JOURNALOF GEOPHYSICALRESEARCH, VOL. 87, NO. Bll, PAGES9389-9407, NOVEMBER 10, 1982 Structure and Origin of the J Anomaly Ridge, Western North Atlantic Ocean BRIAN E. TUCHOLKE Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 WILLIAM J. LUDWIG 1 Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 The J Anomaly Ridge is a structural ridge or step in oceanic basementthat extends southwestfrom the easternend of the Grand Banks. It lies beneaththe J magneticanomaly at the youngend (M-4 to M- 0) of the M series magnetic anomalies. Its structural counterpart beneath the J anomaly in the eastern Atlantic is the Madeira-Tore Rise, but this feature has been overprinted by post-middle Cretaceous deformation and volcanism. In order to study the origin and evolution of the J Anomaly Ridge- Madeira-Tore Rise system, we obtained seismicrefraction and multichannel reflection profiles across the J Anomaly Ridge near 39øN latitude. The western ridge flank consistsof a seriesof crustal blocks downdropped along west-dipping normal faults, but the eastern slope to younger crust is gentle and relatively unfaulted. The western flank also is subparallelto seafloorisochrons, becoming younger to the south. Anomalously smooth basement caps the ridge crest, and it locally exhibits internal, eastward-dippingreflectors similar in configurationto those within subaeriallyemplaced basalt flows on Iceland. When isostaticallycorrected for sedimentload, the northern part of the J Anomaly Ridge has basementdepths about 1400 m shallower than in our study area, and deep sea drilling has shown that the northern ridge was subaeriallyexposed during the middle Cretaceous.We suggestthat most of the system originated under subaerialconditions at the time of late-stagerifting between the adjacent Grand Banks and Iberia. The excessmagma required to form the ridge may have been vented from a mantle plume beneath the Grand Banks-Iberia rift zone and channelledsouthward beneath the rift axis of the abutting Mid-Atlantic Ridge. Resulting edifice-buildingvolcanism constructed the ridge system between anomaliesM-4 and M-0, moving southwardalong the ridge axis at about 50 mm/yr. About M- 0 time, when true drift began between Iberia and the Grand Banks, this southward venting rapidly declined. The resultswere rapid return of the spreadingaxis to normal elevations,division of the ridge systeminto the separateJ Anomaly Ridge and Madeira-Tore Rise, and unusuallyfast subsidenceof at least parts of these ridges to depths that presently are near normal. This proposed origin and evolutionary sequencefor the J Anomaly Ridge-Madeira-Tore Rise systemclosely matchesevents of uplift and unconformity development on the adjacent Grand Banks. INTRODUCTION in crust dating from slightlyyounger than magneticanomaly Predrift reconstructions of the North Atlantic show the M-1 to slightly younger than M-0, but the amplitude of the southwestern edge of the Grand Banks as a sheared (trans- anomaly also can be accounted for by an increase in the form) margin that developed during the fragmentation of magnetic layer thickness [Sullivan and Keen, 1978]. In the Laurasia and the initial opening of the North Atlantic Ocean western North Atlantic the J anomaly can be traced north- [Pitman and Talwani, 1972; Le Pichon et al., 1977]. The eastward from near the eastern end of the New England SoutheastNewfoundland Ridge has been interpreted as the Seamounts to the eastern edge of the Grand Banks. The trace of a fracture zone in oceanic crust marking the continu- anomalyhas intermediateamplitudes south of 38øNand high ation of this transform margin. The ridge extends about 900 amplitudesfrom there north to the SoutheastNewfoundland km southeastfrom the southern tip of the Grand Banks with Ridge [Rabinowitz et al., 1979]. Keen et al. [1977] also a subsidiary projection, the J Anomaly Ridge (or Spur suggestedthat the J anomaly continues northward acrossthe Ridge), extending to the southwest (Figures 1 and 2). SoutheastNewfoundland Ridge into the Newfoundland Ba- sin, with a 100-km offset; magnetic profiles in this area The J magnetic anomaly [Pitman and Talwani, 1972] is a suggestthat the amplitude of the anomaly decreasesnorth- linear zone of high-amplitude magnetic anomalies, up to about 1000 gammas, associated with crust formed between ward, especially north of the Newfoundland Seamounts. In magnetic anomalies M-0 and M-1 on both sides of the North the eastern Atlantic, the J anomaly follows the Madeira-Tore Atlantic Ocean (Figure 2) [Rabinowitz et al., 1979]. The Rise (MTR) [Luyendyk and Bunce, 1973; Rabinowitz et al., crust therefore is of Barremian-Aptian age (Early Creta- 1979],and it appearsto continuenorthward possibly as far as ceous, 108-113 Ma), according to