Geological Society of AmericaSpecial Paper 338

1999

Chapter 7

THE FRANCISCAN: CALIFORNIA’S CLASSIC SUBDUCTION COMPLEX

John Wakabayashi1329 Sheridan Lane

Hayward, California 94544E-mail: wako@tdl.com

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W. GARY ERNST

W. G. Ernst was born in 1931 in St. Louis, Missouri. Hereceived his bachelor of arts degree from Carleton College in1953, his master of science degree from the University ofMinnesota in 1955, and his doctor of philosophy degee fromJohns Hopkins University in 1959. He was a professor in theDepartment of Earth and Space Sciences and Institute of Geo-physics and Planetary Physics at the University of California,Los Angeles (UCLA), from 1960 to 1989. At UCLA, heserved as chair of the Department of Geology from 1970 to1974, chair of the Department of Earth and Space Sciencesfrom 1978 to 1982, and director of the Institute of Geophysicsand Planetary Physics from 1987 to 1989. Since 1989 he hasbeen a professor in the Department of Geological and Envi-ronmental Sciences at Stanford University, and was the deanof the School of Earth Sciences at Stanford from 1989 to1994. Ernst is a member of the National Academy of Sciencesand served as president of the Geological Society of Americain 1986.

Ernst made one of the major contributions to the plate tec-tonics revolution by relating high-pressure–low-temperaturemetamorphism (especially blueschist facies metamorphism)to subduction (in the “classic paper” reviewed here). In sub-

sequent research, he has continued to advance concepts linking tectonics and metamorphism; he is widely regarded as one of, if notthe, most accomplished geoscientists at relating these two subdisciplines of the earth sciences. His research in experimental petrol-ogy, some of which formed part of the foundation for the “classic paper,” has continued to provide new data constraining keymetamorphic reactions and phase stabilities. Although he has had a long research career as an experimental petrologist, Ernst hasalways valued, and actively participated in, geologic field studies. It is probably this dual background that has made him so effec-tive at linking metamorphic petrology and tectonics. In addition to his research accomplishments, Ernst has supervised numerousoutstanding doctoral students, many of whom have gone on to make a significant impact in geologic research.

JUSTIFICATION FOR CHOOSING ERNST (1970) AS THE CLASSIC PAPER FOR THIS CHAPTER

Ernst’s (1970) paper was chosen as the “classic paper” for the Franciscan Complex because it made the connection betweenthe high-pressure–low-temperature (high P-T) metamorphism and subduction, in particular the explanation given for the low geo-thermal gradients associated with high P-Tmetamorphism. This paper was one of the key breakthroughs in the plate tectonic rev-olution, and it helped establish the Franciscan Complex as the type subduction complex. Variations of Figure 3 of Ernst (1970)have become the textbook model of subduction-zone metamorphism. Geologists now regard high P-T (including blueschist facies)metamorphism as the strongest evidence of an exhumed subduction complex.

John Wakabayashi

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114 W. G. Ernst

Tectonic Contact between the Franciscan Mélange and the Great Valley Sequence 115

116 W. G. Ernst

Tectonic Contact between the Franciscan Mélange and the Great Valley Sequence 117

118 W. G. Ernst

Tectonic Contact between the Franciscan Mélange and the Great Valley Sequence 119

120 W. G. Ernst

Tectonic Contact between the Franciscan Mélange and the Great Valley Sequence 121