Volcanic Arc Region Research Articles

The Mono Arch, eastern Sierra region, California: dynamic topography associated with upper-mantle upwelling?

A broad, topographic flexure localized east of and over the central and southern Sierra Nevada, herein named the Mono Arch, apparently represents crustal response to lithospheric and/or upper-mantle processes, probably dominated by mantle upwelling within the continental interior associated Pacific–North American plate-boundary deformation. This zone of flexure is identified through comparison between the topographic characteristics of the active Cascade volcanic arc and backarc regions with the analogous former arc and backarc in the Sierra Nevada and eastern Sierra Nevada. Serial topographic profiles measured normal to the modern Cascade backarc reveal an accordance of topographic lows defined by valley floors with an average minimum elevation of ∼1400–1500 m for over 175 km to the southeast. Although the accordance drops in elevation slightly to the south, the modern Cascade backarc region is remarkably level, and is characterized by relief up to ∼750 m above this baseline elevation. By contrast, serial topographic profiles over the former arc and backarc transitions of the eastern Sierra region exhibit a regional anticlinal warping defined by accordant valley floors and by a late Miocene–early Pliocene erosion surface and associated deposits. The amplitude of this flexure above regionally flat baseline elevations to the east varies spatially along the length of the former Sierran arc, with a maximum of ∼1000 m centred over the Bridgeport Basin. The total zone of flexure is approximately 350 km long N–S and 100 km wide E–W, and extends from Indian Wells Valley in the south to the Sonora Pass region in the north. Previous geophysical, petrologic, and geodetic studies suggest that the Mono Arch overlies a zone of active mantle upwelling. This region also represents a zone crustal weakness formerly exploited by the middle-to-late Miocene arc and is presently the locus of seismic and volcanic activities. This seismic zone, which lies east of the Sierra Nevada block, includes the northern and central Walker Lane in Nevada and the eastern California shear zone to the south in California. Vertical patterns of surface deformation over this region are characterized by west tilting of the Sierra Nevada block and by block faulting of a late Miocene erosion surface east of the Sierra Nevada.

Melting of carbonated pelites at 2.5–5.0 GPa, silicate–carbonatite liquid immiscibility, and potassium–carbon metasomatism of the mantle

Melting experiments on a Fe-rich carbonate-saturated pelite were performed at 850–1300 °C and 2.5–5.0 GPa to define melting relations, melt compositions, and the conditions under which carbonates remain residual. In the selected fertile bulk composition, 30 wt.% potassic granite (2.5 GPa) or phonolite (5.0 GPa) melt is generated at the fluid-absent solidus. The temperature of the latter increases from 900 °C at 2.4 GPa to 1070 °C at 5.0 GPa. Phengite+quartz/coesite control initial silicate melting and melt productivity through the reaction phengite+quartz/coesite+clinopyroxene+calcite=silicate melt+kyanite+garnet, which leaves most of the Fe–Mg–calcite in the residue. Na remains compatible in clinopyroxene (DNacpx/melt=3.1 to 7.3 at the fluid-absent solidus), resulting in silicate melts with K2O/Na2O wt-ratios of 5.8–8.6. Such highly potassic carbonated silicate melts represent ideal metasomatic agents for the source mantle of group II kimberlites.From 3.7 to 5.0 GPa, Fe–Mg–calcite disappears only through the formation of Ca–carbonatite at 1100 °C. The experiments provide a possible source for Ca–carbonatites in combination with alkaline granitic to phonolitic melts at temperatures unlikely to be achieved during ongoing subduction. Large scale carbonate transfer to the subarc mantle can thus only be achieved when burying rates slow considerably down or subducted crust becomes incorporated into the mantle. Consequently, it is likely that carbonates will not be extensively mobilized in a typical subarc region, thus extending and confirming earlier results from subsolidus studies (Connolly, J.A.D., 2005. Computation of phase equilibria by linear programming: a tool for geodynamic modelling and its application to subduction zone decarbonation. Earth Planet. Sci. Lett. 236, 524–541.), that >70–80% of the subducted carbonate will bypass the volcanic arc region and get buried to larger depths.

