CO-AXIAL FOLD INTERFERENCE PATTERNS AND INVERSE METAMORPHISM IN THE TONGA FORMATION: EVIDENCE FOR LARGE-SCALE THRUST TECTONICS IN THE CASCADES CRYSTALLINE CORE

The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretaceous plutonism, contact and regional metamorphism, and multiple episodes of folding related to east-west contractional deformation. The Tonga Formation is exposed in a fault-bounded, north-south elongated tectonic domain that comprises pelite-psammite metasediments ranging from greenschist to amphibolite grade. Metamorphic grade increases from south to north across acute metamorphic isograds, indicating an inverted metamorphic gradient relative to a major westward verging antiform that dominates the internal architecture of the formation.

Sedimentary structures are remarkably well-preserved in the Tonga Formation, unlike in the neighboring metasediments to the east (Chiwaukum Schist). Recognizable depositional features include graded bedding, laminae, rip-up clasts, and flute casts, which allowed for the determination of younging directions throughout the unit. Using facing directions and bedding-cleavage relationships, detailed field mapping indicates a stratigraphically overturned section that forms a large-scale antiformal syncline whose core occupies the southern low metamorphic domain of the area. The overturned nature of the strata and the geometry of gently north-plunging folds imply upsection a pre-existing tight to recumbent anticline refolded into a co-axial (type III) fold interference pattern. The core of this early anticline, exposed in the northern domain, corresponds with the higher metamorphic conditions of the inverted metamorphic gradient.

The co-axial, superposed folding in the Tonga Formation suggests a strong component of east-west shortening in the foreland of the Cascades core. Fold geometries account for the inverted metamorphic zonation and also control the localization of plutons, which are also elongate parallel to the regional fold axes. The southern portion of Tonga Formation is interpreted to be a lower-grade equivalent of the Chiwaukum Schist, as protoliths of each unit are also similar. Exposure of the Evergreen fault is limited, but the observed structural and metamorphic relationships suggest that it may be an east-dipping thrust fault that places the deeper, higher-grade Chiwaukum Schist structurally above the lower-grade Tonga Formation.