Paleochannels, Stream Incision, Erosion, Topographic Evolution, and Alternative Explanations of Paleoaltimetry, Sierra Nevada, California

Origin and Evolution of the Sierra Nevada and Walker Lane themed issue

Paleochannels, stream incision, erosion, topographic evolution, and alternative explanations of paleoaltimetry, Sierra Nevada, California

John Wakabayashi Department of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA

ABSTRACT of late Cenozoic uplift and stream incision 2008; Cassel et al., 2009, 2012; Chamberlain suggests a relationship with triple-junction et al., 2012), and interpreted steep paleogradi- Geologic relationships in the Sierra migration, possibly associated with slab ents of Oligocene and Eocene stream deposits Nevada, California, show negligible stream window development, with a second uplift (Cassel and Graham, 2011; Cassel et al., 2012). incision between Eocene and Late Miocene– pulse related to delamination and limited These interpretations directly contradict pro- Pliocene time. Stream incision of up to ~1 km to the southern Sierra (San Joaquin River posals for late Cenozoic rock and surface uplift began at (from south to north) ca. 20 Ma in drainage and southward). Basement features based on thermochronology (McPhillips and the Kern to Kings River drainages, between may have signifi cantly infl uenced along- and Brandon, 2010, 2012), geodetically measured 6 and 10 Ma in the San Joaquin River drain- across-strike differences in Cenozoic tec- current uplift rates of the range (Hammond age, 3.6–4 Ma in the Stanislaus and Moke- tonics and geomorphic response. et al., 2012), and a combination of geologic lumne River drainages, and ca. 3 Ma in the fi eld relationships and evaluation of geomorphic American and Feather River drainages. INTRODUCTION process (Lindgren, 1911; Hudson, 1955; Bate- These differences in incision timing greatly man and Wahrhaftig, 1966; Christensen, 1966; exceed the time of knickpoint retreat, based For over a century, geomorphologists have Huber, 1981, 1990; Unruh, 1991; Wakabayashi on the example of the North Fork Feather used the Sierra Nevada of California as a natu- and Sawyer, 2001; Jones et al., 2004; Stock River, where the knickpoint may have ral laboratory to explore general geomorphic et al., 2004, 2005; Clark et al., 2005; Figueroa retreated over 100 km in less than 300 k.y. process (e.g., Lawson, 1903; Matthes, 1937; and Knott, 2010; Kemp, 2012). based on ages of interfl uve-capping ande- Wahrhaftig, 1965). In the past few decades, the This paper will fi rst review data on stream sites and an inset basalt fl ow. The knickpoint Sierra Nevada has attracted attention as a local- incision, erosion, and evidence for pre–late in the Stanislaus River may have retreated ity for evaluating climatically versus tectonically Cenozoic relief (“paleorelief”) following Waka- over 50 km in less than 400 k.y. based on driven geomorphic evolution (Small and Ander- bayashi and Sawyer (2001), but with signifi cant somewhat looser constraints. Eocene paleo- son, 1995), an example of epeirogenic uplift updates and additions on the timing of incision, channels show lowest gradients parallel to (uplift not associated with major shortening evaluation of knickpoint migration speed, fault- the range axis, steepest ones perpendicular, or collision) and its causes (e.g., Unruh, 1991; ing, exhumation and erosion rates, along-strike and reaches with signifi cant “uphill” gra- Ducea and Saleeby, 1998; Jones et al., 2004; Le differences, and paleorelief east of the current dients that rise in the paleo-downstream Pourhiet et al., 2006), as well as a location for Sierra Nevada. I will then revisit the relationship direction. Modern Sierran rivers lack this evaluation of erosion and weathering processes between paleochannel gradients and azimuth relationship. The azimuth-gradient relation- and their forcing mechanisms (e.g., Small and and “inverse” (uphill in present-day setting) ships of paleochannels, especially the uphill Anderson, 1995; Granger et al., 2001; Riebe gradients (from Lindgren, 1911; Hudson, 1955; gradients, require late Cenozoic tilting and et al., 2000, 2001; Stock et al., 2005; Clark et al., Jones et al., 2004) that starkly contrasts with uplift. Incision began in spite of decreasing 2005; Phillips et al., 2011). modern stream reaches. The stream incision discharge and increasing sediment load and A debate has emerged as to whether or not late history and paleogradient-azimuth relation will must have resulted from steepening associ- Cenozoic rock and surface uplift has occurred in then be discussed in the context of supporting ated with tilting and uplift. Stable-isotope the Sierra Nevada, and this debate has implica- evidence for late Cenozoic uplift, followed by paleoaltimetry apparently records a profi le tions for analysis of general geomorphic process presentation of alternative interpretations of the similar to that of the modern range and areas and evaluation of recently developed methods paleoaltimetry data, and speculations on con- east of it, in spite of signifi cant vertical defor- in paleoaltimetry. Some suggested a lack of late nections between tectonic mechanisms, uplift, mation that postdates the age of the sampled Cenozoic uplift on the basis of thermochronol- and topographic evolution. deposits, suggesting fairly recent reequilibra- ogy (e.g., House et al., 1998), or on the basis of tion, in contrast to the published interpre- the generally thin Sierra Nevada crust and the GENERAL GEOLOGIC FRAMEWORK tations of closed-system behavior since the tectonic setting of the Sierra Nevada (Wernicke AND TECTONIC HISTORY Oligocene or Eocene. Such apparent open- et al., 1996). More recently, researchers have system behavior agrees with studies showing proposed a lack of late Cenozoic uplift on the The 600-km-long Sierra Nevada is the most progressive hydration of volcanic glass and basis of paleobotany (e.g., Hren et al., 2010), prominent mountain range in California. The the correspondence between weathering and isotopic paleoaltimetry (e.g., Poage and Cham- Sierra Nevada and the Central Valley belong erosion rates. Northward-younging initiation berlain, 2002; Mulch et al., 2006; Crowley et al., to the Sierra Nevada microplate, an element of

Geosphere; April 2013; v. 9; no. 2; p. 191–215; doi:10.1130/GES00814.1; 7 ﬁ gures; 1 table. Received 30 April 2012 ♦ Revision received 1 December 2012 ♦ Accepted 23 December 2012 ♦ Published online 5 February 2013

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the broad Pacifi c–North American plate bound- lacks the simple block topography of regions to inson and Snyder, 1978). The resultant crustal ary (Argus and Gordon, 1991) (Fig. 1). The the north, and in the southernmost part of the thickening apparently resulted in a broad region Sierra Nevada microplate displays little inter- Kern drainage, the highest elevations are on a of elevated topography, comparable to the nal deformation north of the Kern River drain- divide west of the crest, and topography of this Altiplano or Tibetan Plateau (Christiansen and age and moves northwestward relative to stable region is controlled more by faults along the Yeats, 1992; Dilek and Moores, 1999), with a North America at ~10–14 mm/yr, a fourth or Kern River than the FFS (Saleeby et al., 2009). drainage divide hundreds of kilometers east of fi fth of the dextral motion of the plate boundary Crestal elevations vary from 2100 to 2700 m the current crest of the Sierra Nevada (Chris- (Argus and Gordon, 1991; Dixon et al., 2000). in the northern part of the range to 4000–4400 m tiansen and Yeats, 1992; Henry, 2008; Henry The microplate is bounded on the west by an in the central to southern part of the range, with and Faulds, 2010; Henry et al., 2012). Follow- active fold-and-thrust belt that marks the east- the highest elevations in the headwaters of the ing cessation of Cretaceous arc magmatism, ern margin of the Coast Range province (e.g., Kings and Kern Rivers, and maximum elevations exhumation resulted in surface exposure of the Wentworth and Zoback, 1989), and on the east decreasing to the south. The height of the eastern batholith. Eocene stream gravels, sourced well by a prominent east-facing escarpment that escarpment varies from ~1000 m in the northern- east of the present Sierran crest (e.g., Garside marks the Sierra Nevada Frontal fault system most part of the range to nearly 3300 m at Lone et al., 2005), were deposited on the exhumed (referred to as Frontal fault system or FFS herein), Pine (Fig. 2). granitic and older metamorphic rocks in what a zone of normal, normal-dextral, and dextral The axis of the range that parallels smoothed became the northern Sierra Nevada (Bateman faulting (e.g., Clark et al., 1984; Beanland and (fi tted across drainages) elevation contours on and Wahrhaftig, 1966). Clark, 1994; Unruh et al., 2003; Le et al., 2007) the western fl ank varies from about N35°W Regional-scale Cenozoic volcanism began (Fig. 2). Whereas the down-to-the-east compo- in the Feather River drainage to N25°W in the with 31–23 Ma rhyolitic ash fl ow tuff deposi- nent of deformation across the FFS fault strands southern Yuba River and American River drain- tion in channels crossing the Sierra (Garside results in prominent east-facing topographic ages, to N40°W from the Stanislaus River south- et al., 2005; Henry and Faulds, 2010; Henry escarpments ranging from 500 to 3000 m in ward. This along-strike change in the range axis et al., 2012), and culminated with widespread height, dextral shear predominates along this orientation mirrors along-strike changes in the and voluminous andesitic volcanism from 16 transtensional fault system (Unruh et al., 2003). strike of the Frontal fault system (Fig. 2) (Waka- to 3 Ma, associated with the ancestral Cascades The FFS separates the Sierra Nevada microplate bayashi and Sawyer, 2001; Unruh et al., 2003). arc (e.g., Busby et al., 2008a, 2008b; Busby and from the Basin and Range province to the east, Defi nitions of the eastern boundary of the Putirka, 2009; Cousens et al., 2008). The south- or, alternatively, the FFS represents the western- Sierra Nevada vary, but I will follow the defi ni- ernmost active volcano of the Cascades arc is most strand of the dextral Walker Lane belt. The tion used by Wakabayashi and Sawyer (2001) Mount Lassen (Figs. 1, 2). Ancestral Cascades northernmost part of the FFS curves to a WNW wherein the Sierra Nevada is defi ned as being arc activity resulted in voluminous deposition strike, forming the boundary between the northwest of the western strands of the FFS. This of volcanic rocks as far south as the northern ern margin of the Sierra Nevada microplate and defi nition places ranges such as the Diamond Tuolumne River drainage; this volcanism shut the Cascades (Fig. 2); this has been called the Mountains (headwaters of the North Fork off in the wake of the migrating southern edge Sierra Nevada–Cascade Range boundary zone Feather and Middle Fork Feather Rivers) and of the subducted slab as the Mendocino triple (Sawyer, 2009, 2010). Carson Range (range east of Lake Tahoe) east junction migrated northward and the conver- The Sierra Nevada slopes gently westward of the Sierra proper. Note that the strike and gent plate margin was replaced by a transform and abruptly eastward from its crest from the location of basement units associated with the one (e.g., Atwater and Stock, 1998; Busby and Kings River drainage to the northern termina- range, such as the Sierra Nevada batholith, Putirka, 2009). tion of the range (Fig. 2). The northernmost diverge from that of the current mountain range Estimates of Cenozoic landscape evolution part of the range slopes westward at a gradi- (Wakabayashi and Sawyer, 2001). and deformation are best constrained where ent of ~38 m/km (gradient 0.038 or 2.2°) over Prior to becoming part of the transform plate Cenozoic deposits are present in the Sierra a distance of ~50 km. In the Yuba River drain- margin, arc magmatism occurred in what later Nevada (Fig. 1). Widespread Cenozoic depos- age the westward gradient averages ~28 m/km became the Sierra Nevada. Mesozoic arc activ- its are limited to the area north of the Tuolumne (gradient 0.028 or 1.6°), with a lower-gradient ity, associated with east-dipping subduction, River (Fig. 1). The oldest of the Cenozoic cover western portion of ~20 m/km (gradient 0.020 included the emplacement of the Sierra Nevada strata are Eocene gold-bearing gravels, com- or 1.1°) for the western foothills averaged batholith and ended at ca. 85 Ma (Saleeby monly referred to as the “auriferous gravels,” over ~40 km, and ~35 m/km (gradient 0.035 and Sharp, 1980; Stern et al., 1981; Chen and and their fi ne-grained equivalents, the Ione For- or 2°) for the eastern higher-elevation portion Moore, 1982; Saleeby et al., 2008). The Meso- mation, along the western base of the northern averaged over ~50 km. From the San Joaquin zoic plutons intrude Mesozoic and Paleozoic, and central part of the range (e.g., Bateman River drainage to the Kaweah River drainage, metaigneous and metasedimentary rocks (e.g., and Wahrhaftig, 1966; Garside et al., 2005) the foothills region has a steep middle- to high- Schweickert, 1981; Moores and Day, 1984; (paleochannels shown in Fig. 2). These gravels elevation slope that levels off eastward. For Sharp, 1988; Saleeby et al., 1989). Collectively, fi lled drainages that fl owed westward from a example, the San Joaquin River–Kings River the Mesozoic plutons and the Mesozoic to divide ~400–500 km (present-day distance) east divide area has a western foothills region with a Paleozoic metamorphic rocks will be referred of the present Sierra Nevada crest (Henry, 2008; slope of ~27 m/km over ~20 km (gradient 0.027, to as “basement” in this paper. Henry and Faulds, 2010; Henry et al., 2012). 1.5°), then 80 m/km for the next 25 km (gradi- The cessation of magmatic activity in this The Oligocene (31–23 Ma) rhyolite tuffs, ent 0.080, 4.6°), and ~18 m/km (gradient 0.018, region apparently coincided with a shallowing including the Valley Springs and Delleker For- 1.0°) for the remaining 40 km or so to the crest of subduction dip that resulted in the migration mations, overlie the Eocene gravels (Wagner of the range, with an overall average slope of of the locus of magmatism far eastward, as well et al., 1981; Saucedo and Wagner, 1992; Henry ~38 m/km (Fig. 2). The southernmost part of the as signifi cant shortening in the foreland region and Faulds, 2010). These apparently erupted range (mid–Kern River drainage southward) associated with the Laramide orogeny (Dick- from calderas 200–400 km (present-day

192 Geosphere, April 2013

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Mt. Lassen Cenozoic Volanic Rocks q: Quaternary deposits q t: Eocene-Pleistocene sedimentary deposits Lake Almanor g g: granitic rocks (pre Cenozoic) CA q NV N m: metamorphic rocks (pre Cenozoic) m g 40° 40° Quincy geologic contact q 122° g q g 0 70 km g crest of Sierra Nevada

g q q t river g g m g m q Oroville g g t g g q

Feather R. Feather Yuba R. m k/t g m Lake m g Gorda Tahoe B q Plate Basin and m 39° 39° W Range L-ECSZ 122° g g q NV m CA S C e n t r a l g N 38 North g –40 mm 10 t American g Pacific PlateS -14 mm/yr AFS Plate g /yr q American R. Sacramento g g 119° g g g t SN: Sierra Nevada microplate WL-ECSZ: Walker Lane/Eastern m g m g q California shear zone Mokelumne R. SAFS: San Andreas fault system 121° q Mono 38° L. 38° g g q g Stanisl g g aus R. g m m m 118° Modesto t q Tuolumne R. g m g S

Merced R. i t e Bishop

V a l l e y m r r q a m g Undifferentiated A m Geology 120° 37° q Klamath122° Cascades 120° t m Owens Plate Mountains Modoc quin R. C San Joa Plateau r

C o a s t e Gorda Sierra Nevada Basin 118° s m W a l k e r Fresno 40° R. t Lone Pine

40° q m m g Val F r o n t a l m Kings ley g And R. San L a n e m 116° Kaweah q Andreas 38° F a u l t 38° Microplate Range m 119° Pacific m 124° S y s t e m 36° m Fault R a n g e s m m Plate q g System N 36° q 36° V C A q t g m 122° AZ CA q Transverse q Ranges 34° 34° Kern R. Peninsular Bakersfield Ranges m Undifferentiated q q Geology 119° 118° 120° 118° 116° g Mexico 35° m

Figure 1. Sierra Nevada showing distribution of Cenozoic and basement rocks along with the current plate-boundary context (inset B) and regional context (inset C). Adapted from Wakabayashi and Sawyer (2001). CA—California; NV—Nevada; AZ—Arizona.

