ACCOMMODATION OF EOCENE CRUSTAL EXTENSION THROUGH ISOSTATICALLY-INDUCED DECOUPLED CRUSTAL FLOW, IN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX (IDAHO-US)

Isostatically-induced decoupled lower crustal flow has been proposed as one of the mechanisms that accommodate crustal extension during formation of metamorphic core complexes. Few studies have conclusively shown synchronous decoupled flow at different levels. The Pioneer metamorphic core complex (PMCC) has a unique set of features to test models for, and document the conditions and characteristics of isostatically induced decoupled flow.

The PMCC in central Idaho formed during Eocene NW-SE extension. The Wildhorse gneiss complex, which represents the deepest footwall exposures, was pervasively deformed during extension, and hosts high-T fabrics with NW lineations recording the regional extension direction. The deepest exposed levels host a lineation transition zone (LTZ), where stretching lineation trends change abruptly down-section from NW to NNE. The fabrics above and below the LTZ are intruded by ~55-44 Ma meter-scale granitic dikes, displaying the full range of relative timing relationships with strain (isoclinally folded and/or boudinaged to planar cross-cutting).

We have performed LA-ICP-MS U-Pb zircon geochronology of these intrusions to understand the timing relationships of high-T ductile strain, and test whether the fabrics at the different levels formed synchronously or sequentially.

Our results show that strain was synchronous above and below the LTZ. 49-47 Ma represents the time frame were most strain was accumulated, although strain likely began before this. Titanite U-Pb ages indicate that high-T metamorphism took place at 48Ma, synchronous with pluton emplacement.

Our model for the PMCC suggests that; 1) MCC Eocene formation was driven by a mid-crustal shear zone active before ~49Ma, and 2) high-T metamorphism took place at 48Ma, synchronous with pluton intrusion in the shear zone, doming, and mid-crustal flow (49-47Ma). We estimate that the depth of decoupling was shallower than the depths range of theoretical models of crustal flow, suggesting that pluton emplacement supplied enough heat to weaken and localize the strain at this crustal level. Future P-T work in metamorphic rocks, will establish the depth and temperature of the decoupling zone before and during pluton emplacement, to better understand the controls on decoupling.