JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 99, NO. B2, PAGES 3175-3200, FEBRUARY
10, 1994
Rheological evolution of the ocean crust: A microstructural view SusanM. Agar GeologicalSciencesDepartment,NorthwesternUniversity, Evanston,Illinois
Abstract. Rheologicalstudiesof the oceaniclithospherehave useddirectobservations of rocks andinferences basedonthermal,mechanical, andexperimental models.Modelingstudieshelpto constrainan averagecrustandmantlerheologyandthedeep-seated processes thatcontrolregionalscalestrengthand stressfield variations.The smoothingof fine-scalevariationsin suchmodels canobscuremanyof the geologicalprocesses thatinfluencestrainlocalizationanddeformation partitioningin the oceancrust.Examinationof deformationmechanisms andhistoriesin ocean crustrocksprovidesa complementary approachto modeling.Examplesof structures anddeformation historiesin diabasesfrom Deep SeaDrilling Project/ OceanDrilling Programsite504B, the HayesandAtlantisfracturezones,(Mid-AtlanticRidge),andtheTroodosophiolite,(Cyprus),are presentedin conjunctionwith a synthesis of microstructural studiesof the oceancrustoverthe last 25 years.A surveyof brittle,quasi-plastic,and synmagmaticviscousdeformationis usedto demonstrate theinfluenceof primarycompositional andtexturalcharacteristics andvariablemagmatic andhydrothermalhistorieson deformationmechanismsand strainlocalizationin the ocean crust.Geologicalevidenceindicatesthathydrothermalfluidsstronglyinfluencethe natureof deformationand that effectivestresses may be low dueto fluid overpressures. Melt distributionwill stronglyinfluencestrainlocalizationat the baseof thecrustandsynkinematic hydrationduring crystalplasticdeformationplaysa key role in the relativestrengths of polyphaseoceaniclithologies.A schematicdistributionof failure mechanisms in the oceancrustis usedto discussthe controlson variationsin lateralandverticalstrengthprofilesandtheirpossiblerelationto spreading rates.Althoughmicrostructural studiesof oceancrustare still in theirinfancy,theyprovidevaluableconstraintsfor rheologicalmodelsandfurtherinsightsto explainthe distributionof seismicity at spreadingcentersandacousticsignatures.
Introduction
Constraining the detailed rheological evolution of the oceancrust at spreadingcentersis a key aspectfor developing models of constructivemargin initiation, rift propagation,and seafloor spreading. Direct and indirect observationsover the last 25 years have revealed the diversity of igneous and metamorphic assemblagesand the variable intensity of both distributed and localized deformation generated at spreading centers.The mechanicalresponseof the heterogeneousocean crust, combined with the varying thermal, stress, and strain rate conditions during seafloor spreading are key aspectsfor determining time-dependentocean crust rheology (Figure 1). The reconstruction of deformation histories using combined microstructural, petrological, and geochemical studies can provide valuable constraintson the timing, distribution, and mode of failure as oceancrustis generatedand movesoff-axis. For this special section on ocean crust evolution, this paper synthesizesstudiesfrom the last 25 years to assessthe spatial and temporal variations of deformation histories in the ocean crust from a microstructural viewpoint. New examples from Ocean Drilling Program (ODP) site 504B (Costa Rica Rift), the Atlantis and Hayes fracture zones (30 ø and 34øN on the MidAtlantic Ridge (MAR))and the Troodos ophiolite (Cyprus) are used to provide further insightsto the controlson deformation mechanismsand failure in oceanic and ophiolitic crust. The implicationsof these data are discussedin the context of ocean Copyright 1994 by the AmericanGeophysicalUnion. Paper number 93JB02953. 0148-0227/94/93JB-02953
$05.00 3175
crust rheology and are used to highlight areas where future progresscould be made. The term "microstructure"is usedhere in a broad
sense to refer
to structures
evident
in cores or hand
samples either by the naked eye, optical or electron microscope.
Approaches to Understanding Ocean Crust Rheology Investigations of oceanic lithosphere theology have proceededalong two complementarypaths (Figure. 2): direct observations
of rocks
and inferences
based
on thermal
and
mechanicalmodels which employ experimentaldata. Regional models for entire ridge segments (>50 km) have used geophysical data (e.g., gravity, bathymetry, seismic reflection, seismic refraction, and earthquake seismicity) in combination with estimatesof theological properties derived from rock deformation experiments [Tapponnier and Francheteau, 1978; Sleep and Rosendahl, 1979; Watts, 1982; Weins and Stein, 1983, 1984; Bergman and Soloman, 1984, 1990; Morton and Sleep, 1985; Buck, 1986; Phipps-Morgan et al., 1987; Lin and Parmentier, 1989; Chen and Morgan, 1990; Hayes and Kane, 1991]. Such models have provided constraintsfor average ocean crust and mantle theology and for the deep-seatedprocesseswhich control strengthand stress field variations on length scalesof 50-100 km and over depth scales of several kilometers. Assumptions regarding spatial and temporal variations in theology, however, limit the resolution of these models and their predictions. Smoothing of fine-scale (tens of meters to about 1 km) geological variations in these models can obscuremany of the processes which influence the theology of the ocean crust. Data for
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EVOLUTION
OF OCEAN
CRUST
4. Magmatic pulses, limited hydrothermal •
•1. Nomagmatism, limited hydrothermal fluidflow
circulation r•• _____
•
•
/
Mineralogy !
/
St•eady down-tempe rata r•
L orrapid cooling atseafloo• ß tedstresse•
•....
Porosity / Crack Lengths /
/• VariaUons in porefluidpressure,• ( porefluidchemistry, fluctuations
Cohesion •
•, inthermal .gr.adients, effective •
12.Periodic upwelling and
3. Hydrothermalcirculation and magmaticpulses
I
I I
I
I
Examples of magmaticand hydrothermal historiesexperiencedby a rockduring either on axis or duringmotionoff axis Examplesof environmental variationsassociated with schematicmagmaticandhydrothermal histories that will alter pdmaryphysicaland chemical characteristicsand modify the mechanicalresponse of the rock
waning of hydrothermal
circulation withvariable
permeability
[-•magmatic pulse • hydrothermal circulation
Fig. 1. Different environmentsat spreadingcentersand their impact on deformationhistories.Four schematic magmatic and hydrothermalhistoriesare shownto illustratethe varying environmentalconditionsthat a newly crystallizedrock may experienceduringoceancrustevolution.(1) No subsequent magmatismperturbslocal thermal gradientsas the rock coolsrapidlycloseto the seaflooror followsa steadydowntemperaturepathat deptheitheron axis or as it movesoff-axis. (2) Local thermalgradientsandeffectivestressesvary with the multipleupwelling and waning of hydrothermalcirculation(shownschematicallyby black arrows)and permeabilityvariationswhich may, in turn, be influencedby deformation.(3) The rock is locatedclose to subsequentmagmaticintrusions(shown schematicallyby open arrows),vigoroushydrothermalcirculationcausesextensivealteration,modifying thermal gradients and effective stresses.(4) The rock is located close to subsequentmagmatic intrusionsbut remains impermeableto hydrothermalfluids. rheological properties are derived from experiments which have focused mainly on monomineralic rocks, proposedto be representativeof bulk mantle properties,or pristine, unaltered polyphase assemblages [e.g., Goetze, 1978; Chopra and Paterson, 1981; Caristan, 1982; Karato and Paterson, 1986].
Using experimentallyderived flow laws requires extrapolation of results to geological strain rates [Paterson, 1987; Ruttar and Brodie, 1991] and does not take into account
any modification in the mechanicalpropertiesof rocks during deformation and metamorphism. An improved understanding of ocean crust rheology requires answers to the following questions: (1) What are the spatial strength variations in ocean crust and how do they change as newly generatedcrust moves away from a spreadingcenter? (2) How is deformation partitioned and what are the controls on strain localization in the ocean crust?
and magmatic histories which modify rock textures and physical properties, microstructural studies can help to constrain the time-dependent mechanical response of the ocean crust. Well-defined deformation histories provide constraintsfor rheological model assumptions,including the partitioning of deformation and the spatial and temporal variations in stress states. Paleostress and strain estimates,
together with kinematic indicators acquired from microstructural analysis, provide essential comparative data for instantaneous,present-daystressand strain measurements acquiredfrom borehole breakout data [Zoback and Anderson, 1983; Newmark et al., 1984; Moos and Zoback, 1990], geodetic measurements [Shimada et al., 1990; Stein et al., 1991], and sourcemechanismstudiesof earthquakes[Toomay et al., 1985; Huang and Solomon, 1988; Bergman and Solomon, 1990, 1992; Wilcock et al., 1990].
