Boulder, Colo., USA - Matteo Maggi and colleagues from Italy and Brazil present a new model of the development of fractures showing a stairway trajectory, commonly occurring in finely laminated rock, such microbialites and travertines. These fractures strongly enhance permeability by connecting several highly porous zones enveloped in tight impermeable levels. Understanding and predicting this fracture pattern geometry, distribution, and interconnection is valuable not only for locating water supplies, but also for oil, gas, and geothermal exploration.
Major discoveries of hydrocarbon have recently been made in continental (lacustrine) microbial carbonates in the Brazilian South Atlantic margin, some of which exhibit a texture similar to those usually observed in travertines. Understanding of these lacustrine carbonates is still at an early stage. Given that in modern rift settings, vent-related thermal (travertine) and non-thermal (tufa) carbonates are a major component, the proposed conceptual model of staircase fracture localization contributes to the preparation of a model for the occurrence of high-permeability pathways in hydrocarbon and geothermal microbial reservoirs.
Staircase fractures in microbialites and the role of lamination-related mechanical anisotropy: The example of the Acquasanta Terme travertine deposits (central Italy)
M. Maggi et al., Università degli Studi Roma Tre, Roma, Italy. Published online ahead of print on 22 Jan. 2015; http://dx.
Other new GSA BULLETIN articles (see below) cover such topics as
- 1. The San Gorgonio Pass structural knot, California, USA;
2. Rocks of the Lake Mead region of southern Nevada, USA, including strata of the Rainbow Gardens Formation; and
3. Results from the Boulder Creek Critical Zone Observatory, Colorado, USA.
GSA BULLETIN articles published ahead of print are online at http://gsabulletin.
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Late Quaternary slip history of the Mill Creek strand of the San Andreas fault in San Gorgonio Pass, southern California: The role of a subsidiary left-lateral fault in strand switching
K.J. Kendrick et al., USGS Earthquake Science Center, Pasadena, California, USA. Published online ahead of print on 22 Jan. 2015; http://dx.
This report examines the movement history of the San Andreas fault in a geologically complex region of southern California, the "San Gorgonio Pass structural knot." Over the last few years, San Gorgonio Pass has been the focus of considerable scientific scrutiny because the seismologic community is wrestling with whether (and how) a large earthquake rupture on the Coachella Valley segment might (or might not) work its way through this structural complexity -- with accompanying implications for earthquake hazards throughout southern California. This study specifically examines the slip history of the Mill Creek strand of the San Andreas in San Gorgonio Pass and develops an evolutionary sequence of geologic events that supports our understanding of real-time earthquakes. This report also evaluates the role that the sinistral Pinto Mountain Fault may have played in diverting plate motion away from the Mill Creek strand, and thus in increasing the structural complexity of the region.
Late Oligocene-early Miocene landscape evolution of the Lake Mead region during the transition from Sevier contraction to Basin and Range extension
M. Lamb et al., University of St. Thomas, St. Paul, Minnesota, USA. Published online ahead of print on 22 Jan. 2015; http://dx.
Wedged between the Colorado Plateau and the Basin and Range Province, at the mouth of the Grand Canyon, rocks of the Lake Mead region of southern Nevada contain critical evidence that shed light on the evolution of all these parts of the modern landscape. Strata of the Rainbow Gardens Formation demonstrate that, near the end of the Oligocene, about 24 million years ago, this region experienced a profound geographic shift from erosion of earlier highlands to deposition in lakes, wetlands, and rivers. Our study paints a detailed picture of the geography at this critical time, when tectonic compression gives way to tension, and suggests that volcanic eruptions to the north provided the impetus for this shift. Our work also provides important information on the controversy over the age of the Colorado River, where we rule out the existence of an older Colorado River exiting the Grand Canyon during this time.
Hillslope lowering rates and mobile-regolith residence times from in situ and meteoric 10Be analysis, Boulder Creek Critical Zone Observatory, Colorado
M.A. Foster et al., Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado, USA. Published online ahead of print on 7 Jan. 2015; http://dx.
