Dikes, sills, and stress-regime evolution during emplacement of the Jagged Rocks Complex, Hopi Buttes Volcanic Field, Navajo Nation, USA

Abstract

The dikes and related intrusions formed below small volcanoes in volcanic fields are remnants of the simplest volcanic plumbing systems. Their geometry is controlled by interaction of magma-driven cracks with country rock, and reveals regional structural and stress patterns at the time of their emplacement. The shallow stress field, however, may change during the time an intrusion complex is emplaced, in response to addition or removal of magma and country rock during associated surface eruptions. The Jagged Rocks Complex, in the Miocene Hopi Buttes Volcanic Field, Navajo Nation, Arizona, is exposed 300–350m below the pre-eruptive surface. It comprises a group of generally NW–SE striking dikes, punctuated in places by buds, a saucer-like intrusion, larger pyroclastic massifs and a diatreme. We made measurements of 13 dikes, divided into 172 segments, with thicknesses from 8 to 122cm (mean 43cm) and lengths of 60 to 780m. Several sills and inclined sheets in places are thicker than dikes, having mean thicknesses of 48cm and 73cm respectively. Dikes typically show straight, parallel, and en echelon patterns, while sills and inclined sheets are curved. The northwestern dikes differ from the rest in containing large mafic crystals, and are inferred to have been emplaced after the others. We find that the strike of the overall complex (dikes and other sheets, elongate massifs and aligned sub-cylindrical bodies) reflects a crystalline-basement control that is evident throughout Hopi Buttes. Over the period of the complex's emplacement, local stress patterns were not stable. We infer that excavation of deep maar craters, and perhaps the construction of a scoria cone at the surface, modified the local stress patterns to favor emplacement of sills and en echelon dikes later in the complex's evolution, in contrast to simple straight dikes as the complex first formed.