Review of mid-Mesozoic to Paleogene evolution of the northern and central Californian accretionary margin
Published:December 28, 2018
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W.G. Ernst, 2018. "Review of mid-Mesozoic to Paleogene evolution of the northern and central Californian accretionary margin", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
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Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.
This paper synthesizes some recent investigations of the regional geology of California, focusing on Triassic–Jurassic terranes making up the western parts of the Klamath Mountains and the Sierran Foothills belt, as well as the Cretaceous–Paleogene Great Valley Group forearc strata and coeval outboard Franciscan trench units. It presents my personal view of the mid-Mesozoic to Paleogene plate-tectonic evolution of northern and central California. The geology of the area, illustrated in Figure 1, constitutes a relatively clear example of a 175–20 Ma accretionary margin that hosted substantial additions of juvenile sialic crust due to subduction-induced partial fusion at magmagenic depths, and ascent and emplacement of primary calc-alkaline igneous arc rocks. Crustal growth also involved secondary processes that provide additional constraints on the regional plate-tectonic history. Combined with geologic, tectonostratigraphic, metamorphic, and petrofacies studies, recent detrital-zircon U-Pb isotopic studies have informed our understanding of the emplacement and significance of clastic strata deposited during a period typified mainly by transpressive to convergent plate motions. Ages of sedimentation, provenance, and postdepositional metamorphic pressure-temperature (P-T) histories yield important insights regarding the Middle Jurassic to Paleogene petrotectonic evolution of northern and central California.
Late Paleozoic to Middle Jurassic terranes in the Klamath Mountains and Sierra Nevada Foothills consist mainly of fault-bounded ophiolite-chert-argillite oceanic crust and igneous arc sequences. Mafic-ultramafic complexes ± overlying cherty units are dominantly far-traveled (exotic) and chiefly oceanic in nature, whereas superjacent fine-grained, deep-water to coarser-grained, shallow-water clastic sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes (Irwin, 2003). Early Mesozoic felsic igneous rocks are volumetrically minor in the Klamath and Sierran realms, but they reflect episodes of transpressive to convergent Triassic–earliest Jurassic plate motions. The Klamath accretionary stack of thrust sheets roots gently to the east; in marked contrast, correlative lithologic entities in the Sierran Foothills belt stand nearly vertically. The Late Jurassic evidently was a time of profound transition in north and central California, when rifting, translation, and stranding of ophiolitic terranes along a continuous curvilinear sialic margin gave way to nearly head-on Cretaceous subduction that resulted in the construction of a massive calc-alkaline arc. Moreover, by ca. 140 Ma, the Klamath salient, composed of a stack of allochthons invaded by chiefly Jurassic granitoids, appears to have migrated westward relative to the magmatic belt, abandoning its position over the deep-seated magmagenic subduction zone. Thus, during Cretaceous time, voluminous calc-alkaline igneous activity was chiefly confined to the Sierra Nevada part of the arc. Erosion of the igneous arc generated the marginal Cretaceous–Paleogene Great Valley Group forearc basin and, more seaward, the dominantly trench and trench-slope basin deposits of the Franciscan Complex.
REGIONAL GEOLOGY OF NORTHERN AND CENTRAL CALIFORNIA
The Klamath Mountains of NW California and SW Oregon preserve a rock record reflecting multiple stages of Phanerozoic convergent, divergent, and transform continental- and arc-margin processes (Fig. 2). Irwin (1960) defined four major lithotectonic units from east to west: the Eastern Klamath plate, the Central metamorphic belt, the Western Paleozoic and Triassic belt, and the Western Klamath belt. The latter includes the Upper Jurassic overlap Galice Formation and slightly younger Myrtle Formation (the term overlap is used here for strata unconformably overlying an older terrane, in contrast to the lithologic assembly comprising an accretionary prism). Each Klamath accretionary belt or terrane assembly constitutes a separately formed lithotectonic sequence, with tops facing east; in general, the oldest clastic rocks flooring each terrane become younger progressively from east to west. Most accretionary map units are juxtaposed along gently east-dipping, west-vergent thrust faults of mid-Paleozoic to Late Jurassic age, and the sections are not overturned (Irwin, 1960; Burchfiel and Davis, 1981). The imbricate stack of Klamath allochthons contains abundant mafic igneous rocks and continent-derived clastic strata, as well as deep-sea Tethyan-affinity chert and limestone, typically resting on structurally dismembered ophiolites (Harper et al., 1994; Frost et al., 2006). Terranes were serially accreted by paleo–Pacific plate subduction during late Paleozoic to mid-Mesozoic time (Irwin, 1981; Hacker et al., 1993, 1995; Wallin and Metcalf, 1998). The western vergence and younging of the tectonostratigraphic units reflect a major eastward component of oceanic plate subduction. Postkinematic granitoid plutons of mid-Jurassic to earliest Cretaceous age (Lanphere et al., 1968; Irwin and Wooden, 1999) then invaded the accretionary orogen.
The Western Paleozoic and Triassic belt is the largest of the four major belts (Fig. 2). It consists of regionally metamorphosed sedimentary, volcanic, plutonic, and ultramafic rocks. In the southern Klamath Mountains, Irwin (1972) subdivided the Western Paleozoic and Triassic belt from east to west into the N-S–trending North Fork, Hayfork, and Rattlesnake Creek terranes. Later, other Western Paleozoic and Triassic belt map units were recognized (e.g., Eastern and Western Hayfork terranes of Wright, 1982; Stuart Fork terrane of Goodge, 1989). Hacker et al. (1993) showed that these map units extend throughout the broad realm of the Western Paleozoic and Triassic belt in the Klamath Mountains. These accreted terranes have long been correlated with analogues in the NW Sierran Foothills based on similar rock types, structures, ages of the lithologic packages, oceanward assembly of successively younger geologic units, and their comparable times of deformation (e.g., Davis, 1969; Davis et al., 1980; Wright and Fahan, 1988; Wright and Wyld, 1994; Irwin, 2003).
