Implications for the Structure of the Hat Creek Fault and Transfer of Right-Lateral Shear from the Walker Lane North of Lassen Peak, GEOSPHERE; V

Home , Hat Creek (California), Klamath Mountains

Research Paper THEMED ISSUE: Origin and Evolution of the Sierra Nevada and Walker Lane

GEOSPHERE Implications for the structure of the Hat Creek fault and transfer of right-lateral shear from the Walker Lane north of Lassen Peak, GEOSPHERE; v. 12, no. 3 northern California, from gravity and magnetic data doi:10.1130/GES01253.1 V.E. Langenheim, R.C. Jachens, L.J.P. Muffler, and M.A. Clynne 8 figures; 1 table; 1 supplemental file U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

CORRESPONDENCE: zulanger@usgs.gov ABSTRACT Valley block, into this region has been attributed to northward migration of CITATION: Langenheim, V.E., Jachens, R.C., Muf- the Mendocino triple junction 200–300 km to the west (Faulds et al., 2005), fler, L.J.P., and Clynne, M.A., 2016, Implications for the structure of the Hat Creek fault and transfer of Interpretation of magnetic and new gravity data provides constraints on along with northwest translation of the Sierra Nevada–Great Valley block right-lateral shear from the Walker Lane north of the geometry of the Hat Creek fault, the amount of right-lateral offset in the (Unruh et al., 2003). Many publications (e.g., Stewart, 1988; Muffler et al., Lassen Peak, northern California, from gravity and area between Mount Shasta and Lassen Peak (northern California, USA), and 2008; Busby, 2013) do not show the Walker Lane extending north of Lassen magnetic data: Geosphere, v. 12, no. 3, p. 790–808, doi:10.1130/GES01253.1. confirmation of the influence of preexisting structure on Quaternary faulting. Peak (Fig. 1). Yet others have argued that right-lateral shear continues north Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of Lassen Peak, perhaps as part of the enigmatic Northern California shear Received 21 August 2015 of both normal and reversed polarity, whereas a markedly smoother magnetic zone (Wesnousky, 2005, fig. 2 therein), which includes the predominantly Revision received 8 January 2016 field occurs over the Klamath Mountains and Paleogene cover there. Although normal Hat Creek and McArthur faults. The amount of right-lateral offset Accepted 18 February 2016 the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek is unknown for the shear zone, although faults in the northern Walker Lane Published online 24 March 2016 fault, which is one of the most prominent normal faults in the region and have as much as 30 km of displacement (Wesnousky, 2005; Faulds et al., forms the eastern margin of the Hat Creek Valley, is marked by the eastern 2005). Right-lateral shear may step to the west, producing uplift and folding edge of a north-trending magnetic and gravity high 20–30 km long. Model- through the Inks Creek fold belt of Harwood and Helley (1987) (ICF in Figs. ing of these anomalies indicates that the fault is a steeply dipping (~75°–85°) 1 and 2) in the northern Great Valley (Unruh et al., 2003) and very specula- structure. The spatial relationship of the fault as modeled by the potential-field tively to the Grizzly Peak anticline in the eastern Klamath Mountains (GPA data, the youngest strand of the fault, and relocated seismicity suggest that in Fig. 2; Sawyer, 2013). Alternatively, the Walker Lane has been depicted as deformation continues to step westward across the valley, consistent with a extending as far north as the Klamath graben (KG in Fig. 1; Oldow and Cash- component of right-lateral slip in an extensional environment. man, 2009), with extensional faults overprinted by possible young strike-slip Filtered aeromagnetic data highlight a concealed magnetic body of Meso movement (Waldien, 2012; Waldien and Meigs, 2013). Dextral shear in and zoic or older age north of Hat Creek Valley. The body’s northwest margin north of the Hat Creek region might be expected, given oblique subduction strikes northeast and is linear over a distance of ~40 km. Within the resolution of the Gorda plate beneath the North American plate (Fig. 1), and, although of the aeromagnetic data (1–2 km), we discern no right-lateral offset of this geodetic data indicate right-lateral shear north of Lassen Peak, it is clearly body. Furthermore, Quaternary faults change strike or appear to end, as if to accommodated by clockwise crustal rotation north of the California-Ore- avoid this concealed magnetic body and to pass along its southeast edge, gon border (McCaffrey et al., 2007, 2013; Fig. 1 inset) rather than by discrete suggesting that preexisting crustal structure influenced younger faulting, as strike-slip faults. The boundary between the Oregon coast block, which has previously proposed based on gravity data. been rotating since the Miocene, and the Walker Lane is poorly defined, and the amount, if any, of right-lateral shear and how it may be accommodated in this region is not well known. INTRODUCTION The most studied fault north of Lassen Peak is the Hat Creek fault (Muffler et al., 1994; Walker, 2008; Blakeslee and Kattenhorn, 2013; Kattenhorn et al., The Hat Creek fault is located at or near the junction of major tectonic 2016), which is well expressed geomorphically and offsets units as young as provinces in northern California (USA): the Sierra Nevada–Great Valley the Hat Creek Basalt, dated as 24 ± 6 ka (Turrin et al., 2007; refined to 23.8 ± block, southernmost Cascade arc, Basin and Range, Oregon coast block, 1.4 ka by Rood et al., 2015) and ca. 15 ka periglacial deposits (Muffler et al., For permission to copy, contact Copyright and northern end of the Walker Lane (Fig. 1). The encroachment of the 1994). Although the Hat Creek fault is a significant normal fault that may reflect Permissions, GSA, or [email protected]. Walker Lane, a zone of right-lateral shear east of the Sierra Nevada–Great westward encroachment of Basin and Range extension (Muffler et al., 2008),

GEOSPHERE | Volume 12 | Number 3 Langenheim et al. | Hat Creek fault and transfer of right-lateral shear Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/790/4092661/790.pdf 790 by guest on 30 September 2021 Research Paper

INSET

COLUMBIA 50 EMBAYMENT

45 X Juan de Fuca OREGON Plate COAST Fig. 8 44 BLOCK

C A S C A D E KG A R C KLAM Oregon LEGEND Idaho 42° Gorda MTNS Nevada spreading ridge MS Plate ATH BASIN subduction zone LP ICF AND W 40° transform fault

MENDOCINO A

GREAT SIERRA NEL TRIPLE K RANGE extension direction from

JUNCTION E

R Unruh et al. (2003)

VALLEY BL L 38° rotation sense of Oregon San A

VADA Coast Block Pacific Andreas Fault N

E Plate Californi region of dextral shear - OCK a Quaternary Cascades 36° volcanic arc (Hildreth, 2007) gravity lows discussed in Blakely et al. (1997) conductive features

34°N (Bedrosian and Feucht, 2014) Eocene to Miocene tear 0 100 km in subducting slab (Colgan et al., 2011) 126° 124° 122° 120° 118° 116°W

Figure 1. Index map showing plate tectonic setting of the western U.S. Study area (white box; Figs. 2–6) is at the junction of the Cascade Arc, Walker Lane, Basin and Range, the Sierra Nevada–Great Valley block, the Klamath Mountains, and the Oregon coast block. ICF—Inks Creek fold belt of Harwood and Helley (1987); KG—Klamath graben; LP—Lassen Peak; MS—Mount Shasta. Inset shows global positioning system velocities of the western U.S. relative to North America (modified from McCaffrey et al., 2013). Blue x marks the rotation pole of Cascadia.

122°15′ 122° 121°45′ 121°30′ 121°15′W Qs 41°30′N Qv Mt. ACB Tv Qv Shasta Ts Ks Js Qs TRs mv

oph Redding Pzs

McCloud y oph init

89 SUBTERRANES

Tr 41°15′ Tv Tv Figure 2. Geologic map of the study area Pzs (modified from Jennings et al., 2010) Qs superposed on shaded-relief topogra- GPA phy. A–A′—location of profile modeled Big Valley in Figure 7. Qs—Quaternary sediments;

McAr Qv—Quaternary volcanic rocks; Tv—Neo- TRs Qs gene volcanic rocks; Ts—Paleogene and

Js thur F 299 Neogene sedimentary rocks; Ks—Creta- Ts ceous sedimentary rocks; Js—Jurassic mv River sedimentary rocks; TRs—Triassic sedimen- SM ault Big CM FRM P tary rocks; mv—Mesozoic metavolcanic Klamath Bend BS 41° and intrusive rocks; Pzs—undifferentiated Mountains Paleozoic rocks (note Js, TRs, mv, and Pzs Tv Pi t are part of the Redding subterrane); oph— Br Ordovician ophiolite (Trinity subterrane). White lines—major highways; black CB lines—Quaternary faults (U.S. Geological Bu Tv Survey and California Geological Survey, Hat Creek F 2006); heavier black lines—Hat Creek mv Hat fault. ACB—Ash Creek Butte; BB—Bogard BM TRs Creek Buttes; BM—Burney Mountain; Br—Brush Mountain; BS—Burney Springs Mountain; Valley CM—Chalk Mountain; CB—Cinder Butte; A ault A’ ′ FP—Freaner Peak; GPA—Grizzly Peak anti Ts FP 89 40°45 TRs cline; SM—Saddle Mountain; ICF—Inks MV Creek fold belt; MV—Magee volcano; PP— 299 Prospect Peak; WPP—West Prospect Peak. Ks Qv Towns: Bu—Burney; FRM—Fall River Mills; P—Pittville.

WPP BB Ks PP 44 122°00122°1222°022°2 000'' 44 404 °303 ' Lassen Peak 40°30′

Qs Tv Qv ICF

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its left-stepping geometry indicates a small right-lateral, strike-slip component that correlate with positive topographic features in Fig. 3). Figures 3 and 4 also (Wills, 1991; Blakeslee and Kattenhorn, 2013). Estimates of Quaternary normal include an isostatic correction that removes long-wavelength effects of deep slip rates range from 1.2 to 1.9 mm/yr (Turrin et al., 2007) to 2.2–3.6 mm/yr crustal and/or upper mantle masses that isostatically support regional topog- (Blakeslee and Kattenhorn, 2013), whereas the most recent estimate from mod- raphy. The resulting isostatic residual gravity maps (Figs. 3 and 4) reflect lateral eling of global positioning system data is <1 mm/yr (Thatcher et al., 2014). variations of density within the middle to upper crust. The accuracy of these The slip rates are based on an assumed fault dip of 60° because information data is estimated to be better than ±0.5 mGal. on subsurface fault geometry is lacking. Where and in what manner this fault Aeromagnetic data consist of a survey flown in 1980 along east-west flight extends to the north is of interest, particularly for state, federal, and private lines spaced 1600 m apart at an altitude of 2740 m (Couch and Gemperle, infrastructure, such as dams along the Pit River. 1982). These data were processed to account for the inclination (65°) and The area has been relatively little studied, as compared to the Walker Lane. declination (17°) of the Earth’s magnetic field in this region and to shift the Most of the area has been mapped at only reconnaissance levels, except for anomalies over their causative sources. This operation is referred to as re- a map of Lassen Volcanic National Park in the southern part of the study area duction to the pole, and transforms the magnetic anomaly to that that would (Clynne and Muffler, 2010), a map of the outline of a lava flow that extends be measured at the magnetic north pole where the Earth’s magnetic field is from the Medicine Lake volcano area into the north-central part of the study directed vertically down. Although aeromagnetic profiles were flown closer area (Donnelly-Nolan, 2010), and detailed mapping (Muffler et al., 2012) that to the ground (120 m) by the National Uranium Resource Evaluation Program builds upon and extends earlier mapping of Hat Creek Valley (Muffler et al., (Hill et al., 2009), these lines were flown too far apart (4800 m) to map effec- 1994; Turrin et al., 2007). Little is known about pre-Pliocene geology in this tively the magnetic field between the lines. area because of extensive Quaternary volcanic cover (Fig. 2). The subsurface We used gradients of gravity and magnetic fields to map the locations of geometry of the faults in this area is poorly known because seismicity is gen- geologic contacts (dashed gray and black lines, Figs. 3 and 4; magenta lines, erally dispersed, even when hypocenters are relocated with double-difference Fig. 4; black dots, Figs. 5 and 6). The method (Cordell and Grauch, 1985; Blakely methods (Waldhauser and Schaff, 2008). Previous work in the southern Cas- and Simpson, 1986) exploits the fact that gravity anomalies and magnetic cade arc, based on regional gravity lows that transect the study area, proposed anomalies converted to magnetic potential have steepest horizontal gradients that the patterns of Quaternary faulting and volcanism are influenced by pre- directly over the contacts. A matched filter (Phillips, 2001) was applied to the

Below are processing details and gravity data collected by U.S. Geological Survey 2011-2015. existing crustal structure (Blakely et al., 1997; Fig. 1). As part of a larger effort aeromagnetic data to separate the data into different wavelength components The data were referenced to the International Gravity Standardization Net 1971 (IGSN71) datum and the reference ellipsoid is the Geodetic Reference System 1967 (GRS67). The free-air anomalies were calculated using a modified version of formulas to characterize seismic hazard in the region north of Lassen Peak, we collected that can then be related to depth. This is achieved by modeling the observed from Swick (1942). The complete Bouguer anomalies were calculated from the free-air anomalies using the Bouguer correction, terrain corrections, a curvature correction, and a reduction density of 2670 kg/m3. Isostatic corrections were calculated using an Airy-Heiskanen model of isostatic gravity data and reexamined the existing aeromagnetic data in order to refine anomalies as a sum of anomalies from distinct equivalent source layers (fic- compensation. The depth of the crust-mantle boundary was controlled using the following parameters: a crustal thickness at sea level of 25 km, a density contrast of 400 kg/m3 between the crust and mantle, and a crustal the subsurface structure of this region. In doing so, we examine constraints tional layers below the observation surface where the distribution of mag- density of 2670 kg/m3.

Elevation control was from postprocessing of Trimble handheld GPS devices and accuracy is estimated to be 1 meter or less (generally 0.3 m or 1 ft). Data were tied to the base station at Burney, on the amount of right-lateral slip, revisit the role of preexisting structure on netization produces the observed magnetic field) at increasing depths. The California that we established by tying into Chapman base 17 in Fall River Mills (Chapman, 1966, p. 18; ch17 in data table below). The base station (burney) is located at the entrance of the Burney cemetery near the intersection of Mountain View and Erie streets in Burney. Place meter at the base of the west column of the entrance to read. See pictures at end of document. Quaternary faulting, and investigate the geometry of the Hat Creek fault north matched-filter produced dipole-equivalent source layers at depths of 1.1 km

Terrain corrections were calculated using 10- and 30-m digital elevation models out to a distance of 2000 m (code M). The horizontal and vertical locations for the stations are on the North of Lassen Peak. (shallow), 2.6 km (intermediate depth), and 19.8 km (deep) (Fig. 6). Compari- American Datum 1927 (NAD27) and the North American Vertical Datum 1929 (NAVD29), respectively. son of these fields with the geology in the following suggests that the upper EXPLANATION OF FORMAT FOR GRAVITY MEASUREMENTS Item:Explanation two layers generally reflect anomalies caused by the Neogene volcanic rocks, STATION:An alphanumeric combination of up to 8 characters used for station identification. Columns 0-8 DATA AND METHODS whereas the deepest layer is sensitive to sources in the underlying basement. LATD: Degree latitude. Columns 10-11 LATM: Minute latitude. Columns 13-17 Note that the actual source depths for the layers may be shallower, given that LOND: Degree longitude. Columns 19-21 LONM: Minute latitude. Columns 23-27 We collected 1090 new gravity measurements (solid black circles, Fig. 3; broader wavelength anomalies can be fit by both deep and shallow sources. ELEV: Elevation. Columns 29-35 1 OG: Observed gravity. Columns 37-45 Supplemental File ) and added them to the regional database (solid black tri- We also show gradients derived from these filtered versions of the magnetic FAA: Free-air anomaly. Columns 53-58

SBA: Simple Bouguer anomaly. Columns 60-66 angles, Fig. 3), which consists of nearly 2000 measurements in the study area data in Figure 6.

ITC: Inner terrain correction out to a radius of various distances (see TC CODE) from the station, for a density of 2.67 g/cc. Columns 68-73 (Chapman et al., 1977; Robbins et al., 1976). Standard formulas for the free-air, TC: Total terrain correction from the station to 166.7 km for a density of 2.67 g/cc. Columns 75-80 Bouguer, curvature, and terrain corrections were used (Blakely, 1996; Telford CODE: Letter denoting the extent of the inner-zone correction, according to the Hayford-Bowie and Hammer templates (M=2000 meters). See Spielman and Ponce (1984) for additional explanation. Column 82 et al., 1990). We used a standard reduction density of 2670 kg/m3 (typical of RESULTS CBA: Complete Bouguer anomaly reduced for a density of 2.67 g/cc. Columns 84-90 ISO: Isostatic residual anomaly values. Columns 92-98. basement rocks, such as those exposed in the Klamath Mountains) shown in Example of format for gravity file Figure 3; this assumes that crust above the geoid, including the terrain, has a Gravity Anomalies 11hc001 41 0.54 121 26.08 3323.7 979914.18 -32.38 -145.74 0.08 0.32 M -146.54 -18.22 density of 2670 kg/m3. However, much of the terrain, excluding the Klamath 1Supplemental File. Processing details and gravity Mountains, is composed of Cenozoic volcanic rocks with lower average den- The dominant features in the gravity field are the pronounced lows data collected by U.S. Geological Survey 2011–2015. sities of ~2500–2600 kg/m3 (LaFehr, 1965; Table 1). Thus we also show, in Fig- over the Shasta and Lassen volcanic centers and Caribou volcanic field Please visit http://dx.doi .org /10 .1130/GES01253 .S1 3 or the full-text article on www.gsapubs.org to view ure 4, the gravity field reduced with a density of 2500 kg/m in order to atten- and the 30-km-wide band of gravity highs that extends east-northeastward the Supplemental File. uate anomalies that may be artifacts of reduction density (e.g., gravity lows from exposed pre-Cenozoic rocks of the Klamath Mountains (Figs. 3 and 4).

