Extensional deformation and paleomagnetism at the western margin of the Gulf extensional province, Puertecitos Volcanic Province, northeastern Baja California, Mexico

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

ABSTRACT uncertainties. Geologic relationships show west of the peninsula, accommodated plate mo- that pre–6 Ma accommodation-zone struc- tion. The latter scenario requires ~500 km of During the late Miocene east-northeast–di- tures identified in this study did not mark the northwest-southeast–directed right-lateral dis- rected extension in the Gulf of California ex- southern boundary of later rotational defor- placement within the Gulf extensional province tensional province, the western rift margin in mation documented to the north. The bound- as constrained by late Cenozoic offsets across northeastern Baja California, Mexico, was ary of Pliocene to Holocene rotations may be a the San Andreas fault system to the north (e.g., segmented at the northwest-striking Matomí broad, diffuse zone of extensional shear en- Atwater, 1989), whereas the strain-partitioning accommodation zone. The accommodation compassing the northeastern Puertecitos Vol- model places half of this displacement along the zone passed through the northern Puertecitos canic Province, which accommodated small offshore transform faults. Geologic relationships Volcanic Province and separated pre–6 Ma rotations. in Baja California suggest a middle to late extension to the north from the unextended Miocene age (ca. 12–15 Ma) for the initiation of region to the south. Pre–6 Ma northeast-side- Keywords: accommodation zones, Baja Cali- extension along the western rift margin of the down displacement is documented across the fornia, extension tectonics, paleomagnetism, Gulf extensional province (e.g., Gastil et al., accommodation zone, which may have under- plate divergence, volcanics. 1975; Stock and Hodges, 1990; Lee et al., 1996). gone dextral oblique-slip motion. The rift East- to northeast-directed extension continued margin migrated westward during the latest INTRODUCTION through Pliocene time in much of northeastern Miocene or Pliocene, bypassing accommoda- Baja California (see Nagy and Stock, 2000, for a tion-zone structures and incorporating the Late Cenozoic extension associated with Pa- detailed extension history of northeastern Baja). Puertecitos Volcanic Province into the region cific–North America plate interactions is distrib- Field studies that identify the timing, amount, of Gulf extensional province deformation. uted over a broad region in much of western and direction of extension in the Gulf of Califor- East-northeast– to east-directed extensional North America (e.g., Zoback et al., 1981; Henry, nia region are necessary to clarify the contribution deformation is at least post–6 Ma in the north- 1989). In Mexico, the Gulf extensional province of such extension to overall plate boundary defor- ern Puertecitos Volcanic Province and defor- encompasses the region of high-angle normal mation. In this study I summarize the deformation mation currently continues. Pliocene changes faults bordering the Gulf of California (Fig. 1, in- history within the northern Puertecitos Volcanic in the Gulf of California spreading center sys- set) and has been the location of a divergent por- Province located on the western Gulf extensional tem may have triggered incorporation of the tion of the plate boundary since at least ca. 3.5 Ma province rift margin in northeastern Baja Califor- Puertecitos Volcanic Province into the Gulf ex- (anomaly 2A) when seafloor spreading began in nia (Fig. 1). Detailed field mapping (Nagy, 1997) tensional province as recently as 2–3 Ma. the mouth of the Gulf of California at a full plate within volcanic deposits spanning pre–17 to 6 Ma Paleomagnetic analyses of 6.3–6.6 Ma pyro- rate of ~45–50 mm/yr (e.g., DeMets, 1995). De- (Nagy et al., 1999) provide the basis for struc- clastic flow deposits show no consistent evi- tails are still emerging regarding the earlier history tural interpretations. Paleomagnetic analysis of dence for rotational deformation, although of Gulf extensional province deformation and its 6.3–6.6 Ma pyroclastic flow deposits investigates minor (~10°–15°) clockwise rotation is possi- relationship to the evolving plate boundary and the possibility of relative rotations between fault- ble given anomalous declination directions associated southern Basin and Range extension. bounded structural blocks and provides a compar- recorded at some sites. Comparisons with During the late Miocene, Pacific–North ison with regions a few kilometers to the north, paleosecular variation models and the late America plate motion was probably partitioned where Pliocene to Holocene vertical-axis block Miocene paleopole for stable North America between “proto-Gulf” normal faults accommo- rotations are documented (Strangway et al., 1971; imply no statistically significant rotation rela- dating east-northeast–directed extension (Stock Lewis and Stock, 1998a; Stock et al., 1999). tive to geomagnetic north, although minor and Hodges, 1989) and dextral, offshore trans- (~10°) clockwise rotation is permissible given form faults, such as the Tosco-Abreojos and San GEOLOGIC SETTING Benito faults (Fig. 1, inset; Spencer and Normark, *Present address: Department of Earth Sciences, 1979). Gans (1997) offered an alternative inter- The sharp western margin of the Gulf exten- Syracuse University, Syracuse, New York 13224-1070, pretation in which northwest-striking transform sional province is exposed along the eastern side USA; e-mail: [email protected]. faults within the Gulf of California, rather than of the Baja California peninsula (Main Gulf Es-

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

857 E.A. NAGY

115° 00′W Mártir and Sierra San Felipe faults (e.g., Gastil

San Valle de et al., 1975; Stock and Hodges, 1990; Lewis and

Valle Santa Rosa 01020 Stock, 1998b). Pre–6 Ma deformation south of Ar- Basin royo Matomí (Fig. 1) is documented for the first Pedro kilometers time in this study (see also Nagy, 1997). N Post–6 Ma east-northeast– to east-directed ex- tension continued along structures north of Sierra San Arroyo Matomí as well as on approximately Felipe Mártir San Felipe north-striking normal faults in the Puertecitos Chico Gulf Volcanic Province (Dokka and Merriam, 1982; Sierra fault Lewis and Stock, 1998b; Nagy et al., 1999). San Pedro fault of Mártir Felipe Sierra San Faulted 3 Ma volcanic rocks in the easternmost fault Fermín Puertecitos Volcanic Province may have under- San California A gone less extensional deformation than the 6 Ma MC ° ′ volcanics (e.g., Stock et al., 1991; Martín-Barajas

30 30 N Sierra Arroyo Matomí et al., 1995). Northeast-striking, sinistral strike- ′ UEFZA slip faults also displace 3 Ma volcanic rocks in Figure 2 the Sierra San Fermín (Lewis and Stock, 1998b). fault fault Paleomagnetic studies show that the Sierra San

A" Puertecitos Fermín and Santa Rosa basin have undergone Negro Puertecitos Isabel ~30° of Pliocene to Holocene vertical-axis clock-

Sierra wise rotation relative to Mesa Cuadrada in the Cuervo

Santa Santa Isabel southern Sierra San Felipe (Fig. 1; Strangway Volcanic Arroyo Los Heme et al., 1971; Lewis and Stock, 1998a; Stock et al., Volcan Prieto 1999). Recent scarps along the San Pedro Mártir Province fault and the dextral, strike-slip Valle de San Felipe fault indicate that these structures are ac- tive (Gastil et al., 1975; Grover et al., 1993). The west- to northwest-striking Matomí accom- Alluvium U.S.A. modation zone was first hypothesized near Arroyo MEXICO and gravel Main Baja Matomí on the basis of changes in the style and