the Larson and Hilde the western margin of Galicia Bank [Groupe Galice, 1979]. [1975] time scale used throughout this paper. Rabinowitz et These studies suggest that anomaly amplitudes decrease al. interpreted the J anomalies as isochrons formed at the north and south of the Azores-Gibraltar Ridge in the same fashion and over about the same distances as in the western Mid-Atlantic Ridge axis. They modeled the anomalies by North Atlantic. assumingthat large increasesin magnetizationintensity exist A basement ridge or step generally is coincident with the J anomaly in the western North Atlantic, althoughit does not •Now at Gulf Oil Explorationand ProductionCo., Houston, Texas 77099. strictly parallel isochrons. The ridge is asymmetrical and is defined in seafloor morphology only north of 40øN, where Copyright 1982 by the American GeophysicalUnion. the name 'Spur Ridge' also has been applied [Gradstein et Paper number 2B1215. al., 1977]. To the south between 38ø and 40øN, the J 0148-0227/82/002B- 1215505.00 Anomaly Ridge (JAR) is buried by sedimentsof the Sohm 9389 9390 TUCHOLKEAND LUDWIG: STRUCTUREAND ORIGIN OF J ANOMALY RIDGE Fig. 1. Locationmap of theJ AnomalyRidge and Madeira-Tore Rise in theNorth Atlantic. AbyssalPlain, and the ridgecrest commonly exhibits a relief alone are not sufficientto accountfor the amplitudeof smoothsurface (Figures 2 and 3). This part of the ridge the J anomalyif a magneticlayer of constantthickness is slopesgently eastward but sharplywestward in a seriesof assumed [Rabinowitz et al., 1979]. 0.5- to 1.5-kmwest-facing scarps. The changesin basement A crustalridge also occurs beneath the Madeira-ToreRise 65ø 60ø 55ø 50ø 45ø t i i i !•,•"-,L•i I I I f' I '/' I I I I (,.,,• it,• I I I I( i I I/ I"----• ,. , , ,- .•..--:.... ? ß...:...•.....,•.,, .• -•'•-•/J :i;'' .... ' 45 ø • c- •.'- • •. /" ., • SON,• ' . - 40 ø i-'"..•o' %,, 0 I ß;; ß 33 :•, 65 ø 60 ø 55 ø 50 ø 45 ø Fig. 2. Locationmap of Conrad21-11 MCS tracks (heavy lines). Bathymetry inmeters. Black dots are locations of magneticanomalies, asidentified by Rabinowitz etal. [1979].J anomalynorth of SoutheastNewfoundland Ridge from Keenet al. [1977].Inset shows three profiles across the high-amplitudeJ magnetic anomaly, located by dottedlines acrossJ AnomalyRidge. Detail of surveywork (boxed area) is shownin Figure4. Starlocates DSDP site 384. TUCHOLKE AND LUDWIG: STRUCTURE AND ORIGIN OF J ANOMALY RIDGE 9391 M2 WEST • EAST6 _ [-•_•••_, ß •• • -• ............. •• / I•/ ' v•"ø' / • • •; ....... •' • v2701 I 1 z M4 M2 MO o o ._ :.:,-:;:.•:;,/ ' '- "L'::'-- -%½ • / " Vl713 5 •--• • '• , V2301 • M4I M2I ' MOI _ o U -• ' v2302 • ß M4 M2 : MO . 58-w 55- 50- •0'L ,• • : •• • • • V2301 ' 5 - EAST.]4 Fig. 3. Tracingsof single-channelseismic profiles (1-9) acrossJ Anomaly Ridge, alignedalong ridge crest (dotted line), and with M series magnetic anomalies located (modified from Tucholke and Vogt [1979]). Inset shows profile locationson map of depth to basement(in kilometers). Bottom profile showscorrelative basementridge in the eastern North Atlantic; profile is 200 km long, crossingMadeira-Tore Rise 70 km north of Madeira. 9392 TUCHOLKEAND LUDWIG: STRUCTUREAND ORIGIN OF J ANOMALY RIDGE 56 ø 5 5 ø 54 ø 53 ø 52 ø 40 ø 5280 39 ø - ISOBATH ß•, AIRGUN-sONOBUOY STATION 0 EXPLOSIVES-SONOBUOYSTATION:•i'iI:•.... ...5560-- LINE •SCS LINE ALPINE MO 38 ø Fig. 4. Locationmap of Conrad21-11 track andMCS, OBH, andsonobuoy measurements over the J AnomalyRidge. Bathymetry in corrected meters. at the position of the conjugateJ anomaly in the eastern netic anomaliesas part of the MesozoicM series[Rabino- North Atlantic (Figure 1). However, the structureof at least witz et al., 1978, 1979] and the occurrenceof the conjugate parts of the Madeira-ToreRise has been modifiedby late Madeira-Tore Rise in the eastern North Atlantic seem to Cretaceous-Tertiarycompressional deformation and Neo- demonstrateconclusively that the J AnomalyRidge is devel- genevolcanism [e.g., Laughton and Whitrnarsh,1974; Wat- opedin oceaniccrust. Vogt andEinwich [ 1979]also assumed kins, 1973]. Although no detailed study and mapping of an oceaniccomposition, and they suggestedthat the magne- seismic reflection data has been made for this region, our tizationchanges and (or) thickenedmagnetic layer associat- examinationof a variety of seismicprofiles suggeststhat at ed with the J anomalyoriginated in responseto the transition leastparts of the Madeira-ToreRise are a mirrorimage of the from a long period of relatively slow seafloorspreading (M J Anomaly Ridge in terms of general configurationand seriesanomalies) to a period of faster spreading(Cretaceous presenceof smoothbasement (Figure 3, bottom). magnetic quiet zone). In 1975, the northern J Anomaly Ridge was drilled