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3 Needed Investigations There are many gaps in our understanding of the structure and evolution of North American continent-ocean transitions. The gaps and uncertainties have been translated to problems whose solutions will cause major progress in the science of the evolution of North America and of the growth and destruction of continents in general. Focused investigations are necessary to achieve the solutions in a timely and efficient manner. Moreover, they are required to bring together the broad spectrum of scientific backgrounds and resources that must be pooled to address these complex problems. Studies of continent-ocean transitions must bring together terrestrial and marine scientists from varied disciplines: geologists, geophysicists, and geochemists for observational, data processing, theoretical, and laboratory approaches. Four principal avenues for focused investigations are outlined below: syntheses, topical investigations, processes, and techniques. SYNTHESES Transect analyses show the value of regional synthesis and interpretation of existing data by scientists of diverse background acting in concert. The gains in perspective, problem definition, and in many cases, new ideas of continental margin evolution, have been enormous. Equally important, each transect has conceived and displayed graphically a mode] of the continent-ocean transition to Moho depths. Such models should be tested by new data gathering, which in turn will lead to improved models and further refinement in our understanding of continental evolution. The USGC advocates continuing syntheses of North American continent-ocean transitions in two modes: (1) with existing data as in the Transects Program, and (2) with new data acquisition. 1. Syntheses with existing data: There are many regions of the North Ameri- can continent-ocean transition that have not been sampled by the 23 transects of the Transects Program. Studies of such regions comparable to those of the Transects Pro- gram would produce a significantly more comprehensive understanding and problem definition than exist now. Such regions contain features that may be unique and vital 11
12 to continental science and that are fundamental to analysm of lateral gradients and discontinuities in orogenic belts. Candidate regions for a new phase of transect studies are: Aleutian Islands, north- western British Columbia, Santa Barbara-southern Sierra Nevada, northern Labrador to Greenland, northern Mexico across Gulf of California, west Texas to the Gulf of Mexico, Florida across southern Blake Plateau, Yucatan to Cayman Trench, Bahamas to Puerto Rico, Mackenzie Delta to Canada Basin, and the eastern Baffin Islands. 2. Syntheses with data acquisition: Some or Al of the models of the 23 tran- sects should be tested and refined by acquisition and synthesis of new, comprehensive datasets that are profiled along the transects. The new investigations should include seismic profiling with source-receiver parameters designed for penetration and resolu- tion that can maximize analysis of model features: potential field data with uniform high- and low-frequency contents; isotopic dating that will resolve protolith ages and tectonic events; detailed analyses of kinematics and conditions of deformation in dim placement zones and strained rocks; and determinations of latitude anomalies and rotations of well dated magnetizations. Data from scientific drilling on land and at sea should be incorporated. TOPICAL INV1:STIGATIONS The USGC recommends new investigations that focus on problems in three broad tectonic divisions of North American continent-ocean transitions: (1) forearcs, (2) lower crust and Muffs in t.r?~.n~it.inn~1 r-~ir~n~ ~nr1 {.~1 t.-rr~n~c~ and f~r~lan~o rep the orogenic belts. ~ ^ ~ __ ace_ ~~~ _ ~ ~~_~ ~~ ^~^ _~ ~~ user 1. Forearcs: There should be concentrated data acquisition that will lead to greatly enhanced understanding of the deep structure and processes of forearcs of North America, both those active today and those in the Cenozoic. The objective is to evaluate what may be the prime sites of tectonic progradation and/or attrition of the continental edge and at which the continental edge appears to thicken. A prime tech- nique is seismic profiling, which may reveal the characteristics of underplated packets, underthrustin~ sediments. basement reliefs ramrod anr1 It. of rh~na" of ~l~t.z~rkrn,~nt. level, out of sequence faults, diapirs, duplexes, lateral velocity changes indicating rhe- ologic gradients, and the downdip length of sediment cover on the downgoing plate below the forearc and into the crystalline subduction zone. Other important studies of forearcs should include seismicity, fluid and heat fluxes, progressive deformation, and deflection of the downgoing plate. 