T13A-1121
The Enigma that is Ontong Java: The Giant of Unknown Origin
The Ontong Java Plateau (OJP) in the southwest Pacific represents the world's largest Large Igneous Province and represents an enigma in terms of its origin and evolution. Origins through plume(s), meteorite impact, or perisphere/upper mantle models all fail to explain all the features observed on this Greenland-size LIP (e.g., Tejada et al., 2004, Geol Soc. London Spec. Pub. 229, 133). Over the last 25-30 years, more information on the OJP has been obtained, yet the observations and interpretations have brought us more questions than answers. Therefore, it is time to look at what we know about the OJP and evaluate what other information is needed, and where we need to go to get this, in order to better understand how this LIP formed and evolved, as well as its impact on the environment. So what do we know? The first and most noticeable feature of the OJP is its immense size, which could be even larger if the Manihiki and Hikurangi plateaus represent rifted portions of the OJP (Taylor, 2006, EPSL 241, 372). The OJP has a seismically distinct mantle root extending up to 300 km into the mantle that appears to be attached to the OJP and moving with it (Richardson et al., 2000, PEPI 118, 29; Klosko et al., 2001, EPSL 186, 347). Fieldwork on the obducted portions of the OJP outcropping in the Solomon Islands and two Ocean Drilling Program legs have demonstrated that the OJP is remarkable in its monotony, with the Kwaimbaita Basalt (Tejada et al., 2001, J. Pet. 43, 449) dominating the interior of the plateau. What little variability there is in lava composition is found at the plateau margins (Singgalo and Kroenke type basalts: Tejada et al. ibid.; Fitton and Goddard, 2004, Geol Soc. London Spec. Pub. 229, 151). In addition, the majority of lava flows erupted in ~5 m.y. around 122 Ma. However, there is evidence at the plateau margins (Site 803 of ODP Leg 130; the islands of Malaita, Santa Isabel, San Cristobal) of a minor eruptions at ~90 Ma; remarkably, these younger basalts are compositionally indistinct from the ~122 Ma Kwaimbaita type. The topography of the OJP is also enigmatic: plume theory predicts that much of the plateau would have been erupted subaerially, yet evidence suggests the eruption environment was predominantly deep marine. Volcaniclastic debris cored at Site 1184 on the Eastern Salient of the OJP is the only evidence for shallow water/subaerial eruptions. Likewise, the OJP has not subsided as far as is predicted from oceanic crust relaxation models. Rapid and massive eruptions must have occurred during the formation of the OJP yet very few extinctions are coincident with its ~122 Ma age. However, the eruption of the OJP is approximately coincident with the Selli oceanic anoxic event (OAE-1a). Given the information we now have, focused ocean drilling is needed to understand the origin and evolution of the OJP and the consequences its formation had for the environment. This presentation will present a rationale for a new ocean drilling expedition in order to probe further the enigma that is the Ontong Java Plateau.
T13A-1122
Tectonic Setting of the Lyra Basin, west of the Ontong Java Plateau
We present the preliminary results of the geophysical investigation and rock sampling in the Lyra Basin, the deep ocean basin west of the Ontong Java Plateau. The Ontong Java Plateau (OJP) is one of the most voluminous large igneous provinces. The evidences from drilling samples indicate that most of the OJP formed rapidly about 120 Ma at mid-southern latitude in the Pacific Basin. Deep ocean basins are contiguous with the OJP. For settlement of the argument about the origin of the OJP, the tectonic setting of the deep ocean basins around the OJP must be revealed. Tectonic histories of the Nauru, East Mariana, and Pigafetta basins, were revealed using the Mesozoic magnetic anomaly lineations. That of Lyra basin is, however, still unknown because of lack of magnetic anomaly lineations. The depth of the seafloor in the Lyra Basin deepens west, from 4000 to 5000 m. The Lyra Trough, crossing the Lyra Basin, is a broad and deep graben. In December 2006, we conducted the geophysical investigation and rock sampling in the Lyra Basin using R/V KAIREI, Japan Agency for Marine-Earth Science and Technology. Our bathymetric measurement revealed the topographic expression of the Lyra Trough. The depth of the trough is around 5500 m. The bottom of the trench as a whole is very smooth except for several lineated ridges. The height of the lineated ridges is about 500 m. The length of the lineated ridges is around 20 km. The strike of some ridges is N35°W and parallel to the trough strike. That of others is N50°W and oblique to the trough strike. Seamounts are located in the eastern rim of the Lyra Trough south of 1°30'N. The height of the seamounts is more than 2500 m. We obtained volcanic samples from the lineated ridge in the Lyra Trough and the seamount in the eastern rim of the trough. Most of igneous rocks are volcanic breccia, being coated by thick Mn crust. Although many volcanoclastic rocks are highly altered, some fresh samples were recovered. They are olivine-titanaugite phyric alkaline basalt or picrite. This type of volcanic rocks is different from that of the OJP's volcanic rocks.
