ORIGIN AND PROPAGATION OF FRACTURING IN THE VULCANO-LACUSTRINE BASIN OF QUERETARO

We present a geological and geomechanical analysis of the origin and propagation of the Central Fault (CF), a fracture-fault system that affects the Queretaro Valley. The origin of fracturing in lacustrine sedimentary basins is closely related to different factors; specifically, in urbanized areas the mechanical and hydraulic equilibrium in the subsoil is broken by anthropic activities, such as overexploitation of groundwater and overloading of high compressible grounds. The propagation of fractures in this volcanic basin is determined by its highly heterogeneous stratigraphy, consisting of 50 to 150 m width alternating layers of clays and silts, sand, gravel, volcanic ashes and other miocenic pyroclastic rocks. The contrasting physical and mechanical properties (such as density, permeability and compressibility) of the shallow sedimentary sequence evidence their different potential of deformation. Thus, the morphology of the near surface fracturing depends upon the spatial relation between regional faulting and the depositional conditions of sediments. The CF shows at the surface a linear trace that joins two ~N-S segments of normal faulting cutting volcanic rocks observed in the north and south margins of the basin with more than 100 m of vertical displacements. Although the main geometry of the CF is related to the regional structural pattern, we propose that one of the main factors of propagation is the differential settlement of soils because of the variation of vertical displacements and orientation of the main NS trace. In the north zone of the basin a single linear feature with vertical displacements of more than 2 m can be observed. The fracture shows a discontinuous semi-arch bend following the west flank of the Cerro de las Campanas in the middle of the basin. In the southern segment fracturing distributes in three parallel discontinuous paths and the vertical displacements decrease. The traces disappear in the south part of the basin as the ground slope increases. The analysis of multiple GPR and soil profiles, perpendicular and distributed along the fracture, allowed the characterization of the variability of physical properties.