David A. Foster

Jim Vogl, David Foster, David Rogers, & Paul Link

Of fundamental importance to continental extensional tectonics is an understanding of the rheological layering in the crust and the extent to which this layering controls the development of strain and structures at different crustal levels. In this project we are documenting the strain history through a range of crustal levels exposed in south-central Idaho to better understand the behavior of the middle crust during extension. The Pioneer metamorphic core complex, provides a vertical transect from faulted upper crustal rocks, through a curviplanar brittle-ductile extensional detachment. Excellent exposures beneath the detachment include a 5-6 km thick mid-crustal section with two structural domes, and an important rheological boundary, which separates rocks that were pervasively strained during Eocene extension from rocks that preserve Cretaceous shortening-related fabrics. The strain across these features is being documented through detailed field mapping and structural analysis complemented by anisotropy of magnetic susceptibility (AMS) analysis. The temporal component of the strain evolution is being established through a combination of U-Pb geochronology, and 40Ar/39Ar and fission-track thermochronology. Together these data will allow us to compare in detail, the kinematic and strain histories at the various structural levels and investigate the relationships between the mid-crustal rheological layering, domes, and curviplanar detachment. This work will add to our understanding of the nature and controls on rheological transitions in the middle crust and provide field-based tests of three current hypotheses for extensional orogens:

(1) The middle and lower crust is sufficiently weak that it decouples and flows in response to lateral pressure gradients, independently of the upper crust and tectonic forces, as suggested by numerical and conceptual models.

(2) Mid-crustal domes may result from decoupling of weak middle/lower crust that flows inward beneath extensional detachments with flow compensating for denudation of overlying crust.

(3) Vertical thinning during orogenic collapse may be localized within weak rheological layers that are controlled by the presence of partial melt in middle/lower crust.

Many of the current models involving a weak decoupled middle/lower crust are largely derived from inferences drawn from indirect observation rather than from direct field observations, highlighting the need for field-based testing integrated with detailed geochronology.