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Kinematics and timing of three superposed extensional systems, east central Idaho: Evidence for an Eocene tectonic transition

Article in Tectonics · December 1992 DOI: 10.1029/92TC00334

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KINEMATICS AND TIMING OF TItREE activerifting in drivingCenozoic extension have been SUPERPOSED EXTENSIONAL SYSTEMS, investigatedrecently in the North AmericanCordillera EAST CENTRAL IDAHO: EVIDENCE [Armstrong,1982; Coney and Harms, 1984; Sonder et al., 1987;Gans et al., 1989;Armstrong and Ward, 1991]. FOR AN EOCENE TECTONIC Particularattention has focused on the interplayof TRANSITION extensionand magmatismin the Great Basinregion and the CanadianCordillera [Parrish et al., 1988;Gans et al., Susanne U. Janecke' 1989;Taylor et al., 1989]. East centralIdaho is an Departmentof Geologyand Geophysics, University of excellentlaboratory for investigatingthe relationship Utah, Salt Lake City betweenextension and magmatismand for researching possibledriving mechanisms of extensionbecause Idaho Abstract. Cenozoic crustal extension in east central hassuch a longand rich historyof extensionpunctuated Idaho beganabout 50 Ma and continuesat present.Three by discretemagmatic events. In addition,better distinctepisodes characterize one of the longestintervals constraintson the geometryand magnitudeof extensionin of Cenozoicextension yet documentedin the North southern and central Idaho are needed because recent America Cordillera. Crosscuttingrelationships between kinematicmodels of the PacificNorthwest imply greater NE strikingnormal faults and volcanic rocks, regional dike amountsof extensionin Idaho than havebeen previously trends,and slickenlinedata indicateNW-SE extension documented[Heller et al., 1987;Wells andHeller, 1988]. duringpeak Eocene volcanism about 49-48 Ma (episode Late CenozoicBasin and Range extension has left a 1). NE strikingnormal faults, with at mosta few clearimprint on the landscapeand geologyof eastcentral kilometersof offset,formed in an intraarcsetting during Idaho [Scottet al., 1985;Crone et al., 1987;Crone and rapidNE subductionof oceanicplates under the Pacific Hallet, 1991;Turko and Knuepfer,1991]. Prominent Northwest.North to NNW strikingand west dipping Pleistoceneto Holocenefault scarpsand facetedspurs normalfaults, with offsetsup to 10-15km, formedduring define the SW flank of the Lost River, Lemhi, and a youngermiddle Eocene to Oligocenebasin-forming Beaverheadranges (Figure 1) [Scottet al., 1985;Crone event(episode 2). Thisnewly documented episode was andHallet, 1991]. In thispaper the term Basinand the mostimportant extensional event in eastcentral Idaho Rangerefers to late Cenozoicfaults that boundthe andbegan during the waningphases of Challisvolcanism. presentbasins and rangesor normalfaults that were active WSW-ENE to SW-NE extensionduring episode 2 was in late Cenozoic time. nearlyperpendicular to the extensiondirection during In contrastto Basinand Rangefaults, the geometry, episode! and perpendicularto the grain of the kinematics,and timingof pre-Basinand Range extension Idaho-Montanafold andthrust belt. The flip in extension are poorlydocumented in eastcentral Idaho. Hait [1984] directionbetween episode 1 andepisode 2 is tightly wasthe first to arguefor post-middleEocene and constrainedby 'eAr/39Arage determinations to have taken pre-Basinand Range extension in thisarea on the basisof placeat the end of EoceneChallis magmatism about 46-48 westdipping normal faults that offsetEocene volcanic Ma. I infer that plateboundary forces controlled the rocksbut did not deformnearby Miocene rocks. geometryof normalfaults and dikes during episode 1, Previously,only widely scattered exposures of Tertiary whereasinternal stresses within previously thickened crust sedimentaryrocks had been described in the region drovemajor SW to WSW directedextension during [Ruppeland Lopez, 1988], and therewas tittle episode2. A dropin convergencerates between the stratigraphicevidence to suggesta mid-Tertiaryepisode of NorthAmerican and Farallon plates between 59 Ma and extension.Newly identified Eocene to Oligocene 42 Ma (Stockand Molaar, 1988) may coincide with the syntectonicdeposits in the LostRiver-Lemhi area record onsetof gravitationalspreading during episode 2 andmay thismajor orogenlcevent and providecritical timing alsoexplain the abruptend of Eocenemagmatism in the constraintsfor large-displacementnorth to NNW striking PacificNorthwest. Mioceneand youngerSW dipping normal faults. Basinand Range faults (episode 3) extendedthe regionin An evenolder episodeof extensionaffected the a NE-SW direction.Although fatfits formed during Trans-Challis zone in central Idaho and the Pioneer and episode2 andepisode 3 are notparallel, slickenlines Bitterrootmetamorphic core complexesduring Eocene indicateonly small changes in slipvector trends, time(Figure 1) [Mcintyreet al., 1982;Garmezy and suggestingtittle rotationof the extensiondirection in east Sutter, 1983;Wust, 1986;O'Neill and Pavlis,1988; central Idaho since 46 Ma. Silverberg,1990]. The magnitude,areal extent, and timingof Eocenenormal faulting in adjacentareas has INTRODUCTION remaineduncertain, however, in spiteof earlywork by The relativeimportance of plateboundary forces, Ross[1947] and Baldwin [1951]. gravitationalspreading of previouslythickened crust, and Each extensionalepisode outlined above is well expressedin the LostRiver-Lemhi area of eastcentral Idaho(Figure I andPlate 1). NE, NNW, andNW 'Nowat Departmentof Geology,Utah State strikingnormal faults are interpretedto haveformed University,Logan, UT 84322-4505. sequentiallyduring the Cenozoic,although some Copyright1992 by the AmericanGeophysical uncertaintyremains about the agesome NE strikingfaults. Union. The crustextended initially in a NW-SE direction,then Papernumber 92TC00334. E-W to ENE-WSW, and finallySW-NE. The extension 0278-7407/92/92TC-00334510.00 directionin eastcentral Idaho flipped almost 90 ø during 1122 Janecke:Cenozoic Extension, East CentralIdaho

115 ø 114 ø I

113 ø

115'

Boulder Mountains

White Knob .ø

ß IDAHO

0 10 20 30 40 50 km ! ! ! , , , Pioneer core complex • WKH•ntai,113 ø

114 ø Fig. 1. Locationmap of centralIdaho showing selected Cenozoic normal faults. Solidtriangle shows locationof tiltedTertiary conglomerates in the footwallof thePass Creek fault system.Widely-spaced diagonalrule showsTrans-Challis zone. SelectedTertiary plutons are cross-hatched.Small dots outline late Cenozoicbasin fill. NumerousNE strikingnormal faults in the centralLost River Rangeare omittedfor clarity. BPH is BorahPeak horst; WKH is White Knobhorst; PCWC is PassCreek-Wet Creek reentrant.

