Stream profile analysis of the lower Kern River and its tributaries help to constrain the landscape response to late Cenozoic tectonics in the southern Sierra Nevada of California. In this study, we identify two relict landscapes that have been offset from the Kern Plateau by periodic displacement along the southern Sierra Nevada fault system starting ca. 20 Ma. These remnants provide context from which to evaluate existing models of rock uplift and exhumation in the region. Reconstructed channel profiles on the relict landscapes indicate a slow incision rate of ∼0.07 mm/yr throughout most of the Miocene. An increase in incision ca. 6 Ma resulted in the formation of the Kern Canyon. This increase in incision likely occurred in response to east-west delamination of a lithospheric root beneath the southern Sierra Nevada and is reflected in a pulse of vertical incision (∼0.11 mm/yr) that has propagated upstream at a rate of ∼7.3 mm/yr to its current position near Isabella Lake. Another two pulses of rapid incision were likely generated by increased rock uplift associated with the northward migration of the delamination hinge after 1 Ma. Incision rates of 0.58–1.2 mm/yr have propagated up the lower Kern River resulting in formation of the Kern River gorge. These new constraints on the incision history of the lower Kern River provide further corroborating evidence for existing models of late Cenozoic mantle delamination and associated epeirogenic processes that have helped shape the landscape of the southern Sierra Nevada.
The Sierra Nevada is a prominent NW-trending mountain range coupled with the Great Valley in central California (Fig. 1). The mountain-valley system makes up the Sierra Nevada microplate, which is a semi-rigid crustal block measuring ∼600 km long (north-south) and 250 km wide (east-west). The Sierra Nevada is an exhumed Mesozoic batholith emplaced within largely Neoproterozoic–Mesozoic metavolcanic and metasedimentary pendants and wall rock (Saleeby et al., 2008; Chapman et al., 2012; e.g., Saleeby et al., 2008; Chapman et al., 2012). The mean peak elevations gradually increase from north to south to Mount Whitney (4421 m) and then rapidly decrease southward (Wakabayashi and Sawyer, 2001). Internal deformation within the Sierra Nevada microplate is largely concentrated between 35°N and 36.5°N latitude (Mahéo et al., 2009). This structural and topographic complexity is coincident with greater amounts of exhumation and higher rock uplift rates in the southern Sierra Nevada compared to those in the northern Sierra Nevada (Wakabayashi and Sawyer, 2001; Clark et al., 2005; Mahéo et al., 2009). Previous studies have used low-temperature thermochronometry and/or basin stratigraphy to place constraints on the nature and timing of this exhumation (e.g., House et al., 1997, 1998, 2001; Clark et al., 2005; Mahéo et al., 2009; Saleeby et al., 2012, 2013; Cecil et al., 2014). Several of these studies have identified the northwestward migration of a delaminating lithospheric root as a first-order control on late Cenozoic tectonic activity in the southern Sierra Nevada.
The Sierra Nevada has been deeply incised by major rivers that drain its western flank. Major rivers of the southern Sierra Nevada include, from south to north, the Kern, Tule, Kaweah, Kings, and San Joaquin Rivers. Several studies have used constraints on incision history, stream profile analysis, and tectonic geomorphology to investigate the landscape response to late Cenozoic tectonic activity (e.g., Stock et al., 2004, 2005; Clark et al., 2005; Figueroa and Knott, 2010). Stock et al. (2004, 2005) used cosmogenic 26Al/10Be burial dating of cave sediments to constrain pulses of incision and the development of relief along the Kings and Kaweah Rivers. Clark et al. (2005) used low-temperature thermochronometry and stream profile analysis of tributaries along the upper Kern and Kings Rivers to constrain surface uplift and incision of the high-elevation, low-relief landscape of the high Sierra. Figueroa and Knott (2010) used Sierran river longitudinal profiles and other geomorphic indices to identify an overall southward increase in relative tectonic activity since the late Pliocene. Results from each of these studies support a recent topographic response associated with the delamination of a lithospheric root beneath the southern Sierra Nevada.
The lower Kern River and its tributaries are uniquely positioned to best capture the landscape response to delamination of mantle lithosphere beneath the southern Sierra Nevada. While tributaries of the upper Kern River (upstream of Isabella Lake) primarily drain the high-elevation, low-relief landscape of the Kern Plateau, tributaries of the lower Kern River are confined to the western flank of the range (Fig. 1). Downstream from Isabella Lake, the lower Kern River continually steepens as it flows through the narrow Kern River gorge, an inner gorge of the Kern Canyon, and traverses the southern Kern arch region as it enters the San Joaquin Basin (Fig. 1). The Kern arch is a topographic promontory located at the transition between the western Sierra Nevada and San Joaquin Basin that has been interpreted to reflect late Quaternary epeirogenic uplift (e.g., Saleeby et al., 2012, 2013). The proximity of this major river network to the Kern arch means that the topographic signal of relative base-level fall and increased rock uplift potentially associated with lithospheric delamination should be recorded. In this study, we conduct stream profile analysis on the lower Kern River and its tributaries to: (1) quantitatively constrain the shape and steepness of longitudinal stream profiles, (2) identify topographic signatures associated with relict landscapes, and (3) locate knickpoints that may reflect transient signals associated with changes in rock uplift. These analyses allow us to place new constraints on the incision history of the lower Kern River and evaluate the landscape response to proposed mechanisms of rock and surface uplift in the southern Sierra Nevada.
GEOLOGY OF THE SOUTHERN SIERRA NEVADA
The Sierra Nevada Microplate
The Sierra Nevada and Great Valley make up an extensive coupled mountain-basin system known as the Sierra Nevada microplate. The Sierra Nevada microplate primarily consists of Late Cretaceous granitic-tonalitic plutonic rocks in the Sierra Nevada batholith and Cretaceous–Quaternary sedimentary rocks and deposits in the Great Valley (e.g., Saleeby et al., 2008; Chapman et al., 2012). The microplate is a generally westward-tilted block, which has experienced both counterclockwise (ca. 36–24 Ma) and clockwise (24 Ma to present) rotation due to the interaction between the Pacific and North American plates (Unruh, 1991; Unruh et al., 2003; McQuarrie and Wernicke, 2005). It is bound to the east by the Sierra Nevada frontal fault zone, a normal fault system that has generated a high-relief eastern escarpment. The Owens Valley fault system and Walker Lane are located immediately to the east and accommodate up to 25% of the relative Pacific–North American plate motion (e.g., Dixon et al., 2000; Bennett et al., 2003; Frankel et al., 2007; Lee et al., 2009). The rest of the relative plate motion is taken up along the western margin of the microplate by the right-lateral San Andreas fault system, which includes the San Andreas, Hayward-Calaveras, Rinconada, and San Gregorio–Hosgri faults (e.g., Dixon et al., 2000; Argus and Gordon, 2001; Bennett et al., 2003). The southern boundary of the Sierra Nevada microplate is defined by the left-lateral Garlock fault.
Internal deformation within the southern Sierra Nevada is more complex than in the rest of the microplate. South of ∼36.0°N latitude, the Sierra Nevada has a southward-sloping topographic gradient and local topographic relief caused by Cenozoic displacement along faults of the southern Sierra Nevada and Kern Canyon fault systems (e.g., Wakabayashi and Sawyer, 2001; Clark et al., 2005; Mahéo et al., 2009). The development of this modern landscape is thought to have been preconditioned by gravitational collapse of the southern Sierra Nevada during the Late Cretaceous (Chapman et al., 2012). Structural, topographic, thermobarometric, and thermochronometric data indicate a profound southward increase in tectonic and erosional exhumation that is largely attributed to this Late Cretaceous tectonic history (e.g., Clark et al., 2005; Mahéo et al., 2009; Chapman et al., 2012). Some of the exhumation, however, is attributed to erosion driven by transient epeirogenic uplift and subsidence as well as reactivation of Late Cretaceous structures during the Cenozoic (Stock et al., 2004; Clark et al., 2005; Mahéo et al., 2009; Cecil et al., 2014). This Cenozoic activity is interpreted to reflect the landscape response to progressive delamination of a lithospheric root beneath the southern Sierra Nevada and San Joaquin Basin (e.g., Saleeby et al., 2012, 2013; Cecil et al., 2014).
Lithospheric Root Delamination
Constraints provided for Neogene–Quaternary faulting, subsidence, and uplift within the southern San Joaquin Basin and Sierra Nevada have pointed to the delamination of a lithospheric root into the mantle. Volcanism, geophysical data, and sedimentary and topographic records suggest that a colder and denser lithospheric root is being replaced by hotter, more buoyant mantle as the delamination hinge migrates toward the northwest (Jones et al., 1994, 2004; Zandt et al., 2004; Saleeby et al., 2012, 2013; Cecil et al., 2014). Beneath the Tulare sub-basin is the Isabella anomaly, a vertical feature with high-velocity seismic structure and an anomalous gravity signature that is interpreted as the still-attached lithospheric root (Jones et al., 1994; Zandt et al., 2004). Adjacent to the Isabella anomaly is the Kern arch. The Kern arch is a prominent dome-like structure along the mountain-basin transition that is interpreted to be a region experiencing uplift and exhumation caused by mantle upwelling to replace the recently detached lithospheric root. The delamination hinge trace shown in Figure 1 corresponds to the surface trace of the hinge about which mantle lithosphere delamination has progressed to the present. Currently the hinge trace lies in the transition zone between the Tulare sub-basin and (1) the Kern arch to the southeast and (2) the Sierran foothills to the east and northeast (Saleeby et al., 2012).
Ample stratigraphic data from drilling operations in the San Joaquin Basin have been used to help constrain the geodynamic history of the southern Sierra Nevada region. Thermochronometric analyses of detrital apatite recovered from well core samples of Miocene sediments provide constraints on the subsidence and exhumation history of the Kern arch. Partially to completely reset (U-Th)/He ages and inverse thermokinematic modeling suggest that subsidence and sedimentation occurred between 6 Ma and 1 Ma and was immediately followed by rapid exhumation (Cecil et al., 2014). Focused exhumation within the Kern arch resulted in partitioning of sedimentation in the southern San Joaquin Basin, where erosional unroofing redistributed sediment to the west, south, and north (Saleeby et al., 2012, 2013, 2016). North of the Kern arch, thermochronometric, geomorphologic, and geodetic constraints indicate that the Tulare sub-basin has experienced continuous subsidence to present (Stock et al., 2005; Mahéo et al., 2009; Saleeby et al., 2013, 2016; Cecil et al., 2014). The cause of this subsidence is interpreted to be the result of a still-attached lithospheric root (e.g., Saleeby et al., 2012, 2013).
