Paleomagnetic Analysis of 

Miocene Basalt Flows in the 

Tehachapi Mountains, California 

U.S. GEOLOGICAL SURVEY BULLETIN 2100 
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Paleomagnetic Analysis of 

Miocene Basalt Flows in the 

Tehachapi Mountains, California 

By J.B. Plescia, Gary J. Calderone, and Lawrence W. Snee 

U.S. GEOLOGICAL SURVEY BULLETIN 2100 

New paleomagnetic directions and 40 Arf39 Ar extrusion ages from 

Miocene basalt flows on the north flank of the 

Tehachapi Mountains, Southern California 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Plescia, Jeffrey B. 

Paleomagnetic analysis of Miocene basalt flows in the Tehachapi Mountains, 

California I by J.B. Plescia, Gary J. Calderone, and Lawrence W. Snee. 

p. em.-(U.S. Geological Survey bulletin: 2100) 

Includes bibliographical references (p. ). 

Supt. of Docs. no.:" 119.3 2100." 1. Basalt-California-Tehachapi Mountains. 

2. Geology, Stratigraphic-Miocene. 3. Paleomagnetism-California-Tehachapi 

Mountains. I. Calderone, Gary J. II. Snee, Lawrence W. III. Geological Survey 

(U.S.) IV. Title. V. Series. 

QE75.B9 no. 2100 

[QE462.B3] 

557.3 s-dc20 94-22229 

[552' .26' 0979488 CIP 
CONTENTS 

Abstract........................................................................................................................... 1 

Introduction..................................................................................................................... 1 

Geologic Setting . .. . . .................. ... .. . . . ... .. . .. . . .................. ........... ... . .. . ............... ..... . .. . . . . ..... 1 

Geochronology . . . . ..................... ... ... ............ .................. ...... . .... . . . . . . . ........................... ..... 5 

Field and Laboratory Methods........................................................................................ 5 

Paleomagnetic Analysis.................................................................................................. 6 

Discussion and Conclusions ...................................................... ..................................... 10 

Acknowledgments.......................................................................................................... 10 

References Cited............................................................................................................. 10 

FIGURES 

1. Generalized geologic map showing location of Tehachapi Mountains and vicinity................................................... 2 

2. Generalized geologic map of the study area................................................................................................................ 3 

3. 40Arf39

Ar age-spectrum diagram for flows DCB4, DCB5, and DCB13 ...................................................................... 6 

4. Alternating field demagnetization results of samples from flows DCB 1 and DCB 19 .................... ............................ 7 

5. Mean directions and virtual geomagnetic poles from Miocene basalt flows in the Tehachapi Mountains ................ 9 

TABLES 

1. 40 Arf39 Ar age-spectrum data for Miocene basalt flows from sites DCB4, DCB5, and DCB 13 .................................. 4 

2. Mean remanence directions, virtual geomagnetic poles (VGPs), and Fisher statistics of Miocene basalt flows 

from the Tehachapi Mountains, California.................................................................................................................. 8 

III 

PALEOMAGNETIC ANALYSIS OF MIOCENE BASALT FLOWS 

IN THE TEHACHAPI MOUNTAINS, CALIFORNIA 

By J. B. Plescia1

, Gary J. Calderone2, and Lawrence W. Snee3 

ABSTRACT 

In an attempt to constrain the timing and extent of 

clockwise vertical-axis tectonic rotation of the Tehachapi 

Mountains, we have determined paleomagnetic directions 

and isotopic dates from Miocene basalt flows exposed along 

the northern flank of the range. 40 Arf39 Ar spectrum-analyses 

yield an average extrusion age of 22.6±0.6 Ma. The paleomagnetic data set of 22 flow-mean directions and virtual 

geomagnetic poles (VGPs) probably does not adequately 

average the secular variation of the Miocene geomagnetic 

field. Consequently, no tectonic significance may be 

attached to the mean pole (in-situ: long 350.6°£., lat 

54.9°N., Fisher (1953) semi-angle of 95% confidence, 

A95=5.4°, Fisher (1953) best estimate of the precision 

parameter, K=34.2; after tilt correction: long 9.6°E., lat 

59.3°N., A95=5.7°, K=30.6) of the 22 site mean VGPs. 

