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 73
7
ACTIVE MARGINS: GROUP 1
DYNAMICS OF SHORT-TERM DEFORMATION AT ACTIVE MARGINS
THE SINGLE MOST IMPORTANT SCIENTIFIC OBJECTIVE
At the workshop, the title of this working group was
"Mechanics of plate motion." During its discussions, the group
decided to emphasize deformation along the convergent margin
megathrust and in the accretionary wedge, and the vertical and
horizontal distribution of forces and displacements at active
continental margins. Consequently, the working group established
that its single most important scientific objective is to
understand the dynamics of short-term deformation at convergent
and transcurrent margins e Within this broad objective, the group
identified the three major areas of investigation discussed
below.
THREE MAJOR AREAS FOR INVESTIGATION
What Controls Aseismic Deformation and the-Recurrence
Characteristics and Locations of Catastrophic Earthquakes?
Relative plate motion at active continental margins is
accommodated by both seismic and aseismic processes. Some pi ate
boundary segments have no historical record of great earthquakes,
ant] in some cases the record covers a sufficiently ~ ong period of
time that it appears likely that the subduction zone never s] ips
seismically. Other segments rupture at regular or irregu~ ar
intervals with events of varying size and frequency. For still
others, it is not at all clear what fraction of relative motion
occurs seismic ~ y or aseismically.
The factors that control the mode of subduction slip and the
space-and-time characteristics of earthquake recurrence at
subduction zones are as yet poorly known. Progress in
understanding these processes is important both for assessing
long-term seismic risk and in making progress towards short-term
prediction and hazard mitigation.
73
OCR for page 74
The principal factors that are believed to control the
recurrence rate, size, and location of catastrophic earthquakes
are as foil OWSe
1. The relative velocities of the adjacent plates. This is
most important in the recurrence rate since it controls the time
needed to build up the stress to a level that will overcome the
static frictional resistance between the plates.
2. Physical properties of earth materials. The
temperature, rock type, porosity, and Theological behavior of
material in the active margin determines how the earth responds
to plate displacements and whether energy is released
catastrophically or aseismically. For this reason, knowledge of
the physical properties is of the utmost importance.
3. Area of contact between the adjacent plates. This
parameter controls the maximum possible size of the earthquake,
or the amount of energy released. It depends on the local plate
geometry.
4. The temperature and pore fluid pressure in the active
margin. These are among the critical parameters controlling
whether the materials respond in a ductile or brittle fashion.
They cannot be measured in.situ and must be inferred from other
physical properties, such as seismic velocity, heat flow
measurements, and electrical conductivity.
5. Deep geologic structure. The distributions of plate
boundary asperities, mantle inhomogeneities, faults, -and-other
defor~national structures must be better understood if we are to
explain the response of the active margin to the applied
stresses. They are often complicated and affect the local stress
and strain fields in ways that make it difficult to interpret
measurements of stress and strain. However, these features must
be understood if the earthquake processes are to be understood.
It is well known that convergent margins exhibit different
types and levels of seismicity. Strategically, it may be
important to design integrated geophysical and geological
investigations both at margins that are characterized by great
earthquakes and at margins that lack such events. Thus, it will
be necessary to include studies both at margins that produce long
(> 200 km) ruptures and at those that have no historical record
of great earthquakes (rupture 1 ength < 100 km) .
Strongly focused integrated studies of several active
margins, including structural, seismic reflection, geodetic,
bathymetric, and seismic network data -are needed to identify
those f eatures that are d iagnostic of se i smic and ase i smic
accommodation of red ative pi ate motion.
74
OCR for page 75
What Controls Accretion, Nonaccretion, and Eros ion of
the Upper P1 ate at Convergent Margins?
There is a remarkable range of tectonic styles at the front
of convergent margins, ranging frown accretionary to erosional
Margins of the Lesser Antilles and Cascadia, for instance, grow
by accretion, whereas the Middl e America margin varies locally
from accretionary to erosional, and the northern Japanese margin
is erosional . The causes of this variability are not obvious,
but they bear on the fundamental problem of how mass is added to
and remover] from continents.
