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6
PASS IVE MARGINS: GROUP 3
DIVERGENT CONTINENTAL ~GINS--
POST-RIFTING INTERNAL PROCESSES
SUGARY
Post-rifting divergent margin basins are the principal low
to moderate temperature chemical factories on our planet. It
primarily in these basins that erosional debris from the
continents accumulates with organic debris from the oceans and
slowly "cooks." These chemical reactors produce massive
mineralogical changes in the basin sediments, most of the worId's
reserves of oil and gas, and mineral deposits. It is important
scientifically, economically, and environmentally that we
understand how these giant reactors operate.
Because fluid flow is fundamentally involved in almost all
the reactor processes and products, the single most important
scientific objective is to understand the causes and interactions
that control fluid flow in post-riftina divergent margins.
The processes involved are principally the mechanics of
sediment accumulation, faulting, and other kinds of tectonic
remobilization (such as diapirism), and burial diagenesis.
As with any chemical reactor, the critical problems or
principal steps required to understand sedimentary basins on
divergent margins are: (1) characterization of the reactor at
various instants of time, particularly the fluid and chemical
_ _ ~
fluxes within and across the boundaries of the reactor;
(2) documentation and interpretation of the cumulative products
of the reactor over appropriate intervals of time; and
(3) construction of a process model capable of simulating and
predicting the operation of the reactor.
The studies that are needed are case history investigations
carried out with a view toward the refinement and testing
process model of margin reactors that joins tectonic,
stratigraphic, structural, chemical, and fluid flow processes and
accounts for their interactions.
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The strategy required is the proper coordination of process
model development and case history investigations. Process model
development and case history investigations should proceed in
parallel. Case studies addressing, first, present fluid and
chemical fluxes in basins, and then the integrated effects of
such fluxes as they have operated over the history of basin
development appear to be particularly pertinent.
We believe that sedimentary basins on divergent continental
margins are especially amenable to effective study at the present
time because highly evolved and complimentary land- and sea-based
technologies and observations can be merged today in these
locations. At least partial joining of land and sea
investigations on administrative and funding levels could produce
dramatic scientific returns. Post-rift divergent margins are a
good place to initiate such coordination because they are
geologically simpler than convergent margins, and because land
and sea are tectonically contiguous at divergent margins.
BACKGROUND
The diversity and scale of chemical processes that occur in
divergent continental margins are impressive.
At the top of a basin, bacteria consume organic matter and
produce CO2. As the sediments are buried and move through the
normal geothermal gradient,-they are warmed and as a result they
slowly "cook." Especially within and below the "oil window" at
60 to 90° C, the organic matter "matures 'I to produce hydrocarbon
fluids. These reactions have positive ~ V and consequently are
expelled from their host strata in a process called primary
migration. Inorganic reactions that expel water also occur at
about the same temperatures. The principal inorganic reactions
involve the progressive crystallization of clays (e. g., the
tranformation of smectite to illite).
Thermal expansion of pore fluids and positive TV reactions
such as mentioned above cause large volumes of divergent margin
basins to become "overpressured" below about 800m depth. Fluid
pressures approach lithostatic values in the Gulf of Mexico and
the Niger Delta, for example. However, the evolution of fluid
pressure and fluid flow within the overpressured zones is not
well understood. There is some evidence that specific subzones
within a basin behave as independent flow systems (pressure
bottles or bladders), but it is not understood how the
transition zone between "hard" overpressure and hydrostatic
pressure evolves so as to remain at a depth of -80Om, and the
implications of this evolution for chemical diagenesis of the
entire basin are not clear. Transition zones are important sites
of hydrocarbon entrapment in some basins (e.g., the top of
geopressure in the Gulf of Mexico) and not in others (e.g., the
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North Sea ~ ~ There is some suggestion f ram base metal
mineralization in salt domes and in Mississippi Ball By-type
lead-z inc deposits at basin margins that large, overpressured
regions in sedimentary basins episodically rupture and t' squirt"
brines out escape structures on margin edges . If true, such
sul f ide accumulations could tell us a great deal about the f luid
dynamics of basins.
Fluids moving within divergent continental margins often
precipitate minerals on preexisting sediment grains or dissolve
these grains. This chemical alteration can completely plug or
completely remove entire sedimentary units. Mineral overgrowths
are referred to as "cements" because they tend to cement the pore
space and change loose sediments into competent rock. Careful
petrographic, chemical and fluid inclusion studies of cements,
particularly in cases where "cement stratigraphy'' or identifiable
zoning in the cements al lows a sequence to be deduced, provide a
record of mineral precipitation from basin pore fluids and thus a
record of fluid movements within the basin.
Fluids escaping from divergent sedimentary basins can
produce dramatic effects, particularly at sea. If the fluid
expulsion is rapid enough, near surface temperatures and heat
fluxes may be affected. Gas hydrates accumulate in the upper
sediment prism of many continental margins where methane and
other small gases combine with water to form a crystalline solid
that fills the interstitial pore space. Hydrates form under
conditions of low temperature, high pressure and high gas
saturation. They naturally break down at depth in the sediment
column because of geothermal heating. The base of the hydrate
layer thus migrates through the sediment column as sedimentation
continues. Pressure changes associated with changes in sea level
also cause the hydrate boundary to migrate. Such migration may
cause the generation of fluid overpressures and promote strain
(faulting) at the base of the hydrate layer. Progressive
migration of the hydrate zone through the upper sediment column
alters the fabric of the strata in ways that could profoundly
change the porosity and permeability structure of whole sections
of a margin.
Perhaps the most dramatic consequence of fluid venting is
the chemosynthetic-communities it supports. The first
chemosynthetic communities in the deep sea were discovered at
hydrothermal vents at mid-ocean ridges. The communities live off
the oxidative contrast between reduced vent waters and oxygenated
seawater. In the last few years chemosythetic communities have
alto been discovered in continental margin settings. The first
site discovered was along a fault zone off San Diego. Later,
abundant communities were found around saline seeps at the base
of the Florida Escarpment. Since then, communities have been
found on accretionary prisms off Oregon and Japan, around oil
seeps in the Gulf of Mexico, around slump scars off Nova Scotia,
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and in. several submarine canyons. Along one careful ly surveyed
2 On long segment of the Florida Escarpment, chemosynthetic
communities ~ ine ~10% of the escarpment base. Major box canyons
developed in carbonate strata in the surrounding area may have
f ormed as the result of rock dissolution associated with pore
water discharges at the canyon heads. Sing ar canyons are common
along the eastern margin of North America.
THE S INGLE MOST IMPORTANT SCIENTIFIC O=ECTIVE
The above discussion shows that a rich variety of recent
observations bear on f luid movements and chemical processes in
post-rift continental margins. The problem of comprehending and
describing post-rifting changes in cli~rergent continental margins
can be usefully focused by the important parameter of fluid flow,
because fit uid flow is directly involved in many of the changes
and refit ects nears y al ~ the others. We are really dealing with a
giant, natural, fixecl-bed fluid chemical reactor. The single
most important scientif ic obey ective for post-rift divergent
margins can be stated as understanding the interactions that
control fluid flow and its chemical ant] tectonic consequences as
a function of time.
Processes Involved
A large number of processes interact to control f luid f low
in post-rift divergent margins (and post-rift sedimentary teas ins
in general ~ . These are illustrates} schematically in Figure ~ .
Cal
KgedImentatio~
Folding
Faulting, Fracturing, Jointing
Salt
Heat Flow
Fluid Generation
Rock Alteration
metamorphism
diagenesis
Seal Formation
gas hydra~ces
C~3
Overpressure
- Density Differences
salinity
therma1
Topographic
FIGURE 1 Interrelated processes controlling fluid flow in
divergent margin teas ins .
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The movement of pore fluids in a sedimentary teas in is driven
by a number of mechanisms, but principal ly by the fluid
overpressures produced by compaction, heating, and diagenetic
reactions that occur as the sediments are buried. The geometries
and grain size of the sediment packages are the first control s on
pernT eabil ity and thus on the movements of fluids, but as the
margin evolves, faulting, fracturing, j Dinting, and salt and mud
do apiri sm impose additional control s. The movements of fluids
nemselves cause permeabil ity modi f ications through chemical
alteration. All these processes exert first order infix uences on
fig uid movement. None can be disregarded!.
Critical Probe ems
There are two especially critical problems : ( 1~ visual-
izing the effects of the operation of such a large-scale
thermo-chemical-structural reactor; and (2) developing an
integrated process model that can describe the operation of such
a reactor. The steps required to address these problems are: (~)
definition of the present distribution of fluid flux in divergent
margin basins (fluid flux snapshots); (2) definition of the
cumulative effects of past fluid fluxes; and (3) development of
the interpolation and process models required to interpret and
understand the observed fluid fluxes and their products.
