Discordant paleomagnetic data for middle ... - Semantic Scholar
TECTONICS, VOL. 21, NO. 5, 1049, doi:10.1029/2001TC001298, 2002
Discordant paleomagnetic data for middle-Cretaceous intrusive rocks from northern Baja California: Latitude displacement, tilt, or vertical axis rotation? Harald Bo¨hnel Centro de Geociencias, Universidad Nacional Auto´noma de Me´xico (UNAM), Campus Juriquilla, Queretaro, Mexico
Luis A. Delgado-Argote Departamento de Geologı´a, CICESE, Ensenada, Baja California, Mexico
David L. Kimbrough Department of Geological Sciences, San Diego State University, San Diego, California, USA
Received 9 May 2001; revised 5 March 2002; accepted 19 June 2002; published 25 October 2002.
[1] Paleomagnetic results and U/Pb zircon dating from
the San Marcos dike swarm and the El Testerazo pluton in the Cretaceous Peninsular Ranges batholith of northern Baja California are used to evaluate alternative pre-Neogene paleogeographic reconstructions of the Baja California peninsula. The San Marcos dike swarm is a dense, northwest striking, regional dike swarm that is exposed over an 100 km long segment of the batholith and has yielded a U/Pb zircon crystallization age of 120 ± 1 Ma. Dike attitudes from the swarm suggest a regionally consistent average 320E strike and 79NE dip. The El Testerazo pluton is a younger tonalite intrusion that truncates the northern end of the dike swarm. All but one of 36 sites sampled in this study show remanence of normal polarity. Paleopoles for the San Marcos dike swarm and El Testerazo pluton are indistinguishable and were combined into a paleopole at 248.1E, 86.6N, A95 = 4.8, which is displaced with respect to the 122 Ma reference pole for stable North America at 198.2E, 72.3N, A95 = 3.3. The displacement may be described by an apparent clockwise rotation of 18 ± 6 and an apparent northward shift of 8 ± 5. Restoring a northward shift of about 3, related to the separation of Baja California from North America since 10 Ma, only a marginal northward displacement of 5 ± 5 is left. The clockwise rotation may be the result of crustal block rotations within the right-lateral shear systems in northern Baja California, although there is no geological evidence that supports this possibility. Alternatively, the difference between paleopole and reference pole may be due to tilting of the study area. Restoring a northeastward tilt of 11, based on the mean dip measured for the San Marcos dike swarm in the Copyright 2002 by the American Geophysical Union. 0278-7407/02/2001TC001298$12.00
study area, yields a paleopole at 187.6E, 70.8N, A95 = 5.6, which is indistinguishable from the 122 Ma North American reference pole. The tilting hypothesis suggested previously as a potential explanation for low inclination of paleomagnetic data from Baja California therefore seems to be viable for the discordant paleopoles from this segment of the batholith. Data obtained previously about 150 km further south and along the western edge of the batholith defined a concordant paleopole without any tilt correction, indicating that regional tilting may not be valid for all INDEX TERMS: 1525 Geomagnetism of Baja California. and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8105 Tectonophysics: Continental margins and sedimentary basins; 8157 Tectonophysics: Plate motions— past (3040); 9350 Information Related to Geographic Region: North America; 1035 Geochemistry: Geochronology; KEYWORDS: paleomagnetism, Baja California, paleogeography, Cretaceous, intrusive rocks, geochronology. Citation: Bo¨hnel, H., L. A. Delgado-Argote, and D. L. Kimbrough, Discordant paleomagnetic data for middle Cretaceous intrusive rocks from northern Baja California: Latitude displacement, tilt, or vertical axis rotation?, Tectonics, 21(5), 1049, doi:10.1029/2001TC001298, 2002.
