RESEARCH Subduction, Accretion, and Exhumation of Coherent

Home , Blueschist, Coast Range ophiolite, Franciscan Complex, Greywacke, Pelite

RESEARCH

Subduction, accretion, and exhumation of coherent Franciscan blueschist-facies rocks, northern Coast Ranges, California

William L. Schmidt and John P. Platt UNIVERSITY OF SOUTHERN CALIFORNIA, EARTH SCIENCES DEPARTMENT, 3651 TROUSDALE PKWY, LOS ANGELES, CALIFORNIA 90089, USA

ABSTRACT

We present structural data and cross sections from four transects that together cover much of the Eastern Belt of the Franciscan accretionary complex. The westernmost, Middle Eel transect includes jadeite-lawsonite facies rocks (the Taliaferro Metamorphic Complex, TMC) intercalated with lawsonite-albite facies metagreywacke. The TMC shows subduction-related imbricate thrusting, refolded by upright folds with amplitudes of 100–1500 m, and it is cut by abundant normal faults that contributed to exhumation. East of the Coast Ranges divide, three linked transects along Thomes Creek cover the transition from lawsonite-albite facies metagreywackes to the blueschist facies South Fork Mountain Schist. The western section shows thick-bedded metagreywacke intercalated with broken formation, deformed by NW- vergent folds and associated thrusts and pressure-solution cleavage, and intensively dissected by abundant low-angle normal faults. In the central section, thin-bedded metagreywacke, broken formation, and conglomerate show an early foliation and folds overprinted by E-vergent folds and crenulation cleavage. The South Fork Mountain Schist forms the easternmost section and records the most intense deformation. The dominant foliation is a differentiated crenulation cleavage that has been refolded by NW vergent folds with amplitudes of millimeters to hundreds of meters. Structural relationships in the South Fork Mountain Schist exposed in Cottonwood Creek farther north are similar to those in Thomes Creek, indicating that our observations have regional significance. All the contractional structures and ductile deformational fabrics in these transects formed under high-P low-T metamorphic conditions during subduction and accretion, and the dominant deformation mechanism was pressure solution. Exhumation was achieved primarily by intensive normal faulting on the outcrop scale, and normal sense motion on the Coast Range fault. This paper provides the first documentation of syn-subduction normal faulting within the Franciscan Complex.

LITHOSPHERE; v. 10; no. 2; p. 301–326 | Published online 1 March 2018 https://doi.org/10.1130/L697.1

INTRODUCTION may produce large-scale thrust-sheets (Wakabayashi, 1992) or duplexes (Kimura et al., 1996). Exhumation has variously been attributed to return The internal structure of accretionary complexes is poorly known and flow in a subduction channel (Cloos and Shreve, 1988), wedge extrusion understood: most active examples are largely or completely submerged, (Maruyama et al., 1996), normal faulting in the upper part of the accre- and ancient examples are commonly strongly modified by later events tionary wedge, or in the overlying fore-arc basin (Platt, 1986; Jayko et al., such as continent or arc collision. The problem is compounded by the fact 1987; Harms et al., 1992; Wakabayashi and Unruh, 1995; Constenius et that accretionary complexes are commonly largely composed of relatively al., 2000; Schemmann et al., 2008; Unruh et al., 2007), or erosion (Feehan monotonous greywacke sandstone and shale sequences, without well-devel- and Brandon, 1999; Ring and Brandon, 1999; Ring, 2008). oped lithostratigraphy, and with complicated and disruptive structural styles. The internal structure of the Franciscan Complex in California is This hinders field investigations of emergent examples as well as seismic particularly poorly known, in part because of the abundance of highly studies of currently active complexes in submerged fore-arcs. In spite of this, disrupted rocks generally referred to as mélange, and in part because of some excellent seismic studies have demonstrated that the frontal regions poor exposure. The aim of this paper is to present detailed field relation- of accretionary wedges are dominated by imbricate thrusting (Davey et al., ships along a transect across the relatively coherent eastern belt of the 1986; Davis and Hyndman, 1989; Moore et al., 1990; Morgan and Karig, Franciscan in well-exposed river sections in the northern Coast Ranges, 1995), and this has been confirmed by detailed studies of some well-exposed and to discuss the significance of the structure in terms of subduction, emergent examples (Moore and Karig, 1980; Wahrhaftig, 1984; Platt et al., underplating, and exhumation. 1988; Meneghini and Moore, 2007, Wakabayashi, 2017). The structure of the more deeply buried interiors of accretionary wedges is less well docu- GEOLOGIC SETTING mented, and in particular the processes driving exhumation of these rocks remain controversial. The structure in the base of the accretionary wedge is The Franciscan Complex is the archetypal accretionary complex likely to be dominated by the process of subcretion or underplating, which formed at a convergent plate boundary (Bailey et al., 1964; Ernst, 1970;

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Wakabayashi, 1999), and reflects the subduction of tens of thousands of The Eastern Belt km of oceanic lithosphere along the western margin of North America from mid-Jurassic to mid-Tertiary time. The oceanic lithosphere carried The Eastern Belt of the Franciscan includes large tracts of lawson- with it seamounts and a pelagic sedimentary cover, and at times accumu- ite-albite facies siliciclastic rocks with a fairly coherent structure, some lated great thicknesses of clastic sediment in a trench environment, and substantial bodies of mélange, and a number of large sheets or slabs of much of this material was scraped off and accreted to form the accre- blueschist-facies metasediment and metabasalt (Suppe, 1973; Brown and tionary wedge. Some was accreted at shallow depths near the trench, but Ghent, 1983; Bröcker and Day, 1995). The largest of these thrust sheets some was carried beneath the wedge, and underplated at depths of 20–40 is the South Fork Mountain Schist (Blake et al., 1967), which is several km, where it was metamorphosed under high-pressure–low-temperature km thick and extends ~250 km along strike on the eastern margin of the (high‑P/low‑T) conditions (Ernst, 1971). Overall, the Franciscan comprises Franciscan. This is the largest coherent body of blueschist-facies rock in ~80% greywacke sandstone and shale, the remainder being predominantly the northern Coast Ranges, and is characterized by a very strong schistose mafic volcanic rocks and minor amounts of radiolarian chert and pelagic fabric in both metapelitic and metabasaltic rocks (Blake et al., 1967). limestone (Bailey et al., 1964). Perhaps as much as 30% of the outcrop Suppe (1973) mapped a large area of the eastern belt, encompassing area of the Franciscan shows a block-in-matrix texture, generally referred most of the area discussed in this paper. He distinguished two broad to as broken formation or mélange (Hsü, 1968). The matrix is commonly “facies” within the siliciclastic rocks: predominantly thick-bedded grey- shale, usually with a scaly fabric; the blocks, which may vary in size from wacke sandstones and shales, and scaly clay mélange or broken formation. a few mm to tens or even hundreds of m, consist mainly of greywacke He also identified a fault-bounded sheet of jadeite-bearing blueschist- sandstone in broken formation, but may include volcanic rocks, chert, facies metasediments and metabasalt, which he named the Taliaferro eclogite, garnet amphibolite, and blueschist, in which case the rock is metamorphic complex. This lies to the west of, and appears to be distinct referred to as mélange. Mélange and broken formation have variously from, the South Fork Mountain Schist, and appears to be intercalated with been interpreted as olistrostromes, mass flows, or debris flows of sedi- lower-pressure lawsonite-albite facies rocks. Suppe identified the lower mentary origin (Cowan, 1985; Wakabayashi, 2011, 2015; Platt, 2015), tectonic boundary of the South Fork Mountain Schist as a major thrust, or as a result of tectonic processes such as return flow in the subduction the Log Spring thrust, separating it from lower grade rocks beneath it. channel (Cloos, 1982). Suppe’s cross sections show the South Fork Mountain Schist and its The Franciscan Complex is bounded to the east by the mid-Jurassic basal thrust dipping steeply east beneath the Coast Range fault, but flat- Coast Range ophiolite. This represents the oceanic crust on the deformed tening westward, so that they intersect the topography near the crest of leading edge of the North American plate (Hopson et al., 2008), and is the Coast Range (Fig. 2). overlain by fore-arc basin sediments of the Great Valley Group (Dickin- Subsequent mapping (e.g., Worrall, 1981; Blake and Jayko, 1983) led son et al., 1996). Neither the Coast Range ophiolite nor the Great Valley to the distinction of a number of lithotectonic units, made up of greywacke Group shows significant metamorphism. They are separated from the sequences, mélange, or broken formation, separated by major faults, and Franciscan Complex by the Coast Range fault, which dips steeply E Jayko and Blake (1989) then subdivided the Eastern Belt as a whole into along the eastern margin of the Franciscan, but scattered outliers of Coast the Pickett Peak terrane (which includes the South Fork Mountain Schist) Range ophiolite and Great Valley Group across the northern Coast Ranges and Yolla Bolly terranes. These were distinguished in part by their primary suggest that it is regionally gently dipping (see Wakabayashi, 2015, for a lithological characteristics, and partly by their textural grade, which refers review). The Coast Range fault was originally identified by Ernst (1970) to the intensity of fabric development and the degree of recrystallization as the Mesozoic paleo-subduction zone, but it is now generally recognized of clastic sedimentary grains in the greywackes. There is clearly an over- that the fault has been cut or reactivated by later normal sense motion, all increase in metamorphic and textural grade eastward and structurally allowing exhumation of the underlying subduction complex (Platt, 1986; upward across the Eastern Belt (see also Blake et al., 1967; Suppe, 1973), Jayko et al., 1987). We note that Ring and Brandon (1994, 1999) and but it is not clear whether this is an appropriate basis for dividing the belt Ring (2008) have argued that exhumation of the Franciscan Complex into separate terranes. As noted below, we were not able to confirm the was accomplished by erosion of an emergent fore-arc high, and that the locations shown by Jayko and Blake (1989) for some of the major faults Coast Range fault is a later out-of-sequence thrust. The consistency of and unit boundaries, and for this reason we do not use their terrane clas- hanging wall and footwall rock sequences along the Coast Range fault sification in this paper. Geothermometry on rocks from the Eastern Belt suggest, however, that it still closely approximates the original subduc- using laser Raman spectrometry on carbonaceous material is in progress, tion zone contact. and may help in providing a clearer definition of the tectonic units. The Franciscan in the northern California Coast Ranges has tradition- Detrital zircon dating (Dumitru et al., 2010) has established that pro- ally been divided into three belts, based on the timing of accretion, grade tolith ages of the clastic sedimentary rocks in the Eastern Belt are Early of metamorphism, and the overall structural style (Fig. 1) (Berkland et Cretaceous (137–111 Ma); the South Fork Mountain Schist is likely to al., 1972; Ernst, 1975). The character and boundaries of these belts are be the oldest. Ar-Ar ages on white mica from the South Fork Mountain loosely defined and somewhat controversial, but they provide a useful Schist are around 121 Ma; these may be crystallization or cooling ages, framework for discussion of the internal structure of the Franciscan. The but in either case are likely to be close to the time of accretion (Dumitru Eastern Belt is the oldest, shows widespread high‑P/low‑T (lawsonite- et al., 2010). The timing of exhumation is less certain, but Eastern Belt albite and blueschist-facies) metamorphism, and a relatively coherent rocks are likely to have cooled through the apatite fission-track annealing structural style. The Central Belt shows a very disruptive structural style, window (corresponding to a depth of ~10 km) by Late Cretaceous time containing large volumes of scaly-clay mélange with a low metamor- (Dumitru, 1989; Tagami and Dumitru, 1996). phic grade (lawsonite-albite and prehnite-pumpellyite facies), but with The eastern boundary of the Franciscan in the northern Coast Ranges relatively abundant tectonic blocks of eclogite, garnet amphibolite, and shows some complexity. The South Fork Mountain Schist lies in tectonic blueschist. The Coastal Belt is the youngest, includes significant tracts of contact with slivers of low-grade volcanic and siliciclastic rocks, which deformed but coherent siliciclastic sedimentary rock, and is essentially may be correlative with the mid-Jurassic Galice Formation of the Klamath unmetamorphosed (zeolite facies). Mountains (Jayko and Blake, 1986). These low-grade rocks are overlain

