Chronological constraints on the thermal and tilting history of theSierra San Pedro Mártir

ABSTRACT

The tectonothermal history of the four major phases of the SierraSan Pedro Mártir pluton and surrounding metamorphic rocks of theMesozoic Peninsular Ranges batholith of Baja California is presentedon the basis of U/Pb, 40Ar/39Ar step-heating, and fission-track dating,in combination with Al-in-hornblende geobarometry. A previous modelproposed up to 90° of east-side-up tilting of the pluton, exposing >20 kmof crustal section to account for its crescent shape, asymmetrical zon-ing, internal structure, the eastward younging of K-Ar dates across theintrusion and eastward increase in the metamorphic grade of the coun-try rocks, from greenschist to amphibolite facies.

The U/Pb data suggest that the different phases of the pluton wereemplaced sequentially from west to east between 97.0 +4/–1 Ma and93.8 +1/–1 Ma. All except one of the 105 40Ar/39Ar age spectra have well-defined plateaus and are interpreted as cooling ages. Samples from thepluton give hornblende and biotite 40Ar/39Ar plateau dates and apatitefission-track dates that young from west to east; thus, hornblende datesdecrease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, andapatite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagio-clase from the same rock sample collected at the easternmost phase ofthe pluton yield concordant 40Ar/39Ar dates of 88 Ma. The exposed partof the pluton underwent rapid cooling (≈40 °C/Ma) down to ≈250 °C inthe first 10 m.y. after intrusion. Modeling of track-length distributionin apatite is consistent with monotonic slow cooling from ca. 80 Ma tothe present.

The data do not support a history that includes major tilting of thepluton. Eastward younging of 40Ar/39Ar and fission-track dates may beexplained by ≈15° of east-side-up tilting of the pluton at or after 88 Maabout a north-south horizontal axis. Furthermore, the fission-trackdata suggest that part or all of this tilting may have taken place at orafter 57 Ma, and therefore may be a consequence of regional-scalecrustal extension associated with the opening of the Gulf of Californiain Neogene time. Such tilting is in agreement with the Al-in-hornblendegeobarometry for the hornblende-biotite intrusive phase that yields pres-sures of 5.2 ± 0.6 kbar. An ≈15° northeast-side-up tilt of the crustal blockcontaining this pluton would explain the apparent paleomagnetic in-clination discrepancies with cratonic North America and militatesagainst large-scale northerly transport of Baja California.

INTRODUCTION

This study is part of a regional 40Ar/39Ar dating program (Fig. 1) inves-tigating the thermal history of the Peninsular Ranges batholith of Alta andBaja California, and the apparent eastward migration of granitoid emplace-ment within it (Silver et al., 1969; Krummenacher et al., 1975).

The crescent-shaped Sierra San Pedro Mártir pluton is one of many gran-itoid bodies that compose the composite Mesozoic Peninsular Rangesbatholith of southern California, United States, and Baja California, México(Fig. 1). On the basis of field, geochemical, geochronological, and isotopicstudies (Eastman, 1986; McCormick, 1986; Gastil et al., 1990; Walawenderet al., 1990), Gastil et al. (1991) proposed that the 400 km2 zoned pluton(Fig. 2) may have been tilted about a horizontal axis parallel to the trend ofthe batholith to reveal deeper levels to the east, and that the sequence of

728

Chronological constraints on the thermal and tilting history of theSierra San Pedro Mártir pluton, Baja California, México, fromU/Pb, 40Ar/39Ar, and fission-track geochronology

Amabel Ortega-Rivera*E. FarrarJ. A. Hanes

Department of Geological Sciences, Queen’s University, Kingston, Ontario K7L 3N6, Canada

D. A. Archibald}

R. G. GastilD. L. Kimbrough } Department of Geology, San Diego State University, San Diego, California 92182-1020

M. Zentilli Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

M. López-Martínez División de Ciencias de la Tierra, Centro de Investigación Científica y Educación Superior deEnsenada, Km 107 Carretera Tijuana-Ensenada, 22860, Ensenada, Baja California, México

G. Féraud Institut de Géodynamique, Unité de Recherche Associeé, Centre National de la Recherche G. Ruffet } Scientifique, 1279, Université de Nice, France

GSA Bulletin; June 1997; v. 109; no. 6; p. 728–745; 11 figures, 5 tables.

*Corresponding author; E-mail: [email protected]

Data Repository item 9727 contains additional material related to this article.

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Geological Society of America Bulletin, June 1997 729

modal zones mapped from west to east and eastward metamorphic gradientof the country rocks from greenschist to amphibolite facies may representa 20 km crustal section through the pluton. To test the proposal that the plu-ton may have been substantially tilted, the cooling and exhumation historywas determined using U/Pb dating of zircons and monazites, detailed40Ar/39Ar conventional and laser step-heating analyses on hornblende, mus-covite, biotite, and plagioclase, fission-track dating of apatite, and Al-in-hornblende geobarometry on samples from the pluton and its host rocks.Evidence of tilting of the batholith would have a significant bearing on theplausibility of the model of large-scale (100–2500 km) northward tectonictransport of Baja California with respect to the North American craton asproposed, for example, by Hagstrum et al. (1985) on the basis of their pale-omagnetic studies. Butler et al. (1991) calculated that a westward 15°–20°tilt of the whole peninsula about its north-northwest longitudinal axis wouldrestore the apparent poles for western Baja to concordance with cratonicNorth American poles.

REGIONAL GEOLOGIC SETTING

The Peninsular Ranges batholith of southern California, United States,and Baja California, México (Fig. 1), is a well-exposed array of granitoidplutons that trends northwest-southeast for 1600 km, has an average widthof ≈100 km, and extends southward from lat 34°N in Alta California to lat28°N in Baja California (Gastil, 1975, 1990; Krummenacher et al., 1975;

Silver, 1979; Silver and Chappell, 1988; Todd and Shaw, 1979). South of lat28°N, the range is generally considered to extend to the southern tip of theBaja California Peninsula beneath the Cenozoic cover (Gastil et al., 1975).The plutons are predominantly tonalitic and granodioritic, although com-positions range overall from granitic to gabbroic (Gastil et al., 1975).

Available U/Pb zircon ages range from 120 to 90 Ma, indicating that thebatholith mainly formed during a continuous and prolonged period of mag-matic activity in the late Mesozoic, the locus of intrusion migrating east-ward with time (Silver et al., 1969). Krummenacher et al. (1975) showed(1) that K-Ar dates on coexisting hornblende and biotite from the batholithare typically discordant (hornblende dates as much as 15 m.y. older thanthose for biotite); (2) that these dates are substantially younger (up to25 m.y.) than the U/Pb zircon dates (Silver et al., 1979); and (3) that K-Ardates of hornblende and biotite decrease systematically to the northeast,subparallel to the trend of the batholith.

On the basis of geochemical, geophysical, and lithological relationships,the Peninsular Ranges batholith has been divided into western and easternzones (Gastil et al., 1975). The eastern zone, in which lies the Sierra SanPedro Mártir pluton, is dominated by several large and concentrically zoned(in texture, structure, and composition) plutons ranging in compositionfrom tonalite to granodiorite (the “La Posta-type” plutons of Walawender etal., 1990) that have invaded Phanerozoic metasedimentary, metavolcanic,and metaplutonic rocks as young as Cretaceous (Gastil et al., 1975, 1991;Walawender et al., 1990). The Sierra San Pedro Mártir pluton has a surface

Figure 1. Map showing the lo-cation of the Peninsular Rangesprovince, regional 40Ar/39Ar studyarea, and the composite SierraSan Pedro Mártir pluton.

ORTEGA-RIVERA ET AL.

730 Geological Society of America Bulletin, June 1997

area of 400 km2 and is the third-largest of the La Posta–type intrusions(Walawender et al., 1990).

Sierra San Pedro Mártir Pluton

This intrusion (Fig. 2) was emplaced along the boundary between themoderately deformed volcanic and volcaniclastic arc terrane of the Aptian-Albian Alisitos Group (Santillán and Barrera, 1930; Eastman, 1986) andearlier plutonic rocks to the west, and migmatitic metasedimentary andgarnet- to sillimanite-bearing metaplutonic rocks of unknown age and affin-

ity to the east. The metamorphic grade in the country rocks increases fromgreenschist to upper amphibolite facies from west to east across the 20 kmwidth of the pluton (Gastil et al., 1975).

The Sierra San Pedro Mártir pluton is compositionally zoned, and itscomponent facies are distributed asymmetrically (Fig. 2). Contacts arelargely gradational and have been mapped on the basis of modal mineral-ogy (McCormick, 1986; Eastman, 1986; Gastil, 1990; Gastil et al., 1990,1991; Walawender et al., 1990). From west to east (Fig. 2), hornblende-biotite tonalite, biotite granodiorite, and muscovite-biotite granodioritezones are distinguished (McCormick, 1986; Eastman, 1986; Gastil, 1990;

Figure 2. Simplified geologyof the Sierra San Pedro Mártirpluton and environs (after Mc-Cormick, 1986; Eastman, 1986)showing sample locations usedin this study.



Gastil et al., 1990, 1991; Walawender et al., 1990). The last-named domainhas been divided in the field into two parts on the basis of muscovite grainsize. The western margin of the pluton exhibits a foliation defined by flat-tened inclusions, and biotite and hornblende are aligned parallel to theinward-dipping (50° to 90° to the east) contact with the country rocks(Fig. 2). Rocks of the eastern zone are massive and lack pronounced in-ternal structure. Although the mineralogy, texture, and chemistry are ingeneral gradational across the pluton, the muscovite-biotite granodiorite lo-cally truncates the hornblende-biotite tonalite and therefore is younger(McCormick, 1986). Sr, Pb, and O isotopic and rare earth element (REE)data support a model of differentiation from a single parental melt with in-creasing crustal source rock toward the east (Gastil et al., 1994).

Tonalite from the western contact of the pluton yielded a U/Pb zircon dateof 96 ± 1 Ma (McCormick, 1986), and K-Ar dates (Krummenacher et al.,1975) young toward the east (Fig. 3). Walawender et al. (1990) showed thatU/Pb zircon data from four separate samples define a chord to concordia,suggesting that these zircons were derived, in part, from a ca. 1.3 Ga sourceregion, and that emplacement of the pluton occurred at 94 Ma (Walawenderet al., 1990). Conventional biotite K-Ar dates (McCormick, 1986; Walawen-der et al., 1990) from the three major facies of the Sierra San Pedro Mártirpluton decrease eastward, from 87 ± 3 Ma in the hornblende-biotite facies,to 83 ± 3 Ma in the biotite facies, to 72 ± 2 Ma in the most easterly,muscovite-biotite facies (Fig. 3). In a fission-track study of the late uplift his-tory of the entire Sierra San Pedro Mártir region, Dorsey and Cerveny (1991,and R. Dorsey, 1993, personal commun.) showed that zircon ages decreasefrom 105 to 76 Ma and apatites decrease from 76 to 35 Ma over the elevationrange of 2800 to 500 m along the Sierra San Pedro Mártir fault escarpment.

SAMPLING AND ANALYTICAL METHODS

We selected 26 samples to provide an east-west transect across all phasesof the pluton (Fig. 2); 13 samples were also selected from the surroundingcountry rocks, 3 from the western contact, and 10 from the country rocks atthe northeastern contact of the pluton. From these 39 samples, 2 sampleswere chosen for U/Pb analysis, 35 for 40Ar/39Ar conventional step-heating,4 for 40Ar/39Ar laser step-heating, 4 for fission-track analysis, and 6 forAl-in-hornblende geobarometry.

U/Pb Methods

Zircon and monazite U/Pb isotopic analyses (samples 4 and 17, Fig. 2)were carried out at San Diego State University. Isotopic ratios were measured

using a VG Sector 54 multicollector mass spectrometer. Analytical methods,uncertainties, blanks, and common Pb corrections are outlined in Table 1. Zir-con fractions were analyzed following mild leaching in hydrofluoric acid(HF) as described by Kimbrough et al. (1992). The HF leaching techniquehas the effect of removing common Pb from zircon, and by that reducing un-certainties in 207Pb*/206Pb* dates, while also preferentially dissolving moresoluble high-U domains most strongly affected by recent Pb loss.

Figure 3. Previous geochronological results and sample locations forthe Sierra San Pedro Mártir pluton (from McCormick, 1986; Walawen-der et al., 1990) and sample location of new U/Pb dates.

TABLE 1. U-PB DATA FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON

Sample Size Weight Pb U Lead isotopic comp. corrected for fractionation Radiogenic ratios Apparent ages (Ma)

name fraction (g) (ppm) (ppm) 206/208 ( ± ) 206/207 ( ± ) 206/204 ( ± ) 206*/238 ( ± ) 207*/235 ( ± ) 207*/206* ( ± ) 206*/238 207*/235 207*/206*

Sample 4WC-2MZircon (L) <100 0.0045 5.38 348.4 7.432000 (1) 19.914 (7) 6631 (66) 0.01516 (4) 0.01003 (3) 0.04799 (4) 97.0 ± 1 97.1 ± 2 99.0 ± 20

(mesh)Sample 17SSPM-7-25-4Zircon (L) >200 0.0030 9.65 538.8 6.516000 (1) 13.156 (2) 542 (4) 0.01513 (4) 0.1019 (6) 0.04885 (22) 96.8 ± 1 98.5 ± 2 140.0 ± 11

(mesh)Monazite Bulk a 0.0013 195.27 757.0 0.055610 (3) 14.954 (7) 712 (6) 0.01522 (7) 0.0968 (6) 0.04614 (21) 97.4 ± 1 93.8 ± 2 5.0 ± 11Monazite Bulk b 0.0023 98.58 792.7 0.057102 (6) 15.040 (4) 732 (6) 0.01516 (11) 0.0968 (8) 0.04632 (19) 97.0 ± 1 93.8 ± 2 14.0 ± 10

Note: Zircon (L) indicates leaching of fractions with HF on hot plate at ≈100 °C for two days prior to dissolution, following methods outlined by Kimbrough et al. (1992). Separation of U and Pb was done usingHCl column chemistry. Concentrations were determined using a mixed 208Pb/235U spike. Lead isotopic compositions corrected for ≈0.10% ± 0.05% per mass unit mass fractionation, based on replicate analyses ofNBS981 and NBS983. Errors in 206Pb/204Pb measurements were minimized by use of an ion-counting, Daly-multiplier system, for detection of small 204Pb signal, and are typically <1%. Decay constants used1.55125E–10 a = 238U and 9.8485E–10 a = 235U. Present-day 238U/235U = 137.88. Corrections for common lead were made using the model of Stacey and Kramers (1975). Total lead blanks average ≈25 pg. Pb*= radiogenic lead. The 2σ value for analytical errors is shown in parentheses behind radiogenic ratios; e.g., 0.02246 (6) means 0.02246 ± 0.00006. Errors were computed using the data reduction program PBDATof Ludwig (1989). Accuracy of 206*Pb/238U dates is better than ±0.5% (ca. ±1 m.y.) based on long-term reproducibility of a standard zircon sample (OU491271). Uncertainties in the 207*Pb/206*Pb dates are stated atthe 2σ level and assumed a ±0.1 uncertainity in the common 207Pb/204Pb.



40Ar/39Ar Method

The 99 mineral separates (24 hornblende, 61 biotite, 13 muscovite, and1 plagioclase) dated at Queen’s University (Table 2, Fig. 2) were purifiedusing a Frantz magnetic separator and heavy organic liquids. Separates,replicates, and flux-monitors (LP-6) and samples of known age (i.e., inter-laboratory and intralaboratory monitors) were individually packaged indisc-shaped Al-foil pouches and stacked in an aluminum irradiation cans(11.5 cm long and 2.0 cm in diameter). The positions of samples and mon-itors were carefully measured before loading into the irradiation cans.

The cans with the samples and monitors were then irradiated with fast

neutrons in position 5C of the McMaster University Nuclear Reactor(Hamilton, Ontario) for 14.5 hr. Typically 10 or more monitors and 2 to 4replicates were used to determine the neutron-flux. J-values for individualsamples were determined by a second-order polynomial interpolation. Inaddition, two or more replicate samples were loaded into each irradiationcan to aid in intercan comparison. To accommodate all samples, 10 separateirradiations were required. After irradiation, mineral separates, replicates,and monitors were loaded into niobium crucibles and heated in a pure-silicatube (GE214) within a Lindberg furnace. The bakeable, ultrahigh vacuum,stainless steel, argon-extraction system was operated online in a substan-tially modified, Associated Electrical Industries MS-10 mass spectrometer

TABLE 2. 40AR/39AR DATES FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON AND ITS COUNTRY ROCKS

Sample Sample Mineral Lab Size C/P Integrated 2σ error Plateau 2σ error Volume Initial 2σ errornumber name run mesh date date 39Ar 40/36

(Ma) (Ma) ( % )

Country rocksWestern contact1 CONT-2-90 hb AOR-288 80/100 104/44 97.4 1.0 97.4 1.0 100.0 343.4 67.11 CONT-2-90 hb AOR-1114 80/100 104/45 98.7 2.3 98.7 2.3 100.0 344.2 1284.21 CONT-2-90 bt AOR-291 80/100 104/42 94.6 0.9 95.5 0.9 86.1 374.1 128.71 CONT-2-90 bt AOR-1106 80/100 104/43 94.5 1.0 95.6 1.0 93.9 294.0 4.62 CONT-1.1 hb AOR-746 80/100 113/30 96.0 1.7 96.6 1.4 87.2 155.3 1804.02 CONT-1.1 bt AOR-760 80/100 113/40 93.1 1.2 94.1 1.1 87.7 366.6 1526.23 CONT-1.A hb AOR-757 100/120 113/10 95.7 1.7 95.7 1.7 100.0 266.4 353.33 CONT-1.A bt AOR-766 100/120 113/20 93.1 1.0 93.8 1.0 90.5 201.8 98.9

Sierra San Pedro Mártir plutonHornblende-biotite tonalite zone4 WC-(2M) *hb AOR-10 25/40 79/54 94.4 1.0 94.7 0.8 97.7 * *4 WC-(2M) hb AOR-181 25/40 89/16 97.2 2.0 94.6 0.6 99.7 296.5 18.94 WC-(2M) Hb AOR-709 25/40 A104/25A 94.7 2.6 94.6 2.7 95.1 295.3 91.04 WC-(2M) Hb AOR-1085 25/40 A105/8A0 93.4 1.4 94.4 1.2 94.4 291.4 11.54 WC-(2M) hb AOR-1740 25/40 A143/4A0 92.4 5.5 94.5 4.2 55.3 297.2 161.54 WC-(2M) bt AOR-38 25/40 88/14 92.8 0.7 93.0 0.7 99.4 271.4 93.34 WC-(2M) bt AOR-313 25/40 B104/25B 92.8 1.0 93.5 1.0 93.1 295.8 2.34 WC-(2M) bt AOR-1157 25/40 B105/8B0 94.2 1.2 94.7 1.2 92.4 292.8 7.14 WC-(2M) bt AOR-1178 25/40 88/13 92.9 0.8 93.7 0.8 90.0 244.3 122.64 WC-(2M) bt AOR-1743 25/40 B143/4B0 93.2 2.0 93.2 2.0 100.0 294.6 26.95 SSPM-7-26-7-88 hb AOR-90 25/40 88/69 92.9 1.6 93.0 1.6 99.3 265.8 217.25 SSPM-7-26-7-88 bt AOR-49 25/40 88/35 91.4 0.7 92.0 0.7 82.8 332.5 168.7

