35. 



Volcanism broke out on the west in Sonora with thick accumulations 

 grading into the miogeosynclinal types on the east. The extent of vol- 

 canism is not well known, but altogether during Cretaceous time the 

 deposits probably extended to the Pacific across what is now Baja Cali- 

 fornia. ( See Chapters 18 and 30. ) Intense deformation of the geanticlinal 

 area also occurred especially in the Early Cretaceous along the northern 

 part, and coarse conglomerates were derived from the uplifted region, so 

 we cannot characterize the area west of the miogeosyncline entirely as 

 eugeosynclinal. Parts of it probably were eugeosynclinal, however, as indi- 

 cated by the San Fernando formation of the northern part of Baja Cali- 

 fornia. The eruptives are said to be andesite flows, tuffs, and agglomerates. 

 The extent of the volcanic area and geanticline is shown by the legend. 

 pre-batholithic volcanic rocks, on the map of Fig. 35.1. 



IGNEOUS AND TECTONIC 

 PROVINCES IN MEXICO 



3EOSYNCLINE 



Very little is known of Mexico in Paleozoic time. In fact, it is not until 

 Late Jurassic that much can be said of paleotectonic conditions when the 

 Mexican geosyncline (Plate 10) began to form. It occupied central Mexico 

 ind extended longitudinally from Arizona to Mexico City (see Fig. 35.1). 

 [t is presumed to have been flanked on the north, west, and south by land 

 ireas, with the western land known as the Occidental geanticline. Up to 

 5000 feet of sediments accumulated in it, in large part an evaporite se- 

 quence. During Early Cretaceous time the geosyncline sank in places 

 12,000 feet to receive additional sediments of the miogeosynclinal type. 



BATHOLITHIC BELT OF THE FIRST CYCLE 



The Nevadan orogenic belt with its great granodioritic batholiths de- 

 veloped in the region of Baja California. This was the western margin of 

 the eugeosynclinal and geanticlinal belt. The Lower and Middle Creta- 

 ceous sediments were folded and invaded by the batholiths and deeply 

 eroded before the Upper Cretaceous sediments were deposited. The 

 plutons are of immense size but have only been studied in northwestern 

 Baja California, where they are typically quartz diorite. Reconnaissance 

 reports generally refer to "granite." The metamorphic rocks have already 

 been described in Chapter 30, and the belt may be summarized as typical 

 of the Sierra Nevada in California and a continuation of it. 



POST-BATHOLITHIC VOLCANISM 



Minor disturbances and general uplift of Baja California, the Gulf of 

 California, and adjacent Sonora followed, leaving a broad land area 

 in this region. New volcanic outpourings occurred in the region of 

 parallel ranges and valleys which are the foothills to the lofty es- 

 carpments of the Sierra Madre Occidental and in the Sierra Madre Occi- 

 dental itself. These are the volcanic rocks that build the extensive Sierra 



549 