T13A-1123
R/V Sonne Cruise SO193: New Insights Into the Geodynamic History of the Manihiki Plateau, SW-Pacific
The Manihiki Plateau represents a Cretaceous Large Igneous Province (LIP) in the SW-Pacific. In May/June 2007, R/V Sonne cruise SO193 conducted ~4,700 nm of bathymetric mapping in key areas of the Manihiki Plateau region, and the first hard rock sampling of all major geomorphological units of the plateau as well as of seamounts on adjacent oceanic crust. The research project SO193 MANIHIKI aims to reconstruct the origin as well as spatial and temporal evolution of the Manihiki Plateau and to characterize the relationship between Manihiki and the other LIPs in the western Pacific. SO193 recovered magmatic or sedimentary rocks from 77 locations in the Manihiki Plateau region. Olivine-bearing sheet and pillow lavas dominate, but various types of volcaniclastic rocks are also common, some of them indicating subaerial or shallow water volcanic activity. Minor lithologies are, among others, picritic lavas, serpentinites, evolved lavas, and subvolcanic intrusives. Unexpectedly, the upper, directly accessible portions in some areas of the Manihiki Plateau (e.g., Suvorov Trough, North Plateau, the NE corner of the High Plateau) appear to mainly consist of solidified, indurated or lithified sediments which suggests secondary heating and/or intense tectonic movements. In contrast to the previously surveyed Hikurangi LIP east of New Zealand, guyot-type seamounts only exist in a few restricted areas of the Manihiki Plateau. The recent depth of their erosional platforms vary unsystematically between 1,600 m and 2,500 m below sea level (b.s.l.), implying different ages of these volcanoes or non-uniform subsidence rates. On the other hand, uneroded seamounts rising up to ~600 m b.s.l. occur on and close to the Manihiki Plateau. Pillow lavas dredged at a ~600 m high ridge on the High Plateau, which is believed to have formed under subaerial conditions, also indicate late-stage volcanic activity, after subsidence of the High Plateau. Taken together, the preliminary results of mapping and sampling of SO193 suggest a complex geodynamic history for the Manihiki Plateau, including long-term or repeated volcanism and intense tectonic movements (see also Coffin et al. abstract).
T13A-1124
Extensional and Transtensional Tectonics of the Manihiki Plateau, Western Equatorial Pacific Ocean
Standing several kilometers above surrounding seafloor, the submarine Manihiki Plateau, an oceanic large igneous province (LIP), encompasses ~800,000 km2 of seafloor in the western equatorial Pacific Ocean. Of Early Cretaceous (~120 Ma) age, the plateau comprises three major structural highs. The High Plateau to the east contains several islands, including the eponymous Manihiki atoll. To the west, the Western Plateaus lie approximately one kilometer deeper; they are bifurcated by elongated, overall northeast-trending, bathymetric lows known as the Danger Islands troughs (DITs). North of the Western Plateaus is the small, nearly separate North Plateau that is separated from the High Plateau (and contiguous NE portion of the Western Plateaus) by the High-North Basin. In May-June 2007, we acquired extensive multibeam bathymetry and reflectivity over all three structural highs and their flanks during the 40-day F.S. Sonne cruise 193. On the basis of these new as well as pre-existing data, we propose a tectonic model for post-emplacement, probable Late Cretaceous deformation of the Manihiki Plateau involving both extensional and transtensional deformation. In our model, the High-North Basin probably formed by seafloor spreading; the northwestern margin of the contiguous NE portion of the Western Plateaus-High Plateau and the southeastern margin of the North Plateau are conjugate rifted margins, whose separation approximately equals the right-lateral offset between the southern flanks of Western Plateaus on either side of the DITs. The curvilinear southwest boundary of the High-North Basin consists of several pull-apart basins that abut the steep northeast flank of the Western plateaus; LIP and normal oceanic crust are juxtaposed along this boundary. The right-lateral relict plate boundary continues uninterrupted to the south of the deep ocean basin as the DITs, which in the study area comprise a series of major en echelon, right- lateral faults that step to the right, producing extensional relay zones and pull-apart basins between fault tips where displacement was transferred. Geochronology of igneous rock samples dredged from the flanks of the High-North Basin and the DITs during the Sonne expedition may yield information on the timing of deformation, which would in turn contribute to a better understanding of regional Cretaceous plate kinematics.
