Global crustal formation age map project

The determination of a volume-age curve for continental crust is a first-order priority for understanding crustal growth rates through geologic time, and thereby constraining Precambrian tectonic regimes and mantle dynamics. The rapidly growing body of Sm-Nd isotopic data in conjunction with other geochronologic and geologic data, now makes it feasible to determine crustal formation ages for large portions of the continents. With regard to crust-formation events, most Nd data on granitoid rocks suggest that there are two main types: (1) admixture of a small (less than or equal to 30%) fraction of older crust in predominantly newly-formed crust, and (2) addition of small amounts of new crust to the pre-existing material in intra-cratonic magmatic events. The inferred proportion of older crust involved depends on its age and the Nd concentration in old versus young material which may not be determinable. The authors are experimenting with data treatment procedures which will minimize bias in the data but preserve the desired information, and with different methods of presentation. The resulting maps of crustal age provide the soundest available means of evaluating continental evolution, although many more data re needed. The authors can presently illustrate out methods and provide inference on lateral andmore » vertical crustal growth histories for the Western and central United States, southeastern China, and the Arabian shield.« less