GSA Bulletin: Physical Processes of Shallow Mafic Dike Emplacement

Physical processes of shallow mafic dike emplacement near the San Rafael Swell, Utah Paul T. Delaney* U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001 Anne E. Gartner† U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 ABSTRACT crustal extension during the magmatic interval relieved compressive stresses localized by intrusion. Some 200 shonkinite dikes, sills, and breccia bodies on the western Colorado Plateau of south-central Utah were intruded from ap- INTRODUCTION proximately 3.7 to 4.6 Ma, contemporaneous with mafic volcanism along the nearby plateau margin. Thicknesses of dikes range to about A Pliocene dike swarm, almost 60 km long and nowhere more than 30 6 m; the log-normal mean thickness is 85 cm. Despite the excellent ex- km wide, is exceptionally well exposed on the western Colorado Plateau posures of essentially all dikes in strata of the Jurassic San Rafael (Figs. 1 and 2). The dikes are mostly less than 1 m thick and have almost Group, their number is indeterminate from their outcrop and spac- 300 km of outcrop within nearly flat-lying clastic and fine-grained strata. ing because they are everywhere greatly segmented. By our grouping Plug-like bodies, or necks, formed along many dikes and usually contain of almost 2000 dike segments, most dikes are less than 2 km in out- breccias that reflect their mode of development. Together, they fed some crop length; the longest is 9 km. Because the San Rafael magmas were sills and, probably, some eruptive rocks now removed by no more than primitive and probably ascended directly from the mantle, dike about 2 km of erosion. The intrusions likely exemplify the shallow subsur- lengths in outcrop are much less than their heights. The present ex- face beneath volcanic fields of mafic maars, cinder cones, and small-vol- posures probably lie along the irregular upper peripheries of dikes ume lava flows, such as are common throughout western North America. that lengthen and merge with depth. Orientations of steps on dike This paper summarizes observations and measurements collected to contacts record local directions of dike-fracture propagation; about constrain some physical processes of mafic dike emplacement in a setting half of the measurements plunge less than 30°, showing that lateral of relatively simple igneous and host-rock geology. We report on length, propagation at dike peripheries is as important as the vertical propa- thickness, strike, and dip of dikes and on the character and strikes of joints gation ultimately responsible for ascent. The San Rafael dikes, now in adjacent strata of the San Rafael Group and the Navajo Sandstone of exposed after erosion of about 0.5–1.5 km, appear to thicken and the underlying Glen Canyon Group. We also report on directions of dike- shorten upward, probably because near-surface vesiculation en- fracture propagation. These measurements characterize the means of hanced magmatic driving pressures. Propagation likely ceased soon magma ascent and the state of regional stress and ongoing crustal exten- after the first dike segments began to feed nearby sills or vented to ini- sion during the magmatic interval, which lasted about 1 m.y. tiate small-volume eruptions. Among the modes of igneous intrusion, dikes are perhaps the best un- Most of the dikes are exposed in clastic strata of the Jurassic San derstood from a theoretical perspective. Many aspects of the San Rafael Rafael Group. They probably acquired their strikes, however, while dikes seem to confound theory, however, primarily by displaying attrib- ascending along well-developed joints in massive sandstones of the utes seemingly obtained from underlying units and by revealing only underlying Glen Canyon Group. Rotation of far-field stresses during hints of their complex three-dimensional forms and propagation paths. the emplacement interval cannot account for disparate strikes of the The dikes are also of interest for the volcanic processes recorded by the dikes, which vary through 110°, most lying between north and breccia bodies, and the sills for in situ differentiation of shonkinite as re- N25°W. Rather, the two regional horizontal principal stresses were vealed by the syenites localized within; both are of interest for pervasive probably nearly equal, and so the dominant N75°E direction of dike hydrothermal alteration of the host and wall rocks. 