FOR EDUCATIONAL USE ONLY J. GEOPHYS. RES., VOL.102, NO. B2, 3167-3181, FEB. 10, 1997

Tectonic reconstruction of the Clipperton and Siqueiros FractureZones: Evidence and consequences of plate motion changefor the last 3 Myr

Robert A. Pockalny1, Paul J. Fox1,2, Daniel J. Fornari3

Ken C. Macdonald4, and Michael R. Perfit5

Abstract. Bathymetry, side-looking sonar and magnetics data from the northern East Pacific Risehave been analyzed to determine the tectonic history of the Clipperton Fracture Zone (CFZ) andthe Siqueiros Fracture Zone (SFZ) over the last 2-3 m.y. Results of tectonic reconstructionsindicate a series of counterclockwise changes in spreading direction at ~2.5 Ma (1-2°), ~1.5 Ma(1-2°), and ~0.5 Ma (4-5°). Along the right-stepping Clipperton Transform, the most recentchange in spreading direction has resulted in fracture zone normal compression, which wepropose has created a median ridge and transform-parallel troughs along the active transformfault. Volcanic intersection highs located near the ridge-transform intersections (RTI) of the CFZare interpreted to be the result of fracture-zone-normal extension that has created pathways formagma emplacement into/onto the older lithosphere across from the RTI. Along the left-steppingSFZ, we propose the changes in spreading direction have generated extension across thetransform and have resulted in the formation of intratransform spreading centers and flexuraltransverse ridges. Tectonic reconstructions indicate a single Euler pole is unable to describe themotion of the Pacific-Cocos plate boundary between the CFZ and SFZ during periods ofspreading direction change since ~3 Ma. Transtensional transforms will adjust to a new spreadingdirection almost immediately, while transpressional transforms will experience compression for aperiod of time comparable to half the age offset of the compressional transform.

Introduction

The plate tectonic paradigm defines transform faults asboundaries where adjacent plates slide past each other without thecreation or destruction of oceanic crust [Wilson, 1965]. Thissimplistic view is adequate if plate motions are constant overtime; however, the ensemble of lithospheric plates whichcompose the surface of the Earth frequently change theirdirection of motion. Although plate tectonic theory generallyassumes that changes in plate motion are instantaneous, morerealistic models and field relationships require a finite period ofadjustment. During periods of plate readjustment, transformfaults become regions where complex compressional andtensional stress regimes affect steady state shear along the plateboundary.

The right-stepping Clipperton Fracture Zone (CFZ) and left-stepping Siqueiros Fracture Zone (SFZ) are separated by ~200km and offset the East Pacific Rise (EPR) axis in differentdirections (Figure 1). Recent changes in spreading direction

1Graduate School of Oceanography, University of Rhode Island,Narragansett, Rhode Island.

2Now at Ocean Drilling Program, Texas A & M University, CollegeStation, Texas.

3Department of Geology and Geophysics, Woods Hole OceanographicInstitution, Woods Hole, Massachusetts.

4Department of Geology, University of California, Santa Barbara,California.

5Department of Geology, University of Florida, Gainesville, Florida.

Copyright 1997 by the American Geophysical Union.Paper number 96JB03391.0148-0227/97/96JB-03391$09.00

documented by magnetic anomaly patterns, morphostructuralanalyses and tectonic reconstructions indicate that acounterclockwise rotation (4-8°) of the ridge axis has occurredover the last 3 m.y. [Perram and Macdonald, 1990; Carbotte andMacdonald, 1992, 1994]. The geometry of the Clipperton-EPR-Siqueiros plate boundary, and documented changes in platemotion in this region provide a unique opportunity to examine theresponse of oceanic lithosphere along transforms to changes inspreading direction.

This study focuses on the reconstruction of the tectonic historyof the Clipperton-EPR-Siqueiros plate boundary over the last ~3m.y. A key objective of this paper is to determine howdifferences in morphology and structure between the CFZ andSFZ reflect local tectonics and/or regional changes in platemotion. Our analysis is carried out within the context of thediscrete changes in geometry of spreading which have created adistinct structural fabric in the seafloor along the CFZ and SFZ,and within the transform domains of these major offsets of theEPR axis.

Tectonic Setting

The majority of the northern EPR is defined by the Pacific-Cocos plate boundary which is bound to the north by the RiveraFracture Zone (~19°N) and to the south by the Galapagos TripleJunction (3°N) (Figure 1). Present-day spreading rates,calculated from plate motion models [DeMets et al., 1990, 1994],range from 68-71 km/m.y. at the Rivera Fracture Zone to 125-131km/m.y. at the Galapagos Triple Junction.

The axis of the northern EPR along the Pacific-Cocos plateboundary is offset by three major transforms (Figure 1): Orozco,

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Siqueiros

Fig. 2a

82 km/Myr

101 km/Myr

107 km/Myr

PacificPlate

CocosPlate

Rivera70 km/Myr

Fig. 2b

Fig. 4a Fig. 4b

Fig. 4c

Figure 1. Location map of the northern East Pacific Riseshowing existing bathymetric data coverage [Macdonald et al.,1992] (with kind permission from Kluwer Academic Publishers)and the present plate boundary geometry. Representativespreading rates obtained from the NUVEL-1a relative platemotion model [DeMets et al., 1994] are shown near the majortransforms of the Pacific-Cocos plate boundary. Also shown bythe boxed areas are the locations of the indicated figures.

Clipperton, and Siqueiros. The left-stepping Orozco Transform(15°25'N) offsets the EPR about 45 km and represents an ageoffset of ~1 m.y. The right-stepping Clipperton Transform(10°15'N) has an offset of about 85 km and represents ~1.5 m.y.age offset. The left-stepping Siqueiros transform (8°15'N) offsetsthe EPR by about 145 km and represents an age offset of ~2.5m.y. Between the major transforms, smaller ridge axisdiscontinuities are present along the EPR and offset the ridge axisby up to 15 km [Macdonald et al., 1992]. Numerous othersmaller ridge axis offsets or deviations from axial linearity (e.g.DEVALs of Langmuir et al. [1986]) with variable offset sensesare observed along the length of the northern EPR, but themagnitude of their offset is typically less than 1 km.

