Review of Late Jurassic-early Miocene sedimentation and plate

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Chin. J. Geochem. (2015) 34(2):123–142
DOI 10.1007/s11631-015-0042-x
Review of Late Jurassic-early Miocene sedimentation
and plate-tectonic evolution of northern California: illuminating
example of an accretionary margin
W. G. Ernst
Received: 21 January 2015 / Revised: 23 January 2015 / Accepted: 23 January 2015 / Published online: 7 February 2015
Ó Science Press, Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2015
Abstract Production of voluminous igneous arc rocks,
high-pressure/low-temperature (HP/LT) metamafic rocks,
westward relative migration of the Klamath Mountains
province, and U–Pb ages of deposition, sediment sources,
and spatial locations of Jurassic and younger, detrital zirconbearing clastic rocks constrain geologic development of the
northern California continental edge as follows: (1) At
*175 Ma, transpressive plate underflow began to generate
an Andean-type Klamath-Sierran arc along the margin. (2)
Oceanic crustal rocks were metamorphosed under HP/LT
conditions in an inboard, east-inclined subduction zone from
*170-155 Ma. Except for the Red Ant blueschists, such
lithologies remained stored at depth; most HP/LT mafic
tectonic blocks returned surfaceward only during mid- and
Late Cretaceous time, chiefly entrained in circulating,
buoyant Franciscan mud-matrix mélange. (3) By *165 Ma
and continuing to *150-140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle
overlap strata. (4) At *140, immediately prior to the onset
of paired Franciscan and Great Valley Group (GVG) ?
Hornbrook deposition, the Klamath salient was deformed
and displaced *100–150 km westward relative to the
Sierran arc, stranding pre-existing oceanic crust on the south
as the Coast Range Ophiolite (CRO). (5) After the end-ofJurassic seaward step-out of the Farallon-North American
convergent plate junction, terrigineous debris began to be
deposited in the outboard Franciscan trench and intervening
Great Valley forearc. (6) Voluminous sedimentation and
accretion of Franciscan Eastern ? Central belts and GVG
detritus took place during paroxysmal igneous activity and
W. G. Ernst (&)
Department of Geological & Environmental Sciences, Stanford
University, Stanford, CA 94305-2115, USA
e-mail: [email protected]
rapid, nearly orthogonal plate convergence at *125-80 Ma.
(7) Sierran arc volcanism-plutonism ceased by *80 Ma in
northern California, signaling a transition to shallow, nearly
subhorizontal eastward plate underflow attending Laramide
orogeny far to the east. (8) Presently exposed Paleogenelower Miocene Franciscan Coastal Belt sedimentary strata
were deposited in a tectonic realm unaffected by HP/LT
subduction. (9) Grenville-age detrital zircons are absent
from the post-120 Ma Franciscan section. (10) Judging from
petrofacies and zircon U–Pb data, the Franciscan Eastern
Belt contains debris derived principally from the Sierra
Nevada and Klamath ranges; detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in
some Central Belt sandstones, whereas clasts from the Idaho
Batholith, Challis volcanics, and Cascade Range appear in
progressively younger Paleogene-lower Miocene Coastal
Belt sediments. (11) Gradual NW dextral offset of the
Franciscan trench deposits of as much as *1,600 km may
have occurred relative to the native GVG forearc and basement terranes of the American Southwest.
Keyword Post-Paleozoic subduction Franciscan-Great
Valley strata California crustal evolution JurassicMiocene accretion
1 Geologic introduction
This review summarizes some recent studies of clastic strata
exposed in the Sierran Foothills, the eastern and western
edges of the Klamath Mountains, and main units comprising
the Sacramen to Valley ? outboard California Coast Ranges—rocks deposited during a period typified by mainly
transpressive to convergent plate motions. The times of
sedimentation, provenance of these strata, and their post-
Chin. J. Geochem. (2015) 34(2):123–142
magmagenic depths followed by subsequent ascent and
emplacement of primary calcalkaline igneous rocks (i.e.,
continental growth). Crustal construction also involved the
generation of secondary products of clastic sedimentation
and HP/LT metamorphism, both of which provide additional
information regarding the regional plate-tectonic history.
Figure 1 presents the broad geologic framework of the region, including most of California. Figure 2 is a more
depositional recrystallization P–T histories provide insights
regarding the Late Jurassic through early Miocene petrotectonic evolution of northern California. This review is
aimed particularly at Asian Earth scientists unfamiliar with
northern California, because the area constitutes a relatively
clear example of a 175-20 Ma convergent to transpressive
plate margin that hosted a substantial infusion of new sialic
crust, chiefly through subduction-induced partial melting at
125-80 Ma granitic rocks
bduction zo
Ca Arc
Hornbrook Fm
Cascade su
Myrtle Fm
Jurassic metased. + metavolc.
rocks (incl. Mariposa-Galice)
Paleozoic-Triassic metased.
and metavolcanic rocks
Co ges
175-140 Ma granitic rocks
Paleozoic and Mesozoic
serpentinized ultramafic rocks
ls B
t Vall
End of CretaceousMiocene Coastal Belt
Mainly Cretaceous Great
Valley Group (Lower K tan)
ey G
Cenozoic sedimentary
and volcanic rocks
Paleozoic intrusive rocks
Upper Cretaceous
Franciscan Central Belt
Lower to Upper Cretaceous
Franciscan Eastern Belt
200 km
Fig. 1 General geologic map of most of California, depicting the 175-140 Ma Klamath-Sierranand 125-80 Ma Sierran volcanic-plutonic arcs,
Great Valley Group forearc strata, and Franciscan trench belts, simplified from the U. S. Geological Survey and California Division of Mines and
Geology (1966) geologic map, terrane map of Silberling et al. (1987), the Klamath Sierran map of Irwin (2003), land coastal maps of Dickinson
et al. (2005). Shasta Bally pluton = SB. The South Fork and Coast Range faults juxtapose rocks of the Franciscan Complex against those of the
Klamath province and the GVG respectively (e.g., Blake et al. 1999). Labeled fault zones: Oak Flat-Sulphur Spring = OF-SS; Cold Fork-Elder
Creek = CF-EC; on-land and offshore segments of the Nacimiento, = N and N? San Gregorio-Hosgri = SGF; Mendocino = MF; San
Andreas = SAF
Chin. J. Geochem. (2015) 34(2):123–142
Klamath Mountains units
Western Klamaths incl. Galice Fm
Condrey Mountain terrane
Rattlesnake Creek terrane
Western Hayfork terrane
Eastern Hayfork terrane
Hayfork terrane undivided
North Fork terrane
Stuart Fork terrane
Central Metamorphic terrane
E. Klamath terrane undivided
ly n
ain ca
M cis x
an ple
Fr om
100 Km
Red Ant
Younger overlap strata (Klamaths)
M = Myrtle Formation
H = Hornbrook Formation
GVG = Great Valley Group
Sierran Foothills units
Upper Jurassic incl. Mariposa Fm
Slate Creek complex
utb lex
do p
an Com
ata n
str ca
G ncis
GV Fra
Jura-Triassic arc belt
Calaveras complex
Feather River terrane
Northern Sierra terrane
Fig. 2 Simplified lithostratigraphic terranemap of the Klamath Mountains and the western Sierran Foothills ignoring plutons, after Irwin (1981,
2003), Sharp (1988), Edelman and Sharp (1989), Ernst (1998), and Snow and Scherer (2006). The Galice and Mariposa formations are of Late
Jurassic age. Klamath-margin locations of the northernmost Great Valley Group, Myrtle (M), and Hornbrook (H) formations are indicated. The
Myrtle is of latest Jurassic-earliest Cretaceous age, whereas the GVG and Hornbrook are chiefly of mid- and Late Cretaceous age. Also shown is
the Oak Flat-Sulphur Spring sinistral fault zone (OF-SS), but not the slightly younger Cold Fork-Elder Creek fault zone of Fig. 1. The
Klamaths were displaced oceanward *100–150 km relative to the northern extension of the Jurassic Sierran arc but separation across the OF-SS
is only *80–100 km because of viscous drag induced curvature ofthe imbricate salient
detailed view of the study area, and the disposition of
Klamath, Sierran Foothills, Sacramento Valley, and northern Coast Range clastic sedimentary units treated in this
1.1 Jurassic crustal growth
The late Paleozoic-early Mesozoic development of northern
California was typified by chiefly margin parallel slip,
episodic suturing of far-traveled ophiolitic complexes, and
deposition of superjacent chert-argillite deep-marine
sedimentary units (Saleeby 1981, 1982, 1983; Ernst et al.
