August 2011 Geology highlights

(Press-News.org) Boulder, CO, USA - Topics in the August GEOLOGY include banded iron formations, the San Andreas fault, the 12 Jan. 2010 Haiti earthquake, a shorter "dead interval" for marine organisms after the end-Permian mass extinction, fossil reef framework-forming cold-water corals, the Holocene sea-level history for the U.S. Atlantic coast, digital image correlation and the 1980 Mount St. Helens collapse, paleotemperature estimates from exquisitely preserved fossil bivalve shells and sediments, and an investigation of earthquakes in the Tonga-Vanuatu region.

Mineral ecophysiological data provide growing evidence for microbial activity in banded-iron formations
Yi-Liang Li et al., Dept. of Earth Sciences and School of Biological Sciences, University of Hong Kong, Hong Kong, China; doi: 10.1130/G32003.1.

Banded-iron formations are one of the biggest mysteries of Earth's evolution and of its biosphere. The formations are made of alternating iron- and silica-rich bands and have long been believed (without direct evidence) to be the records of long-term interactions between the geosphere and the young biosphere. After careful examination of the extremely fine minerals in the 2.48-billion-year-old banded-iron samples from Western Australia, Yi-Liang Li of the University of Hong Kong and colleagues have obtained "less ambiguous evidence for microbial activity that helps resolve an important question in Earth's history," as commented by a scientific reviewer of their article. They found that phosphorus—the most important nutritional element—formed nanocrystal apatite, just like those found in sedimentary rocks after the origin of animal life on Earth. Li and colleagues are still working on the tiny minerals from ancient rocks to uncover more mysteries of early Earth.

Using vertical axis rotations to characterize off-fault deformation across the San Andreas fault system, central California
Sarah J. Titus et al., Dept. of Geology, Carleton College, One North College Street, Northfield, Minnesota 55057, USA; doi: 10.1130/G31802.1.

The San Andreas fault system marks the boundary between two tectonic plates. Instead of envisioning this plate boundary as a single fault, however, it is important to recognize that plate boundary deformation is accommodated within a wide deforming system comprised of numerous faults and other geologic structures that develop adjacent to faults. Titus et al.'s work is focused near one section of the San Andreas fault in central California, known as the creeping segment, where the two plates move past each other at a steady rate without generating large earthquakes. Titus et al. used three separate methods to quantify deformation in off-fault regions near the creeping segment. Their results are important for understanding where plate boundary deformation is taken up across central California, which is useful for characterizing earthquake hazards. Further, their results suggest that fault creep is not a recent style of deformation for the creeping segment, but has been occurring for several million years.

Porosity redistribution enhanced by strain localization in crystal-rich magmas
Mickael Laumonier et al., CNRS Institut des Sciences de la Terre d'Orleans, UMR 6113, Universite d'Orleans, Campus Geosciences, 1A Rue de la Ferolerie, 45071 Orleans cedex 2, France; doi: 10.1130/G31803.1.

Some volcanoes erupt the same magma in either an explosive or effusive manner. These two opposite behaviors are linked to the way magma loses the gases it contains. Mickael Laumonier of CNRS Institut des Sciences de la Terre d'Orleans and colleagues present a new mechanism by which significant gas escape is achieved. It is based on an experimental study involving sheared magmas with a high crystal fraction and gas bubbles. Shearing in the volcanic conduit is known to favor the connection of gas bubbles. Experiments by Laumonier and colleagues replicate such shearing and show that this process is complicated by the presence of numerous crystals in the magma. Shearing localizes the deformation in narrow bands where the gas bubbles concentrate. Such highly permeable shear bands may allow the magma to degas, potentially turning an explosive eruption into an effusive one.

The not-so-simple effects of boundary conditions on models of simple shear
Marcel Frehner et al., Dept. of Geodynamics and Sedimentology, University of Vienna, 1010 Vienna, Austria; doi: 10.1130/G31957.1.

