RECONSTRUCTING THE GEOLOGIC HISTORY OF ERODED MOUNT YANA BY USING THE GEOMETRY AND ORIENTATION OF EXPOSED DIKES

The Yana Volcanic Center (YVC) is an eroded stratovolcano that was part of the Ancestral Cascadian Volcanic Arc. Dated at 2.8 Ma +/- 0.5, the Pliocene volcano lies 40 km south of the Lassen Volcanic Center and is characterized by layered lava flows and breccias that extend westward to the Central Valley of California. Erosion exposed a complex of sheet intrusions that cut through the breccia host rock. Understanding the emplacement history of the dikes can reveal the eruptive history of the volcano and evolution of principal stresses over time. In Fall 2023, we mapped YVC dikes recording their geometry, orientation, and outcrop descriptions while collecting samples for both hand sample and thin section descriptions. Results indicate that the YVC contains a radial dike sequence with multiple potential central intrusive complexes. Dikes found in the region are generally vertical and range from centimeters to meter-wide features. Interspersed inclined sheets and sills are also seen throughout the area but are grouped in the northwestern region of the volcano cutting a lapilli breccia. Most sheeted intrusions are composed of pyroxene, plagioclase and olivine phenocrysts in a fine-grained plagioclase matrix. The radial dike patterns are seen predominantly higher in the stratigraphy of the volcanic complex and sheeted intrusions lead gradationally to lava flow autobreccias. This leads us to believe that the majority of eruptions that happened in later stages of Mount Yana were flank eruptions similar to those seen at Mount Shasta today. These dike orientations also give us insight into the stress field due to sheeted intrusions forming orthogonal to the maximum principal stress at the time of emplacement. Dike orientations indicate that gravitational loading generated by a conical-shaped edifice was likely the largest contributor to the stress field with perhaps little influence by the regional stress field.