Fundamental controls on mid-crustal 'escape' flow in orogenic systems

In large mountain belts that result from continent-continent collision, the tremendous accumulation of rock mass that creates the high mountainous topography must be removed from the system by a variety of mechanisms. In the shallow crust, this is mostly accomplished by the erosive action of rivers and glaciers. At greater depths the middle and lower crust can be weakened by partial melting and then may flow out of the collision zone in a process known as channel or escape flow. In the Himalayan-Tibetan system, it has been proposed that crustal escape flow dominates deformation in the lower crust beneath the eastern Tibetan margin and may even contribute to slip and seismicity along upper crustal strike-slip faults in northeastern India, northern Myanmar, and southern China. Understanding if these crustal escape flows are truly present and how they operate in large mountain belt systems remains as a fundamental challenge for tectonics research, particularly because they cannot be directly observed at depth. Critical to our understanding of how escape flow may operate is the nature of the boundaries that direct such flow and how they dynamically interact with the channel system, so ideally a study of this type should be carried out by examining a crustal channel that has had its upper crustal 'lid' removed. This project is examining a possible crustal escape flow system that is exposed at the surface in the southern Appalachian Inner Piedmont of North Carolina, South Carolina, and Georgia. In addition, this work is producing a critical suite of geochronologic and petrotectonic analyses for a classic eastern U.S. field area that lacks much of this basic data. This project contributes to the advancement of desired societal outcomes by supporting advanced field and laboratory training for a graduate student in an important science, technology, engineering and mathematics (STEM discipline), and is providing support for an early career investigator. Most importantly, this project will also support research training and collaboration experience for at least six undergraduate researchers under the guidance of the principal investigator.

Although the classic channel flow model quite elegantly explains a number of seemingly unrelated features in the Himalayan system, the condition(s) that may have led to deactivation and/or redirection of the channel at the southern front of the Tibetan Plateau remains as a persistent enigma for this model. Specifically, it is necessary to understand: (1) what conditions or mechanistic changes might lead to a switch from an orogen-normal channel flow dominated system to one that accommodates shortening by critical wedge-style slip along major thrusts at the front of an orogen, (2) could the same conditions(s) and mechanisms be responsible for a switch to orogen-parallel escape flow in large hot orogens that undergo this transition, and (3) what is the nature of the boundaries that direct such flow and how do they dynamically interact with the orogen-parallel channel system? To do this, it was necessary to identify a potential channel system now exposed at the surface. In the southern Appalachian Inner Piedmont, previous studies proposed the presence of a Neoacadian (Devonian-Mississippian) crustal escape flow system based on the observation of a map scale curved lineation pattern and preservation of an extensive area of pervasive migmatization, sillimanite I-II metamorphism, and numerous anatectic plutons. This study is addressing the questions above in the Inner Piedmont using integrated metamorphic and structural petrology, chemical diffusion modeling, and geo- (monazite and titanite laser ablation split stream technique) and thermochronology (40Ar/39Ar hornblende and muscovite) combined with paleopiezometry and quartz fabric-derived deformation temperatures to establish Pressure-Temperature-temperature-stress paths and system residence times for representative structural levels across the interpreted channel system.

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