the Cenozoic,possibly in responseto changingplate al., 1991],and observationsof Tertiarysedimentary rocks boundaryforces in a regionof gravitationallyspreading in the PassCreek, Donkey and Samill Canyonbasins crust. [Janecke,1991; Janecke and Snee,1990, submitted manuscript].Plate 1 is a compositegeologic map distilled PREVIOUS WORK AND NEW DATA from Janecke[1992a, b, c, d] andJanecke and Wilson [1992]. All geochronologicresults are fromJanecke and This studybuilds on geologicmaps of Ross[1947], Snee(submitted manuscript) unless otherwise noted. Baldwin[1951], Mapel et al. [1965],Nelson and Ross EoceneChallis volcanic rocks lap acrosscontractional [1969],Mapel and Shropshire[1973], Ruppel and Lopez folds and thrust faults in the Lost River-Lemhi area that [1981],Skipp et al. [1988],and Susong[1987] and utilizes formedduring the Sevierorogeny [Janecke, 1991; Ross, revisedPaleozoic stratigraphy [Mamet et al., 1971;Skipp 1947]. K-At datingsummarized by Fisheret al. [1992]and et al., 1979;McCandless, 1982]. New data usedin this morerecent •øAr/39Ar ages of Janeckeand Snee studyinclude geologic and photogeologic 7.5' quadrangle (submittedmanuscript) and Snider and Moye [1989]show mapsof the centralLost River and centralLemhi ranges, that Challisvolcanism throughout the regionand in the andDonkey Hills [Janecke,1992a, b, c, d; Janeckeand studyarea lasted less than 5 m.y.(51 Ma to 46 Ma). Wilson,1992], kinematic data collectedalong normal faults Three distinct sets of normal faults offset the volcanic in the centralLost River Range,geologic cross sections, rocks and folded Paleozoic rocks. stratigraphicand •øAr/39Ar investigations of volcanic rocks (S.U. Janeckeand L.W. Snee,Age andStratigraphy of NE STRIKING NORMAL FAULTS (EPISODE 1) EoceneChallis volcanic rocks and youngerconglomeratic deposits,central Idaho, submittedto U.S. Geological NE strikingnormal faults pervade the centralLost Survey,1992; hereafter referred to asJanecke and Snee, RiverRange between the PassCreek-Wet Creek reentrant submittedmanuscript) paleomagnetic studies [Janecke et and the northwesternedge of the BorahPeak horst Janecke:Cenozoic Extension, East Central Idaho 1123

(Figure1 andPlate 1). FromNW to SE the largerfaults the northwestside of the BorahPeak horst [Mapel et al., of thisset include the MahoganyCreek, Elkhorn Creek, 1965;Skipp and Hair, 1977;Rember and Bennett, 1979]. LeathermanPass, Mount Breitenbach, Dry Creek,Long Most NE strikingnormal faults dip gentlyto moderately Lost,Massacre Mountain, Hell Canyon,Castle Peak, northwest(20ø-50ø), but a few dip steeplysoutheast or MountMcCaleb, Swauger Gulch, and perhaps the ESE (LeathermanPass, Castle Peak, Swauger Gulch, and ancestralHanging Valley faults (Plate 1 andFigure 1). In WhiteKnob faults) (Figure 2 andPlate 1)). The apparent the centralLost River Range the spacingbetween NE shallowerdips of NW dippingfaults relative to SE dipping strikingnormal faults is 5 km or lessin mapview. A faults(Figure 2) mayrecord subsequent east tilting during coherentunfaulted block about 15 km wideappears to episode2. There are too few SE dippingfaults to be separatethe HangingValley fault from the nearestNE conclusive,however, and it not dear whetherall or only strikingfault to the southeastand a similarunfaulted part of the centralLost River rangetilted to the east blocklies northwest of the MahoganyCreek fault along duringepisode 2.

MahoganyCreek fault Elkhorn Creek fault

--t- N=15

b Leatherman Pass fault Massacre Mountain fault

d Hell Canyonfault SW end Hell Canyonfault NE end

Fig. 2. Lower-hemisphere,equal-area stereograms of slickenlines(open squares) and fault surfaces (greatcircles and solid dots) along NE strikingnormal faults in (a-i) theLost River-Lemhi area, plus (j) the WhiteKnob fault (along the southeastern margin of theWhite Knob Horst, Figure 1) showinga consistentWNW to NW extensiondirection. In all localities(Plate 1) faultsplace younger rocks on olderrocks, indicating normal offset. N1 is numberof planes,N2 is numberof slickenlines,N is planes and slickenlines.In Figure2a, slickenlinesfrom subsidiaryfaults are alsoshown, but associatedfaults are not included.(k) Combinedslickenline data from NW dippingfaults. (1) Data in Figure2k correctedfor 35•E tilt duringepisode 2. Note thatextension directions are similarin eithercase. 1124 Janeeke:Cenozoic Extension, East Central Idaho

Mount McCaleb fault, NE end Mount McCaleb fault S W

_'•,, /

g •

½'

\ / ,•///•' // ß

i j

= 1 = 1

Fig. 2. (continued)

Displacementsacross NE strikingfaults range from NW dippingfaults (Figure 2k) for 35ø of easttilt during tensof metersto severalkilometers (Figure 3). A typical episode2 yieldsan averageN45øW-S45øE extension NE strikingnormal fault cutsout one or two Paleozoic direction(Figure 21) directly downdip to veryslightly formationsalong its trace. The largestNE strikingnormal right-obliqueon the averagetilt-corrected fault plane. fault, the LeathermanPass fault, juxtaposes Mississippian Kinematicanalyses of the fault-slipdata consistently rocksagainst Ordovician rocks and recordsan estimated4 yieldNW-SE extensiondirections (Figure 4). Note that km of throw. Extensionacross NE strikingfaults was the extensiondirection inferred for the oblique-slip modest,of the orderof 30% (Figure3). If apparently SwaugerGulch fault is similarto that alongNE striking unextended blocks NW and SE of the central Lost River dip-slipfaults (Figure 4). NW-SE extensionare further Rangeare included,estimated extension drops to 10-20%. supportedby paleomagneticdata that showno verticalaxis rotation, and thereforeno reorientationof structures,in Kinematic Data: Faults and Dikes the centralLost River and centralLemhi ranges [Janecke et al., 1991]. Slickenlineson eightNE strikingfaults indicate a Dikes are excellentpaleostress indicators [Anderson, consistent WNW-ESE to NW-SE extension direction and a 1951],unless dikes intruded along preexisting structures. smallcomponent of strike-parallelmovement (Figures Dikes are not abundant in the Lost River-Lemhi area 2a-2j). The nearlyvertical Swauger Gulch fault is unique exceptin a broadNE to ENE strikingswarm near the in this fault setbecause of its largestrike-parallel MountMcCaleb and Swauger Gulch faults (Plate 1). componentof slip (Figure2i). Correctingslickenlines on Most dikesindicate a NW trendingleast principal stress in Janeeke:Cenozoic Extension, East Central Idaho 1125

