Contact metamorphism and laboratory experiments demonstrate that sedimentary rocks can be metamorphosed by heating. In the case of contact metamorphism, the heating happens because of a magmatic intrusion: a heat source is brought to the sedimentary rocks. What is the heat source for regional metamorphism?
Figure 2.06. Continental geotherms. Gulf coast drill hole temperatures (GM), and continental model geotherms from heat flow data (HC, PC). Click on the image to see a larger version with more information.
Temperature increases with depth in the earth. This is known from temperatures in deep mines and from temperatures measured in countless drill holes. Figure 2.06 shows average temperatures from oil and gas drill holes in the Gulf of Mexico as a function of depth (GM) and also two model continental geotherms (HC, PC). If sediment deposited at the earth's surface can be buried or transported into the lower continental crust, it would be heated.
Deep burial by accumulation of sediment is what has happened in the Gulf of Mexico, and the drill hole data show that the sediment has been heated. However, the trajectory of the Gulf of Mexico measured temperatures with depth does not reach the T-P conditions of the aluminosilicate triple point (see Figure 2.06), conditions commonly achieved during regional metamorphism. Instead, sediment accumumation leads to only modest heating and recrystallization termed burial metamorphism by the IUGS (Fettes and Desmons, 2011).
Subduction transports ocean crust and sediment to depth, so it is reasonable to look at subduction zone processes as a way to produce metamorphic rocks.
Figure 2.07. Subduction Zone Temperatures. Temperature contours in degrees Celsius based on a subduction zone model by Kelemen and others (2003, figure 6B). Click on the image to see a larger version with more information.