Quaternary southern AREA GULF Basin and Range OF amount of faulting to the north and south (Dokka Post-batholithic MAP P Province and Merriam, 1982; Stock and Hodges, 1990), and G volcanic and 30°N Gulf OF Sonora San Benito fault its presence may have influenced Pliocene sedi- Tertiary sedimentary rocks Isla California Tiburón mentation rates in the region (Stock et al., 1996; Batholithic rocks CALIFORNIA Martín-Barajas et al., 1997). The Matomí accom- PACIFIC and prebatholithic OCEAN Tosco-Abreojos fault Escarpment Sinaloa modation zone may also mark the southern bound- metasedimentary ary of rotational deformation (Lewis and Stock,

rocks W ° pre-Cenozoic GEP 1998a). Nevertheless, field evidence for an accom- 115 modation zone south of the Sierra San Fermín had Figure 1. Geologic map (simplified from Gastil et al., 1975) of a portion of northeastern Baja not been identified prior to this study. California, Mexico, showing principal localities and faults discussed in text. Ball and bar are on downthrown side of normal faults; arrows indicate sense of slip on strike-slip faults; faults are EXTENSIONAL DEFORMATION IN THE dotted where concealed. The Matomí accommodation zone (A–A′, after Stock, 1999; and A′–A′′, SANTA ISABEL WASH REGION 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—Mesa Lithology and Extensional Structures Cuadrada; UEFZ—Ultima Esperanza fault zone. Inset: GEP—Gulf extensional province; G— Gonzaga Bay; P—Puertecitos. Faulting and erosion expose as much as 700 m of volcanic deposits along the northern margin of the Puertecitos Volcanic Province in the Santa Is- carpment in Fig. 1, inset) and separates the Creta- ward to ~800 m in southern Valle Chico (Stock abel Wash study area in the Sierra Santa Isabel ceous Peninsular Ranges batholith to the west and Hodges, 1990) at the northern margin of the (Fig. 1). Geologic mapping (scale 1:20 000), litho- from downdropped fault blocks to the east of Puertecitos Volcanic Province (Fig. 1), a late logic and petrographic study, 40Ar/39Ar geochron- batholithic rocks overlain by Cenozoic subduc- Miocene–Pliocene ignimbrite field. The San Pe- ology, and electron microprobe analyses of pheno- tion- and rift-related deposits. In much of north- dro Mártir fault has not been recognized south of cryst compositions define 21 lithologic units that eastern Baja California the rift margin is repre- Valle Chico; the western edge of the Gulf exten- are combined into six Miocene volcanic groups sented by the 100-km-long, east-dipping, San sional province is thus poorly defined within the and one pre-Miocene group (Fig. 2). Lithologic, Pedro Mártir fault (Fig. 1). Although as much as Puertecitos Volcanic Province. Northeast- to east- stratigraphic, and geochronologic details given by 5 km of normal separation occurs on the fault directed extension began 12–6 Ma along major Nagy (1997) and Nagy et al. (1999) are summa- (Gastil et al., 1975), displacement decreases south- east-dipping normal faults such as the San Pedro rized here (oldest to youngest): pre-Miocene

858 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE

Geologic Map of Santa Isabel Wash

A BCDEFGH I J K L MNOPQRST UV 1 2 ′ Picacho 3 B s Canelo N 6 Lithologic Age ( 2 σ) in Ma ′ s 5 4

25 s Group: ° A′ s s 5 Tmr 30 can 5.9 ± 0.8, 6.0 ± 0.4 Santa Isabel 6 7 Tmagem, Tmaugl, Tmahem s s 7 Tmr s fp s s 8 Tmrbs fault Tmr Wash 10 9 ec 6.1 ± 0.5 ? Oculto Tmrao 6.4 ± 0.3 fault 6 4 10 Tmr4 Negro ′′ B 11 Tmr3 6.2 ± 0.1, 6.5 ± 0.2 Tmbnew 4 12

Tmrsiw 6.5 ± 0.3 - 6.7 ± 0.3 Cuervo 13 5 Tmatoro 9.3 ± 0.8 14 Tmr

4 sf 12.7 ± 0.5 Isabel Arroyo A′′ 15 Tmdtomb 15.5 ± 0.7 - 16.7 ± 1.0 3 16 Tmblol, Tmbkc 16.3 ± 1.0, 17.1 ± 2.2 Pico 17

Tmrbio 17.1 ± 2.4 Santa Los Heme 2 18 Tmvs N 19 1 pre-Miocene metasediments 1 km and batholithic rocks 20

114°55′W 114°50′W

Figure 2. Simplified geologic map of Santa Isabel Wash in the northern Puertecitos Volcanic Province. The 40Ar/39Ar ages were determined by laser 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 map that differentiates the 21 lithologic units.) Fault symbols: dashed where approximately located, dotted where concealed (inferred), and illustrated with the letter s where lineaments in the Quaternary alluvium are interpreted to be fault scarps; symbols indicating fault motion are as described in 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) thicken considerably from southwest to northeast. The pre–6 Ma topographic slopes are interpreted to be fault controlled and mark the southwestern margin of the Matomí accommodation zone. Post–6 Ma faults at B′–B″ appear to have reactivated these earlier structures.

(probably Paleozoic) metasedimentary rocks in- Extensional deformation is classified as pre–6 pre–15 Ma rocks (groups 2 and 3) and in one lo- truded by granitoids related to the Peninsular and post–6 Ma based upon the relationship be- cality 12.5 and 9 Ma rocks (groups 4 and 5, re- Ranges batholith (group 1); pre–17 Ma subduc- tween faults and the 6.3–6.6 Ma series of tuffs. spectively); thus deformation is post–9 Ma in tion-related volcaniclastic breccias, debris flows, The majority of structures are north-striking, east- some places but possibly as old as 15 Ma in oth- lava flows, and pyroclastic flow deposits (group and west-dipping, high-angle normal faults. Pla- ers. About 100 m of north-side-down normal sep- 2); ca. 15–17 Ma dacitic and mafic lava flows, nar fault surfaces and striations are rare; thus in aration occurred across two east-striking faults which are also probably subduction-related some cases strike-slip or oblique-slip faulting may (I12 in Fig. 2); the amount of offset is otherwise (group 3); the synrift ca. 12.5 Ma Tuff of San Fe- have produced apparent normal displacement. poorly constrained. lipe (group 4); minor andesitic volcanism ca. 9 Ma There are 14 faults that preserve measurable sur- Thickness and elevation variations of the (group 5); as much as 400 m of 6.3–6.6 Ma pyro- faces and 3 that preserve slickenlines (Fig. 3). The 6.3–6.6 Ma tuffs imply the presence of a discontin- clastic flow deposits or ash-flow tuffs (hereafter re- best indicators of tilt in the region are within the uous, yet significant, pre–6 Ma northeast-facing ferred to simply as tuffs), and an intervening mafic 6.3–6.6 Ma tuffs. Thick Tmr3 deposits generally topographic slope across the entire area. The ap- lava flow, which filled irregular topography to pro- filled preexisting topography and preserved a pla- proximate position of two subparallel, northwest- duce the plateau-like, upper surface of the Puer- nar upper surface; overlying tuffs are roughly tab- striking topographic breaks is shown in Figure 2 tecitos Volcanic Province (group 6); and as much ular in shape. In general these uppermost tuffs are (A′–A′′ and B′–B′′) based upon the geometry of the as 500 m of rhyolite (ca. 6 Ma) and andesite (un- horizontal to slightly west dipping (≤10°). onlapping tuffs. Specifically, individual tuffs occur dated) lava flows (group 7). In addition to modern at lower elevations and are an order of magnitude arroyo sedimentation in Santa Isabel Wash and Ar- Pre–6 Ma Deformation thicker northeast of the slopes relative to deposits royo Oculto, older, high-standing alluvium occurs on the volcanic plateau. For example, basal eleva- along the margins of the washes and surrounds With one significant exception pre–6 Ma faults tions of Tmr4 and Tmrec are 350–390 m and isolated hills in the north. Modern playa deposits are rare in Santa Isabel Wash. East- to east-north- 240–320 m lower, respectively, to the northeast of within closed basins are up to 2 km in diameter in east–striking, north-dipping normal faults, which A′–A′′ relative to the southwest, and the tuffs the washes as well as on the volcanic plateau. do not offset the overlying 6.3–6.6 Ma tuffs, cut change from 1–3 m thick southwest of A′–A′′ to at

Geological Society of America Bulletin, June 2000 859 E.A. NAGY

Measured fault planes and paleomagnetic sampling least 70–80 m thick in Arroyo Oculto. There is less sites in Santa Isabel Wash pre–6 Ma displacement across B′–B′′ (~80–100 m). Downdip lineations within the Tuff of El Canelo

(Trec) indicate flow to the north-northeast across A′–A′′, and steep northeast-dipping contacts of the Picacho Canelo rocks that core the plateau (groups 2 and 3) disap- 30°25′N 55° A D pear below the surface at A′–A′′ and B′–B′′, further 78° C Santa ° supporting a change in base level. 66° B 55 J Isabel ° The deposition of 6.3–6.6 Ma tuffs along the 68° 75 E 76° topographic breaks obscures possible pre–6 Ma 80° M fault ° 83 Wash faults traces; thus erosion could explain the topo-

fault 58° G I O ° Oculto 68° F 59 P graphic variations. Alternatively, this zone may ° 40° H Negro K 20 N 75° have undergone northeast-side-down displace- 75° Isabel ment along northwest-striking normal faults prior

