Late Cenozoic subduction and continental margin truncation along the northern Middle America Trench D. E. KARIG I R. K. CARD WELL [ Department of Geological Sciences, Cornell University, Ithaca, New York 14853 G. F. MOORE J D. G. MOORE* Naval Undersea Center, San Diego, California 92132 ABSTRACT tently between Late Cretaceous and late arc basin compared to other arc systems Miocene time. Possible mechanisms include with long subduction histories (Karig and The narrow inner trench slope and the subduction of continental crust (tectonic Sharman, 1975). Moreover, the distance truncated igneous and metamorphic terrane erosion), left-lateral translation associated from the trench to the Cretaceous plutons along the west coast of Mexico between with the Caribbean-North American plate that crop out along the coast, and probably Cabo Corrientes and the Gulf of Tehuan- boundary, and right-lateral translation as- represent the roots of a volcanic arc, is very tepec indicate that part of the continental sociated with oblique subduction between much less than that of any contemporary margin has in some way been removed dur- the Farallon or Cocos plates and the North arc system. ing the process of subduction. However, a American plate. Geological data favor These observations have led to the con- detailed marine geophysical survey of the right-lateral offset and suggest that some of clusion that a former continental margin in inner trench slope near Acapulco indicates the missing margin may be the slivers of this region has somehow been removed (de that this removal is not occurring now. subduction complex found along the west Czerna, 1960; King, 1969; Karig, 1974), South-southwest-trending magnetic coast of Baja California and possibly even but the mechanism of this truncation has anomalies produced by the Xolapa meta- farther north. been debated. The more generally accepted morphic complex extend seaward only 20 scheme to explain subduction-related trun- to 30 km. Oceanic magnetic anomalies that BACKGROUND cation is "tectonic erosion" — the subduc- trend N50°W extend as much as 30 km tion of the continental margin during some landward of the trench. The boundary be- Plate convergence between the Mexican conditions of plate convergence (Scholl and tween these two magnetic patterns lies section of the North American plate and others, 1970; Katz, 1971). Alternatively landward of the trench-slope break and be- either the Cocos or other lithospheric plates there may have been lateral transport of neath the upper-slope sediment pile. The within the Pacific Ocean has been occurring slivers of the continental margin along nonmagnetic material forming the acoustic at least intermittently for more than 100 strike-slip faults resulting from oblique basement trenchward of the metamorphic m.y. and probably for several times that subduction (Karig, 1974) or by other proc- rocks is interpreted to consist of late long (de Czerna, 1960; King, 1969; Coney, esses. Miocene to Holocene deformed trench- 1972). It is quite anomalous, then, that We report here on a detailed marine floor turbidites. Deformation associated much of the Mexican continental margin so geophysical study of a section of the Mexi- with subduction has reversed the gradients poorly reflects the maturity normally as- can continental slope near Acapulco (Fig. 1, of several submarine canyons and tilted the sociated with persistently convergent plate 2), undertaken during the 1974 Cocotow seaward edge of the upper-slope sediment boundaries. The contrast between the Pa- cruise of the Scripps Institution of Ocean- pile away from the trench. The morphology cific margin of Mexico, from Cabo Cor- ography (SIO). The purpose of our investi- and structure of the inner trench slope is rientes to the Tehuantepec Ridge, (Fig. 1) gation was to test whether tectonic erosion typical of accreting trench-arc systems, and eastern Aleutian or southwestern Japan is occurring along the Mexican coast and, if although the morphotectonic units in this margins, which clearly show the charac- possible, to determine the characteristics system are smaller than usual. teristics of mature arc systems, is striking. and conditions associated with tectonic Accretion since late Miocene time is No belts of subduction complexes, at erosion. The Acapulco region was chosen suggested by the age of dredged slope sedi- least of Mesozoic or younger age, parallel because it clearly displays geologic ments and by analysis of offshore magnetic the arc system. Instead, the basement rocks anomalies associated with truncation of the anomalies, which indicate a change from exposed along this part of the Mexican continental margin and because there is a right-lateral oblique to perpendicular sub- coast consist largely of an igneous-meta- relatively large amount of pertinent data duction at that time. Removal of the conti- morphic terrane with structural trends available from both the continental and nental margin probably occurred intermit- that often intersect the present arc at high oceanic sectors. The Cocotow study has angles (King, 1969; de Czerna, 1971; been augmented by other data from the Kesler, 1973). The Middle America Trench Middle America Trench, especially dredge * Present address: DSDP, A-031, Scripps In- stitution of Oceanography, La Jolla, California in this sector lies less than 75 km offshore hauls from the Gulfmat cruise of the Naval 92093. and has a very narrow upper slope or fore- Undersea Center. Geological Society of America Bulletin, v. 89, p. 265 -276, 9 figs., February 1978, Doc. no. 80211. 