Splay-Fault Origin for the Yakima Fold-And-Thrust Belt, Washington State 2 3 Thomas L

Home , Basin and Range Province, Frenchman Hills, Horse Heaven Hills, Saddle Mountains, Umtanum Ridge, Yakima Ridge

1 Splay-fault origin for the Yakima fold-and-thrust belt, Washington State 2 3 Thomas L. Pratt, United States Geological Survey, School of Oceanography, Box 357940, 4 University of Washington, Seattle, WA 98115 5 6 ABSTRACT 7 The Yakima fold-and-thrust belt (YFTB) is a set of anticlines above reverse faults in the 8 Miocene Columbia River Basalt (CRB) flows of Washington State. The YFTB is bisected by the 9 ~1100-km-long Olympic-Wallowa geomorphic lineament (OWL). There is considerable debate 10 about the origin and earthquake potential of the YFTB and OWL, which lie near six major dams 11 and a large nuclear waste storage site. Here I show that the trends of the YFTB anticlines relative 12 to the OWL match remarkably well the trends of the principal stresses determined from Linear 13 Elastic Fracture Mechanics (LEFM) modeling of the end of a vertical strike-slip fault. From this 14 comparison and the termination of some YFTB anticlines at the OWL, I argue that the YFTB 15 formed as splay faults caused by an abrupt decrease in the amount of strike-slip motion along the 16 OWL. If this hypothesis is correct, the OWL and YFTB are likely interconnected, deeply-rooted 17 structures capable of large earthquakes. 18 19 20 INTRODUCTION 21 The Yakima fold and thrust belt (YFTB) of central Washington State is a set of 22 prominent anticlines in the Miocene Columbia River Basalt flows (CRB; figure 1). The YFTB 23 anticlines form three distinct sets (Riedel et al., 1989 and 1994; Watters, 1989). The Frenchman 24 Hills, Saddle Mountains (Riedel, 1984; West et al., 1996) and east part of Umtanum Ridge 25 (Gable Mountain) in the northeast YFTB are west-trending anticlines above south-dipping 26 reverse faults. To the south the Columbia Hills, Horse Heaven Hills, Ahtanum, Yakima and 27 Toppenish Ridges (Campbell and Bentley, 1981) form a fan-shaped pattern of southwest- to 28 west-trending ridges. In the northwest, Manastash, Umtanum and part of Yakima Ridge (Price 29 and Watkinson, 1989) trend northwest. YFTB structures are growing under N-S or NNE-SSW 30 compression (Hooper and Conrey, 1989; Reidel et al., 1994; McCaffrey et al., 2007), but their 31 variety of trends suggests that they developed in a more complex stress field.

32 The Olympic-Wallowa Lineament (OWL) is a northwest-trending, ~1100-km-long 33 alignment of topographic and structural features that cuts through the YFTB (figure 1; Raisz, 34 1945). In the YFTB 1 the OWL includes the Wallula fault zone (Mann and Meyer, 1993; Riedel 35 and Tolan, 1994; Mann, 1994) and the fault-bounded Manastash and Umtanum ridges. The OWL 36 has been interpreted as the northernmost strike-slip “megashear” bounding Basin and Range 37 extension (Hooper and Conrey, 1989; Mann and Meyer, 1993), but geologic evidence for 38 substantial strike-slip motion is hotly debated (Reidel and Tolan, 1994; Mann, 1994; Reidel et 39 al., 1989 and 1994; Hutter et al., 1994). The OWL itself extends across the Cascade Range 40 (Raisz, 1945; figure 1), possibly as a system of active faults (Blakely et al., 2011). 41 The YFTB clearly is related to the OWL. First, the northwest YFTB ridges form part of 42 the OWL. Second, the markedly (~25°) different YFTB trends on each side of the OWL show 43 that it influenced their formation. Third, the Columbia Hills and Horse Heaven Hills anticlines 44 terminate against the south side of the Wallula fault portion of the OWL. Formation of these two 45 anticlines required substantial (kms?) shortening south of the OWL, and the lack of comparable 46 shortening on the north side requires strike-slip motion along the Wallula fault (figure 1). 47 Their earthquake potential makes the origins of the YFTB and OWL more than just 48 academic questions. Within or near the YFTB on the Columbia and Snake Rivers are six major 49 dams. Adjacent to the OWL and within the YFTB is the Hanford nuclear site, with 56 million 50 gallons of liquid radioactive and chemical waste on site (DOE, 1996). Seismic hazard analyses 51 assume maximum earthquake magnitudes of 6.4 or less if the YFTB faults are assumed to be 52 restricted to the ~4-km-thick CRB (“thin-skinned” or “decoupled” models), but reach 53 magnitudes of 6.5 to 7.9 if the faults penetrate deep the CRB (“thick-skinned” or “coupled” 54 models) (Geomatrix Consultants, 1996). 55 Here I use computer models to show that the trends of the YFTB anticlines in relation to 56 the OWL match remarkably well the orientations of stress trajectories near the ends of vertical 57 strike-slip faults as computed using Linear Elastic Fracture Mechanics (LEFM) methods 58 (Fletcher and Pollard, 1981; Rispoli, 1981; Willemse and Pollard, 1998; Kattenhorn and 59 Marshall, 2006; Kim and Sanderson, 2006). The models suggest that the YFTB anticlines formed 60 as splay faults caused by diminishing strike-slip motion on the OWL in the Miocene, and the 61 YFTB faults remain the easiest path for slip despite the modern stress field being rotated slightly

1 (also known in the YFTB area as the Cle Elum-Wallula Lineament [CLEW])

62 since the Miocene. Although some strike-slip motion on the OWL clearly continued to the 63 northwest (Blakely et al., 2011), I hypothesize that a substantial amount of slip on the OWL 64 terminated within the YFTB, where it was absorbed to form the anticlines. 65 66 67 COMPARISON OF MODEL STRESS TRAJECTORIES AND THE YFTB 68 I used a 3-dimensional boundary element code (Gomberg and Ellis, 1993) to model stress 69 trajectories at the tip of a 400-km-long, vertical strike-slip fault assuming LEFM behavior 70 (Figure 2; Pollard and Segall, 1987). The model simulates an abrupt decrease in the amount of 71 displacement along the OWL near Rattlesnake Mountain at the center of the YFTB. The fault in 72 the model was a stress-free surface divided into 80 segments to accommodate decreasing slip at 73 the fault ends. The stress-free condition assumes that faults are weak enough to relieve long-term 74 crustal strains. To determine the stress trajectories, I modeled about 9 m of dextral strike-slip 75 fault motion driven by a uniaxial remote stress oriented at 45° to the fault. This displacement 76 simulates a large earthquake and the resulting stress orientations. The model fault extended to 40 77 km depth, which is the approximate depth to Moho beneath the Columbia basin (Gao et al., 78 2011; Yang et al., 2008). The fault below seismogenic depths is assumed to be a ductile shear 79 zone. 80 The model shows that preferred fault trends distant from the main strike-slip fault are 81 nearly perpendicular (contractional) and parallel (extensional) to the regional stress direction 82 (figure 2). Near the fault tip the stress field rotates so that the maximum compressive stress is at 83 a large angle to the fault in the compressional quadrant and at a small angle in the extensional 84 quadrant. The compressional quadrant is an area of uplift and the extensional quadrant is an area 85 of subsidence. 86 There are remarkable similarities between the trends of the model stress trajectories and 87 the trends of the YFTB ridges northeast and southwest of the OWL (figure 2). I rotated the 88 model so the fault overlays the OWL, resulting in a direction of maximum compression (N9°W) 89 that is consistent with the NNW-SSE principal stress direction inferred for the early Miocene 90 YFTB from dike orientations (Hooper and Conrey, 1989). The Columbia and Horse Heaven 91 Hills south of the OWL are parallel to the model’s stress trajectories, and they terminate against 92 the OWL at angles close to the 70.5° predicted by fracture mechanics. Portions of Toppenish,

