JOURNAL OF GEOPHYSICAL RESEARCH, VOL 103, NO. B1, PAGES 663-676, JANUARY 10, 1998

Dike orientations, fault-block rotations, and the constructionof slow spreading oceaniccrust at 22 ø40'N on the Mid-Atlantic Ridge

R6isfn M. Lawrence and Jeffrey A. Karson Divisionof Earthand Ocean Sciences, Duke University,Durham, North Carolina

StephenD. Hurst DepartmentofGeology, The University ofIllinois, Urbana, Illinois

Abstract. The firstpalcomagnetic results from oriented dike samples collected on the Mid- AtlanticRidge shed new light on thecomplex interplay between magmatic accretion and mechanicalextension at a slowspreading ridge segment. An uppercrustal section about 1.5 km thickis exposed along a west-dippingnormal fault zone that defines the eastern median valley wall of thesouthern segment of theMid-Atlantic Ridge south of theKane fracture zone (MARK area). Twodistinct groups of dikesare differentiated onthe basis of orientationand palcomagnetic characteristics.One group, on the basis of thepalcomagnetic data, appears to bein itsoriginal intrusionorientation. This group includes both ridge-parallel, vertical dikes as well as dikes in otherorientations, calling into question assumptions about uniform dike orientations at oceanic spreadingcenters. The second group consists ofdikes that have palcomagnetic directions that are distinctfrom the predicted dipole direction, and we interpret them to have been tectonically rotated. Thesealso occur in manyorientations. The spatial relations between rotated and nonrotated dikes indicatethat intrusion, faulting, and block rotation were contemporaneous beneath the median valleyfloor. Nonrotated dikes exposed onthe eastern median valley wall indicate that there has beenno net rotation of thisupper crustal assemblage since magmatic construction ceased. Hence slipand associated uplift probably occurred inthe fault zones' present orientation. These results providethe basis for a generalmodel of mechanical extension anddike intrusion forthis segment ofthe Mid-Atlantic Ridge. Initially, a portionof crustforms beneath the median valley by synkinematicdikeintrusion into laterally discontinuous faultblocks. Slip and associated uplift alongacataclastic normal fault zone later exposes this crustal section onthe valley margin. As spreadingcontinues, thisvalley-bounding cataclastic normal fault zone is abandoned infavor of a newfault system thus passively moving the exposed crustal section away from the median valley.

1. Introduction associatedwith seafloorspreading at this segmentof the Mid- Atlantic Ridge. The Mid-Atlantic Ridge south of the Kane Transform, The southemMARK (SMARK) areais locatedapproximately between22ø40'N and 23ø35'N (MARK area)(Figure 1) is oneof !00 km south of the Kane Transform (Figure 1). The themost intensely studied parts of themid-ocean ridge system. bathymetryof this spreadingsegment defines a highly Previouswork highlightsthe diversityin morphology, asymmetricha!f-graben morphology, similar to manyother geology,and geophysics [Purdy and Detrick, 1986; Karson et spreadingsegments along the Mid-Atlantic Ridge (MAR). The al., 1987;Kong et al., 1988;Morris and Detrick,1992]; segmentmorphology is dominatedby an approximately35 ø however,many details of seafloorspreading remain to be westwardfacing eastem median valley wall (EMVW) whichhas investigated.Here we report structural and pa!eomagnetic data a vertical relief of over 2500 m. Microseismic and teleseismic relevantto the creationand evolution of theuppermost oceanic focalmechanisms suggest that normalfaulting extends 4-8 km crustin thisslow spreading (-25 mrn/yr, full rate) segment. We into a relativelycool, brittle, lower crustand uppermantle use measurementsof in situ dike marginand fault surface beneath the median valley [Tooracy et al., 1985]. This orientationsand stepwise demagnetization data of orientedseismicity has beeninterpreted by Tooracyet al. [1988] to dikesamples collected using the Alvin submersible to map the representslip along a planarnormal fault which could be the distributionof magneticallynormal- and reversed-polaritysubsurface continuation of a large fault zoneexposed along the dikesand to determineif the dikes have been tectonically EMVW. rotatedsince the acquisitionof their magneticremanence. Dikes in the oceaniccrust are emplacednormal to the least Thesedata provide important constraints onthe orientation of compressirestress and therefore record the orientationof the dike intrusion and the kinematicsof extensionalfaulting stress field at the time of intrusion as well as subsequent tectonic rotations.Dikes are commonly rotated within fault- Copyright1998 by the American Geophysical Union. boundedcrustal blocks. The magnitudeand kinematics of these rotationsare controlledby the geometryof the bounding Papernumber 97JB02541. 0148-0227t98/97JB.02541$09.00 faults. Orienteddike samplescollected with a submersible- 663 664 LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST

44ø5: Kane Transform

Neowdcanic Ridge

\ 1

ß_3') o 15'N u Segment Boundary Zone

AI 2569

Segment

AI •71 22O55N I 22o40'N

Segment Boundary Zone B

Segment 3 •) Volcano •" Ridge Fissures o lO I I Fault k.m 45ø00'W

Figure 1. Location and generalizedgeology of the Mid-Atlantic Ridge at Kane (MARK) and southernMARK (SMARK) areas. (a) Generalized geology of the MARK area showing the setting of the SMARK area. (b) Location map of the SMARK area. The central portion of the median valley floor is defined as the area enclosed by the 3500 m isobath(shaded gray). Alvin dives are shown as bold lines. Bold, straight lines with tick marks (on hangingwall) indicate the generaltrace of the major Pault-linescarps.

operated orientation tool (Geocompass)have previously been by a faulted monoclinal structure. The gently sloping valley ßused to study paleomagneticand structural aspectsof dikes margin is cut by a right-stepping en echelon array of faults exposedat the seafloor [Hurst et al., 1994a]. In this study the which create a seriesof southwardfacing relay ramps.In some Geocompasswas also usedto collect orientationdata from fault places, pillow lavas and sheeted lava flows appear to flow over surfaces for structural studies and from margins and joint the faulted surfacesalthough faulting generally appearsto post surfaces of dikes to obtain oriented samples. We use these date volcanic activity. paleomagneticand structuraldata to constrain the orientations The EMVW escarpmentis continuousalong strike for over and rotation histories of dikes and faults in the SMARK Area. 30 km (22ø40'N - 22ø60'N). It rises from the median valley floor at --4500 meters below sea level (mbsl) to a ruggedcrest at -2200 mbsl. The escarpmentis essentiallya major fault-line 2. Geology of the SMARK Area scarpproduced by variably degradednormal faults which have The SMARK area representsa single spreadingsegment of created a "tectonic window" into the upper -1.5 km of the the MAR. It is highly asymmetrical with a relatively deep oceaniccrust (Figure 2). We have identified three lithologic median valley floor, a gently sloping western wall, and a units exposedalong the EMVW: a lowest unit of variably steeper,higher easternmedian valley wall. The central part of deformed, massive metadiabase, a middle unit of fractured the medianvalley of the SMARK area is broadlydefined by the metabasalts, and an upper unit of relatively fresh area enclosed by the 3500 m isobath (Figure lb). constructionalbasaltic lava flows [Lawrence et al., 1994, Photogeologicobservations suggest the medianvalley floor is Karson et al., 1996]. Dike exposuresare found mainly between not magmatically active. There is no evidence for a 3400 and 2450 mbsl and there is no evidence for a sheeteddike neovolcaniczone, and much of the valley floor is heavily complex in this section of exposed oceanic crust. sedimentedand/or fissuredand faulted [Lawrence et al., 1994; Variablydeformed and metamorphosed massive diabase is Waterset al., 1996]. The medianvalley floor is boundedto the exposedbetween 4500 and3300 mbsl.Very few (three)dikes eastby the bathymetricallyprominent EMVW. To the west, we were observedin this unit; theseare between3400 and3300 interpretthe more gently slopingvalley margin to be formed mbsl. Discrete faults and intervals of centimeter-spaced LAWRENCE ET AL.: CONSTRUCTIONOF SLOW SPREADINGOCEANIC CRUST 66>

