Simple Shear of Southern California During Neogene

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Dec 10, 1985 - lowed by a late Miocene or Pliocene counterclockwise rotation of 15 °. ... rotated but Santa Catalina has undergone about 90 ° of clockwise rotation. ... plate interactions both rifted the continental crust to create this pattern and rotated the western- .... interpretation of their origin may be that they formed in.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. B14, PAGES 12,454-12,466,DECEMBER 10, 1985

Simple Shear of Southern California During Neogene Time Suggested by PaleomagneticDeclinations BRUCEP. LUYENDYK, ]V[_ARC J. KAMERLING, 1 RICHARDR. T ERRES, 2 AND J. SCOTT HORNAFIUS3 Departmentof GeologicalSciences,Universityof California, Santa Barbara

We have studiedthe paleomagnetismof variousNeogeneage rocks in southernCaliforniaas a means of determiningthe amounts of Oligoceneand youngertectonicrotation and translation which has occurredin this region. Our resultssuggestthat fully 25% of this area, in particularthe TransverseRanges,has undergoneextreme clockwiserotation. Work in southeastern California impliesthat 40ø of clockwiserotation has occurredhere, althoughpaleomagnetic declinationsadjacent to a major right lateralfault are apparentlyrotatedover 200ø. The crustalblockboundedby the San Gabriel and San Andreas fault has undergonea net clockwiserotation of 35ø althoughthe data here can be interpretedto show that an originalearly Miocenerotation of about 50ø was followed by a late Mioceneor Pliocenecounterclockwise rotationof 15ø. Paleomagnetic resultsfrom the offshoreislandssuggests that San Clemente, SantaBarbara,and San Nicolasislandshave not rotated but Santa Catalina has undergoneabout 90ø of clockwiserotation. All of the northern Channel Islands,includingAnacapa,SantaCruz, SantaRosa,and San Miguel, are impliedto have rotated 70ø or 80ø clockwise. This result is also found for the Santa Monica Mountains east of

AnacapaIsland. In the SantaYnez Rangenorth of the ChannelIslands,paleomagnetic studyof the Monterey Formationalsoindicateslargedockwiserotationsof near 90ø. Thesedata alsosuggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphiccontrolon the paleomagnetic data from the MontereyFormation impliesthat the rotationbeganabout 16 m.y. ago and may be continuingtoday in the western

region. Paleomagnetic inclinationdatafrom our studyshowthat the northernChannelIslands,in particular,may have translated15ø northwardsincemiddleMiocenetime. However,equallyvalid interpretations of thesesamedata are that the low inclinations are due to the combinedeffectsof erroneousstructuralcorrections,non dipole magneticfield behavior and right offset on the San Andreas fault system. Palinspasticreconstructionof southernCalifornia regionsfor the early Miocene impliesthat partsof the TransverseRangesstructureswere oncealignedwith north trend-

ing extensionalstructuresin the southwestern United States. We proposethat Pacific-American plate interactionsboth rifted the continentalcrust to createthis pattern and rotated the westernmost structureswithin a dextral simple shearzone whichhad a half width of about 400 km.

dated into clockwise tectonic rotation: (1)

INTRODUCTION

Since

1975

we

and our

associates have

studied

the

continuum

mechanicsin the form of penetrativeshear,(2) rotationof

paleomagnetismof Neogene (mainly Miocene) volcanic, small crustal blocks loosened by extension in pull-apart hypabyssal, and sedimentary rocks throughout the basins,(3) rotationof blocksboundedby conjugateshear Transverse Ranges in southern California. Paleomagnetic faults, (4) distributedshear in the ductilesubcrustalmantle, (5) rotationduring breakupand accretionof oceanic directions from wide areas in southern California show deflections suggestive of clockwise tectonic rotations. plates, (6) differential back arc spreadingand basin and These areas include the western TransverseRanges, some rangeextension,and (7) translationand rotationof conof the offshore islands, and parts of the eastern tinental microplates. Originally, we proposedmechanism 3 or 7 for rotation TransverseRanges(Figure 1). The purposeof this article is to review and update the results of our work over the past several years. This apparent clockwise rotation is believed to be due in

some way to the northwest-southeastoriented right-shear couple which has existed between the Pacific and North

and northward

motion

of the Santa

Monica

Mountains

region [Kamerlingand Luyendyk, 1979]. However, as more results became available, we noticed that

the

apparentrotation was confined(largely) to crustalblocks bounded by east-westtrending left-slip faults. This led us to favor a mechanism similar to 3 above and to predict

American plates since the Oligocene [Beck, 1980; Cox, 1980; Luyendyket al., 1980]. Cox [1980] reviewed seven

other regionswhere Neogene rotations may have occurred

methods by which this dextral shear could be accommo-

[Luyendyket al., 1980].

1Now at ARCO ExplorationCompany,Denver, Colorado. 2Now at Chevron OverseasPetroleum,Incorporated,San _

Ramon, California.

To discriminate among the various models, much more information is needed on the amount, areal extent, and timing of the rotations, together with more information on the regional geology, especially the sense, amount, and timing of fault slip and on the timing of basin formation

3Nowat Mobil Oil Corporation, Denver,Colorado.

(see Luyendyket al. [1980] for discussion).It has been

Copyright 1985 by the American GeophysicalUnion.

our goal since our initial discovery of anomalous paleomagneticdeclinations in the Santa Monica Mountains

Paper number 4B5166 0148-0227/85/004B-5166505.00

[Kamerlingand Luyendyk,1977] to definethe amount and areal extent of rotation and translation as indicated by 12,454

LUYENDYKET AL.. NEOGENESHEARIN SOUTHERNCALIFORNIA

56 ø N

-Jr

12,455

I

+

FAULT

MJ Morro

'%. '%

Ba

Mojave Desert

35 ø

+

+

LH San

Gabriel ck

sY

••.,•L, anta

Basin



•,

SRI

'•

SCIN

•e-%•/•.•_•

34 ø



••

+

••c,s.

