Farallon Spreading Center North of the Mendocino Fracture Zone

Home , Juan de Fuca Ridge, Mendocino Fracture Zone

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B4, PAGES 2813-2838, APRIL 10, 1988

Eocene Reorganizationof the Pacific-FarallonSpreading Center North of the Mendocino Fracture Zone

DAVID W. CARESS AND H. W. MENARD

ScrippsInstitution of Oceanography,University of California, San Diego, La Jolla

R. N. HEY

Hawaii Instituteof Geophysics,University of Hawaii, Honolulu

During a 1984 Sea Beam and magneticssurvey in the North Pacificwe collecteddata in the area north of the Mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducteddetailed surveys of the bends in the Surveyor and Mendocino fracture zones associatedwith this change in plate motion. We have analyzedthese data combined with all available magneticsand bathymetricdata from previous cruisesto determine whether the ridge reorientation and reorganizationoccurred by way of rift pro- pagationor rift rotation through differential asymmetricspreading. The reorganizationbegan just after anomaly 24 to the north of the Surveyor fracture zone but did not begin between the Mendo- cino and Surveyor fracture zones until the time of anomaly 22, a 2.5-m.y. delay produced by the old Mendocino transform trend constrainingthe Farallon plate lithospherebetween the Mendocino and Surveyor transforms to move in the old spreadingdirection until the fracturing through of a new Mendocino transform fault in the new orientation. Most of the reorientationof spreadingrift segmentsoccurs within 2-3 m.y.; the entire reorganization requires about 7 m.y. We find direct evidence for seven propagationepisodes accounting for the primary reorientation of 75% of the length of the ridge system,and nowhere is the data more consistentwith rift rotation than with rift propagationas the principalmeans of reorientation. We concludethat the primary mechanismby which the reorganizationoccurred was rift propagation.Assuming that rift propagationcan account for all of the ridge reorganization,we presentforward models of magneticanomaly profileswhich succeedin fitting the data well. We then presenta model for the tectonicevolution of the reorganization which fits the data well using 11 episodesof both northward and southwardpropagation.

INTRODUCTION almost due east and initiating the Sedna and Sila fracture zones. The rift rotation model has also been used by Vogt It has been recognizedsince early in the developmentof et al. [1969] to explainsimilar anomalypatterns found in plate tectonicsthat significantchanges can occur rapidly in the North Atlantic and by Carlson[1981] to explain the both the magnitudes and directions of plate motion. reorientation of the Juan de Fuca spreadingcenter and has Menard and Atwater [1968; 1969] used fracture zone been extended and applied to the Cretaceous evolution of trends and magnetic anomaly lineations in the northeast the Pacificby Menard [1984]. Pacific(Figure 1) to showthat changesin the directionof More recently, rift propagation[Hey et al., 1980] has seafloor spreading have occurred and that the ridge- been proposedas an alternative mechanismfor ridge reor- transform systemsaffected reorganizedand reoriented to ganization and reorientation in response to changes in regain the familiar orthogonal ridge-transformspreading plate motion. Shihand Molnar [1975] explainedthe elimi- center geometry. They proposedthat the reorientationof nation of the Surveyor fracture zone between anomalies ridgesoccurred through the rotation of ridge segmentsby 12 and 5 by a sequentialseries of rift jumps now under- differential asymmetric spreading,as shown in Figure 2a. stoodto be rift propagation.Hey [1977] suggestedthat rift This mechanismexplained the apparent fanning (called propagation could explain the complex magnetic anomaly "zed patterns")of anomaliesseen in the magneticsdata pattern found in the area of the Juan de Fuca ridge and then available and showed how a single ridge segment subsequentlyHey and Wilson[1982], Wilsonet al. [1984], could break up into several smaller segments, thus creat- Nishimuraet al. [1984], and Wilson[1985] succeededin ing new transformfaults. Atwaterand Menard [1970] modeling the recent tectonic history of the Juan de Fuca presenteda more comprehensivecompilation of magnetics ridge as a series of rift propagation episodes associated data in the northeast Pacific. This study confirmed that with changesin the directionsof relative plate motion. the direction of spreadinghad changedbetween magnetic An example of how rift propagation might reorient a anomalies 23 and 21, changing the trend of the Mendo- ridge systemand initiate new transform faults is shown in cino and Surveyor fracture zones from east-northeastto Figure 2b. If we take Figure 2 as a schematic view of a section of the Pacific-Farallonspreading center with the Copyright1988 by the American GeophysicalUnion. Pacificplate to the west and the Farallon plate to the east, then we see that northward propagation will leave the Paper number 6B6169. failed rifts, the sheared zones, and the inner pseudofaults 0148-0227/88/006B-6169505.00 on the Pacific plate while the outer pseudofaults are left

2813 2814 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

data, magneticsdata, and watergun seismicreflection data during a 26 day cruise on the R/V ThomasWashington in July and August 1984 (leg 4 of the Marathon Expedition: hereafter referred to as the "Zed Expedition") between $5øN Kodiak Island, Alaska, and Honolulu, Hawaii. As shown in Figure 1, we concentrated on the region between anomalies24 and 21, and we conducteddetailed surveys of the Mendocino and Surveyor fracture zones in the areas where they bend in associationwith the change in plate motion. OverlappingSea Beam swath surveyswere also carried out just to the north of both fracture zone .... surveys. Since all of the Farallon plate createdat the time of the ridge reorganizationhas been subducted,the area we surveyed bears the only remaining record of the event. In this paper we present our analysisof our new Sea Beam

45• and magneticsdata combined with a compilation of magnetics and bathymetry data from previous cruises in the area, and we conclude that the primary reorientation and reorganization of the Pacific-Farallon spreading center between the Mendocino fracture zone and the Sila fracture SurveyorFractureZone•0 zone was accomplished by severalepisodes of rift propaga- 4GN Mendocino Fracture Zone tion. We then apply our analysisto develop and presenta propagating rift model of the tectonic evolution of the ridge reorganization which fits the prominent features of the data well.

3S'N DATA ACQUISITIONAND ANALYSIS

Sea BeamData and NavigationAdjustments

30N Our navigationon the Zed Expeditioncame primarily from transit satellites,but we were also able to obtain reliable Global PositioningSystem (GPS) navigation for about 3-4 hours a day. The navigationwas adjustedon the basisof offsetsof bathymetricfeatures in overlapping or crossingSea Beam swathsusing a least squaresoptimi- :.--',•:i•o •' / zation algorithm. In this approachthe adjustments are optimized by fitting the observedrelative offsetsexactly while minimizing in a weighted least squaressense the 2•N- magnitudesof the adjustmentsand changesin ship track , I I I I / length betweenadjustment points. The adjustmentpoints 16gW 15• 15• 14• 140W consistof all points of swath overlap or crossingwhere Fig. 1. Shiptrackof the Zed Expedition superimposedon the relativeoffsets are calculatedand all pointswith navigation regional tectonics. fixes. Navigationadjustments are linearly interpolatedin time betweenthe adjustmentpoints. At adjustmentpoints with navigationfixes the adjustmentsare weightedby the on the Farallon plate. If propagationis initiated south- 95% confidenceerror ellipsesof the fixes; reasonableesti- ward, then the failed rifts, the sheared zones, and the mates of equivalent uncertainties are used for other outer pseudofaults will end up on the now subducted adjustment points. Changes in the lengths of ship track Farallon plate. segmentsbetween adjustmentpoints are weightedby dis- The rift propagationmodel differs from the rift rotation tances correspondingto an uncertainty of 1 knot in ship model in that changesin the orientations of isochronsare speedover the ship track segments. The major and minor abrupt rather than gradual. Since abyssal hill lineations axes of the satellite fix error ellipsesare usually oriented are generally formed parallel to the spreadingridges, nearly east-westand north-south, respectively,because of changes in the abyssal hill fabric of the seafloor should the polar orbits of the transit satellites;this allows us to also be abrupt in the caseof propagatingrifts and gradual treat adjustmentsin latitude separatelyfrom adjustments in the case of the zed pattern model. A combination of in longitude. magnetic anomaly data and high-resolutionswath mapped No direct estimates of the uncertainties in our GPS surveysof seafloortopography is thus an ideal way to dis- fixes could be obtainedat the time of the Zed Expedition; tinguish between the two models. In order to test the two in view of the expectedaccuracy of GPS navigationwithin hypotheses with regard to the change in direction of a few tens of meters we treated the GPS fixes as being spreading around anomalies 24-21 in the northeast completely accurate. Software recently installed on the Pacific, we collected Sea Beam multibeam bathymetric R/V Thomas Washingtonnow gives estimatesof the 95% CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2815

A. Rift Rotation

1. 2. 3.

B. Rift Propagation

1. 2. 3.

Old Spreading Direction

New Spreading Direction Fig. 2. Schematicview of spreadingcenter reorientation through rift rotationand rift propagation.(a) Evolutionof a steppedridge from a straightone through asymmetricspreading following a changein the directionof spreading: (1) initial rift configuration,(2) fast and slowspreading occur at oppositeends of eachnew rift segmentuntil each becomesorthogonal to new spreadingdirection, and (3) new stablerift configurationcontinues spreading. (b) Prop- agatingrift mechanismfor spreadingcenter re-orientation: (1) nucleationof propagatingrifts orientedN-S, orthogonal to new spreadingdirection, (2) activepropagation stage ends; thin dashedlines indicatepseudofaults and thick dashedlines indicate failed rifts, and (3) followingtermination of propagation,fracture zones form.

confidence error ellipses of GPS fixes with typical major the adjustments exceeded the estimated error ellipses of axes of 50-100 m, implying that we overemphasizedthe the original navigation at the adjustment points. Most of GPS fixes in our adjustment optimization scheme. How- the adjustmentswere less than 750 m, although a few ever, this problem does not seem to have adversely adjustmentsof as much as 1500 m were required in places affected the self-consistencyof our results since none of lackingreliable navigationfixes for severalhours. 2816 CARESSET AL.' PACIFIC-FARALLONRIDGE REORGANIZATION

• 3 ø00'.IV' !$$o00 '

Plate la

44ooo9v

F 4

152ø40'W 152020' w 152• 00' W

Plate lb Plate1. Bathymetriccontour maps based on our Surveyorfracture zone Sea Beam survey. (a) 400-mcontour interval.(b) 100-mcontour interval. (The color version and a completedescription of this figure can be found in the separatecolor section in this issue.) CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2817

Plate 2a

4oooW

40 øoo W

155000'W 152 040'W 152 ø20 '• Plate 2b Plate2. Bathymetriccontour maps based on our Mendocinofracture zone SeaBeam survey. (a) 400-m contour interval. (b) 100-m contour interval. (The color versionand a completedescription of this figure can be found in the separatecolor section in thisissue.)