35 SSPM-13 +hb M686-S N.D. N.D. 92.9 4.1 94.1 3.8 94.5 * *35 SSPM-13 +bt M714-S N.D. N.D. 92.6 0.7 92.8 0.7 90.9 * *6 CORONA hb AOR-141 25/40 88/40 91.5 1.0 92.1 0.8 99.2 285.4 38.66 CORONA bt AOR-52 25/40 88/39 90.5 0.6 91.0 0.6 97.9 301.5 18.77 SSPM-7-26-6-88 hb AOR-841 25/40 113/24 94.7 2.8 94.7 2.8 100.0 299.4 34.77 SSPM-7-26-6-88 bt AOR-840 25/40 113/25 91.5 0.8 92.1 0.8 93.5 314.6 208.88 SSPM-H.9-14.5 bt AOR-123 40/60 87/8 90.9 0.8 91.1 0.8 98.2 295.7 30.59 SSPM-7-26-4-88 hb AOR-32 25/40 88/21 94.9 4.4 94.0 1.8 99.6 296.8 38.09 SSPM-7-26-4-88 bt AOR-41 25/40 88/26 89.9 0.6 89.9 0.6 99.5 309.8 48.8

10 E.8-20.6 *hb AOR-6 40/60 79/49 94.7 5.2 94.0 2.5 90.1 * *10 E.8-20.6 hb AOR-177 40/60 88/21 92.1 0.7 92.4 0.7 84.6 275.9 112.510 E.8-20.6 *bt AOR-7 40/60 79/48 89.5 1.1 90.7 0.5 87.7 * *10 E.8-20.6 bt AOR-210 40/60 89/8 89.0 0.2 89.3 0.2 95.1 205.6 20.813 G.4-90-3 hb AOR-947 25/40 113/26 91.7 3.8 94.3 1.4 76.8 238.5 404.013 G.4-90-3 bt AOR-941 25/40 113/27 88.4 0.9 89.0 0.8 95.2 290.3 107.536 SSPM-18 +hb M684-S N.D. N.D. 92.8 1.9 91.7 1.7 96.4 * *36 SSPM-18 +bt M688-S N.D. N.D. 87.2 0.3 87.9 0.3 86.9 * *16 SSPM-LV-8 *hb AOR-8 25/40 79/50 91.0 3.0 92.0 1.8 88.4 * *16 SSPM-LV-8 hb AOR-178 25/40 89/19 91.4 0.8 92.2 0.8 94.6 279.9 89.716 SSPM-LV-8 *bt AOR-4 25/40 79/53 88.0 0.6 89.2 0.5 79.3 * *16 SSPM-LV-8 bt AOR-234 25/40 89/10 86.1 0.3 87.1 0.3 82.4 179.9 92.518 SSPM-G.0-20 bt AOR-1096 40/60 104/80 87.4 1.0 87.7 1.0 91.2 296.4 7.618 SSPM-G.0-20 bt AOR-1124 >10 104/50 88.2 1.3 88.2 1.3 100.0 441.0 572.321 SSPM-7-24-1-88 *hb AOR-22 25/40 79/60 91.9 3.3 92.9 2.3 92.7 * *21 SSPM-7-24-1-88 hb AOR-191 25/40 89/24 93.8 0.6 93.8 0.6 89.3 316.7 212.821 SSPM-7-24-1-88 bt AOR-56 25/40 88/47 92.3 0.5 92.3 0.5 99.7 287.7 28.422 SSPM-7-24-2-88 *hb AOR-13 40/60 79/59 90.4 1.2 90.4 1.0 99.0 * *22 SSPM-7-24-2-88 hb AOR-188 40/60 89/25 91.8 0.8 91.8 0.8 100.0 317.6 76.622 SSPM-7-24-2-88 bt AOR-201 40/60 87/57 86.4 0.7 86.7 0.7 95.2 223.2 19.822 SSPM-7-24-2-88 bt AOR-1173 40/60 87/58 88.7 1.8 88.3 2.2 76.8 602.4 374.823 SSPM-7-24-3-88 hb AOR-691 40/60 104/13 91.4 2.2 91.4 2.2 100.0 290.3 276.323 SSPM-7-24-3-88 hb AOR-1092 40/60 104/14 90.0 2.0 90.8 1.9 97.4 263.3 145.623 SSPM-7-24-3-88 bt AOR-695 40/60 104/16 88.1 1.2 87.9 1.2 97.1 382.7 127.923 SSPM-7-24-3-88 bt AOR-1119 40/60 104/15 89.0 0.7 89.0 0.7 100.0 233.9 160.1



run in the static mode. For the isochron correlation analyses, step-heatingblank runs were measured. The 40Ar blank volume (less than 2% for horn-blendes and 1% for biotites) varied between 0.4 × 10–10 (lowest temperaturesteps) and 0.9 × 10–10 cm3 STP (highest temperature steps), the 37Ar and39Ar blanks were very small compared to the signals measured in the analy-ses of the samples, and the 36Ar blanks were at or below the limit of detec-tion of the MS-10.

Six mineral separates were analyzed by single-grain laser step-heating atNice (two hornblende, four biotite; samples 35–38, Fig. 2). The separateswere prepared using a Frantz magnetic separator and handpicking to select

the freshest grains. Five or six grains of the mineral separate were wrappedin 11 × 5 mm aluminum envelopes. Samples and six similarly wrappedflux-monitors were loaded into an irradiation can; the samples were verti-cally arranged in one level and six samples of monitor MMhb were distrib-uted horizontally within this level. The can was irradiated for 69.5 hr in po-sition 5C of the McMaster University Nuclear Reactor. After irradiation thesamples were mounted on a copper sample holder, beneath the Pyrex win-dow of a stainless steel chamber, connected to an ultra-high vacuum purifi-cation system, and were uniformly heated with a defocused 5.5W CoherentInnova 70-4 continuous argon-ion laser. The evolved gas, after purification,

TABLE 2. (Continued)

Sample Sample Mineral Lab Size C/P Integrated 2σ error Plateau 2σ error Volume Initial 2σ errornumber name run mesh date date 39Ar 40/36

(Ma) (Ma) ( % )