It is difficult to collect and synthesize data in a million
cubic kilometers of sediments so that the interlinked processes
that affected them can be seen. Furthermore, interrogating such
a volume of rock and determining the cumulative products of
reaction is a daunting prospect. Unusual audacity is required to
build ~ model that incorporates and integrates all the diverse
processes that influence operation of the reactor. Nonetheless,
it is our view that progress in understanding post-rifting
processes in divergent margin basins can best be promoted by
attempting just such a large-scale, process-oriented synthesis.
With regard to process modeling, each of the individual
processes circled in Figure 1 are relatively well understood.
What is particu~arly-needed at this point is a quantitative
understanding of the linkages between individual processes, and
the formulation of an integrated model that can describe
post-rift internal processes in divergent continental margins as
a whole.
NEEDED STUDIES
The studies required to meet our principal scientif ic goal
of understanding the interactions that control f luid f low and
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its chemical and tectonic consequences as a function of time can
best be performed as a series of case studies that:
1. define the present flow rates crossing basin surfaces,
2. define the cumulative products of fluid flow in
representative rock volumes (e.g., the total fluxes),
3. develop methods to extend geographically sparse data to
geologically reasonable volumes, and
4. integrate components of a numerical model (Figure 1)
that will eventually be capable of simulating the history of
fluid flow and chemical reactions in a sedimentary basin.
Obtaining Snapshots of Fluid and Chemical Far uses
in Divergent Margin Basins
Understanding the present rates and directions of fat uicI flow
In divergent margins is the most direct approach to determining
overall flux rates and the evolution of fluid flow through time.
We strongly endorse studies that measure or def ine the
velocities of fluid flow across any surface within a given basin.
Studies that determine average flux rates through 100 to 10,000
square meter areas should be especially encouraged.
Several examples of projects that might merit special
attention are:
Fluid Seeps on Flanks and Toes of Divergent Margins
Mapping chemosynthetic community distributions could provide
a measure of the present-c3ay fluid efflux from divergent margins
and is an example of a basin survey that can only be made at sea.
Geopressured Fluid Flow
Much could be learned from comprehensive measurements of
the present pressure distribution within an active overpressured
basin.
Heat Flow
Surface heat flow surveys on continental margins constrain
models of fluid movement. To filter out the effects of seasonal
bottom water temperature variations, surveys using shallow
boreholes or a deeper insertion of temperature and pressure
probes are needed. Heat flow surveys within basins could address
the question of whether fluid flaw between the underlying
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basement and the overlying sediments is an important agent of
mass transfer over time, or only in the initial stages of a
basints evolution. Heat flow surveys within a basin could reveal
the rate and distribution of fluid movement through pressure
seals.
Determining present fluid fluxes in divergent margins will
require substantial resources. As indicated in Tabs e 1,
mapping of submarine seeps wil ~ require ship time, and
sophisticated instrumentation like deep tow sidescan sonars with
photographic calibration. Submersible time will al so be required
to obtain reliable samples of subsurface fluids. Much of the
data on geopressured areas have been collected by the petroleum
industry, and strong academic-industry ties wil ~ be required! to
make use of this important database . Better heat f l ow and pore
pressure measurements will require modification and development
of new technologies to penetrate below the zone of biannual
fit uctuations.
Determining the Cumulative Effects of Fluid
and Chemical Fluxes in Divergent Margin Basins
The previously suggested studies would provide a current
snapshot of fluid movements and chemical fluxes. Studies of the
cumulative products of diagenesis and fluid flow would
potentially provide a record of all post-rifting alteration and
fluid flow. We encourage any study that can measure the
cumulative consequences of sustained fluid flow. Constraints
should be sought on the interval of time over which particular
phases of alteration takes place, so that chemical reaction rates
and chemical fluxes can be deduced and compared with the
determinations of the snapshot studies. Establishing
chrono~ogic constraints is especially important.
Examples of possibly significant integrated studies include:
Diagenetic Cements
Studies that determine the volumes and spatial
distributions of diagenetic products, and the timing of chemical
reactions would provide a direct record of movements of basin
pore fluids. The main challenges to such studies are: (1) the
labor intensiveness of the required petrographic, chemical and
fluid inclusion analyses; (2) difficulties in properly
extrapolating from local drill hole analyses to obtain estimates
of the average alteration over a broader region; and (3) problems
dating specific cement "strata." New chemical logging
technologies may facilitate both data collection and
extrapolation and help solve the first two of these challenges,
especially if selected core is available to calibrate the data
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TABLE 1 Required Resources for Studies of Fit uid F1 ow in
Divergent Margins
Seafloor surveying ant} sampling
1. Deep-tow surveys.
2 ~ Submers ibles-sampl ing e f f luents .
