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 85
8.
ACTIVE MARGINS: GROUP 2
GEOLOGIC EVOLUTION OF ACTIVE CONTINENTAL MARGINS
THE SINGLE MOST IMPORTANT SCIENTIFIC OBJECTIVE
The Active Margins Group 2 considered ''the geological
evolution of active continental margins."' The critical problems
we identified, as discussed below, embody a strong emphasis on
the geological development of margins through time. Our single
most important objective is to understand how convergent and
transcurrent plate motions fabricate, deform,-redistribute, and
waste lithosphere at continental margins.
How are Terranes Formed, Moved, and Accretec]?
A fundamental breakthrough in understanding~how continents
form came with the realization that much of the crust is
constructed of an amalgamation of distinct structural blocks, or
terranes, which originated in widely different locations.
Terranes, for example, are important components of the northeast
margin of the Indian Ocean, the entire northern Pacific rim, the
Caribbean islands, and the eastern Mediterranean. Ancient
terranes are being recognized in mountain belts-throughout the
world. Present day terranes are closely associated with active
plate margins, where the processes that create, fragment, move,
and amalgamate terranes are clearly manifested. These processes
can be best studied in active marginal settings of Mesozoic and
Cenozoic age. The creation, movement, and docking of terranes
significantly influence basin formation and magmatism at the
continental margins. The understanding of terrane histories is
therefore essential in the search for mineral deposits and
hydrocarbons.
Although exciting advances have been made, a number of
fundamental questions still remain. For example:
1. How distinct are the various accreted terranes that
compose the active margins of the continents? In other words,
are these terranes fragments of a smaller group of "mother"
85
OCR for page 86
terranes, or were they derived from unrelated sources?
2. Do terranes typically form within the ocean basins
~ e . g ., oceanic plateaus , island arcs , and seamounts ~ or by
rifting anchor translation of fragments from the surrounding
continent margins?
3 . What is the relative importance of strike-sl ip
translation versus subduction in transporting and accreting
terranes?
4 . How common is it for a terrane to be far-tra~relled, that
is, thousands of kilometers from its source, as opposed to teeing
local ly derived?
5. Are the processes of terrane formation and terrane
accretion episodic or steady-state?
6. Can the study of terranes be used to identify the
existence and movement histories of subducted oceanic plates?
Present day processes of terrane fragmentation, motion r and
amalgamation can be readily studied in southeast Asia' where many
terranes are currently forming and others are in various stages
of amalgamation. Modern oceanic island arcs, such as the
Mariana-Bonin and Tonga-Kermadec arcs, are composite terranes
that have formed by imprinting of new volcanic arcs on crust
rifted from older arc terranes. The Palau-Kyushu Ridge, a
remnant arc with a similar history, is moving westward into the
Ryukyu-Philippine subduction zone. This terrane is destined to
collide with the subduction zone, and it may become incorporated
into the subduction complex.
Western North America includes numerous oceanic and
continental crustal terranes that were accreted to the North
American craton during the late Paleozoic and Mesozoic. These
terranes appear to have been formed at, and moved northward
along, a complex plate-boundary shear zone periodically
estabI ished between ache North American and Pacif ic plates . These
older terranes, which provide crucial info ~ mation on the
evolution-of the Mesozoic Pacific and] its margins, can be
pro f itably studied along the -northern rims of the Pacif ic .
What are the Consequences of Changes in Plate
Motion for Active Plate Boundaries?
The ef feats on plate margins caused by changes in plate
motion can be used to evaluate how stress and strain are
transmitted in the lithosphere. At a fundamental level, the
tectonic processes occurring at active margins are controlled by
basic plate kinematic parameters, such as the rate and relative
direction of convergence and the motion of the overriding plate
relative to a deep mantle (hotshot) reference frame. As these
parameters change through time, the style of tectonic deformation
along the plate boundaries also changes. Some large changes in
plate motion have had profound effects on the geology of the
86
OCR for page 87
circ -Pacific region. The 4S° shift in the direction of Pacific
plate motion in the late Eocene, recorded in the trend of the
Hawaii-Emperor Seamount chain, was accompanied by major changes
in the tectonics of active margins along much of the perimeter of
the Pacific basin. However, even relatively small changes in
plate motion can lead to major changes in geology. For example,
the minor shift in the direction of the Pacific plate around 4 Ma
led to a major change in the amount of convergence across fault
systems in California, and it increased magmatism in the Aleutian
arc in the Pliocene.