Magnetic anomaly data indicate the plate boundary geometryof the northern EPR has experienced significant reorganizationover the last 3-4 m.y. [Klitgord and Mammerickx, 1982;Mammerickx et al., 1988; Atwater and Severinghaus, 1989;Edwards et al., 1991; Lonsdale, 1995]. The majority of thereorganization has involved the northernmost portions of thePacific-Cocos plate boundary between the Orozco and RiveraFracture Zones and includes the abandonment of seafloorspreading along the now extinct Mathematician Ridge.

Plate boundary reorganization along the EPR near the CFZ andSFZ has been limited to an overall counterclockwise change inspreading direction for the similar time period [Perram andMacdonald, 1990; Carbotte and Macdonald, 1992, 1994]. High-resolution magnetics and structural fabric studies of the EPRbetween 8 and 12°N [Perram and Macdonald, 1990; Carbotteand Macdonald, 1992] suggest that counterclockwise (4-8°)

changes in the spreading direction have occurred within the last1-2 m.y. and may be coincident with the reestablishment of truestrike-slip motion along the Rivera Transform about 1.5 Ma, asproposed by Lonsdale [1995]. An even more recentcounterclockwise change in spreading direction (3-6°) within thelast 1 m.y. has been documented with structural trend analysis ofridge-parallel lineations of the EPR between the CFZ and SFZ[Carbotte and Macdonald, 1994].

Data

The majority of the data used in this study were collectedduring many field programs presented by Macdonald et al.[1992]. Sea Beam data from the JOI synthesis [Tighe, 1988] andSeaMARC II data from more recent cruises [Fornari et al., 1989;Macdonald et al., 1992] have been combined and gridded with a300 m grid spacing to produce the bathymetric maps (Figures 2-5). SeaMARC I and II data from several cruises [Kastens et al.,1986; Fornari et al., 1989; Macdonald et al., 1992] have beencombined to produce the side-looking sonar mosaics (Figures 2-5). The majority of the magnetic isochron and crustal ageinformation are taken from the three-dimensional magneticinversion study of Carbotte and Macdonald [1992] withadditional magnetic anomaly identification information fromAtwater and Severinghaus [1989].

We first present the morphological data as resolved by theside-looking sonar mapping conducted on a number of cruises tothis area. We then use the structural fabric defined by the sonarimages to reconstruct the tectonic history, including episodes ofdeformation that created the seafloor features flanking the CFZ,SFZ, and the active transforms. Finally, the implications of theseprocesses are discussed in terms of the temporal and spatialconstraints on lithospheric plate response to changes in plategeometry.

Transform and Fracture Zone MorphologyAlthough the CFZ (Figure 2) and SFZ (Figure 4) are located

only about 200 km away from each other along the EPR (Figure1), the morphology of the active transform, the ridge-transformintersection (RTI), and fracture zone traces are remarkablydifferent.

Active Transforms

The active Clipperton Transform (Figure 2) contains aprominent 50-km-long, transform-parallel ridge composed of aseries of four elongate mounds that are interpreted to becompressional in origin [Gallo et al., 1986; Eisen et al., 1988;Pockalny, 1996]. This compressional ridge is bisected along itslength by a through going depression that is assumed to be theactive fault trace. The strike of the central portion of thetransform has a nearly E-W trend, while the outer portions nearthe RTIs have a more NE-SW trend. Bounding thecompressional ridge to the north and south are anomalously deeptransform-parallel troughs.

The region between the RTIs of the Siqueiros Transform(Figure 4) contains a series of three intratransform spreadingcenters (ITSC) connected to the main axis of the EPR and to eachother by a series of left-stepping transform faults [Fornari et al.,1989; Perfit et al., 1996]. The ITSCs located in the western(ITSC A) and central (ITSC B) portion of the transform domainare well developed and have a sigmoid fabric. ITSC C located inthe eastern portion of the transform domain is poorly defined but

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Fig. 3a

"Relict"Intersection

High

Figure 2a. Side-scan sonar mosaics (top) and shaded bathymetry maps (bottom) of the seafloor bounding thewestern half of the Clipperton Transform and fracture zone extension. The location of scarps and lineamentsidentified from the side-scan images are overlain on the bathymetry, and the location of magnetic isochrons (inmillion years) and the key morphologic features are identified. The focal mechanism of an earthquake with acomponent of compression (CMT database, 1996) is observed along the active trace of the Clipperton Transform.The open dots indicate the location of the fracture zone trace predicted from stage poles determined from aprevious study [Macdonald et al., 1992]. The solid dots show the location of the fracture zone determined from thetectonic reconstructions of the present study. Note that there is only a modest difference in the predictions of thetwo models. The dashed white box indicates the locations of enlarged areas shown in Figure 3.

contains a zone of active volcanism (D. Fornari and M. Perfit,unpublished data, 1991) located within a depression bounded byroughly symmetric, “C”-shaped oblique bathymetric highs. ITSCC is connected to the eastern Siqueiros RTI by a series of shearzones that cut through some of the shallowest topography in thetransform domain.

A pair of fracture-zone-parallel, linear ridges are located alongthe inactive trace of the transform connecting ITSCs A and B(Figure 4b) and appear to have been bisected during the initiationof spreading at ITSC B. On the flanks of the ridge to the north ofthe active transform near 103°25'W and 102° 00’W, fracture-zone-parallel ridges with steep slopes facing the transform andgentle slopes facing away are observed (Figure 5) and have amorphology similar to transverse ridges believed to have aflexural origin along other fracture zones (e.g., Kane FractureZone [Pockalny et al., 1996]). South of the transform domain,the topography has a blocky character with abyssal hill fabricoriented oblique to the spreading direction. This topographybecomes more ridge-parallel near the eastern Siqueiros RTI.

Ridge-Transform Intersections

The western RTI of the Clipperton Transform (Figures 2a and3a) is characterized by a relatively shallow ridge crest thatextends onto the older plate across from the RTI and creates a

region of shallow topography with a plan view morphologyreminiscent of a rooster's comb [Gallo et al., 1986; Kastens et al.,1994; Barth et al., 1994]. The typical ridge-parallel fabric isreplaced by more subtle, oblique lineations and the intersectionhigh is inferred to have a volcanic origin [Barth et al., 1994]. Acomparable feature is observed along the eastern RTI (Figures 2band 3b), but the areal extent on the older plate is more limited andseems to follow the trace of the eastern extension of the fracturezone. This bathymetric high is volcanic in origin [Barth et al.,1994] and is decorated with obliquely trending lineations.

At the western Siqueiros RTI, the intersection high is a broadtongue-like feature [Barth et al., 1994] with over 300 m of reliefthat spills across the transform domain, obliterating much of therelict topography on the west side of ITSC A. The presence ofthis feature suggests that the EPR tip has attempted to propagatesouthward at the RTI boundary in the recent past. The prominenttopographic features at the eastern Siqueiros RTI consist of thetwo deep inside-corner depressions which are aligned in a NEdirection and form the eastern boundary of the complextopography of the western portion of the transform domain. Thesouthern hole is deepest with over 900 m of relief, compared tothe northern hole which has only ~400 m of relief. Thealignment and characteristics of these holes may be interpreted asthe result of recent changes in the geometry of the shear zone at

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Fig. 3b

Figure 2b. Same as Figure 2a, except bounding the eastern half of the Clipperton Transform and fracture zoneextension.

the eastern end of the transform and may suggest that the easternRTI has migrated northward. This would complement theinterpretation of the seafloor fabric mapped north and south of thewestern Siqueiros transform which suggests a southward openingof that portion of the shear zone [Fornari et al., 1989].

Fracture Zone Traces

The fracture zone traces of the CFZ and SFZ have verydifferent characteristics (Figures 2 and 4). The CFZ has a singleoff-axis trace that has persisted for the last 2 m.y., while the SFZexhibits a much more complicated history with multiple fracture

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Transform

Fault

Fracture

Zone

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Zone

Ridge Axis

Ridge Axis

a b

Figure 3. Enlarged side-scan images of the (a) western and (b) eastern RTIs of the Clipperton Transform with overlainbathymetric contours (200 m contour interval). The location of the active transform, the fracture zone extensions, and theridge axes are indicated. The 3000 m depth contour on the north side of the fracture zone near the western RTI (Figure 3a)indicates the boundary between abyssal hill fabric and more recently emplaced volcanic fabric associated with the rooster-comb shape of the intersection high. Near the eastern RTI (Figure 3b), the intersection high is also defined by the 3000 mdepth contour on the south side of the fracture zone, but the obliquely oriented volcanic fabric appears to persist along theENE-trending fracture zone trace. The location of these enlargements are indicated on Figure 2.

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3171

{{

8

-105.5 -105 -104.5

8.5

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-105.5 -105 -104.5

2.6 1.8 1.0

3.5 2.65.0 Fossil SpreadingCenters

0.88°20′N Seamount ChainRidgeAxis

AbandonedTransform Fault

Figure 4a. Side-scan sonar mosaic (top) and shaded bathymetry map (bottom) of the seafloor bounding the westernextension of the Siqueiros Fracture Zone. The location of scarps and lineaments identified from the side-scanimages are overlain on the bathymetry, and the location of magnetic isochrons (in million years) and keymorphologic features are identified. The open dots indicate the location of the fracture zone trace predicted fromstage poles determined from a previous study [Macdonald et al., 1992]. The solid dots show the location of thefracture zone determined from the tectonic reconstructions of the present study. Note the large discrepancy betweenthe two plate motion models, especially along the western extension of the fracture zone and the active transform.

zone traces for the last 2-3 m.y. The western trace of the CFZ(Figure 2a) is a fairly broad (~10 km) region of obliquelineaments and curvilinear features that asymptotically approachthe trend of the fracture zone. A single, continuous trace of thefracture zone is not immediately apparent in this region butappears to be located somewhere within the region of the obliquelineaments. The eastern extension of the CFZ (Figure 2b) isdefined by a 5-km-wide zone of oblique lineaments that decorateseveral fracture-zone-parallel ridges and troughs. Along both theeastern and western traces of the CFZ, two distinct bathymetrichighs resembling relict intersection highs are located on the oldersides of the fracture zone on ~3 m.y. old crust. Thesebathymetric highs are ~200-400 m high and have an orientationand structural fabric that is oblique to the ridge axis.

The western trace of the SFZ (Figure 4a) is a single lineationon crust older than ~3-4 m.y., but evolves into three fracture-zone-parallel swaths of terrain near 105°45'W (~2.5 Ma).

The three fracture-zone-parallel lineaments merge with the distalends of the swaths of seafloor formed at ITSCs A and B (Figure4b). The more northerly fracture zone trace becomes poorlydefined in the bathymetry and side-looking sonar data near105°W (~1.5 Ma) and evolves into a series of oblique trendinglineaments similar to that observed at the present western RTI ofthe Siqueiros Transform. The southernmost fracture zone tracebecomes poorly defined at about 104°15'W, where it abuts theblocky, oblique topography south of the active transform. Thecentral fracture zone trace appears to be continuous to the presentsouthern termination of ITSC A, but a change in the trend from083° to 078° appears to occur at ~104°10'W.

The eastern trace of the SFZ (Figure 4c) evolves from a fairlynarrow zone (<10 km) on older crust near 101°W to a broaderzone (~20 km) of disrupted terrain between 101°30' and102°30'W, and then to a narrower, structurally complex regionnear the present eastern RTI. Within the broad zone of disrupted

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RidgeAxis

A B C

FlexuralRidges

AbandonedTransform Faults

{{

{Fracture Zone-Parallel Ridge

Inside-CornerDepressions

Fig. 5

Figure 4b. Same as Figure 4a, except bounding the western extension of the Siqueiros Fracture Zone. The dashedwhite box indicates the location of enlarged area shown in Figure 5.

seafloor, a more random pattern of oblique lineations is observedwhen compared to the western fracture zone trace. One or twofracture-zone-parallel lineaments are observed just to the south ofthe northern extent of this broad zone. Some of these featurescan be traced back into the region of the ITSCs and appear to beassociated with the fracture-zone-parallel ridges on the north sideof the transform. On oceanic crust older than ~1.5 m.y., theabyssal hill fabric south of the fracture zone intersects the fracturezone at a very high angle and is abruptly terminated at thefracture zone. On younger crust (<1.5 m.y.), the trend of theabyssal hill fabric changes and the separation between thefracture zone and abyssal hill fabric becomes poorly defined. Avery similar pattern of changing abyssal hill fabric is observed onthe north side of the western extension of the SFZ at about thesame time (Figure 4a). On oceanic crust older than ~1.5 m.y. onthe western extension of the SFZ, the abyssal hill fabric to thenorth intersects the fracture zone at a very high angle and isabruptly terminated at the fracture zone. The separation betweenthe fracture zone and abyssal hill fabric on younger crust ispoorly defined and is characterized by oblique lineaments in theside-scan imagery.