Freshbasaltic pillow la,,as with minorsheet flows & diabase dike; 25OO Normalfaults cutting factured pillow lavasand dike;

Altered & f'actured basaltic

pillow lavaswith - 10% 3000 • di•ase dikes

Altered massive diabase

3500 '•

..-I Cataclastic Fault

4000

7• Freshand lavaBasalucPillowlavas fiov. s DiabaseDikes • BasalticPilbxv lavas Scale (km) PervasiveShear Zones•ttaclastic •,• AlteredMassive Diabase High-AngleNormal Faults L4500 0 novertical exaggeration I

Figure 2. Compositecross section and columnar section of theeastern median valley wall (EMVW) transect compiledusing data from dives 2568, 2570-2, 2878-9, and 2887. A thicknessof-1.5 km of uppermost oceaniccrust is exposedalong -35 ø fault-linescarps. Fault anddike orientationsare schematic.

cataclastic shear zones are oriented parallel to the slope predominantlyin the middle unit (3300 to 2600 mbsl; Figure creatingextensive dip-slope surfaces. These fault surfaces 4). These faults have dips in the range 55o-75ø and 200-45ø , commonlydisplay down-dip oriented slickenlines,grooves, respectively,and scatteredstrikes. They typicallyform scarps and striae. The relatively unaltered dikes that cut this unit a few meters high and commonly have relatixely fresh talus sueoestthat deformation and metamorphism of the massive accumulations.Some of the high-angle faults from this second diabasetook place near the locus of magmatismbeneath the groupclearly crosscutthe cataclasticfault zonesof the first median valley floor. group, impartinga step-like morphologyon parts of the Between 3300 and 2600 mbsl, intensely fractured and EMVW slope. variablyaltered basaltic pillow lavas are cut by individual In summarythe EMVW appearsto havebeen formed by the dikes and small swarms of dikes (with up to six individual interaction of two sets of structures. We infer that the slope- dikes).Diabase dikes comprise about 10% of the sequenceand parallelfault surfacesand cataclastic shear zones collectively are generallyless fracturedthan the basaltichost (Figure 3). define a singlelarge-displacement fault zone.We interpretthe Samplesof both dikes and lavas show moderate, patchy first-ordermorphology of this segmentto be the resultof slip replacementof the igneousmineralogy by greenschistfacies alongthe fault zonewhich we referto as the "TheEMVW fault assemblages. zone". The later, high-an•le normal faults appear to have Above 2600 mbsl, constructionalbasaltic pillow lavas and played a secondaryrole in accommodatinguplift and sheetedlava flows comprise>95% of the exposures,with modifyingthe scarp morphology. subverticaldiabase dikes forming the remaining-5% (Figure 2). Dikes in this area are relatively fresh and most lack 3. Samples,Data, and Analysis substantialalteration. The crest of the EMVW has significant 3.1 Oriented Samples sediment accumulations (-1 m) and is apparently not significantlytectonized. Twenty-one oriented samples were collected during 10 Faultsare divided into two groupson the basisof their submersibledives made along three transectsof the EMVW. orientations and structural characteristics. The first group The three transectsare spacedover a 10 km length of the consists of cataclastic shear zones and faults in the EMVW away from any apparent oblique-trending metadiabaseand metabasaltof the lower two units (below 3000 discontinuities (dives 2568, 2570-2, 2878, 2879, 2885, mbsl). Geocompassmeasurements were made at 30 fault 2887-9;Figure 1). Samplingof dikeswas concentrated within surfacesthat have an averagedip of-35 ø andstrike of 350ø- the middle fractured metabasalt unit; however, the full suite of 020ø (Figure 4). These fault surfacesform slope-parallel orientedsamples spans the interval 3500 to 2100 mbsl and outcropsone to severalmeters across. The secondgroup of includessamples from all three units.Oriented samples were structurescomprises high- and low-anglefaults located collected from 19 dike outcrops(Table 1). The samplesare 666 LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST

Figure 3. Alvin hull-mountedcamera photograph of a swarmof 5-6 parallel,weakly jointed dikes (each -0.5 m wide) in brecciatedbasaltic material. The planardike marginsdip about 50ø. Note the cameraperspective is 30 ø downward. generallymassive, joint-bounded blocks with plumosehackle vary by less than +5 ø. Dikes exposed along the EMVW are marks. More than 40 Geocompass measurements of dike relatively planar features, and analysis of dive videos suggests marginsshow a largescatter in the strikesand dips (Figure 4). a given surface or dike margin may have up to 10ø variation. The accuracyof Geocompassmeasurements depends on the Therefore the combined error associated with Geocornpass roughnessand regularityof the exposuresurfaces. Repeated estimates of a dike margin orientation are considered to be Geocompassmeasurements of individual surfacestypically +15 ø

22ø40'N

44ø55'N

Figure4. Mapof theEMVW studyarea showing individual dive tracks, sample locations, the dikes' present orientations,and a highlygeneralized trace of fault-linescarp. Lower hemisphere stereographic projections showthe poles to fault surfaces measured with the Geocompass; these are grouped with respect to the three main lithologicunits defined in thetext (a, variablydeformed massive metadiabase; b and c, fracturedmetabasalt; andd, relativelyfresh constructional basaltic lavas). Solid circles are polesto surfacesmeasured on the cataclasticnormal-fault zones; open circles are poles to surfacesmeasured on the later, generally more steeply dipping faults. LA-WRENCEET AL.: CONSTRUCTION OF SLOW SPREADINGOCEANIC CRUST 667

Table1. Table of Dike SampleDepths, Petrographic Descriptions, and Paleomagnetic Principal-Component Results

Mean Principle number Component

Depth, Dike of _ Rotation Sample mbsl strike/dip Petrology minicores Dec Inc •95 Predicted Polarity