1. Introduction [2] The Peninsular Ranges of southern and Baja California play a central role in the debate over Cretaceous paleogeography of the western Cordillera. Cowan [1994] highlights the uncertainties and conflicts between models based on geologic versus paleomagnetic data. Paleomagnetic studies in the Peninsular Ranges, conducted at many widespread sites on both plutonic and sedimentary rocks, suggest that the Baja peninsula was located 1200 km southward of its present position off the southern margin of Mexico between 200 and 50 Ma [e.g., Hagstrum and Filmer, 1990; Lund and Bottjer, 1991; Hagstrum and Sedlock, 1998]. This restoration represents the 300 km of separation necessary to close the Neogene Gulf of
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
California plus another 900 km of displacement to the south. Geologic evidence on the other hand appears equally strong that simply closing the Gulf of California restores the peninsula to its original position against the craton throughout the Phanerozoic [Gastil, 1993]. Two very different solutions to the discrepancy between paleomagnetic versus geologic restorations of Peninsular California involving offsets along hypothetical faults have been proposed [Hagstrum et al., 1987; Beck, 1991]. However, Gastil [1993] regards these solutions improbable based on regional geology. [3] Butler et al. [1991] and Dickinson and Butler [1998] made the intriguing observation that the paleomagnetic inclinations from plutonic rocks in the Peninsular Ranges [e.g., Teissere and Beck, 1973; Hagstrum et al., 1985], which require 1200 km of northward transport, can be reconciled by westward tilting of the batholith about a horizontal axis. The ‘‘restored’’ paleomagnetic directions would then require minimum northward translation of the batholith, consistent with Gastil’s [1993] geologic restoration. Paleomagnetic results from sedimentary rocks, which also indicate 1200 km of northward transport, are further interpreted by Butler et al. [1991] to potentially reflect anomalously shallow inclination data resulting from unrecognized burial compaction. Such inclination shallowing has occurred in southern California marine sediments [Tan and Kodama, 1998] and sediments from the California margin [Kodama and Ward, 2001], thereby reducing or eliminating the need of paleolatitudinal offsets proposed previously for the corresponding terranes. Significantly, data from Jurassic and Cretaceous sediments in Baja California have failed fold and conglomerate tests [e.g., Hagstrum and Filmer, 1990], indicating a complex magnetic history that makes tectonic interpretation of the magnetic record difficult. [4] We have initiated the study of intrusive complexes from throughout the length of the Baja California peninsula. Our approach is to sample in detail those batholiths where detailed geological, structural and geochronologic data are available. The goal is to obtain reliable individual paleopoles, which in turn should provide a coherent pattern, if the rocks have been affected by simple latitude shift or coherent tilt. Varying records observed from different regions might additionally indicate differential tilting or other even more complex deformation processes. So far results are variable and not yet conclusive: a concordant high quality paleopole was determined for the San Telmo intrusive complex in the north (present latitude 31N [Bo¨hnel and Delgado-Argote, 2000]), in contrast to a 35– 45 clockwise rotated paleopole for the Los Cabos Block in the south (present latitude 23– 24N [Schaaf et al., 2000]). Indeed, these much differing results roughly correspond to the range of dispersion reported previously for paleopoles from sedimentary rocks [e.g., Hagstrum and Filmer, 1990]. It is noteworthy that the San Telmo complex is located in a tectonically undisrupted part of the Peninsula along the western edge of the Peninsular Ranges batholith, while the Los Cabos Block at the tip of Baja California has been strongly affected by Neogene tectonics related to the opening of the Gulf of California [Fletcher et al., 2000].
[5] Here we present new data from the San Marcos dike swarm and the adjacent El Testerazo pluton in northern Baja California. The San Marcos dike swarm was targeted for study because it has a regionally consistent average 320E strike and 79NE dip which potentially represents a strain marker recording westward tilting of the batholith as proposed by Butler et al. [1991].
2. Geology [6] The areas of San Marcos and El Testerazo are located west of the Pinal Pluton in the northwestern part of the Baja California peninsula (Figure 1). This region is part of the magnetite-rich western province of the Peninsular Ranges batholith [Gastil et al., 1990] located west of the magnetiteilmenite boundary (Figure 1). The eastern province of the Peninsular Ranges batholith is predominantly magnetitefree. Plutons of the western province include ring complexes emplaced in dike-rich sub-volcanic environments of the Cretaceous Alisitos Formation [Gastil et al., 1975]. [7] The San Marcos-Testerazo area is located in the south central part of the Southern California Shear Zone defined by Legg et al. [1991]. This regional shear zone is bounded by the right-lateral San Clemente fault on one side and the San Andreas-Gulf of California fault system on the other (Figure 1). In such a tectonic scenario, large-scale counterclockwise rotation of semi-rigid blocks, such as has been documented in the Transverse Ranges of the southern California farther to the north, is a possibility although it has never been recognized in this region. In the area of San Marcos-Valle Seco (Figure 2), north-northwest trending dikes form a well-exposed swarm-like complex which we refer to here as the San Marcos dike swarm. The dikes intrude both the granitoids and metamorphic rocks. Dikes range from basalt to rhyolite but are mostly of intermediate composition (plagioclase is oligoclase-andesine) showing a wide variety of textures ranging from aphanitic to micro granular, with porphyritic textures the most common. All dikes show pervasive to selective deuteric alteration (phyllic and propylitic) with very low silica content. Dike thicknesses range mostly from 1 to 10 m. The attitude of individual dikes strike consistently north-northwest, parallel to the structural grain of the Peninsular Ranges batholith, and are characterized by steep northeast dips. The dikes are hosted by plutonic rock as well as greenschist facies metasedimentary sandstone-siltstone sequences. Plutons that are crosscut by the dikes range in composition from tonalite to monzonite to monzodiorite in composition. Dioritic bodies less than 1.5 km across are reported in the same area by Comisio´n de Estudios del Territorio Nacional (CETENAL) [1977]. Similar dikes, which we interpret to be part of the San Marcos swarm, occur from this region southward to near the Agua Blanca fault. [8] We conducted a systematic paleomagnetic sampling of dikes cropping out along the Agua Caliente Canyon and part of the Valle Seco Valley (Figure 2). A total of 20 dikes were sampled for paleomagnetic and petrographic analyses (Figure 2 and Table 1). Fracturing and attitude of the dikes were measured mainly along the Agua Caliente Canyon and
¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
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Figure 1. Study area and major geologic features of northern Baja California. Most of the reported active faults are indicated [after Legg et al., 1991; Sua´rez-Vidal et al., 1991; Frez and Frı´as-Camacho, 1998a]. Black areas show some of the main plutons identified from satellite images [Romero-Espejel and Delgado-Argote, 1997] and the shaded zone is the magnetite-ilmenite boundary after Gastil et al. [1990]. M, Mason Valley; ET, El Topo; LP, La Posta; P, Pinal; LJ, Laguna Juarez; ST, San Telmo; SJ, San Jose; Z, La Zarza; SPM, San Pedro Martir. around the Valle Seco Valley. The mean orientation of the dikes is 320 and their mean pole plunges 11, with its azimuth oriented toward 230 (Figure 3a). [9] As noted above, the San Marcos dikes dip steeply and consistently to the northeast in the San Marcos area. Theoretical [Delaney et al., 1986] and field studies [Pollard et al., 1983] indicate that dikes in the upper crust typically form in a near-vertical orientation by hydraulic fracturing and either vertical or lateral flow. Assuming that the San Marcos dikes were vertically emplaced and subsequently tilted, then restoration to their original orientation would require rotating 11 to the southwest around a horizontal axis striking 320, the mean orientation of the dikes (Figure 3b).
[10] Fractures in the 17 sampled dikes as well as the host rocks in the area of San Marcos almost parallel the orientation of the dikes. Most of the horizontal fractures are almost perpendicular to the orientation of vertical fracturing (Figure 3c), suggesting that simple cooling formed the set of orthogonal fractures. No border deformation due to forced emplacement was observed in the dikes or the enclosing rocks, indicating a passive emplacement. [11] El Testerazo Pluton is an approximately 10 km wide intrusive body of hornblende-biotite tonalite. There is significant variation of modal hornblende and biotite within the pluton. Figure 2 illustrates curvilinear fractures that characterize the southern part of the pluton; the northern part is partially covered by Eocene conglomerates of the Las
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO Table 1. Site Mean Paleomagnetic Data for San Marcos Dikes and Testerazo Plutona Number
Site
1 SM-1 2 SM-2 3 SM-3 4 SM-4 5 SM-5b 6 SM-6b 7 SM-7 8 SM-8 9 SM-9 10 SM-10 11 SM-11b 12 SM-12b 13 SM-13 14 SM-14b 15 SM-15 16 SM-16 17 SM-17 18 SM-18 19 SM-19 20 SM-20b 21 PT-1 22 PT-2 23 PT-3 24 PT-4 25 PT-5 26 PT-6 27 PT-7 28 PT-8 29 PT-9 30 PT-10 31 PT-11 32 PT-12 33 PT-13 34 PT-14 35 PT-15b 36 PT-16 MEAN POLE
Latitude (N)
Longitude (W)
n
r
R
k
a95, deg
3206.090 3206.090 3206.140 3206.160 3206.160 3206.130 3206.130 3206.320 3206.320 3206.320 3206.320 3211.070 3207.200 3207.140 3207.400 3207.400 3207.400 3206.250 3206.250 3206.250 3218.800 3219.170 3219.500 3219.770 3219.800 3219.800 3219.870 3219.450 3218.370 3218.170 3217.670 3218.140 3218.260 3218.770 3218.810 3218.880
11626.670 11626.670 11626.650 11626.630 11626.630 11626.660 11626.660 11625.410 11625.410 11625.410 11625.410 11626.220 11626.400 11627.460 11625.900 11625.900 11625.900 11625.420 11625.420 11625.420 11632.450 11632.140 11631.560 11631.210 11630.400 11630.700 11631.100 11631.750 11632.330 11632.270 11631.680 11628.990 11630.280 11630.890 11631.210 11631.770
7 6 5 8 3 5 2 7 5 4 3 4 6 6 7 7 9 6 7 6 9 8 8 7 8 8 7 8 8 9 7 7 7 9 7 8 26
1 0 2 0 3 0
6.97763 5.89092 4.94532 7.67605 2.95589 4.82690
268.2 45.8 73.2 21.6 45.3 23.1
3.7 10.0 9.0 12.2 18.5 16.3
0 0 1 1 3 2 2 0 0 0 1 1 0 0 0 1 0 0 0 0 1 1 0 0
6.97995 4.97510 3.97549 2.88120 3.75554 5.79629 5.61327 6.75139 6.96820 8.99216 5.99194 6.96837 5.71408 8.93190 7.91917 7.94115 6.91512 7.85957 7.77470 6.85348 7.68589 7.95750 8.80881 6.98465
299.2 160.7 122.4 16.8 12.3 24.5 12.9 24.1 188.7 1020.4 620.3 189.7 17.5 117.5 96.6 119.0 70.7 49.8 31.1 40.9 22.3 164.7 41.8 391.0
3.5 6.1 8.3 31.0 27.3 13.8 19.4 12.5 4.4 1.6 2.7 4.4 16.5 4.8 6.0 5.1 7.2 7.9 10.1 9.5 12.0 4.3 8.1 3.1
1
6.87656
48.6
8.7
2 0 6
6.31477 7.85943 25.29542
8.8 49.8 35.5
21.6 7.9 4.8
Dec, E
Inc, deg
342.5 56.7 359.1 51.2 350.7 54.6 346.7 53.9 354.2 35.9 348.5 47.9 not calculated 346.5 49.3 345.6 47.5 334.4 58.2 344.3 55.3 311.1 55.4 328.9 48.4 338.0 42.9 150.6 55.6 356.8 49.9 19.7 65.6 343.6 35.0 352.4 62.1 319.0 56.9 14.6 50.6 17.9 43.0 9.0 64.0 343.8 61.1 3.9 58.9 30.7 56.4 350.2 49.7 355.3 57.4 6.5 51.6 1.6 49.0 9.1 50.0 unstable 333.9 53.5 unstable 19.6 46.4 333.0 62.1
Lat, N
Long, E
83.4 80.1 85.9 87.6 76.4 86.7
207.1 331.8 284.9 229.7 35.4 46.5
