New in Geosphere: From Fractal-Sized Fragments to a Large-Footprint LiDAR Survey
Geosphere articles posted online 26 June 2012
Boulder, CO, USA – New Geosphere postings include the first use of virtual fieldwork to study faulting and other terrain data collected by LiDAR after the 2010 Haiti earthquake; an addition "CRevolution 2: Origin and Evolution of the Colorado River System II"; development of the Mexican fold-and-thrust belt; provenance of sandstones in the Colton Formation of the Green River's Desolation Canyon; and interactions of the Seattle and Saddle Mountain faults.
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Interactive terrain visualization enables virtual fieldwork during rapid scientific response to the 2010 Haiti earthquake
Eric Cowgill et al., Dept. of Geology, University of California, Davis, CA 95616, USA. Posted online 26 June 2012; doi: 10.1130/GES00687.1.
The magnitude (Mw) 7.0 Haiti earthquake on 12 Jan. 2010 was the first major earthquake for which a large-footprint LiDAR survey was acquired within several weeks of the event. This new study by Cowgill and colleagues describes the first use of virtual-reality visualization to analyze massive terrain data during the rapid scientific response to a major natural disaster, demonstrating the potential for virtual-reality-based data visualization to transform rapid scientific response by enabling virtual field studies and real-time interactive analysis of massive terrain data sets. To conduct their study, this team of geologists and computer scientists from the University of California at Davis developed a method for conducting virtual fieldwork using both desktop computers and a four-sided immersive virtual-reality environment. This system enabled virtual fieldwork that yielded remote observations of the topographic expression of active faulting within an approx. 75-km-long section of the eastern Enriquillo-Plantain Garden fault spanning the 2010 epicenter. Virtual field observations indicate that the geomorphic evidence of active faulting and ancient surface rupture varies along strike, with clear evidence of repeated surface rupture east of the 2010 epicenter. To the west, the fault was well defined by displaced landforms but was not as clear as in the east. The authors found that the 2010 epicenter lies within a transition zone between these sections, within which there is little evidence of recent surface rupture along the Enriquillo-Plantain Garden fault. Based on their virtual-reality based study, Cowgill and colleagues propose that the 2010 event occurred within a step-over between two separate fault sections and that this step appears to have served as a long-lived boundary between rupture segments, explaining the lack of 2010 surface rupture.
Provenance of the Paleogene Colton Formation (Uinta basin) and Cretaceous-Paleogene provenance evolution in the Utah foreland: Evidence from U-Pb ages of detrital zircons, paleocurrent trends, and sandstone petrofacies
William R. Dickinson et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Posted online 26 June 2012; doi: 10.1130/GES00763.1.
Sandstones of the Colton Formation form the picturesque red cliffs in the Green River's Desolation Canyon, incised across the southern flank of the Uinta basin in northeastern Utah. Colton strata formed an immense delta, with a sediment volume approaching 750 cubic miles, built into Lake Uinta from the south during the interval from 58 to 47 million years ago (Paleocene to Eocene time). Uranium-lead ages for detrital zircon sand grains in Colton sandstone, when coupled with information on the direction of streamflow in delta distributary channels and the composition of Colton sandstone, indicate that sediment in the Colton delta was transported to the Uinta basin by a paleoriver with its headwaters in the Mojave region of southern California nearly 500 miles to the south. Colton sedimentation was the culmination of a pattern of sediment delivery northward within the intermountain region that began approximately 100 million years ago.
The western limits of the Seattle Fault Zone and its interaction with the Olympic Peninsula, Washington
A.P. Lamb et al., Dept. of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA. Posted online 26 June 2012; doi: 10.1130/GES00780.1.
The Seattle fault in the Puget Lowland of Washington State, USA, produced a magnitude 7-7.5 earthquake approximately 1000 to 1300 years ago. During the same time frame, another magnitude 6.5 earthquake occurred on the Saddle Mountain fault in the Olympic Peninsula. The proximity of these two faults systems suggests that these faults may be linked. We present new geophysical data near the juncture of these two faults that suggest the Seattle fault may link to the Saddle Mountain fault system to the west and to the Tacoma fault to the south. These faults extend across the Seattle and Tacoma metropolitan area and understanding their connectivity is of importance to improving earthquake risk assessments for the region.
Interpreting two-dimensional cuts through broken geologic objects: Fractal and non-fractal size distributions
Allen F. Glazner and Ryan D. Mills, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA. Posted online 26 June 2012; doi: 10.1130/GES00731.1.
Fragmented objects are common in geology. Fragmentation generally results in a fractal size distribution of particles, which means that the number of particles in each size class is roughly 300 times more abundant than in the size class 10 times larger. However, theory says that if such a distribution of particles is viewed in a two-dimensional cut, such as an outcrop surface, smaller particles are underrepresented and the resulting factor is 10 times smaller than above (roughly 30). This leads to a size distribution in outcrop that is greatly skewed with respect to the true distribution. Observations of naturally fragmented object in two and three dimensions, coupled with numerical simulations, verify theoretical predictions.
Gravel-capped benches above northern tributaries of the Escalante River, south-central Utah
David W. Marchetti et al., Geology Program, Western State College of Colorado, 600 N. Adams Street, Gunnison, Colorado 81230, USA. Posted online 26 June 2012; doi: 10.1130/GES00772.1.
Andesitic boulder deposits mantle straths cut in sedimentary bedrock high above the northern tributaries of the Escalante River in south-central Utah. The andesitic gravel deposits are derived from the southern escarpments of Boulder Mountain and Aquarius Plateau. The sedimentology and geomorphic expression of these deposits suggest they are from slurry-flow mass movements that have been reworked by fluvial processes. The andesitic boulders are significantly tougher than the local sedimentary bedrock and cause boulder armoring and topographic inversion. Using the oldest exposure age from each surface, David W. Marchetti and colleagues estimate maximum Escalante River northern tributary incision rates of 151–323 meters per million years for the period since 0.6-1.4 million years ago.
The role of folding in the development of the Mexican fold-and-thrust belt
Elisa Fitz-Díaz et al., Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55454, USA; and Depto. de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, Avenida Universidad 3000,C.U., Coyoacán, D.F., México. Posted online 26 June 2012; doi: 10.1130/GES00759.1.
The Mexican fold-and-thrust belt in central Mexico has overall characteristics that fit the critical tectonic wedge model. It is thin-skinned, forward propagating, tapers toward the toe (the east), and displays an overall decrease in deformation toward the toe. The internal structures and heterogeneity of deformation are not typical of fold-and-thrust belts, however, due to the presence of two large carbonate platforms, flanked by more thinly bedded basinal carbonates. Kilometer-scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Elisa Fitz-Díaz and colleagues estimate strain and strain history from mesoscopic analysis of fold geometry and internal strain distribution at several locations across the basin and used this information used to assess the overall kinematics and progressive deformation in the basins, which involve both shortening and shear components. They then discuss the implications of this for the kinematics of the fold-and-thrust belt.