Southern Sierra Nevada Fault System
Internal deformation within the Sierra Nevada microplate is unique to the southern Sierra Nevada fault system (35.0°N to 36.5°N latitude), a Neogene–Quaternary fault system comprising the N-S–trending, west-side-up Breckenridge–Kern Canyon fault zone and NW-trending, west-side-down normal faults that propagate into the basin (Fig. 1) (Mahéo et al., 2009). The Breckenridge and Kern Canyon faults display evidence for normal displacement within the early Neogene and Quaternary and play an important role in contributing to the westward tilt of the southern Sierra Nevada region (Mahéo et al., 2009; Nadin and Saleeby, 2010). The NW-trending normal faults within the southern Sierra Nevada and Kern arch are interpreted to be in large part remobilized from Late Cretaceous basement shear zones, having been reactivated with west-side-down normal displacement in response to the passage of the slab window and ensuing delamination of the lithospheric root (Mahéo et al., 2009).
Prominent faults within the study area include the Breckenridge–Kern Canyon fault system and the West Breckenridge and Kern Gorge faults. The Breckenridge–Kern Canyon fault system is a west-side-up normal fault that has controlled the position of the upper Kern River and ponded sediments to form the Isabella and Walker intermontane basins (Mahéo et al., 2009; Nadin and Saleeby, 2010; Amos et al., 2010). The west-side-down West Breckenridge and Kern Gorge normal faults are interpreted to have accommodated extensional deformation in response to the passage of the slab window (Mahéo et al., 2009).
Geomorphology of the Southern Sierra Nevada
The Kern River watershed traverses much of the southern Sierra Nevada. The headwaters are proximal to the tallest peaks of the Sierra Nevada and flow southward, draining the high-elevation, low-relief landscape of the Kern Plateau. Downstream from Isabella Lake, the lower Kern River crosses the Kern Canyon fault and switches to a southwest flow direction; similar to the other major Sierran rivers to the north. Channel gradient along the lower Kern River increases with increasing drainage area, contrary to what we would expect from a channel in equilibrium with uplift rates (Whipple and Tucker, 1999). The lower Kern River continues to steepen as it flows through a narrow inner gorge (Kern River gorge) and traverses the Kern arch region before rapidly shallowing as it debouches into the San Joaquin Basin. Tributaries along the lower Kern River drain two prominent, perched, low-relief topographic domains: (1) a high-elevation domain within the Breckenridge, Greenhorn, and Piute Mountains, and (2) a lower-elevation domain near the mountain front–basin transition. Because it traverses both the Kern arch and western flank of the southern Sierra Nevada, trunk and tributary longitudinal channel profiles of the lower Kern River are uniquely suited to record landscape evolution potentially driven by lithospheric delamination.
STREAM PROFILES AND LANDSCAPE EVOLUTION
While this power-law relationship explains a channel system in steady state, deviations from a smooth concave-up form can be approached with the aim to extract signals of changing boundary changes (Wobus et al., 2006). The form of channel adjustment to tectonic perturbation (i.e., an increase in uplift rate) is through channel steepening, and thus a change in concavity, and can give insight into transient versus adjusted landscape within a drainage network. For instance, a channel profile that displays two distinct reaches with differing ks and θ values can be interpreted as the river displaying a relict, steady-state form within the upstream reach and a transient form along the downstream reach. The point that separates them, termed a knickpoint, migrates up the drainage network as the channel adjusts to the new set of boundary conditions (Snyder et al., 2000; Wobus et al., 2006; Kirby and Whipple, 2012).
Channel adjustment through trunk channel steepening and resultant erosional lowering establishes a knickpoint which then migrates upstream (Crosby and Whipple, 2006; Wobus et al., 2006). Channel lowering on the trunk channel serves as a local base-level fall for tributary channels. This produces a knickpoint that is initiated at the tributary junction and is translated upstream throughout the tributary watershed (Wobus et al., 2006). Although knickpoint migration rates are largely controlled by contributing drainage area, the vertical rate of knickpoint migration is constant, and therefore knickpoints located at similar elevations should reflect the same transient topographic signature (Wobus et al., 2006). The position of knickpoints within a fluvial network therefore allows for identification of relict topography, remnants of landscapes that have not yet adjusted to the new boundary conditions (Clark et al., 2005). In this study, we analyze the lower Kern River trunk channel and its tributaries to constrain the evolution of the southern Sierra Nevada landscape during the Quaternary.
Stream profile analyses of trunk and tributary channels of the lower Kern River were conducted to help characterize the potential landscape response to delamination of a lithospheric root beneath the southern Sierra Nevada region. Tributary channels were analyzed to constrain pulses of trunk channel incision along the lower Kern River. These tributary channels allow us to spatially and quantitatively identify how adjustment of the lower Kern River is translated throughout the drainage basin.
