Echo soundings recently made in Bah¿a de Banderas show that this region is a graben with steeply dipping walls and several basins; it is the offshore continuation of the Valle de Banderas graben, and of a branching rift (R¿o Ameca rift) originating in the Tepic-Zacoalco rift zone. The general trend of the three structures is ENE with some NE trending offsets, and they have a total length of 150 km; this Banderas Rift Zone is proposed as the NW limit of the Jalisco block. The existence of this limit suggests that there is another platelet, or block, between the Jalisco block and a portion of the Rivera plate, probably bounded by the Tres Mar¿as escarpment, the Jalisco block and the North America plate.