Paleomagnetic evidence for en echelon crustal extension and crustal rotations in western Montana and Idaho

TECTONICS,

VOL. 11, NO. 3, PAGES 663-671, JUNE 1992

PALEOMAGNETIC EVIDENCE FOR EN ECHELON CRUSTAL EXTENSION AND CRUSTAL ROTATIONS IN WESTERN MONTANA AND IDAHO

P. Ted Doughty • AmocoProductionCompany,Denver, Colorado Steven D. Sheriff

Departmentof Geology,Universityof Montana,Missoula

Abstract. The Bitterrootmetamorphiccore complexis one of many Eocene core complexes in the northern Cordillera.

A middle Eocene dike swarm cuts the Bitterroot

metamorphiccore complexand its hangingwall, the Skalkaho slab. A 26ø changein strike of the dikes acrossthe core complexsuggestseither a refractedstressfield or rotation duringEoceneor youngerextension. The nearbyBitterroot Valley andBig Hole Basinalsoindicatetectonicrotation. To distinguishbetween the hypothesesof refractedstressand tectonicrotation, we collectedpaleomagneticdata from 28 dikes. Nineteen sitesprovide reliable results. Eight sites from thehangingwall eastof the mylonitezonegive Dec = 46ø, Inc = 69ø, a95 = 13ø;11 dikesto the west yield Dec = 334ø, Inc = 64ø, a05 = 6ø. The declinations divergefrom the expecteddirection;the inclinationsdo not. The hangingwall

The age of the Bitterrootcomplexis early to middleEocene [Bickfordet al., 1981; Chaseet al., 1983; Garmezy, 1983]. Chase and Talbot [1973], Hyndman et al. [1975], and Hyndman[ 1980] dividedthecorecomplexinto threeparts:(1) the Bitterrootdome, (2) the Bitterrootmylonite,and (3) the upperplate (Figure lb). Hyndmanet al. [1975] referredto the upper plate as the Sapphire tectonic block, a Late Cretaceousfeature. We refer to a similar crustal domain, of greater regionalextent and Tertiary in age, as the Skalkaho slab.

The Bitterroot mylonite's kinematicsshow that the Skalkaho slab/Sapphiretectonic block once overlay the Bitterrootdome. However, the timing and tectonicsof the separationare equivocal. Hyndman et al. [1975] and Hyndman[ 1980]proposedthattheSapphiretectonicblockslid to the east during the Late Cretaceous.Yet, Chase et al. [1983] and Garmezy [1983] suggestthat the Bitterrootdome was pulledwestwardduring the Eocene. A dike swarmof middleEoceneage(Figure lb) intrudes theBitterrootdomeandthe Skalkahoslab. The averagestrike of the dike swarm is N21øE in the Bitterroot dome and N47øE

in the Skalkahoslab. The divergentstrikesimply thateither (1) the Bitterrootdome and/or the Skalkahoslab moved since emplacementor (2) the dikes intruded in a refractedstress system. To test these hypotheses, we conducted a paleomagnetic study of the dikes. Our resultsindicatethe Skalkahoslabrotatedafter emplacement of the dikes.

rocks are rotated 58 ø + 32 ø clockwise at the 95 % confidence level. The footwall shows 14ø + 11ø of counterclockwise

REGIONAL

rotation. Both normaland reversedpolaritydikesare present in eacharea, indicatingthatthe dike swarmsmay adequately averagepaleosecularvariation. Vertical contactsreveal that the dikes have not tilted since emplacement. These paleomagnetic resultsandavailablegeochronologic datashow thattheBitterrootdomewaspulledfrombeneaththeSkalkaho slab in the Eocene. As the dome slid toward the northwest, it experienceda slight counterclockwise rotation,while the upper plate moved clockwisearounda vertical axis near its northernedge. Rotationof the Skalkahoslab occursbecause

The Bitterroot metamorphic core complex is the northernmost metamorphiccore complexassociated with the Basin and Rangeprovince. The core complexdeveloped alongthe easternflank of the Idaho batholithas the northern partof theIdahobatholithandassociated countryrocksmoved to the northwestduring Tertiary extension[Hamilton and Myers, 1966]. Left behind was thinned continentalcrust [Sheriffand Stickney,1984] dicedwith extensional basinsup to 5 km deep. The Bitterrootmylonite,LewisandClark fault zone, and Tertiary basins all participatedin the crustal extension. Granites, rhyolites,and minor andesiteswere emplacedacrosswesternMontanaandIdahoat aboutthe same

the slab lies between en echelon extensional detachments of

the corecomplexand thenextbasinto the east,the Big Hole Basin. The BoehlsButtearea, northcentralIdaho, occupies a stepin the Lewis and Clark fault zone and is probablya core complex related to right-lateralmotion on the fault system. Similaritiesin boththe styleandtimingof extension suggestthat Eoceneductile/brittleextensionoccupiesa swath from BritishColumbia,pastthe Bitterrootmetamorphic core complex,to the Pioneercore complexin southcentralIdaho. INTRODUCTION

The Bitterrootmetamorphiccore complexand Tertiary basinsof westernMontanaare part of an Eoceneextensional systemstretchingfromBritishColumbiato Idaho(Figurela).