2568-1428 3389 - Coarse-porphyriticmetadiabase; Plagioclase phenocrysts in a 4 210.0 -14.2 1.3 Y R groundmassof recrystallizedmaterial including plagioclae, actinolite, chlorite, and minor oxides. 2571-1152 2843 032/50E Very fresh, medium-grained,holocrystalline, metadiabase with 5 299.9 25.4 2.5 Y N plagioclase,olivine and gabbroicglomerocrysts in a subophitic groundmass. 2572-1020 2604 107/25S Very fresh, fine-grainedsubophitic basalt/diabase; contains sparse 4 140.3 -26.8 9.4 N R plagioclaseand olivine microphenocrystsand -10% vesicles. 2878-1440 2344 263/89N Relatively fresh,vesicular, medium-grained plagioclase-phyric 5 347.3 -34.2 6.7 N R basalt. The heterogeneousgroundmass is variably altered,fine- grainedoxides minerals are common. 2878-1500 2316 019/79W Fresh, vesicular,medium-grained plagioclase-phyric basalt similar 5 225.3 -27.1 11.3 Y R to 2878-1500. 2884-1214 2378 344/14E Moderatelyto slightly alteredaliabase. 5 161.3 56.7 5.2 N N 2884-1302 2302 096/40S Slightlyaltered diabase with plagioclaseand olivine phenocrysts in 5 165.0 43.4 12.2 N N a matfix of plagioclase,clinopyroxene, and olivine. 2885-1305 2973 345/22E Basalt/ diabasewith minor alteration. 5 266.6 -22.6 14.6 Y R 2885-1331 2972 357/65E Slightly to moderatelyaltered fine-grained basalt. 2 231.6 -30.5 9.2 Y R 2885-1333 2972 357/65E Moderatelyaltered diabase with smallphenocrysts of plagioclase 3 223.0 -29.9 10.6 Y R and olivine. 2885-1351 2956 288/43N Plagioclase-olivinephyric basalt with highlyresorbed zoned 5 15.4 47.7 0.6 N N plagioclasephenocrysts. 2887-1157 2595 040/58E Plagioclase-phyricvesicular basalt. 5 154.3 25.7 0.5 Y N 2887-1211 2593 055/71E Moderatelyaltered, holocrystalline diabase. 5 337.9 -18.6 10.3 Y R 2887-1229 2599 266/88N Moderatelyaltered holocrystalline diabase. 5 148.6 -36.4 10.6 N R 2887-1244 2600 052/68E Slightlyto moderatelyaltered subophitic diabase. 5 99.8 -73.1 9.9 Y R 2887-1303 2582 032/83W Moderatelyaltered diabase. Sparse plagioclase and olivine 5 176.4 -49.8 2.6 N R phenocrystsin a matrixof holocrystalline,subophitic plagioclase, clinopyroxene,and olivine. 2887-1313 2565 005/84WModerately altered diabase similar to 2887-1303. 5 58.4 2.3 1.3 Y N 2887-1315 2566 090/14S Moderatelyto slightlyaltered basalt with sparse plagioclase 3 59.2 3.1 0.9 Y N microphenocrysts. 2887-13562555 013/90 Moderatelyaltered, medium-grained, holocrystalline diabase. 5 244.5 - 15.5 6.7 Y R 2888-113! 2955 013/54E Very freshplagioclase-phyric basalt with (3-4 mmzoned) 5 247.2 -35.2 2.3 Y R phenocrystsin a groundmassof plagioclase and olivine. 2889-1201 3331 330/55E Holocrystallinemetadiabase to metabasalt. Subophitic plagioclase 5 120.2 72.6 8.4 Y N lathesin a vailablyaltered groundmass of plagioclaseand clinopyroxene. ' Alterationtocla• minerals i•typically restricted tothe groundmass andfraCiu;e fillingsi "' In rn0derateiy altered"samp'ies, n'•inoi'"•Iteration0'•' phenocrystsisobserved. Alteration products ofmetabasalts andmetadibases typically include actinolite andohiorite. The strike and dip of dikes ,me determinedfrom Geocompass measurements. Themagnetic polarity and presence orabsence ofa predictedrotation are determined from the palcomagneticprincipal components. Dec,declination indegrees; Inc, inclination indegrees; mbsl, meters below sea level.

3.2 Paleomagnetic Methods and Results remanencecomponent which decays to the origin was calculatedfor eachsample using principal-component analysis Five or six 2.5 cm diameter,2.5 cm lengthcores were drilled [Kirshvink, 1980]. A mean remanenceand associated95% from each oriented dike sample. These were stepwise sphericalconfidence limit (1x95value) was calculated from the demagnetizedby either alternatingfield (AF) or thermal five to seven remanencedirections per sample. This mean demagnetizationtechniques at Scripps Institution of direction is referred to as the "characteristic remanence" of a Oceanography.Three of the five coresamples were thermally dike. Six of the samples have characteristicremanence demagnetized,and two or threesamples were AF demagnetized. directions that are close to either the predictedpresent-day Stepwisethermal demagnetization was conducted at 50øC normal-polaritydipole direction (declination ! inclination= incrementsfrom 0 ø to 550øC with an additional 575øC step. 000ø/40ø) or the predictedreversed-polarity dipole direction Thenormalized intensity decay curves show two end-member (180ø/-40ø) for the SMARK area (Figure7 and Table 1). The trends, both of which indicate two unblocking temperatures: remaining samples have principal components of one at --120øC and a secondat 550ø-575øC(Figure 5). The magnetizationwith very shallow (inclinations <10ø) dominant550ø-575øC unblocking temperature is compatible directionswhich are distinctfrom either the predictednormal- withmagnetite being the primary magnetic carrier, whereas the or reversed-polaritydipole field directions. smaller, lower, unblocking componentis likely to be Sea-surfacemagnetic data place the EMVW withinthe early geothite. Brunhesepoch [Schulz et al., 1988].Therefore, samples with ProgressiveAF and thermaldemagnetization results show normal-polaritymagnetic remanence are interpreted as having predominatelyunivectorial magnetizations (Figure 6), The acquiredtheir remanencein the Brunhesepoch (0 - 0.78 Ma 668 LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST

1 a: 2884-1302al [Candeand Kent, 1995]), and samples with reversed-polarity b: 2885-1305bl characteristicremanences are interpreted to be Matuyama-age c: 2571-1152al rocks(older than 0.78 Ma). Normal-and reversed-polarity d: 2568-1428cl dikes have a complex spatial distribution,occurring in close proximity to one another and not defining a simple linear 0.6 boundary between areas with reversed-polarityand normal- polarityintrusions. The distributionof differentpolarity dikes appearsnonsystematic with respect to thesea-surface anomaly O.4 boundary(Figure 8). Three samplesfrom east dippingdikes have two, opposite polarity, componentsof magnetization.These sampleshave 0.2 reversed-polarity characteristic remanence directions and a small normal-polarity overprint (Figure 6d). The characteristic 0 I remanence of each is similar to the predicted reversedfield 0 100 200 300 400 500 600 dipole direction suggesting they have experienced no resolvable tectonic rotation since acquisitionof remanencein Temperature,øC a reversed-polarity field. Possible errors associatedwith the palcomagneticdata fall Figure 5. Normalized intensitydecay curvestbr the thermal into two broad categories:mineral alteration of the dikes and demagnetization of four representative samples. The unblocking temperaturesare -120øC and between 550ø and magneticfield variations.An assessmentof the reliabilityof 575øC. The dominant unblocking temperature between 550ø the presentmineralogy to reflect the magneticfield at the time and 575øC is consistentwith magnetite as a magnetic carrier in of dike intrusioncan be made usingrock magneticexperiments the samples. and petrographic observations. Susceptibility versus

