Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California

Abstract

Mxico) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommo dation across the obliquely divergent Pacific-North America plate boundary. Marine strata exposed on southwest Isla Tiburn (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Mio cene interpretation for SWIT marine deposits has played a large role in subse quent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ~1300 km of the plate boundary, and the timing of marine incursion into the Gulf of Califor nia. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextraloblique La Cruz fault. We integrate these results with previ ously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maxi mum thickness of ~300 m. The 6.44 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ashflow tuff stratigraphically below the oldest marine strata, and the 6.01 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ashflow tuff, is interbed ded with marine conglomerate near the base of the marine section. A dikefed rhyodacite lava flow that caps all marine strata yields ages of 3.51 0.05 Ma (Ar/Ar) and 4.13 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 1.81 Ma (KAr) from the flow top and (KAr) 3.7 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2-4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geo chronology thus constrain earliest marine deposition on SWIT to ca. 6.2 0.2 Ma, coincident with a regionalscale latest Miocene marine incursion into the northern proto-Gulf of California. This regional marine incursion flooded the northernmost, >500kmlong portion of the Gulf of California shear zone, a narrow belt of localized strikeslip faulting, clockwise block rotation, and subsiding pullapart basins. Oblique Pacific-North America relative plate mo tion gradually localized in the >1000kmlong Gulf of California shear zone ca. 9-6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway.