Boulder, Colo., USA - When clayey materials are compressed and sheared, they commonly develop a "scaly fabric" wherein the clay is divided by braided shear surfaces into lentil-shaped chips. Although such scaly fabrics are found at the bed of glaciers, the base of landslides, and in gougey faults, little is known about how they form or how they deform. In their article for Geology, Matthew Tarling and Christina Rowe use dry lentils to help explore this process.
Tarling and Rowe built a shear box with a sliding floor and filled it with lentils. In a series of experiments, they show that the lentils have a tendency to shift constantly against one another when the bulk is shearing, prohibiting the development of long-lasting faults.
Each lentil follows a slightly different path, at different speeds, to accomplish the broad deformation of the whole. This "delocalization" behavior seems to be intrinsic to the system. This observation suggests an explanation for why scaly layers of clay may grow so broad in faults, landslides, and the beds of glaciers, and why these types of shear networks do not grow into localized fault zones.
Experimental slip distribution in lentils as an analog for scaly clay fabrics
Matthew S. Tarling, Dept. of Earth and Planetary Sciences, McGill University, 3450 University Street, Montréal, Quebec H3A 0E8, Canada; and Christie D. Rowe Dept. of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand. This article is online at http://dx.
Other recently posted GEOLOGY articles are highlighted below:
Did the A.D. 365 Crete earthquake/tsunami trigger synchronous giant turbidity currents in the Mediterranean Sea?
Alina Polonia et al., ISMAR-CNR (Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche), Via Gobetti 101, 40129 Bologna, Italy. This article is online at http://dx.
The Ionian Sea is the world's oldest oceanic basin. It is located between the Calabrian and Hellenic arcs, where remnants of the Mesozoic Thetis Ocean are subducting into the mantle, causing destructive earthquakes. The effects of such events may be recorded in the deep basin as sediment mass-flows and turbidites. The recent-most, over 25-m-thick, megaturbidite is known as Homogenite or Augias Turbidite (HAT). Forty years after its discovery, the HAT was radiometrically dated and correlated for an area larger than 150,000 square kilometers. These new data refute the hypothesis that relates its emplacement to the Santorini caldera collapse (~3,600 B.P.), but point to a large tsunami initiated by the AD 365 Crete megathrust earthquake. The devastating effects must have been similar to those of the recent Sumatra and Tohoku tsunamis. Correlation and accurate dating of the HAT over the Mediterranean Sea suggests that the AD 365 Crete earthquake and tsunamis, described in historical chronicles and recently revised by geological studies onshore, has resulted in widespread massive sediment remobilization. The HAT is thus the thickest and largest seismo-turbidite tsunamite yet known, and can be used to define proxies to reconstruct super-quake recurrence time also in other megathrust regions.
Linking gas fluxes at Earth's surface with fracture zones in an active geothermal field
Egbert Jolie et al., Helmholtz Centre Potsdam-GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany. This article is online at http://dx.
Geothermal reservoirs in hydrothermal systems are characterized by the natural circulation of hot fluids. Fluids migrate within the reservoir rock, but also along existing networks of fracture zones. The location and occurrence of fractures in geothermal reservoirs are not always known, and important characteristics of faults, such as permeability, are commonly uncertain. Fracture networks provide permeable pathways and enable fluids to reach Earth's surface, where they form well-known geothermal surface manifestations, such as hot springs, fumaroles, mud pots, and others. However, in many cases geothermal fluids reach the surface and form invisible degassing anomalies. Such anomalies are analyzed in this paper by Egbert Jolie and colleagues, using a case study at the Brady's geothermal field in the Basin and Range province (Nevada, USA). Jolie and colleagues demonstrate how invisible parameters (e.g., carbon dioxide, hydrogen sulfide, and radon emissions) can be analyzed at the surface and linked to the geothermal system at depth. The linkage is realized by combining gas fluxes with structural-geological information, such as three-dimensional fault models. Further information on the tendency of fractures to slip and dilate help to assess the connectivity between surface and subsurface.
Magma-slurry interaction in Surtseyan eruptions
C. Ian Schipper, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand; and James D.L. White, Geology Dept., University of Otago, P.O. Box 56, Leith Street, Dunedin 9016, New Zealand. This article is online at http://dx.
Surtseyan eruptions are dynamically unmistakable, as their phreatomagmatic jets emerge from shallow water to produce steam-laden plumes and build new land. Surtseyan eruptions are driven by interaction between magma and external water, but descriptions of the interaction have long remained only qualitative. In this study, Ian Schipper and James D.L. White investigate textures of coarse particles -- bombs and lapilli -- that are ubiquitous but scarce and rarely investigated, in Surtseyan deposits. Examination in hand sample, thin section and by X-ray microtomography reveals that coarse Surtseyan particles are composites, with vesicular host magma enclosing smaller particles. Key textures are repeated at a variety of scales, including void spaces and densified magma around entrained material, and post-entrapment crystallization of the entrained clasts. All features can be explained if jet-initiation is not driven by the hydrodynamically unfeasible interaction between magma and pure water, but instead by mingling of a water-saturated slurry generated as particles and water periodically flood the vent. These findings are consistent with a classical model of Surtseyan eruptions that was based on field observations. The textures presented here, and the dynamic consequences implied by their mode of formation, provide a direct link between the pyroclast textures and the form of Surtseyan jets and plumes.
Storage thresholds for relative sea-level signals in the stratigraphic record
Qi Li et al., Dept. of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA. This article is online at http://dx.
Sedimentary deposits preserve the most complete record of changing sea level over the greatest fraction of Earth's history. However, deciphering this record is difficult due to the intermingling of random processes, such as rapid changes in river course, with signals of changing sea level and climate. Here, using laboratory experiments and theory, Qi Li and colleagues define the attributes that changes in sea level must have in order to be preserved in the largest deposits on Earth's deltas. Reported observations will aid our interpretation of deltaic response to past climate change, thus enhancing our ability to predict their response to on-going climate change.
GEOLOGY articles are online http://geology.