Description: The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Paraná and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (〈100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures)

Description: The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust décollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build mélange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as 〈1–20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands.

Description: Back-scattered electron investigations of Argille Scagliose, mudstones from the Barbados forearc sampled during Ocean Drilling Program Leg 110, and the matrix of the Okitsu melange, southwest Japan, indicate a variety of microfabric geometries and deformation mechanisms despite the similarity of their mesoscopic scaly fabrics. In the Okitsu melange, the scaly fabric is the mesoscopic expression of an anastomosing microfabric, whereas the detailed geometry of scaly fabrics is unrelated to the microfabric in the Argille Scagliose and Leg 110 mudstones. Care must be taken in the classification and interpretation of scaly fabrics in light of these data, and future studies should focus on microstructural investigations.

Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.

Description: Ecosystems damaged by distal volcanic ash and sulfur deposition usually recover within decades. However, sediment, stalagmite, and pollen records from the southernmost Andes indicate a 2000 yr impact on forest and aquatic ecosystems after deposition of a thin tephra layer. SO2 released from altering pumice produced intense soil and lake acidification in a 〉150,000 km2 area. Acidification led to nutrient leaching and affected soil microorganisms, causing plant decay and increased soil erosion in an area larger than 8000 km2. We conclude that weakly buffered soils in humid environments are extremely vulnerable to volcanic and anthropogenic acidification, causing long-lasting ecosystem damage and perturbations of paleoclimate proxy records.

Description: Devonian-Carboniferous sedimentary and volcanic rocks are exposed in the Badenweiler-Lenzkirch Zone (BLZ) of the Southern Schwarzwald and in the Southern Vosges (Central Europe). Several lithostratigraphic units are recognized. Sedimentary or tectonic relations between some of the units can be realized by comparing sedimentary and volcanic facies associations as well as petrographic and geochemical datasets. During the Variscan orogenesis the sediments were deposited in different kinds of basins along an active margin setting. Their deformation was studied in the Markstein area of the Southern Vosges and is presented here in detail. Some arguments favour a northward underthrusting of a narrow and oceanic pre-Upper Devonian back-arc basin beneath the crystalline units of the Central Vosges and Schwarzwald. Accretion of continental crust continued from the Upper Devonian to the late Lower Carboniferous along with a characteristical evolution of a deep marine retro-arc foreland basin. It has accumulated about 3500 m of turbidite sandstones, shales and conglomerates of volcano-plutonic origin, assembling now the Markstein Group in the Southern Vosges. To the south of the Markstein Group, deep marine mud-rich turbidite systems and a shallowing-upward succession of fluvio-deltaic sediments filled in the Oderen Basin, which subsided in the late Tournaisian and early Viséan. A subduction type volcanism occurred in form of tholeiitic basalts, K-calcalkaline island arc andesites and continental arc sandstones. In contrast to former models, a genetical relationship between the Markstein and the Oderen Group is rejected here. Instead, different subduction systems in space and time have caused the development of the active margin basins. The Variscan deformation of the exposed sedimentary units started in the upper Viséan. Structural analysis of kinematic indicators and macroscopic fabrics in the Markstein Group give evidence for thrusting and dextrally transpressive shearing, forming a positive flower structure along the southern border of the Central Vosges crystalline units. This was broadly synchronous with dextrally transpressive deformation along the Lalaye-Lubine/Baden Baden fault at the northern border of the Central Vosges and Schwarzwald and along the "Transition Complex" between the Central and the Southern Schwarzwald. The high-K-calcalkaline Metzeral granite intruded postkinematically into the principal displacement zone and has a position similar to the dextrally sheared Randgranite association along the northern border of the BLZ. A juxtaposition between the Oderen and the Markstein Groups was achieved in the upper Viséan due to the dextral transpressive motion along the ESE-trending Markstein Fault. The exhumation of high grade metamorphic rocks of the Central Vosges and Schwarzwald during the upper Viséan is discussed in the context of long lasting accretionary tectonics and the two-sided dextral transpression, favouring crustal extrusion as a main exhumation process. A thorough examination of the supracrustal rocks and their mode of deformation in the Southern Vosges helps us to determine the relationships with the BLZ and contributes also to the paleogeographic reconstruction of the Armorica - Proto-Alpine - Gondwana connection.

Description: Like the larger Cyclades Islands (e.g. Naxos, Paros, Ios) the Small Cyclades Islands form part of the Attic-Cycladic crystalline basement belt. Subduction in the Paleogene was associated with high-pressure metamorphism, followed by a Barroviantype overprint, local partial melting and granitic plutonism in the Neogene. On the islands of Iraklia and Schinoussa Neogene metamorphism did not exceed mid-greenschist grade, and there are some well preserved older high-pressure/low temperature metamorphic assemblages. The rocks and structural geology of both islands had previously not been analysed in detail. Rocks present are calcite and dolomite marbles, in part with preserved paleosoils (metabauxites and metalaterites), and a suite of metapelitic schists with associated glaucophane schists and piemontite quartzites. There is a ductile deformation history comprising four events. The two older ones (D1, D2) are associated with high strains, and have resulted in a subhorizontal foliation, a N-S trending stretching lineation, and recumbent isoclinal folds with variable axial orientations. At least D1 is bracketed by the growth of glaucophane, with uncertain kinematics. D2, syn-blueschist grade on Iraklia, however, has a clear top-to-north sense of shear. D3 and D4 caused upright folds with N-S and E-W trending fold axes, respectively. This is evidence for late, two-phase horizontal shortening. As the deformed rocks show only a slight static metamorphic overprint, we have estimated flow stresses from dynamically recrystallized grain sizes of quartz and calcite. Coincident flow stress estimates are 28-62 MPa in quartzites, and 31-56 MPa in the marbles. Together with syntectonic temperature extimates, and applying published flow laws, this leads to estimated deformation rates on the order of 10−12 to 10−14 sec−1. Even though deformation history and kinematics is very similar to that on Naxos Island, the clear relation of the earlier part to high-pressure metamorphism on Iraklia and Schinoussa speaks in favour of a subduction-related setting here. This is in contrast to the later formation of metamorphic core complexes in the north (Naxos) and south (Ios).