67. LITHOLOGIC EVIDENCE FOR CONVERGENCE OF THE NAZCA PLATEWITH THE SOUTH AMERICAN CONTINENT

L.D. Kulm, William J. Schweller, A. Molina-Cruz, and Victor J. Rosato,School of Oceanography, Oregon State University, Corvallis, Oregon

INTRODUCTION

Geophysical data (Minster et al., 1974) indicate thatthe Nazca plate is presently converging with SouthAmerica at a rate of 10 cm/yr. The sediments and un-derlying crystalline rocks apparently are being trans-ported across the plate and subducted beneath the conti-nent or scraped off the descending slab and accreted tothe continental slope. Recent studies confirm that someof the Nazca plate sediments are being accreted to thecontinental slope (Kulm et al., 1974; Rosato, 1974) andindicate that the tholeiitic basalt crust is being rupturedto form long axial ridges in the trench axis (Kulm et al.,1973; Prince, 1974; Prince and Kulm, in press).

If the Nazca plate is indeed converging with the conti-nent at a fast rate, there should also be some record ofthis convergence in the sedimentary deposits on theplate, especially along its eastern boundary wherepelagic sedimentation interfaces with hemipelagic sedi-mentation of the coastal regions. Turbidity current de-posits, with the exception of uplifted turbidite basinsseaward of the Peru Trench (Prince et al., 1974), are ab-sent on the Nazca plate (Rosato el al., in press; Rosato,1974; Chapters 4 and 5, this volume); and hemipelagicsedimentation from the continent dominates the regionnear the continental margin. We should see a significantreduction in terrigenous components downhole gradinginto essentially pelagic sedimentation in the lower partof the core section if the Nazca plate is converging withthe continent. The pelagic and hemipelagic deposits atSite 321 do show this trend in sedimentation which sug-gests convergence for the past 5-10 m.y.

In order to recognize lithologies of oceanic origin inthe continental margin, it is essential that we establishthe nature and composition of the sedimentary portionof crust on the descending oceanic plate. Deep-sea drill-ing on Leg 34, along the eastern edge of the plate,provides this information in sufficient detail so thatcomparisons can be made between those materialsrecovered in the drill holes and those suspected exoticoceanic materials recovered by coring in the PeruTrench and on the adjacent continental margin.

The objectives of this study are to: (1) utilize thenature and composition of the materials obtained byLeg 34 to confirm the oceanic character of materialsrecovered in previous studies in the vicinity of the con-vergent zone; and (2) test the existence of the fast con-vergence rate using downhole variations in late Ceno-zoic sedimentary components such as clay minerals,opal, quartz, and organic carbon content.

PREVIOUS STUDIESA recent investigation by Rosato (1974) of the clay

mineralogy and organic carbon content of the Peruvian

continental margin and adjacent Nazca plate sedimentsshows that the surface sediments can be resolved intothree factors (oceanic, continental A, and continental B;see Table 1) using Q-mode factor analysis. The hemi-pelagic and pelagic sediments of the Nazca plate(oceanic factor) are characterized predominantly by theclay minerals smectite (Figure 1) and illite (Figure 2) (re-ferred to as montmorillonite and mica by DSDP) withminor amounts of kaolinite and chorite and a loworganic carbonate content. Mixed-layer clays are alsoabsent in the plate sediments. Upper continental marginsediments are characterized by the continental A factor(mixed-layer clays, illite, chlorite, and kaolinite) or bythe continental B factor (abundant illite and absence ofsmectite and mixed-layer clays).

SEDIMENTARY FACIESThe sedimentary facies recovered at Site 321 are de-

scribed in Chapter 5, Site 321, this volume. The earlyCenozoic iron-rich, calcareous nannofossil ooze of Unit4 (124-58 m) was deposited near a spreading center, pre-sumably the fossil Galapagos Rise and passed throughthe calcium carbonate compensation depth (CCD) inlate Oligocene time as the plate moved eastward towardSouth America (Chapter 5, Site 321, this volume). TheMiocene brown clays (Units 2 and 3, 58-34.5 m) repre-sent pelagic deposition below the CCD with the occa-sional ash falls in Unit 2 being carried from the AndeanMountains by easterly winds. The present lower tropho-spheric winds (below 10 km) in this region flow in acounterclockwise pattern around the subtropical highpressure cell (Rosato et al., 1975; Lamb, 1965; Rumney,1968) and transport eolian material from the continentto the plate. A similar wind system may have been pres-ent during the late Cenozoic when the continental ashlayers were deposited with the clays of Unit 2. The rela-tive abundance of diatoms in the Pleistocene sedimentsof Unit 1 suggests that either Site 321 was approachingthe upwelling zone off South America or upwellingbegan at this time. Kulm et al. (1974) found a cool waterplanktonic foraminiferal assemblage in Pliocene andearly Quaternary carbonates recovered from the lowercontinental slope off Peru which indicates an upwellingsystem was probably present in late Cenozoic time.

Several aspects of the sedimentary facies point tomovement of the Nazca plate toward South America.However, the rate of transport cannot be estimated fromthe gross characteristics of facies alone.

ANALYSIS OF SITE 321 LITHOLOGIES

Clay MineralogyThe clay mineral data used in this study were obtained

from DSDP analyses (Zemmels and Cook, this vol-

795

L. D. KULM, W. J. SCHWELLER, A. MOLINA-CRUZ, V. J. ROSATO

TABLE 1Percent Clay Minerals and Organic Carbon in End Members of Surface Sediments

of the Nazca Plate and South American Margin off Peru (Rosato, 1974)

Factor

123

Name

OceanicContinental AContinental B

Sample

R77-la

23-lb

V33-lc

Smec.

50.80.00.0

M.L.

0.040.8

0.0

Clay Minerals (%)

IHite

26.026.461.5

Kaol.

10.614.812.2

Chlo.

12.618.026.3

OrganicCarbon

(%)

0.884.155.96

Note: Smec. = Smectite; M.L. = Mixed Layer; Kaol. = Kaolinite; Chlo. = Chlorite.a17°59.8'S, 79°09'W.b15°11.9'S, 76°14.7'W.c10°40'S, 78°10'W.

SITE 320

SITE 321

% SMECTITE +MIXED-LAYER CLAYS

CONTINENTAL MARGINTRENCH AXISNAZCA PLATEZEN (1959)

Figure 1. Surface distribution of smectite and mixed-layered clay. Circled symbols indicate sediments with mixed-layeredclays (after Rosato, 1974). Note locations of Sites 320 and 321.

ume). DSDP sample preparation and X-ray techniquesare similar to those used by Rosato (1974). Although theclay mineral analyses may give slightly different values

because they were done in two different laboratories, wefeel that the clay data for surface samples (Rosato, 1974)are comparable to the clay subsurface data obtained at

796

EVIDENCE FOR CONVERGENCE

A CONTINENTAL MARGIN

• TRENCH AXIS

• NAZCA PLATE

* ZEN (1959)

Figure 2. Surface distribution of illite (after Rosato, 1974). Note locations of Sites 320 and 321.

Site 321, because the clay mineral species are the sameand their relative abundances (smectite > mica >kaolinite = chlorite and mixed-layer clays absent) are inreasonably close agreement.

Looking upsection at Site 321, the smectite contentdecreases markedly from the late Miocene to the Pleisto-cene (Figure 3). Smectite concentrations are variable inthe yellow-brown clays of Unit 2 (49-34.5 m), but aresimilar to values in the lower part of silty clays of Unit 1above. Other studies (Heath et al., 1974; Rosato, 1974)show that smectite dominates the clays on the Nazcaplate (Figure 1), especially those near the equator and inpartially altered subsurface ash layers. This latter obser-vation may account for the fluctuations in the smectitecontent in the ash-rich portion of the brown clays inUnit 2. The brown clays of this unit and zeolitic brownclays of Unit 3 (58-49 m) are typical of pelagic depositsfar removed from terrigenous sources.

Illite increased markedly in the Pleistocene at Site 321(Figure 3). According to Rosato (1974), the concentra-tion of illite (Figure 2) decreases rapidly seaward as

fluviatile terrigenous sediments settle out of the watercolumn, but is is slightly greater on topographic highsand at some distance from shore on the Nazca plate dueto atmospheric transport. The surface distribution of il-lite implies both an eolian and fluvial input. The strati-graphic distribution of illite at Site 321 shows relativelylow concentrations from 10 to 2 m.y. During this periodabundant Andean volcanic ash was transported fromthe continent to the site. Illite concentrations doubledbetween 2 and 1 m.y. ago, presumably as a result of in-creased continental input due to plate convergence. Al-ternatively, eolian transport also may have increasedduring the Pleistocene as a result of intensified at-mospheric circulation.

Kaolinite and chlorite are minor components in theclay mineral assemblage at Site 321 and remain essen-tially unchanged during late Cenozoic time (Figure 3).