A popular method to study the deformation of rocks is to reproduce this deformation in the laboratory using rock-analog materials such as polymers, clay, or sand. Marcel Frehner of the University of Vienna and colleagues demonstrate that the quantification of such analog models can be inaccurate, with errors up to 100%, depending on the design of the laboratory deformation apparatus. Particularly in shear experiments, on which the focus, the bounding walls of the modeling box strongly influence the deformation of the rock-analog material. Using a numerical model, different types of analog modeling boxes could be reproduced and the effects of the bounding walls on the model interior could be quantified. A key finding is that large errors not only occur close to the model boundaries, but throughout the entire model volume. This leads, and most probably led in the past, to misinterpretation of kinematic parameters in published laboratory deformation experiments, which hinders the comparison with natural deformation structures. Based on these results, Frehner and colleagues give advice for future designs of laboratory shear deformation machines. For example, it is demonstrated that the length-to-width-ratio of the deformation machine does not reduce the boundary effects within the model.

Offshore sedimentary effects of the 12 January 2010 Haiti earthquake
Cecilia M. McHugh et al., Earth and Environmental Sciences, Queens College, City University of New York, Flushing, New York 11367, USA; doi: 10.1130/G31815.1.

Cecilia M. McHugh of the City University of New York and colleagues show that offshore sedimentation effects of the catastrophic 12 January 2010 Haiti earthquake reveal the marine signature for a large earthquake and the potential for obtaining a subaqueous paleoseismic record.

Lhasa terrane in southern Tibet came from Australia
Di-Cheng Zhu et al., State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China; doi: 10.1130/G31895.1.

Paleogeography is the study of physical geography of the geologic past. The Lhasa Terrane in southern Tibet is one of the largest crustal blocks that make up the composite Greater Tibetan Plateau. It has long been accepted as part of the Qiangtang-Greater India-Tethyan Himalaya continental margin system in the Paleozoic reconstruction of the Indian plate and, therefore, came from the Indian Gondwana. However, Di-Cheng Zhu of China University of Geosciences and colleagues have found that the Lhasa Terrane has a detrital zircon signature and Hf-isotope compositions differing significantly from the Qiangtang and Tethyan Himalayan terranes, but resembling those of zircons from the Albany-Fraser Belt in southwest Australia. This new finding allows Zhu and colleagues to propose that the Lhasa Terrane is actually exotic to the widely accepted "Greater Tibetan Plateau system," and correlated paleogeographically with Australia during the late Precambrian-early Paleozoic. It follows that the Lhasa Terrane experienced its late Precambrian-early Paleozoic evolution as part of Australia in a different paleogeographical setting than that of the Qiangtang-Greater India-Tethyan Himalaya system. Zhu and colleagues bring a refreshing perspective to the Tibetan crustal evolution and show that Tibet is more heterogeneous in its geological heritage than previously thought.

Zircon crystallization and the lifetimes of magmatic-hydrothermal ore systems
Albrecht von Quadt et al. (Christoph A. Heinrich, corresponding), Institute of Geochemistry and Petrology, Dept. of Earth Sciences, ETH Zurich, Clausiusstrasse 25, 8092 Zurich, Switzerland; doi: 10.1130/G31966.1.

Zircon crystals in subvolcanic porphyry intrusions record the lifetime of large underlying magma reservoirs that provide the fluids for generating giant copper-gold ore deposits. The youngest zircon grains, dated by uranium decay to lead, best define the short time interval of porphyry emplacement and ore formation.

Recovery tempo and pattern of marine ecosystems after the end-Permian mass extinction
Haijun Song et al., Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China; doi: 10.1130/G32191.1.

Following the end-Permian mass extinction, the greatest crisis in the history of life, it has traditionally been thought that it took up to five million years before there was even the vestige of a recovery. Song et al. show that this "dead interval" was, in fact, of much shorter duration and that marine life has a much better bounceback capability with geological time scales. Studies in south China show that the fossil record of tiny organisms, such as the single-celled forams, reveal that radiation and diversification began only a million years after the mass extinction. However, the recovery was not even throughout all marine habitats. Ecosystems in the water column recovered almost immediately, suggesting plankton productivity levels were fine at this time, while the reefs did not reappear until nearly 10 million years after the extinction.

Northeastern Atlantic cold-water coral reefs and climate
Norbert Frank et al., Laboratoire des Sciences du Climat et de l'Environnement, Institute Pierre Simon Laplace (LSCE/IPSL), Unite Mixte de Recherche, UMR8212, Avenue de la Terrasse, 91198 Gif/Yvette, France; doi: 10.1130/G31825.1.

