Boulder, Colo., USA - Highlights include new entries to the special issues "Seeing the True Shape of Earth's Surface" and "Origin and Evolution of the Sierra Nevada and Walker Lane." Also online: 3-D modeling of the area in the Pacific Ring of Fire affected by the magnitude 8.1 earthquake on 29 Sept.; another article comparing three different 3-D modeling software packages; and the identification of ancient marine terraces in areas of dense tree cover using airborne LiDAR.
Abstracts for these and other Geosphere papers are available at http://geosphere.
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Emergent and animated COLLADA models of the Tonga Trench and Samoa Archipelago: Implications for geoscience modeling, education, and research
Declan G. De Paor et al., Physics Dept., Old Dominion University, Norfolk, Virginia 23529, USA. Posted online 20 Mar. 2012; doi: 10.1130/GS758.1.
On 29 Sept. 2009, a deadly tsunamigenic earthquake struck American Samoa. The magnitude 8.1 quake occurred near a point where the Pacific "Ring of Fire" goes through a sharp bend. To the south and west, the Pacific tectonic plate is torn in the subsurface with a part of it subducting under the Tonga region. Authors Declan G. De Paor of Old Dominion University and colleagues have created 3-D models of the complex crustal structure of the region, suitable for viewing in Google Earth. Their models represent a number of working hypotheses that explain the tectonics of the region. Some models, presented in Geosphere's unique online platform, emerge from the Google Earth terrain and are animated to help students understand what's going on. Others permit the user to view the subsurface down to lower mantle levels.
Assessing the impact of program selection on the accuracy of 3D geologic models
Kelsey E. MacCormack, University Guelph, 50 Stone Rd. East, Guelph, Ontario N1G 2W1, Canada; and Carolyn H. Eyles, McMaster University. Posted online 20 Mar. 2012; doi: 10.1130/GS732.1.
The increased demand for 3-D geologic models in recent years has led to a rise in the number of 3-D modeling software packages. This article by Kelsey E. MacCormack and Carolyn H. Eyles examines the differences between 3-D geologic models produced using three different modeling software packages (ArcGIS, ROCKWORKS 2006, and VIEWLOG). To ensure that program selection was the only variable influencing the results, identical datasets were interpolated by each of the three modeling programs, using the inverse distance weighting (IDW) and ordinary kriging (OK) algorithms. The results indicate that the 3-D models produced by each of the modeling programs using the OK and IDW interpolation algorithms are inconsistent. MacCormack and Eyles conclude that program selection has a particularly significant influence on the model results when modeling complex subsurface environments with clustered data, a common situation when working with geologic and/or environmental data.
Discovering marine terraces using airborne LiDAR along the Mendocino-Sonoma coast, northern California
Christopher J. Bowles and Eric Cowgill, University of California, One Shields Avenue, Davis, CA 95616-8605, USA. Posted online 20 Mar. 2012; doi: 10.1130/GS702.1.
Marine terraces are planar features in the landscape that record the extent to which Earth's surface has been uplifted above sea level over time. Researchers Christopher J. Bowles and Eric Cowgill present a method for discovering previously unidentified marine terraces in areas of dense tree cover by using data from airborne laser scanning surveys. To test their method, they investigated a 70-km-long section of the northern California coast in Mendocino and Sonoma counties between Fort Ross and Point Arena. In this area, the method reveals 851 individual platforms that Bowles and Cowgill interpret to be a suite of marine terraces. These platforms range in elevation from 100 to 450 meters above sea level, are generally elongate parallel to the modern coastline and one another, and have elevations that increase systematically from west to east in a step-like fashion. Matching the elevations of these platforms with those predicted from sea-level curves suggests the surface-uplift rate for the northern coast of California has been ~0.2 mm/yr over the past two million years. This rate is similar to, but lower than the previously determined uplift rate of ~0.3 mm/yr for a ~23 m-high marine terrace in the area.
Neogene deformation and granite emplacement in the metamorphic units of northern Apennines (Italy): Insights from mylonitic marbles in the Porto Azzurro
Giovanni Musumeci, Universita di Pisa, Dept. of Earth Sciences, Via S.Maria 53, Pisa, Pisa I-56126, Italy and Luca Vaselli, Instituto di Geoscienze e Georisorse-Sezione di Pisa. Published online 20 Mar. 2012; doi: 10.1130/GS665.1.
