Paleomagnetism of Franciscan Ultramafic Rocks fromRed Mountain, California

Results from the study of the magnetism in serpentinized ultramafic rocks of the RedMountain intrusion southeast of San Francisco, California, suggest that these rocks possess achemical remanent magnetization acquired during serpentinization. The stability of mag-netization decreases with serpentinization, owing to the growth of larger magnetic grains. Thus,in highly serpentinized peridotites (serpentinites), most of the CRM is destroyed and theNRM becomes mainly viscous remanent magnetization, rendering these rocks unsuitable forpaleomagnetic work. On the other hand, in partially serpentinized peridotites, pyroxenites,and dunites, the CRM is highly stable and paleomagnetically reliable. A study of the stabledirections of magnetization indicates that the Red Mountain ultramafic body was emplacedand serpentinized prior to the folding of the Franciscan formation. The stability of magneti-zation and removal of viscous components were verified by the results of a storage test andalternating-field demagnetization experiments, by the divergence of the mean directions ofmagnetization from that of the present field, and, most important, by the convergence ofthe directions after tilt correction. Paleomagnetic pole positions calculated from the meandirections suggest that the magnetic pole rapidly migrated southward into the Atlantic Oceanduring late Mesozoic, possibly more than once, as proposed previously by Gromme andGluskoter.

INTRODUCTION

A study of ultramafic rocks covering differ-ent degrees of serpentinization was conductedfor the purpose of investigating their magneticproperties. Samples were collected from RedMountain, Mount Boardman Quadrangle, Cali-fornia, a large ultramafic body intruding theuppermost part of the exposed Franciscan sec-tion in the Diablo Range (Figure 1) . The in-vestigation consisted of two parts: (1) a studyof intensity of induced and remanent magnet-ism, and (2) a study of directions of remanentmagnetism.

Results of the first study, described in an-other paper [Saad, 1969], indicated that thesusceptibility and intensity of remanent mag-netism are highly variable, depending largelyon the degree of serpentinization and, to alesser extent, on the original rock composition.the repanent magnetization in, these ultra-

). mafic r~ks was found to be mainly a chemicalremanent magnetization (CRM) acquired dur-

1Now at the Department of Geology, Univer-sity of Missouri, Rolla, Missouri 65401.

ing the process of serpentinization wherebyiron atoms, relea'sed from the silicate structureof the paramagnetic olivine and pyroxene, areoxidized to form ferrimagnetic magnetite. Asserpentinization increases, the natural remanentmagnetization increases in intensity but de-creases in stability because of the growth ofmagnetite grains from single- to multi-domainsize and of their oxidation, in some cases, tomaghemite.

This. paper deals with the paleomagnetic re-liability and significance of the directions. Themain objectives of the study were (1) to inves-tigate more thoroughly the stability of the nat-ural remanent magnetization (NRM) observedin these rocks and to confirm its origin, (2) totest whether ultramafic rocks can be used reli-ably for paleomagnetic work, and (3) to findthe paleomagnetic pole position and examine itscompatibility with. previously reported polesfrom North America. The paleomagnetic datain this paper are analyzed in connection with.the general problem of easterly directions foundfirst by Cox [1957] and later by Gromme andGluskoter [1965] from late Mesozoic and earlyTertiary rocks of the western United States.The problem is of some interest since these

.p~~ '? 2? 3? 1Q

MILES ~

Fig. 1. Index map showing location of Red Mountain (small rectangle) in relation to thetectonic features of California Coast Ranges. Striped areas are Franciscan eugeosynclinalrocks of upper Jurassic to mid-Cretaceous.

anomalous directions may reflect real field ex-cursions rather than tectonic rotations of thesampling sites.

Geologic setting. The geology of the area(Figure 2) is discussed in detail by Maddock[1955] and summarized elsewhere [Saad, 1968].The Red Mountain intrusion is probably a sillor laccolith in the center of a large synclineroughly parallel to the strike of the intrudedFranciscan sediments. The dip of the sedimen-tary beds near the contact is variable but gen-erally greater than 60°. The axis of the RedMountain syncline, which strikes N700W onthe average, is demarcated only approximatelyby a general reversal of dip over a broad area.The major folds in the area are not tightlycompressed along their middle flanks and haveaverage dips of around 40° [Maddock, 1964].The intrusion is bounded by the Tesla-Ortiga-lita fault on the eastern side. On the othersides, the contact is believed to be a fault-contact also [Bodenlos, 1950]. Contact meta-morphism along the margins is very slight, in-dicating that the body was intruded at fairlylow temperature.