•On leavefrom I)epaxtment of Geological Sciences, Queen'sUniversity,Kingston,Ontario,Canada. Copyright1992 by the AmericanGeophysical Union. Paper number91TC02889. 0278-7407/92/91TC-02889510.00

GEOLOGY

time.

BitterrootMetamorphicCore Complexandthe SkalkahoSlab

The Bitterrootmylonite[Chaseand Talbot, 1973] is the dominate structurein the Bitterroot metamorphiccore complex. The east dippingmyloniteseparates deformed, highlymetamorphosed rocksbeneath themylonitefromlowgraderocksin the hangingwall. The mylonitehas a basal amphibolite-facies sectionthatpasses discordantly upwardinto the greenschist facies. Chloriticbrecciacapsthe mylonites [Hyndman andMeyers,1988]. All threefaciesdisplaytop-tothe-east(110ø)shear[Garmezy,1983;HyndmanandMeyers, 1988]. The Bitterroot dome, on the northeastern flank of the

Idahobatholith,is themetamorphic corebeneaththemylonite. Mesozonalgranodioritesand granitesof the Idaho batholith are prevalentin the core; they rangein age from 81 to 46.5 Ma [Bickfordet al., 1981; Ferguson,1975]. Countryrocks are in theupperamphibolitefaciesandmayincludecrystalline basementrocks. An arcuateline of middle Eocene(51 Ma)

664

Doughtyand Sheriff: Paleomagnetism and en EchelonExtension

ß

,, ß

=oil, = x la.i

x la.i

._1

o o

ill::

•x

o

0

DoughtyandSheriff:Paleomagnetism anden EchelonExtension epizonalgranitesand associated rhyolitedikescrudelydefine the westernedge of the dome [Lund, 1980]. Hyndman et al. [1975] named the structurallyhigher region eastof the mylonite the Sapphiretectonicblock. As defined, the Sapphire tectonic block is a supracrustal allochthonboundedby Late Cretaceousfaults. However, new radiometric dates, regional relationships, and our paleomagneticdata show that faultswith significantTertiary displacementbound the rocks abovethe Bitterrootmylonite. We wish to avoidthe confusioncausedby a new definitionfor an establishedname. Hence we call the Tertiary tectonic block the Skalkahoslab;Figure lb showsits boundaries. Someargue that the amphibolite-facies myloniteis Late Cretaceous,otherssay Eocene. To resolvethis, Bickford et al. [1981] and Chase et al. [1983] obtainedU-Pb zircon and monaziteages of synkinematicgranodioritesand pegmatites [Latour and Barnett, 1987] from the shear zone.

Lower

interceptagesfrom theserocksare 48 + 1 Ma to 57 + 4 Ma [Chase et al., 1983; Toth, 1983]. Most are discordantlower interceptagesfrom zirconswith detritalcoresand euhedral overgrowths[Bickfordet al., 1981]. One shearedpegmatite yieldsa concordantU-Pb zirconage of 52 + 1 Ma [Chaseet al., 1983]. The ages from zircons, monazites,and field relationshipsare all consistent. This arguesfor a faithful representationof the crystallizationage [Bickford et al., 1981]. Also, •øAr/39Ardatesfrom hornblendeand biotitein the mylonite zone reveal rapid cooling and uplift with mylonitizationfrom 43.5 to 45.5 Ma [Garmezy,1983]. These data demonstratethat the amphibolite-faciesmylonite was active during at least the early to middle Eocene. The greenschist-facies myloniteandchloriticbrecciacannotbe any older.

