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4 PASSIVE MARGINS: GROUP _1 MECHANICS OF RIFTING AD ASSOCIATED MAGMATISM PREAMBLE By the late lg60s, there was widespread agreement among earth scientists that ocean basins are born by the separation of plates of continental lithosphere, and that many crusty fragments found in compressional erogenic belts were initially fashioned during one or more phases of extensional tectonism accompanying continental breakup. Given the drama of these advances, one might reasonably ask whether another major breakthrough will occur in our understanding of the mechanics of rifting and the formation of passive margins. However, the kinematic theory of plate tectonics describes the motions of rigid plates on a sphere, whereas rifts are a deforming continuum of continental lithosphere governed by an array of processes that are either unaddressed or not predicted by pa ate tectonics per se. In the last decade, researchers have used a diverse range of approaches to studies of the rifting process that have resulted in a number of major advances. Each was developed more-v-1 ess independently. Many of these-advances thoroughly eclipsed those of the previous century of research into how rifts evolve. The Margins Workshop represented-a rare forum for leading scientists from many individual disciplines to exchange ideas and develop research directions. The need to coordinate methodologies emerged as a strong consensus of the group. Within Passive Margin Working Group 1, which was composed of a number of investigators from several subfields, it was unanimously felt that, because each subdiscipline had accelerated over the past ten years, even greater progress would result from an integration of subdisciplines. Below, we identify six areas where distinct disciplinary or methodological investigations have resulted in the recognition of a first-order aspect of rift evolution that was either contrary to prevailing thought in the mid-1970s or was unanticipated by the paradigm of plate tectonics. No notion of ranking these advances is intended. 29
Surface Geological Mapping in the Basin and Range Province A revolution in thinking about the kinematics of normal fault systems has been brought about by detailed field studies in the Basin and Range. The studies demonstrate the existence of basement-penetrating normal fault systems in the upper part of the continental lithosphere that have accommodated hundreds of kilometers of extensional strain across the province. The faul systems are characterized by 5 0 Han or more displacement and generally emplace thin, steeply rotated fault slices of upper crustal rock onto mid-crustal metamorphic tectonite . This f ieJ d association is known as a metamorphic core complex. Ten years ago, it was widely hell] that extensional tectonism in the Basin and Range was moderate (in the range of 100-150 km) and accommodated along widely spaced, steeply dipping normal faults, each with displacements on the order of ~ km. This view differed little from the conceptualizations of geologists such as G. K. Gilbert and W. M. Davis at the turn of the century. The last ten years have witnessed changes in thinking about Basin and Range extensional tectonism as profound as those a century ago when inquiry into the origin of the province began, including the discovery of a class of fault systems as significant to extensional tectonics as-the discovery of overthrusts was to compressional tectonics. Deep Seismic Reflection Profiling on Extended Continental Lithosphere and Laboratory Measurements of the Strength of Lithospheric Materials Marine- and land-based reflection surveys in many parts of the world show that the lower continental crust is often highly reflective in rifts, and that the Moho tends to be flat beneath rifted areas. Although there is no consensus as to the origin of the reflectivity, its mere existence represents a first order observation of rift architecture that must be explained. In addition to these findings, laboratory experiments on the strength of lithospheric materials suggest that the lower crust is probably very weak relative to the upper mantle, possibly giving rise to a ''jelly sandwich" rheology for the lithosphere as a whole. These two independent observations are complementary, suggesting that the observed reflectivity may be the result of a highly mobile lower crust during extension, and that the Moho may be a dynamic feature capable of reforming into a sub-horizontal configuration during and after the rift process. In any event, the combination of laboratory and reflection results has opened up rich new avenues of investigation that were not presaged by plate tectonics. 30