Serpentinized peridotite is the main rocktype in the Red Mountain. The degree of ser-pentinization ranges from 20 to 95% and aver-ages 65%. Dunite, which is 85% serpentinizedon the average, occurs as small bodies, thelargest of which forms a core in the easternpart of the intrusion (Figure 2). Pyroxenite

occurs locally as small pods and veins and isabout 20% serpentinized. Serpentinite, in thisstudy, refers to a 100% serpentinized rockwith no remanent grains of olivine or pyroxene.Most of the samples belonging to this rocktype were collected from a small sill south ofthe main body. Outlines of some pyroxenegrains were observed in thin sections, suggest-ing that these rocks were originally peridotitesrather than dunites.

The slight deformation of fabrics in partiallyserpentinized peridotites and dunites suggeststhat serpentinization occurred after final em-placement of the ultramafic rocks.

Age considerations. Most of the ultramaficbodies in the California Coast Ranges were em-placed either during or subsequent to Francis-can time but prior to upper Cretaceous [Baileyet al., 1964; Burch, ID68; Hawkes et al., 1942].Maddock [1964] deduced that the Franciscanrocks in the Diablo Range are pre-uppermostJurassic age. The Red Mountain ultramaficrocks are intrusive into the uppermost part ofthe Franciscan section, indicating that they areupper Jurassic to upper Cretaceous in age andmore probably upper Jurassic. This conclusionis supported by the fact that no ultramaficbodies intrude the adjacent upper Cretaceoussediments.

The upper Cretaceous can be chosen also asan upper limit for the age of folding since theRed Mountain syncline is truncated by the

Tesla-Ortigalita fault and apparently does notextend into the upper Cretaceous sediments.Serpentinization occurred after final emplace-ment of the ultramafic rocks, as mentionedabove, and prior to folding as indicated by thedirections of magnetization discussed later.

Field and laboratory procedures. A total of17 sites were chosen for obtaining orientedsamples along road cuts and trails in the RedMountain area (Figure 2). At several localitieswithin each site, one to five .samples were ob-tained by means of a portable gasoline-powered,water-cooled diamond core drill. The coreswere oriented in situ with an accuracy of ±2°(s.d.) using a technique described by Doell andCox [1967a]. Within each site, localities areseparated by 30/100 meters and within eachlocality samples were cored 8 cml1 meter apart.Several unoriented samples were obtained also.The cores were 2,49 em in diameter and werecut into one to three specimens each (about2.28 cm long) for remanent magnetizationmeasurements, giving a total of 177 orientedspecimens and 54 unoriented ones.

The intensity and direction of remanent mag-netization were measured on a spinner mag-

D PD Peridotite

IiliJ DU Dunite• SP Serpentiniteo PX Pyroxenite

[ill GB Gabbro

E'J JF Franciscan Formation(Upper Jurassic)

Red Mountain syncline

Strike 8< Dip af beds

netometer of the type described by Doell andCox [1967b]. A digital computer was used toreduce the magnetometer readings to directionand intensity values with estimated accuraciesof ±1.3° (s.d.) and ±6% (s.d.) respectively.

The apparatus used for demagnetization ex-periments is a three-axis tumbler that rotatesthe specimen in an alternating field smoothlyreduced to zero from a peak value [Doell andCox, 1967c]. The purposes of these experimentswere to eliminate secondary soft components ofviscous and isothermal remanent magnetiza-tion [Cox, 1961] and to establish the stabilityof magnetization. In order to reduce any anom-alous magnetization added by the demagnetiz-ing apparatus, most of the specimens demag-netized at fields of 200 oe or more were invertedin the tumbler and redemagnetized at the samefield. The duplicate measurements were aver-aged and given unit weight in the analysis.

Polished sections and thermomagnetic mea-surements were used to identify the magneticminerals in 20 specimens. The apparatus usedfor thermomagnetic measurements is an auto-matically recording quartz torsion balance de-scribed by Clark [1967]. The accuracy of the

o,

MILE

Fig. 2. Generalized geologic map of the Rad Mountain area showing locations of collect-ing sites. Numbers associated with solid circles refer to sites of oriented samples. Crosses aresite locations of unoriented samples. (Geology taken from Maddock [1964], Bodenlos [1950],and H awkeset al. [1942]).

