A REVIEW OF THE TECTONIC SETTING AND CHARACTERISTICS OF QUATERNARY SILICIC CALDERAS

Silicic calderas occur in a wide variety of tectonic settings, from oceanic volcanic arcs to rifting continents, and are the surface expressions of large magma chambers within the upper crust. In order to understand why silicic calderas form in different tectonic settings, we here undertake an empirical, global investigation of Quaternary silicic calderas. In particular we focus on the relationships between caldera size, composition of the caldera-forming unit, and tectonic setting. Such analysis will aid in determining tectonic provenance of exposed plutons. In addition, with respect to hazard analysis, this research will address the question of whether any volcanic area with silicic output can produce an explosive eruption of sufficient magnitude to produce a caldera, or whether there needs to be a specific confluence of factors that allows the growth of a large volume, silicic magma chamber.

This project incorporates data from a vast literature review and pre-existing datasets. Tectonic, compositional, and physical data were compiled for all known Quaternary silicic (trachytic or dacitic to rhyolitic) calderas greater than 5 km in diameter, a data set of ~150 calderas. The vast majority of silicic calderas meeting our criteria occur in arc settings (77%), with the remainder related to rifting/mid-ocean ridges, hotspots, or post-orogenic deformation.

Specific tectonic settings favor larger and more silicic calderas. In all tectonic settings most of the silicic calderas occur on continental crust, although it is important to note that approximately 20% of all silicic calderas are underlain by oceanic crust. Silicic calderas of a variety of compositions are present on all types of crust (oceanic, transitional, and continental), but the vast majority of calderas lying on continental crust are rhyolitic, as are most calderas >15 km wide. There is a stronger relationship between the type of underlying crust and the size of the caldera -- calderas >30 km in diameter are exclusively located on continental crust, and there are no calderas >20 km in diameter in oceanic settings. Calderas in rifting environments (oceanic or continental) tend to be more alkalic and have caldera-forming eruptions consisting of trachyte or peralkaline rhyolite, whereas arc-related calderas are rarely alkalic.