Although the result would permit post-Miocene clockwise 

vertical-axis rotation of the Tehachapi Mountains, it neither 

supports nor requires such rotation. Although not tectonically significant, it is interesting to note that the rotational 

discordance (27 .2°±5.5° after tilt correction) is consistent 

with rotation of the underlying Cretaceous granitic rocks. 

INTRODUCTION 

The Tehachapi Mountains of southern California are 

the southernmost part of the Sierra Nevada batholithic terrane and may represent the upended, structurally deepest 

level of the batholith (Ross, 1985). Several geologic patterns 

1

Currently at Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. 

ZU.S. Geological Survey, MS913, P.O. Box 25046, Denver Federal 

Center, Denver, CO 80225. 3

U.S. Geological Survey, MS963, P.O. Box 25046, Denver Federal 

Center, Denver, CO 80225. 

in the Tehachapi Mountains trend northeast, apparently 

deflected in a clockwise sense relative to the Sierra Nevada 

where the trends are north-northwest (Ross, 1980; Burchfiel 

and Davis, 1981). Paleomagnetic data (Kanter and McWilliams, 1982; McWilliams and Li, 1983, 1985) suggest that 

these directional changes are the result of a tectonic rotation 

of the Tehachapi Mountains. 

Paleomagnetic analysis of the crystalline basement 

rocks (Kanter and McWilliams, 1982; McWilliams and Li, 

1983, 1985) suggests increasing clockwise rotation southwestward across the range. Virtually no rotation is indicated 

north of Tehachapi Valley, whereas rotation of more than 

90° may have occurred to the southwest near Grapevine Canyon (McWilliams and Li, 1983). 

The timing of the rotation has been only broadly constrained to postdate the rotated 80-120 Ma plutonic rocks. 

These ages are based on U/Pb, K./Ar, and Rb/Sr analyses 

(Sams and others 1983; D. B. Sams, oral commun., 1985; 

Ross, 1983, 1989). Near Tehachapi Valley, the rotation must 

have occurred prior to emplacement of the unrotated 16-18 

Ma Kinnick Formation and associated volcanic rocks (Evernden and others, 1964; corrected for new decay constants, 

Dalrymple, 1979; Kanter and McWilliams, 1982; McWilliams and Li, 1985). It is uncertain to what extent this minimum age also constrains the timing of rotation farther to the 

southwest. In an attempt to constrain the timing and extent of 

the rotation, a section of Miocene basalt flows which crop 

out along the northern flank of the Tehachapi Mountains was 

sampled for paleomagnetic analysis and for isotopic dating 

(figs. 1 and 2). 

GEOLOGIC SETTING 

The study area is along the northern flank of the 

Tehachapi Mountains, Calif., between Pastoria Canyon 

to the southwest and Tunis Canyon to the northeast 

(figs. 1 and 2). Within this area the volcanic rocks are 
35.25° 

35° 

119° 

San Joaquin 

Valley 

~~ 

..• .M··.,. ~~ ~~ 

,,'~ Tejon 

Embayment 

118.5° 

Mojave 

Desert 

0 20 KILOMETERS I I I I I 

EXPLANATION 

c;:)Pre-Tertiary rocks 

High-angle fault--

Dotted where 

concealed 

.._ Thrust fault- -

Dotted where 

concealed 

Major drainage 

Figure 1. Generalized geologic map of the Tehachapi Mountain region showing pre-Tertiary, Tertiary, and Quaternary rock units. Also shown 

are geographic features mentioned in the text. The box outlines the study area shown in detail in figure 2. WWF, White Wolf fault; GF, Garlock 

fault; EF, Edison fault; SF, Springs fault. 