The study of hundreds of fossil reef framework-forming cold-water corals along a nearly 6000-km-long continental margin sector, extending from off Mauritania to the southwestern Barents Sea, has revealed a strong climate influence on the geographical distribution and sustained development of these ecosystems. Norbert Frank of LSCE/IPSL and colleagues have found evidence that over the past three glacial-interglacial cycles (i.e., the past 250,000 years), cold-water coral species Lophelia pertusa and Madrepora oculata populated predominantly reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. In contrast, above 50 degrees N, corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from ~50 degrees N to ~70 degrees N upon Northern Hemisphere climate warming. Based on the regional coral age patterns and marine climate records, Frank and colleagues propose that north-south oscillations of the biogeographic limit of reef developments are paced by the ice ages, and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity potentially related to changes of the polar front. Future ocean warming may promote a further northward invasion of such cold-water coral ecosystems into the Arctic Ocean. However, anthropogenic ocean acidification may limit the ability of corals to calcify in Arctic waters.

Rheology and microstructure of experimentally deformed plagioclase suspensions
D. Picard et al., UMR CNRS, 6113 Institut des Sciences de la Terre d'Orleans, 1a, rue de la Ferollerie, 45071 Orleans, France; doi: 10.1130/G32217.1.

Magmas are directly implied in major geological processes such as mantle dynamics and volcanic eruptions. From their source to their final emplacement, their ability to flow depends on external parameters such as pressure and temperature. During the ascension in the volcanic conduit, a decrease of these parameters induces the crystallization of a solid phase (crystals). This phase plays an important role in the flow of magmas by increasing their viscosities. Moreover, magmas can undergo deformation at the rim of the conduit or at the dome and lava-flow base. This deformation leads to an orientation of the crystals, which is influenced by their characteristics (length, width, and shape ratio). To better constrain the flow of magmas, Picard et al. present the first deformation experiments at high temperatures and pressures of magmatic suspensions of plagioclase, a representative crystalline phase with a high shape ratio. Results highlight the interplay between the flow behavior and the orientation of the crystals. They also demonstrate the critical importance of the shape of crystals on the flow of magmas.

Holocene relative sea-level changes and glacial isostatic adjustment of the U.S. Atlantic coast
S.E. Engelhart et al. (B.P. Horton, corresponding), Sea-Level Research, Dept. of Earth and Environmental Science, University of Pennsylvania, Hayden Hall, 240 South 33rd Street, Philadelphia, Pennsylvania 19104, USA; doi: 10.1130/G31857.1.

S.E. Engelhart of the University of Pennsylvania and colleagues have described the Holocene sea-level history for the U.S. Atlantic coast, which exhibits temporal and spatial variability. From 10,000 years ago to 4,000 years ago, sea level rose rapidly as ice sheets melted and the land subsided from isostatic adjustment. During the past 4,000 years, this rate slowed because the major mid-latitude ice sheets had disappeared. Spatially, the highest rates of relative sea-level rise were found in the mid-Atlantic region, coincident with the area of greatest ongoing glacial isostatic adjustment-related subsidence. A comparison with a model of this process (ICE-5G/6G VM5a) illustrates misfits with the geological data. A revised Earth model (VM5b) that decreases the upper mantle and transition zone viscosity by 50% was able to remove most of the discrepancies with the data. The results may hint toward the importance of laterally heterogeneous viscosity in the upper mantle.

Aptitude of modern salt marshes to counteract relative sea-level rise, Venice Lagoon (Italy)
Federica Rizzetto and Luigi Tosi, Institute of Marine Sciences, National Research Council, Arsenale-Tesa 104, Castello 2737/F, 30122 Venice, Italy; doi: 10.1130/G31736.1.

