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A Summary of Deep Sea Drilling Project Leg 67. Shipboard Results

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A Summary of Deep Sea Drilling Project Leg 67. Shipboard Results ________________________________________-= o~C~E A~N~O~LO~G~I~C~A~A~C~T~A~.~l~9~Bl~.~N~.~S~P_1~~----_ DSDP Guatemala A summary of Deep Sea Subduction TeclOnic TrCllch DSDP Drilling Project Leg 67. Guatemala Subduction Tectonique Shipboard results F.. ~ from the Mid-America trench transect off Guatemala J . Aubouin . , R. von Huene ", J. Al.éma ", G. Slackinton <. J . A. Carier ' . W. T . Coulbourn d, h l D.S.Cowan ", J.A.Curiale', C.A.Dengo·, R. W, Faas , W.H,\rrÎson', R, Hesse • D, M. Hussong <. J . w. Ladd i, N. MuzyJev k. T . Shiki', P. R. Thompson J, J . Weslbcrg d • • Département de Géologie Stru cturale. Univer ~i té Pierre et Marie Curie. 4, place Jussieu. 75230 Paris. b US Geologiea! Survcy, Menlo Park, Califorlli« 94025. USA. < Department of Geology and Geophysics, Universit y of Hawaii , Honolulu. Hawaii 96822. USA. d Scripps Institution o f Oceanography, La Jalla, California 92093. USA . • Department of Geological Sciences. Uni versit y of Washington. Seanle. Washington 98195. USA. r School of Oeology and Geophysics. University of Oklahoma. Norm'In. Oklahoma 73019. USA. 1 Center for Teclonophysics. Texas A & M Uni versity. College Stalion. Texas 77843. USA. and ICAITI . Guatemala. Central America. h Department of Geo108Y . Lafayene College. Easton. Pennsylvania 18042. USA. 1 Technische UniversÎtiit . Munchen. fRG. 1 Lamont-Doherty Geological Observatory. Palisades. New York 10964. USA. l Geological InstÎlute o f USSR. Academy of Sciences. Moscow. USSR. , Departmcnt o f Geology and Mineralogy. Kyoto University. Kyoto. Japan. ABSTRACT The Middle America Trench off Guatemala was transected by 24·channcl seismic-reflection surveys. seismic-refracti on surveys. ;Ind drilling with the G/()mar Chfll/flllger. The drilling was done al three sites on the oceanic Cocos plate and fou r si tes on the Caribbean plate. Thcse plaies converge m about 10 cm/yr as indicated by global plate reconstruction. At ail drill sites sediment of upper miocene through Quaternary age is almost entirely hemipelagic mud with interbedded thin volcanic ash. except in the trench where mud and fine sand turbidiles less than 400.000 ye:us old are ponded. However . the undcrlying rocks are very diffcrent. On the oeeanic Cocos plate a bas'II chalk sequence of lower and middJe Miocenc age is overlain by a thin inlerv;,1 of abyssal clay. In contras! is Ihe Crelaceous 10 lower Miocene clayslonc sequence recovered only at a sile 3 km landward of the trench axis where drilJing penetrated the hemipelagic slope deposÎ ls. A large amount of sediment along with ocean crust has been subducted ouring the present (Miocene 10 Quaternary) episode of subduction. and parts o f the continenlal fmmework may have bee n subducted as weIl. No current tcclonic model salisfa ctoril y explains the surprising occurence of Cretaceous to Miocene daYSlone atthe fool of the continenl. Oceaflo{. A cta. 1981. Proceedings 26'· International geotogical Congress. Geology or COnli nental mllrgins symposium . p ~,r i s. July 7-17. 1980. 225-232. 225 J. AUBOUIN et al. RESUME Bil an des résultats de bord du Lcg 67 (Deep Sen Drilling Projcct : fosse d'Amérique centrale a u large du Guatemala). La fosse d'Amérique centrale a été étudiée selon un profil au large du Guatemala. p;lr sismique réfle:lOion (24 canaux). sismique réfraction. et forage du Glomar Challenger. Les forages ont été effectués il 3 sites sur la plaque de Cocos et 4 sites sur la plaque caraïbe. Le taux de convergence . déduit du mouvement global des plaques. est de 10 cm par an. A tous les site~. les sédiments. du Miocène supérieur au Quaternaire. sont presque entièrement des boues hémipélagiques interstr atîr îée~ de fines couches de cendres \'otcanique~ : sauf dans la fosse où se sont déposées des IUrbidites de boues et sables fin~. d'âge plus récent que 400 000 an~. Cependant les roches sous.jacentes sont très différentes. Sur la plaque de Cocos. une séquence basale de craie d'âge Miocène inférieur et moyen. est recouverte par un fin ni veau d 'argiles abyssales. Au contraire. une séquence de schistes argileux. d'âges allant du Crétacé supérieur au Miocène inférieur. a été forée. à moins de 3 km de la fosse. sous les dépôts hémipélagiques du bas de la pente continentale. Une partie importante des sédiments ont été entraînés avec la croûte océanique pendant le présent épisode de subducti on. du Miocène au Quaternaire. de même. probablement. qu' une partie de la pente continentale elle·même. Aucun modèle tectonique courant n'explique de manière satisfaisante l'existence de la série crétacée-miocène au pied de la marge continentale. OcelUlOl. Acttl. 1981. Actes 26" Congrès Internati onfll de Géologie, colloque Géologie des marges conti nentales. Paris. 7-17 juil. 1980. 