NW SE

3 km

I i i

overlap

NW SE

paleotopography(?) IPMs

•*'••,•.•.• 3km gap Fig. 3. Northwest-southeastcross section of threeNE strikingnormal faults. Volcanic rocks are stippled.Location of crosssection is shownin Plate1. Restorationindicates 30% extension during synvolcanicfaulting in thisarea. Explanation of thegap and overlap is notclear, but the Long Lost fault may havebeen reactivated. the LostRiver-Lemhi area during volcanism. Some ESE dippingnormal faults had offset the tuffaceousrocks, strikingdikes crop out near Burnt Creek [Skipp et al., offsetswould be in the oppositedirection. 1988;Janeeke and Wilson, 1992; Janeeke, 1992b], and Additionalevidence for synvolcanicnormal to similarstrikes have been noted in partsof theWhite Knob normal-obliqueslip faulting exists along the Mount Mountains(L. G. Snider,oral communication, 1988). McCaleb,Hell Canyon,Leatherman Pass, and Swauger ESE strikingdikes near Burnt Creek do not necessarily Gulchfaults. Displacement diminishes northeastward recordNNE-SSW extensionbecause they may have alongthe Mount McCaleb fault as it penetratesChallis rotatedin a counterclockwisesense near the Long Creek volcanicrocks. The sub-Tertiaryunconformity is offset paleomagneticlocality of Janeekeet al. [1991]. morethan 1500 m in mapview, whereas the top of a distinctivedacite flow is offsetonly about 500 m, and TimingConstraints for NE StrikingFaults tuffaceousrocks are virtually unfaulted (Plate 1). Similar relationshipsexist along the Hell Canyonfault, which dies Crosscuttingrelationships between several NE striking out withinandesitic flows of the ChallisGroup, and along faultsand Eocene volcanic rocks suggest that NW-SE the LeathermanPass fault, whichcuts some lava flowsbut extensionbegan and ended during Challis volcanism. appearsto be lappedby youngerlava flows and a 46.1Ma MostNE strikingnormal faults cut andesitic to daciticlava tuff at its northernend (Plate 1). The SwaugerGulch flowsof the EoceneChallis Group (Tc in Plate1). The fault cutsbasal Challis conglomerates but apparentlydoes faultsconsistently displace the sub-Tertiary unconformity not offsetyounger lava flows (Plate 1). exposedin the centralLost River Range in a left-stepping It is temptingto arguethat all NE strikingnormal sense,and as a resultChallis volcanic rocks wrap around faultsin the Lost River-Lemhiarea were activeduring from the Pass Creek-Wet Creek reentrant to the Burnt peakChallis volcanism. However, the minimum age of Creekarea (Plate 1). NW-SEextension was short-lived, faultingis not well constrainedfor theElkhorn Creek, however,because few NE strikingfaults offset younger MahoganyCreek, Mount Breitenbach, Castle Peak, and tuffsand tuffaceous sedimentary rocks near the topof the LongLost faults, and along other faults some slip could Challisstratigraphy. postdateChallis volcanism. In theSawmill Canyon area of The minimumage of faultingis bestconstrained in the LemhiRange, for example,a NE strikingfault may the PassCreek-Wet Creek reentrant,where east dipping displacepostvolcanic sedimentary deposits. Some slip tuffaceousrocks dearly lap acrossthe projected trace of alongthe NE strikingDry Creekfault may postdate the severalNE strikingfaults. The northerlytrace of the OligoceneDonkey fault (Plate1) and a post-latestEocene tuffaceousrocks is interruptedonly by youngerNW SE dippingnormal fault can be inferredto existnear the strikingBasin and Range faults, and these offset the rocks northerntip of theBorah Peak horst. As a generalrule, in a right-steppingsense (Plate 1). If NE strikingand NW however,NW and NNW to north strikingfaults cut NE 1126 Janecke:Cenozoic Extension, East Central Idaho

Hell Canyonfault, SW Hell Canyonfault, NE MassacreMountain fault LongLost fault El E1 E1 E1

White Knob fault Pass fault Creekfault MountMcCaleb fault E1 E1 E1 E1

ßP P q- mi.-

.p

SwaugE Elkhorn Donkeyfault at Buck Donkeyfault at LongCreek Gulch fault fault Creek E2 E1 E1 E2

N = 10 C.I. = 2.0 sigma HangingValley fault ElbowCanyon fault LostRiver fault at E2 E2 ElkhornE3Creek SummaryEpisode1=whiteofextension Episode directions2=black

