T12A-01
The Manihiki Plateau, Hikurangi Plateau, Wishbone Scarp, and Osbourn Trough: A Review and Analysis
The extinct Osbourn Trough spreading system in the southwestern Pacific played a key role in separating the once joined Hikurangi and Manihiki Plateaus in Cretaceous time. Recent studies by Downey et al. [2007] and Taylor [20006] have provided new data and concepts on that history. Studies by Larson et al. [2002] describe Cretaceous histories adjacent and east of the Osbourn system and Eagles et al. [2004] describe a history for the southwest Pacific just after the Osbourn system became extinct. The lack of identifiable magnetic anomalies allows tectonic events during separation of the plateaus and spreading on the Osbourn Trough to occur between about 124.6 and 84 Ma (Chron 34). Satellite gravity maps of the region identify what are interpreted as Cretaceous fracture zone trends. Swath bathymetry data reveal at least four provinces of abyssal hill trends. What is known are the following: Minimum age of the Manihiki High Plateau at 123.4 Ma from DSDP Site 317, the age of seafloor, 115 Ma, from a dredge sample from the southern segment of the West Wishbone Scarp 300 km northeast of the Hikurangi Plateau, the east-west trend and extent of the extinct Osbourn spreading center at latitude 26° S, the trends (NNE-SSW to NE-SW) and extent of portions of the West Wishbone Scarp east of the Hikurangi Plateau, the N-S trend of the East Wishbone Scarp that appears to truncate the West Wishbone Scarp, the NNE- SSW trend and extent of the prominent Manihiki (Eastern) Scarp at the eastern boundary of the Manihiki High Plateau, the NW-SE trend and extent of the Rapuhia Scarp on the northwestern Hikurangi Plateau, and trends in abyssal hills; E-W near the Osbourn Trough and WNW-ESE closer to the plateaus. Differences in abyssal hill morphology suggest one or more changes in spreading rates. The Osbourn Trough began rifting apart a Large Igneous Province formed at 123.5 +/- 1.5 Ma, into the separate Ontong-Java, Manihiki, and Hikurangi Plateaus by 121 Ma. After an initial 6 Myr interval of slow spreading at 4.7 cm/yr (full rate -fsr), the rate at 115 Ma increased to ~18.3 cm/yr fsr at an azimuth of 197.4° (holding Manihiki Plateau fixed) and continued for 10 Myr to 105 Ma. At 105 Ma, the Hikurangi Plateau entered a subduction zone at the north edge of the present Chatham Rise. It is assumed that this caused a change in spreading direction to an azimuth of 183.5° as observed from abyssal hill strike data. Fast spreading is assumed to have continued in this new direction for another 3.5 Myr until 101.5 Ma when a change in abyssal hill morphology suggests a large decrease in rate. From this time until the extinction of the Osbourn Trough at 97-93 Ma, slow spreading proceeded at a rate between 2.6-4.9 cm/yr fsr. Spreading rate calculations are based on assuming the changes in spreading direction seen in abyssal hill trends reflects a change in the tectonic regime of the Chatham subduction zone at 105 Ma. This history accounts for the common origin of the Manihiki Eastern Scarp and West Wishbone Ridge and adjoins the Manihiki High Plateau with deeper portions of the east Hikurangi Plateau. The East Wishbone Ridge is a fracture zone that postdates the end of Osbourn Trough spreading and is responsible for the southward removal of missing Osbourn system fracture zones and ridge segments east of it.
T12A-02 INVITED
Large Igneous Provinces of the Pacific: Insights into Mantle Dynamics and Environmental Consequences
A rich mosaic of disparate crustal types characterizes the Earth beneath the sea, and although normal oceanic crust approximately seven kilometers thick is by far the most prevalent, abnormally thick oceanic-type crust of large igneous provinces (LIPs) also forms a significant component of the marine realm. LIPs are massive and rapid crustal emplacements of predominantly Fe- and Mg-rich (mafic) rock that have not formed by seafloor spreading or subduction. LIPs may represent the dominant form of magmatism on other terrestrial planets and moons of the solar system. On Earth, LIP rocks are readily distinguishable from mid-ocean ridge and subduction- related arc rocks on the basis of petrology, geochemistry, geochronology, physical volcanology, and geophysical data. LIPs occur within both continents and oceans, and include continental flood basalts in the former, and volcanic passive margins, oceanic plateaus, submarine ridges, and ocean basin flood basalts in the latter. Transient LIPs and associated persistent, age-progressive volcanic chains, or hot spot tracks, are commonly attributed to decompression melting of hot, buoyant mantle ascending from the Earth's interior, and thus provide a window into mantle processes. Magmatism associated with LIPs currently represents approximately 10% of the mass and energy flux from the Earth's deep interior to its surface. This flux shows distinct episodicity over geological time, in contrast to the relatively steady-state mode of crustal accretion at seafloor spreading centers. LIP observations therefore suggest that dynamic, non-steady- state circulation within the Earth's mantle has been important for at least the last 250 million years and probably much longer, and a strong potential for LIP emplacements to contribute to, and perhaps even instigate, major environmental changes. In particular, research on Cretaceous Pacific oceanic plateaus - Ontong Java, Manihiki, Hikurangi, Shatsky, Hess, Magellan - and ocean basin flood basalts - Nauru, East Mariana, Pigafetta - has stimulated many ideas about mantle dynamics and the environmental consequences of LIPs.
T12A-03 INVITED
Gravity, Bathymetry and Submarine Volcanism in the Mesozoic Pacific Ocean
Submarine volcano loading studies suggest that the effective elastic thickness, Te, of oceanic lithosphere increases with age at the time of loading. Therefore, a seamount formed on a ridge crest will be characterised by a lower Te than a similar size feature that formed off-ridge. Compilations of data where both crustal and sample ages are known show that Te is given approximately by the depth to the 450° oceanic isotherm, based on plate cooling models. By comparing observed bathymetry and gravity anomalies to predictions based on simple elastic plate models it is possible to estimate Te and hence the age of oceanic lithosphere at the time of loading at bathymetric features of unknown tectonic setting. Early results based on ~100 features suggested that Hess Rise, Necker ridge, Line Islands, and Manihiki Plateaus formed on-ridge and, hence, that there was a major period of volcanism in the central Pacific ~90- 120 Ma. This ‘event' appears to have been accompanied by deep-water volcanism, as shown by the pioneering work of Roger L. Larson in the Nauru Basin. Recently, Watts et al. (2006) used a bathymetric prediction technique to estimate the Te at >9000 seamounts in the Wessel (2001) database. Plots of Te Vs. age at features of known age, however, revealed considerable scatter with many lower values at old ages than expected. Te maps show that these low values form a broad swath from East Pacific Rise crest in the SE, through the Tuamotu Plateau region, to the Line and Marshall Islands and Mid-Pacific Mountains in the NW. The SE end of the swath includes the region dubbed the South Pacific Isotopic and Thermal Anomaly (SOPITA) and some features (e.g. Marcus Wake Guyots, Lines Islands) at the NW end backtrack into the SOPITA. Therefore, some of the scatter maybe caused by a regional shallowing of the controlling isotherm. This has been verified using a moving window admittance technique which suggest controlling isotherms of <~350° as the SOPITA region is approached. These new constraints on the controlling isotherms are used here to estimate the distribution of submarine volcanism in the Mesozoic Pacific Ocean through time.
