Do Microplates in Subduction Zones Leave a Geological Record? Joannm

Home , Explorer Plate, Fracture zone, Gorda Plate

TECTONICS, VOL. 13, NO. 6, PAGES 1472-1487, DECEMBER 1994

Do microplates in subduction zones leave a geological record? JoannM. Stock and Jeffrey Lee• SeismologicalLaboratory, California Institute of Technology,Pasadena

Abstract. Active microplate boundaries in ocean-conti- Introduction nent subductionzones may induce deformation of the over- As a spreadingcenter approachesa trench in a subduction lying plate and spatial or geochemical variations in the setting, the intervening subducted plate may fragment into volcanic arc. We discuss two modern cases. The first is the smaller pieces down to linear dimensions of--100 km South Gorda-Juande Fuca plate boundary in the Cascadia (microplates). A recent example of this processis provided subduction zone, where there is little or no effect on the by the Farallon plate, which broke into progressively overridingplate and the oceanicplate takes up much of the smaller pieces as the Pacific-Farallon ridge approachedthe deformation. The secondcase is the Cocos-Riveraplate subduction zone west of North America [Atwater, 1970; boundaryin the Middle America trench,where the overlying Menard, 1978; Lonsdale, 1991], following the history il- Colima graben contains substantial deformation in a zone lustrated in Figure 1. Some of the small plates that resulted extending from the trench to the volcanic arc and the sub- from a recent period of fragmentationare the Explorer plate duction-relatedvolcanism is spatially and geochemically and the South Gorda plate. These started moving indepen- complex. We apply these observationsto boundariesof the dently at -5 and -3 Ma at the northernmostand southern- Arguello, Monterey,Guadalupe, and Magdalenamicroplates, most edges of the Juan de Fuca plate along the Cascadia which existedin the subductionzone west of Baja California subductionzone (Figure 2). Another small plate, the Rivera at varioustimes from 20 to 12.5 Ma. The past positionsof plate, recently (-5 Ma) separated from the much larger these boundariesrelative to Baja California are constrained Cocos plate at the northern end of the Middle America by global plate reconstructions, closure of the Gulf of trench (Figure 3). Examples of similar plate fragmentation California, and an estimate of extension in the Mexican of Miocene age lie in the marine geology offshore Basin and Range province. Existingregional mapping and California and Baja California [Lonsdale, 1991; Atwater, our additional reconnaissancemapping show that Paleocene 1989] (Figures 4a-4d). to Eocene fluvial and marine sedimentary rocks south of The fragmentation of a subducting plate creates a new Ensenada along the western Baja California peninsula and eastward to the mid-Miocene volcanic arc are undeformed. triple junction, two limbs of which are along the trench. If the two subducting plate fragments then diverge rapidly Limited availabledata reveal no major spatialor geochemi- enough, a spreadingcenter may form between them, so that cal variationsin the mid-Miocene volcanic arc that might the newly formed triple junction quickly evolves to a correlate with the past positions of the microplate bound- trench-trench-ridge (TTR) geometry. (For example, the aries. Thus these microplate boundarieshad little to no ef- fragmentation of the Farallon plate into Cocos and Nazca fect on the overriding continental plate. The nature of plates created a new spreading center, the GalapagosRise, Guadalupe and Magdalena interactions with North America between Nazca and Cocos.) If the two plate fragments may have been closer to the South Gorda-Juan de Fuca ex- converge rapidly, then, presumably, intraoceanic subduction ample, with possible internal deformation of the mi- might result, although we know of no well-documented croplates. The Monterey and Arguello microplatesmay examples of this in the geological record. However, if the have behavedlike the modernExplorer plate, with largely relative motion between the two downgoing plate fragments strike-slip motion relative to North America during their is slow, as in the modern examples discussedhere, then a last stagesof existence. Tectonic patternssimilar to these more complex deformationpattern is observedbetween the examples may be expected from other plate boundaries two subducting plates. As discussed below, the relative where a plate is fragmenting as it enters a subductionzone motion may be taken up within a wide zone of internal (e.g., the Aluk plate in the trench beneath West Antarctica deformation and seismicity of the downgoing plates in early Tertiary time). Whether these microplatessubse- themselves (e.g., the South Gorda deformation zone within quently becomeattached to the overridingcontinental plate or to a larger oceanicplate and whetherthis causesdeforma- the Juan de Fuca plate) or by formationof localized grabens and transtensional structures within the downgoing plate tion in the region of the former subductionzone may depend (e.g., Rivera-Cocos boundary). Triple junctions must still on the velocities of the nearby major plates and the relative be present between the plates, but they may not be as orientationsof the microplate boundaries. simple as those usually envisioned. For example, the modern Cocos-Rivera-NorthAmerica triple junction at the Middle America Trench has two limbs that are trenches and INowat Geology Department, Central Washington University, Ellensburg. one limb that is an oblique extensional zone between the Cocos and Rivera plates. Kinematically, this configuration Copyright1994 by the AmericanGeophysical Union. is equivalent to a TTR triple junction but, in practice, is quite different since there is diffuse oblique extension,not Papernumber 94TC01808. seafloor spreading. 0278-7407/94/94TC01808510.00 The responseto plate fragmentation may also depend on

1472 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 1473

GENEALOGY OF THE FARALLON PLATE (modified from Lonsdale, 1991)

! ! 60 Ma -- Farallon i I

Farallon Vancouver

40 Ma --

.•-: ...... • ...... Argo, ello ! Juan de Fuca Monterey Guadalupe ! 20 Ma -- I

Gorda Juan de Fuca Explorer C era

0 Ma south ?? north

Figure 1. Time line showing the genealogyof breakup of the Farallon plate into smaller fragments, following the detailedmicroplate histories of Atwater [ 1989] and Lonsdale [ 1991]. the size of the resulting plates. A large plate may be driven viewed as end-member cases' (1) tectonic settingswhere the by slab-pull and ridge-push stresseswhich may not change presenceof microplate boundariesin the subductionzone much if the plate is broken into two, equal size pieces. has little to no effect on the geology of the overriding plate However, if a small fragment breaks off a large plate, then and (2) tectonic settingswhere substantialfragmentation of it may lose its slab-pull force and becomedriven entirely by the overriding plate results. We then apply these observa- different conditions. In addition, small plate fragmentsare tions to interpret the kinematics of past microplate bound- generally broken off larger plates in the region near the aries along the westernmargin of Baja California. We do spreading ridge. Thus the average age of smaller plate not discussprocesses that occur after microplate death, al- fragments is generally much younger than the average age though these affect the later geological developmentof the of larger plates, such that smaller plates are thinner, weaker, regionnear the subductionzone [e.g., Nicholsonet al., 1994; and more easily deformed. As a result, smaller plates expe- R. G. Bohannon and T. Parsons, Tectonic implications of rience less slab-pull force, such that their motions may be post-30 Ma Pacific and North American plate motions, able to change quite rapidly. submittedto GeologicalSociety of America,Bulletin,1994]. Small sizes and slow rates of relative motion are charac- teristic of the fragments of the former Farallon plate that Present-Day Subduction' North America were subducted beneath western North America in late Tertiary time. In many cases the boundariesbetween these Cascadia Subduction Zone: Little to No plates, where they entered the subductionzone, were proba- Continental Deformation Correlating bly not spreading ridges but, rather, transform faults, zones With Microplate Boundaries of diffuse extension, or perhaps distributed shear. Here we evaluate kinematics and expected processeswhile these mi- At the Cascadia subduction zone the Juan de Fuca plate, croplateswere still moving relative to the major plates. We the Explorer plate, and the "Gorda deformation zone" discuss the two following examples from the present-day [Wilson, 1989] or South Gorda plate [Riddihough, 1984] are western margin of the North America plate that may be caught betweenthe larger North America and Pacific plates. 1474 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES

NORTH 5O N AMERIC PLATE

45 N

"6f_•ji_s9n[.1...993]...... :.. • - • - - - *:.,-. Mendocino-- - 40 N

...v : ! .?. ' ß•'::"• ...... ::.'.'...... •.•:• ...... •4i:-.

