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Summary: JOURNALOF GEOPHYSICALRESEARCH,VOL.93,NO. B10,PAGES11,845-11,861,OCTOBER10,1988
Formation of Curved SeafloorFabric by Changes in
Rift PropagationVelocityand SpreadingRate'
Applicationto the 98.8ø'\¾GalapagosPropagator
GARY ACTON AND SETH STEIN
DepartmentofGeologicalSciences,NorthwesternUniversity,Evanston,Illinois
JOSEPH F. ENGELN
DepartmentofGeology,UniversityofMissouri,Columbia
Thegeometryofoceanicspreadingcenterscanchangebyriftpropagation,inwhicha newridgesegmentpropagates
andpreemptsapreexistingone.Asthelocusofrelativemotionbetweenthetwoplatesshifts,thedyingandgrowingridge
segmentsarejoinedbyaboundary,usuallyatransform,whichmigrateswithtime,eausingseafloortobetransferredfrom
oneplatetotheother.Asaresult,originallyridge-parallelisoehronsinazonebetweenthegrowingandfailedriftscanbe
reorientedto trendsobliqueto boththeridgeandspreadingdirection.Anattractivefeatureofthismodelisthatthe
reorientationoccursbystrictlyrigidplatetectonics,sincerelativemotionoccursonlybetweenpointsondifferentplates.
RecentsurveysoftheGalapagospropagatingriftsystemat95øWshowseafloorfabriccurvingawayfromthegrowingrift
andtowardthe failedrift. Theseobservationshavebeeninterpretedasrequiringa zoneof distributedsimpleshear
betweenthetworifts.Suchbehaviorwouldrepresenta localdeviationfromrigidplatetectonics,sincerelativemotion
wouldbedistributedovera finiteregionnotpartofeitherofthetwoplates.Hereweexplorepossiblerigidplatemodels
fortheformationofcurvedseafloorlineamentsbyrift propagation.Wefindthatsuchfabriccanbegeneratedbychanges
eitherinthevelocity(rateordirection)ofriftpropagation,orinthespreadingrateduringriftpropagation.Thecurva-
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