Boulder, Colo., USA GSA Bulletin articles posted online ahead of print over the last month study (1) a Carboniferous collision in central Asia; (2) crystal xenoliths in the Bolivian Altiplano; (3) The Tsakhir Event; (4) Onverwacht Group and Fig Tree Group contact, Barberton greenstone belt, South Africa; (5) iron oxide deposits in the Paraba Basin, NE Brazil; (6) the southern Alaska syntaxis; (7) paleotopography of the South Norwegian margin; and (8) the Cheyenne belt suture zone, USA.
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Early Carboniferous collision of the Kalamaili orogenic belt, North Xinjiang, and its implications: Evidence from molasse deposits
Yuanyuan Zhang et al., Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, China 100871. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30779.1.
The Central Asian Orogenic Belt extends from the Urals to the coast of northern China, of similar age and complexity to the Appalachian mountain belt in eastern North America. It was formed by the subduction of ancient oceans and the resulting collision of continental plates. Erosion of mountains formed by plate collision leads to thick conglomerates being deposited close to the mountain front and sandstone farther away -- a sediment package known as molasse. One critical piece of the puzzle in the Central Asian Orogenic Belt was the closure of an ocean and collision resulting in the Kalamaili Mountains in northern Xinjiang, China. Yuanyuan Zhang et al. have dated zircon mineral grains and pebbles of volcanic rocks from near the base of the molasses, and they have also dated lava flows that overlie the molasse. These ages confine the age of the molasse, and hence of the rapid uplift of the Kalamaili Mountains, to between 343.5 Ma and 345 Ma. These ages also help in the understanding of how nearby continental blocks fit into the mountain-building puzzle.
Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia
T.M. Gibson et al., Dept. of Geology, Colorado College, 14 E. Cache La Poudre Street, Colorado Springs, Colorado 80903, USA. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30746.1.
Southern Mongolia is centrally located within the Central Asian Orogenic Belt, which is a mosaic of crustal fragments that amalgamated during one of the largest periods of crustal growth on Earth. However, the precise timing and nature of the events that formed this region remain poorly constrained. T.M. Gibson and colleagues provide the first detailed study of the Lower Devonian Tsakhir Formation in the Shine Jinst region of southern Mongolia. They interpret a stratigraphic transition from quiet water carbonate deposits to coarse siliciclastic marine deposits as a record of a tectonic event, which they have titled "The Tsakhir Event." Their sedimentological analysis, in combination with detrital zircon geochronology data, provides important insights into the depositional history and the tectonic evolution of the Gobi-Altai zone, and more generally, the Central Asian Orogenic Belt.
Characterizing the continental basement of the Central Andes: Constraints from Bolivian crustal xenoliths
Claire L. McLeod et al., NCIET, Dept. of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30721.1.
Critical to understanding the development of active continental margins is knowledge of the crustal basement on which magmatic arcs are built. This study by Claire L. McLeod and colleagues reports results from a whole-rock geochemical and zircon U-Pb geochronological study of a suite of crustal xenoliths from the Bolivian Altiplano, Central Andes, that provide new insight into the evolution and composition of the continental basement beneath the region. The xenoliths comprise both igneous and metamorphic lithologies, including diorites, microgranites, gneisses, garnet-mica schists, granulites, quartzites, and dacites. The xenolith suite exhibits significant Sr-isotopic heterogeneity (87Sr/86Sr from 0.7105 to 0.7368) whilst Pb isotopic signatures reflect crustal domains previously constrained from scattered surface exposures of basement rocks. Ion microprobe U-Pb dating of zircon reveal Early Phanerozoic, Late Mesoproterozoic and Paleoproterozoic age peaks. The presence of these age peaks in the detrital zircon population record demonstrates the important role of crustal recycling in the construction of the modern day Andean margin. The lithological character of the xenoliths and their detrital zircon ages are inconsistent with current understanding of the eastern extent of the Arequipa-Antofalla Basement block beneath the Bolivian Altiplano and instead indicate that it terminates farther to the west than previously assumed.
