Geology of a portion of the north wall of the Puerto Rico Trench

- Elsevier Tectonophysics Printed in The Netherlands

Publishing

GEOLOGYOFAPORTIONOF TRENCH

Company,

Amsterdam

THENORTHWALLOFTHE

PUERTORICO

A.J. NALWALK Marine

Research

Laboratory,

University

of Connecticut,

Noank, Conn. (U.S.A.)

(Received August, 1968) (Resubmitted February, 1969) SUMMARY

Rocks dredged from the north wall of the Puerto Rico Trench represent a thick accumulation of rock types consisting of siliceous mudstones, cherts, basalts, serpentinites and minor amounts of volcanic wackes, carbonaceous material and limestones, all of which formed in a submarine environment. A tentative stratigraphic sequence of the north wall in ascending order is Cenomanian limestone, Upper Cretaceous siliceous mudstones and cherts, Upper Cretaceous-Eocene limestones, Upper CretaceousEocene serpentinites, EocenePliocene siliceous mudstones and cherts, pre-Pliocene volcanic wackes, Pliocene basalts and Pliocene-Recent unconsolidated sediments. Two main fault patterns are apparent in the area of study and trend east-west and northwest-southeast. The east-west faults are older and are terminated by those trending northwest-southeast. Magnetic susceptibility, remanent magnetism and Qn values obtained from the rocks indicate that permanent polarization is responsible for the geomagnetic anomalies measured at sea. INTRODUCTION

Island arcs and associated deep-sea trenches are of geological interest because they are seismically active erogenic areas, and because their development has a direct bearing upon the hypothesis of continental growth by lateral accretion. The sea-ward part of the Caribbean Island Arc has been chosen for this study, which is primarily concerned with rocks collected from the north wall of the Puerto Rico Trench. The objectives of the study are to determine the st*atigraphic sequence of rocks exposed on the north wall of the Puerto Rico Trench and to determine whether the rocks represent an oceanic or a continental environment.

Tectonophysics,

8 (1969) 403-425

403

PREVIOUS

WORK

The Puerto Rico Trench was first studied by Vening Meinesz and Wright (1930) at which time they incorrectly referred to it as the Nares Deep. Hess (1932, 1937, 1938) referred to it as the Brownson Deep. Their studies consisted of sonic depth and gravity determinations. Subsequent geophysical studies were made by Hersey et al. (1952), Ewing and Heezen (1955), Ewing et al. (1957), Officer et al. (1957, 1959), Heezen et al. (1959), Talwani et al. (1959), Hess (1960), Bunce and Fahlquist (1962), Ewing and Ewing (1962), Hersey (1962), Donnelly (1964), Savit et al. (1964), Van Voorhis and Davis (1964), Sykes and Ewing (1965), Griscom and Geddes (1966), Bunce and Hersey (1966) and Conolly and Ewing (1967). The work performed prior to 1950 consisted of gravity and depth determinations and established the general outline and depths of the trench and established that a negative gravity anomaly was associated with the trench. Several hypotheses have been postulated in order to explain the origin of the island arcs. Vening Meinesz and Wright (1930) suggested that light material of the crust was downbuckled into heavier material of the mantle in order to account for the large gravity anomalies associated with the deep trenches. Hess (1932, 1938) and Hess and Maxwell (1953) applied Vening Meinesz’s tectogene hypothesis to the West Indies and introduced the concept of large scale strike-slip movement as part of their model. Ewing and Worzel (1954), Ewing and Heezen (1955) and Worzel and Shurbet (1955) stated that the trenches are sites of tension and thinning of the crust rather than the axes of compression and thickening. Their hypothesis resulted from work done in the Puerto Rico-Virgin Island areas. Donnelly (1964) stated that the boundary between the contrasting “plates” serves to localize a generally applied force. Geophysical and geological data collected since 1960 do not completely substantiate these hypotheses.

~

ATI;ANTIC

OCEAN

Fig.1. Location map. The shaded portion shows the area of study; dashed line represents the 3,400 fathom curve. 404

Tectonophysics,

8 (1969) 403-425

AREAOFSTUDY

The Puerto Rico Trench is the seaward part of the Caribbean Island Arc system. It is a deep-sea trough located on the seaward (Atlantic Ocean) side of the northeastern part of the West Indies. The floor of the trench is about 12 nautical miles in its widest part, greater than 200 nautical miles in length, and lies between an “outer ridge” and the continenta slope of the Greater Antilles (Heezen et al., 1959, p.36). To the east of the Lesser Antilles, the trench shallows gradually and to the west, north of the Dominican Republic, it terminates abruptly. The trench is the deepest part of the Atlantic Ocean. North of the island of Puerto Rico it exceeds 4,400 fathoms. The area of study is a portion of the north wall of the Puerto Rico Trench located about 90 nautical miles north of Puerto Rico, between 64’55’W-66”55’W and 19°45’N-20030’N (Fig.1). Along the face of the wall the water depths vary from 3,200 fathoms to more than 4,200 fathoms.

RESULTS Submarine

topography

The topography on the north wall of the Puerto Rico Trench in the area of study is shown in Fig.2 and Fig.3. West of 65O15’W the north wall has gentle to moderate slopes with the main scarp (Bowin et al., 1966) having an average slope of 15’. In contrast, the area east of 65’15’W is more irregular with slopes greater than ‘70°. Two major features predominate in the eastern area. The first is an east-west trending valley and ridge in the vicinity of 20”05’N. The ridge has a topographic relief of 580 fathoms (Fig.3,4). The valley north of the ridge has a maximum depth of 4,300 fathoms. The second feature consists of a north-south trending scarp which abuts against the east-west trending valley (Fig.3). The scarp has slopes in excess of 68’ and a local relief of 700 fathoms. This area contains the steepest slopes so far known on the north wall of the Puerto Rico Trench. The western part of the area of study has average slopes of 5O-8’ with local steepening to 25“. It is extensively covered with unconsolidated material, and rock outcrops are scarce. The main topographic features consist of east-west trending valleys and ridges that probably represent normal faults (Hersey, 1962). Dating of the samples

Several small pieces of limestone were collected from dredge sites Ch 34-D3 and AII-ll-D5. Samples from Ch 34 are Middle Cenomanian (Todd and Low, 1964). One small piece of limestone obtained from AII-ll-D5 also indicates a Cenomanian age. D. Ericson (written communication, 1966) states that: “The three slides marked AII-11 -D5-c contain well preserved Foraminifera, in particular Rotalipora sp., cf. R. appenninica (Renz). The genus has a short range, from Upper Albian through Cenomanian into Tectonophysics, 8 (1969)403-425

405

Y/y

Fig.2. Bathymetry of a portion of the north wall of the Puerto Rico Trench. contour interval = 100 fathoms; scale = one minute = one nautical mile.

2dlS’N

~0‘90’15

-4

19O45”N

20°00’N

20“lS’N

_^.a..^,_.