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123°6.000 122°39.626 40°9.109 40°9.109

Quaternary

E Great Valley Group

A Klamath terrane S

Tehama-Colusa Melange E

N Eastern Belt

Franciscan Central Belt Complex Coastal Belt Cottonwood Creek C

C R

O A

A L

S Log Springs Thrust

T A

L B

T E

L T

L South Fork T Undi erentiated lawsonite - Mountain Schist albite facies rocks

Taliaferro Metamorphic Coast Range Fault Complex Middle Fork Eel River

10 km Thomes Creek

39°49.256 39°49.256 123°6.000 122°39.626 Figure 1. Franciscan Eastern Belt location map, adapted from Jayko and Blake (1989) and Blake et al. (1992, 1999). Studied portions of river transects are marked in red. Inset: Sketch tectonic map of the northern California Coast Ranges, adapted from Dumitru et al. (2010).

Coast Ra W nge F E ault Taliaferro Metamorphic South Fork Complex Coast Great Mountain Range Valley Schist Ophiolite Group

Undifferentiated lawsonite-albite facies rocks 10 km

Figure 2. Sketch profile across the Eastern Belt of the Franciscan around latitude 40N to show the overall structural relationships.

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in turn by a highly disrupted assemblage of ophiolitic rocks, which has metabasalt through metachert with deformed radiolarians, followed by a been referred to as the Tehama-Colusa mélange (Hopson and Pessagno, rapid transition into metagreywacke (Fig. 5A). 2004; Shervais et al., 2011). The Tehama-Colusa mélange lies in the Metabasalt carries the assemblage glaucophane + lawsonite + sphene same structural position relative to both the Franciscan and the overlying (Fig. 6A); primary igneous textures have been largely destroyed, and Great Valley sequence as the Coast Range ophiolite, but its affinities are it has a variably developed deformational fabric. It is internally imbri- controversial, and it lies in fault contact with the Great Valley sequence, cated, and on the NE limb of the major synform in the Middle Eel there which is tilted steeply east near the boundary. The lower part of the Great are four thrust imbrications, each one repeating the chert-shale section Valley sequence is cut by several large faults, including the Paskenta fault. above the basalts (Fig. 5B). The slices dip and young SW on this limb of These are sinistral in their current orientation, but may originally have the synform, and kinematic data from the thrusts indicate NE-directed been normal faults prior to late Cenozoic tilting (Constenius et al., 2000), thrusting (Fig. 4B). On the SW limb of the synform only two imbrica- and appear to sole onto the Coast Range fault. tions are identifiable: these young and dip to the NE, but we found no kinematic data. The orientations of the faults and the slip lineations are OVERVIEW OF THIS STUDY likely to have been modified by the folding, and given the NE direction of motion identified on the NE limb, it is likely that the slip direction on In common with much of the Franciscan Complex, exposure in the the SW limb is down-dip in its present orientation. The synform itself northern Coast Ranges is for the most part very poor, and is largely trends NW-SE, but must be a younger structure. Stretching lineations in restricted to road-cuts, rivers, and the summit regions of some of the higher the metachert layers trend around E-W (Fig. 4A). mountains. This, combined with the lack of established stratigraphy, and The graphitic phyllite that lies above the basalt-chert section carries the complexity and intensity of the deformation, explains the uncertain- chlorite + white mica + lawsonite + jadeitic pyroxene. It has a strong ties in the geological relationships, as discussed above. The river sections, stylolitic pressure-solution cleavage, and carries numerous sheeted quartz however, can provide good to outstanding exposure over long distances, veins that are tightly folded and transposed parallel to the cleavage (Fig. although they are difficult to access. This study has been confined to the 5C). These veins are particularly well developed along and close to the river sections, as they provide the only way to carry out detailed analysis thrust faults that repeat the metabasaltic section (Fig. 5D). The veins may and correlation of deformational structures. form up to 50% by volume of the rock, and reflect the precipitation of We present a detailed cross section and structural analysis of the rocks quartz dissolved during pressure solution, testifying to the importance of of the Eastern Belt along roughly E-W–trending segments of two major dissolution-precipitation creep during subduction and accretion. Some drainages: Thomes Creek on the eastern side of the Coast Ranges, and of these veins contain coarse prismatic jadeite and lawsonite (Fig. 6B), the Middle Fork of the Eel River on the western side of the main divide. demonstrating that they formed at the time of peak pressure. The overly- A shorter section in Cottonwood Creek, to the north of Thomes Creek, was ing greywacke sandstones in Beaver Creek have relict detrital textures, also investigated in less detail, to check whether the structures observed and carry lawsonite and fibrous jadeite replacing detrital sodic plagioclase in Thomes Creek are regionally consistent and not local or anomalous. (Fig. 6C). All these rocks are locally deformed by one or more sets of While these sections do not provide a continuous transect, we believe that small-scale folds with axial-plane crenulation cleavage. they provide sufficient information for us to make well-substantiated state- The lower boundary of the TMC is well exposed on the north limb of ments about the character of the protolith assemblage, the origin of the the synform, where metabasalt lies in fault contact with lawsonite-albite broken formation, and the structures associated with subduction, accretion facies greywacke sandstone and shale beneath. This contact is marked and exhumation. The locations of these detailed sections are shown on a by an extensive sheeted vein complex, and brecciation and veining of the regional map and cross section (Figs. 1 and 2). After a description of the metabasalt. It must be a post-metamorphic thrust, as it places higher pres- two sections, we integrate our observations to interpret the deformational sure TMC rocks above lower pressure lawsonite-albite facies metasedi- history of the Eastern Belt. ments. On the south limb of the synform the contact has been cut or reactivated by normal faulting (see below). The upper boundary of the TMC THE MIDDLE EEL SECTION is not exposed in the Middle Eel section, but is exposed in Beaver Creek, where it is defined by an array of normal sense shear zones (see below). The section (Fig. 3) extends ~4 km NE along the Middle Fork of the The TMC as a whole, together with the lawsonite-albite facies rocks Eel River from the bridge where forest road FH7 from Covelo to Elk Creek that lie below it, are folded on scales of 100–1500 m by NW-trending, crosses the river. The average orientation of the section is approximately approximately upright folds. These structures may not all have formed at normal to the strike of the structures, and the data have been projected the same time. The major synform in the TMC clearly refolds the imbri- onto a section line that is 3 km long. The section includes rocks of the cate structures within it, and appears to predate the widespread normal Taliaferro Metamorphic Complex (TMC), together with lawsonite-albite faulting (see below). This suggests that it occurred late in the accretion- facies greywacke sandstone, shale, and broken formation, which lie struc- ary process, or early during exhumation, as a mechanism of continued turally below it. We also constructed a short section through the TMC shortening and thickening of the underplated rocks. farther north, in Beaver Creek, which is a small tributary to the Eel River. Both the TMC and the lawsonite-albite facies rocks around it are The Beaver Creek section provides useful additional information on the affected by intensive normal faulting. These faults define many of the structure and stratigraphy of the TMC. Structural data from the Middle present-day boundaries between different rock units, and clearly cut Eel sections is presented in Figure 4. across layering and foliation in many locations. The lower boundary of The Taliaferro Metamorphic Complex in the Middle Eel River forms the metabasalts on the SW side of the major syncline in the TMC is a sig- a tight synform 1.5 km across, with a clearly defined internal stratig- nificant NE-dipping normal fault, which crosscuts or reactivates an earlier raphy and structure. The stratigraphy consists of massive metabasaltic thrust that placed the metabasalts above graphitic phyllite (Fig. 3). A set rocks metamorphosed in blueschist facies, overlain by metachert with of SW-dipping normal faults offset graphitic phyllite above the metaba- pelitic interlayers, followed by dark organic-rich phyllite and greywacke salts on the same limb of the syncline (Fig. 7A). These are distinctive sandstone. In Beaver Creek there is a perfectly exposed section from because the faults are occupied by quartz veins that carry lawsonite, but

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123°5.00′ 123°4.00′ 35 65 7D 39°50.80′ 35 26 5 52 synform 80 antiform with plunge

shear zone: sense as indicated 26 7E,F

reverse fault / thrust 82

normal fault 5D, 7C 39°50.40′

89 cleavage / schistosity 53 5B 66 bedding / compositional layering 58 1 km

64 40

90 50 Lawsonite-albite facies rocks 70 35 thick-bedded greywacke l 4 Middle Ee88 43 6B, 7A greywacke, slate, and 75 broken formation 61 83 FH7 5C FH7 Taliaferro Metamorphic Complex

metagreywacke and slate

metabasalt and metachert (ch)

SE NW ch ch ch ch ch

gwke

Figure 3. Map and cross section along part of the Middle Fork of the Eel River (see Fig. 1 for location). gwke—greywacke.