87.9 86.2 77.4 85.5 59.8 73.9 79.1 75.5 81.9 61.0 76.7 78.1 65.8 77.6 72.7 74.9 73.8 82.0 64.4 81.5 83.1 84.5 87.2 82.1
85.6 88.1 189.9 209.9 171.0 148.8 114.9 354.4 341.0 291.4 79.4 261.5 174.5 334.1 353.9 268.3 195.6 265.6 314.4 144.1 210.9 332.3 33.3 342.1
68.1
165.2
72.4 66.1 86.6
343.6 188.9 248.1
a SM, San Marcos, PT, Testerazo pluton (PT). Statistical parameters are defined as follows: n/r, samples used/rejected for site mean calculation; R, vector sum; k, precision parameter; a95, 95% confidence limit [Fisher, 1953]; Dec, Inc, site mean declination and inclination; Lat, Long, coordinates of corresponding VGP. b Site means with a95 > 15, which are not considered for the mean pole given in the last line.
Palmas Gravels [McDonough and Abbott, 1989]. In the northern and central part of the pluton systematic paleomagnetic sampling was conducted. Dikes related to the San Marcos swarm are notably absent in this pluton indicating that the El Testerazo Pluton is younger. So far no age determination is available, but according to the age contours defined by Ortega-Rivera [1997], El Testerazo would fall into the 100 – 110 Ma age range. From satellite images, the most conspicuous structures of the El Testerazo Pluton are curvilinear lineaments paralleling its borders (Figure 2). In this pluton we measured 34 characteristic penetrative frac-
tures in 16 sites of paleomagnetic sampling. Fractures are radially distributed and lineations of mafic minerals, mostly plunging horizontally in the same sampling sites show a very consistent orientation towards 342 indicating magma flowage in that direction (Figures 3d and 3e). No argument referring to the potential tilting can be derived from fracture data of El Testerazo. [12] The study area is divided by the northwest trending Vallecitos Fault (Figure 1), which is considered active although seismicity is not well defined along its trace. Frez and Frı´as-Camacho [1998a, 1998b] have recently shown
Figure 2. (opposite) Geologic map of the study area. Fractures and faults interpreted from aerial photographs are indicated with thin lines, dikes are shown with thicker and shorter lines and the regional right-lateral Calabazas and Vallecitos fault zones are the widest lineaments. Dots indicate sites of sampling (see Table 1). Lithologic units are as follows: Psch = Paleozoic schist, Pgn and Pq = quartzite (light shaded area) after Gastil et al. [1975], Kgr = undifferentiated granitoids, Kt = tonalite, Kqm = quartz-monzonite, Km = monzonite, Kd = diorite, Kr = rhyolite and undifferentiated acidic volcanic rocks of the Alisitos Fm., Tcg = Eocene conglomerates of the Las Palmas Gravels. Blank areas are Quaternary alluvium. Note: The rhyolite dike for zircon U/Pb geochronology is located in site 12.
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
Figure 3. Structural data from the San Marcos and El Testerazo areas. (a) Schmidt equal-area stereo diagram (SEASD) of the pole azimuths of dikes from San Marcos indicating with solid square the mean orientation. (b) SEASD showing eastward rotated San Marcos dikes. (c) SEASD of fractures in dikes and enclosing rocks of the San Marcos area. (d) Mineral lineation rose diagram of El Testerazo Pluton (data type is bidirectional). (e) SEASD of fracturing in the El Testerazo Pluton.
