Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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7 Active Margins: Group 1
7 Active Margins: Group 1
Pages 73-84
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From page 73... ...
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.
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From page 74... ...
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.
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From page 75... ...
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.
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From page 76... ...
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 .
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From page 77... ...
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.
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From page 78... ...
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.
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From page 79... ...
~~ 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.
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From page 80... ...
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.
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From page 81... ...
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.
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From page 82... ...
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.
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From page 83... ...
Some of the major rock mechanics issues related to the tectonics of active plate margins are discussed in Chapter 10.
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