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 accretion- ary 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 accre- tion, 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; LITHOSPHERE© 2018 The Authors. | Volume Gold 10Open | Number Access: 2 This | www.gsapubs.org paper is published under the terms of the CC-BY-NC license. 301 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/2/301/4098600/301.pdf by guest on 26 September 2021 SCHMIDT AND PLATT 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