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LITHOSPHERE Covers Canada, California, the Alps, and the Scandinavian Caledonides
29 March 2013
Geological Society of America, The (GSA)
The April 2013 issue of Lithosphere is now available. Four classic research papers cover the Saint Elias Mountains of Yukon and British Columbia, Canada; the Nacimiento fault near San Simeon, California, USA; the western Alps; and the Caledonides in Scandinavia. An invited review relays the significance of dynamic topography to long-term sea level change. This month's research focus article, which is open access online, discusses the revolution in remote sensing-LiDAR-laser altimetry swath mapping.
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Detrital zircon Hf isotopic compositions indicate a northern Caledonian connection for the Alexander terrane
L. Beranek et al., Stockholm University, Geological Sciences, Svante Arrhenius väg 8, Stockholm, Stockholm 106 91, Sweden. Issue: April 2013. Originally posted online 19 Dec. 2012; http://dx.doi.org/10.1130/L255.1.
Earth's plate tectonic history during the Silurian and Devonian periods, approx. 400 million years ago, was dominated by the closure of the Iapetus Ocean and subsequent continent-continent collision between Laurentia (ancestral North America) and Baltica (ancestral northern Europe). This collision led to the rise of the Appalachian-Caledonian Mountains and the assembly of supercontinent Laurussia. To test ancient stratigraphic connections between the northern Caledonian mountains of Laurussia and crustal fragments now located in the North American Cordillera, Luke Beranek and colleagues acquired new analytical data from Silurian and Devonian sedimentary rocks of the Alexander terrane in the Saint Elias Mountains of Yukon and British Columbia, Canada. Their datasets indicate that terrestrial and shallow-marine rocks of the Alexander terrane, including distinctive red-bed sandstones, were sourced from northern Caledonian granitoids and are analogous to sedimentary units of the Old Red Sandstone in the present-day North Atlantic region. These data have major ramifications not only for the paleogeography and displacement history of the Alexander terrane, but also the proposed Caledonian affinities of other terranes in the North American Cordillera that underlie much of Alaska, British Columbia, and western United States.
Kinematic analysis of mélange fabrics in the Franciscan Complex near San Simeon, California: Evidence for sinistral slip on the Nacimiento fault zone?
J. Singleton, Dept. of Atmospheric, Oceanic, and Earth Sciences, George Mason University, Fairfax, Virginia 22030, USA; and M. Cloos, Dept. of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712 USA. Issue: April 2013. Originally posted online 19 Dec. 2012; http://dx.doi.org/10.1130/L259.1.
A controversial topic in California geology is the tectonic significance of the Nacimiento fault, a major structure that was active approximately 75 to 60 million years ago during subduction of oceanic crust beneath California. This fault juxtaposes granitic rocks similar to the Sierra Nevada batholith on the northeast side of the fault (the Salinian block) against rocks that formed within a subduction zone on the southwest side of the fault (the Franciscan Complex). Most previous studies have interpreted the Nacimiento fault either as (a) a left-lateral strike-slip fault along which the Salinian block granitic rocks moved 500-600 km northwestward with respect to the Franciscan Complex rocks; or (b) a thrust fault along which the Salinian block rocks were displaced more than 100 km southwestward over the Franciscan rocks. This study by John Singleton and Mark Cloos presents new structural data from Franciscan Complex rocks exposed along beach cliffs near San Simeon, California. These rocks have undergone left-lateral shearing parallel to the Nacimiento fault. Singleton and Cloos suggest this shearing was related to movement on the Nacimiento fault, supporting the tectonic interpretation of the Nacimiento fault as a major left-lateral structure.
Short-lived fast erosional exhumation of the internal Western Alps during the late Early Oligocene: constraints from geo-thermochronology of pro- and retro-side foreland basin sediments
S. Jourdan et al., ISTerre, Grenoble, 38110, France. Issue: April 2013. Originally posted online 25 Feb. 2013; http://dx.doi.org/10.1130/L243.1.
The Oligocene is a key period in the evolution of the western Alps during which the mountain belt evolved from an accretionary wedge (Late Cretaceous to Eocene) to a relatively high-elevation mountain belt, similar to the central Alps today. Studying the sediments and sedimentary rocks deposited in basins adjacent to this mountain belt helps in reconstructing the orogenic evolution. During this period, relatively fast erosion is seen as a result of rapid surface uplift coupled with increasing orographic precipitation during this phase of orogenesis. Surface uplift may have been caused and sustained by different plate-tectonic processes such as a change in convergence direction, intermediate-depth slab breakoff, and emplacement of the Ivrea body during continental collision. The occurrence of contemporaneous volcanic activity on the pro-side of the western Alps on the subducting European plate between 36 and 30 million years ago is seen in connection with slab rollback of the Apennine slab and upwelling of hot mantle material beneath the western Alps.
Subduction along and within the Baltoscandian margin during the closing of the Iapetus Ocean and Baltica-Laurentia collision
D. Gee et al., Uppsala University, Earth Sciences, Uppsala, 752 36, Sweden. Issue: April 2013. Originally posted online 19 Dec. 2012; http://dx.doi.org/10.1130/L220.1.
There are few places in the world where it is possible to trace a hot allochthon for 200 km across a continental margin, demonstrate its lateral displacement to have been more than twice this distance, infer that it was generated in an outer-margin subduction system during the final stages of ocean closure, and show that emplacement onto the platform occurred during subsequent continent collision. As a result of good exposure in the Scandian mountain belt and erosion to middle-crustal levels, the Caledonides in Scandinavia provide one of the best opportunities on the planet to study these aspects of mountain building.
INVITED REVIEW ARTICLE
A review of observations and models of dynamic topography
N. Flament et al., The University of Sydney, School of Geoscience, Madsen Building F09, Room 416, Eastern Avenue, The University of Sydney, NSW 2006, Australia. Issue: April 2013. Originally posted online 4 Feb. 2013; http://dx.doi.org/10.1130/L245.1.
It has been known since the early 1960s that moving tectonic plates shape the Earth's surface, forming mountain belts and rift valleys. In addition to this tectonic topography, the more subtle deformation of the Earth's surface due to mantle flow in the Earth's interior, called dynamic topography, has been an active research topic since the mid-1980s. Dynamic topography has received increased interest over the last few years because it challenged the well-established view that long-term sea level change can be deduced from the rock record of "stable" continental shelves. In this review article, Nicolas Flament and colleagues show that there is good agreement between long-wavelength (greater than 5,000 km) observations and models of dynamic topography. Their work confirms the significance of dynamic topography to long-term sea level change and reinforces that comparing the predictions of mantle flow models to the geological record constrains the physical properties of the mantle. Larger data sets and increasing computing power will enable progress in this field in the coming years.
RESEARCH FOCUS ARTICLE
Active tectonics and LiDAR revolution
A. Meigs, College of Earth, Ocean, and Atmospheric Sciences, 104 CEOAS Administration Hall, Oregon State University, Corvallis, Oregon 97331, USA. Issue: April 2013; free access at http://dx.doi.org/10.1130/RF.L004.1.
A revolution in remote sensing, light detection and ranging (LiDAR) laser altimetry swath mapping, reveals the details of topographic features at such high resolution that they have transformed our understanding of tectonic forcing of the shape of the Earth's surface. Meter-scale DEMs (digital elevation models) capture fault offsets, fault zone structure, off-fault deformation, and landscape properties at microgeomorphic scale, highlighting that the surface faithfully records the complexity and sensitivity of deformation in detail.