Digression on pre-1970 "Structural Petrology"

In the 1970s, when modern structural geological mapping and analysis were just beginning to be applied to terranes in the United States that were to become known as metamorphic core-complexes, relatively few geologists were paying close and informed attention to the structure-tectonic significance of what we now would refer to as “fault-rock fabrics” (especially mylonites and cataclasites) or to shear zones, and certainly not to “sense-of-shear” criteria in any way comparable to what would soon be the case.

It is my impression that prior to the 1970’s in the U.S., theoretical and practical knowledge of fabrics was held by a very small number of “structural petrologists.” Field-oriented structural geologists on the outside of this subdiscipline ‘looking in’ may have felt that microscopic petrofabrics held promising insights into interpreting structures and fabrics, but most may have concluded that functioning in this subdiscipline would have required becoming expert in petrofabrics, and incorporating routinely the use of the U-stage, or a modern pole figure device. After all, the tour de force literature seemed to be anchored in the work of Bruno Sander (1930), and not even Knopf and Ingerson (1938) were able to extract advantageous insights and applications that could be harnessed productively in the course of geological mapping and detailed structural analysis. It proved to be very difficult for many field-oriented structural geologists and tectonists to extract from the general quartz-petrofabrics literature some practical methodologies for use in their ‘real world’ of field-based structural geology and tectonics. Within this literature there is an overarching emphasis on the orthorhombic symmetry of quartz fabrics, and accordingly a proverbial gulf existed between interpreting such fabrics and interpreting shear or slip kinematics. Turner and Weiss (1963) opened the door to providing deeper insights regarding the origin and significance of mesoscopic (outcrop) and microscopic fabrics, but in the end U.S. field-oriented structural geologists found themselves in an environment where “structural petrology,” whatever that expression might have meant, remained largely in the purview of metamorphic petrologists.

Of course the literature on “mylonite” and “cataclasite” had been established very early, with publications on these rocks and fabrics going back into the 19th century, thanks to the seminal insights by Lapworth (1885). It was not a shortage of pre-1970 papers on “mylonite” and “cataclasite” that was the problem. Rather, the problem was due to a combination of other factors.

First, as noted above, few investigators addressing the structural significance of metamorphic core complexes in the late 1960’s and during most of the 1970’s had a commanding understanding of these fabrics and their significance. Notable exceptions were Peter Misch (1960), to whom GSA Memoir 153 was dedicated, Bob Compton (e.g., Compton, 1980), and Vicki Todd (e.g., Todd, 1980).

Second, the developing of a commanding understanding was made difficult by so many individual contributions in the literature on “mylonites” and “cataclasites” that were tantalizingly scant and/or vague and/or inaccurate. Higgins (1971, p. 81), in reference to the work of Conley and Drummond (1965) carried out in the Appalachian Piedmont, writes: “The terms mylonite and ultramylonite are incorrectly used for microbreccias and cataclasites. Except for this inaccuracy, the descriptions are good.”

Third, at the other extreme, descriptions could be elaborate but still confusing, with taxonomy and jargon that would delight John McPhee and his penchant for capturing the ways in which geologists love to name things in multisyllabic ways (McPhee, 1981). For example, in Higgins’ (1971, p. 89) annotation of the work by Reed and Bryant (1964) along the Brevard zone, we read: “ ‘The nomenclature of polymetamorphic cataclasite rocks is complex and confusing, so we therefore define the following terms…(p. 1180):’ …blastomylonite, blastomylonitic gneiss, phyllonite, and phyllonitic schist…They use phyllonite for rocks that should be called diaphthoritic phyllonite.” And on the work of Williams, Turner, and Gilbert (1954) in their introduction to the study of rocks in thin sections, Higgins (1970, p. 94-95) writes: “Divides cataclastic rocks into three categories...: cataclasites, mylonites, phyllonites. Defines: mylonite, pseudotachylite, augen gneiss, cataclasite, phyllonites. Discusses flaser granite and flaser gabbro. All definitions relatively general. “

Fourth, the literature, when viewed as an entire body, can seem internally contradictory even at first-order. When Higgins (1971) published his U.S. Geological Survey Professional Paper on “cataclastic” rocks, he caught the fresh attention of the broad structural geologic community, particularly because he reviewed all the literature, provided a glossary of terms as well as annotations for most of the individual contributions, and laid out a classification system that taught how to use the terminology in an informed and systematic way. But even this work held its own first-order confusing dimensions. Higgins used “cataclastic rocks” as a general term for all fault rocks ranging from fault breccias and fault gouge; through microbreccia and cataclasite; through protomylonite, mylonite, and ultramylonite; to mylonite gneiss and blastomylonite. Yet Higgins observes this was in many ways opposite to the taxonomy of Christie (1960), who applied the expression “mylonitic rocks” as the overarching term for all of what Higgin’s (1971) later described as “cataclastic rocks.”

Fifth, few investigators took advantage of the insights clearly and accurately available in the older literature. There were missed opportunities in leveraging what had been learned about the character, and changing character, of mylonites and cataclasites in certain belts of mylonite. It was as if field geologists attempting to understand mylonites and cataclasites were focusing too much on the taxonomies and microscopic properties and insufficiently on some critical observations on map patterns of these fault rocks.” There are some illustrations which apply particularly to core complex terranes. For example, Peach and others (1907), based on their work on the Moine thrust, concluded that “flinty crush-rocks and mylonites were…formed simultaneously in different parts of the same line of movement’” (Higgins, 1971, p. 87). Termier and Boussac (1911) “describe a complete transition between undeformed granite and mylonite” (Higgins, 1971, p. 92). Phillips (1937) notes “In a crushed quartzose rock, lenticular grains of highly strained quartz lie in a fine-grained mylonitic matrix. … this mylonite presents an interesting example of the differential yield of quartz grains to the shearing according to the attitude of their internal structure in relation to the impressed shear planes” (Higgins, 1971, p. 88). Armstrong (1941) describes zones that “outline lenticular masses of less sheared rocks and in some places attain a thickness of a quarter of a mile” (Higgins, 1971, p. 78). Based on their work in Tanganyika, Sutton and Watson (1959) distinguish and map “wide ‘shear belts” and thin ‘mylonite belts’. The mylonite gneisses can be arranged in a series showing increasingly complete mechanical breakdown, while the noncataclastic types form a second series which appears to show increasing degrees of crystallization’ “ (Higgins, 1971, p. 92).