TECTONICS OF THE BACKARC OF THE CASCADIA SUBDUCTION SYSTEM AND THE ROLE OF TECTONIC INHERITANCE

This presentation is focused on the tectonics of the backarc region of the Cascades arc. Clockwise rotation of the Blue Mountains pivots on a location near Orofino, ID. This pivot is the site of a Precambrian rift (000 oriented) – transform (090 oriented) intersection inherited from Rodinia breakup; this same rift-transform system controls mid Cretaceous deformation in Idaho. Recent rotation of the Blue Mountains starts at ~18 Ma, driven by the capture of western California by the San Andreas fault system. The northward motion of the Sierra Nevada block drives the rotation of the Blue Mountains. The result, in western Idaho and eastern Oregon, is a series of NS-oriented normal faults that accommodate EW elongation. This domain has a fan shape, with the fan widening to the south, and elongation decreasing northward towards Orofino. This extension is on the backside of the Blue Mountains “flipper”. The front side of the Blue Mountains “flipper” is NS-oriented shortening, also forming a fan shape that gets wider to the west. The western part of this shortening zone is the Yakima fold-and-thrust belt, and the eastern part is a series of monoclines in westernmost Idaho. Again, shortening decreases toward Orofino.

Why does this matter to arc magmatism? As described by other models, the southern Cascade arc is translated toward the northwest. The northern Cascades arc seems to rotate on a second pivot point (in Washington State or southern British Columbia). In this system, the Mesozoic Coast Plutonic batholith seems to form the backstop for rotation. This system is also ultimately driven by the movement of the Sierra Nevada block. Outboard of the Yakima fold-and-thrust system (e.g. in the Mt St Helens & Mt Adams area), however, the Cascades arc must transfer from one block rotation to the next. At this location, the strike-slip component of the southern Cascade arc must abruptly end, which provides sites for tectonically accommodated magmatism in features such as large-scale fault terminations. We speculate that this backarc tectonism is a useful vantage point to view Cascadia arc magmatism.