:i: I ...... ',PLAE .. ..:.. .:.•.:

: ... ..• ..

130 W 120 W

Figure 2. Present plate and microplate configuration along the Cascadia subduction zone showing Juan de Fuca plate and related microplates(Explorer and South Gorda). Thick lines indicate spreading ridges; thin lines are transform faults. Microplate geometry is from Riddihough [1984], Wilson [1993], and other referencesin text. Ridge geometryis from Atwater and Severinghaus[1989]. Young volcanic centers in the Cascades,shown as triangles, are from Wood and Kienle [1990]. Segmentation of the active Cascades volcanic arc, shown by lines indicating the position of the volcanic front (where defined) and correspondingsegment numbers in circles, is from Guffanti and Weaver [1988]. Diagonal shadingin oceanicplates showsbroad zones of deformationaccording to Wilson [1993]; dotted shading north of the South Gorda deformation zone indicates the northernmost, well-located, intraplate mainshock-aftershocksequence in this region [Smith et al., 1993].

The Juande Fuca and Gorda platesare being subductedunder not interpreted in detail [Rohr et al., 1993]. The Gorda North America; the Explorer plate is being translatedNW ridge, located between the Juan de Fuca/SouthGorda plate parallel to the margin (Figure 2). The boundariesbetween and the Pacific plate, and its well-developed magnetic the Explorer and Juande Fuca platesand betweenthe Juande anomalies, change in strike from north to south (Figure 2). Fuca and South Gorda regionsare zonesof diffuse deforma- These changesare of interest becauseif there were no data tion. Juan de Fuca, Explorer, and South Gorda are all from east of the spreading center, one might attribute the spreadingaway from the Pacific, but the magnetic anomaly change in strike of the Gorda Ridge and of west (Pacific) record from Pacific-Explorer motion is quite recent and is side magnetic anomalies2A and younger (3 Ma to present) STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 1475

Gulf of Mexico

20N

El Gordo Graben

15N PAClFIC PLATE

i 110 W 105 W 100 W

Figure 3. Current plate configurationin western Mexico and major belts of onshoredeformation and volcanism. Major plates are Pacific, Cocos, Rivera, and North America. MVB is Mexican Volcanic Belt; FZ, fracture zone. Continental blocks that may be moving independently relative to the North America plate include Jalisco(Jal.), Michoacfin(M), and Guerrero (G). Also shown are 2000-m and 500- m bathymetric contours. Magnetic anomalies are centers of normal polarity stripes [Atwater and Severinghaus,1989] and have the following ages [Cande and Kent, 1992]: 2, 1.87 Ma; 2A, 3.08 Ma: 3, 4.54 Ma; 3A, 6.04 Ma; 4, 8.70 Ma; 4A, 9.28 Ma; and 5, 10.21 Ma. "V" in the open squareis Volefin Colima. to the existenceof two different plates east of the ridge. independentlyrelative to the more northern Juan de Fuca However, the magnetic anomalies on the eastern side show plate between -3 and -5 Ma [Wilson, 1993]. that two distinct plates are not presentin this region. The boundary between the south Gorda plate and the Juan The Juan de Fuca plate appearsto be somewhatcoupled to de Fuca plate, located at the change in strike of the spread- the overriding plate in northern Oregon, as indicated by ing ridge and magnetic anomalies,is not a discretestructure. folds and faults that cross the trench, affecting both the In addition, young faulting and diffuse seismicity continues oceanic plate and the continental shelf of the overriding northward into a region where the magnetic anomalies are (North America) plate [Goldfinger et al., 1992]. Farther not visibly deformed [Smith et al., 1993]. This raisesdoubt south, adjacent to southernOregon and northern California, whether the South Gorda plate should be consideredan inde- a 200- x 200-kin area of the oceanicplate east of the Gorda pendent microplate or, rather, an internally deforming sec- ridge and adjacentto the Mendocinotransform fault is expe- tion of the Juan de Fuca plate [Wilson, 1989, 1993]. A riencing internal deformation,as indicatedby diffuse seis- knowledge of the slab configuration("one plate or two?") in micity and major, active strike-slip faults [Silver, 1971; the subduction zone would resolve this problem. Wilson, 1989; Smith et al., 1993] and by distortionof the Unfortunately, there is little recorded seismicity from the seafloor magnetic anomaly patterns within much of the Wadati-Benioff zone, such that it is not known whether the region [Wilson, 1986, 1993] (Figure 2). This region has Wadati-Benioff zone is warped, planar, or broken into sepa- been labeled the "SouthGorda plate" [Riddihough, 1980] or rate Gorda and Juan de Fuca slabs. the "Gorda deformationzone" [Wilson, 1989] and comprises We note that the Mendocino Triple Junction and the in- the southernhalf of a "Gorda plate" that may have moved ternal features of the subduction zone (such as the trace of 1476 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES

ANOMALY 6 (old edge), 20.4 MA

._ .' // ., ,• ., __ • •', PLATE '..%L % :;;/.. Y Shirley• __½--*,-,?-2 ,½;,// PACIFIC.- PLATE., (• Transform ', ,.- ::::...... 95%confidence ellipseforposition of• Arguello Pacificplate points relative to fixed • MOFFO North America microplate

Figure 4a. Configuration of microplates west of Baja California at 20.4 Ma, based on global plate reconstructionsof the Pacific plate to the North America plate [Stock and Molnar, 1988]. Time of magnetic anomalies (6 in Figure 4a) is given according to the Decade of North American Geology (DNAG) timescale; on the timescale of Cande and Kent [1992] this is the old edge of anomaly 6, 20.2 Ma. Marine magnetic anomaliesindicating microplate configurationsare from Lonsdale [1991] and Atwater and Severinghaus[1989]. Pseudofault traces,shown as oblique solid lines with dashedcounter- parts, are from Atwater and Severinghaus [1989]. Locality abbreviations are SF, San Felipe; ENS, Ensenada;SR, Santa Rosalfa;BC, Bahfa Concepci6n;LOR, Loreto; and LP, La Paz. The 23-16 Ma and 24-11 Ma andesitebelts are after Sawlan [1991]; vents south of San Felipe are from Martœn-Barajasand Stock [1993] and Stock [1989]. Note that transform faults active at the time of a given reconstruction are labeled "transform," even though they are now fracture zones since they are no longer active. In addition to the reconstructionsof rigid Pacific plate to a fixed North America plate, an additional 130 km of ENE directed extension has been removed from the Mexican Basin and Range Province. Rotations used are as follows: closure of Gulf of California, 48.6øN, - 75.15øW, 4.69ø; and closure of Mexican Basin and Range, 52øN, 130øE, 1.2ø. the South Gorda-Juan de Fuca deformation zone) have had a tion in the geology of the overlying North America plate. component of trench-parallel motion of-48 mm/yr north- Although there is a zone of internal deformation in the sub- ward relative to the North America plate (e.g., NUVEL 1 ducted oceanic plate, there is no correlative structurein the model of DeMets et al. [1990]). Thus the "microplatebound- overlying plate, such as a graben or other obvious trans- ary" (if it is consideredas such)has a componentof motion verse structurereaching from the trench into the active vol- parallel to the trench, so that it does not influence the ge- canic arc. Certainly, in this region there is nothing analo- ology at any position of the upper plate for any great gous to the presentColima Rift as describedbelow. There length of time. may be a more subtle signal in the volcanic arc, however. It appears that the present Gorda deformation zone and The active Cascades arc is divisible into five major seg- the earlier Juan de Fuca-Gordaplate boundarypostulated by ments [Guffanti and Weaver, 1988] on the basis of geo- Wilson [1993] have had little to no recognizablemanifesta- chemistry and location of vents <5 Ma in age (Figure 2). STOCK AND LEE: MICROPLATES IN SUBDUC'I•ON ZONES 1477

ANOMALY 5D, 17.7 MA

Arguello microplate

Figure 4b. Same as Figure 4a, except for magnetic anomaly 5d, on DNAG timescale, 17.5 Ma.