L N Cuervo to 6 Ma. Evidence for pre–6 Ma north-side-down 80° Arroyo faulting in other parts of Santa Isabel Wash con- 1 km firms that the area underwent some pre–6 Ma de- Santa formation, and minor post–6 Ma displacement Pico Los Heme along northwest- to north-northwest–striking A–P: paleomagnetic sites faults at A′–A′′ and B′–B′′ might represent reac- plunge of lineation tivation of older structures. As elaborated in the dip of fault plane Discussion section below, these pre–6 Ma paleo- W W ′ ′ Present-day drainage topographic features may be associated with the 50 55 ° ° Matomí accommodation zone. 114 Abandoned drainages 114 Post–6 Ma Deformation Figure 3. Orientations of 14 measured fault planes in Santa Isabel Wash (alluvium-bedrock contact is shown for simplicity; see Figure 2 for distribution of geologic units). Three fault planes Most normal faults in Santa Isabel Wash offset that preserve slickenlines indicate sinistral oblique slip. Other structures in the area are high- the 6.3–6.6 Ma tuffs. The closely spaced, approx- angle normal faults; see Figure 2 for inferred sense of offsets. Present-day and older drainage pat- imately north-striking Cuervo Negro and Santa terns suggest that Santa Isabel Wash was structurally created by east- to northeast-side-down de- Isabel fault systems are the most continuous struc- formation along north- to northwest-striking normal faults that diverted northeast-directed flow tures in the area, although the northernmost expo- to the southeast. Paleomagnetic sampling locations are also shown. Site J marks the location of sures are complicated by north-northwest–strik- samples collected from the ca. 12.5 Ma tuff of San Felipe (described in Stock et al., 1999). ing splays and cross-structures (C6 in Fig. 2). The faults accommodate ~500 m of east-side-down normal 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 fault plane (Figs. 3 and 4) indicate dip-slip motion with a sinistral component; thus extension may have been slightly oblique. Similar amounts of dis- placement of the two lithologic groups suggest that the faults did not accommodate extension be- tween 12 and 6 Ma. However, the lowest group 6

tuff (Tmrsiw) is absent west of the Santa Isabel fault, 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 these relationships, as could a fault-parallel topographic low east of the fault developed from hanging-wall rollover. Other post–6 Ma north-northwest– to north- northeast–striking normal faults are considered secondary to the Cuervo Negro and Santa Isabel faults on the basis of discontinuous, arcuate out- Figure 4. The approximately north-striking, east-dipping Santa Isabel fault plane (view to the crop patterns of fault traces, relatively small west). Slickenlines (subparallel to hand lens string), which trend N42°E and plunge 58°, record amounts of normal displacement, and the pres- oblique (sinistral) dip-slip motion. ence of extensional relay ramps between subpar-

860 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE allel faults. The density of secondary faults in- the Santa Isabel fault plane and assuming normal zation plots of three pairs of sister samples (i.e., creases a few kilometers east of the Santa Isabel dip slip across the north-northwest– to north- cut from the same core) are shown for compari- fault, where they become spaced fairly regularly northeast–striking faults throughout the area. The son in Figure 5. Both demagnetization proce- every ~1 km. These synthetic and antithetic secondary faults may also have undergone an dures yield similar results; however, secondary structures in the hanging wall of the Santa Isabel oblique (sinistral) component of slip, as observed overprints are in some cases incompletely re- and Cuervo Negro faults may terminate at depth along the Santa Isabel fault. A maximum of 4% moved with thermal techniques (e.g., Fig. 5, A against the east-dipping fault planes. A con- post–6 Ma east-northeast–directed extension was and B), whereas a primary direction is resolved cealed fault is geometrically required between determined by restoring cross sections across the using AF demagnetization. In addition, AF de- two offset, west-dipping outcrops of the region shown in Figure 2 (Nagy, 1997). Although magnetized samples show a more regular and 6.3–6.6 Ma tuffs (F4 in Fig. 2) and accommo- uncertainties in extension direction are large, es- greater drop in magnetic intensity than do ther- dates ~200–300 m of post–6 Ma east-side-down pecially for pre–6 Ma deformation, these results mally demagnetized samples. Both low coerciv- displacement. This fault may be continuous with imply no appreciable change in extension direc- ity and high blocking temperature minerals are the northwest- to north-northwest–striking faults tion from pre–6 to post–6 Ma. This is in agree- present and appear to record the same remanent at location H9 (Fig. 2) which appear to have re- ment with the extensional history recorded else- direction (e.g., Fig. 5C). On the basis of these activated pre–6 Ma structures along B′–B′′. Re- where in northeastern Baja California (e.g., Gastil preliminary tests, the remaining samples were cent motion along these faults may have con- et al., 1975; Dokka and Merriam, 1982; Stock and demagnetized using the AF procedure. trolled the development of the Santa Isabel Hodges, 1989; Lewis and Stock, 1998b). Wash, as described next. Analysis PALEOMAGNETIC STUDY OF LATE Recent Deformation MIOCENE VOLCANIC TUFFS Characteristic remanent magnetization (ChRM) vectors were calculated using principal component Ongoing extensional deformation is sup- Field and Laboratory Methods analysis (Kirschvink, 1980) to selected demagne- ported by the presence of small, structurally con- tization steps for each sample. Low coercivity vis- trolled pull-apart basins that localize sag pond We sampled 4 of the 6.3–6.6 Ma tuffs for pale- cous magnetization was commonly removed by sedimentation in the washes and on the plateau. omagnetic analysis at 15 locations in Santa Isabel the 15 mT step. Single-component, primary Evidence that the Santa Isabel fault is still active Wash (Fig. 3). Cooling breaks within some of the ChRM directions were fit with lines in vector includes sinistral stream offsets across fresh fault tuffs were used to further divide the 4 tuffs into space; eight samples showing only partial removal scarps. Two 2-km-long lineaments in the modern 11 cooling units. All samples record normal po- of an overprint were fit with planes. Paleomagnetic 40 39 alluvium (M7 and O4 in Fig. 2) may be fault larities. The Ar/ Ar ages determined for Tmrao, results for the 11 cooling units are given in Table 1. scarps, although the sense of motion along them Tmr3, and Tmrsiw span 6.2 ± 0.1 Ma to 6.7 ± A Fisher (1953) distribution function is used to is not apparent. Additional evidence for Quater- 0.3 Ma (Nagy et al., 1999); thus the tuffs most calculate the uncertainties for the tilt-corrected site nary deformation includes a change in flow di- likely erupted during subchron C3An.2n means. I discarded 13 analyses on the basis of rection of the Santa Isabel Wash drainage sys- (6.269–6.567 Ma; Cande and Kent, 1995). Be- poorly preserved ChRM directions or as extreme tem, from northeast to southeast directed. The tween 4 and 8 cores were drilled at each site for a outliers from the rest of the samples at a site (>15° locations of the present wash, as well as aban- total of 217 samples (see Nagy, 1997, for collec- from site mean). ChRM directions cluster well at α doned drainages determined from sediment pat- tion and preparation details). Samples were meas- individual sites with 95 < 10°. Three exceptions α α terns in inactive washes, are indicated in ured at the Caltech Paleomagnetics Laboratory in occur at site D (Tmrao [ 95 = 17.6°], Tmr4 [ 95 = α Figure 3. North- to northwest-striking normal a cryogenic SQUID (superconducting quantum 12.7°], and Tmrsiw (t3) [ 95 = 10.8°]), where the faults south of the wash may have accommo- interference device) magnetometer in a magneti- tuffs are less welded than at most other sites, al- dated east- to northeast-side-down displacement, cally shielded µ-metal room. Anhysteretic and though there is no clear correlation between the thereby creating the northwest-southeast–trend- isothermal remanent magnetization tests and degree of welding and the amount of dispersion in ing topographic low (i.e., hanging-wall rollover) magnetic susceptibility measurements were per- ChRM directions. Small tilt corrections were ap- that forms the present-day wash between E5 and formed to discern the nature of the grains carrying plied based on contact dips between tuffs, and not J9 in Figure 2. the magnetic remanence in the rocks. Results in- on the basis of eutaxitic foliation planes, although dicate a predominance of ferrimagnetic minerals, the orientations of such planes were measured. In- Amount and Direction of Extension such as single-domain or pseudosingle-domain clined eutaxitic foliation planes developed above magnetite, and smaller amounts of antiferro- the Curie temperature of the remanence-carrying The direction of pre–6 Ma extension is poorly magnetic minerals, such as hematite. Superpara- minerals and do not represent paleohorizontal in constrained. Pre–6 Ma faults are east- to east- magnetic grains were detected in Tmrsiw(t2) (see this study (Nagy, 1997). northeast–striking and accommodate north-side- Nagy, 1997, for details). down normal separation; however, fault-plane Alternating field (AF) and thermal demagneti- Results striae on one of these indicate a significant com- zation techniques were compared in a pilot study ponent of sinistral strike-slip motion (Fig. 3). Dip- of 46 samples. Half of the samples were sub- The paleomagnetic data summarized in slip motion along the inferred pre–6 Ma approxi- jected to progressive AF demagnetization, typi- Table 1 suggest that the Santa Isabel Wash region mately northwest-striking structures at A′–A′′ and cally including 13 steps to 80 mT and in some may have undergone a small amount of clock- B′–B′′ implies northeast-directed extension. In cases to 260 mT. These samples were also de- wise vertical-axis rotational deformation. Unfor- any case, most of Santa Isabel Wash did not magnetized thermally after the AF procedure. tunately, this interpretation relies heavily on di- undergo significant pre–6 Ma deformation. The other half of the sample were thermally de- rections recorded at only two sites at the east and Post–6 Ma east-northeast– to east-directed exten- magnetized in about 12 heating steps between west limits of the region sampled. Furthermore, sion is inferred on the basis of slip indicators on 100 °C and 690 °C. AF and thermal demagneti- typical variations within a given cooling unit be-