Geographical Distribution of 3He/4He Ratios in the Chugoku District, Southwestern Japan

We have collected 34 hot spring and mineral spring gases and waters in the Chugoku and Kansai districts, Southwestern Japan and measured the 3He/4He and 4He/20Ne ratios by using a noble gas mass spectrometer. Observed 3He/4He and 4He/20Ne ratios range from 0.054 Ratm to 5.04 Ratm (where Ratm is the atmospheric 3He/4He ratio of 1.39 × 10−6) and from 0.25 to 36.8, respectively. They are well explained by a mixing of three components, mantle-derived, radiogenic, and atmospheric helium dissolved in water. The 3He/4He ratios corrected for air contamination are low in the frontal arc and high in the volcanic arc regions, which are consistent with data of subduction zones in the literature. The geographical contrast may provide a constraint on the position of the volcanic front in the Chugoku district where it was not well defined by previous works. Taking into account the magma aging effect, we cannot explain the high 3He/4He ratios of the volcanic arc region by the slab melting of the subducting Philippine Sea plate. The other source with pristine mantle material may be required. More precisely, the highest and average 3He/4He ratios of 5.88 Ratm and 3.8±1.6 Ratm, respectively, in the narrow regions near the volcanic front of the Chugoku district are lower than those in Kyushu and Kinki Spot in Southwestern Japan, but close to those in NE Japan. This suggests that the magma source of the former may be related to the subduction of the Pacific plate, in addition to a slight component of melting of the Philippine Sea slab.

05/01366 Representing energy technologies in top-down economic models using bottom-up information

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920 mg/kg) and Nb (245 mg/kg), in the coastal sand. The low REE concentration (ΣREE + Y = 240 mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE.The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions.

Sedimentary processes and development of the Zenisu deep-sea channel, Philippine Sea

Zenisu deep-sea channel originated from a volcanic arc region, Izu-Ogasawara Island Arc, and vanished in the Shikoku Basin of the Philippine Sea. According to the swath bathymetry, the deep-sea channel can be divided into three segments. They are Zenisu canyon, E-W fan channel and trough-axis channel. A lot of volcanic detritus were deposited in the Zenisu Trough via the deep-sea channel because it originated from volcanic arc settings. On the basis of the swath bathymetry, submersible and seismic reflection data, the deposits are characterized by turbidite and debrite deposits as those in the other major deep-sea channels. Erosion or few sediments were observed in the Zenisu canyon, whereas a lot of turbidites and debrites occurred in the E-W channel and trough axis channel. Cold seep communities, active fault and fluid flow were discovered along the lower slope of the Zenisu Ridge. Vertical sedimentary sequences in the Zenisu Trough consist of the four post-rift sequence units of the Shikoku Basin, among which Units A and B are two turbidite units. The development of Zenisu canyon is controlled by the N-S shear fault, the E-W fan channel is related to the E-W shear fault, and the trough-axis channel is related to the subsidence of central basin.

Geochemistry of arc volcanic rocks in Central Luzon, Philippines

The different volcanoes and volcanic centers in Central Luzon, Philippines define two volcanic chains - the Western and Eastern Volcanic Chains. The Western Volcanic Chains include volcanic rocks generated in the forearc and main volcanic arc regions with respect to the Manila Trench. The Eastern Volcanic Chains, on the other hand, were extruded on the back arc side. Across- and along-arc variations are present in these two volcanic chains. These variations can be attributed to the interplay of several geochemical processes that occuned in source regions that manifest island arc affinity. An adakite-tholeiiticlcalc-alkaline-adakitic rock across-arc variation is recognized in Central Luzon.

The upper mantle beneath the Philippine Sea region from waveform inversions

We present a three‐dimensional S‐velocity model for the upper mantle beneath the Philippine Sea region. It was derived from inversions of 281 broad band vertical‐component seismograms recorded in the area at the Global Seismological Network (GSN) and SKIPPY portable array stations. We have been able to obtain high‐resolution tomographic images spanning the depths down to 200–300 km and locally down to the upper transition zone. High‐velocity subducting slabs and low‐velocity volcanic arc regions are the dominant features of the model. Fast, thin lithosphere of back‐arc basins is underlain by a prominent low‐velocity zone. Low velocities at lithospheric depths are observed beneath the extinct Central Basin Ridge in the West Philippine Basin and close to the Eauripik Ridge that separates the East and West Caroline basins. High upper‐mantle heterogeneity and resulting scattering presents a difficulty and limits the resolution, especially below 200–300 km. Explicit modeling of seismic wave diffraction may be necessary for a significant improvement in resolution.