The Bitterrootdome, like othercore complexes[Parrish et al., 1988], experiencedthermal disruption during the Eocene. In the northernBitterrootdome,K-Ar andnøAr/39Ar datesfrom biotite clusterat 40 to 44 Ma [Armstronget al., 1977; Williams, 1979; Ferguson, 1975; Garmezy, 1983; Garmezy and Sutter, 1983; Hayden and Wehrenberg,1960]. To the south,nøAr/39Ardatesshow that hornblendecooled throughits blocking temperatureat 51 + 5 Ma [Garmezy, 1983]. Biotite cooling ages from the same rocks are much youngerat 42 to 45 Ma [Garmezy, 1983]. In contrast,the $kalkahoslabhasLate Cretaceous andPaleocenecoolingages [McDowell andKulp, 1969; Hyndmanet al., 1972]. Thus the coolinghistory of the Bitterrootdome reveals rapid uplift during Eocene mylonitization, while the Skalkaho slab's coolingagesdo not. Lewis and Clark Fault Zone

The Lewis and Clark fault zone was an important elementduring the Tertiary extensionof westernMontana. This systemof strike-slipfaultsextendsfrom the PriestRiver complex,in easternWashington,to centralMontana, where strike-slip motion passes into several north trending extensionalbasins[Reynolds,1979] (Figure la). En route, the swarmpassesnorth of the Bitterrootdome and forms the northernedgeof the Skalkahoslab. Before the Tertiary, the LewisandClark faultzonewasa left-lateralstrikeslipsystem [Hyndmanet al., 1988]. Tertiary movementon the fault zone is fight lateral;thecumulativedisplacement is between35 and 81 km [Harrisonet al., 1974; Sheriff et al., 1984]. The fault systemwasactivethroughout thedevelopment of theBitterroot metamorphic core complex.

665

Epizonal EoceneMagmatism and the EoceneDikes

A swarmof rhyolitic,hypabyssal dikesis part of middle Eocene(51-44 Ma) plutonismand volcanismin Montana and idaho [Armstrong,1974; Armstronget al., 1977; Hyndmanet al., 1977]. The dikes are apophyses of shallowgranitesand fed overlyingvolcanicrocks [Bennett,1980; Badley, 1978]. The physicalrelationshipbetweenthe dikesand the Bitterroot myloniteis notwell understood [cf. Holloway, 1980]. Yet the geochronologic datashowthat theserocksare coevalwith the synkinematicgranodioritesand pegmatitesinjectedinto the Bitterrootmylonitezone. Thus epizonalEocenemagmatism alsoaccompanied motionon the Bitterrootmylonite. In the Skalkahoslab, the dike swarm strikesN47øE. The dikesandan Eoceneplutonarewell aligned. In theBitterroot dome the dikes strike N21øE; this averagestrike has more variance than that for the Skalkaho slab.

The western swarm

is perpendicularto the 110ø stretchinglineation in the Bitterrootmyloniteand probablyformedin the samestress field.

The 26 ø difference

in the strike of the Eocene dike

swarmsoccursacrossthe Bitterrootmylonite. The dikes convergetowardthe south(Figure lb); the BitterrootValley also tapersto the south. These relationssuggestthat some separationof the Bitterrootdomeand Skalkahoslabis postmiddle Eocenein age. Such divergenceis consistentwith tectonic rotation about a nearby vertical axis during displacementon the mylonite. PALEOMAGNETIC

FIELD

AND

LABORATORY

WORK

To testthe hypothesisof differentiallyrotateddikes,we collectedsamplesfrom28 paleomagnetic sites.Fourteensites, from two samplingareas, are in the Bitterrootdome. The remaining14 sitesare from two areas in the Skalkahoslab (Figure lb). At 20 siteswe drilled sampleswith a portablerock drill and orientedthem in situ with magneticand sun compasses. Most of thesesites, sampledperpendicularto strike, are in roadcuts. However, eight sites in the Bitterroot dome are from the Selway-BitterrootWildernessarea. At thesesites, we collectedone or two hand samplesfrom eachdike. For each site, we subjectedone pilot specimento stepwisethermaldemagnetization and one to alternatingfield (af) demagnetization. Usually, thermal demagnetization isolatedthe samedirectionas af demagnetization.A Molspin tumbling demagnetizerperformed the af demagnetization. Thermal demagnetization was completedwith a homemade device; inductionin the cooling chamberis less than 6 nT. We used the techniquethat best isolated the characteristic remanentdirectionto progressively demagnetize theremainder of the site. For each specimen,10-25 stepswere measured with a SchonstedtSSM-2A magnetometer. Orthogonalprojectionssimplified the interpretationof each specimen's progressive demagnetization. Most specimens had single-componentcharacteristic remanent directions. Only 13 of 170 specimenscontainedmore than one persistentmagneticcomponent. We used intersecting remagnetizationcircles [Halls, 1978] to isolate the common componentof magnetizationin these specimens. Figure 2 showsrepresentative demagnetization diagramsfor ourreliable specimens. Site mean directionswere calculatedusing the methods of Fisher [1953], Kirschvink [1980], and Onstott[1980]. For hand-sampled sites, we combined directions from each

Doughtyand Sheriff:Palcomagnetism and en EchelonExtension

666 N Up

N _Up

alteration. Rapid acquisition of isothermal remanent magnetization(IRM) [Doughty, 1990] showsthat magnetites are the main magneticcarrier. A weak secondary component of magnetization existsin 56 specimensfrom 12 sites. For sevenof thesesitesthe componentis a viscous remanent magnetization(VRM) parallelto the presentgeomagnetic field. For the remaining five sites, the secondarycomponentshave more dispersion. Magnetic cleaning removes the secondarycomponentsby

NRM

/ H

300øC or 50 mT.