• ETR' DB (NE) SBI

•CAI

+

33 ø N

121 ø

120 ø

119 o

118 ø

+

+%% • +

117 ø

116 ø

I1• øW

Fig. 1. Paleomagnetic declinationsmeasured in Neogene age rocks at sites throughout southern California. The mean declinationat the site is shown along with the 95% confidencelimit on the mean. Faults are from Jennings

[1973]. Site a is from Greenhaus and Cox [1979], b is the Mioceneresultfrom Kanterand McWilliams[1982], and c is from J. Morton and J. Hillhouse(unpublishedmanuscript,1983). Site keys are MJ*, QuaternaryMojave; ETR, easternTransverseRanges;DB (NE), northeastDiligenciaBasin;SB, SoledadBasin;PR, Plush Ranchformation; CM, Chico Martinez; SLI, San Clemente Island; CAI, Catalina Island; SNI, San Nicolas Island; SBI, Santa Barbara

Island;SMI, San Miguel Island;SRI, SantaRosaIsland;SCIN, north SantaCruz Island;'SCIS, southSantaCruz Island; ANI, Anacapa Island; SMM, Santa Monica Mountains; SY, Santa Ynez Range; LH, Lions Head. LA is the city of Los Angeles.

paleomagnetic results from Neogene rocks. However, information on the timing of the rotation has also appeared from our work in the Santa Monica Mountains

and San Gabriel region [Terreset al., 1981; Terres,1984] and Santa Ynez Range [Hornafiuset al., 1981], and it appearsthat the rotation began in early to middle Miocene time and may be continuing today. Besides declination anomalies, we have also measured

significantlydifferent from the Miocene reference pole. In the following text we review our results from three regions within southern California where we have focused our work over the past several years: the interior basins, the offshore islands, and the western TransverseRanges. INTERIORBASINS

Work

in the interior basins represents the work of

anomalously low inclinations (compared against the

Terres [1984]. Three localitieswere studied: the Diligen-

Miocene and Oligo-Miocene reference poles of Irving [1979]). These inclination anomalies may imply north-

cia Basin on the east side of the San Andreas fault (DB

ward translationsfor several regions (Figure 2). These translations are greater than would be predicted from known geologicrelations but in some ways support terran e models for California [Blake et al., 1982; Vedderet al.,

1983; Bottjerand Link, 1984]. The possiblecausesof the

(NE) in Figures 1 and 2), the SoledadBasin west of the San Andreas(SB in Figures1 and 2 [Terresand Luyendyk, this issue], and the LockwoodValley north of the Big Pine fault and west of the San Andreasfault (PR in Figures 1 and 2). These basinscomprisesequencesof Oligoceneto early Miocene continental deposits and Oligocene and

anomalouslow inclinations along with causesof the scatter early Miocene volcanicrocks. Crowell [1962] and Bohanof declinations

non [1976] recognizedthat the basinscould be juxtaposed

are discussed later.

In addition to volcanic and hypabyssalrocks we have worked extensively with the Miocene Monterey Formation in the Santa Ynez Mountains and Santa Maria Dolostones from this formation have shown

basin. stable

by removing Miocene and later right slip on the San

Gabriel and San Andreas faults. Also, Bohannon[1976] suggestedthat the basinswere controlled by approximately east-westtrending normal faults which formed in response

remanence, and both fold tests and reversal tests have to Oligocene north-south extension. Becausewe believe been applied successfully. The results from sites in the that these basins have been rotated clockwise, the correct Santa Ynez Range and from Santa Cruz Island agree interpretation of their origin may be that they formed in closelywith the clockwisedeclination anomaliesmeasured responseto approximately east-west extension. in volcanic rocks in the Santa Monica Mountains and In the Diligencia Basin we concentrated our work on northern Channel Islands. Inclination data for the Monlava flows within the Diligencia Formation. The volcanics terey Formation are slightly too low but are not usua!ly have been dated at 23.0 to 19.1 Ma. [Crowell, 1973;

12,456

LUYENDYKET AL.: NEOGENESHEARIN SOUTHERNCALIFORNIA

56 ø N

+

+

FA U LT

Morro

Mojave

CM

35 ø

t MJ

%,

+

San

Gabriel Block

'•'• Santa M •-• Basin

16,

tl

34 ø

(•'•'• • SRI

degrees -) •o•,h

DB (NE)

½_SBI ••,, SLI +

degrees

north 33 ø

+

+•

117ø

116ø

i

1

119 ø

118"

Fig. 2. Impliedtranslations calculated from the inclinationanomalies measured for the sitesin Figure1. The error barsare the 95% confidencelimit on the translationcalculatedaccording to the methodof Cox [1980]. Solidarrows are thosedatawith statistically significanttranslations.Site keysare as in Figure 1.

Figures1 and 2). Basaltflowsin the Vasquezformation Spitgler and Arthur, 1982• The sampling area includes 70range in agefrom 24.9 to 20.9 Ma [Crowell,1973; Croweet km"

on the east side

the Clemens

Well

fault.

This

fault is a major Neogene fault which may have up to 100 km of right-offset [Powell,1981]. It is subparallelto the

al., 1979]. Seven sitesfrom the centralregionof the

San Andreas fault and located about 20 km east of it. Our

significanttranslation[Terreset al.; Terres,1984]. Closer

sampling sites are distributed between 1 and 6 km perpen-

to the San Andreas fault the declination anomalies are

dicular to the fault trace.

greatly different. Two sites adjacent to the fault show a

All of the volcanic rocks here are normally magnetized, although some of the sedimentary interbeds have a reversed polarity. Both the fold test and baked contact

test were passedby the paleomagneticdata [Terres,1984]. The paleomagneticdata show very large apparent clockwise rotation and significant inclination flattening. Close to the fault, clockwise rotations of over 200ø are suggested,decreasingto 100ø 4 km away [Termset al., 1980]. Deformation

associated with the Clemens Well fault must

be responsible for some or all of the apparent rotation.

study area show 37.0 ø of clockwise rotation and no

more extreme clockwise rotation of between 80 ø and 130 ø.

Possiblythis local anomaly is due to dextral shear near the fault as we saw in the DiligenciaBasinnear the Clemens

Well fault. However, anothersite adjacentto the fault, with six basaltflows, showseither a slight counterclockwise rotation or a clockwise rotation of over 300 ø. These

results also may be from younger flows which have been rotated counterclockwiseafter being rotated clockwise. The Mint Canyon Formation in the Soledad Basin comprises thin volcanic ash beds and continental sedimen-

Terres[1984] has separatedthe resultsinto two groups: tary rocks. Tuff beds in the upper Mint Canyon Formaone result for sites closer to the fault and another farther

away.

The average rotation indicated for the farther

tion have been dated at 11.6 and 10.1 Ma (J. D. Obrado-

vich and T.M. McCulloh, personalcommunication,1982).

group,DB (NE), is 109.4ø [Terres,1984] (Figure1).

Declinations from six welded ash beds at three sites show

Several sites in the Eagle and Pinto Mountains, about 30 km north of the DiligenciaBasinare now being studied (ETR in Figures 1 and 2). Here we measured the

16.0ø _+ 30.0ø of apparent counterclockwise rotation

paleomagnetismof Cretaceous dikes and lava flows of late

[Terres,1984;TerresandLuyendyk, this issue].Although this resultis statisticallyrather weak, it is not inconsistent with other data. Garfunkel, [1974] suggestedthat the Mojave blocks, east of this study area, experiencedcounterclockwise rotation in Neogene time. This rotation

Miocene and Pliocene age. These data suggestthat this region has undergone about 40ø of net clockwiserotation since the late Miocene [Terres,1984; Carteret al., 1983] and also imply that the large declination anomalies seen for the Diligencia Basin are a local effect [Carter et al.,

which is observedtoday. Preliminarypaleomagnetic data from the Mojaveregionsuggestthat this bendingoccurred

1985]. In the region of the Soledad Basin between the San Gabriel and San Andreas faults, volcanic rocks from the Vasquez and Mint Canyon formations were studied (SB in

prior to Pliocene time, when the San Gabriel block was south of its presentlocation[Burkeet al., 1982; J.L. Morton and J.W. Hillhouse, unpublishedmanuscript,1983]. Therefore the entire San Gabriel regionmay have had an

would have bent the San Andreas fault into the left bend

LUYENDYK ET AL.: NEOGENE SHEARIN SOUTHERNCALIFORNIA

12,457

older clockwise rotation followed by a younger clockwise Islands. Kamerling [1985] and Kamerlingand Luyendyk rotation as it moved north into the bend region. This [this issue] noted that this difference is statistically suppositionimplies that the clockwise rotation was origi- significant and that southem Santa Cruz Island is a separate tectonic block, in the sense that it has behaved nallyas muchas 49.2ø (37.0ø + 12.2ø).

The PlushRanch formation (PR in Figures1 and 2) is

differently from its structural surroundings (see also

a sequenceof upper lower Miocene basalt flows and continental sedimentary rocks in Lockwood Valley north o f the Big Pine fault. Several flows from this formation were

Howell et al. [1974]. The paleomagnetic data from all the northern islands and the Santa Monica Mountains are almost equally

studied;they range in age from 19.6 to 17.4 Ma [Crowell, 1973]. The paleomagneticresultsfrom these flows indicate 6.4ø + 12.5ø of counterclockwiserotation [Terres, 1982, 1984]. Three possibleexplanationsfor this result are suggested:(1) that the LockwoodValley is not rotated as are the other basinsto the southeast,(2) that the Lock-

dividedas to polarity[Kamerlingand Luyendyk,this issue]. When the resultsfrom south SantaCruz Island (reversely magnetized)and from tuff beds on SantaRosa Island are

eliminated, 27 normal and 25 reversed units remain. When these different polarity groups are averaged separatelya remarkable result is obtained. The average of wood Valley has been subjectedto counterclockwiserota- reversedand normal units is exactlythe •ame (27 normal tion during northward translation into the left bend region units have I = 35.6_+8.2; D - 71.3_+ i0.1; 25 reversed of the San Andreas fault, after an earlier episode of units have I = -36.5_+6.3;D -- 253.3_+7.8[Kamerling, regional clockwise rotation similar to that undergone by 1985; Kamerlingand Luyendyk,this issue]. This analysis the Soledadand Diligencia regions;this would require the speaks to the high quality of these data or to a remarkable later counterclockwiserotation to be larger than the earlier coincidence: the reversal test is passedfor units separated clockwise rotation so that the earlier rotation was erased, by over 100 km, units which are in different structural setand (3) that the LockwoodValley is clockwiserotated like tings and which differ in age by more than 10 m.y. the other basins but the structural corrections which we Besides the strong declination anomaly evident for this employedare incorrect. The flowswe sampledare all northern region, the paleomagneticresults show significant steeply dipping and strike east-west. We did not observe inclination flattening, implying from 8ø to 20ø of apparent any significantplunge to structures in the region, so we northwardtranslation(Figure 2). The southern islands, with the exception of Catalina, do did not correct for it. If in fact any plunge does exist, then our declination result would be greatly affected. At not demonstrate discordant paleomagnetic declination or present, the Plush Ranch results must be considered inclination [Kamerlingand Luyendyk,1981]. Data from Santa Barbara, San Clemente, and San Nicolas Islands ambiguous. yield a mean direction which is statisticallyidentical to the OFFSHOREISLANDSAND SANTA MONICA MOUNTAINS Miocene expected paleomagnetic direction. However, A paleomagneticstudy of mainly Miocene volcanic, Santa Catalina Island yields a direction which is rotated

hypabyssal, and sedimentary rocks frcrn the islands clockwisefrom the expecteddirection by 'over 100ø and offshore southern