ANOMALY 5C, 16.2 MA

•••-••s••:,...... •j :•i•'"'•'•'"•••:J'•"'"'"':' ' ' ....• ...... •:""•••:•.,-.-•.••!!i TM'-':"i.'.-"-".'•':•'i':?--:'"- ...... "•••....-'•i"'"'•"•'""'"•:"'"•:'•, •'""'" """"" ,."" ,

:"'•-••.-•...... •'";"••':' ' •'•'•'-----'"• --i-•..... •:,•••i•...... ••'•'"*'•••'•••a...... •"'"'"'"'"'"'"'•''' '"'"'•;'"'"•::••••:--""' ' -•--••ji•"• ....•'!Z•'"""••••:•'•••,,•,-.•.--•.-•'.....•.• B LOFI ...... •J'"=•='=";...... •'•:•:•••"-'=-::•"•"'""""••••"••------•••!ii •1'i''"•••• "•";':""e'"'""'"'""'"•-'''•'"•' ""'" '":'"ii ••'•'•...... EN ß•" "'•:.-.-'.'•" ' •:'.'.:.'...-'ii ...... •..:.....:...... •...... ,•• ...... '"•••--"•••••••...... ":•"'""••"'-••":••.•.• •...... *'"e""•• •'"...... •'•'•'"•':' '-' .•' .•' "'•'""'"'"'"'•'"""•"""••"•'"•'"""""'•"'"'•"':•;•:••'•i•['" "' ' • • "'""•'""•'"••••••"i"'"'""'"'"'""

...... • ---•-e-.... - ...... -...... ---i::•- :" ...... - -'---••-• ...... • '"• - • "'"'"''•.-' • "'"'"'"•'•:••••••'• - •,;::__ / , ... x,X ,,•,o',>-".;-...... ! / •.,,•rguez!.o ,'.•?.*b:------.',, . ', /i,.--• .,! FZ , • / ':. • ' 'q"-.,--..•x \ ' ,' -•,.(.x,•..,•,, '.',,,,, • ,,, '.\,',,, ,\•! •, •/. -•'•,• ,, Z'• -:•,•., -..• C0C.'05. PLATE ", ":X / '•V',",, •\ xx. .•. ,"' ".x / ',,x '•, 'Guad•upe•N. d•,./,,• ----•"'-•-•• 01arion • 'x, ,, ' ,,y/-..x, ¾ '•• '• •'•'.,, ,, Sh,'ley .-' ,_ • Transform •-•,•F•CP,,,,,T,=,;• ', •'ran•,or•n.•+..' "..% "•,, / .: /, '.. :•

Figure 4c. Same as Figure 4a, except for magnetic anomaly 5c, 16.2 Ma. 1478 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES

ANOMALY 5AA, 12.92 MA

...... ••"•'"'"•'"'••=••'•:••••il== '•:"-:'-'"•..... '...... ?'•;.-.::..-'•'%•:.;'.!:•• ½.'="..A:•..ME-I'::•!--.'----'•--•-'-••••i"•"""'"'"'"'•"'"•'"•-•"-•-'-•••••i:i....•'i:•..=•.-'•'"'"'"'"'•'"•;•--"••;•••..-'ii ...... '-""""'"'"'""";•'•'"='•'"•'"••••i• •"'"'"'"'""""""•'"'"'"'"'•"=••----':••.••'•'"'"'"'"'""•"'"'•'•:••:••••i...... ••...... •"'"'"'"""• ...... ••i••.-'• ,• - - •i.--..•i ...... •--.:'...... •'"•'"••'------"•••;•,....:•• '"""•'•'"'•*"ø'••*•'-'••••.-,.•.=:'.--.'-':•'-"""'""'"""•••'**••• '--'-•:••'.-• I•'""'"'""'•'•-'"""••-••••••...... •••••i'.'..•!'•"•"'•'"'"'••':':n•'-'•••:•...... •1:=""'"'""• ...... •'"'"'•••'"'•"•'='••";••••••I i•.... •--'•--.•-',o-'--'•-..---•:•••.:.-'...-'..-• ...... • ...... *'•"•'•::••••• •'"'"';"•"-"'?'":*""--'"•*•*-'- ...... •""=•'•'":'•••:•i ...... •"-"•-":• '='"•"'•'--",••v-•,.:'.-•i•'"'""•'"'•"'•'"'...... •"'"'"'"'""'•'"•'•:'"•'""••••.• '•'•••••.••...... '•"'"'""• •'•' '"'"'"'"'"'" '.,.•L•.-• ::','-"-'.':'-"-':-'.'•-:':'-"-"-";':-":....';;'-...'i::c.-'.... .-.'-.'-.'•:••••i•:i...... -'-"-'•-'^'-x,--'--•,• ' "'"•':"" "'-'" '-'-'"'""-'" .'-..•:•.-...:.•.:.'.b.,.?•:•-.x...... '.-.'.-.'•.-•.• ...... •'"'""'"'•"•'"'""•"•'"••'"•••••..'.'ii "'"'"'"''""""••:'"•••• '...... :• •'•'"'•'"•!•:"'•*"* :• "="* ' ...... •:•..,..-•..'•:,•"•'--"--,- -'";-'--'•-'s••••'.•••• :•----"'"--"-':...... ,¾.•--•,•-••••.....-••••,•• ':'•...... • ø '' ' ' ' ' ' '• ':' :'•*":'" '-•.-'.•'.•''---": i•'-:.-'.-'i'-' '•-•.• .-'..'.-'. .' .....•..•.,•..••.•,...... •...•:•••••

\\\\ '%. , /•:,•'\ ".•,\ sGu•al•e / •/ia Maodalena , •': /_".'•,, ,• \ \ Transfor• • 2•;•:x/ •• ', .-' • COCOS \ -_•,' \ ?'.Z?OL,:, >', ,\ ,_'\ '1 %. •.??/ .....:,__._;-_,,,, _ t PlaT; ß•.L , nI - '.-a.',. ¾, •.\ .\',, ', •,ß Shirley. -- -' . .. - ,, o•,. ,,',x d/ -, ' v • ..- ; • .- , • , , ß'// ,I mnt',,•-• , • nn'_,"• iranstorm q•_,• .- ' Ca!, ton ! , u! rt•,vlr•xw. DL-• • .'r... •'-•-..... --- ..... -,-_..__'.___ , ,,. .. :::-..... , . -//.

Figure 4d. Same as Figure 4a, except for magnetic anomaly 5aa, on DNAG timescale, 12.92 Ma and on Cande and Kent [1992] timescale, 13.0 Ma.