Fig.3. Bathymetry Trench. contour interval mile.

Lower

of a portion of the north wall of the Puerto Rico = 50 fathoms; scale: one minute = one nautical

Turonian.

Very probably the age of your specimen is Cenomanian; or a form close to that species occurs in the Del Rio Formation (Cenomanian) of Texas according to H.T. Loeblich and A.R. Loeblich, Jr. (U.S. National Museum Bull., 215, 1957, p.41). The other Foraminifera are in part Praeglobotruncanas: a genus ranging from Early Aptian to the end of the Cretaceous”. In conclusion: The age of the Foraminifera is certainly basal Upper Cretaceous and probably Cenomanian. Radiolaria obtained from, the siliceous sedimentary rocks have been studied by W.R. Riedel and range in age from Upper Cretaceous to Lower Tertiary (Bowin et al., 1966). All of the Radiolaria obtained from the rocks

R. appeminica

Tectonophysics,

8 (1969)403-425

407

r

Fig.4. Profiles Fig.3 (V.E. = 2/l).

A-A’

and B-B’

east of 65’ 15’ W. Data taken from

used in this study were too altered for identification. One sample of chert, AII-ll-D3-104, was sent to Mr. J.F. Grayson of the Pan American Petroleum Corporation Research Center, Tulsa, Cola., for pollen determinations. He has sent the following written communication (1966): “The number of palynomorphs present in the sample was surprisingly low. This fact makes an age interpretation difficult because of the possibility of contamination. Three dinoflagellate identifications suggest that the age of the chert is Cretaceous, and possibly Upper Cretaceous. The few pollen grains observed suggest a Miocene-Pliocene age. Dr. G.L. Williams has suggested that the Cretaceous chert may have been reworked and finally incorporated in a breccia of Miocene-Pliocene age. Threebasalt samples (AU-11-D2-01’7, D4-03 and D8-02) were dated by Geochron Laboratories, Inc., Cambridge, Mass. The three samples give dates of (D2-017) 7.0 2 1.8 m.y.; (D4-03) 4.8 !: 1.8 m.y.; (D8-02) less than 12 m.y. H. Krueger (written communication, 1966) reports that the first two ages are finite ages, while the age for the D8-02 represents an upper limit. The samples used for age dating are fresh, relatively unaltered and equivalent in age. The altered plagtoclase crystals within the samples contain smectite (montmorillonite) and sericite and a few of the mafic minerals are altered to chlorite. The percentage of alteration minerals in a given sample are: (1) (AR-ll-D2-017) 0.5-l% ; (2) (AU-ll-D4-03) ~-12%; and (3) (AR-11-D8-02) c: 3%.

Rock types

and their

distributio,z

Location of sample sites The dredging sites were chosen on the basis of the steepness of slope and apparent variation in rock types as evidenced by continuous seismic profiler data. It is difficult to determine precisely the location of the sample sites because of water depths in excess of 3,200 fathoms. It is AS0 difficult 408

Tectonophysics. 8 (1969) 403-425

to determine which samples are of material collected in situ as contrasted to samples which may have tumbled downslope. The following criteria are used to indicate which rocks were collected in place: (1) preponderence of rock type; and (2) freshly fractured material. The above criteria, while helping to place each rock type in its stratigraphic position, do not allow precise relationships to be determined. If a horizontal layering and normal-type faulting is assumed, then it is possible to give a tentative stratigraphic sequence for the rocks dredged from the north wall (Fig.5). This sequence is tentative because the navigational information and depth relationships of the dredge locations to the ships’ positions are not known too well. Fig.2 and Fig.3 show the best estimate of where the rocks were collected (black squares) and the areas traversed by the ships during the dredging operations (dashed rectangles).

Fig.5. Possible stratigraphic sequence on the north wall of the Puerto Rico Trench. A. .Interpretation west of 65O 15’W by Bowin et al. (1966). B. Alternate interpretation west of 65O 15’W (this report). C. Interpretation east of 65* 15’W (this report). Tectonophysics,

8 (1969) 403-425

409

The order of presentation used here is: (1) the sediments from oldest to youngest; (2) basalts; and (3) serpentinized peridotite. (See Table I for rock types collected.) Sedimentary

vocks *

The oldest rocks collected are small pieces of foraminiferal limestone of Cenomanian age (Todd and Low, 1964). Todd and Lcw (1964) state that the fossils indicate an area of deposition far from land and at a depth greater than 1,000 fathoms, possibly as deep as at present. The limestones have been collected from dredging sites (Ch 34-D3) and (AII-ll-D5). (See Fig.2 and Table I.) One sample of elastic limestone was collected from 66O31’57”W, 20°00’00”N in about 3,350 fathoms of water near the top of the north wall. L. Shishkevish (personal communication, 1966) reports, on the basis of microfossils, that it is Upper Cretaceous to Eocene in age. The shallower

TABLE

I

Location Dredge no.

and rocks haul

Ch

19:D2

Ch

19:D3

Ch

19:DlO

Ch

34:D2

Ch

34:D3

Ch

34:D4

AD-11:Dl AII-11:DZ AII-ll:D3 AII-ll:D4 AII-ll:D5 AII-11:DG AII-11:DB

collected

in each dredge

Location 66“24’33” 20°00’00” 66”25’00” 19”58’55” 66p31’57” 2O“OO’OO” 66°28’OO” 19*57’00” 65Q41’24” 20a15’58” 65”41’15” 20a16’00” 65a08’31” 2OOO5’42” 65°03’OO” 20°07’02” 65”07’28” 20*05’56” 65“10’09” 20n03’54” 65”14’00” 19”58’51” 65“01’39” 20”00’15” 65”05’27” 20”06’06”

haul

Rock present’ W N W N W N W N W N W N W N W N W N W N W N W N W N

serpentinized chert2

peridotite2

, siliceous

mudstone2,

basalt2

(one piece)

serpentinized peridotite22 chert2, siliceous mudstone and limestone talc rock2 (one piece), altered basalt (one piece) basalt2 limestone2, chert2, siliceous carbonaceous mudstone (siliceous) siliceous mudstone2, basalt chertl, basait2,

siliceous

mudstone,

basalt2,

volcanic

wacke

basalt2,

chert2,

siliceous

chert,

limestone2

volcanic

mudstone2,

wacke

mudstone

basalt2 basalt2, siliceous basalt2,

mudstone2, siliceous

chert2

mudstone,

chert

lFor the serpentinized peridotite, see Bowin et al. (1966). 2Rock types, considered to have been collected in place.