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO Table 2. U/Pb Isotopic Data for San Marcos Dike, Site SM12, Determined From Four Zircon Fractionsa Pb Isotopic Compositions (Unspiked, Multiplier Corrected) Fraction Weight, g Pb, ppm
U, ppm
206/208
206/207
206/204
>200
0.0016
6.670
327.7
5.5854
19.329
5207.0
CG
0.0022
7.397
351.7
5.4496
18.303
2717.6
>200
0.0038
2.541
127.7
5.8433
19.992
10848.9
Discordant paleomagnetic data for middle-Cretaceous intrusive rocks from northern Baja California: Latitude displacement, tilt, or vertical axis rotation? Harald Bo¨hnel Centro de Geociencias, Universidad Nacional Auto´noma de Me´xico (UNAM), Campus Juriquilla, Queretaro, Mexico
Luis A. Delgado-Argote Departamento de Geologı´a, CICESE, Ensenada, Baja California, Mexico
David L. Kimbrough Department of Geological Sciences, San Diego State University, San Diego, California, USA
Received 9 May 2001; revised 5 March 2002; accepted 19 June 2002; published 25 October 2002.
[1] Paleomagnetic results and U/Pb zircon dating from
the San Marcos dike swarm and the El Testerazo pluton in the Cretaceous Peninsular Ranges batholith of northern Baja California are used to evaluate alternative pre-Neogene paleogeographic reconstructions of the Baja California peninsula. The San Marcos dike swarm is a dense, northwest striking, regional dike swarm that is exposed over an 100 km long segment of the batholith and has yielded a U/Pb zircon crystallization age of 120 ± 1 Ma. Dike attitudes from the swarm suggest a regionally consistent average 320E strike and 79NE dip. The El Testerazo pluton is a younger tonalite intrusion that truncates the northern end of the dike swarm. All but one of 36 sites sampled in this study show remanence of normal polarity. Paleopoles for the San Marcos dike swarm and El Testerazo pluton are indistinguishable and were combined into a paleopole at 248.1E, 86.6N, A95 = 4.8, which is displaced with respect to the 122 Ma reference pole for stable North America at 198.2E, 72.3N, A95 = 3.3. The displacement may be described by an apparent clockwise rotation of 18 ± 6 and an apparent northward shift of 8 ± 5. Restoring a northward shift of about 3, related to the separation of Baja California from North America since 10 Ma, only a marginal northward displacement of 5 ± 5 is left. The clockwise rotation may be the result of crustal block rotations within the right-lateral shear systems in northern Baja California, although there is no geological evidence that supports this possibility. Alternatively, the difference between paleopole and reference pole may be due to tilting of the study area. Restoring a northeastward tilt of 11, based on the mean dip measured for the San Marcos dike swarm in the Copyright 2002 by the American Geophysical Union. 0278-7407/02/2001TC001298$12.00
study area, yields a paleopole at 187.6E, 70.8N, A95 = 5.6, which is indistinguishable from the 122 Ma North American reference pole. The tilting hypothesis suggested previously as a potential explanation for low inclination of paleomagnetic data from Baja California therefore seems to be viable for the discordant paleopoles from this segment of the batholith. Data obtained previously about 150 km further south and along the western edge of the batholith defined a concordant paleopole without any tilt correction, indicating that regional tilting may not be valid for all INDEX TERMS: 1525 Geomagnetism of Baja California. and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8105 Tectonophysics: Continental margins and sedimentary basins; 8157 Tectonophysics: Plate motions— past (3040); 9350 Information Related to Geographic Region: North America; 1035 Geochemistry: Geochronology; KEYWORDS: paleomagnetism, Baja California, paleogeography, Cretaceous, intrusive rocks, geochronology. Citation: Bo¨hnel, H., L. A. Delgado-Argote, and D. L. Kimbrough, Discordant paleomagnetic data for middle Cretaceous intrusive rocks from northern Baja California: Latitude displacement, tilt, or vertical axis rotation?, Tectonics, 21(5), 1049, doi:10.1029/2001TC001298, 2002.
