Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics

Jane Lock† Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Harvey Kelsey‡ Department of Geology, Humboldt State University, Arcata, California 95521, USA Kevin Furlong§ Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Adam Woolace# Department of Geology, Humboldt State University, Arcata, California 95521, USA

ABSTRACT the double-humped pattern of uplift and our understanding of the lithospheric forces subsidence migrates, and the Coast Ranges that have built the orogen, and recognizing the A landscape records the surface response emerge. Smaller drainages develop and tectonic signal recorded by the landscape. We to tectonics at time scales intermedi- evolve by stream capture and fl ow reversal, describe how the surface responds to tectonics ate between short time-scale information and the two main divides migrate in concert in northern California, and we use the tectonic derived from seismic imaging and global with the triple junction. In contrast to the signal contained in the landscape to test and positioning systems and the long-term geo- systematic development of the small streams, develop our understanding of the geodynamics. logic record. We link late Neogene and the largest trunk streams can maintain grade A variety of mechanisms has been proposed Quaternary deposits and landforms in the through regions of high uplift, and coastal to explain the timing of uplift of the northern northern California Coast Ranges to the tec- river mouths remain stationary despite the California Coast Ranges. Dumitru (1989), using tonics of the Mendocino triple junction. In uplift moving north. Before ca. 2 Ma, the results of fi ssion-track analyses, argued that the northern California Coast Ranges, the majority of the Coast Range drainage fl owed primary uplift of the Coast Ranges occurred Mendocino crustal conveyor geodynamic to a southern coastal outlet near the present in association with Cretaceous subduction. model describes crustal thickening, thin- mouth of the Russian River. At 2 Ma, facili- Abundant late Neogene marine sediments that ning, and dynamic topography that produce tated by headwater stream capture at key outcrop throughout the area indicate that while a “double-humped” pattern of uplift that locations, the drainage direction reversed, Cretaceous uplift may refl ect a period of sub- migrates northward with the Mendocino tri- and the majority of Coast Range rivers now stantial exhumation, it cannot be responsible ple junction. The tectonics are manifest in the drain into the north-fl owing Eel River. The for the development of the present area of high drainage system and elevation pattern of the major drainage reorganization at 2 Ma high- elevation. Alternatively, uplift of the Coast Coast Ranges. At long wavelengths, the ele- lights the potential for complexity in geomor- Ranges could be driven by transpression along vation pattern closely matches the predicted phic response to tectonics. the developing San Andreas fault system, in a double-peaked shape of Mendocino crustal fashion similar to the Southern Alps of New conveyor topography, and the high points of Keywords: northern California Coast Ranges, Zealand (Walcott, 1998). However, at latitudes uplift control the location of drainage divides. landform evolution, Mendocino triple junction, north of the San Francisco Bay, plate motion on Presently, the divide between the Russian drainage evolution, geodynamics, tectonic geo- the San Andreas fault system is almost entirely and Eel Rivers and the divide between the morphology. parallel to the trend of the fault (DeMets et al., Eel and Van Duzen Rivers approximately 1990) (Fig. 1A). correspond to the peaks of uplift predicted INTRODUCTION Rather than transpression-driven or subduc- by the Mendocino crustal conveyor model. tion-related uplift, the underlying cause for the As the triple junction migrates northward, An orogen and its landscape develop in direct formation of the Coast Ranges is more likely response to underlying tectonic driving forces. processes associated with the passage of the †Present address: Department of Earth and As a result, the geomorphology and geology of Mendocino triple junction (Zandt and Furlong, Space Sciences, University of Washington, Seattle, a region record a tectonic history that contains 1982; Furlong et al., 1989; Merritts and Bull, Washington 98195, USA; e-mail: janelock@ess. information about deep-seated geodynamic 1989). The Mendocino triple junction lies at the washington.edu. ‡E-mail: [email protected]. processes. In this paper, we explore the links junction between the Pacifi c, North American, §E-mail: [email protected]. between the surface and tectonics in the north- and Juan de Fuca (or Gorda) plates (Fig. 1A). #E-mail: [email protected]. ern California Coast Ranges (Fig. 1A) using The triple junction is migrating to the northwest

GSA Bulletin; September/October 2006; v. 118; no. 9/10; p. 1232–1246; doi: 10.1130/B25885.1; 9 fi gures; 1 table.

1232 For permission to copy, contact [email protected] © 2006 Geological Society of America Landscape evolution of the northern California Coast Ranges at ~5 cm/yr (Sella et al., 2002). Zandt and Fur- A Mad River A Gor Eureka long (1982) proposed that the high elevations Plate Tr i nity R da ive of the Coast Ranges could be a response to the Van Duzen R. r Eel infl ux of asthenosphere and resulting high tem- MTJ River peratures in the “slab window” that forms in Nor M the wake of a triple junction. Since these initial King Rangeattole R N. Fk. Eel River thern C studies, seismic images have shown that crustal iver 40º N thickness beneath the Coast Ranges varies spa- S.

Fk. tially and reaches thicknesses of up to 40 km 2 alifornia Coas Ma E (Beaudoin et al., 1996, 1998; Villasenor et al., el R

1998; Verndonck and Zandt, 1994). Furlong iver M. Fk. and Govers (1999), in an attempt to explain Pacific Eel River

Plate Nor

the variable crustal structure, developed the th Mendocino crustal conveyor (MCC) model 4 Ma t Range

Pla

(Fig. 1B). The MCC is built on a numerical Ame geodynamic model in which uplift is driven te

0 40 rican by a combination of crustal thickening and s Russian River Clear dynamic topography, with a minor component km 6 Ma Lake of thermally driven uplift. San Andreas Fault B The purpose of this paper is to depict the landscape evolution of the northern California Coast Ranges by integrating the MCC geody- San namic model with geomorphic and geologic Francisco data from the Coast Ranges. We review geologic Santa Rosa Los Angeles and geomorphic data that provide evidence of 123ºW paleocoastlines, drainages, and topography in the late Neogene Coast Ranges, then outline the Southern Edge Area of active Pacific-Sierra Nevada salient points of the MCC geodynamic model. of Gorda Slab strike-slip seismicity relative plate motion We test whether the MCC model predictions (Jennings, 1994) calculated from are refl ected in the topography and geomorphic Line of MCC REVEL (Sella et al, 4 Ma Location of triple evolution of the Coast Ranges. First, the MCC model 2002) ~ 5 cm/yr junction through model predicts that there is a double-peaked Drainage divide time (after Atwater uplift in the Coast Ranges that follow in the and Stock, 1998) wake of the triple junction. Second, streams should respond to the MCC predicted topogra- MTJ SE NW phy by lengthening longitudinally (in a N-NW– S-SE direction) in the wake of the triple junc- B Crust Thins North American Crust tion. Third, E-W–trending stream divides in the Crust Thickens Coast Ranges should migrate N-NW in the wake of the triple junction. Finally, the Coast Ranges Viscous Coupling ate should sequentially emerge with passage of the a Pl ord triple junction such that remnant marine sedi- G ments that outcrop in the now-emergent Coast Upwelling Mantle Ranges young in age to the north. If the topog- Asthenosphere raphy and geomorphic evolution is inconsis- Looking from the northeast. Gorda plate is subducting out of the tent with the MCC model and more consistent page. with Coast-Range-axis-normal convergence, then there will be no northward-younging age progression to drainage development or Coast Figure 1. (A) Location map of the northern Coast Ranges of California showing the Men- Ranges emergence and no evidence for N-NW docino triple junction (MTJ), which marks the intersection of the Pacifi c, North America, stream lengthening or the migration of E-W– and Gorda (or Juan de Fuca) plates. The triple junction has been migrating to the northwest trending stream divides. at a steady rate for the last 8 m.y. Line A–B delineates the location of the two-dimensional The discussion integrates the geologic and Mendocino crustal conveyor (MCC) model (Furlong and Govers, 1999), which predicts a geomorphic attributes of the Coast Ranges with pattern of uplift of the northern California Coast Ranges. (B) Schematic cross section show- our understanding of the tectonics to depict the ing the main geodynamic processes in the MCC model along line A–B in part A. Deforma- landscape evolution of the northern California tion occurs as hot asthenosphere fi lls the gap left by the migrating Gorda plate, causing Coast Range. By making the link between the viscous coupling between the Gorda slab and the base of the North American crust. As the surface and tectonics, we achieve two goals. Gorda plate moves to the north, the North American plate thickens, then thins, driving One, we can use the paleogeomorphology isostatic uplift. Flow in the mantle causes dynamic topography that adds to the uplift and inferred from the preserved geologic deposits subsidence.

