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The Tectonics of A Discussion to Accompany the Tectonic Map of North America Scale 1:5,000,000

GEOLOGICAL SURVEY PROFESSIONAL PAPER 628

The Tectonics of North America A Discussion to Accompany the Tectonic Map of North America Scale 1:5,000,000 By PHILIP B. KING

GEOLOGICAL SURVEY PROFESSIONAL PAPER 628

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 DEPARTMENT OF THE INTERIOR

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY

Dallas L. Peck, Director

First printing 1969 Second printing 1970 Third printing 1970 Fourth printing 1978 Fifth printing 1981

For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS

Page "The Tectonic Map of North America" Continued Pw Abstract...______1 foldbelts Continued Introduction ______1 Sedimentary units______4C Tectonic maps defined ______1 Geosynclinal deposits.... __ 46 Historical sketch ______2 Eugeosynclinal deposits _ _ 47 Appraisal of existing tectonic maps ______7 Miogeosynclinal deposits. ______47 Representation of platform areas______8 of geosynclinal deposits..__ 4P Representation of foldbelts______8 Deposits of successor basins....______4P "Tectonic Map of United States"_____ ^_ 8 Younger basinal deposits.______50 "Tectonic Map of ".______9 Thick deposits in structurally negative "Tectonic Map of U.S.S.R. and Adjacent Areas," areas.______50 and "Tectonic Map of "______9 Volcanic units______50 "Tectonic Map of "______11 Plutonic units______51 "Tectonic Map of Mexico" and "Tectonic Map Granitic rocks.______51 of U.S.S.R."______15 Mafic plutonic rocks_____ 5? Representation of subsea areas.______17 Ultramafic rocks______5? "The Tectonic Map of North America".-______18 Description of foldbelts______5f Compilation of the map______18 (K) Innuitian foldbelt______Specifications of the map______19 (J) East foldbelt______Platform areas.______21 (L) Appalachian foldbelt..______56 (A) Platform areas within the . ___ 21 Northern Appalachians.______56 (B) Platform deposits on Precambrian base­ Southern Appalachians.______5f ment ______22 (M) Ouachita foldbelt______61 (C) Platform deposits on . _ 24 Structural systems related to Ouachita (D) Platform deposits on Mesozoic basement. _ 27 foldbelt-...______. 64 (E) Volcanic rocks and associated sediments of (O) Cordilleran foldbelt______64 North Atlantic province.______27 Northern Cordillera______65 (F) Icecaps______29 Central Cordillera. ______6r Foldbelts of Precambrian age______30 Southern Cordillera______.___ 7? Stratigrapbic classification.______30 (P) Pacific foldbelt______74 Tectonic classification. ______32 Alaska. ______7ft Foldbelts of Canadian Shield______33 Washington and Oregon. 7ft Foldbelts of Greenland______36 California and Baja California.__... 76 Foldbelts south of Canadian . ______37 (Q) Antillean foldbelt______77 Precambrian of northern South America______42 (N) Andean foldbelt______7f Phanerozoic foldbelts ______43 Subsea areas______80 The tectonic cycle______43 zones______81 Terminology ______44 Glossary.______8f Geosyncline______..______44 References cited______87 Orogeny______44 Index of units on "Tectonic Map of North America" Names of ...______45 which are referred to in text______9f m IV CONTENTS ILLUSTRATIONS

Page FIQTJRE 1. Structural map of Bighorn uplift, Wyoming and Montana, by N. H. Darton______2 2. Structural maps of the Jura Mountains, , after Albert Heim______3 3. Tectonic map of the , by Rudolf Staub____--_-____- ______4 4. Tectonic sketch map showing the chief belts of folding in the American Cordilleran system, by Hans StillS__ 6 5. Maps of eastern Tennessee and western North Carolina showing three methods of representing the geology.__ 13 6. Tectonic map of north-central Nevada showing "structural stages" in an area of complex superposed rocks and structures.-_____-_--__-_-_-_------_------_------______14 7. Map of Gulf Coastal Plain in Texas, Louisiana, and Mississippi showing structure contours on two horizons in the strata of the platform cover______-______-__-_-_____-_--__-______26 8. Map of North Atlantic Ocean and adjacent lands showing plateau of North Atlantic province. ______28 9. Map of the showing provinces that contain Precambrian rocks of different ages.______34 10. Map of North America showing inferred extent of exposed and buried provinces of Precambrian rocks__ 39 11. Map of northeastern Greenland showing inferred extent of the late Precambrian Carolinidian foldbelt.______55 12. Tectonic maps of the two larger areas of exposure of the Ouachita foldbelt______62 13. Map of northern Nevada showing extent of Antler of middle Paleozoic time______71 14. Map showing transverse structural features in the southern part of the area of the "Tectonic Map of North America," and in adjacent regions______-_-_---_---_--__-______82

TABLES

Page TABLE 1. Comparison of various classifications of the Precambrian-__.______31 2. Classification of orogenies in Canadian Shield______-_____--_--_-_-___---_-__-__-_- __ ___ 33 3. Summary of Precambrian rocks and events south of Canadian Shield______, ______._ 40 THE TECTONICS OF NORTH AMERICA A DISCUSSION TO ACCOMPANY THE TECTONIC MAP OF NORTH AMERICA, SCALE 1:5,000,000 By PHILIP B. KING

ABSTRACT use and understanding of the map, by providing expla­ The "Tectonic Map of North America," on a scale of nations more lengthy than could be included in th^ 1: 5,000,000, has been compiled by the United States Geologi­ cal Survey in collaboration with other national geological sur­ legend of the map itself. Such a discussion is the mor> veys, and with the assistance of various individuals. The desirable because the map embodies innovations derived- compilers made use of tectonic maps of some of the countries partly from techniques of tectonic mapping that hav maps that have been published or are in process of publication. been developed during the last few decades by geologists In addition, many other basic maps and reports were consulted. in other countries, and that may not as yet be well North America is divided technically into foldbelts of dif­ known, or be well understood in North America. ferent ages and platform areas where flat-lying or gently tilted rocks lie upon basements of earlier foldbelts. The two most TECTONIC MAPS DEFINED extensive platform areas are those with Precambrian basement in the central , and those with Paleozoic basement in the A tectonic map portrays the architecture of the up­ Atlantic and Gulf Coastal Plains. Configuration of the upper per part of the 's crust that is, the features prc - surface of the basement beneath the platforms is shown by contours on a 500-meter interval. duced by deformation and other earth forces and rep­ The foldbelts include three of Precambrian age whose principal resents this architecture by means of symbols, pattern1" exposures are in the Canadian Shield, but which also emerge and colors. Such a map differs from the more familir.r in various outlying areas. The foldbelts of younger ages lie areal geologic map, whose primary aim is to represent nearer the edges of the , and include four that are the surface distribution of rocks of various kinds and mainly of Paleozoic age, two that are mainly of Mesozoic age, and two that are mainly of age. Each foldbelt was ages. Nevertheless, most tectonic maps contain some in­ formed during a geotectonic cycle many geologic periods in dication of the ages and kinds of rocks from which th^ length, beginning with a geosynclinal phase, passing through structures were made, and areal geologic maps contain a time of , and ending with a postorogenic phase. some indication of the structures of the rocks repre­ On the "Tectonic Map of North America" the foldbelts are sented, so that distinctions between the two kinds of distinguished by different colors according to age; where several significant times of deformation occurred within them, these maps are not absolute. Tectonic maps are nearly synony­ are represented by tints of the prevailing colors. The different mous with structural maps, just as the subject of teu­ kinds of rocks which make up the foldbelts are shown by pat­ tonics is nearly synonymous with that of structural terns of these colors. geology. Nevertheless, geologists commonly make a In the foldbelts, the principal units on vague distinction between structural geology and struc­ the map are those which formed in the eugeosynclinal and tural maps, which deal primarily with the description, miogeosynclinal areas. In some foldbelts, deposits are preserved which were laid down in successor basins during or shortly representation, and analysis of structures, mostly on a after the main orogenies. In the foldbelts of western North restricted scale, and tectonics and tectonic maps, which America various subdivisions are also shown in the Cenozoic synthesize these data over greater areas, inevitably with sedimentary rocks and terrestrial volcanic rocks. a larger amount of interpretation. Among the plutonic rocks of the foldbelts, granitic rocks Many other kinds of maps are being made which are most extensive; they form large to small masses mainly in the eugeosynclinal areas. More mafic and more alkalic vari­ show earth features, some of which resemble and some eties are separately indicated. Ultramafic rocks are important of which differ from tectonic maps as here defined: in places, especially near the Pacific Coast and in tine Caribbean 1. Paleotectonic maps show geologic and tectonic ; most of these were emplaced in their present positions features as they existed at various times durir^r by tectonic rather than by magmatic processes. the geologic past, rather than the sum of the tev INTRODUCTION tonics as it exists today. Most of the paleotectonic maps that nave been made portray in much detail This discussion is a companion to the "Tectonic Map the sedimentary facies and the thickness of strata of North America," which is being published separately in the cratonic areas, but show few of these details by the U.S. Geological Survey. It is intended to aid the in the more intensely deformed areas. THE TECTONICS OF NORTH AMERICA

2. Neotectonic maps are a kind of paleotectonic map in in the "Bulletin of the American Association of Pe­ that they represent the tectonic features produced troleum Geologists." A parallel evolution of structural during one part of geologic time, in this case the and tectonic maps occurred in Europe, where notable Quaternary or at most the Quaternary and latest maps have portrayed, for example, the folds and faults Tertiary. Because of the epeirogenic nature of of the Jura Mountains (fig. 2), and the superposed much of this latest deformation, neotectonic maps and structural layers in the Alps (fig. 3). emphasize the broad upwarps and downwarps of the crust. 3. Paleogeographic maps show the probable extent of lands and seas as they existed at various times dur­ ing the geologic past. They thus resemble paleotec­ tonic maps, but involve a much greater element of interpretation. 4. Paleogeologic maps show the areal geology of a sur­ face of unconformity that has been covered by a younger body of strata. This buried areal geology has tectonic significance, but it is generally not feasible to represent it on a tectonic map; tectonic maps and paleogeologic maps should supplement rather than duplicate each other. 5. Geophysical maps show the instrumentally deter­ mined values of gravity, magnetic intensity, or other physical properties of the earth, generally by means of contours. The contours express nu­ merical values produced by the summation of many earth processes, not all of which are known. The data are not themselves tectonic, although many of them have ultimate tectonic causes. Interpreta­ tions of these geophysical data are frequently helpful in making tectonic maps.

HISTORICAL SKETCH Geologists have been making structural and tectonic maps since the early days of the science. Even some of the early structural maps showed folds, faults, and structure contours in much detail, but most of them dealt with rather small areas on large scales. On the other hand, the earlier tectonic maps, covering larger areas such as whole countries, , or the , were on small scales and were primarily intended to por­ tray the tectonic or the historical-geological predilec­ tions of their authors. Only in recent decades have tec­ tonic maps approached the scope and refinement of areal geologic maps. In the United States, many excellent structural maps appeared during the first part of the century in the FIGURE 1. Structural map of Bighorn Mounts ins uplift, folios and professional papers of the U.S. Geological Wyoming and Montana, by N. H. Darton (Darton and Survey; among these, the structural maps by N. H. Salisbury, 1906, p. 13). Configuration of uplift shown by Barton of various areas in the Western States are contours on base of Madison . Dashed lines show classic (fig. 1). Since 1916, many excellent structural approximate configuration where all sedimentary rocks have been removed by . Heavy lines are faults. maps of small to large areas have also been published Stippled, areas are covered by Tertiary deposits. HISTORICAL SKETCH

FIGURE 2. Structural maps of the Jura Mountains, Switzerland, showing folds and faults: A. Part of a detailed map. B, Generalized map of the whole area. Copied from Albert Heim (1919, pi. 20 and fig. 103). Longitude is in degrees east of Paris. THE TECTONICS OF NORTH AMERICA

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W o HISTORICAL SKETCH One of the most ambitious of the early tectonic maps basins in areas of unfolded rocks (possibly by structure con­ was the "Carte tectonique de 1'Eurasie" on a scale of tours) ; salt domes and anticlines; monoclinal folds. Time relations should be indicated, so far as practicable, by 1:8,000,000, that was compiled and hand colored by conventional patterns or colors. If conventions are chosen judi­ fimile Argand. He presented this at the 13th Interna­ ciously, they may be superposed in areas that have experienced tional Geological Congress in Brussels in 1922 as a com­ repeated diastrophism. It may even be feasible to indicate im­ panion of his -making treatise on "Le tectonique portant vertical movements in folded belts, such as the lat? de 1'Asie" (Argand, 1924). A colored reproduction of uplift of the Appalachian region. Considerable ingenuity will be required to represent all the complex disturbances in som° this map on a scale of 1:25,000,000 was published by western areas, even where adequate information is available. the 13th Congress in 1928. The state of tectonic knowl­ It does not seem practicable to show the geologic ages of indi­ edge at the time is suggested, however, by the large vidual faults. However, faults that are recognized as 'active' blank areas which were left in various parts of the map. can be distinguished from those supposedly 'dead'; and it may Argand's map had a great seminal influence among be desirable to indicate that faults in an important group an essentially contemporaneous. geologists, as it created a desire for comparable maps of other areas or continents. Thus, in 1922 the Com­ Many of these aspirations were realized on the "Tec­ mittee in Tectonics was organized in the Division of tonic Map of the United States" as it finally evolved, Geology and Geography of the U.S. National Kesearch but some had to be discarded as infeasible. Nevertheless, Council, and the first report of its chairman, Rollin T. representation of most of the items listed has been at­ Chamberlin (1923, p. 3), stated: 1 tempted on various tectonic maps made subsequently. Final specifications for the "Tectonic Map of the United One of the great needs in our field is a series of tectonic maps of the different continents. An important advance in this direc­ States'? developed as the work of compilation pro tion has recently been accomplished by Argand, whose magnifi­ grossed, and were discussed and adopted by members cent structural map of was one of the outstanding of the committee during periodic meetings. The worJ- exhibits at the International Geological Congress at Brussels of compiling the map was divided among the committee last summer. Our Committee voted to commence work on a tec­ members, each assuming responsibility for one part of tonic map of North America which would bring out the trends of folding, the principal lines of faulting, the axes of doming, the country; in the end, 14 different parts were thus and related structural features. Messrs. Willis and Mansfield compiled. Compilation of the map was largely com­ are to undertake this very important project pleted by 1939, but there were inevitable discrepancies Later, the Committee on Tectonics realized that there between the results of the various compilers so that the were still insufficient tectonic data available to represent results required review and editing} this was done b^ all of North America, and decided to restrict its objec­ Philip B. King under Longwell's direction. Drafting tive to preparation of a tectonic map of the United and printing of the map were delayed by the exigencies States. The task of compiling this map began in 1934 of World War II, and it was not published until 1944. when Chester R. Longwell assumed chairmanship of Completion of the "Tectonic Map of the United the committee. Early aspirations of the committee in States" prompted the making of the comparable "Tec- regard to the map are suggested in Longwell's tonic Map of Canada," which was compiled by a com­ prospectus (Longwell, 1934, p. 3): mittee of the Geological Association of Canada under the chairmanship of Duncan R. Derry (Derry, 1950). It is suggested that the map represent the following features: On this map, many of the same specifications as thos^ trend-lines in folded belts, with axes of individual major folds so far as the scale of the map permits; direction and degree of for the United States map were followed, but they in­ important overturning of folds; cross folds or important cluded some innovations required by the differing tec­ changes in pitch of major axes; direction and degree of tonic features of that country. regional dips; all important faults, with appropriate symbols In Europe, in the meantime, Hans Stilte had been and figures showing, so far as known, direction and degree developing his philosophy of geotectonics and his of dip, amount of throw and nature of displacement whether classification of tectonic features in a lengthy series of normal, reverse or strike-slip; major thrusts (with a special publications, which were mostly illustrated by tectonic convention or color to designate overthrost masses); belts of en echelon faults; important areas of , with sketches (fig. 4). A desire was felt among many of strike and attitude of cleavage so far as it can be shown; Stille's European colleagues to represent his and other areas of Precambrian rocks related to erogenic zones, as dis­ tectonic concepts in the form of more precise and elabo­ cussed recently by Bucher; all major igneous masses; swells and rate tectonic maps, and in this effort leadership wa^ assumed by the Soviet geologists. A. D. Arkhangelsk:7 1 For the subsequent actions of this committee, see the a«""n1 reports of the Division of Geology and Geography of the National Kesearch (1941) had developed "an historico-morphologioal Council. The history of the committee has also been narrated by Long- method, which enables the earth's crust to be tectonic * well (1944a, p. 1767-1760), from whose account the succeeding para­ graphs are largely abstracted. ally zoned according to the degree of completion of th« 6 THE TECTONICS OF NORTH AMERICA

ATLANTIC

Age of climax of alpinotype deformation 1. jjyuu Neocimmerian (Nevadan) 2. JHIMUL Subhercynian (Andean) 9 ' '»' ' Laramide 4. faoeg Late Tertiary .-.-.'. . . Continental shelves

FIGURE 4. Tectonic sketch map showing the chief belts of folding in the American Cordilleran system, by (1936, p. 138). processes of folding of the geosynclinal under­ to elaborate these methods and make possible the going transformation into a platform." - He prepared a preparation of tectonic maps on larger scales, "Tectonic scheme of Eurasia" on a scale of about 1:40,- The first map by Schatsky, published in 19£3, covered 000,000 that was published in the "Proceedings of the the Soviet Union and adjacent areas in color on a scale 17th International Geological Congress" (Arkhangel- of 1:4,000,000, but in a very generalized manner. This sky, 1939, pi. 2, p. 304). It remained for N". S. Schatsky map was preliminary to a more detailed compilation that was made in collaboration with N. A. IMiaevsky, 2 Brief summaries of the Soviet work on tectonic maps are given by A. A. Bogdaiioff, and M. V. Muratov, with the aid of Spizaharsky and Borovikov (1966) and by Yanshin (1966a), from which most of this and succeeding remarks are taken. 41 contributors. The resulting map was published in APPRAISAL OF EXISTING TECTONIC MAPS 1956, in color on a scale of 1:5,000,000, and represented amount and quality of the information available. Qthr^ not only all the Soviet Union, but also those parts of variations indicate wide divergences in the objectives neighboring countries that lay within the area of the of the compilers. To evolve specifications for the "Tec­ map base. tonic Map of North America," all the varieties of exis*- At the 20th International Geological Congress in ing tectonic maps were reviewed, in an effort to find an 3 Mexico in 1956, a Subcommission for the Tectonic Map emulate the better features of each. of the World was organized within the Commission for The basic components of all tectonic maps are two the Geologic Map of the World. Preliminary to prepa­ fold: Those of the first order indicate the nature of ration of the world map, the subcommission encouraged the rocks from which the structures are made by means the compilation of tectonic maps of the various conti­ of patterns and colors. Those of the second order repr**- nents. One of the first products was the "Tectonic Map sent the structures themselves (folds, faults, and tl <* of Europe" on a scale of 1:2,500,000 prepared by geolo­ like) by means of symbols. On all tectonic maps there gists of many European countries, which was issued is a further subdivision (either stated or implied) of in 1964 (Schatsky, 1962); its specifications closely fol­ the first-order components between those of the plat­ lowed those of the tectonic map of the Soviet "Union form or cratonic areas, where the surface strata are and adjacent areas of 1956, but with elaborations. gently tilted or warped, and those of the foldbelt- Ait the 21st International Geological Congress in Co­ where the rocks are much deformed and intruded. This penhagen in 1960, the U.S. Geological Survey agreed subdivision appears on the early tectonic map t y to assume leadership in preparing a tectonic map of Argand (1928), where Eurasia is subdivided into "pays North America, in collaboration with other national tabulaires" and <4pays pliss&s" by means of color geological surveys in North America, and with the co­ patterns. operation of various research institutions and interested The following examples indicate some of the ways individuals. To facilitate this collaboration a committee in which first-order components are shown on tectonic on the map was established, with George V. Cohee as maps: chairman. Philip B. King was designated as chief com­ 1. Some color patterns are used, mainly in the fold- piler. Work on the tectonic map was begun by the com­ belts, and the remaining areas, mainly in the piler in June 1961, and was carried to completion in platforms, are uncolored. Example, "Tectonic December 1966. The completed map was exhibited at Map of United States," 1944,1962. the 23d International Geological Congress in Prague 2. Color patterns show stratigraphic units that have in 1968, and printed copies were available in 1969. some rudimentary tectonic significance. Ex­ Tectonic maps of many other countries or continents amples, "Tectonic Map of Canada," 1950; "Teu­ tonic Map of Australia," 1960; "Geologic- have now been published or are in preparation, com­ tectonic Map of Northern Venezuela," 1962. plete listing of which would be tedious here. Also, work 3. The platform areas are colored with layer tint"; by the subcommission is now far advanced on the long- the foldbelts are colored according to a few planned "Tectonic Map of the World" (see Bogdanoff widespread epochs of climactic orogeny (for and others, 1966). example, Caledonian, Variscan), the rocks of the foldbelts being subdivided in turn in+o APPRAISAL OF EXISTING TECTONIC MAPS tectonic units ("structural stages") that formed The specifications of the tectonic maps previously prior, during, and after the orogenies. Ex­ mentioned, and others not listed, differ from each other amples, "Tectonic Map of U.S.S.R. and Ad­ in many particulars. These differences reflect evolving jacent Areas," 1956; "Tectonic Map of Europe," and diverse methods of tectonic mapping, as the meth­ 1962; "Tectonic Map of Eurasia," 1966. ods have not been stabilized like those used for conven­ 4. Colors in the foldbelts represent primarily the tional areal geologic maps. To some extent these varia­ ages of folding of the respective units, the^ ages being more minutely subdivided than in tions are useful; little future progress can be made in (3). Example, "Tectonic Map of Canada," 19f>. tectonics if the subject is to be ruled by a single set of 5. Colors in the foldbelts show rock sequences claprn- dogmas, nor can progress be made in tectonic mapping fied according to "geotectonic cycles" the a^e if the maps are forced to adhere to a single set of speci­ spans of the "geotectonic cycles" differing frcTi fications. Some of the variations reflect special condi­ one foldbelt to another. Examples, "Tectoric tions in the different regions, not only as to the kinds Map of Mexico," 1961; "Tectonic Map of of tectonic features to be represented, but also as to the U.S.S.R.," 1967. 8 THE TECTONICS OF NORTH AMERICA Representation of second-order structural features, shown on the map referred to, and are distinguished by shown by symbols, offers fewer problems in tectonic layer tints of different colors; basements of o*her ages, mapping than representation of first-order components. in other colors, are shown on subsequent maps The con­ Most of the features to be represented and the symbols figuration of strata within the sedimentary cover is used for them are reasonably standardized, so that they shown by colored contour lines that are superposed on are much the same from one map to another. New or the basement contours and layer tints. This method of unusual symbols for special structural features, to suit representation produces a very effective wall ir ap, as the the desires of the compiler are not objectionable or configuration of the base of the sedimentary cover is confusing, because these symbols can be identified in visible at a glance, leaving the configuration of the the legend. strata within the cover to be ascertained by more de­ tailed study. REPRESENTATION OF PLATFORM AREAS Besides the features discussed, many recert tectonic The objectives in showing platform areas are similar maps differentiate "foredeeps" or "marginal homo- on all tectonic maps, however much they may differ in clines" at the edges of the platforms by means of special appearance namely, to express the gentle tilting and color patterns. Many maps show the "foredeeps" by warping of the covering strata, the different structural means of stripes of the same color as the adjacent fold- layers of which they are composed, and the configura­ belt, alternating with stripes in the same color as that tion of their basements. of the underlying basement; this is intended to indicate On the two versions of the "Tectonic Map of the the historical significance of the "foredeep." Neverthe­ United States (Longwell, 1944b; Cohee, 1962), plat­ less, "foredeeps" and "marginal homoclines" are actu­ form areas are largely left uncolored, except for inliers ally parts of the platform, in which the strata and their of Precambrian rocks and for narrow bands of color basement have been deeply downwarped or steeply tilted which indicate the edges of structural layers that are against the adjoining foldbelts. The present reviewer significantly unconformable on the layers beneath (for questions the need for distinguishing these foatures by example, bases of , , lower separate color patterns. The method might be helpful in Tertiary, and upper Tertiary). Among other purposes, regions of scanty information, but where much infor­ these colored borders differentiate the platform of the mation is available their nature is evident from the con­ Atlantic and Gulf Coastal Plains from the platform of touring of the basement and the higher strata. the central craton. Configuration of the strata is indi­ cated by brown structure contour lines at a uniform REPRESENTATION OF FOLDBELTS interval of 500 feet, an effort being made to select con­ "TECTONIC TMTAP OF UNITED STATE"*" toured horizons that can be extended as widely as pos­ On the two versions of the "Tectonic Map of the sible ; on the 1944 version of the map 19 horizons were United States" (Longwell, 1944b; Cohee, 1962) the mio- used, on the 1962 version 36. In selecting horizons for geosynclinal and other areas of folding and faulting contouring on the first version of the map, the highest are illustrated mainly by structural symbols in black, horizon that would give good results over a wide area without colored overprint, the strength of the deforma­ was chosen; this was due to the state of knowledge at tion being suggested by the relative crowding of the the time, when configuration of the deeper horizons was symbols. Other tectonic features in the foHbelts are largely speculative. This consideration was abandoned indicated by color patterns: (1) Precambrian rocks; in the second version, on which some of the areas were (2) metamorphic rocks, mainly eugeosynclinal (of contoured on the top of basement rocks rather than on strata in the cover. On the two maps, portrayal of the Paleozoic age in east, of Mesozoic age in west) ; (3) in­ trusive rocks, the ages being indicated by different platform areas is effective for detailed study, but the colors; (4) sedimentary rocks in postorogenic depres­ general absence of colors detracts from their usefulness as wall maps. sions (of age in east, of late Cenozoic age in west) ; (5) terrestrial volcanic rocks in the. western fold- A different method of representing platform areas was introduced on the "Tectonic Map of the U.S.S.R. belts, mainly postorogenic and of Cenozoic age. and Adjacent Areas" (Schatsky and others, 1956), and Ages of deformation in the foldbelts are not indicated has been closely followed on many subsequent maps. on the "Tectonic Map of the United States." The gross Contours are drawn on the surface of the basement rocks ages of the foldbelts in the United States have long been at 500- or 1,000-m intervals, and the whole area filled in familiar to geologists the Paleozoic deformation in the with layer tints that indicate the depth to basement. Appalachian region on the east, and the Mewzoic and Basements of Precambrian and Paleozoic ages are Cenozoic deformation in the Cordilleran regain on the APPRAISAL OF EXISTING TECTONIC MAPS 9 west and to attempt further refinements requires an western part of the continent is suggested mainly by untangling of many superposed deformations, or of ex­ the distribution of the color patterns, rather than by trapolation beyond available data (see also Longwell, structural symbols. 19Ma, p. 1772). The gross 'ages of deformation are im­ plied by the representation of metamorphic and intru­ "TECTONIC MAP OF TT.S.S.R. AND ADJACENT AREAS," sive rocks of Paleozoic age in the east, and of Mesozoic AND "TECTONIC MAP OF EUROPE" and Cenozoic ages in the west. This method of repre­ Representation of first-order components in the fold1- sentation, although satisfactory for showing the tec­ belts on the "Tectonic Map of U.S.S.R. and Adjacert tonics of a single country, is less well adapted to Areas" (Schatsky and others, 1956) has served as a pro­ representation of continents or other large regions, totype for many subsequent tectonic maps, and notably where the ages of deformation in the foldbelts are much for the "Tectonic Map of Europe" (Sohatsky, 1962); more diverse. the features of these two maps will be considered hero. "TECTONIC MAP OF AUSTRALIA" On both maps, foldbelts are colored according to th^ age of the assumed climatic orogeny, although the rock^ On the "Tectonic Map of Australia" (Geological So­ within the foldbelts themselves may be much older o^ ciety of Australia, 1960) the whole continent is shown much younger. On the "Tectonic Map of the U.S.S.B., in color patterns, which represent stratigraphic units. and Adjacent Areas" eight ages (or regions) of folding The classification of the units is as follows: are distinguished, three in the Precambrian and fiv*. Stratified rocks. Sedimentary and volcanic rocks, tbe latter in the Phanerozoic: namely, , , Bai- shown by black overprinted patterns, where present. kalian (Riphean), Caledonian, Hercynian, Mesozoic of Cenozoic: Undifferentiated; Tertiary. Mesozoic: Undifferentiated. Pacific border, Alpine, and Cenozoic of Pacific border. Paleozoic: Upper ( to Upper Carboniferous) ; Mid­ On the map of Europe eight ages (or regions) of fold­ dle (Upper Carboniferous to Middle ) ; Lower ing are distinguished, five in the Precambrian and thre^. (Middle Devonian to ). Proterozoic: Undifferentiated; Upper (subdivided into upper in the Phanerozoic: namely, Archean, Svecofennian and lower) ; Lower (subdivided into upper and lower). (Karelian), Gothian (Daslandian), , Baikalian Archean: Undifferentiated; gneiss; sediments and meta- (Cadomian, Assyntian), Caledonian, Variscan (Her­ sediments. cynian), and Alpine; there are additional categories for Intrusive igneous rocks. (Each rock type shown in same color throughout, the ages of the different bodies being indicated Precambrian folding undivided and Paleozoic folding by letter symbols: acid ( and porphyry); alkaline undivided. (mainly syenite) ; intermediate to basic (hypabyssal) ; Each foldbelt is subdivided in turn into many map ultra basic. units, of which the units on each map for the Variscan Each stratified unit is shown by the same color pattern or Hercynian regions are given below as samples: over the entire continent; the units are not themselves "TECTONIC MAP OF U.S.S.R. AND ADJACENT AREAS" tectonic, as their tectonic significance varies from one Regions of Hercynian folding region to another. However, the tectonic significance of 19. Precambrian of cores of anticlinoria. each is indicated in an accompanying table, of which 20. Lower structural stage undivided (China and Taimyr, Cair- the entries for the lower Paleozoic unit are a sample: brian; Tien-Shan, Riphean-). Queensland, folded Tasman geosynclinal zone in east, gently 21. Lower structural stage, lower substage (Kazakhstan, Rip] - folded sediments in west; New South Wales, Tasman geosyncli­ ean-Cambrian; Altai, Cambrian-Ordovician; Ural, Ripl - nal zone; Victoria, Tasman geosynclinal zone; Tasmania, geo­ ean-Cambrian; east slope of Ural, Riphean-Ordovician). synclinal deposits, moderately to strongly folded; South Aus­ 22. Lower structural stage, upper substage (Altai, Ordovician; tralia, folded Adelaide , with gently warped strata Kazakhstan, Cambrian3-Ordovician; Ural, Ordovician an-l to northwest; Western Australia, marine and continental de­ locally Cambrian). posits, gently folded; Northern Territory, intracratonic basins, 23. Middle structural stage, lower substage (Tien-Shan an* gently folded. Kazakhstan, -Devoniani-a; Ural, Silurian-Devor- ian and locally Ordovician; China, Silurian-Devonian; The method of representation of the first-order com­ Altai and Taimyr, Ordovicians-Silurian). ponents used on the "Tectonic Map of Australia," while 24. Middle structural stage, upper substage (Tien-Shan, Kazakl - differing from that on the "Tectonic Map of the United stan and Altai, -Carboniferousi; China, Devo- States," is equally objective. It is well adapted to a niani-CarboniferouSi; Taimyr, Devonian-Permian; Ura* Devonians; elsewhere Devonian-rCarboniferouSi). continent where detailed tectonic information is only 25. Upper structural stage, interior basins (China, Carbonifer- partly available; thus, the existence of many basins, ousi-Permian; Tien-Shan ana Kazakhstan, Carboniferous" uplifts, and other features in the poorly known north­ Permian; Ural, Carboniferousi-TriaseiCi). 10 THE TECTONICS OF NORTH AMERICA 26. Upper structural stage, regional depressions ( Ural foreland, time.8 The climactic orogeny of the f oldbelt is selected OarboniferouSj-Triassici; Kuznets, Carboniferous-Triassic as the one during which a previous geosynclinal region Taimyr foreland, Permian-Triassic; Donetz, Carbonifer­ ous-Permian). was consolidated into a platform a time wh<*Ti forma­ Intrusive igneous rocks: Ultramafic intrusives, Caledonian tion of alpinotype structures gave place to formation granitoids, granitoids undivided, late Hercynian granitoids, of germanotype structures. Each orogenic era (Cale­ and alkalic intrusives. donian, Variscan, Alpine, and so forth) is conceived of "TECTONIC MAP OF EUROPE" in broadest terms. Each orogenic era affected extensive Region* of Variacan or Hercynian folding areas along linear belts and had a wide time range; thus, KV. Reworked massifs of Karelian and older folding (Sudetes, the climactic orogeny in the "regions of Vari^an fold­ Armorica, Bohemia). ing" might have occurred several periods earlier or later BV. Reworked massifs of Baikalian folding. at one place in the f oldbelt than in another. CV. Reworked massifs of Caledonian folding. Evolution of a f oldbelt is expressed by a sequence of VB, etc. Ancient cores reworked by Variscan folding (various "structural stages" ("etages structuraux"), vhich suc­ ages indicated by second index letter). Migmatized older Paleozoic and Precambrian of Ural*. ceed each other in an invariable order, which are ex­ plained as follows (Schatsky and Bogdamff, 1957, Bugeosynclinal (interior) zones p. 16 1959, p. 8). eV. Undivided. The long history of geosynclinal regions is generally charac­ eVl. Lower structural stage (western Europe, Cambrian-Silu­ terized by inherited development. Their cross-sections there­ rian ; Great Caucasus, Riphean-Paleozoic i; Ural, Paleo­ fore lack such sharply distinguished structural streres as the zoic i-Devonian a) . basement and blanket of . However, close analysis of eV2. Middle structural stage (western Europe and Or eat Cau­ the structure and history of development of geosynclinal regions casus, Devonian-Carboniferousi; Balkans and Anatolia, also uncovers a series of clearly defined structural phases; each Silurian-Carboniferousi; Ural, Devonian»-Carbonif er- of these phases, corresponding to a given stage of development OUSl). of the geosynclinal region, consists of a group of formations Miogeosynclinal (exterior) zones that is often separated from those above and below by regional unconformities. Deep-seated (lower) structural phases are usu­ mV. Undivided (south Ireland and Trans-Caspian region, De­ ally dominated by volcanically derived and sedimentary forma­ vonian-Carboniferous). tions (of the spilite-keratophyre type and others), correspond­ mVl. Lower structural stage (western Europe, Cambrian- ing to early stages of geosynclinal development. The middle Devonian; Armorica, Cambrian-Devonian; Morocco, structural phases often contain carbonates, shales and gray- Cambrian-Carboniferous; Or eat Britain, Silurian-De- wacke formations, pierced by granitoid intrusions. Upper struc­ voniani; Little Caucasus, PaleozoiCi; Ural, Ordovician- tural phases contain flysch, molasses, coal-bearing basins and Devoniaih; Moravia, Devonian*.*). mV2. Middle structural stage (western Europe, Devonian; other formations. Armorica, Carboniferous i; Little Caucasus and Swient- Thus, the distinction between successive "structural okrzysky Mountains, Devonian-Carboniferous i; Ural, stages" is two-fold; each "stage" is, first of all, a distinc­ Devonian ^-Carboniferous 2 ; southern France and Mora­ tive body of rocks whose facies is closely related to the via, Devonian 2-Carboniferous i; Morocco, Carbonifer- OUSi). tectonic evolution of the foldbelt, and second, each mV2-3. Middle and upper structural stages, undivided (mostly body of rocks is separated from those above and below Devonian-Carboniferous t ; Great Britain, Devonian «- by regional unconformities. Carboniferous.). Use of the term "structural stage" for subdivisions All zones of the sequence in foldbelts creates an unfortunate con­ V3. Upper structural stage (western Europe, Carboniferous- fusion with the "stratigraphic stage," which is defined Permiani; Balkans, Anatolia, and Bwientokrzysky as a time-stratigrrapMc unit next m rank below a series Mountains, Carboniferous ^Permian; Morocco, Qaxfoon- (American Committee on Stratigraphic Nomenclature, iferoust-Penniani; Caucasus, Paleozoic*; Mangyshlak, 1961, article 31, p. 658-659; see also Gignoux, 1955, p. Permian j-Triassic). 12-15), hence a unit which is essentially of th°> same age Ya. Foredeepe. KZ and scope at all places (for example, Oxford*an, Ceno- -Foredeeps covered by Mesozoic and Oenozoic sediments. Ve manian, Maestrichtian). Study of the excerpts from the Intrusive igneous rocks legends which are quoted above makes it abundantly Varisoan granitoids: (a) Synorogenic, (b) postorogenic. clear that "structural stages" are rock-8tratig*«vpJtic and not time-stratigrrapMc units; the range o* a single The type of representation used on these two maps is "stage" may be from Upper Carboniferous to Lower claimed by its proponents to be "an historical-morpho­ logical method," in that it shows not only the morpho­ This and the next paragraph are paraphrased from Schatsky and Bogdanoff (1967) and from several pamphlets that have been prepared logical (structural) features, but also how these features by them for the Snboommiwion for the Tectonic Map of the World. and the foldbelt as a whole developed through geologic See especially Schataky and Bogdanoff (1960) and Bogd-noff (1962). APPRAISAL OF EXISTING TECTONIC MAPS 11 Triassic in one place, from Permian to Triassic in an­ further, the ones where the structural features repre­ other, or from Carboniferous to Permian elsewhere, yet sented by symbols are most crowded, thus adding to th^ the rocks of the "stage" are presumably of much the complexity of the map pattern (fig. 5). The surfac^ same sedimentary facies from one locality to another. area occupied by a "stage" in a deformed region may The legend for the "Tectonic Map of the U.S.S.R. have little relation to its relative lithologic or historica I and Adjacent Areas" does not seem to provide for longi­ significance; moreover, outcrops of the "stages" may b^ tudinal subdivisions of foldbelts, even though such sub­ interrupted by later superposed structures, or by over­ divisions might have had different histories. This is lapping postorogenic deposits (fig. 6). The true signifi­ rectified on the "Tectonic Map of Europe," in which the cance of any "structural stage" can only be represented foldbelts are divided into eugeosynclinal (internal) and on a paleotectonic map which will show its original ex­ miogeosynclinal (external) zones, each with its own tent beyond its present outcrops, both where the "stage" sequence of "structural stages" (see excerpt from legend has been removed by erosion, and where it has been quoted above). buried beneath younger strata. The concept of "structural stages" in the sequence of A final comment can be made on the method of color­ strata of foldbelts is valid, although it is questionable ing the foldbelts on the two maps. The colors used ex­ whether unconformities make significant boundaries be­ press the climactic times of orogeny in the foldbelts, dif­ tween them. "Stages" can be recognized in most of the ferent units of the sequence being shown by tints of th^ foldbelts of North America, an ideal example being prevailing colors, whatever the actual ages of the units. in the Paleozoic rocks of the Ridge and Valley province The maps thus vividly emphasize the foldbelts, produc­ of the southern Appalachians: ing an eloquent picture on the wall, but the technique 4. Coarse clastic deposits, continental or brackish water, makes detailed study of the foldbelts difficult without with coal measures; broadly "molasse." (Mainly constant reference to the legend. Precambrian basement Pennsylvanian, with Lower Permian locally at rocks are shown in many different colors, from one fold- top; base varies downward from base of Pennsyl­ belt to another, and postorogenic deposits of the inter­ vanian to as low as Upper Devonian). nal and external basins are give the same color as th<* 3. Fine-grained marine elastics, including both typical climactic orogeny, although they may be many period 0 "flysch" and broad sheets of deposit. (Largely younger. In some foldbelts the outcrops of the actusJ middle Paleozoic; base varies from base of Middle deformed strata are rather small, as they are nearly Ordovician to base of Upper Ordovician, or even buried by extensive postorogenic deposits. Many cf higher.) these postorogenic deposits are a platform cover, which 2. Carbonate sequence (largely Cambrian and Lower is coextensive with the platform cover of surrounding Ordovician, but with top at variable levels, as in­ regions. Drawing a boundary between foldbelt color and dicated above). platform color in postorogenic deposits becomes highly 1. Basal clastic sequence; , arkose, shale, basal subjective, unless there are abundant subsurface date. conglomerate on Precambrian rocks. (Earliest fos- siliferous Cambrian, but extending downward into "TECTONIC MAP OF CANADA" unfossiliferous strata classed as Cambrian?). The specifications of the two maps just discussed coir- Sequences of "stages" with very similar lithologic bine several disparate items the nature of the rock?, characters but with widely variable time ranges, can be the evolution of the rock sequence, and the ages of defor­ recognized in parts of the other foldbelts of North mation. Many later tectonic maps use similar specifica­ America. In some foldbelts are incomplete or repeated tions, but with significant divergences in the relative sequences of "stages," and in still others are sequences emphasis given to the different items. of "stages" of very different lithologic character (as in The "Tectonic Map of Canada" (Stockwell, 1969) the California Coast Ranges). thus places primary emphasis on the ages of deformr- Nevertheless, the present compiler has doubts as to tion. On this map, 15 orogenies are differentiated, five in. the value of "structural stages" in tectonic mapping. the Precambrian and 10 in the Phanerozoic, namely: Does representation of the outcrops of the "stages" on Kenoran (late Archean),Hudsonian (late Aphebianc1" a tectonic map give a true picture of the historical de­ Early Proterozoic), Elsonian (late Paleohelikian c* velopment of the foldbelt? Is it possible to represent early Middle Proterozoic), Grenvillian (late Neohelilr- adequately both the morphology of a foldbelt and its ian or late Middle Proterozoic), East Kootenay (middle history of development on a single sheet of paper? The Hadrynian or Late Proterozoic), Taconic (early Late "structural stages" in foldbelts characteristically crop Ordovician), Acadian (Middle to Late Devonian), E^- out where the rocks are much deformed, hence their out­ lesmerian (Early ), Appalachian (Lat"> crops may be very narrow. These deformed areas are, Mississippian to Early Pennsylvanian), Melvillean 12 THE TECTONICS OF NORTH AMERICA

