Geology of the North Slope of the Puerto Rico Trench

Deep-Sea Research, 1968, Vol. 15, pp. 297 to 317. PergamonPress. Printed in Great Britain.

Geology of the North Slope of the Puerto Rico Trench* R. L. CHASE'~ and J. B. HERSEY+ (Received 31 January 1968) Abstract--We use bathymetric and continuous seismic reflection profiles and rocks dredged in 1962 and 1964 to describe topography, stratigraphy and struc~re of the North Slope of the Puerto Rico Trench between 65 ° 00'W and 66 ° 30'W. Abundant basalt, chert, pink and green claystone and minor limestone and soft mudstone were dredged from 3200-3700 fms at 20 ° 16'N, 65 ° 42'W. The basalt has porphyritic, asnygdular or diabasic texture, is partly altered to montmorillonitic clay, contains plagioclase which is mainly labradorite or bytownite, is low in potassium and titanium, and is normalty tholeiitic. The tentative stratigraphic succession of the North Slope is as foUows: (1) Cenomanian basalt flows, possibly interbedded with chert, claystone and limestone, form the oceanic basement, and have compressional velocity near 5 km/sec. (2) Consolidated sediment, with compressional velocity of 3"0--4"5 km/sec, possibly not present everywhere, lies unconformably on the basement. Age is Late Cretaceous and Early Tertiary, and maximum thickness 300 m. (3) Acoustically transparent unconsolidated sediment mantles the basement and the consolidated sediment. Age is Late Tertiary and Quaternary, compressional velocity 1.5-2.6 km/sec, and maximum thickness 400 m. (4) Acoustically layered flat-lying unconsolidated sediment lies in east-west valleys in the North Slope, and forms the abyssal plain at the bottom of the Trench. Maximum thickness in the valleys is 500 m. Present evidence from echo-sounding and seismic reflection profiles is insufficient to indicate whether the scarps, ridges and valleys of the North Slope were formed by thrusting, normal faulting strike-slip faulting or gravity sliding or from a combination of these processes. INTRODUCTION

THE PUERTO Rico Trench lies between the Greater Antilles and the Outer Ridge (Fig. 1). The Trench trends east-west where it is deepest, north of Puerto Rico. Further east it curves southeast around the northern islands of the Lesser Antilles and becomes shallower. The North Slope of the Trench is more than 700 km long, extending from about 62 ° 00'W and 69 ° 30'W. Only the portion between 65 ° 00'W and 66 ° 30'W is discussed here (Fig. 2). Geophysical observations were made over the Puerto Rico Trench and the Outer Ridge, and rocks were dredged from three places on the North Slope of the Trench during Cruise 34 of the R.V. Chain (October-December, 1962). The object of the cruise was to further the understanding of the structures of the Outer Ridge, and to utilize the suggestion, gained from seismic refraction studies and bottom photographs, that the rock layers beneath the Outer Ridge crop out on the North Slope of the Trench (BuNcE and FAHLQUIST, 1962; HERSEY, 1962). In this paper, we present the results of the study of the echo-soundings, the continuous seismic reflection profiles, and the igneous rocks from the North Slope taken during the cruise. Also utilized are a continuous seismic reflection profile and echo-soundings recorded on Cruise 11 of R.V. Atlantis II in 1964 during operation DEEPSCAN. *Contribution No. 2026 from the Woods Hole Oceanographic Institution. tWoods Hole Oceanographic Institution, Woods Hole, Massachusetts. +Office of Naval Research, Washington, D.C.

297

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Seismic reflection profiles were made from the top of the North Slope to the floor of the Trench, in order to locate outcrops of subsurface layers. A rock dredge was then dragged upslope at the locations selected. A report on rocks dredged from the

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Fig. 2. Bathymetry of portion of North Slope of the Puerto Rico Trench based on echo soundings prineil~dly from Chain alad Atlanti~ II. Contour interval is t00 fma, uncorrcet¢~l,for velocity of sound in water, Hatched areas are sediment ponds, Trimlsles ~ t e dredge sites of Chain cruise 19, squares, those of Chain cruise 34. Continuous s e i ~ c profiles ~ from Woods Hole Oceanographic I n s t i ~ ~ p s up to Jemtlary 1966 are ~ a t e d as thin continuous lines. Letters refer to profiles Of Figs. 3-6, ~ t parts of abyssal plain indicated by spot depths.

Geology of the North Slope of the Puerto Rico Trench

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Fig. 4. Seismic reflection profiles CC’, DD’, EE’ of the North Slope of the Puerto Rico Trench. Approximate vertical exaggeration (V. E.) is indicated for each profile. The location of each profile is indicated in Fig. 2.

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North Slope on a previous cruise (Chain 19) contains a preliminary note on dredgings reported in this paper (BOWIN, NALWALKand HERSEY, 1966).

1. Bathymetry

METHODS USED

All soundings for the chart (Fig. 2) are in fathoms* based on an assumed speed of sound in water of 800 fms/sec; slope corrections have not been applied. Bathymetric profiles and continuous seismic profiles of the North Slope (Figs. 3-5) were used to construct true scale profiles of the North Slope (Fig. 6). Bathymetric information for these profiles was read from echo-sounding records of traverses across the slope, plotted on a scale of 1 nautical mile to the inch, and adjusted for the speed of sound in water by means of MATTI-IEWS' (1939) tables. Slopes were then corrected by the method of swinging arcs (HERSEY and RUTSTEIN, 1958) on the assumption that the ship proceeded on a course normal to the slope contours.

