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5 Major Topical Problems Tectonic syntheses of the North American transects have allowed participants of the Transects Program to recognize many fundamental problems in understanding the Phanerozoic evolution of North American continent-ocean transitions. A comprehen- sive survey of such problems is presented below under 12 titles. These general problems (listed below) form the basis for developing new, coordinated investigations aimed at major strides in understanding continental-margin tectonics. 1. Processes of modern active margins. 2. Processes of passive margins. 3. Tectonic heredity. 4. Tectonic significance of magmas and magmatic rocks. 5. Identification and processes of terrane boundaries. 6. 7. Kinematics of orogenic belts. Implications of high-grade metamorphic rocks in erogenic belts. 8. Dating of events in erogenic belts. 9. Foreland deformation and tectonic coupling. 10. Diagnostic geophysical expressions of tectonic units. 11. Deep continental crustal structure and origin. 12. Phanerozoic changes in Proterozoic North America. These general problems include certain common themes, the most frequently occurring of which are four specific problems (listed below) . These specific problems are considered (by the majority of authors) to be priority problems for new investigations. 1. Underplating and tectonic attrition. 2. Origin of the Moho and lower crust. 3. Absolute chronology of tectonic events. 4. Strike-slip components of terrane displacement paths. These 12 general problems and 4 specific problems are discussed in the sections that follow. 21
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22 GENERAL PROBLEMS 1. Processes of Modern Active Margins This problem addresses the relative motions and material transfer along the Pacific and Caribbean edges of North America where the continental margin and North American plate boundary are more or less coincident. Material transfer concerns the sites, processes, rates, and controls of the tectonic transfer of rock and sediment across the plate boundary zone (either way). Accretion of material to the continent may arise by attachment at a subduction trace, overthrusting above the continental edge, and underplating of material below the continental margin. Underplating is the attachment of rock from the downgoing plate to the underside of the overriding plate. Attrition (also called tectonic erosion) of active continental margins occurs by the transfer of continental material to adjacent displacing plates, whether in strike-slip, divergent, or convergent mode. Tectonic accretion at the toe of the continental foreland (offscraping) is moderately well understood, with some exceptions. First, the factors that control the vergence of offscraped packets, whether arcward or oceanward, are not clear. Further, conju- gate asymmetric structures exist at some places, implying the absence of systematic vergence. This problem needs solution in order to interpret subduction polarity in ancient offscraped rocks. Second, it has recently been discovered that in some forearcs upper intervals of the incoming sediment are offscraped whereas Tower intervals pass with oceanic crust below the forearc. What controls the partition of the incoming sediment column between offscraping and underriding intervals? Third, some margins have undergone accretion over substantial durations but attrition at other times. What are the Tong-term conditions that favor the dominance of one process over the other? Fourth, the process of underriding of sediment and rock below the continental forearc is poorly understood. Undeformed sediments have been tracked in seismic sections at least 27 km below the forearc in the Aleutians and over 100 km in other forearcs of the world. What controls the extent of underthrusting, and what happens to underthrust sediment? Ideas on the latter question are: (a) accretion to the base of the forearc or more landward regions of the continental margin (underplating); and (b) subduction into the mantle. It is vital to understand the fate of underthrust sediment to evaluate the degree to which continents have thickened at their margins from massive under- plating, and how much sediment is transformed to magma in continental arcs. How commonly does the underplating process attach ophiolite to the base of the forearc and what process generates ophiolite diapirs? Another aspect of accretion that remains a problem is the path and magnitude of progressive deformation of material during its initial transfer at a deformation front, and afterward, within the forearc. What are the tectonics of continental forearcs that cause progressive landward thickening (such as imbrication, continuum contraction, and underplating) but without deformation of upper slope cover? What tectonics cause the commonly seen progressive contraction between the forearc basin and accretionary prism and antithetic wedging in the inner forearc zone? How much of the deformation in rocks of ancient orogenic belts was acquired during early accretionary phases? Tectonic attrition at active North American margins is poorly understood because the material removed presumably by strike slip, rifting, or subduction—is no longer present locally. Attrition has been recognized from the great volumes of missing material and by massive subsident events in the Aleutian and Mexican forearcs. The .r ~ Z~ Or ° . . . ~ . .
