STRAIN DISTRIBUTION, LOCALIZATION, AND POSSIBLE MID-CRUSTAL DECOUPLING IN THE FOOTWALL OF A CORE COMPLEX: THE ROLE OF LITHOLOGY, MAGMA, AND MIGMATITES

The Eocene Pioneer core complex (PCC) in central Idaho is bound by the Wildhorse detachment and exposes a footwall of mid-crustal rocks that are locally migmatitic and folded into an elongate dome. NW-directed extensional deformation in the footwall is recorded by an intra-footwall detachment fault system and by a complex distribution of ductile fabrics. Using structural observations tied to U-Pb geochronology, we document the evolution of structures and fabrics formed during development of the PCC. This work shows that the spatial/temporal evolution of footwall strain was controlled by lithology, pre-existing structural geometries, the distribution of melt, and by the 3D thermal structure related to large-scale intrusions.

An early intra-footwall detachment omits section and is intruded by a 48-49 Ma granodiorite sheet. Although the granodiorite sheet follows this detachment, observations show that fault motion largely pre-dated intrusion and thus the detachment location was not magmatically controlled. Following granodiorite solidification, strain was partitioned into the heated weaker overlying quartzite and marbles, which were dramatically thinned locally.

At the deepest exposed levels, linear fabrics in paragneisses trend N/NE, but abruptly transition to the dominant NW orientations in the overlying orthogneiss. This transition may represent a mid-crustal zone of decoupling, where flow in the lithologically weaker and more migmatitic paragneiss resulted from compensatory inflow in response to extensional excision of overlying crust. Alternative explanations, however, are also permissible.

Cooling following 47-49 Ma intrusions led to widespread footwall stiffening (indicated by cross-cutting ~46 Ma dikes) and initiation of the bounding Wildhorse detachment. The detachment position was controlled by the solidified ~47 Ma stock in the eastern footwall; the distribution of ductile strain in the detachment zone was controlled by heat from the footwall intrusions, with areas away from the intrusions recording only brittle deformation.

On the regional scale, the PCC represents an area of localized extension, where extension amounts may be several times greater than that recorded in adjacent cross-sections. The causes and structures accommodating these gradients remain speculative.