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EXECUTER STY
OVERVIEW
The geology of continents is to a large degree the geology
of continental margins. The margins mark the site where
continental crust is distilled from the earth's mantle and where
continents grow through plate interactions that progressively
incorporate these margins into the continental mass. Thus, much
of the continents consist of the remnants of ancient margins that
offer an extensive and detailed record of continental evolution
over time and space. This record is essential for understanding
processes of global change' its diversity, and its long-term
history.
.
Approximately 70 percent of the worId's population is
concentrated in the coastal borderlands, which geologists
recognize to be the present continental margins. ~
. . .
Virtuallv all
of society depends on the products of the vast chemical
reactors
that underlie the margins wick transform organic matter into
petroleum resources. Working against these benefits are a
plethora of geologic hazards, ranging from the obvious
destructive effects of earthquakes and volcanoes to subtle
threats of sea level change and climate modification.
The paradigm of plate tectonics developed in the 1960s
created a revolution in our understanding of both the present
ocean basins and the ancient continental interiors. It allowed
us to place continental margins in their proper kinematic
context. Its basic tenets have been conf irmed during two decades
of intense exploration of the earth using increasingly
sophisticated techniques. Plate tectonic theory r however, is
essentially a kinematic model describing the motion of rigid
plates on a sphere. It does not provide any direct insight into
specific processes of pl ate boundary interactions, and hence of
margin evolution.
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In an effort to better understand continental margins,
there has been a recent shift in emphasis away from
phenomenological descriptions to an approach that focuses on
modeling the fundamental physical processes associated with
margin evolution. The construction and testing of such models
involves the same sort of innovation that spawned the plate
tectonic revolution itself.
Seeing the potential for rapid and dramatic advances in our
understanding of the continental margins, the Ocean Studies Board
and the Board on Earth Sciences jointly convened a workshop on
continental margins. This report is the product of that
workshop.
The outcome of the workshop indicates widespread support and
enthusiasm for a new direction in margins research that would
focus on interdisciplinary studies of the fundamental processes
of margin evolutions Interdisciplinary studies, as a new
direction, are demonstrated by the working group reports, which
show a remarkable similarity in focus despite the wide diversity
o f top i as addres s ed and the degree of separation usually present
between investigators from different disciplines. Many of the
scientific problems that were identified, and their suggested
solutions, were common to several working group topics. These
similarities define a commonality of direction that belies
traditional boundaries based on discipline, geography, or
methodology. The principal reason for these similarities is that
many of the processes, such as deformation, seismic rupture,
fluid flow, magmatism, and sedimentation, are not unique to
either divergent or convergent margins. Instead they are
fundamental global processes that control the ways in which
continents grow and deform with time.
The Margins Initiative is proposed as a means of nurturing
this new direction. The primary scientific objective of the
initiative will be to develop programs aimed at understanding the
processes that control the initiation and evolution of
continental margins. The examples below, extracted from the
working group reports, highlight some of the processes that might
be studied as components of this initiative.
Mechanics of Faulting
Faults form a fundamental aspect of the architecture of all
continental margins, as well as extensional and contractional
orogenic belts that form within the continents. The mechanics of
faults, and low angle faults in particular, remain poorly
understood. For instance, high fluid pressures have often been
invoked to explain how thrust sheets overcome the large
frictional resistance to fault slip. This explanation is
compelling where fluid-saturated sediments and oceanic crust are
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thrust rapidly beneath the continental margins, but, observations
at some maj or strike-slip faults and ~ ow-angle normal fault ts
indicate that faults can have very ~ ow shear resistance in the
absence of anomalous fl uid pressures Another probe em is the
slip behavior of low-ang~ e faults: some faults are seismically
active, moving in a stick-slip fashion, whereas others appear
to slip in a steady, aseismic fashion. Some seismologists argue
that this behavior is controlled by plate parameters (erg., rate
of subduction, age of the plate), whereas rook mechanicians argue
that slip behavior is a consequence of the rheology of materials
in the fault zone.
Fluic] Flow
Fluid flow affects almost all geologic processes active at
continental margins. Continental margin sediments represent the
main low-temperature chemical reactor at the earth's surface.
The circulation of fluids in margin sediments is responsible for
the extraction, transport, and concentration of much of the
world's petroleum and mineral resources. High fluid pressure can
drastically reduce the frictional strength of rocks. Rapid
circulation of fluids can enhance pressure-solution and may
result in dissolution and removal of a large portion of the
deforming rock mass. Water from compaction of sediment (both
tectonic compaction at subduction zones and depositional
compaction at continental margin basins) and from subduction of
hydrated oceanic crust is released at at estimated global rate of
1.5 km /yr (assuming a density of 1 g/cm ). Thud these expelled
fluids are probably important for metamorphic processes in the
subduction wedge and magmatic processes beneath the volcanic arc.
