Personal Reflections

The ‘new consensus’ about metamorphic core complexes that had emerged by 1983 contained the following notions: 1/ a crustal-extension origin, 2/ in the mid-Tertiary, (3) with mylonites and cataclasites fashioned in character and geometry (4) by normal-slip ductile-brittle shearing. Paper upon paper published in 1983 and later affirmed time and time again these interpretive conclusions. That literature, and all that followed to the present, devotes itself to both fundamental and specialized inquiries that are adding needed clarifications of the details of the new consensus. One of the most profound and important of these is trying to fully understand the mechanics of origin of detachment faulting, and what causes such concentrated localization of strain. A breakthrough paper was Wernicke’s (1981), who “saw” the formation of metamorphic core complexes and detachment faulting at the crustal scale. His tectonic insights profoundly influenced the way in which crustal-scale regional cross sections of terranes of core complexes and detachment faults would be drawn from then on (e.g., G.A. Davis, Lister, and Reynolds, 1986).

I too tumbled on “shear zones,” but at a different scale, as the context to explain what I was mapping and analyzing in individual metamorphic core complexes, especially the Catalina-Rincon complex. How did I get there, exactly? For me, as a structural geologist and in the context of this volume on “sense-of-shear,” it has been interesting, and sometimes unsettling, to track vocabulary and concepts to see when “shear zones” and “sense of shear” and “fault rocks” and correct usages of “cataclasite” and “mylonite” entered my life as one of those engaged in the splendid frenzy.

In 1977 (Davis, 1977) I was using the language “augen gneissic cores,” “metasedimentary carapace,” “decollement,” and “decollement zones” in describing the structural components of core complexes. I had adopted “decollement” from Peter Misch’s original language for the Snake Range detachment, and Peter Coney’s (1974) follow-up usage. (It took me a long time to set that term aside). I emphasized that the “decollement zones” were composed of mylonite and mylonite breccia. I used the expression “mylonitic schist” in reference to strongly folded, blackish, fine-grained rocks (ultramylonite) full of folds and located at the interface between two varieties of gneisses. My interpretations emphasized mid-Tertiary deformation through a profound flattening and extension, transposition, and ductile through brittle extension and flow in the direction of mineral lineation. My analogue was megaboudinage on a crustal scale (i.e., coaxial strain). I still viewed folding in the cover rocks as having been produced by mid-Tertiary gravity induced folding (Davis, 1973, 1975). I imagined that the decollement zones were localized by ductility contrast at the great unconformity (sigh). I recognized “ductile normal shearing” that projected deep into basement on one side of the core complex.

In 1978 (Davis, 1978) I see that I was characterizing a fault stratigraphy within the so-called decollement zone, distinguishing the omnipresent ledge of microbreccia directly beneath the fault surface itself from what typically lay below: chlorite breccia, transitioning into breccia with preserved patches of thinly banded mylonite, and then downward into mylonitic gneiss with only localized brecciation or chloritic alteration.

In 1979 (Davis and Coney, 1979) I was gaining a better handle on using terms such as “mylonite,” “blastomylonite,” and “microbreccia,” but with tell-tale relapses into expressions such as “low dipping cataclastic foliation.” My dominant interpretive theme remained ductile-through-brittle extension and flow in the direction of mineral lineation, but still driven largely by vertical flattening and subhorizontal extension. “Shear zones” was not part of my working vocabulary as applied to core complexes, and thus I struggled with language in trying to describe what I was mapping and imagining (e.g., p. 123): “The normal faults…constitute a new concept in large-scale faulting, one that we term herein ‘ductile growth faulting.’ The faults are ‘growth’ faults in the sense that the crystalline surface on which metamorphic carapace materials are plated increases in area during the life of the fault. They reflect a mode by which the surface area of the crystalline basement can be increased during extension.”

