ABSTRACT
Arc-trench systems of the circum-Pacific orogenic belt contain voluminous sandstones within fore-arc terranes that include complexly deformed subduction complexes and relatively undeformed fore-arc basins of several types. Turbidites incorporated within subduction complexes include not only axially transported trench fill, but also abyssal-plain sediments deposited on the sea floor beyond the trench, and slope-basin deposits perched on the accretionary trench slope. Mean framework modes of circum-Pacific sandstone suites can be expressed adequately by the use of QFL and QmFLt plots. Supplementary QmPK and QpLvLs plots depict partial modes of mineral grains alone and polycrystalline lithic fragments, respectively. Source rocks were mainly volcanic and plutonic terranes of the circum-Pacific magmatic arcs, although some recycled sedimentary and metasedimentary debris is also present.
Analysis of point counts for 2,587 rocks grouped into 104 suites from California, the Pacific Northwest, southern Alaska, southwest Japan, and New Zealand indicates that the characteristic fore-arc sandstones display similar compositional ranges everywhere, and can be termed the “circum-Pacific volcano-plutonic graywacke suite.” Knowledge of their petrology may aid the recognition of analogous tectonic settings in the geologic past. Frameworks vary across a compositional spectrum that ranges chiefly from feldspatholithic sands derived mainly from volcanic cover in undissected magmatic arcs to lithofeldspathic sands derived largely from plutonic batholiths in dissected magmatic arcs. Quartz contents are typically intermediate and the feldspar-to-quartz ratio is semiconstant. Plagioclase is the dominant feldspar and volcanic rock fragments are commonly the most abundant lithic fragments.
Interpretations of compositional trends within subduction complexes and adjacent fore-arc basins imply that simple transverse delivery of sediment from the arc orogene to the fore-arc region is not the rule. Instead, sand evidently is distributed along subduction zones by longitudinal turbidity currents initiated at widely spaced sediment delivery points, where sediment derived from tectonic culminations along the arc-trench system can bypass the sediment traps of fore-arc basins. Moreover, turbidites shed from dormant or transform segments of arc orogenes are locally spread far into the ocean basin as abyssal plains, which are later rafted bodily into the subduction zone. Finally, no sedimentologic explanations of compositional contrasts across fore-arc graywacke terranes can be accepted without considering the alternative of postdepositional tectonic transport by strike slip.