Fig. 4. Summarydiagrams of faultkinematics for datain Figures2, 6, and 8 plusdata from subsidiary faultsthat are not shown. Kinematicdata are displayedas focalmechanisms using the Binghamstatistic averageof the shortening,and extension axes determined with the FaultKin3.25a programof R. Allmendinger,et al., CornellUniversity, Ithaca, New York. Data for episode1 and 2 were correctedfor 35ø of possiblesubsequent east tilt. Extensiondirections without tilt correctionare similar. Note that the extensiondirection inferred for the oblique-slipSwauger Gulch fault is similarto thatalong dip-slip faults. E 1 is episode1 (light shading),E2 is episode2 (mediumshading), and E3 is episode3 (solid). Summarydata in last stereogramshows the nearly90 ø change of extensiondirection between episodes 1 and 2. Janecke:Cenozoic Extension, East Central Idaho 1127 strikingnormal faults in the LostRiver-Lemhi area, strikingnormal faults. Three faultsof thisset are exposed suggestingthat NE strikingfaults are the oldestnormal in the Lost River-Lemhiarea; the PassCreek fault system, faults in the Lost River-Lemhi area. Later reactivation of the SawmillCanyon fault, and the Donkeyfault (Plate 1 NE trendsremains a possibility. andFigure 1). Dacitic to andesitic lava flows in the Lost River-Lemhi area cut by NE strikingnormal faults range in agefrom Pass CreekFault System about49 Ma to 48 Ma (Janeckeand Snee, submitted manuscript).Tuffaceous rocks of the ChallisVolcanic West dippingnormal faults of the PassCreek fault Groupthat lap NE strikingfaults in the PassCreek Wet systemdef'me the easternmargin of the PassCreek-Wet Creekreentrant are 48.0to 47.7Ma (Janeckeand Snee, Creek reentrant,a major north trendingstructural and submittedmanuscript). North of BorahPeak the volcanic topographiclow in the LostRiver Range. North striking rocksthat lap acrossthe LeathermanPass fault are more and eastdipping Challis volcanic rocks form a continuous than46.1 Ma (Janeckeand Snee, submitted manuscript). belt alongthe westernedge of the reentrant,and younger Togetherthese timing constraints indicate that nonresistantconglomerates and gravelsunderlie the center displacementon some,and perhaps most, NE striking of the reentrant. TheseTertiary volcanic and sedimentary normal faults occurred between about 49 and 48 Ma. rocksdip 10ø-80ø into the PassCreek fault system.Dips Can this incrediblytight timingconstraint on NW-SE on the PassCreek fault are as low as 11ø but typically extensionbe correct?Slip rateson somefaults would rangefrom 20ø to 40ø . Dip separationacross the Pass havebeen of the orderof severalmillimeters per year,and Creekfault is about8 to 11 km (Figure5) basedon extensionwould have spanned little more than 1 m.y. projectedcontacts at depth. Extensionwithin a volcanicarc, particularlyone asrobust The Pass Creek fault branches near its southern end as the Challisarc, is likelyto havebeen extremely rapid, into the Elbow Canyonand Hanging Valley low-angle however,due to thermalweakening of the crust. The normalfaults (Plate 1). The ElbowCanyon fault is regionaltectonic setting of eastcentral Idaho during structurallybelow the HangingValley fault andjuxtaposes middleEocene Challis volcanism is compatiblewith Mississippianrocks in its hangingwall againstDevonian to NW-SEextension (see below), and there is goodevidence Ordovicianrocks in its footwall. The Elbow Canyonfault for NW-SE extension elsewhere in Idaho at this time dips200-25 øWSW, andlast movement was downdip to the [Mcintyreet al., 1982;Ekren, 1985]. WSW (Figure6d). The southernend of theElbow Canyonfault coincides with a sharpbend in the range StructuralS•yle of NE Stn'kingFaults boundingLost River fault (Plate1). The HangingValley fault placesupper Paleozoic The NW dip of mostNE strikingfaults in theLost strataand Tertiary volcanic and sedimentaryrocks in its River-Lemhiarea suggests that they may be rotated hangingwall againstUpper Devoniandolomites in its dominofaults above a gentlyNW dippingdetachment footwall. The fault dipsabout 40 ø WSW in a 20 m long fault. NE strikingfaults are restrictedto a beltbetween exposureof the faultnear its southernend, but to the the ancestralHanging Valley fault and the Mahogany north the fault dipswest or NW. Most slickenllnes Creekfault, suggesting that the ancestralHanging Valley preservedon the sharp,planar fault surfacenear its faultmay have been a breakawayfault for the inferred southernend plunge SW (Figure6c). The ElbowCanyon detachmentfault. Major slipacross the HangingValley andHanging Valley faultsbound an extensionalhorse fault is inferredto datefrom the secondphase of faulting, about!km thick(see also Figure 6 of Link et al. [1988]) however.Northwest of the MahoganyCreek fault the in the southernpart of the PassCreek-Wet Creek inferreddetachment may havepenetrated to greaterdepth reentrant.Dip separationacross the HangingValley and allowingthe hangingwall to slipas a singlecoherent ElbowCanyon faults is about9-10 km, basedon projected block. Alternatively,NE strikingnormal faults may not hangingwall and footwallcutoff points. soleinto a detachmentfault at depth.The data are The largeamount of slip acrossthe Elbow Canyon, insufficient to discriminate between these two models. HangingValley, and Pass Creek faults and their present low dip are typicalof regionallyextensive detachment NNW STRIKING FAULTS (EPISODE 2) faults. In contrast,three lines of evidencesuggest these faultsare rotatedhigh-angle faults. First,Tertiary The largestand most spectacular normal faults in the conglomeratebeds in boththe hangingwall andfootwall Lost River-Lemhiarea formedduring a youngerepisode of eachfault dip east(Figure 1). Second,enveloping of extensionand basinsubsidence (episode 2). Hait [1984] surfaces of folded Proterozoic to Paleozoic rocks in the identifiednormal faults formed during this extensional footwallof the PassCreek fault systemdip eastward episode,but he did not rec%mizegenetically related [Mapeland Shropshire,1973; Rember and Bennett, 1979], conglomerateand gravel sequences preserved in the whereasin an east-vergentfold and thrustbelt enveloping hangingwalls of manyof the northto NNW strikingfaults. surfacesshould dip gentlywestward. Third, the hanging This episodeof extensionproduced a seriesof Basinand wall of the fault systemhas none of the internalfaulting Range-stylehalf grabenand easttilted mountain blocks typicalabove detachment faults. Altogetherthese data throughouteast central Idaho and southwesternMontana suggestthat the Elbow Canyon,Hanging Valley, and Pass in Eoceneto Oligocenetime [Janecke,1991; Fields et al., Creekfaults rotated 250-40 ø duringCenozoic extension. 1985]. Coarsesyntectonic conglomerates derived in large An enormoussurficial gravity slide block intercalated part from upliftedfootwall blocks are preservedin hanging in syntectonicbasin f'dl in the hangingwall of the Pass wall basins and date movement on the north to NNW Creekfault (Figure5) showsthat there was a steep 1128 Janecke:Cenozoic Extension, East Central Idaho

