41. CRUSTAL ACCRETION IN THE GULF OF CALIFORNIA:AN INTERMEDIATE-RATE SPREADING AXIS1

Paul T. Robinson, Department of Geology, Dalhousie University, Halifax, Nova Scotia, CanadaBrian T. R. Lewis, Department of Oceanography, University of Washington, Seattle, Washington

Martin F. J. Flower, Department of Geological Sciences, University of Illinois at Chicago Circle, Chicago, IllinoisMatthew H. Salisbury, Geologic Research Division, Scripps Institution of Oceanography, La Jolla, California

andHans-Ulrich Schmincke, Institut für Mineralogie, Ruhr-Universitàt Bochum, Bochum, West Germany

INTRODUCTION

An important objective of Deep Sea Drilling Project(DSDP) Leg 65 was to study crustal accretion at an oceanridge axis with an intermediate-spreading rate for com-parison with previously studied sections displaying slow-and fast-spreading rates. The southern Gulf of Californiawas selected for this purpose because the basement dis-plays high seismic velocities (comparable to those ob-served for Cretaceous basement in the western NorthAtlantic) and high ambient sedimentation rates, whichfacilitated penetration of zero-age basement. Four siteswere drilled, forming an axial transect immediately southof the Tamayo Fracture Zone (Figs. 1 and 2) and provid-ing a series of characteristic sections into the crust. Thischapter attempts to provide a brief synthesis of the re-sults from Leg 65, focusing particularly on the lithol-ogy, geochemistry, and paleomagnetic properties of thecored basement material. From these data, we presentan interpretation of the processes of magmatic evolu-tion and crustal accretion occurring at the Gulf of Cali-fornia spreading axis.

The Gulf of California represents a young riftedocean basin in which spreading is believed to have beeninitiated between three and four m.y. ago. During earlyTertiary time, prior to approximately 25 m.y. ago, theentire western margin of North America was marked bya subduction zone along which oceanic lithosphere wasbeing underthrust beneath the continent (Atwater,1970). Subduction of the Farallon Plate continued untilthe North American and Pacific plates collided in thevicinity of present-day San Francisco. The San Andreastransform fault then developed, as the triple junctionmigrated southward from the initial point of plate colli-sion. Approximately 4 to 4.5 m.y. ago, the spreadingcenter jumped inland, causing Baja California to beginrifting away from mainland Mexico.

The present structure and bathymetry of the Gulf ofCalifornia express this evolving tectonic pattern and inparticular reflect a northward transgression from a pre-dominantly dilating to a predominantly transform tec-tonic regime. The crest of the East Pacific Rise extendsnorthward more or less continuously to the TamayoFracture Zone at the mouth of the Gulf. North of the

30° N

25° N

Deeper than 1500 m -

Spreading axis andtransform faults

: Pacific Ocean

4(EastPacific Rise

115°W 110°W

Lewis, B. T. R., Robinson, P., et al., Init. Repts. DSDP, 65: Washington (U.S. Govt.Printing Office).

Figure 1. The Gulf of California, showing the Tamayo Fracture Zoneand region investigated by drilling at Site 474 (Leg 64) and Sites482, 483, and 485 (Leg 65).

Tamayo Fracture Zone, the Rise is broken into a seriesof small spreading centers offset by increasingly fre-quent transform faults oriented obliquely to the axis ofthe Gulf (Fig. 1). The spreading centers are expressedbathymetrically as deep basins which are partly filledwith variable thicknesses of sediment derived from theadjacent land masses. New oceanic crust is currently be-ing formed along the crest of the East Pacific Rise andalong the short spreading segments within the Gulf it-self. Axial spreading in the Gulf occurs at a rate of about6 cm/y.

BASEMENT TOPOGRAPHYDetailed surveying across the northern end of the East

Pacific Rise during the International Phase of OceanDrilling (IPOD) site surveys revealed a pattern of alter-nating ridges and flat-floored valleys, symmetrically dis-posed along opposite sides of the Rise crest (Fig. 3)

739

23°40'N

23°20'N

23°00'N

22°40'N

I r

MEXICO

109°00'W 108°00'W

Figure 2. Location of Site 474, drilled on Leg 64, and Sites 482 through 485, drilled on Leg 65. (Contours shown in corrected m.)

NW

- 4

Figure 3. Seismic reflection profiles across the East Pacific Rise in the vicinity of 23°N. Profile 8 lies approximately along the Leg 65 transect. Profiles are arranged sequentially fromnorth (12) to south (2) (after Lewis et al., this volume).

P. T. ROBINSON ET AL.

(Lewis et al., this volume). The ridges are discontinuouslinear features extending parallel to the rise crest for dis-tances of 5 to 30 km. They range from about 3 to 10 kmin width and are asymmetrical in cross section, typicallyhaving a relatively steep scarp facing the ridge axis and amore gradual outward-facing slope. The ridges are sepa-rated by broad, flat-floored valleys of similar width.The valleys are underlain by variable thicknesses ofsediments which show relatively little evidence of tec-tonic disturbance. The basement/sediment interface issmooth, tilts slightly away from the rise axis, and showsonly minor evidence of faulting (Fig. 3).

In the area at the mouth of the Gulf, the axis of theEast Pacific Rise exhibits considerable variation in to-pography. Immediately south of the Tamayo FractureZone, the axis is marked by a distinct valley whichbecomes narrower and shallower to the south and even-tually passes into an axial high. The ridge axis immedi-ately north of the Tamayo Fracture Zone is also a topo-graphic high (Fig. 3). Along this segment of the EastPacific Rise, the axial highs are irregular, somewhatrounded features, rather than steep-sided horsts such asthose described along the Reykjanes Ridge (e.g., Tal-wani et al., 1971).

The topography of the axial zone was also observeddirectly during submersible studies of the Tamayo Frac-ture Zone and the rift valley to the south (CYAMEX,1980). This work revealed that within the axial zone, theridges consist primarily of pillow lavas, whereas the in-tervening valleys are floored by massive, ponded lavaflows with a light, intermittent sediment cover.

