The Jalisco block of southwestern Mexico records the initial stages of continental rifting superimposed on a convergent continental margin. Pliocene- Quaternary eruptions in north-northwest-trending graben systems within the block produced extraordinarily diverse lava types, including minette, leucitite, absarokite, and andesite. Interspersed with these are several basaltic shield volcanoes that have compositional similarities to lavas from oceanic islands. Evidence of Miocene Basin and Range extension, which is seen around much of the Gulf of California, is found only along the northern margin of the Jalisco block. Large volumes of alkali basalt were also erupted during the late Miocene at several locations along this northern margin. We propose that Pliocene- Quaternary extension and associated volcanism within the confines of the Jalisco block are consequences of complex plate-boundary reorganizations that are causing the block to rift away from the Mexican mainland

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