Along the western edge of the orogen, the Upper Jurassic turbiditic Galice Formation (Gray, 2006; MacDonald et al., 2006) displays chlorite-zone greenschist-facies metamorphism and penetrative deformation. Its volcanogenic lithologies are similar to those of the Mariposa Formation, a slaty unit in the Upper Jurassic belt of the western Sierra Nevada Foothills (Fig. 2). Miller and Saleeby (1995) reported a 153 Ma depositional age for the upper Galice Formation, but sedimentation may have started during or before earliest Oxfordian time (164–157 Ma), based on biostratigraphic data summarized by Saleeby and Harper (1993). This conclusion is supported by local interdigitation of Galice metaturbidites with pillow lavas of the subjacent 164–162 Ma Josephine Ophiolite (Harper, 2006; MacDonald et al., 2006). Provenance of Galice sandstone evidently was a combination of ancient SW North American basement and mid-Paleozoic to mid-Mesozoic ophiolite and chert-argillite terranes, as well as younger, nearly coeval arc rocks (Snoke, 1977; Frost et al., 2006).
Scattered erosional remnants of uppermost Jurassic–lowermost Cretaceous Myrtle Formation rest unconformably on the Galice Formation in SW Oregon (Imlay et al., 1959; Dickinson, 2008). Like the underlying Galice Formation, detritus in the Myrtle Formation apparently was derived from both the landward Klamath-Sierran arc ± minor, old SW North American continental basement.
Sierra Nevada Foothills
The contractional, amalgamated metamorphic belt of the Sierran Foothills consists of five distinct east-to-west younging lithotectonic belts (Fig. 2): the Northern Sierra terrane, the Feather River terrane, the Calaveras Complex, the Triassic–Jurassic igneous arc belt, and the Middle to Late Jurassic volcanogenic sequence (Snow and Scherer, 2006; Schweickert, 2015). In contrast to correlative terranes in the Klamath Mountains (Davis, 1969; Burchfiel and Davis, 1981), most map units in the Sierra Nevada Foothills are steeply dipping and are juxtaposed across nearly vertical faults. The Northern Sierra terrane, Feather River terrane, and Calaveras Complex were sequentially stranded against the continental-margin forearc by paleo–Pacific plate subduction (Schweickert, 2015). These progressively westward accretionary terranes were sutured before development of the outboard ca. 200 Ma Triassic–Jurassic volcanic belt and younger units. Interpreted as an offshore island arc, these seaward units evidently were sutured against landward Sierran terranes attending consumption of a Molucca Sea–type plate (Ingersoll, 2008, 2012; Schweickert, 2015). The outboard Middle and Late Jurassic arc sequence includes the Mariposa Formation. Like the Galice Formation, Mariposa slates exhibit chlorite-zone greenschist-facies metamorphism and are penetratively deformed. The Mariposa and associated strata represent a postvolcanic sedimentary sequence overlying the Northern Sierra terrane, Calaveras Complex, and Triassic–Jurassic arc belt (Duffield and Sharp, 1975; Bogen, 1985; Sharp, 1988).
Studies in the NW Sierran Foothills have clarified the timing and styles of folding through detailed geochronologic and field investigations (e.g., Duffield and Sharp, 1975; Schweickert and Cowan, 1975; Schweickert et al., 1984; Saleeby et al., 1989; Tobisch et al., 1989; Graymer and Jones, 1994). The Sierran Foothills evidently underwent a protracted history of deformation, with distinct events during the Early to Middle Jurassic (Girty et al., 1995), Late Jurassic (Schweickert et al., 1984; Tobisch et al., 1989), and Early Cretaceous (Tobisch et al., 1989; Wolf and Saleeby, 1995). These contractional stages have been interpreted as due to arc-margin collision (Schweickert et al., 1984; Schweickert, 2015), oceanic-ridge margin collision (Shervais et al., 2004), or changes in relative plate motions between the paleo–Pacific and paleo–North American plates (Saleeby and Dunne, 2015). The paleogeography and tectonic setting of the Mariposa depositional basin have been studied by many workers. Behrman and Parkinson (1978) suggested that the Middle to Upper Jurassic Logtown Ridge and Gopher Ridge volcanic rocks, as well as the Mariposa slates, represent continental-slope deposits overlying the older arc; Bogen (1983) instead proposed that the latter might be a trench-slope deposit. In any case, the Middle to Late Jurassic arc margin consists of a superjacent sequence that formed in the Sierran Foothills after the accretion of Early Jurassic and older terranes (Sharp, 1988; Ingersoll, 2012, his figure 2; Schweickert, 2015, his figure 22D).
Age data presented herein help to define compressional metamorphic stages of the Nevadan orogeny, and they support conclusions of researchers favoring a long-lived event (Tobisch et al., 1989; Wolf and Saleeby, 1995; Schweickert, 2015). Snow and Ernst (2008) analyzed detrital zircon by secondary ion mass spectrometry (SIMS) methods from five volcanogenic metaturbidite layers of the upper Mariposa Formation and reported that Mesozoic U-Pb age populations are dominated by zircon exhibiting a broad unimodal distribution from ca. 175 to 155 Ma. These zircon U-Pb igneous ages from metasandstone suggest that the clastics were sourced mainly from the Jurassic Klamath-Sierran orogen, especially the mid-Paleozoic to mid-Mesozoic terrane collage, and associated younger volcanic plus granitic arc rocks (Snow and Ernst, 2008). This conclusion agrees with paleocurrent data, indicating an overall southward transport direction for the Mariposa Formation (Bogen, 1985). Accumulation apparently began by ca. 165 Ma and continued until at least 150 Ma (Ernst et al., 2009a). The Late Jurassic depositional ages of the Mariposa slates thus constrain suturing of older terranes in the western Sierran Foothills to have occurred during Middle to Late Jurassic time (Edelman and Sharp, 1989; Snow and Ernst, 2008; Ingersoll, 2008, 2012).
Contrasting Architectures of Klamath and Sierra Nevada Foothills
Terrane assemblies in both the Klamath and Sierran provinces consist of correlated oceanic-margin accretionary prisms surmounted by coeval to later magmatic arcs, but important contrasts in plate-tectonic histories exist. As noted, the Klamath Mountains orogen consists of a gently east-dipping imbricate collage of dominantly right-side-up thrust sheets; ages of formation of the stack of allochthons decrease systematically seaward. Magmatic arcs are sited landward of coeval subduction complexes. Klamath architecture justifies the interpretation of eastward subduction of paleo-Pacific oceanic lithosphere (Irwin, 1960, 1981, 2003; Davis, 1969; Burchfiel and Davis, 1981; Saleeby, 1983).
