Von Herzen, R. P., Robinson, P. T., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118

23. PARTIAL REORIENTATION OF THE DEFORMATIONAL STRUCTURESAT SITE 735 USING PALEODECLINATION MEASUREMENTS1

Mathilde Cannat2 and Janet Pariso3

ABSTRACT

We propose a reorientation of the deformational structures at Site 735 relative to one another, and to theremanent magnetic vector. This reorientation is based on the assumption that the in-situ stable remanent magneticvector (SRMV) of each core piece pointed in the same direction before drilling. The consistency of the resultssupports this assumption. The dominant dip azimuth of the earliest deformation structures (magmatic foliation andsome granulite-facies, solid-state foliations) is northwest of the SRMV. These structures tend to make a 45° anglewith the latter solid-state shear zones. The dominant dip azimuth of these shear zones is west of the SRMV, and thestretching lineation is downdip. The true orientation of these structures in the geographic framework is not knownbecause the in-situ azimuth (declination) of the SRMV is still undetermined.

INTRODUCTION

Site 735 lies on a shallow platform on the east wall of theAtlantis II Fracture Zone (Fig. 1). Its position in the magneticanomaly pattern (Dick et al., this volume) suggests a crustalage of about 11 Ma. This is supported by a zircon age of 11.3Ma (J. Mattinson, pers. comm., 1989).

The 500 m of gabbros drilled at Site 735 (435 m recovered)comprise metagabbronorites, olivine-bearing and olivine gab-bros, Fe-Ti oxide gabbros, and small volumes of troctolitesand of evolved late magmatic intrusives (Shipboard ScientificParty, 1989; Fig. 2). About 30% of these gabbros are plasti-cally deformed (Fig. 2) along flat to moderately dipping normalshear zones (Cannat et al., this volume). Some gabbros arealso thought to have been deformed in the magmatic state asa result of viscous laminar flow of the incompletely solidifiedmagma (Cannat, this volume). The synkinematic assemblagesin the normal shear zones correspond to metamorphic condi-tions ranging from granulite to lower amphibolite facies(Stakes et al., this volume). There is no significant ductiledeformation in the core below the stability temperature ofactinolitic hornblende (Stakes et al., this volume), and brittledeformation is limited. The extensional ductile deformation atSite 735 thus occurred when the gabbros were still hot,probably in the immediate vicinity of the Southwest IndianRidge (Fig. 1). The fresh gabbros with preserved magmatictextures, and the deformed and metamorphosed gabbros,have identical positive remanent inclinations, suggesting mag-netization during a single reversed magnetic interval (Ship-board Scientific Party, 1989). This is another indication of ashort time interval between the initial crystallization of thegabbros at the ridge axis and their deformation.

1 Von Herzen, R. P., Robinson, P. T., et al., 1991. Proc. ODP, Sci.Results, 118: College Station, TX (Ocean Drilling Program).

GDR "Génése et Evolution des Domaines Océaniques," UBO, 6 av. LeGorgeu, 29287 Brest Cedex, France.

* School of Oceanography, University of Washington, Seattle, WA 98195,U.S.A.

Note: Although it is assumed in this study that the stable remanent magnetic'ector (SRMV), to which all structural element directions are referenced, has a

north azimuth, other studies in this volume (Kikawa and Pariso, Pariso et al.)point out that it is reversed from that of the present field. Therefore, to comparedirections in geographic coordinates with other directional data from Site 735, itseems most appropriate to assume that the SRMV points South.

The position of the crust at Site 735 relative to the AtlantisII transform valley suggests that its accretion took place some11 m.y. ago, about 18 km to the east of the ridge/transformintersection (Fig. 1). Large-scale normal faulting of the newlycreated oceanic lithosphere in ridge/transform intersectiondomains is documented by submersible observations at theKane Fracture Zone/Mid-Atlantic Ridge intersection (Dick etal., 1981; Karson and Dick, 1984; Mével et al., in press). Theextensional ductile shear zones observed in the Site 735gabbros may be the deep-seated expression of such normalfaults (Cannat et al., this volume).

