Abstract

The Precambrian core of the southern Wet Mountains consists in part of metasedimentary gneiss and schist concordantly foliated with granite gneiss and composite rocks—migmatite and lit-par-lit gneiss. These rocks developed by alteration during intrusion of the San Isabel Granite batholith and associated smaller plutons. Migmatite borders the batholith and grades outward into lit-par-lit gneiss as a result of more intensive reconstitution and metasomatism nearer the igneous mass. Foliation in the metamorphic and composite units tends to “wrap around” the plutons and is discordant to the foliation (planar flow structure) of these bodies. Lineation, which developed in the metamorphic and composite rocks during folding and neocrystallization of the sequence, plunges northward. Lineation (linear flow structure) of the granite plutons also generally plunges northward and supports the belief that the plutons are of common origin.

Field relations and petrographic investigations, including heavy-mineral studies, indicate a genetic relation among the granite bodies; the smaller plutons are probably contaminated differentiates of the magma which formed the San Isabel batholith. Lead-alpha age dating of zircon from the San Isabel Granite indicates that this batholith represents part of the orogenic activity which occurred about 1500 m. y. ago.

The southern Wet Mountains form a southeast-plunging anticline with the dominant fracture pattern striking N. 60° W., parallel to the range axis, and a minor trend N. 40° E. Complex structural relations reflect both Precambrian deformation and later superimposed fracturing. Late Paleozoic uplift (Ancestral Rocky Mountains) occurred along pre-existing lines of weakness; Laramide deformation and Tertiary block faulting, although influenced by these trends, formed additional fracture patterns. The Wet Mountains were uplifted to present heights from mid-Tertiary to Pleistocene. Probably the Wet Mountain massif acted as a resistant block to regional compression and yielded by upward movement, although uplift may be the result of solely vertical stresses. Movement occurred in stages along prominent high-angle boundary faults and by slight displacements along innumerable joints.

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