gravityslide block •

complexlyfaulteO•sp %•/. ,---,PMs 3km % %%Mba Mm Msc

W 60 ø Ts

gravityslide

Tc

x IPMs IPMs \ \ \

Mba Msp 'Msc

DSu 3 km

Restored section

Fig. 5. Crosssection of the PassCreek fault with slideblock and restoredcross section showing synextensionaltilting of the PassCreek fault. Thereis about10 km of dip separationand 69% extensionof the line of section.A smallnormal fault in the hangingwall of the PassCreek fault was omitted.Ts is Eoceneconglomeratic deposits; Tc is Eocenelava flows and tuffs of ChallisGroup; IPMs is SnakyCanyon formation; Mba is BluebirdMountain and Arco Hills formations;Msu is Surrett CanyonFormation; Msc is SouthCreek Formation; Msp is ScottPeak Formation; Mm is Middle Canyonand McGowan Creekformations; DSu is undividedSilurian to Devonianrocks; and QT is Quaternaryand Tertiarydeposits. topographicfront and highrelief alongthe easternmargin Slip alongthe PassCreek fault systemdearly of the basinwhen sediment was being deposited. controlleddeposition of grabenfill successionsin its Althoughthe late Paleozoiccarbonate rocks in the 3 km hangingwall. The oldestsyntectonic deposits thus date by 15 Ion gravityslide block could have been emplaced the onsetof extensionacross the PassCreek fault system, from west to east across several kilometers of volcanic and youngercoarse sediments record continuing slip. Two rocks,it is muchmore likely that the slideblock originated plateauages and onetotal gas age from thin ashflow and in correlativerocks that were upliftedin the immediate air fall tuffs interbedded in the lower half of the footwallof the PassCreek fault system. sedimentarysection show that mostof the syntectonic conglomeratesand gravelsin the hangingwall of the Pass In general,Eocene volcanic rocks in the hangingwall Creekfault systemwere laid downduring middle Eocene of the PassCreek fault systemdip more steeplyeastward time. A 45.7+_. 0.1 Ma (plateau,sanidiae) ash flow tuff a thando overlyingEocene to Oligocene(?) sedimentary few hundredmeters above the base of the sedimentary rocks. Locally,volcanic rocks dip as muchas 80ø , whereas section(Janecke and Snee, submitted manuscript) suggests the stratigraphicallyhighest beds in the youngerTertiary that extensionbegan about 46 Ma. An air fall tuff sedimentaryrocks are flat lyingto gentlytilted. The intercalatednear the middleof the sedimentarysequence systematicallychange of clastlithology from dominantly is somewhatyounger (45.4 + 0.1Ma, plateau,saddine), volcanicin older sedimentaryrocks to dominantly anda discordanttotal gas age on biotite from the base of limestonein youngersedimentary rocks records tectonic the uppermember of the sedimentarysequence is 43.6+_. uplift and extensiveerosional unroofing of the footwallof 0.2 Ma (Janeckeand Snee, submitted manuscript). the PassCreek fault system[Janecke and Snee, 1990]. Extrapolatedsedimentation rates suggest that theyoungest The data indicatecoeval faulting, block tilting, basin preservedTertiary sedimentary rocks may be Eocenein subsidence,slide block emplacement, and sedimentation. age. Someslip musthave continued on the PassCreek Janeeke:Cenozoic Extension, East Central Idaho 1129

Donkeyfault in LongCanyon Donkeyfault near BuckCreek

J •e•e

N1=9 N2=11

HangingV,alley fault

Fig. 6. (a-d) Lower-hemisphere,equal-area stereograms of slickenlines(open squares) and fault surfaces (greatcircles and poles to faultplanes) along north to NNW strikingnormal faults. Samplelocalities are shownin Plate 1. (e) Compositeof slickenlines,excluding six NW plunginglineations on the HangingValley fault. N=68. (f) Data in Figure6e correctedfor 35øEtilt. fault after depositionof the youngestsedimentary rocks Sawmill CanyonFault becausethese deposits are faultedagainst lower Paleozoic and Proterozoicdolomites and quartzites yet containno The SawmillCanyon fault in the centralLemhi Range dolomiteand quartziteclasts. Thus the sedimentaryrocks juxtaposedTertiary conglomerates of Eocene to Oligocene do not constrainwhen the PassCreek fault systemstopped (?) age againstlower Paleozoic and Precambrian rocks moving. (Plate1). The SawmillCanyon reentrant is probablya The Basinand Range Hawley Mountain fault offsets continuationof the PassCreek-Wet Creek reentrantand, the PassCreek fault (Plate 1). I arguebelow that Basin like the PassCreek-Wet Creek reentrant, is a major andRange faults are 17 Ma andyounger in eastcentral topographicand structural low in the range. Dip Idaho,implying that slipon thePass Creek fault ended separationacross the SawmillCanyon fault is about6.5-8 beforelate earlyMiocene time. The availablerelative and kin. Paleozoicto Tertiaryrocks in thehanging wall dip absolutetiming data show that extension across the Pass 20"-70"east into the westdipping Sawmill Canyon fault. Creek fault systembegan about 46 Ma, duringmiddle The surfacetrace of the poorlyexposed fault suggestsa Eocenetime, and mayhave continued into Oligoceneor moderate400-60 ø westdip. The onlypossible source for possiblyearliest Miocene time. distinctiveclasts within one memberof a thick syntectonic 1130 Janeeke:Cenozoic Extension, East Central Idaho

Tertiaryconglomerate sequence in its hangingwall is in DonkeyHills the Donkeyfault is covered,but a generally the footwallof the Samill Canyonfault, again NNW trendingbedrock escarpment and locallywell demonstratingthe synchroneityof basinsubsidence and developedfault rockspartially constrain the traceof the slip acrossnorth strikingnormal faults. Basinand Range fault. A NW trendingportion of the escarpmentcoincides reactivationof the Samill Canyonfault producedseveral with the projectedtrace of the HawleyMountain fault, discontinuousQuaternary fault scarps[Janecke, 1991]. suggestingthat the HawleyMountain fault cut acrossthe Donkeyfault and offsetit downto the SW. If so,slip on DonkeyFault the Donkeyfault endedbefore late CenozoicBasin and Rangeextension began across the Hawley Mountain fault A newlyrec%•nized low-angle normal fault in the (Plate 1). Nowheredo NE strikingnormal faults offset DonkeyHills juxtaposesa thick sectionof eastand ENE the Donkeyfault, exceptpossibly in the Dry Creekarea dippingOligocene Donkey Fanglomerate and Eocene describedabove. Rather the Donkeyfault truncatesmost volcanicrocks in its hangingwall againatProterozoic NE strikingfaults (Plate 1), indicatingthat slip was middle throughPaleozoic rocks in its footwall. Donkey Eoceneor younger. Fanglomeratebeds dip 20ø to 30ø eastor NE into the fault The Donkeyfault musthave been active during (Plate1). The Donkeyfault loses displacement and cuts depositionof the DonkeyFanglomerate because lower upsectionin its footwallto the south. At its southeastern Paleozoicto Proterozoicquartzites exhumed by the fault end it juxtaposesvolcanic rocks in both its hangingwall alongthe easternmargin of the Donkeybasin were the and footwalland the stratigraphicseparation drops to sourcefor the vastmajority of clastswithin the Donkey zero. Three-pointsolutions of the map traceof the fault, Fanglomerate.There is no sourcefor theseclasts in and field observationsconsistently indicate 00-30 ø westor volcanicand carbonaterocks in thehanging wall of the SW dip (Figures6a and6b). Eastto ENE dipping Donkeyfault. Localizedmegabreccia blocks in the Tertiary rocksin both the hangingwall and footwallof the sedimentarysection attest to hightopographic relief along Donkeyfault suggestthat this fault, like the PassCreek the basinmargin during conglomerate deposition. A fault system,may be a rotatedhigh-angle fault. The f'me-grainedtuff interstratifiednear the baseof the extensiondirection across the southernpart of the Donkey syntectonicDonkey Fanglomerate in the DonkeyHills is faultwas west to WSW (Figures6a and6b). Dip 30-32Ma, indicatingmiddle Oligocene slip across the separationacross the Donkeyfault is more than 10 km in Donkeyfault. When slipbegan and ended is uncertain the DonkeyHills (Figure7). This estimateis imprecise, becausethe top of the DonkeyFanglomerate could not be however,because footwall and hangingwall rocksdiffer in datedand the baseof the DonkeyFanglomerate is not strikeand the depositionalcontact between Challis exposed.There is no evidencefor Quaternaryreactivation volcanicrocks and the DonkeyFanglomerate is not of the Donkeyfault. exposedin the westernDonkey Hills. Paleomagnetic and geologicinvestigations summarized by Janeckeet al. [1991] StructuralStyle of NNW Stn'kingFaults showthat the hangingwall of the Donkeyfault hasrotated in a counterclockwise sense north of about 44o15' to The systematicwest to WSW dip of the PassCreek, accommodatea northward increase in displacementacross SawmillCanyon, and Donkey faults and other Eocene and the fault. The Donkeyfault thusbounds the easternedge Oligocenefaults in the regionsuggest they have tilted of a north plungingand southtapering Paleogene domino-stylein the hangingwall of a regionalwest to extensional basin. WSW dippingdetachment fault. The inferreddetachment Crosscuttingrelationships suggest that the Donkey fault may extendbeneath the Idahobatholith farther west. fault is youngerthan NE strikingnormal faults but older Somefaults of set2 are only1-2 m.y.younger than than NW strikingBasin and Rangefaults. In the southern faultsof set 1, yet their strikesand slip vectors differ