During drilling and coring operations at Site 735, each pieceof core was free to rotate around the vertical axis. Only the dipof the deformational structures therefore can be directlymeasured in the core. Because the stable remanent magneti-zation of the gabbro section as a whole appears to have beenacquired during a single magnetic interval (Shipboard Scien-tific Party, 1989), the paleomagnetic vector in each piece ofcore probably pointed in the same direction before drilling.Based on this assumption, the orientation of the deformationalstructures measured in distinct pieces of core relative to oneanother, and their trend relative to the paleomagnetic vector,can be reconstructed using the paleomagnetic measurements.This chapter presents the results of this reconstruction. De-termination of the azimuth of the deformational structures atSite 735 in the geographic framework involves estimating thein-situ paleomagnetic vector direction, using magnetic loggingdata (Pariso, unpubl. data). The results presented here showthat the shear zones drilled at Site 735 have consistent azimuthand dip directions. This indicates that the tectonic evolution atSite 735, involving viscous flow of the incompletely solidifiedmagma, followed by solid-state ductile extension at progres-sively decreasing temperatures, occurred in a coherent strainfield. This is also an a posteriori indication that the reorienta-tion procedure adopted here is valid.

REORIENTATION PROCEDUREThe dip (inclination) and the azimuth (declination) of the

stable remanent magnetic vector (SRMV) were measured in158 samples (Shipboard Scientific Party, 1989; Pariso et al.,this volume). Eighty-two of these samples show deformationalstructures (foliation, lineation, and/or veins), which have beenmeasured using the ODP coordinate system (Fig. 3). Theestimated error of these measurements is ±10°. The stabledeclinations in these 82 samples, measured in the same

409

M. CANNAT, J. PARISO

32°OO'S

33°00'S

Africa

RIDGE

O Site 735

RIDGE

TEXTURAL TYPE

57°00'E

Figure 1. Bathymetric map of the Atlantis II transform fault, showingthe location of Site 735. Contour intervals at 1000 m (survey fromConrad cruise 27-09, 1986; H. Dick, Chief Scientist, with D. Gallo andR. Tyce).

coordinate system, are listed in Table 1 (estimated error:±15°). Our working hypothesis is that the SRMV in all thesesamples pointed in the same direction before drilling. Becausethe true orientation of this remanent vector is unknown, it hasbeen set to an arbitrary value of 360° (north). A correctionfactor equal to 360°-D (stable remanent declination in the ODPcoordinates) was added to the foliation, lineation, and veindirections measured in each sample (Table 1). The resultingcorrected values are relative to the direction of the in-situremanent vector (Fig. 4). When we know the azimuth of thisvector relative to geographic north (Pariso, unpubl. data), the

oQ

ICO

CDCLCL

z>

ü

<X)

oQ

Iü

üco

Io

Figure 2. Lithological and structural stratigraphy at Site 735 (Cannat etal., this volume). From left to right: depth below seafloor; corenumbers; lithostratigraphy (Unit I: metagabbronorites, and mixedgabbronorite and olivine gabbro; Unit II: olivine gabbro with intervalsof oxide gabbro; Unit III: mixed, disseminated oxide olivine gabbroand olivine gabbro with intervals of oxide gabbro; Unit IV: oxidegabbro; Unit V: olivine gabbro; Unit VI: olivine gabbro with troctolitesand intervals of oxide gabbro; M.: mylonitic shear zone; small trian-gles: late magmatic intrusives); textural types, corresponding to in-creasing finite strain (type I: weakly recrystallized, unfoliated; type II:moderately recrystallized, foliated; type III: extensively recrystallized,well foliated; type IV: extensively recrystallized, with millimeter-thickmylonitic bands; type V: mylonitic). Upper, Middle, and LowerStructural domains: see text.

410

DEFORMATIONAL STRUCTURES AT SITE 735

ARCHIVE HALF

Figure 3. Sketch of the ODP coordinate system, used to measure theazimuth and dip of the deformation structures (foliation, lineation, andveins), and of the stable remanent magnetic vector in each studiedsample (see Table 1).

true azimuth of the measured structures can be readily ob-tained by a simple rotation around the vertical axis.

Cored pieces were cut in half (archive and working halves;Fig. 3) on board the JOIDES Resolution. Foliated pieces wereusually cut perpendicular to the foliation. As a result, mostuncorrected foliations in Table 1 have a 180° azimuth in theODP coordinate system (Fig. 3). The reorientation procedureobviously does not apply to horizontal planar structures.Consequently, the relatively frequent horizontal foliationsthroughout the hole (Cannat et al., this volume), are notrepresented in Table 1, nor in the stereoplots of Figure 4.Precise measurements of the lineation azimuth and dip (in theODP coordinates) are only possible in pieces cut or brokenparallel to the foliation plane. This explains the limited num-ber of lineation measurements presented in Table 1. However,the stretching and mineral lineation is near downdip in mostdeformed samples: the mineral elongation is much stronger insections cut perpendicular to the foliation and parallel to itsdowndip direction, than in sections cut perpendicular to thefoliation and parallel to its strike.

