JOURNAL OF GEOPHYSICAL RESEARCH, VOL. tOO, NO. Bt0, PAGES 20,025-20,035, OCTOBER t0, 1995 Hydrogeologicproperties of a thrust fault within the Oregon Accretionary Prism Elizabeth J. Screaton and Bobb Carson Department of Earth andEnvironmental Sciences, LehighUniversity, Bethlehem, Pennsylvania Gerard P. Lennon Department of Civil andEnvironmental Engineering, LehighUniversity, Bethlehem, Pennsylvania Abstract. Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prismfault zone. The first setof tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992),while the second set of tests were constant-drawdown andconstant-discharge tests conducted in fall 1993using the submersible Alvin. Pressure response during thefirst set of tests suggests thatfractures remained open until excess fluidpressure (relative to hydrostatic) dropped below0.315to 0.325 MPa ( •,* -0.53 to 0.54,where •,* = (pore pressure - hydrostatic)/(lithostatic-hydrostatic)). Analysis of thepacker test data suggested an apparent background pressure of 0.25 MPa (•,* - 0.42 to 0.50). Because theborehole hadbeenopen for 12 hours prior to thepacker tests, formation porepressures may haveexceeded thisvaluepriorto drillingof theborehole. These overpressures dissipated by the time the second setof tests were conducted. One possible explanation for thisdecay is that the borehole may provide a vertical conduit between the overpressured zoneandoverlying or underlying sediments thathadpreviously been hydraulically separated fromtheoverpressured zone. The second set of tests wereconducted at pressures ( _< 0.019 MPa or •,* • 0.03) belowthat estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less thanestimated for thepacker tests (whenfractures were open). Constraints on fluid flow rate along thefault areprovided by observed displacement in a bottom-simulating reflector (BSR) at its intersection with the fault zone. The closed-fracture transmissivities are insufficient to produce flow rates capable of displacing theBSR; therefore open-fracture transmissivities under conditions of elevated porepressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the faultzoneare2 to 3 orders of magnitude lowerthandischarge measured at an associated seafloor vent site;fluid flow must become spatially or temporally focused asit moves up the fault zonetoward the seafloor. Introduction Focused fluid flow along fault zones within accretionary prisms hasbeeninferredfrom submersible observations of vent sites concentrated along thrust fault traces on the Cascadia margin [Moore et al., 1990, 1991 ], from patterns of diagenetic deposits mapped off Oregon [Carson et al., 1991; 1994;Tobin et al., 1993], and from chemical and thermalanomalies observed within fault zones in the toe of the Barbados Accretionary Complex on Ocean Drilling Program (ODP) Leg 110 [Fisherand Hounslow, 1990; Gieskes et al., 1990; Vrolijk et al., 1990]. Numerical models [$hi et al., 1989; $creaton et al., 1990] suggest that a 3 to 5 order-of-magnitude permeability contrast between fault zonesand surrounding sediments is sufficientto channel flow along a fault that is tensof meters in thickness. However, little is known about how fault zones act as fluid conduits or the interaction between deformation, pore pressures, and fluid flow. Copyright 1995 by the American Geophysical Union. Paper number95JB02152. 0148-0227/95/95JB-02152505.00 Laboratory studies on sediment samples constrain permeability values outside fault zones [Moran et al., 1995; Brown, 1995; Horath, 1989; Taylor and Leonard, 1990; Taylor and Fisher, 1993]. Unfortunately, quantifying fracture permeability using laboratory measurements on samples is more difficult, because the sample may notbe largeenough relative to fracture spacing to be representative. In addition, natural pressure conditions are poorly constrained, and fracture permeability may be pressure- dependent. On the basis of experimental studies, Arch and Maltman [ 1990] suggest that anisotropy in clay fabrics during deformationcould create permeabilities parallel to fault zones sufficient to create a fluid conduit. In contrast, Brown andMoore [ 1993] indicate that fabricanisotropy is insufficient andinfer that fracture dilation associated with increased fluid pressures is necessary to createfluid conduits. Investigators [Moore, 1989; Moore et al., 1991; Vrolijk, 1987] hypothesize that the relationship between pore pressure and fluid flow is episodic, with variations in fault-zone behavior (fracture dilation-increased permeability-fluid flow-fracture collapse) associated with pressure fluctuations. Resultsof a successful packer experiment conducted at Site 892 during ODP Leg 146 yielded the first in situ estimates of 20,025