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23 existence of old continental rocks now fronted by recently accreted oceanic sediment implies that materials are missing. Subsidence is implied by the depression of erosional unconformities to depths of 5 km and by the recovery of in-place neritic faunas from great water depths. The subsident events indicate a withdrawal and presumed further landward transport or total subduction of a large tract of the base of the forearc. The kinematics of modern active continental margins are also a fundamental ingredient with which to interpret the development of ancient erogenic belts. Over what range of widths of continental plate boundary zones are horizontal displacements taken up relative to cratonal North America, and what parameters control the width: slab age and/or dip, lithospheric thickness, material strengths, inherited continental structure, obliquity of convergence? Further, how does contemporary active margin structure relate to obliquity and velocity of convergence? What controls partitioning of displacement components into ~ ~ .^ _ .^ ~ ~ ·~ ~ ~ ~ ~ · · 1 ~ ~ · ~ TO ~ _____ _1_ 1 _ ~ _ spaced str~ke-sl~p zones and continuous marg~n-normal contractions! now much plate boundary motion is taken up within the displacing terranes and within oceanic litho- sphere? Finally, how do rigid rotations occur within active margin zones? What sizes of fragments undergo rotations? What controls the size-frequency? What determines the rotation sense? How does the rotation continue with depth? 2. Processes of Passive Margins This major problem concerns the processes and kinematics of passive margin generation. Solutions require improved resolution of the architecture and constitution of the deep region between the continental hinge and oceanic lithosphere with normal crustal thickness in the Canada Basin, Labrador Trough, Atlantic Basin, Gulf of Mexico, and Gulf of California. Especially interesting topics are: (a) the nature, origin, and post-rift history of outer ridges; tb) the origin and uniformity of occurrence of anomalously thick oceanic crust and/or diapiric mantle at the juncture with continental crust; (c) whether brittle structures of rifted crust are predominantly half graben, symmetric graben, low-angle detachment fault nappes, or other extensional structures and whether the predomi- nant structures vary with position; (~) deformation mechanisms and distribution of displacements in the ductile zone; (e) crustal strain and strain gradients during rift phase tectonics; Off roles of magmatism and sedimentation in rift phase tectonics; (g) the volume of mantle material added to the rifted continental crust; (h) controls on siting of oceanic ridge development and the occurrence of ridge jumps that lead to isolated continental plateaus within oceanic lithosphere. 3. Tectonic Heredity The idea that earlier structures influence the distribution of displacements and structures developed in a later deformation has long been appreciated but has gained greater importance in geological thinking with the maturation of plate tectonic theory. Early structures yield rheologic anisotropy to rock masses and extensive surfaces of discontinuity or weakness (contacts, faults). In principle, these guide the types and orientations of later structures that might occur during rifting, collision, or transcur- rent events. One result of this inheritance is that brittle failure may occur first on reactivated rather than virgin fault planes such that the causal stress field may not
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24 be resolved without a very large data set of fault orientations and slip directions. Second, the width of new tectonic zones may be dictated by the distribution of initial discontinuities in the crust. Third, strain rates in the ductile zones may be controlled by inherited anisotropy, and finite strains may be difficult to interpret in terms of orientations of successive tectonic events. The problem of reactivation of old faults is crucial. Why do some fall whereas others that seem to have been in a propitious orientation for reactivation do not? To what degree do normal faults become reactivated as thrust faults and vice versa In sequential deformation? Some examples illustrate the point. Triassic/Jurassic grabens in the eastern United States commonly exhibit initial extensional failure by reactivation of thrust and ramp faults of the Paleozoic Appalachian orogen. No magmatism accompanied this initial phase of deformation. Subsequently, Jurassic movements created swarmers of vertical diabase dikes oriented at high angles to the normal faults in the southeastern United States and fanning into near parallelism with the normal faults in the northeastern United States. Thus, the early brittle failure seems strongly guided by a pre-ex~sting fabric, whereas the later comagmatic failure appears to have been independent of structural fabric control. Similarly, thrust surfaces established during an early erogenic event may be reacti- vated as thrusts during subsequent erogenic events. An example is the mylonite of the Brevard zone of the southeastern Appalachians. Here mylonitization occurred in the Taconian event (ca. 450 Ma) and again in the "Acadians (ca. 350 Ma in the southern Appalachians). The mylonites may be part of the Piedmont sole fault brought to the surface during Late Paleozoic brittle ramp faulting. The major Piedmont/Blue Ridge allochthon itself probably experienced intermittent reactivation during the Taconian, ~Acadian," and Alleghanian progenies. Also, early thrust faults appear to have reac- tivated as strike-slip faults. Examples are the Huntington Valley and Martic faults of Pennsylvania. An example of a normal fault on a rifted margin guiding later thrusting during subsequent collision appears to be the Precambrian(?) Rockfish Valley fault of the Virginia Blue Ridge, which became active as a thrust fault during the Taconian and younger deformations. Rifted continents tend to break along the grain provided by the last major orogeny (as in the North Atlantic) but not always (as along the southern Atlantic margin). In either case, available anisotropies serve to guide the details if not the major pathways of rifting. 