DRILLING RESULTS

Basement Lithology

During Leg 65, three principal sites (482, 483, and485) were drilled on the flanks of the ridge at 23 °N,together with a less successful site (484) in the TamayoFracture Zone (Fig. 2). These sites, together with onesite (474) drilled in older crust on Leg 64 (Curray,Moore, et al., in press), represent an axial transect alongan approximate spreading flow line (Fig. 1). Sites 482and 485 lie east of the ridge axis, whereas Sites 474 and483 lie to the west. The basement ages for these sites areapproximately 0.5 m.y at Site 482, 0.74 m.y. at Site 485,1.5 to 2.0 m.y. at Site 483, and 4 m.y. at Site 474. All ofthe sites were located in sediment-filled valleys in orderto stabilize the drill string while spudding into base-ment. Several basement holes were drilled at Sites 482and 483, but only a single hole penetrated into the base-ment at each of the other two sites (Fig. 4).

The basement sections at Sites 474 and 483 are similarand consist of an upper sequence of massive basalt unitsand interlayered sediments underlain by a lower se-quence of pillow basalts and thin massive flows (Table1). The upper basement sequence is about 41 metersthick at Site 474 and about 50 meters thick at Site 483.At Sites 482 and 485, only the upper sequence was pene-trated, and this had a minimum thicknesses of approxi-mately 80 and 170 meters, respectively, at the two sites.

The sediments interlayered with the upper massive unitsare lithologically similar to those overlying the acousticbasement and consist largely of hemipelagic materialand fine-grained turbidites. The average rates of sedi-ment accumulation above the basement range from aminimum of 7.1 cm/1000 y. at Site 483 to a maximumof 33.4 cm/1000 y. at Site 485, but the rates for individ-ual intervals were often much higher (Benson, this vol-ume). The sedimentation rates during construction ofthe upper basement sequence are assumed to have beensimilar.

The massive basalts in the upper basement sectionrange from about 2 to 26 meters in thickness based ondrilling rate logs and downhole geophysical logs. Thethickest units may comprise several cooling units al-though internal cooling breaks were rarely recovered.The basalts are aphyric to sparsely phyric, fine to me-dium grained, and generally display intersertal to sub-ophitic textures. Quenched glassy margins are often pre-served at both the upper and lower contacts. Some ofthe very thick massive basalts in Hole 485A have coarse-grained, glass-free interiors with ophitic textures. Mostof the massive units are nonvesicular, but a few (e.g.,Unit 7, Hole 483 and Unit 2, Hole 483A) become vesicu-lar toward the margins.

The lower basement sequence penetrated at Sites 483and 474 consists of pillow and massive basalts with rela-tively little interlayered sediment. The massive units aresimilar to those in the upper basement sequence but areusually thinner, ranging from about 2 to 12 meters inthickness. They consist of fine- to medium-grained,aphyric to moderately plagioclase-phyric basalt with in-tersertal to subophitic textures. Most contain smallpatches of interstitial glass, partly altered to smectite.Upper glassy margins are often present, but lower sel-vedges, if they existed, were not recovered. The vesiclesare small and sparse, always making up less than 1% ofthe total volume of the rock. The interlayered pillowbasalt sequences are up to about 30 meters thick andshow no discernable breaks, although thin sedimentarylayers separate pillow from massive units in all cases(Fig. 4). The pillow lavas are characterized by abundantcurved glassy margins and by very fine-grained, oftenquenched, mesostases. The pillow and massive units aremineralogically and chemically distinct (e.g., at Site 483)but appear to be components of related magmatic cycles(Flower et al., this volume).

The pillow basalts are clearly extrusive in origin.Most of the massive units at Sites 482, 483, and 485 arealso interpreted as lava flows because of the lack of ob-vious intrusive contacts, the common presence of glassymesostases, and the wide lateral extent of individualunits. Indurated sediments directly overlying some ofthe massive units have undergone moderate recrystalli-zation, leading to decreased porosity, the in-situ growthof analcite, and the transformation of detrital smectitesinto saponite and mixed-layer chlorite-smectite (Ranginet al., this volume). Oxygen-isotope data from carbon-ate material in these zones suggest maximum paleotem-peratures of about 65°C. However, the 18O content of

742

CRUSTAL ACCRETION

the carbonates could reflect isotope exchange betweenrock and water rather than baking along the contacts(Rangin et al., this volume).

A likely depositional contact is recorded at the top ofLithologic Unit 7b in Hole 483. The indurated sedimentimmediately above this contact is clearly imprinted withthe vesicular texture of the underlying massive basalt,providing strong evidence for later deposition of thesediment. The base of the succeeding basalt unit, Unit7a, is also well preserved and consists of a thin (0.5 cm)layer of wrinkled, devitfified glass. The wrinkled con-tact also implies an eruptive origin for this unit. In con-trast, clear evidence for an intrusive origin is observed atSite 474 in the massive units of the upper basement se-quence, which is distinctly younger than the underlyingpillow sequence (1.8 vs. 3.2 m.y) (Saunders, this vol-ume). At Site 483, a shorter time hiatus is associated witha marked chemical change between the lavas of the up-per and lower basement sequences. A few massive unitsat Site 485 are tentatively interpreted as sills because oftheir coarse grain size. Units 4 and 5 in Hole 485A areboth relatively thick and have groundmass crystals up to1 cm in length. The sedimentation rates were probablygreatest at Site 485 (about 33 cm/1000 y.). For magmat-ic emplacement in this type of environment, the distinc-tion between flows and sills may be largely semantic:flows that erupted onto unconsolidated sediments mayhave burrowed laterally into the sediment to producemassive sill-like bodies, chilled above and below againstthe sediment. Such processes have been documented inthe Columbia Plateau, where flows have spread into sed-imented lakes (Schmincke, 1967).

Chemical and paleomagnetic studies of the core ma-terial have permitted detailed subdivision of the base-ment into very distinct stratigraphic units. The magma"types" established on shipboard (and refined by Flow-er et al., this volume) represent compositional groupingswhich, for the most part, are consistent with groupingsbased on stable magnetic inclinations (Day, this volume).The average compositions of these magma types are giv-en in Table 2.

