EARLY HISTORY OF SUPRA-SUBDUCTION ZONES (SSZ) - INSIGHTS FROM THE MARIANA TRENCH AND ZAMBALES OPHIOLITE

Many North American Cordillera terranes have ophiolite components. Many have arc affinity (e.g., Trinity, Josephine, Sparta, Smartville, Preston Peak). Conventional wisdom is that SSZ arcs are built on older ocean crust, yet “roots” of arcs are rarely found; exceptions are Tonsina, AK and Zambales, Luzon. These show that nascent island arcs form above mantle diapirs that rise into rift systems above subduction zones owing to crustal extension that is a consequence of trench “roll back."

Evidence for mantle diapirism is the penetrative ductile deformation of Cr-rich harzburgite/dunite, at near solidus P-T; (e.g., Canyon Mt., Josephine, Trinity, Zambales). Thick Ol- Opx- Cpx cumulates, (700 m in Zambales) show similar high-T deformation effects;Zambales has up to 1 km of meta-norite. Higher level norite/gabbro, tonalite, diorite, basalt (roof rocks of detachments ?) usually have only low T metamorphic overprint. The Zambales ophiolite comprises rocks of a boninite forearc; “MORB”, gabbro, troctolite from backarc basin; and thick primitive arc series (cumulates, norite, tonalite, basalt). All overlie depleted harzburgite.

The Mariana forearc has been translated eastward more than 1200 km by trench roll-back from the Palau - Kyushu Ridge (PKR) since Eocene. Debris flows dredged on Mariana Trench wall, NE and SW of Guam, sampled roots of a forearc and arc (from PKR ?). Near solidus, high P, penetrative ductile deformation formed gneissose, porphyroclastic, blasto-mylonite and mylonite textures in harzburgite, dunite, norite, and hbl anorthositic gabbro. Boninite and higher level gabbro/norite, tonalite, basalt, silicic tuffs and clastics are undeformed.

New crust of forearcs, volcanic arcs, and backarc basins includes all ophiolite rock types as well as distinctive rocks , e.g., boninite, island arc tholeiite series, tonalite-trondhjemite, and silicic tuffs - clastics. Arc-derived tuffs and clastics are deposited on nearly coeval ocean crust in the early rift stage of backarc basins, and in the forearc. Basalt flows, sills, and dikes may be interspersed with hemipelagic sediments and arc-derived clastics. Parts of backarc basins, starved of coarse clastics, may have pelagic and metalliferous sediments, “umbers”, hydrothermal vent deposits, and silicic tuffs (protolith for cherty argillites). Mature arcs may be underlain by plagiogranites and tonalites complementary to high-Ca, low-K silicic arc volcanism.