Geological Society of America Bulletin, September/October 2006 1233 Lock et al. and landforms to constrain and extend our Deposits and Landforms of the Paleo-Russian late Pliocene “Merced” fauna (Travis, 1952), knowledge of the Mendocino triple junction River and based on its interfi ngering relationship with geodynamic processes. Two, our analysis pro- The once-extensive Russian River gravel, Sonoma volcanic rocks (2.6–7.9 Ma) (Gealey, vides an integrated explanation of the complex which extends 110 km upvalley from Wilson 1950; Travis, 1952; Fox et al., 1985). In the drainage, sedimentation, and topographic pat- Grove to north of Ukiah (Fig. 3), records a north, the gravel in Little Lake Valley contains tern of the northern California Coast Ranges. paleo-Russian River larger in size than its mod- the ca. 0.6 Ma Rockland and the 0.7 Ma Ther- ern namesake. The river at one time extended as mal Canyon tephra (Meyer et al., 1991; Lan- GEOLOGIC CONSTRAINTS ON far north as the Little Lake Valley, which pres- phere et al., 1999; Woolace, 2005). NORTHERN COAST RANGES ently drains to the north into the Eel River basin. The geomorphic setting of the Russian River EVOLUTION Remnants of alluvial fi ll, deposited by a once- gravel is consistent with a late Pliocene and extensive paleo-Russian River, are exposed Pleistocene age. The gravel is young enough The late Neogene and Quaternary history of discontinuously from the northwestern margin such that most of the paleoriver valley in which the northern Coast Ranges, recorded by rem- of the Santa Rosa Basin northward through the it was deposited still exists as the modern Rus- nants of Miocene and younger sediments that Alexander Valley (Glen Ellen Formation of sian River valley, but the gravel is old enough overlie the Mesozoic Franciscan complex, is Weaver, 1949, and Fox, 1983) to the Hopland, to be cut by faults and has a regional dip of 5° largely one of rock uplift and erosion (Irwin, Ukiah, and Redwood valleys (“continental” to 7° north (Treasher, 1955; Cardwell, 1965). 1960; Bailey et al., 1964; Wahrhaftig and Bir- deposits of Cardwell [1965]) (Table 1). We Its original depositional morphology has been man, 1965). Cover sediment that survives as depict deposits in the Alexander Valley as the destroyed by erosion and drainage divide migra- isolated remnants (Table 1) partially chronicles Glen Ellen gravel and deposits in the Hopland, tion, and late Pleistocene fl uvial terraces are cut this uplift and erosion history (Fig. 2). The Ukiah, and Redwood valleys as the Russian into the gravel. mostly metasedimentary Franciscan rocks were River gravel (Figs. 2 and 3). The modern Rus- accreted to the North American continent as sian River fl ows through the Alexander, Hop- Humboldt Basin Shallow-Marine and Fluvial parts of subduction zone complexes and sub- land, Ukiah, and Redwood valleys. Sediment sequently translated northwestward to their Based on records of dozens of well logs, The fi rst appearance of the fl uvial Hook- present latitudinal position (Blake et al., 1985). deposits of the paleo-Russian River are tens to ton Formation (Ogle, 1953) signals the emer- With the possible exception of the King Range hundreds of meters thick. In northwesternmost gence of the lower Eel River drainage basin terrane (McLaughlin et al., 1982; Fig. 2), the Santa Rosa Basin and in the valleys of the Rus- in response to rock uplift (“Humboldt basin Franciscan rocks in northern California were all sian River, gravel deposits are over 500 m thick fl uvial,” Fig. 2). The Hookton Formation lies in place by 10 Ma. A portion of the overlying (California Department of Water Resources, unconformably above a deformed late Neogene cover sediments was deposited when the triple 1956; Cardwell, 1965). In an exploration bore- marine Humboldt basin section (“Humboldt junction was located to the south of the present hole for a dam site near Ukiah, the gravel is basin marine,” Fig. 2) (Ogle, 1953; Woodward- Coast Ranges (Fig. 1A); these sediments are 450 m thick (Treasher, 1955). Clyde Associates, 1980) (Table 1). These fl uvial largely marine and older than 5 Ma. The rest Valley fi ll deposits of Little Lake Valley, deposits chronicle erosion from the Eel River of the cover sediment, and the volcanic rocks, 3 km north of the northern extent of the Russian drainage. As the Bruhnes-Matayama boundary record the passage of the triple junction. River gravel, are also part of the paleo-Russian (780,000 yr B.P.) is near the top of the marine The present Coast Ranges drainage is charac- River drainage. Pleistocene Little Lake Valley Eel River group but not within the overlying Eel terized by N-NW–trending trunk streams linked deposits, which are 30 m to at least 140 m thick River alluvium (Woodward-Clyde Associates, by E-W–trending streams (Fig. 2). Two major (Cardwell, 1965), presently are within the Eel 1980), initial deposition of alluvium at the mod- river systems drain the Coast Ranges between River drainage and are separated from the Rus- ern coastline began shortly after 0.8 Ma. Strata San Francisco and Humboldt Bay: the Eel River sian River gravel by a low divide (Figs. 2 and 3). of the Humboldt Basin are time transgressive and the Russian River. Streams of the Eel River However, all paleofl ow indicators in the Pleisto- and have equivalent facies that are progressively drainage are mainly north-fl owing and drain cene deposits are to the south, opposite in fl ow older to the east (Woodward-Clyde Associates, into the Pacifi c Ocean north of Cape Mendocino direction to the modern surface drainage in the 1980); therefore, the base of the fl uvial section (Fig. 2). The majority of streams in the Russian valley (Woolace, 2005). Little Lake Valley fi ll would be older eastward up the Eel River val- River system fl ow south to an outlet at latitude deposits are similar to the Russian River gravels ley. Earliest fl uvial deposits in the Eel Basin, 39.3°N (Fig. 2). At present, the Eel River drains in that they are fi ne to coarse fl uvial sediment although now eroded, probably date from an area approximately three times that of the deposited as part of a through-going fl uvial sys- ca. 2.0 Ma. Shallowing and emergence of the Russian River. However, using the late Neogene tem, in contrast to the geographically confi ned, Humboldt Basin at ca. 2 Ma is consistent with a Coast Ranges cover sediments, we show that single-drainage-outlet valley that characterizes late Neogene and Quaternary geohistory analy- the Russian River drained a far greater area of the modern Little Lake Valley (Woolace, 2005). sis of the basin (McCrory, 1989), which indi- the northern California Coast Ranges in the late Therefore, the paleo-Russian River drainage cates rapid emergence of the basin ca. 2.5 Ma. Neogene and early Quaternary. extended farther to the north than its modern counterpart. Uplift Rates Derived from Ohlson Ranch Geologic Constraints on Paleodrainage Limited age data indicate that fl uvial sedi- and Fort Bragg Marine Terrace Deposits ment of the paleo-Russian River gravel as a The most signifi cant late Neogene Coast whole may be time transgressive with younger Cover sediments between the Russian River Ranges cover sediment sequences that record deposits to the north (Fig. 3). In the south, the mouth and Fort Bragg (Fig. 2) provide broad paleodrainage are deposits associated with the gravel is likely late Pliocene based on its inter- constraints on coastal uplift rates over time scales paleo-Russian River and deposits associated fi ngering relationship with the marine strata of of hundreds of thousands to a few million years. with the emergence of the Eel River Basin. the Wilson Grove Formation, which contains The Ohlson Ranch Formation (Fig. 2; Table 1) is

