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PALEOMAGNETIC METHODS AND RESULTS
Six 2.54 cm diameter cores were drilled at 27 sites and oriented in situ with a solar
compass or rarely with a magnetic compass (Fig. 2). Ten to 16 specimens, 2.20 cm long, were
sliced from these cores per site, for a total of 333 specimens. The bulk susceptibilities of the
specimens were measured using a Bartlington MS2B meter, giving a median magnitude of M = 1.6
x 102 and first and third quartiles of Q1 = 3.4 x 103 and Q3 = 2.6 x 104. The specimens were stored
in a magnetically-shielded room for five months to allow their viscous remanent magnetization
components to decay. All subsequent magnetic measurements were done in the room.
The natural remanent magnetization of each specimen was measured using an automated
vertical 2G Enterprises three-axis cryogenic magnetometer, giving intensities of M = 3.7 x 10-1
A/m, Q1 = 4.3 x 10-2 A/m, and Q3 = 1.9 A/m. Such large magnitudes usually indicate magnetite in
felsic rocks. Three typical specimens per site were used in pilot tests to determine an effective
demagnetization protocol. The first specimen was alternating field (AF) demagnetized in 15 steps
up to 135 mT using a Sapphire Instruments SI-4 alternating field demagnetizer. Most specimens
decayed rapidly up to ~20 mT, then more slowly to 135 mT, and dropped to <25% of their initial
intensity (Fig. 4A, Fig. DR3A), whereas a few specimens decayed more slowly. Virtually all
specimens isolated a northerly intermediate-downward direction above ~20 mT, which is the
characteristic remanence direction for the collection. The second pilot specimens were thermally
demagnetized in 20 steps to 600 °C using a Magnetic Measurements MMTD 80 demagnetizer.
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Most steps were in the 80 to 130 °C, 260 to 330 °C, and 520 to 580 °C unblocking temperature
ranges of goethite, pyrrhotite and magnetite, respectively. Most specimens showed a slow linear
decay to >500 °C, and then a rapid decay to near zero by 580 °C (Fig. 4B), indicating that
relatively low-titanium titanomagnetite is the principal remanence carrier. Nearly all specimens
isolated a stable direction between ~480 and 580 °C. Occasionally, some pyrrhotite is present (Fig.
DR3B). The third pilot specimens were AF demagnetization in 7 steps to 130 mT and then
thermally demagnetized in 12 steps from 260 to 600 °C. Mostly, these specimens decreased in
intensity rapidly to ~20 mT, remained unchanged to ~520° C, and then decreased rapidly to 580 °C
(Fig. 4CD, Fig. DR3C). This pattern shows that the stable remanence is carried by relatively pure
single-domain magnetite. Based on pilot specimen results, the remaining specimens were
demagnetized using steps of 30 mT and 400, 450, 500, 520, 540, 560 and 590 °C.
Ten specimens were tested using saturation isothermal remanent magnetization to
examine domain size in the minerals carrying the remanence. These specimens were pulse
magnetized in a direct field in 14 steps to 900 mT using a Sapphire Instruments SI-6 pulse
magnetizer and showed a rapid increase to saturation by 350 mT (Fig. DR4A). Next, the specimens
were AF demagnetized in 10 steps to 165 mT, giving an intensity decay pattern that is typical of
mixed multidomain, pseudosingle domain, single domain magnetization in magnetite (Fig. DR4B).
The determined crossover points for the saturation intensity acquisition and decay curves provide a
good fit to the type curves for magnetite (Fig. 5).
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The stable remanence for most specimens came from thermal demagnetization data above
480 °C using vector component plots (Zijderveld, 1967) and least-squares component analysis
(Kirschvink, 1980). Five specimens failed to give a stable remanence direction, leaving 328
directions for analysis (Table DR2). The site mean remanence directions were obtained using
Fisher (1953) statistics (Fig. 6). Only the mean from site 27 proved poorly determined, and was
excluded from further analysis.