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3 
4 PALEOMAGNETIC ANALYSIS OF MIOCENE BASALT FLOWS, TEHACHAPI MOUNTAINS, CALIFORNIA 

Table 1. 40 Ar,89 Ar age-spectrum data for Miocene basalt flows from Tehachapi 

Mountains, Calif. 

[T °C = degassing temperature. The total of 39 Art and 40 Arr expressed in percent. 39 Ar 

expressed in moles X 1o-13. K/Ca expressed as apparent K/Ca in mole/mole. Date expressed 

as apparent date X 106 years and error as one standard deviation from the mean. J = 

irradiation dosage J 

DCB4 

300 

450 

600 

750 

1000 

1450 

DCB5 

300 

450 

600 

750 

1000 

1450 

DCB 13 

300 

450 

600 

750 

1000 

1450 

Ar40/39 Ar37/39 Ar36/39 39Art K/Ca Date 

Whole-rock basalt; sample weight= 0.9960 g; J = 0.005795 

25.900 0.6773 0.0839 0.6 4.4 0.39 0.77 11.9±0.3 

4.087 0.7524 0.0075 6.5 47.3 4.17 0.69 20.1±0.1 

3.082 0.8840 0.0029 10.6 74.2 6.78 0.59 23.8±0.1 

2.359 1.1870 0.0009 18.9 92.8 12.00 0.44 22.7±0.1 

2.245 0.8874 0.0005 41.8 95.6 26.60 0.59 22.3±0.1 

2.635 3.2900 0.0018 21.5 89.3 13.70 0.16 24.4±0.1 

TOTAL GAS DATE 22.8 

PREFERRED DATE (600 oc- 1450 °C): 23.0±1.0 

Whole-rock basalt; sample weight= 0.9709 g; J = 0.005780 

11.730 0.1845 0.0336 11.9 15.4 7.79 2.80 18.8±0.1 

5.462 0.4776 0.0112 11.6 40.1 7.59 1.10 22.7±0.2 

4.197 0.8088 0.0068 21.5 53.5 14.00 0.64 23.3±0.2 

3.308 0.9989 0.0042 16.3 64.3 10.70 0.52 22.1±0.1 

2.864 1.5780 0.0029 19.5 73.9 12.70 0.33 21.9±0.1 

3.360 4.1600 0.0054 19.2 62.0 12.50 0.13 21.6±0.1 

TOTAL GAS DATE 21.8 

PREFERRED DATE (600 oc -1450 °C): 22.3±0.7 

Whole-rock basalt; sample weight= 0.9971 g; J = 0.005750 

14.990 1.195 0.0442 1.5 13.4 1.09 0.44 20.6±0.3 

2.949 0.8718 0.0034 8.9 68.4 6.35 0.60 20.8±0.1 

2.930 0.9573 0.0022 9.5 79.7 6.77 0.54 24.1±0.1 

2.445 1.2350 0.0008 16.5 93.7 11.80 0.42 23.6±0.1 

2.291 1.1920 0.0005 38.7 97.0 27.70 0.44 22.9±0.1 

2.371 2.8280 0.0015 25.0 90.6 17.90 0.18 22.1±0.1 

TOTAL GAS DATE 22.7 

PREFERRED DATE (750 oc- 1450 °C): 22.9±0.9 

well exposed, the geology is well mapped, and the structure is relatively simple. The structure becomes increasingly complex to the southwest. The volcanic rocks of 

the southern San Joaquin Valley are within the upper part 

of the Tecuya Formation east of Pleito Creek and within 

the upper part of the Temblor Formation west of Pleito 

Creek (Nilsen, 1973; Nilsen and others, 1973; Nilsen 

and Clarke, 1975). The Tecuya and Temblor Formations 
FIELD AND LABORATORY METHODS 5 

interfinger and are coeval marine and continental deposits, respectively. 