Data Collection and Preparation
We used digital elevation model (DEM) data obtained from the U.S. Geological Survey (USGS) 3D Elevation Program (3DEP) (www.nationalmap.gov). Multiple National Elevation Dataset (NED) 1 × 1 degree tiles with a resolution of 1/3 arc-second were mosaicked to create a DEM that covered the entire Kern River watershed. This raster data set was then projected using a North American Datum of 1983 geographic coordinate system and a Universal Transverse Mercator Zone 11S projected coordinate system. The resulting raster was resampled to produce an equidimensional DEM with a resolution of 10 m. The DEM was then prepared for stream profile analysis following the procedures outlined below and detailed by Whipple et al. (2007). The hydrology toolbox within ArcGIS software was used to fill pits in the DEM and generate flow direction and flow accumulation rasters for the study area. Pour points were selected for the Kern River trunk channel and tributary channel junctions that had flow accumulation values of at least 20,000 cells (2 × 106 m2 drainage area). A model builder tool was then developed in ArcGIS to generate watershed-specific folders, delineate watershed boundaries, clip DEM and flow accumulation rasters for each watershed, and convert from raster to ASCII format. The resulting .txt files were then converted to .mat format for stream profile analysis using MATLAB software.
Stream Profile Analysis
A stream profiler tool (http://geomorphtools.geology.isu.edu/) was utilized for stream profile analysis. The stream profiler tool integrates ArcGIS and MATLAB software to obtain θ, ks, and normalized steepness index ksn values along river channel profiles through regression of slope-area data extracted from a DEM (e.g., Wobus et al., 2006; Whipple et al., 2007). This analysis requires specification of a reference concavity θref. While the chosen θref value can drastically change the absolute values of ksn within an individual watershed, the relative value of ksn between watersheds remains consistent as long as the same θref is used. All analyses in this study utilized a θref of 0.45 to allow for inter-watershed comparison and to facilitate comparison with other studies (Kirby and Whipple, 2012). The units for ksn values (m0.9) originate from dimensional analysis of the slope versus drainage area relationship and are tied to the value of θref (Wobus et al., 2006; Ouimet et al., 2009).
The stream profiler tool was run in two separate modes for the Kern River and each of the lower Kern River tributary watersheds. The automatic ksn mode (batch profiler code) automatically calculates ksn values through regression of slope-area data within a window that moves along the channel network. These calculations are performed along all channels within the channel network that drain a user-specified minimum critical area. In this study, we used a minimum critical area of 1 × 106 m2 for analysis of the entire Kern River watershed and each of the lower Kern River tributary watersheds. Maximum ksn and watershed averaged ksn values were determined for each of the watersheds analyzed.
The manual ksn mode calculates ksn and θ values through regression of slope-area data between user-specified regression bounds along individual channels. This mode also allows for the identification and marking of knickpoint locations for later importation into ArcGIS. Regression bounds and knickpoint locations were selected based on visual inspection of the channel longitudinal profile and plots of channel gradient versus drainage area and channel gradient versus distance from the mouth. To identify the presence of relict topography, we chose to focus on prominent knickpoints that separate channel reaches with markedly different characteristics. These knickpoint locations were marked, and the bounds of the upstream reach were selected for regression. This analysis was performed on the trunk channel of the Kern River watershed and each of the lower Kern River tributary watersheds.
Channel Profile Reconstructions
Trunk channel profiles of the Kern River and lower Kern River tributaries were reconstructed using methods similar to those employed by Schoenbohm et al. (2004) and Clark et al. (2005). We used the relationship between contributing drainage area and distance from the mouth of the current channel profile, along with a θref of 0.45 and the ksn value obtained for the channel reach immediately upstream of each prominent knickpoint, to calculate the shape of the reconstructed channel profile. The reconstructed profile was then adjusted to match the elevation of the prominent knickpoint. This method allowed us to assess the fit of the reconstructed profile along the entirety of the current channel profile and was used to determine the elevation of the reconstructed trunk channel profile at the mouth of its respective watershed. The resulting difference in elevation of the reconstructed profile and modern profile represents the minimum total incision of the Kern River associated with each knickpoint (Clark et al., 2005; Kirby and Whipple, 2012).
The Kern River watershed drains an area of 5957 km2 with elevations between 210 m and 4412 m. Automatic ksn analysis of the entire Kern River watershed yielded a maximum ksn value of 3125 m0.9 with an average ksn value of 93 ± 1 m0.9 (2σx̅; twice the standard error of the mean). Analyzed on its own, the trunk channel of the Kern River yielded a maximum ksn value of 1062 m0.9 with an average ksn of 185 ± 9 m0.9 (2σx̅) (Table 1). High ksn values within the lower Kern River watershed are concentrated both along the trunk channel and on tributary channels located near the mouth of the Kern River gorge (Fig. 2). Tributary channels that drain the high Sierra also have high ksn values upstream of their junctions with the upper Kern River (Fig. 2).