The site mean remanence directions for the various units were compared following
McFadden and Lowes (1981). The mean directions from the Alpine, Las Bancas and Japatul
Valley units were compared first in pairs and found to be indistinguishable at 95 % confidence, and,
therefore, all three unit populations may be drawn from one population (Table 1), supporting the
contention of Miller (1935) and Lombardi (1992) that these units are simply variations within the
Alpine Complex. The direction for the one Green Valley site of the Ramona Complex falls within
the Lakeview unit population, and so may be added to it. When the Ramona and Alpine complexes
are compared, their directions are not significantly different.
Other Paleomagnetic Results
Paleomagnetic results have been published from other Cretaceous igneous plutons in the
Baja California peninsula that have not been used in this tectonic analysis. Their omission stems
either from the lack of some critical information such as the pluton’s age or sampling details, or
from a significantly discordant paleopole that has been attributed to movement on an adjacent
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major fault.
Schaaf et al. (2000) has reported paleomagnetic results from the Los Cabos Block at the
southern tip of the Baja California peninsula that utilized data also from three sites reported
originally by Hagstrum et al. (1985). The block has been considered to be either part of the Alisitos
terrane (Campa and Coney, 1983) or to be the independent Pericú Terrane (Sedlock et al., 1993),
although recent magnetic and gravity data (Langenheim and Jachens, 2003) suggest that the block
might be the southern extension of the Caborca Terrane. The ChRM age is uncertain with the
deformed northern and undeformed southern “halves” of the pluton giving Rb-Sr and whole rock
biotite ages of 129±15 and 116±2 Ma and of 115±4 and 90±2 Ma (1σ errors), respectively. Further
8 of the 17 accepted site means incorporated into the unit mean ChRM direction have a reversed
polarity, suggesting strongly that remanence acquisition postdates the Cretaceous Normal Polarity
Superchron from 119 to 83 Ma (Opdyke and Channell, 1996). Corrected for Neogene northward
translation and the latitude difference, the Los Cabos Block ChRM direction is significantly
different at >99% confidence from either the North American reference direction or the Yuma
Terrane direction.
In an abstract Böhnel et al. (2003) have reported partial paleomagnetic results from
selected sites in the Aguaje del Burro, Nueva York, San José, El Potrero and San Pedro Mártir
plutons to the east and east-southeast of the San Telmo pluton (Fig. DR1). The ages of the Aguaje
del Burro (D = 358.5°, I = 54.8°, α95 = 4.1°, 7 sites) and Nueva York (D = 356.2°, I = 54.8°, α95 =
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5.2°, 8 sites) plutons in the Alisitos Terrane are not given but are likely ~105±10 Ma based on the
chrontours of Ortega-Rivera (2003). Assuming these plutons are mid Early Cretaceous, then their
mean ChRM directions are concordant at 95% confidence with those of the Yuma Terrane to the
north and indicate an ENE-side-up tilt of ~10° for the Alisitos Terrane. San José (D = 20.6°, I =
29.9°, α95 = 3.0°, 15 sites) and El Potrero (D = 34.6°, I = 25.7°, α95 = 5.5°, 9 sites) are both ~102±2
Ma tonalitic plutons emplaced along the eastern side of the Alisitos Terrane into a thrust slice
between the Main Mártir thrust fault – the terrane’s eastern boundary – and the subsidiary Rosarito
thrust fault (Chávez Cabello, et al. 2006). From petrologic fabric, structural and paleomagnetic
data, Chávez Cabello et al. (2006) concluded that the ChRM direction of the El Potrero pluton has
been tilted away from the North American reference direction by ~35° from an ENE-side-up
rotation about a near-horizontal orogen-parallel axis. They attributed the rotation about equally to
drag during vertical diapiric ascent of the large adjacent San Pedro Mártir pluton and to rotation
during active thrusting on the Rosarito fault between ~100 and ~85 Ma. Further, Chávez Cabello et
al. (2006) suggested that the San José pluton was rotated also by ~15° by activity on the Rosarito
fault. Our calculations suggest that required tilt corrections for the El Potrero and San José pluton
are both on the order of ~50° ENE-side-up. The 95±2 Ma San Pedro Mártir tonalite pluton is in the
eastern zone of the Peninsular Ranges batholith on its westernmost boundary next to the Main
Mártir thrust fault. The pluton’s ChRM direction (D = 18.9°, I = 47.7°, α95 = 5.5°, 15 sites) requires
an ENE-side-up tilt correction of ~25° to bring it into concordancy with the North American
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reference direction. From age dating, structural and Al-in-hornblende geobarometry,
Ortega-Rivera et al. (1997) suggested that the pluton had undergone a NE-side-up tilt of ~15°.
