Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 29
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
OCR for page 30
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
OCR for page 31
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
OCR for page 32
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
OCR for page 33
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
OCR for page 34
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
OCR for page 35
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
OCR for page 36
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
OCR for page 37
- 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
OCR for page 38
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
OCR for page 39
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
OCR for page 40
(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
OCR for page 41
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
OCR for page 42
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
OCR for page 43
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
OCR for page 44
Representative terms from entire chapter:
continental lithosphere