Microstructural studies provide a complementaryapproach to ocean crust rheology by focusing on the deformation mechanismsand their influence on crustal strength[Hanks and
Microstructural
Raleigh, 1980; Schmid, 1982; Poirier, 1985; Twiss, 1986; Knipe, 1989]. By examining the deformation, hydrothermal,
Most of the early models for ocean crust evolution focused mainly on geochemical and petrological data and the nature
Studies
of Ocean
Crust
AGAR: RHEOLOGICAL
EVOLUTION
OF OCEAN CRUST
3177
I Observations and Analysis e.g. lithologies, physical properties, mineralogy,
Regionalobservations Bounda•j conditions
•
œexture/fabric,
e.g. thermal s•ructure, magma chamber geometry,
characœerisfic microscruct•res
I Interpretatiøns I
dro•herrnal circulation
deformation mechanisms,
Rheolocjical Assumptions [
conditions of deformation,
e.g. œemperaœure,
• e.g. lithologies, feilure mechanisms, frtcœional/plasrJc
scrain and strain ral:e, deviaœoric s•ress
flow la•,s
Microstructural Evolution
Rheological Models
Rheological Evolution
Fig. 2. Complementary approaches to understanding oceancrustrheology. and origin of magmaticproductsat spreadingcenters.Less than 10% of over 200 studiesof oceancrustsamplesprior to
intersection,and older oceancrust) with variousspreading
rates(Table1 on microfiche ]). Thedatabase is biasedtoward
the early 1970s [Cann and Sirnkin, 1971; Christensen et al.,
comparatively well-exposed sectionsin the walls of transform
1973] incorporatedmicrofabric descriptionsof any kind. Much of the microfabric work on oceanic and ophiolitic sampleshas focusedon the high-temperaturedeformationof
faults and ridge-transformintersectionsat slow spreading
lower crust and mantle lithologies [Mercier and Nicolas, 1975; Nicolas and Poirier, 1976; George, 1978; Gueguen and Nicolas, 1980; Nicolas et al., 1980; Nicolas, 1989]. The combinedresultsof such studies,however,do not approach the extent and detail of microstructural investigations in continental crust [e.g., Carter and Tsenn, 1987], and brittle failure mechanisms in the oceancrusthavebeenneglected. The presentdatabasefor microstructuralstudiesof the ocean
crust and possible analogs in ophiolites contains numerous isolated (but useful), descriptive, and semiquantitative analysesof microfabrics.Only recentlyhave the significance of thesemicrostructures in termsof the mechanicalresponse of the ocean crust and the time-scales on which deformation
centers.The natureof samplesrecoveredis alsobiasedby the sampling technique. Dredging and submersiblesare more likely to acquire lithologies weakened by alteration and fracturing in exposedfault scarps,while drill core will be biasedtowardmorecoherent,lessdeformedsections. Many of the sampledsectionsmay not be representativeof "normal" ocean crust generatedwithin a long, stable spreadingcenter segment.They provide, however, the lithelogical diversity essential to a comprehensive deformation mechanisms databasefor the oceancrust.If the rocksexposedin transform faults and ridge-transformintersectionsdo representsections of lower oceancrust [Auzendeet al., 1989], then they may provide insights to variations in deformation mechanisms with depth.It has been suggestedthat interpretedsectionsof lower oceanic crust exposedin axial segmentsof the MidAtlanticRidge(MAR) do not differ fundamentally from crustal sectionsexposedin transformfaults [Lagabrielle and Cannat,
historiesoccurbeen considered[WoodsHole Oceanographic Institution (WHOI), 1989]. The interrelationshipsamong magmatic, hydrothermal, and deformation processesat 1990]. spreadingcenters are also becomingincreasinglyapparent This paper focusesmainly on the microstructures in ocean [Delaney et al., 1988; Mutter and Karson, 1992]. The crustlithologiesfrom present-dayoceanbasins.Oceancrustis increasinglyextensiveoceancrustsamplecollectionsthat are used in a petrologicaland geophysicalsenseto includerocks now available present an opportunity to examine the which have crystallizedfrom a magmaand thosewith "crustal' microstructural evolutionof oceancrustover a wide rangeof velocitiesup to 7.8-8 km/s [Purdy and Ewing, 1986; Brocher et al., 1985; McClain et al., 1985]. Ultramafic rocks which are crustal conditions and compare these studies with experimentaldata. The majority of oceancrust sampleshave thoughtto have been emplacedeither by diapirismor fault beenrecoveredby dredging.They are thereforeunorientedand controlled uplift [Thayer, 1969; Francheteau et al., 1990; their local geological settings are not tightly constrained. Lagabrielle and Cannat, 1990] are included in this definition. These samples,however, can still yield valuable insightsto deformationmechanismsand historiesprovided by textural Examples from Ophiolites and petrological analyses. Drill core and submersible Ophiolite exposures provide a view of the spatial samplingcan provideat leastpartiallyorientedsamples,but they still representonly a small proportionof ocean crust relationships among structures, lithologies, and metamorphismwhich cannotbe acquiredby presentseafloor samples. No synthesis exists of the results to date for microstructural studiesof the oceancrustand their rheological implications.
]Table 1 is availablewith entire articleon microfiche.Order from
Microstructuralstudieshavebeenconductedon a spectrum American Geophysical Union, 2000 Florida Avenue, N. W., of oceancrust lithologiesrecoveredfrom a range of tectonic Washington,DC 20009. DocumentB93-012; $2.50. Paymentmust settings (e.g., ridge axis, transform zone, ridge-transform accompanyorder.
