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This report describes the geometry, emplacement history, petrography, and chemistry of a basaltic intrusive complex in the San Rafael and Capitol Reef areas, Utah. The complex consists of diabasic dikes, breccia bodies, and diabasic and syenitic sills exposed in Jurassijc sedimentary strata of the San Rafael Group on the northwest edge of the Colorado Plateau between the San Rafael and Waterpocket monoclines. The complex is 60 km long in a N. 20° E. direction and has a maximum width of 22 km. Thickness of cover at the time of emplacement determined from stratigraphic relations was no more than 2.5 km, but likely considerably less. K-Ar radiometri.c dates of a diabase (3.8±0.2 m.y.) and a syenite (4.6±0.2 m.y.) from the complex are similar in age to a nearby basalt flow (3.810.2 m.y.), indicating that the complex may have been the plumbing system of a now-eroded volcanic terrane. Relative order of mineral abundances of diabase in the dikes are: augite , labradorite, olivine, biotite, orthoclase, and kearsutite. Accessory and alteration minerals include analcite, apatiteJ titano-magnetite, thomsonite , natrolite, and varying amounts of serpentine, chlorite, calcite, and clay minerals. Hydrothermal alteration is extensive throughout the complex. The intrusions are resistant to erosion, a^nd details of dike morphology are remarkably well preserved. Field data include numerous measurements of dike thickness (average 120 cm), outcrop length (highly variable and map-scale dependent), strike (typically between N. 25° Vi. and N. 15° E., average N. 13° W.), local dike-propagation direction (50 percent range between 0° and 25° of horizontal), occurrences of breccias, and strikes of systematic joints in the host rock. Outcrop patterns of the 153 dikes identified at the map-compilation scale of 1:48,000 are discontinuous and composed of about 1200 smaller dike segments. Dike emplacement may have occurred as either magma flow into, and dilation of, preexisting fractures or flow into cracks both opened and guided by the pressure of magmatic injection relative to principal stress orientations. In the study area evidence exists for emplacement echelon geometry of the dikes can occur when &.n initial crack, propagating in a principal stress plane, becomes rotated about an axis parallel to the propagation direction. The en echelon cracks surfaces into a new plane also roughly normal by both processes. The en can then grow along twisting to the minimum principal stress. The pattern of dike emplacement provides a reference frame for interpretation of stress fields in the transition region between the Colorado Plateau and the Basin and Range Province during a part of Pliocene history.

Geometry, emplacement history, petrography, and chemistry of a basaltic intrusive complex, San Rafael and Capitol Reef Areas, Utah