2. Continuous seismic reflection profiles The sound sources used for the seismic profiles were a 13,000 joule stored-energy boomer and 25,000joule stored-energy sparker. Reflected energy was received by both single and arrayed hydrophones towed at various depths beneath the surface, amplified, filtered and recorded on electrosensitive paper by a Precision Graphic Recorder (PGR). Sub-bottom reflectors apparent on the PGR records were plotted beneath the corrected natural scale bathymetric profiles. To do this, it was necessary to assume a seismic velocity in the material between the reflector and the sea bottom. SAVlT, KNOX, BLUEand PAITSON(1964) found in their oblique reflection profile B-2, on the Outer Ridge north and northwest of the seismic profiles under discussion, that several hundred meters of material with compressional wave velocity of 1.5-1.7 km/sec overlies material of 3.0 km/sec. E. T. BUNCE and M. H. SALISBURY(personal communication) obtained similar velocities and thicknesses in oblique reflection profiles in the same area. J. E. NAFE and C. L. DRAKE (unpublished) determined a sound velocity of 2-1 km/sec in sedimentary rock from an east-west refraction profile near 20° 30'N, between 66° and 67°W (BUNCEand FAHLQUIST,1962, p. 3966), Continuous seismic reflection profiles in this area of the Outer Ridge show variations in the thickness of sedimentary layers, particularly near the north lip of the Puerto Rico Trench. Reflectors apparently within the "transparent" layer in reflection profiles of the North Slope may be within either the material of 1.7 km/sec velocity or that of 3.0 km/sec. Thus in drawing up the geologic cross sections from the profiles, the authors used an arbitrary velocity of 2.1 km/sec for material between bottom and sub-bottom reflectors. An exception was made for the layered sediments of the ponds, for which a velocity of 1.7 km/see was used.

3. Dredging A rock dredge (NALWALK,HERSEY, REITZELand EDGERTON, 1962) was used to collect rocks. A pinger was attached to the trawl-wire 100 m above the dredge in order to maintain the dredge on bottom. The position and depth of the dredge behind the ship was determined by means of a horizontal array of three receiving hydrophones, one at the stem of the ship, and two on outriggers 50 ft outboard from the sides of the ship; a similar technique is described by BAXamR(1964). By this means we knew the position of the dredge relative to the ship to within 200 m. *1 fathom = 1-8288meters.

Geology of the North Slope of the Puerto Rico Trench

301

RESULTS

Topography

Bathymetry is based on precision echo-soundings from cruises of Chain, Atlantis, Atlantis 11, Bear and Caryn up to 1964 (Fig. 2). Also marked on the chart are locations of seismic reflection profiles recorded during cruises Chain 34 and Atlantis II. Interpretations of three of these profiles (CC', GG', AA') are reproduced without vertical exaggeration in Fig. 6. Photographs of five of the profiles are reproduced in Figs. 3-5. In these the vertical exaggeration varies, due to changes in ship speed, from 8 to 22. The width of the North Slope, measured between the 3000 fms and 4350 fms contours, varies between 40 and 60 km. Where least disturbed, the average slope is less than 3 ° (profile CC', Fig. 6). However, in most places, the slope is not smooth and uninterrupted, but is broken by steep scarps and has superimposed upon it hills and valleys of lesser magnitude, most of which appear to trend east-west (Fig. 2). The most striking topographic feature of the sector of the North Slope under discussion is a steep scarp falling from 3500 to 4200 fms (6700 to 8100 m, corrected) at 66 ° 45'W. Serpentinite dredged from the scarp was described by BOWIN, NALWALK and HERSEY (1966). Of the east-west valleys, the southern three, which are below 4000 fms, have flat floors of layered sediment which forms " p o n d s " (HERSEY, 1962, Fig. 1). Shallower valleys to the north appear to contain no measurable thickness of ponded sediment. The southernmost valley broadens to the west, forming the floor of the trench. The east-west ridges have comparatively fiat tops. On the chart and on uncorrected profiles they appear to have slopes convex upward. Corrected profiles, however, (Fig. 6, profile AA') show the slopes to be almost straight or, in the northern large valley, possibly concave.

Stratigraphy The complete stratigraphic succession of the North Slope and the Outer Ridge cannot be determined until the crust is drilled. Seismic refraction information, however, indicates a sequence of rock layers whose velocities and thicknesses are indicated 24"N

