The Late Cretaceous Mono Creek granite has a pronounced NW-SE elongate shape, 60 km long by 10 km wide, characteristic of plutons from the eastern Sierra Nevada batholith. An 8 km-wide bulge exists on the NE side of this pluton, which exhibits evidence of forceful emplacement (or in-situ ballooning), such as deflection of metamorphic wallrock and igneous foliation, and the orientation of fracture patterns. Three-dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge, a recurring problem in studies of plutonic terranes. We suggest that emplacement of the Mono Creek bulge was accommodated by all components of the three-dimensional displacement field-including translation, rotation, and pure strain (shape change)-of the surrounding units. Classical strain analysis only addresses the rotation and pure strain components, and is incapable of quantifying the translation component. However, our analysis suggests that translation plays the dominant role in the emplacement process. A shell model of translation of the surrounding igneous and metamorphic units is proposed for the Mono Creek bulge, which suggests that the translation component decreases dramatically away from the intrusion, consistent with the observed geology and finite strain analysis. We propose that translation is the solution to the recurring pluton 'space' problem, either through tectonically controlled (passive) or magmatically controlled (active) movement of the wallrocks. Translation is generally the neglected component of the displacement field, but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction.
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We wish to thank D. Greene for sharing his knowledge of the Mt Morrison pendant, D. Schultz-Ela for help using his tryell program for three-dimensional strain analysis, and S. Morgan and R. Law for continued discussion about pluton emplacement. Helpful reviews by J. Evans, J. Spray, and J. White are acknowledged. Fieldwork was supported by NSF EAR-9305262 and CNRS-INSU DBT grants.