fault propagation, exhumation and triangle zones in analog ... · exhumation of basal layers is...
TRANSCRIPT
nalog sandbox modeling is used to study the impact of syntectonic erosion and sedimentation on fault geometry and exhumation rates in thrust wedges with one and two frictional décollement levels. The
diversity of exhumation patterns in eroded thrust wedges is controlled by the mode of fault propagation, depending on the basal friction (high or low).
In the thrust wedges with one and two décollements, different accretion mechanisms are activated depending on interactions between surface pro-cesses and wedge mechanics: frontal accretion, backthrusting, underthrus-ting and underplating due to décollement-induced duplex formation at depth. These mechanisms may function simultaneously, being located at different parts across the wedge.
Presence of one décollement in the accreted series allows underplating of thrust units developing an anticlinal stack, whose growth and location is favored by erosion. The cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of the thrust wedge.
Erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation. At the advanced stages of erosion, the development of major backthrusts is observed in the thrust wedges without or with one décollement, but no backthrusts is formed in the wedges with two décollements. Variations in the erosion taper lead to changes in duplex geometry and exhumation rate in the thrust wedges with two décollements.
Syntectonic sedimentation induces forward propagation of flat detach-ments, resulting in the formation of a piggyback basin.
Syntectonic erosion and sedimentation acting simultaneously on the thrust wedge promote formation of a triangle zone between the frontal imbri-cate faults of the wedge and the foreland basin.
The experimental results support the observations on structural evolu-tion and erosion in the Alberta Foothills of the Canadian Rockies and in the Quebec Appalachians.
Structural Geology and Tectonic Forum 2012Williamstown, MA June 14-16, 2012
Fault propagation, exhumation and triangle zones in analog models of thrust wedges: influence of syntectonic erosion and sedimentation
Elena Konstantinovskaya
Institut National de la Recherche Scientifique490 de la Couronne, Quebec (QC) G1K 9A9 Canada
46o30'
46o45'
71o30'
71o00'
72o00'
Logan's Line
St. Lawrence River
Queenston Group (Becancour Fm)
Sainte-Rosalie Gr. (Lotbiniere Fm)
Utica Shale
Pontgrave Fm.
Nicolet Fm.
Black River and Trenton groups
GRENVILLE BASEMENTNNormal fault
Thrust fault
Syncline axial plane
Anticline axial plane
Sainte-Rosalie Gr. (Les Fonds Fm)
Slope, rise siliclastics
Reversely reactivated normal fault
0 5 10 km
Chaudière Nappe
Boyer Rive
r Nappe
Bacchus N
appeNeu
ville F
ault
Jacques Cartier Fault
Deschambault FaultCFS Chambly-Fortierville Syncline
Montmorency Fault
Saint P
rospe
r Fau
lt
Aston Fault
+ + + +
+ +
+ + + ++
+
?
TZ
Backthrust
CFS
LT
PQT
St. Lawrence Lowlands Platform
Appalachian thrusts domain
Parautochthonous domain
Lorraine Group
Grondines
Portneuf
Neuville
Chateau Richer
Quebec City
Saint Alban Fault
78-824
MW4. One décollement, erosion 6o MW5. Two décollements, erosion 6o MW6. Two décollements, erosion 8o
Scheme of the experimental device
105 cm
1 2
critical taper 8°
1 particle paths
25°
43°31°
20 cm
125 cm
20 cm1 particle paths
3
1
2
underthrusting
domain of maximum exhumation
critical taper erosion 8°
20°40°
60°50°
20 cm
29%
77%
100%
a
exhumation path1
FT1
two décollements
b
FT1FT2
erosion taper 8° c
ramp-anticlines
FT1 FT2 FT31
d
small-scale duplex
32°43°- 5°54°
domain of maximum exhumation
0% of shortening15O
two décollements
two décollements
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100 120
Shortening, %
Rat
io o
f bas
al u
nder
thru
stin
g, R
und,
%
MW6
MW5
MW3
MW1
MW4
MW2 (b)
no décollements
with décollements
no erosion
Shortening, %
0
10
20
30
40
10 20
Eeros, %
R und
, %
(c)
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100 120
MW6
MW5
MW3
MW1
MW4
MW2
Exte
nt o
f ero
sion
al re
mov
al, E
eros
, %
(a)
no décollements
with décollements
(no erosion)
Sfinal Sinit + Sinp
Serod
Sinit
non-eroded wedgeeroded wedge
proto-wedge15o
Eeros = Serod / (Sinit + Sinp) * 100%,
Sfinal
SundRund = Sund / Sfinal * 100%,
α'
α+β
α
β
critical taper (α+β) is equal (α') when β = 0
MW2. No décollements, erosion 8o
MW1. No décollements, no erosion
0% of shortening
15 o
100%
23%
59%
a
20 cm
exhumation paths1
1 2
antiformal stack