2008). Scattered sialic igneous activity characterized the Late
Triassic margin of California, but most lithologic sections of
late Paleozoic-early Mesozoic age are oceanic in their genesis.
However, a major Andean-type arc began to form in the Sierra
Nevada and Klamath Mountains by *175 Ma attending the
transpressive eastward underflow of oceanic lithosphere
(Dunne et al. 1998; Irwin 2003; Dickinson 2008). This volcanic-plutonic arc shed clastic detritus into the ophiolitic
realm of the Jura-Triassic arc belt ? Early and Middle
Jurassic Eastern Hayfork ? North Fork terranes, and the
earliest Late Jurassic continental margin Mariposa ? Galice
formations of the Klamath and Sierran ranges, respectively
(Miller and Saleeby 1995; Scherer et al. 2006). The Sierran
Jura-Triassic arc belt and Klamath chert-argillite-rich North
Fork and Eastern Hayfork units were laid down at *175165 Ma (Snow and Ernst 2008; Scherer and Ernst 2008),
whereas the near-shore Mariposa-Galice formations began
accumulating by *165-160 Ma (Ernst et al. 2009a).
Transpressive plate convergence also generated HP/LT
basaltic eclogites and garnet-glaucophane schists along the
Middle and Late Jurassic convergent plate junction
at *170-155 Ma (Cloos 1986; Wakabayashi 1990; Tsujimori et al. 2006; Ukar et al. 2012).However, except for
the *174 Ma Red Ant blueschists (Hacker and Goodge
1990; Hacker 1994), such HP/LT metamafic rocks remained
at depth, and fragments apparently returned surfaceward
only during mid- and Late Cretaceous time as tectonic ± olistostromal blocks were sheared and gravity-fed into
low-density Franciscan mud-matrix mélange. These
relatively high-grade metamafic blocks formed much earlier
during construction of the emergent arc. They were not intensely overprinted by low-grade metamorphic phases, so
evidently were stored at rather shallow upper mantle depths
as indicated by the common spatial association of HP/LT
blocks with serpentinized peridotites—never with deepseated xenoliths of continental crust. The arc-derived Mariposa-Galice volcanogenic strata predate formation of the
Great Valley Group forearc and subparallel Franciscan
trench depositional realms (Ernst 2011).Similar to the disaggregated fragments of transported oceanic lithosphere and
capping deep-sea chert, the relatively old HP/LT tectonic
blocks (e.g., Anczkiewicz et al. 2004) represent exotic additions to the Franciscan trench.
1.2 Formation of the Klamath Mountains salient
Paleozoic, fault-bounded Klamath Mountain units on the east
are structurally high in the accretionary stack, whereas the
formational ages of successively added lower allochthons
Chin. J. Geochem. (2015) 34(2):123–142
Fig. 3 Palinspastically restored Sierra Nevada-Klamath volcanic- c
plutonic arc prior to hypothesized differentialslip, including 20°
clockwise rotation of the Klamath salient back to a contiguous
Jurassic arc configuration. Underflow of a segmented Farallon plate
beneath the North American margin at *140-136 Ma was proposed
(Ernst 2012) to account for the present outboard location of the
Klamath Mountains relative to both the Sierra Nevada on the SE and
the Blue Mountains on the NE (Snoke and Barnes 2006, Fig. 1). The
Cretaceous batholith belt trends NNW compared to the NW-trending
Jurassic Andean arc
decrease progressively toward the west (Irwin 1972, 1994).
The tectonized, imbricate collage of west-vergent lithostratigraphic terranes consists of basal ophiolitic units, chiefly
overlain by cherts and fine-grained terrigenous strata (e.g.,
Frost et al. 2006), all invaded by Jurassic calc-alkaline arc
plutons. The accreted terrane assembly of the Klamath
Mountains has long been correlated with the northern Sierran
Foothills based on similar rock types, structures, ages of the
rock packages, the progressive oceanward assembly of successively younger geologic units, and their times of deformation (Davis 1969; Davis et al. 1980; Wright and Fahan
1988; Wright and Wyld 1994; Irwin 2003). However, the
accreted Sierran Foothill terranes stand nearly vertically,
whereas the Klamath thrust sheets root gently to the east.
Figure 1 shows that the Klamath Mountains concave-tothe-east contractional assembly now lies well outboard of
the trend of the Sierra Nevada Range. This salient appears
to be situated *100–150 km west of the formerly contiguous Sierran segment of the curvilinear arc. North of the
Klamath promontory, a major eastward jog toward apparently correlative lithologic units in the Blue Mountains
(LaMaskin 2011; LaMaskin et al. 2011; Schwartz et al.
2011) suggests the possibility of a greater oceanward offset
of the Klamath Mountains relative to the late Mesozoic
accretionary continental margin of eastern Oregon (Snoke
and Barnes 2006). The manner in which this tectonic offset
of the Klamath Mountains collage was accomplished remains obscure. However that may be, Fig. 3 provides a
geologic restoration of what probably was an original, preCretaceous, continuous arc in northern California.
At the end-of-Jurassic time, the Klamath accretionary
terrane assembly was deformed and displaced *100–150 km
westward relative to the Andean arc, gradually removing the
accretionary stack of Klamath allochthons from a site over the
deep-seated magmagenic zone stoking the Sierra Nevada igneous belt (Ernst 2012). Abundant granitic plutons intruded
the Klamath Mountains during 170-140 Ma (Hacker et al.
1995; Irwin and Wooden 1999; Irwin 2003; Snoke and Barnes 2006), with emplacement ages generally younging eastward. This igneous activity in the Klamaths ceased at the
beginning of Cretaceous time. The youngest such body is the
apparently *136 Ma Shasta Bally pluton (Lanphere et al.
1968; Lanphere and Jones 1978; Irwin and Wooden 1999).
However, this date may represent the time of cooling and
Chin. J. Geochem. (2015) 34(2):123–142
Cenozoic sedimentary
and volcanic rocks
125-80 Ma granitic rocks
175-140 Ma granitic rocks
Jurassic metased. + metavolc.
rocks (incl. Mariposa-Galice)
Paleozoic-Triassic metased.
and metavolcanic rocks
Paleozoic and Mesozoic
serpentinized ultramafic rocks
Paleozoic plutonic rocks
200 km
annealing of the pluton rather than the time of its emplacement. Geologic mapping by Blake et al. (1999) documented
Hauterivian GVG strata resting with angular unconformity on
exhumed, eroded Shasta Bally rocks, supporting a
Valanginian or older age for the intrusive (SB locates the
plutonon Fig. 1). Thus, prior to offset of the Klamath
Mountains, a continuous Klamath-Sierra Nevada volcanicplutonic arc was sited above the mantle hearth supplying melt
to the crustal superstructure (Fig. 3).
The hypothesized westward step-out of the convergent plate
junction at *140 Mare positioned the trench directly offshore
from the Klamath orogen. To the south, the new suture trapped
pre-existing, far-traveled oceanic crust-capped lithosphere on
the landward side as the *175-165 Ma Coast Range
Ophiolite ± overlying tuffaceous and distal oceanic strata
(Shervais et al. 2005; Hopson et al. 2008). Slab rollback involving coeval suprasubduction-zone generation of the CRO
would have produced an ophiolitic basement younger than the
rifted apron of terrigenous sediments. Because this mafic crust
is actually *25 Myr older than the basal GVG sediments (e.g.,
Stern 2004; Stern et al. 2012; Shervais and Choi 2012), the
postulated plate-junction step-out stranding pre-existing
oceanic lithosphere appears to more fully accounts for the
geologic relationships than would a gradual slab rollback.
Chin. J. Geochem. (2015) 34(2):123–142
1.4 Paleogene-early Miocene crustal growth
Strongly deformed, weakly metamorphosed Paleogenelower Miocene Franciscan Coastal Belt rocks (McLaughlin
et al. 2000; Dumitru et al. 2013), still sourced in part from
the now-inactive Sierranarc, contain significant amounts of
quartzofeldspathic debris derived from yet more northerly
arc terranes (Dumitru et al. 2013, in press). These Coastal
Belt strata exhibit the effects exclusively low-pressure recrystallization (Bachman 1978; Underwood et al. 1987;
Terabayashi and Maruyama 1998; Ernst and McLaughlin
2012). Reflecting near-surface accretionary offloading,
such units apparently were never deeply subducted.
Although important members of the Franciscan lithotectonic assemblage, the low-P transformation of Coastal Belt
strata stand in marked contrast to much of the rest of the
Franciscan Complex. Underplating of younger sections
beneath older slabs of the Franciscan undoubtedly aided
the buoyant ascent and erosional decapitation of the latter,
so the farthest inboard sections have been exhumed to the
greatest extent. It is thus possible that HP/LT tracts of the
Coastal Belt currently are stored deep within the imbricate
accretionary prism.
1.3 Cretaceous crustal growth
2 Late Jurassic time
At *140 Ma, volcanic-plutonic detritus from the KlamathSierran arc started to accumulate on mafic basement within
the Great Valley depositional basin, whereas clastic debris
carried past the forearc came to rest on the descending Farallon oceanic plate as the outboard Franciscan deep-sea
trench fill. Because the lowermost Cretaceous, relatively
continuous GVG strata were laid down on the stable North
American plate, protected from both surface erosion and
subcrustal tectonic removal, inauguration of this forearc
proximal-to-distal terrigineous sedimentation also signaled
the onset of coeval deposition in the yet more distal, coeval
Franciscan Complex, *25–30 Myr after Middle Jurassic
initiation of the Andean arc (Dumitru et al. 2010; Ernst
2011). Voluminous sedimentation and accretion of Franciscan and GVG rocks took place during the 125-80 Ma
flare-up of the Sierran arc (Surpless et al. 2006; Snow et al.
2010; Dumitru et al. 2010; Sharman et al. in press). The
youngest Sierran granites are *80 Ma, reflecting Late
Cretaceous quenching of the magmagenic zone beneath
northern California. The end of continental margin arc activity generally has been ascribed to a lessening of the subduction dip and relatively shallow, subhorizontal oceanic
plate underflow attending Laramide orogeny well to the east
(Coney and Reynolds 1977; Dickinson and Snyder 1978;
Bird 1988).
2.1 Mariposa Formation clastic sedimentation
Snow and Ernst (2008) analyzed zircons from five volcanogenic metaturbidite layers from the upper part of the
Mariposa Formation of the western Sierran Foothills using SIMS methods. Mesozoic U–Pb age populations are
dominated by zircons exhibiting a broad unimodal distribution from *175-155 Ma. The aggregate zircon U–Pb
ages for these metasandstones suggest that the zircons
were derived mainly from the Jurassic Klamath-Sierran
orogenic belt, especially the mid-Paleozoic to mid-Mesozoic terrane collage, and the spatially associated
younger Andean-type arc volcanic rocks ? granitoids.
This interpretation is compatible with Mariposa paleocurrent data indicating an overall southerly transport
direction (Bogen 1985). Accumulation began at *165160 Ma and continued until at least 150 Ma (Ernst et al.
2009a). Indistinct age culminations at 1,000–1,200,
1,600–1,800, and C2,500 Ma (Snow and Ernst 2008)
suggest minor derivation of the studied Oxfordian and
younger Mariposa sandstones from Grenville, MazatzalYavapai and older SW North American cratonal rocks,
and/or younger, multicycle clastic strata sourced from
these basement terranes.
Chin. J. Geochem. (2015) 34(2):123–142
2.2 Galice Formation clastic sedimentation
The turbiditic Galice Formation is the NW continuation of
Mariposa-type volcanogenic lithologies in the western
Klamath Mountains (Gray 2006; MacDonald et al. 2006).
Miller and Saleeby (1995) reported a 153 Ma depositional
age for the upper Galice, but sedimentation may have begun during or before earliest Oxfordian time based on
biostratigraphic data summarized by Saleeby and Harper
(1993). This is supported by the local interdigitation of
Galice metaturbidites with pillow lavas of the subjacent
164-162 Ma Josephine ophiolite (Harper 2006; MacDonald
et al. 2006). Like the Mariposa Formation, the provenance
of Galice sandstones evidently was a combination of both
ancient SW North American basement rocks and mid-Paleozoic to mid-Mesozoic oceanic ophiolite ? chert-argillite complexes, and the younger, nearly coeval Andean arc
(Snoke 1977; Frost et al. 2006).
2.3 Myrtle Formation clastic sedimentation
Scattered erosional remnants of uppermost Jurassic to early
Lower Cretaceous Myrtle detrital strata rest on the Galice
Formation in SW Oregon (Imlay et al. 1959; Dickinson
2008, Fig. 3a). By analogy with the underlying Galice,
Myrtle detritus plausibly was derived from both the landward Klamath-Sierran arc ± minor old SW North American continental basement sources.
3 Earliest Cretaceous time
3.1 Pacificward relative displacement of the Klamath
Mountains salient
Figure 4a shows that the Klamath Mountains stack of allochthons currently is located well outboard of the trend of
the Sierra Nevada Range of northern California, as well as
the Blue Mountains of SE Oregon. Palinspastic reconstruction combined with clockwise rotation of 20° of the
Klamath salient as shown in Fig. 4b, would restore the
complex to the end-of-Jurassic, presumably curvilinear
Andean margin. Such a back-rotation also would help
minimize the seemingly larger offset separating the Klamaths from the Blue Mountains. However, this simplified
restoration does not address the arcuate bulge (i.e., eastward concavity) which likely reflects *50–100 km of
additional strain induced by differential slip of the Klamath
Mountains salient relative to the adjacent segments of the
once-continuous volcanic-plutonic arc.
Just how relative displacement of the Klamath
tectonostratigraphic belts took place remains obscure, but
constraints provided by the sedimentary section suggest a
mainly earliest Cretaceous (*140-136 Ma) westward offset and minor counterclockwise rotation of the salient
relative to the Sierran Foothills (Ernst 2012). Upper
Jurassic Galice-Mariposa, and uppermost Jurassic-lowermost Cretaceous Myrtle strata rest unconformably on the
western flanks of the Klamath Mountains and Sierran
Foothills segments of the emergent Andean arc whereas, as
pointed out below, post-140 Ma Great Valley ? Hornbrook clastic units lie outboard of the Sierran arc but inboard of the Klamath salient. Whether the Klamath
Mountains moved westward, or the Sierra Nevada ? Blue
Mountains moved eastward, or both tracts were displaced,
is unclear. Here, only the differential slip and its timing are
Geologic relationships among units of the Klamath
Mountains province, the Franciscan Complex, and the
GVG in the vicinity of the Yolla Bolly triple junction
(Jones and Irwin 1971; Sliter et al. 1984; Blake et al.
1999) include several important fault systems. In addition
to the major terrane bounding, NW-trending South Fork
and Coast Range faults, Blake et al. (1999) mapped numerous transverse breaks transecting GVG stratigraphic
units in this area. Significant faults include, from north to
south, the Oak Flat, Sulphur Spring, Cold Fork, and Elder
Creek structures. The Oak Flat-Sulphur Spring fault zone
strikes ENE and is properly oriented to accommodate the
conjectured earliest Cretaceous sinistral slip in northern
California, although the offset is only *80–100 km. To
the south, Wright and Wyld (2007) interpreted the NWtrending Cold Fork-Elder Creek fault zone as an important discontinuity defining several hundred kms of dextral
slip. Judging by geologic field relations documented by
Blake et al. (1999), the Cold Fork-Elder Creek fault zone
and subparallel structures truncate the Oak Flat-Sulphur
Spring faults. Thus, oceanward relative displacement of
the Klamath Mountains salient along the Oak Flat-Sulphur Spring fault zone apparently was mainly completed
in Valanginian-Hauterivian time, prior to the right-lateral
motion described by Wright and Wyld—which therefore
represents Barremian and younger offset. Thicknesses of
the Lower Cretaceous GVG units in the Yolla Bolly triple
junction area monotonically increase northward, so these
breaks underwent at least some slip to as late
as *125 Ma (Constenius et al. 2000; Wright and Wyld
Perhaps due to the complex sequence of strike-slip and
subduction-exhumation compound motions that occurred
in the vicinity of the Yolla Bolly triple junction, the published aeromagnetic anomaly map of this area (Roberts and
Jachens 1999, Fig. 2) fails to show clear paleomagnetic
evidence supporting a major discontinuity that could be
linked to a postulated deep-seated sinistral offset along the
Oak Flat-Sulphur Spring zone.
Chin. J. Geochem. (2015) 34(2):123–142
(a) post-136 Ma:
(b) pre-140 Ma:
(thick oceanic
sprea rc
(future path
of thin,
(thick oceanic
160 km
160 km
Fig. 4 Diagrammatic sketch of the hypothesized underflow at *140-136 Ma of a segmented Farallon plate beneath the North American margin (Ernst
2012). a At the beginning of Cretaceous time, a thin, warm slab of oceanic lithosphere slidbeneath the Klamath Mountains, largely decoupled from the
overlying section of gently east-dipping CCW-rotating crustal allochthons. Thicker, older Farallon plate segments on both north and south were strongly
coupled to the continental margin, resulting in contraction and deformation of the accreted collages into relatively steeply dipping sections. Arrows show
direction of relative crustal slip ± possible backarc extension. Bounding transforms of the Farallon oceanic lithosphere have subparallel, ENE trends
constrained by fault offsets in the pre-existing curvilinear arc. b Inferred palinspastically restored original Sierran-Klamath-Blue Mountains arc prior to
the end-of-Jurassic offset, including 20° clockwise rotation of the Klamath salient (such CW rotation reduces the apparent offset between the Klamaths
and the Blue Mountains). No attempt has been made to undo the accumulated strain caused by frictional drag during the postulated slip that produced the
westward arcuate bulge of the salient
3.2 Hypothesized Klamath salient displacement
The Early Cretaceous Farallon lithospheric plate evidently
approached the western edge of California in a largely
convergent but dextral transpressive fashion (Engebretson
et al. 1984; May and Butler 1986; Schettino and Scotese
2005; Sager 2007; Doubrovine and Tarduno 2008). It appears to have fragmented into several smaller east-dipping
plate segments (Wang et al. 2013). Plate-tectonic models
accounting for the structure and offset of the Klamath
Mountains relative to the Andean arc require impingence
of far-traveled oceanic plate transporting, for example:
(a) a mantle plume; (b) a spreading ridge; (c) a thermal
high generating backarc extension; (d) a microcontinent or
island arc; or (e) an oceanic plateau. After consideration
and rejection of this diverse set of geologic models, Ernst
(2012) instead proposed a different scenario: (f) two subparallel transform faults bounding a thin, relatively warm
oceanic slab.
Shown schematically in Fig. 4, this model postulates the
underflow of a relatively young slab of the Farallon plate
Chin. J. Geochem. (2015) 34(2):123–142
beneath the Klamath Mountains beginning at *140 Ma.
The warm, plastically deformable platelet, bordered on
both north and south across hypothesized ENE-trending
transform faults by old, cold, much thicker, stiffer oceanic
lithosphere, would have been largely decoupled from the
Klamath stack of gently east-rooting crustal allochthons.
Collision of thick oceanic lithosphere on both north and
south could have been responsible for contraction and
eastward displacement of the North American continental
marginal relative to the Klamath Mountains, which would
thus assume its salient configuration. Moreover, shortening
in the Sierran arc might have caused rotation of the Foothills terrane collage to the present stack of near-vertical
imbricate sheets. Whatever the origin of outboard relative
displacement of the Klamath Mountains at *140-136 Ma
relative to the Sierra Nevada Range and the Blue Mountains, it seems likely that segmented eastward underflow
responsible for the architecture that developed in the crust
of northern California was sited in the end-of-Jurassic to
earliest Cretaceous upper mantle lithosphere and subjacent,
flowing asthenosphere.
4 Early Cretaceous-Miocene
4.1 Great Valley Group clastic sedimentation
West of the Cretaceous Sierran arc, the slightly to somewhat younger Great Valley Group clastic strata exhibit a
largely Sierran-Klamath provenance based on sedimentary
petrofacies analysis (Ingersoll 1978, 1979, 1983; Linn et al.
1992). The well-developed GVG forearc, laid down on the
Coast Range Ophiolite and the inboard edge of the Klamath province, began receiving detritus by Valanginian
time. Except for far-traveled basal sandstones that contain
distinctive CRO debris ± a cap of very-fine-grained deepsea turbidites of Tithonian age, the younger quartzofeldspathic section was derived chiefly from the Middle Jurassic
to Late Cretaceous igneous arc. Abundant volcanic clasts
typify many of the lower GVG beds, whereas in general,
Upper Cretaceous strata are richer in plutonic quartz and
alkali feldspars (Dickinson and Rich 1972; Dickinson et al.
1982; Surpless in press). The largest volume of Great
Valley sedimentary strata formed during Late Cretaceous
time, especially in the San Joaquin section (Mansfield
1979; Moxon 1990). GVG sandstones include widespread
but small numbers of zircon grains, typified by igneous U–
Pb ages mostly in the *175-140 and 120-60 Ma ranges
(DeGraaff-Surpless et al. 2002; Surpless et al. 2006;
Wright and Wyld 2007; Sharman et al. in press). Minor
concentrations of Early Cretaceous magmatic zircons in the
basal clastic sediments suggest that shallow intrusives and
the more voluminous Jurassic extrusive arc units in the
Sierra Nevada-Klamath belt supplied debris to the initial
forearc. Great Valley Group deposition evidently began
at *140 Ma, with detritus largely derived from the inboard igneous arc. Because the Sierran magmatism died
by *80 Ma, some of the youngest GVG igneous zircons
probably had a more northerly source.
A few pre-Mesozoic, multicycle grains were reported from
basal GVG sandstones by DeGraaff-Surpless et al. (2002) and
Wright and Wyld (2007). These zircon ages provide Precambrian peaks at 1,000–1,200, 1,400, 1,800–2,000, and
2,600 Ma, suggesting that minor ultimate sources including
the Grenville, Mazatzal-Yavapai, and Wyoming or Superior
4.2 Hornbrook Formation clastic sedimentation
The mid- and Upper Cretaceous Hornbrook Formation,
correlative with the far more voluminous Great Valley
Group, rests with angular unconformity on the landward
margin of the Klamath Mountains near the CaliforniaOregon border (Sliter et al. 1984; Nilsen 1993; Surpless
and Beverly 2013). Like the GVG strata of northern
California, the Albian and younger Hornbrook Formation
rests on the eastern edge of the province, so clearly accumulated after seaward relative displacement of the Klamath
promontory. Detrital zircon U–Pb spectra indicate that
clastic materials were supplied chiefly by the nearby
Sierran and Klamath volcanic-plutonic arcs ± possible
igneous sources in the Pacific Northwest, with only minor
contributions from recycled debris originally sourced in
Grenville, Proterozoic and latest Archean basement
(Surpless and Beverley 2013).
4.3 Franciscan Complex clastic sedimentation
Three major, fault-bounded tectonic belts of the Franciscan
Complex, consisting dominantly of clastic sedimentary
strata, crop out in northern California—the Eastern, Central
mélange, and Coastal belts (Bailey et al. 1964; Blake et al.
1988; Jayko et al. 1989; McLaughlin et al. 1994, 2000).
The Eastern Belt comprises two principal lithotectonic
units, the Pickett Peak and structurally lower Yolla Bolly
terranes, whereas the Coastal Belt contains three major
entities, the inboard, structurally higher Yager, medial
Coastal, and outboard, structurally lower King Range terranes (Blake et al. 1988). Although tectonic and olistostromal mélanges characterize the Central Belt, mélanges
also are present in all three Franciscan belts (Cowan 1978;
Raymond 1984, in press; Aalto 2014). Rocks of these three
accretionary belt assemblies apparently were deposited on
far-traveled oceanic crust as it neared the continental
margin (Ernst 1965, 2011). Petrofacies analyses of graywacke-shale and rare conglomeratic units of the Central
and Eastern belts indicate derivation chiefly from the
northern Californian Andean arc (Dickinson et al. 1982;
Seiders 1983), similar to clastic strata of the directly inboard GVG. Terranes of the Coastal Belt contain similarly
sourced debris but also include clasts derived in part from
the Pacific Northwest (see below).
Most detrital zircons separated from Eastern Belt
sandstones possess Jurassic igneous SIMS U–Pb ages
of *180-160 Ma (Dumitru et al. 2010), but a fewigneous
zircon grains occupy the *120-80 Ma age range (Joesten
et al. 2004; Tripathy et al. 2005; Unruh et al. 2007). The
oldest Franciscan clastic unit is the Skaggs Spring Schist
(Wakabayashi and Dumitru 2007; Snow et al. 2010), so
deposition of the Franciscan apparently began
at *140 Ma. Prior to recent work, Late Jurassic-Early
Cretaceous age assignments for the Eastern and Central
belt strat a relied on the presence of the bivalve, Buchia,
but these occurrences probably reflect redeposited macrofossils, as documented by Dumitru (2012). The source of
such transported specimensmight have included Upper
Jurassic proximal facies rocks of the Mariposa-Galice
overlap sequence. In any case, most of the Eastern Belt
Yolla Bolly quartzofeldspathic units accumulated during
the mid- and Late Cretaceous (120-80 Ma; Ernst et al.
2009b; Dumitru et al. 2010), and were exhumed and exposed shortly thereafter (Mitchell et al. 2010). Sited in
progressively more seaward positions, the Central and
Coastal belts, for which zircon U–Pb age data have now
become available (Dumitru et al. 2013, in press), have Late
Cretaceous (*90-60 Ma) and Tertiary (65-20 Ma) maximum depositional ages, respectively. The presence of
young igneous zircons in clastic rocks of the Coastal Belt
indicate progressive sedimentary supply to the Yager,
Coastal, and King Range terranes of detrital zircons
derived from the Idaho Batholith, Challis volcanic pile, and
Cascade arc.
In addition to post-Paleozoic arc sources, analyzed zircons from Franciscan rocks exhibit small Precambrian age
peaks as follows. Pickett Peak terrane: 1,000, 1,400-1,600,
1,800, and 2,200 Ma (Dumitru et al. 2010). Yolla Bolly
terrane: Pacheco Pass (Ernst et al. 2009b) 1,350, 1,800,
2,600–2,900 Ma; San Francisco Bay area (Snow et al.
2010) 1,500–1,700, 2,000, 2,500 Ma; NW Coast Ranges
(Dumitru et al. in press) 1,300–1,400, 1,800, [ 2,500 Ma.
Except for a few Grenvillian zircons in analyzed Eastern
Belt Skaggs Spring-Pickett Peak metasandstones, these age
data suggest ultimate derivation of the more voluminous
Yolla Bolly strata in part from the Mazatzal-Yavapai and
Wyoming or Superior basement provinces. Central Belt
strata include zircon ages of 1,300–1,400, 1,600–1,750 Ma,
indicating minor Mazatzal-Yavapai Middle Proterozoic
sources, but lacking Late Archean provenance; Coastal
Belt zircons yield Precambrian ages peaking at
Chin. J. Geochem. (2015) 34(2):123–142
1300-1400 Ma, suggesting an orogenic granite sources
from the Mazatzal-Yavapai realm. Thus, except for
the *140 Ma Skaggs Spring and slightly younger Pickett
Peak metagraywackes, Franciscan clastic rocks lackzircon
grains of Grenvillian affinity.
5 HP/LT recrystallization of the Franciscan Complex
Franciscan Eastern and Central belt sandstones display
pervasive effects of HP/LT subduction-zone metamorphism, as widely documented in the Cretaceous Franciscan
sections of northern California (Cloos 1982, 1986; Blake
et al. 1988; Jayko and Blake 1989; Jayko et al. 1989;
Wakabayashi and Dumitru 2007). In contrast, presently
exposed clastic units of the chiefly Tertiary Coastal Belt
only show the effects of feeble, low-T, low-P recrystallization (Bachman 1978; Underwood et al. 1987; Blake
et al. 1988; Dumitru 1989; Tagami and Dumitru 1996;
Ernst and McLaughlin 2012). Deeply buried Lower Cretaceous strata of the Great Valley Group and the moderately recrystallized volcanogenic Mariposa-Galice units
exhibit neoblastic mineral parageneses similar to those
typifying Franciscan Coastal Belt rocks (Dickinson and
Rich 1972; Gray 2006; Snow and Scherer 2006). However,
the Mariposa-Galice units show strong grain flattening and
platy metamorphic cleavage, unlike weakly metamorphosed strata of the Coastal Belt.
Prograde phase relations and schematic physical conditions for Franciscan Eastern, Central, and Coastal belt
rocks, and essentially also for the GVG, Mariposa and
Galice formations, are illustrated in Fig. 5. The
metasedimentary rocks of the Franciscan Eastern coherent
and Central mélange belts display prograde HP/LT
geothermal gradient paths of 100–300 °C 5–8 kbar as
typically followed by units subjected to subduction-zone
P–T conditions, whereas in contrast, other sandstone sections summarized in this review simply appear to show the
effects of diagenesis common in rocks involved in low-P
burial. Because subduction-zone refrigeration continued
during the episodic return toward the surface of Eastern
and Central belt Franciscan sections, their retrograde (i.e.,
decompression) P–T trajectories more-or-less followed
their prograde paths in reverse, but at slightly lower pressures for a given T.
In addition to the in situ post-Jurassic, dominantly
metasedimentary Franciscan section, lenses of much higher
grade eclogite and garnet glaucophane schist are present as
rare, but mineralogically spectacular phase assemblages.
These high-grade blocks of mid- to Late Jurassic metamorphic age are relatively well studied (Coleman and
Lanphere 1971; Wakabayashi 1992; Anczkiewicz et al.
2004; Wakabayashi and Dumitru 2007; Ukar et al. 2012).
Chin. J. Geochem. (2015) 34(2):123–142
Ar c
Pressuere in kbar
B l
An + Qtz
Temperature in °C
Fig. 5 Phase diagram for northern California Franciscan metagraywacke compositions, partly after Terabayashi and Maruyama (1998,
Fig. 7). Pfluid is assumed equal to lithostatic pressure. Stability fields for
heulandite, laumontite (Laum), lawsonite and wairakite are from Liou
(1971), the calcite-aragonite (Cc-Ar) transition is from Carlson (1983)
and the low albite-jadeite ? quartz phase boundary (LAb-Jd ? Qtz) is
from Newton and Smith (1967). Also shown are the Frey et al. (1991)
computed P–T stability fields for prehnite (Preh) and pumpellyite
(Pum) in metabasaltic rocks (Liou et al. 1983). An = anorthite.
Prograde metamorphic P–T paths for the Franciscan belts are from
Ernst (1993) and Ernst and McLaughlin (2012), extended to include P–
T conditions attending recrystallization of the high-grade metabasaltic
blocks. Retrograde P–T paths are not shown. Deeply buried GVG strata
and weakly transformed Mariposa-Galice units have new phase
assemblages comparable to those of the Franciscan Coastal Belt
Initially solidified as Farallon oceanic crust far from the
North American margin, these rocks recrystallized in a
relatively young, warm oceanic-continental convergence
zone along an unrefrigerated, relatively warm mantle
hanging wall, as reflected by HP/LT mineral parageneses
indicating counterclockwise decompression P–T-time trajectories (Cloos 1982, 1986; Wakabayashi 1990, 1999;
Saha et al. 2005; Page et al. 2007; Ukar and Cloos 2014).
Most high-grade tectonic blocks were transformed
at *10–12 kbar, and *400–600 °C (Fig. 5), but some
evidently formed at even higher P–T ranges (e.g., Krogh
et al. 1994; Tsujimori et al. 2006).
In contrast to their relatively well-understood petrogenesis, field occurrences of these high-grade metamafic
blocks are problematic, reflecting an obscure geologic
context. Most such coarse-grained, penetratively deformed
rocks rest on the Earth’s surface with no apparent genetic
relationship to the regionally extensive, distinctly lower
metamorphic grade Franciscan lithologies. In some cases,
the bedrock consist of serpentinite, more commonly of
mud-matrix mélange, or a mixture of pelitic and serpentinitic matrix materials. How the high-grade metamafic
blocks and spatially associated Franciscan, chiefly
metasedimentary section were exhumed is a matter yet
debated (e.g., Ernst 1971; Platt 1986, 1993; Ring and
Brandon 2008). Some HP metabasalts exhibit nearly
complete rinds of actinolite ± chlorite ± talc that are
slightly younger than the high-grade blocks (Moore 1984;
Catlos and Sorensen 2003; Ukar 2012; Ukar et al. 2012). In
the rare cases where Jurassic metamafic blocks are clearly
enveloped in surrounding fine-grained mud-matrix or serpentinite bodies, the latter are substantially younger (e.g.,
mid- to Late Cretaceous). Detailed histories of the exotic
HP/LT blocks provide additional constraints on the Jurassic-Cretaceous convergent margin evolution of northern
California and development of the accretionary Franciscan
Complex during a period typified by oblique-to-orthogonal
plate convergence (Ernst 2011).
6 Clastic strata of northern California: ages
and provenance
A relatively continuous record of mid-Jurassic through
early Miocene sedimentation is preserved in the sandstones
cropping out in northern California. Approximate maximum depositional age spans of these units, largely constrained by detrital zircon U–Pb data, are: North
Fork ? Eastern Hayfork, 175-165 Ma; Mariposa ? Galice, 165-150 Ma; Myrtle, 150-140 Ma; Great Valley
Group ± Hornbrook, 140-60 Ma; Franciscan Eastern
Belt, *140-80 Ma; Central Belt mélange, *90-60 Ma;
Coastal Belt, 65-20 Ma. Except for GVG and Hornbrook
strata situated directly inboard from the Klamath Mountains salient, maximum depositional ages of these
tectonostratigraphic units systematically decrease seaward.
Detrital zircons separated from the studied rocks were
sourced dominantly from pre-existing igneous rocks, chiefly
Mesozoic granitoids and their comagmatic volcanic
equivalents. Generalized temporal relationships between
these primary igneous sources and derived sedimentary
products considered here are shown on Fig. 6. Precambrian
zircon grains are rare, but of those present, most are well
rounded, and undoubtedly underwent multiple cycles of
erosion, transportation, and deposition. Prior to deposition in
their present host rock, most probably last resided in Paleozoic clastic strata derived from the Precambrian basement. Interpreted ultimate source terranes for the described
detrital units, illustrated in the map of Fig. 7, are as follows:
Mariposa-Galice ± Myrtle = Sierran-Klamath arc, Grenville, Mazatzal-Yavapai, and older SW North American
cratons; basal Great Valley Group and Franciscan Skaggs
Spring Schist-Pickett Peak terrane = Sierran-Klamath arc,
Chin. J. Geochem. (2015) 34(2):123–142
Cascade arc rocks
ldsp e rock
qua ous so
Challis volcanic rocks
Idaho Batholith rocks
Cretaceous Sierran arc rocks
Jurassic Klamath + Sierran arc rocks
Coast Range Ophiolite
Cenozoic GVG + Franciscan Coastal Belt
Main GVG + Yolla Bolly + Central Belt
Mariposa + Galice formations
y un
Basal GVG + Skaggs Spring-Pickett Pk
North Fork + Eastern Hayfork terranes
Age in Ma
>2.50 Ga
Fig. 6 Schematic diagram of mid-Mesozoic and younger California-Pacific Northwest igneous arcs inferred to have provided much of the
quartzofeldspathic debris to the sandstones treated here. Ages of volcanic-plutonic generation, the CRO, and largely arc-derived clastic
sedimentary strata are summarized from Irwin (2003), Shervais et al. (2005), Wakabayashi and Dumitru (2007), Hopson et al. (2008), Scherer
and Ernst (2008), Snow and Ernst (2008), Snow et al. (2010), Ernst et al. (2009a, b, 2010), and Dumitru et al. (2010, 2013, in press). Zircon U–Pb
age data are not available for the uppermost Jurassic to lowermost Cretaceous Myrtle clastic strata, but on this diagram it would lie
stratigraphically above the Mariposa-Galice formations, and below-to-coincident with the basal GVG
Superior >2.50 Ga
~35 Ma
1.70 1.70 G
1.03-1.19 Ga
0. ppa
36 la
-0 ch
.7 ian
~70 Ma
0. og
27 e
-0 c
.6 lin
4 e
~100 Ma
Fra plex
0.40-0.58 Ga
1000 km
0.54-0.68 Ga
Ancestral Rockies
1.40-1.48 Ga granitoids
1.34-1.40 Ga granitoids
1.00-1.25 Ga
terranes and
arcs <0.20 Ga
Mexico arc
0.23-0.29 Ga
Fig. 7 Basement map of some North American geologic provinces and their ages of formation, after Dickinson and Gehrels (2009, Fig. 1) and
Gehrels et al. (2011, Fig. 7). Heavy black arrows show inferred westward transport directions of erosional debris shed from Cordilleran volcanicplutonic arcs during the *140-25 Ma accumulation of the Franciscan Complex. Detrital zircon ages suggest possible NW drift of the trench fill
as sedimentation-accretion continued and new volcanic-plutonic arcs lit up on the north. A progressive increase over time in the arrival of
detritus from younger NW igneous arcs could equally well explain the changing source patterns recorded in native (i.e., non-drifting) Franciscan
Chin. J. Geochem. (2015) 34(2):123–142
Grenville, Mazatzal-Yavapai, and late Archean basement;
main GVG ± Hornbrook = Sierran-Klamath arc, Idaho
Batholith, minor Grenvillian, Mazatzal-Yavapai, and Wyoming or Superior cratonal sources; Franciscan Yolla Bolly
terrane = Sierran-Klamath arc, Mazatzal-Yavapai and
Wyoming or Superior basement; Franciscan Central Belt
mélange = Sierran arc ± Idaho Batholith, MazatzalYavapai basement; Franciscan Coastal Belt = Sierran arc,
Idaho Batholith, Challis-Cascade volcanic units.
7 Mid-Jurassic-early Tertiary evolution of northern
The new detrital zircon U–Pb data summarized here support previous radiometric, geologic, structural, and paleontologic studies on a broad range of post-Paleozoic
sandstones, further documenting the crustal growth of
northern California. Middle and Upper Jurassic clastic
strata draped over the western flanks of the Klamath ? Sierra Nevada orogenic belts, the Cretaceous GVG lying
along the westernmost margin of the Sierra Nevada but
along the eastern edge of the Klamath Mountains, and the
yet farther outboard Tertiary sedimentary sections all attest
to a gradual seaward growth of the continent. The Mariposa-Galice ± Myrtle formations define the Pacific margin
of sialic crust at 165-150 Ma to possibly as young
as *140 Ma. In northernmost California, Hauterivian
Great Valley Group and Albian Hornbrook strata lying
inboard from the Klamath Mountains document the time of
essential completion of oceanward relative displacement of
the salient. To the south, the suture between the basal GVG
forearc and the Franciscan trench complex defines the
position of the convergent plate junction after *140 Ma.
Varying degrees of convergence regionally lasted into the
Miocene. As known from prior work (e.g., Bailey et al.
1964), sedimentary ages of the Franciscan imbricate
tectonostratigraphic terranes decrease toward the Pacific
Ocean. Maximum times of deposition and nearly coeval
tectonic accretion range from Early to Late Cretaceous for
the east-rooting Pickett Peak and tectonically underlying,
younger Yolla Bolly terrane metagraywackes of the Eastern Belt, through overlapping latest Cretaceous deposition
and recrystallization for Central Belt metasandstones interstratified with mud-matrix mélanges, to Paleogene-early
Miocene for structurally lower Coastal Belt sandstones.
Jurassic volcanic-plutonic ? Cretaceous Sierran Batholith, Late Cretaceous-Paleocene Idaho Batholith, early
Eocene Challis volcanics, and Oligo-Miocene Cascade arcs
supplied most of the igneous zircons to the clastic strata
described here. Although the Middle and Upper Jurassic
proximal sequences contain substantial contributions
derived from Grenvillian source terranes, 1,000–1,200 Ma
zircons appear to be rare in most GVG strata (Surpless in
press), and are absent from all but the oldest Franciscan
metasedimentary units—i.e., the Skaggs Spring-Pickett
Peak terrane. Cretaceous emergence of the growing Sierranvolcanic-plutonic arc may have blocked the outboard
regional supply of recycled Grenvillian materials from the
continental interior. Overthe course of time, the Franciscan
Complex clearly began receiving greater proportions of
young igneous arc detritus from more northerly sources.
GVG sandstone petrofacies analyses show an analogous
south-to-north trend from basement uplift—[ dissected
arc—[ transitional arc—[ undissected arc (Ingersoll
2012). Older Franciscan bedrock sources on the SE included the Mazatzal-Yavapai 1,400 Ma anorogenic granites and 1,700–1,800 Ma basement terranes. Grenville and
Appalachian igneous zircons are missing from the Franciscan section deposited after *120 Ma, suggesting the
possibility of as much as 1,600 km for progressive, postdepositional NW offset of the trench deposits relative to the
Great Valley Group forearc and craton of the SW conterminous U. S. and NE Mexico. Figure 7 shows the hypothetical extent of dextral offset of the Franciscan Complex
attending its post-120 Ma accumulation (see also Jayko
and Blake 1993).
Other explanations instead may account for the absence
of Grenville zircons in Franciscan mid-Cretaceous to lower
Miocene clastic sediments. These include: (1) non-erosion
of Grenvillian rocks due to cover and/or low elevation; (2)
intervening topographic divides diverting the transport of
detritus away from the Pacific margin; (3) channeling of
arc debris by a few river systems that fluctuated in flow
direction over time; (4) sample bias and/or SIMS analytical
error. Provenance studies described here for northern
California indicate coeval deposition and possible later
NW translation of the Franciscan section; *1,600 km is
merely a reasonable value for the conjectured offset. Alternatively, a systematic NW increase in the supply of
detritus from progressively younger volcanic-plutonic arcs
might have been solely responsible for the changing source
patterns recorded in these tectonostratigraphic terranes as
igneous belts to the south died whereas others to the north
became active.
8 Mid-Jurassic to Miocene northern California
Before Middle Jurassic time, chiefly ophiolite ? chertargillite terranes arrived at, and were stranded along the
continental margin, reflecting dominantly transform ± transpressive plate motions during the mid-Paleozoic to Early Jurassic assembly of northern California
(Saleeby 1981, 1982, 1983; Silberling et al. 1987; Ernst
(a) ~165 Ma:
Mariposa-Galice fms
Andean arc
converging CRO
N. Amer
m igh
et -g
ab ra
as de
(b) ~150 Ma:
converging CRO
± Myrtle fms
Andean arc
N. Amer
m igh
et -g
ab ra
as de
have been a crustal response in the Klamaths to the arrival and
underflow of a segmented Farallon plate. Most of the differential slip occurred prior to Early Cretaceous deposition of the
GVG-Hornbrook strata along the landward SE ? NE edges
of the Klamath Mountains, respectively (Figs. 1, 2). Modest
additional sinistral displacement evidently continued across
the Oak Flat-Sulphur Spring fault zone during Early Cretaceous time (Ernst 2012). Schematic relationships among the
Jurassic volcanic-plutonic Andean arc rocks, and depositional
histories of the Mariposa-Galice ± Myrtle sediments are
diagrammed in the cross-sections of Fig. 8, reflecting the
tectonic configuration of northern California prior to the
postulated end-of-Jurassic oceanward step-out of the Farallon
lithospheric plate. After this *140 Ma step-out, spatial relationships involving the Cretaceous Sierran Batholith, Great
Valley-Hornbrook, and Franciscan clastic strata are depicted
in the cross-sections of Fig. 9.
Late Jurassic sequestration of HP/LT metamafic schists
at moderate depths along the mantle wedge and later tectonic transference of exotic blocks and lenses into buoyant
mélanges ascending along the subduction channel (Cloos
1982, 1986; Cloos and Shreve 1988) of the progressively
less steeply east-dipping oceanic plate well after
the *140 Ma step out are also illustrated schematically.
et al. 2008). This older oceanic terrane collage was capped
by mid-Jurassic and younger sedimentary and volcanic
rocks derived from an emergent igneous arc constructed
along the continental edge. Middle and Late Jurassic Andean volcanism-plutonism in the Klamath ? Sierra Nevada ranges (Dunne et al. 1998; Irwin 2003; Dickinson
2008) reflects an important component of eastward subduction of the Farallon oceanic plate commencing no later
than *175 Ma, and continuing until *140 Ma. Underflow also resulted in the generation and storage at depth of
HP/LT metamafic rocks, now present as tectonic ± olistostromal blocks in mélanges chiefly of the Franciscan
Central Belt (Cloos 1982, 1986; Wakabayashi et al. 2010).
Middle and Late Jurassic oceanic plate motion probably
involved oblique convergence rather than nearly orthogonal subduction (Ernst et al. 2008), because if a major
forearc basin and trench had formed during this stage,
evidence of this subparallel couplet has since disappeared
completely. Alternatively, a substantial component of
transform slip or subcrustal erosion (Wright and Wyld
2007; Scholl and von Huene 2007; Stern and Scholl 2010;
Dumitru et al. 2010) might removed all but the farthest
inboard Mariposa-Galice ± Myrtle overlap strata.