265 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/265/3444321/i0016-7606-89-2-265.pdf by guest on 01 October 2021 266 KARIG AND OTHERS Figure 1. Map of northern Middle America Trench region, showing plate boundaries and geologic features relevant to this study. Box indicates Cocotow survey area (shown in Fig. 2). GEOLOGIC FRAMEWORK tradict other age data. Volcanism, appar- tepec junction, a broad continental shelf ently associated with the present subduc- overlies a filled upper slope (fore-arc) basin Rock types reported along the coastal tion pattern, occurs along the diffuse (Ross and Shor, 1965; Seely and others, mountains from Cabo Corrientes to the Trans-Mexican volcanic belt (Fig. 1), which 1974). Gulf of Tehuantepec include Precambrian is thought to have become active approxi- Shallower trench depths northwest of the to Mesozoic metamorphics and shelf sedi- mately 30 m.y. ago (Mooser, 1973). This Tehuantepec junction, as well as the pro- ments of presumed Cretaceous age, all of volcanic arc is anomalous in that it is paral- gressive southeasterly deepening along this which have been intruded by plutons that lel neither to the trench nor to isodepth con- section, can be correlated with the age and are, at least in part, of Cretaceous age (de tours of the seismic zone (Molnar and depth of the oceanic lithosphere that is Czerna, 1971; Kesler, 1973; Salas and Sykes, 1969). In fact, most of the volcanic being subducted (Truchan and Larson, others, 1974). In the Acapulco area, the centers lie east of the deep-focus earth- 1973) Larger wedges of trench-floor tur- Xolapa metamorphic complex of presumed quakes. bidites northwest of the Tehuantepec Ridge Precambrian and (or) early Paleozoic age The general morphology and structure of are attributed primarily to the greater has been intruded by middle to upper Cre- the continental margin and trench off drainage area landward of the northwestern taceous granitic rocks (Kesler and Heath, Mexico has been adequately presented by section where the continental divide lies 1970; de Czerna, 1971; Salas and others, Fisher (1961), Shor and Fisher (1961), Ross more than 300 km inland. South of the 1974). Foliation and structural axes in the and Shor (1965), and others; only salient Tehuantepec junction the continental divide metamorphic terrane have a regional trend points need be elaborated here. A major is seldom more than 50 km inland, and of N20°E (de Czerna, 1971). change in character of the Middle America drainage is consequently very much less. A The incomplete state of mapping in the arc system occurs where the Tehuantepec southeasterly increase in subduction rate western part of Mexico does not yet permit Ridge meets the trench (Fig. 1). Northwest from about 6.5 cm/yr at the Rivera triple a reliable reconstruction of Cenozoic events of this junction, the inner trench slope is junction to 9 cm/yr near the Caribbean— along the west coast. Some plutonic and narrow, with a small but generally well- Cocos—North American triple junction volcanic rocks may be early Tertiary (Salas defined trench-slope break and upper-slope (Molnar and Sykes, 1969; Larson and and others, 1974), but these reports are not sediment accumulation (Ross and Shor, Chase, 1970) would have an additive but yet substantiated and in some cases con- 1965). In contrast, south of the Tehuan- smaller effect on the size of the trench fill. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/265/3444321/i0016-7606-89-2-265.pdf by guest on 01 October 2021 LATE CENOZOIC SUBDUCTION ALONG NORTHERN MIDDLE AMERICA TRENCH 267 The trench is locally deeper and has less sediment fill near Acapulco, because that area lies between long sediment fans ex- tending along the trench axis from the Rio Balsas to the northwest and several sizable rivers southeast of Acapulco (Fisher, 1961, p. 708). Only relatively minor streams flow into the study area. (Fig. 3). At present the west coast of Mexico ap- pears to be a frontal arc in the trench-arc system. Elevations in the basement terrane exceed 3,000 m within 60 km of the coast (Fig. 3), suggesting that there has been strong uplift during late Cenozoic time, but a well-developed coastal plain and shelf (Fig. 3) and the lack of well-preserved or persistent terraces (Brand, 1958) suggest that most of the uplift is pre-Quaternary in age.