Sierra San Pedro Mártir plutonBiotite granodiorite zone11 SSPM-7-26-3 *bt AOR-12 25/400 79/56 89.3 1.9 89.6 1.6 82.7 * *11 SSPM-7-26-3 bt AOR-221 25/400 89/22 89.6 0.3 89.9 0.3 92.5 265.5 31.937 SSPM-20 +bt M704-S 25/400 N.D. 88.6 0.7 89.1 0.6 85.8 * *12 SSPM-7-26-2-88 bt AOR-46 25/400 88/31 90.6 0.6 90.6 0.6 100.0 309.6 54.714 SSPM-G.4-24.3 bt AOR-45 25/400 88/37 90.7 0.9 90.7 0.9 100.0 289.0 79.015 SSPM-G.1.26 bt AOR-127 25/400 87/15 89.9 0.9 90.4 0.9 93.2 417.4 181.219 SSPM-G.0-29 bt AOR-176 40/600 87/31 88.5 1.1 90.1 1.2 88.0 325.9 168.919 SSPM-G.0-29 bt AOR-1246 80/100 112/18 90.3 1.5 90.8 1.5 85.8 189.7 223.9Sierra San Pedro Mártir plutonMuscovite-biotite granodiorite zone29 SSPM-7-23-4 ms AOR-98 25/400 88/68 87.4 0.7 87.5 0.7 99.0 276.3 29.538 SSPM-21 +bt M706-S N.D. N.D. 88.4 0.9 89.0 0.8 91.0 * *17 SSPM-7-25-4(88) ms AOR-60 60/800 88/58 90.0 0.5 90.0 0.5 100.0 320.9 18.917 SSPM-7-25-4(88) bt AOR-50 25/400 88/29 89.8 0.6 90.0 0.6 97.0 311.7 43.917 SSPM-7-25-4(88) bt AOR-133 25/400 87/18 89.9 1.1 90.4 1.1 94.9 304.1 121.917 SSPM-7-25-4(88) bt AOR-496 25/400 87/19 89.5 1.0 89.5 1.0 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-497 25/400 87/19 89.6 0.6 89.6 0.6 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-501 25/400 87/19 90.2 0.9 90.2 0.9 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-502 25/400 87/19 90.0 0.5 90.0 0.5 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-503 25/400 88/28.1 89.1 0.6 89.1 0.6 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-504 25/400 88/28.1 89.6 0.2 89.6 0.2 *100.0* N.D. N.D.17 SSPM-7-25-4(88) bt AOR-545 25/400 88/28.2 89.0 0.5 89.0 0.5 *100.0* N.D. N.D.20 CT-47 ms AOR-81 80/100 88/72 88.5 0.7 88.5 0.7 100.0 327.3 34.220 CT-47 ms AOR-957 40/100 113/19 90.2 1.5 90.4 1.5 98.1 236.2 44.320 CT-47 bt AOR-83 40/600 88/66 87.3 0.7 88.3 0.9 74.5 338.2 201.120 CT-47 bt AOR-756 40/600 113/20 91.5 0.9 91.5 0.9 68.8 186.3 126.824 CORE ms AOR-750 80/140 113/21 93.2 1.3 88.3 1.1 90.5 311.6 225.324 CORE bt AOR-27 25/400 88/24 87.8 0.6 88.3 0.6 99.2 294.3 17.524 CORE bt AOR-274 25/400 88/25 88.7 0.6 89.2 0.6 96.5 191.0 152.224 CORE bt AOR-751 25/400 113/23 87.9 0.8 88.5 0.8 95.8 474.0 491.624 CORE pg AOR-763 25/400 113/22 89.4 2.2 88.2 1.1 73.1 299.9 16.2Country rocksNortheastern contact25 SSPM-VO-9 ms AOR-34 80/100 88/22 88.6 0.6 88.7 0.6 98.4 301.3 58.325 SSPM-VO-9 bt AOR-40 80/100 88/15 88.6 0.8 88.2 0.8 90.5 332.4 150.726 SSPM-VO-9-5 ms AOR-218 60/800 87/12 92.4 1.4 91.1 1.2 80.6 547.5 844.726 SSPM-VO-9-5 bt AOR-131 60/800 87/6 91.1 0.9 91.4 0.9 98.6 272.3 70.826 SSPM-VO-9-5 bt AOR-271 60/800 87/7 91.8 0.9 90.9 0.9 98.3 227.6 1223.927 SSPM-VO-15 ms AOR-1129 40/600 104/32 90.1 1.1 90.1 1.1 100.0 329.4 65.427 SSPM-VO-15 bt AOR-47 40/600 88/19 89.7 0.6 89.6 0.6 97.7 471.8 143.927 SSPM-VO-15 bt AOR-1109 40/600 104/33 90.6 0.9 90.8 0.9 99.2 293.7 27.128 SSPM-VO-15-1 ms AOR-59 80/100 88/62 85.6 0.5 85.2 0.5 90.4 315.8 39.128 SSPM-VO-15-1 bt AOR-43 60/800 88/17 87.9 0.6 87.9 0.6 100.0 324.9 51.728 SSPM-VO-15-1 bt AOR-244 60/800 88/43 87.5 0.5 87.3 0.5 88.4 298.9 79.230 SSPM-VO-15-4 ms AOR-1103 60/800 104/52 90.5 0.9 89.6 1.0 82.3 294.0 62.030 SSPM-VO-15-4 bt AOR-724 60/800 104/46 91.2 0.9 90.8 0.9 80.6 495.2 497.931 SSPM-VO-16.1 ms AOR-96 80/100 88/65 85.4 0.6 85.5 0.6 98.6 279.1 63.931 SSPM-VO-16.1 bt AOR-213 25/400 89/2 86.3 0.3 86.5 0.3 88.8 277.8 14.232 SSPM-VO-16A bt AOR-57 80/100 88/48 88.3 0.5 88.3 0.5 100.0 318.9 41.632 SSPM-VO-16A bt AOR-269 25/400 89/1 88.5 0.8 88.5 0.8 100.0 314.0 643.840 SSPM-VO-16.D bt AOR-1093 60/800 104/10 84.1 1.5 84.3 1.5 99.6 293.3 8.534 SSPM-VO-17-2 ms AOR-952 60/800 112/39 89.0 4.8 84.2 1.4 95.4 296.8 30.534 SSPM-VO-17-2 bt AOR-835 60/800 11/37 85.1 1.2 85.4 1.2 91.7 279.1 94.533 SSPM-VO-17-A ms AOR-300 60/800 104/30 90.4 0.9 88.3 0.9 79.9 452.7 71.633 SSPM-VO-17-A bt AOR-296 60/800 104/31 60.6 0.9 70.6 1.7 36.7 97.2 54.5

Note: C/P = Can/position; minerals: hb—hornblende, pg—plagioclase, ms—muscovite, bt—biotite, +bt—laser step-heating, *bt—no isotopecorrelation data.The Ar isotope blanks were not well constrained, and no correlation plots were possible. N.D.—no data. An asterisk in the last twocolumns means that no isotope correlation data are available.



was transferred to an online, 120° mass spectrometer, consisting of anM.A.S.S.E. flight tube, a Bäur Signer source, and a Balzers SEV217 elec-tron multiplier (total gain: 5 × 1012), and analyzed in static mode. Blanks,made routinely each first and third run, were subtracted from the subse-quent sample gas fractions. Typical system-blank values were 4.0 × 10–12,4.0 × 10–14, 1.4 × 10–13, and 6.3 × 10–14 cm3 STP for masses 40, 39, 37, and36, respectively. Ruffet et al. (1991) documented the laser-fusion techniquein detail. All samples were irradiated in the same level. J-uncertainties foreach level were less than 0.15%, and the average was used to calculate theages at that level.

Measured mass spectrometric ratios for both systems were extrapolatedto zero time, normalized to the 40Ar/36Ar atmospheric ratio, and correctedfor neutron-induced 40Ar from potassium, and 39Ar and 36Ar from calcium.Dates and errors were calculated using formulae given by Dalrymple et al.(1981) and the constants recommended by Steiger and Jäger (1977). All er-rors shown represent the analytical precision at 2σ, and include the analyt-ical uncertainties of the monitor analyses (J-uncertainties), but assume thatthe error for the age of the monitor is zero. A conservative estimate of 0.5%in the error of the J-value should be added for comparison with samplesusing different a monitor. The Queen’s University dates are referenced tothe LP-6 biotite standard at 128.5 Ma (Roddick, 1983), and the Nice’s datesare referenced to MMhb1 hornblende standard at 520.4 Ma (Samson andAlexander, 1987). Mineral replicates (e.g., samples 4 [hornblende and bio-tite], and 17 [biotite]; see Table 2 and Data Repository1) irradiated withinthe same can and in different cans were analyzed to monitor reproducibil-ity. The standard deviations of 0.11, 0.64, and 0.48 Ma, respectively, deter-mined for samples irradiated in different cans supports our estimates of theanalytical precision quoted for individual analyses. (For information onsize, blanks, and errors in J, see footnote 1.)

Fission-Track Method

Apatite separates from samples 2, 4, 10, and 24 (Fig. 2) were obtainedusing standard heavy liquid and magnetic mineral separation techniques atQueen’s University. The fission-track analyses were done at the Fission-track Research Laboratory of Dalhousie University. Mineral separation,grain mounting, polishing, etching, irradiation, and counting were all doneby standard techniques using the external detector method and are docu-mented in Ravenhurst and Donelick (1992) and Ravenhurst et al. (1994).

Microprobe Analysis and Aluminum-in-Hornblende Geobarometry

Mineral analyses for hornblende barometry (Hammarstrom and Zen,1986) were carried out at Queen’s University on only six samples fromrocks of the hornblende facies (Fig. 2, samples 4, 5, 6, 10, 16, and 22) thatcontain the required assemblage quartz + plagioclase + K-feldspar + horn-blende + biotite + titanite + an oxide phase (magnetite or ilmenite). Five dif-ferent calibrations were used to estimate pressures (for the empirical cali-brations used, see Hammarstrom and Zen, 1986, and Hollister et al., 1987;for the experimental calibrations, see Johnson and Rutherford, 1989, Thomasand Ernst, 1990, and Schmidt, 1992). Mineral analyses were done with anARL-SEMQ electron microprobe using an energy-dispersive spectrometer(EDS). Operating conditions were maintained at an accelerating voltage of15 kV and a beam current of 75 nÅ, with a carbon collimator with an ef-fective diameter of 0.32 cm (0.125 in.), so the detector area is less than thenormal area suggested by the manufacturer. Kaersutite (Smithsonian Insti-

tution USNM 143965) was used as a primary standard for the major elementsin the hornblende analyses. Mineral compositions were determined from anaverage of at least three rim analyses (100 s count time) from differentpoints on the same grain or neighboring grains. Structural formulae werecomputed using MINPROBE software developed by D. M. Carmichael(Queen’s University), with Fe3+/Fe being the average of an upper and lowerlimit imposed by amphibole stoichiometry.

DISCUSSION OF RESULTS

U/Pb Geochronology

U/Pb isotopic data (Table 1, Fig. 3) are reported here from two samplesof the Sierra San Pedro Mártir pluton. Sample 4, representative of thehornblende-biotite tonalite, was collected at the western margin of the plu-ton where it is in contact with pervasively mylonitized wall rock. Sample 17,representative of the muscovite-biotite granodiorite, was collected fromnear the geographic center of the pluton, ≈2 km east of the contact with thebiotite granodiorite zone. A single <100 mesh, HF-leached, zircon fractionfrom sample 4 and two bulk fractions of monazite and a single >200 mesh,HF-leached, zircon fraction from sample 17 were analyzed (Fig. 2). The dataand calculated dates are reported in Table 1. The zircon fraction from sam-ple 4 was concordant, with a 206Pb*/238U date of 97.0 Ma and a 207Pb*/206Pb*date of 99 ± 2 Ma. A single >200 mesh, HF-leached, zircon fraction fromsample 17 yielded discordant results, with a 206Pb*/238U date of 96.8 Maand a 207Pb*/206Pb* date of 140 ± 11 Ma.

Two U/Pb analyses of monazite from sample 17 plotted above the con-cordia. Such discordance in relatively young monazites has been attributedto the preferential incorporation of Th relative to Pb in the monazite crystallattice (Parrish, 1990), leading to an excess of 206Pb (derived from initiallyincorporated 230Th, an intermediate decay product of 238U). The 207Pb*/235Udates would have been unaffected by the incorporation of 206Pb 207Pb*/235Udates from both fractions are identical at 93.8 Ma (Table 1).