By studying the response of an active margin to changes in
plate motion, it is possible to define the critical boundary
conditions for many of the tectonic processes that affect the
margin. Determination of these boundary conditions is essential
for the correct construction of theoretical models. An example
is the response of an active margin to a change from orthogonal
to oblique convergence. Oblique convergence can induce
strike-slip faulting in the forearc and arc and, in some cases,
can ~ ead to the creation of small terranes in the forearc region.
In other cases, there is no obvious change in forearc tectonics.
The variation in response probably reflects a difference in the
coupling between the overriding and subducting pi ate, which may,
in turn, be related to dif ferences in the absolute motion of the
overriding plate. Investigations of this type have been very
important in deducing the large-scale tectonic processes
operating at active margins. For these investigations to remain
successful, it will be necessary to link closely the results of
geochronologic, mode, ing, and field studies.
What Controls Vertical Motions in the Forearc
Region of Convergent Margins?
Vertical motions provide an important measure of both
elastic and permanent deformation at convergent margins. The
release of elastic strain during the rupture of a subduction
thrust can produce dramatic effects, resulting in nearly
instantaneous subsidence and uplift of several meters e
Land-based geodetic studies have demonstrated a predictable cycle
of elastic loading and flexure that precedes and follows a large
rupture event. Both seismogenic and steady-slip subduction zones
also show evidence of long-term permanent deformation. Permanent
uplift is an expected result of contraction along low-angle
faults and ductile shortening within subduction complexes. Large
subsidence, however, is an unexpected feature, and its occurrence
has been cited as evidence of tectonic erosion or abrasion of the
margin. The magnitude of vertical motions ranges from 1 to 6 km
in the submarine portion of the margin to as large as 20 to 30 km
in the rearward region of some subduction complexes where
high-pressure metamorphic rocks have been uplifted and exposed.
Locally, the rate of vertical motion can approach values on the
~ A~ ~ I_ _ _ _ _ _—~
87
OCR for page 88
order of 10 to 50 mm/yr (10 to 50 km/My). The examples below
illustrate how measurement of past and present vertical motions,
using both geodetic and geologic methods, can provide critical
quantitative data about a variety of Reformational processes at
convergent margins.
Specific Problems
l. The zone of frontal accretion at many convergent margins
is marked by the development of large anticlinal folds, which
form above steps or ramps in the subduction thrust. The rate of
uplift of the crest of the anticline is directly related to the
rate of slip along the ramp fault that underlies it. Thus,
measurement of the uplift of the anticline can be used to
determine how plate slip is distributed across the lower slope of
an actively accreting subduction zone. Furthermore, it may
provide information about the state of the subduction thrust at
depth; that is, whether it is creeping at a steady rate and not
accumulating elastic strain, or whether it is locked and storing
elastic strain to be released in a future rupture event.
2. The existence of underplating (the addition of material
beneath an accretionary wedge) is postulated on the basis of
anomalous uplift in the forearc of some accretionary wedges
berg., the middle America trench off Mexico). The uplift and
exhumation of high pressure metamorphic rocks in the rearward
part of some subduction complexes may be caused by this
underplaying process. At present, however, there are only
fragmentary data on uplift rates. Geodetic surveys at
steady-slip (aseismic) subduction zones could provide important
information about the rates and deformation patterns associated
with this process. On-land field studies of uplifted
accretionary complexes represents another method for examining
the causes of large-scale uplift.
3. Some margins show evidence of large amounts of
subsidence. The northeast Japan margin has experienced about 6
km of subsidence in the mid-slope region Recent drilling at the
Peru margin has documented 3.5 km of tectonic subsidence. The
subsidence cannot be attributed to changes in the age or geometry
of the downgoing plate, and thus it must be due to some process
of thinning or abrasion of the overriding plate. Virtually
nothing is known, however, about this process.
One possible explanation is for one or more thrust faults to
develop within the wedge (as out-of-sequence faults) so that
slices of sediments at the front of the wedge are subducted to
deeper levels. A second possibility is that the entire base of
the wedge is abraded in a planar fashion, perhaps due to the
subduction of a lower plate with a "rough" surface (e.g., a
thinly sedimented plate with horst-and-graben topography). A
third possibility is large-scale mass wasting of the surface of
the wedge, with the mass-wasting deposits subducted beneath the
88
OCR for page 89
wee Ige . The f irst option would produce a distinctive pattern of
vertical motion and upli ft followed by subsidence as the so ices
moved rearward beneath the wedge. There is-no evidence of
initial uplift preceding the two cases of large subsidence
discussed above. An important question is whether this pattern
of only subsidence is representative of other tectonically
abraded convergent margins.