The longitudinal channel profile of the Kern River displays a concave-up profile upstream of Isabella Lake, where the river switches from a south-trending to a southwest-trending flow direction. This switch in flow direction is concomitant with channel steepening that yields a convex-up channel profile downstream of Isabella Lake (Figs. 2 and 3). Manual ksn analysis of the Kern River trunk channel resulted in the identification of three prominent knickpoints at elevations of 1891 m, 556 m, and 404 m (Table 2). These knickpoints were used to separate the Kern River into the High Sierra, Kern Canyon, upper gorge, and lower gorge reaches respectively (Fig. 3). The knickpoint located between the Kern Canyon and upper gorge reaches marks the upstream extent of consistently high ksn values obtained along the lower Kern River (Figs. 3 and 4). The results from manual regression of slope-area data along each section of the Kern River are presented in Table 2. These results were used to reconstruct channel longitudinal profiles and determine projected channel elevations at the mouth of the Kern River watershed. All errors for θ, ksn, and projected channel elevations are presented as ±2σ. We were unable to obtain a reasonable fit to the longitudinal profile of the upper Kern River using the ksn value obtained along the High Sierra reach. Regression of slope-area data along the Kern Canyon reach resulted in a θ of 0.54 ± 0.20 and a ksn of 147.5 ± 3.7 m0.9. These ksn data produced a channel profile reconstruction that was a good fit for the Kern Canyon reach and resulted in a projected channel elevation at the mouth of the Kern River watershed of 442 ± 3 m. This reconstructed channel profile also provided a more reasonable fit to the High Sierra for the upstream Kern Canyon reach. Manual ksn analysis of the upper gorge and lower gorge reaches resulted in ksn values of 349.4 ± 36.0 m0.9 and 574.7 ± 42.9 m0.9 respectively. The reconstructed channel profile for the upper gorge reach has an elevation of 285 ± 12 m at the mouth of the Kern River watershed.
Lower Kern River Tributaries
Forty-eight (48) tributary watersheds that feed the lower Kern River were analyzed to elucidate the dynamic response of the landscape to the downstream channel steepening observed between Isabella Lake and the southern San Joaquin Basin (Table 1; Fig. 4). These tributary watersheds drain the western half of the southern Sierra Nevada with watershed relief ranging between 306 m and 1853 m. Automatic ksn analyses yielded minimum and maximum ksn values that range from 0 m0.9 to 591 m0.9, respectively. Watershed-averaged ksn values for individual tributary watersheds range from 12 ± 1 m0.9 (2σx̅) to 142 ± 8 m0.9 (2σx̅). Manual ksn analyses of tributary trunk channels resulted in the identification of 72 prominent knickpoints (Table 2; Fig. 4). All errors for θ, ksn, and projected tributary channel elevations are presented as ±2σ. Eight tributary trunk channels had no discernible knickpoints. Tributary knickpoint elevations range between 641 m and 2094 m, with the lower-elevation tributary knickpoints located along the lower gorge and upper gorge reaches. Higher-elevation tributary knickpoints are primarily located along the Kern Canyon reach. The wide range of θ and ksn values obtained along tributary channel segments immediately upstream of each tributary knickpoint are presented in Table 2. Channel profile reconstructions, based on these data, were created and used to determine the elevation of each reconstructed profile at the modern channel’s confluence with the lower Kern River. This information was then used to estimate the magnitude of river incision that is associated with each tributary knickpoint (Table 2). Our estimates of minimum total incision along the lower Kern River range between 27 ± 1 m and 1298 ± 17 m.
We identify two domains of low-relief topography that have been incised by the lower Kern River and its tributaries. These domains are interpreted as relict landscapes that formed under previous steady-state conditions between rock uplift and erosion. These domains function as markers from which we constrain the magnitude and spatial distribution of incision along the lower Kern River. We compare our estimates of incision with existing models of surface uplift and exhumation within the southern Sierra Nevada.
Relict Landscape of the Kern Plateau
High-elevation, low-relief topography is observed to the north and south of the Kern River in the Breckenridge, Piute, and Greenhorn Mountains. Maximum relief within this domain is 1264 m with a mean elevation of 1700 m. A concentration of tributary knickpoints located at an elevation of ∼1275 m separates this landscape from the steep slopes found within the Kern Canyon (Fig. 5). The elevations of all tributary knickpoints within this domain range between 1275 m and 2094 m with most knickpoints located more than ∼650 m above the present position of the Kern River. Higher-elevation tributary knickpoints are typically located farther from the river (Fig. 4). Stream profile analyses on tributary reaches immediately above each tributary knickpoint yielded ksn values that range from 16.2 ± 4.5 m0.9 to 135.2 ± 3.1 m0.9 with a mean value of 47.0 ± 9.9 m0.9 (2σx̅). Concavity index (θ) values determined along these reaches have a mean value of 0.53 ± 0.14 (2σx̅). Clark et al. (2005) identified the Kern Plateau as a similar high-elevation low-relief relict landscape drained by tributaries of the upper Kern River. Numerous studies interpret this extensive low-relief surface to be relict topography resulting from a period of slow exhumation that followed the end of Sierran arc magmatism in the Late Cretaceous (e.g., Clark et al., 2005; Cecil et al., 2006; Pelletier, 2007; Mahéo et al., 2009). We consider the Breckenridge, Piute, and Greenhorn Mountains to contain remnants of the Kern Plateau relict landscape that have been dissected and differentially tilted by the Breckenridge and Kern Canyon normal faults (Mahéo et al., 2009). These remnants have since been incised by the lower Kern River and its tributaries.
