Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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8 Active Margins: Group 2
8 Active Margins: Group 2
Pages 85-94
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From page 85... ...
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.
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From page 86... ...
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 .
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From page 87... ...
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.
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From page 88... ...
. 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.
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From page 89... ...
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.
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From page 90... ...
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.
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From page 91... ...
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.
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From page 92... ...
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.
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From 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.
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