TECTONICS, VOL. 13, NO. 4, PAGES 755-768, AUGUST 1994 Statistical examination of the existence and relative motion of the Jalisco and Southern Mexico Blocks William Bandy and Mario Pardo 1 Insfituto de Geoftsica, Universidad Nacional Aut6noma de Mdxico,MdxicoCity Abstract. The continental lithosphere of Mexico south of the Trans-Mexican volcanic belt may consist of two litho- spheric blocks; the Jalisco andSouthern Mexico Blocks. The existence of these two blocks can be examined employing the statistical F test (formulated to testfor the presence of significant misclosure around a plate circuit) to platemotion data derived from marine magneticanomaly lineations, earthquake slip vectors, and transform fault azimuths. The result of the F test applied to the Pacific-Cocos-North American plate circuitindicates that there is no significant (at the 99% confidence level) misclosure around this plate circuit. Therefore if the Southern Mexico Block exists,its motion relative to the surrounding lithospheric plates is too small to be resolved from these data. In contrast, the result of the F test appliedto the Rivera-Pacific-North American plate circuit indicates thepresence of a significant misclosure about the Rivera-Pacific-North American plate circuit,the causeof which is uncertain. One interpretation of the misclosure around the Rivera-Pacific-North American plate circuit is that it is due to the presence of an independent Jalisco Block. However,it is more likely that this mis- closure is, instead, primarily due to the effects of recent changes in the relative motion between the Pacific and Riveraplates. Assuming the second explanation is correct, a new Rivera-North American Eulerpole location (21.8øN, 110.4øW) is determinable solely from earthquake slip vectors located along the Rivera-North American boundary. Introduction The continental lithosphere of southern Mexico south of the Trans-Mexican volcanic belt (TMVB) may consist of two small lithospheric plates or blocks. These are the Jalisco Block located northwest of the Colima Graben and the Southern Mexico Block located east of the Colima Graben (Figure1). Studies indicate thatthe Southern Mexico Block exhibits a left-lateral motion relative to the North American (NA) platealong the TMVB [PasquarO et al., 1986;Sudrez and Singh, 1986; Urrutia-Fucugauchi and BOhnel,1988; Urrutia-Fucugauchi and Rosas-Elguera, 1994], whereas the motion of the Jalisco Blockrelative to the NA plateis less clear. Several studies [Gastil and Jensky, 1973; Shurbet and •Alsoat Departmento de Geof/siea, Universidad de Chile, Santiago. Copyright 1994 by the American Geophysical Union. Papernumber 94TC00594. 0278-7407/94/94TC-00594510.00 Cebull, 1984; Luhr et al., 1985; Nieto-Obreg6n et al., 1985; Nieto-Obreg6n, 1990; Serpaet al., 1989, 1992; Carmichael, 1990; Allan et al., 1991; Bourgois and Michaud, 1991; Bandy, 1992] indicate a NW oriented, right-lateral motionat a rateof 1.0 to 2.0 mm/yr, while other studies indicate either little or no motion [Bohnel and Negendank,1988; Nieto- Obreg6n et al., 1992] or a SW oriented relative motion [Johnson and Harrison, 1989; Rosas-Elguera et al., 1993]. Presently, no Euler vectors have beendetermined for either the Jalisco or Southern Mexico Blocks relative to any of the surrounding major lithospheric plates. The purpose of this study is to determine if the motions of both the Jalisco and Southern Mexico Blocks are of suffi- cientmagnitudes asto be resolvable using platemotion data derived frommarine magnetic anomaly lineafions, earthquake slip vectors, and the azimuths of transform faults. Further, if their motions are resolvable employing these data, what are the Euler vectors describing their motions relative to the surrounding majorlithospheric plates; andcanthese vectors be used to resolve the ambiguities in the previous studies of the relative motion between the Jalisco Block and the NA plate? The results of the analysis suggest thatthemotions of both the Jalisco and Southern Mexico Blocks relative to the NA plate are too smallto be resolved from these data. Thusno Euler vectorsdescribing their motionsrelative to the sur- rounding plates can be determined. However, if there is only insignificant motion between the Jalisco Block and the NA plate,the location of the Rivera-NA Eulerpoleis determina- ble from earthquake slip vector data located along the Rivera-NA boundary. This pole location can be further constrained by comparing the morphology of the Rivera- Cocos plate boundarywith Rivera-Cocos relative motion predicted employing thispole. Data and Methods F Tests and Plate Motion Inversions The first step in the analysis is to determine if the motions of the Jalisco and Southern Mexico Blocks are resolvable using plate motion data derived from marne magnetic anomalylineations, earthquake focal mechanism solutions, and transform azimuths. To accomplish this, the statistical F test as formulated by Gordon et al. [1987] is used. This formulation tests for the presence of a significant misclosure around a plate circuit by comparing the magnitude of the weighted residuals (thedifference between the observed data andthe corresponding datapredicted from the best fit Euler vectors divided by their assigned uncertainty) produced by two different models or sets of Eulervector parameters. The