Subsequently Chávez Cabello et al. (2006) suggested from the paleomagnetic data that thrust
motion on the Rosarito fault might have been responsible for the ~15° tilt. Full documentation of
the results from these paleomagnetic collections along the 31°N transect across the batholith
would be a valuable contribution to understanding the tectonic history of the Alisitos Terrane.
Finally, the authors have unpublished preliminary paleomagnetic data from the massive
Cibbet Flat pluton (Walawender et al. 1991) that is located in the Yuma Terrane between the
Alpine and La Posta plutons beside the Cuyamaca – Laguna Mountain shear zone (Fig. DR1).
Dated by the U/Pb zircon method at 105±1 Ma (L.T. Silver, as reported in Todd and Shaw, 1979),
this ~40 km2 pluton consists mainly of pyroxene-biotite tonalite and gives a mean ChRM direction
of D = 15.2°, I = 44.7° (α95 = 9.6°, k = 50.0, 6 sites). Its discordant paleopole, corrected for
Neogene opening of the Gulf of California is located at 78°N, 356°E (A95 = 10°). The pluton’s
discordancy with the North American and Yuma Terrane reference directions is attributed to some
combination of ENE-side-up tilt in the Yuma Terrane and dextral rotation caused by dextral
displacement on the Cuyamaca – Laguna Mountain shear zone, however, additional sites require
measurement to reduce the 95% confidence limits in order to better quantify the tilt and dextral
rotations.
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GEOTHERMOBAROMETRY
Nine and seven plagioclase-amphibole pairs from the Alpine and Ramona complexes, respectively,
were analysed (Table DR4, Table DR5) using a JEOL JXA-8600 Superprobe. The calibration
standards and determination methods follow Symons et al. (2000, 2003). The procedure for
calculating the amphibole structural formulae follows Schumacher (1997) because it can be used
with the internationally recommended amphibole classification (Leake et al. 1997) and the
hornblende-plagioclase geothermometer of Blundy and Holland (1990).
Amphibole Geochemistry
The amphiboles plot in the magnesiohornblende field of Leake et al. (1997) except for
sample 8 in the adjacent corner of the ferrohornblende field (Fig. DR2A). Figure DR2B shows a
plot of the temperature-sensitive edenite exchange reaction, albite + tremolite = 4 quartz + edenite
(Si + [A]… = [4]Al + [A](Na + K), where [A]… stands for an A-site vacancy in the amphiboles. Figure
3 is a plot of the pressure-sensitive tschermakite-forming reaction (2 quartz + 2 anorthite + biotite
= orthoclase + tschermakite) ([4]Al +
[6]
Al = Si + R2+). There is considerable overlap of the
amphibole fields of the units and all samples plot on a single linear trend of decreasing Si with
increasing [4]Al + [A](Na + K) (Fig. DR2B). The amphiboles, however, from the Alpine Unit extend
to much higher
[4]
Al +
[A]
(Na + K) values than any of the other units. Although few samples
represent each unit, their overlapping trends and defined pressures appear different on the
tschermakite exchange plot (Fig. 3). The Alpine, Japatul Valley, and Lakeview-Green Valley units
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all show decreasing Si + R2+ with increasing
[4]
Al +
[6]
Al, but imply differing crystallization
pressures of 400-550 MPa, 350-450 MPa, and 400-500 MPa, respectively. Conversely, the Las
Bancas Unit amphiboles define a trend of increasing Si + R2+ with increasing
[4]
Al +
[6]
Al, with
crystallization pressures of 400-450 MPa. The differences in these amphibole chemical trends
suggest each unit of the Alpine Complex crystallised from a separate intrusive phase. If so, the
gradational boundaries among the Alpine Complex units, described by Todd (1977) and Lombardi
(1992), imply that the units were emplaced nearly synchronously as the cited U/Pb zircon ages for
the Alpine and Las Bancas units indicate. Also, the crystallization temperatures (787 to 725°C,
Table 3) are similar, and the crystallization pressures define coherent trends when the three Alpine
Complex units are considered together. This suggests their geothermobarometric data can be
combined to estimate the original orientation of the complex. Similarly the amphiboles from the
Lakeview and Green Valley units of the Ramona Complex lie along the same trends on the edenite
and tschermakite diagrams (Fig. 3, Fig. DR2B) and yield crystallization temperatures between 752
to 709°C (Table 3), suggesting that the chemical and physical conditions of crystallization for both
units can be combined into one population to reconstruct the complex’s original attitude.