,

L

SD

Doughty [1990] presentsthe details of some rock magneticexperiments thatlead to the followingresults. High NRM:IRM(s) ratios from our specimens show the magnetization is of primaryorigin[Fuller et al., 1988]. Many of the dikes have large, possibly multidomain, grains. However, Lowrie and Fuller [1971] domain-statetests show

SD

Fig. 2. Vectorendpointdiagramsof typicalspecimens during demagnetization. Circles (triangles)representdeclination (inclination). Open(closed)symbolsrepresentupper(lower)

hemisphere. Onescaledivision equals10rsemu/cc. specimen in a hand sample to get a mean direction. Averagingthesemeandirectionsfromhandsamples produced

that the specimensalso have single-domaingrains. These grains probably carry the characteristic remanent magnetizationsin our sp•imens. Wu et al. [1974] and Murthy et al. [1971] investigatedsimilar situations. Their resultsalsoshowthatsingle-domain andpseudosingle-domain magnetitegrainsexist in coarse-grained igneousrocks. Determiningpaleohorizontalis difficult with igneous rocks, yet it is necessaryfor accuratereconstructions.We maketheassumption thatdikesintrudevertically. Our support for this assumption is basedon the fact that the dike swarms outcroplinearlyover tensof kilometersalongstrikeandmost

site mean directions.

dikes have vertical contacts. However, some dikes have

To get site mean directionsthat accuratelyreflect the original magnetic field, it is common to apply arbitrary rejectioncriteriato specimens andsites. Our specimens had to be within two angularstandarddeviations of the sitemean direction.They alsohadto havemaximumangulardeviation valueslessthan 5ø or 15ø for linear and arcuatedecaypaths, respectively. These criteria eliminated40 specimens,130

inclinedcontacts. Where thesedikesare well exposed,they are rolling over to feedcontiguous sills. Thustheir dip is not a postemplacement feature. On the basisof thesecriteria,we infer that tectonictilt of the dike swarmsis unlikely.

survived.

We alsodroppednineof 28 sitesfrom furtherstatistical analysis.Specimens from five sitesweretooweak(M0 < 10' 4A/m) to measurewith our equipment.Two sitesrecordboth normalandreversepolaritiesin transects acrossthedike. One of thesehas inclinationsrangingfrom 60ø to -10ø acrossthe dike. Thesedikesprobablygainedtheir magnetization over enoughtime to recordsomeanomalousfield behavior. By analogy,most of our dikes likely averagedsomesecular variation.

The

sites we drilled

in the field have 95%

confidencelimits (o•95)less than 15ø. However, our handsampledsiteshavefew independent blocks. Thuso•95 is not an appropriate rejectioncriterion. We usedan angularstandard deviation(•63)of 15ø as the arbitrarycutofffor hand-sampled sites. Our reliabledrilled sitesalsohad •63below 15ø. This costus two hand-sampled sites. Table 1 presentsthe results from each site. PALEOMAGNETIC

RESULTS

Most samplesloseabout60 % of their magnetization by 300øC or 50 mT, the rest disappearsnear 580øC. This magnetizationresidesin magnetite. However, for 27 of 79 thermally cleaned specimensabout 15% of the natural remanent magnetization(NRM) persistsuntil 680øC, the unblocking temperatureof hematite. The direction of magnetization carriedby hematiteusuallyparallelsthat in the magnetite. Such hematite likely grew during deuteric