V>~ 60:>!UUJ11.

'" 40...oa:UJ

~ 20:::>z

,. Serpentinitefm DuniteIiKI Anomalous peridotite

{low J a KlEl Peridotiteo Pyroxenite

0.5 1.0 1.5Log,o(J/J,l10 2'0 J./J,

rg (b)

~ 20

~15 10a:UJen:>!:::>z

Fig. 3. Storage test histograms showing changesin (a) intensity of NRM, and (b) direction ofNRM, with repeat measurements. J, and J2, firstand second measurements; Cio, angular deviationbetween repeat measurements. Average time in-terval between measurements, 1.5 years.

Curie temperatures is estimated as ±5°C.Magnetic separates from one sample were ana-lyzed by X-ray diffraction method.

Directions of magnetization were plotted onLambert equal-area projections. All the datawere treated statistically using the methods de-scribed by Fisher [1953J and Wilson [19?9J.

Intensity of Natural Remanent Magnetization

The intensity of NRM (J) measured for allspecimens varies from 2.7 X 10-3 to 2.9 X 10--emu/cc. The average intensities for the fourrock types in emu/cc are: 1.79 X 10-<for pyr-oxenite, 2.45 X 10-<for peridotite, 2.26 X 10-'for dunite, and 13.5 X 10'" for serpentinite.The average values of Koenigsberger ratio (Q)are: \.49 for pyroxenite, 1.40 for peridotite,0.83 ror dunite, and 0.59 for serpentinite. Someperidotite samples, referred to as 'anomalous'peridotites, have relatively low values of in-tensity of NRM (J) and susceptibility (K).The possible causes of their anomalous mag-netization are discussed in another paper [Saad,1969J.

Polished sections, X-ray diffraction, and ther-momagnetic measurements showed that magne-tite,' with Curie temperature of 550° to 580°C,is the main magnetic mineral in these ultramaficrocks.

Directions of NRM

The directions of the natural remanent mag-netization of all specimens from the Red Moun-tain exhibit considerable initial scatter as awhole and within. each site. The largest scatterexists within the serpentinite sites (e.g. SP03,Figure 6a). The most important causes ofscatter are the acquisition of viscous magnetiza-tion, weathering, and tectonic movements ofrocks after being magnetized. The following ex-periments were done to determine which ofthese processes had occurred.

Stability and Reliability Tests

Storage test. The initial magnetization of allspecimens was measured twice in an averageperiod of 1.5 years. Any significant change inmagnetization indicates the existence of softmagnetic components that can be realigned bythe earth's field. The results of the storage testare summarized in Figure 3.

Most of the serpentinites are unstable, withvalues of angular deviation 8 up to 120° andwith J changing by a factor of up to 20. Thepyroxenites and most of the peridotites anddunites are stable with 8 values of 1 to 15°and minor changes in intensity, indicating aconsiderable stability of NRM on the labora-tory time scale. The minor changes in J areprobably due in large part to magnetometerdrift. Figure 3 also shows that the 'anomalous'peridotites have varying stabilities.

In almost all cases, a change in direction wasaccompanied by a comparable change in inten-sity between repeat measurements; on theaverage, the directional changes in degrees wereapproximately equal to the intensity changes inpercent.

AlternaUng field demagnetization. A total of35 specimens were subjected to progressive stepdemagnetization in peak alternating fields of12.5, 25, 50, 100, 200, 400, and 600 oe [As andZijderveld, 1958J. Figure 4 shows some exam-ples of typical demagnetization curves for thevarious rock types and degrees of serpentiniza-tion. The curves are divided into three types.

Type 1 is characteristic of stable magnetizationwhereby more than 50% of NRM remains afterdemagnetization in the 200-oe alternating field.This indicates that most of the magnetizationresides in domains with high .coercive forces.Curves of this type are typical of CRM orTRM [Kobayashi, 1959J. Type 2 curves ex-hibit moderate stability with 10 to 50% ofNRM remaining after demagnetization in the200-oe field, but type 3, with less than 10%of the NRM remaining at 200 oe, shows highinstability. The pyroxenites, in general, arehighly stable (type 1). The peridotites anddunites have curves of type 1 and 2, whereasmost of the serpentinites exhibit maximum in-stability (type 3). The 'anomalous' peridotites,on the other hand, have various degrees of sta-bility (Figure 5); although some are highlyunstable, others are considerably stable (e.g.specimen PD09A1 in Figure 4). In general, thestability of NRM decreases with density. Itwas found that, in almost all cases, the sampleswith unstable ac demagnetization characteristicsalso change magnetically during storage. Theseexperiments indicate that a varying portion of

1.00 •.. _.\ ..•.......•... ~......•..

0.50 '.',," ....•...•.._-,.,.:~~.

0.20 "... . :: ..•.....•...•....•

0.10 •. --~.,., ..•

- Type I: Stobie \0.05 _._._ Type n: Inlermediale \

--- Type m: Unstable ~0.02

~o

l~ 1.00~

SPECIMEN p, gmtcc-9--

DU51BI 2.70

SP33B2 2.63

SP32B2 2.56

SP3BA2 2.59

P031BIPD09AISP40AI

PD24BI

li:.:·~\\"~,\ ..\....•.....\ \ •....'•. 'lj ••••,

...•.~; ....•\ \, , '0

\, '"It '"

\, '. SPI3A2

i i 'r i i SPI4A I25 50 100 200 400 800

ii (OEI

Fig, 4, Typical ac demagnetization curvesshowing stability of NRM for the different ultra-mafic rock types and various degrees of serpen-tinization. Ordinate is the fraction of original in-tensity of NRM remaining after demagnetizationin alternating field of peak value Hoe. P., graindensity; PX, pyroxenite; PD, peridotite; DU,dunite; SP, serpentinite.

15

10

trJ 5zUJ::EU 0UJ0-trJ

u-°20a::lil 15::E~ 10

25 .50 .75 1.00

J (IOO-oe J/Jo

Fig. 5. Histograms of the fraction of originalintensity of NRM remaining after demagnetiza-tion in 100- and 200-oe alternating fields. (Seelegend for Figure 3).

NRM is of viscous temporary OrIgm and re-sides in the lower part of the coercive forcespectrum. '

To remove the unstable temporary compo-nents, all the specimens were subjected to par-tial demagnetization in an alternating field de- .termined by us;ng the criterion of minimumscatter for each site [Irving et 01., 1961J, asillustrated in Figure 6. A field of 100 to 200oe was sufficient to remove the temporarycomponents, as suggested by the reduction inthe scatter within all sites after demagnetiza-tion. Figures 6 and 7 show some typical ex-amples of changes during alternating field de-magnetization.

Consistency among directions, Although thedirections of magnetization within each siteare generally well grouped after partial demag-netization, a few specimens have divergent di-rections (Figure 7b). Of these, five from sitePD05 have a stable magnetization, whereasthe rest are very unstable. The divergence ofthe directions in the case of stable magnetiza-tion is due to weathering. The directions ofmagnetization are displaced toward the presentfield direction, the highly weathered specimensshowing maximum displacement. During sur-face weathering, new magnetic minerals aregenerated or pre-existing minerals are oxidized.Thus the rock acquires a new secondary com-ponent of chemical origin (CRM) that can bevery stable and is in the direction of the

ORIGINAL /13~1MAGNETIZATION

I I I I I + I I I L I I

SITESP03

Fig. 6. Changes in direction of NRM during alternating field demagnetization. (a) Initialdirections of site SP03. (b) Selection of an optimum demagnetizing field for site SP03;progressive demagnetization of three specimens shows that the scatter in their original direc-tions is minimum at 100 oe. (c) Directions of site SP03 after acpartial demagnetization at100 oe; the reduction in initial scatter indicates the removal of viscous components of mag-netization.

present field [Robertson and Hastie, 1962].The unstable specimens, however, have diver-gent directions due to the acquisition of com-ponents of anhysteretic remanent magnetization(ARM) during the demagnetization process .

This is indicated by the repeat-demagnetizationexperiments explained earlier and by the in-crease in intensity of magnetization' and thewandering in directions at higher fields (e.g.SP40A1, Figure 4).