Temperatures and Pressures
The Al-in-hornblende geobarometer and the amphibole-plagioclase geothermometer are
used to estimate the pressures and temperatures of equilibration for the two complexes, and show
that they had similar, but not identical, crystallization environments (Table 3). Preliminary
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pressures calculated using the Schmidt (1992) geobarometer range from 375 to 531 MPa and from
412 to 483 MPa, and the temperatures from the Blundy and Holland (1990) amphibole-plagioclase
geothermometer range from 725 to 787 oC and from 709 to 752 oC in the Alpine and Ramona
complexes, respectively. The temperatures, which lie close to the solidus of tonalitic systems
(Green 1982), are used to correct the preliminary pressures using the Anderson and Smith (1995)
geobarometer, yielding final pressures ranging from 247 to 367 MPa and from 324 to 397 MPa in
the Alpine and Ramona complexes, respectively (Table 3).
Although parts of the Alpine Complex have been designated as Green Valley and Bonsall
tonalites because of their petrographic similarity to the Ramona Complex units (Todd 1982),
perhaps implying the existence of only one complex, the pressures are generally higher and
temperatures generally lower during crystallization in the Ramona Complex than in the Alpine
Complex. Also, the trends defined by the amphiboles on the tschermakite diagram for the two
complexes differ (Fig. 3). Thus we conclude that, despite petrographic similarities, the two
complexes evolved separately. The crystallization depths determined from the pressure data vary
irregularly from 13.0 km in the west to 8.8 km in the east in the Alpine Complex, and from 14.1 km
in the north to 11.5 km in the south in the Ramona Complex. Although the absolute accuracy of the
crystallization depth determination is thought to be only about ±1 kbar (~±3.0 km), the relative
accuracy when both the analyst and experimental conditions are kept constant is thought to be
much better at about ±0.3 kbar (~±1 km). Thus the study area for a given unit needs to be ~400 km2
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at least to get a reliable trend surface fit to the depth determinations. For example, the nine Alpine
Complex depth determinations represent an area of only ~80 km2. When best fitted using the least
squares method, the plane has a strike of N7°E, dip of 8°W and low goodness-of-fit parameters of
r2 = 0.491, Fstat = 2.90. Conversely, the 6 depth determinations for the Ramona Complex cover only
~35km2 but give an attitude of N58°E, 14°NNW with extremely high goodness-of-fit parameters
of r2 = 0.975, Fstat = 58.8. The substantial differences of the attitudes and fit parameters for the two
complexes, and of the depths between adjacent sites within each complex suggest that significant
vertical fault displacements are present within both complexes.
DATA REPOSITORY REFERENCES CITED
Anderson, J.L. and Smith, D.R., 1995, The effects of temperature and fO2 on the Al-in-hornblende
barometer: American Mineralogist, v.80, p. 549-559.
Blundy, J.D., and Holland, T.J.B., 1990, Calcic amphibole equilibria and a new
amphibole-plagioclase geothermometer: Contributions to Mineralogy and Petrology, v. 104, p.
208-224.
Böhnel, H., Molina Garza, R., Contreras Flores, R., Ortega-Rivera, A., and Delgado Argote, L.,
2003, A paleomagnetic transect of the Peninsular Ranges Batholith near the 31st parallel, Part
I: Geological Society of America, Abstracts with Programs, v. 35, p. 74.
Campa, M.F., and Coney, P.J., 1983, Tectono-stratigraphic terranes and mineral resource
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distributions in Mexico: Canadian Journal of Earth Sciences, v. 20, p. 1040-1051.
