44. SUMMARY: LEG 84, MIDDLE AMERICA TRENCH TRANSECT OFF GUATEMALA ANDCOSTA RICA1

Jean Aubouin, Département de Géotectonique, Université Pierre et Marie Curie, Parisand

Roland von Huene, U.S. Geological Survey, Menlo Park, California2

INTRODUCTION AND PREVIOUS WORK

At the beginning of the IPOD (International Pro-gram of Ocean Drilling) program in 1973, the MiddleAmerica Trench transect was assigned a high priority asa primary area for testing the accretionary convergentmargin model based on an industry multichannel seis-mic record (Seely et al., 1974). The conditions favorablefor testing accretion are present in the area; on the basisof earlier deep-sea drilling experience along convergentmargins, these favorable conditions include good ageresolution from a diverse community of microfossils, amoderate supply of terrigenous sediment, and a rela-tively rapid rate of convergence. The study of Seely etal. (1974) was based on one of the first published multi-channel seismic records tied to a deep drill hole at theedge of the shelf (Seely, 1979). The case for accretionappeared convincing, and the Guatemalan segment ofthe Middle America Trench became a type example ofconvergent margin tectonism.

The first studies of the Guatemalan margin to followthose of Seely were the site surveys done in preparationfor the drilling of Leg 67. A grid of multichannel seis-mic records made by the University of Texas (Ladd etal., 1978; Ibrahim et al., 1971) led to the selection of atransect about 70 km southeast of Seely's data that cen-tered about San José Canyon where the cover of sedi-ment above basement was thinnest. These were also theonly records to show a landward-dipping reflection, whichwas the principal basis for the interpretation of imbrica-tion and accretion. Ibrahim et al. (1979) and Ladd et al.(1982) emphasized the landward-dipping reflection andthe rocks of high acoustic velocity that might be slicesof igneous oceanic crust in the imbricated complex.

The Middle America Trench off Guatemala was firstdrilled in 1979 during Leg 67 (Fig. 1). The drilling pro-ceeded smoothly on the Cocos Plate and in the Trench.For the first time the Trench itself was drilled down tothe oceanic crust. Combined with the results of a post-Leg Seabeam cruise by the Jean Charcot (Renard et al.,1980; Aubouin et al., 1981), the Leg 67 data made itclear that the Cocos Plate, with an inherited horst and

von Huene, R., Aubouin, J., et al. Init. Repts. DSDP, 84: Washington (US. Govt.Printing Office).

2 Addresses: (Aubouin) Département de Géotectonique, Université Pierre et Marie Cu-rie, 4 Place Jussieu, 75320 Paris Cedex 05, France; (von Huene) U.S. Geological Survey, Men-lo Park, CA 94025.

graben structural trend oblique to the Trench axis, issubducted beneath the continental margin without anyevidence of compressional structure. Extensional struc-ture remains in the oceanic plate up to the very toe ofthe continental slope (Site 500), where the oceanic crustwas drilled beneath a normal fault zone 75 to 135 mshallower than the middle of the Trench. This surprisingresult was explained by the Seabeam survey: Site 500was located on the southwestern margin of a horst en-tering into the subduction zone (Fig. 2).

The presence of gas hydrates on the continental slopecaused termination of Leg 67 drilling above the targetedobjective for reasons of safety where the testing for im-brication required penetration of the slope deposits andsampling of the acoustic basement. There was one ex-ception at the base of the slope where the drilling wassuccessful in reaching basement, although this fact wasnot recognized at the time. A surprising sediment se-quence from the Quaternary to Upper Cretaceous wasrecovered. Because of logistics, it was impossible to par-allel the hole and sample well enough to show that thebasement had been reached (an objective finally accom-plished on Leg 84). Thus the scientific party of Leg 67was repeatedly frustrated in reaching the tectonic targetsplanned for testing of an accretionary origin for the mar-gin, and yet one hole displayed a section that precludedaccretion at least in the Neogene. The tantalizing evi-dence for lack of accretion or even strong tectonism atthe base of the landward slope of the Trench (unde-formed trench fill continuing into the subduction zone,rocks as old as Late Cretaceous against the subductionCocos Plate, and a cover of coherent slope sedimentthat extended into the early Neogene made this an obvi-ous target for further drilling if the problems of safetycould be overcome.

As a result of Leg 67 studies and some added geo-physical work after the leg, geophysical data showed sub-duction of the Cocos Plate beneath Guatemala, but withonly a few of the commonly observed features of accre-tion or compressional deformation. The present arc-trench system left a good chronology of volcanic ashlayers in the Neogene and this system had achieved ma-turity. According to the accretionary hypothesis thereshould be no remnants from the destruction of a pre-vious margin. Nothing regarding the Trench morpholo-gy or the configuration of the forearc basin, the historyof volcanism, seismicity seemed unusual, compared toother trenches of the Pacific. But clearly, the section ofUpper Cretaceous to Quaternary rock at the front of the

939

J. AUBOUIN, R. VON HUENE

Figure 1. Tectonic setting of the Santa Elena and Nicoya peninsulas (Costa Rica) and of Legs 67 and 84 off Guatemala. Present-day plate motionsfrom Minster and Jordan (1978). 1: Pliocene and Pleistocene volcanism; 2: Oligocene and Miocene; 3: North American Plate; 4: South AmericanPlate; 5: Cenozoic formations of Andes and southern Central America; 6: Mesozoic and Cenozoic ophiolitic complexes; 7: subduction zones; 8:magnetic anomalies.

margin seemed to require some tectonic process otherthan accretion, and only one hole had penetrated theslope sediment. Despite recovery at only one hole, theold rock at the front of the Trench off Guatemala couldnot be dismissed out of hand as a local slump block orsome other kind of accident.

Therefore, the objectives of Leg 84 were twofold.The first was to penetrate the slope deposits to examinethe age and structure of the framework that forms thelandward slope of the Trench off Guatemala. Becausethis objective had been frustrated during Leg 67 by therecovery of gas hydrate, a safe way to deal with the hy-drate was required. It was assumed that the gas hydrate,an icelike substance, might form a seal beneath whichfree gas is trapped. The free gas could be pressured, andif a hole penetrated the seal a blowout might occur. Thusthe Glomar Challenger was not allowed to drill throughthe level of gas hydrate. The seismic data of the Univer-sity of Texas was reprocessed to bring out the reflectionsformed at the base of gas hydrate. These reflections arerecognized by their configuration, which simulates theconfiguration of the seafloor, but are overprinted on thereflective sequence at some depth below it. Sufficient

base of gas-hydrate reflections were thus identified toestablish this depth with certainty so that it could beprojected within the site survey area even where no baseof hydrate reflector was observed. In addition, tempera-ture was measured at the ESSO Petrel and Leg 67 drillholes. The general depth of the hydrate reflection couldalso be predicted from temperature because the stabilityfield of methane gas hydrate recovered on Leg 67 is afunction of depth and the temperature gradient. Drillingsites were therefore located safely above the base of thegas hydrate on basement highs.

The second objective of Leg 84 was the study of theorigin and occurrence of the gas hydrate in the marineenvironment. Although hydrate had been drilled on landin the high latitudes, marine gas hydrates had been sam-pled and studied in limited quantity only on Leg 67. Inaddition, a single site on the lower slope off the NicoyaPeninsula of Costa Rica (Site 565) was drilled with thesame objectives as those off Guatemala.

GEOPHYSICAL DATA

Geophysical studies have continued ever since the Uni-versity of Texas predrilling site survey. Prior to Leg 67

940

LEG 84 SUMMARY

1 2°50'

91°07' 91°00' 90°50'

Figure 2. Bathymetric map made with the Seabeam instrument aboard the J. Charcot (from Aubouin, Stephan, et al., 1981).

