West Spanish Peak and Dike Mountain in south-central Colorado are stocks which cut Tertiary sedimentary rocks near the axis of the La Veta syncline, the structural trough of the Raton basin. Associated with these stocks are radial dike swarms. The outline of the West Spanish Peak dike swarm is elliptical. The Dike Mountain swarm is more radial, and its outline is oval. Both systems are elongated normal to the axis of the La Veta syncline.

The dikes of the West Spanish Peak swarm are of diverse rock types and represent several separate magmatic phases. Those of the Dike Mountain swarm are facies of syenodiorite and probably represent a single phase of magmatic invasion.

The dikes occupy vertical joints that have been generally attributed to radial fissuring during doming of the sedimentary rocks by the emplacement of the stocks. Structural studies, however, indicate that several systems of shear and tension joints resulted from intermittent orogenic stresses of varying direction and magnitude during folding of the syncline before invasion of the magmas, and the writer suggests that selective intrusion into this joint complex accounts for the dike patterns.

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