DOWNHOLE DISTRIBUTION OF THEDEFORMATION AT SITE 735

The downhole distribution of the ductile deformation in theSite 735 gabbros allows one to distinguish three structuraldomains (Fig. 2; Cannat et al., this volume):

1. The Upper Structural Domain extends from Core 118-735B-1D to Core 118-735B-36R (0-171 m below seafloor[mbsf]). It is characterized by extensive ductile deformationthought to be associated with movements along subhorizontalmylonitic shear zones, located in the lower part of Unit I (Fig.2; Cannat et al., this volume)

2. The Middle Structural Domain extends from Core 118-735B-36R to the second third of Core 118-735B-56R (171-272mbsf). It is characterized by the development of a foliationthat probably results from laminar viscous flow in the gabbroicmagma (Cannat, this volume). This magmatic foliation isoverprinted by solid-state ductile deformation in discreteintervals; and

3. The Lower Structural Domain extends from the lowerthird of Core 118-735B-56R to the bottom of the hole at Core118-735B-88N (272-500.7 mbsf). Only the lower part of thisdomain (Cores 118-735B-77R to 88N; 403-500.7 mbsf) isplastically deformed.

The solid-state ductile deformation in these three do-mains occurs in metamorphic conditions ranging from gran-ulite facies to lower amphibolite facies (Stakes et al., thisvolume). An increase in the ductile strength of the gabbros isinferred to have accompanied this temperature decreasebecause deformation appears to have occurred at increasingdeviatoric stress conditions (Cannat, this volume). In theUpper Structural Domain, shearing at lower temperature(lower amphibolite facies) overprints the higher temperature(granulite and upper amphibolite facies) with the samegeometry. In the Middle and Lower Structural domains, thelower-temperature shear zones frequently cut the earlier,higher-temperature, shear zones at a significant angle (Can-nat et al., this volume).

The displacements accommodated by brittle failure in theSite 735 gabbros are limited (Cannat et al., this volume). Themacroscopic features resulting from brittle deformation in thecore are millimeter- to centimeter-thick veins, filled with avariety of hydrothermal minerals (Stakes et al., this volume).These veins are most abundant in the Upper StructuralDomain, where they are mostly filled with hornblende andoften contemporaneous with the last stages of the ductiledeformation (Cannat et al., this volume). In the Middle andLower Structural domains, the veins are often filled withhydrothermal plagioclase and clinopyroxene and mostly post-date the end of the ductile deformation.

RESULTS

The Upper Structural DomainThe declination of the SRMV was measured in 62 samples

from the Upper Structural Domain (Shipboard ScientificParty, 1989; Pariso et al., this volume). Forty-two of thesesamples have been plastically deformed and/or cut by veinsfilled mostly with green-brown hornblende (Stakes et al., thisvolume). The stereoplots of Figures 4A and 4B show theorientation and dip of the foliation, lineation, and veins inthese 42 samples (Table 1), relative to the direction of theSRMV (V = vector).

The mylonitic shear zone at Cores 118-735B-6D (Fig. 2)has a subhorizontal foliation (not represented in Fig. 4A) anda lineation trend of V29O0 (Core 118-735B-6D; Fig. 4A).Above and below this shear zone, the foliation's dip azimuthis variable, with roughly three groups of orientations: V0° toV20°, V310°, and V200° to V260° (Fig. 4A). Conjugate dips,directed toward V130°, were also measured in a few pieces(Fig. 4A). The attitude of the foliation in the Upper StructuralDomain is represented schematically in Figure 5A. Thestretching lineation is dominantly downdip in all samples. In

411

Table 1. Reorientation data for 82 studied samples, Site 735.

Core, section,interval (cm)

118-735B-1D-1, 191D-1, 141

2D-1, 113

2D-1, 139

2D-2, 993D-1, 60

4D-2, 76D-1, 1117D-1, 74

7D-2, 98D-1, 459D-1, 120

10D-1, 2412R-1, 3212R-2, 36

12R-3, 813R-1, 10213R-2, 5513R-3, 14114R-1, 1114R-1, 3514R-2, 2215R-2, 128

16R-1, 13916R-4, 7716R-5, 2422R-3, 118

24R-3, 5024R-4, 2625R-2, 526R-1, 6226R-3, 3628R-2, 11429R-2, 4630R-4, 1430R-5, 9131R-2, 12031R-4, 118

32R-3, 1434R-1, 10335R-1, 2935R-5, 131

Stabledeclin.