Sea Beam Bathymetry color versions can be found in the separate color section of this issue.) This bathymetryis based entirely on Zed In the course of analyzing the Sea Beam bathymetry we Expedition Sea Beam data, with the sole exception of a identified several"omega" artifacts in the data as described small area in Plate l a as described in the caption. Solid by deMoustier and Kleinrock[1986]; thesefeatures were contours are Sea Beam data; interpolations between Sea not incorporated into contour or lineation maps and did Beam swaths are dashed. A color shaded relief map of an not influence the interpretation of the data. Contour maps overlapping swath Sea Beam survey just to the northwest of the bathymetry of the bends in the Surveyor and Men- of the bend in the Mendocino fracture zone is shown in docino fracture zones are presented in Plates 1 and 2. Plate 3. (Plate 3 is shown here in black and white. The (Plates 1 and 2 are shown here in black and white. The color version can be found in the separate color section in 2818 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

' -25 ,•; ,••.:•::•:-%: ...... :..%• .:•-/% :,,•,•, -•.- .....

...... Plate3. Shadedrelief map of overlappingswath Sea Beam survey to northeastof the bendin the Mendocinofrac- ture zone. (•e color versionand a completedescription of this figure•n be found in the separatecolor sectionin this issue.) this issue.) The bathymetryof the overlappingswath sur- that magnetic anomaly lineations and abyssal hill linea- vey to the north of the bend in the Surveyorfracture zone tions are both created parallel to spreading rifts, we use is discussedin detailby Hey et al. [this issue]. the orientations of abyssal hills observed in Sea Beam swaths to interprete the magnetic anomaly data in greater Sea Beam Lineations detail than would otherwise be possible. After describing In order to aid in the analysisof the magnetic anomaly the general characteristicsof the ridge systembefore, dur- data, topographiclineations observed in the adjusted Sea ing, and after the reorganization,we discussfour critical areas in detail. Beam contours were digitized and are presented in Figure 3. The swath envelope in Figure 3 is 3 times the actual Sea Beam swath width to enhance visibility. Clear abyssal The Ridge SystemBefore and After the Reorganization hill trends are extrapolated to the envelope as straight line Prior to anomaly 24.1 the sectionof the Pacific-Farallon segments, and large scarpson the Mendocino and Sur- spreadingcenter south of the Pacific-Farallon-Kulatriple veyor fracture zones are also extrapolatedto the envelope. junction and north of the Mendocino fracture zone con- Other structural features such as scarps associated with sisted of two linear ridge segmentstrending N17øW and fracture zones, seamounts, or areas with very rough or offset approximately320 km (a 8.6 Ma age contrast)in a disturbed bathymetry are shown as they appear in the right-lateral sense by the Surveyor transform. The offset data. acrossthe Mendocino transform at this time was certainly left lateral and large, but the magnitudeis more difficult to Magnetic Anomaly Data gauge because of a paucity of identifiable anomalies In addition to the magneticsdata collected on the Zed immediatelyto the south. Atwaterand Menard [1970] ten- Expedition, we compiled digitized magnetic anomaly and tatively identified anomaly 32 at about 150øW, which bathymetry data from 45 previous cruises in the area implies an offset (and age contrast) of about 1000 km between 37øN and 52øN, 160øW and 148øW, obtaining the (27 Ma). However, the complexity of the topography data from the Scripps Geological Data Center and from south of the bend in the Mendocino fracture zone sug- the National GeophysicalData Center. Selectedmagnetic gests that these values should be treated with caution, as anomaly profiles are presented in Figure 4 along with will be discussedlater in the section dealing with fracture some of the anomaly correlations;our interpretation of zone bathymetry. the magneticanomaly lineations is presentedin Figure 5. After the end of anomaly 21 the reorganized and Forward models of 23 magnetic anomaly profiles are reoriented ridge system consistedof nearly north-south shown in Figures 6-9 and are discussedin the magnetic trending linear ridge segmentsoffset left laterally by the anomaly profile modeling section of this paper. The new Sila and Sedna transforms, right laterally by the numbering of the anomalies follows the convention of reoriented Surveyor transform, and left laterally by the Hadand et al. [1982], so that the younger of the two reoriented Mendocino transform. The offsets (and age periods of normal polarity which constitute anomaly 24 is contrasts) on the Sila and Sedna transforms were 155 km referred to as anomaly 24.1 and the older interval as (4.8 Ma) and 210 km (5.6 Ma), respectively.The offset anomaly 24.2. We will wish to denote separately the on the Surveyor transform had decreased about beginningand ending reversalsdefining the single period 150-170 km (4.0 Ma). The Mendocino transform offset of normal polarity comprisinganomaly 21, and following had becomeapproximately 1150 km (20 Ma). Nesset al. [1980], we will refer to the youngerreversal as 21(y) and to the olderreversal as 21(o). General Character of the Ridge Reorganization Contrary to previous interpretations in which the ridge INTERPRETATION OF MAGNETIC ANOMALY reorganizationoccurred simultaneouslyon both sides of AND SEA BEAM DATA the Surveyor transform, Figures 4 and 5 show that north In this section we analyze the magnetic anomaly data, of the Surveyor fracture zone the ridge reorganization the Sea Beam lineations, and the Sea Beam bathymetry began between anomalies 24.1 and 23, but between the together in order to develop a new interpretation of the Surveyor and Mendocino fracture zones the change did magnetic anomaly lineations in the area of the ridge reor- not begin until just before or during anomaly 22, a delay ganization, which is presentedin Figure 5. By assuming of about 2.5 m.y. CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2819

1 16 15 Sila FZ

2,4: •'o, SOøN

4gN 22 [ Sedna•' '2, 21(o)

46øN 2 20 21(o) q

44N

Surveyor FZ

42øN 20 211(y) 2•1(o)[

40øN

3gN 15•W 15GW 154øW 15 2W 15(•W 148øW

Fig. 3. Lineation map generatedfrom Zed Expedition Sea Beam data. Lineation swath widths are 3X Sea Beam swath widths. Clear abyssalhill trends have been extrapolatedas line segmentsto the edgesof the swaths;large scarpson the Mendocino and Surveyor fracture zones are also extrapolated to the swath envelopes. Other structural features(such as seamountsor smallerscarps) are shownin their actualextent. 2820 CARESSET AL.: PACIFIC-FARALLON RIDGE REORGANIZATION

158• 156W 154W 15 ffW 150øW 148•W 52•

• $ilaFZ ! !

21 18 SOøN

4•N

46øN

22 21 2O

44N

Surveyor FZ

18 I 21

Mendocino FZ I

40øN 44:40N

•• [500 n T 44%20N

38øN I i 44•-00N

Fig. 4. Magneticanomaly profiles projected perpendicular to shiptrackwith positivetowards the north. Someof the moreobvious anomaly correlations are shown.Labeled profiles are modeled in Figures6-9 or discussedin text. Z indicatesdata obtained on theZed Expedition; all otherprofiles are taken from digitized data from previous cruises, exceptfor N1 andN2, whichare taken from Raft[1966]. The area of a detailedsurvey enclosed in therectangle around44øN and 153øWis insetat a largerscale in the lowerright corner. CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2821

52øN 1 615 Sila FZ

50øN

48øN .•,/(,o (y)I0 9

46øN

44øN •••- SurveyorFZ ...---'T , I •1,•1 i I.o1,'1,,11 $ 2,2,2•1(o)

I ...:-:.;:• •"' •'::...-•. / $8øN!I I i I 'i i i i i• - 158•/ 156•/ 154øW 152øW 150øW 148øW

Fig. 5. Magneticanomaly interpretation. All numberedlines correspondto the center of the named normal polarity interval, exceptfor anomalies18, 20, and 21, where the linesdenote the reversalsat the beginningand end of the respectivenormal polarity intervals. Dashed lines representfracture zones. 2822 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATIO•½

o -o CARESSETAL.' PACIFIC-FARALLON RIDGE REORGANIZATION 2823

/ ,

o 2824 CARESSET AL.' PACIFIC-FARALLONRIDGE REORGANIZATION

PROFILE Z8 PROFILE Z14

24.2 23 22 21 22 21 24.1

PROFILE ZlO PROFILE Z15 I I

PROFILE Zll PROFILE Z16 I

I -- 4.2••232 22 21

PROFILE Z13 PROFILE Z18 I I 2.4.2I " I I I I I

0 I 0 20 30 km

Fig. 8. Magnetic data and models between44øN and 45ø30'N. Pseudofaultsare shown as dashedlines.

As discussedby Menard and Atwater[1969], a clockwise Thus we proposethat the delay in the rift reorganization rotation (of less than 90ø ) of the directionof spreading between the Mendocino and Surveyor transforms will cause right-lateral transform faults such as the Sur- representsthe amount of time requiredfor the fracturing veyor transform to be in tension and open or become through of the new Mendocino transform fault. For lack "leaky," whereas left-lateral offsets such as the Mendocino of a better existingterm referring to the processthrough transform will be in compressionand will tend to constrict which a transform fault trending obliquelyto the current or lock. When the change in the direction of spreading spreading direction (rotated clockwise relative to the occurred in the northeast Pacific, the piece of Farallon transform'strend) constrainsthe adjacentblocks of litho- plate lithosphere lying between the Mendocino and Sur- sphere to move parallel to its trend rather than in the veyor transforms was constrained to move in the old directionof overall plate motion, we will use"trammeling" spreading direction by the Mendocino offset. It was not to refer conciselyto this phenomena. until a new Mendocino transform fault was fractured Presumably,some deformation resultingin north-south through in an east-west trend that the direction of diver- shortening occurred along the old, unfavorably oriented gence along the spreading center between the Mendocino Mendocino transform prior to the creation of the new and Surveyor transforms could follow the new relative transform. This shortening could have been accommo- plate motion and begin the spreadingcenter reorientation. dated by deformationor convergencealong the Surveyor CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2825

PROFILE Z20 PROFILE S9

I _ _ _

21 21 24.1 i i 24.1 • •

PROFILE Z21 PROFILE Z24 I

I 24.2 23 22 24.1 i i i Ll•;• 24.1 ' ,

PROFILE Z22 PROFILE Z25

I I I I •l• II I I

24.1 i i • 24.1 I I

PROFILE Z23 PROFILE SlO

24.1 I 24.1 ' I ' I 'I l.._l (•{•) I I I I II I I I

0 I 0 200 3 0 400

Fig. 9. Magneticdata and modelsbetween 39øN and 43ø30'N. Failed rifts are shownas short dashedlines and, pseudofaultsare shownas long dashedlines.

