STRUCTURE &GEODYNAMICS
Rapid Cooling and Post-Orogenic Collapse Scenarios of the Caledonides of Northwest Scotland
Brandon SpencerPhD Candidate, UK Structure and Geodynamics Group
Co-authors: J. Ryan Thigpen (UK), Richard Law (VT), Kip Hodges (ASU), Sean Gallen (CSU), Jason Dortch (KGS), Calvin Mako (Bates)
STRUCTURE &GEODYNAMICS
How are orogens destroyed? How fast?
Rapid to very slow: multiple orders of magnitude
Dependent on lithology, climate, structure, etc.
Hodges, 2016
Mechanism 1: Surface Processes
Mechanism 2: Geodynamic CollapseGravitationally-driven ductile flow and brittle
extension
Metamorphic core complexes and
channelized flow
Modified from Rey et al., 2001
Modified from Spencer et al., 2020
35 km
STRUCTURE &GEODYNAMICS
The Scandian Orogenic RetrowedgeScandian (ca. 435-415 Ma) phase of broader Caledonian orogeny
Early onset deposition of Old Red Sandstone (ca. 407-403 Ma)
Apparent rapid collapse of wedge—thermochronology to determine timing
From Spencer et al., 2020
STRUCTURE &GEODYNAMICS
Why Scotland?Lower Emsian? (407-403 Ma)
(Wellman 2014)
Muscovite Ar/Ar at post-420 Ma?(Dallmeyer et al. 2001)
STRUCTURE &GEODYNAMICS
Cooling Rates from Thermochronology Cooling from muscovite closure (400 °C) to surface (~25 °C): ~35-90 °C
Myr-1
1.9-3.8 mm yr-1 unroofing rate!
Orogen core cooling from amphibole (550 °C) to muscovite (400 °C)
closure: ~15-45 °C Myr-1
Spencer et al., 2020
Slow cooling… Fast cooling!
Constrained by previous work
Constrained by THIS work
STRUCTURE &GEODYNAMICS
Typical unroofing/erosion rates?
Appalachians: 0.01-0.05 mm/yr average, hillslope rates ~0.05-0.1 mm/yr
Nepal Himalaya:4-5 mm/yr
Photo: https://www.maxpixels.net/Landscape-Himalayan-Steep-Mountain-Nepal-994863
STRUCTURE &GEODYNAMICS
Exhumation/Unroofing of the Scandian Retrowedge—Erosion?
Why don’t the Scottish Caledonides follow the “expected” path?Did it ”gain” erosional potential by some means? Or was there another mechanism at work?
Spencer et al., 2020
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Matlab 1D Stream Power and Isostasy Modeling of Post-Collision Timeframe
10 Myr runtime with 1.5 mm/yr vertical uplift at
onset of decay from 5000 meters max. elevation
Still over 2 km of relief at orogen core at presumed
end of collapse!
STRUCTURE &GEODYNAMICS
Preliminary Modeling: Topography Remains…
Python-based Landlab models: erosion and isostatic response
Preliminary models agree with inability to denude orogen—more refinement to be done
Tectonic uplift
Erosion and Isostasy
STRUCTURE &GEODYNAMICS
Rapid Exhumation—Surface Processes or Geodynamics?
Spencer et al., 2020
Spencer et al., 2020
Evidence for geodynamic mechanism—extension of East Greenland Caledonides• Extensional deformation in upper crust• Existence of high temperatures during orogenesis (>700 °C)• Rapid cooling/unroofing and inconsistency with current models
STRUCTURE &GEODYNAMICS
• Hodges, K.V. 2016. Crustal Decoupling in Collisional Orogenesis: Examples from the East Greenland Caledonides and Himalaya. Annual Review of Earth and Planetary Sciences, 44, 685-708, http://doi.org/10.1146/annurev-earth-060115-012412.
• Rey, P., Vanderhaeghe, O. & Teyssier, C. 2001. Gravitational collapse of the continental crust: definition, regimes and modes. Tectonophysics, 342, 435-449, http://doi.org/10.1016/S0040-1951(01)00174-3.
• Spencer, B.M., Thigpen, J.R., Law, R.D., Mako, C.A., McDonald, C.S., Hodges, K.V. and Ashley, K.T. 2020. Rapid cooling during late-stage orogenesis and implications for the collapse of the Scandian retrowedge, northern Scotland. Journal of the Geological Society, jgs2020-022. https://doi.org/10.1144/jgs2020-022.