Geosphere, April 2013 193

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distance) east of the present Sierran crest and the Sierra Nevada, but minor accumulations changed local base levels (e.g., Huber, 1990). fi lled the same west-fl owing drainages previ- of these deposits are found along the eastern This can be viewed as aggradation (deposition) ously fi lled by the Eocene gravels (Henry and margin of the Central Valley as far south as the that raised the elevation of the thalweg, followed Faulds, 2010). These rhyolites are overlain by Kings River (e.g., Bartow, 1985, 1990). Scat- by incision through the deposits (or through ca. 16–3 Ma andesites, andesitic mudfl ows, tered outcrops of Miocene to late Quaternary basement adjacent to such deposits) to the for- and associated volcanic sedimentary rocks of volcanic rocks occur south of the Tuolumne mer equilibrium profi le of the stream after the the ancestral Cascades arc that erupted from River (Moore and Dodge, 1980). These volcanic sediment load dropped and aggradation ceased. vents in the vicinity of, but generally east of, rocks represent local eruptive events rather than Volcanic deposition represents exceptionally the present crest (Wagner et al., 1981; Saucedo the Cenozoic volcanic arc and, prior to erosion, fast aggradation, and in the most extreme cases and Wagner, 1992; Bartow, 1979; Busby et al., constituted a much smaller volume than ances- this aggradation is associated with signifi cant 2008a, 2008b; Cousens et al., 2008). The tral Cascades rocks to the north (Ducea and steepening of the stream owing to the construc- term “Mehrten Formation” has been applied Saleeby, 1998). tion of volcanic edifi ces. The canyons carved in to describe these andesitic rocks, excluding the fl anks of the modern Cascade volcanoes and those north of the North Fork Feather River STREAM INCISION IN THE associated fl ows illustrate this principle well. A (for example, Unruh, 1991; Wakabayashi and SIERRA NEVADA similar, but smaller-scale, example of incision Sawyer , 2001), following the definition of following aggradation is the incision of streams Curtis (1954). Andesites of the ancestral Cas- Late Cenozoic Stream Incision into the alluvial plains, but not into bedrock, on cades arc blanketed the northern and central the western margin of the Sierra Nevada, where Sierra, covering all but a few scattered basement Canyons are a fi rst-order topographic feature rapid deposition during glaciation was followed highs, with extensive deposits preserved as far of any mountain range, and the incision of these by incision through alluvium during the present south as the northern Tuolumne River drain- canyons represents a signifi cant geomorphic interglacial period (e.g., Janda, 1966). Accord- age (Slemmons, 1966; Durrell, 1966; Guffanti response to climatic, depositional, or tectonic ingly, I will refer to incision only as that into et al., 1990, Busby et al., 2008a, 2008b; Busby processes. Accordingly, the evidence of stream basement beneath the base of the Cenozoic and Putirka, 2009; Cousens et al., 2008). incision serves as an important component in deposits (“basement incision” of Wakabayashi The age of the youngest strata of the ancestral the evaluation of the Sierra Nevada uplift. and Sawyer, 2001). Cascades in any part of the Sierra is important, The base of late Cenozoic deposits, includ- Incision in the major drainages increases because the initiation of signifi cant incision fol- ing deposits representing the local thalwegs upstream from zero at the western base of the lowed the deposition of these units. Andesitic of major paleochannels, crop out hundreds of range to the deepest parts of the canyons then fl ows in the headwaters of the North Fork Amer- meters above canyon bottoms incised into base- decreases to zero near the headwaters of the ican River have K-Ar and Ar-Ar ages as young ment and thus record incision that postdates streams just west of the crest or drainage divide as 3.0 Ma (Harwood, 1981; Cousens et al., 2008, the deposition of the late Cenozoic deposits (Wakabayashi and Sawyer, 2001) (Fig. 2). The 2012) data in Saucedo and Wagner, 1992); these (Fig. 3). Although the streams have locally North Fork Feather and Middle Fork Feather are the youngest ages obtained from andesites incised up to several hundred meters into late Rivers are exceptions to this because they cross south of the North Fork Feather River. Between Cenozoic, mainly volcanic, deposits as well the crest of the range, so their deepest incision these rocks and the North Fork Feather River, (Bateman and Wahrhaftig, 1966; Busby et al., corresponds to the point where they cross the the youngest K-Ar age from andesitic rocks is 2008a, 2008b; Busby and Putirka, 2009), the crest of the range. Post-Mehrten incision in 6.8 Ma (Saucedo and Wagner, 1992), but age incision through the late Cenozoic deposits or the northern and central Sierra Nevada ranges data in this region are sparse. in basement to the level of the bottom of older, up to ~1200 m (Table 1) (Wakabayashi and The youngest andesitic rocks in the Sierra Cenozoic paleochannels can be driven by depo- Sawyer, 2001) and the deepest incision in most proper, yielding 2.4–3.3 Ma Ar-Ar and K-Ar sition of these late Cenozoic deposits alone, drainages occurs downstream of the limit of ages (Guffanti et al., 1990; Wakabayashi and because their deposition may have drastically glaciation. Sawyer, 2000; Kemp, 2012; M.A. Clynne, unpublished data), cover basement north of the North Fork Feather River. These rocks are associated with the Yana volcanic cen- Figure 2 (on following page). The Sierra Nevada and adjacent areas showing topography, ter and are called the Tuscan Formation on major rivers, faults, and Eocene–Oliogene paleochannels. Although the elevation color scale the western fl ank of the northernmost Sierra shows elevation for the entire fi gure, 250 m elevation contours are shown for the Sierra Nevada (Guffanti et al., 1990). Northward younging only. Of the minor internal faults within the Sierra, only those with signifi cant long-term of the youngest late Cenozoic volcanic rocks slip rates (~0.1 mm/yr or more) or the continuation of en echelon Frontal faults across the in the Sierra Nevada may be expected because crest are shown. Also shown on the map are the depths (magenta numbers) of paleochannels, of the northward migration of the Mendocino which are Eocene–Oligocene with the exception of the Miocene paleo–San Joaquin River; triple junction and associated shutoff of sub- and late Cenozoic incision recorded in modern river canyons (blue numbers). Note that the duction and arc volcanism, but existing data do depth of the paleocanyons is somewhat different than the paleorelief recorded by Waka- not show such a pattern except from the Yana bayashi and Sawyer (2001) in that it considers average canyon depth rather than recording volcanic center northward to the active Lassen the elevation difference between isolated paleohighs and paleothalwegs. Paleochannel depths volcano (Guffanti et al., 1990; M.A. Clynne, are from this study, Wakabayashi and Sawyer (2001), Gonsior and Dilles (2008), and Henry unpublished data). and Faulds (2010). Late Cenozoic incision is revised from Wakabayashi and Sawyer (2001). The Eocene gravels, Oligocene rhyolites, and Paleochannels are from Garside et al. (2005) and Henry et al. (2012). Faults are from Waka- Mio-Pliocene andesitic volcanic rocks mainly bayashi and Sawyer (2001), Faulds and Henry (2008), Maheo et al. (2009); Amos et al. (2010), crop out north of the Tuolumne River within U.S. Geological Survey (2011), and Sawyer and Page (2010).

194 Geosphere, April 2013

Mt. Lassen N.Fork.

110010

000

0 L. Almanor

0 Honey Lake fault zone NEVADA 250225 505 CALIFORNIA 0 Mill Creek 700 15015 40° 40° Deer Creek505 Quincy 0 E BranchF Mohawk Valley fault zone 122° 11900 >1000 200 r250 180 o Sacramento R. 830 M.Fork. n N.Fork. 620 450 700 280 t Oroville 210 N.Fork.250 a 450 Feather R. S. YubaS.Fork. 750775 910 l 505 180 0 600 L. Yuba R. 620 1200 122° 430 450 Tahoe 39° 39° 250252 N.Fork American505 N.Fork. 120° 0 200paleo- C o a s t R. azimuth, channel150151 C e n t r a l 50 gradient, Fig.6 140 0 S.Fork. 0 F

250252 a

0 u American R. 700 240 980 Sacramento l 119° 210 N.Fork. 300 300 600 520 710 t Mokelumne R. 270 250252505 00 121° Mono Carquinez 15015 38° 100110 505 890 L. 38° 000 0 Strait 0 0 130 0 White Mtns. fault zone Hayward fault 410 580 Stanislaus R. 118° San Modesto Francisco Tuolumne R. Tuolumne 1050 S 0 R. azimuth, 9700 Fish 15015 50 y 505 250225 gradient,250252 Fig.6 0 550 0 1100 Bishop Lake Valley-Death Valley fault zone Merced R. 0 San Joaquin s R. azimuth, 100 760 Calaveras fault 600 gradient, Fig.6 t 225025 Owens Valley fault 270 500 0 e 37° San Andr 120° m R a n g e s V a l l e y e San Joaquin R. as fault Fresno Kings R. 250 Lone 750775 505 50 00 Pine N 0

l Mt Whitney

Elevation (m) Kaweah R.

4400 250252 505 Contour interval 250 m 0 Canyon fau 250250 for Sierra Nevada only 00

0 36° Kern Eocene/Oligocene paleochannel 119° fault

Sierra Nevada crest

270 Late Cenozoic incision (m), Garlock fault modern river, at blue dot Kern R. 0 70 km Bakersfield 100 Paleocanyon depth (m), at magenta dot

Figure 2.

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A. Deposition of Eocene gravels most cases. The age of the oldest units inset beneath base of the interfl uve-capping deposits Local Paleohigh provides the minimum age of incision initiation. To compare timing of incision range-wide, the Pre-Cenozoic basement speed of headward migration of an incision pulse Paleorelief (associated with knickpoint migration) in a given B. Deposition of Oligocene drainage warrants consideration. Field relation- rhyolite ashflow tuffs 31–23 Ma ships in the North Fork Feather River drainage suggest rapid knickpoint retreat, or near-synchronous incision initiation along a signifi cant Pre-Cenozoic basement length of the trunk stream. Andesites of the Yana volcanic center cap upland surfaces on the north rim of the North Fork Feather River canyon, and C. Deposition of Miocene-Pliocene andesites these deposits extend to the western fl ank of the (Mehrten Formation) 16–3 Ma range (Guffanti et al., 1990). Andesite capping Depositionally-driven channeling the ridge north of the East Branch North Fork– North Fork confl uence yielded an Ar-Ar age of 2.8 Ma (Wakabayashi et al., 1994; Wakabayashi Pre-Cenozoic basement and Sawyer, 2000). K-Ar and Ar-Ar ages of similar andesites in this part of the drainage range D. Present Day between ca. 2.8 and 3.0 Ma, including rocks Interfluves capped by late Cenozoic volcanic rocks capping the crestal highlands north of the North Fork Feather River (Kemp, 2012; M.A. Clynne, Youngest Mehrten Formation: maximum incision initiation age unpublished data; WLA, 1996). This suggests that incision along the North Fork Feather River began after 2.8–3.0 Ma. Over 20 km upstream of the North Fork Feather River–East Branch North Fork Feather Pre-Cenozoic basement River confl uence, a 2.1 Ma basalt fl ow is inset Inset Plio-Pleistocene channel-filling below an erosion surface that makes up the west unit: minimum incision rim of the North Fork Feather River canyon

Basement Incision initiation age (Wakabayashi et al., 1994; Wakabayashi and Figure 3. Schematic diagram showing Cenozoic depositional and Sawyer, 2000). This surface is overlain by Yana incision history in the northern Sierra Nevada. In this schematic volcanic rocks ~10 km north of the basalt expo- view, the stream is fl owing out of the plane of the fi gure (i.e., toward sure (WLA, 1996). The base of the 2.1 Ma basalt or away from the viewer). is 180 m below the erosion surface and 385 m above the canyon bottom. These fi gures are revised from those in Wakabayashi and Sawyer Initiation of Late Cenozoic Incision fl uves, and dividing the elevation difference by (2000) because the earlier incision amounts were and Rates the age of the youngest of the deposits (Fig. 3). based on what I consider a somewhat oblique Such rates are minima, because time required projection to the canyon bottom (with respect to Long-term late Cenozoic incision rates can be for the stream to cut through the volcanic rocks the position of the 2.1 Ma basalt remnant). estimated by measuring the elevation difference is not accounted for and the time elapsed after If the incision rate from 2.1 Ma to present between the present channel bottoms and the deposition of interfl uve-capping units and the (0.18 mm/yr) is applied to the 180 m of pre– base of late Cenozoic deposits capping the inter- initiation of incision is poorly constrained in 2.1 Ma incision beneath the erosion surface, the

TABLE 1. MAXIMUM LATE CENOZOIC INCISION AMOUNTS AND RATES Incision through Maximum incision Minimum incision Preferred incision Incision rate (mm/yr) Stream basement (m) initiation age initiation age initiation age [time interval] North Fork Feather River 1190 2.8–3.3 Ma 2.1 Ma 3 Ma 0.40 Middle Fork Feather River 830 — — 3 Ma 0.28 North Fork Yuba River 700 6.8 Ma — 3 Ma 0.23 North Fork American River 910 3.3 Ma — 3 Ma 0.30 North Fork Mokelumne River 980 (g) 4 Ma — 3.6–4 Ma 0.26 [3.8 Ma to present] 670 Stanislaus River 710 6 Ma 1.6 Ma 3.6–4 Ma 0.19 [3.8 Ma to present] Tuolumne River 890 — — 4 Ma 0.23 [4 Ma to present] San Joaquin River 970 (g) 10 Ma 3.6 Ma 6–10 Ma 0.097–0.16 [10–6 Ma to present] San Joaquin River 580 — — — 0.091–0.24 [10–6 Ma to 3.6 Ma] San Joaquin River 390 (g) — — — 0.11 [3.6 Ma to present] Eo-Miocene <30 — — — <0.001 [34–14 Ma] Note: (g) indicates glaciated.

