Colleagues and their Impact

I now distinguish two stages of my ‘core-complex’ life, insofar as related to the interaction with special colleagues. There were first the early “heroic years,” which is what Peter Coney would call them, when the challenges and opportunities abounded in the ‘tracking down’ and mapping of these complexes, …even spotting them from the air when landing at Sky Harbor Airport in Phoenix. The list of colleagues and student-colleagues that particularly impacted my life and my thinking while working on the ground in southern Arizona is very long, and includes the likes of Peter Coney, Greg Davis, Lawford Anderson, Steve Reynolds, Tom Anderson, Bill Rehrig, Eric Frost, and Stan Keith. It was my good fortune to benefit directly from these individuals, and others, and to work closely with the many students who would address core complex problems (e.g., Terry Budden, Bob Varga, Monte Swan, Chuck Kiven, Gene Suemnict, Greg Benson, Steve Lingrey, Jim Hardy, Jean Crespi, Bob Krantz, Jerry Hansen, Lee DiTullio, Steve Naruk, Veronica Martins, Ann Bykerk-Kauffman).

Then there was a second stage of connection with special colleagues, ones who unveiled deformation mechanisms and sense-of-shear-criteria. I conclude by describing several such examples.

After the Penrose Conference on mylonites, in 1981. Gordon Lister came to visit. We had been corresponding about the quartzite tectonite fabrics beautifully exposed in the Coyote Mountains metamorphic core complex in southern Arizona. I had reported on discovering and interpreting these rocks and fabrics (Davis, 1977, 1980), and Anne Gardulski had carried out thesis work involving larger-scale geological mapping and quartz-petrofabric analysis (Gardulski, 1980). Anne and I ‘showed off’ these rocks and structures on a GSA field trip (Davis, Gardulski, and Anderson, 1981). Frankly, Anne and I could not make sense of the quartz petrofabrics, and at the suggestion of Jan Tullis, we sent samples to Gordon Lister. He proclaimed them at once to be the finest quartzite mylonites he had ever seen, and as gently as Gordon knows how, he told us that our quartz c-axes were misplotted. He had to see these rocks, and this is what caused him to make a visit. We went to the Coyote Mountains together, and he introduced me firsthand to “mica fish,” “stair-stepping mica fish trails,” and the use of these fabrics in determining sense-of-shear. Through the quartz fabric plots he had prepared for these same rocks, he showed how they conformed beautifully to deformation under conditions of non-coaxial shear (Davis, Gardulski, and Lister, 1987). Later, Lister and Snoke (1984) featured these particular Coyote-Mountains quartzites as their example of type II S-C mylonites.

In May, 1982, Gordon held forth in seminar at The University of Arizona, and walked us through crystal-plastic behaviors, strain partitioning, the vicissitudes of strain hardening and strain softening, S-C mylonites, pole figures connoting coaxial versus non-coaxial shear, and dynamic recrystallization. Again, in only the way that Gordon can do, he introduced us to “banana fish,” which he explained are fashioned by the presence of mica veneers that give the impression that the fish are wrapped in banana leaves. He also delighted in debunking the “myth” of orthorhombic fabric.

At some time during Gordon’s visits, he connected again with Steve Reynolds at Arizona State University, where Steve tested – to Gordon’s delight – a new sense-of-shear indicator that he had discovered, namely “fish flash!” (see Davis and Reynolds, 1996, p. 523).

Carol Simpson visited in late 1982, or early 1983, …we cannot quite recall. Bill Dickinson and I gave Carol a tour of the Rincon-Catalina core complex. When we got to Saguaro National Park (East), she riveted on “the very fine grained, black phyllonites and mylonites …with recumbent isoclinal intrafolial folds and splendid mica fish and asymmetrical feldspar porphyroclasts in gneissic layers that all indicated down-dip movement on the detachment faults” (Carol’s notes). She talked to us about S-C, shear bands, grain-size reduction, delta-type porphyroclasts with soft recrystallized materials in their tails, sigma-type porphyroclasts and their ‘teapot’ shapes, and mica fish with recrystallized tails. She pointed out domino-faulted feldspar grains, urging us to concentrate only on those that are cut by fractures at very high or very low angles to the zone, emphasizing that those at 45º, when rotated, become misleading. She emphasized that the dynamically recrystallized quartz creating oblique foliations yields only the final incremental deformation, so beware! She warned us to keep clear of the feldspar clasts. Carol’s language, always appropriately nuanced, described circumstances under which we would surely draw incorrect conclusions if we did not understand and appreciate the deformation mechanisms. We examined together the so-called “mylonitic schists” and left these outcrops calling them “ultramylonites.” We examined the extraordinary ultrafine-grained shiny black rocks I had mapped at the detachment fault position on Tanqe Verde antiform in the Rincon Mountains, for I wanted to know whether Carol thought any of these might be pseudotachylite. I was relieved when she agreed they were cataclasites, and we left those outcrops calling them “ultracataclasites.”

