The Southern Sierra Nevada Pediment, Central California GEOSPHERE

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

GEOSPHERE The southern Sierra Nevada pediment, central California Francis J. Sousa1, Jason Saleeby1, Kenneth A. Farley1, Jeffrey R. Unruh2, and Max K. Lloyd1 1Division of Geological and Planetary Sciences, California Institute of Technology (Caltech), 1200 E. California Blvd., Pasadena, California 91125, USA GEOSPHERE; v. 13, no. 1 2Lettis Consultants International, Inc., 1981 N. Broadway, Suite #330, Walnut Creek, California 94596, USA

doi:10.1130/GES01369.1 ABSTRACT Nevada pediment (SSNP). We will first introduce the long history of studies 12 figures; 3 tables; 1 supplemental file using low-temperature thermochronologic data to constrain the evolution of The southern Sierra Nevada foothills, central California (USA), expose a the southern Sierra Nevada, and then describe the SSNP by presenting field, CORRESPONDENCE: fsousa@gps.caltech.edu fossil pre–40 Ma bedrock pediment which we call the southern Sierra Ne- geochemical, and mineralogical data. Next, we will use these data to constrain vada pediment. We document this landscape with multiple types of data, and a chronology of landscape evolution and tectonic activity along the SSNP. CITATION: Sousa, F.J., Saleeby, J., Farley, K.A., also report new apatite 4He/3He, (U-Th)/He, and zircon (U-Th)/He data from Finally, we will interpret this chronology within the broader context of the Unruh, J.R., and Lloyd, M.K., 2017, The southern Sierra Nevada pediment, central California: Geosphere, v. 13, the pediment that significantly expand the spatial extent of southern Sierra southern Sierra Nevada–Great Valley system and discuss its implications for no. 1, p. 82–101, doi:10.1130/GES01369.1. low-temperature thermochronology data westward into the foothills. Apply- regional tectonics and landscape evolution. ing recently published thermal modeling software for thermochronologic data, The regional additions to basement thermochronologic data from the south- Received 24 May 2016 which uses a transdimensional Bayesian Monte Carlo Markov chain statisti- ern Sierra that we present here improve our understanding of the post-mag- Revision received 17 August 2016 cal approach, we tightly constrain the thermal history of the southern Sierra matic evolution of the southern Sierran arc. These data include new bulk Accepted 20 October 2016 4 3 4 3 Published online 10 November 2016 Nevada pediment. Integrating this thermal history with numerous previously apatite (U-Th)/He data (Ap-He), apatite He/ He data (Ap- He/ He), and zircon published data sets from across the southern Sierra, we present a chronology (U-Th)/He data (Z-He), all from locations significantly farther west than those of tectonic and landscape evolution of the southern Sierra Nevada. For the of any previously published data from this part of the mountain range (Fig. 1). first time we cover the entire width of the range, integrate the numerous pub- This spatial expansion of basement thermochronometric data bears signifi- lished data sets into a single coherent geologic story, and link each phase of cantly on the debate in the literature about the geomorphic evolution of the this story to a potential mechanism. southern Sierra Nevada (House et al., 1998, 2001; Clark et al., 2005; McPhillips Modeling results are consistent with a three-phase cooling history for the and Brandon, 2012; Wakabayashi and Sawyer, 2001; Wakabayashi, 2013, 2015) southern Sierra Nevada pediment. Rapid exhumation ca. 95–85 Ma resulted in and, more importantly, on the assumptions that underlie the key arguments cooling to between 55 °C and 100 °C. Following this, slow cooling to surface in these studies. In the context of the large body of research regarding the conditions occurred from 85 Ma to 40 Ma at rates consistent with those esti- topographic evolution of the southern Sierra and recent constraints on Eocene mated for the axial southern Sierra during the same time period by previous uplift (Sousa et al., 2016), we piece together a chronology of tectonic and land- studies. Little if any additional cooling occurred post–40 Ma. We hypothesize scape evolution of the southern Sierra Nevada. Furthermore, we present the

that a thin sedimentary cover protected the 40 Ma bedrock landscape through first application of the (U-Th)/He chronometer to the TiO2 mineral anatase. much of the last 40 m.y., and that this cover eroded away post–10 Ma, re- exhuming the southern Sierra Nevada pediment as a fossil pre–40 Ma landscape. Each of these three phases of cooling links to a distinct tectonic or GEOLOGIC SETTING geomorphic regime, including the profound rapid exhumation of the southern Sierra Nevada–Mojave segment of the Cretaceous arc due to subduction of a The SSNP runs ~150 km along the western edge of the southern Sierran large oceanic plateau, the formation of the low-relief landscape of the high-el- foothills from near 36°N at Fountain Springs, California, in the south to near evation areas of the southern Sierra Nevada with more limited tectonic forc- 37° N at Friant, California, in the north (Fig. 1). Along the pediment, bedrock ing, and Eocene activity on the Western Sierra Fault System. lithology consists of plutonic rocks of the composite Sierra Nevada batholith as well as pre-batholithic wall rocks. Locally the batholith consists of Early Cretaceous plutonic rocks emplaced ca. 115 ± 10 Ma (Chen and Moore, 1982; INTRODUCTION Saleeby and Sharp, 1980; Lackey et al., 2005; Clemens-Knott and Saleeby, 1999) at pressures of 3–4 kb (Ague and Brimhall, 1988; Ague, 1997; Nadin et al., Basement outcrops along the boundary between the southwestern Sierra 2016). Secondary to plutonic rocks are pre-batholithic wall rocks of the Kings- For permission to copy, contact Copyright Nevada foothills and the San Joaquin Valley (central California, USA) ex- Kaweah ophiolite belt that runs along nearly the entire length of the SSNP; this Permissions, GSA, or [email protected]. pose a bedrock pediment landscape that we refer to as the southern Sierra belt consists of the Paleozoic Kings River ophiolite and Kaweah serpentinite

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120°W 119°W Bates, 1945; Creely and Force, 2007; Palmer, 1978; Palmer and Merrill, 1982). !( !( !( 115°W At its type locality near Ione, California, in the northern Sierra Nevada, the !(!(!( Superjacent !(!(!(!( Series !(!( Ione Formation is of middle Eocene age, based on a limited molluscan fauna San!( Joaquin !( 40°N as well as stratigraphic correlations to the Domengine Formation in the Coast !(!( !(!( !( Ranges and Great Valley subsurface and to the auriferous gravels of the north- !(!( !( !(!( !( !( ern Sierra Nevada (summarized in Creely and Force, 2007). The Ione Formation !( !( !( !( !( 35°N 120°W in our study area is the southernmost extent of the Ione and correlates to the !( !( !(!( !( !( !( non-marine facies of the Ione at its type location (Palmer and Merrill, 1982; !( !(!( !( !( F !( !( !( 37° N Creely and Force, 2007). These outcrops (Fig. 3B) sit directly on the 114 Ma !(!(!( !( !( !( !( S !( !(!( !( Kings !(!( tonalite of Blue Canyon (Busacca, 1982; Bateman et al., 1983), which is locally an Joaquin !( !( !( !( !( A !( !( deeply weathered beneath the Eocene nonconformity (Fig. 3D). !( !( !( !( B !( !( !(!(!( !( !( !(!(!(!( The southern terminus of the pediment, 150 km to the south, abuts the !( !(!( !( !( !(!(!( !( C !(!(!(!( !( northernmost edge of the Kern Arch, a crescent-shaped active uplift along the !( boundary between the San Joaquin Valley and the southern Sierran foothills D !( Kern (Cecil et al., 2014; Fig. 1). Analogous to the stratigraphic relationship at the !( !( Va !(!(!( !( !( northern end of the pediment, Cenozoic strata of the Kern Arch are Eocene E !( !(

!( 36°N TB !( and younger, with the basal Walker Formation, containing a 40.1 ± 0.3 Ma tuff, !(!( !(!( lley FS !(!( !( !( !( !(!( !( !( !(!( !(!( !( deposited nonconformably on deeply weathered Sierran basement (Saleeby !( !( Kern !( !( !( !(!( et al., 2016; Fig. 3A and 3C). !( !( !( Arch !(!( !(!( !(!( !( !( !( !(!(!(!(!(!( Along the western edge of the SSNP, soils and sediments of the eastern ¹ !( !( !( !( San Joaquin Valley shallowly cover low-relief bedrock outcrops, with soil ! !(!( New Ap-He samples !(!( ( !( depths on the order of meters to tens of meters (Sousa et al., 2013; Saleeby Pub. Ap-He data et al., 2013b; this study). This area hosts widespread agriculture, which makes WSFS detailed geological observations difficult. Nonetheless, field and remote sens- Bedrock pediment Mojave 35°N ing observations of bedrock tors interspersed amongst orchards, as well as