Intrasite Stratigraphic CorrelationMultiple holes were drilled at Sites 482 and 483, pro-

viding an opportunity to monitor tectonic displacementsin the basement. The excellent between-hole correlationestablished at these sites provides strong evidence forcoherent upper basement structure, apparently undis-turbed by faulting. At Site 482, individual lithologicunits can be correlated petrographically, chemically, andmagnetically between three holes (482B, C, and D), lo-cated about 1 km apart along a flow line roughly per-pendicular to the axis of spreading (Fig. 4A). Eightlithologic units are recognized in Hole 482B, two inHole 482C, and four in Hole 482D. The uppermost unitin each hole is a massive basalt of Magma Type A(Flower et al., this volume). In Hole 482D Unit 2 alsoconsists of a Magma Type A basalt, but the unit isseparated from Unit 1 by a thin sediment layer, whereasin Hole 482C, Unit 2 consists of a Magma Type Bbasalt. This compositional sequence is the same in both

Holes 482B and 482D except that Types A and B areseparated by massive units of Type F (Flower et al., thisvolume).

The correlation of lithologic units and magma typesbetween holes is supported by stable paleomagnetic in-clination data (Day, this volume). Lithologic Units la(Hole 482B), lb (Hole 482C), and 2 (482D), all ofMagma Type A, have the same stable inclinations towithin 1 standard deviation. Unit 1 in Hole 482D, on thebasis of a single inclination measurement of 24.0°, istentatively correlated with Unit la of Hole 482C, whichhas an average inclination of 29.0° ± 1.6°. Litholog-ic Units 3 (Hole 482D) and lb (Hole 482B), both ofMagma Type F, also have the same average inclinationsto within one standard deviation. Units 2 (Hole 482C)and 4 (Hole 482D), both of Magma Type B, also haveidentical average paleomagnetic inclinations. These twounits correlate compositionally and magnetically withUnit 2 in Hole 482B, although the latter displays morevariability in inclination. The upper 50 meters of base-ment at Site 482 thus reflect a marked lithologic, chemi-cal, and magnetic coherence over at least 2 km. Thislateral continuity is far greater than that observed atother DSDP sites, especially those in the Atlantic (Halland Robinson, 1979). The correspondence of stable in-clination data to predicted dipole values in the Leg 65sequences (Day, this volume) is a further indication ofthe lack of faulting or tectonic rotation during and sub-sequent to crustal formation. This lateral continuity isalso consistent with an extrusive rather than an intrusiveorigin for most of the massive units.

Similar correlations may be drawn between the holesdrilled at Site 483 (Fig. 4B). The upper four units inHoles 483 and 483B, for example, may be correlated onthe basis of lithology, chemical composition, and inter-calated sedimentary layers. Lithologic Unit 5 in Hole483, which belongs to Magma Type E, is equivalent to anonrecovered basalt interval intercalated with sedimentsin Hole 483B. Likewise, a nonrecovered basalt intervalrecorded by logging in Hole 483 corresponds to a pillowsequence (Magma Type I) in Hole 483B. At lower levelsin the section, several massive units of Magma Type Hmay be traced between holes. A stratigraphic synthesisof Site 483 thus provides excellent documentation oflateral continuity in the upper levels of the basementformed along this part of the East Pacific Rise.

PROCESSES IN AXIAL MAGMA SYSTEMSCrustal construction processes at spreading ridges are

reflected in the geochemical stratigraphy of the eruptedlavas. The observed geochemical patterns and the recog-nition of upper and lower basement sequences at theLeg 65 sites provide a basis for modeling and interpret-ing the processes of magmatic evolution which operateat the axial zone of accretion. Most current models ofmagma generation and evolution at spreading axes re-late chemical variation in basalts to variations in crustalspreading rate (e.g., Cann, 1974; Sleep, 1975; Flower,1980; 1981). Studies of crustal sections formed at theMid-Atlantic Ridge suggest that accretion at such slow-spreading axes is strongly episodic and is accompanied

743

P. T. ROBINSON ET AL.

West

I 483| | water£ D depth

3070 m

483B

Oi σmudline

£ .-ao c

j

waterdepth

3070 mMagmaType

EruptiveCycle

50

100

150

EoVo

-Q

3CO

200

250

300

- 4

5

6

10

EastPacific

Rise

"T"

5 B

o.2 Cü —n __ _

0) __ _ _

(I)

L

M

Q.

αD

CO

o

Figure 4. Stratigraphic sections showing correlation of basement lithologic units, magma types, and eruptivecycles at Sites 482, 483, and 485.

744

CRUSTAL ACCRETION

East

•a 482A,BJj g water

Magma £ 13 depthType 'J 2998 m

•a 4 8 2 C

.2 ±: water| 3 depthj 2996 m

a-2 .t! water. C D depth Magmaj 3008 m Type

•a 4 8 5 A

£ .* water^ 3 depth MagmaJ 2981 m Type

A

C--4D--5A__6

7

Legend

hemipelágic clay

sand and silt

pillow basalt

diatomaceousooze

carbonateconcretions

no recovery

massive basalt

dolomite

Figure 4. (Continued).

(F-G)•

.üT--j-T

A-E

z —

745

P. T. ROBINSON ET AL.

Table 1. Correlation of lithologic units, cooling units, and magma types at Sites 482, 483, and 485.