A chain of seamounts located on the northern side of thewestern SFZ (Figure 4a) extends from near the western RTI outto the edge of the survey area (3.6 Ma). The overall strike of the

seamount chain (269°) does not follow an absolute motion trend(279°) but appears be more closely associated with the relativemotion trend (264°) or the trace of the western fracture zoneproximal to the RTI. Changes in the trend of the volcanic chainare observed at ~1.8 and 0.8 Ma. There is no comparablevolcanic chain along the eastern Siqueiros Fracture Zone.

Tectonic Reconstructions

Independent tectonic reconstructions of the CFZ since 2 Ma(Figure 6) and SFZ (Figure 7) since 3-4 Ma were accomplishedby integrating the seafloor lineations from side-looking sonarwith the age control provided by magnetics data from the specificregions. The location and sense of offset of key seafloorlineations were then digitized from side-looking sonar mosaics(Figures 2 and 4). The location of magnetic isochrons fromthree-dimensional inversions [Carbotte and Macdonald, 1992]and other identified magnetic anomalies [Atwater andSeveringhaus, 1989] were also digitized. Both the side-scan andmagnetic lineations were then overlain on gridded bathymetricdata using the Generic Mapping Tool system [Wessel and Smith,1994].

An algorithm was developed to draw the trend of a fracturezone or transform fault emanating from a point on the ridge for a

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{Abandoned

Transform Faults

RidgeAxis

5.0FlexuralRidge Broad Fracture Zone

Abyssal HillTrend Change

Figure 4c. Same as Figure 4a, except bounding the eastern extension of the Siqueiros Fracture Zone.

given stage pole. The amount of rotation in either spreadingdirection could be modified to correspond with digitizedmagnetic lineations. The results of a given fracture zone modelwere overlain on the actual bathymetry, side-scan and magneticdata to compare the correspondence of the trend with the data.For multiple stage poles, the trend of a given stage wascalculated, rotated, and drawn at the present-day location.

To determine the assemblage of stage poles that best describethe recent history of the individual fracture zones (Table 1),previously calculated stage poles for the CFZ and SFZ[Macdonald et al., 1992] were used as a starting point for eachreconstruction scenario. These initial stage pole locations androtation rates were then iteratively and methodically adjusted toobtained stage poles that provided the qualitative best fit with theavailable structural and age controls. Additional informationconcerning pole changes from other parts of the Pacific-Cocosand Pacific-Rivera plate boundaries [Perram and Macdonald,1990; Madsen et al., 1992; Lonsdale, 1995] were also used toconstrain the relative timing and duration of a given stage pole.The general results of the tectonic reconstructions using thismethod describe a similar temporal history for both fractureszones with an overall counterclockwise change in spreadingdirection over the last 2-3 m.y.

Clipperton Fracture Zone

The resulting tectonic reconstruction of the CFZ fromavailable bathymetry data is limited to the last 2 m.y. but appearsto be relatively simple (Figure 6). During this time, at least twocounterclockwise changes in local spreading direction causedcompression to occur along the transform at ~1.5 and 0.5 Ma.The change in spreading direction at 1.5 Ma was relatively minor(1-2°) and led to the southward retreat (<5 km) of the westernRTI and a minor amount of northward retreat of the eastern RTI.A change in spreading direction is not well constrained by theoff-axis trace of the fracture zone but is based primarily on thepresence of anomalous bathymetry or relict intersection highs onthe older sides of the fracture zone across from 1.5 Ma crust(Figure 2).

The change in spreading direction at 0.5 Ma was moresignificant (4-5°) and a recent earthquake with a compressionalcomponent along the active transform (Figure 2a) suggests theClipperton plate boundary is still in the process of readjusting[Macdonald et al., 1992]. This change in spreading directioncaused the southward retreat (<5 km) of the western RTI andnorthward retreat (<5 km) of the eastern RTI. The best evidencefor this change in spreading direction are the lineations in the

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FlexuralCross Section

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Profile

ObservedPlate ThicknessExtension

= 30˚= 10 km= 2 km

Figure 5. Enlarged side-scan image (left) of the fracture-zone-parallel transverse ridge on the north side of theSiqueiros Transform with overlain bathymetric contours (200 m contour interval). (right) Also shown are modeledflexural cross sections (shaded area) [Pockalny and Buck, 1995] overlain by bathymetric cross-sectional profiles(solid lines) of transverse ridges from the Siqueiros and Kane fracture zones. The location of the Siqueiros profile isindicated (dashed, solid line) on the side-scan image. Note the similarity in shape between the two bathymetricprofiles and the flexural profile associated with 2 km of extension of a 10 km thick plate along a 30° fault dip.

transform that are assumed to be the active trace of the transformfault. In the central portion of the transform, the fault strike isnearly E-W, while near the two RTIs the fault strike is rotatedcounterclockwise (Figure 2). The combination of these twochanges in spreading direction also produces off-axis lineamentsthat better correspond to the observed side-looking sonarlineation pattern, especially between 0.5 and 1.0 Ma.