It is important to understand the structure of accretionary
wedges at two different seal es. At a regional scale, the shape
of the wedge is important because of its dependence on the
rel ative strengths of the material s within and below the wedge e
Determination of the smaller-scale internal structures in the
wedge is important because of the information they contain on the
stress, strain rate, temperature, ant} fluid conditions under
which the deformation has taken place.
Orogenic-wedge model s imply that accretion and erosion at
convergent margins are influenced by temporal and spatial
variable ity in the frictional coupling between the downgoing and
overriding plates. It has been suggested that an important
mechanism for accretion or erosion of sediments is the upward or
downward Lisps acement of the master clecoll ement. If this is the
case, it is unclear what alters the material strengths in such a
way as- to make it mechanical ly preferred for the basal thrust to
shift position. Frictional coupling is a poorly constrained]
function of the rheology of the accretionary deposits, and it is
particularly sensitive to variations in pore fig uid pressure.
Unfortunately, the Theological behavior of accretionary materials
is poorly known under the combination of temperatures, pressures,
porosities, and pore-f~uid pressures prevalent in most
accretionary wedges.
The margins most likely to permit critical assessment of the
relationship between the mechanics of the accreted materials and
the structural configuration of the decollement are those in
which the effects of variability in the sediment input can be
isolated. Thus, margins with considerable 1 atera~ variation in
both sediment intake and decollement structure, such as
Peru-Chile and the Lesser Antilles, would provide promising
targets. Margins undergoing active erosion (e.g., the eroding
portions of Peru-Chile and northern Japan) may provide drilling
targets at reachable depths, permitting direct measurements of
both mechanical properties and stress.
The geometry of the descending oceanic plate has been
suggested as another primary control on the tectonics of the
frontal part of the overriding plate, although no clear causal
75
OCR for page 76
frontal part of the overriding plate, although no clear causal
relationship has been demonstrated. The geometry of the Benioff
zone appears to be related to the age of the downgoing
lithosphere, the relative velocities of the two plates' and the
tectonic history of the upper plate.
Topography on the converging ocean crust, when inserted into
the subduction zone, disturbs the tectonics of that region. Even
when the basement red ief is insufficient to erode the upper
plate, it may wed ~ perturb the stress field by an amount
suf f icient to alter the conf iguration of the decollement ~
The subduction of a 3500-m-high seamount beneath northern
Honshu produces ups ift of the upper plate above the leading
se amount flank. Another seamount, further into the subduction
zone, has produced the coil apse of the margin down its trailing
f lank ~ In less than a million years ~ the impact and subduction
~ f the seamount caused a change f ram accretion to eras ion and
back to accretion again. This example if lustrates a process
which must af feet convergent margins at many scales . Swath
mapping (two-dimensional bathymetric surveys) and high-reso~ution
se ismic surreys now provide powerful tools for the study of
collisions at subduction zones.
What Determines the Partitioning of Deformation
In P1 ate-Boundary Zones?
The concept that the lithosphere acts as a high-strength
stress guide is fundamental to plate tectonics. The concept
works well in plate interiors, where the crust may move with the
(nearly unifo ~) velocity of the underlying high-strength mantle
lithosphere. However, plate boundaries are not simple, localized
zones of shear. This is particulary true at continental margins,
where deformation can be distributed over zones ranging up to
~1, 000 km in width. The mechanics of this departure from simple
plate behavior represents a fundamental aspect of the geodynamics
of plate boundaries.
A starting point for understanding strain partitioning at
plate boundaries is to recognise the general variation of
Reformational properties within the lithosphere. There are
alternating strong and weak layers that have prompted the analogy
of a peanut butter and jelly sandwich. In the shallower half of
the crust, rocks support substantial shear stress. At these
depths, rocks ultimately deform by brittle failure and can
produce earthquakes. Relating earthquakes to the general strain
release at plate boundaries has considerable societal as well as
scientific value. Unlike the rocks nearer the surface, the lower
crust is very weak, deforming continuously by any of several
creeping processes. The uppermost mantle is at a lower fraction
of its men ting temperature. It is again structurally strong, and
76
OCR for page 77
in many areas earthquakes occur within it. The details of the
distributions of deformation modes and shear stresses depend on a
number of factors. Low temperature and low confining pressures
favor brittle failure, while high temperatures promote creep.
High deviatoric stress' pore fluids and vocatives also have
weakening effects.
