Lateral variation of basalt magma type across continental margins and Island Arcs
TL;DR: In this paper, the lateral variation of quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkaloic and less silicerous type (alkali olivine basalt) on a continental side.
Abstract: Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina basalt, and alkali olivine basalt are arranged parallel to the Pacific coast in the order just named, whereas in the southwestern part of the Japanese Islands, the Aleutian Islands, northwestern United States, New Zealand, and Indonesia, zones of high-alumina basalt and alkali olivine basalt are arranged parallel to the coast. In the Izu-Mariana, Kurile, South Sandwich and Tonga Islands, where deep oceans are present on both sides of the island arcs, only a zone of tholeiite is represented. Thus the lateral variation of magma type is characteristic of the transitional zone between the oceanic and continental structures. Because the variation is continuous, the physico-chemical process attending basalt magma production should also change continuously from the oceanic to continental mantle. Suggested explanations for the lateral variation assuming a homogeneous mantle are: 1) Close correspondence between the variations of depth of earthquake foci in the mantle and of basalt magma type in the Japanese Islands indicates that different magmas are produced at different depths where the earthquakes are generated by stress release: tholeiite at depths around 100 km, high-alumina basalt at depths around 200 km, and alkali olivine basalt at depths greater than 250 km. 2) Primary olivine tholeiite magma is produced at a uniform level of the mantle (100–150 km), and on the oceanic side of the continental margin, it leaves the source region immediately after its production and forms magma reservoirs at shallow depths, perhaps in the crust, where it undergoes fractionation to produce SiO2-oversaturated tholeiite magma, whereas on the continental side, the primary magma forms reservoirs near the source region and stays there long enough to be fractionated to produce alkali olivine basalt magma, and in the intermediate zone, the primary magma forms reservoirs at intermediate depths where it is fractionated to produce high-alumina basalt magma.
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TL;DR: In this paper, a system was presented whereby volcanic rocks may be classified chemically as follows: Subalkaline Rocks:A.B. Tholeiitic basalt series:Tholeitic picrite-basalt; tholeiite, tholeitic andesite; dacite; rhyolite.
Abstract: A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.
6,269 citations
TL;DR: In this paper, it is proposed that mountain belts develop by deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins, resulting from the events associated with the rupture of continents and the expansion of oceans by plate generation at oceanic ridges.
Abstract: Analysis of the sedimentary, volcanic, structural, and metamorphic chronology in mountain belts, and consideration of the implications of the new global tectonics (plate tectonics), strongly indicate that mountain belts are a consequence of plate evolution. It is proposed that mountain belts develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These assemblages result from the events associated with the rupture of continents and the expansion of oceans by lithosphere plate generation at oceanic ridges. The earliest assemblages thus developed are volcanic rocks and coarse clastic sediments deposited in fault-bounded troughs on a distending and segmenting continental crust, subsequently split apart and carried away from the ridge on essentially aseismic continental margins. As the continental margins move away from the ridge, nonvolcanic continental shelf and rise assemblages of orthoquartzite-carbonate, and lutite (shelf), and lutite, slump deposits, and turbidites (rise) accumulate. This kind of continental margin is transformed into an orogenic belt in one of two ways. If a trench develops near, or at, the continenal margin to consume lithosphere from the oceanic side, a mountain belt (cordilleran type) grows by dominantly thermal mechanisms related to the rise of calc-alkaline and basaltic magmas. Cordilleran-type mountain belts are characterized by paired metamorphic belts (blueschist on the oceanic side and high temperature on the continental side) and divergent thrusting and synorogenic sediment transport from the high-temperature volcanic axis. If the continental margin collides with an island arc, or with another continent, a collision-type mountain belt develops by dominantly mechanical processes. Where a continent/island arc collision occurs, the resulting mountains will be small (e.g., the Tertiary fold belt of northern New Guinea), and a new trench will develop on the oceanic side of the arc. Where a continent/continent collision occurs, the mountains will be large (e.g., the Himalayas), and the single trench zone of plate consumption is replaced by a wide zone of deformation. Collision-type mountain belts do not have paired metamorphic belts; they are characterized by a single dominant direction of thrusting and synorogenic sediment transport, away from the site of the trench over the underthrust plate. Stratigraphic sequences of mountain belts (geosynclinal sequences) match those asciated with present-day oceans, island arcs, and continental margins.