Federica Rizzetto and Luigi Tosi of the Institute of Marine Sciences, Venice, Italy, point out the response of a part of the Venice lagoon tidal flats to sea-level changes in the past 70 years. In the past century, in contrast with the general erosional trend, some salt marshes in the northern lagoon preserved their original characteristics and showed accretion and development of the tidal creek network. The evolution of one of them, considered as an example of intertidal accretionary landform that responded rapidly to sea-level variations, was sketched by the interpretation of ultrahigh-resolution aerial photographs taken from A.D. 1938 to 2006, and the results were compared with relative sea-level rise trend and high tide frequency. The long-term investigation pointed out the most significant morphological changes that occurred over the entire period, whereas the short-term analysis showed in detail the subsequent phases of salt marsh evolution and their relations with sea-level variations. Even if in the past 70 years the salt marsh underwent low accretion rates, it did not disappear, probably because the remobilization of sediments eroded from the marsh front and the lagoon bottom by tides and other local hydrodynamic processes, and their accumulation on the marsh surface favored by vegetation, were sufficient to offset sea-level rise.

Preservation of an extreme transient geotherm in the Raft River detachment shear zone
R. Gottardi et al., Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA; doi: 10.1130/G31834.1.

When the continental crust deforms in extension, it divides into two layers with very different behaviors: the cold upper crust thins by brittle faulting, while the hot ductile lower crust flows. Those two contrasted layers are separated by an interface called a detachment zone, which is an important mechanical (from brittle to ductile) and thermal (from cold to hot) transition. By analyzing the chemistry of minerals sampled across a detachment zone exposed in northwest Utah, this study by R. Gottardi of the University of Minnesota and colleagues reveals an abrupt increase in temperature from the upper to lower crust. Other chemical analysis shows that meteoric fluids derived from the Earth's were circulating down to the detachment and may have played an important role in cooling the rocks during their deformation. Gottardi and colleagues point out three mechanisms that could contribute to sustain such abrupt increase of temperature across a detachment: large displacement across the detachment system, thinning of the rocks, and, more critically, circulation of surface fluids.

Coarse-grained sediment waves in hyperpycnal clinoform systems, Miocene of the Austral foreland basin, Argentina
Juan Jose Ponce and Noelia Carmona, Instituto de Investigacion en Paleobiologia y Geologia, Universidad Nacional de Rio Negro, Isidro Lobo y Belgrano (8332) General Roca, Rio Negro, Argentina, and Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Buenos Aires, Argentina; doi: 10.1130/G31939.1.

Until now, identification, description, and modeling of coarse-grained sediment waves were mostly based on high-resolution side-scan sonar and 3-D seismic-reflection imaging, and sediment core samples. However, there are only a few examples of coarse-grained sediment waves in outcrops because they are difficult to recognize due to their large size and low preservational potential. Juan Jose Ponce and Noelia Carmona of CONICET Argentina present an outcrop example of coarse-grained sediment waves generated by extraordinary fluvial discharges entering the sea as hyperpycnal flows, in Miocene clinoform successions exposed in the northeast Atlantic coast of Tierra del Fuego, Argentina. The sedimentation processes related to the generation of an individual set of coarse-grained sediment waves in a complete cycle of hyperpycnal discharge from waxing to waning phase are deciphered. Ponce and Carmona also analyze how the successive hyperpycnal discharges control the stacking pattern of such sediment waves. They provide new insight into the depositional mechanisms that generate coarse-grained sediment waves in channel-lobe transition zones, which are controversial topics in sedimentology.

Structural architecture of the 1980 Mount St. Helens collapse: An analysis of the Rosenquist photo sequence using digital image correlation
Thomas R. Walter, Dept. 2, Physics of the Earth, Helmholtz-Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam 14473, Germany; doi: 10.1130/G32198.1.

The accurate detection of volcanic activity, and of the dynamic developments that occur immediately prior to eruptions, is among the main scientific results that are achieved using modern monitoring instrumentation. In many cases, however, simple photographs are the only information available from erupting volcanoes. Thomas R. Walter of the German Research Centre for Geosciences describes the technique of digital image correlation that can be applied to almost any photographic time lapse dataset at active volcanoes. Similarly, older archives might be reanalyzed, as this study exemplifies for Mount St. Helens, which collapsed more than 30 years ago. Through the digital image correlation method, Walter shows that photographs of this event can be analyzed at a high level of detail, revealing complexly slumping and rotating rock masses.

Warm, not super-hot, temperatures in the early Eocene subtropics
Caitlin R. Keating-Bitonti et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA; doi: 10.1130/G32054.1.

