TECTONICS OF THE CASCADE MAGMATIC ARC

The Cascade magmatic arc, like the rest of the Pacific Northwest, is caught in the Pacific-North America dextral shear couple. As a result, the trend of the arc, its vent distributions, and composition change dramatically along strike in response to tectonic segmentation and rotation of the arc massif. The northern segment from Meager Mt. to Mt. Rainier consists of widely separated vent complexes and associated stratovolcanoes resting on uplifted older basement intruded by late Cenozoic plutons. South of Mt. Adams, the arc is dominantly extensional. In Oregon and California, the major stratovolcanoes lie within discontinuous axial graben and are surrounded by thousands of basaltic and basaltic andesite vents. The change coincides with greater rotation of the Oregon forearc, and the calculated rotation poles from paleomagnetic and GPS data fall just inboard of the diffuse boundary between the northern compressional and southern extensional segments.

Because rotations are larger in the forearc than in the backarc, long-term deformation models have treated the extensional arc as a fundamental tectonic boundary between the rotating forearc and the extending backarc. In these models, the southern arc performs a dual role, magmatically accommodating some of the extension on the trailing edge of the rotating forearc blocks, and acting as the leading edge of the Basin and Range, which expands northwestward behind the rotating forearc. Hot, dry basaltic magmatism associated with the Basin and Range occurs within the southern arc and appears to be propagating northward behind the migrating forearc block. Reconstruction of past positions of the forearc blocks using current rates are consistent with the westward migration of backarc magmatism and modest extension in the arc of about 1-2 mm/yr.

Short term deformation of the arc derived from GPS is in marked contrast to that inferred from geology and paleomagnetism. In the GPS data, the arc is under compression along most of its length. This may reflect viscoelastic response to prolonged compression of the upper plate, as might be expected late in the 500 yr Cascadia subduction zone seismic cycle. The current relative seismic and volcanic quiescence of the Oregon arc may thus be temporary, with activity more likely in the centuries immediately following a great subduction earthquake.