fracture zone on the Farallon plate, along the Mendocino old trend) in two places,once around 49øN and oncejust transform,or in the blockof lithospherelying betweenthe to the north of the Surveyor fracture zone. These anom- two boundaries. Since the breaking through of the new aly patterns are consistent with southward propagation Mendocino transform transferred the wedge of lithosphere leaving the outer pseudofaulton the Pacific plate (as containingalmost all of the old transformto the Farallon shownin Figure 2). Areas of disturbedbathymetry which plate, the entire recordof where and how the lithospheric are interpreted as shearedzones associatedwith northward shortening occurred has been subducted under North propagationare seenin the SeaBeam lineations (Figure 3) America. between anomalies 24.1 and 23 at 47ø30'N and on the Together the magneticsand Sea Beam data strongly eastern parts of profilesZ6 and Z7. South of the Surveyor indicate that the primary mechanismfor the ridge reorgan- fracture zone anomaly 22 is cut by an oblique anomaly ization was rift propagation,although rift rotation cannot due to northward propagation,and distinctly north-south be excluded as a possible mechanism for several minor orientedabyssal hill lineations(Figure 3) indicatethat the instancesof reorientation. North of the Surveyor fracture apparent fanning of anomaly 21 seen in Figure 4 is better zone anomaly23 (with the new N-S trend) can be seenin explainedby successiveepisodes of northward and perhaps Figure 4 to cut into and throughanomaly 24.1 (with the southward propagation. 2826 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

Initiation of the Sedta Fracture Zone trend (seen at 48ø20'N and 153ø30'Win Figure 3), but this may be an artifact of increasingsediment thicknessto Profiles S2, Z1, Z2, Z3, and S3 in Figure 4 distinctly the north. This northward propagationepisode may have show anomaly 23 successivelycutting into and through initiated the Sednatransform around the time of anomaly anomaly 24.1 from the north with a southward trend. 23, or it may have extended the reoriented rift segment Abyssal hill lineations are consistent with the observed farther north to be cut off by a later southwardpropagat- orientation of anomaly 23 but are covered by sediment to ing rift during anomaly 21 which served to initiate the the west. Although the reoriented anomaly 23 lineation Sedna transform at that time. Certainly the reoriented extends from 50ø30'N to 49øN, the width of anomaly 24.1 ridge segment between 49øN and 51øN must have does not vary systematicallyor fan to the north of profile extended southwardsomewhere between anomaly 22 and Z1 (about 49ø30'N) as would be expectedin rift rotatation. the end of anomaly 21, presumablyby rift propagation. Noting that any rift rotation explanation for the observed The magneticsbetween anomalies 24.2 and 21 on profiles magnetic anomaly pattern must thus involve at least two S3 and Z4 are difficult to interpret in any simpleway, sug- separate segments which rotate at different times before gesting that this processwas quite complicatedand may coalescinginto a single segment and also a period of 100% have involved multiple episodes of both northward and asymmetric spreading to explain the complete disap- southward propagation. pearenceof anomaly 24.1 on profile S3, we think that such a scenario is unreasonably complicated compared to the Northward PropagationInto the Sedna Fracture Zone simple alternative of a single southward propagatingrift. Thus we interpret the gradual cutting off of anomaly 24.1 Clear abyssal hill lineations with the trends of the old as evidence for a southward propagating rift between ridge system are found to the east of anomaly 24.1 anomalies 24.1 and 23 which accounts for the primary between46øN and 47øN (Figure 3), indicatingthat oblique reorientationof the ridge between 50ø30'N and 49øN. spreadingpersisted here from the change in plate motion However, several complexities beyond a single episode past anomaly 22, which is seen on the west end of profile of propagationare evident in the tectonic history of this Z6. An anomaly 23 lineation segment with the old trend area, especiallywith regard to the initiation of the Sedna is inferred to lie between anomalies 24.1 and 22 at about fracture zone. Between 51øN and 48ø30'N anomaly 24.2 47øN, as shown in Figure 5. Farther to the east around decreases in width to the south and is slightly rotated 151øW disturbed but recognizable northeast-southwest clockwisein trend relative to 24.2 farther south (Figures4 lineations (Figure 3) are evidence for a sheared zone and 5). There appears to be about a 15 km left-lateral resulting from northward propagationduring anomaly 21. offset in 24.2 between profilesZ4 and S4, and the width of The rift jump associatedwith this northward propagating 24.2 abruptly increases to the south of the offset to the rift is confirmed by the double humped magneticanomaly same width seen north of 51øN. Abyssal hill lineations found on profile S6 where anomaly 21 would have been are largely obscured by sediment, and thus the variations expected (see Figure 7), and the inner pseudofault in 24.2 can be explained either by southward rift propaga- expressedas the western edge of the more eastern part of tion during 24.2 along a trend rotated slightly clockwise the double anomaly can be seen intersecting the Sedna from the rest of the ridge system or by a localizedarea of fracture zone at 150øW on profile S5 in Figure 4. The asymmetric spreading producing a zed pattern. To the angle between the inner pseudofault and the reoriented east, anomaly 22 is rotated slightly clockwiserelative to 23 ridge between profiles S6 and S5 appears to be about half into a trend largely preserved by succeeding anomaly the angle observed to the south of profile S6, implying lineations. The Sea Beam lineations, though less obscure, that the propagationrate increased and/or the spreading again fail to unambiguouslydistinguish between rift propa- rate decreasedsometime during anomaly 21. gation and asymmetric spreadingas the mechanism of this minor reorientation. SouthwardPropagation Into the SurveyorFracture Zone Anomaly 23 can be identified on profile S4 at 153ø40'W Between 45øN and 46øN the magneticsand Sea Beam and its beginning(the westwardor older end) can be seen data are insufficient to determine the mechanism of rift on the short profile just to the south. The lack of Sea reorientation. To the south, Zed Expedition profiles Beam data in this area and the uncertainty over the total Z8-Z18 show anomaly23 with a north-southtrend cutting width of the second identification of anomaly 23 makes it into anomaly 24.1 from the north, eventually cutting 24.1 unclear whether this short magnetic lineation has the old off entirelyby profileZ16 (see inset in Figure4). Abyssal or the new orientation. In order to avoid arbitrarily com- hill lineationsfrom an east-westswath at 45ø15'N (profile plicating the spreading center reorganization by introduc- Z8) appearto smoothlyfan from the old to the new orien- ing reoriented anomaly lineations not required by the data, tation between anomalies 24.1 and 23, but an oblique we prefer the former interpretation and show this short swath just to the south shows an abrupt change in the lineation in Figure 5 as having the old orientation. abyssalhill orientations across a trough that is consistent Farther to the south, an area of disturbed but fairly with a pseudofault[Hey et al., this issue].Following the coherent lineations oriented northeast-southwest is seen same reasoningapplied north of the Sedna fracture zone, around 48øN and 153ø30'Win the Sea Beam data (Figure the gradual terminationof 24.1 providesstrong evidence 3) to the west of the north-southtrending anomaly 23, a for a southward propagationepisode between anomalies pattern we interpret as a sheared zone associated with a 24.1 and 23 which accomplishedthe primary reorientation northward propagatingrift between anomalies 24.1 and 23 of the ridge between45øN and the Surveyorfracture zone. from about 47øN up to 48ø30'N. This sheared zone The Sea Beam bathymetry between 44ø35'N and the Sur- appears to be terminated along a northwest-southeast veyor fracture zone shows that the inferred location of the CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2827 outer pseudofaultleft by this propagatingrift goes through second northward propagatingrift. Looking at the mag- an area of numerous small seamounts which obscure the netics data of Figure 4 alone, anomaly 21 appearsto form abyssalhill texture. These seamountscan be seen around a perfect zed pattern by fanning so that it is wider to the 44ø25'N and 152ø50'W in Figure 3 and Plate la. north than it is to the south. However, clear north-south Directly to the east of the seamountsbeginning around abyssalhill lineations seen throughout this area in Figure 152ø40'W is an area of very clear abyssal hill lineations 3 are evidence against rift rotation, and we suggestthat with the northwest-southeasttrend of the old ridge system one or more propagationepisodes are responsiblefor the which are terminated abruptly on the east by north-south variationsin width of anomaly 21. abyssalhills, the boundary forming an outer pseudofault Profile Z26 (Figure 4) lies within the triangularregion associatedwith a secondsouthward propagating rift around just to the west of the bend in the Mendocino fracture the end of anomaly 23. This second propagation episode zone that was transferred from the Farallon plate to the seems to be limited to the area between 44ø40'N and the Pacific plate when the new Mendocino transform fractured Surveyor fracture zone and apparentlyreoriented a section through. We would expect the failed rift that forms the of ridge left in the old orientationby the first propagation western boundary of the transferred lithosphere to be and then continued south into the Surveyor fracture zone. flanked by symmetric magnetic anomalies allowing us to Because the trammeling along the old Mendocino determine when the new transform was created. Although transform implies convergencealong or between the Faral- the anomalies observed in profile Z26 appear distorted and lon plate side of the Surveyor fracture zone and the Men- are consistent with the new Mendocino transform break- docino transform, it is not surprising that the history of ing through either during the time of anomaly 22 or just the ridge immediately to the north of the Surveyor after the time of anomaly 24.1, the most reasonablewest- transform should be complicated,and it may be that this ward extension of the new Mendocino fracture zone convergence produced temporary perturbations to the crosses profile S10 to the north