Structural control on late Miocene to Quaternary volcanism in the NE Honshu arc, Japan
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Volcanological and structural field data are used to define the tectonic control on the N–S volcanic arc of NE Honshu (Japan) since late Miocene. During late Miocene‐Pliocene, bimodal products were mainly erupted from along‐arc and NE–SW‐aligned and elongated calderas. The deformation pattern mostly consisted of N–S dextral faults and subordinate NE–SW extensional structures produced by NE–SW compression. This pattern, because of the indentation of the Kuril sliver, is similar to that of oblique convergence settings. Magma rose and extruded along NE–SW areas of localized extension created by the dextral faults. These extensional areas were uncoupled with regard to those, ∼E–W trending, inferred to have focused the rise of melts from the subducting slab in the mantle. During Quaternary, a larger amount of andesite was mainly erupted from along‐arc and ∼E–W‐aligned and elongated stratovolcanoes. The deformation pattern mostly consisted of N–S thrust faults and subordinate ∼E–W extensional structures, produced by ∼E–W compression, resulting from orthogonal convergence due to the variation in the absolute motion of the Pacific Plate. The ∼E–W extensional structures are the shallowest expression of ∼E–W‐trending hot mantle fingers, suggesting mantle‐crust coupling for the rise of magma. Such a coupling ensures (1) higher extrusion and (2) mixing between a deeper mafic and a shallower felsic magma, generating the andesites. The significantly larger volumes (Ma −1 200 km −1 of length of the arc) of magma erupted during Quaternary show that pure convergence conditions do not necessarily hinder the rise and extrusion of magma.Keywords:
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<p>The Neogene volcanism in the western part of Romania is confined to the Apuseni Mountains and surrounding areas. The largest volcanic area is mostly developed in the WNW-ESE oriented, ca. 120 km in length Z&#259;rand-Brad-Zlatna Basin.</p><p>The Bont&#259;u Volcano (Seghedi et al., 2010) is located inside the western part of the Z&#259;rand-Brad-Zlatna Basin and it is strongly affected by erosional processes, being crossed in its northern part, from east to west, by the Cri&#537;ul Alb River.</p><p>The Bont&#259;u Volcano is known to be active roughly between 14-10 Ma (according to the available K/Ar data) and it has been characterized as a composite or stratovolcano volcano associated with dome complexes, built by calc-alkaline andesitic lavas and pyroclastic deposits (andesite to basaltic andesite). The long-lasting volcanism developed in the Bont&#259;u area has a complex build up stages that we recently have found were interrupted by a series of destructive failure events. Several important volcanic collapses of the volcanic edifice took place producing large volcanic debris avalanches followed by numerous debris flows which produced various secondary volcaniclastic deposits that can be observed in different places all around the Bont&#259;u volcano. The debris avalanches deposits have not yet been known up to this study. The distribution of the debris avalanche deposits and associated volcaniclastic deposits is the main target of this study. In order to reconstruct Bont&#259;u Volcano activity and reconstruct its original morphology we done field observations and sampled the main lithologies to perform petrographic observations and geochemical and isotopic analyses (for the main lithologies).</p><p>During our field observations we tried to identify the relationships between debris avalanche deposits and older volcanic bodies (lavas, domes, volcaniclastic). One main important remark is related with the presence of several small basins at the margin of the volcano consisting of a succession of thin planar and cross-bedded sandstone in an alternation of coarse and fine layers associated with discontinuous lapilli trains (including pumices); The deposits are poorly to moderately sorted; with low angle cross lamination in lenses or pockets. Such deposits, as closely associate with debris avalanche deposits have been interpreted as small intra-hummocky basins formed after debris avalanche generation; they are mostly situated at the margins of the volcano.</p><p>The presence of multiple debris avalanche deposits can be connected with volcano growing in an extensional environment. We may assume that the long-lived Miocene rift graben system of the Z&#259;rand-Brad-Zlatna Basin experienced numerous changes in the fracture propagation and vertical movements that promoted repeated dyke intrusion and facilitated generation of numerous debris avalanches.</p><p>Acknowledgements: This work was supported by a grant of the of Ministry of Research and Innovation, CNCS &#8211; UEFISCDI, project number PN-III-P4-ID-PCCF-2016-4-0014, within PNCDI III.</p>
