U.S. Geological Survey
By Ralph A. Haugerud1
As part of a project to understand the crustal dynamics of the Cascadia ridge-trench-arc system, we are planning a series of digital maps of the region extending from the south of the Mendocino triple junction to north of the Nootka triple junction, and from west of the Juan de Fuca ridge to the back-arc region in western Idaho.
The first map in the series is a 250-m resolution digital elevation model (DEM). It differs from previous efforts by extending across both the US-Canada border and the water's edge. It is compiled from numerous sources: 60% of the map area is sub-sampled from higher-resolution primary sources, while 40%, mostly offshore, is over-sampled from lower-resolution sources. Data quality thus varies widely. Mismatches at boundaries between different sources are mostly minor, which inspires confidence in the overall quality of the DEM. Niether CRT display nor plotter output does justice to a map of this size and detail (32" X 35" with ~200 data points per linear inch at 1:2,000,000 scale), thus we are endeavoring to print a color- shaded relief map derived from the DEM.
The map is stunning. Clearly visible are conspicuous rift-accretion topography generated at the spreading ridge; the sediment blanket that covers most of the Juan de Fuca plate; mega-landslides that dominate much of the accretionary prism; topographic segmentation of the volcanic arc; high, smooth, constructional topography of the southern back-arc region; and active tectonic topography of the northern Basin and Range, the Yakima fold belt, and a fold belt developed in the accreted part of the Astoria fan. Questions provoked by the map include: Were the Westcoast fault and the Olympic-Wallowa lineament a single structure? Is the bulge in the continental shelf off the Chehalis River a Pleistocene delta? How much do along-strike variations in the continental margin reflect local erosion, large sediment supply, and cyclic loading at the southern limit of Cordilleran glaciation?
By mid-1996, the DEM, 60+ MB in binary format, should be online at http://wrgis.wr.usgs.gov
1 U.S. Geological Survey, Box 351650, Geophysics Program, University of Washington, Seattle, Washington, 98195-1650