Relict Landscape of the Kern River Gorge
A second domain of low-relief topography is identified along the western flank of the southern Sierra Nevada (Figs. 4 and 6). Tributary watersheds located near the mouth of the Kern River gorge drain this surface and have elevations that range between 288 m and 1089 m. Stream profile analyses on tributaries that drain only this surface yielded ksn values that range from 19.1 ± 0.8 m0.9 to 70.8 ± 1.2 m0.9 with a mean value of 32.4 ± 14.3 m0.9 (2σx̅). The average θ value obtained along these tributaries is 0.36 ± 0.11 (2σx̅). This low-relief domain is spatially correlated with the steepest reaches of the Kern River. Knickpoints within tributary watersheds downstream of the upper gorge knickpoint mark the abrupt transition between this low-relief landscape and the steep channels and hillslopes within the Kern River gorge. Tributary knickpoint elevations decrease from 891 m to 676 m over a distance of 12.9 km upstream toward the upper gorge knickpoint (Fig. 5). An additional two prominent tributary knickpoints were observed at elevations below the low-relief landscape and within the walls of the Kern River gorge. These tributary knickpoints decrease in elevation from 704 m to 641 m over an upstream distance of 2.5 km toward the lower gorge knickpoint. We interpret this low-relief topographic surface as a remnant of a relict landscape related to the high-elevation low-relief relict landscape of the Kern Plateau. Several studies have interpreted this surface to be an exhumed early Tertiary nonconformity that extends from the foothills of the Sierra Nevada into the subsurface in the San Joaquin Basin (e.g., Saleeby et al., 2013, and references therein; Cecil et al., 2014; Sousa et al., 2017). In the Kern arch region, surface uplift and erosion are interpreted to have stripped off and redistributed upwards of 1000–1800 m of Tertiary sediments from this surface (Cecil et al., 2014). Mahéo et al. (2009) interpreted the Eocene nonconformity surface as a continuation of the high-elevation low-relief relict landscape. Differences in the present-day elevation of these surfaces are explained by late Cenozoic offset along normal faults of the southern Sierra Nevada fault system (Mahéo et al., 2009; Saleeby et al., 2016). The Kern Gorge, West Breckenridge, and Breckenridge faults offset the relict surface within the study area (Mahéo et al., 2009).
Incision of the Kern River
Incision estimates derived from the difference in elevation of reconstructed tributary channel profiles and their confluence with the lower Kern River reveal several discrete periods of incision. The first period of incision is related to departure from the slow and steady erosional degradation of the high-elevation relict landscape of the Kern Plateau (e.g., Clark et al., 2005; Cecil et al., 2006; Pelletier, 2007). This period is loosely constrained to have initiated ca. 20 Ma in response to activation of the southern Sierra Nevada fault system (Mahéo et al., 2009; Saleeby et al., 2016). Reconstructed channel profiles from the relict landscape yielded minimum total incision estimates that range between 77 ± 15 m and 1298 ± 17 m (Fig. 7). These incision estimates are similar to the 360 m to 1360 m of minimum total incision determined by Clark et al. (2005) along the upper Kern River. Our data correspond to a maximum long-term incision rate of ∼0.07 mm/yr. This is similar to the results of thermochronometric age-elevation measurements that yielded long-term erosion rates of 0.04–0.06 mm/yr from 73 to 32 Ma in the Kern and Kings River low-relief regions (Clark et al., 2005). Cosmogenic radionuclide–derived basin-averaged erosion rates of the northern Sierra low-relief surfaces also range from 0.015 to 0.075 mm/yr (Riebe et al., 2001). If we consider only incision that occurred prior to formation of the present-day Kern River Canyon, the lower Kern River is likely to have incised ≥1000 m into the relict landscape of the Kern Plateau during this period (Fig. 7). This estimate, however, does not account for any Tertiary sediments that may have blanketed the relict landscape (Mahéo et al., 2009; Saleeby et al., 2016).
We are unable to resolve the position of any knickpoints on the upper Kern River that are associated with the initiation of this period of incision along the Kern River (Fig. 3). The uppermost knickpoint identified on the Kern River occurs at an elevation of 1891 m and is a prime candidate. A poor match between the longitudinal channel profile and the reconstructed channel profile based on measured ksn values, however, precludes us from this interpretation. This uppermost knickpoint roughly corresponds with the lower-elevation limit of major glaciers in the Kern River watershed during the Tahoe glaciation (Fig. 3) (Moore and Mack, 2008; Moore and Moring, 2013). Upstream of this knickpoint, automatic ksn analyses show tributary channels with anomalously high ksn values that enter a section of the upper Kern River with low ksn values (Fig. 2). This pattern is suggestive of a hanging-valley setting where the channel form has been extensively impacted by glacial processes. Other factors may also complicate both the identification of a single knickpoint and the reconstruction of a fluvial bedrock channel profile along this reach. The upper Kern River, above Isabella Lake, roughly follows the strike of the Kern Canyon fault zone and closely parallels the eastern flank of the Breckenridge-Greenhorn horst (Mahéo et al., 2009; Amos et al., 2010; Nadin and Saleeby, 2010). The presence of highly fractured and fault-damaged rocks within this region is likely to cause significant variation in the erodibility of the channel bed (e.g., Stock et al., 2004; Crosby and Whipple, 2006; Wobus et al., 2006). Fracture spacing may have also influenced the overall shape of the valley during glaciations as a result of focused plucking and quarrying (Dühnforth et al., 2010). These complicating factors may have caused the upper Kern River to deviate from the power-law slope-versus-area relationship expected for equilibrium channel profiles in fluvial systems. Our analysis also does not consider the likely role that older, pre-Neogene paleochannels may have had on the incision history of the Kern Plateau (e.g., Wakabayashi and Sawyer, 2001; Wakabayashi, 2013; Saleeby et al., 2016). Because of these difficulties, we focus our attention on the remaining periods of incision interpreted along the lower Kern River.
