No abstract

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.

No abstract

A review of the structures mapped across the Columbia River flood-basalt province reveals a consistent strain pattern from the beginning of the eruptions of the Columbia River Basalt Group (CRBG) until the end of the eruptions of Grande Ronde Basalt (-17.5 to 15.5 Ma). The observed strain is one of north-northwest shortening and west-southwest extension. The degree of strain is small (i.e., extension «1 percent) and resulted in north-northwest tensional fissures (feeder dikes), approximately east-west folds associated with steep reverse faults, and northwest (right-lateral) and northeast (left-lateral) strike-slip faults. This strain pattern is present from the Brothers fault zone in central Oregon to the northern margin of the CRBG in northern Washington and varies only in its intensity, a factor that can be correlated with the nature of the underlying crust. From approximately 15.5 Ma to the present the same stress pattern continued to deform the flows of the CRBG as they formed north of the Olympic-Wallowa Lineament (OWL), which bisects the Columbia Plateau in a west-northwest direction. South of the OWL the structural pattern changed abruptly at -15.5 Ma. North-northwest feeder dikes, east-west folds, and northwest and northeast strike-slip faults are replaced by well-developed north-northwest-trending grabens, which indicate a much greater degree of crustal extension (-20 percent) accompanied by crustal thinning. The change in strain corresponds to a change in the type of volcanicity: from the pre-15.5 Ma flood eruptions of tholeiitic basalt to post-15.5 Ma small volumes of olivine basalt and intermediate to alkalic and silicic compositions erupted locally along the graben faults.The increased crustal extension south of the OWL at -15.5 Ma implies right-lateral strike-slip motion along that zone, and it is shown that structures previously mapped along the OWL are similar to those mapped along the Brothers fault zone to the south. In both zones the structures are consistent with their interpretation as right-lateral megashears. A tentative model is introduced in which the deformation pattern of the whole Columbia River flood-basalt province is related to oblique subduction to the west and the back-arc spreading associated with the Basin and Range crustal extension to the east.

This paper responds to the expanding interest in archaeology in the use of portable X-Ray fluorescence (pXRF) technologies. Accurate analysis using pXRF requires correction for absorbance and secondary enhancement of the excited element X-rays by the other elements present. Several correction methods are widely used, including fundamental parameters, influence coefficients, Compton ratioing, multi-variate statistical analysis, and dilution. Most pXRF calibrations use either fundamental parameters or multi-variate statistics. However, influence coefficients are known to be the most certain calibration method for XRF analysis of geological materials. Portable XRF calibrations using influence coefficients in the analysis of obsidian, flint, mudbrick, and sediment have far less bias and include a wider range of elements (Mg through Ce) than multi-variate statistical or fundamental parameter calibrations using beam filtered spectra. Bias v. wavelength dispersive XRF data using influence coefficients is mostly less than 1 % for obsidian and flint, and less than 2 % for mudbrick and sediment, in contrast with the large biases (up to 36 %) found using fundamental parameters or multi-variate statistical methods.