Geological Society of America Bulletin, June 2000 861 C ° C C C ° ° ° 300 300 590 590 NRM E, N E, E, N E, NRM NRM NRM 80 mT 80 mT N, up N, N, up N, S, down S,

S, down S, (t5) W, S W, S

siw mT mT Tmr -3 -5 C) °

(

300 500

100 700 mT mT C) ° -3

-4 (

300 500 Final intensity = 3.1 x 10 NRM intensity = 3.4 x 10 Demagnetization step Demagnetization step (mT) Sample from

100 700 20 40 60 80 NRM intensity = 3.4 x 10 Final intensity = 4.3 x 10

NRM NRM E E mT mT mT mT -3 -6 -3

-7 C) °

( NRM NRM

100 300 500 700 ao N C) N °

(

300 500 700 Tmr Final intensity = 5.7 x 10 NRM intensity = 2.5 x 10 NRM intensity = 2.7 x 10 Final intensity = 8.6 x 10 Demagnetization step Demagnetization step (mT) ized to the initial magnetic intensity. The histograms show the fraction of total intensity re- The histograms show ized to the initial magnetic intensity. AF demagnetization fol- subjected to demagnetization step. Samples were at a given moved heating by thermal demagnetization (top) or thermally demagnetized incremental lowed magnetization. (bottom). NRM—natural remanent

100 W W

NRM NRM 20 40 60 80 E, N E, NRM NRM NRM E, N E, Sample from NRM ° ° 450 450 10 mT 10 mT N, up N, N, up N, S, down S, S, down S, W, S B. W, S E mT mT mT mT

C) -3 -5 ° -3 -4 (

300

(t4) 100 500 700 N N

C)

° siw (

300 500 NRM NRM intensity = 4.0 x 10 Final intensity = 1.0 x 10 NRM NRM intensity = 5.0 x 10 Final intensity = 2.0 x 10 Demagnetization step

40 (mT) Tmr Demagnetization step WE

100 700 W NRM 20 60 80

NRM E, N E, 15 mT 20 mT 20 mT E, N E, 15 mT ° ° N, up N, Sample from S, down S, 300 S, down S, 300 N, up N, NRM NRM NRM NRM W, S W, S A. C. AF and thermal demagnetization Thermal demagnetization Figure 5. Pairs of sister samples from three cores (A, cores three of sister samples from 5. Pairs Figure B, of alter- C) illustrate the results nating field (AF) and thermal demagnetization techniques by Zijderveld plots (closed/open (AF) and thermal demagnetization techniques by Zijderveld nating field projections), horizontal/vertical symbols represent plots (iso- equal-area hemisphere lower at sampling location), field is present-day lated circle and magnetic intensity plots normal-

862 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE

TABLE 1. PALEOMAGNETIC DATA FROM THE PUERTECITOS VOLCANIC PROVINCE, BAJA CALIFORNIA, MEXICO N Location Bedding N/Nc Geographic Stratigraphic Fisher κα Strike Dip Dec Inc Dec Inc 95 Tmrao (one cooling unit) Site B* 187 5.5 5/5 12.1 51.8 5.1 52.0 213 5.4 Site C 187 5.5 5/5 4.9 42.7 359.9 42.2 192 5.5 Site D 187 5.5 3/5 9.3 45.7 3.6 45.6 50 17.6 Site 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 P Tmr4 (two cooling units) Tmr4 Site B* 187 5.5 5/5 6.5 42.4 1.5 42.1 236 5.4 Site 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 5.3 Tmr3–4 Site B 187 5.5 5/5 0.8 42.3 355.9 41.4 175 5.8 Site C* 187 5.5 5/6 2.1 33.8 358.5 33.1 53 10.9 W E Site 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) Tmrao Upper Tmr3 (type II) Tmr4 Site L 0 0.0 8/8 29.3 52.1 29.3 52.1 135 4.8 Tmr3-4 Mean direction Middle Tmr3 (type II) Tmr3 - Upper Type II Site L 0 0.0 7/7 31.6 58.2 31.6 58.2 410 3.0 Tmr3 - MiddleType II Site N 0 0.0 7/7 21.3 53.5 21.3 53.5 435 2.9 Tmr3 - LowerType II P Middle (type II) mean: 14/14 26.1 56.0 26.1 56.0 225 2.7 Tmr3 - Type I Lower Tmr3 (type II) Tmrsiw - t5 Present-day field Site E 250 8.0 5/5 31.2 61.8 21.5 56.3 1085 2.3 Tmrsiw - t4 Site I 180 4.0 6/6 30.8 66.3 22.3 68.0 544 2.9 Tmrsiw - t3 Site L 0 0.0 7/7 28.2 58.5 28.2 58.5 206 4.2 Site M* 0 0.0 5/7 24.0 34.7 24.0 34.7 66 9.8 Tmrsiw - t2 Site N 0 0.0 7/7 28.5 47.2 28.5 47.2 331 3.3 Site O 250 8.0 4/4 36.2 64.8 24.5 59.7 229 6.1 Figure 6. Equal-area projection of tilt-cor- Lower (type II) mean: 34/36 29.4 55.6 25.9 54.4 48 3.6 rected mean directions (with projected Fisher Tmr3 (type I) α confidence cones) from 11 volcanic cooling Site A* 182 6.5 5/5 354.5 67.5 340.7 65.7 258 5.1 95 Site D 187 5.5 4/5 354.0 66.7 342.4 64.9 116 8.6 units sampled for paleomagnetic analysis Site K 186 4.0 3/6 321.5 54.9 317.8 52.0 291 7.2 from Santa Isabel Wash. The 2σ confidence Tmr3 (type I) mean: 12/16 342.3 64.1 333.0 61.7 68 5.3 limits for paleosecular variation around a Site K removed: 9/10 354.1 67.1 341.4 65.3 185 3.9 Miocene geomagnetic pole (D = 0°, I = 50°) Tmr (four cooling units) siw were calculated using model C1 (solid ellipse) Tmr (t5) siw and the volcanic database (dashed ellipse) of Site A 182 6.5 5/5 17.9 40.7 12.3 42.2 116 7.1 Site E 250 8.0 5/5 20.0 46.4 15.4 40.1 3849 1.2 Quidelleur and Courtillot (1996). Standard Site F 0 0.0 6/6 28.1 47.2 28.1 47.2 546 2.9 deviations of the declination and inclination Site G 0 0.0 4/6 20.4 36.1 20.4 36.1 152 7.5 σ σ Site H 312 10.0 5/5 13.1 49.2 17.6 40.3 330 4.2 ( D, I) between lat 30°N and 40°N are (12.5°, Site I 180 4.0 6/6 31.3 42.7 27.9 44.7 184 5.0 13°) from model C1 and (15°, 12.5°) from the Site K 186 4.0 6/6 9.0 42.2 5.3 42.3 149 5.5 database, and were used to calculate the 2σ Site M 0 0.0 7/7 6.9 42.0 6.9 42.0 126 5.4 α Site N 0 0.0 8/8 25.7 38.9 25.7 38.9 418 2.7 confidence limits for declination ( 95 [dec]) Tmr (t5) mean: 52/54 19.3 43.1 17.8 42.0 88 2.1 α α σ siw and inclination ( 95 [inc]) via: 95 (dec) = (2 Site K removed: 46/48 20.7 43.1 19.5 41.9 99 2.1 α σ D) cos (50°) and 95 (inc) = (2 I). With the ex- Tmrsiw (t4) Site D 187 5.5 5/5 26.9 24.8 24.4 26.6 461 3.6 ception of Tmrsiw (t3) and Tmrsiw (t4), all Tmrsiw (t3) mean directions overlap with expected paleo- Site D 187 5.5 5/5 26.6 29.5 23.5 31.0 51 10.8 secular variation limits, suggesting that rota- Site P 312 10.0 6/6 37.9 59.5 38.8 49.5 107 6.5 tion of the entire region relative to geomag- Tmr (t3) mean: 11/11 31.3 46.1 30.8 41.4 31 8.3 siw netic north is not likely. The mean direction of Tmrsiw (t2) Site D 187 5.5 4/5 17.2 38.5 12.7 39.2 112 8.7 the 11 cooling units is dec = 14.4°, inc = 46.1°, Site G 0 0.0 5/5 20.0 35.0 20.0 35.0 723 2.8 α Fisher 95 = 8.7°. Site N 0 0.0 7/7 25.1 29.1 25.1 29.1 834 2.1 Site 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. inferred fault east of site K, concealed beneath Santa Isabel Wash near E10 in Figure 2, could be tween relatively unrotated sites suggest that the also a 15° difference in declination recorded in a structure that separates site K from a region of

amount of rotation is at the threshold of resolu- Tmrsiw (t3) between sites D and P (Fig 3). If the rotation to the east. Site P is separated from the tion. Specifically, declinations recorded at site K directions recorded at sites K and P are accurate, other sampling sites by a major north-striking,

(Fig 3) in Tmrsiw (t5) and Tmr3 (type I) are it implies that the entire area rotated 10°–20° west-dipping normal fault (N8–N18 in Fig. 2), 10°–25° counterclockwise of most directions clockwise relative to site K, and that site P under- which could mark a boundary between regions of preserved in the same units at other sites. There is went an additional 15° of clockwise rotation. An differential rotation.

Geological Society of America Bulletin, June 2000 863 E.A. NAGY

° or natural variations of the preserved magnetiza- 90 tion directions. Incorrect structural corrections could account for anomalous results, although re- alistic uncertainties in bedding tilts do not signifi- cantly change the mean directions. 180° 0° An argument for rotational deformation would be more convincing if more of the stratigraphic package had been sampled at sites K and P. With the 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 from sites K and P are thus removed from the following discussion in order to examine the directions recorded in the rest of the study area without the complication of possible rotations relative to these

two sites. Mean directions for Tmrsiw (t5) and Tmr3 (type I) without site K are listed in Table 1.

Virtual Geomagnetic Pole Direction from Santa Isabel Wash

study site 270° An equal-area plot of the 11 cooling unit mean directions, without data from sites K and P, is shown in Figure 6. Secular variations of the Earth’s magnetic field are the most likely reason North America paleopole (3–10 Ma) from Global Paleomagnetic database for the 11 different directions. A comparison be- tween the mean directions and statistical studies of North America paleopole (0–20 Ma) after Besse and Courtillot (1991) paleosecular variation (Quidelleur and Courtillot, Virtual Geomagnetic Pole from Santa Isabel Wash (uncorrected) 1996) shows that most mean directions are within expected limits of paleosecular variation around Virtual Geomagnetic Pole from Santa Isabel Wash (corrected for post-5.5 Ma the Miocene geomagnetic pole. The 2 σ confi- displacement relative to North America) dence limits are shown in Figure 6 centered on D = 0° and the expected geocentric axial dipole value Figure 7. Northern hemisphere (equal area) projection showing virtual geomagnetic pole di- of I = 50° at the study latitude. Shallow inclina- rections (with 2 σ uncertainty ellipses) from Santa Isabel Wash. The average direction of the 11 tions recorded in some of the Tmrsiw cooling units cooling units gives a virtual geomagnetic pole at lat 77.6N°, long 341.0E° (dp = 22.2°, dm = 9.9°). are from a single site (D) and may be due to rota- In order to account for divergence between Baja California (on the Pacific plate) and the North tion of magnetic grains during emplacement (e.g., America plate since the latest Miocene, the virtual geomagnetic pole is rotated 4.7° (e.g., Stock Rosenbaum, 1986) rather than tectonic in origin. and Hodges, 1989) about a pole at lat 48.7°N, long 281.8°E determined from the global plate mo- Because most mean directions are within ex- tion model NUVEL-1 (DeMets et al., 1990; DeMets, 1995). The corrected virtual geomagnetic pected paleosecular variation limits, rotation of pole for Santa Isabel Wash is at lat 80.2N°, long 335.8E°. The virtual geomagnetic pole directions the entire region relative to geomagnetic north is are statistically indistinguishable from a stable North America paleopole (lat 97.2N°, long not likely. To further test this interpretation, aver- α 86.8E°, 95 = 3.8°) calculated from selected sites within the 1997 Global Paleomagnetic Database aging the 11 directions may average the secular ≤ (McElhinny and Lock, 1995) for studies in 3–10 Ma rocks. Selection criteria included dp 15°, variation signal recorded in the different cooling n (sites) > 3, and N (samples) > 35. Results from Santa Isabel Wash are also statistically indis- units, thereby providing a virtual geomagnetic α tinguishable from a stable North America paleopole (lat 87N°, long 57E°, 95 = 7°) calculated pole direction that can be compared to the stable from 0–20 Ma by Besse and Courtillot (1991) in their study of apparent and true polar wander. North America paleopole. As illustrated in Fig- A small (~10°) amount of clockwise rotation of Santa Isabel Wash relative to north would not be ure 7, a virtual geomagnetic pole direction from detectable given uncertainties. Santa Isabel Wash, corrected for divergence be- tween Baja California and the North America plate since the latest Miocene, overlaps within Other examples of anomalous directions are results from site M are in good agreement with 95% confidence limits with the stable North not consistently recorded between different cool- mean directions in overlying lower Tmr3 (type II) America paleopole (Besse and Courtillot, 1991; ing units, which casts some doubt on the results and Tmr3–4. Declinations recorded at site I also McElhinny and Lock, 1995). The angular differ- from sites K and P. For example, the mean decli- differ inconsistently relative to the mean direc- ence between the sampling site and the virtual ge- nation recorded at site M in Tmrsiw (t5) is coun- tions in Tmrsiw (t5), lower Tmr3 (type II), and omagnetic pole directions from Santa Isabel Wash terclockwise of the directions recorded within this Trao. These examples suggest that in some cases (50.3°) and the North America paleopoles calcu- cooling unit at most other sites and is very similar anomalous directions are probably not tectonic in lated from McElhinny and Lock (1995) (60.1°) to results from site K in Tmrsiw (t5). In contrast, origin but rather due to sampling procedure errors and Besse and Courtillot (1991) (58.0°) are 9.8° ±

864 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE

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 α95

6.3–6.6 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.