A modified tilt test [McFadden and Lowes, 1981] on

seven inclined dikes also shows that the tilt is original. Restoringtilted dikes from the Skalkahoslab to vertical is inconclusive.The dispersion of sitemeandirectionsincreases, but only at the 80% confidencelevel. However, for tilted dikes in the Bitterrootdome, the increasein dispersionis significantat the 95 % confidencelevel. The tilted dikeswere probablymagnetizedin situ. Thus the paleomagnetic results in Table 1 do not requireor includetilt corrections. To apply paleomagneticresultsto tectonicproblems, paleosecularvariationmust be averagedto a constantvalue. Paleosecular variation model G [McFadden et al., 1988]

predicts17.5øof angularstandarddeviation(•o) from virtual geomagneticpoles (VGP) at our sampling paleolatitude. Because the dikes are as much as 20 m thick and cooled

slowly, each averagessome paleosecularvariation. Thus betweensite scatterof VGP shouldbe less thanpredictedby model G. The Bitterrootarea yields •e3 = 12.8øwith 95 % confidencelimits of -9.3 ø and + 19.9ø [Cox, 1969]. Sites in the Skalkahoslabgive •e3 = 27.0ø (-20.4ø, +40.0ø). Neither are directly on the expectedvalue. For two additional reasons,we believethe Bitterrootand Skalkaholocalitiesmay adequatelyaveragepaleosecularvariation. First, normaland reversedpolarity dikes are presentin both areas. In the Skalkahoslab, three dikes have normal polarity and five reversedpolarity. The Bitterrootdome has eight normal polarity dikes and three of reversepolarity. Thus the sites representa time spangreaterthanthelongestperiodof secular variation. Second, as we discuss below, some of our declinations are anomalous, yet the inclinations are concordant.If theunusualdeclinations resultfrom incomplete

Doughtyand Sheriff:Paleomagnetism anden EchelonExtension

Site

Lat

TABLE 1. Site Mean PaleomagneticData, BitterrootDome/SkalkahoSlab Long N/Nc Inc Dec a95 663 k Plat

667

Plong

Bitterroot Localities

B01

46.7

114.6

4/6

65.7

336.9

9.5

8.3

94.5

74.2

169.6

B02

46.7

114.6

4/8

66.9

342.8

11.9

10.4

60.4

78.3

175.6

B03 B05 B06 B09 A B B10 Bll

46.7 46.7 46.7 46.1

114.6 114.6 114.6 115.2

46.1 46.1

115.2 115.1

7/8 5/6 3/5 2/2# 2/4 2/3 3/4 2/2#

59.0 59.2 50.9 -64.6 -72.5 -56.7 71.4 78.1

324.8 331.4 327.1 171.7 166.5 174.5 344.0 328.1

3.8 11.1 10.3 35.9 19.1 16.5 17.9 47.3

5.0 11.6 6.7 11.4 6.2 5.3 11.6 14.8

256.8 48.6 144.3 50.5 172.9 231.4 48.5 30.0

63.8 69.4 60.7 -84.2 -75.7 -80.3 75.9 63.2

153.3 150.2 136.3 -17.6 34.6 -88.2 -154.3 -142.2

A

4/5

81.8

268.3

13.3'

10.9'

43.3

-137.5

B B12

115.1

4/5 2/2#

69.2 -57.7

348.4 163.0

11.0 6.8

7.2 2.2

80.0 -75.5

-159.3 -50.8

46.1

-31/-.9

126.0 1362.9

A

5/5

-57.7

160.1

4.2

4.4

333.0

-73.4

-45.7

B

3/3

-57.6

165.9

2.0

1.3

3905.3

-77.0

-56.9

2/2#

-66.9

120.3

40.4

12.7

40.4

5/5

-64.3

99.3

B15

46.1

115.1

A B

6.9*

-50.9

2.5

6.3*

-120/-.3

-36.6

-56.5

2/3

-66.4

143.2

3.8

1.2

4447.8

-65.2

-5.2

B18

46.1

115.1

8/8

-54.8

161.8

2.3

3.3

599.5

-72.6

-56.8

S03

45.8

113.9

7/8

-71.9

270.1

5.7

7.6

113.4

-36.8

108.3

S04

45.9

113.9

7/8

-60.3

233.7

6.1

8.1

99.7

-51.5

142.8

S07

45.9

113.8

6/8

67.8

350.9

5.7

6.9

138.0

82.2

-161.5

S10

45.9

113.7

5/8

75.0

100.5

6.4

6.8

142.8

35.3

-79.8

S12

45.9

113.8

6/8

79.1

90.6

4.3

5.1

247.3

41.8

-84.8

S13

45.9

113.8

5/7

-54.5

197.2

11.0

11.5

49.5

-73.3

-170.6

S16

46.1

114.0

3/7

-64.1

186.2

14.0

9.2

78.2

-86.1

154.8

S17

46.1

114.0

6/8

44.9

247.8

2.8

3.3

596.3

-33.8

-19.5

Skalkaho Localities

A andB denotehandsamplesfromhand-sampled sites(only onehandsamplewasusedat sitesB 18 and B10); Lat and Long are samplinglatitudeand longitude,positivenorth and west, respectively; N/Nc is number of specimens averaged/collected(# denotes number of hand samples averaged/collected); Inc. andDec. are inclinationanddeclinationof sitemeandirectionin degrees;a95 is radiusof 95 % confidenceconein degrees;663 is,circularstandarddeviationin degrees;k is Fisher

[1953]precision parameter; Plat.andPlong.areV'GPlatitudeandlongitude.The asterisk denotes averageof major and minor axesof a95 ellipseand 663 oval from analysisof remagnetization circles, concentration parameters[Onstott,1980] for suchdataare tabulatedas kt/k2.

