in Alaska, magnetic trends can be traced landward beneath the outer 

 edge of the margin, supporting postulated underthrusting of oceanic 

 crust beneath the continents. Deformed sedimentary layers on the 

 outer margin suggest that sediments from the oceanic plate are being 

 accreted to the northwest U.S. margin as they are in Alaska. 



The accretion process, and the resulting deformation on the inner 

 wall and shelf along trenches, continues to be an area of active 

 research. Through support by NSF, scientists have been able to 

 examine this process along the Peru-Chile trench, using mul- 

 tichannel seimsic reflection techniques. Standard reflection techni- 

 ques seldom allowed more than a few hundred meters of seismic 

 penetration in the sediments along the inner wall of this trench. The 

 multichannel technique, with its larger sound sources and improved 

 signal-to-noise ratio, has allowed the oceanic lithosphere to be 

 traced for over 50 kilometers beneath the continental margin. 

 Preliminary analysis of these data shows a changing style of 

 deformation along the trench. Off the coast of northern Peru active 

 accretion of oceanic sediments is taking place, with numerous thrust 

 faults visable in the accreted section of sediments. Off the coast of 

 southern Peru, on the other hand, accretion is much less apparent, 

 and there is some suggestion that continental crust is perhaps being 

 fractured and carried down with the descending oceanic plate. 



The multichannel reflection system is perhaps the single most 

 exciting tool for examining trenches. USGS will initiate surveys of 

 the Aleutian trench using deep-penetration multichannel systems to 

 understand the processes occurring along that trench-continental 

 margin. Multichannel work to provide data on deep-trench structure 

 along very complicated margins is being planned in an NSF- 

 sponsored program in Southeast Asia. Both multichannel and 

 seismic refraction techniques will be used to select sites along 

 trenches for deep drilling under the IPOD program. 



Although the plate-tectonic theory deals primarily with large- 

 scale horizontal movements, the destruction of oceanic crust is 

 apparently the cause of large vertical movements in trenches. 

 Scientists studying the Peru-Chile trench have found uplift rates on 

 the order of a few tens of centimeters per year, when averaged over 

 the last few hundred thousand years. This uplift apparently occurs 

 in response to compression of the accreted sedimentary material 

 along the inner wall of the trench. 



Closely associated with studies of trenches are those of marginal 

 seas that lie behind trench-island arc complexes in the western 

 Pacific. With NSF and ONR support, seismic refraction and 

 petrologic studies have shown that the crust of marginal seas is 

 oceanic in character. Several scientists have suggested that these 

 marginal seas are formed during episodes of extension behind island 



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