The early Eocene, 48-55 million years ago, is considered one of the warmest periods of the past 65 million years of Earth's history. Caitlin Keating-Bitonti of the University of Wisconsin and colleagues combine new and traditional geochemical techniques to reconstruct sea-surface temperatures of the U.S. Gulf Coast from exquisitely preserved fossil bivalve shells and sediments. Four independent techniques all yield consistent paleotemperature estimates of ~27 degrees Celsius. These early Eocene temperatures are 2 to 3 degrees warmer than temperatures in the region today and are similar to or even cooler than published early Eocene temperature estimates from higher latitudes. These results have bearing on the ongoing efforts to understand Earth's climate during pronounced greenhouse times, particularly how the equator-to-pole temperature gradient changes with increasing global warmth. If the low to mid latitudes exhibit paleotemperatures of approximately 27 degrees Celsius, it becomes more difficult to explain recently suggested estimates in excess of 30 degrees Celsius at high latitudes.

Molybdenum isotopic records across the Precambrian-Cambrian boundary
Hanjie Wen et al., State Key Laboratory of Ore Deposit Geochemistry (SKLODG), Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China; doi: 10.1130/G32055.1.

The redox-sensitive metal molybdenum and its isotopes have been used increasingly in the past decade as proxies for changing redox conditions in the oceans, in which how to trace the Mo isotopic composition in the paleo-seawater is crucial question. Commonly, the Mo isotopic composition of euxinic sediments with [H2S] greater than 11 microM is believed to reflect the coeval seawater record. However, tracking the isotopic variations of ocean Mo through time still remains difficult because such anoxic/euxinic sediments are not ubiquitous throughout geological times. Hanjie Wen of the Chinese Academy of Sciences and colleagues found that in analogue to non-euxinic, non-skeletal carbonates, the "pristine" phosphorites also can preserve the ambient fluid, which makes the "pristine" phosphorites a new potential fingerprinting tool to record the coeval paleo-seawater. This finding demonstrates the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records, and largely expands the application of Mo isotopes as geochemical proxy. Based on this observation, result of two Early Cambrian formations in southern China suggest that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the Early Cambrian.

Did the great dying of life take 700 k.y.? Evidence from global astronomical correlation of the Permian-Triassic boundary interval
Chunju Huang et al., Faculty of Resources, China University of Geosciences, Wuhan 430074, P.R. China; doi: 10.1130/G32126.1.

The cause of the great Permian-Triassic boundary (PTB) mass extinctions remains unknown. A crucial step in identifying the cause involves a precise timing of the mass extinction interval (MEI) in order to reconstruct the pattern of biotic evolution and the chronologic record of potential triggers. Magnetic susceptibility data from Shangsi, south China, provide evidence for strong 405,000-year orbital eccentricity forcing throughout the PTB interval. This allows development of an astrochronology for the PTB interval based on the 405,000-year orbital eccentricity metronome that has been proposed for the Mesozoic time scale. Radioisotope dating combined with 405,000-year tuning provides an absolute time scale through the PTB interval at unprecedented high resolution. An estimated ~700,000-year duration for the MEI at Shangsi is supported by eccentricity-tuned estimates of three other sections in China (Meishan, Huangzhishan, and Heping), and one Alpine section (Gartnerkofel, Austria) from the eastern and western margins of the Palaeo-Tethys Ocean during PTB time. This suggests that the PTB mass extinctions were not the result of a single catastrophic event. Siberian trap volcanism was largely synchronous with the MEI and appears to be the most likely cause of the mass extinctions; astronomically paced climate change may also have played a role.

Why is the North America Cordillera high? Hot backarcs, thermal isostasy, and mountain belts
R.D. Hyndman Pacific Geoscience Centre, Geological Survey of Canada, Sidney V8L 4B2, British Columbia, Canada; and C.A. Currie; doi: 10.1130/G31998.1.

Global mountain belts are commonly thought to be a consequence of crustal shortening and thickening resulting from continental collision. The thick crust floats higher, like a thick-root iceberg. The Himalayan Mountains, resulting from the collision of India with Asia, are a type case. However, accumulating data indicate that the North American Cordillera mountain belt generally has quite thin crust; thinner than the adjacent stable areas of eastern North America. R.D. Hyndman of the Geological Survey of Canada and C.A. Currie of the University of Edmonton show that this high elevation, in spite of its thin crust, is explained by systematically higher than normal temperatures in the crust and upper mantle in the Cordillera. Thermal expansion buoyancy is found to contribute 1600 meters of elevation, explaining the high average mountain belt elevation even with thin crust. The high temperatures are found to be a characteristic of subduction zone backarcs (i.e., the region landward of the Cascadia volcanic arc), and recent, now extinct, arcs to the north and south.

