MID-CRUSTAL STRAIN DURING EPISODIC EXTENSION IN SOUTH-CENTRAL IDAHO: RESULTS OF PRELIMINARY WORK IN THE FOOTWALL OF THE PIONEER CORE COMPLEX

In the Pioneer core complex (PCC) of central Idaho, the Wildhorse detachment separates upper crustal rocks from mid-crustal igneous/metamorphic rocks. The mid-crustal exposures comprise a dome of gneisses flanked by a sheet of the ~48 Ma Pioneer intrusive suite (PIS), which in turn is overlain by suite of metasedimentary rocks on the SW flank of the dome along a proposed extensional shear zone. Previous interpretations suggest that the metamorphic core developed as a result of syn-PIS SSW-directed extension along this shear zone followed by latest Eocene WNW-directed extension along the Wildhorse detachment. In this model extensional strain is limited to these two shear zones and doming is attributed to extensional unroofing. More recent data from upper crustal rocks in adjacent areas conflict with the direction and timing of these proposed extensional episodes. We have initiated a study to test current interpretations and gain a better understanding of the behavior of the middle crust during episodic extension, including processes associated with dome formation. Here we present preliminary results and observations from this study.

One of our main conclusions is that the temporal/spatial distribution of mid-crustal strain is more complex than previously recognized. For example, a new U-Pb zircon age of 48 Ma on a foliated syn-extensional migmatite from the gneisses below the PIS, indicates that extensional strain was not limited to the two shear zones. Some of this strain records WNW-ESE extension as evidenced by some lineations within the gneisses that are parallel to those within the Wildhorse detachment. Within these gneisses, however, lineations are highly variable and may record two episodes of extension, as well as pre-extensional shortening. AMS (anisotropy of magnetic susceptibility) lineations from the syn-extensional PIS have NW-SE trends and may indicate that, contrary to previous interpretations, the PIS was emplaced during NW-SE extension, rather than SW-NE extension. Further magnetic analysis is being performed to test models of timing and mechanisms of dome formation. For instance is the doming related to a component of horizontal shorting rather than to 2D extension? More detailed mapping and analytical work will provide a better understanding of the extensional strain history of the region and the mid-crustal response to episodic strain.