California

and from the Santa Monica

Mountains northwest of Los Angeles was undertaken by Kamer#ng[1980, 1985]. The islands of Anacapa, Santa Cruz, Santa Rosa, and San Miguel Form an east-westtrending structural ridge and are considered part of the Transverse Range Province. The islandsof Santa Catalina, Santa Barbara, San Nicolas, and San Clemente generally have northwest-southeast trending structuresand are part of the Peninsular Range

from the mean of the northern Channel Islands (exclud-

ing south SantaCruz Island) by over 25ø. Paleomagnetic directions from Eocene sedimentary rocks from San Nicolas Island and Miocene

volcanic

rocks frcrn Santa Barbara

Island are deflected counterclockwiseslightly from the expected directions, but the deflection is not statistically significant. These results suggest that the northern Channel Islands have been tectonically rotated into place during Miocene

Province. Oligocene (?) through Miocene volcanicscrop

time

as the outer

borderland

area translated

northwest-

out extensively in the Santa Monica Mountains and on all of the islands except San Nicolas Island. Due to this wide distribution and the typically excellent paleomagnetic results from volcanic rocks, these rocks were selected for

ward within a large shear zone between the Pacific and

North American plates [e.g., Crouch, 1979]. Perhaps

Santa Catalina Island rotated as a ball bearing would if caught between blocks within this shear system. Southern paleomagnetic study. In addition,Eocenesandstones were Santa Cruz Island may have behaved similarly. The shalsampled extensively on San Nicolas Island and less so on low paleomagnetic inclinations suggest a northward latitude translation of about 15 ø for the westem Transverse San Miguel and Santa Cruz islands. The results from units which passedthe fold, reversal, Ranges and 3ø for the southern islands (excludingSanta and baked contact tests revealed three and possibly four Catalina). The southern islands data are more consistent main groupings of paleomagnetic directions. Directions with the amount of northward translation calculated from from the northern Channel Islands and Santa Monica plate tectonic reconstructions. _

Mountains

are deflected

clockwise

68 ø to 131 ø from

the

I•ESULTSFROMSANTAYNEZ RANGE

expecteddirection and are too shallow in inclination by 8ø

to 27ø [Kamerlingand Luyendyk,1979, this issue•Kamerling, 1985]. The rotation anomalyfrom SantaCruz Island south

of

the

Santa

Cruz

Island

Fault shows

a much

strongerclockwisedeflection• 131ø, or over 50ø more than the

mean

declinations

for

the

other

northern

Channel

ANDSANTA M Am B•.sIN(MONTEREY FORMATION) The

Santa Ynez

Range portion of

the western

TransverseRanges (SY in Figures1 and 2) extendsfrom Ventura west to Point Conception. It comprisesa south dipping homocline of Mesozoic and Tertiary sedimentary

12,458

LUYENDYK ETAL.: NEOGENE SHEAR IN SOUTHERN CALIFORNIA

rocks which were uplifted in Plio-Pleistocene time [Dib- significanceof the inclination anomalies. The average

blee, 1966]. A study of the paleomagnetism of the rotationindicated for the westemTransvers6 Rangesis Miocene Monterey Formation was undertakenby lessthan90ø. However,a rotationof 120øto 130ø maybe Hornafius[1984, this issue]. The Monterey Formation in coastalsouthern California is several hundred meters thick and from about 16 to 6 m.y. old (about middle to late Miocene [Obradovich and Naeset, 1981;Isaacs, 1983]. It consistsprimarilyof diato-

neededto restorethe trendsof Paleogenerocksin these rangesto a configurationparallelto the alignmentof other lithic belts in California [Joneset al., 1976; Crouch, 1979; Howelland Vedder,1981]. Someregionsexhibit an anomaly this large, for instance,South Santa Cruz Island.

maceous shales deposited under deep marine anoxic con-

The reasons a larger anomaly is not generally detected

ditions [e.g., Pisciottoand Garrison,1981]. A reconnais- may be that (1) therewassignificant rotationpriorto midsancestudyrevealedthat only bedsof secondarydolomite dle Miocenetime, (2) that localcounterclockwise rotations within the formation possessa stable remanencedirection.

due to left-lateral wrenching are superimposed on the

The quality of the magneticresults from the Monterey older clockwiserotation [Kamerlingand Luyendyk,this Formation are unusuallygood. In most of the sections issue], (3) that the rotationoccurredduringthe eruption whichwere studied,fold or reversaltestswerepassed,and and depositionof the units we studied,or (4) simplythat in several instancesboth tests were successfullypassedat

the samesite [Hornafius,1984]. In all, 20 siteswere studled, comprising106 individual dolomite beds.

most regionswere never rotated more than 90ø.