85° 82° KENTUCKY 84° v ___ __ 83° VIRGINIA EXPLANATION TENNESSEE

36°

35°

Sedimentary contac*.

852 _ _ KENTUCKY _ ___ 84 83° VIRGINIA ' ' '.'.'.TENNESSEE' ' "^ " EXPLANATION

36°

35'

Sedimentary contact

B APPRAISAL OF EXISTING TECTONIC MAPS 13

85° 82° JCENTUCKY 84° ^______. 83" VIRGINIA EXPLANATION TENNESSEE

Axis of anticline

Axis of

Precambrian crystalline rocks

35°

100 KILOMETERS

G FIGURE 5 (left and above). Three maps of eastern Tennessee and western North Carolina showing different methods of representing the geology in a region of strongly deformed rocks. In figures .4. and 7> the representation of time- stratigraphic and rock-stratigraphic units much obscures the representation of structural features, even though their scale is about 3 times that of the ''Tectonic Map of North America." Compiled from geologic maps of Tennessee and North Carolina, .i, Map showing time-stratigraphic units (geologic systems). /?, Map showing rock-stratigraphic units ("structural stages"). C, Structural map, showing folds and faults.

(Late Pennsylvania!!), Tahltaniaii (Early to Middle Hudsonian orogeny (H) ; folding and granitic intrusions dur­ Triassic), Inklinian (Late Triassic to Early ), ing late Aphebian. Nassian (Middle Juracsic), Columbian (Late Jurassic Hg, Granitic intrusions emplaced during the Hudsonian, including highly granitized gneisses. Hgd, discordant to ), and Laramide (Late Cretaceous intrusions. to ). Hb, Basic intrusions. Hb', mainly gabbro. Ha, mainly Each orogeny is correlated over the whole country; anorthosite. for example, rocks aft'ected by the Appalachian orogeny Hn, Aphebian sedimentary and volcanic gneiss and scl ist. are mapped not only in the Appalachians but also in Hug, mixed with granitic material. Hr, and charnockite. the Arctic Islands. The areas aft'ected by the different Hm, Aphebian miogeosynclinal deposits. He, Apliel ian orogenies are shown by separate colors, the color of the eugeosynclinal deposits. dominant orogeny being used also for areas reworked Heu, Aphebian and (or) Archean slightly metamorphosed from earlier orogenies and for unconformably overly­ eugeosyuclinal deposits. Him, gneissic equivalents. ing deposits. The rocks affected by each orogeny are Hu, Aphebian and (or) Archean sedimentary and volcanic gneisses, commonly mixed with granitic material. Fun, subdivided in turn according to their lithology and ori­ metamorphic rocks in largely unmapped areas, with gin, their degree of metamorphism, and the. deforma­ undivided granitic intrusions. tion to which they have been subjected; some have HKg, Granitic intrusions, emplaced during the Kenoran undergone two or more prior orogenies. and re\yorked during the Hudsonian. HKg, intrusions Two examples of the subdivisions of the rocks of the emplaced during the pre-Kenoran and reworked during foldbelts which are made on the ''Tectonic Map of Can­ the Kenoran and Hudsonian. HKe, Slightly metamorphosed Archean eugeosynclinal de­ ada" are quoted below, one for the Precambrian, one for posits, folded during the Kenoran and reworked during the Phanerozoic: the Hudsonian. EXPLANATION

Tertiary volcanics and sediments

Upper Mesozoic volcanics and sediments NEVADAN OROGENY

Lower Mesozoic shelf deposits

Permian and Triassic volcanics SONOMA OROGENY

Eugeosynclinal Transitional Miogeosynclinal Lower Paleozoic deposits

NEVADA \ 39°30' 39°30' \

20 KILOMETERS

20 MILES

FIGURE 6. Tectonic map of north-central Nevada illustrating the difficulty of telescoped by low-angle thrusting; later, their rocks were invaded by plutons, making a meaningful representation of "structural stages" on small-scale partly covered by postorogenic deposits, and disrupted by block-faulting, so tectonic maps in an area of complex superposed rocks and structures. The that they are now exposed in small, disconnected areas. Compiled from Paleozoic and Mesozoic rocks consist of three sequences, each divisible ini:o county geologic maps of Nevada, and from various U.S. Geological Survey "structural stages." During Paleozoic and Mesozoic time the sequences were publications. APPRAISAL OF EXISTING TECTONIC MAPS 15

HKn, Gneissic equivalents, mixed with granitic material. orogenies recognized in Canada would be merely phases HKr, Granulite and charnockite. of the three or four orogenic eras recognized in Europe (A) ; folding and granitic intrusions mainly and the U.S.S.R., whose climaxes occurred at different during the late Middle to early Late Devonian. A', folding during middle Early Devonian. A", folding possibly during times in different places. Very likely the widely spaced Late Silurian. Precambrian orogenies recognized in Canada are ac­ Ag, Granitic intrusions. tually groups of orogenies like those in the Phanerozoic, An, Upper Ordovician to Middle Devonian miogeosynclinal for which the. individual records have become blurred. deposits. Am', Cambrian to Middle Devonian. Ae. Silurian The classification of the Precambrian rocks and orog­ to Middle Devonian eugeosynclinal deposits. Ae', Ordo­ vician to Middle Devonian eugeosynclinal deposits. An', enies in the Canadian Shield on the "Tectonic Map of derived gneiss and schist. Canada" is useful and expressive. It blocks out larr^s A'm, Hadrynian to Early Devonian miogeosynclinal regions or provinces which have been subjected to oro­ deposits. genic events of widely varying ages, showing within A"g, Granitic intrusions. A"b, basic intrusions. each province the intrusive and supracrustal rocks that A"e, Lower Ordovician to Middle Silurian eugeosynclinal deposits. A"n, derived gneiss and schist. formed during, or only a little before, the climactic ATg, Granitic intrusions emplaced during the Taconic and orogeny, as well as the relics of older rocks affected by modified during the Acadian. ATb, basic intrusions em- earlier orogenies that were reworked by the climactic placed during the Taconic and modified during the orogeny. Representation of the Precambrian rocks of Acadian. the Canadian Shield will be discussed at greater length ATin, Cambrian to Ordovician miogeosynclinal deposits folded during the Taconic and refolded during the Aca­ later (p. 33-36). dian. ATM, metamorphic equivalents. ATe, Upper The classification of the Phanerozoic rocks and ore 7- Hadrynian to Middle Ordovician eugeosynclinal deposits, enios is less appealing to the present compiler. T^<* folded during the Taconic and refolded during the Aca­ large number of orogenies differentiated, and the larre dian. ATE, metamorphic equivalents. ATn, derived gneiss and schist number of rock units mapped in each category obscrre AK"M, Hadrynian or older metamorphosed miogeosynclinal the broader relations. The differentiation gives the im­ deposits, folded during the middle or late Hadrynian and pression, perhaps unintended, of many distinct foJd- modified during the Acadian. AK"E, similar metamor­ belts built against each other, whereas the orogenic phosed eugeosynclinal deposits. AK"n, derived gneiss, units are actually not sharply defined in either time or schist and migmatite. AK'g, Granitic intrusions emplaced during the early place; this compiler considers these units to be subdi­ Hadrynian or before and modified during the Acadian. visions or phases of gross foldbelts that evolved during AK'M, Metamorphosed early Hadrynian or older miogeo­ lengthy tectonic cycles. Moveover, unless unusually com­ synclinal deposits folded during the early Hadrynian or plete age data are available, such a classification in­ earlier and refolded during the Acadian. AK'ng, derived volves a large measure of inference and speculation. A gneiss and schist, mixed with granitic material. AK'e, Earlier Hadrynian or older eugeosynclinal deposits comparable classification would be difficult to make ebe- folded during the early Hadrynian or earlier and re­ where in North America, where the available age data folded during the Acadian. are less complete. The orogenies listed above are differentiated partly "TECTONIC MAP OP MEXICO" AND "TECTONIC MAP by geologic evidence stratigraphically dated uncon­ OF U.S.S.R." formities, intrusive relations, and the like for which A divergence in another direction from the specifica­ the record is unusually complete in parts of Canada. To tions of the "Tectonic Map of the U.S.S.R. and A d- a considerable extent, however, they are based on the jacent Areas," and the "Tectonic Map of Europe" is extensive radiometric dating program of the Geological illustrated by two maps the "Tectonic Map of Mexico" Survey of Canada, and especially potassium-argon dat­ (de Cserna, 1961) and the "Tectonic Map of the ing. Most of the rocks that have been dated are either U.S.S.R." (Spizaharsky, 1966). On these maps, em­ intrusive or metamorphic. phasis is not on the climactic orogenies in the foldbelts, The five Precambrian orogenies are widely spaced, nor on details of the orogenic episodes, but on the broad their climaxes being at about 2,400, 1,700, 1,300, 900, tectonic cycles during which the foldbelts were built. and 700? m.y. (million years) ago, or 700, 400, 400 and On the "Tectonic Map of Mexico" the country is in­ 200 ? m.y. apart. The five Paleozoic orogenies lie within terpreted as being composed of "structural belts," erch a span of 200 m.y. (Middle Ordovician to Late Pennsyl- of which became consolidated during a "geotectonic vanian), and the five Mesozoic orogenies within a span cycle," the cycle being the interval of time during which of 155 m.y. (Early Triassic to end of Cretaceous). The an orthogeosynclinal belt became a craton.4 The fol- figures cited might suggest an acceleration of orogeny 4 The theoretical basis for the map is summarized from de Cserna through time, but the large number of Phanerozoic (1960) and from the brief text printed on the map. 16 THE TECTONICS OF NORTH AMERIO lowing structural units, or belts, are recognized in Mex­ rocks produced during its "geotectonic cycle" s.re clearly ico: basement complex (Precambrian); Jaliscoan struc­ displayed; they are classified as follows on tl ^ map: tural belt (early Paleozoic?); Huastecan structural belt (late Paleozoic to early Mesozoic); Sonoran belt, Mexican geotectonic cycle (or structural 6eZ*) poorly defined (early Mesozoic); Mexican structural Sedimentary and metamorphic rocks Magmatic recks belt (Mesozoic and Cenozoic); Gulf Coast geosyncline Clastic volcanic debris. Final volcanic rock*. (largely Cenozoic); Trans-Mexican volcanic belt (Qua­ Postorogenic debris or molasse, Subsequent volcani? rocks and ternary) ; and Quaternary clastic deposits. On the map, continental and marine. subsequent intrur've rocks. rocks of the Jaliscoan belt are shown in tints of purple, Preorogenic clastic or flysch Anatexitic intrusive rocks. those of the Huastecan in tints of blue, and those of the wedge. Sonoran and Mexican in tints of green, the colors sug­ Eugeosynclinal and miogeo­ gesting the relative ages. synclinal deposits. Within the "geotectonic cycles" several phases are A somewhat similar rendering appears on the "Tec­ recognized, namely: (1) an orthogeosynclinal phase, tonic Map of the U.S.S.R." (Spizaharsky, 1966), but in with development of eugeosynclines and miogeosyn- more elaborate form, partly because of the more varied clines; (2) an anatexitic phase, with emplacement of tectonic features of the country, partly for theoretical batholiths and regional metamorphism in the eugeosyn- reasons. The foldbelts of Phanerozoic age in the Soviet clinal area, and deposition of a flysch wedge over the Union are divided into 13 systems, each colored differ­ miogeosyncline; (3) an erogenic phase during which the ently, of these, 2 are called "geosynclinal" and are pre­ miogeosynclinal rocks and the flysch wedge were folded sumably still in process of deformation, the remainder and thrust toward the foreland; (4) a taphrogenic are called "folded" because the cycle of deformation phase, during which the whole was block- has been completed. These foldbelts are: ge^synclinaZ faulted, molasse was deposited, and subsequent mag- systems, Pacific and Alpine; and folded systems, Ural- matic activity produced both intrusions and extrusions. ian, Kazakhstanian, Altay-Sayan, Baikal, Mongolo- The Jaliscoan and Huastecan structural belts are ex­ Amur, Tien-Shan, Zaysan, Sikhote-Alin, Verkhoyansk, posed only in small areas, and their original extent and Chukotka, and Taimyr. subdivisions are hypothetical. The Mexican structural The rocks of each system are subdivided ii turn into belt forms most of the surface of the country, and the tectonic units, of which the following is a sample : Uralian system (or foldbelt)

Tectonic regime Supracrustal rocks Intrusive rocks Age "Orogenic" structures. Undivided structural stages. Late Triassic to Ea rly Jurassic

Superposed structures and None present in this system. intrusives.