Northeast ofPastoria Canyon (fig. 2) the volcanic rocks 

are unconformably overlain by an unnamed conglomerate 

(Dibblee, 1973) which may be correlative with the Bena 

Gravels to the north and northeast (Bartow and Dibblee, 

1981; Bartow, 1984; Bartow and McDougall, 1984). In the 

study area the Tecuya Formation unconformably overlies 

marine rocks of the Eocene Tejon Formation, and they in 

tum overlie compositionally heterogeneous basement rocks 

(Dibblee, 1973; Ross, 1980, 1983, 1989). 

The basalt flows dip 1 Oo- 25° to the northwest and crop 

out along the range front in a series of northeast-striking hogbacks. Because of the patchy nature of outcrops in the 

hogbacks, however, we could not discern an unambiguous 

flow-by-flow stratigraphy within the basalt flows. This lack 

of known internal stratigraphy limits our ability to rigorously 

analyze the paleomagnetic data in this report. 

GEOCHRONOLOGY 

Samples from three flows (DCB4, DCB5, and 

DCB 13; fig. 2) were analyzed for 40 Arf39 Ar dating. These 

relatively unaltered samples were ground and sieved and 

the 180-250 J..Lm whole-rock size fraction was treated 

with 10 percent HCl to remove secondary calcium carbonate. Standard procedures were used in this experiment as 

described in Dalrymple and others (1981) and Snee and 

others (1988). The samples were irradiated to produce 39 Ar from 39K. After irradiation, each sample was progressively heated under vacuum in six temperature steps from 

300 oc to 1,450 °C. For each temperature step, the 

released argon was analyzed by mass spectrometer and an 

apparent age was calculated. Decay constants used are 

those of Steiger and Jager (1977). The geochronologic 

standard used was MMhb-1 hornblende with an age of 

519.4 Ma (Alexander and others, 1978). The argon agespectrum data are presented in table 1 and figure 3. 

In general, all three age spectra show minor internal discordance, and no "plateaus" are present; therefore, a preferred date was calculated for each sample. The preferred 

date is an average of all contiguous temperature steps that 

contain more than 10 percent of the total 39 Ar released from 

the sample and that have greater than 50 percent radiogenic 

argon (

40ArR). The preferred dates for flows DCB4, DCB5, 

and DCB13 are 23.0±1.0, 22.3±0.7, and 22.9±0.9 Ma, 

respectively. Since these dates are statistically indistinguishable from each other, the average date of 22.6±0.6 Ma is the 

best estimate for the extrusion age of these basalt flows. This 

age is statistically identical to that determined by Turner 

(1970), about 22.5 Ma, for samples from the same stratigraphic unit in the San Emigdio Mountains to the west. The 

volcanic rocks in the San Emigdio Mountains have K-Ar 

dates of 22.1-25.2 Ma and an average of 22.5 Ma (Turner, 

1970; corrected for new decay constants, Dalrymple, 1979). 

FIELD AND LABORATORY METHODS 

Twenty-five basalt flows between Pastoria Canyon and 

Tunis Canyon were sampled for paleomagnetic analyses 

(figs. 1 and 2). Seven to fifteen oriented cores were collected 

from each site (flow) using a portable rock drill. Both sun 

and magnetic compasses were used to orient all cores. 

Because solar and magnetic azimuths had local discrepancies of as much as 20°, solar azimuths were used in all orientation calculations. 

Natural remanent magnetizations (NRM) of the samples were measured on a two-axis superconducting magnetometer. Sample NRM intensities average about 5.7 amperes 

per meter (Am-1

). Progressive stepwise alternating field (AF) 

demagnetizations were performed on two or three pilot specimens from each site. Median destructive fields ranged from 

about 15 to >100 milliTesla (mT), averaging about 37 mT. 

