Late Cenozoic Subduction and Continental Margin Truncation Along the Northern Middle America Trench

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

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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.

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The trench is locally deeper and has less sediment fill near Acapulco, because that area lies between long sediment fans extending 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 appears 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. Mareogarph records along the coast show several tenths of a metre uplift over 10-yr periods (Grivel, 1967; Grivel and Arce U., 1971), but this movement could be a reversible effect associated with earth- quakes along the seismic zone as well as with more permanent uplift. lorw i oo° 30' Low headlands and ridges extending to Figure 2. Ship tracks used for magnetic investigation in survey area. Cocotow track (solid) is anno- the coast, as well as calculations of depth to tated as key to reflection profiles in other figures. Dashed lines are other Scripps Institution of magnetic anomaly (see section below) indi- Oceanography cruises: Scan, Iguana, and Cato. cate that only a thin sediment cover under- lies the coastal plain. Outcrops of Xolapa complex have been mapped on the beach ments are tilted landward by relative uplift (Fig. 1), contained several angular clasts of opposite our survey area by Arturo Car- of the trench-slope break (Figs. 3, 5). igneous rocks. One of these, reportedly a ranza E. (1976, personal commun.). In contrast, at the southeastern edge of granodiorite, gave a hornblende age of 88.1 Approximately 10 km offshore is an the survey area, the upper-slope sediments ± 2.0 m.y. (D. Krummenacher, 1971, per- abrupt increase in water depth, which are trapped behind a ridge only 1,000 m sonal commun.). This haul has been cited coincides with a marked drop in the base- below sea level (Figs. 3, 6) that trends more several times (for example, Seely and ment surface (Fig. 3, and section below on westerly than the other arc units. This ridge others, 1974) as evidence for exposure of magnetics). In the survey area, this base- merges southeastward with the main conti- continental crust and for tectonic erosion ment drop-off marks the landward flank of nental block, has the magnetic character of on the lower trench slope. Most of the an upper-slope sediment accumulation that the Xolapa complex, and is most likely a dredge haul, however, consisted of sand- is at least 500 m thick and, on the basis of fault block of continental material. Still stone, shale, and well-indurated massive calculations of depth to magnetic basement, farther to the southeast the trench-slope siltstone of late Miocene or younger age (E. probably is closer to several kilometres morphology again becomes similar to that Allison, 1971, personal commun.; J. F. van thick. in the northwestern area, except that the Sant, 1973, personal commun.). Another The upper slope varies in character along trench-slope break is at a depth near 3 km. Gulfmat dredge (12D) was located just the arc system in the survey area. At the The lower part of the inner trench slope within the survey area (Figs. 1,3) and recov- northwest end there is a well-developed in the Middle America trench is quite steep ered sandstone, well-indurated siltstone trench-slope break at a depth near 2.5 km in comparison to that in most other and mudstone, and minor fragments of and a relatively broad upper-slope area trenches; in the survey area this slope aver- pelagic limestone. Dredge 9D, again on the (Figs. 3, 4). This upper slope is traversed by ages 11°. In addition, and perhaps because lower trench wall at the northern end of the many small canyons, which coalesce of the steep slope, linear ridges on the lower trench, recovered similar siltstones and downslope into several larger systems. slope of the survey area are not easily dis- mudstones of late Miocene or Pliocene age. What appears to be a 20-km-long zone of cernible. At the base of the slope in both A 623-cm piston core (Cocotow-47P), sliding is observed in the upper-slope sedi- Cocotow trench crossings (Figs. 4, 6), there taken on a steep scarp at the base of the ments (Fig. 4). The seaward margin of this is a hyperbolic return that very likely does trench slope (Figs. 3, 6), recovered stiff silt upper slope is marked by a discontinuous represent a ridge of accreted material. of Holocene age (T. A. Walsh, 1975, per- ridge of acoustically opaque material with a The composition of material underlying sonal commun.). very low magnetic susceptibility. As the the lower slope is still a matter of conjec- Piston cores (R. L. Fisher, 1953, unpub- canyons approach this ridge, their gradients ture. Dredge haul 13D from the Gulfmat lished ONR progress report #28, SIO ref flatten. In the best controlled example, the cruise, taken on the lowermost section of 53-52) together with reflection profiles channel is blocked and the ponded sedi- the inner slope southeast of the survey area (Figs. 4, 6) show that the trench-floor sedi-

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 Figure 3. Bathymétrie map of survey area; depths in metres, corrected with Matthews' tables. Topogtaphy, in metres, is simplified from 1:500,000 sheet. Primary bathymétrie control (dots) is that shown in Figure 2, with additional control supplied by Scripps sounding compilations. Locations of Cocotow piston cores 46P and 47P and of Gulfmat dredge 12D (stippled bar) are also indicated.

ment wedge in the survey area consists of continental crust; the indurated sediments continental-crust material. Because the turbidites that average some 500 m in would then be slope deposits. Alternatively, oceanic anomalies have low amplitudes at thickness. These turbidites probably have the indurated sediments could represent this latitude and because the anomaly pat- been transported both through the local uplifted trench sediments, and the igneous terns over metamorphic complexes are not submarine canyons and along the trench clasts could have been transported from ex- nearly as regular as those over the ocean, axis from the large Rio Balsas canyon-fan posures of continental crust farther up- we used an average track spacing of 4 km system. Beneath the trench fill is the pelagic slope. Relative merits of these alternative and oriented these tracks both along and cover of the Cocos plate, which in this re- interpretations are better determined after across the continental margin. Although gion is less than 100 m thick (Figs. 4, 6). the magnetic characteristics of this slope are ship positions were determined by satellite Scattered surface samples (Frazer and discussed. navigation and by a computer smooth plot- others, 1972; Cocotow core 46PG) indicate ting program, there were initial errors of as that this part of the Cocos plate cover con- MAGNETIC CHARACTERISTICS much as 1 km, which were corrected by sists of pelagic clay from the spreading ridge OF INNER hand smooth plotting and by matching to the trench. Very likely the pelagic section TRENCH SLOPE depths at the 19 track crossings. Our final just seaward of the trench is also dominated location errors are estimated to be less than by clay, without a well-developed basal The primary purpose of the Cocotow 0.5 km. carbonate. study was to track and extend the oceanic The total field magnetic readings were The suite of samples from the inner magnetic anomalies landward from the averaged over 1-min intervals and plotted trench slope might be interpreted in several trench and to trace the induced magnetic every 5 min. The International Geomagnet- ways. The igneous rocks could be in situ anomalies expected from the Xolapa com- ic Reference Field (1965.0) was then re- and representative of the leading edge of the plex seaward in a search for the edge of moved, but the dominantly positive anom-

B A

N n- I O I —I Sii Ì O 1 ion

2 —

3 — 4 - x H CL LU O — 5

a: 4 — UJ VE= 2 Ix I 1 — 6 0 10 20 KM 5 —

— 7

6 —1 8 Figure 4. Seismic reflection profile A-B across both inner and outer trench slopes in northwestern half of survey area. Two-part division of inner slope by well-developed trench-slope break is clearly illustrated in this profile. At base of inner slope, possible fold ridge is shown beneath hyperbolic reflection from point farther upslope. Probable gravity slide on upper slope is located between 0415 and 0510 hr. This profile starts seaward of steep drop from continental shelf.

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aly values that resulted indicate that the ref- Therefore, all mismatch was attributed to study by removing the Geomagnetic Refer- erence field is about 200 nT too low in this this factor. ence Field and then linearly fitted to the region. After removal of the reference field, From the difference in magnetic field val- Cocotow survey by adjusting the values at there were magnetic mismatches at track ues at the 19 crossings, a diurnal variation the track intersections. These profiles were crossings of as much as 29 nT. A number of curve was constructed which is consistent used only in developing trends and were not factors can influence the measured magnetic with the observed diurnal variation curves used in the contouring. Contouring was field values, including ship and instrument for this inclination (42°N; Chapman and done following correlation of anomalies on effects, solar storms, the coastal effect, secu- Battels, 1940; Vacquier, 1972). With this profiles, which was relatively straightfor- lar variations, and diurnal variations. The curve, all anomaly values were corrected to ward in the area of oceanic anomalies (Fig. magnitudes of these effects were estimated, a common time to remove the variation 7). Anomaly 5A was identified just seaward and only the diurnal variations appeared to term. Five additional SIO magnetic track- of the trench, and anomalies as old as 14 produce effects over 10 nT in our survey. lines (Fig. 2) were also integrated into the m.y. occur at the boundary with the continental magnetic anomaly pattern. The oceanic anomalies trend approxi- J' J mately N45°W seaward of the trench and can be followed landward 20 to 30 km (Fig. 7), although amplitudes drop as the plate descends. Disappearance of the oceanic anomaly pattern could be caused by one or more of several effects: increasing depth to source, demagnetization of layer 2 by heat- ing or alteration, structural disruption of subducted layer 2, and masking by the shallower continental anomalies. The magnetic anomalies just offshore, which are attributed to the metamorphic basement, have large-amplitude short wavelengths and are often difficult to corre- late between lines even a few kilometres apart. Several of the larger anomalies can be traced across as many as eight tracks and show a trend near N10°E, subparallel to the trend of the structures in metamorphic rocks. These anomalies, and the high- frequency pattern itself, seem to end quite abruptly at their seaward edge and are separated from the oceanic pattern by a narrow band where anomalies are muted and confused (Fig. 7). Neither the seaward edge of the continental anomalies nor the anomalies themselves have any discernible expression in the local bathymetry. Depth-to-anomaly calculations along the larger, more persistent of these anomalies were made using the total field method of Vacquier and others (1951). This method assumes bottomless rectangular prismatic sources with constant magnetization. Along both anomalies the maximum depth in- creases sharply from the coastline across the break in slope mentioned earlier and is greater than 5 km beneath the central part of the upper slope (Fig. 8). However, the I 1 1 calculated maximum depth at the seaward 0 10 20 ends of both anomalies is near 4 km, which KM may reflect an actual reversal in the basement profile. Figure 5. Seismic reflection profile J—J' across inner trench slope in center of survey area. This Thus, continental-crust rocks underlie profile crosses both trench-slope break (at 1130 hrs) and end of oblique basement ridge (at 1315 hr). Landward-tilted strata behind trench-slope break are ponded within structurally blocked submarine the uppermost part of the trench slope and, canyon (see Fig. 3). in particular, the oblique basement ridge

that crosses the upper slope (Fig. 9). On the whereas the granitic plutons mapped in this magnetic and sedimentologic evidence to- other hand, the trench-slope break and the region (de Czerna, 1965) have oblate gether as support for accretion rather than lower slope are underlain by material with- shapes and very irregular boundaries. truncation from perhaps late Miocene to out discernible magnetic characteristics. Moreover, the metamorphic structural Holocene time, associated with near- trends generally wrap around the plutons perpendicular subduction at rates of about DISCUSSION AND rather than being sharply truncated at their 6.5 cm/yr (F. Schilt and M. Truchan, in INTERPRETATION borders. Bottom samples from the lower prep.). Although some of the thin pelagic slope have most commonly been indurated cover of the Cocos plate may be included in Our magnetic survey indicates that the clastic rocks of late Miocene or younger age the accretionary prism, it appears to com- Xolapa metamorphic complex does not un- and are too coarse grained to be hemi- prise predominantly trench-floor turbidites. derlie the lower trench slope in this section pelagic. The angular clasts of igneous rock The accretionary prism that has since de- of the Middle America Trench. It is also in dredge 13D could be material from a developed is small, but all typical features of unlikely that this slope is underlain by bris flow similar to those mapped in up- the inner trench slope can be identified nonmagnetic granitic rocks. The contact be- lifted trench-slope deposits west of Sumatra along the northern section of the Middle tween the two magnetic patterns is very and in the Franciscan terrane of California America Trench. A very simple calculation regular and nearly parallel to the trench, (Cowan and Page, 1975). We interpret the demonstrates that the volume of this accretionary prism can easily be balanced against the amount of material fed to the L K trench since late Miocene time: Cross-sectional area of accretionary prism 0 —, I— 0 (Fig. 9) = 120 km2 Percentage of area consistingof rock (assumed average porosity of 0.2) = 96 km2 I— I Average rate of subduction perpendicular to trench (F. S. Schilt and M. Truchan, in prep.) = 63 km/m.y. Average thickness of trench turbidite fill = 0.5 km Total volume of rock accreted (in km3 per 2 — o km of trench) with average porosity of LU cn 0.6 and duration of 7 m.y. = 86 km2.

