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Nonlinear Viscoelasticity and the Formation of Transverse Ridges

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Nonlinear Viscoelasticity and the Formation of Transverse Ridges JOURNALOF GEOPHYSICALRESEARCH, VOL. 97, NO. B10,PAGES 14,195-14,206, SEPTEMBER 10, 1992 NonlinearViscoelasticity and the Formationof TransverseRidges DAVID BERCOVICI Department of Geologyand Geophysics,University of Hawaii, Honolulu HENRY J. B. DICK Departmento] Geologyand Geophysics,Woods Hole OceanographicInstitution, WoodsHole, Massachusetts THOMAS P. WAGNER Departmentof Earth, Atmosphericand Planetary Sciences, M•assach,usetts Institute of Technology,Cambridge Transverseridges are regionsof anomalouslyhigh uplift that,run parallelto manyof the trans- formfattits at mid-oceanridges. Previous models of the formationof theseridges generally fail to explainthe magnitudeof uphft (between1 and 8 km) or accountfor geophysicalobservations (whichindicate that the ridgesare dynamicallyuphfted by neutralor heavy,but not buoyant, material).However, the potentially large Maxwell relaxation time of thelithosphere and the high strain rates and finite deformationof transformfaults imply that nonlinearviscoelasticity may play an importantrole in the formationof theseridges. We present simple theoretical and exper- imentalmodels of viscoelasticflow beneatha transformfattit to showthat the purelyhorizontal motion of the fattit can generatevertical uplift typical of transverseridges. The theoreticalmodel is of an infinitehalf-space of fluid drivenfrom aboveby two platesmoving parallel and in op- positedirections to one another. The constitutiverelation for the viscoelasticrheology is for a second-orderfluid; i.e., nonlinearviscoelastic effects are treatedas perturbationsto Newtonian, purelyviscous flow. The theoreticalmodel not only predictsthe two transverseridges that are oftenobserved on eitherside of the transformfault, but the centraltransform valley that sep- arates them as well. For typical hthosphericflexural rigidity and deformationassociated with transverseridges, the viscoelasticeffect provides sufficient vertical stressto producethe observed uplift. Further,the dynmnicuplift, is a purelymechanical effect (i.e., it doesnot involvebuoyant materiM)which corresponds to geophysical observations. The experimentalmodel is comprisedof a viscoelasticfluid (2% aqueous solution of highmolecular weight carboxymethylcellulose) driven by a rotatingplate. This is analogousto a t.ransk,rm fattit because the boundaryof a spiroting circulartectonic plate (surroundedby stationaryplates) is onecontinuous transform fattit. The experimentproduces a largeridge of fluid alongthe edgeof the platewith a shghttrough outside of the ridge; this result is qualitatively describedby a cyhnch-icalversion of of the above theoret- ical model. In conclusion,both experimental and theoretical models show that when nonlinear viscoelasticityis accorotted for, uphft characteristicof transverseridges caa• be generatedfrom purely horizontal motion typical of transform faults. INTRODUCTION formationacross the transformfaults, along with the large hthosphericMaxwell relaxation time (i.e., a characteristic Transverseridges are an enigmaticfeature of the Earth's timefor elasticstresses to berelaxed away by viscousflow), mid-oceanspreading centers. They are relatively narrow haveled us to considera new modelconcerning the purely zonesof extremeuphft, with peaksranging between 1 and mechanicaleffects of nonhnearviscoelasticity. We showwith 8 km above the seafloor,flanking many oceanictransform relatively simple theory and a laboratory experimenttl•at faults(Figures 1 a and1 b). Rocksalong these ridges may be nonhnear viscoelasticity can turn the horizontal motion of elevated from beneath the seafloor to above the sea surface transformfaults into verticaluphft that (in shape,size, and to form islets such as St. Paul's Rocks near St. Paul's correlationwith observations)is highlysuggestive of trans- FractureZone in the equatorialAtlantic Ocean [Melson and verse ridges. Thompson,1971]; other instancesof uphft to sealevel are suggestedby wave cut platformsand corm reefs,such as at TransverseRidges: GeologicalObservations the RomancheFracture Zone in the Atlantic[Bonatti and Transverseridges generally occur or are mostpronounced Honnorez,1971, 1976; Honnorezet al., 1975; Bonatti and at slowspreading centers (e.g., the Mid-At]anticRidge with Chermak,1981; Bonatti et al., 1983; Fisher and Sclater, spreadingvelocities of the order of I cm/yr). Theseridges 1983;Dick et al., 1990]. can appearon both sidesof the transformfault (as at the Previousmodels of the formationof theseridges (see be- RomancheFracture Zone) or oneside of the fault (e.g.,the low) do not accordwith eitherthe total amountof uplift or VemaFracture Zone in the AtlanticOcean) (Figure lb). At geophysicalobservations. The high strain rates (i.e., com- slowspreading centers, there is typicallyalso a centraltrans- pared with typical geodynamicstrain rates) and finite de- form valleycentered along the transformfault. The valley is typically 4 to 5 km deepin relationto normalseafloor, Copyright 1992 by the American GeophysicalUnion. with the greatest depths along the active transformfault Paper number 92JB00890. occurringin the nodal basinsloca.ted at or near the ridge- 0148-022? / 92/ 92JB-00890 $ 05.00 transform intersection. 