Effectiveness of Satellite Programs for Technical Updating of Vocational Education Teachers

This article describes the planning, implementation, development and evaluation of a funded project that used distance education technologies for technical updating of vocational education teachers in Florida. The project was completed during the summer of 1992. Eight of nine public universities in Florida worked in collaboration on the project. Delivery of 12, two-hour live satellite programs was completed within a four-week period with participation of teachers from throughout Florida. Analysis of responses from participants in the workshops depicts acceptance of the use of distance education technologies for technical updating

Application of computational modelling to the dynamic and geometric controls on slab earthquakes

© 2018 Dr. Dan SandifordThe rupture mechanisms of intermediate depth earthquakes have led to a view that subducting slabs often exhibit either bulk extension or shortening in the upper mantle. Additionally, bending-related deformation has been invoked to explain the distribution and orientation of seismicity in double seismic zones. However, to date there is no clear understanding of why bending is more or less prominent in different settings. The primary aim of this research is to explore the relationship between intermediate depth seismicity and subduction dynamics, utilising thermo-mechanical modelling. I show that bending, in conjunction with strong thermal restrictions, can produce diverse seismic expression. For instance, flat slab geometries induce zones of positive bending rate within sub- horizontal, low-curvature sections of the slab. This gives rise to downdip extension in the upper part the flat slab, consistent with prevalent normal faulting in flat slab settings in Mexico, Peru and Chile. Deeper earthquakes on the lower side of the bending neutral plane are much less common. Overall, this leads to an apperent state of bulk downdip stretching. These examples, along with other settings explored in this study, suggest that bending has a more prevalent role than previously recognised. Another key theme of this research is how changes in mantle wedge flow can influence slab morphology and the configuration of volcanic arcs. These insights intersect with the seismicity problem due to the strong geometric control on slab bending rates. Systematic differences in the style of slab seismicity, either side of the Pacific, may be largely explicable in terms of these geometric differences. The framework I develop for intermediate depth seismicity is consistent with the idea that slabs are largely supported by mantle drag and transmit relatively little in-plane stress

Geometric controls on flat slab seismicity

The relationship between intraslab seismicity and the dynamics of subduction is a subject of ongoing debate. Uncertainty surrounds (1) the extent to which the stress regime associated with slab earthquakes reflects the driving/resisting forces of subduction, or more localised processes such as metamorphic or thermo-elastic volume change, and (2) the relative contribution of uniform (stretching/shortening) and flexural (bending/buckling) deformation modes in slabs. Because variations in slab curvature are very clear in subduction zones with flat slab segments, such settings allow for assessment of the relationship between slab geometry, bending and seismicity. Using a numerical model that reproduces published flat slab geometries we show how bending rates, which are dominated by the advective component, relate to downdip curvature gradients. Based on published slab geometries, we show that the patterns of seismicity in the Peruvian and Mexican flat slabs, vary systematically with slab curvature gradient. Seismicity is restricted to regions colder than about 600 ◦C. This means that only the upper half of flat slab bending zones are expressed seismically, providing the appearance of a uniform extensional regime.This work was supported by the Australian Research Council (Discovery grant DP150102887). Development of the Underworld2 code (http://www.underworldcode.org/) was supported by AuScope. DS’s postgraduate research at the University of Melbourne was supported by a Baragwanath Geology Research Scholarship. This work was supported by resources provided by The Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. This work was supported by the Nectar Research Cloud, a collaborative Australian research platform supported by the National Collaborative Research Infrastructure Strategy (NCRIS)

Spatially and Geochemically Anomalous Arc Magmatism : Insights From the Andean Arc

