a0. U.S. Science Support Program for Expedition 324 research. Daly, L., and LeGoff, M., 1996, An updated and homogeneous world secular Additional support for Sager and Pueringer was provided by the variation database: I. Smoothing of the archeomagnetic results: Physics of the Earth and Planetary Interiors, v. 93, p. 159–190, doi:10.1016/0031 Jane and R. Ken Williams ’45 Chair for Ocean Drilling Science, -9201(95)03075-1. Education, and Technology at Texas A&M University. This study Day, R., Fuller, M., and Schmidt, V.A., 1977, Hysteresis properties would not have been possible without the Integrated Ocean Drill- of titanomagnetites: Grain-size and compositional dependence: Physics of the Earth and Planetary Interiors, v. 13, p. 260–267, ing Program (IODP), as well as the dedicated technical staff, doi:10.1016/0031-9201(77)90108-X. scientists, and crew on board the D/V JOIDES Resolution. This Doubrovine, P.V., and Tarduno, J.A., 2004, Self-reversed magnetization carried research used samples and data provided by the IODP, which is by titanomaghemite in ocean basalts: Earth and Planetary Science Letters, v. 222, p. 959–969, doi:10.1016/j.epsl.2004.04.009. sponsored by the U.S. National Science Foundation and partici- Doubrovine, P.V., and Tardnuno, J.A., 2005, On the compositional fi eld of pating countries under management by IODP Management Inter- self-reversing titanomaghemite: Constraints from Deep Sea Drilling national. The authors thank two anonymous reviewers for many Project Site 307: Journal of Geophysical Research, v. 110, B11104, doi:10.1029/2005JB003865. helpful suggestions that improved the manuscript. Duncan, R.A., and Richards, M.A., 1991, Hotspots, mantle plumes, fl ood basalts and true polar wander: Reviews of Geophysics, v. 29, p. 31–50, REFERENCES CITED doi:10.1029/90RG02372. Dunlop, D.J., 2002a, Theory and application of the Day plot (Mrs/Mr versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data: Arason, P., and Levi, S., 2010, Maximum likelihood solution for inclination- Journal of Geophysical Research, v. 107, p. EPM 4-1–EPM 4-22, only data in paleomagnetism: Geophysical Journal International, v. 182, doi:10.1029/2001JB000486. p. 753–771, doi:10.1111/j.1365-246X.2010.04671.x. Dunlop, D.J., 2002b, Theory and application of the Day plot (Mrs/Mr ver- Audunsson, H., and Levi, S., 1989, Drilling-induced remanent magnetization sus Hcr/Hc) 2. Application to data for rocks, sediments, and soils: in basalt drill cores: Geophysical Journal International, v. 98, p. 613–622, Journal of Geophysical Research, v. 107, p. EPM 5-1–EPM 5-15, doi:10.1111/j.1365-246X.1989.tb02294.x. doi:10.1029/2001JB000487. Beaman, M., Sager, W.W., Acton, G.D., Lanci, L., and Peres, J., 2007, Improved Dunlop, D.J., and Özdemir, O., 2007, Magnetizations in rocks and minerals, Late Cretaceous and early Cenozoic paleomagnetic apparent polar wan- in Kono, M., ed., Geomagnetism: Treatise on Geophysics, Volume 5: der path for the Pacifi c plate: Earth and Planetary Science Letters, v. 262, Amsterdam, Elsevier, p. 277–336. p. 1–20, doi:10.1016/j.epsl.2007.05.036. Evans, H.F., Channell, J.E.T., and Sager, W.W., 2005, Late Miocene– Holocene Bleil, U., 1985, The magnetostratigraphy of northwest Pacifi c sediments, Deep magnetic polarity stratigraphy and astrochronology, ODP Leg 198, Sea Drilling Project Leg 86, in Heath, G.R., Burckle, L.H., et al., Ini- Shatsky Rise, in Bralower, T.J., Premoli Silva, I., Malone, M.J., et al., Pro- tial Reports of the Deep Sea Drilling Project, Volume 86: Washington, ceedings of the Ocean Drilling Program, Scientifi c Results, Volume 198: D.C., U.S. Government Printing Offi ce, p. 441–458, doi:10.2973/dsdp. College Station, Texas, Ocean Drilling Program, p. 1–39, doi:10.2973/ proc.86.117.1985. odp.proc.sr.198.118.2005. Briden, J.C., and Ward, M.A., 1966, Analysis of magnetic inclinations in Foulger, G.R., 2007, The “plate” model for the genesis of melting anomalies, borecores: Pure and Applied Geophysics, v. 63, p. 133–152, doi:10.1007/ in Foulger, G.R., and Jurdy, D.M., eds., Plates, Plumes, and Planetary BF00875164. Processes: Geological Society of America Special Paper 430, p. 1–28, Butler, R.F., 1992, Paleomagnetism: Magnetic Domains to Geologic Terranes: doi:10.1130/2007.2430(01). Oxford, UK, Blackwell Scientifi c Publications, 399 p. Gee, J., and Nakanishi, M., 1995, Magnetic petrology and magnetic properties of Carvallo, C., 2014, Data report: Hysteresis measurements on basalts from Holes western Pacifi c guyots: Implications for seamount paleopoles, in Haggerty, U1346A, U1347A, and U1350A on Shatsky Rise, in Sager, W.W., Sano, J.A., Premoli Silva, I., Rack, F.R., et al., Proceedings of the Ocean Drilling T., Geldmacher, J., et al., Proceedings of the Integrated Ocean Drilling Program, Scientifi c Results, Volume 144: College Station, Texas, Ocean Drill- Program, Volume 324: Tokyo, Integrated Ocean Drilling Program Man- ing Program, p. 615–630, doi:10.2973/odp.proc.sr.144.020.1995. agement International, Inc., doi:10.2204/iodp.proc.324.205.2014. Geldmacher, J., van den Bogaard, P., Heydolph, K., and Hoernle, K., 2014, Carvallo, C., and Camps, P., 2013, Data report: Magnetic properties of basalts The age of the Earth’s largest volcano: Tamu Massif on Shatsky Rise from Shatsky Rise, in Sager, W.W., Sano, T., Geldmacher, J., et al., Pro- (northwest Pacifi c Ocean): International Journal of Earth Sciences, v. 103, ceedings of the Integrated Ocean Drilling Program, Volume 324: Tokyo, p. 2351–2357, doi:10.1007/s00531-014-1078-6. Integrated Ocean Drilling Program Management International, Inc., Gogorza, C.S., Sinito, A.M., Ohlendorf, C., Kastner, S., and Zolitschka, B., doi:10.2204/iodp.proc.324.201.2013. 2011, Paleosecular variation and paleointensity records for the last mil- Carvallo, C., Camps, P., Ooga, M., Fanjat, G., and Sager, W.W., 2013, Pal- lennium from southern South America (Laguna Potrok Aike, Santa Cruz, aeointensity determinations and rock magnetic properties on basalts from Argentina): Physics of the Earth and Planetary Interiors, v. 184, p. 41–50, Shatsky Rise: New evidence for a Mesozoic dipole low: Geophysical doi:10.1016/j.pepi.2010.10.011. Journal International, v. 192, p. 986–999, doi:10.1093/gji/ggs100. Harrison, C.G.A., 1980, Secular variation and excursions of the Earth’s Chenet, A.L., Fluteau, F., Courtillot, V., Gérard, M., and Subbarao, K.V., magnetic fi eld: Journal of Geophysical Research, v. 85, p. 3511–3522, 2008, Determination of rapid Deccan eruptions across the Cretaceous- doi:10.1029/JB085iB07p03511. Downloaded from specialpapers.gsapubs.org on April 20, 2015

170 Sager et al.

Heaton, D.E., and Koppers, A.A.P., 2014, Constraining the rapid construction Pueringer, M., Sager, W., and Housen, B., 2013, Data report: Paleomagnetic of Tamu Massif at a 146 Myr old triple junction [abs.]: 2014 Goldschmidt measurements of igneous rocks from Shatsky Rise Expedition 324, in Conference, 8–13 June, Sacramento, California, http://goldschmidt Sager, W.W., Sano, T., Geldmacher, J., et al., Proceedings of the Inte- .info/2014/abstracts/abstractView?abstractId=4093. grated Ocean Drilling Program, Volume 324: Tokyo, Integrated Ocean Heller, P.L., Anderson, D.L., and Angevine, C.L., 1996, Is the middle Cretaceous Drilling Program Management International, Inc., doi:10.2204/iodp pulse of rapid sea-fl oor spreading real or necessary?: Geology, v. 24, p. 491– .proc.324.202.2013. 494, doi:10.1130/0091-7613(1996)024<0491:ITMCPO>2.3.CO;2. Sager, W.W., 2005, What built Shatsky Rise, a mantle plume or ridge tecton- Johnson, C., Constable, C.G., Tauxe, L., Barendregt, R., Brown, L.L., Coe, ics?, in Foulger, G.R., Natland, J.H., Presnall, D.C., and Anderson, D.L., R.S., Layer, P., Mejia, V., Opdyke, N.D., Singer, B.S., Staudigel, H., and eds., Plates, Plumes, and Paradigms: Geological Society of America Spe- Stone, D.B., 2008, Recent investigations of the 0–5 Ma geomagnetic cial Paper 388, p. 721–733, doi:10.1130/0-8137-2388-4.721. fi eld recorded by lava fl ows: Geochemistry Geophysics Geosystems, v. 9, Sager, W.W., 2006, Cretaceous paleomagnetic apparent polar wander path for Q04032, doi:10.1029/2007GC001696. the Pacifi c plate calculated from Deep Sea Drilling Project and Ocean Kent, D.V., Wang, H., and Rochette, P., 2010, Equatorial paleosecular varia- Drilling Program basalt cores: Physics of the Earth and Planetary Interi- tion of the geomagnetic fi eld from 0 to 3 Ma lavas from the Galapagos ors, v. 156, p. 329–349, doi:10.1016/j.pepi.2005.09.014. Islands: Physics of the Earth and Planetary Interiors, v. 183, p. 404–412, Sager, W.W., 2007, Divergence between paleomagnetic and hotspot-model- doi:10.1016/j.pepi.2010.08.010. predicted polar wander for the Pacifi c plate with implications for hotspot Kirschvink, J., 1980, The least squares line and plane and the analysis of pal- fi xity, in Foulger, G.R., and Jurdy, D.M., eds., Plates, Plumes, and Plan- aeomagnetic data: Royal Astronomical Society Geophysical Journal, etary Processes: Geological Society of America Special Paper 430, v. 62, p. 699–718, doi:10.1111/j.1365-246X.1980.tb02601.x. p. 335–357, doi:10.1130/2007.2430(17). Kono, M., 1980a, Statistics of paleomagnetic inclination data: Journal of Geo- Sager, W., and Han, H.-C., 1993, Rapid formation of the Shatsky Rise oceanic physical Research, v. 85, p. 3878–3882, doi:10.1029/JB085iB07p03878. plateau inferred from its magnetic anomaly: Nature, v. 364, p. 610–613, Kono, M., 1980b, Paleomagnetism of DSDP Leg 55 basalts and implications doi:10.1038/364610a0. for the tectonics of the Pacifi c plate, in Jackson, E.D., Koizumi, I., et al., Sager, W.W., and Horner-Johnson, B.C., 2004, Paleomagnetism of basaltic Initial Reports of the Deep Sea Drilling Project, Volume 55: Washington, rocks cored at ODP Site 1179: Implications for mid-Cretaceous Pacifi c D.C., U.S. Government Printing Offi ce, p. 737–752, doi:10.2973/dsdp plate paleolatitude and paleomagnetic pole, in Sager, W.W., Kanazawa, .proc.55.135.1980. T., Escutia, C., et al., Proceedings of the Ocean Drilling Program, Sci- Koppers, A.A.P., Sano, T., Natland, J.H., Widdowson, M., Almeev, R., Greene, entifi c Results, Volume 191: College Station, Texas, Ocean Drilling Pro- A.R., Murphy, D.T., Delacour, A., Miyoshi, M., Shimizu, K., Li, S., gram, doi:10.2973/odp.proc.sr.191.002.2004. Hirano, N., Geldmacher, J., and the Expedition 324 Scientists, 2010, Mas- Sager, W.W., Handschumacher, D.W., Hilde, T.W.C., and Bracey, D.R., 1988, sive basalt fl ows on the southern fl ank of Tamu Massif, Shatsky Rise: Tectonic evolution of the northern Pacifi c plate and Pacifi c- Farallon- A reappraisal of ODP Site 1213 basement units, in Sager, W.W., Sano, Izanagi triple junction in the Late Jurassic and Early Cretaceous T., Geldmacher, J., and the Expedition 324 Scientists, Proceedings of (M21–M1): Tectonophysics, v. 155, p. 345–364, doi:10.1016/0040 the Integrated Ocean Drilling Program, Volume 324: Tokyo, Integrated -1951(88)90274-0. Ocean Drilling Program Management International, Inc., doi:10.2204/ Sager, W.W., Polgreen, E.L., and Rack, F.R., 1993, Magnetic polarity rever- iodp.proc.324.109.2010. sal stratigraphy of Hole 810C, Shatsky Rise, western Pacifi c Ocean, in Korte, M., Constable, C., Donadini, F., and Holme, R., 2011, Reconstructing Natland, J. H., Brass, G.W., Stokking, L.B., et