1234 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges and Carver (1988) Kelsey and Carver (1988) Kelsey (1965), Kelsey (1977) (1965), Kelsey Associates (1980), McCrory (1989) (1952), Mankinen (1972), Bartow et (1976), (1973), Sarna-Wojcicki al. (1985), Sarna- et al. (1983), Fox Fox (1992) Wojcicki (1952), Treasher (1955), California (1955), California Treasher (1952), Resources Water Department of (1983) (1965), Fox (1956), Cardwell (1989) (1981), Rymer (1981), Rymer et al. (1981), Rymer et al. (1988) Resources (1956), Jennings and (1960) Strand (1978), Kennedy and Lajoie (1982), (1978), Kennedy Merritts and Bull (1989) MacGinitie (1943), Menack (1986), MacGinitie (1943), Menack Ogle (1953), Woodward-Clyde Woodward-Clyde Ogle (1953), Higgins (1960), Peck (1960), Prentice Higgins (1960), Peck McLaughlin and Nilsen (1982) Brice (1953), Donnelly-Nolan et al. Brice (1953), Donnelly-Nolan et al. California Department of Water Water DepartmentCalifornia of Pliocene (based on mollusks and diatoms) Pleistocene invertebrate marineinvertebrate fossils younger mammal remains marine deposits to terrace the west Miocene and Pliocene Travis (1950), Johnson (1934), Gealey Pliocene and Pleistocene Travis (1950), (1949), Gealey Weaver Ca. 3.5–1.7 Ma, based on Ca. Inferred to be 4.0 Ma or Inferred 1.8–3.0 Ma, based on fossil 1.8–3.0 Ma, based on fossil Coeval with the Fort Bragg Coeval Pleistocene and Birman (1965), Kennedy Wahrhaftig Pleistocene (1999) Koehler uvial and estuarine sediment Miocene Pliocene and ne sand; mantles a wave-cut platform, platform, mantles a wave-cut ne sand; and on the west side of the Santa Rosa valley. The ca. 6 Ma Roblar tuff (Fig. 3) tuff (Fig. 6 Ma Roblar The ca. side of the Santa Rosa valley. and on the west ngers eastward Marine sediment interfi is interbedded within the marine section. At their eastern 2.6–7.9 Ma) deposits. (Sonoma volcanic, uvial and volcanic with fl (late Miocene to late Pliocene) marine margin, these deposits record a long-lived to nonmarine transition. northwestern margin of the Santa Rosa Basin northward to the Little Lake Valley northwesternValley margin of the Santa Rosa Basin northward to the Little Lake (Willits) 250–470 m above sea level. A fi ssion-track age on an interbedded tuff is 3.3 ssion-track A fi sea level. 250–470 m above ± 0.8 Ma. along the propagating Maacama fault zone. along the propagating Maacama fault Minimum age constrained by overlying 1.66 ± 0.10 Ma Clear Lake volcanics. 1.66 ± 0.10 Ma Clear Lake overlying by age constrained Minimum out at elevations of 90–190 m elevation along ~12 km of Navarro and Anderson along ~12 km of Navarro of 90–190 m elevation out at elevations Valleys. Water Resources, 1956). Resources, Water to the divide with the Van Duzen River. Duzen Van to the divide with Marine strata unconformably overlain by Pleistocene fl by overlain Marine unconformably strata TABLE 1. NEOGENE AND QUATERNARY COVER SEDIMENT IN THE NORTHERN CALIFORNIA COAST RANGES CALIFORNIA COAST THE NORTHERN SEDIMENT IN COVER NEOGENE AND QUATERNARY 1. TABLE and fl uvial and fl Marine sand and silt Fossiliferous Late early Miocene to Fluvial and marine sand, and coaluvial gravel, Marineand silt interbedded with fl and estuarine clay Miocene Clark (1940), Kelsey Marine sand and silt Fossiliferous Pliocene and Birman Wahrhaftig Irwin (1960), Marine estuarine Fluvial and lacustrine from the ll that outcrops discontinuously alluvial fi Remnants of a once-extensive Fluvial basin deepening and extending Fluvial sediments deposited within an actively Fluvial crops in places, 70 m thick over with interbedded lacustrine clay, Sand and gravel Marine Department (California as 15 m thick as much of Beach and nearshore deposits, Garberville area deposits near Covelo deposits near Covelo Duzen and northernDuzen Basin Eel River Basin sediment Russian River and Little Russian River sediments Valley Lake Creek Anderson Valley Anderson Valley deposits terrace deposits terrace Deposits of the Unit or formation nameUnit or formation Robinson Creek Formation: Temblor Environment Marine Oyster-bearing conglomerate Comments Miocene age Probable Orchard (1979) Age References Scattered deposits, Van Van Scattered deposits, Humboldt (Eel River) Humboldt (Eel River) Wilson Grove Formation Formation Wilson Grove Fluvial and marine south of the Russian River uves at-topped interfl Sediment is preserved on broad, fl Glen Ellen Formation; Glen Ellen Formation; Ohlson Ranch Formation Ohlson Ranch Formation Marine Beach and nearshore deposit, primarily fi Deposits of Little Sulfur Cache Formation Cache Formation Fluvial ~4000 m thick. deposits; occurrences of quiet water Fluvial deposits with rare Navarro River and River Navarro Fort area marine Bragg Kettenpom gravel gravel Kettenpom Fluvial sequences that step up from the Northll terrace Eel River Fork fi Three ~30-m-thick

Geological Society of America Bulletin, September/October 2006 1235 Lock et al. a fi ne sandy beach and nearshore marine deposit cliff landscape that developed on a late Pliocene age on zircons separated from a tephra within that mantles one or several elevationally closely coast. We infer a rock uplift rate of 0.08–0.2 m/ the deposit (Prentice, 1989) rather than on fos- spaced wave-cut platforms (Higgins, 1960). The k.y. using the current elevation of the deposit sils (Higgins, 1960). eastern margin of this deposit is bounded by a (250–470 m) and assuming a shoreline age of Extending along 60 km of the northern Cali- paleo-sea cliff. The underlying composite plat- the highest Ohlson Ranch beach of ca. 3 Ma fornia coast near Fort Bragg (Fig. 2), beach form is tilted and faulted with a structural relief (Table 1; Peck, 1960; Higgins, 1960; Prentice, and nearshore marine deposits as much as of ~200 m. The Ohlson Ranch deposits are part 1989). The ca. 3 Ma age for the Ohlson Ranch 15 m thick (California Department of Water of an uplifted wave-cut platform and paleo-sea Formation is based mainly on a fi ssion-track Resources, 1956) are preserved on interfl uves as four to six marine terraces. The terraces are up to 8 km wide and range in elevation from 12 124° 123° to 240 m (Wahrhaftig and Birman, 1965; Mer- 41° 41° ritts et al., 1991). In this area, we infer a rock uplift rate around 0.1–0.4 m/k.y. using the low- Eel River est three marine terraces, which were formed during oxygen isotope stage 5 highstands Humboldt (~125–75 ka) (Kennedy, 1978; Kennedy and Humb. basin Lajoie, 1982; Merritts and Bull, 1989; Merritts basin marine fluvial et al., 1991), and the California sea-level curve Van Duzen R (Muhs et al., 1992). CM * Scattered Pliocene marine Coastal Drainage Outlet Positions S *Eel Rive Mattole River . F k . E r e At least since the late Miocene, the time l Kettenpom span over which remnant cover sediment is gravel N preserved, there have been only two coastal out- King Range .

* F terrane k lets for rivers draining the interior of the north- .