In contrast, whereas the latest Jurassic and younger structural development of the Sierra Nevada Foothills and magmatic arc exhibits a similar polarity, older units allow the interpretation of a Molucca Sea type of double convergence, with outboard lithotectonic units showing the effects of an initial westward subduction, whereas eastern units reflect landward underflow comparable to that of the Klamath Mountains province (Moores, 1970; Schweickert and Cowan, 1975; Schweickert, 1981, 2015; Ingersoll and Schweickert, 1986; Ingersoll, 2000, 2008). It is conceivable that pre–Late Jurassic double convergence of the oceanic plate characterized assembly of the post–Western Paleozoic and Triassic belt Klamath accretionary complex, but available geologic data do not require such an interpretation.
Great Valley Group
West of the Cretaceous Sierran magmatic arc but southeast of the Klamath Mountains, Great Valley Group clastic strata (Figs. 1 and 2) display a largely Klamath-Sierran provenance (Ingersoll, 1978, 1979, 1983, 2012; Linn et al., 1992; Surpless, 2015). These little-disturbed Great Valley Group forearc basin strata, which overlie the SE edge of the Klamath province, the Great Valley Ophiolite (Godfrey et al., 1997), and the western margin of the Sierra Nevada batholith plus its roof pendants, began receiving arc and arc-margin (native) detritus by Valanginian time (ca. 140 Ma: DeGraaff-Surpless et al., 2002; Surpless et al., 2006). Older, far-traveled coarse basal breccia and volcanic sandstone containing distinctive mafic igneous debris, ± a Tithonian cap of fine-grained deep-water distal turbidites, rest on the Great Valley Ophiolite (Shervais et al., 2004, 2005; Hopson et al., 2008; Johnston, 2013). These Upper Jurassic sediments probably had local sources in oceanic crust well outboard of the continental margin, so they are allochthonous in the present setting. The main stage of overlying, more proximal Great Valley Group strata was derived mainly from the nearby Jurassic to Late Cretaceous igneous arc and its country rocks, and is native (DeGraaff-Surpless et al., 2002; Surpless et al., 2006). The lower Great Valley Group consists of recycled orogenic and volcaniclastic sandstone, whereas the Upper Cretaceous strata are dominated by granitic units rich in plutonic quartz and alkali feldspars sourced from the dissected Sierra Nevada magmatic arc (Dickinson and Rich, 1972; Dickinson et al., 1982; Ingersoll, 2012; Surpless, 2015). The largest mass of preserved Great Valley Group formed during Late Cretaceous time, especially in the San Joaquin basin (Mansfield, 1979; Moxon, 1990). Great Valley sandstone includes widespread detrital zircon typified by igneous U-Pb ages mostly in the ca. 175–140 Ma range, with fewer grains in the 120–75 Ma range (DeGraaff-Surpless et al., 2002; Surpless et al., 2006; Wright and Wyld, 2007; Sharman et al., 2015; Dumitru et al., 2015, p. 787). Minor amounts of Early Cretaceous igneous zircon in the basal clastic Great Valley Group rocks suggest that upper-crustal stocks and the more widespread Jurassic extrusive arc units in the Klamath–Sierra Nevada magmatic belt supplied debris to the initial forearc. Great Valley Group deposition apparently began at ca. 140 Ma, with detritus largely from the inboard ingeous arc and arc margin. Sierran plutonic igneous activity died by ca. 85 Ma (Saleeby and Dunne, 2015), so some of the youngest Great Valley Group zircon probably had a more northerly arc source (e.g., the Idaho batholith).
The mid- and Upper Cretaceous Hornbrook Formation (Figs. 1 and 2) correlates with the more voluminous Great Valley Group and rests with angular unconformity on the inboard margin of the Klamath Mountains near the California-Oregon border (Sliter et al., 1984; Nilsen, 1993; Surpless and Beverly, 2013). The Hornbrook Formation is a remnant of a depositional basin that extended across much of the Klamath Mountains before Late Cretaceous exhumation and erosion (Batt et al., 2010). Like the Great Valley Group of NW California, Albian and younger Hornbrook strata overlap the eastern edge of the Klamath province and accumulated after apparent seaward relative offset of the salient (Fig. 2). Detrital zircon U-Pb age spectra indicate derivation chiefly from the nearby Klamath and Sierran calc-alkaline arcs ± igneous sources in the Pacific Northwest (Surpless and Beverly, 2013).
Three major, fault-bounded Franciscan belts, consisting chiefly of clastic tectonosedimentary strata, crop out in northern and central California: the Eastern, Central, and Coastal belts (Bailey et al., 1964; Blake et al., 1988; Jayko et al., 1989; McLaughlin et al., 1994, 2000). The accretionary complex consists of a series of west-vergent, fault-bounded terranes; individual map units are mostly right-side-up, but the stacks of thrust sheets become younger oceanward. The relatively coherent Eastern belt consists of two main lithotectonic units, the Pickett Peak ± Skaggs Spring terrane and the structurally lower Yolla Bolly terrane. Olistostromal and tectonic mélanges typify much of the Central belt, but chaotic mud-matrix units are present in all three Franciscan belts (Cowan, 1978; Raymond, 1984, 2015; Wakabayashi, 2011, 2015; Aalto, 2014). The Coastal belt contains three main map units, the inboard, structurally high Yager terrane, the medial Coastal terrane, and the outboard, structurally lower False Cape–King Range terrane (Blake et al., 1988).
These major accretionary assemblies were deposited in or near the trench formed by subduction of far-traveled Farallon oceanic crust as it arrived at the North American margin (Ernst, 1965, 2011). The petrologic compositions of graywacke (a term commonly used to denote first-cycle Franciscan sandstone consisting chiefly of quartz, feldspars, and rock fragments set in a muddy matrix) and rare conglomerate of the Central and Eastern belts indicate derivation mainly from the northern and central Californian magmatic arc and spatially associated country rock (Dickinson et al., 1982; Seiders, 1983). The Coastal belt contains similarly sourced debris, but it also includes clasts likely derived from the Pacific Northwest, as noted later herein.
Detrital zircon in Franciscan Eastern belt metasandstone possesses chiefly Jurassic U-Pb, mostly igneous, ages of ca. 180–160 Ma (Dumitru et al., 2010), but a few grains occupy the ca. 120–85 Ma range, indicating a Cretaceous maximum age of sedimentation (Joesten et al., 2004; Tripathy et al., 2005; Unruh et al., 2007). Based on its youngest detrital zircon, the oldest known Franciscan clastic unit is the Early Cretaceous Skaggs Spring Schist ± the Pickett Peak terrane (Wakabayashi and Dumitru, 2007; Snow et al., 2010). Franciscan deposition apparently began by ca. 140 Ma. Prior to recent geochronologic U-Pb studies of detrital zircon, Late Jurassic and Early Cretaceous age assignments for Eastern and Central belt strata relied on occurrences of the bivalve Buchia. However, some (possibly all) of these fossils in the Eastern belt were redeposited, having been eroded from Jurassic marine deposits deformed during the Nevadan orogeny (Dumitru, 2012; Dumitru et al., 2015). Such transported specimens might have been derived from the Upper Jurassic Mariposa-Galice overlap sequence. Although Franciscan sedimentation could have begun in Late Jurassic time, no older trench strata have yet been documented by U-Pb geochronology. In any case, most Eastern belt Yolla Bolly sandstone accumulated during mid- and Late Cretaceous time (ca. 120–85 Ma; Ernst et al., 2009b; Dumitru et al., 2010) and were exhumed soon thereafter (Mitchell et al., 2010). Sited in progressively farther outboard positions, the Central and Coastal belts have yielded Late Cretaceous (ca. 90–60 Ma) and Tertiary (ca. 65–20 Ma) maximum depositional zircon U-Pb ages, respectively (Dumitru et al., 2013, 2015, p. 787). These workers showed that young detrital (igneous) zircon ages in Coastal belt strata indicate sequential supply of clasts derived from the Idaho batholith, the Challis volcanic pile, and the Cascade Andean-type arc to the Yager, Coastal, and False Cape–King Range terranes.
METAMORPHISM OF CLASTIC UNITS
Metamorphic characteristics of sedimentary rocks yield important information about their postdepositional physico-chemical conditions, and geologic environmental histories. Metasandstones of the Franciscan coherent Eastern and Central mélange belts display clockwise prograde high-pressure–low-temperature (HP/LT) geothermal gradients of 100–300 °C and 5–8 kbar (Terabayashi and Maruyama, 1998; Ernst and McLaughlin, 2012) typical of subduction-zone environments. Most analyzed Eastern belt metagraywackes recrystallized at higher pressures than did Central belt units. In contrast, Coastal belt and other metaclastic sections noted above display the effects of far less intense recrystallization, reflecting low-P burial (Liou et al., 1983; Underwood et al., 1987, 1999). Figure 3 shows prograde metamorphic phase relations and P-T conditions for Franciscan rocks and, by analogy with Coastal belt mineral parageneses (Ernst and McLaughlin, 2012), for the low-grade metamorphosed Great Valley Group and Mariposa-Galice Formations. Limited geochronologic data suggest (1) local culminations in HP/LT metamorphic ages, and (2) intervals of erosion-induced flooding of HP/LT clasts to the trench as the Franciscan Eastern and Central belts were subducted and offloaded, thickening the orogen. Cooling continued during episodic return toward the surface, so decompression P-T trajectories of Eastern and Central belt rocks closely follow their prograde paths in reverse, but at slightly lower pressures for a given temperature.
In addition to Cretaceous and younger, chiefly metasedimentary Franciscan strata, lenses of high-grade metabasaltic eclogite, garnet-amphibolite, and garnet-blueschist occur as rare, mineralogically famous rocks. These intensely recrystallized metabasalts are of mid- to Late Jurassic recrystallization age (Coleman and Lanphere, 1971; Wakabayashi, 1992; Anczkiewicz et al., 2004; Wakabayashi and Dumitru, 2007; Ukar, 2012). Formed as paleo-Pacific oceanic crust, such basalts were transformed in a relatively young oceanic-continental-margin convergence zone along an initially unrefrigerated, warm-mantle hanging wall. HP/LT mineral parageneses mark counterclockwise P-T trajectories attending their subsequent subduction-zone cooling (Cloos, 1982, 1986; Wakabayashi, 1990, 1999; Saha et al., 2005; Page et al., 2007; Ukar and Cloos, 2014). Most exotic high-grade metamafic tectonic blocks recrystallized under conditions of ~10–12 kbar and ~400–600 °C (Fig. 3), but consideration of garnet-omphacite-phengite equilibria plus THERMOCALC computations suggest that some may have formed at even higher P-T values (Krogh et al., 1994; Tsujimori et al., 2006).
PRE–LATEST JURASSIC CRUSTAL GROWTH
The late Paleozoic to mid-Mesozoic development of northern and central California was typified by chiefly margin-parallel slip, episodic stranding of far-traveled ophiolitic complexes, and deposition of overlying deep-marine chert-argillite units (Saleeby, 1981, 1982, 1983, 2011; Ernst et al., 2008; Saleeby and Dunne, 2015). Scattered intermediate and felsic igneous activity also typified the Triassic crustal margin of California, but most late Paleozoic to mid-Mesozoic lithologic sections are oceanic in genesis and mafic in bulk-rock chemistry. However, a major igneous arc began to form in the Sierra Nevada–Klamath realm prior to ca. 175 Ma, attending the eastward transpressive descent of oceanic lithosphere (Dunne et al., 1998; Irwin, 2003; Dickinson, 2008). This volcanic-plutonic belt shed first-cycle clastics into the seaward, oceanic realm of the coeval Sierran latest Triassic–Jurassic arc plus Klamath Eastern Hayfork–North Fork terranes, and the Upper Jurassic arc-margin Sierran Mariposa plus Klamath Galice Formations (Miller and Saleeby, 1995; Scherer et al., 2006). The Sierran latest Triassic–Jurassic arc belt and derived Klamath chert-argillite–rich North Fork and Eastern Hayfork units formed before ca. 175 Ma (Snow and Ernst, 2008; Scherer and Ernst, 2008; Ernst et al., 2017), whereas sedimentation of the more proximal Mariposa and Galice overlap formations began by ca. 165–160 Ma (Ernst et al., 2009a). These volcanogenic overlap strata were derived from both magmatic-arc and country-rock sources; they predate Cretaceous deposition of the native Great Valley Group forearc and subparallel, coeval Franciscan trench complex (Ernst, 2012).