While most volcanic arcs show a distinctive spatial relationship to subducting plates, there are many examples where volcanoes occur in anomalous locations. These are commonly also geochemically anomalous relative to the composition of more typical subduction-related rocks. Using Holocene volcanoes in South America as a case study, we document the spatial and geochemical patterns along the Andean volcanic belt. To determine whether spatial variations are also geochemically anomalous, we assess a series of geochemical indices that provide information on the depth and degree of melting, and the role of metasomatic subduction inputs in melt generation. We use these parameters to develop a scoring system, with the lowest and highest scores indicating “typical” and “anomalous” arc melting processes, respectively. Typical arc magmatism is defined as melts generated in the sub-arc mantle wedge through slab-derived fluid metasomatism, with or without contributions from subducted sediments. In contrast, we show that anomalous volcanism in South America appears to relate to geometric anomalies in the subducting Nazca plate (e.g., beneath Sumaco, Laguna Blanca and Payun Matru), or to areas affected by variations in mantle flow due to the proximity to the slab edge (Crater Basalt Volcanic Field). By establishing relationships between anomalous magmatism and slab structure, we propose that similar geochemical fingerprints could be used to explore the magmatic response to slab deformation and/or tearing in older arc systems, particularly in cases where the three-dimensional slab structure is no longer detectable.</p

Spatially and Geochemically Anomalous Arc Magmatism: Insights from the Andean Arc

While most volcanic arcs show a distinctive spatial relationship to subducting plates, there are many examples where volcanoes occur in anomalous locations. These are commonly also geochemically anomalous relative to the composition of more typical subduction-related rocks. Using Holocene volcanoes in South America as a case study, we document the spatial and geochemical patterns along the Andean volcanic belt. To determine whether spatial variations are also geochemically anomalous, we assess a series of geochemical indices that provide information on the depth and degree of melting, and the role of metasomatic subduction inputs in melt generation. We use these parameters to develop a scoring system, with the lowest and highest scores indicating “typical” and “anomalous” arc melting processes, respectively. Typical arc magmatism is defined as melts generated in the sub-arc mantle wedge through slab-derived fluid metasomatism, with or without contributions from subducted sediments. In contrast, we show that anomalous volcanism in South America appears to relate to geometric anomalies in the subducting Nazca plate (e.g., beneath Sumaco, Laguna Blanca and Payun Matru), or to areas affected by variations in mantle flow due to the proximity to the slab edge (Crater Basalt Volcanic Field). By establishing relationships between anomalous magmatism and slab structure, we propose that similar geochemical fingerprints could be used to explore the magmatic response to slab deformation and/or tearing in older arc systems, particularly in cases where the three-dimensional slab structure is no longer detectable.</p

Kinematics of footwall exhumation at oceanic detachment faults: solid-block rotation and apparent unbending

Abstract Seafloor spreading at slow rates can be accommodated on large-offset oceanic detachment faults (ODFs), that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). Footwall rock experiences large rotation during exhumation, yet important aspects of the kinematics - particularly the relative roles of rigid block rotation and flexure - are not clearly understood. Using a high-resolution numerical model, we explore the exhumation kinematics in the footwall beneath an emergent ODF/OCC. A key feature of the models is that footwall motion is dominated by solid rotation, accommodated by the concave-down ODF. This is attributed to a system behaviour in which the accumulation of distributed plastic strain is minimized. A consequence of these kinematics is that curvature measured along the ODF is representative of a neutral stress configuration, rather than a 'bent' one. Instead, it is in the subsequent process of `apparent unbending’ that significant flexural stresses are developed in the model footwall. The brittle strain associated with apparent unbending is produced dominantly in extension, beneath the OCC, consistent with earthquake clustering observed in the Trans-Atlantic Geotraverse at the Mid-Atlantic Ridge

Kinematics of Footwall Exhumation at Oceanic Detachment faults: Solid‐Block Rotation and Apparent Unbending

