The Tahoe-Sierra Frontal Fault Zone, Emerald Bay Area, Lake Tahoe, California: History, Displacements, and Rates

Research Paper

GEOSPHERE The Tahoe-Sierra frontal fault zone, Emerald Bay area, Lake Tahoe, California: History, displacements, and rates

1 2 1 3 4 4 GEOSPHERE, v. 15, no. 3 R.A. Schweickert , J.G. Moore , M.M. Lahren , W. Kortemeier , C. Kitts , and T. Adamek 1Department of Geological Sciences, MS 172, University of Nevada, Reno, Nevada 89557, USA 2U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA https://doi.org/10.1130/GES02022.1 3Western Nevada College, 2201 West College Parkway, Carson City, Nevada 89703, USA 4Robotic Systems Laboratory, Santa Clara University, 500 El Camino Real, Santa Clara, California 95053, USA 18 figures; 3 tables; 1 set of supplemental files

CORRESPONDENCE: [email protected] ABSTRACT zones along the eastern edge of the Sierra Nevada microplate. The Lake Tahoe basin (Figs. 1A and 1B), a complex half-graben, is part of the Walker Lane belt, CITATION: Schweickert, R.A., Moore, J.G., Lahren, M.M., Kortemeier, W., Kitts, C., and Adamek, T., 2019, The location and geometry of the boundary between the Sierra Nevada possibly an incipient plate boundary, and a region of dextral transtensional de- The Tahoe-Sierra frontal fault zone, Emerald Bay area, microplate and the transtensional Walker Lane belt of the Basin and Range formation between the internally unfaulted Sierra Nevada and the extensional Lake Tahoe, California: History, displacements, and Province in the Lake Tahoe area have been debated. Two options are that Basin and Range Province to the east. Numerous studies in the Walker Lane belt rates: Geosphere, v. 15, no. 3, p. 783–819, https://doi .org/10.1130/GES02022.1. the active structural boundary is (1) a few km west of Lake Tahoe, along the have established the following (see, for example, Bormann et al., 2016; Busby, northwest-trending Tahoe-Sierra frontal fault zone (TSFFZ) or (2) within Lake 2013, 2016; Carlson et al., 2013; Dixon et al., 2000; Faulds and Henry, 2008; Science Editor: Raymond M. Russo Tahoe, along the largely submerged, north-trending West Tahoe–Dollar Point Hammond et al., 2011; Hammond and Thatcher, 2007; Lifton et al., 2013; Rood Associate Editor: Jose M. Hurtado fault zone (WTDPFZ). et al., 2011a; Schweickert et al., 2004; Surpless et al., 2002; Taylor and Dewey, Emerald Bay, a famous scenic locality at the southwest end of Lake Tahoe, is at 2009; Unruh et al., 2003; Wesnousky et al., 2012): The Walker Lane currently Received 12 June 2018 the juncture between the TSFFZ and the WTDPFZ. There, utilizing high-resolution, takes up about one fifth of the dextral displacement between the Pacific and Revision received 29 August 2018 Accepted 14 January 2019 multibeam-echosounder maps and derived bathymetric profiles, detailed field North American plates. Kinematic complexity characterizes the Walker Lane, studies on land are integrated with bathymetric data and remotely operated with various parts dominated by normal faults, dextral or sinistral strike-slip Published online 19 March 2019 vehicle observations to clarify the existence and activity of faults and sedimen- faults, and/or vertical-axis rotations of crustal blocks. Additionally, in some tology of the bay. Results include the most detailed structural maps of glacial parts of the Walker Lane, discrepancies have been noted between long-term, moraines and the bottom of Lake Tahoe ever produced. Glacial moraines on both geologically determined slip rates and those calculated from instantaneous sides of Emerald Bay clearly have been deformed by normal displacements on GPS geodetic studies. faults within the TSFFZ and the WTDPFZ. Tectonic geomorphic features include The Lake Tahoe basin, a normal-fault domain, is bounded to the north scarps along moraine crests, locally back-tilted crests, and tectonic reversal of and south by domains that have conjugate dextral and sinistral faults and moraine crests, where older, higher moraines locally lie at lower elevations than earthquake focal mechanisms (Fig. 1B; Schweickert et al., 2004). This study younger, lower moraines. The alignment of crests of lateral moraines shows focuses primarily on the Tahoe-Sierra frontal fault zone (TSFFZ), a complex, that dextral slip has not occurred during or since late Pleistocene glaciations. northwest-trending zone of faults one to five km west of Lake Tahoe (Fig. 1B). On the floor of Emerald Bay, submerged youthful faults that correspond to Observations are also provided on the southern part of the north-trending West onshore faults that displace glacial moraines have numerous distinct, well-pre- Tahoe–Dollar Point fault zone (WTDPFZ), a largely submerged and relatively served, postglacial fault scarps, for which the vertical component of slip (ver- simple zone of normal faults within and adjacent to Lake Tahoe. tical separation) is estimated. This study clearly demonstrates that the TSFFZ is the active structural boundary of the Sierra Nevada microplate and that the TSFFZ has a higher rate Statement of the Problem of slip than the WTDPFZ. It also provides evidence for complex range-front evolution, with both zones of normal faults active concurrently at various times. Normal faults along the western edge of the Lake Tahoe basin provide insights into the evolution of range-front faults along an incipient, transtensional plate boundary. The nature and continuity of faults in the area within and south INTRODUCTION of Emerald Bay, however, have been debated for more than a decade (Sch- weickert et al., 2000a, 2000b, 2004; Kent et al., 2005; Schweickert and Lahren, This paper is published under the terms of the New studies at Emerald Bay, a world-famous, scenic locale in the Lake Tahoe 2006; Dingler et al., 2009; Brothers et al., 2009; Howle et al., 2012; Maloney CC‑BY-NC license. basin, California-Nevada, clarify the relationship between two important fault et al., 2013; Kent et al., 2016; Pierce et al., 2017). Due in part to this ongoing

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Figure 1. (A) Tectonic sketch map of the Sierra Nevada microplate and Walker Lane belt, showing location of Lake Tahoe basin (T) (modified from Busby et al., 2016, and Unruh et al., 2003). Limit of Walker Lane belt from Faulds and Henry (2008). Heavy lines represent major faults. ECSZ—eastern California shear zone; MTJ—Mendocino triple junction. (B) Sketch map of the Lake Tahoe region, showing kinematic relations of faults and historic earthquakes (modified from Schweickert et al., 2004, with fault additions from Hunter et al. (2011; PF—Polaris fault); and Gold et al. (2014; MVF— Mohawk Valley fault). Basins are white; mountainous areas are gray. Outline of Figure 2 is shown. Abbreviations for fault zones include: WTDPFZ—West Tahoe–Dollar Point fault zone; NTIVFZ—North Tahoe–Incline Village fault zone. SN:NA—Sierra Nevada motion relative to Colorado Plateau. Modern stress axes from earthquake focal mechanisms: T-axis (= least principal stress) is horizontal, E-W; P-axis (= greatest principal stress) is vertical in normal-fault domains and is horizontal, N-S in areas with conjugate strike-slip faults.

debate, along with the structural importance of the faults and public interest high-standing bedrock facets (Fig. 2A) that are underlain mainly by granitic in the Emerald Bay area, the locale deserves a full, accurate characterization. bedrock. Two million-year-old and older lacustrine sediments, which were In the Lake Tahoe basin, which fault zone—the northwest-trending TSFFZ or deposited in a lake that predated modern Lake Tahoe are found in the hang- the north-trending WTDPFZ—represents the active structural boundary of the ing wall of the TSFFZ over much of its length (Fig. 2B; Kortemeier et al., 2018; Sierra Nevada microplate (Fig. 1A). The location and orientation of the active Lopez et al., 2004; Moore et al., 2006; Schweickert et al., 2005). structural boundary are important because kinematic models for transtension The north-trending West Tahoe–Dollar Point fault zone (WTDPFZ), as used (Dewey, 2002; Dewey et al., 1998; Taylor and Dewey, 2009) indicate that they set in this report, is submerged along a 25-km-long stretch northward from near a kinematic boundary condition (e.g., they determine the direction of maximum Emerald Bay to Dollar Point (Fig. 2A), north of which it continues for about

instantaneous extension, Xi) for transtensional deformation in the Walker Lane another 10 km on land (Howle et al., 2012; Schweickert et al., 2004; Schweickert belt to the east. The active boundary may also pose a significant seismic hazard. et al., 2000a, 2000b, 2004; Sylvester et al., 2012). Criteria used here to identify the active structural boundary are that it should The combined 1380 m height of footwall facets on both fault zones two to be marked by faults that have large displacements and are currently active. five kilometers north of Emerald Bay gives that area the greatest structural Additionally, what is the kinematic history of these normal fault systems relief of any area within the Lake Tahoe basin. along the eastern edge of the microplate? Have they experienced exclusively dip-slip displacement, or have they experienced components of dextral slip? Has activity along these faults migrated progressively from the range front out Geology of the Emerald Bay Area into the basin, as observed along many range fronts in the Basin and Range Province (e.g., Koehler and Wesnousky, 2011; McCalpin, 2009; Personius et al., Emerald Bay, a submerged glacial valley, is well known for its glacial ge- 2017; Wesnousky et al., 2005), or has fault activity alternated back and forth ology and scenery. During parts of its history, it has been a moraine-dammed between the range front and the basin (e.g., Wallace, 1987)? When both the lake, although at the current level of Lake Tahoe, the two lakes have merged. TSFFZ and the WTDPFZ are considered, are long-term, geological slip rates Prominent lateral moraine complexes that trend N35–45°E (035–045° AZ) form

1 SCHWEICKERT ET AL. DATA RECOVERY DOCUMENT for the Lake Tahoe basin consistent with instantaneous slip (or strain) rates, the steep slopes along the sides of Emerald Bay and Cascade Lake (Figs. 3–5) 2 or does a discrepancy exist? and are ~2.2 km in length. Glacial till within the moraines consists principally 3 NOTES ON GEOLOGIC SETTING

4 Emerald Bay and Cascade Lake (Figs. 3, 4; S1) lie within a structurally low area of boulders of granodiorite with a sandy matrix. Scenic, State Route 89 (CA 5 referred to here as the Emerald-Fallen Leaf (EFL) tectonic depression (Figs. 2, S1). The 89) traverses high-standing glacial moraines that enclose the bay. 6 depression formed along a large, complex relay ramp between the Mt. Tallac fault, on its 7 western margin, and the Rubicon Peak fault to the east (Fig. 2). A west-side-down normal Regional Setting Emerald Bay and Cascade Lake, together with their glacial moraines, span 8 fault, here called the Tahoe Mountain fault (Fig. S1), bounds the east side of the

9 depression east of Fallen Leaf Lake. The EFL tectonic depression trends south-southeast most of the faults of the TSFFZ (Figs. 4 and 5). Emerald Bay lies along a 10 from Emerald Bay to the upper Truckee River canyon and appears to be a small graben in The Tahoe-Sierra frontal fault zone (TSFFZ), whose trend varies from N26° to prominent right step in the TSFFZ, where the steep range front steps 1.8 km 11 en echelon relation to the main Lake Tahoe half-graben.