In the Franciscan Complex, the famous 170-155 Ma highgrade metabasaltic eclogites and garnet-blueschists formed
during early stages of the underflow that generated the
emergent mid- and Late Jurassic Klamath-Sierran arc as well
as the derivative Mariposa-Galice proximal sedimentary
aprons. The HP/LT mafic tectonic blocksevidently were
sequestered at moderate depths, predating onset of the
lengthy period of subduction that produced the paired GVG
and Franciscan sedimentary belts. In general, intrusion
of *170 Ma granitoid bodies in the western Klamath
Mountains and progressive geographic restriction of
younger plutons to the more easterly Klamath lithotectonic
belts (Hacker et al. 1995; Irwin and Wooden 1999; Irwin
2003) suggest that seaward migration of the salient might
have started earlier, perhaps at *155-140 Ma, as the crustal
assembly of ophiolitic terranes and superjacent clastic ? volcanogenic rocks gradually began to migrate off the
deep-seated magmagenic zone along or directly above the
descending paleo-Pacific plate. This west-directed oblique
offset of the imbricated Klamath belts apparently occurred
during a relatively brief period characterized by widespread
left-lateral slip along the western margin North America
(Saleeby 1992; Saleeby et al. 1992).
Attending Late Jurassic termination of Mariposa-Galice ± Myrtle sedimentation, the Klamath promontory rotated *20° counterclockwise and moved outboard *100–
150 km (? arcuate deformation of 50–100 km) relative to the
formerly contiguous Sierran arc (e.g., Coleman et al. 1988;
Constenius et al. 2000; Ernst 2012). The manner in which the
tectonic offset was accomplished remains unclear, but may
Chin. J. Geochem. (2015) 34(2):123–142
Fig. 8 a Diagrammatic depths of recrystallization, and b later, ascent
and modest-depth storage of high-grade metabasaltic rocks of the
descending Farallon plate. Relationships exaggerated for clarity are
shown before stranding of pre-existing oceanic lithosphere as the
Coast Range Ophiolite inboard from the ~140 Ma plate junction. The
mechanism allowing the sequestered HP metabasaltic material to
ascend along the plate junction (arrows) may have involved
transportation as blocks in buoyant serpentinite
Chin. J. Geochem. (2015) 34(2):123–142
(a) ~90 Ma:
Klamath salient
me h-gra
tab de
Eastern Franciscan
Central Franciscan
High-grade blocks
(b) ~90 Ma:
Sierran flare-up
N. Amer
me h - g r a
tab d e
Fig. 9 Schematic introduction of high-grade metamorphosed oceanic
crustal blocks into the Franciscan Complex outboard from a the
Klamath Mountains and b the central Sierra Nevada Range. Sustained
underflow of progressively younger, warmer Farallon lithosphere
resulted in a gradually decreasing lithospheric plate dip. Two-way
flow within the subduction zone is indicated. Note insertion of HP
tectonic blocks into the voluminous, low-density, Upper Cretaceous
Franciscan circulating mud matrix and net upward transport.
Although the thickness of the circulating mélange zone in the Central
Belt is exaggerated for clarity, it probably consisted of a progressively seaward-younging series of much thinner subduction channels,
judging from mapped tectonic imbrications. Also sketched are
olistostromal blocks probably carried surfaceward by serpentinite
diapirs (not shown), and introduced into the Franciscan section
through erosion, transportation, and sedimentary deposition
Insertion of the high-grade tectonic blocks involved a
process whereby traction of circulating subduction-zone,
low-density mélange against an overlying stable lithospheric plate induced shearing and plucking of HP/LT
tectonic blocks and lenses previously stored along the
mantle hanging wall into a Cloos-type subduction channel.
How these Jurassic HP/LT units previously had ascended
to shallower storage depths remains unclear, but metasomatic actinolitic rinds on many tectonic blocks suggest
their early-stage inclusion in buoyant serpentinites. Prior to
the *125 Ma onset of nearly orthogonal, rapid subduction
and return flow of large volumes of Central Belt mudmatrix mélange, plate-margin shearing apparently was insufficient to cause the tractive insertion of dense metamafic
blocks into the subduction channel.
During mid-Paleozoic to mid-Jurassic time, predominantly margin-parallel differential slip involved the
episodic docking of ophiolitic complexes along the
continental edge. In contrast, Andean-type arc magmatism
(Barth et al. 2013) was vigorous over the period *175140 Ma, and became paroxysmal during mid- and Late
Cretaceous time (*125-80 Ma) during the change from a
southward to a northward tangential component of drift of
the Pacific-Farallon plate junction (Engebretson et al. 1984;
May and Butler 1986; Schettino and Scotese 2005; Sager
2007; Doubrovine and Tarduno 2008). U–Pb ages of detrital zircons document the presence of relatively small
volumes of Lower Cretaceous clastic strata, and in general,
much larger masses of Upper Cretaceous sediments in the
Franciscan Complex. Based on these relationships, Dumitru et al. (2010) proposed that, reflecting the change in
relative plate motions, coastal California transitioned from
a non-accretionary to an accretionary margin at *123 Ma.
Although plausible, nearly head-on convergence also
would result in larger tracts of lithosphere descending
through the magmagenic zone—and increasing generation
of arc magmas—hence rapid, nearly orthogonal mid- and
Late Cretaceous plate convergence might equally well have
been responsible for the *125-80 Ma flair-up in igneous
arc activity and consequent production of voluminous
Upper Cretaceous GVG and Franciscan clastic units (Blake
et al. 1988; Ernst et al. 2008; Cloos and Ukar 2010).
The Middle to Late Jurassic Andean ? Cretaceous
Sierran arc, and Paleocene-Miocene Idaho Batholith,
Challis complex, and Cascade volcanic rocks supplied
most of the igneous zircons to the Franciscan clastic strata
discussed in this paper. Although Middle and Upper
Jurassic continental margin proximal sequences contain
substantial contributions derived from Grenvillian source
terranes, 1,000–1,200 Ma zircons appear to be sparse in
Cretaceous GVG strata, and are absent from all but the
oldest Franciscan trench units. With the passage of time,
the Franciscan Complex began receiving greater proportions of younger arc detritus from more northerly source
areas. This change in provenance raises the possibility, but
does not require up to *1,600 km of post-120 Ma northwesterly transport of the Franciscan trench complex during
deposition, relative to the Great Valley Group forearc and
tracts of the SW North American basement terranes.
9 A final word
Like many other parts of the world, northern California has
received the detailed investigative attention of geologists,
geochemists and geophysicists for more than a century.
Accordingly, much of the geologic-geophysical framework
and geochemical-petrotectonic development of the crust
briefly sketched in this summary has been well understood
for decades. However, the advent of TIMS and SIMS microanalyses of individual detrital zircon grains and their
aggregate populations has made possible a relatively independent way to study the diverse igneous, metamorphic
and sedimentary lithologic assemblages. Integrated with
spatial knowledge of the region, these data provide new
constraints on the geohistory (i.e., plate tectonic development). This review of independent age and provenance
constraints reflected in the zircon U–Pb data represents just
one example of how the study of new geochemical systems
can increase our understanding of a critical portion of the
Earth—in this example, the post-Triassic convergent plate
margin of northern California. The attempt has been to
elucidate a few principles involving accretionary crustal
growth for Asian Earth scientists unfamiliar with California, because every area is different. However, although the
geologic histories of east Asia and NW California contrast
markedly, the governing principles are likely the same.
Acknowledgments Stanford University supports my field and
analytical studies of Californian lithotectonic belts. The National
Science Foundation provided additional aid through grant NSF EAR
0948676 to Marty Grove. Various workers carried out the detrital
zircons U–Pb age determinations on which this summary in based,
chiefly employing the SHRIMP-RG at the Stanford-USGS MicroAnalysis Center and the LA-ICPMS at the University of Arizona
LaserChron Center. Many of of these works are already published, but
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