TERRANE CONCEPT, HELP OR HINDRANCE? AN ALASKA PERSPECTIVE

A flurry of major mapping projects in the 1970’s and 1980’s in Alaska led to the recognition that certain packages of rocks were fault-bounded, had distinct lithologies and unique geologic histories. This in turn led to the development of the terrane concept as a modification of 1960’s plate tectonics. An important impact of the terrane concept is that it served to focus studies in Alaska on what were perceived as “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.

However, in the years since, a number of insights call into question the definition and application of the terrane concept in Alaska. (1) Application of the definition of what constitutes a terrane has been spotty; bounding faults are commonly left undefined, included stratigraphy only loosely defined, ultimately resulting in no fixed or even necessarily agreed upon definition of any given terrane. (2) As more information becomes available, linkages between disparate terranes are apparent, suggesting common geologic histories. (3) The poorly defined terms, overlap sequence and basement, used with respect to terranes add confusion. 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?

Application of the terrane concept in Alaska is regionally controlled. It is most commonly and unreservedly applied in southern Alaska; moving northward there are fewer well-defined terranes, until virtually the entire northern quarter of the state is one terrane having uncertain boundaries and limits. Yet, it is unlikely that the regional tectonic complexity of Alaska markedly changes, rather it reflects the challenges in applying the terrane concept to the poorly constrained 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.