BIOTITE INCLUSION TRAIL EVIDENCE OF DEFORMATION IN THE PEQUOP MOUNTAINS, NEVADA

The Pequop Mountains are a north-south trending mountain range in northeastern Nevada. Rocks metamorphosed by crustal thickening during the Cretaceous Sevier orogeny were exhumed during Basin and Range extension. The low-grade metamorphism that occurred in the Pequops provides conditions where evidence of multiple deformations still exists in the rocks. Our study focused on the Dunderberg Shale, a schist layer with biotite porphyroblasts. The Dunderberg Shale shows a dominant foliation (S₂), visible in the matrix as aligned muscovite micas and also in outcrop. S₂ is roughly parallel to the compositional layering. In some samples, evidence of an earlier foliation (S₁) is visible in the matrix and as inclusion trails in biotite porphyroblasts. S₁ is also visible in rare outcrops in the field. We think that S₁ formed as a result of crustal thickening, with the evidence of S₁ surviving the crustal extension that formed S₂. The goal of this study is to determine the orientation of S₁ prior to extension in order to learn something about the kinematics of crustal thickening.

During the summers of 2005 through 2007, we collected 12 samples of the Dunderberg Shale and made thin section pairs for analysis: one perpendicular to the foliation and one parallel. So far we measured S₂, S₁ (where visible in the matrix), and inclusion trails within the biotite porphyroblasts in 4 sets of thin sections. We used stereonets to reconstruct the orientation of S₁ included in the biotite porphyroblasts in these samples. Two yield shallow-dipping S₁ consistent with burial under rocks thrust from the west. In these samples S₂ is very well developed and S₁ is close to S₂ and only preserved as inclusion trails. The low angle between S₂ and S₁ measured as inclusion trails in two of the samples suggests that the biotite porphyroblasts grew after S₁ began to rotate toward S₂ as S₂ formed. Two of the samples yield a very steep S₁ at a moderate angle to S₂, consistent with northeast-southwest compression. In these samples S₂ is a crenulation cleavage spaced widely enough for S₁ to still be visible in the matrix.