The upper mantle part of the lithosphere is nor-~ally the
structurally strongest layer in the lithospheric "sandwich." For
this reason it usually constitutes the primary stress guide of
the lithospheric plates. The mushy nature of plate boundaries in
continental margins may result from the weakening of this
normally strong member of the lithospheric sandwich due to high
temperature.
The western United States provides an example of distributed
plate-boundary deformation. In this case, strain is broadly
distributed across a region extending from the continental margin
to the Rocky Mountains. The maximum displacement in central and
southern California over recent geologic time is on the San
Andreas fault, but substantial deformation, on both geological
and seismological timescales, occurs throughout the region. The
pattern of deformation involves more than the simple shear
expected as the result of transform motion between the adjacent
rigid plate interiors. For example, strike-slip motion on the
San Andreas fault contrasts sharply with the nearly orthogonal
horizontal contraction indicated by many structures in the
Transverse Range, including the Big Bend of the San Andreas.
Many of the largest historic earthquakes in California, in fact,
have been associated with these contractional features.
It has been suggested that the Big Bend and associated
convergence are the result of a distributed zone of weakness in
the area along the San Andreas fault. This weakness might be
related to the large total slip of the fault. Forces due to the
str~ke-slip relative motion of the plate interiors would thus be
transmitted through the lithosphere and converted to convergent
motion simply as a kinematic consequence of the weak zone being
curved.
An alternative hypothesis is that all the major faults in
the upper crust, as well as the underlying lithospheric mantle
this region, are relatively weak and incapable of transmitting
large shear tractions over large distances. In this case, the
substantial forces needed to drive convergence and build the
actively growing Transverse Ranges must be provided locally,
perhaps by stresses associated with convective flow in the
underlying mantle.
Partitioning of deformation is also observed at some
obliquely convergent margins, such as the the western Aleutian
and northern Sumatra margins. In such settings, contractional
77
OCR for page 78
structures apparently accommodate the component of convergence
orthogonal to the plate boundary, and the motions along
transcurrent faults accommodate the boundary-parallel part of the
relative plate motion.
Space-based geodetic techniques (e. g., Gl obal Positioning
System) will be crucial in determining (on time scat es of years
to decades) the pattern of deformation across continental
margins. Seismic tomography is a rapidly developing tool used to
determine the structure of the crust and upper mantle.
Determination of variations in the state of stress using borehole
measurements or focal mechanisms is important for discriminating
between tectonic hypotheses, as is the accurate resolution of the
structure:of the lithosphere. The comparison of geologic strain
with geodetic strain is important in determining seismogenic
potential of structures. Relating seismic activity at depth to
geologic structure would shed light on the nature of the seismic
rupture process. Finally, numerical modeling is essential for
the quantitative testing of hypotheses against data.
NEEDED STUDIES
Necessary studies include those required to determine the
three-dimensional geometry of the downgoing plate and to isolate
the factors that control whether plate-boundary deformation is
seismic or aseismic. The studies will require the acquisition of
structural, seismic reflection, geodetic, bathymetric, and
seismic network data.
Geometry of the Downgoing Plate
It has been suggested that the geometry of the descending
oceanic plate strongly influences the deformation of the frontal
part of the overriding plate. This geometry is difficult to
resolve accurately in the upper-50 to 75 km, and requires
integration of seismic tomography, gravity studies, anc}
hypocentral determinations made us ing local seismic networks . A
maj or impediment to seismic studies at convergent margins has
been the fact that seismic networks have been larger y restricted
to the continent. Even when stations can be pa aced on islands,
the islands are commonly wider y dispersed and are commonly
1 ocated atop structures with locally anomalous seismic velocities
which complicate the interpretation of data. For these reasons,
a combination of both OBS (ocean-bottom seismometer) and
land-based stations will be needed in future seismic networks at
convergent margins. Seismic tomography provides a three-
dimensional view of the subducting ocean crust down to mantle
depths. This reconnaissance information can be augmented with
high-energy seismic reflection images of the crust and Moho.
Such studies could be carried out at any sufficiently
78
OCR for page 79
well-studied margin. Examples providing contrast would include
the Andean, Marianas, Lesser Antilles, and Alaskan margins .
