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Deformation in the Peru Trench, 6⁰-10⁰S

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Abstract

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

Year: 1973
OAI identifier: oai:ir.library.oregonstate.edu:1957/28793
Provided by: ScholarsArchive@OSU
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