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Landscape evolution and hazards

March 31, 2016

Boulder, Colo., USA: Landscapes are formed by a combination of uplift and erosion. Uplift from plate tectonics raises the land surface; erosion by rivers and landslides wears the land surface back down. In this study, Georgina L. Bennett and colleagues examine the interplay of uplift and erosion along the coast range of Northern California to understand how the modern topography is built.

This region is unique in that a wave of uplift is sweeping north through the Coast Range, allowing geoscientists to document the erosional response and assess the timescale of the process. Bennett and colleagues find that rivers cut down through the uplifting land surface, steepening surrounding hillslopes and triggering landslides when hillslope angles reach a limit.

Landslides are the main erosional process balancing uplift in the region. However, intriguingly, they may also have a negative feedback to ongoing erosion, through the delivery of large resistant rocks to rivers that act to armor the riverbed from ongoing erosion. Thus the erosion of parts of the Coast Ranges in response to uplift may be delayed. These findings have implications for understanding landscape evolution, as well as hazards such as landslides.

FEATURED ARTICLE Landslides, threshold slopes, and the survival of relict terrain in the wake of the Mendocino Triple Junction
Georgina L. Bennett et al., Dept. of Geological Sciences, University of Oregon, 1275 E 13th Ave, Eugene, Oregon 97403-1272, USA. This article is online at http://geology.gsapubs.org/content/early/2016/03/30/G37530.1.abstract.

GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Other recently posted GEOLOGY articles highlighted below:




In-situ oxygen isotope records of crustal self-cannibalization selectively captured by zircon crystals from high-δ26Mg granitoids
Hui Je Jo et al., Dept. of Isotope Geochemistry, Korea University of Science and Technology, Daejeon 305-350, Republic of Korea. This paper is online at http://geology.gsapubs.org/content/early/2016/03/30/G37725.1.abstract.

Granitoids, the chemically evolved quartzofeldspathic plutonic rocks, are the most abundant rock types in Earth's continental crust. Although there is still no single universally accepted solution to the problem of granite petrogenesis, it becomes increasingly manifest that most granitoid rocks contain the recycled crustal component. The robust mineral zircon is an excellent recorder of the recycling process by virtue of its famous physicochemical durability. When combined with biotite magnesium isotopes, zircon oxygen isotopes of Mesozoic-Cenozoic granitoids in the southeastern margin of the Korean Peninsula provide compelling evidence for the involvement of surface rocks into the shallow magma system. Single magmatic zircon crystals became progressively enriched in the light oxygen isotope toward their rims, selectively in granitoid rocks containing biotite enriched in the heavy magnesium isotope. Such a concomitant isotopic variation is explained by the recycling of weathered surface rocks that experienced hydrothermal alteration after the crystallization of the zircon core. This situation would have been best achieved through assimilation of roof rocks around the volcanic center, which is conventionally referred to as "crustal cannibalization." This research shows that the combined stable isotope data from plutonic rocks sensitively detect the input of surface rocks that have now been eroded away.




Rapid ice sheet retreat triggered by ice stream debuttressing: Evidence from the North Sea
Hans Petter Sejrup et al., Dept. of Earth Science, University of Bergen, Allegaten 41, 5007 Bergen, Norway. This article is online at http://geology.gsapubs.org/content/early/2016/03/30/G37652.1.abstract.

Warming air and ocean temperatures are causing losses in the mass of existing ice sheets and it is thought that the long-foretold collapse of part of the Antarctic Ice Sheet has now been triggered and expected to be irreversible over the next hundreds of years. Such predictions, via numerical ice sheet modelling, are sensitive to the formulation of the mechanics of flow and the parametrisations chosen, and yet few opportunities exist for assessing model robustness at timescales beyond the observational record of tens of years. In this study, Hans Petter Sejrup and colleagues use fresh discoveries of glacial landforms in the North Sea to reconstruct the maximum extent of the ice sheet during the last glacial stage, finding that it underwent dramatic disintegration at around 18,500 years ago. They reconstruct collapse, triggered by grounding line retreat of its primary ice stream which de-buttressed adjacent British and Norwegian ice masses, and whose unzipping was accompanied by drainage of a large ice-dammed lake. The discovery of these major events provides an opportunity to improve understanding and modelling of the disintegration of marine-based ice sheets, and the complex interplay between ocean circulation and the cryosphere.




