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PART III BACKGROUND PAPERS These papers express the views of the authors, not necessarily those of the National Research Council. As is customary with reports of this kind, the background papers are reproduced here, for the reader's convenience, as they were received from the authors without the NRC review and editorial attention given to the preceding sections of this report.

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CONTENTS Section 1: Active Continental Margins I. Mechanics of P1 ate Motions A. Mechanics, Kinematics, and Deformation Modes at Convergent Margins; Mark Te Brandon and Dan M. Davis. ~ ~ ~ e ~ ~ ~ ~ e B. Driving Forces: Slab Subduction and Mantle Convection; Bradford Hager . . . . . ~ ~ . e II. Geologic Evolution of Active Continental Margins A. Initiation of Subduction; Dan Karig. e ~ ~ ~ ~ ~ 14 6 B. Intraoceanic Convergent Margins; Brian Taylor. . 160 C. Collision of Seamounts, Ridges, and Continental Fragments: Their Effects on Convergent Margins; Roland van Huene. . . . . . ~ . . . . . ~ ~ . . . 169 . . 117 . . 131 TIT. Mass and Chemical Transfer A. Mass Flux and Crustal Evolution at Convergent Margins; R.W. Kay and S. Mahiburg Kay . . . e Section 2: Passive Continental Margins I. Mechanics of Rifting of the Lithosphere 181 A. Comments on Rifting and Passive Margin Evolution in Light of Some Recent Studies; John Mutter and Brian Wernicke. . . . . . . e ~ ~ 208 B. Igneous Processes and the Evolution of Rifted Continental Margins; Jeffrey Parson and Carolyn Zehnder e ~ e ~ e ~ ~ ~ ~ ~ e e e e e 230 II. Rift and Passive Margin Basins -- The Sedimentary Record A. Investigating the Sedimentary Record: Sequence Stratigraphy--the Record of Tectonism and the Global Ocean Environment; Joel S. Watkins. . . . 247 B. Post Rifting Evolution of Passive Margin Basins; Dale S. Sawyer. . . . . . . . . . . . . . . . . . 269 115

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Dative ~i~ttal Mars ME=P,2~CI; OF HE Gallon 116

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MECHANICS, KINEMATICS, AND DEFORMATION MODES AT CONVERGENT MARGINS Mark T. Brandon and Dan M. Davis INTRODUCTI ON Much of the deformation that affects the crustal portion of the lithosphere occurs at convergent plate boundaries, which in the broadest sense encompass oceanic subduction zones, oniand thrust belts, and everything in between. Horizontal convergence at these boundaries is accommodated by a combination of two processes. The first is pervasive erogenic shortening within the leading edge of the overriding plate, as exemplified by accretionary wedges and collisional mountain belts such as the Himalayan or Taiwan thrust belts. In this case, excess mass is accommodated by continental growth and by uplift and erosion. The second is wholesale plate subduction, as illustrated by the non-accreting or eroding subduction zones such as the Mariana and southern Middle America trenches. In this case, excess mass is removed by assimilation into the asthenosphere. Modern and ancient convergent boundaries populate the spectrum between these two end- members. As a result, convergent boundary processes can have important and variable effects on the growth of continents and the chemical evolution -of the mantle. Moreover, convergence-related deformation can also give rise to a variety of important tectonic processes, such as regional metamorphism resulting from a perturbed thermal regime, and frequent large earthquakes due to episodic slip on a decollement tenet. We have not sought to provide an exhaustive review of the literature on convergent margins; such reviews exist (e.g., van Huene, 1984; Jarrard, 1986; Kanamon, 1986; Moore and Silver; 1987~. Instead, we hope to raise what we believe to be some of the most pressing current issues related to convergent margin deformation at all scales, up to and including those observed geodetically and seismically. Our understanding of Reformational processes at convergent boundaries has advanced considerably in the last ten to fifteen years. This improvement is due to a number of factors, both observational and theoretical. Increasingly sophisticated models have been developed to explain the geometry, kinematics 117

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and mechanics of thrust belts. These models are broadly encompassed by the concept of an erogenic wedges as originally proposed by Elliot ( 1976) and Chapple (1978~. The leading edge of the overriding plate at a convergent boundary deforms into a wedge-shaped profile. The base of the wedge. is bound by an active sole thrust or decollement which accommodates most of the horizontal convergence. Mechanical models dictate that the wedge must maintain a critical taper angle for decollement slip to occur (Davis et ale, 19831. This critical taper is apparently maintained by deformation within the wedge and accretion at the base and front of the wedge. Several aspects of these models for the mechanics of erogenic wedges remain unproven and are still being critically assessed. Most of the questions concern Reformational processes and decollement structure within the deeper, more internal part of the wedge (e.g., PavIis and Brown, 1983; Platt, 1986; lamieson and Beaumont' 1988~. Unfortunately, the internal regions of most oniand thrust belts are commonly obscured by post-orogenic metamorphism (due to thermal relaxation) and by younger superimposed erogenic events. Furthennore, because the base of the wedge commonly dips more steeply than the erosional section, the deeper part of the decollement is commonly not exposed. As a result, our understanding of the large-scale structure of these boundaries remains incomplete. We contend that modern subduction zones (Figure ~ ), where the downgoing plate is oceanic, provide a unique natural laboratory for the study of erogenic deformation at a variety of scales. At the largest scale, we can examine the relations between plate motions and intra-plate deformation. At some convergent margins, the ovembing plate acts as a stress guide, resulting in large-scale contractional deformation many hundreds of kilometers landward of the trench (examples fabled "C" in Figure ~ ). The Andean thrust belt (Iordan et al, 1983; Suirez et al., 1983) is a prime -example. At other margins, such as the Marianas (Mrozowski and Hayes, ~980; Hussong and Uyeda, ~ 982), structural features suggest that extensional deformation may dominate the entire margin, virtually all of the way to the trench (examples fabled "X" in Figure ~ ). At intermediate scales, we can study tectonic processes associated with the development of an erogenic wedge. Subduction complexes can be used to critically test and to extend existing models for erogenic wedges. 118

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At the small scale, we can examine the effects of a variety of low temperature Reformational processes. Rocks and sediments in this tectonic setting are subjected to a wide range of stresses, strain rates and confining pressures while under all degrees of lithification (Figure 2~. Modern subduction complexes provide well-controlled "laboratories" for the study of erogenic deformation at both intermediate and small scales: a) The Reformational environment is relatively steady and uniform over long periods of time (up to, or in some cases exceeding 10 million years). b) Temperatures are generally sufficiently low that only a limited number of microscale Reformational mechanisms are active. c) Growth of the wedge is not complicated by subaerial erosion. d) Deformation involves a relatively simple range of lithologies. e) The amount and rate of overall convergence is generally well known. f) The position of the downgoing plate is usually easily resolved by Benioff zone seismicity. g) The elastic properties of the downgoing plate are well understood. The more than 20 modern subduction complexes that presently populate the surface of the Earth (Figure 1 ) show a marked range in tectonic setting and defonnational behavior (e.g., Uyeda and Kanamori, 1979; Uyeda, 1982; larrard, 1986~. Convergence rates range from 10 to 1 00 mm/yr, and sedimentary cover on the downgoing plate from 0.2 to 10 km thick. Subduction complexes vary widely in their accretionary behavior: some grow by accretion of sediment and possibly oceanic crustal rocks, whereas others show evidence for long periods of non-accreting, possibly accompanied by loss of material (subduction erosion). The seismogenic character of the subduction thrust also shows considerable variability between margins and along strike within a single margin (e.g., Kanamori, 1971; Uyeda and Kanamori, 1979; Ruff and Kanamori, ~ 980~. In some cases fault slip occurs in an "aseismic" manner, progressing at a relatively steady rate without a significant rupture events, whereas in others berg., Sykes and Quit~meyer) slip occurs in an episodic or. seismogenic fashion, marked by large thrust earthquakes (Mb greater than 7) and long repeat times (greater than 50 years). Aseismic and seismic slip may occur at different levels in the same subduction zone (Figure 2), with seismic slip limited to the region between the shallow sediment-rich and the deeper high-temperature portions of the boundary. 119

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The main limitation on research at modern subduction complexes is that their submarine setting makes them relatively inaccessible. This places restrictions on the type of research problems that can be addressed and the methods that can be used to address them. In the summary below' we highlight some important problems and research opportunities. SELECTED PROBLEMS: (1) What factors control the accretionary behavior of subduction complexes? Why do some subduction complexes grow by frontal accretion (e.g., Barbados, Cascadia, Nankai), whereas others show no evidence' of frontal accretion (e.g., southern Middle America)? Subduction erosion of the overriding plate has been proposed at a few margins (e.g., Mariana, Peru-Chiley, but this potentially important process remains poorly documented. How does such erosion take place? Does accretion occur at a deeper level at "nonaccreting" and "eroding" margins? Is accretionary behavior directly influenced by such factors as the internal structure and lithological composition of the overriding plate or the subduction geometry of the downgoing plate? (2) What are the dominant microscale Reformational mechanisms and the associated structural responses for sedimentary materials within the forearc wedge? Our understanding of microscale Reformational processes under low temperatures and variable pore fluid pressures is relatively limited compared with that for high temperature metamorphic settings. In particular, there is a wide range of opinions on the rheological behavior of unlithified sediments under high pore fluid pressures: Do these sediments deform as a low viscosity fluid (e.g., Cloos, 1982) or do they fail in a brittle fashion? What is the role of solution mass transfer (stress solution)? The temperature range and deviatoric stress conditions under which this mechanism dominates are very poorly resolved. A better understanding of this mechanism would greatly improve our ability to interpret structures and Reformational histories at ancient, uplifted subduction complexes. The deep limit of interplate thrust earthquakes is apparently controlled by temperature. However, the factors controlling the trenchward limit of such earthquakes are less well understood. Wedges at seismogenic subduction zones may show a mix of Reformational mechanisms, 320

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associated with alternating high and low strain-rate regimes (co-seismic versus inter-seismic deformation). (3) How does an accreting wedge maintain a critical wedge t a p e r ? Accretion at the front of a thrust wedge must be balanced by thickening at the rear of the wedge. Does thickening occur by basal accretion (e.g., thrust imbrication) or by ductile flow within the wedge? What fraction of shortening occurs in the immediate "toe" area of the wedge, as opposed to farther upslope? How does the wedge respond to and recover from the passage of a bathymetric feature such as a seamount, transform fault, or ridge? Also important is the potential presence of a backstop within the wedge, which would appear as a distinct kinematic boundary marking the transition from an actively deforming portion of the wedge seaward of the boundary to a more stagnant region landward of it. It is uncertain how such a boundary might relate to major structural boundaries or lithologic transitions within the wedge, or to the pattern of seismicity on the master thrust beneath the wedge. (4) What factors control the localization of the subduction zone decollement and how deeply does this decollement incise into the downgoing plate? Reflection seismic profiles have demonstrated the presence of well developed decollement horizons at the front of several subduction complexes. Thrust seismicity shows that the subduction thrust remains a localized and nearly planar feature to depths on the order of 50 km. The geometry and position of this master thrust probably exerts an important control on accretionary processes and ultimately determine the flux of crustal and sedimentary materials in and out of the wedge. (5 ~ What is the nature of heat and fluid flow within subduction complexes? These fluxes can strongly influence diagenetic and metamorphic processes, and can also greatly accelerate ductile defo'~ation due to solution mass transfer (e.g., Ethendge et al., 1983; Shi and Wang, 1984, Reck, 1987~. Some important questions are: What are the major sources of fluid within the accretionary wedge: sediment compaction, dewatering of ocean crust, or gravity-driven flow? How do faults, stratigraphic units, and surface recharge affect the pattern of fluid flow? What geological conditions are ~21

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required to produce and maintain high excess fluid pressure within the wedge? (6) What determines the Reformational response of the overriding plate at convergent margins? Can the development of onland erogenic zoned' far from the trench, be related to plate boundary interactions, such as changes in the rate or type of sediment accreted to the wedge, or in the geometry or rate of plate subduction? Empirical studies (e.g., Ruff and Kanamor~' ~ 980; larrard, ~ 986) have shown that several factors, including speed, dip, and age of the subducting plate are strongly correlated with the Reformational style of the overriding plate, whether contractional or extensional. Several explanations have been posed for this correlation, but we are still far short of resolving the mechanics of this interaction. Furthennore, some margins show evidence of substantial Reformational gradients, changing from active accretion at the front of the wedge to within-arc or back-arc extension (e.g., Ryukyus). Margins showing this behavior indicate that several competing processes may be involved in determining the state of stress and defonnational response in the upper plate. (7 ~ What factors govern seismic slip behavior at subduction b o u n ~ a r i e s ? It has been proposed that slip behavior, whether convergence occurs by steady slip or seismic rupture, is controlled by large-scale attributes of the plate boundary, such as the age of the downgoing lithosphere and the rate of subduction (e.g., Ruff and Kanamori, 1980; Ruff,- 1983; Jarrard, 1986~. Rock mechanicians consider large shallow earthquakes to - be caused not by rupture of new rock, but rather by a velocity weakening instability that is dependent upon rock type (e.g., Dieterich, 1978; Stuart and Mavko, 1979; Rice, 1980; Tse and Rice, 1986). Are these two points of view compatible? Does the presence or absence of subducted sediment influence the slip behavior of the subduction thrust (Byrne et al., ~ 988~? What is the mechanical behavior of rocks associated with deep subduction thrust earthquakes (at depths of about 50 km or greater, where the expected temperatures may suggest ductile flow)? How much "aseismic" slip occurs at seismogenic subduction zones? What is the physical significance associate with spatial variations in moment release during subduction thrust earthquakes (i.e., asperities)? Can these be related to geometric factors such as ramps or bends in the master thrust fault, or 122

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basement structures, such as seamounts? Finally, can we achieve reliable assessment of the seismic risk at those margins that have not had instrumentally observed great earthquakes? ~ ~ ~ How is the seismogenic character of a subduction zone related to geodetically measured strain? To the first order, the elastic dislocation model can account for the temporal changes in geodetically measured strain at seismogenic convergent margins (e.g., Savage, 1983; Thatcher and Rundle, 1984~. However, there remain important gaps in our understanding of the relationship between seismicity (paleoseismic, historic, and instrumentally observed) and the results of geodetic studies. There appears to be no strain accumulating in the Shumagin Gap (Alaska Peninsula - Figure ~ ~ where there is good evidence that great earthquakes have occured. In contrast, geodetic data from the Cascades subduction zone suggest ~ buildup of strain (preseismic ?), but there is no historic evidence of large thrust earthquakes. Further complicating this problem is the presence of tectonic processes unrelated to the seismic cycle that can produce long-term secular strain. It has been proposed that short-term geodetic behavior of subduction zones is related to their seismogenic character. How are these two Reformational processes related to each other, and what are the implications of this relationship for the tectonic development of convergent margins? Finally, does the seismic character of a margin have any clear relation to the geodetically measurable secular strain? FUTURE RESEARCH DIRECTIONS Many of the problems above are being actively pursued using conventional research methods, such as field studies of uplifted subduction complexes (structure, metamorphism and fluid flow history)' routine marine surveys of modern subduction complexes (multichannel seismic profiles and swath mapping methods), and analysis of local and global seismic data associated with subduction thrust earthquakes (precise event locations, source mechanisms and rupture histories). These studies have contributed to the bulk of our present understanding of subduction complexes and will undoubtedly continue to do so. However, the direction these studies take and the rate at which they advance will be strongly affected by future "high tech" studies. For instance, results from the relatively limited suite of deep ocean drilling 123

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-POST RIFTING EVOLIJ~IION OF PASSIVE MARGIN BASINS Dale S. Sawyer Department of Geology and Geophysics Rice University P.O. Box 1892 Houston,TX 77251 (713)285-5106 Introduction Rifted or Impassive" continent margins are sites-of tremendous sediment accumulation. After their deposition, sediments are far from passive and undergo a broad spectrum of physical and chemical changes. The chemical changes include diagenesis, metamorphism, hydrocarbon generation, and interaction with or contributions to the seawater system. In addition to the obvious economic importance of these phenomena, the influence of passive margins on the global geochemical balance is perhaps large, but largely unknown. The physical environment of the sediments, including pressure and temperature, along with the mechanics of the sediments, control physical properties changes and sediment tectonics. Crustal processes such as subsidence rate and isostatic response to sediment loading also affect the conditions in passive margin sediments. The proposed Continental Margin Workshop is an opportunity to examine the interactions among these diverse phenomena and devise strategies for studying ~em. Physical Processes Fluid flow in rocks affects diagenesis, mineralization, metamorphism and tectonics in basins. While this is well known, studies of fluid circulation have been largely confined to looking at the effects of fluid circulation rather than directly observing it. Fluid flow is 269

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probably the primary means of redistributing elements in passive margin basins. Mass balance calculations in the Gulf of Mexico Coast region pertaining to such diverse sedimentary phenomena as sandstone cementation Wand and Dungeon, 1979), formation of sediment hosted mineral deposits (Price and others, 1983), development of secondary porosity (Schmidt and McDonald, 1979) and carbonate loss from muds tone ~undegard and Land, 1986), an require large amounts of matenal and energy transfer up through the sediment column. Fluid flow basins produces chemical fluxes into the oceans* Although not as intense as the fluxes at m~- ocean ridges, those at margins may be greater. The character of fluids entering the sea water from passive margin basins is likely to be quite anomalous and variable. Biological systems, including those associated with hydrocarbon seeps and cold saline seeps, can be affected or even owe Weir existence to fluid c~ulanon (Bright et ale 1980; Kle~nschmidt and Tschauder, 1986). Mechanisms of fluid flow in passive margin basins include compaction, differential loading, hydrothermal convection, and gravitational flow of meteoric and saline water. Compaction produces an upward flow of water when permeabilities permit, but leads to the formation of ove~pressure when water camlot escape. The thickness and physical properties of sediments deposited in passive margin basins are often laterally heterogeneous leading to significant lateral fluid flow. Under appropriate circumstances hydrothermal circulation may serve to move large quantities of water and heat through sediments. This may pareiaDy explain puzzling results from diagenetic studies that require tremendous amounts of water, much more than can be attributed to compaction dewatering, to deposit or remove dissolved elements. Water flow in passive margin basins also results from density differences between pore fluids such as meteonc and saline water. The migration arid accumulation of oil arid natural gas from source regions to resenro~s, or probably more frequently to escape into the ocean or atmosphere, are poorly understood. Migration and accumulation are probably at least partly a result of water circulation Trough sediments of varying porosity and permeability. Migration can be constrained using 270

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geochernical signatures that in some cases allow sources to be identified and pathways determined. A better understanding of hydrocarbon migration win have significant economic value as wed as improve our knowledge of the hydrogeology of margin basins. A massive database for addressing these problems lies in the oil industry and is currently mostly unavailable to academic researchers. Isostanc response of the margin lithosphere affects the s~augraphy md configuration of sediments. To study other processes, including subsidence and sea level variation, it is important to be able to remove the effects of the isostatic response. The isostatic response at a margin is a manifestation of the rheology of continental, extended continental, and ocean crust. Of those 3 types of crust, it is only in oceanic crust that we have much understanding of the isostatic response. Each type of crust or lithosphere flexes when loaded. The wavelength and amplitude of Me flexurE vanes spatially In a basin and wad time. Subsidence In a marginal basin is the result of tectonic processes initiated when the margin formed and We isostatic response to sediment loading. We are usually interested in separating the sediment loading induced subsidence from the tectonic subsidence by observing the sediment loading history and predicting its isostatic response. The sediments on margins have served as recorders of the subsidence history. Tectonic subsidence history may be used to constrain me mechanisms Mat formed the margin. Two end-member models of continental rifting and subsequent passive continental margin formation are currently popular. In the first, the pure shear model (McKenzie, 1978 for example), conjugate passive margin subsidence is predicted to be symmetrical (although I suspect that this is false; Dunbar and Sawyer, 1988). The second, the simple shear model (Wernicke, 1981, 1985; Lister et al., 1986), predicts that subsidence on conjugate margins will be highly asymmetrical. Subsidence studies, along with seismic reflection smbies, will play a role in distinguishing between these models. Temperature is an important control on the rates of many chemical reactions in margin basin sediments ~opatin, 1971). Cooling of the lithosphere causes its contraction which is a principal cause of tectonic subsidence in margin basins. Rock physical properties are sensitive 271