Abstract Seafloor spreading at slow rates can be accommodated on large-offset oceanic detachment faults (ODFs), that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). Footwall rock experiences large rotation during exhumation, yet important aspects of the kinematics - particularly the relative roles of rigid block rotation and flexure - are not clearly understood. Using a high-resolution numerical model, we explore the exhumation kinematics in the footwall beneath an emergent ODF/OCC. A key feature of the models is that footwall motion is dominated by solid rotation, accommodated by the concave-down ODF. This is attributed to a system behaviour in which the accumulation of distributed plastic strain is minimized. A consequence of these kinematics is that curvature measured along the ODF is representative of a neutral stress configuration, rather than a 'bent' one. Instead, it is in the subsequent process of `apparent unbending’ that significant flexural stresses are developed in the model footwall. The brittle strain associated with apparent unbending is produced dominantly in extension, beneath the OCC, consistent with earthquake clustering observed in the Trans-Atlantic Geotraverse at the Mid-Atlantic Ridge

Geomorphic and cosmogenic nuclide constraints on escarpment evolution in an intraplate setting, Darling Escarpment, Western Australia

The ~900km long Darling Scarp in Western Australia is one of the most prominent linear topographic features on Earth. Despite the presence of over-steepened reaches in all westerly flowing streams crossing the scarp, and significant seismic activity within 100km of the scarp, there is no historical seismicity and no reported evidence for Quaternary tectonic displacements on the underlying Darling Fault. Consequently, it is unclear whether the scarp is a rapidly evolving landform responding to recent tectonic and/or climatic forcing or a more slowly evolving landform. In order to quantify late Quaternary rates of erosion and scarp relief processes, we obtained measurements of the cosmic-ray produced nuclide beryllium-10 (10Be) from outcropping bedrock surfaces along the scarp summit and face, in valley floors, and at stream knickpoints. Erosion rates of bedrock outcrops along the scarp summit surface range from 0·5 to 4·0mMyr-1. These are in the same range as erosion rates of 2·1 to 3·6mMyr-1 on the scarp face and similar to river incision rates of 2·6 to 11·0mMyr-1 from valley floor bedrock straths, indicating that the Darling Scarp is a slowly eroding 'steady state' landform, without any significant contemporary relief production over the last several 100kyr and possibly several million years. Knickpoint retreat rates determined from 10Be concentrations at the bases of two knickpoints on small streams incised into the scarp are 36 and 46mMyr-1. If these erosion rates were sustained over longer timescales, then associated knickpoints may have initiated in the mid-Tertiary to early Neogene, consistent with early-mid Tertiary marginal uplift. Ongoing maintenance of stream disequilibrium longitudinal profiles is consistent with slow, regional base level lowering associated with recently proposed continental-scale tilting, as opposed to differential uplift along discrete faults. Cosmogenic 10Be analysis provides a useful tool for interpreting the palaeoseismic history of intraplate near-fault landforms over 105 to 106 years

Contrasted East Asia and South America tectonics driven by deep mantle flow

East Asia and South America have both experienced long-term subduction since at least the Jurassic, but they have had contrasting tectonic evolutions since the Late Cretaceous. East Asia was dominated by extensional tectonics with many marginal sea basins forming during the Cenozoic while South America was dominated by compressional tectonics building the Andean mountains. The mechanism controlling this contrast in tectonic style on opposite sides of the Pacific has been unclear. We suggest that the deep mantle flow revealed by seismic tomography, plate reconstructions, and geodynamic models contributed to the pan-Pacific tectonic disparity. Our geodynamic models suggest that the Atlantic Ocean opening plays an important role in promoting compression-dominated tectonics and Andean building along the South American margin by allowing fast trench-ward motion of South America. On the other hand, the long-standing downwelling flow beneath the interior of Asia since Asian assembly in the Paleozoic and the early Mesozoic helps to inhibit Atlantic-type ocean opening in Asia and restrain trench-ward motion of East Asia, promoting extension-dominated tectonics along the Asian margin. The restrained trench-ward motion suggests a low probability of flat-slab subduction occurrence in East Asia during the late Mesozoic