1. Introduction [2] The Peninsular Ranges of southern and Baja California play a central role in the debate over Cretaceous paleogeography of the western Cordillera. Cowan [1994] highlights the uncertainties and conflicts between models based on geologic versus paleomagnetic data. Paleomagnetic studies in the Peninsular Ranges, conducted at many widespread sites on both plutonic and sedimentary rocks, suggest that the Baja peninsula was located 1200 km southward of its present position off the southern margin of Mexico between 200 and 50 Ma [e.g., Hagstrum and Filmer, 1990; Lund and Bottjer, 1991; Hagstrum and Sedlock, 1998]. This restoration represents the 300 km of separation necessary to close the Neogene Gulf of
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
California plus another 900 km of displacement to the south. Geologic evidence on the other hand appears equally strong that simply closing the Gulf of California restores the peninsula to its original position against the craton throughout the Phanerozoic [Gastil, 1993]. Two very different solutions to the discrepancy between paleomagnetic versus geologic restorations of Peninsular California involving offsets along hypothetical faults have been proposed [Hagstrum et al., 1987; Beck, 1991]. However, Gastil [1993] regards these solutions improbable based on regional geology. [3] Butler et al. [1991] and Dickinson and Butler [1998] made the intriguing observation that the paleomagnetic inclinations from plutonic rocks in the Peninsular Ranges [e.g., Teissere and Beck, 1973; Hagstrum et al., 1985], which require 1200 km of northward transport, can be reconciled by westward tilting of the batholith about a horizontal axis. The ‘‘restored’’ paleomagnetic directions would then require minimum northward translation of the batholith, consistent with Gastil’s [1993] geologic restoration. Paleomagnetic results from sedimentary rocks, which also indicate 1200 km of northward transport, are further interpreted by Butler et al. [1991] to potentially reflect anomalously shallow inclination data resulting from unrecognized burial compaction. Such inclination shallowing has occurred in southern California marine sediments [Tan and Kodama, 1998] and sediments from the California margin [Kodama and Ward, 2001], thereby reducing or eliminating the need of paleolatitudinal offsets proposed previously for the corresponding terranes. Significantly, data from Jurassic and Cretaceous sediments in Baja California have failed fold and conglomerate tests [e.g., Hagstrum and Filmer, 1990], indicating a complex magnetic history that makes tectonic interpretation of the magnetic record difficult. [4] We have initiated the study of intrusive complexes from throughout the length of the Baja California peninsula. Our approach is to sample in detail those batholiths where detailed geological, structural and geochronologic data are available. The goal is to obtain reliable individual paleopoles, which in turn should provide a coherent pattern, if the rocks have been affected by simple latitude shift or coherent tilt. Varying records observed from different regions might additionally indicate differential tilting or other even more complex deformation processes. So far results are variable and not yet conclusive: a concordant high quality paleopole was determined for the San Telmo intrusive complex in the north (present latitude 31N [Bo¨hnel and Delgado-Argote, 2000]), in contrast to a 35– 45 clockwise rotated paleopole for the Los Cabos Block in the south (present latitude 23– 24N [Schaaf et al., 2000]). Indeed, these much differing results roughly correspond to the range of dispersion reported previously for paleopoles from sedimentary rocks [e.g., Hagstrum and Filmer, 1990]. It is noteworthy that the San Telmo complex is located in a tectonically undisrupted part of the Peninsula along the western edge of the Peninsular Ranges batholith, while the Los Cabos Block at the tip of Baja California has been strongly affected by Neogene tectonics related to the opening of the Gulf of California [Fletcher et al., 2000].
[5] Here we present new data from the San Marcos dike swarm and the adjacent El Testerazo pluton in northern Baja California. The San Marcos dike swarm was targeted for study because it has a regionally consistent average 320E strike and 79NE dip which potentially represents a strain marker recording westward tilting of the batholith as proposed by Butler et al. [1991].
2. Geology [6] The areas of San Marcos and El Testerazo are located west of the Pinal Pluton in the northwestern part of the Baja California peninsula (Figure 1). This region is part of the magnetite-rich western province of the Peninsular Ranges batholith [Gastil et al., 1990] located west of the magnetiteilmenite boundary (Figure 1). The eastern province of the Peninsular Ranges batholith is predominantly magnetitefree. Plutons of the western province include ring complexes emplaced in dike-rich sub-volcanic environments of the Cretaceous Alisitos Formation [Gastil et al., 1975]. [7] The San Marcos-Testerazo area is located in the south central part of the Southern California Shear Zone defined by Legg et al. [1991]. This regional shear zone is bounded by the right-lateral San Clemente fault on one side and the San Andreas-Gulf of California fault system on the other (Figure 1). In such a tectonic scenario, large-scale counterclockwise rotation of semi-rigid blocks, such as has been documented in the Transverse Ranges of the southern California farther to the north, is a possibility although it has never been recognized in this region. In the area of San Marcos-Valle Seco (Figure 2), north-northwest trending dikes form a well-exposed swarm-like complex which we refer to here as the San Marcos dike swarm. The dikes intrude both the granitoids and metamorphic rocks. Dikes range from basalt to rhyolite but are mostly of intermediate composition (plagioclase is oligoclase-andesine) showing a wide variety of textures ranging from aphanitic to micro granular, with porphyritic textures the most common. All dikes show pervasive to selective deuteric alteration (phyllic and propylitic) with very low silica content. Dike thicknesses range mostly from 1 to 10 m. The attitude of individual dikes strike consistently north-northwest, parallel to the structural grain of the Peninsular Ranges batholith, and are characterized by steep northeast dips. The dikes are hosted by plutonic rock as well as greenschist facies metasedimentary sandstone-siltstone sequences. Plutons that are crosscut by the dikes range in composition from tonalite to monzonite to monzodiorite in composition. Dioritic bodies less than 1.5 km across are reported in the same area by Comisio´n de Estudios del Territorio Nacional (CETENAL) [1977]. Similar dikes, which we interpret to be part of the San Marcos swarm, occur from this region southward to near the Agua Blanca fault. [8] We conducted a systematic paleomagnetic sampling of dikes cropping out along the Agua Caliente Canyon and part of the Valle Seco Valley (Figure 2). A total of 20 dikes were sampled for paleomagnetic and petrographic analyses (Figure 2 and Table 1). Fracturing and attitude of the dikes were measured mainly along the Agua Caliente Canyon and
¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
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Figure 1. Study area and major geologic features of northern Baja California. Most of the reported active faults are indicated [after Legg et al., 1991; Sua´rez-Vidal et al., 1991; Frez and Frı´as-Camacho, 1998a]. Black areas show some of the main plutons identified from satellite images [Romero-Espejel and Delgado-Argote, 1997] and the shaded zone is the magnetite-ilmenite boundary after Gastil et al. [1990]. M, Mason Valley; ET, El Topo; LP, La Posta; P, Pinal; LJ, Laguna Juarez; ST, San Telmo; SJ, San Jose; Z, La Zarza; SPM, San Pedro Martir. around the Valle Seco Valley. The mean orientation of the dikes is 320 and their mean pole plunges 11, with its azimuth oriented toward 230 (Figure 3a). [9] As noted above, the San Marcos dikes dip steeply and consistently to the northeast in the San Marcos area. Theoretical [Delaney et al., 1986] and field studies [Pollard et al., 1983] indicate that dikes in the upper crust typically form in a near-vertical orientation by hydraulic fracturing and either vertical or lateral flow. Assuming that the San Marcos dikes were vertically emplaced and subsequently tilted, then restoration to their original orientation would require rotating 11 to the southwest around a horizontal axis striking 320, the mean orientation of the dikes (Figure 3b).