(uncorr.)

333135

137

320

31561

250190360

33025015923153

208

6152

153142109126199261

15219461

214

325112

19248

22725344

181232196

6648

195159

Fol. andlin.

(uncorr.)

135S25180E45L:120SE42180WS50L:90W50180W55L:90W55180W20

180W35L:120-0180W35L:145NW30180W20180W20L:205-0180E15130NE40180E25L:90E25180E25180EN40180E40180E50180E40180E40180E45180E25L:30N10180E30180E25180E30

180E30

180E50

180W25

180E40

180E30L:90E30180WS30

Fol. andlin. (corr.)

342W25225NW45L:160N42223SE50L:133SE5040W55L:310W5545W55

110N35L: 11060180W35L:145NW3030W20110N20L:176-0129S15257N40152W25L242W25299N25308N40207W40218W50251N40234NW40161W4599S25L:127E10208W30166W25299N30

35E30

112S50

307SW25

179W40

164W30L:254W30294S30

Vein(uncorr.)

180E70180W40180E35

150W55

180W50

180E75180E75

180W70180W60

180W50180W35

90S3090N35

300S60290N55180E70

180E75180W65

180E60

180W55

225-90180E60

180W60180E30

Vein(corr.)

299N70299S40110S35

277S55

299S50

251N75234NW75

161E7099N60

208E50166E35

236NW30236SE35155W60358E55341E70

338NE75288S65

316NE60

128N55

339-90312N60165E60

201W30

Core, section,interval (cm)

36R-2, 11

36R-3, 3637R-1, 1137R-3, 8039R-1, 145

39R-3, 2140R-2, 6240R-3, 9641R-4, 6842R-4, 6243R-4, 1743R-4, 6445R-1, 145R-2, 1546R-2, 2146R-2, 128

48R-4, 8249R-2, 8952R-4, 6953R-3, 15

55R-3, 13058R-2, 3363R-3, 80

63R-6, 2866R-2, 8668R-3, 1570R-3, 476R-3, 5077R-4, 7078R-4, 6579R-2, 65

79R-6, 2779R-7, 9980R-1, 131

82R-1, 1883R-4, 95

85R-4, 9

86R-6, 14387R-5, 20

87R-6, 28

Stabledeclin.

(uncorr.)

189

94140152120

13016311921035383

141309262108352

11794

35403

171355133

321331

3815017082

120146

134231211

112126

128

189188

315

Fol. andlin.

(uncorr.)

180E60 (m)180E60 (m)180E15 (m)180E55L:90E55180E40220SE30180E40

180W40 (m)180E20 (m)180E45 (m)180W40 (m)

180E30 (m)180W30L:90W30180E25 (m)180E20 (m)

L:90-0180E25190E15

90S20180E45180E35180W25L:115W22180E25180E35180E15L:210NE06180E50180E20L:90E20180E35L:90E35180E20180E15L:90E15180E20

Fol. andlin. (corr.)

266N60 (m)220NW60 (m)208W15 (m)240NW55L:150NW55230NW40237NW30241NW40

07W40 (m)227N20 (m)219W45 (m)51NW40 (m)

252N30 (m)08W30L:278W30243NW25 (m)266N20 (m)

L:267-0357E25189W15

188E20240NW45214W35214E25L:150SE22226NW25129S35149SW15L:179S09248NW50234NW20L:144NW20232NW35L:142NW35171W20172W15L:262W1545E20

Vein(uncorr.)

180W40180W15

180-90

180W50180-90180W30245NW45

60SE80340NE80

180E40

180E50220E45140SW50

180E75160E70155E70180E45180W2090S60180W45180W4090-90

180W60

Vein(corr.)

171E40171E15

220-90

230SE50197-90241SE30215E45

157E8019W80

98S40

06E5037E45317SW50

05E75207W70202W70221SE4529W20232SE60210E45190E40188-90

232SE60

Note: The correction factor is equal to 360°-D (uncorrected stable remanent declination). The corrected orientation and dip of the deformation structures are expressed in acoordinate system in which North is parallel to the remanent magnetic vector (see text).