Superjacent Series 35°N 50 km shallow depths to basement in local water wells, confirm that this boundary 119°W 118°W is generally a low-relief bedrock landscape (this study). A few kilometers far- Figure 1. Overview map of the southern Sierra Nevada region (central California, USA). Pre- ther west, in the San Joaquin Valley subsurface, Late Cretaceous to Eocene viously published apatite (U-Th)/He data (Ap-He) from House et al. (1997, 1998, 2001), Clark sedimentary rocks overlie Sierran basement (Reid, 1988). East-west–trending et al. (2005), and Maheo et al. (2009) are plotted as white circles. Scarps of the Western Sierra Fault System (WSFS) are mapped as black lines. Extent of the Superjacent Series is mapped channels with hundreds of meters of relief and a deeply weathered zone tens in orange with dashed outlines, after Bateman and Wahrhaftig (1966). The locations of new of meters thick mark this sub–Upper Cretaceous basement nonconformity in Ap-He data presented in this paper are plotted as black circles. Friant, California, is located the San Joaquin Valley subsurface (Reid, 1988). with a white F at 37°N and is adjacent to sample 11SS6 at the north end of our study area. Fountain Springs, California, is located with a white FS and is adjacent to sample 11SS1 at East of the SSNP, the southern Sierra rises rapidly to ~2000 m elevation the south end of our study area. New mapping of bedrock peneplain exposures is shown across a series of topographic steps. Sousa et al. (2016) showed that one of in green along the foothills-to-basin transition. Locations of cross sections A through E are these steps is an eroded fault scarp of the Western Sierra Fault System (WSFS) shown (cross sections are included in the Supplemental File [see footnote 1]). For geographic and posited that the rest of the system was also active ca. 45–40 Ma (Fig. 1). reference, the San Joaquin Valley, Kern Arch, Tulare Basin (TB), Mojave Desert (Mojave) and San Joaquin, Kings, and Kern Rivers are located. An inset map of a California digital elevation model (DEM) outlining the figure extent is shown at the upper right. Base imagery is a hillshade DEM derived from 10 m U.S. Geological Survey National Elevation Dataset. PREVIOUS WORK

mélange and encloses ophiolitic blocks and infolds of nonconformably over- Southern Sierra Nevada Pediment lying upper Paleozoic–lower Mesozoic slaty marine strata (Fig. 2; Saleeby and Sharp, 1980; Saleeby, 2011). Prior to this study, much of the research in the southern Sierra Nevada North of the SSNP, a section of Eocene and younger deposits known as the (western) foothills focused on pre- and syn-batholithic petrology, geochemis- Superjacent Series nonconformably overlies Sierran basement (Bateman and try, and tectonics (e.g., Saleeby and Sharp, 1980; Clemens-Knott and Saleeby, Wahrhaftig, 1966). The southernmost outcrops of the Eocene Ione Formation 1999; Saleeby, 2011). However, some studies have considered the geomorphol- (Fig. 1) occur at the northern end of our study area near Friant (Lindgren, 1911; ogy of the southern Sierran foothills (Hake, 1928; Wahrhaftig, 1965; Saleeby

r 119°30′W

6 SJ Rive 37°N

Great Central Va LT

ll e y Sa n Andrea

8 s faul Fig.2

t Fresno err 180 Kings River Central California 9 Index Map 1 0 m 7 e 5 te m rs10 119O 00’ W e t e

r s

S 11 Figure 2. Geologic map of the southern Sierra Nevada foothills (central California, 12 USA) from ~36°N to ~37°N, modified after Saleeby (2011). Each sample site is labeled

with white on black circles. Bedrock pedi

San Joaquin 30 ′N ment is shown in a light red shade. The 36° Legend first four rows of the legend under Bed- 5 rock Pediment each show two columns Index Map representing the map unit of the under- Foothills Metamorphic Belt Va lying lithology and the right-hand column lley indicates the shade that appears on the Contiguous Sierra Nevada VH batholith map where the bedrock pediment over- Kaweah laps the lithology at left. State highways River Geologic Map of the Southern Sierra Nevada Foothills 180, 198, and 190 are drawn as thin black lines, and labeled with the corresponding Bedrock Pediment 198 highway number. SSNP—southern Sierra Under shallow San Joaquin Valley sediments Nevada pediment; SJ River—San Joaquin River. Eroded basement outcrops 4

10 m Structure contours on depth to basement 75 m from shallow water well data CH San Joaquin Valley sediments shallowly covering SSNP near foothills Eocene Ione Formation Tertiary strata of the Kern Arch 2 Early Cretaceous Sierra Nevada batholith: gabbro, diorite, tonalite and granodiorite Western Foothills Metamorphic Belt Jurassic intrusive complexes 190 Upper Paleozoic to Lower Cretaceous overlap strata Tule e r Paleozoic Kings-Kaweah ophiolite belt Rive 36°N

2 11SS Sample location and number S S = Smith Mountain, CH = Chrysoprase Hill, VH = Venice Hills, LT = Little Table Mountain

0 5 10 15 20 25 km 1

A B

Figure 3. (A) Field photo of 40.1 ± 0.3 Ma tuff from the Walker Formation along the White River, California (USA), a few kilometers southeast of sample location 11SS1. Outcrop is ~2 m in total height. (B) Field photo of laterically weathered unit at the base of the Ione Formation, deposited on deeply weathered basement near Friant, California. Outcrop is in a road cut along California State Highway 145 just west of Little Table Mountain. Rock hammer C D in the figure is 32 cm long. (C) Weathered granitic outcrop from the vicinity of sample location 11SS1. Corestone weathering out on the right side of the frame is ~2 m across. (D) Deeply weathered basement at the nonconformity beneath the Ione For- mation at the same outcrop as B. Scale shown is in millimeters.

and Foster, 2004; Pelletier, 2007; Figueroa and Knott, 2010). While Figueroa and range-parallel topographic steps. An early study by Hake (1928) described Knott (2010) and Pelletier (2007) each focused on much larger areas than the a set of these steps running from the San Joaquin to Kern Rivers as intra- SSNP, Saleeby and Foster (2004) did focus on this area. They interpreted this batholithic west-down normal fault scarps (Fig. 1). However, workers ne- segment of the southern Sierran foothills as dominated by steep faceted topog- glected this idea in the literature after Wahrhaftig (1965) dismissed it based raphy buried by active sedimentation in the eastern San Joaquin Valley. This on flawed geologic interpretations and with very little mention of Hake’s description does not bear out fully. Rather than steep topography being actively (1928) observations. These erroneous interpretations include incorrect cor- buried, the topographic features within this landscape commonly rise from a relation of units across topographic steps (Huber, 1981) as well as the detail low-relief peneplain which is only shallowly, if at all, covered by sediments that the allegedly unfaulted unit (the ca. 10 Ma Kennedy Table Mountain (Sousa et al., 2013; Saleeby et al., 2013; this study). Saleeby and Foster (2004) flow) was too young to test for the existence of pre–10 Ma Cenozoic faults. contended that the geomorphic differences between this and other segments of Without addressing the descriptions presented by Hake (1928), Wahrhaftig the Sierran foothills result from the surficial response to the actively foundering argued that the steps of the southern Sierra are of a purely erosional origin. mantle lithospheric phenomenon known as the Isabella anomaly, which lies Jessup et al. (2011) tested this conclusion by measuring cosmogenic ero- beneath this segment of the southern Sierra foothills (e.g., Zandt et al., 2004). sion rates on step treads versus risers and found their data to be generally The findings of this current study, as well as more recent modeling work on the inconsistent with Wahrhaftig’s interpretation. Recently, Sousa et al. (2016) surficial effects of mantle lithospheric dynamics beneath the SSNP, support this used thermochronometric data from the main trunk and north fork of the interpretation (Le Pourhiet el al., 2006; Saleeby et al., 2012, 2013b). Kings River to show that at least one of the steps described by Hake (1928) is an eroded fault scarp. This fault accommodated kilometer-scale west-down Western Sierran Slope displacement in Eocene time and is a part of the WSFS. In this context, the swath of Sierran basement that composes the SSNP is the hanging wall of A fundamental topographic characteristic of the Sierra Nevada is the fact the WSFS. Figure 1 shows that with the exception of a few samples pre- that in the north, the western slope is a continuous ramp, while the south- sented in Sousa et al. (2016), all of the previous Ap-He studies in the southern Sierra rapidly attains an elevation of 2000 m across a set of roughly ern Sierra are entirely east of the WSFS.