90°43'

only the University of Texas seismic, magnetic, and OBS(ocean-bottom seismometer) refraction data were avail-able (Ibrahim et al., 1979; Ladd et al., 1982). After Leg67, a Seabeam survey (Renard et al., 1980; Aubouin,Stephan, et al., 1981) and a deep-tow survey (Moore etal., 1982) were made; and after Leg 84, another Sea-beam survey and a high-resolution seismic reflection sur-vey (T. Shipley et al., personal communication, 1983) weremade. The seeming inability to predict what would beencountered in the drilling from the initial geophysicaldata provided impetus for a continuing search for defin-itive geophysical resolutions.

Topography, especially high-resolution topography asprovided by the Seabeam instrument, was used to ad-dress the tectonic problems of initial subduction at thebase of the landward slope of the Trench, the origin ofbenches displayed prominently on the main seismic re-cord of the transect, and the nature of the midslope ter-race beneath which most of the base of gas-hydrate re-

flections were found. From the initial processed seismicrecords it was obvious that slope sediment covers muchof the basement topography. Thus much of the tectonicsthat configured the basement upper surface could bestudied in plain view rather than only in widely spacedprofiles.

The initial Seabeam survey made by the Charcot re-vealed that the Cocos Plate has a structural fabric trend-ing 30 to 35° to the Trench axis (Renard et al., 1980; Au-bouin, Stephan, et al., 1981; Aubouin, Stephan, et al.,1982). This structural fabric governs the trend of horstand graben formed where the Plate is flexed downwardinto the Trench. Thus the Trench floor is sectioned intoa series of diamond-shaped basins separated by ridges(Fig. 2). The basins are graben in the Cocos Plate, andthe ridges are horsts. This structure is a consequence ofthe progressive burial of the Cocos Plate beneath thesediment filling the Trench. This angular relationshipbetween the structural fabric of the Cocos and Carib-

941

J. AUBOUIN, R. VON HUENE

bean plates contrasts well in the contours of the Sea-beam maps. The lowermost landward slope of the Trenchis formed by local fault-bounded steps, but the natureof the bounding faults is not clearly revealed in the orig-inal seismic records because diffractions obscure struc-ture. The trend of these bounding faults bears little rela-tion to that of the ridges and troughs in the subductingCocos Plate. The local distribution and irregular topog-raphy of the steps were used to argue for normal ratherthan reverse or thrust fault origin for the benches.

A second Seabeam survey (T. Shipley et al., personalcommunication, 1983) extended the area of coverage toinclude the midslope area and San José Canyon (Fig. 3).The first and second surveys were merged and combinedwith conventional bathymetric data to provide a blockof data for analysis with perspective diagrams (Fig. 4).With the perspective diagrams three topographic divi-sions of the slope become distinct: the upper slope be-gins as a sharp break at the edge of the shelf and ends atthe smooth gently dipping midslope terrace; the terracegives way to a lower slope of hummocky morphologyterminated at the Trench by a straight basal scarp. In thecentral part of the mapped area are two benches with ir-regular surfaces. The benches and bounding scarps arefeatures local to the mapped area in a subtle reentrantand are suggestive of a collapsed slope topography.However, the basal scarp has maintained its linear tracealong the Trench axis.

San José Canyon is a prominent transverse feature ofthe shelf and the slope. The Canyon reaches its maxi-mum 1.8-km depth of erosion as it crosses the shelfedge. Its course is not disrupted by tectonism until itreaches the benches of the lower slope. Drilling at Site566 indicates a Miocene age for the San José Canyon.From the seismic data and the drilling it is clear thaterosion in the Canyon has stripped the basement of itscover near the thalweg. The age of sediment redepositon the basement is the late Miocene, as indicated at Site566 (Site 566 report, this volume). The Canyon showslittle disruption except at the benches of the lower slope.The morphology and sedimentary record along San JoséCanyon suggest tectonic stability along the slope sincethe late Miocene.

Seismic reflection records GUA-13 and -18, the mainrecords along the transect, were completely reprocessedwith programs not available during the original survey(Fig. 5). Diffractions that obscured many tectonic rela-tions in the original site survey data were eliminated bymigration. The reprocessed records display the trunca-tion of beds along the scarps bounding the benches ofthe lower slope; the relation between slope deposits andthe basement was clarified locally, and the resolution ofbeds in the forearc basin and the sediment sequence be-low it provided not only deeper imaging but also resolu-tion of as little as 100-m vertical displacements alongfaults, details not visible in the initial site survey data.The greater detail strengthened interpretation and madethe geophysical observations more consistent with thedrill results.

Sediment of the Neogene forearc basin and the un-derlying Paleocene and Cretaceous base of continental

margin sediment have been little deformed by conver-gence. The beds are locally broken by faults dipping bothlandward and seaward with less than 250 m vertical dis-placement. Within the reflective sequence are many con-tinuous beds that are interspersed with a few local un-conformities and pinching of beds. These irregularitiesare concentrated on the seaward side of the basin. Areconstruction of the depositional history indicates ac-cumulation of the Cretaceous and Paleocene sedimentin a tabular body probably more than 100 km wide andat rates generally found in terrigenous deposits. Theshapes of the overlying Eocene and Oligocene bodies in-dicate uplift of the front of the margin, and, by the Ne-ogene, a well-defined forearc basin had developed. Thesoutheastern part of the surveyed area was subaeriallyeroded before the early Miocene and, in a less extensiveway, at various times during the Neogene, but the north-western part appears to have remained more or less sub-merged. Uplift continued during the Neogene at theedge of the shelf but at a slower rate than during the Eo-cene and Oligocene. At the same time, the forearc basinsubsided until at its center basement may now be deeperthan 10 km. The most active tectonism was at the edgeof the shelf and was not a regional event that involvedthe whole forearc area. Therefore the general tectonichistory developed by Seely (1979) from a paleontologi-cal summary of the ESSO Petrel drill hole is confirmedfor the forearc by a regional analysis of seismic stratig-raphy. But this history does not fully apply to the land-ward slope of the Trench. The tectonic histories of theshelf and the landward slope were diverse in the pre-Eo-cene. A fundamental tectonic boundary separates a Cre-taceous-Paleocene sedimentary section on the shelf fromanother on the slope and each seems to have developedin a different environment. Displacement on the upperslope boundary juxtaposed these rocks just before orduring the tectonic episode that initiated developmentof the present arc-trench system. The present system be-gan to form in the Eocene and by the late Oligocene ithad become structured much like it is at present.

One of the last tectonic events was the failure of thelower slope, as recognized by Aubouin, Stephan et al.(1981). The slope is underlain by an altered ophioliticbasement that failed and thus produced two rough bench-es. Such failure suggests little transmission of compres-sive horizontal stress into the upper plate. The high de-gree of decoupling across the subduction zone suggestedby the low compressive strain is consistent with the over-pressured pore fluids observed directly on Leg 84. Thecontrast between the collapse morphology of the bench-es and the straight escarpment at the Trench axis em-phasizes the passive origin of the benches and the dy-namic process by which the Trench axis maintains itsposition. The slope sediment structure and the longevityof San José Canyon show that only the front of themargin and the edge of the shelf appear to be subject torapid tectonism.

The margin off Guatemala has been surveyed exten-sively with refraction seismic techniques including twoOBS surveys. The first was done during the site surveyand was reported by Ibrahim et al. (1979), and the sec-

942

LEG 84 SUMMARY

13°30'

1 3°20'

1 3°1 0'

13°00'

1 2°50'

1 2°40'

1 2°30'90°40' 90°30'

Figure 3. Topography across the DSDP Middle America Trench transect area. Contours, at 100-m inter-vals, are solid where controlled by Seabeam data (Aubouin, Stephan, et al., 1981; T. H. Shipley et al.,personal communication, 1983), and are dashed where controlled by conventional soundings. Linesare adjusted ships tracks of conventional data and the regular grid of tracks at the right margin arefrom the Glomar Challenger, made during a standby period. Conventional data positions were adjust-ed by fitting conventional bathymetric profiles to the Seabeam data in areas of overlap. The trackthrough the center of the map is GUA-13 and -18, dots are DSDP drill sites, and lines are dredgetracks from the pre-drilling survey (see von Huene et al., this volume).

ond was done during Leg 67 and is reported here (Am-bos and Hussong, this volume). The rocks comprisingthis margin have relatively low acoustic velocities andfew strong magnetic anomalies. The weathered igneousocean crust that comprises this margin has low acoustic

velocity and magnetic susceptibility. Thus geophysicalmethods are poorly suited to clearly differentiate con-solidated and tectonized sediment from igneous oceaniccrust. The acoustic velocity of the igneous oceanic crustoff Guatemala is in the same range as that of the Creta-

943

J. AUBOUIN, R. VON HUENE

San José Canyon

2000

4000

6000

Figure 4. Perspective diagram of the bathymetry in Figure 2 at an exaggeration of ×7.5.

ceous limestone recovered at Site 567 or the Cretaceousshale recovered from beneath the Japan Trench margin(Murauchi and Ludwig, 1980).