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302

R.L. CHASEand .I.B. I-tm~E*

in Fig. 7. Layers seen in seismic reflection profiles of the Outer Ridge and North Slope (Figs. 3-5) can be correlated with the upper layers of this sequence (BoNcw and Hm~sEY, 1966). The uppermost layer of the reflection profiles, an acoustically transparent layer 0.5-1 km thick, can be correlated with the layer to which BtrNCE and FArlLQUIST (1962) ascribe a velocity of 2.1-2.6 kin/see and to which SAWT, KNOX, BLUEand PAITSON(1964), and BUNCE, SALISBURYand CLOUGH(personal communication) give velocities of 1.5-1.7 kin/see. Layered rocks of variable thickness beneath the transparent layer are correlated with a discontinuous layer of velocity 3.0--4.5 km/sec (the" stratified zone " o f SAWT, KNOX, BLtr~ and PAITSON, 1964). They found low velocity material (2.1 kin/see) in one place beneath material of 4.5 kin/see in this layer. The layered rocks rest with apparent unconformity on a strongly reflecting irregular surface, the deepest seen in the reflection profiles. This surface is taken to be the top of the "basement," a thicker more eontinuons layer up to 2 km thick with a velocity of 5-1-5.4 km/sec. Underlying these comparatively thin layers, and not discernible on the reflection profiles, is the lowest layer of the crust, 3-5 km thick, with velocity 6.5--6.6 km/sec. The continuous seismic reflection profiles of the North Slope (Figs. 3-5) show the following stratigraphic sequence from youngest to oldest: 3. Flat-lying sedimentary layers make up the floor of the trench itself and form sediment ponds in depressions which run generally along the North Slope. The greatest thickness observed on the records of the ponds on the North Slope is about 0.5 km (0.6 see two-way travel time). 2. The acoustically transparent layer which forms the top of the Outer Ridge succession continues down the slope of the trench with some thinning and interruption, and in profiles CC' and DD' (Fig. 4) can be seen dipping beneath the flat-lying sediments which form the floor of the trench. 1. Only the top of the next layer, a strong reflector, is visible on CSP records in most places, leading one to surmise that here the transparent layer may lie directly on the 5.I-5.4 kin/see "basement." In some places, however, (e.g., profile BB', Fig. 2) irregular layering is seen below the transparent layer, both on the lip of the North Slope, and beneath the Slope itself. This layered material presumably is a part of the discontinuous 3.0-4.5 kin/see layer mentioned above. Petrology Sites from which rocks were dredged during cruise Chain 34 are given in Table 1 and marked in Fig. 1. Dredge Chain 34D2, the only one in which serpentinite was raised, was obtained from the steep escarpment mentioned above (about 20 ° 00'W, 66° 28'N; Fig. 1). The serpentinite together with serpentinite recovered here in earlier dredge hauls (Chain 19, D-2, -3 -10) was described by BowrN, N~WALK and H~a~Y (1966). The other two hauls, dredged some thirty miles northeast, from a steep slope near the top of the Slope, (profile CC', Fig. 2) contained no serpentinite and were dominantly basalt. Haul Chain 34D3 consisted o f about t50 kg of rock, including abundant basalt, chert, and pink and green claystone. Minor rocks were limvstone, carbonaceous material, and soft gray mudstone.

Geology of the North Slope of the Puerto Rico Trench

303

Table 1. Locations of the dredge sites Dredge No.

Latitude

Longitude

Depth (fms)

Date

Rocks recovered

CHN34D2

19° 58'N

66° 28'W

3850

3 Dec 62

CHN34D3

20° 16'N

65° 42'W

3200-3700 4 Dec 62

CHN34D4

20° 13'N

65° 47'W

3250-3500 4 Dec 62

one fragment serpentinite and one fragment basalt 150 kg basalt and sedimentary rocks one fragment mudstone and several fragments basalt

Characteristics of the sedimentary rocks will be dealt with elsewhere. Basalts Texturally, basalts of this haul are of three types: porphyritic and amygdular basalt are the main types; a third type, of minor volumetric importance, is coarser, has rare vesicles and phenocrysts, and is termed here, for convenience, diabasic basalt. Larger basalt fragments weigh over 5 kilograms, are angular, and are coated on exterior surfaces with soft green clay. The clay also coats interior fractures. Freshly broken surfaces of the basalt are olive green or green-brown, porphyritic varieties having numerous dull white plagioclase phenocrysts, some of which have soft altered centers. Amygdular types contain spherical amygdules ranging from 0.3 to 2 mm in diameter which may make up around 5 700 of the rock volume. Some amygdules are filled by white, dirty-yellow, or olive green friable material; in others a central void is surrounded by a shell of fibrous or platy yellow clay. Black matter presumed to be manganiferous occurs on interior fractures and exterior surfaces. The basalt breaks easily along the clay-coated internal fractures. Petrography 1. Porphyritic basalt. The texture of the porphyritic basalt based upon thin-section analysis is glomeroporphyritic. Groups and single crystals of plagioclase and rare phenocrysts of olivine lie in a ground mass consisting of microlites of plagioclase, altered glass and mafic silicates. The olivine is almost completely altered to serpentine and montmorillonite; the glass has been altered to yellow, brown and green sheetsilicates, principally nontronite. Veinlets of green nontronite cut through both groundmass and phenocrysts. Rare irregular patches of sheet-silicates lacking plagioclase microlites may be altered clumps of ferromagnesian minerals. The rare amygdules are filled by radially oriented aggregates of nontronite. 2. Amygdular basalt. The texture of the amygdular basalt is intersertal. Amygdules and rare phenocrysts of zoned plagioclase and altered olivine are set in a groundmass ofplagioclase and mafic minerals. Plagioclase laths in the groundmass average 0.4 mm in length. The mafic minerals are extensively altered to sheet-silicates. 3. " Diabasic" basalt. In the volumetrically minor diabasic basalt, the texture is intersertal, and in comparison with the other types, plagioclase laths are larger (1 mm average length), clinopyroxene is more abundant and less altered, and glass forms a smaller percentage of the total rock. The glass is completely altered to green or brownish nontronite, and is shot through in places with needles of an opaque mineral, presumably magnetite.

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R.L. CHASEand J. B. I ~ E Y

A model analysis gave the following volume percentages--plagioclase 36, pyroxene 35, altered glass, opaque minerals and clay 27, vesicles 1. Mineralogy 1. Feldspar. Anorthite content of feldspar was determined by taking the extinction angle normal to X on the universal stage. Feldspar laths are normally zoned. Phenotrysts in the porphyritic and diabasic varieties have calcic cores (up to 85 An)and more sodic rims. Mierolites in the porphyritic varieties range from 38 to 53 An and in the amygdular basalts from 54 to 79 An. Laths in the diabasic varieties range in composition from 44 to 80 An. 2. Pyroxene. Pyroxene was not determinable in the porphyritic and amygdular basalt because of fine grain size and extensive alteration. The pyroxene of the diabasic variety is an augite (2 V 44 °-55 °, Nb 1.693 ~ 0.007). Orthopyroxene and pigeonite were not found. 3. Olivine. Olivine was too altered for optical determination of composition. 4. Nontronite or saponite. Samples of the green clay coating exteriors, interior fractures, and filling some amygdules were put into distilled water, agitated with an ultrasonic agitator, and drops of the suspension containing the fine fraction applied to porous porcelain slides mounted on the mouth of a flask to which a vacuum pump was attached. X-ray diffraction traces of the resulting oriented clay laminae show the following dominant peaks: (a) (b) (c) (d)

Untreated samples: 13-13,5 A Soaked for 24 hr at 70°C in methylglycolate: 16.7-17/~ Heated for 1 hr at 400°C: 9.8-9.93 A Collodion-mounted sample (060): 1.53 A.