Hole

482B

482C

482D

483

483B

483C

485A

LithologicUnit

Topa

(m)Basea

(m)Cooling

UnitsPhenocrystAssemblage

Core-Section(level in cm)

MagmaType b

1 Massive basalt 136.52 Massive basalt 158.0Sedimentary intercalation

Massive basaltMassive basaltMassive basaltMassive basaltMassive basalt

184.1193.5199.5201.5205.0

Sedimentary intercalation8 Massive basalt 224.7

1 Massive basalt2 Massive basalt

135.5157.0

1 Massive basalt 138.0Sedimentary intercalation2 Massive basalt 141.7Sedimentary intercalation3 Pillow basalt? 154.14 Pillow basalt? 169.61 Massive basalt 110.0Sedimentary intercalation2 Massive basalt 115.0Sedimentary intercalation3 Massive basalt 127.0Sedimentary intercalation4 Massive basalt 142.2Sedimentary intercalation5 Massive basalt 156.5Sedimentary intercalation6a Pillow basalt?Sedimentary intercalation6b Pillow basalt? 169.06c Massive basalt 186.56d Pillow basalt? 188.5Sedimentary intercalation7a Massive basalt 191.5Sedimentary intercalation7b Massive basalt 200.41 Massive basalt 110.0Sedimentary intercalation2 Massive basalt 111.3Sedimentary intercalation3 Massive basalt 133.0Sedimentary intercalation4 Massive basalt 137.6Sedimentary intercalation5a Pillow basalt 169.05b Pillow basalt 175.0Sedimentary intercalation6a Massive basalt 194.0Sedimentary intercalation6b Massive basalt 199.0Sedimentary intercalation6c Massive basalt 204.66d Massive basalt 206.2Sedimentary intercalation7a Pillow basalt 211.17b Pillow basalt 213.0Sedimentary intercalation8a Massive basalt 227.7Sedimentary intercalation8b Massive basalt 232.6Sedimentary intercalation8c Massive basalt 237.0Sedimentary intercalation9 Pillow basalt 249.5

1 Massive basalt 109.5

1 Massive basalt 153.5Sedimentary intercalation2 Massive basalt 180.5Sedimentary intercalation3 Pillow basalt 201.5Sedimentary intercalation4 Massive basalt 212.0Sedimentary intercalation5 Massive basalt 239.5Sedimentary intercalation6 Massive basalt 277.5Sedimentary intercalation7 Massive basalt 293.6Sedimentary intercalation8 Massive basalt 314.5

158.0 1-4 Aphyric174.7 10-11 Plagioclase

186.2 12 Plagioclase199.5 13 Plagioclase201.5 14 Plagioclase-Clinopyroxene205.0 15 Plagioclase-Clinopyroxene-Olivine220.3 16 Plagioclase-Clinopyroxene-Olivine

229.0 17 Aphyric

157.0 1-2 Aphyric184.0 3-7 Plagioclase-Clinopyroxene-Olivine

139.6 1-2 Aphyric

154.0 3 Aphyric

169.6 4-10 Aphyric186.5 11-16 Plagioclase

111.0 1 Plagioclase-Olivine

127.0 2a, b Aphyric

135.8 3 Aphyric

145.0 4 Aphyric

160.1 5 Aphyric

186.5 6-27 Plagioclase-Clinopyroxene-Olivine188.5 28 Plagioclase-Olivine190.0 29-30 Plagioclase-Clinopyroxene-Olivine

200.3 31-34 Plagioclase-Olivine

10-7, 8 to 15-1, 11515-1, 115 to 17-2, 150

18-1, 90 to 18-2, 10719-1, 60 to 20-2, 8320-2, 83 to 20-3, 13020-3, 130 to 21-3, 5021-3, 50 to 23-1, 30

24-1, 25 to 24-3, 130

9-1. 60 to 12-1,012-1, Oto 15-4, 145

8-1, Oto 8-2, 30

9-1, 15 to 10-3, 60

24-1, 5 to 26-1, 40

A, FA

BCDED

D

AF, B

A

F, E

10-3, 65 to 12-1, 13012-1, 130 to 13-3, 3513-4, 5 to 13-4, 95

14-1, 5 to 15-2, 128

15-2, 128 to 16-3, 12

17-1, 18 to 17-3, 24

18-4, 130 to 19-1, 10

unrecovered

20-1, 5 to 22-4, 6022-4, 60 to 23-2, 2023-2, 20 to 23-2, 150

FBA

B

C

D

E

F?

FF-Plagioclase

F

204.5111.3

127.0

136.4

146.8

175.0184.6

197.3

204.6

206.2210.5

213.0227.7

232.4

236.9

249.5

267.0

114.0

159.4

184.0

202.0

226.2

270.4

294.0

298.7

328.5

351

2

3

4

6-1011-22

23

24

2526

2728-56

57

58

59

60-82

1

1-5

7

7

8

9

10

11

12

PlagioclasePlagioclase-Olivine

Aphyric

Aphyric

Aphyric

Plagioclase-Olivine-ClinopyroxenePlagioclase-Olivine-Clinopyroxene

Aphyric

Aphyric

AphyricPlagioclase-Olivine-Clinopyroxene

Plagioclase-OlivinePlagioclase-Clinopyroxene-Olivine

Plagioclase-Clinopyroxene-Olivine

Plagioclase-Olivine

Plagioclase-Olivine

Plagioclase-Olivine-Clinopyroxene

Aphyric

Plagioclase-Olivine-Clinopyroxene

Plagioclase

Plagioclase

Plagioclase-Olivine

Plagioclase

Plagioclase-Olivine

Plagioclase

Plagioclase

26-1, 50 to 26-3, 1502-7, Oto 3-1, 75

3-1, 75 to 4-7, 20

7-1, 7 to 7-3, 75

8-1, 10 to 9-1, 44

12-1, Oto 13-1, 1013-1, 10 to 15-1, 6

17-1, Oto 17-3,45

18-1, Oto 19-1, 65

19-1, 65 to 19-2, 8519-2, 85 to 20-2, 65

20-2, 130 to 20-3, 3021-1, Oto 24-1, 145

24-1, 145 to 25-2, 5

25-2, 20 to 26-1, 150

26-2, 5 to 29-1, 65

29-1, 65 to 32-3, 82

4-2, 47 to 4-5, 120

11-3, 55 to 13-1, 140

17-1, 0 to 18-1, 58

22-1, Oto 22-1, 3

23-1, 50 to 26-1, 20

29-1, Oto 33-2, 95

34-1, 56 to 35-6, 55

36-3, 64 to36-cc , 31

38-2, 2 to 39-5, 60

HA

B

C

D

IF

H

H

HH

H?J

K

K

K

L, M

A

A-E

Z

7

H

I

J

K

L

a Calculated from core log and corrected for spacers.b See Table 2.

