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In situ photo of Galapagos Rift hydrothermal vent. First hand evidence of hydrothermal plume. Fluids contain H 2 S. 



Nazca Plate Study 



The Nazca Lithospheric Plate lies adjacent to the event edge 

 of the great metallogenesis province of the Andes. This area was 

 the subject of major field programs from 1972-75 by Oregon 

 State University and the Hawaiian Institute of Geophysics in 

 cooperation with scientists from Chile, Columbia, Peru, and 

 Ecuador. (For a list of projects, see table 12.) The results were 

 synthesized into comprehensive models of the Nazca Plate that 

 served as site surveys for subsequent drilling on Offshore Drill- 

 ing Project (OSDP) Leg 34 by the Glomar Challenger. 

 Three holes were drilled through the sedimentary sequence into 

 basement rocks, one in the Bauer Basin (metalliferous sedi- 

 ments) and two on the seawater side of the Peru-Chile Trench. 



Based on seismic refraction velocities, the western edge of the 

 plate has a relatively thin ocean crust thickening to 30 km near 

 the trench. Oceanic magnetic anomaly patterns make it possible 

 to follow the history of plate origin and movement during the 

 past 26 million years. The information obtained here is expected 

 to be applicable to understanding the process along subduction 

 zones around the circum-Pacific belt. 



Sediments on the Nazca Plate are derived from four sources: 

 1) hydrothermal (direct precipitation from seawater hydrother- 

 mal systems); 2) biogenous; 3) detrital (weathering and erosion 

 of crustal rocks); 4) hydrogenous (precipitation from normal 

 bottom waters). Most metalliferous sediments were deposited by 

 hydrothermal fluids emanating from sources of basaltic mag- 

 matism along the East Pacific Rise (EPR). Deposition, how- 



ever, took place in normal low temperature seawater. 



Analysis of the sedimentary sequence cored at Deep Sea 

 Drilling Project Site 319 (Leg 34) in the Bauer Basin shows a 

 change in composition through time parallel to the changes in 

 surface sediments, as one moves from west to east across the 

 Nazca Plate. Metal accumulations are highest near the base- 

 ment layer, suggesting strong hydrothermal contribution during 

 the early history of this site. Abundant manganese nodules and 

 crusts were also recovered from the Bauer Basin. Analysis 

 shows a distinct mineralogical and chemical difference between 

 them. Normal precipitation from seawater controls the miner- 

 alogy and chemistry of the crusts, while that of the nodules 

 appears to be governed by small-scale reactions in the under- 

 lying sediments. The presence of biogenic opal appears to be 

 a critical factor in nodule formation; a factor that should be 

 useful in explaining concentration of nodules in other areas of 

 the sea floor. 



Study of formation of large grabens in the Chile Trench, a 

 result of bending of the Nazca Plate, may explain the tectonic 

 erosion of sections of the Chile continental margins. Other evi- 

 dence suggests that the bending has produced rapid tectonic 

 uplift along the trench axis. A detailed set of bathymetric maps 

 of the Peru-Chile Trench, including all the data collected during 

 the Nazca Plate project, was completed and has been accepted 

 for publication by the Geological Society of America. The maps 

 synthesize the results of this project, are in the final stages of 

 editing, and are scheduled for publication late in 1978. 



59 



