Boulder, Colo., USA - New Geology research reports on data suggesting that existing knowledge on the composition, structure, and dynamics of the upper mantle needs to be re-examined; observations providing groundwork for creating new quantitative models for reaction texture formation to place better constraints on the rates of metamorphic processes; fossilized fungi observed in samples representing a depth of 150 m below the seafloor; newly released reflection seismic data; and a transient change in groundwater temperature after earthquakes.
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Fe3+-rich augite and high electrical conductivity in the deep lithosphere
Xiaozhi Yang and Catherine McCammon, Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany. Posted online 6 Jan. 2012; doi: 10.1130/G32725.1.
Electrical anomalies in the uppermost mantle have usually been attributed to the presence of highly interconnected melts or hydrous olivine. Xiaozhi Yang and Catherine McCammon show through laboratory experiments that such anomalies could also be explained by regionally abundant Fe3+-rich augites (e.g., with a volume fraction of at least 10 percent) and potentially other pyroxenite assemblages. Moreover, the effect of mantle flow on the spatial distribution of augites (and other pyroxenites) relative to wall peridotites may result in substantial electrical anisotropy in the upper mantle. Their model is consistent with mantle petrology regarding the regional occurrence of augites (and other pyroxenites) in the form of dykes and/or veins, and efficiently reconciles some of the conflicting arguments on several properties of the uppermost mantle. Their data suggest that existing knowledge on the composition, structure, and dynamics of the upper mantle needs to be re-examined.
Corona networks as three-dimensional records of transport scale and pathways during metamorphism
Eric T. Goergen and Donna L. Whitney, Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA (Goergen) and Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA (Whitney). Posted online 6 Jan. 2012; doi: 10.1130/G32452.1.
Mafic gneisses from the Thor-Odin gneiss dome (British Columbia, Canada) preserve a record of chemical transport during metamorphism in the form of interconnected networks of coronal reaction textures. Coronal textures are the remnants of metamorphic reactions that have not gone to completion. The preservation of these textures allows Goergen and Whitney to explore the dynamics of chemical processes that occur as a rock endures changes in pressure and temperature during a tectonic cycle. They report results from high-resolution X-ray tomography that reveal the architecture of reaction diffusion pathways that permitted long length-scales of chemical communication between different reactions occurring concomitantly in a rock volume. Pathways focus chemical diffusion as direct paths linking different reaction sites. Three-dimensional interconnection is also evident within the reaction textures themselves. Electron backscattered diffraction data reveal the fine-scale structure of intergrown phases in their reaction textures as three-dimensional dendrites. These data suggest the mineral growth mechanisms in open chemical systems, as described in this study, share mechanistic similarities to closed system nucleation and growth processes. These observations provide the groundwork for creating new quantitative models for reaction texture formation that may place better constraints on the rates of metamorphic processes.
Fossilized fungi in subseafloor Eocene basalts
Magnus Ivarsson et al., Department of Palaeozoology and Nordic Center for Earth Evolution, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden. Posted online 6 Jan. 2012; doi: 10.1130/G32590.1.
Ivarsson et al. report on fossilized fungal hyphae in subseafloor basalts from the Emperor Seamounts, Pacific Ocean. The fossilized fungi are observed in carbonate-filled veins and vesicles in samples that represent a depth of 150 m below the seafloor. Based on morphological characteristics and the presence of chitin, fossilized filamentous microfossils are interpreted as fungal hyphae rather than filamentous prokaryotes. To visualize the morphological characteristics, synchrotron-radiation X-ray tomographic microscopy (SRXTM) was used. The presence of fungi in subseafloor basalts challenges the present understanding of the deep subseafloor biosphere as being exclusively prokaryotic.
Detecting earliest shortening and deformation advance in thrust belt hinterlands: Example from the Colombian Andes
Mauricio Parra et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Posted online 6 Jan. 2012; doi: 10.1130/G32519.1.
Chronological and structural geologic relationships identified in Parra et al.'s newly released reflection seismic data reveal a previously unrecognized zone of early shortening deformation in the northern Andes of Colombia. Low-temperature thermochronological data (apatite fission-track results) and thermal modeling help define the onset of rapid exhumation about 60-50 million years ago along the boundary between Magdalena Valley and the Eastern Cordillera. Subsurface angular unconformities above fold-thrust structures indicate sediment deposition above a deforming wedge with doubly vergent reverse faults. Restoration of a geologic cross section indicates early Cenozoic shortening and exhumation along a broad zone of east- and west-directed reverse faults, with later deformation focused on west-directed inversion structures. These relationships reveal that deformation did not proceed systematically from west to east, but varied spatially through time.