Chávez Cabello, G., Molina Garza, R., Delgado Argote, L., Contreras Flores, R., Ramirez, E.,
Ortega Rivera, A., Böhnel, H., and Lee, J., 2006, Geology and paleomagnetism of El Potrero
Pluton, Baja California: Understanding criteria for timing of deformation and evidence of
pluton tilt during batholith growth: Tectonophysics, v. 424, p. 1-17.
Engebretson, D.A., Cox, A., and Gordon, R.G., 1985. Relative motions between oceanic and
continental plates in the Pacific Basin: Geological Society of America, Special Paper 206, 59
pp.
Fisher, R.A., 1953. Dispersion on a sphere: Royal Astronomical Society of London Proceedings, v.
A217, p. 295-305.
Green, T.H., 1982, Anatexis of mafic crust and high pressure crystallization of andesite. in R.S.
Thorpe, ed., Andesites, John Wiley and Sons, New York, p. 465-488.
Kirschvink, J.L., 1980.The least squares line and plane and the analysis of paleomagnetic data.
Geophysical Journal of the Royal Astronomical Society, v. 62, p. 699-718.
Langenheim, V.E., and Jachens, R.C., 2003, Crustal structure of the Peninsular Ranges Batholith
from magnetic data; implications for Gulf of California rifting: Geophysical Research Letters,
v. 30, p. 51/1-51/4
Leake, B.E. and 21 others, 1997, Nomenclature of amphiboles: Report of the sub-committee on
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amphiboles of the International Mineralogical Association, Commission on New Mineral
Names: Canadian Mineralogist, v. 35, p.219-246.
Lombardi, M. 1992, The Alpine tonalite - image of a gabbroic source: M.Sc. Thesis, San Diego
State University, California, 178 pp.
McFadden P.L., and Lowes, F.J., 1981, The discrimination of mean directions drawn from Fisher
distributions: Royal Astronomical Society Geophysical Journal, v. 67, p. 19-33.
Merriam, R.H., 1946, Igneous and metamorphic rocks of the southwestern part of the Ramona
Quadrangle, San Diego County, California: Geological Society of America Bulletin, v.57,
p.223-260.
Miller, W.J., 1935, Crystalline rocks of southern California: Geological Society of America
Bulletin, v. 57, p. 457-540.
Opdyke, N.D., and Channell, J.E.T., 1996, Magnetic stratigraphy: Academic Press, San Diego,
346 pp.
Ortega-Rivera, A., 2003, Geochronological constraints on the tectonic history of the Peninsular
Ranges batholith of Alta and Baja California: Tectonic implications for western Mexico, in
Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty, G.H., Kimbrough, D.L. and Martin-Barajas,
A., eds., Tectonic evolution of Northwestern Mexico and Southwestern U.S.A., Geological
Society of America Special Paper 374, p. 297-335.
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Ortega-Rivera, A., Farrar, E., Hanes, J.A., Archibald, D.A., Gastil, R.G., Kimbrough, D.L., Zentilli,
M., Lopez-Martinez, M., Féraud, G., and Ruffet, G., 1997, Chronological constraints on the
thermal and tilting history of the Sierra San Pedro Mártir Pluton, Baja California, Mexico,
from U/Pb,
40
Ar/39Ar, and fission-track geochronology: Geological Society of America
Bulletin, v. 109, p. 728-745.
Schaaf, P., Böhnel, H., and Pérez-Venzor, J.A., 2000, Pre-Miocene palaeogeography of the Los
Cabos Block, Baja California Sur: geochronological and palaeomagnetic constraints:
Tectonophysics, v. 318, p. 53-69.
Schmidt, M.W., 1992, Amphibole composition in tonalite as a function of pressure: An
experimental calibration of the Al-in-hornblende barometer: Contributions to Mineralogy and
Petrology, v. 110, p. 304-310.
