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The AAPG/Datapages Combined Publications Database
Studies of active continental margins show the diversity of sedimentation patterns in trenches and on trench slopes. Many variables produce a range of distinctive deposits on the seaward trench slope, the trench axis, and the landward trench slope which can be used to interpret modern and ancient depositional settings.
Deposits along the trench axis are often described as a trench wedge, but the geometry and volume of these deposits vary extensively. The volume is dependent upon a balance between the rate of plate convergence and the rate of sedimentation. Sedimentation rate is a function of many factors such as climate, onshore drainage patterns, sea level fluctuations, the width of the continental shelf, and the amount of sediment trapped in fore-arc basins of behind smaller tectonic ridges along the landward trench slope. Variations in the geometry of trench axis deposits are due to irregularities on the oceanic plate and to sediment transport systems. Turbidites from a single point source may build a massive fan or a long wedge via an axial channel, while a series of smaller submarine canyons ma build a set of small fans or a chain of ponded basins.
Sedimentation along the landward trench slope depends on many of the same variables, particularly the size and spacing of canyons and the distribution of basins and ridges on the slope. An apron of hemipelagic muds may cover much of the slope, while locally slope basins receive terrigenous turbidites. Large deep canyons
can divert coarse clastics directly to the trench axis, bypassing basins on the lower slope.
A model of trench and trench slope facies must account for the many variables recognized in modern subduction complexes. The model proposed here, based on the facies defined for deep sea and channel deposits, relates associations of facies to various trench and trench slope settings.
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