Boulder, Colo., USA – Geology highlights include understanding new evidence for rock pulverization by catastrophic events near major faults in California and Japan; modern-day examples of active arc-continent collision in Taiwan; discovery and study of the highest-latitude coral reefs presently known on Earth, located in Japan; the puzzling record of the changing isotope ratio of calcium in seawater over the last 500 million years; and a possible refutation of hypotheses concerning shallow-water methane seep fauna.
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Dynamic branched fractures in pulverized rocks from a deep borehole
Amir Sagy and Dorit Korngreen, Geological Survey of Israel, 30 Malkhei Israel Street, Jerusalem 95501, Israel. Posted online 12 July 2012; doi: 10.1130/G33194.1.
Both detection of paleo-catastrophic earthquakes and distinguishing between faults that generated earthquakes to more "quiet" faults have always been challenging tasks for geoscientists. Recently, zones of pulverized rocks were found in outcrops near major faults in California and Japan and it was hypothesized that they are products of massive dynamic fragmentation that took place during mega-earthquakes within the upper part of Earth's crust. However, it was quite possible that weathering and other near-surface processes strongly affected the original pulverization structure. New supporting evidence for the origin of the pulverization by catastrophic events was discovered in rocks at a ~5 km depth that were retrieved by drilling through a deep fault zone. The rock samples included fine-grained carbonates that were well-compacted and homogenous. Analysis of these samples revealed a unique pattern of micro-fractures that are characteristic of hierarchical branching of tight tensile cracks. This unique branching pattern is a well-known effect of instability in rapid fractures that propagate near elastic wave velocities. Thus, the deep burial setting, which has preserved these fracture patterns, now provides direct evidence that pulverization is indeed an effect of dynamic loading and fracturing. The significant depth at which these fractures were found indicates that a large part of earthquake energy may be absorbed by off-fault deformation.
Active extension in Taiwan's pre-collision zone: A new model of plate bending in continental crust
Ryan Lester et al., Institute for Geophysics, The University of Texas, J.J. Pickle Research Campus, Building 196, 10100 Burnet Rd., Austin, Texas 78758-0000, USA. Posted online 12 July 2012; doi: 10.1130/G33142.1.
Collisions between continental margins and volcanic arcs mark the closing of ocean basins, the rise of mountain belts, and the growth of continents. However, there are only a handful of modern-day examples of active arc-continent collision. Taiwan is one such example, where the southern Chinese continental margin is currently colliding with the Luzon arc at the Manila trench. A new seismic image and numerical models indicate that such collisions may be preceded by bending and stretching of the continental margin as it approaches the arc in subduction zones. This plate-bending reactivates a deep-seated normal fault near the continental shelf break southwest of Taiwan that penetrates deep into the crust and creates a significant fault scarp at the seafloor. Authors Ryan Lester and colleagues conclude with an argument that a similar normal fault may have evolved into the Lishan fault, a major structural and topographic boundary in the mountains of Taiwan.
Coral reefs at 34°N, Japan: Exploring the end of environmental gradients
Hiroya Yamano et al., National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan. Posted online 12 July 2012; doi: 10.1130/G33293.1.
Hiroya Yamano and colleagues discovered the highest-latitude coral reefs presently known on Earth, located in Japan at 34°N, and describe their architecture and development. Although coral reefs occur generally in warm, clear-water settings, the reefs are distributed within turbid inner bays and experience winter seawater temperatures that fall to 13 degrees Celsius, which is well below the generally accepted lower limit (18 degrees Celsius in winter) of tropical coral reef formation. Despite low seawater temperatures and high turbidity, coring indicated reefs ranging to 555 cm in thickness since ca. 4300 years ago. The corals were dominantly massive Favia, in contrast to tabular and branching Acropora that tend to dominate low-latitude, tropical-subtropical reefs. The reefs provide a unique opportunity to examine the structural changes of coral reefs along latitudinal and turbidity gradients.
Shallow-water methane-seep faunas in the Cenomanian Western Interior Seaway: No evidence for onshore-offshore adaptations to deep-sea vents
Steffen Kiel et al., Courant Research Center Geobiology, Georg-August University Göttingen, Goldschmidtstrasse 3, 37077 Göttingen, Germany. Posted online 12 July 2012; doi: 10.1130/G33300.1.
Sulfide-rich environments in shallow water were considered to be sites where animals acquired pre-adaptations enabling them to colonize deep-sea hydrothermal vents and seeps or where they survived extinction events in their deep-sea habitats. In this paper, Steffen Kiel and colleagues document late Cenomanian shallow-water seep communities from the Tropic Shale in the Western Interior Seaway, United States, as a possible refutation of these hypotheses.
Explaining the Phanerozoic Ca isotope history of seawater
Clara L. Blättler et al., Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. Posted online 12 July 2012; doi: 10.1130/G33191.1
This work addresses the puzzling record of the changing isotope ratio of calcium in seawater over the last 500 million years, as reconstructed from fossil material. Seawater chemistry has at times favored the precipitation of different minerals of calcium carbonate, either aragonite or calcite, which exhibit distinct isotope ratios, but shifting between these intervals cannot entirely explain the calcium isotope data. The most significant trends in the data can be explained using a modern budget of calcium isotopes in marine carbonate minerals. Characteristic isotopic signatures are found for three main depositional groups: deep-sea calcite, shallow-water calcite, and shallow-water aragonite. The evolving proportions of these three groups, in ways that are consistent with the fossil record, can successfully explain the major features of the calcium isotope record of ancient seawater.
Journal
Geology