In 1980 (Davis, 1980b) I am now almost always preceding “augen gneiss” with “mylonitic;” I am reporting that metasedimentary “carapace tectonites” are “smeared,” “welded,” or “plated” onto the underlying crystalline rocks; and I am noting that decollement zones are composed of “fine grained cataclastically deformed rocks.” I reemphasize that the structural fabric of mylonitic gneisses and carapace tectonites are intimately coordinated, and that decreasing ductility through time is recorded by features such as “folding of lineation and refolding of folds along ductile normal fault zones and superimposition of brittle normal-slip faults upon ductile faults.” I see that I had tacitly backed off on gravity-induced deformation (whew), stating that cover rocks became denuded both during and after the formation of lineated tectonite, and that “late-stage mid-Miocene listric normal faulting further denuded the cover… .”

By November, 1980, my thinking about the kinematic significance of metamorphic core complexes and decollement (i.e., detachment faults) had seriously shifted from dominantly coaxial strain to non-coaxial shear. “…, the mylonitic tectonite acquired its strain during progressive simple-shear rotational deformation and not by pure shear (although pure shear probably occurred locally). …With respect to present attitudes, the mylonitic tectonites appear to be zones of ductile normal slip (flow). If so, these tectonites may be partial exposures of ductile normal shear zones of regional extent. Mapping the tectonites as parts of regional ductile shear zones, and mapping both the sense of simple-shear and the slip-line path are necessary to assess the tectonic significance of Cordilleran metamorphic core complexes as a whole. …The position of the ductile shear zone represented by mylonitic tectonite appears to be well below the great unconformity” (Davis, 1980a, p. 157).

Consequently, in Davis, Gardulski, and Anderson (1981) the fault-rock language that I was using seems to have come together properly. By this time I have largely dropped “core,” “carapace,” and “cover” from my vocabulary (Davis , 1980a), and succeeded in staying away from references to “lower plate” or “upper plate.” I used “detachment” not as the fault itself but as what most would describe as “upper plate,” for I was increasingly concerned that we may never know the number and character of “plates” involved nor the number of decollement zones at depth. My use of “detachment” in this way was short lived. Primarily I emphasized that “the tectonite fabric is interpreted to have formed by ductile normal shearing within gently dipping curvitabular zones of simple-shear. As cooling of the system took place, some already-formed mylonitic tectonite was disrupted, rotated, and microbrecciated by closely spaced, mesocopically penetrative, normal-slip listric (?) faults… The kinematic coordination is perfect” (Davis, 1981a; Davis and Hardy, 1981).

I was impressed with the fact that even the low-angle faulting of Miocene fanglomerates translated these rocks in a direction perfectly parallel to the direction of penetrative lineation in the closest mylonite tectonites, and that this held for southeastern Arizona as well as the southern part of the western Cordillera (Davis, 1980a). This is the theme I chose to emphasize, and illustrate, in my presentation at the GSA “Frontiers of Structural Geology Symposium,” seminar, which coincided with the launching of the Structure-Tectonics Division of the Geological Society of America (Davis, 1981b), and which in turn was the basis for my paper on the shear zone origin of metamorphic core complexes (Davis, 1983).

The challenge I experienced in coming to recognize shear zone deformation at work was related importantly to my inability at that time to ‘read’ the mylonite fabrics for sense of shear. Determining the sense and direction of movement of faults in the “upper plate” was reasonably straightforward, especially where the rocks consisted simply of tilted Miocene fanglomerates and the low-angle normal faults were adequately exposed. It was just matter of analyzing bed strike, bed dip, fault dip, and slickenline orientations. For the Catalina-Rincon Mountains metamorphic core complex, I determined that the direction and sense of movement was ~S60°W. Determining direction and sense of shear in the tectonite carapace of mylonitic marbles and calc-silicate rocks proved to be manageable using Hansen’s (1971) separation arc method in the analysis of the penetrative folding; direction and sense-of-shear once again proved to be ~S60°W. But defining sense-of-shear within the mylonites would have been impossible for me, had it not been for the bands of ultramylonites within the mylonitic gneisses, and being able once again to harness the folds contained therein using Hansen’s separation arc-method. But this was spotty and slow going. Most challenging was discerning that there is a decipherable sense of fault transport in the cohesive breccias, and then determining what it was. Thankfully for me there is a beautiful set of exposures in Saguaro National Park (East) containing faulted and rotated mylonitic fabrics (foliation and lineation) that lended themselves to fine-scale mapping and stereographic analysis to work out rotations (Davis, 1980a).