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Figure 4. Equal area lower hemisphere plots. (A) Middle Eel Taliaferro Metamorphic Complex (TMC); open cir-

cles—poles to S1 parallel to bedding; points—stretching lineation. (B) Middle Eel TMC; open squares—poles to thrusts; points—thrust lineations. (C) Middle Eel whole section; open diamonds—poles to normal faults; points— normal sense lineations. (D) Beaver Creek, upper contact CDof TMC; open squares—poles to conjugate normal- sense shears; points—normal sense lineations (green is E-directed; blue is W-directed). (E) Thomes Creek (Slab

section); open circles—poles to S1; red points—stretch-

ing lineation; black points—bedding/S1 intersection. (F) Thomes Creek (Slab section); open squares—poles to normal faults; points—normal sense lineations. (G)

Thomes Creek (Lanz section); open circles—poles to S1; red points—stretching lineation. (H) Thomes Creek (Lanz

section); open triangles—poles to S2; points—D2 fold axes and crenulation lineation.

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ch mb mb

mb ch mb mb

aA 2 m bB 5 m

cC dD Figure 5. (A) Chert-greywacke section, Taliaferro Metamorphic Complex (TMC), Beaver Creek (N39°56.184, W122°59.377), looking N. Graded bedding in the metagreywake indicates that the section youngs to the east. The section overlies bluechist facies metabasalt. (B) Imbricated metabasalt/chert sequence, TMC, Middle Eel (N39°50.248, W123°03.960), looking E. Metachert (ch) lies stratigraphically south (right) of metabasalt (mb), and is dupli- cated by an early thrust (highlighted green), now tilted vertical. Imbricated sequence is cross-cut by a late low-angle normal fault (highlighted red), displacing NE. (C) Tightly folded sheet-vein complex in pelitic schist near lower contact of TMC, Middle Eel (N39°49.660, W123°04.858), looking S. (D) Sheet-vein complex along basal thrust of metabasalt, TMC, Middle Eel (N39°50.314, W123°03.911), looking S.

not jadeite; the sheeted veins in the graphitic phyllites, however, carry greywacke sandstone, together with significant bodies of broken forma- both lawsonite and jadeite (Fig. 6B). This suggests that the normal faulting tion. The greywacke sandstones generally show low internal strain, well- was accompanied by significant decompression. Normal faults cut and developed detrital textures, and a weak disjunctive cleavage produced offset the imbricate thrusts on the NE limb of the syncline (Fig. 5B), and by pressure-solution (Ring and Brandon, 1999; Bolhar and Ring, 2001) an array of conjugate normal-sense shear zones defines the upper bound- (Fig. 7D). The pelites have clearly accommodated most of the strain, ary of the TMC in Beaver Creek (Figs. 4D, 6D, and 7B). Where normal and generally show a simple slaty cleavage, at a low or moderate angle faults cut shaly or slaty rocks, in both the TMC and the lawsonite-albite to bedding, that appears to be axial planar to the large-scale folds. Much facies rocks, they create a scaly-clay fabric with shear bands (Fig. 6D), of it, however, shows a scaly-clay fabric indicative of deformation under whereas in the greywackes the faults are discrete and commonly marked semi-brittle conditions, associated with the abundant faulting. The broken by quartz veins. Normal faults mostly have gentle dips, very variable slip formation consists largely of fragmented beds of greywacke sandstone directions, and commonly occur as conjugate sets. The majority dip NE, floating in a dark pelitic matrix. The sandstone fragments are commonly with a mean dip of 28/041, and a mean slip direction of 22/089, with a aligned, defining a fabric that may be folded (Fig. 7E). Locally, there are very large scatter from SE to NE (Fig. 4C). A smaller number of W or small, irregular bodies of metavolcanic rock, which commonly show a NW dipping normal sense shears have a NW sense of slip. fragmental texture (Fig. 7F). These may have been emplaced as individual A few minor strike-slip faults are present. Dextral faults strike 300– slides or olistostromes of pyroclastic material. 330; sinistral faults strike 084–116. These are reasonably interpreted as As noted above, the TMC is bounded below by a thrust, which must minor structures related to faults of the San Andreas system. have been active after peak metamorphic conditions were reached in the The lawsonite-albite facies rocks lying structurally below the TMC TMC, and in Beaver Creek it is bounded above by an array of normal comprise interlayered dark organic rich pelite and massive thick-bedded faults. Its present structural position, as a slice bounded above and below

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laws

jd laws

A B 100 µm

C 100 µm D 500 µm

Figure 6. (A) Glaucophane (blue) and lawsonite (colorless) in metabasalt, Taliaferro Metamorphic Complex (TMC), Beaver Creek (N39°56.184, W122°59.377). Plane light. (B) Prismatic jadeite (jd) and lawsonite (laws) in quartz vein from foliation-parallel sheet-vein complex in pelitic schist (see Fig. 7A), TMC, Middle Eel (N39°49.765, W123°04.204). Plane light. (C) Metagreywacke with fibrous jadeite replacing detrital plagioclase (center of image), TMC, Beaver Creek, (N39°56.184, W122°59.377). Plane light. (D) Shear bands in sheared metapelite associated with normal faults (see Fig. 7B), TMC, Beaver Creek (N39°56.234, W122°59.227). Plane light.

by lower-grade rocks, appears to have resulted from two separate tectonic (Fig. 2), with the Lanz and South Fork Mountain Schist sections lying processes: late-stage thrusting during the underplating process, and normal progressively higher in the sequence. The Slab section is separated from faulting during exhumation. the Middle Eel section described above by a broad zone of poor exposure along the crest of the Coast Ranges. According to the Willows quadrangle THOMES CREEK map (Blake et al., 1992) this area is occupied by mélange, together with a body of TMC rocks several km across. Given the overall easterly dip The Thomes Creek section is conveniently divided into three parts, of the Franciscan (Fig. 2), these rocks probably lie structurally below the with different structural characteristics, which are described separately Slab section described here. here. The western, upstream part (Slab section) comprises lawsonite-albite facies metasediments lithologically similar to those in the Middle Eel Thomes Creek: Slab Section section. The central part (Lanz section) is made up of a similar sequence with a better-developed cleavage, and with a fairly systematic set of late The Slab section (Fig. 8), named after the concrete slab where Forest folds associated with a crenulation cleavage. The eastern (downstream) Road 24N01 crosses Thomes Creek, consists of thin- to thick-bedded part corresponds to the South Fork Mountain Schist. These three sections greywacke sandstone and shale sequences and numerous thick beds of are separated by prominent bodies of metabasaltic rocks, which may lie pebble conglomerate, all interlayered on various scales with broken for- along major thrust boundaries. The Slab section is structurally lowest mation. Sandstone beds locally show grading, with sharp bottoms and

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C D

E F

Figure 7. (A) Normal fault offsetting sheet-vein complex in pelitic schist, Taliaferro Metamorphic Complex (TMC), Middle Eel (N39°49.765, W123°04.204), looking E. This is one of several normal faults displacing SW. (B) Conjugate sets of normal faults cutting pelitic schist near upper contact of TMC in Beaver Creek (N39°56.234, W122°59.227), looking N. (C) Sheeted vein complex in pelitic schist, TMC, Middle Eel (N39°50.314, W123°03.911), looking S. Vein complex is folded, with axial plane crenulation cleavage. (D) Bedding (horizontal) and cleavage (dipping right) in lawsonite-albite facies metagreywacke, Middle Eel (N39°50.815, W123°03.650). Strongly deformed shale rafts are present near the stratigraphic top of the lower turbidite bed. (E) Folded broken formation with aligned sandstone blocks and rafts in a shaly matrix, lawsonite-albite facies, Middle Eel (N39°50.553, W123°03.783). (F) Raft of brecciated metavolcanic material in broken formation, lawsonite-albite facies, location as for (E).

