Detailed surveys of several segments of the Peru Trench show
that the region between 6° to 10⁰ S is an area of recent deformation.
Seismic reflection records across the axis of the trench show faulting,
uplift, and tilting of the sedimentary fill and the acoustic basement.
Uplift of the acoustic basement beneath the trench is greatest
at 7°40'S and 9°20'S where ridges are elevated above the trench floor.
Turbidites occur on top of the ridge at 9°20'S and seaward of the
ridge in a basin which is elevated 300 m above the main trench floor.
Based upon a hemipelagic sedimentation rate of 1.7 cm/1000 yr, the
age of uplift of the ridge is dated at less than 10,000 yrs. B.P.
Similarly, the age of uplift of the elevated basin seaward of the
ridge is dated at less than 34,000 yrs. B.P. near the ridge and at
less than 53,000 yrs. B.P. at the seaward edge of the basin.
The trench shoals and turns eastward as one proceeds from
south to north along the axis. It divides naturally into three segments
separated by the axial ridges at 7°40'S and 9°20'S. The southern
segment trends N31W and has an axial depth of 6300 m; the middle
segment trends N24W at 6200 m; and the northern segment trends
N11W at 5800 m.
The upper continental slope is characterized by submarine
canyons which funnel sediments into the trench axis. The lower
slope is characterized by benches. These benches may define old
imbricate thrust sheets. Ridges in the axis are thought to be new
imbricate thrust sheets which are forming at the boundaries between
segments of the subducted lithosphere.
An apparent fracture zone trending N45E enters the area from
the southwest. Two turbidite basins (B1 and B2) trending N9E occur
northeast of this fracture zone. Turbidite deposition ended in these
basins 5100 yrs. ago. The basins intersect the trench axis just north
of the ridge at 7°40'S and are presently 700 m above the trench axis.
This relative difference in depth is attributed to a combination of subsidence
of the trench and uplift of the oceanic plate upon initiation of
thrust faulting which presumably occurred 5100 yrs. B.P. There is
still insufficient data to determine the exact origin of these basins.
From the regional structure, it appears that the lower continental slope of South America is underthrusting the upper continental
slope along old imbricate thrust faults beneath the Peruvian continental
slope. This overthrusting has caused uplift and accretion of the
continental slope and shelf edge and subsidence and sediment infilling
of the area between the shelf edge and the coastline.
The author suggests that the seismic gap (present lack of large
magnitude shallow earthquakes in this area) may be in part due to the
highly fractured and deformed nature of the subducted Nazca Plate.
Finally, using variable motion along old imbricate thrust faults, the
imbricate thrust model provides mechanisms for reorientation of the
trench and for episodic subduction of the oceanic plate beneath the
trench axis. / Graduation date: 1974
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28793 |
| Date | 28 December 1973 |
| Creators | Prince, Roger Allan |
| Contributors | Kulm, L. D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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