12 At the north end of the EFL tectonic depression, an immense, steep, 1 km-tall- 45°W (334–315° AZ), extends over 100 km from near Echo Lakes to the Mohawk eastward from Eagle Falls toward Emerald Point (Howle et al., 2012; Figs. 2–5). 13 bedrock escarpment, trending N 350 E (0350 AZ), extends south from the summit of Valley fault (Figs. 1B and 2; Howle et al., 2012; Schweickert et al., 2000a, 2000b, The Mount Tallac segment (as used here) of the TSFFZ, which includes 14 Stony Ridge. This escarpment is as tall and steep as bedrock escarpments along the Mt. 15 Tallac and Rubicon Peak segments of the TSFFZ, suggesting it is somehow a fault- 2004; Schweickert, 2009; Sylvester et al., 2012). The footwall of the fault zone at least five subparallel branches or splays within granitic bedrock, extends 16 related escarpment. The escarpment possibly formed by normal slip in pre-glacial times

17 on a large, northeast-striking normal fault along the south side of Stony Ridge. This consists of granitic rocks of the largely unfaulted Sierra Nevada microplate ~20 km N30°W (330° AZ) along the prominent, 1-km-high range front from 18 would have been a cross fault within the TSFFZ, possibly a connecting fault breaching (Howle et al., 2012; Saucedo, 2005; Schweickert et al., 2000b, 2004; Schweick- near the south end of Fallen Leaf Lake northwestward past the southwest 19 the relay ramp (see Peacock et al., 2000). There is no direct surface evidence for this fault

20 where depicted in Figure S1, although east-west striking faults and fractures occur in ert, 2009; Surpless et al., 2002; Unruh et al., 2004). In places, the granitic rocks end of Emerald Bay (Howle et al., 2012; Figs. 2, 4, and 5; Fig. S1 in the 21 granitic bedrock northwest of Eagle Falls (Fig. S1; Fig. 3). This connecting fault would are nonconformably overlain by cover sequences of Paleogene and Neogene Supplemental Material1). Eagle Falls and Cascade Falls (Figs. 3 and 4) both lie 22 have been cross-cut by the Stony Ridge and Rubicon Peak faults, and subsequently 23 buried beneath the left-lateral moraine complex of Emerald Bay. volcanic rocks, Pleistocene glacial deposits, and, locally, lacustrine deposits along this prominent escarpment. It is noteworthy that both deep, U-shaped, (e.g., Saucedo, 2005; Sylvester et al., 2012). glacial valleys of Eagle and Cascade creeks upstream from the falls do not 1 Between ca. 3.5 and 2 Ma, east-side-down displacement on numerous align with the downstream parts of the valleys. The upstream valleys (Figs. 3 subparallel branch faults of the TSFFZ beheaded various west-flowing Sier- and S1 [text footnote 1]), which have been carved into granitic bedrock, ap- 1 Supplemental Material. Notes on geologic setting, ran drainages leaving wind gaps along the present crest (Fig. 2B; Schweickert, pear to have been displaced in a dextral sense relative to the valleys below mapping criteria for glacial moraines, tectonic geomor- 2009). Normal displacement along the TSFFZ established the steep, east-facing the falls. This geomorphic relationship suggests that faults within the Mount phology, new mapping of moraines of Emerald Bay Sierran escarpment that controlled the development of younger, east-flowing Tallac segment experienced some dextral displacement during Pleistocene and Cascade Lake, high-resolution bathymetry images, ROV observations. Figures show topography and drainages and Pleistocene glacial valleys (Fig. 2A). time. Work in progress suggests that a similar relationship exists along many bathymetry, comparisons of submerged scarps with The TSFFZ includes several overlapping, northwest-trending, en echelon of the glacial valleys north of Emerald Bay (Fig. 2B). subaerial scarps, and photos of submerged scarps. segments (Howle et al., 2012). From south to north, these include Twin Peaks, The Stony Ridge fault (SRF) lies between the Mount Tallac fault and the Please visit https://doi.org/10.1130/GES02022.S1 or access the full-text article on www.gsapubs.org to view Ellis Peak, Rubicon Peak, Mount Tallac, and Echo Peak segments (Figs. 2A and Rubicon Peak fault (Howle et al., 2012; Figs. 4 and 5). Northwest of Emerald the Supplemental Material. 2B). Each segment, through long-term normal displacements, has developed Bay, the SRF lies entirely within granitic bedrock, although it displaces glacial

A B

SV SV

TC TC N TR TR N 39°10’ 39°10’

Figure 2. Generalized tectonic maps of the western part of the Lake Tahoe basin (modified from Howle et al., 2012) showing Head N35–45°W (135–145° AZ)–trending Tahoe-Sierra frontal fault zone scar (TSFFZ) and N-S–trending West Tahoe–Dollar Point fault zone MKB MKB (WTDPFZ). Abbreviations: BW—Blackwood Canyon; CL—Cas- cade Lake; DP—Dollar Point; EB—Emerald Bay; EL—Echo Lakes; ELP—Ellis Peak; EP—Echo Peak; EPT—Emerald Point; FLL—Fallen Leaf Lake; GC—General Creek; MKB—McKinney Bay; MC—Meeks Creek; McK—McKinney Creek; MT—Mount Tallac; NTFZ—north ELPELELPP ELPELELPP Tahoe fault zone; RP—Rubicon Peak; RPT—Rubicon Point; SPP— SPP SPP Sugar Pine Point; SR—Stony Ridge; SV—Squaw Valley; TC— Tahoe City; TR—Truckee River outlet; TW—Twin Peaks; WC—Ward WTDPFZ WTDPFZ Creek. EB and CL denote the Emerald Bay–Cascade Lake area, the subject of this report. Heavy dashed blue lines show limits x of 12,000–21,000-year-old McKinney Bay landslide. Pale-yellow x Rubicon Peak Rubicon Peak shading—Emerald–Fallen Leaf tectonic depression, described in RPT segment RPT segment document in Supplemental Material [text footnote 1]. (A) Axes of major glacial valleys are marked by heavy red lines; bedrock RP facets forming escarpments along major faults are depicted by various colors; black numerals give maximum heights of the SRSR facets. Small, red X’s denote localities where glacial moraines have been dated by Howle et al. (2012) and Pierce et al. (2017). area of (B) Same base map as (A), depicting reconstructed geometry this of former west-flowing Sierran drainages truncated by east- CL study side-down normal displacement along the Tahoe-Sierra frontal x VLVL Mt. Tallac x Mt. TaTallacllac fault zone (TSFFZ) (Schweickert, 2009). Thick, orange dashed segment of lines—former and present South Fork American River; thick, gold segment of segment of dashed lines—former and present tributaries of Rubicon River. the TSFFZ the TSFFZTSFFZ Red hexagons—sites of truncation of west-flowing valleys. Red dashes—sites along the TSFFZ showing apparent right-oblique Bedrock facetsets separation of glacial valleys, discussed in text. (heights in m) Ancestral A west-flowing AL TSFFZ segmentsts channels Twin Peakaks Rubicon River Ellis Peak Sierra Rubicon Pk crest American River Mount Tallac channel Echo Peak cutoff (windgap) WTDPFZ Echo Peak Echo Peak Dollar Point segment of segmentsegment off West Tahoe the TSFFZ thethe TSFFTSFFZZ

Figure 3. Oblique, shaded, bare-earth view of the Emerald Bay–Cascade Lake area showing some features referred to in text (compare with fig. 4 of Howle et al., 2012); water is removed from Em- erald Bay, and shorelines are depicted by thin, black lines. Vertical exaggeration (VE)—1.5; distance across the bottom of the image is ~4.3 km (3 mi). Abbreviations: CA—California; CC—Cascade Creek; EB—Emerald Bay; EBCC—Emerald Bay–Cascade Creek. Note apparent right-separation of both glacial valleys of Cascade Creek and Eagle Creek along the range front, where branches of the Mount Tallac fault (Figs. 2A and 4) traverse the image, relative to Cascade Lake and Emerald Bay. This separation is ~485–600 m (1600–1980 ft). Also note irregular, glaciated topography above and left (south) of both Cascade and Eagle falls, where Tahoe and Tioga glaciers spread from upper parts of glacial canyons into offset lower parts. See text and Figure 2B for discussion.

Figure 4. Generalized geologic map of Emerald Bay–Cascade Lake area (modified from Howle et al., 2012, with additions from this study); water is removed from Emerald Bay and thin, black lines depict the shoreline. Faults and fault zones shown with white lines include: MTF—Mount Tallac fault (several branches); SRF—Stony Ridge fault; RPF—Rubicon Peak fault; WTF—West Tahoe fault. Other abbreviations: EB—Emerald Bay; EP—Emerald Point; EPT—Eagle Point. Small, white x’s mark localities on Stony Ridge where scarps along the SRF cut glacial moraines.

Rubicon Peak Jakes Peak

WW N S N E

MTF

MTF RPF MTF SRF

MTF

SRF

WTF (Fig. 7) LAKE TAHOE RPF Emerald Bay Emerald (water Point removed) WTF

Eagle Point (Fig. 9)

Figure 5. Oblique, shaded, bare-earth view of Stony Ridge and Emerald Bay (EB) showing topographic expression of major fault segments (compare with fig. 9 of Howle et al., 2012). Abbreviations: MTF—Mount Tallac fault (several branches); SRF—Stony Ridge fault (two branches); RPF—Rubicon Peak fault (two branches); WTF—West Tahoe fault (two branches). Vertical exaggeration (VE)—1.4; horizontal distance across the bottom of the figure is 3.2 km (1.9 mi). Water is removed from Emerald Bay, and shoreline is depicted by thin, black lines.

