Graduate Thesis Or Dissertation
 

Chile trench : extensional rupture of oceanic crust and the influence of tectonics on sediment distribution

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  • Extensive new trackline coverage of the Chile Trench between 23°S and 34°S, including more than 60 bathymetric and seismic reflection profiles across the trench axis, allows a much more detailed study of the tectonics and sedimentation of this feature than previously possible. Sediment distribution along the axis shows a remarkable variation from over a kilometer of turbidites in the axis south of 33°S to a barren axis in places north of 27°S. Turbidity currents originating on the outer continental margin in the south carry sediment northward along the axis. Ponding behind structural barriers created by plate convergence restricts the amount of sediment reaching northern trench sections. Horst and graben blocks are the dominant structural features on the seaward trench slope, with fault offsets of 500 to 1000 meters. Grabens range in width from 4 to 8 km, while the faulting probably extends down into oceanic layer three. Faulting is most pronounced in the deeper northern parts of the trench, but can also be seen in the basement beneath undeformed axial sediments in the south. This crustal rupture can be related to extensional stress in the upper oceanic crust due to the downbending of the Nazca Plate prior to subduction. Most of the active normal faulting occurs soon after the plate begins its descent into the trench, and not within the trench axis. Using structure, sediment distribution, bathymetry, and morphology, the trench and outer continental margin can be divided into three provinces (Northern, 23°-27°S; Central 27°-33°S; and Southern, 33°-34°S) separated by distinct tectonic transition zones at 27°S and 33°S. These boundaries coincide with breaks in onshore geologic trends and correlate less well with seismic zone segmentation. An analysis of potential strain due to subduction along a non-arcuate trench concludes that segmentation in the trench is probably controlled more by continental block structure than by the linearity of the trench. A narrow, continuous pond of sediment partially fills the Central Province trench axis between 32°30'S and 27°S. Trench axis morphology and piston core samples indicate there is transport of terrigenous sediment down the axis from the abundant sediment supply regions of the Southern Province. A model is formulated from this data which accounts for the sediment wedge in the Central Province by supply of turbidites from the south. A steady-state of axial fill can be maintained by one typical flow every 14 to 27 years. Uplifted axial turbidites are present on the seaward trench slope at 30°35'S, elevated 350 meters above the axis. Radiocarbon dating puts the age of initial uplift at 5380 ± 350 years B.P., which results in a minimum vertical movement rate of 6.5 cm/yr. Reversed faulting due to compressive stresses generated by plate convergence is the presumed mechanism of uplift. A model is proposed to explain the differences in the Chilean continental margin morphology in each of the three provinces. The radical differences in the amount of sediment available to the trench axis appears to be a prime influence in the development of the margin. An abundance of axial sediments provides a buffer zone along the major interplate contact (slip) zone, plus material to be accreted into the lower continental slope. If the amount of axial sediments is limited, excessive frictional resistance to slippage between the converging plates may tectonically erode the margin by slowly wearing away the underside of the continental slope.
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