Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study

The Papoose Flat pluton of eastern California is commonly cited in the geol ogic literature as a classic example of a "forcibly" emplaced pluton, altho ugh the relative importance attributed to magmatic versus tectonic processe s in controlling the structural evolution of the pluton has been controvers ial. Reexamination of this Late Cretaceous (83 Ma) pluton, using a combinat ion of new structural, petrographic, microstructural, and anisotropy of mag netic susceptibility (AMS) data on the pluton-wall-rock system, has shown t hat the pluton is an inclined and internally zoned tabular structure, assem bled by forcible intrusion of successive pulses of magma at a crustal depth of 12-15 km. Initial pluton formation involved magma ascent in a vertical west-northwest-striking feeder dike, which was arrested at a stratigraphica lly controlled mechanical discontinuity in the overlying Cambrian metasedim entary rocks, leading to formation of a southwest-dipping sill. Subsequent sill inflation, accompanied by horizontal infilling from the feeder dike at the base of the sill, resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof, facili tated by thermal weakening as the wall-rock temperatures progressively rose during emplacement of successive magma pulses. Cooling from the roof of th e pluton downward resulted in cessation of vertical inflation on the west s ide of the pluton and promoted lateral expansion toward the northeast and f loor depression below the eastern part of the pluton. We have been unable to document any regional-scale structures (e.g., of equ ivalence to similar-age synplutonic strike-slip shear zones in the Sierra N evada batholith to the west) that may have controlled emplacement of the Pa poose Flat pluton. However, this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time; indeed, tectonic overpressuring was probably essential for produ cing sufficiently high magma pressures for laccolith formation at this midc rustal level. Simple thermal modeling, using microstructural and thermobaro metric data, indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr. This rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional-scale deformation.