PART II— D'kNAMICS OF THE SOLID EARTH 



geophysical evidence. The new 

 "global tectonics" appears to provide 

 an adequate explanation of the struc- 

 ture and continental-margin processes 

 of most of the circum-Pacific belt. 



Along the California coast the pat- 

 tern is different, however. No oceanic 

 trench lies seaward of California, and 

 the earthquakes are confined to nar- 

 row, vertical zones beneath the San 

 Andreas fault system to depths that 

 do not exceed about 15 kilometers. 

 Thus, the brittle behavior of the 

 crust in coastal California is confined 

 roughly to the upper half of the crust. 

 Fault-plane solutions of the earth- 

 quakes occurring along the San An- 

 dreas fault system are predominantly 

 right-lateral strike-slip, but some solu- 

 tions indicating vertical fault move- 

 ments are also obtained. As already 

 noted, both geological and geophysi- 

 cal studies suggest that the Mesozoic 

 Franciscan formation of the Coast 

 Ranges was deposited in an oceanic 

 trench. These observations and in- 

 ferences are compatible with the con- 

 cept of a westward-drifting continent 

 colliding with an eastward-spreading 

 Pacific Ocean floor, resulting in conti- 

 nental overriding of the Franciscan 

 Trench and the East Pacific Rise and 

 development of the San Andreas sys- 

 tem as a complex transform fault. The 

 pattern of these relations is not tidy, 

 however, and many problems remain 

 to be solved in unraveling the struc- 

 ture and continental-margin processes 

 of California. 



Isotopes and the Evolution and 

 Growth of Continents 



Lead and strontium isotopic studies 

 of continental and oceanic rocks com- 

 pleted during the past decade have 

 contributed greatly to a better under- 

 standing of processes involved in the 

 growth and development of conti- 

 nents through geologic time. The 

 studies of continental igneous rocks 

 indicate addition of primitive (mantle- 

 derived) material and hence support 

 the concept of continental growth. 



Lead isotope studies of feldspars rep- 

 resenting significant volumes of crus- 

 tal material place constraints on the 

 rate of transfer of uranium, thorium, 

 and lead from the mantle to the crust 

 and suggest early development (3,500 

 to 2,500 million years ago) of a sig- 

 nificant portion of the crust. Geochro- 

 nologic studies of Precambrian rocks 

 show that at least half of the North 

 American crust was present 2,500 

 million years ago, lending support to 

 this thesis. 



Lead and strontium data obtained 

 on young volcanic rocks in the oceanic 

 environment have provided direct 

 information on the existence of sig- 

 nificant isotopic and chemical hetero- 

 geneities in the upper mantle. Sys- 

 tematics provided by these decay 

 schemes allow estimates on the times 

 of development and preservation of 

 these chemical heterogeneities, many 

 of which must have been generated in 

 Precambrian time. If a dynamic crust- 

 mantle system is assumed, the data 

 for the oceanic environment can be 

 interpreted as reflecting events related 

 to the development and growth of 

 continental regions. 



Studies of volcanic rocks being 

 erupted at the continental margins 

 allow an isotopic evaluation of the 

 concept of ocean-plate consumption 

 in this environment. Lead isotopes in 

 volcanic rocks of the Japanese arc are 

 compatible with partial melting of 

 the underthrust volcano-sedimentary 

 plate. Strontium isotopes in calc- 

 alkaline rock series have placed sig- 

 nificant constraints on the basalt- 

 hybridization theory and the concept 

 of partial melting of older crust, how- 



Local studies of lead and strontium 

 in continental igneous rocks have al- 

 lowed evaluation of the involvement 

 of crustal material in the genesis and 

 differentiation of these rocks. The 

 studies are circumscribed, however, 

 by the lack of chemical and isotopic 

 knowledge of the lower crust. If the 

 isotopic anomalies of some conti- 



nental rocks are related to generation 

 in, or assimilation of, the lower crust, 

 this region must be characterized 

 by low uranium/lead and rubidium/ 

 strontium ratios relating to earlier 

 depletion of uranium and rubidium, 

 perhaps at the time of initial crustal 

 formation. 



Although these isotopic studies 

 have provided many answers, they 

 have also generated new questions 

 and problems. Continued work on 

 oceanic volcanic rocks and ultra- 

 mafic rocks of mantle mineralogy are 

 needed. High-pressure experimental 

 work to determine trace-element par- 

 titioning in the mantle is needed to 

 make full use of the isotopic variations 

 that have been observed. Chemically 

 and isotopically, less is known about 

 the lower crust than the upper mantle 

 and upper continental crust. Direct 

 sampling of this environment is a dis- 

 tinct possibility with modern drilling 

 technology; it would provide sorely 

 needed information not only from the 

 isotopic standpoint but also for many 

 other earth-science disciplines. 



Tectonics and the Discovery of 

 Mineral Deposits 



Adequate supplies of mineral raw 

 materials are essential to our econ- 

 omy, but they are becoming increas- 

 ingly difficult to find as we are forced 

 to seek ore deposits that offer only 

 subtle clues to their existence and lo- 

 cation. The science of ore exploration 

 is advancing rapidly, however. And 

 as it does, more is being learned of 

 the basic principles controlling the oc- 

 currence and distribution of ore de- 

 posits and their relation to continental 

 structures. Economic geologists are 

 increasingly adept at predicting where 

 deposits are apt to occur — where in 

 terms of geologic and tectonic envi- 

 ronment and where in terms of geo- 

 graphic areas. 



The essential first step toward in- 

 creasing our knowledge in this field is 

 to plot known mineral deposits and 



30 



