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Overview INTRODUCTION The North American Continent-Ocean Transects Program is a study of the struc- ture and Phanerozoic evolution of the transitional region of the North American hemisphere between its craton (Figure 1, zone 1) and bordering oceanic lithosphere (Figure 1, zone 5~. The U.S. Geodynamics Committee (USGC) places strong emphasis on the im- portance of the transition zone between continental and oceanic lithosphere. In 1978, the USGC recommended that a series of transects be prepared to set forth existing geological, geochemical, and geophysical data along a series of corridors around the U.S. coast from the continental craton across the transition zone to the oceanic lithosphere. The USGC initiated the Transects Program early in 1979. The USGC appointed Robert C. Speed as reporter of the USGC to coordinate the Transects Program. On behalf of the USGC, he provided leadership in developing and carrying out the program. Transect groups were organized for a series of corridors on the U.S. margins. Specific plans for the conduct of the program and presentation and publication of the results were developed. Within two years after its initiation, the program expanded to include Canadian and Mexican corridors; thus it became a North American Continent-Ocean Transects Program. The program includes about 200 geologists and geophysicists from Canada, Mex- ico, and the United States. It is jointly sponsored by the U.S. Geodynamics Committee, the Canadian Committee for the International Lithosphere Program, and the Institute of Geology, University of Mexico. The Transects Program has received direct support from the National Science Foundation, the Geological Society of America, the Uni- versity of Mexico, the Geological Survey of Canada, and the U.S. Geological Survey and indirect support from the many institutions represented by the participants in the program. The written and graphic products of the Transects Program are published mainly by the Geological Society of America within the Decade of North American Geology series (see Appendix A). 3

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4 Arctic Basir, 60~ ~ ; ~ \ i /~ ' I A> ~(~ t)C)~ \ 4. displacing terranes _~ :~ ~ A. oceanic lithosphere Yakutat4 ~ \ / 5 terrane ~ .\ / 1. Proterozoic N. America, little deformed in Phanerozoic 2. Proterozoic N. America, deformed in Phanerozoic 3. ancient displaced terranes B 1: \ A/ A> '\ \' Atlantic O (A J~7,D4 uan de R T H ! 4~ Fuca PA Pacif ic Plate A M E R I C A N ~ Atlantic \ ~ ~ ~ ~ ~ ~ C, , L | P 1 a t e H = ~ _ |-~ . ~ active margin 1 2 0 O Caribbean,' Plate I ;~ I,/ 5 HO FIGURE 1 Map of North American continent and plate showing positions of 23 corridors of the Transects Program. PROGRAM OBJECTIVES The objectives of the Transects Program are threefold: 1. To analyze the structure, motions, and processes of the transitional zone between the North American continent and adjacent oceanic and other lithospheres and the evolution of that zone over Phanerozoic time (approximately the last 700 Ma).

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5 The analysis is concentrated in 23 corridors that cross the transitional zone between oceanic and cratonal lithospheres where possible (Figure 1) and employs existing data. The analysis includes requirements for common presentation formats, with a gom or effective comparison of continental margin structure around North America. 2. To recognize the major gaps in understanding the character and origins of the North American continent-ocean transition, to identify the problems and avenues of study most likely to lead to breakthroughs, and to recommend to the scientific community and government the priorities and nature of research programs that can effectively solve such problems. 3. To provide coordination among major ocean- and land-based scientific pro- grams. --rig . ~ ~ ~ In partial fulfillment of these objectives, especially the second and third, the USGC is issuing the present report. PROGRAM ACHIEVEMENTS The Transects Program achieved certain ends prior to the publication of its prod- ucts. The program has demonstrated the ability of diverse earth scientists to collabo- rate successfully in the pursuit of new, broader, and more fundamental definitions of problems and interpretations of the structure and evolution of North American conti- nental crust between the craton and oceanic lithosphere. This collaboration, together with new data, has extended former lirn~ts of thought in such topics as downward continuation of structure from surface to mantle; compatibility of motions between the oceanic terranes and the continental foreland; and event correlations around the con- tinent's edge. Further, the Transects Program has conceived new display techniques to enhance comparability among the transects so that all members of the earth sci- ence community can recognize the important similarities and differences of continental structure and evolution around the entire margin. Perhaps most important, the interpretations of crustal structure of the Transects Program have already stimulated the acquisition of new data by geophysical and geological profiling along several transects with the goal of testing hypotheses. Completion and publication of many of the transects by the Tn~d-1980s led to two unforeseen, but related, developments: recognition of the importance of digitization of future transects and the development of a Global Geoscience Transects (GOT) Project as part of the International Lithosphere Program. (See Chapter 4, "New Developments." ~ NORTH AMERICAN CONTIN1 :NT For the Transects Program, the North American continent is defined as the region currently bounded by oceanic lithospheres of the Atlantic, Arctic, and Pacific basins and by the North American-Caribbean plate boundary (Figure 1~. The continental edges adjoining the Atlantic and Arctic oceanic lithospheres are passive margin types; they occur within the North American plate. Greenland and an undefined region off- shore of western Alaska belong to the North American continent under this definition, but the Transects Program has not included them in its study. The edge of North America bordered by the Pacific basin is mainly a series of active margins between the

