Miles from ridge axis Plate Tectonics: a breakthrough Brian Mason (Scripps) led a group that studied a 2-D area spanning the Mid-Atlantic Ridge in detail An explanation of the curious magnetic anomaly pattern
Convergent boundary Three sub-types l Ocean-Continent l Ocean-Ocean l Continent-Continent Can you name an example of each?
Convergent boundary l Ocean-Continent: Andes, Cascades l Ocean-Ocean: Aleutians, Japan l Continent-Continent: Himalaya, Alps
Convergent boundary l Trench and subduction zone l Earthquakes l Linear chain of andesitic volcanoes (granites below) l Creation of mountain ranges (also linear chains) F Andean type - continental arc F Himalayan type - collisional (a terminal type)
“Andean-type” orogenesis Continental crust thickens by addition of magma from the subduction zone Compression due to plate convergence
“Himalayan-type” orogenesis Begins as Andean-type
“Himalayan-type” orogenesis How do you locate the suture zone today? How can you determine the “polarity” of subduction?
Slivers of oceanic crust and upper mantle (ophiolites) become incorporated into the “mélange” in the accretionary wedge of deformed sediments The “suture zone” is marked by the mélange and particularly by the occurrence of ultramafic rocks composing the mantle portion of the ocean lithosphere
Chain of ultramafic bodies in Vermont indicating a suture zone of the Ordovician Taconic Orogeny. The ultramafics mark a closed oceanic basin between North American rocks and an accreted island arc terrane. From Chidester, (1968) in Zen et al., Studies in Appalachian Geology, Northern and Maritime. Wiley Interscience.
Appalachia n History Can “accrete” island arc terranes as well as continents
Igneous Processes l Decompression partial melting at divergent zones
Igneous Processes l Partial melts: low-T fraction is always more Si-Al-Na- K-rich and Fe-Mg-poor than source rock. l Leaves behind Mg-Fe-rich refractory residue l Decompression partial melting at divergent zones
Igneous Processes l Hot spots and mantle plumes l Subduction zones: Conveyor of basalt to melt andesite F Water lowers melting point of mantle wedge
Igneous Processes l Subduction zones: water lowers melting point