40Ar/39Ar Geochronology

For almost all the analyses, the age spectra are either completely flat, orhave dates that climb over the first ≈10% of 39Ar released to a well-definedplateau for the remaining ≈90% of the spectrum. Integrated and plateaudates for all analyses are presented from west to east in Table 2; represen-tative age spectra for samples from the country rocks and the pluton are il-lustrated in Figure 4 from west to east. There is excellent agreement be-tween laser and conventional step-heating results. There is also excellentagreement for replicate analyses from samples irradiated within the samecan and in different cans (e.g., samples 4 and 17) (Table 2; see also foot-note 1). The isotope correlation analyses agree with, but are less reliablethan, the plateau dates due to data clustering. The complete 40Ar/39Ar dataset, sample localities (in UTM coordinates, Zone 11R), elevations, isotopecorrelation plots, and age spectra are available from the GSA Data Reposi-tory (see footnote 1).

The plateau dates for hornblende, biotite, muscovite, and plagioclase areplotted on the sample location map in Figure 5 (a–c). Replicate results forindividual samples have been averaged. These plateau dates are also plottedwith respect to east-west locations ion Figure 6 (a–c).

Hornblende. The plateau dates for hornblende range from 98 to 90 Ma.From west to east, the oldest plateau dates (samples 1, 2, and 3, Fig. 2)range from 98 to 95 Ma (Table 2, Fig. 4a) and belong to the metaplutonicrocks of the western contact zone. Hornblende plateau dates of the horn-blende-biotite tonalite zone (outer zone) range from west to east from 95 to90 Ma (Figs. 4, b–d, 5a, and 6a), the oldest hornblende dates (95 Ma) being

1GSA Data Repository item 9727, index table and 40Ar/39Ar data and spectra,is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder,CO 80301. E-mail: [email protected].



from rocks at the western contact of the pluton (Fig. 4b). The hornblendedates are older in the country rocks to the west of the pluton, supporting theinterpretation of postmetamorphic emplacement (Figs. 4a and 5, a and b;Table 2; and Data Repository [see footnote 1]).

Biotite and Muscovite. In general, biotite dates vary smoothly fromwest to east from 96 to 88 Ma. Biotite plateau dates for the country rocks atthe western contact range from 96 to 94 Ma; across the pluton they decreasefrom 94 Ma to 88 Ma; and those from the country rocks northeast of thepluton decrease from 92 Ma at the contact to 70 Ma toward the north(Figs. 4, 5b, 6b; Table 2; and Data Repository [see footnote 1]). The plateaudates for the three muscovite-biotite pairs from the easternmost phase of thepluton are essentially concordant (Fig. 4e). Most plateau dates for biotite-muscovite mineral pairs from the country rocks northeast of the pluton arealso essentially concordant (Fig. 5, b and c; Table 2; and Data Repository[see footnote 1]). Due to their low closure temperatures they would not beexpected to retain metamorphic ages corresponding to the probable depthof emplacement, and they do not show older dates toward the north. Among

these eight biotite-muscovite mineral pairs, only two, both from the vicin-ity of the Sierra San Pedro Mártir fault (Fig. 2), are discordant. Theyoungest biotite date is from a sample from the country rock at the north-ernmost end of the north-south transect and yields the only substantiallydisturbed spectrum with an anomalously young integrated date of61 ± 1 Ma (Figs. 2, 4g, and 5b; sample 33, Table 2).

Plagioclase. One plagioclase separate from the muscovite-biotite gran-odiorite zone near the eastern end of the transect yields a well-definedplateau date of 88 Ma (Fig. 2, sample 24). This date is concordant with bio-tite and muscovite plateau dates for the same sample.

Fission-Track Geochronology

Analytical data for the four apatite samples (dates and confined track-length data) are presented and summarized in Table 3. The dates are plottedwith respect to sample location in Figure 7. Dates for three apatite separatesfrom the pluton decrease from 72 ± 8 Ma to 57 ± 15 Ma from west to east.

Figure 4. Representative 40Ar/39Ar age spectra of mineral pairs from (a) the western country rocks; (b) the western contact of the pluton; (c, d,and e) west to east across the Sierra San Pedro Mártir pluton; (f and g) the eastern country rocks. AOR number—lab number; S number—sam-ple number; PD—plateau date (2σ errors); Hb—hornblende, Bt—biotite, Ms—muscovite. Plateau segment is indicated by arrows. Data Repos-itory (see text footnote 1) has complete data set.



A sample from the metaplutonic country rocks near the western margin ofthe pluton yields an apatite date of 59 ± 10 Ma. Track lengths average13.7 µm. There is no sensible correlation between present elevations of thesample localities and uncorrected apatite fission-track ages (Fig. 7), whichare very similar within errors. Fission-track annealing in apatite by track-length reduction is a thermally controlled process. The kinetics of thisprocess have been studied experimentally and several annealing modelshave been developed (e.g., Laslett et al., 1987; Carlson, 1990). Using theannealing model of Green et al. (1989), modified by Crowley et al. (1991),Willet (1992) designed an algorithm to predict apatite fission-track age andlength distributions. The algorithm produces a theoretical age and track-length distribution for a given thermal history, and compares these to mea-sured age and track-length distributions using a Kolmogorov-Smirnov(K-S) statistic at an acceptable significance level (i.e., 0.95) (for more de-tails see Ravenhurst et al., 1994). The inverse model has been used to modelthe track-length distributions and calculates apatite fission-track ages for thesamples with the most track-length data. Track-length data show that theapatites have undergone moderate annealing.

Sample 2 (Fig. 2, Table 3), which has sufficient track-length measure-ments to yield satisfactory statistics, has been modeled to elucidate the pos-sible cooling history of the suite using the Willet algorithm. The actualtrack-length distribution (histogram) and modeled distributions (curve) forthe mean are shown in Figure 8, a and b, respectively. A time-temperature

envelope (upper and lower curves) was generated by the model for the 250statistically acceptable solutions (Fig. 8b). The middle curve in Figure 8b isthe exponential mean solution. The model was constrained to begin at95 Ma and 500 °C to match the 40Ar/39Ar hornblende age from the samesample and to be an ambient temperature of 25 °C at present. Because thetrack-length distribution appears unimodal (Fig. 8a), cooling-only solutionsare provided.

Aluminum-in-Hornblende Geobarometry

The empirical correlation between the pressure of emplacement of calc-alkaline granitic plutons and the total aluminum content of hornblendeequilibrated with quartz was investigated by Hammarstrom and Zen (1986)and Hollister et al. (1987). Experimental calibrations were presented byJohnson and Rutherford (1989), Thomas and Ernst (1990), and Schmidt(1992). The geobarometer is only valid when (1) the assemblage quartz +plagioclase + K-feldspar + hornblende + biotite + titanite + an oxide phase(magnetite or ilmenite) is coexistent within the rock; (2) the plagioclase inthe sample has a constant rim composition in the range An25–35; (3) theanalyses are limited only to the rim composition of hornblende; and (4) thepressure of crystallization is above 2 kbar (Hammarstrom and Zen, 1986;Hollister et al., 1987).

From the 40 samples collected for this study, only 6 samples (Fig. 9) had

Figure 5. Maps showing 40Ar/39Ar plateau dates; (a) hornblende,(b) biotite, and (c) muscovite and plagioclase. Plateau dates for repli-cate analyses have been averaged. Samples are listed in Table 2 fromwest to east.



the correct mineral assemblage (i.e., quartz + K-feldspar + hornblende + bio-tite + titanite + an oxide phase [magnetite or ilmenite] + primary epidote +plagioclase with a composition constant between An24 and An36). Alu-minum contents for the rim of the hornblendes were determined for thesesix samples. On the basis of these compositions (Table 4), pressures werecalculated employing five different calibrations (Table 5). These resultsshow, regardless of the calibration used, that within the typical 2σ errorlimit of ±1.2 kbar, there is no appreciable difference in pressure at the timeof crystallization across the exposed surface of the pluton (Fig. 9). Usingthe highest and lowest estimates (Table 5), depths between 12 and 20 kmare used in the discussion.

INTERPRETATION OF RESULTS

Age of Emplacement

The maximum crystallization age of the hornblende-biotite tonalite isgiven by the 99 ± 2 Ma 207Pb*/206Pb* zircon date from sample 4. Couplingthis with the concordant 206Pb*/238U date of 97.0 Ma, a conservative esti-mate of the age of emplacement, allowing for the possibility of slight Pbloss, is 97 +4/–1 Ma. The coincident 93.8 +1/–1 Ma 207Pb*/235U monazitedates from sample 17 are interpreted as reflecting the time of closure of themonazite system and most probably representing the crystallization age ofthe muscovite-biotite granodiorite. This supports the contention of Mc-Cormick (1986) from preliminary U/Pb zircon dates that these phases crys-tallized diachronously and appears to contradict the synchronous emplace-ment model of Walawender et al. (1990). An alternative interpretation of theyounger monazite dates is that the muscovite-biotite granodiorite cooled

Figure 6. 40Ar/39Ar plateau dates, with 2σ errors, projected onto aneast-west line. (a) Hornblende, (b) biotite, and (c) muscovite and pla-gioclase; ellipse—hornblende; triangle—biotite; rectangle—mus-covite; white circle—plagioclase; and white squares—laser probedates. Samples are listed in Table 2 from west to east.

Figure 7. Fission-track results for apatite: location, date (2σ error),and elevation in meters.



more slowly than the marginal hornblende-biotite tonalite. Thus, synchro-nous crystallization of all phases of the pluton cannot be completely ruledout. The 207Pb*/206Pb* date of 140 ± 11 Ma for zircon sample 17 from themuscovite-biotite granodiorite indicates an inherited crustal component(similar to results reported in Walawender et al., 1990); therefore, the206Pb*/238U date from this fraction represents only a maximum crystalliza-tion age for the sample, assuming no lead loss for the zircon. U/Pb agesfrom the two samples reported here suggest a difference in crystallizationage of ≈3 m.y. for these two zones of the Sierra San Pedro Mártir pluton.However, the ages just overlap within the stated uncertainties and therefore,the data do not clearly distinguish two separate intrusive events. Thus, theresults from these samples do not resolve the debate.

