2. SUMMARY OF THE GEOLOGY AND GEOPHYSICS OF THE EAST PACIFIC RISE IN THEVICINITY OF THE SIQUEIROS FRACTURE ZONE

Bruce R. Rosendahl, Department of Geology, Duke University, Durham, North Carolinaand

Leroy M. Dorman, Scripps Institution of Oceanography, La Jolla, California

ABSTRACT

The results of four expeditions to the Siqueiros area of the EastPacific Rise, including a pre-drilling site survey cruise, have beencombined to yield an excellent knowledge of the areal geology andgeophysics and some insight into the processes responsible for theproduction of sea-floor morphology, crustal structure, and chemicalvariations in basalts. These factors, combined with logistical andclimatical considerations, were ultimately responsible for both theselection of the area for crustal drilling and many of the targets andtactics employed on Leg 54.

INTRODUCTION

Prior to Leg 54 drilling on the East Pacific Rise, theSiqueiros area had been the target of no fewer than fourseparate expeditions of the Scripps Institution of Ocean-ography: Quebrada (1969), Siqueiros (1974), Cocotow(1974), and Deepsonde II (1976) — the IPOD site surveyfor Leg 54. The first three expeditions were discussed indetail by Batiza et al. (1977), Dorman (1975), Orcutt etal. (1975, 1976), Rosendahl (1976), and Rosendahl et al.(1976). Deepsonde II results have so far gone unpub-lished, and it is one of the goals of the present paper toremedy this situation. In the interests of both brevityand relevance, we shall use a format that integrates theresults of all of the above expeditions into categoriesthat bear directly on the Leg 54 drilling results. Henceour discussion of, say, morphology and structure repre-sents the combined results from the above and othersources, not specifically or exclusively those from Deep-sonde II. We hope this will provide the reader with acomprehensive overview of what was known about theregion prior to any drilling.

On another level, this report represents the geograph-ical "package of knowledge" available to the Leg 54scientists during their drilling operations. In this sensethe paper has a direct bearing on the methods used onLeg 54, providing both an historical perspective and ameans of evaluation. To this end, we have included asection on the strategies, targets, and recommendationsthat were made on the basis of the information pre-sented. Finally this paper, in combination with the fol-lowing chapters, represents the regional framework intowhich the Leg 54 results ultimately must be interwoven.

REGIONAL AND TECTONIC SETTINGThe geographic area dealt with by this paper and

targeted for drilling on Leg 54 is the Siqueiros region ofthe equatorial eastern Pacific, or "Survey Area PT-4"

in DSDP jargon. The area of interest is defined approxi-mately by Plate 1 (in jacket at back of volume) and in-cludes the axis and western flank of the East PacificRise (the Pacific plate) and the Siqueiros fracture zoneregion.

In terms of plate kinematics, the East Pacific Risehere represents the accreting edge of the Pacific andCocos plates. As nearly as we can determine, the spread-ing has remained uniform and symmetric with respect tothe rise axis for at least the past several million years,and the rate has remained essentially constant at about60 mm/y (half-rate) for the past 5 m.y. (Larson andChase, 1970; Herron, 1972). On the basis of the azi-muths of flow-line lineaments, it appears that thespreading direction is somewhat oblique to the rise axis,with both the Pacific and Cocos plates moving on anazimuth that is 15 to 20° north of a projected or-thogonal arrangement. This means that the true spread-ing rate here is probably a few mm per year higher thanindicated above.

The plate configuration and geometry before about 5m.y. ago are not well known, but there is no reason tobelieve that it deviates significantly from the above pat-terns during the interval of 5 to 10 m.y. On crust olderthan about 10 m.y., the Siqueiros fracture zone cannotbe identified morphologically (Truchan and Larson,1972), and Herron (1972) postulates a major reorganiza-tion of plate motions at about this time.

MORPHOLOGY AND STRUCTUREThe morphology of the Siqueiros region is essentially

that of an accreting plate boundary offset by one majorand at least one minor fracture zone. In addition, anumber of small- to moderate-sized seamounts aresuperimposed on the plate fabric (here "plate fabric"refers to the general north-south arrangement of faultblocks that constitute the sea-floor relief away from theimmediate vicinity of the ridge axis and which appear as

23

B. R. ROSENDAHL, L. M. DORMAN

rounded hills on shipboard echo-sounder records). Thebathymetric coverage of the region is unusually good(Figure 1) and has permitted us to produce a relativelydetailed and reliable contour chart (Plate 1). The re-mainder of this section will deal with the individualmorphologic features shown in Plate 1 and their rele-vance to the tectonic evolution of this part of the PacificOcean.

Rise Crest Province

The East Pacific Rise crest in this portion of thePacific Ocean is characterized by a 10- to 20-km-wideaxial block (Figure 2), which stands about 200 to 400meters above the surrounding sea floor (Anderson andNoltimier, 1973; Rosendahl, 1976). From about 8° 15' Nto 8°30' N, this axial block is relatively flat-topped andsymmetric, but further north it exhibits considerablevariability. Nonetheless, the minimum depth along thetop of the axial block is remarkably constant betweenthe Siqueiros and Clipperton fracture zones and prob-ably varies by less than about 70 meters (excluding theimmediate environs of the small unnamed fracture zoneat 9° N, discussed later). Lonsdale (1977) suggests agradual and systematic shoaling southward toward the

10°N

Siqueiros fracture zone, but the work of the presentauthor (e.g., Rosendahl efal., 1976) does not supportsuch a relationship. Regardless, the constancy in depthminima and the preservation of an axial block-typemorphology between the Siqueiros and Clipperton frac-ture zones must signify a constancy of process, bothfrom the standpoint of the hydraulic head of lavas thatupwell along the top of the block and from that of themechanisms that produce axial block topography. Acorollary of this observation is the implied lack ofpresent-day seamount production along the axis of theEast Pacific Rise.

