Physical processes of shallow mafic dike emplacement near the San Rafael Swell, Utah

Abstract

Some 200 shonkinite dikes, sills, and breccia bodies on the western Colorado Plateau of south-central Utah were intruded from approximately 3.7 to 4.6 Ma, contemporaneous with mafic volcanism along the nearby plateau margin. Thicknesses of dikes range to about 6 m; the log-normal mean thickness is 85 cm. Despite the excellent exposures of essentially all dikes in strata of the Jurassic San Rafael Group, their number is indeterminate from their outcrop and spacing because they are everywhere greatly segmented. By our grouping of almost 2000 dike segments, most dikes are less than 2 km in outcrop length; the longest is 9 km. Because the San Rafael magmas were primitive and probably ascended directly from the mantle, dike lengths in outcrop are much less than their heights. The present exposures probably lie along the irregular upper peripheries of dikes that lengthen and merge with depth. Orientations of steps on dike contacts record local directions of dike-fracture propagation; about half of the measurements plunge less than 30°, showing that lateral propagation at dike peripheries is as important as the vertical propagation ultimately responsible for ascent. The San Rafael dikes, now exposed after erosion of about 0.5–1.5 km, appear to thicken and shorten upward, probably because near-surface vesiculation enhanced magmatic driving pressures. Propagation likely ceased soon after the first dike segments began to feed nearby sills or vented to initiate small-volume eruptions. Most of the dikes are exposed in clastic strata of the Jurassic San Rafael Group. They probably acquired their strikes, however, while ascending along well-developed joints in massive sandstones of the underlying Glen Canyon Group. Rotation of far-field stresses during the emplacement interval cannot account for disparate strikes of the dikes, which vary through 110°, most lying between north and N25°W. Rather, the two regional horizontal principal stresses were probably nearly equal, and so the dominant N75°E direction of dike opening was not strongly favored. Across the center of the swarm, about 10 to 15 dikes overlap and produce 15–20 m of dilation. Many are in sufficient proximity that later dikes should be thinner than earlier ones if neither the magma pressures nor regional stresses were changing during the emplacement interval. However, dike thicknesses vary systematically neither along the length of the swarm nor in proportion to the number of neighboring dikes. It appears that crustal extension during the magmatic interval relieved compressive stresses localized by intrusion.