Most of the segmentboundaries lie above the rigid part of slip faults, the Tepic-Zacoalco graben (to the west) and the the subducting Juan de Fuca plate, but the southernmost Chapala and Citala Grabens(to the east). Active volcanoes boundary, where there is a slight SE jump in the volcanic continue eastward from here within the MVB across Mexico front, may correspondto the postulatedlimit between the and westward along the Tepic-Zacoalco graben. South Gorda and Juan de Fuca plates. Some of the other The Colima Rift and Manzanillo Graben are boundedby segment boundaries of the Cascadia arc are much more dra- high-angle faults, but the geometry and structuralexpression matic than this one. Thus from the geology of the upper of extension vary along strike within the grabens. The plate alone, one would not interpret this location as a likely Colima rift is composed of three different structural basins microplate boundaryin the subductionzone. from north to south [Allan eta/., 1991]. The northern basin varies from 20 to 65 km in width, has >2500 m of normal Middle America Trench: Deformation Above separationon its western bounding faults, and has extended a Subducting Plate Boundary by 1.5 to 3.3 km since -4 to 5 Ma [Allan, 1986]. East and The Colima rift and its offshore extension, the west of the northern basin are minor N-S striking grabens. Manzanillo Trough, constitute one of the best modern ex- The central basin has a steep normal fault and substantial amples of upper plate deformationcoincident with the pres- topographicrelief on its western margin and containsthree ence of a plate boundary within a subductionzone (Figure large volcanoes, the southernmost one of which (Volc•n 3). They lie within the North America plate, at a high angle Colima) is presently active (Figure 3). The southernstruc- to the trench, above the inferred location of the Cocos- tural basin is a composite basin, 100 km wide, inferred to Rivera slab boundary, and extend from the Middle America contain up to 8 km of sedimentaryfill [Bandy et al., 1993]. trench to the volcanic arc (the Mexican Volcanic Belt or Although evidence for significant or active extension in MVB). The Colima Rift is a distinct topographiclow, this region is debated [Serpa et al., 1992], normal faults strikingjust east of north, with -1000 m of topographicre- striking N40øE in the upper slope of the trench (the lief on its western wall and a bit less on its eastern wall. Manzanillo Graben) are active [Bourgoisand Michaud, 1991; Adjacent to it offshore, a depressionin the upper slope of Bandy, 1992]. the trench (the Manzanillo Graben) producesan embayment A variety of compositionally and spatially anomalous in the trench axis. Together, the Colima Rift and the young volcanism occurs within the northern and central Manzanillo Graben separate the Jalisco block (on the structural basins and along the bounding fault systems. The northwest) from the Michoacin-Guerrero block (on the calc-alkaline volcanismfound here is not surprising,as it is southeast) for a distance of -260 Fan. The Colima rift ends typical of subduction-relatedvolcanic arcs. However, peral- northwardat the MVB in a junction with two other grabens kaline and alkaline lavas, more typical of continental rift defined by topographiclows and active normal and strike- zones, have erupted in close proximity to and coeval with STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 1479 the calc-alkaline rocks [Allan et al., 1991]. Some of these On the basis of marine magnetic anomalies of the adja- compositionsare similar to those found in the East African cent Pacific seafloor [Lonsdale, 1991; Atwater and rift and are not known from any other subduction setting Severinghaus, 1989] and global plate reconstructions[e.g., [Allan et al., 1991]. Volcanism also extends 120 km closer Stock and Molnar, 1988], the evolution and locations of off- to the trench than is typical in the main MVB; Volc•n shore plates in this region are fairly well understood. The Colima and numerous cinder cones have developed within southernend of the Pacific-North America plate boundary, 170 km of the trench. This spatially and compositionally corresponding to the northern end of the Middle America anomalous volcanism is not confined to the Colima Riff. Trench (Figure 3), has occupiedapproximately the same po- Within the Jalisco block, numerous small Plio-Quaternary sition relative to North America since --t0 Ma. From t0 volcanoesare found within grabensat similar distancesfrom Ma to 5 Ma this was the location of a triple junction be- the trench [e.g., Wallace et al., 1992; Righter and tween three major plates, the North America, Cocos, and Carmichael, 1992]. Pacific. By ~5 Ma, owing to a progressivespreading center The main deposits that have accumulatedin the Colima transfer causing the brief existence of the "Mathematician Rift since extension began in early Pliocene time are vol- paleopiate"[Mammerickx eta/., 1988, Williams and Lonsdale, canic deposits from vents within the rift and off the main 1993], the spreadingsegment just south of the Rivera trans- trend of the MVB. These include eruptiveproducts of large form fault relocated to within ~lee km of the trench [Stock, calc-alkaline stratovolcanoes, calc-alkaline domes and cinder 1993a]. By this time, part of the Cocos plate north of this cones, small alkaline cinder or lava cones, and mudflow and transform fault had begun to move independently of the debris avalanche deposits from the stratovolcanoes,some of main Cocos plate. This northern plate fragment became the which flowed all the way to the trench. Additional nonvol- modern Rivera plate. It is important to note that the posi- canic deposits include lacustrine deposits from ephemeral tion of the Cocos-Rivera-North America triple junction has lakes, alluvium, and gravels. The sedimentaryfill in vari- not moved much with respect to the North America plate ous parts of the rift ranges from 1 km to 8 km in thickness since its inception because subduction of both plates has [Allan et al., 1991; Bandy et al., 1993]. These depositshave been nearly orthogonal to the trench. In sharp contrast to accumulated in a zone that varies from ~20 km to ~lee km the Cascadia subduction zone, the microplate boundary in width. This structural and depositional association is within the Middle America trench appearsto have been in- unique with respect to the surrounding,mainly Cretaceous fluencing the geology of the overriding plate at about the basementrocks. This associationis the type that we would same location for ~5 m.y. (5 Ma to present). This corre- look for elsewhere, at a high angle to the trench or lates with the ~5 Ma age for the oldest recognized volcan- palcotrench, as evidence for the possible position of a ism within the main part of the Colima graben [Allan, microplate boundary within downgoing plates at a 1986]. subduction zone. The Colima Graben-Manzanillo Graben extensional struc- The Rivera and Cocos plates, presumedto exist beneath tures are generally viewed as an upper plate response to di- this region, are unquestionably independent plates. vergence between the Rivera and Cocos plates at depth Magnetic anomalies from ridge segmentsnorth and south of within the subduction zone. However, there is continued the Rivera transform fault yield demonstrably different disagreementregarding the motion of these oceanic plates Pacific-Cocos and Pacific-Rivera poles [DeMets and Stein, relative to the overriding North America plate. Some 1990; Bandy, 1992]. However, the boundary between the authorshave suggestedthat subductionof the Rivera plate is Rivera and Cocos plates, extending from the eastern end of obliquely dextral and that subductionof the Cocos plate is the Rivera Transform Fault to the trench, is a complicated normal to the trench; this would imply that the Jalisco zone of deformation. The type and orientation of structures block (within the North America plate but above the Rivera vary along strike and do not yield direct informationon rel- plate) is moving a few millimeters per year right laterally ative plate velocities. The pole of relative motion obtained with respect to the rest of North America and that there may by combining Cocos-Pacific and Rivera-Cocosdata lies very be an incipient jump of the Pacific-North America spreading close to this boundary, but its position has a large uncer- center southeastwardinto the Colima Graben [e.g., Luhr and tainty. Any pole must be compatible with the observedex- Carmichael, 1981; Luhr et al., 1985; Allan, 1986]. An al- tension between the Cocos and Rivera plates at the eastern ternative model in which subductionof the Rivera plate is end of their mutual boundaryin the E1 Gordo Graben[Bandy, normal to the trench and subduction of the Cocos plate is 1992; Bandy and Pardo, 1994] (Figure 3). The configura- obliquely sinistral relative to North America implies that tion of the subductedslab of the Rivera and Cocosplates is the Jalisco block is fixed to North America but that a not well resolved. Seismicity suggeststhat the Cocos plate coastal block in the Michoac•n-Guerrero area is being dis- steepenstoward the NW, reaching a dip of 50ø and a depth placed left laterally at a velocity of a few millimetersper of 130 km beneath Volc•n Colima [Pardo and $udrez, year relative to North America [DeMets and Stein, 1990]. 1993]. However, existing data do not define the slab at Current models of regional plate velocities [DeMets and depth farther west beneath the main part of the Jalisco Stein, 1990; Bandy, 1992] permit both possibilities,within block; this part of the Benloft zone is seismically much the uncertainties. In either case the rate of subduction of quieter than the Cocos-North America Benioff zone. the Cocos plate is faster than the rate of subductionof the Becauseof the lower seismicity of the Rivera slab, it is not Rivera plate, so that the componentof Rivera-Cocosmo- known whether the Rivera plate slab connectsto the Cocos tion is dominantly strike slip along their mutual boundary. plate slab, nor is it known at what depth these slabs under- Regardlessof which model provesto be correct,there is a lie the MVB in the Jalisco region. striking correlation of upper plate structuraldisruption, as 1480 STOCK AND LEE: MICROPLATES IN SUBDUC-q'IONZONES well as upper plate volcanism and consequentsedimentation, The positionsof these boundariesrelative to North America with the position of the Rivera-Cocosboundary. and relative to geological observations on the Baja In this case the presence of an (extensional) microplate California peninsula can then be estimated by using plate boundary in the subduction zone has dramatic geological reconstructions. Three major ingredients are necessary in consequences.First, it correlateswith a major, topographic order to do this properly. First, one must remove the ex- low and series of basins associated with numerous active tension and strike-slip faulting in the Gulf of California, volcanoes of a variety of compositions in the overriding thereby returning Baja California to its early Miocene posi- plate [Allan et al., 1991]. Second, the connection of this tion relative to the shoreline of mainland Mexico. Second, structure to grabens in the MVB, at a type of "triple junc- one must move the Pacific plate into the appropriatepast tion," suggeststhat the differential movement of the sub- positionrelative to North America. Third, one must remove ducted plates may be causing the detachment and lateral the deformation that occurred between the continental mar- translation of a piece of the overriding North America plate gin and the stable interior of North America (principally in the forearc region. Basin and Range extension) since the time of interest. Although neither the configuration of the closed Gulf of California nor the reconstructions of the Pacific plate to NeogeneMicroplates in the Subduction Zones North America is precisely known, if several assumptions of Western North America are made and the uncertainties are included, then a reasonablereconstruction can be attempted[e.g., Stock and