Compositional trends in the Permian sandstones from the Denison Trough, bowen basin, queensland reflect changing provenance and tectonics

Point-count data of sandstone samples collected from six sedimentary formations encountered in boreholes in the Permian sequence of the Denison Trough (an Australian backare/ retroarc foreland basin) provide information about the tectonic evolution of their source areas and the depositional sites. Plots of the detrital compositions of these Permian sandstones against borehole depths indicate that the Reids Dome Beds sandstones, containing highly abundant lithic/volcanolithic grains, mark the onset of intense compressional tectonics and arc volcanic activities to the east accompanied by extensional tectonics and subsidence in the trough. Folding and thrusting owing to intense compressional tectonics in the New England Fold Belt and Arc volcanism in the Camboon Volcanic Arc region to the east generated a great supply of recycled sediment and volcanic detritus that led to a rapid infilling of the subsiding Denison Trough during formation of the Reids Dome Beds sandstones in Early Permian time. The detrital composition of the sandstones of the overlying Cattle Creek Formation, Aldebaran Sandstone, and the Freitag Formation indicate gradually decreasing compressional tectonics and arc volcanism accompanied by increasingly dominant sediment-supply from the stable continental craton to the west. The Freitag Formation sandstone compositions mark the quietest tectonic and volcanic episode in the region, accompanied by subsidence of the Denison Trough owing to thermal cooling during the Middle Permian. The Peawaddy Formation and the Bandanna Formation sandstone compositions represent a Late Permian renewal of compressional tectonics and arc volcanism to the east accompanied by uplift and folding of the sedimentary strata, resulting in the formation of largely fluvial depositional environments in the Denison Trough. Modal compositions of the Bandanna Formation sandstones indicate that renewed arc volcanism and compressional tectonic activity attained maximum intensities during the Late Permian.

Regional geochemistry of sediments from the Hellenic volcanic arc in regard to submarine hydrothermal activity

Some 600 surface sediment samples from the Hellenic Volcanic Arc region have been analyzed by bulk and HCl selective leach geochemical techniques. Geostatisti‐cal analysis, followed by further selective leaches on a number of samples, has been used in order to identify any regional hydrothermal influence on the sediments and/or any local hydrothermal metal enrichments. The two main sediment components affecting the geochemical variability in the region are biogenic carbonate and volcaniclastic/terrigenous detritus. Proximity to islands appears to be the primary factor controlling the variability in these components. Sediments from deeper water, to the south of the Hellenic Volcanic Arc and in the Anydros Basin, exhibit some elevated levels of Mn, Co, and Cu, probably of hydrogenous origin. No widespread hydrothermal influence to the sediments in the region is recognized, although this could in part be due to the high sedimentation rates in the region. Localized hydrothermal inputs to sediments are, howeve...

Stress Fields of the Overriding Plate at Convergent Margins and Beneath Active Volcanic Arcs

Tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales. Volcanic arcs are a common element of overriding plates. Stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes. Analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc. A process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region.

Relationship between tectonics, volcanism, sedimentation and basin development, Late Cenozoic, central part of Northern Honshu, Japan