of where a very clear stressregime which deflectedthe first propagatingrift into anomaly 23 is observed,indicating that the failed rift must the old rift orientation for a time. Alternatively, the lie farther to the east and thus that the new transform oblique spreading center segment could have been the most likely broke through during anomaly 22. remnant of the first southward propagator's curved tip (propagatingrift tips typicallycurve toward the failing rift FRACTURE gONE BATHYMETRY [Hey et al., 1980, 1986]) if that propagatingrift stopped when it intersected a northward propagating rift nearly SurveyorFracture Zone head-on. The northward propagatorwould have been ini- As seen in Plate la, to the west of its bend at 152ø40'W tiated from a point on the Surveyor transform about the Surveyor fracture zone is expressed as a 5600- to 30 km to the west of the riff-transform intersection. The 6000-m-deep and approximately 5-km-wide trough only propagatingrift documentedas being initiated from a bounded on the north by a 200- to 600-m-high ridge and transform fault eliminated the Surveyor transform around then a 10- to 20-km-wide band that lies about 5600 m 36 Ma [Shihand Molnar, 1975; Wilson,1985]. The bathy- deep. To the south of the trough the seafloorshoals within metric evidence for rift propagationfound in the overlap- 15 km to abyssalhill topographybetween about 4800 m ping swath Sea Beam survey in this area between 44ø20'N and 5200 m deep. The shallowest bathymetry occurs in and 44ø30'N is discussedin detail in the companion paper the immediate area of the bend in the fracture zone by Hey et al. [this issue]. (shown in detail in Plate lb) where the trough is terminated at 152ø45'W by a northeastward curving structure Betweenthe Surveyorand MendocinoFracture Zones that shoalsto 3900 m; the eastwardend of this structure The trends of the magneticlineations between the Sur- corresponds with the point where the magnetics data veyor and Mendocino fracture zones rotate clockwise implies the first pseudofaultcomes in from the north. To about 1ø-2 ¸ between anomalies 24.1 and 22. This minor the north this pseudofault is inferred to run nearly north- episode of apparently smooth fanning is consistent with south about 152ø30'Walong the easternedge of an area of rift rotation through differential asymmetric spreadingbut numerous small seamounts between 44ø10'N and 44ø30'N can also be explained by a slight clockwiserotation of the which obscure the abyssalhill structure. block of Farallon plate lithosphere lying between the two As discussedabove, to the east of the seamountsis an transforms during the period of trammeling along the old area of abyssalhills with the trend of the old ridge system. Mendocino transform. These lineations are terminated at 152ø15'W by the second The primary magnetic lineation reorientation begins pseudofault, which can be clearly seen in Plates l a and lb where the northwest-southeast trend of anomaly 22 to trend southward into a deep cleft which breaks through between the Surveyor and Mendocino fracture zones is the Surveyor fracture zone at 152ø00'W. The bottom of interrupted at 42øN by a north-south trending anomaly the cleft is nearly flat at 5660 m but dips slightly toward whichextends north to the Surveyorfracture zone (Figure the center; it has apparently been filled in by turbidity 4) and correspondsto a zone separatingabyssal hill linea- currents flowing in along the trend of the Surveyor frac- tions of the old and new trends (Figure 3). We interpret ture zone from the Tufts Abyssal Plain to the east. The this oblique anomaly as evidence for a northward episode cleft is bounded on the west by a 1400-m scarp forming of propagationduring anomaly22 which reorientedhalf of the east wall of a right triangular seamount. This curi- the ridge between the two large transforms. A secondrift ously shaped structure shoals precipitously from linear jump at the beginning of anomaly 21 can be seen in scarps to the south and east to 4200 m at its southeast profiles Z20 and Z21 in Figure 4; this probably reflects a corner and then deepens gradually to the northwest, 2828 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION appearing as though the southeast corner of a flexible wasting may have buried the scarp, or perhaps a steep plate has been thrust upward. (The only other seamount scarp never existed. of similar morphology known to the authors is the Steel South of the Mendocino fracture zone a single Sea Vendor Seamount, which is located just to the north of Beam swath shows a series of four scarps parallel to the the Mendocino fracture zone at 129ø30'W and is indented fracture zone's old trend with depth offsets of on the northeastby two scarpsmeeting at a right angle.) 400-1100 m. The northernmost of these scarpsshallows To the east of the deep cleft the Surveyor fracture zone is to the north, and the other three scarpsshoal to the south. expressedmainly as a turbidite filled trough in the midst We interpret these scarpsas fracture zones with the north- of a gradual shallowing of 100-200 m from north to ernmost having a left-lateral offset and the southerlythree south. scarps having right-lateral offsets, and we present the Detailedstudies [Hey et aL, 1980, 1986;Naar andHey, inferred locations and trends in Figure 5. Prior to the 1986] show that propagating rift tips are typically change in plate motion a series of both right-lateral and expressed as deep holes, and PhippsMorgan and Parmen- left-lateral transforms apparently existed just to the south tier [1985] have successfullymodeled these depth varia- of the left-lateral Mendocino transform; the fracture zones tions as resulting from viscoushead loss of the upwelling left by these transforms were cut off when the new trend asthenosphere to the surrounding walls of cold litho- of the Mendocino transform broke through. These frac- sphere. One would expect this effect to be accentuated as ture zones lie within the Cretaceousmagnetic quiet zone, a propagating rift approaches a transform fault across so it is difficult to estimate the age offsets acrossthem. which the lithosphere is significantlyolder and colder than Since the profile on which Menard and Atwater [1969] that through which it is propagating. A deep axial depres- identified anomaly 32 south of the Mendocino fracture sion causedby viscoushead loss is accompaniedby notica- zone lies to the south of these newly proposed fracture ble uplift of the lithosphere flanking the depressionform- zones, a further implication is that we have no reliable ing a deep axial rift (median valley) characteristicof slow way of estimating the age offset across the Mendocino spreading ridges. The conjunction of the right triangular fracture zone prior to the ridge reorganization. In princi- seamount with the deep cleft could be formed as the result ple, age offsets across fracture zones can be estimated of increasedviscous head loss as the propagatingrift came from the depth offsets[Sandwell and Schubert,1982], but into the immediate vicinity of the Surveyor Transform, the variability of fracture zone topographyand the defor- with the most extreme bathymetry being produced as the mation and faulting that we have documented around the propagator intersected the transform and stopped propa- bends in the Surveyor and Mendocino fracture zones lead gating. us to conclude that this relationship is tenuous in our study area. Mendocino Fracture Zone MAGNETIC ANOMALY PROFILE MODELING To the west of its change in trend at 152ø20'W, the Mendocino fracture zone is expressedas a linear ridge The purpose of this section is to test the reasonableness bounded on the south by an 800-m scarpand on the north of our propagatingrift model for the ridge reorganization by a 200-300-m scarp(Plates 2a and 2b). The shallowest and to determine the magnitudes and timing of the rift feature in our survey is a seamount which sits astride the jumps involved in the rift propagation. In the previous fracture zone at 152ø50'W and shoals to 3700 m. To the sections we have presented evidence for several episodes north is a triangular area of abyssal hills with the of rift propagationwhich account for the primary reorien- northwest-southeasttrend of the old ridge system. An tation of about 75% of the length of the Pacific-Farallon east-west trending contact seen in Plates 2b and 3 between spreading center between the Mendocino fracture zone these abyssal hills and rough bathymetry to the north and 51øN. We have also noted several instances where forms the northern boundaryof the triangle; we interprete the data could not distinguishbetween rift propagationand this contact as the line along which the new east-west asymmetric spreading, and we recognize that numerous trending Mendocino transform fractured through. changesin spreadingrates could conceivablyhave compli- Becauseour Sea Beam survey does not extend far enough cated the tectonic history of this region. However, for the to the west to show where the new Mendocino transform purposesof developinga history of the ridge reorganiza- began, we are unable to unequivocally determine whether tion, we chooseto use what we considerto be the simplest the new Mendocino transform broke through at the time possiblemodel. Having establishedthat most of the reor- of the plate motion changeor 2.5 m.y. later as we propose. ganization was accomplishedthrough rift propagation,we Around the point where the new and old trends of the will assume that rift propagation can explain all of the fracture zone intersect, the scarp disappears(Plate 2a). reorganization, neglect asymmetric spreading entirely, and To the east an overall depth change of 800-1000 m is use the simplest spreadingrate model allowed by the data. constant, but the steep scarp appears to be buried by lobate structures except for intervals between 150ø00'W GeneratingSynthetic Anomaly Profiles and 150 ø10'W and between 149ø05'W and 149 ø15'W. Not- Synthetic one dimensional magnetic anomaly profiles ing that the south side of the fracture zone was probably are calculatedusing the bathymetry, a 1.7-km-thick mag- left heavily fractured and faulted when the transform netic layer, and an uniform effective susceptibilityof 0.01. broke through on the new trend, we speculatethat these All of the magnetic anomaly profile models presented in lobate structures may be voluminous basalt flows that Figures 6-9 assumeeast-west oriented spreading,and all spilled out the end of the ridge north of the transform as of the data profiles have been projected onto an east-west it moved east past the fractured area. Alternatively, mass line. In order to model anomalies created on northwest- CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2829