93 Ahtanum and Yakima Ridges likewise are parallel to the model stress trajectories. The 94 Frenchman Hills, Saddle Mountains, and Gable Mountain anticlines north of the OWL strike 95 nearly perpendicular to the model’s principal compressive stress direction. The north-trending 96 stretch of the Columbia River north of Rattlesnake Mountain is parallel to the predicted strike 97 direction for extensional fractures, suggesting that the fault inferred along the river (Reidel, 98 1984) may have normal motion. 99 The contrast in elevations on opposite sides of the OWL also is consistent with the model 100 (figure 2). Rattlesnake Mountain has some of the highest elevations in the area and an elongate 101 shape similar to that of the model’s uplift area. Southeast of Rattlesnake Mountain, uplift south 102 of the OWL between 8 and 6 Ma forced the Columbia River southeast to Wallula Gap (Reidel et 103 al. 1994), where the model predicts decreasing amounts of uplift. The Pasco Basin on the north 104 side of the OWL has been a locus of subsidence since the Miocene, and the approximate 105 dimensions of the basin are similar to that predicted by the fault termination model (figure 2; 106 Reidel et al., 1989 and 1994). 107 Umtanum and Manastash Ridges in the northwest part of the YFTB are also consistent 108 with formation near a fault tip, but they lie in the quadrant dominated by shear stresses and 109 strike-slip branch faults (figure 2; Kim and Sanderson, 2006). Both ridges form part of the OWL 110 and are bounded by reverse or oblique-slip faults that are nearly parallel to the orientation of 111 maximum shearing in the model. The Saddle Mountains and Frenchman Hills also change trend 112 to run parallel to the direction of maximum shearing west of the Columbia River. The 113 comparison suggests that the northwest YFTB ridges formed as flower structures (Harding, 114 1985; Sylvester, 1988; Naylor et al., 1986) bounded by oblique-slip faults, although reverse 115 motion may now dominate in the modern NNE stress field. 116 117 118 DISCUSSION AND CONCLUSIONS 119 The model for YFTB formation proposed here requires substantial strike-slip motion on 120 the OWL during YFTB ridge growth 17.5 to 6 Ma (Reidel et al., 1989 and 1994). YFTB growth 121 coincided with Miocene CRB extrusion, Basin and Range extension, and westward migration of 122 the Oregon coast range (Reidel et al., 1989 and 1994; Hooper and Conrey, 1989; Wells and 123 Heller, 1988). Extension in terranes south of the OWL may have been as great as 20%, which

124 should have caused right-lateral slip on the OWL (Hooper and Conrey, 1989; Atkinson and 125 Hooper, 2000). However, there is a decades-long debate about the geologic evidence for or 126 against large amounts of strike-slip motion on the OWL (Hooper and Conrey, 1989; Reidel et al., 127 1989 and 1994; Mann and Meyer, 1993; Reidel and Tolan, 1994; Mann, 1994; Hutter et al., 128 1994). The remarkable similarity between the YFTB and the modeled stress trajectories 129 presented here bolsters the tectonic arguments in favor of strike-slip motion on the OWL during 130 the Miocene formation of the YFTB. 131 It is unclear how much strike-slip motion is occurring on the OWL at present. The N9°W 132 principal stress direction in the fault model differs by 12°-15° from the current NNE stress field 133 (Reidel et al., 1994; McCaffrey et al., 2007), which indicates slight rotation of the stress direction 134 over the past 17 Ma (Barrash et al., 1983; Hooper and Conrey, 1989). The hypothesis presented 135 here is that the YFTB structures formed in the Miocene under a NNW stress direction and, once 136 formed, have remained the easiest route for contraction despite the rotation of the stress field. 137 The N to NNE compression inferred today is oriented about 60° to the OWL (Reidel et al., 1994; 138 McCaffrey et al., 2007), which suggests both reverse and right-lateral slip may be occurring. 139 Most of the faults within the YFTB are favorably oriented for thrust motion, but there is some 140 evidence of right-lateral strike slip motion from earthquake focal mechanisms (Pezzopane and 141 Weldon, 1993; McCaffrey et al., 2007). 142 A rheologic or structural boundary could cause partial termination of slip on the OWL 143 near Rattlesnake Mountain. An increased thickness of low-density material below the CRB is 144 inferred from gravity data (Saltus, 1993; Lutter et al., 1994), and a rift structure is hypothesized 145 beneath the YFTB (Catchings and Mooney, 1988). The Hog Ranch-Naneum anticline west of the 146 Columbia River may mark a crustal boundary (Campbell, 1989; Reidel et al., 1989), and the 147 edge of the pre-Miocene continent may lie near Wallula Gap (e.g., Reidel et al., 1994). Little is 148 understood about these deep crustal features, however, because seismic imaging through the 149 CRB is notoriously difficult (Catchings and Mooney, 1988). 150 If the OWL is a major strike-slip fault system as inferred here, its length implies that it 151 penetrates the seismogenic crust. Compilations of earthquake data indicate that the downdip 152 widths of strike-slip fault ruptures span the entire seismogenic portion of the crust when their 153 length exceeds 35 to 60 km (Leonard, 2010). Large strike-slip earthquakes (>60 km rupture 154 length) on long fault systems like the OWL thus likely rupture the entire seismogenic zone.

