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The 40 Ar/ 39 Ar investigations of a large suite of fi ne-grained basaltic rocks of the Boring volcanic fi eld (BVF), Oregon and Washington (USA), yielded two primary results. (1) Using age control from paleomagnetic polarity, stratigraphy, and available plateau ages, 40 Ar/ 39 Ar recoil model ages are defi ned that provide reliable age results in the absence of an age plateau, even in cases of signifi cant Ar redistribution. (2) Grouping of eruptive ages either by period of activity or by composition defi nes a broadly northward progression of BVF volcanism during latest Pliocene and Pleistocene time that refl ects rates consistent with regional plate movements. Based on the frequency distribution of measured ages, periods of greatest volcanic activity within the BVF occurred 2.7-2.2 Ma, 1.7-0.5 Ma, and 350-50 ka. Grouped by eruptive episode, geographic distributions of samples defi ne a series of northeast-southwest-trending strips whose centers migrate from south-southeast to north-northwest at an average rate of 9.3 ± 1.6 mm/yr. Volcanic activity in the western part of the BVF migrated more rapidly than that to the east, causing trends of eruptive episodes to progress in an irregular, clockwise sense. The K 2 O and CaO values of dated samples exhibit well-defi ned temporal trends, decreasing and increasing, respectively, with age of eruption. Divided into two groups by K 2 O, the centers of these two distributions defi ne a northward migration rate similar to that determined from eruptive age groups. This age and compositional migration rate of Boring volcanism is similar to the clockwise rotation rate of the Oregon Coast Range with respect to North America, and might refl ect localized extension on the trailing edge of that rotating crustal block.

The juxtaposition of disparate geologic terranes in southern Alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the Denali fault system in Cenozoic time. Recent geologic investigations in the Healy quadrangle strongly suggest that Cenozoic horizontal displacements of such magnitude along the Denali fault system do not exist. In the Healy quadrangle, isograds and metamorphic facies boundaries of an early Late Cretaceous metamorphic belt trend across the Cenozoic McKinley strand of the Denali system without significant horizontal offsets. The present geologic makeup of most of southern Alaska is primarily the result of the Talkeetna superterrane, consisting of the previously assembled Peninsular terrane and Wrangellia, colliding with and subsequently being thrust upon the Yukon‐Tanana and Nixon Fork terranes of the ancient North American continent in about middle Cretaceous time. The leading edge of the Talkeetna superterrane faces a wide, complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with Jurassic and Cretaceous flysch. The flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks. The postcollisional Denali fault system developed in Cenozoic time across the already accreted continental margin, in eastern Alaska along an older, Cretaceous suture.

Visible and near‐infrared (0.35 to 2.5 μm) bidirectional reflection spectra were recorded for a suite of particulate samples from mineralogically well‐characterized serpenatinized ultramafic rocks. The reflection spectra typically exhibit well‐defined minima due to electronic and vibrational processes in the individual mineral constituents. The contrast of near‐infrared spectral features of primary magnesian silicate minerals and secondary hydrous‐serpentine group minerals can be used to indicate the degree of serpentinization of the rock, provided less than about 1 percent of finely divided magnetite is present. The effect of magnetite, apparent in rocks with more than 50 percent serpentine, is to reduce the overall reflectance and the contrast of spectral bands. Near‐infrared spectrometry is potentially a rapid and reliable technique for detecting the highly serpentinized rocks which constitute target areas for asbestos exploration.