The terminus of the Middle America Trench has been traditionally represented as an arc, concave towards the continent. TresMarías Islands are located at the terminus of the Middle America Trench in western Mexico, and their location is not only intriguing but also a key to the re-construction of the position of Baja California peninsula before separation from the North America plate. Previous re-constructions suggested various places around the location of TresMarías Islands for the position of the tip of the peninsula, and several converge to a position that invades the area occupied by the islands, offering no explanation for the overlap. Before peninsular separation from North America, the Guadalupe trench followed a smooth curve; recreating the position of this paleo-trench yields a baseline for fixing the position of the peninsula, as well as the original position of the TresMarías Islands fragment. A new tectonic view of the structure of the Middle America Trench terminus is proposed, replacing the traditional arc representation with a series of en echelon blocks, the northernmost terminates at the TresMaríasEscarpment. The long sides of the blocks correspond to previously identified geological faults in the Bahía de Banderas region, while their offshore continuation is supported by topographic observations. As a test of this model I show the corresponding re-construction of the position of Baja Californiaprior to separation from the North America plate and the positional evolution of the peninsula and the TresMarías fragment from Chron 4n.2 (7.90 Ma) to Chron 3n.4 (5.12 Ma).
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