al., Proceedings of the the Holocene geomagnetic fi eld: Earth and Planetary Science Letters, Ocean Drilling Program, Scientifi c Results, Volume 132: College Sta- v. 312, p. 497–505, doi:10.1016/j.epsl.2011.10.031. tion, Texas, Ocean Drilling Program, p. 47–55, doi:10.2973/odp.proc Larson, R.L., and Lowrie, W., 1975, Paleomagnetic evidence for motion of .sr.132.304.1993. the Pacifi c plate from Leg 32 basalts and magnetic anomalies, in Larson, Sager, W.W., Kim, J., Klaus, A., Nakanishi, M., and Khankishieva, L., 1999, R.L., and Moberly, R., et al., Initial Reports of the Deep Sea Drilling Proj- Bathymetry of Shatsky Rise, northwest Pacifi c Ocean: Implications for ect, Volume 32: Washington, D. C., U. S. Government Printing Offi ce, ocean plateau development at a triple junction: Journal of Geophysical p. 571–577, doi:10.2973/dsdp.proc.32.122.1975. Research, v. 104, p. 7557–7576, doi:10.1029/1998JB900009. Larson, R.L., and Sager, W.W., 1992, Skewness of magnetic anomalies M0 to Sager, W.W., Evans, H.F., and Channell, J.E.T., 2005, Paleomagnetism of M29 in the northwestern Pacifi c, in Larson, R.L., Lancelot, Y., Fisher, A., Early Cretaceous (Berriasian) sedimentary rocks, Hole 1213B, Shatsky et al., Proceedings of the Ocean Drilling Program, Scientifi c Results, Vol- Rise, in Bralower, T.J., Premoli Silva, I., Malone, M.J., et al., Proceed- ume 129: College Station, Texas, Ocean Drilling Program, p. 471–481, ings of the Ocean Drilling Program, Scientifi c Results, Volume 198: doi:10.2973/odp.proc.sr.129.137.1992. College Station, Texas, Ocean Drilling Program, doi:10.2973/odp.proc Larson, R.L., Steiner, M.B., Erba, E., and Lancelot, Y., 1992, Paleolatitudes .sr.198.117.2005. and tectonic reconstructions of the oldest Pacifi c portion of the Pacifi c Sager, W.W., Sano, T., and Geldmacher, J., and the Expedition 324 Scientists, plate: A comparative study, in Larson, R.L., Lancelot, Y., Fisher, A., et 2010, Proceedings of the Integrated Ocean Drilling Program, Volume al., Proceedings of the Ocean Drilling Program, Scientifi c Results, Vol- 324: Tokyo, Integrated Ocean Drilling Program Management Interna- ume 129: College Station, Texas, Ocean Drilling Program, p. 615–631, tional, Inc., doi:10.2204/ iodp.proc.324.2010. doi:10.2973/odp.proc.sr.129.144.1992. Sager, W.W., Sano, T., and Geldmacher, J., 2011a, How do oceanic plateaus Lund, S.P., Schwartz, M., Keigwin, L., and Johnson, T., 2005, Deep-sea sedi- form? Clues from drilling Shatsky Rise: Eos (Transactions, American ment records of the Laschamp geomagnetic fi eld excursion (~41,000 Geophysical Union), v. 92, p. 37–44, doi:10.1029/2011EO050001. calendar years before present): Journal of Geophysical Research, v. 110, Sager, W.W., Sano, T., Geldmacher, J., and the Expedition 324 Scientists, B04101, doi:10.1029/2003JB002943. 2011b, IODP Expedition 324: Ocean drilling at Shatsky Rise gives clues Mahoney, J.J., Duncan, R.A., Tejada, M.L.G., Sager, W.W., and Bralower, about oceanic plateau formation: Scientifi c Drilling, v. 12, p. 24–31, T.J., 2005, Jurassic-Cretaceous boundary age and mid-ocean-ridge-type doi:10.5194/sd-12-24-2011. mantle source for Shatsky Rise: Geology, v. 33, p. 185–188, doi:10.1130/ Sager, W.W., Zhang, J., Korenaga, J., Sano, T., Koppers, A.A.P., Widdowson, G21378.1. M., and Mahoney, J.J., 2013, An immense shield volcano within the McFadden, P.L., and Reid, A.B., 1982, Analysis of paleomagnetic inclination Shatsky Rise oceanic plateau, northwest Pacifi c Ocean: Nature Geosci- data: Royal Astronomical Society Geophysical Journal, v. 69, p. 307–319, ence, v. 6, p. 976–981, doi:10.1038/ngeo1934. doi:10.1111/j.1365-246X.1982.tb04950.x. Sharp, W.D., Turrin, B.D., Renne, P.R., and Lanphere, M.A., 1996, The Nakanishi, M., Sager, W.W., and Klaus, A., 1999, Magnetic lineations within 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, Hawaii Shatsky Rise, northwest Pacifi c Ocean: Implications for hot spot-triple Scientifi c Drilling Project: Journal of Geophysical Research, v. 101, junction interaction and oceanic plateau formation: Journal of Geophysi- p. 11607–11616, doi:10.1029/95JB03702. cal Research, v. 104, p. 7539–7556, doi:10.1029/1999JB900002. Shipboard Scientifi c Party, 2002, Site 1213, in Bralower, T.J., Premoli Silva, I., Özdemir, Ö., and O’Reilly, W., 1982, Magnetic hysteresis properties of syn- Malone, M.J., et al., Proceedings of the Ocean Drilling Program, Initial thetic monodomain titanomaghemites: Earth and Planetary Science Let- Reports, Volume 198: College Station, Texas, Ocean Drilling Program, ters, v. 57, p. 437–447, doi:10.1016/0012-821X(82)90162-5. doi:10.2973/odp.proc.ir.198.109.2002. Downloaded from specialpapers.gsapubs.org on April 20, 2015