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sampling and observation techniques. Although many ophiolites may have formed as small aborted oceans in marginal basins or in arc settings, their early geodynamic framework
is inferred
to be similar
to that of the extensional
regime at mid-ocean ridges [Miyashiro, 1973; Moores et al., 1984; Pearce et al., 1984; Lagabrielle and Cannat, 1990]. Marked similarities do exist between the lithostratigraphiesof ophiolites and sections of ocean crust interpreted from
OF OCEAN
CRUST
data suggestthat lateral and vertical lithological complexities are common, occurring both in fracture zone and axial ridge segments.For example, along strike of axial segmentson the Mid-Atlantic Ridge the crust is locally extremely thin and discontinuous, exposing gabbros and serpentinized peridotites [Karson and Dick, 1983; Karson et al., 1987; Karson, 1990; Lagabrielle and Cannat, 1990; Mevel et al., 1991; Gente et al., 1991]. Intercalated basalt and residual
borehole and seismic data [Coleman, 1977; Brown and Mussett, 1981; Gass et al., 1984]. In this respect, ophiolites still form useful structural analogs for the ocean crust and provide valuable insights to the physical property variations and rheology of ocean crust [Karson, 1990; Mascle et al., 1991]. The value of ophiolite studies to constrainingocean crust
mantle peridotites have been recovered in core at DSDP site 395 together with repeated intrusive contactswhere dikes are intruding older gabbros [Sinton, 1978]. Within the Oceanographer, Kane, Vema, and Romanche fracture zones [Bonatti et al., 1974; Karson and Dick, 1983] texturally and compositionally diverse assemblages of basalts, gabbros, serpentinites, and mylonites are exposed on numerous fault rheology relies on whether spreading center related, ridgescarps.Even though the south wall of the Vema fracture zone flank deformationcan be distinguishedfrom obduction-related exposes a crustal section which correlates closely with that deformation. Assessment of the conditions and timing of predicted by seismic models [Auzende et al., 1989], the hydrothermalmetamorphismrelative to structurescan help to gabbroic section is disrupted by serpentinized slivers. distinguish spreading-centerdeformation [e.g., Schiffman et Serpentinized protrusions which disrupt ocean crust al., 1987]. An oceanic origin has been inferred for the Carrick "stratigraphy"and permeatealong fault zoneshave been found Luz shear zone in the Lizard Complex [Gibbons and in several regions [Cann and Funnell, 1967; Bonatti, 1976; Thompson, 1991], where doleritic dikes predate and postdate Barrett and Aumento, 1970; Bonatti and Honnorez, 1981; mylonitization. In the Josephine Ophiolite, mylonite zones Francis, 1981 ]. have been "tentatively interpreted" [Alexander and Harper, Heterogeneous strain during brittle or quasi-plastic 1992, p.15] as preobductionand oceanicin origin [Norrell and deformation as a consequenceof lithological and alteration Harper, 1988; Norrell et al., 1989; Harper et al., 1990]. variations within major crustal units or acrosstheir boundaries Crystal plastic deformation of olivine, which occurred at has been documented in both ophiolitic and ocean crust temperaturesin excessof the temperatureattainedduring and outcrops [Aumento and Loubat, 1971; Delong et al., 1978; after obduction, and shear sense indicators (which are CA YTRO UGH, 1979; Fox and Stroup, 1981; Girardeau and incompatible with ophiolite emplacementdirections) provide Mevel, 1982; Francheteau et al., 1990; Alexander and Harper, the basis for this interpretation. Multiple criteria for 1992] and core sections[Agar, 1990; Cannat et al., 1991a,b]. recognizingoceanic faults in the Josephin e ophiolite have The time-varying relief on the transition between dikes and been discussedby Alexander et al. [1993]. plutonic rocks, formed in magmatic lenses or larger magma Ophiolite studies still provide the main sourceof oriented chambers, is an important control on near-ridge axis ocean structuraldata to constrainthe relationshipsof microfabricsto crust rheology: In the Semail and Norwegian Caledonides the three-dimensional structural geometries generated at a ophiolite complexes, the "dike-pluton" boundary is spreadingcenter. The complex temporal and spatial relations representedby a transition zone of variable thickness and that arise from polyphase magmatic, deformation, and relief. This zone was generated by multiple generations of hydrothermal histories are exposed in ophiolitic sections. gabbroic intrusions feeding dikes which were intruded by later Observations through drilling, dredging, submersibles, or plutons [Rothery, 1983; Pederson, 1986]. Contrasting remote sensingof the ocean floor are currently too restricted textures and grain size of dikes and plutonics, combinedwith to provide equivalentdata. This paper drawson examplesfrom the variable relief of this lithological junction, are potential ophiolites where suitable examples in the ocean crust are sourcesof inhomogeneousstress and strain. For example, in lacking, but ophiolitesare not intendedto be the main focus. the Ayios Ioannis region, Troodos ophiolite, crystal plastic
Influence of Ocean Crust Deformation Histories
Architecture
on
The seismic model for ocean crust of Engel and Fisher [1975] has been significantly enhanced by subsequent seafloor and ophiolite studies that reveal some of the magmatic, tectonic, and hydrothermal influences on crustal generation[Karson, 1990; Karson and Rona, 1990; Mutter and Karson, 1992]. The polyphase intrusive and hydrothermal histories of the ocean crust and ophiolites generatea complex crustal infrastructure,varying in composition,texture, and grain size [Moores and Vine, 1971; Bonatti et al., 1975; Pallister and Hopson, 1981; Browning, 1982; Rothery, 1983; Pealerson 1986; Bloomer et al., 1991; Mevel and Cannat, 1991; ODP Leg 147 Scientific Party, 1993] (Figure 3). Near-
axis processesjuxtapose shallow and deep crustalunits which contain major lithological and structural discontinuities' Interfingering of pillow basalts and lava flows, diabasedikes, and gabbroic rocks (broadly related to seismic layers 2A, 2B, and 3, respectively), protrusion of serpentinite diapirs (mantle?) up to the seafloor, and exposures of deep crustal/uppermantle rocks in the footwalls of low angle faults have been documented in ocean crust and ophiolites. These
shear zones, and mylonites have localized within gabbros along dike contacts[Agar et al., 1992, 1993] (Figure 4a). The juxtapositioning of diabase dikes and gabbros along the Kakopetria detachment at Lemithou in the Troodos ophiolite [Varga and Moores, 1985] and in the Agros region [Agar et al., 1992] (Figure 4b) suggest similar lithological controls on strain
localization.
Superimp6Sedon the magmatic heterogeneitiesare equally heterogeneous alteration assemblages resulting from extensive fluid-rock interaction up to temperaturesof about 750øC [e.g., Alt et al., 1986; Delaney et al., 1987; Rona, 1988; Mevel and Cannat, 1991]. Contrasting patterns and intensities of alteration resulting from variable penetration depthsof seawaterinto ophiolitic and oceaniccrust [Bonatti et al., 1975; Gregory and Taylor, 1981; Ito and Clayton, 1983; Schiffman et al., 1987; Schiffman and Smith, 1988; Vanko, 1988; Nehlig and Juteau, 1988a,b; Mevel et al., 1991] modify the physical characteristics of ocean crust lithologies. Evaluations of the impact of these primary magmatic and secondaryalteration characteristicson strain localization and the diversity of post-magmatic deformation histories has only recently been incorporated into microstructuralinvestigations
[Mevel and Cannat, 1991]. Constraining the detailed infrastructureof the ocean crust is thereforea critical aspectof ocean crust rheology studies.
AGAR: RHEOLOGICAL
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OF OCEAN CRUST
!
1
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i
[
[
!
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OF OCEAN
CRUST
AGAR:
Deformation
Mechanisms
RHEOLOGICAL
in the Ocean
EVOLUTION
Crust
The rheological evolution of the ocean crust will be strongly influenced by the impact magmatic, hydrothermal, and deformation histories on the physical and mineralogical properties of oceanic lithologies and deformation environments (Figure 1). Contrasting magmatic and hydrothermalhistoriesproposedfor different spreadingrates (Macdonal, 1982, 1986; Rona, 1988) suggestthat there could be
distinct
histories
and
differences the
in
distribution
their of
associated failure
deformation
mechanisms.
For
example, the episodic magmatic activity proposedfor slow spreadingcentersis likely to generatea heterogeneous crustin which stress and strain rate patterns are correspondingly complex. Some generalizationshave been proposed,relating the depth of a "brittle-ductile" transition to the thermal gradients modeled for different magma chamber depths and geometriesfor a fixed strain rate. A rigorous correlation of deformation histories to spreading rates, however, requires comprehensive knowledge of deformation partitioning through the crust and the factors affecting microstructural preservation. As calculated spreading rates represent an average displacement over long time periods and do not differentiate between magmatic crustal generation and crustal extension by faulting, their correlation with deformation histories remains speculative. Individual rock samples from any given spreadingcenter may have deformed over a broad spectrum of strain rates, reflecting transient responsesto magmatic and hydrothermal processes,heterogeneousstrain, and macroscopic fault and shear zone geometries. Interpretationsof the pressure,temperature,strain, and strain rate paths followed by rocks during deformation requires an understanding of the environmental changes imposed by magmatismand hydrothermalfluid flow. Macroscopicmodels for ocean crust theology at spreading centerspredict large strengthvariations dependenton changes in the thermal regime controlled by magma chamber history and assumedspreadingrates [e.g., Harper, 1985; Chen and Morgan, 1990]. Such models clearly illustrate the importance of varying environmental parameters during ocean crust deformation but still present instantaneousviews for fixed assumptionsof the mechanical properties of the ocean crust.