Transpressive plate motions generated metabasaltic eclogites, garnet-hornblendites, and garnet-glaucophane schists along the convergent junction at ca. 170–155 Ma (e.g., Anczkiewicz et al., 2004; Lu-Hf ages of garnets from five central Californian eclogites). These relatively high-grade HP/LT metamafic blocks formed at considerable subduction depths (~30–40 km) during inboard crustal construction of the calc-alkaline arc. With possible exception of the Red Ant blueschists in the northern Sierran Foothills (Fig. 2; Hacker and Goodge, 1990; Hacker, 1993; Schweickert, 2015), the HP/LT metamafic rocks remained sequestered at depth. Only weakly overprinted by low-grade neoblastic minerals, they likely were stored at shallow upper-mantle depths, as suggested by the common spatial association of such HP/LT rocks with serpentinized peridotites, and never with deep-seated xenoliths of continental crust (Ernst, 2015). These distinctive metamafic rocks returned surfaceward in mid- and Late Cretaceous time as tectonic and olistostromal blocks that were sheared and/or gravity-fed into low-density Franciscan mud-matrix mélanges (Cloos, 1982, 1986; Wakabayashi, 2011, 2015). Similar to dispersed fragments of far-traveled oceanic lithosphere and capping deep-sea chert in the clastic sections, the Jurassic high-grade metabasaltic blocks are exotic to the Cretaceous Franciscan trench assembly.
Paleozoic, fault-bounded Klamath lithotectonic units on the east crop out structurally high in the accretionary stack, whereas formational ages and times of thrusting and exhumation of successively accreted lower allochthons decrease monotonically oceanward (Irwin, 1972, 1994). The imbricate collage of west-vergent tectonized sheets consists of basal ophiolitic units, chiefly overlain by cherty and fine-grained clastic strata (Frost et al., 2006), and were invaded by discrete, mainly Jurassic calc-alkaline plutons. Klamath terranes are generally correlated with analogues in the NW Sierran Foothills based on similar rock types, most (but not all) major structures, formational ages of the lithologic packages, oceanward assembly of successively younger geologic units, and their times of deformation (Wright and Fahan, 1988; Wright and Wyld, 1994; Irwin, 2003; Saleeby and Dunne, 2015).
OFFSET OF THE KLAMATH MOUNTAINS SALIENT
The Klamath Mountains concave-landward contractional assembly of thrust sheets lies ~80–100 km west of the former, likely contiguous Sierran segment of the curvilinear magmatic arc (Fig. 1). North of this Klamath promontory, a major northeastward jog toward possibly correlative units in the Blue Mountains of eastern Oregon (Jones and Irwin, 1971; LaMaskin, 2011; LaMaskin et al., 2011; Schwartz et al., 2011) suggests an even greater oceanward offset of the Klamath province relative to the late Mesozoic accretionary continental margin. Palinspastic restoration of an original, continuous pre-Cretaceous arc in northern California, including a 20° clockwise rotation of the Klamath salient (Ernst, 2012), shows that such a back rotation would reduce the large gap between the Klamaths and the Blue Mountains. However, part of the present offset may reflect post–mid-Cretaceous NE dextral transpression of the Blue Mountains along the western Idaho shear zone (Gaschnig et al., 2017, their figure 12).
As Figure 2 depicts, the Upper Jurassic Mariposa, Galice, and Myrtle Formations all lie outboard of the main Klamath-Sierran arc. Likewise, in most of California, Cretaceous Great Valley Group strata lie seaward of the Sierran Foothills and landward igneous arc, whereas in NW California and SW Oregon, the Great Valley Group and Hornbrook Formation rest on the eastern margin of the magmatically extinct Klamath salient. Thus, the Klamath Mountains collage was displaced relatively westward before deposition of these Cretaceous strata, as shown in the sketch map of Figure 4. By the end of Jurassic time, the Klamath terrane assembly evidently had been deformed and migrated oceanward relative to the Sierran igneous arc, gradually removing the accretionary stack of allochthons from a crustal site above the upper-mantle magmagenic zone stoking the arc (Ernst, 2012). Numerous granitoid bodies intruded the Klamath Mountains during the period 175–140 Ma (Hacker et al., 1995; Irwin and Wooden, 1999; Irwin, 2003; Snoke and Barnes, 2006). Pluton emplacement ages roughly decrease eastward, but igneous activity in the Klamaths ceased in earliest Cretaceous time. The youngest intrusive unit is the ca. 136 Ma Shasta Bally pluton (Lanphere et al., 1968; Lanphere and Jones, 1978; Irwin and Wooden, 1999). However, these data may represent the time of cooling and annealing of the pluton rather than its emplacement age, because geologic mapping by Blake et al. (1999) documented Hauterivian Great Valley Group strata resting with angular unconformity on exhumed Shasta Bally rocks; this supports a Valanginian or older age (139–134 Ma) for the pluton (SB indicates the location of the granitic pluton on Fig. 1). Thus, prior to transportation of the salient, a continuous Klamath–Sierra Nevada volcanic-plutonic arc evidently existed along the edge of North American crust.
Westward initiation of a new convergent plate junction by ca. 140 Ma (Fig. 4) would have sited the Franciscan trench complex directly offshore from the Klamath orogen (Ernst et al., 2008; Ernst, 2012). To the south, the new suture apparently trapped far-traveled oceanic lithosphere on the hanging-wall side of the plate junction as the mid-Jurassic Great Valley Ophiolite plus its overlying tuffaceous and distal oceanic strata (Shervais et al., 2005; Hopson et al., 2008). This mafic crust formed at ca. 175–161 Ma, and so it is ~25 m.y. older than the primarily arc-derived siliciclastic Great Valley Group (ca. 140 Ma: DeGraaff-Surpless et al., 2002; Surpless et al., 2006). Step-out formation of a new plate junction stranding preexisting, older paleo-Pacific lithosphere on the hanging-wall side seems to more fully account for geologic relationships than would suprasubduction slab rollback (e.g., Stern, 2004; Stern et al., 2012; Shervais and Choi, 2012; Snow and Shervais, 2015).