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to temperature. Present temperature can be measured by drilling. Present surface heat flow can be measured. Paleotemperatures can be estimated by observing the progress of temperature sensitive chemical reactions. Most often these give us values of the integral of a function of time and temperature rather than temperature directly. Temperature distribution within passive margin sediments is largely a function of thermal conductivity, permeability, porosity, and supply of heat to the bottom of the basin. In basins with low permeability sediments the temperature distribution may be controlled by heat conduction alone. When fluids circulate freely, however, the temperature distribution may be completely controlled by thermal convection. This or other mechanisms are required in many basins to move energy upward faster than conduction should allow. The physical properties of margin basin sediments and the lithosphere below influence literally all of the geophysical observations: gravity, geoid height, seismology, magnetics, well logs, and physical processes we seek to understand: fluid flow, heat flow, deformation, fracture, sediment compaction, and etc. The physical properties of sediments change, in some cases dramatically, as they are compacted and/or chemically altered during burial and aging. Knowledge of the history of these changes is critical to understanding of every other physical process we discuss here. Physical properties are observed or inferred from studies of surface samples, well samples, well logs, seismic experiments, and potential field observations. Distinct physical properties are then linked by, often empirical, mathematical relations. Often porosity and Ethology are used as variable parameters in these relations. Then porosity can be linked though Ethology to depth of burial and then incorporated into geodynamic models to make a suite of testable or useful predictions about processes. Sediments are often deformed or faulted after deposition. Further, they develop cracks on a variety of scales that influence fluid flow. Growth faulting due to differential loading is an important class of fault. These are common where sediment deposition is locally rapid. The dynamics of growth fault formation are poorly understood. The role of fluids in lubricating the fault plane and the use of fault planes as conduits are also unknown. Salt mobility is common 272

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In margin basins. It takes several forms including the formation of pillows, diapirs, walls and sills. The intricacies of diapir growth are becoming better known due to physical and numerical model studies and observation. The stress environment in margin basins is unknown although methods now exist to make measurements In wells. Chemical Processes The generation of oil and natural gas in margin basins is an economically important process. Hydrocarbons are generated by heating kerogens, biological products deposited sufficiently quickly or in anoxic seas. Within bounds, time and temperature can be interchanged to achieve a particular level of kerogen maturation (Lopatin, 1971; Wapples, 1980). Important questions remain about the affect of other chemicals on hydrocarbon generation, the types of kerogen source beds and the hydrocarbons they may produce, the relations between time, temperature and the many ways to measure hydrocarbon source maturity. We must understand the mineralogical, chemical and textural changes that sediments undergo with increasing burial, fluid pressure and temperature. This will improve prediction of the distribution of hydrocarbon reservoir rocks. It win allow us to build quantitative models of diagenes~s. The magnitude of inorganic chemical fluxes into the ocean through passive margin basins are largely unknown. The weathering cycle is particularly important in establishing the chemistry of the oceans. Since the bulb of the solid products of weathering end up in margin basins, it is possible that significant interactions exists Some Key Questions What mechanisms condor the hydrogeology of passive margin basins? What are the nature and relative importance of proposed mechanisms of sediment ove~pressuring? 273

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How does the lithosphere at passive continent margins respond to sediment loading? What are He dynamics and kinematics of salt diap~nsm? What are the present temperatures in passive continental margin basins and how important is hydrothermal circulation in influencing He temperature field? How are sediments of different Ethology modified by burial and chemical interaction in passive condiment margin basins? What are the chemical fluxes into the global seawater system at passive continental margin basins? How, when, and where do hydrocarbons mature in passive continental margin basins? How and when do hydrocarbons migrate in passive continental margin basins? What are He mass and energy balances In evolving passive continental margin basins? Suggestions for Future Research The types of methods that will be required to approach these problems are as diverse as the problems themselves. A combination of seismic reflection methods and driving will be required to establish the structural and s~atigraphic Stonework of particular marginal basins. In the case of some marginal basins, such as the Gulf of Mexico or North Sea basins, large quantities of geophysical data are available. Data are quite sparse in most other areas. Although large numbers of exploratory wells have been drilled, because of the methods used to drill them, relatively few high quality scientific studies were, or can be, performed in them or on samples from them. Deepening or reusing exploratory wells, although it sounds economical, is rarely a viable approach because He bottom hole diameter is usually too small to allow further casing and drilling. Most of the questions of sediment chemist and chemical flux can only be addressed using uncontaminated samples of rock and pore fluid. This is not usually possible if conventional industry drilling practice has been employed. In some cases these problems will require the installation of long term downhole systems to monitor temperature, pressure and allow fluid sampling. Methods of studying margin hydrogeology 274