at Sample2885-1351al b Sample2887-1313al W, Up w, Up N N

H 10e-l A/m NRM

Sample2572-1020al d Sample2887-1305al W, Up w, Up

N

NRM H 10e-1A/m H 10e-1A/m

Figure 6. Vector-end-pointdiagrams for four sampleswhich representthe main stylesof magnetization observed.Open circles represent the projectionof the remanencein the verticalplane, and solid circles representthe projectionof the remanencein the horizontalplane. The data are shownin geographic coordinatesafter reorientation using the Geocompassdata. (a) A single-componentnormal-polarity sample. Declinationis nearnorth and inclination is -40 ø, closeto the presentday predicteddipole direction. (b) A single-componentmagnetization sample, the remanencedirection of which is shallowand distinctfrom the predictedpresent dipole field direction. (c) A reversed-polarity,single-component sample which is closeto the predictedreversed-polarity field. (d) A samplewith two componentsof magnetization,a reversedpolarity component(350ø-575øC) and a normal-polarityoverprint (0ø-300øC). A normal-polarity overprint on a reversed-polarityremanence was observedin three of the samples. LAWRENCEET AL.:CONSTRUCTION OF SLOWSPREADING OCEANIC CRUST 669

N N that the reinanent magnetizationaccurately records the 2887-13 2885-135 geomagnetic field at the time of intrusion. The EMVW is located near the boundarybetween the Brunhesand Matuyama epochs(Figure 8) [Schul:.,et al., 1988], and both normaland reversed-polarityfield dikeswere sampled.Secular variation and field variability during a polarity transition may result in dikes with anomalous remanencedirections. Therefore it is possiblethat samples N N with nondipolarremanent magnetizations record transitional 2884-13,. 2878-15 field directions.Studies of the Brunhes-Matuyamaboundarv indicate the transitionoccurred over less than 10,000 years [l•ett and Obradivich, 1991' Tauxe et al., 1992; Baksi et al., 1992]. Using an averagefull spreadingrate at the SIXlARKarea of 2.5 cm/yr and assuming that the dike intrusion rate is constant and that all spreading is accommodated by dike intrusion, the total width of crust created during the transitional interval would be about 250 m. The width of the Figure 7. Stereographic projections of principal componentsof magnetizationfor five subsamplesfrom each median valley wall crustal section over which both normal- and orientedsample, the associatedmean direction,and ot95 reversed-polaritydikes were sampledis much wider, about 3 to confidencelimits. (a) and(b) Normal-polaritysamples. Figure 4 kin. Assuming the dikes are about I m wide and intruded 7a has a mean characteristic remanence direction throughouta 3 to 4 km wide area during a 10,000 year field indistinguishablefrom the presentdipole direction. The mean transition, one in 12 to 16 dikes will record a transitional remanencedirection shown in Figure 7b is shallow (2.3 ø) and field. This gives a probability of 6 to 8% that any one dike clearly distinct I¾omthe 15ø angular variation ellipse estimated sampled records a transitional field. Twenty one oriented dike to representsecular variations about the dipole direction. (c) sampleswere used in this study, 13 of which haxe nondipole and (d) Reversed-polarity samples. Figure 7c has a mean characteristic remanence directions. If all the transitional field remanence direction which is (within errors) not distinct from the reversed dipole direction. Figure 7d shows a mean dikes intruded the 3 to 4 km wide sample region and all the remanencedirection distinct l¾omthe dipole. Figures 7a and 7c dikes with nondipolar remanencesrepresent transitional dikes, are interpreted as samples for which no rotation can be the probability of sampling 13 transitional field dikes by assumed.Figures 7b and 7d are interpreted as samplestaken random selection is 7 x 10-•Sto 2 x 10-•6%. Hence we from rotated dikes. conclude that the magnetization of all the nondipolar dikes sampled cannot be attributed to Brunhes-Matuyama transitional field variations. Instead, we interpret the 3 to 4 km temperature and isothermal remanent magnetization (IRM) wide region of mixed-polarity dikes to be a reflection of the studieswere done on five of the samplesat the paleomagnetics minimum width over which dike intrusion occurred across the laboratory at University of California, Davis to determine the median valley at the time this crustal assemblage was nature of the magnetic carriers. constructed. An IRM was induced in each sample twice, initially a saturationIRM (SIRM) at a peak field of 1200 mT, then a reversed lower field of 300 mT was applied to preferentially 4. Quantifying Tectonic Rotations remagnetize only the 1ow-coercivity minerals. The ratio of IRM300mT/SIRMI2oO mT intensities gives an indicationof the Paleomagnetic data can be used to identity and quantity coercivity of the magnetic carriers [Verosub and Roberts, rotations of dikes intruded at seafloor spreading centers 1995]. The samples have IRM/SIRM ratios approximately [Alletlon and Vine, 1987; Hurst et al., 1994a]. Certain equal to one (0.92-0.98), indicating the principal magnetic assumptionsmust be made in order to e'•aluate paleomagnetic carrier has a low coercivity, such as magnetiteor maghemite data in this way: (1) The Earth's magnetic field can be (Table 2). Susceptibility as a function of temperaturewas approximatedby a simpledipole. (2) The magneticremanence measuredusing a Kappabridge KLY-2 with a CS-2 furnace vector is "locked" at the time of intrusion. (3) Dikes are tabular attached. The results of which (Figure 9) indicate that rigid bodies, and hence the angle betv,een the normal to the magnetite and/or titanomagnetite are the main magnetic dike margin and the magnetic remanence vector remains carriers. constant. Petrographicanalysis shows that the diabasedike samples As a first step, we considerif a dike samplehas experienced have subophitictextures with equantolivine and range from post-intrusiontectonic rotation. The errors on the magnetic relatively fresh to moderately altered. Alteration typically directionsand secularvariations about the predicteddipole are consistsof replacementof the mafic mineralswith chloriteand both estimatedto be about 15ø . If thesetwo errors overlap, we actinolite and crosscuttingveins of chlorite, epidote, and interpret that there has been no paleomagneticallyresolvable smectite.Overall, the petrographic,IRM, susceptibility,and rotation. On this basis, seven of the 20 sampleshave not been demagnetizationdata are consistent with relatively coarse- rotated.These dikes are importantbecause they show the range grained magnetite as the main magnetic carrier. Because of possibleintrusion orientations. In addition, faults and other magnetite is a primary mineral formed on cooling, no structures that these dikes cut cannot have been rotated since significantcomponent of the magnetizationis carried by intrusion. The remaining 13 samples have characteristic magnetic grains that formed later, and both AF and thermal remanencedirections distinct from either the predictednormal- stepwisedemagnetization results show the majorityof samples or reversed-pt.•laritydipole directiona and are interpreted to haveonly a singlecomponent of magnetization,we believe have been tectonically rotated. 670 LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST ø00••'W 02•2ø50'N/

22ø45'N ,,',:,,'.-.. 22ø45'NI

22ø40'N

13o; Figure 8. Distribution of oriented samples with respect to the Brunhes - Matyama magnetic polarity boundary. The magnetic polarity of the characteristic remanence of samples is shown by solid (normal polarity) and open (reversed polarity) circles. The location of the Brunhes/Matyama boundary is determined from sea-surhce anomalies [ Schulz et al., 1988]. The Brunhes- Matuyama boundary is inferred to be near the top of the EMVW at approximately2500 metersbelow sea level. The positive anomaly area (shaded) is widest in the central portion of the SMARK spreadingsegment. The negative anomaly region is shown without shading.