Introduction Hodges, 1989]. We use these techniques to obtain the constraints, dis- The approach of the Pacific-Farallon ridge toward North cussedbelow, on the timing and positions of the microplate America and the creation of the Pacific-North America trans- boundaries near Baja California. The reconstructionspre- form margin resulted in the formation of two triple junc- sented in this paper use the same plate circuit and the same tions, the Mendocino (between Pacific, North America, and closure of the Gulf of California as did Stock and Hodges North Farallon fragment) and the Rivera (between Pacific, [1989]. However, here we use an updatedand more detailed North America, and South Farallon fragment) [Atwater, version of the offshore magnetic anomalies and the mi- 1970]. Continued fragmentationof the Farallon plate cre- croplatehistory [Lonsdale, 1991]. Also, Stock and Hodges ated various, short-lived microplatesin the subductionzone [1989] did not attempt to remove late Tertiary extension adjacent to both of these triple junctions, in turn, creating within Mexico. They noted that this yielded a clearly unac- other triple junctions of different types along the margin ceptable overlap of present Pacific seafloor and North (Figure 1). Magnetic anomaly patterns and bathymetric American continental crust in some of their reconstructions. data, preserved offshore coastal California and Baja The Neogene extensionalhistory of western Mexico is still California, constrain the history of these microplates and too poorly known to accuratelyreconstruct Baja California have been most recently summarized by Lonsdale [1991], and the Sonora-Sinaloa coast relative to the stable North Atwater [1989], and Atwater and $everinghaus [1989]. The America plate. Nevertheless, in our reconstructionswe 'microplates identified thus far include, from oldest to have removed the minimum amount of extension, in an youngest and from north to south, the Monterey ENE-WSW direction (130 kin), that is needed to eliminate (independent from --30 Ma to --19 Ma), Arguello (-20 to this overlap. This direction of extensionis perpendicularto -18 Ma), Guadalupe (--20 to --14 Ma), Magdalena (14 to 12 strike of Neogene normal faults in the Mexican Basin and Ma), and Rivera (--5 Ma to present)(Figure 1). Range province,many of which may have been active dur- These microplates did not always get completely sub- ing the time period of these reconstructions[Henry and ducted beneath North America. Dead ridges, symmetric Aranda-Gomez, 1992]. This introducesan additional uncer- magnetic anomaly patterns, and age of basementpenetrated tainty into the reconstructions(see Figure 4 caption),but it in ocean drill holes indicate that remnants of these mi- is still smaller than the overall geographicalextent of the croplatesbecame suturedonto the Pacific plate and are still featureswe are seeking (deformationrelated to presenceof preserved. When these microplates died, they may have microplateboundaries). In orderto reducethe uncertainties, been larger than the areas presently preserved offshore. it is vital to obtain the necessary geological data to Pacific plate ocean floor (and presumablyalso fragmentsof accuratelyaccount for the Tertiary extensionwithin main- dead ridges and oceanic crust belongingto microplates)may land Mexico, but that task is beyond the scopeof this pa- have been locally overridden by continental North America per. [Page and Brocher, 1993], although the importanceof such processesis strongly debated [e.g., Nicholson et al., 1994; Murray and Morro Fracture Zones (Southern R. G. Bohannon and T. Parsons, submitted manuscript, Boundary of the Monterey Plate) 1994]. In order to examine the possible effects of these mi- The southernedge of the Monterey plate was the Murray croplate boundaries on the tectonics of North America, we fracture zone (FZ) from the time of anomalies9-7 and the would like to know the location and timing of activity of Morro FZ thereafter,from -25 to -19 Ma (Figure 4a). The these structures in the subduction zone. These can be con- obliquityof thesefracture zones to the continentalmargin strainedby the magnetic anomaly patterns [Lonsdale, 1991; and to the coeval Pacific-Guadalupe FZs that were present Atwater, 1989; Atwater and $everinghaus, 1989; Klitgord farther south clearly shows that the Monterey plate was and Mammerickx, 1982; Mammerickx and Klitgord, 1982]. movingin a directiondifferent than the region southof the STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 1481