These visits by Gordon and Carol were similar to what was happening in any number of geosciences departments, where the transfer of knowledge was taking place at ballistic speed. Undergraduate and graduate students were in on the action, and they took up the language as if (in today’s term) they were hooking up their IPODS. Just as geomajors and graduate students today, they would have had no idea how much flailing had taken place before the introduction of practical, reliable sense-of-shear indicators.

The impact of these particular visits, the connectedness between research and teaching, and the literature that was rolling out, were profound. I re-read a tiny abstract by Naruk and Davis (1983, p. 650), where we write: “Structural and petrofabric analysis of mylonitic gneiss of the Pinaleno Mountains metamorphic core complex demonstrates 1/ that the mylonites represent a low-dipping zone of heterogenous simple shear, locally modified by pure shear and/or volume loss; 2/ that the shear strain within such zones can be quantified as a function of the angle between the mylonitic foliation and the shear zone boundary; and 3/ that the total translation across such zones can be quantified from the calculated shear strains and the inferred original thickness of the zone.” Moreover. “Microscopic schistosité (S) and cisaillement (C) planes, elongate subgrains, and asymmetric porphyroclasts are everywhere consistent with normal displacement, non-coaxial shear.” The changed language, the broader scope, the higher-order problems captured in these fragments reflect the results of the cross-talk that flourished in 1981 and 1982.

There are two other connections with colleagues that I wish to mention. Rick Sibson visited in November, 1984, and his timing was perfect for me and my students. It was important to learn firsthand from him his ‘take’ on fault rocks, and particularly the mechanisms of formation of same (Sibson, 1977, 1982). His descriptions of breccia formations took us into “fault sucking” and “implosion.” He graphed the granite rigidity as a function of depth and temperature. He drew longitudinal profiles of the San Andreas fault, showing the distribution of microseismic focal centers. He emphasized that thrusting effectively decreases the geothermal gradient, thereby resulting in a drop of the brittle-ductile transition; with detachment faulting working the other way around, as I was later to see so clearly along the Moine thrust. He gave us what I would call “pseudotachylite envy” as he described exposures along the Alpine fault, and explained that holding a sample of pseuotachlylite in your hand is like holding an earthquake, …a fossil earthquake.

Through these experiences, and the literature, I became much more aware of the significance of previous work along the South Armorican shear zone in Brittany. To my great pleasure in 1986 I heard from Jean Pierre Brun that he and some of his colleagues wanted to visit. Jean-Pierre, Pierre Choukroune, Jean Van den Driessche, and Gwen Gweron landed at LAX and drove to Tucson, checking out core complex localities and big faults along the way, and being staggered by the vastness of the region. Together we looked closely at mylonities and cataclasites within the Catalina-Rincon-Tortolita core complex, and Gwen succeeded in identifying a thesis area in the Durham Hills at the far north end of the Catalina-Rincon-Tortolita metamorphic core complex, where he worked on S-C-C’ relations in the context of strain and displacement. In 1989 I participated in Gwen’s jury defense at The University of Rennes, and this gave me the opportunity to visit some classic locales within the South Armorican shear zone, in the hands and through the eyes of Jean-Pierre Brun, Philip Davy, Dennis Gapais, Gwen Gweron, and Peter Cobbold. The South Armorican shear zone of course is a right-handed strike-slip shear zone, yet when you pull mylonite from the outcrop and look at it free-standing out-of-orientation, you have mylonite in your hand from the Rincon Mountains. This experience particularly brought home the realization that while structure-tectonics people in the Western Cordillera in the 1970s were working hard to try to unravel the tectonic significance of metamorphic core complexes, French tectonists from Rennes were similarly applying their minds to deciphering the mylonites in Brittany. Cross-connection between the two groups in the 1970’s would have accelerated interpretations of the tectonic significance of metamorphic core complexes.

On the other hand, looking back, I would not have wanted it any other way.