New crust formed at or near the axis of the EastPacific Rise must eventually reach the edge of the axialblock and then undergo a 200- to 400-meter drop in ele-vation over horizontal distances of a few kilometers.Our crossings of the axial block north of the Siqueirosfracture zone strongly suggest that this elevation changeoccurs by faulting; however, the orientations and spac-ings of the faults are difficult to ascertain with sea-surface instrumentation. It appears that individual faultslivers are asymmetric and tilted, with the steeper slopefacing outwards. This implies step-faulting along high-angle faults that dip away from the ridge axis. However,

SI0 Expeditions Siqueiros, Deepsonde

I , Quebrσdα and Cocotow

Continuous echo-sounding data

HI6ond L-D60 Continuous echo-

sounding data.

Non-institutional data.

8 ° N -

104<W 103°W

Figure 1. Bathymetric track line covering the Siqueiros region of the East Pacific Rise. Reflection profiler data wereobtained along both institutional track lines.

24

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

-3

-4

-5

0700Z 0600ZI

0500Z 0400Z- -6

Figure 2. Reflection profiler record across the axial block of the East Pacific Rise. Profile approximately followsDeepsonde II cross-ridge refraction profile shown in Figure 4. Appearance of sediment-like reflectors on this pro-file is artifact of source function.

the mechanism proposed by Rea (1975) for the forma-tion of the rise crest morphology at 10° S seems equallyplausible to us. The reader is referred to Lonsdale andSpiess (this volume) for a near-bottom perspective ofthis problem.

In the region immediately south of the Siqueiros frac-ture zone, the East Pacific Rise is inadequately definedand on some crossings the rise crest cannot be identifiedwith certainty. (This is indirectly illustrated in Plate 1 bythe vague topographic expression of the Rise.) Webelieve this is a consequence of the complexity of the Si-queiros fracture zone, as we shall make clear in subse-quent discussions. South of about 8° N, the rise crestusually is marked by a bathymetric minimum, but a dis-tinct axial block is not apparent on our crossings. It alsoshould be noted that where the rise crest is identifiablesouth of the Siqueiros fracture zone, it is about 175meters deeper than north of the Siqueiros—a dramaticfinding made by Lonsdale (1977). There is some sugges-tion in existing data that the ridge crest depth south ofthe Siqueiros also is more variable than north of thefracture zone, but this is still speculative. It is mostprobable, however, that the Siqueiros fracture zone marksa change in both the hydraulic properties of upwellinglavas and the topography-producing mechanisms thatoperate along the East Pacific Rise. If this is so, then itcould be predicted that this change may be recorded inthe chemistry of the lavas produced on either side of theSiqueiros.

Fracture Zones

The major fracture zone in the area is, of course, theSiqueiros, located at about 8°15'N. Its topographicsignature is well known between about 103°W and

105° 15 'W, and there can be little doubt that the lateralvariability shown in Plate 1 is a valid characteristic ofthe feature. This variability contrasts sharply with thegreat northeastern Pacific fracture zones, which oftenpreserve their cross-sectional forms over great distances(Menard, 1964). On the basis of seismological evidence,the present zone of decoupling apparently follows thenorthernmost trough, at least as far east as 103 °W, butthe width and complexity of this fracture zone indicatethat the decoupling zone has shifted with time. It isprobable that this shift is a progressive one (for exam-ple, south to north) reflecting a recurring adjustment ofspreading directions rather than random "decouplingchatter," perhaps intrinsic to fast spreading along veryshort transform faults. This would explain why the west-ern (northern) segment of the rise crest near the trans-form fault intersection is relatively "clean" and well-de-fined compared with its eastern counterpart, where therise is bathymetrically vague and perhaps offset by sev-eral small fracture zones (Plate 1). Crane (1976) has pro-posed that isolated segments of active rise crest may be"trapped" within the Siqueiros fracture zone betweenwhat she defines as northern and southern troughs. Al-though speculative, such a mechanism would also ex-plain the overall complexity of the Siqueiros, but it doesnot account for the vague bathymetric definition of therise south of the Siqueiros. Regardless of how the mor-phologic complexity of the Siqueiros may have arisen,the conclusion can be drawn that this fracture zone isfar from a simple and singular shear when consideredfrom other than an instantaneous time perspective. Anydrill holes placed within the structured "width of influ-ence" are likely to yield unpredicted age relationships,and perhaps atypical basalt chemistries, as well.

25

B. R. ROSENDAHL, L. M. DORMAN

A small unnamed fracture zone occurs at about 9° N.It is observed in the bathymetry only at the rise crest(Plate 1), but can be traced as an offset in the magneticanomalies as far west as 104°45'W. In terms of offsetand width, this fracture zone is comparable with Frac-ture Zone A in the FAMOUS area. The precise trend ofthis fracture zone cannot be determined from our data,but we are reasonably certain it has developed withinabout the past 1 m.y. This suggests that the rise axis hasundergone (or is still undergoing) a recent reorientationor realignment of the spreading pattern that has his-torically characterized the area.

Plate Fabric

The primary plate fabric is the most pervasive aspectof the regional contour chart north of the Siqueiros frac-ture zone (Plate 1). The wavelength of the fabric (that is,the distance between successive bathymetric peaks) isonly a few kilometers and the average depth variancefrom a smoothed age-depth plot would be about 50 me-ters, yet many individual components of the fabric canbe followed in a north-south direction for tens of kilo-meters. This spatial continuity, combined with the highdegree of parallelism of the fabric, suggests that the pro-cess of fabric production must be fairly uniform alongthis portion of the East Pacific Rise. We believe thisprocess occurs as slivers of crust spall off the margins ofthe axial block; however, we find it surprising that theprocess could maintain spatial continuity over 10 ormore kilometers.

As one proceeds away from the rise crest region, theplate fabric exhibits progressively less relief, linearity,and parallelism—changes that cannot be attributed tosedimentary smoothing alone. This suggests that verti-cal tectonic readjustments continue well after crustalgeneration, probably in isostatic response to a changingthermal regime. The alternative explanation — that therehas been a progressive temporal change in the amountof relief originally produced — seems unlikely in view ofthe uniform seismic structure of the flanks of the rise, aswe shall discuss in another section.