DEFORMATIONAL STRUCTURES AT SITE 735

Figure 4. Reconstructed orientation and dip of the deformationstructures (foliation, lineation, and veins) relative to the direction ofthe stable remanent magnetic vector (see text). Lower hemisphere,equal angle projection. Squares: poles of the planar structures (folia-tion and veins); closed circles: stretching lineation. A. Upper Struc-tural Domain (Cores 118-735B-1D to 118-735B-36R; Fig. 2). Solidstate foliation and lineation. The squares with an oblique line insidecorrespond to poles of foliations with conjugate dips (see text). Thehorizontal lineation in the mylonitic shear zone at Core 118-735B-6D(Fig. 2) is identified by its core number. B. Upper Structural Domain;veins. C. Middle Structural Domain (Cores 118-735B-36R to 118-735B-56R; Fig. 2); magmatic foliations. D. Middle Structural Domain;solid-state foliations and lineations. The squares with an oblique lineinside correspond to the poles of the early, high temperature folia-tions, parallel to the magmatic foliation of the surrounding gabbros.The foliations and lineations in the shear bands at Cores 118-735B-46R and 118-735B-53R to 118-735B-56R (Fig. 2) are identified by theircore numbers. E. Middle Structural Domain; veins. F. Lower Struc-tural Domain (Cores 118-735B-56R to 118-735B-88N; Fig. 2); solid-state foliations and lineations. The lower-temperature, higher stressfoliation and lineation at Core 118-735B-87R are identified by theircore number. G. Lower Structural Domain; veins.

the foliated gabbros, the veins are preferentially perpendicularto the stretching lineation. Most veins shown in Figure 4Bwere measured in the undeformed to moderately deformedintervals between Cores 118-735B-13R and 118-735B-35R(Fig. 2 and Table 1), where the dominant dip direction of thefoliation is V220° to V260° (Fig. 4A and B). The dominant dipof these veins is thus toward V40° to V80° (Fig. 4B).

The Middle Structural DomainThe declination of the SMRV was measured in 36 samples

from the Middle Structural Domain (Shipboard ScientificParty, 1989; Pariso et al., this volume). Twenty-one of thesesamples (Table 1) show a magmatic foliation, or a solid-stateductile foliation and lineation, and/or veins, filled mostly with

Figure 5. Schematic representation of the foliation data presented in thestereoplots of Figures 4A, 4C and 4D. V = direction of the remanentmagnetic vector. A. Upper Structural Domain; thin discontinuous lines= foliation above and below the subhorizontal shear zone of Core118-735B-6D (thick line). B. Middle Structural Domain; thin lines =magmatic foliation (note the progressive downward decrease of the dip);thick line = one of the later solid-state shear zones.

plagioclase, clinopyroxene or hornblende (Stakes et al, thisvolume).

The stereoplots of Figures 4C and 4D show that, as in theUpper Structural Domain, the foliations (magmatic and solid-state) tend to have a northwesterly dip, relative to the SMRV.The dip direction of the magmatic foliation ranges from V28O0

to V36O0, with a dominant dip toward V32O0 (Fig. 4C). The dipof this magmatic foliation decreases progressively downwardfrom Core 118-735B-36R to Core 118-735B-56R. This isrepresented in a schematic way in Figure 5B. In the moder-ately deformed interval between Cores 118-735B-38R and118-735B-40R (Fig. 2), the solid-state foliation dips towardV33O0 (Fig. 4D), parallel to the magmatic foliation in theneighboring samples (Fig. 4C). This is consistent with themicrostructural observations, which suggest that the solid-state episode in this interval resulted from the very last stagesof flow in the progressively solidifying magma (Cannat et al.,this volume). The foliation and lineation in the meter-thicknormal shear zone at Core 118-735B-46R, and in the highlysheared interval between Cores 118-735B-52R and 118-735B-56R (Fig. 2), have the same orientation: the dip direction ofthe foliation is V2660 to V2780, and the lineation is downdip(Fig. 4D). This geometrical similarity is an important con-straint on the tectonic evolution of the Site 735 gabbrosbecause the microstructural observations suggest that thedeformation at Core 118-735B-46R post-dated the normalshearing at Cores 118-735B-52R to 118-735B-56R and oc-curred at lower temperatures (Cannat et al., this volume). Thesketch in Figure 5B shows the crosscutting relationshipsbetween these shear zones and the earlier magmatic foliationof the gabbros. The veins preferentially dip toward V80° toVI40° (Fig. 4E), roughly perpendicular to the magmatic andsolid-state foliations (Figs. 4C and 4D).