746

CRUSTAL ACCRETION

Table 2. Average magma type compositions3 at Sites 482, 483, and485 (after Flower et al., this volume).

MagmaType

Site 482

ABCDEFüH

Site 483

BCDEFHIJKLMz

Site 485

ABCEH1JKLZ

SiO2

50.5850.8551.2050.3550.6750.4750.7950.55

51.0450.7849.8451.0950.3450.3350.0650.4050.3650.4250.4250.87

50.0050.5050.6250.8350.2650.1050.6349.9550.4449.97

TiO2

1.821.351.281.561.891.442.151.43

1.241.391.011.851.862.181.581.731.821.811.621.33

1.542.092.122.042.242.051.792.481.872.33

A12O3

14.7114.9414.7015.5414.6715.1414.4014.96

15.2514.7716.8716.4715.0814.5415.7015.0414.6714.9115.5914.71

17.3613.9313.8314.1814.8314.7114.5919.6115.0015.47

FeO

10.259.94

10.039.88

10.3210.1211.4010.05

9.179.978.278.48

10.7211.4110.3610.2510.7110.529.639.99

9.3711.9312.1111.7211.4011.0710.689.85

10.7412.09

MnO

0.210.180.170.180.180.180.190.18

0.160.170.160.150.200.190.200.210.190.230.200.17

0.130.230.230.210.170.190.180.190.190.19

MgO

7.968.208.167.427.858.046.878.31

8.047.869.097.447.147.287.227.567.677.607.647.75

7.136.836.897.127.077.827.786.417.687.85

CaO

11.6311.9312.0012.2511.5411.9511.0211.90

12.3112.1712.3810.9911.8010.9712.0711.9311.6311.6011.9412.37

11.2511.6211.3811.1110.7611.1211.487.55

11.278.87

Na2O

2.582.462.302.652.672.482.862.46

2.642.732.273.192.612.802.622.652.722.652.682.65

3.082.602.562.542.932.622.603.682.582.98

K2O

0.090.040.060.040.060.060.110.05

0.060.060.040.160.090.100.060.080.090.110.120.05

0.030.090.070.060.140.130.060.060.060.05

P2O5

0.160.110.110.140.170.120.200.11

0.090.100.070.180.170.210.130.150.160.160.150.10

0.100.190.190.190.200.180.150.210.160.20

Note: FeO* = total iron as FeO.a Values given in wt.%; normalized and corrected for carbonate.

by extensive tectonic disruption of the crust (Hall andRobinson, 1979; Flower and Robinson, 1979; 1981b).The resulting crustal segments are composed of overlap-ping lenses with little lateral continuity. Compositionalvariations in these lenses reflect repetitive magmaticcycles, usually passing from most evolved at the baseto most primitive at the top (Flower and Robinson,1981b). In many cases, the magmas appear to have un-dergone extensive crystal fractionation in separate sub-rift magma chambers.

Flower (1980; 1981) has observed that basalts formedat slow-spreading axes may be characterized by the mas-sive accumulation of calcic plagioclase and by wide-spread phenocryst reaction morphologies suggestive ofpolybaric fractionation regimes, while those generatedalong fast-spreading axes appear to have formed in lowpressure (isobaric) systems. Kinematic and thermal mod-els (e.g., Sleep, 1975; 1978) also imply changes relatedto spreading rate, specifically, a tendency toward iso-baric, open-system fractionation along fast-spreadingaxes and polybaric, closed-system fractionation alongslow-spreading axes (Flower 1980; 1981; Nisbet andFowler, 1978). The physical and chemical effects ofthese differences on eruptive magma chemistry havebeen discussed by Sparks et al. (1980), Stolper andWalker (1980), O'Hara (1977), and O'Hara and Math-ews (1981), and provide a perspective from which toview the results from the Gulf of California.

The compositional variation of the Leg 65 basalts isintermediate in type between that observed along fast-spreading segments of the East Pacific Rise and Nazca

Plate and the slow-spreading Mid-Atlantic Ridge (Fig.5). Oxide and trace element studies and least-squaresmass balance models strongly suggest that some of themassive and pillow units recovered (e.g., Magma Types482-D; 483-D, -F, -H, and -K; 485-A, -E, -H, and -I)display evidence of incipient, and in a few cases, sub-stantial plagioclase accumulation (Flower et al., this vol-ume). This accumulation is evident mainly from com-positional variations within magma types and appearsto be superimposed on a generalized "cotectic" liquidfractionation trend represented by variations in glasscomposition (Flower and O'Hearn, this volume) and bytypical "fast-spreading" whole-rock variation patterns(e.g., Clague and Bunch, 1976). The more extensive pla-gioclase accumulation in basalts formed at slow-spread-ing axes tends to obscure liquid-type trends, whereas theintermediate character of the Gulf of California varia-tion pattern allows both effects to be discerned.

It is reasonable to interpret geochemical variations inthe Leg 65 basalts in terms of separate fractionation epi-sodes involving simple phenocryst redistribution in more-or-less cogenetic parent magmas (Flower et al., thisvolume). However, close scrutiny of the petrographicand chemical characteristics of the Leg 65 basalts leadsto some important constraints on the extent to whichopen- vs. closed-system fractionation may operate inthis environment. In comparatively rare phyric lavas,the phenocrysts of clinopyroxene and plagioclase arefrequently resorbed, and the plagioclase shows oscilla-tory zoning. Moreover, basalts with chemical composi-tions reflecting the most extensive accumulation of pla-gioclase are aphyric or, at most, very sparsely phyric.Flower et al. (this volume) interpret these lavas as "cryp-tocumulates," resulting from accumulation of plagio-clase during temporary stagnation of the magma supply,followed by resorption of plagioclase during reheatingas a consequence of mixing with newly injected primi-tive magma. This hypothesis is supported by two linesof evidence: (1) the least phyric "cryptocumulates" arethe most MgO-rich (e.g., Magma Types 483-D; 485-A),and (2) the major and trace element variation withinthese magma types (Fig. 5) is strongly suggestive of mix-ing (as opposed to fractional crystallization).

