California terranes

Two of this volume’s coeditors persuaded the third into commenting on his own paper. There are three clear alternatives as a “classic paper” on California terranes. In the first of these, Hamilton (1969) first mentioned the North American Cordillera as a collage of exotic terranes and argued that the continental margin of North America was built out from the shelf edge by material carried to it on a converging oceanic plate. He argued for a process of “ocean-floor sweeping” caused by “underflow” (later called subduction; White et al., 1970) of Pacific mantle beneath the North American margin. Although it was a revolutionary paper, Hamilton (1969) emphasized exclusively east-dipping subduction beneath North America. The single-subduction tectonic model rapidly became a “ruling theory” (Chamberlin, 1890), a position it has held for the past 30 years. In the second alternative paper, Irwin (1972, p. C103) first defined a tectonic terrane as “an association of geologic features...which lend a distinguishing character to a particular tract of rocks and which differ from those of an adjacent terrane.” We, however, base our discussion on the third paper by one of us (Moores, 1970). This paper had its origins at the second Penrose conference, convened by W. R Dickinson and described briefly earlier in this volume (in the biography of Dickinson). One of us (E. M. M.) attended this conference. In 1969, I (E. M. M.) had been working in ophiolites for some six years, first on the Vourinos complex in northern Greece and subsequently on the Troodos complex in Cyprus (Moores and Vine, 1971). At the Penrose conference I reported on the evidence for sea-floor spreading in the Troodos complex, although I had not yet made the connection to all ophiolites. One of the informal breakout sections at the Penrose conference was devoted to discussion of the problem of ophiolite emplacement. One model discussed during this session was emplacement of ophiolites on continents by collision of a continental margin with a subduction zone dipping away from the continent. I had recently read an abstract from the annual meeting of the GSA by Ralph J. Roberts (Roberts, 1969), in which he described the Phanerozoic tectonic history of the U.S. Cordillera as an alternation of eugeosyncline and miogeosyncline depositional conditions along the margin with orogeny involving thrust from the continental margin toward the interior of the continent. As described earlier in this volume, in the final session of the conference, convenor Dickinson summarized his emerging ideas relating ancient geosynclines to modern tectonic environments, a talk I earlier described as “one of the most exciting moments of my scientific life.” Sitting there greatly inspired after Dickinson’s remarks, I was meditating on the previous evening’s discussions of ophiolite emplacement and Roberts’ account of Cordilleran history, when in a blinding flash of insight, it came to