ALBERT

Description: Rifts are segmented linear depressions that are filled with sedimentary and igneous rocks; they form by extension and often evolve into plate boundaries. Flood basalts, a class of large igneous provinces (LIPs), are broad regions of extensive volcanism formed by sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift (MCR) is an unusual combination, because its 3000-km length formed during a continental breakup event 1.1 Ga, but it contains an enormous volume of igneous rocks that are mostly flood basalt. We show that MCR volcanic rocks are significantly thicker than other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR a rift’s geometry but a LIP’s magma volume. Structural modeling of seismic-reflection data shows that LIP volcanics were deposited during two phases—an initial rift phase where flood basalts filled a fault-controlled extending basin and a postrift phase where LIP volcanics and sediments were deposited in a thermally subsiding sag basin without associated faulting. The crust thinned during the initial rifting phase and then rethickened during the postrift phase and later compression, yielding the present thicker crust observed seismologically. The restriction of extension to a single normal fault in each rift segment, steeply inward-dipping rift shoulders with sharp hinges, and persistence of volcanism after rifting ended gave rise to a deep flood basalt–filled rift geometry not observed in other presently active or ancient rifts. The unusual coincidence of a rift and LIP arose when a new rift associated with continental breakup interacted with a mantle plume or overrode anomalously hot or fertile upper mantle.

Description: The Harney Basin is a relatively flat-lying depression in the northeast corner of the enigmatic High Lava Plains volcanic province in eastern Oregon. A thick blanket of volcanics including flood basalts, rhyolites, tuffs, ash flows, and distinct eruptive centers covers the basin, making it very difficult to study the upper-crustal features. In addition to a portion of the High Lava Plains active source seismic data in the Harney Basin area, we employed geologic, gravity, magnetic, digital elevation, and other geospatial data in our integrated study. We generated an upper-crustal 3D seismic tomographic model of the Harney Basin using a sparse grid of 2D seismic lines and constructed an integrated geophysical model of the upper-crustal structure, which reveals that the basin reaches as deep as 6 km in its central area. The tomographic inversion also detected some unusually high-velocity (〉6.5 km/s) bodies in the upper crust near the central basin area. The presence of several ash-flow tuffs and voluminous rhyolites in the Harney Basin region indicates that the sources of these materials are nearby. We observe two major caldera-shaped features within the basin, which we interpret to be likely candidates for the source of some of these tuffs. These potential calderas are associated with low seismic velocities, low gravity anomaly values, and topographic depressions. We interpret the extent and evolution of these potential calderas based on our integrated analysis.

Description: We have investigated the mode of emplacement of iconic Devils Tower, which is a phonolite porphyry monolith in the state of Wyoming in the western United States. Our field survey of this structure and its geological setting, its radiometric dating, and the tectonomagmatic evolution of the region suggest a new genetic interpretation of the volcaniclastic rocks in the area and provide a basis for a new hypothetical emplacement scenario for Devils Tower. This interpretation was inspired by an analogy of the tower with a similar phonolite butte in the Cenozoic volcanic region of the Czech Republic and analogue modeling using plaster of paris combined with finite element thermal numerical modeling. Our results indicate that Devils Tower is a remnant of a coulée or low lava dome that was emplaced into a broad phreatomagmatic crater at the top of a maar-diatreme volcano.