Flow processes and sedimentation associated with erosion and filling of sinuous submarine channels
David R. Pyles et al., Chevron Center of Research Excellence, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA. Posted online 6 Jan. 2012; doi: 10.1130/G32740.1.
Sinuous submarine channels are common on all of Earth's siliciclastic continental margins. Due to the infrequent and destructive nature of gravity flows, few direct measurements of flow processes and sedimentation in sinuous submarine channels exist. Pyles et al. use measurements from a three-dimensional exposure of a sinuous submarine channel and its fill to document, for the first time, how flow properties and sedimentation differ between erosional and filling stages of the channel. Gravity flows that deepened and laterally migrated the channel resulted in the deposition of point bars, with coarsening-upward profiles, on the inner part of channel bends. These gravity flows were mud-rich with a wide grain-size distribution. Flow heights exceeded the depth of the channel resulting in the deposition of levees. Strong secondary flow is evident with a helical pattern reversed to their subaerial counterparts. Strata in point bars and levees are inclined and deposited primarily by tractive processes. Gravity currents that filled the channel were sand-rich with a relatively narrow grain-size distribution. Flow heights scaled to the depth of the channel, and they contain no evidence for secondary flow. Associated strata are horizontal and deposited primarily from suspension.
Transient change in groundwater temperature after earthquakes
Chi-yuen Wang et al., Earth & Planetary Science, University of California, Berkeley, California 94720, USA. Posted online 6 Jan. 2012; doi: 10.1130/G32565.1.
ABSTRACT: Postseismic decrease in groundwater temperature was documented on the upper rim of a large alluvial fan near the epicenter of the 1999 Mw 7.5 Chi-Chi earthquake (Taiwan). Wang et al. use a model of coupled heat transport and groundwater flow, constrained by documented water-level changes, to interpret this change. They show that groundwater temperature is sensitive to earthquake-induced flow and the observed temperature decrease may be explained by increased groundwater discharge due to earthquake enhanced vertical permeability. The result implies that heat flow near active mountain fronts may be lowered by recurrent earthquakes.
How does the continental crust thin in a hyperextended rifted margin? Insights from the Iberia margin
Emilie Sutra and Gianreto Manatschal, IPGS-EOST (Institut de Physique du Globe de Strasbourg–Ecole et Observatoire des Sciences de la Terre), Université de Strasbourg-CNRS, 1 rue Blessig 67084 Strasbourg-Cedex, France. Posted online 6 Jan. 2012; doi: 10.1130/G32786.1.
Research into the formation of passive rifted margins is incontestably undergoing a paradigm shift. The discovery of exhumed mantle and hyperextended crust is fundamental for the understanding and the prediction of the stratigraphic architecture and tectonic evolution of present-day rifted margins. At present, little is known about age and kinematics of structures and the processes associated with extreme crustal thinning and mantle exhumation observed at many present-day rifted margins. This lack of knowledge is mainly due to the fact that deep-water rifted margins are difficult to image geophysically and have been drilled down into basement only along the Iberia-Newfoundland rifted margins. Emilie Sutra and Gianreto Manatschal attempt to answer to the fundamental questions of how, where, and when the crust thinned along the western Iberia rifted margin. To answer to these questions they used reflection and refraction seismic lines. Their observations suggest that extension is, during early rifting, primarily controlled by crustal inheritance. They also suggest that during final rifting, serpentinization becomes an important process that controls the style of extension.
Amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones
Richard H. Worden et al., School of Environmental Sciences, University of Liverpool, Liverpool L69 3GP, UK. Posted online 6 Jan. 2012; doi: 10.1130/G32661.1.
Deeply buried petroleum sandstone reservoirs typically have little pore space for hydrocarbons after burial and compaction. One mechanism observed for retaining porosity is the occurrence of microcrystalline quartz, which coats the surface of the sandstone grains and inhibits the growth of cements. Although observed, there has been no mechanistic understanding of why or how the microcrystalline quartz grows, or why it maintains high porosity in petroleum sandstone reservoirs. Worden et al. describe the use of high resolution analytical techniques (scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy) to study the microcrystalline quartz in the Late Cretaceous Heidelberg Formation, Germany. They have documented a nanofilm of amorphous silica (50-100 nm) and a layer of chalcedony between the sandstone grains and microcrystalline quartz cement. This amorphous silica appears to insulate the sandstone grains and prevent the growth of quartz cement. The microcrystalline quartz adopts the crystal orientation of the underlying chalcedony, thus preventing growth into the pore. Now that we know what controls microcrystalline quartz growth and why it preserves porosity, it can be used to help identify, rank, and appraise deeply buried petroleum accumulations.
Journal
Geology