CONTINENTAL ARCS IN 4D: THE ROLE OF SHEETED COMPLEXES IN VERTICALLY EVOLVING MAGMA PLUMBING SYSTEMS

The rarely exposed vertical dimension in arcs is typically the direction where the greatest chemical and physical evolution occurs in ascending magmas. One location exposing this vertical direction is in a tilted crustal section located in the Transverse Ranges of southern California, where the Mesozoic Cordilleran arc is exposed from 0-30 km depth. Here, we observe upper crustal granites-granodiorites as both blobby and tabular plutons, a mid-crustal (15-20 km depth) sheeted complex with meter-thick layers ranging from gabbro to garnet granite, and lower crustal diorite-tonalites. Though there is some range in silica, overall, magma compositions beneath and within the sheeted complex are distinctly more mafic than those in the upper crust.

While sheeted complexes are found in other tilted arc exposures (e.g. Cascades), their role, significance, and depth in the crust remain uncertain. Compared to individual plutons, these sheeted complexes are larger, tabular, and contain sheets that vary widely in age, composition (gabbros to granites) and texture (cumulate to aplitic to pegmatitic). Associated with the cores of orogenic belts or significant tectonic boundaries and found at mid-crustal depths, these complexes are potentially fundamental in the structure of arcs.

Vertical differentiation in the Transverse Ranges arc is focused in the sheeted complex. While a variety of differentiation processes are likely occurring simultaneously, filter pressing in sheets may be controlling which magmas rise higher in the crust, as hornblende cumulates within sheets suggest. The large surface to volume ratio of individual sheets may facilitate assimilation of host rock or pre-existing sheets. However, this may also result in rapid cooling, either driving the system below the solidus or creating a critical crystal saturation for filter pressing or any other method of melt-crystal separation to take place. The sheeted complex also has evidence of magma mingling, mixing, and storage. It is not yet clear how the sheets have been emplaced, as exposure is discontinuous from Cenozoic brittle faulting, or how upper-level granite plutons have moved through or emerged from this zone.