When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region
Simon Richards et al., School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia; doi: 10.1130/G31937.1.

Using new technologies and combining these with developing ideas on how Earth works, scientists are now able to virtually "see" into Earth's interior. Using this information, they are able to interpret what triggers earthquakes and earthquake swarms at great depth. Simon Richards of James Cook University, Australia, and colleagues present a new, geologically and geodynamically consistent model that helps explain why large-magnitude earthquakes and earthquake clusters form below Tonga. They suggest that four-million years ago, the base of the west-dipping subducted Australian slab beneath Vanuatu broke off or detached. This a 400- to 500-km-wide strip of slab sunk through the mantle and eventually collided with the west-dipping Pacific slab beneath Tonga and the Lau Basin. Collision of the two slabs and their ongoing subduction has led to the generation of an unusually high number of earthquakes between 400 and 600 km depth below Tonga. The model developed by Richards and colleagues, involving slab breakoff and subsequent collision, also helps explain geodynamic changes in the upper plate, such as changes in the rate of opening of the North Fiji Basin and triggering of rifting in the Lau Basin at the time of collision.

Baltica in the Cordillera?
E.L. Miller et al., Dept. of Geology and Environmental Science, Stanford University, Stanford, California 94305, USA; doi: 10.1130/G31910.1.

The western Cordillera of North America is a long-lived mountain belt, shaped by subduction for hundreds of millions of years. Within it lie various fault-bound terranes whose site of origin is problematic based on exotic fossils and large shifts in latitude determined paleomagnetically. Uranium-lead radiometric ages of the mineral zircon, found in sandstones eroded from granitic rocks, is increasingly used to fingerprint the continent of origin for sediments. Paleozoic rocks of the Alexander terrane of southeastern Alaska have long been known to be far-traveled based on fossils, paleomagnetic data, and zircons in sandstones, traits that are shared by terranes in the Klamath Mountains and Sierra Nevada. Various authors have suggested these terranes originated from the edge of the Baltic Shield or European continent and traveled by plate motions into the Cordillera via the east coast and Caribbean or through the Arctic. E.L. Miller of Stanford University and colleagues test these ideas by providing the first set of zircon ages from Paleozoic sandstones deposited near St. Petersburg and the Polar Urals of Russia. The match of the new data to zircon ages in the Cordilleran terranes is excellent. Miller and colleagues prefer this northern route because it is shorter.

Origin of the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia: Was a sedimentary basin involved?
Jocelyn McPhie et al., ARC Centre of Excellence in Ore Deposits, and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia; doi: 10.1130/G31952.1.

One of the largest known metal-rich ore deposits occurs at Olympic Dam in South Australia. The deposit is mined for copper, uranium, gold, and silver. These metals occur in small grains of sulfide and oxide minerals within granite that has been intensely fragmented. The metal-rich ore minerals were precipitated from hot aqueous fluids that permeated through the fragmented granite. The sulfide and oxide minerals also occur in fine sedimentary rocks that are mainly composed of grains deposited in relatively deep water such as a lake. These sedimentary rocks suggest that the ore deposit was originally covered by a thick (hundreds of meters) layer of sediment and that the sediment was present while the deposit was forming. This new insight into the circumstances that prevailed during ore formation re-opens the debate about the origin of Olympic Dam. Jocelyn McPhie of the University of Tasmania and colleagues propose that the overlying sediment could have been an important source of the metals and fluids required by the metal-rich sulfide and oxide minerals. This hypothesis is consistent with the present and reconstructed architecture of the ore deposit and the rock formations around it, and readily explains both the enormous size and polymetallic character of the deposit.

### Keywords: BIFs, San Andreas Fault, Haiti, dead interval, U.S. Atlantic coast, Venice tidal lagoons, Mount St. Helens, paleotemperatures, molybdenum, Permian-Triassic boundary, North American Cordillera, Tonga, Vanuatu, Baltica, Olympic Dam, Australia, ore deposits

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