122°15′ 122° 121°45′ 121°30′ 121°15′W

41°30′N mGal Mt. Shasta 12

6 89 McC 3 41°15′ 0

GPA –3 Big Valley –6 Figure 3. Isostatic residual gravity map (reduction density, 2670 kg/m3). Solid black 299 –9 dots—new gravity data; black triangles— previously collected data. Blue line—Pit Klamath River; pale blue lines—major highways –12 Mountains P (California state highways are shown by BiB FRM 41° white ovals). Dashed gray lines—gradi- –15 ents marking edges of gravity highs from Blakely et al. (1997); dashed black lines— additional gravity gradient identified from –18 new data from horizontal gradient analy- CB sis. Heavy brown lines delineate Hat Creek Bu –21 fault. Red line labeled A–A′—location of profile modeled in Figure 7. CB—Cinder –24 Butte; GPA—Grizzly Peak anticline; HCF— Hat Creek fault; WPP—West Prospect 89 Peak. Towns: BiB—Big Bend; Bu—Burney; A A –27 FRM—Fall River Mills; McC—McCloud; P— ′ 40°45′ Pittville. HCF –30 299 –33

–36 WPP

44 –39 Caribou Lassen Volcanic 44 –42 Peak 40°30′ Field –45

Lassen Volcanic Center 015 0152025km

TABLE 1. DENSITY AND MAGNETIC SUSCEPTIBILITY MEASUREMENTS Minimum Minimum Maximum Maximum Number of Number of Density Susceptibility density susceptibility density susceptibility density susceptibility Rock type (kg/m3) (10–3 SI) (kg/m3) (10–3 SI) (kg/m3) (10–3 SI) measurements measurements All Neogene rocks (volcanics, diatomite, scoria) 2610± 2586.34± 4.78 1360 0.00 2930 20.73134 230 Andesite 2642± 1187.75± 4.95 2310 1.01 2830 20.7350104 Basalt (all, including weathered) 2665± 1495.33± 4.45 2240 0.50 2930 19.4877115 Basalt (unweathered) 2730± 95 6.26 ± 4.86 2500 0.63 2930 16.085562 Mesozoic metavolcanics 26607.34± 1.67 5.54 8.84 13 Mesozoic sedimentary rocks 2632± 10 0.09 ± 0.02 2610 0.05 2670 0.11 46 (Pit Formation and Hosselkus Limestone)

Pronounced gravity highs also bracket the Lassen volcanic center and Caribou Pit River that is less dense than the reduction density (2670 kg/m3) based on the volcanic field on its eastern and western margins. Although they do not cor- gradient’s alignment with the river and its steepness. However, an erroneous respond with any exposed pre-Cenozoic basement in the study area, the grav- reduction density cannot explain the entire gradient because the northern gra- ity highs likely reflect concealed basement highs (Blakely and Jachens, 1990; dient remains when the reduction density is reduced to a more representative Blakely et al., 1997), as basement is exposed ~10 km to the south of the study density of 2500 kg/m3 (Fig. 4; Table 1), or even lower densities such as 2400 area (Jennings et al., 2010). Although the gravity highs that bracket the Lassen kg/m3. We conclude that part of the northern gradient is caused by a subsur- volcanic center and Caribou volcanic field are of similar amplitude on either face density contrast. The northern gradient is gentler east of California High- side of the gravity low, the basement rocks differ in their magnetic character way 89 and is interrupted by gravity lows associated with thick basin fill in the (Fig. 6C) and may reflect different basement rock types. The alternating bands Fall River Mills area and Big Valley. of gravity highs and lows extend to the east-northeast beyond the study area Superposed on the gravity low that extends north in a subdued fashion from for ~300 km (Blakely et al., 1997; green polygons in Fig. 1). the Lassen volcanic center are higher gravity values that trend north and are The gravity highs extending northeast from the Klamath Mountains are west of the Hat Creek fault (HCF in Fig. 3) and within the eastern part of Hat Creek interrupted by gravity lows extending north-northwest from Fall River Mills Valley (Fig. 4). The east edge of the gravity high coincides with the Hat Creek fault and Big Valley (Figs. 3 and 4). These lows likely reflect low-density basin fill south of Cinder Butte. The gravity high weakens north of Cinder Butte. The in young tectonic basins because the margins of these lows partly coincide presence of a gravity high over most of the valley contrasts with gravity lows with Quaternary faults. Some of the basin fill may be late Neogene in age, that arise from low-density basin fill in the Fall River Mills area, Big Valley, but the ages of the oldest lava flows exposed in the upthrown blocks of these and most basins of the Basin and Range province to the east. This feature is faults (Page and Renne, 1994) suggest that these basins formed primarily discussed more at length in the section on modeling of the Hat Creek fault. during the Quaternary. The gravity low west of Big Bend coincides with the exposed Triassic sedimentary section of the Redding subterrane (Fig. 2) and thus cannot be caused by Cenozoic basin fill. The gravity low results from the Magnetic Anomalies sedimentary rocks being less dense than the mafic and ultramafic rocks of the Trinity subterrane exposed to the west and denser basement rocks con- The magnetic data reveal a complicated magnetic pattern over exposed cealed beneath Cenozoic volcanic cover to the east. These concealed base- Neogene volcanic rocks. The pattern consists of short-wavelength, high-ampli ment rocks likely include metavolcanic rocks and hypabyssal diorite bodies, tude magnetic highs and lows and is in contrast with smoother anomalies such as those exposed along the west bank of the Pit River near Big Bend over the Klamath Mountains (Fig. 5). Although the magnetic pattern is com- (Renne and Scott, 1988). plex over the young volcanic rocks, some of the anomalies can be clearly tied The new gravity data refine the southern boundary of the gravity high be- to geologic features exposed at the surface. Pronounced oblong magnetic tween Big Bend and Fall River Mills. Here the southern boundary is stepped highs are present over young lava cones or shield volcanoes at West Pros- and marked by two gradients. The northern gradient is aligned along the Pit pect Peak (WPP, Fig. 5; ca. 400–300 ka, Clynne and Muffler 2010) and Prospect River for a distance of 20 km west of California Highway 89 (westernmost black Peak (PP, Fig. 5; 247 ± 56 ka, Clynne and Muffler, 2010), Burney Mountain (BM, dashed line in Fig. 3); the southern gradient is generally coincident with the Fig. 5; 40K/39Ar 243 ± 24 ka by Brent Dalrymple and 40Ar/39Ar 280 ± 6 ka by edge of the low defined by Blakely et al. (1997; gray dashed line in Fig. 3). Part Marvin Lanphere, in Muffler et al., 2012), Freaner Peak (FP, Fig. 5), Magee Vol- of the northern gradient in Figure 3 is caused by terrain north and south of the cano (MV, Fig. 5; 210 ± 120 ka; Clynne and Muffler, 2010), and Ash Creek Butte

122°15′ 122° 121°45′ 121°30′ 121°15′W

41°30′N mGal Mt. Shasta 24

McC 15 41°15′ 12 89 GPA 9 Big Valley 6 Figure 4. Isostatic residual gravity map (reduction density 2500 kg/m3). Note that gravity field is smoother over volcanic ter- 3 299 rain than in Figure 3. Blue line—Pit River; Klamath Mc white lines—major highways (California F 0 state highways are shown by white ovals). Mountains BiB FRM P 41° Dashed gray lines—gradients marking –3 gravity high from Blakely et al. (1997); dashed black lines—additional gravity gradient identified from new data. Thick –6 magenta lines are based on horizontal gradient analysis and coincide in part with Bu CB 89 –9 the Hat Creek fault and other Quaternary faults. Red line—location of profile mod- Hat eled in Figure 7. CB—Cinder Butte; GPA— –12 Creek Grizzly Peak anticline; HCF—Hat Creek fault; McF—McArthur fault; WPP—West Valley –15 Prospect Peak. Towns: BiB—Big Bend; A HCF A Bu—Burney; FRM—Fall River Mills; McC— ′ 40°45′ McCloud; P—Pittville. –18 299 –21

–24 WPP 44 –27 Lassen Caribou Volcanic 44 –30 Center Volcanic 40°30′ Lassen Field –33 Peak

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122°15′ 122° 121°45′ 121°30′ 121°15′W

41°30′N nT Mt. ACB Shasta 780

720

660 McC 600 41°15′ 540 89 480 Big Valley Figure 5. Shaded-relief aeromagnetic map 420 of study area (reduced to pole). Black dots—maximum horizontal gradients of b 299 c pseudogravity anomaly that represent 360 Klamath geologic contacts. White arrows denote a linear northeast-trending magnetic gra- Mountains SM P 300 dient discussed in text. Dark blue and CM FRM Big BS 41° white line—Pit River; white lines—major highways (California state highways are 240 Bend shown by white ovals). Red line—loca- Br tion of profile A–A′ modeled in Figure 7. 180 Magenta star marks Hat Creek Basalt vents. ACB—Ash Creek Butte; BB—Bogard CB Buttes; BM—Burney Mountain; Br—Brush Bu 120 Mountain; BS—Burney Springs Mountain; CM—Chalk Mountain; CB—Cinder Butte; River H BM at Creek 60 FP—Freaner Peak; SM—Saddle Mountain; MV—Magee volcano; PP—Prospect Peak; 89 WPP—West Prospect Peak. Magnetic lows 0 A A′ labeled a, b, and c are discussed in text. Pit FP Fa 40°45′ Towns: Bu—Burney; FRM—Fall River Mills; ult –60 McC—McCloud; P—Pittville. MV –120 299

–180 WPP BB 44 PP –240

44 –300 40°30′ –360 Lassen Peak

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122°15′ 122° 121°45′ 121°30′ 121°15′W

41°30′N nT Mt. ACB Shasta 70

60 McC 55 41°15′

50 89 45 Big Valley 40 Figure 6 (on this and following two pages). Matched-filtered magnetic maps (reduced 299 to pole). (A) Anomalies caused by shallow 35 sources. White arrows denote linear north- Klamath east-trending magnetic gradient discussed SM 30 in text. Black dots—maximum horizontal Mountains CM BiB BS 41° gradients (magnetization boundaries). FRM Dark blue and white line—Pit River; white 25 lines—major highways (white ovals are Br California state highways). Red line—loca 20 tion of profile A–A′ modeled in Figure 7; ACB—Ash Creek Butte; BB—Bogard Bu CB Buttes; BM—Burney Mountain; Br—Brush 15 Mountain; BS—Burney Springs Mountain; ver CM—Chalk Mountain; CB—Cinder Butte; Ri Hat Creek F BM 10 FP—Freaner Peak; SM—Saddle Mountain; MV—Magee volcano; PP—Prospect Peak; 89 WPP—West Prospect Peak. Towns: BiB— 5 A A Big Bend; Bu—Burney; FRM—Fall River Pit ′ ′ FP ault 40°45 Mills; McC—McCloud. 0 MV

299 –5

–10 WPP BB 44 PP –15

Lassen 44 –20 Peak 40°30′ –25

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122°15′ 122°121°45′ 121°30′ 121°15′W

41°30′N Mt. ACB nT Shasta

650

600

550 McC

41°15′ 500

89 450

Big Valley 400

350 299

300 Klamath SM Mountains CM BS FRM BiB 41° 250

Br 200 Figure 6 (continued). (B) Anomalies caused by intermediate-depth sources. CB 150 Bu

100 River BM

Hat Creek 50 A A’ Pit FP 40°45′ 0

Fault MV –50 299 –100

WPP BB –150 44 PP –200 Lassen 44 Peak 40°30′ –250

–300

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122°15′ 122° 121°45′ 121°30′ 121°15′W

41°30′N nT Mt. ACB Shasta 280

260

240

McC 220 41°15′ 200 89 180 Big Valley 160

299 140 Klamath SM 120 Mountains CM FRM BiB BS 41° 100

Br Figure 6 (continued). (C) Anomalies caused 80 by deep sources.

