The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete ...sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.
The Hielo Patagónico (Patagonia Icefield), South America, is the largest temperate ice body in the Southern Hemisphere with more than 70 major outlet glaciers and a total area of ca. 17,200 km2. For ...the Hielo Patagónico, two major schemes for the Holocene glaciations (Neoglaciations) have been proposed. Mercer first proposed three Neoglaciations, I at 4500-4000 years before the present (yr BP), II at 2700-2000 yr BP and III at the 17-19th centuries. Aniya later postulated a scheme of four Neoglaciations, I at ca. 3600 yr BP, II at 2400-2200 yr BP, III at 1600-900 yr BP, and IV at the 17-19th centuries, which is congruent with other regions of the Andes. The main differences are the age of Neoglaciation I and the existence of a Neoglaciation at 1600-900 yr BP. After examining the dates given by other studies, although scanty, I propose a new scheme of five Neoglaciations, I at 4500-4000 yr BP, II at 3600-3300 yr BP, III at 2700-2000 yr BP, IV at 1600-900 yr BP and V at 17-19th centuries, that is, the Little Ice Ace (LIA). In addition, there were two earlier Holocene glaciations, probably at 5700-5000 yr BP and 8100-6800 (or 7500) yr BP. Two older ages, 8800-8500 yr BP and 9700-9100 yr BP, are uncertain.
The University of Tsukuba team began investigations of the Neolithisation process in the Slemani area in 2014. Excavations were undertaken at Qalat Said Ahmadan (2014, 2015) in the Pshdar Plain, in ...addition to surveys at Jarmo (Charmo)1 and Turkaka (2016-2018) in the Chamchamal area. Integrated research, including the making of topographical maps and 3D visualisations using the Unmanned Aerial Vehicle (UAV) and geological and paleoenvironmental surveys, along with archaeological sounding excavations, will provide new perspectives on landscape and early farming in the Slemani district. R.J. Braidwood proposed a conception about primitive farming in the Zagros region as “simple rain-fed farming along the hilly flanks.” However, we may get results that are more fruitful if we investigate a series of archaeological sites while adding to new perspectives and the concept of “more complicated farming using springs in the water reservoir area.”
The University of Tsukuba team began investigations of the Neolithisation process in the Slemani area in 2014. Excavations were undertaken at Qalat Said Ahmadan (2014, 2015) in the Pshdar Plain, in ...addition to surveys at Jarmo (Charmo) and Turkaka (2016-2018) in the Chamchamal area. Integrated research, including the making of topographical maps and 3D visualisations using the Unmanned Aerial Vehicle (UAV) and geological and paleoenvironmental surveys, along with archaeological sounding excavations, will provide new perspectives on landscape and early farming in the Slemani district. R.J. Braidwood proposed a conception about primitive farming in the Zagros region as “simple rain-fed farming along the hilly flanks.” However, we may get results that are more fruitful if we investigate a series of archaeological sites while adding to new perspectives and the concept of “more complicated farming using springs in the water reservoir area.”
L’équipe de l’université de Tsukuba a entamé des enquêtes sur le processus de néolithisation dans la région de Slemani en 2014. Des fouilles ont été entreprises à Qalat Said Ahmadan (2014, 2015) dans la plaine de Pshdar, en complément des enquêtes menées à Jarmo (Charmo) et à Turkaka (2016-2018) dans la région de Chamchamal. Les recherches intégrant la création de cartes topographiques et les visualisations 3D (à l’aide du véhicule aérien sans pilote), des enquêtes géologiques et paléo-environnementales, ainsi que des fouilles archéologiques de sondage offriront de nouvelles perspectives sur le paysage et l’agriculture précoce du district de Slemani. R.J. Braidwood a proposé une conception de l’agriculture primitive dans la région de Zagros comme « une simple agriculture pluviale le long des flancs vallonnés ». Cependant, nous pourrions obtenir des résultats plus fructueux en étudiant une série de sites archéologiques via de nouvelles perspectives et le concept d’une « agriculture plus complexe utilisant des ressources de la zone du réservoir d’eau ».
In 2009, petit-spot submarine volcanoes were discovered off the oceanward slope of the central Chile trench, offshore from Valparaiso, Chile, at around 33°S. Ar-Ar dating of mugearite and ...alkali-basalt from the volcanoes yields ages of 10.11 ± 0.22 Ma and 6.69 ± 0.88 Ma, respectively. Back-calculations of plate motion along the present absolute movement direction of the Nazca Plate, conducted using the Ar-Ar age data, indicate that eruption occurred above a zone of plate flexure. The back-calculation results suggest that the mugearite was erupted at a flexural arch prior to arrival at the active site of the Juan Fernández hotspot. In contrast, the alkali-basalt was erupted on a plate flexure at a site of interaction between a flexural moat and an outer-rise area, where the source material was probably influenced by the Juan Fernández hotspot. The geochemistry of the lavas supports this interpretation because the concentration ratios of various rare earth and other trace elements in the mugearite are different from those of the alkali-basalt and Juan Fernández hotspot lavas. Consequently, petit-spot melts could reflect the composition of source materials below the plates at sites of plate flexure.
The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh‐pressure–high‐pressure (UHP–HP) metamorphic belt in this area is composed of four ...subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite‐ and diamond‐bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite‐dominant Unit I on the bottom and the orthogneiss‐dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post‐dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non‐coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top‐to‐the‐northwest motions and those from Unit III provide top‐to‐the‐southeast, south or south‐west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear‐indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large‐scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike‐slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north‐westward extrusion of UHP Unit II.
The ophiolitic remnants of the Upper Mesozoic Rocas Verdes basin in southernmost South America were studied from the perspectives of petrography, chemistry of minerals, bulk-rock geochemistry, and ...U-Pb geochronology. The study aimed to unravel the tectonic, magmatic, and metamorphic evolution of a suprasubduction rift zone that underwent a transition to a back-arc basin. The rifting phase and bimodal magmatism within the Rocas Verdes basin started prior to or during the Late Jurassic, as indicated by a gabbro in contact with pillow basalts that dated at 154 Ma. In the Late Jurassic Capitán Aracena and Carlos III complexes, tholeiitic basalts are geochemically comparable to enriched mid-oceanic ridge basalts. Back-arc basin development continued for 35 myr until the Early Cretaceous, as suggested by the ages of detrital zircons in cherty layers within pillow basalts and metamorphic titanite that crystallized during seafloor metamorphism near the spreading/magmatic axis. In the Early Cretaceous Tortuga Complex, tholeiitic basalts are comparable to normal mid-oceanic ridge basalts. Non-deformative metamorphism converted the primary mineralogy of the ophiolites to low- to intermediate-grade metamorphic assemblages formed during ocean-floor type alteration in a suprasubduction setting. Fossilized bacteria, preserved as rounded aggregates of titanite microcrystals, were identified in the pillow basalts up to the Early Cretaceous. The Rocas Verdes basin closed during the Andean orogeny, which started during the Late Cretaceous, and ophiolites were tectonically juxtaposed and thrust over the sedimentary infill of the quasi-oceanic basin in which they developed. The tectonic emplacement of the ophiolitic complexes was complete before the latest Cretaceous, as indicated by crystallization ages of granites intruded into the ophiolitic complexes.
The Kokchetav Massif of Kazakhstan includes high to ultrahigh‐pressure (HP–UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower ...pressure metamorphic components. We investigated the relationship between the HP–UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu–Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP–UHP metamorphic unit overlies the LP unit across a west‐dipping subhorizontal boundary. In addition, kinematic indicators display top‐to‐the‐north senses of shear along the tectonic contact between the two units, indicating that the HP–UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths.
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