MODERATE CLOCKWISE ROTATION OF CARMEN ISLAND DURING OPENING OF THE GULF OF CALIFORNIA

The Gulf of California has a highly oblique-divergent plate boundary that formed since 12 Ma. The modern system of transform faults and spreading ridges formed since 5.6 Ma based on formation of the Alarcon spreading ridge and related transform fault in the mouth of the Gulf and transtensional basins and faults of Pliocene age on the margins of the Gulf. It has been postulated that the transtensional Loreto fault and related normal and strike-slip faults in the Loreto rift segment were directly linked to major strike-slip faults offshore that became two transform faults. Carmen Island is a horst that is part of the Loreto segment. The elongate island has an anomalous NNE orientation compared to other islands in the Gulf of California. Bedding in Oligocene(?) to middle Miocene(?) volcanic rocks, when compared to bedding in the Loreto segment on the Baja California peninsula, suggests that the island has rotated clockwise 30° to 40°. We conducted a paleomagnetic study of tuffs and andesite flows, and Pliocene strata on Carmen Island to test this proposed rotation. The volcanic rocks are being dated to confirm their correlation to the Comondu Group rocks that have been dated between 29 and 12 Ma near Loreto. The Pliocene strata lay over the volcanic rocks across unconformities and are dated by foraminifera. We collected paleomagnetic samples from 17 sites on both sides of the southern and central parts of the island and in the Perico basin in the northeast part of the island. Fifteen sites are from the older volcanic rocks and two sites are from Pliocene strata. We also have one reliable site from the Loreto segment on the peninsula. Samples have been analyzed and analysis of the sites and domains is ongoing. Preliminary analysis shows a range of clockwise rotation of sites from 15° to 45°. We will present the final analysis. A north-widening graben between Carmen Island and Loreto supports this rotation. Moderate clockwise rotation of Carmen Island likely occurred between en echelon dextral strike-slip faults that linked to the better understand Loreto fault from ~6 – 2 Ma. These strike-slip faults may have been the precursors of the transform faults of the modern plate boundary. The strike-slip faults and rotation likely died when the transform faults linked to the modern spreading ridges at ~2 Ma.