TABLE 2. Locality Mean Palcomagnetic ResultsFrom the BitterrootDome and Skalkaho Slab, Idaho and Montana

Locality

N c•9•

k

Dec

Inc

R

+ R

F

+ F

Bitterroot

11

5.6

68.1

334.1

63.8

-14.4

11.0

1.7

5.2

Skalkaho

8

13.4

18.1

46.4

69.1

57.9

32.0

-3.9

10.7

N is numberof sites;k is Fisher [1953] precisionparameter;m95is radiusof 95 % confidencecone in degrees;Inc. andDec. are inclinationanddeclinationof localitymeandirectionin degrees;R = Do•D•x; 5: R = radiusof 95% confidence cone;F = I•x - Io•; 5: F = radiusof 95% confidence cone. Expecteddeclination (D•0 = 348.5ø;expected inclination(I•0 = 65.5ø [Diehl et al., 1983].

averagingof paleosecularvariation, we expect discordant inclinations

TECTONIC

IMPLICATIONS

as well.

Table 2 presentsmeandirectionsfor the Bitterrootdome and Skalkaho slab.

The BitterrootMetamorphicCore Complex

For the Bitterroot dome, the mean

directionfrom 11 sitesis Dec = 334ø, Inc = 64ø, a95 = 6ø. Eight sitesfrom the SkalkahoslabgiveDec = 46ø,Inc = 69ø, a95 = 13ø (Figure 3).

Diehl et al. [1983] providea North Americanreference pole(at 82.0øNand 170.2øE,a9• = 3.5ø) for the periodfrom 47 to 54 Ma. Using this palcopole, the expected

668

Doughtyand Sheriff.'Paleomagnetism and en EchelonExtension 0

horizontalaxis. However, the regionalplunge of the Bitterrootdomeis to the south,perpendicular to theneeded direction. Thus the counterclockwise deflection of declination

is probablynot from tilting. Most likely the deflected declination measures rotationabouta nearbyverticalaxis. Regardless of rotation,regionalgeologyshowstheBitterroot domemovedto thenorthwest alongtheLewisandClarkfault zone[HamiltonandMeyers,1966]. For example,$ixt [1988] estimates thattheBitterrootmyloniterecordsat least25 km of displacement. AlsoHarrisonet al. [1974]propose 81 km of

REFERENCE DIRECTION

fight-lateraloffset on the Lewis and Clark fault zone. This is the upper limit of extensionin the area. Becausethe Lewis

and Clark fault zone parallelsa line of latitudearoundthe Eocenepole, our data do not measurethe known translation

90

O.

of the dome. We suggestthat as the domeslid northwest some25 km alongthe fault zone,it rotatedcounterclockwise. The Skalkaho slab shows 58ø +

32ø of clockwise

rotation.To produce themeasured declination withtilting,the Skalkahoslabwouldhaveto dip 18ø to the west. However, Cretaceousmetamorphism within the Skalkahoslabdecreases

toward the northeast,suggesting that the slab plunges northeast.Again,theregionalplungeis perpendicular to the neededdirection. Additionally, the dikesoutcroplinearly, with verticalcontacts,over tensof kilometers of rugged topography. There is little or no tilt to the west. The simplest interpretationis that the Skalkaho slab rotated

clockwiseabouta nearbyverticalaxis. The Skalkahoslab probablyrotatedas a coherentslab, boundedby discrete

9O

extensional faults.

Structuralrelationshipsconstrainthe location of the Fig. 3. Lower hemisphereequal-areaprojectionsof the (a) Skalkahoand (b) Bitterrootlocalitysite meandirections. All reversedpolaritysitemeandirectionshavebeeninvertedinto the lower hemisphereand portrayed as open circles; solid squares represent normal polarity site mean directions; Pentagramand large circle are locality mean direction and 95 % confidencecone in degrees;plus sign is the expected

rotationaxisfor theSkalkaho slab. Suppose therotationaxis lay near the southend of the Skalkahoslab. Then, there wouldbe about44 km of middleEocene,left-lateral,strikeslip motionalongthe northernedgeof the Skalkahoslab. There is no suchmotiondocumented to date. Tertiary

Eocene direction from Diehi et al. [1983]; small cross and asteriskare the current dipole and local geomagneticfield directions,respectively.