moraines near small cirques high on the east flank of Stony Ridge (Figs. 2A the right-lateral moraines (south side) are free standing and separate Emerald and 4). Howle et al. (2012) reported that branches of the fault southeast of Bay and Cascade Lake. Moraines of different ages have been differentiated Stony Ridge displace lateral moraines on the north side of Emerald Bay and during field mapping using criteria described by McCaughey (2003) and Howle then enter Emerald Bay. Beneath the bay and described in detail here are what et al. (2012) (see document in Supplemental Material [footnote 1]). In this study, appear to be fault scarps that cut through and around Fannette Island and that Tahoe-age moraines (Qta) and two sets of Tioga-age (Qti) moraine crests have also form a steep, submerged bedrock escarpment on the northeast side of been distinguished, an older Qti-1 crest and a younger, inset Qti-2 moraine crest. the island. About 200 m southeast of Fannette Island, these faults are buried The Tioga-age crests have similar weathering characteristics and subequal beneath modern sediment and apparently form a broad flexure. Southeast- heights (Howle et al., 2012; see document in Supplemental Material [footnote 1]). ward from there, several scarps representing branches of the SRF cross and Structural maps and topographic profiles of the moraines (Figs. 6–9) have displace the medial moraine complex south of Emerald Bay. been prepared using established criteria for tectonic geomorphology of faulted In the Rubicon Peak segment of the TSFFZ, Howle et al. (2012) reported that moraines (Howle et al., 2012; Schweickert et al., 2004; see document in Sup- two main branches of the Rubicon Peak fault (RPF) cross and displace moraines plemental Material [footnote 1]). Careful field examination has allowed recog- on the north side of Emerald Bay, and they interpreted a large landslide on the nition of very small scarplets (1–2 m in height) that are not evident in the light bottom of the bay to have been cut by youthful scarps (Figs. 4 and 5). Where detection and ranging (LiDAR) imagery. The terminology used for scarps, scarp the RPF continues southeastward across the medial moraine complex on the height (SH), and vertical separation (VS) is explained in Figure S4 (footnote 1). southeast side of Emerald Bay, moraine crests define a monoclinal flexure Scarp heights for subaerial scarps cutting moraines have been estimated (Fig. 4). About 200 m southeast from the Emerald Bay moraine complex, the in the field using a tape and compass and using topographic profiles, as dis- probable RPF passes beneath postglacial fluvial deposits northeast of Cas- cussed in later sections. All estimates are reported to the nearest half meter cade Lake. One kilometer farther southeast, prominent scarps along the RPF and to the nearest foot. cut moraines on the south side of Cascade Lake (Figs. 4 and S3 [footnote 1]). Landslides are present along much of the range front defined by the TSFFZ, particularly along the Mount Tallac and Rubicon Peak segments (Howle et al., Faults and Scarps Cutting the Left-Lateral Moraines (Figs. 6 and 7) 2012; their figures DRF2, DRF5, and DRF16). East-facing fault scarps and backtilted moraine crests are prominent where the left-lateral moraines have been cut and displaced by the Mount Tallac, Stony DETAILED STUDIES ALONG THE TSFFZ AT EMERALD BAY Ridge, and Rubicon Peak faults (Howle et al., 2012; Fig. 6A). Near the southwest end of the Qta moraine, large gullies along the eastern The following sections present new maps, profiles, and descriptions of: branch of the Mount Tallac fault have removed the crest; the remnant of the (1) glacial moraines, (2) modern sedimentology, and (3) submerged scarps crest east of the large gullies is ~27 m (89 ft) lower than the part southeast on the floor of Emerald Bay. Significantly, the moraine crests at Emerald Bay of the gullies, reflecting post-Tahoe normal displacement along an eastern provide plentiful evidence for dip-slip normal displacements, while the sub- branch of the Mount Tallac fault. merged scarps allow vertical separation and extension rates to be estimated The Stony Ridge fault (SRF) has two distinct branches that have produced for many of the normal faults. scarps of similar height in the Qti-1 moraine (Figs. 6A and 6B). A few tens of Reliable age control on glacial moraines in the region that encompasses meters northeast of the Stony Ridge fault, the Qta moraine has been buried Emerald Bay includes results of Howle et al. (2012) at Meeks Creek, 8 km north- beneath the Qti-1 moraine (this is called tectonic reversal, which indicates sig- west, and Pierce et al. (2017) on right-lateral moraines at Cascade Lake, 2 km nificant fault displacement occurred between the deposition of Qta and Qti-1 south (Fig. 2A). Additionally, Rood et al. (2011b) determined ages on postglacial moraines; see document in Supplemental Material [footnote 1]). deposits on the eastern flank of the Carson Range, 25 km east of Emerald Bay. As Along both branches of the Rubicon Peak fault (RPF), prominent, northeast- discussed in the document in the Supplemental Material [footnote 1], we adopt side-down scarps are within the Qta and Qti-1 moraines (the Qti-2 moraine crest ages of 23.5 ± 3 ka to 20.5 ± 0.6 ka for Tioga-age moraines (Howle et al., 2012). where crossed by the RPF has been removed by landsliding [Figs. 6A and 6C]). Field estimates for heights of two scarps in the Qti-1 moraine are 8 and 6 m (~26 and 20 ft), respectively (the topographic profile in Fig. 6C gives estimates Structure of the Glacial Moraines of 9 and 6 m [30 and 20 ft] for heights of the two scarps), in general agreement with the Howle et al. (2012) estimate of VS for the scarps cutting Qti-1. Lateral moraines are referred to as left and right lateral, respectively, when The submerged West Tahoe fault (WTF; the southern part of the WTDPFZ) viewed downstream or down canyon (Howle et al., 2012). The left-lateral mo- continues southward onshore near the northeast end of the left-lateral moraine raines of Emerald Bay (north side) are draped upon granitic bedrock, whereas complex (Fig. 7). There, a bedrock escarpment ~90 m (297 ft) high separates

1200 06’W EXPLANATION A Kgr Qti-1 Shoreline at 1888 m (HWM)

2061 m Emerald x Spot elevaon, m Pre-Tahoe Point (174 m) (height above HWM) moraine 1897 m y Qal Holocene alluvium 1988 m x East-side-down x 2 scarp, height in m Qal Qti-2 Qls Landslide deposits Kgr 2 Figure 6. Detailed structure of left-lateral East-side-down glacial moraines, faults, and scarps along Q Tioga-age moraine normal fault; dashed, northwest side of Emerald Bay. (A) Struc- (Q-1, Q-2) approximate; doed, Qta B’ tural map. Figure 7 is outlined at upper Qta Tahoe-age moraine concealed right. In this and subsequent figures, spot elevations are provided for comparison of Qti-1 2 heights of Tahoe-age and Tioga-age mo- 2 pQta Pre-Tahoe moraine Moraine crest 2 Kgr raines. (B) and (C) Topographic profiles graded down canyon along crest of left-lateral Qti-1 moraines, . Quaternar Qti-2 extracted from light detection and rang-

et Moraine crest 6 Figure 7 ing (LiDAR) data using QT Mapper; vertical Kgr Cretaceous 2061 m Cr backlted up canyon and horizontal scales in feet. VE—vertical granodiorite (174 m) 9 exaggeration. Thick, blue lines emphasize parts of crest with normal down-canyon Ends of proﬁle A-A’ in Fig. 6B x B slopes, and thick, red lines highlight parts

Ends of proﬁle B-B’ in Fig. 6C A’ Service road of moraine crests that have been back- to boaters campground tilted up canyon. Faults are assumed to 6.5m x 2062 m dip 60°, and dips have been corrected for scarp vertical exaggeration. Scarp height (SH) Qti-1 (175 m) is shown with thin, vertical, red lines. Qti-2 (B) Profile A–A′ along crest of left-lateral Emerald Bay Qti-1 moraine where crest has been dis- 1.5 2 (water removed) placed by normal slip on branches of Stony 16m 1.5 Qls Ridge fault (SRF). (C) Profile B–B′ along

scarp crest of left-lateral Qti-1 moraine where 2109 m crest has been displaced by normal slip x 5.5m pier on branches of Rubicon Peak fault (RPF). (221 m) scarp A HWM—high-water mark. (Continued on 57.5’N Qta x 2101 m following page.) 0 1 2 Kgr (213 m) Post-Tioga 38 Qls N 27 landslide

x Southeast 2154 m ridge (266 m) Kgr Parson 1 km Rock Fannette Island

Figure 6 (continued).

Qta, Qti-1, and Qti-2 crests to the southwest from much lower Qti-1 and Qti-2 branch of the Mount Tallac fault (Fig. 8A). The Qta remnant has several small, moraine crests near Emerald Point; Qta is not exposed in the hanging wall 2–3-m-high scarps along its crest, and at its northeast end, the moraine ter- east of the escarpment, due to tectonic reversal. In contrast, the Qti-1 moraine minates in a 15-m-(50-ft)-high triangular facet along the eastern branch of the crest projects across this escarpment with little apparent displacement. Two Stony Ridge fault (SRF). possible explanations are: (1) that at least 90 m of post-Qta and pre-Qti-1 Near CA 89 and Inspiration Point (Figs. 8A and 8B), a 10-m-(33-ft)-high normal displacement occurred along the fault at the base of the escarpment, scarp along an unnamed fault separates granodiorite capped by Qti-2 in the or (2) the Qta glacier flowed down a preexisting bedrock escarpment, and footwall from backtilted Qti-2 moraine in the hanging wall. This scarp may further fault displacement occurred prior to deposition of the Qti-1 moraine. mark a separate branch of the MTF. We tentatively favor the second hypothesis. A few tens of meters northeast of Along the western branch of the SRF, the Qti-2 crest has been removed the bedrock escarpment (Fig. 7), an eastern branch of the WTF has noteworthy, where a deep gully cuts into the moraine (Figs. 8A and 8B). The Qti-2 crests fresh scarps, as mapped by Howle et al. (2012). on opposite sides of the gully have a difference in elevation of ~10.5 m (35 ft). The Qti-2 crest has not been displaced or disrupted adjacent to the large triangular facet at the end of the Qta moraine (Figs. 8A and 8B). This observa- Faults and Scarps Cutting the Right-Lateral Moraines (Figs. 8 and 9) tion strongly suggests that major displacement on the eastern branch of the SRF, which resulted in tectonic reversal of Qta, was post-Qta and pre-Qti-2. The The high-standing, 1-km-long, remnant of the Qta moraine near the range Qti-2 moraine is the only moraine exposed for a distance of ~240 m northeast front has a strongly backtilted segment in the hanging wall of the eastern of the SRF, where it is surmounted by a very narrow stretch of CA 89.

RPF Kgr

6 pQta Qta Triangular Triangular Qti-1 2 2 facet & scarp facet & scarp x 1988 m Qti-2 Kgr x 1897 m Q Tioga-age moraine (Q-1, Q-2) Qti-1 Qta Tahoe-age WTF moraine pQta Pre-Tahoe 3 moraine Kgr 2 3.5 1 Lake Tahoe Kgr Cretaceous 1 (bottom not shown) granodiorite WTF 2 Shoreline at Backtilted Triangular 1888 m (HWM) facet Backtilted Qti-2

6 East-side-down 2 scarp, height in m Qti-2 Fault mapped 1.5 East-side-down by Howle et al. recessional normal fault; dashed, (2012) moraines approximate; doed, concealed Moraine crest graded down canyon Emerald Bay (bottom not shown) Moraine crest backlted up canyon Emerald Point

Figure 7. Structural sketch of area near Emerald Point (oblique, shaded, bare-earth image using light detection and ranging [LiDAR] data), showing branches of West Tahoe fault (WTF). North is to right; vertical exaggeration (VE) ~1.5:1. Distance along the bottom of the figure is ~1 km (0.6 mi). The smooth, subdued topography between the bedrock escarpment and Emerald Point is interpreted to reflect beveling of glacial moraines by tsunamis generated by the McKinney Bay landslide (Schweickert et al., 2000b, 2004; Moore et al., 2014), between 12,000 and 21,000 years ago. HWM—high-water mark; RPF—Rubicon Peak fault.

A EP

Figure 8. Detailed structure of right-lateral moraines. (A) Structural map. Out- line of Figure 9 is at upper right. Thin, red lines delineate bathymetric profiles (profiles 1, 7, 15, 16, and 17) shown in later figures. Map of moraines enclos- ing Cascade Lake is in Figure S3B and S3C (text footnote 1). Topographic profiles along crests of right-lateral Qti-2 and Qti-1 moraines, extracted from light detection and ranging (LiDAR) data using QT Mapper; vertical and horizontal scales in feet. Abbreviations and line ornament as in Figures 6B and 6C. (B) Profile C–C′ along crest of right-lateral Qti-2 moraine where crest has been displaced by normal slip on unnamed faults at Inspiration Point and western branch of the Stony Ridge fault (SRF). (C) Profile D–D′ along crest of right-lateral Qti-2 and Qti-1 moraines where crest has been deformed by normal displacement on branches of the Rubicon Peak fault (RPF; here, a blind fault), West Tahoe fault (WTF), and an unnamed fault near Eagle Point (EP). MTF—Mount Tallac fault; SH—scarp height; grd—granodiorite. (Continued on following page.)

Figure 8 (continued).