196 Geosphere, April 2013

estimated age of incision initiation is 3.1 Ma, 2009), possibly signaling initiation of dip-slip gravels in the crestal region from the American but this is older than the age of the andesite on faulting along this part of the FFS. However, the River drainage northward (Wakabayashi and the rim of the East Branch North Fork–North initiation of faulting along this part of the FFS Sawyer, 2001). These geomorphic-stratigraphic Fork confl uence. A pre–2.1 Ma incision rate need not coincide with incision west of the crest relationships apply in the above drainages from faster than the post–2.1 Ma rate (required to ini- at this time, given the local examples of Frontal the crest to the downstream limit of incision and tiate incision at 2.8 Ma or later) would refl ect faulting postdating initiation that I will review thus limit the area of differential Eocene–Mio- a temporal pattern of incision rate opposite of below in the section on faulting, as well as the Pliocene tilts to a relatively narrow zone along that recorded in other reaches of the North Fork uncertainty of knickpoint retreat time noted the eastern margin of the Central Valley. Feather River (Wakabayashi and Sawyer, 2001). above. Given these uncertainties, I will consider Initiation of incision in the Tuolumne River Collectively, these fi eld relationships suggest incision to have initiated in the American River is even more poorly constrained owing to a pau- that the time of knickpoint retreat is shorter than drainage at ca. 3 Ma. city of age dates on the youngest andesites in the the collective uncertainty of the age of ridge- The youngest andesitic volcanic rocks drainage. For the Tuolumne River, I will adopt capping andesites and the estimate of incision (Mehrten Formation) capping the interfl uve an initiation of incision age of ca. 4 Ma for the time prior to deposition of the 2.1 Ma basalt. between the Mokelumne River and Calaveras Tuolumne based on proximity to the Stanislaus These relationships suggest knickpoint retreat River (a comparatively small stream between and Mokelumne drainages. The Merced River of over 100 km from the mouth of the North the Mokelumne and Stanislaus Rivers) in the drainage lacks dated late Cenozoic deposits, Fork Feather River in less than ~300 k.y. For the western foothills region are ca. 4 Ma, based on precluding a reliable estimate of the timing of purposes of discussion, I will adopt a time of ca. K-Ar ages of two dacitic plugs that intrude the late Cenozoic incision initiation. 3 Ma for the initiation of incision in the North Mehrten, one of which is overlain by uppermost In the San Joaquin River drainage, incision Fork Feather River canyon. Mehrten strata (Bartow, 1979). Thus, incision of began sometime between 10 Ma and ca. 3.5 Ma, The estimated knickpoint retreat rate for the the Mokelumne River appears to have initiated based on the 3.4–3.9 Ma (K-Ar age; Dalrymple, North Fork Feather River may be as fast as that after 4 Ma, and this constraint appears applicable 1964) volcanic rocks inset as much as 580 m determined by Crosby and Whipple (2006) to the Stanislaus River, owing to the proximity of below the reconstructed position of the 10 Ma (tens of kilometers of retreat within 18 k.y.) for the latter to the dated interfl uve deposits. Cosmo- paleochannel (Huber, 1981). Recent Ar-Ar dat- the Waiapaoa River of New Zealand, and that genic nuclide dating of cave sediments indicate ing, fi eld mapping, and paleomagnetic data have estimated for the upstream reaches of the Yel- that ~390 of 490 m of late Cenozoic incision of confi rmed the inset relationship of the volcanic low River in China (over 250 km within 500 a reach of the Stanislaus River canyon had taken rocks in the San Joaquin River drainage and k.y.) by Burbank and Anderson (2012) based on place prior to 1.6 Ma (Stock et al., 2005). If an allowed assignment of an age of ca. 3.6 Ma to studies by Harkins et al. (2007) and Craddock incision rate of 0.20 mm/yr, signifi cantly higher those rocks (Carlson et al., 2009). Similar to the et al. (2010), to as slow as an order of magnitude than the long-term average rate for this part of approach followed for the Stanislaus River, if slower, within the limits of age constraints. The the canyon (0.12–0.13 mm/yr), is adopted for a the highest incision of central-southern Sierra estimated knickpoint retreat rate for the North maximum rate of pre–1.6 Ma incision, then inci- streams (0.27 mm/yr for the 2.7–1.4 Ma period in Fork Feather River is one to two orders of mag- sion had begun by ca. 3.6 Ma at this point in the the Kings River; Stock et al., 2004) is applied for nitude faster than that estimated by Berlin and Stanislaus River canyon. Accordingly, incision the San Joaquin prior to 3.6 Ma, then late Ceno- Anderson (2007) for streams draining the Roan in the Mokelumne and Stanislaus River drain- zoic incision began before ca. 6 Ma. Accordingly, Plateau to the Colorado River in Colorado, ages initiated between ca. 3.6 and 4 Ma. late Cenozoic incision probably began in the San one to two orders of magnitude faster than the These relationships also provide a constraint Joaquin River between 10 and 6 Ma. rates estimated by numerical modeling of Stock on knickpoint migration along the Stanislaus Maximum incision amounts are given in et al. (2004) for the Kings River, and two to River. The 4 Ma age of the top of the ande- Table 1 for other major Sierran rivers, where three orders of magnitude faster than rates esti- sitic volcanics should apply to the surface constraints on timing are scarce or lacking. For mated by numerical models of Pelletier (2007) of the interfl uve extending to near the mouth of those drainages, the timing of incision is esti- for southern Sierra rivers. If the North Fork the river (downstream limit of incision). If so, mated by comparison to adjacent drainages for Feather River knickpoint migration rates apply incision in the Stanislaus River migrated over which better geochronologic constraints are to other Sierran drainages, then incision may 50 km upstream in less than 400 k.y. These con- available. have propagated from the mouths of the trunk straints are looser than those in the North Fork Temporal variation of incision rate apparently streams to near their headwaters within a few Feather River, owing to the larger age difference differs along strike. The Kings River incised at hundred thousand years. Somewhat looser age between interfl uve-capping and inset deposits. 0.27 mm/yr between 2.7 and 1.4 Ma, and 0.02 constraints in the Mokelumne and Stanislaus Along the western fl ank of the Sierra Nevada, mm/yr thereafter (Stock et al., 2004). In con- River drainages (see below) are consistent with Cenozoic strata of Holocene to Eocene age trast, one reach of the North Fork Feather River this conclusion. show progressively greater dip (tilt perpen- (east of the crest and upstream of the point of Incision in the North Fork American River dicular to range axis) with greater age (Unruh, greatest incision and rates) incised at 0.08–0.09 drainage appears to have begun after ca. 3.3 Ma, 1991). Projection of such tilts toward the mm/yr from 2.8 to 1.1 Ma, accelerated to based on the age of the andesites capping a crest would suggest progressive incision from 0.25–0.40 mm/yr from 1.1 Ma to 0.6 Ma, then ridge southeast of Donner Summit (Shriver Eocene to modern time (Huber, 1981; Unruh, slowed to 0.12–0.16 mm/yr after 0.6 Ma (Waka- and Wakabayashi, 2010). A minimum age of 1991) and would predict that the progressively bayashi and Sawyer, 2000, 2001). For the major incision initiation there cannot be determined younger units should be inset below older ones drainages from the Kings River northward, late because of the lack of dated inset units. Ini- in canyons. In contrast, Mio-Pliocene andesitic Cenozoic basement incision rates associated tiation of tilting of Pliocene strata in the Boca rocks overlie Oligocene rhyolites at the crest of with the deepest parts of the canyons range from Basin directly east of the FFS and the Donner the range from the Stanislaus drainage north- 0.10 to 0.40 mm/yr, averaged over the entire Summit area began at ca. 2.7 Ma (Mass et al., ward, and these units in turn overlie Eocene time span of incision (Table 1). The Kern River

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exhibits post–3.5 Ma incision rates of 1.1 mm/yr sion (referred to as paleorelief), because some the Tuolumne River southward, the tops of the (Saleeby et al., 2009), the fastest long-term inci- erosional lowering of topographic highs has andesitic deposits are inset below basement rims sion rate recorded in the Sierra. occurred during the late Cenozoic. The amount of paleocanyons. Paleorelief exceeds 1000 m of lowering of these topographic highs in the in parts of the San Joaquin drainage, and the Eocene to Miocene Incision late Cenozoic is probably small based on the southward increase in paleorelief coincides with extremely low erosion rates measured for bed- the southward increase in elevation in the range Incision between Eocene and Pliocene time is rock upland surfaces (erosion surfaces) of the (Figs. 2, 4). Paleorelief appears to greatly exceed recorded by the position of post-Eocene paleo- Sierra Nevada, ~0.004 mm/yr since ca. 11.8 Ma, 1000 m in the Kings River drainage based on thalwegs versus Eocene paleothalwegs (Fig. 3). based on Ar-Ar dating of volcanic rocks depos- the age of cave deposits at different levels above The maximum incision of Oligocene or Mio- ited on low-relief, high-altitude surfaces of the the canyon bottom (Stock et al., 2004). In addi- cene volcanic rocks below the base of Eocene Sierra (Phillips et al., 2011). The long-term tion, the steeper western slope along the south- channels is no more than ~30 m (Bateman and rates are similar to the shorter-term rates deter- ern Sierra corresponds to an abrupt eastward Wahrhaftig, 1966). The minimum age differ- mined by cosmogenic nuclide geochronology increase from 100 to over 600 m in paleorelief ence between the youngest Eocene gravels and for upland low-relief surfaces in the northern along the San Joaquin drainage (Huber, 1981). the end of ancestral Cascades (Mehrten Forma- (Riebe et al., 2000) and southern/central Sierra In addition to direct evaluation of paleorelief tion) deposition is ~30 m.y. (e.g., Garside et al., (Small et al., 1997; Stock et al., 2005) that range from late Cenozoic deposits, thermochronlogic 2005; Henry and Faulds, 2010). Averaging 30 m from ~2 to 20 m/m.y. (0.002–0.019 mm/yr) over studies (House et al., 2001; Clark et al., 2005; of incision over 30 m.y. yields 0.001 mm/yr as a the last 30–236 k.y. The estimates of paleorelief McPhillips and Brandon, 2012) suggest large maximum Eocene to Miocene incision rate. using Cenozoic stratigraphic-geomorphic rela- magnitudes of paleorelief (1 km or greater) for Incision of Miocene paleochannels through tionships are best constrained northward from the southern Sierra Nevada. Oliocene or Miocene volcanics or through base- the San Joaquin River drainage, owing to the The distribution of paleorelief suggests that ment to levels at or above the thalwegs of older scarcity of Cenozoic deposits to the south. the major along-strike differences in topo- paleochannels apparently took place in the Paleorelief in the Sierra Nevada increases graphic expression of the range are largely a crestal region of the Sierra Nevada (Busby et al., from north to south, with a signifi cant increase consequence of greater paleorelief in the south- 2008a, 2008b; Busby and Putirka, 2009). As south of the Stanislaus River drainage (Waka- ern (south of Stanislaus River) compared to noted above, incision that does not cut below the bayashi and Sawyer, 2001); this section of this the northern part of the range (Wakabayashi base of the previous paleochannel refl ects rapid paper largely reviews the earlier analysis with and Sawyer, 2001). This is consistent with the aggradation and constructional-depositional addition of new analysis of paleochannel depths hypothesis of Wahrhaftig (1965), who argued topography, followed by decrease in sediment associated with the largest paleochannels. Most that the west-facing topographic escarpments load that leads to incision back toward the pre- of the region north of the American River has of the south-central and southern Sierra were aggradational base level, rather than tectonic paleorelief of less than 400 m (Figs. 2, 4). In this erosional in origin and not late Cenozoic fault base level changes (i.e., uplift) or a discharge region, the paleorelief defi nes paleochannels that scarps. West-facing topographic steps in the increase in a stream, so this intra-formational deepen eastward toward the crest, with isolated Tuolumne River drainage coincide with pos- incision will not be considered in this discussion. paleohighs composed mainly of resistant meta- sible down-west offsets in the paleothalweg of In general, the incision history of the Sierra morphic rock (Fig. 4). North of the Tuolumne Miocene paleochannels, but the long distance Nevada (San Joaquin River drainage and north- drainage it is primarily the isolated basement between paleochannel deposits may alterna- ward) shows exceedingly low rates of stream highs that exceed the elevation of ridges capped tively permit a paleochannel reach of steeper incision from Eocene to Miocene–Pliocene by Miocene ande sites (Fig. 4), whereas from gradient, possibly controlled by basement erodi- time, followed by diachronous initiation of late Cenozoic incision at between 6 and 10 Ma in the San Joaquin River and ca. 3 Ma in the American and Feather River drainages. Late Cenozoic inci- Figure 4 (on following page). Photos showing paleorelief in the Sierra Nevada. (A) Photo of sion in the Kings to Kern River drainages of the Spanish Peak, northern Sierra Nevada, taken from the town of Quincy at the base of the southernmost Sierra began earlier (20 Ma) than east-facing escarpment of the Frontal fault system. Eocene (?) to Miocene paleochannel and drainages to the north, based on thermochrono- andesitic volcanic rocks cap granitic Spanish Peak, which is the highest peak in the area. Such logic data (House et al., 2001; Clark et al., 2005; a relationship illustrates the comparative lack of paleorelief in this area. (B) Photo northward Clark and Farley, 2007; Saleeby et al., 2009). from near the summit of Lyons Peak (approximate elevation of viewpoint 2680 m), North Fork American River drainage. The paleochannel (Soda Springs) fi lled with Oligo cene rhyo- PALEORELIEF AND DEPTH lite shows ~450 m of paleorelief at this point just west of the crest of the range. Local base- OF PALEOVALLEYS ment highs stand up to 700–800 m above the local basement surface and up to 1 km above the base of the deepest nearby paleochannels. This region is not far west of the crest and the Minimum topographic relief that existed at amount of late Cenozoic incision is minimal. This is illustrated by the difference in elevation the time of the deposition of Cenozoic deposits between the North Fork American River and the base of the Soda Springs paleochannel may be estimated by comparing the elevation of (~200 m). This area has been glaciated. (C) San Joaquin River drainage viewed southward basement topographic highs relative to the eleva- from Foerster Peak (3676 m) on the San Joaquin River–Merced River divide. All of the high tion of the local base of Cenozoic strata (Fig. 3) points in the photo are composed of basement. All elevations above this constitute paleorelief (Lindgren, 1911; Bateman and Wahrhaftig, in this glaciated terrain. Some of the highest peaks in the Sierra Nevada are seen in this view. 1966; Wakabayashi and Sawyer, 2001). The The highest peak within the San Joaquin River drainage or on one of the drainage divides of elevation difference is a minimum estimate of the San Joaquin River in this view is Mount Humphreys. Mount Whitney, Mount William- relief that predated late Cenozoic stream inci- son, and North Palisade are the three highest peaks of the Sierra Nevada.

198 Geosphere, April 2013

Figure 4.

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bility contrasts or actual west-down faulting canyons. For example, the Nine Hill paleovalley tion vastly exceeded erosion for that region for a (Wakabayashi and Sawyer, 2000). If west-down is 7 km wide with ~600 m of paleorelief near signifi cant part of the exhumation path (Saleeby late Cenozoic faulting is associated with the the crest in the present North Fork American et al., 2007; Chapman et al., 2012). topographic steps in the Tuolumne River drain- River drainage (Henry and Faulds, 2010) and Late Cenozoic exhumation of ranges east age, the maximum permissible vertical separa- the 9-km-wide Soda Springs paleovalley shows of the Sierra Nevada in the Basin and Range/ tion of paleochannels across the steps is much ~450 m of paleorelief near the crest in the head- Walker Lane signifi cantly exceeds (by several less than half of the height of any associated waters of the South Fork Yuba River drainage kilometers) late Cenozoic exhumation of the step. No signifi cant offsets are observed in the (Sylvester et al., 2007) (Fig. 4). Sierra Nevada, and appears to refl ect pri marily reconstructed 10 Ma San Joaquin paleochannel Further west of the crest, near the region of tectonic denudation by normal faults, rather of Huber (1981). In contrast, thermochronologic maximum late Cenozoic incision, the northern than erosion (Stockli et al., 2002, 2003). The and structural data presented by Maheo et al. Sierra paleovalleys appear somewhat broader. dip-slip component of late Cenozoic faulting (2009) showed that some of the major topo- For example, the Eocene South Fork Yuba along the eastern margin of and in the interior graphic escarpments of the Kern River drain- paleochannel (downstream of the confl uence of of the Sierra is normal, so Cenozoic exhumation age and vicinity are late Cenozoic fault scarps, the Nine Hill and Soda Springs paleovalleys), rates within the Sierra probably exceed erosion rather than pre-Neogene erosional escarpments. presently located in the Middle Fork American rates. Accordingly, the Cenozoic exhumation Paleorelief defi nes paleochannels that crossed River drainage, exhibits about ~450 m of paleo- rates shown on Figure 5 should be considered the Sierra prior to fi lling of these channels relief across a 20-km-wide paleovalley 25 km maximum erosion rates. with Miocene to Pliocene andesitic volcanic west of the crest, and 200 m of paleorelief across Comparing the crystallization depth of the rocks. The deepest paleochannels are the old- a paleocanyon of the same width 50 km west of youngest plutons (Ague and Brimhall, 1988) est (Eocene) channels. Oligocene paleochannels the crest. The paleo–Yuba River (downstream overlapped by Eocene deposits with their age, appear to have largely followed the Eocene of the confl uence of the major paleovalleys) Wakabayashi and Sawyer (2001) estimated a drainages (Henry, 2008; Henry and Faulds, exhibits ~180–280 m of paleorelief across long-term averaged exhumation rate, equated 2010; Garside et al., 2005) (Fig. 2). At the crest, a 20-km-wide paleovalley. Within the broader to erosion rate on the basis of transpressional preserved paleochannel relief appears to be paleovalleys, narrower inner paleochannels are tectonics that prevailed at the time (see above), ~200 m for the paleo–Feather River, 450 m for also observed (Garside et al., 2005). of 0.26–0.35 mm/yr between 100 and 57 Ma the middle branch of the paleo–Yuba River, and (Fig. 5). Clark et al. (2005) showed a strong 450–600 m for the southern branch of the paleo– EROSION RATES IN THE SIERRA linear relationship between apatite (U-Th)/He Yuba River (Henry, 2008; Henry and Faulds, NEVADA AND DEPOSITION RATES age (closure temperature ~65 °C) and eleva- 2010; and this analysis). This paleorelief associ- IN THE GREAT VALLEY tion relative to a regional erosion surface for ated with these paleochannels apparently con- samples from the Tuolumne to northern Kern tinues to increase east of the crest. For example Incision rates represent the erosion rate along River drainages, and calculated an exhumation the southeast (or southernmost) tributary of the the bottom of a stream valley. As noted above, rate of 0.04 mm/yr, integrated over canyons and paleo–Yuba River (southeast tributary of Nine studies have determined low erosion rates for the interfl uves, from ca. 80 Ma to 20 Ma (younger Hill paleovalley) has a paleorelief of 1200 in the upland surfaces of the Sierra Nevada on scales age limit revised from 32 to 20 Ma following vicinity of Yerington, ~100 km east of the pres- of the last tens to hundreds of thousands of years Saleeby et al., 2009) (Fig. 5). Using samples ent Sierran Frontal faults, and the paleorelief and from ca. 12 Ma to the present. Longer-term from the Yuba River drainage, Cecil et al. (2006) decreases to ~600 m directly east of the crest in erosion rates can be obtained from barometry estimated a post–ca. 65 Ma exhumation rate of the Tahoe Basin area (Henry and Faulds, 2010) of the youngest plutons, thermochronology of 0.02–0.04 mm/yr from apatite (U-Th)/He ages, (Fig. 2). Another major paleochannel, the Gol- the basement, and sedimentary overlap rela- and ca. 90–65 Ma exhumation rates of 0.2–0.8 conda Canyon paleovalley of the Tobin Range tionships, with the assumption that exhumation mm/yr based on comparison of zircon (closure of Nevada, exhibits over 1000 m of paleorelief recorded by these data equals erosional denuda- temperature modeled as 173 °C) and apatite ~200 km east-northeast of the Frontal faults tion. Such an assumption holds if reverse and (U-Th)/He ages. Apatite (U-Th)/He ages from of the Mohawk Valley fault zone (Gonsior and thrust faulting drove exhumation, but not nor- the southernmost Sierra, related to sample Dilles, 2008) (northeast part of Fig. 2). mal faulting; in the latter case exhumation can elevations corrected for late Cenozoic faulting, Paleorelief associated with paleochannels greatly exceed erosional denudation (e.g., Platt, suggest exhumation rates of 0.06 mm/yr after within the Sierra defi nes channels broader than 1986). During the main stage of exhumation of ca. 80 Ma (Maheo et al., 2009). the present-day canyons (Henry et al., 2012). the basement (pre-Eocene), the dominant mode A comparison of pluton crystallization ages For example, the Middle Fork San Joaquin of dip-slip deformation north of the Kern River (U-Pb, zircon) and barometry, overlap relation- paleocanyon exhibits ~1 km of paleorelief with drainage is likely to have been reverse faulting ships of late Cretaceous strata in the western ~30 km of distance between the paleocanyon within a transpressional regime (e.g., Renne et Central Valley, hornblende and biotite Ar-Ar rims. In contrast, the post–10 Ma incision al., 1993; Tobisch et al., 1995; Tikoff and de and K-Ar cooling ages, and zircon and apatite deepened the canyon by ~1 km with ~5 km of Saint Blanquat, 1997; Pachell et al., 2003), with (U-Th)/He ages show that the southern Kern width associated with this younger incision. In comparatively minor additional exhumation in and adjacent drainages experienced rapid exhu- the northern Sierra, the deepest late Cenozoic the late Cenozoic (≤1 km associated with the mation at rates of ~2 mm/yr between ca. 90 and canyon, the North Fork Feather River, exhibits deepest canyons as noted above). For the Kern 80 Ma (Saleeby et al., 2010; Chapman et al., 1.2 m of incision across a canyon 6 km wide. River drainage and environs, exhumation of 2012) (Fig. 5). The northern Sierra Nevada The Eocene paleovalleys of the northern Sierra the batholith, from depths of up to 30 km, was may have also experienced a similar history of appear to have been narrower than those of the initially associated with erosion (and presumed rapid late Cretaceous exhumation of short dura- south in the region near the present Sierran crest, shortening), followed by late Cretaceous normal tion, so that the interpreted 65–90 Ma period of although still broader than the late Cenozoic faulting associated with extension, so exhuma- high exhumation rates (Cecil et al., 2006) may

200 Geosphere, April 2013

Exhumation rate from pluton crystallization ages, cooling ages, stratigraphic overlap, zircon and apatite (U-Th)/He ages, southern Sierra ~2.0 mm/yr

break in scale Sediment accumulation rates, Great Valley: latitude color key 0.8 ~36.1°N 0.8 Exhumation rate ~36.8°N ~36.85°N 0.7 from zircon and ~37.0°N 0.7

apatite (U-Th)/He Ma 112-65 ~37.4°N ages, N. Sierra 0.6 ~36.8°N 0.6 Exhumation rate ~37.5°N

from Eocene overlap 65-0 Ma ~38.1°N 0.5 of plutons, N. Sierra Frontal faulting, 0.5 Major stream incision 0.4 N./Central Sierra 0.4 Mehrten 0.3 Exhumation rate Formation 0.3

(mm/yr) from apatite (U-Th)/He ages, southernmost Sierra 0.2 0.2 Exhumation rate from apatite (U-Th)/He 0.1 ages, N. Sierra 0.1

0 Erosion Exhumation or erosion rates, Sierra Sediment accumulation rates, Great Valley surface 110 100 90 80 70 60 50 40 30 20 10 0 denudation active arc Laramide Time (Ma) rate deformation Franciscan Coastal Belt accretion

Figure 5. Erosion rates in the Sierra Nevada and deposition rates in the Great Valley. Revised and updated from Wakabayashi and Sawyer (2001), with exhumation rates derived from thermochronologic data by Clark et al. (2005), Cecil et al. (2006), Saleeby et al. (2009), Saleeby et al. (2010), and Chapman et al. (2012), and upland surface erosion rates of Phillips et al. (2011).

have been briefer, with higher rates, but exist- McPhillips and Brandon (2012) presented a a direct assessment of erosion rates within the ing thermochronologic data are not suffi cient to thermo-kinematic model, incorporating pub- Sierra Nevada as reviewed above, so the Great confi rm this. lished hornblende barometry from granitic plu- Valley sedimentation rates are not needed to All areas of the Sierra appear to have expe- tons, apatite fi ssion-track and apatite (U-Th)/He estimate Sierran erosion through time. How- rienced low exhumation and erosion rates of ages from the Merced River to the Kern River ever, the Great Valley deposition rates give 0.06 mm/yr or less from the late Cretaceous to drainages, and physical properties of the Sierran insight into the total sediment load carried the onset of late Cenozoic incision. Exhuma- crust. Their results show a decrease in surface by Sierran streams, whose drainage basins tion rates for this period appear to have been elevation from ca. 100 Ma to the lowest paleo- included signifi cant area east (upstream) of the slightly higher in the southernmost Sierra (0.06 elevations in mid- to late Cenozoic time, followed Sierra. The analysis of sediment load of Sierran mm/yr) compared to the northern Sierra (0.02– by onset of surface and rock uplift and associ- streams is relevant to assessing aspects of 0.04 mm/yr) (Maheo et al., 2009; Cecil et al., ated stream incision between 30 and 10 Ma. stream power connected with the incision his- 2006). The greater amount of erosion below Wakabayashi and Sawyer (2001) compiled tory of these streams. the presumed early Cenozoic erosion surface in stratigraphic thickness versus age from pub- Wakabayashi and Sawyer (2001) acknowl- the southernmost Sierra appears to refl ect this lished studies in the Great Valley and pos- edged that some sediment carried by trans- (Chapman et al., 2012). In addition, the Kern tulated that the accumulation rate recorded Sierran streams bypassed the forearc basin River drainage appears to be associated with there was approximately representative of the (Great Valley) and reached the trench (accreted higher slip rates and cumulative vertical sepa- erosion rates in the Sierra Nevada region that as part of the Franciscan subduction complex ration of intra-Sierran faults, compared to other constituted the main source for those clastic or completely subducted), but proposed that parts of the Sierra (Maheo et al., 2009; Nadin sedimentary rocks (Fig. 5). In the years since the Great Valley deposition rates were pro- and Saleeby, 2010; Amos et al., 2010). 2001, thermo chronologic studies have allowed portional to the erosion rates in the drainage

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basins of those streams. Great Valley sedimen- down vertical separation of Mehrten Formation uphill across the fault scarps. Consistent with tary rocks record high Paleocene and earlier equivalents along the FFS, in the northernmost this relationship, the major river drainages of deposition rates and low Eocene to Miocene Sierra along the Mohawk Valley fault zone and the range, excluding the Feather River, such as rates, consistent with the estimates of Sierran related faults in the Feather River drainage, those of the San Joaquin (Matthes, 1930; Huber, exhumation and erosion rates presented above, ranges between 600 m and 1000 m, and may 1981), Stanislaus, and Yuba Rivers (Lindgren, followed by a notable increase in deposition reach 1500 m along the western margin of the 1911), have been beheaded by movement along rates in the Pliocene that coincided with late Lake Tahoe Basin, apparently greater than that the Frontal faults. The relationship of Frontal Cenozoic incision initiation (Fig. 5). The Fran- along the western strands of the FFS to the north faulting to beheaded drainages and the distribu- ciscan subduction complex records signifi cant or south of this basin (Wakabayashi and Sawyer , tion of volcanic rocks suggests that the FFS and frontal accretion in Eocene time, represented 2001). In the headwaters of the Stanislaus River Walker Lane belt have encroached westward by the accretion of the Coastal Belt (Dumitru drainage, the vertical separation of the ca. 9 Ma in the late Cenozoic (Slemmons et al., 1979; et al., 2013). Although this may suggest a sig- Eureka Valley tuff is ~1100 m (Noble et al., Dilles and Gans, 1995; Jones et al., 2004). The nifi cant infl ux of detritus from Sierran streams 1974; Slemmons, 1966). At the headwaters of timing of initiation of dextral faulting along that bypassed the forearc basin and reached the the San Joaquin River a 2.2–3.6 Ma volcanic some parts of the fault system may have dif- trench, detrital zircon ages and regional strati- unit is vertically separated by ~980 m across the fered from the initiation of signifi cant dip-slip graphic correlations indicate that the bulk of Frontal faults in areas that do not appear to have faulting alone (e.g., Cashman et al., 2009). In this clastic sediment originated from well north additional subsidence related to the Long Valley the summary below, I will focus on evidence of the Sierran proper and was not transported to caldera (Wakabayashi and Sawyer, 2001, inter- for timing of dip-slip (normal) faulting because the trench-forearc basin region by trans-Sier- preted from Bailey, 1989). this faulting directly bears on landscape evo- ran streams (Dumitru et al., 2013). Note that Whereas the topographic escarpment along lution through development of topographic although most of the Coastal Belt is located the southern Sierra Nevada reaches 3.3 km in escarpments, base level changes for streams, north of the San Francisco Bay, restoration of height, the range is capped by granitic base- and beheading of streams. dextral San Andreas fault system slip (~200– ment rather than late Cenozoic volcanic rocks, In the Feather River area, movement on the 250 km, depending on position of specifi c and Owens Valley has 1.1 or 2.1 km of late Honey Lake fault zone initiated between ca. 3 Coastal Belt exposures) restores these expo- Cenozoic basin fi ll according to different inter- and 6 Ma, based on regional kinematic consid- sures southward (Wakabayashi, 1999) so that pretations of gravity data (Pakiser et al., 1964, erations and the ages of basinal deposits along they are directly west of the Carquinez Strait in and Bachman, 1978, respectively). The total the northern Walker Lane (Faulds et al., 2005; Eocene time, the approximate position of the height of the bedrock escarpment in Owens Henry et al., 2007; Hinz et al., 2009), whereas submarine canyons that drained the northern Valley is thus 4.4–5.4 km, but the lack of Ceno- movement on the present FFS in the Mohawk Great Valley forearc basin to the trench during zoic deposits atop the crest precludes a straight- Valley area can be constrained only as post- Eocene and later time (e.g., Dickinson et al., forward estimate of fault separation and timing. Mehrten or post–ca. 5 Ma, based on identi- 1979). Geochemical similarities of Francis- Jayko (2009) correlated erosion surfaces in the cal offsets of the 16 Ma Lovejoy Basalt and can and coeval Great Valley Group rocks also Sierra and White Mountains and used them to overlying Mehrten Formation (Wakabayashi appear to point to a common source of trench estimate Cenozoic vertical separation across and Sawyer, 2001). Movement on some of and forearc-basin sediments during the sub- FFS and other Walker Lane fault strands to the most signifi cant faults of the Frontal fault duction regime (Ghatak et al., 2011). the east. Because erosion surfaces occur at or system crossing the North Fork Feather River near the summit of high peaks on the crest of canyon probably did not begin until after LATE CENOZOIC FAULTING the range, the height of the escarpment may be 600 ka (Wakabayashi and Sawyer, 2000). directly tied to vertical separation on the FFS Thus, the western margin of the Walker Lane Late Cenozoic faulting on the margins of and (~3 km plus the buried part of the escarpment belt appears to have stepped 50 km westward within the Sierra Nevada block provides infor- in Owens Valley). Warping of an 11.7 Ma basalt from the Honey Lake area to the present east- mation on the potential linkage of tectonics and fl ow across Frontal fault deformation associ- ern escarpment of the northern Sierra Nevada topographic evolution within the range. Within ated with the Coyote warp near Bishop records within the last 5 m.y., and has encroached into this context, I will review information on the ~1600 m of vertical separation, and consid- the northernmost part of the range since the nature and evolution of the Frontal fault system eration of stratigraphy in the northern Owens mid- to late Quaternary. (FFS), deformation within the Sierran block, Valley fi ll indicates at least 600 m more verti- Westward encroachment of the western and spatial-temporal comparisons between fault cal separation valleyward of the lowest basalt Walker Lane belt margin occurred along much activity and incision. outcrop for a total >2200 m of post–11.7 Ma of the tectonic boundary. The western edge vertical separation (Phillips et al., 2011). of the central Walker Lane belt from 38°N to Late Cenozoic Faulting Along the 39°N progressively stepped 100 km westward Frontal Fault System (FFS) Timing of Late Cenozoic Vertical from 15 to 7 Ma, based on detailed struc- Separation Along the FFS: Westward tural, stratigraphic, and geochronologic stud- In this this section, I will focus on: (1) the Encroachment During the Late Cenozoic ies (Dilles and Gans, 1995). East of the Lake amount of late Cenozoic vertical separation Tahoe area, major east-down faulting began across the FFS, with emphasis on its western Many of the late Cenozoic volcanic rocks before 10.3 Ma in the Verdi Basin, based on strands, (2) the timing of faulting, and (3) the of the central and northern Sierra Nevada had ages of syntectonic basinal sediments (Henry long-term evolution of this system, whereas I will sources east of the FFS (e.g., Durrell, 1966; and Perkins , 2001), then encroached ~20 km not review some of the details of the physical Slemmons, 1966), suggesting that Frontal westward to the Boca Basin at ca. 2.7 Ma (Mass characteristics of the FFS along strike presented faulting did not begin until after the eruption of et al., 2009). Slemmons et al. (1979) also sug- in Wakabayashi and Sawyer (2001). East- these deposits; the fl ows could not have fl owed gested westward encroachment of the Walker