What Controls Arc Rifting and the Development
of Backarc Basins?
One of the enigmas of convergent margin evolution is the
existence of periodic phases of extension that result ~ in arc
rifts and backarc basins e The cause of this extension is poor y
untlerstootl. Many of the word ~ ' s metal deposits formed during
such periods of convergent margin extension. These include
Kuroko-type (volcano hostel]) ant] Besshi-type sediment hosted)
massive sulphide deposits, as well as suiphide, nickel, and other
deposits associated with ophiolites. Moreover, many abducted
ophiolites are interpreted to have formed in back-arc settings.
Three classes of model s have been proposed to explain
back-arc rifting and spreading:
I. Mantle diapirism: material coming of f the subducted
slab induces melting in the overlying mantle, result ting in the
bouyant rise of a mantle diapir.
2. Forced convection: mantle convection driven by the
motion of the subducting slab induces rifting, either by forcing
hot mantle into the arc region or by shearing the base of the
overriding arc. -
3. Global kinematics: far-field kinematic boundary
conditions, together with coupling of the forearc region to the
subducting slab, cause the arc to rift apart.
The first two models invoke mechanical processes that should
be local to all subduction zones, whereas the third model is
kinematically based and operates on a much larger scale.
Although considerations of arc rheology and temperature correctly
predict the region along which the arc should split, none of the
above models has successfully explained what initiates and what
stops backarc basin spreading.
In further evaluating these models and in generating new
models, it will be necessary to determine the variation In stress
and strain, seismicity, gravity, kinematics, geochemistry, heat
flow, and geologic structure at convergent margins that are being
actively rifted. It will then be necessary to compare these
observations with the results of geodynamic modeling.
89
OCR for page 90
What Causes the Initiation of Subduction?
The initiation of subducting plate boundaries--where, why,
and with what characteristics--is an important but often
neglected problem with implications for almost every aspect of
active margin studies. The location of the initial subduction
zone and its mode of propagation are disputed issues that bear on
the strength of the lithosphere as well as the dynamics of plate
behavior. The initial setting of subduction is also critical to
the origin of ophiolites and the basement of forearc basins.
Determination of the type of magmatism and its timing along
initiating subduction zones should provide insights into both the
thermal evolution of subduction zones and the signif icance of the
enigmatic boninite suite. In general ~ if we are to understand
the error ution of active pi ate margins, we need to develop a more
accurate model of the initial conditions of these boundaries.
Current problems in subduction initiation fall into two
categories: the geological evolution of active margins and the
geodynamics of the lithosphere and asthenosphere. Perhaps the
most important of the geological objectives is the location of
the initiation zone relative to preexisting crustal structure,
such as rifts at passive margins and} transcurrent faults at
active margins . In particular, we must determine whether
initiation of subduction is a step in a progressive evolution or
"cycle" of a margin (e.g., part of the Wilson Cycle), or whether
it might instead represent a fracture, emanating from some locus
of high stress along a preexisting structural boundary.
Determination of the location of the initial break relative to
the preexisting crustal framework should help resolve whether
forearc basins are f, gored by oceanic crUSte Data on the
seismicity and differential uplift associated with the
propagation of a subducting boundary would provide important
clues concerning the state of stress and mechanical behavior of
the lithosphere and asthenosphere in this critical setting.
Determining the nature of the initial magmatism along
subducting boundaries is another important objective for several
reasons. Most immediate is the need to interpret the volcanic
history of evolved arcs, some of which contain unusually broad
tracts of boninitic lavas. More fundamental, perhaps, is that
the knowledge of initial magma character can be used as a tool
for probing the chemical evolution of the upper mantle in this
setting. The thermal structure of the mantle could also be
probed if the surface distribution of the initial eruptive
centers and the time lag between initiations of magmatism and of
mechanical rupture were known.
90
OCR for page 91
NEEDED STUDIES
The problems identified here require a variety of
cross-disciplinary studies of terranes, backarc basins, forearcs
and propagating subduction zones. Representative studies are
described below.
