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J. C. Gutmanis 《Geological Journal》1989,24(3):183-192
A tectonic study of the Newberry Crater region of central Oregon has been based on the interpretation of Landsat Thematic Mapper imagery. Two major faults, the Brothers-Tumalo and Eugene-Denio Faults, pass NW-SE through the region and step to the right at the eastern margin of the Cascades Range. Dextral wrench faulting on these structures during the Tertiary controlled the formation of the La Pine Basin, a pull-apart structure containing Tertiary and Quaternary sediments and volcanics. Tertiary wrench faulting appears to have been associated with rotations of crustal blocks at a plate margin, but was superseded in the Quaternary by extensional faulting of the Basin and Range province. Newberry Crater and other major bimodal volcanic centres in the NW Cordillera (Crater Lake, Medicine Lake, Mt. St. Helens) seem to have a similar tectonic setting in crustal pull-aparts. A relationship between magma type and fault trend at Newberry and Medicine Lake is suggested. 相似文献
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The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%).
The volatile (H2O and CO2) contents and textures of these clasts vary considerably. FTIR measurements of H2O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO2 contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO2 source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed.
Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the
eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from
dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated
into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion
of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow
conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls.
Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders
of magnitude, with maxima of 88 kPa and 10−2.3 s−1, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the
shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and
8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates
and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption
and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude
that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent. 相似文献
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Daniel Dzurisin 《Bulletin of Volcanology》1999,61(1-2):83-91
Personnel from the U.S. Geological Survey's Cascades Volcano Observatory conducted first-order, class-II leveling surveys
near Lassen Peak, California, in 1991 and at Newberry Volcano, Oregon, in 1985, 1986, and 1994. Near Lassen Peak no significant
vertical displacements had occurred along either of two traverses, 33 and 44 km long, since second-order surveys in 1932 and
1934. At Newberry, however, the 1994 survey suggests that the volcano's summit area had risen as much as 97±22 mm with respect
to a third-order survey in 1931. The 1931 and 1994 surveys measured a 37-km-long, east–west traverse across the entire volcano.
The 1985 and 1986 surveys, on the other hand, measured only a 9-km-long traverse across the summit caldera with only one benchmark
in common with the 1931 survey. Comparison of the 1985, 1986, and 1994 surveys revealed no significant differential displacements
inside the caldera. A possible mechanism for uplift during 1931–1994 is injection of approximately 0.06 km3 of magma at a depth of approximately 10 km beneath the volcano's summit. The average magma supply rate of approximately 1×10–3 km3/year would be generally consistent with the volcano's growth rate averaged over its 600,000-year history (0.7–1.7×10–3 km3/year).
Received: 10 September 1998 / Accepted: 4 December 1998 相似文献
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