JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B7, PAGES 13,851-13,868, JULY 10, 1994 A theoretical model for thrust-induced deep groundwater expulsion with applicationto the Canadian Rocky Mountains Shemin Ge Department of Geological Sciences, University of Colorado, Boulder Grant Garven Morton K. Blaustein Department of EarthandPlanetary Sciences, Johns Hopkins University Baltimore, Maryland Abstract. This paper presents a numerical modelfor simulating deformation and induced fluid flow in fold-and-thrust belts.Unfractured rock strata are modeled as poroelastic mediawhile fault zones are treated as plastic-elastic media.We introduce the slip element technique into a finite element code to accommodate largedeformations along faults. Thrustdisplacements, stress field changes, and the effects of thrust faultingon groundwater flow are investigated by solving the coupled stress andflow equations numerically. The calculation shows that whena thrust sheet is displaced along its fault surface, the displacement-induced stress generates highporepressure zones nearthe tectonic stress boundary andbeneath low permeability ramps. These overpressures cause transient fitfid flow across the thrust belt. Sensitivity studies on the hydrologic properties of the fault zonesuggest thathydraulic conductivities within a fault play important rolesin initiating slip deformation andin determining the extent of transient disturbances to the flow field. Low permeability canresult in rapidporepressure buildup in the fault, thereby reducing the effective strength of the fault whichleads to earlier failures. A low-permeability fault canalso impede the movement of flow into the footwall, thereby limitingthe tectonic impact on the flow system withinthe hanging-wall. Application of the model to the McConnell Thnmt in the Canadian Rockies indicates that the total volume of fluid flow induced by tectonic compression could have been of the order of 105 to 10 • m 3 overa time period of tens to hundreds of years accompanied by an average 100 m of thrust movement. Introduction The importance of pore pressure in faulting has longbeen recognized. From classic effective stress theory [Terzaghi, 1923] to the latest greasing fault theory [Evans, 1992], elevated pore pressures areconsidered to be a primary factor in weakening rock and causing fault slip. Past hydrologic studies of sedimentary basins have been conducted mainly on the fluid flow domainwithin regionally continuous strata [Garven,1989].The mechanics of flow neara thrust fault in thefold-and-thrust margin of a basin, especially during active faulting, are less well explored. In earlierstudies [ Ge and Garven, 1989; Ge and Garven, 1992, Garven, et al., 1993] we presented the basic theory of coupled mechanical deformation and fluid flow, theresults of numerical modeling on a generic basin, andapplication to the Arkoma Foreland Basin. As a follow-up study,this paper focuses on the coupling between the deformation alonga thrust fault and fluid flow nearthe marginof basins (Figure 1). A number of studies haverelated high porepressures to thrust movement. The theoretical study by Hubbert and Copyright 1994 by the American Geophysical Union. Paper number 94JB00687. 0148-0227/94/94JB- 00687505.00 Rubey [1959] introduced the effectivestress concept to stru• geology and recognized that frictional resistance of rock is a function of effective normal stressrather than total normal stress. The buildup of porepressure weakens the rock mass by reducing effective stress and the frictional resistance so that displacements are possible. Gretener [1972, 1981] qualitatively described the interaction between pore pressure buildup and therock displacement asa self-perpet- uating process. In that process, high pressures aid thrust displacemere while advancement of a thnlstsheet loads underlying strata, thereby increasing pore pressures. The thrust sheet may move like a caterpillar, one segment at a time. This view has gainednew support by a seriesof laboratory sliding experiments reported in a recem study by Blanpied et al. [1992]. From the studies of vein systems associated with fault zones, Sibson [1981, 1987]proposed a seismic pumping model which described a process whereby pore pressure increases cause hydraulic fracturing, while opening of fractures results in pressure drops. P/att [1990] studied thn• mechanics in the Makranaccretionary wedge and suggested that rapid tectonic loading maycause sudden pressure increases within saturated sediments which leads to dilatant fracturing. Advancing of thrustsheets can cause compaction of strata and propagation of thin-skin ramping of rocksequences [Elliot, 1976]. 13,851