The recent recognition of fresh- and saline-water seeps at the
base of some continental margins suggests that gravity-driven
flow from the Continents may be responsible for an average global
flux of 100 km /yr. The interactions and effects of these
different fluid fluxes remain virtually unknown.
Growth and Modification of Continental Crust
Magmatism at continental margins, such as basaltic magmatism
at divergent margins and especially arc magmatism at convergent
margins, is the primary mechanism for the formation of new
continental crust. The process of magma genesis is strongly
dependent on the interaction of several distinct chemical
reservoirs, such as subducted oceanic crust and sedimentary cover
and a variety of mantle reservoirs. We now have the ability to
quantify the fluxes of mass and fluid between these different
reservoirs using geochemical tracers. An excellent example is
the short-lived cosmogonic isotope ,°Be, which provides a useful
tracer for the presence of subducted sediment in arc magmas.
s
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An important problem to be addressed is how continental
crust attains its chemical character. The average composition of
continental crust appears to be "ndesitic, whereas the melts
extracted from the mantle wedge at convergent margins are
dominantly basaltic. This observation suggests that the more
mafic component of juvenile continental crust is stripped away.
Deposit~onal Systems at the Edge of the Continents
Sedimen is are the most abundant constituent of continental
margins. They typical ly form thick, extensive deposits over ~ ying
the zones of rifting or convergence that def ine the continental
edges . The depositional processes respons ~ ble for transport =~d
a ccumu ~ a t i on o f these s ed iments repre s ent a c omo ~ e x, dynam i c
system that is controlled by eustatic, tectonic, and climatic
conditions. The sedimentary record itself has great practical
signif icance because it provides a direct measure of past
deformation over a long interval of time. The record further
provides a rich source of inf formation about evolution o f the
environment and of life itself, which is Ingram uable in
understanding current global change. An important and continuing
goal is to develop realistic models of erosion, transport, and
accumulation across the entire width of the continental margin.
DEVELOPMENT OF A SCIENCE PLAN
This report presents a scientific rationale for a new
conceptual approach to continental margins research. The central
thrust o f this approach is the acknowledgement that we have
passed well beyond the main phase of the plate tectonics
revolution in which the new paradigm was erected and its tenets
were tested largely by studies that were discipline- and region-
· .
speck: : 1C.
We have now reached a stage characterized by basic enquiry
into the fundamental processes associated with plate
interactions. Results of the workshop show that many of these
processes are common to all margins, whether formed by extension,
contraction, or translation. This conclusion suggests a unified
approach to margins research that emphasizes the critical role of
process-oriented inter-disciplinary programs. Several specific
corollaries become apparent. One is that research on any one
type of margin can be of significance to research on all margins.
Another is that research in intra-continental erogenic belts,
where continental extension is initiated and ancient margins are
ultimately preserved, represents an essential component of the
study of continental margins. Thus, marine and land-based
studies need to be further integrated. Yet another corollary is
that laboratory and theoretical studies of the processes of plate
interaction provide a basic contribution to the study of
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continental margins. The process-based approach advocated here
gives promise that a more aggressive and coherent strategy for
margins research can be defined.
The workshop took the first few steps toward defining this
strategy by identifying the essential scientific objectives of a
new margins research direction. Several further steps toward
developing and implementing a concrete science plan are outlined
below.
Formation of a Margins Initiative Committee
The workshop participants concluded that an independent
scientific oversight committee, such as those organized under the
auspices of the National Research Council, would be useful as a
means to foster and ensure a greater degree of coordination and
communication between land and marine communities in establishing
process-oriented research programs. It would also advise U.S.
funding agencies on mechanisms necessary for implementing such
programs.
Organization of Workshops
The committee would be charged to organize some workshops by
the end of 1990 involving a broad cross section of the community.
The workshops would focus on the following topics:
1. Mechanics of Low Angle Faults
2. Fluids and Fluid Flow
3. Magmatism and the Growth of Continental Crust
4. Continental Margin Sedimentary Record
The objective of the workshops would be to take the broad
scientific objectives defined by the Irvine workshop and to
develop and implement from them a focused program of research in
the four areas named above.
The plans formulated by these workshops and the results of
discussions with the funding agencies will be combined into a
so i ent i f ic pi an to be f ormul ated by the end of 1990 to chart a
course for margins research for-the following decade.
7
Representative terms from entire chapter:
continental crust
