But Things Changed Fast

Perhaps the most significant agent of change, as well as the most tangible symbol of the changes taking place, was the Penrose Conference held in 1981 in San Diego, California, on the “Significance and Petrogenesis of Mylonitic Rocks.” The conveners were Jan Tullis, Art Snoke, and Vicki Todd (Tullis, Snoke, and Todd, 1982). As a result of this Penrose Conference, knowledge of practical field methods for discerning sense-of-shear in outcrop spread like wildfire. I did not attend, but lore has it that at the first good exposure of outcrops of mylonites Carol Simpson, Stephan Schmidt, and Gordon Lister ‘decoded’ for others how to ‘read’ sense-of-shear, through various methods and fabrics! In an instant those of us doing structural geology in the Western Cordillera became aware of how to determine sense-of-shear on the basis of outcrop examination, so fast did the word spread. “Mica fish,” “S-C,” “C’,” “shear bands,” “delta-type porphyroclasts,” and “sigma-type porphyroclasts” became part of the working vocabulary. Soon these techniques, and the deformation-mechanisms behind them, would become broadly available in the literature. The work of Simpson and Schmid (1983) in particular had immediate major impact. A byproduct of this paper was their calling clear and appropriate attention to the seminal discoveries by French tectonists, who first introduced the concept of “S-C fabrics,” one of the finest sense-of-shear indicators in mylonites derived from granite (Bérthe, Choukroune, and Jégouzo, 1979a; Bérthe, Choukroune, and Gapais, 1979b). The French cohort of structural geologists and tectonists in the mid to late 1970’s had been picking-apart shear-zone deformation within and along the South Armorican shear zone in Brittany (Jégouzo, 1980). With the passing of time we have had, of course, additional essential and broadly accessible references on shear-sense indicators (e.g., Simpson, 1986; Passchier and Simpson, 1986; Hanmer and Passchier, 1991).

The report on the ‘mylonites’ Penrose is worth reading carefully (Tullis, Snoke, and Todd, 1982), as is the overview of fault rocks (Snoke and Tullis, 1998), which also was an outgrowth of the conference. These contributions include detailed summaries of the tortuous pathways along which our present understanding of “fault rocks” and “mylonites” have traveled from the time of Lapworth (1885). To be sure, the decision to hold the conference was motivated by the “continuing controversy over the nomenclature of fault rocks” (Tullis, Snoke, and Todd, 1982). We learn as well from Tullis, Snoke, and Todd (1982, p. 230) that participants concluded that “ ‘cataclastic rock’ should be abandoned as a general term,” and that the use of the term “mylonite” should be applied to rocks marked by concentrated strain, grain size reduction, narrow planar zones, enhanced foliation and/or lineation, and a deformation mechanism that was ductile, with little or no microcracking. I particularly appreciate their closing observation (Tullis, Snoke, and Todd, 1982, p. 230): “As structural geologists, we are interested in the processes and conditions of deformation; we need to determine criteria for the recognition of faults and shear zones; rather than arguing about terminology.”

The photographic atlas of fault rocks (Snoke, Tullis, and Todd, 1998), though published 13 years following the Penrose Conference, filled an enormous void in the literature. It organizes fault rocks smartly (by brittle behavior, semi-brittle behavior, and ductile behavior), provides stunning photographic images of all kinds of fault rocks, conveys the structure-tectonic context of each of the examples, and presents a comprehensive bibliography.