Setting the Stage

This volume on “sense-of-shear” causes me to look back on the discovery and interpretation of perhaps the most impressive category of shear zones exposed today on the surface of the globe, namely the brittle-ductile shear zones for which metamorphic core complexes in the Basin and Range Province of the American Southwest owe their existence (Crittenden, Coney, and Davis, 1980). The impressiveness of this brand of shear zones is not simply a function of thickness (up to at least 4 km), or trace length (up to at least 125 km of strike length in exposed parts of in individual core complexes), or regional extensiveness (~30 core complexes in the Basin and Range), or bold clarity of exposure in a vast semi-arid desert region. Instead it derives largely from (1) the stark contrasts between rocks, structures, and fabrics in upper versus lower ‘plates,’ and (2) the telescoped structural gradients at and immediately beneath the detachment faults, which separate upper from lower ‘plates.’

It is becoming more and more uncommon for major discoveries in geology to emerge strictly from fieldwork, but I have heard it argued that the discovery and elucidation of metamorphic core complexes by the structure-tectonics cohorts working in the Western Cordillera is one of the compelling examples. In a brief period of time in the mid-1970s, more than 40 metamorphic core complex terranes were identified in the North American Cordillera, and the fundamental structural characteristics were identified (Crittenden, Coney, and Davis, 1980). Steps to discovery included field rendezvous of structural geologists and tectonists with graduate students meeting-up in the field at locales marked by an arrangement of fabrics, structures, and contact relationships that taken together, at the time, seemed unique in the world, and were unaccounted for in the structure-tectonics literature. Debates and discussions were loud, strong, sustained, and enjoyable.

Agreement on the basic physical and geometric characteristics associated with metamorphic core complexes proved easier than interpreting them. Thinking back on how challenging it was for the structure-tectonics community to interpret the tectonic significance of “metamorphic core complexes,” I imagine that some would say that the main hurdle was the illusiveness of the timing of ‘what happened when,’ and especially the difficulty of establishing the age(s) of protolith of the igneous rocks now overprinted by core complex deformation. This point is underscored in the last sentence of Peter Coney’s introduction to Geological Society of America Memoir 153 on metamorphic core complexes (Coney, 1980b, p. 6), where he stated: “It is extremely important to date the terranes better, and if preliminary results are any indication, the dating of these basement terranes will be a geochronological nightmare which only detailed, multiple-attack methods combined with very careful field control will resolve.”

Each geoscientist who was engaged in the splendid frenzy of field investigations of metamorphic core complexes in the late 1960’s, 1970’s, and early 1980’s will have his/her own ‘take’ on the challenges of interpretation. My emphasis here is that they derived primarily from not recognizing clearly in the very early going that the mylonitic tectonites of metamorphic core complexes were expressions of shear zones, and that rigorous understanding of shear-zone theory would be a critical guide to interpretation. Part of the barrier was the very real mapping-and-visualization challenges associated with determining the three-dimensional shape(s) of the bodies of mylonites in the metamorphic core complexes. Because of the great size and breadth of metamorphic core complexes, and the fact that high-angle (bona fide) Basin and Range faulting had pulled them apart, parts are buried beneath deep broad basins, and thus it was never quite sufficient to work within the confines of single mountain ranges to grasp the full geological picture of metamorphic core complexes. Furthermore, there was a steep learning curve in recognizing mylonites and cataclasites for what they were, and another steep learning curve for grasping the relationship(s) between mylonites and shear zones.

When the shear-zone nature of important dimensions of metamorphic complexes finally ‘clicked’ into place, investigators were able to turn almost immediately to a burgeoning, converging new literature that gave insight regarding “fault rocks,” “shear zones,” “deformation mechanisms,” and yes, “sense-of-shear” criteria. Full utilization of these concepts and topics required understanding the close interrelations among all four of these phenomena.

I call on the thoughts of Peter Coney again to underscore this set of points. Consider what he chose to emphasize in the last paragraph of his introduction to the GSA metamorphic core complex volume (Coney, 1980b, p. 5-6): “The most important controversy still remaining is the origin and significance of the mylonitic gneiss fabrics so characteristic of the basement cores of the complexes. Without question, the dramatic resemblance of these fabrics to those produced along deep-seated thrust faults in other parts of the world is the last remaining obstacle to what might be called a new consensus. Some workers have concluded that even this aspect of the complexes was produced by Tertiary regional extension. This fact will demand and inspire much-needed intense wok in the future. It is remarkable how little detailed petrography and petrology have been done in these terranes.”

Coney emphasized this for a reason. The first mention of “metamorphic core complexes” in the literature was in 1977 (Crittenden, Coney, and Davis, 1977). Between then and at least 1980 there were strikingly contrasting working-views on the tectonic significance of the fabrics and structures associated with metamorphic core complexes. Identifying promising avenues of interpretation would depend importantly on discerning not ‘simply’ the ages of the fabrics and structures, but the tectonic transport directions embodied in them, as well as the depth-temperature environment(s) in which these fabrics and structures developed.

Permit me to provide a sampling of the diverse inclinations on tectonic significance of core-complex structures and fabrics, as reviewed by Coney (1980a, p. 8-12). Back in the 1960’s in eastern Nevada and western Utah, Peter Misch and his students were trying to prove a connection between the Snake Range decollement and the Sevier-Laramide ‘eastern” thrusts,’ but could not (Misch, 1960). Roberts and Crittenden (1973) and Hose and Danes (1973) viewed the decollement and younger-on-older faulting as the result of extensional (gravity-driven) faulting of cover off of the hinterland core region, eastward, and producing in the toe region the folding and thrusting marking Mesozoic Sevier fold and thrust structures. Armstrong and Hansen (1966) and Armstrong (1968) were predisposed to a mid-Mesozoic orogeny that remobilized and metamorphosed basement rocks and produced a Caledonian-type remobilized core zone and infrastructure. Coney (1974) advocated middle Tertiary low-angle gravity sliding of unmetamorphosed cover rocks off metamorphic basement on a decollement surface. Further north in the Albion-Raft River-Grouse Creek ranges, Compton and others (1977) concluded that the characteristic core complex fabrics had been imprinted on a mid-Tertiary pluton.

Further south Greg Davis and I saw things differently from one another at that time, especially in regard to whether mylonities exposed in the lower plate beneath detachment faults were formed in Mesozoic and/or early Tertiary (G. A. Davis and others, 1977) as a result of compression and thrusting, or in the mid-Tertiary in an environment of regional extension (G.H. Davis, 1977). Based upon mapping within the Tortolita Mountains in southeastern Arizona, my students and I concluded that the characteristic core-complex lineation and foliation was Tertiary in age and of extensional origin (Davis and others, 1975). We visited one another’s field areas to compare and contrast, in the same way that Peter Coney and I visited one another’s field areas in 1974, where we confirmed what we thought was the case: amazing similarities between structures and fabrics in the Snake Range of Utah and the Rincon Mountains of southern Arizona (Davis, 1973, 1975; Coney, 1974).

In short, there was tremendous excitement related to recognizing enormous regional tracts of common tectonic properties, and a lot of confusion in trying to assess regional tectonic significance. Amidst the confusion was a sense that interpretations of “tectonic transport directions” held promise in sorting things out.