SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA FLOOR 



263 



more than 20,000,000 years ago. In the section of the Murray Fracture Zone 

 nearest the continent, the rehef becomes more subdued, but there are several 

 short troughs more than 500 m deep and the regional change in depth still 

 exists. A sea-borne magnetic survey (Vacquier et al., 1961) of this region shows 

 a north-trending pattern of anomalies with magnitudes of about 500 y. The 

 anomaly jDattern is disturbed along the east-trending line of the Murray 

 Fracture Zone. The pattern can be matched on opposite sides of the disturbance 

 and it appears that it has been offset 150 km, which would place it among the 

 largest fault displacements known. Even so, the displacements on the Pioneer 

 and Mendocino fracture zones are much larger (Fig. 26). The transverse ridges 

 of California lie on the trend of the Murray Fracture Zone and, at least super- 



730 miles left lateral 



MENDOCINO FR/^CTURE 



.170 miles 

 left lateral 



PIONEER RIDGE 



97 miles 

 right Icterol 



WIURR^'^ 



TrStureTone 



_:hv_ 



Fig. 26. Displacement of a prominent magnetic anomaly by three fracture zones. Distances 

 in statute miles. (After Vacquier et al., 1961.) 



ficially, they seem to be an extension of the zone. Fracture zones are difficult to 

 detect without continuously recording echo-sounders, and it is probable that 

 many more will be found (see Chapter 16). 



h. Minor lineations 



Widely distributed minor lineations on the deep-sea floor frequently lie in 

 pinnate patterns relative to the major lineations such as island arcs, archi- 

 pelagos and fracture zones. Individual minor lineations are generally 20 to 

 200 km long and consist of: (1) elongate submarine volcanoes, atolls on volcanic 

 platforms and volcanic islands ; (2) lines of closely spaced circular submarine 

 volcanoes ; (3) asymmetrical ridges ; (4) narrow troughs ordinarily associated 

 witli parallel asymmetrical ridges ; and (5) straight troughs resembling sub- 

 marine canyons on continental and insular slopes. 



