SPECULATIONS ON SIERRA NEVADA BATHOLITH EVOLUTION

Batholiths are central components of the Earth's architecture because they signify crustal growth and recycling and commonly form strong, high-standing terrains that influence regional tectonics, surface processes, and climate. In spite of their important role in continental geology, however, several key aspects of batholith formation are poorly understood. This talk will summarize evidence pro and con for several conjectures about the evolution of the Sierra Nevada batholith (SNB), including:

(1) The SNB probably never attained Andean crustal thickness (60+ km) because since the Jurassic it has resided on a windward margin subject to mid-latitude westerlies and consequent glacial and fluvial erosion. Great crustal thickness in the central Andes occurs at trade wind latitudes where erosion is minimal and volcanic cover can accumulate to great thickness. The southern Andes, where westerlies prevail and erosion may limit crustal thickness, is perhaps a more appropriate analog for the SNB. (2) The unbroken, high-standing nature of the SNB and other batholiths results from their large grain size. Field data commonly show that coarse-grained "weak" minerals (e.g., quartz) exhibit greater strength in shear zones than fine-grained "strong" minerals (e.g., plagioclase). Thus, the SNB may form the unbroken, high-standing, rain-capturing western boundary of the Basin and Range because of protracted cooling and consequent formation of large crystals. (3) Many plutons in the SNB were emplaced at roughly the same time as movement across nearby contractional shear zones, yet many modern arcs are extensional. The shear zones may record shortening during transient subduction of asperities on the downgoing slab, with the plutons themselves exploiting the shear zones during ensuing extension. (4) Mass balance of SNB magmas requires input of large amounts of continental lithospheric mantle supplied by antithetic subduction of the backarc. Rapid Late Cretaceous batholith growth coincided with large-magnitude backarc shortening in the Sevier belt. (5) Plutons in the SNB settled into the crust under the load of overlying plutons and volcanic rocks, displacing wall rocks laterally; mass balance in such systems is a fundamentally 3-dimensional problem that cannot be solved in map view.