EXTENSION SUPERPOSED ON SHORTENING IN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX, SOUTH CENTRAL IDAHO

New mapping and thin section study of middle plate rocks from the Pioneer Mountains metamorphic core complex (Idaho) provides evidence of three deformational events (D1, D2, D3). The transect is located in the headwaters of Hyndman and North Fork Hyndman creeks, between the overlying Wildhorse detachment fault (to the west) and the underlying Pioneer Intrusive Suite (to the east).

Middle plate strata generally form a SW-dipping section but are involved in three map-scale NE-vergent folds: two overturned synclines and one intervening anticline. Compositional layering (S0) and a parallel metamorphic foliation (S1) define these folds. The folds are subhorizontal and generally NW-trending, although in one location two folds bend sharply to become W-trending. Downsection, in the upright limb of the eastern syncline, formations dip progressively more steeply to the west and contain numerous outcrop-scale, NE-vergent folds.

Inspection of hand specimens indicates a downsection increase in ductile strain. In the SW, immediately beneath the detachment fault, Kinnikinic Quartzite and Ella Marble appear weakly foliated and unlineated. To the NE, in the crest of the overturned anticline, the same formations contain a moderate foliation and weak WNW-trending lineation, while the moderately foliated Clayton Mine Quartzite preserves original cross-beds. Further downsection, underlying formations (banded calc-silicate, gniessose quartzite, pelite) abruptly become more strongly foliated, WNW-lineated, and appear to have experienced grain-size reduction via crystal-plastic flow mechanisms. Thin section analysis will be completed to document the minerals that define foliation and lineation and whether the lineation is formed by stretching or intersection. Our preliminary interpretation is that foliation (D1) was deformed into NE-vergent folds (D2), which were then weakly (SW) to strongly (NE) superposed by WNW-directed extension (D3). Because D3 strain increases with proximity to the underlying Pioneer Intrusive Suite, it is an Eocene (49-47 Ma) event. D2 may be Cretaceous or more speculatively Eocene (50-49? Ma) if an undated dike, which contains axial planar foliation to D2 folds, yields an Eocene U-Pb age.