Spatial control provided by the Mariposa–Galice–Myrtle–Great Valley Group–Hornbrook sedimentary sections suggests westward offset and ~20° counterclockwise rotation of the Klamath salient relative to the NW Sierran arc margin near the end of the Jurassic (Ernst, 2012). Figure 4 does not remove the accumulated arcuate strain caused by shearing during the ca. 140 Ma sinistral slip that produced the westward bulge of the salient. Upper Jurassic Galice-Mariposa and uppermost Jurassic–lowest Cretaceous Myrtle strata rest unconformably on the western flanks of the Klamath Mountain and Sierran Foothill margins of the igneous arc, so they were deposited prior to seaward migration of the salient. In contrast, the post–140 Ma native Great Valley Group and Hornbrook Formation lie outboard of the Sierran arc but inboard of the Klamath province. Thermochronologic research in the Western Klamath terrane by Batt et al. (2010) provides possible support for this scenario. They documented postoffset exhumation and 40Ar/39Ar-based cooling-degassing ages of Western Klamath belt rocks and constituent minerals at ca. 135–126 Ma. This episode may reflect the earliest Cretaceous departure of the Klamath salient from the Sierra Nevada arc, followed by gradual uplift.
Geologic relations among map units of the Klamath Mountains, the Franciscan Complex, and the Great Valley Group in the vicinity of the Yolla Bolly triple junction (Jones and Irwin, 1971; Sliter et al., 1984; Blake et al., 1999) include several major fault systems. Besides the major NW-trending South Fork and Coast Range thrust faults, Blake et al. (1999) mapped several smaller breaks transecting the Great Valley Group in this area, some of which are syndepositional normal faults (Constenius et al., 2000). Significant breaks include, from north to south, the Oak Flat, Sulfur Spring, Cold Fork, and Elder Creek faults. The Oak Flat–Sulfur Spring fault zone trends ENE and is properly oriented to accommodate the proposed earliest Cretaceous sinistral slip in northern California. However, the young apparent offset is much less than required to explain the ~80–100 km left-lateral slip. To the south, Wright and Wyld (2007) regarded the NW-trending Cold Fork–Elder Creek fault zone as an important discontinuity localizing several-hundred kilometers of dextral slip. Judging by geologic field relations presented by Blake et al. (1999), Cold Fork–Elder Creek structures and subparallel breaks truncate the Oak Flat–Sulfur Spring faults. Thicknesses of Lower Cretaceous Great Valley Group strata in the Yolla Bolly triple junction area monotonically increase northward, so these faults evidently underwent minor slip as late as ca. 125 Ma (Constenius et al., 2000; Wright and Wyld, 2007).
Speculative Displacement Mechanism for the Klamath Salient
Models generating offset of the Klamath Mountains relative to the Sierra Nevada require subduction of oceanic lithosphere to account for continuing magmatism in the Sierran arc during the Cretaceous. By ca. 140 Ma, the oceanic plate evidently approached the western edge of North America in a convergent but slightly dextral transpressive fashion (Engebretson et al., 1984; May and Butler, 1986; Schettino and Scotese, 2005; Sager, 2007; Doubrovine and Tarduno, 2008). Ernst (2012, 2015) advanced a scenario explaining outboard migration of the Klamath orogen as requiring subparallel transform zones bounding a medial, relatively warm oceanic slab—somewhat analogous to the Cenozoic arrival and subduction of a fragmented Farallon–Juan de Fuca–Gorda plate as described by Wang et al. (2013). Figure 5 illustrates the posited subduction of a relatively young plate segment beneath the Klamath Mountains by ca. 140 Ma. This warm platelet, bordered on both north and south across ENE-trending transform faults or zones of ductile deformation by old, cold, thicker, and stiffer oceanic lithosphere, evidently was decoupled from the overlying Klamath stack of gently east-rooting crustal allochthons. Impaction of thicker, more coherent oceanic crust on both the north and south could have caused contraction and eastward displacement of the North American margin relative to the Klamath orogen, which would thereby assume its salient configuration. Shortening of the active Sierran arc also might have rotated the Foothills terrane collage into its current stack of near-vertical imbricate sheets. The ductile (?) deformation zone containing this ~80–100 km offset in the North American crustal margin apparently lies at depth in the northern Great Valley forearc basin, covered by postslip Great Valley Group strata, perhaps in the basement beneath the Oak Flat–Sulphur Spring fault zone (Fig. 2). Whatever the reason for seaward relative migration of the Klamath orogen by ca. 140 Ma, it seems possible that ENE subduction of a segmented oceanic plate was involved.
CRETACEOUS–PALEOGENE CRUSTAL GROWTH
After ca. 140 Ma, recycled orogenic and magmatic-arc detritus derived from adjacent Klamath-Sierran terranes began to accumulate on the newly stranded Great Valley Ophiolite basement within the Great Valley Group forearc basin (Ingersoll, 1979, 1983, 2012). Clastic debris carried beyond the forearc came to rest on the descending Farallon oceanic plate as the Franciscan Complex (Ernst, 1970). Lower Cretaceous relatively continuous Great Valley Group strata were deposited on the western edge of the North American plate, largely protected from both surface erosion and subcrustal tectonic removal. Inauguration of forearc deposition presumably signaled onset of coeval deposition in the more distal, coeval Franciscan Complex, at least 30–35 m.y. after earliest Jurassic emergence of the igneous arc (Dumitru et al., 2010; Ernst, 2011). Franciscan trench deposition could have commenced in Late Jurassic time, but if so, such older clastic deposits have yet to be recognized. Voluminous sedimentation and accretion of the Franciscan Complex and Great Valley Group took place during the 125–85 Ma flare-up of Sierran arc magmatism (Surpless et al., 2006; Dumitru et al., 2010; Snow et al., 2010; Sharman et al., 2015). The youngest Sierran granites are ca. 85 Ma, reflecting Late Cretaceous extinction of the deep-seated magmagenic zone beneath northern and central California. The end of Sierran magmatic arc activity has been ascribed to a lessening of the subduction dip, arrival at the margin and subduction of overthickened oceanic crust, and flat-slab subduction attending Laramide orogeny far to the east (Coney and Reynolds, 1977; Dickinson and Snyder, 1978; Bird, 1988; Liu et al., 2010; Chapman et al., 2012).