In order to evaluate the validity of these rotations, we use and we are unable to analyze the possibility of multiple two reorientationmethods guided by simple models of oceanic rotations about different axes. The first method is used to test crust formation. In the first, we assume vertical dike intrusion the hypothesis that originally vertical dikes have been and determinethe requiredrotation axis and amount of rotation. systematically rotated in a manner consistent with the In the second, we use a single rotation axis defined by the strike of regional fault orientations to determine the original orientations of the dikes and the amount of net rotation. In 6001 • • I i I I both, we can only consider net rotations about a single axis, 500b• \'•---\ 5oøC - 2887-130562 ' I•,,oc5 --- 2878-1440a3 400 Table 2. SIRM at 1200 mT and IRM at 300 mT and Ratio Data From Five RepresentativeSMARK Dike Samples , , oo5 200 Stunpie S1RM12{•} IRM3{•} IRM/SIRM Intensity (A/m) Intensity (A/m)

..... 2568-1428c2 7.58 6.95 0.92 0 i i i I ! i 0 1O0 200 300 400 500 600 700 1878-1440a3 296.8 275.8 0.93 1878-1500c2 242.5 238.3 0.98 Te•nperature(øC) 1884-1302a2 129.2 126 0.98 1887-130562 260.8 253.3 0.97 Figure9. Susceptibilityas a functionof temperaturegraphs for two representativesamples. These curves are compatible IRM, Isothermal remanent magnetization; SIRM, Saturation with magnetite and/or titanomagnetitebeing the main isothermalreinanent magnetization magnetic carriers. LAWRENCEET AL.: CONSTRUCTIONOF SLOWSPREADING OCF_A•C CRUST 671 observedfault geometries.The secondallows us to evaluate either 156ø or 204ø , both of which are approximatelyridge- originaldike orientationsforced to fit the observedfault parallel (010ø-190ø).However, the steeprotation axes imply geometries.These results can be comparedto Geocompassblock rotationscontrolled by oblique-slipor strike-slipfaults; measurementsof dikes with dipolar characteristicremanence no evidenceof which was seenwithin the studyarea. directionsinterpreted to be in their original orientations. In addition, a number of nonvertical dikes with remanence directionsthat are coincidentwith eitherthe predictednormal- 4.1 Rotation of Initially Vertical Dikes or reversed-polaritydipole direction were identified. These The "Penrosemodel" [Penrose ConferenceParticipants, dikes are interpretedon the basis of their palcomagnetismto 1972]for the structureof the oceancrust, together with many be in their original orientations,and hencethe assumptionof investigationsof oceaniccrust and ophio!ites,and theoretical vertical dike intrusionis not necessarilyapplicable to all the modelsof dike intrusionnear the Earth's surfacesuggest dikes SMARK dikes. are commonlyintruded vertically. Inclined dikes have been observedin crustal exposuresof slow spreadingenvironments; 4.2 Rotation by Faulting however, these are generally interpreted to have been A secondpossible reorientation method sets the orientation tectonicallytilted within crustal blocks boundedby normal of the rotation axis based on a simple fault model for the faults [Auzende,et al., 1989; Pariso and Johnson, !989; EMVW. Geocompassmeasurements show that exposedfault Karson and Rona, 1990; Karson et al., 1987; Juteau et al., surfacesstrike 010 ø +10ø, and slickenlinedata show only dip- 1995]. slip motions.From these data we infer an averagehorizontal If the original orientation is taken to be vertical, the rotation axis of 010ø/0ø. Unlike the previousexercise, this magnitudeand amount of rotationare determinedby rotating method makes no assumptions regarding the original the normal to the dike margin to horizontal and the orientationof the dikes. The magnetic direction is returnedto characteristicremanence back to the predicteddipole position, within error of the geocentricaxial dipole direction along a while maintaining their angular relationship[Allerton and rotation path about the axis defined. By maintaining the Vine, 1987]. This techniquetypically providestwo possible angular relationship between the dike margin and the results.Outcrop observationscan be used to help choose paleomagneticdirection, the pre-rotation orientation of the which, if either, is geologicallyreasonable. In this case the dike is determined. preferredresults would be the restoreddike orientationmost The possible rotations calculated by slip along west- nearlyparallel to the ridgeaxis. dipping approximately N-S striking normal faults yield The possibletectonic rotations determined are highly rotation magnitudes between 20 ø and 100ø (Table 3). The variablewith widely scatteredaxes and the majoritybeing results suggestmany of the possible rotations are less than steeplyinclined (>60ø), the magnitudeof the rotationsare in 40ø. Most of the nondipolardike data could be the result of therange 24 ø to 178ø (Table3, all rotationsare described using variable amountsof rotation about the predictedregional fault right-handrule notation). For example,sample 2878-1500 is a axis (Figure 10). Thus the majority of nondipolardikes (9 out subverticaldike (strike/dip: 019ø/79øN) with a nondipole of 13) may have experiencedtectonic rotationsresulting from characteristicremanence (declination/inclination: 213ø/-33ø), the tilting of crustalblocks boundedby normal faults which the rotations which return the dike to vertical and the parallelthe EMVW and hencethe regionalspreading axis. The remanenceto the dipole are 42 ø about an 054o/69ø results of this reorientation method suggest that six were (trend/plunge)axis and 48ø about an 090ø/74ø axis. These intrudedin an inclinedorientation (dips <75ø), but the majority reorientationssuggest a vertical dike was intrudedstriking (10) were intruded subvertically. Four of the dike samples

Table3. PalcomagneticRotation Analysis for SamplesWhere a Rotationis PredictedFrom the Paleomagnetic Principal Components

vertical dike reodentation I vertical dike reorientation II structural re-orientation

Sample Dike Magnetic Rotation Amountof Dike Rotation Amountof Dike Amountof dike orientsion strike/dip polarity AxisI RotationI OrientationI Axis !I RotationII OrientationI! Rotation Strike/dip

