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How old are these rocks, how were they made, and how long ago did these geologic changes happen?

Boulder, CO, USA - New GSA BULLETIN science published online 9-20 March includes studies in the western Aleutians, south-central Alaska, Canada, Iceland, the Southern Pyrenees, and the western Gulf of Mexico. Topics cover the crystallization process of granophyre, marine outcrops in south-central Chile, characterizing the source and age of Wilcox Group sediments, sediments transported to the deep-sea trench, pieces of mid-oceanic ridge found above ground, and large wedges of crust added to the edges of existing continents.

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Geology and 40Ar/39Ar geochronology of the medium- to high-K Tanaga volcanic cluster, western Aleutians

Brian R. Jicha et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Posted online 9 Mar. 2012; doi: 10.1130/B30472.1.

The Tanaga volcanic cluster, located in the western Aleutian Islands, Alaska, consists of three closely spaced, active volcanoes (Sajaka, Tanaga, and Takawangha). Geologic mapping and dating establish an eruptive chronology for the past 295,000 years. The eruptive activity has been mostly continuous for the past 150,000 years, which is unlike most other well-characterized arc volcanoes that tend to grow in discrete pulses. Chronostratigraphic reconstruction by Brian R. Jicha and colleagues indicates that a sector collapse analogous to the lateral blast at Mount St. Helens in May 1980 occurred within the past few thousand years on the western side of Sajaka volcano. In addition, Jicha and colleagues find that a deep-seated edifice failure dramatically altered the landscape of the cluster between 41,000 and 91,000 years ago. Trace-element correlations provide insight into the crystallization histories and plumbing system architecture beneath the three volcanoes. Modeling of major and trace element data suggest that all magmas erupted from the Tanaga volcanic cluster may share parent magmas of similar composition.

Two detrital zircon signatures for the Cambrian passive margin of northern Laurentia highlighted by new U-Pb results from northern Canada

T. Hadlari et al., Geological Survey of Canada, 3303-33rd St NW, Calgary AB, T2L 2A7. Posted online 9 Mar.; doi: 10.1130/B30530.1.

T. Hadlari of the Geological Survey of Canada and colleagues describe new U-Pb ages of detrital zircon within Cambrian sandstones from northern Canada that were derived from two distinct sediment sources. Published detrital zircon ages from Neoproterozoic and Cambrian strata spanning Canada and Greenland were filtered and then subdivided into two distinct groups that Hadlari and colleagues find to be remarkably consistent across northern Laurentia, even though a spatial pattern is not readily apparent. Widespread distribution of those sources is attributed to (1) similar crust forming events in the Precambrian Shield at the core of Laurentia, and (2) post-orogenic uplift of the Grenville orogen resulting in a discrete phase of pan-continental sediment transport and deposition of Grenvillian grains within Neoproterozoic basins. Both of those sources were then eroded and recycled into Cambrian sandstones. Characterization of Laurentia in the Cambrian period allows identification of detrital zircon in younger rocks that were sourced from exotic crust made available during subsequent collisional events; for example, 500-million-year-old zircon found in the Devonian clastic wedge of northern Canada.

Evolving heavy mineral assemblages reveals changing exhumation and trench tectonics in the Mesozoic Chugach Accretionary Complex, south-central Alaska

Peter D. Clift et al., School of Geosciences, University of Aberdeen, Aberdeen, AB24 3UE, UK. Posted online 9 Mar. 2012; doi: 10.1130/B30594.1.

Earth's crust has been built up over long periods of time above subduction zones by the addition of magma and by the scraping off of sediment from oceanic crust as it passes through the deep sea trenches. These sediments form large wedges of crust added to the edges of existing continents. In southeastern Alaska, Peter D. Clift and colleagues observe how this process initiated starting about 160 million years ago and which now forms one of the largest such prisms worldwide. Sediment began to be offscraped after a chain of volcanic islands struck the old edge of North America, pushing up mountains that were eroded. The sediments in the rivers draining these mountains were transported to the deep sea trench and then at least partially added back to the continent, and the Pacific Plate continued to pass under SE Alaska, as it continues to do today. This study emphasizes the role that collision events have in controlling the tectonic state of active subducting margins around the Pacific Rim.

Sub-volcanic subsidence and caldera formation during subaerial seafloor spreading in Iceland

Drew L. Siler and Jeffrey A. Karson, Nevada Bureau of Mines and Geology, MS 187, University of Nevada, Reno, NV 89557, USA. Posted online 9 Mar. 2012; doi: 10.1130/B30562.1.

The global mid-ocean ridge system is an 80,000-km-long mountain range that primarily occupies Earth's ocean basins. Precious few segments of the mid-ocean-ridge system are above sea level and accessible for detailed geologic characterization. As a result, questions about the local- and regional-scale magmatic and tectonic processes that occur at mid-ocean ridges remain unanswered. Iceland is a relatively rare natural laboratory where both subaerial exposures and deep glacial erosion permit detailed geologic investigation of the oceanic crust. Detailed field geologic characterization by Drew L. Siler and Jeffrey A. Carson enables them to describe a caldera-like structural basin in northern Iceland, juxtaposed within the ubiquitous gently titled basaltic lava flow sections. This feature, along with others like it, is evidence for a tens-of-kilometers-scale along-strike variation in the magmatic and tectonic processes that accommodate plate spreading and crustal construction at mid-ocean ridges. Volcanic centers undergo kilometer-scale focused subsidence, in order to form these caldera-like features, while a few to tens of kilometers away, evidence for focused subsidence is absent, and wholly different geologic processes accommodate plate spreading. Given the structural similarities between Iceland and other magmatically robust extensional settings, segmentation of the spreading axis in this manner is likely a defining characteristic of magmatic rifting processes.