Sedlock, R.L., 2003, Geology and tectonics of the Baja California Peninsula and adjacent areas, in
Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty, G..H., Kimbrough, D.L., and
Martin-Barajas, A., eds., Tectonic evolution of northwestern Mexico and the southwestern
USA: Boulder, Colorado, Geological Society of America Special Paper 374, p. 1-42
Sedlock, R.L., Ortega-Gutiérrez, F., and Speed, R.C., 1993, Tectonostratigraphic terranes and
tectonic evolution of Mexico: Geological Society of America, Special Paper 278, 153 pp.
Symons, D.T.A., and Cioppa, M.T., 2001, Crossover plots: a useful method for plotting SIRM data
in paleomagnetism: Geophysical Research Letters, v. 27, p. 1779-1782.
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Todd, V.R., 1977, Geologic map of the Aguna Caliente Springs 7.5’ quadrangle, San Diego County,
California: United States Geological Survey Open-file Report 77-742, 20 pp. scale 1:24,000.
Todd, V.R., 1982, Geologic map of the Tule Springs 7.5’ quadrangle, San Diego County,
California: United States Geological Survey Open-file Report 82-221, 23 pp., scale 1:24,000.
Todd, V.R. and Shaw, S.E., 1979, Structural, metamorphic and intrusive framework of the
Peninsular Ranges batholith in southern San Diego County, California, in, Abbott, P.L. and
Todd, V.R., eds., Mesozoic crystalline rocks, San Diego, California, San Diego State
University, Department of Geological Sciences, p. 177-231.
Umhoefer, P.J., Mayer, L., and Dorsey, R.J., 2002, Evolution of the margin of the Gulf of
California near Loreto, Baja California Peninsula, Mexico: Geological Society of America
Bulletin, v. 114, p. 849-868.
Walawender, M.J., Girty, G.H., Lombardi, M.R., Kimbrough, D., Girty, M. and Anderson, C., 1991,
A synthesis of recent work in the Peninsular Ranges batholith, in, Walawender, M.J. and
Hanan, B.B., eds., Geological Excursions in Southern California and Mexico, Geological
Society of America Guidebook, p. 297-312.
Zijderveld, J.D.A., 1967, A. C. demagnetization of rocks: Analysis of results, in Collinson, D.W.,
Creer, K.M., and Runcorn, S.K., eds., Methods in paleomagnetism: Amsterdam, Elsevier, p.
254-286.
Table DR1. Estimated mineral percentages of selected samples of the Alpine Igneous Complex and the Ramona
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Igneous Complex.
Site # Quartz Plagioclase K-Feldspar Amphibole Biotite Clinopyroxene* Orthopyroxene* Epidote** Titanite***
ALPINE IGNEOUS COMPLEX
Alpine Tonalite Unit
4
25
45
6
5
20
40
8
45
40
Las Bancas Tonalite Unit
10
10
50
21
15
40
5
25
20
55
Japatul Valley Tonalite Unit
2
20
50
3
25
50
27
20
60
RAMONA IGNEOUS COMPLEX
Lakeview Tonalite Unit
13
20
55
14
15
60
15
15
50
17
20
60
18
20
65
Green Valley Tonalite Unit
16
15
60
10
20
5
15
10
12
5
5
Tr
5
10
5
15
15
10
10
10
10
5
10
5
15
5
10
10
10
5
Tr
10
10
10
15
10
5
15
0.1
0.5
10
0.5
10
0.5
15
15
10
10
10
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
10
Tr
Tr
* Trace reported where very minor relics of the pyroxenes are preserved in the cores of amphiboles.
** Primary epidote.
*** Other accessory minerals typically include opaque oxides, apatite, and zircon, and epidote and chlorite are
common alteration products, in all units.
Table DR2. Site mean characteristic remanence directions
Site Location
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Lat.
N
32.770
32.800
32.808
32.835
32.838
32.838
32.835
32.842
32.837
32.842
32.887
32.987
32.987
33.042
33.048
33.057
33.112
33.105
33.065
32.835
32.842
32.837
32.840
32.823
32.823
32.818
32.745
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Site Mean Direction
Long.
W
116.687
116.742
116.780
116.750
116.682
116.703
116.718
116.795
116.623
116.638
116.828
116.797
116.797
116.825
116.792
116.763
116.788
116.788
116.810
116.740
116.783
116.672
116.635
116.612
116.602
116.583
116.673
No. Spec.
n
13
12
12
12
12
10
12
12
12
12
12
12
11
12
11
11
12
12
12
11
12
12
12
12
12
16
12
Decl.