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? Forest Road 24N01 Road Forest e 30 41 bedding, normal and overturned S1 normal fault reverse faul shear zone synclin anticline overturned fold 66 1 km 32 48 56 10 e e 25 67 122°49.50’ metaconglomerat broken formation slate/phyllit metagreywacke 26 25 64 10B 22 40 20 cross-section 12B 16 10A, C 72 16 52 ,12A 41 9A 34 9B,10D, 11 74 86 52 61 46 20 50 9C 39 28 29 map 51 40 18 27 20 60 46 9D Figure 8. Structural map and cross section along the Slab section in Thomes Creek (see Fig. 1 for location). 1 for (see Fig. section along the Slab in Thomes Creek map and cross Structural 8. Figure 63 34 68 20

Y Thomes Creek: Slab section Y 52.60’ 52.40’ 53.20’ 39°53.00’ 52.80’

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diffuse tops, and the thicker beds commonly contain abundant shale rip-up The greywacke sandstones show relict detrital textures and a simple clasts concentrated near the top of the bed. Sandstone dikes are common pressure solution cleavage, accompanied by crystallization of quartz and (Fig. 9A), indicating fluid overpressuring prior to lithification. Interlay- white mica in pressure shadows (Fig. 10C). Pelites carry a simple slaty ered broken formation is clearly stratigraphically bound, with a crude cleavage. This is clearly overgrown by small lawsonite tablets (Fig. 10D), alignment of sandstone clasts parallel to the bedding in the surround- indicating that it formed early in the subduction history. Both rock types ing rocks (Fig. 9B), and is locally quite strongly folded (Fig. 9C). Clast locally show sheeted vein complexes parallel to the cleavage, some of morphology is generally irregular with rounded edges and re-entrants, which carry abundant aragonite, partly altered to calcite. The folds and suggesting that the sand was unlithified at the time of disruption (Fig. cleavage may therefore reasonably be attributed to early stages in the 9C), but locally forms bedding-parallel slabs with planar surfaces that subduction process, most likely contraction associated with underplating. may have been partly lithified. At the north end of the section, changes in younging direction revealed The sequence is folded on scales from a few cm to several hundred by grading in massive NW-dipping greywacke sandstone beds appear to meters (Figs. 8, 10A, and 10B), and bedding is highly variable in orienta- predate the visible folding and the S-dipping cleavage (Fig. 8). This may tion. Folds trend NE-SW, are predominantly N- to NW-vergent, and are be a result of early folding in a nonmetamorphic environment at the wedge accompanied by a moderately well-developed slaty cleavage in pelites, and front, or soft-sediment slump folding. a spaced pressure-solution cleavage in sandstones (Figs. 10C and 10D). In the center of the section a zone of broken formation a few tens of Some of the folds are accompanied by minor SE-dipping thrust faults. The meters thick is strongly deformed and foliated, in contrast to the rest of cleavage generally dips moderately SE, but has locally been tilted into a the sequence, which shows only a weak to moderately developed slaty NW dip: these variations may be a result of the extensive disruption by cleavage. Clasts in the broken formation are elongate, defining a NE- the later faults that cut the sequence (see below). trending stretching lineation (mean orientation of 064°), and some of the

A 5 cm B

C D

Figure 9. (A) Sandstone dikes cutting lawsonite-albite facies slate, Thomes Creek (Slab section, N39°52.612, W122°50.006). Bedding is horizontal, parallel to the slaty cleavage. Sandstone dikes are folded and shortened normal to the cleavage. (B) Broken formation, overlain concordantly by thick-bedded greywacke sandstone, lawsonite-albite facies, Thomes Creek (Slab section, N39°52.547, W122°49.952), looking S. Note alignment of clasts parallel to bedding in the overlying sandstone. (C) Broken formation showing aligned clasts defining a fabric parallel to bedding in surrounding rocks, which has been folded. A weak axial planar cleavage dips 45° to right. Thomes Creek (Slab section, N39°52.764, W122°50.236), looking N. (D) Strongly foliated broken formation with asymmetric clasts indicating top-NE shear sense. Thomes Creek (Slab section, N39°52.874, W122°50.377), looking SE.

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aA 30 cm bB 1 m

100 µm

cC 100 µm dD

Figure 10. (A) Small-scale NW-vergent folds and axial-plane cleavage in greywacke-slate sequence, Thomes Creek (Slab section, N39°52.455, W122°49.833), looking NE. (B) Outcrop-scale NW-vergent fold in greywacke-slate sequence, Thomes Creek (Slab section, N39°52.441, W122°49.751), looking NE. (C) Photomicrograph: pressure solution cleavage in metagreywacke. Thomes Creek (Slab section; location as for A). Mica “beards” form asymmetric tails around detrital feldspars. (D) Photomicrograph: slaty cleavage in pelite, upper Thomes Creek, showing lawsonite tablets growing across the cleavage. Thomes Creek (Slab section, location as for 9B).

clasts are asymmetric, with shapes suggesting a top-NE sense of shear very poorly exposed over 1 km, and the heavily degraded material seen (Fig. 9D). The kinematics of the deformation in this zone appear incom- in the canyon walls suggests the presence of one or more major faults. A patible with the overall NW-vergence of the folds and cleavage, and we discontinuous body of metavolcanic rocks ~200 m across occupies the suggest it represents a localized zone of ductile shear post-dating the center of this zone of faulting. A simple interpretation is that the metavol- main folding, but predating the brittle normal faulting that follows (see canics mark the stratigraphic base of the metasedimentary sequence to the below). The foliation in these rocks is overgrown by lawsonite, so that it seems likely that it formed during the subduction process, perhaps as a zone of backthrusting. EW The Slab section as a whole is intensively disrupted by normal faults (Fig. 8). The faults dip gently either N or SE, and locally form sets soling grey onto near horizontal slip surfaces (Figs. 11 and 12). The mean orienta- wacke sandston tion of the measured faults is 15/112. Kinematic data are not easy to find e and vary widely in orientation, but the mean of slip lineations we have measured is NE, around 055°.

Thomes Creek: Lanz Section broken formation 1 m

The Lanz section (Fig. 13) is named after the Lanz pack-trail (no longer Figure 11. Field sketch of normal fault array, Slab section, Thomes Creek. passable), which crosses Thomes Creek near the eastern boundary of the Broken formation is stratigraphically overlain by thick-bedded greywacke section. The western boundary of the Lanz section in Thomes Creek is sandstone, and disrupted by SE-dipping normal faults (see Figs. 9B and 12A).

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A B

C D 10 cm

Figure 12. (A) Normal faults cutting massive greywacke, and soling into a subhorizontal detachment, Thomes Creek (Slab section, N39°52.547, W122°49.952), looking SW (see Fig. 11). Whitney Behr for scale. (B) Listric normal fault cutting greywacke sandstones and shale, Thomes Creek (Slab section, N39°52.579, W122°49.981), looking NE. John Platt for scale. (C) Metabasalt “knocker” surrounded by dark-colored broken formation with a slaty cleavage, Thomes Creek (Lanz section, N39°52.181, W122°46.609), looking SE. The “tail” of the knocker, to left, is cut by a minor normal fault. Will Schmidt for scale. (D) Fine grained broken formation, showing fragmental texture, in contact with overlying metabasalt knocker. Location as for (C).

east, and that they were disrupted by a major accretion-related thrust that derived from the bluffs on either side of the canyon. We suggest that a placed the Lanz section above the Slab section to its west. layer ~100 m thick of mafic schist lies at or near the structural base of The eastern boundary of the Lanz section with the South Fork Moun- the SFMS, and that the fault exposed in the creek is in fact a late normal tain Schist (SFMS) is a discrete planar fault dipping 56/025, perfectly fault that cuts out this layer and the true Log Spring fault (Fig. 15). We exposed on a strath terrace on the south side of Thomes Creek (Figs. 13 assume that the Log Spring fault itself is a major thrust fault formed dur- and 15). The boundary was named the Log Spring fault by Suppe (1973), ing underplating of the Lanz section beneath the SFMS. who traced it north into the Tomhead Mountain area. Our location for the The Lanz section is made up largely of thin-bedded metagreywacke boundary in Thomes Creek is 700 m east of that shown by Suppe (1973), sandstone and shale, some metaconglomerate, broken formation, and one and 1300 m west of the contact shown by on the Willows geological sheet or more bodies of metabasalt apparently floating as large blocks, or knock- (Blake et al., 1992). The exposed contact separates very similar protoliths: ers, in broken formation (Figs. 12C and 12D). The metabasalt commonly dark graphitic pelite with dispersed fragments of greywacke sandstone, shows well-developed pillow structure; it is moderately deformed, and typical of the broken formation facies we have described from the Middle forms slabs that thin out at either end. It is locally cut by narrow shear Eel and Slab sections. The rocks on either side are readily distinguished, zones in which it has been converted to fine-grained glaucophane schist however, most obviously by the strongly differentiated character of the (Fig. 14A), but the bulk of the rock consists of relict igneous plagioclase primary foliation in the SFMS, which is absent in the Lanz section. laths in a matrix of fine-grained chlorite and sphene. For several hundred meters west of the contact the stream bed is filled The metasediments show a pervasive foliation, phyllitic in character, with boulders up to several tens of meters across of massive mafic blue- but detrital grains are still preserved in the metasandstones (Fig. 14B). schist. Just east of the contact, several blocks have selvedges of pelitic Lawsonite tablets are abundant in the pelitic rocks, and lie parallel to schist and metachert that are clearly derived from the SFMS. This blue- the foliation, suggesting that the fabric formed during or after the high- schist body is not exposed in the stream, but the blocks appear to be P/low-T metamorphism. Metaconglomerate and broken formation are

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Log Spring thrus 21 s 20 25 40 s 56 41 14A 14C, D 42 30 D1/D2 interference structure 20 12C, D 18 enveloping surface of S1 72 D1/D2 interference structure 40 shear zone 35 bedding/compositional layering S1 S2 25 1 km 22 40 20 122°47.00’ 54 38 55 26 enveloping surface of S2 13 45 30 51 s

30 1 72

-vergent minor folds

35 Road 24N0 Road

N- to NW

28 Forest 58 D1/D2 interference structure -vergent minor folds 39 32 N- to NW Thomes Creek 62 122°48.00’ 46 Figure 13. Structural map and section along the Lanz section in Thomes Creek (see Fig. 1 for location). SFMS—South Fork Mountain Schist. SFMS—South Fork location). 1 for (see Fig. map and section along the Lanz in Thomes Creek Structural 13. Figure 30 40 43 20 39°52.00’

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aA bB 500 µm

cC 10 cm dD 1 mm

E F

Figure 14. (A) Glaucophane schist layer in metabasalt, looking N, folded by second-phase E-vergent fold with axial-planar crenulation cleavage. The change in orientation of the crenulation cleavage with increasing strain toward the top of the layer suggests top-E shear sense. Thomes Creek (Lanz section, N39°52.155, W122°46.414). (B) First-phase pressure solution cleavage in Lanz section phyllite, showing detrital quartz grains, recrystallized lithic fragments, and metamorphic mica. Thomes Creek (Lanz section, N39°52.080, W122°45.754). Plane light. (C) Interference structures between upright first phase folds and inclined second generation folds, Thomes Creek (Lanz section, N39°52.089, W122°46.320), looking E. (D) Crenulation cleavage associated with second-phase folds. Location as for (C). Plane light. (E) Large-scale second-generation fold, Thomes Creek (Lanz section, N39°52.216, W122°45.484), looking S. Note the more intense minor folding and stronger cleavage in the top limb, indicating that the fold is overturned toward the E. (F) Detail of hinge area of fold shown in (E).

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strongly deformed, so that the clasts define a shape fabric (foliation and contain abundant shear surfaces, which provide some insight as to the stretching lineation). The foliation is for the most part parallel to bedding, nature of the Coast Range fault. and together with bedding has predominantly gentle easterly dips through Pervasively developed reverse-sense shears exposed in the M4 forest the section, although orientations are very variable on the outcrop scale. road south of Thomes Creek dip moderately E, and slip lineations plunge The mean stretching lineation is 14/074. Thin-bedded greywacke sand- toward 073°, giving a shear sense directed toward 253° (Fig. 18G); later stones show tight early folds that are likely related to the main foliation: more localized normal-sense shears dip NE, with slip lineations directed these are strongly overprinted by later folds (Fig. 14C) and have very toward 023° (Fig. 18H). variable orientations, but predominantly easterly plunges. The SFMS includes pelitic mica-schists, massive metagreywacke, The primary foliation, together with bedding, has been crenulated and minor metachert, the ~900 m thick Chinquapin metabasalt member near folded through most of the section on scales ranging from mms to ~50 m, the structural top of the unit, and thinner slices of metabasalt within and defining a series of predominantly E- to NE-vergent secondary folds (Fig. at the base of the unit. A few meters of broken formation, composed of 14A). These have a differentiated crenulation cleavage developed parallel sandstone rafts in a pelitic matrix, are present close to the Log Spring to their axial planes, but the new fabric is only locally strong enough to fault near the structural base of the unit (Fig. 16A). Massive thick-bedded become the dominant foliation (Fig. 14C). The mean orientation of the greywacke units have been distinguished on the map and section (Fig. secondary fold hinges is 09/135 (Fig. 4H), but they become very widely 15), but in many places pelitic schist and greywacke are interlayered on a dispersed on the overturned limbs of major folds. Fold axial planes, and variety of scales. Proportions of greywacke to pelite were not rigorously the crenulation cleavage, are regionally subhorizontal (mean 15/232), but defined in this study and these interbedded sequences have been mapped in the eastern part of the section commonly dip gently east, less steeply as pelite. The metasedimentary units include quartz, white mica, albite, than the main foliation, consistent with the easterly vergence of the folds chlorite, lawsonite, calcium carbonate, and framboidal pyrite. Metaba- (Fig. 13). The larger folds produce distinctive steep zones where bedding salts include glaucophane, stilpnomelane, and Fe-rich epidote. Jadeitic and the early foliation are around vertical, and in the eastern part of the pyroxene has not been observed in the SFMS in this study or by previous section they locally have overturned limbs with W-vergent minor folds. workers. Brown and Ghent (1983) report that pyroxenes from metabasalts The variation in orientation of the cleavage with strain (Fig. 14A), and in the Ball Rock and Black Butte areas are rich in acmite (50%–75%) and the more intense thinning and disruption of bedding on the overturned poor in jadeite (10%–30%). limbs of the E-vergent folds, suggest that this deformation event was There are four recorded episodes of deformation. The earliest detect- associated with E-directed shear. able foliation, S1, consists of 1–2-mm-thick differentiated bands of quartz In the western half of the section, secondary folds are much more vari- and white mica. It is preserved predominately within crenulation arcs at able in orientation and vergence. The variability may partly be a result of a high angle to the main foliation, S2 (Fig. 17A). S1 is preserved at the superposition on the earlier set of folds: interference structures are com- outcrop scale in limited locations within the westernmost massive grey- mon in several parts of the section (Figs. 13 and 14C). wacke unit and in just a single location east of the greywacke, where a Within 500 m of the boundary with the Log Spring fault, the second- green chert layer contains D2 folds whose axial planes can be seen to ary folding becomes more intense, with overturned limbs on the major form the main S2 foliation of the surrounding rock (Fig. 17B). Although folds, and the crenulation cleavage locally becomes the dominant foliation S1 is the result of the oldest discernible deformation episode, it may visible in the field. This suggests that the deformation may be related in not represent the first deformation event experienced by the SFMS. It is some way to the fault, a point discussed further below. possible that the deformation of the SFMS was intense enough to erase evidence of earlier episodes. Thomes Creek: South Fork Mountain Schist S2 is the main fabric and forms the foliation visible at outcrop scales. It is primarily visible as differentiated quartz and mica domains. Complete The South Fork Mountain Schist (SFMS) section is the structurally transposition has left S2 parallel to bedding in the SFMS. Lawsonite is highest and easternmost tectonic unit in the Eastern Belt of the Francis- commonly parallel to S2 and is particularly abundant in the mica-rich can. In the Thomes Creek transect it comprises two or more internally domains parallel to S2 (Fig. 17C). Where lawsonite and the foliation are at coherent thrust sheets discussed further below. Metamorphic grade in the high angles to each other, S2 is observed to wrap around lawsonite grains, Franciscan increases from westward, and the SFMS is the highest grade indicating that S2 was formed after the SFMS had reached blueschist unit in this transect. It also shows the highest intensity of ductile defor- facies conditions (Fig. 17D). The westernmost greywacke unit within mation. The section through the SFMS extends from its lower boundary the SFMS (Fig. 15) contains multiple outcrops with preserved tight to at the Log Spring thrust (described above) for 8 km downstream to its isoclinal D2 folds (Fig. 16C). Hinges are rarely exposed and are difficult upper boundary, 1.2 km west of the Thomes Creek Gorge near Paskenta, to measure, but appear to be at an orientation which does not match that giving a total structural thickness of ~3.5 km. Due to the variable strike of folds in nearby pelitic material. Fold axial planes are commonly sub- of the stream over this distance, the section has been projected onto parallel to the transposed bedding. Also present in this massive greywacke multiple section lines (Fig. 15). Along its upper boundary at this point section are thin interlayers of pelitic material which host both D2 and D3 it lies in sharp contact with metasedimentary and volcanic rocks, attrib- structures. D2 structures are expressed as mm and cm crenulations at a uted tentatively to the Galice Formation of the Klamath Mountains. The high angle to, and clearly refolded by, cm scale D3 crenulations (Fig. structurally higher portion of the Galice slice is coarse greywacke while 20B). Stretching lineations on S2 are defined by deformed quartz granules the lower portion is a silty shale. The entire slice is less deformed and and pebbles in coarse-grained metagreywacke (Fig. 16D), and deformed lower grade than the adjacent SFMS, as evidenced by a lesser degree rock fragments in broken formation. Amphibole mineral lineations in of recrystallization and differentiation. The contact between the Galice metabasalt are parallel to the stretching lineation, and are likely to have and the SFMS appears to be parallel to the foliation in both rock units, the same significance. Taken together, these lineations have an average and dips ~60° ENE. We did not find any clear evidence for the nature of orientation of 49/013 (Fig. 18E). Kinematic indicators in the SFMS are this contact, which must be a fault within the Coast Range fault system. sparse and largely restricted to asymmetrically boudinaged quartz veins. Serpentinite in the overlying Tehama-Colusa mélange, however, does These are commonly ambiguous or give conflicting shear senses. Three

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/2/301/4098600/301.pdf by guest on 26 September 2021 Eastern Franciscan accretionary wedge deformation and evolution | RESEARCH E E 21 D 38 5m 17 C D 36 D D 73 65 78 C 70 21A, B, 63 78 68 67 70 54 122°42.00’ 54 53 C C 33 47 20 C 38 43 62 51 C 26 17 A 17 B d te 27 correc 20 tilt- 35 42 122°43.00’ 20 23 10 1km 30 m 20 36 25 26 16 B 71 23 89 21 62 86 67 69 68 Are a t 15 25 17 D lt Corrected 30 Ti Broken Formation Metapelite Metagreywacke Metachert Metabasal 16 30 B 13 21 19 17 8 17 d s te 8 rgence B correc tilt- 29 16 D Interpreted S3 D3 Fold Ve Measured S3 Measured Bedding/S2 Interpreted Bedding/S2 Figure Location 85 60 20 A 16 A 87 16C, 19C 69 20 B B 45 , olds 19 B 31 olds 2 nk-f ki 43 19 A 32 46 19 D 65 erence: abundant D2 isoclinal f rf 16 A ld inte Fo and conjugate sets of post-D2 A A 39°52.00’ 39°51.50’ 39°51.00’ Figure 15. Map and composite cross section of Thomes Creek South Fork Mountain Schist transect (see Fig. 1 for location). Structural data have been projected onto lines AB to DE, which DE, lines AB to onto been projected data have Structural location). 1 for (see Fig. Mountain Schist transect South Fork section of Thomes Creek cross Map and composite 15. Figure the down a view directly for allowing section, the SW in cross 25° to been tilted have the structures areas In the tilt corrected of S2. the strike to as possible normal as far drawn were the orientation of S2 in each section line. show Stereoplots lines of D3 folds. hinge

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A B

C D

Figure 16. (A) Broken formation in South Fork Mountain Schist (SFMS), consisting of sandstone rafts in a pelitic matrix. The schist shows chaotic folding and strong disrupted quartz veins, due to its proximity to the Log Spring thrust (N39°52.240, W122°45.4519). (B) Trains of asymmetric D3 crenulations which verge to the NW form kink bands which dip to the SE. This is representative of the NW-vergent, flat limb geometry which dominates the SFMS at a variety of scales. View looking NE (N39°51.758, W122°43.549). (C) D2 fold pair in massive greywacke, delineated by thin quartz veins parallel to S1 (N39°52.309, W122°44.878). (D) Stretching lineation defined by deformed clastic quartz grains on an S2 surface in massive metagraywacke (N39°52.372, W122°44.621).