Near the switchbacks along CA 89 (Figs. 8A and 8C), the slope of the Qti-2 Each fault in the glacial moraines shows evidence of significant post-Tahoe crest steepens markedly down canyon. This steeper section of the moraine displacement and postglacial displacement (Table 1). As discussed later, faults crest was interpreted by Howle et al. (2012) as a fault-propagation fold, formed and scarps cutting the moraines align closely with scarps on the floor of the bay. during the post-Qti-2 interval, along concealed (or blind) branches of the RPF. A few tens of meters northeast of the fold, the Qti-1 and Qta crests are at a slightly lower elevation than the Qti-2 crest, and then both Qti-1 and Qta moraines rise Bathymetry and Sedimentology of Emerald Bay gradually northeast to elevations greater than that of Qti-2. The latter observation indicates growth of the fold (or fault displacement) occurred during the interval Procedures between deposition of the Qti-1 and Qti-2 moraines, as well as in post-Qti-2 times. A fault or faults cut through the Qta moraine 0.6 km (0.4 mi) northeast of High-resolution bathymetry of Emerald Bay has been mapped in two cam- the switchbacks on CA 89 (Figs. 8A and 8C). More than one interpretation of paigns, the first in 2009, using techniques described by Howle et al. (2012), and the faults is possible. One option is that a branch of the West Tahoe fault (WTF) the second, in 2011, 2012, and 2013, during which bathymetric mapping used may continue southeastward from Emerald Point across the bay (where a sub- an autonomous vessel, “SWATH,” built by C. Kitts and students, Santa Clara merged scarp is present; see below) toward the large Qta moraine remnant University, Mechanical Engineering Group (see document in Supplemental northeast of CA 89 (Howle et al., 2012; Figs. 8 and 9). Alternatively, the fault Material for details [footnote 1]). cutting the Qta moraine may be a continuation of a submerged fault marked Continuous video images of the bottom of Emerald Bay were scanned by the southeast ridge within Emerald Bay (discussed later, Figs. 8 and 9). along ~34 traverses across scarps along the bottom of Emerald Bay over 11 Faults near Eagle Point (Fig. 9), which lie in the hanging wall of the WTF, days in May and September 2011, 2012, 2013, and 2014, utilizing the remotely are not discussed further; more work is necessary in that area (see document operated vehicle (ROV) Triton (Mechanical Engineering Department, University in Supplemental Material [footnote 1]). of Santa Clara) (Table 2, Fig. 10).

Figure 9. Structural map of right-lateral moraines near Eagle Point, showing Qta, Qti‑1, and Qti-2 moraine crests and probable faults lying northeast of West Tahoe fault (WTF). HWM—high-water mark.

TABLE 1. STRUCTURAL OBSERVATIONS ON FAULTS CUTTING LATERAL MORAINE COMPLEXES AT EMERALD BAY Moraines Fault Left-lateral Right-lateral Implications Mount Tallac fault* ~27 m elevation difference in Qta across eastern branch of the fault Major backtilt of Qta towards the faultSignificant post-Tahoe fault displacement

Stony Ridge fault* ~16 m elevation difference in Qta and tectonic reversal across fault; Tr uncation of Qta and tectonic reversal of crests; 12 m elevation Significant post-Tahoe and post-Tioga fault displacement scarps >5 m in Qti-1 and backtilt of Qti-1 and Qti-2 difference in Qti-1 across fault, backtilting; 5 m of post-Qti-2 elevation difference

Rubicon Peak fault* Vertical separation (VS) 30 m in Qta moraine (Howle et al., 2012); Large fault-propagation fold in Qti-2 crest with about 30 m of relief Significant post-Qta and post-Qti-2 displacement 8- and 6-m-high scarps cut Qti-1; VS estimate 15 m (Howle et al., and tectonic reversal of Qti-1 and Qti-2 2012)

West Tahoe fault* Qta—tectonic reversal across major bedrock escarpment; Qti-1 and Qta—prominent scarps; Qti moraines have minor scarps Significant post-Qta and post-tsunami displacement§ Qti-2 have several 2-m-high scarps on eastern splay; Qti-1 and in left-lateral moraines; displacement decreasing Qti-2 spacing increases across bedrock escarpment southward

Faults near Eagle Point*† Qta—significant scarps cut Qta, Qti-1, and Qti-2 Significant post-Qta and post-Qti-2 displacement *Tahoe (Qta) moraines (ca. 70 ka) always show larger and more numerous scarps than Tioga (Qti-1 and Qti-2) moraines (ca. 23 and 21 ka); together with several cases of tectonic reversal, this indicates that considerable displacement occurred along all faults during interglacial times. †Only exposed on right-lateral moraines. §Tsunami estimated to have occurred betwean 12,000 and 21,000 years ago (Moore et al., 2014).

Results strata thickening to the southwest. These depocenters are interpreted here to be fault-bounded half grabens in the hanging walls of major normal faults. In the Multibeam-echosounder bathymetry for Emerald Bay (Figs. 10 and S5A subbasin between the SRF and RPF, Maloney et al. (2013; their figure 10; shown and S5B [footnote 1]) reveals a shallow bottom near the mouth of the bay; the schematically in Fig. 13) reported that the sediments may reach thicknesses bottom slopes gently (slightly over 3°) southwest toward the deep (>65 m [>200 greater than 25 m (82 ft) (see document in Supplemental Material for discussion). ft]), relatively flat, central part of the bay. The southwestern end of Emerald East of the RPF, one subbasin may have a sediment thickness greater than ~10 Bay between the SRF and the mouth of Eagle Creek is a shallow shelf (~3–30 m (33 ft), and a more easterly subbasin along the WTF may have a thickness m [10–100 ft] deep) upon which a shallow, sandy delta is developing nearshore, greater than ~5 m (17 ft) (see also Dingler et al., 2009). New interpretations of and muddy sediment is accumulating south of Fannette Island. Nearly all Eagle relations among these depocenters and mapped faults are discussed below. Creek sediment that bypasses the delta is routed through the channel south of Fannette Island, because the channel to the north is partially blocked by bedrock promontories and by submerged recessional moraines (Fig. 11; see below). Maps and Bathymetric Profiles on Submerged Scarps within Both glacial till and talus exist in places, especially near steep escarpments. Emerald Bay In the central basin, near the presumed trace of the RPF (Figs. 10 and S5 [footnote 1]), measured water depths range from ~60–65 m (198–215 ft). The Submerged scarps within Emerald Bay were first reported by Howle et al. basin floor in that area is flat and featureless and is blanketed by water-rich, (2012). Because several authors cited earlier have maintained that submerged muddy sediment, derived from currents passing south of Fannette Island. In faults are not evident within Emerald Bay, however, and to verify the existence the northeastern part of the basin, sand derived from the subaerially exposed and nature of scarps and active faults as reported by Howle et al. (2012), all lateral moraines blankets the bottom. of these features have been examined and mapped using ROV dives, and The deep, central basin is interrupted along its northern margin by a large, bathymetric profiles have been constructed from the multibeam-echosounder post-Qti-2 landslide (Figs. 4 and 8; Howle et al., 2012). This landslide and sev- data using QT mapper. Profiles were constructed perpendicular to scarps at eral other smaller landslides in Emerald Bay developed along mapped faults their highest points. and resulted from failure of steep sidewalls of lateral moraines. ROV observations reveal that the submerged scarps are generally better Published seismic-reflection profiles (Dingler et al., 2009; Maloney et al., 2013) preserved than subaerial scarps, probably due to a lack of slope wash and depict depocenters in places including subbasins east of the SRF and subba- mass wasting of scarps in the lacustrine environment. Debris slopes, col- sins east of the RPF. In profile section, each depocenter is wedge shaped, with luvial wedges, and wash slopes (Fig. S4B [footnote 1]), which are common

TABLE 2. DIVES WITH REMOTELY OPERATED VEHICLE ROV Dives and Scarp Profiles along a Submerged Branch of the Mount TO INVESTIGATE FAULTS IN EMERALD BAY Tallac Fault (MTF) Objective Date Dive number Two ROV dives and a bathymetric profile revealed a probable scarp at the Mount Tallac fault (3 dives) eastern edge of the Eagle Creek delta (Figs. 8, 11, and 12A). Along dive 2011-H, 7 September 2011 2011-H the flat, sandy bottom of the shelf, at 22–25 m (73–82 ft) depth, rises gradually 2011-I 2011-J westward to 15 m (50 ft) depth. Between 15–10 m (50–33 ft) depth, a possible scarp is in loose sand. Profile 1 (Figs. 8, 11, and 12A) was constructed across the Stony Ridge fault (18 dives) submerged scarp ~75 m (248 ft) south of dive 2011-H, where the scarp height East branch 27 May 2011 2011-1 reaches a maximum. The scarp face has a maximum slope angle of 27°, a re- 2011-2 2011-3 markably steep face in loose sand, much steeper than typical delta fronts (which 6 September 2011 2011-B rarely exceed 5°–6°; Patruno et al., 2015), suggesting the scarp is a youthful 2011-C feature. A rough estimate of maximum scarp height from the profile, which 18 May 2012 2012-E-2 includes the (unknown) height of the delta front is ~17 m (~56 ft; discussed 2012-M later). This youthful, submerged scarp at the edge of the Eagle Creek delta 13 September 2012 2012-2A aligns with the eastern branch of the MTF to north and south (Figs. 6 and 8). 29 May 2013 2013-5 5 May 2014 2014-3 Dive observations and profile 1 suggest that significant postglacial, even Ho- West branch 6 September 2011 2011-A locene, normal displacement has occurred on the eastern branch of the MTF. 7 September 2011 2011-D 2011-E 2011-F ROV Dives and Scarp Profiles along Submerged Parts of the Stony Ridge 2011-G Fault (SRF) 18 May 2012 2012-E-3 2012-E-4 29 May 2013 2013-2 The granodiorite bedrock high marked by Fannette Island resembles a clas- sic roche moutonnée (Easterbrook, 1999), with a relatively gentle up-canyon Rubicon Peak fault (9 dives) slope and a very steep, down-canyon slope. Howle et al. (2012) indicated that 27 May 2011 2011-4 2011-5 the steep slope facing down canyon also has a tectonic origin, however, be- 18 May 2012 2012-L cause they interpreted two prominent sets of scarps around Fannette Island as 19 May 2012 2012-L-2 fault scarps along the SRF. Maloney et al. (2013; like Howle et al., 2012) depicted 13 September 2012 2012-1A a short fault segment along the east side of Fannette Island on their maps but 29 May 2013 2013-1 referred to it as part of the “West Tahoe–Dollar Point fault” (a designation with 1 June 2013 2013-3-1 which we disagree, as discussed below). The scarps were examined in this 2013-3-2 study to determine whether they are related to faults. 2013-3-3 Eastern (main) scarps. The eastern scarps, the more prominent of the scarps, Southeast ridge (2 dives) were imaged in four ROV dives (Fig. 11), including, from north to south, 2011 11 September 2011 2011-K dive E, 2012 dive 2A, 2013 dive 5, and 2011 dive A (Fig. 11; Table 2). Bathymetric 13 September 2012 2012-4A profiles 4–6 were constructed across this scarp (Figs. 12D–12F). West Tahoe fault (2 dives) The eastern scarp (Fig. 11) is ~200 m (660 ft) in length and extends well beyond 14 May 2014 2014-1 the limits of granodiorite bedrock; it dies out ~100 m (330 ft) north and 50 m (165 ft) 2014-2 southeast of the island. The central part of the scarp is dominated by a steep, east-facing, wall of granodiorite (see photos in Figs. S6A and S6B [footnote 1]) that extends from ~64 m (210 ft) depth up to lake level and continues nearly 30 m (66 ft) to subaerial scarps (McCalpin and Nishenko, 1996; McCalpin, 2009; Wallace, above lake level. Glacial polish on the granodiorite surface was observed at 24 m 1977), are not developed on most of the submerged scarps and very poorly (80 ft) depth in one dive, indicating that Tioga glaciers covered parts of the scarp. developed on others. Talus has accumulated locally near the bases of some The nearly horizontal surface in the hanging wall of the eastern scarp is high bedrock scarps (Fig. 11), probably a result of freeze-thaw frost wedging underlain by mud at depths of ~53–64 m (175–213 ft; Fig. 11). This deposit along episodically exposed upper parts of the scarps. extends directly to the base of the nearly vertical wall of granodiorite, with

Figure 10. Shaded relief map of Emerald Bay utilizing light detection and ranging (LiDAR) and echosounder data, with water removed, showing areas covered by 2011–2014 remotely operated vehicle (ROV) dive tracks and outlines of geologic maps in Figures 11 and 14. Areas mapped with ROV are in yellow. Green, dotted lines marked M and D are approximate locations of seismic-reflection profiles (Dingler et al., 2009; Maloney et al., 2013). Core site EB2 (Dingler et al., 2009) is approximately located by white X.

