arcs. This extensional process is apparently related to high 

 temperature gradients encountered in the vicinity of island arcs. Not 

 all marginal seas, however, may owe their origin to extensional 

 processes. Scientists have found magnetic lineations, apparently 

 generated by sea-floor spreading, which trend at high angles to 

 active island arcs in both the Bering Sea and Philippine Sea. This 

 distribution of magnetic anomalies is difficult to interpret in terms of 

 simple extensional models. These observations have led to the 

 suggestion that some marginal basins form by entrapment of oceanic 

 crust when an island arc-trench system develops in an oceanic plate. 

 The discrepant trends in the Bering Sea appear to be relatively 

 ancient features of a region that may best be classed as a passive 

 continental margin today. The change of character has resource 

 implications, especially with regard to petroleum occurrence. As a 

 result, USGS has initiated detailed studies, including plans for the 

 examination of deep structures using deep-penetration seismic 

 techniques. 



Structure and History of Passive Continental Margins 



Passive continental margins are formed by sea-floor spreading 

 when initial rifting has served to split a preexisting land mass. These 

 margins then drift "passively" away from the spreading center and 

 are typically free of major tectonic activity. Their dominant 

 characteristics are therefore the lack of seismic activity and a thick 

 cover of sediments. This thick cover of sediments significantly 

 impairs the examination of passive continental margins. Programs 

 in these areas must, therefore, use an integration of gravity, 

 magnetic, and seismic refraction and reflection techniques to 

 examine the deeply buried structures. Indeed, multichannel siemsic 

 reflection techniques were developed by the petroleum industry for 

 studies of thick sediments along passive margins. 



Critical problem areas in the study of passive margins are 



(1) Delineation: to what extent can this boundary actually be 

 delineated and by what means? How much has the boundary been 

 modified by postrifting processes? 



(2) Subsidence: What are the rates of subsidence of the continental 

 edge, and how do these rates change with time? Are the rates related 

 primarily to thermal contraction and subsidence following the initial 

 rifting? Where is the hinge line seaward of which subsidence takes 

 place, and how does it change through time? To what extent is the 

 rate of subsidence affected by sediment loading? 



(3) Formation: After stripping away the effects of the postrifting 

 geological development, what can be learned about the processes 

 associated with the initial rifting? Do fracture zones buried beneath 



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