Physical transport processes determine the ways in which magmas traverse the upper crust, how they are modified, where they are deposited, whether and how they erupt, and what their final products will be. This project will investigate the physical mechanisms that transport and distribute magmatic materials using a superb natural laboratory: the Colorado River region south of Las Vegas, where relatively young (~15 million years old) magmatic systems are spectacularly exposed through a combination of steep tilting during rift-related faulting, high relief, and sparse vegetation. Numerous plutons, extensive dike swarms, and thick volcanic sequences document a relatively brief but highly productive episode of magma influx. A key part of the project will be devoted to understanding the plutons (large bodies of intrusive rock, solidified beneath the surface) and what they say about the nature and history of magma chambers (temporary melt-rich zones at depth that solidify to form plutons and may expel magma to erupt at the surface). Plutons, the magma chambers that formed them, dikes that fed the chambers, and dikes that fed volcanic eruptions at the surface all must have dynamic histories, but the physical processes that move crystals, melts, and fragments of solid or near-solid rocks within these systems are not well understood. General distributions of volcanic and intrusive rocks in the study area and details of their internal structure suggest fluid dynamic models that are analogous to fluid and particle transport in surface environments - notably sedimentation and mass wasting processes. The project will address this fundamental, general issue:
How do physical transport processes in the upper crust determine the eventual distribution and physical and compositional characteristics of the products of a magma system?
To evaluate this broad problem, the focus will be on the following more specific questions: (1) How do depositional processes dictate the construction and ultimate architecture of plutons? (2) How do external factors (recharging, roof collapse, eruption, tectonism) influence internal physical processes within magma chambers? (3) How much, and why, do melt fraction and volume vary over the lifetime of a pluton (~magma chamber)? (4) What determines whether deep-level magmas transit the upper crust unmodified or are modified during residence in magma chambers?; Do magma chambers serve as effective filters, selectively inhibiting further ascent of certain magmas or magmatic materials? (5) What determines whether magma pools to form plutons, remains restricted to dikes, or erupts?
Field studies of key areas will characterize critical structures and map patterns that provide evidence for mechanical transport and deposition processes and provide samples for further analysis. These samples will be used for detailed radiometric dating and microanalysis of tracer elements and isotopes in minerals. Complementary lab experiments on liquid + particle analogs of magma will evaluate plausible fluid dynamic processes in the magma systems. Together, the sample geochemistry and geochronology and lab experiments will test field-based magma transport hypotheses. This project will provide ideal research training and constitute the basis for theses for 7 Master's and 12 undergraduate students. The students will present and publish their results and gain experience in combining careful field observation with advanced analytical work, in collaborative research, and in integrating their local efforts into a global framework. This work prepares them not only for further research in petrology, geochemistry, and tectonics, but also for a broad range of Earth science problem solving and teaching. Based on past experience a large proportion of these students will be female and participation by minority students is likely. The study will stimulate exchange of ideas between the PI's and their students and several informal collaborators who will add their own perspectives and expertise and in some cases analytical methodologies. These collaborations, formal and informal field trips, and presentations to specialist and general audiences as well as published work will disseminate developing ideas, foster discussion in a variety of forums, and showcase this spectacular example of magmatic processes. Much of the research will take place in Lake Mead National Recreation area, which will provide an excellent venue for presentation of new ideas to the public. The project will also contribute to a new cooperative program at Vanderbilt involving geology and environmental engineering. Although principally a basic science project, the results of this work will contribute to understanding of eruptive processes and thus will bear upon volcanic hazards.