WSW ENE LostRiver Range PahsimeroiValley DonkeyHills

Proterozoicquartzite Y

3 km

Fig. 7. Crosssection of the DonkeyHills showingthe Donkey and Goldburg faults. Amountof offset acrossthe Goldburgfault is conjectural.IPMu is undividedMississippian to Pennsylvaniancarbonates; Ts is Eocene(?) to Oligoceneconglomeradc deposit; Tc is Eocenelava flows;Tct is Eocenetuff; Y is middleProterozoic quartzites; and QT is Quaternaryand Tertiary deposits. 1131 Janecke:Cenozoic Extension, East Central Idaho

radically(Plate 1 andFigures 2, 4, and6). WSWto west Wyomingshows that initial Basin and Range extension plungingslickenlines on theDonkey, Elbow Canyon, and datesback to as far as 17 Ma [Fieldset al., 1985; HangingValley faults suggest that the extensiondirection Hannemanand Wideman, 1991; Burbank and Barnosky, mayhave changed by roughly 90" between 48 Ma (when 1990]. I prefer the 17 Ma estimatefor the onsetof Basin NW extensionended) and 46 Ma (whensediment began and Range extensionin eastcentral Idaho becausethe to accumulatein the hangingwall of the PassCreek fault meagersedimentary record of lateOligocene to early system)(Figure 4). Miocenedeposits in eastcentral Idaho summarized by Ruppeland Lopez [1988]is consistentwith thisscenario BASIN AND RANGE EXTENSION AND [Janecke,1991] and becauseinitial extensionwould be EMERGENCE OF NW TRENDING RANGES synchronousacross a largeregion. Further work is clearly neededto resolvethis important issue. ExtensionDirection and Magnitude DISCUSSION

Basinand Range faults (episode 3) in theLost CenozoicExtension in NearbyAreas River-Lemhiarea include the Lemhi, Barney, Hawley Mountain,Goldburg, and Lost River faults [Janecke, The middle Eocene was a time of extensionassociated 1991].Most of thesefaults strike northwesterly and dip with magmatismin Idahoand western Montana. Ductile SW. SW plungingslickenlines measured along the Lost deformationand ESE-WNW tectonic unroofing of the River fault near Elkhorn Creek are consistentwith the Bitterrootand perhaps the Pioneer metamorphic core NE-SW extensioninferred by Stickneyand Bartholomew complexeswere roughly coeval with middleEocene Challis [1987]for SW Montanaand easternIdaho and with magmatism[Garmezy and Sutter, 1983; Wust, 1986; previousmeasurements made along the Lost River fault by O'Neill andPavlis, 1988]. In the areanorth and west of R. C. Bucknamand A. J. Crone(Figures 8a and8b). Challis,NE strikinggraben and half graben developed East central Idaho extended 12 to 20% in a NE-SW duringvolcanism and filled with thick sectionsof directionas a resultof Basinand Range faulting, assuming pyroclasticand volcaniclastic rocks [Mcintyre et al., 1982; the amount of horizontal extension across the Lost River Ekren,1985]. Caldera margins, plugs, and stocks (such as fault(Table 3 of Janeckeet al. [1991])is similarto the elongateCastro pluton) are alignedalong the NE extension across the Lemhi and Beaverhead faults. trendingTrans-Challis zone (Figure 1) [Bennett,1986; Kiilsgaardet al., 1986;Fisher et al., 1992],and the Initiationof Basinand Range Faulting in East Central overwhelmingmajority of Eocenedikes in thisregion Idaho strikeNE [Nelsonand Ross, 1969; Fisher et al., 1983;K. Johnson,oral communications 1990]. Some NW striking The ageof initialBasin and Range extension is poorly younger-on-olderstrike-slip faults northwest of the constrainedin east central Idaho, and the relationship (or Pioneermetamorphic core complex may also date from lackthereof) between extension and the Yellowstone thistime [Burton et al., 1989;Batatian, 1991]. Kinematic hotspotis debated[Scott et al., 1985;Fields et al., 1985; datafrom supracrustalNE strikingfaults in centralIdaho Smithet al., 1985;Anders et al., 1989;Rodgers et al., are sparse,but the persistentNE strikeof the normal 1990]. Basinand Range extension may date mostly from faultsand dikes suggests NW to WNW extensionduring the last4 to 7 m.y.[Scott et al., 1985]or mayhave begun middleEocene Challis magmatism. The extension before10.3 Ma [Anderset al., 1989].Stratigraphic directionis in excellentagreement with that inferred for evidence from southwestern Montana and northwestern episode1 in theLost River-Lemhi study area (Figure 4).

kk N2:19//

Lost River fault near Elkhorn Creek

Fig.8. Lower-hemisphere,equal-area stereograms of slickenlines (open squares) on the Lost River fault (greatcircles and solid dots). The extension direction ranges from N45øE to N50øE. (a) Sample locality in Plate1. Co)Data of R. C. Bucknamand A. J. Crone(A. J. Crone, written communications 1988). N=15. 1132 Janeeke:Cenozoic Extension, East Central Idaho

Unroofmgof the lowerplate of the Pioneer 1990]. The kinematicdata alone, in the absenceof metamorphiccore complex may date from thisperiod geochronologicdata, would suggest formation of WNW [Wust,1986; O'Neill andPavlis, 1988], but the mostrecent directednormal faults during a singleextensional episode. and detailedstudy suggests WNW directedextension in Basinsubsidence and roughlyE-W to NE-SW latestEocene to earliestOligocene time [Silverberg,1988, extensionafter Challismagmatism affected the Salmon 1990]. Silverbergdated metamorphic rocks in the lower Basin[Harrison, 1985] and basins of southwestern plate of the Pioneermetamorphic core complex and the Montana[Dunlap, 1982; Fields et al., 1985;Hanneman thin sliverof mylonitesalong the NW flank of the complex andWideman, 1991]. The geographicextent, kinematics, and arguedfor two phasesof ductileextension in the andtectonic causes of thisphase of extensionhave not complex:the first phasewas characterized by NNE-SSW been explored. extensionduring emplacement of the Pioneerintrusive suite48-45 Ma and the secondphase, in latestEocene to TectonicSem3g of Extension earliestOligocene time, involvedWNW directedextension alongthe Wildhorsedetachment fault [Silverberg,1990]. NE strikingnormal faults in the centralLost River If Silverbergis correct,WNW directedextension in the Rangeand the Trans-Challiszone formed in an intraarc Pioneermetamorphic core complex (the mainphase of settingin the midstof the middleEocene Challis volcanic extension)would be about10 m.y.younger than the arc. The unusualwidth of the Challisvolcanic arc, from youngestwidespread volcanism in the Challisfield and Washingtonto northwesternWyoming [Armstrong, 1979] wouldbe distinctlyyounger than episodeI in the Lost (Figure9), is probablydue to highconvergence rates River-Lemhiarea. This is an unexpectedresult because betweenoceanic plates and North America in middle extensiondirections on synvolcanicNE strikingfaults in Eocenetime. Ratesof convergenceranged from 110 to the LostRiver Range (Figure 2) are strikinglysimilar to 200 mm/yr at the latitudeof eastcentral Idaho the extensiondirections along the Wildhorsedetachment [Engebretsonet al., 1985;Stock and Molnar, 1988]. NE fault [Wust,1986; O'Neill and Pavlis,1988; Silverberg, strikingnormal faults were perpendicular to the axisof the

N

Kulaplate

/ '"' '.• ! ] : !

.:

:',. I I ; I I

: Idaho batholith

o lOO 200 km ß . i ß .

ß

ß

ß Farallonplate

:

Fig. 9. Paleogeographicmap of thePacific Northwest at 55+_5Ma showingnearly trench-parallel extensionin eastcentral Idaho. Map is modifiedfrom Heller et al. [1987]. Locationof Eocene volcanicand intrusive rocks (random dash) is afterArmstrong and Ward [1991]. Frontalfaults of the Sevierthrust belt in Canadaand Montana were inactive by thistime [Harlanet al., 1988],but contractioncontinued farther to the south[Dickinson et al., 1988].

