Compositional Trends and Eruptive Cycles at Mount St. Helens

The 40,000-year eruptive history of Mount St. Helens reveals an overall compositional trend from rhyodacite to andesite, with basalt at ~1.9 and ~1.6 ka. A cyclic eruption pattern is superimposed on this trend. Cycles comprised a repose interval, when compositional and thermal gradients developed in the underlying magma body, followed by an eruption interval in which progressive tapping of magma beheaded these gradients. Recovery of gradients varied with duration of the ensuing repose period. Eruption sequences follow the pattern: (1) eruptive progression from Plinian eruptions to dome growth accompanied by pyroclastic flows and tephra, followed (in some cases) by lava flows punctuated by pyroclastic outbursts; (2) a mineralogic progression from hydrous Fe-Mg phenocrysts (hb, cm, bi) toward pyroxenes; (3) a magmatic compositional progression from rhyodacite or dacite to andesite. Progressions 1 and 2 stem mainly from volatile gradients in the magma reservoir whereas progression 3 (and to some extent 2) reflects gradients of melt composition and crystal content. Three eruption cycles within the last 4,000 years follow this pattern. Earlier cycles are probable but only dimly perceived, mainly from the partial record of tephras and pyroclastic flows

Alternate origins of the Coast Range Ophiolite (California); introduction and implications

Correctly interpreting the tectonic evolution of the California continental margin requires understanding the origin of the Jurassic Coast Range Ophiolite, which represents a fragment of mafic-to-ultramafic crust of oceanic character lying depositionally beneath the western flank of the Great Valley forearc basin in fault contact with the Franciscan subduction complex of the California Coast Ranges. Three contrasting hypotheses for genesis of the ophiolite as seafloor are each based on internally consistent logic within the framework of plate tectonics, but are mutually exclusive and lead to strikingly different interpretations of regional tectonic relations, even though each assumes that the Sierra Nevada batholith to the east represents the eroded roots of a magmatic arc linked to subduction along the Mesozoic continental margin. To encourage the further work or analysis needed to develop a definitive interpretation, summary arguments for each hypothesis of Coast Range Ophiolite genesis in mid- to late Jurassic time are presented in parallel: (1) backarc spreading behind an east-facing intraoceanic island arc that then collided and amalgamated with the Sierran continental- margin arc; (2) paleoequatorial midocean spreading to form oceanic lithosphere that was then drawn northward toward a subduction zone in front of the Sierran continental-margin arc; and (3) forearc spreading within the forearc region of the Sierran continental-margin arc in response to transtensional deformation during slab rollback