A pronounced interplay between tectonics, volcanism and sedimentation can be established in the volcanic arc region of the central part of Northern Honshu during the Early Miocene to Quaternary interval. Four stages are recognized with the type of volcanism and sedimentation varying according to the tectonic stress regime. 1. (1)| Rifting stage (22?–15 Ma). NS-trending half grabens were formed under an extensional stress regime related to the rifting of the Sea of Japan. Basins were rapidly filled by intermediate to felsic volcanic products, which are often associated with subaerial fluvial and lacustrine sediments. 2. 2)| Back-arc Basin Opening Stage (15-13 Ma). With more advanced stagess of rifting, subsidence from crustal stretching resulted in marked marine incursion, and an overall transgressive succession developed. During this stage, the back-arc side of Northern Honshu subsided rapidly to middle bathyal environments. At the same time, associated with the opening of the Sea of Japan, large volumes of regionally extensive mafic lavas and associated hyaloclastites were erupted within submarine grabens. In contrast, in the present Backbone Ranges, felsic volcanism occurred within bathyal to shallow marine environments. Igneous rock types in this stage display a bimodal mafic-felsic chemistry. 3. (3)| Transitional Stage (13-2.4 Ma). (a) Thermal Subsidence Substage (13-8 Ma). An intermediate stress regime developed, which included phases of both weak extension and compression. Consequently, grabens along the Sea of Japan ceased to form. Later, lithospheric cooling eventually led to thermal subsidence of the back-arc region, and hemipelagic sediments mantled the previously active mafic volcanic apron. Volcanism at this time was mainly restricted to a narrow zone along the present Backbone Ranges where felsic to intermediate volcanic products accumulated within shallow marine environments. (b) Incipient Compressional Substage (8-2.4 Ma). Hemipelagic sedimentation continued on the back-arc side of Northern Honshu. Along the present Backbone Ranges, the intermediate stress regime continued, with formation of many large caldera structures associated with subaerial felsic volcanism. 4. (4)| Shortening Deformation Stage (2.4 Ma-present). The change of the absolute direction of motion of the Pacific plate at the Early to Middle Pliocene changed the regional stress regime of the volcanic arc from intermediate to compression. Movement of back-arc thrusts along the Sea of Japan coast began 2–3 Ma and led to uplift of the volcanic arc. Continued compression in the volcanic arc produced narrow uplifted zones, with associated basins bounded by reverse faults. These basins were filled rapidly by terrestrial sediments containing upward-coarsening sequences. Under this highly compressional tectonic stress regime, volcanism became intermediate-mafic in composition, with large stratovolcanoes becoming prominent.

Helium isotopes and tectonics in southern Italy

Geodynamic evolution of southern Italy can be understood within the framework of the Mediterranean‐Alpine System. Subduction of a plate along the Sicily‐Calabrian forearc under the Tyrrhenian Sea has been suggested by many geophysicists, although it is not yet confirmed and remains somewhat controversial. Helium isotope ratios provide useful information on the geotectonic structure of the region. We report here the ³He/4He ratios of terrestrial gas samples from southern Italy. The observed ³He/4He ratios are relatively high in the Eolian volcanic arc region and low in the other areas. Dichotomous explanations are presented. Firstly volcanic arc‐forearc hypothesis suggests the subduction along the Sicily‐Calabrian forearc. Secondly horizontal transport hypothesis is described based on the relationship between the ratios and radial distance from the recent spreading basin in Southern Tyrrhenian Sea.

Helium isotope ratio and heat discharge rate in the Hokkaido island, Northeast Japan.

We have collected 21 gas samples of various kinds such as volcanic gas, hot spring gas and CH4-rich natural gas in the Hokkaido Island. Helium and neon in the samples were purified in a vacuum line and the 3He/4He and 4He/20Ne ratios were measured by a mass spectrometer. Observed 3He/4He and 4He/20Ne ratios vary significantly from 0.242 × 10-6 to 9.58 × 10-6 and from 0.53 to 96, respectively. There is no correlation between the 3He/4He and 4He/20Ne ratios. Taking into account present data as well as published ones, geographical distribution of 3He/4He ratios indicates a clear contrast, high in the volcanic arc and low in the forearc region. The distribution pattern agrees well with the data of Northeast Japan (Sano and Wakita, 1985) and those of the North Island of New Zealand (Sano et al., 1987). Higher 3He/4He ratios than air in the volcanic arc region can be well explained by the presence of an uprising magma. Lower 3He/4He ratio in the forearc region is attributed to the radiogenic helium accumulated in crustal rocks. A positive correlation is found between the common logarithms of 3He/4He ratio and heat discharge rate of hot springs in the area. The tendency can be explained by a combination of magma aging and the dilution of magmatic fluids by local groundwater.

