Morphological Interpretation of Seamounts in American Samoa: Inferring Genesis Mechanisms through Shape and Distribution Analysis Jed Roberts Master’s Candidate in Geography Department of Geosciences Oregon State University AAG San Francisco - April 19, 2007 Presentation Overview Study Area Research Questions Data Description Shape Statistics Distribution Analysis Morphological Interpretation Future Work Acknowledgements Study Area Image produced by the U.S. National Park Service Study Area Western Volcanic Province Eastern Volcanic Province (Samoa) (American Samoa) Image produced by the U.S. National Park Service Why This Study Area? Data availability Intrigue of controversy regarding volcanic regime No previous comprehensive investigation of geomorphology in the eastern volcanic province Tectonic Setting Image modified from Sandwell and Smith Controversy Artwork by Jayne Doucette, Woods Hole Oceanographic Institution Artwork by Naoto Hirano, Scripps Institution of Oceanography Hart et al. suggest Natland suggests primary volcanic lithospheric flexure mechanism is a at plate boundary mantle plume results in shallow (hotspot) magma upwelling Research Questions Will shape and distribution analyses reveal new clues about seamount origin in the absence of corresponding geochemical data? Will the findings support one volcanic regime, both, or neither? How will predicted seamount distributions compare with previous studies? Data Description Multiple datasets collected during separate research cruises (1999-2005) Cruises operated by Scripps Institution of Oceanography, HURL, Oregon State University, and University of South Florida Data collected by various shipboard multibeam sonar systems with differing quality Data has been merged at a resolution of 210m with depths of up to 6 km below sea level covering an area of 27,181 square km Multibeam Data Merged with Sandwell and Smith 1km resolution predicted bathymetry Image created using Fledermaus Data source: The Seamount Catalog www.earthref.org Multibeam Data With 210m resolution swaths isolated Image created using Fledermaus Data source: The Seamount Catalog www.earthref.org Methods | Identifying Seamounts Create slope surface for multibeam data Candidate seamounts are visually circumscribed by slope Avoid island and large seamount flanks, select seamounts near or on abyssal plain 100 meters or more in height, due to resolution constraints Completeness of data Methods | Identifying Seamounts Slope Surface Map created in Fledermaus Data source: The Seamount Catalog www.earthref.org Methods | Identifying Seamounts 51 Seamounts Selected Map created in Fledermaus Data source: The Seamount Catalog www.earthref.org Methods | Characterizing Seamounts Assume an elliptical base and summit Approximate seamount shape as a conical frustum Methods | Characterizing Seamounts Plan View Cross-sectional View Images created in Fledermaus Methods | Characterizing Seamounts Azimuth Angle Summit Width Slope Left Slope Right Height Base Width Base Depth Images created in Fledermaus Methods | Seamount Statistics Base and Summit Areas Height Slope Base Depth Flatness (ratio of summit to base area) Elongation (ratio of base minor axis to base major axis) Volume Results | Seamount Statistics ------------------------- Mean St. Dev. Min. Max. Total Base Area (km2) 6.7633 5.5800 1.7064 36.5213 344.9299 Summit Area (km2) 0.0891 0.2828 0.0044 2.0487 4.5453 Height (m) 323 152 105 850 N/A Slope (%) 13.3 3.3 5.9 19.7 N/A Base Depth (mbsl) -4245 738 -2640 -5380 N/A Flatness 0.0118 0.0161 0.0014 0.1021 N/A Elongation 1.28 0.24 1.00 2.10 N/A Volume (km3) 1.01 1.58 0.09 10.76 51.73 Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Results | Relational Statistics Methods | Distribution Analysis Negative Exponential Distribution (from Smith and Jordan ) Distribution of seamounts is modeled as: v(H) = v0exp(-ßH) Where v(H) is the # of seamounts per unit area with a height greater than H, v0 is the total # of seamounts per unit area, and ß is the negative of the slope of the line fitting ln(v(H)) against H The characteristic height of the seamount sample is equal to negative reciprocal of ß Methods | Distribution Analysis Define appropriate sample 100 meter height bins containing at least three seamounts were included 48 seamounts in all, within 100-600 meter height range Define appropriate areal value Total area of data set is 27,181 km2 Reduced to 22,745 km2 by including only depths below -2640 m This area approximates only the near- lithosphere abyssal plain Methods | Distribution Analysis 100-600 m range Methods | Distribution Analysis Define appropriate sample 100 meter height bins containing at least three seamounts were included 48 seamounts in all, within 100-600 meter height range Define appropriate areal value Total area of data set is 27,181 km2 Reduced to 22,745 km2 by including only depths below -2640 m This area approximates only the near- lithosphere abyssal plain Methods | Distribution Analysis Calculation of Area by -2640 m Cutoff Total area before depth cutoff: 27,181 km2 Total area after depth cutoff: 22,745 km2 Map created in Fledermaus Data source: The Seamount Catalog www.earthref.org Results | Distribution Analysis ν0 = 2.6 ± 0.2 (per 1000 km2) ß-1 = 138 m Results | Distribution Analysis Comparison with previous studies Study Region (Latitude) Height Range (m) Seamount Density Characteristic (per 103 km2) [v0] Height (m) [ß-1] This Study ASSC (13º-15ºS) 100 – 600 2.6 ± 0.2 138 Jaroslow et al. (2000) MAR (25º-27ºN) 70 – 350 58.3 ± 1.6 92 Rappaport et al. (1997) ESC (27º-29ºS) 200 – 1000 2.7 ± 1.5 308 Schierer et al. (1996) Southern EPR (15º-19ºS) 200 – 1200 4.8 ± 0.2 421 Magde and Smith (1995) Northern MAR (57º-62ºN) 50 – 250 310 ± 20 68 Schierer and MacDonald Northern EPR (8º-18ºS) 200 – 800 1.9 ± 0.2 240 (1995) Kleinrock and Brooks Galapagos (2ºN, 95ºW) 50 – 350 370 ± 30 29 (1994) Bemis and Smith (1993) Southern Pacific (9º-22ºS) 300 – 700 13 ± 2 233 Smith and Cann (1990, MAR (24º-30ºS) 50 – 210 195 ± 9 58 1992) Abers et al. (1988) Southern Pacific (7º-22ºS) 100 – 1000 12.6 ± 0.8 174 Smith and Jordan (1987), Eastern Pacific 400 – 2500 5.4 ± 0.7 285 and Smith (1988) ASSC is the American Samoa Seamount Chain, MAR is the Mid-Atlantic Ridge, ESC is the Easter Seamount Chain, EPR is the East Pacific Rise Results | Interpretation Relational shape statistics are in agreement with those observed in previous studies Elongation and azimuth reveal slight directional trends that may support lithospheric flexure Distribution analysis demonstrates seamount population densities typical of southern Pacific Small seamount chains trend northeast- southwest, while large seamounts and islands trend east-west Results | Interpretation Directional Trends Map created in Fledermaus Data source: The Seamount Catalog www.earthref.org Significance Lithospheric flexure is not ruled out as volcanic mechanism for production of small seamounts Initial identification of seamounts Volume and other shape statistics never before calculated Locations and distribution of seamounts important for biological studies and habitat protection Future Work Re-grid dataset at slightly higher resolution Add data collected by NOAA in 2006 to regional dataset compilation Examine shape statistics and distributions based on natural geographic partitions Submit seamount locations and morphologies to the Seamount Catalog Compare findings with forthcoming geochronological data Acknowledgements Dr. Dawn Wright, Oregon State University Graduate Advisor Dr. Anthony Koppers, Oregon State University Seamount Catalog Webmaster Scripps Institution of Oceanography, Hawaii Undersea Research Lab, Oregon State University, and University of South Florida Data Sources Dr. Deborah Smith, Woods Hole Oceanographic Institution Dr. Thomas Jordan, Massachusetts Institute of Technology Distribution Analysis Methods You can download this presentation here: http://oregonstate.edu/~robertje/projects/aag2007 Contact me via e-mail at: email@example.com References Abers, G. A., Parsons, B., and Weissel, J. K. 1988. Seamount abundances and distributions in the southeast Pacific. Earth and Planetary Science Letters. 