Siqueiros Fracture Zone

The tectonic reconstruction of the SFZ over the last 3-4 m.y. ismuch more complex and is characterized by a series ofcounterclockwise changes in spreading direction at about 3.5, 2.5,1.5, and 0.5 Ma (Figure 7). These spreading direction changesresulted in extensional events across the active transform that

~1.5 Ma

Present

~0.5 Ma

> 1.8 Ma

-105o -103o-104o

10o

1.0 0.81.82.6 1.0 1.80.83.5

0.81.8 1.0 0.8 3.51.0 1.8 2.6

Figure 6. Sequence of plate boundary geometries describing a scenario in which a series of counterclockwisechanges in spreading direction at ~1.5 Ma and ~0.5 Ma produced the present plate boundary geometry observedalong the Clipperton Fracture Zone. A spreading direction change of 1° was used for the event at ~1.5 Ma and alarger 3° change was used for the more recent event at ~0.5 Ma. For simplicity, the changes in spreading directionwere assumed to be punctuated. The results of the tectonic reconstruction are overlain on the bathymetry.

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3175

Propagation

AbandonedSpreading Centers

PRESENT

~0.5 MA

~1.5 MA

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Figure 7. Sequence of plate boundary geometries describing a scenario in which a series of counterclockwisechanges in spreading direction at ~2.5, ~1.5 Ma and ~0.5 Ma produced the complicated plate boundary geometryobserved along the Siqueiros Fracture Zone. Spreading direction changes at ~2.5 Ma (1°), ~1.5 Ma (2°), and ~0.5Ma (5°) were used for the tectonic reconstruction. For simplicity, the changes in spreading direction were assumedto be punctuated, except for the extended period of propagation associated with the intra-transform spreadingcenters between 1.5 and 2.0 Ma. The results of the tectonic reconstruction are overlain on the bathymetry.

also produced a series of flexural transverse ridges and ITSCsalong the active transform fault trace.

The small change in spreading direction at ~3.5 Ma is poorlyconstrained, but fracture-zone-parallel lineations along theeastern trace of the fracture zone suggest a counterclockwisechange in spreading direction (~1°). This caused the scissor-likeopening along the transform, leading to the southwardpropagation of the western RTI and the relocation of the activetransform fault to a more southerly location. The position of theeastern RTI, meanwhile, remained relatively unchanged. There isalso the subtle indication of the presence of a fracture-zone-parallel transverse ridge along the northern side of the fracturezone near 102°W (Figure 4c).

The counterclockwise change in spreading direction at ~ 2.5Ma also appears to have been fairly modest (1-2°), but the effectson the tectonics and morphology of the SFZ were verysignificant. The most dramatic effect was the formation of twoleft-stepping ITSCs near the western RTI (Figure 4b). Thespecifics of the initiation and controls on the location of theseITSCs are still uncertain, but the presence of ITSCs isdocumented by the ridge-parallel fabric within the westernfracture zone complex. Also associated with this platereadjustment is the continued southward propagation of the

western RTI, the abandonment of the transform to a much moresoutherly location, and the formation of the fracture-zone-paralleltransverse ridge near 102°50'W.

The ITSCs continued to propagate southward until about 1.5Ma, when another 1-2° counterclockwise change in spreadingoccurred (Figure 7). The spreading direction change caused asingle ITSC to form (ITSC A), replacing the two previous ITSCs.This occurred either by forming an entirely new ITSC or byabandonment of the more southerly segment. These plateadjustments caused the western RTI to propagate slightlysouthward and resulted in the relocation of the transform faultand eastern RTI farther north. This change in spreading directionmay have also formed the distinct fracture-zone-paralleltransverse ridge located at 103°30'W (Figure 4). The presence ofabyssal hill fabric overprinting this transverse ridge and thedistinctive cross-sectional morphology suggests a flexural originfor this feature (Figure 5). Other evidence for the platereadjustment is observed in the change in abyssal hill fabric nearthe fracture zones of both RTIs. A change in the trend (~6°) ofthe volcanic chain west of the western RTI is also observed about1.5 Ma, but the significance of this trend change is uncertainsince the proposed spreading direction change is only 1-2°.

The most recent change in spreading direction appears to have

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3176

occurred about 0.5 Ma and represented a much more significantcounterclockwise change (4-5°) in spreading direction (Figure 7).This event is proposed to have formed another left-stepping ITSC(ITSC B) near the center of the Siqueiros transform domain. Thisalso caused the transform fault between the western RTI andITSC A to be rotated and relocated farther south. An anomalousbathymetric high interpreted to be a flexural transverse ridge canbe seen to the north of the fracture zone near 103°55'W. A newtransform fault was also established linking ITSC B and theeastern RTI. To accommodate the new transform, the easternRTI propagated northward.

Since 0.5 Ma, the SFZ has continued to adjust to the newspreading direction by forming an additional left-stepping ITSC(C) in the eastern half of the transform (Figures 4 and 7). Theanalysis of multibeam and side-looking sonar data and visualobservations made during ALVIN dives in ITSC A indicate thatduring this time, ITSC A separated into two left-steppingsubsegments, with volcanism occurring on short (3-5 km long)ridges located at the north and south margin of the pull-apartbasin [Casey et al., 1992].

Structures Associated With Transpression andTranstension

A comparison of the counterclockwise changes in spreadingdirection (~2.5, 1.5 and 0.5 Ma) and the various morphologicfeatures observed along the right-stepping CFZ suggests that both

compressional median ridges and the RTI volcanic ridges(intersection highs) may be intimately associated with periods oftranspression along a transform fault. When a similar exercise isperformed for the left-stepping SFZ, both ITSCs and flexuralridges bordering the fractures zone appear to be associated withperiods of extension along the transform fault.

Intratransform Compressional Ridges

Counterclockwise changes in spreading direction along a right-stepping fracture zone produce a component of compressionalong an active transform. If the change in spreading direction issignificant, the compression can cause crustal shortening thatresults in the formation of compressional ridges along the lengthof the active transform (Figures 8 and 9). The compression andgrowth of the median ridge will continue until the change inspreading direction has been accommodated along the entiretransform. It is expected that deformation of the younger, thinnerplate should occur preferentially and result in an asymmetricdistribution of the compressional ridges. Once the spreadingdirection change has been accommodated, the growth of thecompressional ridges would cease and the relict features wouldbe transported along the older sides of fracture zones.