Figure 11. Geologic map (plotted on multibeam-echosounder image) of floor of Emerald Bay near Fannette Island. Bathymetric profile 1 (60 m [200 ft] south of the south edge of this map) is shown in Figure 8A. Abbreviations: MTF—Mount Tallac fault; ROV—remotely operated vehicle; SRF—Stony Ridge fault; HWM—high-water mark.

Figure 12. Bathymetric profiles across eastern branch of Mount Tallac fault (MTF) and branches of Stony Ridge fault (SRF). Profiles extracted from multibeam-echosounder data (see Figs. 8A and 11 for locations). V (vertical) and H (horizontal) scales are in feet; VE—vertical exaggeration; grd—granodiorite. For profiles 2–6, fault is projected to toe of scarp, as discussed in Figures S4C and S4D [text footnote 1]. Vertical red lines delineate scarp height (SH) and vertical separation (VS). In profiles 1, 2, and 5, surface slopes are approximately parallel, and, therefore, the VS estimate is independent of position of fault. Conversely, because surface slopes are non-parallel in profiles 3, 4, and 6, VS estimate depends upon correct location of fault. (A) Profile 1 across Mount Tallac fault (MTF) eastern branch. Part of profile marked by thin, black dashed line is a data gap where surface has been extrapolated. See text for discussion. (B) Profile 2 across western branch of SRF north of Fannette Island. (C) Profile 3 across western branch of SRF on south side of Fannette Island. Continued( on following two pages.)

Figure 12 (continued). (D) Profile 4 across eastern branch of SRF near northeast tip of Fannette Island. (E) Profile 5 across eastern branch of SRF near east end of Fannette Island. (F) Profile 6 across eastern branch of SRF near southeastern side of Fannette Island. (Continued on following page.)

Figure 12 (continued). (G) Profile 7 across eastern branch of SRF southeast of Fannette Island.

Figure 13. Line drawing of seismic-reflection profile in figure 10 of Maloney et al. (2013), as reinterpreted here; see Figures 10 and 11 for approximate location. Vertical- ex aggeration (VE) reportedly ~20:1. TWTT—two-way travel time in seconds; sound velocity assumed to be 1450 m/s for both water and sediments (see document in Supple- mental Material [text footnote 1] for discussion). “Data omitted” refers to parts of profile in which Maloney et al. (2013) omitted data from their figure 10; the entire profile is shown in figure 15 of Dingler et al. (2009). Various reflectors are shown with yellow, orange, green, blue, and white lines. Yellow reflector, interpreted as Tsoyowata ash (7930–7790 cal. yr B.P.; Bacon, 1983; Sarna-Wojcicki et al., 1991), terminates abruptly at the Stony Ridge fault, suggesting some displacement postdates this horizon. Ge- ometry of reflectors near the RPF may suggest west-side-down displacement along a west-dipping fault, but this is an artifact of great vertical exaggeration. Bathymetry images (Figs. S5A, S5B [see text footnote 1]) do not support such a fault. The orange reflector may mark a 5.0–5.4 ka horizon (Maloney et al., 2013). The black line separates sediment with multiple reflectors above from material lacking reflective layering below. See text for discussion.

no evidence of a colluvial wedge; however, in some places, loose granitic (postglacial, e.g., post–14 ka) displacement along the scarps. The escarpment boulders dislodged from the cliff above rest upon mud. formed largely by displacement along the SRF, however, as indicated by the Near the eastern tip of Fannette Island (Fig. 11), the main scarp splits into presence of crush breccia along its lower parts. Higher parts of the escarpment two, with a high-standing bedrock scarp striking S40°W (220° AZ; past points comprise a sheer wall of granodiorite, parts of which are glacially polished. 2011-A-8 and A-10) and a less prominent bedrock scarp continuing due south; The presence of youthful scarps both north and south of the gently sloping the south-trending scarp face consists of granodiorite and, then, a few tens escarpment in the mid-channel (Figs. 8A and 11) supports the conclusion that of meters farther south, of bouldery till. the slope is indeed an expression of a fault-propagation fold. In some places, lower parts of the main bedrock scarp are mantled with A published seismic-reflection profile (Maloney et al., 2013; Fig. 13), in ad- postglacial, bouldery talus (Fig. S6C [footnote 1])—some with angular boulders dition to depicting a depocenter in the hanging wall of the SRF, also appears to up to 1.5 m (5 ft) in diameter. Over-steepened slopes have developed in the show that the inferred Tsoyowata ash horizon has been displaced. If so, post–ca. talus at the base of the scarp, suggesting that the most recent fault displace- 7.7 ka (and pre–5.0 ka) displacement has occurred on this branch of the SRF. ment postdates accumulation of the talus. Western scarps projecting through Fannette Island. Prominent scarps both At sites 2011 A-5, A-8, and A-10 at the bases of both scarps, slabs of dark, red- north and south of Fannette Island (Fig. 11), ~120 m (400 ft), west of the main dish, iron-oxide–cemented breccia lie within mud (Fig. S6D [footnote 1]). These scarps, are described briefly below. rocks lack foliation and are interpreted to be spalled-off slabs of crush breccia South side of island. A submerged granodioritic promontory projects south- formed by frictional slip along the fault (see Sibson, 1977). In dive 2012-2A eastward from the western part of Fannette Island (Fig. 11). Its eastern margin along the main escarpment (Fig. 11), a similar breccia was observed as a steeply is a prominent fault-related scarp >11.5 m (>39 ft) in height (profile 3, Fig. 12C). dipping veneer on the steep granodioritic face at a depth of ~45 m (~150 ft). The hanging wall of this scarp is a gently sloping, nearly flat surface at 30 m At its northern tip (site 2011-E-3), the main scarp is developed entirely in (100 ft) depth, underlain by sand. The lower part of the scarp face exposes talus shed from the north edge of the island, some with angular boulders up bouldery talus resting in places upon till. Locally, the talus forms an over-steep- to 1.8 m (6 ft) across. Again, an over-steepened slope in talus indicates that ened slope and appears to have been displaced during a youthful slip event. the talus has been displaced by the most recent rupture along the fault. In North side of island. A steep, east-facing escarpment (Fig. 11) lies beneath profile 4 (Fig. 12D) a few meters south of this site, the scarp face in talus has the north channel on the north side of Fannette Island. This scarp, which is a slope angle near 60°, and the scarp has a vertical separation of ~5.5 m (18 underlain by granodiorite, has a height of 15.5 m (51 ft) and a VS of 11 m (36 ft) ft). One hundred meters (330 ft) south of profile 4, the same scarp has vertical (profile 2, Fig. 12B). The lower part of the scarp face is a steep, bouldery talus separations of 4.5–5 m (15–16 ft) (profiles 5 and 6; Figs. 12E and 12F). slope (with a slope angle up to 35°). As on the scarp to the south, the steep, About 100 m (330 ft) southeast of Fannette Island, beneath the south chan- sloping surface of the talus in places suggests that the deposit has been cut nel, a probable monoclinal (fault-propagation) fold developed in unconsoli- or deformed by displacement on the fault. dated sediments along the SRF (Howle et al., 2012). Water depths range from On Fannette Island, exposed granodiorite is intensely fractured but lacks ~70 m (231 ft) in the hanging wall to ~30 m (100 ft) in the footwall, with a scarp- any obvious topographic expression of a west-dipping normal fault. This ob- like slope ~30–40 m (100–132 ft) in height. Several ROV dives traversed this servation suggests that any pre-glacial bedrock scarp on the upstream side of slope in mid-channel southeast of Fannette Island, where the hanging wall at the roche moutonnée surface may have been eroded smooth by Tahoe and ~61–64 m (200–210 ft) depth consists of flat-lying muddy sediment. The entire Tioga glaciers, and that post-Tioga displacements are not detectable in the slope (profile 7, near the southern edge of Fig. 11; Figs. 12G and S5 [footnote 1]) fractured granodiorite. is smooth and draped with muddy sediment indistinguishable from that of As with the eastern (main) set of scarps, the submerged scarps projecting the hanging wall. The mud is covered with algal masses, is water-rich, lacks through the western part of Fannette Island clearly represent fault scarps both be- strength, and is easily disturbed by propellers on the ROV. Abrupt steps or cause they cut talus deposits and because they align with prominent scarps devel- slope breaks were not observed in the slope. Either the muddy deposits have oped in moraines along the western branch of the SRF (see Figs. 6A, 8A, and 11). been draped across a preexisting fault, or the muds have been warped during displacement, as proposed by Howle et al. (2012). The eastern scarps are indeed fault scarps developed along the eastern ROV Dives and Profiles on Submerged Scarps along the Rubicon Peak branch of the SRF. This is borne out by the facts that the scarps extend north Fault (RPF) and south well beyond the limits of the bedrock, align with mapped branches of the SRF in the moraines, and sediment in the hanging wall adjacent to The large, post-Qti-2 landslide near the north shore of Emerald Bay (men- the scarp consists of muddy sediment rather than glacial till. In a few places, tioned earlier; Fig. 14) has two sets of submerged, north-trending, east-fac- postglacial, bouldery talus from Fannette Island stands in sharp relief against ing, scarp-like features developed in glacial till that was displaced from the muddy deposits of the hanging wall (Fig. S6C [footnote 1]), arguing for youthful left-lateral Qti-2 moraine. These scarps have been examined with the ROV to

Figure 14. Geologic map (plotted on multibeam-echosounder image) of fault scarps along branches of Rubicon Peak fault (RPF), developed in large landslide in north part of Emerald Bay. HWM— high-water mark. Figures S7A–S7D are in the Supplemental Material (see text footnote 1).

