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Accretionary Mesozoic–Cenozoic expansion of the Cordilleran continental margin in California and adjacent Oregon

William R. Dickinson Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT Franciscan was largely coeval motion of the Pacifi c plate along the evolving with intrusion of the dominantly Cretaceous San Andreas transform. My method is to present The Mesozoic–Cenozoic Cordilleran oro- batholith into the roots of the summary geologic maps showing the accretion- gen of California includes multiple accre- Cordilleran magmatic arc, but Franciscan ary belts in plan view and accompanying chro- tionary belts incorporated sequentially into accretion was just a late phase of continuing nostratigraphic diagrams that display transverse the continental margin since Middle Trias- tectonic expansion that spanned more than age relationships of rocks both within the accre- sic time. Accreted tectonic elements include 200 m.y. along the California continental tionary belts and superimposed upon them. The subduction complexes assembled along the margin. coordinated maps and diagrams spanning the Cordilleran margin, intraoceanic island full width of California Mesozoic–Cenozoic arcs attached to the continental margin by Keywords: accretion, California, Klamath accretionary tracts were compiled from mul- Jurassic arc-continent collision, and subduc- Mountains, Sierra Nevada, tectonics. tiple sources cited in fi gure captions, and have tion complexes associated with the fl anks of no counterparts of equivalent scope in the litera- the exotic island arcs. Systematic analysis INTRODUCTION ture. The ages of the rock assemblages within of areal relations and geochronological data the accretionary belts, and of the stitching plu- displayed on subregional geologic maps and Ever since the seminal papers of Hamilton tons and sedimentary cover sequences that tie summary chronostratigraphic diagrams (1969) on “underfl ow” (subduction) of Pacifi c the belts together, set constraints on the times allows the punctuated but quasi-continuous mantle beneath California, and of Moores (1970) of amalgamation of the successive belts into the pattern of tectonic accretion to be discerned. on the accretion of intraoceanic arc structures to edge of the continent. Systematic interpretations Stitching plutons and sedimentary overlap the continental margin, geoscientists have been of the maps and diagrams also call attention to successions constrain the times that succes- trying to comprehend the scope and geometry of shortcomings in the currently available database, sive accretionary belts were juxtaposed and tectonic accretion at ancient subduction zones in and thereby indicate the kinds of information amalgamated into the edge of the continental California (e.g., Ernst, 1983, 1984). This paper needed to resolve uncertainties in the tectonic block. In the and Sierra is an appraisal of the Mesozoic–Cenozoic accre- history of the region. Nevada, a continental-margin magmatic arc tionary expansion of the California continental The northwest-trending Mesozoic–Cenozoic of Triassic–Jurassic age includes volcanic and margin based on a fresh synthesis of incremen- lithotectonic belts of California are separated plutonic components built upon and intruded tal geologic mapping and geochronological from the interior of the Laurentian craton by the into deformed Paleozoic assemblages that studies by many geoscientists during the past deformed Cordilleran miogeocline overthrust were accreted to the Laurentian margin several decades. Traverses along key transects by Paleozoic accreted before Middle before Middle Triassic time. The native arc across all the accretionary belts during the inter- Triassic time and discussed in detail elsewhere assemblage is separated from intraoceanic val 1998–2004 gave me personal impressions of (Dickinson, 2000, 2006). Westward toward the Triassic–Jurassic arc assemblages exposed each as a context for understanding descriptions coast, multiple subparallel tectonic assemblages farther west by a compound suture belt of by others in the literature. This overview does of diverse origin were thereafter added to the mélange and broken formation derived from not extend southward into the California Trans- fl ank of during Mesozoic–Cenozoic the remnant ocean basin that separated the verse Ranges, where onshore transrotation and subduction (Fig. 1): (1) a native Mesozoic arc east-facing intraoceanic arc system from the offshore rifting have transposed and disrupted succession built on pre-Mesozoic rocks of the Cordilleran margin. Polarity reversal after key tectonic elements, but does extend north- evolving continental margin; (2) a suture zone Middle to Late Jurassic arc-continent colli- ward to include the Oregon extension of the of tectonic mélange and broken formation sion was followed by accretion of a disrupted Klamath Mountains. immediately seaward; (3) accreted Mesozoic arc ophiolitic belt forming mafi c in My aim is to outline the pattern of punctu- structures seaward from the mélange belts; and the subsurface of the Great Valley forearc ated but quasi-continuous accretion of diverse (4) both Mesozoic and Cenozoic components of basin. Subsequent forearc sedimentation oceanic elements to the California continen- the Franciscan subduction complex farther west accompanied the assembly of multiple belts tal margin from mid-Triassic to mid-Tertiary near the coast. of mélange and broken formation that form time. Subsequently, tectonic accretion ended as Following introductory passages addressing the Mesozoic–Cenozoic Franciscan subduc- subduction of the Farallon and derivative oce- key topical issues, my areal treatment begins tion complex of the California Coast Ranges. anic plates was gradually supplanted by lateral with a discussion of the Klamath Mountains

Geosphere; April 2008; v. 4; no. 2; p. 329–353; doi: 10.1130/GES00105.1; 9 fi gures.

For permission to copy, contact [email protected] 329 © 2008 Geological Society of America

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z B o rotated isb n ee e Transverse PRb SI ri Ranges f ft ba sin 0 100 200 US M A EXI scale in km CO 120° 115° ABCDEFG

T-Fran K-Fran is arcs sut belt acc Pz nat arc batho

Figure 1. Position of California continental margin (west of Great Valley forearc basin) in tectonic framework of southwest Laurentia (adapted after Reed et al., 2005). Regional tectonic relations after Dickinson (2000, 2004, 2006), Dickinson et al. (2005), and this paper, with Jurassic–Cretaceous Bisbee rift basin after Dickinson and Lawton (2001b) and Triassic–Jurassic backarc basin in Nevada adapted after Wyld (2002). Barbed lines are principal thrusts (solid barbs where active). Heavy lines are strike-slip faults (Gaf—Garlock; Naf— Nacimiento; Rif—Rinconada; SAf—San Andreas; SHf—San Gregorio–Hosgri; SIf—San Isidro) with associated Mendocino triple junction (MTJ) and Salinian block (SB). Symbols: A (T-Fran)—Tertiary–uppermost Cretaceous Franciscan subduction complex and Paleogene Siletz-Umpqua assemblage; B (K-Fran)—Cretaceous–latest Jurassic Franciscan subduction complex; C (is arcs)—accreted intraoceanic Triassic–Jurassic island arcs; D (sut belt)—mid-Mesozoic suture belt (pre-Franciscan mélanges); E (acc Pz)—accreted Paleozoic crustal elements (oceanic allochthons and island arcs); F (nat arc)—native Triassic–Jurassic continental-margin arc assemblage (not shown to southeast where crosses miogeocline and southwestern prong of craton); G (batho)—major batholiths of largely Cretaceous age (IDb— Idaho; PRb—Peninsular Ranges; SNb—Sierra Nevada).

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and adjacent California-Oregon coastal ranges Golconda thrust of Nevada, to the subduction transport estimated from paleofaunal interpre- where the various accretionary belts are most zone associated with a Permian–Triassic (284– tations as thousands of kilometers of longitude completely exposed, and then proceeds south- 232 Ma) magmatic arc in eastern Mexico to was required to bring the Tethyan limestones to ward along the Sierra Nevada and the Califor- the southeast (Dickinson and Lawton, 2001a). the Cordilleran continental margin (Stevens et nia Coast Range. Original relationships of the Major strike slip postdated terminal juxtaposi- al., 1990, 1991; Belasky and Stevens, 2006). accretionary belts are progressively obscured tion of against Laurentia along the southward by intrusion of the Mesozoic Sierra diachronous Ouachita-Marathon suture belt that Lateral Translation Nevada batholith, deposition of overlying Meso- closed in earliest Permian time in west Texas zoic or Cenozoic sedimentary cover, and post- (Ross, 1986). Interpretations of the California segment of accretion deformation including disruption by The line of truncation was oriented northwest- the Cordilleran orogen developed for the Geo- multiple strands of the Neogene San Andreas southeast at a high angle to the northeast-south- logical Society of America Decade of North transform fault system near the coast. west trend of Paleozoic tectonic elements cross- American program infer that many of ing Nevada (Dickinson, 2000), and is delineated the accretionary assemblages treated in this paper BACKGROUND in California by the eastern limit of subsequently were displaced hundreds of kilometers during accreted Mesozoic tectonic elements. The trun- Mesozoic time by lateral translation parallel To avoid repetitious discussions of the same cated continental margin that formed near the to the continental margin before reaching their issues, selected aspects of regional tectonics that Paleozoic–Mesozoic time boundary became present positions relative to the interior craton infl uence interpretations of multiple tectonic the locus for subsequent circum-Pacifi c subduc- (Oldow et al., 1989; Saleeby and Busby-Spera, belts in California are discussed here before tion of seafl oor beneath California (Hamilton, 1992; Saleeby, 1992). Most postulated lateral specifi c treatment of the various accretionary 1969). The diverse post–Middle Triassic accre- motions do not directly affect interpretations of assemblages. tionary belts underlying about half the width of this paper bearing on the times of amalgamation California accumulated sequentially against the of different accretionary belts into the continen- Continental Truncation truncated continental margin. tal margin because the nature and magnitude of Paleozoic limestone blocks are embedded in tectonic movements that preceded accretion are Mesozoic accretion along the Cordilleran some Mesozoic mélanges west of the line of not addressed by evidence for the times of fi nal continental margin of California was preceded truncation, but the structurally isolated lime- amalgamation. It is noteworthy, however, that by oblique truncation of the continental block stone blocks are afl oat in matrices of deformed paleomagnetic data from Mesozoic rock assem- by late Paleozoic–earliest Mesozoic strike slip Triassic–Jurassic argillite and chert, and the age blages of the Klamath Mountains and Sierra (Hamilton, 1978; Davis et al., 1978). Sinistral of formation of a tectonic mélange is given by Nevada support no inferred latitudinal motions strike slip displaced crustal blocks by ~950 km the age of its youngest, not its oldest, lithic com- that exceed the inherent paleomagnetic uncer- along the California-Coahuila transform (Fig. 2) ponent (Hsü, 1968). Tethyan faunas in some of tainties in paleolatitude (Mankinen and Irwin, during the interval from mid-Carboniferous to the limestone blocks were pantropic forms that 1990). In any case, the times of amalgamation Middle Triassic time (Stevens et al., 1992, 2005; migrated into island chains of the proto-Pacifi c of accreted tectonic elements into the California Dickinson, 2000). The transform connected Ocean west of Mesozoic (Miller margin as indicated by ages of stitching plutons orogenic trends on the northwest, including the and Wright, 1987; Newton, 1988). Tectonic and sedimentary overlaps provide constraints on

1 1 N 110° W N 0 2 ° 0 0 2° ° Gulf of Mexico ° 4 34 W W (filled by Yucatan) Ouachita-Marathon suture belt Permian-Triassic magmatic arc V Cordilleran ANGolconda miogeocline GONDWANA C thrust California-Coahuila transform truncating continental margin LAURENTIA A S U X E ° N M 42 subduction zone displaced miogeoclinal 1 0 0 continental ° W continental fragment G truncation ulfof California N (closed) displaced or 500 km accreted terranes 1 11 displaced or 2 N 0° N 0 ° accreted terranes ° ° Baja California 6 W 34 W 2

Figure 2. Regional tectonic relations of Permian–Triassic truncation of the California continental margin along the California-Coahuila transform adapted after Dickinson (2000) and Dickinson and Lawton (2001a). Abbreviations: CA—California; MEX—Mexico; NV— Nevada.