The clay is either a dioctahedral or trioctaherdral member of the montmorillonite group (WARSHAWand RoY, 1961). Although the refractive indices of the clay, (Nv 1.56-1.58, N~ 1.55-1.57) are toohigh for saponite, the likely member of the trioctahedral montmorillonites, and fit well nontronite, the iron rich dioctaherdral montmorillonite, (c) above indicates that the mineral is probably saponite. The pleochroism is yeUowgreen to green. The range of values of refractive index may indicate either variable iron: magnesium ratio, or variable hydration, or both. A similar alteration product of submarine basalt is described by BANKSand MELSON(1966). Chemical composition Chemical analyses and norms of three basalts are given in Table 2, The first is porphyritic and extensively altered, the second diabasic and slightly altered, and the third is amygdular and extensively altered. The notable features of the analyses are low silica content, moderately high alumina, high ferric iron; ferrous iron ratio, low potassium, and high water content. The high oxidation state of the iron and the high hydration are no doubt not primary characteristics of the basalt, but reflect the secondary alteration to clay minerals. Even the least altered rock has a much higher proportion of its iron in the ferric state than the average basalts of NOCtCOLDS(t954). As a consequence of the alteration of the bulk of the ferrous iron to ferric iron the normative compositions are not indicative of the original degree of silica saturation. Thus, although the norms indicate that the basalts are quartz tholeiites, the rocks may have

305

Geology of the North Slope of the Puerto Rico Trench

Table 2.

Chemical analyses and norms of basalt, Puerto Rico Trench, Dredge No. CHN34D3 Porphyritic Amygdular extensively altered

SiOe TiOz AI2Oa

Fe2Oa FeO MnO MgO CaO Na20 K20 P205 H~O + H~O -

COz total q

or ab an di hy mt hm

il tn] ap cr cc c

Diabasic

extensively altered

34-3-41

34-3-50

34-3-68

47"25 1"14 19'34 6.24 1.42 0.06 5"47 7.44 2-77 1.27 0.15 3'20 4"19 0"04 99'96

48"49 1"37 15"46 5.07 3'75 0'12 7"25 11 "41 2"75 0'36 0'26 1"35 2"00 0"05 99"69

47"09 1'49 17"02 7.48 I'16 0-09 6"60 9-75 3.22 0-64 0"23 1"56 3"19 0'55 100.07

2"69 7-50 23 "43 35.77 13"62 -1.39 5-28 2"17 -0.33 0"05 0.09 0.30

0"77 2-13 23 "26 28-77 20"29 9.21 7.35 -2"60 -0.57 0'06 0"11 --

0"19 3'78 27.24 30'09 10" 11 11.74 -7-48 2"58 0.32 0.50 0"06 1-25 --

all been olivine tholeiites or even undersaturated nepheline basalts before alteration. However, the absence of feldspathoids in thin section suggests that the composition of the magma was that of quartz or olivine tholeiite. As would be expected, the porphyritic basalt with abundant plagioclase phenocrysts has highest alumina content of the three. Titaniam content of basalts partly altered to clay can be taken as the same as or greater than the original titanium content (GoLDSCnMIDT, 1954, p. 419). The titania and magnesia contents of the Puerto Rico Trench basalt are low enough to exclude it from "oceanic island" basalt under the criterion of CHAVES and VELDE (1965). The alteration of the basalt to montmorillonitic clay is doubtless accompanied by a change of volume. When alteration takes place along previously formed fractures, the expansion should result in the extension and elongation of the fractures, which in turn should allow further penetration of water and alteration to clay. In a place where basalt crops out on steep slopes, the alteration and its resultant fracturing should result in fragments of basalt periodically breaking loose, tumbling downslope and accumulating as a talus of non-rounded weathered fragments where the slope decreases. Such deposits may be the source of the rocks hauled up in dredges. P A L E O N T O L O G I C A L E V I D E N C E FOR AGE OF ROCKS

TODD and Low (1964) identified Foraminifera from limestones of haul CHN34D3 as Cenomanian. In consolidated siltstone of the same haul W. R. RIEDEL(personal

306

R.L. CHASEand J. B. I-I~.sEy

communication from H. S. LADD) found Cretaceous Radiolaxia. RIEDELand D. B. ERICaON (/n BOWIN, NALWALKand HERSEY, 1966) identified fossils from sedimentary rocks dredged further west as Upper Cretaceous (CHN19D3) and Eocene-Oligocene (CHN19D10). BOWlN, NALWALKand HERSEY(1966) noted that bedded sedimentary rock is observable in photographs of the scarp above the sites of the CHN 19 dredgings. ERICSON (in BOWlN, NALWALKand HERSEY, 1966) remarked on the unusual degree of compaction of the fossiliferous sediments of the dredge haul CHN19D3. T. SAITO (1964) reported Cenomanian Foraminifera associated with volcanic ash and siliceous rock in a fragment from a dredge haul of R.V. Conrad on the North Slope of the Trench. Volcanic rock fragments are reported from the same haul (ibid). Dredge hauls in the same area by R.V. Atlantis II in 1 ~ also yielded basalt, limestone, chert, and red claystone, the last similar in appearance to the claystone raised some 65 km east during cruise Chain 34. Thus, the assembly basalt-chert-red claystone-limestone may be representative of a formation of Cenomanian age which crops out along the North Slope of the Trench. SEISMIC V E L O C I T I E S I N D R E D G E D R O C K S A N D P O S S I B L E S I G N I F I C A N C E

F. BIRCH (personal communication) determined compressional wave velocity in cores taken from selected fragments of dredge-haul CHN34D3 summarized in Table 3 (for method, see BIRCH, 1960). The most appropriate values for comparison with velocities in rock layers beneath the Outer Ridge and North Slope determined by seismic refraction studies are those measured while the rocks were subjected to quasihydrostatic pressures of 1 kilobar and more.