Chronostratigraphy of the Boltaa anticline and the Ainsa Basin (Southern Pyrenees)

Tania Mochales et al., Instituto Geolgico y Minero de Espaa, Unidad de Zaragoza, c/Manuel Lasala 44, 50006 Zaragoza, Spain. Posted online 20 Mar. 2012; doi: 10.1130/B30418.1.

The importance of the Ansa Basin lies in its well-preserved sedimentary sequences and configuration of its structures, which are oblique to the main Pyrenean orientation. These structures formed in a context where sedimentation and tectonic deformation took place synchronously. The geologic period involved in this study ranges within the Eocene. This work, by Tania Mochales and colleagues aims to date the rocks cropping out in this area by means of magnetostratigraphy, a well-known technique based on the reversals of the Earth's magnetic field, also supported on paleontologic data (shallow benthic foraminifera and charophytes).

Provenance of the Paleocene-Eocene Wilcox Group, western Gulf of Mexico basin: Evidence for integrated drainage of the southern Laramide Rocky Mountains and Cordilleran arc

Glen N. Mackey et al., University of Utah Dept. of Geology and Geophysics, Frederick A Sutton Bldg., 115 S 1460 East Room 383, Salt Lake City, Utah 84112, USA. Posted online 20 Mar. 2012; doi: 10.1130/B30458.1.

The Wilcox Group, deposited between 61 and 49 million years ago, represents the first major delivery of sand to the western Gulf of Mexico following a protracted period of carbonate deposition along this margin. Because it is contemporaneous with Laramide uplift of the Rocky Mountains, the large volume of sand that composes the Wilcox Group has long been believed to have been derived from erosion of these uplifts. To test this hypothesis, Glen Mackey and colleagues assess the source of the Wilcox Group sediments by determining the modal grain composition of Wilcox Group sandstones from sites across South Texas. Additionally, they determine radiometric ages on zircon grains from several of these sites to compare with the known age distributions in potential source rocks. Mackey and colleagues note that mineral composition and radiometric ages indicate that in addition to the Rocky Mountains, the California and Mexico portions of the Cordilleran Arc (a magmatic arc near the west coast of North America much like the modern Andean volcanic belt in South America) was an equally important source. These new findings indicate that sediment dispersal systems feeding the Gulf of Mexico drained a more extensive region than previously thought.

Major forearc subsidence and deep-marine Miocene sedimentation in the present Coastal Cordillera and Longitudinal Depression of south-central Chile (3830'45'S)

Alfonso Encinas et al, Depto. de Ciencias de la Tierra, Universidad de Concepcin, Casilla 160-C, Concepcin, Chile. Posted online 20 Mar. 2012; doi: 10.1130/B30567.1.

Neogene marine outcrops in the Coastal Cordillera and Longitudinal Depression of south-central Chile between 3830' and 4145'S indicate the onset of a major marine transgression that covered most of the forearc in this area. Integrated sedimentologic, ichnologic, and micropaleontologic studies on samples from oil wells and outcrops in the region indicate that these successions were deposited at lower-bathyal depths (greater than 2000 m) during the middle to late Miocene. Shallow-marine deposition followed in the southwestern part of the study area during the Pliocene(?). Alfonso Encinas and colleagues attribute deep-marine sedimentation in this area to a major event of subsidence in the Miocene that affected the entire forearc and that was caused by basal subduction erosion. They suggest that the anomalously thin crust that characterizes this area may have facilitated forearc subsidence and allowed the Miocene transgression to advance much farther inland here than in other regions of Chile. Subsequent uplift of the forearc is ascribed to basal accretion or underplating of sediments. Their conclusions contradict previous studies that favor a stable margin at these latitudes since the Jurassic. Deep-marine sedimentation in this area during the Miocene implies that the present Coastal Cordillera and Longitudinal Depression were probably submerged during that epoch.

Process of granophyre crystallization in the Long Mountain granite, southern Oklahoma

George B. Morgan VI and David London, ConocoPhillips School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Suite 710, Norman, OK 73019, USA. Posted online 20 Mar. 2012; doi: 10.1130/B30569.1.

George Morgan and David London describe the crystallization process of granophyre, a rock defined by fine-grained irregular to cuneiform intergrowth of alkali feldspar and quartz. Although texturally and mineralogically well described, and recognized to form where silica-rich liquids cool rapidly, the physico-chemical factors responsible for producing granophyric fabric previously remained unconstrained. By modeling the liquid line of descent for the whole rock as well as the model mineralogy of progressive formational increments of granophyric intergrowth in the A-type Long Mountain Granite, Morgan and London demonstrate a causative link between magmatic undercooling and sluggish diffusion in the production of granophyric fabric. By relation to formation in natural and experimental systems, the formation of granophyre is found to form at conditions similar to those that produce graphic granite, and place granophyre in the cooling regime between quenched/spherulitic felsite and hypidomorphic granular granite.

Contact: Christa Stratton
Geological Society of America

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