4. Tectonic Significance of Magmas and Magmatic Rocks It is a goal to interpret uniquely the tectonic environment of the generation and emplacement of magmas from their composition and the physical characteristics of magmatic rock masses. For example, how can ophiolites and basalts of midocean ridge, offridge oceanic, island arc, and continental sites be discriminated? Do calcalkaline magmatic rocks uniquely indicate a contemporaneous subjacent stab, or can such rocks be generated in other tectonic circumstances? What range of chemical and isotopic modifications do igneous rocks undergo in oceanic environments and under multiple metamorphic events in erogenic belts? What are the principal magma sources: base of lithosphere, base of crust, within crust, subducted slab, subducted ocean floor sediments?
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25 Another question is the volume addition to the crust by magmatic rocks. How is stretching taken up in rifted crusts between constant volume on the one hand and constant (or increased) thickness due to magma addition on the other; what controls the proportionation and spatial variation of these two stretching modes? Do island and continental magmatic arcs emerge from a line source of magma or as spaced point sources, and, if the latter, what is the evolution of the lithosphere in between the points? Contamination and/or assimilation affects the composition and physical properties of magmas at active continental margins. It is important to discriminate contamination at the magma source and by fluids from the downgoing slab from that within the lithospheric column above the magma source. 5. Identification and Processes of Terrane Boundaries Orogenic belts of the North American continent contain displaced terranes that make up much if not all of zones 3 and 4 (Figure 1~. Moreover, the deformed margin of Precambrian North America (zone 2, Figure I) is partly overlain by terranes. Terraces are tectonic units that have been displaced or are displacing with respect to North America and to one another. Some terranes may be nappelike and underlain by other terranes, whereas others probably have deep lithospheric underpinnings and are thus microplates. At least 100 terranes have been identified to date in North America by gross differences in tectonostratigraphic histories. Of identified terranes, however, many are almost certainly composites of two or more smaller terranes with more subtly different tectonostratigraphic histories. In general, the resolution and delineation at the surface of discrete terranes remains a major tectonic problem in need of additional criteria. Moreover, the subsurface resolution of vertically stacked terranes is a problem hardly touched upon. A major advance in terrane identification as well as in understanding terrane kinematics may come from study of terrane boundary phenomena. In general such boundaries must be considered zones of major displacement. What determines the width of terrane boundary zones and the types of structures developed with position across and with depth in the zone? What are the relative roles of total displacement, displacement gradients and rates, initial rock types, and fluid activity and permeabil- ity? Processes at terrane boundaries may create unique structural imprints in the rocks involved that inform us about the physical environment which dominates the process during the formation of the boundary. This imprint goes even beyond transforming pre-existent rocks but also may create rocks of its own (anatectites) or create avenues for the emplacement of ascending magmas and mineralizing fluids. The kinematic study of mylonite belts which may identify important sutures is capable of establishing the sense, type, and even amount of displacement and distortions that occur along many of these boundaries. The analyses of individual fault zones at terrane boundaries in areas where differ- ential uplift has created structural relief of, say, 10 km or more, should be considered extremely important in establishing probable changes in structural behavior or major faults as they descend into the crust. The presence of fragments of mylonite within my- lonite, the complex microfolding of mylonitic foliation, and the involvement of granites during the process of mylonitization indicate complex evolution and changing condi- tions during the process of accretion. Furthermore, it seems that once established, the
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26 structurally weakened zone at a terrane boundary will be the locus of later breakup under completely different tectonic environments. 