8.6° and 7.7° ± 9.7°, respectively. Results from mín and Santa Rosa basin have undergone ~30° been suggested in several studies (Dokka and Santa Isabel Wash are thus statistically indistin- vertical-axis clockwise rotation relative to Santa Merriam, 1982; Stock and Hodges, 1990; Axen, guishable from the stable North America paleo- Isabel Wash since the time of deposition. A slight 1995), and the diffuse nature of upper crustal de- pole, although a small amount of clockwise rota- difference in declination direction (~10°) is formation along the rift margin further favors a tion (~10°) is permissible within uncertainties. recorded in the tuff of San Felipe between Santa subhorizontal basal shear zone. Isabel Wash and Mesa Cuadrada, again support- Regional Distribution of Rotational ing a small (~10°) amount of clockwise rotational DISCUSSION Deformation deformation in Santa Isabel Wash as discussed above. Given results from Tmr4 and the tuff of Matomí Accommodation Zone Lewis and Stock (1998a) presented paleomag- San Felipe, lithologic correlations between inter- netic evidence for 31° ± 15° vertical-axis clock- vening sampled tuffs are problematic. A series of Dokka and Merriam (1982) first proposed a wise rotation of the Sierra San Fermín relative to tuffs designated Tmr3 in these studies, based west-striking accommodation zone in the vicin- Mesa Cuadrada in the southern Sierra San Felipe upon correlations with stratigraphy defined in the ity of Arroyo Matomí (Fig. 1) to explain the (Fig. 1). This amount of relative rotation is an av- southern Valle Chico (Stock, 1989), is similar in northward termination of late Miocene–Pliocene erage value found within ca. 12.5 and 6.3–6.6 Ma mineralogy, field appearance, stratigraphic posi- faults in the eastern Puertecitos Volcanic tuffs and agrees with the sense and amount of tion, and age throughout the region. Given paleo- Province. Stock and Hodges (1990) further in- rotation found in Pliocene sediments in the east- magnetic results from the underlying and over- ferred that the accommodation zone separated ern Sierra San Fermín (Strangway et al., 1971). lying tuffs, however, lithologic correlation of any pre–6 Ma east-northeast–directed extension Lewis and Stock (1998a) also identified ~25º–30° Tmr3 units between Santa Isabel Wash and the north of Arroyo Matomí from the undeformed re- clockwise rotation of a 3 Ma ash-flow tuff in the other sampling locations implies significantly gion to the south. In Axen’s (1995) regional Sierra San Fermín relative to a reference point in different amounts and relative senses of rotation analysis of segmentation and vergence reversal of Arroyo Matomí. Because some of the same pyro- (Table 2). It is likely that there are at least six the Main Gulf Escarpment, he assigned the entire clastic flows were sampled at Santa Isabel Wash, cooling units in the Tmr3 position throughout the Puertecitos Volcanic Province region to the role comparisons between these distant sites can be region (four Tmr3 cooling units in Santa Isabel of a late Miocene west- to west-northwest–strik- made without concern for secular variations of Wash and two in the Sierra San Fermín and ing accommodation zone separating an east-di- the Earth’s magnetic field. southern Sierra San Felipe). Tmr3a and Tmr3b rected normal fault system to the north from a A summary of the regional paleomagnetic re- from the Sierra San Fermín are unlikely to be west-directed system to the south. A Pliocene age sults and lithologic correlations (after Nagy et al., correlative with the tuffs of Santa Isabel Wash for the accommodation zone has also been in-

1999) is given in Table 2. Correlations are most (Tmrsiw), which are restricted to the region south ferred. For example, Lewis and Stock (1998a) robust between the tuff of San Felipe (Tmrsf in of Arroyo Matomí (Nagy et al., 1999). claimed that the Matomí accommodation zone Fig. 2) and Tmr4. Note that the tuff of San Felipe Lewis and Stock (1998b) suggested that rota- marks the southern boundary of Pliocene to was also sampled in Santa Isabel Wash (site J in tional deformation in the Sierra San Fermín is ei- Holocene rotational deformation in the Sierra Fig. 3) and in Santa Rosa basin (Fig. 1); paleo- ther (1) restricted to a region above a subhorizon- San Fermín, and the style of Pliocene rift margin magnetic data were given in Stock et al. (1999). tal detachment or basal shear zone or (2) the sedimentation in the eastern Puertecitos Volcanic Inclination values recorded in Tmr4 and the tuff surface expression of a vertical shear zone link- Province and the Sierra San Fermín is attributed α of San Felipe agree within 95 between Santa Is- ing major transform faults to the north and south. to an accommodation-zone setting (Stock et al., abel Wash and the other regions. Different decli- Upper plate detachment faulting along the rift 1996; Martín-Barajas et al., 1997). In summary, nation directions indicate that the Sierra San Fer- margin east of the San Pedro Mártir fault has these studies imply the existence of a pre–6 Ma

Geological Society of America Bulletin, June 2000 865 E.A. NAGY segmentation along the Gulf extensional of southern Valle Chico, Stock and Hodges Pliocene to Holocene rotations because Mesa province rift margin that continued to mark the (1990) described a west-northwest–striking ex- Cuadrada, directly to the north, is not within the approximate location of a rift margin accommo- tensional accommodation zone (Ultima Esper- rotated region. The Ultima Esperanza fault zone dation zone during the Pliocene. anza fault zone after Stock, 1993), which they as- terminates westward against a high-angle north- Paradoxically, field evidence for pre– or sociated with the Matomí accommodation zone. northwest–striking fault that might be the contin- post–6 Ma accommodation-zone structures is The Ultima Esperanza fault zone (Fig. 1) is com- uation of the Valle de San Felipe fault (Fig. 1; scarce. Most significantly, accommodation-zone posed of anastomosing strike-slip and dip-slip Stock and Hodges, 1990). A large alluvial plain structures have not been identified within 3 and faults along which the sense of offset is not evi- north of Santa Isabel Wash conceals the area east 6 Ma volcanic rocks extensively faulted by north- dent (Stock, 1993). Displacement along the fault of the Ultima Esperanza fault zone, where it may northwest–striking normal faults in Arroyo zone occurred after deposition of the ca. 12.5 Ma have been offset by a Pliocene northeast-striking Matomí (Stock et al., 1991), or within 3 and 6 Ma tuff of San Felipe (unit Mr1 of Stock, 1993; sinistral strike-slip fault (Fig. 1; Stock, 1999). rocks between Arroyo Matomí and Arroyo Los geochronology updated by Stock et al., 1999), The pre–6 Ma approximately northwest- Heme (Fig. 1; Stock et al., 1991; Martín-Barajas but is otherwise unconstrained. The fault zone striking normal faults inferred in the Santa Is- et al., 1995). Farther west, in the southeast corner cannot correspond to the southern boundary of 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- A Pre-6 Ma ated with the Matomí accommodation zone. Be- cause these fault zones are not aligned along strike, the region of accommodation-zone struc- NE- to ENE-directed extension tures either broadened southeast of the Ultima Esperanza fault zone or formed an en echelon southwestern margin of series of faults stepping southward into Santa Matomí accommodation zone Isabel Wash. Differential extension across the N two segments in Santa Isabel Wash suggests a zone of deformation consisting of multiple sub- A′ within GEP ~ 12.5 Ma tuff relatively region of parallel structures rather than a single thorough- undeformed ′′ deformation going fault. region A The Matomí accommodation zone may have ? formed in a zone of weakness separating two off- “future” Cuervo Negro fault ? set portions of the north-northwest–striking rift “future” Santa Isabel fault margin during east-northeast–directed extension (Dokka and Merriam, 1982; Stock, 1999). One of these rift margin structures was the early San Ped- ro Mártir fault, and the other one was perhaps lo- B Post-6 Ma cated near Gonzaga Bay (Fig. 1, inset; Stock, 1999). Stock and Hodges (1990) suggested that dextral strike-slip motion along the Matomí ac- ENE- to E-directed extension commodation zone transferred slip from the south- ern end of the San Pedro Mártir fault onto Gulf of California structures to the southeast by dextral reactivation along W-dipping volcanic rocks pre-6 Ma NW-striking oblique-slip motion. Geologic relationships across in hanging wall of structures major E-dipping faults inferred accommodation zone structures in Santa N Isabel Wash, which document down-to-the-north- present-day east normal separation, further support an oblique 6.3-6.6 Ma tuffs Santa Isabel Wash sense of motion. The Matomí accommodation ~ 12.5 Ma tuff Cuervo Negro fault Santa Isabel fault zone may be structurally similar to oblique-slip transfer faults on the eastern margin of the Gulf of slickenlines with 60-80° rake Suez (Red Sea rift system), which developed be- ′′ pre-6 Ma Matomí A accommodation zone tween originally offset, main-rift normal faults ? (McClay and Khalil, 1998). An irregularity in the early to middle Miocene arc may have been ex- synthetic and antithetic structures in hanging wall ploited at the time of rift margin development, of major E-dipping faults causing the initial offset (Stock, 1999). The loca- Figure 8. Schematic block models of pre–6 and post–6 Ma deformation in Santa Isabel Wash. tion and position of accommodation zones along (A) Prior to 6 Ma most of the northern Puertecitos Volcanic Province was located southwest of other rift margins, such as in East Africa, similarly the northwest-striking Matomí accommodation zone (simplified; see Fig. 2 for segmentation), followed preexisting structural discontinuities or which separated the Gulf extensional province (GEP) to the northeast from the Puertecitos Vol- zones of weakness (e.g., Bosworth, 1992; canic Province. Northeast- to east-northeast–directed extension is inferred. (B) Most faults in Kilembe and Rosendahl, 1992; Scott et al., 1992). the study area displace all mapped units and are thus post–6 Ma in age. Slickenlines and fault Most northwest-striking accommodation zone orientations indicate east-northeast– to east-directed extension. structures in Santa Isabel Wash were bypassed af-

