Mid-Crustal Strain in the Footwall of the Pioneer Core Complex, Idaho: New Working Models for Testing the Extent of Coupling with the Upper Crust

The Pioneer core complex in south-central Idaho contains a NW-directed brittle/ductile detachment that separates an unmetamorphosed upper plate recording NW-SE syn-Challis (50-45 Ma) extension from mid-crustal igneous/metamorphic rocks. The mid-crustal section, including a ca. 48 Ma granodiorite sheet intruded along the middle/lower plate boundary, is folded into an elongate NNW/SSE-trending dome, providing exposure of ~5-6 km of structural section. New observational and analytical data challenge many aspects of existing interpretations and allow the relationship between upper crustal and mid-crustal strain to be investigated.

A new U-Pb age of ~48 Ma from a syn-deformational leucogranite in the lower plate suggests that the middle/lower plate boundary may represent an important rheological transition zone separating rocks that were pervasively strained during Eocene extension from rocks that preserve ductile Cretaceous shortening-related fabrics. SSW sillimanite lineations and top-SSW shear were noted by Silverberg (1990) at the base of the lower plate. Thus, the strain directions are oblique to apparently synchronous NW-SE syn-Challis extension in the upper plate. This raises the question of whether there was pervasive flow of the middle crust that was decoupled from the upper crust during extension.

Based on available data, we present working models for the mechanical-kinematic significance of the middle/lower plate boundary, dome formation, and the extent of coupling between mid-crustal strain and upper crustal extensional structures. Two end-member decoupled models are possible. One model involves unidirectional flow of weak decoupled lower plate in response to topographic gradients related to previous crustal shortening. The second model invokes inward flow of weak decoupled middle crust beneath the detachment driven by localized extensional denudation. In contrast, coupled models attribute mid-crustal strain to motion along deep levels of the detachment system. These models are currently being tested with field observations and geochronological data.