Janecke:Cenozoic Extension, East Central Idaho 1135

Challisarc and nearly perpendicular to the trenchat that beingsubducted beneath each area (shown schematically time (Figure9). NW-SE synvolcanicextension in Idaho in Figure9). Platereconstructions and geologic and thuslengthened as well aswidened the volcanicarc. paleomagneticdata indicate that the Kula-Farallonridge NE strikingnormal faults are nearlyperpendicular to wasnear northern Washington in Eocenetime [Coeet al., Mesozoicfold axesin the LostRiver-Lemhi area (Plate 1) 1990]. If so,rapid oblique subduction of theKula plate and the overall trend of the Sevier fold and thrust belt beneathBritish Columbia [Engebretson et al., 1985]may (Figure9). ThusNE strikingnormal faults did not simply have resulted in a reduction in the stress exerted normal reversea previousepisode of E to ENE directedcrustal to the belt of thickenedcrust, triggering widespread E-W thickening.Gravitational spreading alone shotfid be in an extension in southern British Columbia and northern E-W to NE-SW direction,perpendictfiar to the overall Washingtonin earlyto middleEocene time [Friedman and trendof the Idaho-Wyoming-Montanafold andthrust belt Armstrong,1988]. Southof the triplejunction plate and/or perpendicularto the continentalmargin. boundaryforces prevailed and intraarc NW-SE extension Gravitationalspreading in a NW directionis expectedonly developedbecause convergence was normal to thebelt of if a thickenedwelt is constrainedalong its southwestern thickened crust in the Idaho-Montana fold and thrust Ix:It. marginand the welt is unableto spreadin that direction. The extensiondirection may have depended to some Extensionparallel to the axisof a volcanicarc has degreeon the angleand speedof oceanicconvergence. been documentedin modernarcs and is typically Faultsof set2 are roughlyparallel to (1) Mesozoic expressedby normalfaults, dikes, joints, and flank fold axesin the LostRiver-Lemhi area, (2) theSevier fold eruptionson volcanoesthat are alignedparallel to the andthrust belt, and (3) thereconstructed continental convergencedirection across the adjacentsubduction zone marginof the PacificNorthwest during Eocene to [Nakamura,1976; Nakamura et al., 1977]. The northern Oligocenetime (Figure 9) [Helleret al., 1987,Figure 9]. andsouthern margins of theAltiplano-Puna Plateaus in The largeEocene to OligoceneNNW strikingnormal the centralAndes are presentlyextending in a roughlyN-S faultsof eastcentral Idaho (episode 2) mayrecord directionalong oblique-slip normal faults [Sebrier et al., collapseand lateral gravitational spreading of thickened 1988;Allmendinger et al., 1989]. This small-magnitude crustin responseto a decreasein horizontaltectonic extensionis roughlyperpendicular to the convergence forces,as outlinedby Coneyand Harms [1984], Molnar vectorof the adjacentNazca plate. Althoughgravitational and Chen [1983],and manyothers. Convergencebetween spreadingmay causeextension in the centralAndes the Farallonand North Americaplates slowed at the [Molnar and Lyon-Caen,1988], it is rapid eastward latitude of east central Idaho some time between 59 and convergenceof the Nazcaplate at about100 mm/yr that 42 Ma, possiblyaround 49 Ma [Stockand Molnar, 1988]. probablycontrols the directionof extension(see The nearly50% decreasein the convergencerate may Allmendingeret al., [1989],for a differentinterpretation). explainboth the suddenextinction of the Challisarc in Syn-Challisextension in centralIdaho at 49-48Ma Idahoabout 45 Ma [Janecke,1991] and the major shift in maybe analogousto youngextension in the central extensiondirection and structural style between episode 1 Andes. Convergenceof theFarallon and Kula plates(if. it andepisode 2. RapidFarallon-North America stillexisted at 50 Ma) relativeto NorthAmerica was very convergenceduring episode 1 led to bothintense arc rapidand was toward the northeast in middleEocene time magmatismin the overridingplate and sufficient (Figure9) [Engebretsonet al., 1985;Stock and Molnar, horizontalstress to preventW or WSW directed 1988]. The coincidenceof plate convergencevectors gravitationalspreading of a preexistingcrustal welt. After (N10øEto N85øE)and strikes of dikesand normal faults, thedrop in convergencerates around 49 Ma, however,the andthe highangle between preexisting contractional weltspread outward from its north to NNW trendingaxis, structuresand the earliestnormal faults suggest that andperpendictfiar to the continentalmargin. Thermal duringepisode 1 the geometryof extensionwas controlled weakeningduring Tertiary magmatism probably played an primarilyby plateboundary forces and not by forces importantrole in triggeringthe WSW to SW directed engenderedby thickenedcrust. Rapid NE convergence extensionduring episode 2; however,some other producedNE strikingnormal fatfits in the middleof the mechanism must have caused the extension because volcanicarc duringpeak magmatism. The crustin east episode2 continuedlong after magmatismended in the centralIdaho may have been unable to thickenedfurther region. andbegan to collapse,in itsthermally weakened state, in a The extension direction in the Lost River-Lemhi area directionperpendictfiar to the convergence vector between flippedfrom NW-SE to roughlyNE-SW between48 and North Americaand subducting oceanic plates. Regions to 46 Ma as the forcenormal to a previouslythickened the south in the Great Basin did not extendat this time, crustalwelt droppedbelow a thresholdlevel. The near possiblybecause they had not beenthermally weakened by 90ø changein extensiondirection and kinematic axes Tertiarymagmatism [Armstrong and Ward, 1991]. betweenepisode 1 and2 (Figures2, 4, 6, and8), suggests NW-SE extensioncontrolled by plateboundary forces that the intermediateand minimumcompresslye stresses in eastcentral Idaho was slightly younger than roughly mayhave exchanged as the magnitudeof the NE trending E-W gravitationalspreading in southernBritish Columbia principalstress dropped below the magnitudeof theNW and northeasternWashington [Parrish et al., 1988]. The trendingprinciple stress. differentextension directions can be explainedif therewas Strikesof Basinand Range normal faults in east more crustalthickening in BritishColumbia than in centralIdaho are counterclock•seof two previoussets of centralIdaho before the onsetof extension,if Mesozoic normalfaults. The overalldirection of extensionduring extensionwas prominent in the U.S. Cordillera[Miller, episodes2 and 3 were not dramaticallydifferent, however 1990;Wells et al., 1990],or if differentoceanic plates were (Figures4, 6f, and8), andeast central Idaho continued to 1136 Janeeke:Cenozoic Extension, East Central Idaho

extendin a NE-SW direction. Extensionduring the Basin droppedbelow a thresholdlevel. Becausepaleomagnetic and Range eventwas more modestthan extensionduring investigationsshow no significantvertical axis rotation in episode2. the centralLost RiverRange, this change in extension directionmust be dueto an evolvingstress field. The CONCLUSIONS minimumand intermediatecompressire stresses may have exchanged. The earliestwidespread normal faults in eastcentral Thefinal Basin and Rmage episode of NE-SW Idaho are perpendicularto preexistingfold axesand extension(episode 3) probablybegan about 17 Ma and thereforeare not easilyexplained by gravitationalcollapse resultedin roughly12-20% extension of eastcentral Idaho. modelsof extension.Rather, the strikesof synvolcanic Extension directions have been consistent since 45 Ma. normalfaults are subparallelto the middleEocene convergencevector between oceanic plates and North Americaand are roughlyperpendicular to the trench. Acknowledgments.The authoris gratefulfor support Plateboundary forces associated with rapid convergence from the NationalSalenee Foundation (grant playedan importantrole in determiningthe kinematicsof EAR-8721100 to Ronald L. Bruhn and John W. earlyintraarc extension in eastcentral Idaho. Geissman),the donorsof the PetroleumResearch Fund, About 48-46 Ma, Challisvolcanism waned and administeredby theAmerican Chemical Society (grant to extensionon north to NNW strikingnormal faults began. RonaldL. Bruhn),and Sigma Xi. The U.S. Geological Normal faultsof this set are widelyspaced, rotated Surveyprovided logistical assistance. Many individuals high-anglenormal faults, and slip acrossindividual faults helpedin thisendeavor. I thankJohn Bartley, Frank exceeded 10 km in the Lost River-Lemhi area. West to Brown,James Evans, Donald Fiesinger, John Geissman, SW directedslip acrossthese faults began during middle M. H. Ha_it,Jr., RobertHium, PaulLink, BettySkipp, Eocenetime, continuedthrough Oligocene time, and Larry Snee,Sandy Soulliere, Loudon Stanford, Kirk producedBasin and Range-styleextensional basins Vincent, Anna Wilson,and RonaldWorl for ideas,advice, throughouteast centralIdaho. reviews,and logisticalassistance. Matt Barrett,lames This major episodeof extensionmay represent Evans,Beth Geiger,Russell Griffin, Joan Holloway, and gravitationalspreading of thickenedand thermally David Holmes assisted in the field. I thank Rick weakenedcrust. The onsetof north to NNW striking Allmendingerfor his Stereonet4.3a and FaultKin3.25a normalfaulting and the abruptend of Challisvolcanism programs,and Anthony Crone for datain Figure8b. bothmay have been triggered by a roughly50% reduction Robert Ennisof the U.S. ForestService, Mackay, Idaho, in the Farallon-NorthAmerica convergencerate between kindlyloaned me hundredsof colorair photographs, 59 and42 Ma [Stockand Molnar, 1988]. The extension greatlyfacilitating this study. I especiallyacknowledge directionin the LostRiver-Lemhi area flipped from Ron Bruhnfor suggestingeast central Idaho as a field NW-SE to roughlyNE-SW between48 and 46 Ma as the area. Tectonics reviewers Michael Wells and Rick forcenormal to a previouslythickened crustal welt Allmendingersuggested improvements to the manuscript.

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