Southern Sierra Region used a numerical model to test different bedrock erosion models in the southern Sierra, and the results of his preferred model indicate that the southern Although many models exist for the antiquity and evolution of southern Sierra Nevada experienced range-wide surface uplift in the latest Cretaceous Sierra Nevada topography, little consensus has emerged in the literature re- and late Miocene. garding the timing of generation of the high elevations and large-relief can- McPhillips and Brandon (2012) integrated published Ap-He and apatite yons that compose the modern southern Sierra. fission-track thermochronometry and igneous geobarometric data into a numer- The assumption of Cenozoic rigid-block down-to-the-west tilt of the Sierra ical landscape evolution model encompassing much of the modern Sierra. Their Nevada mountain range underpins the analysis presented in several previous preferred model finds onset of range-wide uplift and incision at ca. 30–10 Ma. studies. Some of these studies explicitly stated this assumption, and some Studies in the western foothills and eastern San Joaquin Valley subsurface used its implications to extrapolate geologic data over long distances and ar- reported direct measurements of Late Cretaceous and Paleogene paleo-relief. gue for a late Cenozoic origin of most of the present-day topography, partic- This includes a minimum of 500 m of paleo-relief in the Kaweah River drainage ularly north of the Kings River canyon (Huber, 1981; Unruh, 1991; McPhillips near the Sierra–Great Valley transition based upon interpretation of Ap-He data and Brandon, 2012). Wakabayashi and Sawyer (2001) used long-distance pro- and bedrock pediment geomorphology (Saleeby et al., 2013b; Sousa et al., jection of tilted volcanic units to argue for only a minor departure from rigidity 2013, 2014; this study). Reid (1988) measured the same scale (500 m) of relief due to east-down faulting near the Sierra crest. They also extended this rigid- on the Late Cretaceous basement nonconformity in the eastern San Joaquin block idea westward into the Great Valley, where sedimentation is balanced Valley subsurface. with erosion of the Sierra uplands during rigid west tilting (Wakabayashi and The recent documentation of normal faulting along the western slope of Sawyer, 2001). the southern Sierra (Sousa et al., 2016) undermines the fundamental assump- On the other hand, the correlation of Ap-He ages with the location of major tion of down-to-the-west rigid tilt included in many of these previous studies. river canyons along two horizontal transects from the axial Sierra supports a This assumption is most critical in the western foothills, where data have been Late Cretaceous antiquity of the large-amplitude, long-wavelength relief pat- lacking and long-distance extrapolation using the rigid-block model has been tern common to these river canyons (House et al., 1998, 2001; Braun, 2002a, necessary. By filling this gap in the basement thermochronometric data along 2002b). Furthermore, vertical transects of Ap-He data from the southern Sierra the SSNP, we obviate the need for such an assumption and test the new model show a consistent age-elevation slope of 40–60 m/m.y. and lack clear inflec- put forth by Sousa et al. (2016) for Eocene faulting, extension, and uplift. tions that would record canyon-incising events. This implies that the high- In summary, the findings of previous studies clearly require a polyphase elevation, low-relief interfluvial plateaus mimic the landscape that developed evolution of southern Sierra topography, with distinct topographic patterns in the Late Cretaceous and was slowly exhumed at roughly this same rate until linked to specific periods of tectonic activity and erosion. This includes 40 Ma or later (Clark et al., 2005; Mahéo et al., 2009; House et al., 1997, 2001). large-relief river canyons dating back to Late Cretaceous time, and two Ceno- Together these interpretations imply that low-relief highlands and high-relief zoic phases of uplift and incision of these canyons in Eocene (ca. 45–40 Ma; canyons were both part of the Late Cretaceous landscape. In this view, much Sousa et al., 2016) and Plio-Pleistocene time (Stock et al., 2004, 2005). of the form of the modern Sierran landscape mimics regional morphology that was established in Late Cretaceous (e.g., House et al., 1998), at which time the Sierra Nevada mountains were the western flank of a high-elevation plateau THE BEDROCK PEDIMENT referred to as the Nevadaplano (DeCelles, 2004). In contrast, Stock et al. (2004) identified a pulse of late Cenozoic river inci- Description of Pediment Morphology sion in the southern Sierra using cosmogenic radionuclide burial dates from vertical transects of quartz-bearing sediments deposited on abandoned fluvial- In contrast to other types of pediments that form due to differences in cut terraces in carbonate caves. These data resolve late Pliocene to Pleistocene erodibility caused by lithologic or structural boundaries, bedrock pediments incision of the lowest ~20% of total relief of several central to southern Sierra form within monolithologic areas (Oberlander, 1974; Twidale, 1981; Dohren- river canyons (e.g., 400 m in Kings Canyon). Stock et al. (2004) pointed out wend and Parsons, 2009). This type of morphology remains unexplained by that they do not constrain the age of the upper 80% of relief (e.g., 1600 m in theory, but modeling efforts to understand bedrock pediment formation agree Kings Canyon). in the requirement of an extended period (roughly 106–107 yr) of erosion and Clark et al. (2005) identified two knickpoints in stream long profiles of the tectonic quiescence (e.g., Pelletier, 2010; Strudley et al., 2006). main trunks and tributaries of the Kings and Kern Rivers and argued that these The principal components of a bedrock pediment are a low-relief peneplain, knickpoints correspond to two pulses of incision responsible for most of the hillslopes rising from the peneplain, and, most critically, the piedmont angle relief in these canyons. The authors asserted that these events must have post- where slope changes rapidly from peneplain to hillslope without any structural dated the youngest Ap-He age on the Kings River (ca. 32 Ma). Pelletier (2007) or lithologic boundary (Oberlander, 1974; Twidale, 1981; Pelletier, 2010; Strudley

et al., 2006). We define the SSNP as the bedrock landscape exposed along the the Horse Creek Campground (Fig. 4), the current channel of the Kaweah River transition from the San Joaquin Valley to the Sierra Nevada foothills, which opens onto a bedrock peneplain averaging 500–1000 m wide. Seasonally and consists of these three morphologic components: (1) low-relief bedrock out- in wet years this area floods, but low water levels in Lake Kaweah (e.g., during crops within the peneplain; (2) bedrock hillslopes rising from the peneplain by drought years) expose the low-relief bedrock peneplain. There are abundant meters to hundreds of meters; and (3) the transition between these two zones low-relief bedrock outcrops across the pediment that show that the alluvium is where slope rapidly changes, called the piedmont angle (Oberlander, 1974; shallow. Around the edge of the pediment, a transition to hillslope is exposed, Twidale, 1981; Pelletier, 2010, Strudley et al., 2006). We document these ele- where without any lithologic or structural boundary the bedrock landscape ments of the landscape using multiple methods. Where access was possible, rises over 500 m to local peaks. we made field observations which are the basis for the mapping shown in Fig- ure 1. We complemented field work with aerial and satellite images, hillshade Mineralogical and Paleosol Occurrences on the SSNP models derived from a 10 m U.S. Geological Survey digital elevation model, and published geologic maps (Matthews and Burnett, 1965; Clemens-Knott, In the southern portion of the SSNP where bedrock lithology locally includes 2011; Macdonald, 1941; Saleeby and Sharp, 1980; Saleeby, 2011; Busacca, 1982; Kaweah serpentinite mélange of the Kings-Kaweah ophiolite belt, there are Bateman et al., 1983). several mineralogical and paleosol occurrences that are important indicators

A A' N28W S28E 1000 1000 Smith Mountain Depth-to-basement data from shallow water wells allows us to extend our of the pre–40 Ma landscape. Specifically, these include mineral occurrences 900 (projected from west) 900 Venice Hills 800 Intersection Intersection Intersection Intersection 800 Campbell of of Kaweah of of 700 Mountain B - B' Orosi Cutler C - C' River D - D' Lindsay E - E' 700

600 600

500 Todd's 500 mapping of the SSNP beyond the accessible exposures and into the subsurface distinctive of nickel laterites formed by chemical weathering of serpentinites Mountain 400 400

300 300

200 200

? 100 ? ? 100

? ? ? Elevation (feet) ? Elevation (feet) 0 ? ? 0 west of the foothills-to-basin transition (Fig. 2). We averaged individual water (e.g., Vasconcelos and Singh, 1996; Eggleton et al., 2011). At Chrysoprase Hill, ? ? ? ? -100 ? ? -100 Explanation ? ? ? -200 Bedrock encountered -200 Bedrock not encountered -300 Buried channel -300 250 feet Most shallow point Most shallow well of Kaweah River? 2 bedrock encountered in section, -400 -400 no bedrock encountered ? Average depth of bedrock in section well data over 1 mi sections and compiled the data into five cross sections cov- Venice Hills, and Smith Mountain (Fig. 5), nickel-rich chalcedony (the gemstone -500 -500 Deepest point in 0 Deepest well section, bedrock not -600 in section, -600 0 10,000 feet encountered (bedrock no bedrock encountered encountered if circle -700 40X vertical exaggeration is black) -700

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000110,000 120,000 130,000 140,000 150,000 160,000 170,000 180,000 190,000 200,000 210,000 220,000 230,000 220,000 Distance (feet) ering a large portion of our study area; Figure 1 shows the locations of these chrysoprase; Fig. 6) occurs in conjunction with deeply weathered and silicified cross sections, which are included in the Supplemental File1. These cross sec- bedrock that outcrops as an erosion-resistant ferruginous silcrete (Fig. 6). At 1 ′ Supplemental File. Five cross sections (A–A through tions generally support our field based observations of the SSNP, documenting Chrysoprase Hill and the Venice Hills there are also occurrences of hydrous E–E′), two tables listing individual grain apatite and zircon (U-Th)/He data, and eight thermal modeling sum- areas where the floors of small valleys along the foothills-to-basin transition are Ni-Mg–rich silicates (garnierite), a nickel ore common to lateritically weath- maries for individual samples along the pediment. low-relief bedrock landscapes covered by only tens of meters of regolith (e.g., ered ultramafic rocks (e.g., Thorne et al., 2012). Prior to this study, mentions Please visit http://dx .doi.org/10.1130/GES01369.S1 near the town of Orange Cove, California, and in the valley of Cottonwood Creek; of these occurrences in the literature appeared only in bulletins and reports of or the full-text article on www.gsapubs.org to view the Supplemental File. cross sections B and C, respectively; see the Supplemental File [footnote 1]). the mineral resources of California (e.g., Goodwin, 1958; Pemberton, 1983) and A good example of this morphology is immediately upstream of Terminus popular mention of chrysoprase as an economic gemstone mined along the Dam along the Kaweah River. At an elevation of ~210 m above sea level near SSNP for several decades in the late nineteenth and early twentieth centuries

Hillslope Relief Figure 4. Bedrock pediment exposure Piedmont angle just upstream of Lake Kaweah, Califor- nia (USA). Terminus Dam is visible in the background in the central region of the figure. Bedrock pediment morphology is annotated showing the low-relief bedrock peneplain, hillslope relief (locally >500 m), and the piedmont angle where slope rapidly changes in the absence of any structural or lithologic boundary. Abundant bedrock outcrops across the pediment Bedrock Peneplain show that there is only shallow alluvium covering the bedrock pediment.

119°30′W 11SS6 A !( San Joaquin !( 10 cm 37° N

Little Table 40° N !( !( Mountain !( !( !( !( Figure 6. (A) Photograph of a thick !(

35° N section cut through a ferruginous !( 120°W 115°W !( silcrete sample taken from Venice 11SS7 Hills, California (USA), near the !( Kaweah River. (B) Photograph of !( !( !( !( a Tulare County chrysoprase from 11SS8 !( the Caltech mineral collection. !( !( !( Chrysoprase forms during deep Kings !( weathering of ultramafic rocks !(!( N B (Vasconcelos and Singh, 1996) ′ !( 11SS9 and was mined as a gemstone from beneath ferruginous silcrete !( !(!( outcrops along the southern Sierra 11SS10 !( !( !( 37° N !( th 36°30 !( Nevada pediment in the late 19 Smith !( !( !( !( 11SS11 and early 20th centuries (e.g., New Mountain !(!( York Times, 1902). W

′ !(

!( 11SS5!( !(

119°30 Venice Hills !( !( 10 cm N

Kaweah ′

Chrysoprase (New York Times, 1902). Together with the other data presented in this paper, 11SS!( 4 ¹ Hill these minerals and paleosols support our interpretation that the modern SSNP !( 36°30 4000 m!( !( is a fossil pre–40 Ma landscape.