2568-1428 - R ! 12/23 ø 45 180/vertical 013/60ø 96 167/vertica! 32 035186ø E 2571-1 !52 032/50E N 025/43 ø 25 145/vertical 0561450 100 140/vertical 43 024/88ø E 2878-1500 0!9/79W R 050/69 ø 42 156/vertical 045/70ø 48 204/vertical 26 19'9/76ø W 2885-1305. 345/22E R 210/16ø 80 182/vertical 047145ø !38 178/vertical 82 174/76ø W 2885-1331 357/65E R 145/57 ø 43 148/vertical 050/25ø 112 19g/vertical 24 357•89ø E 2885-!333 357/65E R 137/66 ø 40 152/vertical 026/47ø 120 208/vertical 31 177/84ø W 2887-!157 040/58E N 254/11 ø 72 !41/vertical 102/69ø 178 2 !91vertical 22 034•76ø E 2887-!211 055/71E R 164/560 64 ! 84/vertical 026/!6ø 130 176/vertical 112 262/51ø N 2887-1244 052/68E R 282/16 ø 48 226/vertical 152/16ø 152 134/vertical 87 241t57ø N 2887-1313 005/84W N 249/22 ø 108 129/vertical 222174ø 134 231/vertical 45 187/39ø W 2887-1315 090/14S N 251/30 ø 92 112/vertical 220/68ø 120 230/vextical 45 225/38ø N 2887-1356 013/90 R 317/56 ø 34 260/vertical 328•52ø 36 100lve•ca! 37 196/54ø W 2888-113! 013/54E R 048/65ø 60 197/vertical 027/2ø 72 163/vertical 82 194/45ø W 2889-1201 330/55E N 032/72ø 24 141/vertical 046/48ø 120 219/vertical 63 153168ø W - 'Rotationsarecal•ul.at•'ii bytWO methods; a•Sumingan"'ødginal verticaldikeorientation (with2possible'so'iUtions:' I andII)and bystmc•ura¾' re-orientationassuming arotation axis defined bythe strike ofridge-parallel, west-dipping normalfaults. Dash indicates datanot available. 672 LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST

rotations have occurred about a ridge-parallel axis. The Rotation Axis variabilityin rotationssuggest individual dikes and groupsof ø1ø/øøø dikes are within discretefault-bounded blocks. On the basisof the geologicalobservations in the SMARK area[Karson et al., 1996], the MARK area [Brown and Karson, 1988] and elsewherealong the MAR [Macdonaldand Luyendyk,1977; Verosub and Moores, 1981; Karson and Rona, 1990] we consider the results of this reorientation method (rotations generally<40 ø ) to be reasonablein this tectonicsetting. Hencewe prefer this methodto the first approachwhich (1) assumesinitially vertical dikes (an assumptionnot supported by ourdata) and (2) resultsin muchlarger rotations (commonly >100ø). Consideringthe results of the two methodswe suggestthe following: (I) Dikes presently exposed along the EMVW in Figure 10. Stereographicprojection of mean characteristic the SMARK area were not necessarilyintruded vertically. (2) remanence directions for all the samples. Reversed polarity The magnetic remanence of the majority of samples with samples(open circles) are plottedin the upperhemisphere, and nondipole remanence directions (i.e., rotated) can be reoriented normal polarity samples(solid circles)are plotted in the lower hemisphere.The predictedregional fault axis of 010ø/00ø and to a predicted original dipole by moderate rotations about a normal and reversed dipole directions are marked. A circle ridge-parallel, horizontal axis. (3) Several differently rotated around the dipolaf directionsshows the predicted 15ø secular blocks were sampled. (4) Uplift and faulting of the crustal variation errors. Any rotationabout the predicted010ø/0 ø axis assemblage,which was created during faulting and intrusionin (and estimatederror limits) will follow a path within the shaded the median valley to its present position in the EMVW, took areas; therefore any characteristicremanence direction within place without significant rotations. the shadedarea can be modeled by a single rotation about the predictedpole back to a dipolaf position or within secular variation errors. 5. Discussion

