Steady rotation of the Cascade arc
Ray E. Wells and Robert McCaffrey write that the clockwise geologic displacement of the 16 million-year-old (m.y.) ancestral Cascade volcanic arc from the presently active volcanic chain in the northwestern U.S. and Canada is in the same sense and at nearly the same rate as the motions calculated from Global Positioning System (GPS) over the past 15 years.
Motion of the ancestral arc can be explained by clockwise rotation of the leading edge of North America at 1.0 degree/m.y. over the magma source generated by the subducting plate, which is itself moving westward 1 to 4.5 km/m.y. as it slowly sinks into the mantle.
The remarkable similarities between post-16 m.y. arc migration, paleomagnetic rotation, and modern GPS block motions indicate that the block motions from decadal GPS can be used to calculate meaningful long-term crustal strain rates and earthquake hazards.
Silica gel formation during fault slip: Evidence from the rock record
J.D. Kirkpatrick and colleagues discusses the dynamic reduction of fault strength as a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature.
Here, Kirkpatrick and colleagues report on the first documented occurrence of a natural silica gel coating a fault surface at the Corona Heights fault slickenside in San Francisco, California, USA.
Comparison of microstructures in superplastically deformed synthetic materials and natural mylonites: Mineral aggregation via grain boundary slidingTakehiko Hiraga et al., Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. Published online 22 July 2013, http://dx.doi.org/10.1130/G34407.1
Very similar microstructure, that is, the same phase aggregation is found in both experimentally deformed synthetic materials and natural ultramylonite. The stress-strain-rate relationship, grain-size dependent flow strength, and the achievement of large tensile stain on the synthetic samples indicate that the samples creeped due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase oriented in the direction of compression. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, the results presented here by Takehiko Hiraga and colleagues contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.
Sea-level-induced seismicity and submarine landslide occurrenceDaniel S. Brothers et al., U.S. Geological Survey, Coastal and Marine Science Center, 384 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34410.1.
The relationships between global climate change and marine geohazards remains poorly understood. This study by Daniel S. Brothers and colleagues investigates compelling linkages between rapid sea-level rise, bending of the lithosphere, and stress loading of crustal faults. Rupture of such faults may induce slope failure and generate submarine landslides, thus offering a new explanation for the temporal coincidence between many submarine landslides and rapid sea level rise between 16,000 and 8,000 years ago.
Reconciling disparate estimates of total offset on the southern San Andreas faultMichael H. Darin and Rebecca J. Dorsey, Dept. of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34276.1.
The total amount of motion along a fault can be estimated by offset geologic markers that were once continuous across the fault. There are various pairs of cross-fault markers along the San Andreas fault in southern California that suggest different amounts of offset ranging from 160-240 km. These estimates assume that the markers were offset solely by sliding along the fault. In this paper, Michael H. Darin and Rebecca J. Dorsey reinterpret one pair of offset markers and use a simple geometric model to show that fault block rotation adjacent to the fault can account for a small but significant amount of the apparent offset, thus reducing the amount of sliding required to displace the formerly continuous markers. Their model provides a new estimate of ~200 km of total offset on the San Andreas fault within the Salton Trough region, which is consistent with all other geologic data from various offset markers. This new lower estimate of total slip on the southern San Andreas fault implies that more slip is required on other faults in California and Arizona, in order to satisfy estimates of the total amount of motion between the Pacific and North American plates.
Erosion of the Tsangpo Gorge by megafloods, Eastern HimalayaKarl A. Lang et al., Dept. of Earth and Space Sciences, and Quaternary Research Center, University of Washington, Box 351310, Seattle, Washington 98195, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34693.1.
Karl Lang and colleagues present new detrital zircon U-Pb provenance data from large magnitude "megaflood" deposits immediately downstream of the Yarlung-Tsangpo River Gorge in the easternmost Himalaya. These data support the previous hypothesis that Tibetan lakes restrained by glacial ice and debris within the Yarlung-Tsangpo drainage episodically evacuated through this steep, narrow gorge. These extreme flood events were capable of transporting a considerable amount of material from steep channel-adjacent hillslopes, focusing erosion within the gorge. Although megaflood frequency remains unconstrained, these data support a mechanism to substantially contribute to the Quaternary erosion of the Yarlung-Tsangpo Gorge.
The role of multiple glacier outburst floods in proglacial landscape evolution: The 2010 Eyjafjallajökull eruption, IcelandStuart A. Dunning et al., Geography, Engineering and Environment, Northumbria University, Newcastle upon Tyne NE1 8ST, UK. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34665.1.
In 2010, Eyjafjallajökull volcano in Iceland erupted, causing global disruption due to the dispersing ash cloud. Less well known are the series of floods (known by the Icelandic term "jökulhlaup") generated by the melting of parts of the volcano ice-cap. Stuart A. Dunning and colleagues had a unique opportunity to survey the flood routing prior to the flooding as monitoring indicated an imminent eruption, and to then return after the eruption ended. They used laser scanning and time-lapse camera imagery to create detailed 3D models of the landscape to quantify change. The jökulhlaups completely in-filled a lake at the foot of Gigjökull glacier with sediment totaling more than 17 million cubic meters -- enough to cover nearly 315 NFL pitches 10 m deep in a mix of ash, rock, and ice. Contrary to prior models assuming that the largest events dominate the proglacial landscape, much change occurred during a series of comparatively small jökulhlaup events. It is erosion and deposition from these events that dominate the current surface landscape.
Globally synchronous Marinoan deglaciation indicated by U-Pb geochronology of the Cottons Breccia, Tasmania, AustraliaC.R. Calver et al., Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australia. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34568.1.
The apparently global distribution of Marinoan glacial deposits inspired the "snowball Earth" hypothesis, and prompted designation of the top of the type Marinoan glacials in South Australia as the Global Stratotype Section and Point ("golden spike") for the base of the terminal Proterozoic, Ediacaran System. However, horizons suitable for radioisotopic dating are lacking in the stratotype section and correlated sequences on mainland Australia. Ash beds suitably placed to directly and precisely date the Cryogenian-Ediacaran transition have so far been found only in Namibia (635.5 plus or minus 0.8 million years ago) and south China (635.2 plus or minus 0.8 million years ago). In this paper, C.R. Calver and colleagues show that a probable reworked volcaniclastic horizon at the very top of the Cottons Breccia, a Marinoan glacial correlative on King Island, Tasmania, has yielded an abundant population of juvenile zircons dated (by U-Pb on zircon, using chemical abrasion-thermal ionization mass spectroscopy) at 636.4 plus or minus 0.5 million years ago. Equivalence to the ash bed dates from Namibia and China supports correlation of those strata to the Australian type sections, and globally synchronous deglaciation at the beginning of the Ediacaran Period, and is consistent with the "snowball Earth" hypothesis.
A hematite-bearing layer in Gale Crater, Mars: Mapping and implications for past aqueous conditionsA.A. Fraeman et al., Dept. of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34613.1.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter collected data in a spatially overlapping mode that show hematite, an iron oxide mineral, is present on the top of a layered ridge located about 3 km away from the Curiosity rover's proposed arrival point at Mount Sharp's base. The hematite formed either through leaching of local material in neutral to acidic waters or through mixing of anoxic groundwaters with a more oxidizing water body or atmosphere. These formation hypotheses can be tested using Curiosity's payload, and both scenarios indicate that the ridge was a site of past active iron oxidation. In similar environments on Earth, iron oxidation is almost exclusively mediated by microorganisms. This hematite ridge therefore represents a specific site where concentrated and localized iron oxidation occurred, and is a prime location to search for signs of past habitability.