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Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera
Abstract Subduction zones are highly productive seismic sources both at the interface and within the slab, and have produced many of the highest-impact earthquakes throughout history. Several current probabilistic seismic hazard analyses model interface sources as faults with rates constrained by only seismicity or tectonics singularly, and intraslab sources as gridded points, sometimes smoothing rates according to former occurrences. Here, we propose a subduction source modelling methodology that improves on current standards for both source types. We use cross-sections of earthquake catalogues, focal mechanisms and geophysical models to define the locked subduction interface and slab volume, and to classify seismicity. We divide these geometries along-strike to impose segment boundaries. For the interface, we combine tectonic and seismicity components to assign occurrence properties, considering fault area, convergence rate and seismic coupling. We use ruptures that are confined to the slab volume to demarcate intraslab sources, enhancing their utility with ground motion prediction equations that use site-to-rupture distance. We demonstrate the proposed approach on the Nazca subduction zone of South America, validating the source model parameters against observed earthquakes and palaeoseismic information when available, and showing that the resulting magnitude–frequency distributions better fit the observed occurrences than traditional approaches.
Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event
Abstract The Gulf of Suez Basin is a classic extensional rift basin of Miocene age, with a number of syn- and pre-rift hydrocarbon plays. Exploration started in 1886, targeting areas around documented oil seeps, and was highly successful. During the boom in offshore exploration in the 1950s and 1960s, a combination of diligent geology and serendipity resulted in the discovery of a number of giant fields whose reserves form a major part of some 10 billion barrels discovered to date. However, the pursuit of smaller fields, both structural and stratigraphic, has been hampered by the poor quality seismic data characteristic of the basin. The poor quality of the seismic data is due to the interbedded shales and evaporites of shallow post-rift Zeit and South Gharib formations that create massive reverberation and severe attenuation of the seismic signal. Incremental progress in imaging the deeper horizons, including the key pre-rift Nubia Sandstone reservoir, has been achieved through low frequency enhancement and 3D seismic data acquisition. However it is still common for exploration and development wells to miss the Nubia objective due to the poor imaging and consequent misinterpretation of the seismic data. Exploration is now directed towards the smaller targets and subtle plays of the Gulf of Suez. To improve seismic imaging the focus has been on improved acquisition, with 3D ocean-bottom cable seismic data. This has the advantages of reduced multiple energy, higher fold and a broader bandwidth over streamer 3D data. Combined with detailed models of the tectonically controlled sedimentation that characterizes the syn-rift section, this has allowed the development of a re-invigorated exploration programme.
Mn oxide as a contaminated-land remediation product
On-site repository construction and restoration of the abandoned Silver Crescent lead and zinc mill site, Shoshone County, Idaho
Abstract From the early 1900s through the 1950s the Silver Crescent mine and mill processed lead, zinc, and silver from ore found in the Precambrian metasedimentary rocks of the Belt Supergroup. Approximately 150,000 cubic yards of tailings and waste rock were deposited in the floodplain of Moon Creek less than 2 miles upstream of what is now a residential area. The actively eroding tailings impoundments were a source of heavy metal contamination to the surface and groundwater flowing through the site. The U.S. Forest Service began a CERCLA non-time-critical removal action at the Silver Crescent mine in 1998. Removal action goals included reduction of particulate and dissolved metal loading into Moon Creek and local groundwater. These goals were successfully achieved in part by incorporating the tailings and waste rock dumps into an on-site capped repository. The nearly $2 million Silver Crescent removal action construction phase was completed in late 2000 with the final habitat restoration phase scheduled for completion in 2007.
Geology is among the most visual of the sciences, and spatial reasoning takes place at various scales and in various contexts. Among the spatial skills required in introductory college geology courses are spatial rotation (rotating objects in one's mind) and spatial visualization (transforming an object in one's mind). Geologic curricula commonly require students to visualize Earth in many ways, such as envisioning landscapes from topographic maps, the interaction of layers and topography, and the progressive development of geologic features over time. To facilitate learning in introductory college geology laboratories, we created two geologic modules— Visualizing Topography and Interactive 3D Geologic Blocks. The modules were developed as learning cycles, where students explore first, are then introduced to terminology and concepts they have observed, and finally apply their knowledge to different, but related problems. Both modules were built around interactive QuickTime Virtual Reality movies that contain landforms and geologic objects that students can manipulate on the computer screen. The topography module pairs topographic maps with their three-dimensional (3D) representations on the same screen, which encourages students to visualize two-dimensional maps as three-dimensional landscapes and to match corresponding features on the map and 3D perspective. The geologic blocks module permits activities that are not possible with normal paper-based curricula, such as interactively rotating, slicing into, eroding, and faulting the blocks. Students can also make the blocks partially transparent to reveal the internal geometry of layers, folds, faults, intrusions, and unconformities. Both modules encourage active participation by having students describe, draw, and predict, and both modules conclude with applications that require the students to extend and apply key concepts to novel situations. Assessment of the modules using control and experimental groups shows that the modules improved student performance on a geospatial test, that general spatial ability can be improved via instruction, and that differences in performance between the genders can be eliminated by a semester-long laboratory. “To go out into the field with a geologist is to witness a type of alchemy, as stones are made to speak. Geologists imaginatively reclaim worlds from the stone they're trapped within.” – Frodeman (1996 , p. 417).