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<p>The Erzurum-Kars Volcanic Plateau (EKVP) was formed by volcanic eruptions during the Messinian-Zanclean (~5.5 Ma) period, related to a continental collision event between Eurasia and Arabia, initiated ~15 Ma ago. The EKVP unconformably overlies a series of older sedimentary formations spanning in age from Cretaceous to Miocene. It starts with a ~400 m thick pyroclastic-rich layer at its bottom, named the Akkoz basal tuff, consisting of rhyolitic and dacitic ignimbrites, pyroclastic fall and surge deposits, which are intercalated with andesitic and dacitic lavas. Upper layers of the plateau are dominated by andesitic and basaltic andesitic lavas (~100 m).</p><p>In the northwest of the study area, an eroded stratovolcano, named Hamaml&#305; volcano, which is possibly coeval with the plateau volcanism is present. It covers ~280 km<sup>2</sup> area and consists of a thick sequence of rhyolitic lavas, tuffs, ignimbrites, perlites and obsidians. The best preserved volcanic edifice in the study area is the Greater Alada&#287; Stratovolcano with a footprint of ~230 km<sup>2</sup>. It is composed of intermediate lavas with andesitic, dacitic, trachy-andesitic compositions, erupted ~3.55 Ma in Piacenzian. A small volcanic cone, named in this study as the Lesser Alada&#287; volcano, sits on the northern flank of the Greater Alada&#287;. Lesser Alada&#287; has an elliptical shape and is composed of basaltic-andesitic and basaltic trachy-andesitic lavas. Three semi-circular shaped rhyolitic domes called the Odalar rhyolite sit on the southern and eastern slopes of the Greater Alada&#287;. In the N and NE, the Alada&#287; volcanic sequence is unconformably overlain by a younger (~2.7 Ma) sequence of olivine basalts and basaltic andesites, which is known as the Kars volcanic plateau.</p><p>All volcanic products in the study area are calc-alkaline in character with a clear subduction signature. Results from our petrological modelling studies indicate that the magmas that fed the Alada&#287; volcanic system were evolved in a chamber, which was periodically replenished by fresh and primitive basaltic magma. Our assimilation model results based on the equations of DePaolo (1981) and Aitcheson and Forrest (1994) show that fractional crystallization was more important than crustal assimilation process in evolved lavas of the Alada&#287; system. Interestingly, EC-AFC model results indicate that some of the youngest basalts from the Kars volcanic plateau contain higher degrees of crustal assimilation relative to more evolved lavas.</p><p>Crystal chemistry of amphiboles by EMP from the amphibole-bearing lavas of the Akkoz basal tuff layer indicates that they had experienced crystallization pressures between 5.63 and 6.45 kbar and temperatures between 949 and 1026 &#176;C during their magma chamber evolution. On the other hand, pyroxene thermo-barometry of the Alada&#287; units has given crystallization pressures between 0.8 and 4.8 kbar, and temperatures from 1025 to 1078 &#176;C, implying polybaric fractionation. Calculated crystallization pressures and temperatures from the younger lavas of the Kars volcanic plateau are ~8.8 kbar and ~1179 &#176;C respectively. Our partial melting models indicate that the primitive basaltic magmas might have been derived from a metasomatised spinel peridotite source with varying melting degrees from 0.7% to 2%.</p>
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The Quaternary Zao volcano is situated on the Tohoku Backbone Ranges, Northeast Japan. It is known that activity of the volcano started about 1 Ma and continues to the present. The volcanic history of the Zao volcano can be divided into four stages as follows. Stage 1: A relatively small-scale volcano was formed. It is composed predominantly of pyroclastic materials of basalt and basaltic andesite. Stage 2: A stratovolcano was constructed by lavas and pyroclastic materials of andesite and dacite. The stage 2 is further subdivided into two substages, 2 a and 2 b. Stage 3: Lavas and pyroclastic materials from two vents situated near the summit widely distributed around the flank of the volcano. Rocks consist of basaltic andesite and andesite with a small amount of basalt. Stage 4: After formation of caldera, Goshikidake pyroclastic cone was formed in the caldera. The eruptive products comprise pyroclastic materials and lavas of basaltic andesite. The rocks of stage 1 belong to low-K series, and all the others to medium-K series. The medium-K rocks from different stages show contrasting trends; the rocks of stage 3 have slightly higher K 2O than those of stage 2 and stage 4 at the same SiO2 content. The basalt of stage 3 is medium-K and a contrast to the low-K basalts of stage 1.
