The junction of the western Mexican Volcanic Belt and the Gulf of California represents a superposition of subduction and continental rifting tectonic regimes in the late Cenozoic. Subduction of the oceanic Rivera plate has caused late Cenozoic uplift of western Mexico, forming the Jalisco Block. This paper addresses three questions: Where is the northern boundary of the Jalisco Block; how is the continental fracture system related to that offshore; and what is the spatial distribution and composition of subduction-related lavas?

40Ar/39Ar dates on Cretaceous to Paleogene silicic ash flows show that the northern boundary of the Jalisco Block may be defined by the abrupt change in basement age from Cretaceous to Miocene. 40Ar/39Ar and K-Ar dates on faulted lavas from the Nayarit region indicate that extension at the edge of the Jalisco Block has occurred since ca. 4.2 Ma. The least principal stress (σ3 direction associated with these faults has had two different orientations from 4.2 Ma to the present: Several Pliocene to Holocene (4.20, 1.05, and 0.65 Ma) lava flows, and aligned cinder cones have a N45°W associated least principal stress direction, whereas three Pliocene (3.36, 3.38, and 3.11 Ma) lava flows are cut by faults indicating a north to north-northeast least principal stress direction. The two different stress directions may arise either from structural features in the basement of the arc or from changes in offshore plate boundaries (e.g., 2.5 Ma when the Rivera and Mathematicians plates were locked together).

Whole-rock major and trace element analyses of lavas from the coastal Nayarit region reveal three different lava types: an alkali basalt series, basaltic andesite, and andesite. The alkali basalts show FeO and TiO2 enrichment and have low Ba/Zr and Ba/La ratios, consistent with derivation from an oceanic-island-type mantle. The basaltic andesites and andesites are both alkaline and calc-alkaline, contain hydrous phenocrysts, and have high Sr/Zr and La/Nb ratios, consistent with a subduction-related source. Among these lavas there is a correlation between Sr/Zr, oxygen fugacity, and H2O contents, suggesting that an oxidized, hydrous fluid is involved in the genesis of the subduction-related lavas. The distribution of subduction-related volcanic centers in the coastal Nayarit region, and areas southeast within the Jalisco Block, defines front that parallels the Middle America Trench and is consistent with a 45° dip on the subducted Rivera plate.

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