New Directions

In the 1970s, profound changes appeared in the ways in which deformational fabrics were being addressed and interpreted. The clearest signal of this to the broader structure-tectonic community occurred as an awakening,’ with the publication of the new structural geology textbook written by Hobbs, Means, and Williams (1976). This text bore the unprepossessing name: An Outline of Structural Geology. Inside this ‘outline’ were riches for the structure-tectonics community in the United States, including those of us working on metamorphic core complexes. The book contained detailed and accessible overviews of “principles of microstructural development,” “microstructures developed in rocks undergoing deformation,” “crystallographic preferred orientations in deformed rocks developed by slip and rotation,” “crystallographic preferred orientations in deformed rocks developed by recrystallization,” and “mylonite zones.” Some of the new insights were built upon a surge of new 1970’s contributions on mylonites, mica fabrics, and quartz fabric deformation mechanisms (e.g., Bell and Etheridge, 1973, 1974; Elliot, 1972; Lister, 1974; Lister and Hobbs, 1974; Lister and Paterson, 1974; Etheridge, Paterson, and Hobbs, 1974; and Tullis, Christie, and Griggs, 1973).

Compton (1980) had grasped this before he framed an important part of his work on deformed quartzites in the Raft River Mountains (Utah) through addressing deformation mechanisms and fabrics. His research there included attempts to partition “pure shear” versus “simple shear” based on quartz-petrofabric analysis. I now note with some interest that his contribution to GSA Memoir 153 included just three bibliographic citations related to deformation mechanisms and the behavior of quartz: Lister’s (1977) discussion on fabrics in quartzites plastically deformed by plane strain and progressive simple shear; Tullis’ (1977) paper on preferred orientation of quartz produced by slip during plane strain; and Lister, Paterson, and Hobbs’ (1978) model for fabric development in plastic deformation and its application to quartzite.

Most investigators of metamorphic core complexes in 1977 had not yet been personally impacted by the new opportunities, methodologies, and concepts that were coming available through the emerging literature. I can illustrate this with a simple, single observation. Max Crittenden, Peter Coney, and I wrote a brief “Penrose Conference Report” which was published in Geology in 1977. In that 2-page write-up there is not one reference to “mylonite” or “cataclasite.” Furthermore, the post-Penrose papers presented (just 5 months after the conference) at the 1977 National Meeting of the Geological Society of America in Seattle contain just two abstracts in which “mylonite” fabrics are referred to as such (G. A. Davis and others, 1977; G. H. Davis, 1977), in spite of the fact that all of the investigators had clear first-hand awareness of the penetrative foliations, lineations, and zones of cohesive breccias so dominant in the core complexes and so essential to interpretive models.

However, things were changing fast. By the time the GSA Memoir on core complexes was published in 1980, “foliations” and “lineations” were prefixed abundantly by the term “mylonitic,” and references to various types of cataclasites, including “microbreccias,” were being made with much greater discernment and confidence. The Citation Index reveals and calibrates certain aspects of when things began to change, and how fast. It is fascinating to harness the Citation Index and explore the frequency of investigators citing one another with respect to the topics of “shear zones,” “sense of shear,” and “fault rocks,” and then to compare these with frequency of citations for the topic “metamorphic core complexes.” There are literally no citations of the topics “fault rocks” and “sense of shear” before 1985, and essentially no references to the topic of “shear zones” before 1985. In 1991, the citations for each of these three topics ramped up profoundly. By the year 2007, “shear zones” enjoyed ~10,000 citations, “fault rocks” ~8000, and “sense of shear” ~3000 per year. The topic-citation numbers for “metamorphic core complexes” is quite similar to that for “shear zones” (except for raw numbers) in that the first citation is 1979 and the jump-off point is 1991. In 2007 total topic citations for “metamorphic core complexes” numbered ~1300.

It is not sufficient to view these Citation Index records as indicating that coherent new literatures had arrived, which they had. Rather, the Citation Index reveals a sustained surge in ‘cross-talk,’ one that began happening in ~1980.