Organic CarbonThe organic carbon content of sediment samples was

determined by the Leco induction furnace method

797

L. D. KULM, W. J. SCHWELLER, A. MOLINA-CRUZ, V. J. ROSATO

CLAY MINERALS «2µ)SMECTITE

40 60 80KAOLINITE CHLORITE0 20 0 20 0

- 1 0

ORGANICCARBON

BULKOPAL

QUARTZ(OPAL FREE)

I0 20 0 I0 20%

5 0 J

30^

4 0 -

45-

J

LITHOLOGY

GREENISH-GRAY

SILICEOUS

RICH CLAY

AND

SILTY CLAY

LIGHT YELLOW

BROWN CLAY

WITH CLEAR

VOLCANIC ASH

ZEOLITE BROWN CLAY

AGE(ray.)0.0

-0.5

-1.0

-2.0

-3.0

-4.0

-5.0

-6.0

-70

-8.0

-9.0

üJ

Figure 3. Percent clay mineral composition, organic carbon, opal (calcium carbonate free), and quartz (opal andfree) in late Cenozoic sediments of Units 1 and 2 at Site 321.

-10.0

carbonate

(Cameron, Carbon-Carbonate Results, this volume),which is the same method used by Rosato (1974) forNazca plate and continental margin sediments. At Site321 the organic carbon content is quite low and essen-tially constant, ranging from 1.2% to 0.4% of the totalsediment. Although the upper Miocene sediments havethe lowest values, trends are probably not significant be-cause the variations are close to the analytical pre-cision. Organic carbon values in the surface sedimentsrange up to 0.5% for the Peru and Chile basins along theeastern edge of the plate (Rosato et al., 1975). Thesevalues are similar to those obtained at Site 321. Thehighest organic carbon values in the area (up to 6.9%)are found beneath the upwelling region along the coastand along the equator (Rosato, 1974; Rosato et al.,1975; Figure 3).

These high organic carbon values in the surface sedi-ments (Rosato, 1974) and recent Oceanographic studiesof upwelling in the area (Smith et al., 1971) suggest thatthe main part of the upwelling region is situated over thecontinental margin. A large portion of the organic car-bon is probably derived through biological productivity.Although the bulk of the upwelling region lies over triemargin, the radiolarian assemblages in the surface sedi-ments (Molina-Cruz, 1975) and the relative abundanceof diatoms about 1.0 to 0.5 m.y. ago at Site 321 indicatethat the site is receiving fauna and flora typical of theupwelled waters at a distance of about 200 km beyondthe main upwelling region. The Pleistocene eustaticlowering of sea level to minus 125 meters would shift theupwelling zone approximately 100 km seaward of itspresent position, but this still would not bring Site 321

798

EVIDENCE FOR CONVERGENCE

within the confines of the main part of the upwellingregion. Apparently some of the fauna and flora in thesurface waters either diffuse seaward from the main up-welling zone or are transported farther seaward by sur-face currents. Based upon this study, it is clear that morework is required to understand the complicated sedi-mentation patterns associated with upwelling regions.

OpalOpal determinations (Table 2, Figure 3) were made at

Oregon State University using the techniques described(Calvert, 1966; Goldberg, 1958). The bulk opal content(carbonate-free basis) at Site 321 is about 12% inPliocene sediments (5.0-3.0 m.y.) when sedimentationrates were low (0.33 cm/1000 yr). Opal decreased toabout 10% from 3.0 to 0.8 m.y. with a further decreaseto 5% from 0.8 m.y. to the present. The opal content islowest in the Pleistocene deposits despite the excellentpreservation of siliceous fauna and flora and theappearance of diatoms about 1.0-0.5 m.y. ago (Chapter5, Site 321, this volume). Dissolution may be an impor-tant contribution to the low opal content, but it appearsthat the siliceous components are being diluted by thehigher input of terrigenous silts and clays during thepast 1 m.y., with perhaps the highest input suggested bythe marked decrease in opal during the past 0.6 m.y. Thelatter interval includes several eustatic glacial loweringsof sea level with a consequent increase in sediment loadcarried by rivers because of the increased gradients. Us-ing the surface and subsurface distribution of clayminerals, Rosato (1974) detected a seaward shift in theshoreline which was correlated with the eustatic lower-ing of sea level.

