September-October GSA Bulletin media highlights

Boulder, Colo. – The September-October issue of the Geological Society of America Bulletin includes several newsworthy geology articles. Topics include a subduction earthquake record for New Zealand's Hikurangi zone and the relationship between northern California's Mendocino triple junction and the shape of California Coast Ranges and rivers.

Paleoecological insights into subduction zone earthquake occurrence, eastern North Island, New Zealand

Ursula Alyson Cochran, Institute of Geological and Nuclear Sciences, Hazards Group, Lower Hutt, North Island 6009, New Zealand; et al. Pages 1051–1074.

Keywords: Holocene, Wairoa, Mahia Peninsula, Hawke's Bay, Lachlan Fault, accommodation space, earthquakes, microfossils, sea level, subduction zone, subsidence, tsunami.

The world's greatest magnitude earthquakes generally occur at subduction plate boundaries on the interface between the subducting and overriding plate. These subduction earthquakes are known from recent examples to have devastating consequences, e.g., Chile 1960, Alaska 1964 and Sumatra 2004. Geological studies of relative land-level change can be used to identify evidence for prehistoric subduction earthquakes enabling estimates of magnitude and frequency to be calculated. Such studies along the west coasts of North America and Canada have greatly refined seismic hazard estimates based on historical seismicity alone. This article outlines the first stage in establishing a subduction earthquake record for the Hikurangi subduction zone off the east coast of the North Island of New Zealand. Evidence for two large earthquakes involving subsidence and tsunami generation is identified from the mid-early Holocene coastal sequences. Further sites are required to determine whether the source of these earthquakes was a large offshore fault and/or the subduction interface.

Combined stratigraphic and isotopic studies of Triassic strata, Cuyo Basin, Argentine Precordillera

Guilherme Mallmann, Australian National University, Research School of Earth Sciences, Canberra, ACT 0200, Australia; et al. Pages 1088–1098.

Keywords: Nd isotopes, provenance, chronostratigraphic horizon, Triassic, South America.

Combined stratigraphic and isotopic studies are herein presented in order to constrain the sedimentary infilling, source areas, and age of Triassic strata related to the active faulted margin of the Cuyo Basin (northwest Argentina). The results suggest that sources of sediments were restricted to units with Grenvillian age associated to the Cuyania Terrane. U-Pb dating of volcanic deposits interlayered within the basal deposits provides a potential chronostratigraphic horizon for future correlations.

Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard

Adam C. Maloof, Princeton University, Geosciences, Princeton, NJ 08544, USA; et al. Pages 1099–1124.

Keywords: polar wandering, paleomagnetism, Svalbard, Neoproterozoic, carbon cycle, paleogeography.

On the mostly ice-covered Svalbard archipelago in the Norwegian Arctic, a thick pile of 800 million year old carbonate sediments are exposed in rare ridges peaking out between glaciers. The carbonates likely formed on a tropical or subtropical platform, similar to the modern Bahamas and equally sensitive to global change. Magnetic minerals found within the carbonates have yielded a new record of the ancient geomagnetic field and suggest that Svalbard experienced two rapid latitudinal shifts during deposition. Recorded in the same carbonate sediments, and coincident with the paleogeographic shifts, are large changes in the way the global oceans cycled carbon, as well as transient changes in sea level. One hypothesis consistent with the range of data involves true polar wander (TPW), where the entire solid Earth rotates with respect to the spin axis (instead of the more familiar picture of plate tectonics, where individual plates wander with respect to each other and the spin axis) in response to some form of rotational disequilibrium involving unstable configurations of mass anomalies (such as those found associated with plumes and subduction zones) driven by normal mantle convection. The hypothesis is testable because a TPW event will affect every continent differently, but predictably, depending on the continent's position relative to Earth's paleo-spin axis.

Erosion of steepland valleys by debris flows

Jonathan David Stock, U.S. Geological Survey, WESP, Menlo Park, CA 94025, USA; and William E. Dietrich. Pages 1125–1148.

Keywords: Geomorphology, erosion, debris flows, river incision, landscape evolution.

Stock and Dietrich investigated erosion by debris flows, mixtures of soil, boulders, and water that flow violently down steep valleys during episodic large storms. In the mountains of the western United States and Taiwan, Stock and Dietrich show that valleys that are eroded by debris flows have a different shape than those cut by rivers. Using field measurements and physics, they construct a model of debris flow incision that reproduces the distinct topographic signature of debris flow erosion. They argue that many of the world's steep valleys are cut by debris flows, not ice or running water. If this new view is correct, many of the world's mountains owe much of their relief and shape to this newly understood incision process.

Tectonic geomorphology of the southeastern Mississippi Embayment in northern Mississippi, USA

Randel Tom Cox, University of Memphis, Department of Earth Sciences, University of Memphis, Memphis, TN, USA; et al. Pages 1160–1170.

Keywords: Neotectonics, drainage basins, rivers, fault blocks, tilt blocks, North America.

Transverse asymmetry of drainage basins within a region peripheral to the New Madrid seismic zone was quantitatively analyzed in order to recognize areas showing lateral migration of stream channels in a preferred direction. Transverse basin profiles were converted to two-dimensional vectors that denote channel position, and a spatially-averaged vector field was generated for the region. Trends in this vector field show areas of uniform lateral stream migration controlled by inclined bedrock as well as areas of recent ground tilting. Results indicating ground tilting suggest that geologically young faulting in the region is not confined to the New Madrid seismic zone, but may extend into Mississippi.