5.1 Dike Intrusion Within the Median Valley cannot be reoriented by rotation about the predicted axis A minimum width of magmatismbeneath the medianvalley (Figure 10) and may have experiencedmultiple rotationsabout of 3-4 km can be estimated from the spatial distribution of different axes. normal- and reversed-polarity dikes sampled. This represents the minimum width of the dike intrusion zone that was active The results of dive 2887, which made a traverse along the strike of the EMVW at -2600 mbsl (Figure 1), provide an during the Brunhes/Matuyama magnetic field transition, a example of the results of this reorientation method. We period of 10,000 years. These observationsand interpretations interpret some of the younger (<0.78 Ma, Brunhes normal- are comparable to or greater than estimates of the width of polarity) dikes to have been tectonicallyrotated, whereas some of the older (>0.78 Ma, Matuyama reversed-polarity)dikes are in their original orientations.Seven of the samplescollected during this dive are groupedaccordingly, two reversed-polarity rotated dikes, the individual rotations of which suggest both dikes were intruded in inclined orientations (-250ø/55øN) and rotated -100 ø to east-dippingorientations (-055ø/70øE: Table 3). Intermingled with these are two reversed-polarity, nonrotated samples that were intruded in -2500/80 ø orientations. Less than 1 km along strike from these are three dikes we interpret to have experienced a -40 ø rotation. We Accommodafi'•on suggestthese rotated dikes lie within a separatefault block and structure that a fault bounding that block, perhaps a minor accommodationzone, separatesthe two groupsof dikes (Figure •). Figure 11. Schematicblock diagramof interpreteddike and The dive 2887 dikes have original orientations of fault relationshipsfrom dive 2887. We interpretdikes A, B, 262ø/51øN and 241ø/57øN; 212ø/81øN and 266ø/88øN; and C (samples2887-1313, 2887-1315, and 2887-1356)to be 196ø/54øW, 187ø/39øW, and 225ø/38øN (for the three groups, originally subverticalor west-dippingdikes which have been respectively)and were intruded at a variety of approximately rotated-40 ø aboutan axis definedby the strikeof localfaults. Dikes I and 2 have different rotation histories from dikes A, B, ridge-parallel, inclined orientations(Table 3). We infer from the mixed assemblageof younger/older, nonrotated/rotated andC; we interpretthese to be withina separatefault-bounded block and there to be an accommodation zone between the tv,o dikes that faulting and associatedblock rotation within the areas. Both dikes 1 and 2 have reversed-polarit) median valley was active at the time of dike intrusion. magnetizations;dike 1 hasbeen rotated -100 ø abouta similar Therefore it appears that fault-bounded crustal blocks were axis as dikes A, B, and C (samples2887-1211 and 2887- intrudedby dikes both before and after tectonicrotations. 1244), whereasdike 2 is in its originalorientation (samples Dike reorientations using a single rotation axis defined by 2887-1229 and 2887-1303). The intrusion and present the regional fault geometry indicate it is possible that orientations of these dikes is shown in Table 3. LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST 673 present-daymagmatic accretion in the SMARK area [Lawrence less rotatedor nonrotateddikes within a single fault-bounded et al., 1994;Waters et al., 1996]and elsewhere along the MAR block (Figure 11). Also, dike assemblagesin adjacentfault [Macdonald,1982; Karson et al, 1987;Kong et al., 1988; blocks are likely to have different rotation/intrusion Smithand Cann, 1993;Bryan et aL, 1994;Hussenoeder et al., relationships. This stems from our interpretation that 19961. relatively older dikes(reversed polarity) are apparentlyin their It is commonly assumed that dikes beneath oceanic original orientations, whereas nearby normal-polarity spreadingcenters are intrudedin a ridge-parallel,vertical (younger) dikes have been rotated. These two sets of orientationand form sheeteddike complexes[e.g., Cann, relationshipsare specifically highlightedin the resultsfrom 1970].This assumptionis intuitivelyappealing in termsof dive 2887, discussedin section 4.2 (Figure 1 I). These data manysimple models for crustalaccretion and the mechanics of suggestthat normal faults within the median valley form a dikeintrusion at the Earth'ssurface [e.g., Vetosuband Moores, complexpattern &limiting fault-boundedb!ocks of the order 1981]. However, many factors could influenceintrusion of a few hundredmeters in length parallel to the ridge axis geometryat shallow crustal levels, such as variations in crustal (Figure 12). density,pre-existing fault structure,or topography.The 40 5.2.2 The EMVW fault zone. The upper crustal SMARK area dike margin measurementshave an average assemblagedescribed above has beenuplifted and exposed,in orientationof 199ø/54øE. Many dikes observedfrom Alvin theEMVW. The majorscarps in thisarea appear to be theresult also have east dipping orientations. These observations of the interplayof a west-dippingcataclastic fault zone (the contrastwith the predicted average ridge-parallel vertical EMVW fault zone) and generallylater, more steeplydipping orientation. faults.The palcomagneticdata suggest some of the oldestdikes Nonvertical intrusions have been documented in the form of are in their original orientations.These data suggestthat cone sheets and ring dikes around central volcanoes in crustalblocks have been uplifted into the EMVW with little or extensionaltectonic settings [e.g. Gudrnundsson, 1983; no net rotation. This includes blocks that experienced no Walker,1993]. Bathymetric and camera tow dataat theSMARK rotationsin the medianvalley floor as well as rotatedblocks. area indicate a number of conical features we interpret to be The presenceof nonrotateddikes along the EMVW indicates isolated volcanoes [Waters et aI., 1996], that may be the that slip alongthe EMVW fault zonehas not resultedin a location of similar shallow-level intrusions. The systematicrotation of the footwall.With no net rotationof the palcomagneticresults suggest some of theinclined dikes are in footwall the EMVW fault zone is unlike!y to have rotatedand theiroriginal orientations and could be fragmentsof tingdikes thereforemust have slipped in its presentorientation of-35 ø or cone sheetsintruded at shallow crustal levels around these westdipping. This interpretationis compatiblewith that of typesof volcanicedifices. Overall, the SMARK areadikes Toomeyet al. [1988],who suggestedthat activemechanical appearto be a mixedassemblage of vertical dikes and inclined extensionat SMARK is dominatedby a west-dippingplanar sheets.Similar patterns of dikeswith variabledips have been normalfault zonebeneath the medianvalley floor, a part of the documentedand are interpretedas intrusionsduring the waxing footwallof whichis exposedat the surfacealong the EMVW. andwaning of the magmasupply, for examplearound Kilauea Volcano,Hawaii [Ryan, 1988]. Isolatedconical volcanoes 5.3 Construction of Slow Spreading Oceanic havealso been described along other parts of the MARK area Crust [Konget al., 1988,Bryan et al., 1994]and other segments of Numerousmodels for seafloorspreading at slow rates(<50 theMAR [Smithand Cann,1992, Smith and Cann,1993] and mm/yr full rate) have been proposedbased on the internal appearto be an importantaspect of magmaticaccretion at slow structureof ophiolites[Harper, 1984; Allenon and Vine, 1987; spreadingrates. Hurst et aL, 1994b; Varga, 1994] or on observedsurficial We suggestthat mixed vertical and nonverticaldike structuresalong the MAR [Macdonald,1982; Karson et al., intrusionmay be commonalong slow spreadingridges and 1987; Karsonand Winters,1992; Mutter and Karson, 1992; concludethat both ridge-parallel vertical dikes (such as dikes Cannat, 1993; Tucholke and Lin, 1994]. On the basis of feedingfissure eruptions) and inclinedring dikes(such as shallow-level dikes around individual volcanic edifices) are geologicaland palcomagnetic constraints we suggesta model for the accretion and structural evolution of the uppermost intrudedbeneath the medianvalley floor (Figure12). If this is oceaniccrust in the SMARK area (Figure 12) that may be true,then dikes or evenassemblages of dikesintruded beneath applicableto other segments of the MAR. For the purposes of the medianvalley may not providereliable palcoverticalthis discussionwe identifytwo distincttectonic regimes: (!) referencemarkers in slow spreadingoceanic crust. the magmaticand mechanicalprocesses active within and beneaththe median valley floor and (2) the mechanical 5.2 Geometry and Kinematics of Faults processesalong the medianvalley marginthat appearto 5.2.1 Faults within the median valley. The controlthe first-order morphology of thisslow spreading ridge reorientationmethod based on the strike of faults suggeststhat segment. the SMARK area dikes were intruded into a number of fault- New oceanic crust forms within the median valley by bounded blocks. In addition, the rotation results are magmaticaccretion in a zonewhich is >3-4km wide.Plutons compatiblewith theseblocks having experienced variable coo! at shallow levels (!-2 km) producing the massive rotationsabout approximately ridge-parallel axes. Nonrotated diabase/microgabbro unitobserved along the lower portions of andv •ariably rotated dikes occur in closeproximity in ourstudy the EMVW. Thesesmall plutons are thesource for thedikes and area,suggesting that dike intrusionoccurred before, during, lava flowswhich make up the upper 1-2 km of crust(Figure andafter normal faulting beneath the medianvalley floor. 12a).The uppermostcrust is comprisedof smallswarms of Althoughcrosscutting relationships among dikes were not dikes which cut older lava flows, There is no sheeteddike observed,we wouldexpect that older rotated dikes are cut by complex.Intrusions occur in a variety of orientations 674 LAWRENCE ET AL.' CONSTRUCTION OF SLOW SPREADING OCEANIC CRUST

normal Medianvalley Volcano A faults faults Fissureeruptions, Cone sheets, vertical dikes inclined dikes

N

3

5•

7 0 Scale(kin) 5

No vertical exaggeration

Median valley B volcano (i) Originally verticaldike (ii)Originally inclined dike (v) (iii) Originally verticaldike, now inclined (iv) Originally inclineddike, now vertical i original (v) Originally inclineddike, tilted but still inclined (iv) (ii) vertical 0 Scale (kin) i (i) I l

