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The Genesis of Oceanic Crust: Magma Injection, Hydrothermal Circulation

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The Genesis of Oceanic Crust: Magma Injection, Hydrothermal Circulation JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B4, PAGES 6283-6297, APRIL 10, 1993 The Genesis of Oceanic Crust: Magma Injection, HydrothermalCirculation, and CrustalFlow JASON PHIPPS MOROnN Instituteof Geophysicsand PlanetaryPhysics, Scripps Institution of Oceanography,La Jolla, California Y. JOHN (2HEN Collegeof Oceanography,Oregon State University,Corvallis, Oregon In this studywe constructa thermaland mechanicalmodel for the genesisof oceaniccrust. Magma is halted in its ascentwithin the oceaniccrust when it reachesa freezing horizon, where the dilational volumechange associated with magmafreezing leads to viscousstresses that favor magmaponding near the freezinghorizon. To modelthe steadystate thermal impact of crustalaccretion via dike injectionand pillow flows, we treat all crustalaccretion in rockscooler than a magma"solidus" to occurin a narrow 250-m-wide dike-like region centeredabout the ridge axis. The rest of the oceaniccrust is modeled to be emplacedas a steadystate magma lens directlybeneath the "solidus"freezing horizon where the steadystate emplacement rate is determinedby the constraintthat this lens supplyall crust that is not emplacedthrough diking/extrusion above the magmalens. If hydrothermalheat transportwithin crustal rocks cooler than 600øC removesheat 8 times as efficiently as heat conduction,then we find that a steadystate magma lens will only exist within the crustfor ridgesspreading faster than a 25 mm/yr half rate. The depthdependence of the magmalens with spreadingrate is in good agreementwith seismic observations.These results suggest that a fairly delicatebalance between magmatic heat injection during crustalaccretion and hydrothermalheat removalleads to a stronglydifferent crustalthermal structureat fast and slow spreadingridge axes. Our resultssupport the hypothesisthat medianvalley topographyis due to extensionof strongridge axis lithosphere;it is the differencein thermalregime that is directly responsiblefor the strikingdifference between the typical medianvalley seenat slow spreadingridges (e.g., Mid-AtlanticRidge) and the axial high seenat fast spreadingridges (e.g., EastPacific Rise). This paradigmfor the origin of a medianvalley at a slow spreadingridge predictsthat along-axisvariations in medianvalley topographyof a slow spreadingcenter reflect variations in recentmagmatic heat input alonga segment,that is, that the axial topographyis a goodtime-averaged indicator of the relative importanceof hydrothermalcooling and magmaticinjection along a given sectionof a ridge segment. We determinethe accumulatedcrustal strain associatedwith lower crustal flow which supportsthe hypothesisthat the Oman Ophiolitecrust was createdat a paleo-analogueto a fast spreadingridge and alsosuggests that crustalstrain, and not cumulatelayering, may be the dominantphysical process that generates"layered gabbros" within the Oman Ophiolite. INTRODUCTION Rise (EPR) have yielded intriguingclues to the depth, thick- Recentseismic studies of fast and slow spreadingridges ness,and along-strikecontinuity of the axial magmachamber suggestthat a quasisteady state magma chamber exists at shal- at a fast spreadingridge [Detrick et al., 1987; Mutter et al., low • 1.5 km depthsbeneath the axis of a fast spreadingridge 1988; Harding et al., 1989; Vera et al., 1990; Kent et al., 1990; Detrick, 1991]. MCS data collected on the northem EPR [De- but is not presentwithin the crustat a slow spreadingcenter. This observationmay be a fundamentalclue to the physical trick et al., 1987; Mutter et al., 1988] show a bright reflector origin for the characteristicdifference in axial topographybe- interpretedas the top of an axial magmachamber along much tweenthe medianvalley topographyof a slow spreadingridge of the spreadingcenter. Figure 1 showsa typical across-strike andthe axial high topographyof a fast spreadingridge. Obser- MCS profile. The reflectoris continuouswhere the axial depth vationsof crustallayering in the Oman Ophiolitecan alsobe and morphologypoint to strongmagmatic activity, and it dis- used to constrainthe patternof magma injection and crustal appearsas the axis deepensand narrows. The magmachamber flow at a spreadingcenter. After reviewing these observa- reflectoris found along roughly60% of the rise axis between tionalconstraints we presenta simplemodel for heatand mass the Siqueirostransform and 13ø30'N. After correctionfor the transportwithin oceaniccrust that suggeststhat a fairly del- diffractioneffects associated with the magmachamber reflec- icate balancebetween magmatic heat injectionduring crustal tor, the inferred across-strikewidth of the magma chamber accretionand hydrothermalheat removalleads to a