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. / Graduation date: 1976
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28345 |
| Date | 27 February 1976 |
| Creators | Schweller, W. J. (William John) |
| Contributors | Kulm, LaVerne D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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