The cosmogenic radionuclide beryllium-10 (10Be) is useful in constraining the timing of a landscape. We use both in situ and meteoric 10Be to calculate residence times and production rates for mobile regolith on hillslopes in a non-glaciated catchment within the Colorado Front Range. We also develop analytical models of 10Be concentrations to test the steady-state assumption underlying our calculations. Close agreement between meteoric and in situ results indicates that upper and middle hillslope positions are eroding both steadily and uniformly; mobile regolith residence times are 10-20 ka and production rates are 3.1 cm/ka. Comparison with our steady state analytical model indicates that these numbers are likely no closer than plus or minus 25% of the geomorphic reality. Our data also reveal that large quantities of meteoric and in situ 10Be are stored not only in mobile regolith but in the underlying saprolite, highlighting the importance of careful field identification of the mobile regolith-saprolite boundary.
Detrital zircon geochronology of Neoproterozoic-Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave-Snow Lake fault hypothesis
Alan D. Chapman et al., Stanford University, Stanford, California, USA. Published online ahead of print on 22 Jan. 2015; http://dx.
The southwestern margin of North America developed after breakup of the supercontinent Rodinia some 750 million years ago. Multiple phases of mountain building and volcanic activity subsequently disrupted the primary architecture of this continental margin. This paper combines new insights into the origin of Upper Precambrian and Lower Cambrian sedimentary rocks deposited at the location of the White-Inyo Range (east-central California) with existing information from adjacent areas to provide an updated synthesis of the post-Rodinia breakup paleogeography of the southwestern United States.
Cyclic steps along the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea
G. Zhong et al., State Key Laboratory of Marine Geology, Tongji University, Shanghai, China. Published online ahead of print on 7 Jan. 2015; http://dx.
This article by G. Zhong et al. reports the discovery of cyclic steps mapped with multibeam bathymetric and multichannel seismic data along the South Taiwan Shoal and West Penghu canyons on the northeastern continental slope of the South China Sea. The features align in trains extending a remarkable length of up to 100 km and may be the longest ever reported. Single train may consist of up to 19 continuous cyclic steps, which is interesting considering the flow oscillated between subcritical and supercritical around 40 times. These steps are usually kilometers in wave length and tens of meters in wave height that are comparable to the highest falls on our planet. Rough estimations indicate that the paleo-flows responsible for the cyclic steps are 100-300 m thick with maximum velocities of up to 10 m per second, and that the estimated paleo-discharges are approx. 7-23 x 105 cubic meters per second, equivalent to ten times the discharge of the modern Amazon River. Submarine cyclic steps represent a class of newly recognized, large-scale step-like features associated with supercritical flows. They have proven to be a common sea-floor geomorphology and to play an important role in canyon evolution.
Synchronous crustal growth and reworking recorded in late Paleoproterozoic granitoids in the northern Tarim Craton: In-situ zircon U-Pb-Hf-O isotopic and geochemical constraints and tectonic implications
R. Ge et al., State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing, China. Published online ahead of print on 7 Jan. 2015; http://dx.
Extensive granitoids, or granitic rocks, are a unique feature of Earth's continental crust, thus their origin is crucial for our understanding of continental growth. This paper reports a zircon U-P-Hf-O isotopic and geochemical study for the late Paleoproterozoic granitoids in the northeastern Tarim Craton, NW China. The results indicate a relatively short period of magma emplacement approx. 1.93 to 1.94 billion years ago, immediately followed by metamorphism approx. 1.91 to 1.92 billion years ago. In-situ zircon Hf-O isotopic modeling is applied for the first time, and the results, when combined with geochemical results, document that most granitoids in the study area were formed as result of synchronous juvenile input and crustal reworking in a continental arc setting, a conclusion that has great implications for models of continental growth. Combined with previous studies, a late Paleoproterozoic Tarim-North China Craton connection is proposed for the reconstruction of supercontinent Comlumbia/Nuna.
Primary hematite in late Archean to Paleoproterozoic oceans
Si Sun et al., University of Hong Kong, Hong Kong, China. Published online ahead of print on 22 Jan. 2015; http://dx.
Banded iron formation is a sedimentary record of the first two billion years history of the biosphere. The "primary" hematite in banded iron formations can be preserved for more than 2.5 billion years with little change. The primary hematite is the mineral record the Fe(II)-oxidizing photosynthesis in Archean biospheres. The primary hematite crystals of submicrometer size is not a record of the oxygenated Archean atmosphere.