The question remains, then, as to how, LU when, and why the truncation of the Mexi- 3 — 1—4 - can margin did occur. It apparently oc- X curred after the emplacement in Late Creta- I- CL ceous time (about 80 m.y. B.P.) of plutons LU LlJ > now exposed along the coast (Larsen and O — 5 < CE others, 1958; Jensky, 1974; Salas and CU 4 — others, 1974). If the extensive igneous ter- LU rane of silicic to mafic rocks (Gastil and >- < others, 1974) represents the roots of a Cre- — 6 taceous volcanic arc, then it now lies much 00 too close to a trench that would have been 5 — located at the truncated continental edge. — 7 Future investigations may narrow this post-Cretaceous but pre-late Miocene win- dow, but at present the early Tertiary subduction history along this part of the arc 6 — — 8 remains virtually unknown. Tectonic erosion of the continent during underthrusting may have been responsible for the truncation during that interval, but it must be proposed on an ad hoc basis, be- 7 —J cause such a process does not yet have any Figure 6. Seismic reflection profile K—L across inner trench slope in southeastern half of survey convincing documentation in currently ac- area. On profile, no trench-slope break can be identified. Ridge at 2100 hr is basement ridge that ob- liquely crosses slope, as explained in text. Small benchlike feature with sediment cover at 2130 hr is tive trenches. Moreover, the subduction oblique crossing of submarine canyon (see Fig. 3). Line-drawing section of profile represents slower parameters that might result in tectonic crossing of steep drop from shelf. erosion are unknown. It is commonly as-

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sumed that a lack of sediment in the trench gins is inevitable. The truncated continental Nicaragua is assumed to be a result of is a cause of erosion, but thinly sedimented margin along western Mexico has been in- strike-slip faulting and is applied to the downgoing plates are associated with ac- terpreted as a result of either left-lateral (de western margin at the Tehauntepec offset, cretion in the southeastern section of the Czerna, 1971; Malfait and Dinkleman, the original position of the west end of the Middle America Trench and in the western 1972) or right-lateral (Karig, 1974) shift. Guatemalan cratonic block would have Pacific trenches (Karig, 1974). The left-lateral shift has been associated been less than halfway up the truncated On the other hand, the lateral shifting of with motion along the North American- part of the Pacific margin. In addition, part marginal terranes as a result of oblique Caribbean plate boundary (Malfait and of the left-lateral offset along the North subduction is known to occur in several Dinkleman, 1972), but for several reasons, American—Caribbean boundary apparently areas today (Fitch, 1972; Karig and Mam- we do not think that the truncation in is resolved into east-west extension within merickx, 1972). If such activity persists for question can be attributed to left-lateral Central America (Malfait and Dinkleman, long periods, even intermittently, then shear. Even if all the offset of the eastern 1972; Muehlberger and Ritchie, 1975; long-distance transfer of continental mar- continental margin from Yucatan to Plafker, 1976), further reducing the fraction

I6°30 N—

Figure 7. Contoured magnetic anomalies in survey area, illustrating distinctive oceanic and continental patterns and sharp boundary between them.