TABLE 1. Adjustmentsto Magnetic Anomaly Time Scale over the interval would fit the data well. The maximum adjustment of a reversal boundary is 0.41 m.y. Noting Normal Polarity Intervals a that the control on the adjusted part of the time scale is Anomaly Adjusted Berggrenet al. [1985] minimal [Berggrenet al., 1986], we suggestthat our adjust- 20 44.66 to 46.17 44.66 to 46.17 ments as presented in Table 1 may actually constitute an 21 48.75 to 50.34 48.75 to 50.34 improvement to the magnetic polarity time scale. Between 22 51.69 to 52.49 51.95 to 52.62 23 53.51 to 53.62 53.88 to 54.03 anomalies21 and 18 we find that the spreadinghalf rates 23 53.68 to 54.29 54.09 to 54.70 increase southward from 34.8 km/m.y. at 50øN to 24.2 55.14 to 55.37 55.14 to 55.37 46.4 km/m.y. at 41øN. 24.1 55.66 to 56.14 55.66 to 56.14 25 58.64 to 59.24 58.64 to 59.24 SouthwardPropagation Between 48ø30'N and 51 øN 26 60.21 to 60.75 60.21 to 60.75

aAll times in m.y. Figure 6 compares our magnetic anomaly models with the data from profilesS2, Z1, Z2, and Z3; rift jump times and distances are listed in Table 2. In addition to the southward propagatordocumented by the gradual elimina- southeast oriented ridges we adjust the spreading rates to tion of anomaly 24.1, we model the thinning and rotation account for both the azimuth of the anomaly lineations of anomaly 24.2 by an earlier episodeof southwardpropa- and the azimuth of the shiptrack along which the data gation. In order to fit variationsin the width of anomaly were collected. The timing, sense, and magnitude of rift 23 we add a third southward propagating rift between jumps are constrainedby the amounts of normal and anomalies 22 and 23, in agreement with the observation reverselypolarized crust which must be added or removed that the azimuth of anomaly 22 is rotated slightly clock- in order to make a syntheticprofile fit a data profile. Rift wise relative to 23. jumps which crossa reversal boundarycan usually be well The rift jump times obtained for all three propagation constrained, but rift jumps that occur entirely within a episodesscatter over periodsof 0.06-0.17 m.y. insteadof normal or reversed polarity interval can only be con- becoming sytematically younger to the south, implying strained to lie within that interval and cannot be dis- that the propagationsoccurred instantaneouslywithin the tinguishedfrom asymmetricspreading on the basisof the resolutionof the magneticprofile modeling. The rift jump magnetic anomaly modeling alone. distancesof 10-11 km used for the first propagationare arbitrary becauseof the lack of any reversalto the west of SpreadingRates and Adjustmentsto the Magnetic Time Scale anomlay 24.2 close enough to fit in these short profiles. In forward modeling the magnetic anomalies using the The westwardjump of the second propagationincreases Berggrenet al. [1985] time scale, we find that with the southward to about 23 km at profile Z3 and the jump dis- exception of anomalies 24.1 where it is cut off by south- tance of the third southwardpropagation is 20-23 km on ward propagation and 24.2 where it narrows between the three southerly profiles and about 13 km on profile S2. 48ø30'N and 51øN, all of the anomalies older than and Reliable estimates of the propagation rates cannot be including 24 shown on profiles in Figure 4 can be well fit obtainedfrom the resultspresented in Table 2, but we can by a single spreadinghalf rate of 37 km/m.y. Between use estimatesof the uncertainty in rift jump times to cal- anomalies 24 and 21, however, we find that when we culate maximum periods over which the propagation neglect all of the parts of magnetics profiles that are episodes could have occurred, and these can in turn be known to involve rift propagation,a systematicpattern of used to obtain estimates of minimum propagationrates. apparent changesin spreadingrate appearsthroughout our In cases where propagation rates can be calculated, esti- data set. On average, we obtain half rates of mates of the uncertainty in rift jump times can be used to 24.2 km/m.y. ñ3.7 between anomalies 21 and 22, 34.5 obtain uncertainties in the propagation rates. Because km/m.y. ñ 2.4 during 22, 23.2 km/m.y. ñ 3.9 between 22 one-dimensionalforward modeling of magnetic anomaly and 23, 27.5 km/m.y. ñ 8.5 during 23, and 55.3 km/m.y. profiles makes many simplifying assumptionsabout the ñ 5.5 between 23 and 24.1. In order to simplify the mag- seafloor spreadingprocess, evaluating how well the models netic anomaly modeling we have adjusted the anomaly generated fit the data is always a subjective process, and time scaleof Berggrenet al. [1985] betweenanomalies 21 rigorousestimates of the uncertaintyin the model parame- and 24 so that a singlespreading half rate of 28.8 km/m.y. ters are difficult to obtain. However, assuming that our

TABLE 2. Ridge Jump Times and DistancesFrom Figure 6

First Ridee Jumt• Second RidRe Jumo Third RidRe Jumo Profile Timea Distanceb Timea Distanceb Timea Distanceb

S2 55.90 11.00 55.00 5.30 53.24 12.70 Z1 55.91 10.00 54.70 16.50 53.30 22.70 Z2 55.90 10.00 54.62 19.70 53.17 22.30 Z3 55.85 11.00 54.70 23.40 53.13 20.70

aAll times in Ma. ball distancesin km. Positivedistances imply westward jumps, negative distancesimply eastwardjumps. 2830 CARESSET AL.: PACIFIC-FARALLON RIDGE REORGANIZATION

TABLE 3. Ridge Jump Times and DistancesFrom Figure 7 Sedna fracture zone. The primary eastward rift jump increasesfrom 20 km at profile S7 to 48 km at profile S6, First Ridge Jumt• •;econdRidge Jumt• and the second propagationinferred at profile S6 has an Profile Timea Distanceb Timea Distanceb eastwardjump of 13 km. The increasein the propagation S6c 49.66 -56.70 rate and the initiation of the secondnorthward propagating S6d 49.51 -48.10 49.18 -12.80 rift may be related to the increase in the spreadinghalf Z6 49.81 -35.50 S7 54.00 -20.50 rate from 28.8 to 39.10 km/m.y. at 47øN which occurred sometime around anomaly 21(o). Alternatively, the aAll times in Ma. apparent increase in propagation rate could be due to a ball distancesin km. Positivedistances imply westward fast second propagatingrift overtaking the slower first jumps, negativedistances imply eastwardjumps. CSinglepropagating rift model. propagatingrift and then continuing northward to the dDoublepropagating rift model. Sedna transform.

SouthwardPropagation Into the SurveyorFracture Zone adjusted time scale, our spreadingrate model, and our Two propagationepisodes (Table 4) are usedto model placementof rift jumps are correct,we conservativelyesti- the profilesshown in Figure 8. The first is associatedwith mate that the rift jump times and distanceswe present in the gradualcutting away of anomaly24.2 towardthe south Tables2-5 are goodto within +_0.15m.y. and 12.5 km. seen in Figures 4 and 5 and the secondis documentedby For the first and third propagationepisodes we have the abrupt change in abyssalhill lineations seen in Figure maximum propagationtimes of 0.36 Ma and 0.47 Ma, 3 and discussedin detail by Hey et al. [this issue]. A respectively;from these estimates we obtain respective further complicationinvolves the narrowing of anomaly 24 minimum propagation rates of 340 km/m.y. and in profiles Z10-Z18 relative to areas to the north and 260 km/m.y. Taking the earliest possibletime of the south. Unlike the narrowing of anomaly 24.2 between secondrift jump on profile S2 to be 55.00 Ma and using 51øN and 48ø30'N, here all of anomaly24 (includingthe our uncertainty estimatesfor the other profiles yields a reversed polarity interval between 24.1 and 24.2 and minimum propagationrate of 230 km/m.y. We conclude excluding24.1 where it is cut off by propagation)is uni- that the propagationrates were at least of the order of formly narrower than predicted by a spreadingrate of 200-300 km/m.y. and were quite possiblymuch faster. 37 km/m.y. This apparent spreadingrate changecould be It is possiblethat the ridgejumps of one profile might explained by asymmetricspreading and is consistentwith a not be associatedwith the same propagatingrifts responsi- small amount of rift rotation, although certainly not ble for the rift jumps on other profiles; in particular, it is enough to account for the primary reorientationof the possiblethat the rift jumps of profile S2 are unrelatedto ridge. Another possibilitymight that the changein plate those of profilesZ1, Z2, and Z3. If, as shownin Figure 2, motion had occurred by the beginning of anomaly 24.2 we assumethat the propagatingrifts all begin on the old and that the trammeling along the Mendocino transform ridgeand groworiented perpendicular to the new direction influenced the area to the north sufficiently to slow the of spreading,then a straightsection of ridge in the new overall spreadingrate along the ridge just to the north of orientation can only be producedby a single propagation the Surveyor transform. Because the variation in the episode (two episodesif we allow both northward and apparentspreading rate uniformly involves all of anomaly southwardpropagation from a single point on the ridge, 24 and becauseit can be explained by the independently but for the cases consideredhere all of the rift jumps are supportedhypothesis of trammeling along the Mendocino westward and thus consistent with southward propaga- transform, we prefer the secondinterpretation and model tion). Large deviations in propagationorientations are anomaly 24 in profilesZ10-Z18 using a spreadingrate of neccessaryto allow multiple propagatingrifts to create a 32 km/m.y. straightsection of reorientedridge, and thus we believe The rift jump times obtained for the first propagation that it is unlikely that the secondand third sets of rift episode modeled in Figure 8 scatter over a period of jumps represent more than one propagatingrift each. 0.25 m.y. and are no more consistentwith a model involv- Becausethe first propagationepisode only reorients the ing a northward as well as a southward propagation ridge segmentby about 1ø and sincewe have no way of knowingthe detailsof the ridge'sevolution in the 2.5 m.y. between anomalies 25 and 24.2, we note that multiple propagationsare a distinctpossibility in this case. TABLE 4. Ridge Jump Times and DistancesFrom Figure 8

First Ridee Jumt• Second Ridee Jumt• NorthwardPropagation to the SednaFracture Zone Profile Time" -Distanceb Timea I•istanceb

Figure 7 and Table 3 presentthe resultsof our magnetic Z8 54.50 6.60 anomalymodeling of profilesS6, Z6, and S7. We model Z10 54.65 14.50 53.20 13.20 profile S6 using both a single westwardrift jump and two Zll 54.73 15.00 53.16 12.60 Z13 54.75 17.10 53.11 13.50 westwardrift jumps; the double rift jump model is pre- Z14 54.71 18.60 53.06 12.00 ferred because it fits the width of the anomalies better and Z15 54.64 20.50 53.04 12.10 also nicely places the two pseudofaultson conspicuous Z16 54.60 30.60 53.04 13.80 scarps.The propagationrate inferred betweenprofiles S7 Z18 54.56 29.70 52.49 17.90 and Z6 is 19 km/m.y. +_1.5, but betweenprofiles Z6 and aAll times in Ma. S6 the propagationrate increasesto 180 km/m.y. +_90, ball distancesin km. Positivedistances imply westward apparently maintaining a similar rate on north to the jumps, negativedistances imply eastwardjumps. CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2831

TABLE 5. RidgeJump Times and DistancesFrom Figure 9

Third Ridge Jumr• First Ridge Jumt> Second Ridge Jumr• _ _ _ Profile Timea Distanceb Timea Distanceb Time'• Distanceb Z20 52.12 -23.80 50.27 - 10.20 Z21 52.09 -23.70 50.26 - 11.40 49.26 -8.60 Z22 52.21 - 16.40 50.54 -9.00 49.40 -4.40 Z23 52.33 -10.30 50.68 -12.20 49.47 -3.50 S9 52.50 - 7.00 50.80 - 10.00 Z24 50.92 -8.90 Z25 c 49.50 4.00 S 10c 49.50 17.60

aAll times in Ma. ball distancesin km. Positivedistances imply westward jumps, negative distancesimply eastwardjumps. CFourthpropagation episode.

