ABSTRACT

During the late Miocene east-northeast–di-rected extension in the Gulf of California ex-tensional province, the western rift margin innortheastern Baja California, Mexico, wassegmented at the northwest-striking Matomíaccommodation zone. The accommodationzone passed through the northern PuertecitosVolcanic Province and separated pre–6 Maextension to the north from the unextendedregion to the south. Pre–6 Ma northeast-side-down displacement is documented across theaccommodation zone, which may have under-gone dextral oblique-slip motion. The riftmargin migrated westward during the latestMiocene or Pliocene, bypassing accommoda-tion-zone structures and incorporating thePuertecitos Volcanic Province into the regionof Gulf extensional province deformation.East-northeast– to east-directed extensionaldeformation is at least post–6 Ma in the north-ern Puertecitos Volcanic Province and defor-mation currently continues. Pliocene changesin the Gulf of California spreading center sys-tem may have triggered incorporation of thePuertecitos Volcanic Province into the Gulf ex-tensional province as recently as 2–3 Ma.Paleomagnetic analyses of 6.3–6.6 Ma pyro-clastic flow deposits show no consistent evi-dence for rotational deformation, althoughminor (~10°–15°) clockwise rotation is possi-ble given anomalous declination directionsrecorded at some sites. Comparisons withpaleosecular variation models and the lateMiocene paleopole for stable North Americaimply no statistically significant rotation rela-tive to geomagnetic north, although minor(~10°) clockwise rotation is permissible given

uncertainties. Geologic relationships showthat pre–6 Ma accommodation-zone struc-tures identified in this study did not mark thesouthern boundary of later rotational defor-mation documented to the north. The bound-ary of Pliocene to Holocene rotations may be abroad, diffuse zone of extensional shear en-compassing the northeastern Puertecitos Vol-canic Province, which accommodated smallrotations.

Keywords: accommodation zones, Baja Cali-fornia, extension tectonics, paleomagnetism,plate divergence, volcanics.

INTRODUCTION

Late Cenozoic extension associated with Pa-cific–North America plate interactions is distrib-uted over a broad region in much of westernNorth America (e.g., Zoback et al., 1981; Henry,1989). In Mexico, the Gulf extensional provinceencompasses the region of high-angle normalfaults bordering the Gulf of California (Fig. 1, in-set) and has been the location of a divergent por-tion of the plate boundary since at least ca. 3.5 Ma(anomaly 2A) when seafloor spreading began inthe mouth of the Gulf of California at a full platerate of ~45–50 mm/yr (e.g., DeMets, 1995). De-tails are still emerging regarding the earlier historyof Gulf extensional province deformation and itsrelationship to the evolving plate boundary andassociated southern Basin and Range extension.

During the late Miocene, Pacific–NorthAmerica plate motion was probably partitionedbetween “proto-Gulf” normal faults accommo-dating east-northeast–directed extension (Stockand Hodges, 1989) and dextral, offshore trans-form faults, such as the Tosco-Abreojos and SanBenito faults (Fig. 1, inset; Spencer and Normark,1979). Gans (1997) offered an alternative inter-pretation in which northwest-striking transformfaults within the Gulf of California, rather than

west of the peninsula, accommodated plate mo-tion. The latter scenario requires ~500 km ofnorthwest-southeast–directed right-lateral dis-placement within the Gulf extensional provinceas constrained by late Cenozoic offsets acrossthe San Andreas fault system to the north (e.g.,Atwater, 1989), whereas the strain-partitioningmodel places half of this displacement along theoffshore transform faults. Geologic relationshipsin Baja California suggest a middle to lateMiocene age (ca. 12–15 Ma) for the initiation ofextension along the western rift margin of theGulf extensional province (e.g., Gastil et al.,1975; Stock and Hodges, 1990; Lee et al., 1996).East- to northeast-directed extension continuedthrough Pliocene time in much of northeasternBaja California (see Nagy and Stock, 2000, for adetailed extension history of northeastern Baja).

Field studies that identify the timing, amount,and direction of extension in the Gulf of Califor-nia region are necessary to clarify the contributionof such extension to overall plate boundary defor-mation. In this study I summarize the deformationhistory within the northern Puertecitos VolcanicProvince located on the western Gulf extensionalprovince rift margin in northeastern Baja Califor-nia (Fig. 1). Detailed field mapping (Nagy, 1997)within volcanic deposits spanning pre–17 to 6 Ma(Nagy et al., 1999) provide the basis for struc-tural interpretations. Paleomagnetic analysis of6.3–6.6 Ma pyroclastic flow deposits investigatesthe possibility of relative rotations between fault-bounded structural blocks and provides a compar-ison with regions a few kilometers to the north,where Pliocene to Holocene vertical-axis blockrotations are documented (Strangway et al., 1971;Lewis and Stock, 1998a; Stock et al., 1999).

GEOLOGIC SETTING

The sharp western margin of the Gulf exten-sional province is exposed along the eastern sideof the Baja California peninsula (Main Gulf Es-

857

Extensional deformation and paleomagnetism at the western margin of theGulf extensional province, Puertecitos Volcanic Province, northeastern BajaCalifornia, Mexico

Elizabeth A. Nagy* Division of Geological and Planetary Sciences, California Institute of Technology,Pasadena, California 91125, USA

GSA Bulletin; June 2000; v. 112; no. 6; p. 857–870; 9 figures; 2 tables.

*Present address: Department of Earth Sciences,Syracuse University, Syracuse, New York 13224-1070,USA; e-mail: eanagy@syr.edu.

carpment in Fig. 1, inset) and separates the Creta-ceous Peninsular Ranges batholith to the westfrom downdropped fault blocks to the east ofbatholithic rocks overlain by Cenozoic subduc-tion- and rift-related deposits. In much of north-eastern Baja California the rift margin is repre-sented by the 100-km-long, east-dipping, SanPedro Mártir fault (Fig. 1). Although as much as5 km of normal separation occurs on the fault(Gastil et al., 1975), displacement decreases south-

ward to ~800 m in southern Valle Chico (Stockand Hodges, 1990) at the northern margin of thePuertecitos Volcanic Province (Fig. 1), a lateMiocene–Pliocene ignimbrite field. The San Pe-dro Mártir fault has not been recognized south ofValle Chico; the western edge of the Gulf exten-sional province is thus poorly defined within thePuertecitos Volcanic Province. Northeast- to east-directed extension began 12–6 Ma along majoreast-dipping normal faults such as the San Pedro

Mártir and Sierra San Felipe faults (e.g., Gastilet al., 1975; Stock and Hodges, 1990; Lewis andStock, 1998b). Pre–6 Ma deformation south of Ar-royo Matomí (Fig. 1) is documented for the firsttime in this study (see also Nagy, 1997).

Post–6 Ma east-northeast– to east-directed ex-tension continued along structures north ofArroyo Matomí as well as on approximatelynorth-striking normal faults in the PuertecitosVolcanic Province (Dokka and Merriam, 1982;Lewis and Stock, 1998b; Nagy et al., 1999).Faulted 3 Ma volcanic rocks in the easternmostPuertecitos Volcanic Province may have under-gone less extensional deformation than the 6 Mavolcanics (e.g., Stock et al., 1991; Martín-Barajaset al., 1995). Northeast-striking, sinistral strike-slip faults also displace 3 Ma volcanic rocks inthe Sierra San Fermín (Lewis and Stock, 1998b).Paleomagnetic studies show that the Sierra SanFermín and Santa Rosa basin have undergone~30° of Pliocene to Holocene vertical-axis clock-wise rotation relative to Mesa Cuadrada in thesouthern Sierra San Felipe (Fig. 1; Strangwayet al., 1971; Lewis and Stock, 1998a; Stock et al.,1999). Recent scarps along the San Pedro Mártirfault and the dextral, strike-slip Valle de SanFelipe fault indicate that these structures are ac-tive (Gastil et al., 1975; Grover et al., 1993).

The west- to northwest-striking Matomí accom-modation zone was first hypothesized near ArroyoMatomí on the basis of changes in the style andamount of faulting to the north and south (Dokkaand Merriam, 1982; Stock and Hodges, 1990), andits presence may have influenced Pliocene sedi-mentation rates in the region (Stock et al., 1996;Martín-Barajas et al., 1997). The Matomí accom-modation zone may also mark the southern bound-ary of rotational deformation (Lewis and Stock,1998a). Nevertheless, field evidence for an accom-modation zone south of the Sierra San Fermín hadnot been identified prior to this study.