The fabric south of the Siqueiros fracture zone is not-ably less continuous and linear than that to the north.Admittedly, this difference is biased by the fact that ourdata base is much poorer in this region, but qualitativelythe difference appears to be a real one. Presumably thefabric here reflects the ill-defined character of the risecrest south of the fracture zone and is perhaps related tothe lack of an axial block along this segment of the EastPacific Rise.

Seamounts

There are a number of seamounts and ridges in theSiqueiros region associated with both the plate fabricand the fracture zones. The group of lineated seamountsshown in Plate 1 and Figure 3 are of particular concern,because they dissect the area targeted for drilling on Leg54. This chain of seamounts, which was dubbed "OCPRidge" in honor of the JOIDES Ocean Crust Panel,was discovered during the Deepsonde II site survey ofthe PT-4 sub-area and lies within about 6 km of the Leg

54 prime target (e.g., Site 423). We will return to thistopic in the following section. As a point of interest,normal reconnaissance profiling failed to reveal anypockets of sediments on the seamounts thick enough toprovide spud-in sites for drill holes, with the exceptionof several of the seamounts or seamount ridges withinthe Siqueiros fracture zone.

The seamounts away from the fracture zones are ob-viously of volcanic origin, but there is considerable un-certainty regarding the location of their formation, thedepths of magma genesis, and the stages of their mag-matic evolution. With the discovery of a shallow magmareservoir beneath the axial block of the rise crest (seesection on Crustal Structure), one might suspect that theseamounts form essentially on the rise axis from reser-voir magmas which are themselves derived from deeperlevels in the asthenosphere (see also Lonsdale andSpiess, 1979). As spreading progresses, the feeder sys-tems linking the magma reservoir with the seamountswould eventually become disconnected and construc-tional volcanism would cease. Alternatively, it could beargued that small jumps in the spreading center loca-tions may isolate a crustal magma reservoir and its con-duits to the surface. If the reservoir continues to eruptlavas onto the sea floor for even a short period, a sea-mount would, of course, be formed on the flank of therise crest. It is also possible that the seamounts are notspatially related to the subaxial reservoirs but rather arefed directly from asthenospheric sources away from theridge axis.

The total absence of any seamounts on the rise axis atthe present time and the occurrence of only one set ofseamounts at equal distances on each side of the rise axis(Plate 1) argue against the first hypothesis. Dynamic orisostatic problems argue against the second hypothesis,at least with respect to large seamounts. Finally, the ap-parent occurrence of fundamental chemical dissimilar-ities between at least some of the seamount basalts andthe rise crest/fabric basalts (Batiza and Johnson, thisvolume) argues in favor of the third hypothesis. We willreturn to this matter in the Petrology section of thisoverview.

PT-4 Sub-Area

After considerable reconnaissance surveying in AreaPT-4, the Siqueiros site survey was concentrated onroughly a 1° square, called the sub-area, centered at105° 10'W and 9°15'N. The decision to concentrate onthis sub-area was made for three reasons: (1) sedimentponds were known to exist here from previous data col-lections; (2) about 10 days of reconnaissance surveyinghad failed to locate more attractive young sites else-where in Area PT-4; and, (3) a prime site had to be se-lected by about the 15th day of the site survey if aseismic refraction study were to be conducted.

The morphology of the sub-area essentially consistsof the north-south lineated spreading fabric and thewest-northwest-trending ridge of seamounts (Figure 3),referred to as the OCP Ridge. The existence of the1300-meter-high seamount located at 1O5°35'W and9° 10 'N was suspected prior to the site survey (on the ba-

26

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

105-30' 105°00'

9" 30'

9 β00'

CONTOUR INTERVALS00m uncorrected)

9°30'

105*30' 105*00'

Figure 3. Bathymetric chart of the PT-4 sub-area with dredge location and possible drill targets.

sis of poorly navigated Navy soundings), and the exist-ence of the small (800-m) seamount at 105°W and9°05'N became known toward the end of the sub-areasurvey. However, the fact that these features form acontinuous ridge about 100 km long did not come tolight until the bathymetry of the sub-area was con-toured. Had this been known earlier, the location of theseismic experiment — and hence the Leg 54 drilling op-erations — would perhaps have been different. As it is,the preferred prime site for multiple re-entry (PT-4A)was located about 11 km from the base of the easternseamount, but only about 6 km from the base of theRidge. Moreover, the OCP Ridge is surrounded on both

sides by a "moat" that is particularly well developed onits northwestern side. The preferred prime site was closeto the edge of the lineated fabric, just north of the mar-gin of this moat. The proximity of this feature to thepreferred site caused concern among most of the scien-tists involved in Leg 54 crustal drilling and was, in part,responsible for the formulation of the original drillingstrategy adopted by the Leg 54 scientific party.

The ridge connecting the eastern and western sea-mounts is less than 100 meters high in some localitiesand probably represents minor fissure eruptions along arift zone connecting the main vents that formed the sea-mounts. The orthogonal alignment of the OCP Ridge

27

B. R. ROSENDAHL, L. M. DORMAN

with a short offset of the Rise Crest at about 9°N (Plate1) and its parallelism to some Siqueiros fracture zonefeatures suggest that the Ridge is associated with thesmall unnamed fracture zone discussed earlier. Howev-er, the Ridge does not offset the topography or magnet-ics and clearly is not continuous east of about 105° W.Moreover, the feature appears to overprint (postdate)the fabric in the sub-area, suggesting that it formedaway from the spreading axis. The lack of any sedi-ments on the Ridge supports this inference. Taken inthis context, the OCP Ridge probably forms a flow-linelineament with the same plate motion connotations thatthe Hawaiian Islands provide. The orthogonal align-ment of the Ridge with the unnamed fracture zone isthus a fortuitous event without genetic implications.