6. Kinematics of Orogenic Belts The erogenic belts that nearly envelope cratonal North America and include most if not all of Mexico and Alaska have been studied for many years as to their con- stituents and the relative motions among them and North America. Current study of North American erogenic belts indicates that they have arisen by a combination of processes: extensional rifting, parautochthonous accretion and magmatism, attach- ment and removal (attrition) of displaced terranes, and parautochthonous deformation (that is, deformation of marginal North America and attached terranes). Although understanding is accelerating, the sequence of processes and associated motions with place and time remains the key to an accurate Phanerozoic history of North American continent-ocean interactions. Some questions of particular importance within this topic are as follows: Paleomagnetic and other data show that margin-parallel displacement components of thousands of kilometers have occurred between North America and some terranes in western Canada and the United States. Moreover, some terranes have moved relatively north and some south (but not at the same time). Have large strike slip components been similarly important in the evolution of the orogenic belts of Mexico, Florida, the Appalachians, and the Arctic? If so, what were the timing, senses of movement, and magnitude of displacement? What are the kinematics of terrane migration and emplacement within erogenic belts? Did each currently attached terrane migrate as a discrete entity on an oceanic conveyor? Or, were terranes chipped off and stuck to North America from a passing co- herent ensialic plate, either as slices at strike-slip boundaries or as nappes at collisional boundaries? If terranes were mainly discrete, were they strongly amalgamated to one another before attachment to North America? How much fragmentation and further displacement by strike-slip faults have terranes undergone after first attachment to North America? For example, did the terranes in the assemblage that constitutes most of Alaska arrive unit by unit at their current positions, or at the other limit, did the assemblage migrate as a unit? Are the many arc terranes in the Appalachians derived from a now-fragmented single arc, or do they represent mainly diverse original arcs? What are the kinematics of continent-continent collisions? Did many discrete microplates exist in the collision zone, or do the continents maintain coherency? How is the obliquity of collision taken up in underriding and overriding continent? How much of North America was subducted in collisions? What arrests the convergence? What was the precise history of closure of North America with other continents that led to Pangea? Last, the timing and ejects of superposed deformations must be clear at all positions. How much of Precambrian North America was removed between the times of Late Proterozoic passive margin formation and the present edge of sialic North America that is mainly below the erogenic belts? Were the losses, if any, mainly by rifting and ridge jump, strike-slip plucking, or by collisional subduction? What are the vertical motions in the development or erogenic belts? Do these reflect ramping over pre-existing declivities, thermal/magmatic doming, regional ho- mogeneous contraction, or other phenomena?
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27 What is the origin of the regional heterogeneities of deformation in erogenic belts? Are they due to attachment of terranes, rotation of allochthonous or parautochthonous units, different vertical displacements, initial paTeogeographic complexities, rheologic changes, or to complexity of superpositions? 7. Implications of High-Grade Metamorphic Rocks In Orogenic Belts Relatively narrow belts of anomalous high-grade metamorphic rocks occur in most North American erogenic belts. What were the vertical and horizontal motions that brought these belts to their current positions, and what tectonics caused the mo- tions? There are many hypotheses for such metamorphic rocks: (a) roots of magmatic arcs exposed by deep erosion, (b) suture-generated rocks in zones of thickened crust, (c) nappes of either North American continental basement or collided continents, and (~) unroofed regions of large crustal extension. By far the most widespread development of Phanerozoic metamorphic rocks now exposed in the North American continent is in the Appalachian mountain system. A close temporal and spatial relationship may exist between the timing of accretion of large terranes and the three major erogenic episodes that were at least locally accompanied by regional high-grade metamorphism and in some instances by anatectic magmatism. A similar mode! has been recently proposed for two metamorphic belts in British Columbia. Metamorphism is attributed to the doubling of the continental crust by the overthrusting of two superterranes onto the North American continental margin. In the Klamath Mountains of northern California and southern Oregon, it has recently been proposed that four tectonostratigraphic terranes were imbricated and accreted to the continental margin, producing a tectonic stack ~18 km thick in which there is an upward increase in metamorphism through the prehnite-pumpellyite and greenschist facies into highly deformed garnet-bearing schists and gneisses. A special case concerns the Cordilleran metamorphic core complexes which all ap- pear to lie within the deformed portion of siaTic North America from Arizona to British Columbia, well east of the zone of displaced terranes (zone 3~. The metamorphism is Hated in several areas as Middle Jurassic (~160 Ma) and all the core complexes were subsequently involved in Cenozoic detachment faulting that resulted in their tectonic deroofing. 8. Dating of Events in Orogenic BeIte It has been commonplace in geological sciences to consider that events in erogenic belts occurred in distinct periods separated by durations of little activity and that events occurred uniformly over wide areas. Critical analysis indicates, however, that existing data are commonly insufficient to justify these long-held views. In fact, deformation at some places could have been continuous over hundreds of millions of years and heterogeneous over distances of 100 km or less. The Appalachians provide a conspicuous example. The old view of Ordovician Taconian orogeny, Devonian Acadian, and Pennsylvanian-Permian Alleghanian has been questioned by age data. For example, Cambrian and Late Proterozoic Ap- palachian extension are recognized in the central Appalachians and Maine, and a deformation continuum from Cambrian to Mississippian may occur in displaced ter- ranes of the southern Appalachians.