Morro FZ. If spreading were symmetric, then net While the eastern counterpartto the Shirley FZ was an ac- convergence is implied between the Monterey microplate tive microplate boundary, it would have intersectedthe sub- and Guadalupe (or Arguello) plate to the south. However, it duction zone along Baja California Sur, from about the lati- has been speculated [Lonsdale, 1991; Atwater, 1989] that tude of La Paz at -20 Ma (Figure 4a) to about the latitude of the Monterey plate was "stagnant" and possibly was Loreto at -12.5 Ma (Figure 4d) when Guadalupe-Pacific moving with the North America plate (see discussion by spreadingceased. Given the -150 km uncertaintyin the Nicholson et al. [1994] and R. G. Bohannon and T. Parsons plate reconstructions(parallel to the presentcoastline), any (submitted manuscript, 1994)). If so, it seems likely that geologiceffect on the overridingplate due to the presence the Monterey-North America boundary may have been of the Shirley FZ as a microplate boundary may be as far geometrically analogous to the modern Explorer-North north as the latitude of Bahia Concepci6n in southern Baja America boundary, a complex transcurrentfault zone where California (Figure 4). there was little to no subduction of the Monterey plate Magdalena-Cocos Plate Boundary beneath North America. In this case the microplate boundary might not induce deformation of the adjacent The Magdalena Rise, a series of en echelon bathymetric North America plate. The Monterey-Guadalupe(Arguello) highs present south of the Shirley FZ along the length of convergencemay thus have been accommodatedentirely by Baja California, has been interpreted as a dead ridge-trans- internal deformationof the Monterey plate. Any effects due form system of about the age of the Guadalupe Ridge, al- to the Monterey-Guadalupeboundary should be soughtin the though directly relevant age data are lacking (Figure 4d) Continental Borderland region between Ensenada and the [Lonsdale, 1991]. The difference in orientation between the Vizcaino Peninsula. Magdalena Rise and the Cocos-Pacific magnetic anomalies farther south suggeststhat the Magdalena Rise was formed Arguello FZ (Arguello-Guadalupe Plate Boundary) by spreadingof the Pacific plate away from a separatemi- croplate (Magdalenaplate of Lonsdale [1991]) from -14 Ma The southern boundary of the Arguello plate was the to 12.5 Ma (Figure 4d). Thus an active Magdalena-Cocos Arguello FZ from the time of anomalies6 to 5D (20 Ma to boundary would have been entering the trench during this 18 Ma, Figure 4a). Slight offset of isochrons 5E and 5D time interval. The Magdalena-Cocos-Pacifictriple junction immediately south of the Arguello FZ may be related to lay at the tip of a northward migrating offset north of the Arguello-Guadalupemotion, as the southernboundary of the Clarion FZ (Figure 4d). Thus the Magdalena-Cocosbound- Arguello microplate jumped farther south during the last few ary migrated north relative to the North America plate faster million years of its lifetime. Arguello-Pacific spreadingwas than did the Pacific plate. However, it remained entirely intermediate in direction and rate between Cocos-Pacific and south of the latitude of Baja California. It would have inter- Monterey-Pacific spreading. The configuration of the sected the Middle America Trench near Jalisco or possibly youngest known Arguello-Pacific anomalies (5E and 5D, opposite the region that later became the mouth of the Gulf 18.8 to 17.3 Ma) suggeststhat Arguello, also, was becom- of California. ing stagnant(moving nearly with North America) [Atwater, Northern Edge of the Guadalupe Plate (Pacific- 1989]. Its southern boundary would probably have been en- Guadalupe Plate Boundary) tering the subductionzone offshore northernBaja California The northern edge of the Guadalupe plate must have been (Figures 4a and 4b). However, by analogy with the modern a microplateboundary from 20 to 12.5 Ma, but the precise Explorer plate, at this point there may have been virtually geometry and location of this boundary relative to North no Arguello-North America convergence, so that this mo- America and Baja California probably varied (Figure 4). No tion between Arguello and North America may have been dead oceanic ridge has been identified between the Arguello largely transcurrent. FZ and the northern end of the Soledad Ridge (Figures 4c and 4d). Rather, the youngest magnetic anomalies trend Shirley FZ (Guadalupe-Cocos Plate Boundary) obliquely into the trench, gettingyounger southward. These The Guadalupe-Cocosplate (Figure 1) had probably bro- represent the maximum ages of cessation of spreading ken into northern (Guadalupe) and southern (Cocos) frag- [Atwater, 1970]. The youngest possible time of cessation ments by the time of anomaly 6 (20.45 Ma on the Decade of spreading could be a bit younger if magnetic anomalies of North American Geology (DNAG) timescale), judging have been overrun by the continent during later conver- from differences in spacing and orientation of anomalies 6A gence. The northernmostpossible position of this bound- and younger across the trace of the Shirley fracture zone ary was the Arguello FZ at 20 Ma (Figure 4a). The boundary [Lonsdale, 1991]. The ridge segmentsbetween the Pacific may have moved southwardwith time relative to the Pacific plate and the Guadalupeplate rapidly rotated anticlockwise plate, and it could have been as far south as the northern into approximateparallelism with the trench, possibly due end of the Soledad ridge at 12.5 Ma when spreadingceased to pivoting subduction as described by Menard [1978] (Figure 4d). Thus this boundary could have lain anywhere caused by stronger slab pull in the south due to its older from Ensenada southward and might have stayed in a rela- plate age. This reoriented the fracture zones north of the tively constant location, or it might have migrated south- Shirley FZ counterclockwise 10-15 ø and increased their ward as far as the Vizcaino Peninsula by 12.5 Ma. curvature(i.e., the local Euler pole moved•1oser to the re- The Guadalupe-Pacific-NorthAmerica triple junction pre- oion• relative to those south of the Shirley FZ. During this sent in this region, after the Arguello microplate became at- time the easterncounterpart of the Shirley FZ may have had tached to Pacific at -17 Ma, would have connected north- a component of extension across it. ward with the Pacific-North America boundary,possibly rep- 1482 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES resented by a set of "fossil spreading centers" tentatively 5). They correspondto the 17 to 22 Ma andesitc belt of identified within the Southern Borderland Rift, from the Gastil et al. [1975] and in southernBaja California they are Soledad Ridge northward [see Crouch and Suppe, 1993]. If often referred to as the "Comondfi Formation." (See they reflect Pacific-North America motion, then they indi- Hausback [1984] for a detailed discussion of the strati- cate that the Guadalupe plate was south of this position at graphic nomenclatureof volcanic and sedimentaryrocks of the time they were active. However, they have not been this age in Baja California Sur.) In Baja California Sur dated. some volcanic vents crop out along the eastern side of the From these data it appearsthat the easterncounterparts of peninsula; others are inferred to have lain farther east and the Arguello FZ, the Shirley FZ, possibly the GuadalupeFZ, are now offshore. Numerous K-Ar ages constrain the vents and structuresin the zone between the Arguello FZ and the to be from -24 to 12 Ma in age [Hausback, 1984; Sawlan northern Soledad Ridge may all have been active as mi- and Smith, 1984; Sawlan, 1991]. These rocks range from croplate boundarieswest of Baja California at various times calc-alkaline andesitc to rhyolite, but are dominantly an- during the Miocene. However, we find no evidence that desitc, and have typical subduction-relatedchemical signa- they induced any throughgoing deformation zones in the tures [e.g, Sawlan, 1991]. No known rift-related volcanic overriding plate between the trench and the volcanic arc rocks of this age have been described. (that is, within the crust of Baja California before it sepa- In southernBaja California these rocks are generally flat rated from the North America plate). After these plates lying and undeformed, except near the Gulf of California "died" by becoming attached to the Pacific plate there were where they have been affected by normal faulting and local, strong effects along the continental margin in the vicinity soft sediment deformation [e.g., Hausback, 1984]. of the old trench (the California Continental Borderland) but Regional geological studies [McLean, 1988; McLean et al., not extending east into the region of the old volcanic arc 1985, 1987; $awlan and Smith, 1984; Fenby and Gastil, [Nicholson et al., 1994; R. G. Bohannon and T. Parsons, 1991] show this lack of deformationclearly and make it dif- submitted manuscript, 1994]. We describe the principal ficult to postulate the presence of any throughgoing evidence for geological stability of much of the Baja Miocene fault zones or grabenscrossing the Baja California California Peninsula during Miocene time, as follows: peninsula at this latitude. The only major throughgoing continuity and lack of deformation of the Miocene structure is the north striking graben along the La Paz fault subduction-relatedvolcanic are and continuity and lack of zone [Hausback, 1984] which is attributedto left slip during deformation of pre-Miocene sedimentary rocks between the late Miocene extension at the mouth of the Gulf of volcanic arc and the trench. California [Lonsdale, 1991]. Elsewhere, the early-to-middle Miocene volcanic belt of southern Baja California is sur- Guadalupe FZ (Boundary Within the Guadalupe prisingly flat lying and undeformed. Plate) In northern Baja California the rocks of the early to middle Miocene volcanic arc do not crop out continuously. Pacific-Guadalupespreading ceased along the Guadalupe North of 28øN, they are only discontinuously exposed, Ridge, betweenthe Shirley FZ and the GuadalupeFZ, during chron 5A (12.5 Ma on the DNAG timescale, Figure 4d). owing partly to erosional removal and to burial by younger volcanic rocks. Dacite and andesitcvents in this age range This ridge death caused a 220- x 220-kin fragment to be- are found beneath late Miocene to Pliocene rocks at come sutured to the Pacific plate. The Soledad Ridge, an- 30ø16.5'N, 16 Ma (A. Martin-Barajas et al., Arc-rift other small (120 km long) topographic high north of the transition volcanism in the Puertecitos Volcanic Province, GuadalupeFZ, is interpretedas a similarrifted volcanicridge northeastern Baja California, Mexico, submitted to that may have died at the sametime (Figure4d) [Lonsdale, Geological Societyof America Bulletin, 1994) and at 30ø28'N, 1991]. The magneticanomalies immediately adjacent to the 17-20 Ma [Stock et al., 1991]. Other dacite-andesitevents of Soledad Ridge are difficult to interpret and look different unknown but probably similar age occur at 31ø27'N (J. Lee, from those surrounding the Guadalupe Ridge to the south. unpublishedmapping, 1993) (Figures 4b and 4c). Relict This suggests a different spreading history across the volcanic necks farther west at this latitude, such as Pico boundaryor to a zone of diffuse deformationwithin the re- Matomi and Pico Los Heme, are speculatedto also be within gion north of the GuadalupeFZ. If the GuadalupeFZ or a diffuse zone to the north of it had been active as a mi- this age range [Gastil et al., 1975] but have not been dated. GuadalupeFZ, possibly correspondingto anothermicroplate Related mass flow depositsand epiclastic rocks, mapped as fluvial conglomerates by Gastil et al. [1975] and labeled croplate boundary,then it would have been only for a rela- "Comondfi Formation" by Dorsey and Burns [1994], crop out tively short time (1-2 m.y.) and it would have intersected in the Sierra Ju/rez area (locality B, Figure 5) [Miller et al., the subduction zone somewhere within -100 km (parallel to 1992; Lee and Miller, 1994] and from the southern end of the margin) of the Vizcaino Peninsula. the Sierra San Pedro M•rtir (locality C, Figure 5) [Stock, 1989] to 29ø40'N (locality A, Figure 5) [Gastil et al., 1975]. Geologic Stability of Baja California These volcaniclastic sedimentaryrocks are extensive on the During Miocene Time Baja California peninsulaand can thus be used as a guide to Geometry and Geochemistry of the Early to Middle the location of the early to middle Miocene volcanic belt Miocene Volcanic Arc in Baja California and for recognition of syn- or postvolcanicdeformation. Calc-alkaline volcanic rocks and related sedimentary Insofar as is known from existing geological mapping in rocks of early to middle Miocene age are exposed along Baja California, the western exposuresof these arc-related much of the length of the Baja California peninsula (Figure deposits are not significantly deformed along the length of STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 1483

245 • 250 ø

U.S.A.