202 Geosphere, April 2013

Lane into the Sierran block in the late Ceno- have started until ca. 8–11 Ma along southern AZIMUTH VERSUS GRADIENT zoic, with somewhat broader age constraints. Owens Valley, and until ca. 3.5 Ma along north- OF MODERN RIVERS AND Based on published analysis of volcanic depos- ern Owens Valley. PALEOSTREAMS its and gravels (Huber, 1981; Bailey, 1989), Wakabayashi and Sawyer (2001) proposed Late Cenozoic Internal Deformation of A comparison of the gradients of Cenozoic that the Frontal fault system encroached at the Sierra Nevada paleochannel deposits and modern Sierran least 40 km westward after 3 Ma to its present rivers gives insight into the landscape evolu- position in the region of the headwaters of the Late Cenozoic internal deformation of the tion of the mountain range. Lindgren (1911) Middle Fork San Joaquin River. Sierra Nevada, recorded by faulting and local examined the present gradients of the thalwegs There are exceptions to the progressive west- tilting of late Cenozoic deposits, is minor com- of paleochannels of the gold-bearing Eocene ward encroachment of faulting into the Sierran pared with faulting along the FFS (e.g., Lind- gravels and showed a systematic relationship block. For example, signifi cant faulting occurred gren, 1911; Christensen, 1966; Bateman and between the azimuth of the paleochannel reach at ca. 10 Ma in the Sierran crestal region at the Wahrhaftig, 1966) excluding the Kern River area and the gradient. The steepest reaches have azi- headwaters of the Mokelumne and Stanislaus and the northernmost part of the range. Intra- muths at right angles to the axis of the range, Rivers (Busby et al., 2008a), long before many Sierran faults have slip rates of hundredths of a whereas the lowest-gradient reaches have azi- of the westward jumps reviewed above. millimeter per year or less, and vertical separa- muths parallel to the axis of the range (Fig. 6A). Bachman (1978) suggested that the Sierran tion of less than ~40 m, except in the areas near Hudson (1955) and Jones et al. (2004) refi ned escarpment in the Owens Valley area did not and directly west of the crest (Wakabayashi and and reaffi rmed this relationship (Fig. 6B). The form until 2.3–3.4 Ma. Bachman’s (1978) data Sawyer, 2000). Internal deformation is distrib- variation of azimuth of paleochannel reaches and observations constrain timing of the uplift of uted fairly evenly across the range, with more does not correspond to differences in basement the White Mountains (the range east of Owens closely spaced faults and faults with greater geology when compared to general compila- Valley), but do not directly constrain movement displacements (up to ~200 m on some faults) in tions of basement geology, although the long on the FFS. Jayko (2009) proposed that the FFS the area directly west of the crest (see detailed (~55 km) north-trending section of the paleo– in the Owens Valley area began movement ca. discussions in Wakabayashi and Sawyer , 2000). South Yuba River that records the widest range 10 Ma. Based on the paleo drainage history across The internal faulting near the crest appears to of paleochannel azimuth variations has an aver- the range crest in the Bishop area (northern be related to en echelon east-down Frontal faults age trend parallel to the north-trending struc- Owens Valley), and the position of erosion sur- that cross the crest (Wakabayashi and Sawyer, tural grain within primarily metasedimentary faces covered by volcanic rocks of 11.7–11.8 Ma 2000) (Fig. 2). Because of these faults down- rocks (e.g., Saucedo and Wagner, 1992). and 3.4 Ma age, Phillips et al. (2011) concluded dropping the crest, projection of the tilts of Some of the Eocene paleochannel reaches that most of the vertical separation along that the Lovejoy Basalt and Table Mountain Latite along the paleo–South Yuba (location of this part of the FFS took place after 3.4 Ma. Zircon from the western margin of the Sierra Nevada section of paleochannel noted on Fig. 2) have and apatite (U-Th)/He ages from a vertical tran- to the crest appears to overestimate the actual negative gradients that gain elevation in the sect on the eastern escarpment 20 km south of crestal elevations of these strata at the crest by paleo-downstream direction (Lindgren, 1911; Mount Whitney, the highest point (4419 m) on 315–365 m compared to their actual outcrop Hudson, 1955; Yeend, 1974; Jones et al., 2004). the Sierran crest, show that movement on the elevations. Similar deformation near the crest Hudson (1955) confi rmed the paleofl ow direc- FFS in that area did not begin until after 11 Ma may be expected in much of the Sierra north tion in these reaches by observation of pebble (Maheo et al., 2004). An investigation of the El of the Kings River headwaters, owing to the en imbrication and cross-bedding orientations. Paso Basin, east of the southernmost Sierra and echelon geometry of FFS strands (Wakabayashi These reaches range in length from 2.4 to south of Owens Valley, indicates that drainage and Sawyer, 2001). 11.5 km (consecutive uphill reaches combined) eastward from a rising Sierra began by 8 Ma, A west-down warp and/or zone of distrib- (Hudson, 1955). The most signifi cant uphill suggesting the beginning of east-down Frontal uted faulting, known as the Chico monocline, reach gains 66 m over a distance of 11.5 km, and faulting by that time at that latitude (Loomis and deforms Pliocene (Yana volcanic center/Tuscan the second-most signifi cant uphill reach gains Burbank, 1988). Formation) volcanic rocks along the western 44 m over 2.4 km in the paleo-downstream direc- Initiation of FFS movement did not coincide margin of the northernmost part of the range, tion (Hudson, 1955). All of these uphill reaches with initiation of incision along much of the north of the Feather River (Harwood and Helley, have azimuths trending more easterly than the length of the Sierra. For example, in the Feather 1987). Internal faulting east of the Chico mono- axis of the range (Figs. 6A, 6B). River drainage, incision may have begun at ca. cline has higher slip rates (tenths of a millimeter The systematic relationship between steep- 3 Ma, whereas many of the western strands of per year or greater) than in regions to the south ness and azimuth exhibited by the Eocene the Frontal faults may not have begun move- and is part of a broad fault zone that forms the paleochannels starkly contrasts with modern ment until after 0.6 Ma. Faulting and incision northern margin of the Sierra Nevada micro- Sierran rivers, which show no correspondence may have begun in the North Fork American plate (Sawyer, 2009, 2010). The Sierra of the between azimuth and gradient for reaches of headwaters at ca. 3 Ma, whereas faulting began Kern River drainage and adjacent areas to the approximately equivalent length (1.4–9.1 km) to at ca. 10 Ma in the Sonora Pass (Stanislaus west is cut by multiple late Cenozoic faults, those examined for the Eocene paleo channels, River headwaters), but incision apparently ini- some exhibiting higher (tenths of a millimeter including reaches that trend easterly of the tiated at 4–3.6 Ma. In the San Joaquin River per year) slip rates, and larger cumulative late axis of the range (Fig. 6C). However, the lack drainage, incision began between 10 and 6 Ma, Cenozoic vertical separation (1 km or more) of correspondence between bedrock lithology whereas the Frontal faults appeared to have ini- than internal faults in any other part of the and azimuth is common to both paleochannels tiated at ca. 3 Ma. In the Kern to Kings drain- microplate excluding the northern boundary and modern rivers. The North Fork American ages, the fi rst episode of late Cenozoic incision zone (Maheo et al., 2009; Amos et al., 2010; River and Tuolumne River reaches plotted in began at 20 Ma, but Frontal faulting may not Nadin and Saleeby, 2010; Brossy et al. 2012). Figure 6C have incised into phyllites, with a

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A Map View Schematic of Paleochannel strong structural grain imposed by the steep Azimuth-Gradient Relationships foliation and steep lithologic contacts (Wagner

Paleochannel et al., 1990). In contrast, the San Joaquin River reaches with the wide range of azimuths plotted

“Uphill” gradients, in Figure 6C cut primarily granitic basement. Steepest gradients, for reaches with axis-perpendicular paleoflow azimuth more easterly than range DISCUSSION axis

Shallowest gradients, Results or Predictions Common to axis-parallel Range axis ori Competing Models of Landscape Evolution

Many lines of evidence and interpretation enation bear on the debate over whether late Cenozoic uplift of the Sierra occurred or not. Much of Range axis orientation Range axis orientation paleoflow direction the evidence and interpretations accommodate Perpendicular to Range axis orientation either model, whereas some more clearly support one position at the exclusion of the other. Before discussing the issue of late Cenozoic Sierran uplift in detail, I will brieﬂ y list the evidence and interpretations that support both com- B Azimuth and Gradients of Thalweg of peting interpretations rather than supporting or 24 South Yuba Paleochannel challenging one at the exclusion of the other. 22 1.20° 20 High paleoelevations east of the present 18 Sierra Nevada, as exempliﬁ ed by the Nevada- 16 plano concept (e.g., Christiansen and Yeats, 14 0.80° 12 1992; Dilek and Moores, 1999; Henry, 2008) do 10 not contradict late Cenozoic uplift of the Sierra 8 0.40° 6 Nevada, contrary to some published statements, 4 positive ‘downhill’ gradients

Gradient m/km such as those of Wolfe et al. (1997, 1998) who 2 Gradient, degrees postulated a post-Miocene decrease in Sierran 0 no gradient 0 –2 elevations based on high interpreted paleoeleva- –4 tions for samples collected east of the range. –6 –0.40° –8 The various lines of evidence and resultant –10 negative interpretations merely show that the area east of ‘uphill’ gradients –12 the Sierra was higher than the Sierra in Eocene –14 –0.80° –16 to Miocene time, so that streams fl owed west- –18 ward from what is now Nevada across what has –20 –1.20° become the Sierra Nevada to the Pacifi c Ocean (Henry, 2008; Henry and Faulds, 2010; Henry 180° 270° 360° 90° et al., 2012). Such models and observations do 90° to axial orientation axial orientation Azimuth of Thalweg of Paleochannel not preclude post-Miocene uplift of the Sierra (from Jones et al., 2004) Nevada, and the concept of higher elevations Azimuth and Gradients of Modern River Reaches east of the Sierra Nevada was accepted by advo- C cates of late Cenozoic uplift who agreed that 14 North Fork American River 1 0.80° Tuolumne River pre–Mio-Pliocene streams fl owed westward 12 San Joaquin River 3 across the area that later became the Sierra (e.g., 10 2 1 8 4 Huber, 1981; Wakabayashi and Sawyer, 2001). 4 3 2 8 0.40° 6 3 2 5 (U-Th)/He apatite ages within most of the 4 Gradient m/km 6 7 2 1 Sierra Nevada exceed 32 Ma, indicating less

9 Gradient, degrees 5 than ~2–4 km of exhumation since then (House 90° 180° 270° 360° et al., 2001; Clark et al., 2005; Cecil et al., 2006).

