STRAIN ANALYSIS OF METAMORPHIC ROCKS IN THE LATE GALICE FORMATION, KLAMATH MOUNTAINS: INSIGHTS FROM MICROSTRUCTURAL AND PETROGRAPHIC STUDIES

This research focuses on analyzing the results of strain analysis along the overturned isoclinal fold limbs of the Late Galice Formation within the Klamath Mountains. The samples were collected along the GO road, which provided well-exposed road structures of the Galice Formation. To understand the relationship between axial planar cleavage, bedding cleavage, and accommodated strain in a heterogeneous medium with grains of varying viscosity, we conducted microstructural and petrographic analysis on thin sections. The initial part of the study involves measuring strain markers using monocrystalline quartz crystals. In the subsequent phase, we will measure strain based on the preferred orientations of phyllosilicate minerals. The analyzed samples, which are metasandstones, exhibit fine-grained muscovite, chlorite, and layers rich in opaque minerals. These layers form continuous anastomosing cleavage, including quartz-mica beard structures growing along the cleavage planes. The axial planar cleavage (S1), which is continuous along the fold limbs, is at a low angle to the bedding-parallel cleavage (S0). Considering the shallow angle (~10°) between the axial planar and bedding-parallel cleavage, as well as the upward direction from the load structures, we concluded that the samples belong to the east-dipping limb of an overturned isoclinal fold within the Galice Formation. By performing a 3D strain analysis on the samples, we identified a consistent northeast-southwest trending pattern with a moderate northeast dip of ~40-60°. According to Hsu plot, the samples lie close to plane strain, with a strain type range of v = -0.09 to 0.45 and a moderate to high fabric magnitude (ε = 0.42, 0.63). The strain analysis results further support the conclusion that the study area is subject to moderate strain, with the orientation of strain ellipsoids overlapping the northeast-southwest trend of bedding and bedding-parallel cleavage. The highest strained samples were located near the east side of the fold axis of the eastern fold limb, indicating that they are the youngest in age, as the upward direction was from west to east.