Boulder, Colo., USA - New GSA BULLETIN articles posted online ahead of print on 7 June cover granite, granitoids, and kimberlite; Garwood Valley Antarctica; Death Valley, California, USA; Esan Volcanic Complex, Japan; and Little Lake, California, USA. Some questions addressed include how melting affects granite emplacement; "how do you bury an ancient remnant ice sheet?"; how glaciation affected the evolution of Death Valley; and the risk of eruption at the Esan Volcanic Complex.
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Granite: From genesis to emplacement
Michael Brown, Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, Maryland 20742-4211, USA. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30877.1.
Earth's crust melts at low temperatures (less than 700 degrees Celsius) if water is available, but most granite is derived by water-absent melting involving breakdown of mica and amphibole with quartz and feldspar. These reactions produce 20 plus 70 vol. % melt according to source composition at temperatures up to 1,000 degrees Celsius. Preservation of unretrogressed granulites in the deep crust requires that most of the melt was extracted. Migmatite-granite complexes represent an intermediate level in the crust where melting occurred and was drained, but also where melt accumulated on its way through to upper crustal granites. Evidence from migmatites shows that crust may become porous at only a few vol. % melt allowing the source to become permeable as melt segregates into veins. The network of veins allows melt migration to ascent conduits. Upper crustal granites are built from multiple batches of magma, implying that melt is drained from the source in multiple extraction events. Buoyancy-driven magma ascent occurs via dikes in fractures or via high-permeability zones controlled by tectonic fabrics. Emplacement of upper crustal granite plutons occurs around the ductile-to-brittle transition zone, whereas steep tabular sheeted and blobby plutons deeper in the crust represent back freezing of melt during ascent.
Garwood Valley, Antarctica: A new record of Last Glacial Maximum to Holocene glaciofluvial processes in the McMurdo Dry Valleys
Joseph S. Levy et al., Institute for Geophysics, University of Texas, Austin, Texas 78758-4445, USA. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30783.1.
How did Antarctic ice sheets rise and ebb during the last ice age? How do you bury an ancient remnant ice sheet beneath one of the largest river deltas in Antarctica? Joseph Levy and colleagues attempt to answer these questions by analyzing newly discovered ice-age deposits from Garwood Valley -- one of the McMurdo Dry Valleys of Antarctica. The buried ice sheet is shown to date from the last glacial maximum. Damming of the paleo-Garwood River, one of Antarctica's most powerful seasonal streams, by the Ross Sea Ice Sheet ice sheet resulted in the stranded ice sheet being buried under river and lake sediments. Detailed dating of this ice and sediment provides new insight into the rise and collapse of Antarctic ice. Ice-sheet dammed lakes are shown to have persisted in Garwood Valley from the last glacial maximum through warmer conditions in the last several thousand years. This research raced against the clock to understand these key paleo-climate deposits before erosion and permafrost melting destroys them forever.
The Laurentian record of Neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death Valley, California
Francis A. MacDonald et al., Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30789.1.
Francis MacDonald and colleagues refine regional correlations both across Death Valley and throughout Laurentia and construct a new age model for glaciogenic strata and microfossil assemblages. Particularly, their mapping shows for the first time that glacial deposits of both the Marinoan and Sturtian glaciations can be distinguished in southeastern Death Valley, and that beds containing vase-shaped microfossils are slump blocks derived from the underlying strata. With these data, MacDonald and colleagues conclude that all of the microfossils that have been described to date in Neoproterozoic strata of Death Valley predate the glaciations and do not bear on the severity, extent, or duration of Neoproterozoic Snowball Earth events.
Eruption history, conduit migration, and steady discharge of magma for the past 50,000 yr at Esan volcanic complex, northern Japan
Daisuke Miura et al., Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko 270-1194, Japan. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30732.1.
Many previous studies in volcanology significantly indicate the long-term rate of magma discharge is one of the best parameter that reflects processes in the magma plumbing system at deep crustal levels; this information can be integrated and modeled to forecast future volcanic activity. To this end, Daisuke Miura and colleagues have undertaken a detailed field, petrological, geochronological, and modeling study of Esan Volcanic Complex (EVC), northern Japan, in order to establish its eruptive history over the past 50,000 yr. The provocative points in this paper are (1) to demonstrate the complete history in major eruption records at the EVC, and (2) to study variations in long-term magma discharge, of which a simple elastic model is utilized. This paper reveals that at the EVC, the magma storage system was attributed to the upward migration of magma batch in early eruption episodes with a shorter recurrence interval. Given this finding, and the fact that the last eruption occurred at 9000 years ago, there is currently a risk of a large eruption at the EVC.
Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California
Colin B. Amos et al., Geology Department, Western Washington University, Bellingham, Washington 98225, USA. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30803.1.
Despite previous satellite-based measurements of relatively rapid modern-day shear along the Little Lake fault in southeastern California, new geologic constraints reveal steady and modest rates of fault slip over the past several hundred thousand years. Colin Amos and colleagues identified, mapped, and dated a series of landforms and deposits within an ancient channel of the former Owens River in the Little Lake area. The age of these features confirms previous suggestions that this portion of the Owens River channel formed in response to repeated volcanic eruptions into the river canyon, potentially resulting in periodic damming of the channel and subsequent catastrophic outburst flooding. Ground-based laser scanning of these deposits reveals high-resolution measurements of the total offset along the Little Lake fault over several geologic time intervals. These new measurements of slow and steady fault slip may indicate that contemporary shear from satellite measurements is transitory, and potentially related nearby recent earthquakes in the Mojave Desert. Whether or not this rapid modern-day shearing brings faults closer to failure remains unknown.
Initiation and growth of strike-slip faults within intact metagranitoid (Neves area, eastern Alps, Italy)
Giorgio Pennacchioni et al., Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, 35131 Padova, Italy; and Istituto Nazionale di Geofi sica e Vulcanologia (INGV), Via Vigna Murata 605, 00143 Roma, Italy. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30832.1.
Exhumed strike-slip faults in the Neves area of the Tauern Window (Eastern Alps, Italy) formed in the lower brittle crust within intact granitoids. Faults initiated as en-echelon fractures delineating shear bands. Due to the initial en-echelon pattern, stepovers between fault segments were contractional during subsequent slip accumulation. Synthetic slip on the segments was initially associated with the development of antithetic faults in the stepovers, oriented at 30-45 degrees to the bounding faults. In larger faults (slip on the order of a few meters), the antithetic faults within the stepover were crosscut by synthetic sigmoidal faults that connected the overstepped fault segments and accommodated most of the subsequent displacement transfer. This stage of fault linkage is well documented in the Mesule fault, which has a current maximum offset of about 10 meters. The Neves area provides unusually detailed field constraints on fault initiation, linkage and displacement accumulation within non-bedded and relatively isotropic granitoid rocks at the base of the brittle crust, where neither a free upper surface nor substantial volume change (e.g., by veining and pressure solution) were controlling factors in accommodating fault linkage and displacement transfer.
Physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record
Richard J. Brown, Department of Earth Sciences, Durham University, Durham DH1 3LE, UK; and Greg A. Valentine, Department of Geology, 411 Cooke Hall, University at Buffalo, Buffalo, New York 14260, USA. Posted online ahead of print 7 June 2013; http://dx.doi.org/10.1130/B30749.1.
Eruptions of kimberlite magmas, the main transporters of diamonds to the surface of the Earth, have been extremely rare over the past 30 Ma. As a result most kimberlite volcanoes are heavily eroded and in many cases only their subterranean parts are preserved (mostly dikes and pipes, or diatremes). Erosion poses problems for interpreting the nature of kimberlite eruptions from the rock record. This paper presents new size data on eroded kimberlite volcanoes and compares them with more common and less eroded types of monogenetic basaltic volcanoes that comprise dikes that fed effusive and weakly explosive surface eruptions, and diatremes generated during phreatomagmatic eruptions. Richard J. Brown and Greg A. Valentine illustrate the many similarities between basaltic and kimberlite volcanic fields. The data suggest that the selective removal of surface volcanic structures and deposits by erosion may have distorted the geological record of kimberlite volcanism. Selective mining of preferentially large, diamondiferous kimberlite pipes and underreporting of small kimberlite pipes and dikes may have added further bias. Reassessment of published data reinforces the notion that kimberlite magmas can erupt in a variety of ways and that most published data, taken from the largest kimberlite pipes, may not be representative of kimberlite volcanism as a whole.
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