90° to axial orientation The maximum amount of late Cenozoic exhu- axial orientation axial orientation Azimuth of Reach of Stream mation is recorded by the deepest stream incision of ~1 km. Thus, the lack of late Cenozoic Figure 6. Azimuth versus gradient relationships of paleochannels and modern Sierran rivers. exhumation ages do not preclude the small mag- (A) Schematic map view showing the relationship between paleochannel azimuth and gradi- nitude of late Cenozoic rock uplift and incision ent compared to the range axis orientation, as documented for the Eocene South Yuba River that has been proposed (~1–2 km). Wakabayashi paleochannel (Lindgren, 1911; Hudson, 1955; Jones et al., 2004). (B) Azimuth compared and Sawyer (2001) and Clark et al. (2005) pre- to gradient for the Eocene South Yuba paleochannel from Jones et al. (2004). (C) Azimuth sented evidence for paleorelief of up to 1–2 km compared to gradient for modern rivers keyed to reaches on Figure 2. in the highest part of the Sierra Nevada, and

204 Geosphere, April 2013

proposed moderately high paleoelevations in azimuth of the paleochannel. The systematic rela- by Wakabayashi and Sawyer (2001), and I will the southern Sierra. Such paleorelief and paleo- tionship between paleochannel azimuth and gra- update some of their arguments here. Stream elevations of the southern Sierra do not preclude dient, with the lowest-gradient reaches parallel to power, which governs incision or aggradation, 1–2 km of subsequent rock and surface uplift the range axis and the steepest reaches perpen- is a function of discharge, gradient, erodibility (Wakabayashi and Sawyer, 2001; Clark et al., dicular to the range axis, demonstrates westward of the bedrock substrate, and the amount of sedi- 2005), although estimates of large amounts of tilting and rock uplift of the Sierra Nevada with ment transport load (e.g., Sklar and Dietrich, paleorelief have been used to argue against late the tilt axis parallel to the range axis (Lindgren, 1998). During the history of incision of Sierran Cenozoic uplift (e.g., House et al., 1998). 1911; Hudson, 1955; Jones et al., 2004) (Figs. streams, the bedrock geology, and hence erodi- An apparent post–Oligo-Miocene decrease 6A, 6B). Most signifi cantly, the uphill reaches bility, in any one position of the stream can be in mean elevation east of the Sierra Nevada has of the paleochannels cannot refl ect original gra- regarded as constant, because it comprises either been proposed on the basis of paleobotanical dients and must have resulted from post-deposi- metamorphic rocks with steep lithologic con- interpretations (e.g., Wolfe et al., 1997, 1998), tional westward tilting. The tilting of the uphill tacts and foliation dips, or granitic rocks (e.g., stable isotopic data (Horton and Chamberlain, paleochannel reaches is not a product of local- Bateman and Wahrhaftig, 1966; Schweickert, 2006), and evidence of signifi cant extensional ized west-down tilting or faulting, for profi les of 1981; Moores and Day, 1984; Sharp, 1988; collapse (thinning) of the lithosphere in this Cenozoic paleochannels in these drainages show Saleeby et al., 1989). Greater discharge and region (Wernicke et al., 1996). As noted by no evidence of a locally tilted block or west-down higher gradients raise the stream power and abil- Wakabayashi and Sawyer (2001), this does faulting (Wakabayashi and Sawyer, 2000). More- ity of the stream to incise, whereas a large sedi- not contradict interpretation of post-Miocene over, the inverse gradient of these reaches is con- ment load above a threshold amount will retard uplift in the Sierra to the west. Although the sistent with their azimuth that trends east of the the stream’s ability to incise (Sklar and Dietrich, late Cenozoic faults of the FFS show dip-slip range axis (Figs. 6A, 6B). 1998). Progressive westward encroachment of displacements that reach or exceed 3 km with As noted previously, there is no relationship down-to-the-east faulting since the Oligocene, the range side up, this does not demonstrate or between paleochannel or modern river azimuth alluded to earlier, has progressively beheaded preclude late Cenozoic uplift because the verti- and basement rock type, an explanation pro- Sierran stream systems and reduced their drain- cal movement of the Sierran side relative to sea posed by Cassel et al. (2012) for relationship of age area. Climate from the Late Miocene to the level cannot be directly determined from the paleochannel gradient and azimuth (but not for present has not been wetter than earlier times displacement alone. inverse gradients). Cassel et al. (2012) pointed and in fact, the climate that prevailed during The proposal of a rain shadow in the west- out that a paleochannel thalweg does not con- the deposition of the Eocene gravels is postu- ern Basin and Range at least as far back in time nect channel bottom points formed at the same lated to be warmer and considerably wetter than as 16 Ma, based on stable isotopic data from time during a river’s history. However, azimuth- Pliocene to recent climate (e.g, Zachos et al., authigenic minerals (Poage and Chamberlain, gradient relationships reviewed above do not 2001; Minnich, 2007). Accordingly the progres- 2002), also does not refute late Cenozoic Sier- depend on short-term time equivalence of differ- sive reduction of drainage area indicates a cor- ran uplift because (1) all but one of the samples ent points on the paleothalweg. Rather, the rela- responding reduction in stream discharge from are taken from positions east of at least one set tionship refl ects the history after the formation Eocene to Pliocene time. of normal faults that are themselves east of FFS and abandonment of the paleochannel. Detailed The record of erosion in the Sierra Nevada, (the eastern boundary of the Sierran microplate mapping and fi eld inspection by previous work- and of deposition in the Great Valley that shows predating late Cenozoic uplift was east of the ers (e.g., Lindgren, 1911; Hudson, 1955; Gar- a signifi cant increase in deposition rate in Late present FFS as noted earlier), and (2) propo- side et al., 2005) demonstrates that segments Miocene to Pliocene time, suggests an increase nents of uplift do not dispute the possibility of a with “uphill” gradients are a not consequence in sediment load in Sierran rivers at the time inci- relatively high southern Sierra Nevada at 16 Ma of fi lling of a paleochannel and spilling over a sion began. Accordingly discharge should have (e.g., Wakabayashi and Sawyer, 2001; Clark divide, for the paleochannel network is well con- progressively decreased since Eocene–Oligo- et al., 2005; Saleeby et al., 2009). strained over these reaches. The latter scenario cene time, and sediment load increased since would also demand a fortuitous relationship the Pliocene. Both of these factors should reduce Late Cenozoic Rock Uplift: Yes or No? between the spillover points and buried gullies stream power, so only an increase in stream gra- with (northward) azimuths east of the range axis. dients could have caused the initiation of inci- The western margin of the Sierra Nevada Whereas the amount of exhumation in the late sion in Pliocene time, and only rock uplift and (eastern margin of the Central Valley) from about Cenozoic (≤1.2 km) is too small to be recorded tilting could have increased the stream gradient. the San Joaquin River northward has maintained by apatite (U-Th)/He ages (House et al., 2001; In contrast to the interpretation summarized approximately the same elevation from early to Clark et al., 2005; Cecil et al., 2006), transverse above, Cassel and Graham (2011) proposed high mid-Cenozoic time, a view endorsed by those apatite (U-Th)/He age gradients have been inter- sediment loads for streams crossing the Sierra both in favor and against the premise of late preted to indicate signifi cant westward tilting of in the Oligocene and Eocene. Such a proposal Cenozoic uplift in the Sierra (compare Cassel the range since 5 Ma (McPhillips and Brandon, would suggest that more sediment from the et al., 2012, to Wakabayashi and Sawyer, 2001, 2010) and 4He/3He dates record late Cenozoic Sierran streams bypassed the Great Valley depo- for original sources). Accordingly, if late Ceno- incision of the Kings River (Clark and Farley, center during Oligocene and Eocene time than zoic rock and surface uplift of the Sierra has 2007). Numerical modeling of multiple sets before or after, so that the Great Valley deposi- not taken place, then the original gradients of of thermochronologic data suggests ~2 km of tional record as shown in Figure 5 poorly rep- the Eocene paleochannels are recorded by the crestal uplift in the region from the Merced to resents relative sediment delivery rates from modern-day exposures, whereas if late Cenozoic Kern River drainages that began at 30–10 Ma Sierran streams at that time. However, the Oligo- uplift took place, the associated westward tilt- (McPhillips and Brandon, 2012). cene sediment load, represented mostly by ash- ing would have modifi ed the original gradients The late Cenozoic initiation of incision fl ow tuff deposition in paleochannels (e.g., Henry with the degree of modifi cation dependent on the strongly supports late Cenozoic uplift as noted and Faulds, 2010), pales in comparison to the

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much larger amount of andesitic material gener- 2.6 Ma (Zachos et al., 2001) as well the onset for ent incompatibility of the stable isotopic and ated by the ancestral Cascades arc, and Miocene glaciation in the Sierra, for which the earliest pre- botanical paleoaltimetry, and related interpreta- onset of deposition of these andesites is refl ected served deposits may be ca. 1.5 Ma (Huber, 1981; tion of steep paleogradients, invites additional by an increase in sedimentation rate in the Great reviewed in Clark et al., 2003). Thus, increased explanation. In this section, I will present alter- Valley (Fig. 5). In spite of the large volumes of upland erosion rates associated with glaciation, native interpretations of the stable isotopic and deposited andesitic material, incision between and resultant isostatic uplift, did not trigger the paleobotany data as well as alternative explana- eruptive depositional pulses did not progress late Cenozoic landscape rejuvenation, and tec- tions for sedimentologic data interpreted to show below the former thalweg channel positions, tonic uplift is the more likely cause. steep paleogradients of Oligocene to Eocene nor did basement incision take place during the Estimated rock and surface uplift exceeds paleochannels. In making these arguments I will 7 m.y. following the emplacement of the young- incision throughout the range (Wakabayashi and focus the discussion on works based on data est Oligocene ash-fl ow tuffs and prior to initial Sawyer, 2001), as noted above, and it is useful to from within the Sierra Nevada, instead of those eruption of Miocene andesitic rocks (23 Ma to revisit the long-term rates of uplift owing to new that have based conclusions on samples east of 16 Ma) that saw virtually no deposi tion in the age constraints on incision introduced herein. the range. Accordingly, I will focus on Mulch paleocanyons as well as low deposition rates in For the Feather River drainage, the uplift rate et al. (2006) and Cassel et al. (2009, 2012) for the Great Valley. Accordingly, initiation of late of the crest averaged since 3 Ma is ~0.6 mm/yr, stable isotopic paleoaltimetry, and Hren et al. Cenozoic incision corresponds to an increase, and for the Stanislaus River drainage it is ~0.5– (2010) for paleobotany. rather than a decrease, in sediment load in 0.6 mm/yr averaged since 3.8 Ma, using uplift Stable isotopic paleoaltimetry relies on two streams, whereas multiple episodes of decreased estimates (Wakabayashi and Sawyer, 2001) critical assumptions. The fi rst assumption is sediment load following volcanic depositional based on the Lovejoy Basalt and Table Moun- that the samples record closed-system behavior pulses in Oligo cene and Miocene time were not tain Latite, respectively. since the time of interest, or since Oligocene or followed by basement incision (e.g., Busby et al., The diachronous timing of incision initiation Eocene time in the case of the Sierran studies 2008b; Henry and Faulds, 2010). indicates diachronous initiation of late Ceno- (Mulch et al., 2006; Cassel et al., 2009, 2012). The estimates for uplift presented on the basis zoic uplift in the Sierra. In addition to differ- These studies assume that volcanic glass in of reconstructed marker horizons (e.g., Lind- ences in timing of uplift, the magnitude of uplift Oligocene rhyolites hydrated and equilibrated gren, 1911; Hudson, 1955; Christensen, 1966; may vary along strike from the San Joaquin with surface water within 5 k.y. after erup- Huber, 1981; Unruh, 1991; Wakabayashi and River northward, but with no systematic pattern tion and there has been no isotopic exchange Sawyer, 2001; Jones et al., 2004) record rock (Waka bayashi and Sawyer, 2001). For example, since then (Cassel et al., 2009, 2012), or that rather than surface uplift, but, as pointed out by the estimates of crestal rock uplift for the Feather kaolinite formed after Eocene deposition in Wakabayashi and Sawyer (2001), the erosion River and Stanislaus River areas (~1700–1900 m paleochannels and prior to covering by Oligo- rates on upland erosion surfaces are so low that and ~1800–1900 m, respectively; Wakabayashi cene rhyolites (Mulch et al., 2006). The second rock uplift very closely approximates surface and Sawyer, 2001) and the San Joaquin River assumption is that the Oligocene climate and air uplift for the highest elevations. In the years (2150 m; Huber, 1981) exceed that for the circulation patterns in the region are known well since 2001, other studies have published data Yuba River–northern American River region enough that the isotopic ratios can be reliably showing low erosion rates on the upland erosion (1200–1500 m; Jones et al., 2004). These dif- related to temperature and hence surface ele- surfaces, ranging from rates averaged over tens ferences are large enough that they probably vations. Molnar (2010) challenged this assump- to hundreds of thousands of years from cosmo- exceed differences resulting from applying dif- tion on the basis of the poorly known paths of genic nuclides (Stock et al., 2005), to Ar-Ar data ferent methods or the uncertainties associated air masses, and hence atmospheric water, in the that show extremely slow erosion rates averaged with those methods. Oligocene and Eocene, but regardless of the over the last 12 m.y. (Phillips et al., 2011). Col- In summary, late Cenozoic rock and surface validity of the paleoclimate model, the uncer- lectively, the low erosion rates of the upland ero- uplift of the Sierra Nevada is indicated by the tainty envelope given by Cassel et al. (2009, sion surfaces, and the much faster incision rates azimuth-gradient relationships observed in 2012) is broad enough to permissibly support of the streams, indicate an increase in relief in the paleo–Yuba River and supported by the rec- the amounts of surface uplift proposed by pro- the late Cenozoic in addition to a signifi cant ord of erosion and stream incision in the Sierra. ponents of late Cenozoic uplift (e.g., those of increase in surface elevation. Thermochronologic data also support the prem- Unruh, 1991; Wakabayashi and Sawyer, 2001; In addition to peak surface elevation increase, ise of late Cenozoic Sierran uplift (Clark et al., Jones et al., 2004). signifi cant mean elevation increase has occurred 2005; McPhillips and Brandon, 2010, 2012). The preferred Oligocene paleoelevation (including over 1 km of elevation increase for GPS and interferometric synthetic aperture profi le of Cassel et al. (2009, 2012) progres- the bottoms of the deepest canyons) indicating radar (InSAR) data show present-day uplift of sively increases in elevation eastward to the that tectonics has driven most of the elevation the Sierra at rates of 1–2 mm/yr along the entire location of the current Sierran crest, then levels increase and tilting (Wakabayashi and Sawyer, length of the range (Hammond et al., 2012), off eastward, a pattern remarkably similar to 2001; Jones et al., 2004; Stock et al., 2004) consistent with late Cenozoic (and ongoing) present-day topography. The present-day topo- instead of isostatic uplift owing to erosion (e.g., uplift of the range. graphic profi le, however, has been strongly Small and Anderson, 1995). infl uenced by post-Oligocene faulting that has The initiation of signifi cant stream incision Alternative Explanations of Isotopic downdropped blocks east of the Sierran crest from ca. 3 Ma in the Feather to American River and Paleobotanical Paleoaltimetry and along east-down faults (e.g., Wakabayashi drainages, to 3.6–4 Ma in the Mokelumne to Interpreted Steep Paleogradients and Sawyer, 2000, 2001; Faulds et al., 2005; Stanislaus River drainages, to 6–10 Ma in the San Henry et al., 2007; Faulds and Henry, 2008; Joaquin River drainage, and 20 Ma in the Kings Whereas geologic evidence appears to Hinz et al., 2009). Restoration of these faults, to Kern River drainages, signifi cantly predates demand late Cenozoic rock and surface uplift and associated restoration of Basin and Range the onset of North American glaciation at ca. and westward tilting of paleochannels, the appar- lithospheric thinning, should result in higher

