SUMMARY
This article explains the structure and history of Crevice pluton. The structures found inside the pluton are observed which include the moderate to steep penetrative foliation, top-to-west shallow ductile shear zones and subvertical northwest-striking mode I fractures of regional extent. A research was conducted to examine the inside architecture and age of deformation of Crevice pluton, near the main fork of the Salmon River.
This investigation also attempts to examine the relation that is assumed to exist between the different fabric patterns of Crevice Pluton. Laser ablation technique was also used to analyze zircon from Crevice Pluton and Looking Glass pluton, which is adjacent to it. The methods used for the research involved the collection of different rocks, which were examined at the University of Idaho, Moscow. The analysis of these zircons was done at the Geo-Analytical Laboratory of Washington State University.
The findings of Crevice Pluton zircon analysis showed that most of zircons had a bright core surrounded by dark rims. The age of the rims and cores were sometimes not recognizable but a few of them had old cores. This pattern determined the crystallization age of the pluton to be Early Cretaceous age.
In case of the Looking Glass pluton, most of the zircon grains did not show any cores. The analysis recorded the age of crystallization for the Looking Glass pluton to be Late Cretaceous age. Later in the paper the Pluton ages, relations, and deformation constraints are also discussed. As per this discussion, the age of the Crevice pluton makes it a part of biotite-granodiorite and megacrystic orthogenesis, which was originally found in the Little Goose Creek.
The Zircon from the Looking Glass Pluton is found to be within error of the Payette River tonalite found across the Little Goose Creek. This pluton also overlaps the Payette River tonalite in its age as well. The combination of the ages of these two plutons, along with the already existing Ar–Ar and Sm–ND data from the Salmon River canyon, define the time of ductile deformation along the segment of Riggins region boundary.
The analysis of pluton structures resulted that no evidence has been found which would support the assumption of overlapping tectonic belts in the Riggins Region of the West- Central Idaho. The paper concluded that the data collected from the analysis of zircons collected from Crevice Pluton and Looking Glass Pluton determines that respective crystallization ages were the first ones to be reported from the Salmon River Canyon.
As the paper already explained that there is no such overlapping between the tectonic belts, the study also suggests that during the process of contractional deformation, an accumulation of high strain fabrics occurred. The analysis of the Salmon River Canyon’s architecture, led to the suggestion that distinguished changes occur between the Riggins Region and the northern and southern areas. At the end, the research explained and reinterpreted the arc-craton boundary as it aimed to study the orogenic belts and their overlapping relation in west-central Idaho.