The youngest part of the Great Valley Group rests nonconformably on rocks of the Sierra Nevada batholith and associated Foothill terranes (May and Hewitt, 1948; Ingersoll, 1979; Saleeby, 2014). The oldest part of the Great Valley Group was deposited on the Great Valley Ophiolite and overlying far-traveled distal turbidites. Native Great Valley Group sandstone was derived from accreted oceanic terranes, and volcanic-plutonic sources primarily in the Sierra Nevada region (Dickinson and Rich, 1972; Ingersoll, 1978, 1983, 2012; Mansfield, 1979; Linn et al., 1991, 1992; DeGraaff-Surpless et al., 2002; Surpless et al., 2006; Surpless and Beverly, 2013; Surpless, 2015; Sharman et al., 2015). Most graywacke of the Franciscan Eastern belt (Ernst et al., 2009b; Dumitru et al., 2010) was deposited during mid- and Late Cretaceous time (ca. 120–85 Ma) and had similar sources. The more westerly Central and Coastal belts possess Late Cretaceous–Paleocene (ca. 90–60 Ma) and Tertiary (ca. 65–20 Ma) maximum depositional ages, respectively (Dumitru et al., 2013, 2015). Some Central belt clastic strata contain zircon with U-Pb ages similar to the Idaho batholith. Young igneous zircon in Coastal belt strata indicates progressive sedimentary supply derived from the Idaho batholith, Challis volcanic pile, and Cascade Range to the Yager, Coastal, and King Range lithotectonic units, respectively (Dumitru et al., 2013, 2015). Figure 6 shows the more northerly provenance of the younger, farther outboard Franciscan terranes.
SOURCES OF JURASSIC–PALEOGENE CLASTIC ROCKS
Before deposition of Sierran-Klamath igneous zircon in Mariposa-Galice, Great Valley Group, and Franciscan sandstone, most terrigenous detritus probably was shed from pre-Jurassic clastic strata of the rifted North American sialic margin (e.g., Calaveras, Shoo-Fly, and related units, as well as arc volcanics) and/or the Precambrian basement. Based on regional relationships, petrofacies, and detrital-zircon U-Pb age data, interpreted ultimate sources for the clastic rocks considered here are as follows: Mariposa-Galice and Myrtle = Sierran-Klamath arc, Grenville, Mazatzal-Yavapai, older SW North American cratons; basal Great Valley Group and Franciscan Skaggs Spring Schist–Pickett Peak terrane = Sierran-Klamath arc, Grenville, Mazatzal-Yavapai, late Archean basement; main Great Valley Group and 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, Wyoming or Superior basement; Franciscan Central belt = Sierran arc ± Idaho batholith, Mazatzal-Yavapai basement; Franciscan Coastal belt = Sierran arc, Idaho batholith, Challis volcanics, and Cascade arc units.
The mid-Jurassic to Cretaceous magmatic arc plus its regional basement collage, and the Late Cretaceous to Miocene Idaho batholith, Challis complex, and Cascade volcanic arc supplied most of the igneous zircon to the clastic strata discussed in this paper (Fig. 6). Although Middle and Upper Jurassic continental-margin proximal sequences contain important detrital contributions derived from Grenvillian source terranes, ca. 1100 ± 100 Ma zircon appears to be sparse in Cretaceous Great Valley Group strata, and is absent from all but the oldest Franciscan Eastern belt rocks. Over time, the Franciscan trench began receiving greater amounts of younger arc detritus from more northerly sources. This change in provenance raises speculation for, but does not require, up to ~1600 km of post–120 Ma NW transport of the Franciscan Complex during deposition, relative to the Great Valley Group forearc and SW North American basement (Ernst, 2015).
Deformed, weakly recrystallized Paleogene Franciscan Coastal belt rocks (McLaughlin et al., 2000; Dumitru et al., 2013) display only low-P mineral transformations (Bachman, 1978; Underwood et al., 1987; Terabayashi and Maruyama, 1998; Ernst and McLaughlin, 2012), indicating that they were not deeply subducted. As important members of the Franciscan lithotectonic assemblage, the low-P metamorphism of Coastal belt strata stands in marked contrast to the rest of the trench complex (Fig. 3). Underplating of younger sections beneath older slabs of the Franciscan Complex aided buoyant ascent and erosional decapitation of the latter; the farthest east, oldest sections have been exhumed to the greatest extent. Thus, HP/LT rocks of the Coastal belt may be stored deep within the imbricate accretionary prism (Ernst and McLaughlin, 2012; Chapman et al., 2016).
JURASSIC–PALEOGENE HISTORY OF NORTHERN AND CENTRAL CALIFORNIA
During mid-Paleozoic to Middle Jurassic assembly of northern and central California, ophiolite and chert-argillite terranes arrived and were stranded along the sialic margin, reflecting mainly transform ± transpressive plate motions (Saleeby, 1981, 1982, 1983; Silberling et al., 1987; Ernst et al., 2008). This older oceanic terrane collage provided an accretionary backstop for the offloading of Upper Jurassic and younger sedimentary and volcanic rocks derived principally from the igneous arc sited along the continental margin. Middle and Late Jurassic volcanism-plutonism in the Klamath Mountains and Sierra Nevada Range (Dunne et al., 1998; Irwin, 2003; Dickinson, 2008) reflects a major component of eastward subduction of paleo-Pacific oceanic plates commencing by ca. 175 Ma, and continuing at least until ca. 140 Ma. Subduction also resulted in the generation and storage at depth of HP/LT metamafic rocks, now present as tectonic and olistostromal blocks in mélanges chiefly of the Franciscan Central belt (Cloos, 1982, 1986; Wakabayashi et al., 2010). Jurassic oceanic plate motions probably involved oblique convergence rather than orthogonal subduction (Ernst et al., 2008), because if a major forearc basin and trench had formed during this stage, all evidence of this subparallel couplet has since disappeared without leaving a trace—an unlikely situation. Of course, an important component of transform slip and/or subcrustal erosion (Scholl and von Huene, 2007; Wright and Wyld, 2007; Dumitru et al., 2010; Stern and Scholl, 2010) might have removed all but the farthest inboard overlap strata (e.g., the Mariposa-Galice and Myrtle Formations).
In the Franciscan Complex, 170–155 Ma high-grade metabasaltic eclogites, garnet-amphibolites, and blueschists formed during older stages of oceanic-plate subduction that generated the growing mid- to Late Jurassic Klamath-Sierran igneous arc as well as derivative Mariposa-Galice ± Myrtle proximal sedimentary aprons. Exotic HP/LT metamafic tectonic blocks evidently were then sequestered at modest depths, predating onset of a lengthy period of subduction that produced the paired Great Valley Group and Franciscan clastic sedimentary belts. In general, intrusion of ca. 175–170 Ma granitoid plutons in the western Klamaths and progressive geographic restriction of younger intrusive units to more easterly lithotectonic belts (Hacker et al., 1995; Irwin and Wooden, 1999; Irwin, 2003) suggest that seaward migration of the salient might have begun slightly earlier, perhaps at ca. 155–140 Ma. In any case, the Klamath assembly of ophiolitic terranes and superjacent volcanic and clastic rocks gradually began to migrate off the deep-seated magmagenic zone sited along or above the descending paleo–Pacific plate. This west-directed, sinistral offset of imbricated Klamath thrust sheets took place during a relatively brief period typified by widespread left-lateral slip along the western margin of North America (Saleeby, 1992; Saleeby et al., 1992).