Table 3. Velocity of compressional waves (V_p) in rocks dredged from the North Slope of the Puerto Rico Trench (CHN34D3)---determination by Francis Birch Density amygdular basalt porphyritic basalt red claystone chert laminated gray limestone

2.69-2.70 2.58,2.70 2.08-2.18 2.50--2.55 2.13

1 atm

Velocity (Vp) (km/sec) 1 kbar

4.37-4,57 4.39-4.96 3'59-4.28 4.99-5-32 3.53

4.89--4-95 4.60-5.15 3.78-4-29 5.31-5-41 4.81

4 kbar 5.42-5.48 4,90-5.50 3.96-4.30 5'40-5-50 4.93

Of the sedimentary rocks of the dredge-hauls, the red claystone, at one kilobar, has velocitiesaround 4 kin/see, and the basalt, chert, and limestone have velocities around 5 kin/see. The chert and basalt have velocitiesin the same range as the 5.1 km/sec layer which underlies the discontinuous layer. Thus the Ccnomanian formation may be the 5.I km/sec layer, The basaltrecovered from the North Slope has a compressional wave velocitylower than those recorded for relativelyfresh basalts from the Mid,Atlantic Ridge. The latter have velocitiesat I kilobar of around 6.2 km/sec (F. BIRCH,personal communication), The difference may be attributed to the considerable amounts of montmoriUonitic clay present as alteration products in the basalt from the Trench. Serpentinite ~ g e d from 66 ° 30'W during Chain ~ s e 19 also has sound velocities near 5 kin/see (Bowr~, NALWALK and HSRSEY, 1966). This rock, which

Geology of the North Slope of the Puerto Rico Trench

307

crops out about 7 km below sea-level, may be from the upper part of the 6.5 km/sec layer (Fig. 7). The latter can be expected to be made up of more or less altered material of the upper mantle (serpentinized peridotite) and diabase or gabbro dikes which were feeders for the basalts of the overlying 5.1 km/sec Cenomanian formation. Compressional wave velocities in diabase under one kilobar pressure lie between 6-7 km/sec (PRESS, 1966, Table 9-2). The combination of serpentinized peridotite and diabase may well yield a layer with compressional velocities in the range 6.5-6.6 km/sec. Possibly, as BOWIN, NALWALKand HERSEY(1966) noted, the serpentinite comes not from a layer but from an intrusion. The intrusion could have taken place along a fault, the surface expression of which is the steep scarp at the foot of which the serpentinite was dredged. Association of serpentinite with faults is common in orogenic areas, and in the Greater Antilles is reported from Hispaniola (BOWIN, 1966) and from Puerto Rico (MATTSON, 1960). Strata overlying the 5.1-5-4 km/sec layer may be represented by the less consolidated sedimentary rocks of the dredge-hauls, whose velocities were not successfully measured. The Tertiary sedimentary rocks described by BOWIN, NALWALK and HERSEY (1966) presumably come from these units. It can be deduced (NAFE and DRAKE, 1963; PRESS, 1966) that they are characterized by velocities in the range of those of the transparent layer and the well-layered material beneath it described by BUNCE and FAHLQUIST(1962) and by SAVIT, KNOX, BLUE and PAITSON (1964), i.e., 1.7-4.2 km/sec. Material from these layers could have reached the dredge sites, which were all on slopes below outcrops of the transparent layer, by slumping downslope. The following sequence is postulated to compose the upper crustal layers cropping out on the North Slope : 3. Unconsolidated sediments of the transparent layer. Upper Tertiary and Quaternary. Overlying (2). 2. Consolidated sediments, with or without minor basalt flows, possibly not present in the succession cropping out on the North Slope in all places, lying unconformably on the rugged surface of (1). Probably Upper Cretaceous and Lower Tertiary. 1. A succession of basalt flows, with interbedded chert, claystone, and limestone of average velocity near 5 km/sec. Cenomanian in age, possible extending down into the Lower Cretaceous. This can be correlated because of its age with horizon beta of EWING, WORZEL,EWING and WINDISCH(1966).

STRUCTURE Figure 6 shows three profiles of the North Slope on which sub-bottom reflecting surfaces revealed in the continuous seismic profiles have been plotted. The reflectors lie usually within, or at the base of, the transparent layer which can be traced northward to the Outer Ridge, where the characteristic velocity of compressional waves within it has been determined by seismic refraction and reflection studies to be 1.5-2.6 km/sec (BuNcE and FAHLQUIST,1962; SAVIT,KNOX, BLUEand PAITSON, 1964). A velocity of 2.1 km/sec was assumed for the material above and between the reflectors when their positions were plotted on the corrected profiles. Beneath the abyssal plain at the bottom of the Trench and in valleys on the North