Garberville E e l ern California Coast Ranges: a southern outlet 40° 40° sediments R Point Delgada . near the modern Russian River mouth and a northern outlet in the vicinity of the Humboldt Temblor Delgada Fm Basin (Fig. 3). The Delgada fan (Fig. 2), now Fan Laytonville displaced to the north along the San Andreas . approximate position Noyo Canyon Valley R . F l of Miocene fault, was at the latitude of the Wilson Grove M k. Ee (ca. 10 Ma) Formation at 5–6 Ma (Sarna-Wojcicki, 1992). shoreline The Delgada fan grew rapidly between 6 and Little 2 Ma (Drake et al., 1989). Sandy facies in the Fort Bragg Lake Willits Valley Wilson Grove Formation have been interpreted Redwood as turbidite fl ows feeding into the fan (Allen and Fort Bragg Russian Valley Holland, 1999). Thus, the marine and fl uvial marine terraces River gravel Ukiah Wilson Grove Formation and the Delgada fan Valley may have been deposited synchronously and San Andreas Robinson Cr fault so may record the paleo-Russian River mouth 39° Anderson 39° as a long-lasting marine-nonmarine transition between 2 and 6 Ma. Sediments in the Delgada Hopland Valley Cache fan (Fig. 2) indicate rapid fan growth starting at Russia Sulphur Cr Fm ca. 6 Ma (Drake et al., 1989), which is when we Marine infer the paleo-Russian River began to deliver n Rive Marine Alexander sediment from the interior Coast Ranges through terrace sand Ohlson r Valley the southern outlet (Fig. 3). Ranch Fm The interior location of the marine Wilson Fluvial Glen Ellen gravel Grove Formation (Fig. 2) means the modern Russian River mouth is located 20–25 km west 0 50 km Santa Rosa Russian River Wilson of the paleomouth. Higgins (1952) argued that Grove Fm the Russian River mouth migrated westward as 38.25° 38.25° the Wilson Grove Formation became emergent 124° 123° in the last 2 m.y. and the river incised a can- Figure 2. Map depicting all postsubduction accretion cover sediment in the Coast Ranges yon across the emerging coastal plain. There- on the North American plate east of the San Andreas fault in the area drained by the Rus- fore in the interval ca. 6–2 Ma, paleo-Russian sian and Eel Rivers and by intervening coastal streams (see compiled references, Table 1). River–derived sediment must have been trans- Boundaries of cover sediment are smoothed and consolidated in cases where actual cover ported across a 20–25-km-wide shelf, which sediment is patchy due to erosion. CM—Cape Mendocino. was a low-lying coastal plain at times of lower

1236 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges

coastal marine marine/ marine outlet for fluvial fluvial volcanic tephra sediment sediment terrace rocks interior transition sand drainage 10 Clear Lake volcanics Roblar tuff 9 Temblor Fm, 9 Covelo 8 (marine) 8 Robinson Sonoma Creek cgl

k basin k volcanics 7 Humboldt Garberville (marine) Glen 7 basin sediment Ellen (marine) Temblor Fm, 6 (marine) Covelo gravel 6 Pliocene (fluvial) Ohlson tuff 5 remnants 5 (marine) Cree Sulphur Wilson 4 Grove 4 * * fm * (marine) 3 3 arine Cache Years before present (Ma) present before Years Latitude-time trajectoryLittle for MTJsian Fm 2 Lake Rus 2 shallow m Laytonville River n basin Valley 1 Humboldt basi gravel Ohlson 1 Kettenpom Anderson Ranch Fm 0 gravel 0 Eel River Fort Wilson Santa 41 alluvium Point 40Bragg 39 Cape terraces Ukiah Grove Rosa Delgada Willits Mendocino Rockland ash Incision of Russian Latitude (degrees north) River canyon

Figure 3. Latitude-time distribution of Neogene cover sediments and location of the Mendocino triple junction (MTJ). Fluvial-marine transitions record a northward-progressing emergence of the northern California Coast Ranges in concert with the northward migration of the Mendocino triple junction.

relative sea level, to the site of the Delgada Timing and Pattern of Coast Ranges Miocene in age. North of 39.75°N, marine cover fan. The permanent emergence of the shelf at Emergence remnants in the now-emerged Coast Ranges are 2 Ma promoted incision of the lower Russian Pliocene (Garberville and other scattered marine River canyon. Using the current elevation, we Defi ned by the age and location of the marine remnants) or as young as early Pleistocene in estimate uplift rates that accompanied canyon and fl uvial sediments described in the previous the case of the most northerly sediments in the incision to be on the order of 0.1–0.4 m/k.y., sections, the submerged western part of what Humboldt Basin (Figs. 2 and 3; Table 1). similar to the uplift rate over the last 125 k.y. is now the Coast Ranges emerged in the late Another indication of northward time-trans- for the Fort Bragg marine terraces ~100 km to Neogene. Emergence started in the south and gressive emergence is that at ca. 3 Ma, the coast- the north. progressed northward. We delineate the approx- line in the south was already situated where it is Alluvial fi ll of the Cache Formation (Figs. 2 imate position of the Miocene (ca. 10 Ma) today, whereas the coastline in the north had not and 3; Table 1) represents another outlet to shoreline (bold shaded line, Fig. 2) as that N- yet started to retreat westward. The paleo-sea the south that drained the interior of the Coast NW–trending boundary east of which there is cliff for the ca. 3.0 Ma Ohlson Ranch Formation Ranges in the late Neogene. The Cache Forma- no evidence of late Neogene marine rocks. West shoreline is only 8 km east of the modern coast tion outlet probably drained southeastward to of this line, remnant marine cover sediment at latitude 38.5°N to 38.7°N (Fig. 2), whereas the Central Valley of California rather than to indicates that the Coast Ranges were submerged in the latitude range 40.1°N to 40.6°N, the the coast. until emergence began as the triple junction ca. 3.0 Ma paleoshoreline for the Garberville The Humboldt Basin (Fig. 2) is the north- migrated northward (Fig. 3). and other Pliocene marine sediments is at least ern drainage outlet for the interior of the Coast The age and position of marine cover strata 60 km east of the modern coast (Fig. 2). Ranges. While the basin has been a marine dep- argue for a time-transgressive, younging-to-the- The onset of fl uvial deposition at the mouth ocenter since the Miocene, on the basis of allu- north emergence of the Coast Ranges. South of of the northern Eel River outlet, recorded by vial sediment at the top of the Humboldt Basin, 39.75°N, marine sediment remnants in the now- the emergence of the Humboldt marine basin it has been the down-valley end of a large inte- emerged Coast Ranges (Temblor Formation at ca. 2 Ma (Ogle, 1953; Woodward-Clyde rior drainage only since ca. 2 Ma. and Robinson Creek; Figs. 2 and 3; Table 1) are Associates, 1980; McCrory, 1989), marks the

Geological Society of America Bulletin, September/October 2006 1237 Lock et al. uplift and westward movement of the northern age network fl owed primarily to the south. The north of the gap). The wind gaps may connect part of the coastline to reach its present location upper reaches of the North Fork Eel River, the a formerly through-going trunk valley that con- (Fig. 2). Middle Fork Eel River, and the main Eel River tained a through-fl owing stream. For instance, all fl ow south before turning to fl ow to the north the Kettenpom gravel (Figs. 2 and 3; Table 1) is Evidence of Drainage Reorganization (Fig. 4); in each of the three cases, the bends in evidence that the North Fork Eel River and the the streams (fi shhooks) are trunk stream valleys headwater reach of the current Van Duzen River Geomorphic indicators of stream capture and that fl ow E-W (henceforth called cross-streams) were once a joined, through-fl owing drainage. drainage reversal in the Coast Ranges are con- in a trend cutting across the N-NW structural The Kettenpom gravel can be traced northward sistent with a decrease in the volume of sedi- grain (Fig. 4). toward the Hettenshaw wind gap (Fig. 4), which ment discharged from the southern outlet at the The headwaters of several major tributar- drained headwater streams southward into the same time as an increase in fl uvial sediment dis- ies in the Eel River Basin initiate in wind gaps North Fork of the Eel River prior to abandon- charge at the northern outlet. Two topographic (Fairbridge, 1968), that is, low divides that were ment of the gap as a result of headwater capture attributes of the predominantly north-fl owing formerly occupied by a water course (located by by the north-fl owing Van Duzen River (Koehler, modern Coast Ranges drainage network, fi sh- triangles in Fig. 4). The wind gaps are between 1999). Presently, recapture is imminent at several hooked streams (streams that initially fl ow south N-NW–trending trunk valleys containing streams of the low divides depicted in Figure 4, notably before turning 180º to drain to the north) and that fl ow in opposite directions (south-fl owing Railroad and Potter Valley, where south-fl ow- wind gaps, are indications that the paleodrain- to the south of the gap and north-fl owing to the ing headwater streams will capture and redivert north-fl owing headwater reaches. As explained next, a pattern of capture and recapture at wind gaps is a consequence of drainage adjustment to 41°N the Mendocino crustal conveyor.