Structures, Geodetics, and Seismicity of Subduction Margins
It is well known that convergent margins exhibit dif ferent
types and levels of seismicity ~ Strategically, it may be
important to design integrated geophysical and geological
investigations both at margins that are characterized by great
earthquakes and at margins characterized by smaller magnitude
events. Some possible candidates are:
1. Great earthquakes ( > 200 km ruptures)
- Chile (virtually all its coast from Arica to Chonos
Archipelago)
Central Aleutians
- Kamchatka
- Alaska
2. No historical great earthquakes (> 100 km ruptures)
- Izu-Bonin-Marianas (no events M > 7~4)
- Ryukyu
- Tonga-Kermadec (may have experienced a few
earthquakes, ~200 km ruptures)-
Strongly focused integrated studies of several active
margins are needed to identify features diagnostic of seismic and
aseismic accommodation of relative plate motion. Observational
programs aimed at delineating structural features and determining
contemporary patterns of crustal deformation and seismicity will
provide correlations and constraints. The age, geometry,
structure, and deformation styles of accretionary wedges are
expected to play an important role in determining the seismic-or
aseismic character of convergence. High-resolution seismic
imaging at specific convergent margins will contribute to the
mechanical understanding of wedges, and it can complement the
results of laboratory studies designed to determine the
constitutive properties of wedge materials and to define their
failure modes under a range of pressure and temperature
conditions. ~~ -
New measurements of present-day crustal deformation are
needed to determine the spatial and temporal patterns of movement
and relate them to seismic and aseismic processes and to the
long-term deformation of active margins e Use of Global
Positioning System COPS ) geodetic surveying onshore and new
ocean-bottom techniques in offshore environments can provide the
data needed to define the present geodetic movement pattern from
the magmatic arc to the oceanic outer rise and to monitor its
temporal variations. Integration of these data with structural
information supplied by multichannel seismic, high-resolution
79
OCR for page 80
bathymetric surveys, and small earthquake locations and focal
mechanisms obtained from onshore/offshore seismic networks will
provide a context for relating present geodetic movements to the
recent geologic record.
Although much can be learned from dry-land geodetic surveys,
the development of seabottom measurement techniques is vital for
obtaining a complete picture of active margin deformation.
Experience with land-based measurements in Japan, California, and
New Zealand indicates considerable breadth (100 to 200 km or
more) in the zone of contemporary deformation. A similar range
Bill probably be found in other active margin settings. Offshore
geodetic measurements are needed to bound the regi on of
present-day deformation, to relate it to the seismic or aseismic
character of each active margin, and to determine the
partitioning between elastic straining, which will ultimately be
relieved by earthquakes, and permanent deformation, which will be
preserved in the geologic record. Seismic observations have
suggested a linkage between subduction zone earthquakes, outer
rise seismicity, and lateral migration of great underthrust
earthquakes. Geodetic monitoring, both onshore and offshore, can
critically constrain this strain migration process.
Developments in several research areas are particularly
important in understanding short term deformation at active
margins. Those singled out here include numerical modeling,
enhanced geophysical instrumentation, and rheological studies of
geologic materials.
Numerical Modeling
Numerical modeling is essential for the quantitative testing
of hypotheses against data. More sophisticated models are
needed, incorporating realistic three-dimensional geometries and
coupling of rheological variables. Such models require
sophisticated software, as well as access to powerful computers.
Modeling of the distribution Of stress and strain at active
continental margins can be used to relate observations of
deformation to rheological laws obtained in the laboratory.
Modeling can also suggest improvements in observational
strategies and indicate those laboratory studies that are likely
to prove most critical.
Geodetic observatories using GPS and surface techniques will
provide maps of surface deformation at the centimeter level
within the next decade . Breakout studies in existing wells can
provide stress directions in many areas. Two-dimensional and
three-dimensional finite-element models will be required to
interpret these observations. Models should include elastic,
fault related, plastic, and nonlinear viscous rheologies.
80
OCR for page 81
Several goals are clear, among them:
1. to distinguish those margins where deformation is
relatively monotonic from those where it is cyclic due to the
accumulation of elastic strain energy and fault rupture; and
2. to provide a better understanding of the cyclic
accumulation and release of strain associated with great
earthquakes.