W Geologic mapping of plutons and ophiolitic wall rocks exposed along the

!( ′ 11SS2 SSNP (Fig. 2; Saleeby and Sharp, 1980; Clemens-Knott and Saleeby, 1999; 36° N Saleeby, 2011, Saleeby et al., 2013a), in conjunction with our geomorphic map- !( Tule 0 m ping of the pediment surface, shows that the area lacks transverse faults, indi- !

New Ap-He samples 118°30 cating structural continuity along this swath of Sierran basement. Accordingly, ( Pub. Ap-He data we conclude that the bedrock exposed at the sub-Eocene nonconformities 11SS1!( near Fountain Springs and Friant represents two ends of a single strip of base- Fountain W. Sierra Flt. Sys. ment that composes the SSNP. We interpret the bedrock pediment geomor- Springs 20 km Bedrock pediment !( phology and the distinctive lateritically weathered paleosols and mineralogical 119°W 36°N 118°30′W occurrences along the SSNP to be remnant elements of a pre–40 Ma (sub–Ione

Figure 5. Map of our study area (central California, USA). Yellow stars indicate loca- and Walker) landscape. tions of deeply weathered basement exposures; the name of each location is labeled on the figure (see text for details). Purple shading shows new bedrock pediment map- THERMOCHRONOLOGY ping. Black circles show new sample locations and names, and white circles show the locations of previously published apatite (U-Th)/He (Ap-He) data (same as Fig. 1). Scarps of the Western Sierra Fault System (W. Sierra Flt. Sys.) are drawn as black Sampling Approach and Methods lines. The San Joaquin, Kings, Kaweah, and Tule Rivers are located for geographical reference; an inset California digital elevation model (DEM) at upper right outlines the Samples come from outcrops of Early Cretaceous plutonic rocks of the figure extent. Base imagery is an overlay of a hillshade and elevation-classified DEM from the U.S. Geological Survey National Elevation Dataset colored according to the Sierra Nevada batholith along the westernmost bedrock exposures of the scheme shown at right. Sierran foothills from the towns of Fountain Springs to Friant (Fig. 1). After

crushing, sieving, and standard heavy mineral separation, apatite and zircon lowing the same criteria as for bulk age determination, with particular attention grains from each sample were selected with a stereoscopic microscope for paid to the absence of birefringent inclusions and complete euhedral morphol- analysis. Grains were selected for euhedral habit and checked to exclude any ogy. Each individual grain was step-wise degassed using a halogen lamp as grains with birefringent inclusions (examined with cross-polarized light and heat source (Farley et al., 1999). 4He and 3He were measured at each degassing immersed in ethanol). The dimensions of each grain were then measured and step using a GV Instruments SFT sector field mass spectrometer at Caltech. recorded. For each sample, four to seven individual grains were first analyzed Ap-4He/3He data are included in the Supplemental File [see Footnote 1]. for Ap-He and Z-He age determination. Helium was measured with a Pfeiffer Prisma quadrupole mass spectrometer. After standard mineral digestions, (U-Th)/He and Ap-4He/3He Data parent concentrations were measured via isotope dilution on an Agilent 7500 inductively coupled plasma–mass spectrometer (ICP-MS) (e.g., Farley, 2002). A A single-grain (U-Th)/He age is generally compatible with a diversity of corrected age was calculated for each grain using the alpha-ejection correction thermal histories. A more restricted range of thermal histories is constrained factor (Ft) based on the measured grain dimensions (after Farley et al., 1996). by combining multiple bulk ages from different minerals like apatite and zircon Table 1 shows average bulk Ap-He and Z-He ages. (e.g., Reiners et al., 2000), multiple grains with variations in effective U concen- For the samples chosen for 4He/3He analysis (samples 11SS1 and 11SS6), tration (eU = U + 0.235Th) (e.g., Flowers et al., 2009), or by measuring single additional apatite grains were proton irradiated to make a uniform distribution grain 4He rim-to-core concentration profiles via 4He/3He studies (Shuster and of 3He (Shuster and Farley, 2004, 2005). Individual grains were then picked fol- Farley, 2005). Variations in radiation damage result in closure temperatures

TABLE 1. AVERAGE BULK APATITE (U-Th)/He AND ZIRCON (U-Th)/He DATA eU Average Average range† raw age corrected§ age 4He/3He Latitude Longitude Elevation Sample N* (ppm) (Ma) (Ma) present? Pluton (°N) (°W) (m)

ap 6 13–38 44.2 ± 3.766.0± 3.5Yes 102Ma quartz diorite 11SS1 35.889 118.939 239 zr 4 59–216 74.8 ± 4.184.9± 4.9No (Lackey et al., 2005) ap 4 98–118 45.3 ± 1.670.4± 2.4No 11SS2 Unmapped 36.144 118.958 182 zr 4 59–119 67.9 ± 7.386.6± 6.5No

ap 4 29–43 49.5 ± 3.078.0± 3.9No 120Ma granodiorite 11SS4 36.268 118.995 198 zr 4 50–102 67.1 ± 2.383.3± 1.9No (Lackey et al., 2005)

ap 7 201–383 46.8 ± 3.269.1± 4.0No 120Ma bt-hbl tonalite 11SS5 36.473 119.156 137 zr 4 64–171 105.4 ± 4.8122.8 ± 4.6No (Saleeby and Sharp, 1980)

ap 7 21–66 63.1 ± 3.391.6± 4.4Yes 114Ma tonalite of Blue Canyon 11SS6 37.011 119.772 165 zr 4 32–116 80.3 ± 4.897.3± 4.8No (Bateman et al., 1983)

ap 4 39–202 51.4 ± 2.574.7± 2.1No 120Ma Academy norite 11SS7 36.905 119.518 195 zr 4 22–31 102.9 ± 5.3123.1 ± 8.6No (Saleeby and Sharp, 1980)

ap 4 30–66 50.0 ± 1.880.1± 2.2No 114Ma hbl-bt tonalite 11SS8 36.814 119.493 152 zr 3 86–194 76.4 ± 9.791.2± 12.8 No (Saleeby and Sharp, 1980) ap 4 42–447 51.2 ± 3.475.2± 2.1No 11SS9 Unmapped 36.716 119.354 153 zr 4 32–143 105.9 ± 9.9122.1 ± 8.3No 114Ma quartz diorite 11SS10 ap 4 25–29 47.1 ± 0.970.9± 1.2No 36.668 119.305 143 (Chen and Moore, 1982)

ap 4 15–29 41.2 ± 3.070.1± 4.0No 120Ma gabbro 11SS11 36.598 119.287 136 zr 4 43–65 90.9 ± 5.2102.5 ± 5.5No (Clemens-Knott and Saleeby, 1999) Note: ap—apatite; zr—zircon; bt—biotite; hbl—hornblende. *N is the number of single-grain He analyses used for each sample. †eU is effective uranium: U (ppm) + 0.235 * Th (ppm). §Uncorrected age is corrected for alpha-ejection after Farley et al. (1996).

that vary with eU, and different time-temperature (t-T ) paths result in significantly different 4He concentration profiles based on the time-integrated bal- ance between alpha-particle in-growth and loss by both ejection and diffusion. A The Ap-4He/3He method allows us to mine the 4He rim-to-core concentration profile (Shuster and Farley, 2004, 2005), and subsequent thermal modeling plag allows us to constrain t-T paths. cc

Sample 11SS6, near Friant brk an At the northern end of the study area near Friant, the southernmost out- an crops of the Eocene Ione Formation overlie Sierran basement at an elevation of 165 m. At this location, the SSNP coincides with the sub-Ione noncon formity. Where this nonconformity crops out, bedrock is deeply weathered and nearly unrecognizable as a plutonic rock (Fig. 3D). Basement at this location is the 114 Ma tonalite of Blue Canyon (Busacca, 1982; Bateman et al., 1983). The mean zircon (U-Th)/He age from sample 11SS6 from this location is 97 ± 5 Ma (1 standard error [s.e.], n = 4), and the mean apatite (U-Th)/He age is 92 ± 4 Ma (1 s.e., n = 7) with eU ranging from 21 ppm to 66 ppm. It is worth noting that chl to our knowledge, sample 11SS6 has yielded the oldest Ap-He age from the Sierra Nevada batholith. Ap-4He/3He data from this sample are presented later 2 mm in the paper together with thermal modeling results.