...-.•..•. :l ......•....•• Ie

Circles- Site PD05Triangles _ Site OUI2

-.-.••• ••

DUI2 ,•. --~7,,181[ A- A~I .1i .•.& J

at 2000e .•. /, I

I '1~'"; +! I I I I I I I I"" __~/ ~)( Highly Weathered

"Unstable ~---'i::-,~'Slightly Weathered

(~ \-e. "".1at 200 oe\ • /.

"L-PD05

SOUTH SOUTH

SOLID, LOWER} * MEAN DIRECTIONOPEN, UPPER HEMISPHERE 181 PRESENT FIELD

Fig. 7. Typical examples of grouping of directions after ac partial demagnetization andrejection of unstable (crossed triangles) and weathered (crossed circles) specimens.

All specimens within each site, excluding thosewith divergent directions due to any of theabove criteria, show consistency among thcirdirections. Such inte~al consistency, togetherwith the divergence of the mean directions ofeach site from that of the present earth's field,are indications of stability and nonviscous originof magnetization .

.Tilt test. Although the previous criteria as-sisted in testing the stability of the directionsand in reducing their scatter within each site,the mean directions of the sites are significantlyscattered as shown in Figure 9 (open circles).However, the rocks· from which the sampleswere obtained are considerably tilted and folded.A plot of these directions along a profile per-pendicular to the synclinal axis of the RedMountain shows a pronounced systematic changein the mean directions across the structure(Figure 8), thus· suggesting the necessity ofapplying Graham's tilt test [Graham, 1949].The test has great significance in providingvaluable information regarding the time of theacquisition of. magnetization, its stability andpossibly its origin~

The average strike and dip of the rocks ateach site were determined from the regionalgeology of the area by interpolation from zerodip over the axis of the syncline to the strikeand dip of the neighboring sediments at themargin of the concordant intrusion. Using thestrikes and dips estimated by this procedure andgiven in Table 1, Graham's tilt test was appliedto the mean directions of all sites. The resultsof the test are shown in Figure 9. With theexception of five sites (Figure 9b), the scatterof the mean directions is reduced markedly on

..:..<:..:~:::F~'on'~is~'dn·.Formation·.

.... .

.<..< .

>:',~;,,;4Fig. 8. Change in mean directions of NRM

across the Red Mountain syncline. Structure isinferred. Arrows indicate directions of magnetiza-tion projected onto a plane perpendicular to thestructural axis. Numbers refer to sites (Figure 2).

Fig. 9. Graham's tilt test applied to mean sitedirections of magnetization. Open circles, beforetest; solid circles, after test; all points exceptcrossed ones, lower hemisphere. Tilt of site PD16,uncertain.

correcting for the inferred dip and strike. Thisindicates that the magnetization of these rockspredates the folding and has not changed sincethen, thus providing substantial evidence forthe high stability of the magnetization. It isevident that these results exclude any post-folding thermoremanent magnetization (TRM)as an origin of the magnetization. A pre-foldingTRM can be acquired under two circumstances:

1. During the cooling of the original perido-tite magma below the surface of the crust, theultramafic body acquired a stable but weakTRM due to the scarcity of magnetic mineralsin the fresh rocks. Howdver, the directions ofmagnetization would be expected to be scattered(after the tilt test) due to tectonic emplaCE?ment.

2. The intrusion was strongly magnetizeddue to serpentinization but then was reheatedto its Curie temperature and acquired a TRMon later cooling. This is unlikely since reheatingof the rocks above 500°C causes deserpentiniza-tion. Moreover, a second episode of sepentiniza-tion will cause scattering in the directions, incontradiction to the results of the tilt test.

In summary, the results of the tilt test ex-clude TRM as the origin of magnetization .Thus, the remanent magnetization of the RedMountain ultramafics is probably CRM de-veloped during the serpentinization of the bodyprior to its folding. Except for the pyroxenites,the effect of the initial TRM is negligible either