Humboldt MENDOCINO TRIPLE JUNCTION Bay TECTONICS AND THE MENDOCINO 0 50 km CRUSTAL CONVEYOR

CM Van Duzen The Mendocino triple junction marks a fun- River damental change in tectonic regime along west- Hettenshaw Northern peak ern North America from the subduction zone of of MCC uplift and the Cascadia margin north of the triple junction North Fork drainage divide Eel River to the transform zone between the Pacifi c and Point Eel River Delgada North American plates south of the triple junc- 40°N tion (Dickinson and Snyder, 1979; Zandt and Salt Cr. Middle Fork Eel River Furlong, 1982; Furlong et al., 1989). A variety draina of seismic studies (Verdonck and Zandt, 1994; Beaudoin et al., 1996, 1998; Henstock et al., Black Butte Island ge 1997) has shown that crustal structure varies Ridge headwaters, substantially throughout northern California. Tomki Eel River Crustal thicknesses double from the initial Railroad Southern peak ~20 km in the accretionary margin of northern Potter of MCC uplift and Russian River drainage Valley drainage divide California and reach maximum thicknesses Cache exceeding 40 km near the southern edge of the subducting Gorda plate (Beaudoin et al., 1996, 39°N Point Arena 1998; Villasenor et al., 1999). Because the spa- Cache Creek tially varying crustal thickness is attributed to triple junction tectonics, triple junction migra- tion has driven an ephemeral thickening of the Pacific Ocean North American crust along coastal California (Furlong and Govers, 1999). Based on a fi nite-element numerical model, Furlong and Govers (1999) proposed that the observed thickness variation of the North Amer- ican crust is a consequence of the migration of the Mendocino triple junction. In the model, 38°N crustal thickening occurs by viscously coupling 124°N 123°N the southern edge of the Gorda slab to the base Figure 4. Prominent wind gaps (triangles) and fi shhooked rivers (shaded curved arrows) in of the overlying North American crust (Fig. 1B). the northern California Coast Ranges. The two main drainage divides (dashed shaded lines) Coupling the overlying North American crust are located at the peaks of Mendocino crustal conveyor (MCC) uplift. The E-W–trending to the migrating Gorda slab causes the North sections of rivers are what we refer to as cross streams. CM—Cape Mendocino. American crust above the triple junction to be

1238 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges

pulled into itself to the north, causing thicken- the crust thins. Actual uplift rates will depend on thickest crust, reducing the net uplift, while the ing, and stretched away to the south, causing whether local isostasy or fl exure is the dominant positive buoyancy force causes uplift and delays crustal thinning. The geodynamic processes response to crustal thickening and thinning. The the subsidence produced by crustal thinning. described by the Mendocino crustal conveyor details of the crustal thickening and thinning When the dynamic topography is superimposed (MCC) model are predicted to thicken the North rates are secondarily dependent on the initial on the broad domal uplift generated by crustal American crust in advance of the triple junction crustal thickness and whether the crustal defor- thickening and thinning, the model result is a and subsequently thin the crust in the wake of the mation is uniform throughout the thickness of “double-humped” surface pattern (Fig. 5C; Fur- Mendocino triple junction (Furlong and Govers, the crust or is localized at certain depths. long and Govers, 1999). The “double-humped” 1999; Furlong and Guzofski, 2000; Furlong and Although in the MCC model uplift is mostly pattern migrates with the triple junction, driving Schwartz, 2004). The model predicts that the the response to crustal thickening (Fig. 5A), uplift and subsidence rates up to ~1.5 mm/yr crust should be thickened and then thinned back the model predicts an additional component (Fig. 5D). to its original thickness over ~10 m.y. as the tri- of uplift from dynamic topography (Fig. 5B). ple junction passes (Fig. 1B). Such substantial Dynamic topography is a consequence of the Spatial Extent of the MCC Model changes in crustal thickness will cause isostatic migrating geodynamic processes. Two modes of uplift, and thus MCC-described processes can dynamic topography exist in the MCC model. As a two-dimensional model, the MCC drive the northward progressing uplift and emer- First, subhorizontal fl ow induced in the slab model describes processes and predicts uplift gence of the northern Coast Ranges recorded in window by migration of the Gorda plate pro- along a line that trends NW-SE in the direction the cover sediments. duces a “low pressure” in the slab window that of plate motion (Fig. 1A). The model does not results in a downward fl exure of the overlying place limits on the aerial extent of the predicted Mendocino Crustal Conveyor Model Uplift crust (Fig. 5B). The model predicts fl exure with uplift. However, because the primary effect of Rates an amplitude of up to 2 km and a wavelength MCC processes is to thicken and thin the crust, of ~100 km centered on the southern edge of we use the crustal thickness in the Coast Ranges The MCC model describes rock uplift that the slab. The second component of dynamic to place limits on the lateral extent of the region is driven by two mechanisms: the isostatic topography occurs ~100–150 km south of the affected by MCC modeled processes. Seismic response to crustal thickening and a dynamic model slab edge, where asthenospheric mantle imaging, both active source (e.g., Beaudoin response to mantle fl ow. The model predicts fl ows into the slab window, producing surface et al., 1998; Henstock et al., 1997) and local that crustal thickening rates of ~3–5 mm/yr in uplift. In the model, the domal uplift caused crustal tomography (e.g., Villasenor et al., 1998; advance of the triple junction will double the by the upwelling mantle fl ow is up to 1 km Fig. 6) provide useful constraints on the pattern thickness of the crust over just 5 m.y. (Furlong in magnitude and has a 100+ km wavelength and magnitude of crustal thickness variation and Govers, 1999; Fig. 5). For typical crustal (Fig. 5B). The two components of dynamic beneath the northern Coast Ranges. The tomog- and upper-mantle densities, 3–5 mm/yr crustal topography combine with the isostatic uplift to raphy (Fig. 6) shows a relatively complex pat- thickening equates to isostatic uplift rates of produce the total predicted uplift (Fig. 5C). The tern of crustal thickness with two main areas of ~0.5–1 mm/yr, with similar subsidence rates as downward fl exure is located near the region of thickened crust. The thickest crust lies beneath

4 4 4 Location A Isostatic Uplift B Dynamic C of MTJ Topography 3 3 3

2 2 2 Downward Flexure 1 1 1 MTJ Migration Rock Uplift (km) Rock Dynamic Uplift (km) Isostatic Uplift (km) 4 SE NW 0 0 0 200 400 200 400 200 400 Distance (km) Distance (km) Distance (km) -1 -1

Dynamic 2 D -2 -2 Uplift 1 0 200 400 -1 (mm/yr) Uplift Rate

Figure 5. The combination of isostatic uplift (A) and dynamic topography (B) results in a double-humped pattern of uplift (shown at 1 Ma and present) (C), which migrates to the northwest at a rate of ~40–50 mm/yr driving rock uplift and subsidence rates up to 2 mm/yr (D). The peaks in the pattern of uplift correspond closely to the location of the two major divides in the Coast Ranges depicted in Figure 4. MTJ—Mendocino triple junction.

Geological Society of America Bulletin, September/October 2006 1239 Lock et al.