Sample 11SS1, near Fountain Springs B

Our southernmost sample (11SS1) is from near Fountain Springs at an elevation of 290 m. Bedrock outcrops at the sample location include meter- chl to 10-m-scale corestones eroding out of the landscape, a distinct element of deeply weathered granitic rocks (Fig. 3C; e.g., Shaw, 1997). Sample 11SS1 is an altered felsic plutonic rock (Fig. 7). The nearest published U-Pb zircon age is cc from a 102 Ma quartz diorite a few kilometers to the east (Lackey et al., 2005; Saleeby and Sharp, 1980). A few kilometers south of this location along the cc White River, a 40.1 ± 0.3 Ma tuff sits on deeply weathered basement (Laser an Ablation ICP-MS zircon U-Pb from Saleeby et al. [2016]). The mean Z-He age from sample 11SS1 is 85 ± 5 Ma (1 s.e., n = 4), and the mean Ap-He age is 66 ± chl 4 Ma (1 s.e., n = 7), with eU ranging from 13 ppm to 38 ppm. Ap-4He/3He data from this sample are presented later in the paper together with thermal mod- brk eling results. The bulk anatase (U-Th)/He age from this sample is 97 ± 13 Ma. plag cc The Horizontal Transect 2 mm We report bulk (U-Th)/He analyses from eight additional samples along the Figure 7. (A and B) Thin section photographs of sample 11SS1 (35.889 °N, 118.939 °W), which SSNP between Fountain Springs in the south and Friant in the north (Fig. 8). is an altered felsic granitic rock. Mineral assemblage is quartz, plagioclase (plag), calcite (cc), The samples were all taken from plutonic outcrops near the western edge of chlorite (chl), anatase (an), and brookite (brk). Photos were taken with crossed polarizers and the southern Sierra foothills. Some samples are from isolated bedrock out- ambient light flooding the field of view to highlight the orange anatase. Tetragonal cross section of anatase is visible in central portion of B, where anatase (blocky) and brookite (blady) crops interspersed amongst shallow soils of the San Joaquin Valley, and are in a calcite matrix. Stable and clumped isotope data from this sample are discussed in others are slightly farther east in the Sierran foothills. Widespread agricul- the text. tural land use in this area commonly masks the foothills-to-basin transition,

Mono Lake 20 km Owens Valle S y an Jo h ¹ a s a Figure 8. New apatite (U-Th)/He (Ap-He) g e

qu n K

in i w ern and zircon (U-Th)/He (Z-He) data with 1σ K a

T standard errors are plotted versus distance K ul e along the length of the southern Sierra Nevada pediment. Distances are projected 9 10 11 5 4 2 directly down from an oblique Google 7 8 Earth aerial image of the southern Sierra 1 6 Nevada, shown above with locations of samples (e.g., 1 = 11SS1, 6 = 11SS6) and Fresno San Joaquin Valley the San Juaquin, Kings, Kaweah, Tule, and Kern Rivers. View is to the northeast. 120 Squares show Ap-He data; circles show 110 Z-He data, with closed circles represent- 100100 ing samples with a Z-He age significantly Age (Ma) younger than the local pluton ages, and 90 90 open circles representing ages overlapping 80 80 Z-He with local pluton ages (ca. 120 Ma), as dis- 70 70 cussed in the text.

(U-Th)/He 60 Ap-He 0 km 50 100 150 km Distance (km)

but in several locations, we sampled from isolated bedrock outcrops scattered cantly younger than the local pluton ages (Table 1). The second group (n = 3) amongst orchards. Published U-Pb zircon ages along this transect are gener- has an average Z-He age of ca. 123 Ma. Two of these samples come from Early ally 115 ± 10 Ma and range from 102 Ma to 125 Ma (Saleeby and Sharp, 1980; Cretaceous plutons with emplacement age ca. 120 Ma, and the third is not near Chen and Moore, 1982; Clemens-Knott and Saleeby, 1999; Lackey et al., 2005). a published zircon U-Pb age. We assume that these Early Cretaceous helium Average bulk Ap-He ages along this transect range from 69 Ma to 80 Ma ages were set during conductive cooling of their host plutons (ca. 120 Ma) and (Fig. 8), with an overall average of 74 ± 4 Ma (1 standard deviation, s.d.). Aver- remained cooler than the Z-He PRZ during the later plutonism (ca. 115–105 Ma).

age eU amongst these samples is unusually high at 116 ppm. A few samples 500 contained grains with highly divergent eU concentrations (see the Supplemen- tal File [footnote 1]). The best such example is sample 11SS9, which includes 400 four individual ages averaging 75 ± 2 Ma (1 s.e.), with a range in eU from ) 42 ppm to 447 ppm (Fig. 9). According to the radiation damage accumulation and annealing model (RDAAM of Flowers et al., 2009), the large difference in 300 eU amongst these grains means that they must have substantially different closure temperatures owing to variations in accumulated radiation damage. 200 On its face, the agreement amongst the ages of sample 11SS9 apatite grains fective Uranium (ppm

(all are within ~10% of the mean; see the Supplemental File [footnote 1]) sug- Ef gests that cooling through the helium partial retention zone (PRZ) occurred 100 quickly. However, even though the range is small, 11SS9 is in fact the only sample in our suite that shows a compelling age versus eU correlation (Fig. 9). 0 We incorporate the RDAAM model into our thermal modeling to extract de- 65 70 75 80 85 tailed t-T information from the large variation in eU of individual grains from Corrected Age (Ma) this sample. This modeling is presented in a later section of this paper. All Figure 9. Age versus effective uranium (eU) concentration individual grain zircon and apatite (U-Th)/He data are tabulated in the Supple- for individual grain analyses from sample 11SS9 (36.716°N, mental File [footnote 1]. 119.354°W). Note the very large spread in eU concentration and positive correlation between age and eU. Raw data are dis- Mean Z-He ages from these samples fall into two distinct populations cussed in the text and included in the Supplemental File (see (Fig. 8). The first (n = 4) has an average age of 91 ± 8 Ma (1 s.d.) and is signifi- footnote 1).

THERMAL MODELING The model outputs show the results of this post–“burn in” period. QTQt can simultaneously apply this iterative process to find a most likelyt -T path with Modeling Approach and Setup multiple different data inputs (Ap-He, Z-He, Ap-4He/3He). For each sample we input all of the available helium data into QTQt. For a detailed review of QTQt To extract quantitative information from the helium data, we utilize the and its relation to other thermal modeling software packages, see Vermeesch thermochronologic modeling software QTQt to obtain t-T histories of individ- and Tian (2014). ual samples (Gallagher, 2012). QTQt employs a trans-dimensional Bayesian We also impose a minimal set of manually controlled thermal his- Monte Carlo Markov chain (MCMC) statistical approach to find the bestt -T tory constraints. Where available, we use published zircon U-Pb ages as paths for a sample by employing a large number of iterative perturbations high-temperature constraints (650 ± 100 °C; for citations, see Table 1); else- (we use at least 106 iterations). After each perturbation, the model compares where we used 115 ± 10 Ma, which encompasses the observed range of U-Pb the proposed path to the initial path and chooses the better-fitting of the two zircon ages. We input a reasonable bounding box of temperature and time according to an acceptance criterion (Gallagher, 2012). The model converges for the model to explore (150 ± 135 °C, 120 Ma to present) and a rough esti- on the best-fitt -T path through this process during the “burn in” period mate of modern mean annual surface temperature (20 ± 5 °C) as a present- (Gallagher, 2012). For each of our model runs the “burn in” period consists day temperature constraint. For samples 11SS1 and 11SS6 we also input a of at least 5 × 105 iterations (after Vermeesch and Tian, 2014). After the model low-temperature constraint corresponding to the age of the overlying rock has converged on the best-fitt -T path, we run a set of 5 × 105 post–“burn units (40 ± 5 Ma; 20 ± 5 °C). Table 2 lists the details of the inputs for each in” model iterations to document the distribution of best-fitt -T histories. model run.

TABLE 2. SUMMARY INFORMATION FOR ALL MODEL RUNS Modern No. of # of post– 4He/3He Time-temperature High temperature Low temperature temperature pre–“burn in” “burn in” Sample N* present? bounding box constraints constraints constraints iterations iterations ap 6 (average) Yes 120–0 Ma 102± 5 Ma 40 ± 5Ma (Walker Fm.) 11SS1 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C20± 5°C ap 4No 120–0 Ma 115± 10 Ma 11SS2 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C ap 4No 120–0 Ma 120± 5 Ma 11SS4 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C ap 7No 120–0 Ma 110± 5 Ma 11SS5 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C ap 7 (average) Yes 120–0 Ma 114± 5 Ma 40 ± 5Ma (Ione Fm.) 11SS6 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C20± 5°C ap 4No 120–0 Ma 120± 5 Ma 11SS7 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C ap 4No 120–0 Ma 114± 5 Ma 11SS8 None 20 ± 5°C ≥500,000500,000 zr 3 (average) No 150 ± 135°C650 ± 100°C ap 4No 120–0 Ma 115± 10 Ma 11SS9 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C 120–0 Ma 114± 5 Ma 11SS10 ap 4No None 20 ± 5°C ≥500,000500,000 150 ± 135°C650 ± 100°C ap 4No 120–0 Ma 120± 5 Ma 11SS11 None 20 ± 5°C ≥500,000500,000 zr 4 (average) No 150 ± 135°C650 ± 100°C Note: See Table 1 for geographic location of each sample. See text for explanation of pre–“burn in” and post–“burn in” model iterations. ap—apatite; zr—zircon. *N is number of single-grain He analyses used for each sample.