Opal concentrations (Molina-Cruz, 1975) in surfacesediments of the northeastern part of the Nazca plategradually increase seaward of the Peruvian marginnorth of 15°S latitude and also increase northward onthe plate toward the equator (Figure 4). Near-surfaceopal contents at Site 321 are close to those found in thesurface sediments nearby. The effects of the glacialperiods are believed to be minimal at the location (12° S)of Site 321 and the present-day climatic conditions ap-parently are typical of the past 5 m.y. It appears thatduring the Pliocene and Pleistocene, Site 321 was west ofits present position. Brown clays of Units 2 and 3 aretypical of pelagic sedimentation several hundredkilometers from the continent; there is no doubt thatSite 321 was farther to the west prior to the Pliocene.

Quartz

The quartz contents (Table 2, Figure 5) of Site 321sediments have been determined simultaneously withopal by X-ray diffraction. Values have been convertedto opal-free concentrations for comparison with surfaceconcentrations determined by Molina-Cruz ( 1975) forthe northeastern part of the Nazca plate (Figure 5). Thesurface and downcore analyses have been made in thesame laboratory, so the results are comparable. The sur-face data show a tongue of high quartz concentrationsextending from the continent northwesterly toward theequator. The quartz probably represents both eolianand fluvial sources with the more distant sedimentsprobably composed mainly of eolian quartz.

TABLE 2Weight Percent Opal and Quartz (Carbonate-free)

in the Pliocene-Pleistocene Sedimentsat Site 321

QuartzSample Bulk Opal Bulk Quartz (Opal Free)

(Interval in cm) (%) (%) (%)

1-1,85-872-4, 82-843-1,136-13834, 96-984-2, 125-1274 4 , 119-1215-2, 100-1025-3, 133-135

5.196.215.43

10.458.02

12.5912.6612.64

12.019.297.418.594.956.256.425.65

12.679.907.839.595.387.157.355.47

70°

Figure 4. Percent opal (calcium carbonate free) in surfacesediments of the northeastern Nazca plate. Samplelocations indicated by dots and open circles (radiolariansabsent). Contour intervals are irregular. Note locationof Sites 320 and 321.

In Pliocene sediments, the quartz (opal and carbonatefree) content ranges from 5.4% to 7.4% (Figure 3). It in-creased during the Pleistocene to 12.7% near the surface,which is approximately the value found in surface sedi-ment in the vicinity of Site 321 (Figure 5). The change inSite 321 quartz values through time corresponds to thegradient of surface quartz values away from the conti-nent, suggesting, again, that during late Cenozoic timethe Nazca plate was farther west than it is today.

SUMMARYThe sedimentary facies at Site 321 indicate that

deposition began near a spreading center. Late Eocenemetalliferous sediments lie directly above the basalt andhigher in the section at this site and at Site 320 to thenorth. The late Miocene metalliferous sediments at Site

799

L. D. KULM, W. J. SCHWELLER, A. MOLINA-CRUZ, V. J. ROSATO

NO

Figure 5. Percent quartz (opaland carbonate free) in surfacesediments of the northeastern Nazca plate. Samplelocations are indicated by dots. Contour interval 2.5%.Note locations of Sites 320 and 321.

321 indicate that the site was still influenced by ridgeprocesses even after its formation 40 m.y. ago. The fossilGalapagos Rise was the probable source of metal-richmaterial found in these sediments (Chapter 5, this vol-ume). Because the middle to late Miocene brown claysof Units 2 and 3 were deposited below the CCD, Site 321was some distance out on the flank of the ridge at thattime. Wind-transported volcanic ash from the AndeanMountains was deposited in higher concentrations inthe late Miocene and Pliocene deposits than in youngerdeposits.

Terrigenous sedimentation began to influence the latePliocene to early Pleistocene deposits of Site 321 (Fig-ure 3). The illite content increased abruptly, implying anincreased input of clay from terrigenous sources, eitherfrom fluvial or eolian transport. Sedimentation ratesalso increased dramatically approximately 1 m.y. agofrom 0.33 to 2.1 cm/1000 yr, suggesting that influx ofsilts and clays from rivers was more important thaneolian contributions.

The smectite content also decreased significantly inthe late Pliocene, and this may have resulted from eithera diminished ash contribution or a shift in the locationof Site 321.

The opal content of Site 321 sediments decreased be-tween 3.0 and 2.0 and 0.75 and 0.5 m.y. ago. Althoughthe rate of opal influx in unknown during these periods,the preservation of delicate radiolarian tests and dia-tom frustrules is excellent, particularly within the latterinterval (Chapter 5, this volume). The lower opal per-centages in the sediment suggest that the high terrigen-ous sedimentation rates tend to dilute the siliceous con-

tribution. Prince et al. (1974) calculate a hemipelagicsedimentation rate of 1.7 cm/1000 yr for the Holocenealong the seaward wall of the Peru Trench at 8°S lati-tude. The 2 cm/1000 yr rate at Site 321 is averaged overthe past 1 m.y.; it includes both glacial and nonglacialperiods. Increased hemipelagic sedimentation rates usu-ally mark glacial episodes.

