TECTONICS, VOL. 17,NO. 5, PAGES780-801, OCTOBER 1998 Kinematic history of the Laramide orogeny in latitudes 35ø-49øN, western United States Peter Bird Departmentof Earthand Space Sciences, University of California,Los Angeles Abstract. The kinematic history of the Rocky Mountain First, it occurredin a plate interior,so it hasno simpleexpla- forelandand adjacent areas is computedback to 85 Ma, using nationin termsof platetectonics. Second, it occurredin a re- virtually all the structural,paleomagnetic, and stressdata in gion with a continuouscover of Phanerozoicsection whose the literature.A continuousvelocity field is fit to the data in surfacewas initially near sea level, so that all strainsand up- eachtime stepby weightedleast squares, and this velocity is lifts are especiallywell recorded.Third, the literatureon this integratedback throughtime. As proposedby Hamilton subject is mature, and the chance for future discoveriesof [1981], the net movementof the ColoradoPlateau was a first-orderstructures seems small. The Laramideorogeny is clockwiserotation about a pole in northernTexas; but the ro- the primaryfocus of this paper,although the calculationnec- tationwas less(3 ø) than somehave inferredfrom paleomag- essarily includesthose parts of the Sevier orogeny and the netism.The Laramide orogenyoccurred during 75-35 Ma, Tertiary extensionthat overlappedit. withpeak Colorado Plateau velocities of 1.5 mm yr -1 during During this time, easternNorth America was stable.Oce- 60-55 Ma. The mean azimuth of forelandvelocity and mean anic lithospherewas subductingat the westernmargin of directionof forelandshortening was stableat 40ø for mostof North America, as it had been since the Jurassic.The Kula the orogeny,increasing to 55ø in 50-40 Ma; the counter- plate was subductingalong the north part of the margin,and clockwiserotation of shorteningdirections proposed by some the Farallon plate was subductingalong the southpart; the previousauthors is incorrect.Comparing the computed histo- past history of the triple junction betweenthem is controver- riesof forelandflow speedand direction with the knownmo- sial [Engebretsonet al., 1985]. This subductionmay have in- tions of the Kula and Farallon plates confirmsthat the fluenced the Rocky Mountain foreland by generating Laramideorogeny had a differentmechanism from the early horizontalstresses in the lithosphereat the margin.Alterna- Sevierorogeny: it was drivenby basaltraction during an in- tively, there may have been an episodeof horizontalsubduc- terval of horizontal subduction,not by edge forces due to tion of one or both oceanic plates, allowing direct stress coastalsubduction or the spreadingof the westerncordillera transferto the baseof the lithospherein the foreland[Dickin- or by accretionof terranesto the coast.Tentatively, a minor son and Snyder, 1978]. A major goal of this paperis to refine clockwiserotation of shorteningdirections at 50 Ma may rec- the kinematicsso asto permittesting of thesehypotheses. ordthe passageof an activeKula-Farallon transform within In this paper, I apply a new algorithm to computethe the subducted slab. kinematicsof the orogenyin time stepsof 5 m.y. back to 85 Ma. The detailsof the methodare describedin AppendixA. Broadly,however, the algorithmhas the five followingsteps: 1. Introduction (1) Geologic and paleomagneticdata concerningfinite dis- The west centralpart of the United States(Figure 1) has placements,rotations, or strainsover long time periodsare been deformedby three overlappingevents since Cretaceous convertedto rate estimates.An uncertaintyis assignedto each time. The Sevier orogenylasted from about 119 Ma [Heller rate estimate.(2) The velocitiesof all the nodes in a finite and Paola, 1989] to 50 Ma [DeCellesand Mitra, 1995] and elementgrid are determinedby solvinga linear system,which resultedin displacementof thick platesof sedimentaryrock is based on weighted least squaresfitting of the velocity eastwardfor tensof kilometerson bedding-planethrusts with modelto the tentativerates. (3) Thesevelocities are integrated westdipping ramps. The Laramideorogeny lasted from about backwardover time, using 5 m.y. steps.The programmoves 75 to 35 Ma [Dickinsonet al., 1988] and involvedthrusting the nodesof the finite elementgrid, the presentstate lines, or of the Precambrianmetamorphic basement in a varietyof di- otherfiducial markersand the positionsof all dataconcerning rectionson faults of 25o-30ø dip with throwsup to 13 km. earlier times. In particular,paleostress indicators are restored Extensionbegan about 49 Ma [Constenius,1996] in the for- to their original azimuths.