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the times that any lateral translation may have of greater offset (e.g., Wyld and Wright, 2001) Some descriptions of Klamath and Sierran occurred. are viewed as questionable, and not incorpo- mélange belts dating to 30–35 yr ago infer an rated into the interpretations of this paper. olistostromal character (Cox and Pratt, 1973; Snow Lake Fault Schweickert et al., 1977; Saleeby et al., 1978), Mélange Origin implying sedimentary origin, but subsequent Offset across the Early Cretaceous Snow descriptions in later years reinterpret the same Lake fault is signifi cant, however, for my analy- Key interpretations of this paper rest upon exposures as mélange and broken formation sis because the fault displaced pre-Cretaceous the premise that mélange and broken formation of tectonic origin (Wright, 1982; Goodge and rock assemblages previously accreted to central are formed by pervasive structural disruption of Renne, 1993; Schweickert et al., 1999). A par- California. The fault trace was along the axis oceanic facies and trench fi ll by intricate of allel evolution in thinking occurred globally of the Sierra Nevada batholith before intrusion progressively dewatering strata within subduc- over the past few decades as a growing appre- of Late Cretaceous plutons that form the bulk tion zones. Mélanges are standard components ciation of processes within subduction zones of the batholith, and is now a cryptic structure of the accretionary wedges developed along led to the understanding that many disrupted overprinted by intrusion of the batholith. The the fl anks of magmatic arcs (Hamilton, 1988). stratal assemblages regarded as olistostromal present areal pattern of preexisting tectonic ele- By analogy with lubrication theory, their origin before the advent of were actu- ments cannot be understood, however, without has been ascribed to fl ow within a deforming ally formed as tectonic mélange. attention to Snow Lake fault offset. subduction channel of fi nite thickness devel- The local generation of massive olistostromes The existence of the Snow Lake fault was oped between overriding and subducted plates from failure by mass movement of dislocated detected by recognition of the Cambrian (Shreve and Cloos, 1986; Cloos and Shreve, strata exposed on trench slopes at subduction Zabriskie Quartzite within the Snow Lake roof 1988a, 1988b). zones where mélange forms is nevertheless pendant of the central Sierra Nevada, where The distinction between mélange and bro- widely appreciated (Pini, 1999). Outcrop dis- its exposures are separated from stratal coun- ken formation lies in the greater lithologic tinction between mélange formed from previ- terparts in the southeastern or Death Valley heterogeneity of the former. Intricately dis- ously undisturbed bedded sequences and from facies of the Cordilleran miogeocline by a wide rupted domains derived entirely from seafl oor dislocated strata of redeposited olistostromes is expanse across which only the northwestern or or turbidite sedimentary successions, without therefore challenging (Raymond, 1984; Under- Inyo facies is exposed. The structure was origi- admixture of igneous or blueschist blocks, are wood, 1984; Cowan, 1985; Lash, 1987). For nally termed the Mojave–Snow Lake fault from termed broken formation (Hsü, 1968), but could either mélanges or olistostromes, however, the the interpretation that it displaced Zabriskie equally well be described as chert-argillite or age of the youngest block or matrix component Quartzite from its western extension into the graywacke-argillite mélange (Cowan, 1974). constrains the oldest possible age of formation Mojave block south of the Sierra Nevada, and Both mélange and broken formation display the of the disrupted rock mass (Hsü, 1968). a displacement of 400–500 km was initially same characteristic phacoidal or lentiform fab- suggested (Lahren and Schweickert, 1989; Sch- ric of internal dislocation from outcrop scale to Crustal Collision weickert and Lahren, 1990, 1993a). Reconsid- map scale (Dickinson, 1977). The characteristic eration of regional stratigraphic and structural fabric is well illustrated by outcrop photographs Usage of the term “collision” is currently relations indicates, however, that Zabriskie from multiple worldwide settings (Moore and inconsistent in the tectonic literature. The text of Quartzite was more probably offset into the Wheeler, 1978; Moore and Karig, 1980; Cowan, this paper follows the recommendation of Cloos Snow Lake roof pendant from exposures of the 1982; Nelson, 1982; Bell, 1987; Barnes and (1993) to reserve the term crustal collision for Death Valley facies in the Inyo Mountains east Korsch, 1991; Ujiie et al., 2000; Onishi et al., juxtaposition of tectonic elements that are buoy- of the Sierra Nevada, and that its presence in the 2001; Fukui and Kano, 2007). ant enough to jam subduction zones, and thereby Snow Lake pendant requires fault transport of Elongate tracts of mélange or broken for- to change either the positions of subduction only 210 ± 15 km (Dickinson, 2006). Moreover, mation, or both in combination, can be termed zones or the patterns of plate motion (or both). that total displacement can be viewed as the mélange belts as convenient shorthand, and are In the context of California tectonics, the crustal net offset across a family of dextral faults, one inferred here to mark the sites of former ocean structures of intraoceanic island-arc complexes being the Snow Lake fault, offsetting miogeo- basins of indeterminate width that did not close attached to the Cordilleran continental margin at clinal strata of the eastern Sierra Nevada (Ste- until after their youngest lithologic components intervals over time are the only features bulky vens and Greene, 1999, 2000). The inferred net were formed. Tectonic elements that are seaward enough to merit the term “collision” to describe slip of 210 km across the fault system is com- of mélange belts but include rocks of the same their accretion. Ongoing subduction of oceanic patible with the distortion of strontium isotope age are inferred here to have formed within oce- plates, to form incrementally accreted mélange and other geochemical isopleths along the axis anic realms west of the continental block. They belts and to underthrust ophiolitic slabs, is not of the Sierra Nevada batholith (Kistler, 1993), were not accreted to the continental margin until described herein as “collision” even though con- whereas the greater supposed Mojave–Snow formation of the intervening mélange belts was vergent plate motions are involved. Lake fault offset of 400–500 km is seemingly completed by subduction that closed interven- precluded by the limited geographic extent of ing tracts of underlying remnant KLAMATH MOUNTAINS the isopleth anomaly. ocean basins. Mélange belts between more To avoid confusion, the lesser structure with intact crustal blocks are taken to be suture zones The principal accretionary belts of the Klamath offset not exceeding 225 km is here termed the marking the sites of former ocean basins, and Mountains and adjacent Northern Coast Ranges Snow Lake fault, rather than the Mojave–Snow commonly include tectonic slivers or thrust pan- (Figs. 3 and 4) were fi rst delineated as discrete Lake fault with its postulated offset of 400– els of deformed serpentinite representing scraps lithic belts by Irwin (1960) before we had the con- 500 km, twice the amount inferred here as valid. of oceanic lithosphere incorporated structurally cepts of plate tectonics to help interpret them. Now Tectonic interpretations that accept the postulate into growing subduction complexes. commonly termed terranes (Snoke and Barnes,

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124° R 123° W Umpqua iv er A e

s

t Oregon e

r

n Coast

C Range a s SU c forearc a CB d Cape Arago basin e s (OCRfb) a n Ro d

H

i

SU g

WJ h

43° C 43°

a

s MG ver Ri c Umpqua a

OCRfb d

e CS 156 Cape DF s

Blanco v

ol River WJ c

ano RC Colebrook MG Schist CS g (CS) e eni u g 160 o A R WJ 139 c GP

c

r ov Rogue Rive

Cape 154 e

M r Sebastian

Hf (

WH C CHf

v

c

WH ) DF 160 Hf 174 148 161 N Cvc 146 RC B OR 42° 42° CA Hf 162 RC CMW r e WJ iv RC NF R h 25 km CM CM at HC 162 165 Klam Point Saint George CC 124° 153 123°

Figure 3 (continued on next page). Pattern of Mesozoic–Cenozoic accretionary tectonic belts fl anking eastern Klamath Paleozoic terranes (overlapping segments: A—northern, mostly in Oregon; B—southern, mostly in California). Barbed subregional thrusts (CMW—Con- drey Mountain window) between successive accretionary belts (see Fig. 4 for symbols and ages) modifi ed locally by younger faults (not shown to preserve indications of initial structural stacking). Exposures of native eastern Klamath Triassic–Jurassic arc assemblage of Fig- ure 4 overlie Paleozoic rocks just beyond eastern margin of B. Ages of stitching (post-accretion) plutons (shaded) after Irwin and Wooden (1999) and Allen and Barnes (2006). Pre-Oligocene sedimentary cover stippled (and yellow) after Figure 4, but post-Oligocene volcanic and sedimentary cover is blank. Letter designations of tectonic belts (refer to Fig. 4): CHf—Cape Sebastian–Hunters Cove Formations; CM—metavolcanic rind of CMW; CoB—Coastal Belt Franciscan (see Figs. 5 and 6); CS—Colebrook (~Pickett Peak) Schist; DF—Dothan- Franciscan subduction complex; EH—Eastern Hayfork belt; GVG—Great Valley Group; HC—Happy Camp window; Hf—Hornbrook Formation; MG—Myrtle Group; NF—North Fork (–Sawyers Bar) belt; RC—Rattlesnake Creek belt; SF—Stuart Fork (–Fort Jones) belt; SU—Siletz-Umpqua belt; WH—Western Hayfork belt; WJ—Western Jurassic belt; Yf—Yager Formation. Small klippen, fensters, granitic bodies, and patches of Cenozoic cover are omitted for reasons of scale. Adapted after Peck (1961), Dott (1971), Coleman (1972), Baldwin (1974), Irwin et al. (1974), Klein (1977), McLaughlin et al. (1982, 1994), Underwood (1983), Ryberg (1984), Blake et al. (1985a), Cashman et al. (1986), Donato (1987), Wagner and Saucedo (1987), Saleeby and Harper (1993), Irwin (1994), Wright and Wyld (1994), Goodge (1995), McCrory (1995), Ernst (1998), Irwin and Mankinen (1998), Blake et al. (1999), Wells et al. (2000), Dickinson (2000), Snoke and Barnes (2006), and Allen and Barnes (2006). Selected towns (for orientation): A—Agness; B—Brookings; CB—Coos Bay; CC—Cres- cent City; E—Etna; Eu—Eureka; GP—Grants Pass; HC—Happy Camp; M—Medford; R—Redding; Ro—Roseburg; SB—Sawyers Bar; W—Weaverville; WC—Willow Creek; Y—Yreka.

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124° WH 160 Hf WH 174 B DF 148 161 Cvc B 146 RC 42° N OR 42° CA 162 RC Hf CMW r e RC iv R WJ CM h 25 km a t 144 HC 162 167 Klam CC 148

Point Saint George Hf 162 Scot R t 146 iver 150 Cvc SF K 162 l NF a m EH E a t 167 h

Redwood Creek 164 Schist WJ DF WH SB 159

R pre-Middle iv 137

e r SF Triassic

accretionary 142 assemblage T 170 NF r in EH i 136 Trinidad Head ty (Paleozoic rocks) 41° 138 41° WC 168 141

Riv r 151 er e iv R

y t i

n i r Humboldt Eu T basin W

170 GVG DF R Yf 136 South Fork GVG WH Mtn. Schist Cape RC 169 Mendocino GVG Mattole CoB Great basin GVG Valley Yf 193 alluvium backthrust King Range belt 123° (post-mid Miocene) 124°

Figure 3 (continued).

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in Ma Scale

C re a t c e o u s P le a i r T o g n s s e a c e i N r e J u o e a g s s e i c n

Miocene aeEryMLt al aePEcn P O Eocene P Late Early Late M Early Late M E

arc (native) arc eastern Klamath volcanic Klamath eastern basin

forearc

Fort Jones Fort ilsen (1993), Pessagno et al. Saleeby and

Lanphere et al. (1978), Irwin (1977, 1978, Lanphere Stuart Fork- Stuart m lls et al. (2000), and Dickinson (2000). Time scale Time lls et al. (2000), and Dickinson (2000). (U-Pb zircon ages), and synaccretion ages), and synaccretion (U-Pb zircon

rg (1983), Harper (1984), Harper and Wright (1984), Wright and (1984), Harper rg (1983), Harper East

Ages plotted for accretionary belts and their sedimen- belts and their accretionary Ages plotted for Legend

North Fork-Sawyers Bar Fork-Sawyers North Eastern Hayfork mélange Hayfork Eastern

Hornbrook Formation (Hf)

Great Valley Group (GVG) Western Hayfork belt Hayfork Western post-Oligocene overlap pre-Oligocene overlap post-accretion plutons pre-accretion plutons metavolcanic rind of Condrey Mtn window P

P

Klamath Mtns

Rattlesnake Creek belt Creek Rattlesnake Western Jurrasic belt Jurrasic Western

West Formations (CHf) belt schist Peak Pickett (MG) Myrtle Group m Cape Sebastian-Hunters Cove basin

Franciscan trench-slope assemblage Dothan-Franciscan

Fm (Yf)

Yager Coastal Belt Franciscan Belt Coastal

Coast Ranges

Humboldt basin belt Range King

SW-NW offset in traverse in offset SW-NW Siletz-Umpqua assemblage Siletz-Umpqua

(NW) OCRfb

Miocene aeEryMLt al aePEcn P O Eocene P Late Early Late M Early Late M E

Cretaceous Paleogene Neogene Triassic Jurassic 0 in Ma Scale 25 50 75 100 125 150 175 200 225 250 Coleman et al. (1988), Wright and Fahan (1988), Wyld and Wright (1988), Alexander and Harper (1992), Hacker and Ernst (1993), N (1992), Hacker and Harper Alexander (1988), Wright and Wyld and Fahan (1988), Wright Coleman et al. (1988), We (1995), Constenius et al. (2000), et al. (1995), McCrory (1994), Hacker Wyld and Wright et al. (1994), (1993), Harper Harper Gradstein et al. (2004). after tary cover (symbols same as Fig. 3) include sedimentary constituents (biostratigraphic ages), pre-accretion igneous components (symbols same as Fig. 3) include sedimentary constituents (biostratigraphic ages), pre-accretion tary cover (1975), Hotz et al. (1977), Dott (1971), Evitt and Pierce Age data from basin. Coast Range forearc ages). OCRfb—Oregon (m, K-Ar and Rybe Ando et al. (1983), Blome and Irwin Heller 1982, 1983), Ingersoll and Dickinson (1981), Saleeby et al. (1982), Figure 4. Ages of Klamath (and westward) accretionary belts (Fig. 3) with stitching (post-accretion) pluton ages from Figure Figure 3. belts (Fig. pluton ages from Ages of Klamath (and westward) accretionary 3) with stitching (post-accretion) Figure 4.