T13A-1125
The Cretaceous OAE1a-Submarine Plateau Link: Additional Geochemical Evidence from Marine Sedimentary Sections
We explore the proposed link between submarine plateau volcanism associated with construction of the Ontong Java-Manihiki-Hikurangi plateau (122 Ma) and early Aptian Ocean Anoxic Event 1a (OAE1a) through biostratigraphic data and trace metal abundance anomalies in marine sedimentary sections recovered in cores from DSDP Site 167 (Magellan Rise), ODP Sites 463 and 866 (Mid-Pacific Mountains), and at the Cismon and Piobbico on-land drillsites (Belluno and Umbria-Marche Basins, Italy). Sections were correlated using bio- and magneto-stratigraphic data, and the global d13C isotope anomaly associated with OAE1a. Bulk sediment samples that bracket the OAE1a interval at each site were analyzed by ICP-MS methods. After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that precedes the abrupt positive climb in the d13C isotope excursion, beginning near magnetic chron M0, continuing through the organic-rich interval of anoxic conditions. The metal abundance anomalies (e.g., Sc, Cu, Co, Sn, Cr, Ni, V, Cd, Ag, Bi, Se, W, Mo, Sb, Pb up to 100x background), variable in intensity and pattern of elements, indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that turnover in plankton communities and increases in isotopically light organic carbon burial occurred. The stratigraphic position of the intervals of trace metal anomalies matches events prior, during and after OAE1a and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in biota, carbon burial and deep ocean oxygen contents. Calcareous nannofossil abundance and composition display major changes in biogenic paleofluxes, temperature and fertility of surface waters, coeval with metal enrichments. Thus, hydrothermal activity during plateau construction seems vital for primary productivity as well as for biocalcification. The geographical variation in abundances and patterns of trace metals is consistent with a source in the south- central Pacific.
T13A-1126
Subsurface Structure of the "petit-spot" Volcanoes on the Northwestern Pacific Plate
A seismic reflection survey was conducted in the northwestern Pacific to investigate subsurface structure of small volcanoes considered to be formed by the newly-discovered "petit-spot" intra-plate volcanism. In contrast with an acoustically transparent sedimentary layer in the ambient northwestern Pacific, sedimentary layers are acoustically opaque with interbedded strong reflections beneath the volcanoes. These reflections are possibly caused by inhomogeneous structure due to intrusion of central vent. Subhorizontal strong reflections at base of the sedimentary layer, identified in the vicinity of volcanoes, are probably reflections from sills of dense lavas flowing out within the sedimentary layer. These volcanoes could be monogenetic volcanoes produced by small amount of magma intrusion, <1 km3 in volume. Although such small volcanoes have never been discovered unless through a high-resolution bathymetric mapping, the "invisible" intra-plate volcanism affects successive evolution/modification of the crustal architecture of old-age oceanic plates.
