Pennrose Conference on "Metamorphic Core Complexes"

GSA Memoir 153 was a product of the (1977) Penrose Conference on “Tectonic Significance of metamorphic core complexes in the North American Cordillera,” sponsored by the Geological Society of America, and convened by Max Crittenden (U. S. Geological Survey) and Peter Coney and I (The University of Arizona) (Crittenden, Coney, and Davis, 1980). In 1976, while planning for the conference, we targeted a sweeping descriptive review of eight subregions of enigmatic but apparently homogeneous structure/tectonic character comprising two-mica garnet-bearing granitic intrusions; zones of medium- to high-grade metamorphism; augen gneisses; Tertiary-on-older fault relations (which none other than P. B. King took particular note of during the first evening of the Penrose); low-angle decollement; high-standing domical topography; subhorizontal foliations; consistently oriented penetrative mineral lineations; tracts of geology profoundly rotated (about horizontal axes); and expansive regions of reset K/AR ages. Because the reset K/AR ages were found to be consistently Eocene in the metamorphic core complexes north of the Snake River Plain but Miocene in core complexes to the south of it, we wanted to make certain that both regions were represented in invitations. We chose to cover the following terranes to the north of the Snake River Plain: Frenchman’s Cap, Thor-Odin, Pinnacles, Valhalla, Okanogan, Kettle, Selkirk, Bitterroot, and Pioneer. The terranes we chose to cover to the south of the Snake River Plain were Albion-Raft River-Grouse Creek, Ruby, Snake Range, Death Valley Turtlebacks, Whipple, Harcuvar, Harquahalla, South Mountains-White Tank, Picacho, Tortolita, Catalina-Rincon, Santa Teresa-Pinaleno, Comobabi-Coyote, Pozo Verde Magdalena, Madrea, and Mazatan to the south (Crittenden, Coney, and Davis, 1977).

Seventy-two earth scientists and ten student associates from Canada, the United States, and Mexico attended, and based on what we knew then this seemed to be a sufficient range of invitations in terms of expertise, disciplines, and topics represented. (A list of participants is at Appendix A.) Foremost we knew that the invitation list emphasized geologists who had done the mapping of these complex systems, and had paid attention to tectonic setting and history. However, back in 1976 during the planning stage for the conference, we did not appreciate the degree to which the deciphering of the tectonic significance of metamorphic core complexes would depend upon developing an understanding of “shear zones” and “fault rocks,” in fact an understanding that could be applied practically in the field based on outcrop-scale observations. Looking back to 1977, “shear zone deformation” per se was not a part of the conversation at Tanque Verde Guest Ranch in Tucson, even though many of the participants had a reasonable working knowledge of the seminal work by Ramsay and Graham (1970) on shear belts and strain variations within them. (Not until 1980 would the clear and comprehensive review of shear zones by John Ramsay appear, an article that made fundamental understanding of shear zone geometry much more accessible to the structure-tectonics community than it had been to date).

Furthermore, most of us who participated in the core-complex Penrose Conference were not really clear on the distinction between “mylonite” versus “cataclasite,” nor the specific fabrics to which these terms applied. Part of this can be explained by the fact that the pre-1970 literature on what we now call “fault rocks” did not lend itself to ease of understanding, a view I will expand upon in the next section of this essay. Part of this was due to “mylonites” and “cataclasites” hardly being foremost on the minds of the tectonics community, and thus many of us were simply not paying sufficient attention to a new literature that was taking shape. Bob Compton (Stanford University) was one of the notable exceptions, and thus as conveners of the Penrose Conference we urged him to bring thin sections with him to Tucson to instruct on taxonomy and fabrics. “Fault rocks,” which structural geologists now regarded as the essential fourth category of major rock types (after igneous, sedimentary, and metamorphic), were simply not yet an integral part of the experience and vocabulary of most. Ironically, Rick Sibson’s classic paper on fault rocks and mechanisms appeared precisely in 1977.

Had we as conveners been prescient, I suspect that the invitation list would have been altered and broadened to include a number of additional geoscientists, and notably more from outside of the U.S., who had been productively engaged in field-mapping and analysis of well exposed shear zones in other parts of the world, and investigators who quite literally were in the process of illuminating the physics of fundamental “deformation mechanisms,” and developing practical methodologies and insights regarding how to interpret fabrics in terms of “sense-of-shear.”