THE TERRANE CONCEPT – NEED IT EVOLVE OR HAS IT OUTLIVED ITS USEFULNESS? A PERSPECTIVE FROM ALASKA

The development of plate tectonic theory in the 1960’s and recognition that certain packages of rocks had distinct lithologies and unique geologic histories led to development of the terrane concept in the 1970’s. As defined, terranes are fault-bounded and have distinctive geologic histories relative to their neighbors, having been tectonically juxtaposed. An important impact of this concept is that it serves to tear Alaska apart into an assortment of exotic blocks, literally derived from around the globe. Ultimately, by the early 1990's, more than 70, sometimes overlapping, terranes had been defined in Alaska. Yet, in the years since, a number of insights call into question the utility of the concept as applied. (1) Although there is a specific definition of what constitutes a terrane, application has been spotty. (2) As we collect more information, we discover linkages between disparate terranes, suggesting non-unique geologic histories. (3) Unlike stratigraphic nomenclature, there is no fixed or even necessarily agreed upon definition of any given terrane. (4) And finally, confusion ensues as the poorly defined terms, overlap sequence and basement, become part of the conversation. Is the basement part of a terrane or is it a part of some other pre-existing terrane? When does an overlap sequence become a terrane and when does a terrane become basement? In detail, often the bounding faults of a terrane are not apparent or mapped, so the boundaries are difficult to define.

In Alaska, the concept is most strongly and commonly applied in its southern part; moving northward there are fewer, less well-defined terranes, until virtually the entire northern quarter of the state is one terrane having uncertain limits. Yet, it is unlikely that the tectonic complexity markedly changes, rather it reflects challenges in applying the terrane concept to the known geology.

As a tool for tectonic reconstructions, terranes are commonly used as building blocks; serving as generalizations. However, when applied at mapping scales, the difficulties and challenges of applying fuzzy definitions are apparent. In the end, the existing terrane concept serves to maintain a focus on how things are different, rather than how they might go together. As utilized, this handicaps our overall understanding of the geology of Alaska and ultimately our planet.