Graduate Thesis Or Dissertation
 

Peruvian deep-sea sediments : evidence for continental accretion

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  • 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.
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