The Cascadia subduction zone has been characterized as a typical Chilean-type
subduction zone based on qualitative comparisons of plate age and convergence rate, with
simple forearc structure. However, the discovery of unusual structural styles of
deformation, variations in the morphology of the forearc, and its absence of seismic activity
suggest differences from the Chilean analog. The manuscripts presented here (McNeill et
al., 1997, in press, submitted) illustrate this complexity and provide examples of
contrasting deformation throughout the offshore forearc. The Washington and northern
Oregon shelf and upper slope are characterized by extension in the form of listric normal
faults. These faults have been active since the late Miocene and are driven by detachment
and extension of the underlying overpressured m��lange and broken formation. This region
of the forearc is partly to wholly decoupled from convergence-driven compression which
dominates deformation elsewhere in the forearc. One exception to convergence-driven
compression is a region of N-S compression of the inner shelf and coastal region which
reflects the regional stress field. N-S compressional structures apparently influence the
positions of coastal lowlands and uplands and may contribute to the record of coastal marsh
burials interepreted as the result of coseismic subsidence during subduction zone
earthquakes. Modeling of subduction zone earthquake characteristics based on marsh
stratigraphy is likely to be inaccurate in terms of rupture zone position, magnitude, and
recurrence interval. The Cascadia shelf and upper slope are underlain by a sequence of
deformed basinal strata which reflects the tectonic evolution of the margin. The surface of
a regional late Miocene angular unconformity (7.5-6 Ma: a global hiatus) indicates
deformation by uplifted submarine banks and subsided synclines (coincident with low
recent uplift onshore), which control the current shelf break position. The basin is
currently filled behind a N-S-trending outer-arc high, which uplifted in the early-middle Pliocene following truncation and erosion of the seaward edge of the basin. Breaching of the outer-arc high occurred in the early Pleistocene leading to the formation of the Astoria Submarine Fan and increased growth rates of the accretionary wedge. / Graduation date: 1999
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33569 |
| Date | 12 October 1998 |
| Creators | McNeill, Lisa C. |
| Contributors | Yeats, Robert S. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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