T12A-04
Petrologic Aspects of Seamount and Guyot Volcanism on the Ancestral Mesozoic Pacific Plate: a Review
Hundreds of large seamounts and guyots are widely scattered almost in a "shotgun-blast" arrangement in an area about the size of the United States west of the Mississippi River on the Mesozoic Pacific plate between the Mariana Trench and the Gilbert Islands. Most of these formed between ~160-100 Ma while the Pacific plate was surrounded by spreading ridges and growing outward in all directions. There is little to no indication that the seamounts and guyots formed along linear seamount chains; existing radiometric-age data show no age progressions. The volcanoes appear to have formed in response to a uniform stress configuration across the plate, which was either not moving or moving very slowly at the time (1, 2), much like the modern Antarctic plate. When the growing plate started to encounter subduction systems in the western Pacific at ~90 Ma, consistent stress patterns began to develop, and the broad linear Gilbert and Line volcanic ridge systems began to form. Even then, however, considerable overlapping of volcanism occurred, and only the most general age progressions are evident in existing data. Petrologic data from samples obtained from dozens of volcanic summits by dredging and beneath several carbonate platforms by drilling reveal considerable diversity in development of differentiated alkalic magmatic lineages rooted in diverse parental basaltic rocks. These include transitional, alkalic and basanitic compositions, with differentiates of hawaiite, mugearite, trachyte and one phonolite. Many of the basaltic rocks are partly to significantly transformed by alteration under oxidative conditions (dredged rocks) and both oxidative and non-oxidative conditions (drilled rocks). This can make estimations of mantle geochemical provenance difficult. Nevertheless, the province has been linked by backtracking techniques to the modern SOPITA region of the South Pacific (3), and its rocks show enrichments in trace elements and isotopic characteristics similar to lavas from the Cook-Austral, Marquesas, Society, and Samoan linear volcanic chains. Significantly, Hawaiian- type tholeiite has not been sampled in the region, and the diversity of basaltic rocks and differentiates has always been high. Even unusual potassic nephelinites (K2O > Na2O) with phenocrysts of kaersutitic amphibole or phlogopite occur in the Wake and Line Seamounts. These resemble lavas of portions of the East African Rift, but also have counterparts in the Samoan and Society chains, and resemble very young basalts obtained on the outer trench swell of the Pacific plate near Japan. I suggest that variably and often strongly enriched material was originally supplied to the shallow upper mantle beneath a broad region of the Pacific plate during the Mesozoic; that partial melts of this material were subsequently tapped along major fracture systems that developed to form linear island chains as stress configurations changed on the Pacific plate; and that narrow plume conduits of ascending mantle have never figured in the emplacement of the broadly distributed enriched SOPITA volcanoes. 1) Natland, J. H., and Winterer, E.L., 2005, GSA Spec. Paper 388: 687-710. 2) Larson, R.L., et al., 1992, Proc. ODP, Sci Results, 129: p. 615-631; 3) Staudigel, H., et al., 1991, EPSL, 102: 24-44.
T12A-05
Seamount Moats in the Central Equatorial Pacific Ocean: Origins and Implications
Numerous seamounts in the central equatorial Pacific Ocean are surrounded by shallow, circular depressions, which we term seamount moats. These moats are typically 50-150 m deep, 1-2 km wide, and surround relatively small seamounts (e.g., relief less than 500 m). The majority of the seamount moats are located on crust of Cretaceous age (70-130 Ma) at depths greater than 4500 m. Perhaps the most interesting aspect of the moat distribution is the predominance of these features within 5 degrees of the equator. Any moats found beyond this equatorial zone occur on seafloor shallower than 3500 m and are typically associated with more recent volcanic events. Several possible origins for these features have been explored (e.g., lithosphere flexure, sediment scour, tectonic collapse); however, we prefer a sediment dissolution model similar to that proposed by Bekins et al. (2007). In this model, carbonate-rich seawater enters the fractured, permeable ocean crust and begins to warm. The retrograde solubility of carbonate results in the distributed precipitation of carbonate minerals within the crustal aquifer. The remaining water then exits the crust through the seamounts and cools again to ambient seawater temperatures. At these cooler temperatures, the water is under-saturated with respect to carbonate and begins to dissolve the carbonates within the overlying or adjacent sediments. There are several interesting implications for this carbonate dissolution scenario to explain the origin of the seamount moats. First, significant hydrothermal circulation occurs in crust much older than previously assumed (e.g., 70-130 Ma). Second, discharge rates calculated for the missing sediment volume are comparable or slightly less than previous estimates for younger crust (e.g., 1 m/yr). Finally, the distributed precipitation of carbonates suggests the ocean crust may be a significant reservoir for the global carbon budget.
