TECTONIC RESTORATION OF THE BAJA CALIFORNIA MICROPLATE AND THE GEODYNAMIC EVOLUTION OF THE PACIFIC-NORTH AMERICAN PLATE MARGIN

The southern Gulf of California contains key relationships that define microplate interactions and the magnitude of Pacific-North American plate motion since 12.5 Ma. Transtensional shearing initiated on either side of the Baja microplate, but only in the gulf did the rifting progress to seafloor spreading. Across the mouth of the gulf, 400 to 300 km of late Neogene oceanic crust separate correlative batholithic rocks in Puerto Vallarta and southernmost Baja. However, closing this ocean basin also requires restoration of the extended continental margins, which raises the total separation across the gulf to at least 450 km parallel to 305. Faults in the pacific borderland likely accommodated much less displacement based on the lack of seafloor spreading and numerous correlations across the movement zone. Farallon-derived microplates are found along the entire length of the unfaulted Baja microplate suggesting that they underplate and contribute to the strength of this thin sliver of continental crust. Moreover, transpeninsular faults (e.g., Agua Blanca and Carrizal) are only found adjacent to the northern and southern limits of Farallon-derived microplates. The boundary between the Magdalena and Guadalupe microplates coincides well with profound geologic transitions within the Baja microplate including major north-south changes in the magnitude of basement uplift and the character of Miocene arc volcanism. Lastly, the Magdalena submarine fan is not significantly displaced from El Cien, a likely source terrane on Baja. These correlations indicate a maximum of 150 km of transtension across faults west of Baja.

This tectonic reconstruction places the northernmost Farallon-derived microplate very close to an abrupt change in modes of extension and width of the mainland rifted margin. This suggests that seafloor spreading in the southern gulf required narrow rifting above a deep window in the Farallon slab, whereas a broader rift and metamorphic core complexes could only develop in Sonora, which experienced different boundary conditions and had a thermal evolution that could generate midcrustal asthenosphere. In contrast to the widely-proposed model of orthogonal rifting in the protogulf, this reconstruction requires integrated transtentional shearing in the gulf from the start (~12.5 Ma).