Many specimens contained both low- and high-coercivity 

components of magnetization (fig. 4). The low- to moderatecoercivity components have steep inclinations (about 60°) 

and northerly declinations (about 350°), and they are removable by AF demagnetization at a peak field of 20 mT. These 

components are most likely viscous remanent magnetizations (VRM) acquired during the present-day geomagnetic 

field. The stable high-coercivity components, isolated by AF 

demagnetization to peak fields of >20 mT, are almost certainly high-temperature thermal remanent magnetizations 

(TRM) acquired at the time of extrusion. 

Stepwise progressive thermal demagnetizations, at 

temperatures as high as 600 oc, were also run on pilot specimens for comparison with the AF demagnetization behavior. The directions determined by principal component 

analysis (Kirschvink, 1980) of the thermal trajectories are 

indistinguishable (at the 95 percent confidence level) from 

trajectories determined by AF analysis. The similarity of the 

AF and thermal demagnetization data supports the interpretation that the high-coercivity components of magnetization 

are indeed TRM. The thermal demagnetization analysis indicates unblocking temperatures of 400-580 oc, suggesting 

that magnetite is the principal carrier of remanence. 

On the basis· of the demagnetization behavior of the 

pilot specimens, all samples were AF demagnetized in peak 

fields of 20-40 mT, a level sufficient to completely remove 

the secondary VRM. The few specimens for which a meaningful direction could not be isolated were deleted from the 

data set. Samples from three sites in this same section were 

independently collected and analyzed by M.O. McWilliams 

(written common., 1985) and exhibit directions similar to 

those reported here. 
6 PALEOMAGNETIC ANALYSIS OF MIOCENE BASALT FLOWS, TEHACHAPI MOUNTAINS, CALIFORNIA 

ca 

~ 1.0 

w 

" <( 

1-

z 

w 

a: 

<( 

D. 

c. 

<( 

25 

20 

15 

r--

EXPLANATION 

-- DCB4 

DCBS 

DCB13 

10~--~--~--~--~~~~--~--~--~~~ 

0 20 40 60 80 100 

39ArK RELEASED (PERCENT) 

Figure 3. Argon age-spectrum diagram for Miocene basalt flows DCB4, DCB5, and DCB 13. The upper 

panel shows the K/Ca ratio as a function of percent total Ar released for each temperature step. The lower 

panel shows the apparent age for each temperature step. 39 ArK is the reactor-produced 39 Ar from 39K (see text 

for details). 

After magnetic cleaning, a mean direction and Fisher 

(1953) statistics were calculated for each flow (see table 2 

and figure 5A) in both tilt corrected and in-situ coordinates. 

Using the mean directions and the standard dipole formula, 

a mean virtual geomagnetic pole (VGP) was calculated for 

each flow (table 2 and figure 5B). 

PALEOMAGNETIC ANALYSIS 

Critical examination of the data in figure 5 and table 2 

reveals several problems with the data set of site mean 

paleomagnetic directions and VGPs both before and after 

tilt correction. 

(1) There are several groups of flows within which individual flow directions are statistically indistinguishable at 

0.99 probability using the test of McFadden and Lowes 

(1981). This coincidence of directions within groups of 

flows may be the result of (a) rapid extrusion of multiple 

flows recording the same geomagnetic field direction; (b) 

multiple flows recording similar but independent directions 

of the geomagnetic field as would be common near the mean 

of Fisher-distributed vectors; or (c) both causes. Lacking 

firm control of the internal stratigraphy we cannot 
PALEOMAGNETIC ANALYSIS 

Site DCB1 NRM 

w 

(A) 

Site DCB1 30 mT 

N 

I 

+ 

I 

s 

N 

w ~------------+ 

{B) 

s 

Site DCB19 Sample 5 

1--1 

0.2 

Amps/meter 

(C) 