OF OCEAN
CRUST
3181
By assessingthe impact of the preceeding deformation and alteration histories on the mechanical response, it will be possible to construct more detailed, time-dependent theological models. The first part of this synthesis considers fracture, cataclasis, and frictional sliding on faults. Synmagmatic viscous flow and quasi-plastic flow are then discussedwith high-temperature (->600øC) "semibrittle" deformation. Following Rutter [1986] the term ductility is used here to refer to the capacity for more or less uniformly distributedflow and is a scale-dependent term that does not imply particular deformation mechanisms.The terms brittle and plastic are used here to indicate deformation mechanisms,the former relating to cataclasis involving dilation and the latter referring to a constant volume flow by crystal plasticity and diffusion mechanisms.
"Semibrittle"
is used
to refer
to deformation
where both brittle and plastic failure mechanisms have operated synchronously. Fractures,
Cataclasis,
and
Faulting
Morphological and seismic studies in conjunction with inferences from ophiolites have helped to constrain the macroscopicexpression of brittle deformation in the ocean crust [Macdonald et al., 1993; Mutter and Karson, 1992]. Physical property studies have provided useful data on the dimensions and geometries of fractures, but the strain paths represented by fracture fill assemblagesand fabrics [e.g., Ramsay, 1980] and fracture mechanisms [Atkinson, 1982, 1987] are limited for the ocean crust. The role of the microscopic brittle deformation in basalt has been investigatedexperimentally [Rutter et al., 1985]. Changesin porosity, grain size, and stress statesinduced during brittle failure are known to influence the velocity structure [Carlson and Gangi, 1985; Christensen, 1978]. An understandingof brittle
failure
mechanisms
and their
distribution
in the ocean
crust is therefore important for constrainingseismic velocity models as well as rheological models. Fracture histories will be influenced by stress variations related to magmatism, cooling, pore fluid pressurevariations, and faulting, superimposedon far-field stress regimes. The modeof fracturing(modesI, II, or III or mixed mode [Lawn and
Figure 4. (a) Strainlocalizationat a dike margin,AyiosIoannis,Troodosophiolite,Cyprus.Gabbro(G) is deformed againstthedikechilledmargin(D). A shape-preferred orientation of plagioclase andamphiboledefinesa 5-10 cm wide, foliated zone alongthe chill. Thin sectionexaminationof this contactshowsmixed crystalplasticityand cataclasisin feldsparswith asymmetricalshearsin porphyroclasts. Myloniteshave been foundin other, similar contactsin the sameregion.Dike injectionpostdatedat leastsomeof the deformation.The shearzone may have provided a weaknessfor melt migration;alternatively,back-veiningduring dike margin shearingmay have occurred.(b) Contrastingfracturesin sheeteddikes and gabbrosjuxtaposedby an oblique,normal fault, Agros, Troodosophiolite,Cyprus.Hammerlies on the fault rock which comprisespredominantlybrecciateddolerites, sealedby quartz,epidoteandsulfides.Gabbro(G) veiningconsists of an irregularmeshworkof thin (600øC) result in high strain gradients towards the base of the crust deformation, followed by a discussionof the relationshipsof where grain boundary sliding in partially molten material hydrothermal fluid flow to high temperature subsolidus would grade into zones of penetrative crystalline plasticity deformation. [George, 1978; Bloomer et al., 1991]. Several observations have highlighted the interplay between magmatism and crystalline plasticity: For example, dunites and harzburgites Synmagmatic Deformation within an interpreted crust-mantle transition in the Garrett Regional-scalemodelsfor spreadingcentershave alludedto transform fault [Cannat et al., 1990; Hebert et al., 1983] were magmaticand amagmaticspreadingmechanismswhere strain deformed under similar high-temperature conditions and is accommodatedby faulting during periods of low magmatic subsequentlyimpregnated with melt. In the Bay of Islands activity and axial intrusionsdominate faulting when a magma ophiolite, high-temperaturedeformation intensifies downward through an interlayered sequence of cumulate gabbros and chamber is present [Schouten and Klitgord, 1982; Harper, 1985; Karson et al., 1987; Mutter and Karson, 1992]. There ultramafic rocks, also localizing strain in the crust-mantle must also exist a spectrum between the two end-members "transition zone". Cross-cutting relations suggest that the where deformation and magmatism contribute to seafloor deformation was synchronouswith the generationof "veins" spreadingto variable degrees.Experimentson the rheologyof of dunite and pyroxenite [Casey et al., 1981]. In the Antalya partial melts suggestthat small changesin melt proportions Complex, Reuber et al., [1982] interpret "hydraulic" fracturing in (20%+/-10% - 35%) can causerapid viscosityvariationsof at the crust-mantleboundarywhere gabbroshave been injected severalordersof magnitude[Shaw, 1969; Arzi, 1978; Van der downward, cutting across penetrative foliations. Molen and Paterson, 1979]. Nicolas et al. [1993] have since Hydrofracturing within peridotites has also been proposed suggestedthat hypersolidusflow can be achieved with melt [Nicolas and Jackson, 1982]. Strain rate increases and/or fractions as low as 10%. The percentage of melt and the permeability changes associated with lower crustal anisotropyof its distributionare thereforecritical to modeling deformation may have generated these magmatic ocean crust strength variations. hydrofractures.A similar effect is seen at the dike-pluton Magmatic flow textures in partially crystallized magmas transition in Troodos where multiple magmatism has have been inferred from fabrics in peridotitesin both oceanic generated local pods of angular breccias, sealed by a later and ophiolitic settings [Ave Lallement and Carter, 1970; generation of gabbros (Figure 4f). These features clearly Thayer, 1980; Nicolas et al., 1980; Girardeau and Nicolas, indicate deviations from a homogeneous, steady state 1981; Cannat, 1991; ODP Leg 147 Scientific Party, 1993]. A hypersolidusflow. lack of significant intracrystalline deformation, Magmatism and shear zone generation may also be recrystallizationtextures with rare subgrainboundaries,and contemporaneous. In Mid-Atlantic Ridge (6øN) samples, deformation twins has been cited as evidence of laminar Bonatti et al. [1975] observed narrow zones of banding at viscous flow of an incompletely crystallized magma. diabase-gabbrocontactsdefined by --lmm zonesof amphibole Postdeformation annealing of crystal plastic deformation andplagioclase. The• suggested thatthesecloselyresembled however could remove many of the characteristic shear zones localized at the dike-gabbro contacts in the microstructures.Supportingevidence for magmatic flow has Rosignano Ophiolite. In the Troodos ophiolite, igneous also included shape-preferred orientations of plagioclase, textures in gabbros have been modified into local shape-
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preferred orientations with narrow [up to 10 cm] mylonite zones localized at the margins of dikes (Figures 4a and 4e). Preexisting shear zones may have facilitated dike intrusion [Bonatti et al., 1975], but it is also possible that some of these examples were generated solely by compactionof the wall rock, softened by local heating during intrusion. The viability of dike-induced faulting, however, is supportedby modeling [Rubin and Pollard, 1988]. Conversely, it has also been noted, in the Josephineophiolite that some small faults are "pinned"by dikes [Alexanderand Harper, 1992]. Crystalline (~>600øC)
Plasticity and Brittle Failure
High-Temperature
Strain localization in the ocean crust was first describedby Charles Darwin, who reported the "highly sheared"rocks of the St Paul's rocks [Tilley, 1947; Melson et al., 1967], since classified as ultramylonites [Nicolas et al., 1980]. High temperature, subsolidus crystallographic fabrics have been found at numerous ocean crust localities [e.g., Hekinian, 1970; Aumento and Loubat, 1971; Francheteau et al., 1990;
Ito and Anderson, 1983; Cannat, 1991], and high temperature, oceanic shear zones have been inferred from ophiolites [e.g., Casey et al., 1981; Norrell and Harper, 1988; Nicolas, 1989]. The recoveryof plasticallydeformedplutonicsand the relative abundance of metamorphic rocks exposed on fault scarps correlate closely with locations at slow spreading centers, particularly transforms or ridge-transformintersections.This correlationhas been usedto suggestthat metamorphicrocksat slow spreadingcentersare exhumedby crustalextensionalong listric faults that detach in low angle ductile shear zones at depth [Karson et al., 1987; Mutter and Karson, 1992]. As yet these geometries have not been clearly demonstrated, although recent seismic data have been used to suggestthat such faults represent major structural controls on the segmentationof ridges at slow spreadingcenters[Mutter and Karson, 1992]. While microfabric and kinematic indicator data from core samples [e.g., Cannat et al., 1991a] provide valuable
constraints
on
the
orientation
of
discrete
shear
zones, some caution is required when extrapolatingkinematic indicators
in core to crustal scales. A near-vertical
borehole
will favor the penetrationof shear zones with shallow dips, and the fabric orientations may not be simply related to macroscopic structural geometries. Although existing boreholes and sampling do not provide adequate threedimensional resolution to identify fault geometries,recovered samples do provide valuable insights to the deformation mechanismpathsover wide temperaturerangesand can be used to constrain deformation partitioning in the lower oceanic crust.
