Tectonic, plutonic, and metamorphic history of the central Kootenay Arc, British Columbia, Canada

Tectonic, plutonic, and metamorphic history of the central Kootenay Arc, British Columbia, Canada

BOOK REVIEWS 351 This is inevitably a large book, coffee-table rather than bookcase in size. It is beautifully written in an easy-moving style that ...

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BOOK REVIEWS

351

This is inevitably a large book, coffee-table rather than bookcase in size. It is beautifully written in an easy-moving style that holds the reader's attention like a spy story. We are constantly reminded of the influence of personalities as well as tectonic history in the evolution of the present geological picture of Europe. In his preface Professor Rutten warns us of the dominating influence on European geology of the grand "maRres", and their submissive schools. Though some of us like to think that we are not so cowed by our old professors as in the traditional schools of continental Europe, this is still undoubtedly a problem. At an international conference recently a distinguished and a liberal-minded continental professor boasted proudly "all my students agree with me", and the mainly British audience thought this very funny. Dogmatism is far from dead. Professor Rutten himself is not afraid to air his opinions and prejudices, but he would be the last to suggest that this is the final word on the major features of European geology. His book is very much up-to-date but every year new evidence (especially from deeper borings and geophysics) produces fundamental changes in the basic geological picture of our continent. Let us hope that this book and others like it, will encourage Americans and Europeans alike to come and judge and synthesise for themselves. It is important to look at the mountains through microscopes; it is important to map on the largest scale possible, but it is also important to see the mountain range, to see the continent, as a whole. D. V. AGER (Swansea)

Tectonic, Plutonic, and Metamorphic History of the Central Kootenay Arc, British Columbia, Canada. P. CROSBY. Geol. Soc. Am., Spec. Papers, 99, New York, N.Y., 1968, 94 pp., $ 6.00. Crosby's paper on the Kootenay Arc analyzes the geology of a segment of the vast Cordilleran eugeosynclinal belt and is another in the Regional Studies series of the Geological Society of America. The Kootenay Arc of southeast British Columbia is an intruded metasedimentary belt critically situated between the classic thrusted miogeosynclinal Rockies and the high grade plutonic metamorphic "Shuswap"-type terranes of the eugeosyncline to the west. A 20,000-ft. plus section of mostly clastic metasediments ranges in age from Late Proterozoic to Triassic and contains metavolcanics throughout. Lower Paleozoic and Triassic rocks form the bulk of the section, although age control is poor and is largely based on lithologic correlation with tenuously dated sparsely fossiliferous units elsewhere. By structural and fabric analyses, Crosby shows that these rocks were metamorphosed as high as sillimanite grade during primary isoclinal folding and strike faulting, subsequently refolded, and then intruded by several complex grano-

Palaeogeography, Palaeoclimatol., Palaeoecol., 8 (1970) 345-352

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BOOK REVIEWS

dioritic batholiths and numerous minor intrusives. Crosby believes that these events took place as one protracted period of deformation, metamorphism, and intrusion spanning 100 million years or more of Jurassic-Cretaceous time. The shortcomings of the paper are few and familiar: a seven year lapse between completion of work and publication (references are excellently up-todate); maps which fall out of the paper; frequently verbose. The insertion of a good regional tectonic sketch map would have clarified the critical regional setting for outsiders. Why are many modal analyses of metamorphic rocks published? They are hardly used and the author declares he will cover chemical aspects of metamorphism in a future paper. Some readers will object to the use of "arc" as a structural element since it may be interpreted to mean island arc. Crosby has produced a good study of a difficult area and attempted a reasonable synthesis where surrounding areas remain largely unmapped. He has shown that metamorphism is synkinematic and not related to later batholiths, and that two important phases of folding preceded intrusion. He emphasizes the importance of distinguishing between the tectonic history of the cores vs. the margins of plutons. His major contribution, however, is the detailed description of a zone of high metamorphic gradient and complex structure which fits into the emerging picture of an "Abscherungszone" separating a Cordilleran infrastructure from its suprastructure. Crosby's limited (250 square miles) study naturally leaves many unresolved problems for future workers. Many questions remain about the Shuswap terranes of granulite migmatites. Why do they have such a great range of radiometric ages, from 11 to 140 m.y.? Do anomalously low K-Ar dates represent "cooling" dates reflecting episodes of erosion or gravity gliding? Why did the Shuswap domes stay active for such long periods? Are they the locus of great crustal shortening? Other questions equally significant concern pre-Mesozoic tectonism, sources of eugeosynclinal sediments, the sources of granitic plutons, and the regional extent of Proterozoic crust. Further forays into the orogenic belt from the sure ground of the miogeosyncline may help to answer these questions. J. HELWIG(Cleveland, Ohio)

Palaeogeography, Palaeoelimatol., Palaeoecol., 8 (1970) 345-352