depth (Km)
0 -1000
0
100 20
strength (MPa)
Moho
1500
compression extension
0 temperature (°C) 1500 geotherm
-1000
strength (MPa)
depth (Km)
0 -1000
0
100
30 Moho
1500
compression extension
0 temperature (°C) 1500 geotherm
-1000
Figure 1: Strength pro�iles modi�ied after Ziegler & Cloetingh 2004.
depth (Km)
-1000
20
strength (MPa)
0
-1000 0
100
Moho
1500
compression extension
0 temperature (°C) 1500
geotherm Cooling
&
Strengthening Heating
&
Weakening
Extended cratonic lithosphere
(75 Ma after rifting) Extended cratonic
lithosphere (0 Ma after rifting) Unextended normal
cratonic lithosphere (age 100 Ma)
OLD RIFT
STRONG HETEROGENEITY YOUNG RIFT
WEAK HETEROGENEITY REFERENCE
LITHOSPHERE Figure 3: a) Sketch of the experimental set-up; b) representative strength pro�iles calculated for a convergence rate of 1 cm/h, describing the very initial deformation stage.
Brittle upper crust (feldspar sand) Viscous lower crust (silicone 1)
Brittle upper mantle (quartz sand) Viscous upper mantle (silicone 2)
Length scale 1 cm = 20 Km
Lower mantle lithosphere
& asthenosphere (low viscosity �luid)
EXP NATURE
Plexiglas box
Engine = convergence velocity
Asthenosphere
36 cm 42 cm
Moving w all
Old rift
A
B
VLC BUM
BUC
VUM
A
b) a)
1,0 0 0,5
1,5 2,0 2,5 3,0 3,5 4,0
0 200 400 600 800
De pt h (c m )
(σ1 -σ3) Pa B
Increase in brittle crustal strength increase in bulk lithospheric strength
simulates
0 200 400 600 800
De pt h (c m )
(σ1- σ3) Pa A
1,0 0,5
1,5 2,0 2,5 3,0 3,5 4,0
0
STRONG
HETEROGENEITY REFERENCE
LITHOSPHERE
(1) Department of Earth Sciences, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands (2) Department of Geosciences, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway (3) Géosciences Rennes UMR 6118 CNRS/Rennes 1 University, 35042 Rennes Cedex, France (4) Geologisches Institut, ETH-Zürich and Universiteit Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
Contact: E.Calignano@uu.nl
Lateral strength variation in the lithosphere: a key parameter for the localization of intra-plate deformation (T31C-2520)
E. Calignano, D. Sokoutis, J.P. Brun, E. Willingshofer & J.P. Burg
1 1,2 3 1 4Aknowledgments: The research project was funded by the European Union FP& Maire Curie ITN “Topomod”, contract n. 264517
References
Willingshofer E., Sokoutis D. & Burg J.P., 2005 - Lithospheric scale analogue modelling of collision zones invoking a pre-existing weak zone. In “ Deformation Mechanisms, Rheology and Tectonics: from Minerals to the Lithosphere” (Eds. Gapais D., Brun J.P. & Cobbold P.R.). Geological Society, London, Special Publications 243, 277-294 Ziegler P.A. & Cloetingh S., 2004 - Dynamic processes controlling evolution of rifted basins. Earth Science Reviews, 64, 1-50
2. Experimental parameters
3. Analogue experiments: set-up
Assumptions and simpli�ications
- Simpli�ied rheology of viscous layers: analogue materials are characterized by depth-invariant viscosity.
- Erosion and sedimentation are not included in the experiments.
- Lateral strength variation in the mantle lithosphere are not in- vestigated; lateral changes in bulk lithospheric strength are sim- ulated with variation in the thickness of crustal layers.
- Despite the above simpli�ications the presented experiments are considered representative for �irst order deformation and as- sociated topography in presence of a laterally heterogeneous lithosphere under compression.
6. Summary and Conclusions
In presence of a mechanically stronger old rift subject to compressional stresses deformation localizes along the basin margin facing the compression direction.
Strain rate governs the geometry of the deep lithospheric structure. An increase in convergence velocity results in a progressive increase in aymmetry of the lithospheric root underlying the pop-down.
The brittle-ductile ratio in the lithospheric mantle determines the absence (low B/D) or presence (high B/D) of faults in the upper brittle mantle (Experiment 2, Experiment 3). For a low B/D ratio deformation in the mantle is accommodated by shear zones (Experiment 2).
Underthrusting along the margin of the old rift is the main deformation mechanism in case of low strength brittle mantle and high convergence velocity (Experiment 1).
Folding and formation of a mountain root are the main deformation mechanisms in case of low strength brittle mantle and low convergence velocity (Experiment 2).
A weak viscous upper mantle allow the development of a major pop-down prismatic structure in the upper crust; a strong viscous upper mantle prevents the formation of a pop-down basin in the crust and underthrusting along the basin margin (Experiment 4).
In presence of a mechanically weaker young rift subject to compressional stresses deformation starts along the rift margins and remains localized inside the weak plate leading to the development of pronounced topography, which is compensated by a lithospheric root (Willingshofer et al., 2005; Figure 5).
1. Introduction and objectives
Lateral variation of strength in the lithosphere is an important factor controlling the localization of intra-plate deformation. Pre-existing heterogeneities can become reactivated in extension as well as in compression, governing the spatial and temporal development of intra-plate deformation.