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2006), the various accretionary belts include two provide minimum ages (170–140 Ma) for the ing the underthrust subduction channel at the fundamentally different types (Wright, 1982). time frames during which disparate accretionary inception of subduction in comparison to later, On the one hand are intraoceanic arc assem- belts of the Klamath Mountains were stitched when underplating of cooler subducted materi- blages that were constructed offshore, and then together along the continental margin (Fig. 4). als insulates the active subduction channel from transported as intact though deformed entities Older increments (240–170 Ma) of the conti- a structural lid of pre-accretion lithosphere. to accrete in bulk to the continental margin. On nental-margin arc assemblage are represented the other hand are intricately disrupted stratal by Triassic–Jurassic volcanic rocks overlying North Fork–Eastern Hayfork Belts assemblages composed of mélange and broken previously accreted Paleozoic terranes of the formation assembled in subduction zones at the eastern Klamath Mountains (Fig. 4). Granitic Internally dislocated belts of mélange and continental margin or along the fl anks of arriv- plutons that intrude the Paleozoic basement rep- broken formation were successively underthrust ing intraoceanic island arcs. The subduction resent younger increments (<140 Ma) of the arc from the west beneath the blueschist-bearing complexes are not necessarily exotic to the con- assemblage (Fig. 3B). Stuart Fork–Fort Jones assemblage to form the tinental margin, although most include far-trav- The successive accretionary belts of variably North Fork and Eastern Hayfork belts sand- eled lithic components. The accretionary assem- deformed seafl oor and intraoceanic wiched between the previously expanded con- blage as a whole includes ophiolitic seafl oor or arc assemblages in the western Klamath tinental margin and accreted intraoceanic arc of remnant-ocean and interarc-basin origins, Mountains and adjacent coastal ranges form a assemblages farther west (Figs. 3 and 4). Both deformed seamount edifi ces, seafl oor sediments, record of punctuated but continuing subduction the North Fork and Eastern Hayfork belts are seamount cappings, and intraoceanic island arcs along the continental margin from Late Trias- composed predominantly of disrupted seafl oor (Wright, 1982; Coleman et al., 1988). The per- sic to early Paleogene time. Each has a lithol- chert-argillite successions and metavolcanic spective that the magmatic-arc components of ogy and internal structure differentiable from assemblages derived from seafl oor and oceanic the accretionary assemblages were native to the the adjoining belts, and is separable from other , with minimal clastic input of sandy continental margin (Gray, 1986) is discounted belts as a mappable entity. The tectonic entities terrigenous sediment (Wright, 1982). Protoliths here because mélange belts intervene between are denoted here simply as belts, rather than are inferred here to have formed the ophiolitic them and the native magmatic arc of the eastern as terranes, because the latter usage tends to fl oor and sediment fi ll of a remnant ocean basin Klamath Mountains (Wright, 1982). imply wholly disparate origins, and thereby to that was between the Triassic–Jurassic Cordille- Arcuate Mesozoic–Cenozoic accretionary prejudge whether there are genetic connections ran margin and an intraoceanic arc complex that belts are draped in plan view around an east- between selected pairs or groups of the tectonic evolved from an initial position some indeter- ern Klamath core of Paleozoic rocks (Fig. 3B), belts. Moreover, mélange belts assembled incre- minate distance offshore. The contact between which were accreted to Laurentia before Middle mentally by subduction along the continental the North Fork and Eastern Hayfork belts may Triassic time and are among the tectonic assem- margin or the fl anks of intraoceanic island arcs mark the cryptic join between a North Fork sub- blages truncated by the California-Coahuila were formed in those positions, and did not exist duction complex to the east that was associated transform to establish the trend of the Cordille- as coherent tectonic entities before assembly. with a paleotrench along the active Cordilleran ran margin (Dickinson, 2000). Mélange fabrics To term a subduction complex a risks continental margin and an Eastern Hayfork sub- in the Jurassic–Cretaceous Dothan-Franciscan the implication of pre-assembly existence that duction complex to the west related to a paleo- belt (Fig. 3AB) near the coast are apparent in would be misleading. trench that was along the fl ank of the accreted outcrops because mélange matrix of deformed The belts are discussed here from east to west, arc complex. Occurrences of late Paleozoic as scaly clay is largely unmetamorphosed, and in the order they were accreted to the continental well as early Mesozoic fossils within both belts blocks suspended in the matrix display contrast- margin. (Irwin, 1972, 1981; Wright, 1982; Blome and ing metamorphic grades (Irwin et al., 1974). Irwin, 1983; Mortimer, 1984; Hacker and Ernst, Triassic–Jurassic mélange belts exposed farther Stuart Fork–Fort Jones Belt 1993; Hacker et al., 1993, 1995; Miller and into the interior of the Klamath Mountains have Ernst, 1998) imply derivation of mélange from acquired metamorphic overprints that make their Upper Triassic blueschists of the Stuart Fork subduction of long-lived seafl oor that underlay initial mélange fabric more diffi cult to discern. or Fort Jones belt (Figs. 3B and 4) mark the onset the remnant ocean basin. Discrete pods of lime- Regional Klamath metamorphism developed of post-truncation Mesozoic accretion west of stone encased in mélange probably represent within the post-accretion arc massif into which the eastern Klamath Paleozoic assemblages either structurally detached cappings of oceanic multiple Jurassic–Cretaceous granitic plutons (Goodge, 1989a). The blueschist assemblage seamounts or olistoliths that slid down the fl anks were emplaced (Ernst, 1983). Granitic intrusions includes phyllitic metachert, metasedimentary of seamounts (Wright, 1982; Miller and Wright, are absent from the Dothan-Franciscan belt to the semischist, and mafi c metavolcanic rocks of an 1987; Miller and Saleeby, 1991). west because the plutons represent the ancestral oceanic assemblage including igneous seafl oor Metasedimentary strata of the North Fork belt roots of the Cordilleran magmatic arc for which and its pelagic to hemipelagic sediment cover are broken formation derived from seafl oor chert- the Dothan-Franciscan assemblage was the (Goodge, 1989b, 1990). The occurrence of argillite successions that are locally preserved paired subduction complex. The anomalously the oldest and most intensely developed blue- intact as structural enclaves encased in more close proximity (<25 km) of the westernmost arc schist metamorphism along the inland fl ank of severely deformed domains. Subordinate sea- plutons to the eastern edge of the Dothan-Fran- the Mesozoic accretionary belts of the western fl oor limestone is present as thin beds intercalated ciscan subduction complex suggests signifi cant Klamath Mountains is in harmony with the gen- within successions of chert and argillite. Metavol- post-pluton thrust telescoping of the Klamath eral observation that global blueschists are high- canic strata form large slab-like tectonic rafts accretionary assemblages (Wright and Fahan, est in grade along arcward fl anks of accretionary laced internally by narrow bands of crushed rock 1988; Coleman et al., 1988). prisms built at ancient subduction zones (Ernst, or broken formation, but depositional contacts The ages of the intrusions that crosscut con- 1975). This relationship may refl ect the higher between chert and metavolcanic rock are locally tacts between accretionary assemblages (Fig. 3) temperature of rock masses structurally overly- preserved as a record of sedimentation on either

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the seafl oor or the fl anks of seamounts. Multiple disruption in the deepest structural horizons of a structural overprint imposed during accretion tectonic sheets of sheared serpentinite derived the Rattlesnake Creek arc assemblage (Wright of an intraoceanic arc assemblage to the edge of from oceanic lithosphere slice through the North and Wyld, 1994; Miller and Ernst, 1998) implies the growing continent. Limestone cobbles in the Fork belt at widely spaced intervals. The belt as that the arc structure was built in part on previ- Western Hayfork belt contain McCloud fauna a whole can be viewed as a disrupted ously assembled mélange. (Hacker and Ernst, 1993), native to tropical (Ando et al., 1983), but without any implication The polarity of the accreted are complex is island settings, which from paleofaunal analy- that all its components formed at the same locale not known with certainty, but it was probably sis were initially at least 1000 km and possibly in simple vertical succession. The general pau- east facing, with subduction downward to the >5000 km from the continental margin of Lau- city of volcaniclastic strata suggests that most west beneath an as it approached the rentia (Stevens et al., 1990). metavolcanic components of the North Fork belt continental margin. No coeval subduction com- were seafl oor accumulations or seamount edifi ces, plex is known to the west of the deformed arc Stitching Plutons rather than arc structures. One bulky and relatively structure, whereas coeval mélanges of the East- intact metavolcanic component of the North Fork ern Hayfork belt are to the east (Fig. 4). The Plutons of Middle Jurassic age (170–160 Ma) belt termed the Sawyers Bar terrane has been postulated polarity of the Rattlesnake Creek arc stitch all the accreted Klamath terranes together interpreted, however, as an immature intraoceanic implies that mélanges and broken formations of as far west as the Western Hayfork and Rattle- island arc of uncertain polarity (Ernst, 1990, 1991, the central Klamath Mountains are the record snake Creek belts (Fig. 4). There is a hint from 1999; Ernst et al., 1991; Hacker and Ernst, 1993; of a suture zone between the west-facing conti- detailed age relationships that the Western Hacker et al., 1993). nental-margin arc and an offshore island arc of Hayfork–Rattlesnake Creek arc structure may The Eastern Hayfork belt is a subduction opposed polarity. Plutons injected into the roots have been stitched to the paired Eastern Hay- complex or accretionary prism (Wright, 1982; of the Rattlesnake Creek arc assemblage overlap fork subduction complex along its eastern fl ank Hacker and Ernst, 1993) that includes large in age with components of the native magmatic somewhat earlier than the combined exotic arc tracts of argillite-rich metasedimentary broken arc overlying Paleozoic rocks in the eastern assemblage was stitched across the collisional formation, overprinted by metamorphism, with Klamath Mountains (Fig. 4). Oceanic protoliths mélange belts of the central Klamath Mountains phacoidal slivers of chert encased in a pelitic of the mélange terranes in the central Klamath to the blueschist-bearing Stuart Fork–Fort Jones matrix at multiple scales. Also present is hetero- Mountains are coeval with both arc assemblages belt fl anking the ancestral Cordilleran margin to geneous mélange including lensoidal blocks of (Fig. 4). The width of the ocean basin that was the east. The wide distribution of Middle Juras- greenstone and slivers of serpentinite, together between the Cordilleran margin and the offshore sic plutons (Fig. 3) indicates that the Cordilleran with intact enclaves of ribbon chert and lime- intraoceanic arc is indeterminate, but the bulk magmatic arc had migrated seaward across all stone. Minor quartzose turbidites in the litho- and complexity of intervening mélange belts the accreted pre–Late Jurassic Klamath belts logic assemblage show that distal pulses of con- (Fig. 3) imply a broad remnant ocean before by the time of pluton intrusion (Wright and tinentally derived sediment were spread across arc-arc collision. Fahan, 1988). This relationship indicates that the fl oor of a remnant ocean basin before its clo- Following accretion of the intensely deformed the remnant ocean basin between the Cordille- sure by subduction. Rattlesnake Creek arc structure of intraoce- ran margin and the exotic offshore island arc of anic character, reversal of arc polarity may have Triassic–Jurassic age now exposed as the Rattle- Western Hayfork–Rattlesnake Creek Belts occurred before eruption of the less deformed vol- snake Creek–Western Hayfork arc complex in canic rocks of the Western Hayfork belt, which is the western Klamath Mountains had closed by West of the mélange belts in the central younger than any mélange protoliths to the east Middle Jurassic time. Both the mélange belts of Klamath Mountains, an internally deformed (Fig. 4). Abundant volcaniclastic strata in the the central Klamath Mountains and the exotic island-arc complex of intraoceanic origin was Western Hayfork belt and its petrology clearly arc structure to the west were by then incorpo- accreted as the Triassic to Lower Jurassic Rat- refl ect formation within a magmatic arc (Wright, rated into the expanding continental margin. tlesnake Creek belt overlain depositionally by 1982; Hacker and Ernst, 1993), which may have the less deformed Lower to Middle Jurassic been related to subduction downward to the east Western Jurassic Belt Western Hayfork belt. The two arc assemblages (Wright and Fahan, 1988; Wright and Wyld, have locally been telescoped by post-accretion 1994). If so, Western Hayfork volcanism can be The pre–Late Jurassic Western Hayfork and thrusting (Fig. 3B), but the contact between taken to mark initiation of a west-facing mag- Rattlesnake Creek arc assemblages are locally them is commonly obscured by metamorphism. matic arc built on the newly expanded continen- overlain depositionally and elsewhere widely The Rattlesnake Creek arc assemblage was tal margin (Donato, 1987). Older components of underthrust by the Western Jurassic belt (Fig. 3) erupted upon an ophiolitic oceanic substratum the Western Hayfork arc overlap in age, however, of Late Jurassic age (Fig. 4). The Western of deformed and gabbro with no ves- with waning phases of volcanism along the native Jurassic belt was formed by post-accretion arc tige of continental basement in its roots (Donato, continental-margin arc of the eastern Klamath magmatism and incipient backarc spreading 1987; Coleman et al., 1988; Hacker and Ernst, Mountains 100 km to the east (Fig. 4). that produced west-facing frontal and remnant 1993). Multiple Triassic–Jurassic U-Pb ages This space-time relationship suggests the pos- arcs separated by an interarc basin fl oored by (212–193 Ma) of constituent plutons (Wright sibility that the Western Hayfork arc represented ophiolitic crust along the expanded Cordilleran and Fahan, 1988; Hacker and Ernst, 1993; a waning phase of intraoceanic arc evolution continental margin (Harper, 1984; Harper and Wright and Wyld, 1994) indicate emplace- before collision of an east-facing intraoceanic Wright, 1984; Wright and Fahan, 1988; Saleeby ment during an interval of time when seafl oor arc complex (Rattlesnake Creek–Western Hay- and Harper, 1993; Hacker et al., 1995; Wyld and successions were still being deposited within a fork) with the subduction zone along the con- Wright, 1988; Yule et al., 2006; MacDonald et remnant ocean basin to the east, and a separate tinental margin. Locally severe dislocation of al., 2006). Plutonic arc roots, frontal and rem- arc assemblage was being erupted along the Western Hayfork volcanic rocks to the condition nant arc edifi ces, the ophiolitic interarc basin continental margin (Fig. 4). Widespread stratal of broken formation can then be understood as fl oor, and the interarc basin fi ll are commonly