Incl.
α95
358.0
3.3
351.3
9.5
4.8
354.2
12.8
352.9
359.7
340.7
7.3
4.0
357.9
355.2
349.3
0.4
14.4
354.4
2.2
2.3
359.3
10.1
346.3
7.1
3.1
9.7
30.3
63.3
41.3
57.1
43.5
55.9
53.2
59.9
49.5
43.0
56.5
56.4
61.1
60.9
68.0
44.2
50.2
49.2
53.8
55.0
52.5
49.2
50.6
44.2
41.8
43.2
43.1
28.0
5.9
3.5
4.9
3.1
5.6
6.2
6.7
4.9
5.2
2.5
4.4
6.6
7.3
7.2
8.8
5.1
3.3
8.6
5.0
4.2
3.5
5.2
4.0
3.8
3.0
2.1
27.2
Igneous
Complex
k
Unit
Ajv
49.6
Ajv
158
Ajv
80.8
Aap
200
Aap
61.9
Aap
61.5
Aap
43.4
Alb
78.6
Alb
71.3
Alb
314
Alb
96.2
Rlv
44.4
Rlv
40.1
Rlv
37.4
Rlv
27.6
Rgv
82.2
Rlv
177
Rlv
26.6
Rlv
77.6
Aap
119
Alb
151
Aap
71.4
Alb
120
Alb
132
Alb
205
Alb
296
*
Ajv
3.5
Notes. Location - Latitude (Lat.) and Longitude (Long.) in degrees ( ) north (N) and
west (W), respectively. Mean Direction specified by number of specimen end point
directions (n), declination (Decl.), inclination (Incl.), radius of cone of 95% confidence
( α95 ), and precision parameter (k) of Fisher (1953). Pluton: A - Alpine (Miller, 1935;
Lombardi, 1992) [with tonalite units of Todd (1977) of ap - Alpine, lb - Las Bancas,
and jv - Japatul Valley]; R - Ramona Complex [with tonalite units of Merriam (1946)
of gv - Green Valley, and lv - Lakeview]. * omitted from averages because of poor
clustering of directions.
Table DR3. Lithology, cratonic reference pole and Neogene-corrected paleopoles for Cretaceous
DR2008103
igneous units in Baja California.
Terrrane
Entry, Unit, Lithology
Caborca Terrane
1, La Posta Pluton, tonalite-monzogranite
Alisitos Terrane
2, San Telmo Pluton, gabbro-granodiorite
Yuma Terrane
3, Ramona Igneous Complex, biotite-hornblende tonalite
4, Peninsular Ranges plutons, granitoids
5, Testerazo Pluton, hornblende-biotite tonalite
6, Alpine Complex, hornblende-biotite tonalite
7, San Marcos Plutons, gabbro
8, San Marcos Plutons, gabbro
9, San Marcos Dikes, basalt-rhyolite
10, Santiago Peak Volcanics, basalt
North American
NeogeneCratonic Pole
Corrected Unit
Lat. Long. Radius Paleopole
N
E
N
E α95
α95
75.9 202.8
5.7
69 192
8
76.1 201.6
5.9
72 176
4
76.6
76.2
75.8
75.4
73.1
73.1
73.1
71.9
6.7
4.7
5.5
4.5
2.4
2.4
2.4
2.6
85
88
84
88
88
84
76
80
195.8
195.2
194.8
194.4
193.9
193.9
193.9
193.0
216 8
117 10
234 7
160 4
030 8
359 4
171 6
202 7
118 W
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117 W
120 110
100
N
90
10
80
4
8
8
PACIFIC OCEAN
33 N
3
116 W
6
33 N
1
U.S
ME .A.
XIC
O
32 N
5, 9
117 W
AG
UA
B
FA LAN
UL
CA
T
2
?
?
?