The experiences of Legs 67 and 84 show the level ofinterpretive constraint even with gridded geophysical sur-veys and modern techniques. Resolution in seismic re-flection records is poor in deep water and along steepslopes because many of the assumptions in the seismictechnique are violated. The original site survey providedsuch loose constraints that major tectonic features weremisinterpreted. In hindsight, the Seabeam topography,deep-tow surveying, a well-processed seismic reflectionsurvey, and a good map of magnetic anomalies wouldhave indicated the lack of a classical accretionary struc-ture. However, a single multichannel seismic record acrossthe margin, magnetic anomalies, gravity anomalies, andreconnaissance bathymetry provide insufficient data toidentify a nonaccretionary origin of the Guatemalan mar-gin. The drilling that showed the existence of a pre-Eo-cene or even pre-Late Cretaceous basement everywherebeneath the slope is an even better argument. Neverthe-less, until a more detailed survey is done along the rec-ord published by Seely et al. (1974), it is still possiblethat within a generally nonaccretionary margin, localaccretion has been occurring at the foot of the slope.

SEDIMENTARY ROCKS

Upper Oligocene to Quaternary

Most of the sediment recovered during Leg 84 wasslope sediment; when considered together with the slopedeposits recovered on Leg 67, the five sites and nine

cored holes represent the greatest amount of such mate-rial collected from drilled holes along any IPOD activemargin transect (Fig. 6). The sampled material is parti-ally from an area of detailed Seabeam bathymetry andwithin a grid of seismic data. The sediment sources werefrom land, and as sea level lowered, from local insularareas at the edge of the shelf and along the coast. Theclastic sediment was transported by channels of varioussizes that shifted location with time. Thus as one depo-center was accumulating sediment rapidly, another mayhave lost its feeder channel and temporarily have been inan interchannel area where hemipelagic sedimentationwas dominant, or perhaps even in an erosional areawhere material was removed to produce a local uncon-formity. Where the slope was steep, downslope trans-port and a variety of mass transported materials weredeposited along local topographic flats, benches, or ul-timately in the Trench. Thus we recognized no simplelithofacies distinction between deposits in slope basinsand in the Trench off Guatemala.

The observations on which we base this general modelare from the study of core, seismic, and bathymetric in-formation. Although the main sediment type recoveredis hemipelagic mud and mudstone, considerable sandand calcareous mudstone were also recovered. In somesections the amount of sand, including coarse sand, ex-ceeded that found in the Trench (Coulbourn et al., 1982).Along San José Canyon, sand included rock fragmentsfrom the arc and from the basement that floors parts ofthe Canyon; transport through San José Canyon is par-ticularly rapid and involves materials that have traveledcomparatively little since leaving the land. Some stretches

944

I I I I I I I l _ I I I I J LJ I I I I I I I I

Figure 5. Seismic record GUA-13, landward slope of the Middle America Trench, Guatemala, migrated time section.

570Water depth

1718m

Guatemala slope key

Siliceous mud—mudstone

Hemipelagic mud—mudstone

Breccia andconglomerate layers

Sand—sandstone

Basic and ultrabasicslope basement

566Water depth

3675 m^ C r ^ ~.upper

"---Pleist.upper Plio.

upper— Mio.

Basement

/_/

10I

1 5

km

//

/ / "/ / / upper

7 / Oligocene middle—lower Miocenelower Eocene

-' L Basement

J3asement 364.9 m417.7 m /

136.6 m

501.0 m499 500

Water depth __ Water depth6127 m 6123 m

0

500"

400

Basalt

ΦC

\ o\ 2

\ \ .2

\ \^liocene

Vliocene

J , Λ I 4

-100

/ /-200

//

165.5 m

Pleistocene

lower Mio.

Basalt

286.5 mBasalt

446.5 m

Cocos Plate key

Trench-fill turbidites withsiliceous biogenic admixture

Hemipelagic mud withsiliceous biogenic admixture

Brown abyssal clay

Nannofossil chalk

Manganiferous chalk

BasaltCocos Plate basement

13°00

Guatemala

Middle AmericaTrench axis

50 km

6 km

91 "00 W

Figure 6. Simplified stratigraphic columns summarizing the results of Legs 67 and 84. Note that the oceanic plate, the Trench floor, and the continental margin were drilled to their basement. Anotable feature is the presence of the pre-early Eocene (Sites 569, 570), pre-Late Cretaceous basement (Site 567) on the slope, namely at the very toe, which disagrees with the accretionary model.

LEG 84 SUMMARY

of the Canyon have overbank deposits, others are erod-ed, and the upper reaches are marked by a sharp V-shaped thalweg. A possible abandoned channel of SanJosé Canyon (von Huene, Miller, et al., this volume)suggests that tectonism not only causes small channelsto shift, but also causes large canyons to shift positionand depocenters with time. San José Canyon ends at thebenches of the lower slope, and, seen with Seabeam da-ta, its channel cannot be traced through this area of tec-tonic disturbance. Thus San José Canyon is probablyolder than the benches.

Contrasting in size with San José Canyon are themany smaller channels in the Seabeam data or in seis-mic or bathymetric records that run parallel to the re-gional trend. Sharp V-shaped rills are seen in the recordswith a high vertical exaggeration. Some begin and endin a short distance on the slope, others are tributaries oflarge channels or an integrated channel network. Theshort segments may be part of abandoned channels.

In addition to sediment transport in channels are thedownslope mass movements shown by displaced fossils.All depth diagnostic microfossil assemblages are mostlydisplaced from upslope. A general creep of the cover ofslope deposits seems to prevail except on local benchesor terraces. More catastrophic are the thick (ca. 50-m)Miocene boulder breccias near the present base of theslope that may be from slumping or perhaps from accre-tion of sediment represented by the extensive reflectivehorizons in seismic records across the slope.

Sediment accumulation rates are variable. At somesites sediment accumulates at rates of 300 m/m.y. andmore, above a long hiatus or unconformity. Many chan-nels appear to have supplied sediment to one depocenterfor 2 to 5 m.y. and then to have shifted course. Erosionis well documented along the length of San José Can-yon. The lengthy period of sustained slope deposition atSite 570, for instance, provides a quantitative indicationof tectonic stability during which channels and a depo-center remained undisrupted.

The large middle and late Miocene hiatus in the slopesediment cover at Sites 567, 568, and 569 may have beencaused by bottom-water currents or by erosion along anabandoned channel of San José Canyon. Stone and Kel-ler, and McDougall (both this volume) point out thecorresponding general Pacific Miocene hiatuses and thedetection of changing bottom waters in the benthic fau-na. However, the large abandoned canyon in the Sea-beam bathymetry (von Huene, this volume) suggestsdownslope erosion in the vicinity of the sites, which isanother possible mechanism. No distinct unconformitywas noted in seismic records corresponding to the hia-tus, and at Sites 568 and 495 (Leg 67) the hiatus ap-peared to correspond to a change in sedimentation.Thus the relative importance of different erosional pro-cesses is difficult to assess along the IPOD Guatemalantransect.