Within the Clipperton Transform, the present compressionalmedian ridge is slightly biased toward the western RTI. Theasymmetric distribution of the compressional ridge may indicatethat a majority of the compression has occurred near the westernRTI. The maximum relief is located in the central portion of the

Clipperton

Siqueiros

Calculated

New

t1 t2 t3

Old

Siqueiros

Clipperton

New

VolcanicRooster Comb

MedianRidge

FlexuralRidge Abandoned

Transform Fault

Intra-TransformSpreading Center

Transpression

Transtension

New

Old

Figure 8. Series of time sequences illustrating the position of "apparent" Euler poles associated with a counterclockwisechange in spreading direction along the Clipperton and Siqueiros Fracture Zones. Also shown is the formation andevolution of morphologic features observed along the Clipperton and Siqueiros Fracture Zones including compressionalmedian ridges, volcanic intersection highs, flexural transverse ridge, ITSC, and abandoned transform fault traces. Priorto the spreading direction change, plate motion along both transforms may be described by a single Euler pole (Old). Atthe very onset of the counterclockwise spreading direction change (t1), there exists a component of tension (openarrows) along the Siqueiros Transform and a component of compression (solid arrows) along the Clipperton Transform.After the plate motion change has begun (t2), plate motion along the Siqueiros Fracture Zone changes to the newspreading direction immediately. Along the Clipperton Fracture Zone, however, the convergence and nonrigid behaviorof the plates along the active transform causes plate motion to be intermediate between the new and old spreadingdirections. After the plates have adjusted to the new spreading direction (t3), plate motion along both transforms may bedescribed by a single Euler pole (New) again. During the change in spreading direction, extension along the SiqueirosTransform produced the ITSC, the flexural transverse ridge and the abandoned transform fault trace. Compressionacross the Clipperton Transform generated a compressional median ridge, and extension along the Clipperton FractureZone near the RTI resulted in the formation of the volcanic intersection highs.

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3177

iqueiros Fracture Zone Clipperton Fracture Zone

Ridge Axis

Ridge Axis

Ridge Axis

CompressionalRidge

Volcanic Ridge"Rooster Comb"

Flexural Ridge

ObliqueSpreading Centers

VolcanicChain

Melt Melt

"Leaky" Transform

Transform-ParallelTrough

Figure 9. Three-dimensional perspective views of the (left) Siqueiros and (right) Clipperton Fracture Zonessummarizing the internal anatomy of intratransform spreading centers, flexural transverse ridges, compressionalmedian ridges, transform-parallel troughs, and intersection highs.

transform where the strike of the transform fault is parallel to theolder spreading direction. The location of the compressionalridge and the presence of a compressional earthquake focalmechanism (Figure 2a) suggest that the change in spreadingdirection has not been completely accommodated. Althoughprevious episodes of compression have occurred recently (1.5Ma) along the CFZ , features that may representpaleocompressional ridges are not observed along the fracturezone trace. The lack of older compressional ridges may be due tothe smaller changes in spreading direction (1-2°) proposed in thereconstructions (Figure 6). Perhaps a larger change in spreadingdirection (>5°) is required to generate a compressional ridge.

Transform-Parallel Troughs

The causal mechanism for the creation of the bathymetrictroughs bordering the compressional ridge along the ClippertonTransform is not known, but the morphologic character isreminiscent of the flexural moat that surrounds the HawaiianIslands [Walcott, 1970] or large seamounts [Watts et al., 1975].In the case of the Clipperton Transform, the source of the flexureis an endload represented by the compressional median ridge(Figure 9). To test the plausibility of this model, themethodology of Turcotte and Schubert [1982] was used tocalculate the flexural response of an elastic plate to an end loadsimilar in size to the compressional ridge (Figure 10). The resultsindicate a very good correspondence between the profilescalculated with a thin plate thickness (0.5-0.7 km) and thebathymetric profile on the southern side of the transform on the

Cocos plate. Although the calculated plate thickness values areless than previously calculated values of young lithosphere (1-6km) [Watts, 1978; Wilson, 1995], the addition of a horizontalcomponent of stress associated with compression across thetransform will produce a flexural profile representative of a largerendload or thinner plate. The bathymetric profile on the northernside of the transform on the Pacific plate is more difficult tomodel because of the lack of data, but available data suggest thatthe end load is smaller or that the plate thickness is greater.These observations are consistent with the preferred deformationof the younger lithosphere during compression across thetransform.

An interesting observation is the absence of transform-paralleltroughs along the fracture zone extensions of the CFZ, eventhough other periods of compression have occurred as recently as1.5 Ma. This may indicate that these features are dynamicallysupported, rebounding to pretranspression elevations once thespreading direction change has been accommodated. A morelikely explanation, however, may be that these troughs aredependent on the magnitude of a spreading direction change,similar to the explanation given above for the presence of thecompressional median ridge.

RTI Volcanic Ridges

While compression is occurring along a right-steppingtransform during a counterclockwise change in spreadingdirection, a significant component of fracture-zone-normalextension should be present along the fracture zones near the RTI

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3178

10 10 20

1.0 km0.7

0.5

Transform Fault

NCompressional Ridge

Trough Trough

02030

500

0

-500

-1000

Bat

hym

etry

(m

)

Distance from Clipperton Transform (km)

Cocos Plate Pacific Plate

Figure 10. Bathymetric cross section (grey) across the Clipperton Transform near 104°05'W showing the resultsof calculations to determine the topography associated with the flexure of an elastic plate with an endload similar indimensions to the observed median ridge. The actual and modeled topography match very well, especially forplate thicknesses between 0.5 and 0.7 km.

(Figure 8) [Pockalny et al., 1996]. The extension may besufficient to produce fissures in the crust and upper mantle thatserve as conduits for melt, especially if the RTI is associated witha ridge segment that has a shallow melt lens [e.g., Sinton andDetrick, 1992] or robust magmatic potential which would cause itto attempt to propagate across the RTI boundary. The meltemanating from these conduits will flow out onto the older,deeper crust as surficial flows and produce the rooster-combmorphology near the RTIs of the CFZ (Figures 2 and 3).