1,462 citations
TL;DR: In this paper, the authors used petrologic diagrams applied to analyses of volcanic rocks for construction of discriminant lines between rock series and provided coordinates for sufficient points to enable accurate plotting of the boundary lines within seven diagrams, viz.
1,150 citations
TL;DR: In this paper, the authors used plate theory to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle, and demonstrated that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature.
Abstract: Summary Plate theory has successfully related sea floor spreading to the focal mechanisms of earthquakes and the deep structure of island arcs. It is used here to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle. Comparison with observations demonstrates that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature. The flow and the stress heating in the mantle can maintain the high heat flow anomaly observed behind island arcs. Plate theory also suggests a new approach to the convection problem. The most obvious mechanism causing surface motion is the force on the plates due to the sinking lithosphere. This does not appear to be the way in which the motions are maintained. However, the input of large volumes of cold material can control convection and cause general downward movements in the mantle near island arcs. This input of cold lithosphere must cease when the island arc tries to consume a continent, since the light continental crust cannot sink through the denser mantle. Attempts to assimilate continental crust in this way can produce fold mountains, and also permit a rearrangement of convection cells.
988 citations
TL;DR: In this paper, the authors regarded the alkalic series as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics.
Abstract: The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics The alkalic series as a whole are characterized by higher Na2O+K2O content than the subalkalic series in the alkali vs SiO2 diagram At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type) Alkalic rocks of these three trends are higher not only in Na2O+K2O but also in Rb, Ba, Sr and Zr than subalkalic rocks The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite
509 citations
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01 Jan 2012TL;DR: In this article, the relative seismicity of various parts of the earth during the limited period for which accurate information is available, and identify and discuss the geographical and geological relationships of the principal zones and areas of seismic activity.
Abstract: This paper is intended: (1) to give an account of the relative seismicity of various parts of the earth during the limited period for which accurate information is available, and (2) to identify and discuss the geographical and geological relationships of the principal zones and areas of seismic activity.
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TL;DR: The boundary lines between the tholeiite and alkali petrographic provinces are located very closely to those between the areas where earthquakes occur at depths shallower than about 200 km and those for deeper ones as mentioned in this paper.
Abstract: Three petrographic provinces can be recognized in the Cenozoic volcanic fields of Japan and surrounding areas A province of a tholeiite series lies on the Pacific side of the Japanese Islands and includes the Izu Islands, whereas that of an alkali rock series occupies the Japan Sea side of the Islands with a narrow offshoot extending across central Honsyū (Honshū) and a continuation westward to Korea and Manchuria A province of a calc-alkali rock series is superposed on the two provinces and occupies the greater part of the Japanese Islands exclusive of the Izu Islands and the islands in the Japan Sea southwest of Honsyū and north of Kyūsyū (Kyūshū) The boundary lines between the tholeiite and alkali provinces are located very closely to those between the areas where earthquakes occur at depths shallower than about 200 km and those for deeper ones It is suggested that the parental tholeiite magma is produced by partial melting of the periodotite layer at depths shallower than 200 km In the Izu Islands, except Nii-zima(Nii-jima) and Kōzu-sima(Kōzu-shima) close to Honsyū, the magma erupts to the surface without assimilating granitic material because the granitic layer is absent, resulting in volcanoes made up exclusively of the tholeiite series The parental alkali olivine basalt magma is produced by partial melting of the peridotite layer at depths greater than 200 km In the Japan Sea region, Korea, and Manchuria, it erupts to the surface without assimilating the granitic material, although it passes through a thick granitic layer, resulting in volcanoes made up exclusively of the alkali series However, in the Cenozoic orogenic belt of the Japanese Islands, both types of parental magma assimilate granitic material during passage to the surface and erupt to form volcanoes of the calc-alkali series
361 citations