Emplacement of shallow dikes and sills beneath a small basaltic volcanic center - The role of pre-existing structure (Paiute Ridge, southern Nevada, USA)

Late Miocene sills and dikes in the Paiute Ridge area of southern Nevada were emplaced in an extensional setting beneath a small volume, alkali basaltic volcanic center. Dikes (400-5000 m long, 1.2-9 m wide) mostly occupy pre-existing E-dipping normal faults. Elastic deformation of the wall rocks alone cannot explain dike dimensions; inelastic deformation, wall rock erosion by flowing magma, and syn-emplacement extension of the host structural system also contributed to dike widths. After primarily subvertical emplacement, flow focused toward the southern end of one of the dikes to form a volcanic conduit. This dike and a fault-hosted radial dike subsequently were subject to high pressures due to transient volcanic processes. Three small sills (extending laterally up to ∼ 500 m, and 20-46 m thick) and two larger sills (each having lateral dimensions ∼ 1 km) locally branch off some dikes within ∼ 250 m of the paleosurface. Individual small sills extend only into the hanging wall blocks of the faults that host their parent dikes, and are connected to the dikes by stems that are only a few tens of meters wide; elsewhere along their strikes the parent dikes extend above the sills. This mode of sill emplacement was caused by local rotation of principal stresses related to the intersection of the dike-hosting fault planes with the complex contact between relatively strong Paleozoic carbonates and weak Tertiary tuffs. Orientation of bedding planes in the tuffs controlled the direction of sill propagation. The three most areally extensive sills formed lopoliths with sagging roofs, indicating interaction with the free surface.