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denoted as separate though generically related subduction complex. Their distribution perhaps into an anomalous position west of exposures of subterranes (Alexander and Harper, 1992; refl ects higher crustal temperatures that fostered the Franciscan subduction complex (Fig. 7). Hacker and Ernst, 1993; Hacker et al., 1995). more penetrative deformation beneath the initial Accretionary tectonic relationships south of Combined isotopic and biostratigraphic ages lid of the Franciscan subduction channel when the Klamath Mountains are discussed here from indicate a time span of only 10–15 m.y. for it was positioned directly below amalgamated east to west along the length of California, with evolution of the entire arc-interarc assemblage arc complexes forming the western edge of the treatment of the Sierra Nevada fi rst, then the (Fig. 4); both the arc edifi ces and the ophiolitic Klamath accretionary assemblages. As the sub- mafi c basement of the Great Valley to the west, fl oor of the interarc basin formed during the duction complex evolved, older components of and fi nally the Franciscan subduction complex interval 165–155 Ma and sediment cover of the the Dothan-Franciscan accretionary prism would of the Coast Ranges. The Nacimiento fault interarc basin was deposited during the interval have served to insulate younger components of along the western side of the Salinian block, 160–150 Ma (Saleeby et al., 1982; Harper and the accretionary prism as they were drawn into and displaced components of the Franciscan Wright, 1984; Wyld and Wright, 1988; Harper the active subduction channel. Cretaceous– subduction complex west of the fault, have been et al., 1990, 1994; Alexander and Harper, 1992; Paleogene components of the growing accretion- discussed elsewhere (Dickinson, 1983; Dickin- Saleeby and Harper, 1993; Hacker and Ernst, ary prism were capped over time by the overlap son et al., 2005), and are not treated here. 1993; Hacker et al., 1995). The compound successions of various forearc and trench-slope assemblage was structurally telescoped by Late basins (Fig. 4), except where the King Range Sierra Nevada Jurassic thrusting along the continental margin terrane is present as a Miocene increment to (Wright and Fahan, 1988; Harper et al., 1990), the Franciscan subduction complex along one At the northern end of the Sierra Nevada and was stitched into place by post-thrust plu- short segment of the present coast (Figs. 3B and block where the western edge of the main tons during the interval 155–145 Ma near the 4). Paleogene structural relationships northwest batholith trends eastward off the range crest, close of Late Jurassic time (Fig. 4). of the Dothan-Franciscan belt in Oregon were the native Cordilleran arc assemblage of Juras- Underthrusting of the Western Jurassic belt clarifi ed by Ryberg (1984), who showed that sic age (Fig. 6) overlies an accreted Devo- beneath the accretionary assemblages of the Paleocene to lower Eocene Siletz seamount vol- nian–Permian arc assemblage that rests depo- central Klamath Mountains by tectonic tele- canics and Umpqua turbidites were imbricated sitionally on imbricated thrust panels of the scoping during structural evolution of the con- together by accretionary tectonism along the lower Paleozoic Shoo Fly subduction complex tinental margin is revealed by exposures within continental margin. These deformed strata are (Fig. 5), also accreted to Laurentia before Mid- the Condrey Mountain and Happy Camp tec- overlain by lower to middle Eocene strata of the dle Triassic time (Dickinson, 2000). Southward, tonic windows, or fensters (Figs. 3 and 4). A forearc basin that developed along the length of the native Jurassic arc assemblage onlaps the tectonic rind of metavolcanic rock that discon- the Oregon Coast Range parallel to the present Shoo Fly complex (Fig. 5), and both assem- tinuously bounds the windows is viewed here coastline (Heller and Ryberg, 1983). blages continue southward into the southern as a tectonic sliver derived from the structurally Sierra Nevada, where increasingly voluminous overlying Rattlesnake Creek–Western Hayfork SIERRA NEVADA AND COAST RANGES batholithic intrusions progressively obscure the arc complex, and has yielded pre–Late Juras- relationship between them (Fig. 7). The native sic isotopic ages (Helper, 1986; Coleman et al., Tectonic relationships across the North- Jurassic arc succession further onlaps south- 1988; Saleeby and Harper, 1993). The absence ern Coast Ranges and northern Sierra Nevada ward over deformed Paleozoic strata of the Cor- of any Middle Jurassic intrusions in the tectonic (Figs. 5 and 6) are similar to those across the dilleran miogeocline both to the east and to the windows indicates that the Middle Jurassic plu- Klamath Mountains to the north, i.e., succes- west of the cryptic Early Cretaceous Snow Lake tons cutting structurally overlying accretionary sive accretionary belts becoming younger to the fault (Fig. 7). The Snow Lake fault is inferred assemblages are now rootless, truncated down- west (Saleeby et al., 1989a). Exposures of the to have dextrally offset a facies transition within ward by the Late Jurassic thrust system that Franciscan subduction complex are much wider, the miogeocline by ~215 km from Lone Pine bounds the windows (Wright and Fahan, 1988; however, leading to subdivision of the Francis- to Snow Lake before intrusion of the Late Cre- Coleman et al., 1988). The position of the Con- can belt into multiple subunits. Moreover, the taceous Sierra Nevada batholith (Dickinson, drey Mountain window indicates that the arc- tectonic stacking of accretionary assemblages 2006). The Shoo Fly Complex is inferred to be interarc assemblage of the Western Jurassic belt in the Sierra Nevada foothills is more complex, truncated southward along sinistral fault strands was underthrust for at least 75 km and probably and a belt of mafi c basement not present in the of the California-Coahuila transform (Fig. 2) ~100 km beneath the continental margin. Klamath Mountains intervenes between Sierran that are also cryptic due to batholithic intrusion and Franciscan belts in the subsurface of the (Fig. 7). Offset counterparts of the Shoo Fly Franciscan Subduction Complex Great Valley. Across the Southern Coast Ranges Complex are present on the Kern Plateau at the and southern Sierra Nevada (Fig. 7), patterns southern end of the Sierra Nevada block (Dunne Continued subduction through Cretaceous of tectonic accretion can only be discerned by and Suczek, 1991), where they are overlain by and into Cenozoic time west of the Klamath analogy with relations farther north. Massive clastic strata of Paleozoic age not delineated Mountains progressively assembled the various Late Cretaceous granitic intrusions of the Sierra separately here (Fig. 7). accretionary belts of the Dothan-Franciscan sub- Nevada batholith have partly obliterated eastern West of exposures of the Shoo Fly complex, duction complex (Fig. 3) and the related Siletz- accretionary belts, Cenozoic deformation asso- bound in the northern Sierra Nevada by the Umpqua assemblage to the north (Fig. 3A). ciated with development of the Neogene San Paleozoic Feather River peridotite belt (Figs. 5 Strongly foliated schists (Colebrook, Redwood Andreas transform system has folded western and 6), the Calaveras mélange belt is the oldest Creek, South Fork Mountain) of the Pickett Peak accretionary belts and their bounding thrusts on principal component of the post–Middle Trias- schist belt (Fig. 4) occur along the eastern fringe a large scale, and the San Andreas fault has trun- sic accretionary assemblages west of the locus of the Dothan-Franciscan belt, or as klippen cated the accretionary terranes on the west and of continental truncation along the Permian– representing the highest structural levels of the translated the Salinian block of Sierran affi nity Triassic California-Coahuila transform (Fig. 2).

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c r y i e r n v

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i Tahoe HLV t c c r l o Mz e v

T S i e r a Mz N e v a d a c belts (black): FRp— b a t h o l i SF nyford; SR—Santa Rosa; 120 mation. Adapted after Max- Adapted after mation. arc; SF—Shoo Fly Complex; arc; 166 B 164 Mz native arc Pz accreted arc 170 SF 170 Pz 133 FRp CaB 373 iddle Creek (–Tuolumne River) belt; (–Tuolumne iddle Creek southern tip of Klamath Mountains 170 RA tively (Jayko, 1990). Letter designation tively (Jayko, 1990). Letter skenta. Ultramafi SF CaB FRp Day and Bickford (2004), Hopson CaB 987), Edelman et al. (1989a), Ernst (1990b), uction faults overprinting contacts between 143 ), Wagner and Bortugno (1982), McLaughlin Wagner ), FC FC 166 n and Pessagno (2004). Selected towns (for ori- n and Pessagno (2004). Selected towns (for SC 151 121° 167 SC GV 121° 158 rrow enclaves of disparate rock within Franciscan enclaves of disparate rock rrow

