Magnetotelluric Studies of Active Continent–Continent Collisions     

Martyn Unsworth 《Surveys in Geophysics》2010,31(2):137-161

Continent–continent collisions are an important tectonic process and have played a fundamental role in the evolution of the modern continents. A combination of geological and geophysical data has provided new constraints on the structure and temporal evolution of these orogens. Magnetotelluric (MT) studies have been an important part of these studies since they can constrain the fluid content and thermal structure which are key parameters for defining the rheology of the crust and upper mantle. MT studies of the Himalaya have defined the geometry of active faults associated with continued plateau growth. Orogen scale MT studies have shown that both the India–Asia collision (Tibetan Plateau and Himalaya) and the Arabia–Eurasia collision (Eastern Anatolia) have developed a low resistivity mid-crustal layer with upper surface at 10–20 km that is likely due to a combination of partial melt and associated aqueous fluids. The properties of this layer are consistent with a strength contrast that permits crustal flow over geological timescales. The upper mantle from the Moho to at least 100 km beneath both Northern Tibet and the Anatolian Plateau is characterized by low resistivity values (10–30 Ωm) indicating the presence of shallow asthenosphere. Future integrated seismic and MT studies of collision zones are needed fully to explore the 3D structures associated with deformation and further constrain geodynamic models.  相似文献   

The Geochemistry of Supraglacial Streams of Canada Glacier,Taylor Valley (Antarctica), and their Evolution into Proglacial Waters     

Sarah?K.?FortnerEmail author  Martyn?Tranter  Andrew?Fountain  W.?Berry?Lyons  Kathleen?A.?Welch 《Aquatic Geochemistry》2005,11(4):391-412

We have investigated the geochemistry of supraglacial streams on the Canada Glacier, Taylor Valley, Antarctica during the 2001–2002 austral summer. Canada Glacier supraglacial streams represent the link between primary precipitation (i.e. glacier snow) and proglacial Lake Hoare. Canada Glacier supraglacial stream geochemistry is intermediate between glacier snow and proglacial stream geochemistry with average concentrations of 49.1 μeq L⁻¹ Ca2+, 19.9 μeq L⁻¹ SO₄²⁻, and 34.3 μeq L⁻¹ HCO₃⁻. Predominant west to east winds lead to a redistribution of readily soluble salts onto the glacier surface, which is reflected in the geochemistry of the supraglacial streams. Western Canada Glacier supraglacial streams have average SO₄²⁻:HCO₃⁻ equivalent ratios of 1.0, while eastern supraglacial streams average 0.5, suggesting more sulfate salts reach and dissolve in the western supraglacial streams. A graph of HCO₃⁻ versus Ca2+ for western and eastern supraglacial streams had slopes of 0.87 and 0.72, respectively with R2 values of 0.84 and 0.83. Low concentrations of reactive silicate (> 10 μmol L⁻¹) in the supraglacial streams suggested that little to no silicate weathering occurred on the glacier surface with the exception of cryoconite holes (1000 μmol L⁻¹). Therefore, the major geochemical weathering process occurring in the supraglacial streams is believed to be calcite dissolution. Proglacial stream, Anderson Creek, contains higher concentrations of major ions than supraglacial streams containing 5 times the Ca²⁺ and 10 times the SO₄²⁻. Canada Glacier proglacial streams also contain higher concentrations (16.6–30.6 μeq L⁻¹) of reactive silicate than supraglacial streams. This suggests that the controls on glacier meltwater geochemistry switch from calcite and gypsum dissolution to both salt dissolution and silicate mineral weathering as the glacier meltwater evolves. Our chemical mass balance calculations indicate that of the total discharge into Lake Hoare, the final recipient of Canada Glacier meltwater, 81.9% is from direct glacier runoff and 19.1% is from proglacial Andersen Creek. Although during a typical, low melt ablation season Andersen Creek contributes over 40% of the water added to Lake Hoare, its overall chemical importance is diluted by the direct inputs from Canada Glacier during high flow years. Decadal warming events, such as the 2001–2002 austral summer produce supraglacial streams that are a major source of water to Lake Hoare.  相似文献   

Mechanisms of fine extinction band development in vein quartz: new insights from correlative light and electron microscopy     

Tine?Derez  Tom?Van Der?Donck  Oliver?Plümper  Philippe?Muchez  Gill?Pennock  Martyn?R.?Drury  Manuel?SintubinEmail author 《Contributions to Mineralogy and Petrology》2017,172(7):56

Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz.  相似文献