MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



513 



CONSTITUTION 



VELOCITY 



CRUST 



OLD MOHO 



TRANSI- 

 TIONAL 



NEW MOHO 

 MANTLE 



14.' jUi'I-uI'li 



1 HI MOBILJZED_BASEMEN_T i_N 1 I 1 I 

 i BASALT_- BAS_EMENT TRANSITION^, 

 BASALT 



liii, 



LL. '_L t_i_ 



ft. 



g 



1 



PARTIALLY MELTED PERIDOTITE 

 OR 



PHASE TRANSITION 

 ECLOGITE TO BASALT 



PERIDOTITE OR ECLOGITE 



6 5r KM/SEC 



7.59 KM/SEC 



8.0 KM /SEC 



KN 



Fig. 31.26. Postulated constitution of velocity layers under eastern part of Great Basin. 



viewed. These must certainly be considered in visualizing the constitution 

 of the crustal layers and the role of the 7.5 layer in tectonism. The writer's 

 ideas of the arrangement in the eastern part of the Great Rasin are shown 

 in Fig. 31.26, and are discussed as follows. 



The mantle is regarded as either peridotite or eclogite. If the 7.5 layer 

 is a transition layer, as seems necessary from its seismic velocities, then if 

 the mantle is peridotite, the transition layer would be one of peridotite 

 and its early melt product, basalt. If eclogite, then basalt or gabbro would 

 result as a phase transition. In either case, Cook's name, mantle-crust mix, 

 would be suitable. The writer favors the peridotite-basalt mix, because he 

 sees in it a means of bringing molten basalt in large quantities upward 



Fig. 31.25. Late Cenozoic uplift in western United States. An attempt is made to portray the 

 broad vertical movements of the silicic crystalline basement layer. Contours in thousands of 

 feet. 



