l15 - 1 Metamorphism and Tectonics Definitions: Steady...

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1 Metamorphism and Tectonics Definitions: Steady state geotherm : Is that curve defining the change in T as a function of depth in an area that is not experiencing any tectonic activity such as a stable shield or continental interior. Transient geotherm : Is the geotherm in a tectonically active area, and will only prevail for a limited time period that depends on the duration and type of this tectonic event. Geothermal gradient : Is the slope of the geotherm at a particular time in the history of the study area. Metamorphic field gradient : Is a trajectory connecting the P-T conditions at the maximum T calculated for each metamorphic zone. According to England and Richardson (1977), the metamorphic field gradient has little physical meaning because peak temperatures in each metamorphic zone were reached at different times during the metamorphic history of the area. An overview of metamorphism in relation to tectonic regimes: The metamorphic facies series encountered in different tectonic regimes or settings can be summarized as follows, and are shown schematically on Figs. 1 and 2: (a) Ridges and rift valleys: characterized by high geothermal gradients contact and ocean floor metamorphism. (b) Areas of magmatic activity; volcanic - plutonic complexes: greenschists amphibolites granulites. (c) Areas of crustal thickening and mountain building: greenschists amphibolites granulites and type B eclogites (particularly if there are magmatic intrusions). (d) Subduction zones: Characterized by low geothermal gradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies type C eclogites. A- Convergent Plate Boundaries: I- Subduction Zone metamorphism: Rocks of the subducted plate are usually metamorphosed following "clockwise" P-T paths in which peak pressures are attained before peak temperatures. According to tectonic setting, subduction is of two "types": (a) B-type subduction: where the oceanic crust is subducted beneath a continental or another oceanic plate. This type usually results in the formation of the
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l15 - 1 Metamorphism and Tectonics Definitions: Steady...

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