of truncated coast which could be explained ridge at the mouth of the Gulf of California 1976). On Cedros Island, this fault zone, by left-lateral shear. Furthermore, Kesler (Larson, 1972) and is not applicable to the which here appears to have a thrust com- (1973) has questioned the match in struc- truncation, because the marginal slivers ponent, and others juxtapose rocks of the tural trends and observed offsets along the could not have moved across the East Pac- volcanic arc with metamorphosed sub- assumed zone of shear across Central ific Rise after it moved into the Gulf (4 m.y. duction-zone rocks of the same approxi- America (Cuilco-Chixoy-Polochic and B.P.). However, there have been numerous mate age (Kilmer, 1969; Jones and others, Motagua fault zones). suggestions of right-lateral slip on faults 1976; Minch and others, 1976). Again, Right-lateral shear and northward dis- farther to the west and within the Basin and both of these rock types are anomalously placement of the marginal slivers would be Range province which must be added to the close to the trench. attributed to the general and persistent San Andreas offset in order to calculate From Vizcaino Bay to Magdalena Bay, northward motion of the Pacific plates rel- total displacement of the margin (Atwater both Triassic and Jurassic subduction ative to North America (see references in and Molnar, 1973; Howell and others, complexes are exposed (Minch and others, Karig, 1974), and to strongly oblique sub- 1974; Johnson and Normark, 1974; How- 1976; Yeats and others, 1971) in which a duction associated with the break-up and ell, 1974, 1976). Estimates of total dis- number of northwest-trending right-lateral rapid rotation of the Cocos plate during placement, exclusive of that associated with faults have been mapped (Minch and Miocene time (F. Schilt and M. Truchan, in the East Pacific Rise at the Gulf of Califor- others, 1976). The displacement along these prep.). Analysis of fanning magnetic nia, range from 700 km (Howell, 1976) to faults is unknown, but in places the older anomalies in the segment of the Cocos plate more than 1,000 km (Clarke and Nilsen, Triassic complex is west of the Jurassic that was in contact with the survey area 1973; Gastil and others, 1974; Johnson complex (Fink and Abbott, 1976; D. G. shows oblique subduction from about 7 to and Normark, 1974). Howell, 1976, written commun.), just op- 15 m.y. B.P. Along the northern end of the Evidence of translated terranes along the posite the normal stacking sequence. This subplate, nearly pure strike-slip motion is west coast of Baja California may be more tectonic "enigma" (Jones and others, 1976, indicated. directly related to the missing margin to the p. 349) could most rationally be explained As appealing as this Miocene period of southeast. From Vizcaino Bay to Mag- by large-scale strike-slip faulting. The tim- oblique subduction might be, it did not dalena Bay, scattered areas of subduction ing of the faulting in the Baja region is not persist long enough to cause the 1,000-km complex are exposed, flanked to the east by well understood, but apparently it spans the offset necessary to completely remove the a basin deeply filled with Cretaceous clastic period from before Late Cretaceous to more margin south of Cabo Corrientes. If right- rocks and farther east by a plutonic com- recent than Miocene (Minch and others, lateral shear did cause truncation, it must plex. This appears to be a perfectly normal 1976). have been active, at least intermittently, convergent margin pattern until it is exam- The total northwesterly component of throughout much of early Tertiary time. ined more closely. displacement between the Farallon or Pa- The evidence for these displacements has On the San Benito islands, for instance, cific plates and the North American plate not been found or has largely been removed which lie only 40 km from the recently de- determined from finite plate tectonics re- from the truncated part of the Middle funct trench (Cedros Deep; Fisher, 1974), constructions approaches 1,000 km for the America arc, but we can point to a number there are Franciscan-like rocks, including past 30 m.y. and possibly several times that of observations and supporting studies metamorphics of blueschist facies (Cohen since Cretaceous time (Atwater and Mol- from more northerly sectors of the conti- and others, 1963). Such highly metamor- nar, 1973). In view of the strike-slip fault- nental margin. phosed rocks are unique and distinctly ing associated with most convergent plate In California, the San Andreas fault sys- anomalous at the surface this close to a boundaries that show oblique subduction tem has had approximately 300 km of trench with a normal accretionary prism. (Fitch, 1972), we suggest that much or most right-lateral offset in the last 15 to 20 m.y. This complex is cut by several right-lateral of this component is accommodated on (Huffman, 1972; references in Clarke and shear zones, one of which appears to be a shear zones parallel to the plate margin. Nilsen, 1973) and may have had as much as major shear extending several hundred Although the details are very hazy, there are 600 km of displacement since Late Creta- kilometres along the San Benito Ridge many indications that very large northwes- ceous time (Suppe, 1970; Clarke and through Cedros Island and into the terly displacements of marginal terranes Nilsen, 1973; and others). Of this, 200 to Vizcaino Peninsula (Cohen and others, have been occurring along the Pacific coast 240 km is associated with the spreading 1963; Krause, 1965; Minch and others, of North America throughout Cenozoic