MEXICO 23-16 Ma andesite belt

30 ø 30 ø

24-11 Ma andesite

Early to middle Miocene volcanic arc

Clastic rocks from the early to middle Miocene andesitic arc , 25 ø 25 ø

marineUpper Cretaceousand fluvial rocks(postbatholithic) /'-••..

fluvialPaleocene-Eocene strata (flat-lying)marine and "•

245 ø 250 ø

Figure 5. Distributionof pre-Mioceneflat-lying strata on the Baja CaliforniaPeninsula [from Gastil et al., 1975;Fenby and Gastil, 1991]; early to middle Miocenevolcanic rocks of the andesitebelt, inferredto be related to subduction[from Sawlan, 1991; Gastil et al., 1979]' and early to middle Miocene sedimentaryrocks of the clasticapron of the andesiticarc [from Gastilet al., 1975;Dorsey and Burns, 1994; Stock, 1993b]. Localities A, B, and C are discussedin the text. The lack of significant deformationrecognized in any of theseunits west of the Main Gulf Escarpmentprecludes the existence of any major zonesof early to middleMiocene deformation reaching from the volcanicarc westwardto the former trench. 1484 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES the peninsula(some 1500 km). They are generallyflat ly- oped on the Baja Californiapeninsula owing to thesemi- ing west of the Gulf of California extensional province croplateboundaries, then it shouldbe visible in the exist- [e.g., Dorsey and Burns, 1994]. Their outcrop pattern is ing geologicaldata. The lack of any suchstructure and the fairly linear in map view, implying no big excursionsof final spreading histories of these microplates with the contemporaneousvolcanism to locationsmuch closer to the Pacific plate resemblethe situationat the Juan de Fuca- trench. Significant faulting and tilting has affected them Gorda boundaryor the Juande Fuca-Explorerboundary, not within the Gulf of California extensional province, but the Rivera-Cocosboundary. The boundariesbetween these where this deformation has been dated, it postdates subduc- microplatesmay have been,in somecases, zones of diffuse tion and is attributed to extensional faulting accompanying deformationand high strain within the oceanicmicroplates, the opening of the Gulf of California [e.g., Stock and rather than discrete structural boundaries. (Collection of fur- Hodges, 1989; Lee and Miller, 1994]. ther marine geophysicalinformation over the last remaining seafloor of these microplatesmight help to resolve this is- sue.) Paleocene to Eocene Sedimentary Rocks Along In addition, the direction of the last stages of spreading the Pacific Coast of the Baja California of the Arguello and Montereymicroplates from the Pacific Peninsula plate suggeststhat they were adoptingthe motion of the largerNorth Americaplate [Atwater, 1989; R. G. Bohannon Tertiary sedimentaryrocks crop out west of and/or be- and T. Parsons, submittedmanuscript, 1994], in much the neath the depositsof the Miocene volcanic arc along much same manner that the Explorer plate is moving nearly with of the western side of the northern Baja California North America at present [Rohr et al., 1993]. If so, Peninsula. These Paleocene to Eocene [Gastil et al., 1975] subduction of these two plate fragments beneath western marine and fluvial deposits are flat lying to very shallowly North America might have effectively stopped when these west dipping (<5 ø) and unconformablyoverlie Cretaceous microplates were born and the spreadingdirection changed, rocks. These rocks are found continually along the coast several millions of years before the microplates actually from 28øN to 29ø30'N and in a few isolated patches farther ceased to exist. Presumably, if subduction of these plate north (30øN, 31øN) (Figure 5). These flat-lying rocks can fragmentshad stagnated,then their mutual boundary would be followed eastward as far as the Main Gulf Escarpment. In not have gone very far down the trench and hence would not between these exposures and north to the U.S. border there have been likely to cause much deformation of the are continuous exposures of postbatholithic, Upper overlying North America plate. Cretaceous, marine and fluvial strata [Gastil et al., 1975]. The orientation and magnetic anomaliesof the Guadalupe Within these coastal exposures there are no mapped struc- Rise indicate that the Guadalupeplate did not adopt the mo- tures that could be attributed to Miocene grabens at high tion of the North America plate. Rather, it was still con- angles to the west coast of the Baja California peninsula. verging with the North America plate at the time Pacific- There are no major thicknesses of Miocene sedimentary Guadalupe spreading stopped. Nevertheless,the differential rocks, nor is there any evidence of significant deformation motion across the Shirley FZ (the southernboundary of the of the older rocks. Thus we conclude that in northern Baja Guadalupeplate) may have been fairly small, except for the California the available geological evidence supportsstruc- last few million years of its existence,when the ridge orien- tural coherence of the peninsula between Ensenada and La tations north and south of the Shirley FZ appear to diverge. Paz. We can find no evidence for any major structural During this stage of plate motions we might expect some breaks extending from the early to middle Miocene volcanic effect of the Shirley Transform's eastern counterpart on the arc into the region of the former trench. geology of the overriding plate. None is known, however. This raises the possibility that (1) the eastern counterpartof the Shirley Transform correspondedto a zone of diffuse de- Discussion formation between the subducting Guadalupe and Cocos/Magdalenaplates or (2) the easterncounterpart of the Shirley Transform fault was a finite structure, but it swept Plate Configurations During Waning Subduction along the subductionzone at a rate that was too fast to af- The geological data from the Baja California peninsula fect the overriding plate. suggest that the microplate boundaries present west of Baja California during the final stagesof Neogene subductionhad Styles of Microplate Death no major effect reaching into the interior (that is, from the trench to the volcanic arc) of the overriding plate. There All of the microplates discussed above (Monterey, appears to be no structural or deformational pattern that Arguello,Guadalupe, and Magdalenaplates) became attached would correlate with these boundaries; rather, the Paleocene- to the Pacifi• platewhen their ridge boundaries with the Eocene sedimenta.ry rocks along the west coast of the Pacific plate stopped spreading. By this process the peninsula are surprisinglyundisturbed, and within the middle Pacific-North America strike-slip margin jumped progres- Miocene volcanic arc and related sedimentary rocks to the sively southward. After becomingsutured to the Pacific west, no major structural discontinuities have been recog- plate some of the former microplatesmay have dragged nized. The level of geological knowledge in many of these piecesof continentalmaterial with them [Nicholsonet al., areas is still reconnaissance in nature, but nevertheless, if a 1994; R. G. Bohannon and T. Parsons, submitted structure analogousto the present Colima Graben had devel- manuscript, 1994], transferring bits of continental North STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES 14 85