124°W 122°W 120°W 42°N 42°N Vp

7.5

C A Gorda Slab km/s 7.0

6.5

40°N 40°N

Pioneer Fragment Region of Thickened Crust

25 km Depth 38°N D B 38°N 124°W 122°W 120°W AB MCC Crustal Thickening 0 5.4 -10 6.2 -20 Vp 7.0 -30 Gorda Base of Crust Depth (km) Slab 7.8 -40 8.0 0 50 100 150 200 250 300 350 Position Along Profile (km) 7.0 km/s 6.0 C Pioneer Crustal Thickening D 0 5.4 5.0 -10 6.2 -20 7.0 -30 Pioneer Fragment Base of Crust Depth (km) 7.8 -40 0 50 100 150 200 250 300 350 Position Along Profile (km) Figure 6. Tomography in the northern California Coast Ranges (from Villasenor et al., 1998) at a depth of 25 km and NW-SE–oriented cross sections. The tomography data image the high-velocity bodies of the subducting Gorda plate in the north and the Pioneer fragment farther south. These features are moving to the northwest, causing crustal thickening of the overlying North American crust. MCC—Mendocino crustal conveyor.

1240 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges

the center of the northern Coast Ranges, and a terraces (Merritts and Bull, 1989) is a local- intersection of the Pioneer fracture zone and more subdued region of thick crust lies beneath ized uplift driven by the transport of this ter- the North American margin (Atwater, 1970). A the Mendocino Range (Figs. 6 and 7). MCC rane onto the western margin of North America short ridge segment linked the Pioneer and Men- modeled processes can explain the thickened (McLaughlin et al., 1982). The crustal structure docino transform faults/fracture zones. As the crust beneath the central Coast Ranges, which beneath the King Range is not anomalously ridge segment approached the margin, spread- shows that the spatial extent of effects of the thick, and MCC-predicted isostatic uplift and ing ceased, and the triple junction jumped to its migrating slab window extend from the Rus- dynamic topography do not extend this far west present position at the end of the Mendocino sian River–Eel River corridor to just less than (Fig. 6). fracture zone (Atwater and Stock, 1998). The 100 km to the east. The high elevation in this The tomography illuminates an area of thick- abrupt jump of the triple junction likely led to area is the isostatic response to the thickened ened crust beneath the Mendocino Range that the abandonment of a small subducted fragment crust. In the N-S direction, MCC-driven crustal is spatially correlated with a high-velocity body (the Pioneer fragment) that was attached to the thickening begins ~150 km north of the triple just north of the Mendocino Range (Figs. 6 Pacifi c plate along the extinct ridge segment. junction (Figs. 5A and 6). The effects of the and 7). This area of thickened crust cannot be The subducted Pioneer fragment has been MCC model extend south to the latitude of driven by processes related to the slab window translating beneath the margin of North America Clear Lake. In total, processes described in the left by the Gorda slab, because the thickened since its capture by the Pacifi c plate at ca. 25 Ma. MCC model modify an ~4000 km2 area of the crust is located too far south of the Mendocino The fragment, which would cause crustal thick- northern California Coast Ranges. triple junction. We consider the high-veloc- ening and dynamic topography similar to the Although the processes modeled by the MCC ity body north of the thickened crust to be the migrating edge of the subducted Gorda slab, is explain much of the uplift and emergence of the “Pioneer fragment,” and we infer that the Men- presently located in the vicinity of the coastal Coast Range, they cannot be used to explain docino Range uplift is driven by the fragment’s bight slightly north of Fort Bragg. There is an uplift of the Mendocino Range west of the pres- migration. The Pioneer fragment represents a associated crustal thickening and thinning in the ent Russian River valley nor can they explain remaining bit of the Farallon plate. In the early fragment’s wake, albeit of reduced magnitude uplift of the King Range (Fig. 7). Rapid uplift stages of the development of Mendocino triple (maximum crustal thickness of 30–35 km com- of the King Range terrane recorded by marine junction, the triple junction was located at the pared with 40 km in the Coast Ranges) (Fig. 6).

DISCUSSION: INTEGRATING CRUSTAL DYNAMICS AND SURFACE M PROCESSES T Eureka J Migr Evolution of Coast Ranges Morphology North Van Duzen ati Mattole River on In an active orogen, crustal thickness is the American MTJ Plate result of tectonically driven crustal thicken- ing and thinning, modifi ed by erosion or other Eel River surface processes that redistribute crustal mass. 40°N A weakness of geodynamic models such as the MCC is that they do not typically include the effects of erosion or any other modifi cation of Pacific the crust by surface processes. Without incorpo- Plate rating surface processes, the prediction of crustal 0 40 thickness from a geodynamic model should km have misfi ts with the observed crustal thickness

Rus Clear and isostatic topography. In order to compare MCC predictions to the elevations observed sian R Lake in the Coast Ranges and to place robust con- Central Range San straints on the geodynamic model, we use the Mendocino Range Andreas crustal structure imaged by seismic tomogra- King Range phy (Fig. 6). Assuming typical crust and mantle Fault densities, isostatic elevation is calculated from Western extent of tomography-derived crustal thickness along the MCC processes transect of the MCC model (Fig. 8A). If the height of the Coast Ranges is controlled solely 124°N 123°N by the isostatic response to crustal thickness, the observed elevations should more or less Figure 7. Map of northern California Coast Ranges showing hypothesized zones of uplift in mimic the calculated isostatic pattern. Making response to different tectonic drivers. Uplift of the Central Range is driven by the migration the comparison between the observed and iso- of the southern edge of the Gorda slab. Uplift of the Mendocino Range occurs in response static elevations, there is a reasonable match in to the migration of the Pioneer fragment. King Range uplift is due to docking of a crustal the overall wavelength, but the observed eleva- fragment (McLaughlin et al., 1982) and not due to the Mendocino triple junction (MTJ) tions are signifi cantly less than those obtained migration. MCC—Mendocino crustal conveyor. assuming local isostasy (Fig. 8A). If we instead

Geological Society of America Bulletin, September/October 2006 1241 Lock et al.