Sediment distribution in the immediate vicinity of theOCP Ridge is predominantly a consequence of pondingin the surrounding moat. The sediments here arerelatively uniform in thickness and show a high degreeof acoustic stratification (see, for example, East PacificRise Site Report). They are considerably more reflectivethan sediments elsewhere in Survey Area PT-4. Sedi-ment distribution away from the OCP moat is variableand unpredictable. Profiling at standard reconnaissancespeeds (approximately 10 knots) revealed only a spottydistribution of sediments that was less related to topog-raphy than to the occurrence of patches of smooth sea-floor. Careful profiling at slow speeds (2 to 4 knots)showed, in fact, that a smooth blanket of poorly strati-fied, moderately transparent sediment covers most ofthe sea-floor in the sub-area. There is a tendency to-wards ponding in topographic lows or against broadtopographic bumps, although somewhat less than onemight expect, and occasional occurrences of "puffed-up" mounds of sediments, which are probably current-controlled.

On the basis of the acoustic character, the lithologyof gravity cores, and the description of cores from thegeneral PT-4 area, the sediment blanket away from theOCP Ridge is probably composed of a poorly sortedforaminifer-nannofossil ooze. Accumulation rates ap-parently average about 20 meters/m.y. Given the age ofthe sea floor in the vicinity of the potential drilling local-ities (approximately 1.5 to 2.0 m.y.), a two- to threefoldincrease in accumulation rate is needed to meet the spud-in requirements for multiple re-entry. Only five such ar-eas could be identified: PT-4A located at the center ofthe seismic array on a thickened portion of the regionalsediment blanket, PT-4B, 4C, and 4D on sediment pondsat the bases of broad topographic fabric highs, and PT-4E on sea floor characterized by sediment mounding(Figure 3). Of these five potential drilling localities, PT-4A and PT-4C were selected as the preferred and alter-native targets for multiple re-entry drilling.

CRUSTAL STRUCTURE

Before the IPOD Survey of Area PT-4, the SiqueirosExpedition had yielded a knowledge of the gross crustalstructure of the area (Orcutt et al., 1975; Rosendahl etal., 1976). The most publicized discovery was, of course,that the axial block is underlain by a crustal low-velocity

zone which probably represents a reservoir or "restingplace" for tholeiitic magmas generated at deeper levelsin the asthenosphere. An equally important result of theexpedition has been the realization that crustal structureis the seismological expression of a complex interplay ofvarious factors, such as lithostratigraphy, thermal gra-dient, age, and degree of fracturing. The key to under-standing the geologic relevance of youthful crustalstructure lies in deciphering the nature of this interplay.

In the following sections we present an overview ofthe Siqueiros experiments (Figure 4), with particularemphasis on results that have a bearing on the drillingoperations.

Type Seismic Section

Integrating the work of Rosendahl et al. (1976), Ro-sendahl (1976), Orcutt et al. (1975), Orcutt et al. (1976),and Garmany (personal communication), we have de-rived a standard model of the seismic structure of AreaPT-4. In brief, the model shows a structure consisting ofthree compressional-wave velocity domains. The upper-most domain is a region of very rapid increase in Vpwith depth extending from the sea floor down to about1.5 to 2.0 km. It overlies a domain characterized byrelatively high and uniform velocities (about 6.8 to 7.0km/s) that increase with depth at rates less than 0.2km/s per kilometer. The lowest domain is characterizedby velocities greater than 7.5 km/s. These three domainsrepresent the classical inferred basalt-gabbro-peridotitelithostratigraphy of oceanic crust, and constitute themineralogic framework upon which effects such as ther-mal gradient are superimposed.

The domains are roughly coincident in depth with thestandard Layer 2-Layer 3-mantle terminology; and, as afirst approximation, they can be treated in an equivalentmanner. However, it must be emphasized that the veloc-ity domains are not true seismic layers separated by dis-continuities, but are regions of a characteristic gradientseparated by narrower zones of rapid change in gradi-ent. It is for this reason that we do not adopt the con-ventional layer terminology here.

Axial Low-Velocity Zone

The interplay beteen lithostratigraphy and thermalgradient is an extreme one at the ridge axis, where thethermal gradient apparently intersects the crustal melt-ing curve at two different depths producing a finite re-gion of partial melt. This region manifests itself in theseismic data as a triangularly shaped low-velocity zonesandwiched between the middle and lower velocity do-mains (that is, between Layer 3 and the mantle).

The seismological evidence for the existence of thiszone is overwhelming. First, the travel-time data show acontinuous subaxial shadow zone between 7 km/s crust-al arrivals (the middle velocity domain), which abruptlyterminate at moderate ranges, and 7.8 km/s mantle ar-rivals. It seems that this shadow zone can be producedon the requisite geographic scale only by a velocity in-version. Secondly, the mantle arrivals are associatedwith anomalously large intercept times (approximately

28

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

10°N

9°N

8°N

SIQUEIROS REFRACTION LINES

DEEPSONDE π

OBR-19

>QBR-17

105°W 104°W 103°W

Figure 4. Seismic refraction profiles solid and dotted lines (– ) and rock dredges f D D D D j in the Sique-iros area. Profiles SR-4, 10S, and 7S were shot approximately along the 1.4-, 2.4-, and 4.1-m.y. isochrons south ofthe Siqueiros fracture zone. Profiles SR-ION and SR-7N were shot approximately along the 0- and2-m.y. isochronnorth of the Siqueiros.

1.5 s after removal of the water column) which producegreat apparent crustal thicknesses when a low-velocityzone is not introduced into standard layered-types ofanalyses. Independent evidence of this crustal velocityinversion comes from synthetic modeling of the ob-served wave amplitudes and patterns of energy distribu-tion. The best models of the ridge axis record sections —those that produce the best correlation between ob-served and synthetic data — contain a low-velocity zoneof some sort, whereas the models away from the ridgedo not require or even permit this inversion.

