Orientations of dikes and hydrothermal veins indicate that east-northeast compression occurred during the main pulse of volcanism in west Texas 39 to 32 m.y. ago. Direction of compression was essentially the same as that during Eocene Laramide deformation. The observations support a continental-arc tectonic setting. The beginning of extension between 32 and 30 m.y. ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in Texas and to bimodal but dominantly rhyolitic volcanism in northern Mexico.

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