Paleomagnetism of igneous rocks from the Shatsky Rise 171

Stolper, E.M., DePaolo, D.J., and Thomas, D.M., 2009, Deep drilling into a the downhole magnetic architecture of the uppermost lava sequences mantle plume volcano: The Hawaii Scientifi c Drilling Project: Scientifi c at Tamu Massif: Geophysical Research Letters, v. 39, L21301, Drilling, v. 7, p. 4–14, doi:10.5194/sd-7-4-2009. doi:10.1029/2012GL052967. Tominaga, M., Sager, W.W., and Channell, J.E.T., 2005, Paleomagnetism of Turner, G.M., and Thompson, R., 1981, Lake sediment record of the geomag- the igneous section, Hole 1213B, Shatsky Rise, in Bralower, T.J., Premoli netic secular variations in Britain during Holocene times: Royal Astro- Silva, I., Malone, M.J., et al., Proceedings of the Ocean Drilling Program, nomical Society Geophysical Journal, v. 65, p. 703–725, doi:10.1111/ Scientifi c Results, Volume 198: College Station, Texas, Ocean Drilling j.1365-246X.1981.tb04879.x. Program, doi:10.2973/odp.proc.sr.198.113.2005. Tominaga, M., Evans, H.F., and Iturrino, G., 2012, “Equator Crossing” of MANUSCRIPT ACCEPTED BY THE SOCIETY 26 NOVEMBER 2014 Shatsky Rise?: New insights on Shatsky Rise tectonic motion from MANUSCRIPT PUBLISHED ONLINE XX MONTH 2015

Printed in the USA Downloaded from specialpapers.gsapubs.org on April 20, 2015 Downloaded from specialpapers.gsapubs.org on April 20, 2015

Geological Society of America Special Papers Online First

Paleomagnetism of igneous rocks from the Shatsky Rise: Implications for paleolatitude and oceanic plateau volcanism

William W. Sager, Margaret Pueringer, Claire Carvallo, et al.

Geological Society of America Special Papers, published online March 24, 2015; doi:10.1130/2015.2511(08)

E-mail alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article

Subscribe click www.gsapubs.org/subscriptions to subscribe to Geological Society of America Special Papers

Permission request click www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA.

Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

Notes