Flow Behavior and Emplacement of the Northwest Dike Swarm in the Jagged Rocks Complex, Hopi Buttes Volcanic Field, Arizona

The Jagged Rocks Complex in the Hopi Buttes Volcanic Field provides excellent exposure of shallow dikes and sills as well as massifs and buds that fed three to five maar-diatreme volcanoes. The northwest dike swarm consists of a series of six en-echelon dikes as well as five massifs and many smaller buds. The excellent exposure of these shallow plumbing structures provides a unique opportunity to study the role of shallow dikes in supporting maar-diatreme eruptions. Anisotropy of magnetic susceptibility (AMS) and other rock magnetic methods were used to investigate the timing of the intrusions, the characteristics of their magnetic mineralogy, and the flow history preserved within the dikes. Curie point analysis shows the magnetic minerals have a low Curie point indicative of high-titanium titanomagnetite. The isothermal remanent magnetization (IRM) measurements indicate that the grains are dominantly pseudo-single domain (PSD) edging on single domain (SD). This is supported by anisotropy of anhysteretic remanent magnetization (AARM) measurements, which also indicate the influence of SD grains on some sites’ anomalous AMS fabrics. AMS shows dominantly horizontal flow in the majority of the sites. Only half of the margin pairs provide reliable imbricated K1 lineations for flow direction interpretation. The use of the K3 method to define the main foliation plane does not reveal any imbricated fabrics beyond those seen with the K1 method. The interpretable margin pairs all show flow to the northwest, providing flow directions for five out of the six dikes analyzed. This direction is generally away from the other main series of intrusions to the southeast, indicating a lateral diversion of magma away from a magma-supply system located somewhere to the southeast. The lateral intrusion of the northwest dike swarm could have been a response to overpressure in the shallow subsurface due to the formation of a volcanic edifice or due to overpressure in the magma supply system. Coherent dikes formed first, with limited water/magma interactions, and later reduction in flux or withdrawal of magma initiated the phreatomagmatism that formed the massifs