206 Geosphere, April 2013

paleoelevations east of the Sierra rather than an hydration rate of volcanic glass, they did not The studies of Mulch et al. (2006) do not eastward leveling-off of elevations. If the Oligo- question the general principle of progressive include samples (or modeling) east of the Sierra cene topography consisted of an eastern plateau hydration with time over geologically signifi - Nevada, so they cannot be examined in light of with a marked westward steepening at the pres- cant time scales. Basin and Range faulting as above. However, ent Sierran crest, then the Eocene–Oligocene Friedman et al. (1993) proposed that small the uncertainties in paleoclimate models noted paleochannels should display the highest paleo- glass shards in volcanic ash completely hydrate above equally plague interpretations of stable relief in the vicinity of the crest. In contrast, the (and isotopically exchange with water) within isotopic composition of kaolinite in the Eocene paleorelief of these channels increases east of 5 k.y. after eruption, a situation that differs from gravels, and the question of isotopic closure the crest (Henry and Faulds, 2010; Henry et al., obsidian or glassy basalt that feature a vastly applies equally to kaolinite as to volcanic glass. 2012) as part of a progressive eastward increase larger effective radius of glass domains in con- Mulch et al. (2006) interpreted Eocene growth in paleorelief reviewed earlier. Accordingly, the tact with water. The samples used for isotopic and isotopic closure of the kaolinites in the interpreted Oligocene paleoaltimetry profi le paleoaltimetry come from ignimbrites (Cassel Eocene gravels on the basis of the less-intense appears inconsistent with the post-Oligocene et al., 2009, 2012) that cannot be compared to weathering of the Oliogene rhyolites compared faulting and extension history of the Basin and loose volcanic ash, owing to the degree of weld- to the Eocene gravels, and on the assumption Range and the distribution of paleorelief in ing of glass shards that decreases the perme- of rapid Eocene weathering owing to a warm Oligo cene paleochannels. ability of the material and increases the effective and moist climate. However, the discussion Because of signifi cant post-Oligocene modi- radius of the glass domains. Thus obsidian is a above strongly suggests the possibility of iso- fi cation of topography by Basin and Range better analog of the hydration behavior of glass topic reequilibration of the Oligocene rhyolites faulting, the close correspondence between the in the rhyolite ash-fl ow tuffs through time. and the potential for later growth of kaolinite interpreted paleoaltimetry profi le and modern Even if part of the surface of an ignimbrite and isotopic exchange. Moreover, the compari- topography suggests post-Oligocene isotopic had originally completely hydrated and equili- son of the degree of weathering between the exchange between surface water and hydrated brated with surface water soon after erup- rhyolites and the gravels does not account for volcanic glass. This brings into question the tion, the slow advance of the weathering front the vast difference in lithology, permeability, and assumption of geologically instantaneous hydra- through time into unweathered and unhydrated consequent susceptibility of weathering of the tion and subsequent closed-system behavior material, associated with progressive denuda- two units. Finally, as noted above, recent studies, used by Cassel et al. (2009, 2012), citing the tion, must be considered. Chemical weathering conducted within the Sierra and other localities work of Friedman et al. (1993), who proposed rate depends most strongly on erosion rate, and in the world, have demonstrated that chemical complete hydration of small glass shards within climate exerts but a weak infl uence (Riebe et al., weathering rates show minimal dependence on 5 k.y. (discussed further below). In fact, the 2001, 2004). Accordingly, the weathering front climate but strong dependence on erosion rates progressive hydration (open-system behavior) advances downward through rock as erosion/ (Riebe et al., 2001, 2004). Thus the conclusions of volcanic glass has been well established in denudation proceeds. Even in the most slowly of high Eocene paleoaltitudes of Mulch et al. numerous studies, to the extent that the thickness eroding areas on the Sierran interfl uves, where (2006) can be alternatively interpreted to refl ect of hydration rinds on obsidian has been cali- the Oligocene rhyolites (and Eocene gravels) uncertainty in paleoclimatic models, open-sys- brated as a geochronometer, employed for dat- are preserved, erosion has removed a signifi cant tem isotopic behavior, progressive advance of ing of archaeologic artifacts (e.g., Friedman and amount of material since the Oligocene. For the weathering front associated with denudation, Smith, 1960; Friedman and Long, 1976; Mor- example, the amount of denudation in the past or a combination of these factors. Stable isotopic genstern and Riley, 1974; Rogers, 2010). Studies 25 m.y. is 100 m extrapolating the 0.004 mm/yr studies of authigenic smectite (e.g., Poage and show progressive hydration of volcanic glass for erosion surface rates of Phillips et al. (2011) to Chamberlain, 2002), in addition to not being a samples at least as old as 15 Ma (Morgenstern 25 Ma, and 50–475 m extrapolating the 0.002– defi nitive test of Sierran uplift owing to sample and Riley, 1974). 0.019 mm/yr erosion rates of upland surfaces locations as noted above, are also subject to Volcanic glass as old as 1.1 Ga has been found determined by Small et al. (1997) and Stock the uncertainties with respect to paleoclimatic (Palmer et al., 1988), and volcanic glass from et al. (2005). Thus, the weathering front began models, open-system isotopic behavior, and the ophiolites at least as old as Jurassic has been advancing downward into rock from a land sur- possibility of new authigenic minerals form- coveted in studies of ophiolite petrogenesis, face >50 m above the present level of exposure ing progressively downward in bedrock as the owing to the fact that the rarely preserved glass >25 m.y. ago and may not have reached some weathering front advances. retains the original chemistry of the volcanic or many of the rhyolite samples until compara- Hren et al. (2010) interpreted a high paleo- rock that has otherwise been altered to some tively recently. altitude of the Eocene Sierra Nevada based on degree (e.g, Robinson et al., 1983; Shervais In conclusion, the similarity of the preferred hydrogen isotope ratios associated with organic and Hanan, 1989). The progressive hydration Oligocene paleoaltimetry profi le of Cassel et al. compounds within leaf fossils associated with of volcanic glass has been demonstrated empiri- (2009, 2012) to the modern topographic pro- Eocene deposits in the northern Sierra Nevada. cally by comparison of rind thicknesses and fi le, appears incompatible with post-Oligocene As for the paleo-isotopic studies of kaolinite independent age dates on the rocks (e.g., Fried- Basin and Range faulting and extension, and and hydrated volcanic glass noted above, the man and Smith, 1960; Morgenstern and Riley, suggests geologically recent reequilibration same problems regarding paleoclimatic mod- 1974; Rogers, 2010), by experimental studies of the stable isotopic system of the hydrated els and open- versus closed-system behavior (e.g., Friedman and Long, 1976; Delaney and glasses studied in their investigations. The lat- apply. For the latter issue Hren et al. (2010) Karsten, 1981; Karsten et al., 1982; Zhang ter conclusion is consistent with numerous proposed that leaf compounds can be preserved et al., 1991; Zhang and Behrens, 2000; Steven- studies demonstrating the open-system behav- in sedimentary rocks over geologic time scales son et al., 1998), and chemical kinetics (e.g., ior of volcanic glass, as well as the general cor- without isotopic exchange, citing the work of Doremus, 2000). Although Anovitz et al. (1999) respondence between denudation and chemical Schimmelmann et al. (1999). The experimental challenged the nature of equations governing weathering rates. study of Schimmelmann et al. (1999), however,

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which focused primarily on isotopic fi nger- rected for Basin and Range extension based on cene volcanic rocks of major streams north of printing of the source rocks of oil, pointed out Henry and Faulds (2010) for these reaches of the the North Fork Feather River (Deer Creek and the complexities in the various mechanisms of paleo–Yuba River, and a mean grain size of Mill Creek; Fig. 2) and (2) the related decrease in isotopic exchange with organic material and 0.5 m, results in extremely low paleogradients elevation of the top of basement from ~2100 m a corresponding variability in preservation of of ~0.4–0.5 m/km (0.0004–0.0005), an order on the high upland surface north of the North original (pre–lithifi cation/burial) isotopic ratios. of magnitude lower than the estimates of Cassel Fork Feather River, to ~1100 m in an erosional The possibility of isotopic exchange in the fos- and Graham (2011). Christensen (1966) used window on Deer Creek, to below 900 m in sil leaf materials with water during the weather- data by Leopold and Miller (1956) and the U.S. Mill Creek (which has not incised to basement ing process associated with denudation should Geological Survey (1960) to estimate paleo- northwest along the trend of the basement high) be considered. In addition, the streams in which gradients based on drainage area alone. This (Lydon et al., 1960). Thus, the northward young- the leaves were deposited were fl owing in paleo- relationship has more scatter than that of Hack ing of uplift (incision initiation) may indicate a valleys. There is some possibility of downhill/ (1957), possibly because the mean bed-load size relationship between uplift and the northward- downstream movement of leaves both in the is not accounted for. Nonetheless, application of migrating southern edge of the subducted Gorda trunk stream and from the slopes bounding the the Christensen (1966) gradient–drainage area slab, tied to the Mendocino triple junction, as paleochannel (e.g., Greenwood, 1991). relationship results in an estimated paleogradient suggested by Crough and Thompson (1977). Interpretation of high paleoelevations of the range of ~0.4–1.8 m/km (0.0004–0.0018) incor- The resolution of the two models (triple- Sierra based on stromatal density (Kouwenberg porating the full range of scatter in the calibrat- junction-related and delamination-related) may et al., 2007, 2010) are subject not to issues of ing data. The interpreted braided nature of the come from the synthesis of thermochronologic, isotopic closure, but of the microscopic struc- Eocene paleochannels was used by Cassel and structural, and geomorphic evidence from the ture of fossil leaves. The stromatal density Graham (2011) to support steep paleogradients, Kings to Kern River drainages, where a fi rst

should reﬂ ect a decline in CO2 partial pressure but multiple studies show that the gradients for uplift and incision event apparently occurred with altitude but the signiﬁ cant variation in braided streams vary inversely with discharge at ca. 20 Ma and a second one after ca. 3.5 Ma

atmospheric CO2 content through time poses (e.g., Leopold and Wolman, 1957; Ferguson, (Clark et al., 2005; Saleeby et al., 2009). The complications for distinguishing the altitude 1987). Accordingly, sedimentological data do fi rst event may have been triggered by slab win- from the climatic variation signal. In addition, not require steep paleogradients of Eocene and dow formation in the wake of the northward- the downslope/downstream transport of leaves, Oligocene rivers, whereas evidence for signifi - migrating slab edge, whereas the second event as noted above, is a possibility. cant tilting of the paleo channels from azimuth may have been the response to delamination Cassel and Graham (2011) proposed that versus gradient relationships precludes such beneath the southern Sierra (Maheo et al., 2009). Oligocene and Eocene stream paleogradients of steep paleogradients. Many streams of the Kern River drain- the paleo–Yuba River were steep, similar to the age have two knickpoints (Clark et al., 2005), preserved gradients of the deposits, on the basis Cenozoic Rock Uplift and Landscape which appears to be characteristic of the Kings of sedimentologic features. As noted above, the Response Revisited: Reassessment River drainage as well (Clark et al., 2005; Stock relationship between azimuth and gradient of of Tectonic Models et al., 2005) and possibly the San Joaquin River these paleochannels strongly refutes the notion (Huber, 1981), whereas northern and central of steep paleogradients and supports signifi cant The previous sections have discussed the Sierra streams appear to have a single knick- tilting (steepening) of the paleochannels in the evidence supporting late Cenozoic uplift in point (Kemp, 2012). The two knickpoints found late Cenozoic. Cassel and Graham (2011) based the Sierra Nevada and the most signifi cant geo- in many of the streams of the southern Sierra their interpretation of steep paleogradients on morphic response to it: stream incision. New, drainages appear to refl ect two late Cenozoic the mean grain size of the coarser paleochannel primarily geochronologic data require revision incision events that may have been triggered by deposits (~0.5 m), compared to a physical model of the conclusions presented in Wakabayashi two episodes of uplift (Clark et al., 2005; Stock relating channel slope to stream gradient pre- and Sawyer (2001), particularly on the timing et al., 2005). The second incision event appears sented by Paola and Mohrig (1996). Paola and and along-strike variability of uplift, and these to be synchronous, within uncertainty in ages, in Mohrig (1996), and Cassel and Graham (2011) revisions also indicate the need to revisit the tec- these southern drainages, from post–3.5 Ma in their interpretation derived from it, took the tonic models proposed for this uplift. in the Kern River drainage and post–3 Ma in the position that channel gradient is independent As presented above, the late Cenozoic inci- Kings River, to post–3.6 Ma in the San Joaquin of discharge and drainage basin area, an asser- sion resulted from rock and surface uplift, so the River. In contrast, the older incision event is tion that contradicts research dating back over a timing of incision can be taken as approximat- clearly earlier (beginning at ca. 20 Ma) in the century that has demonstrated a strong inverse ing the timing of uplift and base-level forcing. Kern and Kings Rivers than it is in the San relationship between channel gradient and drain- The timing of incision and uplift varies, with an Joaquin, where the older incision event began age basin area (and therefore discharge) (e.g., older initiation time in the south than the north. between 6 and 10 Ma. Gilbert , 1877; Leopold and Maddock, 1953; Although delamination or foundering of a dense Thus, the geomorphology of the Kern to San Hack, 1957, 1973; Christensen, 1966; Flint, root of the Sierra has been proposed as a driv- Joaquin drainages appear to refl ect two late 1974). This fundamental relationship is best ing mechanism for uplift, the direct evidence Cenozoic uplift events, the fi rst of which youngs illustrated by the typical longitudinal profi le of a for this event, from xenoliths in volcanic rocks, northward and may have been a consequence of stream that increases in gradient headward. exists only from the San Joaquin River drainage slab window development in the wake on the Hack (1957) showed a strong dependence of southward (Ducea and Saleeby, 1996, 1998). northward-migrating Mendocino triple junction the mean grain size of bed load on both the gradi- Late Cenozoic Sierran uplift appears to as proposed by Maheo et al. (2009), whereas the ent of the stream and its drainage area. Using the decrease to near zero near the southern edge second, which began at about the same time in relationship of Hack (1957), a range of estimated of the subducted Gorda plate. This is shown by these drainages, may have resulted from delami- drainage basin area of 30,000–55,000 km2 cor- (1) the negligible incision beneath Plio-Pleisto- nation (e.g., Ducea and Saleeby, 1996, 1998;

208 Geosphere, April 2013

Saleeby and Foster, 2004; Le Pourhiet et al., A Ca.90 Ma B 57 to 35 Ma (Ca. 50 Ma) 2006). North of the San Joaquin River drainage, the geomorphology apparently reﬂ ects a single late Cenozoic uplift event that may be related N to slab window development in the wake of the migrating triple junction.

Thompson and Parsons (2009) proposed that Future Crest Position isostatic uplift of the Sierran crest, in response to footwall unloading by FFS normal faulting, may have driven late Cenozoic uplift in the range. Their model generated about the same amount Future Crest Position of rock uplift as estimated by proponents of late Cenozoic uplift in the Sierra (1200–1300 m). C 35 to 21 Ma (Ca. 25 Ma) D 21 to 3 Ma (Ca. 10 Ma) If footwall unloading was the primary cause of late Cenozoic Sierran uplift, Sierran uplift

should have taken place much earlier in the t Position northern and central Sierra, in response to faulting along the western margin of the Basin and

Future Cres Range. Perhaps mantle upwelling, associated Future Crest Position with slab window formation in the wake of the triple junction, acted to enhance footwall isostatic response to normal-fault unloading.