Enhanced Geophysical Instrumentation
- Certain specific new measurement capabilities will be
needed. They include: the ability to make extensive geodetic
measurements on very short notice; improved abilities to conduct
underwater geodetic and seismic network studies; and improved
techniques for stress measurements and deep seismic reflection.
Some of these future needs are outlined below.
Submarine and Rapidly Deployed Geodetics
Recent developments in space-based geodetic techniques
Berg., GPS) have opened the possibility of accurate geodetic
observations on a scale far larger than has been heretofore
feasible. These techniques will be crucial in determining, on
timescales of years to decades, the general pattern of
deformation across continental margins. Rapid deployment of
instruments to the near-field region after earthquakes would
allow the observations of temporal variations in postseismic
strain that are crucial for constraining the Theological
properties of fault zones, the lower crust' and the upper mantle.
Extension of these techniques to allow accurate positioning on
the seafloor appears technically feasible and is extremely
important for the obvious reason that most deformation at active
margins spans the border between land and sea.
Ocean-Bottom Seismometers
Seismic tomography is a rapidly developing too] that is used
to determine the structure of the crust and upper mantle. For
example, in the Transverse Ranges of southern California,
regional tomographic studies have revealed a curtain of high
velocity material extending to a depth of 250 km. This feature
has been interpreted as the convective downwelling of the cold,
dense base of the thermal lithosphere. The tomographic image
loses resolution in the offshore region because of the lack of
seismic stations there. This is a good example of an important
tectonic problem that could be addressed much more effectively
with the deployment of OBS to fill in gaps in coverage. In this
and other tectonically active regions along continental margins,
81
OCR for page 82
the Kept oyment of dense seismic arrays should be a high
scientific priority.
Measurements of Stress anti Deep Structures
Determination of the in situ state of stress using seismic
focal mechanisms and at selected locations using boreho~ e
measurements is important for discriminating among tectonic
hypotheses. Accurate description (in three dimensions) of the
geologic structures is also crucial for understanding the
mechanics of tectonic processes, both to determine crustal
kinematics on geologic timescales and to identify the structural
units that are important mechanically. Modern structural
geology, utilizing data from seismic reflection surveys and
boreholes, provides an additional approach to this problem.
Rheological Studies
Because orogenic-wedge models suggest that accretion and
erosion at convergent margins are inf luenced by variate! ity in
coupling along the zone of major deformation, studies to define
the rheological behavior of accretionary materials and to assess
the hydrology of pore fluids are needed. However, the
mechanical properties of sedimentary rocks are much less well
known under accretionary wedge conditions than within the high
pressure-low porosity conditions generally studied in rock
mechanics or the low pressure-high porosity conditions studied in
soil mechanics. Laboratory studies are needed of the conditions
representative of those associated with the processes of
subduction erosion and underplaying (0 to 200 MPa and 10° to
500°C), in order to determine the material behavior associated
with these processes. Laboratory samples, however, cannot
include larger-scale (>10 cm) features , such as veins and
fractures, which are ubiquitous in accretionary complexes and
which may control the strength of these materials in situ.
Although the frictional coupling between lithospheric plates is
difficult to quantify from direct observations, the integration
of downhole geotechnical (stress-strain, pore pressure)
determinations, laboratory results, and numerical modeling could
provide realistic constraints on material behavior.
Porosity, intergranular and fracture permeabilities, and
pore pressures within the prism are parameters that are needed to
understand fluid flow. Although we need to determine in situ
values for these parameters, such measurements are difficult and
expensive. Therefore, hydrologic modeling, constrained by a few
high-quality field measurements, may prove to be the most
efficient means to improve our understanding of fluid flow and
the water budget within the prism.
82
OCR for page 83
A variety of microscale Reformational processes operates in
the crust and mantle. They are responsible for the wide range of
structures seen in both active and passive continental margins.
Many of these processes can be studied by experiments in the
laboratory, where it is possible to obtain quantitative
information on rheology under controlled Reformational
conditions. This in turn makes possible a better understanding
of the mechanics of crustal-scale deformation because it permits
estimates of stresses if strain rates are known, and vice versa.
Some of the major rock mechanics issues related to the tectonics
of active plate margins are discussed in Chapter 10.
~3
OCR for page 84
Representative terms from entire chapter:
continental margins