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6 North American continent and a number of oceanic plates. In fact, the continent occu- pies the Pacific plate at two places; thus, today's North American continent contains plate boundaries. The southern edge of the North American continent is less clearly delineated because much of it is below sea level and because of the two closed oceanic basins, the Gulf of Mexico and the Yucatan basin. The Transects Program has arbitrarily assigned to the continent all regions of the North American plate not floored by oceanic lithosphere as far south as its boundary with tectonic borderlands of the Caribbean plate (Figure 1~. PRESENT AND PAST CONTINENT-OCEAN TRANSITION Today's transitions of the North American continent to adjoining oceanic litho- sphere mark the sites of the most recent plate boundary tectonics to have shaped the continent in each region. At active margins, such tectonics are ongoing, whereas at passive margins, they range from modern (Gulf of CaTifornia) to as old as mid-Mesozoic (central Atiantic). Much is learned at existing continent-ocean transitions regarding instantaneous relative motions, earthquake-derived kinematics, and concurrent geo- physical expressions of the way continental and oceanic lithospheres interact. Such studies show, moreover, that relative motions between plates are widely distributed and heterogeneous and that a broad zone spanning the continental edge must be studied to address the interactions fully. It is now well known that most oceanic lithosphere is returned to the mantle after a brief residence (<200 Ma) at the surface whereas continental and other types of nonoceanic lithospheres survive, at least in some proportion, at the surface. It is also now evident that continents, their shapes and volumes as well as positions, have evolved markedly through time. Plate boundary motions have transferred small and large fragments from position to position within a continent and probably from continent to continent; moreover, they may have caused some loss of continent to the mantle by subduction and addition of mantle to continent via magmatism and underplating. The history of these and other processes associated with continent- ocean interactions, therefore, is recorded within the continents. A full understanding of the evolution of the North American continent requires a synthesis of the structure and processes across the continent-ocean transitional zones as a function of position and time over geological history. The Transects Program addressed this sizeable task by studying the structure of 23 corridors around the North American continent, each 100 to 200 km wide and aligned normal to, and crossing, the local modern margin. The analysis is carried to depths that include (as a minimums the top of the mantle. The historical duration adopted IS Phanerozoic time, loosely defined as about 700 Ma. This choice was based On the occurrence of widespread plate tectonic events near the end of Proterozoic time that gave rise to the general size and shape of cratona] North America today. North American Continental Zonation The present North American continent can be divided into four tectonic zones (1-4, Figure 1~. Zones 2-4 constitute what is here called the transitional regionthe part of the continent that has been shaped by continent-ocean tectonic interactions

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in Phanerozoic time. The transitional region lies between cratonal and oceanic litho- spheres except in southernmost North America (Figure 1~. Some of the boundaries and tectonic components of zones of Figure 1 are not yet well documented or agreed upon; it has been a goal of the Transects Program to improve regional tectonic divisions and delineations. The following paragraphs define the tectonic zonation. Zone 1: Cratonal North America: This is the undeformed sialic nucleus of North America, which has behaved as a continuous, nearly rigid unit at lent since the Proterozoic. Cratonal North America is a single tectonic unit, and it provides the reference frame for the history of relative motions of units in the tectonically active outboard zones in the last 700 Ma. The periphery of zone 1 is a post-Precambrian deformation front. Zone 2: Deformed North America: This zone contains Proterozoic North America and its cover, which has undergone significant deformation related to Phanerozoic continental margin tectonics. Unlike zone 1, zone 2 consists of multiple tectonic units, which presumably reflect different loci, times, and mechanisms of plate boundary events that affected the margin of Precambrian sialic North America. Although zone 2 is probably well fragmented, the pieces have probably not moved great distances with respect to one another or to zone 1. However, zone 2 may have been the source of continental fragments that have undergone large displacements and now exist in zones 3 or 4 or among accreted terranes of other continents. Examples of tectonic units in zone 2 are the nappes of the Valley and Ridge province and Blue Ridge province, and the Rocky Mountain fold and thrust belt. Of special interest are the reactivation of old discontinuities by new phases of margin tectonics and the correlation of style of structures with type of margin tectonics. Zone 3: Ancient Displaced Terranes: Zone 3 comprises terranes that are tectonic units that have certainly or probably undergone large transport but that are now fixed with respect to cratonal North America. Terranes include fragments of continents, arcs, oceanic lithospheres, deformed oceanic strata, and rocks of uncertain origin. Zone 3 also includes cover strata and igneous rocks that formed in place after attachment of displaced terranes to North America or earlier accreted units. Examples include the Avalon and Meguma terranes of the Canadian Appalachians, the Pericu and Zapoteco terranes of Mexico, and Stikinia and Wrangellia of the Pacific Northwest. Important considerations are the sources of such terranes, transport histories, times of attachment, coalescence with other displaced terranes before accretion to North America, internal tectonic history, and deep structure. Zone 4: Displacing Terranes: The modern continent of North America incorporates parts of the Pacific plate as well as the North American plate. These parts (zone 4, Figure 1) are evidently moving with respect to zone 1 and are joined to the rest of North America by active margins. Displacing terranes are Baja and Alta California west of the San Andreas fault system and the Yakutat terrane west of the Queen Charlotte fault. To understand the structure, tectonics, and Phanerozoic evolution of the continent- ocean transition of North America, the Transects Program has proceeded as follows: (1) analyzed tectonic zones 2-4 in terms of their constitutive tectonic units, which reflect past or present relative motions, (2) synthesized an evolution of these units into their current configuration as North America, and (3) interpreted the processes involved.

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8 Co~Tidore Study corridors have been chosen to transect the tectonic zones from cratonal North America to oceanic lithosphere (zones 1 to 5~. The exceptions are Mexico and Alaska, where the transects do not intersect Proterozoic North America (zones 1 and 2~. The 23 corridor locations are selected to provide hemispheric representation of North American continent-ocean transitions, but more important, they incorporate the most significant examples of modern and ancient continental margin structures. An unport ant element in understanding the evolution of the transition is the ability to compare its structure corridor by corridor. To achieve comparability, corridor displays and discussions in the Transects Program are standardized by the inclusion of mandatory entries and formats (Figure 2~. The mandatory entries include: 1. A tectonostratigraphic strip map of the corridor, colored according to protolith age. 2. A cross section depicting tectonic units that are colored according to a stan- dardized hemispheric code; this section goes at least to the Moho. 3. Factual geological cross section that depicts rock stratigraphic units that are colored according to protolith age. 4. Geophysical profile data. 5. A tectonostratigraphic event diagram that shows kinds of tectonic events as functions of corridor position and age. Preparation of the transects was predicated on the use of existing data. Coop- eration among academia, government, and industry was an important element in the success of the program. There was coordination with other major geoscience programs. In particular, the results of deep seismic reflection profiling were integrated into the transects. 1 INDEX 1 MAP ~ MAP ~ ~.' ~ ins- I SPACE' | DIAGRAN1 1 , , Legend for Figure 2.

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