S2 stretching lineations were also measured in Cottonwood Creek. They indicating that they were not rotated (Fig. 18F), and we interpret the have an average orientation of 30/302, significantly more westerly trend- stretching lineations as being a result of D2 deformation. ing than the lineations in Thomes Creek (Fig. 18E). The vergence of the asymmetric D3 folds is different on the opposing The third episode of deformation folded S2 into kink bands, crenula- limbs of large-scale folds (Figs. 19A, 19B, 14E, and 14F), allowing for tions, and asymmetric folds, which are pervasive throughout the SFMS the identification of asymmetric folds up to hundreds of meters in ampli- (Fig. 16B). D3 folds are most strongly expressed in pelitic units and are tude. The large-scale folds are interpreted to have the same kink band or weakest in sections which are dominated by metagreywacke or metabasalt. asymmetric fold structure as the smaller folds that are visible at outcrop With the exception of an ~30-m-long section observed in Cottonwood scale. The long limbs of these large-scale folds tend to have gently dipping Creek, the D3 folds are generally absent from the metabasalts. D3 fold foliation and NW-vergent minor folds while the short limbs have steeper, hinges in the Thomes Creek section of the SFMS primarily plunge toward E-dipping foliation, E-vergent folds, and more intense deformation. the northeast, with an average orientation of 32/050 (Fig. 18A). Although In the extreme easternmost portion of the SFMS D3 folds are both more the fold hinges have a preferred orientation, there is a fair amount of scat- intense and more chaotic. The western end of the transect is dominated by ter and the trend varies by ± 35 degrees while the plunge varies by ± 30 the 0.7-km-thick steep limb of a major, SW-plunging D3 fold. Straddling degrees. In places where the hinges plunge at a shallow angle they often the location where the steep limb breaks the vertical and becomes over- break the horizontal, resulting in hinges which plunge in opposite direc- turned are outcrops with fold interference patterns. On the upright portion tions despite having similar trends. of the limb, W-vergent, SW-plunging D3 folds with 15 cm amplitudes are Stretching lineations in the SFMS and D3 fold hinges have similar seen to cross E-vergent and NE-plunging folds with 5 cm amplitudes. On average orientations. If this were due to the folds being rotated into the the overturned portion of the limb, the vergences appear to be reversed due extension direction then W- and E-vergent folds would have different to the inversion of the layering (Fig. 20D), but the two phases of folding average orientations. They are seen to have the same average orientation, are readily identified by the differences in their orientations and amplitudes

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A S1 B

C D

Figure 17. (A) S1 and S2 in the South Fork Mountain Schist (SFMS). Both fabrics are defined by differentiated quartz and mica bands. S1 is preserved as crenulation arcs at a high angle to S2, the dominant foliation (N39°51.395, W122°42.703). (B) S2 (dominant foliation in pelitic schist to right) is axial planar to D2 (folds in the green chert layer to left) (N39°51.492, W122°42.811). (C) Abundant lawsonite tablets in pelitic schist of the SFMS lie parallel to S2 (N39°51.058, W122°41.206). (D) S2 foliation in SFMS wraps around a lawsonite tablet, indicating that S2 formed after subduction to blueschist facies depths (N39°52.169, W122°44.140).

(Fig. 20A). Timing relationships between the two sets of folds are not suggests the possibility that the outcrop scale structure of the two sections clear. West of this transition zone, within the overturned limb, is another was formed simultaneously and that a later, as yet unobserved, structure outcrop with two phases of folding. The dominant phase is defined by cm has rotated the Cottonwood Creek section around a vertical axis ~50–70° to m scale, symmetric to E-vergent D3 folds. A smaller cm-scale set of counterclockwise relative to the Thomes Creek section. W-vergent folds is also present and may predate the D3 folds (Fig. 19D). The fourth and most recent episode of folding affects only limited Portions of the overturned limb contain a measurable, gently dipping S3. portions of the SFMS, locally producing fold interference patterns (Fig. Within the dominantly E-vergent, overturned limb is a small, W-vergent, 20C) and folds with undulatory hinges. and apparently upright outcrop. Folds at the transition from the E-vergent A large fault with predominately reverse sense kinematic indicators, section to the W-vergent section record opposing vergences on opposite but also some normal sense indicators, bisects the Chinquapin member limbs (Fig. 19A). Axial planes throughout this section are gently dipping (Figs. 21A and 21B). We interpret this as evidence of thrust faulting in a manner consistent with the measurable S3 present at nearby outcrops followed by more minor normal sense reactivation. It is likely that this (Fig. 19B). In places, S3 can be seen to fan out around D3 folds, contrib- is the continuation of the Tomhead fault described by Worrall (1981), a uting some variability to its orientation (Fig. 19C). fault which bisects the Chinquapin in Cottonwood Creek. The portion of D3 fold hinges in Cottonwood Creek have an average orientation the fault exposed in Thomes Creek is oriented at 63/080 and is accom- of 43/349 (Fig. 18B), ~70° counterclockwise relative to the hinges in panied by a stretching lineation oriented at 36/016. A sequence of cherty Thomes Creek. In both Thomes Creek and Cottonwood Creek, D3 fold metasediments (Fig. 21C) structurally above metabasalt was observed hinges trend ~40° clockwise relative to the stretching lineations in the directly below the Tomhead fault, indicating that the two thrust slices same transect. Additionally, the dip direction of the main S2 foliation making up the Chinquapin are upright and young to the east. Chert is in Cottonwood Creek is oriented ~50° counterclockwise from the dip also present adjacent to the structurally highest and eastern most limit of direction of the S2 foliation in Thomes Creek (Figs. 18C and 18D). This the Chinquapin. The eastern portions of both the Cottonwood Creek and

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Figure 18. Equal area lower hemisphere plots. (A) D3 fold hinges in Thomes Creek South Fork Mountain Schist (SFMS). (B) D3 fold hinges in Cottonwood Creek SFMS. (C) Poles to S2 in Thomes Creek SFMS. (D) Poles to S2 in Cottonwood Creek SFMS. (E) SFMS stretching lineations in Thomes Creek (points) and Cottonwood Creek (squares). (F) Comparison of W-vergent D3 fold hinges (red triangles), E-vergent D3 fold hinges (blue CDsquares) and D3 fold hinges with no recorded vergence (points). W- and E-vergent hinges share an average orientation, indicating that they were not rotated into the direction of extension. (G) Poles to thrust sense shear planes (open diamonds) and slip lineations (filled circles) from serpentinite in the Tehama-Colusa mélange (Forest Road M4, N39°49.154, W122°38.993). (H) Poles to normal sense shear planes (open squares) and slip lineations (filled circles) from serpentinite in the Tehama-Colusa mélange (Forest Road M4, N39°49.156, W122°39.233). (G) and (H) represent deformation associated with the Coast Range fault.

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Figure 19. (A) Meter-scale D3 fold in South Fork Mountain Schist (SFMS) with opposite vergences of minor folds on opposite limbs. Looking SW (N39°52.227, W122°45.186). (B) Gently dipping axial planes are commonly associated with D3 folds in steep limbs of major D3 folds. Opposite limbs contain smaller folds with opposite vergences, SFMS (N39°52.239, W122°45.050). (C) Differentiated crenulation cleavage S3 fans out around a minor D3 fold in SFMS (N39°52.309, W122°44.878). (D) Near the western limit of the SFMS, two phases of folding are present. The dominant phase is an E-dipping, E-vergent to symmetrical D3 set at scales of cm to m. Small cm-scale W-vergent folds are present as well; they appear to be older and only locally developed (N39°52.226, W122°45.335). Viewed looking S.

the Thomes Creek transects include faults with a reverse sense of motion, truncate quartz grains, suggesting that precipitated quartz was locally which juxtapose pelitic schist with metabasalt (Fig. 21D). Faults tend sourced. Dilational microcracks are pervasive throughout the SFMS and to dip E to NE, though there is some variability in Cottonwood Creek are filled with quartz. Different quartz veins have experienced different exposures, where both normal and thrust faults are present. At the western degrees of dynamic recrystallization, likely reflecting different timing end of the SFMS, a thrust fault with a markedly different orientation of relative to ductile deformation (Figs. 22C and 22D). The coexistence of 52/208 truncates the hinge of a major D3 fold. Extensive fault parallel the microcracks and their varying degrees of ductile overprint indicates quartz veining is present, in places up to 25 cm thick. Based on the offset that the SFMS was experiencing brittle and ductile deformation simul- of the fold hinge, the fault appears to have accommodated a somewhat taneously while under blueschist facies conditions, and at temperatures minor amount of motion. Minor normal faults are present throughout the near the brittle-ductile transition. SFMS; kinematic data are difficult to obtain, but the sense of motion can be recognized by Riedel shears and by fault drag folding. DISCUSSION Veining and pressure solution at a variety of scales are common throughout the SFMS. At the meter scale, younger veins can be seen Eastern Belt rocks in our transect contain a variety of structures that cross cutting the foliation while older veins have been transposed and together record the stages of subduction, accretion, and exhumation. are parallel to the foliation. At the thin section scale, quartz has grown Deformation was likely continuous and it is not always possible to say in pressure shadows around pyrite (Fig. 22A), in the dilational arcs of with certainty which structures belong to which stage of the evolution of fold hinges (Fig. 22B), and in microcracks where the quartz has locally the accretionary complex, even when they can be chronologically ordered. grown orthogonal to the walls. Pressure solution has differentiated the Here we interpret structures that predate blueschist facies mineral growth rock into quartz-rich and mica-rich domains and pressure solution seams to be related to subduction, and structures that are synchronous with