Modeling Results Calcite Clumped Isotope Thermometry and Stable Isotopes

For samples 11SS1 and 11SS6, we input the mean Z-He age as well as the We measured the clumped isotope composition of carbonate from an al- Ap-4He/3He data linked to the mean Ap-He age into the QTQt model (Table 2). tered bedrock sample (11SS1) using a well-documented general procedure The thermal modeling results from sample 11SS6 show rapid cooling ca. for determination of the temperature dependent mass-47 anomaly (D47) of 95–85 Ma to <55 °C followed by slow cooling to surface conditions by 40 Ma carbonate samples by automated digestion, online purification, and measure- (Fig. 10). This period of slow cooling corresponds to an erosion rate of roughly ment by dual-inlet gas-source mass spectrometry (e.g., Eiler, 2011; Dennis et al., 30 m/m.y. (55 °C to 20 °C from 85 to 40 Ma with an assumed geothermal gradi- 2011). Two samples of whole-rock material, 63.0 and 99.7 mg, were powdered ent of 25 °C/km [Brady et al., 2006]). The model for sample 11SS1 is consistent to <106 µm and reacted under vacuum in separate McCrea-style vessels with with rapid cooling from 95 to 85 Ma to ~100 °C and slow cooling from ~100 °C 10% phosphoric acid for 24 h at 25 °C to react all calcite in the sample (McCrea,

to ~20 °C from 85 Ma to 40 Ma, implying an erosion rate of roughly 70 m/m.y. 1950). Evolved CO2 was extracted from the vessels and separated from water (Fig. 10). Both of these model results are consistent with zero additional cool- by conventional cryogenic methods on a glass vacuum line. A second reaction

ing after exhumation to the surface ca. 40 Ma. step for 24 h at 50 °C yielded no CO2, indicating that no dolomite was present We ran an individual model for each of the other eight samples along the in the sample (Al-Aasm et al., 1990). Based on manometric measurements of 4 3 SSNP, none of which have Ap- He/ He data. Individual single-grain Ap-He CO2, carbonate contents of the 63.0 mg aliquot and the 99.7 mg aliquot were ages and the mean Z-He age for each sample were input into these models calculated to be 3.39 wt% and 3.35 wt%, respectively. This calculation assumes (Table 2). Figure 11 shows a compilation of the acceptable t-T paths for each of that all carbonate was stoichiometric calcite and digestion of calcite proceeded these model runs, and the results from these individual models and their fits to completion. Due to the excellent agreement of the percent carbonate values, to Ap-He data are included in the Supplemental File [footnote 1]. The model re- we conclude that these assumptions are correct.

sults are consistent with the results of samples 11SS1 and 11SS6, with samples In order to obtain sufficient CO2 for a single mass-spectrometric measure- cooled rapidly from temperatures hotter than the Z-He closure temperature ment, these separate gas aliquots were combined into a single break-seal. The

(~190 °C) to between 100 °C (11SS1) and 55 °C (11SS6) from 95 to 85 Ma. Slow composite sample CO2 was purified on an automated system that includes cooling to the surface at rates consistent with the erosion rates determined multiple cryogenic steps and a pass through a Poropak-Q 120/80 GC column from samples 11SS1 and 11SS6 (30–70 m/m.y.) occurred from 85 Ma to 40 Ma. in a He carrier gas to remove potential organic contaminants, and measured on a Thermo Scientific MAT 253 gas-source mass spectrometer at Caltech. The results were projected into the absolute reference frame using standard equili ADDITIONAL DATA FROM SAMPLE 11SS1 brated gases measured during the same week-long analytical session (Dennis et al., 2011). 13 Anatase (U-Th)/He Chronometry The composition of carbonate in sample 11SS1 is: d Cvpdb = –10.70‰ ± 18 0.01‰ (vpdb, Vienna Pee Dee belemnite), d Ovsmow = 14.22‰ ± 0.01‰ (using One of our samples (11SS1) is an altered granitic rock hosting a mineral the carbonate-acid fractionation from Swart et al. [1991]; vsmow, Vienna stan- assemblage of quartz, plagioclase, calcite, chlorite, anatase, and brookite. dard mean ocean water), and D47 = 0.509‰ ± 0.012‰ (all 1s s.e.). Using our

Crystalline anatase (TiO2) grains were separated from this sample using the in-house high-temperature calibration, this corresponds to a crystallization same procedures as for apatite. A stereomicroscope was used to pick indi- temperature of 103 ± 8 °C (Bonifacie et al., 2011). We infer from the texture of vidual grains roughly 100 mm wide and 200 mm long, chosen based on size, the sample (Fig. 7) that the calcite, anatase, and brookite likely grew together morphology, and lack of visible inclusions. Euhedral grains had a tetragonal during the same period of alteration. dipyramidal morphology and an orange color. Individual grain degassing fol- The occurrence of anatase and calcite in sample 11SS1 from the southern- lowed the same procedure as for bulk apatite analyses. Due to our inability to most exposures of the SSNP surface offers another datum for the t-T history recover individual grains after degassing, we used separate grains for measur- of the surface. d18O of carbonate is dependent on growth temperature and the ing U and Th content, using the same dissolution and measurement procedure d18O of the water from which it grew. Assuming that the D47 value of this sam- as for zircon. We calculated a raw age using the average He, U, and Th con- ple was not modified by burial heating or rock-buffered recrystallization, this 18 centrations determined from several aliquots. An alpha correction was then calcite was in equilibrium with a fluid with a d Ovsmow of –0.5‰ to –2.4‰ (Kim applied using Ft calculated using a surface area–to–volume ratio determined and O’Neil, 1997). Such a composition is intermediate between low-latitude from the grains used for the analyses, a density of ~3.9 g/cm3, and our calcu- meteoric water (~–10‰ to –5‰; Sheppard, 1986) and plutonic rocks (5‰–12‰; lated Th/U ratio (after Farley et al., 1996; Ketcham et al., 2011). The He diffu- Taylor, 1968) and could have been produced by isotopic exchange of mete- sion kinetics of anatase are presently unknown, so we treat this as a minimum oric water with bedrock. The temperature of calcite formation and the isotopic anatase formation age. Table 3 lists anatase U, Th, and He data. composition of the carbonating fluid strongly suggest that the sample was

11SS6 Model Results Time (Ma) 11SS1 Model Results Time (Ma) 100 Ma 80 60 40 20 0 Ma 100 Ma 80 60 40 20 0 Ma

15°C 15°C ) ) °C °C ( ( 40°C 40°C

60°C 1 60°C 1 y 0.8 0.8 y 80°C 0.6 80°C 0.6 Temperature Temperature Probabilit 0.4 0.4 Probabilit

0.2 0.2 100°C 100°C Figure 10. QTQt thermal model results 0 0 from samples 11SS6 (left) and 11SS1 (right). The upper panels show the prob- Model 4He/3He Fit Model 4He/3He Fit ability of passing through each pixel in time-temperature (t-T ) space as deter- He 1.4 He 1.4 mined by the acceptable t-T paths during 3 3 the post–“burn in” phase of the model run. 1.2 1.2 Note that light blue shaded regions at He /

He / right of each upper panel indicate the 4 4

1 1 possibility of some amount of reheating after 40 Ma (to 40 °C for sample 11SS6 0.8 0.8 and to 50 °C for sample 11SS1). The black line plotted on each of the upper panels 0.6 0.6 shows the average t-T path resultant from the model. This average t-T path directly results in the model-fit apatite 4He/3He 0.4 0.4 spectra shown as thick lines in the middle Measured Spectrum panels. For comparison with the model fit, Bulk Normalized Measured Spectrum 0.2 Bulk Normalized 0.2 black outlined boxes show the measured Model Fit (expected t-T) Model Fit (expected t-T) spectra. The lower panels show the histo- 0 0 grams of the accepted (Acc.) ages during 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 the post–“burn in” model iterations (each Fraction 3He Fraction 3He totals 500,000 iterations). For compari- Model Ap-He Age Fit Model Ap-He Age Fit son, the average bulk apatite (U-Th)/He (Ap-He) age for the sample is overlain as a vertical black line with two standard error Measured Age Measured Age (s.e.) bars plotted as dashed black lines. 91.6 ± 8.8 Ma (2 s.e.) 66.0 ± 7.0 Ma (2 s.e.) Relative Probabilty Relative Probabilty

Histogram of Histogram of Acc. Model Ages Acc. Model Ages

58.0 87.0 115.9 45.5 60.6 75.8 90.9 Corrected Age (Ma) Corrected Age (Ma)

Time (Ma) formation and Z-He closure during 95–85 Ma rapid exhumation. Saleeby et al. 100 Ma 80 60 40 20 0 Ma (2010) hypothesized that ca. 90 Ma major west-dipping low-angle normal faults regions of acceptable t-T paths drove rapid exhumation along the west margin of the Sierra Nevada batholith. 20°C We further posit that such an extensional regime would have fostered hydro- )

°C thermal alteration of the actively exhuming basement surface as large normal ( S6 40°C 11S faults penetrated plutons along the west margin of the batholith that were still warm from primary heat, and such faults climbed further upwards in the crust 1 S to tap meteoric water sources. 60°C S Near Surface 1 1 40 Ma–0 Ma