308

R . L . C~tASEand J. B. HERSEY

Slope the seismic profiles show fiat-lying " p o n d e d " sediments (H~RSEY, 1965). The bottom limit of reflections from these sediment layers has been plotted on the corrected profiles, and serves to give an idea of the shape and extent of the ponds. A velocity of sound of 1.7 km/sec was assumed for the sediments of the ponds. The simplest of the three profiles is CC' (Figs. 4 and 6) at approximately 65 ° 47'W. Here are no deep valleys nor sediment ponds on the North Slope. Flat layers of the abyssal plain which forms the floor of the Trench appear at the south end of the profile. Reflectors in the transparent layer underlie almost the entire slope except the stretch between 20 ° 16'N and 20 ° 19'N. The transparent layer extends out under the sediments of the trench floor for at least 1 km. The apparent angular unconformity of reflectors at 20 ° 02'N is probably due to lateral, i.e. east-west, irregularity of the seafloor. The deepest reflectors sounded lie about 1 km below the sea-floor, A lensingout of the transparent layers occurs at the highest part of the North Slope, around 20 ° 20'N. Such lensing-out occurs in all three profiles, usually just below a sudden steepening in gradient. Profile GG', approximately along the meridian 66 ° 32'W (Figs. 5 and 6), includes a section of very steep slope which falls 1500 m in less than 6 kin, a gradient greater than 1 in 4, between 19° 58'N and 20 ° 0I'N. Serpentinite was dredged from this slope in 1961. The transparent layer lenses out at the top of the slope. The apparent absence of reflectors beneath considerable parts of the area north of the steep slope, and beneath the crest of the low ridge which Hes to the south, is due to a temporary malfunction of profiling equipment. A profile made from Vema on the same meridian (EwINO and EWIN6, 1962, Fig. 4) shows continuous reflectors beneath both these areas. The thickness of the ponded sediments beneath the abyssal plain here is at least 650 m (350 fins). The profile AA', approximately along meridian 650 08'W near the eastern end of the Trench (Figs. 3 and 6), passes through four sediment-ponds, the southernmost being the abyssal plain which forms the trench floor. A very small pond is perched on the ridge just north of the floor; to the north, two larger ponds, occupying depressions between east'west ridges, lie 8173 m (4255 fins) and 7917 m (4128 fms) below sea level. The deeper of these has sediments about 650 m thick, and appears to have rugged steplike slides. The other has a plane,convex cross-section. The sediments which form the floor of the Trench appear here to be about 350 m thick, and lie in a depression which has an almost vertical south side and a steeply sloping north side. The transparent layer pinches out below two increases in slope in the northern part of the North Slope and covers only ridge tops in the southern part. The reflection profile shows that it does not underlie the floor of the Trench, nor is it, nor any other subbottom reflector, visible on the lower part of the South Slope, which is devoid of sediment ponds. O R I G I N OF T R E N C H AND T O P O G R A P H Y OF NORTH SLOPE

VENING MEINESZ (1930) proposed the concept of the crustal down-buckle (for which KUENEN, 1936, coined the term "teetogene ") to account for the large negative gravity anomalies he found over trenches in the East and West Indies. EWING and WORZEL (1954), combining gravity data with early refraction data, explained what appeared to be thin crust beneath the Puerto Rico Trench as a " necking ', of the crust below the Trench due to crustal tension. TALWANI, SUTTON and WORZEL (1959),

Geologyof the North Slope of the Puerto Rico Trench

309

utilizing later refraction data and gravity measurements, showed that the crust below the Puerto Rico Trench is intermediate in thickness between continental and normal oceanic crust, and suggested that the trench originated by a downdropping of several kilometers of a crustal block about as thick as the crust of a continental margin, under conditions of crustal tension. TALWANI(1964) summarized arguments for and against these hypotheses. HESS(1962) and DIETZ (1962) put forward the hypothesis of seafloor spreading. A locus of downgoing of mantle convection cells is necessary to match the upwelling beneath mid-ocean ridges. Seismological evidence (SYKESand EWlNG, 1965; SYKES,1966) suggests a plane of earthquake foci dipping at 45 ° to 60 ° under trenches and adjacent island arcs and continental margins. Geodetic, radiogenic and seismological studies of the area affected by the Alaskan earthquake of 1964 (PFLAKER, 1967) suggest that continental crust is being thrust over oceanic crust along a fault dipping 15° under the Alaska peninsula. Seismological studies suggest similar movements occur under other Pacific island arcs (STAUDER,1967; ISACKS,SYKESand OLIVER, 1967, OLIVER and ISACKS, 1967) and support the hypothesis that the downturn of mantle convection cells takes place beneath trenches and adjacent continents and island arcs. If this hypothesis is true, what should the seaward side of an oceanic trench be like ? HESS (1962) supposed that the " oceanic layer," the lowest and thickest component of the oceanic crust, is principally composed of serpentinized peridotite. This material should give up its water of hydration and be reconverted into mantle peridotite when it is dragged down through the 500°C isotherm by the downgoing mantle convection current on which it rides. The several kilometers of material above the oceanic layer, however, because of their relatively low density, may not be dragged down with the convecting mantle, but may be added to the crust of the island arc or continent. This should occur under the landward slope of a trench. The seaward slope should be normal oceanic crust warped down as the convecting mantle beneath it sinks. Whether the seaward slope of a trench is under tension or compression, and what structural modifications of the slope will occur to relieve such tension or compression are not obvious from the spreading sea-floor hypothesis. The Pacific arcs beneath which evidence suggests that convective sinking of mantle material takes place are parallel or oblique to appropriate segments of the Pacific oceanic ridge, and are sites of Quaternary or Late Tertiary volcanism. In contrast, the Puerto Rico Trench is normal to the Mid-Atlantic Ridge, and the Greater Antilles have not seen volcanism since the Paleogene. These differences indicate that earth-movements beneath the Puerto Rico Trench may differ from those beneath Pacific trenches. Possibly, as suggested by WILSON(1966), the Caribbean crustal block is overriding the Atlantic crust to the east of it, and is bounded on the north by a strike-slip fault along the Cayman (Bartlett) Trough. EWINGand HEEZEN(1955) suggested that the Puerto Rico Trench is an eastern continuation of the Trough. Gravity measurements (BOwlN, 1968), however, indicate that the extension of the Trough is into the Gulf of Gonave on the west coast of Hispaniola, so that it does not connect with the Puerto Rico Trench. Nevertheless, some of the left-lateral movement on the northern side of the Caribbean may be taken up by faults beneath the Puerto Rico Trench. Four possible mechanisms by which the topography and structure of the North Slope of the Trench could have originated are discussed below. They are: (1) thrusting from the south (2) normal faulting (3) transcurrent faulting and (4) gravity sliding.