Desormier, 1975]. TheseTertiarynormalfaultsonlyslightly offset Cretaceousstructures and reveal little strike-slip

palcomagnetic directionin our studyareais Dec = 348.5ø, Inc = 65.5ø. When comparedto the referencedirection throughstandardtechniques [Beck, 1980; Demarest,1983], the sites in the Bitterroot counterclockwise

rotation

at

dome show the

95%

14ø _+ 11ø of

confidence

level.

Flatteningof inclinationfor the Bitterrootregionis 2ø __+_ 5ø (positiveis shallower). The Skalkahoslabshows58ø _ 32ø of clockwiserotationand-4ø + 11øof flattening.Perhapsour sites in the Skalkaho slab do not completely average

palcosecular variation.However,theconcordant inclinations makeit unlikelythat all the differencein declinationis from incompleteaveragingof palcosecular variation. If the dikes swarms intruded with a 26 ø difference

in

strike, their palcomagneticdirectionswould be the same. They are not. The divergence of the dikesis mostlikely the resultof tectonicrotationduringpost-middleEocenecrustal extension. Thus muchof the developmentof the Bitterroot corecomplexis post-middleEocenein age. The Bitterroot dome shows 3ø of significant counterclockwise rotation at the 95 % confidence level but no

flattening. The anomalous declinationcouldbe from the footwallbeneaththe mylonitetilting arounda northstriking

displacement on the Lewis andClark faultzoneis on down-tothe-southwest normal faults [Nelson and Dobell, 1961;

motion. The rotation axis for the Skalkaho slab cannot be at its southend; it must be toward the northern end.

En EchelonExtension andTertiaryBasins

TheBitterroot myloniteandTertiarybasinsalsodisplay evidencefor crustal rotation. At its northernend, the Bitterroot myloniteseparates upperamphibolite-facies rocksin

the footwallfrom low-graderocksin the hangingwall. Within100km to thesouth,themylonite juxtaposes rocksof similarmetamorphic grade. At least25 km of displacement waneswithin 100 km. This, androtationof thedome,causes

thesimilarsouthward narrowing of theBitterroot Valley. Counterclockwise

rotation of the Bitterroot

dome can

accountfor 10 km (40%) of differentialdisplacement on the mylonite.At thenorthendof themylonite,thereremains15 km of regionalextensionalongthe mylonite. At the south end, all the regional extension(at least 25 km) must be accounted for on other structures.

The mostlikely candidateto take up regionalextension is theBig Hole Basinto the southeast (Figurelb). Thisbasin is a half-grabenthat may containan east dipping Eocene detachmentalong its westernedge. The Big Hole Basin narrows toward the north, oppositefrom the taper of the

Doughtyand Sheriff: Paleomagnetism and en EchelonExtension Bitterroot Valley. Such tapering reflects the fact that extensionin the Bitterroot Valley decreasessouthward, whereas extension in theBigHoleValleydecreases towardthe north (Figure 4). Consideringboth basins, extensionis constantacrossthe region. The Big Hole and Bitterroot

valleysresultfrom en echelonfaultsthat lack a connecting transform

fault.

Acceptingthe en echelon relationshipbetween the

Bitterroot ValleyandBi• HoleBasinexplains theclockwise rotation of the Skalkaho slab. As the Bitterroot dome moved

frombeneath theSkalkaho slabtheBig Hole andDeerlodge basinsformedto the southeast.The Skalkahoslab,caught betweenthesebasinsand the dome, rotatedclockwiseabouta verticalaxis at its northernend(Figure4).

Boehls

Butte •• •

•'

.......

I•

"% '

•

•

R?_er..ro_o• ---•,•-•,

•

•,,

Bitterroot

'AMylonite

/,x,•

Skalkaho

669

Northwestof the Bitterrootcore complexis a wide zone of Eocene(58-45 Ma) crustalextensionin British Columbia and northeastWashington[Parrishet al., 1988; Bickford et al., 1985]. Crustal extensionand clockwise rotation occur betweentwo en echelon,right-lateral, strike-slipfaults that strike northwest[Price, 1979] (Figure la). Our synthesisof the regional geology is that similar crustal rotation and extensionoccurseastof theBitterrootcorecomplexandsouth into eastcentralIdaho. This rotationand extensiondeveloped in responseto movementof the Idaho batholithextensional terranetowardthe northwest[Hamiltonand Meyers, 1966] at the sametime (57-44 Ma) as extensionto thenorth. The two areasof extension are coupled,in echelon,by theright-lateral Lewis and Clark line which behaved as a continental transform