CB Bu 60

Hat Creek River BM 40

A A 20 ′ F ′ Pit FP a 40°45 u 0 lt

–20 299

–40 WPP BB 44 PP –60

–80 Lassen 44 40°30′ Peak –100

015 0152025km

(ACB, Fig. 5; 227 ka, Andrew Calvert, 2015, personal commun.), consistent with island-arc volcanic rocks and intruded by plutons (Irwin, 1994). This sequence their formation during a normal polarity chron. Inverse modeling of the Ash was affected by burial metamorphism up to greenschist facies (Renne and Creek Butte magnetic high indicates that the bulk of the magnetization (decli Scott, 1988). This subterrane is not a major magnetic source, although the nation of ~23°, inclination of ~33°) arises from at or below the topographic filtered magnetic data that enhance shallow sources (Figs. 6A, 6B) highlight base of the butte, possibly as an intrusive plug (Christiansen et al., 1977). The anomalies associated with the igneous parts of the subterrane. The Redding hydrothermally altered core of Brokeoff Volcano (Clynne and Muffler, 2010) subterrane (Fig. 2) is faulted on the west against Ordovician gabbro and ultra- may be the source of a subdued magnetic low 5 km southwest of Lassen Peak. mafic rocks of the Trinity subterrane, which is considered to be the source of A pronounced magnetic low coincides with the dacite of Bogard Buttes (BB, the prominent northeast-trending magnetic high in the northwest corner of the Fig. 5; 2350 ± 70 ka and 2235 ± 49 ka, Clynne and Muffler, 2010), consistent with study area (Griscom, 1977; Blakely et al., 1985). The magnetic high increases in its having been erupted during a reversed polarity chron. The magnetic high amplitude over outcrops of Trinity subterrane along the west edge of the map over Burney Spring Mountain (BS, Fig. 5) suggests that it should have erupted area (oph in Fig. 2), and the magnetic field filtered to enhance deeper sources during a normal polarity chron. An age of 2556 ± 142 ka (Andrew Calvert, in (Fig. 6C) is consistent with the Trinity subterrane extending northeastward in Muffler et al., 2012) makes it slightly younger than the Gauss normal chron the subsurface to beneath Mount Shasta (Blakely et al., 1985). Interpretation of (Cande and Kent, 1995), suggesting that the true age may be within the older a seismic refraction profile just north of the study area also supports the pres- range of the age uncertainty. Additional dates (Andrew Calvert, 2015, personal ence of an 8–10-km-thick slab of ultramafic rocks extending beneath Mount commun.) and paleomagnetic data (Duane Champion, 2015, personal com- Shasta at 5 km depth (Fuis et al., 1987). mun.) support its age within the Gauss normal chron and are consistent with Southeast of the smooth magnetic pattern associated with the Redding the magnetic high. subterrane is a prominent magnetic gradient that extends over a distance of Circular to oblong magnetic lows over Saddle Mountain, Brush Mountain, ~40 km northeast from Big Bend (northwest edge shown by white arrows in and Chalk Mountain suggest reversed polarity for these volcanoes. Although Figs. 5 and 6). Although many of the short-wavelength magnetic highs south- the flows at the top of Brush Mountain have normal polarity, the magnetic east of the magnetic boundary are clearly caused by Neogene volcanic rocks, low suggests that the bulk of the edifice may have formed during a reversed these anomalies are superposed on a broad magnetic high (Fig. 6C). The north- polarity chron, a conclusion supported by several paleomagnetic samples with west edge of the roughly rectangular high has a steeper gradient than the reversed directions taken from the base of the mountain (Duane Champion, southeast edge, suggesting that the source dips to the southeast. Two sources 2015, personal commun.). have been proposed for the magnetic high: exposed Quaternary volcanic rocks Another set of magnetic anomalies can be attributed to juxtaposition of rock (Glen et al., 2004) or a block of mafic or ultramafic rock buried at shallow depth of differing magnetic properties by Quaternary faults. The faults bounding Hat (Blakely et al., 1985), probably 2–3 km deep, based on the width of the gradi- Creek Valley provide the best example in the study area. The Hat Creek fault ent on its northwest edge. We prefer the latter interpretation for four reasons. coincides with the east edge of magnetic high that extends north from the inter (1) The northwest edge of the magnetic high is present in the same place in all section of California Highways 89 and 44 (Fig. 5). The magnetic high broad- of the filtered magnetic maps, and the high extends across outcrops of weakly ens and appears to step slightly to the west, ~5 km south of Cinder Butte. Just magnetic Montgomery Creek Formation, an Eocene nonmarine sandstone and north-northeast of the source vents for the Hat Creek Basalt (star in Fig. 5) is a conglomerate sequence as much as 800 m thick (Sanborn, 1960) near Big Bend. localized high superposed on the linear high bound by the fault. This high coin- (2) Blakely et al. (1985) showed that uniformly magnetic volcanic terrain could cides with a shift in fault activity that Kattenhorn et al. (2016) hypothesize may not account for the broad magnetic high. (3) The inferred southeast dip of the be related to underlying magmatic influences. A magnetic low straddles the body is consistent with southeastward younging and structure within this part western margin of the valley, also marked locally by faults, from Highway 44 to of the Redding subterrane (Fig. 2). (4) Regional seismic tomography (Thurber the latitude of Burney over a distance of 25 km, and coincides with a belt of vol- et al., 2009) shows higher velocities at depths of 4–14 km in the area of the mag- canic rocks that are considerably older than those to the east and west (Fig. 2). netic high. The age of the concealed body is unknown, but likely to be Mesozoic The short-wavelength magnetic anomalies associated with young volcanic or even older, if, for example, it is a fragment of Ordovician Trinity subterrane. rocks contrast with the smoother pattern over the western third of the study area. Broader magnetic anomalies are present in the southwest part of the study area, despite outcrops of Neogene volcanic rocks. The smoother pattern Geometry of the Hat Creek Fault in this area results from the survey altitude, which placed the magnetic sen- sor more than 2000 m above the ground surface. The smoother pattern over We used the gravity and magnetic fields to model the geometry of the the Klamath Mountains, however, results from extensive outcrops of weakly Hat Creek fault, the most prominent fault in the study area. We used a 2.5 magnetic Redding subterrane, which is composed of a Devonian to Jurassic dimensional simultaneous gravity and magnetic modeling program based on sequence consisting of sandstone, shale, tuff, and limestone intercalated with generalized inverse theory. The program requires an initial estimate of model

