1. Deep crustal structure, processes, and properties from xenoliths, basement
exposures, and seismic observations in the northern Rocky Mountain region
Hellroaring Creek area, Northern Madison Range2019 EarthScope workshop
Mahan, K.H.1, Schulte-Pelkum, V.1, Condit, C.B. 2, Barnhart, K.R.1, Butcher, L.3,
Blackburn, T.J.4, Bowring, S.A.2, Jones, C.1, Flynn, C. 1, O.F. Orlandini1, Ault, A.5, Möller,
A.6, Flowers, R.M.1, and Farmer, L.1
1University of Colorado-Boulder
2Massachusetts Institute of Technology
3Univ. Of California –Santa Barbara
4Univ. Of California – Santa Cruz
5Utah State University
6Univ. Of Kansas

2. Big Questions
What is the nature of the modern
deep crust of the Wyoming Craton
and how did it evolve?
What were the roles of tectonic
processes along the northern margin
of the Wyoming Craton during
amalgamation with Laurentia?
How can we use deep exposures of
basement in southwestern Montana
to discover new insight into
deformation processes in general?

3. Lower crustal high-velocity zone
 20-25 km thick
Modern Lithospheric
Structure
Gorman et al., 2002

4. A composite origin for
Wyoming craton’s 7.x layer
• Archean: 2.9-2.7 Ga
• 2.0-2.2 Ga
• 1.86-1.7 Ga
• 1.5-1.3 Ga
After Barnhart et al., 2012
Suggested by several studies:
Chamberlain et al., 2003 – surface geology
Barnhart et al., 2012 – crustal xenoliths
Mahan et al., 2012 – crustal xenoliths
Foster et al., 2012 – Hf zircon in Cret. plutons

5. P-wave S-wave
Comparisons to seismic structure and
different observational methods
From Barnhart et al., 2012, Mahan et al., 2012, Re-compiled Schulte-Pelkum et al., 2017

6. Comparisons to seismic structure and
different observational methods
P-wave S-wave
Re-compiled Schulte-Pelkum et al., 2017, Tectonics

7. P-wave S-wave
Comparisons to seismic structure and
different observational methods
Re-compiled Schulte-Pelkum et al., 2017, Tectonics
An interface
within the
HVLC?
Seemingly
supported by
xenoliths?
~10% Grt
>30% Grt

8. P-wave S-wave
Comparisons to seismic structure and
different observational methods
More likely
if deeper
Moho
contrast is
weaker?
Seemingly
supported by
mantle
xenoliths.
Re-compiled Schulte-Pelkum et al., 2017, Tectonics
From Downes et al., 2004

9. P-wave S-wave
Comparisons to seismic structure and
different observational methods
Real Moho
Fake Moho
Re-compiled Schulte-Pelkum et al., 2017, Tectonics

10. Archean and
Proterozoic basement
in southwest Montana
Big Sky Orogeny
ca. 1.8-1.7 Ga
thermotectonism in SW
Montana
Harms et al., 2004
A
A’

11. Orogenic Growth
Condit et al., 2015
Arc Magmatism
Partial Melting
Metamorphism
Metamorphism
SE-directed propagation
of the metamorphic core
~100km, ~80 myr
Condit et al., 2015, Lithosphere

12. Shear zones in metagabbro dikes
Condit & Mahan, 2018, JMG
Opportunity to study fundamental deformation
processes – shear zone evolution

13. Shear zone evolution
Initial nucleation from fractures highlights importance of brittle deformation even
in nominally ductile environments (here 0.9 GPa, 700 °C) Condit & Mahan, 2018, JMG

14. 1: igneous crystallization
2: (dry) M1 (Grt1 + Cpx1 + Pl1)
3: fracturing (Grt + Hbl)
4: static M (Hbl + Pl )
Condit, 2017
Condit & Mahan, 2018, JMG
Shear zone evolution

15. 1: igneous crystallization
2: (dry) M1 (Grt1 + Cpx1 + Pl1)
3: fracturing (Grt + Hbl)
4: static M (Hbl + Pl )
Undeformed
Microfracturing, dislocation creep,
synkinematic reactions
Diffusion-accommodated granular flow
Condit, 2017
Condit & Mahan, 2018, JMG
Shear zone evolution Variation in deformation
mechanisms means
differences in CPO
strength –translates to
properties like seismic
anisotropy