80°C Temperature y. DISCUSSION /m. m 0 7 100°C y. North and South Ends of the Horizontal Transect 30 m/m. Slow Cooling Rapid Cooling 85 Ma–40 Ma Thermochronometric and stratigraphic data tightly constrain the thermal 95 Ma–85 Ma history of the bedrock exposed at the north end of the horizontal transect near Figure 11. Compilation of time-temperature (t-T ) regions acceptable to each of Friant (sample location 11SS6). The tonalite of Blue Canyon was emplaced ca. the individual sample model runs. Gray areas on the plot show the two standard 114 Ma (Chen and Moore, 1982). Thermal modeling results are consistent with deviation interval around the expected t-T path of the model result for each of the samples along the southern Sierra Nevada pediment, excluding the two ends rapid cooling through Z-He and Ap-He partial retention zones ca. 95–85 Ma. (samples 11SS1 and 11SS6). For each of these two ends, the average expected From 85 Ma to 40 Ma, slow cooling to the surface occurred at a rate of roughly t-T path is overlain (same as plotted in the upper panels of Fig. 10). Black arrows 30 m/m.y. Around 40 Ma this bedrock was deeply weathered and at earth-sur- annotated below the plot show the three main phases consistent to all of our face conditions when nonconformable deposition of the Ione Formation be- thermal modeling results: rapid cooling ca. 95–85 Ma, slow cooling 85–40 Ma, and very little to no cooling 40–present. See text for further discussion. gan. Since 40 Ma, no basement exhumation has occurred at this location. The modeling results (Fig. 10) and the lack of age versus eU correlation (see the Supplemental File [footnote 1]) within Ap-He data at this location are strong subject to substantial alteration through interaction with a hot fluid of meteoric evidence that the overlying Tertiary section at this location was never thick origin. This is consistent with the mineralogy and fabric of the sample, which enough to disturb Ap-He ages in the underlying bedrock. Thermal modeling is highly altered, hosting a mineral assemblage of quartz, plagioclase, chlorite, shown in Figure 10 indicates that samples were not heated above 40–50 °C and calcite intergrown with anatase and brookite (Fig. 7). after 40 Ma, which corresponds to a maximum possible thickness of roughly We interpret this hydrothermal alteration to be related to the early rapid ex- 1 km of cover. humation of the sampled area ca. 95–85 Ma. Our field observations and those Data from the southern end of the horizontal transect near Fountain Springs of Saleeby and Sharp (1980) indicate structural and petrologic continuity be- similarly constrain the thermal history of the bedrock at this location. Pluton tween the 11SS1 sample site and the sites of the 102 Ma U-Pb zircon ages for emplacement occurred at ca. 102 Ma, followed by rapid cooling to ~100 °C the Fountain Springs tonalite (Lackey et al., 2005; Saleeby and Sharp, 1980). If ca. 85 Ma, after which slow cooling to surface conditions occurred at a rate we then bracket this igneous crystallization age with the 97 ± 13 Ma anatase of roughly 70 m/m.y. until deposition of the overlying tuff at 40.1 ± 0.3 Ma (U-Th)/He age and the 85 ± 5 Ma Z-He age, we find that based on thermal (Saleeby et al., 2016). modeling of the conductive cooling of the Sierra Nevada batholith (Barton The bulk ages from samples at the northern and southern ends of the and Hanson, 1989), the hosting tonalite pluton (at 3–4 kb conditions) retained SSNP are quite different (average Ap-He is 92 Ma at sample location 11SS6 enough primary heat to render the thermal conditions for anatase + calcite and 66 Ma at 11SS1). Despite this difference, the QTQt thermal modeling

TABLE 3. ANATASE (U-Th)/He DATA FOR SAMPLE 11SS1 Uranium Thorium Helium Raw age Sphere equivalent radius Corrected age (ppm) (ppm) (nmol/g) (Ma) (µm) Ft (Ma) 18.7 13.2 8.772± 10 44.7 0.74 97 ± 13 Note: See Table 1 for geographic location of sample 11SS1. Ft—alpha ejection correction factor.

allows us to interpret them both in the context of the same general t-T history. Because of the old Ap-He ages and the presence of 500-m-scale relief within In conjunction with Ap-4He/3He data for each sample, the models reveal that the modern landscape, we prefer a different model. Integrating the results of their thermal histories are both consistent with the same three phases: rapid Saleeby and Foster (2004), Stock et al. (2004, 2005), Saleeby et al. (2012, 2013a), cooling 95–85 Ma, slow cooling 85–40 Ma, and no cooling 40 Ma to present. and Cecil et al. (2014), we hypothesize that ca. 40 Ma, Cenozoic sediments cov- The thermal modeling indicates that the significant divergence in their ages is ered the SSNP and preserved the ca. 40 Ma landscape. A sedimentary thick- due to the different rates of slow cooling from 85 Ma to 40 Ma (~70 m/m.y. in ness on the order of several hundred meters could have completely buried the the south and ~30 m/m.y. in the north), rather than a different timing of rapid modern relief without resetting Ap-He ages. This thickness is of the same order cooling. These different slow erosion rates resulted in an additional ~2 km of of magnitude as that of the Cenozoic section in the foothills of the northern erosion at 11SS1 from 85 to 40 Ma compared to 11SS6. While the data and Sierra (Bateman and Wahrhaftig, 1966). modeling results do not constrain the mechanism for this difference in slow The overlying sediments were likely removed during late Pliocene–Pleisto- erosion rates, it is noteworthy that these erosion rates bracket the estimates cene erosion consistent with model predictions by Saleeby at al. (2012, 2013a) from the axial part of the southern Sierra during the same period of time, 40–60 and in conjunction with the pulse of river incision documented by Stock et al. m/m.y. (House et al., 1997, 2001; Clark et al., 2005; Mahéo et al., 2009; Sousa (2004, 2005). This erosion may have been due to a combination of factors in- et al., 2016). We interpret this close agreement to indicate that the difference cluding climate change related to ice age onset (e.g., Bintanja and van der between these slow erosion rates is minor relative to the major phases of ther- Wal, 2008) and surface uplift potentially driven by dynamic processes related mal history that are clearly present along the entire SSNP. to the Isabella anomaly (Zandt et al., 2004). This erosion removed the overlying sediments and weathered basement which are significantly more erodible Thermal History of the Southern Sierra Nevada Pediment than intact basement rocks (Sklar and Dietrich, 2001). This revealed the more resistant bedrock pediment and its ferruginous silcrete carapace preserved as All of the thermal modeling results are consistent with the same three- the fossil landscape below, re-exposing it as the modern landscape. phase style of cooling history. Our primary conclusion about this history is that the SSNP was rapidly cooled to between 100 °C and 55 °C ca. 95–85 Ma, Potential Cause and Mechanism of Early Rapid Exhumation and then slowly cooled and exhumed to near the surface by 40 Ma. On average, if the entire length of the pediment were at the surface ca. 40 Ma, then From ca. 95 to 85 Ma, the southernmost Sierra Nevada–Mojave segment the cooling rate from 85 Ma to 40 Ma would have been roughly 30–70 m/m.y. of the Cretaceous arc gravitationally collapsed and the batholith was rapidly (35 to 80 °C of cooling over 45 m.y. with a geothermal gradient of 25 °C/km). exhumed to depths equivalent to ~10 kb (e.g., Chapman et al., 2012; Saleeby, In our interpretation, the same batholithic swath exposed for an extended 2003; Saleeby et al., 2007). Liu et al. (2010) argued for a dynamic link between period of erosion and chemical weathering prior to 40 Ma is again exposed this event and the subduction of a large oceanic plateau which impinged on as the modern bedrock landscape; i.e., it is a Paleogene fossil landscape. This the Cretaceous subduction zone ca. 90 Ma. Based on the mating of basement raises the question: How could this landscape have survived this time interval core petrography and geochronology, and deep seismic data for the Great Val- without significant erosion occurring? ley subsurface immediately west of the SSNP, Saleeby et al. (2010) hypothe- The climatic conditions conducive to the chemical weathering required sized that ca. 90 Ma major west-dipping low-angle normal faults drove rapid to form the types of nickel laterite occurrences along the SSNP are roughly exhumation along the west margin of the Sierra Nevada batholith. The rapid >1000 mm/yr annual precipitation with cold month mean temperature rang- exhumation required by our thermal modeling from 95 to 85 Ma is in gen- ing from 15 to 27 °C (Thorne et al., 2012). Modern conditions along our study eral agreement with the timing hypothesized by Saleeby et al. (2010). Together area are not warm or wet enough to meet these criteria. However, previous these multiple lines of evidence suggest that the deep exhumation of the Cre- workers invoked warmer and wetter global conditions in the Eocene (Pear- taceous arc to the south was not spatially limited to the southernmost Sierra– son et al., 2007) to explain the formation of middle Eocene paleo-Oxisols Mojave region. We hypothesize that rapid exhumation of the SSNP from 95 to within the Ione Formation in central California (Yapp, 2004) and developed 85 Ma is a northern, lower-magnitude manifestation of this same event. on bedrock beneath the middle Eocene section in Baja California, Mexico (Abbott et al., 1976). In drier and cooler climates, these paleosols can be re Regional Implications for Southern Sierra Nevada Evolution sistant to weathering and could potentially last for millions of years at the earth surface (e.g., Bierman and Turner, 1995). However, where plutonic bed- With the exception of the area south of the Kern River (Mahéo et al., 2009), rock crops out we expect that erosion would have been too fast to preserve prior to this study no low-temperature thermochronometric data had been the landscape for ~107 yr. In other words, if this landscape had been exposed published from the southern Sierra foothills. The westernmost published data continuously since 40 Ma we would expect the Ap-He ages to be younger that were available for the region between 36°N and 37°N were a few Ap-He due to continued cooling. ages along the main trunk and north fork of the Kings River, ~30 km east of our