The source of the fracture-zone-normal extension proposed forthe intersection high formation is not immediately evident butmay be the result of different plate motion vectors on either sideof the fracture zone near the RTI. On the younger side of thefracture zone, plate motion has become oriented with the newspreading direction, while the older side of the fracture zone has aplate motion vector that is somewhat intermediate to the new andold spreading directions (Figure 8). The source of theintermediate spreading vector would be directly related to thecompression occurring along the transform. At some distanceaway from the RTI, the plate motion vectors on either side of thefracture zone would become parallel again, and fracture-zone-normal extension would not occur at this location. Thisexplanation of intersection highs is consistent with seismicrefraction studies which suggest a crustal thickening below theintersection high at the eastern RTI of the Clipperton Transform[Barth, 1994] and may also be responsible for similar featuresobserved at other RTIs of fast spreading ridges (e.g., GarrettFracture Zone [Gallo et al., 1986]).

On the older crust across from the western RTI of the CFZ(Figures 2a and 3a), a very characteristic intersection high ispresent. At the eastern RTI (Figures 2b and 3b), the volcanicridge south of the transform has a fracture-zone-parallel shape.Similar features with a distinctly rooster-comb shape are seen onthe older sides of the fracture zones along the flanks of the CFZ(Figure 2). On both the eastern and western flanks, "relict"intersection highs are present across from crust that is ~1.5 -1.8Ma, coinciding with our proposed change in spreading direction.

Intratransform Spreading Centers

A counterclockwise change in spreading direction along a left-stepping transform will result in a component of extension along

the active transform (Figure 8). The amount of extension alongthe trace of the transform fault may be sufficient to generate pull-apart basins that evolve into ITSCs . The formation of ITSCsappear to be the result of tears or propagation events initiatednear the trace of a transform fault, similar to that proposed byBird and Naar [1994] and Lonsdale [1995] for microplateformation.

The detailed evolution of ITSCs is uncertain, but severalfeatures associated with the present ITSCs may provide keyinformation about their formation (Figure 4). The initial stages ofITSC formation may begin with a leaky transform (Figure 9).Submersible studies have documented that young maficvolcanism has occurred along the transform fault connectingITSCs A and B [Perfit et al., 1996]. If the transform continues toleak, a transform-parallel volcanic ridge may form. This mayaccount for the transform-parallel ridges (103°10'W and103°55'W) located along the northern trace of ITSC B (Figure4b). These volcanic ridges were initially a continuous feature,but subsequent formation of ITSC B bisected the feature. Asextension across the transform continues, the leaky transformevolves from a highly oblique rifting valley to a mature ITSC.Evidence for this evolution can be seen in the rotation ofstructural trends from oblique to nearly ridge-parallel trendswithin the swath of terrain generated at ITSC B (Figure 4b). Theinitial stages of this process may be occurring at ITSC C.

The ultimate fate of the ITSCs are likely to be dependent onfuture changes in spreading direction. If a counterclockwisechange in spreading direction continues, the ITSCs may lengthenor produce additional ITSCs. If no future change occurs, thentheir character and distribution may persist. If a clockwisechange in spreading occurs, then compression will likely lead tothe demise of the present ITSCs. Complex terrain composed ofridge-parallel and transform-parallel-to-oblique features has beenfound to be quite common in fracture zones associated withintermediate to superfast spreading ridges. Fracture zonesassociated with the Blanco [Embley and Wilson, 1992], Orozco[Madsen et al., 1986], Wilkes [Cochran et al., 1993], Eltanin[Lonsdale, 1986], Gofar, Quebrada, Discovery [Searle, 1983],and Garrett [Fox and Gallo , 1989] transforms have thischaracteristic topographic fabric which can be used to reconstructepisodes of plate reorganization.

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3179

Flexural Transverse Ridges

The presence of fracture-zone-parallel transverse ridges alongthe northern side of the active transform and eastward extensionof the SFZ appear to be related to the sudden southwardpropagation of the western RTI and the relocation of strike-slipfaulting to more southerly locations. The best example of thisfeature is located at 103°10'W-103°50W (Figures 4a and 5) and asimilar example may be located at ~102°00’W-102°30'W (Figure4c). In both cases, the features are bound to the south by steeplysloping south facing scarps and to the north by more gentlysloping topography. These features resemble flexural ridgeswhich would be produced if fracture-zone-normal extensionoccurred along a southward dipping normal fault coincident withthe steep, south facing scarp (Figures 5 and 9). These features arealso associated with mantle Bouguer anomaly highs [Weiland andMacdonald, 1995]. This association suggests a regionalcompensation mechanism and is in agreement with a flexuralmodel of formation. A similar bathymetry and mantle Bouguergravity anomaly is seen along the northern side of the activetransform along the left-stepping Orozco Fracture Zone. Thismechanism of transverse ridge formation has been modeled byPockalny and Buck [1995], and very good correlations existbetween bathymetric profiles and models with plate thicknessesof 2-10 km and amounts of extension of <2 km (Figure 5).

Nonrigid Plates Versus "Apparent" Euler Poles

The observation that a single Euler pole may not be able todescribe the relative plate motion history of an entire plateboundary during a spreading direction change is a veryinteresting result of the tectonic reconstructions of neighboringfracture zones (Figures 6 and 7 and Table 1). According to strictplate tectonic theory, the relative motion of two internally rigidplates along a common plate boundary should be defined by asingle Euler pole. To explain this apparent departure from platetectonic theory, either additional plates must be formed during thespreading direction change or nonrigid behavior of the platesmust occur. Although it may be difficult to discriminate fullybetween these two mechanisms, the presence of anomalousfeatures such as median ridges, transverse ridges, intersectionhighs, and intratransform spreading centers seems to favor anexplanation of nonrigid plate behavior. Regardless of themechanism producing the two "apparent" Euler poles, theseobservations suggest neighboring transforms with different offsetsenses adjust to changes in spreading direction at different rates.