America onto the Pacific plate and moving the Pacific-North data on relative plate and block motions from the Rivera- America strike-slip zone east with time. However, this de- Jalisco-NorthAmerica region. However, the available data formation was limited to areas west or north of the present suggestthat the motion of Rivera is intermediatebetween Baja California Peninsula (i.e., the ContinentalBorderland). that of North America and that of Cocos, permitting the The peninsula appears to be quite coherent and intact. possibility that the Rivera plate is slowly becoming at- Deformation in the borderlanddid not reach all the way from tached to the North America plate. the trench into the volcanic arc but was restricted to a zone Whether the Rivera plate can fully attach and then remain within --100 km of the trench (e.g., R. G. Bohannon and T. attachedto the North America plate for any length of time Parsons, submitted manuscript, 1994). will depend on the interplay between plate boundaries and None of these microplates became attached to the North relative plate motions in the region of the present Rivera- America plate. This is especially surprising in the case of Pacific boundary (the Rivera Transform fault, Figure 3). If the northern (Arguello and Monterey) plates, which were this fault is reasonably optimally oriented parallel to or moving nearly with North America at the time they died (see transtensive with respect to future North America-Pacific also R. G. Bohannon and T. Parsons[submitted manuscript, motion, then the Rivera plate may be able to suture to 1994]). These relations could be explained if during the North America. However, if this boundary becomes trans- final stagesof existence of these microplates, Pacific-North pressive,then the resultingdeformation and shearmay rede- America motion were obliquely compressiverelative to the tach the Rivera fragment from the North America plate and Monterey-Pacific and Arguello-Pacific transform faults. strand it permanently as part of the Pacific plate. Such geometry could have sheared off these young oceanic If the Rivera plate is able to attach to the North America plates at the continental margin, so that these plates ceased plate, then it is extremely important as a possible analogy spreadingrelative to and became suturedto the Pacific plate. to past plate motions. The setting where a ridge-transform- However, such a scenario must remain highly speculative trench triple junction is migrating laterally along a trench until the Pacific-North America plate motion history is and the trench is getting shorter provides a mechanismfor known at resolution high enough to compare directions of suturingoceanic plate material to the formerly, entirely con- relative motion at ~l-m.y. time steps. This is not possible tinental, overriding plate. In particular, it provides a mech- at the present time. anism by which the mouth of a continental rift (in this case These observationslead to speculationabout the eventual the Pacific-North America rift in the Gulf of California) can fates of the Gorda microplate (or South Gorda deformation contain magnetic anomalies that were not formed by the zone, Figure 2) and of the Rivera plate (Figure 3). Much same plate pair but, rather, by a different spreadingsystem will depend on the future velocity of the Pacific plate rela- (in this case, Pacific-Rivera spreading) (Figure 3). tive to the surrounding plates. The Gorda microplate is al- ready deforming because its southern boundary, the A similar plate configurationwith microplate tectonicsin Mendocino Transform, is constrained by the geometry of a shorteningsubduction zone appearsto have existedduring the larger Pacific plate to the south. If the Gorda microplate early rifting of the Campbell Plateau-Chathamrise region does become fixed to North America, then the Gorda ridge from West Antarctica during late Cretaceoustime. The Aluk will be free to reorient into the correct configuration for plate was being subductedbeneath the west Antarctic mar- Pacific-North America spreading, but the southern boundary gin, and the Pacific-Aluk ridge was approachingthe trench, of the Gorda microplate (the Mendocino transform fault) will in a manner analogousto that of the Cocos plate and the then become transpressive because of its orientation with East Pacific Rise in the Middle America trench today. In the respectto the Pacific-North America plate motion vector. If wake of the ridge-trench interaction, continental material the Pacific-North America plate motions maintain their was transferred from West Antarctica to the Pacific plate overall orientation, then it seems more likely that this (analogousto the transfer of Baja California to the Pacific small deforming region would be shearedoff, perhaps along plate). Despite the fact that the continentalmaterial adja- the paleotrench, in an event that would abruptly extend the cent to this rift system belongs to the Pacific and Antarctica San Andreas fault from the Mendocino triple junction along plates the oldest magnetic anomalies in the mouth of the the present trench up to its junction with the Blanco fracture rift system were not formed by Pacific-Antarctic spreading zone. Once such shearingoccurs, the Gorda fragment would but, rather, by spreading between the Pacific plate and a end up attached to the Pacific plate, by ridge death, just as fragment that probably broke off the Aluk plate (labeled in the case of the Monterey and Arguello microplates. "Bellingshausen" by Stock and Molnar [1987]). The As noted earlier, the Rivera plate may be strongly cou- Bellingshausenplate may have continued to converge with pled to the overriding Jalisco block. Others [e.g., Luhr and West Antarctica, even after rifting began, analogous to the Carmichael, 1981] have suggestedthat both of these blocks modern Rivera plate within the Middle America trench. are in the processof becomingattached to the Pacific plate Further studies of the region of the Bellingshausen plate as the East Pacific Rise jumps into the North American con- will help to clarify the degree to which this analogy is tinent at the location of the Colima Rift. This would con- valid. From our detailed knowledge of the microplate flict with the inferences from global plate motions that kinematics along the western margin of the North America Rivera-NorthAmerica velocity is probablysmall [Bandy and plate we expect that the West Antarctic margin may, in Pardo, 1994; DeMets and Stein, 1990] and that, in fact, future studies, reveal evidence for continued breakup of the Rivera may be stagnating and slowly becoming attached to Aiuk plate and possible preservatioraof microplate remnants the overriding North America plate. Final reconciliation of (as well as their geological effects on the continent) along these two points of view must await more precise kinematic the former trench there. 1486 STOCK AND LEE: MICROPLATES IN SUBDUCTION ZONES

Conclusions fiat lying and undeformed. More geologicaldata may reveal some subtle details, but existing data suggest that these 1. Subduction of active microplate boundaries of non- plate boundariesdid not have any major effect on the geol- ridge type sometimes correlates with deformation of the ogy of the overridingplate. Thus they were more likely to overriding continental plate (e.g., formation of grabens), have been of the Juan de Fuca-South Gorda or Juan de Fuca- anomalousvolcanism above the microplate boundary in the Explorertype than of the Rivera-Cocostype. If internalde- subductionzone, and spatial or geochemical variations in formation occurred within these microplates, then the exist- the subduction-relatedvolcanic arc. Some active microplate ing level of marine surveyingis still too sparseto identify boundaries(e.g., Rivera-Cocos)exhibit many or all of these it. effects, whereas others (e.g., Juan de Fuca-Gorda, Juan-de- 3. The ultimate fate of the microplatesin the subduction Fuca-Explorer)exhibit none of them. We recognizethese as zone may be strongly dependenton local plate geometries two end-member plate tectonic settings as follows' mi- relative to changesin motion between the major plates of croplates whose boundariesinduce substantialdeformation the region. Although all of thesemicroplates died by trans- of the overlying plate and microplates whose boundaries fer onto the Pacific plate, the modern Rivera plate may be leave no apparent trace in the geology of the overlying an exception;it may be in the processof becomingtrans- plate. The difference between these may be due to the ferred onto the North America plate. If so, this is the only trench-parallelvelocity componentof the microplatebound- known modern example of a process, inferred elsewhere ary and to the degree of coupling betweenthe variousplates from the geologicalrecord (e.g., West Antarcticmargin and in a given region. It may also dependon the size and age Bellingshausen-Pacificspreading), by which a microplate distribution of the microplate being subductedsince a very generatesmagnetic anomalies in the mouth of a rift while small microplate with insufficient slab-pull force may stag- still being subductedbeneath one of the boundingcontinen- nate or attach itself mainly to the overriding plate. tal platesof the rift. If this microplatelater stopssubduct- 2. The Monterey, Arguello, Guadalupe, and Magdalena ing, then the seafloorit createdmay be later mistakenfor microplates were present in the subductionzone west of seafloorgenerated by spreadingbetween the two surrounding Baja California at varioustimes from 20 to 12.5 Ma. Their continentalplates during the early stagesof rifting. past locations and relative motions can be constrainedby global plate reconstructions. Paleocenethrough Miocene Acknowledgments.This workwas supported by NSF grantEAR-92- rocks are extensively exposedin westernBaja California in 96102. We thank ik>b Bohannonand Don Argus for helpful reviews. the region above the former microplate boundariesand are Contribution5437, CaltechSeismological Laboratory.

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