Analogue models investigating the deformation pattern and topography development of compressional intra-plate settings are presented. The initial scaling conditions are designed to analyse the effects of �irst order lateral strength variations. The reference lithosphere is characterized by a uniform four-layers brittle-ductile rheological structure. An increase in upper crustal thickness, and thus strength, has been used to approximate a strong lithospheric section, representative for an old rift setting. The introduced lateral heterogeneity is striking perpendicular to the compression direction. All experiments have been deformed under normal gravity �ield. Other investigated parameters have been the strain rate, the thickness of the brittle mantle and the rheology of the viscous upper mantle.
4. Results: intra-plate deformation in presence of a strong lithospheric section
5. Comparison with previous experiments:
intra-plate deformation with a pre-existing weak lithospheric section
Experiment 1 Experiment 4
Experiment 2 Experiment 3
Con ver genc e v elocit y (cm/h)
Thickness brittle mantle (cm)
Brittle mantle faulting
5 cm 100 Km nature
A VLC BUM
BUC
Weak VUM
Elevation (cm)
0 + 0,85
- 0,15
5 cm 100 Km nature
A
Elevation (cm)
0 + 0,85
- 0,15
VLC BUM BUC
Strong VUM
Elevation (cm)
0 + 0,85
- 0,15
BUMVLC BUC Weak VUM
Asthenosphere
5 cm
100 Km nature
A
Asymmetric deep lithospheric structure
Shear z one
5 cm
100 Km nature
A
Symmetric deep
lithospheric structure
BUMVLC BUC
Weak VUM
Shear zone Shear z
one
Elevation (cm)
0 + 0,85
- 0,15
Brittle mantle folding
Figure 4: representative cross sections and DEM (Digital Elevation Model) of the experiments’ surface at 20% BS (bulk shortening).
1,0 0,5
0 0 5,0
1,0
A Lower
crust
Viscous upper mantle
5 cm
100 Km nature
Elevation (cm)
0 + 1,5
- 0,2
Reduced brittle crustal strenght
absence strong + brittle upper mantle
simulates
decrease in bulk lithospheric strength
VLC BUM
BUC
VUM
Figure 5: intra-plate deformation with a pre-existing weak zone.
Modi�ied after Willingshofer et al., 2005.
0 1.01.5
2.5
22 c m
18 c
m Lateral Confinement
13
brittle upper mantle
dry feldspar sand brittle crust
viscous crust strong viscous
upper mantle
brittle crust viscous crust silicone mix I
viscous mantle
silicone mix I dry quartz sand
silicone mix II low viscosity lower mantle lithosphere +
asthenosphere (liquid)
16 cm 10 cm 16 cm
depth (cm)
C D
C
depth (cm)
0 200 400 600 800
1,0 0,5
1,5 2,0 2,5 3,0 3,5
0 1000
D
(σ1 - σ3) Pa
depth (cm)
0 200 400 600
1,0 0,5
1,5 2,0 2,5 3,0 3,5 0
(σ1 - σ3) Pa
REFERENCE
LITHOSPHERE WEAK
HETEROGENEITY
GEOMETRICAL and KINEMATICAL PARAMETERS
Experiment Convergence velocity
(cm/h) Strain rate
(s-1) Bulk shortening
(%) H brittle upper mantle (cm)
Experiment 1 5,0 3,31E-05 20 0,5
Experiment 2 1,0 6,61E-06 20 0,5
Experiment 3 1,0 6,61E-06 20 1,0
Experiment 4 5,0 3,31E-05 20 1,0
Parameters valid for all models
Model length 42 cm Model width 36 cm Old rift width 4 cm
h UC (reference lithosphere) 1,0 cm h UC (old rift) 1,3 cm
h LC (reference lithosphere) 0,5 cm h LC (old rift) 0,2 cm
h viscous UM 1,3 cm Gravity 9,81 m/s2
Upper brittle crust Lower viscous crust Upper brittle
mantle Lower viscous
mantle
Nature
σ − σ1 3
Depth (Km)
Experiment
Depth (mm)
Byrlee (brittle) law
Viscous flow law
Sand Silicon 1 Sand
Silicon 2 σ − σ1 3
Temperature (i.e. depth) dependent
viscous behaviour
Constant viscosity with depth
Figure 2: strength pro�iles for continental lithosphere in nature (left) and in analogue experiments (right).
RHEOLOGICAL PARAMETERS
Layer Material Experiment Density Coeff. friction Cohesion Stress exponent Material constant Effective viscosity
ρ (kg m-3) µ C (Pa) n A η (Pa s)
Brittle upper crust dry feldspar sand 1 to 4 1300 0.4-0.7 15-35
Viscous lower crust silicon 1 1, 2 1400 1,16 1,00E-05 1,06E+05
Viscous lower crust silicon 1 3, 4 1400 1,16 1,00E-05 8,48E+04
Viscous upper mantle silicon 2 1 1578 1,06 1,00E-05 9,35E+04
Viscous upper mantle silicon 2 2,3 1578 1,06 1,00E-05 1,02E+05
Viscous upper mantle silicon 3 4 1550 1,6 7,00E-07 3,37E+05
Lower lithosphere Na Polytungstate+glycerol 1 to 4 1600 1,2
Brittle upper mantle dry quartz sand 1, 2 1500 0.6 30-70