~16–3 Ma andesite Summary Model of Topographic E 3 Ma to present (present) Evolution of the Sierra Nevada since ~31–23 Ma rhyolite

Late Cretaceous Time Eocene gravels

I conclude with a summary model for the topographic evolution of the Sierra Nevada since system

the Cretaceous (Fig. 7). Peak erosion rates in the fault Sierra and deposition rates in the Great Valley

coincide with the fi nal stages of the emplace- W a l k e r L a n e ment of the Sierra Nevada batholith from 100 Frontal to 85 Ma (Fig. 7) (Wakabayashi and Sawyer, 2001). Owing to the lack of preserved landscape Figure 7. Schematic model for topographic evolution of the Sierra Nevada. The southern- surface markers, the topography of the late Cre- most drainage shown represents a topographic-stratigraphic relationship similar to that taceous Sierra Nevada is poorly constrained, but of the San Joaquin River drainage, whereas the drainages to the north represent relation- it is unlikely that the crest of the range coincided ships such as those found from Stanislaus River northward. Revised from Wakabayashi and with the current one. Thermobarometry of plu- Sawyer (2001). Each frame represents a time interval in tectonic-topographic evolution of tons suggests greater exhumation in the western the Sierra, but during each time interval the landscape of the Sierra was evolving so that the part of the range compared to near the present specifi c time shown by each frame is given by the age in parentheses. Note that in frames D crest (Ague and Brimhall, 1988; Saleeby, 2007; and E there is signifi cant along-strike variation in timing of events. This is particularly true Saleeby et al., 2010; Chapman et al., 2012). The of the age of Frontal faulting and andesitic volcanism. See text for details. locus of exhumation progressed from west to east (Tobisch et al., 1995) prior to a signifi cant late Cretaceous exhumation event in the southernmost Sierra (Saleeby, 2012; Chapman et al., At ca. 84 Ma, arc magmatism shut off in the within the Sierran region after rock uplift ceased 2012). This may have led to eastward migration vicinity of the present Sierra Nevada, appar- (Fig. 7). The beginning of the Eocene brought a of the crest of the range. The location of the ently in response to a low-angle subduction that dramatic decrease in sedimentary accumulation Cretaceous magmatic arc and the batholith does resulted in greater coupling between the subduct- in the forearc basin. The paleorelief preserved in not correspond to the boundaries of the present ing plate and North America, inducing crustal the modern Sierra dates to this time and earlier. mountain range. In the south, the Sierra Nevada thickening and increase of elevation to the east This paleorelief (relief at the time of formation; batholith includes many early Cretaceous plu- as part of the Laramide orogeny (e.g., Dickin- paleorelief as preserved today) was progressively tons beneath Great Valley sediments west of the son and Snyder, 1978). Exhumation rates slowed reduced from late Cretaceous time (McPhillips range (Saleeby, 2007; Saleeby et al., 2010) as dramatically in the Sierra after ca. 80 Ma, but and Brandon, 2012). well as some plutons east of the range, and north sedimentation rates in the Great Valley remained From the Stanislaus River drainage north- of Lake Tahoe the batholith strikes more north- relatively high until the end of Paleocene time ward, what became the Sierra comprised the erly than the range so that the majority of the (Fig. 5), possibly as a consequence of sedi- western fl ank of a broad upland region, whose batholith diverges eastward from the mountain ments sourced further inland and/or as a result drainage divide lay ~300 km east of the present range (Van Buer et al., 2009). of continued headward erosional propagation range crest, restoring subsequent extension of

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the Basin and Range (Henry, 2008; Henry and based on restoration of estimated late Cenozoic out insetting from the lowest incised reaches of Faulds, 2010). Streams fl owed westward from uplift that accounts for the internal complexity in rivers to the crest, from the Feather River to at this divide and deposited the Eocene gravels in this region. least as far south as the Stanislaus River drain- broad paleovalleys that deepened upstream and There is no direct evidence of paleochannels age (Wakabayashi and Sawyer, 2001). This sug- east of the present Sierra (Garside et al., 2005; crossing the Sierra south of the San Joaquin gests long-lived west-side-down warping along Henry, 2008; Henry and Faulds, 2010; Henry River, although such evidence may have been the eastern margin of the Central Valley from et al. 2012). The largest amount of paleorelief eroded away (Henry et al., 2012). This may the northernmost part of the range to at least as north of the Stanislaus River (600 m) was asso- indicate a late Cretaceous to Eocene drainage far south as the San Joaquin River drainage. The ciated with one of these paleovalleys at the divide near the position of the present crest of position of this warp along the eastern margin position of the present crest; this paleorelief the southern Sierra (Fig. 7), as proposed by of the Central Valley basin fi ll suggests that this increases further upstream east of the pres- Saleeby et al. (2009). warp may have resulted from fl exural response ent range. Much of the paleorelief in the crest The difference in paleorelief and present sur- to sediment loading, with localization of defor- region in this region is 200 m or less. Eocene face elevations between north and south may mation related to a zone of weakness within the paleoelevations north of the Stanislaus River, refl ect along-strike differences in the nature of underlying basement. estimated by restoring late Cenozoic uplift the lithosphere. These differences may refl ect In Oligocene time, the period of low erosion and tilting, ranged from ~800 to 1400 m at the the divergence of the modern range from the rates and a stable landscape continued in the present crest (Wakabayashi and Sawyer, 2001). Sierra Nevada batholith, and the position of the Sierra, as refl ected by geologic relationships and Erosion of upland surface regions has occurred (pre-Paleozoic) margin of the North American estimated erosion rates. Ash-fl ow tuffs issued since Eocene time, and the amount of erosion continent, as marked by the 0.706 initial 87Sr/86Sr from calderas well east of the present crest may be significant, given the comparatively isopleth (Kistler and Peterman, 1973). The latter and fl owed down the drainages already partly large amount of elapsed time (~50 m.y.). For coincides with the position of basement eleva- fi lled with Eocene gravels (Henry, 2008; Henry example, if the 0.004 mm/yr erosion surface tion increase; continental initial 87Sr/86Sr ratios and Faulds, 2010; Henry et al., 2012) (Fig. 7). lowering rate of Phillips et al. (2011) is restored of >0.706 are found south of the increase in Erosion rates and incision rates remained low (to 50 Ma), the estimated paleoelevations of basement surface elevation. The position of the in the Sierra through the Miocene. Basin and the crest summit regions of the north Sierra in old continental margin may have also affected Range normal faulting began and the west- Eocene time becomes 1000–1600 m. Elevations the subsequent position of the Miocene arc ern edge of this faulting migrated westward to the east of the Sierra should have been signifi - whose axis was located along or just east of the (e.g., Slemmons et al., 1979; Dilles and Gans, cantly higher, for the area that became the north present crest of the range from the Stanislaus 1995; Surpless et al., 2002), beheading stream Sierra represented the western fl ank of an uplifted River drainage northward, but was much further systems and diminishing their drainage areas region to the east (e.g., Henry, 2008; 2012). east of the range to the south (Christiansen and (Wakabayashi and Sawyer, 2001). This faulting South of the Stanislaus River drainage, the Yeats, 1992; Busby et al., 2008a, 2008b; Busby did not involve footwall rock uplift of western basement surface rises, and the paleorelief and Putirka, 2009; Busby, 2012). The position strands, even though exhumation associated increases in corresponding fashion to 1.5 km of the old continental margin may have also with at least some of these faults is much greater from the southern San Joaquin River drainage to infl uenced the petrogenesis and resultant chem- than that associated with the FFS (e.g., Stockli the northern Kern River drainage; the basement istry of Miocene volcanic rocks erupted across et al., 2002), otherwise the tilts of streams drain- surface elevation and paleorelief decreases south- this boundary (Putirka and Busby, 2007). ing across the Sierra should have increased lead- ward in the southern Kern River drainage (Waka- The unique aspects of the southernmost ing to incision, contradicting the observation of bayashi and Sawyer, 2001; Clark et al., 2005; Sierra, with far greater internal deformation no incision between Eocene and Pliocene time Saleeby et al., 2009; Chapman et al., 2012). than any other part of the Sierra, may have (Wakabayashi and Sawyer, 2001). Accordingly, Eocene paleoelevations in the highest part of the been infl uenced by a late Cretaceous exten- the incision record in the Sierra Nevada suggests range have been estimated at 2000–2500 m, based sional event that affected only that part of the that Basin and Range faulting has resulted in a on restoration of late Cenozoic uplift extrapolated Sierra (e.g., Maheo et al., 2009). Similarly, the decrease in mean elevation (because the hang- southward from the San Joaquin River drainage northern termination of the Sierra Nevada at ing-wall side would decrease in elevation with (Wakabayashi and Sawyer, 2001) and analysis the Sierran-Cascade boundary zone, character- respect to the footwall) in the Basin and Range, of geomorphology and thermochronology (Clark ized by active faults that strike west-northwest consistent with interpretations of paleoelevation et al., 2005). However, both of these estimates are (Sawyer, 2009, 2010), appears controlled by based on paleobotany (Wolfe et al., 1997, 1998) subject to signifi cant uncertainty. The difference the basement structural grain that curves to a and expectation of elevation decrease associated in internal deformation and timing of uplift from west-northwest strike and may be related to an with lithospheric thinning (e.g., Buck, 1991; the Kern River to the San Joaquin River drain- earliest Cretaceous or older transform bound- McKenzie et al., 2000). ages, as well as the along-strike variability in ary between the Sierran and Klamath Mountain Beginning at ca. 16 Ma and continuing to uplift magnitude, renders the extrapolation of late regions (Dilek and Moores, 1992; Ernst, 2012). ca. 3 Ma, andesitic volcanism associated with Cenozoic uplift estimates southward from the lat- There is no evidence of older, analogous, late the ancestral Cascades arc covered much of the ter drainage dubious. In addition, the presence of Cenozoic structures in the Sierra Nevada south Sierra from the Stanislaus River drainage north- signifi cant dip-slip internal faulting in the Kern of the present boundary zone. ward (Fig. 7) (Busby et al., 2008a, 2008b; Busby River drainage (e.g., Maheo et al., 2009; Amos The progressively steeper dips of Quaternary and Putirka, 2009; Cousens et al., 2008). These et al., 2010; Nadin and Saleeby, 2010) compli- to Eocene strata along the eastern margin of the andesitic fl ows and mudfl ows issued from vol- cates estimates of uplift from geomorphology Central Valley (Unruh, 1991) appears to be a canic centers along or east of the present crest of and thermochronology in the Kern River drain- feature localized along the margin of the Cen- the range (Christiansen and Yeats, 1992; Busby age. Saleeby et al. (2009) proposed paleoeleva- tral Valley fi ll, because Cenozoic strata overlie et al., 2008b) and fi lled existing paleovalleys so tions exceeding 3000 m for the southern Sierra, each other in normal stratigraphic order with- that only isolated basement highs rose above

210 Geosphere, April 2013

them. Some faulting, locally associated with ize many streams in these drainages, as well as Bartow, J.A., 1990, The late Cenozoic evolution of the San signifi cant hanging-wall tilting, began near the the inner canyons of the major streams of this Joaquin Valley, California: U.S. Geological Survey Professional Paper 1501, 40 p. current position of the crest and Frontal fault region (Huber, 1981; Clark et al., 2005; Stock Bateman, P.C., and Wahrhaftig, C., 1966. Geology of the system at ca. 10 Ma (Busby et al., 2008a, 2008b; et al., 2004, 2005; Carlson et al., 2009). Sierra Nevada, in Bailey, E.H., ed., Geology of North- ern California: California Division of Mines and Geol- Busby and Putirka, 2009), but this did not result Frontal faulting continued to encroach into ogy Bulletin 190, p. 107–172. in rock uplift of the Sierran block and incision the Sierran microplate after the initiation of late Beanland, S., and Clark, M.M., 1994, The Owens Valley of Sierran streams. The eruption and deposi- Cenozoic uplift, so the initiation of slip on the fault zone, eastern California, and surface rupture associated with the 1872 earthquake: U.S. Geological tion of large volumes of volcanic rock resulted present set of Frontal faults postdates initia- Survey Bulletin 1982. in an extremely rapid temporary increase of the tion of uplift and incision from the San Joaquin Berlin, M.M., and Anderson, R.S., 2007, Modeling of knick- elevations of the streambeds, followed by rapid drainage southward, and in the Feather River point retreat on the Roan Plateau, western Colorado: Journal of Geophysical Research, v. 112, F03S06, incision to the former equilibrium position of the drainage. Although northward migration of the doi:10.1029/2006JF000553. channel after volcanic deposition ceased for each onset of uplift and incision in the Sierra Nevada Brossy, C.C., and 11 others, 2012, Map of the late Qua- ternary active Kern Canyon and Breckenridge faults, pulse. South of the Stanislaus drainage, the axis may have been related to slab window evolution southern Sierra Nevada, California: Geosphere, v. 8, of the magmatic arc swung much further east. In tied to ridge collision and subsequent migration p. 581–591, doi:10.1130/GES00663.1. addition, a preexisting drainage divide may have of a triple junction, the northern and southern Buck, W.R., 1991, Modes of continental lithospheric extension: Journal of Geophysical Research, v. 96, prevented andesitic mudfl ows and other depos- termination of the Sierra may be fi xed and con- p. 20,161–20,178, doi:10.1029/91JB01485. its from reaching the central and southern Sierra trolled by preexisting basement structure. Burbank, D.W., and Anderson, R.S., 2012, Tectonic Geo- Nevada in areas south of the San Joaquin River morphology (second edition): West Sussex, UK, Wiley- ACKNOWLEDGMENTS Blackwell, 454 p. drainage (Fig. 7) (Henry et al., 2012). Busby, C., 2012, Extensional and transtensional continental During this period, arc magmatism in the arc basins: Case studies from the southwestern U.S. I thank C. Busby and K. Putirka for inviting me and Mexico, in Busby, C., and Azor, A., eds., Recent Sierra shut off from south to north as the south to participate in the Penrose Conference on the Sierra Advances in Tectonics of Sedimentary Basins: West edge of the subducting Gorda plate migrated Nevada and to submit this contribution. I have benefi t- Sussex, UK, Wiley-Blackwell, p. 382–404. northward with the Mendocino triple junction ted from discussions of Sierran landscape evolution Busby, C.J., and Putirka, K., 2009, Miocene evolution of (Atwater and Stock, 1998). Rock uplift in the with many, including G. Stock, C. Henry, F. Phillips, the western edge of the Nevadaplano in the central and Cliff Riebe, J. Saleeby, and students who have worked northern Sierra Nevada: Paleocanyons, magmatism, Sierra Nevada began sometime after the north- with me on these matters, C. Kemp, C. Carlson, and and structure: International Geology Review, v. 51, ward migration of the southern slab edge, pos- A. Shriver. I thank J. Saleeby and F. Phillips for their p. 670–701, doi:10.1080/00206810902978265. Busby, C.J., Hagan, J.C., Putirka, K., Pluhar, C.J., Gans, sibly in response to slab window development thorough and thoughtful reviews that led to signifi cant P., Wagner, D.L., Rood, D., DeOreo, S.B., and Skill- (Maheo et al., 2009). 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