| Summary: | In order to determine whether the sediments found on the landward
wall of the Peru Trench are accreted Nazca Plate sediments,
the clay mineralogy and organic carbon contents of 52 surface
samples were submitted to factor analysis. Q-mode factor analysis
resolved the data from the Nazca Plate and Peru continental margin
into three factors. The most important factor (oceanic assemblage)
is strongly associated with Nazca Plate sediments and is comprised
of smectite and aeolian illite. In contrast, upper continental margin
sediments are dominated by either of the two continental factors (A or
B). The principal difference between the continental factors is that
mixed-layer smectite-chlorite clays are characteristic only of continental
assemblage A. Lower continental margin sediments are
characterized by either an oceanic or continental factor dominance.
The boundary between sediments dominated by the oceanic
factor and those dominated by the continental factor was as much as
100 km to the west of its present position earlier in the Quaternary.
The seaward shift in the boundary is attributed to westward shoreline
displacement in response to glacially-induced sea level changes,
increased erosion rates on land during more humid times, and
deposition of continental factor dominated sediments seaward of the
present Peru Trench axis.
Quaternary sediments from 27 cores reveal minor fluctuations
with time in factor loadings in Nazca Plate and upper continental
margin cores and significant variations in some areas near the trench
axis and on the middle to lower continental slope. Displacement of
oceanic sediments into areas with continental sediments is determined
with respect to the factor dominance boundary. Using this
method, continental accretion is indicated for five cores, located up
to 3000 m above the trench floor. One core on the middle continental
slope off Lima, Peru, contains diatom-rich Quaternary dolomite
that probably originated as calcareous sediment on the Nazca Ridge.
If this is true, left-lateral strike-slip motion of the Nazca Ridge along
the Peru Trench axis is indicated.
The bulk of the 28 cores recovered from the acoustically complex
landward wall of the Peru Trench contain sand-silt turbidites of
continental origin. Even though there is a distinct overprint of
terrigenous sedimentation, accreted oceanic sediments can be
recovered in a tectonically active convergent plate boundary. === Graduation date: 1974
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