determine whether they are fault-related topographic features (Howle et al., faces are steep, no colluvium mantles their lower parts, and postglacial sand 2012) or are related to landslide lobes. mantles both hanging walls and footwalls but not the scarp faces themselves The western set of east-facing scarps (Fig. 14) begins ~20 m (66 ft) south- also attest to youthful, postglacial displacement. east of the pier at the boaters’ campground continues, southward within the central part of the landslide, and includes three southwest-trending splays near its southern end. About 125 m (412 ft) east of the western scarps, the eastern ROV Dives and Profiles across the Southeast Ridge scarps, which are near the eastern exposed limits of the landslide, include three discontinuous, arcuate, east-facing segments. Both sets of scarps continue to the A bathymetric feature in the east-central part of Emerald Bay and here southern exposed limit of the landslide but cannot be traced into deeper water, referred to as the “southeast ridge” (see Figs. 6A, 8A, and 10) was interpreted suggesting that the scarps predate the late Holocene muds of the central basin. as a north-facing fault scarp by Howle et al. (2012). The ridge is linear, ~0.2 km ROV dives and profiles on the western set of scarps. The scarps are abrupt, (660 ft) in length, and trends west-northwest, perpendicular to Qti recessional east-facing walls of bouldery till, typically 3 m (10 ft) or more in height (see pho- moraines along the right-lateral moraine complex (Fig. 8A). ROV observations tos in Figs. S7A and S7B [footnote 1]). The hanging wall east of the scarps is a support the interpretation of Howle et al. (2012), because bouldery Qti till is smooth, flat, sandy surface, with a gentle slope to the east and south, developed exposed along the scarp face. Several unusual features of this scarp include its upon the landslide. Westward, beyond the top of the scarp, the bottom (footwall), short, exposed length, its linearity, and the fact that it strikes at a high angle to which is the surface of the landslide, is again level, smooth, and mantled by sand. the RPF. It also shows a gently south-tilted footwall slope. LiDAR images suggest Bathymetric profiles indicate that all of the scarps have typical fault geom- that a continuation of this fault to the southeast may disrupt and offset crests of etry, with flat, smooth surfaces on the hanging walls and footwalls and abrupt, Qti-2 recessional moraines (Fig. 8A; discussed earlier); conceivably, the south- steep scarp faces. Scarp profiles 8 and 9 (Figs. 14, 15A, and 15B) reveal scarp east ridge could be a link between the RPF and the southern part of the WTF. faces with maximum slopes of 18° to 39° and vertical separations (VS) of 3.5 m (11 ft) and 2.5 m (8 ft). The scarps near the southern edge of the exposed landslide are less prominent (Figs. 15C and 15D). Profiles and ROV Dives along Scarp Possibly Related to the West Tahoe ROV dives and profiles on the eastern set of scarps. The scarp faces are Fault (WTF) held up by abrupt, steep walls of granodioritic boulders varying from ~0.5–2.5 m (1.6–8 ft) in diameter (Figs. S7C and S7D [footnote 1]) in glacial till displaced The bedrock escarpment along the West Tahoe fault (WTF) near Emerald onto the floor of the bay by the large landslide in Figure 14. In places, loose Point lacks bathymetric expression within Emerald Bay. However, Howle et al. boulders have tumbled down from the scarp face and rest upon sand at the (2012) mapped a short, ~140-m-(460-ft)-long, submerged, northwest-striking base of the scarp. A possible bevel at the top of one scarp (Fig. S7D) sug- scarp (shown in Figs. 8A and 9) on the flat bottom of the bay ~0.8 km (0.5 mi) gests that more than a single slip event may have produced the scarp. A flat, northeast of the submerged landslide discussed above, as part of the WTF. Our smooth, sandy bottom east of the arcuate scarps (e.g., in the hanging wall) observations support that conclusion. In plan view, the scarp is slightly arcuate, slopes gradually to the east and south. The footwall west of the scarps is a with at least two subtle steps. ROV dives (2014 dives 1 and 2) revealed that this flat, smooth surface underlain by sand. In bathymetric profiles (Figs. 15E–15G), scarp has a steep northeast flank (scarp face) exposing fresh Qti till, with prom- these scarps, like the western scarps, all have geometry typical of fault scarps, inent granodiorite boulders, and a gently sloping southwestern flank. Smooth, with flat surfaces on hanging walls and footwalls and with abrupt, steep scarp postglacial sand mantles both the footwall and hanging wall. Profile 17 (Figs. 8A faces (maximum slope angles of scarp faces range from ~21° to 32°). The ar- and 15J) shows a vertical separation of only ~1 m (3 ft) and a scarp-face slope cuate scarp segments have vertical separations of 2 m (7 ft), 4 m (13 ft), and angle of 20°. A few tens of meters north of the profile, the scarp height is 2 m 6.5 m (21 ft), increasing in magnitude from north to south. (7 ft). Development of this scarp postdated Qti-2 (post–21 ka), but it is unknown Interpretation of the scarps in the large landslide. The ROV data and bathy- if it postdated complete deglaciation at 14 ka. Along strike ~200 m (660 ft) south- metric profiles support the conclusion that these submerged scarps are fault east of the scarp, a kink along the crest of an onshore Qti recessional moraine scarps. The surface of the landslide, which is mantled by sand, is relatively flat (see Fig. 9) may represent deformation related to displacement on the WTF. and smooth, with no evidence for lobes within the landslide. The submerged scarps also correlate favorably with mapped scarps and faults along the RPF within moraines to the north, and to a feature in a profile of Maloney et al. Summary of ROV Dives and Detailed Maps (2013; Fig. 13) interpreted here as a buried branch of the RPF. Because the scarps developed in the large landslide derived from the left-lateral Qti-2 moraine ROV dives, together with echosounder bathymetry, confirm the fault map- (Howle et al., 2012), the scarps postdate at least part of the deglaciation (e.g., ping and conclusions of Howle et al. (2012) and in particular the continuation they are post–21 ka and possibly post–14 ka). Furthermore, the facts that scarp of active branches of the MTF, SRF, RPF, and WTF beneath the floor of Emerald

Figure 15. Bathymetric profiles extracted from multibeam-echosounder data across scarps along Rubicon Peak fault (RPF), southeast ridge, and West Tahoe fault (WTF; see Figs. 8A and 14 for locations of profiles). In all profiles, fault surface is projected toward toe of scarp (see Figs. S4C and S4D [text footnote 1]). V (vertical) and H (horizontal) scales all in feet. For profiles 8, 9, 11, 13, and 14, hanging-wall and footwall slopes are non-parallel; estimates of vertical separation (VS) depend on correct position of fault. (A) Profile 8 across western branch of RPF near central part of scarp. (B) Profile 9 across western branch of RPF. (C) Profile 10 across western branch of RPF near south end of scarp. SH—scarp height. VE—vertical exaggeration.Continued ( on following three pages.)

Figure 15 (continued). (D) Profile 11 across small scarps west of profile 10. (E) Profile 12 across eastern branch of RPF, along northern scarp. (F) Profile 13 across eastern branch of RPF, along central scarp. (Continued on following two pages.)

Figure 15 (continued). (G) Profile 14 across eastern branch of RPF, along southern scarp. (H) Profile 15 across western end of southeast ridge. (I) Profile 16 across central part of southeast ridge. (Continued on following page.)

Figure 15 (continued). (J) Profile 17 across arcuate scarp along WTF.

Bay (Fig. 16). Youthful scarps exist along all the faults, and crush breccia is horizontal layering in other parts of the seismic profiles consists dominantly of preserved along the SRF. sediment younger than ca. 8 ka (Maloney et al., 2013). From this comparison, the seismic-reflection profiles appear to be compatible with the existence of the scarps and faults described by Howle et al. (2012) and this study. Comparison of Results with Seismic-Reflection Profiles in Emerald Bay

As noted previously, some authors have stated that active faults do not Estimates of Slip Rates exist within Emerald Bay (Dingler et al., 2009; Kent et al., 2006), based upon unfaulted, horizontal layering in seismic-reflection profiles. Although precise Heights of scarps and vertical separations have been estimated in many locations and scales for the profiles were not provided, enlargements of the places in this study. Most scarps in the glacial moraines have been excluded seismic profiles have been compared with new bathymetric images and the from VS analysis, however, because, in most cases, the hanging wall has been locations of scarps described above. backtilted toward the fault scarp. In such cases, VS cannot be determined The seismic-reflection profile sketched in Figure 13 (from Maloney et al., precisely (Howle et al., 2012; McCalpin, 2009). 2013), as noted earlier, appears to image the eastern branch of the SRF and There are many uncertainties in determinations of VS, including correct place- possibly contains evidence for post-Tsoyowata displacement. This profile also ment and dip of faults (see discussions in Rood et al., 2011a; Wesnousky et al., appears to intersect a branch of the RPF and depicts a wedge-shaped deposit 2005; also see Fig. S4 [footnote 1]). In this study, normal faults are assumed to in the hanging wall of the fault. Other seismic profiles of Dingler et al. (2009) dip an average of ~60°, following many other published works (e.g., Gilbert, 1890, and Kent et al. (2006) image the large postglacial landslide cut by the RPF dis- 1928; Howle et al., 2012; Personius et al., 2017; Rood et al., 2011a; Wesnousky et cussed above (Fig. 14) as an amorphous mass with no internal reflections. The al., 2005); this value is consistent with bedrock scarp faces in Emerald Bay that lack of a well-layered succession in the landslide makes recognition of faults dip 50°–75°. Howle et al. (2012) used 3D modeling and three-point solutions to problematic, because in seismic-reflection profiles, the existence of steeply obtain a mean of 62 ± 12° for seven measurements of fault dip along the RPF. dipping faults is usually inferred from offsets of planar layering (Alcalde et The exact point of intersection of a fault surface with a scarp also may affect al., 2017; McCalpin, 2009; Yeats et al., 1996). Subtle bathymetric steps in the the VS estimate, especially in cases where hanging-wall and footwall surfaces top surface of the landslide are evident in the seismic profiles, however, and are not parallel. Many other studies in unconsolidated deposits have assumed these most likely are the scarps shown in Figure 14. Prominent, unfaulted, that fault surfaces project to midpoints or steepest parts of scarps (Howle et

Figure 16. Oblique view looking northwest across Emerald Bay (water removed) showing major faults of the Tahoe-Sierra frontal fault zone (TSFFZ) and West Tahoe fault (WTF) traversing Emerald Bay and locations discussed here and in Howle et al. (2012). Vertical exaggeration (VE)—1.4. Distance along the bottom of figure is 2.4 km (1.4 mi). Shoreline of Emerald Bay depicted with thin, black lines. Scarps along Mount Tallac fault (MTF) where it cuts the Emerald Bay right-lateral moraines are a few hundred meters off the figure at far left. VSR—vertical separation rate; RPF—Rubicon Peak fault; SRF—Story Ridge fault; WTDPFZ—West Tahoe–Dollar Point fault zone.

al., 2012; Koehler and Wesnousky, 2011; Personius et al., 2017; Wesnousky et Eight km (4.8 mi) southeast of Emerald Bay, Howle et al. (2012) concluded al., 2005). In this study, most submerged scarp faces expose bedrock, talus, that the submerged faults at the head of Fallen Leaf Lake are splays of the or glacial till, and colluvial wedges do not cover the lower parts of scarps. MTF. On one of these faults, an estimated “vertical deformation” rate of 0.4–0.7 Therefore, fault surfaces are projected in most cases to intersect the profiles mm/yr was reported by Brothers et al. (2009; Fig. 18). near the bases or toes of the scarps (Figs. S4C and S4D [footnote 1]). Below and in Table 3, for estimation of vertical separation rates (VSR), an age of 21 ka is assumed for Qti-2 moraines, and 14 ka is assumed for the age Stony Ridge Fault (SRF) of final deglaciation. Approximate vertical separation rates are provided below, using estimated No previous estimate of VSR has been made for the SRF. Some scarps along vertical separations and broad age estimates, together with previously pub- the submerged eastern branch of the SRF are younger than the 14 ka deglaci- lished rates. The estimates from this study, which are conservative minimum ation. The vertical separation of 5.5 m (18 ft) at the north end (Figs. 11 and 12D, values, are approximate, owing to uncertainties in age constraints and factors profile 4) and a limiting age of 14 ka produce a minimum VSR of 0.4 mm/yr along mentioned above. Rates therefore are reported to one significant figure only the fault (Table 3). Actual rates may be somewhat higher, because Holocene (e.g., 0.36 mm/yr is reported as 0.4 mm/yr, etc.). Nevertheless, these data displacement may have occurred (see discussion above). The morphological provide a basis for comparison with other published data for slip rates within similarity of scarps along the submerged western branch of the SRF to those the Lake Tahoe basin and the Walker Lane belt. on the eastern branch suggests that both branches have similar VSRs. If so, then the combined, minimum, VSRs from both branches would be ~0.8 mm/yr.