EXTENSIONAL DEFORMATION IN THESANTA ISABEL WASH REGION

Lithology and Extensional Structures

Faulting and erosion expose as much as 700 mof volcanic deposits along the northern margin ofthe Puertecitos Volcanic Province in the Santa Is-abel Wash study area in the Sierra Santa Isabel(Fig. 1). Geologic mapping (scale 1:20000), litho-logic and petrographic study,40Ar/39Ar geochron-ology, and electron microprobe analyses of pheno-cryst compositions define 21 lithologic units thatare combined into six Miocene volcanic groupsand one pre-Miocene group (Fig. 2). Lithologic,stratigraphic, and geochronologic details given byNagy (1997) and Nagy et al. (1999) are summa-rized here (oldest to youngest): pre-Miocene

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858 Geological Society of America Bulletin, June 2000

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Figure 1. Geologic map (simplified from Gastil et al., 1975) of a portion of northeastern BajaCalifornia, Mexico, showing principal localities and faults discussed in text. Ball and bar are ondownthrown side of normal faults; arrows indicate sense of slip on strike-slip faults; faults aredotted where concealed. The Matomí accommodation zone (A–A′, after Stock, 1999; and A′–A′′,after Nagy, 1997, and this study) separated a region of pre–6 Ma east-northeast–directed exten-sion to the north from an unextended region to the south. Abbreviations: MC—MesaCuadrada; UEFZ—Ultima Esperanza fault zone. Inset: GEP—Gulf extensional province; G—Gonzaga Bay; P—Puertecitos.

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Geological Society of America Bulletin, June 2000 859

(probably Paleozoic) metasedimentary rocks in-truded by granitoids related to the PeninsularRanges batholith (group 1); pre–17 Ma subduc-tion-related volcaniclastic breccias, debris flows,lava flows, and pyroclastic flow deposits (group2); ca. 15–17 Ma dacitic and mafic lava flows,which are also probably subduction-related(group 3); the synrift ca. 12.5 Ma Tuff of San Fe-lipe (group 4); minor andesitic volcanism ca. 9 Ma(group 5); as much as 400 m of 6.3–6.6 Ma pyro-clastic flow deposits or ash-flow tuffs (hereafter re-ferred to simply as tuffs), and an intervening maficlava flow, which filled irregular topography to pro-duce the plateau-like, upper surface of the Puer-tecitos Volcanic Province (group 6); and as muchas 500 m of rhyolite (ca. 6 Ma) and andesite (un-dated) lava flows (group 7). In addition to modernarroyo sedimentation in Santa Isabel Wash and Ar-royo Oculto, older, high-standing alluvium occursalong the margins of the washes and surroundsisolated hills in the north. Modern playa depositswithin closed basins are up to 2 km in diameter inthe washes as well as on the volcanic plateau.

Extensional deformation is classified as pre–6and post–6 Ma based upon the relationship be-tween faults and the 6.3–6.6 Ma series of tuffs.The majority of structures are north-striking, east-and west-dipping, high-angle normal faults. Pla-nar fault surfaces and striations are rare; thus insome cases strike-slip or oblique-slip faulting mayhave produced apparent normal displacement.There are 14 faults that preserve measurable sur-faces and 3 that preserve slickenlines (Fig. 3). Thebest indicators of tilt in the region are within the6.3–6.6 Ma tuffs. Thick Tmr3 deposits generallyfilled preexisting topography and preserved a pla-nar upper surface; overlying tuffs are roughly tab-ular in shape. In general these uppermost tuffs arehorizontal to slightly west dipping (≤10°).

Pre–6 Ma Deformation

With one significant exception pre–6 Ma faultsare rare in Santa Isabel Wash. East- to east-north-east–striking, north-dipping normal faults, whichdo not offset the overlying 6.3–6.6 Ma tuffs, cut

pre–15 Ma rocks (groups 2 and 3) and in one lo-cality 12.5 and 9 Ma rocks (groups 4 and 5, re-spectively); thus deformation is post–9 Ma insome places but possibly as old as 15 Ma in oth-ers. About 100 m of north-side-down normal sep-aration occurred across two east-striking faults(I12 in Fig. 2); the amount of offset is otherwisepoorly constrained.

Thickness and elevation variations of the6.3–6.6 Ma tuffs imply the presence of a discontin-uous, yet significant, pre–6 Ma northeast-facingtopographic slope across the entire area. The ap-proximate position of two subparallel, northwest-striking topographic breaks is shown in Figure 2(A′–A′′ and B′–B′′) based upon the geometry of theonlapping tuffs. Specifically, individual tuffs occurat lower elevations and are an order of magnitudethicker northeast of the slopes relative to depositson the volcanic plateau. For example, basal eleva-tions of Tmr4 and Tmrec are 350–390 m and240–320 m lower, respectively, to the northeast ofA′–A′′ relative to the southwest, and the tuffschange from 1–3 m thick southwest of A′–A′′ to at

1 km

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Figure 2. Simplified geologic map of Santa Isabel Wash in the northern Puertecitos Volcanic Province. The 40Ar/ 39Ar ages were determined bylaser fusion of feldspar mineral separates (Nagy et al., 1999). Unit labels (after Nagy et al., 1999) indicate Tertiary (T), Miocene (m), basalt (b), an-desite (a), dacite (d), rhyolite (r), or volcaniclastic sediment (vs), followed by a unit name abbreviation. (See Nagy, 1997, for 1:20000 geologic mapthat differentiates the 21 lithologic units.) Fault symbols: dashed where approximately located, dotted where concealed (inferred), and illustratedwith the letter s where lineaments in the Quaternary alluvium are interpreted to be fault scarps; symbols indicating fault motion are as describedin Figure 1. Horizontal bedding is indicated by a circle and cross symbol. Two large arroyos are named Santa Isabel Wash and Arroyo Oculto.A′–A′′ and B′–B′′ mark the positions of two approximately northeast-facing topographic slopes across which 6.3–6.6 Ma tuffs (group 6) thickenconsiderably from southwest to northeast. The pre–6 Ma topographic slopes are interpreted to be fault controlled and mark the southwesternmargin of the Matomí accommodation zone. Post–6 Ma faults at B′–B″ appear to have reactivated these earlier structures.

least 70–80 m thick in Arroyo Oculto. There is lesspre–6Ma displacement across B′–B′′ (~80–100 m).Downdip lineations within the Tuff of El Canelo(Trec) indicate flow to the north-northeast acrossA′–A′′, and steep northeast-dipping contacts of therocks that core the plateau (groups 2 and 3) disap-pear below the surface at A′–A′′ and B′–B′′, furthersupporting a change in base level.

The deposition of 6.3–6.6 Ma tuffs along thetopographic breaks obscures possible pre–6 Mafaults traces; thus erosion could explain the topo-graphic variations. Alternatively, this zone mayhave undergone northeast-side-down displace-ment along northwest-striking normal faults priorto 6 Ma. Evidence for pre–6 Ma north-side-downfaulting in other parts of Santa Isabel Wash con-firms that the area underwent some pre–6 Ma de-formation, and minor post–6 Ma displacementalong northwest- to north-northwest–strikingfaults at A′–A′′ and B′–B′′ might represent reac-tivation of older structures. As elaborated in theDiscussion section below, these pre–6 Ma paleo-topographic features may be associated with theMatomí accommodation zone.

Post–6 Ma Deformation

Most normal faults in Santa Isabel Wash offsetthe 6.3–6.6 Ma tuffs. The closely spaced, approx-imately north-striking Cuervo Negro and SantaIsabel fault systems are the most continuous struc-tures in the area, although the northernmost expo-sures are complicated by north-northwest–strik-ing splays and cross-structures (C6 in Fig. 2). Thefaults accommodate ~500 m of east-side-downnormal separation based upon the relative dis-placement of the ca. 12.5 and 6.3–6.6 Ma tuffs.Slickenlines preserved on the Santa Isabel faultplane (Figs. 3 and 4) indicate dip-slip motion witha sinistral component; thus extension may havebeen slightly oblique. Similar amounts of dis-placement of the two lithologic groups suggestthat the faults did not accommodate extension be-tween 12 and 6 Ma. However, the lowest group 6tuff (Tmrsiw) is absent west of the Santa Isabelfault, and the overlying cooling unit (Tmr3;type I) is geographically restricted to regions im-mediately east of this fault (see paleomagnetic re-sults below). A pre–6 Ma east-facing scarp form-ing a topographic boundary could explain theserelationships, as could a fault-parallel topographiclow east of the fault developed from hanging-wallrollover.

Other post–6 Ma north-northwest– to north-northeast–striking normal faults are consideredsecondary to the Cuervo Negro and Santa Isabelfaults on the basis of discontinuous, arcuate out-crop patterns of fault traces, relatively smallamounts of normal displacement, and the pres-ence of extensional relay ramps between subpar-

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860 Geological Society of America Bulletin, June 2000

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Figure 3. Orientations of 14 measured fault planes in Santa Isabel Wash (alluvium-bedrockcontact is shown for simplicity; see Figure 2 for distribution of geologic units). Three fault planesthat preserve slickenlines indicate sinistral oblique slip. Other structures in the area are high-angle normal faults; see Figure 2 for inferred sense of offsets. Present-day and older drainage pat-terns suggest that Santa Isabel Wash was structurally created by east- to northeast-side-down de-formation along north- to northwest-striking normal faults that diverted northeast-directed flowto the southeast. Paleomagnetic sampling locations are also shown. Site J marks the location ofsamples collected from the ca. 12.5 Ma tuff of San Felipe (described in Stock et al., 1999).

