VOLCANO-TECTONIC EVOLUTION OF THE CENTRAL BAJA CALIFORNIA GULF MARGIN: 40AR/40AR GEOCHRONOLOGICAL CONSTRAINTS

Volcanic rocks of central Baja California have been divided into subduction-related calc-alkaline andesites (~24-12 Ma), and post-subduction “Bajaites” (<12 Ma), enriched in incompatible REEs, further subdivided into magnesian andesite (with 50-58% SiO2 and high Mg) and adakite (>56% SiO2 and Mg<3%)^1,2. The Bajaites correlate spatially with a fossil slab imaged under central Baja, and are inferred to record post-subduction slab melting by asthenospheric upwelling associated with rifting or slab breakoff³.

In the Santa Rosalía area we have mapped and dated calc-alkaline volcanic rocks at 13.32 to 9.83 Ma, rift transitional volcanic rocks at 9.83 to 9.34 Ma, and a magnesian andesite intrusion and lavas previously included in the Bajaite suite¹, which we date at 6.1 Ma ± 0.3 Ma. The magnesian andesites are contemporaneous with Boleo mineralization in the Santa Rosalía basin, and may have provided hydrous fluids for mineralization in the basin.

In the Mulegé area, the oldest rocks are down-to-the west graben filling andesitic breccia and sandstone red beds with interstratified proximal adakite block-and-ash-flow tuffs. These are cut by an E-W dike swarm with andesite and magnesian andesite compositions, dated at 16.34 ± 0.20 Ma and 16.66 ± 1.01 Ma. This was followed by eruption of andesite and magnesian andesite lava shields, dated at 12.39 ± 0.62 Ma and 10.86 ± 0.06 Ma, respectively. The sequence was then cut by a N-S fault >15 km long, with ~800 m down-to-the-east offset (lateral offset unknown). This was followed by development of a peneplanation surface, draped by a >200 m thick sequence of areally-extensive magnesian andesite lavas (dating in progress). Finally, the extensive magnesian andesite lavas were tilted westward about Gulf Escarpment down-to-the east normal faults, with the lavas forming a very broad west-dipping footwall monocline that extends from the escarpment to the Pacific margin of Baja California.

This work shows for the first time that not all “Bajaites” are post-subduction in age, but instead range back to 16 Ma (magnesian andesite dikes) and older (adakite block-and-ash-flow tuffs cut by the dikes).

¹ Calmus et al., 2003, Lithos 66 (77-105); ² Calmus et al., 2011, Pure and Applied Geophysics 168 (1303-1330); ³ Wang et al., 2013, PNAS, 5342–5346.