225-232. INTROD UCTI ON The Acapulco segment of the Middle America Trench lies along the North American continent : the Guatemalan The Mid-America Trench transect off Guatemala is the segment runs along Central America, whîch is basicaJJy a second of two geophysical and drilling transects recommen­ tectonic edifice of old oceanic crust covered by tertiary ded for the Internatiomtl Program o f ocean DrilJina (11'00) volcanics. The Acapulco segment abruptly truncates the across the convergent margin of southern Mexico and Sierra Madre structures; the trench axis is ;It 4S ~ to the Central America. The first. a tra nsect off Oaxaca. Mexico. Miocene tectonics axis of the Sierra Madre dei Sur and was drilled on Leg 66 of the Glomar ClrallenRer to in vesti­ there is only a very narrow continental shelf along tho Co.1St. gate a convergent marglO where the leading edge is a The GU>l temala segment does nOI eut the Centnll American complex tecto nically accreted during the Neogene and from structure: the trench lIxis is parnllel 10 the tectonic edifice the mid-slope landward the terrdne is a truncated Precam­ that crops out for instances in the Nicoya peninsul a (Costa brian a nd PaJeozoic continental framework (Moore et al .. Rica) and there is a wide continental shelf. which is 1979). Of( Guatemala our principal objecti ve was to study underlain by a tertiary basin with a thick sedimentary this mit rgin where geophysical data indicated çont i n uou ~ sequence (Seely. 1979). accrelion and imbri cation (Seely et al .. 1974). Seely and his Il has been inferred Ihat the Mesm;oic igneous rock çrop­ colteagues made a case for the imbricate thrust model that is ping out in the Nicoya pc ninsula underlies the Tertiary basin widely accepted and th i~ model is supported by the multi­ o n the continental ~ h el r. A sUong magnetic anomaly, channel seismic refleçtion s ite"Sur vey~ (Ladd el (II. 1978; running alonK the continental ~h~lr. can be projected from Ibrahim el al. 1979). the Nicoy~ peninsula along the edge of the shelf (Ladd This paper is patterned after a more complete report et ai .. 1978). Mesozoit: sequences crop oui. from north to prepared on the prelimin,try results of Leg67. in which the south. in the Santa Elena. Nicoya, Osa. Azuero peninsulas. data from the cores are discussed in greater detail (Aubouin They consist of ; et al.• 1979: von Huene ft al .. 1980). 1) the Nicoya complex. a sequence of oceanic ~ed i ments (graywackes. cheriS, r:.tdiolarites) associated with volcanics (diabasis. pillow htvas). containing rocks of uppermost Jurassic, lower Cretaceous. middle Cretaceous. and upper GEOLOGIC SETIING Cretaceous age: 2) the S'lOta Elena complex. composed of a thick ultramafic The Middle America trench is separated by the Tehuantepec body thrust upon a \'olc~lOo- s ed im enta r y sequence with a ridge in two segmenlS (Auboui n. Tardy, 1980) : the Aca· litho-facies si milar to that o f the Nicoya complex complex pulco segment in the norlh and the Guatemala segment in and consisting of uppermost Crelaceous sedimentary rock the south. A corresponding fealure on land. is the active (upper Campa ni an.Mae~trichlian) which reSh unconforma· Polochic-Motagua fault zone. which separates the mexican bly upon pe rid oti t e~ (Azéma. Tournon. 1980): cordillera (S ie rr<l Madre) to the north. from the true Central American terrai ne 10 the south . The Polochic·Motagua has 3) the Azuero complex. showing green-schist facies rock been a fundamental boundary of the southern end of the derived from volcano-sedimentary ~eq uence and underlying North American continent for a long time: the present an unconformable uppermost Cret3ceous ~edimentary moti on is a sinistral west-east stike Slip. but at the end of the Çover (upper Campanian·Maestrichtian). Cretaceou~ it was a North-South compression with over· A feature of great significance is the upper Campaniun thrusting of (Caribbean) ophiol iles over the end of the Sierra unconformity : a great tectonic evenl before the end of the Madre and the Peten platform ('" Yucatan platforml. Cretaceous affected ail Central America. The Mesozoic 226 • LEG 67 : MID·AMERICA TRENCH (GUATEMALA) sequences of Central America are quite differecl trom the data ind ieate that the oeeanie crust is sub<!ueted be neath thc age equivalent rock of the Mexicnn Sierra Madre norlh of treneh landward slope. T he subducted erust Îs overl ain by Il the Polochic-Motagua fault zone. They are sîmîlar 10 the thiek sequence of sediment eontaini ng some landward franciscan facies cropping oui in Baja California, bul d ip pi ng refleetions (Fig. 2). Near Ihe top of the treneh slope different in de tail ; they can be compared to the weSler­ promine nt landward d ipping refleeti ons become the sea­ nmost facies o f Colombian cordillera, but the data are ward flank of the foreare basin that is fîlled with sediment a.c; insuffident for a positive correlati on.
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