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Figure 20. (A) 10-cm-scale W-vergent kink band in South Fork Mountain Schist (SFMS) intersects 2-cm-scale E-vergent kink band. Timing relationships are difficult to establish, and it is likely that these kink bands represent conjugate sets. Looking NW (N39°52.322, W122°44.847). (B) D2 crenulations in SFMS refolded by D3 crenulations (N39°52.267, W122°44.941). Viewed looking S. (C) Intense differentiated foliation (S2) in SFMS is tightly folded by D3 folds and crenulations with axial surfaces dipping to the right; these are refolded by more open D4 folds with axial surface dipping to the left. View looking S (N39°51.216, W122°42.271). (D) Equal-area lower hemisphere plot showing S2 planes from two adjacent outcrops in the steep zone of a major D3 fold in SFMS. Plane shown in black is vertical; plane shown in red is overturned. Closed circles—D3 fold hinges from the two limbs; squares—hinges of later kink folds.

blueschist facies metamorphism are likely related to accretion or the early same way as the surrounding bedded sequences of greywacke sandstone. stages of exhumation. Structures formed during the later stages of exhuma- These features are consistent with an origin of the broken formation by tion are identified by their predominantly brittle nature, relation to exten- surficial sliding and mass transport in a trench environment, as suggested sion (vertical shortening), and lack of associated high-P/low-T minerals. by Wakabayashi (2011) and Platt (2015). A potentially controversial issue is the age and origin of the disrup- Structures related to subduction are widely developed but commonly tive deformation that produced the broken formation found throughout overprinted by later deformation. These structures include early thrust the region, including the westernmost (structurally lowest) part of the repetitions of basalt-chert-greywacke sequences in the TMC. Limited SFMS. Critical to this discussion are the following observations. (i) Clasts kinematic data from these structures suggest roughly NE-directed shear, in the broken formation primarily comprise rafts or fragments of grey- which is not obviously consistent with the likely direction of thrusting in wacke sandstone that have irregular edges and embayments. Fragments the Franciscan accretionary wedge. They may have formed by backthrust- of metavolcanic rocks are uncommon, and we found no blocks with meta- ing during subduction, but given the complexity of the later deformation, morphic grade higher than their surroundings. (ii) The degree of fragmen- and the likelihood that convergence was oblique during at least part of tation in the broken formation appears to be unrelated to the intensity of Franciscan history, vertical-axis rotation of early formed structures can be the deformational fabric, and some bodies of broken formation have little expected (see, for example, Platt, 2000). Vertical-axis rotation is also sug- or no fabric. (iii) Many of the bodies of broken formation appear to form gested by the systematic differences in orientation in the SFMS between stratigraphically bound interlayers in bedded greywacke sandstone. (iv) the Thomes Creek and Cottonwood transects. Layers of broken formation commonly contain elongate clasts oriented N- to NW-vergent asymmetric folds accompanied by a slaty cleavage parallel to their boundaries, and the fabric in the mélange is folded in the are common in the lawsonite-albite facies rocks of the Middle Eel and

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Figure 21. (A) Tomhead fault cutting the Chinquapin metabasalt member of the South Fork Mountain Schist (SFMS) in Thomes Creek, looking S. Metabasalts have been placed structurally above a 2-m-thick red chert layer. To the right and structurally below the chert layer are cherty metasediments which in turn conformably overlie metabasalt. John Platt for scale (N39°51.176, W122°41.744). (B) Asymmetric clasts in metachert directly beneath the thrust in (A) give a reverse sense of motion (N39°51.176, W122°41.744). (C) Cherty metasediments 5 m below the Tomhead fault. Shear bands and asymmetric chert fragments give a top-down-to-the-right sense of motion. This is top NE, suggesting normal-sense reactivation of motion on the thrust (N39°51.176, W122°41.744). (D) An E-dipping thrust fault close to the Coast Range fault system has placed metabasalt of the SFMS (left) structurally above pelitic schist (right). Looking S (N39°51.040, W122°41.159).

Slab transects; the cleavage is overgrown by lawsonite, suggesting that is in thrust contact with underlying lawsonite-albite facies rocks, and was these structures are related to subduction. A NE-directed zone of ductile likely emplaced under those conditions, as the tectonic boundaries are shear a few tens of meters wide in broken formation in the Slab section marked by sheeted vein complexes containing lawsonite. The original also predates lawsonite growth, and may be a backthrust formed dur- Log Spring thrust, and the likely thrust contact between the Lanz and ing subduction. The earliest foliations found in all parts of the transect Slab sections, are not exposed in Thomes Creek, but we think they are are likely related to subduction. Sheet vein complexes parallel to early likely to have formed during continued subduction-related convergence. foliation in the TMC carry jadeite and lawsonite; the first cleavages in The large-scale synform in the TMC postdates subduction related thrusts, lawsonite-albite facies rocks of the Middle Eel and Slab sections are but predates exhumation-related normal faulting, and may have formed overgrown by lawsonite; the first cleavage in the Lanz section contains during the accretion stage. oriented stable lawsonite and is overgrown by lawsonite porphyroblasts. The second-generation folds in the Lanz section, and both D2 and D3 Early folds in the Lanz section are coeval with the first foliation, but we in the SFMS, all demonstrably formed under high-P/low-T conditions. were not able to determine their vergence. The earliest deformational Secondary folds and associated crenulation cleavage in the Lanz section fabric in the SFMS likely predates lawsonite growth, but is heavily over- deform a lawsonite-bearing fabric (S1), but formed during glaucophane printed by later deformation. growth in metabasaltic rocks. Lawsonite in the SFMS formed before or Contractional structures that formed during the high-P/low-T meta- during the main foliation (S2), and lawsonite porphyroblasts locally show morphism represent deformation during subduction or accretion at depth rotational inclusions suggesting continued growth during D2. Hence these (underplating). These likely include the fault boundaries between the structures all formed after the initial phase of subduction, but are likely to various large-scale tectonic units in the region. The TMC, for example, represent continued subduction-related shear during or after accretion. The

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Figure 22. (A) Quartz growth in the pressure shadow of pyrite in South Fork Mountain Schist (SFMS) (N39°52.169, W122°44.140). (B) Quartz has been precipitated in the void space of the hinges of tightening folds in SFMS. Pressure solution seams on the limbs of the folds suggest that the quartz was locally sourced (N39°51.999, W122°43.997). (C) Quartz vein in SFMS has experienced a small amount of crystal-plastic deformation as evidenced by minor bulge nucleation and undulose extinction, but grain size is still large and fluid inclusion stacks still visible (N39°52.241, W122°45.419). (D) A quartz vein which has been almost entirely dynamically recrystallized by subgrain rotation (N39°52.241, W122°45.419).

same is true of the Log Spring fault, which is clearly a contractional fault, the initial stages of underplating, and that these structures were cross- but which juxtaposes rocks with significantly different metamorphic and cut by the W-directed Log Spring fault, which accomplished the final deformational histories. It is therefore likely to represent the subduction emplacement of the Lanz section beneath the SFMS. D3 in the SFMS zone interface during underplating of the Lanz section under blueschist likely accompanied motion on the Log Spring fault. facies conditions. This therefore raises the question of its relationship to The intensity and complexity of the deformation in the SFMS also the deformational history of the SFMS and Lanz sections on either side increases on the east side of the section, and includes several syn-meta- of the fault. morphic thrusts, including a major thrust that duplicates the mafic section The E-vergent secondary folds that are dominant in the eastern part in the Chinquapin metabasalt. This suggests that subduction-related shear of the Lanz section increase in intensity toward the Log Spring fault, and continued along the eastern tectonic boundary of the Franciscan Complex they have the same orientation and style as E-vergent folds in the adjacent as a whole throughout the underplating history. western part of the SFMS. The E-vergent folds in the Lanz section are Normal faults related to exhumation of the high-P/low-T rocks are related to E-directed shear, however, whereas those in the SFMS formed widespread and locally very abundant. Normal faults in the TMC cut struc- on overturned limbs of the major W-vergent D3 folds that are predominant tures related to subduction and accretion, and are associated with sheeted throughout the SFMS, and are likely to be related to W-directed shear. A vein complexes that carry lower pressure assemblages (lawsonite + albite) possible solution to these paradoxical relationships is that the E-vergent than those formed at peak pressure (lawsonite + jadeite). The precise tim- folding in the Lanz section was related to E-directed backthrusting during ing and geometrical relationships between these normal faults and tectonic