Mount Tallac Fault (MTF) Rubicon Peak Fault (RPF) Several strands of the MTF lie within bedrock west of Emerald Bay, but youthful markers have not been identified along these branches, and therefore it is un- For a conservative estimate of VSRs, the submerged scarps are assumed known if youthful displacements have occurred. However, latest Quaternary to to be younger than 21 ka, the age for Qti-2. On the western branch of the RPF Holocene displacement likely occurred along the eastern branch of the MTF where (Figs. 14 and 15A–15C), scarps have VS up to ~3.5 m (11 ft), yielding a 0.16 (0.2) it bounds the Holocene delta of Eagle Creek (Figs. 4, 8A, and 11). There, the promi- mm/yr (post–21 ka) VSR. On the eastern branch (see Figs. 14 and 15E–15G), nent scarp (Fig. 12A, profile 1) is assumed to be post–14 ka (e.g., postglacial retreat). submerged scarps have VS of up to 6.5 m (21 ft), yielding a minimum VSR of Because the height or relief of the unfaulted or unmodified delta and the 0.3 mm/yr (post–21 ka). The estimated VSRs on the submerged scarps combine foreset slope are unknown, VS on this scarp is tentatively approximated as to give an aggregate minimum VSR of ~0.5 mm/yr (post–21 ka). This result is follows (see profile 1, Fig. 12A). The slope break is reconstructed by extending consistent with the estimate of Howle et al. (2012) where the RPF displaced the footwall surface and the scarp face until they intersect. Two 6° sloping the left-lateral Qti-1 moraine of Emerald Bay. lines (defining an upper limit to delta foreset slopes [Patruno et al., 2015]) are constructed from the slope break and base of the scarp. An approximate VS of ~13 m (44 ft) is obtained as the vertical height between the 6° sloping Southeast Ridge lines. A rough VSR estimate of ~0.9 mm/yr is then obtained. This is three times the VSR of 0.3 ± 0.1 mm/yr (post–23.5 ka) at a youthful scarp along Scarp heights along the southeast ridge, a scarp cutting post–Qti-2 till, the MTF, ~5 km southeast of Emerald Bay, between Cascade Lake and Fallen range from 3.5–5.5 m (11–18 ft; see Figs. 15H and 15I). VS cannot be estimated Leaf Lake (Fig. 18; Howle et al., 2012). At Emerald Bay, owing to the uncer- directly, however, because the footwall has been tilted to the south. A very tainties in estimating VS, half of the estimated VSR, 0.45 mm/yr (rounded rough estimate of VS of ~3 m (10 ft) and a maximum limiting age of 21 ka for to 0.5 mm/yr), is taken as a conservative estimate of minimum VSR for the Qti-2, yield a minimum VSR of 0.1 mm/yr (with large uncertainties). eastern branch of the MTF. A VSR estimate of 1.4 ± 0.7 mm/yr was reported for the MTF where it forms a very high scarp on the right-lateral Tioga moraine of Cascade Lake (Pierce et al., West Tahoe Fault (WTF) 2017; Fig. S3 [footnote 1]; those authors considered this scarp to be the “West Tahoe fault”). At that site, we mapped three scarps ranging from 1–7 m (3–23 ft) Estimates of VSR have not previously been reported for the southern part high in the Qti-2 moraine (Fig. S3; Howle et al., 2012). As discussed in the document of the WTF. The Qti-2 moraine northwest of Emerald Point (Fig. 7) has at least in the Supplemental Material, it seems likely that the very high bedrock scarp three scarps with approximate heights ranging from ~2–3.5 m (7–11 ft). The discussed by Pierce et al. (2017) represents both pre-and post-Tioga displacement. hanging wall is backtilted, however, and no estimate of VSR is made there. The

GEOSPHERE | Volume 15 | Number 3 Schweickert et al. | The Tahoe-Sierra frontal fault zone, Emerald Bay Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/3/783/4708655/783.pdf 812 by University of Nevada Reno user on 01 August 2019 on 01 August 2019 by University of Nevada Reno user Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/3/783/4708655/783.pdf Research Paper Rubicon P St Unnamed f Mount A. Fa SUMMAR on ultB y Ridgefa T allac fa eak f ault Y OFES ult (SRF) ult (MTF) ault (RPF) TIMA TES OF F (pr East F (pr East Fi (pr East F (pr F (pr F (pr SE ofF (pr W Fi East Fi (pr SE ofF East F (pr NE endofFa W F (pr W F (pr East F (pr south ofF W F (pr nor East W (pr Near InspirationP F W f pr VER igur igs igs igs igs igs igs igs igs igs igs igs ranch, gs gs gs of est est est est est est of of of of of of of of of of of of of of th ofF T ile, . 14and15G) . 14and15F) . 14and15E) . 14and15D) . 14and15D) . 11and12F) . 14and15C) . 11and12E) . 11and12D) . 14and15B) . 14and15A) . 11and12C) . 11and12B) . 8,11,12A) e(s) er er er er er er er ile 14, ile 13, ile 12, ile 16, ile 11, ile 6, ile 10, ile 5, ile 4, ile 9, ile 8, ile 3, ile 2, ile 1, ABLE 3. er er er er er er TIC n branch n branch n branch n branch n branch n branch n branchatdelt n branch n branch n branch n branch n branch n branch annett annett AL SEP annett annet SUMMARY nnet e Island e Island te e Island ARA Island( te oint (F Island TION RA a~ ig OF . 8) VER TES ONSUBA TIC Scar AL SEP 15.5 m1 (51 ft (33 ft 10 m- (SH) (3 ft 1 p height m2 ERIAL ARAT ) )( ) ION AND SUBMERGEDSC AND EXTENSIONRA Ve rt ical separatio (16.5 ft 11 (21 ft (13 ft (15 ft (18 ft (11 ft (44 ft 6.5 m2 1.5 m2 4.5 m1 2.5 m 5.5 m1 3.5 m2 (7 ft (7 ft (5 ft (8 ft (VS) 39 ft 36 ft 4 m2 2 m2 2 m2 5 m1 13 m1 1 .5 -2 m* m* ) ) ) )2 ) ) ) ) ) ) ) )0 ) n ARPS TES AL Limiting age ONG FA (k a) 10 10 10 1< 1< 40 1 40 40 10 10 4* 1 UL TS AT EMERALDBA separation ra 0.9 mm/yr 0.1 mm/yr 0.1 mm/yr .3 mm/yr .2 mm/yr .1 mm/yr .2 mm/yr .2 mm/yr .4 mm/yr .1 mm/yr .2 mm/yr .5 mm/yr Ve (VSR) *N.A. N.A. N.A. (continued rt ical te Y )

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GEOSPHERE | Volume 15 | Number 3 Schweickert et al. | The Tahoe-Sierra frontal fault zone, Emerald Bay 814 Research Paper

Figure 17. Line drawing of published bathymetric profile across West Tahoe fault (WTF) near Sugar Pine Point (Fig. 18 of Brothers et al., 2009). New interpretations are in gray boxes with italic typeface. VS—vertical separation; VSR—vertical separation rate; VE—vertical exaggeration.

youngest Qti-2 recessional moraine (Fig. 7) has been cut by two scarps, ~2 m combined vertical separation rate up to three times greater than that for and 1.5 m (7 ft and 5 ft) in height (because the crests are approximately horizon- the WTF. Available data also suggest that VSRs for both the RPF and the tal, SH there approximates VS). Using a limiting age of 21 ka and a combined WTF decrease southward; this is consistent with the fact that these two VS of 3.5 m, a minimum VSR of 0.2 mm/yr is obtained for the WTF at Emerald fault zones die out as mappable scarps a few kilometers southeast of Point. The post–21 ka submerged scarp along the WTF, with a VS of ~1 m (3 ft) Emerald Bay. from profile 17 (Fig. 15J), yields an approximate minimum VSR of <0.1 mm/yr. Minimum extension rates along two transects of the TSFFZ, one near Meeks For the WTF, ~12 km (7 mi) north of Emerald Bay near Sugar Pine Point, Bay and the other at Emerald Bay, calculated from VSR data (Table 3) are a “vertical deformation rate” of 0.43–0.81 mm/yr has been reported (location comparable, ~1.1–1.2 mm/yr, in a direction N50–55°E (050–055° AZ; Fig. 18, shown in Fig. 18; Brothers et al., 2009; Dingler et al., 2009). If the fault surface inset). How do these estimates based upon geologic data compare to mod- is projected toward the midpoint of the scarp (Fig. 17), an improved estimate of ern GPS results? VS is ~8 m (26 ft). This fan surface, its associated canyon, and incised channels Modern strain rates across the Lake Tahoe basin based on GPS geodesy are formed by backflow during and after a major tsunami (Moore et al., 2014), which ~0.8–1.1 mm/yr in a direction S45°E (145° AZ) (Hammond et al., 2011; Wesnousky gives limiting ages of 21–12 ka. These data (~8 m VS in 21 ka) yield an improved et al., 2012). Notably, the extension directions for the two geological transects estimate of minimum VSR of ~0.38 (rounded to 0.4) mm/yr for the WTF at this site. are at right angles to strain directions calculated from geodesy. Wesnousky et al. (2012) noted that the GPS data provide evidence for ongoing dextral displacement between the Sierra Nevada microplate and ranges to the east Rates for TSFFZ versus WTF and GPS Geodesy of the Lake Tahoe basin, despite the fact that there is little direct evidence for strike-slip displacements in the region. They also observed that directions and At Emerald Bay, each major fault in the TSFFZ has a VSR greater than the magnitudes of fault slip from geologic data do not agree with GPS results, a WTF. Taken together, the various segments of the TSFFZ have an estimated conclusion supported by results here.

Figure 18. Tectonic map of west half of Lake Tahoe basin with estimates of vertical separation rates (VSRs) and extension rates and directions (white boxes) for two transects across the Tahoe-Sierra frontal fault zone (TSFFZ) and West Tahoe–Dollar Point fault zone (WTDPFZ); northern transect at General and Meeks Creeks utilizes results of Howle et al. (2012), and southern transect at Emerald Bay uses data reported here (methods used for these estimates are summarized in Table 3B [footnote]). Abbre- viations: EBLL—Emerald Bay left-lateral moraine; EXR—extension rate; GMCM— General and Meeks Creek medial moraines; MCRL—Meeks Creek right-lateral moraines; RPF—Rubicon Peak fault; SER—southeast ridge in Emerald Bay; SRF—Stony Ridge fault; VSR—estimated vertical separation rate; WTF—West Tahoe fault; other abbreviations as in Figure 2. Inset: Comparison between extension rates for TSFFZ and WTF from Howle et al. (2012) and this study (N and S transects, red arrows) and GPS strain rate estimated for Lake Tahoe basin (blue arrow; Wesnousky et al., 2012).

