Boulder, CO, USA – The February Geosphere, The Geological Society of America's e-journal, is now online. Topics include studies of the San Andrea fault in southern California; Africa as a collage of ancient crustal blocks; and three-dimensional visualization of the High Plains aquifer.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at cstratton@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Christa Stratton for additional information or other assistance.
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Basin geometry and cumulative offsets in the Eastern Transverse Ranges, southern California: Implications for transrotational deformation along the San Andreas fault systemV.E. Langenheim, U.S. Geological Survey, 345 Middlefield Road, Mail Stop 989, Menlo Park, California 94025, USA; and R.E. Powell
We use small variations in the Earth's gravity and magnetic fields to investigate the architecture of deformation southeast of the big bend in the San Andreas fault in the Eastern Transverse Ranges, southern California, home of Joshua Tree National Park. This region has rotated about 40 degrees clockwise as a result of shear on the San Andreas fault; however, the amount of offset on east-striking faults that traverse the region may not account for the amount of rotation, assuming that the region acts like a rigid block. Our results indicate that this discrepancy is resolved if the main faults are linked by subsidiary faults and by internal deformation as wide as 15 km adjacent to the San Andreas fault.
The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolutionG.C. Begg et al., Minerals Targeting International, 26/17 Prowse Street, West Perth, Western Australia 6005, Australia; and GEMOC ARC National Key Centre, Dept. of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia; corresponding author: W.L. Griffin.
This study records a geological safari down more than 300 km beneath the surface of the African continent and back to 3.6 billion years ago. A novel combination of chemical data gleaned from fragments of the deep Earth carried to the surface in volcanoes, and remotely sensed gravity, magnetic, and seismic data on the physical characteristics of the upper 300 km of this region, has provided a new window into the past and the depths. We show that Africa is a collage of ancient crustal blocks (cratons, more than 2.5 billion years old) with mantle roots that extend much deeper than previously recognized. These ancient fragments are stitched together by younger fold belts (less than 0.5 billion years). The ancient cratons have split apart and reassembled several times since they originally formed, and fragments of these cratons are stranded within the Atlantic Ocean Basin, providing evidence about how continents rift to form ocean basins. The boundaries of the ancient cratonic roots have focused later magmas and fluids, and this observation provides a predictive tool for finding magma-related ore deposits (including diamonds, nickel, and platinum).
New insights into the hydrostratigraphy of the High Plains aquifer from three-dimensional visualizations based on well recordsP. Allen Macfarlane, Kansas Geological Survey, Lawrence, Kansas 66047, USA
The High Plains aquifer is the dominant source of irrigation water for the Great Plains region of the United States and is under intense development pressure, resulting in large saturated thickness declines in many areas. Adequate management of the remaining supplies will require the assembly and use of groundwater flow models to evaluate alternative strategies and predict future availability. The aquifer consists of variable amounts of gravel, sand, silt, and clay. Characterization of the complex distribution of permeable pathways and zones that hinder the movement of ground water is challenging because of the lack of high quality sources of geologic information. However, thousands of drillers' logs are available and were used to produce three-dimensional visualizations of the High Plains aquifer in a pilot study in southwest Kansas, using commercially available software. The visualizations reveal the existence of regional scale, highly transmissive portions and barriers to flow embedded within a matrix that consists of more localized, thin permeable zones randomly interlayered with thin zones that are much less permeable. Unlike previous investigations into the structure of the aquifer framework, these results show that the aquifer does not consist solely of local, randomly distributed zones of varying permeability.
Source: Geological Society of America