We know that the rotationwas occurringwhilethe units we studied were being deposited. Therefore, in the

The paleomagnetic resultssuggest that the entireSanta averagingof unit directionsto eliminatesecularvariation, Ynez Range has been rotated clockwisea large amount the declinationanomaly is also averagedand the result is

during Miocene time, similar to the northernChannel lessthan the maximumdeclinationanomaly[seeKamerIslands to the south. Studies on lava flows from the ling and Luyendyk,1979]. Results from some individual Miocene TranquilIon volcanicson the north side of the units, which also include secularvariation,show apparent

Santa YnezRange alsoshow large apparent clockwise rota- clockwise rotations of over 90ø [Kamerling, 1985; tion. Hornafius, 1984, this issue]. However, resultsfrom rocks One site at Lions Head in the Santa Maria Basin (LH in older than middle Miocene do not show generally larger

Figures1 and 2) showsno significantrotation.Hornafius declinationanomalies.Eocenesandstones on San Miguel et al. [1981] suggested that the north boundaryof the Island have only about 80ø or 90ø [Kamerling,1985; rotated terrane of the Transverse Ranges is the Santa Kamerling and Luyendyk, this issue; Championet al., Ynez River fault at the south edge of the Santa Maria 1981], concordantwith the results from the younger Basin. This suggestionis compatiblewith the dramatic rocks. It may be that the maximum rotation is in fact contrastin stratigraphyacrossthis fault [Sylvester and Dar- about 90ø; therefore either the northwestalignment of row, 1979]. North of the SantaMaria Basin,Greenhaus lithic belts cannot be restoredby undoingthe rotation and Cox [1979] found clockwise discordant directions in

alone, or the belts were never northwest trending. Possi-

lower Miocene volcanicplugsnear Morrow Bay ("a" in Figures1 and 2). They suggestthat this rotationmay be localizedalonga fault zone. It may not be relatedto the generalrotationof the TransverseRangeswe are describing here but rather to rotation of smallercrustal blocks

bly the pre-Miocenecontinentalmargin had an existing right bend. If this was the case,the marginwould have presentedan asperityto the north slippingPacificplate, facilitatingthe initiationof rotationin southemCalifornia. Somescattercan be seen in the declinationresultsfor

alongthe centralCaliforniacoast.

the variouscrustaldomains(Figure1). Thisscattercan

Close stratigraphiccontrol in the Monterey Formation be due to severalcausesincludingexperimentalerror, secallows the declinationanomalyto be describedas a func- ular variation, and rotation during the deposition of tion of time. The rotationbeganat leastby 16 Ma and sequencesof units as discussedabove. Another likely

proceededat 10ø/m.y. for 5 m.y. At that time it apparentlyslowedor stoppedand resumedagainat 6 Ma at a rate of 5ø/m.y. [Hornafius,this issue]. The rotation may still be continuingin the westernSantaYnez Range and may be accommodated by slip on the Hosgrifault and folding in the SantaMaria Basin[Hornafius,this issue]. During the rotation event the entire SantaYnez Range

causeof scatteris localsmallblockrotation[Kamerling, 1985]. This can be either clockwise or counterclockwise

dependingon the senseof shear. Like the declination anomalies, the low-inclination anomalies have significanttectonic implications. For example,the low inclinationsfor the northernChannel Islandsand Santa Monica Mountainssuggestan initial rotated as a coherent unit. In addition, the declination paleolatitudeof about 20ø for this regionand 15ø of north-

data suggest that the SantaYnez Rangehasbeenorocli- ward transportsinceearly Miocenetime. The implied nally beiatconcavenorthward.The westernmost sites transportrate is near 10 cm/yr [Kamerling, 1985]. The showmore declinationanomalythan the easternsites,and this changein trend parallelsthe changein trend of the SantaYnez River fault and the SantaYnez fault (see Figure 4 below).

DiscussioN

implicationsfor the DiligenciaBasinare generalythe same (Figure 2). Other workers have made similar observations. Marshallet al. [1979] found 10ø of northwardtransport suggestedby paleomagneticresults from Miocene

lavas in southern andBajaCalifornia. Champion etal. [1981] computed 19ø of northward transportfor Creta-

From the above observations, several points deserve ceousthrough Eocene sedimentaryrocksfrom San Miguel discussion: the maximum amounts of declination Island; their results agree closelywith ours. These sugges-

observed, the scatter in declination results, and the tions of northward transport imply tectonic accretion

LUYENDYKET AL.: NEOGENESHEARIN SOUTHERNCALIFORNIA

12,459

30SCIS

SMI

DB(NE)

2O

NORTH

AMERICA

WEST OF SAg

MR

c-Mdo c-Mdc

o

MJ•

b-TEM

-IO

0

.5

I0

I

AGE,

MILLION

20

25

30

YEARS

Fig. 3. Flattening,expectedinclinationminusobservedinclination,versusage. Soliddotsdenotelocalitieseastof

the SanAndreasfault. The limitingbarsare the 95% confidence levelson flattening,and the knownagerange from K-Ar or stratigraphic determinations.Sitekeys are as in Figure 1, with MCN, Mint Canyonformation; SCINM, Montereyformationfrom North SantaCruz Island;SRIt, tuffs from northSantaRosaIsland;and SRIi, intrusivesfrom southSantaRosa Island. SWAa, b, c, southwestArizona results[Calderone and Butler,1984] recomputed from their datafor the middleMiocene(SWAa, Growlerand Buckskin Mountains),andearlyMiocene

(SWAb,Plomosa andSauceda Mountains; SWAc,LittleAjo Mountains).SYa-e areresultsfromHornafius [this

issue];-MJa through-MJc are recomputedMojave resultsfrom Burkeet aL [1982] and J. L. Morton and J. W. Hillhouse(unpublished manuscript, 1983);a-MR is the Morro Rocksresultand b-TEM the Tehachapi Mountains result.

