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