See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/253193927 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 CITATIONS READS 41 74 1 author: Susanne Ursula Janecke Utah State University 112 PUBLICATIONS 1,760 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: The East Shoreline strand of the southern San Andreas Fault and its implications for the sites of the next big earthquakes along the SAF in southern California View project Quaternary tectonics and landscape evolution, northeast Great Basin View project All content following this page was uploaded by Susanne Ursula Janecke on 09 July 2015. The user has requested enhancement of the downloaded file. TECTONICS, VOL. 11, NO. 6, PAGES 1121-1138, DECEMBER 1992 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