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SEATTLE, UNIVERSITY of WASHINGTON DEPARTMENT of OCEANOGRAPHY Seattle, Washington 98105 ~. " "'::\". ' H ,/ "~'" ., ./~' :~/ SEATTLE, UNIVERSITY OF WASHINGTON DEPARTMENT OF OCEANOGRAPHY Seattle, Washington 98105 Technical Reports Nos. 200, 201, 202, 203, 204, and 205 A CO~~ILATION OF ARTICLES REPORTING RESEARCH SPONSORED JOINTLY BY THE u. S. ATOMIC ENERGY COMMISSION and THE OFFICE OF NAVAL RESEARCH -_.--'/,""'j .'~~ u.S. Atomic Energy Commission .//{U:/~'t./ rt~ Contract AT{45-1)-1725 #. '. , and RICHARD H. FLEMING U Office of Naval Research Chairman Contracts Nonr-477(37) and Nonr-477(lO) ... .:) /...i ./~"J""/' :_/"./....)':' . '/IJ / 'tjl(/(JA! CLIFFPab A. BARNES Prindi~al Investigator Reproduction in whole or in part is permitted for any purpose of the United States Government ARTICLES REPORTING RESEARCH SPONSORED JOINTLY BY THE U.S. ATO~rrC ENERGY COl~·rrSSION AND THE OFFICE OF NAVAL RESEARCH Technical Report No. 200 NOTES ON PATAGIill1 IN THE RADIOLARIAN GENERA HYNENIASTRUM AND DICTYASTRUl1, by Hsin-Yi Ling. Micropaleontology, 12(4): 489-492. 1966. Technical Report No. 201 PHYSIOGRAPHY OF COBB AND GORDA RISES, NORTHEAST PACIFIC OCEAN, by Dean A. McHanus. Geological Society of America Bulletin, 78:527-546. 1967. Technical Report No. 202 ORGANIC CARBON IN SURFACE SEDIMENT FROr." THE NORTHEAST PACIFIC OCEAN, by rI. Grant Gross. International Journal of Oceanology and Limnology, 1(1): 46-54. 1967. Technical Report No. 203 CURRENTS AT THE COLL11BIA RIVER MOUTH, by Alyn C. Duxbury. Photogrammetric Engineering, 33(3):305-310. 1967. Technical Report No. 204 SEDINENT MOVEMENT ON THE CONTINENTAL SHELF NEAR ~JASHINGTON AND OREGON, by M. Grant Gross and Jack L. Nelson. Science, 154(3750):879-885. 1966. Technical Report No. 205 TIDAL PERIOD OSCILLATIONS OF AN ISOHALINE SURFACE OFF THE l-fOUTH OF THE COLUMBIA RIVER, by Alyn C. Duxbury and Noel B. McGary. International Journal of Oceanology and Limnology~ 1(2):71-84. 1967. UNIVERSITY OF WASHINGTON DEPARTM~NT OF OCi:ANOGRAPHY TECHNICAL REPORT NO. 2 0 1 i DEAN A. McMANUS Dept. Oceanography, University of Washington, Seattle, Washington .-if Physiography of Cobb and ~. Gorda Rises, Northeast Pacific Ocean Abstract: Two oceanic rises, present near the coast provinces, particularly those of Gorda Rise, are of the northwestern United States, are bordered on more similar to those of the Mid-Atlantic Ridge the east by the continental rise, on the west by than to those of the East Pacific Rise. abyssal hills and plains, and on the north and south In previous studies, Cobb and Gorda rises have by fracture zones. A third fracture zone separates been interpreted as northern extensions of the East and smaller zones transect the rises. Physiographic Pacific Rise in the southeast Pacific. Alternative in­ provinces on the rises include a crest province with terpretations are that the northern rises may be are· a median valley, a flank province, and a transition juvenated segment of an older arcuate rise off the province. The crest of Gorda Rise has greater relief west coast of the United States, or that they may and a better-developed median valley and associated form an auxiliary ridge associated with the Mendo. positive magnetic anomaly than the Cobb Rise. cino Fracture Zone and are possibly unrelated to Both may be Miocene or older. Their physiographic the East Pacific Rise. CONTENTS Introduction 527 2. BoHom topography of Gorda Rise and part of Acknowledgments . 528 Cobb Rise, northeast Pacific Ocean ... 530 Gorda Rise. ... 528 3. Physiographic provinces on Gorda and Cobb Boundaries . 528 rises, northeast Pacific Ocean ...... 531 Crest province ....... 528 4. Profile across crest of Gorda Rise, northeast Flank and transition provinces. 533 Pacific Ocean. .......... .. 532 Cobb Rise .......... 533 5. Generalized bottom topography of continental Boundaries. ........ 533 rise, oceanic rises, and deep-sea floor in part Crest province ....... 534 of northeast Pacific showing earthquake Flank and transition provinces. 535 epicenters and ocean floor heat·flow meas­ Regional setting. 536 urements ............. .. 534 Discussion 537 6. Bottom profile from continental slope to Tufts Gorda Rise . 537 Plain, northeast Pacific Ocean. ..... 536 Cobb Rise 539 7. Three interpretations of regional relationships Age. 540 of Cobb and Gorda rises, northeast Pacific Development. .... 541 Ocean 542 Summary and conclusions. 543 References cited. 544 Table Figure 1. Comparison of Mid-Atlantic Ridge rift valley I. Physiographic provinces in northeast Pacific and Escanaba Trough, northeast Pacific Ocean. 529 Ocean 538 area was designated by ~fenard and Dietz INTRODUCfION (1951) as the Ridge and Trough province and ~10st studies of the oceanic rise system in the later as a northern extension of the East Pacific eastern Pacific have dealt wi th the East Pacific Rise by Menard (1960; 1964), who gave a Rise in the southeast Pacific. The continental recent summary of work in the area. rise off the coast of the northwestern United Menard (1962) identified the Blanco Frac­ States, however, is bordered on the seaward ture Zone which crosses the oceanic rise, and a side by an area of hills, ridges, seamounts, more detailed study (McManus, 1965c) has troughs, and generally rough topography. This suggested that the oceanic rise is segmented Geological Society of America Bulletin, v. 78, p. 527-546, 7 figs., April 1967 527 528 D. A. McMANUS-COBB AND GORDA RISES, NORTHEAST PACIFIC OCEAN into two rises, the Cobb Rise and Gorda Rise (Fig. 1), by the fracture zone. A similar con· ACKNOWLEDGMENTS elusion was reached by Wilson (1965). This research was supported by Office of The relatiomhip between the East Pacific Naval Research Contract 477(37), Project NR Rise of the sou theast Pacific and the oceanic 083 012, and U.S. Atomic Energy Commission rises off Washington and Oregon is problemati· Contract AT(45-1)-1725. cal. According to one interpretation (Menard, 1960), the northern rises are the extension of GORDA RISE the East Pacific Rise, which is considered to merge with the North American continent Boundaries in the Gulf of California and extend under the The margins of both Gorda Rise and Cobb southwestern part of the United States. It then Rise are indistinct tectonically, and are, there· reappears through transcurrent faulting as a fore better described topographically. The topographic feature of the sea floor off the southern margin of the topographic rise on Oregon coast. A recent interpretation by Gorda is a sharp boundary, as oceanic rise Talwani and others (1965) is similar, but does topography is present on the sea floor north of not call on offset by transcurrent faulting to em· the Mendocino Fracture Zone (Menard, 1960), place the rises. The present pattern, instead, but is absent on the sea floor immediately south reflects original sites of development. Another ofit. The eastern margin of the rise topography recent interpretation (Wilson, 1965) considers is also distinct, for it is bordered on the east the northern rises to be a continuation of the by the continental rise (Fig. 1); the northern East Pacific Rise, but the emplacement is by limit is the Blanco Fracture Zone. The western means of transform faul ting along the San boundary, however, is ill-defined at the present Andreas fault. In this interpretation the East time. To the west there is an extensive area of Pacific Rise does not continue under the con­ abyssal hills, and just to the north of the hills, tinent of the western United States. It is ap· Tufts Abyssal Plain. The border between the parent, therefore, that the rises off the north­ rise topography and, the hills indicated in western United States are, as yet, inadequately Figure 1 may be moved west of 130 0 \V. when related to the remainder of the oceanic rise detailed studies are made of the area. system of the world ocean. Gorda Rise, as presently defined by topog­ Topographic and sedimentologic studies in raphy, represents a somewhat equidimensional 2 the region of Gorda and Cobb rises (referred to area of about 58,000 km • It is about 220 km by Wilson, 1965, as Juan de Fuca Ridge) have wide in the south and 330 km wide in the been made by the Department of Oceanow north. At the northeastern corner the rise is raphy, University of Washington, since 1956. separated from the continental slope by only Some results of the sedimentologic studies 30 km of the continental rise passing through (Enbysk, 1960, Ph.D. dissert., Univ. Washing· Blanco Gap. Even at the western margins of the ton, Seattle, Washington; Enbysk and Budin· rise, the continental slope is only about 370 ger, 1960; Enbysk and Cooper, 1965; Mc­ km distant. l\1anus, 1965a; Nayudu, 1959, Ph.D. dissert., Univ. Washington, Seattle; 1964; Nayudu and Crest Province Enbysk, 1964), and a compilated topographic Two categories of physiographic provinces chart covering part of the area (Fig. 2) are recognized on Gorda and Cobb rises, as on (McManus, 1964) have been published. the Mid-Atlantic Ridge (Heezen and others, Bathymetric studies of Cobb and Gorda 1959) : a crest province and flank province (Fig. rises on the sea floor off the northwestern 3). The crest province on Gorda Rise is about United States have resulted in the recognition 150 km wide and continues eastward to within of several physiographic provinces. A pre· 100 km of the Oregon coast. The most promi· liminary description was given by McManus nent feature of the province is the median (1965b), and they are of particular interest Escanaba Trough (Fig. 2) of500 fathoms relief, because they have not been reported from the trending as an almost linear feature slightly East Pacific Rise in the southeast Pacific east ofnorth. The trough is 280 km long, about (Menard, 1964, p. 121); yet, they are similar 30 km wide, and appears to be open at both to those reported from the :Mid-Atlantic ends. The southern end is open to the con­ Ridge (Heezen and others, 1959).
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