Figure 4. The approximately north-striking, east-dipping Santa Isabel fault plane (view to thewest). Slickenlines (subparallel to hand lens string), which trend N42°E and plunge 58°, recordoblique (sinistral) dip-slip motion.

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Geological Society of America Bulletin, June 2000 861

allel faults. The density of secondary faults in-creases a few kilometers east of the Santa Isabelfault, where they become spaced fairly regularlyevery ~1 km. These synthetic and antitheticstructures in the hanging wall of the Santa Isabeland Cuervo Negro faults may terminate at depthagainst the east-dipping fault planes. A con-cealed fault is geometrically required betweentwo offset, west-dipping outcrops of the6.3–6.6 Ma tuffs (F4 in Fig. 2) and accommo-dates ~200–300 m of post–6 Ma east-side-downdisplacement. This fault may be continuous withthe northwest- to north-northwest–striking faultsat location H9 (Fig. 2) which appear to have re-activated pre–6 Ma structures along B′–B′′. Re-cent motion along these faults may have con-trolled the development of the Santa IsabelWash, as described next.

Recent Deformation

Ongoing extensional deformation is sup-ported by the presence of small, structurally con-trolled pull-apart basins that localize sag pondsedimentation in the washes and on the plateau.Evidence that the Santa Isabel fault is still activeincludes sinistral stream offsets across fresh faultscarps. Two 2-km-long lineaments in the modernalluvium (M7 and O4 in Fig. 2) may be faultscarps, although the sense of motion along themis not apparent. Additional evidence for Quater-nary deformation includes a change in flow di-rection of the Santa Isabel Wash drainage sys-tem, from northeast to southeast directed. Thelocations of the present wash, as well as aban-doned drainages determined from sediment pat-terns in inactive washes, are indicated inFigure 3. North- to northwest-striking normalfaults south of the wash may have accommo-dated east- to northeast-side-down displacement,thereby creating the northwest-southeast–trend-ing topographic low (i.e., hanging-wall rollover)that forms the present-day wash between E5 andJ9 in Figure 2.

Amount and Direction of Extension

The direction of pre–6 Ma extension is poorlyconstrained. Pre–6 Ma faults are east- to east-northeast–striking and accommodate north-side-down normal separation; however, fault-planestriae on one of these indicate a significant com-ponent of sinistral strike-slip motion (Fig. 3). Dip-slip motion along the inferred pre–6 Ma approxi-mately northwest-striking structures at A′–A′′ andB′–B′′ implies northeast-directed extension. Inany case, most of Santa Isabel Wash did notundergo significant pre–6 Ma deformation.Post–6 Ma east-northeast– to east-directed exten-sion is inferred on the basis of slip indicators on

the Santa Isabel fault plane and assuming normaldip slip across the north-northwest– to north-northeast–striking faults throughout the area. Thesecondary faults may also have undergone anoblique (sinistral) component of slip, as observedalong the Santa Isabel fault. A maximum of 4%post–6 Ma east-northeast–directed extension wasdetermined by restoring cross sections across theregion shown in Figure 2 (Nagy, 1997). Althoughuncertainties in extension direction are large, es-pecially for pre–6 Ma deformation, these resultsimply no appreciable change in extension direc-tion from pre–6 to post–6 Ma. This is in agree-ment with the extensional history recorded else-where in northeastern Baja California (e.g., Gastilet al., 1975; Dokka and Merriam, 1982; Stock andHodges, 1989; Lewis and Stock, 1998b).

PALEOMAGNETIC STUDY OF LATEMIOCENE VOLCANIC TUFFS

Field and Laboratory Methods

We sampled 4 of the 6.3–6.6 Ma tuffs for pale-omagnetic analysis at 15 locations in Santa IsabelWash (Fig. 3). Cooling breaks within some of thetuffs were used to further divide the 4 tuffs into11 cooling units. All samples record normal po-larities. The 40Ar/39Ar ages determined for Tmrao,Tmr3, and Tmrsiw span 6.2 ± 0.1 Ma to 6.7 ±0.3 Ma (Nagy et al., 1999); thus the tuffs mostlikely erupted during subchron C3An.2n(6.269–6.567 Ma; Cande and Kent, 1995). Be-tween 4 and 8 cores were drilled at each site for atotal of 217 samples (see Nagy, 1997, for collec-tion and preparation details). Samples were meas-ured at the Caltech Paleomagnetics Laboratory ina cryogenic SQUID (superconducting quantuminterference device) magnetometer in a magneti-cally shielded µ-metal room. Anhysteretic andisothermal remanent magnetization tests andmagnetic susceptibility measurements were per-formed to discern the nature of the grains carryingthe magnetic remanence in the rocks. Results in-dicate a predominance of ferrimagnetic minerals,such as single-domain or pseudosingle-domainmagnetite, and smaller amounts of antiferro-magnetic minerals, such as hematite. Superpara-magnetic grains were detected in Tmrsiw(t2) (seeNagy, 1997, for details).

Alternating field (AF) and thermal demagneti-zation techniques were compared in a pilot studyof 46 samples. Half of the samples were sub-jected to progressive AF demagnetization, typi-cally including 13 steps to 80 mT and in somecases to 260 mT. These samples were also de-magnetized thermally after the AF procedure.The other half of the sample were thermally de-magnetized in about 12 heating steps between100 °C and 690 °C. AF and thermal demagneti-

zation plots of three pairs of sister samples (i.e.,cut from the same core) are shown for compari-son in Figure 5. Both demagnetization proce-dures yield similar results; however, secondaryoverprints are in some cases incompletely re-moved with thermal techniques (e.g., Fig. 5, Aand B), whereas a primary direction is resolvedusing AF demagnetization. In addition, AF de-magnetized samples show a more regular andgreater drop in magnetic intensity than do ther-mally demagnetized samples. Both low coerciv-ity and high blocking temperature minerals arepresent and appear to record the same remanentdirection (e.g., Fig. 5C). On the basis of thesepreliminary tests, the remaining samples weredemagnetized using the AF procedure.

Analysis

Characteristic remanent magnetization (ChRM)vectors were calculated using principal componentanalysis (Kirschvink, 1980) to selected demagne-tization steps for each sample. Low coercivity vis-cous magnetization was commonly removed bythe 15 mT step. Single-component, primaryChRM directions were fit with lines in vectorspace; eight samples showing only partial removalof an overprint were fit with planes. Paleomagneticresults for the 11 cooling units are given in Table 1.A Fisher (1953) distribution function is used tocalculate the uncertainties for the tilt-corrected sitemeans. I discarded 13 analyses on the basis ofpoorly preserved ChRM directions or as extremeoutliers from the rest of the samples at a site (>15°from site mean). ChRM directions cluster well atindividual sites with α95 < 10°. Three exceptionsoccur at site D (Tmrao[α95 = 17.6°], Tmr4 [α95 =12.7°], and Tmrsiw (t3) [α95 = 10.8°]), where thetuffs are less welded than at most other sites, al-though there is no clear correlation between thedegree of welding and the amount of dispersion inChRM directions. Small tilt corrections were ap-plied based on contact dips between tuffs, and noton the basis of eutaxitic foliation planes, althoughthe orientations of such planes were measured. In-clined eutaxitic foliation planes developed abovethe Curie temperature of the remanence-carryingminerals and do not represent paleohorizontal inthis study (Nagy, 1997).

Results

The paleomagnetic data summarized inTable 1 suggest that the Santa Isabel Wash regionmay have undergone a small amount of clock-wise vertical-axis rotational deformation. Unfor-tunately, this interpretation relies heavily on di-rections recorded at only two sites at the east andwest limits of the region sampled. Furthermore,typical variations within a given cooling unit be-

862 Geological Society of America Bulletin, June 2000

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agne

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EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE

Geological Society of America Bulletin, June 2000 863

tween relatively unrotated sites suggest that theamount of rotation is at the threshold of resolu-tion. Specifically, declinations recorded at site K(Fig 3) in Tmrsiw (t5) and Tmr3 (type I) are10°–25° counterclockwise of most directionspreserved in the same units at other sites. There is

also a 15° difference in declination recorded inTmrsiw (t3) between sites D and P (Fig 3). If thedirections recorded at sites K and P are accurate,it implies that the entire area rotated 10°–20°clockwise relative to site K, and that site P under-went an additional 15° of clockwise rotation. An

inferred fault east of site K, concealed beneathSanta Isabel Wash near E10 in Figure 2, could bea structure that separates site K from a region ofrotation to the east. Site P is separated from theother sampling sites by a major north-striking,west-dipping normal fault (N8–N18 in Fig. 2),which could mark a boundary between regions ofdifferential rotation.

