Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology

Abstract

Ophiolites are on-land remnants of oceanic lithosphere, and most of the more extensive ophiolites apparently formed above a subduction zone, a tectonic setting known as a suprasubduction-zone setting. Thin sheets of high-grade metamorphic rocks, known as metamorphic soles, crop out structurally beneath many suprasubduction-zone ophiolites. Such rocks may have formed during the inception of subduction beneath young and hot oceanic lithosphere. Disagreement exists as to whether suprasubduction-zone ophiolites are emplaced over the same subduction zone over which they once formed or over a later one. High-grade metamorphic rocks (blocks-in-mélange and coherent sheets) from the Franciscan Complex may represent a metamorphic sole beneath the suprasubduction-zone Coast Range ophiolite. Trace-element and isotopic data indicate that the Franciscan high-grade metamorphic rocks formed in a suprasubduction-zone envi ronment, requiring the existence of a pre-Franciscan sub duction zone, whereas later-subducted, lower-grade oceanic rocks are of mid-ocean-ridge or oceanic-island basalt affi ni ties. The Coast Range ophiolite and Franciscan high-grade rock protoliths formed over a pre-Franciscan subduction zone that may have dipped westward. The high-grade Franciscan rocks were metamorphosed at the inception of east-dipping subduction beneath the Coast Range ophiolite, and the ophiolite was subsequently emplaced over this later subduction zone. Suprasubduction-zone protolith signatures have been obtained for other metamorphic soles beneath suprasubduction-zone ophiolites, suggesting that our proposed model of suprasubduction-zone ophiolite generation over one subduction zone and emplacement over a second one may be globally applicable. Regional geology suggests that this dual subduction-zone model may also apply to suprasubduction-zone ophiolites with midocean-ridge and/or oceanic-island basalt soles.Generation of ophiolite over first subduction zone and emplacement over a later one.

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

“…Sierra Nevada ranges (Dunne et al 1998;Irwin 2003;Dickinson 2008) reflects an important component of eastward subduction of the Farallon oceanic plate commencing no later than *175 Ma, and continuing until *140 Ma. Underflow also resulted in the generation and storage at depth of HP/LT metamafic rocks, now present as tectonic ± olistostromal blocks in mélanges chiefly of the Franciscan Central Belt (Cloos 1982(Cloos , 1986Wakabayashi et al 2010). Middle and Late Jurassic oceanic plate motion probably involved oblique convergence rather than nearly orthogonal subduction ), because if a major forearc basin and trench had formed during this stage, evidence of this subparallel couplet has since disappeared completely.…”

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…The nearcoastal region of the former forearc broadly coincides with the surface distribution of the serpentinite tectonic blocks and slivers and diapirs in forearc crustal rocks (Figure 2) as well as 'sedimentary' or 'mud' serpentinites that were thought to be part of a mélange of serpentinites erupted on the seafloor in the oceanic basins (Barnes et al 2013), like those observed in some modern subduction zones, such as the Marianas (Fryer et al 1985;Murata et al 2009;Wakabayashi 2011). Field relations in California indicate some of the diapiric or fault-bounded serpentinites were emplaced in the late Cenozoic (Bailey and Everhart 1964;Dickinson 1966;Coleman 1971Coleman , 1977Coleman , 2000Murata et al 1979;Page et al 1999;Wakabayashi 2004Wakabayashi , 2005Wakabayashi , 2012Wakabayashi and Dilek 2011;Wakabayashi et al 2010). The common local occurrence of blocks of mantle peridotite, blueschists, and/or eclogites in such Neogene emplacements suggest that these serpentinite bodies may have originally come from deep crustal and/or mantle depths and were entrained with serpentinite ascent during the formation and lengthening of the San Andreas Fault System.…”

Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting

“…Dupuis et al, 2005;Polat and Kerrich, 1999;Saha et al, 2005;Taira et al, 1992;Tankut et al, 1998;Wakabayashi et al, 2010;Xu et al, 2012).…”