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include determination of patterns of fluid circulation, diagenetic patterns of continental margin sediments, and nature of deposits formed by sea floor seeps. These observations should be incorporated into hydrogeological and geochemical models of greater sophistication than are available today. It is likely that such work will be most productive if pursued in a few basins where ~e" most data am available, the Gulf of Mexico and Norm Sea. Smaller study areas within the basins should be the subject of new data acquisition aimed at determining the seafloor fluxes of fluids and chemicals. Drilling will eventually be required but is not useful until more survey work is complete. The principal means of study of the isostatic response of the lithosphere under passive continental margin basins involve comparing observations of gravity, geoid height, topography, sediment density and distribution, crust thickness, density and li~ology' and subsidence history' using geodynamic models. lleferences Because He range of subjects to be addressed In this document is so great, ~ drew heavily on ideas presented ~ previous workshop documents. ~ particular ~ used the report of a DOSECC sponsored! workshop on Ul~adeep Scientific Drilling In He Texas Gulf of Mexico Coast and He report of He Second Conference on Scientific Ocean Drilling. Bnght, Tot., Larock, P.A., Issuer, R.D., and Brooks, I'M., 1980, A brine seep at the East Flower Garden Bank' northwestern Gulf of Mexico, Int. Revue gesamt. Hydrobiol. v. 65, p. 535-549. Dunbar, J. A. and Sawyer, D. S., 1988, Continental rifting at preexisting lithospheric weaknesses, Nature, v. 333, p. 450-452. Kle~nschm~dt, M., and Tschauder, R., 1986, Shallow-water hydrothermal vent systems off the Palos Verdes Peninsula, T-os Angeles County, California, Biol. Soc. Wash. Bull., v. 6, p. 485-488. Land, L.S. and Dutton, S.P., 1979, Cementation of Sandstones: reply, J. Sedimentary Petrology' v. 49, p. 13S9-1361. Lister, G.S., Etheridge, M.A., and Symonds, P.A., 1986, Application of the detachment fault model to He formation of passive continental margins. Geology' v. 14' p. 24~250. Lopatin, N.V., 1971, Temperature and geologic time as factors in coaliBlcation (in Russian), Akad. Nauk SSSR {zv. Ser. Geol., no. 3, p. 95-106. 275

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L4undegard, P.D., and Land, L.S., 1986, Carbon Dioxide and organic acids: Weir role in porosity enhancement and cementation, Paleogene of the Texas Gulf Coast, SOce of Econ. Paleontologists and Mineralogists Spec. Publication No* 38, p. 129-146. McKenzie, D.P., 1978, Some remarks on the development of sedimentary basins, Earn and Planetary Science Letters, v. 40, p. 25-32. Pnce, P.E., Kyle, I.R., and Wessel, G.R., 1983' Salt dome related zinc deposits, in Kisvarsanyi, G., Grant, S*K., Pratt, W.P., and Koenig, J.W., (eds.~' Int. Conf. Mississippi Valley type lead-zinc deposits, Proc. Vol. Umv. Missoun Rolla' p 558-577. Schmidt, V., and McDonald, D.A., 1979, The role of secondary porosity in the course of sandstone diagenesis, Soc. of Econ. Paleontologists and Mineralogists Spec. Publication No. 26, p. 175-207. Wapples, D W., 1980' Time and temperature in petroleum formation: Application of Lopadnts method to petroleum exploration, American Association of Petroleum Geologists Bulletin, v. 64, p. 916-926. Wernicke, B., 1981, Insights from Basin and Range surface geology for the process of large- scale divergence of continental lithosphere (abstracts, ~ Papers Presented to the Conference on Processes of Planetary Rifting, Lunar and Planetary Institute, Houston, p. 90-92* Wernicke, B., 1985, Uniform-sense normal simple shear of the continental lithosphere, Canadian Journal of Earth Sciences, v. 22, p. 108-125. 276