TABLE 1. PALEOMAGNETIC DATA FROM THE PUERTECITOS VOLCANIC PROVINCE,BAJA CALIFORNIA, MEXICO

Location Bedding N/Nc Geographic Stratigraphic FisherStrike Dip Dec Inc Dec Inc κ α95

Tmrao (one cooling unit)

Site B* 187 5.5 5/5 12.1 51.8 5.1 52.0 213 5.4Site C 187 5.5 5/5 4.9 42.7 359.9 42.2 192 5.5Site D 187 5.5 3/5 9.3 45.7 3.6 45.6 50 17.6Site I* 0 0.0 6/6 4.1 39.6 4.1 39.6 81 7.6

Tmrao mean: 19/21 7.3 44.6 3.4 44.4 86 3.7

Tmr4 (two cooling units)Tmr4Site B* 187 5.5 5/5 6.5 42.4 1.5 42.1 236 5.4Site D 187 5.5 4/5 8.3 44.2 2.9 44.0 53 12.7

Tmr4 mean: 9/10 7.2 43.2 2.1 43.0 99 5.3Tmr3–4

Site B 187 5.5 5/5 0.8 42.3 355.9 41.4 175 5.8Site C* 187 5.5 5/6 2.1 33.8 358.5 33.1 53 10.9Site M 0 0.0 7/7 352.9 39.8 352.9 39.8 562 2.5

Tmr3–4 mean: 17/18 357.9 38.8 355.6 38.4 112 3.4

Tmr3 (four cooling units)Upper Tmr3 (type II)Site L 0 0.0 8/8 29.3 52.1 29.3 52.1 135 4.8Middle Tmr3 (type II)Site L 0 0.0 7/7 31.6 58.2 31.6 58.2 410 3.0Site N 0 0.0 7/7 21.3 53.5 21.3 53.5 435 2.9

Middle (type II) mean: 14/14 26.1 56.0 26.1 56.0 225 2.7Lower Tmr3 (type II)Site E 250 8.0 5/5 31.2 61.8 21.5 56.3 1085 2.3Site I 180 4.0 6/6 30.8 66.3 22.3 68.0 544 2.9Site L 0 0.0 7/7 28.2 58.5 28.2 58.5 206 4.2Site M* 0 0.0 5/7 24.0 34.7 24.0 34.7 66 9.8Site N 0 0.0 7/7 28.5 47.2 28.5 47.2 331 3.3Site O 250 8.0 4/4 36.2 64.8 24.5 59.7 229 6.1

Lower (type II) mean: 34/36 29.4 55.6 25.9 54.4 48 3.6Tmr3 (type I)Site A* 182 6.5 5/5 354.5 67.5 340.7 65.7 258 5.1Site D 187 5.5 4/5 354.0 66.7 342.4 64.9 116 8.6Site K 186 4.0 3/6 321.5 54.9 317.8 52.0 291 7.2

Tmr3 (type I) mean: 12/16 342.3 64.1 333.0 61.7 68 5.3Site K removed: 9/10 354.1 67.1 341.4 65.3 185 3.9

Tmrsiw (four cooling units)Tmrsiw (t5)Site A 182 6.5 5/5 17.9 40.7 12.3 42.2 116 7.1Site E 250 8.0 5/5 20.0 46.4 15.4 40.1 3849 1.2Site F 0 0.0 6/6 28.1 47.2 28.1 47.2 546 2.9Site G 0 0.0 4/6 20.4 36.1 20.4 36.1 152 7.5Site H 312 10.0 5/5 13.1 49.2 17.6 40.3 330 4.2Site I 180 4.0 6/6 31.3 42.7 27.9 44.7 184 5.0Site K 186 4.0 6/6 9.0 42.2 5.3 42.3 149 5.5Site M 0 0.0 7/7 6.9 42.0 6.9 42.0 126 5.4Site N 0 0.0 8/8 25.7 38.9 25.7 38.9 418 2.7

Tmrsiw (t5) mean: 52/54 19.3 43.1 17.8 42.0 88 2.1Site K removed: 46/48 20.7 43.1 19.5 41.9 99 2.1

Tmrsiw (t4)Site D 187 5.5 5/5 26.9 24.8 24.4 26.6 461 3.6Tmrsiw (t3)Site D 187 5.5 5/5 26.6 29.5 23.5 31.0 51 10.8Site P 312 10.0 6/6 37.9 59.5 38.8 49.5 107 6.5

Tmrsiw (t3) mean: 11/11 31.3 46.1 30.8 41.4 31 8.3Tmrsiw (t2)Site D 187 5.5 4/5 17.2 38.5 12.7 39.2 112 8.7Site G 0 0.0 5/5 20.0 35.0 20.0 35.0 723 2.8Site N 0 0.0 7/7 25.1 29.1 25.1 29.1 834 2.1Site O 250 8.0 7/7 18.0 46.3 13.6 39.8 114 5.7

Tmrsiw (t2) mean: 23/24 20.7 37.3 18.6 35.5 97 3.1

*Planes and lines used to calculate site average. Lines shown in bold are group mean directions.

N

TmraoTmr4Tmr3-4Tmr3 - Upper Type IITmr3 - MiddleType IITmr3 - LowerType IITmr3 - Type ITmrsiw - t5Tmrsiw - t4Tmrsiw - t3

Tmrsiw - t2

W E

P

P

Present-day field

Mean direction

Figure 6. Equal-area projection of tilt-cor-rected mean directions (with projected Fisherα95confidence cones) from 11 volcanic coolingunits sampled for paleomagnetic analysisfrom Santa Isabel Wash. The 2σ confidencelimits for paleosecular variation around aMiocene geomagnetic pole (D = 0°, I = 50°)were calculated using model C1 (solid ellipse)and the volcanic database (dashed ellipse) ofQuidelleur and Courtillot (1996). Standarddeviations of the declination and inclination(σD, σI) between lat 30°N and 40°N are (12.5°,13°) from model C1 and (15°, 12.5°) from thedatabase, and were used to calculate the 2σconfidence limits for declination (α95 [dec])and inclination (α95 [inc]) via: α95 (dec) = (2σD) cos (50°) and α95(inc) = (2σ I). With the ex-ception of Tmrsiw (t3) and Tmrsiw (t4), allmean directions overlap with expected paleo-secular variation limits, suggesting that rota-tion of the entire region relative to geomag-netic north is not likely. The mean direction ofthe 11 cooling units is dec = 14.4°, inc = 46.1°,Fisher α95 = 8.7°.

Other examples of anomalous directions arenot consistently recorded between different cool-ing units, which casts some doubt on the resultsfrom sites K and P. For example, the mean decli-nation recorded at site M in Tmrsiw (t5) is coun-terclockwise of the directions recorded within thiscooling unit at most other sites and is very similarto results from site K in Tmrsiw (t5). In contrast,

results from site M are in good agreement withmean directions in overlying lower Tmr3 (type II)and Tmr3–4. Declinations recorded at site I alsodiffer inconsistently relative to the mean direc-tions in Tmrsiw (t5), lower Tmr3 (type II), andTrao. These examples suggest that in some casesanomalous directions are probably not tectonic inorigin but rather due to sampling procedure errors

or natural variations of the preserved magnetiza-tion directions. Incorrect structural correctionscould account for anomalous results, although re-alistic uncertainties in bedding tilts do not signifi-cantly change the mean directions.

An argument for rotational deformation wouldbe more convincing if more of the stratigraphicpackage had been sampled at sites K and P. Withthe possible exception of these two sites,post–6 Ma relative rotations between fault-bounded structural blocks are not apparent be-tween the different sampling sites. Samples fromsites K and P are thus removed from the followingdiscussion in order to examine the directionsrecorded in the rest of the study area without thecomplication of possible rotations relative to thesetwo sites. Mean directions for Tmrsiw (t5) andTmr3 (type I) without site K are listed in Table 1.

Virtual Geomagnetic Pole Direction fromSanta Isabel Wash

An equal-area plot of the 11 cooling unit meandirections, without data from sites K and P, isshown in Figure 6. Secular variations of theEarth’s magnetic field are the most likely reasonfor the 11 different directions. A comparison be-tween the mean directions and statistical studies ofpaleosecular variation (Quidelleur and Courtillot,1996) shows that most mean directions are withinexpected limits of paleosecular variation aroundthe Miocene geomagnetic pole. The 2 σ confi-dence limits are shown in Figure 6 centered on D =0° and the expected geocentric axial dipole valueof I = 50° at the study latitude. Shallow inclina-tions recorded in some of the Tmrsiw cooling unitsare from a single site (D) and may be due to rota-tion of magnetic grains during emplacement (e.g.,Rosenbaum, 1986) rather than tectonic in origin.