Late Cretaceous to Eocene

The few cores in which Late Cretaceous and Eocenesediment was recovered are very significant because insome cases they represent the only examples of the East

Pacific open ocean and lower continental slope stratig-raphy preserved in Central America (Fig. 6). Two dis-tinct Eocene domains can be recognized. Landward un-der the shelf is a basin with a thick clastic sediment sec-tion documented at the Esso Petrel Well and traced overan extensive area in seismic records. The limestones,sandstones, and black and red conglomerates recoveredat Site 570 may represent distal equivalents of this faciesthat rest directly on a basement high in the vicinity ofthe shelf edge as seen in seismic records. Seaward, be-neath the middle and lower slope is a deep open oceansequence represented by dark gray siliceous mudstonesdeposited below the CCD (Sites 494 and 569). These arenot slope deposits but have ocean plate affinities.

The same distinction between environments is indi-cated in the upper Campanian to Maestrichtian sedi-mentary rocks. Beneath the shelf is a thick base of slopeclastic sediment section whereas beneath the slope arethin pelagic limestones deposited above the CCD awayfrom any terrigenous influence. These two distinct envi-ronments may extend along much of the Middle Ameri-ca Trench slope and shelf from the Gulf of Tehuantepecto the Nicoya Peninsula.

Far less clear is the tectonic history that gave rise tothe present juxtaposition of these depositional environ-ments. Was the Eocene basin bounded seaward by achain of islands in the vicinity of the present shelf edge?Simple reconstruction of the Eocene basin edge in thenorthern part of the IPOD transect area indicates suchislands, and the Nicoya Peninsula was also insular at thetime. What were the Cretaceous and Eocene morpho-logical and tectonic environments seaward of the basin?The scant litho- and biostratigraphy suggest Cretaceousdeposition far from a continent and the Eocene rocks,although deposited in deeper water, received distal ter-rigenous material. Would the apparent deepening be-tween the Cretaceous (above the CCD) and Eocene (be-low the CCD) represent the approach of an oceanic platetoward land? A similar but continuous sequence was re-covered in the Neogene on the Cocos Plate, document-ing the transit of the Plate toward the Middle AmericaTrench (Aubouin, von Huene et al., 1982). What wasthe character of the Paleocene tectonic event that uplift-ed the inshore basin and juxtaposed a terrain of oceanicaffinity against it? The constraints are insufficient to fa-vor an environment dominated by either transcurrent orconvergent motions.

These questions, crucial for the interpretation of Cen-tral American tectonic history, serve as directions for fur-ther research.

IGNEOUS ROCKSThe acoustic basement of the Middle America Trench

landward slope was recovered at four sites drilled duringLeg 84 (Fig. 6). Harzburgites (cumulative-texture peri-dotites), gabbros, dolerites, basalts, and amphibolite fa-cies rocks were recognized by petrologic, X-ray fluores-cence, and microprobe analyses. Some of these rocksbelong to an ophiolitic sequence. A few basalt sampleshave also been studied with ^K-^Ar isotopic ratios (Bel-Ion et al., this volume).

947

J. AUBOUIN, R. VON HUENE

The ultramafic rocks were sampled from Holes 567A,566, 566A, 566C, and 570. In Hole 567A the ultramaficrocks are in three distinct positions: (1) reworked ser-pentine blocks and pebbles in the lower Miocene sedi-ments, between 260 and 330 m; (2) a thick block of ser-pentinized peridotites above the Upper Cretaceous lime-stones; and (3) between 380 and 500 m—recoveredbasement rocks under the Upper Cretaceous limestones.

All the ultramafic rocks recovered during Leg 84 arewidely serpentinized, however, their primary mineralogyappears as relicts in some samples. Because the serpen-tinization of the pyroxene is different from that of oliv-ine, it was possible to identify the original textures. Thustwo groups of ultramafic rocks were identified: (1) peri-dotites (harzburgites) with a xenomorphic texture (Holes567A, 566, and 566C) that is generally serpentinizedcompose the first group. (The olivines are Fo88-Fo92 andthe orthopyroxenes are En86-En89, the clinopyroxenechemical compositions are Wo = 47.9, En = 48.4, Fs= 3.7, and from one sample to another clinopyroxenesshow obvious variations in Al and Ca contents. The spi-nels have a positive correlation between Cr/Al and Mg/Fe ratios. The rocks and the mineralogical chemicalcompositions of Leg 84 harzburgites are similar to thoseof many ophiolitic complexes, for example, from Troo-dos and from Oman.) (2) The second group of ultra-mafic rocks comprises cumulative-texture peridotites thatwere recovered at Site 570. (The olivine appears in auto-morphic crystal aggregates cemented by xenomorphicpyroxene. The olivine and the pyroxene are completelyserpentinized, but the cumulative texture is still obvi-ous.)

Gabbro, recovered only in Hole 567A, is mainly com-posed of Plagioclase, green amphibole, and occasionalclinopyroxene. The paragenesis is: (1) Plagioclase + cli-nopyroxene + pseudomorph orthopyroxene; (2) Plagio-clase + clinopyroxene + amphibole. Opaque mineralsare absent. Magmatic layering is obvious in one sample.The chemical compositions of the clinopyroxene (W044,En45, Fs10) and Plagioclase (An86_76) resemble those ofOman and Antalya ophiolites.

Dolerite, recovered from several levels of Hole 567A,has a homogeneous chemical composition that showshigh Mg-Fe and Ti contents and low concentrations ofK. The clinopyroxene is calcic and zoned, the Plagio-clase is An80 to An40, the opaque minerals are ilmeniteand titanomagnetite. The dolerite has tholeiitic affini-ties.

Basalt was only recovered from the basement of Hole567A. One sample (567A-19,CC) gives an age of 78.7+ / - 3 . 9 m.y. No major secondary processes have beendetected in this basalt that suggest the ^K-^Ar age mayreflect the true crystallization age of this lava. This ageis in good agreement with the stratigraphic position ofthe sample just under the upper Campanian-lower Maes-trichtian limestones, that is to say around 72.1 m.y. Thissample is a quartz normative basalt with low TiO2 andhigh A12O3 contents. Its major element geochemistry isvery similar to that of the andesites and basalts recov-ered from the same stratigraphic level during Leg 67(Hole 494A). The trace elements analysis shows low

amounts of Cr, Ni, Sr, Zr, Ba, and hydromagmaphileelements. The ratio Ti/V is in the orogenic lava field ac-cording to Shervais (1982). The trace element contentsare nearly identical to those of the samples from Hole494A. The pyroxenes from Sample 567A-19,CC havehigh Mg/Fe ratios, and high SiO2 and low A1O3 andTiO2 contents. In the discrimination diagram (Ti/Ca +Na) proposed by Leterrier et al. (1982), samples plot in-to the orogenic basalt field. The plagioclases are An72 toAn50 and do not contain significant amounts of K2Oand MgO. Thus Sample 567A-19,CC shows mineralogi-cal and geochemical characteristics similar to those ofthe basalt from Hole 494A that provide evidence for anactive volcanic island arc during the Santonian and ear-ly Campanian.

Four other samples from Hole 567A were also stud-ied. They are clearly different from the 567A-19,CC oro-genic basalt and from oceanic basalts from the CocosPlate. These rocks show an enrichment in light rare earths,high Ba and Sr contents, and a wealth in Ti and norma-tive nepheline. On the other hand, the pyroxenes andplagioclases are typical of alkaline basalts. Three 40K-40Ar ages were obtained: (1) the youngest is 91 + / - 4 . 5m.y. from Sample 567A-29-2, 147-149 cm; (2) the sec-ond is 132 + / - 6.6 m.y. from Sample 567A-25-3, 74-78cm; (3) the oldest, from Sample 567A-25-2, 90-96 cm, is169 + / - 8 . 4 m.y. The oldest radiometric age is ob-tained from the basalt that has the less developed sec-ondary paragenesis, that is to say, the oldest age seemsto be the nearest to the true crystallization age.

Thus a significant difference was observed betweenthe orogenic basalt ages that are young and the alkalinebasalt ages that are significantly older. At the momentthese oldest ages (132-168 m.y.) must be considered withcare; in particular they must be compared with the on-shore Costa Rican ophiolitic complex of the Santa ElenaPeninsula and the Santa Clara ophiolitic complex of Guat-emala. As a matter of fact, potassic alkaline rocks ofthe lower structural unit of the Santa Elena complexgive radiometric ages of 130-140 m.y., and fossils insedimentary inclusions in pillow basalts of the SantaClara ophiolitic complex of Guatemala yield an age rangeof 100 to 141 m.y.