[10] Fractures in the 17 sampled dikes as well as the host rocks in the area of San Marcos almost parallel the orientation of the dikes. Most of the horizontal fractures are almost perpendicular to the orientation of vertical fracturing (Figure 3c), suggesting that simple cooling formed the set of orthogonal fractures. No border deformation due to forced emplacement was observed in the dikes or the enclosing rocks, indicating a passive emplacement. [11] El Testerazo Pluton is an approximately 10 km wide intrusive body of hornblende-biotite tonalite. There is significant variation of modal hornblende and biotite within the pluton. Figure 2 illustrates curvilinear fractures that characterize the southern part of the pluton; the northern part is partially covered by Eocene conglomerates of the Las
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO Table 1. Site Mean Paleomagnetic Data for San Marcos Dikes and Testerazo Plutona Number
Site
1 SM-1 2 SM-2 3 SM-3 4 SM-4 5 SM-5b 6 SM-6b 7 SM-7 8 SM-8 9 SM-9 10 SM-10 11 SM-11b 12 SM-12b 13 SM-13 14 SM-14b 15 SM-15 16 SM-16 17 SM-17 18 SM-18 19 SM-19 20 SM-20b 21 PT-1 22 PT-2 23 PT-3 24 PT-4 25 PT-5 26 PT-6 27 PT-7 28 PT-8 29 PT-9 30 PT-10 31 PT-11 32 PT-12 33 PT-13 34 PT-14 35 PT-15b 36 PT-16 MEAN POLE
Latitude (N)
Longitude (W)
n
r
R
k
a95, deg
3206.090 3206.090 3206.140 3206.160 3206.160 3206.130 3206.130 3206.320 3206.320 3206.320 3206.320 3211.070 3207.200 3207.140 3207.400 3207.400 3207.400 3206.250 3206.250 3206.250 3218.800 3219.170 3219.500 3219.770 3219.800 3219.800 3219.870 3219.450 3218.370 3218.170 3217.670 3218.140 3218.260 3218.770 3218.810 3218.880
11626.670 11626.670 11626.650 11626.630 11626.630 11626.660 11626.660 11625.410 11625.410 11625.410 11625.410 11626.220 11626.400 11627.460 11625.900 11625.900 11625.900 11625.420 11625.420 11625.420 11632.450 11632.140 11631.560 11631.210 11630.400 11630.700 11631.100 11631.750 11632.330 11632.270 11631.680 11628.990 11630.280 11630.890 11631.210 11631.770
7 6 5 8 3 5 2 7 5 4 3 4 6 6 7 7 9 6 7 6 9 8 8 7 8 8 7 8 8 9 7 7 7 9 7 8 26
1 0 2 0 3 0
6.97763 5.89092 4.94532 7.67605 2.95589 4.82690
268.2 45.8 73.2 21.6 45.3 23.1
3.7 10.0 9.0 12.2 18.5 16.3
0 0 1 1 3 2 2 0 0 0 1 1 0 0 0 1 0 0 0 0 1 1 0 0
6.97995 4.97510 3.97549 2.88120 3.75554 5.79629 5.61327 6.75139 6.96820 8.99216 5.99194 6.96837 5.71408 8.93190 7.91917 7.94115 6.91512 7.85957 7.77470 6.85348 7.68589 7.95750 8.80881 6.98465
299.2 160.7 122.4 16.8 12.3 24.5 12.9 24.1 188.7 1020.4 620.3 189.7 17.5 117.5 96.6 119.0 70.7 49.8 31.1 40.9 22.3 164.7 41.8 391.0
3.5 6.1 8.3 31.0 27.3 13.8 19.4 12.5 4.4 1.6 2.7 4.4 16.5 4.8 6.0 5.1 7.2 7.9 10.1 9.5 12.0 4.3 8.1 3.1
1
6.87656
48.6
8.7
2 0 6
6.31477 7.85943 25.29542
8.8 49.8 35.5
21.6 7.9 4.8
Dec, E
Inc, deg
342.5 56.7 359.1 51.2 350.7 54.6 346.7 53.9 354.2 35.9 348.5 47.9 not calculated 346.5 49.3 345.6 47.5 334.4 58.2 344.3 55.3 311.1 55.4 328.9 48.4 338.0 42.9 150.6 55.6 356.8 49.9 19.7 65.6 343.6 35.0 352.4 62.1 319.0 56.9 14.6 50.6 17.9 43.0 9.0 64.0 343.8 61.1 3.9 58.9 30.7 56.4 350.2 49.7 355.3 57.4 6.5 51.6 1.6 49.0 9.1 50.0 unstable 333.9 53.5 unstable 19.6 46.4 333.0 62.1
Lat, N
Long, E
83.4 80.1 85.9 87.6 76.4 86.7
207.1 331.8 284.9 229.7 35.4 46.5
87.9 86.2 77.4 85.5 59.8 73.9 79.1 75.5 81.9 61.0 76.7 78.1 65.8 77.6 72.7 74.9 73.8 82.0 64.4 81.5 83.1 84.5 87.2 82.1
85.6 88.1 189.9 209.9 171.0 148.8 114.9 354.4 341.0 291.4 79.4 261.5 174.5 334.1 353.9 268.3 195.6 265.6 314.4 144.1 210.9 332.3 33.3 342.1
68.1
165.2
72.4 66.1 86.6