which is in some way compatiblewith the terrane interpre-

at least since the Early Tertiary. West of the San Andreas

tations for California by Blake et aL [1982], Vedderet aL

the Pacificplate has of coursebeen moving northward for several million years. Besidesthis fact, analysesby Blake

[1983], andBottjerand Link [1984]. For example,it has been suggestedthat parts of the northern Channel Islands amalgamated to other parts of southem California as recently as the Late Tertiary.

et al. [1982] and Bottjerand Link [1984] suggestthat a

paleomagnetic inclination if the rocks from these localities

Disregarding for the moment the confidencelimits on F

separate terrane, incorporating crust or lithosphere west of

the San Andreas and south of a line bisectingthe Santa In Figure 3 we have plotted the flattening F, of Barbara Channel and Ventura basin may have accretedto paleomagnetic inclinations at various localities as a functhe SantaYnez Mountains (Sur-Obispocompositeterrane tion of geologic age. The flattening is the expected of Vedderet aL [1983]) in Neogenetime.

formed in situ, minus the observedpaleomagnetic inclina- shown in Figure 3, two conclusionsare apparent: first, tion, so that positive values of F may be interpreted as that there is a wide range in values, from -7ø (southward northward transport [Beck, 1980]. In addition, we have transport?)to +28 ø (northwardtransport?),and second, discriminated between localities east and west of the San that virtually all localitiesshow positiveflatteningregardAndreas fault. The rationale for this is that the eastern less of age or terrane assignments.The data from cratonic region is most clearly attached to cratonic North America,

North America (solid dots) showan interestingpattern:

12,460

LUYENDYKET AL.: NEOGENESHEARIN SOUTHERN CALIFORNIA

36 ø + 122 ø

+

+

+'.36

+

ø

116 •

CENTRAP-

EHACHAPI

MOdAVE

BLOCK

BLOCK

MOR BLOCK

SAN

GABRIEL

BLOCK

EAST

TRANSVERSE BLOCK

•nto Mln. Fault

WESTERN Fault

TRANSVERSE BLOCK

33ø+ 121ø

0

50

moo

MILES ß

Fig. 4. Regionsin southernCaliFornia(shaded)where apparentclockwiserotationis predictedFromgeometryand geologyor From palcomagneticdata. Question marks denote regionsnot yet tested by palcomagneticwork.

large and significantflattening is evident for localitiesolder than about 20 M a; this flattening decreaseswith decreasing age but is still positive. Some localitiesin the northern Channel Islands show a flattening larger than North America. These include Anacapa Island, Santa Cruz Island, and the north side of Santa Rose Island. This observation implies that the northern Channel Islands are allocthonous.

A

closer

look

at the

data

reveals

some

disturbing inconsistencieswith this premise. Results from the Monterey Formation on northern Santa Cruz Island

the flattening anomalies. The offset dipole hypothesis of Wilson [1971] could explain several degreesof latitude anomaly for all terranes due to an apparent farsided pole. In an analysis of long-term nondipole components of the earth's field, Couplandand Van der Voo [1980] showed that significant quadrupole and octupole components existed for the past 50 m.y. and that these components can be interpreted in part as an offset dipole. The effect of these components is to make inclinations shallower so that fixed Neogene sites in southern California would appear to have undergoneabout 4ø of northward motion. Epp et al.

are not anomalously fiat (SCINM); the result for the Santa Rosa Island intrusives (SRI, south of the Santa [1983] found that paleolatitudesfor Miocene-agedPacific Rosa Island fault) is not fiat; the SantaMonica Mountains sediment cores are too low by about 5ø. Wilson and (SMM) result, which is from what is probablythe same McElhinney[1974] have computeda late Tertiary (0 -- 25 volcanic formation as on Anacapa Island, is not Ma) paleomagneticpole for westernNorth America which anomalous. Further,

the southern Channel Islands of

is far-sided. Using this pole as a reference, the northern

Santa Barbara(SBI), SantaCatalina (CAD, San Clemente Channel Islands have been translated about 10ø compared (SCI), and San Nicolas (SNI) do not show a flattening to 15ø for the Irving [1979] referencepole, and the Diliwhich is either significantly positive or different from North America. From Figure 3 it is evident that the terrane hypotheses alone cannot explain the flattening data

gencia Basin has been translated north about 11ø compared to 15.5ø.

and that other explanations should be considered.

PacificNorthwest, concludedthat nondipoleeffects are not significant in explaining anomalous F since the reference

Nondipole magnetic field behavior may explain part of

Beck [1984] in an analysisof flatteningdata from the

LUYENDYK ETAL.: NEOGENESHEAR IN S:)UTHERN CALIFORNIA

12,461

42øN

."

40 o

\

:.

X,



"

i

,,, ,,

,•1 •

r"l

/

,,,18 o

':•.. ,,,1Go

TRANSVERSE Ranges

EaSTERn

34o

32øN

124ø •/

I

122 ø

I

120 ø

lib 0

IIGø

114 ø

112ø

II0ø•1

Fig 5. Map of extensional faultsin the westernU.S. of probably Neogene age(adapted from GableandHatton, 1980]. The faultsin the Transverse Rangesare interpreted hereashavingcomponents of dip slipin theirearlyhis-

tory. Forthe mostpartfaulting in California andArizonabeganearlier(lateOligocene-early Miocene)thanfaultingin theGreatBasin-Basin andRange(earlyandmiddleMiocene).

pole would also have the same flattening. This is true if about 90% of his units from the northern Channel Islands the reference pole is computed from localities near the and Santa Monica Mountains have north or northeast dips study area; if nondipole effectswere significantin the past near 20ø. On the other hand, his units from the southern then there would be significant disagreement between

islands which show little or no inclination anomaly, are

same age referencepolesfrom widely separatedlocalities. near horizontal. Therefore the indications are strong that

