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Shephard, L.E., 1981

Geotechnical properties of select convergent margin sediments

Bibliographic Reference

Shephard, L.E., 1981, Geotechnical properties of select convergent margin sediments: College Station, Texas, Texas A&M University, Ph.D. dissertation, 168 p.


Geotechnical property analyses on sediments recovered by DSDP drilling on select convergent margins were integrated to determine the variability in these properties along the lower trench inner slope and to delineate the effects of convergence on these properties. Consolidation states range from very underconsolidated to highly overconsolidated with preconsolidation pressures exceeding 46,000 kPa in Quaternary sediments. Underconsolidated sediments are attributed to high sedimentation rates, low sediment permeability relative to the length of the drainage path, laterally applied stresses, induced pore pressures resulting from the subduction of high water content pelagic sediments and physicochemical interparticle bonding. Factors contributing to the state of overconsolidation include tectonically induced overpressures, removal of overburden by mass movement processes, low sediment accumulation rates, and great age. Vertical gradients of index properties also vary greatly with the maximum gradient associated with overconsolidated sediments. Values of porosity exceed 30% at all margins studied suggesting this represents the minimum value attainable solely by the effects of convergence. Geotechnical property results and site-specific parameters allow classification of the convergent margins studied as either clastic dominated or pelagic dominated margins. Clastic dominated margins formed the basis for the initial geotechnical property model for convergent margin sediments. These margins are characterized by thick trench sediment sequences which are folded and faulted and become progressively more deformed upslope. Convergence rates are low and no well-developed faults or horst and graben structures are evident on the downbending oceanic plate. Highly overconsolidated sediments and index property values which change rapidly with depth result from the slow progressive deformation of trench and lower slope sediments. Examples of clastic dominated margins include the Nankai Trough, Aleutian Trench and Washington continental margin. Pelagic dominated margins are characterized by thin clastic trench sediment sequences overlying pelagic sediments on a rapidly converging oceanic plate. Well-developed faults and/or horst and graben structures are evident on the downbending oceanic plate. Sediment deformation along the trench and lower slope appears limited. These sediments are underconsolidated with gradual and often irregular changes in index property values with depth. Japan Trench and the Middle America Trench-Guatemala margins are considered pelagic dominated. Middle America Trench-Mexico appears to represent an intermediate case having characteristics of both margin types.

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