Shallow Marine Reflection Profiling and Surface Geologic Studies of the East African Rift System Although East Africa has been recognized as an example of the earliest stages of continental rifting for over a century, detailed characterization of the major rift valleys in three dimensions has begun only within the last ten years. The result of these studies demonstrate that the rift is asymmetric and segmented along strike, and that the sense of asymmetry periodically "flip-flops'' along the strike of the rift from east-facing to west-facing master normal faults, in harmony with map-view sinuosity. In addition, there appears to be magmatic as well as structural segmentation. These new findings provide an observational base to which mechanical models of rift initiation must conform. Given that the sinuosity of hinge zones on passive margins occurs at a variety of scales' the segmented character of the East African rifts represents a significant new starting point for understanding processes at all later stages of continental separation. Marine Geophysical Studies of the Deep Structure of Passive Margins in the Central and North Atiant~c Oceans :s Major technological advances in shipboard acquisition of multichannel seismic reflection and refraction profiles, complemented by potential field studies, drilling, dredging, and submersible observations, have revealed -strongly contrasting end-products of continental separation. Off Norway and East Greenland, for example, rifting was accompanied by the production of volumes of igneous rocks totalling a large fraction of the thickness of the pre-rift crust (more than 20 km). These margins have been termed "magmatic," as it is clear that huge volumes of melt accompanied the culmination of rifting and the onset of seafloor spreading. The erupted units form relatively symmetric wedges of seaward-dipping reflections that extend to substantial depth, a geometry that led to the hypothesis of "subaerial seafloor spreading." These observations spurred theoretical studies of factors that may control melt volumes during rifting, which suggest that the volume of magma erupted from the mantle is highly sensitive to the temperature of the asthenosphere before rifting and may be influenced by the actual rift configuration as well . Rapid, cn~stal-scale mass transfer from asthenosphere to lithosphere, the kinematics of subaerial seat loor spreading, and the strong contrasts in eruptive histories on passive margins are unexpected, potent food for thought in a number of earth science disciplines, and they are topics whose pre-1980s literature is rather thin. Another end-product of rifting appears to be block-faulted, "amagmatic" margins where magmatic activity appears to have had little or no role in the rift history. The southwestern European 31
margin Ireland to France) and Grand Banks of the Newfoundland margin provide some of the best examples of sediment-starved amagmatic margins, where brittle clefor~ation and rotation of crustal blocks dominates the crustal record. Similarities between these margins and the continental extensional regimes, such as the Basin and Range province and the North Sea, provide the opportunity to- examine the role of faulting in regions where the amount of extension and operation environment (marine vs. Onshore ) are substantial ly di f f Brent . Detailed regional studies of the U. S ~ and Canadian Atlantic margins have mapped a segmented character and asymmetry in deep crustal fault structures similar to those noted in the East African Rift System. Crustal structure along the landward edge of the deep marginal basins of the IJ. S ~ margin is similar to the block-faulted structures of the amagmatic margins, but the seismic structure on the oceanic sidle of these basins resembles the magmatic margins. The similarities in structures among margins and their relationship to rift systems are emerging as unifying concepts in extensional tectonics. Theoretical Modeling of Subsidence and Thermal History in Sedimentary Basins and the Lithosphere as a Continuum Concomitant with-the measurement of large extensional strains of the continental lithosphere in areas such as the Basin and Range and the starved passive margin in the Bay of Biscay, a number of workers have-turned their attention to theoretical studies of the relationship between heat and mass transfer during crustal stretching and the syn- and post-rift vertical motion history recorded in sedimentary basins. Basin stratigraphic descriptions based on-multichannel seismic data and exploration drilling in regions like the U.S. East Coast margins and-the North Sea provided an extensive data base for the application of these models. The investigators developed a set of techniques whereby the sedimentary cover of the rift can be "backstripped,'t so that parameters, such as the relative contributions of crustal and mantle lithospheric thinning beneath the basin, the thermal evolution of the sedimentary cover, and the contribution of mass additions to the lithosphere can be predicted. The studies have produced a powerful set of techniques for constraining hypotheses of rift evolution, whose applications to a wide spectrum of new problems in rift mechanics (e.g., "simple shear" vs. "pure shear'' kinematics) have only begun in the last few years. In addition, recent continuum models that simulate lithaspheric strain in three dimensions make possible an examination of the nature of the forces that control extension, including those derived from the gravitational potential of the lithosphere itself, as well as those applied to the base of the lithosphere via plate interaction. 32