parameters (depth, shape, magnetization, and density of suspected sources) model assumes that the gravity gradient results from juxtaposition of dense and then varies selected parameters in an attempt to reduce the weighted root (2800 kg/m3) basin fill (brown body in Fig. 7), presumably basalt, against less mean square error between the observed and calculated potential fields. The dense (2590 kg/m3) volcanic rocks east of the fault. A good fit of the steep initial model estimate is based on mapped geologic relationships and physical gravity gradient is achieved if the fault dips 75° and the basin fill is as much as property information. The amplitude of the anomaly is not the only attribute to 2 km thick. The fill must include basalt flows older than the Hat Creek Basalt, match; matching its gradients and inflections are critical parameters that pro- because the Hat Creek Basalt has a maximum thickness of ~50–75 m (Walker, vide important information on the depth to the top of the source and its shape. 2008; Kattenhorn et al., 2016). The dense basin fill, if also magnetic, can also The interpretation of potential-field data yields nonunique solutions be- account for the magnetic high in the eastern part of the valley, although the cause an infinite number of geometrical models will have an associated field predicted gradient is somewhat steeper than observed. that closely matches the measured field. For example, increasing the density The second model attempts to fit the gravity and magnetic data using a while decreasing the thickness of a proposed basalt body will generally not concealed, normally magnetized, steeply dipping body beneath the eastern produce an appreciable change in the computed field. Although potential-field part of the valley (red body, bottom panel in Fig. 7). The steeply dipping body modeling is nonunique, the final models are based on numerous iterations model fits the shape of the magnetic high better (red line, top panel in Fig. 7), and have been extensively tested to produce a model that is geologically rea- but the resulting gravity gradient (red line, middle panel in Fig. 7) is apprecia- sonable with minimum structural complexities. The synthesis of the poten- bly less steep than observed. A geologic interpretation of this steeply dipping tial-field data and independent constraints described in the following leads to a body could be a dike swarm, presumably older than the Hat Creek Basalt and family of similar models, regardless of the starting model, with characteristics Cinder Butte (24 ± 6 ka and 38 ± 7 ka; Turrin et al., 2007), because no dikes are that support our major conclusions on the subsurface geology. exposed in that part of the valley. The vents that erupted the Hat Creek Basalt To determine the geometry of the Hat Creek fault, a profile was modeled are located only near the southern margin of the magnetic high (magenta star along section A–A′ (see Figs. 2–6). This profile was chosen because it coincides in Fig. 5). For both the basin-fill and steeply dipping model geometries, a 60° with detailed gravity measurements where the gravity anomaly associated dipping interface for the gravity and magnetic highs bound by the Hat Creek with the fault is most pronounced and where the magnetic anomaly associ- fault produces gradients that are significantly less steep than observed. Re- ated with the fault is more two-dimensional in character. gardless of the model, the potential-field data suggest that the Hat Creek fault We first tested whether the gravity anomalies result from topographic ef- dips steeply (75°–85°) in the upper 2–10 km. fects. Rock of uniform density 2000 kg/m3 is needed to fit the amplitude of the gravity gradient across the Hat Creek fault (blue dashed line, center panel, Fig. 7). Although the sharpness of the gradient argues for a near-surface source, DISCUSSION a density of 2000 kg/m3 is significantly lower than density measurements of the rocks in this area (Table 1), seems unlikely to account for all the observed We discuss the possible implications of the potential-field data for the style gravity gradient across the Hat Creek fault, and does not fit the observed grav- and evolution of Neogene deformation in the region north of Lassen Peak. ity anomalies away from the fault. The magnetic anomalies that span the valley also do not result from topographic effects, as indicated by the general lack of agreement between observed magnetic variations and those predicted from Hat Creek Fault uniformly magnetic terrain along the profile (top panel in Fig. 7). We modeled the profile using reasonable densities and magnetic proper- Although the Hat Creek fault is clearly a normal fault, the potential-field ties to fit anomalies of interest, starting with a deep magnetic source (pink data suggest that it dips more steeply than a typical Basin and Range normal body, bottom panel, Fig. 7) to fit a broad magnetic high west of the valley (Fig. fault, which dips 40°–60° as documented by Stein and Barrientos (1985). Nor- 6C); its exact geometry is not well constrained, but it may be related to the mal faults can dip more steeply, for example, in volcanic rocks. As mapped, deep, oblong magnetic anomaly to the northwest (Fig. 6C). Its density is not the youngest scarps of the fault are subvertical (Muffler et al., 1994). The extent constrained, but assumed to be higher than the surrounding crust and could of these subvertical scarps into the subsurface, however, would be minimal, if result from partly serpentinized ultramafic rock. they displace only the Hat Creek Basalt (~50–75 m thick; Walker, 2008; Katten- To account for the long, narrow magnetic low along the western margin horn et al., 2016) along preexisting near-vertical cooling joints (Muffler et al., of the valley (Figs. 5, 6A, and 6B), we model a reversely magnetized, steeply 1994; Blakeslee and Kattenhorn, 2013; Kattenhorn et al., 2016) that are assumed dipping body (dark blue body, bottom panel in Fig. 7). Given the length and at depth to merge into a 60° dipping master normal fault. Near-vertical scarps width of the magnetic low and the modeled depth extent of its source, it could produced by surface-breaking normal faults have been documented elsewhere, reflect a concealed system of dikes. We have two possible simple models for such as Hawaii, Iceland, and the East African Rift, but fissures associated with the magnetic high and gravity gradient of the eastern margin of the valley. One these scarps are considered to result from slip on more shallowly dipping nor-

WEST EAST A A′ Observed Calculated 200 (red, steeply-dipping body) 0

–200

–400 Calculated Calculated Magnetics (nT) assuming rocks are (magnetic –600 S=0.126 dense fill) Figure 7. Gravity and magnetic model –10 Calculated assuming along profile A–A′ (location shown in Figs. 2–6). Dashed blue lines in upper two pan- –15 uniform density Calculated 3 els are predicted magnetic and gravity vari- of 2000 kg/m (red, steeply- ations if the terrain is uniformly magnetic –20 Observed dipping body) or of low density; red lines are predicted magnetic and gravity variations if the mag- –25 netic and gravity highs in Hat Creek Valley are caused by a 75° dipping feature (red Gravity (mGals) –30 Calculated body in bottom panel); brown lines are pre- (magnetic dense fill) dicted magnetic and gravity variations if –35 gravity and magnetic highs are caused by flt flt flt Hat Creek HCF flt flt Valley a tabular body (fill or flow; brown body in bottom panel). HCF—Hat Creek fault; flt— D=2800 Quaternary fault. D—density (in kg/m3); D=2470 S—magnetic susceptibility (in 10–3 SI); M— S=0.025 S=0.075 D=2600 magnetization (in A/m). Magenta crosses 0 S=0.025 in bottom panel are double-difference re- D=2570 D=2570 S=0.025 D=2570 located seismicity (1984–2011) within 1 km M=10 A/m 2800 S=0.025 of the profile from Waldhauser and Schaff I=–64 S=0.126 ? d=180 (2008; version 20112.1). V.E.—vertical exag- S=0.025 geration; I—inclination; d—declination. Depth (km) 5

D=2670 S=0.088

V.E.=1

0 10 20 30 40

Distance (km)

mal faults at depth (Grant and Kattenhorn, 2004). However, relocated seismic- could reflect dikes and accommodate at depth the extension manifested at the ity (Waldhauser and Schaff, 2008) projected onto our model profile suggests surface as fissures or brittle faulting. Such behavior has been documented in that deformation is accommodated along steeply west-dipping planes that the Rahat Lunayyir in Saudi Arabia (Pallister et al., 2010), but not in Hat Creek extend through the seismogenic crust beneath the central part and west of the Valley (Muffler et al., 1994, 2012). Furthermore, the amount of throw on the valley (Fig. 7). The eastern alignment of seismicity using a best-fit regression Hat Creek fault strands is probably too high to be attributed to a near-surface does not project up to the surface at mapped strands of the Hat Creek fault, but phenomenon above the underlying dikes. Although the relocated microearth- 1 km west of its westernmost strand at a dip of ~72° (r2 = 0.85). The seismicity quakes are precisely located with respect to each other (errors of 100–300 m), alignments are adjacent to our modeled steeply dipping body, a feature that the absolute locations may be less accurate. The systematic 1–2 km westward

offset of the seismicity alignments with respect to our modeled features and suggesting that it is not simply a near-surface feature, but extends at least the surface traces of the Hat Creek fault may result from errors in the veloc- through the upper crust. The linearity of this magnetic boundary precludes ity model; no direct information on velocity variations at the scale of these any discrete right-lateral offsets of the causative body >1–2 km (Fig. 6A). Thus, features is available. Comparison of relocated earthquakes clustered near no significant discrete right-lateral offset associated with the Walker Lane has a quarry at Brush Mountain suggests that there is no systematic bias in the propagated from the southeast across this boundary. Furthermore, given that locations. In this case, the offset of the microseismicity locations is real and this boundary is interpreted as arising from the Klamath basement, no signifi thus could be interpreted in terms of deformation stepping westward through cant right-lateral offset has propagated across this area since the Mesozoic. time. No surface expression of such a fault has been found, which implies that Other interpretations, which we posit as less likely because of the absence the fault has not yet propagated to the ground surface. Westward-stepping of supporting data in the study area, could also explain the absence of dis- faulting has been proposed for the Hat Creek fault, which consists of three crete right-lateral offsets across the Eastern Klamath boundary, such as distrib- subparallel scarps, the youngest scarp being the westernmost strand (Walker, uted right-lateral shear. The absence of discrete offsets of the linear magnetic 2008; Blakeslee and Kattenhorn, 2013; Kattenhorn et al., 2016). The distribution boundary could result from slip on many closely spaced faults. South of the of microearthquakes beyond the plane of profile A–A′, although diffuse and Pit River, the Hat Creek fault becomes one of many faults distributed between primarily south of profile A–A′, suggests a more westward trend than that of Burney and Pittville. Given this distributed nature of faulting, we consider an the various strands of the Hat Creek fault. alternate interpretation of the Eastern Klamath boundary as having been ro- The left-stepping geometry of the fault suggests some component of tated into its current orientation by distributed right-lateral shear. This shear right-lateral slip in an extensional setting (Walker, 2008; Blakeslee and Katten- could be related to the clockwise rotation of the Oregon coast block. Although horn, 2013); this is supported by focal mechanisms of microearthquakes in Hat there are no paleomagnetic data that pertain to Neogene and younger clock- Creek Valley (Lahontan Geoscience, Inc., 2012). Other faults in the study area wise rotation in the study area, clockwise rotation has been documented in also exhibit left-stepping geometry, such as the McArthur fault (Wills, 1991), mid-Oligocene to early Miocene volcanic rocks to the north near the California the northern part of which forms the east margin of the Fall River Valley basin. border (14° ± 9°; Beck et al., 1986) and Miocene to mid-Pliocene volcanic rocks The western margin of the Fall River Valley basin, as defined by steep grav- to the east near the Nevada border (11.9° ± 4.5°; Ritzinger et al., 2014). The ity gradients (Fig. 4), is also left stepped, suggesting that these older, mostly amount of rotation in these rocks overlaps with the amount based on a few concealed faults may also have accommodated some component of right- sites scattered north and west of our study area in Cretaceous sedimentary lateral slip. rocks (11.5° ± 15.8°; Mankinen and Irwin, 1982). However, paleomagnetic data A kinematic analog for the Hat Creek fault may be the Sierran frontal fault from the volcanic rocks in this area are needed to document timing and evo- system between Bishop and Lake Tahoe. Although individual fault segments lution of any rotation near the southern boundary of the Oregon coast block. are normal faults, they form a left-stepping pattern (Unruh et al., 2003) with The absence of discrete offsets of the magnetic boundary may indicate dominantly strike-slip focal mechanisms (Oldow, 2003). Although the Hat that right-lateral slip associated with the Walker Lane and other faults steps Creek fault has predominantly normal displacement, the steeply dipping struc- to the west in the area between Lassen Peak and Mount Shasta along a trans- tures implied by the potential-field modeling and seismicity suggest that it is pressional belt, as suggested by Sawyer (2013); his interpretation is based on active in a transtensional setting. the apparent change in strike of faults near the Pit River to a more western An 80° dip on the Hat Creek fault implies slightly slower slip rates as com- direction and the east-west prong of pre-Cenozoic basement near the crest pared to a typical fault with 60° dip, for a given vertical separation (~12%). of the mountains north of Big Bend that has been attributed to an enigmatic However, if the basin fill model is correct, the amount of cumulative offset on structure called the Grizzly Peak anticline (GPA in Fig. 2) (Lahontan Geoscience, the Hat Creek fault would be greater than the throw previously documented Inc., 2012; Mushroom Rock anticline; Austin, 2013). Eocene Montgomery Creek using topographic profiles across the fault scarps (at least 570 m; Kattenhorn Formation is exposed on the southern limb of the anticline and appears to et al., 2016); the throw measured from the topography does not account for wrap around the Redding subterrane rocks (Fig. 2). Some support for a com- offset concealed by young lava and thus represents a minimum estimate. pressional stress regime in this area comes from analysis of focal mechanisms that indicate transpression (and transtension) in this area (Lahontan Geosci- ence, Inc., 2012), and a very detailed study of faulting style in the Dicalite mine Northeast-Trending Magnetic Gradient north of Burney Springs Mountain that indicates that the stress state there has evolved from predominantly normal to strike slip to reverse during the past The boundary between the smooth magnetic pattern of the Klamath Moun- 1 m.y. (Austin, 2013). A narrow, small-amplitude gravity high coincides with tains and the complex magnetic pattern to the southeast is very linear (white the proposed Grizzly Peak anticlinal structure and may extend east as far as arrows in Fig. 5). The linearity of this boundary, which we name the Eastern California Highway 89 (Figs. 3 and 4), although the timing of basement uplift Klamath boundary, persists in all filtered versions of the magnetic field (Fig. 6), suggested by this anomaly is not constrained by the potential-field data. It is

likely that the Grizzly Peak structure formed earlier than 1 Ma, given Neogene sources of the easternmost lows are not associated with volcanic edifices and doming and uplift documented in other parts of the Klamath Mountains, such are concealed beneath surficial deposits or volcanic rocks. We speculate that as the Condrey Mountain area 100–150 km to the northwest (Mortimer and the distribution of these older volcanoes may also have been influenced by the Coleman, 1984), and thus is not deformation attributable to the Walker Lane. preexisting crustal structure as delineated by the northeast-striking regional Recent geodetic modeling in the area (Thatcher et al., 2014) suggests a compo- gravity high discussed by Blakely et al. (1997). Alternatively, the change in nent of compression across the southern Klamath Mountains (Wayne Thatcher faulting style and distribution of volcanic rocks could result from a change and Robert Simpson, 2014, written commun.), so it is possible that there is a in kinematic boundary conditions at this latitude. Quaternary component to the development of the anticline. Detailed mapping The change in faulting character also coincides with the southeastern edge of the volcanic rocks on either side of the proposed structure that provides of the deep magnetic body that produces the prominent long-wavelength amount, sense, and age of any tilting might address whether there is a mea- magnetic high in Figure 6C. If the magnetic body indicates more mafic rock, surable Quaternary component to the formation of the Grizzly Peak anticline. such as ophiolite, it may be more resistant to brittle deformation and thus influence the location and style of faulting. If this is the case, it is reasonable to examine the relationship of magnetic anomalies and faulting over a broader Influence of Preexisting Structure region than our study area. Such an examination (Fig. 8) shows that Qua- ternary faulting appears to avoid the most prominent magnetic highs. Most Preexisting structures appear to influence Quaternary faulting and vol recently, based on its focal mechanism and aftershock distribution, the 2013 canism in this region, as pointed out by Blakely et al. (1997) based on grav- M5.7 Canyondam earthquake ruptured a 70° northeast-dipping fault (http:// ity anomalies. Quaternary and Neogene volcanoes and vents appear to be earthquake.usgs.gov/earthquakes/eqarchives/poster/2013/20130524CA.php, concentrated in the gravity lows associated with major volcanic centers of accessed 8/12/2014) that is aligned along the southwest edge of a prominent the southern Cascade Range (e.g., Lassen Peak, Mount Shasta, and Mount magnetic high. Strings of serpentinite and large bodies of metavolcanic rocks McLoughlin). These broad lows extend to the northeast 300–400 km (Fig. 1). are exposed to the southeast of the epicenter (Jennings et al., 2010) and may East-west extension, as characterized by north-striking faults, is concentrated be the source of the magnetic high. in the lows, and the lows are interpreted to be areas that are thermally weak- Northeast-trending conductive features roughly coincident with the gravity ened. In the intervening gravity highs, which coincide with areas that lack lows are also imaged by magnetotelluric data (orange lines in Fig. 1; Bedrosian young volcanism and thus may possess greater elastic strength, faults strike and Feucht, 2014) at middle to lower crustal depths (~32 km). Given that the northwest and are more optimally oriented for right-lateral slip. Here we ex- gravity data reflect structure within the upper and middle crust, the colocation pand on this motif by examining how preexisting structure, as manifested in of the northeast-trending zones in the lower crust suggests that these geophysi the potential-field data, coincides with the character of the Hat Creek fault. cal structures may be related to each other and thus penetrate the entire crust. The Hat Creek fault north of Cinder Butte is one of many mapped faults The geophysical features are also parallel to, and envelop, a proposed north- distributed between Burney and Pittville. The gravity and magnetic character east-trending Eocene to Miocene tear in the subducting slab (Colgan et al., 2011; associated with the fault also changes in this area, with the weakening of the dashed blue line in Fig. 1). The potential-field features also are on strike with the gravity high northward along the eastern margin of the valley (Fig. 3) and a structure or structures responsible for the westward translation of the Klamath change in strike of the magnetic high to the northeast that is parallel to the terranes from correlative terranes in the northern Sierra Nevada foothills. The course of the Pit River (Fig. 5). The source of the lower gravity values is un- translation of these terranes must have occurred during the Early Cretaceous, known but may be basin sediments that are concealed by the young volcanic based on assemblages that overlap terranes in both the Klamath Mountains cover. A possible component of that basin fill could be diatomite lake-bed de- and Sierran foothills and on the absence of post–125 Ma plutons in the Klamath posits, which are exposed in an east-west belt along the Pit River and suggest Mountains (Ernst, 2012). The evolution of these older features, however, is a that this area has been low-lying for ~1–1.5 m.y. (Page and Renne, 1994). The discussion beyond the scope of this paper, but has led to an interesting and low-lying area may have resulted from distributed normal faulting. This region enigmatic superposition of preexisting structures that extend into the Hat Creek is bracketed by two gravity gradients: the southern gradient roughly coinci- fault region and that appear to have influenced Quaternary deformation. dent with that identified in Blakely et al. (1997), and the northern gradient at the latitude of the Pit River identified by our new data where faults have more of a northwest strike (Fig. 2), as strain is deflected into the even stronger crust CONCLUSIONS as indicated by the gravity highs. The change in faulting style is roughly located along a weak, northeast-strik- Modeling of gravity and magnetic highs that extend 20–25 km along the ing alignment of prominent, semicircular magnetic lows (Br, SM, a–c in Fig. 5) eastern margin of the Hat Creek Valley suggests that the Hat Creek fault is that may reflect volcanoes that erupted during reversed polarity epochs. The a 75°–85° dipping structure in the upper crust. The potential-field anoma-

124° 123° 122° 121° 120°W nT

260

240

43°N 220 Figure 8. Shaded-relief magnetic map 200 (filtered to enhance deep sources and reduced to pole) with Quaternary faults 180 (brown lines) from U.S. Geological Survey and California Geological Survey (2006). 160 Red dashed line is outline of study area (Figs. 2–6). Gravity lows from Blakely et al. 140 (1997) are shown by gray shaded regions. 120 Faults change strike from north-south in OR the gravity lows to northwest-southeast 100 in the intervening gravity highs (Blakely CA 42° et al., 1997). The change in strike also ap- 80 pears to coincide with the southern edge of magnetic high between Mount Shasta 60 (MS) and Lassen Peak (LP). Quaternary faults appear to avoid magnetic highs, 40 which here reflect older crustal structure, MS such as ophiolite and mafic parts of the 20 Klamath–Sierra Nevada crust. Focal mechanism represents the 2013 M5.7 Canyon- 0 dam earthquake (http://earthquake.usgs .gov/earthquakes/eqarchives/poster/2013 –20 /20130524CA.php, accessed 8/12/2014) 41° and aftershock distribution indicates –40 oblique right-lateral normal slip on a 70° –60 dipping, northwest-trending fault plane.