Because most mean directions are within ex-pected paleosecular variation limits, rotation ofthe entire region relative to geomagnetic north isnot likely. To further test this interpretation, aver-aging the 11 directions may average the secularvariation signal recorded in the different coolingunits, thereby providing a virtual geomagneticpole direction that can be compared to the stableNorth America paleopole. As illustrated in Fig-ure 7, a virtual geomagnetic pole direction fromSanta Isabel Wash, corrected for divergence be-tween Baja California and the North Americaplate since the latest Miocene, overlaps within95% confidence limits with the stable NorthAmerica paleopole (Besse and Courtillot, 1991;McElhinny and Lock, 1995). The angular differ-ence between the sampling site and the virtual ge-omagnetic pole directions from Santa Isabel Wash(50.3°) and the North America paleopoles calcu-lated from McElhinny and Lock (1995) (60.1°)and Besse and Courtillot (1991) (58.0°) are 9.8° ±

E.A. NAGY

864 Geological Society of America Bulletin, June 2000

study site

Virtual Geomagnetic Pole from Santa Isabel Wash (uncorrected)

Virtual Geomagnetic Pole from Santa Isabel Wash (corrected for post-5.5 Ma displacement relative to North America)

North America paleopole (3–10 Ma) from Global Paleomagnetic database

North America paleopole (0–20 Ma) after Besse and Courtillot (1991)

180° 0°

90°

270°

Figure 7. Northern hemisphere (equal area) projection showing virtual geomagnetic pole di-rections (with 2 σ uncertainty ellipses) from Santa Isabel Wash. The average direction of the 11cooling units gives a virtual geomagnetic pole at lat 77.6N°, long 341.0E° (dp = 22.2°, dm = 9.9°).In order to account for divergence between Baja California (on the Pacific plate) and the NorthAmerica plate since the latest Miocene, the virtual geomagnetic pole is rotated 4.7° (e.g., Stockand Hodges, 1989) about a pole at lat 48.7°N, long 281.8°E determined from the global plate mo-tion model NUVEL-1 (DeMets et al., 1990; DeMets, 1995). The corrected virtual geomagneticpole for Santa Isabel Wash is at lat 80.2N°, long 335.8E°. The virtual geomagnetic pole directionsare statistically indistinguishable from a stable North America paleopole (lat 97.2N°, long86.8E°,α95= 3.8°) calculated from selected sites within the 1997 Global Paleomagnetic Database(McElhinny and Lock, 1995) for studies in 3–10 Ma rocks. Selection criteria included dp ≤≤15°,n (sites) > 3, and N (samples) > 35. Results from Santa Isabel Wash are also statistically indis-tinguishable from a stable North America paleopole (lat 87N°, long 57E°,α95 = 7°) calculatedfrom 0–20 Ma by Besse and Courtillot (1991) in their study of apparent and true polar wander.A small (~10°) amount of clockwise rotation of Santa Isabel Wash relative to north would not bedetectable given uncertainties.

8.6° and 7.7° ± 9.7°, respectively. Results fromSanta Isabel Wash are thus statistically indistin-guishable from the stable North America paleo-pole, although a small amount of clockwise rota-tion (~10°) is permissible within uncertainties.

Regional Distribution of Rotational Deformation

Lewis and Stock (1998a) presented paleomag-netic evidence for 31° ± 15° vertical-axis clock-wise rotation of the Sierra San Fermín relative toMesa Cuadrada in the southern Sierra San Felipe(Fig. 1). This amount of relative rotation is an av-erage value found within ca. 12.5 and 6.3–6.6 Matuffs and agrees with the sense and amount ofrotation found in Pliocene sediments in the east-ern Sierra San Fermín (Strangway et al., 1971).Lewis and Stock (1998a) also identified ~25º–30°clockwise rotation of a 3 Ma ash-flow tuff in theSierra San Fermín relative to a reference point inArroyo Matomí. Because some of the same pyro-clastic flows were sampled at Santa Isabel Wash,comparisons between these distant sites can bemade without concern for secular variations ofthe Earth’s magnetic field.

A summary of the regional paleomagnetic re-sults and lithologic correlations (after Nagy et al.,1999) is given in Table 2. Correlations are mostrobust between the tuff of San Felipe (Tmrsf inFig. 2) and Tmr4. Note that the tuff of San Felipewas also sampled in Santa Isabel Wash (site J inFig. 3) and in Santa Rosa basin (Fig. 1); paleo-magnetic data were given in Stock et al. (1999).Inclination values recorded in Tmr4 and the tuffof San Felipe agree within α95 between Santa Is-abel Wash and the other regions. Different decli-nation directions indicate that the Sierra San Fer-

mín and Santa Rosa basin have undergone ~30°vertical-axis clockwise rotation relative to SantaIsabel Wash since the time of deposition. A slightdifference in declination direction (~10°) isrecorded in the tuff of San Felipe between SantaIsabel Wash and Mesa Cuadrada, again support-ing a small (~10°) amount of clockwise rotationaldeformation in Santa Isabel Wash as discussedabove. Given results from Tmr4 and the tuff ofSan Felipe, lithologic correlations between inter-vening sampled tuffs are problematic. A series oftuffs designated Tmr3 in these studies, basedupon correlations with stratigraphy defined in thesouthern Valle Chico (Stock, 1989), is similar inmineralogy, field appearance, stratigraphic posi-tion, and age throughout the region. Given paleo-magnetic results from the underlying and over-lying tuffs, however, lithologic correlation of anyTmr3 units between Santa Isabel Wash and theother sampling locations implies significantlydifferent amounts and relative senses of rotation(Table 2). It is likely that there are at least sixcooling units in the Tmr3 position throughout theregion (four Tmr3 cooling units in Santa IsabelWash and two in the Sierra San Fermín andsouthern Sierra San Felipe). Tmr3a and Tmr3bfrom the Sierra San Fermín are unlikely to becorrelative with the tuffs of Santa Isabel Wash(Tmrsiw), which are restricted to the region southof Arroyo Matomí (Nagy et al., 1999).

Lewis and Stock (1998b) suggested that rota-tional deformation in the Sierra San Fermín is ei-ther (1) restricted to a region above a subhorizon-tal detachment or basal shear zone or (2) thesurface expression of a vertical shear zone link-ing major transform faults to the north and south.Upper plate detachment faulting along the riftmargin east of the San Pedro Mártir fault has

been suggested in several studies (Dokka andMerriam, 1982; Stock and Hodges, 1990; Axen,1995), and the diffuse nature of upper crustal de-formation along the rift margin further favors asubhorizontal basal shear zone.

DISCUSSION

Matomí Accommodation Zone

Dokka and Merriam (1982) first proposed awest-striking accommodation zone in the vicin-ity of Arroyo Matomí (Fig. 1) to explain thenorthward termination of late Miocene–Pliocenefaults in the eastern Puertecitos VolcanicProvince. Stock and Hodges (1990) further in-ferred that the accommodation zone separatedpre–6 Ma east-northeast–directed extensionnorth of Arroyo Matomí from the undeformed re-gion to the south. In Axen’s (1995) regionalanalysis of segmentation and vergence reversal ofthe Main Gulf Escarpment, he assigned the entirePuertecitos Volcanic Province region to the roleof a late Miocene west- to west-northwest–strik-ing accommodation zone separating an east-di-rected normal fault system to the north from awest-directed system to the south. A Pliocene agefor the accommodation zone has also been in-ferred. For example, Lewis and Stock (1998a)claimed that the Matomí accommodation zonemarks the southern boundary of Pliocene toHolocene rotational deformation in the SierraSan Fermín, and the style of Pliocene rift marginsedimentation in the eastern Puertecitos VolcanicProvince and the Sierra San Fermín is attributedto an accommodation-zone setting (Stock et al.,1996; Martín-Barajas et al., 1997). In summary,these studies imply the existence of a pre–6 Ma

EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE

Geological Society of America Bulletin, June 2000 865

TABLE 2. REGIONAL PALEOMAGNETIC RESULTS FROM NORTHEASTERN BAJA CALIFORNIA, MEXICO

Age* Unit† Santa Isabel Wash§ Mesa Cuadrada# Sierra San Fermín# Santa Rosa Basin**(Ma) Dec Inc α95 Dec Inc α95 Dec Inc α95 Dec Inc α956.3–6.6 Tmrao 3.4 44.4 3.7 N.D. N.D. N.D.6.3–6.6 Tmr4 2.1 43.0 5.3 N.D. 33.7 50.6 19.1 N.D.6.3–6.6 Tmr3–4 355.6 38.4 3.4 N.D. N.D. N.D.6.3–6.6 Upper Tmr3 (type II) 29.3 52.1 4.8 N.D. N.D. N.D.6.3–6.6 Middle Tmr3 (type II) 26.1 56.0 2.7 N.D. N.D. N.D.6.3–6.6 Lower Tmr3 (type II) 25.9 54.4 3.6 N.D. N.D. N.D.6.3–6.6 Tmr3 (type I)†† 341.4 65.3 3.9 N.D. N.D. N.D.6.3–6.6 Tmr3b N.D. 9.5 53.9 8.9 39.2 51.5 7.2 N.D.6.3–6.6 Tmr3a N.D. 52.4 52.2 12.9 77.8 52.3 22.3 N.D.6.3–6.6 Tmrsiw (t5)†† 19.5 41.9 2.1 N.D. N.D. N.D.6.3–6.6 Tmrsiw (t4) 24.4 26.6 3.6 N.D. N.D. N.D.6.3–6.6 Tmrsiw (t3)§§ 23.5 31.0 10.8 N.D. N.D. N.D.6.3–6.6 Tmrsiw (t2) 18.6 35.5 3.1 N.D. N.D. N.D.~12.5 Tmrsf 229.7** –9.1** 2.0** 218.3 –6.9 4.1 259.4 –3.7 10.4 259.4 –11.5 2.7

Note: N.D.—no data.*Based upon 40Ar/39Ar geochronology, stratigraphic position, and comparison with geomagnetic polarity time scale.†Stratigraphic order of Tmr3a and Tmr3b in Mesa Cuadrada and Sierra San Fermín relative to the four Tmr3 cooling units in Santa Isabel

Wash is unknown.§This study.#Lewis and Stock (1998a).**Stock et al. (1999).††Site K removed from group mean.§§Site P removed from group mean.

segmentation along the Gulf extensionalprovince rift margin that continued to mark theapproximate location of a rift margin accommo-dation zone during the Pliocene.

Paradoxically, field evidence for pre– orpost–6 Ma accommodation-zone structures isscarce. Most significantly, accommodation-zonestructures have not been identified within 3 and6 Ma volcanic rocks extensively faulted by north-northwest–striking normal faults in ArroyoMatomí (Stock et al., 1991), or within 3 and 6 Marocks between Arroyo Matomí and Arroyo LosHeme (Fig. 1; Stock et al., 1991; Martín-Barajaset al., 1995). Farther west, in the southeast corner

of southern Valle Chico, Stock and Hodges(1990) described a west-northwest–striking ex-tensional accommodation zone (Ultima Esper-anza fault zone after Stock, 1993), which they as-sociated with the Matomí accommodation zone.The Ultima Esperanza fault zone (Fig. 1) is com-posed of anastomosing strike-slip and dip-slipfaults along which the sense of offset is not evi-dent (Stock, 1993). Displacement along the faultzone occurred after deposition of the ca. 12.5 Matuff of San Felipe (unit Mr1 of Stock, 1993;geochronology updated by Stock et al., 1999),but is otherwise unconstrained. The fault zonecannot correspond to the southern boundary of

Pliocene to Holocene rotations because MesaCuadrada, directly to the north, is not within therotated region. The Ultima Esperanza fault zoneterminates westward against a high-angle north-northwest–striking fault that might be the contin-uation of the Valle de San Felipe fault (Fig. 1;Stock and Hodges, 1990). A large alluvial plainnorth of Santa Isabel Wash conceals the area eastof the Ultima Esperanza fault zone, where it mayhave been offset by a Pliocene northeast-strikingsinistral strike-slip fault (Fig. 1; Stock, 1999).

The pre–6 Ma approximately northwest-striking normal faults inferred in the Santa Is-abel Wash region (A′–A′′ and B′–B′′ in Fig. 2)are the first structures identified east of the Ul-tima Esperanza fault zone that might be associ-ated with the Matomí accommodation zone. Be-cause these fault zones are not aligned alongstrike, the region of accommodation-zone struc-tures either broadened southeast of the UltimaEsperanza fault zone or formed an en echelonseries of faults stepping southward into SantaIsabel Wash. Differential extension across thetwo segments in Santa Isabel Wash suggests azone of deformation consisting of multiple sub-parallel structures rather than a single thorough-going fault.

The Matomí accommodation zone may haveformed in a zone of weakness separating two off-set portions of the north-northwest–striking riftmargin during east-northeast–directed extension(Dokka and Merriam, 1982; Stock, 1999). One ofthese rift margin structures was the early San Ped-ro Mártir fault, and the other one was perhaps lo-cated near Gonzaga Bay (Fig. 1, inset; Stock,1999). Stock and Hodges (1990) suggested thatdextral strike-slip motion along the Matomí ac-commodation zone transferred slip from the south-ern end of the San Pedro Mártir fault onto Gulf ofCalifornia structures to the southeast by dextraloblique-slip motion. Geologic relationships acrossinferred accommodation zone structures in SantaIsabel Wash, which document down-to-the-north-east normal separation, further support an obliquesense of motion. The Matomí accommodationzone may be structurally similar to oblique-sliptransfer faults on the eastern margin of the Gulf ofSuez (Red Sea rift system), which developed be-tween originally offset, main-rift normal faults(McClay and Khalil, 1998). An irregularity in theearly to middle Miocene arc may have been ex-ploited at the time of rift margin development,causing the initial offset (Stock, 1999). The loca-tion and position of accommodation zones alongother rift margins, such as in East Africa, similarlyfollowed preexisting structural discontinuities orzones of weakness (e.g., Bosworth, 1992;Kilembe and Rosendahl, 1992; Scott et al., 1992).

Most northwest-striking accommodation zonestructures in Santa Isabel Wash were bypassed af-

E.A. NAGY

866 Geological Society of America Bulletin, June 2000

?A′′

N

pre-6 Ma Matomíaccommodation zone

present-daySanta Isabel Wash

synthetic and antitheticstructures in hanging wallof major E-dipping faults

6.3-6.6 Ma tuffs~ 12.5 Ma tuff

ENE- to E-directed extension

W-dipping volcanic rocks in hanging wall of

major E-dipping faults

slickenlineswith 60-80° rake

Post-6 Ma

reactivation alongpre-6 Ma NW-str iking

structures

B

Cuervo Negro fault

Santa Isabel fault

NE- to ENE-directed extension

N

southwestern margin ofMatomí accommodation zone

?

Pre-6 Ma

~ 12.5 Ma tuff

?

A′

A′′relatively

undeformedregion

within GEPregion of

deformation

A

“future” Cuervo Negro fault

“future” Santa Isabel fault

Figure 8. Schematic block models of pre–6 and post–6 Ma deformation in Santa Isabel Wash.(A) Prior to 6 Ma most of the northern Puertecitos Volcanic Province was located southwest ofthe northwest-striking Matomí accommodation zone (simplified; see Fig. 2 for segmentation),which separated the Gulf extensional province (GEP) to the northeast from the Puertecitos Vol-canic Province. Northeast- to east-northeast–directed extension is inferred. (B) Most faults inthe study area displace all mapped units and are thus post–6 Ma in age. Slickenlines and faultorientations indicate east-northeast– to east-directed extension.

ter 6 Ma when the rift margin migrated westwardonto approximately north-striking extensionalfaults such as the Santa Isabel and Cuervo Negrofaults (Fig 2). Although some accommodationzone structures were reactivated, paleomagneticresults show that no relative rotation occurredacross these faults since 6 Ma. For example,paleomagnetic sampling sites B, C, and D (Fig. 3)are northeast of the northwest-striking faults reac-tivated at B′–B′′ (Fig. 2). Thus the southern struc-tural boundary of Pliocene to Holocene rotationsin the Sierra San Fermín, inferred by Lewis andStock (1998a) to be the west-northwest–strikingMatomí accommodation zone, cannot coincidewith the pre–6 Ma accommodation zone struc-tures inferred in this study. Pliocene accommoda-tion-zone structures may be concealed below thelarge alluvial plain north of Santa Isabel Wash, al-though where they project farther to the east is un-known. It is interesting that an unidentifiedPliocene structural boundary must also occurwithin the Sierra San Felipe, separating the ro-tated Santa Rosa basin from Mesa Cuadrada.Overall paleomagnetic results from Santa IsabelWash, and especially results from site P, hint thatthe eastern Puertecitos Volcanic Province mayhave in fact undergone a small amount(~10°–15°) of rotational deformation. Rather thanbeing represented by a discrete fault zone, it ispossible that the southern boundary of rotationaldeformation is a broad, diffuse zone involving ex-tensional structures in the northeastern PuertecitosVolcanic Province, including Santa Isabel Wash,which accommodated small amounts of shear (ro-tational) deformation. This scenario is not sup-ported, however, by results from the ArroyoMatomí sampling site in the study by Lewis andStock (1998a), where declinations recorded in a3 Ma tuff are 24°–30° counterclockwise of direc-tions found at two sites in the Sierra San Fermín.