Is stoping a volumetrically significant pluton emplacement process?

Allen F. Glazner, University of North Carolina, Department of Geological Sciences, Chapel Hill, NC 27599-3315, USA; and John M. Bartley. Pages 1185–1195.

Keywords: igneous petrology, structural geology, tectonics, pluton, xenolith.

Plutons are frozen accumulations of unerupted magma and are a basic building block of the continental crust. Plutons have long been assumed to form commonly from large accumulations of magma that rise from the lower crust by a process known as stoping. In this process, chunks of a magma chamber's roof break off and fall to the floor, and thus magma rises to take their place. This paper shows that stoping fails many tests of its viability as a significant process in the ascent of magma.

Mid- to late Paleozoic K-feldspar augen granitoids of the Yukon-Tanana terrane, Yukon, Canada: Implications for crustal growth and tectonic evolution of the Northern Cordillera

Stephen John Piercey, Laurentian University, Mineral Exploration Research Centre, Sudbury, Ontario P3E 2C6, Canada; et al. Pages 1212–1231.

Keywords: tectonics, Yukon-Tanana terrane, Nd-isotope geochemistry, augen granitoid, crustal growth, magmatism.

This paper is a combined field- and laboratory-based study that examines ancient granites from the Stewart River region along the Yukon-Alaska border. These granites represent remnants of the western edge of the ancient Pacific continental margin of North America from ca. 365 to 260 million years ago. These granites record episodic magmatism within an evolving magmatic "arc" that extended from Alaska to northern British Columbia. Two groups of granites reflect 362–347-million-year-old magmatism associated with a volcanic/magmatic arc chain that was building along the distal edge of the North American continent at this time. Coincident with this arc chain was the development of an ocean basin between this arc and the North American continent in a geometric relationship very similar to present day Japan relative to the Chinese mainland with the Japan Sea in between. This arc and ocean basin persisted until ca. 270 million years ago when the ocean basin closed resulting in a new magmatic arc and the formation of a younger 260-million-year-old group of granites.

Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for mendocino triple junction tectonics

Jane Lock, University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195, USA; et al. Pages 1232–1246.

Keywords: Northern California Coast Ranges, landform evolution, Mendocino triple junction, drainage evolution, geodynamics, tectonic geomorphology.

North of San Francisco, the northern California Coast Ranges separate the Californian Coast from the Central Valley. Until recently, the cause of this area of high topography was unknown. Lock et al. explain how the migrating Mendocino triple junction controls the shape of the ranges and its rivers. For the last 10 million years, the triple junction has been moving north along the coast of northern California. As the triple junction moves, flow in the mantle beneath the earth's crust causes the surface to be squashed and stretched creating two high "bumps" in the landscape that move north with the triple junction. These bumps are seen in the elevation and the drainage divides, and over time, these moving bumps shift the location and direction of the rivers. The Russian River used to be the biggest river in northern California, but 2 million years ago, the Eel River became the biggest river by stealing streams from the Russian River causing a major shift in the rivers and drainage pattern in northern California.

Cretaceous and Triassic subduction-accretion, HP/LT metamorphism and continental growth in the Central Pontides, Turkey

Aral I. Okay, Istanbul Teknik Üniversitesi, Department of Geological Engineering, Istanbul TR-80626, Turkey; et al. Pages 1247–1269.

Keywords: Pontides, Turkey, HP/LT metamorphism, exhumation, continental growth, orogeny.

The Pacific mountain belts along the western North American margin have grown oceanward by subduction-accretion and by incorporation of oceanic edifices, such as island arcs, oceanic islands and oceanic plateaus. On the other hand, the Alpine-Himalayan orogen is generally considered to be the result of the collision of the megacontinents Laurasia, Gondwana, and India during the Tertiary. In contrast to this prevailing view, this paper shows extensive precollisional growth of continental crust along the southern margin of Laurasia in northern Turkey by accretion and underplating of oceanic rocks in the Late Triassic and Late Cretaceous. Prior to the terminal continental collision, the northern Tetyhan margin was not too different from the present day Pacific margins.

Deformation of Gondwana margin turbidites during the Pampean orogeny, North-central Argentina

Aranzazu Piñán-Llamas, Boston University, Earth Sciences, Boston, MA 02215, USA; and Carol Simpson. Pages 1270–1279.

Keywords: Pampean orogeny, Andean orogeny, Chevron folds, Turbidites, Gondwana margin.

The northern and eastern Sierras Pampeanas of north-central Argentina contain >1000 km by ~100 km of almost continuous exposure of deformed and metamorphosed turbidites that record major deformational events in the tectonic evolution of the western Gondwana margin. A series of transects compares the structural style and deformational sequence in northern, unmetamorphosed sections of the Vendian-aged Puncoviscana Formation with that in intermediate and southern sections, in metasedimentary rocks from deeper structural levels. These changes in structural style are shown to be gradational. All mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Piñán-Llamas and Simpson also describe distinctive effects of late-stage brittle folds and thrusts that, although more marked in the northern sections, do not significantly alter the overall structural style or geometry of the basement rocks. They apply their observations and previously reported data to the development of a tectonic model in which a mainly orthogonal contraction of the turbiditic sequence during the Cambrian Pampean orogeny is followed by shear zones and plutonism of the Famatinian and Achalan orogenies, and culminates in a major thrusting event, most likely related to the upper Tertiary Andean orogeny.


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