« Fourth structural stage. . Permian to Early Triassic. Third structural stage. Alkaline rocks. Granites and granodiorites. Carboniferous Geosynclinal and parageosyn- Gabbro and diorite. clinal structures (second Ultramafics. group). Second structural stage. Granites and granodiorites. Middle Devonian to lower Gabbros. Carboniferous Ultramafics. (Tournaisian) First structural stage. Alkaline rocks. Granites and granodiorites. Ordovician to Early Devonian Plagiogranites. Gabbro. Ultramafics. Third structural stage. Granites. Late Proterozoic to Early Gabbros. Cambrian. Geosynclinal and para- Second structural stage. Middle Proterozoic. geosynclinal structures (first group). First structural stage. Alkaline rocks. late Early Proteroroic to early Middle Prcterozoic. Basement structures. Undivided. Granites. Early Proterozoic to Archean. Gabbros. APPRAISAL OF EXISTING TECTONIC MAPS 17 The theoretical basis for the representation on this these are obscured by the generalizations made on othei" map and its legend has been presented at length by maps, where tectonic features from widely separated Spizaharsky and Borovikov (1966). In brief, the earth's regions are grouped together and correlated. crust is considered to be heterogeneous, and to be divided REPRESENTATION OF SUBSEA AREAS into gross blocks along abyssal fractures; if so, each block would undergo its own independent history, and On most of the tectonic maps just reviewed only the universal times of orogeny would be impossible. The sea-bottom configuration is shown, by topographic cor- tectonic cycle in each folded system began with a geo- tours and bathmetric tints. Even by themselves, subsea synclinal period; the supracrustal rocks that were topographic contours are more expressive of tectonics formed during the period are divisible into a sequence of than topographic contours on the land, as the structures "structural stages" (rock-stratigraphic units like those of the sea bottom have been less modified by erosion?.! on previous Soviet maps), and there is a parallel se­ and depositional processes. Hence, fault scarps, fault quence of intrusive rocks (shown in an adjoining col­ blocks, upwarps, and downwarps are evident merely umn in the legend). Folding was largely completed by from their topographic configuration. the end of the geosynclinal period, but orogenic cli­ Although interpretation of features beneath the sea maxes are not mentioned. In the folded systems the geo­ on tectonic maps cannot, as yet, be made with as much synclinal period was followed by an "orogenic" and assurance as features on the land, recent oceanographic "koilogenic" period, during which the crust was epeiro- advances have encouraged proposals for more specify genically unwarped and downwarped. These authors tectonic representation of subsea features. Such repre­ use "orogenic" in a geomorphological rather than a sentation, especially by Soviet geologists and oceanog-- tectonic sense that is, for the formation of mountainous raphers, has appeared on some of the more recent tee- topography by uplift, rather than by deformation. In tonic maps. On the "Tectonic Map of Eurasia" the geosynclinal systems, the "orogenic" and "koilo­ (Yanshin, 1966b) the ocean bottoms are subdivided into genic" period has not yet been attained. first-order units shown by color patterns, as follows: According to the map legend, the geosynclinal period (and presumably also the periods of folding) ended in Structure?, of the Sea and Ocean Floors one folded system during the Cambrian, but in others Regions of pre-Cenozoic folding; continental platforms [=Cor- during the Permian, the Triassic, the Early Cretaceous, tinental shelves] : 1. Regions of epi-Mesozoic and older platforms. and the Late Cretaceous; in the geosynclinal systems Cenozoic folded and geosynclinal regions : the geosynclinal period (and period of folding) has i?. Folded and geosynclinal systems. not yet ended. Were it not for the theoretical predilec­ 3. Regions of pre- folding. tions of the compilers, the folding in these systems could 4. Deep basins without granitic layer. be assigned, as on the "Tectonic Map of the U.S.S.R. 5. Deep ocean trenches. 6. Deep trenches of inland seas. and Adjacent Areas" and the "Tectonic Map of Eu­ Regions of oceanic platform [= abyssal plains and ridges] : rope," to the broadly defined Baikalian, Variscan, Meso- 7. Arched oceanic elevations of the basaltic crust; swell'. zoic, and Alpine orogenies. 8. Marginal swells of oceanic platforms and Philippine On the tectonic maps of Mexico and the U.S.S.R. the basin. supracrustal rocks are thus subdivided in a manner like 9. Oceanic ridges of block structure. 10. Midoceanic ridges, and graben structures of Bay of that on some of the tectonic maps hitherto considered, Aden and Red Sea. into a sequence of "structural stages" (although these 11. Old oceanic plates between zones of elevation (parts are not so designated on the map of Mexico). Placing thickly covered by sediments shown by overprinted the intrusive and (or) magmatic units in a column in pattern). the legend adjacent to the supracrustal sequence is a 12. Oceanic plates without granitic layer, originating iu Paleozoic and Mesozoic. worthy innovation, as it illustrates the parallel evolu­ Volcanicity: tion of the two classes of rocks during the tectonic cy­ Submarine volcanic ridges and plateaus (shown by over­ cles in the foldbelts. The resulting legend is fairly printed patterns) : a, basaltic; b, mixed composition simple on the "Tectonic Map of Mexico." On the Tec­ mostly andesitic. tonic Map of the U.S.S.R." the great variety of features Besides the first-order units, second-order symbols ar^ to be represented produces a legend vastly larger and used on the map to indicate morphological or tectonic more complex than on any tectonic map hitherto con­ boundaries, faults, folds, flat-topped seamounts, atollr sidered. This increases the difficulty of using the map and the like. and legend, but it certainly presents a truer picture of Many of the first-order units used on the map am the local peculiarities and histories of each foldbelt; merely morphological or descriptive, such as deep 18 THE TECTONICS OF NORTH AMERICA oceanic trenches, marginal swells, and mid-ocean ridges; script, is the chief basis for representing nearl^ half of these would probably be nearly as evident from detailed the land area of North America. The part of this map topographic contours alone. Other units imply rock covering the Canadian Shield was published first composition, such as granitic or basaltic crust and thick (Stockwell, 1965), the whole map in 1969. As will be sedimentary cover, and have been verified by geophys­ seen later, the tectonic classification of the Pre^ambrian ical measurements at least in places. Still others seem used on this map was the principal basis for classifica­ more speculative, such as units involving specific times tion of the Precambrian used elsewhere on the North of folding, as these times are not verifiable by present America map. Additional data on western Car ada were oceanographic methods. obtained from a tectonic map of the western Cordillera, Even more radical proposals for representing subsea in British Columbia and neighboring areas by William tectonics have been made by Soviet geologists. These H. White (1966b, fig. 10-1). involve classification of areas by rock composition and All of Greenland is shown on a manuscript tectonic ages of folding, placing heavy reliance on particular map by Asger Berthelsen prepared for the Geological theories of oceanic origin and evolution. Survey of Greenland. Eastern and northern Green­ Today, oceanographic and marine geological investi­ land are also shown on maps by John Haller of the gations are revealing not only subsea topography to Danish East Greenland Expeditions. Many of the latter considerable depths, but results are as yet available only maps have been published in various formr and on in a few places. Accurate portayal of sea-bottom tec­ various scales (Haller, 1961b; Haller and Kulp, 1962, tonics in the manner of the more radical proposals fig. 3 and pi. 4; Haller, 1968, maps 1-3), but others are mentioned above must await a much wider extension of still in press. The easternmost part of the U.S.S.R., these investigations. lying within the area of the North America nap base, is shown on a tectonic map by S. M. Tillmar and his "THE TECTONIC MAP OF NORTH AMERICA" associates of the Siberian Section of the Acr demy of COMPILATION OF MAP Sciences of the U.S.S.K. (Tillman and others, 1966), which was generously made available by itr authors Compilation of the "Tectonic Map of North America" prior to publication. was facilitated by the existence of tectonic maps of many Mexico is shown on a tectonic map compiled by Zol- of the areas or countries involved some printed and tan de Cserna (1961), which was the chief basis for available at the time the compilation began, others in representing that country on the North America map. progress at that time and printed subsequently, and In addition, as indicated below, the scheme of classi­ others still in manuscript at the time of this writing. fication adopted by de Cserna in Mexico greatly aided The maps in manuscript or in progress during the time the present compiler in working out a general tectonic of compilation were made available to the compiler classification for North America. South of Mexico, the through the courtesy of their authors. next published tectonic maps are of Venezuela (Bucher, A major source of information was the "Tectonic 1950; Smith, 1962). The tectonics of the intervening Map of the United States, exclusive of Alaska and Ha­ regions of and the Antilles was filled waii" (Longwell, 1944b; Cohee, 1962), but configuration in from geologic maps and miscellaneous data, both of the surface of the basement rocks in this region was published and unpublished. For Central America, ma­ taken from the "Basement Map of North America Be­ jor assistance was obtained from Gabriel Dengo of the tween 24° and 60° N.," compiled by a committee of the Organization for Economic Integration of Central American Association of Petroleum Geologists (Flawn, 1967). The tectonics of the State of Alaska is shown on America. A tectonic map of Cuba did not become avail­ a manuscript map assembled in 1958 by George Gryc able until after completion of the North America map and his associates in the U.S. Geological Survey, but for but representation of the country on this map is much the North American map the data shown there were the same as on the present map (Puscharovsky and greatly amplified by information obtained subsequently others, 1966). by these same geologists. The subsea topography surrounding North America Canada is covered by a published tectonic map was assembled from contour charts of the U.S. Navy (Deny, 1950), but this is superseded by a new tectonic Hydrographic Office, the U.S. Coast and Geodetic Sur­ map compiled by the Geological Survey of Canada and vey, the U.S. Bureau of Commercial Fisheries, and the a committee of Canadian geologists, under the direction U.S. Geological Survey, extensively supplemented by of Clifford H. Stockwell (Stockwell, 1969). The new publications of the staffs of Scripps Institution of map, which was made available to the compiler in manu­ Oceanography, Lament Geological Observatory, and SPECIFICATIONS OF MAP ir Woods Hole Oceanographic Institution. These com­ tectonic units, rather than stratigraphic units. But what pilations were corrected by Henry W. Menard and are tectonic units? The preceding review of existing Robert L. Fisher of Scripps Institution of Ocean­ tectonic maps indicates a wide diversity of opinion ography and by Bruce C. Heezen of Lament Geological among makers of tectonic maps as to what a tectonic Observatory. The subsea topography of the Arctic unit should be a diversity which reflects botl Ocean and its surroundings was obtained from charts philosophical and practical considerations. by the Marine Sciences Branch, Canadian Department In the judgment of the compiler, the ages of deforma­ of Mines, Energy, and Resources, which were furnished tion of the rocks should not be the major criterion for in manuscript through the courtesy of M. M. de Leeuw. distinguishing tectonic units. Many contrasting kinds of Preparation of the North America map was ma­ deformation can occur nearly simultaneously, in differ­ terially aided by the opportunity afforded to the com­ ent rocks, and in different environments; these rockr piler to view tectonic features in the field during many and their environments are as significant as the timer excursions, including excursions to such remote areas when they were deformed. Moreover, although specific as Alaska, Newfoundland, southern Mexico, and Guate­ ages of deformation can be determined in some local mala. The compiler also had several mutually helpful areas, regional ages of deformation are seldom clear- conferences with Clifford H. Stockwell and others at cut. Regional ages of deformation are seemingly clearest the offices of the Geological Survey of Canada, and with in the Precambrian rocks, which are readily divisible, Zoltan de Cserna and others at the offices of the In- into gross provinces or foldbelts, but this is largely be­ stituto de Geologfa de Mexico. He* further benefited cause of the great length of Precambrian time and tho from attendance at conferences abroad which dealt with incompleteness of the record in these ancient rocks tectonic problems. Further, the compiler acknowledges Within the Phanerozoic rocks, many deformational with pleasure his rewarding association with Professor events can be recognized, which not only can be more A. A. Bogdanoff of Moscow University, U.S.S.R., a specifically dated than those in the Precambrian but world leader in the subject of tectonic mapping. which are more closely spaced in time. Differentiation of Finally, the "Tectonic Map of North America" could all such events on a tectonic map is of dubious signifi­ not have become a reality without the great and good cance, at least on a map of continental scope; the extent help of Douglas M. Kinney, geologic map editor of the of many of the individual events is very local, or can be U.S. Geological Survey, first for his sage counsel and specifically proved only in local areas. Much more sig­ advice during the work of compilation, and second for nificant is the gross time (that is, the tectonic cycle) his assistance in obtaining a printed product that faith­ during which the structures in each foldbelt were fully reproduced the original specifications. created. The units shown by color patterns on the "Tectonic SPECIFICATIONS OF THE MAP Map of North America" should, then, combine several When planning the specifications of the "Tectonic significant tectonic items. Within the deformed regions Map of North America," the compiler considered the the gross units should be the f oldbelts, which evolved by legends and the philosophical bases of the maps previ­ continuing deformation during lengthy periods of geo­ ously discussed, and adapted their best features to the logic time and each of which had its own distinctive new map. Rendering of second-order features on the history and time span; these foldbelts should be dis­ North America map closely follows that on the "Tec­ tinguished by different colors. Subdivisions of each fold- tonic Map of the United States" (Longwell, 1944b; belt should represent the kinds of rocks involved, which Cohee, 1962), but many innovations are made in the are, to a greater or lesser degree, products of this evolu­ rendering of the first-order features. All the land areas tion such as eugeosynclinal, miogeosynclinal, and other are subdivided into tectonic units, shown by color pat­ sedimentary rocks, and various metamorphic, plutonic, terns, instead of being only partly colored. A practical and volcanic rocks. These kinds of rocks can be differ­ reason for this change is that, whereas structural and entiated by patterns of the prevailing colors. Finally, tectonic data are complete enough within the contermi­ nous United States for many features to be effectively deformations of specific ages should be indicated only represented by symbols alone, data are less complete where they imply significant pulses in the evolution of elsewhere and many tectonic features in poorly known the foldbelt as a whole; they can be shown by various areas can only be blocked out by color patterns. tints of the prevailing color. These concepts correspond Clearly, the units to be shown by color patterns on the closely to the concepts that were the basis for tho "Tectonic Map of North America" must differ from "Tectonic Map of Mexico" (de Cserna, 1961), and the those on conventional areal geologic maps; they must be present compiler acknowledges his indebtedness to thoso 20 THE TECTONICS OF NORTH AMERICA concepts in evolving specifications for the "Tectonic (B) Platform deposits on Precambrian basement (cratonic Map of North America." area of central United States, western Crnada, and the Arctic Islands). These philosophical considerations are all very well, (C) Platform deposits on Paleozoic basement (Atlantic and but they require modification in order to be represented Gulf Coastal Plains). on a map of North America on a 1:5,000,000 scale. Many (D) Platform deposits on Mesozoic basement (Arctic- tectonic items of theoretical significance are difficult or Coastal Plain of Alaska and northern Onada). impossible to map at all, and others would require repre­ (E) Volcanic rocks and associated sediments of North At­ sentation on a scale of 1:2,500,000, or even larger. The lantic province, on Precambrian and younger base­ ment (Iceland, Greenland, and part of Baffin "structural stages" which adorn many recent tectonic Island). maps are a case in point. The "stage" concept is valid in (F) Icecaps of Quaternary age, on Precambrian and young­ expressing the evolution of the f oldbelts through time, er basement (Greenland and parts of Arctic and "stages" can be recognized in the rock sequences of Islands). the f oldbelts of North America; to map them is another Foldbalts of Precambrian age: matter. Sequences of "stages" are generally best marked (G) Kenoran foldbelt (Canadian Shield and elsewhere). (H) Hudsoniaa foldbelt (Canadian Shield and elsewhere). in strongly deformed areas, thus, both crowded outcrop (I) Greenville foldbelt (Canadian Shield and elsowhere). patterns and crowded structural symbols would coincide Foldbelts mainly of Paleozoic age : on the map (figs. 5 and 6). The present compiler believes (J) East Greenland foldbelt (northern east Greenland). that the structures in such areas are more significant (K) Innuitian foldbelt (Arctic Islands and no*th Green­ than the rock sequence, hence that representation of the land). latter must be sacrificed. Nevertheless, as will become (L) Appalachian foldbelt (southeastern North America). (M) Ouachita foldbelt (southern North Americr). apparent later, many of the subdivisions of the f oldbelts Foldbelts mainly of Mesozoic age : used on the "Tectonic Map of North America" are gross (N) Andean foldbelt (, in southeastern cor­ "structural stages" in the sense of existing maps, but ner of map). these are broad enough in scope to be represented clearly (O) Cordilleran foldbelt (western North America). on the 1:5,000,000 scale. Foldbelts mainly of Cenozoic age: A final disclaimer is in order: Tectonic classification (P) Pacific foldbelt (west coast of North Amer'ca). (Q) Antillean foldbelt (Antilles, southern Central America, will always be much more subjective than stratigraphic and Caribbean coast of South America). classification and will always be greatly influenced by Subsea areas. the judgment and predilections of the map compiler. On the map and its legend, these tectonic units and No two compilers will make identical tectonic classifica­ their subdivisions are indicated by colors that follow tions of the rocks of the same area for example, as to a prismatic scale, according to the ages of tl Q, geotec- what rocks are eugeosynclinal or miogeosynclinal, or as tonic cycles during which they were formed brown to what rocks are synorogenic or postorogenic. For the and red for Precambrian, purple for older Paleozoic, "Tectonic Map or North America," the present compiler blue for younger Paleozoic, green for Mesozoic, and yel­ has appraised the source data and the opinions of geolo­ low and orange for Cenozoic. The units and their sub­ gists that relate to each region, but judgment as to the divisions are also indicated by symbols. Prefires of the final representation on the map was made by him alone. symbols are the capital letters in the preceding list. What he has shown on the map will differ more or less These letters are followed by numerals or letters which from what would be shown by another compiler, and designate the subdivisions thereof numerals for the may be erroneous in places; every feature shown on sedimentary and metasedimentary subdivisions, Greek the map is controversial in some degree. Nevertheless, letters for the plutonic and volcanic subdivisions.5 in making this map a single compiler has viewed North However, some of these subdivisions occur in more America in all its parts, and has compared the tectonic than one of the foldbelts. For example, "thiclr deposits features of each part with each other part, thus hope­ in structurally negative areas" are characteristic not fully arriving at a balanced judgment of the relative only of the Cordilleran foldbelt, but also of tt^ adjoin­ significance of each. ing Pacific and Antillean foldbelts. Moreover, rocks of On the tectonic map, North America and its surround­ various Precambrian foldbelts come to the surface in ings are divided into the following tectonically signifi­ 8 Nine Greek letters are used in the legend, which are placed in con­ cant major units: ventional order (alpha, beta, gamma, delta, and so forth), reading from bottom to top of each column; however, certain letters in the sequence are omitted that would not reproduce clearly on the map (s"ch as zeta, Platform areas: eta, kappa, and nu). The letters are used indiscriminately IT whatever (A) Little deformed Precambrian deposits overlying more rock type appears in the sequence, so that granite, for example, is designated in one place as alpha, in others as beta, delta, and theta. deformed earlier Precambrian rocks (Canadian This usage differs from that on some European maps where all granites are designated as gamma, all basalts as beta, and so forth, regardless of Shield and elsewhere). their place in the sequence. PLATFORM AREAS 21 the foldbelts of later ages. On many of the maps pre­ Precambrian age, which lie on truncated surfaces of viously reviewed, such subdivisions are colored, pat­ rocks-that were strongly deformed during earlier Pre­ terned, and symbolized in an entirely different manner cambrian orogenies. These orogenies are of diverse agee. from one foldbelt to another, and it is difficult to observe and the ages of the deposits that overlie the rocks de­ their obvious kinships. In order to express such kinship formed by them are probably equally diverse; on the on the "Tectonic Map of North America," the same map, these deposits are classed as of Early, Middle, and colors and patterns are used for the subdivisions in all Late Proterozoic ages (Al, A2, and A3).e Most of these places but they are repeated in the legends of each fold- flat-lying or gently tilted rocks are now preserved in belt, only their symbols differing from one foldbelt to relatively small areas, but such rocks are very likely the another. last remnants of what were originally much more ex­ PLATFORM AREAS tensive platform deposits. One well-known Precambrian platform deposit is the Platform areas are those parts of the continents in Keweenawan Series of the Lake Superior region a which flat-lying or gently tilted deposits, mainly sedi­ very thick sequence of mafic lavas interbedded with and mentary, are underlain at varying depths by a basement succeeded by red continental (A2). The se­ of rocks that had been consolidated, not only by earlier ries has been downwarped into a broad syncline, faulted, deformation, but in part by metamorphism and pluto- and invaded by thick sheets of gabbro (Aa) but it r nism. This definition, while seemingly clear, is not every­ unmetamorphosed and is tilted rather than folded, so where easy to apply. In North America there is much that its structure contrasts with the underlying Pre­ debate among geologists and geophysicists as to which cambrian which has undergone thorough erogenic de­ rocks should be called basement and which should not; formation and moderate to strong regional metamor­ in compiling the "Tectonic Map of North America" phism (James, 1955). Radiometric dating of the gabbro some arbitrary decisions have been made in places. The sheets indicates an age of about 1,100 m.y. (Goldich and most extensive platform areas in North America are others, 1961, p. 96), so that the part of the Keweenawar those with a Precamorian basement in the central craton Series intruded by them is Middle Proterozoic; the- and those with a Paleozoic basement in the Atlantic higher parts of the series might extend into the Upper and Gulf Coastal Plains. The other platform areas are Proterozoic (Stockwell, 1964, p. 12). Other Precam­ less extensive or otherwise less typical, but they can brian continental and volcanic rocks which occur ir. more appropriately be placed in this category than in more remote parts of the shield, such as the Athabaska, any other. Dubawnt, and Coppermine River Groups, are even lesr The tectonic features of the platform areas are most disturbed than the Keweenawan Series, and like it f onr effectively portrayed by means of contours on the up­ a platform cover over more disturbed earlier Precam­ per surfaces of their underlying basements. These sur­ brian rocks. Toward the northwest, on the shores of the faces were produced by erosional truncation before the mainland and in the southern parts of the Arctic Islands, platform deposits were laid over them, but erosion or­ the Precambrian platform deposits include marine car­ dinarily had advanced far enough so that any residual bonates and a few evaporite units (Blackadar and topographic features are scarcely apparent in the con­ Fraser, 1961, p. 363-368). Many of these groups, liko tours on a small-scale map. Most of the configuration the Keweenawan Series, have been radiometrically of the surfaces is therefore the sum of all the deforma­ dated as Middle Proterozoic. tions that were imposed on them after they were trun­ Besides the extensive Middle Proterozoic platform cated. Contours on the surfaces of the underlying deposits there are smaller areas of Precambrian plat­ basements are available for all of the central cratonic form deposits of earlier and later ages. No platform de­ area, for a large part of the Atlantic and Gulf Coastal posits of Archean age remain, if such ever existed. How­ Plains, and for the Greenland icecap; these are repre­ ever, Lower Proterozoic supracrustal rocks extend in a sented on the "Tectonic Map of North America." Foi few places as platform deposits (Al) over the deformed1 the remaining platform areas configuration of the un­ Archean, as in the region between Lake Huron anc^ derlying basement is either unknown or is known only Lake Temiskaming. More commonly, they are eroded in small areas. These platform areas are relatively in­ back to the edges of their foldbelts, where they form consequential and are not contoured. tilted sequences that lie against the deformed Archean (A) PLATFOBM ABEAS WITHIN THE PBECAMBBIAN (the "marginal homoclines" of Stockwell, 1965; here in­ cluded in unit H5). The deformed rocks of the Middle In parts of the Canadian Shield are areas of flat- Classification of the Precambrian used on the "Tectonic Map o* lying or gently tilted sedimentary or volcanic rocks oi North America" is discussed on pages 30-32. 22 THE TECTONICS OF NORTH AMERICA Proterozoic Grenville foldbelt are overlain by a few tion is indicated by layer tints. During the last few small patches of flat-lying Upper Proterozoic deposits decades the top of the basement in the cratoi south of (A3), such as the Double Mer near Hamilton the 60th parallel has been penetrated at numerous places Inlet; the Upper Proterozoic attains much greater bulk by wells drilled for water, oil, or gas, so thrvt control in parts of North America outside the shield, as will be for contouring is excellent except near the centers of seen later. the deeper basins (Flawn, 1967); here, the basement Included with the Upper Proterozoic platform de­ configuration must be extrapolated from the structure posits on the "Tectonic Map of North America" are the of overlying strata. North of the 60th parallel, in Yukon pre-Upper Cambrian rocks of the Wichita Mountains Territory and the District of Mackenzie, very few deep of Oklahoma. Most of the pre-Upper Cambrian rocks wells have been drilled, but to complete the map, the that are exposed are intrusive granites and gabbros, but sparse data from these wells have been used to extend subsurface data in surrounding areas demonstrate that the basement contours hypothetically to tl ?. Arctic these are not basement in the usual sense, but are sheets Ocean. that intrude a thick and extensive terrane of lavas and The paleogeology of the buried Precambrian surface graywackes (Ham and others, 1964, p. 21-37). Isotopic is of much tectonic interest, as it indicates the extensions dating of the intrusives indicates ages of about 500 m.y. of the Precambrian foldbelts away from their surface suggesting an Early Cambrian rather than a Precam- exposures, in the Canadian Shield and in outlying areas. brian age; the host rocks are of about the same age or The paleogeology can be deduced with much confidence slightly older. Regardless of an eventual decision as to in many places by means of well data. For the most part, the age classification of these pre-Upper Cambrian however, it is not feasible to show the paleog?«logy on rocks, they are more closely related tectonically to the the "Tectonic Map of North America," except where later Precambrian (A3) than to the Paleozoic, and are exposed boundaries are extended for short distances so treated here (see p. 64). beneath the cover as dotted lines. The paleoglogy of Shown on the "Tectonic Map of North America" by the cratonic area in the United States is indicated on the same color patterns as the Middle and Upper Pro­ another map, so far as it can be deduced (Bayley and terozoic platform deposits are strata of similar ages in Muehlberger, 1968); data from that map, and from parts of the Appalachian and Cordilleran foldbelts. other sources, are summarized in figure 10. These strata are properly geosynclinal rather than era- The paleogeologic data show that in most places the tonic, yet they resemble the platform deposits in that Precambrian surface is a true basement of met amorphic they overlie deformed basements of earlier Precam­ and plutonic rocks. Nevertheless, in some areas the Pale- brian rocks, and were themselves not materially de­ ozic cover is separated from the basement by m oderately formed except by the Phanerozoic orogenies. They will deformed younger Precambrian sediments an^ lavas be discused more specifically later. themselves ancient platform deposits like those in the (B) PLATFORM DEPOSITS ON PRECAMBRIAN Canadian Shield. Buried extensions of the Ker^eenawan BASEMENT Series and the related occ^r in the states immediately south and southwest of Lake Su­ In the central craton of North America the exposed perior, and there are areas of rhyolitic lava in Okla­ Precambrian rocks of the Canadian Shield are sur­ rounded by areas in which they are covered by varying homa, north Texas, and adjacent states (Mu^hlberger thicknesses of platform deposits of Paleozoic and and others, 1966, p. 6422). At present, it is impracticable younger ages. These platform deposits cover the inte­ to represent the configuration of the lower srrfaces of rior lowlands south of the shield in the United States, these Precambrian platform deposits, and the basement from whence they extend northwestward through the contours are perforce carried over their tops. plains of western Canada into the Arctic Islands and The basal deposits of the platform cover a TO Paleo­ northern Greenland. A broad outlier of similar plat­ zoic, the earliest being mostly Late Cumbrian in age, form deposits covers the center of the shield southwest but the Cambrian is overstepped in many areas by of . Between the shield and the Appala­ younger strata. As indicated on the various geologic chian foldbelt on the southeast, the platform deposits maps of North America, Paleozoic rocks also lie at the make only a discontinuous, narrow strip along the St. surface over extensive parts of the craton, especially Lawrence River. toward the southeast, but they are covered r^estward, On the "Tectonic Map of North America" the upper in the plains east of the Cordillera of the United States surface of the Precambrian in the central craton is con­ and Canada, by thick Mesozoic deposits and thinner toured on a 500-m (1,640-ft) interval, and its configura­ Tertiary deposits. PLATFORM AREAS 23 The structure of the strata in the platform cover is Faults occur at different levels in the platform cover of much tectonic interest and is known in detail, espe­ and its basement in the central craton and pose problens cially as a result of drilling for oil, gas, and water. of three-dimensional representation. The faults are of Within each area the sequence of strata form a "layer three kinds: (1) those which displace units in the baro- cake." "Each layer is separated from the other by an ment, but not the overlying strata; (2) those which dis­ unconformity; each layer of geology is completely inde­ place the basement surface, but which may or may not pendent of other layers above and below; there is no extend upward to the ground surface; (3) those which clue in the upper layer of either the existence or char­ displace the surface strata, but which may or may not acter of the next layer below; and each layer has its own extend downward to the basement surface. On the "Te < > oil and gas geology, completely independent from each tonic Map of North America," faults which display of the other layers" (Levorsen, 1943, p. 912). Some of units in the buried parts of the basement are mostly the unconformities between the layers are local; others not shown; they are more appropriate as components of extend over much or all of the craton, and the strata a paleogeologic map. Faults which displace the surfa^ between them can be integrated into gross "sequences" of the basement but which do not extend to the ground (Sloss, 1963, p. 95). Four gross sequences have been surface are shown by dotted lines (as concealed faults). recognized in the Paleozoic, the 'boundaries between Faults which displace the surface strata are shown by them lying in the lower part of the Ordovician, the solid lines; some of these extend downward to the bar

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ti g 5 S «w "E^ S§ w3 PLATFORM AREAS 27 reservations; de Cserna (oral commun., 1964) has lies on Paleozoic platform deposits, but farther north­ pointed out to the compiler that other ranges in the east on disturbed Mesozoic strata of the Sverdrup basin eastern part of the Mexican Cordillera have similar (a part of the Innuitian foldbelt). features, although perhaps to a lesser degree, so that distinctions are not absolute. (E) VOLCANIC BOCKS AND ASSOCIATED SEDIMENTS OF THE NORTH ATLANTIC PROVINCE (D) PLATFORM DEPOSITS ON MESOZOIC BASEMENT The plateau basalts and associated volcanics and sedi­ Classed as an area of "platform deposits on Mesozoic ments of the northeastern part of the area of the "Tec­ basement" (D) is the Arctic Coastal Plain a well- tonic Map of North America" are part of an extensive marked but somewhat discontinuous ^eomorphic fea­ petrographic province variously termed the North J t- ture that extends across northern Alaska, past the lantic, Brito-Arctic, or Thulean province, which in­ mouth of the Mackenzie River in Canada, and along the cludes some of the British Isles, the Faeroe Islands, and northwestern edge of the Arctic Islands nearly to Axel Spitzbergen, beyond the map area (Wenk, 1961, p. 27f). Heiberg Island. This coastal plain differs technically Within the map area these rocks form Iceland, Jan from those just discussed, and is more heterogeneous, Mayen Island, parts of the east and west coasts of so that its validity as a true platform is somewhat Greenland, and a small part of the east coast of Bafn doubtful. Island (fig. 8). In northern Alaska the coastal plain lies between The basalts and associated rocks are not entirely com­ the front of the Cordillera (Brooks Range) and the parable to the platform deposits previously discussed, , and extends from west of Point Barrow but they do lie with little deformation on basements nearly to Yukon Territory, where the Cordillera im­ with more complex histories. The basalts and associated pinges on the coast (Gryc, 1959, p. 107-110). Much of rocks of Greenland and Baffin Island were spread ov°,r it is masked by Quaternary deposits, which lie in part continental crust mainly metamorphic and plutonic on Upper Cretaceous, in part on marine Tertiary. Un­ rocks of the Hudsonian and earlier Precambrian fold- like the strata of the Atlantic and Gulf Coastal Plains, belts, but northward along the east coast of Greenland those of this coastal plain slope landward from the upon the diverse rocks of the Paleozoic East Greenland coast, and thicken into a foredeep along the front of the foldbelt. The basalts in Iceland and the other islards Brooks Range; basement is within 720 m (2,500 ft) of may have been erupted on oceanic crust. the surface near Point Barrow, and stratigraphic evi­ Whether the basalts and associated rocks in the widely dence suggests at least the ephemeral existence of a base­ separated parts of the North Atlantic province were ment massif offshore beneath the present continental ever originally connected is a question which has impli­ shelf. The structure of the base of the coastal plain cations as to the origin of the North Atlantic Ocean deposits in northern Alaska is indicated on the tectonic whether by continental foundering, , map by partly hypothetical contours on a basement or some other mechanism but this question is beyond which may be equivalent to the metamorphosed lower the scope of the present discussion. The occurrence of Paleozoic rocks of the core of the Brooks Range. these rocks near the 70th parallel on both the east and On the Canadian mainland, in Yukon Territory and west coasts of Greenland, as well as in Baffin Islard, the District of Mackenzie, another segment of the Arctic suggests former connections, even though they are now Coastal Plain includes the delta of the Mackenzie River separated by the Greenland icecap and the waters of and some areas on either side, all mantled by Quater­ Davis Strait. The peculiar configuration of the base of nary deposits. Here, the underlying bedrock and its the Greenland icecap near the 70th parallel may have basement slope steeply seaward, as attested by a single some relation to the volcanic structures. Nevertheless a deep oil test well in the delta which failed to pass connection across Greenland beneath the icecap has be°-n through the Lower Cretaceous at a total depth of 3,841 questioned, because the sediments and lavas overlap in­ m (12,668 ft) (British-American, Shell, and Imperial land from the coasts (Wager, 1947, p. 29; Wenk, 19^1, Oil Companies, Reindeer D27 well, completed 1966). p. 279). In the northwest part of the Arctic Islands the Arctic The basal sediments in Greenland (El) include bcth Coastal Plain is formed by the Beaufort Formation Lower and Upper Cretaceous on the west coast, and (Craig and Fyles, 1961, p. 406-408) of preglacial Plio­ high Upper Cretaceous on the east coast; these are fol­ cene or earliest Pleistocene age a deposit several hun­ lowed by Paleocene sediments with volcanic components dred feet thick that was laid down by streams draining that foreshadow the succeeding volcanic episode. Similar northwestward toward the Arctic Ocean. On Banks basal sediments occur on Baffin Island (Wilson and Island and part of Prince Patrick Island the Beaufort Clarke, 1965), but are not shown separately on the mrp. THE TECTONICS OF NORTH AMERICA

Probable areas of plateau beneath icecap

Tertiary plateau basalt Quaternary volcanics Midocean ridge

FIGURE 8. Map of Xorth Atlantic Ocean and adjoining lands, including areas beyond the limits of the "Tectonic Map of North America," showing plateau basalts of North Atlantic province, and related features. PLATFORM ABBAS 29 The overlying plateau basalts (Eft) have a thickness icecap that covers the bedrock in all but the coastal of nearly 8,000 m (26,000 ft) in east Greenland (Wager, parts of Greenland, and the smaller icecaps in the north­ 1947, p. 21) and are as thick or thicker in west Green­ eastern part of the Arctic Islands are therefore repre­ land (Bosenkrantz and others, 1942, p. 55). In east sented as platform deposits on the tectonic map. Most Greenland they are capped by remnants of late Eocene geographic maps overemphasize the ice-covered areas at to Miocene marine sediments, a feature indicating that the expense of the ice-free areas, but while the ice is the eruptions were completed during Eocene time. As­ geographically interesting not all of it is a platform sociated with the plateau basalts in east Greenland are cover in a tectonic sense. On the tectonic map, minor ice plutons of various sizes and compositions (Ea), includ­ patches, valley glaciers, and ice areas with many r

TABLE 1. Comparison of various classifications of the Precambrian [In the table the rock terms'' Lower," "Middle," and" Upper" are used, although some of the sources quoted use the time terms "early" ("earlier"), "middle," and "late" riater")]

U.S. Geological Survey Geological Survey of Canada Tectoric Minnesota Map of Geological (5) NortI Before 1933 (1) After 1933 Survey, 1961 (2) 1957, 1962 (3) After 1964 (4) America (presert report.)

