In the last two decades, crustal channel and escape flow, wherein long-wavelength ductile flow of lower crustal material transports mass and heat out of the collision zone, have remained among the most impactful ideas proposed to explain shortening accommodation in continental collisions. In the Inner Piedmont (IP), southern Appalachians, channel and escape flow have been previously proposed for the Devonian-Mississippian Neoacadian orogeny, and the deep exhumational level of the IP relative to other orogens in which channel flow has been proposed makes it ideal for testing the channel and escape flow models. In the IP channel flow model, the Brevard fault zone (BFZ) footwall is interpreted to buttress orogen-normal crustal flow of the hot IP in northwestern North Carolina and drive escape flow to the southwest. However, the polymetamorphic and deformational history of the southern Appalachians has made it difficult to isolate the spatial and temporal extent of thermal and deformational events driving flow of the interpreted channel. To address this, we use in situ laser ablation split stream monazite (Mz) U-Pb geochronology and geochemistry coupled with quantitative P-T data to define the extent and conditions of Paleozoic metamorphic events in the southern Appalachians of North Carolina. In this area, northwest of the BFZ, Mz dates indicate mostly Taconic (~462 Ma) and minor Neoacadian
metamorphism (~368 Ma) whereas IP data show Neoacadian metamorphism (~363–330 Ma) with no Taconic ages. IP Mz also records a transition over time from HREE-poor to HREE-rich compositions, indicating Mz growth associated with both garnet growth and breakdown, respectively. This, along with diffuse chemical profiles and resorption textures in garnet, suggests that IP Mz records prograde to retrograde metamorphism. Furthermore, P-T estimates from the eastern Blue Ridge of northwestern NC are 5–9 kbar and 565–730 °C, whereas peak Neoacadian metamorphism in the IP core reached 5–8 kbar and 750–850 °C. We interpret this to indicate that the BFZ footwall acted as both a thermal and rheological boundary in northwestern NC during Neoacadian metamorphism, supporting earlier interpretations. Future work will assess the timing and conditions of metamorphism further south into the Blue Ridge and IP of South Carolina, Georgia, and Alabama.
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Variations in the P-T-t of deformation in a crustal-scale shear zone in metagranite
Deformation in crustal-scale shear zones occurs over a range of pressure-temperature-time (P-T-t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P-T-t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn-kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal-sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high-T shearing preserved far into the footwall, to low-T shearing adjacent to the hanging wall. The Ti-in-quartz and Si-in-phengite thermobarometers yield deformation P-T conditions, as both were reset syn-kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite-facies conditions (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist-facies conditions (∼305°C, 1.5 kbar) at ∼11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P-T-t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P-T.
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- Award ID(s):
- 1650173
- NSF-PAR ID:
- 10276617
- Date Published:
- Journal Name:
- Geochemistry geophysics geosystems
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1029/2020GC009384
