DUCTILE-BRITTLE TRANSITION IN HOST ROCK RHEOLOGY DURING PLUTON EMPLACEMENT: OBSERVATIONS FROM THE NORTHEASTERN GREAT BASIN, USA

New field observations from two mid- to shallow-crustal plutons indicate a sequential variation in host rock rheology attending magma emplacement. The plutons intrude carbonate and siliciclastic rocks of the Cordilleran miogeocline. The White Horse pluton, Goshute Range, NE Nevada, intruded Devonian marble and quartzite. Marbles have a bedding parallel, spaced compositional banding (S1). Along the eastern margin of the pluton, quartzite marker beds define a set of overturned folds that verge towards the pluton contact and occur within the hanging wall of a west-directed thrust fault. The amplitudes of these folds decreases to the east and define an apparent structural aureole that is approximately 500 m thick. Locally S1 appears to wrap around curviplanar portions of the pluton contact. However, our in progress mapping suggests that the pluton both deflects and cross cuts S1, the overturned folds, and the thrust. While it is possible that fold and thrust development occurred prior to pluton emplacement, we note that the spatial proximity of these ductile structures to the pluton and the apparent increase in intensity of structural development approaching the pluton is compatible with contraction within the aureole during pluton emplacement. Yet, all of the ductile structures appear to be truncated by the intrusive contact. The Crater Island pluton, W Utah, intruded Upper Paleozoic, homoclinally west-dipping limestones. A 500 m thick contact aureole formed along the northern margin and consists of a penetrative, compositional banding that is subparallel to the enveloping surface of the curviplanar pluton contact. This foliation is axial planar to three west-southwest plunging folds with 50 to 100 m wavelenghts. At the mesoscale, the foliation is discordant to the pluton contact. We suggest that these observations reflect rapid changes in host rock strain rate as ductile creep mechanisms are outpaced by the rate of magma intrusion (i.e., magmatic overpressure + buoyancy), so that brittle fracture accommodates continued host rock displacement. These obesrvations indicate that a sequence of ductile to brittle deformation mechanisms apparently accompanies magma chamber construction.