EPISODIC MAGMATISM IN THE MESOZOIC SIERRA NEVADA ARC: MAGMA ADDITION RATES AND PLUTONIC/VOLCANIC VOLUME RATIOS

To explore arc-scale, temporal magmatic histories, firm geochronologic control is needed on all aspects of arc evolution. Both ICPMS U/Pb dating of in situ and detrital zircons indicate that the Mesozoic Sierran arc initiated at ~250 Ma, three magmatic flare-ups occurred with peaks at 225, 161, and 98 Ma, the arc never shut off, but migrated eastward at 85 Ma. Magma flare-ups and lulls are depth independent, although volcanism during the Jurassic and Cretaceous flare-ups reached their maximum ~5 Ma before plutonism.

True magmatic fluxes are not available for any arcs. In the Sierra Nevada, however, we have determined both plutonic and volcanic magmatic addition rates (MARs) through time by combining in situ igneous ages with (1) maps showing areal distributions of units; (2) retro-deformation of map units through use of 667 finite strain measurements and tectonic data; and (3) estimates of pre-deformation 3D volumes. The biggest unknowns are (1) pre-tectonic areal distribution of volcanic rocks plus the amount of volcanic ash deposited outside of arcs, and (2) true 3D volumes of plutonic rock versus depth. We maximized estimates of volcanic volumes by connecting isolated volcanic pendants with similar ages and assuming ½ as much ashfall outside the arc. In regards to plutonic volumes, conservative estimates of total percent of plutonic versus host rock increases from ~65% at crustal levels < 10 km to ~90% at 30 km depths. Plus huge volumes of mafic to ultramafic plutonic materials likely occur in unexposed crustal roots. Even by minimizing plutonic MAR calculations, assuming 30 km arc thickness, we obtained plutonic/volcanic volume ratios in excess of 20/1. For a 70 km thick arc, we obtained ratios >30/1: true ratios that include roots are even higher.

For 1^o arc length, Cretaceous MARs reach ~300,000 km³ per 10 m.y. interval with total volumes swamping both Jurassic and Triassic flare-ups combined (peak MAR’s <75,000 km³ per 10 m.y. interval) although the latter are underestimated due to recycling of plutonic and volcanic materials during subsequent flare-ups. These huge MAR flare-ups significantly rework crustal columns, change crustal rheologies, and along with equally episodic tectonism (see Cao et al., this session), drive surface uplift, form large crustal roots and cause remelting of crustal materials.