IGNEOUS AND TECTONIC PROVINCES IN MEXICO 



551 



Madre Occidental proper (see Fig. 34.5, section E-E'-E"). They are 

 thought to be early Tertiary by King (1939) but the new geologic map 

 of Mexico (1956) indicates them "principally as andesites of the Oligo- 

 cene and rhyolites of the Miocene with corresponding pyroclastics." King 

 also indicates that the Tertiary volcanic rocks are more acidic and more 

 varied than the older beveled Cretaceous volcanic rocks upon which they 

 rest in places in central Sonora. They contain a considerable thickness of 

 rhyolite and some flows of basalt. In northeastern Sonora, Imlay ( 1939 ) 

 notes that the lavas aggregate more than 5000 feet in thickness, and basalt 

 predominates toward the top of the deposits but rhyolite and andesite 

 are the most common. Basalt appears more abundant than in the region 

 studied by King. Howell Williams (personal communication) recognizes 

 large sheets of welded tuffs and thinks that these may be very extensive 

 in the Sierra Madre Occidental. Much of the volcanic rock, heretofore 

 called flows, at the north end of the Sierra Madre Occidental are welded 

 rhyolitic tuffs (Enlows, 1955). The volcanic flows appear to be the result 

 of fissure eruptions (King, 1939), but tuffs and pyroclastics indicate the 

 occurrence of central vent eruptions also. The accumulations are thickest 

 in the eastern Sierra Madre Occidental. 



BATHOLITHIC BELT OF THE SECOND CYCLE 



Along the western margin of the Sierra Madre Occidental, particularly 

 in the region of parallel ranges of west-central Sonora, a mid-Tertiary 

 |( post-volcanic) orogeny occurred, and the volcanic and older rocks were 

 folded in a measure exceeding the previous Laramide folding there. Ac- 

 companying the folding were vast intrusions of granite, diorite, and 

 granodiorite which ascended through the Paleozoic and Mesozoic strata 

 and in places penetrated the Oligocene and Miocene volcanic rocks. 

 Granites predominate (King, 1939). These are the black areas on the 

 map of Fig. 35.1 along the western margin of the Sierra Madre. In the 

 Sonoran Desert geomorphic province the granites are carved to broad 

 pediments, and the plutons are so extensive there that one may infer that 

 the whole region is underlain by a vast batholith or series of large related 

 plutons. 



METAMORPHIC AND INTRUSIVE BELT 



Extending across southern Mexico from Banderas Bay to the Isthmus 

 of Tehuantepec is a belt of metamorphic rocks and various intrusive 

 bodies. In width the belt extends from the coast to the Tertiary volcanic 

 rocks of the Mesa Central which cover it irregularly on the north (Chap- 

 ter 43). 



Considerable parts of the belt shown on the map of Fig. 35.1 are covered 

 with Jurassic and Cretaceous strata as well as fields of volcanic rocks 

 whose age is not well known. 



Although very little can be learned about the belt of orogeny, it seems 

 evident that a pre-Jurassic and probable late Paleozoic age for most of it 

 must be recognized. The Sierra Madre del Sur with its numerous post- 

 metamorphic intrusions is regarded as a continuation of Baja California 

 and therefore, of the Nevadan belt. 



RELATION TO DEPRESSED BELTS 



The Gulf of California is regarded as a depressed area along a zone of 

 faults (Shepard, 1950). The faults in places have displacements com- 

 parable to those along the east side of the Sierra Nevada, and if the slope 

 of their submarine escarpments has not been reduced since faulting, then 

 the fault planes dip at rather low angles, which seems unusual. It is also 

 observed that the San Andreas fault system extends through southern 

 California to the head of the Gulf of California, and thence continues 

 southward as the fault zone of the depressed Gulf area. Since the San 

 Andreas and related fractures are generally recognized as a system of 

 strike slip or wrench faults (Hill and Dibblee, 1953), a conflict in interpre- 

 tation of the nature of faulting is evident. It is postulated in Fig. 31.22 

 that the block of Baja California has moved northwestward about 300 

 miles along the San Andreas fault zone and in so doing, has pulled away 

 from the mainland somewhat, leaving the Gulf of California floored with 

 oceanic crust. There can be no doubt, however, that the Gulf is a zone 

 of subsidence in late Tertiary and Quaternary time. Anderson (1950) 

 observed the faulting on islands in the Gulf to have extended from 



