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Extension in the Aegean nappe-stacks: numerical modelling and their geological validationE. Lecomte1, B. Huet1, L. Le Pourhiet1, L. Labrousse1 and L. Jolivet2
1: iSTeP-UPMC, Université Pierre et Marie Curie, Paris 6, France2: Université d’Orléans, France
P-T-t data after Parra et al. [2002], Martin [2004], Duchêne et al. [2006]
The setup
Sequential evolution of a MCC in a inherited crustal wedge
Influence of the dip and comparison with natural data
0
10
20
30
40
50
60
70
NS
100101010
20202020
modeled part of the Hellenic wedge
Quaternary volcanics
Attic-Cycladic Blueschist unit
Miocene granitoids
Pelagonian unit Apulian slab
Cretan nappe stack
Cycladic basement
Main structural units
fast slab retreat
MOHO
0 1000500 1500
20
40
60
80
Dep
th [
km]
Temperature [°C]
0
810
°C
topT = 0 °C
botT = 1300 °C
= 3300 kg.mm-3
= 2700 kg.mc-3 T
x= 0T
x= 0
-1xv = 0 cm.yr
yv : free
-1xv = 1 cm.yr
yv : free
210 km
50 k
m50
km
40 k
m
CRUST
MANTLE
lithostatic pressure
MOHO
0 250 500
20
40
60
80
Dep
th [
km]
Strength [MPa]
0
mantle
lower nappemiddle nappeupper nappe
MOHO
10 km10 km
30 km
40 km
10 kmupper nappe
middle nappe
lower nappe
mantle
50 km
-16 -13-15 -14
log (ε [s ])-1.
napp
es
visc
ous
stre
ngth
upper
lower
middle
0.1 Myr
2 Myr
5 Myr
10 Myr
16 Myr
20 Myr
Geometry and active structures Strain-rate
D1
D1
D1
D2D1
D1
f2
vc
vc
vc
vc
vc
f3
sz1
f1a f1b f1c
d1
d2
d3f3
d3
f1d
d1
BDT
BDT
BDT
BDT
BDT
BDT
1
4
7
6
5
3
2
1
4
7
6
5
3
2
0
Time [Myr]
6
10
8
4
2
0 6 1210842 14 16 18 20
0°
15°
25°
35°
45°
Initial dip
0° 15° 25° 35° 45°
Computed values, initial dip Mesured values
Mea
n ex
hum
atio
n ra
te[m
m.y
r ]-1
Max
. exh
umat
ion
rate
[m
m.y
r ]-1
0
1
0
1
0
1
500200 800 500200 500200800 800T [°C] T [°C] T [°C]
P [G
Pa]
0
1
0
1
0
1
500200 800 500200 500200800 800T [°C] T [°C] T [°C]
P [G
Pa]
0
1
0
1
0
1
500200 800 500200 500200800 800T [°C] T [°C] T [°C]
P [G
Pa]
0
1
0
1
0
1
500200 800 500200 500200800 800T [°C] T [°C] T [°C]
P [G
Pa]
0
1
0
1
0
1
500200 800 500200 500200800 800T [°C] T [°C] T [°C]
P [G
Pa]
lower nappe(Naxos)
middle nappe(Naxos)
middle nappe(Tinos)
flat
mod
elw
edge
mod
els
lower nappe(Naxos)
middle nappe(Naxos)
middle nappe(Tinos)
flat
50 km
45°
35°
25°
15°
π φ??
−
ψ = φSimilar to Riedel shears (=Byerlee, 1992) Sibson model (=Sibson, 1990)
Associated plastic flowNon-associated plastic flow
R
Y
R’
Shear fractures developed within the fault zone for a friction coefficient of 0.6:
Direction of the principal stresses within the faultzone due to stress rotation
π ψ??
?
α-shear band
β-shear band
ψ = −φ
ψ = 0σ?
σ?σ?in
σ?in
σ?in
σ?in
σ?in
σ?in
out
out σ?