T13A-1127
A discovery of another petit spot volcanic field in the Cretaceous Pacific Plate
Petit spot is a volcanic micro-knolls recently discovered in the NW Pacific (Hirano et al., 2006). According to Hirano et al. (2006), this new kind of intra-plate volcanism is explained by reaking of small fraction of upper mantle melt from asthenosphere through flexure-induced fractures in the subducting oceanic plate. Thus, they suggest it is a non-plume related intra-plate. Two volcanic fields of petit spot volcanoes have discovered in the NW Pacific on a single flow line of the current plate motion since a JAMSTEC R/V Kairei cruise (KR04-08) in June 2004. They erupted at 0.1~1.0 Ma on one site and 4.2-8.5 Ma on the other site. Petit spot volcanism is still enigmatic phenomenon. We do not know the eruption mechanism, the melt formation in the upper mantle, nor their uniqueness. On the other hand, another possible volcanic field of petit spot volcanoes was previously shown by multi-narrow beam survey (Hirano et al., 2006). This field is also in the Cretaceous Pacific Plate, but about 200km away from the same flow line with the previously discovered petit spot volcanic field. If it is also petit spot volcanic field, it will imply that this kind of melt leaking volcanism is not unique, but occurs anywhere in the similar tectonic settings of the oceanic plates. During R/V Kairei cruise in June 2007 (KR07-07), three dives by JAMSTEC ROV KAIKO 7000II took place the field, called -Chocochip knolls-. We sampled petitspot-like young alkaline basalt from two knolls of them, and the other one knoll has a large volcanic caldera with fresh outcrops of mid-ocean ridge basalt and dolerite. Their eruption ages have not measured yet, but they are certainly unaltered recent erupted basalt in the Cretaceous Pacific Plate as it was expected. Furthermore, these knolls are overlapped with an aftershock distribution of one large outer rise earthquake (M7.1) in 2005. That is sub-parallel to the trench axis, and horst & graben structure on the Pacific Plate. The relationships between the petit spot volcanism and the outer rise earthquake is not clear, but the petit spot activity certainly disturbs the oceanic plate strutigraphy and may change its physical property.
T13A-1128
Seamounts, knolls and petit spots on the NW Pacific Plate represent intra-plate volcanism from the Cretaceous to present
Most of seamounts of the western Pacific formed before 70 Ma in the so-called West Pacific Seamount Province (WPSP) which is characterized by relatively short seamount chains maybe indicating a significant short-lived hotspot system (Koppers et al., 2003). As for the NW Pacific Plate offshore of Northeast Japan, the Joban and Japanese Seamount Trail are also composed of middle Cretaceous seamounts, which are erupted on the northern margin of WPSP. The 120 to 100 Ma seamounts in the Joban seamount chain do not show a middle Cretaceous hotspot track, whereas the Japanese seamount chain shows a well-established ENE to WSW trend in this age range. On the other hand, the unnamed knolls, which are well-circular and flat-topped in shape, are scattered on the NW Pacific Plate and are not aligned to any volcanic chains. These were correspond to eruptive ages of 70 Ma based on Ar-Ar ages of a second volcanic event in the NW Pacific. As the last stage, we should note that Hirano et al. (2006) reported the 0-1, 2, 4.2, 6.0 and 8.6 Ma volcanoes, called petit spots, in the Japan Trench on the outer-rise system. The petit spot volcanoes imply episodic eruptions of magma over a distance of 600 km of plate motion on the flexural Pacific Plate before its subduction but with low volumes of magma production. The volume of volcanic edifice of the petit spot volcanoes certainly is several orders of magnitude less than the Cretaceous seamounts and knolls. Therefore, we can interpret that the petit spot volcanoes are not related to any mantle plumes and hotspots. Evidence includes the geochemical data and the tectonic alignment of the volcanoes which show that the petit spot lavas escaped along fractures in the lithosphere and were sourced from small pockets of asthenospheric melts. The bathymetric map and sidescan sonar imagery of the ocean-ward slope in the Tonga Trench also shows a possible presence of young volcanoes. Such small volcanoes, therefore, may be ubiquitous on the ocean floor, where incipient melts in the asthenosphere can be squeezed out by tectonic forces associated with plate flexure of outer-rise. http://www.geo.titech.ac.jp/lab/takahashi/staff/nhirano/
T13A-1129
Upper Mantle Composition Beneath the Petit-Spot Area in Northwestern Pacific: Insights From Electrical Conductivity