310

R.L. CHASeand J. B. HERSEY

Thrusting from the south This mechanism is discussed because itis a possible outcome of mantle convection. The argument goes as follows: If the Puerto Rico Trench and Antilles Island Arc lie over the descending limb of a convection current in the mantle, then the Trench's existence may be due to a downdrag of the crust. The disappearance downward of the substratum beneath the crust and uppermost mantle lead to horizontal compression north of the downwell, The horizontal pressure is then relieved by crustal shortening, achieved by the thrusting of slices of crust from the south over those north of them. An upside-down s h i n ~ g might result, as illustrated in Fig. 8. If the North Slope had been formed by this process, and if the transparent layer had been deposited before thrusting took place, the transparent layer of any thrust slice should dip southward under the slice overthrusting it from the south, and possibly show contortion immediately to the north of the thrust face. Such features are not observed on the North Slope so if thrusting did take place, it preceded the deposition of the transparent layer. Another possibility is that thrusting takes place further south, beneath the South Slope. s

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Normal faulting H~s~Y (1962) and EWrNG and EWlNG (1962) interpreted the features on the North Slope to be the result of normal faulting. In Fig. 9, faults have been drawn in on a replica of structural sections GG' and AA'. The faults are not evident in the seismic reflection profiles, thus they are purety hypothetical. For such normal faulting to occur, the North Slope must have been in a state of tension at the time of faulting. Normal faults produced by tension should dip steeply in the upper brittle part of the crust, then become more gentle at depth as they pass through rock nearer the melting

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point. Movement on such curved fault-surfaces must cause the tops of the faultblocks to tilt away from the direction in which the fault-surfaces bend at depth. Thus, if the fault-surfaces beneath the North Slope curve southward toward the Trench at depth, the dip of the initially flat-lying transparent layer should be northward after movement has occurred. Only in one place is a northward dip of the transparent layer observed. This is immediately below the greatest escarpment on the North Slope (Profile FF'). In other places, the dip is fiat or southward. The implications of this argument will not be explored beyond this point.

Transcurrent faulting Left-lateral strike slip faults trending in general WNW are known throughout the pre-Oligocene terrain of Puerto Rico. BRIGGSand PEASE(1960) described horsts and grabens in east-central Puerto Rico, formed between some of these faults, along which there was vertical as well as horizontal movement. If such faulting has occurred in the Greater Antilles, it may have been active recently enough to have influenced the present morphology of the North Slope. The topography of the North Slope shows that any set of faults beneath it must trend ENE rather than WNW as in Puerto Rico. The steep scarp from which serpentinite was dredged (Fig. 2, Fig. 5, GG') could be the topographic expression of a major fault trending northeasterly. The almost latitudinal boundaries of the two high ponds near 20 ° 00'N, 65 ° 15'W, the northeast-trending scarp which follows the 4000 fins contour through 66 ° 00'W, and a parallel scarp a few miles north of this (Fig. 1), could mark other faults. Still other faults could be postulated on the basis of topographic scarps, but the writers consider this a pointless exercise considering the inaccuracy with which the topography is known.

Gravity sliding Some Italian geologists now suppose that in the Tertiary Period blocks of rock. kilometers in extent, travelled many kilometers by sliding into migrating troughs that existed where the Appenines now stand (MAXWELL,ed., 1964). Considerable areas of the Island of Timor, which is situated on the axis of the Java Trench, have been interpreted as exposures of chaotic deposits formed by gravity sliding in the Miocene (AUDLEY-CHARLES, 1965). MOORE(1964) proposed large-scale landsliding to explain several thousand square-kilometers of irregular topography on the northeast slope of the Hawaiian Ridge. Features of the continental rise off eastern North America have been attributed to gravity sliding (RONA and CLAY, 1967). Tertiary slump breccias containing large exotic blocks of older rocks occur in Hispaniola (NAOLE, 1966). In Puerto Rico, blocks--up to 30 km ~ in outcrop area--slid into a graben during the Early Tertiary (GLovER, 1967). Thus, slumping has occurred previously in orogenic belts and island arcs, and it is not unreasonable to assume that slumping could take place down the slopes of the Puerto Rico Trench. From Figs. 2 and 6 (cross-section AA') we can see that blocks up to 25 km long, 5 km wide, and 1 km thick must have moved southwards as units if the topography were formed by gravity-sliding. Figure 10 shows the sequence of events which might have led to the pond-and-ridge topography on the North Slope between 65 ° and 65 ° 30'W. We assume that the initial slope of the Trench was unbroken and simple, similar to profile CC' (Fig. 4). Blocks of the

313

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middle part of the slope then broke away and overrode the bottom part of the North Slope and possibly part of the south slope. Smaller-scale slides on the north and south sides of the blocks softened their outlines. Later slumps and turbidity currents formed the flat-lying, ponded sediment lenses between the blocks and at the bottom of the trench. Sedimentary interbeds between basalt flows could provide surfaces along which gravity-sliding occurred. An east-west normal or transcurrent fault at the south margin of the original position of the southern slide-block might have made it easy initially for the blocks to break away and override the lower part of the slope. Discussion