σ?out
out
New compaction model
Predicted newly formed micro/meso-structures after the stress rotation within the fault zone
Partial reactivationSteady state
Reactivation and tension
Analytical solution limiting partial reactivation and steady state
Analytical solution limiting reactivationand tension in steady state
Analytical solution limiting reactivationand tension in partial reactivation
symbol
mode I II III IV
shear strain
0 2%
outCoout *Co = σv
μin
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.7
0.6δ = 24°μ = 0.85ψ = -10°
out
At steady state, strain must be infinite
Plastic strain rate obtained in partial reactivation before locking
Comparison to the classical brittle models
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Co = 0.22out *
φ = 30°out
Analytical solution limiting partial reactivation and steady state:
in ψ ≠ φ
in ψ ≠ φ
in ψ = φ
in ψ = φ (= Collettini and Sibson, 2001)
μin
0 10 20 30 40 50 60 70 80 900
1
2
3
4
μ = 0.6in
θr ( ° )
θr ( ° )
θr
σ1o
ut
σ1o
ut
σ3o
ut
-
σ1
τ σ3
Well-oriented fault Badly-oriented fault
Steady state
partial reactivation
45 50 55 60 65 70 75 80 9085
-30°
-20°-10°
0°10°Steady state Partial reactivation
No
ten
sio
n fa
ilure
Ten
sio
n fa
ilure
II
2δ
φeff
σ
φ
ψ
φin
out
xx n
σ1σ
τ
2δσ
φ
φin
out
xx n
σ1σ
τβ
φeff
III
?
φin
φeff
β
?
IV
nσ
τ
σ3out
σ3out
τ
2δσ
φ
φin
out
xx n1
σ
φeff
outCo
outCo
ψ
σ
I
β > ψ
β > ψ
σ3out
σ3out
2δσ
φout
xx1
outCo
outCo
σ
Four modes of reactivation possible including a complete or partial reactivation of the shear zone with or without tensile failure in the surrounding medium.
Setup: extensional reactivation of a shallow-dipping inherited heterogeneity
outside the fault
inside the fault
v 1σ σ= =ρgzout
computed
x
y
h σσ = out3
δ
τ
nσ1inσ
3inσ
xxoutσ
Onset of the reactivation
β
σvσhσn
τ
2δ
τ =tan(φ ).σ n
in
σn
τ fault plane
Steady state (ss)
σxx σvσhout σn
τ
τ
σn
τ =tan(φ ).σ neff
2δφeff
φin
σn
τ
τ
σnσxxinσ3
in σ1in
ψ
τ =tan(φ ).σ n
in
2δinss
σ*
τ*
Continuum rock mechanics :- continuity conditions : σ = σ = σ and τ = σ = σ- compatibility conditions : ε = ε
yynout
outout
xxout
xxin
yyin
in
xyin
xyout
Parameters:- outside the fault zone: Co , φ and z - within the fault zone: φ , ψ and δ
Mykonos island, Cyclades
20%
30%
1) High contrast 2) Intermediate contrast 3) Small contrast
Plastic strain (ps) Second Invariant of stress tensor (tau)
0 1 >2 < 1E+5 2E+7 >5E+7
10km 10km10km
10%
kmkm km
ps
tau
tau
tau
0
5
10
15
0
5
10
15
0
5
10
15
0
5
10
15
0
5
10
15
0
5
10
15
0
5
10
150
5
10
15
0
5
10
15
0
5
10
15
0
5
10
15
0
5
10
15
Varying the viscous strength contrast
Brittle ductile transition
Effect of a weak viscous nappe
Long maxwel relaxation time
Short Maxwell relaxation time
Strength Contrast
(t1)(t1)
(t2)
(t3)(t2)
(t2)
(t2) (t2)
scale for plastic strain (ps)
0 1 >2
ps
0
5
10
15
0
5
10
15
Kalavrita DoumenaMamousia
Helike Aigion
Gulf
10 km
Kalavrita DoumenaMamousia
Helike Aigion
Gulf
10 km
Comparison with Microseismicty
age of the faults t1 > t2 > t3
Effective elasto-plastic behavior
Effective visco-elastic behavior
Hypocentre of main earthquakes
Orientation of maximum compressive stress
In order to reproduce natural data set available in the Gulf of Corinth at least one extremly weak viscous nappe must be present in the basement
Accounting for a weak fault the friction must be very close to zero in order to explain the faulting pattern
What Can be the cause for such weakness at the brittle ductile transition ?
Weak viscous contrast
Strong vicous contrast Strong frictional contrast
Weak frictional contrast
The models are constrained by a large set of natural data:- structural geology in the Gulf of Corinth and the Cycladic islands,- microseismicity in the Gulf of Corinth,- microstructure within the gouges of Mykonos detachments,- P-T paths and exhumation rates of the metamorphic units exposed in the Cycadic MCCs,- kinematics of the ductile deformation in the footwall of the detachments.
The Cycladic MCCs open windows in the deformation of the lower
crust below the Gulf of Corinth
The Gulf of Corinth reworks and roots in the Cretan HP-LT metasediments (Phyllite-Quartzite nappe).