Figure 12. Schematic diagram depicting the evolution of the SMARK area. (a) Schematic diagram of the median valley and adjacent valley margins. The overall segment morphology is defined by a large west-bacing, fault-line scarp (the EMVW) interpretedas the surface expressionof an active normal-fault zone that projects into the subsurface-5-7 km beneath the median valley floor. Lower relief, east-facing normal fault-line scarps occur along the western valley margin. (b) Schematicdiagram of-5 km of the median valley floor within the region of active magmatic accretion. We suggestthe following evolution for a portion of crust I which initially forms by magmatic constructionand mechanical extension beneath the median valley floor. Dike intrusion is diffuse and there are large individual volcanoes. Rotation of individual fault-bounded blocks within the median valley results in variably rotated dikes within the upper crust as dikes are intrudedinto the blocksboth before and after tectonic rotations.This crustal assemblagewill later be exposed on the valley margin as a result of uplift associatedwith slip along a major normal fault zone like the inferred EMVW fault zone. This occurs in the same way that the similar but older crustal assemblageII (Figure 2) has been uplifted and exposedalong the EMVW. As spreadingcontinues, this valley-boundingfault zone may be abandonedas a new fault zone forms closer to the ridge axis. Crust II and its boundingfault-line scarpwill then be passivelymoved away from the medianvalley just as crust lII was previously. including ridge-parallel vertical dikes as well as vertical and Considerable extension is accommodated by slip along a inclined dikes in a range of azimuths (Figure 12b). Ring dikes cataclasticnormal fault zone which dips approximately35 ø and cone sheets are likely to be common in the vicinity of from the EMVW westward beneath the median valley flool conical volcanoes within the median valley. (Figure 12a). Slip along this cataclasticfault zone and the In addition to magmatic accretion within the median valley associateduplift of crustalblocks created in the medianvalley some of the plate separation is accommodatedby normal floor area have resultedin the bathymetricasymmetry of the faulting and associatedrotations of small 'fault-boundedblocks ridge segment(Figure 12a). Later steepnormal faulting has (Figure 12b). Dikes intrude fault-boundedblocks before and resultedin final uplift and geometricmodification of the after tectonic rotations. In this way, dikes are intruded and then EMVW. We infer fromthe datapresented in thispaper that the rotated within small (hundred meter scale) normal-/hult bounded majorityof uplift has occurredwithout net rotationof the blocks which may later be intrudedby more dikes. This results footwall; the simplest kinematics to result in the exposureat in variably rotated dikes within individual fault blocks. In the surfaceof a nonrotatedupper crustal section is a planar addition, dikes in different t:ault blocks may have very different normal fault zone. In time the active fault zone will be rotation histories, which can result in the case of older supersededby a laterfault zone to the west,and the crustal nonrotated dikes in one fault block in close proximity to sectionpresently exposed along the EMVW will bepassively younger rotated dikes within an adjacent fault block (Figure movedaway from the axis (Figure 12a). In thisway, parts o! •). uppercrust, such as those depicted at I, 11,or 111in Figure12a, LAWRENCE ET AL.: CONSTRUCTION OF SLOW SPREADINGOCEANIC CRUST 675 areprogressively accreted and moved away from the locus of Gudmundsson,A., Form and dimensionsof dykesin easternIceland, Tectonophysics,95, 295-307,1983. magmaticaccretion. Harper,G.D., The JosephineOphiolite, northwestern California, Geol. Soc.Am. Bull., 95, 1009-1026, 1984. 6. Conclusions Hurst,S.D., J.A. Karson,and K.L. Verosub,Palcomagnetism of tilted dikesin fastspread oceanic crust exposed in the HessDeep Rift: We have shownthat palcomagneticanalysis of oriented Implicationsfor spreadingand rift propagation,Tectonics, 13, 789- 802, 1994a. samplesfrom the ocean crust can provide new information on Hurst,S.D., E.M. Moores,and R.J. Varga, Structuraland geophysical theprocesses of magmatic accretion and mechanical extension expressionof the Solea Graben, TroodosOphiolite, Cyprus, in the SMARK areathat may be applicableto otherslow Tectonics,!3, 139-156, 1994b. spreadingridge segments. Wehave used measurements offault Hussenoeder,S.A., M.A. Tivey, H. Schouten,and R.C. Searle,Near- orientationscombined with palcomagneticdata to help bottommagnetic survey of theMid-Atlantic Ridge axis, 24ø-24•40'N: Implicationsfor crustalaccretion at slowspreading ridges. J. of determinethe timing and kinematics of faultingand associated Geophys.Res., 101, 22,051-22,069, 1996. fault-blockrotations in the SMARK Area.The resultsof this Izett, G.A., and J.D. Obradivich,Dating of the Matuyama-Brunhes studycan be usedto makea numberof specificconclusions boundarybased on 40Ar-39Ar ages of theBishop Tuff and Cerro San Luisrhyolite: Geol. Soc. Am. Abstr. with Progams., 23(5), A 106,1991. regardingmagmatic accretion and mechanical extension inthis Juteau,T., D. Bideau,O. Dauteuil,G. Manach,D.D. Naidoo,P. Nehlig, spreadingsegment: H. Ondreas,M.A. Tivey, K.X. Whipple, and J.R. Delaney,A 1. Ourdata indicate that dike intrusionin the SMARKarea submersiblestudy in thewestern Blanco Fracture Zone, N.E. Pacific: tookplace over at least3-4 km wideregion and that the Structureand evolution during the last 1.6Ma, Mar. Geophys.Res., orientationsin which dikes were initially intrudedwere 17, 399-430, 1995. variable,probably including both ridge-parallel vertical dikes Karson,J.A., S.D. Hurst,R.M. Lawrence,and SMARK '95 Cruise Participants,Upper crustal construction and faulting at a segment- andinclined ring dikesor conesheets. scalehalf graben on the Mid-Atlantic Ridge at 22ø30'N(SMARK 2. Thepresence of both rotated and nonrotated dikes in close Area)(abstract), Trans A GU, 77(17), SpringMeet. Suppl., S271, 1996. proximityindicates that dike intrusion was concurrent with Karson,J.A., and A.T. Winters,Along-axis variations in tectonic faultingbeneath the median valley. extensionand accommodationzones in the MARK Area, Mid- 3. Nonrotateddikes provide evidence that crustalblocks AtlanticRidge 23øN latitude. OphioIites and Their Modern Oceanic exposedin the EMVW have experienced nonet rotation since Analogues,edited by L.M. Parson,B.J. Murton, and P. Browning, dike intrusion. Geol.Soc. Sp. Pub., 60, 107-116,1992 4. Nonrotateddikes on theEMVW constrainnearby cataclastic Karson,J.A. and P.A. Rona, Block-tilting, transfer faults and structural controlof magmaticand hydrothermal processes on theTAG area, faultzones to haveslipped in theirpresent orientations (dip Mid-AtlanticRidge 26øN, Geol. Soc. Am. Bull., 102, 1635-I645, 1990. -35ø to the west).These fault zonesmay be relatedto the Karson,J.A., et al., Along-axisvariations in seafloorspreading in the seismogeniczone previously identified beneath the median MARK Area, Nature,328, 681-685, 1987. valley floor. Kirschrink,J.L., The least-squaresline andplane and analysisof palcomagneticdata,Geophys. J.R. Astron. Soc., 62, 699-7!8, 1980. Kong,L.S., R.S. Derrick, P.J. Fox, L.A. Mayer, and W.B.F. Ryan, The Acknowledgments.Support was provided bythe National Science morphologyand tectonics of the MARK Area from Sea Beam and FoundationMarine Geology and Geophysics Program (grant OCE 92- SeaMARC ! observations(Mid-Atlantic Ridge 23•N), Mar. Geophys. 02661).We are indebted toJ.S. Gee at Scripps and K.L. Verosub atUC Res., I O, 59-90, 1988. Davisfor the use of themagnetics laboratories, to D. Kelleyfor the Lawrence,R.M., J.A.Karson, & SMARKCruise Participants, Geology initialpetrography of the dive samples, and to W.P. 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