The mantle composition beneath the petit-spot area, where is about 500 km offshore from Japan Trench in northwestern Pacific, is discussed through electrical conductivity obtained by seafloor magnetotelluric (MT) survey. The seafloor MT data were collected using ocean bottom electromagnetometers (OBEMs) at four sites with the spacing of 100-150 km, between May and August, 2005. The survey was conducted as a part of the petit-spot multidsciplinary project. The petit-spot is young volcanic activity on very old (~130 Ma) oceanic plate characterized as a clump of small knolls which erupted strong to moderate alkaline basalt. This volcanic field is associated with neither any plate boundaries nor hot spots. To elucidate the magma generation process of this new-type volcanic activity, a collaborative study of various geophysical and geochemical approaches has been carried out. The MT survey aims to constrain the physical state of the lithosphere and asthenosphere where the petit-spot melt is probably generated. The acquired electromagnetic field variation data were analyzed and the MT responses, which is the transfer function between the electric and magnetic fields, were obtained. The effect for the ocean-land distribution and seafloor topography on the MT responses was modeled and stripped. As the result, the corrected responses indicate that the lateral heterogeneity in electrical conductivity is less significant beneath the survey area. One- dimensional inversion study shows that the data require a peak in conductivity (0.05 S/m) at about 200 km depth. The mantle temperature may be calculated from the conductivity using an experimental result for dry olivine (Constable et al., 1992). The resultant temperature is about 1750 °C which is lower than the dry solidus for garnet peridotite. Instead, assuming the temperature as GDH1 model (Stein and Stein, 1992) for 130 Myr old mantle, we calculate water content in olivine using an experimental result by Wang et al. (2006). The resultant water content is about 0.001 wt%, which is again too small to decrease solidus and to initiate partial melting. Consequently, the conductivity model does not support the melt generation in the asthenosphere. The inconsistency between the conductivity model and the existence of the petit-spot volcanoes may be explained by contribution of material like eclogite which the solidus is much lower than garnet peridotite.
T13A-1130
Calibration of Pre-M25 Marine Magnetic Anomalies: Magnetic Polarity Composite of ýLate Callovian Through Kimmeridgian
Ammonite-zoned successions have yielded a composite magnetic polarity pattern ýspanning latest Callovian (lamberti ammonite Zone) through Late Kimmeridgian ýý(acanthicum Zone) that confirms marine magnetic anomalies M37 through to M24 ýinterpreted by deep-tow and other magnetic surveys in the western Pacific. This pattern ýwas constructed after thermal demagnetization of over 1000 samples from over 30 ýsections in Poland, British Isles, France and Spain. Polish sections include thick ýammonite-zoned limestone formations of the Krakow-Czestochowa-Wielun Upland and ýHoly-Cross Mountains. British limestone and clay formations were investigated in ýEngland (Dorset and Yorkshire) and in Scotland (the Isle of Skye). The sections include ýcandidates for the global stratotypes for the Callovian–Oxfordian and Oxfordian–ýKimmeridgian stage boundaries. All British and most of the Polish-French-Spanish ýsections are calibrated to ammonite biostratigraphy at the subzone level (Boreal-ýSubboreal realm and Sub-Mediterranean realm, respectively) and to regional sequence ýstratigraphy. The independent Boreal-Subboreal and Sub-Mediterranean composites of ýmagnetic polarity are consistent, and the main features of the modeled pre-M25 marine ýmagnetics can be calibrated. ý The Callovian–Oxfordian boundary (base of Quenstedtoceras mariae Zone) occurs in a ýnarrow normal-polarity subzone correlated to polarity subchron M36a of the western ýPacific magnetic polarity pattern. The beginning of the Middle and the Late Oxfordian ýsubstages as defined in the Sub-Mediterranean province in Poland correspond ýapproximately to M33 and M29 of the Pacific M-sequence. The placement of the ýOxfordian- Kimmeridgian boundary in the Sub-Boreal ammonite zonation (base of ýPictonia baylei Zone) is at the beginning of the M27r polarity zone. This is significantly ýolder than the traditional placement of the Oxfordian–Kimmeridgian boundary in the ýSub-Mediterranean zonation (base of Sutneria platynota Zone) near the base of the M25r ýpolarity zone.ý This project is a collaboration with A. Wierzbowski, M. Lewandowski, E. Glowniak, J. ýGutowski, P. Ziolkowski, M. Sidorczuk, and Z. Zlonkiewicz (Poland); A. Coe and J. ýWright, (England); and N. Nowaczyk (Germany)ý
T13A-1131
A Robust Compilation of the Pacific M-anomaly Geomagnetic Polarity Sequence
The current M-anomaly geomagnetic polarity time scale (GPTS) is mainly based on the Hawaiian lineation set in the Pacific Ocean. Furthermore, existing studies of this GPTS rely on a relatively small number of magnetic profiles. This limited basis is contrary to the fact that the GPTS is assumed to represent a global reversal record. Compiling an anomaly record from a larger array of magnetic profiles from all ocean basins is desirable because consistency and repeatability support the global nature of each polarity reversal. We present a new compilation of the M-anomaly GPTS derived from a Pacific-wide set of Late Jurassic to Early Cretaceous anomalies as a first step toward a global model. The anomalies were collected from three lineation sets (Hawaiian, Japanese, and Phoenix) across the western Pacific basin and resampled for subsequent modeling. Both inverse and forward calculations were applied to estimate polarity reversal boundaries. Polarity block models were established within each lineation set separately