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28 The need for precise knowledge of dates of erogenic events with age and position is evident, both to comprehend erogenic processes ant! to detect times of collision of exotic terranes that may no longer exist locally. Good resolution of ages of an erogenic phase at a given place would include ages of onset of movements, of maximum depth penetration/metamorphism, of rapid uplift, of attachment to North America, and completion of deformation. It is also important to resolve the timing of superimposed deformations. 9. Foreland Deformation and Tectonic Coupling
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29 contraction in Wyoming be related to the concurrent highly oblique Kula plate mo- tion at the active North American margin? How does the apparently marg~n-normal contraction of the Appalachians relate to the diachronous closure of Africa and North America from Ordovician to Pennsylvanian times? Perhaps foreland contraction arises by intracontinental stress systems. 10. Diagnostic Geophysical Expressions of Tectonic Units It is important to be able to identify the tectonic units that constitute most of the outer zones of the North American continent between the Proterozoic craton and the ocean basins. Many of these units can be delineated by surface structure and tectonostratigraphy, but geophysics provides the necessary data for determining their vertical extent, their areal extent (where submarine or buried by younger cover), and possible evidence for their deformation history. Most units are constructed of a number of specific geological and structural elements, and therefore each unit has a number of geophysical signatures which together identify the particular tectonic unit. Examples of tectonic environments are: mid-ocean ridge, island arc subduction zones, continental edge subduction zones, passive rift and continental margins, continent-continent colli- sion zones, and continent-continent transform zones. Island arcs, with their subducted oceanic crust, accretionary sedimentary wedge, voIcanicIastic sedimentary deposits mixed with intrusives in the magmatic arc, and backarc basins, provide distinctive tectonic megaunits. Passive rifted continental margins have continental crust and oceanic crust separated by a rifted crust of faulted blocks and intrusives, overlain by evaporite, carbonate, and ciastic sedimentary rocks deposited in shelf, slope, and rise paleoenvironments . Many geological features have characteristic geophysical signatures based on Heist mic, magnetic, gravity, and heat flow studies. For example, oceanic crust has a char- acteristic seismic P and S wave velocity structure. Peralkaline granitic (magnetite- bearing) bodies usually produce positive magnetic anomalies and negative gravity anomalies, whereas calcalkaline granites (ilmenite-bearing) produce magnetic and gravity Tows. (;ranitic bodies are generally associated with zones of high heat flow. Rhyolitic and gabbroic intrusives also have distinctive magnetic and gravity patterns. Island arc sedimentary-voIcanic units produce a chaotic short wavelength magnetic signature that is clearly distinguished from that of zones of platonic rocks and lineated patterns associated with foliated rocks. On a regional scale, there are prominent magnetic, gravity, and seismic signatures that mark major crustal boundaries. For example, the East Coast Magnetic Anomaly is a linear feature that appears to separate oceanic from rifted continental crust. Foreland basins have a characteristic seismic structure due to sediment distribution. Fault plane geometries are important characteristics for identifying foreland-hinteriand relations and the distinction between shear zone and collision zones. Distribution of plutons, batholiths, block-fault structures, etc., are important indicators of collision types and subduction directions. Other examples are: (a) the steep gravity gradient that runs the length of the Appalachian orogenic belt and (b) the correspondence of positive gravity anoma- lies with basement exposures in New England. It is important to evaluate fully the interpretation that the gradient reflects the eastward-thinning wedge of continental
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30 crust at the Atlantic passive margin. Example (b) points up the need for careful evaluation of geophysical signatures because, although they are associated with positive anomalies, the basement rocks are less dense than their tectonic and sedimentary cover. The erosion and metamorphism of units change their geophysical signature, often producing signatures characteristic of a particular event. The uplift and erosion of plutons gradually expose more mafic material, changing the geophysical signature of a magmatic arc. Compressive tectonism will alter random magnetic patterns into more lineated patterns as rocks are reheated and foliated. To analyze erogenic belts by means of geophysics, it is vital to have comprehensive and spatially dense data sets: gravity, magnetics, heat