2. Lower crust and Moho: The scientific objectives to be addressed include the origin of reflector sets in the lower crust of the transitional region, nature and thickness of the Moho, existence of motions along and mass transfer across the Moho, protoliths on both sides of the Moho, and age of the Moho. Also included here are the structures of the deeply buried, greatly extended North American crust of the Mesozoic and Cenozoic passive margins and the nature of specific features such as the outer ridge, possible mantle protrusions at the continent-ocean join, continental microplate structure (Orphan Knoll, Flemish Cap, Blake Plateau), rift basin configurations, and estimation of amount of magma added to the crust. Investigations of this problem may follow several lines, which should be coordi- nated: (a) geophysical studies, chiefly seismic, that are tuned to desired depths, are set out in three-dimensional arrays so as to determine true dips and velocity anisotropies and are flexible enough to track deep structures to shallow depths; (b) studies of
13 magmas and xenoliths derived from to the surface of plunging lower crustat structures, ana fat lower crustal or upper mantle levels; (c) tracking ~ 'I' study of greatly exhumed rocks that were once at depths equivalent to today's Moho in transitional regions. 3. Terranes and forelands of orogenic belts: There should be continuing study of the erogenic belts of North America with traditional goals of defining basic tectonic units and dating their times of movement and attachment. Added emphasis should be placed on analysis of kinematics of erogenic belts, on the relationships between motions in the zone of terranes and those of the continental foreland, and on subsurface structure in the zone of terranes. Further goals of this topical study are a comprehensive picture of the closure history of Laurasia and Gondwana to form Pangea and the Tosses of Precambrian North America in tectonic interactions at active and passive margins since the Precambrian. The reconstruction of terrane migration in Mesozoic Mexico and western North America is also a goal. Also included here is the objective of understanding the widths of modern and ancient boundary zones of the North American plate over which boundary-related motions occur. The methods to be employed in this investigation include all those for kinematic analysis: brittle and ductile fault slips, strain to displacement calculation, satellite geodesy, paleomagnetics, seismic parameters, and barometry of metamorphism. They include extensive dating by multiple techniques, and identification of protolith kindred by isotopic signatures (Nb, Nd, Sm. Sr, Pb, etc.~. They also include geophysical studies, mainly seismic, and drilling to identify and track subsurface horizons that may be terrane boundaries, especially those horizons that lead from the zone of terranes onto the edge of sialic North America. PROCESSES OF CONTINENT-OCEAN TRANSITIONS Aside from the areal investigations of the synthesis and topical types discussed above, studies of certain basic processes have been recognized to play major roles In tectonic zones. Examples of such processes are detachment, unroofing, reactivation in fault zones, underthrusting and wedging, accretion, attrition, diapirism, intrusion, foliation generation, defluidization, extensional failure, anatexis, initiation of subduc- tion, onset of drifting, predrift elevation, and transform faulting. Another process that requires extensive research Is continental separation and the definition of the rift-drift transition from sedimentary successions and tectonic structures. The objectives are improved comprehension of the conditions under which each process occurs and the mechanics and kinematics. The study of processes should include observational, theoretical, and experimental means. TECHNIQUES Major advances in understanding come not only from the recognition of new problems or new insights, but also from the development or ~rnprovement of techniques that yield new data. The study of the continent- ocean transitions has benefited greatly in the twentieth century from new techniques, such as paleomagnetem and isotopic dating. Therefore, research on improvement of techniques is an important component of the study of continental evolution. Specifically, advances in techniques in the following areas would be important: dating of protolith ages; dating of thermal ~d straining events and of unroofing times; resolution of displacement components, especially of rotations and margin-paraBel an
14 latitude-parallel components; measures of bulk finite strain and small-domain strains; barometry and thermometry; detections of major rise of fluids and fluid sources; methods of delineating structures in and tracking displacement zones to Arid- and lower crustal depths; improved analysis of the structure of the Moho; and more precise geodetic analyses. Advances should be made in techniques of digitization of data and in software that are compatible with commonly used computers and that allow combination and manipulation in three dimensions.