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 GEOLOGY are in sufficient proximity that later dikes should be thinner than ear- lier ones if neither the magma pressures nor regional stresses were Tectonics changing during the emplacement interval. However, dike thicknesses vary systematically neither along the length of the swarm nor The form of the San Rafael dike swarm is irregular but trends slightly in proportion to the number of neighboring dikes. It appears that east of north, roughly parallel with the nearby boundary of the Colorado Plateau and the Basin and Range provinces about 20–50 km to the west (Fig. 1). The swarm lies between the Waterpocket monocline and the an- *e-mail: [email protected] ticlinal San Rafael Swell, although it parallels neither of these Laramide †e-mail: [email protected] folds nor other nearby flexures and faults (Fig. 2). The mean N14°W GSA Bulletin; September 1997; v. 109; no. 9; p. 1177–1192; 16 figures; 2 tables 1177 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/9/1177/3382791/i0016-7606-109-9-1177.pdf by guest on 03 October 2021 DELANEY AND GARTNER strike of the dikes approximately parallels an inferred N20°W direction of fractures in Precambrian basement of the Colorado Plateau (Davis, 1978); the trend of the northern laccolithic intrusions of the Henry Moun- tains and of the Henry basin to the south-southeast, for example, are aligned along this direction with the younger San Rafael swarm (Fig. 1). The San Rafael intrusions resemble other Tertiary alkaline mafic rocks that crop out in sparse, widely spaced groups along the western margin of the North American craton. Such rocks are locally exposed along the tran- sition zone between the Colorado Plateau and Basin and Range provinces (Thompson and Zoback, 1979; Kempton et al., 1991) and were discussed by Tingey et al. (1991); the nearest, the minettes and melanephelinites of the Wasatch Plateau, are about 40–80 km north of the San Rafael swarm (Fig. 1). The Colorado Plateau margin has been a locus of volcanism since the Tertiary Period. The San Rafael swarm is at the eastern limit of a broad zone of basaltic volcanism that crosses to the plateau from the Basin and Range Province of southern Utah (Fig. 1). Although this volcanism dates back to middle Miocene time, most is of Pliocene and Qua- ternary age, including the alkalic and tholeiitic basaltic rocks of the adjacent high plateaus (Nelson, 1989; Mattox, 1991, 1994). Judging from the primitive nature of the shonkinites, they probably ascended directly from a source region in the mantle. No geophysical anomaly suggestive of a crustal magma reservoir has been identified near the San Rafael swarm. The swarm lies on the flanks of an elongate re- Figure 1. San Rafael shonkinite dike swarm shown in relation to gional aeromagnetic high and a positive Bouger-gravity anomaly identi- other melanocratic rocks of the western Colorado Plateau margin; fied from locally sparse data (Zietz et al., 1976; Cook et al., 1989). The the Wasatch Plateau swarm is the closest. Also shown are the Henry nearby aeromagnetic high extends northeasterly from the Waterpocket Mountains and the zone of Tertiary and Quaternary volcanism along monocline to the San Rafael anticline; the gravity anomaly is centered the margin of the Colorado Plateau (after Kempton et al., 1991; over the San Rafael anticline. Tingey et al., 1991; S. Nelson, 1996, personal commun.). Plateau uplift was waning during dike emplacement at about 4 Ma (Thompson and Zoback, 1979) and much of the post-Mesozoic strata was already stripped by erosion. Dikes at 2200–2600 m elevations on the west inites is typically chloritic, and calcite and zeolites, mostly thomsonite, side of the San Rafael swarm are within 6 km of trachybasalt flows of occur as veins, fillings of abundant vesicular and amygdaloidal cavities, similar age and composition at elevations above 3000 m (Nelson, 1989). and replacements of phenocrysts. From intergrowths and textural rela- Dikes occur as much as 30 km east of the these lavas, at elevations as low tions of analcite, which generally composes 3%–4% of modal abun- as 1500 m. Although no paleotopographic reconstruction is available, dances, Gilluly (1927) concluded that it is a primary mineral. Contact modern exposures probably correspond to no more than about 2 km of metamorphism and alteration of wall rocks occurs locally along many emplacement depth, consistent with the probable stratigraphic thick- dikes and is pervasive around the sills. nesses of overlying units (Smith et al., 1963). About 45 dikes contain abundant breccias of admixed shonkinite and comminuted host rock (Gartner and Delaney, 1988). In most instances, Igneous Rocks brecciation was accompanied by wall-rock erosion, leading to local widening of dikes during magma flow. Although the breccia bodies prob- The dike rocks are dark gray, locally porphyritic, microshonkinites and ably merged upward to volcanic necks, now removed by erosion, no un- alkali diabases containing clinopyroxene, biotite, sanidine, olivine, am- ambiguous scoria, tephra, agglutinate, or tuffaceous facies of the shonk- phibole, plagioclase, and magnetite (Gilluly, 1927; Williams, 1983). SiO2 inites were identified. ranges from 44 to 48 wt%, total alkalies range from 5.2 to 7.2 wt%, and magnesium numbers range from 0.72 to 0.75.

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