4000 Isostatic elevation 4000 assume that fl exure is the response to crustal calculated from loading, the result is even higher elevations, A crustal 3000 structure 3000 B Observed Elevation and such overestimation shows that isostasy is (tomography) a more dominant control on elevation than fl ex- 2000 2000 ure. Thus, the crust in northern California has 1000 1000 Elevation (m) Elevation a relatively small effective elastic thickness, in (m) Elevation 0 0 agreement with the small effective elastic thick- Observed elevation -1000 -1000 MCC-predicted ness found by McNutt (1983). dynamic SE NW topography The substantial differences between the cal- -2000 -2000 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 culated isostatic and actual elevation (Fig. 8A) Distance (km) Distance (km) mean there must be forces in addition to isos- tasy controlling elevations. The misfi t between Location 4000 Addition of isostatic the isostatic elevation and actual elevation has elevation and 4000 of MTJ C MCC-predicted a shape, wavelength, and magnitude compara- 3000 3000 D MCC-predicted dynamic elevation ble to the dynamic topography force predicted topography 2000 2000 by the MCC model, suggesting that dynamic X3 topography is the missing force needed to 1000 X2 X1 1000 Elevation (m) Elevation Elevation (m) Elevation explain the height of the Coast Ranges. Add- 0 0 ing the MCC-predicted dynamic topography (Fig. 8B) to the isostatic elevation calculated -1000 -1000 SE NW from the observed crustal structure (Fig. 8A) -2000 -2000 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 produces a pattern that has a shape, wavelength, Distance (km) Distance (km) and magnitude comparable to Coast Ranges ele- vation (Fig. 8C). The good fi t between the sum Figure 8. (A) Isostatic elevation calculated from crustal structure derived from seismic of the isostatic and dynamic elevations and the tomography and maximum observed elevations taken along a 50-km-wide swath trending observed elevations in Figure 8C provides sup- N24W. If the Coast Ranges were purely isostatically compensated, the observed elevations port for the MCC model–determined viscosity would match the isostatic elevation. (B) Dynamic topography as predicted by the Men- in the slab window and the 5−10 km effective docino crustal conveyor (MCC) model. (C) The solid black line shows the superposition elastic thickness of the North American crust of MCC-predicted dynamic topography and the isostatic elevation. X1, X2, and X3 mark (Furlong and Govers, 1999). the locations of E-W–trending cross streams. X1 is the cross stream of the North Fork Eel If the MCC does indeed explain the mechanism River, X2, the South Fork Eel River, and X3, the main Eel River headwaters. The triangles driving crustal thickening and uplift of the north- mark the two main drainage divides that approximately correspond to the peaks of the ern Coast Ranges, the amount of exhumation in MCC-predicted uplift. (D) Difference between the MCC-predicted elevation and the actual the Coast Ranges is the difference between MCC- elevation; the difference between these two lines is the amount of exhumation. MTJ—Men- predicted uplift (i.e., using the MCC model-pre- docino triple junction. dicted uplift, not the isostatic uplift derived using the seismically imaged crustal thickness) and the observed elevations. This difference between predicted uplift and observed elevations reaches The two geodynamic processes that drive drainage divides migrate in concert with the a maximum of ~2 km, consistent with Creta- uplift, isostatic uplift and dynamic topogra- triple junction, as their current location implies, ceous (i.e., unreset) apatite fi ssion-track appar- phy, combine to produce the predicted double- migration rates are on the order of 40 mm/yr. ent ages of Dumitru (1989), which indicates less humped elevation pattern in the Coast Ranges. We infer that tectonically imposed gradients that than ~4–5 km of erosion. The location and separation of the two major lead to fl ow reversal within established valleys Although we cannot make a quantitative drainage divides—between the Eel and the Rus- (other than the major trunk streams) and leave comparison of MCC-predicted uplift rates with sian Rivers in the south and the Eel and Van the observed wind gaps. Although the drainage uplift rates based on geomorphic constraints, Duzen Rivers to the north (Fig. 2)—correspond divides delineated by smaller streams migrate there is agreement between the northward pro- to the MCC-predicted double hump (Figs. 5C with the triple junction, the highest-order, major gressing emergence of the Coast Ranges and and 8D). Evidence for stream capture and fl ow trunk stream of the Eel River has suffi cient the northward migration of the Mendocino reversal provides additional support that the stream power to cut through the northern peak triple junction. MCC uplift begins ~150 km in location of the drainage divides represents the of uplift. But the same northern peak of uplift advance of the triple junction and northward- migrating peaks of uplift. For example, at the marks the transition between the two lower- progressing emergence of the Coast Ranges southern divide, which separates the Russian order rivers, the upper reaches of the north- moves with the northward-migrating Men- River and Eel River, recapture of several north- fl owing Van Duzen River, and the south-fl owing docino triple junction. Currently, uplift driven fl owing headwaters of the Eel River by the smaller upper reaches of the Eel River. by triple junction tectonics is responsible for south-fl owing headwaters of the Russian River the continued emergence of Humboldt Bay. We is imminent at the Railroad and Potter Valley Evolution of Coast Ranges Drainage interpret the emergence of the fold-and-thrust low divides (Fig. 4). At the northern divide at belt near Humboldt Bay (McCrory, 2000) to the Hettenshaw gap, capture occurred relatively In most convergent orogenic belts that share be the onset of the predicted NW-SE–directed recently (within the past million years), and the same long linear shape of the Coast Ranges, MCC-driven crustal shortening. recapture is imminent (Koehler, 1999). If the streams fl ow predominantly perpendicular to

1242 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges

the axis of the orogen, as in the Coast Ranges between the two uplift peaks (our interpretation Streams that are initiated in advance of the of Oregon or the Southern Alps of New Zealand is that it was subsiding or at least experiencing triple junction are long-lived, and their channels (Kelsey et al., 1994; Hovius, 1996; Burbank only a small amount of uplift), accommodat- are subsequently occupied by either NW- or SE- and Anderson, 2000). In contrast, major Coast ing Russian River outlet formation at that loca- fl owing streams. The NW-SE pattern is likely Ranges rivers trend NW-SE, parallel to the trend tion. As the triple junction migrated northward, in part a consequence of streams preferentially of the orogen (Figs. 2 and 4), a drainage trend the Russian River, a large established drainage eroding along N-NW–trending subduction- that suggests the Coast Ranges landscape is system, could downcut through the uplifting related faults in the Franciscan rocks. The length driven by more than simple convergence. The southern peak, and the location of the coastal of streams formed in advance of the triple junc- NW-SW–trending rivers include many shorter outlet remained stationary in spite of the migrat- tion should be on the order of 75 km, because reach streams and are separated by low divides ing uplift. The main stem of the paleo–Russian that is the distance that MCC-driven uplift cur- that are oriented NW-SW, perpendicular to the River continued to downcut through the uplift rently operates to the north of the triple junc- trend of the mountain belt (Fig. 1). peak in the same way that the trunk stream of tion. It also is the distance between the northern Our interpretation of drainage evolution in the the Eel River currently cuts through the north- divide and the present coastline at Humboldt Coast Ranges integrates geological and geomor- ern peak of the uplift predicted by the MCC Bay. Prior to 2 Ma, the northern (Eel River) out- phic constraints with geodynamic predictions of (Figs. 9B and 9C). The stream capture and fl ow let drained the emerging coastal region north of the MCC model. In general, a drainage pattern reversal observed at the headwaters of the Eel the fi rst divide. We envision that the paleo–Eel should mimic elevation change with fl ow from River today (Railroad and Potter Valley wind River northern outlet resembled the drainage high elevation to low elevation. If rivers followed gaps, Fig. 4) is a process that has continued pattern of the modern-day Van Duzen River this pattern over an MCC double-hump shape through time, causing steady divide migration. system. (Fig. 5), streams fl ow down the topographic gra- From 6 Ma to perhaps as recently as 3 Ma, the From the regular 35–40 km spacing of E-W– dient and migrate along with the triple junction. Russian River lengthened by capturing streams fl owing cross streams, we infer that cross stream In advance of the triple junction, the develop- in its headwaters to reach a maximum length of formation may be controlled in part by the MCC ment of a northwest topographic gradient would more than 100 km, spanning from Wilson Grove pattern of uplift. With triple junction migration form northwesterly fl owing streams. Between north to Little Lake Valley (north of Ukiah) and rates of 40–50 mm/yr, a distance of 35–40 km the two uplift peaks, overall fl ow would be depositing the extensive Neogene fl uvial gravels corresponds to an E-W trunk stream being ini- internal with both northwest and southeast fl ow of the Glen Ellen, Russian River, and Little Lake tiated or occupied every 0.75–1 m.y. Rather directions. After passage of the second peak, gravels (Figs. 2, 9A, and 9B). than behaving in a buzz-saw fashion, moving rivers would fl ow to the southeast. This pattern At 2 Ma, the upper reaches of the Russian northward as the triple junction migrates, the of stream fl ow is seen in the smaller, lower-order River could no longer cut through increasing cross streams maintain their latitudinal position. streams in the Coast Ranges and in the capture uplift of the second migrating uplift peak, per- The northernmost and youngest cross stream and recapture scenarios at wind gaps, but not haps because its average slope had decreased is located just north of the area of thickened in the highest-order trunk streams of the Eel as the river lengthened or because the river was crust in the trough of the uplift (Fig. 8A). If River (Fig. 4). The drainage areas of the trunk trying to defeat two and not just one uplift high. this youngest cross stream formed recently, and streams where they cut through the uplift highs As a result, the upper portion of the Russian if formation of E-W stream segments of main are ~2800 km2 (South Fork Eel River), and River started to be captured by the north-fl ow- trunk streams occurs in the trough of the MCC- ~6800 km2 (main fork Eel River). It seems that ing Eel River system with its outlet at Hum- uplift pattern, then a new cross stream will not while larger reaches of the trunk streams con- boldt Bay (Fig. 9B). Because a large segment of form for another ~1 m.y. As an alternative, a tinue to downcut with increasing uplift, smaller the high-elevation drainage is linked to a main preexisting feature, for instance the E-W–fl ow- upper reaches with smaller drainage areas are stem by a few E-W–fl owing cross streams, the ing section of the Van Duzen River, may become unable to keep pace with uplift. Smaller streams major drainage reversal from a south to a north an E-W–fl owing cross stream (Fig. 8A). cannot modify their slope and erode as fast as the draining system required only a few breaches changing uplift and instead are passively draped of the northern divide to capture the major- Pioneer Fragment and Uplift of the on the local topographic gradient created by the ity of Coast Ranges drainage. Such an event Mendocino Range tectonics. The history of the drainage divide appears to have occurred at ca. 2 Ma, based on between the Van Duzen and the North Fork of the onset of rapid sedimentation in Humboldt The post–3 Ma emergence of the Ohlson the Eel River, the Hettenshaw drainage divide Basin. Following this major capture at 2 Ma, it Ranch Formation (Fig. 4) and post–late Pliocene (Fig. 4), exemplifi es the systematic migration of took more than 1 m.y. for the Eel River drain- incision of the Russian River canyon (Higgins, drainage divides in lower-order drainage as the age to extend southward and capture the north- 1952, 1960) (Fig. 3) chronicle late Pliocene to triple junction translates to the north. ernmost headwaters of the Russian River drain- early Pleistocene uplift of the Mendocino Range In contrast to the evolving drainage pattern age, because streams in the Little Lake Valley (Fig. 6). The marine Ohlson Ranch Formation in lower-order streams, the long-lived rela- (which now fl ow to the north) were still fl ow- (Fig. 2) emerged 3–4 m.y. after passage of the tive stability of the paleo–Russian River outlet ing to the south at ~0.5 m.y. (Woolace, 2005). triple junction (Fig. 4). Similarly, downcutting causes a more complex response to triple junc- At 2 m.y., the drainage area of the Russian of the Russian River canyon between Healds- tion migration by the two major trunk streams River at the location where it cut through the burg and the coast, and formation and uplift of of the Eel and the Russian Rivers. By 6 Ma, southern peak of uplift was ~1800 km2. Based the Fort Bragg marine terraces, has occurred in the paleo–Russian River drainage system was on the modern drainage pattern, the southern the last 2 m.y. established (Fig. 9A). Initiation and subsequent uplift peak defeated the upper reaches of the We suggest that emergence of the Ohlson anchoring of the main outlet at the Russian Russian River at ~0.5 m.y., when the drainage Ranch Formation, uplift of the Fort Bragg River between 8 and 6 Ma occurred at a time area upstream of the southern peak had dimin- marine terraces, and downcutting of the Rus- when the Russian River Basin lay in the trough ished to less than 800 km2. sian River canyon were all driven by migration