Amphibolites. Hole 569A encountered metamorphicbasement at a depth of 350 m beneath the seafloor cov-ered by lower Eocene sediments. Only 2 m of metamor-phic rocks were recovered in Cores 10 and 11. All ofthese rocks are amphibolites, with evidence of low-tem-perature retrograde metamorphism in the greenschist fa-cies. But the primary high-temperature paragenesis (mag-nesiohornblende, basic Plagioclase, ilmenite) of amphi-bolite facies is partially preserved. The SiO2, A12O3, FeO,MgO, and TiO contents of these amphibolites are thoseof basalts. The K2O content is very low and suggeststholeiitic affinities. The Na2O content is very high andthus the amphibolites of Hole 569A may correspond totholeiitic basalts.

Summary—Igneous Rocks

From the drilling of Leg 84, igneous basement wasfound to consist of mafic and ultramafic rocks includ-

948

LEG 84 SUMMARY

ing basalt, dolerites, gabbros, and peridotites and rocksof metamorphic-grade amphibolite facies. The harzburg-ites and the ultramafic cumulates may be linked to thesame ophiolitic complex as the gabbros because of theirmagmatic layering, their weak Fe and Ti contents, andthe lack of opaque minerals. The dolerites and the ba-salts differ from the gabbros by their chemical composi-tion, their Fe and Ti contents, and the mineralogicalevolution of the clinopyroxenes and plagioclases. Theamphibolites of Hole 569A are high-temperature meta-morphic rocks. Such rocks are sometimes found associ-ated with ophiolitic complexes as a tectonic sole underor inside the ophiolitic complexes. Thus the basementrock of the landward slope of the Middle America Trenchoff Guatemala could be a disrupted ophiolitic complex,except for the basalts in Sample 567A-19,CC, which couldbe linked to a volcanic island arc.

Ophiolite outcrops appear on land in Central Ameri-ca. They are linked to the Polochic-Motagua zone ofGuatemala and crop out in the Santa Elena Peninsula ofCosta Rica. These two outcrops of mafic and ultramaficrocks have the following similarities with the drilled base-ment of the landward slope of the Middle AmericaTrench off Guatemala: (1) the major component is harz-burgites; (2) the plutonic rocks are ultramafic cumulatesand layered gabbros; (3) they include basalts, dolerites,and amphibolites; and (4) the lower Eocene-Upper Cre-taceous sediments overlie the ophiolites.

This interpretation of basement rocks as part of anophiolitic complex is supported by radiometric (^K-^Ar)data that indicate a Mesozoic age for the igneous base-ment—at least pre-Late Cretaceous (pre-Turonian, 91 m.y.)or at most pre-Late Jurassic (pre-Callovian, 168 m.y.).The igneous basement of Central America is old; its for-mation is certainly unrelated to Neogene accretion.

BIOSTRATIGRAPHY ANDPALEOENVIRONMENT

Microfossil assemblages recovered on Leg 84 includeforaminifers, nannoflora, radiolarians, diatoms, and pol-len that ranged from Late Cretaceous to Holocene. Thesparse Cretaceous record consists of an Albian-Aptianradiolarian assemblage in chert, and a Maestrichtian fo-raminiferal and nannoflora assemblage in deep-waterlimestone. This fragmental Mesozoic record was recov-ered in dredge samples and in 2 drill holes (494A, Leg67; 567A, Leg 84); at 3 sites on the slope where drillingpenetrated to the basement the Mesozoic was absent.

Paleogene sediment was also discontinuous and thin-ner than that of the Neogene, and it was broken by localunconformities or hiatuses. No Paleocene was recoveredfrom the slope despite the thick Paleocene section at theedge of the shelf (Seeley, 1979). The Eocene and Oligo-cene sections were present but incomplete at Sites 569and 570, and at Sites 556 and 567 the entire Paleogenewas missing. However, the discontinuity of the Paleo-gene sediment was probably overestimated by the selec-tion of drill sites on basement highs.

Neogene sequences, although more continuous thanthe older ones, were also broken by hiatuses and a singlelarge unconformity. Along the main transect of drill holes,

the planktonic foraminiferal biostratigraphy in particu-lar indicates absence of the middle Miocene (Sites 567,568, 569; and Leg 67 Sites 494 and 496). Elsewhere,away from the main transect as at site 570, the middleMiocene sequence is relatively complete. Local erosionin the transect area is a possible explanation, becauseSan José Canyon has been eroding its present coursesince at least the late Miocene and its predecessor wasprobably the abandoned channel parallel to the transectof drill holes. The abandoned channel was not apparentuntil the Seabeam survey of the middle slope. If this istrue, the abandoned channel eroded much of the middleand lower Miocene section during the late Miocene andwas beheaded during the late Miocene and the Pliocenewhen its trace across the upper slope tectonic zone wasdisrupted by tectonism.

Abundant microfossils and broad species diversity re-sulted in very good biostratigraphic resolution. At nosite was a biostratigraphic zone repeated by thrust fault-ing, and the Eocene to Holocene section was drilled innormal succession at all sites.

The environments of deposition are particularly wellindicated by the benthic foraminifers. So far as the mod-ern assemblages show, the fauna on the slope are char-acterized by downslope movement with most of the as-semblages being transported from upslope. The oceanicand shelf assemblages on the other hand are character-ized by few transported forms and are shallow or deepassemblages. Applying the modern analogy to the an-cient sediment indicates a continuous slope paleoenvi-ronment back to the Eocene except at Site 494 (Leg 67),where the Eocene is composed of terrigenous lithologiesbut does not have the transported fauna. All Cretaceousfauna have open ocean lithostratigraphies and biostrati-graphies.

The quantitative analysis of up to 60 benthic fora-miniferal species per sample (McDougall, this volume),indicates both the paleodepth and also the identity ofwater masses along the Guatemalan slope during theNeogene. The paleodepths indicate little vertical changethroughout the Neogene within the 1500-m-thick zonesthat can be distinguished by benthic assemblages. Sitesat the base of the slope (5000 m) show only abyssal ben-thic assemblages from 3500-2000 m and have generallyremained in that depth range. Sites in the lower andmidslope area were at abyssal depths until the Pliocene-Pleistocene when they were uplifted to their present depthsat the top of that depth zone (Site 568, and Leg 67 Site496). Site 570 rose from abyssal depths through the lowerbathyal to its present middle bathyal depth in the Plio-cene and Pleistocene.

During the Neogene, changes in the depths and com-positions of water masses flowing by the Guatemalanslope caused periods of dissolution and changes in fau-nal composition. The shifting water masses first showthe increasingly restricted communication between theCaribbean and the Pacific deep water masses during themiddle Miocene; gradually the shallower water masseswere affected until establishment of the present circula-tion system in the Pliocene and Pleistocene. The historyof these shifts show superposition of the effects of the

949

J. AUBOUIN, R. VON HUENE

emergence of Panama on the Antarctic refrigeration anddevelopment of Antarctic bottom waters. Pleistocene gla-ciation and interglacial periods are seen in the pollen rec-ord by the spread of conifer forests in the lowland areasinterspersed with flora typical of dry but not savannavegetation.

A major void in the IPOD biostratigraphy of the Cen-tral American margin is the absence of a stratigraphy atthe edge of the shelf. The drilling of a site in the SanJosé Canyon at the edge of the shelf was planned as thelast hole of the leg. However, the time allotted to drillingat this site was required instead to transit to a U.S. portfor replacement of the 5000 m of drill string lost at Site567. In the decade since the ESSO Petrel hole was drilledat the edge of the shelf, progress in biostratigraphic anal-ysis has reached a higher level of refinement and the useof cores rather than drill cuttings adds further to theprecision of results. Thus the suggested 1000-m upliftduring the Pliocene-Pleistocene based on the differencesin paleobathymetry at the ESSO drill hole and just 20km away at Site 570 do not necessitate a correspondingvertical offset along the upper slope tectonic zone. Thetectonic and the biostratigraphic value of a core fromthe edge of the shelf has greatly increased since the Gua-temalan transect was first proposed.