Strike, Dip, Field,deg deg oe N

BeforeDR IR

AfterDR IR

PDlA N35W 14NE 200 4 3.965 91.8 4.6 92.0 -8.0 86.2 10.0 7.6 3.8PDlB .N35W 14NE 100 12 11.643 42.8 67.6 45.0 56.0 30.8 7.9 14.0 4.0P002 rejected 100 4 2.709 220.5 29.9 2.3 79.5 53.2 26.6SP03 N75W 60NE 100 5 4.936 87.8 9.8 87.5 -9.5 62.1 9.8 9.2 4.1PD04 N58W 14NE 100 16 15.339 50.2 68.5 44.0 56.5 22.7 7.9 16.5 4.1P005 N74W 72NE 200 5 4.940 182.4 42.9 38.6 62.0 66.8 9.4 8.9 4.0PD06 N75W 55NE 200 2 1.998 171.6 56.9 45.0 61.5 413.5 12.3 2.8 2.0PD07 N77W 30NE 200 2 1.989 100.6 75.3 41.1 55.0 89.1 26.8 6.1 4.3PD08 N80W lONE 200 2 1.996 354.6 75.7 359.0 65.8 241.9 16.1 3.7 2.6DU9A N80W 25SW 200 1 3.8 47.4 353.1 71.8DU9B N70W 32SW 200 4 3.969 37.1 34.9 54.8 64.2 96.1 9.4 8.3 4.1PXlO 0 200 2 1.987 113.3 9.9 113.3 9.9 76.5 29.0 6.6 4.6DUll N75W lOSW 100 2 1.985 1.2 65.4 354.1 74.5 65.2 31.5 7.1 5.0DU12 N87W 5SW 200 9 8.794 334.6 73.4 329.9 78.0 38.8 8.4 12.3 4.1PX13 N80E lOSE 200 5 4.911 52.4 29.1 58.0 33.8 44.8 11.5 10.8 4.8DU14 N70E 30SE 200 2 1.998 339.0 50.3 339.0 81.5 483.7 11.4 2.6 1.8PDl5 N60E 70SE 200 1 42.3 37.4 93.2 25.-5PD16 N40E 80SE 200 2 1.999 315.1 -6.2 329.5 74.0 968.2 8.0 1.8 1.3SP17 rejected 0 30 21.717 23.1 86.9 3.5 16.6 43.3 7.9

Notes.N' Number of samples (number of specimens for rejected sites).R Vector sum of N unit vectors.DR Declination of the mean vector.IR Inclination of the mean vector.k Fisher preeision parameter = (N - 1)/(N - R).A95 Radius of 95% confidence cone about direction of R.ASD Angular standard deviation [Wilson, 19;)91.SDM Standard deviation of the mean [Wilson 1959].Before and After refer to tilt test.Field refers to peak value of demagnetizing field in oe.

because it is weak compared with the strongCRM, or possibly because it was destroyedduring serpentinization. The pyroxenites, beingslightly serpentinized, may have an initial TRMstronger than the later CRM.

PALEOMAGNETIC ANALYSIS

Analysis of average directions. The averagedirections of magnetization and precision pa-rameters for each site were computed using themethods described by Fisher [1953] and Wilson[1959]. The statistical methods were appliedtwice lQn two levels. On the first level, samplemeans' were obtained by using the methods onspecimen observations from a sample givingeaeh unit weight. On the second level, eachsample mean from a site was given unit weightto obtain site means. This procedure was usedbecause specimen-to-specimen variations aregenerally smaller than variations among sam-

pIes although in some cases the first are as largeas the latter [Irving, 1964, chap. 4]. Otherprocedures have been tried, e.g. giving unit·weights to each specimen to find site means. Inall cases, the differences between the site meansoptained were not more than 20

• Table 1 showsthe statistics on the mean site directions ob-tained by giving a unit weight to each samplemean.

The mean site directions, in general, are foundto fall into three classes. The first class includestwo sites (PD02 and SP17) with widely scat-tered directions, as indicated by a very low pre-cision parameter k. The cause is uncertain forsite PD02, although large errors in measuringthe small moments of these samples and largeamounts of ARM acquired during demagnetiza-tion may be responsible. Site SP17 includes allserpentinite specimens collected from the smallsill south of the main body (Figure 2). These

specimens are characterized by unstable inten-sities, scattered directions, and very rapidchanges in both direction and intensity of NRMwhile being measured in the spinner magneto-meter; the changes are more pronounced as therock becomes more demagnetized. The phe-nomenon is due to a very soft component ofmagnetization, with short relaxation time ofthe order of minutes or seconds superimposedon other components in the rock. Anotherfeature of the serpentinites is the pronouncedwandering of directions from one hemisphereto the other during the process of demagnetiza-tion at higher fields. .