A conceptual model of what may be occurring along the EPRnear the CFZ and SFZ during a spreading direction change is

Table 1. Stage Pole Parameters Model Age Interval Latitude Longitude Angle

Ma °N °W deg/my

Clipperton (this study) 0-0.5 39.0 110.0 2.00.5-1.5 36.4 107.7 2.1

Siqueiros (this study) 0-0.5 41.0 113.5 2.00.5-1.5 36.4 106.4 2.1

Macdonald et al., [1992] 0-0.7 37.4 108.5 2.00.7-1.7 36.4 107.1 2.1

DeMets et al., [1994] 0-3.2 36.8 108.6 2.0

shown in Figure 8. Prior to the change in spreading direction, asingle Euler pole is able to describe the plate motion of both theClipperton and Siqueiros transforms. A far-field change in theglobal circuit of plate motion vectors is postulated to have causeda counterclockwise change in the overall spreading direction ofthe Pacific-Cocos plate boundary. Near the Siqueiros Transform,the change causes a component of extension to occur along thetransform which permits the plate motion on either side of thetransform to adjust rapidly to the new Euler pole. Near theClipperton Transform, however, the counterclockwise change inspreading direction results in a component of compression alongthe transform. This prevents spreading in this region to adjust tothe new Euler pole, resulting in nonrigid plate behavior and platemotion that is described by an "apparent" Euler pole intermediatebetween the old and new pole position. Our model predicts the"apparent" Euler pole will remain for as long as a component ofcompression exists along the Clipperton Transform. Once thechange in spreading direction has been accommodated, the platemotion along both transforms will be described by a single Eulerpole again. The time required to accommodate this change inspreading direction is not well constrained, but an upper limitshould be about one half of the age offset of the compressionaltransform. This period of time will allow crust generated at theRTIs during the initial spreading direction change to slide pasteach other along the transform. This period of adjustment mayalso be dependent upon the magnitude of the spreading directionchange, small changes (1-2°) may be accommodated almostimmediately, while intermediate pole changes (>5°) may requirelonger periods of adjustment.

Conclusions

1. Tectonic reconstructions using available bathymetry, side-scan and magnetics data indicate that a series of counterclockwisechanges in spreading direction have occurred along the Pacific-Cocos plate boundary about 2.5 Ma, 1.5 Ma, and 0.5 Ma. Thesespreading direction changes caused a series of compressionalevents to occur along the right-stepping CFZ and a series ofextensional events to occur along the left-stepping SFZ.

2. Several bathymetric features present along the trace of theCFZ appear to be associated with periods of compression acrossthe transform during changes in spreading direction. The medianridge located along the active trace of the transform appears to bethe result of crustal shortening associated with compression alongthe younger portions of the bounding plates. The transform-parallel troughs surrounding the compressional median ridgeappear to be the flexural response of the lithosphere to the endload of the median ridge. The bathymetric highs near the RTIshave a volcanic character and appear to be the result of meltemanating from conduits produced by fracture-zone-normalextension near the RTIs.

3. Along the SFZ, the documented changes in spreadingdirection have caused the transform to be subjected to a series ofextensional events that have led to the formation of ITSCs andflexural transverse ridges. The formation and evolution of theITSCs are likely to be causally related to specific episodes ofplate reorganization, and available evidence suggests that theymay begin as leaky transforms and evolve into small well-developed spreading centers if the change in plate geometry issufficiently large and persistent. The formation of flexuraltransverse ridges appear to be associated with the propagation ofthe RTI and the relocation of the active transform fault. The old

POCKALNY ET AL.: CLIPPERTON AND SIQUEIROS FRACTURE ZONES 3180

trace of the transform represents a crustal weakness that is thelocus of normal faulting that occurs in response to the extensionacross the transform. The transverse ridges are the flexuralresponse of the lithosphere to the normal faulting. Although thedocumented changes in plate motion do not appear to beresponsible for the formation of the seamount chain along thewestern trace of the SFZ, the overall trend and changes in thetrend of the seamount chain suggest a relationship exists betweenthe seamounts and the western Siqueiros RTI.

4. Attempts to find a series of common stage poles that wouldexplain the plate motion associate with both the CFZ and the SFZduring changes in spreading direction have been unsuccessful.This has led to the observation that a single Euler pole is unableto describe plate motions along an entire plate boundary during achange in spreading direction. Near a transform experiencingextension, the plate motions will adjust to the new Euler polealmost immediately. Near a transform experiencing compression,however, plate motions will be somewhat intermediate to the newand old spreading directions. This pattern of plate motions willpersist until the change in spreading direction has beenaccommodated. The amount of time required for thisaccommodation to occur is probably dependent on the magnitudeof the spreading direction change and the age-offset of thecompressional transform.

Acknowledgments. The authors wish to thank Ginger Barth, RobBird, Roger Buck, Dwight Coleman, Suzanne Carbotte, Neil Driscoll,Dave Gallo, Pascal Gente, Kim Kastens, John Madsen, Roger Larson, andBill Ryan for their helpful discussions over the years. We would also liketo thank Suzanne Carbotte, David Naar, and an anonymous reviewer fortheir helpful reviews of the manuscript. One of us (R.P.) would also liketo extend a special thanks to Carolyn Kincaid for her help withinterpreting the side-scan data and to Mark Eisen for his initial work withthe side-scan data in the Clipperton area. Over the years, portions of thiswork were supported by ONR grants N00014-90-J1645 and N00014-93-10108 (K.M.), ONR grants N00014931003 and N0001490J1448 (J.F. andR.P.), NSF grant OCE9020404 (D.F.), and NSF grant OCE9019154(M.P.).

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D.J. Fornari, Department of Geology and Geophysics, Woods Hole

Oceanographic Institution, Woods Hole, MA 02543 (e-mail:fornari@tone.whoi.edu)

P.J. Fox, Now at Ocean Drilling Program, Texas A & M University,College Station, TX 77840-1917 (e-mail: fornari@tone.whoi.edu)

K.C. Macdonald, Department of Geology, University of California,Santa Barbara, CA 93106 (e-mail: ken@magic.geol.ucsb.edu)

M.R. Perfit, Department of Geology, University of Florida,Gainesville, FL 32611 (e-mail: perfit@geology.ufl.edu)

R.A. Pockalny, Graduate School of Oceanography, University ofRhode Island, Narragansett , RI 02882-1197. (e-mail:robp@bobbyorr.gso.uri.edu)

(Received April 22, 1996; Revised September 16, 1996;accepted October 31, 1996.)