flow, seismic refraction velocity structure, and seismic reflection layering and velocities. Geophysical surveys of North American erogenic belts are necessary to develop a coherent geological mode! of the tectonic evolution of the continent in Phanerozoic time. 11. Deep Continental Crustal Structure and Origin Deep seismic reflection profiling shows that the continental crust contains reflec- tors at all depths above the Moho, but with varied characteristics and concentrations Such reflectors occur in the zone of deformed siaTic continent as well as In the cra- tonal crust, and moreover, in displaced terranes. Individual reflectors are commonly discontinuous, subhorizontal, and in sets that are extensive laterally and vertically over tens and several kilometers, respectively. Variations include sets of discontinuous dipping reflectors and, locally, a long continuous reflector or set of reflectors. At some places, sets of reflectors occur densely throughout the crust, whereas at others, they are uniformly but sparsely recorded, and at yet others, they are concentrated in the lower crust below a nonreflective upper crust. The base of reflector sets seems to be a laterally continuous surface that can be called the reflection Moho. These crustal reflectors are at the same time a prime control and a puzzlement in the formulation and interpretation of deep geological structure. What are the nature and origin of the velocity layering that causes them? Some candidates for the nature are little deformed strata, compositionally differentiated metamorphic rocks, mylonite and nonmylonite zones, intrusions or anatectites, and thrust zones. It is essential to learn what gives rise to the reflectors before the origin and tectonic history of the continental crust can be interpreted. The reflection Moho may be coincident with the more commonly known refraction Moho at most places, but at some sites the two differ in depth with any reasonable velocity structure employed. By definition, the reflection Moho is the maximum depth of finely scaled velocity layering, below which exists a more homogeneous upper mantle. In some regions, young structures deform the reflection Moho whereas in others (for example, Basin and Range), Late Neogene throws at the free surface of up to 5 km are apparently absent at projected positions of the reflection Moho. Are the reflection and refraction Mohos in fact due to the same structural phe- nomenon? What is the continental Moho? Is it a compositional boundary preserved from early continental formation? Or, at the other extreme, is it a modern density- rheology boundary above which tectonics cause zonal impedance contrasts? The problem of the nature and origin of the continental Moho is inextricably linked to studies of the tectonic evolution of North America.
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31 12. Phanerozoic Changes in Proterozoic North America Current theory suggests that near the end of Proterozoic time, North America was located completely or mostly within a supercontinent. The Late Proterozoic breakup of the supercontinent and growth of oceanic lithosphere led to the isolation of a protocontinent of Phanerozoic North America. A Phanerozoic tectonic history therefore must account for the changes in extent and configuration between the Late Proterozoic protocontinent and the part of today's North American continent cored by Precambrian rocks that have remained contiguous or nearly so (Figure 1, zones 1 and 2~. Necessary ingredients to approach the problem of Phanerozoic changes in Pros terozoic North America are the size and shape of the protocontinent and an accurate picture of the extent of Precambrian North America today. The latter (outer edge zone 2, Figure 1) is uncertain because today's edge of Precambrian North America is tectonically buried by displaced terranes at most places, and its subsurface locus has been inferred by inexact techniques. It is reasonable to suppose that Phanerozoic tectonics have reduced the girth of the protocontinent, and this is supported by lo- cally truncated I,ate Protozoic passive margin structures. Conceptual mechanisms for removal of Precambrian material are rifting and plucking out of chunks of the edge of North America during oblique convergence (as with Baja CaTifornia) and subduction during collision, with loss via sinking of a detached slab, and/or by melting. A com- plete history of the changes should incorporate times, places, and tectonics of removal of fragments of Precambrian North America. Another basic question is how far south (into what is now Mexico) Precambrian North America once extended and how this continental salient, if it existed, related to the confluence of the Appalachian and Cordilleran erogenic belts in northern Mexico. It is generally assumed that the zone of Precambrian North America deformed in the Phanerozoic (zone 2, Figure 1) incorporates no large internal displacements (e.g., >100 km). The assumption may be incorrect and should be tested. The existence of such Phanerozoic displacements within the Precambrian would be an important factor in models of the dynamics of continental deformation. SPECIFIC (PRIORITY) PROBLEMS From consideration of the 12 genera] and comprehensive problems set forth above, the following four specific problems are identified as the most fundamental to an improved understanding of the Phanerozoic evolution of North American continent- ocean transitions. Part ~ of this report deals with investigations needed to solve these and other problems. 