3 . Flux measurement devices and experiments e *
4 . Longer heat flow probes (10-20 m ~ ong) to
avoid biannual temperature wave on shed yes,
instrumented with pore pressure capability. *
5 . Geophys ical surveys to image stratigraphy and
alteration. cletailec] 3-D seismics.
Fat ux
Snapshots
Ef fects of
Cumulative Fluxes
Interpolation
and Process
Models
Downhole devices
1. In-situ pore water samplers. *
2. Pore pressure anti rock stress surveys.
3 . Downhole flowmeters; tracer tests at inj ection
wells.
4. Wirel ine re-entry tools. Long-term
observatories and/or repeated sampling. *
5. Core acquisition.
6, Logging (routine and special tool s*, detailed
cumulative surveys).
Other resources
1. Technology to date alteration minerals,
strata, anc} f luid inclusions with higher
revel ution. ~
2 . Access to case histories and/or -pi 1 ot tests 0 f
sparse data interpolation methods.
3. Access to supercomputer facilities; software e*
Models workstations for database/model
comparison.
4. Physical and chemical models of basin
processes (tectonic, structural,
stratigraphic, diagenetic)~*
* requires new technology and/or development
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petrographically and stratigraphically, and augment it by
providing paragenetic information. Current methods of
determining absolute ages of cement strata are only applicable in
very restricted instances, but there is hope that these methods
may be broadened. More generally applicable cement dating
techniques are urgently needed.
Gas Hydrates
Hydrates may provide a direct measure of the f flax of
methane, carbon dioxide, and other gases from large portions of
continental margins. Possible interactions between fluctuating
sea levels, gas generation at the base of the hydrate layer, and
detachment fault ting promoted by fluid overpressures illustrate
the kinds of interactions between fluid overpressure and
tectonics that also occur at the larger, whole-basin scale
(Figure 1~. Studies of such slumping could provide valuable
insights into much larger scale basin phenomena.
Hydrocarbon Maturation
Hydrocarbon maturation is controlled by the time-temperature
history of the source rock. Predicting this thermal history is
of critical importance to successful hydrocarbon exploration.
The existing indicators of time-temperature history (vitrinite
reflectance, thermal alteration index, 40Ar/39Ar, fission tracks,
hydrocarbon isomerization and aromatization) need to be extended,
carefully calibrated, and broadly debated and reviewed. Spatial
and temporal variations in thermal maturation relate to the
integrated values of many of the fluxes discussed above.
Maturity variations may directly ref lect integrated thermaal
ef fects of fluid flow, although conductive heat transfer usuall
dominates.
The three approaches to measuring integrated effects of
fluid flow sketched above illustrate the potential diversity and
richness of various measures of integrated flux. They are
illustrative only. We anticipate that many approaches will be
possible. The approaches listed require the use of conventional
tools as well as-the development of new tools and methodologies,
as indicated in Table 1.
Developing Integrated Models to Simulate Fluid-Chemical
Processes in Divergent Margins
Snapshot and integrated fluxes measured must be interpolated
and simulated by process models. Local interpolation models are
needed to convert borehole measurements, chemical logs, or
seismic images to measures of alteration over relatively large
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(~lOOm~ x ~100 m) volumes of sediment. Integrated process moclels
are needed to bring data from different basins to bear on common
problems of understanding basin reactors.
Local Models
Sectimentol ogists are enjoying considerable success with
''dif fusion" models that simulate how highlands erode and
depressions fit 1. Moclels of sedimentation that take into account
the geometry of lakes and seas, ant] the effects of storm wind
stress on currents r erosion, and sedimentation are remarkable in
their abil ity to s imulate observed sediment patterns . What now
needs to be done is to cieter~ine the permeability impl ications of
these models so that measured alteration can be extrapolated
taking into account the paths of likely fluid movement.
Structural geologists have long recognized a scale
invariance which they have encapsulated in Pumpelly~s Rule that
small-scale structures reflect large-scale structures. Minor and
major structures exert a tremendous influence on fluid flow, and
it is vital to develop the means to describe=the average effects
of structures of all scales. Tracts theory offers the
possibility of doing this, and it is an actively developing
field.
Salt domes and tongues are manifestations of salt tectonics
which, in many divergent-margin basins, are the principal agents
of deformation. The impermeability of salt and its ability to
rotate and fracture surrounding rocks strongly constrains fluid
movement. Salt tectonics--have been simulated-realistically by
centrifuge experiments and finite element modeling.
., . . . .. ~
Local models and~modeling~approaches--can-be expected to be-
diverse. Numerical, analog, and statistical methods are all
needed. In addition, they need to be tested and calibrated in a
variety of case studies. -Material properties contributed by
local models and studies-will feed-directly into an integrated
process model. - -
. .. . .
The Integrated Process Model
The final litmus test of the understanding of any reactor is
the demonstration that an integrated process model can simulate
its operation. We cannot rest until we have developed such a
model for the economically most significant geological reactors
on our planet--sedimentary basins.
The components of the integrated mode] are shown in Figure
1, and they have been discussed previously. Models of virtually
all individual components have already been developed. What
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remains is to fuse these components together into a model of the
entire reactor* This can be done using supercomputers,
especially machines that allow parallel computation of large
three-dimensional models such as sedimentation, structural
deformation, and fluid, chemical, and thermal flux. We envision,
again, that this will be done in stages, with the first step
combining and testing pairs of models, such as structure and
fluid flow. Later models will stitch together and test model
pairs* Many pilot or case history analyses will be required. To
a considerable degree, the greater the number of independent
efforts that can be mounted, the faster and surer the progress
that can be expected. It is important to recognize that as
models are combined, each constrains the others. For example, if
sedimentological models predict premeabilities and temperature
distributions that, when combined with realistic fluid-drive
mechanisms and chemical alteration models, predict cement
distributions similar to those observed, we will be encouraged
that we are on the right track.
STRATEGY REQUIRED TO ACHIEVE GOALS
The most appropriate strategy for achieving the scientific
goal of understanding the interactions that control fluid flow
and its chemical and tectonic consequences as a function of time
is to fund a series of case studies that are loosely coordinated
around the objective of developing an overall process model.
Observational case studies will undoubtedly and very
properly occur on a variety of divergent basins, as exposures,
data availability, and investigator interest dictate. Case
studies should be connected by the common goal of defining,
through observations or models, the operation of post-rift
divergent margin basins as giant thermo-tectonic-structural-
chemical fluid reactors.
At the same time, it will be important to fund theoretical
or modeling efforts that provide a basis for interpolation of
observations and that work toward a fully integrated mode] of
basin processes. ~
Perhaps the most important need is for funds in support of
the interdisciplinary collaboration that is essential to
developing and testing both local and integrated models.
Individuals must be encouraged to address problems that lie
largely outside their specialties.
BROADER IMPLICATIONS
Study of the movement of overpressured fluids in divergent
margin basins, where resources of land and sea technologies can
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be brought to bear in an unusually effective fashion, can
contribute to the solution of a diverse set of important
geological problems.
For example, overpressured volatiles in magmas at convergent
marg ins in f luence the so ructura ~ evo lut ion o f intrus ions and have
prodll~::ed signif icant ore deposits ~ e . g ., porphyry copper , and
molybdenum and gold deposits) . overpressure al so allows
hydrocarbons to escape f rom the very impermeable, organic rich
beds in which they are generated. This primary migration is
followed by secondary migration into structural or stratigraphic
traps where economic quantities of hydrocarbons may accumulate.
Overpressure along faults may control earthquakes in ways not
currently understood. Finally, fluid overpressures are thought
to fundamentally control the shape and tectonics of accretionary
prisms. Progress in detecting, modeling, and predicting
overpressured fluid flow in divergent margins will contribute
directly to similar research efforts in convergent continental
margins where the processes involved are more complex.
SELECTED BIBLIOG~PHY
Abbott , D . H ., ~ . W . Embley I and M ~ A e Hobart . 19 8 S .
Correlation of shear strength, hydraul ic conductivity, and
thermal gradients with sediment disturbance: South Pass
region, Mississippi Delta. Geo-Marine Letters 5: 113-119 .
Bethke , C . M., W. J . Harrison, C. Epson , and S . P. Altaner .
1988. Supercomputer analysis of sedimentary basins. Science
239: 261-267.
Cathies, L. M., anc] A. T. Smith. ~ 983 ~ Thermal constraints on
the formation of Mississippi Valley-type lead-zinc deposits
and their implications for episodic basin dewatering and
deposit genesis. Econ. Geoff ~ 78: 983-1002 e
Paull , C. J., and A. C. Neumann. 1987 . Continental margin brine
seeps: their geological consequences. Geology 15 : 545-548 .
Talbot, C. J ., and M. P. A. Jackson. 1987 . Salt tectonics ,
Scientific American 2 SS ~ 8 ~ : 70-79
72
Representative terms from entire chapter:
divergent margins