Characteristic microstructuresof crystalline plasticity and high-temperature brittle deformation in gabbros have been identified in several oceanic locations [Francheteau et al., 1990; Helmstaedt, 1977; Quon and Ehlers, 1963; Nicolas et
al., 1980; Cannat, 1991] (Figure 8). These structuresinclude core and mantle textures, commonly in plagioclase porphyroclasts,lobate grain boundariesdue to grain boundary migration, subgrains, and neoblasts, all indicative of dislocation creep (Figures 8a and 8b). A steady state deformation is approachedas these recovery processesare counteractedby work hardening,due to dislocationtanglesand pileups, indicated optically by undulose extinction [Nicolas and Poirier, 1976]. High-temperature intracrystalline deformation in olivines and pyroxene frequently involves kinking and fracturing together with the development of strong undulatory extinctions subgrainsand mechanicaltwins [Mercier and Nicolas, 1975; Malcolm, 1981]. Synkinematic mineral assemblages,the activation of diffusion-controlled, temperature-dependent deformation mechanisms, and the activity of specific crystallographicslip systems[Guegen and
OF OCEAN
CRUST
Nicolas, 1980] provide evidence of high-temperature deformation conditions. For example, temperaturesin excess of 1000øC at moderate stresslevels (1000øC] mantle conditions. Comparison between stress conditions derived from dislocationspacingand grain size relations were found to give highly inconsistentresults [Cannat, 1991]. Low stress conditions over decreasing temperatures[1000-800øC] were interpretedfor the Xigaze Ophiolite, Tibet [Girardeau and Mercier, 1988] using activation of olivine glide systems. Where olivine is not present, relative estimates of deviatoric stress intensity, using the dynamically recrystallized grainsize of plagioclase in gabbros,have been used [Cannat, 1991]. Plagioclasepaleopiezometershave not been calibrated though and recrystallized grain size measurementsmake the assumption that no grain growth after deformation has occurred
and
do
not
differentiate
between
different
3190
AGAR:
RHEOLOGICAL
EVOLUTION
recrystallization mechanisms. Although the stress conditions during high-temperature, subsolidusdeformation can only be broadly constrained, such estimates are valuable for comparisons with present-day stress estimates in shallow boreholes.They also provide a comparisonfor experimentally derived flow stresses.Experimental data suggestthere may be up to an order of magnitude difference in the flow stress dependingon the mineralogy [Hacker and Christie, 1990] (see discussion, below).
Synkinematic Fluid
and
Postkinematic
Hydrothermal
Flow
Hydrothermal assemblages (vein fillings, coronas, pseudomorphsand bulk hydrous phases)togetherwith direct and remote monitoring of hydrothermal activity clearly indicate the ubiquitouspresenceof aqueousfluids in the ocean crust [Staudigel et al., 1981; Rona 1988]. The penetrationof water during high temperaturedeformationplays an important
OF OCEAN
CRUST
role in the patterns of ocean crust metamorphismand strain localization [Mevel and Cannat, 1991]. Syn-kinematic hydrothermalphase relations and texturesprovide constraints on the conditions
of deformation
and kinematics.
Shear zones
may enhancefluid accessto the lower crust [Stakesand Vanko, 1986; McCaig, 1988; Mevel and Cannat, 1991], but minor strain rate variations, controlled by primary lithologies, or pore fluid pressurevariationscould generatemixed brittle and crystal plastic deformation at high temperaturesand enhance permeability. Some
of
the
alteration
and
deformation
characteristics
resulting from water-rock interaction during an overall downtemperature deformation path are exemplified by gabbro samplesrecoveredfrom the east end of the Hayes fracture zone [Silantiev et al., 1991] (Figure 9). Plagioclaseporphyroclasts display weakly defined subgrains, serrated grain boundaries and undulose
extinction
but neoblasts
and core and mantle
structuresare absent(Figure 9a). Lower temperaturesduring crystal plastic deformation, relative to the Atlantis fracture
Fig. 9. Crystalplasticandcataclasticdeformationin a gabbrofrom the HayesFractureZone, sample(Petrov 16-17). (a) Plagioclaseexhibits serratedgrain boundariesindicativeof grain boundarymigration.Unduloseextinction andweak subgrainboundariesare present.Core andmantlestructures andpolygonalsubgrainsandneoblastsare not present.Mechanicaltwins are dissectedandoffsetby discretemicrofractures containingbrownamphibole(arrow). Photomicrograph field of view is 2.5 mm, crosspolarizedlight. (b) Cataclasticfailure in plagioclaseand magnetite (black) sealed by chlorite and amphibole. An albite vein post-datesand seals the cataclastic deformation. Photomicrograph field of view is 2.5 mm, crosspolarizedlight. (c) BSE image showingmarginof plagioclase porphyroclast (P) in Figure9d below.The darkerphaseis albite (arrow)whichfills 'tensiongashes'adjacentto the shearzone.The geometryof the tensiongashessuggests that themarginof the porphyroclast wasextendedparallel to the amphibole/chloriteshearzone,with detachedfragmentsdispersedin the amphibole/chlorite matrix. (d) BSE image showing the disaggregationof plagioclaseporphyroclasts(P) along zonesof albite alteration which were focusedalong subgrainboundaries."A" is an amphibole-richhorizonwhere strainlocalizationis suggestedby a strongpreferredorientationof amphibole.
AGAR:
RHEOLOGICAL
EVOLUTION
zone sample (Figure 9b), may have limited thermally activated recovery processes. Dislocation glide was probably the dominant deformation mechanism, which would have resulted
in work hardening. Work hardening may have assistedfluid accessby generating grain boundary and intergranularcracks, now sealed by brown hornblende (Figure 9a). Synkinematic greenschistfacies mineralsindicate a lower temperatureduring cataclasiswhich overprints crystalline plasticity (Figure 9b). Similar to examples from the 504B diabases (Figure 7), plagioclase porphyroclasts have dismembered along their albite/chlorite alteration zones and are dispersedwithin the amphibole shear zones suggesting continuing deformation under greenschistfacies conditions(Figures 9c and 9d). Fracture zone samples may not be representative of "typical" fluid flow and deformation histories in the ocean crust due to the ease with which water may penetrate these regions [Mevel and Cannat, 1991]. Interactions of metamorphism,fluid flow, and high-temperaturedeformation, however, vary considerably both within small regions at individual sites and among different settings: Penetration of hot seawater during progressive, down-temperature deformation paths occurred at DSDP hole 556 (700øC to 400øC) with hydrothermal recrystallization ceasing at about 400øC and postdating high-temperature penetrative deformation [Mevel, 1987]. In contrast, the textural relations between microstructures and hydrous phases in the metagabbrosof the Cayman Trough, Vema fracture zone, and Gorringe Bank suggestthat deformation and hydrothermal alteration occurred together [Malcolm, 1981; Ito and Anderson,
1983; Honnorez
et al., 1984; Mevel,
1987].