Cambrian Cambrian Cambrian Cambrian Cambrian Cambrian Cambrian

Hadrynian Sparagmite Series ("Eoeambrian") Upper Divided Divided Grenville Algonkian infor­ into: orogeny Daslandian 850-1,200 System mally Grenville ending 880 m.y. into: orogeny m.y. Upper, 1,100 m.y. Upper, lower Middle, Neohelikian Lower PROTEROZOIC PROTEROZOIC PROTEROZOIC Middle Boundary Elsonian Gothian platform not or fc orogeny 1,200- deposits clearly a or ending 1,500 1,300 PROTEROZOICERA denned PRECAMBRIAN & 1,280 m.y. m.y. m.y. |PRECAMBRIAN Paleohelikian Rapikivi Upper, Upper, granites Middle, Penokean Lower 1,600 Lower orogeny Hudsonian m.y. Archean 1,700 m.y. orogeny Karelian and Sveco- Lovar System ending fennian 1,500-1,900 _CJ 1,640 m.y. m.y. T3 -a Aphebian 3 Belomorian 1,900- 2,100 m.y. fc Algoman Kenoran orogeny < orogeny ending 2,390 W 2,500 m.y. m.y. W Saamian 2,150- 0 2,900 m.y. ARCHEAN ARCHEAN PH ARCHEAN <5 Laurentian < Katarchean 2,770- | orogeny 3,950 m.y. 3 Age??

1. Wilmarth (1926. p. 42,103,127. and pi. 1). 2. Qoldich and others (1961, p. 5). 3. Harrison (1957, p. 27); Stocfrwell (1962, p. 126 and fig. 3). 4. Stockwell (1964, p. 7-9; 1965; 1966. p. 33-34). 5. Compiled from Holtedahl and others (1964), Magnusson (1960), Simonen (I960). Polkanov and Qerling (196(0, and other sources. The names used seem to apply inter­ changeably to rock units, to the tectonic cycle during which they were formed, and to the terminal orogenies; for the foldbelts created by the orogenies the suffix "-ides" is commonly substituted for "-ian." The Katarchean occurs only as relics in the Saamian gneisses of the Kola Peninsula. The Gothian and Daslandian infra- crustal rocks occur only in the southwest part of the shield, and the Jotnian supracrustal rocks only in the central part. suits had become so incongruous by 1933 that formal "early late," but these are so offensive and confusing subdivisions of the Precambrian were abandoned, after that they do not merit serious consideration. which only informal subdivisions were used, described In 1964 the Geological Survey of Canada adopted five as "lower" and "upper" ("older" or "younger") or named subdivisions of the Precambrian "Areher.n," "lower," "middle," and "upper"; these have been ap­ "Aphebian," "Paleohelikian," "NeoheliMan," and plied locally, seldom with any implications of general "Hadrynian" (see table 1). The last four names, wlich correlation. The English language provides only three are used for parts of an inclusive "Proterozoic," are such relative descriptive terms, whereas modern knowl­ derived from Greek words which indicate relative de­ edge demonstrates that the Precambrian contains four grees of maturity (aphebos, heUhis, and hadrynes^ or or more major subdivisions. Additional categories might past maturity, maturity, prematurity). The meritr of be provided by expressions such as "lower upper" or these names and the acceptance which they will recftive 32 THE TECTONICS OF NORTH AMERICA remain to be determined. The novelty of the names long, separated by periods as long or longer with few or militates against comprehending either their meaning or no dates (for example, see Gastil, 1960, figs. 1 and 2; the sequential relations of the units. Stockwell, 1964, fig. 2). Most of the datings h*ve been On the "Tectonic Map of North America" the interim made on plutonic and metamorphic rocks, and the clus­ classification of the Precambrian therefore reverts to the ters of dates express times of plutonism and metamorph- traditional terms "Archean" and "Proterozoic," which ism. Inferentially, the clusters also express times of have been hallowed by long usage in geological text­ orogeny, of which the plutonism and metamorpHism are books and among geologists. The subdivisions thereof partial manifestations. This is confirmed in places by can be expressed by descriptive terms, and the Protero­ structural unconformities between the rocks tl *.t yield zoic is so divided into "Lower," "Middle," and "Upper" the dates and overlying less deformed rocks. TTie times ("Early," "Middle," and "Late"); a similar subdivision which yield clusters of radiometric dates are thus com­ of the Archean might be possible at a later time, but is monly interpreted by geologists as erogenic times, and not attempted on the map. These traditional terms are they are so treated on the "Tectonic Map cf North open to many objections it is true ambiguities in their America." earlier definitions and how they were applied, and later Nevertheless, a qualification is needed. The clusters of questions as to how they could be redefined more pre­ dates have definite climaxes, but the whole spread of a cisely on the basis of radiometric dates. As suggested by cluster may be as great as 400 m.y. or as long as all table 1, there seems to be little agreement between North geologic time since the beginning of the Devonian. Dur­ American and European geologists as to the age of the ing Devonian and later time, geologists hav?i distin­ termination of the Archean. This interim classification, guished many separately named orogenies, the number whatever its deficiencies, at least makes possible a sub­ depending on the predilections of different geologists. division of the Precambrian into more than the three Thus, the Precambrian "orogenies" are probably com­ descriptive categories available in the English language, parable, not so much to the orogenies within the fold- and provides the subdivisions with names whose ages belts of later times, as to the tectonic cycles which and sequential relations can be inferred by the geologist created the f oldbelts as a whole; probably the Precam­ who uses the map. brian "orogenies" themselves consisted of imny such Besides the general time-stratigraphic units just dis­ lesser orogenies, now blended together by imperfections cussed, the captions in the legend of the "Tectonic Map of the record. of North America" mention the names of a few local The nature of the Precambrian orogenies and tectonic Precambrian rock units (the Keewatin, Temiskaming, cycles has been debated, opinions being colorec1 by con­ Huronian, Animikie, Grenville, and Keweenawan Se­ flicting theories as to the origin and evolution of the ries, the Sudbury Norite and Duluth Gabbro). These earth's crust. Many geologists in the past have assumed are included merely for mnemonic purposes and as ex­ that crustal behavior during much of Precambrian time amples ; all are from the southern part of the shield in differed substantially from that of Phanerozoic time. Canada and the United States, where the Precambrian An ancient fallacy that has caused much mischief is an is familiar to geologists. Their use does not imply that assumption of the universality of Precambrian oroge­ the names can be extended to rocks outside their typical nies; remnants of this fallacy persisted even into later areas, although such extensions have been proposed in times. Bucher (1933, p. 419) thus summarized the the past. Elsewhere in the shield the rock units have thinking of his day: properly been given other local names, but those in The best evidence of the diminution in the course cf geologic remote places and those recently proposed are less famil­ time of the areas occupied by the orogenic belts ir the pro­ iar to geologists. Many of the names for other rock units, gressive decrease in the width of the zones of crystalline and the probable ages of the units, have been tabulated schists and gneisses produced in successive eras. * * * For in publications by Stockwell (1962, fig. 3; 1964, table 3) Archeozoic time, crystalline schists and gneisses occupy ap­ parently 100 percent of the folded belts so far as they are and by Goldich and others (1961, table 2). accessible to view today. For the Proterozoic, the percentage is still large, but much less than 100 percent. The contrast be­ TECTONIC CLASSIFICATION tween the Proterozoic and the Paleozoic is still greater. * * * Tectonic classification of the Precambrian rocks of A modern variant of these views is expressed by North America on the basis of radiometric dating and Stockwell (1966, p. 34-35): other evidence is clearer than the stratigraphic classifi­ The orogens of the shield characteristically cover very broad cation. As more radiometric dates of Precambrian rocks regions quite unlike the chains of later times. An­ become available, it is increasingly evident that they other difference is the long time interval between deposition cluster during periods several hundred million years of sediments and their involvement in orogeny (about 400 POLDBELTS OF PRECAMBRIAN AGE 33 m.y. in two instances), as contrasted with the close time se­ Hudsonian orogeny quence of geosynclinal deposition, sinking, and mountain building in younger rocks. Still another difference is the very Churchill province extensive overlapping of a younger orogen on an older one and Southern province this occurs in regions not likely ever to have been the site of Bear province intervening geosynclinal deposition. On the whole, the thesis that the orogenic development of the shield differed from that Kenoran orogeny of the younger rocks seems worth considering and, it may be suggested, resulted from deep crustal or subcrustal movements Superior province unrelated to the sinking of geosynclinal belts. Slave province At least some of the alleged contrasts between Pre- Nain province (minor) cambrian and Phanerozoic orogenies may be more ap­ The restriction of orogenic effects during Precam­ parent than real. The depth of denudation of a foldbelt brian time to specific foldbelts (provinces) is illustrated is progressively greater the older the rocks; hence the by the passage of deformed supracrustal rocks in the exposed areas of crystalline rocks are also greater with foldbelts into the platform deposits that extend o^er age. Moreover, the universality of crystalline rocks in earlier foldbelts, as described under an earlier heading the Archean is probably not a product of any single (p. 21-22). Platform deposits of Lower Proterozoic age world-wide orogeny, but results from the great length (Al) thus define Kenoran foldbelts, those of Middle of Archean time during which many orogenies built Proterozoic age (A2) Hudsonian foldbelts, and those foldbelts in different places. Also, if the foldbelts men­ of Upper Proterozoic age (A3) Grenville foldbe'ts. tioned by Stockwell are actually products of gross tectonic cycles rather than single orogenies they are Such relations are most dramatically expressed during more comparable with those of Phanerozoic time; the Middle Proterozoic time, when the rocks of the Gr?n- Cordilleran foldbelt of western North America which ville foldbelt in the southeastern part of the shield w*»re was built during the last 150 m.y. of geologic time com­ subjected to deep-seated deformation and accompany­ pares favorably in breadth and length to the Pre- ing metamorphism and plutonism, whereas in the gre«t- cambrian foldbelts, and likewise contains relics of er part of the shield to the northwest, supracrustal pb.t- earlier foldbelts that were reworked during the Cor­ form deposits (A2) were being laid down over rocks dilleran orogeny. Even allowing for probable changes already consolidated by earlier orogenies the de­ in the nature and behavior of the earth's crust with formed rocks and the platform deposits both yielding time, it is reasonable to assume (as Sederholm did more nearly contemporaneous radiometric dates. than half a century ago) that the resemblances between the Precambrian and the Phanerozoic foldbelts are greater than the differences (see Holmes, 1963, p. xxi- TABLE 2. Classification of orogenies in Canadian Shield xxii). Climax of Rang" of FOLDBELTS OP CANADIAN SHIELD Orogenies orogeny ' orogeny * Rock sequence In the Canadian Shield, three great clusters of radio- Canada ' Minnesota ' Million years metric dates have been obtained in the Precambrian Upper Proterozoic rocks, whose times probably express major orogenies; (minor iu shield). these times are so designated on the "Tectonic Map of (I) Grenville Keweenawan 945 880-1,000 orogeny.4 igneous North America," following previous usage. As ex­ activity. plained earlier, the so-called orogenies of Precambrian Middle Proterozoic. («) time are more properly gross orogenic times or tectonic (II) Hudsoninn Penokean 1, 735 1.640--1.820 cycles in terms of Phanerozoic events, and this will be orogeny. orogeny. implicit in the discussion henceforth. The orogenies Lower Proterozoic. (Q) Kenoran Algoman 2.490 2.390-2.600 have been classified as shown in table 2. orogeny. orogeny. These dates, and the orogenies which they express, Archean. occur in well-defined areas the "provinces" of Cana­ dian geologists which are here considered to be the 1 Modified from Slock we ^1 (1964, tables 1 and 2) and other papers. : Modified from Goldich and others (1961, table 2; 1966, table 5). foldbelts produced by the orogenies (fig. 9). The prov­ 'The climaxes and ranges of the orogenies are inferred from statistical summaries of recorded radiometric dates in Canada (Stockwell, 1964, p. 2-7); the clitnaier and ranges of the orogenies are based on the abundance of the radiometric dates', the inces (or foldbelts) of each age are listed below, in order ends being assumed to be the mean minus the standard deviation. 4 Pertinent objections have been raised by Qilluly (1966, p. 104-108) to such terms of increasing age: as" Grenville orogeuic belt" and "." Rightly or wrongly ,-the terms are now so firmly entrenched in usage that it is undesirable to substitute some new Grenville orogeny and unfamiliar nomenclature. « Stockwell distinguishes an additional Elsonian orogeny between the Hudsonian Grenville province and Grenville; this is appraised on p. 56. 34 THE TECTONICS OF NORTH AMERICA

130 120° 110° 100° 90° 80° 70° 60° 50° 40°

pPf^X] & V^^\,

50

1000 KILOMETERS 0 1000 MILES

:::::::::::::::: * * ' " * * ^ * ^\\^ Province containing Province containing Province containing Province containing Little deformed Edge of Phanerozoic Kenoran foldbelts Hudsonian foldbelts Kenoran and Hudsonian Grenville foldbelt supracrustal strata platform co^er elements, overprinted mainly middle by Elsonian event Proterozoic FIGURE IX Map of Canadian Shield showing provinces that contain different assemblages of Precambrian rocks. Modified from Stockwell (19&4, fig. 1). FOLDBELTS OF PRECAMBRIAN AGE 35 Besides the major orogenies, there are indications of Nevertheless, the significance of the Elsonian eveit other events of possible erogenic significance during remains dubious. Even in the eastern part of the shield, Precambrian time in the Canadian Shield, two of which radiometric dates within this age range are few, and merit discussion: have no prominent peak of abundance like those of the Archean time was prolonged, and it is presumed that other Precambrian orogenies; radiometric dates within some or many orogenies occurred before the terminal this age range are virtually lacking elsewhere in the Kenoran orogeny. One of these, the "Laurentian" orog­ shield, except in the Southern province (Michigan and eny is suggested by geological evidence in the Lake Wisconsin). Besides the anorthosites and gabbros, there Superior region. As early as 1885, A. C. Lawson recog­ are no obvious geological manifestations of an orogeny nized that the Keewatin greenstones of the region were and the event can hardly have produced a foldbelt in the intruded by the so-called "Laurentian" granite (a mis­ usual sense. The compiler infers that the Elsonian is a nomer, as the original "Laurentian" is in the Grenville minor event in the Canadian Shield that produced no province, and very much younger), and that the eroded more than an overprint on rocks already consolidated by surfaces of both were overlain by younger strata, now the Hudsonian orogeny. On the "Tectonic Map of Nor+h classed as later Archean (the Knife Lake); he ascribed America" the rocks of the Nain province are assign**! to this event a major role in Precambrian history. Sub­ to the Hudsonian foldbelt, the overprint of the Eko- sequent investigations indicate, however, that the nian event being indicated, where appropriate, by a younger Kenoran orogeny and the accompanying Algo- superposed diagonal ruling (H2). man granites are much more prominent in the region, Although the Laurentian, Elsonian, and other prob­ and the "Laurentian" has been relegated to a lesser rank lematical Precambrian events have only minor signifi­ (Goldich and others, 1961, p. 73-74). The age of the cance in North America, they may reflect orogenies of "Laurentian" event is still indeterminate, as no radio- greater significance in other continents. Thus, the Goth- metric dates from it have survived; very likely they ian deformation in the southwestern part of the Baltic were overwhelmed by the Kenoran orogeny. Similar Shield seems to be nearly contemporaneous with the (but not necessarily contemporaneous) events may have Elsonian event in North America (table 1). occurred elsewhere in the Kenoran foldbelts during Subdivision of the rocks in the Precambrian fold- Archean time, as suggested by the occurrence of granite belts of the Canadian Shield on the "Tectonic Map of clasts in many of the sediments. Radiometric data indi­ North America" largely follows the classification of cate relics of a 3,550 m.y. event in the gneisses of the Stockwell (1965). Within the Kenoran foldbelts little Minnesota River valley (Goldich, Lidiak, and others, subdivision can be made, except to separate the supra- 1966, p. 5395-5396), but their relation, if any, to the crustal sedimentary and volcanic rocks (G2) from the "Laurentian" event is unknown. granitic rocks (Gy); the migmatites (Gl) probably The record of pre-Kenoran tectonic events during the represent partial conversions of the former into the Archean is so imperfectly preserved that Stockwell latter. More complex classifications are possible in the (1965) was unable to map their effects in the Canadian Hudsonian and Grenville foldbelts, although even here Shield, and these effects are accordingly not indicated it is necessary to show areas of undifferentiated rocks on the "Tectonic Map of North America." (HI and II), where the are little mapped or Stockwell (1964, p. 2-3) proposed an "Elsonian orog­ poorly understood. It has been proposd that the closely eny" on the basis of a scattering of radiometric dates accordant radiometric dates in each of these foldbeJts in the Nain province in the northeastern part of the indicates not only the ages of their climactic orogenies Labrador Peninsula with a mean age of about 1,370 m.y. but also the general age ranges of all the rocks of ea^-h The area from which the dates were obtained is a ter- foldbelt (J. T. Wilson, 1950, p. 107-108). Stockwell's rane of gneisses and embedded granitic plutons, appar­ more detailed review indicates greater complexities. ently not differing from the adjoining Hudsonian fold- Each foldbelt includes not only geosynclinal supracrrs- belt except for the occurrence of bodies of anorthosite tal rocks that formed during the tectonic cycle that gave and gabbro like those in the Grenville foldbelt to the rise to the foldbelt, but also plutonic and supracrustal south (I£); Stockwell suggests that all the anorthosite rocks which are relics of earlier cycles reworked by t\e and gabbro bodies are of Elsonian age, those in the later ones; these reworked earlier rocks are indicated Grenville foldbelt having been reworked during the sub­ on the tectonic map by superposed diagonal rulings. TT- e sequent Grenville orogeny. Credence in the existence of Hudsonian foldbelts thus contain reworked relics of an "Elsonian orogeny" is enhanced by the occurrence of the Kenoran foldbelt (H3, Ha); the Grenville foH- radiometric dates within the same age range elsewhere belt contains reworked relics of both the Kenoran and in North America outside the shield. Hudsonian foldbelts (12,13, la). 36 THE TECTONICS OF NORTH AMERICA The famous "Grenville front," or prominent north­ parts are virtually unexplored, especially on the east western border of the Grenville foldbelt, truncates at coast, south of the 68th parallel. Less radiometric dat­ high angles all the structures of the adjoining Kenoran ing has been done on the Precambrian rocks of Green­ foldbelt, and there has been much speculation as to its land, compared with that of the Canadian Shield, but meaning. Certainly throughout its length it marks a the available results are consistent with those of the significant contrast in style of deformation and meta- shield. morphism, and certainly through parts of its length it The oldest clearly recognizable foldbelt in Gr-^nland is a zone of major faulting and northwestward thrust­ is the Ketilidian, which forms all the west coas* as far ing. Its actuality as a structural feature is confirmed by north as the 63d parallel. Most of it consists ci deep- the occurrence through part of its length of a pro­ seated metamorphic and plutonic rocks (Gl) tlat have nounced linear negative gravity anomaly (Canada undergone long and complex histories. These have been Dominion Observatories, 1957). Nevertheless the divided into several named complexes of varying com­ "front" does not, as has sometimes been assumed, juxta­ position and metamorphic grade, which are separated in pose a terrane of younger rocks in the Grenville fold- part by strike-slip faults. A few radiometric dates be­ belt on the southeast against terranes of older rocks in tween 2,100 and 2,700 m.y. are available which indicate the foldbelts to the northwest. Stockwell (1964, p. 18- a correlation of the foldbelt with the Kenoran fold- 21) has been able to trace relics of Archean and Lower belts of the mainland (fig. 10). Supracrustal rod's (G2) Proterozoie rocks involved in the Kenoran and Hud­ occur in a few places, notably in the Ivigtut ar^ near sonian orogenies through the Grenville foldbelt (12, the 61st parallel, where a sequence of sediments and 13), and he proposes that even the Grenville Series (14) volcanics as thick as 4,000 m (13,000 ft) is preserved. of the southern part of the foldbelt (from which the All the metamorphic and plutonic rocks of the east names of the foldbelt and the orogeny are derived) is coast south of the plateau basalts at the 68th parallel not younger than the Early Proterozoic, and hence is not are also probably part of the Ketilidian foldbelt (Gl- simply a geosynclinal precursor of the much later H3a) but they are little known. Farther north, similar Grenville orogeny. rocks form the autochthone of the East Greenland fold- belt at the head of Scoresby Sound and have yielded FOLDBELTS OF GREENLAND maximum radiometric dates of 2,300 m.y. The island of Greenland is obviously a detached On the west coast at S0ndre Str0mf jord near the 67th northeastern extension of the Canadian Shield, with parallel, rocks of the Ketilidian foldbelt are su "-ceeded analogous Precambrian rocks and structures. Incom­ northward by other metamorphic and plutonic rocks of plete geophysical surveys suggest that the intervening the Nagssugtoqidian foldbelt (H4), which have yielded Baffin Bay and Davis Strait are floored by oceanic radiometric dates of about 1,500 to 1,650 m.y., hence are crust, so that the detachment was more likely by drift correlative with the Hudsonian foldbelts of th*. main­ than by foundering of shield rocks. Nevertheless, the land. The Nagssugtoqidian foldbelt, like the Ketilidian, geometry of the Precambrian rocks and structures does is divided into named complexes of varied nature. Su­ not match convincingly from one coast to another so pracrustal rocks (H5), called the Agpatides, are re­ that the original fit of the two shield areas is still ported around Umnak Fjord north of Disko Islr.nd, but uncertain. they have been little studied. Precambrian shield rocks underlie much of Green­ In southernmost west Greenland the Ketilidirn fold- land, but they are covered in small part by the plateau belt has been overprinted by the Sanerutian e^ent, so basalts and associated rocks (E/J), and in much larger that all radiometric dates here are 1,600 m.y. or less, part by the central icecap (F), so that they only emerge hence (like the Nagssugtoqidian) being correlative with near the coasts. Along the east and north coasts, besides, the Hudsonian orogeny of the mainland. The area in­ the shield is impinged by the East Greenland and In- cludes the extensive Julianehaab Granite (Ha), pctually nuitian foldbelts (J and K), whose Precambrian com­ a heterogeneous complex of various granites with, relics ponents have been reworked by Paleozoic orogenies. of sedimentary and volcanic suprastrata; although Berthelsen and Noe-Nygaard (1965) have made an ex­ originally formed during the Ketilidian it was immobil­ cellent summary of the Precambrian rocks of Greenland. ized during the Sanerutian. The area also includes dis­ Parts of these rocks are known in much detail, espe­ cordant bodies of postorogenie granite (H8) which yield cially in the shield area on the west coast as far north as Sanerutian dates. On the east coast the Ketilidip n fold- the 73d parallel (Berthelsen, 1961) and in the East belt is overprinted by the Sanerutian event as fa r north Greenland foldbelt between the 70th and 80th parallels as Angmagssalik at the 66th parallel. (Haller, 1961a, b; Haller and Kulp, 1962); some other Southernmost west Greenland also contains the still FOLDBELTS OF PRECAMBRIAN AGE 37 younger Gardar Group continental sandstones and lift), the Arbuckle Mountains, and the Llano uplift. volcanics (A2), block-faulted but otherwise little dis­ In the Appalachian foldbelt, metamorphic and plutonic turbed, and contemporaneous alkalic intrusives (Aa). rocks form some of the ranges near the central ayis, These are platform rocks like the Keweenawan Series but the remainder of the Precambrian here consists of of the continental interior, and like them yield radio- younger supracrustal strata. In the northern part of metric dates of about 1,000 m.y. They are not compar­ the Oordilleran foldbelt only younger Precambrian able to the infracrustal rocks of the Grenville foldbelt supracrustal strata come to the surface, but in the Cen­ which strike out to sea on the opposing coast in Lab­ tral and Southern Rocky Mountains metamorphic and rador, even though they yield similar dates, nor are they plutonic rocks are extensive in the cores of the rang<\s. correlative with the remnants of Upper Proterozoic They reappear in the Basin and Range province w; uplift, the St. Francois Mountains (in the Ozark up­ Lidiak and others, 1966; Muehlberger and others, 196?). 38 THE TECTONICS OF NORTH AMERICA t v:.1/..-?.'^,,*;' V::;U/i/, "^C-:-, /7 >

130 EXPLANATION

Late Precambrian to early Cambrian (about 500 my ago)

Mainly middle Proterozoic (about 800-1300 my ago)

Outcrops of Precambrian rocks

Contacts between Roeks involved in Avalonian_ Precambrian units event (550-650 my ago)

Fronts of Phanerozoic Rocks involved in Grenville foldbelts orogeny (880-1000 my ago)

120° Rocks involved in Hudsonian

(1280-1460 my ago)

Rocks involved in Hudsonian orogeny (1640-1860 my ago)

Rocks involved in Kenoran orogeny (2400-2600 my ago)

110 FIGUKE 10. Map of North America showing inferred extent of exposed and buried provinces of Precambrian rocks. Compiled from many sources, including Stockwell (1964, fig. 1), and Bayley and Muehlberger (1908).

CO 40 THE TECTONICS OF NORTH AMERICA

TABLE 3. Summary of Precambrian rocks and events south of Canadian Shield

Canadian Appalachian Northeastern North-central South-central Mexico (Fries, Shield (Modi­ foldbelt United States United States United States Western United States 196^; Fries fied from (Rodgers, (Lidiak and (Goldich, Lidiak (Muehlberger (Greatly generalized and Riac6n- Stockwell, 1967, and others, 1966) and others, and others, from many sources) Orta, 1965) 1964) other sources) 1966) 1966)

Wichita igne­ PALEOZOIC ous activity 500 m.y. 600 m.y. Avalonian event (coastal region from Eastern Mex­ Newfound­ ico (Tama- UPPER land south­ ulr>as) PROTER- ward) 710 m.y. OZOIC Holy rood granite 580 m.y. Pikes Peak Granite, Metamorphism Intrusives Colo- east of Keweenawan in Apache rado; Southern Grenville Grenville "Grenville igneous Llano orogeny Group, Southern Mexico orogeny orogeny - front," Ohio - - activity, - - 1,000-1,150 - - 1,100 m.y. Cali- - (Oaxaca) - ~ 880-1,000 (in base­ 800-1,000 1,000-1,200 m.y. Dates on fornia 770-980 m.y. ment of m.y. m.y. Belt Se- pluton- m.y. interior Panhandle and ries 900- ics, 1,000 O Appalachi­ H-1 Folding of Spavinaw 1,300 m.y. m.y. O ans) 800- Sioux Quartz- igneous (S3 1,000 m.y. ite, 1,200 activities, tfO m.y. 1,100-1,300 W m.y. tfO OH Nemaha igne­ St. Francois ig­ Apparent ages, ous activity Sherman Granite, Colo­ Elsonian (No earlier neous activ­ 1,200-1,400 1,350-1,450 rado; younger Eastern Mex­ event dates re- ity, 1,200- m.y. m.y. granites of Arizona, ico - 1,280- - corded) - - 1,350 m.y. - -Granites, - - 1,300-1,500 in.y. - (Hidalgo) - 1,460 m.y. Plutonic com­ Arbuckle 1,210 m.y. plex of Iowa, Mountains, W 1,3007-1,500? 1,320-1,400 m.y. m.y. Q Event, 1,490 (No earlier ip i m.y. dates re­ corded) Mazatzal orogeny, Ari­ Nortl -"western Hudsonian Penokean and zona; final meta- Mexico orogeny - -Relics, 1,600- - Black - _ morphism _ (Soiora) 1,640-1,820 1,800 m.y. Hills oroge­ in Front Range, Colo­ 1,320-1,710 m.y. nies 1,600- rado, and prfr-Belt m.y. 1,800 m.y. basement, Montana, 1,650-1,750 m.y. LOWER PROTER- OZOIC

Kenoran orogeny Granites and metamor- _ 2,400-2,600 ~ 2,400-2,750 phics, NW. Wyoming, m.y. m.y. 2,400-2,600 m.y. Minnesota Valley relics ARCHEAN Relics, NW. Wyoming 3,550 m.y. 3,100 m.y. FOLDBELTS OF PRE CAMBRIAN AGE 41

The oldest radiometric dates outside the Canadian The supracrustal rocks of this region are probably Shield in North America have been obtained from Pre- Middle Proterozoic like the Belt Series in the nor*h; cambrian rocks in the mountains of northwestern diabase sills in the Apache Group have been dated at Wyoming and adjacent Montana (Beartooth Moun­ 1,100 m.y. (Shride, 1967, p. 76-77). tains, Bighorn Mountains, Wind River Mountains, and Special problems attend the Precambrian metamor­ other mountain ranges); they are consistently more phic and plutonic rocks of the Southern Rocky Moun­ than 2,400 m.y. old, with a few relics as great as 3,100 tains in Colorado and nearby Wyoming; clearly, tHy m.y. The dates obtained from the outcrops, and from have had a complex history. The older paraschists r.nd drill samples in surrounding areas, define a province paragneisses (Idaho Springs Formation and other comparable in age to those produced by the Kenoran units) were originally laid down as geosynclinal sfii- orogeny in the Canadian Shield; nevertheless, these ments and volcanics and were afterwards deformed r.nd provinces are not connected, as younger dates have been metamorphosed. Still later, they were plastically folded, found in the intervening area. The rocks of the province and another regional metamorphism was superposed; are mainly granites and granite gneisses (Gy), but they this event is dated at 1,750 m.y. ago (Pearson and others, include the mafic Stillwater Complex of the Beartooth 1966; Hedge and others, 1967, p. 554), or at about the Mountains (Ga), and some schist and iron-formation time of the Hudsonian orogeny. These rocks, and arso- in central Wyoming and elsewhere (G2). In the Black ciated plutonic rocks, are classed on the tectonic map Hills, where most of the Precambrian is Lower Protero- as Kenoran reworked by Hudsonian orogeny (H3, Ha). zoic, 2,500-m.y. dates have been obtained in the small Sequences of Lower Proterozoic supracrustal strata Nemo district, from rocks probably representing an (H5) are preserved in places, as in the Medicine low earlier basement. Mountains of southern Wyoming and the Needle Mo'\n- The remaining Precambrian metamorphic and plu- tains of southwestern Colorado, and there are several tonic rocks of the western Cordillera all yield younger sets of younger plutonic rocks. Anorthosite (Ift) like radiometric dates. Dominant dates from Montana to that in the eastern part of the Canadian Shield forms Arizona are between 1,500 and 1,800 m.y.; the rocks the central part of the Laramie Range, Wyo. (New- which yield them are classed on the tectonic map as house and Hagner, 1957); extensive granites such as Hudsonian (H5 or O2; Hy or Oa), but this is a gross the Sherman and Silver Plume were emplaced 1,200- generalization of varied rock assemblages, all of which 1,300 m.y. ago, and the less extensive Pikes Peak Granite have had complex histories. In the north, the basement was emplaced 1,000 m.y. ago. rocks below the Belt Series of Montana and comparable Plutonic activity in the Southern Rocky Mountains supracrustal strata in the Uinta and Wasatch Mountains thus extended well past the time of the Hudsonian orog­ of Utah have yielded dates within this age range (but eny, into the times of the Elsonian event and the Gr-m- relic earlier dates occur in the Little Belt Mountains, ville orogeny. A similar situation obtains farther south­ Mont.); the Belt Series itself has been dated by a variety west, in Arizona, where granites emplaced at the c^ose of methods as between 1,300 and 900 m.y. old (Obrado- of the Mazatzal orogeny are dated at 1,650 m.y. r.^o, vich and Peterman, 1967), hence is Middle Proterozoic and are followed by later granites dated at 1,375 ruy. or somewhat younger. ago (Silver, 1966); various stray younger Precambr^an In the south, the basement beneath the Grand Can­ dates reported in Arizona are probably an overprint re­ yon Series and Apache Group, and beneath the Precam- lated to these younger granites. The history implied by brian supracrustal strata near Caborca, northwestern radiometric dates is also complex in the Transverse Sonora, was deformed by the Mazatzal orogeny, dated as Ranges of southern California (Silver and others, If ^3; 1,650-1,750 m.y. ago (Wasserburg and Lanphere, 1965, Silver, 1968); gneisses and plutonics in the San Gabriel p. 755), although it was once ascribed a younger ("El- Mountains are dated at 1,650-1,700 m.y. ago (hence sonian") age. Actually, two closely spaced events can be Mazatzal or Hudsonian); they were followed by a meta­ discriminated in the orogeny, one 1,650-1,715 m.y. ago, morphic event 1,420-1,450 m.y. ago, and later by intru­ another 1,715-1,750 m.y. ago, the younger dominating sion of an anorthosite-syenite complex (Oft) 1,220 r^y. toward the south (Silver, 1967). In several different ago. The Precambrian rocks were long afterwards in­ parts of Arizona the rocks deformed and metamorpho­ vaded by Permian to Triassic and by later Mesorcoic sed by the Mazatzal orogeny include sequences of sedi­ plutonic rocks. ments and volcanics as thick as 6,000 m (20,000 ft) Radiometric dates intermediate between the Hud­ (Anderson, 1966, p. 6-8). A basal unconformity has been sonian and Grenville orogenies have been determined in observed beneath one of these sequences (Blacet, 1966), many of the Precambrian basement rocks penetrated by but its regional significance remains to be established. drilling east of the Cordillera, in the central CK ton 42 THE TECTONICS OF NORTH AMERICA (Goldich, Lidiak, and others, 1966, p. 5384-5386). Some attested by the recent discovery of unconformaHy over­ are merely apparent dates or an overprint, but others lying fossiliferous strata of Late Cambrian and younger pertain to actual plutonic or volcanic events, as shown Paleozoic ages (Pantoja-Alor and Robison, 1967). on table 3. Thus, rhyolitic volcanic rocks were spread There was thus a wide and lengthy belt along much widely in Missouri, Oklahoma, and northwestern Texas, of the southeastern edge of North America that was and have been dated from place to place as 1,200-1,350 subjected to metamorphism, plutonism, and prcbably to m.y., 1,150-1,300 m.y., 1^100-1,200 m.y., and 500 m.y. orogeny about 1,000 m.y. ago, or at a time comparable ago (Muehlberger and others, 1966, p. 5421-5422); there to the Grenville orogeny and the formation of the Gren­ was also granite emplacement during some of these ville foldbelt in the southeastern part of the Canadian times. In the compiler's judgement, these events do not Shield (fig. 10). This was during a part of Precambrian justify classing the basement rocks of the southeastern time when the remainder of North America to tl <». north­ part of the craton as an extension of the proposed El- west had been largely stabilized, and was receiving only sonian foldbelt of the shield.7 supracrustal cratonic or geosynclinal deposits. The youngest Precambrian metamorphic and plutonic Some final Precambrian events deserve notice, al­ rocks south of the Canadian Shield are in the south­ though they are inadequately represented on the "Tec­ eastern part of the United States, most of which are tonic Map of North America" (data regarding them broadly correlative with those of the Grenville foldbelt. were received too late for inclusion). An Avalonian In the Appalachian foldbelt, rocks yielding dates of event (or orogeny) is now well documented in south­ about 1,000 m.y. (LI) form the basement of the Paleo­ eastern Newfoundland (Rodgers, 1967, p. 409-410; zoic miogeosynclinal and eugeosynclinal sequences and, Poole, 1967, p. 14-17), where the Holyrood Granite where present, that of the Upper Proterozoic supra- (La) intrudes the Harbour Main Volcanics and these crustal strata (L2) (King, 1969, p. 61-62); they emerge are succeeded by as much as 6,000 m (20,000 ft) of Up­ along the axis of the foldbelt in the Long Range of New­ per Proterozoic strata that are topped by foseiliferous foundland, the Green Mountains and other uplifts in Lower Cambrian; the granite has yielded a radio- New England, and in the Blue Ridge as far south as metric date of 575 m.y. (McCartney and others, 1966). North Carolina. They can be extended westward by Comparable very late Precambrian events may have drill data to a well-defined boundary (probably a pro­ occurred in the basement rocks below the Lover Cam­ longation of the "Grenville front" of the shield) that brian in the Maritime Provinces and possibly in south­ is traceable southward from Michigan, through Ohio, eastern New England (Isachsen, 1964, p. 812-816), al­ into Kentucky (Bass, 1960). though on the tectonic map these rocks are shown either Rocks affected by this event are recognizable again as Grenville metamorphics (LI) or as Paleozoic plu- in Texas, where the Llano orogeny of the Llano uplift tonics (L8). Dates comparable to those of the Holyrood has been dated at 1,000 to 1,150 m.y. (Muehlberger and Granite have been obtained from drill samples near others, 1966, p. 5422-5423); comparable dates have been the Atlantic Coast in North Carolina (Denison and others, 1967), and perhaps in Florida (fig. 10). Farther obtained from nearby drill samples and from west Texas outcrops (Wasserburg and others, 1962). Ap­ northwest in North America, the only significant very late Precambrian to early Paleozoic events were the parently the belt continues southward with much the same trend into eastern Mexico, although details are Wichita igneous activity (A3), referred to earlier (p. 22, 40) and the unconformity between the Belt and uncertain because of sparse outcrops (which are labeled Windermere Series and their equivalents (O3 and O4) O2 on the map, a symbol used elsewhere for rocks af­ fected by the Hudsonian orogeny). Precambrian in- in the northern part of the Cordilleran fokHMt. liers in Tamaulipas have yielded a minimum date of The Avalonian event, although scantily represented 770 m.y., and those in Hidalgo a date of 1,210 m.y. The in North America, is of interest because it is nearly metamorphic complex in Oaxaca in the far south has contemporaneous with the Pan-African or Damaran yielded nine or more dates with a scatter between 700 orogeny, dated at 450-550 m.y. ago, during which many and 1,100 m.y. (Fries and others, 1962, p. 45-52; Fries foldbelts throughout were deformed and meta­ and Rincon-Orta, 1965, p. 81-87). The Precambrian morphosed (Kennedy, 1964; Clifford, 1967). An orog­ age of the metamorphic rocks in Oaxaca is further eny of about the same age is also reported in the Pre­ cambrian rocks of the Atlantic coastal area in Brazil. 7 W. R. Muehlberger has contributed a dissenting comment (written commun., April 1968) : "All the events yon cite are on volcanic rocks and their associated shallow granitic intrnsives which are clearly PRECAMBRIAN OF NORTHERN SOUTH AJTERICA younger than the basement on which they rest or into which they were Intruded. This basement almost everywhere in the region from New Mexico eastward to Ohio has Isotopic ages of 1,350-1,450 m.y. This Brief mention should be made of the Pre".ambrian basement I would include within the Eteonlan of the exposed Canadian Shield. This is the event which we named the Nemaha to distinguish it of the Guyana Shield in Venezuela and adjacent coun­ from the Elsonian inasmuch as there is no clear connection between the two because of the later Grenville events." tries of northern South America, whose edge projects PHANEROZOIC FOLDBELTS 43 into the southeastern corner of the "Tectonic Map cles consolidated originally mobile parts of the crust into of North America." The part of the shield shown in the cratons, the structures formed during successive phases map consists of metamorphic and plutonic rocks, from changing from alpinotype to germanotype in the s^iase which radiometric dates of 2,000-2,500 m.y. have been of Stille. The orogenic phase was the climax of mobUity obtained (McConnell and others, 1964), hence are in the foldbelt, when the folds, faults, and plutonic classed as of Archean age and part of the Kenoran fold- structures that were produced exceeded in magnitude belt on the tectonic map (G2 and Gy). Other dates, both and complexity any that were produced before or after. older and younger, have been reported from the The rocks that formed in the foldbelts during the crystalline rocks of various parts of the Guyana Shield, tectonic cycles also changed with time. During the errly, outside the area shown on the tectonic map. To the or preorogenic phase, the sites of most foldbelts v^ere southeast, also outside the area shown on the map, the , or tectonically unstable linear trougl s in crystalline rocks are overlain unconformafoly by the which sediments and volcanics accumulated, commonly nearly undef ormed sandstones of the Roraima Forma­ to a greater thickness than in the adjoining cratons. tion, once thought to be of Paleozoic or Mesozoic age. Preorogenic and orogenic phases of the foldbelts over­ However, mafic sills in this formation have yielded lapped; parts of the geosynclines were being oro^-^ni- radiometric dates of 1,700 m.y. (McConnell and others, cally deformed while other parts were still receiving: de­ 1964), showing that it is likewise Precambrian and is posits. The net effect of the orogenic phase was to reduce of Lower Proterozoic age. The prolonged stability of the areas of accumulation of supracrustal rocks and to the Guyana Shield, as indicated by these data, is break the earlier broad geosynclinal tracts into irore noteworthy. localized basins, many of which were surrounded by Northwest of the shield, in the Venezuelan Llanos, areas of much topographic relief; localized deposition nearer the front of the , small areas of crystalline continued into the postorogenic phase, partly in fault- rocks (Nl and N/3) project near El Baul, but are prob­ block depressions. In most foldbelts, marine deposition ably all of Paleozoic rather than of Precambrian age; decreased progressively from the orogenic into the post­ Cambrian fossils have been found in some of the orogenic phase, with a corresponding increase in phyllites, and the intrusive granites have yielded nonmarine deposition. A still later phase occurs in the radiometric dates of 270 m.y. Phanerozoic foldbelts that formed earliest; they became so stable that their truncated surfaces were overspread PHANEROZOIC FOLDBELTS by platform deposits. In North America, the foldbelts of Phanerozoic age During the tectonic cycle the foldbelts also underwent lie farther away from the center of the continent than a magmatic evolution. During the preorogenic phase the Precambrian shield and its bordering platforms, and volcanics were erupted in submarine environment^ in nearer the surrounding oceans. The continent is thus parts of the geosynclines; during the orogenic and p ost- almost ideally symmetrical; however, this apparent orogenic phases the environment of volcanism became symmetry may not be a valid generalization, as it is progressively more terrestrial; and during the Hter much less perfectly expressed, if at all, in other phases in many of the foldbelts the deformed bedrock continents. was nearly or wholly concealed by broad sheets of vol­ The Phanerozoic foldbelts evolved through time from canic products surmounted by chains of volcanic cones. early in the Paleozoic to the present, the evolution of Deep-seated crustal activity, of which the volcanism some of them having been completed long ago, of others may be a surface manifestation, reached its climax more recently, whereas in a few the evolution still con­ during the orogenic phase, when various suites of tinues. Their sedimentary and tectonic histories become plutonic rocks were emplaced, some in great volume. plainer the younger their age, but the histories of all During the postorogenic phase plutonic rocks v^ere of them are better preserved than those of the Precam­ emplaced in lesser volume, but they intruded both the brian foldbelts. This makes it possible to classify their original geosynclinal tracts and the adjacent disrupted components in more detail than those of the Precam­ parts of the original cratonic areas. brian, but it creates correspondingly greater problems The foregoing paragraphs summarize the tectonic as to how to represent them on the "Tectonic Map of cycles that have been observed in many of the foldbelts* North America." of the world, details of the history and terminology THE TECTONIC CYCLE being purposely suppressed so as to give the sumrrary All foldbelts evolved through time, passing through its widest possible application. More elaborate vers: ons what is here termed a tectonic cycle, consisting of pre- of the cycles have been proposed, but these obviously orogenic, orogenic, and postorogenic phases. These cy­ apply only in places; even the present version may be 44 THE TECTONICS OF NORTH AMERICA too particular. There are no "invariable laws" govern­ piler (King, 1959, p. 56-57), rather than undertake a ing the evolution of foldbelts, as has sometimes been precise definition, listed ten "geosynclinal attributes," claimed; the histories of many of the foldbelts fail to which were intended to encompass these variations. conform, in a few or in many respects, to the classical In the preceding section (p. 43), it was stated that concepts. the preorogenic phase of the tectonic cycle in a foldbelt Nevertheless, tectonic cycles of the kind outlined oc­ was also commonly a geosynclinal phase, and tH inevi­ curred widely enough hi North America, and are suffi­ tability of a geosynclinal phase as a prelude to orogeny ciently similar from one foldbelt to another to serve has been proclaimed by many geologists, beginning with as a frame on which to build the classifications adopted James Hall in his first statement of the concept. Accord­ on the "Tectonic Map of North America." Although the ing to de Sitter (1956, p. 351), geosynclines ar^ "those cycles in each of the foldbelts are thus broadly similar, accumulations of sediments of great thieknes^ which they and the rocks that formed during the cycles are have beien severely folded." Other geologists 1

103

EXPLANATION

Leptogeosynclinal deposits Older flysch Younger flysch Synorogenic and Platform deposits Volcanic rocks Devonian and older Paleozoic deposits deposits postorogenic deposits Cretaceous and and intrusives la lowest exposed units, Mississippian Pennsylvanian Younger Pennsylvanian Tertiary Cretaceous and separately shown in part in A; Permian in B Tertiary

__i__ Thrust fault Normal fault Synclinal axis 30°

FIGURE 12. Tectonic maps of the two larger areas of exposure of the Ouachita folclbelt showing the tectonic subdivisions of its rocks. These maps supplement the "Tectonic Map of North America," where the subdivisions are not shown because of the small scale. A, Ouachita Mountains of Oklahoma and Arkansas. B Marathon region in western Texas. Compiled from geologic maps of T*»xa«, Oklahoma, and Arkansas, with additions from other sources. OUACHTTA FOLDBELT 63 The course of the Ouachita f oldbelt, like that of the 5,200 m (17,000 ft) or more thick, but both here jvnd Appalachian f oldbelt, is marked by salients and recesses, in the Marathon region this part wedges out alirost but of even greater amplitude. The Ouachita Mountains entirely at the front of the foldbelt. The early Pennsyl- are at the apex of one salient, the Marathon region at vanian part (Atoka Formation) is 6,000 m (19,000 ft) another; a deep recess lies between the Ouachita and thick at the front of the Ouachita Mountains, and Appalachian foldbelts in Mississippi and Alabama, and wedges out almost entirely near the Ozark uplift in the another adjoins the Llano uplift in central Texas. craton to the north. In west Texas a similarly great but Throughout its course, the foldbelt lies against the cen­ younger flysch sequence occurs along the front, mainly tral craton, whose Paleozoic rocks have a different char­ in the Val Verde basin, and includes earliest Permian. acter and structure, commonly with thrust contact; in Successive pulses of orogeny in the Ouachita foldHlt part it is bordered by deep basins containing upper are thus suggested, progressing outward from the in­ Paleozoic synorogenic clastic deposits the Black War­ ternal zones and westward along its trend Mississip- rior basin of Alabama and Mississippi, the Arkoma pian and early Pennsylvanian pulses in both areas, a basin of Arkansas and Oklahoma, and the Val Verde late Pennsylvanian pulse in the western area. Related basin of western Texas. to the younger pulses are structural unconformitier in The relation of the Ouachita foldbelt to the Appala­ the higher Paleozoic foreland strata. chian foldbelt is incompletely determined. Trends of the As a result of these orogenies, the exposed rocks of two structures converge nearly at right angles, and drill the Ouachita foldbelt were deformed in much the sr.me data show that the frontal structures curve sharply from manner as the rocks of the miogeosynclinal belt of the one trend to the other, but the manner of junction of southern Appalachians, into asymmetrical folds over­ the internal zones is unknown. The Ouachita foldbelt turned toward the north, with many low-angle to also meets the rocks and structures of the Wichita sys­ high-angle thrust faults. Thrusts are especially nu­ tem nearly at right angles in southern Oklahoma; the merous and crowded in the western half of the Oua­ junction is complexly faulted, but the Ouachita rocks chita Mountains, near the impingement of the foldbelt are thrust over the Wichita rocks. In western Texas the on the Wichita system. The eastern half was formerly Ouachita foldbelt again meets at right angles the believed to be rather simply folded, even though the younger Cordilleran foldbelt, which largely overwhelms deepest parts of the structure and those parts fartHst it in Mexico. south were known to be conspicuously metamorphosed; The Paleozoic rock sequences of the exposed parts of it is now known that frontal thrusts extend across all the Ouachita foldbelt in the Ouachita Mountains and this part to the Mississippi Embayment, and that the Marathon region are remarkably alike, especially con­ structures behind the front include major recumbent sidering their wide geographic separation and the very folds or nappes (Viele, 1966). different nature of the contemporaneous rocks exposed In the buried parts of the foldbelt, drill holes h

(P) are bordered for their entire length by the Pacific NORTHERN CORDILLERA Ocean, without coastal plains and with only narrow The Cordilleran foldbelt trends northwestward in continental shelves. In the northwest, the Pacific Coast Canada with a nearly constant width of about 800 km is closely adjoined by the Aleutian Trench and in the (500 miles). A little east of the Alaska-Yukon boundary south by the Middle America Trench, both with depths the belt is constricted by a deep recess along its north­ greater than 6,000 m (20,000 ft). In the more lengthy eastern side. In Alaska the belt turns westward, widen­ intervening area the ocean bottom next to the coast ing and branching, one branch extending out to sea lacks trenches and lies at depths of 3,000-4,000 m southwestward to form the Aleutian . Despite (10,000-15,000 ft). the constriction and some tectonic differences on its t wo In the Cordilleran foldbelt, the climactic orogenies sides, the cross section of the foldbelt in both Canrda took place during Mesozoic time, but it had a long ante­ and Alaska is much the same, and longitudinal strips- of cedent history of sedimentation and deformation. It like rocks and structures extend through the whole seg­ arose from several geosynclines, some of which took ment, including a belt of miogeosynclinal rocks on the form early in Paleozoic time, as in the Appalachian northeast and a belt of eugeosynclinal rocks on the foldbelt, others as late as Mesozoic time. Early def orma- southwest. In addition, a belt of younger rocks snd tional features have been largely overwhelmed by the structures fringing the Gulf of Alaska is distinguisl <*! later ones, so that their patterns are seldom clear; some as a part of the Pacific foldbelt (P) and described under were probably precursors of the later structures, others a later heading. The segment extends a short distance seem to have been quite discordant. The postorogenic southward across the 49th parallel into Washington, history of the foldbelt was much more eventful and Idaho, and Montana, where it terminates against over­ complex than that in the Appalachians, with orogenic lapping volcanic rocks and the transverse faults of the and epeirogenic events extending through most of the Lewis and Clark zone. Cenozoic. In many places the imprint of these later Easternmost , opposite Alaska, contains fold- events is so great as to disrupt or obscure the more fun­ belts that are the obvious extensions of the Cordilleran damental Mesozoic structures. and Pacific foldbelts of North America into the corti- These later events are also largely responsible for nent of Asia. Indeed, the rocks and structures of the two the present system of Cordilleran Mountains. Through continents are separated only by the very shallow nor+h- most of the length of the Cordillera these events have ern part of the Bering Sea on the south and the Chukchi produced two major mountain chains, one near the Pa­ Sea on the north, underlain by continental crust. It is cific Coast on the west, the other fronting the continen­ evident from published maps (notably Tilman and tal interior on the east, with lower plateaus and less others, 1966) that rocks, structures, and orogenic tines continuous ranges between. These are only indirectly occur in Siberia which are analogous to those in Alas!-a; related to the original Cordilleran foldbelt, and in much nevertheless, their positions and trends do not clos^y of the western United States the eastern mountain chain match. There has been much speculation as to precise (Central and Southern Rocky Mountains) is actually a connections by Soviet and American geologists (among reactivated part of the original craton (see p. 23-24). the latter, Ostenso, 1968; Michael Churkin, Jr., unpub. As might be expected from the great length of the data), but more precise comparisons are desir­ Cordilleran foldbelt, its geology, tectonics, and history differ from one part to another, which makes generaliza­ able. Further information will no doubt become avail­ tion difficult, both on the tectonic map and in the present able as a result of oceanographic investigations now in text; this is reflected by the large number of units into progress on the sea-bottom geology of the intervening which the foldbelt is divided on the map legend, some areas. of which occur only in one part or another. Nevertheless, Between the 46th and 60th parallels the outer, or mio­ a certain degree of unity is evident for long distances in geosynclinal, part of the northern Cordillera forms the the foldbelt, with rather abrupt contrasts in tectonic Rocky Mountains, a bundle of parallel ranges with little 66 THE TECTONICS OF NORTH AMERICA curvature, about 120 km (75 miles) broad, which rise part of the stratified sequence, hence are shown sepa­ abruptly west of the plains of the continental interior, rately on the tectonic map (O3,04). generally along a zone of thrusting (Bally and others, The Middle Proterozoic part of this supracrustal se­ 1966, p. 340-345). The Lewis thrust near the 49th paral­ quence (O3) is exemplified by the Belt (Purcell) Series lel carries Middle Proterozoic strata (O3) eastward at that is widely exposed near the 49th parallel, but whose least 30 km (20 miles) over the rocks of the plains (Ross, equivalents also occur in the outer ranges as far north 1959, p. 76-78); the similar but not identical McConnell as Yukon Territory. The Belt is formed of shallow- thrust farther north carries Paleozoic strata eastward in water, fine-grained clastic rocks and interbecMed car­ the same manner. Behind the frontal thrust the rocks bonates; toward the east it overlies a Hudsonian (Lower of the mountains are closely crowded into folds and Proterozoic) crystalline basement (H5, O2), but within thrust blocks, probably accompanied by decollement the foldbelt its base is not visible even though it attains over the basement. East of the frontal thrusts the rocks a thickness of 15,000 m (45,000 ft). It is everywhere un- of the plains are themselves intensely disturbed in a conformable beneath younger rocks, and in the interior foothills belt 30 km (20 miles) wide. of the foldbelt is succeeded by Upper Proterozoic supra­ North of the 60th parallel, the Rocky Mountains give crustal rocks (O4), exemplified by the Windermere place to the Mackenzie Mountains, also a bundle of Series near the 49th parallel, a sequence nearly as thick, ranges of folded and faulted rocks, but broader, more but coarser and with some volcanic components, that strongly arcuate, and offset a little to the east. An outer is conformable or nearly so with the succeeding Paleo­ strand, the Franklin Mountains, deforms the rocks of zoic. The Upper Proterozoic includes the "grit unit" of the plains still farther east, nearly to Great Bear Lake the internal zones in Yukon Territory, a poorly sorted and the edge of the Canadian Shield. Much farther arkose, apparently derived from crystalline rocks far­ northwest is the Brooks Range of northern Alaska, with ther southwest. a structure like that of the Rocky Mountains and Mac­ The Middle Proterozoic supracrustal rock? are geo­ kenzie Mountains, but trending nearly westward. Be­ synclinal deposits that formed in a trough antecedent tween the Mackenzie Mountains and the Brooks Range, to the main Cordilleran geosyncline, yet one which near­ the deep recess referred to above extends nearly to the ly corresponded in position to its successor. Most of its Yukon River, into which structures curve from the sediments were seemingly derived from the continental ranges on either side. Within the recess are blocklike interior. This early geosynclinal phase was terminated uplifts, such as the Richardson Mountains, separated by crustal movements, most of which were ep^irogenic, by deep structural basins. but which attained erogenic proportions, at least in It should be evident from the "Tectonic Map of North Yukon Territory. The succeeding Upper P-oterozoic America" that the Brooks Range is an integral part of supracrustal rocks are more closely related to the main the Cordilleran foldbelt (Gates and Gryc, 1963, p. 265- Cordilleran geosynclinal phase, and form its basal de­ 266), and is not an independent tectonic element, sepa­ posits where present. Their coarse sediments were de­ rated from the true Cordilleran foldbelt farther south rived from mixed sources, and part of them from lands by a "Yukon stable block," as has been claimed (Jeletz- farther in the interior of the foldbelt. Neithe^ of these Proterozoic sequences was materially disturbed until ky, 1962, p. 62-66). The recess between the arcs of the the much later Cordilleran orogenies, and t]i?ir meta- Brooks Range and Mackenzie Mountains has features morphic phases (O3a, O4a) date from that period. of a stable block, but this block does not extend indefi­ On the tectonic map, strata shown as iniogeosynclinal nitely westward across Alaska. Large parts of the pre- (O9) in the northern Cordillera extend from Cambrian Mesozoic rocks south of the Brooks Range are Paleozoic to Jurassic in Canada, and from Upper Devonian to geosynclinal deposits, partly metamorphosed, which Jurassic in Alaska where the lower Paleozoic is shown have participated in two or more of the Cordilleran separately (O5). The Cretaceous is generally excluded, orogenies. but is mostly preserved in the adjoining platform area No Precambrian crystalline rocks emerge anywhere (B) rather than in the foldbelt. On the map, the bound­ within the ranges of the miogeosynclinal belt, nor ap­ ary between the miogeosynclinal and platform area is parently elsewhere in the northern Cordillera. However, thus commonly drawn at the contact between the younger Precambrian supracrustal rocks are extensively Cretaceous and older strata, which in manr places is exposed in the higher uplifts in the miogeosynclinal also the outer limit of strong deformation. The deformed belt and the adjoining part of the eugeosynclinal belt Paleozoic rocks in the outer strand of the foldbelt in the (Reesor, 1957, p. 152-162; Gabrielse, 1967, p. 271-275). Franklin Mountains are therefore shown FS miogeo­ They form significant "structural stages" in the lower synclinal, although they might, with much propriety, be NORTHERN CORDILLERA 67 considered as deformed platform cover. Also shown as but thick limestone bodies also accur, although they are miogeosynclinal are the pre-Cretaceous rocks in the mostly of local extent. Coarser clastic rocks, including recess between the Mackenzie Mountains and the Brooks conglomerate, attest the uplift and probable deforma­ Range, although not all of their complex sequence is tion of adjacent areas. truly geosynclinal (Jeletzky, 1962, p. 62-66). In places, On the "Tectonic Map of North America," the eug'°x>- miogeosynclinal rocks contrast greatly with adjacent synclinal rocks that were involved only in the Meso­ eugeosynclinal rocks of the same ages (O6, O7), mostly zoic orogenies, and which are primarily of Mesozoic age where they are juxtaposed along faults. Elsewhere, the (O7) are separated from an earlier part, primarily of boundary is indefinite, different parts of the sequence Paleozoic age (O6) which was involved in one or more partaking of one f acies or the other, so that the contact previous orogenies and reworked later. Besides these, a is arbitrarily drawn. still older unit of early Paleozoic age (O5) is showr in In general, the Paleozoic miogeosynclinal rocks are Alaska a unit which was involved in middle Paleozoic dominantly carbonates,, and the Mesozoic dominantly or earlier orogenies, and not all of whose rocks are of clastic wedge deposits, but the sequence varies greatly eugeosynclinal character. from one part of the belt to another (Bally and others, Rocks of the last-named category (O5) are extensive 1966, p. 361-369; Gabrielse, 1967, p. 275-283; Brosge in the interior of Alaska, where they include phyllites and others, 1962). Unconformities occur, especially in and schists (O5a) that underlie little metamorphosed the upper part of the Devonian, but they were mostly rocks of known Mesozoic age. On the flank of the Brooks produced by epeirogenic rather than erogenic move­ Range the metarnorphic rocks are traceable northward ments, and angular discordances are local. The array of into unmetamorphosed formations beneath Upr»er unconformities and the epeirogenic movements which Devonian conglomerates; elsewhere, their ages are they express is especially complex in the recess between known only from fossils in scattered limestone units. the Mackenzie Mountains and the Brooks Range (Je­ Clearly, these rocks have been involved in one or more letzky, 1962, p. 62-71). However, true orogenic deforma­ orogenies during Paleozoic time, and perhaps early in tion seems to have occurred in the core of the eastern that time. In the Seward Peninsula the metamorpMc Brooks Range, where rocks below the Upper Devonian and partly fossiliferous rocks were folded along north- have been much metamorphosed (O5a); granites in the south axes during the Paleozoic, and were refolded vicinity have been dated radiometrically between 370 along east-west axes during the Mesozoic or later (D. M. and 220 m.y., hence are middle to late Paleozoic (Oe). A Hopkins, oral commun., 1966). possible relation between these deformed rocks and the Southeast of Alaska older Paleozoic rocks are ex­ Innuitian foldbelt to the northeast has already been posed only along the northeastern and southwestern suggested (p. 53). edges of the eugeosynclinal area; those on the northeast Most of the Mesozoic clastic wedge deposits of the are gradational into the miogeosynclinal deposits, those miogeosynclinal sequence are related to orogenic activ­ on the southwest (in the islands of the Alaska Panhan­ ity in the eugeosynclinal part of the foldbelt, rather dle) are more truly eugeosynclinal (Brew and others, than in the miogeosynclinal area itself. Nearly all the 1966, p. 157-166). In the northeastern belt, where th^se deformation in the miogeosynclinal part of the foldbelt rocks are not separated from the younger Paleozoic, was late in the Cretaceous or still later, hence is Lara- significant orogeny near the close of the Devonian has mide in the broad sense; in the southern part of the been recorded in the Cassiar area and elsewhere segment Paleocene strata are deformed equally with (Wheeler, 1966, p. 34). Plutonic rocks of early Paleo­ the Cretaceous in the foothill belt east of the Rocky zoic age (Oc) occur in the southwestern area. Mountains. In the Brooks Range, however, Laramide Later Paleozoic and early Mesozoic eugeosynclinal deformation was preceded by an Early Cretaceous deposits (O6, O7) extend the length of the northern (Aptian) time of orogeny and thrusting. Cordillera, and many sequences attain thicknesses of Much of the remainder of the northern Cordillera in more than 7,600 m (25,000 ft). Proportions of the dif­ both Canada and Alaska was a eugeosyncline during ferent components volcanics, chert, argillite, grr»y- large parts of Paleozoic and Mesozoic time. Its supra- wacke, and limestone vary greatly from one place and crustal rocks and deformational events were as varied from one part of the sequence to another, but the Me^o- as those in other eugeosynclines, and at least the younger zoic generally contains coarser elastics than the Pal?.o- of these are well documented in many places. Volcanic zoic. Deposition was widely interrupted during early rocks are common, especially in certain parts of the Triassic time, mostly by epeirogenic movements, hut sequence for example, from Devonian into Mississip- locally by moderate deformation (White, 1966a, p. 187). pian and from Permian into Triassic in some areas Mesozoic time was one of nearly continuous orogeny 68 THE TECTONICS OF NORTH AMERICA from place to place in the eugeosynclinal area, and it is Other metamorphic rocks have been transformed by difficult to make any meaningful generalizations as to recrystallization and plastic flowage to such an extent erogenic climaxes; the statement on the legend of the that most indications of their original nature r.nd rela­ tectonic map that the eugeosynclinal deposits were de­ tions have been lost, and they are therefore mapped as formed mainly during a middle Mesozoic (Nevadan) metamorphic complexes (Ol). The complexes include orogeny is a gross oversimplification (Misch, 1966, p. the Birch Creek Schist of interior Alaska, the equiva­ 119). In many areas all the Mesozoic, and even part of lent Yukon Group of Yukon Territory, the Shuswap the Tertiary, is steeply folded. Nevertheless, a signifi­ complex of southern British Columbia, and various cant part of the deformation had been accomplished intervening bodies all in the northeastern plutonic well before the terminal Mesozoic (Laramide) orogeny and metamorphic belt. in the miogeosynclinal belt to the northeast. The Earlier geologists interpreted the metamorphic com­ younger Mesozoic and the Tertiary rocks (OlO, Oil), plexes as an Archean protaxis that separated younger even where steeply folded, did not participate in many geosynclines on the two sides, but subsequent investi­ of the deformations that affected the older rocks; they gations reveal a much more eventful history, involv­ are less metamorphosed and plutonized, and they con­ ing heating and recrystallization in place, penetrative tain larger volumes of terrestrial deposits and of coarse flowage, and superposed deformations through pro­ immature elastics. longed periods of time. Most radiometric dates in the During Mesozoic time granitic rocks were emplaced complexes are surprisingly young 150 m.y. or less in the eugeosynclinal area on a vast scale (O0). The but those record only the latest thermal events and sub­ great Coast batholith of western Canada is nearly con­ sequent cooling, after burial at great depths in the tinuous for 1,800 km (1,100 miles), from the 49th past crust. Parts of the complexes may be highly altered the 60th parallel, and is 80-200 km (50-120 miles) wide, phases of Paleozoic or even Mesozoic supracrustal but' it is inhomogeneous internally and of several ages rocks, but other parts may once have been Proterozoic (Roddick, 1966, p. 76-79). Other sizeable but less con­ supracrustal rocks or an even earlier basement, now tinuous batholiths occur in Alaska to the northwest and remobilized. in Canada to the east, especially in a belt along the During Late Jurassic and through most of the Cre­ northeastern side of the eugeosynclinal area (Cassiar taceous time, the deformed eugeosynclinal rocVs of the Mountains southward to Monashee Mountains). Geo­ northern Cordillera were partly covered by deposits of logic relations and radiometric datings both indicate a successor basins (OlO), which were themselves vari­ prolonged time of emplacement from 250 m.y. to less ably deformed later. The deposits are coarse elastics than 70 m.y., or through all of Mesozoic and into Terti­ and interbedded volcanics, partly marine, partly con­ ary time. By far the most extensive granitic emplace­ tinental, most of the clasts having been derived from ment, especially in the Coast batholith, occurred during surrounding uplifted areas; the deposits also include Cretaceous. The batholiths of early Tertiary age are fragments from granite plutons that had Hen em- late phases of the Mesozoic plutonism, but a few near placed only a little earlier (Grantz and otheT-s, 1963, the 49th parallel are as young as middle Tertiary (OA). p. B58-B59). The successor basins are most extensive Radiometric dating suggests pulses of granitic emplace­ in Alaska (Gates and Gryc, 1963, p. 269-272), but the ment during the Mesozoic at intervals of 30 m.y., but Bowser basin in central British Columbia is 200 by 320 while the pulses lie within the general period of orog­ km (120 by 200 miles) across (Souther and Armstrong, eny, they have little obvious relation to the known 1966, p. 178-180). Smaller basins of continental Ter­ times of deformation in the supracrustal rocks (Ga- tiary deposits (Oil) occur here and there in the interior brielse and Reesor, 1964, p. 127-128). of the northern Cordillera. Parts of the eugeosynclinal rocks have been strongly Parts of the eugeosynclinal rocks are broken 17 faults, metamorphosed, mainly in areas near granitic plutons, some of which originated during the geopynclinal which were also areas of long-persistent heating and period itself, but extensive low-angle thrusts h*.ve been recrystallization. Metamorphic rocks thus lie within described only in the northern Cascade Range of Wash­ and around the Coast batholith, as well as in the plu- ington (Misch, 1966, p. 120-136). Of greater interest tonic belt along the northeastern side of the eugeosyn­ are the very lengthy longitudinal faults, which are clinal area, where they are more continuous than the commonly expressed as topographic trenches o^* valleys granitic rocks themselves. Many of the metamorphic on the land and as fiords between the offshore islands. rocks in these belts can be identified as equivalent to less The fault zone of the Rocky Mountain Trend and its altered Proterozoic, Paleozoic, and Mesozoic forma­ continuation into the Tintina Trench extends with a tions, and have been so mapped (O3a, O4a, O6a, O7a). few offsets from south of the 49th parallel across Can- CENTRAL CORDILLERA 69

ada into Alaska, and the Denali fault zone extends fornia it extends as far west as the front of the Sierra through the high mountains of southern Alaska into Nevada, but in the Mojave Desert to the south its con­ the southeastern panhandle. The straight, or gently tinuity is lost in the confused ranges of the southern curved courses of these and other fault zones, the inter­ transverse zone. change of apparent upthrown and downthrown sides In western Wyoming and southeastern Idaho the along their courses, and the frequent absolute differ­ frontal part of the miogeosynclinal belt consists of close­ ences between rocks on the opposite sides all suggest ly crowded folds and thrust slices, very much like those major strike-slip displacements; strike-slip displace­ in the Canadian Rocky Mountains (Armstrong and ments have been proved in a few segments, but the ac­ Oriel, 1965). Southward in Utah, the frontal part is de­ tual displacement in much longer segments remains flected into the eastern edge of the Great Basin, aromd enigmatic. The Fairweather fault near the coast of the western end of the axis. The ranges Alaska is active today and has given rise to major of the eastern Great Basin contain low-angle thrust earthquakes, but most of the remainder have long been faults of great lateral displacement, many of which su­ dormant. perpose strata near the base of the Paleozoic over Meso­ CENTRAL CORDILLERA zoic strata; the thrusts can be correlated from one range The Cordilleran Mountains occupy most of the to the next, through Utah into southern Nevada, in such United States west of the 104th Meridan, but the cen­ a manner as to indicate that they are continuous fea­ tral segment of the Cordilleran foldbelt (O), as here tures (Longwell, 1960; Crittenden, 1961; Armstrong, defined, forms only part of them. The foldbelt is lim­ 1968, pi. 1). Behind the frontal belt in western Utah and ited northward by the Lewis and Clark transverse zone eastern Nevada, the thrusts within the ranges difl'er; near the 46th parallel, and southward by another trans­ higher parts of the miogeosynclinal sequence have verse zone near the 34th parallel the Transverse moved over the lower parts, and the whole has moved Ranges and the Texas Lineament of southern Cali- over a major decollement near the base of the Paleozoic lyrnia and Arizona. The eastern part of the moun­ (Misch, 1960). Correlation from one range to the next tain system (Central and Southern Rocky Moun­ indicates that the decollement is a continuous feoture tains and Colorado Plateau) is classed as a reactivated like the frontal thrusts to the east. It is tempting to infer craton (B), and the western part (Cascade Range and that the decollement is the root zone of the frcutal Coast Ranges) is placed in the Pacific foldbelt (P). As thrusts, although there is some evidence that the here defined, the Cordilleran foldbelt is 1,100 km (700 former originated somewhat before the latter (Arm­ miles) wide near the 42nd parallel, but it is much nar­ strong, 1968, p. 444). Frontal thrusts and decolle­ rower to the north and south. ment are exposed across nearly the whole width of the Even within the central segment of the foldbelt as miogeosynclinal belt in this latitude, and if they are thus restricted, the fundamental Mesozoic and older parts of the same feature there was an eastward trans­ orogenic structures are greatly obscured by younger port of very broad sheets of miogeosynclinal strata over rocks and structures, so that their gross pattern is by their basement although for distances by no means as no means as evident on the tectonic map as it is in the great as the observed breadth of exposure. northern Cordillera. Superposed on the Mesozonic and The crystalline basement (O2), probably part of the older structures are Cenozoic volcanic fields and a sys­ Hudsonian foldbelt, emerges near the eastern edg«. of tem of block faults which trend in other directions, and the miogeosynclinal area in Utah, but it extends ae far on the tectonic map these varied features play against west as Death Valley in southeastern California. It is each other in counterpoint. In the northern part of the succeeded in places by Middle Proterozoic strata (O3), segment, the continuity of the Mesozonic and older comparable to the Belt Series farther north and the structures is interrupted entirely by a transverse belt Grand Canyon Series farther east, and more widely by of volcanic rocks that extends from the Snake River Upper Proterozoic strata. The latter attain imprersive Plain of Idaho into the Absaroka Mountains of thicknesses in the south (Noonday, Johnnie, Wyman, Wyoming. and other units), and like the Upper Proterozoic of the The eastern, or miogeosynclinal part of the foldbelt northern Cordillera are nearly conformable with the is poorly defined north of the transverse volcanic belt, Paleozoic above; in this segment of the Cordillera their in Idaho and Montana, where it is closely crowded be­ areal extent is not great enough to separate them from, tween cratonic and internal elements. It widens farther the higher miogeosynclinal strata on the tectonic map. south in the Great Basin, where it extends westward The miogeosynclinal sequence of the central Cordil­ 400 km (250 miles) from the edge of the Colorado lera (O9) is largely Paleozoic, but includes remnants of Plateau in Utah into central Nevada. In eastern Cali­ lower Mesozoic (Triassic and Jurassic) at the top, as 70 THE TECTONICS OF NORTH AMERICA well as some infolded Cretaceous along the eastern edge. 800 km (500 miles); its principal manifestation is an Farther back in the foldbelt, as at Eureka, Nev., the Cre­ eastward transport along the low-angle Roberts thrust 9 taceous is postorogenic and continental, and was laid of the eugeosynclinal sequence over a miogeosynclinal down in successor basins whose present remnants are sequence of the same age (fig. 6 and 13). Windows and too small to map. Throughout the Great Basin, the klippen of the thrust occur from one range to tl ^ next volume of the Paleozoic carbonate rocks is impressive. across a breadth of 95 km (60 miles), which is the. mini­ To the south, near Las Vegas, Nev., 5,500 m (18,000 ft) mum distance of transport. The known manifestation of dominantly carbonate rocks, extend from Cambrian of the Antler orogenic belt involved only shallow parts to Permian. Farther north, near Eureka, Nev., carbon­ of the crust, but its deep-seated origin is sugges+ed by ate rocks from Cambrian to Devonian alone are 4,500 m its influence on subsequent tectonic history. The o**ogen- (15,000 ft) thick, but in this latitude they are succeeded ic belt was the site of lesser disturbances durirg suc­ by a Mississippian clastic wedge that expands westward ceeding Paleozoic time, and the upper Paleozoic and toward the middle Paleozoic Antler orogenic belt in the lower Mesozoic eugeosynclinal deposits were laic1 down eugeosynclinal area (Nolan and others, 1956); the thin­ only farther west (Silberling and Roberts, 1962, p. 7- ner, higher Paleozoic, again contains carbonate strata. 25). The lower Mesozoic eugeosynclinal rocks (O7) Further complications develop in the eastern part of pass eastward into near-shore shelf deposits toward the the miogeosynclinal area in the same latitude, where the site of the orogenic belt. upper Paleozoic thickens to 9,000 m (30,000 ft) in the Elsewhere in the western part of the Cordilleran fold- Oquirrh basin nearly 10 times its value in nearby belt eugeosynclinal conditions were long persistent. areas in an exceptional area of extreme but transitory Volcanic rocks as old as Silurian and Devonian have subsidence. On the other hand, in Idaho and Montana, been identified in parts of the and ephemeral positive areas have caused large gaps in the the Sierra Nevada, and volcanic rocks are prominent in Paleozoic sequence, and the Lower Cambrian is missing the remaining Paleozoic and the older Mesozoic, but the at the base. Klamath Mountains sequence contains several prominent The eugeosynclinal belt of the central Cordillera ex­ Paleozoic carbonate formations (Irwin, 1966, p. 23; tends westward from the miogeosynclinal belt to the Clark and others, 1962). The Triassic and Jurassic con­ Pacific f oldbelt (P) near the coast, which had a differ­ tain great volumes of andesitic basaltic submarine lavas ent tectonic history. In California the boundary with and pyroclastics (Dickinson, 1962). Partial sequences the Pacific foldbelt is along the western edge of the of eugeosynclinal rocks in various areas are 9,000 m Sierra Nevada and Klamath Mountains, which in the (30,000 ft) or more thick; however, the total of the latter area is a zone of east-dipping thrusts (Irwin, whole eugeosynclinal column is undetermined. Struc­ 1966, p. 33-36; Davis, 1968). In Oregon and Washing­ tural unconformities at various levels indicate pulses of ton the boundary curves eastward in the Columbia arc orogeny from one place to another at various timos dur­ (not labeled on the map), as shown by deflection of both ing the Paleozoic and early Mesozoic, but their extent structural trends and the quartz diorite petrographic and magnitude is not as clearly defined as it is in the boundary (Moore, 1959, p. 199). Whether the Columbia arc was indigenous, or whether it was created by oro- Antler orogenic belt. clinal bending during the Cenozoic (Hamilton and The climactic middle Mesozoic orogeny in the Sierra Myers, 1966), is beyond the scope of this discussion. Nevada has been dated as late in the Jurassic, and1 is the As in the northern Cordillera, the eugeosynclinal classic Nevadan orogeny; the climax may havo been supracrustal rocks of the central Cordillera are divided earlier or later in other parts of the eugeosynclinal belt on the map into a unit of Triassic and Jurassic age (O7) (Gilluly, 1963, p. 146-150). Deformation was progres­ deformed only during the Mesozoic orogenies, and a sively younger eastward across the foldbelt, and in the unit of Paleozoic age (O6) that was deformed during miogeosynclinal area there is no clear separation be­ several earlier orogenies. tween a middle Mesozoic orogeny and a terminal Meso­ In Nevada, the eastern part of the eugeosynclinal belt zoic orogeny; most of the events were at intermediate is dominated by a lower to middle Paleozoic (Ordovi- times. At Eureka, Nev., deformed miogeosynclinal rocks cian to Devonian) sequence of cherts and volcanics as are overlain by remnants of Lower Cretaceous successor much as 15,000 m (50,000 ft) thick that was deformed basin deposits, but at the eastern edge in the same during the Antler orogeny of middle Paleozoic (Early latitude the coarse synorogenic deposits are Upper Mississippian) time (Roberts and others, 1958, p. 2816- Termed the "Roberts Mountains thrust" In many publications, but 2821). The orogenic belt can be traced through the later the shorter form proposed by Gilluly (1963, p. 141-142) to preferred by structures that are superposed on it for a distance of the compiler. CENTRAL CORDILLERA 71

116° 114° 120 118° EXPLANATION

OREGON [lDAHO__ _ £_ _ _ "" NEVADA" '/ 3 § fc

Outcrop of eastern (miogeosynclinal) sequence

Outcrop of Roberts thrust

Inferred trace of Roberts thrust, where covered by younger deposits

Boundaries between tectonic regions

-38°

200 KILOMETERS 200 MILES

FIGURE 13. Map of northern Nevada showing extent of Antler orogenic belt of middle Paleozoic time. The Paleozoic and Mesozoic rocks of northern Nevada are divisible into three tectonic regions with the following characters: Region A, Lower and upper Paleozoic eugeosynclinal deposits not affected by Antler orogeny, deformed by Sonoma orogeny during Peririan time, overlapped from west by Triassic shelf deposits, and deformed by middle Mesozoic orogenies. Region B, Antler orogenic belt and area involved in Roberts thrust; lower Paleozoic eugeosynclinal deposits deformed by Antler orogeny in middle Paleozoic time and overlapped from east by upper Paleozoic shelf deposits. Region C, Lower Paleozoic miOTeo- synclinal deposits, upper Paleozoic clastic wedges derived from Antler orogenic belt, and minor Mesozoic deposits; deforned during late Mesozoic time, with development of thrusts and decollements. Compiled from many sources, including Roberts and others (1958).

Cretaceous (Spieker, 1946). Major thrusting in the gently to steeply dipping tabular bodies, some of great miogeosynclinal belt in Utah was completed by the mid­ thickness and extent. Commonly these bodies are ? long dle of Late Cretaceous time (Armstrong, 1968, p. 451). fault planes, and whatever their original history they Farther north, in southeastern Idaho and western Wyo­ clearly have been emplaced into their present positions ming, successive thrust blocks formed from west to east by tectonic processes. between Late Jurassic and Paleocene time (Armstrong Granitic rocks (O0) are as extensive in the eugeo­ and Oriel, 1965, p. 1857-1861). The classic Laramide synclinal belt of the central Cordillera as they are in the orogeny of Paleocene time occurred primarily in the northern Cordillera. They coalesce into two major reactivated craton, east of the Cordilleran foldbelt as batholiths the Sierra Nevada batholith in the routh here defined. and the Idaho batholith in the north, the latter offset Ultramafic rocks (Oy) are prominent in the western eastward from the former in accordance with the curve part of the eugeosynclinal belt, especially in the Sierra of the Columbia arc (p. 70). Elsewhere, granitic rocks Nevada and Klamath Mountains, where they form form smaller plutons in the eugeosynclinal terrare, or 72 THE TECTONICS OF NORTH AMERICA inliers in areas of younger rocks. Both of the larger rane was split longitudinally into broad to narrow batholiths are composite, being composed of many dis­ blocks, whose displacement produced the present topog­ crete plutons, and in the Idaho batholith including mig- raphy of disconnected ranges and intervening basins; matized host rocks. The granitic rocks become more larger, longer fault blocks along the edge of tho Colo­ silicic eastward, changing from quartz diorite to quartz rado Plateau to the east and the Sierra Nevada to the and granodiorite (Moore, 1959, p. 205-206). west are more coherent manifestations of the same struc­ With some exceptions, they are also younger from to ture. The block faulting is a mountain-creating process, west to east; all are Mesozoic, but they yield radiometric hence one of the few deformationa] events in the Cordil­ dates between 180 and 80 m.y., or from late Triassic lera to which the term "orogeny" can be applied in the through the Cretaceous (Kistler and others, 1965, p. narrow sense of Gilbert; in the broader sense of the term 162-164). Most of those west of the Sierra Nevada bath­ the block faulting is more properly postorogenic. olith are Jurassic or older, whereas most of the dated The faults at the edges of the mountain blocks are plutons in the Sierra Nevada and Idaho batholiths are shown by normal fault symbols on the tectonic map, so of Cretaceous age. The Boulder batholith near the front far as they are known. Some of the known faults can be of the foldbelt east of the Idaho batholith invades Late observed directly, the existence of others is proved by Cretaceous rocks. In the Great Basin Mesozoic plutons the even baselines of the mountains and the steep escarp­ are mingled with smaller Tertiary intrusives (OA); the ments behind them. Other mountains may also have been latter extend thence across the miogeosynclinal belt and fault blocks, but they were uplifted so long ago that the into the reactivated craton east of it. The age range of fault scarps have been destroyed by erosion, and the the Mesozoic granitic rocks broadly coincides with the positions of the faults themselves are lost. The down- time of erogenic deformation of the eugeosynclinal faulted blocks, or basins between the ranges, have been rocks, but those of Cretaceous age are later than the filled by debris eroded from the upfaulted areas, shown erogenic climax; the Sierra Nevada batholith breaks on the map as "thick deposits in structurally negative into and shoulders aside eugeosynclinal rocks that areas" (O12). Coarse fanglomerates were deposited near had already been steeply upended and regionally the edges of the basins and fine sands and silts near metamorphosed. the centers, the whole accumulation of deposits attain­ During Cenozoic time, wide areas of the deformed ing thicknesses of 3,000 m (10,000 ft) or more. On the geosynclinal rocks were covered by terrestrial volcanic tectonic map the distribution of these debris-filled basins rocks (Oft, Oir). The cover is largely intact through suggests the pattern of latest deformation in tlv<^ cen­ Oregon into southern Idaho, but farther south it is tral Cordillera, even where faulting is not apparent. dismembered by block faulting and erosion. Volcanic history was prolonged; the older Tertiary volcanics are SOUTHERN CORDILLERA much deformed and mineralized, the more extensive The southern Cordillera is separated from tH cen­ middle Tertiary volcanics are block faulted, the Quater­ tral Cordillera by the so-called Texas Lineament nary volcanics are merely tilted in the transverse Snake (Albritton and Smith, 1957; King, 1965, p. 118), a trans­ River downwarp. The varied compositions and origins verse zone that extends east-southeast from the Trans­ of the volcanic rocks cannot readily be indicated on the verse Ranges of coastal California, south of the Colorado tectonic map, and are therefore not differentiated in the Plateau and a little north of the United States-Mexican generalized units adopted. Basaltic and andesitic vol­ boundary, into western Texas. The zone is a strip oi canics are prominent to the north and west, but silicic country as much as 160 km (100 miles) wide that sepa­ volcanics cover large parts of the southern Great Basin, rates two parts of the Cordillera with different topog­ and include widely spread sheets of ignimbrite; where raphies, geologic histories, and styles of deformation. known, the calderas from which they were erupted are South of the zone the Cordilleran foidbelt exterds 800 shown by smbols on the map. km (500 miles) farther east than on the north side, and During Cenozoic time also, large parts of the central for long distances its deformed rocks closely adjoin Cordillera were disrupted by block faulting, caused by little deformed rocks in the Colorado Plateau and the crustal distention. Block faulting has occurred in other block mountains of New Mexico, which are reactivated foldbelts of the world late in the tectonic cycle, but its or disrupted parts of the former craton. These contrasts magnitude here is without parallel. Block faulting and have not been produced by transverse faulting, a nd the volcanism partly coincide in time, but the faulting con­ Texas Lineament is not a through-going fault zone, as tinued later; major faulting occurred during the early has sometimes been assumed. It is true that the Trans­ Pleistocene, and minor faulting continues today in verse Ranges on the west contain eastward-trending places. In the Great Basin, the previously deformed ter- high-angle faults, some with left-lateral displacement, SOUTHERN CORDILLERA 73 but there are few faults parallel to the zone farther east, present Sierra Madre Oriental, the Mesa Central on the although it contains many faults trending in other di­ west, part of the Sierra Madre del Sur on the south, and rections (as elsewhere in the Cordillera). its extension into Central America. The miogeosynclinal The southern Cordillera includes all of Mexico except rocks are dominantly later Mesozoic (mapped as O9, the Gulf Coastal Plain on the east, an area of platform like the more heterogeneous miogeosynclinal deposits deposits (C), and the parts of Baja California on the in Sonora). Jurassic and Lower Cretaceous carbonate west which belong to the Pacific foldbelt (P). From rocks form most of the ranges, and are succeeded by southeastern Mexico it extends eastward into northern Upper Cretaceous marine clastic wedge deposits. P«v- Central America, where it forms much of Guatemala, neath the carbonates are earlier Jurassic and Triarsic Belize, and Honduras. Except in the extreme south (see red elastics, which include evaporites in some places r.nd p. 74), most of the surface of the southern Cordillera volcanics in others. In the interior ranges of central md is formed of Mesozoic and Cenozoic rocks, and older southern Mexico the deformed Mesozoic miogeosyncli­ rocks emerge only as small inliers in the higher folds nal rocks are overlain by thick accumulations of synoro- and fault blocks, so that the pre-Mesozoic structural genic and postorogenic coarse continental deposits of pattern is largely undetermined or speculative. Very Eocene and younger age, the so-called red conglonr^r- likely the Mesozoic and Cenozoic rocks and structures ates (O10) (Fries, 1960, p. 91-101). In other ranges, are superposed on a very different set of Paleozoic and fronting the Gulf Coastal Plain, marine lower Tertiary older geosynclines and foldbelts, along with their fore­ deposits, including Oligocene and even younger a#es, lands and backlands. are folded with the earlier rocks. Deformation of the In the far northwest, in the Altar district of Sonora, miogeosynclinal belt of the southern Cordillera was thus are incomplete exposures of a geosynclinal sequence, primarily Late Cretaceous and early Tertiary, or very much like that in the Great Basin of the central broadly contemporaneous with the Cordillera (O9), consisting of carbonate rocks that in­ of the central Cordillera. clude Lower Cambrian and the higher Paleozoic sys­ Deformation of the miogeosynclinal rocks produ?-ed tems, topped by Triassic and Jurassic marine shales; a fine display of folds (Alvarez, 1949, p. 1330-13? 3), beneath are Upper Proterozoic supracrustal rocks like some long and closely crowded, others shorter and dis­ those in the Great Basin (O4), which rest on a crystal­ continuous; these are accompanied by few low-an^le line (Hudsonian) basement (O2) (Cooper and Arel- thrusts. The closely crowded folds formed over a decol- lano, 1946; Damon and others, 1962). The relations of lement in the weak layers of the lower part of the this geosynclinal sequence to rocks of surrounding Mesozoic sequence, especially where evaporites are inter- areas in Mexico is unknown, and it is separated from bedded ; the more open folds involve the basement, wHch its analogue in the Great Basin to the northwest by emerges in the cores of the uplifts. South of the 22d 650 km (400 miles), in which no trace of it is preserved. parallel the deformed miogeosynclinal rocks front Farther east, the front of the foldbelt of the southern the little disturbed rocks of the Gulf Coastal Plain ir an Cordillera overwhelms the Ouachita foldbelt (M), abrupt mountain face, but farther north domical uplifts whose characteristic rocks and structures are exposed in stand in front (shown by structure contours in the the Marathon dome and some other areas in western coastal plain rocks on the tectonic map) the Sienr. de Texas. In northeastern Mexico the Coahuila peninsula Tamaulipas, the Sierra de San Carlos, the Sierranias notably distorts the Cordilleran folds, and is a massif del Burro, and the Marathon dome in Texas. The rrain that was evidently consolidated during the Ouachita bundle of folds is strongly deflected westward near the orogeny. Inliers of Paleozoic and Precambrian rocks 25th parallel around the south end of the Coahuila pen­ occur, both near the Coahuila peninsula and farther insula, producing a prominent salient near Monterrey, south along the Sierra Madre Oriental, but are too frag' Nuevo Leon. A lesser branch of the foldbelt, with or>en mentary to afford much of an idea of the true pre-Meso­ folds, extends northwest from Monterrey to the Big zoic structural patterns. The continuation of the Oua­ Bend of the Rio Grande (Weidie and Murray, 1967, p. chita foldbelt (or Huastecan belt) within the southern 689). South of the 20th parallel the frontal folds b^.nd Cordillera remains speculative. Some geologists (for eastward from Chipas into Guatamala and Belize, vdth example de Cserna, 1960, p. 598-601) have proposed that a south-projecting recess in midcourse. the late Paleozoic foldbelt follows the younger Cordil­ The Mesozoic miogeosynclinal rocks of the Mesa Cen­ leran structures southeastward through Mexico into tral in Mexico pass westward beneath little deformed Central America. Tertiary volcanics (Oft), including extensive sheets of Much of the southern Cordillera is a Mesozoic and ignimbrite, which form the broad dissected plateau0 of younger feature. Its miogeosynclinal belt includes the the Sierra Madre Occidental. Relations between the prer 74 THE TECTONICS OF NORTH AMERICA volcanic external and internal zones of the foldbelt are include equivalents of the upper Paleozoic itself thus largely concealed, and eugeosynclinal rocks are (Williams and others, 1964, p. 2-5). poorly known on the mainland. Metamorphic rocks of The topographic Sierra Madre del Sur of southern uncertain age (mapped as O6a) are exposed west of the Mexico trends east-southeast parallel with the Pacific volcanic plateau in Sinaloa. The prevolcanic rocks on Coast and the Middle America Trench immediately off­ the south coast of Mexico west of the 102d meridian shore, but the structures of both its miogeosyi^.linal (mapped as Ol and O9) are now known to include Meso- Mesozoic rocks and its metamorphic complex extend zoic eugeosynclinal slates and volcanics and embedded southeastward across it to the coast, where they are ultramafic rocks (Zoltan de Cserna, written commun., remarkably truncated. In Guatemala and Belize the 1968). The main occurrence of eugeosynclinal rocks is structures are alined more toward the east, ard are in Baja California, where they and their granitic plu- traversed by a bundle of closely spaced high-angle faults tons (O7, O7a, O0) form the backbone of the northern with the same trend. The high-angle faults extend east­ half of the peninsula and its southern tip. In the north ward into the faults that bound the Cayman Trench on they are submarine lavas, pyroclastics, and volcanic the floor of the Caribbean Sea; their westward extension sediments, steeply upended and partly metamorphosed; toward the Pacific is concealed. They enclose slices of they are known to include strata of Jurassic and Lower diverse and contrasting rocks Paleozoic, Mesozoic, and Cretaceous (Aptian) age, and may include older com­ Tertiary sediments, metamorphic complex, granitic ponents (Allison, 1964, p. 12-15). Both the eugeosyn­ rocks, and ultramafic rocks (the latter possibly up^hrust clinal rocks and the plutons are overlain by nearly mantle material). To the compiler, this set of faults has undeformed latest Cretaceous (Campanian) sediments every indication of being a zone of major strike-slip dis­ (OlO), indicating that the erogenic climax in the south­ placement, yet little tangible field evidence for such ern Cordillera was later than the classic Nevadan orog­ displacement has been reported (Walper, 1960, p. 1312- eny of analogous parts of the central Cordillera. 1313; McBirney, 1963, p. 212-213). In the Sierra Madre del Sur of southern Mexico, and Immediately north of the Sierra Madre del Sur the eastward into northern Central America, pre-Mesozoic transverse Mexican volcanic belt (On ) extends from the rocks and structures are more widely exposed than else­ Pacific to the , and contains a dozen lofty where in the southern Cordillera. A sequence of Penn- volcanic cones (including Pico de Orizaba or Citlal­ sylvanian and Permian carbonate rocks and their basal tepetl, 5,698 m or 18,696 ft) and a host of lesser cones elastics extends from Chiapas across Guatemala into and craters, some still vigorously active. It originated Belize; it was deformed by variable but undetermined late in Cenozoic time and no doubt follows a deep-seated amounts before the Mesozoic miogeosynclinal deposits line of weakness, but it is accompanied by little surface were laid over it, hence is shown on the tectonic map as rupture and it does not perceptibly deflect the structures a lower "structural stage" (older miogeosynclinal de­ of the miogeosynclinal rocks that plunge beneath it. from posits, O8). A much greater part of the pre-Mesozoic of either side. this region is a metamorphic complex (Ol), varying (F) PACIFIC FOLDBELT from paraschist with interbedded marble and amphibo- Cenozoic erogenic activity is especially pronounced in lite to highly migmatized gneisses (McBirney, 1963, p. that part of the Cordilleran Mountain System ne^.r the 187-197; de Cserna, 1965, p. 15-19). It contains em­ Pacific Coast, and on the tectonic map it is appropriate bedded granitic plutons that have yielded radiometric to separate this part from the main Cordilleran foldbelt dates from 300-90 m.y., or from middle Paleozoic to late (O) as a rather vaguely defined Pacific foldbelt (P). A Mesozoic age (Oe, O0), thus indicating (as in the meta­ similar distinction has been made in the Asiatic ectuiva- morphic complexes of the northern Cordillera) pro­ lent of the North American Cordillera, where Soviet longed heating and plutonic activity. The complex itself geologists separate a "region of Cenozoic foHing" is probably of heterogeneous ages. Much of it in Oaxaca (sometimes called "Kamchatkan") from a "region of is Precambrian, as it has yielded radiometric dates be­ Mesozoic folding" farther inland. The Pacific foMbelt tween 700-1,100 m.y. and is overlain unconformably by is characterized by late erogenic, volcanic, and seismic Cambrian strata (Fries and others, 1962, p. 45-52; Fries activity, by Cenozoic rocks and structures, and by a base­ and Rincon-Orta, 1965, p. 81-87; Pantoja-Alor and ment of eugeosynclinal rocks that are younger than Robison, 1967). In Guatemala and Belize the complex is those of the main Cordillera. The Pacific foldbelt was a unconformable below the upper Paleozoic strata, but late to the edge of the North American conti­ may include earlier Paleozoic (Dixon, 1956, p. 13-17), nent, and part or all of it may have formed on what whereas in southern Guatemala and Honduras it may was originally oceanic crust. WASHINGTON AND OREGON 75 On the "Tectonic Map of North America" the Pacific of the Cordilleran foldbelt, and is succeeded by Tertiary foldbelt is shown as including the Aleutian Islands, the volcanics and volcanic-derived sediments (PS, P4), coastal mountains of southern Alaska, the Coast Ranges which are surmounted by young andesitic volcanic cor0® of Washington, Oregon, and California, part of the (Pe), some still vigorously active (Burk, 1965, p. 119- volcanic province of Washington and Oregon, and part 122). Very little of the pre-Tertiary basement (PI) of Baja California. Its boundary with the Cordilleran emerges in the Aleutian Islands, and they are mostly foldbelt is well defined in places, but indefinite in others, formed of Tertiary and younger volcanics, the young where it has been necessary to overlap the units distinc­ andesitic cones lying on the northern side of the arc, tive of the two foldbelts. Thus, granitic rocks of the away from the Aleutian Trench. Cordilleran foldbelt (O0) are shown in the California Coast Ranges, and plateau basalt of the Pacific foldbelt WASHINGTON AND OREGON (Py) is shown in the interior of British Columbia. The northwestern States of Oregon and Washington differ both in geology and tectonic style from nearby ALASKA parts of the Cordilleran region, for here Cenozoic vol­ The characteristic rocks and structures of the internal canic rocks and structures dominate the scene nearly part of the Cordilleran foldbelt extend westward across to the exclusion of all others (Waters, 1955, p. V04-71S; southern Alaska, south of the Alaska Range and into the Snavely and Wagner, 1963) lava plateaus, volcanic Alaska Peninsula Mesozoic eugeosynclinal rocks (O7) cones, and eruptive ranges that were produced by up­ with their embedded granitic plutons (O0), and younger building, and folded or block-faulted ranges that were Mesozoic and Tertiary successor basin deposits (OlO, produced by deformation of the volcanic materials. Cen­ Oil). Between them and the coast, in the Chugach ozoic volcanic rocks, and the sediments associated with Mountains, Kenai Peninsula, and Kodiak Island, are them, extend along the Pacific Coast for 480 km ("% younger eugeosynclinal deposits a vast sequence of miles) and extend inland for as much as 650 km (4-00 steeply upended and partly metamorphosed gray- miles). Within the volcanic areas pre-Cenozoic rocks wackes, containing granitic debris from the middle emerge only as occasional inliers, although they form Mesozoic plutons farther inland, but themselves in­ most of its periphery. truded by small plutons of Tertiary age (P/3) (Miller, The pre-Cenozoic rocks of the Inliers and of the pe­ 1959, p. 20-21). Sparse fossils indicate that the greater riphery of the volcanic area belong to the eugeosynclinal part of this sequence is of Cretaceous age (PI), but it is part of the Cordilleran foldbelt, whose structures are overlain near Prince William Sound by similar deposits recessed eastward in the Columbia arc (see p. 70); of Paleocene age (P2). The same sequence extends the crust within the recess probably differs from that southwestward beneath the continental shelf off the of the Cordilleran foldbelt, and may have been oceanic Alaska Peninsula, where it reappears in the Shumagin during its early history. On the site of the present Co^-st Islands (Burk, 1965, p. 63-71). The base of the Cre­ Ranges within the recess, eugeosynclinal deposits (P2) taceous eugeosynclinal deposits is undetermined, and were laid down during Eocene time, and are interbedded they may have been laid down off the edge of the orig­ with large volumes of submarine pillow basalt (P£a). inal Cordilleran foldbelt, at oceanic depths at the foot During Miocene time the eastern part of the recess v^as of the continental slope. Overlying the eugeosynclinal covered by great floods of plateau basalt of the Columbia deposits in their eastern segment is 7,600 m (25,000 ft) River Group (Py), and at about the same time andesitic of younger Tertiary shallow water marine deposits lavas and pyroclastics (PS) accumulated along the site (P4). of the Cascade Range to the west. Building of the Cas­ Northwest of the Cretaceous-Pa leocene eugeosyncli­ cade Range continued into late Tertiary and Quater­ nal belt is the younger Aleutian (Coats, nary time, when a chain of volcanoes (Pe) grew along 1962), which extends 960 km (600 miles) through the its north-south axis. During Tertiary time, small mafic Alaska Peninsula, ending northeastward at Mount to silicic plutons (P/?) were intruded in the volcanic Spurr in the Alaska Range. West of the peninsula, the rocks of the Cascade Range. arc runs out to sea for another 1,600 km (1,000 miles) The structure of the volcanic area was produced by through the Aleutian Islands; the islands surmount a a combination of volcanic construction and of moderate submarine ridge between the deep Aleutian Trench on deformation during Cenozoic time. The Coast Ranges the south and the shallower Bering Sea on the north; the on the west are an anticlinorium, openly folded except ridge continues westward through the Kommandorsky in the steeply upthrust Olympic Mountains at the Islands to the coast of Kamchatka. The basement of the north end. Between them and the Cascade Range to the Alaska Peninsula is formed of Mesozoic and older rocks east is a belt of downwarping expressed as a chain of 76 THE TECTONICS OF NORTH AMERICA lowlands extending from Puget Sound southward to strip of the Coast Ranges that extends northward past the Willamette Valley, whose deeper parts are thickly San Francisco Bay. The basement of the Transverse covered by late Tertiary and Quaternary deposits (P5). Ranges includes Precambrian plutonic rocks ancf Paleo­ The plateau basalts within the recess of the Columbia zoic metamorphic rocks (Oa, O/?, O6a). In the Coast arc form a broad , little deformed in Ranges to the north are extensive granitic rocks (O0) places, but in others crossed by anticlinal folds. and metamorphic rocks of probable early Paleozoic age (O6a); the granitic rocks have yielded radiometric CALIFORNIA AND BAJA CALIFORNIA dates of 80-90 m.y., hence are Cretaceous and are as The Coast Ranges of California extend south-south­ young or younger than the Franciscan Formation, but eastward along the Pacific Coast for 925 km (575 miles) they and the metamorphic rocks are everywhere faulted from Cape Mendocino past San Francisco Bay to Point against the Franciscan, so that the original relations Conception, and are separated by the Great Valley between them are unknown. from the Cordilleran foldbelt in the Sierra Nevada to The Cenozoic strata of the Coast Ranges and Trans­ the east. South of Los Angeles, the Peninsular Ranges verse Ranges have been described in many publications extend south-southeastward into Baja California. In (Reed and Hollister, 1936, p. 1559-1596; Taliaferro, the intervening area are the eastward-trending Trans­ 1943, p. 135-149; Page, 1966, p. 263-268). On the tec­ verse Ranges. Cenozoic strata (P4), largely of marine tonic map they are shown as a single unit (P4), al­ origin, are thick and extensive in the Coast Ranges though their record is so complex that they could be south of San Francisco Bay, as well as in parts of the divided into a number of "structural stages" or a map Transverse Ranges and adjacent basins, and have un­ of larger scale. Deposits of Miocene and earlier ages dergone several periods of erogenic deformation. are extensive and mainly marine, but most cf them In the coastal belt of California great thicknesses of grade eastward into continental equivalents. Pliocene clastic rocks accumulated from latest Jurassic through marine deposits occur in more restricted coastal areas, Cretaceous time, or mainly after the climax of the Ne- such as the Los Angeles and Ventura basins, and in vadan orogeny in the Cordilleran foldbelt. To the east the southern part of the Great Valley. Marine Pliocene these rocks are fossiliferous shelf deposits (P3), but strata attain thickness of 4,500 m (15,000 ft) in some of to the west they are a. contemporaneous sequence of the coastal areas, and nonmarine Pliocene in the interior poorly fossiliferous eugeosynclinal deposits (Pi) the basins is even thicker. In the coastal areas the marine graywackes, shales, cherts, and pillow basalts of the Pliocene is succeeded conformably by thick lower Pleis­ Franciscan Formation and its kindred (Bailey and tocene marine and brackish water deposits, which are others, 1964). The Jurassic and Cretaceous shelf depos­ strongly unconformable beneath upper Pleistocene its are thrust to the west over the Franciscan deposits, marine terrace deposits and alluvium. and no transitional phases between them are known The Cenozoic sequence of the Coast Ranges and (Page, 1966, p. 268-272). The Franciscan deposits were Transverse Ranges records an eventful tectonic history, probably laid down at oceanic depths at the foot of the with times of deformation indicated at so many levels continental slope, in a manner similar to the Cretaceous and with such varying magnitude from place to place and Paleocene eugeosynclinal deposits along the south that they are difficult to generalize into any erogenic coast of Alaska. climaxes. Nevertheless, a group of deformations during Rocks similar to the Franciscan Formation probably middle and late Miocene time appears to have been im­ extend southward along the continental shelf west of portant, to have deformed the earlier Cenozoic strata the Peninsular Ranges as far as southern Baja Cali­ widely, and to have restricted the areas of subsequent fornia, as they emerge in many of the offshore islands sedimentation. A final climax of deformation occurred and coastal promontories of the peninsula, such as Cata- during Quaternary time, mainly during the gap be­ lina Island in the north and Sebastian Vizcaino Penin­ tween the lower and upper Pleistocene deposits (Stille, sula farther south (Allison, 1964, p. 14-15). Here, in 1936, p. 867-868; Bailey, 1943, p. 1562-1564). contrast to farther north, they appear to be of about The sequence of Tertiary marine deposits along the the same age as the eugeosynclinal rocks of the Cordil­ western side of Baja California is nearly as complete as leran foldbelt to the east (O7), rather than younger as in California (Allison, 1964, p. 16-19), but neitHr they in California. Their sedimentary facies is decidedly nor the late Cretaceous deposits beneath them hr.ve been different, and they contain no granitic plutons. as much deformed as farther north. In the southern In other parts of the coastal mountains of California half of the peninsula the upper Tertiary marir^ depo­ a basement of crystalline rocks emerges, not only in the sits pass eastward into a thick mass of terrestrial vol- Peninsular and Transverse Ranges, but also in a long canics, pyroclastics, and volcanic sediments (Comondu CALIFORNIA AND BAJA CALIFORNIA 77 Formation, P8) a mass which is broken off on the east movements were progressive, so that each topographic by escarpments that face the Gulf of California. feature and each bedrock formation is more offset the The structure of the Coast Kanges, Tranverse greater its age. Ranges, and northern ( Peninsular Ranges is confused The peninsula of Baja California, with eugeosyn- by a multitude of faults. The widespread westward clinal rocks and granitic plutons in its basement, is .a thrusting of Jurassic and Cretaceous shelf deposits over long sliver of continental crust, separated from the the Franciscan Formation has already been mentioned. Mexican mainland by the deep water of the Gulf of Much more prominent and generally younger are the California. The northern half of the gulf is thickly high-angle faults, many of which are of great length fille.d with sediments, but the southern half is floored and have dominant components of strike-slip displace­ by oceanic crust; the gulf is evidently a rift, across ment (Crowell, 1962). The high-angle faults cross the which the peninsula drifted northwestward from the grain of the ranges at an acute angle, mostly in a north­ continent during Cenozoic time (Rusnak and others, westward direction, but with a conspicuous east-trend­ 1964, pp. 68-74). The faults which were formed during ing set in the Tran§verse Ranges. Strike-slip displace­ the drifting do not, however, extend directly down the ment on the northwest-trending set is dominantly gulf, and are not direct extensions of the high-angle right lateral and on the east-trending set dominantly faults of California. The latter trend southeastward left lateral. The most lengthy high-angle fault is the into Mexico, each dying out on the Sonoran mainland. San Andreas, which crosses all the coastal ranges from They are succeeded farther south by other faults, northern California to the United States-Mexican bor­ plainly indicated by the bathymetry of the gulf floc^, der with a nearly straight southeastward course, except which also trend southeastward, with an en echelon for an eastward deflection in the Transverse Ranges. pattern. The northwestward drift of Baja California Nearly every aspect of the high-angle faults of the and the strike-slip displacement of the high-angle coastal ranges has been diversely interpreted the time faults of California are not independent phenomena, of their inception, the magnitude and kind of their but are parts of a single grand movement of the western movements, and their role in the geologic history of the coastal urea of North America (Hamilton, 1961). region (Hill and Dibblee, 1953; Oakeshott, 1966; (Q) ANTILLEAN FOLDBELT Dibblee, 1966). The San Andreas fault has been vari­ Another region of pronounced Cenozoic erogenic ously estimated to have originated in early Cenozoic activity, here called the Antillean foldbelt (Q), lies be­ time or before and to have been displaced laterally as tween North and South America in the lands and islands much as 560 km (350 miles), or to have originated dur­ surrounding the Caribbean Sea. On the tectonic m?.p ing the Quaternary and to have been displaced less than the foldbelt is shown as including the islands of the 2 km (1 mile). In view of the marked contrasts between Greater and Lesser Antilles, and the mainland are^s the rocks on the opposite sides of the San Andreas and of coastal Venezuela and southern Central America. most of the other high-angle faults, larger rather than Southern Central America is included for convenience, smaller displacements seem likely; such movements but with equal propriety it could be classed as a southern probably account for the anomalous juxtaposition of the prolongation of the Pacific foldbelt. Franciscan basement against the crystalline basement The Antillean foldbelt differs from those of NorMi in the Coast Ranges. and South America, as it is an oceanic rather than a Along the San Andreas fault and many of the others continental feature. Much of it is far from any larjre are overwhelming indications of marked displacement continental areas, and most of the emerged parts ri^e during Quaternary time, and of continuing activity to­ steeply from oceanic depths, making it one of the mcst day. Modern activity is attested by the clustering of rugged mountain systems of the . The Puerto earthquake epicenters along the faults and by ground Rico Trench is 8,000 m (26,000 ft) deep 130 km (80 breakage along the faults at the times of earthquakes. miles) north of the coast of Puerto Rico; the Caymr.n Fault movements during the last few thousand years Trench is 7,000 m (23,000 ft) deep 40 km (25 miles) are indicated by characteristic landforms along the south of the coast of Cuba, and the Sierra Maestra ris^s fault traces fault trenches or rifts, pressure ridges, another 2,000 m (6,500 ft) behind the coast. Peaks in and sag ponds. Even more striking is the distortion of the central cordilleras of Hispaniola and Costa Ri?.a modern topography along the faults by offset of ridges project above 3,000 m (10,000 ft), but the other lard and stream channels; stream channels crossing the San areas in the foldbelt are lower. As shown by the bathym­ Andreas fault have been offset right laterally 100-1,500 etry on the tectonic map, the Caribbean sea floor is m (325^,500 ft). The record of displacement along the itself diversified by basins and trenches, with ridges ard San Andreas and other high-angle faults indicates that swells between, many of which do not project above 78 THE TECTONICS OF NORTH AMERICA sea level, and whose structure is largely conjectural. east, which are mainly volcanic and rise individually out The land areas thus constitute only a fragment of the of deep water, with a trench extending at least part of tectonics of the whole foldbelt, the remainder of which the way around their Atlantic side. must be deduced from data obtained during oceano- Another item must be added to complete the tectonic graphic surveys. picture the great fault zones on each side of tH Carib­ Because of the fragmentary evidence, the origin and bean Sea, extending through Venezuela and rrinidad the tectonic history of the Antillean foldbelt are less on the south and the Greater Antilles on the ncrth. The certain than those of any of the foldbelts so far con­ Cordillera de la Costa in Venezuela and the North sidered, and have been diversely interpreted. Many of Eange in Trinidad are a line of narrow ridges a long the the early hypotheses required extensive continental sea, composed of metamorphosed Mesozoic roclrs (Q2a) areas in the Caribbean region, which have now van­ unlike the unmetamorphosed Mesozoic rocks south of ished by foundering or other causes. It is true that the them. They have reached their present position Vj right- crust of the Caribbean sea floor is somewhat thicker lateral displacement along a fault zone on the south and less dense than normal oceanic crust, but it is by side of the ridges (Rod, 1956, p. 474-476). no means continental (Donnelly, 1964, p. 683-684); it On the northern side of the Caribbean Sea the Cay­ was probably produced by hydration of normal oceanic man Trench extends diagonally through the Greater crust, rather than by "oceanization" of continental Antilles between Cuba and Jamaica, with steep inf acing crust. The tectogene hypothesis, which was at one time scarps on each side (Meyerhoff, 1962, p. 1-3). Farther applied to the Antillean foldbelt (Hess, 1938) is discred­ east, and not in direct alinement, the Anegada Passage ited by newer geophysical data; as originally proposed, forms another, smaller trench between the Greater and it involved downbuckling of a thicker and more sialic Lesser Antilles. Both these features are certainly fault crust than is now known to exist. bounded, and at least some left-lateral displacement has Perhaps the Caribbean Sea was produced by the drift­ occurred along them (or displacements opposite to those ing apart of the North and South American continents, in Venezuela and Trinidad), but there are complica­ but most of the proposed reconstructions of drifting in tions. The Cayman Trench is floored by an oceanic crust, the region involve implausible shifting, and even com­ much thinner than that beneath the islands and sub­ plete rotation of bits of continental crust to produce the marine rises on either side, suggesting that th? trench present pattern of lands and islands. Other, more is a rift, perhaps comparable to the rift in the Gulf of plausible reconstructions of the Caribbean by the drift California, mentioned above (Ewing and others, 1960, hypothesis might be possible, but they are still in the p. 4095-4096). Whatever the nature of the fault move­ formative stage. Hess writes (1966, p. 5), "It can be ments north and south of the Caribbean Sea, they de­ stated without question that the Caribbean existed dur­ tached the more mature parts of the foldbelt from the ing the whole of the Tertiary more or less with its pres­ unstable parts between, where tectonic evolution is still ent outlines, but how long before that was there a Carib­ in progress. bean?" Present evidence seems to indicate that, what­ No pre-Mesozoic tectonic units have been proved in ever may have been the origin and early nature of the the Antillean foldbelt, and their existence is doubtful Caribbean, the present lands and islands of the Antil­ in most of it. The last exposure of the pre-Mesozoic is lean foldbelt were built up during Mesozoic and Ceno- in northern Central America where its metr.morphic zoic time on an original oceanic crust, by the addition of magmas from the mantle, followed by a complex tec­ complex (Ol) plunges southward beneath Tertiary vol- tonic evolution into the modern foldbelt (Dengo, 1962, canics (Q8) near the Honduras-Nicaragua border, and p. 156-157; Donnelly, 1964, p. 689-695; Bowin, 1966, p. it is cut off eastward by the Caribbean coast. Farther 15-17). south in Central America the basement is th? Nicoya This tectonic evolution is most nearly completed in Complex (Q2), resembling the Franciscan of the Pa­ the large, massive islands of the Greater Antilles on the cific foldbelt, and like it probably no older than Meso­ north and in the coastal area of Venezuela and Trinidad zoic (Dengo, 1962, p. 138-142). The pre-Mesozoic meta- on the south, both of which underwent erogenic defor­ morphic complex seems to extend east of the coast be­ mation during late Mesozoic and early Cenozoic time, neath the Nicaraguan Rise and Cayman Ridge, but and have now attained relative stability. It is less com­ whether it emerges again in the Greater Antilles is un­ plete in Central America on the west, which is bordered determined. The basement of the Greater Antilles is also on the Pacific side by a chain of active volcanoes and by termed a metamorphic complex (Ql), but this is known the Middle America Trench (Dengo, 1967). Even less only to be older than Late Jurassic. Most of if", is prob­ mature are the small islands of the Lesser Antilles on the ably older Mesozoic eugeosynclinal material, although CALIFORNIA AND BAJA CALIFORNIA

pre-Mesozoic components have been proposed in Cuba of the Lesser Antilles to the east, but in the northern (Khudoley, 1967, p. 672-674; Hatten, 1967, p. 780-782). half of the arc the cones are bordered on the Atlantic Mesozoic eugeosynclinal deposits (Q2), with large side by half a dozen lower islands with a foundation of volumes of volcanics and volcanic-derived sediments, oc­ older Tertiary volcanics and diorite plutons, capped l^y cur in all areas of the Antillean foldbelt. In southern later Tertiary limestones and other sediments (Q4). Central America they are overlain by nonvolcanic Up­ (N) ANDEAN FOLDBELT per Cretaceous deposits (Q3), but in the Greater Antil­ Many features like those in the North American Cc iw- les the eugeosynclinal sequence extends in most places dillera are repeated in the South American Cordillera, to the top of the Mesozoic, and in southeastern Cuba or Andean foldbelt (N). Most of the Andean foldbelt is into the Eocene (Q4a). In the Cordillera de la Costa of outside the area of the "Tectonic Map of North Amc * - Venezuela, where all the eugeosynclinal rocks are region­ ica," but its northern prongs extend into it, in Colombia ally metamorphtosed (Q2a), the largely nonvolcanic Ca­ and Venezuela. The tectonic features of the Andean racas Group is bordered on the south by the structurally foldbelt are shown on the map down to the 6th parallel, separated but probably older volcanics of the Villa de mainly to provide a southern frame for the Caribbean Cura Group. Mesozoic miogeosynclinal deposits, includ­ Sea and its Antillean foldbelt (Q). The structures rep­ ing thick carbonates, border the eugeosynclinal rocks on resented are copied from compiled maps, such as the the north in Cuba (Q4), and on the south in Venezuela "Geologic-Tectonic Map of Northern Venezueb" (N3), and are continuous with the platform deposits of (Smith, 1962), and publications on the foldbelt have the Bahama Islands and the Venezuelan interior. Suc­ been studied only sufficiently to establish usable map ceeding older Tertiary deposits (Q4) are varied, largely units (see, for example, Jacobs and others, 196^; clastic in many areas, but including volcanic components Mencher, 1963). The tectonics of the foldbelt and its in southern Central America and thick carbonate units map units will be shown more authoritatively on the in parts of the Greater Antilles. "Tectonic Map of South America," which is now in A significant Middle Cretaceous orogeny occurred in preparation by geologists of the South American Cuba, but throughout much of the foldbelt the main countries. orogenies occurred from Late Cretaceous through Eo­ cene time. They were accompanied by emplacement of In Colombia, the Andes Mountains split northward granitic plutons (Q/?) of middle Cretaceous to Eocene into a Cordillera Occidental, a Cordillera Central, and a age, largely more mafic than those of the other f oldbelts Cordillera Oriental. The first two plunge northward (diorites and quartz diorites). In Cuba the eugeosyn­ beneath the lowlands of the Rio Magdalena and its clinal rocks were carried northward over the miogeo­ tributaries, but the third extends northeastward into synclinal rocks in great thrust sheets or nappes of Al­ Venezuela, bifurcating in turn into the Sierra de Perija pine type (Hatten, 1967, p. 785-788), many of which and the Cordillera de Merida, which enclose the Mar*- have tabular masses of ultramafic rocks (Qa) along caibo Basin. The Sierra Nevada de Santa Marta along their soles. the Caribbean coast in Colombia is an independent In the more mature parts of the foldbelt, deformation massif. The various cordilleras and sierras are composed was largely completed by Eocene or Oligocene time, so of Mesozoic and older rocks, and the lowlands between that the later Tertiary is postorogenic and little de­ them are underlain by Cenozoic rocks. formed (Q5). Deformation continued later in Hispani- Pre-Mesozoic rocks, largely metamorphosed (Fl, ola, and there was significant Miocene orogeny in Costa Na), form much of the Cordillera Central and Sierra Kica, accompanied by emplacement of small granitic Nevada de Santa Marta, and emerge in smaller inliers in plutons in the central Cordillera (Qy) (Dengo, 1962, the Cordillera Oriental and Cordillera de Merida. Th^.y p. 143-147). include fossiliferous rocks of various Paleozoic agos, Young volcanic rocks are lacking in the more mature and probably also Precambrian rocks; the pre-Mesozc^c parts of the foldbelt, but they are important in the less basement of the Cordillera Oriental includes reworked stable intervening parts. Large areas in southern Cen­ parts of the edge of the Precambrian Guyana Shield. tral America are covered by late Tertiary lavas and Orogenies at various times during the Paleozoic are pyroclastics, probably derived from fissure eruptions suggested in places by unconformities or gaps in tV (Q8). On these, on the Pacific side, younger volcanics sequence, but they are too imperfectly known to gi^e (Q«) have been built, surmounted by a chain of vol­ much of an idea of the resulting structures. Embedded canic cones that extends from the eastern border of in the metamorphic rocks are granitic plutons (N/P), Mexico to Costa Kica (Williams, 1952,1960). An arcuate primarily Mesozoic but possibly including Paleozo;<*.. chain of volcanic cones also forms most of the islands Kecent radiometric determinations in Colombia shew 80 THE TECTONICS OF NORTH AMERICA that they are dominantly Mesozoic, the large pluton in sponds to that on most tectonic maps previously the Sierra Nevada de Santa Marta is dated at 160-170 published. m.y. (Jurassic) and the large pluton near Medellin in The topography of the sea bottom is clearly more the Cordillera Central is dated at 70 m.y. (Late expressive of tectonics than the topography on the land, Cretaceous). because the original structural surfaces have bien much Tectonic history becomes plainer in the Mesozoic. The less modified by subsequent erosion and deposition. Cordillera Occidental and the lower ranges along the Hence, fault scarps, fault blocks, upwarps, and down- Pacific coast are formed of Mesozoic eugeosynclinal de­ warps are evident merely from their surface configura­ posits (N2) submarine lavas and associated sediments tion. Moreover, the internal structure of the topographic that are largely of Cretaceous age, but which may also features on the sea bottom cannot, as yet, be deciphered include Jurassic. The Cordillera Oriental and its exten­ with as much assurance as on the land. The features sions into Venezuela is formed of Mesozoic miogeosyn- are not available for field inspection, but must lv> probed clinal deposits (N3), including thick sequences of indirectly by various kinds of oeeanographic surveys. Cretaceous carbonates. Beneath them, however, are These surveys are still incomplete in the areas sur­ continental red elastics (Giron Group), which are rounding North America, and some of the methods of largely of Jurassic age. survey are still being perfected. The Andean Cordilleras were deformed and shaped On the North America map, as on many others, the into much their present outlines by several orogenies subsea areas are not classified into tectonic units like between Late Cretaceous and Eocene time. The miogeo- those on the land. However, subsea tectonic units are synclinal rocks of the Cordillera Oriental and Sierra shown on some of the newer tectonic maps, such as the de Perija were thrown into closely crowded, east-di­ "Tectonic Map of Eurasia" (Yanshin, 1966) discussed rected folds and thrust slices, but the cordilleras of earlier (p. 17-18). Many of the tectonic units shown on metamorphic and plutonic rocks were raised mainly as such maps are merely morphological or descriptive, and block uplifts bordered by high-angle faults, some with correspond to the physiographic provinces which have strike-slip displacement. The Bocono fault, which ex­ been mapped on the sea bottoms around North America tends the length of the Cordillera de Merida, has a large (for example, Hcozen and others, 1959, p. 20), and component of right-lateral displacement, and may be include such features as continental slopes, continental still active (Rod, 1956, p. 463-468). rises, and abyssal plains. Most of these provinces are Basinal deposits of Eocene and younger Tertiary evident in the subsea topography itself. Other tectonic ages (N4), partly marine and partly continental, and units shown on such maps indicate the surface or sub­ both synorogenic and postorogenic, fill the depressions surface composition of the sea bottom, such as areas between the cordilleras, and have been extensively ex­ of oceanic crust or continental crust, and areas with plored for petroleum. In deeper depressions along the thick sedimentary cover. Many data on the composition Magdalena River in Colombia, and in the Maracaibo of the sea bottom have been obtained around North Basin in Venezuela, these deposits are covered in turn America, but no maps have been published of large by late Tertiary and Quaternary deposits (N5). The arear which show instriunentally verified compositions. configuration of the pre-Tertiary basement beneath the Other tectonic units on such newer maps ar^ of more questionable value, as these imply ages of th° units or Maracaibo Basin is shown on the map by 500-m con­ ages of the deformation, which cannot be verified by tour lines. SUBSEA AREAS existing oceanographic methods, and whicl imply a choice between several possible theories of oceanic On the "Tectonic Map of North America" the terrain tectonics. beneath the seas and oceans surrounding North America Some of the existing survey methods offer more pos­ is represented by 500-m (1,640-ft) topographic con­ sibilities than others for refining the tectonic classifica­ tours, with the first 200-m (656-ft) contour added, and tion of the subsea areas. Continuous seismic reflection by bathymetric tints for each 1,000-m (3,280-ft) in­ profiling provides information on the structure of re­ terval. The many sources from which the contours have flecting surfaces more than 1,000 m (3,300 ft) below been compiled were summarized earlier (p. 18-19). In the bottom surface, revealing features that can be inter­ addition, a special symbol is used to represent promi­ preted as flat-lying, tilted, or folded sedimen tary beds, nent faults or fracture zones on the ocean floor, whose as well as faults and unconformities. Many profiles have existence is indicated by topographic scarps, and in now been made on the continental shelves and slopes part by geophysical evidence. The treatment of the sub- around North America for example, off th«, Atlantic sea areas adopted on the North America map corre­ Coast of the United States (Uchupi and Emery, 1967). FAULT ZONES 81 These surveys will aid materially in an eventual tec­ press differences in crustal thickness in the areas between tonic mapping of the shelves and slopes around the con­ the zones. The age of the faulting is undetermined. In tinent, areas which are shown without symbols on the view of the fact that few ages older than Cretaceous present map. have been verified in the oceanic areas, they cannot be Magnetic surveys also have tectonic implications. A very ancient, yet no earthquake epicenters occur along total intensity magnetic survey has been made of a large them except near their intersections with other features. area off the Pacific Coast of the United States (Vacquier Despite the strong expression of the faults on the and others, 1961; Raff and Mason, 1961) which, al­ Pacific Ocean floor and their close approach to the though incomplete, has revealed a remarkable pattern coast of North America, remarkably little indication of of narrow, parallel belts of positive and negative them can be discovered on the land. The only zone di­ anomalies, offset along the subsea fracture zones. Similar rectly traceable to the land is the Mendocino, which parallel belts of magnetic anomalies have been found on offsets the edge of the continental shelf right laterally each flank of the Mid-Atlantic and other mid-ocean about 80 km (50 miles), but even it is not identifiable ridges (Vine and Matthews, 1963), and it has been pro­ in the surface rocks farther inland. It has been proposed posed that all these anomaly belts are products of sea on geological or geophysical evidence that the rone ex­ floor spreading (Wilson, 1965a; Vine and Wilson, 1965). tends long distances eastward across North Amer;",a Parallel anomaly belts, apparently of a different kind, (Gilliland, 1962; Fuller, 1964); however, the proposals are known on the continental shelf and slope off the are as yet unconvincing. The Murray zone disappears Atlantic Coast (Drake and others, 1963), but the west­ near the foot of the continental slope, perhaps bener.th ern Atlantic Ocean basin between Bermuda and the a sedimentary cover. It may be related to the faults w;th continental slope is reported to be magnetically fea­ left-lateral displacement in the Transverse Ranges of tureless (E. R. King and others, 1961; Heirtzler and southern California; these, in turn, are at the western Hayes, 1967). Magnetic surveys, when more complete, end of the Texas Lineament, which is not a fault zone. will have an important influence on the tectonic map­ The Clarion zone disappears far west of the coast, bit ping of the subsea areas around North America. is alined with the Trans-Mexican volcanic belt, which again is not a fault zone. The Clipperton zone disap­ FAULT ZONES pears far west of the coast, but is alined with miror Further discussion is desirable on the only subsea transverse structures in Costa Rica, and these in turn structural features specifically shown on the tectonic with the lengthy faults on the north coast of South map the faults or fracture zones. The most notable of America. A broken, zig-zag connection between the these are off the Pacific Coast of North America; the Clipperton zone and the transverse faults of Guate­ zones have been described and interpreted by Menard mala can also be imagined through the northeast-tren d- and others in various publications (for summary, see ing Tehuantepec fracture zone. The vagueness of all Menard, 1964, p. 41-53). From north to south, those these relations strengthens the belief that the faults of shown on the map are the Mendocino, Pioneer (minor), the Pacific Ocean floor are structures in the ocearic Murray, Molokai (incompletely represented by the crust, and that if they extend eastward into the North Shirley Trough), Clarion, and Clipperton. Still other American continent, it is beneath the upper crust, whose fracture zones extend westward from the coast of South surface has been affected indirectly at most. America, beyond the map area (fig. 14). Where the Mendocino fracture zone leaves the corst The fault zones on the Pacific Ocean floor are re­ it is expressed by a northward-facing scarp, but 120 km markably evenly spaced, about 1,000 km (600 miles) (75 miles) farther west the topography reverses to a apart, and trend nearly westward from the coast along southward-facing scarp as much as 3,000 m (10,000 ft) great circle courses for distances of many thousands high, which continues to the termination of the zone, of kilometers. All of them are expressed topographically showing that displacements on the zone have been com­ by escarpments or lines of ridges and troughs, and at plex. This complexity is more strikingly demonstrated least some of them form the borders of ocean bottoms of by matching magnetic anomalies across it. The anoira- different depths; thus, the ocean bottom south of the lies indicate that west of the coast there has been an r,s- Mendocino zone is at least 1,000 m (3,300 ft) lower than tonishing left-lateral offset of 1,170 km (785 miles), as north of it. The areas between the zones also differ in compared with an 80 km (50 miles) right-lateral offret tectonic behavior, as shown on the map by the nearly of the continental shelf along the same zone. Similar'y, featureless surface between the Mendocino and Murray matching the anomalies across the Murray zone in­ zones, contrasted with the abundant seamounts north dicates as much as 640 km (395 miles) of right-lateral and south of them. Seismic surveys show that these ex­ offset at sea, decreasing eastward, whereas the faults in 00 to