In summary, it is tentatively concluded that fraction-ation of magma ascending beneath this portion of theaxis of the East Pacific Rise occurs neither in a singlecontinuously evolving, steady-state magma chamber norin a simple, linear sequence of discrete, closed systems,We infer that construction of any given vertical sectionof the crust involves the operation of several separatesystems, each of which may represent a steady-statesystem during its own finite existence. Although it is dif-ficult to "invert" geochemical parameters in the erupt-ed products of such systems to define cyclic evolution(O'Hara and Mathews, 1981), real constraints can bederived from stratigraphic, and hence chronologic, rela-tionships among the different eruptive units. For exam-ple, the chemical stratigraphy at Site 483, based on vari-ations in Zr-Ti, Zr-Y, and Ti-Y element pairs with Mg/Mg + Fe2 + , reflect a distinct cyclic pattern defined bysuccessive chemical types or groups of types. Four cy-

747

P. T.ROBINSON ETAL.

14.0

10.0MgQ (wt.%)

25.0

23.0 -

21.0 -

,19.0 -

17.0 -

15.0 -

13.010.0 8.0 7.0

MgO (wt. %)

Figure 5. Oxide variation at Site 483 compared with variations in crust formed at a slow-spreading ridge(Sites 332, 417, and 418 on the Mid-Atlantic Ridge) and a fast-spreading ridge (Site 319 on the NazcaPlate). A. FeO* vs. MgO; B. A12O3 vs. MgO. (Letters represent magma types at Site 483. Data fromFlower et al. [this volume; 1977] and Rhodes et al. [1976].

cles are recognized consisting of Chemical Types E, I-F-H, J-K, and L-M (Flower et al., this volume). Eachcycle comprises a series of massive units succeeded bypillow basalts. The small but consistent differences inlow-KD element abundances between cyclic associations(Fig. 6) may be attributed to one or more of the fol-lowing: (1) a heterogeneous mantle source, tapped vari-ably in time and space; (2) changes in the percent of par-tial melting for successive batches of mantle-derivedmagmas; and (3) differences in the longevity of steady-state (periodically refilled and depleted) magma systems

(O'Hara and Mathew, 1981) and thus, in the durationof cyclic fractionation.

At Sites 474, 483, and, to a lesser extent, 485, a dis-tinct compositional change marks the transition fromthe lower to upper basement sequences (Figs. 5 and 6).At Site 474, this transition corresponds to a significanttime hiatus (Saunders, this volume) and at Site 483, itcorresponds to a paleomagnetic polarity reversal (Day,this volume). The first basalt unit in the upper basementsection at Site 483 (Magma Type 483-D) displays unam-biguous evidence of plagioclase accumulation, magma

748

CRUSTAL ACCRETION

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Figure 6. Trace element variation at Site 483 according to magma type. A. Ti/Y vs. Zr/Y; B. 1/Ni vs. Zr/Ni; C. 1/Sr vs. Zr/Sr. (Deflection of varia-tion from straight-line trends is attributable in A to source-related processes (such as variable partial melting or mantle heterogeneity) and/or tofractionation in a steady-state "open" system. The deflections in B result from mafic phase fractionation. Those in C result from plagioclasefractionation or accumulation. Straight-line variation for all parameters is consistent with binary mixing [cf. Langmuir et al., 1978].) (Data fromFlower et al., this volume.)

mixing and reheating, and possibly the inception of anew cyclic phase. At Site 474, the upper massive unitsmay represent later off-axis volcanism (Saunders, thisvolume), perhaps related to a renewed phase of rifting.At the Leg 65 sites, we interpret the upper basementunits as widespread flows that originated at the spread-ing axis.

CRUSTAL ACCRETION PROCESSES

Slow-spreading ridges such as the Mid-Atlantic Ridgeare usually characterized by distinct and continuous ax-ial rift valleys. Two models have been proposed to ex-plain the continued existence of such rift valleys duringcrustal accretion, the "suction" model of Lachenbruch(1976) and the "necking" model of Tapponier andFrancheteau (1978). Both models assume that the mor-phology of the ridge axis is a steady-state phenomenonand that it reflects a passive response to the rifting ofrigid plates.

It is clear from ridge transect profiles that the axialvalley on the East Pacific Rise at 23 °N is not a continu-

ous feature since it passes into an axial high both to thenorth and south. If the spreading rate is assumed to beconstant, the morphologic variations along the ridgeaxis must reflect either differences in the rate of magmasupply or tectonic disturbance. The axial highs on thissegment of the East Pacific Rise differ markedly in mor-phology from the horst-like features found on the Reyk-janes Ridge (Talwani et al., 1971), and there is no evi-dence that they have formed by faulting or uplift in theaxial valley.

If the axial valley at the Rise crest were a steady-statefeature, the upper crust should everywhere consist ofinterlayered massive basalts and sediments similar to thesections drilled on Leg 65, and the linear ridges on theflanks of the Rise would simply mark upfaulted crustalsegments. Although the ridges have not been sampledby drilling, several lines of evidence strongly suggestthat they are underlain by crust of a different characterfrom that drilled in the sediment-filled valleys: (1) Theridges are irregular, asymmetric, somewhat rounded fea-tures, whereas the upper basement surface beneath the

749

P. T. ROBINSON ET AL.

valleys is always smooth and regular (Fig. 3); (2) Dredg-ing on the flanks of the ridges has recovered glass-richpillow lavas without the interlayered sediments foundby drilling in the valleys; (3) The interlayered massivebasalts and sediments drilled on Leg 65 presumably ac-cumulated in topographic lows, probably by ponding inthe axial valley. Since ponding cannot take place wherethe Rise axis is marked by a topographic high and dredg-ing indicates that the axial highs are composed largely ofpillow piles, the crustal section must vary along the Riseaxis; (4) If the axial valley south of the Tamayo FractureZone is a steady-state feature, as proposed for the Mid-Atlantic Ridge axis, and all crustal accretion occurs with-in the valley, constructional topographic features shoulddisappear once the crust has migrated out of the axialvalley and a uniform layer of sediment would be ex-pected to form above the basement. Instead, the Riseflanks at 23 °N are marked by a highly symmetrical pat-tern of alternating basalt ridges and sedimented valleys(Lewis, 1979).

From these considerations and the geochemical evi-dence discussed above, we conclude that crustal accre-tion along this segment of the East Pacific Rise can bestbe explained in terms of episodic processes involvingvariable rates of magma supply, both from the uppermantle to the magma reservoir systems and from the lat-ter to the surface of the ocean floor. At present, thelongitudinal distance between axial highs and lows is es-timated to be about 90 km (Lewis, 1979). If these fea-tures reflect a propagating magma source (Lewis, 1979),they suggest a longitudinal migration rate for eruptiveactivity of about 50 cm/y., which corresponds to an av-erage periodicity of 0.35 m.y.

Our model for the evolution of this ridge segment issummarized in Figure 7. During periods of low magmasupply, continued spreading produces an axial valleysuch as that observed in the Leg 65 transect (Fig. 7A). Insome cases, this rifting may be accompanied by normalfaulting and uplift of the ridge flanks. Where this occursthe upper basement surface in the adjacent valleys maybe tilted somewhat away from the rift axis. In othercases, where there is no evidence of tilting, the riftingprobably occurred without significant uplift and fault-ing. The resulting crustal sections formed during peri-ods of low magma supply consist of pillow lavas andthin massive flows, corresponding to the lower basementsequences penetrated at Sites 474 and 483. Sediments ac-cumulate rapidly in the axial rift, in part by turbiditycurrent deposition. Continued spreading initiates a newrifting episode within the axial valley, perhaps related toa longitudinally propagating magma source, allowing aseries of relatively rapid eruptions to build an axial ridge(Fig. 7B). During construction of these axial ridges, theeruptions are episodic on a fine scale with variableamounts of magma being emplaced during each epi-sode. Relatively voluminous eruptions produce massiveflows that pond in the axial low to either side and spreadacross the sediment-covered surface (Fig. 7C). In thewaning stages of the more voluminous pulses and dur-ing less voluminous eruptions, pillow basalts accumu-late in quenched mounds close to the eruptive source.Finally, as the magma supply to the axial zone again

diminishes, a new rift forms, starting the cycle anew(Figure 7D).

In this model, the massive units in the upper basementsequences at the Leg 65 sites are related to the sameeruptive episodes that produced the linear axial ridges.This interpretation is supported by the evidence for atime hiatus and a marked geochemical transition at theboundary between the upper and lower basement se-quences. Additional evidence for small scale periodicityresulting in relatively voluminous massive flows andsmall mounds of pillows comes from the following: ob-servations of the axial topography using submersibles(e.g., CYAMEX, 1981), from the chemically definederuption/fractionation cycles (massive flows followedby pillow sequences) drilled on Leg 65, and from paleo-magnetic and sedimentary indications of episodic crus-tal construction.

An episodic magma supply is also interpreted frombasement stratigraphy in the Atlantic Ocean (e.g., Flow-er et al., 1977; Byerly and Wright, 1978; Flower andRobinson, 1979) and from comparative studies of theFAMOUS and AMAR segments of the Mid-AtlanticRidge (Bryan and Moore, 1977; Ballard et al., 1978).Comparisons of chemical and lithologic stratigraphyand longitudinal variations in morphology between in-termediate-spreading rate segments of the East PacificRise and the slow-spreading Mid-Atlantic Ridge lead totwo contrasting dynamic models. At the faster-spread-ing axis, accretionary processes reflect both large andsmall scale variations in magma supply. The morphol-ogy of the ridge axis varies longitudinally, probably inresponse to a propagating magma source. Accreted crus-tal segments are laterally continuous and tectonically un-disturbed. At the slow-spreading axis, small scale fluc-tuations in magma supply are also characteristic, but theabsence of large scale variations allows perpetuation ofan axial rift. Crustal accretion takes place by subsidenceand the periodic burial of eruptive lenses under newlava. These contrasting tectonic patterns are evident pri-marily from the distinctive longitudinal character of EastPacific Rise vs. Mid-Atlantic Ridge topography and theevidence for iterative tectonic rotation in slow-spread-ing, as opposed to faster-spreading crust (Hall and Rob-inson, 1979; Flower and Robinson, 1979; 1981a; 1981b).This model is further supported by the well-known rela-tionship between ridge topography and spreading rate(e.g., Cann, 1974) whereby fast-spreading axes are in-variably characterized by their lack of rifts in contrast toslow-spreading axes.

To summarize, the geometric and temporal characterof magma supply and fractionation systems in the 23 °Nsegment of the East Pacific Rise are intermediate incharacter between those predicted for fast-spreading sec-tions of East Pacific Rise and the slow-spreading Mid-Atlantic Ridge axis. The Gulf of California magma sys-tems resemble those of the Mid-Atlantic Ridge in beingtransient, possibly polybaric, and in giving rise to pla-gioclase-cumulate magmas. In contrast, the emplace-ment of magma appears to take place under relativelystable tectonic conditions, probably more typical offaster-spreading environments in which tectonic adjust-ments to episodic spreading are relatively infrequent.

750

CRUSTAL ACCRETION

Legend:

] Pillow basalt

Massive basalt

l. .•. I Sediments

Figure 7. Schematic diagram illustrating model for crustal accretion along the East Pacific Rise axis at 23 °N. A. Rift-ing during period of relatively low magma supply produces axial valley. B. Sedimentation in axial valley is accom-.panied by renewed volcanism along rift axis. C. Rapid magma production and emplacement builds axial high com-posed largely of pillow lavas. Massive flows and sills are emplaced on flanks of axial high, forming upper basementsection composed of interlayered sediments and massive basalts. D. Renewed rifting during period of low magmasupply forms new axial valley, starting the cycle again.

ACKNOWLEDGMENTS

We thank the captain and crew of the D/V Glomar Challenger fortheir efforts in making Leg 65 a success. Claude Rangin providedmany lucid descriptions of features observed during submersible stud-ies of the Rise crest, and James Mehegan assisted with the drafting ofseveral of the figures.

REFERENCES

Atwater, T, 1970. Implications of plate tectonics for the Cenozoictectonic evolution of western North America. Geol. Soc. Am.Bull., 81:3513-3536.

Ballard, R. D., Atwater, T., Stakes, D., et al., 1978. AMAR 78 pre-liminary results IV. AMAR rift valley—Evidence for cycles in theevolution of rift valleys. Trans. Am. Geophys. Union, Eos, 59:1198-1199.

Byerly, G., and Wright, T. L., 1978. Origin of major-element chemi-cal trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge. /. Vol-canol. Geotherm. Res., 3:229-279.

Bryan, W. B., and Moore, J. G., 1977. Compositional variation ofyoung basalts in the Mid-Atlantic Ridge rift valley near 36°5O'N.Geol. Soc. Am. Bull., 88:556-570.

Cann, J., 1974. A model for oceanic crustal structure developed. Geo-phys. J. R. Astron. Soc, 39:169-187.

Clague, D. A., and Bunch, T. E., 1976. Formation of ferrobasalt atEast Pacific midocean spreading centers. J. Geophys. Res., 81:4247-4256.

Curray, J. R., Moore, D. G., et al., in press, [nit, Repts. DSDP, 64:Washington (U.S. Govt. Printing Office).

CYAMEX Scientific Group, 1981. First manned submersible dives onthe East Pacific Rise, 21 N. (Project RITA): General results. Mar.Geophys. Res., 4:365-379.

Flower, M. F. J., 1980. Accumulation of calcic plagioclase in ocean-ridge tholeiite: An indication of spreading rate? Nature, 287:530-532.

, 1981. A thermal-kinematic model for ocean-ridge magmafractionation. J. Geol. Soc. Lond., 138:695-712.

Flower, M. F. J., and Robinson, P. T., 1979. Evolution of the'FAMOUS' ocean ridge segment: Evidence from submarine anddeep drilling investigations. In Talwani, M., et al., (Eds.), DeepDrilling Results in the Atlantic Ocean: Ocean Crust (Vol. 2):Washington (American Geophysical Union), Maurice Ewing Ser.,pp. 314-330.

, 1981a. Basement drilling in the western Atlantic Ocean 1.Magma fractionation and its relation to eruptive chronology. J.Geophys. Res., 86:6273-6298.

., 1981b. Basement drilling in the western Atlantic Ocean 2.A synthesis of construction processes at the Cretaceous ridge axis.J. Geophys. Res., 86:6299-6309.

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P. T. ROBINSON ET AL.

Flower, M. F. J., Robinson, P. T., Schmincke, H.-U., and Ohnmacht,W, 1977. Magma fractionation systems beneath the Mid-AtlanticRidge at 36-37°N. Contrib. Mineral. Petrol., 64:167-195.

Hall, J. M., and Robinson, P. T., 1979. Deep crustal drilling in theNorth Atlantic Ocean. Science, 204:573-586.

Lachenbruch, A. H., 1976. Dynamics of a passive spreading center. J.Geophys. Res., 81:1883-1902.

Langmuir, C. H., Bender, J. F., Bence, A. E., et al., 1977. Petrogene-sis of basalts from the FAMOUS area, Mid-Atlantic Ridge. EarthPlanet. Sci. Lett, 36:133-156.

Lewis, B. T. R., 1979. Periodicities in volcanism and longitudinalmagma flow on the East Pacific Rise at 23°N. Geophys. Res.Lett., 6:753-756.

Nisbet, E. G., and Fowler, C. M. R., 1978. The Mid-Atlantic Ridgeat 37°N. and 45 °N. and some geophysical and petrological con-straints. Geophys. J. R. Astron. Soc, 54:631-660.

O'Hara, M. J., 1977. Geochemical evolution during fractional crystal-lization of a periodically refilled magma chamber. Nature,266:503-507.

0'Hara, M. J., and Mathews, R. E., 1981. Geochemical evolution inan advancing, periodically replenished, periodically tapped, con-tinuously fractionated magma chamber. J. Geol. Soc. Lond., 138:237-277.

Rhodes, J. M., Blanchard, D. P., Rodgers, K. V., et al., 1976. Petrol-ogy and chemistry of basalts from the Nazca Plate: Part 2—Majorand trace element chemistry. In Yeats, R. S, Hart, S. R., et al., In-it. Repts. DSDP, 34: Washington (U.S. Govt. Printing Office),239-244.

Schmincke, H.-U., 1967. Fused tuff and peperites in south-centralWashington. Geol. Soc. Am. Bull., 78:319-330.

Sleep, N. H., 1975. Formation of ocean crust: Some thermal con-straints. J. Geophys. Res., 80:4037-4042.

, 1978. Thermal structure and kinematics of mid-ocean ridgeaxes: Some implications to basaltic volcanism. Geophys. Res.Lett., 5:426-428.

Sparks, R. S. J., Meyer, P., and Sigurdsson, H., 1980. Density varia-tion amongst mid-ocean basalts: Implications for magma mixingand the scarcity of primitive lavas. Earth Planet. Sci. Lett.,46:419-430.

Stolper, E., and Walker, D., 1980. Melt density and the average com-position of basalt, Contrib. Mineral. Petrol., 74:7-12.

Talwani, M., Windisch, C. C , and Langseth, M. G., Jr., 1971. Rey-kjanes Ridge crest: A detailed geophysical study. J. Geophys. Res.,76:473-517.

Tapponier, P., and Francheteau, J., 1978. Necking of the lithosphereand the mechanisms of slowly accreting plate boundaries. /.Geophys. Res., 83:3955-3970.

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