866 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE ter 6 Ma when the rift margin migrated westward rection. Dextral oblique-slip predominates along The Santa Isabel and Cuervo Negro fault sys- onto approximately north-striking extensional late Miocene, northwest-striking faults in the tems accommodate more than half of the offset faults such as the Santa Isabel and Cuervo Negro Sierra San Fermín (Lewis and Stock, 1998b); thus observed along the San Pedro Mártir fault in faults (Fig 2). Although some accommodation a similar sense of motion along this zone in Santa southern Valle Chico (~800 m; Stock, 1989), sug- zone structures were reactivated, paleomagnetic Isabel Wash does not contradict the late Miocene gesting that they represent principal structures results show that no relative rotation occurred regional pattern of deformation. Other pre–6 Ma accommodating Pliocene to Holocene rift margin across these faults since 6 Ma. For example, approximately east-striking faults accommodate deformation. The northward projection along paleomagnetic sampling sites B, C, and D (Fig. 3) relatively minor north-side-down displacement strike of the Santa Isabel and Cuervo Negro faults are northeast of the northwest-striking faults reac- (not differentiated in Figure 8) and are perhaps re- is ~10 km east of the San Pedro Mártir fault and tivated at B′–B′′ (Fig. 2). Thus the southern struc- lated to accommodation-zone deformation. approximately along strike with the Valle de San tural boundary of Pliocene to Holocene rotations Most extension in Santa Isabel Wash oc- Felipe fault (Fig. 1). The San Pedro Mártir nor- in the Sierra San Fermín, inferred by Lewis and curred after the deposition of 6.3–6.6 Ma tuffs. mal fault and dextral Valle de San Felipe strike- Stock (1998a) to be the west-northwest–striking Faults generally strike north-northwest to north- slip fault were interpreted by Grover et al. (1993) Matomí accommodation zone, cannot coincide northeast, although some faulting localized on to link the Gulf of California transform system to with the pre–6 Ma accommodation zone struc- pre–6 Ma northwest-striking accommodation- transpeninsular faults to the north by partitioning tures inferred in this study. Pliocene accommoda- zone structures (B′–B′′ in Fig. 2). The Santa Isabel of normal and dextral slip. Reconnaissance in- tion-zone structures may be concealed below the and Cuervo Negro fault systems are major, east- vestigations have not revealed major rift margin large alluvial plain north of Santa Isabel Wash, al- dipping normal faults that accommodate ~500 m structures west of the Santa Isabel Wash region though where they project farther to the east is un- of post–6 Ma displacement. Conformable contacts (Gastil, et al., 1975; Dokka and Merriam, 1982; known. It is interesting that an unidentified between 12.5 and 6 Ma tuffs further imply that the J. Stock, 1997, personal commun.), although Pliocene structural boundary must also occur area did not undergo significant pre–6 Ma defor- northwest- to north-northwest–striking structures within the Sierra San Felipe, separating the ro- mation, in contrast to angularly unconformable in the northwestern Puertecitos Volcanic Province tated Santa Rosa basin from Mesa Cuadrada. contacts between the same units north of Arroyo (Fig. 1) probably link extensional deformation at Overall paleomagnetic results from Santa Isabel Matomí (Gastil et al., 1975; Dokka and Merriam, the south end of the San Pedro Mártir fault to the Wash, and especially results from site P, hint that 1982; Stock, J., 1989, 1993; Lewis and Stock, Santa Isabel Wash region. the eastern Puertecitos Volcanic Province may 1998b). The Santa Isabel and Cuervo Negro faults have in fact undergone a small amount may merge at depth to form a listric detachment Influence of Oceanic Plate Boundary (~10°–15°) of rotational deformation. Rather than surface such as interpreted for the San Pedro Structures on Rift Margin Deformation being represented by a discrete fault zone, it is Mártir fault (Dokka and Merriam, 1982; Stock and possible that the southern boundary of rotational Hodges, 1990). A shallow westward dip of There is an ambiguous relationship between deformation is a broad, diffuse zone involving ex- 6.3–6.6 Ma tuffs in the hanging wall of the Santa late Miocene structures developed along the Gulf tensional structures in the northeastern Puertecitos Isabel fault is perhaps the result of reverse drag extensional province margin and those of the Volcanic Province, including Santa Isabel Wash, along the fault, also implying a listric geometry at Pliocene–Quaternary oceanic spreading center which accommodated small amounts of shear (ro- depth. High-angle normal faults in the hanging system within the Gulf of California. The high tational) deformation. This scenario is not sup- walls of the Santa Isabel and Cuervo Negro faults angles between several accommodation zones ported, however, by results from the Arroyo are secondary structures that perhaps intersect the and Gulf of California transform faults, as well as Matomí sampling site in the study by Lewis and east-dipping fault planes at depth. Rotational their relative ages and extension directions, sug- Stock (1998a), where declinations recorded in a deformation is not statistically supported by the gest that there is little relationship between the 3 Ma tuff are 24°–30° counterclockwise of direc- paleomagnetic results for most of the area, al- earlier segmentation and the subsequent geome- tions found at two sites in the Sierra San Fermín. though further sampling at the eastern and western try of the Gulf of California spreading center sys- limits of the area would help clarify some anom- tem (Axen, 1995). Stock (1999), however, found History of Deformation in Santa Isabel Wash alous declination values. A minor amount (~10°) correlations between several accommodation of clockwise rotation of the entire area relative to zones in northeastern Baja California and major The Gulf extensional province rift margin has north is permissible given uncertainties. transform offsets in the Gulf of California. The evolved structurally in the Santa Isabel Wash re- Extensional deformation continues in Santa Is- most convincing relationship is between the gion since late Miocene time. Schematic block abel Wash, as evidenced by sag ponds, offset Tiburón transform fault (Fig. 9) and the Matomí models (Fig. 8) illustrate deformation before and drainages, and fault scarps in the active alluvial accommodation zone, although structures associ- after 6 Ma. Prior to 6 Ma Santa Isabel Wash was surface, and may have controlled recent changes ated with the accommodation zone have not been separated from the extending region to the north in the drainage pattern of Santa Isabel Wash. The identified in the eastern Puertecitos Volcanic by the northwest-striking Matomí accommoda- Puertecitos Volcanic Province may continue to Province. If the pre–6 Ma approximately north- tion zone. Roughly 300–350 m of pre–6 Ma undergo extensional deformation with an in- west-striking structures in Santa Isabel Wash are northeast-side-down displacement occurred creasing amount of dextral shear over time, and related to the accommodation zone, their orienta- across the zone. A strike-slip component of mo- may eventually be fully incorporated into the re- tion is not at a high angle to transform fault ori- tion along the zone is not evident in Santa Isabel gion of rotational deformation. Northeast-strik- entations, although they do differ in relative age Wash; however, dextral oblique slip may have oc- ing faults within the Quaternary alluvium in and extension direction from Gulf of California curred if the structures are related to those that Santa Isabel Wash could represent structures ac- structures. transferred slip from the San Pedro Mártir fault commodating conjugate plate boundary slip such The timing of formation of pull-apart basins onto Gulf of California structures to the southeast as described along post–3 Ma structures in the between offset northwest-striking, dextral strike- (Stock and Hodges, 1990), implying a more east- Sierra San Fermín and Arroyo Matomí regions slip faults in the central and northern Gulf of Cal- erly (e.g., east-northeast–directed) extension di- (Lewis and Stock, 1998b). ifornia is poorly constrained because of the ab-