Diatom frustules appear in the well-preservedsiliceous fraction about 0.75 to 0.5 m.y. ago. We knowthat the Peru current, with its associated upwelling, waspresent during the later Pliocene and early Quaternarybecause of the cool water faunas found in the pelagicdeposits in the lower continental slope opposite the Naz-ca Ridge (Kulm et al., 1974). Therefore, we concludethat Site 321 reached the influence of the upwellingregion during late Pleistocene time.

The sedimentary data and natural remnant mag-netism of both basalts and sediments from Site 321(Chapter 5, this volume), together with the pattern ofsea-floor magnetic anomalies (Herron, 1972), suggestthat the Nazca plate moved relatively eastward with re-spect to the South American continent. The sedimentarydata from the eastern edge of the Nazca plate suggest arather uniform convergence rate during the past 5 m.y.

ACKNOWLEDGMENTSWe thank Drs. Stanley Hart and Robert Yeats, Co-Chief

Scientists of Leg 34, for their cooperation in obtaining the sam-ples for this study. Special thanks go to Drs. G. Ross Heathand Harvey Sachs for their critical review of the manuscript.

This study was done in conjunction with two NSF/IDOEprograms, CLIMAP and Nazca Plate Project, that are beingconducted on the Nazca plate. Partial support for this studywas derived from CLIMAP (Grant GX-28673) and NazcaPlate Project (GX-28675).

REFERENCESCalvert, S.E., 1966. Accumulation of diatomaceous silica in

the sediments of the Gulf of California: Geol. Soc. Am.Bull., v. 77, p. 589-596.

Goldberg, E.D., 1958. Determination of opal in marine sedi-ments. J. Marine Res., v. 17, p. 178-192.

Herron, E.M., 1972. Sea floor spreading and the Cenozoichistory of the east-central Pacific: Geol. Soc. Am., v. 83,p. 1671-1692.

Lamb, H.H., 1965. Climatic changes and variations in the at-mospheric and oceanic circulations: Geol. Rundschau,v. 54, p. 486-504.

Kulm, L.D., Resig, J.M., Moore, T.C., Jr., and Rosato, V.J.,1974. Transfer of Nazca Ridge pelagic sediments to thePeru continental margin: Geol. Soc. Am. Bull., v. 85, p 769-780.

Kulm, L.D., Scheidegger, K.F., Prince, R.H., Dymond, J.,Moore, T.C., Jr., and Hussong, D.M., 1973. Tholeiiticbasalt ridge in the Peru Trench: Geology, v. 1, p. 11-14.

Minster, J.B., Jordan, T.H., Molnar, P., and Haines, E., 1974.Numerical modeling of instantaneous plate tectonics:Geophys, J. Roy. Astron. Seo., v. 36, p. 541-576.

Molina-Cruz, A., 1975. Paleooceanography of the subtropicalsoutheastern Pacific during late Quaternary. A study ofRadiolaria, opal and quartz contents of deep-sea sedi-ments: M.S. thesis, Oregon State University, Corvallis,Oregon.

Prince, R.A., 1974. Deformation in the Peru Trench, 6°-10°S.M.S. thesis, Oregon State University.

800

Prince, R.A. and Kulm, L.D., in press. Crustal rupture andthe initiation of imbricate thrusting in the Peru Trench:Geol. Soc. Am. Bull.

Prince, R.A., Resig, J.M., Kulm, L.D., and Moore, T.C., Jr.,1974. Uplifted turbidite basins on the seaward wall of thePeru Trench: Geology, v. 2, p. 607-611.

Rosato, V.J., 1974. Peruvian deep-sea sediemnts: evidence forcontinental accretion. M.S. Thesis, Oregon State Universi-ty.

801

EVIDENCE FOR CONVERGENCE

Rosato, V.J., 1974. Peruvian deep-sea sediments: evidence forcontinental accretion. M.S. thesis, Oregon State Universi-ty.

Rumney, G.R., 1968. Climatology and the worlds climates:New York (MacMillan Co.).

Smith, R.L., Enfield, D.B., Hopkins, T.S., and Pillsbury,R.D., 1971. The circulation in an upwelling eco-system: thePisco cruise: Investig. Pesquera, v. 35, p. 9-24.