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Earth Systems and Cycles
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Geologists do not agree on the exact number of lithospheric plates. In part, that number depends on whether plates that were once separate, but move as a single unit today, are lumped or split. For example, the boundary between North America and the Atlantic Ocean was once an active subduction zone. Today, however, the North American continent and the Atlantic Ocean, from the east coast of the United States to the Mid-Atlantic Ridge, move together as a single plate (see Fig. 2.24). The two formerly independent plates (North America and the western half of the North Atlantic) are said to be welded together, and the boundary between them is called a passive margin (because no subduction or other tectonic activity is occurring there). Together the two former plates make up most of the present-day North American Plate. Greenland, parts of the Caribbean, a small part of Russia, and a few islands in the Atlantic Ocean are also part of the North American Plate. Today the western edge of the North American Plate is an active margin characterized by subduction and volcanism on the coasts of Washington and Oregon, the San Andreas transform fault in California, and more subduction and volcanism in Mexico. The eastern edge of the North American Plate is an active margin characterized by volcanism and a plate boundary at the Mid-Atlantic Ridge.
Magmatism and Magmatic Rocks
Published in Aurèle Parriaux, Geology, 2018
Let us consider a lithospheric plate made up of oceanic and continental parts connected by a passive margin (for example, India in the case of the formation of the Himalayas). If this plate encounters a continental plate (Eurasia is the chosen example), subduction of the oceanic part will occur first. Afterwards, the subduction will turn into a collision between two continental plates. Their low density prevents one of them from plunging into the mantle. Therefore, subduction ceases. The collision creates large mountain chains located at the heart of continental plates, like the Himalayas and the Alps. This relationship does not create volcanoes because there is no subduction.
Cathaysian slivers in the Philippine island arc: geochronologic and geochemical evidence from sedimentary formations of the west Central Philippines
Published in Australian Journal of Earth Sciences, 2018
C. B. Dimalanta, D. V. Faustino-Eslava, J. T. Padrones, K. L. Queaño, R. A. B. Concepcion, S. Suzuki, G. P. Yumul
The whole-rock geochemical data of the samples were also plotted on tectonic discrimination diagrams. Maynard, Valloni, and Yu (1982) modified the SiO2vs K2O/Na2O diagram first proposed by Roser and Korsch (1986) and used SiO2/Al2O3 instead of SiO2 alone. In this diagram, the island arc setting is further differentiated into A1, which comprises basaltic and andesitic detritus, and A2 consisting mostly of felsic-plutonic detritus (e.g. Imchen, Thong, & Pongen, 2014; Roy & Roser, 2013). The active continental margin (ACM) refers to subduction-related basins in a continental margin magmatic arc, continental collision basins or pull-apart basins associated with strike-slip fault zones, whereas passive margin (PM) refers to basins on continental crust, basins associated with ocean floor spreading, failed rifts and Atlantic-type continental margins (Roser & Korsch, 1986). The clastic rocks collected from Palawan, northwest Mindoro and northwest Panay consistently plot in both ACM and PM fields (Figure 7a). The Fe2O3+MgO and TiO2 are used as discriminants of tectonic setting owing to their low mobility and low residence times in seawater (Bhatia, 1983). The shift from oceanic island arc (OIA) to continental island arc (CIA) to ACM to PM setting is accompanied by a decrease in Fe2O3+MgO and TiO2 (Bhatia, 1983). In this discrimination diagram, the majority of the samples from Palawan and Buruanga plot in the ACM–PM fields. The Mindoro samples, however, occupy the CIA, ACM and PM fields (Figure 7b).