*Detailed descriptions cost of reproduction. 410

of the rocks

can be obtained

from

the author by paying the

Tectonophysics,

8 (1969) 403-425

depth and younger age indicate that it is stratigraphically above the Cenomanian limestones. Siliceous mudstones were collected in water depths of 3,350-4,100 fathoms (Fig.2,3; Table I). They are very fine-grained, porous, and consist almost entirely of quartz and opal. Microfossils indicate that they range in age from Upper Cretaceous to Miocene, and possibly Pliocene. Cherts collected from the north wall range in age from Upper Cretaceous to Miocene-Pliocene. The cherts are very fine-grained, massive, dense and consist predominantly of quartz and opal with minor amounts of chlorite, montmorillonite and pyrite. Radiolaria, Foraminifera, dinoflagellates and pollen grains are replaced by quartz exhibiting wavy extinction. The cherts differ from the siliceous mudstones in that they contain fewer fossils and have been more extensively silicified. Two samples of carbonaceous material were collected from dredge site Ch 34-D3 (Table I; Fig.2). Friedel and Nalwalk (1968) have described these samples and indicate that they closely resemble kerogen. Several small pieces of volcanic wacke were collected from dredging sites AU-ll-Dl and AII-ll-D2 (Table I; Fig.2,3). The basalt fragments are extensively altered to smectite (montmorillonite) and chlorite, and are surrounded by a matrix of clay minerals consisting of montmorillonite and chlorite. Their stratigraphic position is uncertain, but the writer believes that they probably occur beneath the Pliocene basalts. Serptentinites collected from the north wall have been described by Bowin and Nalwalk (1963) and Bowin et al. (1966). They concluded that the samples represent serpentinized peridotites. Basalt. Basalts have been collected from 65’03’OO”W 20’07’02”N to 65’43’15”W 20”16’00”N in depths ranging from 3,400 to 4,000 fathoms and from slopes ranging between 10” and 70° (Fig.2; Table I). Basalts collected from the north wall are tholeiites. They are aphanitic, glassy, plagioclase-rich, generally clinopyroxene-poor and contain minor phenocrysts of olivine and orthopyroxene. Chemically they have low K20 contents and are enriched in MgO. The basalts are distinctly different from those described from Puerto Rico (Lidiak, 1965). In general the north wall basalts contain more glass, fewer phenocrysts, less K20 and are more aphanitic. The textural differences appear to be due to rapid quenching of molten lava. Serpentinites. Bowin et al. (1966) described the serpentinites collected from the north wall of the Puerto Rico Trench. They suggested that the serpentinized peridotites might represent a continuous layer that is pre-Cenomanian in age, but the writer now considers that the serpentinites intruded Upper Cretaceous-Eocene sediments and are post-Cenomanian in age, because the serpentinites have been obtained from an area less than 21 square nautical miles in extent and because sediments collected with some of the serpentinites are Upper Cretaceous-Eocene in age.

Tectonophysics,

8 (1969) 403-425

411

The north wall of the Puerto Rico Trench in the area of study is a south-facing scarp fFig.2,3). The scarp is dissected locally into a series of east-west trending valleys and ridges, which are parallel to the axis of the Puerto Rico Trench and represent the dominant trend. East-west trending valleys and ridges are probably due to normal faults and appear to be partially offset and partially terminated by north-south trending lineaments. The north-south trending structures are also probably due to normal-type faulting. The north-south faults have exposed Pliocene basalts and this may suggest that the faulting is as old as Pliocene, The east-west faults also appear to be associated with Pliocene basalts within the vicinity of the north-south faults. On the basis of bathymetry, the north wall can be separated into two areas: one west of 65O15’W and one east of 65O15’W. The area west of 65O15’W is relatively smooth, has an average slope of 5*-8* with local steepening to 25*, and is extensively covered with unconsolidated sediments. In contrast, the area east of 65Ol5’~ is rugged, has slopes in excess of 68’ and is extensively faulted (Fig.3,4). Area west of 0‘5*15’ W. Several seismic refraction and reflection studies have been made in the area west of 65Ol5’W. These studies were widely separated and conducted primarily north of the trench. Bunce and Fahlquist (1962) determined that a 2.1 km/set-layer directly overlies a 5.1 km/set-layer just to the north of the north wall in the vicinity of 66O31’W. They suggested that the 2.1 km/set- and 5.1 km/seclayers probably cropped out on the north wall, an observation that appears to be substantiated by reflection profiles taken later by Bunce and Hersey (1966). The rock types representing velocities of 2.1 and 5.1 km/set have not been identified. The 2.1 km/set-layer may, however, be represented by siliceous mud&ones as F. Birch (personal communication, 1963) obtained velocities ranging from 1.57 to greater than 2 km/set for some of the siliceous mudstones from the north wall. The 5.1 km/set-layer may be represented by: (1) a combination of basalt and siliceous mudstones; (2) serpentinites; or (3) a rock type that has not been adequately sampled. Steeper slopes are encountered from 65”3O’W to 65O15’W and 20°00’N to 20OlO’N. There is a strong indication of north-south faulting in this area, which suggests that the north-south trending faults are present in the vicinity of 65*2O’W and continue to at least 65OOO’W. This north-south faulting may be the cause of the complexity evidenced in the reflection discussed by Bowin et al. (1966). Ar-ea east of f&S”15 W. The fault patterns east of 65“15’W trend north-south and eastrwest. Fig.4 illustrates two profiles from the area. The profiles indicate that the main ridge is a horst and the valley is a graben. Profile A -A ’ shows a normal fault, a horst, a graben, a possible small horst and a normal fault on the east side of the profile. The profiles and apparent faults were deduced from bathymetric data so that strike-slip components cannot be determined. Pliocene basalts are associated with the faults in this area, especially along 412

Tectonophysics,

8 (1969) 403-425

the north side of the east-west trending valley between 65°10’W-65002’W and 20”05’N. Basalts may also be associated with the south side of this valley, but unfortunately no samples have been collected from that part of the north wall. The east-west trending valley is partially filled with sediments as indicated by sub-bottom penetration on the original echo-sounding records. It is also associated with an east-west trending ridge immediately to the south. This ridge has a gentle surface and is terminated to the south by a steep south-facing scarp that passes into another broad valley, which may be the beginning of the Puerto Rico Trench floor. The east-west trending structures appear to be older than the north-trending structures as most of them are offset or terminated by the north:trending faults. Not all of the north-south faulting is younger than the east-west faulting because the north-south faults are offset or terminated in the vicinity of 65@02’W 20”Ol’N and 65OO3’W 20’07’N. The north-south trending structures evidenced in the areas east of 65°15’W-64055’W appear to be younger, in general, than the east-west trending features. They are quite evident in this region of the north wall but may have been overlooked in other areas. The major north-south trending structure is present between 65”ll’W 20°09’N and 65OO9’W 19O56’N. This structure appears to be a graben which starts north of the main east-west valley, probably crosses the valley and then partially offsets the main east-west trending ridge. The ridge appears to be offset about 4 nautical miles to the south, indicating some strike-slip movement along this fault zone. Another major north-south fault zone is present between 65OOl’W 19’5’7’N and 65OO3’W 20°09’N. Sykes and Ewing (1965, p.5073) noted that the area east of 65”15’W and west of 65O55’W is an active seismic area. This is the general area of intersection of the north-south and east-west fault zones. The main north-south structural trend between 65O12’W 20”13’N and 65”lO’W 19O55’N appears to be the north-south trending ridge in the vicinity of which Sykes and Ewing located three epicenters. The north-south faults appear to be related to the inner arc of the Lesser Antilles. The inner arc and the area east of 65’15’W are seismically active, have a north-south trending fault pattern and associated volcanic activity that is Upper Tertiary or younger in age. Both areas also have an east-west trending set of faults. The relationship of these two structural trends is not readily apparent, but the east-west trending features may represent reactivation of east-west trending structures that are related to the Puerto Rico Trench, whereas the north-south trending fracture may be more closely related to the inner arc of the Lesser Antilles. As far as the writer knows, this is the only area on the north wall with major north-south trending structures.

