CRUSTAL ‘ESCAPE’ FLOW IN HOT OROGENS

In modern day tectonics research, one of the most intriguing and impactful notions to emerge is the idea that long wavelength flow (100’s of kilometers) of weak viscous mid- to lower-crustal rocks can substantially influence shortening accommodation and the redistribution of mass and heat in collisional systems. These models, which are mostly predicated on a number of whole orogen numerical simulations from the Dalhousie University Geodynamics Group, quite elegantly explain a number of seemingly unrelated features in the Himalayan-Tibetan (HT) system. These persistent consistencies between model predictions and field observations not only changed the way many interpreted the evolution of the HT system, but it was also conceptually proposed to explain observations in a number of other collisional worldwide. Despite the strong correlations between model predictions and field and analytical observations, 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. One very intriguing possibility is that the mid- to lower-crustal mass and heat efflux was redirected to the east in a style of ‘escape’ flow as the frontal Himalaya transitioned to critical wedge-style slip along the Main Boundary and Main Frontal thrusts, implying an interesting spatial and temporal partitioning of deformation in these systems.

Because ‘escape’ flow may play a prominent geodynamic role in collisional shortening accommodation, we need to understand: (a) what mechanisms might drive crustal channel flow to be deflected from orogen-normal to orogen-parallel, (b) are the rheological boundaries controlling the geometry of ‘escape’ flow merely a function of pre-existing lithological differences or do they result from transient thermal effects such as heat advection, and (c) do the thermal and rheological conditions recorded in both the presumed ‘escape’ flow channel and in the bounding channel buttress and/or crustal lid match those conditions necessary for flow to occur in numerical simulations (i.e. percent melt present, viscosity, channel thickness, P-T-t particle trajectories)? In this study, we are using the southern Appalachian Inner Piedmont and the modern Himalayan system as natural laboratories to address these questions.

This project is supported by NSF Tectonics grant EAR 1802730 grant to JRT and is being conducted in conjunction with collaborators Kyle Ashley (University of Pittsburgh), Arthur Merschat (USGS), Bob Hatcher (University of Tennessee, Knoxville), Gabe Casale (Appalachian State University), and John Cottle (UCSB).

Nodal temperature results of a finite-element model of a tapered thrust wedge with single fault. In this model, the maximum thrust rate is ~80 km/Myr and the thermal distribution shown is after 6.0 Myr of motion and a total lateral (right-directed) displacement of 250 km. Deformation of the isotherms is the result of very rapid thrusting, which also results in footwall heating at rates up to 160° C/Myr. Although this model yields some first-order constraints on footwall heating rates, it lacks the influence of key processes such as erosion, mechanical weakness due to heating, and isostasy, all of which are currently being developed in our group. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

Nodal temperature results of a finite-element model of a tapered thrust wedge with single fault. In this model, the maximum thrust rate is ~80 km/Myr and the thermal distribution shown is after 6.0 Myr of motion and a total lateral (right-directed) displacement of 250 km. Deformation of the isotherms is the result of very rapid thrusting, which also results in footwall heating at rates up to 160° C/Myr. Although this model yields some first-order constraints on footwall heating rates, it lacks the influence of key processes such as erosion, mechanical weakness due to heating, and isostasy, all of which are currently being developed in our group. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

Rate of temperature change in simple beam models (shown in previous figure) with a maximum thrust displacement rate of ~80 km/Myr at (top) 3-4 Myr after onset of thrusting, (middle) 4-5 Myr after onset of thrusting, and (bottom) 5-6 Myr after onset of thrusting. Note that the maximum heating rates of >120° C/Myr recorded in the bottom figure are confined to the immediate footwall <1 km beneath the thrust. Also note that the spatial extent of rapid heating rates (i.e. >40° C) is much less than that observed in models with slower thrust rates. This indicates that although increasing thrust rate can increase footwall heating rate (and by association hanging wall cooling rate), the predicted spatial extent of the effect is subsequently reduced and in some cases may be difficult to separate from near fault thermal processes such as shear heating. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

Rate of temperature change in simple beam models (shown in previous figure) with a maximum thrust displacement rate of ~80 km/Myr at (top) 3-4 Myr after onset of thrusting, (middle) 4-5 Myr after onset of thrusting, and (bottom) 5-6 Myr after onset of thrusting. Note that the maximum heating rates of >120° C/Myr recorded in the bottom figure are confined to the immediate footwall <1 km beneath the thrust. Also note that the spatial extent of rapid heating rates (i.e. >40° C) is much less than that observed in models with slower thrust rates. This indicates that although increasing thrust rate can increase footwall heating rate (and by association hanging wall cooling rate), the predicted spatial extent of the effect is subsequently reduced and in some cases may be difficult to separate from near fault thermal processes such as shear heating. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.