"Pressureshadows"of amphiboleon porphyroclasticaugenof plagioclase and pyroxenes, with recrystallized tails, also indicate fluid penetration during high-temperaturedeformation [Helmstaedt, 1977; Prichard and Cann, 1982]. Hydration is thought to be contemporaneous with the late stages of deformationin samplesfrom the MAR at 46øN [Aumento and Loubat, 1971], whereas the presence of undeformed hydrothermal minerals in decimeter thick breccia zones of the ODP hole 735B gabbrossuggeststhat the brecciationoccurred prior to sealing by hydrothermalphases[Stakeset al., 1991]. At DSDP hole 334, high temperature (granulite facies) deformation in gabbros is overprinted by static recrystallization, succeeded by the development of breccia zonesat temperaturesof around300øC [Helmstaedtand Allen, 1977]. Mixed brittle and plastic deformation in the Cayman Trough gabbros occurred at about 800øC [Ito and Anderson, 1983] and resulted in variable and timing and degree of fluid penetration. Some samplesfrom the Cayman Trough appear never to have been exposedto significant quantitiesof water as a consequence of the heterogeneous deformation. These examples illustrate the variable presence of fluids during deformation over a wide range of temperaturesthat influence the textural
evolution
and the deformation
histories
of the
ocean crust. Although each of these studies identifies an overall down-temperature path, the location and timing of deformation relative to emplacement and movement off axis are poorly constrained. Such deformation paths may be generated during the freezing of a magma chamber on axis, exhumation on the footwall of a normal fault, and/or cooling as the crust moves away from the spreadingcenter.
Controls on Ocean Crust Strength Variations: Textural Evolution, Fluid Flow, and Deformation Histories
Examples of microstructures from ODP hole 504B, the Hayes and Atlantis fracture zones and the Troodos ophiolite, in conjunctionwith a synthesisof previous studies,illustrate the potential influence of mineralogy, primary textural
OF OCEAN
CRUST
3191
relations, and magmatic and alteration histories on the timedependent mechanical response of the ocean crust. Where deformation has transiently localized along faults and shear zones, the protolith is often extensively altered and deformed so that its physical properties no longer reflect those of pristine ocean crust lithologies. Interpreting softening or work hardeningbehavior of a rock requirescaution [Hobbs et al., 1990], but microstructural evidence from the ocean crust
does provide insights to the controls on the location and nature of ocean crust deformation(Figure 10). Brittle failure in the ocean crust is accommodatedby a variety of discrete dilational and shear fractures, systematic fracture arrays, frictional sliding on fault gouge, and distributed macroscopic and grain-scale cataclasis. Synkinematic mineral assemblages and textural relations indicate multiple phases of deformation and both high temperature and low temperature brittle failure. These structuresand assemblagessuggestthat the cohesive strength of the ocean crust must vary spatially and temporally (Figure 10).
Microstructuresin the 504B diabase sampleshighlight the influence of polyphase assemblages and primary and secondary textural variations on failure mechanisms and locations. For example, where augite has been pervasively altered, the diabase consistsof a network of plagioclase laths surrounding actinolite and chlorite. If the plagioclase grains are closely connected, then they will form a supporting framework and the plagioclase will control the mechanical responseof the diabase.Where augite is more abundantand the plagioclase laths are isolated, the strengthof the diabase may be controlledprimarily by the deformationof augite alteration products, actinolite, and chlorite. When failure has localized, for example, by frictional sliding on a fault gouge or by distributed cataclasis, brittle deformation will be influenced
more by the theological evolution of the fault zone than the bulk propertiesof unalteredbasic lithologies. Primary fracture characteristics and textural relations of constituent phases may partly contribute to temperature-related changes in relative strength between plagioclase and clinopyroxene interpreted from experimental deformation of Maryland diabase [Kronenberg and Shelton, 1980]. The water-rock interactions that influenced the early stages of
strain
localization
in
the
504B
diabases
are
also
inextricably linked to faulting in the ocean crust. During fault slip, dilation resulting from cataclasis or compaction of hydrothermal alteration phases can induce permeability variations. The impact of hydrothermalfluid flow on strength variations will dependon the fluid temperature,composition, and flow rates. Where fluids can permeate a fault rock, replacement by hydrous phasesand precipitation of vein fills will in turn alter the cohesion, crack lengths, porosity, and permeability characteristics.As minerals precipitate on the sidesof large open fractures,they may limit further water-rock interaction, while a low, but evenly distributed permeability may result in extensive alteration due to relatively slow fluid flow. The interaction of mean confining pressure,pore fluid pressure, composition and flow rates, temperature, stress state, and strain rates all influence failure modes on a fault
[Rutter and Brodie, 1991; Etheridge et al., 1983; McCaig, 19881. The evolving compositionof a fault gougeduring fluid flow may also vary its frictional properties.For example, Dengo
and Logan [1981] show that antigorite serpentinitesrequire higher shear stresses to initiate sliding than lizarditedominated serpentine. They also suggest that the smaller grainsize of the antigorite serpentinites will produce an increased frictional strength.Reaction softening, enhancedby the presence of hydrothermal fluids, may reduce strength [White et al., 1980; White and Knipe, 1978]. Ductile creep in fine-grained reaction products has been proposed as an
3192
AGAR: RHEOLOGICAL
EVOLUTION
OF OCEAN CRUST
B
Fig. 10. Schematic section through oceancrustillustrating thevariations in failuremechanisms andthepossible controlson their location. Circular diagramsshow schematicmicrofabricsobservedat different locations.The
stippled,"v" and"+" patternsindicatezonesof progressively youngergenerations of plutonicrocks.A., (1) Thermalcrackdistribution influenced by originalrocktextureandcoolinghistory.(2) Precipitation of alteration phases in thermalcracksandalonggrainboundaries alongwhichphenocrysts andporphyroclasts disaggregate. B., (1) Localized cataclastic flowof diabase onfaultzone.(2) Clay-dominated, hydrothermally altered faultgouge.(3)
Fine-grained reactionproducts thataccommodate ductilecreep.C., (1) Hydrofracturing in brecciasealedby hydrothermal alterationphases,localizedon fault plane.(2) Fracturearraysin processzoneof normalfault, overprinting primarygabbrotextures. D., (1) Dynamicrecrystallization structures overprinted by hydrothermal
alteration andsubsequent cataclasis. (2) Dynamicrecrystallization duringdislocation creep.(3) Grainboundary slidingin meltpocketat baseof crustandsyn-magmatic flow fabricanisotropies. E., (1) Annealedtexturein first
generation gabbro. (2) Modification of igneous textures by staticrecrystallization. F., (1) Antigorite mylonite. (2) Fine-grained olivinegenerated duringdehydration of serpentinite. Bothexamplesin a transform zone.G., (1) Distributed cataclasis. (2) Anastomosing fracture cleavage in lizardite/brucite gouge. Bothexamples in a transform zone.Blockdiagrams belowshowschematic distribution of crustal strength forcartoon above. Verticalstrength profilein leftdiagram (x,y,z,a,b) haveincreasing strength to theright. Solidcontours correspond to locations in cartoonand indicateregionsof relativelyhigh strength.Dashedcontoursare regionsof relativeweakness correspondingto the fault zones,transformzone and melt accumulations.See text for discussion.