S,Down 

Site DCB1 Sample 3A N,Up 

w, -,~--+---~--~--~~--~---+ 

Horizontal 

1----i 

0.2 

Amps/meter 

{D) S,Down 

Figure 4. Alternating field (AF) demagnetization results. Equal-area projection of (A) in-situ initial and (B) postAF demagnetization (at peak fields of30 mT) directions from flow DCBl (solid circle= lower hemisphere projection; 

open circle = upper hemisphere projection). Note scatter of initial natural remanent magnetization (NRM) directions 

along great circle containing present field directions. (C) Vector demagnetization diagram showing univectorial decay 

of remanence toward origin during progressive AF demagnetization; squares represent the projection of the magnetic 

declination into the horizontal plane, circles are the projection of the magnetic inclination into a vertical plane passing 

through the declination. (D) Vector demagnetization diagram showing removal of a northerly component of magnetization having steep inclination probably caused by a present field viscous remanent magnetization (VRM); squares 

and circles are as represented in part C. 

7 
00 

"'0 

~ ti1 

Table 2. Mean remanence directions, virtual geomagnetic poles (VGPs), and Fisher (1953) statistics of Miocene basalt flows from the Tehachapi Mountains, Calif. 0 

~ 

[Lat and Lon are the north latitude and east longitude of the site. D-Level is the alternating field demagnetization treatment in milliTesla. Dec and Inc are the components of the site mean g; 

direction. a95, and k are the parameters of Fisher (1953). Pion and Plat are the coordinates of the VGP calculated from Dec and Inc using the standard pole transformation. Dm and dp are the 

axes of the the 95% confidence ellipse around the VGP. cDec and cine are the components of the mean direction after tilt correction. cPlon and cPlat are the coordinates of the tilt corrected VGP ~ calculated from cDec and cine using the standard pole transformation. N is the number of specimens used to calculate the mean. GN is the cooling unit group to which each flow belongs ] (J 

-

~ Flow Lon( 0 ) Lat( 0 ) D-Level Dec(0

) Inc(0 ) a.95(0 ) k Plon(0 ) Plat(0) dm(0 ) dp(o) cDec(0 ) clnc(0 ) cPlon(0 ) cPlat( 0 ) N/GN ~ 

en 

DCB1 241.17 34.95 30 232.1 -17.1 2.4 330.5 167.1 -35.8 2.3 1.4 225.8 -14.6 173.4 -39.8 1211 -

en 

DCB2 241.20 34.96 20 205.3 -24.1 2.2 497.4 189.3 -58.0 2.3 1.1 199.2 -14.1 204.3 -57.1 10/2 0 

"I1 

DCB3 241.16 34.94 30 204.7 -41.1 1.2 375.1 172.4 -65.7 1.7 1.1 192.4 -24.3 211.3 -65.1 7/3 ~ 

DCB4 

DCB5 241.16 

241.16 34.94 

34.94 20 

40 211.5 

201.8 

-39.3 

-44.4 

2.8 

1.8 587.4 

340.9 167.5 

170.5 

-59.7 

-69.3 2.3 

3.4 

1.1 

1.7 

188.9 

198.4 

-26.6 

-24.7 

217.9 

199.5 

-67.6 

-62.4 

12/5 

9/4 -

~ ti1 

DCB6 241.16 34.94 40 196.4 -41.7 6.1 51.5 184.3 -72.1 7.4 4.5 185.8 -22.6 227.0 -66.2 10/5 ~ DCB8 241.16 34.94 20 200.6 -46.0 2.3 290.4 168.8 -70.9 2.9 1.7 187.4 -27.7 221.1 -68.7 15/5 0:1 

DCB9 241.22 34.97 30 204.3 -27.2 3.0 343.9 188.4 -60.0 3.4 1.7 199.0 -18.9 202.2 -59.4 8/2 > en 

DCB10 241.21 34.97 30 211.8 -26.2 4.4 99.4 179.7 -54.4 4.6 2.3 207.5 -7.7 195.7 -49.8 12/6 > r 

DCB11 241.21 34.97 30 200.1 -31.4 3.1 198.2 191.2 -64.6 3.4 1.7 191.2 -21.8 215.2 -64.2 1217 ..., 

"I1 

DCB12 241.18 34.96 20 346.8 - 3.3 4.3 167.6 82.6 51.4 4.8 2.4 349.5 -20.1 75.5 43.6 8/15 r 