The Lower Structural DomainThe declination of the SRMV was measured in 60 samples

from the Lower Structural Domain (Shipboard ScientificParty, 1989; Pariso et al., this volume). Only 19 of thesesamples (Table 1) show a plastic foliation and lineation, and/orveins, filled mostly with clinopyroxene, or hornblende, andplagioclase (Stakes et al., this volume). This small number ofsamples reflects the lack of deformation in the upper 20 cores

413

M. CANNAT, J. PARISO

(Fig. 2) and the moderate vein density in the Lower StructuralDomain (Stakes et al., this volume).

The solid-state plastic foliation, developed in the lower 11cores (Cores 118-735B-77R to 118-735B-88N; Fig. 2), dipseither toward V330°, or toward V260° to V270° (Fig. 4F). Thelineations are downdip. The two distinct orientations of thefoliations do not strictly coincide with distinct tectonic events.However, most amphibolite facies shear zones from Cores118-735B-85R to 118-735B-88N (Cannat et al., this volume)dip toward V260° to V270° (Fig. 4F), while the earlier granu-lite-facies shear zones in Cores 118-735B-77R to 118-735B-84R dip dominantly toward V330°, parallel to the magmaticfoliation and to some solid-state foliations in the MiddleStructural Domain (Figs. 4C and 4D). The veins, measured indeformed and undeformed intervals, dip preferentially towardV95° to VI35° (Fig. 4G) as in the Middle Structural Domain(Fig. 4E).

CONCLUSIONS

The partial reorientation of the Site 735 deformationstructures relative to the SRMV shows that there is aconsistent dip direction of the different sets of structures:magmatic foliation; early granulite-facies, solid-state folia-tion; later, lower-temperature foliation; and veins. Thisconsistency is an a posteriori indication that the in-situSRMV of each core piece probably did point in the samedirection before drilling.

The orientation of the different sets of structures relative toone another suggests that the earliest deformation structuresin the Middle and Lower Structural domains (magmatic foli-ation and early, granulite-facies, solid-state foliation) tend tomake a 45° angle with the later, lower-temperature, shearzones (see sketch in Fig. 5B). This indicates a shift in theprincipal strain axes orientation during the tectonic evolutionof the Site 735 gabbros near the ridge axis.

The dip azimuth of the solid-state foliation is consistentlywest or northwest (relative to the azimuth of the SRMV). Thestretching lineation in a subhorizontal mylonitic shear zonefrom the Upper Structural Domain trends V290° (Fig. 4A),roughly parallel to the stretching lineation in the main shearzones of the Middle and Lower Structural domains (V260°-

V280°; Figs. 4D and 4F). This suggests a similar shearinggeometry in all these shear zones.

The synkinematic hornblende veins measured in the UpperStructural Domain tend to dip toward the northeast of theSRMV (Fig. 4B), perpendicular to the foliation of the sur-rounding gabbros (Fig. 4A). In the Middle and Lower Struc-tural domains, the veins' (mostly post-kinematic) dip azimuthsare preferentially oriented at 90° to 140° from the SRMV (Figs.4E and 4G). They are also nearly perpendicular to the mag-matic and solid-state foliations measured in these gabbros(Figs. 4D and 4F).

The in-situ azimuth (declination) of the paleomagneticvector at Site 735 is still undetermined. Once this informationbecomes available (Pariso, unpubl. data), we will be able toreorient the deformation structures observed in the gabbros ina geographic frame. This will be done by simply rotating thestereoplots in Figure 4 so that the remanent magnetic vectorpoints toward its azimuth relative to geographic North.

ACKNOWLEDGMENTS

Financial support for this study was provided by ODP-France and by NSF Grant No. OCE86-9260. One of us (J. P.)also benefited from a JOI/USSAC Fellowship.

REFERENCES

Dick, H.J.B., Thompson, G., and Bryan, W. B., 1981. Low anglefaulting and steady-state emplacement of plutonic rocks at ridge/transform intersections. EOS (Abstract), 62:406.

Karson, J. A., and Dick, H.J.B., 1984. Deformed and metamor-phosed oceanic crust on the Mid-Atlantic Ridge. Ofioliti, 9:279-302.

Mével, C , Cannat, M., Gente, P., Marion, E., Auzende, J. M., andKarson, J. A., in press. Emplacement of deep rocks on the westmedian valley wall of the MARK area (M.A.R., 23°N). Tectono-physics.

Shipboard Scientific Party, 1989. Site 735. In Robinson, P. T., andVon Herzen, R., et al., Proc. ODP, Init. Repts., 118: CollegeStation, TX (Ocean Drilling Program), 89-212.

Date of initial receipt: 24 July 1989Date of acceptance: 4 June 1990Ms 118B-162

414