Jolivet et al. [2004]
The Cycladic detachments exhume the HP-LT Attic-Cycladic Blueschists and Cycladic Basement.
The Hellenides are the result of conver-gence between Africa and Europe since the late Cretaceous.
Thickening has been achieved by stack-ing of successive units coeval with HP-LT metamorphism:- Eocene in the Cyclades,- Miocene in Crete and Peloponese.
Retreat of the African slab induced litho-spheric stretching in the back arc:- Oligo-miocene MCCs in the Cyclades,- Mio-quaternaty Gulf of Corinth in the Peloponese.
Jolivet et al. [2010]
Jolivet et al. [2004]
Sibson, R.H. [1990] Rupture nucleation on unfavourably oriented faults, Bulletin of the Seismological Society of America, 80-6, 1580-1604. Byerlee J.D. & Savage J.C. [1992] Coulomb plasticity within the fault zone, Geophysical Research Letters, 19, 2341-2344. Collettini C. & Sibson R. H. [2001] Normal faults,
normal friction?, Geology, 29, 927-930. Jolivet L. et al. [2004] Correlation of syn-orogenic tectonic and metamorphic events in the Cyclades, the Lycian nappes and the Menderes massif. Geodynamic implications, BSGF, 175, 217-238. Le Pourhiet L. et al. [2004] Rifting through a stack of inhomogeneous thrusts (the dipping pie concept), Tectonics, 23, doi:10.1029/2003TC001584. Jolivet L. et al. [2010] Rifting and shallow-dipping detachments, clues from the Corinth Rift and the Aegean, Tectonophysics, 483, 287–304. Lecomte E. et al. [in press] A continuum mechanics approach to quantify brittle strain on weak faults: application to the extensional reactivation of shallow-dipping discontinuities, Geophysical Journal International. Huet B. et al. [submitted to EPSL] Formation of metamorphic core complex in inherited wedges: a thermomechanical modelling study.
1. The Hellenides and the Aegean domain 4. Structural inheritance in the Gulf of Corinth:an active detachment [Le Pourhiet et al., 2004]
Abstract
2. Corinth and the Cyclades: two parallel stories
3. A large dataset to constrain the models
5. Structural inheritance on Mykonos island:an exhumed detachment [Lecomte et al., in press]
6. Structural inheritance in the Cycladic:a crustal wedge reworked by MCCs [Huet et al., subm.]
7. Conclusions
References
The Aegean region is being stretched in a back-arc position for the last 30-35Myr. At large scale, the deformation is distributed but at smaller scale, field observations show that the extension has been localisied along detachments as the Aegean slab was retreating with time. In all the cases, extension has been and is still reworking the initial nappe stack accreted within the accretionary prism of the subduction. In this contribution, we present
attemps of modelling the effect of this structural inheritence at different scales in the crust. In all the cases, different dataset have been used to validate the models. Since all the studied areas are representative of the same process with very similar initial conditions, it is possible to use the example of the Gulf of Corinth has an active analogue for the initiation of the Cycladic detachments and oppositely to use the
exhumed Cycladic MCCs has a field analogue for the lower crust of the Gulf of Corinth. Thanks to those parallel histories, it is possible to use more datasets to validate the mechanical models of the crust and therefore to retrieve the effective rheology of the crust and the lithosphere.
Structural inheritance is present at all scale in the Aegean. Including geological knowledge of the initial complexity of the crust in the set up of mechanical or thermo-mechanical models allows to get more realistic results and therefore to use more data in order to validate quantitatively the models. Inherited dipping heterogeneities such as a 1km thick mica-schist layer (Gulf of Corinth), and inherited weak fault zone (Mykonos) or a several km thick layer of meta-sediments (Cyclades) impose some complexity in the kinematics that allows explaining out of sequence faulting at crustal scale, the distribution of the seismicity at depth and the slip lines within kilometre scale fault zone or the reheating of material during the exhumation without introducing complex parametrisation of the rheology in the models. With this approach, we can use simple rheological models in order to asses effective rheological behaviour of the rocks in situ. Since the boundary conditions have not changed that much and plenty of todays active graben of the Aegean are located in similar geological context as were the Cycladic MCCs during the early Miocene Times, we were able to use similar model set up to model exhumed MCCs, the microstructures in exhumed detachments or an active rift such as the gulf of Corinth. This allows to gather more dataset together in order to constrain the rheology of the crust at different scale across the brittle-ductile transition.