AGAR: RHEOLOGICAL
EVOLUTION
alternative deformation mechanism to frictional sliding and cataclasisin fault zones [Sleep and Blanpied, 1992] (Figure 10, B3). For example, an alignment of amphibole needles or serpentine fibers may enhance strain localization through "geometricalsoftening"where a shape-preferredorientationof recrystallized grains facilitates slip [Kirby and Kronenberg, 1987; Hacker and Christie, 1990] (Figure 10 D1, F1, G2). Experiments on basalt [Rutter et al., 1985] suggest that softening of basalt does not necessarily occur during syntectonic hydration reactions, but they do show that a moderatewater pressurewas essentialto the deformability of the rock. The fluid chemistrymay also causefailure well-below a predicted "fracture strength" by promoting subcritical fracture, where fracturing occursbelow a critical stressvalue for a given material [Atkinson, 1987]. These interrelated metamorphicand deformationprocessesmay causesignificant deviationsfrom a standard"strengthenvelope"representedby Byerlee's law. The influence of primary texturesand mineralogy on strain partitioning and deformation mechanismsin the ocean crust is also apparent during dominantly crystal plastic deformation. In examples from the Atlantis fracture zone, plagioclasehas been extensively dynamically recrystallized during dislocation creep. Deformation in the pyroxenes is restricted to their margins where high-temperature replacement amphibole has recrystallized. Microfractures probably generated by rheological contrasts among pyroxene, plagioclase and Fe-Ti oxides, combined with voids along recrystallized grain boundaries provided the early fluid pathways. Down-temperature continuation of deformation, marked by a transition from crystalline plasticity to semibrittle failure is illustrated by the Hayes fracture zone (Figure 9): Deformation in this specimenwas accommodated by a combination of cataclastic flow in plagioclase and crystalline plasticity with minor fracturing in amphibole/chlorite zones which dismembered the partially altered plagioclase aggregates. Grain boundary sliding may have also accommodated displacements in the amphibole/chloritezones but there is no way to confirm this. Ambiguous textural relations between microstructuressuggest that
some
of
the brittle
deformation
could
have
occurred
OF OCEAN
CRUST
3193
Ocean Crust
Seismicity and the Timing
Distribution
of
Failure
and
Mechanisms
Seismicity data provide valuable constraints on the distribution of failure in the ocean crust but they are sparse, and represent a geologically instantaneous view. The distribution of seismicity surrounding spreading centers has been used to infer crustal thicknesses and the depth to a "brittle-ductile" transition in the ocean crust [Weins and Stein, 1983; Fleitout and Froidevaux, 1983; Bergman and Soloman, 1984;Toomey et al., 1985, 1988; Bratt et al., 1985; Lister, 1986; Huang and Solomon, 1988]. As a generalization, the sustainedsteep thermal gradients proposedfor fast spreading
centers have been used to suggestthat the "brittle-ductile" transition will be located at higher crustal levels than at slow spreadingcenters, where the deepestearthquakeshave been located. Microearthquakes spatially associated with black smokersystemsare estimatedto reach a maximum depth of 2-3 km. These are interpreted as indicators of a possiblelimit to brittle deformation above the magma chamber on the EPR [Riedesel et al., 1982]. Maximum earthquake depths in the MARK area range from 4-8 km on the ridge axis [Toomey et a1.,1988] and from 6-9 km within the transform zone (Wilcock et al., 1990].
Rheological models for spreadingcenters often incorporate failure by depth-dependent, frictional sliding on faults [Byerlee, 1978]. Byerlee's law is consideredto be applicable to a broad range of crustal rocks, but, as Rutter and Brodie [1991] have noted, Byerlee's law was derived primarily from room temperatureexperimentsfor small displacements.At low effective stresses(up to 50 MPa) there is a wide variability in frictional characteristics of rocks [Rutter and Brodie, 1991]. A
spatial association of faulting and hydrothermal activity at spreadingcenters [Williams et al., 1974; Crane and Normark, 1977; Temple et al., 1979; Rona et al., 1976] and ophiolites [e.g., Bettison-Varga et al., 1992; Alexander et al., 1993] indicates that fluid flow is an integral part of the slip history on ocean crust faults. The existenceof pore fluid pressuresin excess of hydrostatic have been proposed by Kelley and Delaney [1985; 1987] while underpressuredhorizons have been found at DSDP hole 504B [Anderson and Zoback, 1982;
Anderson et al., 1983]. A detailed quantitative assessmentof concurrently with the late stages of plagioclase crystal ocean crust pore fluid pressure is limited by existing plasticity (Figure 10, D1). Changingdeformationmechanisms technology, but geological evidence supports at least and strength variations over decreasing temperatures have transiently high pore fluid pressures related to faulting. been interpreted in the Josephine ophiolite. Norrell et al. Localized over pressuresmay result if fluid "pumping" along [1989] document a transition from dynamic recrystallization (up or down) faults is inhibited by a decrease in fracture of olivine through cataclasis of olivine to strain localization permeability or grainsize reduction during gouge generation along serpentinite shear zones (Figure 10 F1 and G2). They [Sibson, 1981; 1989]. Transiently high pore fluid pressures suggestthat syn-kinematicphase changesfrom antigorite to could trigger hydrofracturing, generating the observed lizardite and chrysotile resulted in contrasting fault rock polyphase brecciation or multiple crack-seal fracture fills microstructuresdue to a change in deformation mechanisms, discussedabove (Fig. 10, C1). The significanceof this type of from plastic flow on antigorite mylonites to brittle failure on brittle failure and associated low effective stresses during foliated lizardites. crustal extension at spreading centers has yet to be In previous examples, the rheology of the gabbros was established,but Byerlee's law may not be appropriatefor such cases. initially controlled by the deformation of plagioclase, Variable stress states and strain rates in the ocean crust can involving dislocation creep and dislocation glide, induce transient instabilities and seismogenic failure in a subsequently controlled by amphibolite deformation or a dominantly "plastic" regime. The variable intensities, mixed crystal plastic amphibole/ brittle plagioclase locations, and depths of earthquakesat spreadingcentersmay deformation. Extrapolated experimental data suggest that reflect such instabilities. At fast spreading centers, quasiamphibolite, diabase, albite, and anorthosite should flow at roughly the same stress levels at geological strain rates steady state deformation may dominate where sustainedhigh [Hacker and Christie, 1990]. Flow stresses for basic temperatures promote recovery processes which compete effectively with work hardening (Figure 10 D2). Ocean crust lithologies however can vary by an order of magnitude, samples from slow spreadingcenters, however, indicate that depending on their composition. For example, at strain rates 13 subsolidus deformation during cooling and metamorphic of 10- at 900øC, amphibolite flow stressesare estimated to be about 100 MPa, while flow stresses for wet dunite or histories of gabbroic and ultramafic rocks is not steady state. The interaction of hydrothermalfluid flow and microstructural diopsiderock reach values in excessof 1000 MPa [Hacker and Christie, 1990]. evolution over the brittle-quasi-plastic transition can also
3194 cause
AGAR: transient
variations
RHEOLOGICAL
in deformation
mechanisms.
EVOLUTION For
example, pressure decreases resulting from dilation during cataclasis can induce two phase separation of circulating seawaterat temperaturesabove 350øC [Goldfarb and Delaney, 1988]. The volume increase associated with this two-phase separationcould generate high pore fluid pressures,resulting in brittle failure in a rock that was previously deforming by crystal plastic mechanisms [Goldfarb and Delaney, 1988]. Metamorphism will also influence the distribution of seismic failure: Rutter and Brodie [1989] suggest that fine-grained reaction products resulting from the prograde alteration of serpentinites at ridge-transform intersections may explain anomalously shallow and low levels of seismicity in transform faults [Engeln et al., 1986] (Figure 10, F2). They propose that shear zones lined with ultrafine grained olivine, deforming by diffusion-accommodatedgrain boundary sliding can induce a dramatic weakening and suppresscataclastic faulting. The impact of magmatismon environmentalconditionsand physical characteristics of deforming lithologies will also modify deformation mechanisms. A magmatic pulse (either from an adjacent dike intrusion or proximity to the top of a magma chamber) will modify stressfields and steepenthermal gradients. Hornfels textures would develop in adjacent lithologies as a result of static recrystallizationin the absence of deviatoric stresses.Annealing would heal cracks, which may strengthen the rock. Annealing would also remove dislocations, facilitating intracrystalline slip if shear stresses were present and temperatureswere sufficiently high (Figure 10, El). The temperatureincreasemight be so high that a rock deforming by brittle-failure mechanismswill deform by quasiplastic flow, returning to brittle failure as the intrusion cools. Variations in depths of faulting around the Mid-Atlantic Ridge have been attributed to fluctuationsin magma supply. Parsons and Thompson[1991] suggestthat normal faulting is suppressed at fast spreading centers by magmatic over pressuring and use this to explain the lack of seismicity and relief at fast spreading centers. Vigorous hydrothermal circulation postulatedfor fast spreadingcenters[Rona, 1988] could also generate extensive crustal alteration where creep occurs on fine-grained alteration products, suppressing seismicity. It is also possible that magmatism and plastic instabilities can nucleate seismic instabilities [Hobbs et al., 1986; Knipe, 1989; Bergman and Soloman, 1990; Sykes, 1970; Francis,
1974]. Field and microstructural evidence
presentedin this paper suggestthat aseismic slip and locally high strains may be induced by the presenceof partial melts and late magma pockets, where viscous flow is accommodated by grain boundary sliding (Figure 10, B3). Although spreadingshouldbe accommodatedprimarily by magmatismat fast spreadingcenters, melt accumulationsat the base of "fastspread" crust over broad regions [e.g., 5-20 km, Garmany, 1989; Barth et al., 1991] may cause strain localization toward the base of the crust.