155 Geologic and geophysical evidence also suggest that the OWL penetrates rocks below the CRB. 156 The OWL extends beyond the west edge of the CRB, suggesting it is not restricted to the 157 volcanic rocks. The Straight Creek fault system northwest of the CRB forms part of the OWL 158 and cuts older rocks (Campbell, 1989), and there is debatable evidence that the OWL forms a 159 boundary within rocks beneath the CRB (Barrash et al., 1983; Campbell, 1989; Saltus, 1993; 160 Reidel et al., 1994; Campbell, 1989). A crustal-penetrating OWL also is consistent with its 161 possible role as the northern edge of basin and range extension (Wise, 1963; Lawrence, 1976; 162 Wells and Heller, 1988; Hooper and Conrey, 1989; Mann and Meyer, 1993) and as a major block 163 boundary at present (McCaffrey et al., 2007). 164 The faults forming at least one of the YFTB structures deforms strata below the CRB. A 165 seismic profile across the Saddle Mountains anticline of the YFTB shows folding of strata below 166 the ~3.5-km depth to the base of the CRB (Campbell, 1989), and kinematic models suggest the 167 fault extends to 6-9 km depth (figure 3). This south-dipping fault presumably roots in the OWL 168 south of the profile. Manastash and Umtanum ridges also appear to be bounded by faults that 169 extend into older rocks below and to the west of the CRB (Blakely et al., 2011). 170 The model proposed here is that the YFTB is formed by splay faults that are effectively a 171 large damage zone associated with strike-slip motion on the OWL. Damage zones along strike- 172 slip faults examined in outcrop scale (Kim et al., 2003; McGrath and Davison, 1995), in analog 173 models (Naylor et al, 1986), and on seismic profiles (Harding, 1985; Naylor et al., 1986; 174 Sylvester, 1988) show downward-narrowing, V- or U-shaped geometries in vertical cross 175 section. When strike-slip faults terminate or step over to another fault, however, the damage zone 176 widens and the splay fault dips get shallower to produce a broad ‘V’ (e.g. McClay and Bonora, 177 2001). These analogs imply that the faults forming the YFTB likely intersect the OWL at mid- 178 crustal or lower-crustal depths (figure 3). Seismicity in the YFTB region does not directly 179 delineate specific fault zones in the subsurface (e.g., Reidel et al., 1989; Gomberg et al., 2012), 180 so the precise relationship between the YFTB structures and the modern seismicity remains 181 enigmatic (Reidel et al., 1989; Gomberg et al., 2012). However, the model and data presented 182 here indicate the YFTB faults extend below the CRB and may root in the OWL, making them 183 more consistent with hosting earthquakes with the larger magnitudes (M6.5-7.9) assumed in the 184 hazard analyses (Geomatrix Consultants, 1996).

185 In summary, this report presents a model in which the YFTB formed as splay faults in 186 response to a perturbed stress field caused by a decrease in the amount of strike-slip motion on 187 the OWL. The YFTB structures appear to be directly related to the OWL and the trends of the 188 YFTB, and the areas of uplift and subsidence near Rattlesnake Mountain and the Pasco Basin, 189 matches remarkably well those predicted for splay faults by modeling of the stress field at a fault 190 tip using fracture mechanics theory. The implication is that the OWL is indeed a major strike-slip 191 fault zone, as has been hypothesized in the past but has not been demonstrated based on geologic 192 evidence. Its length suggests that the OWL cuts most or all of the crust, and that the associated 193 YFTB faults are deeply rooted and therefore capable of larger earthquakes than are plausible for 194 “thin-skinned” structures. 195 196 197 ACKNOWLEDGMENTS 198 This paper benefited from discussions with and reviews by Juliet Crider, Brian Sherrod, Joan 199 Gomberg, Rob McCaffrey, Craig Weaver and Rick Blakely. Elizabeth Barnett assisted in the 200 figure preparation. Seismic Exchange, Inc. gave permission to show the seismic profile.