strongly reflector is 1-1.5 km [Kent et al., 1990]. differentcrustal thermal structure at fast and slow spreading Analysisof expandingspre. ad profile data confirmsthat the ridgeaxes, that is thata differencein thermalregime is directly reflectorcoincides with a sharpinterface between high veloc- responsiblefor the observeddifferences in axial morphology. ities above and low velocities below [Harding et al., 1989; Vera et al., 1990]. Analysisof the expandingspread profiles SEISMIC OBSERVATIONS [Harding et al., 1989; Vera et al., 1990] and tomographicin- version of seismictravel time data [Burnett et al., 1989; Caress Multichannelseismic (MCS) studiesalong the East Pacific et al., 1990; Toomey et al., 1990] reveal a broader, lower- Copyright1993 by the AmericanGeophysical Union amplitude,low-velocity zone underlyingthe thin (<100-500 m) axial magma chamberreflector (see Figure 1). This axial Paper number92JB02650. low-velocityzone is roughly6 km wide, extendsfrom a depth 0148-0227/93/92JB-02650505.00 of 1.5-2.0 km down to Moho, and consists of velocities de- 6283 6284 PHIPPSMORGAN AND CHEN: GENESISOF OCEANICCRUST EPR CROSS-AXIS LINE ot 9ø57' N 3.0 MAGMA CHAMBER WIDTH: 700 M -- ,-, 2550 M ~ 4150 M _- _ 5.0 W <--- approx. 5km 2.2 ES-- BRECCIATED DIKI SHEETED DIKES 5 5.0 7.0 5.5 LV Z I GABBROS / 6.0 10 MANTLE ULTRAMAFICS ' -•: -I 0 I 2: :5 4 5 6 7 8 9 10 W DISTANCE{kin) E Fig. 1. (Top)East Pacific Risc (EPR) Multichannel seismic (MCS) reflection structure [figure courtesy of G. Kent]. Depthsshown on the right hand side are relative to sealevel. Most of theapparent "width" of theaxial magma chamber (AMC) reflector(shown by centralline pointer) is dueto diffractioneffects (shown by sideline pointers). The actual magmachamber width here is roughly700 m. (Bottom)Correlation of MCS and expanding spread profile (ESP) seismic datawith inferred geological description of oceanic crust. Velocity contours are in kilometersper second. AMC is shaded. Axial low-velocityzone (LVZ) showsregion of "hotrock" (modified from Vera et al. [1990]). pressedby 0.5 to 1.0 km/s. This smallreduction in velocityis that even the smallest discontinuities form boundaries between consistentwith marie or ultramarierocks containingless than differentmagma sources. However, the combinationof a sur- a 3% melt fraction [Caress et al., 1992]. prisinglysmall volumeof subaxialmelt and the along-axis The pictureof axialstructure that emerges from the northern continuityof the magmachamber for tensof kilometerssug- EPR seismicwork includesa 6-km-wide zone of hot plutonic geststhat melt productionmust be quitesteady and thatthe rockswith a smallor negligibleamount of melt cappedby a magmamust generally cycle through the axialsystem rapidly. thin (<100-500 m thick), narrow(0-2 km wide) lensof high This is a radicallydifferent magma chamber structure than melt fraction. The narrownessof the actual magma cham- that proposedin manyprevious ideas of a crustal-thickness- ber and its further constrictionor interruptionat small axial size chamber,although it is essentiallythe magmachamber discontinuitiesare consistentwith the petrologicalobservation structureenvisioned by Sleep [1975; 1978]. It is consistent PHIPPS MORGAN AND CHEN.' GENESIS OF OCEANIC CRUST 6285 with ophioliteevidence for 100 m scalecryptic geochemical magma lens constraints. variationwithin the crustalgabbros of the Oman Ophiolite [Browning, 1984], and a magma lens of the size seen at the OPHIOLITE OBSERVATIONS EPR is large enoughto easily generatethe major extrusives As noted above, the Oman ophiolite, thought to be cre- recentlymapped along the Cocos-NazcaRidge [Macdonaldet ated at a fast spreadingcenter [cf. Nicolas, 1989], shows al., 1989]. Gravity dataalong the EPR are alsoconsistent with cryptic chemicalvariation which suggeststhat only a small a smallmagma chamber [Madsen et al., 1990],as is along-axis magma body was presentat any time during crustalaccre- petrologic segmentation[Sinton and Detrick, 1992]. Sinton tion [Browning, 1984]. In additionthis ophiolite showsa and Detrick [ 1992] presentan excellentrecent overview of this structuralsequence through the gabbrosection from isotropic emergingparadigm about the axial structureof a fast spread- gabbrosdirectly beneath the sheeteddike/gabbro contact grad- ing ridge axis, and Phipps Morgan [1991] also summarizes ing downwardsto more strongly"layered" gabbros near the recentwork on this subject. J.E. Quick and R.P. Denlinger gabbro/mantleperidotite interface [e.g. Pallisterand Hopson, (Ductiledeformation and the originof layeredgabbro in ophi- 1981; Nicolas et al., 1988]. Changinglayer dips within the olites, submittedto Journalof GeophysicalResearch, 1992) gabbrosection have been usedto infer the shapeof a near- (hereinafterreferred to as Quickand Denlinger, 1992) present ridge magmachamber in which these"cumulate layers" are a petrologicalinvestigation
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