Solute sources and geochemical processes in Subglacial Lake Whillans, West Antarctica
Alexander B. Michaud et al., Dept. of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana 59717, USA. This paper is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/03/30/G37639.1.abstract.

Subglacial Lake Whillans (SLW), West Antarctica, is an active component of the subglacial hydrological network located beneath 800 m of ice. The fill and drain behavior of SLW leads to long (years to decades) water residence times relative to those in mountain glacier systems. Here, Alexander Michaud and colleagues present the aqueous geochemistry of the SLW water column and pore waters from a 36-cm-long sediment core. Stable isotopes indicate that the water is primarily sourced from basal-ice melt with a minor contribution from seawater that reaches a maximum of ~6% in pore water at the bottom of the sediment core. Silicate weathering products dominate the crustal (non-seawater) component of lake- and pore-water solutes, and there is evidence for cation exchange processes within the clay-rich lake sediments. The combination of significant seawater and crustal components to SLW lake and sediment pore waters in concert with ion exchange processes result in a weathering regime that contrasts with other subglacial systems. The results also indicate cycling of marine water sourced from the sediments back to the ocean during lake drainage events.




Crustal accretion at a sedimented spreading center in the Andaman Sea
Aurélie Jourdain et al., Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR7154-CNRS, 1 rue Jussieu, 78238 Paris Cedex 05, France. This article is online at http://geology.gsapubs.org/content/early/2016/03/30/G37537.1.abstract.

Through the interpretation of high resolution seismic profiles in the Andaman Sea, Jourdain and colleagues provide the first complete model of the magmatic and tectonic processes during oceanic crust emplacement below sedimented spreading centers. It is shown that the sediments are incorporated in crustal accretion process, where interaction with magma lenses creates a heterogeneous upper crust between the recent sediments and the lower igneous crust. For the first time on a slow-spreading center, the presence of multiple magma lenses on axis at different depth has been imaged. Also, within the axial valley, faults are steeply dipping (65-75 degrees) in a staircase pattern and their base coincides with shallow dipping (30 degrees) reflections. These low angle faults could define the zone of extension and magmatism. They evolve during accretion, back-tilting the upper crust. Thus, as the sediments mix with sill/dike intrusions, these sequences are rafted away from the axis, rotated and buried due to subsidence and faulting, forming the upper oceanic crust.




Identification of the short-lived Santa Rosa geomagnetic excursion in lavas on Floreana Island (Galapagos) by 40Ar/39Ar geochronology
Andrea Balbas et al., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. This article is online at http://geology.gsapubs.org/content/early/2016/03/30/G37569.1.abstract.

Convecting fluid in Earth's core that generates and controls Earth's magnetic field is known as the geodynamo. Mechanisms that control the goedynamo are not well defined, and Earth's magnetic field varies in strength and orientation through time. Changes in the field can be dramatic, resulting in large losses in field strength and full reversals of the field's orientation. It is well accepted that large changes in field orientation are also accompanied by large reductions in field strength. It is also well accepted that the orientation of the field can change dramatically and then return to its previous polarity state. These large changes in field orientation that do not result in a polarity reversal are known as geomagnetic excursions. In this study, Andrea Balbas and colleagues define that a geomagnetic excursion known as the Santa Rosa Excursion occurred ~926,000 years ago and was accompanied by an 86% reduction in field strength. Their data combined with other natural archives illustrate that this event had global significance. Balbas and colleagues suggest that the entirety of the excursion and recovery occurred in a time interval as short as 3,000 years. This work highlights similarities between the well-known Laschamp Excursion and the Santa Rosa Excursion indicating that dramatic short-lived excursions can occur during both polarity conditions (normal or reversed).

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Geological Society of America

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