343.6 188.9 248.1
a SM, San Marcos, PT, Testerazo pluton (PT). Statistical parameters are defined as follows: n/r, samples used/rejected for site mean calculation; R, vector sum; k, precision parameter; a95, 95% confidence limit [Fisher, 1953]; Dec, Inc, site mean declination and inclination; Lat, Long, coordinates of corresponding VGP. b Site means with a95 > 15, which are not considered for the mean pole given in the last line.
Palmas Gravels [McDonough and Abbott, 1989]. In the northern and central part of the pluton systematic paleomagnetic sampling was conducted. Dikes related to the San Marcos swarm are notably absent in this pluton indicating that the El Testerazo Pluton is younger. So far no age determination is available, but according to the age contours defined by Ortega-Rivera [1997], El Testerazo would fall into the 100 – 110 Ma age range. From satellite images, the most conspicuous structures of the El Testerazo Pluton are curvilinear lineaments paralleling its borders (Figure 2). In this pluton we measured 34 characteristic penetrative frac-
tures in 16 sites of paleomagnetic sampling. Fractures are radially distributed and lineations of mafic minerals, mostly plunging horizontally in the same sampling sites show a very consistent orientation towards 342 indicating magma flowage in that direction (Figures 3d and 3e). No argument referring to the potential tilting can be derived from fracture data of El Testerazo. [12] The study area is divided by the northwest trending Vallecitos Fault (Figure 1), which is considered active although seismicity is not well defined along its trace. Frez and Frı´as-Camacho [1998a, 1998b] have recently shown
Figure 2. (opposite) Geologic map of the study area. Fractures and faults interpreted from aerial photographs are indicated with thin lines, dikes are shown with thicker and shorter lines and the regional right-lateral Calabazas and Vallecitos fault zones are the widest lineaments. Dots indicate sites of sampling (see Table 1). Lithologic units are as follows: Psch = Paleozoic schist, Pgn and Pq = quartzite (light shaded area) after Gastil et al. [1975], Kgr = undifferentiated granitoids, Kt = tonalite, Kqm = quartz-monzonite, Km = monzonite, Kd = diorite, Kr = rhyolite and undifferentiated acidic volcanic rocks of the Alisitos Fm., Tcg = Eocene conglomerates of the Las Palmas Gravels. Blank areas are Quaternary alluvium. Note: The rhyolite dike for zircon U/Pb geochronology is located in site 12.
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO
Figure 3. Structural data from the San Marcos and El Testerazo areas. (a) Schmidt equal-area stereo diagram (SEASD) of the pole azimuths of dikes from San Marcos indicating with solid square the mean orientation. (b) SEASD showing eastward rotated San Marcos dikes. (c) SEASD of fractures in dikes and enclosing rocks of the San Marcos area. (d) Mineral lineation rose diagram of El Testerazo Pluton (data type is bidirectional). (e) SEASD of fracturing in the El Testerazo Pluton.
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¨ HNEL ET AL.: DISCORDANT PALEOMAGNETIC DATA, BAJA CALIFORNIA BO Table 2. U/Pb Isotopic Data for San Marcos Dike, Site SM12, Determined From Four Zircon Fractionsa Pb Isotopic Compositions (Unspiked, Multiplier Corrected) Fraction Weight, g Pb, ppm
U, ppm
206/208
206/207
206/204
>200
0.0016
6.670
327.7
5.5854
19.329
5207.0
CG
0.0022
7.397
351.7
5.4496
18.303
2717.6
>200
0.0038
2.541
127.7
5.8433
19.992
10848.9