–80

LP –100 –120

0100 km 40°

lies can be fit by dense, magnetic fill that is as much as 2 km thick, signifi- along the west margin of the Fall River Mills basin are delineated by steep cantly more than the maximum thickness of the Hat Creek Basalt (~50 m; gravity gradients and suggest a style of deformation similar to that of the Hat Walker, 2008). Alternatively, the anomalies can be fit by a 75° dipping dense Creek fault. magnetic body that is 1–2 km east of relocated seismicity (Waldhauser and Magnetic data limit where right-lateral offsets associated with the Walker Schaff, 2008) that delineates a steep west-dipping structure. Given that the Lane can pass through the area north of the Pit River. The data reveal a 40-km- strands of the Hat Creek fault appear to step left through time, consistent long linear gradient that separates the complex magnetic anomalies present with a component of right-lateral slip in an extensional environment, the lo- over the young volcanic terrain and the smoother pattern over the Klamath cation of seismicity relative to the youngest strand of the fault may suggest Mountains. The source of the gradient is concealed and within the pre-Ceno- that deformation continues to step westward across the valley. Left steps zoic basement. The absence of discrete offset within the resolution of the data

(1–2 km) indicates that no significant right-lateral offset associated with the Busby, C.J., 2013, Birth of a plate boundary at ca. 12 Ma in the Ancestral Cascades arc, Walker Walker Lane has propagated from the southeast across this boundary and, fur- Lane belt of California and Nevada: Geosphere, v. 9, p. 1147–1160, doi:10.1130 /GES00928 .1. Cande, S.C., and Kent, D.V., 1995, Revised calibration of the geomagnetic polarity timescale for thermore, no discrete offsets have been accommodated across this boundary the late Cretaceous and Cenozoic: Journal of Geophysical Research, v. 100, p. 6093–6095, since the Mesozoic. doi:10.1029/94JB03098. Our interpretation of magnetic and new gravity data supports the hypothe- Chapman, R.H., Bishop, C.C., and Chase, G.W., 1977, Principal facts and sources for 1,820 gravity stations on the Alturas 1° by 2° quadrangle, California: California Division of Mines and sis that Quaternary faulting is influenced by preexisting basement structure in Geology Open-File Report 77–17, 15 p. the region between Mount Shasta and Lassen Peak (Blakely et al., 1997). New Christiansen, R.L., Kleinhampl, F.J., Blakely, R.J., Tuchek, E.T., Johnson, F.L., and Conyac, M.D., gravity data indicate that the southern margin of a gravity high in the eastern 1977, Resource appraisal of the Mt. Shasta wilderness study area, Siskiyou County, Califor- nia: U.S. Geological Survey Open-File Report 77-250, 89 p. Klamath Mountains is stepped, with a newly delineated east-northeast–trend- Clynne, M.A., and Muffler, L.J.P., 2010, Geologic map of Lassen National Park and vicinity, Cali ing gradient extending parallel to the Pit River between Big Bend and Cali- fornia: U.S. Geological Survey Scientific Investigations Map 2899, 110 p., scale 1:50,000. fornia Highway 89. Across this gradient, Quaternary faults change to a more Colgan, J.P., Egger, A.E., John, D.A., Cousens, B., Fleck, R.J., and Henry, C.D., 2011, Oligocene and Miocene arc volcanism in northeastern California: Evidence for post-Eocene segmentation northwestward strike as strain is deflected into stronger crust, as suggested by of the subducting Farallon plate: Geosphere, v. 7, p. 733–755, doi:10 .1130 /GES00650 .1. higher gravity values. The change in faulting character also coincides with the Cordell, L., and Grauch, V.J.S., 1985, Mapping basement magnetization zones from aeromag- southeast margin of a concealed magnetic high inferred to be a southeast-dip- netic data in the San Juan Basin, New Mexico, in Hinze, W.J., ed., The utility of regional gravity and magnetic anomaly maps: Tulsa, Oklahoma, Society of Exploration Geophysi- ping fragment of ophiolite. Quaternary faulting becomes more distributed in cists, p. 181–192, doi:10.1190/1.0931830346.ch16. nature and appears to avoid the body, presumably because its mafic composi- Couch, R., and Gemperle, M., 1982, Aeromagnetic measurements in the Cascade Range and tion is more resistant to deformation. Modoc Plateau of northern California—Report on work done from June 1, 1980, to Novem- ber 30, 1980: U.S. Geological Survey Open-File Report 82-932, 23 p. Donnelly-Nolan, J.M., 2010, Geologic map of Medicine Lake volcano, northern California: U.S. ACKNOWLEDGMENTS Geological Survey Scientific Investigations Map 2927, 48 p., scale 1:50,000. Ernst, W.G., 2012, Earliest Cretaceous Pacificward offset of the Klamath Mountains salient, north- We acknowledge financial support by the Pacific Gas and Electric Company and the National Co- west California–southwest Oregon: Lithosphere, v. 5, p. 151–159, doi:10 .1130 /L247 .1. operative Geologic Mapping Program of the U.S. Geological Survey (USGS). Carson McPherson- Faulds, J.E., Henry, C.D., and Hinz, N.H., 2005, Kinematics of the northern Walker Lane: An in- Krutsky provided valuable field assistance, generously volunteering for field work after her intern- cipient transform fault along the Pacific–North American plate boundary: Geology, v. 33, ship with the USGS ended, and we thank Katherine (Kyeti) Morgan, a USGS intern, who helped p. 505–508, doi:10.1130/G21274.1. with additional field work and physical property measurements. Reviews by Rick Blakely, Simon Fuis, G.S., Zucca, J.J., Mooney, W.D., and Milkereit, B., 1987, A geologic interpretation of seis- Kattenhorn, Cathy Busby, and an anonymous reviewer greatly helped improve the paper. Any use mic-refraction results in northeastern California: Geological Society of America Bulletin, of trade, firm, or product names is for descriptive purposes only and does not imply endorsement v. 98, p. 53–65, doi:10.1130/0016-7606(1987)98<53:AGIOSR>2.0.CO;2. of the U.S. Government. Glen, J.M.G., McKee, E.H., Ludington, S., Ponce, D.A., Hildenbrand, T.G., and Hopkins, M.J., 2004, Geophysical terranes of the Great Basin and parts of surrounding provinces: U.S. Geological Survey Open-File Report 2004-1008, 303 p. REFERENCES CITED Grant, J.V., and Kattenhorn, S.A., 2004, Evolution of normal faults at an extensional plate bound- Austin, L.J., 2013, Evolution of regional stress state based on faulting and folding near the Pit ary, southwest Iceland: Journal of Structural Geology, v. 26, p. 537–557, doi:10 .1016 /j.jsg River, Shasta County, California [M.S. thesis]: Eugene, University of Oregon, 61 p. .2003.07.003. Beck, M.E., Burmester, R.F., and Craig, D.E., 1986, Paleomagnetism of middle Tertiary volcanic Griscom, A., 1977, Aeromagnetic and gravity interpretation of the Trinity ophiolite complex, rocks from the western Cascade series, northern California: Journal of Geophysical Re- northern California: Geological Society of America Abstracts with Programs, v. 9, no. 4, search, v. 91, p. 8219–8230, doi:10 .1029 /JB091iB08p08219 . p. 426–427. Bedrosian, P.A., and Feucht, D.W., 2014, Structure and tectonics of the northwestern United Harwood, D.S., and Helley, E.J., 1987, Late Cenozoic tectonism of the Sacramento Valley, Califor- States from EarthScope USArray magnetotelluric data: Earth and Planetary Science Letters, nia: U.S. Geological Survey Professional Paper 1359, 46 p. v. 402, p. 275–289, doi:10.1016/j.epsl.2013.07.035. Hildreth, W., 2007, Quaternary magmatism in the Cascades—Geologic perspectives: U.S. Geo- Blakely, R.J., 1996, Potential theory in gravity and magnetic applications: Cambridge, Cambridge logical Survey Professional Paper 1744, 125 p., http://pubs.usgs.gov/pp /pp1744. University Press, 441 p. Hill, P.L., Kucks, R.P., and Ravat, D., 2009, Aeromagnetic and aeroradiometric data for the conter- Blakely, R.J., and Jachens, R.C., 1990, Volcanism, isostatic residual gravity, and regional tec- minous United States and Alaska from the National Uranium Resource Evaluation Program tonics of the Cascade Volcanic Province: Journal of Geophysical Research, v. 95, p. 19,439– of the U.S. Department of Energy: U.S. Geological Survey Open-File Report 2009-1129, http:// 19,451, doi:10.1029/JB095iB12p19439. pubs.usgs.gov/of/2009/1129/. Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or Irwin, W.P., 1994, Geologic map of the Klamath Mountains, California and Oregon: U.S. Geologi gravity anomalies: Geophysics, v. 51, p. 1494–1498, doi:10 .1190 /1 .1442197. cal Survey Miscellaneous Investigations Map I-2148, scale 1:500,000. Blakely, R.J., Jachens, R.C., Simpson, R.W., and Couch, R.W., 1985, Tectonic setting of the south- Jennings, C.W., Gutierrez, C., Bryant, W., Saucedo, G., and Wills, C., 2010, Geologic map of Cali- ern Cascade Range as interpreted from its magnetic and gravity fields: Geological Society fornia: California Geological Survey Data Map 2, scale 1:750,000. of America Bulletin, v. 96, p. 43–48, doi:10.1130/0016-7606(1985)96<43:TSOTSC>2.0.CO;2. Kattenhorn, S.A., Walker, E.L., Krantz, R.W., and Blakeslee, M.W., 2016, Evolution of the Hat Creek Blakely, R.J., Christiansen, R.L., Guffanti, M., Wells, R.E., Donnelly-Nolan, J.M., Muffler, L.J.P., Fault system, northern California: American Association of Petroleum Geologists Bulletin Clynne, M.A., and Smith, J.G., 1997, Gravity anomalies, Quaternary vents, and Quaternary (in press). faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc LaFehr, T.R., 1965, Gravity, isostasy, and crustal structure in the southern Cascade Range: Jour- evolution: Journal of Geophysical Research, v. 102, p. 22,513–22,527, doi:10 .1029 /97JB01516 . nal of Geophysical Research, v. 70, p. 5581–5597, doi:10.1029 /JZ070i022p05581 . Blakeslee, M.W., and Kattenhorn, S.A., 2013, Revised earthquake hazard of the Hat Creek fault, Lahontan GeoScience, Inc., 2012, Seismogenic deformation in the Pit River region, Shasta northern California: A case example of a normal fault dissecting variable-age basaltic lavas: County, California: Final Report contract 2500616652 for Pacific Gas and Electric Company: Geosphere, v. 9, p. 1397–1409, doi:10.1130/GES00910.1. Reno, Nevada, Lahontan GeoScience, Inc., 40 p.

