TIMESCALES OF DUCTILE FLOW IN THE MIDDLE CRUST DURING METAMORPHIC CORE COMPLEX FORMATION:DOMING, DECOUPLED FLOW, AND EVOLVING STRAIN LOCALIZATION

The footwall of the Pioneer metamorphic core complex (PMCC) contains ductile deformation fabrics that are intruded by ~52-44 Ma meter-scale granitic dikes, displaying the full range of relative timing relationships with strain (isoclinally folded and/or boudinaged to planar cross-cutting). Consequently, the age of strain and other structural features may be tightly constrained by dating deformed and undeformed dikes in single outcrops. The PMCC thus offers a unique opportunity to test models for isostatically induced lower crustal flow, and provide unprecedented documentation of the overall spatial-temporal evolution and localization of strain during doming and core complex formation.

The PMCC formed during Eocene NW-SE extension and is bound by a top-NW curviplanar detachment. The Wildhorse gneiss complex (WGC), which represents the deepest footwall exposures, was pervasively deformed during extension. High-T fabrics with NW lineations within the WGC record the regional extension direction; the subhorizontal to shallow W-dipping foliations are folded into subvertical dips on the east flank of the map-scale Wildhorse dome. U-Pb zircon dates from isoclinally folded and undeformed dikes on the east flank of the dome yield ages of 49.0±0.6 and 47.1±0.4 Ma, respectively. Two U-Pb ages from a granitic stock intruding the east flank yield ages of 47-48 Ma. These ages along with map relations indicate that large strains accumulated and the associated fabrics were folded into the dome between ~49 and 47 Ma, prior to emplacement of the stock. At the north end of the dome, fabrics in the Wildhorse gneiss complex are overprinted by greenschist-grade mylonites with abundant dikes where the WGC intersects the Wildhorse detachment.

The deepest exposed levels host a lineation transition zone (LTZ), where stretching lineation trends change abruptly downsection from NW to NNE. Deformed and undeformed dikes occur above and below the LTZ. Thus, dating of these dikes will allow us to test whether these two fabrics formed synchronously and test the model that the LTZ represents a zone of isostatically induced decoupled flow of the deeper crust driven by localized extension. Preliminary dating suggest both levels may have been active between ~53 Ma and ~47.5 Ma, but ongoing work will provide tighter constraints.