(4) When the historyis complete, mer Sevierorogenic belt, and after29 Ma it alsoaffected the eachgeologic and paleomagneticdatum on finite strain,dis- Rocky Mountain forelandin New Mexico and Colorado placement,or rotationis comparedto the historypredicted by where the Rio Grande rift was formed. the model. In general,the model rate of strain,displacement, The Laramideorogeny did not involvelarge strains or dis- or rotation will not be uniform over the time window of the placements,but it is of particularinterest for severalreasons. datum, as initially assumed.New target rates are now as- signed,based on the time-historyfrom the previousmodel but adjustedby a factor to achieve the correct total strain, dis- Copyright1998 by the AmericanGeophysical Union. placement,or rotation.(5) The entirecomputation is now re- Papernumber 98TC02698. peated, beginning with step 2. In all, 50 iterationsof the 0278-7407/98/98TC-02698 $12.00 historywere performed. 780 BIRD' KINEMATIC HISTORY OF THE LARAMIDE OROGENY 7g I 150' 140' 130' 120' 110' 100' 90' 80' I • Canadian Rocky Mountains NORTH Laramide orogen RGR AMERICA Colorado Plateau PLATE 130' 120' 110' 100' Figure 1. Locationof the Laramideorogen in relationto otherCretaceous-Tertiary tectonic provinces in pre- sentcoordinates. The GreatPlains is the upwarpedmargin of the stablepart of the North Americaplate. The Laramideorogen (or Rocky Mountainforeland) is a regionof basementthrusts overlain by forcedfolds. The ColoradoPlateau has similarstructure but underwentless strain. The Sevierorogen (or Sevierbelt or Over- thrustbelt) containseast vergent ramp/flat thrusts in thick sedimentarysequences. The CanadianRocky Mountainsare similar in structureto the Sevierorogen. The Hidalgo orogenis intermediatein characterbe- tweenthe Laramideand the Sevierand may be structurallyconnected to both (dashedlines). The zone of Tertiaryextension includes the Basinand Range province and the Rio Granderift (RGR). The zoneof dis- placedterranes includes Baja Californiaand allochthanousterranes in BritishColumbia and Washington. The Explorer,Juan de Fuca,Gorda, and Cocos plates are remnants of the largeFarallon plate. Bold rectangle showsthe studyarea and locationof Figures2-6. At the end of the computation,we have estimatedhistories This methodis a type of "inverse"tectonic modeling, of the slip on eachfault and the drift and rotationof eachpa- which computesvelocities from geologicdata. It is concep- leomagneticsite. More important,we have an estimatedve- tually and procedurallydistinct from previous efforts to locity field throughtime for the whole region,which can be modelthe Laramideorogeny [Bird, 1988, 1989, 1992],which examinedfor insightsinto the ultimatecauses of the orogeny were "forward"or "dynamic"models based on physicsand and its relationto nearby eventsin North America and/orthe assumedrheologies. Inverse tectonic modeling is relatively Pacific basin. new. Saucier and Humphreys[1993] modeledvelocities in 782 BIRD: KINEMATIC HISTORY OF THE LARAMIDE OROGENY Californiafrom fault slip ratesand geodesy. Holt and Haines eastern North America on the eastern side and on the eastern [1993] usedseismicity to solvefor a continuousvelocity field part of the northernside. The history back to 85 Ma was in Asia, while Avouac and Tapponnier[1993] divided Asia computedusing 5 m.y. time steps.This historywas iterated into four rigid blocks,and Peltzer and Saucier [1996] mod- 50times, using initial rate uncertainties ( cr* ) of 4x 10 -12 m eled its fault network in more detail. In comparison,this s-1 (0.12 mm yr -1) for fault offsets and balanced cross-section methodis uniquein its ability to handlelong histories,finite extensions,2x10 -lø m s-• (0.06ø m.y.-1)for paleolatitude strain,many faults, paleomagneticdata, and stress-direction shifts,and 1.4x10 -•6 radians/s(0.25 ø m.y.-1)for vertical- data. axis rotations.In a typical iteration, about 30% of the rate histories were automatically adjusted; cumulatively, 64% were adjusted.(As discussedin AppendixA, somerate histo- 2. Data and Computation riescannot be adjustedbecause of the dangerof numericalin- The first applicationof this methodis a solutionfor the stability.) The rate uncertaintiesin the final iterationswere Rocky Mountain foreland province of the westernUnited basedonly on the individualuncertainties in displacementor States(latitudes 35ø-49øN, longitudes 103ø-113øW) since 85 rotationtabulated in the files namedin AppendixB. The un- Ma (Santonian).I chosethis regionfor an initial trial because certaintyin the nominally zero strainrate of regionswithout the tectonicsare relatively simple and noncontroversialand activefaults was 5x 10 -17 s-1 (0.16% m.y.-•) in theRocky becausethey are describedin a matureliterature of manage- Mountain foreland and Great Plains but slightly larger able size. (lx10-•6 s-1) in theSevier belt, and was largest (2x10 --•6 I attemptedto collect all the significantinformation