The second class of results is characterizedby magnetization possessing stability and con-sistency within each site yet having a meandirection that is inconsistent with other sitemeans after applying a tilt correction (Figure9b). The divergence of these means can beexplained by the occurrence of one or both ofthe following processes: (l) the magnetizationwas acquired at a time different from that ofthe main body, or (2) the divergent sites wererotated after being magnetized. These sites in-clude two pyroxenite sites (PXlO, a vein, andPX13, a pod) possibly intruded later into themain ultramafic body at different times. Threeother sites that belong to this class are ser-pentinites and serpentinized peridotites. Theyare near the fault contact of the body and mayhave rotated. However, in spite of the differencein rock types and the wide geographic separa-tion between them (Figure 2), all have aboutthe same anomalous direction, which is possiblya reflection of an anomalous paleomagnetic fielddirection.

All other sites (third class) have stable mag-netic directions that are mutually consistentafter the tilt correction (Figure 9a). The meandirections, however, form two slightly separatedgroups, one mostly dunite and the other perido-tite. The F-ratio test was used to compare theprecision parameters of the two groups and alsoto compare their mean directions [Watson andIrving, 1957]. The test showed that the twoprecision parameters are identical and that theseparation of the two mean directions is signifi~cant. This separation supports the view ofHawkes et ai. [1942] that the dunite was in-truded at a time subsequent to the intrusion ofthe peridotite and may reflect also a change inthe paleomagnetic field direction.

Paleomagnetic pole positions and comparisonwith other data. To calculate the paleomag-netic poles for the purpose of comparison withother data, the sites are treated statistically asthree independent groups: the peridotites, thedunites and the divergent sites. The results ofthe statistical analysis shown in Table 2 wereobtained by giving each mean site direction aunit weight. Paleomagnetic poles correspondingto each group-mean direction are given inTable 3, together with three late Mesozoic polesreported from California [Gromme et ai., 1967].

It is evident that the Red Mountain paleo-magnetic poles, particularly poles (1) and (3),have anomalous easterly positions compared toall other published Jurassic and Cretaceous polesfor North America that have longitudes between90° and 2700W [Irving, 1964]. The only com-parable pole for this time with easterly longi-tude has been obtained by Gromme and Glus-koter [1965] from some diabasic and spilitic

(1)(2)

j (3)), . (1) + (2)

59.374.610.868.2

44.5350.089.928.8

5.9834.9724.566

10.702

4.76.7

26.714.0

3.96.426.68.0

1.93.0

11.94.2

Notes.(1) Peridotite sites.(2) Dunite sites.(3) Other divergent sites (including pyroxenite sites).N Number of sites.Other symbols as in Table 1.

·AFIF BANI SAAD

TABLE 3. Paleomagnetic Pole Positions

Pole NLat. WLong. DM DP Age (101Years)

(1) 55.6 49.9 5.8 4.3 Upper(2) 65.6 132.0 11.6 10.5 Jurassic to(3) 3.4 35.8 27.0 13.7 Upper

(1) + (2) 65.5 75.1 13.4 11.3 Cretaceous(80-140,

F 26 50 17.8 11.5 estimated)J 53.7 174.1 9.5 9.5 138C 69 165 9.6 9.6 84

Red Mountainultramafics

Franciscanformation

SieM-aNevadapll1tonS

Notes.(1), (2), (3)F,J,C

Poles calculated in the present study from information in Table 2.Upper Jurassic and Cretaceous poles from California (for comparison). F: Gromm~ andGluskoter [1965]; J, C: Gromm~et al. [1967].Sem4xes of 95% oval of confidence.

bodies in the Franciscan formation (late Juras-sic-late Cretaceous) in California (pole F, Fig-ure 10). Pole (3) from the divergent sites ofthe Red Mountain is located close to the anom-alous Franciscan pole; poles (1) and (2) fromthe peridotite and dunite groups respectively arelocated between this anomalous position and thecurrently accepted positions of the Jurassic andCretaceous poles for western U.S.A. (poles Jand C, Figure 10). Pole (2), however, is veryclose to pole C from the Sierra Nevada granitesof Cretaceous age [Gromme et al., 1967J; theoverlap of their ovals of confidence indicatesthat they are not significantly different at the95% probability level.