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331 332 FIGURES 333

334

335 336 Figure 1: A. Shaded relief map of the U.S. Pacific Northwest (Haugerud, 2004) showing the 337 Olympic-Wallowa lineament (OWL; between red arrows) cutting through the Yakima Fold and 338 Thrust Belt (YFTB) and the area of subdued topography caused by the Columbia River Basalts 339 (CRB). The black rectangle outlines the area of the map in figure 1B. 340 B. Digital elevation model of the YFTB and associated faults (USGS, 2006; Washington State, 341 2010). Blue line is the location of the seismic profile shown in figure 3. Yellow dots are 342 earthquakes greater than magnitude 2 since 1970 from the U. S. Geological Survey’s National 343 Earthquake Information Center (NEIC). Black dashed line is the Columbia River. Formation of 344 the Columbia and Horse Heaven Hills anticlines requires shortening between B and B’ on the 345 south side of the OWL without comparable shortening between A and A’ on the north side; this 346 difference in shortening requires strike-slip motion along the Wallula fault. MR=Manastash 347 Ridge; UR=Umtanum Ridge; GM=Gable Mountain; YR=Yakima Ridge; RH=Rattlesnake Hills; 348 RM=Rattlesnake Mountain; WG=Wallula Gap; HR-N=Hog Ranch-Naneum anticline.

349

350 351 352 Figure 2: A. Outcrop-scale crack with horizontal (strike-slip) motion showing the pattern of 353 extensional cracks and compressional solution features near its tips (from Kattenhorn and 354 Marshall, 2006). B. Displacement versus distance on the modeled fault shown in parts C-F. C. 355 Left half of a model showing the vertical uplift (colors) and the principal stress trajectories (cross 356 symbols) around the tip of a 400-km-long vertical strike-slip fault. The long lines in the crosses 357 are perpendicular to the maximum compressive stress and show the expected strike of thrust 358 faults (e.g., red dashed lines); short lines in the crosses are perpendicular to the minimum 359 compressive stress and show the expected strike of extensional fractures (e.g., black dashed 360 lines). The amounts of uplift and subsidence in the model are related to the displacement on the 361 fault, which was arbitrarily set at about 9 m to simulate a large earthquake. D. Stress trajectories 362 from the fault model overlain on the YFTB. The ridges and reverse faults northeast and 363 southwest of the OWL are nearly perpendicular to the maximum compressive stress. The 364 Columbia River (dashed black line) north of the fault tip trends parallel to the least principle 365 stress and could follow a normal fault. The area of maximum uplift coincides with Rattlesnake 366 Mountain, and the area of predicted subsidence coincides with the Pasco basin (gray dashed line; 367 generalized from Reidel et al., 1989 and 1994). E. Predicted directions of faults oriented for 368 maximum shearing (short black lines) near the fault termination. Background colors indicate the 369 magnitude of shear stress. F. Shear stresses amplitude and fault directions near the modeled fault 370 overlain on a map of the YFTB. Manastash and Umtanum ridges form part of the OWL, and the 371 Frenchman Hills (FH) and Saddle Mountains (SM) anticlines turn parallel to the preferred shear

372 directions west of the Columbia River. Other faults in the area have similar trends to the 373 predicted directions of maximum shearing.

374 375 Figure 3: Uninterpreted (A) and interpreted (B) seismic profile across the Saddle Mountains 376 anticlinorium (2x vertical exaggeration; see figure 1 for location) showing folding of strata 377 below the base of the CRB (dashed line marked ‘B’). The crest of the anticline is at about km 19 378 of the profile, with the thrust fault reaching the surface in the data gap caused by the steep north 379 slope of the anticline. A drill hole near the crest of the anticline (vertical black line, projected 380 about ~11 km to the west) penetrated the base of CRB at a depth of about 3500 m below sea 381 level (Campbell, 1989), which places the base of CRB at a depth of 3850 m on the profile whose 382 top is at 350 m elevation. The base of the CRB and underlying reflectors are folded in the 383 anticline, requiring the fault to be deeper. Reflector truncations are consistent with a fault dip of 384 about 60° to depths of 5 km or more, and kinematic models indicate the fault bends to shallow 385 dips between 6 and 9 km depth. C. N-S topographic profile near the longitude of the seismic 386 profile and a possible pattern of splay faults forming the YFTB. The only control on fault depth 387 is from the seismic profile beneath the Saddle Mountains (SM); the remaining geometries are 388 inferred from generalized studies of strike-slip splay faults (Harding, 1985; Naylor et al., 1986; 389 Sylwester, 1988; McClay and Bonora, 2001). The scales above and below sea level (S.L.) are 390 different. CR=Columbia River. Other letters as in figure 1. Seismic profile is reproduced with the 391 permission of Seismic Exchange, Inc.