The Continental Crust

Though we know quite a lot about the upper crust, there is still quite a lot of uncertainty about the lower crust. Is there a real Conrad discontinuity separating the lower from the upper crust? Is the lower crust made up of dry granulite-facies rocks. Is it more mafic than the upper crust, perhaps as a result of intrusion of mafic magmas into the lower crust (called "underplating"). Or is it more mafic as a result of extraction of silicic granitic magmas from the deep crust?

There a number of regions where we think tectonic activity has brought segments of the lower crust up for inspection. Notable examples are Kapuskasing and Pikwitonei in Canada (Precambrian crust), the Lewisian of NW Scotland (also Precambrian), Calabria in S. Italy and the Ivrea Zone in the Alps (both Phanerozoic).

Fig. 2. Diagram (based on field and geophysical studies) to show how deep crust can be thrust up to high crustal levels. Kapuskasing structure, Ontario, Canada). The Lewisian of NW Scotland can be interpreted similarly, the high-grade lower crustal granulites being thrust over amphibolites >2.5 Gyr ago.


Fig. 3. Estimates as to the extent to which we have sections thru two segments of Archaean crust and one segment of ‘recent’ Alpine crust. There is uncertainty as to whether actually we can directly sample the lowest crust. The Alpine sequence is in a series of thrust slices, but some rocks in region of the Ivrea zone have been down to greater depths and have re-bounded again to the surface. Perhaps the more important question is what causes high-P rocks to exhume?