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 land- scape. 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 © 2016 Geological Society of America GEOSPHERE | Volume 13 | Number 1 Sousa et al. | The southern Sierra Nevada pediment Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/1/82/1000780/82.pdf 82 by guest on 28 September 2021 Research Paper 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 Domen gine 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 !( !( !(!( !( N !( !( non-marine facies of the Ione at its type location (Palmer and Merrill, 1982; !( !(!( !( !( F !( !( !( 37° 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)