episodewhich meet at 44ø27'N than they are with a single Figure 5. No attempt was made to fit in detail areas of southward propagatingrift. In either case, this propaga- poorly understoodcomplexity such as to the west of the tion, like the southwardpropagating rifts discussedabove, initiation of the Sedna fracture zone and just to the north occurredinstantaneously within the resolutionof the mag- of the bend in the Surveyor fracture zone. As with the netic anomaly modeling. As expected,the rift jump distances increase southwards from 7 km at profile Z8 to about 30 km as the propagationnears the Surveyor frac- TABLE 6. Tectonic Evolution Model Parameters ture zone. Using the ridgejump time uncertaintyestimate RotationPoles b of _+0.15m.y. yields a maximum time of propagationof Time Intervala øN øE deg/m.y.c 0.55 m.y., from which we obtain a minimum propagation 61.00- 55.00 66.00 64.00 0.410 rate for a singlesouthward propagator of 240 km/m.y. In 55.00- 50.34 85.60 239.75 0.423 the dual propagation model for this first episode the 50.34 -- 44.00 73.76 215.50 0.775 minimum propagationrates decreaseto 170 km/m.y. for 55.00- 52.51'/ -21.91 31.91 0.750 the southwardpropagator and about 70 km/m.y. for the 52.51-- 50.34a 85.60 239.75 0.423 Prorogation Rate northward propagator. The rift jumps of the secondpro- _ pagation episode are nearly constant at 12-14 km and PR Number Time Intervala deg/m.y.e km/m.y.e occur sequentiallylater toward the south, implying a pro- 1 56.47- 55.80 -3.10 -345 pagationrate of 65 km/m.y. _+20. This value is in agree- 2 55.05 -- 54.60 -3.30 -367 ment with the estimate obtained from geometrical argu- 3 55.03 -- 54.35f -3.00 -334 mentsin the companionpaper [Hey et al., this issue]. 4 55.00 - 54.00 1.70 189 5 54.50 - 50.30 0.17 19 5 50.30 - 49.15f 1.40 156 Betweenthe Mendocinoand SurveyorFracture Zones 6 53.90 -- 53.20 - 1.70 - 189 In addition to the northward propagationepisode which 6 53.20 -- 51.95f --0.60 --67 7 53.68 -- 53.05 --2.40 --267 split anomaly 22 on profilesZ20, Z21, Z22, and Z23, we 8 52.50- 51.80f 1.70 189 model the eight profilesshown in Figure 9 with a second 9 51.20 -- 49.80f 1.44 160 northwardpropagating rift aroundthe beginningof anom- 10 50.50 -- 50.00 - 1.00 - 111 aly 21 anda thirdduring anomaly 21 (Table5). A south- 11 50.20 -- 49.25f -2.00 -222 ward propagationepisode is used on profilesZ25 and S10 Propagation to fit the narrowingof anomaly21 to the south. The east- PR Number Timea Angle Changeg ward rift jumps of the first northward propagationincrease 1 56.47 -1.0 from 7 km at profile S9 to 24 km at profilesZ21 and Z20, 2 55.05 -14.0 implying a propagationrate of 190 km/m.y. _+100. The 2 54.90 5.0 secondnorthward propagatingrift is well constrainedonly 3 55.03 -14.0 3 54.81 6.0 on the northernmost two profiles, where we obtain a pro- 4 55.00 -21.0 pagationrate of 160 km/m.y. _+80 and jump distancesof 5 54.50 -7.0 9-12 km. The third northward propagation and the 7 53.68 -20.0 southwardpropagation are not well constrainedand may 8 52.50 - 18.0 9 51.20 - 19.0 in fact be oversimplificationsof a more complicatedhis- tory involving several northward and southwardpropaga- aAll times in Ma. tion episodesduring anomaly 21. bRotationpoles describe motion of Farallonplate relative to a fixed Pacificplate. A TECTONIC EVOLUTION MODEL CAngular velocity half rates. dpolesused between Mendocino and Surveyorfracture zones Here we present a model for the tectonic evolution of during reorganization. the Pacific-Farallonspreading center during the ridge reor- epositive propagationrates are northward; negative rates are southward. ganization which uses the rift propagationepisodes dis- fApproximatetime that propagatingrift intersectstransform cussedin previoussections to fit the major features of the fault. magneticanomaly and bathymetrydata as interpreteftin gPositiverotations are clockwise;negative are counterclockwise. 2832 CARESSET AL.'. PACIFIC-FARALLONRIDGE REORGANIZATION

52.80 Ma

53.20 Ma

54.20 Ma 7 54.70 Ma

55.00 Ma

56.00 Ma

57.00 Ma

48.50 Ma SO.00 Ma 50.35 Ma 52.20/_Ma ]

9 8 11

Fig. 10. Stepwiseevolution of the ridge system. Propagationepisodes are numberedas in Table 6. The dashed linesat 52.20and 50.35Ma represent"leaky" sections of the Surveyortransform. forward modeling of the magnetic anomaly profiles, we oriented great circle segmentsoffset by transform faults neglect asymmetricspreading and use the simplestpossible which are small circlesabout the rotation pole. The model sequenceof Euler poles. The parametersused in generat- steps forward in time from a specified beginning ing the model are shown in Table 6; the model itself is configurationusing as input a sequenceof Euler rotation presentedin Figure 10 and Plate 4. (Plate4 is shownhere poles and parameterswhich specify the rates and direc- in black and white. The color version can be found in the tions in which ridge sectionspropagate. Isochrons are separatecolor sectionin this issue.) generatedas a series of evenly spacedpoints along the ridgeswhich are moved incrementallyby finite rotationsto Technique their final positions. The forward modeling of the ridge evolution was Poles of Rotation accomplishedusing the program developed and described by Hey and Wilson[1982] and Wilsonet al. [1984]. This Engebretsonet al. [1984] obtained poles of rotation program models two plate spreading on a sphere by describingthe motion of platesin the PacificBasin using approximatingthe ridge system as a series of arbitrarily previouslypublished interpretations of magneticanomaly CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2833

54.70 Ma

55.00 Ma

56.00 Ma

57.00 Ma

52.20 Ma

54.20 Ma

lineations[e.g., Atwaterand Menard, 1970; Hayesand Pit- techniqueand computerprogram of Minsteret al. [1974] man, 1970]. Becausethis studyinvolves a magneticlinea- and Minster and Jordan [1978] and then adjustedby trial tion interpretation(Figure 5) that is significantlydifferent and error to obtain an optimal fit, a procedurenecessitated from those of earlier studies,a new seriesof Euler poles is by the difficulty in obtaining spreadingrates uncontam- requiredin order to successfullymodel the tectonicevolu- inated by rift propagation.The pole used for the period tion of the spreadingcenter reorganization. For the period after the reorganizationwas obtaineddirectly by inversion. between and including anomalies 25 and 24, the pole North of the Surveyor fracture zone a series of three obtainedby Engebretsonet al. [1984] for the time between stage poles with boundariesat the end of anomaly 24.1 anomalies 34 and 25 was found to fit the direction of and the beginningof anomaly21 are sufficientto fit the spreadingwell, but the magnitudeof the angularvelocity data well. Between the Mendocino and Surveyor fracture half rate had to be increased from 0.36ø/m.y. to zonesthe magneticsdata require a fourth pole of rotation 0.41ø/m.y. in order to fit the widthsand separationsof the to describe the motion between the end of anomaly 24.1 anomalies. Poles used to model the periods during the and the beginningof anomaly22, a result which supports reorganizationwere first approximatedusing the inversion trammeling along the old Mendocino transform during 2834 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

50.00 Ma

50.35 Ma

•_"•l,!,l,,,,il::7_ --'!'""44., M,,i•_l_iiil.[ [[ I

Plate 4. (continued)