Figure 3 showsthe very importantresultsof Calderone part of the inclinationanomalyfor the northemChannel and Butler[1984], who studiedlavasof earlyand middle Islands is due to improper structuralcorrectionsfor units Mioceneagein southwest Arizona(SWAa,b,c),a region which had an initial dip. Paleomagnetic data for the DiligenciaFormationfrom presumably well situatedwithin cratonicNorth America. We recomputedtheir data for the three time periods both flowsand sedimentaryinterbedsalsoshowlow inclishown. The large flatteningfor the earlyMiocene result nations(DB (NE) in Figure3; Terres,1984]). The incliagreesvery well with the flatteningfoundfor San Miguel .nation data from the inland basins on North America Island (SMI), implying no resolvablemotion between presenta true predicament.The DiligenciaFormation these two localities since the end of the Oligocene.

shows a clear inclination anomaly, whereas the results

Kamerling[1980, 1985] investigatedthe possibilitythat improperstructuralcorrectionsfor the depositionalatti-

from the Vasquez(SB) and PlushRanch(PR) formations do not (Figure 3). There are compellinggeologicargu-

tudes of lava flows may account for anomalous low incli- ments that these formations were originally deposited in nation results. The conceptis that flowsare not deposited verycloseproximityto one anotherCrowell,1962;Bohan-

horizontallybut with an initial dip. A later improper non, 1976; Powell,1981; Terres,1984]. Thereforethe among theseformations correctionto the apparenthorizontaldecreases the inclina- large inclinationdiscrepancies tion. This effectwould causeapparentlow inclinationsfor cannothave any geologicsignificance.Becausethe formaunits which had undetected initial dips to the north, east, tions are correlativein age, magneticfield effects can be or west. In addition, improper correction for an ruled out; becausethe fold testis passedin the Diligencia undetectedinitial dip would accountfor part of the scatter Formation,improperstructuralcorrectionis alsoin doubt. in the declinationdata. Kamerling[1985] has shown that A remainingpossibilityis that secularvariation is not

12,462

LUYENDYKET AL.: NEOGENESHEARIN SOUTHERN CALIFORNIA

40øN

/

//

38o

36 o

34o

32o

30øN

122øW

I 120o

Fig. 6. A late Oligocene-earlyMiocene reconstructionfor the southwesternUnited States showing the Transverse Ranges back rotated and slip on the San Andreas fault removed.

averaged in the results from the DiligenciaFormation or that both the flows and sediments were deposited and

but suggested that it was related to simple right shear between the Pacific and American plates which began in

magnetized with an initial dip on a regional paleoslope. the late Oligocene[Atwaterand Molnar, 1973]. An attrac-

lowing observationson the inclinationanomalyare valid: 1. Most of the localities studied show significant low

tive concept is that shear in the asthenospheredrives the rotation from below rather than the rotation being driven by contact forces between the surface blocks. It is not known whether the blocks could actively push one another

inclination results; these include almost all localities on

without

cratonic North America.

rotated and untorated regions. Assuming that the blocks are driven by ductile shear implies that they extend in depth to a brittle-ductile transition zone either in the crust or upper mantle. We have no information regarding the

The results here remain an enigma. With the presently available data base we feel the fol-

,

2. Long-periodnondipolefield effectsmay explain much but not all of the anomalies.

3. There is evidence that improper structural corrections

may be responsiblefor the Oligo-Miocene low inclination anomaly in the northern Channel Islands.

thickness

visible

deformation

of the rotated

at the

boundaries

blocks and it remains

between

one of the

swingingof the TransverseRangescan only accountfor 3ø to 4ø of transport or a small part of the maximum inclina-

major questions to be answered before the dynamics of rotation can be effectively studied. Our original geometric model predicted that regions in southern California bounded by east-westtrending left-slip

tion anomaly.

faults have been rotated. We have revised this prediction

4.

Offset

on

the

San Andreas

fault

and northward

5. Finally, the flattening data lack the necessaryresolu- using new data (Figure 4). These data [Kamerling,1985] tion to distinguish between terrane models, especially show that the San Nicolas block is not rotated as was when confidence limits are considered. predicted. Kanter and McWilliams [1982] showedthat the PALEOMAGNETISM AND TECTONICS OF SDUTHERNCALIFORNIA

Geometric Model of Luyendyket al., [1980]

Tehachapi block was rotated as can be predicted from

observationof the structure alone [Locke et al., 1940; Luyendyket al., 1980] but in pre-Miocenetime; this was

In a previous model we proposed that the Transverse evidently due to a shear pattern existing prior to PacificRanges, in addition to several other regions in southern American plate contact. We still lack data which show California, rotated clockwiseas separateblocks surrounded whether the central Mojave block has been clockwise by a set of northwest trending crustal blockswhich were rotated, although preliminary data from the surrounding undergoingright shear [Luyendyket al., 1980]. We did Mojave Desert show that this region has been rotated (J.L. Morton and J.W. Hillhouse, not specify the exact dynamic situation responsiblefor this slightlycounterclockwise

LUYENDYKET AL.: NEOGENESHEARIN SOUTHERNCALIFORNIA

118ow + BFL

12,463

116øW 115øW + + 36øN GF + 35øN

114 W

+

113 ø

+ 34øN

1120 + 33øN

III ø + 320 N

•- 31 øN

Fig7. Prerotation geometry insouthern California about lateOligocene time(modified fromLuyendyk etal.[1980, Figure 3A]andKamerling [1980]).TheEastPacific Risemayhaveserved to extend thecrustin thearea,although someof the faultsexistedbeforethistime. Faultsindicatedare GF, Garlockfault;SAF, San Andreasfault;NF,