T12A-06 INVITED
Progress and Perspectives on the Pacific Plate Apparent Polar Wander Path
Because the Pacific plate has moved rapidly, its apparent polar wander path (APWP) shows a large amount of motion since Cretaceous time. This APWP is useful not only for understanding the tectonics of the Earth's largest plate, but also for comparison with other plate tectonic reference frames, such as the hotspots. Roger Larson made significant strides in defining the Pacific APWP with early studies of seamount magnetic anomalies that showed significant northward drift of the plate, by publishing a study of DSDP cores that implied Late Jurassic southward motion of the plate, and by authoring a study on M-anomaly skewness that illuminated the Cretaceous "hook" in the APWP. The Pacific APWP is coming into better focus owing to new paleomagnetic data and data compilations; although, significant questions about the path remain. Recent results confirm ~40° of northward motion since ~123 Ma, but show an irregular amount of northward motion. During one period (94-80 Ma), apparent polar wander was rapid (~1°/Ma) whereas a subsequent time span (80-47 Ma) shows a still-stand. The overall shape of the APWP appears to correspond, however, to the known history of the Pacific plate and its interactions with surrounding plate boundaries. Despite recent advances, significant uncertainties still exist in the shape and details of the Pacific APWP. The chief problem is that paleomagnetic data are limited because the plate is almost entirely covered by ocean. As a result, the APWP is constrained mostly by "non-standard" paleomagnetic data, including azimuthally-unoriented core data and magnetic anomaly inversions (of seamounts and lineation skewness). Important questions that remain to be resolved are: (1) the direction and amount of early southward drift of the plate, (2) better characterization of the period of Late Cretaceous rapid apparent polar wander and the still-stand, (3) better understanding of why Ontong Java Plateau paleomagnetic data are anomalous with respect to other Pacific data, (4) reconciliation of apparent differences between skewness and other paleomagnetic data, and (5) an integration of paleomagnetic data from the south Pacific.
T12A-07 INVITED
Paleoenvironments of the Jurassic and Cretaceous Oceans: Selected Highlights
There are many themes contributing to the sedimentation history of the Mesozoic oceans. This overview briefly examines the roles of the carbonate compensation depth (CCD) and the associated levels of atmospheric carbon dioxide, of the evolution of marine calcareous microplankton, of major transgressive and regressive trends, and of super-plume eruptions. Initiation of Atlantic seafloor spreading in the Middle Jurassic coincided with an elevated carbonate compensation depth (CCD) in the Pacific-Tethys mega-ocean. Organic-rich sediments that would become the oil wealth of regions from Saudi Arabia to the North Sea were deposited during a continued rise in CCD during the Oxfordian-early Kimmeridgian, which suggests a possible increase in carbon dioxide release by oceanic volcanic activity. Deep-sea deposits in near-equatorial settings are dominated by siliceous shales or cherts, which reflect the productivity of siliceous microfossils in the tropical surface waters. The end-Jurassic explosion in productivity by calcareous microplankton contributed to the lowering of the CCD and onset of the chalk ("creta") deposits that characterize the Tithonian and lower Cretaceous in all ocean basins. During the mid-Cretaceous, the eruption of enormous Pacific igneous provinces (Ontong Java Plateau and coeval edifices) increased carbon dioxide levels. The resulting rise in CCD terminated chalk deposition in the deep sea. The excess carbon was progressively removed in widespread black-shale deposits in the Atlantic basins and other regions – another major episode of oil source rock. A major long-term transgression during middle and late Cretaceous was accompanied by extensive chalk deposition on continental shelves and seaways while the oceanic CCD remained elevated. Pacific guyots document major oscillations (sequences) of global sea level superimposed on this broad highstand. The Cretaceous closed with a progressive sea-level regression and lowering of the CCD that again enabled widespread carbonate deposition in the deep sea.
T12A-08
Upwelling in the Jurassic-Cretaceous Pacific Ocean: Biota Changes and Sedimentary Evidence for Paleoequatorial Crossings of the Pacific Plate
The migration of the Pacific Plate during the Jurassic and Cretaceous can be traced by deciphering the lithological and paleontological composition of sediments deposited under different paleoenvironmental conditions. Several sites drilled during DSDP and ODP legs recovered Cretaceous and, partly, Jurassic sedimentary successions that have been used to reconstruct paleoceanographic changes related to paleolatitudes. In the Pacific Ocean chert, radiolarite and porcellanite are the dominant pelagic lithologies in specific intervals of the Mesozoic, more commonly represented by limestone. The siliceous lithotypes are the expression of biogenic sedimentation in the equatorial fertility belt, whereas limestones result from production of calcareous plankton under stable, oligotrophic conditions. The widespread distribution of massive chert is easily detectable on seismic profiles, where constitutes a strong reflector trough the Northwestern Pacific. Nannofossil quantitative data and sedimentary changes are combined to define the paleoequatorial upwelling belt, characterized by physico-chemico-trophic conditions favoring opportunistic taxa. The results are consistent with previous reconstructions based on shallow-water successions from the tropical Pacific, recording atoll drowning at peri-equatorial locations. The pelagic and neritic carbonate crisis zone approximately between 10° S and 10 °N paleolatidudes may be used to constrain the motions of the Pacific Plate.