Assembly of a dike-fed magma chamber: The Jackass Lakes pluton, central Sierra Nevada, California
The mechanisms of ascent, assembly, and emplacement of granitic magma in the crust are critical to understanding the dynamics of continental margin growth, yet these mechanisms remain controversial and poorly understood. Detailed study of structural and petrologic fabrics in the middle Cretaceous Jackass Lakes pluton-wall-rock system, central Sierra Nevada, California, coupled with U-Pb geochronology, indicates that the pluton formed via sheet-like assembly of a dike-fed magma chamber. Final emplacement of the pluton was facilitated by multiple brittle and ductile mechanisms that were active at different times and places within the system; this supports hybrid viscoelastic emplacement models as realistic alternatives to end-member models (i.e., dike versus diapir). Fracture propagation, which initiated ≈40% of the space required for emplacement, may have been facilitated by a small component of arc-parallel dextral shear that produced north-northwest-striking tension gashes. A combination of ductile wall-rock shortening during lateral expansion of sheets, and return flow of elongate, strongly deformed wall-rock septa, produced an additional ≈25% of the space required. Other mechanisms, including coeval formation of the overlying Minarets caldera and stoping in the subvolcanic part of the magma chamber, must account for the remaining ≈35% space, implying that vertical transfer of material is an important emplacement mechanism at shallow crustal levels.