Evidence of the geometry of the zone comes from thesonobuoy refraction data, which provided coverage ofthe flanks of the axial block and showed a progressivethinning of the zone with distance from the ridge axis.The ocean bottom seismograph analyses (Orcutt et al.,1976) furnished an estimate of the thickness of the crustabove the zone and the mean velocity within the zone,resulting in the simple seismic model of the axial intru-sion zone shown in Figure 5. Evidence that the zonerepresents a region of partial melt is provided by thevery high attenuation of seismic energy penetrating thezone and by theoretical considerations (e.g., Sleep, 1975).

Genesis of the Seismic Section

On the basis of the model shown in Figure 5 and theinference that the low-velocity zone defines a crustalmagma reservoir, it is relatively easy to visualize howthe type seismic section might be generated. The upper-most velocity domain probably represents a dike com-plex capped by basalt flows. The rapid positive velocitygradient is a consequence of the intrinsic velocity differ-ences between the flows and dikes, smeared out by lackof a sharp contact between the two units and a decreasein pore space (that is, fracturing) with depth. The factthat a rapid thermal gradient would tend to decrease thevelocity with depth suggests that temperature plays aminor role in this velocity domain. In fact, it will beshown in a later discussion that its role is not so minoras it is subtle. The loci of dike injection and the distribu-tion of eruptive centers would be determined by thestress field of the magma reservoir, which is conducivein this region to preferential injection and eruptionalong the topographic axis of the crestal block. Theseismic data demand that this process be restricted tothe immediate locality of the ridge axis, because there

29

B. R. ROSENDAHL, L. M. DORMAN

DISTANCE (km)

20 10

R.S.

5,3

6,9 HM

"

Q

i

UJCO

CD1

CQZDCO

0 •

1

2

3

4

5

6

SEISMIC

5 l 1 /

7,0//

/

- /

/ 5 . 01

—1/

7,8

MODEL

y 5,, "

\ 7,0\

\

\ -

5,θ\\V-^

7,8

R.S,

5,3

6,9

Figure 5. Seismic refraction model of the East PacificRise crest (after Rosendahl, 1976). Triangular-shaped,low-velocity zone is believed to represent magma res-ervoir.

are no systematic changes in the thickness of this do-main with age (see next section). A similar restrictionseems to apply to the formation of the lower seismic do-main (the upper mantle), because the depth to Moho ap-parently remains relatively constant beneath the entireaxial block. This suggests that the upper mantle is gener-ated essentially at the ridge axis. It appears certain thatthe 7.8 km/s Moho that floors the reservoir representsan ultramafic cumulate of dense early-formed mineralphases with little or no melt present. The seismic veloci-ty of the mantle is basically a function of original miner-alogy at elevated but sub-melt temperatures.

In contrast to the upper and lower seismic domains,the genesis of the middle domain (that is, Layer 3) is notentirely restricted to the immediate vicinity of the ridgeaxis. The thickness of this domain increases with agefrom a few hundred meters near the ridge axis to about4 km at the edge of the axial block — a rate of thicken-ing of about 500 meters/km of distance — at the ex-pense of the low-velocity zone (Figure 5). Hence, pro-gressively deeper parts of the domain evolve from thepartial melt zone (that is, they pass through the solidus)at progressively greater distances from the ridge axis, al-

though the accretion is completed within the boundariesof the axial block. The top few hundred meters of thedomain still must be formed essentially at the ridge axis,because high compressional-wave velocities are ob-served everywhere above the low-velocity zone. We be-lieve that this early-formed material could be a plagio-clase-rich gabbro which caps the summit of the partialmelt zone and produces a thin, high-velocity lid. Thebase of this domain probably consists of high-densityprecipitates (olivine + pyroxene + opaque minerals)plus plagioclase formed from trapped pockets of inter-granular melt. The velocity in this material would beslightly higher than that in the plagioclase-rich gabbroiclid, accounting for the slight positive velocity gradient inthis domain and the apparent transition zone between itand the mantle. The material sandwiched between thegabbroic lid and the basal precipitates would be a heter-ogeneous mixture of gabbroic rock types with local poc-kets of late-stage differentiates. The maximum velocityin these rocks, although variable, probably does not ex-ceed that in the gabbroic lid, and in some instanceswould be less.

Age Variations in the Seismic Section

The major change in gross crustal structure with ageis the thickening of the middle velocity domain (Layer3) at the expense of the partial melt zone (Figure 5). Thisis a primary change involving the genesis of the litho-stratigraphy of the bulk of the crust. No significantthickness changes were observed in the other velocitydomains elsewhere in the study region. Moreover, thethickness of the middle domain appears to attain con-stancy once it spreads beyond the confines of the axialblock.

In spite of the lack of thickness variations, there doappear to be systematic, albeit subtle, changes in seismicvelocities with age. Both thick-layered and gradientmodel analyses suggest a slight decrease in mantle veloc-ity (parallel to the ridge axis) in the first several millionyears after formation. The variation is about 0.3 km/s,decreasing from a high of about 7.7 to 7.8 km/s at theridge axis to a low of about 7.4 to 7.5 km/s between the2.0- and 2.4-m.y. isochrons (Figure 4). Between the 2.0-and 4.1-m.y. isochrons, the gradient analyses show anincrease in mantle velocity from about 7.4 to 7.5 km/s,to about 8.0 km/s. This 0.5 km/s increase is an actualobservation (that is, greater than the uncertainty limits)and, if generally valid, reflects the lowering of the ther-mal gradient as the mantle gradually cools. The initial0.3 km/s decrease in mantle velocity is at the resolutionlimit and hence may or may not be a real phenomenon.If real, the decrease would imply an initial reversal inmantle cooling with age. We have no explanation forthis possibly anomalous behavior in the first severalmillion years after genesis.

