Palcomagnetic data have been obtained from granitic rocks of the Miocene Grotto andSnoqualmie batholiths in the Central Cascades, Washington. Alternating field magnetic dleaning demonstrated that a wide range of magnetic stability was present and that many specimens were unsuitable for palcomagnetic analysis. A simple statistic (Qa'), defined as the ratio of remanent magnetic intensity of the entire sample after af cleaning to weak-field magnetic susceptibility, proved effective in discriminating between specimens of marginal stability. Experiments with saturation isothermal remanent magnetism indicated that. stable NRM is dominated by magnetic grains with single-domain characteristics, whereas grains with multidomain characteristics dominate the NRM of the unstable samples. After cleaning, a Miocene pole was obtained for the Central Cascades at 208.5øE, 85.5øN (•ip --4.5, tt,• --5.4, and k --360.4). This pole is statistically indistinguishable from other Miocene po.les of North America, which suggests that any major tectonic movement postulated for the Central Cascades relative to North America must have occurred before the Miocene. In a recent study of Cretaceous Central Cascade intrusives Beck and Noson [1972] interpreted their palcomagnetic results to suggest that west central Washington was at anomalously low paleolatitudes during the Cretaceous. Their data indicate that the Cretaceous part of the Central Cascade area was approximately 30 ø farther south than cratonic North America. Tectonic translation has also been postulated by Monger and Ross [1971] in one of their proposed models of the western Cordillera based on the distribution of fusulinaceans. Other earlier theories such as Wise's [1963] application of Carey's orocline concept to the Pacific Northwest require tectonic rotation of the Central Cascades. Palcomagnetic data. from Miocene granitic intrusions summarized in this paper suggest that the Central Cascades have not undergone any post-Miocene tectonic rotation or translation. Therefore any tectonic movements required by these theories must have ended by the Miocene. In addition, preliminary work on an upper Eocene granitic stock shows similar results and suggests that these observations might be pushed back even further into the Tertiary. GEOLOGICAL SETTING The Miocene Grotto and Snoqualmie batholiths are exposed just west of the crest of the Cascade Range, 64 km east of Seattle (Figure 1). These intrusions. are composed predominantly of granodiorite and contain lesser amounts of diorite, quartz diorite, and quartz monzonite. There is no clear petrologic distinction between samples from the two batholiths, and they were thought to be the same unit until radiometric age dating revealed their separate histories [Erickson, 1968]. The K-At ratios from biotite and hornblende date the Grotto batholith at 26.3 --0.8 and 25.0 _ 1.5 m.y. [Yeats, 1970]. The K-Ar ratios on biotites from the Snoqualmie batholith yield an age of 18 m.y. [Baadsgaard et al., 1961] and 17 --0.7 m.y. [Curtis et al., 1961], wher...
Consulting engineer), and Tim Walsh (DGER). Administrative and clerical support by Michelle Davis and Mary Ann Shawver of DGER contributed substantially to the success of the workshop. This workshop represents the effort to synthesize the wealth of new data gathered under the aegis of the NEHRP program for the Puget Sound and Portland areas and to translate it into engineering practice so as to reduce the risk from future earthquakes in the region. Attendance by more than 300 scientists, engineers, emergency planners, and others attests to the strong interest in understanding regional earthquake hazards and reducing future personal and property losses.
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