Tectonothermal History from 40Ar/39Ar Dating

This study was initiated to determine whether the Sierra San PedroMártir pluton has undergone significant east-side-up tilting. Because the40Ar/39Ar results compose the most comprehensive data set, they have beencombined with the sparse U/Pb data to provide a framework to develop pos-sible models of the postemplacement tectonothermal history of the SierraSan Pedro Mártir pluton. These models are then evaluated in the light offission-track and geobarometry studies.

The excellent plateau dates obtained from virtually all hornblende, biotite,muscovite, and plagioclase separates for samples from both the east-westand north-south transects are most readily interpreted as cooling ages(Fig. 4, a to f; Data Repository [see footnote 1]). These 40Ar/39Ar coolingages are combined with estimated argon closure temperatures (hornblende:500 °C [Harrison, 1981]; muscovite: 350 °C [Purdy and Jäger, 1976]; bio-tite: 280 °C [Harrison et al., 1985]; plagioclase: 220 °C [Harrison and

TABLE 3. APATITE FISSION-TRACK DATES AND TRACK LENGTH DATA FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON AND COUNTRY ROCKS

Sample Zone 11R Elevation Sample Lab Mineral G-C Ns Ni Rhos Rhoi Chi-2 Nd F-T ± 2 sigma Tracks Tracks ± St-errorno. UTM-E UTM-N (m) name run date no. length

2 631855 3426843 1620 SSPM- FT92-156 Apatite 12 206 635 10.4 32.0 0.5921 5312.0 59.0 ± 10.0 27 13.8 ± 0.5CONT-1.1

4 632349 3426113 1700 WC(2M) FT92-157 Apatite 11 702 1772 17.0 42.8 0.2468 5312.0 72.0 ± 7.4 88 13.6 ± 0.210 641676 3428342 2440 SSPM- FT92-158 Apatite 11 178 512 4.05 11.9 0.7148 5312.0 62.0 ± 11.2 35 14.0 ± 0.4

E.8-20.6

24 650177 3418481 2100 CORE FT92-159 Apatite 8 82 260 5.36 17.0 0.9182 5312.0 57.0 ± 14.8 16 13.4 ± 0.8

Note: A sumary of the track count data. All samples passed the chi-square test at the 95% confidence level and ages were calculated using pooled statistics. All analyseswere done by G. Li. A value of 106.9 ± 2.4 was used for the zeta factor. Fission-track date error estimates are at the 95% (2-sigma) confidence level. Abbreviatons are asfollows: F-T date = fission-track date in Ma, G-C = grains counted, Chi-2 = chi-square, Ns = number of spontaneous (fossil) tracks, Ni = number of induced tracks, Nd =number of flux dosimeter (CN-1) tracks counted, Rhos = density of spontaneous tracks (10E5/cm2), Rhoi = density of induced tracks (10E5/cm2), Tracks length = meantrack lengths (µm), Tracks (number) = number of tracks counted, and St-error = standard error (µm, 2-sigma).

Figure 8. Results of inverse fission-track modeling of sample 4 (FT92-157 [WC-2M]). (a) Measured (histogram) and exponential-mean mod-eled (smooth curve) apatite confined fission track-length distributions. (b) Time-temperature ranges (upper and lower curves) for 250 statisticallyacceptable solutions generated by the model. The middle curve is the exponential-mean solution. The model was constrained to begin at 95 Maand 500 °C, and to provide cooling-only solutions. The measured fission-track date of this sample is 72 ± 8 Ma. The mean closure age for model(i.e., age corrected for thermally induced length shortening) is 77 ± 5 (2σ errors). Samples are listed in Table 3 from west to east.



Clarke, 1979]) and with the U/Pb data to develop four models, initiallysummarized by Ortega-Rivera et al. (1994) and Gastil et al. (1994) and pre-sented below, that satisfy the data. The first two of these do not requirepostemplacement tilting of the pluton, whereas the other two do. Anymodel of the tectonothermal history of the pluton must satisfy the followingconditions required by the 40Ar/39Ar and U/Pb data and assumptions aboutgeothermal gradients.

(1) At the western margin (Figs. 2 and 4b), the most precise 40Ar/39Arplateau date for hornblende of 94.6 ± 0.5 Ma is close to the 97 Ma U/Pbzircon age of intrusion. Hornblende dates from the rest of the pluton aresimilar (Figs. 4, c and d, 5a, and 6a), ca. 92 Ma, and slightly younger thanthe 94 Ma U/Pb monazite date from the muscovite-biotite granodiorite. Thehornblende dates require that all phases of the pluton must have been em-placed, and the currently exposed surface cooled below 500 °C, by 92 Ma.Assuming a probable minimum paleogeothermal gradient of 30 °C/km, as

Figure 9. Locations of samples used for Al-in-hornblende geo-barometry and pressures obtained using the Schmidt (1992) calibra-tion; pressure is in kbar (2σ errors). Samples are listed in Table 5 fromwest to east.

TABLE 4. ELECTRON MICROPROBE HORNBLENDE ANALYSESFROM THE SIERRA SAN PEDRO MÁRTIR PLUTON

Average Sample no. 4 Sample no. 5 Sample no. 6 Sample no. 10 Sample no. 16 Sample no. 22analysis WC-2M SSPM-7-26-7 CORONA E8-20.6 SSPM-LV-8 SSPM-7-24-2(wt%)

SiO2 44.15 43.35 42.48 43.12 44.06 43.42Al2O3 10.05 10.08 9.6 10.42 9.23 9.92TiO2 0.79 0.91 0.096 1.05 0.65 0.8Fe2O3 3.6 4.05 3.36 3.87 3.7 3.59FeO 15.43 15.21 15.67 13.56 17.42 14.89MgO 10.28 10.1 9.39 9.26 8.96 10.15MnO 0.4 0.43 0.48 0.57 0.6 0.34CaO 12.13 11.85 11.35 11.91 12.08 12.21Na2O 1.23 1.28 1.16 1.25 1.03 1.04K2O 1.07 1.12 0.98 1.1 0.91 1.06Cr2O3 0.05 0.07 0 0.05 0 0.12H2O 2.02 2 1.94 2 1.99 1.99

Total 101.2 100.39 97.37 101.17 100.63 99.52

Average Sample no. 4 Sample no. 5 Sample no. 6 Sample no. 10 Sample no. 16 Sample no. 22structural WC-2M SSPM-7-26-7 CORONA E8-20.6 SSPM-LV-8 SSPM-7-24-2formula

Si 6.559 6.502 6.575 6.462 6.643 6.552Al (4) 1.441 1.498 1.425 1.538 1.357 1.448Al (6) 0.319 0.284 0.327 0.302 0.283 0.317Ti 0.088 0.103 0.112 0.118 0.074 0.091Fe3+ 0.403 0.457 0.392 0.437 0.42 0.408Fe2+ 1.917 1.909 2.029 2.076 2.196 1.879Mg 2.276 2.258 2.166 2.068 2.014 2.283Mn 0.05 0.055 0.063 0.072 0.077 0.043CA 1.931 1.904 1.882 1.912 1.951 1.974Na 0.354 0.372 0.348 0.363 0.301 0.304K 0.203 0.214 0.194 0.21 0.175 0.204Cr 0.006 0.008 0 0.006 0 0.014O 22 22 22 22 22 22OH 2 2 2 2 2 2

Note: The number of aluminum ions in the structural formula of hornblende decreases slightly with increasingFe3+. Each hornblende structural formula is the average of a no-glaucophane formula that minimizes Fe3+ by filling15 sites with all cations except Na and K, and a no-cummingtonite formula that maximizes Fe3+ by filling 13 siteswith all cations except Mn, Ca, Na, and K.



might be expected in a metamorphic terrane on continental basement, thisfurther requires that the currently exposed surface of the pluton must havebeen no deeper than ≈16 km at this time.

(2) The observed decrease in the biotite dates from west to east (Figs. 4and 6b) require that the currently exposed surface of the pluton last passedthrough 280 °C between 94 Ma and 88 Ma diachronously from west to east.No comparable diachroneity exists from north to south. Again assuming aprobable minimum paleogeothermal gradient of 30 °C/km, the present sur-face of the pluton was at a depth of 9 km or less by 92 Ma at its western sideand by 88 Ma at its eastern side. Furthermore, the plagioclase date (Fig. 5c)constrains the eastern side to have been shallower than 7 km at ca. 88 Ma.

MODELS 1 AND 2:NO POSTEMPLACEMENT TILTING REQUIRED

Model 1: Eastward Migration of Plutonism

The pattern of eastward-younging of 40Ar/39Ar biotite dates (Figs. 4and 5b) could be explained by a progressive eastward migration of pluton-ism over time, with no subsequent tilting of the pluton. McCormick (1986)raised this as a possibility to explain his pattern of conventional K-Ar datesfor biotite. The easternmost muscovite-biotite granodiorite phase of theSierra San Pedro Mártir pluton and the garnet and muscovite-bearing ElDiablo granodiorite to the northeast crosscut, and are therefore younger than,the hornblende-biotite tonalite phase (Eastman, 1986; McCormick, 1986).

