Subduction initiation along transform faults: The proto-Franciscan subduction zone John W. Shervais1* and Sung Hi Choi2 1DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, 4505 OLD MAIN HALL, LOGAN, UTAH 84322-4505, USA 2DEPARTMENT OF GEOLOGY AND EARTH ENVIRONMENTAL SCIENCES, CHUNGNAM NATIONAL UNIVERSITY, DAEJEON 305-764, SOUTH KOREA ABSTRACT The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones pres- ent special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduc- tion initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite mélange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that refl ect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that refl ect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite mélange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The mélange matrix represents hydrated refractory peridotites with forearc affi nities, and blocks within the mélange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current mélange refl ects largely upper-plate litholo- gies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault or fracture zone that evolved into a subduction-zone mélange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite mélange. LITHOSPHERE; v. 4; no. 6; p. 484–496 | Published online 14 December 2011 doi: 10.1130/L153.1 INTRODUCTION stable oceanic lithosphere, or adjacent to buoyant continental crust (e.g., Casey and Dewey, 1984; Leitch, 1984; Stern and Bloomer, 1992; Stern, The process of subduction initiation cannot be observed directly but 2004). In either case, the ultimate result is a subduction zone that underlies must be inferred from the rock record after a subduction zone is estab- a volcanic arc, where the arc rests on older oceanic crust or continental lished. “Induced” subduction initiation (Hall et al., 2003; Stern, 2004) crust (Tatsumi and Eggins, 1995). Investigations of early arc volcanism results from far-fi eld plate stresses that force convergence across a zone over the past two decades have shown that primitive early arc volcanics of weakness within a plate during plate-boundary reorganizations (e.g., comprise the same rock assemblages as most ophiolites, and they are com- the Maquarie-Puysegur-Fiordland system near New Zealand; Ruff et al., monly preserved in highly extended forearc regions, which are interpreted 1989) or in response to a collisional event between an existing subduction to represent the products of spontaneous subduction initiation (Bloomer et zone and continental or unusually thick oceanic crust (e.g., Ontong Java al., 1995; Stern and Bloomer, 1992; Hawkins, 2003; Reagan et al., 2010). Plateau collision with the Solomon Islands Trench; Cooper and Taylor, Most extensive ophiolite terranes are believed, on the basis of their 1985). In contrast, “spontaneous” subduction initiation occurs most com- lavas and mantle residues, to have formed in, or passed through, a supra- monly when stable ocean lithosphere, which forms by anhydrous decom- sub duction-zone environment (e.g., Miyashiro, 1973; Alabaster et al., pression melting at mid-oceanic spreading centers and essentially “fl oats” 1982; Shervais, 1982; Pearce et al., 1984; Shervais and Kimbrough, 1985; on top of the underlying asthenosphere, becomes gravitationally unstable Metcalf and Shervais, 2008). While the specifi c environment (backarc, and begins to sink back into the mantle, either adjacent to gravitationally arc, and forearc) is often debated, it is clear that the volcanic rock series found in ophiolites most closely corresponds to lavas now found within the forearc region of active arc terranes (e.g., Metcalf and Shervais, 2008). Further, it has been proposed that at their time of formation, there is little *E-mail: [email protected]. or no evidence for the existence of a volcanic arc (Stern and Bloomer, Editor’s note: This article is part of a special issue titled “Initiation and Termina- tion of Subduction: Rock Record, Geodynamic Models, Modern Plate Boundaries,” 1992; Pearce, 2003; Metcalf and Shervais, 2008). edited by John Shervais and John Wakabaya shi. The full issue can be found at Shervais (2001) showed that suprasubduction-zone ophiolites display http://lithosphere.gsapubs.org/content/4/6.toc. a consistent sequence of events during their formation and evolution, sug- 484 For permission to copy, contact [email protected] | |Volume © 2012 4 Geological | Number Society6 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/6/484/3038916/484.pdf by guest on 26 September 2021 Subduction initiation along transform faults: The proto-Franciscan subduction zone | RESEARCH gesting that they form in response to processes that are common to all such 2004; Metcalf and Shervais, 2008; Whattam and Stern, 2011), the specifi c ophiolites. This sequence includes: (1) birth—formation of the ophiolite setting or circumstances of this event are commonly unclear. Our second above a nascent or reconfi gured subduction zone; (2) youth—continued focus here is on the evidence provided by serpentinite mélange zones, melting of refractory asthenosphere (depleted during birth) in response to which are commonly associated with suprasubduction-zone ophiolites. fl uid fl ux from the subducting slab; (3) maturity—onset of semistable arc The petrologic and geochemical character of blocks within these mélange volcanism; (4) death—the sudden demise of active spreading and ophio- zones provides clues regarding their origin, which may be unrelated to the lite-related volcanism; and (5) resurrection—emplacement by obduction adjacent ophiolite, or may refl ect a linkage among the mélange, subduc- onto a passive margin or accretionary uplift with continued subduction. tion initiation, and ophiolite formation. This sequence of events is similar to that inferred by Reagan et al. (2010) We take as our case example the Coast Range ophiolite of Califor- from their detailed studies of the Izu-Bonin-Mariana forearc, with tholei- nia, which has been studied extensively for over three decades (Hopson itic “forearc basalts” preceding boninites, and with both forming during et al., 1981, 2008; Shervais and Kimbrough, 1985; Shervais, 1990, 2001; an episode of rapid extension in the forearc prior to establishment of the Coleman, 2000; Shervais et al., 2005a, 2005b). The Coast Range ophiolite calc-alkaline volcanic arc. is closely linked to the Franciscan assemblage—one of the world’s most Our primary focus here is on the fi rst two stages proposed by Shervais intensely studied subduction complexes and a model for convergent bound- (2001): (1) birth, characterized by forearc basalts (Reagan et al., 2010), ary plate processes (Bailey et al., 1964; Blake and Jones, 1974; Blake et represented by a variety of early arc tholeiites with strong mid-ocean- al., 1982; Wakabayashi, 1999; Ernst, 1993). In northern California, the ridge basalt (MORB)–like characteristics, and (2) youth, characterized by ophiolite is separated from younger rocks of the Franciscan assemblage by high-Mg basalts and andesites of the boninite suite, which are among the the Tehama-Colusa mélange—a serpentinite-matrix mélange containing most depleted volcanic rocks on Earth (e.g., Metcalf and Shervais, 2008). blocks of peridotite, metabasalt, chert, and high-grade metamorphic rocks These stages are thought to represent the onset and development of sub- that has been interpreted as an oceanic fracture-zone assemblage (Hopson duction initiation, followed by the transition to stable subduction (stage and Pessagno, 2005) linked to subduction initiation (Choi et al., 2008b). three: maturity and calc-alkaline volcanism). Ophiolites that represent the fi rst two stages in this progression (birth-youth) are interpreted to have GEOLOGIC SETTING formed during a subduction initiation event, regardless of their current apparent setting (Fig. 1). Further, these stages can be established