to allow examination of variability among the different lineation sets. Subsequently, a composite polarity block model was constructed by normalizing and averaging the block models from the three lineation groups. The large number of magnetic profiles used in this study allows us to compute errors and test coherence in anomaly correlations. Consistency and repeatability of most of Mesozoic anomalies are excellent amongst the three lineation sets, except for the pre-M25 anomalies. The difficulties with pre-M25 anomaly correlations raise the question whether we can construct a reliable GPTS model for this time period. Owing to lack of reliable direct dating of the M-anomalies on ocean crust, the composite model has few age calibration points. Nevertheless, the polarity block models from this study are a better foundation for future GPTS models. Our compilation represents a wider and statistically more robust analysis of anomalies, giving a more reliable record of changes in the Earth's geomagnetic field.
T13A-1132
Updated Pacific Plate Paleomagnetic Pole for Chron 32 (72 Ma), Uncertainties in Pacific- Hotspot Rotations, and Updated Pacific-Hotspot Plate Reconstructions
A fundamental problem of global tectonics and paleomagnetism is determining what part of apparent polar wander is due to plate motion and what part is due to true polar wander. One approach for separating these is available if the hotspots are tracking the motion of the mantle beneath the asthenosphere. To make progress on these questions and assumptions, high-quality paleomagnetic poles for the Pacific plate are needed, as well as estimates of Pacific plate motion relative to the hotspots and the uncertainties in such motion. Here we present results for all three. First, we present an updated Pacific paleomagnetic pole for chron 32 (72 Ma) determined from the skewness of magnetic anomaly 32 (Petronotis et al. 1999). The updated paleomagnetic pole corrects for the spreading-rate dependence of anomalous skewness (Dyment & Arkani-Hamed 1995, Koivisto et al. 2006). We furthermore build on a new method for objectively estimating plate-hotspot rotations and their uncertainties (Andrews et al. 2005) and present an updated reconstruction of the Pacific plate relative to the hotspots at 72 Ma along with the uncertainties in the reconstructions. We combine paleomagnetic and plate reconstructions to determine the total uncertainty of the 72 Ma Pacific plate paleomagnetic pole reconstructed into the Pacific hotspot frame of reference. The results indicate that the pole is distinctively different from the present spin axis, but consistent with our estimate of the coeval paleomagnetic pole for the Indo-Atlantic hotspots. These results therefore indicate that the hypothesis that the Indo-Atlantic hotspots have been fixed relative to the Pacific hotspots cannot be rejected from paleomagnetic data.
T13A-1133
Evidence for True Polar Wander since mid-Cenozoic time: A Paleomagnetic Investigation of the Skewness of Magnetic Anomaly 12r (32 Ma) Between the Galapagos and Clarion Fracture Zones on the Pacific Plate
In recent years, some researchers have asserted that there has been no motion of the Pacific hotspots relative to the spin axis since the age (ca. 47 Ma) of the elbow in the Hawaiian-Emperor chain (e.g., Tarduno et al. 2003). In contrast, the apparent polar wander of the Indo-Atlantic hotspots shows distinct motion of the hotspots relative to the spin axis over the same time interval (e.g., Morgan 1981; Besse and Courtillot 2002). If this latter shift is due to true polar wander, one would expect to see a similar shift of Pacific hotspots relative to the spin axis. Here we present critical new data and analyses to test these distinctly different hypotheses. Specifically, we present results of an investigation of the skewness of magnetic anomaly crossings of anomaly 12r between the Galapagos and Clipperton and between the Clipperton and Clarion fracture zones on the Pacific plate. We chose to focus on these adjacent regions for three reasons. First, numerical experiments showed that these crossings, of all those available from the Pacific plate, should contain the most information about the location of the 32 Ma paleomagnetic pole for the Pacific plate. Second, many of the available crossings are from vector aeromagnetic profiles, which have superior signal to noise ratios (Horner-Johnson and Gordon, 2003). Third, the rate of seafloor spreading recorded in these crossings exceeds the threshold (half rate of 50 mm/yr) above which no anomalous skewness occurs. Moreover, for the first time, we combine uncertainties in plate- hotspot rotations (Andrews et al. 2005) with paleomagnetic uncertainties to obtain the total uncertainties of our new paleomagnetic pole reconstructed into the Pacific hotspot frame of reference. The results show significant and unambiguous motion of Pacific hotspots relative to the spin axis since 32 Ma. Moreover, when the 32 Ma Pacific plate paleomagnetic pole is reconstructed into the Pacific hotspot reference frame, it is consistent with the paleomagnetic pole of the Indo-Atlantic hotspots. We conclude that the global set of hotspots have mainly moved in unison relative to the spin axis since 32 Ma, which is most simply interpreted as true polar wander.
