DISTRIBUTION AND GEOMETRY OF FEEDER DIKES FROM COLUMBIA RIVER BASALT SUBSWARMS, NE OREGON AND W IDAHO

Voluminous flood basalt flows of the Imnaha and Grande Ronde members of the Columbia River Basalt Group were fed by the Chief Joseph dike swarm, which occupies roughly 30,000 km² of NE Oregon, SW Washington, and W Idaho. Dikes in the Chief Joseph swarm are concentrated into numerous subswarms with 7 to 12 dikes per km². Dikes of the Chief Joseph swarm overall trend N10°W, but each subswarm has a distinctive orientation (Table 1). Dikes preferentially occur in granitoid rocks where they commonly follow local joint patterns. Although dips on single dikes can vary over 35°, overall intersection with topography indicates dikes are near vertical. Linear transects were taken perpendicular to the average strike across three dike subswarms (Table 1); dikes are typically 6 to 14 m thick, but thickness can range from a few cm up to 43 m. Dikes in the Iron Mountain subswarm are thinner (only 5 dikes are >10 m thick) and more closely spaced than dikes from the Big Lookout Mountain subswarm (only 1 dike is < 10 m thick). Two dikes in the Lakes Basin are >10 m thick; Lakes Basin dikes are also the most widely spaced. From dike width and spacing we estimate that local horizontal extension is as much as 6% in the Lakes Basin, 10% at Big Lookout Mountain, and 17% at Iron Mountain, as compared to Taubeneck’s regional estimate of about 1% extension across the entire Chief Joseph dike swarm.

Table 1. Geometry of dikes in Chief Joseph subswarms in granitoid rocks

	Iron Mtn (transect)	Big Lookout Mtn (transect)	Pedro Mtn	Cornucopia	Lakes Basin (transect)
Mean strike	335	016	341	359	348
Strike range	320-342	004-026	317-354	351-009	320-0
No. dikes	16	12	>19	>10	6
Mean thickness	11 m	13 m	--	--	12.5 m
Mean spacing	62 m	122	150 m	183 m	217 m
% extension	17	10	--	--	6