fault duringextension[Hamiltonand Myers, 1966; Ewing, 1980; Sheriff et al., 1984; Rehrig et al., 1987]. Between the Priest River complexand the Bitterroot metamorphiccore complexis Boehls Butte (Figure lb). BoehlsButteis an enigmaticregionof highlymetamorphosed rockswith EoceneK-At coolingages[Armstronget al., 1977; Hiemen, 1984]. BoehlsButte occupiesa right-stepping jog (Figureslb and 4) in the Lewis and Clark fault zone, and crustalextension is expected in suchlocations.We suspect the BoehlsButte area is an unrecognized Eocenemetamorphic core complex[cf. Seyfert, 1984]. SUMMARY

AND

CONCLUSIONS

We obtainedreliable paleomagneticdirectionsfrom 19 sites collected

from

middle

dome and Skalkaho slab.

Eocene

Their

dikes in the Bitterroot

declinations differ from the

expecteddirection, whereastheir inclinationsdo not. The Bitterroot

95%

Fig. 4. Block diagramillustratingthe proposedscenariofor Tertiaryextensionin westernMontanaandIdaho. Crustsouth of the Lewis

and Clark

line and west of the extensional

dome shows 3 ø of counterclockwise

confidence

clockwise

level.

rotation

The at

the

Skalkaho 95 %

rotation at the

slab shows 26 ø of confidence

level.

Unfortunately, our sites in the Skalkaho slab may not completelyaveragepaleosecular variation. Futureeffortswill

detachments (linedpattern)has movedto the westby 25-35

test this and determine whether the Skalkaho slab behaves as

km.

one independent block. The paleomagneticdata and available geochronology [Bickford et al., 1981; Chaseet al., 1983; Garmezy, 1983] show that the Bitterrootmetamorphiccore complexformed duringregionalEocenecrustalextension.The Bitterrootdome was pulled from under the Skalkahoslab and transportedat least25 km to the northwest. During translation,the dome

The Skalkaho

slab rotated clockwise

about an axis

toward the north and the Bitterroot dome rotated slightly counterclockwise as the crustextendedalongthedetachments. BoehlsButte, northwestof the Bitterrootdome, occupiesa right stepin the Lewis and Clark line and may be a small Eocenecore complex.

rotated counterclockwise.

RegionalPerspective Rotating the Skalkaho slab around a northern axis requiresswingingthe southernend towardthe west. Yet, no major strike-slipfaults bound the southernedge. Instead, regional crustal extensionaccommodates the displacement. The Trans-Challisfault zone, a northeasttrendingsystemof Eocenenormal faults, crossesthe region [O'Neill and Lopez, 1985]. Eocene ductile extension,like that in the Pioneer core complex[Wust, 1986; O'Neill and Pavlis, 1988], is exposed

in fault-bounded rangesin southcentralIdaho (Figure la). Like

the

structures

associated

with

the

Bitterroot

core

complex,concurrentbasinsbeganto form duringthe middle Eocene [Janeckeand Snee, 1990]. Some of the uplifted rangesexhibitpaleomagneticevidenceof post-middleEocene crustal rotations as well [Janecke et al., 1991]. The displacementat the south end of the Skalkaho slab is distributedamongtheseextensionalstructures.

At the southern end of the core

complex, crustalextensionsteppedin echelonstyle to the southeastinto the Big Hole Basin. The Skalkaho slab, boundedby these detachmentsand pinned at its northern margin,rotatedclockwise.The regionalgeochronologic dates leavelittle doubtthatthissystemof Eoceneextensionstretches from British Columbiavia the Priest River complex, Boehls Butte, and the Bitterrootcore complexto Tertiary basinsin southwestern

Montana

and central Idaho.

Acknowledgments. The generouscontributions of thelate Ruth Doughty Bradshawand National ScienceFoundation grant EAR-8607902 helped supportthis research. Amoco ProductionCompanyandtheUniversityof Montanaprovided facilitiesanddraftingsupport.J. A. Gunderson, J. W. Sears, and D. W. Winston were very helpful. Selway River Outfitters provided four-leggedlogistical supportfor field work in the Selway-BitterrootWilderness. Reviews of the manuscriptby two anonymousreviewersare appreciated.

Doughtyand Sheriff:Palcomagnetism anden EchelonExtension

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(ReceivedJuly 9, 1990; revisedSeptember 9, 1991; accepted November6, 1991.)