CHARACTERIZATION OF THE CRETACEOUS COLLISION ZONE BETWEEN THE WESTERN AND EASTERN PENINSULAR RANGES BATHOLITH IN THE SIERRA CALAMAJUE, BAJA CALIFORNIA NORTE, MEXICO

The Jura-Cretaceous Peninsular Ranges batholith is commonly divided into western (WPRb) and eastern (EPRb) zones. Geologic, petrologic, geochemical, and isotopic studies indicate more primitive, island arc characteristics for the WPRb, while the EPRb resembles a more evolved, continental-margin arc. South of the Agua Blanca fault, the transition between these zones is marked by Cretaceous inter-arc basins that were deformed when the Alisitos arc (WPRb) collided with North America during the mid-Cretaceous.

In the Sierra Calamajue, the WPRb is composed of chlorite to biotite grade volcanogenic strata, which are juxtaposed with Cretaceous pelitic and carbonate deposits and Paleozoic miogeoclinal and deep-water deposits of the EPRb across SW-vergent reverse and thrust faults. The EPRb is composed of amphibolite grade metasedimentary and local metaigneous rocks. A detrital zircon analysis from this strata matches age spectra of Ordovician quartzites from North America indicating that this sequence most likely represents Paleozoic deep basin deposits of North American affinity.

Deformation in the WPRb increases from west, where rocks generally show a single, weak cleavage, to east, where deformation is intense, field strain measurements range from 60 to 90% shortening and >>100% extension, and tight folds of bedding are transposed by a NW-SE striking, moderately to steeply NE-dipping cleavage (S_T). In the EPRb, moderate to very high strain has strongly disrupted layering and all but completely obliterated depositional features. Abundant isoclinal folds of compositional layering are, in places, transposed by S_T and locally tightly refolded.

We suggest that this section represents the transition between a magmatic arc accreted to North America during the mid-Cretaceous and deep water rocks, which may have been deposited along the Paleozoic passive margin of North America and possibly have correlative sequences in Sonora, MX. More complex deformation in the EPRb may indicate a prolonged deformation history that is not simply explained by Cretaceous deformation alone, but instead may be related to Permo-Triassic deformation along the Paleozoic southern margin of North America as reported from similar strata in Sonora, MX.