nearly parallel to the El Carrizal Fault. Some of these feed the lava flows of the Cedro Formation (Figure 2). During fieldwork, a large amount of vein, fault and slickenline measurements were collected. The vein and fault data show mean orientations that are nearly parallel to the major faults (Figures 9 and 2). Almost all of the observed faults are normal or oblique, with rare exceptions including purely lateral faults, which we did not include in Figure 9C. A paleostress tensor that was calculated using the right dihedron method is shown in Figure 9D (Delvaux and Sperner, 2003). The minimum compressional principal stress is oriented 14°/242° (dip/trend), which agrees with the distribution of slickenlines (Figure 9C), indicating that the main extension was oriented However, a secondary extension direction oriented near NW-SE is inferred from the presence of the dike system and the El Carrizal Fault (Figure 2). These two extension directions have been reported in the Mesa Central (MC) and were interpreted as products of Cenozoic deformation (Nieto-Samaniego et al. 1997; 1999). The general structure of the southern GMD is illustrated in sections A-A’ and B-B’ (Figure 8). Master faults of the El Cubo and Veta Madre half-grabens dip SW, and in both structures beds of the Calderones Formation dip NE indicating that rotation of the fault-limited blocks responded to fault displacements. The strong control of major faults on bed tilting and outcrop patter suggest they are more probably related to normal faulting than to a caldera collapse. We propose that the southern GMD structure is formed of listric southwest-dipping normal faults that could be linked at depth (Figures 2 and 8). To the northeast of the Las Gachas fault the mean dip of the Calderones Formation is 25° NE, whereas Guanajuato Conglomerate beds dip 25°-35° to the E or ENE (Figure 7). The similar
nearly parallel to the El Carrizal Fault. Some of these feed the lava flows of the Cedro Formation (Figure 2). During fieldwork, a large amount of vein, fault and slickenline measurements were collected. The vein and fault data show mean orientations that are nearly parallel to the major faults (Figures 9 and 2). Almost all of the observed faults are normal or oblique, with rare exceptions including purely lateral faults, which we did not include in Figure 9C. A paleostress tensor that was calculated using the right dihedron method is shown in Figure 9D (Delvaux and Sperner, 2003). The minimum compressional principal stress is oriented 14°/242° (dip/trend), which agrees with the distribution of slickenlines (Figure 9C), indicating that the main extension was oriented However, a secondary extension direction oriented near NW-SE is inferred from the presence of the dike system and the El Carrizal Fault (Figure 2). These two extension directions have been reported in the Mesa Central (MC) and were interpreted as products of Cenozoic deformation (Nieto-Samaniego et al. 1997; 1999). The general structure of the southern GMD is illustrated in sections A-A’ and B-B’ (Figure 8). Master faults of the El Cubo and Veta Madre half-grabens dip SW, and in both structures beds of the Calderones Formation dip NE indicating that rotation of the fault-limited blocks responded to fault displacements. The strong control of major faults on bed tilting and outcrop patter suggest they are more probably related to normal faulting than to a caldera collapse. We propose that the southern GMD structure is formed of listric southwest-dipping normal faults that could be linked at depth (Figures 2 and 8). To the northeast of the Las Gachas fault the mean dip of the Calderones Formation is 25° NE, whereas Guanajuato Conglomerate beds dip 25°-35° to the E or ENE (Figure 7). The similar
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