study area (Sousa et al., 2016), and a horizontal transect (referred to as T1 in The first phase of this chronology is the emplacement of the southern Sierra House et al., 1998) between the Kaweah and San Joaquin drainages running Nevada batholith ca. 115 ± 10 Ma in our study area, and ending at 85 Ma in the ~45 km east of our study area (House et al., 1998). eastern part of the range. During the final stages of magmatism (95–85 Ma), The southern Sierra Nevada is significantly different from the northern the bedrock swath along our study area was rapidly exhumed (shown in red Sierra, both geologically and physiographically. With the exception of the shades in Fig. 12) to ~3–4 kb levels and ~55 °C (in the north) and 100 °C (in the Eocene rocks near Friant and Fountain Springs, the southern Sierra almost com- south). This exhumation is roughly contemporaneous with, and likely geneti pletely lacks the Paleogene deposits that are common in the northern Sierra cally related to, the profound tectonic exhumation and gravitational collapse (e.g., Busby et al., 2016). The southern Sierra also lacks the distinctive western of the southernmost Sierra–Mojave region to the south (Chapman et al., 2012). ramp morphology that characterizes the north. Instead, the southern Sierra After the cessation of magmatism and early rapid exhumation, the entire rapidly attains elevations of roughly 2000 m across a series of topographic SSNP slowly cooled at rates roughly the same as those of the axial part of the steps (e.g., Hake, 1928). Despite these differences between the northern and range from 85 to 40 Ma to near-surface conditions (shown in brown shades on southern Sierra and the complete lack of thermochronometric data from the Fig. 12). Combined with igneous barometric emplacement pressures of 3–4 kb western foothills, previous workers leaned on the assumption that throughout (Ague and Brimhall, 1988; Nadin et al., 2016), the thermochronologic data in- the Cenozoic the southern Sierra has behaved similarly to the northern Sierra, dicate that the early rapid exhumation (95–85 Ma) accounted for ~8–9 km of as a rigid west-down tilt block with a hinge line lying close to the western exhumation, while the slow erosion from 85 to 40 Ma accounted for the final foothills–San Joaquin Valley boundary (e.g., Wakabayashi and Sawyer, 2001; 2–3 km of exhumation. Previously published thermochronometric and igneous House et al., 1998). Sousa et al. (2016) showed that the rigid-block assumption barometric data (House et al., 1997, 1998, 2001; Clark et al., 2005; Ague and is incorrect in the vicinity of Kings Canyon, where a kilometer-scale west-down Brimhall, 1988) from higher elevations along the axial southern Sierra Nevada normal fault was active in Eocene time, and suggested that the WSFS was further suggest that the early phase of exhumation also included most of the likely active in 45–40 Ma along the entire span of the southern Sierra from the rest of the southern Sierra Nevada batholith (roughly 3–4 km of early exhuma- San Joaquin River to the Kern River. This fault activity was part of a tectonic re- tion in the axial part of the range). gime marked by uplift and extension within the coupled Sierra Nevada–Great In the axial Sierra, the extended period of slow erosion (85–40 Ma) resulted Valley region, including uplift of the axial southern Sierra and shallowing of the in the initial form of the modern Sierra, including the low-relief interfluvial proximal Great Valley forearc (Bartow, 1992; Sousa et al., 2016). highlands (Clark et al., 2005), and the long-wavelength (>10 km) large-ampli- What was happening in the foothills during this time? We hypothesize tude (>1 km) topographic relief that is visible on digital elevation models (e.g., that contemporaneous with this regional tectonic event ca. 45–40 Ma, some Fig. 5; House et al., 1998, 2001). In the foothills, this resulted in formation of uplift and exhumation should have occurred in the foothills. Because our He bedrock pediment morphology as well as the distinctive nickel laterite occur- ages are all older than the time of this hypothesized exhumation, we conclude rences discussed earlier in this paper. that this exhumation was not of sufficient magnitude to noticeably disturb the Around 45–40 Ma, activity on the WSFS resulted in extension and uplift Ap-He and Ap-4He/3He data along the bedrock pediment. Based on the QTQt of the axial southern Sierras and kilometer-scale incision in the major south- modeling for samples 11SS1 and 11SS6, we estimate that there could not have ern Sierran trunk river canyons (shown in green shades on Fig. 12; Sousa been more than roughly 500 m of exhumation ca. 45–40 Ma. et al., 2016). To the west of the WSFS, in the foothills, a small amount of ex- The overlying Eocene rocks at the northern and southern termini of our humation may have occurred (roughly a few hundred meters) in conjunction study area closely follow the timing of this event (deposition beginning ca. with shallowing of the proximal Great Valley forearc (Bartow, 1992; Sousa 40 Ma). Combining the thermal modeling and the evidence from the overlying et al., 2016). From 40 Ma through the late Neogene, slow erosion continued Eocene deposits, we conclude that a few hundred meters of exhumation could in the axial southern Sierras, and a shallow cover of Cenozoic deposits likely have occurred in the foothills ca. 45–40 Ma in conjunction with shallowing of armored the SSNP. Post–10 Ma, as a result of the convective removal of the proximal Great Valley forearc to the west and axial Sierran fault-controlled dense sub-batholithic mantle lithosphere, epeirogenic deformation partially uplift to the east (Bartow, 1992; Sousa et al., 2016). re-exposed the SSNP to its current state (shown in yellow shades on Fig. 12). The shallow cover armoring the SSNP eroded, exposing the ancient bedrock landscape, and uplift in the axial southern Sierra resulted in the Summary of the Chronology of Southern Sierra Nevada incision of the inner slot canyons common to the major Sierran trunk rivers Landscape Evolution and Tectonic Forcing (Stock et al., 2004). Active upper-mantle dynamic processes are resulting in Pleistocene to Holocene uplift of the Kern Arch and coupled subsidence of Integrating our new data with Eocene activity on the WSFS, as well as the Tulare Basin (Fig. 1) as the most recent phases of epeirogenic deforma- other previously published data, we piece together a chronology of tectonic tion (Zandt et al., 2004; Saleeby et al., 2012, 2013a; Saleeby and Foster, 2004; and landscape evolution for the southern Sierra Nevada outlined in Figure 12. Cecil et al., 2014).

85 Ma incision begins A B large-relief Late K C in WSFS footwall river canyons ca. 45–40 Ma

arc migration 11 5 Ma

WSFS rapid exhumation slow erosion WSFS hanging wall 95–85 Ma 85–40 Ma 45–40 Ma

slow erosion rapid incision of D after WSFS faulting E inner slot canyons, 40–10 Ma <10 Ma Plio-Pleistocene incised inner canyon

Explanation erosion, structures, relief related to: rapid cooling from 95–85 Ma

slow cooling from 85–10 Ma

WSFS activity 45–40 Ma Re-exposure of SSNP <10 Ma mantle lithospheric dynamics Eocene deposition N post–10 Ma surface expression of ca. 40 Ma Isabella anomaly

Figure 12. Cartoon of the summary chronology of tectonic and landscape evolution of the southern Sierra Nevada (central California, USA) as described in the text. This schematic is not intended to represent a specific southern Sierra canyon, but rather to depict the different phases of evolution applicable across the southern Sierra. Erosion (arrows), structures (lines), and relief (shaded areas) are color coded according to which phase of southern Sierra Nevada evolution each is related. Color codes, as shown in explanation at lower right, are: red, 95–85 Ma rapid cooling (this study); brown, slow cooling 85–10 Ma (this study); green, Western Sierra fault System (WSFS) activity 45–40 Ma (Sousa et al., 2016); and yellow, post–10 Ma mantle lithospheric dynamics (Saleeby et al., 2013a; Stock et al., 2004). (A) Late Cretaceous magmatism and pluton emplacement migrating eastward from beneath the Great Valley at 140 Ma to the axial southern Sierra at 85 Ma, with the locus of magmatism passing the southern Sierra Nevada pediment (SSNP) ca. 115–110 Ma. Rapid exhumation and initial generation of large-relief river canyons (e.g., House et al., 1998) occurs between A and B. (B) From 85 to 40 Ma, large-relief river canyons, low-relief highland plateaus (Clark et al., 2005), and main trunk river canyons slowly erode (K—Cretaceous). (C) From ca. 45 to 40 Ma, kilometer-scale west-down normal fault activity on the WSFS occurs, triggering the beginning of fluvial incision. Major incision occurs between C and D. (D) Around 40 Ma, depositional armoring of the foothills and continued slow erosion of the axial southern Sierra occurs. (E) Post–10 Ma, erosional removal of depositional armoring of the SSNP and rapid incision of inner slot canyons result from mantle lithospheric dynamics and possible Plio-Pleistocene climate change related to ice age onset. The axial part of the range includes main trunk river canyons originally established in the Late Cretaceous but rejuvenated in both the Eocene and late Cenozoic. Topographic steps separating the axial part of the range from the foothills are eroded fault scarps of the WSFS. The southern Sierra Nevada pediment was exhumed and eroded pre–40 Ma, armored in mid-Cenozoic time, and re-exhumed as a fossil landscape post–10 Ma.

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This Al-Aasm, I.S., Taylor, B., and South, B., 1990, Stable isotope analysis of multiple carbonate sam- landscape evolved during a prolonged period of erosional modification and ples using selective acid extraction: Chemical Geology: Isotope Geoscience Section, v. 80, chemical weathering from ca. 85 Ma to 40 Ma following a phase of rapid, prob- p. 119–125, doi:10.1016/0168-9622(90)90020-D. Barton, M.D., and Hanson, R.B., 1989, Magmatism and the development of low-pressure meta- ably tectonic exhumation along the western Sierra Nevada batholith between morphic belts: Implications from the western United States and thermal modeling: Geologi 95 and 85 Ma. Little to no net erosion has occurred along the length of the cal Society of America Bulletin, v. 101, p. 1051–1065, doi:10.1130/0016-7606(1989)101<1051: pediment over post–40 Ma time. MATDOL>2.3.CO;2. 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ACKNOWLEDGMENTS Chapman, A.D., Saleeby, J., Wood, D.J., Piasecki, A., Kidder, S., Ducea, M.N., and Farley, K.A., 2012, Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith, Cali- We thank Kerry Gallagher for assistance with setting up QTQt runs and Lindsey Hedges for help fornia: Geosphere, v. 8, p. 314–341, doi:10.1130/GES00740.1. with sample preparation, analyses, and a life full of friendship. Thanks to Guest Associate Editor Chen, J.H., and Moore, J.G., 1982, Uranium-lead isotopic ages from the Sierra Nevada Batho Cathy Busby and two anonymous reviewers for constructive reviews of this manuscript. This work lith, California: Journal of Geophysical Research, v. 87, p. 4761–4784, doi:10.1029 was partially supported by the Gordon and Betty Moore Foundation through grant GBMF #423.01 /JB087iB06p04761. to the Caltech Tectonics Observatory. 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