Structural analysis of a relay ramp in the Querétaro graben, central Mexico: Implications for relay ramp development

Abstract

This work presents a study of the overlapping zone between the Cinco de Febrero and Cimatario normal faults in the Querétaro graben, located in Querétaro, Mexico. The profiles measured from topographic fault scarps indicate that a relay ramp was formed in the southern part of the overlapping zone. In the relay ramp there are fractures striking oblique to the major faults. Those fractures are interpreted as Riedel fractures (R´) formed by minor rotation around a vertical axis. The orientation of bedding is not uniform in the relay ramp; dip directions of bedding are dependent on the sense and amount of rotation around two horizontal axes. The observed geometry of faults and bedding are consistent with published models of relay ramps in which there are three axes of rotation. Two axes are horizontal and one axis is vertical. For small faults, bedding rotations are due to both normal and reverse drags. In the relay ramp, bedding rotation due to simple shear is documented between the two large faults. On the other hand, the extension due to veins and dykes in the overlapping zone can reach 80 % of the total heave of the two overlapping faults. Finally, layer-parallel slip in the normal fault regime is occurring in the overlapping zone, despite the dips of bedding in our studied area being less than 20°.