Mankinen, E.A., and Irwin, W.P., 1982, Paleomagnetic study of some Cretaceous and Tertiary Rood, D., Amidon, W., McKeon, R., Baldwin, J., Gray, B., Page, W., and Farley, K., 2015, Paleo- sedimentary rocks of the Klamath Mountains province, California: Geology, v. 10, p. 82–87, seismic assessment of the Hat Creek fault using cosmogenic He-3 surface exposure dating doi:10.1130/0091-7613(1982)10<82:PSOSCA>2.0.CO;2. in basalt, northeastern California: A proof of concept study: American Geophysical Union McCaffrey, R., Qamar, A.I., King, R.W., Wells, R., Khazaradze, G., Williams, C.A., Stevens, C.W., fall meeting abs. T42A–01. Vollick, J.J., and Zwick, P.C., 2007, Fault locking, block rotation and crustal deformation in Sanborn, A.F., 1960, Geology and paleontology of the southwest quarter of the Big Bend quad- the Pacific Northwest: Geophysical Journal International, v. 169, p. 1315–1340, doi:10 .1111 /j rangle, Shasta County, California: California Division of Mines and Geology Special Report .1365-246X.2007.03371.x. 63, 26 p. McCaffrey, R., King, R.W., Payne, S.J., and Lancaster, M., 2013, Active tectonics of northwestern Sawyer, T.L., 2013, The northern California shear zone—Missing link in the Pacific–North Ameri- U.S. inferred from GPS-derived surface velocities: Journal of Geophysical Research, v. 118, can plate transform margin?: Geological Society of America Abstracts with Programs, v. 45, p. 709–723, doi:10.1029/2012JB009473. no. 6, p. 59. Mortimer, N., and Coleman, R.G., 1984, A Neogene structural dome in the Klamath Mountains, Stein, R.S., and Barrientos, S.E., 1985, Planar high-angle faulting in the Basin and Range: Geo- California and Oregon, in Nilsen, T.H., ed., Geology of the Upper Cretaceous Hornbrook For- detic analysis of the 1983 Borah Peak, Idaho, earthquake: Journal of Geophysical Research, mation, Oregon and California: Pacific Section, Society of Economic Paleonotologists and v. 90, p. 11,355–11,366, doi:10.1029/JB090iB13p11355. Mineralogists Volume 42, p. 51–88. Stewart, J.H., 1988, Tectonics of the Walker Lane belt, western Great Basin: Mesozoic and Ceno- Muffler, L.J.P., Clynne, M.A., and Champion, D.E., 1994, Late Quaternary normal faulting of the zoic deformation in a zone of shear, in Ernst, W.G., ed., Metamorphism and crustal evolution Hat Creek Basalt, northern California: Geological Society of America Bulletin, v. 106, p. 195– of the western United States, Rubey Volume VII: Englewood Cliffs, New Jersey, Prentice 200, doi:10.1130/0016-7606(1994)106<0195:LQNFOT>2.3.CO;2. Hall, p. 683–713. Muffler, L.J.P., Blakely, R.J., and Clynne, M.A., 2008, Interaction of the Walker Lane and the Cas- Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied Geophysics (second edition): Cam- cade Volcanic Arc, northern California: Eos (Transactions, American Geophysical Union), bridge, Cambridge University Press, 770 p. v. 89, no. 3, fall meeting abs. T23B–2032. Thatcher, W.R., Svarc, J.L., and Lisowski, M., 2014, Present-day deformation in northeastern Cali Muffler, L.J.P., Champion, D.E., Calvert, A.T., and Clynne, M.A., 2012, Paleomagnetic, geochrono- fornia, northwest Nevada, and southern Oregon: American Geophysical Union fall meeting logic, and petrologic data discriminate tholeiitic basalts of the northern Hat Creek graben, abs. G11A–0465. northeastern California: American Geophysical Union fall meeting abs. V33B–2868. Thurber, C., Zhang, H., Brocher, T., and Langenheim, V., 2009, Three-dimensional seismic veloc- Oldow, J.S., 2003, Active transtensional boundary zone between the western Great Basin and ity model of the crust and uppermost mantle of northern California: Journal of Geophysical Sierra Nevada block, western U.S. Cordillera: Geology, v. 31, p. 1033–1036, doi:10.1130 Research, v. 114, B01304, doi:10.1029/2008JB005766. /G19838.1. Turrin, B.D., Muffler, L.J.P., Clynne, M.A., and Champion, D.E., 2007, Robust 24±7 ka 40Ar/39Ar Oldow, J.S., and Cashman, P.H., 2009, Introduction, in Oldow, J.S, and Cashman, P.H., eds., Late age of a low-potassium tholeiitic basalt in the Lassen region of NE California: Quaternary Cenozoic structure and evolution of the Great Basin–Sierra Nevada transition: Geological Research, v. 68, p. 96–110, doi:10.1016/j.yqres.2007.02.004. Society of America Special Paper 447, p. v–viii, doi:10 .1130 /2009 .2447(00). Unruh, J., Humphrey, J., and Barron, A., 2003, Transtensional model for the Sierra Nevada Page, W.D., and Renne, P.R., 1994, 40Ar/39Ar dating of Quaternary basalt, western Modoc Plateau, frontal fault system, eastern California: Geology, v. 31, p. 327–330, doi:10.1130 /0091 -7613 northeastern California: Implications to tectonics, in Lanphere, M.A., et al., eds., Abstracts (2003)031<0327:TMFTSN>2.0.CO;2. of the Eighth International Conference on Geochronology, Cosmochronology, and Isotope U.S. Geological Survey and California Geological Survey, 2006, Quaternary fault and fold data Geology: U.S. Geological Survey Circular 1107, p. 240. base for the United States: U.S. Geological Survey, http://earthquakes .usgs.gov/regional Pallister, J.S., et al., 2010, Broad accommodation of a rift-related extension recorded by dyke /qfaults (accessed 22 February 2012). intrusion in Saudi Arabia: Nature Geoscience, v. 3, p. 705–712, doi:10 .1038 /ngeo966 . Waldhauser, F., and Schaff, D.P., 2008, Large-scale relocation of two decades of northern Califor- Phillips, J.D., 2001, Designing matched bandpass and azimuthal filters for the separation of po- nia seismicity using cross-correlation and double-difference methods: Journal of Geophysi- tential-field anomalies by source region and source type: Australian Society of Exploration cal Research, v. 113, B08311, doi:10.1029 /2007JB005479 . Geophysicists, 15th Geophysical Conference and Exhibition, expanded abstracts CD-ROM, Waldien, T., 2012, Active tectonics of the northwestern tip of Walker Lane in southern Oregon: 4 p., doi:10.1071/ASEG2001ab110. American Geophysical Union fall meeting abs. T51B–2589. Renne, P.R., and Scott, G.R., 1988, Structural chronology, oroclinal deformation, and tectonic Waldien, T., and Meigs, A., 2013, Shear development and overprinting in a back-arc basin, Klamath evolution of the southeastern Klamath Mountains, California: Tectonics, v. 7, p. 1223–1242, Falls, Oregon [poster]: Oregon State University Scholars Archive, ir.library.oregonstate.edu doi:10.1029/TC007i006p01223. /xmlui/bitstream /handle /1957 /39017 /CUE _2013 .pdf ?sequence=2 (accessed 24 June 2015). Ritzinger, B.T., Glen, J.M.G., and Egger, A.E., 2014, Paleomagnetic correlation of late Miocene– Walker, E.L., 2008, Evolution of the Hat Creek fault, northern California [M.S. thesis]: Moscow, Pliocene basalt flows in the northwestern Basin and Range: Documenting timing of faulting, University of Idaho, 112 p. volcanism, and vertical-axis rotation in Surprise Valley, northeastern California: American Wesnousky, S.G., 2005, The San Andreas and Walker Lane fault systems, western North Amer- Geophysical Union fall meeting abs. GP21A–3646. ica: Transpression, transtension, cumulative slip and the structural evolution of a major Robbins, S.L., Oliver, H.W., and Sikora, R.F., 1976, Principal facts, accuracies, sources, base sta- transform plate boundary: Journal of Structural Geology, v. 27, p. 1505–1512, doi:10.1016 tion descriptions, and plots for 1794 gravity stations on the Susanville 1° by 2° quadrangle, /j.jsg.2005.01.015. California: U.S. Geological Survey Report, 77 p., http://research .utep.edu/default.aspx?tabid Wills, C.J., 1991, Active faults north of Lassen Volcanic National Park, northern California: Cali- =37229 (accessed October 2010). fornia Geology, v. 44, p. 51–58.