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Wedgeworth BS 1985 Thesis.Pdf Ita Mai Tai Guyot: a comparative geophys 5 15539 • 3 AC . H no . U11111111111111111111111111111111 Wedgworth, Bruce S. SOEST Library • • ITA KAI TAI GUYO'l': A cml'ARATIVB GEOPHYSICAL S'l'ODY 0. WBS!'ERH PACllIC SUMOURl'S • A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT • OF THE REQUIREMENTS FOR THE DEGREE OF MASI'ER OF SCIENCE IN GEOLOGY AND GEOPHYSICS • DECEMBER 1985 • By Bruce Steven Wedgeworth • Thesis Committee: James N. Kellogg. Chairman Loren W. Kroenke • Eduard Berg RETURN TO HAWAI I INSTITUTE OF GEOPHYSICS • LIBRARY ROOM • • ii • • • We certify that we have read this thesis and that, in our opinion, it is satisfactory in scope and quality as a thesis for the degree of • Master of Science in Geology and Geophysics • • THESIS COMMITTEE / • • I \ : \ 1 . ~ i..:.' '.'-I I • v • • • iii • I would like to give special thanks to Dr. James Kellogg who was inspirational in getting this study off the ground and who provided • leadership combined with immeasurable patience along the way. Dr. Kroenke and Dr. Berg both gave umch appreciated criticism and insight over the course of the project • • There never would have been a study without the data provided by the crews and scientific staff of the following cruises: DSDP Legs 20 and 89, and KK810626 Leg 2. Three people working in the Penthouse were • especially helpful in the acquisition and management of geophysical data: Terri Duennebier, Elaine Demian, and Sharon Warlop. Steve Dang and Charlie Myers provided valuable information on illustration drafting • and photography. I am deeply indebted to John Tuttle, Bob Cessaro, John Williams, and Doug Myhre for their assistance in the world of computers, especially the inner workings of the Harris. Karen Ogino contended with • all the crazy general off ice requests I made of her. Karen Re·hbock helped me figure out Muse and Carol Yasui with all the fiscal hassles. Pat Price spent many hours searching for and finding obscure (and not so • obscure) bits of data that I needed. Lynn Ajifu assisted with much of the typing, an area where my skill is definitely lacking. Spec~al and sincere thanks go to all the members of the Bilger • Annex Graduate University (BAG U) community for putting up with me during the long haul, who were there and cared when !•needed them: Wade Bartlett, Glenn Brown, Paul Daugherty, Chuck Ferrall, Mike Knight, Bob • Lauritzen, Patty Lee, Jill Mahoney, Bob Mallonee, Sylvia Newsome, Scott • • iv Rowland, John and JoAnn Sinton, Adam Weiner, Kathy Yamauchi, and Rob • Yonover. Very special thanks to my parents who never waivered in their love and support, who gave me the courage to make it my own way, and who, • with me, never gave up. This project was supported by the Office of Naval Research • • • • • • • • • • v ABSTRACT • Ita Mai Tai is a large, uncompensated seamount on the eastern edge of the East Mariana Basin. A large positive gravity anomaly of 254 mgal • characterizes the summit and a low of -69 mgal, the surrounding moat. Using polygonal prisms to approximate the bathymetry, the observed 3 gravity was inverted to calculate an average density of 2.59 g/cm for • the seamount. Other geologic data and seismic reflection profiles from DSDP Legs 20 and 89 provided further modeling constraints such as the density of the volcanic conduit and the density of sediment bodies. The • drill sites describe a volcanic edifice formed in the Aptian/Albian on Jurassic/Cretaceous crust. The volcanism is recorded in volcanoclastic and epiclastic deposits in the basins nearby. The guyot was covered • initially by a succession of reefal and lagoonal~ediments followed by a thick mantling of pelagic sediments after it subsided. Seismic reflection records indicate two different depths to • basement on either side of Ita Mai Tai which may be explain_ed by eruptions. along or near an unrecognized fossil plate boundary. Gravity models that adequately match the calculated and observed data sets for • Ita Mai Tai show little crustal thickening, suggesting that Ita Mai Tai is almost completely uncompensated isostatically. A study of ninety-six other western Pacific seamounts including Sio Guyot, a completely • compensated seamount in the Mid-Pacific Mountains, show a linear inverse relationship between the maximum free-air anomaly and the minillDlm depths to the tops of .seamounts, approximated by g z • (3800m - z) I 16.0. Ita • Mai Tai and Sio Guyot are seen as end members with their contrasting degree of compensation. To determine whether the observed anomalies could be explained by crustal "roots", theoretical models at 0 and 100 • • vi percent local compensation were derived for all western Pacific • seamounts studied. Most of the observed gravity anomalies for seamounts in this study lie in the field between the theoretical lines of 0 and 100 percent compensation. Twenty-two appear to be more than 100 percent • compensated. A map of isostatic compensation shows a distinct break between the eastern and western Marshall Islands, perhaps indicating different ages • of seamount formation. There is as much as a 200 mgal difference in the gravity field observed for well-surveyed seamounts and islands of the same size in the western Pacific. The sea floor age at the time of • seamount formation seems to vary inversely with the relative amount of compensation. The over-compensated seam.aunts may have been erupted near Cretaceous ridge crests rather than in a mid-plate event as has been • previously thought • • • • • • • vii • TilLB C-. COB'RllrS • lAu. ACKNOWLEDGEMENTS iii ABSTRACT v • LIST OF TABLES viii LIST OF ILLUSTRATIONS ix INTRODUCTION 1 • REGIONAL FBAMEWOBK 4 SEAMOUNT MORPHOLOGY 6 SEDIMENTARY BASIN STRATIGRAPHY 7 EDIFICE STRUCTURE • 14 • -· GRAVITY • 22 MODEL STUDIES 22 COMPARISONS WITH OTHER WESTERN PACIFIC SEAMOUNTS AND ISLANDS 38 • CONCLUSIONS 54 APPENDICES 56 • BIBLIOGRAPHY 86 • • • • viii • LIST (W TAM.JS T&ble • I DSDP LEG 89, SITE 585 DRILLING RECORD 11 II PELAGIC CAP VELOCITIES , TRAVEL-TIMES , THICKNESSES • 17 III TRA VELTIMES AND VELOCITIES FOR SEDIMENT FILLED BASINS • 58 IV LEAST SQUARES FITS . 59 • . • . • . v SEAMOUN'I ISOSTATIC CORRECTION VALUES . • 63 VI H/R AND R /Rl RATIOS 67 2 . • • • • . • . • . • • VII SEA FLOOR/SEAMOUN'I FORMATION AGE VS. FAA - THEOR. LINE 71 • VIII SEAMOUN'I DATA TABLE • • . • • . • • . • • . 74 • • • • • • • ix • LIST C. ILLUSDArIOBS Figure fAil • 1 Bathymetry of North Pacific Ocean . 2 2 Bathymetry of Ita Mai Tai Guyot . 3 3 Seismic reflection record A-A' . 9 4 Seismic reflection record B-B' . 10 • 5 Seismic reflection record D-D' 13 6 Seismic reflection record E-E' . 15 7 Lithologic components of pelagic cap . 20 • 8 Depth to basement map . 21 9 Free-air anomaly map . 23 10 Observed gravity and model A . 24 ""-c • 11 Plan view of model A bathymetric prisms 25 12 Calculated and residual gravity fields (model A) 27 13 Cross-section of model B . 29 • 14 Plan view of model B bathymetric prisms . ~ 30 15 Calculated and residual gravity fields (model B) . 31 16 Observed gravity and model B 32 • 17 Cross-section of model C . 33 18 Plan view of model C bathymetric prisms . 34 19 Calculated and residual gravity fields (model C) 35 • 20 Change in crustal thickness sketch . • 36 21 Observed gravity and model C . 37 22 Maximum free-air anomaly vs. minimum depth plot . 40 • 23 3-D view of Ita Mai Tai and Sio Guyots . 41 • • x • Figure Page 24 . Max. FAA vs. min. depth for three seamount groups 42 25 Regional isostatic compensation map • • 44 26 Five theoretical seamount models 45 • 27 Seamount models' calculated gravity fields 47 28 0 and 100 percent gravity fields 48 29 Ten models on gravity-depth plot 49 • 30 Gravity-depth plot of 96 seamounts 50 31 Sea floor I edifice age vs. max. FAA plot 52 32 Seamount study location map 53 • 33 Dimensions of seamount frustrum • 57 34 Typical prism used in volume calculations 57 • • • • • • • 1 • IBTJUJDUCTIOB A major goal in the application of the theory of plate tectonics in the Pacific has been determining the age and location of the oldest • part of the Pacific plate. An attempt to reach this objective was made in 1971 when DSDP Leg 20 drilled Site 199 in the East Mariana Basin (Fig. 1). It was anticipated that Jurassic lithosphere would be • encountered [Larson, 1976; Hilde et al., 1977; Shipley et al., 1983; and others]. Unfortunately, deep water drilling difficulties prevented this primary objective from being reached, and an alternate • site was chosen farther east in a cluster of shallow seamounts between the Magellan and Marshall seamount chains (Fig. 1). Three holes were drilled at Sites 200, 201 and 202 on one of these seamounts located at • 12°45'N, 156°45'E (Fig. 2). This seamount was ~iven the informal name It a Mai Tai Guyot by Bruce Reezen which means "no damn good" in ' Tahitian [M. Tharp, pers. comm., 1984], possibly because basement was not reached. A second unsuccessful attempt to reach Jurassic basement • ~ was made in 1982 when DSDP Leg 89 drilled Site 5~5 just north of Ita Mai Tai. Other cruises across Ita Mai Tai include those of Conrad • 1205, DDM05 in 1971 and Vema 3401 in 1977 • In 1981 the Hawaii Institute of Geophysics research vessel Kana Keoki surveyed Ita Mai Tai (Fig. 2) and other charted and uncharted • seamounts in the central and western Pacific collecting bathymetric, gravimetric, magnetic, petrologic and seismic reflection profiling • data. Although magnetic data also were obtained, the magnetic field • appears too chaotic to attempt to model [W. Sager, pers. comm., 1984] • • • • • • • • • • • • • Figure 1. Bathymetric and tectonic map of the North Pacific Ocean (AAPG, Circum-Pacific Series, 1981). N • 3 • • A • ' ', \ \ I I • \ \ I \ I I \ I i 13 i • I I I I I I I • I I \ \ I ' II ' \ ~ ~ \ \ \ I I \ • 12 N \ 155/45°E 157 CONTOURS IN METERS X 100 • Figure 2.
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