No significant velocity changes are observed in themiddle velocity domain, except perhaps for a slight de-crease in the gradient value with age. This change is eas-ily explained by a decrease in the temperature gradient,which should also produce an analogous decrease in the

30

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

velocity gradient with age in the upper domain and acorresponding increase in its mean velocity. The latterpoint appears to be supported by the thick-layered anal-yses, which suggest a slight increase in the velocity of the"Layer 2" refractor and in the mean velocity of this do-main between the ridge and the 2.4-m.y. isochron (Fig-ure 4). The gradient analyses, on the other hand, suggestthat the rate of change in velocity with depth in this do-main increases from about 0.75 km/s/km near the ridgeaxis to about 1.75 km/s/km at ages of 2.4 and 4.1 m.y.There is an apparent contradiction here between the twoanalytical techniques, which is difficult to resolve at thistime. The problem is that the thick-layered analyses, al-though normally much cruder than the gradient analy-ses, are based upon averages of many profiles alongeach of the isochrons, whereas the more sophisticatedgradient analyses are based upon single isochronal pro-files with relatively few shots in the range-where the up-per domain refractions appear as first arrivals. If thethick-layered analyses are correct, then the velocity vari-ations in the upper domain are easily attributable tosimple cooling. However, if the gradient values are pro-ven correct then it will be necessary to postulate a radi-cal decrease in the velocity of the near surface rocks inthe first several million years after crustal genesis. Theonly plausible mechanism would be fracturing as thecrust spread away from the ridge axis, perhaps associ-ated with the faulting that must occur at the margins ofthe axial block.

Spatial Variations in the Seismic Section

One of the most surprising results of the Siqueiros ex-periments in Area PT-4 was the uniformity of the crust-al structure along the off-ridge isochron shooting lines(that is, the 1.4- and 2.4-m.y. profiles shown in Figure4). There are no multiple OBS lines along any isochronto provide a check on subtle variations in velocity gra-dient, but thick-layered analyses of the multiple sono-buoy profiles clearly show that first-order isochronalchanges in velocities and thicknesses are absent. Indeed,if the data along each isochron are plotted as thoughthey originated at a single receiver, it is found that thestandard deviation of the arrival times from those pro-duced by the mean structure is equal to or less than theexperimental error. We interpret this lack of isochronalvariation to signify that the process of crustal genera-tion at or near to the ridge axis is uniform along thispart of the East Pacific Rise. Since the process has notvaried significantly between isochronal lines either (seeprevious section), there must be a temporal and spatialstability for crustal generation on the scale of at least afew million years and several hundred kilometers.

PETROLOGY

Regional Summary

Most of the rock samples from the Siqueiros region(Figure 4) range from highly LIL-depleted mid-oceanridge basalt to progressively more fractionated tholeiiticbasalts (Batiza et al., 1977). Taken as a group, the thol-eiites contain 6 to 10 per cent MgO and show continuous

enrichment in Fe, Ti, Mn, Na, K, and P on MgO-varia-tion diagrams (Figure 6), accompanied by progressivedepletion in Ca, Al, Ni, and Cr. Because these trendscan be explained by the subtraction (crystallization) atshallow depth of large amounts of analyzed olivine plusplagioclase plus clinopyroxene from the most Mg-richspecimens, it is likely that these rocks represent variousstages of fractionation within the crustal magma reser-voir that is believed to underlie the rise crest (Batiza etal., 1977). The wide range in degree of fractionationin these rocks and the abundance of relatively frac-tionated specimens is characteristic of lavas erupted ataxial block-type ridge systems (e.g., East Pacific Rise)and contrasts with lavas erupted at axial valley-typeridges (Rosendahl, 1976). We believe this differencecould reflect variations in the size of the magma reser-voirs beneath both types of ridges.

Two groups of rocks dredged in the Siqueiros regiondo not fall into the tholeiite fractionation pattern de-scribed above. One group, from dredge SD-7 (Figure 4),is an assemblage of spectacularly fresh picritic "basalts"containing olivine phenocrysts (Mg90) as large as 5 mmand unzoned plagioclase microlites in a matrix of glass.Pyroxene phenocrysts do not occur in these rocks,which are among the most "primitive" ever recoveredfrom the sea floor. They are the subject of a separatechapter by Schrader et al. in this volume. The othergroup, recovered from dredge SD-8 at the intersectionof the East Pacific Rise and the Siqueiros fracture zone,are alkali basalts with a chemistry not entirely like thatof any other alkali basalts. This unique chemistry sug-gests rather extensive partial melting (20 to 25 %) of ashallow mantle source (P < 10 kbar), followed by en-richment in LIL elements by wall/rock interactionand/or zone refining without significant crystal frac-tionation — an evolutionary scheme that may reflect thepeculiarities of secondary volcanic processes resultingwhen earlier-formed transform fault-bounded litho-sphere re-enters the thermal regime of the western seg-ment of the East Pacific Rise (Batiza et al., 1977).

PT-4 Sub-Area

Based upon partial chemical analyses, the rocks fromdredges D-l and D-5 in the PT-4 Sub-area (Figure 3) areLIL-depleted mid-ocean ridge tholeiites with MgO-vari-ation trends similar to those found by Batiza et al.(1977). Apparently these rocks represent further exam-ples of the ridge-crest tholeiites dredged elsewhere in thearea. Some of the rocks from dredges D-3 and D-4 alsoare normal, LIL-depleted tholeiites, but in additionthese dredges contain basalts that show a tendency to-wards enrichment in P2O5 with respect to TiO2 anddepletion in CaO with respect to A12O3 (Figures 7 and8). The basalts are also enriched in LIL- and light-rare-earth elements compared with the more typical ridge-crest tholeiites, and have a higher 87S/86S ratio (Batizaand Johnson, this volume). This amounts to a trendtoward transitional basalts or even the alkalic basaltsfound in dredge SD-8.

