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Transcript
Crustal genesis: at least a two-step process • Hot subduction zones create ‘granites’ • Weathering & seafloor alteration: removal of some elements to mantle • Recycling of lower crust Hot subduction creates granite If hot, basaltic crust will melt to form ‘granite’ Animations courtesy T. Atwater, UCSB http://emvc.geol.ucsb.edu/ Weathering: Loss of magnesium 2. Mg (and Na, Ca?) absorbed by altered oceanic crust. 1. Chemical weathering releases soluble elements: Ca, Mg, Na, which wash into oceans 3. Mg returns to mantle, Na (and Ca?) returns to crust via arc magmatism. Animations courtesy T. Atwater, UCSB http://emvc.geol.ucsb.edu/ Lower Crustal Removal through density foundering Lithospheric mantle Eclogite From Laubcher Example from the Alps? a) relamination of subducted sediment Lower Crustal Removal through ‘relamination’ upper crust melt b) relamination of subducted intra-oceanic arc melt upper crust lower crust ed ti iz og ec l relaminant delaminant t us r c oc e lat p c ni a e c) relamination of crust removed by subduction erosion relaminant delaminant From Hacker et al.,of 2014 d) relamination subducted continental crust Figure 15. Four tectonic settings for continental refining via relamination. In all cases, the relaminating layer may be thrust directly beneath existing crust, rise en bloc, perhaps in a "subduction channel", or rise as diapirs through the mantle wedge, depending on physical conditions. In all cases, there may be melting that produces a liquid that ascends well above the relaminating layer and there may be residues that are either positively or negatively buoyant with respect to the adjacent mantle. A) Subducted sediment is thrust into or beneath arc lower crust, or is gravitationally unstable and rises to relaminate the base of the crust in upper plate. B) Subducted volcanoplutonic arcs undergo density separation as the mafic lower crust transforms to eclogite while the buoyant upper crust relaminates the base of the crust in upper plate. C) Felsic crustal material removed from upper plate by subduction erosion is relaminated to the http://www.dstaplesphotography.com/INDIANEPALandTIBET/Tibetan-plateau.html base of the crust in upper plate; mafic material transforms to eclogite and sinks within upper mantle. D) Subducted felsic continental crust is relaminated to the base of the crust in upper plate. Any (ultra)mafic material of sufficient size transforms to eclogite and returns to upper mantle. Example from the Pamir & Tibetan Plateau? Recycling of Deep Lithosphere Foundered continent Animations courtesy T. Atwater, UCSB http://emvc.geol.ucsb.edu/ Plate Tectonics is essential for continental crust genesis • Growth from magmas in subduction zones (and at hot spots) • Removal of basaltic parts (weathering & subduction, deep crustal recycling) What do other planets look like? Mercury • Ancient crust (heavily cratered) • Mg-rich Basalt • Tectonic activity • Surprises: sulfur-rich, polar water MESSENGER image Fault scarps Smooth plains 14 January, 2008 MESSENGER image Smooth lava plains 6 October, 2008 MESSENGER image Mercury • No continents • No ocean basins • No plate tectonics MESSENGER image