Figure 8. Composite section across inner trench slope in northwestern section of survey area. Lined pattern = metamorphic rocks of Xolapa complex. Bars with upward-pointing arrows indicate calculated maximum depths to magnetic basement. Shape of seaward boundary of metamorphic basement is highly conjectural, as is boundary between accreted lower-slope sediments and upper-slope sediments. Polarity and extent of oceanic crustal magnetic anomalies is indicated by polarity symbols within oceanic crust.

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time. The pattern of faults along western seems to have occurred sometime between 7 have converged on the idea that marginal Baja and upper California indicates that the and 80 m.y. B.P. and could be attributed to terranes have undergone large right-lateral margin was displaced in slivers rather than tectonic erosion or to either left- or right- displacements along the continental margin as a single unit. lateral translation of marginal slices. of North America. One might suspect that Tectonic erosion is not disproved, but we the Middle America situation is not unique. CONCLUSIONS AND find no reason to invoke such a process nor The truncation of Paleozoic and early SPECULATIONS evidence to support it. We clearly favor Mesozoic marginal belts in the southwest- right-lateral displacement as a result of ern United States (Hamilton, 1969; Burch- A combination of geological and geo- oblique subduction but cannot rule out a par- fiel and Davis, 1972) is another candidate. physical data show reasonably clearly that tial contribution from left-lateral slip at the More important for ongoing research into the Middle America Trench from Cabo southeast end related to motion of the subduction-zone behavior is the possibility Corrientes to the Tehuantepec junction is Caribbean-North American plate bound- that the branching of the Andean chain in not now a locus of marginal truncation. ary. Although combinations of these three Colombia (Case and others, 1971) might Rather, there has been accretion of trench processes might be invoked, this sort of have resulted from a similar process. This turbidites, probably since late Miocene solution seems inherently unsatisfactory. possibility was noted earlier (Karig, 1974), time, and the generation of a small but typi- To us it is relevant that a considerable but has recently been debated by Hussong cal accretionary prism. number of investigators, using different ap- and others (1976) on the basis of their in- The truncation of the continental margin proaches to reach a variety of objectives, terpretation of the velocity structure of the trench slope coupled with calculations of S material accreted to the South American o plate. While it is clear that no reliable conclusion can be reached with available data, it should be pointed out that L. D. Kulm (1976, personal commun.), using much the same data, has interpreted the inner slope to be underlain by high-velocity dewatered and metamorphosed sediment. This difference alone strongly affects conclusions con- cerning the balance of material fed to the trench with that accreted. Also, the reason- ing leading to the postulation of a very large land mass west of the central Andean coast appears to minimize the amount of uplift that might take place without production of coarse clastic rocks and to ignore the marked drainage asymmetry observed in many orogenic uplifts. We remain im- pressed by the multiciplicity of orogenic belts in Colombia and by the large right- lateral shear zones separating these belts, along which major displacements are postulated to have occurred (Case and others, 1971, and references therein). These features cannot but have affected the continental margin farther south.

Another conclusion arising from our survey was that individual or even widely spaced profiles across trench slopes are often misleading with regard to the smaller scale features in this setting. Only quite closely spaced tracks can adequately differ- entiate canyons from structural depressions and reveal structural deviations from two-dimensionality. Clearly, we do not need detailed surveys of all consuming margins, but we do need more such surveys Figure 9. Composite tectonic diagram of survey area, showing relationship between morphotec- in areas representing different subduction tonic features and magnetic anomaly patterns. Magnetic anomalies are shown by stippled bars, with situations. In particular, we need detailed boundary between oceanic and continental anomalies located well landward of trench-slope break (asterisks). V pattern = oblique ridge beneath upper slope, underlain by Xolapa metamorphic rocks; and composite surveys in areas suspected of modified normal fault pattern = drop in basement behind upper slope. undergoing tectonic erosion, so that the

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