143

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er

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T R elds (x): CLv—Clear Lake; SBv—Sutter Buttes. Pz-Mz denotes Butt Valley and Valley Buttes. Pz-Mz denotes Butt Lake; SBv—Sutter elds (x): CLv—Clear n t o 122° m e r a 122° S a c GVG N RB G R E A T TCs V A L E Y A L U V I M Pf ECf CFf GVG So TCs YB uves, and local alluviated valleys are omitted for reasons of scale (broad expanses of Quaternary alluvium of scale (broad reasons omitted for uves, and local alluviated valleys are SR SM CLv VS K CeB CeB

er Riv YB Lake Clear PP ian ss Ru 123° U Co Head Bodega

CeB er Riv Eel lt fau CoB G block Yf Gualala as

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San Scale in km N 124° 124° 050 Miocene) backthrust 39° 40° Kings Range belt (post-mid- Figure 5. Accretionary tectonic belts (see Fig. 6 for symbols and ages) of northern Sierra Nevada Northern Coast Ranges (K— 6 for tectonic belts (see Fig. Accretionary 5. Figure Irwin (2003) and others (see below). Post-subd plutons (shaded) after Ages of stitching (post-accretion) 3B). Fig. from bedrock intra-Franciscan belts not shown separately (to preserve indications of initial structural stacking). Small granitic bodies, na intra-Franciscan belts not shown separately (to preserve on Sierra Nevada interfl volcanic cover Tertiary belts, accretionary and Tertiary volcanic cover are blank). Tilted normal faults (northwestern Great Valley): CFf—Cold Fork; ECf—Elder Creek; Pf—Pa Creek; CFf—Cold Fork; ECf—Elder Valley): normal faults (northwestern Great Tilted blank). are volcanic cover Tertiary and Feather River peridotite belt; TCs—Tehama-Colusa serpentinite mélange. Cenozoic volcanic fi TCs—Tehama-Colusa peridotite belt; River Feather to northern Sierra Nevada and eastern Klamath Mountains, respec Soda Ravine blocks with Paleozoic–Mesozoic stratigraphy similar 6): CaB—Calaveras mélange belt; CeB—Central Belt Franciscan; CoB—Coastal FC—F to Fig. of tectonic belts (refer (–Lake Combie) Ant blueschist; SB—Smartville block; SC—Slate Creek PP—Pickett Peak Schist belt; RA—Red Group; Valley GVG—Great For Yf—Yager Bolly belt; YB—Yolla Springs belt; VS—Valentine serpentinite mélange; TCs—Tehama-Colusa SM—Snow Mountain seamount; well (1974), Cady (1975), Saleeby and Sharp (1980), Ingersoll Dickinson (1981), Schweickert et al. (1980, 1984a, 1988, 1999 et al. (1982, 1994), MacPherson (1983), Blake (1984, 1985b, 1988, 2002), McLaughlin and Ohlin (1984), Jayko (1 (1992), Constenius et al. (2000), Dickinson Blake (2002), Wagner Harwood (1992), Day (1992a, 1992b), Saucedo and Pessagno (2004). Selected pluton ages after Hanson et al. (1996, 2000), Fagan (2001), Day and Bickford (2004), Hopso Pessagno (2004). Selected pluton ages after M—Marysville; Sa—Sacramento; So—Sto Valley; entation): B—Blairsden; Ci—Chico; Co—Covelo; FB—Fort Bragg; G—Garberville; GV—Grass N—Napa; RB—Red Bluff; U—Ukiah.

Geosphere, April 2008 339

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in Ma Scale

C r e t c a e o s u P T l e a i r a g o s s e n i c e N J o u e a r g e e s s n c i

Miocene O M M P Eocene P Late Early Late Early Late E

(color omitted from Fig. 7) Fig. from omitted (color Sierra Nevada batholith Nevada Sierra ghlin and Pessagno (1978), Saleeby

Middle Triassic

River peridotite)

(native) arc volcanic Continental margin (Paleozoic Feather northern Sierra Nevada Sierra northern

s (U-Pb zircon ages), and synaccretion meta- ages), and synaccretion s (U-Pb zircon en (1984), Blake et al. (1984, 2002), Murchey en (1984), Blake et al. (1984, 2002), Murchey (Roberts Mtns or Antler belt mélange Calaveras and Golconda or Sonoma) Cordilleran miogeocline accreted and Permian island arcs Shoo Fly Complex and oceanic allochthons and Jones (1994), Girty et al. (1995), Dickinson ed for accretionary belts and their sedimentary belts and their accretionary ed for d Sharp (1989), Edelman et al. (1989a, 1989b), . of continental margin m pre-Middle Triassic substrate

Red Ant

blueschist Fiddle Creek-Tuolumne River belt River Creek-Tuolumne Fiddle

arc belt arc

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(GVG)

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East

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belt

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Coast Range

Legend

Tehama-Colusa serpentinite Tehama-Colusa

Valentine Spring belts Spring Valentine m and Peak Pickett

strata folded Snow Mtn. seamount (SM) seamount Mtn. Snow

strata Yolla Bolly belt Bolly Yolla

Cenozoic (m) fossils sparse

Franciscan Central Belt Central West

m Fm (Yf)

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Franciscan Complex basin Franciscan Belt Coastal

trench-slope

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Humboldt basin unexposed King Range King Permanente

mutual relations

Miocene O M M P Eocene P Late Early Late Early Late E

Neogene Paleogene Cretaceous Jurassic Triassic 0 in Ma Scale 25 50 75 100 125 150 175 200 225 250 cover (symbols same as Figs. 5 and 7) include sedimentary constituents (biostratigraphic ages), pre-accretion igneous component (symbols same as Figs. 5 and 7) include sedimentary constituents (biostratigraphic ages), pre-accretion cover Figure 6. Ages of Sierra Nevada and Coast Ranges accretionary belts with stitching pluton ages from Figures 5 and 7. Ages plott Figures 5 and 7. belts with stitching pluton ages from Ages of Sierra Nevada and Coast Ranges accretionary Figure 6. (1972), Behrman (1978), and Parkison McLau Armstrong Suppe and Age data from ages). Ar/Ar and morphism (m, K-Ar (1982), Saleeby MacPherson (1983), Bog and Sharp (1980), Schweickert et al. Stern (1981), Chen Moore Gradstein et al. (2004) scale after Time et al. (1996a), Dickinson (2000), Fagan (2001), and Hopson Pessagno (2004). and Jones (1984), Sliter (1984), Bateman et al. (1985), Day (1985, 1988), Sharp (1988), Dilek (1989a, 1989b), Edelman an and Jones (1984), Sliter (1992), Graymer Wakabayashi Saleeby et al. (1989a, 1989b), Dilek (1990), Edelman (1990, 1991), Herzig and Sharp (1992),

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of m 118° r A N D H fo facies B A S I N s R A N G E T an I tr miogeoclinal Death Valley P R O V I N C E

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Lake E a s (QS—Neogene Quien Sabe volcanic fi Mono eB—Central Belt Franciscan; CoB—Coastal N C ? 119° yko and Blake (1984), Schweickert and Lahren yko and Blake (1984), Schweickert Lahren e s (for orientation): B—Bishop; F—Fresno; LP— orientation): B—Bishop; F—Fresno; s (for

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n N Sa O C E A N Scale in km uves, and local alluviated valleys omitted for reasons of scale. Southeasternmost exposures of Paleozoic allochthons (Fig. 6) of Paleozoic allochthons (Fig. of scale. Southeasternmost exposures reasons uves, and local alluviated valleys omitted for CeB P A C I F 050 fault (offshore) offset Nacimiento 37° 36° Reyes Point Nevada interfl Coast Ranges. Quaternary alluvium is blank. Small granitic bodies, narrow enclaves of disparate rock within Franciscan accretio enclaves of disparate rock Coast Ranges. Quaternary alluvium is blank. Small granitic bodies, narrow basement rock and sedimentary-volcanic cover not depicted in east of Sierra Nevada frontal escarpment, east of Sierra Nevada frontal not depicted in Basin and Range province and sedimentary-volcanic cover basement rock Figure 7. Accretionary tectonic belts (offset locally by strike-slip faults) of southern Sierra Nevada and Southern Coast Range Accretionary 7. Figure pendants conjoined) clarity (and some clusters of small roof pendants within Sierra Nevada batholith enlarged for Selected roof Paleozoic strata. Letter designation of tectonic belts (refer to Fig. 6): BH—Burnt Hills terrane; CaB—Calaveras mélange belt; C designation of tectonic belts (refer Paleozoic strata. Letter TR KK—Kings-Kaweah belt; P—Permanente terrane; SF—Shoo Fly arc; Belt Franciscan; CM—Cordilleran miogeocline; FA—Foothills Cady (1975), Nelson (1976), Saleeby and Sharp (1980), Ja Adapted after Asterisk (DP) denotes Del Puerto ophiolite. belt. Creek) and Schweickert (1989), Dunne Suczek (1991), Ernst (1993a, 199 (1987, 1991, 1993b), Schweickert et al. (1988, 1999), Lahren et al. (2002), and Dickinson (2005). Selected town (1999, 2000), Blake et al. (2002), Graymer (1997), Stevens and Greene T—Tulare. PR—Paso Robles; S—Stockton; SJ—San Jose; Mo—Modesto; M—Monterey; Lone Pine; Me—Merced;

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The Calaveras mélange belt, continuous for by mélange fabric. Many other components genetic studies indicate that thick calc-alkalic the length of the Sierra Nevada (Figs. 5 and 7), of the Foothills arc belt, including associated of arc affi nity overlie thinner tholeiitic has a pervasively disrupted internal structure of metasedimentary strata, and much of the Slate basalts of seafl oor affi nity (Snow, 2007) to form phacoidal or lentiform chert bodies encased in Creek–Lake Combie arc belt, are strongly foli- the type of stratigraphic succession expected for argillite representing mélange matrix of initially ated, though undisrupted by mélange fabric. The an intraoceanic arc. scaly clay now overprinted by post-mélange Fiddle Creek–Tuolumne River belt is composed foliation. The belt also includes fault-bound of mélange and broken formation in which Stitching Plutons enclaves of greenstone representing dislocated blocks and lenses of argillite, ribbon chert, and seafl oor or seamount volcanic rock as well as metavolcanic rock are dominant but ultramafi c As in the Klamath Mountains, migration of intact fault duplexes or tectonic rafts composed lenses are also present (Hacker, 1993). the Cordilleran magmatic arc into the domain of chert-argillite successions representing The Smartville block (Figs. 5 and 6) is a of Sierran accretionary belts provides evidence domains of undisrupted seafl oor sedimentary complex of both intrusive and extrusive igneous in the form of stitching plutons for minimum strata. The enclaves of intact stratal succes- rocks representing a dissected magmatic arc that ages of accretion. The Calaveras mélange belt sions individually top to the east, but the belt overlies an ophiolitic substratum, in part Paleo- was fi rmly stitched to the Cordilleran continen- as a whole youngs to the west (Bateman et al., zoic age, that was strongly imprinted by intra- tal margin by post-accretion plutons of Middle 1985), suggesting an imbricate internal structure arc rifting (Saleeby, 1982; Beard and Day, 1987; Jurassic age (170–160 Ma), and the more west- produced by incremental accretion at a subduc- Beiersdorfer et al., 1991; Beiersdorfer and Day, ern accretionary belts exposed in the Sierra tion zone along the continental margin. Chert 1992). Deeper structural levels include metagab- Nevada foothills were conjoined by only slightly phacoids are commonly fl attened to extreme bro cut by sheeted and unsheeted diabase younger Middle to Late Jurassic (160–150 Ma) aspect ratios, and the belt can aptly be described swarms, and higher structural levels include stitching plutons (Fig. 6). The oldest plutons that as metamélange. Paleozoic limestone blocks lavas and pillow lavas grading upward to vol- stitch the western accretionary belts to the Cala- that were probably derived from seamount caps caniclastic successions, with gabbroic to tonal- veras mélange belt are, however, of younger Late are also present within the Calaveras mélange itic plutons an integral part of the intraoceanic Jurassic age (ca. 140 Ma), and this dichotomy in belt (Schweickert et al., 1977; Day et al., 1985, volcanoplutonic complex. The Smartville block the ages of stitching plutons (Fig. 6) leads to an 1988; Dilek, 1989a; Edelman, 1990; Edelman structurally overlies deformed arc assemblages ambiguity of interpretation for times of accre- and Sharp, 1989; Edelman et al., 1989b; Hacker, of the Slate Creek–Lake Combie belt, which in tion. If all the more western plutons are viewed 1993), as they are in central mélange belts of the turn structurally overlies mélanges of the Fiddle as records of the native Cordilleran arc, then the Klamath Mountains. Creek–Tuolumne River belt (Day et al., 1985, pluton ages imply accretion of all the Sierran Southward from the southern tip of the Feather 1988; Edelman and Sharp, 1989; Edelman et al., accretionary belts by Middle Jurassic time (Edel- River peridotite belt, the contact between the 1989a; Edelman, 1991; Dilek et al., 1990; Fagan man and Sharp, 1989; Girty et al., 1995). On the Shoo Fly Complex and the Calaveras mélange et al., 2001; Day and Bickford, 2004). These other hand, if the older of the more western plu- belt is marked by the contrast between coherent structural relationships imply eastward thrusting tons are viewed as the record of igneous activity though internally imbricated metasedimentary within the accretionary assemblages west of the within an offshore intraoceanic arc complex, then strata of the Shoo Fly complex and intricately Calaveras mélange belt (Ricci et al., 1985). The accretion of the exotic arc terranes was delayed dislocated though metamorphosed mélange of Smartville and Slate Creek–Lake Combie arc until Late Jurassic time. In either case, however, the Calaveras belt. In one restricted area of the assemblages are in part coeval in age (Fig. 6), arc crust of intraoceanic origin was incorporated northern Sierra Nevada, an enclave of Red Ant and the contact between them represents struc- into the Sierra Nevada foothills by arc-continent blueschist (Fig. 5) is structurally interleaved tural telescoping within a compound arc com- collision (Moores and Day, 1984; Godfrey and with the Feather River peridotite belt (Sch- plex of intraoceanic origin (Sharp, 1988; Dilek, Dilek, 2000; Ingersoll, 2000). weickert et al., 1980; Saleeby et al., 1989a) as a 1989a, 1989b; Dilek et al., 1990). The Slate The Calaveras mélange belt is here inter- record of metamorphism within the early Meso- Creek–Lake Combie arc assemblage was con- preted as the record of subduction along a paleo- zoic subduction zone, but post-accretion meta- structed on an ophiolitic substratum, with no trench fl anking the continental margin, whereas morphism in greenschist and amphibolite facies evidence of continental basement in the arc the more western mélanges (Fiddle Creek– has elsewhere overprinted all the Sierran accre- roots (Edelman, 1990; Edelman et al., 1989a, Tuolumne River belt) are interpreted as the tionary assemblages. Metasedimentary Red Ant 1989b; Hacker, 1993; Fagan et al., 2001; Day record of a subduction zone along the fl ank of an blueschist blocks are enclosed as phacoidal bod- and Bickford, 2004). offshore intraoceanic island arc as it approached ies within heterogeneous metamélange that also The Foothills arc belt (Figs. 6 and 7) farther the continental margin (Moores and Day, 1984). includes dispersed lentiform blocks of metavol- south is a polygenetic intraoceanic assemblage The intermélange contact (Dilek et al., 1990) canic greenstone. built upon a severely disrupted ophiolitic sub- along the western fl ank of the Calaveras mélange West of the Calaveras mélange belt (Fig. 5) stratum (Schweickert and Bogen, 1983; Edel- belt (Fig. 5) is accordingly interpreted as a suture is a complex assemblage of accreted island arcs man and Sharp, 1989), and is composed of between mélange assembled at the continental (Slate Creek–Lake Combie belt, Smartville structurally interleaved volcanic and volcani- margin and mélange transported to the conti- block–Foothills arc) and associated subduction clastic strata that probably include counterparts nental margin together with an exotic island arc. complexes (Fiddle Creek–Tuolomne River belt) of the Slate Creek–Lake Combie belt as well as Where accreted arc assemblages are in direct that have been progressively attenuated south- the Smartville block to the north (Sharp, 1988; contact with the Calaveras mélange belt south- ward by post-accretion deformation (Fig. 7). Herzig and Sharp, 1992). The least deformed ward along the Sierra Nevada foothills (Fig. 7), Large segments of the Smartville block and its components of the Foothills arc assemblage the placement of the suture between the evolving southern extensions along the Foothills arc are include thick intact successions (1000–5000 m) continental margin and exotic arc terranes is not massive metavolcanic rock with minimal folia- of largely volcanogenic strata (Bogen, 1984, in doubt, but the ambiguity for time of accre- tion and no indication of structural dislocation 1985; Schweickert et al., 1988). Recent petro- tion remains. If arc collision and accretion were

