TERRANES B

                                                    Lopez Island

EARLY EVOLUTION OF THE “TERRANE” CONCEPT

Early on, when I first began to believe that large chunks of crust were allochthonous* with respect to interior North America I simply referred to them as “displaced crustal blocks”.  Some others interested in this line of work called them “micro-continents”, but that particular awkward terminology  quickly disappeared, although it left the term “microplate tectonics” behind as a memento.

The “official” terrane definition arose out of a GSA Penrose Conference held on Lopez Island, Washington, in the summer of 1980.  A report on that conference was published in the GSA journal later that same year:

            Beck. Cox and Jones, Mesozoic and Cenozoic microplate tectonics of western North America, Geology, v. 8, pp. 454-456, 1980.

The order of authorship is alphabetical; Davey wrote most of it.

Anyway, this “official” definition arose from three fun-filled days of argument, field trips and liquid refreshment.  Here it is::

A tectonic (or tectonostratigraphic) terrane is a fault-bounded geologic entity characterized by a distinctive stratigraphic sequence  and/or structural history differing markedly from its adjoining neighbors ….

Later, of course, new terms arose:

            Suspect terrane:  A chunk of crust that may fit the definition given above, but not for sure.

            Exotic terrane: A tectonostratigraphic terrane that didn’t originate as part of North America.

*Two useful words that geologists toss around:

            Autochthonous:  Formed right where it is now.

            Allochthonous:  Formed somewhere else than where          it is now.

Thus, the B.C. terranes Nick is describing are allochthonous, whereas the young cover rocks that partially hide them are autochthonous.