Deformation
Mechanism
Paths
at
Spreading
Centers
The mechanical behavior of a rock is critically dependent upon the deformationmechanismswhich are in turn controlled by parameters such as strain rate, temperatureand material properties [Ashby, 1972; Rutter, 1976]. Constructing a deformation mechanismpath [Knipe, 1986] by establishinga sequence of deformation mechanismsfrom microstructural relations is therefore essential in constrainingthe rheological evolution of the ocean crust. Mapping the deformation mechanism path, in conjunction with experimentally derived data for the flow laws of oceancrustlithologies,can be usedto constrain the conditions (pressure,temperature,strain, strain
OF OCEAN
CRUST
rate) during the deformation history and rheological models [Sibson, 1977, 1983; Hobbs et al., 1986].
Interpretations of deformation mechanism paths and their significance in the ocean crust are in their infancy. Even though the local strain rates interpretedfrom oceanic samples may be extremely variable, contrasting magmatic and hydrothermal histories proposedfor fast and slow spreading end-members (Sinton and Derrick, 1992; Rona, 1988] would generate distinct metamorphic and deformation histories.Fast spreading ridges are characterized by a narrow, long-lived magma conduit while the volcanic edifices at slow spreading ridges suggest that magma conduits migrate within the neovolcanic zone [Macdonald, 1983; 1986; Lister, 1983; Sinton and Derrick, 1992, Smith and Cann, 1993]. Lithologies at slow spreading centers may therefore be subjected to several reheating events as they move off-axis while fast spreadcrust would be more likely to follow a steady down temperature path. Wider strength variations would thereforebe anticipatedfor a slow spreadingcenterthan a fast spreading center, although progressive deformation even under near-steady state thermal conditions will also vary physical properties. Migration of magma conduit(s) at slow spreadingcenters could result in a strong diachroneity of faulting and may promote an asymmetricfault distribution(Karson, 1990). At a slow spreading center, discrete textural overprinting of deformationby annealing or hydrothermalalterationrelated to upwelling of a hydrothermalsystemwould occur with distinct magmatic and amagmatic spreading episodes. Renewed magmatism, however, and its associated hydrothermal circulation could also promote failure within faults and shear zones, as discussed above. Whereas crystal plastic deformation may approach a steady state at depth at a fast spreadingcenter, a steady thermal gradient and hydrothermal circulation may generate persistent, depth-dependent alteration and metamorphic zones, which may subsequently influence crustal decoupling. Support for these speculations requires more detailed sampling for microstructuralobjectives at constructive margins and improved constraints on the timing of deformation. A
transition
in
deformation
mechanisms
with
associated
temperature changes has been commonly interpreted as being synchronouswith the uplift path of deep crustal samplesto a shallow crust location in the footwalls of normal faults [Mevel
and Cannat, 1992]. The deformation path identification is not necessarily unique. Transitions from crystalline plasticity to brittle failure do not necessarilyindicate a drop in temperature unless synkinematic minerals, indicative of lower temperatures, are present. Changes in deformation mechanisms and mineral assemblagesmay reflect varying strain rates, pore fluid pressures, compositions and temperatures,characteristicof slow spreadingcenters,without significant uplift. Most deformation histories are constrained only by temperaturevariations indicated by synkinematicand overprinting minerals whereas geobarometry is less wellconstrained. Depth estimates for the location of deformation are commonly derived from assumed thermal gradients and simple isotherm geometries. Models for large uplift (up to 6 km in some cases)have been largely basedon the preservation of high temperature mineral assemblagesexposed at inner corner highs at ridge-transform intersections of slow spreading centers [Severinghaus and Macdonald, 1988; Karson, 1990; Mutter and Karson, 1992]. These uplift estimates have been used to propose the existence of major low-angle detachments which penetrate the lower crust. A combination of rift flank uplift on planar, high angle faults, shallow level plutonism and mass-wastingcould also expose
AGAR: RHEOLOGICAL
EVOLUTION
mid-crustal rocks on the 2-3 km offset scarpsfound at inner corner highs. Diapirism also provides an additional mechanismfor emplacementof metamorphosed samplesat the surface,many of which are extensivelyserpentinized.
Summary
OF OCEAN CRUST
3195
References Adamson,A.C., Basementlithostratigraphy,Deep Sea Drilling Project hole 504B, Initial Rep. Deep SeaDrill. Proj., 83, 121-128, 1985. Agar, S.M., Fractureevolution in the upper oceancrust:Evidencefrom DSDP hole 504B, in Deformation Mechanisms, Rheology and Tectonics,edited by R.J. Knipe and E.H. Rutter, Geol. Soc. Spec. Publ. London, 54, 41-50, 1990.
This paper has attemptedto demonstratesomeof the factors influencing ocean crust theology that could be usefully incorporated in theological models. It is not possible to incorporate the detailed deformation histories into such models, but the impact of lithologies combinedwith physical property evolution and stressand strain rate variationscould be explored. Although the grain-scale features and processes outlinedmay seemtrivial comparedto the "bulk" behaviorof a spreadingcenter, it is theseprocessesthat influencewhen and where failure occurs. This impact is shown schematically in Figure 10 where contrastingfailure mechanismsin the crust generate local weaknessesrelative to the surroundingcrust. Although no absolutevalues are placed on schematicstrength
envelopesand shearstrength"contours",' they illustratethe possible nature of lateral and vertical variations in shear strengthin the oceancrust.Evaluatingthe magnitudesof such shear strengthvariations is a clearly a high priority for future research.Future work related to ocean crust theology would also benefit from more experimental data on polyphase, texturally diverse assemblages,and investigationsof oriented samplesto provide kinematic interpretationsand palcostress orientations.Focusing on fracture network characteristics,the different origins of fractures (e.g., cooling-related or distributed brittle shear), and their relative timing, may help to constrain fault zone locationand geometry. The detailed strain paths and comparisonsbetween bulk and local strains remain poorly constrained in both brittle and plastic
deformation, although spatialfindtemporal heterogeneity of strain in the ocean crust is apparent from well-documented seafloor outcropsand boreholes. Understandingthe controls on deformation partitioning is
Agar, S.M., Microstructuralevolutionof a fault zone in the upper ocean crust: An example from DSDP hole 504B, J. Geodyn.,13, 119-140, 1991.
Agar, S.M., and F.C. Marton, Microstructural controls on strain localization
from DSDP/ODP
1989.
60, 3-38, 1992.
Alexander, R.J.,G.D. Harper,andJ.R.Bowman,Oceanicfaultingand fault-controlledsub-seafioorhydrothermalalterationin the sheeted dike complexof the Josephineophiolite,J. Geophys.Res.,98, 97319759, 1993.
Alt, J.C., C. Laveme, and K. Muehlenbachs,Alteration of the upper oceancrust,mineralogyand processes in DSDP hole 504B, Leg 83, Initial Rep. Deep Sea Drill. Proj., 83, 217-248, 1985. Alt, J.C., J. Honnorez, C. Laveme, and R. Emmermann,Hydrothermal alterationof a 1-km sectionthroughthe upper oceaniccrust,DSDP hole 504B: The mineralogy, chemistryand evolution of seawaterbasaltinteractions,J. Geophys.Res.,91,309-335, 1986. Anderson,E.M., The Dynamicsof Faulting, 206 pp., Oliver and Boyd, Edinburgh,1951. Anderson, R.N., and R.K. Nishimori, Gabbro, serpentiniteand mafic breccia from the east Pacific, J. Phys. Earth Inter., 27, 467-480, 1979.
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