DCB13 241.17 34.95 30 233.0 -18.1 2.9 519.7 165.9 -35.4 2.9 1.7 223.6 -23.1 170.4 -44.4 6/1 ~ DCB14 241.21 34.97 30 202.0 -22.8 2.7 438.0 195.0 -59.5 2.9 1.7 197.4 -12.1 208.1 -57.0 8/2 sn 

DCB15 241.21 34.97 30 226.0 -27.3 3.3 333.8 165.8 -44.0 3.4 1.7 217.7 -22.9 175.8 -48.8 7/8 ..., 

ti1 

DCB16 241.21 34.97 20 223.5 -21.9 3.1 313.2 171.5 -44.1 3.4 1.7 217.3 -17.0 179.8 -47.0 8/8 ::r: 

> DCB17 241.21 34.97 40 222.6 -22.5 1.4 1151.8 171.6 -45.0 2.3 1.1 216.3 -17.3 180.6 -47.8 9/8 (J 

DCB18 241.21 34.97 30 222.5 -21.9 3.6 236.1 172.1 -44.9 4.0 2.3 216.4 -16.7 180.9 -47.5 8/8 ::r: 

> 

DCB19 241.21 34.96 40 39.6 44.7 3.3 245.4 334.4 55.1 4.0 2.8 20.5 42.2 356.5 69.4 9/9 '"tl 

- DCB20 241.21 34.96 20 32.9 43.2 2.5 604.5 341.3 60.0 2.8 1.7 15.7 38.7 10.7 71.0 7/10 ~ 

DCB21 241.21 34.96 40 28.1 35.9 2.4 621.5 355.1 61.0 2.8 1.7 15.3 30.5 20.9 67.0 7/11 0 

c 

DCB22 241.21 34.96 30 30.5 42.2 2.6 534.8 344.7 61.6 3.4 1.7 14.2 37.1 16.2 71.1 7/12 

DCB23 241.16 34.94 30 231.2 -42.4 3.3 341.3 150.0 -44.8 4.0 2.3 211.8 -45.8 158.2 -61.8 7/13 ~ DCB24 241.17 34.94 30 242.1 -20.1 3.5 302.1 158.8 -28.7 3.4 1.7 233.8 -28.9 159.1 -38.2 7/14 z 

sn 

DCB25 241.18 34.96 20 340.3 -5.6 4.8 114.7 91.4 47.9 4.8 2.4 343.1 -22.1 83.3 40.9 9/15 (J 

DCB26 241.18 34.96 30 341.2 -0.2 3.5 250.2 91.8 50.8 3.5 1.8 342.9 -16.6 84.9 43.7 8/15 > r 

-

2J 

~ > 
A 

w 

0 

0 0 

0 

PALEOMAGNETIC ANALYSIS 

N 

E 

0 

s 

Figure 5. Mean directions and virtual geomagnetic poles (VGPs) from Miocene basalt flows from the Tehachapi Mountains, 

Calif. (A) Equal-area projection of in-situ site mean directions (solid circle = lower hemisphere projection; open circle = upper 

hemisphere projection). (B) Northern hemisphere polar projection of all site mean VGPs from this study. Solid circles are northern 

hemisphere VGPs. Open circles are the antipodes of southern hemisphere VGPs. 

9 
10 PALEOMAGNETIC ANALYSIS OF MIOCENE BASALT FLOWS, TEHACHAPI MOUNTAINS, CALIFORNIA 

distinguish among these possibilities. Rapid extrusion has 

probably produced at least three of the groups (groups 5, 8, 

and 15, table 2). Other groups (group 2), however, cannot be 

interpreted because of inadequate stratigraphic control. 

Consequently, we cannot objectively establish the number of 

independent cooling units represented by the 25 site mean 

directions. This in turn, hampers our ability to establish 

meaningful estimates of both the mean direction and its confidence limits. 

