March 2011 LIP of the Month

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Ocean Drilling at Shatsky Rise Gives Clues About Oceanic Plateau Formation

William W. Sager, Department of Oceanography, College Station, TX, 77843-3146, USA;

sager@ocean.tamu.eduhttp://ocean.tamu.edu/Directory/Faculty/Geo/sager.html

Introduction

Oceanic plateaus are giant volcanic features whose existence implies an extraordinary flux of magma from mantle to lithosphere (Duncan and Richards, 1991; Coffin and Eldholm, 1994).  By understanding their formation, these large igneous provinces can be important indicators of fundamental processes of mantle convection and geodynamics.  Although it is widely thought that oceanic plateaus arise from massive eruptions resulting from the arrival of a rising deep mantle plume head at the lithosphere, this hypothesis does not readily explain all features of plateaus that have been studied in detail.  An alternative explanation is that plateau eruptions are related to decompression melting of unusually fusible mantle beneath fast-spreading ridges (Foulger, 2007).  Shatsky Rise (Fig. 1) is a unique oceanic plateau because existing geologic and geophysical data are consistent with both types of formation mechanism.   On the one hand, initial eruptions apparently formed a huge volcanic edifice (Tamu Massif) and subsequent eruptions waned with time, consistent with formation by a plume head and tail (Sager, 2005).  On the other hand, it is traversed by magnetic lineations showing that it formed at a triple junction of spreading ridges during Late Jurassic and Early Cretaceous times, consistent with a strong tie to ridge tectonics (Nakanishi et al., 1999; Sager et al., 1999; Sager, 2005).  Thus, studies of Shatsky Rise should be able to help in sorting out arguments of plume and non-plume formation mechanisms.  Shatsky Rise is also a monster volcanic construct whose formation style is poorly understood.  The oldest and largest volcanic edifice of Shatsky Rise (Tamu Massif) is a volcano as large in area as any in the solar system.  Recently, two cruises collected new data at Shatsky Rise to learn more about this plateau.  During 2009, IODP Expedition 324 cored Shatsky Rise principally to study the igneous rocks of the volcanic massifs (Sager et al., 2011).  Last year, Cruise MGL1004 of the R/V Marcus G. Langseth collected seismic refraction and reflection data to better describe the structure of TAMU Massif. 

Drilling Results from IODP Expedition 324

Integrated Ocean Drilling Program (IODP) Expedition 324 cored Shatsky Rise at five sites to collect core samples that can be used for geological and geochemical studies (see Proceedings of the IODP, http://publications.iodp.org/proceedings/324/324title.htm).  Cored igneous sections consist mainly of variably evolved tholeiitic basalts emplaced as pillows or massive flows (Fig. 2).  Massive flows are thickest and make up the largest percentage of section on the largest and oldest volcano, Tamu Massif, implying it formed at high effusion rates.   Such massive flows are characteristic of flood basalts and similar flows were cored at Ontong Java Plateau.  Indeed the similarity of igneous sections at Site U1347 with that cored on Ontong Java Plateau implies similar volcanic styles for these two plateaus.  On younger, smaller Shatsky Rise volcanoes, pillow flows are common and massive flows thinner and fewer, implying volcanism waned with time.  Cored sediments from summit sites contain fossils and structures implying shallow water depths or emergence at the time of eruption and normal subsidence since.  Summit sites also show pervasive alteration that implies high fluid fluxes.  A thick section of volcaniclastics cored on Tamu Massif suggests that shallow, explosive submarine volcanism played a significant role in the geologic development of the plateau summit.  Expedition 324 results imply that Shatsky Rise began with massive eruptions forming a huge volcano and that subsequent eruptions waned in intensity, forming large, but not unusual volcanoes.

Seismic Data from Cruise MGL1004

Cruise MGL1004 of the R/V Marcus G. Langseth collected both seismic refraction and multichannel seismic (MCS) reflection data over Tamu Massif.  Two reversed seismic refraction lines were shot with a 6,600 cubic-inch airgun array, with the main line traversing Tamu Massif from NW-SE and extending into adjacent ocean basins (Fig. 3).  A second, shorter refraction line is perpendicular to the first and oriented along the Tamu Massif axis.  Data for the refraction lines were recorded by 28 ocean bottom seismometers resting on the seafloor.  Preliminary examination of the refraction data shows clear wide angle refraction arrivals out to source-receiver distances >200 km.  From these data it should be possible to define the Moho and crustal layers of the plateau even beneath its thickest parts.  Over 2,200 km of MCS reflection data were collected along seismic tracks that include the OBS refraction lines.  These seismic data define the top of the igneous pile (Fig. 4) and display intra-basement reflectors well into the volcanic edifice.  Generally, the seismic profiles show reflectors dipping away from the axis of Tamu Massif and presumably these are stacks of lava flows that emanated from an axial rift zone.  In addition, the MCS data apparently show the Moho beneath oceanic crust adjacent to the plateau.  With further study, the MGL10014 seismic data should provide significant information about the structure of Tamu Massif, which will help us understand its formation and evolution.

Figures


Figure 1: Shatsky Rise, surrounding magnetic lineations and fracture zones (heavy red lines; Nakanishi et al., 1999), Expedition 324 drill sites (red dots) and ODP Site 1213 (blue dot).  Dark area at lower right shows the basal contour of Olympus Mons (Mars) at the same scale.  Inset shows the location of Shatsky Rise relative to Japan. 


Figure 2: Lithology of Shatsky Rise sites, showing both basal sediment and igneous sections. Sites are arranged in transect order, from north (left) to south (right).  Site 1213 from ODP Leg 198 (Shipboard Scientific Party, 2002) is shown at the same scale for completeness.  The vertical scale is in meters below the seafloor (mbsf).  Roman numerals indicate lithologic units, which are described in the Expedition Reports (http://publications.iodp.org/proceedings/324/324title.htm).


Figure 3: Seismic tracks from cruise MGL1004 of the R/V Marcus G. Langseth.  Blue and green dots show ocean bottom seismometer locations used for seismic refraction lines (A-B, C-D).  Heavy red lines indicate multichannel seismic lines.  Dotted lines show planned future MCS lines and purple line shows extra multibeam bathymetry data collection.  Yellow stars indicate sites where drill cores were obtained (see Fig. 1).


Figure 4: Near-trace seismic data plot (top) and interpretation (bottom) from line A-B-B2.  Gray in the interpretation denotes sediment layers whereas black indicates igneous basement.  U1347 is a drill site from IODP Expedition 324. 

References

Coffin, M. F., and Eldholm, O., 1994. Large igneous provinces: Crustal Structure, dimensions, and external consequences. Rev. Geophys., 32:1-36. doi:10.1029/93RG02508

Duncan, R. A., and Richards, M. A., 1991. Hotspots, mantle plumes, flood basalts, and true polar wander. Rev. Geophys., 29:31-50. doi:10.1029/90RG02372

Foulger, G. R., 2007, The “plate” model for the genesis of melting anomalies, in Foulger, G. R., and Jurdy, D. M., (Eds.), Plates, Plumes, and Planetary Processes. Spec. Pap. – Geol. Soc. Am., 430:1-28.

Nakanishi, M., Sager, W. W., and Klaus, A., 1999. Magnetic lineations within Shatsky Rise, northwest Pacific Ocean: Implications for hot spot-triple junction interaction and oceanic plateau formation. J. Geophys. Res., 104:7539-7556. doi:10.1029/1999JB900002

Sager, W. W., 2005, What built Shatsky Rise, a mantle plume or ridge tectonics? in Foulger, G. R., Natland, J. H., Presnall, D. C., and Anderson, D. L. (Eds.), Plates, plumes, and paradigms. Spec. Pap.-Geol. Soc. Am. 388:721-733.

Sager, W. W., Sano, T., and Geldmacher, J., 2011. How do oceanic plateaus form?  Clues from drilling at Shatsky Rise.  EOS, Trans. AGU., 92:37-44.

Sager, W. W., Kim, J., Klaus, A., Nakanishi, M., and Khankishieva, L. M., 1999. Bathymetry of Shatsky Rise, northwest Pacific Ocean: Implications for ocean plateau development at a triple junction. J. Geophys. Res., 104:7557-7576. doi:10.1029/1998JB900009