Chapter 1

Two major fault zones, one coincident with the west-northwest trending Olympic-Wallowa Lineament (OWL-zone) and the other coincident with the north-northeast trending Hite Fault System (HFS), have been identified as potential strike-slip structures of regional significance (Hooper and Camp, 1981; Hooper and Conrey, 1989; Mann and Meyer, 1993; Figure 1). These two structures intersect on the Columbia Plateau in northeast Oregon southeast of Walla Walla, Washington (Figure 1 and Figure 2). Here the exposed geology is composed of horizontal to shallowly dipping basalt flows of the Columbia River Basalt Group (CRBG) overlain by much younger loess with alluvial deposits along the valley floors.

Numerous faults, fractures, and lineaments in this area have been mapped and described by Kienle and others (1979), Kendall (1981), Swanson and others (1981), and Sandness and others (1982). These authors have suggested both normal and strike-slip displacements on some of the various smaller faults within and parallel to the regional fault systems. Unfortunately, the essentially horizontal disposition of the basalt flows cut by these structures and the difficulty in distinguishing one flow from another in the field has led to differing interpretations. Consequently, the specific location, age, and type of translation along the suggested major fault systems has proved controversial. The present study was undertaken to gather more information about these potential fault systems by mapping critical exposures in the area where they meet. Using a combination of field, petrographic, and analytical methods this study attempts to identify each basalt flow or small group of flows and so establish a stratigraphic framework which can be used to correlate flows as stratigraphic markers across the various structures to determine the age and type of displacement.

To establish the sequence and distribution of flows, five principal stratigraphic sections (Figure 3) were sampled and analyzed for twenty-seven major and trace elements by automated X-ray Fluorescence Spectrometry (XRF) in the Geoanalytical Laboratory at Washington State University. The precision and estimated accuracy for each element analyzed by the XRF method used here are described by Hooper and others (1993).

Smaller sections and isolated flow outcrops throughout the study area were analyzed and correlated to the main sections to map out the areal distribution of flows. Correlations between two stratigraphic sections located respectively to the west and east of the Hite Fault in the valley of the South Fork Walla Walla River (Figure 3) were used to study the magnitude and history of vertical displacement across the Hite Fault.

Mapping of individual flows and both previously and newly recognized faults in the vicinity of Tiger Creek and along the Walla Walla River on the existing 1:24000 scale topographic base enlarged to 1:6000 and 1:10000 respectively helped constrain flow distributions and fault offsets. Color aerial photography aided in the location of faults. Particularly good outcrops of faults in the road cuts along Tiger Creek Road and in the Walla Walla River area were studied in detail in order to constrain fault displacements. Mapping of feeder dikes on the south side of the valley of the South Fork Walla Walla River and the distribution of the Umatilla flow in the Table Glade-Target Meadows area (Map 4) is, in part, after Swanson and others (1981). Mapping of the right step of the Hite Fault and flow distributions around Elbow Creek (Map 4) is after Kendall (1981).

Geologic Setting and Previous Work

The study area is located where the Olympic-Wallowa Lineament (OWL) and Hite Fault System (HFS) intersect in northeastern Umatilla County, Oregon, on the western flank of the Blue Mountains Uplift (Figure 2 and Figure 3). Basalt flows and faults in this area were mapped by Kienle and others (1979), Swanson and others (1981), and Kendall (1981). Sandness and others (1982) compiled a map of lineaments and known faults which includes this area. Tholeiitic basalts of the Columbia River Basalt Group (CRBG) constitute the sole rock type present in the study area. The CRBG was erupted during the Miocene between 17 and 6 million years B.P. and CRBG covers 164000 square kilometers in Washington, Oregon, and Idaho (Tolan and others, 1989; Figure 3). A covering of loess obscures the basalts on most hill and ridge tops and gentle slopes with most outcrop occurring on the more steeply sloping valley walls.

The structural framework of the Columbia Basin region began developing before the eruption of the CRBG (Reidel and others, 1994). The regional strain regime of NNW-SSE shortening and WSW-ENE extension, which may have been present as early as the Eocene, is apparent in the approximately east-west trending anticlinal ridges of the Yakima Fold Belt (YFB), east-west trending folds of the Lewiston Structure in the vicinity of Lewiston, Idaho, and north-northwest trending CRBG dike swarms (Figure 1, Figure 2, and Figure 4; Hooper and Camp, 1981; Hooper and Conrey, 1989; Hooper and others, in press). Deformation continued throughout and after the period of eruption of the CRBG (Hooper and Conrey, 1989). Reidel (1984) and Reidel and others (1984) have documented the thinning of many basalt flows over several of the Yakima folds implying contemporaneous eruption and deformation. The topographic ridges of the folds attest to the continued deformation into the present. However, the rate of deformation has decreased since Miocene time (Reidel and others, 1994). Hooper and Conrey (1989) conclude that the orientation of the regional stress regime has persisted unchanged over the last 17.5 million years because of the consistent orientation of structures (most obviously the dikes) that have been produced. Rohay and Davis (1983) document the modern stress pattern. This regional stress is the classic condition for the development of west-northwest right-lateral and north-northeast left-lateral strike slip faults (Hooper and Swanson, 1990). Many structures with these trends are indeed present, including faults of the OWL-zone and HFS (Figure 1).

The observed strain has local variations resulting from crustal inhomogeneities. The form and intensity of the strain observed varies with location probably because of differences in the nature of the underlying crust (Hooper and Conrey, 1989). For example, where the underlying crust is part of the North American craton (Figure 3), there is relatively little deformation (Reidel and others, 1994; Sobczyk, 1994). In the area of the Blue Mountains, the crust is composed mainly of accreted terranes, and there the surface cover of basalts is significantly more deformed (Hooper and Conrey, 1989).

The Blue Mountains Uplift forms a generally northeast trending topographic ridge which extends from the Cascade Range in Oregon to the Snake River in southeast Washington (Figure 4). It is made up of three segments, which differ in structural style, orientation, and probably age (Hooper and Conrey, 1989). The southwestern segment extends from the Cascades nearly to the Hite Fault System and trends east-northeast. The central segment is intimately associated with the Hite Fault System and trends north-northeast. The northeastern segment consists of an east-west trending anticline which dies out eastward as it approaches the Limekiln fault (Figure 2).

The study area is located in the central portion of the Blue Mountains Uplift which contains and parallels the Hite Fault and corresponds with a gravity low (Hooper and Conrey, 1989; Sobczyk, 1994). The gravity low indicates the presence of relatively low density crust and suggests that this portion of the ridge results at least in part from isostatic compensation. This view is held by Kendall (1981) who suggests that relatively low density crystalline basement similar to that of the Wallowa Mountains may have produced the uplift. Hooper and Swanson (1987) and Hooper and Conrey (1989) argue that the central portion of the uplift is principally a horst with gentle anticlinal flexure linked to left-lateral strike slip faults which include faults of the Hite Fault System. Sobczyk (1994) favors a model of combined isostatic uplift and faulting.

The Hite Fault System is a series of north-northeast trending faults which in the study area includes the main Hite Fault, Peterson Ridge Fault, Blalock Mountain Fault, and several unnamed faults. The Hite Fault proper was named after Thomas Hite who did some early mapping of rock types in 1937 which was later published by Wagner (1949). The fault extends at least from the North Fork Umatilla River east of Pendleton, Oregon, to Pomeroy, Washington, a distance of approximately 150 kilometers (Kienle and others, 1979; Kendall, 1981; Figure 2). Mohl and Thiessen (1985) and Mohl (1989), in a gravity study of the cratonic margin, tentatively extended the Hite Fault from around Pomeroy, Washington to the northeast, across the suture zone, and into the craton perhaps as far as Colfax, Washington.

The Hite Fault has been interpreted as a normal fault (Newcomb, 1965, 1970; Meyer and Price, 1979) and as a right-lateral fault (Kienle and others, 1979). Kendall (1981) interprets the Hite Fault as having an early phase of dip-slip movement followed by left-lateral strike slip. Kendall's evidence for left-lateral displacement comes from the geometry of minor faults indicating compression within a one kilometer right step of the Hite Fault in sections 23, 24, 25, and 26 of township 4 north, range 37 east (location 38 on Map 4). Personnel of the U. S. Department of Energy (1988) also suggests early dip slip movement followed by left-lateral strike-slip. Evidence for Quaternary movement on faults of the Hite Fault System is described by Kienle and others (1979). Reidel and others (1994) suggest that the Hite Fault follows the eastern trace of the cratonic margin and draw the western margin across the OWL about as far south as Pendleton where it swings east and joins the Hite Fault System. Sobczyk (1994) proposes that the cratonic margin extends along the Hite Fault to near the Washington-Oregon border east of Walla Walla on the basis of a gravity gradient which is similar to a gradient associated with the well-defined suture zone near Lewiston, Idaho (Mohl, 1989; Mohl and Thiessen, in press; Figure 2). Additional evidence comes from the Darcell Number 1 borehole which penetrated cratonic rocks and is located well north of the OWL (Figure 2). Sobczyk's proposed margin implies about 80 kilometers of left-lateral displacement along the Hite Fault System prior to the eruption of the CRBG. The proposed margin of Reidel and others (1994) would imply even greater left-lateral displacement. The left-lateral displacement also implies that the Hite Fault extends significantly north of the suture zone as previously suggested by the gravity studies of Mohl and Thiessen (1985) and Mohl (1989). Pre-basalt left-lateral displacement of the cratonic margin along the Hite Fault System discounts the simple dip-slip then strike-slip interpretation.