Crustal fracturing and chert dike formation triggered by large meteorite impacts, ca. 3.260 Ga, Barberton greenstone belt, South Africa
Donald R. Lowe, Dept. of Geological and Environmental Sciences, 118 Braun Hall, Stanford University, Stanford, California 94305-2115, USA. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30782.1.
The approx. 3260 million-year-old contact between the largely volcanic Onverwacht Group and overlying largely sedimentary Fig Tree Group in the Barberton greenstone belt, South Africa, is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the Mendon Formation (Onverwacht Group). In the Barite Valley area, these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the Mendon Formation. Spherules that formed during a large meteorite or asteroid impact event occur in a wave- and/or current-deposited unit, spherule bed S2, which widely marks the Onverwacht-Fig Tree contact, and as loose grains and masses within some chert dikes up to 50 m below the contact. Four main types of chert dikes and veins are recognized: (Type 1) irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy; (Type 2) small vertical dikes, most less than one m wide, which are restricted to the lower half of the Mendon chert section; (Type 3) small crosscutting veins, most less than 50 cm across, filled with precipitative silica; and (Type 4) small irregular to bedding-parallel to irregular veins, mostly less than 10 cm wide, filled with translucent precipitative silica. Type 2 dikes formed first and reflect a short-lived seismic event that locally decoupled the sedimentary section at the top of the Mendon Formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower, lithified part of the sedimentary section. Later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section, widespread liquefaction of soft, uppermost Mendon sediments, and flowage of the liquefied sediments and loose impact-generated spherules into the open fractures. Late-stage tsunamis everywhere eroded and reworked the spherule layer. The coincidence of crustal disruption, dike formation, spherule deposition, and tsunami activity suggests that all were related to the S2 impact or impact cluster. Crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact Fig Tree Group, including a small conglomeratic fan delta in the Barite Valley area. Remobilization and further movement of debris in the subsurface continued for some time. Locally, the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike, causing foundering of S2 and ~1-2 m of overlying baritic sediments into the dike. Spherule beds occur at the base of the Fig Tree Group over wide areas of the Barberton belt, marking the abrupt change from approx. 300 million years of predominantly anorogenic, mafic, and komatiitic volcanism of the Onverwacht Group to orogenic clastic sedimentation and associated felsic volcanism of the Fig Tree Group. This area never again returned to Onverwachtstyle mafic and ultramafic volcanism but evolved approx. 100 million years later into the Kaapvaal craton. These results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events approx. 3260 to 3240 million years ago.
Structural control on the formation of iron-oxide concretions and Liesegang bands in faulted, poorly lithified Cenozoic sandstones of the Paraba Basin, Brazil
F. Balsamo et al., Dept. of Physics and Earth Sciences, Parma University, Campus Universitario, Parco Area delle Scienze 157/A, I-43124, Parma, Italy. Posted online 7 March 2013; http://dx.doi.org/10.1130/B30686.1.
In this contribution, F. Balsamo and colleagues describe the occurrence and geometry of different types of iron oxide deposits, which are significant indicators of the mobility of Fe2+ and O2 in shallow groundwater, associated with strike-slip faults developed in the vadose zone in quartz-dominated sandstones of the Paraba Basin, NE Brazil. The development of highly permeable and low-permeability domains along isolated fault segments promoted the physical mixing of Fe2+-rich waters and oxygenated groundwater. This arrangement favors O2 diffusion in flowing Fe2+-rich waters and, consequently, iron oxide precipitation as sand impregnations, small nodular concretions, and well-cemented mineral masses. The formation of hydraulically isolated compartments along more complex strike-slip fault zones promoted the development of Liesegang bands (a classical example of spontaneous self-organization process) in a reaction zone dominated by pore-water molecular diffusion of O2 into Fe2+-rich stagnant water. The structural-diagenetic coupling described in this paper support the role of tectonic activity on near-surface sandstone diagenesis in determining preferential hydraulic pathways for the physicochemical interaction between oxygenated groundwater and iron-rich fluids. Structural setting, fault zone architecture, and related grain size-permeability structures determine the dominant mode of solution interaction and, thus, the type and distribution of iron oxide deposits in deformed sandstones.
Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska
Jeanette C. Arkle et al., Dept. of Geology, University of Cincinnati, P.O. Box 0013, Cincinnati, Ohio 45221, USA. Posted online 5 April 2013; http://dx.doi.org/10.1130/B30738.1.
The Yakutat microplate is subducting at a shallow angle beneath southern Alaska and in the region of maximum curvature of most of the major mountain belts and faults -- the southern Alaska syntaxis. The shallow subduction is thought to be responsible for most of the deformation in the region, as well as for devastating earthquakes such as the 1964 Good Friday M9.2 megathrust earthquake. Most studies of deformation have concentrated on the inboard deformation areas along the Denali fault in the Alaska Range, or on the more outboard collision-related deformation in the St. Elias orogen. This new study by Jeanette C. Arkle and colleagues focuses on the western Chugach Mountains and Prince William Sound area in between the inboard and outboard regions. They use thermochronologic data to infer recent and relatively rapid rock uplift that is focused in the core syntaxial area and interpret this focused rock uplift as being caused by underplating above the shallow subducting microplate. The increase in underplated material may be the result of influx of material derived from erosion of the St. Elias orogen farther outboard, thus attesting to the causal and temporal linking of these orogenic systems and the positive feedbacks between precipitation, glacial activity, and rock uplift.
Linking offshore stratigraphy to onshore paleotopography: The Late JurassicPaleocene evolution of the south Norwegian margin
Tor O. Smme et al., Statoil, Martin Linges vei 33, 1330 Fornebu, Norway. Posted online 5 April 2013; http://dx.doi.org/10.1130/B30747.1.
The link between paleotopography and basin stratigraphy along the South Norwegian margin is a long-debated topic that recently has received new attention. Despite the wealth of data available both onshore and offshore, regional relationships between onshore source areas and offshore depocenters remain to be established. In this study, Tor O. Smme and colleagues use the volume of discrete Upper Jurassic-Paleocene, point-sourced depositional units to estimate corresponding landscape topography at the time of deposition. This is accomplished by comparing the observed volume to an ideal sediment prediction model. The results show that the geometry and volume of offshore sedimentary units are best explained by topography that varies from ~1.6 km in the latest Jurassic, to ~0.5 km in the Late Cretaceous, and ~1.1 km in the Paleocene. Long-term changes in sediment flux to the margin suggests that the onshore topography has experienced recurrent periods of uplift along major fault zones followed by periods of regional denudation. This approach may also be useful for analyzing source-to-sink relationships along continental margins elsewhere.
A new view of an old suture zone: Evidence for sinistral transpression in the Cheyenne belt
W.A. Sullivan and R.J. Beane, Dept. of Geology, Colby College, 5803 Mayflower Hill, Waterville, Maine 04901, USA. Posted online 5 April 2013; http://dx.doi.org/10.1130/B30679.1.
This article revisits a fossil plate tectonic boundary, the Cheyenne belt, which lies near the Wyoming-Colorado boarder. The Cheyenne belt separates an old piece of crust, the 1,650 to 1,800-million-year-old Colorado province, from a very old piece of crust, the 2,500 to 3,100-million-year old Wyoming province. These crustal plates were connected or sutured together about 1,750-million years ago, and they now make up part of the stable core of the North American continent and underlie much of the central and southern Rocky Mountains. The authors present a new detailed dataset collected using modern analysis techniques that were not available when the area was last examined in detail 25 to 30 years ago. This dataset, combined with other new data from the region, indicates that the Wyoming and Colorado provinces collided obliquely rather than head on as previously thought. The authors' conceptual model for the Cheyenne belt suture zone explains a number of different geologic phenomena including the orientations of faults at the boundary between the two plates, the differences in temperatures at which the rocks were deformed across these faults, and a 2 to 10 km difference in the thickness of Earth's crust across the fossil plate boundary.
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