87: 137-51. Bemis, K. G., and Smith, D. K. 1993. Production of small volcanoes in the Superswell region of the South Pacific. Earth and Planetary Science Letters. 118: 251-62. Hart, S. R., Staudigel, H., Koppers, A. A. P., Blusztajn, J., Baker, E. T., Workman, R., Jackson, M., Hauri, E., Kurz, M., Sims, K., Fornari, D., Saal, A., and Lyons, S. 2000. Vailulu'u undersea volcano: The New Samoa. Geochemistry Geophysics Geosystems. 1(12): 2000GC000108. Hart, S. R., Coetzee, M., Workman, R. K., Blusztajn, J., Johnson, K. T. M., Sinton, J. M., Steinberger, B., and Hawkins, J. W. 2004. Genesis of the Western Samoa seamount province: age, geochemical fingerprint and tectonics. Earth and Planetary Science Letters. 227: 37-56. Hirano, N., Takahashi, E., Yamamoto, J., Abe, N., Ingle, S.P., Kaneoka, I., Hirata, T., Kimura, J., Ishii, T., Ogawa, Y., Machida, S., and Suyehiro, K. 2006. Volcanism in Response to Plate Flexure. Science. 313: 1426-28. Jaroslow, G. E., Smith, D. K., and Tucholke, B. E. 2000. Record of seamount production and off-axis evolution in the western North Atlantic Ocean, 25º25'-27º10'N. Journal of Geophysical Research. 105(B2): 2721-36. Kleinrock, M. C., and Brooks, B. A. 1994. Construction and destruction of volcanic knobs at the Cocos-Nazca spreading system near 95ºW. Geophysical Research Letters. 21(21): 2307-10. Magde, L. S., and Smith, D. K. 1995. Seamount volcanism at the Reykjanes Ridge: Relationship to the Iceland hot spot. Journal of Geophyical Research. 100(B5): 8449-68. References Natland, J. H. 1980. The progression of volcanism in the Samoan linear volcanic chain. American Journal of Science. 280-A: 709-35. Natland, J. H. 2004. The Samoan Chain: A Shallow Lithospheric Fracture System. www.mantleplumes.org (last accessed March 11, 2006). Rappaport, Y., Naar, D. F., Barton, C. C., Liu, Z. J., and Hey, R. N. 1997. Mophology and distrubution of seamounts surrounding Easter Island. Journal of Geophysical Research. 102(B11): 24,713-28. Scheirer, D. S., and Macdonald, K. C. 1995. Near-axis seamounts on the flanks of the East Pacific Rise, 8ºN to 17ºN. Journal of Geophysical Research. 100(B2): 2239-59. Scheirer, D. S., MacDonald, K. C., Forsyth, D. W., and Shen, Y. 1996. Abundant Seamounts of the Rano Rahi Seamount Field Near the Southern East Pacific Rise , 15º S to 19º S. Marine Geophysical Researches. 18: 13-52. Smith, D. K. 1988. Shape analysis of Pacific seamounts. Earth and Planetary Science Letters. 90: 457-66. Smith, D. K., and Jordan, T. H. 1988. Seamount Statistics in the Pacific Ocean. Journal of Geophysical Research. 93(B4): 2899-918. Smith, D. K., and Cann, J. R. 1990. Hundreds of small volcanoes on the median valley floor of the Mid-Atlantic Ridge at 24º-30º N. Nature. 348: 152-5. Smith, D. K., and Cann, J. R. 1992. The Role of Seamount Volcanism in Crustal Construction at the Mid-Atlantic Ridge (24º-30ºN). Journal of Geophyical Research. 97(B2): 1645-58. References Walker, G. P. L., and Eyre, P. R. 1995. Dike complexes in American Samoa. Journal of Volcanology and Geothermal Research. 69: 241-54. Workman, R. K., Hart, S. R., Jackson, M., Regelous, M., Farley, K. A., Blusztajn, J., Kurz, M., and Staudigel, H. 2004. Recycles metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end-member: Evidence from the Samoan Volcanic Chain. Geochemistry Geophysics Geosystems. 5(4): 2003GC000623.
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