Incision of the lower Kern River has been punctuated by at least two waves of rapid incision. The first wave of rapid incision in the study area is associated with the formation of the Kern River Canyon. Tributary knickpoints that separate the high-elevation relict topography from the steeper channels and hillslopes along the lower Kern River reveal an upstream decrease in incision along the Kern Canyon reach. Incision estimates range from 501 ± 12 m immediately upstream of the upper gorge knickpoint to 77 ± 15 m near the Isabella Dam (Fig. 7). This decrease in incision occurs over a stream distance of 28.8 km and suggests that incision of the Kern River has progressed through upstream knickpoint migration. The current location of this knickpoint appears to be at the Isabella Dam, in very close proximity to the Breckenridge–Kern Canyon fault zone. The presence of the dam, low channel gradients, and the confluence of the South Fork Kern River make it difficult observe a clearly defined knickpoint in the longitudinal profile and slope-versus-area or gradient-versus-distance plots at this location. Regardless, this period of rapid incision along the lower Kern River appears to have initiated near the western range front and propagated upstream.
The West Breckenridge normal fault defined the western range front of the southern Sierra Nevada throughout the Miocene (Mahéo et al., 2009; Saleeby et al., 2016). Subsidence and sedimentation histories in the adjacent southern San Joaquin Basin indicate mean subsidence rates of ∼0.055 mm/yr between 40 Ma and 6 Ma (Cecil et al., 2014). Observed early Neogene variation in the timing and rate of subsidence from wells within the Kern arch may reflect localized fault control along the southern Sierra fault system (Mahéo et al., 2009; Saleeby et al., 2016). Subsidence rates across the Kern arch significantly increase to 0.2–0.5 mm/yr between 6 Ma and 1 Ma (Cecil et al., 2014). This increase is interpreted to reflect transient epeirogenic uplift and subsidence driven by flexural-isostatic responses to delamination that may also involve local remobilization of preexisting faults due to flexural forces (Saleeby et al., 2012, 2013; Cecil et al., 2014). Epeirogenic uplift and subsidence are likely to have driven increased incision across the transition between the Sierra Nevada and San Joaquin Basin. We therefore investigate the possibility that the lower Kern River experienced an increase in incision ca. 6 Ma. If this is in fact the case, our incision estimates correspond to a maximum long-term incision rate of ∼0.08 mm/yr and a horizontal knickpoint migration rate of >4.8 mm/yr. The observed linear incision-versus-distance relationship suggests a minimum total incision of 642 m, a long-term incision rate of 0.11 mm/yr, and a horizontal knickpoint migration rate of 7.3 mm/yr from the West Breckenridge mountain front. We contend that this wave of incision on the lower Kern River is similar to a period of increased incision observed on major Sierra Nevada rivers to the north ca. 3.5 Ma (Stock et al., 2004, 2005). The extent of this incision event along the western flank of the Sierra Nevada suggests that regional east-to-west delamination of the lithospheric root is the principal driving mechanism (Saleeby et al., 2012, 2013). Minor differences in the timing and magnitude of incision may reflect north-south variations associated with detachment of a megaboudin from the northern segment of the delaminating lithospheric root ca. 3–4 Ma (Saleeby et al., 2012, 2013).
A second wave of increased incision is revealed by tributary knickpoints within the lower Kern River watershed. This wave of incision is responsible for formation of the Kern River gorge. Knickpoints on tributaries located downstream of the upper gorge knickpoint define the abrupt transition between the Kern River gorge relict topography and the steep channels and hillslopes along the lower Kern River (e.g., Fig. 6). Incision estimates consistently decrease upstream of over a distance of 12.9 km toward the upper gorge knickpoint (Fig. 7). These values range from 506 ± 3 m near the mouth of the Kern River at the Kern Gorge fault to 39 ± 8 m near the upper gorge knickpoint. The consistent decrease in incision estimates from tributaries located downstream of the West Breckenridge fault suggest that the upper gorge knickpoint migrated upstream along the Kern River from the range front, rather than being initiated at or modified by displacement on the West Breckenridge fault. Cecil et al. (2014) used detrital (U-Th)/He thermochronometry and other metrics to reveal a ca. 1 Ma pulse of uplift and exhumation in the Kern arch region. Since this time, anywhere from 1000 to 1800 m of sedimentary strata was stripped off of the Kern arch. The resulting exhumation rates of 1–1.8 mm/yr are consistent with contemporary uplift of the Sierra Nevada determined by GPS and interferometric synthetic aperture radar (InSAR) measurements (Cecil et al., 2014; Hammond et al., 2016). We therefore assert that this period of uplift and exhumation is fundamentally tied to exposure of the Kern River gorge relict landscape and formation of the Kern River gorge. Our data, therefore, correspond to an upper long-term incision rate of 0.51 mm/yr and a horizontal knickpoint migration rate of 12.9 mm/yr. The observed incision-versus-distance relationship suggests a minimum total incision of 575 m, a long-term incision rate of 0.58 mm/yr, and a horizontal knickpoint migration rate of 18.3 mm/yr from the modern mountain front. Our results highlight north-south differences in rock uplift and incision along the southern Sierra Nevada at this time. Incision rates leading to the formation of the Kern River gorge are significantly higher than the ∼0.3 mm/yr obtained by Stock et al. (2004, 2005) and attributed to the incision of inner gorges (ca. 3 Ma) along major Sierran rivers to the north. Our incision estimates do not account for the removal of any sedimentary cover from the relict landscape and are therefore minimum values. Stock et al. (2004, 2005) reported a dramatic decrease in incision rates along northern rivers to ∼0.02 mm/yr that began ca. 1.5 Ma and continues to the present. This large difference in incision supports a north-south variation in rock uplift associated with northward migration of a delamination hinge ca. 1 Ma (Saleeby et al., 2012). This pattern is further manifest by uplift and exhumation of the Kern arch and anomalous subsidence in the Tulare sub-basin (Saleeby et al., 2012; Cecil et al., 2014).
