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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
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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
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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
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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
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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
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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
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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 ;
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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
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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
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Representative terms from entire chapter: