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SIO and NEL Exploration of the Middle America Trench B. Index A. Index map: S.I.O. and N.E. L. exploration of the Middle America Trench B. Index map: Sounding lines furnished by the U.S. Hydrographic OfKce ECHO-SOUNDING LINES USED IN THE PREPARATION OF PLATES 2, 3, AND 6 FISHER, PLATE 1 Geological Society of America, volume 72 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 ROBERT L. FISHER Scripps Institution of Oceanography, University of California, La Jolla, Calif. Middle America Trench: Topography and Structure Abstract: From 1952 to 1959, during nine expedi- companying paper (Shor and Fisher, 1961) were tions of the Scripps Institution of Oceanography employed in determining the trench structure. and one of the U. S. Navy Electronics Laboratory, Three refraction stations were taken along the axis research vessels recorded 31,950 miles of echo- of the trench west of Acapulco and two along its sounding traverses in and adjacent to the Middle axis off Guatemala and El Salvador. Another sta- America Trench, which extends from the Islas Tres tion was shot on the shelf and one 60 miles sea- Marias off western Mexico to the Cocos Ridge ward of the trench off Guatemala. Thick sediments southwest of Costa Rica. were found in the Tres Marias Basin off Manzanillo The Middle America Trench is continuous at and at the shelf station off Guatemala. Arrivals depths greater than 2400 fathoms (4400 m) for 1260 from rock with compressional wave velocity of 4-6 miles, except off Manzanillo and Zihuatanejo, km/sec were observed at the Tres Marias Basin and Mexico, where submarine mountains lie in the Guatemala shelf stations. Off Guatemala, on a sec- trench. It is deeper than 3000 fathoms (5500 m) for tion normal to the trench, the depth below sea 380 miles as the Guatemala Deep. Northwest of level to the M discontinuity is interpreted from Acapulco it is generally U-shaped in cross section, these seismic data as about 9 km (Pacific Basin), 10 with a steeper shoreward flank and a flat bottom km (outer ridge), 16 km (trench), and 17 km suggesting sedimentary fill. From Acapulco south- (shelf). Below the sea floor the crust thickens from east to the west side of the Gulf of Tehuantepec, 5-7 to 10-17 km along this section. The M dis- the trench shoals, in a series of basins, to 2700 continuity is deeper and the crust below the sedi- fathoms (5000 m). To the southeast it widens and ments thicker under the two southern stations deepens abruptly to a maximum 3500 fathoms than under the two central trench stations. The (6400 m) off western Guatemala, then shoals gradu- mantle is deeper under the Tres Marias Basin, ally to merge into the sea floor off Costa Rica. The where thick (13/2 km) sediments are found, than southeast segment is also asymmetrical in cross under the central stations. section but is V-shaped with irregular bottom. A The Gulf of Tehuantepec marks a major change northeast-trending band of ndge-and-trough topog- in trench configuration and possibly in age. North- raphy, 60 miles wide, separates the 1800- to 1900- west of Tehuantepec the flat trench bottom de- fathom sea floor outside the trench off southern veloped in most places suggests a greater age. Mexico from the 2100- to 2200-fathom Guatemala Southeast of the gulf the deep V-shaped trench, Basin. This zone has been traced from several with thicker crustal layers but very little fill, hundred miles offshore to an intersection with the borders a volcanically active coast. The zone of trench near the west side of the Gulf of Tehuante- ridge-and-trough topography trending southwest pec. from Tehuantepec may be another evidence of this Seismic-refraction studies reported in an ac- boundarv. CONTENTS Introduction 704 Bathymetric evidence for sedimentary fill in the Regional relations 704 Middle America Trench 713 Scope of the present study 704 Discussion 714 Acknowledgments 706 References cited 717 Previous work 706 Bathymetry 706 Figure Gravity measurements 706 1. Index map: seismic setting of the Middle Amer- Topography 707 ica Trench 705 Preparation of the charts 707 2. View to northwest from Banderas Bay .... 708 Topography within and shoreward of the Middle 3. View to southeast from La Libertad, El Salvador 710 America Trench 708 4. View to northwest from Gulf of Tehuantepec . 712 Topography seaward of the Middle America 5. Schematic structure section off western Guate- Trench 711 mala 715 Topography of the Gulf of Tehuantepec . 