PLUTON EMPLACEMENT PROCESSES INFERRED FROM GEOLOGIC MAPPING OF THE SPLIT MOUNTAIN ROOF PENDANT, SIERRA NEVADA BATHOLITH

The oft-photographed Split Mountain area of the eastern Sierra Nevada (originally mapped by Jim Moore and Paul Bateman of the USGS) bears particularly on emplacement of three plutons: the Jurassic Tinemaha granodiorite and Red Mountain Creek (RMC) leucogranite and the Cretaceous Lamarck granodiorite. Contact and geometric relations of the Lamarck pluton resemble the closely related McDoogle pluton (Mahan et al., 2003, GSA Bulletin) and favor the interpretation that the Lamarck is a dike-like body localized by the same regional shear zone as the McDoogle pluton. The floor of the Tinemaha pluton is exposed over an area of several km², and the gentle dip of the contact, its general concordance with Cambrian metasedimentary strata in the floor, and the lack of xenolith accumulation on the floor favor laccolithic emplacement of at least this southern part of the pluton. Zak and Paterson (2006, J. Struc. Geol.) argued that the spectacular subhorizontal roof of the RMC pluton, featured in John Shelton's famous photograph, reflects emplacement by stoping and diapirism. However, their arguments are based on several incorrect observations. They interpreted dark cliff exposures from afar to be metasedimentary xenoliths, but most are diorite dikes of the 148 Ma Independence swarm ; we have carefully examined large areas of the RMC, and xenoliths are practically absent. Zak and Paterson stated that the flat roof bends over into steep primary walls. Owing to cross-cutting intrusions and Quaternary cover, only the northern and western contacts are exposed; both originally dipped gently but have been strongly modified by a major ductile shear zone (western) and high-angle faults (northern). Ductile deformation of wall rocks mainly predates emplacement of the RMC, locally postdates it, and nowhere appears to be synchronous with intrusion as would be required by diapiric emplacement. The undated Taboose leucogranite, which they interpreted to be a second diapir nested inside the RMC, is probably Late Cretaceous rather than Jurassic; the RMC contains hundreds of Independence dikes whereas the Taboose leucogranite contains only a few dikes that likely are of Cretaceous age. Field relations thus favor laccolithic emplacement of the vertically stacked Jurassic plutons and do not favor a significant role for either stoping or diapirism.