RECONNAISSANCE STUDIES OF PROTEROZOIC CRYSTALLINE ROCKS IN SONORA: 1968-1978: A STIMULUS TO NEW IDEAS

In 1968 a program of field and geochronology studies was begun to assess displacements along the San Andreas fault (SAF), a recently recognized transform. Study of two belts of Proterozoic rocks (Yavapai --1.8-1.72 Ga and Mazatzal -- 1.71-1.63 Ga) and the northeasterly-striking contact between them would provide a principal basis for assessment. A third distinctive stratigraphic element was anorthosite that is intrusive into strongly deformed and metamorphosed gneiss (1.6+/- Ga) and was known at that time principally west of the SAF.

In extending the known distribution of the Proterozoic belts to the southwest, we found an unexpected and perplexing distribution of Yavapai equivalent basement (the Caborca block, CAB) that suggested either a strongly curved Proterozoic plate boundary or a fault offset that was un-related to the SAF. Two factors supported a fault origin of the anomalous position of the CAB: 1) distribution of Phanerzoic cover rocks, and 2) correlations (thicknesses, ages, and rock types) with units to the north, far removed from Sonora. Correlation between the Caborca block (CAB) and crystalline units in the World Beater Complex in Death Valley was suggested by ~2.0 Ga ages from zircon in Proterozoic quartzite from both localities. The detritus from very old rocks suggests proximity to Archean sources, the closest of which are known far north of Sonora.

Additional Proterozoic crystalline units occurring in Sonora are: 1) porphyritic (1.41-1.44 Ga) granitoids that comprise a common, widespread intrusive suite in both Yavapai and Mazatzal provinces of Sonora, and 2) ~1.1 Ga micrographic granite, restricted to the Yavapai basement of the CAB. The CAB Proterozoic crust was strongly metamorphosed at ~1.65 Ga as recorded by zircon in pegmatitic segregations and gneissic rocks in northwestern most Sonora that yield apparent ages of about 1.67 +/- Ga.

The concept of the Mojave-Sonora megashear arose from these objective data; any model of the tectonic evolution of this area must be concordant with these data.