552 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Pliocene to Recent, and the zone is known to be one of modern seismic 

 activity. 



Along the adjacent western margin of Sonora, particularly in the 

 province of parallel ranges and valleys, are fanglomerates with basal 

 basalt flows and agglomerates of late Pliocene and perhaps younger age, 

 the Baucarit formation (King, 1939). These have accumulated in down- 

 faulted intermontane depressions. The basalt flows are generally conspicu- 

 ous on the back slopes of tilted blocks where the overlying fanglomerates 

 have been eroded away. The early or mid-Tertiary eruptives of the 

 Sierra Madre Occidental are generally less basic. 



Renewed orogenic activity resulted in overthrusting of rocks of each 

 of the ranges west of the Sierra Madre westward over the Baucarit beds. 

 This was observed north of the 28th parallel ( Guaymas ) , but south of the 

 parallel the faults are normal (King, 1939). In addition the western 

 Sonoran normal and reverse faults are of the same age approximately as 

 the Gulf faults and, hence, evidently belong to the same system. The 

 reverse faults may be gravity slide phenomena. We have to deal, then, 

 with a complex fault zone 150 miles wide in which submergence of the 

 Gulf area relative to uplift of Baja California and the Sierra Madre was 

 of the order of 10,000 feet. 



Accompanying the faulting was the eruption of a large volcanic field on 

 the southern part of the peninsula of Baja California. The accumulation 

 is known as the Comondu formation which is made up of "many kinds" of 

 volcanic rocks. The volcanism occurred possibly in Miocene time, but 

 stratigraphically the flows seem related to the Baucarit formation of west- 

 ern Sonora of late Pliocene age. Comondu rocks may have been deposited 

 near sea level and now are at elevations of 1000 to 5000 feet, which 

 means adjustment of this order of magnitude along the great fault zone 

 in Pleistocene time ( See Chapter 30 ) . 



Here, in western Mexico, the downfaulted belt has developed along the 



continental side of the batholithic (Nevadan) belt, and evidently on the 

 Cretaceous eugeosynclinal volcanic belt. This is a normal relation in 

 reference to the Andean depressed belts. If the Paleozoic metamorphic 

 belt exists here, it is mostly under the depressed area and covered. The 

 second cycle batholithic belt is partly involved in the faulting, but mostly 

 it is along the east margin of the fault zone. The great early and mid- 

 Tertiary volcanic field of the Sierra Madre Occidental is east of the fault 

 zone and suffered uplift at the time. Within the fault zone and on the 

 west, on top of the first cycle batholithic belt, volcanism was recurrent. 

 The field is of great extent in the southern part of the peninsula. Vol- 

 canoes were active during the Pleistocene and have continued active to ; 

 the present. Isla Tortuga is a very young volcano in the Gulf, and Las 

 Tres Virgenes are said to have been active in historic times. Isla Coro 

 nada is a Pleistocene andesitic volcano. Many cones and flows on the 

 western slopes of Baja California exhibit features of recency (Beal, 

 1948). 



The tectonic and petrologic relations of Baja California, the Gulf, and 

 adjacent Sonora are similar to those of the Andes, but south of the Gulf 

 the relations are less familiar. The Nevadan batholithic belt seems to be 

 continued by the Paleozoic metamorphic belt and Mid-Cretaceous intru- 

 sions. North of the Paleozoic metamorphic belt is the southern termina- 

 tion of the great early and mid-Tertiary volcanic field of the Sierra Madre 

 Occidental, and on top of these post-batholithic volcanoes and on the meta- 

 morphic rocks are great new piles of late Tertiary and Quaternary vol- 

 canics. According to Andean precedent these stratovolcanoes should be 

 accompanied by a fault zone. The southern limit of the Mexican Plateau 

 is said to be marked by a high fault scarp, but its position is not evident 

 on the new geologic map of Mexico. The Balsas basin province may be due! 

 to downfaulting, but the writer has not been able to learn anything of the 

 fault relations there. 