120
100
110
116 W
GULF OF
CALIFORNIA
31 N
32 N
90
115 W
Figure. DR1
Mg/(Mg + Fe 2+)
1.0
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0.8
magnesiohornblende
tschermakite
ferrohornblende
ferrotschermakite
0.6
0.4
0.2
A
0.0
7.2
6.8
6.4
6.0
5.6
Si (atoms per 13 cations)
Si (atoms per 13 cations)
7.0
Edenite exchange:
Si + [A] = [4]Al + [A](Na + K)
15
4
18
3
6.8
16
13
14
10
2
17
25
6
21
27
6.6
8
B
6.4
1.4
1.6
1.8
[4] Al
+
[A] (Na
2.0
2.2
+ K)
Figure. DR2
W, U
W, U
A) 190101
B) 150101
C) 250101
W, U
1
1
1
J/J 0
J/J 0
J/J 0
0.5
0.5
0.5
0
0
0
0
70 mT 140
N
S
E, D
S
E, D
0
030 mT 300 C 600
300 C 600
N
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S
N
E, D
Figure. DR3
1.0
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0.9
J/J900
A
0.5
350
0
0
300
H - mT 600
900
1.0
J/J900
B
0.5
MD
PSD
SD
0
0
50
H - mT 100
150
Figure. DR4
A
CRATON
F
b
W
E
S
T
NORTH
B
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W
E
S
T
E
A
S
T
b
F
CRATON
NORTH
E
A
S
T
a
a
S
SOUTH
S
SOUTH
Figure. DR5
DR2008103
Captions
Figure DR1. Locations of paleomagnetic studies on Cretaceous igneous rocks in Alta and Baja
California and of
40
Ar/39Ar hornblende chrontours in Ma from Ortega-Rivera (2003). The
study areas are:1) La Posta Pluton, 2) San Telmo Batholith, 3) Ramona Complex, 4) Peninsular
Ranges Plutons, 5) Testerazo Pluton, 6) Alpine Complex, 7) San Marcos Plutons, 8) San
Marcos Plutons, 9) San Marcos Dikes, and 10) Santiago Peak Volcanics. The paleomagnetic
data and references are given in Table 4.
Figure DR2. Hornblende compositions from the Alpine and Ramona Complexes. A)
Simplified calcic-amphibole classification after Leake et al. (1997); B) Amphibole edenite
exchange reactions. Identifying symbols for the Alpine Complex are: Alpine Unit ( ); Las
Bancas Unit ( ); Japatul Unit ( ), and for the Ramona Complex are: Lakeview Unit ( );
Green Valley Unit ( ).
Figure DR3. Vector component plots of example specimens showing: A) alternating field (AF),
B) thermal (Th), and C, D) AF then Th step demagnetization. Vectors in the horizontal plane
(north, N; east, E; south, S; west, W) are solid circles and in the vertical plane (north, N; down,
D; south, S; up, U) are open circles. Axial increments are 20% of the specimen’s natural
remanent magnetization (NRM) intensity. The inset figure shows the decay of the NRM
intensity against AF intensity in mT or temperature in °C.
Figure DR4. Saturation isothermal remanent magnetization (SIRM) analyses. A) Acquisition
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of SIRM showing the mean (●) values for the 12 representative specimens for the collection
along with their standard deviations where J/J900 is the ratio of the measured remanence to the
remanence at 900 mT and H is the magnetizing or demagnetizing field intensity. Single-domain
magnetite reaches saturation by ~350 mT. B) Alternating field demagnetization of SIRM with
the fields for single, pseudosingle and multidomain (SD, PSD, MD) magnetite, respectively.
Note that the remanence isolated above ~40 mT is mostly retained by single-domain magnetite.
Figure DR5. Schematic showing: A) poleward (northward) orogen-parallel translation of the
Yuma Terrane along the western coast of the North American craton (a), starting (S) in Central
America or southern Mexico, rotating clockwise (b) enroute, and accreting finally (F) to infill
the Gulf of California beside northern Mexico; and B) poleward (northward) and eastward
orogen-perpendicular translation of an exotic terrane that collides, rotates and accretes to infill
the Gulf of California. The open arrow indicates the required seafloor spreading direction for
the Farallon Plate (Engebretson et al., 1985; Umhoefer et al., 2002) on which the terrane rode.