40e

30

10

Faults and fracture zones Nonfaulted transverse Oceanic trenches Oceanic ridges and rises Seamounts zones on the land FIGUKE 14. Map showing transverse structural features on the lands and beneath the seas in the southern part of the area of the "Tectonic Map of North America," and in adjacent regions. Compiled from "Tectonic Map of North America," and from publications of H. W. Menard, B. C. Heezen, and others. GLOSSARY the Transverse Ranges on the land are offset left places in the text. French equivalents of some of tH laterally. terms are included to facilitate comparison with tH These paradoxes seemed inexplicable until J. T. Wil­ legend of the "Tectonic Map of Europe" (Schatsky, son (1966) proposed the concept of "transform faults," 1962). For brevity, sources and authorships of the terms along which lateral displacement has occurred in op­ are seldom included; for literature references and for posing senses on the two sides of a zone of oceanic history of usage the reader should consult the "Interna­ spreading. The zone of spreading in this case is the tional Tectonic Dictionary." East Pacific Rise, which is supposed to extend north­ Allochthone. A mass of rocks which has been mowi ward from the ocean beneath the continent in the Gulf by tectonic forces from its original site of origin, as in of California and to emerge again in the ocean floor a thrust sheet or . Opposite, autochthone. north of the Mendocino fracture zone. The oceanic Alpinotype tectonics. The tectonics of alpine moun­ spreading along the East Pacific Rise, and the accom­ tain belts, which are produced from orthogeosyncline^. panying transform faulting, if these exist, had a major Characterized in the internal parts by deep-seated pla^- influence on the creation of the younger tectonic features tic folding and plutonism, and in the external parts 17 of the Cordilleran and the Pacific f oldbelts of the North lateral thrusting, which has produced nappes, thru^ American continent. sheets, and closely crowded folds. The existence of lengthy fracture zones on the Pacific Antithetic structures. Structures produced by minor Ocean floor has implications on the origin of the fault movements that oppose the prevailing larger movements zones on the north and south sides of the Caribbean Sea in the area; for example, faults which are downthrown (p. 78), especially since east-west fracture zones, also in a sense opposite to the general uplifting or arching. transform faults, are now known to cross the Mid- Opposite, synthetic structures. Atlantic Ridge between the equator and lat 15° N. Autochthone. A mass of rocks which, though vari­ (Heezen and Tharp, 1%1). The faults in the Carib­ ably deformed, has remained at its original site of origin, bean Sea may therefore not be local features produced where it is still rooted to its basement. Opposite, by a movement pattern in the Antillean foldbelt alone, allochthone. but may be parts of a circumglobal fracture system in Basement rocks. Rocks beneath the platform are^s the oceanic crust (fig. 14). and the f oldbelts that were consolidated during earlier Aside from the subsea faults in the Caribbean Sea, deformations, which are overlain by a less deformed only one other is shown on the tectonic map on the younger structural layer or cover. Ideally, basement eastern side of North America. This is supposed to fol­ rocks are metamorphic and plutonic, and contra^ low the arc of the Kelvin Seamounts ("Bermuda-New strongly with their cover, but parts of the basement are England seamount arc" of Northrop and others, 1962) composed of intermediate rock types so that distinctions onto the continental shelf and slope near the 40th paral­ are blurred in places. In the central part of the conti­ lel. The existence of the fault on the shelf and slope is nent the basement rocks are Precambrian, but they are suggested by a 145-km (94-miles) right-lateral distor­ of younger ages elsewhere. French, socle. (See p. 21.) tion or actual offset of sedimentary basins and belts Batholith. Traditionally used for large, subjacent of magnetic anomalies that have been determined by and discordant plutonic bodies, supposedly floorless in oceanographic surveys (Drake and Woodward, 1963, p. contrast to other intrusive bodies; most batholiths are 53-55). A landward prolongation of the fault across formed of granitic rocks but presumably could be of the Appalachian foldbelt has been proposed on seem­ other compositions. In the western Cordillera of tJ ^ ingly tenuous evidence, although there is admittedly a United States and Canada the term is used specifically discontinuity at about this latitude between the northern for regionally extensive aggregates of granitic rocks, and southern Appalachians, so that the structures of complex internally, and consisting of many individual the former lie eastward of those of the latter (p. 56). plutons of varied compositions and ages; the "Tectonic Map of North America" and the present text defer to tl -> GLOSSARY latter usage. Examples, Sierra Nevada batholith, Idaho This glossary defines the tectonic terms used in the batholith, Coast batholith. (See p. 51-52.) text, not all of which are familiar to the non-specialized Block faulting. The breakup of a previous more reader. The initial sentences under many of the items homogeneous terrane into blocks as a result of faultily are abstracted from the "International Tectonic Dic­ that is mainly normal or tensional. The resulting struc­ tionary" (Dennis, 1967). Succeeding sentences are ex­ ture is an array of uplifted mountains (preserving the;r planations or qualifications by the compiler. Some of the block forms in the early stages), separated by down- terms are discussed at greater length at appropriate faulted depressions. Block-faulting is primarily a pos4-- THE TECTONICS OF NORTH AMERICA orogenic feature and disturbs terranes with varied prior with successor basin, the term preferred in tb* present histories, ranging from those which have undergone text. little earlier deformation to those of thoroughly de­ Eugeosyncline. The internal part of a geosyncline formed rocks. Example, the Basin and Range province (orthogeosyncline), away from the craton, character­ of the southwestern United States, (See p. 24,72.) ized by strong volcanism during its early phases and by Craton. A part of the earth's crust which has at­ synorogenic plutonism during its later phases. Deposits tained stability, and which has been little defonried for of eugeosynclines include submarine volcanics, cherts, a prolonged period. As originally defined, the cratons slates, and graywackes. Counterpart, miogeosyncline. included parts of both the continents and the ocean bot­ (See p. 46-47.) toms, but the proposed examples of the latter are un­ Event. A noncommittal term used for any incident proved or dubious. In the present text the term, is used of probable tectonic significance that is suggested by only in the continental areas, where it includes both geological, radiometric, or other evidence, whose full shields and platforms. implications are unknown. Used especially for minor Decollement. Detachment of rocks along a strati- clusters of radiometric dates whose relations to geologic graphic surface, resulting in independent styles of de­ structures or processes have not been precise1 y deter­ formation in the rocks above and below. The term is mined. merely descriptive, and the detachment could have been Extemides. The external or outer part of a foldbelt, produced by any one of a number of tectonic forces, in­ nearest the craton or foreland, commonly the site of a cluding lateral thrusting and gravity sliding. miogeosyncline during its early phases and sul'jected to Diapir. A fold or dome which contains a core of more marginal deformation later. Opposite, internides. The mobile rock that has pierced and intruded the overlying terms externides and internides express relative po­ less mobile rock. Ordinarily the more mobile rock has a sitions of units within the foldbelts, and not their re­ lower density than its cover, but it may be of many lation to adjacent parts of the continents. Ir. relation kinds, including salt (as in salt domes), other evapo- to the central craton of North America, the extftrnides of rites,orclay. the Phanerozoic foldbelts would be internal, and the Disharmonic folding. Folding in which there are internides external. (See p. 46). notable differences in kinds of structures between ad­ Flysch. A sequence of distinctively interlayered jacent rock bodies, resulting from bedding-plane slip sandstones, mudstones, and marls believed to have ac­ along sedimentary layers. This type of deformation is cumulated in relatively narrow, deep, rapidly subsiding characteristic of stratified sequences and is, in fact, de­ troughs. The term originated in the European Alps, but manded by the geometry of their folds. In extreme form, nearly identical deposits occur in many other foldbelts bedding-plane slip produces decollement, and the two of the world. Flysch is commonly interpreted as a syn­ structures are commonly associated. orogenic deposit, and hence the term has been used im­ Disrupted areas. Used specifically in the text for properly by many geologists for any clastic sediment previously stabilized or cratonic areas that have been that was laid down during the time of the orogeny. (See broken up by block-faulting and related processes. Ex­ p. 48). ample, the part of the Basin and Range province in Foldbelt. A linear belt that has been subjected to southern Arizona and New Mexico. (See p. 24,72.) folding and other deformation during the orogenie Epeirogeny. Broad uplift and subsidence of the phase of a tectonic cycle. Each foldbelt evolved during crust, primarily by vertical movements, which has af­ a time span different from that of any of the others, fected large parts of the continents. Epeirogeny occurs and details of their histories differ, although they can both in the cratons and in foldbelts during the final be grouped broadly according to generalized world­ stages of their tectonic cycles. It has produced the pres­ wide times of orogeny. Individual foldbelts and the ent mountainous topography in many of the foldbelts, tectonic cycles which created them, are used as funda­ and thus would be "orogeny" as some geologists have mental units on the "Tectonic Map of North America." defined the term (but not the present compiler). Even Synonyms, mobile belt, orogenic belt, and (less prop­ as here defined, epeirogenic and orogenie structures erly) mountain belt. French, region de ptissement. grade into each other in detail, although most of them Foredeep. The part of the foreland or craton next to differ distinctly from each other. Counterpart, orogeny. a foldbelt that was deeply downfolded and thhkly filled (See p. 44-45.) with sediments during the climactic orogeny. Epieugeosyncline. A sedimentary trough or basin on Foreland. A stable area marginal to a foMbelt, to­ the site of a former eugeosyncline. Nearly synonymous ward which its rocks were thrust or overf olded. The area GLOSSARY 85 is commonly continental, and is a craton or platform widely throughout the world for any kind of postorv area. genic deposit. Because of the great variety of alleged G&rmanotype tectonics. Tectonics of the cratons and molasse deposits the term is not considered useful out­ the stabilized foldbelts, derived from the structures in side its original area in the present text. (See p. 48). Germany north of the Alps. The milder phases of this Neotectonics. The youngest tectonics of the earth, type of tectonics are epeirogenic, but it also includes which was produced during the later part of the Tc?- broad folds dominated by vertical uplift and high-angle tiary, during the Quaternary, as well as the tectonics faults, block-faulted terranes, and sedimentary basins in process of formation at the present time. deformed within a frame of surrounding massifs. Oroclinal structure. An arcuate structure supposed Infrastructure. Structure produced at a deep crustal to have been formed by bending of the crust in a hori­ level, in a plutonic environment, under conditions of zontal direction, or by "deformation in plan". Some elevated temperature and pressure,, which is character­ geologists propose that deformation of this kind pro­ ized by plastic folding and by granitic and other mig- duced many or most of the arcuate patterns of the matitic and magmatic rocks. This environment occurs in foldbelts. the internal parts of most of the foldbelts, but the term Orogenie phase. The median part of the tectoric is used especially where the infrastructure is clearly con­ cycle in a foldbelt, characterized by the climax of crustal trasted with a less disturbed or altered higher layer, or mobility and erogenic activity, and by the formation of suprastructure. alpinotype structures. The erogenic phase is commonly Intemides. The internal part of a foldbelt, farthest shorter than the preorogenic and postorogenic phaees away from the craton, commonly the site of a eugeosyn- which precede and follow it, and may be less than a cline during its early phases, and subjected later to geologic period in length, although it is usually pro­ plastic folding and plutonism. Opposite, extemides. longed by a succession of erogenic pulsations. RocH KoUogenic. A term, used by Spizaharsky and Bor- formed during the erogenic phase are term**! ovikov (1966) for broad downwarping or subsidence of synorogenic. the crust, of a sort that would be classed as a negative Orogeny. The processes which create the rock stn^?^ epeirogenic movement in the present text. tures within the mountain chains or foldbelts. The wo^d Leptogeosyncline. A part of a geosyncline (ortho- orogeny actually means "the formation of mountains," geosyncline) in which the water was of great depth, but which implies not only the formation of the rock struc­ which received only small thicknesses of sediments over tures in the mountains but also of the mountainous lard- prolonged periods; a form of starved . scapes. When the term first began to be used a century Massif. A massive topographic and structural fea­ ago, these were supposed to be parts of the same process ture in a f oldbelt, commonly formed of rocks more rigid and the distinction between them was only recogniz^xi than those of its surroundings. These rocks may be pro­ later. Most geologists today use the term orogeny as he^ truding bodies of basement rocks, rocks consolidated defined, and consider the formation of the mountainous during early deformations in the foldbelt, or younger landscapes to be a postorogenic or epeirogenic process. plutonic bodies. (See p. 44-45.) Miogeosyncline. The external part of a geosyncline, Orthogeosyncline. A major linear sedimentary where there was little or no magmatic activity during its trough lying outside the craton, generally divisible into depositional phase, and which was little deformed ex­ eugeosynclinal and miogeosynclinal parts. The term is cept during the closing phases of the orogeny. Deposits synonymous with geosyncline as originally defined and of miogeosynclines include carbonate rocks, quartzites, as used here; it was proposed in order to distinguish it and shales; these are identical with those laid down on from sedimentary troughs and basins in the crate ns the cratons, but they were laid down to a much greater which were called parageosynclines^ but these are rot thickness and in a more complete sequence. Counterpart, accepted as geosynclines in the present text. eugeosyncline. (See p. 47-49.) Overprint. The superposition of a younger evmt Miomagmatic geosyncline. A term proposed by on a dominant earlier event, as indicated by a mil or Stille in 1936, but superseded by his later term miogeo- scatter of young radiometric dates in a terrane doim- syncline. (Seep.46.) nated by earlier dates. Where valid, the younger dates Molasse. A term used in the European Alps for a can sometimes be ascribed to renewed orogeny or to re­ Tertiary sequence of marine and brackish water sand­ curring plutonic activity, but the reasons for many of stones, with some layers of gravelly to bouldery con­ them are obscure. glomerate. As the molasse was laid down after the Paleogeographic map. A map which purports to climax of the the term has been used show the extent and outlines of lands, seas, and otl <** 86 THE TECTONICS OF NORTH AMERICA geographic features at a given time in the geologic past. are varied; by common European usage they are called (See p. 2.) molasse^ but this term has little utility. Postorogenic Paleogeologic map. A map which shows the areal plutonic bodies include discordant granitic plutons and geology of the surface beneath an unconformity, which a wide variety of hypabyssal intrusives. In mary of the has been covered by younger strata. (See p. 2.) foldbelts, terrestrial volcanic rocks were spread widely Paleotectonic map. A map which represents the over the deformed terrane during this phase. tectonic features as they existed at a given time in the Preorogenic phase. The initial phase of the tectonic geologic past. (See p. 1.) cycle of a foldbelt, prior to the climactic orogeny. The Parageosyncline. A sedimentary trough or basin in phase is the time of formation of geosynclines, most of the craton. Although different varieties of parageosyn- which are clearly divisible into eugeosynclinal (inter­ clines have been recognized and separately named, none nal) and miogeosynclinal (external) parts, the first of them are considered to be true geosynclines in the characterized by abundant submarine volcanism, the present text. Counterpart, orthogeosynoline. second by little magnatism and by carbonate-cuartzite sedimentation. The plutonic rocks of the preorogenic Pennine-type structure. Structure like that in the phase include ultramafic bodies and rare early granitic Pennine Alps of central Europe, characterized by plas­ plutons. tic deformation of the crystalline basement and its cover, and by the formation of large recumbent folds or Protaxis. An antique term for the central rxis of a nappes. mountain chain, supposedly consisting of the oldest rocks and structures., (See p. 49.) Platform. That part of a continent which is covered Pulsation. A minor time of deformation which is by flat-lying or gently tilted strata, mainly sedimentary, one part or phase of a more prolonged e^xxih of which are underlain at varying depths by basement orogeny. rocks that were consolidated during earlier deforma­ tions. A part of the craton of the continent. French, Reactivated areas. Used specifically in the present plateforme. (See p. 21). text for the parts of a previously stabilized craton that have been involved in one or more renewed deforma­ Pliomagmatic geosyncline. A term proposed by tions. Example, Southern Rocky Mountains. (See p. Stille in 1936, but superceded by his later term eugeosyn- 23-24.) cline. (See p. 46.) Reworked structures. Structures that were formed Pluton. Originally proposed as a noncommittal term during an earlier time of deformation or orogeny, which for a body of intrusive of any shape or have been involved in later deformation that has re­ size, hence including batholiths and many other varie­ folded or otherwise modified them. ties. However, in the western Cordillera of the United Shield. A large area of exposed basement rocks in a States and Canada the term is commonly used for the craton, commonly surrounded by sediment-covered plat­ individual, relatively restricted bodies of particular forms; the term is virtually restricted to expored areas composition and age which are components of the larger of Precambrian basement rocks. French, boucli°r. aggregates, or batholiths. Structural stage. A rock-stratigraphic unit or layer Plutonism. Processes which take place at depth in in a foldbelt, characterized throughout by a common the earth's crust under conditions of elevated tempera­ facies related to the tectonic evolution of the foldbelt. ture and pressure, which produce plutonic rocks that As originally conceived, successive structural stages are is, metamorphic, migmatitic, and magmatic rocks. In supposed to be separated by unconformities, but this the present text the term plutonic rocks is used specifi­ is dubious. Rocks of a single stage may vary consider­ cally for the migmatitic and magmatic varieties, espe­ ably in age span from place to place, hence a structural cially the granitic types. Plutonic processes are promi­ stage is not a stratigraphic stage, or time-stratigraphic nent in the internal parts of foldbelts, and the plutonic unit. French, etage structuraux. (See p. 10-11.) environment can be termed infracrustal. Successor basin. A sedimentary basin of relatively Postorogenic phase. The final phase of the tectonic restricted extent in the internal part of a foldbelt, cycle in a foldbelt, following the climatic orogeny. In some foldbelts the postorogenic rocks and structures are formed during the erogenic phase or early part of the minor and do not obscure the erogenic structures, in postorogenic phase. Its deposits overlie geosynclinal others they nearly overwhelm them; in all foldbelts, rocks that were deformed by the climatic oro,?enies of the postorogenic events have produced the present the foldbelt, but they themselves have been var ably de­ mountainous landscapes. Postorogenic structures are formed during the later phases of the orogeny. Includes germanotype and epeirogenic. Postorogenic sediments epieugeosynclines. (See p. 49-60.) REFERENCES CITED Supracrustal rocks. Sedimentary and volcanic rocks REFERENCES CITED of the suprastmcture in a foldbelt, which have been Albritton, C. C., Jr., and Smith, J. F., Jr., 1957, The Texas linea­ subject only to near-surface deformational processes, ment, in v. 2 of Relaciones entre la tectonic y la sedimenta­ and not to deep burial, plutonism, or metamorphism. tion : Internat Geol. Cong., 20th, Mexico City 1956, sec. 5, p. Suprastrueture. The upper structural layer in a 501-518. foldbelt, subjected to relatively near-surface deforma­ Allison, E. C., 1964, Geology of areas bordering Gulf of Califor­ tional processes, in contrast to an underlying and dif­ nia, in VanAndel, T. H., and Shor, G. G., Jr., eds., Marine ferently deformed infrastructure. geology of the Gulf of California A symposium: Am. Assoc. Petroleum Geologists Mem. 3, p. 3-29. Synorogenic rocks. Rocks formed during the Alvarez, Manuel, Jr., 1949, Tectonics of Mexico: Am. Assoc. erogenic phase of the tectonic cycle of a foldbelt. Syn­ Petroleum Geologists Bull., v. 33, no. 8, p. 1319-1335. orogenic sediments are preserved mainly in the external American Committee on Stratigraphic Nomenclature, 1961, Co-'e parts of the foldbelt; they include flysch as strictly of stratigraphic nomenclature: Am. Assoc. Petrolerm denned, as well as broad sheets of deposit spread far Geologists Bull., v. 45, no. 5, p. 645-665. from their erogenic sources. Synorogenic plutonic rocks Anderson, C. A., 1966, Areal geology of the southwest, in Titley, S. B., and Hicks, C. L., Geology of the porphyry copper include concordant granites which have disrupted or deposits, southwestern North America: Tucson, Ariz., Un'v. replaced the rocks of the internal parts of the foldbelt. Arizona Press, p. 3-16. Synthetic structures. Structures produced by minor Argand Emile, 1924, La Tectonique de 1'Asie: Internat Geol. movements that are in harmony with the major move­ Cong., 13th, Brussels 1922, Comptes rendus, p. 171-372. ments of the area; for example, faults which are down- 1928, Carte tectonique de 1'Eurasie: Internat Geol. Cocg., thrown in the same sense as the general uplifting OT 13th, Brussels 1922, scale 1:25,000,000. arching. Opposite, antithetic structures. Arkhangelsk!, A. D., 1939, Structure geologique et hisforre geologique de 1'U.S.S.B.: Internat Geol. Cong., 17th, Mos­ Tectonic cycle. The interval of time during which an cow 1937, Repts., v. 2, p. 285-304. original mobile area has evolved into a stabilized fold- 1941, Geological structure and geological history of the belt, passing through preorogenic, orogenic, and post- U.S.S.B.: Moscow-Leningrad, State Sci.-Tech. Press Pe­ orogenic phases, each characterized by distinctive struc­ troleum and Solid Fuel Lit, 376 p. [in Russian]. tures and by distinctive suites of sedimentary, volcanic, Armstrong, F. C., and Oriel, S. S., 1965, Tectonic development and plutonic rocks. Called a geotectonic cycle by some of Idaho-Wyoming thrust belt: Am. Assoc. Petroleum Ge^l- ogists Bull., v. 49, no. 11, p. 1847-1866. geologists. (See p. 43-44.) Armstrong, B. L., 1968, Sevier orogenic belt in Nevada and Utah: Tectonic map. A map which portrays the archi­ Geol. Soc. America Bull., v. 79, no. 4, p. 429-458. tecture of the upper part of the earth's crust, or the Askelsson, J., Bodvarsson, G., Einarsson, T., Kjartansson, G., features produced by deformation and other earth and Thorarinsson, S., 1960, On the geology and geophysics forces, and represents them by means of symbols, pat­ of Iceland: Internat. Geol. Cong., 21st, Copenhagen 19?0, terns, and colors. (See p. 1-2.) Guide to Excursion A2, 74 p. Bailey, E. H., Irwin, W. P., and Jones, D. L., 1964, Franciscan Transform, fault. A term applied to a class of faults and related rocks, and their significance in the geology of on the ocean floor; no valid equivalents have been western California : California Div. Mines and Geology BrU. demonstrated on the continents. They are transverse to 183,177 p. mid-ocean ridges (and rises), which are commonly off­ Bailey, T. L., 1943, Late Pleistocene Coast Range orogenesis in southern California: Geol. Soc. America Bull., v. 54, no. set geographically across them (see fig. 14). The axes 10, p. 1549-1567. of the ridges are inferred to be loci of the addition of new Bally, A. W., Gordy, P. L., and Stewart, G. A., 1966, Structure, crustal material, which results in spreading of the ocean seismic data, and orogenic evolution of southern Canadian floor and of strike-slip displacement of the transform Rocky Mountains: Canadian Petroleum Geology Bull., v. faults on the two flanks. However, the geographic off­ 14, no. 3, p. 337-381. Bass, N. M., 1960, Grenville boundary in Ohio: Jour. Geology, v. sets of the ridges themselves are only indirectly related 68, no. 6, p. 673-677. to such displacements. As a result of spreading of the Bayley, R. W., and Muehlberger, W. R., 1968, Map of basement ocean floor from the ridge the strike-slip displacement rocks of the United States: U.S. Geol. Survey, scfle on the faults is in opposite senses on the two flanks. The 1:2,500,000. displacements can be inferred in places from offsets of Berthelsen, Asger, 1961, On the chronology of the Precambrian of western Greenland, in Raasch, G. O., ed., Geology of the sea-bottom topography, but they can be demonstrated Arctic, V. 1: , Ontario, Univ. Toronto Press, p. even more conclusively by offsets of narrow parallel 329-338. belts of magnetic anomaly. 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