Mapping, Paleomagnetics, Petrology, and
Geochemistry of Terranes
Information needed to identify terranes and determine their
movement histories is provided by a combination of onshore and
offshore geophysical and geological studies. Of particular value
are paleomagnetic and biogeographic data to determine histories
of paleolatitude changes and block rotations, and surface mapping
and subsurface reflection and refraction investigations to
identify the tectonic nature and evolution of terrane boundaries
and their overlap sequences. Petrologic and geochemical studies
are necessary to distinguish the protolith, original tectonic
setting, and alteration histories of associated igneous and
metamorphic rocks. Terrane analysis of active margins is
assuredly one of the fundamental tools by which the evolution of
ocean basins and their surrounding continental and island arc
borders can be successfully unravel ed.
Cross-Disciplinary Studies
of Rifting and Backarc Basins
Various cross-disciplinary studies are needed to evaluate
existing models for rifting. Convergent margins that are being
rifted today, such as the Bonin-Marianas, Tonga-Kermadec, and
Okinawa troughs, need to be thoroughly investigated to determine
the variation in stress and strain along and across the margin.
Such studies will include: earthquake studies of strain release
seismic/geoid/gravity studies of lithosphere structure; seafloor
swath mapping and MCS imaging to determine the geometry of
extension and contraction; satellite geodesy to determine
kinematics; ocean drilling to determine rates of uplift,
subsidence and fluid flux; geochemical studies of the arc, rift,
and proto-remnant-arc lavas to-constrain the amount.and source of
magmatism; and heat-flow studies to establish the nature of the
thermal regime.
The rifted passive margins of some backarc basins should be
surveyed to investigate the variability in structural styles and
the nature of the magmatic contribution to completed rifts.
Such observations will make it possible to carry out
meaningful geodynamic modeling of the subduct i on system.
91
;
OCR for page 92
To determine the consequences of changes in plate motion for
active plate boundaries, we need to:
1. identify critical times of plate motion changes; and
2. identify critical areas for investigating the effects of
these changes.
First, the plate motion models that are currently used for
studying the tectonics of active margins are only barely capable
of resolving times of important plate motion changes. Relative
plate motion models are being improved by incorporating a variety
of space-based measurements and through the acquisition of
magnetic anomaly data in critical areas of the oceanic basins,
such as remote parts of the South Pacific e To determine absolute
plate motions with respect to a deep mantle reference frame, it
will be necessary to improve resolution not only of relative
plate motions' but also-of absolute motion, using indicators such
as ho/spot-tracks. Then, with higher resolution plate motion
models, it will be possible to locate plate boundary geometries
where-mechanical models predict-specific consequences as a result
of changes in-relative motion. For example, this historical
approach could be used-to examine-the degree:of obliquity
required at a plate boundary-necessary to fragmentation and
translate a terrane. Case studies of this type will provide
insight into some of the basic mechanical models of the forces
that make plates move.
Geodetic and Geomorphic Studies
of Propagating Subduction
It is surprising that so little effort has been focused on
the problem of the initiation of subduction zones, not only
because the tools and techniques exist,~but also because the
necessary expenditure in time and funds is relatively modest.
logical first step would be the application of-modern but
conventional oceanographic techniques to several carefully chosen
examples-from the half dozen probable cases of initiating
subduction zones. A particularly good example would be the
delineation of the nor~hward-propagating tip of the Philippine
trench east of Luson with swath bathymetric and side scan
systems,-as well as with seismic reflection and gravity
profiling e Coordinated on-land geodetic and geomorphic studies
would provide data concerning the displacement field of the upper
plate. Furthermore, these data could be used to determine the
rates of propagation and compared with~other data on the magmatic
and tectonic history. Examination of initiation of subduction in
a more ensimatic setting, such as the Hunter-Matthew Ridge or
Mussau Trough, might more clearly resolve the initial tectonic
and magmatic evolution of an oceanic forearm
92
OCR for page 93
More ambitious and longer range approaches could include the
determination of the lithospheric stress field around the initial
subduction zone in deep sea drill holes and the use of seismic
tomography to view the changes in the upper mantle associated
with initiation of subduction. Although each of these approaches
would be valuable alone, the integration of geodetic,
gravitational, mechanical, and seismic data would provide a
synoptic view of a fundamental tectonic process.
93
OCR for page 94
Representative terms from entire chapter:
active margins