The occurrence of more alkalic basalts in dredge D-4is not surprising since it partially sampled the base of the

31

B. R. ROSENDAHL, L. M. DORMAN

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Figure 6. MgO variation diagrams for Siqueiros basalt (after Batiza et al., 1977).For all oxide diagrams, open circles refer to dredge SD-3, crosses SD-4, open tri-angles SD-5, solid circles SD-6, circled dots SD-8, solid triangles CD-I, and cir-cled plus signs QBR. See Figure 4 for dredge locations. For Ni, Cr, and V, solidcircles represent tholeiites, circled dots alkali basalts. Dashed lines enclose alkalibasalts.

seamount at the eastern end of the OCP Ridge (Figure3), but the occurrence of transitional to alkalic tenden-cies in some rocks from dredge D-3, which sampled thenorthern tip of the moat surrounding the OCP Ridge,was somewhat unexpected. Apparently, the moat isfloored, in part, by the OCP Ridge lavas originatingfrom fissures off the main trend of the Ridge. Becausedrill target PT-4A is located on the north-south lineatedtopographic fabric, which dredges D-l and D-5 suggestis chemically normal, it is likely that it has not been in-fluenced by the OCP Ridge (or moat) volcanism. Still,target PT-4A is only 2 to 3 km from its nearest approachto dredge D-3, and the possibility of contamination isindeed a real one. The drilling strategy outlined inFigure 9 and Chapter 1 of this volume was designed totest this possibility prior to establishing a multiple re-entry hole at Target PT-4A.

MAGNETICS AND GRAVITYFigure 10 shows a plot of magnetic anomalies along

some of the longer east-west tracks in the PT-4 area.Because of the proximity of the study region to the mag-

netic Equator, the spreading anomalies are of relativelylow amplitude. Nonetheless, the Jaramillo (Anomaly 1)and Gilsa (Anomaly 2) events can be identified in thepart of the Siqueiros area that lies north of the Siqueirosfracture zone and west of the rise crest. The latter eventis particularly well defined and there can be little doubtof its identification in Figure 10, whereas the centralanomaly is poorly developed in the study region. In theabsence of bathymetry, the central anomaly would notbe identifiable to the untrained eye. The small unnamedfracture zone that offsets the rise crest at 9° N may dis-rupt the continuity of Anomaly 1 but not that of eitherthe positive anomaly immediately to the west of it orAnomaly 2. Apparently this fracture zone developed lessthan a million years ago in spite of the fact that the OCPRidge, which rests on crust greater than a million yearsold, may line up orthogonally with it. It also should benoted that most of the seamounts and larger-scale fabrictopography have little expression in the magnetics.

The major result of the gravity studies in Area PT-4has been the demonstration by Dorman (1975) that thedepth of isostatic compensation beneath the axial block

32

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

0.6

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Figure 7. Plot of P2O5 versus TiO2 for all Siqueiros dredge hauls including PT-4sub-area dredges. Siqueiros data from Batiza et al. (1977); sub-area data fromJohnson (unpublished).

20

16

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3 = DS-3 samples4 = DS-4 samples8 = SD-8 samples+ = DS-5 samples• = all other samples

= DS-3—>-SD-8 possiblefractionation trend,through some DS-4 samples

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trend, typified by SD-6,included in samples

12 16CaO (wt. '

Figure 8. Plot of CaO versus A12O3 for all Siqueirosdredge hauls PT-4 sub-area dredges. Siqueiros datafrom Batiza et al. (1977); sub-area data from John-son (unpublished).

region of the East Pacific Rise is about an order of mag-nitude less than that found in the continental UnitedStates, or about 10 km for the axial block. This value isfurther evidence that the rise crest morphology is an iso-static response to the shallow magma reservoir thoughtto underlie the axial block (Rosendahl, 1976).

DRILLING RECOMMENDATIONS BASED ONTHE SITE SURVEY AND PRIOR CRUISES

General

On the basis of the results presented, we recommend-ed that the Siqueiros area of the East Pacific Rise (i.e.,Survey Area PT-4) be chosen as the type-drilling areafor fast-spreading, young oceanic crust. Because thisrecommendation was accepted and ultimately gave riseto Leg 54, it is appropriate to list here the reasons forthe recommendation:

1) The area is very well surveyed and bathymetricallyrepresents the best known segment of an accreting-platemargin in the fast-spreading Eastern Pacific.

2) The seismic structure of the crust is at least betterknown, and probably better understood, than the struc-ture of any comparable area of accreting-plate bound-ary.

3) The pre-drilling petrologic control in the areaexceeded that available to most other ocean crust drill-ing legs and can be used to place the drill-hole data intheir proper perspective.

4) The rise crest in the area is characterized by an ax-ial block—a feature that typifies the East Pacific Risespreading axis.

5) A realistic opportunity exists for relating basaltstratigraphy and chemistry with the physicochemicalprocesses that may be occurring in the axial magma res-ervoir; the latter presumably represents the final "stag-ing area" of the basalts prior to their eruption or injec-tion. The inadequate understanding of the structure ofother ridge crests automatically eliminates a true pro-cess-response type of study elsewhere.

33

B. R. ROSENDAHL, L. M. DORMAN

Test for Suitability ofPT-4 Area for Multiple

Re-entry (via pilot-holes)

Test forChemical Normality

PT-4ANormal

PT-4AAbnormal

Multiple Re-entryat PT-4A

Multiple Re-entryat PT-4C

Multiple Re-entryat Alternative Site

Area PT-4Single-Bit Program

Alternative AreaSingle-Bit Program

Test for Suitabilityat Alternative Site(e.g., Anomaly 5)

Yes No

ContingencyProgram

Figure 9. Flow diagram of recommended drilling strategy for Leg 54.

6) The area is logistically and climatically amenableto multiple re-entry drilling and follow-up studies, yet islocated well beyond any territorial waters.

Targets, Tactics and Specific Recommendations

Although the advantages mentioned in choosingArea PT-4 as the type-drilling area for fast-spreadingyoung crust were relatively obvious, the selection of ac-tual target localities for multiple re-entry was complex.The main reason was that the minimum sediment re-quirements for spud-in (approximately 80 m) dictate thelocations of potential target areas, and among the limit-ed number of resulting targets no single one is ideal. Inorder to choose between the five potential targets thatmeet the sediment requirements (Figure 3), the follow-ing criteria were used, in order of importance:

1) The "normality" of a given target in terms of itssea-floor morphology, igneous petrology and chemis-try, and geophysical signature.

2) The likelihood of correlating remote seismic meas-urements at a given target with its downhole lithology.

3) The youthfulness of a given target and its proxim-ity to the center of a normal or reversed magnetic block.

Site PT-4A was the superior location from the stand-point of seismic correlation, youthfulness, and magneticcriteria, but it was sufficiently close to the OCP Ridge toquestion whether it was structurally and chemicallytypical. Indeed, as indicated in the petrologic discus-sion, some rocks from dredge D-3 (located about 2 to 3km from PT-4A) show chemical affinities with the tran-sitional basalts found in dredge D-4, which was locatedon the northern edge of the OCP Ridge. Even thoughPT-4A appeared to be located on the north-south

lineated spreading fabric and dredge D-3 along themargin of the "moat" surrounding the OCP Ridge, theuncertainty regarding its petrologic typicality requiredthat an alternate multiple re-entry site be chosen. SitePT-4C was that locality. It must be realized that the al-ternative site, although probably normal from a petro-chemical standpoint, is located outside the limits whereseismic control is significant. Hence, if PT-4C were cho-sen, Leg 54 would sacrifice the opportunity of seismo-logical correlation for the sake of presumed petro-chemical normality. This would have been a painfullyhigh price to pay and one upon which not all interestedparties were in full agreement. For this reason andbecause the mechanical suitability of a given re-entrytarget could not be predicted prior to actual drilling, arecommendation was also made that single-bit pilotholes be drilled at both targets PT-4A and PT-4C. Thefour possible results of pilot-hole drilling, outlined inFigure 9, would determine what course of action shouldbe followed by the shipboard party.

In addition to the pilot holes at PT-4A and 4C, werecommended drilling one or more single-bit holes atunspecified localities in the moat between the OCPRidge and PT-4A, and at PT-4D. These holes were to bedrilled on a "time-available basis" with the objective offurther chemical characterization of the OCP Ridge la-vas, both in time and space.

Alternative and Contingency Plan

In the event that the PT-4 sub-area proved unsuitablefor drilling, we recommended that an alternative site bechosen in the vicinity of magnetic anomaly 5. The pro-gram should include multiple re-entry and closely spaced,

34

GEOLOGY AND GEOPHYSICS OF EAST PACIFIC RISE

10°N

105°W 104°W 103°W

Figure 10. Magnetic anomalies plotted along selected east-west profiles in the Siqueiros area. Margin of East PacificRise axial block shown as dotted lines. Numbers refer to probable anomaly numbers.

single-bit holes. If multiple re-entry again proved notfeasible, we recommended that the Pacific flow-linetransect of single-bit holes be continued westward forthe duration of Leg 54. This constitutes the contingencyprogram identified in Figure 9.

As it turned out, the final drilling program of Leg 54differed considerably from the plans presented here.The drilling program, and the reasons for it, are out-lined in the introduction and site chapters of thisvolume.

REFERENCES

Anderson, R., and Noltimier, H. C , 1973. A model for thehorst and graben structure of mid-oceanic ridge crestsbased upon spreading'Velocity and basalt delivery to theoceanic crust. Geophys. J. Roy. Astron. Soc, v. 34, p.137-147.

Batiza, R., Rosendahl, B. R., and Fisher, R. L., 1977.Evolution of oceanic crust: 3. Petrology and chemistry ofbasalts from the East Pacific Rise and Siqueiros transformfault. J. Geophys. Res., v. 8-2, p. 265-276.

Crane, K., 1976. The intersection of the Siqueiros TransformFault and the East Pacific Rise. Mar. GeoL, v. 21, p. 25-46.

Dorman, L. M., 1975. The isostatic compensation of the top-ography at the crest of the East Pacific Rise. EOS Trans.Amer. Geophys. Union, v. 56, p. 1964, Abstract T49.

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

Larson, R. L., and Chase, C. G., 1970. Relative velocities ofplate tectonic history of the mouth of the Gulf of Califor-nia. Ibid., v. 83, p. 3345-3360.

Larson, R. L., and Chase, C. G., 1970. Relative velocities ofthe Pacific, North America, and Cocos Plates in the MiddleAmerica region. Earth Plant. Sci. Lett., v. 7, p. 425-428.

Lonsdale, P., 1977. Regional shape and tectonics of the equa-torial East Pacific Rise. Mar. Geophys. Res., v. 3, p.295-315.

Lonsdale, P., and F. N. Spiess, 1979. A pair of young cra-tered volcanoes on the East Pacific Rise, J. Geol., v. 87, p.157-173.

Menard, H. W., 1964. Marine Geology of the Pacific: NewYork (McGraw-Hill).

Orcutt, J. A., Kennet, B. L. N., Dorman, L. M., and Proth-ero, W. A., 1975. Evidence for a low-velocity zone underly-ing a fast-spreading rise crest. Nature, v. 256, p. 475.

, 1976. Structure of the East Pacific Rise from anocean bottom seismometer array. Geophys. J. Roy. As-tron. Soc, v. 45, p. 305-320.

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B. R. ROSENDAHL, L. M. DORMAN

Rea, D. V., 1975. Model for the formation of topographic fea- crest derived from seismic refraction data. J. Geophys.tures of the East Pacific Rise crest. Geol., v. 3, p. 77-80. Res., v. 81, p. 5294-5305.

Rosendahl, B. R., 1976. Evolution of oceanic crust, 2 Con- N 9J5 F o r m a t i o n o f o c e a n c r u s t . S o m e t h e r m a l

^ ' " ^ T S 1 0 1 1 5 ' m f e r e n C e S L GeoPhys Res> v constraints. Ibid., v. 80, p. 4037-4042.

Rosendahl, B. R., Raitt, R. W., Dorman, L. M., Bibee, L. O., Truchan, M., and Larson, R.L., 1972. Tectonic lineamentsHussong, D. M., and Sutton, G. H., 1976. Evolution of on the Cocos Plate. Earth Planet. Sci. Lett., v. 17, p.oceanic crust, I, A physical model of the East Pacific Rise 426-432.

36