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Middle Jurassic in age in the Sierra Nevada as The Kings-Kaweah ophiolite belt is a tectonic can be inferred during subduction that suc- well as in the Klamath Mountains (Sharp, 1988; megabreccia (Saleeby, 1977) of heterogeneous ceeded accretion of exotic island-arc structures Girty et al., 1995), Upper Jurassic lithic assem- lithology intruded by Lower Cretaceous plu- of the Sierra Nevada foothills in Middle to Late blages of the Smartville block (Figs. 5 and 6) and tons (Saleeby, 1992). The entire ophiolite belt Jurassic time. From that perspective, suturing their continuations southward along the Foothills has been metamorphosed since its emplacement of the island arcs into the Cordilleran margin arc belt can be viewed as direct analogues of the by proximity to the Sierra Nevada batholith along a collision zone within the Sierra Nevada Western Jurassic belt of the Klamath Mountains (Saleeby, 1977). A reconstruction of the struc- foothills was followed by polarity reversal that (Dilek and Moores, 1988). tural fabric of the southern (Kaweah) part of the led to accretion of the Kings-Kaweah belt and For the alternate scenario of Late Jurassic arc- belt before intrusion reveals an array of lozenge- the mafi c Great Valley basement to their west- arc collision between an east-facing intraoceanic shaped tectonic blocks as much as 1000–1500 m ern fl ank. If this interpretation is correct, mod- arc complex and the west-facing continental- long and 250 m wide, but ranging downward in eling of the mafi c Great Valley basement as an margin arc-trench system, upper Oxfordian– dimensions to outcrop scale, encased in an anas- intact ophiolite (Godfrey et al., 1997; Godfrey lower Kimmeridgian (155 ± 2.5 Ma) metasedi- tomosing matrix of sheared serpentinite forming and Klemperer, 1998) can be viewed as the mentary strata are interpreted as the clastic fi ll of outcrop bands as much as 1 km wide (Saleeby, simplest possible model that satisfi es available a suture basin that developed along the collision 1977). The northern (Kings) part of the belt is geophysical constraints, but not a valid model zone (Schweickert and Bogen, 1983). The strata composed of multiple intact slabs of ophiolite of the subsurface. The modeled thickness of the contain quartzolithic detritus derived mainly that are structurally interleaved with only thin subsurface ophiolitic basement and the struc- from subduction complexes caught along the seams of sheared serpentinite separating them tural thickness of the disrupted Kings-Kaweah suture belt. Older Callovian–Oxfordian (160– (Saleeby, 1977). As reconstructed, the structural ophiolite belt are strikingly similar (10–15 km), 155 Ma) strata of both similar and volcaniclastic thickness of the belt is 10–15 km. Blocks and but full reconciliation of the subsurface geo- petrology are interpreted as the sediment fi ll of slabs within the belt include lithologic represen- physical data with the interpretation of the Great the remnant ocean basin and trench present along tatives of all levels of oceanic crust and upper- Valley mafi c basement preferred here is beyond the evolving suture zone before arc-arc collision most mantle: chert, pillow , ophicalcite the scope of this paper. was complete (Ingersoll and Schweickert, 1986; and detrital serpentinite, diabase dikes, gabbro The age range (225–160 Ma) of Mesozoic Dickinson et al., 1996b). and pyroxenite, and , including weh- lithic components of the Kings-Kaweah belt rlite, harzburgite, and dunite. The variety of the and the age (130–125 Ma) of stitching plutons Great Valley blocks closely matches lithologies described that tie the Kings-Kaweah belt to foothills arc from bottom-hole samples of cores that pen- assemblages imply accretion in Late Jurassic– Bedrock under the Great Valley between the etrate mafi c basement rock in the subsurface Early Cretaceous time (Fig. 6). The earlier part Sierra Nevada and the Coast Ranges is masked of the Great Valley, i.e., greenstone, diabase, of that time interval (160–130 Ma) is preferred for a lateral span of 30–90 km over a length of greenschist, gabbro, and serpentinite (Cady, here because accretion of the Kings-Kaweah ~650 km by Quaternary alluvium overlying 1975; Constenius et al., 2000). Plagiogranite and belt logically preceded accretion of Franciscan thick successions of Cretaceous and Tertiary metadiorite of the Kings-Kaweah belt include assemblages in the Coast Ranges to the west, forearc strata (Figs. 5 and 7). Permian as well as Triassic–Jurassic ophiolitic and blueschists of the Franciscan subduction Forearc sedimentation expanded laterally rocks (Saleeby and Sharp, 1980). complex have yielded metamorphic Late Juras- through Cretaceous and Paleogene time as suc- Pervasive deformation of the Kings-Kaweah sic ages as old as ca. 160 Ma (Fig. 7). In a cessive increments of the Franciscan subduction ophiolite belt has been attributed to disloca- regional sense, accretion of the Kings-Kaweah complex were accreted by underthrusting on the tion during strike slip along the transform belt and the mafi c basement of the Great Valley west (Fig. 5), and forearc strata progressively fault responsible for latest Paleozoic to earliest to the continental margin can be viewed as the onlapped the western fl ank of the accreted arc Mesozoic truncation of the continental block result of a short-lived phase of pre-Franciscan assemblages forming the exposed fringe of (Saleeby, 1977, 1992; Saleeby et al., 1978). subduction that followed post-collision polarity the Sierra Nevada block to the east (Ingersoll, This interpretation is not favored here because reversal in the Sierra Nevada foothills. 1982). An elongate band of mafi c basement rock the Kings-Kaweah belt is west of the Calaveras in the subsurface beneath the center of the Great mélange belt (Fig. 7), which was not accreted Coast Range Ophiolite Valley is delineated by a prominent magnetic to the continental margin until Middle Jurassic and gravity anomaly (Cady, 1975). Geophysi- time (Fig. 6), long after continental truncation West of the northern Great Valley (Fig. 5), a cal modeling of the mafi c basement responsible by transform slip. The internal structure of the steeply dipping belt of ultramafi c and associated for the anomalies has suggested a unitary slab Kings-Kaweah belt, marked by structural stack- mafi c rocks exposed west of the Great Valley of obducted but otherwise essentially unde- ing of Kings ophiolite slabs and by dispersal of forearc basin have been termed the Coast Range formed ophiolite emplaced as backarc oceanic elongate lenticular blocks of ophiolitic lithology ophiolite (Fig. 6). Upper horizons of the ophi- crust during arc accretion in the Sierra Nevada in Kaweah serpentinite mélange, can be viewed olitic succession include volcanopelagic strata foothills (Godfrey et al., 1997; Godfrey and instead as structural features imparted by sub- containing a radiolarian fauna inferred to refl ect Klemperer, 1998; Godfrey and Dilek, 2000). duction of oceanic crust and lithosphere. deposition at more southern paleolatitudes than When the subcrop of mafi c basement delineated Spatial correlation of subsurface mafi c base- radiolarian faunas of the adjacent Cordilleran by Cady (1975, his oversize Fig. 2) is traced ment under the Great Valley with the exposed margin (Hopson et al., 1996). An inferred exten- southward, however, the mafi c rocks emerge Kings-Kaweah ophiolite belt implies that the sion of the ophiolite into the subsurface of the at the surface along the fringe of the Great Val- elongate band of buried mafi c rocks is not an forearc basin to the east has been thought to form ley east of Fresno and northeast of Tulare as the intact slab of ophiolite, but instead a deformed obducted basement beneath the Great Valley. The deformed Kings-Kaweah ophiolite belt of the belt of disrupted oceanic crust and lithosphere. ophiolitic assemblage as a whole has long been Sierra Nevada foothills (Fig. 7). Accretion to the California continental margin regarded as a dismembered slab of oceanic crust