Isotope and Trace Element Analysis of Rift Voicanics and Xenoliths The development of combined isotope and trace element studies of rift magmas, in particular the application of neodymium and helium isotopic ratios as tracers for their sources, have added a new dimension to the quantification of mass transfer between enriched and depleted mantle, asthenosphere and old continental mantle lithosphere, and crust and mantle. These techniques, developed principally in the late 1970s and early '980s, have only now begun to be applied in detail to continental rift settings. We can now potentially constrain the time and place at which magmas are extracted from the mantle, the degree to which magmas are recycled lithosphere or new material from the mantle, and the interplay between volatiles in the mantle and the melting process. Although rifts are only one of a number of tectonic settings that can be studied petrologically, the new arsenal of techniques and the ever-increasing resolution and speed of analytical instrumentation foretell a rich harvest of data that will robustly test rift models deduced from independent data sets. Each member of the working group was generally aware of the contributions of the others. Although some cross-fertilization was evident (for example, Basin and Range field studies and continental reflection profiling were coordinated to some degree), most of these developments were accomplished on technique-specific projects rather than problem-specific projects. The barriers are well-defined and traditional: geology vs. geochemistry vs. geophysics, and marine-based studies of these three types vs. land-based studies. The working group felt that gains more impressive than those of the last ten years would be possible if only the community could achieve a higher level of communication among workers with diverse expertise tackling the same problem. The inevitable synergism of meshing what have been highly successful yet independent methodologies into single research projects could-thus be exploited. Below, we address the working group charges from a technical perspective that crosses the boundaries apparent from the nature of the recent advances discussed above. THE SINGLE MOST IMPORTANT SCIENTIFIC OBJECTIVE The formation of sedimentary basins by extensional processes in a variety of tectonic settings is a fundamental expression of deformation on a lithospheric scale. The thermal and mechanical properties of the lithosphere before , during, and after extension are the key factors that determine the physical shape of sedimentary basins and the stratigraphic pattern of sediments that infill them during and after rifting. The Reformational 33
history and resulting crustal structure of such basins also profoundly influence the structural fabric of collision zones, whose interpretation can therefore be greatly facilitated by an understanding of the basin forming processes. Quantitative models of basin evolution require an understanding of the kinematics and dynamics of basin-forming processes at scales from crust to lithosphere. Thus, the single most important scientific Objective is to understand how the thermal and mechanical evolution of rift systems at crust to lithosphere scales controls the variability of continental margins in space and time. The objective is unlikely to be achieved without accepting as a prerequisite that basin forming processes and their expression in the lithostratigraphic record are intimately related. Consequently, a synergism that exploits wider, common use of ostensibly disparate fields in earth science is demanded. To understand the thermal and mechanical evolution of rift systems at crust to lithosphere scales, the working group participants defined three key problems that can be addressed by the study of specific extensional processes. 1. Determine the distribution of three-dimensional strain in the lithosphere. Processes requiring examination/quantification: a. Low-angle normal faulting in the crust. b. The interplay between extension and the rheologica~ stratification of the lithosphere. c. Structural and magmatic segmentation of rifts in the context of the mechanics of rifting and heterogeneities of-the lithosphere. 2. Determine the causes of compositional, temporal, and spatial variations in magmatism and degassing. Processes requiring examination/quantification: a. Vertical and lateral magmatic transport. b. The relationship of the geochemistry of igneous rocks to the thermal and mechanical evolution of rifts. c. The transition from rift to r idge magmat i sm . d. Mobility of the crust-mantle boundary during rifting. 3. Determine the cause for spatial and temporal variability of uplift and subsidence in rift systems. Processes requiring examination/quantification: a. The timing and rate of vertical motion. b. Isostasy during and after rifting. 34
c- Uplift and subsidence as an independent constraint on the overall structure of the rift. e The interplay between rift structure, topography, erosion, and deposition. NEEDED STUDIES In the text that follows, the fundamental problems outlined above are described together with key studies needed to examine and quantify the underlying processes. The discussion follows the outline format, although no particular hierarchy is intended. Determine the Distribution of Three-Dimensional Strain in the Lithosphere Low-Angle Normal Faulting in the Crust One of the most exciting problems in extensional tectonics is the mechanism for producing low-ang~e (dips less than 30 degrees) faults. These structures may be the result of active low-angle fault motion. Rock mechanics theory and earthquake focal mechanism studies are, however, at odds with active slip on low-angle faults. This paradox has led some authors to propose that all low-angle normal fault structures result from rotation of normal faults that were active at a high dip angle. This view, however, challenges that of active slip and shallow initiation angle held by most workers studying metamorphic core complexes. A well-defined set of observations is capable of resolving this controversy. They should include a series of- thermochronologic measurements-on rocks from the lower plate (footwall) of well-mapped low-angle normal fault systems. The dating of such a series of samples holds the potential for determining the rates of vertical motion and rotation of the lower plate rocks and constraining the dip of active fault motion. Such measurements must be combined with structural geologic mapping and finite strain studies of rocks deformed by normal faulting._-Detailed synrift sedimentary records of such zones would provide crucial independent information about the deformation history. Paleomagnetic studies may offer a separate method for constraining any rotation of the lower plate rocks. These field and thermochronologic studies should be complemented by rock mechanics experiments aimed at constraining the properties and deformation mechanism of faulted rocks e In addition, several potentially active low-angle normal fault systems in the Basin and Range (e.g., Sevier Desert detachment and Panamint Valley fault zone) offer the potential for in situ stress measurements adjacent to low-angle normal faults at depths 3S
of a few hundred to a few thousand meters. Such measurements would go a long way toward resolving the mechanical paradox. The Interplay Between Extension and the Theological Stratification of the Lithosphere The rheology of the lithosphere almost certainly controls the restyles of rifting at a margin. The term, style, encompasses the width of the extending margin, the spacing of faults, their degree of rotation, etc. The process of extension also should affect the rheology of the lithosphere during rifting. For example, it is possible that the flexural rigidity of the lithosphere will be reduced as a result of temperature changes, stress state, and strain resulting from extension e This in turn will af feet how strain is localized in the evolving rift. Quantifying the effect of rheology on the style of extension requires that a variety of rifts be studied where geochemica1, heat flow, or other data can be used to infer the average thermal state of the pre-rift ~ ithosphere. Temperature should be a primary control on rheology. We expect to observe systematic variations in rift style as a function of inferred ~ithospheric temperature. Predictive numerical models are needed to elucidate the parameters of extension that control the style of rifting. The models must combine elastic and viscous behavior in studying finite strains resulting from extension. -Software has been developed to deal with aspects of this problem, but many aspects, such as three dimensionality and temporal changes, are not included in the models (for example, faulting of a brittle layer). In the next few years, the models should be developed to the point where they provide useful insights into the link between rheology-and rift structures. Structural and Magmatic Segmentation of Rifts in the Context of the Mechanics of Rifting and Heterogeneities of the Lithosphere A fundamental property of rift systems is variability in structure and geometry between and within them. Some studies (e.g., East African rift) suggest that-certain aspects of rift segmentation are constant (e.g., lengths of rift segments) and others are not (e. g., polarity of half grabens) . A similar segmentation and fault polarity reversal has been recognized on the U. S . East Coast margin. Key questions are: - Does such segmentation ref lect fundamental aspects of how the ~ ithosphere stretches? 36
- Does segmentation reflect geometric control of preexisting structural fabric? - Does segmentation occur at the same scales in different rift systems? Do scales of segmentation change as the rift evolves? Is segmentation comparable on conjugate margins? We need to describe this segmentation in a variety of rift systems such as young rifts, mature rifts, magmatic and amagmatic rifts, and conjugate margins, while simultaneously relating segmentation to other properties of the lithosphere. Many of the data required to characterize segmentation are obtainable with current technologies, but the important aspect is to combine technologies into an integrated, multidisciplinary approach that will achieve the goal of understanding why segmentation occurs. To this end the suite of observational measurement suggested includes: I. Geologic field mapping and sampling to define segmentation and the regional geologic framework (erg e ~ fault geometries, stratigraphic relations, pre-rift structural fabric, etc.~. - 2. Multichannel reflection profiling to define the three dimensional geometry of rift segments and details of accommodation zones separating segments. 3. Wide angle seismic reflection and refraction data (including Vp and Vs data) to define velocity structure (Ethology ?) and thereby determine whether (how?) surface segmentation translates to depth. It is imperative that this data be collected coincidentally with data from (2) above. 4. Seismicity studies in active rifts to define the geometry and deformation of faults that bound (and presumably control formation of) rift segments, and to determine the configuration of the underlying deep mantle. 5. Stratigraphic or geochronologic data from wells or outcrops to constrain the temporal evolution of adjacent rift segments. 6. Landsat and radar imagery, where appropriate, to map regional structures within and outside the zone of extension in subaerial systems, and side-Iooking sonar mapping of submarine systems. 7. Potential field studies (e.g., gravity, magnetics) to constrain the distribution of crustal and mantle rocks beneath rift segments. 8. Geochemical and petrologic analyses of magmatic products. 9. Heat flow and conductivity studies. The studies should occur on young rifts, old starved rifts, old sedimented rifts, magmatic rifts, non-magmatic rifts, and conjugate margins e To complement the observational data, numerical models must be developed for predicting the regular 37
segmentation in places like East Africa. Clay modeling, for example, has not been able to reproduce observed segmentation and changes in he-1 f graben polarity. Determine the Causes of Compositional, Temporal, and Spatial Variations in Magmatism and Degassing Magmatism is so intimately tied to extensional tectonics that an understanding of lithospheric extension is incomplete without a full description of attendant magmatism. Any rift that culminates in seafloor spreading has evolved into a dominantly magmatic system. While virtually all rifts exhibit some evidence of magmatism, the extent of the magmatic contribution is greatly variable, both in the early intra-continental stage and in the final stage resulting in the continental margin and seafloor spreading. The degree to which this variability is related to ---differences in structural style is poorly understood. For example, the temporal relationships between extensional tectonism and magmatism are currently unclear, so we have a poor understanding of whether a stress-driven tectonism induces changes in the mantle that result in magmatism, or whether a mantle perturbation that leads to melting and migration of magmas alters the Theological properties of the lithosphere to such a degree that extension occurs. That is, we have little notion of whether magmatism is the passive response to' or the driving force behind, extension. Vertical and Lateral Magmatic Transport The distribution of magmatic products in space and time throughout an evolving extensional system is one of the most puzzling aspects of rift magmatism. - A fundamental process invo lvec] in th i s distribut ion is the f l ow o f mantl e rocks and the associated processes of melt extraction and migration. Numerical and analytical models of these processes have been created for the particular setting of a mid-ocean ridge spreading center, where a variety of flow mechanisms has been investigated, including non-hydrostatic pressure gradients associated with plate separation, buoyancy-driven circul ation and, most recently, the effect of viscous breakdown due to melting. Consideration of these phenomena in an evolving rift system has been only a minor component of most descriptions of extension, yet it must be of critical importance. There is an urgent need to extend what has been learned from numerical model ing o f f ~ ow and melt migrat i on at spreading centers into the more complex intra-continental rift environment to develop physical models of magmatism that include parameters describing rift geometry, separation rates, initial thermal structure, etc. 38
Mantle flow patterns can be directly mapped by determining seismic anisotropy in three dimensions using dense arrays of seismometers in a region surrounding an active rift. Ideal areas include the Aegean Sea, Red Sea, western Woodiark Basin, Gulf of Cal if ornia, anti the Salton Trough. In active rifts, the distribution of melts can be identified by significantly lowered velocities, especially shear wave velocities. Tomographic inversion of three dimensional seismic array data from active and pass ive experiments can be used to obtain these data . The Transition from Rift to Ridge Magmatism A precise known edge of spatial and temporal red ations between extension and magmatis~a ranging from the earl lest stages of intra-continental rifting to mature seafloor spreading will shed light on the issue of whether the magmatism is solely a passive response to the extension of the lithosphere (i.e., decompressive melting of the upper mantle) or whether there is "active" upwelling of hot mantle intrudes the overlying lithosphere and localizes subsequent extension. Another desirable.objective is to understand the changes in the thermal structure and composition of the upper mantle during the evolution of rift systems. These changes of mantle thermal state and composition determine the distribution in rifts of igneous rocks, both plutonic and volcanic, and their composition. To achieve this objective, the following work is required: 1. Systematic along-strike sampling should be carried out on volcanic rocks in rift systems that are inferred to be at different stages of evolution. An example of a variably evolved rift system is the East African/Afar/Red Sea/Gulf of Aden system, which ranges from a continental rift (E. African rift) to an oceanic rift (Gulf of Aden) with transitional rifts In between (Afar and Northern Red Sea). Sampling of lower crustal ~ gabbro i c ~ and upper mantI e ~ ul tramafic) rocks should also be carried out whenever possible, either from xenoliths or from tectonically uplifted bodies. 2. Systematic analytical programs should be employed to identify magma types, differentiation histories, and depth and extent of melting in the upper mantle. In addition, geothermometry and geobarometry should be attempted whenever possible' particularly of gabbroic and ultramafic rocks. This analytical program should include determination of major and- trace elements, isotopic chemistry, and absolute age. 3. The studies must be integrated with structural and thermal data as obtained by seismic refraction and reflection, gravimetry, magnetometry, heat flow measurements, tomography, etc. Seismic studies are especially important in imaging the middle and lower crust; in particular, detailed reflection and refraction studies of complex three dimensional features 39
(plutons, lava sequences, magma chambers, etc.) would be useful. Seismic tomographic studies of magmatica1ly active regions could provide important constraints on-the mechanics of melt movement. All these studies must have appropriate lateral and vertical resolution. 4. Mantle degassing and related metasomatism are inferred to be very significant in rift systems. These processes can have a strong effect on the conditions of melting of the mantle, and can cause enrichment of incompatible elements in the crust. Mantle degassing can be investigated by determining the composition Of fluid inclusions in plutonic rocks, and by determining He/He, CH4 and H2 in hydrothermal springs being discharged along rift systems. 5. The furtherance of our understanding of the relationship between rift processes and igneous processes involves continued advances in theory, numerical modeling, and technological skills. For example, to test existing models of magma genesis in rift systems, we must utilize methodologies that adequately handle the often weathered rock record. Improved laboratory measurements of Vp and Vs in a variety of igneous samples at a range of confining pressures and percentage partial melt are needed to improve our understanding of velocities identif fed in the crust and upper mantle by the indirect seismic techniques employed. Theoretical and analytical advancements in seismologic studies are needed to image complex structures-better, snd laboratory techniques must be improved to allow for better understanding of lower crust and sub-crustal events. Further developments in fluid dynamics are needed to better understand the movement of melt through the lithosphere. Mobility of the Crust-Mantle Boundary During Rifting The suggestion that the Moho may somehow restore itself continuously during rifting, such that its depth following extension differs little from that before extension, implies processes involving exchange between the crust and mantle materials in an unknown manner. An important aspect of this problem is the relationship between the seismic Moho and the petrologic Moho. To constrain this phenomenon, if indeed it occurs, we need to obtain accurate measures of the depth and nature of the Moho in extensional systems at various ages of development, together with detailed characterizations of crustal structure above Moho. Rifts exhibiting different crustal extension factors and different tectonic styles must be included. One possible mechanism for Moho restoration is infilling by magma in the form of sills and plutons that were derived by decompressional melting during lithospheric extension. This process, however, requires that the precise amount of melt is generated to completely fill the crustal anti-root caused by extension. Numerical modeling to 40
constrain melt vol ume in relation to degree of extension is needed to test the viabi] ity of such a process . I f magmatism is the cause -of the Moho regrowth, one might al so anticipate that some surface volcanism woul ~ accompany the ~ owe r crustal plutonium, the petrology of which should indicate patterns of fractionation associated with the plutonic complex . Systematic sampling and anal ysis of rift voicanics are needled in addressing this probleme Determine the Cause for Spatial and Temporal Variability of Uplift and Subsidence in Rift Systems One of the primary measurable results of the rift process is the vertical motion caused by perturbing the lithosphere. The nature of sedimentary deposits accumulating just prior to, during, and after rifting serves to constrain various aspects of rift dynamics. Depositional sequences and erosional surfaces within and adjacent to the rift record vertical motions' constraining their timing and rate. Pre-rift crustal units, primarily sediments, provide a primary baseline for estimating pre-extensional crustal conditions. Syn-rift deposits provide a spatial and temporal record of variations in short-wavelength subsidence patterns caused by normal faulting, and long-wavelength patterns that reflect processes deeper in the lithosphere. Finally' the post-rift deposits record vertical motions of the crust after rifting, which are controlled principally by the cooling of the lithosphere and associated changes in its mechanical properties. Because processes of strain and magmatism (discussed above) are intimately linked to the timing and rate of vertical motions, it is imperative to learn as much as possible about such vertical motions to constrain hypotheses of rift dynamics. The Timing and Rate of Vertical Motion To constrain the timing and rate of uplift and subsidence, we need high-resolution stratigraphic studies of the pre-, syn- and post-rift deposits. This can be obtained via a combination of field mapping and drilihole sampling of partially exposed systems, meshed with detailed seismic reflection-refraction grids set out with the primary goal of-calibrating the seismic information with characteristics of each of the major classes of deposits. The geological characterization of geophysical signatures in various portions of rifts is an important first step in developing detailed subsidence history. Development of comparative stratigraphic sequences between exposed rift basins (eeg., U.S. East coast early Mesozoic rift basins) clearly points to the possibility of establishing a calibrated comparative seismic stratigraphy between buried rift basins offshore. This would enable us to establish relative timing constraints within 41
buried-rifts without drilihole data, although Armhole data will be essential in a number of areas to establish absolute age control on uplift and subsidence. Geophysical studies must be complete enough to permit examination of both short and long wavelength variations in depositional pasterns e The offshore drilling capability of both the ODP and the petroleum industry should be an essential part of conjugate margins rift system programs that can be integrated with high resolution seismic studies. An important but often neglected aspect of examining overcores is their thermal history: in particular, time-thermochronometry using f ission track and Z0Ar/39Ar techniques. We suggest that these analyses be routinely used for overcore samples. Isostasy During and After Rifting The primary control of the vertical motion history during and after rifting is isostatic equilibrium. In particular, a maj or factor in determining variations in subsidence is the degree to which density contrasts are accommodated locally or regionally. For example, as the lithosphere progressively cools after rifting, its flexural rigidity may increase substantially, which has major implications for the geometry of post-rift- sedimentation. During rifting, observations from regions such as the Basin and Range Province suggest that relatively short-wavelength arches form on mid-crustal detachments,- apparently driven by isostatic rebound of tectonically denuded crust. -The interplay between buoyancy and the apparent fiexural strength of the lithosphere-is a rapidly developing field of investigation in rift systems. Progress in elucidating vertical motion histories will also come from detailed gravity surveys over key portions of active rifts, accompanied by modelling studies. Of particular importance for most models is determining the degree to which isostatic compensation for various features is local or regional, using various admittance techniques on gravity spectra. Uplift and Subsidence as an Independent Constraint on the Overall Structure of the Rift -While detailed mapping and seismic imaging of the upper crust can constrain its structural history, deeper crustal phenomena are more difficult to constrain directly with these methods. The vertical motion history, however, is generally highly sensitive to the redistribution of mass at deep levels in the lithosphere by strain and magmatism. Any model or proposed study of a rift must exploit this relationship. The fact that thinning the mantle part of the lithosphere results in uplift, while thinning the crustal part causes subsidence, permits the 42
relative contribution of extension in each layer to be assessed using uplift and subsidence data. Thus, model studies that generate synthetic subsidence histories should play an important role in the interpretation of deep seismic, gravity, and xenolith data. Subsidence modeling is in effect the glue that unites many disparate data sets, and it must play an interactive role in their simultaneous interpretation. The Interplay between Rift Structure, Topography, Erosion, and Deposition The inf luence of upper crustal structure on the geomorpho~ ogy and sedimentation within rift systems is an important problem that is still poorly understood. For example, it is typically thought that an influx of coarse detritus into a rift valley succession signals an episode of tectonism. In some asymmetric half-grabens, however, the region near the fault scarp subsides so rapidly that coarse detritus is unable to prograde into the valley as alluvial fans, and the sedimentation near the scarp is largely evaporitic. Paradoxically, only when faulting ceases does coarse detritus appear in the basin near the fault scarp. These and other problems are crucial for understanding how to interpret the rock record at a very basic level. There is important feedback in cause and effect with respect to sedimentation and tectonism. Where sediment supply is abundant, rift valleys fill more rapidly and the footwalls of bounding normal faults are less readily deformed compared to areas where sediment is in short supply. Calibration of sedimentary and geomorphic responses to rifting is thus an important factor in studying rifts. Studies of these phenomena in active rifts must be carried out. 43