Chemical

analyses

of selected

basalt

samples

The basalts from the north wall are both fine grained and glassy, and chemical analyses are necessary to characterize the rocks. Sixteen chemical analyses are given in Table II. All of the samples analyzed contain considerable glass except sample 6, which contains less than 3%. Analysis 16, Tectonophysics,

8 (1969) 403-425

413

‘TABLE

II

Chemical compositions Rico Trench1

of basalt samples

Basalt

Element

1 _-

.-. sampies

(weigbt

dredged

from the north wail of the Puerto -_..-._

percentages)

;: ‘1 _~~~_ _.__._ .__.--_~_ . 4...--_

-_ 6

5

p205 CO2

46.94 1.06 lb.11 6.01 3.58 0.15 6.20 11.81 2.68 0.15 1.S7 1.46 0.12 0.14

45.16 0.98 17.34 4.10 3.71 0.19 S.28 10.72 ‘2.80 0.22 3.14 1.42 0.10 0.17

46.38 0.85 17.41 4.57 4.90 0.14 8.24 10.95 1.71 0.13 2.96 1.14 U.11 0.15

48.08 1.02 18.30 4.51 3.50 0.14 6.04 12.71 2.43 0.18 1.41 1.27 0.12 0.12

4s. 83 1.22 17.26 4.63 4.17 0.19 6.19 12.37 2.43 0.17 1.10 1.56 0.14 0.10

Total

100.28

100.33

99.64

99.83

0.30

0.30

1.67 14.67 39.75 6.73 10.77 19.27

1.98 1.11 20.44 38.36 6.50 18.57 7.53

2.10

1.92

SiO TiO?j AI203 Fe203 Fe0 MnO MgO CaO Na20 %C H20+ H20_

Normative minerals (Cross, Iddings, Calcite Ilmenite Orthociase Albite Anorthite Magnetite Diopside Hypersthene Olivine Quartz Apatite Hematite Analyst:

Prieson 0.30 1.93 1.11 22.53 36.97 8.82 16.23 8.00 0.90

_-... .._-

_-L_._.--L._

-.

1.18 16.22 2.57 6.38 0.17 8.33 11.90 1.98 0.10 1.34 0.74 0.14 0.29

48.55 1.29 16.55 4.29 4.94 0.16 5.77 12.33 2.45 0.14 1.27 1.49 0.17 0.18

51.14 1.43 14.X? 3.55 5.66 0.16 7.42 7.70 4.25 0.23 2.77 0.42 0.39 0.18

100.36

99.93

99.58

100. Ih

0.20 2.28 1.11 20.44 35.58 6.73 19.95 8.26

0.60 2.28

0.40 2.43

0.40 2.74 1.67 36.15 20.29 5.10 11.09 18.52

48.59

and Washington) 0.40 1.82 1.11 23.58 34.19 6.03 14.17 0.73 13.48

2.82

16.77 35.31 3.71 17.64 20.14 1.02

20.96 34.19

6.26 20.57 8.22 3.48

1.01

H. Dehn

the most altered basalt analyzed, indicates a possible range of composition for altered versus fresh basalt (analysis 61. Sample numbers, depth of water and location of the dredge sites where the basalts were collected are given in Table III. The most notable characteristics of the samples are the low K20 content and relatively high MgO content; this appears to be characteristic of submarine basalts (Fig.6). Yoder and Tilley (1962, p.352) have devised a method for classifying basalts based on normative mineral contents, as computed from C.I.l?.W. calculations. Analyses 1, 3, 4, 5, 6, 7, 9, 11 and 14 contain normative hypersthene and quartz and belong to the over saturated tholeiite group of Yoder and Tilley (1962). Analyses 2, 10, 12, 13, 15 and 16 contain hypersthene and olivine in their norms and belong to the olivine-tholeiite group of Yoder and Tilley. Analysis 8 contains hypersthene in the norm and no 414

Tcctonaphysics,

8 (1969) 403-425

Basalt 9 Si02 TiO2 Al203 Fe203 Fe0 MnO MgO CaO Nag0 K2C H20+ H20p205 CC2 Total

Calcite Ilmenite Orthoclase Albite Anorthite Magnetite Diopside Hypersthene Olivine Quartz Apatite Hematite

49.66 1.30 16.24 4.43 4.17 0.14 6.92 11.78 2.85 0.22 0.94 1.26 0.17 0.06 100.14

2.43 1.11 24.10 28.08 6.50 23.83 8.12

samples

(weight

10

11

49.66 1.25 15.46 3.63 5.85 0.15 7.61 11.68 2.85 0.33 0.68 0.85 0.14 0.10 100.24

0.20 2.43 2.22 24.10 28.36 5.34 23.10 10.48 2.29

50.52 1.08 15.96 2.20 5.84 0.23 8.01 9.85 2.70 0.64 1.98 0.42 0.11 0.13 99.67

0.30 2.13 3.89 23.06 29.47 3.25 14.98 20.08 0.06

2.34

percentages) 12

13

50.06 1.30 15.37 2.77 5.86 0.14 8.45 8.82 4.25 0.10 2.13 0.91 0.14 0.12 100.42

0.30 2.43 36.15 22.80 3.94 16.00 3.52 11.88

51.01 1.33 15.20 4.28 4.78 0.15 8.08 7.41 4.06 0.66 2.20 0.72 0.15 0.06 100.09

2.58 3.89 34.58 21.13 6.26 12.29 12.98 3.21

14

13

49.14 1.36 15.56 4.69 4.86 0.17 8.02 10.71 3.00 0.20 1.30 0.77 0.14 0.14 100.36

0.30 2.58 1.11 25.15 28.63 6.73 18.65 14.28

16

.51.00 1.x1 14.53 3.93 5.22 0.17 8.03 8.27 4.36 0.3 1 2.15 0.72 0.14 0.34 100.52

0.80 5.58 1.67 36.68 19.46 5.80 15.19 10.81 4.46

4X.38 1.36 17.09 6.07 2.21 0.18 6 n0 10.61 3.44 0 31 1.:40 1.66 II.16 0.10 99.67

0.20 2.58 1.67 29.34 30.30 3.94 16.85 6.30 1.96

0.24 3.36

quartz or olivine; it belongs in the saturated tholeiite group. All 16 of the chemically analyzed basalts are tholeiites; there are, however, some differences within each group. In the over saturated tholeiitic group, analyses 1, 3, 4 and 5 have Al203 contents in excess of 1’7%, while analyses 6, 7, 9, 11 and 14 have less than 16% A1203. The higher Al203 content in analyses 1, 3, 4 and 5 may be due to alteration as the plagioclase in these samples is more altered to sericite and smectite than that in numbers 6, 7, 9, 11 and 14. In the olivine tholeiite group analyses 2 and 16 have Al203 contents in excess of 17% while the rest of the samples contain less than 15.5%. These two samples are also the most altered of this group. The differences between the olivine and the over saturated tholeiite groups are not readily apparent. In general there is more plagioclase present in the over saturated tholeiites and a higher degree of alteration to smectite and sericite. Tectanophysics,

8 (1969)

403-425

415

‘TABLE

III

Rock types, sample numbers, samples from the north wall NO.

1 2 3

4 5 6 7 8 9 10 11 12 13 14 15 16

Hock

type

Sample

water depth of the Puerto no

Depth of water (fathoms)

AII-ll-Dl-015 A&ll-Dl-02 AII-ll-Dl-71 MI-ll-DZ-013 AII-11-D2-015 AII-11-D2-017 AD-ll-DZ-102 AII-ll-D3-3 AII-ll-D4-00 AII-ll-D4-03 AII-ll-DE-00 AII-ll-DE-02 AII-ll-Da-03 AII-ll-D8-015 AII-11-D&016 Ch 34-D4-3

basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt basalt

I

A’

\

‘\A\

\ \

W W W W W W W W W W W W W W W W

20’05’42” 20“05’42” 20“05’42” 20”03’42” 20”05’42” 20“05’42” 20‘305’42” 20’05’56” 20”03’54” 20”03’54” 20”06’06” 20”06’06” 20”06’06” 2OOO6’06” 20”06’06” 20°16’00”

N N N N N N N N N N N N N N N N

\

A

\

0

3,600 :3,6OU 3,600 3,600 3.800 4,000 4,000 3,950 3:950 3,950 3 950 3:950 3.400

analyzed

Location -65U08’31” 65”08’31” 65”08’31” 65“03’00” 65”03’00” 65’=03’00” 65°03’00” 65”07’28” 65”10’09” 65”10’09” 65“05’27” 65*05’27” 65”05’27” 65“05’27” 65“05’27” 65“43’15”

4.000

A

\

\

4.000 -l I000

of chemically

x-9

, IA

and locations Rico Trench

~.~_--’

’ \

\ A/

\

0' I

’ Lor,

K20

Ol /

:t”i;

1

0

+2 A3 a4

X K,O

Fig.6. Plot of MgO versus K20 for oceanic basalts. 1 = Basalts from the north wall; 2 = Mid-Atlantic Ridge basalts (Nicholls et al., 1964); 3 = basalts from “Discovery II” (Muir and Tilley, 1966); 4 = Mid-Atlantic Ridge basal@ Mid-Pacific Rise basalts and Carlsberg and mid-Indian Ocean basalts (A.E.J. Engel et al., 1965; C.G. Engel et al., 1965). 416

Tectonophysics,

8 (1969)

403-425

Magnetic data Magnetic

of the rocks .fyo?n the worth wall

Properties

Magnetic properties of 50 of the samples are given in Table IV. The susceptibilities of the basalts range from 1 . lo-” to 3. lob5 e.m.u./g, with most of the samples being in the range of 5. 10m4 e.m.u.jg. The remanent magnetization of the basalts is much higher than the magnetic susceptibility. remanent magnetism The QH = . induced magnetism TABLE

of the basaIts

range from

0.07 to 162.

IV

Magnetic Sample

values

properties

of rocks

no.

from

the north

wall

of the Puerto

Trench Jr Qn =;KF

Magnetic (e.m.u./g.

Basalts AII-11-Dl-00 AII-ll-Dl-01 AII-ll-Dl-02 AII-ll-Dl-1 AH-ll-Dl-015 AII-ll-Dl-71

1,675.57 699.92 184.67 20.98 1,089.44 229.62

54.04 2,474.46 1,427.38 64.07 4,973.47 5,063.70

AII-ll-D2-2 AII-ll-D2-00 AII-ll-DB-1 AII-ll-D2-010 AII-ll-DB-011 AII-ll-D2-012 AII-ll-D2-013 AII-ll-D2-014 AII-ll-D2-015 AII-ll-D2-016 AII-ll-D2-017 AII-ll-D2-018 AII-11-D2-22-W-14 AII-11-D2-102

222.59 143.06 119.62 294.10 811.42 1,266.99 328.89 528.64 301.10 633.34 449.37 718.89 942.35 88.21

4,149.68 2,489.26 8,735.74 7,502.95 1,858.90 2,901.21 6,432.21 1,988.60 5,542.55 8,181.89 5,460.9h 4,951.29 321.9X 1,254.15

AII-11-D3-3 AII-D3-37 AII-11-D3-38

528.64 918.14 424.59

1,627.16 2,969.76 4,$)25.4X

26

AII-11-D4-00 AII-ll-D4-01 AII-11-D4-03 AII-ll-D4-28 AII-ll-D4-73

235.74 477.80 167.28 141.32 238.45

2,038.08 2,827.60 5,970.81 409. 13 3,028.76

19 13 79 6 28

AII-ll-DS-01 AII-ll-D5-02 AII-ll-D5-2

29.04 571.89 226.76

49.38 1,430.30 534.64

AII-ll-Da-00 AII-11-D&01 AII-ll-D8-02 AII-ll-D8-03 AII-ll-DX-04

29.88 1,402.52 648.42 780.00 478.43

36.60 2,754.56 149.31 680.91 113,059.48

Tectonophysics,

8 (1969)

403-425

moment 10e6)

Rico

Susceptibility K) (e.m.u./g. lo- b )

F = 0.45 0.07 17 10 49 41 39 1162 57 3 3 43 41 29 27 1: 0.76 32

4

3 4 0.5 61

417

Susceptibility

S,ample no.

(e.nl.u.,/g’ 1.356.4:~ 19::.G6 -14U.64 131.65

AII-ll-DX-015 AH-ll-Dfi-OlF AII-II-DR-89 AII-Il-Da-202 Ch 34-D3-53 Ch 34-D4-3 Sedimentary

K) IO- i‘)

21!).34 546.94

Magnetic (e.m.u./g. -. .- - 922.37 931.37 G.72d.54 6,981.OU 1,041.5U

rocks

90

3.5,

AII-ll-DH-UG

Ii.45

52.93

Ch 19-D3-201

42.UU

35.11

Ch Ch Ch Ch

15L . 65 37.36 33.26 20.99

62.62 27.49 78.17 26.88

Ch 34-D3-173

--

JI Qn = rF

4,6386..?‘J

AII-11-D3-fi

19-DlO-28 19-DlO-31 19-DlO-35 19-DlU-36

moment 10-6) _- . .-

51.79

_---

2o.w

21.04

___.

0.9

The susceptibilities of the siliceous sedimentary rocks are all in the range of 5. lo-> e.m.u./g or less and the remanent magnetization ranges from 1.10’4 to 8.10-s e.m.u./g. The Qn values of the measurable sedimentary rocks range from unity to 7. Many of the sedimentary rocks are below the sensitivity of the reversible susceptibility bridge (1 * 10e7 e.m.u./g) and only those samples having susceptibilities high enough to be determined with precision are presented. Somerton et al. (1963) reported magnetic susceptibilities of 10e4 e.m.u./g for two basalt samples from the experimental Mohole Test Sites at Baja, California. They reported that the low magnetic susceptibilities existed because the samples contained less than 0.1% magnetite. Basaltsfrom the north wall having susceptibilities of 10s4 e.m.u./g contain up to 5% opaque minerals consisting of magnetite, ilmenite and rarely sulfides. The writer believes that the low susceptibilities of the basalts may be due to the small magnetite crystals having one magnetic domain, and, therefore, an induced magnetic field of less than 3 Oe will give a small measurement. All of the basalt samples having susceptibilities of 10m5 e.m.u./g contain small skeletal crystals and less than 1% opaque minerals. Submarine sedimentary rocks having Qn values of unity to 7 are reported here for the first time. All of these sedimentary rocks contain at least 1% hematite and a few contain pyrite and/or manganese oxide. In general the sedimentary rocks from the north wall have higher rem&ent magnetism than magnetic susceptibility and, therefore, the unmeasured rocks may also have Qn values in excess of unity. All of the submarine basalts studied and exhibiting low susceptibilities have some or all of the following characteristics: (1) they contain skeletal crystals of opaque minerals; (2) they contain basaltic glass; (3) they were extruded into salt water as molten material; (4) they did not attain equilib41tl

Tectonophysics,

8 (1969)

403-425

rium; and (5) they appear to represent pillows or parts of pillows. It is suggested that basalts having the above characteristics and magnetic susceptibilities less than 10V3 e.m.u./g, indicate a submarine environment of formation. There is not enough data to draw any conclusions from magnetic properties of the submarine sediments. The Q,a values obtained from the rocks from the north wall indicate that remanent magnetism of submarine rocks may be responsible for the geomagnetic anomalies observed over the trench. Phillips (1966) measured a total of ten samples of basalts from the Mid-Atlantic Ridge (3 samples), Barracuda Rise (1 sample) and the Puerto Rico Trench (6 samples) and also believes that remanent magnetism may be responsible for the geomagnetic anomalies observed over the oceans.

Comparison

of the north

wall rocks

with the island

rocks

Rocks from Puerto Rico ranging in age from Albian to Cenomanian consist of andesites, andesitic ash flows and sedimentary rocks, which represent shallow water and subaerial deposits. In contrast, rocks from the north wall indicate deep-ocean deposits. Rocks from Puerto Rico and the island ranging in age from Santonian to Eocene consist of limestones, mudstones, pyroclastics, volcanic conglomerates, graywackes, turbidites, andesitic ash, andesites, spilites, diorites and granodiorites. The rocks on the islands indicate emergence, increasing topographic diversity and persistence of topographic highs and lows for geologically long periods of time (Donnelly, 1964). In contrast, Upper Cretaceous-Eocene rocks from the north wall indicate a deep-sea environment and consist of limestones, mudstones, cherts and minor serpentinite. The limestones and siliceous mudstones of Upper Cretaceous age collected from the north wall may be equivalent in age to the Robles Formation of Puerto Rico (Pease and Briggs, 1960; Pessagno, 1960). It has not been possible, however, to establish a correlation between fossils from the north wall of the trench and those described by Pessagno from the Robles Formation (Todd and Low, 1964). The Upper Cretaceous volcanic and plutonic rocks which occur on the islands do not appear to be represented on the north wall of the trench. This is the primary difference between the Upper Cretaceous rocks on the islands and those from the north wall. The basalts from the Greater Antilles and the outer islands of the Lesser Antilles were extruded during Upper Cretace us-Eocene time, whereas those from the north wall are Pliocene in aJ e or younger; with regard to age, the basalts from the trench are more closely related to those from the inner islands of the Lesser Antilles, where volcanic activity is Late Tertiary and younger in age.

Tectonophysics,

8 (1969) 403-425

419

Fig.7 and Fig.8 illustrate the relationship between the K20, total alkalis and MgO contents of the basalts from the north wall of the Puerto Rico Trench and the published analyses of mafic rocks from Puerto Rico and the Lesser Antilles Islands. Most of the island rocks plot to the right of the arbitrarily drawn line il_B in the diagrams correlating K20, total alkalis and MgO contents. All of the basalts from the north wall. of the trench plot to the left of line .4-E. Only a few of the mafic rocks from Puerto Rico have similar NazO + IQ0 and MgO contents as those from the north wall. There appears to be a general correlation between higher Nag0 c K20 with higher MgO content. This general correlation, however, is because of the NazO content. The KzO content of the north wall samples appears to be consistently tow, while the K20 content of the island rocks is variable. There is some variation, but the maximum K20 content obtained from a submarine basalt is 0.79%. . .

Q

I

l 2

8_ l* *

?_ l 6.

i+

3.

Fig.7. Plot of MgO versus K20 for basalts from the north wall of the Puerto Rico Trench and racks from the Greater and Lesser Antilles. 2 = Basalts from the north wall; 2 = Mdic rocks from the Greater and Lesser Antilles containing less than 52% ,902. Fig.8. wall of the Antilles. I and Lesser 420

Plot of MgO versus Na20 + K20 for basalts from the north Puerto Rico Trench and rocks from the Greater and Lesser = Basal& from the north wall; 2 = Mafic rocks from the Greater Antilles containing less than 52% SiU2. Tectonophysics,

8 (1969) 403-425

The rocks from the islands in general have higher KzO and Si% contents and lower MgU contents than the rocks from the north wall. The data indicate that the basafts from the north wall represent a submarine environment. fn contrast, mafic rocks from the islands appear to have had a different history. The difference may be due to assimilation af K20 and SiO2 with subsequent loss of MgO while passing through the thicker crustal section of the islands or the composition of the source material may have been different. It would be premature to account fox the apparent differences between the rocks from the two areas because chemical data pertaining to the mafic rocks of the islands is scanty, especially from the Lesser Antilles, and does not allow significant averages to be ascertained.

The basalts from the north wall of the trench are very similar to those from the Mid-Atlantic Ridge. Basalts from both areas are tholeiites which are low in KzO and high in MgO. The differences between the two suites is that there are no high aluminum olivine basalts (Nicholls et al., 1964) present on the north wall. Conversely, no saturated tholeiite has been reported from the Mid-Atlantic Ridge. In addition, the north wall basalts appear to be more altered and contain more total H2.0. The basalts from the Indian Ocean are tholeiites and very similar to those from the north wall of the Puerto Rico Trench. Submarine basalts from the Pacific Ocean described by A.E.J. Engel et aL(lQ65) are very similar to those from the north wall of the Puerto Rico Trench. All are tholeiites according to the classification of Yader and Tilley (1962). Fig.6 shows a plot correlating MgO and K20 for basalts collected and analyzed from the north wall, Atlantic, Pacific and Indian oceans. The basalts plot in three narrow fields, i.e., high aluminum basalts (HAB), basalts having high K20 contents and basalts having relatively low K20 contents. The greatest number of analyses plot in the low K20 part of the diagram. The secondlargest field contains basalts having K 20 percentages as high as 0.79%. All of the basalts showing these differences are from the Atlantic Ocean. The apparent fields shown by dashed lines are not meant to suggest separate basalt types present in the ooeans, but are used here to delineate minor differences, The two apparent main fields will probably merge as more analyses are published.

Rocks collected from the north wall of the Puerto Rico Trench indicate that they were deposited in a deep-ocean environment. The relationship of the north wall to the formation of the trench and the Caribbean Island Arc appears to be one of secondary importance because the rocks predominantly represent an oceanic environment. They may, however, Tectonophysics,

8 (1969) 403425

4‘21

reflect in part the history of the islands and, in a minor way, the major tectonic events that occurred. Deposition at the beginning of Upper Cretaceous was quiet. This is indicated by the Cenomanian limestones which are rich in Foraminifera. The limestones indicate a stable deep-sea environment in which the islands exercised little or no control. After Cenomanian time the islands of the Greater Antilles emerged in part (Donnelly, 1964) and the remaining part of the Upper Cretaceous-Early Eocene is represented by rocks that reflect in part the complicated history of the islands. The sediments of the north wall are silica-rich and contain numerous Radiolaria, minor Foraminifera and glass shards. The available silica is evidenced in the siliceous sedimentary rocks obtained from the north wall. Hess (1938) presented evidence from Barbados which suggests that the Puerto Rico Trench started to form in Late Eocene time. He further stated that another major deformation occurred in Late Miocene-Early Pliocene time. King (1959, p.85) stated that the stratigraphic sequence on Barbados suggested that a deep-sea trench formed in Upper Eocene-Lower Oligocene time. Donnelly (1964) presumed that the trench formed when the Water Island Formation was uplifted. He assigned a pre-Albian time of formation for the trench and islands. It appears that workers in the area are divided as to the time of formation of the Puerto Rico Trench with some favoring contemporaneous development of the Greater Antilles and the trench and others favoring development of the Greater Antilles first and then formation of the trench at a later time. The rocks collected from the north wall do not resolve this problem. From Upper Eocene to Miocene time, siliceous mudstones and cherts were deposited in a stabie deep-sea environment. The volcanic wackes indicate that submarine mudflows were active sometime in the Middle or Late Tertiary. Pliocene was a time of north-south faulting on the north wall with subsequent extrusion of tholeiitic basalt. The north wall rocks support Hess’ (1938) proposal of a second deformation in Late MiocenePliocene time. Pliocene-Recent time is represented by unconsolidated sediments that overlie the Pliocene basalt flows.

CONCLUSIONS

(1) The tentative stratigraphic sequence of rocks on the north wall of the Puerto Rico Trench is: (a) unconsolidated sediments (Pliocene-Recent); (b) basalts (Pliocene); (c) volcanic wackes (pre-Pliocene); (d) siliceous mudstones and cherts (Eocen+Pliocene); (e) serpentinite (Upper Cretaceous-Eocene); (f) limestone (one sample; Upper Cretaceous-Eocene); (g) siliceous mudstone and cherts (Upper Cretaceous); (h) limestone [Upper Cretaceous (Cenomanian)]. (2) The rocks from the north wall formed in an oceanic environment. 422

Tectonophysics,

8 (1969)

403-425

The basal& are similar to those collected from the Mid-Atlantic Ridge, Pacific Ocean and Indian Ocean. The sedimentary rocks were deposited in the deep ocean and contain numerous planktonic Radiolaria and Foraminifera. (3) The low magnetic susceptibilities and the high remanent magnetism exhibited by the basalts and sedimentary rocks from the north wall suggest that: (a) the basalts were quenched quickly and much of the magnetite present consists of small crystals with single magnetic domains; (b) the basalts with susceptibilities of low5 e.m.u./g and lower contain less than 1% magnetite; and (c) remanent magnetism is probably the main type of magnetism measured by ship-borne and aeromagnetic surveys. (4) The basalts from the north wall are Pliocene in age. They are separable from the basalts of equivalent age that were extruded along the inner arc because they contain lower K20 and higher MgO. The inner arc basalts may be representative of an island arc environment, while those from the north wall represent a deep-ocean environment. (5) The basalts from the north wall are tholeiites and characteristically (a) contain much volcanic glass, (b) were quenched quickly in salt water, (c) contain two periods of plagioclase - one period contains large euhedral calcic crystals that are partially altered to sericite and smectite (montmorillonite), while the second period contains fresh skeletal crystals that are calcic, but less so than the larger crystals - (d) exhibit lower than normal magnetic susceptibilities, (e) are plagioclase-rich and generally clinopyroxene-poor and (f) are generally much lower in K20 and richer in MgO than rocks from the Greater and Lesser Antilles.

ACKNOWLEDGEMENTS

The writer wishes to thank H.J. Werner, E.G. Lidiak, A.F. Frederickson R. Wyckoff, P. Gray, H. Dehn, J.B. Hersey and E.E. Hays for their help throughout this study. This work was done in partial fulfilment of the Ph.D. degree at the University of Pittsburgh, Pittsburgh, Pennsylvania, in April 1967. REFERENCES Bowin,

C.O. and Nalwalk, A.J., 1963. Serpentinized peridotite dredged from the north wall, Puerto Rico Trench (Abstract). Trans. Am. Geophys. Union, 44:120. peridotite from Bowin, C.O., Nalwalk, A.J. and Hersey, J.B., 1966. Serpentinized the north wall of the Puerto Rico Trench. Geol. Sot. Am. Bull. 77:257-270. Bunce, E.T. and Fahlquist, D.A., 1962. Geophysical investigations of the Puerto Rico Trench and outer ridge. J. Geophys. Res., 67:3955-3972. Bunce, E.T. and Hersey, J.B., 1966. Continuous seismic profiles of the outer ridge and Nares Basin north of Puerto Rico. Geol. Sot. Am. Bull., 77:803--812. Conolly, J.R. and Ewing, M., 1967. Sedimentation in the Puerto Rico Trench. J. Sediment. Petrol., 37:44-59. Donnelly, T.W.. 1964. Evolution of eastern Greater Antilles Island Arc. Bull. Am. Assoc. Petrol. Geologists, 48:680-6!36. of Engel, A.E. J., Engel, C.G. and Havens, R.G., 1965a. Chemical characteristics oceanic basalts and the upper mantle. Geol. SOC. Am. Bull., 76:719-734. Engel, C.G., Fischer, R.L. and Engel, A.E.J., 1965b. Igneous rocks of the Indian Ocean floor. Science, 150:605-609. Tectonophysics.