STRUCTURAL FEATURES IN CORES

Tectonic features observed in the cores include thosein the ophiolitic basement and those in the overlyingsediment. Some of these small features can be related tothe more general tectonics of the margin through thetechniques developed for the analysis of microstructures.

The ophiolitic rocks have two types of foliation: onedeveloped from magmatic processes during emplacementand the other developed during deformation and altera-tion of the rock. Primary foliation from magmatic pro-cesses can be seen in the ultramafic rock recovered atshallow depths in cores from Site 566. Secondary folia-tion from deformation can be seen in the foliation ofserpentinite throughout the Site 567 section. The folia-tion and superposition of various igneous rocks fromdifferent levels indicates there was intense and complexdeformation of the whole section at the front of themargin. In addition to the fragmentation and rotationobserved, there is evidence of injection of ductile mate-rials during the shearing, as suggested by the intense de-formation of the alkali basalt drilled at the base of thehole at Site 567 (Ogawa et al., this volume). Texturally,the peridotites drilled at Site 570 differ from those atother sites and, despite massive serpentinization, the cu-mulative texture is still obviously different from the tec-•tonized harzburgites of Site 567.

Within the massive muddy sediment overlying the ig-neous basement, vein structure develops from the es-cape of water through these impervious sediments asthey become compacted during burial. These vein struc-tures have been observed in the hemipelagic cover ofmost modern margins drilled by DSDP. Cowan (1982)described the dark sigmoidal veins spaced from one toseveral millimeters apart observed during Leg 67. Oga-wa et al. and Helm and Volbrecht (both this volume)

found the vein filling to contain the same clay mineralsas in the adjacent sediment. The fabric of the vein was,however, characterized by fine-grained and closely packedaggregates of clay flakes oriented preferentially parallelto the vein boundaries. The dilational component ofdisplacement perpendicular to the boundaries is evidentfrom the filling of the veins.

The orientation of the shear zones in conjugate setsindicates coaxial deformation with the axis of maximumshortening in a subvertical direction. Therefore the shearscould be related to overburden pressure and downslopecreep. These shears could also be related to large-scaleslumping. The shears appear to be Riedel shears, andthere is no indication of low-angle synthetic Riedel shears.

GAS HYDRATEDuring Leg 67 gas hydrates were found unexpectedly

at three sites; because the base of the hydrate layer wasnot known, drilling could not continue to targeted depths.With subsequent geophysical studies, the base of gas hy-

*drate was determined, and further investigation of thehydrate became a major objective during Leg 84 (Kven-volden et al., this volume).

On Leg 84 gas hydrates were observed visually at Sites565, 568, and 570 and inferred to be present, on the ba-sis of inorganic and organic geochemical evidence, atSites 566 and 569; no evidence of gas hydrates was ob-served at Site 567. Recovered gas hydrates were solidpieces of white, icelike material occupying fractures inmudstone or pores in sandy lithic ash (Fig. 7). A 1.05-m-long core of massive gas hydrate was unexpectedlyobtained at Site 570. Downhole logging indicated thatthe hydrate was actually 3 to 4 m thick (Mathews andvon Huene, this volume), but no sign of this hydrate ac-cumulation could be seen on multichannel or high-res-olution single-channel seismic records. The base of thesediment in this area is shallower than the depth to thebase of the gas-hydrate zone, and so the base of hydratereflection was not visible.

The volume of methane released during the decom-position of the samples clearly showed that gas hydrateshad been found. The distribution of evolved hydrocar-bon gases indicated that structure I gas hydrates werepresent because of the apparent inclusion of methaneand ethane and exclusion of propane and higher mole-cule weight gases. The water composing gas hydrateswas fresh, having chlorinities ranging from 0.5 to 3.2%o.At Sites 565, 568, and 570, where gas hydrates were ob-served visually in cores, the chlorinity of pore watersqueezed from sediment decreased with sediment depth.The chlorinity profiles may indicate that gas hydratesnot visible in cores can nonetheless occur as finely dis-persed small inclusions in sediments. The gas hydratesare not visually observed because of their small size orbecause they did not survive the drilling and recoveryprocess. This observation is supported by studies ofdownhole logging (Fig. 8). The velocity and resistivitylogs were calibrated in the zone of massive hydrate; whenthis calibration is applied to other logged sections of thedrill hole, it appears that gas hydrate is generally presentin concentrations of less than 10% (Mathews and von

950

LEG 84 SUMMARY

10

15

20

25

100 -

105 -

110 1 -

Figure 7. A-C. Photographs of examples of gas hydrates recovered at Sites 568 and 570 of Leg 84 (Sections 568-43-1 [A] and 570-26-5 [B]; and Core570-27 [C]).

Huene, this volume). Methane in the gas hydrates foundon Leg 84 was mainly derived in situ by biogenic pro-cesses, whereas the accompanying small amounts of eth-ane likely resulted from low-temperature diagenetic pro-cesses.

In order to form hydrate, permeability and porosityare required. Along the Middle America Trench, thehemipelagic slope deposits have extremely low permea-bility; therefore most hydrate may be found in sedimentthat has become consolidated enough to fracture. Asfree gas migrates upward through the fractures it be-comes hydrated. The large transverse fault near Site 570may have formed a zone through which gas could mi-grate and collect as hydrate to form the massive bodycored there.

Finding gas hydrates on Leg 84 expands observationsmade earlier on Leg 66 and particularly Leg 67. Theresults of all these legs show that gas hydrates are com-mon in landward slope sediments of the Middle Ameri-ca Trench from Mexico to Costa Rica.

CONCLUSIONS

Leg 84 results complement those of Leg 67 and to-gether these results provide many more observations thanare available along most modern active margins (Fig. 9).The two legs and associated geophysical surveys providea multidisciplinary data set that focuses interpretationsconcerning accretion and sedimentation. During Leg 67the hole that penetrated through the Trench axis sedi-ment pond and into the subducting oceanic crust wasdrilled. Similarly, the sediment of the landward slopehas been penetrated to basement at more sites than alongany other DSDP active margin transect. However, de-spite its status as one of the best-studied modern mar-gins, there are still several critical targets that have notbeen drilled or surveyed with appropriate geophysicalinstruments.

Prior to Leg 84, the results of Leg 67 indicated a Mi-ocene age for the ocean crust. Seabeam mapping showsthe horst and graben structure of the oceanic Cocos

951

J. AUBOUIN, R. VON HUENE

TILE

108

Cali per

(in.)

9 14

0 . 0

-no

GR (GAPI)

SP (mV)

. 0 0

100

f>O.

. 0

on

D.

n.

LLS (ohm/m)

εoooLLD (ohr

2000

εooo.n/m)

εooo.

Figure 8. Schlumberger well log showing the log responses for a portion of the hole at Site 570. From leftto right the logs are caliper, gamma ray, self potential, resistivity (shallow and deep), sonic velocity,neutron porosity, and bulk density. Depth (water plus sediment) scale (m) is on the left. Each divisionrepresents 1 m. The physical properties of gas hydrate beds (V 3.6 km/s, d 1) are very pro-nounced; they are easy to recognize, as, for example, between 1965 and 1970 m. By comparison, thesandstones give a high velocity and a high density close to 2 (examples at 1954 and 1975.5 m).

952

Petrel Well

GUA-14

GUA-13

GUA-2

π=rΛ Λ

S Λ Λ

Miocene

Basalt oceanic crust

b

- i -• i — i -— i —• i — i -

ii i

i i

b — late Miocene toPleistocene

c — late Oligocene toearly Miocene

Basement cover

a — Indistinct

b — Eocene

c — Late Cretaceous

Basic and ultrabasic basement

Figure 9. Block diagram showing the convergent zone between the Cocos Plate and the Caribbean (Central America) Plate in the Middle America Trench off Guatemala (after Aubouin, von Hueneet al., 1982). This diagram includes the locations of the seismic, Seabeam, and deep-tow surveys, and Legs 67 and 84 sites.