History of Deformation in Santa Isabel Wash

The Gulf extensional province rift margin hasevolved structurally in the Santa Isabel Wash re-gion since late Miocene time. Schematic blockmodels (Fig. 8) illustrate deformation before andafter 6 Ma. Prior to 6 Ma Santa Isabel Wash wasseparated from the extending region to the northby the northwest-striking Matomí accommoda-tion zone. Roughly 300–350 m of pre–6 Manortheast-side-down displacement occurredacross the zone. A strike-slip component of mo-tion along the zone is not evident in Santa IsabelWash; however, dextral oblique slip may have oc-curred if the structures are related to those thattransferred slip from the San Pedro Mártir faultonto Gulf of California structures to the southeast(Stock and Hodges, 1990), implying a more east-erly (e.g., east-northeast–directed) extension di-

rection. Dextral oblique-slip predominates alonglate Miocene, northwest-striking faults in theSierra San Fermín (Lewis and Stock, 1998b); thusa similar sense of motion along this zone in SantaIsabel Wash does not contradict the late Mioceneregional pattern of deformation. Other pre–6 Maapproximately east-striking faults accommodaterelatively minor north-side-down displacement(not differentiated in Figure 8) and are perhaps re-lated to accommodation-zone deformation.

Most extension in Santa Isabel Wash oc-curred after the deposition of 6.3–6.6 Ma tuffs.Faults generally strike north-northwest to north-northeast, although some faulting localized onpre–6 Ma northwest-striking accommodation-zone structures (B′–B′′ in Fig. 2). The Santa Isabeland Cuervo Negro fault systems are major, east-dipping normal faults that accommodate ~500 mof post–6 Ma displacement. Conformable contactsbetween 12.5 and 6 Ma tuffs further imply that thearea did not undergo significant pre–6 Ma defor-mation, in contrast to angularly unconformablecontacts between the same units north of ArroyoMatomí (Gastil et al., 1975; Dokka and Merriam,1982; Stock, J., 1989, 1993; Lewis and Stock,1998b). The Santa Isabel and Cuervo Negro faultsmay merge at depth to form a listric detachmentsurface such as interpreted for the San PedroMártir fault (Dokka and Merriam, 1982; Stock andHodges, 1990). A shallow westward dip of6.3–6.6 Ma tuffs in the hanging wall of the SantaIsabel fault is perhaps the result of reverse dragalong the fault, also implying a listric geometry atdepth. High-angle normal faults in the hangingwalls of the Santa Isabel and Cuervo Negro faultsare secondary structures that perhaps intersect theeast-dipping fault planes at depth. Rotationaldeformation is not statistically supported by thepaleomagnetic results for most of the area, al-though further sampling at the eastern and westernlimits of the area would help clarify some anom-alous declination values. A minor amount (~10°)of clockwise rotation of the entire area relative tonorth is permissible given uncertainties.

Extensional deformation continues in Santa Is-abel Wash, as evidenced by sag ponds, offsetdrainages, and fault scarps in the active alluvialsurface, and may have controlled recent changesin the drainage pattern of Santa Isabel Wash. ThePuertecitos Volcanic Province may continue toundergo extensional deformation with an in-creasing amount of dextral shear over time, andmay eventually be fully incorporated into the re-gion of rotational deformation. Northeast-strik-ing faults within the Quaternary alluvium inSanta Isabel Wash could represent structures ac-commodating conjugate plate boundary slip suchas described along post–3 Ma structures in theSierra San Fermín and Arroyo Matomí regions(Lewis and Stock, 1998b).

The Santa Isabel and Cuervo Negro fault sys-tems accommodate more than half of the offsetobserved along the San Pedro Mártir fault insouthern Valle Chico (~800 m; Stock, 1989), sug-gesting that they represent principal structuresaccommodating Pliocene to Holocene rift margindeformation. The northward projection alongstrike of the Santa Isabel and Cuervo Negro faultsis ~10 km east of the San Pedro Mártir fault andapproximately along strike with the Valle de SanFelipe fault (Fig. 1). The San Pedro Mártir nor-mal fault and dextral Valle de San Felipe strike-slip fault were interpreted by Grover et al. (1993)to link the Gulf of California transform system totranspeninsular faults to the north by partitioningof normal and dextral slip. Reconnaissance in-vestigations have not revealed major rift marginstructures west of the Santa Isabel Wash region(Gastil, et al., 1975; Dokka and Merriam, 1982;J. Stock, 1997, personal commun.), althoughnorthwest- to north-northwest–striking structuresin the northwestern Puertecitos Volcanic Province(Fig. 1) probably link extensional deformation atthe south end of the San Pedro Mártir fault to theSanta Isabel Wash region.

Influence of Oceanic Plate Boundary Structures on Rift Margin Deformation

There is an ambiguous relationship betweenlate Miocene structures developed along the Gulfextensional province margin and those of thePliocene–Quaternary oceanic spreading centersystem within the Gulf of California. The highangles between several accommodation zonesand Gulf of California transform faults, as well astheir relative ages and extension directions, sug-gest that there is little relationship between theearlier segmentation and the subsequent geome-try of the Gulf of California spreading center sys-tem (Axen, 1995). Stock (1999), however, foundcorrelations between several accommodationzones in northeastern Baja California and majortransform offsets in the Gulf of California. Themost convincing relationship is between theTiburón transform fault (Fig. 9) and the Matomíaccommodation zone, although structures associ-ated with the accommodation zone have not beenidentified in the eastern Puertecitos VolcanicProvince. If the pre–6 Ma approximately north-west-striking structures in Santa Isabel Wash arerelated to the accommodation zone, their orienta-tion is not at a high angle to transform fault ori-entations, although they do differ in relative ageand extension direction from Gulf of Californiastructures.

The timing of formation of pull-apart basinsbetween offset northwest-striking, dextral strike-slip faults in the central and northern Gulf of Cal-ifornia is poorly constrained because of the ab-

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Geological Society of America Bulletin, June 2000 867

sence of magnetic anomalies in most of the Gulfof California; however, development of theoceanic system probably began ca. 4–6 Ma(Lonsdale, 1989). Motion along the Tiburóntransform fault and extension in the upper andlower Tiburón basins (Fig. 9) was replaced sometime after 2 Ma by northwestward propagation ofthe Guaymas transform fault into the Ballenaschannel and extension in the upper and lowerDelfín basins (Lonsdale, 1989; Stock, 1999).

Deformation recorded in Santa Isabel Washsupports the idea that late Miocene–Pliocenestructural adjustments along the rift margin arerelated to changes in the offshore spreading cen-ter system. In particular, post–6 Ma extension inthe Puertecitos Volcanic Province may have be-gun when dextral strike-slip motion along theGuaymas transform replaced similar motion

along the Tiburón transform. The Guaymas trans-form projects farther south into the rift marginthan does the Tiburón transform (Fig. 9); thusPliocene extension began in the Puertecitos Vol-canic Province when the Guaymas transformlinked northwestward to rift margin structuressuch as the San Pedro Mártir fault. The Matomíaccommodation zone was thus abandoned as riftmargin deformation moved southwestward. It isinteresting that this adjustment exposed thePuertecitos Volcanic Province to east-north-east–directed Gulf extensional province exten-sion, producing N-striking structures such as theSanta Isabel and Cuervo Negro faults, rather thanforming northeast-striking normal faults and/ornorthwest-striking dextral strike-slip faults to ac-commodate northwest-southeast–directed plateboundary motion.

Diffuse Plate Boundary Deformation in theNorthern Gulf of California Region

As documented in this study, evidence for dis-tributed deformation in northeastern Baja Califor-nia includes ongoing east- to northeast-directedextension and Pliocene or younger vertical-axisclockwise rotations. These observations have ledto a new model for the formation and evolution ofthe upper and lower Tiburón and Delfín basins,which is summarized here and detailed elsewhere(Nagy, 1997; Nagy and Stock, 2000). The modelis based on the assumption that Pacific–NorthAmerica plate boundary deformation in the north-ernmost Gulf of California is not accommodatedalong northwest-striking oceanic transform faultsor northeast-striking spreading centers, such as inthe central and southern Gulf of California, butrather occurs over a broad region undergoing dif-fuse deformation since the late Miocene. Dextraloblique slip along submerged structures in theGulf of California north of about 30°N may occuron reactivated north- to north-northwest–strikingnormal faults developed during late Miocene ex-tension. The absence of an oceanic plate boundaryin the northern Gulf of California is supported bygeophysical data, which indicate lower heat flowin the north relative to the central and southernGulf of California, an absence of gravity lowssuch as those associated with extensional basins,and a greater depth to the Moho (20–35 km in thenorth vs. 10 km in the central and southern re-gions) (Henyey and Bischoff, 1973; Gastil et al.,1975; Couch et al., 1991).