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4 ‘3

Ewing, J.I. and Ewing, M., 1962. Reflection profiling in and around the Puerto Rico Trench. J. Geophys. Res., 67:4729-4739. Ewing, J.I., Officer, C.B., Johnson, H.R. and Edwards, R.S., 1957. Geophysical investigations in the eastern Caribbean: Trinidad shelf, Tobago Trough, Barbados Ridge, Atlantic Ocean. Bull. Geol. Sot. Am., 68:897-912. Ewing, M. and Heezen, B.C., 1955. Puerto Rico Trench topographic and geophysical data. In: A. Poldervaart (Editor), Crust of the Earth - Geol. Sot. Am., Spec. Papers, 62:255-268. Ewing, M. and Worzel, J.L., 1954. Gravity anomalies and structure of the West Indies, 1. Buli. Geol. Sot. Am., 65:X65-174. Friedel, R.A. and Nalwalk, A-J., 1968. Characterization of carbonaceous material from the Puerto Rico Trench of the Atlantic Ocean. Nature, 217:345-347. Griscom, A. and Geddes, W.H., 1966. Island-arc structure interpreted from aexomagnetic data near Puerto Rico and the Virgin Islands. Geol. Sec. Am. Bull., 77:153-162. Heeaen, B.C., Tharp, M. and Ewing, M,, 1959. The floors of the oceans, 1. The North Atlantic. Geol. Sot. Am. Spec. Papers, 65:122 pp. Hersey, J.B., 1962. Finding made during the June, 1961 cruise of R.V. “Chain” to the Puerto Rico Trench and Caxyn Seamount. J. Geophys. Res., 67:1109-1116. Hersey, J.B., Officer Jr., C.B., Johnson, H.R. and Bergstrom, S., 1952. Seismic refraction observations north of the Brownson Deep. Bull. Seismol. Sot. Am, 42:2Ql-306. Hess, H.H., 1932. Interpretation of geological and geophysical observations. In: The Bavy-Princeton Gravity Expedition to the West Indies in 1932. U.S. Hydrograph. Office, pp. 27-54. Hess, H.H., 1937. Geological interpretation of data collected on cruise of U.S.S. “Barracuda!’ in the West Indies. Trans. Am. Geophys. Union, 1:69-77. Hess, H.H., 1938. Gravity anomalies and island arc structure with particular reference to the West Indies. Pro?, Am. Phil. Sot., 79:71-96. Hess, H.H., 1960. Caribbean research project: progress report. Bull. Geol. Sot. Am., 71:235-240. Hess, H,H, and Maxwell, J.C., 1953. Caribbean research project. Bull. Geol. Sot. Am., 64:1-6. King, P.B., 1959. The Evolution of North America. Princeton University Press, Princeton, N.J., 190 pp. Lidiak, E.G., 1965. Petrology of andesltic, spilitic and keratophyric flow rocks, north-central Puerto Rico. Geol. SOC. Am., Bull., 76:57-88. Muir, I.D. and Tilley, C.E., 1966. Basalts from the northern part of the Mid-Atlantic Ridge, 2. The Atlantic collections near 30PN. J. Petrol., 7:193-201. Nicholls, G.D., Nalwalk, A.J. and Hays, E.E., 1964. The nature and composition of rock samples dredged from the Mid-Atlantic Ridge between 22”N. and 52QN. Marine Geol., 1:333-343. Officer Jr., C.B., Ewing, J.I., Edwards, R.S. and Johnson, H.R., 1957. Geophysical investigations in the eastern Caribbean: Venezuelan Basin, Antilles Island Arc and Puerto Rico Trench. Bull. Geol. Sot. Am., 68:359-3’78. Officer Jr., C.B., Ewing, J.I., Hennion, J.F., Harkrider, D.G. and Miller, D.E., 1959. Geophysical investigations in the eastern Caribbean: summary of 1955 and 1956 cruises. In: L.H. Ahrens, K. Rankama and SK. Runcorn (Editors), Physics and Chemistry of the Earth. Pergamon, London, 3:17-109. Pease Jr., M.H. and Briggs, R.P., 1960. Geology of the Comerio quadrangle, Puerto Rico. U.S. Geol. Surv., Misc. Invest. Map, 1: 320. Pessagno Jr., E.A., 1960. Stratigraphy and micropaleontology of the Cretaceous and Lower Tertiary of Puerto Rico. Micropaleontology, 6:87-110. Phillips, J.D., 1966. Magnetic properties of oceanic basdts from the Mid-Atlantic Ridge, Barracuda Rise and Puerto Rico Trench. Geol. Sot. Am., Ann. Meeting, 1966:163-164. Savit, C.H., Knox, W.A., Blue, D.M. and Pa&son, Ll., 1964. Reflection and velocity profiles of the Outer Ridge, Puerto Rico. J. Geophys. Res., 69:701-719. 424

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Some&on, W.H., Ward, S.H. and Kine, M.S., 1963. Physical properties of Mohole test site basalt. J. Geophys. Res., 68:849-856. Sykes, L.R. and Ewing, M., 1965. The seismicity of the Caribbean region. J. Geophys. Res., 70:5065-5074. Talwani, M., Sutton, G.H. and Worzel, J.L., 1959. A crustal section across the Puerto Rico Trench. J. Geophys. Res., 64:1545-X155. Todd, R. and Low, I>., 1964. Cenomanian f,Cretaceous) Foraminifera from the Puerto Rico Trench. Deep-Sea Res., 11:395-414. Van Voorhis, G.D. and Davis, T.M., 1964. Magnetic anomalies north of Puerto Rico: trend removal with orthogonal polynomials. J. Geophys. Res., 69:5363-5371. Vening Meinesz, F.A. and Wright, F.E., 1930. The gravity measuring cruise of the U.S. submarine S-21. U.S. Naval Obs., Ser. 2, 8:51 pp. Worzel, J.L. and Shurbet, G.L., 1955. Gravity interpretations from standard oceanic and continental crustal sections. In: A. Poldervaart (Editor), Crust of the Earth - Geol. Sot. Am., Spec. Papers, 62:87-100. Yoder Jr., H.S. and Tilley, G.E., 1962. Origin of basalt magmas: an experimental study of natural and synthetic rock systems. J. Petrol., 3:34‘2-532.

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425