J. AUBOUIN, R. VON HUENE

Plate that develops oblique to the trend of the Trenchand produces a succession of diamond-shaped basins(graben) separated by the ridges (horst). Leg 67 drillingalso revealed the abundant hemipelagic sediment thatcrossed the Trench axis and composes the upper half ofthe section on the subducting Cocos Plate. But on thecontinental slope, drilling was abandoned at all but onehole for safety reasons when gas hydrate was encoun-tered. At only one site 1.5 km landward of the Trenchaxis was Upper Cretaceous (Upper Campanian-Maes-trichtian) sedimentary rock recovered directly on igne-ous rock. This unexpected discovery raised the possibili-ty that a nonaccretionary regime has existed since theLate Cretaceous along the Trench off Guatemala. Butthis conclusion remained in question because at the timefurther drilling was not allowed.

Leg 84 was therefore dedicated to the exploration ofthe continental slope with two major objectives: (1) todrill through the Neogene slope deposits and sample theproposed accretionary complex or, conversely, to reachthe basement; and (2) to study the occurrence of gas hy-drate.

A secondary objective proposed during Leg 67 was todrill off the Nicoya Peninsula of Costa Rica a referencesite for the usual sequence drilled at the base of theslope off Guatemala (Site 494). This objective could notbe realized because of the presence of gas hydrate in avitric ash bed above the target depth; the hole was aban-doned to gain time for the major objectives off Guate-mala. The loss of 5000 m of drill pipe during logging ofthe second site off Guatemala (Site 567) limited the depthof further drilling. It was no longer possible to drill siteson the lower slope as was planned, and during the re-mainder of the leg drilling was restricted to sites on themiddle and upper slope.

The two main objectives off Guatemala were met de-spite the loss of drill pipe. Basement of the continentalslope was recovered at four sites; gas hydrate was alsorecovered at four sites, and a complete set of logs througha 4-m-thick massive methane hydrate at Site 570 provid-ed a quantitative estimate of the in situ volume of hy-drate. The success in reaching targeted drilling objec-tives allows four major conclusions as follows:

1. The pre-Late Cretaceous ophiolitic basement ofthe continental slope was emplaced prior to the presentarc-trench system. At the sites where basement was pen-etrated, the age of the sediment covering the ophioliticrock was early Miocene (567), early Eocene (569 and570), and late Miocene in San José Canyon where thesedimentary section was once completely stripped fromthe basement (566). The sites are distributed transverseto the margin and are laterally separated by as much as100 km. Peridotite (harzburgite), gabbro, dolerite, ba-salt, and abundant serpentinite in various stages of al-teration were recovered. These rocks have a metamor-phic grade from low-temperature to greenschist faciesand were tectonically juxtaposed without particular or-der. The Guatemalan margin is therefore underlain bytectonically disrupted ophiolitic rocks that are at leastpre-Eocene and probably pre-Campanian, in agreementwith the radiometric ages and the Cretaceous limestone

recovered from two sites. The ophiolitic rock was tec-tonized and the resulting tectonic complex was emplacedprior to development of the present arc-trench system.Clasts of ophiolitic rock are found in the Oligocene tolower Miocene slope sediment particularly at the baseof the slope where debris flows contain large blocks (ca.50 m) of serpentinized peridotite.

The ophiolitic and sedimentary rocks off Guatemalaare petrologically, structurally, and chronologically sim-ilar to both the Santa Elena peridotites, the Nicoya com-plex of Costa Rica, and the Guatemalan Santa Claraophiolite. Hence the crust beneath the continental slopeoff Guatemala is probably an extension of the crust be-neath parts of Middle America, but this statement sim-plifies a complicated area. Middle America is broken in-to a number of microplates or blocks, and several occurbetween Costa Rica and Guatemala. Felsic plutonic rocksoutcrop in Honduras and further complicate interpreta-tions of Central American basement.

2. Neogene subduction-accretion, whereby sedimentfrom the oceanic plate is scraped off and attached to theupper plate, is not seen within the area studied. In thenorthern Middle America Trench off Mexico, the Leg66 staff made indirect arguments for accretion of Trenchsediment based on the coarse-grained sediment recov-ered from the Trench axis and the lack of similar sedi-ment in slope deposits. Implied in the interpretation isthe presence of coarse sand only in the Trench and itsexclusion from the slope (cf., Watkins, Moore, et al.,1982). Such an argument cannot be applied to the areastudied off Guatemala. The hollows and topographicflat areas off Guatemala have lower Miocene pondedsediment and prograding sediment bodies that containmore sand than was found in the ponded sediment ofthe Trench axis. Sand was also recovered in channels,demonstrating again that the channels are transport pathsdirectly to the Trench axis. Many local unconformitiesand hiatuses were detected in the slope deposits of sedi-mentary rather than tectonic origin. It seems that as onedepocenter is accumulating sediment, another may loseits feeder channel and temporarily be in an interchannelor erosional area. The presence of coarse sand and rapidsediment accumulation is not an exclusive criterion bywhich trench and slope sediment can be differentiatedoff Guatemala. As a matter of fact, if the cores fromthe Guatemalan margin were studied without knowl-edge of their position on the slope or in the Trench axis,textural criteria alone would be of little use in showingtheir provenance.

As we have emphasized previously, the front of theGuatemalan margin consists of Mesozoic rock coveredby slope sediment (Aubouin, von Huene, Arnott, et al.,1982). No Neogene accreted sediment has been samplednor have any low velocities or densities indicating ac-creted sediment bodies been detected geophysically. Thestrength of the evidence for nonaccretion off Guatemalamake this a tectonic type end-member, however, the mech-anisms whereby accretion or nonaccretion dominate thetectonics of a convergent margin are not known.

3. Despite plate convergence, little compressionalstructure is detected in the rocks cored and in the seis-

954

LEG 84 SUMMARY

mic reflection records along the Guatemalan active mar-gin transect. The seaward side of the Middle AmericaTrench off Guatemala is formed by the downward flex-ure of the Cocos Plate into the subduction zone. Bend-ing of the Plate produces stress that is accommodatedby extensional tectonic failure along the grain inheritedby the crust during its formation at the East Pacific Rise.Thus the horst and graben parallel the magnetic anom-aly pattern of the Cocos Plate that is oblique to theTrench axis. On the other side of the Trench axis, a localcomplementary set of extensional structures signifiesdownslope failure so that along the transect, the Trenchis essentially a graben despite the anticipated dominant-ly compressional forces from subduction.

The secondary extensional structure along this tran-sect, in a system dominated primarily by compressionalforces, requires very low friction along the subductionzone. We were able to test the hypothesis that elevatedpore-fluid pressures exist in the subduction zone with adirect measurement just tens of meters above the sub-duction zone at Site 567. A minimum measured pressureof 350 psi (pounds per square inch) above hydrostaticpressure is an explanation for the high degree of decou-pling that is required to subduct soft sediment beneath amass of ophiolitic rock. Indeed, high pore pressure ap-pears to be common across the Guatemalan convergentmargin, because it was similarly observed by direct mea-surement at the bottom of three other drill holes in themiddle and upper slope, and kilometers above the sub-duction zone.

One feature emphasized in early studies of seismicdata was a landward-dipping reflection beneath the up-per slope (Ladd et al., 1982). This reflection is truncatedby younger reflections produced during the present arc-trench system; thus it was emplaced during a prior tec-tonic regime. The landward-dipping feature could be ei-ther from a sedimentary layer, perhaps Cretaceous lime-stone interleaved with ophiolite, or a thrust related tothe pre-Eocene tectonism of the Cretaceous basement ofCentral America.