DISCUSSION Taylor and Dewey (2009) noted that, with the N35–45°W (320° AZ)–trending TSFFZ setting the kinematic boundary condition and a Sierra Nevada trans- New observations demonstrate that the TSFFZ continues through the Em- port direction of ~N50–60°W (305° AZ; Fig. 1B), the maximum instantaneous

erald Bay area, is active, and owing to its large displacement, the fault zone stretching direction (Xi) should be oriented roughly east-west. As noted by represents the active structural boundary of the Sierra Nevada microplate in Taylor and Dewey (2009), the N-S–trending WTDPFZ is a normal fault zone

the region around the Lake Tahoe basin. These new results contradict state- formed perpendicular to Xi. ments and conclusions of several previous studies (e.g., Brothers et al., 2009; The reconciliation of long-term, northeast-southwest extension along the Dingler et al., 2009; Kent et al., 2006, 2016; Maloney et al., 2013; Schmauder, TSFFZ with northwest-southeast dextral transtension is still problematical. 2013; and Smith et al., 2013) that discounted the existence or importance of For a region ~100 km in width involving seven N-S–trending Walker Lane the TSFFZ. basins, including the Lake Tahoe basin, Wesnousky et al. (2012) invoked a As noted by many previous studies, the TSFFZ includes geologic features Ramsay-style brittle-ductile, dextral shear zone model (Ramsay, 1967; Ramsay typical of major range-bounding normal fault systems throughout the Basin and Huber, 1983). Such a shear zone model involves plane-strain, simple shear. and Range Province. Such features include large topographic relief along the Some modification of that model may be required because northwest-directed eastern front of the Sierra Nevada, high-standing bedrock facets, topographic transtension involves constrictional strain rather than plane strain (Dewey, control of east-flowing glacial valleys, and glacial moraines at the mouths of 2002; Taylor and Dewey, 2009) and requires a N-S component of shortening. bedrock glacial valleys. The authors of the papers cited above were unaware Additionally, in a shear-zone model, all N-S–striking normal faults in the re-

of or disregarded the work of Howle et al. (2012), who made a compelling case gion, which may reflect the east-west iX as in the Lake Tahoe basin (Taylor and for activity along the TSFFZ. Dewey, 2009), should have rotated progressively clockwise. The TSFFZ, which has set the kinematic boundary conditions for transtension Alternatively, transtension in the region may be viewed as heterogeneous, in the Lake Tahoe region, has had a long and complex history, as described with domains of extension alternating with zones of strike slip. Historic earth- briefly in an introductory section. Kinematic changes may have occurred at quakes in the Lake Tahoe region suggest that transtensional deformation is various times along this fault zone and may have led to some of the structural currently partitioned into domains of normal and conjugate strike-slip faults complexity. As interpreted above, during the Pleistocene, after large normal dis- (Fig. 2B; Schweickert et al., 2004). The normal fault domains (such as the Lake placements had formed a high, northeast-facing escarpment, minor dextral dis- Tahoe basin) accommodate east-west or east-northeast extension, and adjoin- placement appears to have occurred along some of the faults. Normal slip then ing domains of conjugate strike-slip faults may accommodate the northerly continued during and after the Tahoe glaciation and continues to the present. component of displacement. The WTDPFZ is clearly younger than the TSFFZ, because 2–2.3 Ma basaltic volcanic rocks and lacustrine sediments are in both hanging-wall and footwall positions on the WTDPFZ (Kortemeier et al., 2018; Schweickert, 2009; Schwe- SUMMARY AND CONCLUSIONS ickert et al., 2004). Yet both fault systems appear to have been active during late Pleistocene and Holocene times (Howle et al., 2012; this study). Extensive field mapping, analysis of LiDAR data and multibeam-echosounder The evolution of these two normal fault systems has important implications bathymetric data, and numerous ROV dives in Emerald Bay have been com- for the migration of activity and evolution of range-front fault systems in the bined to determine the character, relative age, and offsets along scarps of Basin and Range Province, supporting the conclusions of Wallace (1984, 1987). the three faults (and their branches) of the Tahoe-Sierra frontal fault zone Although it is commonly assumed that range-front normal fault systems have (TSFFZ) and the WTF (southern continuation of the West Tahoe–Dollar Point a clear progression of activity from the range front toward the adjacent basin fault zone [WTDPFZ]). These are the most detailed structural maps to date of along with deactivation of the range-front system, Wallace’s (1984, 1987) work glacial moraines and parts of the lake bottom in the entire Lake Tahoe region. and this study reveal that some range fronts have a much more complex pat- The new data confirm previous reports (e.g., Howle et al., 2012) that several tern of evolution, with activity both migrating from range front into the basin important, northwest-striking, normal faults (Mount Tallac [MTF], Stony Ridge but also with concurrent activity occurring within both systems. [SRF], and Rubicon Peak [RPF] faults, etc.) do pass beneath Emerald Bay and Within the Basin and Range Province and, in particular, the Walker Lane displace both lake-bottom sediments and glacial moraines. Additionally, the SRF belt, discrepancies commonly exist between geodetically determined strain is confirmed as an active fault lying in the footwall of the Rubicon Peak segment rates and slip rates determined from geologic measurements along component of the TSFFZ. The TSFFZ now has the most dense array of VSR data of any part range-bounding faults (e.g., Bormann et al., 2016; Gold et al., 2014; Herbert of the microplate boundary. The continuity of the faults mapped by Howle et al. et al., 2013; Lifton et al., 2013; Personius et al., 2017, Wesnousky et al., 2005, (2012) precludes any direct connections between the WTF and the Mount Tallac 2012). This study, like many previous reports, emphasizes the fact that geodetic fault (MTF). Significantly, both glacial moraines and bottom sediments of Emerald strain rates do not necessarily reflect long-term slip rates on discrete faults. Bay provide excellent records of late Quaternary to Holocene fault displacements.

Submerged scarps, in particular, are exceptionally well preserved. Faults of the Brune, J.N., and Anooshehpoor, A., 1999, Dynamic geometrical effects on strong ground motion in a normal fault model: Journal of Geophysical Research, v. 104, p. 809–815, https://doi.org TSFFZ have a combined VSR of about 2.1 mm/yr. /10.1029/1998JB900030. The TSFFZ, a complex zone with several en echelon segments, each with Busby, C.J., 2013, Birth of a plate boundary at ca. 12 Ma in the ancestral Cascades arc, Walker Lane numerous subparallel branches, forms the eastern edge of the rigid Sierra Ne- belt of California and Nevada: Geosphere, v. 9, p. 1147–1160, https://doi .org /10 .1130 /GES00928 .1. vada microplate. Geologic relations and patterns of range-front normal faulting Busby, C.J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., 2016, Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into indicate that the TSFFZ is the older, more long-lived normal fault system. The modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra WTDPFZ is a relatively simple zone with a single main trace. Nevada: Geosphere, v. 12, no. 1, p. 135–175, https://doi.org/10.1130/GES01182.1. The fact that both fault zones have been active during late Quaternary to Carlson, C.W., Pluhar, C.J., Glen, J.M.G., and Farner, M.J., 2013, Kinematics of the west-central Walker Lane: Spatially and temporally variable rotations evident in the Late Miocene Stanislaus Holocene times has important implications for the migration of activity of range- Group: Geosphere, v. 9, no. 6, p. 1530–1551, https://doi.org/10.1130/GES00955.1. front fault systems, emphasizing the importance of conclusions of Wallace (1987), Dewey, J.F., 2002, Transtension in arcs and orogens: International Geology Review, v. 44, p. 402–439, who described examples from the Basin and Range Province (BRP) both where https://doi.org/10.2747/0020-6814.44.5.402. Dewey, J.F., Holdsworth, R.E., and Strachan, R.A., 1998, Transpression and transtension zones, fault activity migrated basinward from range fronts and where fault activity has in Holdsworth, R.E., Strachan, R.A., and Dewey, J.F., eds., Continental Transpressional and jumped back and forth from range front to the basin and back through time. Transtensional Tectonics: Geological Society of London Special Publication 135, p. 1–14. Howle et al. (2012) noted that the TSFFZ is important both for youthful normal Dingler, J., Kent, G., Driscoll, N., Babcock, J., Harding, A., Seitz, G., Karlin, R., and Goldman, C., fault displacements and for potential seismogenic earthquake ruptures in the 2009, A high-resolution seismic CHIRP investigation of active normal faulting across the Lake Tahoe basin, California-Nevada: Geological Society of America Bulletin, v. 121, p. 1089–1107, Lake Tahoe basin. The activity of faults within the TSFFZ is important for another https://doi.org/10.1130/B26244.1. reason. During normal fault earthquakes, ground motion in hanging walls may Dixon, T.H., Miller, M., Farina, F., Wang, H., and Johnson, D., 2000, Present-day motion of the Si- be significantly greater than in footwalls (e.g., Anderson et al., 2000; Brune and erra Nevada block and some tectonic implications for the Basin and Range province, North American Cordillera: Tectonics, v. 19, p. 1–24, https://doi.org/10.1029/1998TC001088. 3 Anooshehpoor, 1999). Because the head scar of the 10–12.5 km McKinney Bay Easterbrook, D.J., 1999, Surface Processes and Landforms (second edition): Englewood Cliffs, (mega)landslide (Fig. 18) lies within the hanging wall of the TSFFZ (and is west of New Jersey, Prentice-Hall, 546 p. the WTDPFZ), one or more large slip events along the TSFFZ may have triggered Faulds, J.E., and Henry, C.D., 2008, Tectonic influences on the spatial and temporal evolution of the Walker Lane: An incipient transform fault along the evolving Pacific–North American plate the large-scale collapse event(s) and resultant megatsunami (Moore et al., 2014). boundary, in Spencer, J.E., and Titley, S.R., eds., Ores and Orogenesis: Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits: Arizona Geological Society Digest, v. 22, p. 437–470. Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 p. ACKNOWLEDGMENTS Gilbert, G.K., 1928, Studies of Basin and Range Structure: U.S. Geological Survey Professional Paper 153, 92 p. Many people contributed to this project. In particular, we gratefully acknowledge James Howle Gold, R.D., Briggs, R.W., Personius, S.F., Crone, A.J., Mahan, S.A., and Angster, S.J., 2014, Latest for his assistance in providing detailed LiDAR and sonar images and in preparing numerous scarp Quaternary paleoseismology and evidence of distributed dextral shear along the Mohawk profiles for us. We also are grateful to Brant Allen, Tahoe Research Group; Jamie McCaughey, for- Valley fault zone, northern Walker Lane, California: Journal of Geophysical Research. Solid mer M.S. student at University of Nevada, Reno (UNR); undergraduate and graduate mechanical engineering students at Santa Clara University; undergraduate students and graduate teaching Earth, v. 119, p. 5014–5032, https://doi.org/10.1002/2014JB010987. assistants in UNR geology field camps in 2003, 2004, 2007, 2008, and 2009; and Brent von Twistern Hammond, W.C., and Thatcher, W., 2007, Crustal deformation across the Sierra Nevada, northern for acquisition of the detailed echosounder map of the floor of Emerald Bay. Financial support Walker Lane, Basin and Range transition, western United States measured with GPS, 2000–2004: for ROV work from Santa Clara University is also gratefully acknowledged. Several anonymous Journal of Geophysical Research. Solid Earth, v. 112, B05411, https://doi .org /10 .1029 /2006JB004625. reviewers and Geosphere editors provided comments that materially improved this report. 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