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The Nasu volcano group is composed of six volcanic edifices, Kasshiasahidake, Sanbon-yaridake, Asahidake, Minamigassan, Futamatayama, and Chausudake, located in the volcanic front of the Northeast Japan arc. Kasshiasahidake (ca. 0.54-0.42 Ma), Sanbon-yaridake (ca. 0.36-0.27 Ma), Minamigassan (ca. 0.21-0.03 Ma) have similar geological sequences. Alternation of thin basaltic andesite lava flows and associated pyroclastics developed in the lower part, whereas thick andesitic or dacitic lava flows and minor pyroclastic flows developed in the upper part. Between these two stages, caldera collapse sometimes occurred. On the other hand, Asahidake (ca. 0.21-0.06 Ma), Futamatayama (ca. 0.14 Ma), and Chausudake(ca. 0.04-0 Ma) are composed of andesitic lavas, lava domes, and pyroclastic flows, lacking thin basaltic andesite sequences. From evolutionary historical point of view, Kasshiasahidake, Sanbon-yaridake, and Minamigassan (including Asahidake and Chausudake edifices) edifices construct individual stratovolcanoes, which have similar evolutionary histories. Futamatayama is distinct from these. Volume and eruption duration of the three stratovolcanoes are as follows; ca. 200 k.y., 16.2 km3 for Kasshiasahidake, ca. 150 k.y., 7.2 km3 for Sanbon-yaridake, ca. 200 k.y., 17.8 km3 for Minamigassan (including Asahidake and Chausudake). These volumes and eruption rates are less than those of large stratovolcanoes (Akagi, Haruna, Hakone volcanoes etc.) in near the triple junction of plate boundary, but are comparable to those of small stratovolcanoes (Quaternary volcanoes in Shin-etsu Highland).
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A field-based study used geologic mapping and a ground-based magnetic survey to investigate the exhumed central intrusive complex (CIC) of Summer Coon stratovolcano in Colorado. The CIC comprises a group of diorite to granodiorite stocks and sub-vertical domains of conduit-filling breccia hosted within basaltic-andesite breccia associated with cone building. The stocks serve as the focal points for 53 basaltic-andesite (mafic) and 36 andesite to rhyolite (evolved) radial dikes mapped within the CIC. The evolved dikes with outcrop lengths (strike length) of <650 m are confined to the central areas of the CIC, cut through the stocks, and have steeply plunging terminal segments. In contrast, most evolved dikes with outcrop lengths >1600 m are excluded from the central areas of the CIC, do not cut through the stocks, and have terminal segments that plunge shallowly toward their focus. Assuming the dikes are blade-shaped and that they originated directly below the CIC, the propagation direction of the evolved dikes was estimated using the plunge angles, spatial distribution, and dike outcrop lengths. The relatively long dikes may have ascended toward the level of exposure along inclined paths. These dikes remained exclusively within the basaltic-andesite breccia and, probably due to the higher relative stiffness of the stocks, were unable to propagate through the stocks as they ascended. Upon approaching the level of exposure, the same dikes encountered a stress barrier likely generated by the gravitational load of the edifice. This barrier altered the dike propagation paths from inclined to sub-horizontal, significantly increasing their outcrop lengths. In contrast, the dikes with short outcrop lengths ascended along primarily sub-vertical paths, intersecting the level of exposure within the central portions of the CIC. To propagate sub-vertically through both the relatively stiff stocks and a potential stress barrier, the magma overpressures within the shorter dikes may have been higher relative to the longer sub-horizontally propagating dikes. It is probable that at active volcanoes, only dikes with sufficiently high overpressures can ascend through the central intrusive complex of mature stratovolcanoes to feed eruptions near the summit. Perhaps more frequently, stress barriers and existing intrusions stall or deflect dikes with lower relative overpressures toward the slopes of the volcano where they may feed flank eruptions.
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The results of experimental measurements on elastic wave velocity (Vp) for rocks of different types from Tarim Basin at high temperatures and high pressures are presented here. The Vp of samples at ordinary temperature ranges from 6. 007 km·s-1 to 6. 803 km·s-1, given the maximum pressure at 2. 0 GPa, and the andesitic volcanoclastic rock has the largest Vp. At the temperature of 600℃, the Vp varies between 5. 871 km·s-1 and 6. 658 km·s-1, and the largest Vp value is obtained also for andesite. Combined with the velocity structure derived from seismic converted wave in study area, here we propose that the crustal structure in the most areas of Tarim Basin is of three layers: the upper crust consists mainly of granitic metamorphic rocks, the middle crust is mainly made of granitic diorite, and the lower crust is mainly composed of andesitic basalts.
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Musa volcano is situated at the central part of the Akan-Shiretoko volcanic chain, which belongs to the Kurile arc, eastern Hokkaido. In this region, Pliocene subaqueous volcanism (Yunosawa, 574m highland, and 626m-peak volcanoes) has changed to terrestrial one (Musa volcano) in early Pleistocene. Musa volcano is composed of a cluster of several stratovolcanoes and lava domes, and is topographically divided into two stages, older and younger. K-Ar age of the andesite from the younger stage has been obtained to be 0.48 ± 0.19 Ma. The eruptive rocks of Musa volcano ranges from basaltic andesite to dacite, and are defined as low-K series of Gill (1981). K2O content of the rocks increases from the volcanic front (Musa and Mashu volcanoes) to the back arc side (Shari, Shiretoko-Iwo volcanoes). The zonal distribution of lava chemistry (eg. K2O) in the southern part of the Kurile arc transverses the echelon arrangement of the volcanoes.
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