A THEORETICAL-MODEL FOR THRUST-INDUCED DEEP GROUNDWATER EXPULSION WITH APPLICATION TO THE CANADIAN ROCKY-MOUNTAINS

This paper presents a numerical model for simulating deformation and induced fluid flow in fold-and-thrust belts. Unfractured rock strata are modeled as poroelastic media while fault zones are treated as plastic-elastic media. We introduce the slip element technique into a finite element code to accommodate large deformations along faults. Thrust displacements, stress field changes, and the effects of thrust faulting on groundwater flow are investigated by solving the coupled stress and flow equations numerically. The calculation shows that when a thrust sheet is displaced along its fault surface, the displacement-induced stress generates high pore pressure zones near the tectonic stress boundary and beneath low permeability ramps. These overpressures cause transient fluid flow across the thrust belt. Sensitivity studies on the hydrologic properties of the fault zone suggest that hydraulic conductivities within a fault play important roles in initiating slip deformation and in determining the extent of transient disturbances to the flow field. Low permeability can result in rapid pore pressure buildup in the fault, thereby reducing the effective strength of the fault which leads to earlier failures. A low-permeability fault can also impede the movement of flow into the footwall, thereby limiting the tectonic impact on the flow system within the hanging-wall. Application of the model to the McConnell Thrust in the Canadian Rockies indicates that the total volume of fluid flow induced by tectonic compression could have been of the order of 10(5) to 10(6) m3 over a time period of tens to hundreds of years accompanied by an average 100 m of thrust movement.