INTR ODUC TION
On the Significance of Weak Layers for Continental Subduction and Collision Processes
INTRODUCTION
Over the last decades of research into mountain building processes, it became evident that the rheology of the plates involved in processes like continent-continent collision or subduction of
continental lithosphere exerts a first order control on the resulting deformation geometries. Analogue and numerical modelling is used to investigate the role of weak, decoupling horizons and lateral
strength contrasts in terms of collision dynamics and orogen geometries.
Our results show that differences in rheology of the crust across and along plate boundaries may lead to a variety of deformation patterns and mountain belt geometries and thus can be used as a proxy for inferring the rheological state of continents during collision.
Ernst Willingshofer 1 , Dimitrios Sokoutis 1 & Katharina Vogt 2
1 Faculty of Geosciences, University of Utrecht, The Netherlands, 2 Hochschule Bochum, International Geothermal Centre, Bochum, Germany Corresponding author: e.willingshofer@uu.nl
CONCL USIONS
ANAL OGUE MODELLING NUMERIC AL MODELLING
1300
1400 4.8 & 1,8 x 104
1550 1.2 x 105
1500 7.2
Faculty of Geosciences
0 50 150 250
1
2
3
Upper crust
Lower crust Strength (Pa)
D epth (cm) Mantle
lithosphere
Weak lower crust
EXPERIMENTS WITH DECOUPLING ZONES
Experiment A3
40 cm
00,8 1,3 cm2,8
brittle crust
Weak plate interface
Velocity: 1.9 x 10
-6(ms
-1); Length Scale: 1cmModel = 30kmNature
0 60
120 180 240km
1 1 2 3
4 5
2
SUMMARY OF MODELLING RESULTS
NATURAL EXAMPLES
Experiment A1
Experiment A4 Experiment A2
20% bs ~ 220 km
Decoupling in the mantle along the plate contact
Propagation of deformation
upper crust weak plate interface / lower crust
mantle lithosphere
strong decoupling at the plate interface favours continental subduction and dominant pro-wedge deformation (A1, B1, C2)
weak layers within the crust of the upper plate facilitate upper plate deformation ( A4, C1)
lateral variations in plate rheology can lead to back-stepping instead of foreland-propagation of deformation (A2)
a strong subducting plate produces antiformal stacks and material transfer onto the upper plate is through a retro-shear zone/fault (B2)
4 1
2 3
Density Viscosity (kg/m
3) (Pa s)
NW SE
0
50 km
Insubric line
Lower Plate Upper Crust & Sediments
Upper Plate Lower Crust
Mantle Lithosphere Sutures
Weak plate Strong plate
upper crust lower crust
mantle lithosphere
weak plate interface
Strenght Profiles Composition
Vogt et al. (2018)
Modified after Pfiffner (2016) Modified after Muñoz (1992)
Conceptual Model of Collision Zones Pro-wedge
Retro-wedge
2 1 3
4
A
0 60
120 180 240km
4 cm
6 2 1 6
0 60
120 180 240km
2 1
3 4 5 6
A
λ 0
60
120 180 240km
Decoupling at the plate interface (A1, A2)
Crust-mantle coupling on the upper and lower plates (A3, A4)
Pro-wedge Retro-wedge
1
2 3
4
Luth et al. (2010)
Luth et al. (2013) Willingshofer et al. (2013)
4 c REFERENCES
Luth S., Willingshofer E., Sokoutis D. & Cloetingh S., 2010. Analogue modelling of continental collision: Influence ofplate coupling on mantle lithosphere subduction, deformation and surface topography. Tectonophysics, v. 484; doi:10.1016/j.tecto.2009.08.043, p. 87-102.
Luth S., Willingshofer E., Sokoutis D., & Cloetingh S., 2013. Does Subduction Polarity Changes below the Alps? Inferences from Analogue Modelling.
Tectonophysics 582,140–161, doi: 10.1016/j.tecto.2012.09.028.
Muñoz, J. A., 1992. Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section. In Thrust tectonics, 235-246.
Springer Netherlands.
Pfiffner, O., 2016. Basement-involved thin-skinned and thick-skinned tectonics in the Alps. Geological Magazine, 153 (5-6), 1085-1109.
Sokoutis D., & Willingshofer E., 2011. Decoupling during continental collision and intra-plate deformation. EPSL, 305 (2011), 435-444, doi: 10.1016/j.epsl.2011.03.028.
Vogt, K., Willingshofer, E., Matenco, L., Sokoutis, D., Gerya, T., & Cloetingh, S. (2018). The role of lateral strength contrasts in orogenesis: A 2D numerical study. Tectonophysics, 746, 549-561, doi:https://doi.org/10.1016/j.tecto.2017.08.010
Willett S., Beaumont C., & Fullsack P., 1993. A mechanical model for the tectonics of doubly-vergent compressional orogens. Geology, 21, 371-374.
Willingshofer E., & Sokoutis D., 2009. Decoupling along plate boundaries: Key variable controlling the mode of deformation and geometry of collisional mountain belts. Geology, v. 37; no. 1; p. 39–42; doi: 10.1130/G25321A.
Willingshofer, E., Sokoutis, D., Luth, S. W., Beekman, F. & Cloetingh, S., 2013. Subduction and deformation of the continental lithosphere in response to plate and crust-mantle coupling: Geology, v. 41; no. 12; p. 1239-1242; doi:10.1130/G34815.1.
ACKNOWLEDGMENTS
Funding of this study by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) is gratefully acknowledged.
Reactivated retro-shear
Significant upper plate deformation above de-
coupling layer (red)
25% bs ~ 300 km 25% bs ~ 300 km
25% bs ~ 300 km
Willingshofer &Sokoutis (2009); Sokoutis & Willingshofer (2011)
wet quartzite anorthosite/diabase
dry olivine
Stage 1 Block Uplift
S Stage 2
Pro-wedge Retro-wedge
S
Stage 3
Minimum Taper Maximum Taper Minimum Taper
S
Stages of basic model development. From Willett et al. (1994).
Lower crustal subduction
Axial Zone
NPF
Orri . x N
S
50 km
25 0
4
6 3 2
1'
8
4
80 km 4 cm
22% bs ~ 182 km
1
5 6 10
24% bs ~ 210 km 9
80 km 4 cm
Initially Inclined Boundary
Brittle CrustViscous Crust and Upper Mantle
Model Moho
Ductile Shear Zone Initially Decoupled Boundary
Viscous Upper Mantle