We observe two additional tributary knickpoints at elevations below the Kern River gorge relict landscape and within the walls of the inner gorge (Fig. 4). These prominent tributary knickpoints reflect modern incision that is in the process of moving through the Kern River watershed. Incision estimates from these tributary knickpoints decrease from 366 ± 4 m to 216 ± 2 m over an upstream distance of 2.5 km toward the lower gorge knickpoint (Fig. 7). Unfortunately, we are currently unable to constrain the initiation of this pulse of incision to better than <1 Ma. Based on the observed incision-versus-distance relationship, we estimate upper long-term incision rates of >0.37 mm/yr and a horizontal knickpoint migration rate of >2.5 mm/yr. The correlation with the lower gorge knickpoint, steepness of the lower gorge reach of the Kern River, and the steepness of the incision-versus-distance relationship, however, suggest that these values should be equivalent to or greater than incision values associated with the initial formation of the Kern River gorge. We interpret these knickpoints to reflect a further increase in rock uplift rates tied to the thermal effects of mantle delamination.
We identified two prominent relict landscapes that have been deeply incised by the lower Kern River and driven in response to late Cenozoic rock uplift within the southern Sierra Nevada and San Joaquin Basin region. The Breckenridge, Piute, and Greenhorn Mountains are remnants of the high-elevation relict landscape of the Kern Plateau. This relict landscape developed during the Late Cretaceous–early Tertiary and has since been offset by normal and strike-slip faults of the southern Sierra Nevada fault system and deeply incised by major Sierran rivers. Long-term incision rates of ∼0.07 mm/yr highlight slow erosional degradation of the landscape following the cessation of Sierran arc magmatism in the Late Cretaceous. This landscape has been periodically offset by displacement along the West Breckenridge, Breckenridge–Kern Canyon, and other faults within the southern Sierra Nevada fault system starting ca. 20 Ma. Stream profile analysis of the lower Kern River and its tributaries reveal several pulses of incision that we attribute to different stages of lithospheric root delamination beneath the southern Sierra Nevada. Prominent knickpoints on the lower Kern River are tied to tributary knickpoints that help to constrain at least three periods of increased incision. Tributary knickpoints that separate the relict landscape of the Kern Plateau and the Kern river Canyon indicate a pulse of incision that moved upstream at a rate of 7.3 mm/yr with a vertical incision rate of ∼0.12 mm/yr. This pulse of incision resulted in >642 m of vertical incision at the mountain front and is attributed to east-west delamination of the lithospheric root ca. 6 Ma. This period of increased incision is contemporaneous with incision observed along major Sierran rivers to the north. Tributary knickpoints near the mountain front help to define the Kern gorge relict landscape. This relict landscape was offset from the Kern Plateau by periodic displacement along the West Breckenridge fault throughout the last ∼20 m.y. Incision estimates from tributary knickpoints on this relict landscape provide evidence for a second pulse of incision along the lower Kern River. This pulse of incision migrated upstream at a rate of 18.3 mm/yr and initiated a vertical incision rate of ∼0.58 mm/yr along the lower reaches of the river. This incision event formed the Kern River gorge and is interpreted to reflect increased uplift and exhumation related to thermal effects on the crust following northward migration of an actively delaminating lithospheric root <1 Ma. Tributary knickpoints within the Kern River gorge loosely constrain a third pulse of incision that occurred within this time frame. These tributary knickpoints are tied to the steepest reach of the lower Kern River and suggest an incision rate of ∼1.2 mm/yr at the mouth of the gorge. These results constrain the incision history of the lower Kern River and provide geomorphic evidence that corroborates existing models of late Cenozoic tectonics and its impact on the evolution of the southern Sierra Nevada landscape.
The authors would like to thank Kristin Koehler and Spencer Schroer for helpful discussions and assistance with using the stream profiler tool. We also thank Jason Saleeby, Greg Stock, and associate editor Jeff Lee for reviews and comments that helped to improve the manuscript. This project was funded by National Science Foundation grants HRD-1137774 and HRD-1547784. Additional support was provided by a student research grant from the Geological Society of America (to Foreshee).