this period. Because the sequence of Euler poles used before in this paper, the magnetics data indicate that the between the Mendocino and Surveyor fracture zones tectonic history of this area was much more complicated differs from that used to the north of the Surveyor frac- and probably involved several episodes of propagation. ture zone, the tectonic evolution of the two areas had to PR 11 is used to reorient the spreadingcenter just to the be modeled separately and then pasted together to create north of the Mendocino fracture zone, but again the Figure 10 and Plate 4 by assuming plate rigidity along actual tectonic evolution could well have involved several anomaly 25 on the Pacific plate. Discrepanciesbetween more propagating rifts in this area. A single southward the northern and southern parts of the model are propagation episode is used to model the narrowing and represented in Plate 4 as overlap or gaps along the Sur- disappearanceof anomaly 24.1 just north of the Surveyor veyor and Mendocino fracture zones. fracture zone. The model is further simplified by neglect- ing the second northward propagation episode inferred from the modeling of profile S6 and the third northward TectonicEvolution Model Summary propagation episode shown in the modeling of profiles The 11 propagatingrifts (PRs) used in modeling the from between the Mendocino and Surveyor fracture tectonic evolution are numbered in Table 6 and Figure 10 zones; the evidence for the latter propagator is not com- accordingto the chronologicalorder of their initiation. Of pelling, and neither propagationepisode has a large ridge the PRs used to generate the model, PRs 10 and 11 do offset or plays a significant role in the reorientation and not derive from direct evidence in the magneticsand Sea reorganizationof the ridge system. Beam data but rather are inferred in order to fit the data The propagation rates used in our tectonic evolution as simply as possible. PR 10 is used to extend the north- model are consistent with the results of the magnetic ernmost section of ridge southward from the termination anomaly profile modeling. The PRs not constrained by of PR 7 to initiate the Sedna transform, but as noted magnetic anomaly data are modeled with reasonablepro- CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 2835 pagation rates which give adequatefits to the data and the trend around 52.5 Ma, allowing spreading between the magnetic anomaly interpretation. The north-south short- Mendocino and Surveyor transforms to move in the new ening resulting from the trammeling along the old Mendo- direction. Reoriention of this ridge segment begins cino transform is represented in Plate 4 as overlap or con- immediately as PR 8 propagatesnorthward from 42øN at vergencealong the Surveyor fracture zone on the Farallon 189 km/m.y. all the way to the Surveyor transform. PR 9 plate and along the Mendocino transform, but as discussed follows PR 8 northward, beginning from 42ø20'N at earlier in this paper, this shortening could just as well have 51.20 Ma at a rate of 160 km/m.y. The segment of the been shown as deformation in the block lying between the Surveyor transform lying to the east of the rift segment Mendocino and Surveyor fracture zones. extended north by PR 9 is neccessarily"leaky" after the We begin the model at anomaly 25 with the ridge sys- new Mendocino transform breaks through and presumably tem consisting of straight segments oriented north- develops some unspecified en echelon spreading center northwest which are offset by the Mendocino and Sur- geometry. Our model assumesthat the reoriented rift seg- veyor transforms. The first PR (PR 1) is initiatedsouth- ment continues to extend north againstthe Surveyor tran- ward at a rate of 345 km/m.y. from 50ø40'N at 56.47 Ma, form as it moves eastwardalong the Surveyor transform, reorienting the ridge clockwise by 1ø as far south as transferring the lithosphere created on the "leaky" 48ø30'N, where it stops propagatingat 55.80 Ma. PR 1 transform to the Farallon plate, where it shown as a blank may represent the first hint of the change in plate motion, area in Plate 4. To the north of the Surveyor transform, but the change in the rotation pole does not occur in the the Sedna transform is initiated when PR 10 starts propa- model until 55.00 Ma. The primary reorganization north gating southward at 50.50 Ma from the offset left by PRs of the Surveyor transform begins essentially simultane- 2 and 7 through the offset left by PRs 1 and 4 to stop at ously with the Euler pole change. Southward propagation 50.00 Ma at about 48ø20'N. episode PR 2 is initiated at 367 km/m.y. from 50ø40'N at A final changein the rotation pole occursat the begin- 55.05 Ma; PR 3 is also initiated southward at a rate of ning of anomaly21 (50.34 Ma), resultingin spreadingrate 334 km/m.y. from 46øN at 55.03 Ma. Both propagators increasesranging from about 5 km/m.y. at 50øN to about are associatedwith the gradual elimination of anomaly 20 km/m.y. at the Mendocino transform. The reorienta- 24.1 seen in Figures 4 and 5 and are oriented so as to tion of the ridge between the Mendocino and Surveyor account for most but not all of the 21ø of clockwise rota- transforms is completed by PR 11, which propagates tion in the reorientation. PR 2 stops at 54.60 Ma before it southward from 42ø20'N beginning at 50.20 Ma and end- reachesthe offset left by PR 1; PR 3 propagatesinto and ing when it intersects the new Mendocino transform ends at the Surveyor transform. A northward propagation around 49.25 Ma. The propagationrate of PR 5 increases (PR 4) begins at 55.00 Ma from 47øN and ends at to 156 km/m.y. at 50.30 Ma (as noted in the magnetic 54.00 Ma at the offsetleft by PR 1 (a point to the north of anomaly profile modeling section, this rate increase may the eventual locationof the Sednatransform). PR 5, the be due to a second faster PR which overtakes the initial last and at 19 km/m.y. the slowestof the initial propaga- slowerPR), and PR 5 then continuesnorthward to end at tion episodes,begins propagatingnorthward from 46øN at the Sedna transform, completing the reorganization at 54.50 Ma and eventually ends the ridge reorganization about 49.15 Ma. The model ends with a stable ridge sys- when it propagatesinto and stops at the Sedna transform tem offset by the Sedna, Surveyor, and Mendocino about 4.35 m.y. later. transforms which has been producedby a ridge reorgani- The second phase of the reorientation involves two zation process requiring about 7 m.y. between the initia- southward propagation episodes (PRs 6 and 7) which tion of PR 1 and the termination of PR 5. finish the reorientations begun by PRs 2 and 3. PR 6 begins from 45øN at 53.90 Ma with a propagationrate of Evaluating the Model 189 km/m.y., but slows to 67 km/m.y. at 53.20 Ma to be The model presentedin Plate 4 fits the primary features consistent with the constraints from the Sea Beam and of the magneticanomaly interpretation (Figure 5) quite magneticsdata of the overlappingswath survey discussed well. In particular, the model fits the trends of the Men- aboveand by Hey et al. [thisissue]. PR 7 is initiatedfrom docino, Surveyor, and Sedna fracture zones, exhibits the 50ø40'N at 53.68 Ma with a propagation rate of gradual elimination of anomaly 24.1 around 49øN and just 267 km/m.y. and stops at 53.05 km/m.y. at the offset left to the north of the Surveyor fracture zone, shows oblique by PR 2. By 53.00 Ma, about 2 m.y. after the change in spreadingpersisting out past anomaly 22 around 47øN, has plate motion, about 70% of the length of the ridge system the distinctiveV-shaped pattern of anomaly 21(o) found north of the Surveyor transform is fully reoriented. Dur- just to the south of the Sedna fracture zone, and shows ing the first two phasesof the reorganization,trammeling the observeddoubling of anomaly22 in the northe•rnhalf along the old Mendocino transform prevents the Farallon of the areabetween the Mendocinoand Surveyorfracture plate lithosphere lying between the Mendocino and Sur- zones. The only significantfailure of the modelinvolves veyor transforms from moving in the new spreadingdirec- its prediction that anomalies 23 and 22 in the area of tion, thus delaying the reorientation along that segment oblique spreadingaround 47øN should be found slightly to and slightly rotating this block of lithosphereclockwise to the east of the observed locations, a result that is perhaps produce the convergence shown in Plate 4 as overlap indicative of local asymmetric spreading or an undocu- along the Mendocino transform and the Farallon plate side mented episode of rift propagation. Overall, our tectonic of the Surveyor fracture zone. evolution model demonstrates that a propagating rift The final phaseof the reorganizationbegins when a new model can quite successfullyaccount for the major ele- Mendocino transform breaks through in an east-west ments of the ridge reorientationand reorganization. 2836 CARESSET AL.'. PACIFIC-FARALLONRIDGE REORGANIZATION

IMPLICATIONS FOR PROPAGATING RIFTS tion at rift-transform intersections. Thus we speculatethat the initiation of propagatingrifts in responseto changesin Consideringthis paper and Hey et al. [thisissue] in con- the direction of spreading may occur at small transform junctionwith the Wilsonet al. [1984]and Wilson[1985] offsets or overlapping spreading centers. Since overlap- papers,detailed studies of the tectonicevolution of the ping spreading centers tend to occur at local axial depth Pacific-Farallonspreading center (later the Pacific-Juande maxima,like rift-transformintersections [Macdonald et al., Fuca spreadingcenter) north of the Mendocinofracture 1984], one consequenceof this modelis that propagation zone now encompassthe entire period between60 Ma and must initially proceed up axial depth gradients. Unfor- the present with the exception of a period of ridge- tunately, our data set does not have sufficient resolution transformstability between 44 and 37 Ma. These studies to allow us to locate small offsetsin the ridge systemprior all successfullyuse rift propagation to explain the to the changein plate motion, so we are unable to test this observedpattern of magneticanomalies and togetheriden- hypothesis. tify at least32 distinctepisodes of rift propagation.Propa- Previouslyreported propagationrates range from about gating rifts have also been identifiedon the Galapagos 40 km/m.y. on the Juan de Fuca ridge [Wilsonet al., spreadingcenter [e.g., Hey et al., 1986], the East rift of 1984; D. S. Wilson, personalcommunication, 1987] up to the Easter microplate[e.g., Naar and Hey, 1986], the a maximum of 350 km/m.y. on the East rift of the Easter southeastrift zone of Iceland[e.g., Schilling et al., 1982], microplate[Naar and Hey, 1986], with most of the rates the Antarctic-Australianspreading center [e.g., gogtet al., being in the range from about 40km/m.y. up to 1983], and between the East Pacific Rise and the 100 km/m.y. With the exception of PRs 5 and 6 and the MathematiciansRidge [e.g., Mammerickx et al., thisissue]. possibilityof a northward propagatorbeing a part of the Clearly,rift propagationcan now be recognizedas one of otherwise southward PR 3, all of the propagation rates the primary mechanismsby which spreadingridge systems inferred from the data in this study are greater than evolve and on occasionundergo major reorganizations. 100 km/m.y., and the propagationrates inferred for four However, the mechanismsby which propagatingrifts of the initial propagationepisodes are of the order of at are initiated and perpetuated are still poorly understood. least 200-300 km/m.y. Within the resolution of our mag- PhippsMorgan and Parmentier [1985] note that propagating netics data, these fast propagationepisodes could actually rifts on the Galapagosspreading center, the Juan de Fuca have been discreteor instantaneousrift jumps. However, ridge, the East rift of the Easter microplate,and south of since detailed studies of rift jumps capableof determining Iceland all grow away from areas of anomalouslyshallow the characterof the phenomenon have invariably obtained seafloor associatedwith hotspots and propose that excess finite propagationrates, we believe that instantaneousrift gravity-spreadingstresses due to the shallow ridge crest jumps are unlikely. The generally fast characterof the initiates and drives the propagation. Although no obvious propagationepisodes involved in this ridge reorganization traces of hotspot activity are found in the study area, axial may result from unusuallylarge stressesdriving the propa- depth variationsalong the spreadingcenter at the time of gationor mayreflect other factors such as spreading rates the reorganizationmay have influenced the initial posi- and lithosphericage. tions and the directions (northward or southward) of the propagating rifts. Since axial depths are observed to SUMMARY AND CONCLUSIONS increase near rift-transform intersectionsand overlapping spreadingcenters [e.g., Macdonaldet al., 1984]), our Our analysis and modeling of the magneticsand Sea interpretation of rift propagation into both the Surveyor Beam data indicatethat the primary mechanismby which and Mendocino fracture zones is consistent with the the Pacific-Farallonspreading center reoriented and reor- gravity-spreadingstress model. ganized was rift propagationrather than rift rotation by An inspection of the bathymetric variations found on differential asymmetric spreading. Consideration of Sea crust created around the time of the change in plate Beam lineation data enhances the magnetic anomaly motion fails to reveal a pattern which can be related to interpretation presentedin Figure 5, and direct evidenceis variations in axial depth along the spreadingcenter at that found for propagationepisodes accounting for the primary time; the lack of a transform along the spreading rift to reorientation of 75% of the ridge system. Nowhere are the north of the Surveyor transform prior to the reorgani- the data more consistent with rift rotation than with rift zation thus makes a correlation of the propagationpaths in propagation as the principal means of reorientation, that area with axial depth gradients impossible. We note although the data are compatible with either hypothesisin that the distancesof up to 250 km over which some indi- several casesof minor reorientations. The reorganization vidual propagations occurred are greater than the began just after anomaly 24 north of the Surveyor wavelengthsof the observedaxial depth variations on the transform but was delayed south of the Surveyor fracture East PacificRise [Macdonaldet al., 1984] and that a propa- zone until anomaly 22 by trammeling along the long left gating rift was initiated from the stableSurveyor transform lateral offset of the Mendocino transform. The ridge at 36 Ma [Shihand Molnar, 1975; Wilson,1985], implying between the two major transforms did not begin reorganiz- propagationup as well as down axial depth gradients. ing by rift propagationuntil a new Mendocino transform If we suppose that starting rift propagation in older, fractured through in the new direction of motion. Most of thicker lithosphere is more difficult than starting it in the reorientation occurredquickly, with about 70% of the young, thin lithosphere,then it followsthat a shift in plate ridge length north of the Surveyor transform being motion which causestrammeling along long offsets such reoriented in about 2 m.y., and all of the reorientation as the Mendocino transform might succeed in beginning between the Surveyor and Mendocino transforms being rift propagationat shorter offsetsthrough stressconcentra- accomplishedwithin 2.5 m.y. of the breaking through of CARESSET AL.: PACIFIC-FARALLON RIDGE REORGANIZATION 2837