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and lithosphere overlain depositionally by basal of ophiolitic rocks depositionally underlying settings, including open-ocean seafl oor, oceanic horizons of the Great Valley succession (Bailey et Great Valley forearc strata along the eastern seamounts, intra-arc rifts, and backarc basins al., 1970; Dickinson and Seely, 1979; Ingersoll, fl ank of the Coast Ranges to the west (Coast (Shervais and Kimbrough, 1985; Shervais, 1982, 2000). Recent evaluation of the ultramafi c Range ophiolite of Godfrey and Dilek, 2000) 1990; Giaramita et al., 1998; Huot and Maury, components of the ophiolitic assemblage has are the outcrop expression of an analogous but 2002; Shervais et al., 2004, 2005). Descrip- shown, however, that the bulk of the ultramafi c younger accretionary belt structurally overlying tions thereby imply a composite origin for the belt is serpentinite mélange (Tehama-Colusa forearc serpentinite mélange and the Franciscan Coast Range ophiolite. Reconciliation of its serpentinite mélange of Hopson and Pessagno, subduction complex. Several lines of argument varied petrologic character with the concept of 2004), viewed here as an eastern component of favor the interpretation that the ophiolitic assem- a coherent, laterally contiguous slab of ophiol- the Franciscan subduction complex analogous to blages described heretofore as parts of the Coast ite is challenging, but unnecessary if the Coast the hydrated mantle described from modern arc- Range ophiolite represent a collage of disparate Range ophiolite constitutes a dislocated collage trench systems as forearc serpentinite (Bostock ophiolitic slabs rather than a coherent and later- of disparate tectonic elements. et al., 2002; Blakely et al., 2005). The protolith ally contiguous ophiolite succession. (4) Several locally exposed ophiolitic slabs for the serpentinite was oceanic upper mantle, (1) With the Tehama-Colusa serpentinite that are structurally beneath the forearc sedi- with blocks of oceanic crust including basaltic mélange removed from the Coast Range ophi- mentary succession, and have been viewed as pillow lava, radiolarian ribbon chert, slaty argil- olite, exposures of the latter are restricted to segments of the Coast Range ophiolite, actually lite, and rare diabase and gabbro enclosed within ~20 geographically isolated remnants of lim- display equivocal relations with the Great Val- the foliated serpentinite matrix (Hopson and Pes- ited individual extent exposed beneath forearc ley Group. Perhaps the best known and most sagno, 2004). strata along the west side of the Great Valley and widely discussed is the Del Puerto ophiolite Recognition that the voluminous ultramafi c within the Coast Ranges to the west (Hopson et (Evarts, 1977; Evarts et al., 1999; Coleman, belt previously regarded as an integral lower al., 1981; McLaughlin et al., 1988; Hopson and 2000), exposed along the eastern fl ank of the part of the Coast Range ophiolite is instead the Pessagno, 2004). Original lateral continuity of Diablo Range west of the Great Valley (Fig. 7). structurally distinct Tehama-Colusa serpentinite the ophiolite remnants is commonly inferred, Variably metamorphosed exposures of ultra- mélange (Fig. 6) challenges past interpretations but cannot be demonstrated on outcrop. Infold- mafi c, plutonic, and volcanic rock distributed of the Coast Range ophiolite. With regard to the ing into the structurally underlying Franciscan along Del Puerto Creek have been interpreted Great Valley subsurface, there is no longer any subduction complex, displacement by detach- as dismembered fragments of a coherent ophio- justifi cation for the inferred extension of the ment faults (Jayko et al., 1987; Harms et al., lite succession depositionally underlying the ultramafi c mass now known to be composed 1992), and lateral translation by strike slip Great Valley forearc succession, and largely of serpentinite mélange eastward beneath the (McLaughlin et al., 1988) have all contributed to undeformed prior to forearc sedimentation. As Great Valley as an obducted slab of undeformed the structural isolation of the various remnants of mapped in detail, however, ultramafi c rocks of ophiolite. My interpretation of mafi c basement the Coast Range ophiolite, but do not preclude the ophiolitic assemblage are faulted against beneath the Great Valley as an analogue along earlier deformation of the ophiolitic assemblage forearc strata, which depositionally overlie only strike of the Kings-Kaweah ophiolite belt formed prior to the deposition of overlying sedimentary the volcanic rocks. The volcanic rocks are cut within an evolving subduction zone obviates the cover forming the Great Valley Group. by small mafi c intrusions and gabbro layers are need to postulate an obducted slab of ophiolite (2) Sedimentary strata, including ophiolitic present within the nearby ultramafi c body, but beneath the Great Valley, but other aspects of the breccias that depositionally overlie the dis- the volcanic and plutonic masses are structurally Coast Range ophiolite as currently conceived persed fragments of Coast Range ophiolite, disconnected on outcrop, and their reconstruc- are called into question as well. rest alternately on basaltic pillow lava, sheeted tion into a single ophiolite profi le is inferential, Alternative suggested origins of the exposed dike complexes, gabbro, serpentinite, metadac- even though widely accepted (Evarts, 1977; Coast Range ophiolite have assumed that related ite, and andesitic breccia (Hopson et al., 1981; Evarts et al., 1999; Coleman, 2000). The actual ophiolite spans the width of the Great Valley Lagabrielle et al., 1986; McLaughlin et al., outcrop relations are compatible with the alter- basement from the Coast Ranges to the Sierra 1988; MacPherson and Phipps, 1988; Robert- nate view that different components of the local Nevada (Dickinson et al., 1996a): (1) backarc son, 1989, 1990). These diverse contact rela- ophiolitic suite were structurally telescoped oceanic lithosphere formed behind an east- tionships indicate that the ophiolitic assemblage along the tectonic contact between the Great facing island-arc complex now accreted to the of the Coast Range ophiolite was deformed Valley Group and the Franciscan Complex, and continental margin in the Sierra Nevada foot- enough before deposition of overlying sedimen- do not defi ne a unitary ophiolite succession. hills (Dickinson et al., 1996b); (2) mid-ocean tary strata to expose locally different horizons (5) Some ophiolite exposures identifi ed in the lithosphere accreted to the continental margin of the ophiolite pseudostratigraphy from place past as remnants of the Coast Range ophiolite west of the Sierra Nevada foothills after plate to place, and are compatible with origin of the are surrounded by exposures of the Franciscan transport from afar (Hopson et al., 1996); or (3) Coast Range ophiolite as a tectonic collage of subduction complex, and need not necessarily oceanic lithosphere formed in place by forearc ophiolitic character. Chronostratigraphic equiv- be termed Coast Range ophiolite (Ingersoll and spreading induced by slab rollback along the alence of the volcanopelagic strata forming the Schweickert, 1986). western fl ank of the continental-margin Sierra uppermost tiers of the local ophiolite succes- In sum, the perspective that the mafi c base- Nevada arc (Saleeby, 1996). sions imply derivation of all exposures from the ment beneath the Great Valley is a complexly The controversy becomes moot if (1) mafi c same oceanic realm, but do not indicate how far deformed accretionary belt of ophiolitic rocks basement beneath the Great Valley (Great Val- apart the various exposed ophiolite remnants suggests that the Great Valley Group of the ley ophiolite of Godfrey and Dilek, 2000) is a originally were within that realm. forearc basin was not deposited on a laterally deformed belt of stacked ophiolite thrust sheets (3) Petrologic studies of various segments coherent ophiolite, but instead overlapped an and ophiolite-bearing mélange akin to the of the Coast Range ophiolite have suggested intricately deformed ophiolitic substrate as it Kings-Kaweah ophiolite belt, and (2) exposures diverse origins across a spectrum of ophiolitic onlapped westward over the growing Franciscan

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subduction complex (Ingersoll and Dickinson, bands of mélange including greenstone blocks nated Cordilleran eugeosynclinal evolution and 1981). Existence of a discrete Coast Range (or lace through the outcrop belt, and its strata included intrusion of the Sierra Nevada batho- Great Valley) ophiolite is an unnecessary and probably represent both trench-fl oor and trench- lith, the idea of Nevadan was extended unlikely postulate that forces tectonic interpre- slope deposits (Bachman, 1982; Bachman et to global scale in the guise of a universal end- tations into contrasting and mutually contra- al., 1984). Large domains of Lower Cretaceous Jurassic event. This original view of the Nevadan dictory avenues of thought that can be avoided metavolcanic rock representing oceanic sea- orogeny became untenable once it was learned with the view that the isolated remnants of the mount edifi ces were locally incorporated into that the bulk of the Sierra Nevada batholith is Coast Range ophiolite were accreted sepa- the Franciscan accretionary prism to form the Cretaceous in age. rately to the continental margin. Snow Mountain block (Figs. 5 and 6) and much When plate tectonic concepts were fi rst of the Permanente terrane (Figs. 6 and 7). applied in detail to the Sierra Nevada foothills Coast Ranges The most eastern components of the Fran- by Schweickert and Cowan (1975), it was sug- ciscan subduction complex are schistose or gested that deformation and metamorphism The bulk of the Coast Ranges west of the semischistose domains (Pickett Peak–Valentine traditionally ascribed to the Nevadan orogeny Great Valley is formed by the vast Franciscan Springs and Yolla Bolly belts of Figs. 5–7) that was associated with Late Jurassic collision and subduction complex coeval with the Sierra appear to be intact though metamorphosed suc- accretion of an exotic intraoceanic island arc in Nevada batholith, which forms the roots of the cessions lacking the mélange fabric imparted to the Sierran foothills. This viewpoint requires paired and coeval magmatic arc (Fig. 6). The the Central Belt by stratal dislocation. Intricate diachroneity for the Nevadan orogeny because Franciscan accretionary prism has been subdi- interleaving of metachert bands with domains arc collision and accretion was Middle Jurassic vided into multiple components of varied lithol- dominated by metagraywacke and the occur- in age along tectonic strike in the western Klam- ogy and internal structure grouped here on maps rence of lensoid metavolcanic bodies commonly ath Mountains. and diagrams into just a few main belts. Each associated with the metachert occurrences sug- Two disparate perspectives have accordingly belt was thrust successively beneath the belt to gest an imbricate internal structure overprinted been adopted for the timing of the Nevadan the east, as seen clearly in the Northern Coast by the metamorphism that developed the schis- orogeny in the Klamath Mountains. On the one Ranges (Fig. 5). The present outcrop distribu- tosity (Ernst, 1993b). The original nature of lith- hand, Hacker and Ernst (1993) argued for a tions of some Franciscan assemblages have ologic contacts within the schistose rocks cannot continuum of structural deformation, metamor- been infl uenced by synaccretion tectonic denu- be discerned through the metamorphic overprint, phism, and plutonism that spanned the interval dation along low-angle normal faults (Jayko but intricate intermingling of graywacke, chert, 170–150 Ma (Middle to Late Jurassic) with no et al., 1987; Harms et al., 1992), probably in and basalt protoliths can best be understood as distinct Nevadan orogeny discernible from the response to thickening of the accretionary prism the result of thrust imbrication of multiple pan- geologic record. Others prefer to regard the by subcretion or underplating (Platt, 1986). The els of oceanic seafl oor (chert-basalt) overlain structural amalgamation of the Western Jurassic tectonic denudation locally attenuated structural by trench-fi ll turbidites (graywacke). It is likely belt with older accretionary belts of the Klam- cover over deeper horizons of the subduction that stratal successions of this kind were formed ath Mountains as a deformational event that complex, but did not alter the fundamental struc- repetitively as successive increments of seafl oor deserves the appellation of Nevadan orogeny tural stacking achieved by underthrusting during were rafted into the Fanciscan paleotrench. (Saleeby et al., 1982; Wright and Fahan, 1988; subduction. Post-accretion deformation associ- To my eye, transitional outcrops between Harper and Wright, 1984; Harper et al., 1990). ated with Neogene evolution of the San Andreas Central Belt mélanges and more eastern schis- Using the best available information for the transform system folded many fault contacts tose rocks in the Northern Coast Ranges suggest timing of the structural telescoping that carried between tectonic belts, and their outcrop patterns that the schistose fabrics were superimposed the Western Jurassic belt beneath the expand- become progressively more complex southward in part on broken formation. If so, the planar ing continental margin, this approach dates the (Fig. 7). Protoliths and times of inferred accre- lithologic interfaces that were ancestral to schis- initiation of Nevadan thrusting as 155–150 Ma tion generally become younger westward within tosity, and commonly interpreted as bedding (Kimmeridgian), as marked by metamorphic the Franciscan complex, although in complex surfaces, may in some cases have been part of ages for sole and roof thrusts, and the termi- and partly overlapping fashion (Fig. 6). the shear fabric of mélange. This interpreta- nation of Nevadan thrusting as 150–145 Ma Detailed discussion of internal relations within tion suggests that some of the schistose eastern (Tithonian), as marked by the emplacement of the Franciscan Complex is beyond the scope of Franciscan rocks may have passed through a crosscutting plutons (Harper et al., 1994). this paper, but the Central Belt and the Coastal premetamorphic phase of mélange formation Depending upon how the complex age rela- Belt have the overall character of mélange and as well as structural imbrication before acquir- tionships within the Sierra Nevada foothills are broken formation, respectively. Undisrupted ing their schistose fabrics. If that viewpoint is interpreted, this time span (155–145 Ma) for the phacoidal blocks encased in a mélange matrix invalid, it remains unclear why the easternmost Nevadan orogeny may be appropriate as well for of scaly clay in the Central Belt range widely Franciscan assemblages were underthrust as Late Jurassic arc collision and accretion in the in dimensions, from the size of a fi st to the size intact stratal successions before the accretion Sierra Nevada (Fig. 6). From an analysis of stratal of a mountain. Graywacke derived from trench of the mélange assemblages exposed so promi- ages and deformational features, Schweickert et turbidites is the dominant block lithology (Bai- nently farther west. al. (1984b) suggested an age of 155 ± 3 Ma for ley et al., 1964), but metavolcanic greenstone of collisional Nevadan orogeny in the Sierra Nevada both seafl oor and seamount origin (MacPherson NEVADAN OROGENY foothills. Others have subsequently argued, how- et al., 1990; Shervais, 1990) is also a ubiquitous ever, that penetrative deformation continued for component, as is associated ribbon chert derived The concept of a Nevadan orogeny looms tens of millions of years thereafter (Saleeby et al., from seafl oor stratal successions. The Coastal large as a historical legacy in our thoughts 1989b; Tobisch et al., 1989). Belt is largely broken formation formed by about California tectonics. Originally conceived Continued usage of the term Nevadan disrupted graywacke-argillite successions, but as a climactic or paroxysmal event that termi- orogeny, originally conceived as marking the