T13A-1134
Polarity Zones and Apparent Polar Wander at the Jurassic /Cretaceous Boundary in the Southern Alps, Italy
The magnetic and nannofossil stratigraphies of several sections spanning the Jurassic/Cretaceous (J/K) boundary have been studied in the Lombardian Basin (Torre de Busi, Colle di Sogno) and Trento Plateau (Colme di Vignole, Foza). The magnetic stratigraphies span the CM16R (late Berriasian) to the CM22 (Kimmeridgian/Tithonian boundary) interval corresponding to the 138-150 Ma interval spanning the J/K boundary (142 Ma). In Lombardy, the facies transition from "Rosso ad Aptici" (siliceous limestone) to Maiolica (pelagic limestone) occurs in the latest Tithonian (CM19N-CM20N), just prior to the J/K boundary, closely synchronous with the transition from Rosso Ammonitico to Maiolica on the Trento Plateau. The identification of polarity zones is based on polarity-zone pattern fit, and the previously established correlations of polarity chrons to nannofossil events/zones. The apparent polar wander path (APWP) for each section is calibrated based on the magnetic stratigraphy and lies close to an estimated African APWP of the same age, implying very little rotation of the Lombardian and Trento plateau regions of the Southern Alps relative to Africa since latest Jurassic time. Our objective is to produce a calibrated APWP (polar wander by polarity chron), and thereby improve the definition of the Adria APWP in this interval of relatively rapid apparent polar wander.
T13A-1135
Mapping Geomagnetic Field Variations in the Cretaceous Quiet Zone with Unmanned Airborne Vehicles
About one quarter of the present seafloor was generated during the constant normal polarity interval from 121 to 83 Ma (Cretaceous Quiet Zone or KQZ), and the lack of temporal markers limits tectonic reconstructions in these areas. Although magnetostratigraphic studies provide strong evidence that the KQZ formed during predominantly normal polarity, there are nonetheless relatively large amplitude variations in many sea surface magnetic anomaly profiles crossing KQZ crust. To evaluate the relative importance of geomagnetic and crustal variables (thickness, geochemistry) in generating these anomalies, we collected multibeam bathymetry and magnetic data on 19 profiles crossing anomaly 34 and extending 500 km into the KQZ in the southwest Pacific. The relatively fast spreading (60 km/m.y. half rate), minimal sediment cover and high paleolatitude of formation make this area ideal for evaluating the magnetic anomaly pattern. An additional 10,000 km of magnetic anomaly data were acquired using an autonomous unmanned airborne vehicle (UAV). Although land-launched UAVs have been used in a variety of research applications, the nine successful flights during our cruise represent the first deployment from a UNOLS research vessel. The UAV (operated by Fugro Airborne) was launched from a pneumatic catapult and captured by a wingtip clip that attaches to a rope suspended from a retractable boom on the fantail. The Cs-vapor magnetometer data from the UAV compare favorably with results from the surface-towed magnetometer, with minor differences related primarily to the higher elevation (120m above sea level) of the UAV. The resulting magnetic coverage indicates that, as with younger seafloor, quasi-linear short wavelength anomalies are present within the KQZ. These anomalies can vary on spatial scales smaller than the multibeam swath width, highlighting the utility of obtaining additional coverage with the UAVs.