Paper No. 330-12
Presentation Time: 4:10 PM
TECTONIC HISTORY OF THE IDAHO BATHOLITH: FABRIC ANALYSIS THROUGH TIME
BYERLY, Ad, Department of Geoscience, University of Wisconsin-Madison, Weeks Hall, 1215 West Dayton Street, Madison, WI 53715, TIKOFF, Basil, Department of Geoscience, University of Wisconsin, 1215 W Dayton St, Madison, WI 53706, GASCHNIG, Richard M., Department of Geology, University of Maryland, College Park, MD 20742, FAYON, Annia, Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455 and VERVOORT, Jeffrey D., School of the Environment, Washington State University, Pullman, WA 99164
The Idaho Batholith is one of the large North American Cordillera batholiths. Recent geochronology indicates relatively continuous magmatism from 100-54 Ma (Late Cretaceous and early Tertiary), making it distinctly younger than the other US batholiths (Sierra Nevada, Peninsular Ranges). The bulk of the batholith – the main phase of the Atlanta Lobe – consists of homogeneous peraluminous two-mica granite, indication derivation from crustal melting. Structural investigations of the Idaho Batholith are mostly lacking due to the absence of mappable contacts and obvious fabrics. In this study, we constrain the first-order structural history of the Idaho batholith using detailed fabric analyses at sites where robust geochronological (Laser ablation ICPMS U-Pb on zircon) data exists. Fabric throughout the Idaho batholith formed dominantly in the magmatic state, as determined by microstructural analysis; these data also constrains the formation of the fabric to be shortly after the recorded the U-Pb zircon age. The orientation and strength of the fabrics were determined using anisotropy of magnetic susceptibility (AMS) and shape preferred orientation (SPO) techniques. This approach allows us to examine fabric evolution through time.
Fabric development varies in space and time. Fabrics within igneous rocks affected by the western Idaho shear zone (suture zone suite: 100-92 Ma) show a well-developed and consistently oriented (NS) solid-state fabric: All younger fabrics are dominantly magmatic. The border zone suite (91-85 Ma) displays weakly consistent orientations. Fabric orientations in the Atlanta lobe (85-65 Ma) are highly variable, which we interpret as due to internal plutonic processes without the influence of significant regional tectonic strain. The northern Bitterroot lobe (65-50 Ma) show consistently oriented (NW-SE) and better developed fabrics. The very weak development of fabrics throughout the Atlanta Lobe suggests that the Idaho batholith existed as a long lasting center for episodic melting of continental crust in a relatively dormant tectonic regime. Yet, Idaho batholith formation occurred during significant foreland deformation in the thrust belt to the east
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