Geological Society of America Bulletin, September/October 2006 1243 Lock et al.

N ABN 6 Ma 4 Ma

Scattered * Pliocene Garberville Garberville * sediments sediments

e

n

i

l

t

s *

a Kettenpom

e

o

c gravel n

i l

o t s e l a a o

p MTJ c leo pa

Russian River Robinson Cr gravel Redwood Valley MTJ Ohlson Ranch Fm Alexander Valley

Glen Ellen Wilson gravel Grove Fm Streams lengthening as northern hump migrates

Main Wind gap produced drainage Coastline by stream capture Location of uplift peak & Mendocino Site of stream small stream drainage divide triple juncition capture N N 2 Ma M 0 Ma C C

Humboldt R o basin c k fluvial U p l if MTJ t

e n i l 4 o t 0 s MTJ a o

c

o

e

Eel River Eel

l a p Laytonville Fort Bragg Valley an marine terraces

LLV ific Oce

Pac

Russia

Ohlson n River Ranch Fm

0 50 km C Paleo coastline D Figure 9. Drainage evolution and emergence of the northern California Coast Ranges. Uplift and emergence of the Coast Ranges propagate north as the triple junction migrates. Bold line in A through C shows the progressive advance of the coastline with the Mendocino triple junction (MTJ). In each time frame, the position of the coastline is constrained by the location and age of fl uvial and marine deposits. The locations of the two small stream drainage divides, controlled by the position of the peaks in the pattern of uplift (black double-humped profi le), migrate as small streams are captured and fl ow reverses. The overall drainage evolution does not migrate smoothly with the triple junction. A major reorganization occurs at ca. 2 Ma, when the Eel River captures much of the upper Russian River to become the primary river draining the Coast Ranges. The switch is recorded by a decrease in sedimentation in the Russian River Basin and an increase in sedi- mentation near the lower Eel River mouth at ca. 2 Ma (“Humboldt basin fl uvial,” frame D). See text for details. LLV—Little Lake Valley; MCC—Mendocino crustal conveyor.

1244 Geological Society of America Bulletin, September/October 2006 Landscape evolution of the northern California Coast Ranges of the Pioneer fragment. The amount of crustal simply records stable migration of the triple Mendocino crustal conveyor migratory uplift thickening observed in the tomographic model junction. Similarly, the San Andreas fault zone and has caused the fundamentally different (Fig. 6), ~14 km, should drive ~2–3 km of iso- develops atop the ephemerally thickening crust, response to the same tectonic forcing between static uplift. However, if the geodynamic effects and the shape of the coastline is driven more by larger compared with smaller rivers. of the Pioneer fragment mimic MCC processes, emergence due to migratory crustal thickening we would expect a dynamic topography to be than it is by the development of the San Andreas ACKNOWLEDGMENTS superimposed on the isostatic component of transform zone in the wake of the triple junc- This research was partially supported by National uplift, reducing the total amount of uplift. If so, tion. In the future, the triple junction will con- Science Foundation (NSF) grants EAR-9902937 dynamic topography associated with migration tinue to migrate in a stable fashion lengthening to K. Furlong, R. Slingerland, and H. Kelsey, and of the Pioneer fragment has a magnitude >1 km, the Coast Ranges to the northwest. EAR-0221133 to K. Furlong. We thank A. Sarna- reducing the net uplift from the 2–3 km isostatic Wojcicki for identifying the 0.7 Ma Thermal Can- response to the ~500 m elevation currently CONCLUSION yon tephra in Little Lake Valley and R. McLaughlin, T. Nilson, and M. Larsen for help and advice in the observed for the maximum elevation of the fi eld. Reviews by K. Grove, J. Roering, D. Merritts, Ohlson Ranch Formation. The smaller ampli- Uplift and emergence of the northern Califor- N. Snyder, and F. Pazzaglia have substantially tude of the dynamic topography associated nia Coast Ranges results from the migration of improved the paper. with the Pioneer fragment (>1 km) compared the Mendocino triple junction. Transient crustal with the magnitude of the dynamic topography thickening and dynamic topography caused by REFERENCES CITED associated with the MCC (~2 km) is compat- the triple junction’s migration drive a northward- Allen, J.R., and Holland, P.J., 1999, Description and inter- ible with the smaller residual isostatic gravity migrating, double-humped pattern of uplift that pretation of pebbly sandstone facies of the Wilson anomaly in the Mendocino Range (Jachens and is the primary control on the topography and Grove Formation, Marin County, California; a possible Griscom, 1983). Based on the current eleva- evolving drainage pattern in the northern Cali- sediment source for the Delgada Fan during late Mio- cene/early Pliocene time: Geological Society of Amer- tion of the Mendocino Range and the Ohlson fornia Coast Ranges and on the shape of the ica Abstracts with Programs, v. 31, no. 6, p. 33. Ranch Formation, we infer that Pioneer-related coastline. Small streams respond to the chang- Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North Amer- dynamic topography is insuffi cient to generate ing uplift by stream capture and fl ow reversal to ica: Geological Society of America Bulletin, v. 81, the trough in the isostatic uplift that is evident cause a systematic migration of divides in tandem p. 3513–3536. in the main Coast Ranges. Rather, we infer with triple junction migration. Although the fl ow Atwater, T., and Stock, J., 1998, Pacifi c–North America plate tectonics of the Neogene Southwestern United that the total uplift in response to the Pioneer direction in the channels reverses, the channels States—An update: International Geological Review, fragment is plateau-like with rapid uplift (rates themselves are long-lived features. Northwest- v. 40, p. 375–402. ~0.5 km/yr) followed by a relative stasis. 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Geological Society of America Bulletin, September/October 2006 1245 Lock et al.

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1246 Geological Society of America Bulletin, September/October 2006