Nacimiento fault;HF, Hosgrifault;SYR andSYRF,SantaYnezandSantaYnezRiverfault;MCF, MalibuCoast fault; SCI, SantaCruz Islandfault; and MIF, Newport-Inglewood fault. Placenamesindicated are BFL, Bakersfield; MRY, Monterey; DB, Diligencia Basin; SB,Soledad Basin; PR,PlushRanch formation; SLO,SanLuis Obispo; SBA,SantaBarbara; SMM, SantaMonicaMountains; PVP,PalosVerdesPeninsula; SAN, SanDiego; ELC, E1 Centro.

unpublishedmanuscript,1983). Originally,we predicted rotation occurredbetween 16 and 10 Ma [Hornafius,1984, that the eastern Transverse Ranges were rotated to the this issue]. The rotation pausedor slowedfor about 4 M a samedegreeas the westernTransverseRanges. However, then resumed and presumably continues today. This the extreme rotations we have found in the Diligencia

pause in rotation at 10 Ma may be related to a change in

Basinappearrelated to local deformationin a major shear zone; we have not yet fully documentedthe regionalrotation here, but existingdata suggestthat it may be about 40ø [Carteret al., 1985]. The present data base shows that the net rotation decreases systematically eastward.Maximum rotationsof

stress orientation

over 90ø are found in the TransverseRangeswest of Ven-

in the southwestern

United

States at this

time [Barrashand IZenkatakrishnan, 1982] or to the initiation

of movement

on the San Gabriel

fault which

would

have moved the dextral shear zone to the east. It may be argued that superimposed on the clockwiserotation is a

Pliocene (?) counterclockwiseevent in the Mojave Desert related to left shear on the Garlock fault system [Gar-

rotation tura (approximately119.40W). East of here the net rota- funke-i,i974]. In this casethe net clockwise tion is near 70ø. In the San Gabriel region the net rota- observed for the San Gabriel region is less than the total tion is near 35ø and the eastern Transverse Ranges also experiencedbecausethis region would have rotated coun-

may be rotated this amount. We have not found the eastern limit to the rotated blocks but results from some

terclockwise as it moved Andreas fault.

north into the left bend in the

Oligo-Miocenelavas in extreme southeasternCalifornia

show no net rotation (M. Marshall, personalcommunica- RelationshipBetween "Basin and Range" Tectonismand

tion, 1983).

Rotation

Many of the rotatedregionswe have found sharesome Much more data on the timing of rotation have become available. It now appearsthat half (50ø?) of the observed common aspectsin their Oligo-Miocenegeology. The

12,464

LUYENDYKETAL.: NEOGENESHEARIN SOUTHERN CALIFORNIA

rotated regionswere in most casessitesof Oligocenenon-

modificationof Figure 3 from Luyendyket al. [1980].

marine sedimentation in fault-bounded elongate and re-

Here the East Pacific Rise is shown impacting the North American margin near the vicinity of the Transverse Ranges. The presence of the spreadingcenter here in Oligocene time could have resulted in tensional stress being transmittedto the overlying plate with concomitant

strictedbasins[McCracken,1972; Bohannon,1976; Terres, 1982; Nilsen, 1984]. Basalticvolcanismaccompaniedthe alluvial sedimentation in many locations. These geologic aspectsare particularlywell recognizedin the Diligencia,

extensionalfaulting [Kamerling,1980]. The fact that the [Bohannon, 1976; Terres,1984a,b]. Many of thesebasins Rivera triple junction remained at this relative position for were formed in Oligocenetime [Nilson,1984]. The fault- severalmillion yearsbefore migratingsouthward[AiwaLet ingwhichdefinedthesebasinswasextensional [Bohannon, and Molnar, 1973] increasesthe likelihoodof profound Soledad and Lockwood (Plush Ranch) formation basins

1976].

tectonic effects on the adjacent North American plate. We

Within the present geographyof southern California these Oligoceneextensionalbasinstrend east-westimplying fracturingand volcanismin a regimeof north-south extension. However, upon considerationof the paleomagnetic data, these elongate basinswere probablyformed

expect that the tectonic style of extension and rotation

with

a north-south

or

northeast-southwest

seen in southern California may not be unique and is presentat other locationsalong plate marginswhere triple junction migration has occurred.

orientation

Acknowledgements. Many peoplehave aidedin our studiesover the years. In particularwe wouldlike to thankJ. C. Crowell,J. R. Dunn, M. Fuller, J. Hickey, M. Marshal, T. Powell, and D. Weaver. We thank A. Cox, J. C. Crowell, andD. G. Howell for reviewsof the manuscript. R.R.T. thanksStanfordUniversity for southwestem United States [Kamerling, 1980; Terres, his use of their paleomagneticfacilities.We alsothank R. Butler for providingthe completedata set of resultsfrom southwest 1982, 1984;Luyendyk et al., 1982;Nilsen,1984]. In Figure 5 we show the patternof late Cenozoicexten- Arizona (Calderone and Butler, 1984). This researchhas been supportedby National ScienceFoundationgrantsEAR76-03768, sional faults in the western United States taken from

[Terres, 1982, 1984]. These interpretationslead to the proposalthat the rotatedbasinsare membersof the familiar north trending Neogene extensionalstructuresof the

Gable and Hatton [1980]. We have added faults of the

EAR78-23426, and EAR81-16507; ARCO oil and Gas Company; Mobil Oil Company;and the Universityof California.

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(ReceivedJune21, 1984; revised July 12, 1985; acceptedJuly 15, 1985.)