(2) Using the graphical methods of Lewis and Fisher 

(1982), flows DCB12, DCB25, and DCB26 (group 15, table 

2) represent statistical outliers to the rest of the paleomagnetic directions. These flows may record a short-term excursion of the geomagnetic field. 

(3) After removal of the outlying directions, the distribution of remaining directions (corrected for tilt) is roughly 

Fisherian, using the tests of Lewis and Fisher (1982) and 

McFadden (1980a). The resulting distribution of site mean 

VGPs, however, is streaked perpendicular to the meridian 

connecting the locality and the mean pole position (figure 

5B). This streaking may be the result of (a) inadequate averaging of paleosecular variation; (b) tilt correction errors; or 

(c) both causes. In this case, however, we can attempt to distinguish between possibilities. The methods of McFadden 

(1980b; see Calderone, 1988, and Calderone and others, 

1990, for details) using the McFadden and McElhinny 

(1984) model of paleosecular variation at 95 percent confidence predicts an angular dispersion between 1 Oo and 20° for 

VGPs from the latitude of the Tehachapi Mountains. The 

total angular dispersion (14°- 18°) of site mean VGPs (insitu or tilt corrected) is well within this predicted range. Only 

about 6° of the observed dispersion, however, is parallel to 

the paleomeridian connecting the Tehachapi Mountains to 

their mean pole. Consequently, it is unlikely that the data 

average the secular variation of the Miocene geomagnetic 

field. Improper structural corrections may also contribute to 

the overall dispersion of VGPs. 

DISCUSSION AND CONCLUSIONS 

Paleomagnetic results from 22.6±0.6 Ma basalt flows in 

the Tehachapi Mountains are inconclusive. The data set of 

directions and VGPs, tilt corrected or not, probably does not 

adequately average the secular variation of the Miocene geomagnetic field. Consequently, tectonic significance may not 

be attached to the mean pole (long 350.6°E., lat 54.9°N., 

A95=5.4°, K=34.2; after tilt correction, long 9.6°E., lat 

59.3°N., A95=5.7°, K=30.6) of the 22 site mean VGPs (outliers from flows 12, 25, and 26 removed). Thus, although 

these results would permit post-Miocene clockwise rotation 

of the Tehachapi Mountains, they would neither support nor 

require such rotation. The rotational discordance (Beck and 

others, 1986; Demarest, 1983) of 37.7°±5.4° before tilt 

correction and 27 .2°±5.5° after tilt correction (using the 

North American reference pole calculated by Hagstrom and 

others, 1987) is consistent with that of the underlying Cretaceous granitic rocks (Kanter and McWilliams, 1982; 

McWilliams and Li, 1985). If the rotational discordance 

could be shown to be tectonically significant, then the age of 

these basalt flows and the age of the unrotated Kinnick Formation (Kanter and Me Williams, 1982) may constrain the 

timing of rotation to be between 23 and 16-18 Ma._Further 

investigation, however, will be required to constrain the timing and extent of rotation in the Tehachapi Mountains. 

ACKNOWLEDGMENTS 

We are grateful to the Tejon Ranch Company, especially Jack Hunt, for permission to conduct field work on 

their property. We thank Nancy Riccio and Cori Hoag for 

laboratory assistance; Don Elston for use of the U.S. Geological Survey (USGS) paleomagnetic laboratory in Flagstaff; 

Bob Butler for advice and the use of the University of Arizona laboratory; and Mike McWilliams, Peter Coney, Clem 

Chase, and Steve Lund for advice and comments regarding 

the data. John Sutter provided access to the argon geochronology laboratory at the USGS in Reston and Mick Kunk 

helped with sample preparation and analysis. Thorough 

reviews by Duane Champion and an anonymous reviewer 

led to significant improvements in the manuscript. Reviews 

of earlier versions by J.A. Bartow, J.W. Hillhouse, L.S. Frei, 

and T.H. Nilsen are greatly appreciated. 

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