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.