SHAPE AND STRUCTURE OF OCEAN BASINS 



31 



rSOOO 





Fusion curves after 



Uffen (1952) « •« »t 

 Simon (1953) x x x 

 Gilvorry (1957) 1+ + + 

 a) p 91 b) p 93j 



Temperature after 



Gutenberg (1951) 



Jeffreys (1952) 



Jacobs (1956a, modelE) 



Verhoogen (1956) 



Gilvorry (1957) 



Lubimova (1958) 



Fig. 12. Calculated fusion temperatures and estimated temperatures in the 

 earth. Some curves are based on graphs preferred by the respective author; others 

 are averages of several graphs or tables. xA.U contain uncertain assumptions (Guten- 

 berg, 1959). 



The low-velocity zone, shown in Figs. 7 and 9, extends from 

 about 50 km for oceans and from 100 km for continents down to 

 about 200 km. Lubimova (1958) has calculated lower thermal 

 conductivities in this region (see Fig. 14), and it is quite likely that 

 temperatures approaching the melting point, and consequently 

 much smaller values of shear modulus, must be found in this 

 region. These lower values of shear modulus will of course affect 

 the shear waves more than the compressional waves (Lehmann, 

 1954). 



In Fig. 7 a straight line has been indicated in both the P and 5 

 curves. The intercepts of these lines indicate extrapolated surface 

 velocities of about 7.3 and 4.2 km/sec. These are suggestive of the 

 seismic velocities found for the upper mantle in anomalous areas 

 such as the Mid-Atlantic Ridge, parts of the Caribbean area, and 

 some continental margins. 



New melting point curves, based on seismic velocity distribu- 

 tions (UfTen, 1952; Gutenberg, 1959), including a region of lower 



