TECTONICS, VOL. 19, NO. 1, PAGES 1-24 FEBRUARY 2000 Present-day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range province, North American Cordillera TimothyH. Dixon,• MeghanMiller, 2 FredericFarina, • Hongzhi Wang • and Daniel Johnson 2 Abstract. Global Positioning System (GPS) data from some continentalareas appear to behave like oceanic five sites on the stable interior of the Sierra Nevada block lithospherein one important respect:deformation is are inverted to describeits angular velocity relative to occasionallyconcentrated in narrow fault zones that stableNorth America. The velocity data for the five sites accommodaterelative motion betweenrigid blocks. The fit the rigid block model with rms misfits of 0.3 mm/yr Sierra Nevada block in the western United States is (north) and 0.8 mm/yr (east), smaller than independently probablya good exampleof a rigid continentalblock estimateddata uncertainty,indicating that the rigid block [Wright,1976]. Seismicityaround its marginsdelineates model is appropriate. The new Euler vector, 17.0øN, an aseismicregion east of the SanAndreas fault and west 137.3øW, rotation rate 0.28 degreesper million years, of the Basin and Range extensionalprovince (Figure 1). predictsthat the block is translatingto the northwest, However,a rigoroustest of the rigidity of this blockhas nearly parallel to the plate motion direction, at 13-14 neverbeen performed (nor, to ourknowledge, for anyother mm/yr, fasterthan previous estimates. Using the predicted continentalblock or microplate). Perhapsthe Sierra Sierra Nevada block velocity as a kinematic boundary Nevadablock is not rigid at all, and the absenceof condition and GPS, VLBI and other data from the interior seismicityindicates a weak block undergoingdiffuse and margins of the Basin and Range, we estimatethe aseismic deformation, or strain accumulation on a few velocitiesof some major boundary zone faults. For a locked faults cutting the block, previously assumed transect approximately perpendicularto plate motion inactiveor activeat low strainrates (e.g., the Kern Canyon throughnorthern Owens Valley, the easternCalifornia shear fault),to bereleased in futurelarge earthquakes. zone(westem boundary of the Basin and Range province) Spacegeodesy can rigorously test the conceptof rigid accommodates11+1 mm/yr of right-lateralshear primarily continentalblocks, just as it can measurethe angular on two faults, the Owens Valley-White Mountain (3+2 velocityand test the rigidity of largerplates [Argus and mm/yr) and Fish Lake Valley (8+2 mm/yr) fault zones, Gordon,1996; Dixon et al., 1996]. Sincethe motionsof basedon a viscoelasticcoupling model that accountsfor rigidblocks or plateson a spherecan be describedby Euler the effectsof the 1872 Owens Valley earthquakeand the (angularvelocity) vectors [e.g., Chase,1978; Minster and rheologyof the lower crust. Togetherthese two faults, Jordan, 1978; DeMets et al., 1990], we can test whether separatedby less than 50 km on this transect,define a velocitydata from siteson a continentalblock are well fit region of high surfacevelocity gradient on the eastern by a givenEuler vector, and whethervelocity predictions boundaryof the SierraNevada block. The WasatchFault onthe marginsof the blockbased on this Eulervector are zone accommodatesless than 3+1 mm/yr of east-west consistent with other observations. An accurate estimate of extensionon the easternboundary of the Basin and Range Sierra Nevada block motion also holds the key to province. Remaining deformationwithin the Basin and understandingcertain aspects of Pacific-NorthAmerican Rangeinterior is alsoprobably less than 3 mm/yr. plateinteraction [e.g., Atwater, 1970; 1989; Dixon et al., 1995; Wellset al., 1998;Hearn and Humphreys,1998]. 1. Introduction Minster and Jordan [1987] first appliedspace geodetic data to estimate motion of the Sierra Nevada block, Plate boundaries within continents are often definingnorthwest motion of the block at a velocityof characterizedby diffusezones of deformation,quite distinct -10+2 mm/yr. Argus and Gordon [1991] estimatedan from the narrowplate boundariesthat characterizeoceanic Euler vector for the block, defining counterclockwise lithosphereand definelarge rigid plates. However,at least rotationof the block about a pole locatedclose to and southwestof the block, and northwest to north-northwest motionof the block at 9-11 mm/yr, dependingon location. Both of thesepioneering studies used very long baseline • RosenstielSchool of Marineand Atmospheric Science, interferometry(VLBI) data from a few sites near the Universityof Miami, Miami, Florida. 2 deformingeastern margin of the block. The smallnumber Departmentof Geology, Central WashingtonUniversity, of sitesavailable at the time precludeda test of the rigid Ellensburg, Washington. block hypothesis,while the site locationslimited the accuracyof the relativemotion estimatesfor the Sierra Copyright2000 by the AmericanGeophysical Union. Nevadablock, sincethey are not actuallylocated on the stableblock (Figure 1). New Global PositioningSystem Paper number 1998TC001088. (GPS)data from siteson the stableinterior and margins of 0278-7407/00/1998TC001088512.00 the block are now available, enabling us to perform a 2 DIXON ET AL ß PRESENT-DAY SIERRA NEVADA MOTION Longitude (W) 120 ø 116 ø 112 ø 40 ø NA • o Z 38 _• 36 ø i. PA ß %. 34 ø 236 ø 238 ø 240 ø 242 ø 244 ø 246 ø 248 ø 250 ø Longitude (E) Figure 1. Regionalneotectonic map for the westernUS, and (inset) major bounding plates: PA, Pacific plate, JF, Juande Fuca plate, NA, stableNorth America. Active faults on margins of SierraNevada block (SNB) are from Jenningsand Saucedo[1994]: bsf, Bartlett Springsfault; dvfcf, Death Valley-Furnace Creek fault zone; gf, Garlock fault; gvf, Green Valley fault; hcf, Hunting Creek fault; hlf, Honey Lake fault; hmpvf, Hunter Mountain-PanamintValley fault zone; mf, Maacamafault zone; mvf, Mohawk Valley fault zone; rcf, Rogers Creek fault zone; ovf, Owens Valley fault zone; saf, San Andreas fault. CNSB, Central Nevada seismic belt. Faults in eastern Basin and Range (BAR) are from Smith and Arabasz [1991] and include active faults and Late Cenozoic faults that are possibly active; wf, Wasatch fault zone. Seismicity from the University of California, Berkeley catalog at http://quake.geo.berkeley.edu/cnss/catalog-search.htmlshows all events after 1960 with magnitude>4.5 and depth less than 30 km. Note paucity of events within SNB. Spacegeodetic sites within the seismically active easternboundary of SNB and Basin and Range interior shown as open triangles with four letter identifiers. Sites on SNB are omitted for clarity here, but are shown in Figure 3 (location outlined by light solid line). rigoroustest of the rigid block hypothesis,and refineour the interiorof the SierraNevada block, on its eastern understandingof Sierra Nevada motion and associated margin,and in theinterior of theBasin and Range province crustal deformation. These data, and the new (Figures1 and3; Table 1); interpretationsthey afford, are the focusof thispaper. 2. Observationswere made at semi-permanent, continuouslyrecording stations in California,on or near 2. Observationsand Data Analysis theSierra Nevada block, including Quincy (QUIN), part of theInternational GPS Service (IGS) network, operating for The dataused in this studywere obtained in several morethan 5 years;Columbia (CMBB) in California,part ways: of theBay AreaRegional Deformation (BARD) network, 1. Annual campaignswere conductedin August- operatingfor about5 years;ORVB, UCD1 and SUTB, Septemberbetween 1993 and 1998, althoughnot all sites alsopart of the BARD network,operating for about2 were occupiedeach year. Most sites were observedfor 24 years.Stations operating for lessthan 2 years(as of March hoursper day for 3-5 days,in campaignsin 1993, 1994, 1999)are not considered in thisreport; 1995 and 1998 (Figure2). Where available,the resulting5 3. Observationswere made at 16 semi-permanent, yeartime seriesprovide accurate site velocity data, as GPS continuouslyrecording stations widely distributed through velocityerrors depend strongly on the total time spanof easternand centralNorth America,operated by various observations[Mao et al., 1999]. These sites are locatedon agencies,used to definea stableregional reference frame for DIXON ET AL.' PRESENT-DAY SIERRA NEVADA MOTION 3 _ '''1'''1'''1'''1'''1''' 50- o_ -50 50- _ o _ 50- _ o -50 - 50- -50 50 -50 OVRO North OVRO East OVRO Height 94 95 96 97 98 99 94 95 96 97 98 99 94 95 96 97 98 99 Year Figure 2. GPS positions(relative to ITRF-96) as a functionof time, with an arbitraryconstant removed. Error bars(Table 3) omittedfor clarity. Slopeof bestfit line from weightedleast squaresgives velocitiesin Table I. 4 DIXON ET AL.: PRESENT-DAY SIERRA NEVADA MOTION 5O -5O 5O -5O 5O ß E -50 ß I::: 50 (" o * i o ß -5o 0 50 -5o 50- ** _ -50 - i i I i i i i 1 i I i i r [ i i I i ' 5O 0 : I -50 , WMTNNoah • WMTNEast • WMTNHeight '''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1'''1 94 95 96 97 98 99 94 95 96 97 98 99 94 95 96 97 98 99 Year Figure 2. (continued) DIXON ET AL.: PRESENT-DAY SIERRA NEVADA MOTION 5 Table 1. GPS Site Velocities Relative to ITRF-96 Latitude, Longitude, V•19•:ity, mm/yr degN degW North West Vertical CEDA (CedarCreek) 35.75 118.59 -3.6 + 0.9 19.3 + 1.2 -1.4 + 2.9 CMBB(Columbia) a 38.03 120.39 -5.1 + 0.7 19.4 + 1.4 -9.6 + 3.1 GFLD (Goldfield) 37.82 117.36 -11.0 + 1.6 10.8 + 2.7 2.6 + 5.8 ELYA (Ely) 39.29 114.84 -11.6 + 0.8 14.8 + 1.1 -2.5 + 2.9 KMED (KennedyMeadows)