Geological Society of America Bulletin, June 2000 867 E.A. NAGY

Wagner Diffuse Plate Boundary Deformation in the Transition North America plate Northern Gulf of California Region Zone As documented in this study, evidence for dis- 114°W32°N 112°W 30°N tributed deformation in northeastern Baja Califor- nia includes ongoing east- to northeast-directed SONORA extension and Pliocene or younger vertical-axis clockwise rotations. These observations have led Wagner to a new model for the formation and evolution of Basin CALIFORNIA Isla Cerro Prieto fault OF transform fault the upper and lower Tiburón and Delfín basins, GULF Tiburón Tiburón Tiburón Delfín Basins which is summarized here and detailed elsewhere Sierra Juarez fault transform fault Basins Guaymas (Nagy, 1997; Nagy and Stock, 2000). The model San Felipe Isla A. de la G. is based on the assumption that Pacific–North Puertecitos transform fault America plate boundary deformation in the north- San Pedro Mártir fault Ballenas ernmost Gulf of California is not accommodated San Miguel fault zone SI-CN faults along northwest-striking oceanic transform faults BAJA Agua Blanca fault CALIFORNIA or northeast-striking spreading centers, such as in the central and southern Gulf of California, but ° rather occurs over a broad region undergoing dif- North America 28 N fuse deformation since the late Miocene. Dextral Pacific oblique slip along submerged structures in the Present-day relative Gulf of California north of about 30°N may occur plate motion N37°W Pacific plate on reactivated north- to north-northwest–striking normal faults developed during late Miocene ex- Figure 9. Present-day Pacific–North America plate boundary structures in the northern Gulf tension. The absence of an oceanic plate boundary of California region. Relative plate motion direction is after Atwater and Stock (1998). Spread- in the northern Gulf of California is supported by ing centers with a solid pattern are actively extending; those with a striped pattern are extinct. geophysical data, which indicate lower heat flow The Wagner transition zone is proposed as a region of diffuse plate boundary deformation that in the north relative to the central and southern includes east-northeast–directed extension (double-headed arrow), dextral shear manifested as Gulf of California, an absence of gravity lows clockwise vertical-axis rotations along the coast, and dextral-oblique normal slip along approx- such as those associated with extensional basins, imately north- to north-northwest–striking faults submerged in the northern Gulf of California and a greater depth to the Moho (20–35 km in the (depicted schematically through Wagner basin; small arrow shows likely trend of slip vector north vs. 10 km in the central and southern re- necessary to meet plate motion vector constraints). A zone of divergence created at the south- gions) (Henyey and Bischoff, 1973; Gastil et al., eastern margin of the Wagner transition zone can explain the positions and jumps of nearby 1975; Couch et al., 1991). spreading centers (upper and lower Tiburón and Delfín basins) since ca. 6 Ma and the timing of This region of diffuse, dextral plate boundary incorporation of the Puertecitos Volcanic Province into the Gulf extensional province in Pliocene shear has been termed the Wagner transition zone time. Santa Isabel Wash (open circle) is currently within the Gulf extensional province. See Nagy (Fig. 9; Nagy, 1997; Nagy and Stock, 1998, 2000) (1997) and Nagy and Stock (2000) for evolutionary plate boundary model of this region since the and consists of a continuum of transtensional de- late Miocene. Abbreviations: Isla A. de la G.—Isla Angel de la Guarda; SI-CN faults—Santa Is- formation north of lat 30°N that varies from east- abel and Cuervo Negro fault systems. northeast–directed extension along dip-slip faults in the west to dextral shear along the coast to dex- tral-oblique slip along north- to north-north- sence of magnetic anomalies in most of the Gulf along the Tiburón transform. The Guaymas trans- west–striking faults in the Gulf of California. This of California; however, development of the form projects farther south into the rift margin structural scenario can account for Pacific–North oceanic system probably began ca. 4–6 Ma than does the Tiburón transform (Fig. 9); thus America plate motion vector constraints at the lat- (Lonsdale, 1989). Motion along the Tiburón Pliocene extension began in the Puertecitos Vol- itude of the northern Gulf of California. The transform fault and extension in the upper and canic Province when the Guaymas transform southeastern limit of the transition zone, which lower Tiburón basins (Fig. 9) was replaced some linked northwestward to rift margin structures roughly coincides with the southern Puertecitos time after 2 Ma by northwestward propagation of such as the San Pedro Mártir fault. The Matomí Volcanic Province on land and projects northeast- the Guaymas transform fault into the Ballenas accommodation zone was thus abandoned as rift ward into the Gulf of California, is a complex re- channel and extension in the upper and lower margin deformation moved southwestward. It is gion where diffuse deformation within the Wag- Delfín basins (Lonsdale, 1989; Stock, 1999). interesting that this adjustment exposed the ner transition zone is juxtaposed against discrete Deformation recorded in Santa Isabel Wash Puertecitos Volcanic Province to east-north- oceanic plate boundary structures in the central supports the idea that late Miocene–Pliocene east–directed Gulf extensional province exten- Gulf of California. Differential motion thus pro- structural adjustments along the rift margin are sion, producing N-striking structures such as the duced a northeast-striking zone of divergence that related to changes in the offshore spreading cen- Santa Isabel and Cuervo Negro faults, rather than localized the positions of the upper and lower ter system. In particular, post–6 Ma extension in forming northeast-striking normal faults and/or Tiburón and Delfín basins (Fig. 9). For example, the Puertecitos Volcanic Province may have be- northwest-striking dextral strike-slip faults to ac- in the early Pliocene the northwest-striking gun when dextral strike-slip motion along the commodate northwest-southeast–directed plate Tiburón transform fault accommodated north- Guaymas transform replaced similar motion boundary motion. west-southeast–directed motion and intersected

868 Geological Society of America Bulletin, June 2000 EXTENSION IN THE PUERTECITOS VOLCANIC PROVINCE the region of ongoing east-northeast–directed ex- gin deformation thus migrated southwestward tween a domain of diffuse deformation in north- tension in the Wagner transition zone. The upper from the Matomí accommodation zone onto new eastern Baja California (and the northern Gulf of Tiburón basin formed and the spreading axis extensional structures such as the north-striking California) and the region accommodating plate translated along the Tiburón transform away from Santa Isabel and Cuervo Negro fault systems, boundary motion on discrete oceanic structures the Wagner transition zone. Because east-north- which accommodate ~500 m of post–6 Ma east- in the central Gulf of California. east–directed extension continued in the Wagner side-down displacement. A similar amount of transition zone, a new zone of divergence was displacement of ca. 12.5 and 6.3–6.6 Ma tuffs ACKNOWLEDGMENTS eventually created, and spreading in the upper across these faults supports the notion that the Delfín basin developed 2–3 Ma. Development of Santa Isabel Wash region remained outside of I thank K. Holt, X. Quidelleur, K. Rostedt, the Tiburón and Delfín basins may correspond to the Gulf extensional province prior to 6 Ma. T. Tyndall, and K. Wertz for invaluable field assis- the 6 and 3 Ma pulses of Puertecitos Volcanic Other north-northwest– to north-northeast–strik- tance, and D. Evans, J. Holt, J. Kirschvink, C. Lewis, Province volcanism (Stock, 1999). The creation ing post–6 Ma faults in Santa Isabel Wash are M. Oskin, X. Quidelleur, and J. Stock for helpful of the lower Delfín basin and the northwestward secondary faults in the hanging wall of these ma- discussions. I thank J. Besse and J.-P. Cogne for as- propagation of the Guaymas transform 2–3 Ma jor structures and perhaps terminate against sistance with virtual geomagnetic pole calculations. may have been facilitated by the zone of diver- them at depth. Fresh fault scarps, active sag Thoughtful reviews by S. Gilder and C. Lewis gence in the southwestern corner of the Wagner ponds, and stream offsets indicate that exten- greatly improved an early version of this manu- transition zone. sional deformation is actively continuing, and script. I thank Bulletin Editor J. Geissman, Associ- North- to north-northwest–striking late Mio- drainage patterns suggest that Santa Isabel Wash ate Editor L. Brown, and reviewers J. Hagstrum cene structures control local topographic and ba- is a structurally controlled topographic low re- and B. Luyendyk for helpful comments and sug- thymetric trends within the Wagner transition cently created by east- to northeast-side-down gestions. This work was supported by National Sci- zone and differ from northwest-striking bathy- normal faults. ence Foundation grants EAR-9218381, EAR- metric features in the central Gulf of California. No measurable relative rotations occurred 9296102, and EAR-9614674. This is Division of The intersection of these different zones of defor- between most paleomagnetic sites within Geological and Planetary Sciences of the Califor- mation are reflected in the ~35° jog in the coast- 6.3–6.6 Ma tuffs, although two sites at the eastern nia Institute of Technology contribution 8575. line north and south of the Puertecitos Volcanic and western limits of the sampled region with Province. Ongoing east-northeast–directed ex- anomalous declination directions could be inter- REFERENCES CITED tension in the Wagner transition zone explains the preted to indicate small amounts (10°–15°) of greater width (east-northeast to west-southwest) clockwise rotational deformation. 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Martín-Barajas, A., Téllez-Duarte, M., and Stock, J.M., 1997, Stock, J.M., 1993, Geologic map of southern Valle Chico and Pliocene volcanogenic sedimentation along an accommo- adjacent regions, Baja California, Mexico: Geological MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 22, 1998 dation zone in northeastern Baja California: The Society of America Map and Chart Series MCH076, REVISED MANUSCRIPT RECEIVED JULY 26, 1999 Puertecitos Formation, in Johnson, M.E., and Ledesma- 11 p., scale 1:25000. MANUSCRIPT ACCEPTED AUGUST 30, 1999

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