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boundaries between the TMC and adjacent lawsonite-albite facies rocks Blake, M.C., Harwood, D.S., Helley, E.J., Irwin, W.P., Jayko, A.S., and Jones, D.L., 1999, Geologic map of the Red Bluff 30′ × 60′ quadrangle, California: U.S. Geological Survey are unclear; they appear to have formed under comparable conditions. Geologic Investigations Series Map I–2542, 1 sheet, scale 1:100,000, https://pubs.usgs Normal faults are particularly abundant in the Slab section, and domi- .gov/imap/2542/. nate the structure. They are less obvious in the Lanz and SFMS sections, Bolhar, R., and Ring, U., 2001, Deformation history of the Yolla Bolly terrane at Leech Lake Mountain, Eastern belt, Franciscan subduction complex, California Coast Ranges: Geo- but we observed a probable normal fault with a displacement of ~100 m logical Society of America Bulletin, v. 113, p. 181–195, https://doi .org/10.1130/0016-7606 that cuts out the Log Spring thrust in Thomes Creek, and normal sense (2001)113<0181:DHOTYB>2.0.CO;2. reactivation of the synmetamorphic thrust in the Chinquapin metabasalts Bröcker, M., and Day, H.W., 1995, Low-grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, Northern California: Journal of Metamorphic Geology, of the SFMS. Large-scale normal-sense shear is likely to have occurred v. 13, p. 61–78, https://doi.org/10.1111/j.1525-1314.1995.tb00205.x. along the Coast Range fault, which played a major role at a late stage Brown, E.H., and Ghent, E.D., 1983, Mineralogy and phase relations in the blueschist facies in the exhumation of the Franciscan Complex as a whole, and we have of the Black Butte and Ball Rock areas, northern California Coast Ranges: The American Mineralogist, v. 68, p. 365–372. documented both reverse and normal sense motion in sheared serpentinite Cloos, M., 1982, Flow mélanges: Numerical modeling and geologic constraints on their from the Tehama-Colusa mélange. origin in the Franciscan subduction complex, California: Geological Society of America Bulletin, v. 93, p. 330–345, https://doi .org /10 .1130 /0016 -7606 (1982)93 <330: FMNMAG>2 The presence of quartz veining in structures related to all phases of .0.CO;2. evolution, and the differentiated character of all the foliations throughout Cloos, M., and Shreve, R.L., 1988, Subduction-channel model of prism accretion, mélange the profile, indicate that pressure solution was a significant deformation formation, sediment subduction, and subduction erosion at convergent plate margins: 1. Background and description: Pure and Applied Geophysics, v. 128, p. 454–500. mechanism throughout the entire evolution of the Eastern Belt and high- Constenius, K.N., Johnson, R.A., Dickinson, W.R., and Williams, T.A., 2000, Tectonic evolution lights its importance in facilitating the deformation of this part of the of the Jurassic-Cretaceous Great Valley forearc, California: Implications for the Franciscan Franciscan Complex. thrust wedge hypothesis: Geological Society of America Bulletin, v. 112, p. 1703–1723, https://doi.org/10.1130/0016-7606(2000)112<1703:TEOTJC>2.0.CO;2. Cowan, D.S., 1985, Structural styles in Mesozoic and Cenozoic mélanges in the western Cor- CONCLUSIONS dillera of North America: Geological Society of America Bulletin, v. 96, p. 451–462, https:// doi.org/10.1130/0016-7606(1985)96<451:SSIMAC>2.0.CO;2. Davey, F.J., Hampton, M., Childs, J., Fisher, M.A., Lewis, K., and Pettinga, J.R., 1986, Struc- The Eastern Belt of the Franciscan Complex includes rocks metamor- ture of a growing accretionary prism, Hikurangi margin, New Zealand: Geology, v. 14, phosed under a variety of high-P/low-T conditions, and progressively p. 663–666, https://doi.org/10.1130/0091-7613(1986)14<663:SOAGAP>2.0.CO;2. Davis, E.E., and Hyndman, R.D., 1989, Accretion and recent deformation of sediments along deformed during a history of subduction, underplating, and exhumation. the northern Cascadia subduction zone: Geological Society of America Bulletin, v. 101, The earliest structures are stratigraphically bound layers of broken forma- p. 1465–1480, https://doi.org/10.1130/0016-7606(1989)101<1465:AARDOS>2.3.CO;2. tion, and isoclinal folds in sandstone that predate the main cleavage; these Dickinson, W.R., Hopson, C.A., and Saleeby, J.B., 1996, Alternate origins of the Coast Range Ophiolite (California): Introduction and Implications: GSA Today, v. 6, no. 2, p. 1–10. features are likely related to surficial processes in the trench environment. Dumitru, T.A., 1989, Constraints on uplift in the Franciscan subduction complex from apatite Subduction-related structures include imbricate thrusting in the blueschist- fission track analysis: Tectonics, v. 8, p. 197–220, https://doi .org /10 .1029 /TC008i002p00197. facies Taliaferro Metamorphic Complex, early W-directed folds and cleavage Dumitru, T.A., Wakabayashi, J., Wright, J.E., and Wooden, J.L., 2010, Early Cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the Francis- in lawsonite-albite facies metagreywackes, and the earliest foliation in the can subduction complex: Tectonics, v. 29, TC5001, https://doi .org /10 .1029 /2009TC002542. blueschist facies South Fork Mountain Schist. Structures formed during Ernst, W.G., 1970, Tectonic contact between the Franciscan mélange and the Great Valley Sequence—crustal expression of a Late Mesozoic Benioff Zone: Journal of Geophysical underplating include E-directed shear zones and E-vergent folding related Research, v. 75, p. 886–901, https://doi.org/10.1029/JB075i005p00886. to backthrusting within the accretionary wedge; major W-directed syn- to Ernst, W.G., 1971, Metamorphic zonations on presumably subducted lithospheric slabs from post-metamorphic thrusts that juxtapose tectonic units of different meta- Japan, California, and the Alps: Contributions to Mineralogy and Petrology, v. 34, p. 43–59, https://doi.org/10.1007/BF00376030. morphic grade; and intensive W-vergent folding and crenulation cleavage in Ernst, W.G., 1975, Systematics of large-scale tectonics and age progressions in Alpine and the South Fork Mountain Schist, which are likely to be associated with the circum-Pacific blueschist belts: Tectonophysics, v. 26, p. 229–246, https://doi.org/10.1016 major syn- to post-metamorphic thrust faults bounding the unit, including the /0040-1951(75)90092-X. Feehan, J.G., and Brandon, M.T., 1999, Contribution of ductile flow to exhumation of low- Coast Range fault system on the eastern margin of the Franciscan. Structures temperature, high-pressure metamorphic rocks: San Juan–Cascade nappes, NW Wash- related to exhumation include locally intensive normal faulting throughout ington State: Journal of Geophysical Research, v. 104, p. 10883–10902, https://doi .org/10 .1029/1998JB900054. the section, and normal-sense reactivation of the Coast Range fault. Harms, T., Jayko, A.S., and Blake, M.C., 1992, Kinematic evidence for extensional unroofing of the Franciscan complex along the Coast Range fault, northern Diablo Range, California: ACKNOWLEDGMENTS Tectonics, v. 11, p. 228–241, https://doi.org/10.1029/91TC01880. This research was funded in part by National Science Foundation grant EAR-1250128 to J. Platt. Hopson, C.A., and Pessagno, E.A., 2004, Tehama-Colusa serpentinite mélange: a remnant of We are grateful to Whitney Behr, Alex Lusk, Ellen Platzman, Daniel Platt, Daniel Schmidt, Franciscan Jurassic oceanic lithosphere, northern California, in Ernst, W.G., ed., Serpen- and Francisco MeldeFontenay for their help in the field, and to Tom MacKinnon for sharing tine and Serpentinites: Mineralogy, Petrology, Geochemistry, Ecology, Geophysics, and the results of his work in Grindstone Creek and for numerous stimulating discussions. We Tectonics, A Tribute to Robert G. Coleman: Geological Society of America International appreciate constructive and helpful reviews by Gary Ernst and John Wakabayashi, and we Book Series, v. 8, p. 301–336. thank Damian Nance for editorial handling. Hopson, C.A., Mattinson, J.M., Pessagno, E.A., Jr., and Luyendyk, B.P., 2008, California Coast Range ophiolite: composite Middle and Late Jurassic oceanic lithosphere, in Wright, J.E. and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hop- REFERENCES CITED son: Geological Society of America Special Paper 438, p. 1–101, https://doi.org/10.1130 Bailey, E.H., Irwin, W.P., and Jones, D.L., 1964, Franciscan and related rocks and their signifi- /2008.2438(01). cance in the geology of western California: California Division of Mines and Geology Hsü, K.J., 1968, The principles of mélanges and their bearing on the Franciscan-Knoxville Bulletin 183, 177 p. paradox: Geological Society of America Bulletin, v. 79, p. 1063–1074, https://doi .org /10 Berkland, J.O., Raymond, L.A., Kramer, J.C., Moores, E.M., and O’Day, M., 1972, What is Fran- .1130/0016-7606(1968)79[1063:POMATB]2.0.CO;2. ciscan?: The American Association of Petroleum Geologists Bulletin, v. 56, p. 2295–2302. Jayko, A.S., and Blake, M.C., 1986, Significance of Klamath rocks between the Franciscan Blake, M.C., and Jayko, A.S., 1983, Preliminary geologic map of the Yolla Bolly–Middle Eel Complex and Coast Range ophiolite, northern California: Tectonics, v. 5, p. 1055–1071, Wilderness and adjacent roadless areas, northern California: U.S Geological Survey Mis- https://doi.org/10.1029/TC005i007p01055. cellaneous Field Studies Map 1565-A, scale 1:62,500, https://pubs.er.usgs.gov/publication/ Jayko, A.S., and Blake, M.C., 1989, Deformation of the Eastern Franciscan Belt, northern Cali- mf1595A (accessed January 2018). fornia: Journal of Structural Geology, v. 11, p. 375–390, https://doi.org/10.1016/0191-8141 Blake, M.C., Irwin, W.P., and Coleman, R.G., 1967, Upside-down metamorphic zonation, blue- (89)90016-3. schist facies, along a regional thrust in California and Oregon: U.S. Geological Survey Jayko, A.S., Blake, M.C., and Harms, T., 1987, Attenuation of the Coast Range Ophiolite by Professional Paper 575, Chapter C, p. C1–C9, https://pubs.usgs.gov/pp/0575c/report.pdf extensional faulting, and nature of the Coast Range “Thrust”, California: Tectonics, v. 6, (accessed January 2018). p. 475–488, https://doi.org/10.1029/TC006i004p00475. Blake, M.C., Helley, E.J., Jayko, A.S., Jones, D.L., and Ohlin, H.N., 1992, Geologic map of Kimura, G., Maruyama, S., Isozaki, Y., and Terabayashi, M., 1996, Well-preserved underplating the Willows 1:100,000 quadrangle, California: U.S. Geological Survey Open-File Report structure of the jadeitized Franciscan Complex, Pacheco Pass: California Geology, v. 24, OF-92-271, scale 1:100,000. p. 75–78, https://doi.org/10.1130/0091-7613(1996)024<0075:WPUSOT>2.3.CO;2.

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