PLASTIC TO CATACLASTIC TRANSITIONAL DEFORMATION OF THE SANTAQUIN BASEMENT COMPLEX IN THE CHARLESTON-NEBO THRUST SHEET, CENTRAL UTAH

In a fold-thrust belt (FTB) the presence of crystalline basement in a thrust sheet indicates a deformation history that is critical to understanding the overall mechanics of the belt. As an example, we examine the deformation history of a Proterozoic basement slice carried by the Charleston-Nebo thrust sheet, the frontal-most exposed sheet in the Provo Salient of the Sevier FTB in Central Utah. This slice, the Santaquin Basement Complex (SBC), has undergone several phases of deformation, including Proterozoic terrane accretion, Cretaceous Sevier thrusting and Tertiary Basin and Range normal faulting. During thrust emplacement and erosion the unroofing of the SBC carried it through the transition between greenschist grade plastic deformation and cataclastic deformation. A microstructural study of the deformation that occurred during Cretaceous thrusting shows an overprinting of plastic features by cataclastic features. Optical microscopy of samples from a transect across a prominent shear zones within the SBC shows the progression of microstructure development. Weakly deformed rocks show greenschist grade features like chlorite vein fill and sericitized and sausseratized plagioclase grains, together with undulose extinction and intragranular cracks in quartz and feldspar. More highly deformed rocks show undulose extinction in quartz and feldspar, continued cracking in feldspars, and subgrain and recrystallized grain development in quartz, together resulting in grain size reduction and development of a protomylonite. The most highly deformed rocks show transgranular cracking in all grains indicating cataclasis overprinting the protomylonite fabric. This transitional deformation during Cretaceous thrusting yields insight into the unique circumstances leading to current placement of the SBC in a foreland thrust sheet, as well as providing information on the process of transitional deformation in granodioritic rocks.