There are two other poles that have beenreported with anomalous easterly directions.The first is from lower Eocene Siletz Rivervolcanics of western Oregon [Cox and Doell,1960J and has a pole position of 37°N, 49°W,similar to the Franciscan pole but with 10°higher latitude. The other pole is from lateMesozoic dolerites from Spitsbergen [Spall,1968J at 600N, 2°E. The question arises as towhether these easterly directions of the mag-netic field found from Siletz River volcanics,Fran9scan formation, Spitsbergen dolerites, andR~ Mountain ultramafics are due to tectonicrotatIon of the sampling sites or real excursionsof the geomagnetic field. The paleomagneticdata from the Red Mountain suggest that thegeomagnetic pole, assuming a mainly dipolarfield, migrated southeastward into the AtlanticOcean and back on one or more occasions dur-

ing late Mesozoic and probably early Eoceneas proposed previously by Gromme and Glus-koter [1965J. In every case mentioned above,there is no evidence other than the magneticdata in support of tectonic rotations. Further-more, the fact that similar data have been ob-

Fig. 10. Paleomagnetic poles from the RedMountain ultramafic body (circles) and other lateMesozoic poles (squares). (1), {2), and (3), polesfrom peridotite, dunite, and pyroxenite and mar-ginal sites, respectively (present study); F, polefrom Franciscan formation [Gromme and Glus-kater, 1965]; J and C, poles from Sierra Nevadaplutons of upper Jurassic and Cretaceous, respec-tively [Gromme et al., 1967], 95% confidenceovals or circles are shown solid for present studyand dashed for others. X indicates sampling loca-tion.

tained from Spitsbergen in another continentindicates a worldwide phenomenon related tothe geomagnetic field itself and not to localtectonic rotations.

Although accurate dating of the Franciscanformation, including the ultramafic rocks, isnecessary for evaluating any paleomagneticdata, some conclusions regarding the emplace-ment and magnetization of the Red Mountainbody can be summarized in light of the abovementioned hypothesis as follows:

1. The margins of the body were magnetizedfirst (by serpentinization) during or immedi-ately after emplacement at a time closer toFranciscan magnetization time when the mag-,netic pole was in one of its excursions into theAtlantic Ocean (pole 3, Figure 10).

2. The main body (peridotites) acquired itsmagnetization at a later time during which themagnetic pole was recessing toward its normalposition (pole 1, Figure 10).

3. The dunites were emplaced and mag-netized at a much later time when the magneticpole was near its normal position, possibly inthe Cretaceous (pole 2, Figure 10).

4. The pyroxenites, which intruded thedunites as veins and pods, were magnetizedafter the dunites, possibly in the Eocene timeduring which the magnetic pole was in anotherexcursion as suggested by Gromme and Glus-koter [1965] (pole 3, Figure 10). It is alsopossible that the anomalous directions of thepyroxenites are due to the stronger effect of theinitial TRM on the later CRM.

CONCLUSIONS

Results of the tilt test, applied to the averagedirections of magnetization, support the con-clusion of the intensity study that the remanentmagnetization observed in the Red Mountainultramafic rocks is CRM acquired during theserpentinization of the body. The results also

~showed that the body was emplaced and ser-), ; pentinized prior to folding, a finding of im-

, portance to the geologic history of the Coast.Ranges but difficult to obtain geologically

[Bailey et al., 1964, p. 83].In using ultramafic rocks for paleomagnetic

work, only the partially serpentinized are reli-able; the highly serpentinized are magneticallyunstable. Paleomagnetic pole positions, found

from the study of partially serpentinized ultra-mafics of the Red Mountain, indicate relatively.rapid changes in the geomagnetic field duringthe late Mesozoic, thus confirming the hypoth-esis advanced by Gromme and Gluskoter [1965].

Acknowledgments. I am gratef1l1 to ProfessorAllan V. Cox for his supervision, valuable sug-gestions, and informative discussions during thestudy, and for his careful reading and editing ofthe manuscript. I would like also to thank Pro-fessor George A. Thompson for his continuousencouragement, advice, and reviewing of the man-uscript.

A portion of this work was supported by fundsfrom the Geophysics Department of StanfordUniversity. .

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(Received October 8, 1968;revised August 4, 1969.)