THE DEADWOOD DEFORMATION ZONE, CENTRAL IDAHO: CONSTRAINTS ON TIMING AND FABRIC DEVELOPMENT

The Deadwood deformation zone is an intra-batholith structure within the 90-54 Ma Idaho batholith. The Deadwood deformation zone is a locally up 2 km wide ductile feature, trends 010, dips steeply, ductilely deforms pre-Tertiary rocks, and forms discrete brittle fault segments in Eocene and older aged rocks. Previous examinations of the mylonites of the Deadwood shear zone reveal evidence for dextral motion. The northern-most extent of the Deadwood shear zone, 1 km directly north of Goldman Cut (25 miles N-NE of Yellow Pine, Idaho), was chosen to constrain the timing of the ductile deformation. Field mapping indicates that mylonitized granite, screens of Paleozoic metasedimentary rock, boudinaged dikes of aplitic white-mica granite, and a cross-cutting porphyritic rhyolite dike are exposed in this area. U-Pb analyses on zircon were conducted at Washington State University, using laser ablation inductively coupled plasma mass spectrometry. The sheared granite, forming the matrix, yielded Ordovician and Neoproterozoic aged cores and whole grains, a few grains that contained rims dated at 79-82 Ma, and a single whole grain of 86 Ma. The boudinaged pods of aplitic granite was dated at 82,8 +/- 1.8 Ma. Finally, the cross-cutting rhyolitic dike provided an age of 47.0 +/- 1.3 Ma, indicating an expected Challis age. The age dating indicates that deformation continued past 82 Ma and ceased by 47 Ma. The zircon rims on the older cores in the sheared granite may be metamorphic, which would suggest that peak metamorphism (and deformation?) occurred at ~80 Ma. To address the effect of pervasive and wide (~2 km) fabric development, we sampled granites adjacent for the shear zones, in an eight-mile traverse perpendicular oriented perpendicular to the strike of the zone. This transect uses anisotropy of magnetic susceptibility (AMS) analyses, a well-established tool for identifying fabrics in granitoid bodies, and shape preferred orientation analyses of mineral phases. The preliminary data indicate that pervasive fabrics occur in the adjacent granites, oriented parallel to the fabric within the shear zone. The relation of these fabrics to regional deformation – rather than being directly attributable for movement on the Deadwood deformation zone – will be discussed.