The Middle America Trench off Guatemala has beenproposed as the model of a convergent-extensional ac-tive margin (Aubouin, Bourgois, et al., 1982, 1984).Such a margin has no accretion and displays extensionalfaulting, much like a passive margin with tilted blocks,but it is above a subduction zone. Such a margin is incontrast to a convergent compressional margin, whereaccretion accompanies subduction. The Barbados Ridgeis the drilled example (see Moore, Biju-Duval, 1984) ofthe convergent compressional margin, where the upperpart of the convergent sedimentary section is detachedand accreted by imbricate thrusting and the lower part issubducted with the igneous oceanic crust. Although bothtypes of margins accompany subduction, the convergentextensional margin seems to have a thin layer of sediment,whereas the convergent compressional margin involvesa relatively thick layer of sediment. This idea could betested along a single margin, such as the Antilles mar-gin, where the southern part is sediment-flooded and thenorthern part is sediment-starved. In fact, the Peru-Chile,the Java, and the Aleutian trenches provide the same type

of contrasting sediment-flooded and sediment-starved set-tings for such investigations.

4. Methane gas hydrate is ubiquitous across the wholeGuatemalan margin even where no base of gas-hydratereflection was seen. Gas hydrate was observed interspersedin fine-grained sediment, occupying fractures in mud-stone, and as a massive unit along a fault. From logs itwas determined that the high acoustic velocity (4 km/s)and low density (1.0 g/cm3) interact to give a low im-pedance; thus the hydrate cannot be detected acousti-cally in massive mudstone because there is no acoustic-impedance difference between mudstone and hydrate.However, hydrate contrasts well with free gas, and hencethe base of gas-hydrate reflection. Estimates of the vol-ume of hydrate from the downhole logging indicatedless than 10% hydrate at Site 570, the site where thegreatest number of visual observations of hydrate in thecore were made. Thus it follows that at Site 568, whereonly few observations of the hydrate were made in thecores, and where the logs gave no indication of gas hy-drate, the volume of dispersed hydrate was also less than10%. Seismic reflection records across Site 568 show awell-developed base of gas-hydrate reflection. There-fore, a base of gas-hydrate reflection may be evidentwhere the amounts of dispersed hydrate are less than10%.

Inorganic geochemical studies on Leg 84 confirm theproposed relationship between low values of salinity andchlorinity and the occurrence of gas hydrate. In all caseswhere gas hydrates were observed, the values of salinityand chlorinity were lower than in sediments where gashydrates were absent. Most gas hydrates were composedof gases from microbial and early diagenetic processes.However, at two sites (566 and 570) gases were found inthe serpentinite. The gases in serpentinite are probably aproduct of the thermal breakdown of organic matter,because they contain high concentrations of ethane aswell as hydrocarbons as large as hexane and heptane.This raises the question of the upward migration of gasfrom subducted sediment along the Wadati-Benioff zone.Serpentinite is not known to generate hydrocarbons. Sed-iment in a subduction zone is subjected to increasingtemperature as it is carried to deeper levels and would bea source of thermogenic gas. Another possible source ofthermogenic gas are the bodies of Mesozoic sedimentthat became folded into the ophiolitic assemblage dur-ing the pre-Eocene development of the present forearcbasement. Such gases would tend to migrate upwardalong tectonic disruptions with any trapped fluids. Suchsources of hydrocarbon could be investigated isotopical-ly at future active margin drilling sites.

THE TECTONIC HISTORY OF THE MARGINAND ITS SIGNIFICANCE IN THE EVOLUTION

OF CENTRAL AMERICAThe results of DSDP drilling, the offshore geophysi-

cal surveys, the data from the ESSO Petrel drill hole onthe shelf, the geologic studies of the Nicoya and SantaElena areas in Costa Rica, and the geologic studies ofon-land Guatemala indicate a three-fold tectonic histo-ry. In the first period, ultramafic rock was thrust over

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autochthonous Jurassic-Cretaceous sediments and wasin turn unconformably overlain by Campanian-Maes-trichtian sediments. This tectonic event, evidenced onthe Santa Elena Peninsula and in the Santa Clara ophio-lite of Guatemala, could correspond with the initial tec-tonism of the Guatemalan ophiolitic rock and its associ-ated Campanian-Maestrichtian sediment. In the secondperiod, the edge of the present shelf began to rise in thelate Paleocene through early Miocene. This uplift formedthe present wide forearc basin in which a 10-km-thicksection of sediment was ponded. During this period theunconformable Eocene sediment section was depositedon the continental slope. In the third period, from earlyMiocene to the present, the existing arc-trench systemwas established, and massive hemipelagic mudstone wasdeposited on the present continental slope. At the edgeof the shelf, local areas emerged above sea level causingthe unconformities now evident in the seismic stratigra-phy. The present arc-trench system was certainly estab-lished in the late Oligocene, as evidenced by the age ofthe first substantial layers of volcanic ash in the slopesediment section. Although Eocene volcanic ash was re-covered offshore and is commonly found on land inCentral America, the tectonic setting of that volcanismcannot be reconstructed off Guatemala. Tectonism ofthe arc-trench type appears to have continued through-out the Neogene punctuated by local uplift of the upperslope in the Pliocene and Pleistocene. San José Canyonhas been the path for sediment transiting the slope tothe Trench since the late Miocene. The lower reaches ofthe Canyon are now disrupted by normal faults that pro-duced benches. The shape and structure of the benchesindicate an area of collapse at the front of the margin.

Four tectonic processes in an arc-trench system areexemplified in the Guatemalan margin. First, as the Pa-cific Ocean crust is flexed downward into the Trench, itfails along the grain inherited during its emplacementon the East Pacific Rise rather than parallel to the axisof the flexure. The resulting divergence in the trend be-tween the horst and graben of the oceanic plate and theTrench axis is probably repeated along many active mar-gins. Such oblique failure is to be expected anywheremagnetic anomalies are less than 30° oblique to the axisof a trench (von Huene and Aubouin, 1982), but a swath-mapping system is required for its detection. Second, anessentially total net subduction of sediment since thelate Oligocene requires a mechanism whereby frictionbetween the upper ophiolitic plate and the lower sedi-mentary sequence is very low. Perhaps the upper plate isessentially floating due to pore pressures at near litho-static levels within the subduction zone. Third, the up-lift at the edge of the shelf and on the upper slope ap-pears local in space and episodic in time. Such upliftcan be accomplished by underplating, whereby sedimentis added above the subduction zone as it passes beneaththe upper slope from the subducting sediment in thesubduction zone. That the crust may be thickened local-ly in the area of uplift by compressional deformation isdifficult to suggest here because the uplifted area is de-void of major compressional structures. And fourth, thetectonic regime at the edge of the shelf is poorly under-

stood, however, the seismic and magnetic results indi-cate greater depths to basement beneath the shelf thanbeneath the upper slope. The exact configuration of thecontact is obscured in seismic records, but the gross ge-ometry can be produced by obduction, normal faulting,or transcurrent faulting. Obduction requires landwardthrusting of the slope ophiolitic complex over the fore-arc sediment sequence. In the absence of compressionalstructure in strata of the forearc basin, the latter twoprocesses are most likely. Normal faulting associated withthe shelf-edge uplift could be tested with a high-resolu-tion multichannel seismic survey.

The results of Legs 67 and 84 and related geophysicalstudies have raised the level of questioning from the clas-sical study of accretionary processes by Seely et al. (1974)to the following: How was the ophiolitic basement ofCentral America assembled into its present form andwhat does the landward-dipping reflection emphasizedby Ladd et al. (1982) signify in terms of that pre-Eoceneor perhaps Cretaceous assembly? What was the role ofoffshore strike-slip faulting prior to and after the lateOligocene? (Strike-slip faulting is common to this partof Central America on land.) Was some form of colli-sion involved in the formation of the Central Americabasement? Such questions can perhaps be answered bycoordinated on-land and offshore studies in the future.

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Aubouin, J., von Huene, R., and 1'équipe Scientifique du Leg 84 duGlomar Challenger, 1982. Subduction sans accretion: la marge pa-cifique du Guatemala: premiers résultats du Leg 84 du Deep SeaDrilling Project (Janvier-Février, 1982). C. R. Acad. Sci. Paris,294:803-812.

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