In the northern Tyrrhenian Sea, late Miocene intrusions (Monte Capanne pluton and Porto Azzurro pluton) were emplaced at upper crustal levels (less than 0.2 GPa) in the thrust systems of Elba Island. Detailed structural analysis in the contact aureole of the Porto Azzurro pluton (eastern Elba) led to discovery of two decameter-scale carbonate shear zones, namely the Calanchiole and Felciaio shear zones. These structures represent two weakened layers in the contact aureole in which west dipping mylonitic foliation, sheath folds, boudinage structures, and upright folds developed under metamorphic conditions retrograde from pyroxene hornfels (thermal peak) to hornblende-greenschist hornfels facies (retrograde cooling). Moreover, geometric and kinematic features indicate that both shear zones correspond to ductile thrusts, which led to internal stacking of the contact aureole. Authors Giovanni Musumeci and Luca Vaselli conclude that because Elba Island is a key area in the northern Apennines-northern Tyrrhenian Sea system, the shear zones give clear indications that, in the northern Apennines, magmatic activity was coeval with crustal shortening with development of out of sequence thrust systems. This suggestion rules out the previous interpretations emphasizing the role of crustal extension and gives a new perspective about tectonic regime and mode of granite emplacement in the Late Neogene evolution of northern Tyrrhenian Sea-northern Apennine system.
Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith, California
Alan D. Chapman et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA, and Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 20 Mar. 2012; doi: 10.1130/GS740.1.
The Sierra Nevada batholith is an ~600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafic to felsic plutonic assemblages. To the south, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. Alan D. Chapman and colleagues assess the magnitude of displacement along this detachment system by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ~5 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. Chapman and colleagues thus infer (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics.
The Fine Gold Intrusive Suite: The roles of basement terranes and magma source development in the Early Cretaceous Sierra Nevada batholith
Jade Star Lackey et al., Geology Dept., Pomona College, Claremont, California 91711, USA. Posted online 20 Mar. 2012; doi: 10.1130/GS745.1.
The Fine Gold Intrusive Suite is one of the largest (more than 2000 square kilometers) and oldest intrusive complexes in the Sierra Nevada batholith (California, USA) and therefore contains a wealth of information about nascent magmatic processes in a convergent margin arc. Because the suite intrudes both accreted oceanic and/or island-arc terranes and continental crust, it provides perspective on how convergent margin magmatism recycles existing crust versus reworking of fringing island arcs into continental crust. Such insight informs our understanding of how continental crust formation may have operated in the Phanerozoic as compared to earlier in Earth history. Jade Star Lackey and colleagues present new data showing that the largely tonalitic suite was emplaced over about 19 million years (124� Ma), in three pulses that young from west to east. Although the Fine Gold Intrusive Suite is comparable to the Late Cretaceous voluminous intrusive suite of the eastern Sierra in terms of duration and age zoning of magmatism, the influence of preexisting basement compositions and differing degrees of organization of the magma sources with age is more pronounced. In addition, the findings show that recycling of fringing arc terranes into continental crust is relatively rapid and that estimates of the growth of Phanerozoic continental crust from such reprocessing should be revised upward.
Magma hybridization in the middle crust: Possible consequences for deep-crustal magma mixing
Calvin G. Barnes et al., Dept. of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA. Posted online 20 Mar. 2012; doi: 10.1130/GS730.1.
The compositions of magmas in many continental arcs -- zones of intense magmatism formed long continental margins above subduction zones -- are commonly explained by equilibrium of the magmas with garnet. Where its chemical effects are observed, magma-garnet equilibrium is traditionally interpreted to occur at or near the base of continental crust, because magnesium-rich garnet is not stable at lower pressures. This study by Calvin G. Barnes and colleagues describes examples from north-central Norway in which the process of magma mixing in the middle crust resulted in garnet stability at lower-than-expected pressure conditions. Garnet became stable because the hybrid magma was both Fe- and Al-rich, owing to mixing of Fe-rich basaltic magmas with Al-rich granitic ones. Mixing of similar end members to produce Fe- and Al-rich hybrids in the lower crust should result in formation of high proportions of garnet in the hybrid. The high-density, garnet-rich hybrids so produced could promote delamination and foundering of the lower crust.