Structure 



The geological structure of a region can 

 often be inferred from its physiography, par- 

 ticularly when the latter is largely of tectonic 

 origin. Information about deep structure 

 must come from indirect geophysical meas- 

 urements. Although such measurements are 

 incomplete off southern California, those 

 that do exist permit the drawing of some 

 conclusions about the composition and ori- 

 gin of the region. 



Faults Inferred from Topography 



The location of faults on the sea floor 

 presents somewhat different problems from 

 those of locating faults on land. Detailed 

 lithologic and stratigraphic evidence is so 

 much harder to obtain on the sea floor than 

 on land that it has been used only in a few 

 nearshore areas where intensive surveys of 

 the bottom have been made mostly by oil 

 companies. Visibihty in the ocean is so poor 

 that fault contacts, slickensides, offsets of 

 bedding planes, and other minor details can 

 rarely be recognized except in shallow water. 

 Topography, however, is a better tool for 

 locating faults on the sea floor than on land. 

 Recognition of fault scarps on land is often 

 made difficult by the masking effect of ero- 

 sion by streams. Locally, of course, erosion 

 on land aids recognition by the selective re- 

 moval of rocks that have been mechanically 



weakened by faulting or chemically altered 

 by weathering where faulting has opened 

 avenues for passage of ground water. On 

 the sea floor weathering is practically non- 

 existent, and erosion is far less important 

 than on land. Consequently, the topography 

 of the sea floor is chiefly of structural origin; 

 exceptions occur in shallow water where 

 wave erosion has truncated the structures or 

 locally in deeper water where submarine 

 canyons have been cut perhaps by turbidity 

 currents. Deposition, a greater factor than 

 on land, eventually buries the fault scarps, 

 but sediments are first deposited only around 

 the lower parts of the topography, leaving 

 the higher parts virtually unaffected. 



The difference in rate of erosion of fault 

 scarps on the land and the sea floor makes 

 difficult any estimation of their comparative 

 ages from physiography alone. For exam- 

 ple, many Pleistocene fault scarps on land 

 have been so eroded as to be practically un- 

 recognizable, whereas some scarps on the sea 

 floor that are believed to be of Late Miocene 

 age appear sharp and clear from sounding 

 data. 



As discussed in the section on topography, 

 the criteria for fault origin of the sea floor 

 scarps is straightness, offsets, height, steep- 

 ness, step-like profiles, and linear depres- 

 sions at the base. Using some of these 

 criteria, Lawson (1893Z>) was one of the first 

 to point out the presence of fault topography 



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