Emplacement of pyroclastic dykes in Riedel shear fractures: An example from the Sierra de San Miguelito, central Mexico

Abstract

Although magmatic dykes are considered to be emplaced at depth, there are very few studies about the emplacement of pyroclastic dykes. There are two commonly accepted mechanisms for dike emplacements; namely, formation of a hydraulic (tension) fracture and filling of pre-existing fractures. In the Sierra de San Miguelito, central Mexico, there is a dike swarm with orientations that do not concur with any of the above two cases. Instead, the dikes have orientations resembling some type of secondary fractures within the fault-bounded blocks.Within a fault-bounded block, the reverse drag of the normal faults can be explained by simple shear. Based on the simple shear mechanism, we establish a series of equations to calculate the extension and the direction of the simple shear within a fault-bounded block. As an application of our methodology, the value of β, which is the angle measured from the vertical to the direction of the simple shear, is calculated from the domino faults in the Sierra de San Miguelito, Mesa Central, Mexico. The results demonstrate that the absolute values of β for the inclined shear are smaller than 34°. The inclined shear can be antithetic (β>0), synthetic (β<0) or vertical (β≈0). The pyroclastic dykes in the study area have most of the strikes between 300° and 330° and are sub-parallel to the major faults. The preferred dips of the pyroclastic dykes vary from 80° to 90°. The distribution of the pyroclastic dykes in the study area indicates that the dikes were primarily emplaced along the R fractures due to simple shear. These results are different from the traditional understanding, which assumes that the dykes were mainly emplaced within the tension fractures.