At the Late Jurassic end of Mariposa-Galice ± Myrtle sedimentation, the Klamath salient evidently rotated ~20° counterclockwise and moved outboard ~80–100 km (plus arcuate strain of ~50 km) relative to the formerly contiguous Sierra Nevadan arc (Coleman et al., 1988; Constenius et al., 2000; Ernst, 2012). The Jurassic Klamath-Sierran magmatic arc trends NW, whereas the Cretaceous Sierran batholith trends NNW, a similar 20° counterclockwise angular difference as proposed for the ca. 140 Ma counterclockwise rotation of the Klamath promontory (Fig. 1). How the termination of Jurassic tectonic offset and rotation was accomplished is unknown, but it may have been a crustal response to arrival and subduction beneath the Klamaths of a segmented Farallon oceanic plate. Most of the slip occurred prior to Early Cretaceous deposition of the Great Valley Group and Hornbrook Formation along the landward SE and NE edges of the Klamath Mountains, respectively (Figs. 1 and 2). Minor sinistral shear may have continued across the NE-trending fault zones in the Great Valley Group, as mapped by Blake et al. (1999), during Early Cretaceous time (Ernst, 2012).
Eastward subduction of the oceanic lithosphere shallowed long after the posited ca. 140 Ma seaward formation of the new convergent plate junction. Insertion of stored high-grade metamafic blocks into the subduction channel likely involved traction-induced shearing and plucking of blocks from the hanging-wall mantle by circulating mud-matrix mélange. The way in which the Jurassic HP/LT rocks previously ascended to shallower mantle storage depths remains obscure, but metasomatic actinolitic rinds on many of the blocks suggest their early-stage inclusion in low-density, buoyant serpentinite bodies. Subsequently, some metamafic fragments may have risen to the surface in serpentinite diapirs and then been eroded and supplied to the Franciscan trench complex as olistoliths. Prior to the ca. 125 Ma onset of nearly orthogonal rapid subduction, arc magmatism, and surfaceward return flow of large volumes of Central belt mud-matrix mélange, shearing apparently was insufficient to promote tractive insertion of dense HP/LT blocks into the Franciscan subduction channel.
During mid-Paleozoic to Middle Jurassic time, largely margin-parallel slip involved the episodic docking of far-traveled ophiolitic complexes along the continental edge. In contrast, magmatic arc activity (Barth et al., 2013) was vigorous over the period ca. 175–140 Ma and became voluminous during the mid- and Late Cretaceous (ca. 125–85 Ma) during an apparent change from a southward to a northward 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). Geologic field relationships and detrital zircon U-Pb ages in both the Great Valley Group and Franciscan Complex document preservation and exposure of relatively small amounts of Tithonian (Upper Jurassic, probably far-traveled) and Lower Cretaceous (native) clastic strata, and much larger preserved volumes of Upper Cretaceous sediments (Mansfield, 1979; Moxon, 1990; Shervais et al., 2004, 2005; Hopson et al., 2008; Dumitru et al., 2010, 2015; Johnston, 2013). Dumitru et al. (2010) explained the greater volumes of Upper Cretaceous Franciscan clastic rocks as reflecting a change in relative plate motions, wherein coastal California changed from a nonaccretionary to an accretionary margin at ca. 123 Ma. However, transition to head-on convergence would result in large masses of oceanic lithosphere descending through the magmagenic zone, and in the increasing generation of arc magmas. Hence, rapid, nearly orthogonal mid- and Late Cretaceous plate subduction might have caused the ca. 125–85 Ma flare-up in arc activity and the production of voluminous Upper Cretaceous Great Valley Group and Franciscan clastic rocks (Blake et al., 1988; Ernst et al., 2008; Cloos and Ukar, 2010). Shallowing to subhorizontal subduction at ca. 85 Ma extinguished the magmatic arc in eastern California, whereas arc and arc-margin erosion continued to supply igneous detritus to the Paleogene margin.
Earth scientists owe an enormous debt to Bill Dickinson, who elucidated and quantified descriptions and plate-tectonic settings of clastic sedimentary rocks, mainly in convergent arc settings. He was a geologic giant without peer. My review is a small tribute to some of the many concepts he pioneered. Stanford University supports my field and analytical studies of Californian geology. The National Science Foundation (NSF) provided early aid through NSF grant EAR-0948676 to Marty Grove. Many workers carried out detrital-zircon U-Pb age determinations on which this summary is based, chiefly using the sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) at the Stanford–U.S. Geological Survey Micro-Analysis Center and the laser ablation–inductively coupled plasma–mass spectrometer at the University of Arizona Laser-Chron Center. Bob McLaughlin and Jason Saleeby provided fiercely insightful reviews that allowed me to pick up the pieces and present a somewhat improved synthesis. Kathy Surpless, an anonymous reviewer, and GSA Special Paper co-editor Ray Ingersoll provided additional critical but hugely helpful feedback. To these colleagues and institutions, I give my thanks, but especially to Bill Dickinson, who showed us the way forward.
Figures & Tables
Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson
- absolute age
- accreting plate boundary
- accretionary wedges
- Cascade Range
- Central California
- Challis Volcanics
- clastic rocks
- Franciscan Complex
- Galice Formation
- Great Valley Sequence
- Grenvillian Orogeny
- high pressure
- Hornbrook Formation
- Idaho Batholith
- igneous rocks
- Klamath Mountains
- Laramide Orogeny
- low pressure
- Mariposa County California
- metaigneous rocks
- metamorphic rocks
- middle Mesozoic
- Northern California
- Pacific Plate
- plate tectonics
- sedimentary rocks
- Sierra Nevada
- Sierra Nevada Batholith
- spatial distribution
- United States
- Upper Cretaceous
- Upper Jurassic
- volcanic belts
- zircon group
- detrital zircon
- Myrtle Group