Scale Contexts

The data acquired and used in this report conveys different information at different scales. For example, a feature not visible at a 1:3,000,000 scale may be crucial for interpretation and modeling at a 1:10,000 scale.

Pacific Northwest Regional Context

An assessment of the available geophysical data should begin at the largest regional scale, which extends from the southern Cascades near Lassen Peak to the U.S.-Canada border. Viewed at this broad scale, magnetic and gravity data display first-order features related to subduction and major geologic terranes.

Pacific Northwest Magnetic Data

Figure 3 shows the magnetic data for the Pacific Northwest. Different wavelengths are clearly visible at the Oregon-California border transition—this is sometimes called a “fence-line anomaly” by geophysicists or a “boundary fault” by geologists and geographic information system specialists and is due to different survey parameters on either side of the border. However, the large-scale arcuate features seen near the coast at the California-Oregon State boundary are not data artifacts. These are related to ophiolitic rocks within the accreted Klamath terrane (for example, Blakely and others, 1997). Larger amplitude magnetic anomalies form a north-south band west of the chain of the Cascades volcanoes, and this is due to a buried both surficial and buried volcanic sources. The uplifted sediments of the Olympic Peninsula (extreme northwest of Washington State) have a notably subdued magnetic character in contrast to the arcuate belt of volcanic rocks to the east. At this scale, Newberry Volcano appears to be part of the same magnetic domain as the rest of the High Cascades volcanoes.

Pacific Northwest Gravity Data

Figure 4 shows the gravity data for the Pacific Northwest. Higher values predominate on the west side of the figure, caused by the proximity of the subducted, denser Juan de Fuca oceanic plate (and thus relatively less underlying silicic crustal material) and the lack of isostatic compensation. The Columbia River Basalt Group manifests as the anomalous high gravity values (orange to red) along the Oregon-Washington border east of the Cascades. The gravity-low character (blue colors) of the extensional Basin and Range Province is obvious from central Oregon southward through northeastern California. There appears to be little correlation between the Cascades volcanoes and the regional gravity data, because the volcanic terrain is relatively superficial on the depth scale (at least 50 km) being sampled.

The Oregon Regional Context

We now move to an intermediate scale between the Pacific Northwest as a whole and the volcano of interest. Here new features become apparent that help us understand the context of Newberry Volcano.

Central Oregon Magnetic Data

Figure 5 shows magnetic data for central Oregon superimposed over shaded-relief topography (Shuttle Radar Topography Mission (SRTM) 30 data). The volcano’s caldera is located north and adjacent to Paulina Peak. The southern boundary of this image and the following three figures is the California-Oregon border. Regional geologic and structural provinces are shown in figure 2 for reference. Different flight-line spacing cannot account for the overall changes observed in the wavelength and orientation of the magnetic anomalies in this figure. One could not successfully outline the Newberry volcanic flows by using magnetic data alone.

The Curie-point isotherm in this region ranges from 5 to 12 km depth (Bouligand and others, 2009). This means that only relatively shallow rocks (a depth less than half the distance from La Pine to Paulina Peak) could contribute to the magnetic anomalies seen on this map. The Curie-Point isotherm simply measures where the magnetic character of ferromagnetic minerals disappears—when magnetic domains disappear in individual mineral grains. This can occur in either of two ways: (1) increased temperature with increasing depth or (2) iron minerals transitioning to a low magnetic susceptibility hydrous phase. The lateral and vertical resolution of the Curie-point isotherm images are both necessarily poor, as each Curie-point estimate is derived using a 100×100-km data window. A large window such as this is necessary because the Curie-point depth is derived by isolating the very longest wavelengths of the magnetic data. Such large windows limit the lateral and vertical resolution possible with any Curie-point method.

Figure 6 shows the magnetic data with CET domain boundaries superimposed over it. In this figure, there are magnetic low “terraces” on both the southeastern and southern margins of Newberry Volcano (see also fig. 10 below); this does not reflect an absence of volcanic material or sparse data but instead is likely due to complex assemblages of normally and reversely polarized lava flows (Jensen and others, 2009). There are also several reversed-polarity lava flows on the northern reach of Newberry Volcano as well. The Western Cascades on the left edge of the figure appear by contrast to be magnetically more uniform. It may be that the Western Cascades lack the coherent large shield volcanoes common in the High Cascades that gives the dimpled pattern. The Western Cascades have been subject to extensive erosion and development of unconformities, deposition of large sequences of sedimentary rocks, faulting, more severe alteration locally, and widespread low-grade alteration.

Central Oregon Gravity Data

Figure 7 displays the publicly available Complete Bouguer Gravity data for central Oregon superimposed over shaded-relief topography (SRTM 30 data). Sources of gravity anomalies are not depth-and-temperature limited in the same sense as are magnetic data; there is no Curie-point equivalent, and longer wavelengths represent deeper sources (or cumulative aggregate shallower sources). Nevertheless, the color changes visible in this figure are caused by lateral density transitions mainly due to sources likely no deeper than 100 km, as the plastic nature of the mantle below this ensures approximate density equilibrium.

Figure 8 shows the regional gravity data with CET domain boundaries superimposed over it. The CET processing in effect creates a “terracing” to make the different gravity domains contrast more clearly against each other.

The Local Newberry Context

The three following figures are provided to allow characterization of data quality at the higher resolution scale of the volcano itself. At this scale, one can see features directly related to splice errors among datasets and discrete features of the volcano itself. In addition, we will see why much of the publically available data do not support two-dimensional modeling or depth to source calculations.

Newberry Area Magnetic Data

Figure 9 is an index map of the different magnetic datasets available in the public domain for the area around Newberry Volcano. Keeping in mind these boundaries, figure 10 shows the local, concatenated or merged magnetic data laid over contoured, shaded-relief topography. These data appear seamless and emphasize the complexity of the magnetic polarization of volcanic flows around on the volcano and its surrounding lava apron. A careful reference to figure 9 shows some edge-effects, due to dataset splicing.

Figure 11 shows the magnetic data after an Analytic-Signal filter has been passed over the dataset. Here the north-south seam between surveys No. 3066 (south half of Newberry) and No. 4108 (eastern edge of Newberry beginning at 121° W., both labeled in fig. 9) can be clearly seen, and the east-west seam between Oregon Survey No. 4108 and the NURE data at 44° N. shows as a band of red (north) over blue (south). These surveys were flown by contractors for the Oregon Department of Geology and Mineral Industries and subsequently archived by the U.S. Geological Survey (Hill and others, 2009). Owing to these seams, it is not possible to combine these data analytically for modeling or depth to source calculations with any confidence.

Newberry Area Gravity Data

Figure 12 shows the local gravity data in and immediately adjacent to Newberry Volcano. The gravity data show poor correlation between Basin and Range topography south and east of Newberry Volcano, but this may be in part an artifact of the coarser station coverage.

Newberry Area NURE Data

For completeness’ sake, five images of radiometric data from the NURE surveys discussed earlier are included. As previously noted, the flight-line spacing for these data is so large (5 km for the northern half of Newberry and 10 km for the southern half) that their usefulness is diminished at less than State-sized presentation scales (1:1,000,000 or greater). Radiometric data provide information on relatively shallow sources, not unlike geochemical sampling, but have an indeterminate effective depth-penetration as the surveys actually measure gaseous decay products of the isotopes of interest. Airborne radiometric surveys are consequently highly susceptible to changes in atmospheric pressure and winds during the survey (Duval, 1990).

Figure 13 shows the apparent potassium channel for the vicinity of Newberry Volcano. The area in the upper left edge of the figure appears to be free of radionuclides of any form, something typical of more mafic rocks in general. However, this may only reflect the fact that maintaining a 400-ft/122-m optimum (terrain draped) flying height in rugged terrain of this area is nearly impossible for a fixed-wing aircraft, and the survey was almost certainly flown higher than this the nominal standard. A sharp band of high-potassium response extends nearly due east of the caldera and correlates closely with the tephra deposit from the 1,300-yr-old Big Obsidian Flow eruption, strengthening confidence in these data.

Figure 14 shows the apparent thorium channel for Newberry Volcano and vicinity. It is remarkably similar to figure 13 (potassium channel).

Figure 15 shows the apparent uranium channel for Newberry and vicinity. Although there are close similarities between the uranium, potassium, and thorium channels, there is a distinct feature not apparent in either of the first two channels—a uranium high around La Pine, Oregon. A sharp east-west edge effect at 43° 20’ N. may be an artifact of crystal calibration or imperfect atmospheric correction between flight lines flown on different days.

Figure 16 represents a ratio of the thorium to the potassium channel for Newberry Volcano and vicinity; only subtle distinctions can be seen between this map and those of the individual channels.

Figure 17 is a ratio of the uranium to the potassium channels for Newberry Volcano and vicinity. Maps showing ratios of radioelements have been helpful in other areas (for example, southwestern Colorado) to distinguish separate ash falls originating from different volcanoes. It does not appear that radioelement ratios have the same utility in the Newberry context, perhaps because there isn’t that great a variation in them in terms of potassium, thorium, and uranium isotopes. The figures are provided here nevertheless for completeness’ sake.

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Suggested citation:

Wynn, Jeff, 2014, Gravity, magnetic, and radiometric data for Newberry Volcano, Oregon, and vicinity: U.S. Geological Survey Data Series 830, https://dx.doi.org/10.3133/ds830.

U.S. Department of the Interior
SALLY JEWELL, Secretary