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endogenous terminal phase of a supposed geo- the faults dipped gently before eastward tilting The trend of the headwall scarp for the forearc tectonic cycle, seems equivocal in an era when of the Great Valley Group led to the suggestion system of normal growth faults is northward in orogenic events are described in terms of the that they were syntectonic thrust faults (Dickin- the subsurface (Constenius et al., 2000). This improved plate model. Those who continue to son and Seely, 1979). Refl ection profi les beneath trend, parallel to the strike of the normal faults use the term have the obligation to specify the the Great Valley that connect to exposures on before they were tilted along with forearc basin structural features and events to which the name outcrop have now shown that the family of fi ll, would carry the scarp into the covered zone Nevadan is attached, for multiple interpretations faults instead formed as a system of low-angle between the Klamath and Sierra Nevada blocks are logically possible. normal growth faults active during deposition (Fig. 9). In effect, foundering of the fl ank of the of the Great Valley Group (Constenius et al., Great Valley forearc basin toward the Francis- KLAMATH-SIERRAN CORRELATION 2000). Strong contrasts in stratal thicknesses of can paleotrench calved the Klamath block off Lower Cretaceous strata are evident across the the trenchward fl ank of the arc massif and dis- The manner in which analogous tectonic individual fault traces, and apparent displace- placed it downward to the west, backtilting at belts of the Klamath Mountains and northern ments across the faults decrease upward in the least the eastern edge of the Klamath block dur- Sierra Nevada connect beneath Cretaceous and stratigraphic succession (Fig. 8). ing its displacement. Cenozoic cover underlying and surrounding the The syndepositional faults carried the trench- This kinematic scheme can account for defl ec- northern end of the Great Valley has long been ward fl ank of the forearc basin progressively tion of pre-Cretaceous tectonic belts of the north- diffi cult to assess (Davis, 1969; Irwin, 2003). downward to the northwest as forearc sedimen- ern Sierra Nevada westward into the Klamath The respective tectonic belts strike subparallel tation continued, and successive intervals of the Mountains. The accreted arc assemblages on the to one another but are not in alignment (Fig. 1). Great Valley forearc succession change thick- west, the central mélange belts, and the Paleo- Recognition that related Cretaceous faults ness in the appropriate sense across strands of zoic assemblages overlain by native Mesozoic (Fig. 8) along the western side of the northern the fault system bounding separate half-grabens. arc successions on the east are all out of align- Great Valley are tilted normal growth faults The half-graben fi lls have been tilted to steep ment by comparable amounts across the forearc (Constenius et al., 2000), active during evolu- eastward dips by Cenozoic uplift of the North- normal fault system (Fig. 9). The original inter- tion of the Great Valley forearc basin, provides ern Coast Ranges, but their initial confi gurations pretation of strike slip along the growth faults as insight into this longstanding question. are recovered by retrodeformation of the Meso- an explanation for offset of the Klamath block zoic succession of the Great Valley forearc basin with respect to the Sierra Nevada fails because Forearc Faulting (Constenius et al., 2000). The northernmost half- the tilted faults at their present steep dips strike graben is fl oored by the pre-Cretaceous bedrock toward the north-central Sierra Nevada rather From their present steep dips, the faults were assemblage of the Klamath Mountains (Fig. 8), than the northern end of the Sierra Nevada. The initially inferred to form a zone of sinistral strike showing that the Klamath block participated in offset of an Early Cretaceous paleoshoreline that slip (Jones and Irwin, 1971). Appreciation that the growth faulting. fi gured in the original interpretation of sinistral

South North Paskenta fault zone Sites anticline and Elder Creek fault zone Fruto syncline Great Valley alluvium Cold Fork fault zone

Sulphur Springs fault

Albian Cenozoic Aptian Great Valley Group Discontinuity in Great Valley Group Santonian Hauterivian-Barremian Franciscan Turonian - Coniacian Valanginian-Berriasian Klamath block complex Cenomanian 10 km Tithonian-Kimmeridgian Coast Range ophiolite

Figure 8. Outcrop confi guration of syndepositional normal fault system displacing strata of the Great Valley Group near the juncture of the Northern Coast Range and the Klamath block (left). View downdip in Great Valley Group adapted after Constenius et al. (2000) to restore bedding to subhorizontal and display faults in original orientation.

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CALIFOR Lower to Middle Jurassic volcanic- PLUTONS 124° W 123° W NIA/OREGON volcaniclastic

Hornbrook C

CC HC Formation E pre-Middle Triassic continental N margin (Paleozoic assemblages) Upper Cretaceous

O Sierra Nevada Y KLAMATH Z compound suture belt of batholith (<125 Ma) MTNS O Triassic-Jurassic mélange I PACIFIC C accreted Jurassic intraoceanic

OCEAN C island arc complexes Lower Cretaceous O (135-145 Ma) V Upper Cretaceous Great E

R Lower Cretaceous Valley Upper Jurassic Group Middle (to Upper) Jurassic F Franciscan Complex 41° N R (170-155 Ma) A (Jurassic-Cretaceous) Humboldt NC WC

basin C

N (Cenozoic) native 121° W A

I E S Mesozoic LI

V

C F Eu arc native A W OR AN D

Mesozoic A

m arc N te

I

ys A E SIERRA Re s A t R l V E NEVADA u C O S a f T I C H l Z O o C E a N O ne R m E y O forearc normal r C o A N n L fault system a S Lake k c e T Almanor RB ar AL e fa A r u o l G f t N f D o 40° N forearc C rp serpentinite a ENO B c s E C belt ll a E L dw Co Z T N a B he T O R Sites I F s C A anticline u N R b Ci L s C u

A r f O a

N c

e V BE C m FB E I a f R S i L c

b C T a Lake s A S e SBv m Tahoe N en 25 km U t GV

Figure 9. Displacement of Klamath Mountains block (on northwest) from Sierra Nevada block (on southeast) by down-to-the-paleotrench forearc normal growth faults (Fig. 8) of Early Cretaceous age (areal geology adapted after Figs. 3 and 5). SBv—Sutter Buttes volcano (Plio- cene—Pleistocene). Selected towns (for orientation): B—Blairsden; CC—Crescent City; Ci—Chico; Co—Covelo; Eu—Eureka; FB—Fort Bragg; G—Garberville; GV—Grass Valley; HC—Happy Camp; RB—Red Bluff; Re—Redding; U—Ukiah; W—Weaverville; WC—Wil- low Creek; Y—Yreka.

strike slip across the faults (Jones and Irwin, (Figs. 3B and 9), and extend into Oregon as Klamath block before and during its displace- 1971) is equally well accommodated through the Myrtle Group exposed along and near the ment, and exposures of Lower Cretaceous strata displacement of the paleoshoreline by normal northwestern fl ank of the accreted Klamath now appear to swing westward out of alignment growth faulting. assemblages (Fig. 3A). By contrast, Upper Cre- with the Great Valley forearc basin to the south; The relative positions of northern extensions taceous increments of forearc sediment forming whereas (2) Late Cretaceous forearc sedimenta- of the Great Valley forearc basin are compat- the Hornbrook Formation (Figs. 3A, 3B, and 9) tion was shunted east of the displaced Klamath ible with syndepositional displacement of the were deposited along the eastern fl ank of the block where exposures of Upper Cretaceous Klamath block. Lower Cretaceous increments Klamath Mountains after displacement and tilt- strata align with the Great Valley forearc basin of forearc sediment overlie accreted arc and ing of the Klamath block. These relations imply to the south (Fig. 9). mélange belts along the western fl ank of the that (1) Early Cretaceous forearc sedimenta- The distribution of arc plutons also refl ects accreted Klamath assemblages in California tion continued along the western fl ank of the displacement of the Klamath block toward the

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Franciscan paleotrench during Cretaceous time. here, accretion of the Smartville block was from A.W. Snoke helped further my interpretations of Klamath relations, clarifi ed by reconnaissance Lower Cretaceous plutons (145–135 Ma) of the delayed until Late Jurassic time, then intraoce- with timely reprints from W.P. Irwin in hand, and arc roots are intrusive into the eastern Klamath anic eruptions and intra-arc extension within the correspondence with J.E. Wright clarifi ed my inter- Mountains (Fig. 3B) as well as the foothills of east-facing Smartville island arc, which was still pretations of arc polarity in the central and western the northern Sierra Nevada (Fig. 5). Upper Cre- offshore in Middle Jurassic time, were contem- Klamath Mountains. Extensive discussions with R.V. Ingersoll and R.A. Schweickert improved my under- taceous plutons (younger than 125 Ma) of the poraneous with interarc rifting to form the Jose- standing of the Great Valley and Sierra Nevada. A main Sierra Nevada batholith occur, however, phine ophiolite after Middle Jurassic arc colli- preliminary oral version of this paper was presented only along the Sierran crest well to the east of sion and polarity reversal along tectonic strike to in the Theme Session on Accretionary Orogens in the Klamath block, which had been displaced the north. Analogy can then be drawn between Space and Time, arranged by K.C. Condie, at the 2005 into the forearc region by mid-Cretaceous time. diachronous arc collision and consequent polar- Geological Society of America meeting in Salt Lake City. Jim Abbott of SciGraphics prepared the fi gures. Continuations of Late Cretaceous plutonism ity reversal in the western Klamath Mountains Reviews by C.A. Hopson, C.H. Jones, J.P. Platt, and northward from the Sierra Nevada are refl ected (Middle Jurassic) and Sierra Nevada foothills J.B. Saleeby improved both text and fi gures, but none by granitic exposures isolated by intervening (Late Jurassic) with diachronous impingement of them share all my perspectives and the conclusions Tertiary volcanic cover in northwestern Nevada of the Luzon arc on modern Taiwan to induce of the paper are my own. and southwestern Idaho, but Upper Cretaceous analogous polarity reversal at the south end of REFERENCES CITED plutons are unknown in the Klamath Mountains the Ryukyu arc in northern Taiwan (Ingersoll, farther west (Fig. 3). 2000; Teng et al., 2000). Alexander, R.J., and Harper, G.D., 1992, The Josephine ophiolite: An ancient analogue for slow-to intermediate- spreading oceanic ridges, in Parson, L.M., et al., eds., Tectonic Correlations SUMMARY AND CONCLUSIONS and their modern oceanic analogues: Geolog- ical Society [London] Special Publication 60, p. 3–38. 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Knowledge of the geometry Beard, J.S., and Day, H.W., 1987, The Smartville intru- intra-arc extension in the Smartville block of the of forearc normal faulting clarifi es Klamath- sive complex, Sierra Nevada, California: The core of a rifted volcanic arc: Geological Society of America Sierra Nevada foothills (Figs. 4 and 6). If the Sierran tectonic correlations. Bulletin, v. 99, p. 779–791, doi: 10.1130/0016- Smartville block was accreted to the continental 7606(1987)99<779:TSICSN>2.0.CO;2. margin in Middle Jurassic time, the Josephine ACKNOWLEDGMENTS Behrman, P.G., 1978, Pre-Callovian rocks west of the Melones fault zone, central Sierra Nevada foothills, in and Smartville ophiolitic assemblages can be Howell, D.G., and McDougall, K.A., eds., Mesozoic Discussions with the late P.J. Coney nurtured my ascribed to analogous intra-arc rifting along tec- paleogeography of the western United States: Pacifi c appreciation of accretionary tectonics. 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MANUSCRIPT RECEIVED 10 MARCH 2007 man, J., 2004, Multi-stage origin of the Coast Range Ujiie, K., Hisamitsu, T., and Soh, W., 2000, Magnetic and REVISED MANUSCRIPT RECEIVED 21 OCTOBER 2007 ophiolite, California: Implications for the life cycle of structural fabrics of the mélange in the Shimanto MANUSCRIPT ACCEPTED 2 NOVEMBER 2007

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