Island arc tectonics of New Zealand manifested in helium isotope ratios

Abstract Thirty gas samples of various types—volcanic gas, geothermal gas, CH4-rich natural gas, and petroleum gas—were collected in and around the North Island, New Zealand. 3 He 4 He and 4 He 20 Ne ratios were measured using a mass spectrometer. The observed 3 He 4 He and 4 He 20 Ne ratios of the samples varied from 0.08 × 10−6to 8.75 × 10−6 and from 4.8 to 2100, respectively. Based on a 3 He 4 He 4 He 20 Ne diagram. He in the samples can be explained by mixing of a subduction-type magmatic, a radiogenic, and an air component. The geographical distribution of 3 He 4 He ratios over the island indicates a clear contrast between forearc and volcanic arc regions: high in the former and low in the latter. A He boundary can be drawn from the Taupo Volcanic Zone to Egmont Volcano in terms of the 3 He 4 He profile. This boundary agrees well with that of high Q/lowQ in the upper mantle as well as with the geographical distribution of terrestrial heat flow values. The high 3 He 4 He ratios in the volcanic arc region are attributed to subduction-type He associated with an uprising magma from the upper mantle and the lower ratios in the forearc region to radiogenic He derived from U and Th in basement and sedimentary rocks. This tendency is very similar to that found in northeast Japan and is understood to be a general signature of the He isotope ratio in an island arc system.

Geographical distribution of 3He/4He ratios in Japan: Implications for arc tectonics and incipient magmatism

The 3He/4He and 4He/20Ne ratios of 115 natural gas samples, including various types of gases from volcanoes, hot springs, mineral springs, water wells, petroleum fields, and natural gas fields, were measured using a mass spectrometer. The observed 3He/4He and 4He/20Ne ratios range from 7.47×10−8 to 9.65×10 −6 and from 0.26 to 1100, respectively. The 3He/4He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3He/4He ratios between the frontal arc (forearc) and volcanic arc (back arc) regions. Lower ratios were found in the trench side region and higher ratios in the back arc side. This result suggests that the mantle‐derived helium in the volcanic arc region is associated with the diapiric uprise of a magma. Lower 3He/4He ratios in the frontal arc region may be due to radiogenic He produced by radioactive decay of U and Th in the crustal and sedimentary rocks. In southwestern (SW) Japan there is no clear geographical contrast in the 3He/4He ratios. Some samples in the frontal arc region show quite high 3He/4He ratios. The tendency of the 3He/4He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3He/4He ratios observed in the frontal arc region in SW Japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab.

Areal distribution of [formula omitted] ratios in the Tohoku district, Northeastern Japan

Natural gas samples, including various types of gases from hot springs, mineral springs and water wells, were collected at nine sampling sites in the Tohoku district, northeastern Japan. The 3 He 4 He and 20 Ne 4 He ratios of these samples were measured using a mass spectrometer. The ranges of observed 3 He 4 He and 20 Ne 4 He ratios of gas samples are from 3.56·10 −7 to 9.01·10 −6 and from 2.9·10 −3 to 5.2·10 −1, respectively. The temporal variations in 3 He 4 He ratios observed for two sampling sites show little fluctuation over a period of 28 months. Samples collected in the forearc region have radiogenic He ratios, being about one-third of the atmospheric value, whereas in the volcanic arc region, the ratios are high with values 2.5–6.4 times the atmosphere. The 3 He 4 He ratios in the Tohoku district show good correlation to the subduction-related tectonic structure of the island-arc system. These observed variations in 3 He 4 He ratios may reflect the state of material flow associated with magma generation.

The Tremadoc series of China

A review of the distribution of Tremadoc successions in China is presented in terms of their occurrences in various depositional settings: shelf regions (Tarim, Yangzi and North China), shelf margins (Jiangnam and NE flank of Tarim), slope regions (South‐east China and NE of Tarim) and ocean basin or volcanic arc regions. The biostratigraphical schemes for correlating the Tremadoc rocks of China are outlined. Brief comments are also made on the position of the Cambrian‐Ordovician boundary in China.

Lead isotopes in island arcs

New lead isotope data for calc-alkaline volcanic rocks from New Zealand and the Lesser Antilles, combined with published data for Japan and the Andes, show that the spread of isotopic composition in each volcanic arc region is small (2–4% range in Pb206/Pb204) compared to the range of values observed (8%). Pb207 and Pb206 increase systematically from Japan to the Andes to New Zealand to the Caribbean. Likewise Pb208 and Pb206 are positively correlated, but there is evidence of long term (108 m.y.) differences of Th/U between the regions studied.