1. Underplat~ng and! Tectonic Attrition Underplating is the process of transfer of sediment and/or rock from a clown am ing plate to the base of the forearc and/or crystalline lithosphere of North America at active margins. Questions are: What are the mechanics that cause sediment to underride the forearc and be attached to the base of the forearc or continental crust? How important has this process been in the thickening and outgrowth of North Amer- ican transitional crust over Phanerozoic time? What are the structures and material changes (strengthening, metamorphism, fluid release) that occur during underplating and how do these affect continental margin tectonics and magmatism? How can the
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32 existence of modern and ancient underplate be identified and characterized at depth by remote techniques and at the surface in orogens by geological and geophysical study? An important counterpart is the negative of underplating, that is, tectonic attrition of the base of North America above a downgoing plate. Attrition should be considered a part of this highest priority problem. Questions are: Is the overriding plate eroded by protuberances on the downgoing plate? Is the process controlled by migration of overpressured fluids or by the thickness of underriding sediment? How is attrition related to obliquity and velocity of convergence? 2. Origin of the Moho and Lower Crust The evolution of the transitional region (and North America as a whole) will remain in part a mystery until we have much improved understanding of its deep, remotely observed reaches. Specific questions are: What material changes cause the acoustic impedance and density contrast across the Moho in the continent-ocean transitional zones? What are the protoliths? What are the age and age variability of the boundary, and is it a continually evolving surface or one that forms mainly during or before major tectonic events? What is the material flux across the Moho? What causes the deep reflection events in lower crusts of the transitional regions (major displacement zones, intrusions, strata processing, etc.~? What are the origins and transport paths of lower crustal rock mainly from the surface, underplating, the mantle, or from primeval processes (autochthonous)? 3. Absolute Chronology of Tectonic Events The times of major events in the evolution of North American continent-ocean transitions are generally known to the first order. These provide a glimpse into the complexity of the development of the transitional region: the superpositions of processes, temporal variability of processes along the margin, large and rapid vertical motions of 30 or more km, and the enigmas of origins of terranes and their approach and attachment to North America. A well understood Phanerozoic evolution of the North American margin requires both more widespread and more highly resolved dating that will indicate ages of deposition, magmatism, maximum depth penetration, uplift times and rates, fluid passage, and the superpositions of these and other effects. More important, the elucidation of major processes of the continental transitions demands vastly greater quantity and improved resolution of dates. 4. Strike-Slip Components of Terrane Displacement Paths The terranes that constitute a nearly encircling zone around Precambrian North America (Figure 11 have generally unknown provenance. It is currentiv a major debate 1 ~ 1 1 ~ r ·. .~ · . ~ whether terranes emerged trom sites near their present one, for example, either by ex- traction from the continent or the local arc and marginal basin development, or on the other hand, whether they have undergone displacements of thousands of kilometers, perhaps with large margin-parallel (strike-slip) components. An understanding of the evolution of the transitional region, of which terranes make up a sizeable proportion, requires knowledge of displacement paths of terranes into and within that region. The relative motions of terranes with respect to North America and to one another provide one of the primary means with which to deterrn~ne obliquities in ancient convergent
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33 margins of the North American plate, ancient global plate reconstructions, and paleo- biological excursions. The detection of major strike-slip components has proved to be elusive thus far, and different techniques sometimes yield greatly conflicting results. This difficulty must be rectified in future research.
Representative terms from entire chapter: