[1] Hydrothermally altered rocks, particularly if water saturated, can weaken stratovolcanoes, thereby increasing the potential for catastrophic sector collapses that can lead to far-traveled, destructive debris flows. Evaluating the hazards associated with such alteration is difficult because alteration has been mapped on few active volcanoes and the distribution and intensity of subsurface alteration are largely unknown on any active volcano. At Mount Adams, some Holocene debris flows contain abundant hydrothermal minerals derived from collapse of the altered edifice. Intense hydrothermal alteration significantly reduces the resistivity and magnetization of volcanic rock, and therefore hydrothermally altered rocks can be identified with helicopter electromagnetic and magnetic measurements. Electromagnetic and magnetic data, combined with geological mapping and rock property measurements, indicate the presence of appreciable thicknesses of hydrothermally altered rock in the central core of Mount Adams north of the summit. We identify steep cliffs at the western edge of this zone as the likely source for future large debris flows. In addition, the electromagnetic data identified water in the brecciated core of the upper 100-200 m of the volcano. Water helps alter the rocks, reduces the effective stress, thereby increasing the potential for slope failure, and acts, with entrained melting ice, as a lubricant to transform debris avalanches into lahars. Therefore knowing the distribution of water is also important for hazard assessments. Our results demonstrate that high-resolution geophysical and geological observations can yield unprecedented views of the three-dimensional distribution of altered rock and shallow pore water aiding evaluation of the debris avalanche hazard.

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Veins and dikes of calcite-rich rocks within the Central Metasedimentary Belt boundary zone (CMBbz) in the Grenville Province of Ontario have been interpreted to be true carbonatites or to be pseudocarbonatites derived from interaction of pegmatite melts and regional Grenville marble. The putative carbonatites have been metamorphosed and consist mainly of calcite, biotite, and apatite with lesser amounts of clinopyroxene, magnetite, allanite, zircon, titanite, cerite, celestite, and barite. The rocks have high P and rare earth element (REE) contents, and calcite in carbonatite has elevated Sr, Fe, and Mn contents relative to Grenville Supergroup marble and marble mélange. Values of δ18OSMOW (9.9–13.3‰) and δ13CPDB (−4.8 to −1.9‰) for calcite are also distinct from those for marble and most marble mélange. Titanites extracted from clinopyroxene–calcite–scapolite skarns formed by metasomatic interaction of carbonatites and silicate lithologies yield U–Pb ages of 1085 to 1035 Ma. Zircon from one carbonatite body yields a U–Pb age of 1089 ± 5 Ma; zircon ages from two other bodies are 1170 ± 3 and 1143 ± 8 Ma, suggesting several carbonatite formation events or remobilization of carbonatite during deformation and metamorphism around 1080 Ma. Values of εNd(T) are 1.7–3.2 for carbonatites, −1.5–1.0 for REE-rich granite dikes intruding the CMBbz, and 1.6–1.7 for marble. The mineralogy and geochemical data are consistent with derivation of the carbonatites from a depleted mantle source. Mixing calculations indicate that interaction of REE-rich pegmatites with regional marbles cannot reproduce selected major and minor element abundances, REE contents, and O and Nd isotope compositions of the carbonatites.

Volcanic rocks (Cenozoic) MESOPROTEROZOIC SEDIMENTARY ROCKS Belt Supergroup (Mesoproterozoic)-Area containing major bodies patterned Yellowjacket Formation and associated metasedimentary rocks (Mesoproterozoic)-Area containing major bodies patterned TRANS-HUDSON OROGEN Volcanic-plutonic arc complexes (Paleoproterozoic)-Includes Archean gneisses TRANS-MONTANA OROGEN (ACCRETED TERRANES) Wallace terrane (Paleoproterozoic)-Covered Biotite-quartz-feldspar gneiss and amphibolite of Wallace terrane (Paleoproterozoic)-Exposed in Bitterroot Range Granite, diorite, and gneiss (Paleoproterozoic)-Crops out in Little Belt Mountains. U-Pb zircon age of 1,860-1,880 Ma (Mueller and others, 2002) Medicine Hat block (Archean)-Covered CONTINENTAL-MARGIN ASSEMBLAGE (FOLD-AND-THRUST BELT) Imbricately intercalated rocks (Paleoproterozoic and Archean)-Covered Meta-sandstone, shale, iron-formation, and graphitic shale intruded by gabbro dikes (Paleoproterozoic)-Crops out in Gravelly Range. Interpreted as foreland basin deposit Marble, quartzite, and schist (Paleoproterozoic)-Inferred to have formed on a rifted, passive margin (about 2.0 Ga). Formerly regarded as Archean Massive to weakly foliated granite to granodiorite (Late Archean) Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988) Mafic to ultramafic rocks in Spanish Peaks area and Tobacco Root Mountains (Archean) WYOMING PROVINCE (CRATON) Stillwater Complex (Late Archean)-2.7 Ga (Wooden and others, 1988). Exposed Foliated and gneissic granitoid rocks (Late Archean)-About 2.74-2.79 Ga (Wooden and others, 1988). Exposed Granitic rocks, undivided (Archean)-Magmatic domain. Covered Gneissic rocks, undivided (Archean)-Gneiss domain. Covered

Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

Basement Domain Map of the Conterminous United States and AlaskaCover: Isoclinally folded Paleoproterozoic tonalite orthogneiss from newly identified Wallace domain basement, northern Idaho.For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit