JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B10, PAGES 24,245-24,250, OCTOBER 10, 1998 Comment on "Relative motions of the Pacific, Rivera, North American, and Cocosplates since0.78 Ma" by Charles DeMets and DouglasS. Wilson W. L. Bandy,V. V. Kostoglodov, C. A. Mortera-Guti6rrez andJ. Urrutia- Fucugauchi Instituto de Geoffsica, Universidad Nacional Aut6norna de M•xico, M•xico D.F., M•xico 1. Introduction Since Atwater [1970] first proposed the existence of the Rivera (RIV) plate, its motionrelative to the adjacent Pacific (PAC), Cocos (COC), andNorth American(NAM) plates has been controversial, and several widely different plate motion models have been proposed [Larson, 1972;Minsterand Jordan, 1979; Klitgord and Mammerickx, 1982; Ness et al., 1985; Bandy and Ycm, 1989;DeMets and Stein, 1990;Bandy,1992; Lonsdale, 1995]. Two factors for thiscontroversy are clearly apparent, namely, (1) significant recent changes in relative plate motions between the RIV and adjacent plates,and (2) close proximityof the RIV-PAC, RIV-COC, and RIV-NAM Euler poles (i.e., the surface locations of Euler vectors) to the RIV plate and its boundaries. The first factor is most problematic when attempting to derivepresent-day instantaneous relativeplate motion models using finite rotation methods [e.g., Hellinger, 1979; Chang, 1988] or instantaneous rotation methods[e.g., Chase, 1972; Minster et al., 1974]. The reason for thisis that to produce an accurate present-day Euler vectorusingthese methods, plate motions musthaveremained constant throughout the Brunhes Chron (i.e., since 0.78 Ma). If plate motions have been changing, thesechanges will introduce systematic errorsinto the data. It is well established [e.g., Bevington, 1969] that statisticalmethods, such as the finite rotation and instantaneous rotation methods, arenotgenerally useful for removing system- atic errors.Thus present-day platemotion models derived from these data,employing eitherof the two methods, may contain systematic biases if the constant pole assumption is violated. Since theaccuracy of theplate motion models depends on how well onecanremove systematic errors, the presence of system- aticerrors mayresult in inaccurate, although precise, models of present-day motions. One could simply assume,as have DeMets and Wilson [1997] (hereinafter referredto as DW97), that the effects of the recentplate motionchanges have not seriously degraded the accuracy of the present-day Euler poles. However, due to the closeproximity of the RIV-PAC, RIV-COC, and RIV-NAM Euler polesto the RIV plate, even a small error in the Euler polescan result in substantial errors in the predicted present- day plate velocities along the boundaries of the RIV plate. Given this,onemust demonstrate thatthese changes havenot seriously degraded the accuracy of the plate motionmodels. Without sucha demonstration, one mustbe very cautious in drawing conclusions about present-day platemotions based on either the results of the finite or instantaneous rotation methods. Copyright 1998by theAmerican Geophysical Union. Paper number 98JB01488. 0148-0227/98/98JB-01488509.00 Given this, we are in disagreement with several of the assumptions, statements, andconclusions presented in DW97. In their study,marine magneticanomalylineations, fracture zone crossings, and conjugate points areused in a finiterotation algorithm todetermine a new model forthe average (0.0to0.78 Ma) relativemotions betweenthe PAC, RIV, NAM, and COC plates. They assume that these finite rotation poles are equiva- lent to the present-day instantaneous Euler poles and draw conclusions asto present-day relative motions, but we believe some of these conclusions areat odds withexisting geologic data. In the following,we address our comments to eacharea of disagreement in turn. 2. Use of 0.78 Ma Averages to Approximate Present-Day Plate Velocities DW97 have assumed that the 0.78 Ma RIV-PAC finite rotation pole determined in their study represents present-day instantaneous plate motions. As the validity of manyof their conclusions rests on this assumption, it is critical to ascertain whether it is reasonable. We are of the opinion that this assumption is invalid for threereasons. First,according to plate tectonic theory a 0.0-0.78 Ma finite rotation poleis only equivalent to a present-day instantaneous Euler vector if plate motions haveremained unchanged during the time period 0.0-0.78 Ma; except givenfortuitous circum- stances [Cox and Hart, 1986]. However, numerousstudies havedemonstrated theoccurrence of significant changes in the relativemotion of the RIV plate with respect to the PAC plate during the past 1.5Myr [e.g., Macdonald et al., 1980; Lonsdale, 1989; Bandyand Yah, 1989;DeMets and Stein, 1990;Bandy, 1992;Lonsdale, 1995], changes which havecontinued into the time period 0.0-0.78 Ma. Along the RIV-PAC spreading center, between the Rivera and Tamayo transforms,these changes have produced (1) a clockwise rotation of the ridge axes, (2) an increase in thegradient of spreading rates along the spreading center, and(3) an increase in spreading rates. At the eastern endof theRiveratransform nearthe boundary between the RIV and COC plates, these changes have produced a counterclockwise reorientation of RIV-PAC motion. This counterclockwise reorientation is clearlyobserved by compar- ingthepresent-day orientation (S86 øE)of theRiveratransform adjacent to theMoctezuma spreading segment (MSS) with the orientation (S63øE to S67øE) of the paleo-Rivera transform to theeast (Figure 1 a). Further, the present-day Rivera transform exhibits a progressive counterclockwise reorientation as it approaches theMSS (Figurelb), indicating thatthiscounter- clockwise rotation hascontinued into the time period0.0-0.78 Ma. These observations haveled to the proposal [e.g.,Bandy, 1992;Lonsdale, 1995] of a southward migration of the instanta- neous RIV-PAC Euler vector during the past1.5 Myr. Thus the 24,245