The U/Pb data from the Sierra San Pedro Mártir pluton indicate that crys-tallization of the pluton may have progressed from west to east over a≈3 m.y. time period (from ca. 97 to ca. 94 Ma). This trend is similar to thatobserved in a sequence of nested plutons in the Sequoia National Park re-gion of the Sierra Nevada batholith (Chen and Moore, 1982). If the 3 m.y.span of crystallization is correct, then the 40Ar/39Ar ages from the SierraSan Pedro Mártir pluton suggest that hornblende-biotite tonalite of the outerzone had cooled below 500 °C (i.e., 94.6 ± 0.5 Ma hornblende age) whilethe muscovite-biotite granodiorite of the inner zone was still crystallizing(94 +1/–1 Ma, U/Pb monazite date).

40Ar/39Ar hornblende and biotite dates (Figs. 4 and 5, a and b) satisfy amodel of such eastward younging of plutonism, in which simple, rapid cool-ing followed emplacement of each successive phase. The oldest 40Ar/39Ardate of 97.4 ± 1.0 Ma (Fig. 4a) from a hornblende in the country rock, nearthe western margin of the pluton, agrees with the 97.0 +4/–1 Ma U/Pb age foremplacement of the biotite-hornblende tonalite. The hornblende from thewestern margin of this tonalite, where cooling would be somewhat slower,yields an age of 94.6 Ma (Fig. 4b). Similarly, the 92 Ma cooling age for the

hornblende farther east (Fig. 4d) is somewhat younger than the 94 +1/–1 MaU/Pb monazite date for the muscovite-biotite granodiorite phase.

In model 1, the decrease of biotite dates from 94 Ma at the western mar-gin to 88 Ma near the eastern margin (Fig. 4) would simply reflect passagethrough ≈280 °C at progressively younger times from west to east as suc-cessively younger phases cooled. If this model is valid, the 92 Ma dates onhornblende and the ≤90 Ma dates on biotite from the interior parts of thehornblende-biotite tonalite (Fig. 5, a and b) must have resulted from some-what slower cooling of the interior relative to the western margin of thisphase, and/or overprinting as a consequence of proximity to the youngermagmatic pulses to the east.

Model 2: Intrusion of All Phases at ca. 97 Ma; Inward Cooling and Faulting

It is possible to explain the pattern of the hornblende and mica dates with-out requiring tilting of the pluton, even if all phases of the pluton had beenemplaced at essentially the same time (probably 97 +4/–1 Ma, the 94 +1/–1 Mamonazite date for the muscovite-biotite granodiorite representing slowercooling of the interior). For this to be the case, it must be argued that the agepatterns are a consequence of more rapid cooling of the pluton at its mar-gins, and progressively slower cooling toward its interior. This would explainwhy the oldest cooling ages for hornblende and biotite are those from sam-ples taken at the western margin of the pluton, and why, in a general way,the mica dates young eastward toward the eastern edge of the pluton.

Model 2 would require the dates for micas from near the northeast marginand from the extreme southeastward part of the pluton to be older than thoseat the center of the pluton; however, they are not. In model 2, the observedage pattern of the biotites is explained by removal of the east and northeastparts of the original pluton by faulting (Fig. 10), leading to the present asym-metric distribution of the phases and biotite dates (Figs. 5b and 6b).

A significant escarpment at the northeast margin of the pluton marks thelocation of the still-active Sierra San Pedro Mártir fault. This fault dipsnortheast at 60° and cannot be responsible for the dismemberment of thepluton, but the possibility of earlier, low-angle faults cannot be ruled out.There is a minor northeastward younging of micas toward the fault. Theonly disturbed age spectrum found in this study is from a biotite from thenorthernmost area, which, if the Sierra San Pedro Mártir fault dips north-eastward at a low angle, would have been close to the fault surface. The>20 m.y. discordance between the biotite and muscovite dates from thisrock (sample 33, Table 2, and Fig. 4g) may have been caused by hy-drothermal fluids from the fault zone.

TABLE 5. ALUMINUM-IN-HORNBLENDE GEOBAROMETRIC RESULTSFOR THE SIERRA SAN PEDRO MÁRTIR PLUTON

Sample UTM Elevation Sample Mineral Al (T) P (1) P (2) P (3) P (4) P (5)number long lat (m) name (kbar) (kbar) (kbar) (kbar) (kbar)

4 632349 3426113 1700 WC-(2M) hb 1.84 5.8 4.3 3.6 5.3 5.64 632349 3426113 1700 WC-(2M) hb 1.70 5.1 3.7 2.9 4.7 4.95 636097 3428718 2240 SSPM-7-26-7-88 hb 1.78 5.5 4.1 3.3 5.0 5.36 637859 3429950 2520 CORONA hb 1.75 5.3 4.0 3.1 4.9 5.1

10 641676 3428342 2440 E.8-20.6 hb 1.78 5.5 4.1 3.3 5.0 5.310 641676 3428342 2440 E.8-20.6 hb 1.84 5.7 4.3 3.6 5.3 5.616 640985 3416545 2040 SSPM-LV-8 hb 1.64 4.8 3.5 2.5 4.3 4.522 649672 3425197 2400 SSPM-7-24-2-88 hb 1.77 5.4 4.0 3.2 5.0 5.222 649672 3425197 2400 SSPM-7-24-2-88 hb 1.77 5.4 4.0 3.2 5.0 5.2

Note: P (1)—pressure determined using Schmidt (1992) calibration [±1.6], P (2)—pressure determined using Johnsonand Rutherford (1989) calibration [±1.0], P (3)—pressure determined using Thomas and Ernst (1990) calibration [±1.6],P (4)—pressure determined using Hammarstrom and Zen (1986) empirical calibration [±6.0], P (5)—pressure determinedusing Hollister et al. (1987) empirical calibration [±2]. Pressures are in kilobar ± 2-sigma errors. Al (T)—total aluminum.



MODELS 3 AND 4:POSTEMPLACEMENT TILTING REQUIRED

Model 3: Intrusion at 97–94 Ma; Major Rotation by 88 Ma; Minor Rotation Possible After 88 Ma

It has been suggested that an east-west transect of the Sierra San PedroMártir represents a 20 km crustal section exposed by 90° of rotation abouta horizontal axis (Gastil et al., 1991). In model 3, significant tilting of thepluton is invoked to explain the 40Ar/39Ar data, but such tilting (Fig. 10)must have occurred by 88 Ma (i.e., within ≈9 m.y. of emplacement). At leastpart of the tilting must have occurred by 91 to 92 Ma (the hornblende clo-sure age) in order for both the formerly deepest parts of the pluton (which,at a depth of 20 km, would otherwise have remained too hot for argon re-tention) and the shallowest parts to have cooled essentially simultaneouslythrough 500 °C. The remainder of this major tilting must have been com-pleted by 88 Ma, in order for the temperature to have fallen below 280 °C(the closure temperature of biotite) and possibly 220 °C (the closure tem-perature of plagioclase). After major tilting, there could have been later mi-nor (≈15°) tilting after 88 Ma (model 4).

Model 4: Intrusion at 97 Ma; Minor Tilting After 88 Ma

The 40Ar/39Ar data can be accommodated if the pluton underwent no tilt-ing before 88 Ma and minor tilting (≈15°) at or any time after 88 Ma (Fig. 9).In this model, following emplacement of the pluton at 97 +4/–1 Ma, horn-blende in the contact zone at the western margin became closed to argon dif-fusion between 98 and 94 Ma. The rest of the currently exposed surfacecooled sufficiently rapidly through 500 °C to close most of the hornblendesto argon loss by 91 Ma. This surface was then tilted (Fig. 10) and cooled di-achronously through the closure temperature of biotite (280 °C) from 93 to88 Ma as the subhorizontal isotherm migrated downward. The 88 Ma cool-ing age for plagioclase from the eastern part of the pluton shows that the tem-peratures of the currently exposed surface may have been as low as ≈220 °Cby this time. East-side-up tilting at, or any time after, 88 Ma would create theobserved biotite age gradient at the currently exposed surface of the pluton.

Assuming a probable minimum paleogeothermal gradient of 30 °C/kmat 88 Ma and closure temperatures of 280 °C and 220 °C for Ar loss frombiotite and plagioclase, respectively, the difference in paleodepths betweenthe western and eastern ends of the pluton prior to tilting is calculated tohave been less than 7 km, and probably less than 5 km. The present-day20 km east-west exposure of the pluton could therefore have resulted from≈15° of tilting any time after 88 Ma.

Figure 10. Geologic map of the Sierra San Pedro Mártir pluton and schematic representation of four possible tectonic cross sections used toexplain the tectonothermal history of the Sierra San Pedro Mártir pluton. (c) Modified from Gastil et al. (1991).



Depth of Emplacement and Aluminum-in-Hornblende Geobarometry

Ideally, the mineralogy of plutons and adjacent metamorphic rocks canbe used to estimate the depth of emplacement. For this pluton, the bulk com-position of the rocks in the contact zone is unsuitable for geobarometry;therefore, few studies of these metamorphic rocks have been attempted.Eastman (1986) inferred that this pluton probably crystallized at mesozonaldepths of between 7 to 15 km because the pluton contains magmatic epidoteand the contact zone has coarse-grained schist and gneisses with sillimanite.Rothstein and Manning (1994) calculated a minimum pressure and temper-ature of 5 kbar and 700 °C for mineral assemblages from regionally meta-morphosed migmatitic, semipelitic schist east of the pluton on the escarp-ment of the Sierra San Pedro Mártir fault. Thus, the depth of emplacementof this pluton is not well constrained. However, six of the dated samplesfrom the pluton have the correct mineral assemblage for Al-in-hornblendegeobarometry.

Zen and Hammarstrom (1984) stated that primary epidote may form atpressures of ≈8 kbar. Ghent et al. (1991) concluded that the presence of mag-matic epidote in calc-alkaline plutons is indicative of a low CO2 activity.Therefore, because this pluton contains primary epidote (Eastman, 1986), itmay be assumed that the coexisting magmatic fluid was H2O dominated.Schmidt’s (1992) experimental calibration of the Al-in-hornblende geo-barometer is based on an H2O-saturated fluid, and is most likely to yield thebest pressure estimates for the Sierra San Pedro Mártir pluton. This calibra-tion yields the highest pressure estimate, averaging 5.3 ± 1.2 kbar (Table 5,Fig. 9), whereas that of Thomas and Ernst (1990) yields the lowest,3.1 ± 1.2 kbar. Thus, if these values are taken as the maximum and minimum,the geobarometry suggests that the currently exposed surface of the plutonwas emplaced at a depth between 12 and 20 km (assuming an average over-burden density of 2.7 g/cm3). The highest pressure estimate is, moreover, con-sistent with the Rothstein and Manning (1994) pressure-temperature esti-mates for the regionally metamorphosed rocks east of the pluton.

Although Al-in-hornblende barometry is still not well understood, thebarometer is theoretically sound if equilibrium was attained at the solidustemperature both in the experiment and in the field. It can be seen that, forthe purpose of this study and regardless of the calibration used to determinethe paleopressures (Table 5), the six analyzed samples equilibrated, withinerror, at the same pressure. This implies that little if any tilting of this sur-face has occurred since equilibration, and hence lends support to models 1and 2, but rules out the major tilting underlying model 3. Errors in the pres-sure determinations allow differences in paleodepth across the currently ex-posed surface of 3.5 km, and therefore the data do not preclude minorpostemplacement tilting of the pluton (model 4). The geobarometry re-stricts the maximum tilt to ≈15°, but does not constrain its orientation.

Apatite Fission-Track Analyses

The fission-track data (Table 3) are too sparse and the errors in the dates toolarge to provide definitive evaluation of the tectonic models proposed above.However, some general inferences are possible. Fission-track dates on apatite(closure temperature of 110 °C; Naeser, 1979) suggest that all parts of the cur-rently exposed surface of the pluton were within ≈3 km of the surface (as-suming a paleogeothermal gradient of 30 °C/km) by ca. 60 Ma. The mea-sured fission-track dates support the hypothesis that these rocks cooledthrough the apatite closure temperature around the Cretaceous-Tertiaryboundary. However, the track-length data (Table 3) for sample 2 show thatthere has been moderate annealing, and that cooling of the plutonic rocks wastherefore protracted. The modeling of sample 2 (Fig. 8b) suggests that a sim-ple cooling of the sample after 77 Ma (closure age of the sample when itbegan to accumulate tracks) represents an adequate model for the Sierra San

Pedro Mártir pluton. The measured apatite fission-track date of this sample is72 ± 8 Ma. The mean closure age from the model (i.e., age corrected for ther-mally induced length shortening) is 77 ± 5 Ma (2σ errors).

Eastward younging of the fission-track dates (ca. 72 Ma near the westernmargin and ca. 57 Ma in the east) across the pluton (Fig. 7) suggests di-achronous cooling through ≈110 °C, although the large errors in the fission-track dates make this conclusion tenuous. The 59 Ma date for apatite froma sample of country rock west of the pluton margin does not fit the appar-ent pattern of eastward younging shown by samples from the pluton. Thismay reflect the large uncertainties in the fission-track dates, or alternatively,a fault may have allowed uplift of a region of country rock relative to thepluton (Gastil, 1990).

If the late diachronous cooling implied by the eastward younging of thefission-track ages across the pluton is real, it must be considered in evaluat-ing the models. Such diachroneity is unlikely to be a consequence of themechanisms proposed in models 1 and 2, because the differential coolingacross the pluton called for in those models would not be sufficiently pro-tracted to yield an appreciable age difference in the apatites 25 m.y. later.Isotherms would have become subhorizontal within 10 m.y. after emplace-ment of the pluton (e.g., Beck, 1992). However, diachronous cooling is im-plicit in models 3 and 4, which accommodate (model 3) or require(model 4) minor tilting after 88 Ma. The fission-track data would suggestthat part or all of this minor tilting occurred at or after 57 Ma.

CONCLUSIONS AND TECTONOTHERMAL IMPLICATIONS

Our data support the following history for the Sierra San Pedro Mártirpluton.

(1) The U/Pb ages indicate that crystallization of the pluton probablytook place diachronously, from ca. 97 Ma for the hornblende-biotite tonalitein the west to ca. 94 Ma for the muscovite-biotite granodiorite in the east.

(2) The 40Ar/39Ar and U/Pb ages show a rapid rate of cooling(≈40 °C/m.y.) in the first 10 m.y. for all parts of the exposed pluton(Fig. 11). This requires that significant uplift and erosion (≈7 km) must haveoccurred by 88 Ma to close biotite and plagioclase to argon loss. The apatitefission-track ages suggest an additional 4 km of erosion by 57 Ma (theyoungest apatite date). Modeling of track-length distribution supports a his-tory of monotonic slow cooling of the pluton from ca. 80 Ma to the present.This is in contrast to the history after 80 Ma of the west-central (Snee et al.,1994) and eastern (George and Dokka, 1994) Peninsular Ranges of south-ern California, which were interpreted to have undergone rapid cooling as-sociated with uplift at ca. 76 Ma for the eastern part and at ca. 62 Ma for thewest-central part of the batholith.

(3) Although tilting of the pluton is not required to explain the monotonicdecrease in 40Ar/39Ar cooling ages of biotites eastward across the presentsurface of the pluton, our preferred interpretation is that minor (≈15°) east-side-up tilting of the pluton about a north-south horizontal axis occurred ator after 88 Ma, satisfying the younging in apatite fission-track dates. Further-more, the fission-track data suggest that part or all of this tilting may havetaken place at or after 57 Ma. Similar patterns of southwest to northeastyounging of K-Ar dates across other La Posta–type plutons (Krummenacheret al., 1975) may similarly be a consequence of late-stage northeast-side-uptilting about an axis subparallel to the trend of the batholith.

(4) Al-in-hornblende geobarometry suggests that the currently exposedsurface of the pluton has not been significantly rotated since its time of for-mation at depths of between 12 and 20 km. The maximum tilt of the plutonis restricted to ≈15°. This result, coupled with our interpretation of the40Ar/39Ar data, indicates that the increase in metamorphic grade of the hostrocks from west to east across the belt is a preemplacement feature.

(5) Butler et al. (1991) suggested that northeast side-up tilting (15°–20°)



of the batholith as opposed to large-scale northward tectonic transport is re-sponsible for the discordant paleomagnetic inclinations obtained fromrocks of western California and Baja California. Such tilting is consistentwith the upper limit of tilting calculated from our geochronological dataand militates against large-scale northerly transport of Baja California, asproposed by, for example, Hagstrum et al. (1985). Moreover, Paleozoic andMesozoic rocks in Baja California are compatible with a position of penin-sular California adjacent to Sonora prior to Cenozoic opening of the Gulf ofCalifornia (Gastil and Miller, 1984).

If the pattern of northeastward younging of the K-Ar and fission-track

dates in the Sierra San Pedro Mártir pluton is a result of tilting after 57 Ma,the tilting may have been caused by regional-scale crustal extension asso-ciated with the opening of the Gulf of California in Neogene time (e.g.,Stock and Hodges, 1989). Such effects are not seen in the more northerlyareas of the batholith. The protracted cooling after 80 Ma suggested by theannealed apatite fission-tracks is in agreement with a component of late butminor uplift, perhaps associated with such tilting. Local expression of thisextension includes the east-dipping, still-active Sierra San Pedro Mártirfault and related listric faults (Gastil et al., 1975) that have rotated crustal-scale blocks in the correct sense to produce northeast-side-up tilting.

Figure 11. Cooling curves forthe currently exposed surface ofthe Sierra San Pedro Mártir plu-ton; dates in Ma and 2σ errors,symbols are larger than errorbars except those for the apatitedata.



ACKNOWLEDGMENTS

This paper is part of Ortega-Rivera’s Ph.D. dissertation. Major fundingfor this research was provided by a grant from the Consejo Nacional deCiencia y Tecnología (CONACyT) of México to Ortega-Rivera, and grantsfrom the Natural Science and Engineering Research Council of Canada toFarrar, Hanes, and Zentilli (8820). We gratefully acknowledge a minor travelgrant from the School of Graduate Studies at Queen’s University and atravel grant from the Department of Geology of San Diego State University,California, to Ortega-Rivera. Logistical support in the field was provided bystaff from the Division of Earth Sciences of the Centro de InvestigaciónCientífica y de Educación Superior de Ensenada (CICESE), Baja Califor-nia, and staff from the Department of Geology of San Diego State Univer-sity. A postdoctoral fellowship from the Commission of European Com-munities and internal grant from CICESE to López-Martínez allowed useof the laser mass spectrometer system at Nice, France. Technical support toLópez-Martínez at CICESE was provided by V. Moreno and G. Mora. Wethank the staff of the McMaster Nuclear Reactor for assistance with sampleirradiation; M. Colpron, H. Sandeman, D. Kempson, and H. Jamieson fortheir assistance with the Queen’s University microprobe analyses; andD. M. Carmichael for helpful discussions on the metamorphic relationshipsand aluminum-in-hornblende geobarometry. We thank G. Li and A. Gristof Dalhousie University for assistance with the fission-track analyses andmodeling, A. H. Clark and D. M. Carmichael for computer and printer fa-cilities, and A. H. Clark for revision of the final manuscript.

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MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 24, 1995REVISED MANUSCRIPT RECEIVED OCTOBER 21, 1996MANUSCRIPT ACCEPTED NOVEMBER 8, 1996