This region of diffuse, dextral plate boundaryshear has been termed the Wagner transition zone(Fig. 9; Nagy, 1997; Nagy and Stock, 1998, 2000)and consists of a continuum of transtensional de-formation north of lat 30°N that varies from east-northeast–directed extension along dip-slip faultsin the west to dextral shear along the coast to dex-tral-oblique slip along north- to north-north-west–striking faults in the Gulf of California. Thisstructural scenario can account for Pacific–NorthAmerica plate motion vector constraints at the lat-itude of the northern Gulf of California. Thesoutheastern limit of the transition zone, whichroughly coincides with the southern PuertecitosVolcanic Province on land and projects northeast-ward into the Gulf of California, is a complex re-gion where diffuse deformation within the Wag-ner transition zone is juxtaposed against discreteoceanic plate boundary structures in the centralGulf of California. Differential motion thus pro-duced a northeast-striking zone of divergence thatlocalized the positions of the upper and lowerTiburón and Delfín basins (Fig. 9). For example,in the early Pliocene the northwest-strikingTiburón transform fault accommodated north-west-southeast–directed motion and intersected

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868 Geological Society of America Bulletin, June 2000

BAJA

GULF OF

Isla A. de la G.

28°N

30°N32°N114°W 112°W

WagnerTransition

ZoneNorth America plate

Tiburón transform fault

Guaymas transform fault

WagnerBasin

North America

Pacific

Present-day relative

plate motion N37°W

TiburónBasinsDelfín

Basins

IslaTiburón

CALIFORNIA

CALIFORNIA

SONORA

Ballenas transform fault

Agua Blanca fa

ult

Sierra Juarez fault

Cerro Prieto fault

San Miguel fault zoneSan Pedro Mártir fault SI-CN faults

Pacific plate

San F

elipe

Puer

tecit

os

Figure 9. Present-day Pacific–North America plate boundary structures in the northern Gulfof California region. Relative plate motion direction is after Atwater and Stock (1998). Spread-ing centers with a solid pattern are actively extending; those with a striped pattern are extinct.The Wagner transition zone is proposed as a region of diffuse plate boundary deformation thatincludes east-northeast–directed extension (double-headed arrow), dextral shear manifested asclockwise vertical-axis rotations along the coast, and dextral-oblique normal slip along approx-imately north- to north-northwest–striking faults submerged in the northern Gulf of California(depicted schematically through Wagner basin; small arrow shows likely trend of slip vectornecessary to meet plate motion vector constraints). A zone of divergence created at the south-eastern margin of the Wagner transition zone can explain the positions and jumps of nearbyspreading centers (upper and lower Tiburón and Delfín basins) since ca. 6 Ma and the timing ofincorporation of the Puertecitos Volcanic Province into the Gulf extensional province in Pliocenetime. Santa Isabel Wash (open circle) is currently within the Gulf extensional province. See Nagy(1997) and Nagy and Stock (2000) for evolutionary plate boundary model of this region since thelate Miocene. Abbreviations: Isla A. de la G.—Isla Angel de la Guarda; SI-CN faults—Santa Is-abel and Cuervo Negro fault systems.

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Geological Society of America Bulletin, June 2000 869

the region of ongoing east-northeast–directed ex-tension in the Wagner transition zone. The upperTiburón basin formed and the spreading axistranslated along the Tiburón transform away fromthe Wagner transition zone. Because east-north-east–directed extension continued in the Wagnertransition zone, a new zone of divergence waseventually created, and spreading in the upperDelfín basin developed 2–3 Ma. Development ofthe Tiburón and Delfín basins may correspond tothe 6 and 3 Ma pulses of Puertecitos VolcanicProvince volcanism (Stock, 1999). The creationof the lower Delfín basin and the northwestwardpropagation of the Guaymas transform 2–3 Mamay have been facilitated by the zone of diver-gence in the southwestern corner of the Wagnertransition zone.

North- to north-northwest–striking late Mio-cene structures control local topographic and ba-thymetric trends within the Wagner transitionzone and differ from northwest-striking bathy-metric features in the central Gulf of California.The intersection of these different zones of defor-mation are reflected in the ~35° jog in the coast-line north and south of the Puertecitos VolcanicProvince. Ongoing east-northeast–directed ex-tension in the Wagner transition zone explains thegreater width (east-northeast to west-southwest)of the northern Gulf of California relative to re-gions farther south. If a system of northwest-striking en echelon strike-slip faults developedwithin the Gulf of California region following thetermination of subduction ca. 11 Ma (e.g., Gans,1997), evidence for a continuation of this systeminto the northern Gulf of California is not cor-roborated by present-day structures and styles ofdeformation.

CONCLUSIONS

Two distinct periods and styles of deformationalong the Gulf extensional province rift marginare recorded in the Santa Isabel Wash region ofthe northern Puertecitos Volcanic Province. Priorto the deposition of 6.3–6.6 Ma tuffs, the north-west-striking Matomí accommodation zone seg-mented the rift margin and separated northeast-to east-northeast–directed extension to the northfrom the undeformed Puertecitos VolcanicProvince to the south. About 300–350 m ofpre–6 Ma northeast-side-down displacementoccurred across accommodation-zone structuresin Santa Isabel Wash. Additional evidence forpre–6 Ma deformation in the region is rare andprobably related to accommodation-zone de-formation.

At some time after the deposition of theplateau-forming 6.3–6.6 Ma tuffs, the Puer-tecitos Volcanic Province underwent <5% east-northeast– to east-directed extension. Rift mar-

gin deformation thus migrated southwestwardfrom the Matomí accommodation zone onto newextensional structures such as the north-strikingSanta Isabel and Cuervo Negro fault systems,which accommodate ~500 m of post–6 Ma east-side-down displacement. A similar amount ofdisplacement of ca. 12.5 and 6.3–6.6 Ma tuffsacross these faults supports the notion that theSanta Isabel Wash region remained outside ofthe Gulf extensional province prior to 6 Ma.Other north-northwest– to north-northeast–strik-ing post–6 Ma faults in Santa Isabel Wash aresecondary faults in the hanging wall of these ma-jor structures and perhaps terminate againstthem at depth. Fresh fault scarps, active sagponds, and stream offsets indicate that exten-sional deformation is actively continuing, anddrainage patterns suggest that Santa Isabel Washis a structurally controlled topographic low re-cently created by east- to northeast-side-downnormal faults.

No measurable relative rotations occurredbetween most paleomagnetic sites within6.3–6.6 Ma tuffs, although two sites at the easternand western limits of the sampled region withanomalous declination directions could be inter-preted to indicate small amounts (10°–15°) ofclockwise rotational deformation. Comparisonswith models of paleosecular variation show thatthe mean directions of 11 cooling units generallyare within permissible bounds (95% confidence)of secular variation of the geomagnetic pole. Avirtual geomagnetic pole calculated by averagingthe 11 mean directions is not statistically differ-ent from the stable paleopole for North America,although a small amount (~10°) of clockwise ro-tation is permissible given uncertainties. Santa Is-abel Wash thus remained predominantly south ofthe region that underwent Pliocene to Holoceneblock rotations. The southern boundary of rota-tional deformation recorded to the north may bea broad, diffuse zone of extensional shear withinthe northeastern Puertecitos Volcanic Provinceand the Santa Isabel Wash region that accommo-dates small vertical-axis rotations. Concealedstructures may also exist below the alluvial plainnorth of Santa Isabel Wash.

The abandonment of portions of the Matomíaccommodation zone, incorporation of thePuertecitos Volcanic Province into the Gulf ex-tensional province, and the commencement ofblock rotations to the north may have occurred2–3 Ma, when strike-slip motion along theTiburón transform fault was replaced by similarmotion along the Guaymas transform fault. Therift margin thus migrated westward onto newstructures as a consequence of changes in thegeometry of the oceanic system. The evolution ofboth rift margin and oceanic structures may bedue to differential movement at the boundary be-

tween a domain of diffuse deformation in north-eastern Baja California (and the northern Gulf ofCalifornia) and the region accommodating plateboundary motion on discrete oceanic structuresin the central Gulf of California.

ACKNOWLEDGMENTS

I thank K. Holt, X. Quidelleur, K. Rostedt,T. Tyndall, and K. Wertz for invaluable field assis-tance,and D.Evans,J.Holt, J.Kirschvink,C. Lewis,M. Oskin, X. Quidelleur, and J. Stock for helpfuldiscussions. I thank J. Besse and J.-P. Cogne for as-sistance with virtual geomagnetic pole calculations.Thoughtful reviews by S. Gilder and C. Lewisgreatly improved an early version of this manu-script. I thank BulletinEditor J. Geissman,Associ-ate Editor L. Brown, and reviewers J. Hagstrumand B. Luyendyk for helpful comments and sug-gestions. This work was supported by National Sci-ence Foundation grants EAR-9218381, EAR-9296102, and EAR-9614674. This is Division ofGeological and Planetary Sciences of the Califor-nia Institute of Technology contribution 8575.

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