712 Geological Society of America Bulletin, v. 72, p. 703-720, 5 figs., 6 pis., May 1961 703 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 704 R. L. FISHER—MIDDLE AMERICA TRENCH Plate Facing 5. Echograms of three crossings of the Middle 1. Echo-sounding lines used in the preparation of America Trench 715 Plates 2, 3, and 6 703 6. Submarine topography, from San Jose, Guate- 2. Submarine topography, from Islas Tres Marias mala, to Punta Burica, Panama 720 to Acapulco, Mexico 706 3. Submarine topography, from Acapulco, Mexico, Table to San Jose, Guatemala 710 1. Maximum depth of fill in the Middle America 4. Echogram of traverse along trench, near Man- Trench, as deduced from bathymetric zanillo, Mexico 714 records 714 2. Velocities of refracting layers 717 INTRODUCTION Along the trench axis and to the east of it, a very active zone of shallow earthquakes follows Regional Relations the Mexican coast from Jalisco to Oaxaca, A narrow depression, 20-100 miles offshore, crosses the Gulf of Tehuantepec, and continues, extends 1550 miles along the west coasts of with reduced activity, to western Panama (Fig. Mexico and Central America, from the Islas 1). Outside the trench shallow activity is most Tres Marias to the Golfo de Dulce (Fig. 1). frequent west of Colima, but several shocks This deep area includes the Acapulco Trench have occurred off Oaxaca. Intermediate shocks or Deep and the Guatemala Trench of various follow the volcanic belts on land. No deep authors (e.g., Agassiz, 1906; Whitcroft, 1944; shocks are known from this area (Gutenberg Fisher and Revelle, 1955). Following Heacock and Richter, 1954, p. 36). The earthquake dis- and Worzel (1955), the writer now favors the tribution has been studied by Benioff; he con- name Middle America Trench for the entire cluded that from the Islas Tres Marias to structure. Panama a "marginal reverse fault complex," Miocene to Recent volcanic rocks, folded with an "intermediate component" extending and faulted Paleozoic, Mesozoic, and Cenozoic to a depth of about 220 km, dips eastward be- sedimentary and metamorphic rocks, and neath the continent (Benioff, 1954, p. 390). granitic rocks largely of late Cretaceous age, Gunn (1947) attempted a quantitative treat- are exposed along the mountainous coasts of ment of the Middle America Trench-Coastal Jalisco, Cohma, Michoacan, Guerrero, and Mountain-Volcanic Chain relations. He as- Oaxaca (Schuchert, 1935, p. 129-131). Coastal sumes a strong elastic lithosphere supported on mountains trend nearly parallel to the shore a weak magma and examines the mechanics of line. The east-west line of recently active a "compressed shear thrust fault" formed in Mexican volcanoes intersects the trench near such a crust. From his calculations, Gunn states 19° N. (Fig. 1). The Pacific coastal plain of that shear faults resulting from horizontal com- Tehuantepec has a thin cover of undeformed pression would be localized at continental Pleistocene marine sedimentary rocks lying on margins and that with continuing compression the pre-Carboniferous basement. The isthmus the higher continental mass would overthrust was downfaulted, probably in Late Miocene the ocean basin and simultaneously form a time, then raised slightly during late Pliocene linear deep at the toe of the overriding block and Pleistocene to its present low elevation and a line of volcanoes, parallel to the deep, (Webber and Ojeda R., 1956, p. 78). From 50-90 km inland from the continental margin. southeastern Chiapas to Lake Nicaragua, the coastal plain is bordered on the east by late Scope of the Present Study Cenozoic to Recent andesitic volcanoes. Rocks This work represents one of the results of re- extruded by these young volcanoes have search carried out under contracts with the buried the western ends of east-trending Office of Naval Research and the Bureau of mountain ranges of folded and faulted upper Ships, U. S. Navy. Paleozoic rocks (Schuchert, 1935, p. 314-320). The bathymetric studies were made during The Pacific coasts of southern Nicaragua and the 1952 U. S. Navy Electronics Laboratory Costa Rica are mountainous and composed Shuttle Expedition (bathymetry, bottom- chiefly of Tertiary and Quaternary volcanic sampling, hydrography) and the Scripps Insti- rocks and igneous detritus. The recently active tution of Oceanography expeditions: Shellback "Volcanic Range" of Costa Rica trends south- (1952; hydrography, bathymetry), Toro (1953; east, and passes inland from the Nicaragua- bathymetry, seismic-reflection studies, bottom- Costa Rica border (Schuchert, 1935, p. 592- sampling), Acapulco Trench (1954; bathy- 600). metry, seismic-refraction studies, heat-flow and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 INTRODUCTION 705 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 SUBMARINE TOPOGRAPHY, FROM ISLAS TRES MARIAS TO ACAPULCO, MEXICO FISHER, PLATE 2 Geological Society of America, volume 72 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/72/5/703/3442060/i0016-7606-72-5-703.pdf by guest on 02 October 2021 708 R.
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