The Olympic-Wallowa lineament (OWL) is a 500 kilometer long west-northwest trending alignment of topographic features originally defined by Raisz (1945), who traced it from the Olympic Mountains of western Washington across the Cascades and Columbia Basin and along the northeast side of the Wallowa Mountains in northeastern Oregon (Figure 1). Raisz regarded the OWL as either a major strike-slip fault zone or a coincidental alignment of topographic features. It parallels pre-CRBG structures along the northwest margin of the Columbia Basin (Campbell, 1989; Reidel and Campbell, 1989; Reidel and others, 1994). Associated with the portion of the OWL which crosses the Columbia Basin is the Cle Elum-Wallula deformed zone (CLEW) which extends from near Cle Elum in the eastern Cascades of Washington to the western margin of the Blue Mountains (Kienle and others, 1977; Reidel and others, 1994). The CLEW, probably the most well-defined zone of deformation along the OWL, consists of three main parts, a zone of apparent bending of folds in the Yakima fold belt, a narrow belt of aligned doubly plunging anticlines from Rattlesnake Mountain to Wallula Gap (sometimes called the Rattlesnake-Wallula alignment or RAW), and the Wallula fault zone which extends from Wallula Gap to the Blue Mountains (Figure 4; Washington Public Power Supply System, 1981; Reidel and others, 1994). Quaternary deformation has been documented at more than fifteen locations along the CLEW (Washington Public Power Supply System, 1981; U.S. Department of Energy, 1988; Tolan and Reidel, 1989; Mann and Meyer, 1993; Reidel and others, 1994). A number of earthquakes have also occurred along this zone including the historic 1936 magnitude 6.1 Milton-Freewater earthquake (Mann and Meyer; 1993; Mann, 1994). Mann and Meyer (1993) and Mann (1994) prefer to associate this earthquake with the Wallula fault zone while Woodward-Clyde Consultants (1980) and Reidel and Tolan (1994) prefer to associate it with faults of the Hite Fault System.

Southeast of the Wallula fault zone, the location of a tectonic lineament is less clear. Here deformation may be distributed among multiple faults or spread over a broad zone (Figure 1 and Figure 4). One possible trace, part of Raisz's original definition of the OWL, is along the northeast side of the Wallowa Mountains, the location of the Wallowa fault. Across this trend, there is apparent right-lateral offset of the Grande Ronde and Cornucopia dike swarms (Figure 2). Another possible trace is the Imnaha fault to the north (Hooper and Conrey, 1989; Hooper and others, in press; Figure 2). Mann and Meyer (1993) prefer to link faults on the south side of the Wallowa Mountains into a principal zone of displacement.

A series of pull-apart grabens is associated with the southeastern portion of the OWL (Figure 4). These grabens are located to the south of the OWL in this area. No similar extensional structures are found to the north of the OWL. Right-lateral strike-slip and Quaternary movement has been documented on some faults of the La Grande graben (Barrash and others, 1979; Gehrels and others, 1980, Gehrels, 1981; U.S. Army Corps of Engineers, 1983). Evidence of late Quaternary movement is also present in the Baker graben (Geomatrix Consultants, 1989). Mann and Meyer (1993) describe seismicity with magnitudes up to 3.8 associated with the Pine Valley graben. Extensional structures are also present in the vicinity of the Weiser River (Hooper and Conrey, 1989; Figure 2)

The OWL roughly parallels a series of northwest-trending fault zones to the north and to the south including some as far south as Nevada and California (Figure 1 and Figure 4; Mann and Meyer, 1993). Several of these zones have been interpreted as right-lateral strike-slip fault systems (Wise, 1963; Lawrence, 1976; Wright, 1976; Walker and Nolf, 1981; Sheriff and others, 1984; Schmidt and Garihan, 1986). Wise (1963) suggested that the series of fault zones is the result of right-lateral shear beginning in the Paleozoic across at least a 180 kilometer (300 mile) wide zone related to similar movement on the San Andreas fault. Sheriff and others (1984) and Schmidt and Garihan (1986) document a large degree of right-lateral displacement in northern Idaho and southwestern Montana across the Lewis-Clark zone, a structure containing faults with displacements as old as late Precambrian. Lawrence (1976) interpreted four major fault zones in Oregon as being right-lateral shears accommodating the northward decrease in Basin and Range extension. In Lawrence's model, the strike-slip faults essentially partition areas with different degrees of extension with the right-lateral displacement resulting from the greater westward displacement of blocks on the southwest side of each zone relative to the northeast side. This model allows for differential extension across the OWL which Hooper and Conrey (1989) argue is the expression of the northernmost of this series of fault zones and marks the termination of significant (>1%) Basin and Range extension in northeast Oregon. Differential extension is compatible with the clockwise rotation of the Cascades and magnetic trends described by Magill and others (1982) and Wells and Heller (1988).

Continue to . . . Chapter 2

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