14,195 14,196 BERCOVlCIET AL.: VISCOELASTICITYAND TRANSVERSERIDGES Atlantis II Fracture Zone '•;;.!!"':-::'.......:..... :,:.:.:•.,::;::;•:'::;.'.'i:;::•..o::.-:-, .-...,.. ....... ......... .•.•= .. .... ß• ... ... • .... • ....;..%• ...... .-*. •. •. ............' .•*:j•=•.•=.•,.•=•....::,...• ........ =•4.=...•:='-'"'--'-*='.............. ' -?-¾•:;;....•"............... -'-*=-';.-•-'=•..;.:. •W•=.f*• ..... .c•',.-•='-•=•-' •. .......... ..i:,?,.::. ?:...s..;: .... .......'":*'-*.... 10 km Fig. I a. A three-dimensionalbathymetric image, made from high-resolutionSea Beam data, of the transverse ridgesflanking the central transform valley at the Atlantis II Fracture Zone in the Indian Ocean. While oceanic fracture zones typically extend beyond the Previous Models activetransform fault regioninto the plate interiors(thereby Since the first observation of transverse ridges, severM xnarkingthe pMcotraceof the transform), transverseridges models have been developed to explain their presence, mor- generally do not. The opposing walls of the fracture zone phology, and anomalous uphft. One of the earbest models valley in these inactive fracture zoneshave very different ori- developed to explain the formation of the nodM basin in- gins: one wall having passedthrough the transform tectonic volvesviscous head lossof a conduit (or sheet)of fluid pas- zone, while the other has not. Transverse ridges, common sivelyupwelhng Mong a spreadingcenter [Sleep and Biehler, on transform and pMeotransfonn walls, are generally ab- 1970]; i.e., the conduit •naintains a negativehydrauhc head sent alongnontransform walls. Thus the origin of the ridges at the surface and thus a pressure low to sustain flow frown is possibly hnked to the shearing •notion Mong transform the asthenosphereinto the conduit (in other words, the faults. ß spreading •notion generates a suction force in the conduit Transverseridges the•nselves may be continuous(e.g., at so that the conduit can be replenished).The negativehy- the Verna Fracture Zone) or a seriesof undulationsMong drauhc head creates the rift valley Mong the spreading center the length of the ridge (e.g., the Kane Fracture Zone in and is enhanced at the transform fault-spreading center in- the North Atlantic). Where the rift valley of the spread- tersection(because additionM •nateriM is being drawn frown ing center and the transform walls intersect, there is fre- the conduit into the transform fault, thus a larger pressure quently great uphft, forming what is know as an "insidecor- low is requiredto feed the conduit) to form the nodM basin. ner high" which can,extend abovesea level (e.g., St. Paul's MateriM carried out of the pressure low rebounds to hydro- Rocks)[Melson and Thompson,1971; Searleand Laughton, static equilibriu•n, creating the uphfted blocks on the rift 1977; Karson and Dick, 1983; Fox and Gallo, 1984; Sevring- valley walls and the transverse ridges the•nselves. This i•n- haus and Macdonald,1988]. The abundant exposureof plu- phes that the transverse ridges are isostatically supported tonic rocks, particularly •nantle peridotites, over large re- and are thus nonnM crust that only appear to be ridges gions along the lengths and crests of •nany of these ridges by virtue of contrast with the centrM transform valley and indicates unusual tectonic processesnot occurring at the ad- nodM basin. This would then i•nply that the ridges should jacent •nid-oceanspreading centers [Dick et al., 1990]. essentially be of the sa•ne elevation as the rift valley walls; BERCOVICI ET AL.: VISCOELASTICITY AND TRANSVERSE RIDGES 14,197 I I . 54'4.6'S NORTHW -EST o.cos • [ SOUTHE^STI - i 10krnI - / i , ROMANCHE FZ (18o4o.w) - VEMA FZ (42ø4.8'W) -- 110km I - VE 2 - t NORTH • • SOUTH NORTH• •'d'•'" SOUTH i Fig. 1 b. Cross-sectionalprofiles of the transverseridge systemsat the Conrad and Islas Orcadasfracture zones in the southwestIndian Ocean[Sclater et al., 1978]and the Vernaand Romanchefracture zones in the Equatorial Atlantic [Bonatti and Honnorez,1976]. as transverseridges rise in someinstances to sealevel, this lantic and the Atlantis II Fracture Zone in the Indian Ocean mechanismis probably not adequateto explain the forma- [Cochran,1973; Robb and Kan½,1975; Loudcn and Forsyth, tion of the ridges. 1982; J. Snow, personalcommunication, 1990] showsthat Basedon the frequentexposure of serpentinizedperi-
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