The evidence available is insufficient to rule out any one of the processes mentioned above as having been active in forming the North Slope of the Puerto Rico Trench. Possibly more than one was involved. The steep high scarp at 66 ° 30'W from which serpentinite has been dredged could be a result either of a normal fault with the southern block thrown down or o f a transcurrent fault with a vertical component. The more complex topography farther east is most obviously explained by normal faulting or gravity sliding, though transcurrent faulting is also a possibility. The small amount of deformation of the sedimentary layers of both trench floor and the ponds on the North Slope indicate little or no recent ubiquitous deformation of the North Slope, though deformation of parts of the Slope away from the ponds is not ruled out. GEOLOGICAL HISTORY

The history of the part of the earth's crust now forming the North Slope of the Puerto Rico Trench began so far as we know in the Cenomanian, with the extrusion of tholeiitic magma. The resulting flows are interbedded with chert and limestone. Also in the Cenomanian, or later, pink sittstones and claystones were deposited. The flows and sediments probably rest on a substrate of serpentinized pcridotite cut by feeder dikes. In the remainder of the Late Cretaceous and in the early part of the Tertiary, more sediments, which now form discontinuous layers on top of the rugged surface of the flows, were laid down. Later, the segment of crust we are discussing became the North Slope of the Puerto Rico Trench, and a layer of sediments called the transparent layer was laid down over the region of Greater Antilles Outer Ridge and the North Slope of the Trench. (The relative age of the transparent layer and the Trench will be discussed in a later paper). Deformation by normal faulting, transcurrent faulting, thrusting from the south, gravity sliding, or a combination of these processes occurred, resulting in formation of steep scarps and east-west valleys on the North Slope of the Trench. Those events probably took place in the later part of the Tertiary. Deposition of layered sediment in the bottom of the Puerto Rico Trench has resulted in the submergence of the lower reaches of the transparent layer on the North Slope beneath almost flat layers of sediment which form an abyssal plain flooring the Trench. Slumps and turbid currents coming off surrounding slopes have partly filled the east-west valleys with flat layers of ponded sediment. CONCLUSIONS

The volcanic rock dredged from the North Slope of the Puerto Rico Trench, though partly altered to montmorillonitic clay, resembles closely the thoIeiites of

Geology of the North Slope of the Puerto Rico Trench

315

mid-ocean ridges, appears to be of Cenomanian age, and to come from outcrops of the oceanic basement of the Greater Antilles Outer Ridge, which is characterized by compressional velocity of 5 km/sec. Sedimentary rocks dredged up with the basalt may be in part correlative with layering which appears in places above the basement reflector on continuous seismic reflection records, and with the discontinuous layer characterized by compressional velocity of 4 km/sec found in seismic refraction profiles of the Outer Ridge. The transparent layer of the Outer Ridge extends down the N o r t h Slope and beneath the abyssal sediments of the Trench floor. Ponded sediments of the Trench floor and of east-west valleys in the N o r t h Slope appear undisturbed, suggesting no ubiquitous recent deformation of the North Slope. Present information from bathymetric and continuous seismic reflection profiles is inadequate to show whether east-west valleys and ridges and ENE-trending scarps which characterize North Slope topography were produced by thrusting, normal faulting, gravity sliding or strike-slip faulting, or by a combination of these processes. Acknowledgements--A. J. NALWALKassisted in dredging and selected rocks for thin sectioning. ELIZABETHT. BUNCEsupplied echo-sounding and continuous seismic reflection data she obtained during the summer of 1964 aboard Atlantis H and gave valuable assistance in many discussions concerning seismic reflection and refraction data from the Puerto Rico Trench and Outer Ridge. S. S. JAcoas (1962, unpublished) adjusted the navigation used for plotting the bathymetric information obtained during cruise Chain 19. FV.ANClSBIRCHdetermined seismic velocities in cores of the rock samples. C. A. BtrgK helpfully discussed the interpretation of the structure of the trench. K. O. EMERYand E. T. BtmCE critically read the manuscript. This work was supported by the U.S. Navy under contracts Nonr 4029(00) and 1367 with the Office of Naval Research, and by N.S.F. grants GP 822, GP 1123 and GP 5355. REFERENCES

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BowrN C. O. (1968) Geophysical study of the Cayman Trough. J. geophys. Res., 72, BOWIN C. O., A. NALWALKand J. B. HERSEY(1966) Serpentinized peridotite from the North Wall of the Puerto Rico Trench. Bull. Geol. Soc. Am., 77, 257-269. BRIGGS R. P. and M. H. PEASE (1960) Compressional graben and horst structures in Eastcentral Puerto Rico. Prof. Pap., U.S. geol. Surv., 400B, B365-B366. BLrNCE E. T. and D. A. FAHLQUIST(1962) Geophysical investigation of the Puerto Rico Trench and Outer Ridge. J. geophys. Res., 67, 3955-3972. BtmCE E. T. and J. B. HERSEY(1966) Continuous seismic profiles of the Outer Ridge and Nares Basin north of Puerto Rico. Bull. Geol. Soc. Am., 77, 803-811. CHAYES F. and D. VELDE 0965) On distinguishing basaltic lavas of circumoceanic and ocean-island type by means of discriminant functions. Am. J. Sci., 263, 206-222. DIETZ R. S. (1962) Ocean-basin evolution by sea-floor spreading. In: Continental Drift, S. K. RtrNCORN, editor, Academic Press, 289-298. EwINo J. and M. EWrNG 0962) Reflection profiling in and around the Puerto Rico Trench. J. geophys. Res., 67, 4729-4739. EwrNo J., J. L. WORZEL, M. EWING and C. WINDISCH(1966) Ages of horizon A and the oldest Atlantic Sediment. Science, 154, 1125-1132.

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