the new Mendocino transform. The entire reorganization Acknowledgments.We thank Captain Johnsonand the crew of of the ridge systemrequired about 7 m.y. the R/V ThomasWashington for theirskillful help, and M. Klein- Detailed Sea Beam surveys of the bends of the Sur- rock and T. Atwaterfor their assistancein collectingthe dataat sea. We alsothank the many peopleat SIO who providedassis- veyor and Mendocino fracture zones corroborate the rift tance,especially, U. Albright,J. Charters,A. Foster,M. Keeler, propagationhypothesis. The bend in the Surveyor frac- I. Kim, M. Kleinrock,D. Naar,J. PhippsMorgan, S. Smith,and ture zone correspondsto the location of a pseudofault W. Smith.We thankM. Kleinrock,D. Naar,J. PhippsMorgan, T. inferred from magneticsdata, and a secondpseudofault is Atwater, D. Wilson, and J. Morgan for adviceand conversations aboutthe area. Specialthanks go to M. Kleinrock,D. Naar,and clearly expressedas an abrupt changein abyssalhill linea- J. Phipps Morgan for critical and insightfulreviews of the tions which trends into a deep cleft abutting the fracture manuscript,to E. Portillofor draftingassistance, to S. Slavenyof zone. The contact along which the new Mendocino WashingtonUniversity for generatingthe shadedrelief map of transformbroke throughcan be seen terminatingthe old Plate 3, and to R.L. Parkerfor suggestingthe use of the word fracturezone trend in the Sea Beam bathymetry. "trammeling."This work was supportedby the NationalScience Foundationthrough grants OCE83-15366, OCE83-15364, and Our forward modelingof magneticanomaly profiles and OCE85-09150. the tectonic evolution of the reorganizationsucceeds in fitting the data well with 11 episodesof propagation, REFERENCES although it is clear that this constitutes a significant oversimplification in some areas. Propagation rates Atwater, T. M., and H. W. Menard, Magnetic lineationsin the inferred from this modelingare in excessof 100 km/m.y. northeastPacific, Earth Planet. Sci. Lett., 7, 445-450, 1970. with two exceptions,and four propagationepisodes had Berggren,W. A., D. V. Kent, and J. J. Flynn, Jurassicto Paleo- rates of the order of at least 200-300 km/m.y., valuesas gene, part 2, Paleogenegeochronaology and chronostratigraphy, edited by N.J. Snelling, The Chronology of the Geologic large as or larger than any previouslyreported. These fast Record,Mem. Geol.Soc. Am., 10, 141-195, 1985. propagation rates may be due to large variations in the Carlson, R.L., Late Cenezoic rotations of the Juan de Fuca stressregime or may involve relationshipsbetween spread- ridge and the Gorda rise: A case study, Tectonophysics,77, ing rates, lithosphericage, and propagationrates that are 171-188, 1981. not well understood. de Moustier, C., and M. C. Kleinrock, Bathymetricartifacts in Sea Beam data: How to recognizethem and what causesthem, Although much has yet to be learned about the J. Geophys.Res., 91, 3407-3424, 1986. mechanismscontrolling the initiation and perpetuation of Engebretson, D.C., A. Cox, and R.G. Gordon, Relative these phenomena, the analysispresented in this paper and motionsbetween oceanic plates of the Pacificbasin, J. Geophys. in other studiesindicates that propagatingrifts constitute Res., 89, 10291-10310, 1984. Harland, W.B., A.V. Cox, P.G. Llewellyn, C.A. Pickton, one of the primarymechanisms of spreadingcenter evolu- A. G. Smith, and R. Walters,A GeologicTime Scale, Cambridge tion and reorganization. However, the reorganizationpro- University Press,New York, 128 pp., 1982. cess revealed in our analysis and incorporated into our Hayes, D. E., and W. C. Pitman III, Magnetic lineationsin the model of the tectonicevolution of the area is quite com- North Pacific,Geological Investigations of the North Pacific, editedby J. D. Hays,Mem. Geol.Soc. Am., 126, 291-314, 1970. plex, being characterized by multiple northward and Hey, R.N., A new classof pseudofaultsand their bearingon southwardpropagation episodes and trammeling along the plate tectonics:A propagatingrift model, Earth Planet.Sci. Lett., long left-lateral Mendocino transform offset. Our tectonic 37, 321-325, 1977. history model suggeststhat the reorientationof rift seg- Hey, R.N., and D. S. Wilson, Propagatingrift explanation for the tectonicevolution of the northeastPacific--The pseudomo- ments occursrelatively rapidly in responseto changesin vie, Earth Planet.Sci. Lett., 58, 167-188, 1982. plate motion but the developmentof a new stable spread- Hey, R.N., F. K. Duennebier,and W. J. Morgan, Propagating ing center geometrycan requireseveral million years. rifts on mid-oceanridges, J. Geophys.Res., 85, 3647-3658, 1980. Hey, R.N., M. C. Kleinrock, S. P. Miller, T. M. Atwater, and R.C. Searle, Sea Beam/Deep-Tow investigationof an active IN MEMORIAM oceanicpropagating rift system,Galopagos 95.5øW, J. Geophys. Res., 91, 3369-3393, 1986. It is fitting that Bill Menard's last oceanographicexpedi- Hey, R.N., H. W. Menard, T. M. Atwater, and D. W. Caress, tion and the project that he was working on at the time of Changesin directionof seafloorspreading revisited, J. Geophys. Res., this issue. his death should concern the topographyof fracture zones Macdonald,K. C., J. C. Sempere,and P. J. Fox, East PacificRise and the evolution of the north Pacificbasin. Bill not only from Siquerirosto Orozco fracture zones: Along-strikecon- discovered and characterized many of the important tinuity of axial neovolcanic zone and structure and evolution of features of the structure of the Pacific basin such as the overlappingspreading centers, J. Geophys.Res., 89, 6049-6069, 6301-6305, 1984. great fracture zones, the ideas regarding these features Mammerickx, J., D. F. Naar, and R. C. Tyce, The Mathemati- that he publishedthrough his career profoundly impacted cian paleopiate,J. Geophys.Res., this issue. our view of the processeswhich shape the ocean floor and Menard, H. W., Evolutionof ridgesby asymmetricalspreading, the Earth's surfaceas a whole. All of us found working Geology,12, 177-180, 1984. with Bill to be a constant intellectual challenge and joy Menard, H. W., and T. M. Atwater, Changesin directionof sea floor spreading,Nature, 219, 463-467, 1968. which taught us much about the role that creativity and Menard, H.W., and T.M. Atwater, Origin of fracture zone enthusiasm play in science. Although Bill passed away topography,Nature, 222, 1037-1040, 1969. before our conclusionscould be committed to writing, he Minster, J. B., and T. H. Jordan,Present-day plate motions,J. actively participated in working up and interpreting the Geophys.Res., 83, 5331-5354, 1978. Zed Expedition data and many of the ideas within this Minster, J. B., T. H. Jordan,P. Molnar, and E. Haines,Numeri- cal modelingof instantaneousplate tectonics,Geophys. J. R. paper are his. We hope that the reader can find a bit of Astron.Soc., 36, 541-576, 1974. Bill Menard in the sciencewe present. Naar, D. F., and R. N. Hey, Fast rift propagationalong the East 2838 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

PacificRise near Easter Island, J. Geophys.Res., 91, 3425-3438, D. R. Bracey, Discontinuitiesin sea-floor spreading,Tectonophy- 1986. sics,8, 285-317, 1969. Ness, G., S. Levi, and R. Couch, Marine magnetic anomaly Vogt, P. R., N. Z. Cherkis, and G. A. Morgan, ProjectInvestiga- timescalesfor the Cenozoicand late Cretaceous: A precis,cri- tor, I, Evolution of the Austral-Antarcticdiscordance deduced tique, and synthesis,Rev. Geophys.,18, 753-770, 1980. from a detailedaeromagnetic study, in AntarcticEarth Science, Nishimura, C., D. S. Wilson, and R.N. Hey, Pole of rotation edited by R. L. Oliver, P. R. James, and J. B. Jago, pp. analysisof present-dayJuan de Fuca plate motion, J. Geophys. 608-613, CambridgeUniversity Press, New York, 1983. Res., 89, 10,283-10,290, 1984. Wilson, D.S., Tectonic History of the Juan de Fuca Ridge, PhippsMorgan, J., andE( M. Parmentier,Causes and rate limit- Ph.D. thesis,105 pp., StanfordUniv., Stanford,Calif., 1985. ing mechanismsof ridge propagation: A fracture mechanics Wilson, D. S., R.N. Hey, and C. Nishimura, Propagationas a model, J. Geophys.Res., 90, 8603- 8612, 1985. mechanismof reorientationof the Juan de Fuca Ridge,J. Geo- Raft, A.D., Boundaries of an area of very long magnetic phys.Res., 89, 9215-9225, 1984. anomalies in the northeast Pacific, J. Geophys.Res., 71, 2631- 2636, 1966. D. W. Caress, Scripps Institution of OceanographyA-025, Sandwell,D, and G. Schubert,Lithospheric flexure at fracture University of California, San Diego, La Jolla, CA 92093. zones, J. Geophys.Res., 87, 4657-4667, 1982. R. N. Hey, Hawaii Institute of Geophysics,University of Schilling, J.-G., P.S. Meyer, and R. H. Kingsley, Evolution of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI 96822. the Iceland hotspot,Nature, 296, 313-320, 1982. Shih, J., and P. Molnar, Analysis and implications of the sequenceof rift jumps that eliminated the Surveyor transform (ReceivedJune 30, 1986; fault, J. Geophys.Res., 80, 4815-4822, 1975. revisedNovember 4, 1986; Vogt, P. R., O. E. Avery, E. D. Schneider,C. N. Anderson, and acceptedFebruary 8, 1987.) CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 3507 3508 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 3509 3510 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

œ7 CARESSET AL.' PACIFIC-FARALLONRIDGE REORGANIZATION 3511 3512 CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION

54.70 Ma

55.00 Ma

56 O0 Ma \!

57.00 M•

52.20 Ma

'-•2.80 Ma

54.20 Ma

Plate 4. Step by step model of the evolutionof the Pacific-Farallonspreading center north of the Mendocino transformbetween 57 and 44 Ma. Thick linesrepresent transform faults or fracturezones, thin linesrepresent iso- chronsassodated with geomagneticreversals, pseudofaults are shownas dashedlines, and activespreading ridge segmentsare shown as very thick dashed lines. (Thecolor version and a completedescription of thisfigure can be found in the separatecolor sectionin this issue.) CARESSET AL.: PACIFIC-FARALLONRIDGE REORGANIZATION 3513

50.00 Ma

50.35 Ma

44.00 Ma

Magnetic Anomaly Sequence 24.2 23 21 20 20 21 23 242.

25 24,1 22 ! 22 24.1 25 49.00 Ma Spreading Center at 44.00 Me

Plate 4. (continued)