synformal klippe
domain of maximum exhumation
d
two décollements
two décollements c
FT2FT1
erosion taper 6°
51 cm
two décollements
FT1
b
PU1 PU218%
59%
100%
a
underplating frontal accretion
FTn
FT1 erosion taper 6°antiformal stack c
underplating frontal accretionduplexing
antiformal stacke
décollementsynformal klippe1 2 1+2
Accretion mechanisms
0% of shortening15o
20 cm
décollement
décollement
FT1 d
exhumation paths31
321domain of maximum exhumation
FTn FTn’
frontal accretionduplexing
FT1b
décollement
Experiment 2
Experiment 1
Experiment 2
Experiment 1
backthruststriangle zone
forward propagated detachmentsyn-orogenic sand layers
basal detachment
29%
2 cm
Scheme of the experimental device
5 cm
29%
backthrust
5 cm
38%
model cross-section
blind forethrusts
backthrust
motor
h=0 cm
h=1 cmh=1.6 cm
h=0.5 cm
b
b′
60o
140o
LF
LF
LF
2o
HF
2.2 cma-a′
DL
0.6 cm
3 cm
motor
h=0 cm h=1 cm
5 cm
a
a′
140o140o
h=1.6 cm
HF/DL LF
LF
46o30'
46o45'
71o30'
71o00'
72o00'
0 5 10 km
St. Flavien
Two-way time (s)
N Light angle 300o
Declination 25o
Vertical exaggeration 10x
Syncline axial planeCFS Chambly-Fortierville Syncline
0.10.51.0
2.0
3.03.5
1.5
2.5
GRENVILLE BASEMENT
Montmorency Fault
Neuvill
e Fau
lt
Jacques-Cartier Fault
Deschambault Fault
Saint-P
rospe
r Fau
lt
Quebec
+ + + +
+ +
+ + + ++
+
Aston Fault
Backthrust
CFS 78-824
Grondines
Neuville
Chateau Richer
TZ
Portneuf
Structural cross-section across the southern Alberta Foothills
SW NE
SW NELine JR-103 Line JR-1092 km
Roof Thrust
LD
Low detachment
TWT
(s)
0
1
2
TWT
(s)
0
1
2
2
4
0
SW NE
Moose Mountain Thrust
McConnell Thrust
00
SW NE
McConnell Thrust
Moose Mountain Thrust
MD
CM
D
C
MD
C
Cretaceous
Cretaceous and Jurassic
Mississippian
Devonian
Cretaceous
Cambrian
M
D
C
Jurassic to Cretaceous
Mississippian
Devonian
Cambrian
M
D
C
DM
M DC
M
D
C C
5 km
5 km
Tertiary
Lower Cretaceous and Jurassic
Upper Cretaceous
Mississippian and Upper DevonianM-D
CambrianC
PRECAMBRIAN BASEMENT
PRECAMBRIAN BASEMENT
PRECAMBRIAN BASEMENT
M
DC
M
D
C0
2
4
PALEOZOIC STRATA
MD
McConnell Thrust Raven River
Thrustkm
kmM-D
C
M-DC
Roof Thrust
Main Detachment Level
α = 2otaper angle
β = 3obasal detachment angle
basal detachment
basal detachment
Triangle zone
Triangle zone
South of the Bow Valley
Jumping Pound gas fields
Moose Mountain Culmination
Limestone Mountain Culmination
after Ollerenshaw (1978), Bruce, et al. (1995), Newson and Sanderson (1996), Soule and Newson (2000), Price and Monger (2000), Begin and Spratt (2002)
M
D
C
Tertiary
Upper-Lower Cretaceous and Jurassic
Upper Cretaceous
Mississippian
Cambrian
Upper Devonian
The thrust wedge retains the same geometry through shortening. Exhuma-tion of basal layers is observed at the rear part of the wedge at the end of shortening.
proto-wedge
plastic sheet
erosion angle
sand layers
2 m
10 cmrigid base
15back
stop
sand
glass side wall
high basal friction
α’
REFERENCES
1. Konstantinovskaya, E., and Malavieille, J., 2011. Thrust wedges with décollement levels and syntectonic erosion: A view from analogue models. Tectonophy-sics, 502: 336–350.2. Konstantinovskaya, E.A., D., Rodriguez, D., Kirkwood, L.B., Harris, and R., Thériault. 2009. Effects of basement structure, sedimentation and erosion on thrust wedge geometry: an example from the Quebec Appalachians and analogue models. Bulletin of Canadian Petroleum Geology, 57 (1): 34–623. Konstantinovskaya, E.A. and Malavieille, J. 2005. Erosion and Exhumation in Accretionary Orogens: Experimental and Geological Approaches. Geochemistry, Geophysics, Geosystems, 6, N2, Q02006.4. Malavieille, J., 2010. Impact of Erosion, Sedimentation and Structural Heritage on the Structure and Kinematics of Orogenic Wedges: Analog Models and Case Studies, Geol. Soc. Am. 20 (1), 4–10.
ACKNOWLEDGMENTS One part of the present study was supported by a French MENRT grant to Elena Konstantinovskaya to fund the Associate Professor position in Montpellier (2000–2003). Another part of the study was realized in the INRS-ETE laboratory for physical simulations, which was funded by a Canada Foundation Innovation, Ministère de l’Education, de Loisir et du Sport grant, with contribution from INRS-ETE, BEG (University of Texas at Austin), Sun Microsystems, Seismic Microtech-nology and Norsar to Lyal Harris. Open File seismic lines and well data from the Ministère des Ressources naturelles et de la Faune, Quebec, formed a basis for regional geological studies.
2o
LF
2.2 cmb-b′0.6 cm
3 cm
syntectonic erosion and exhumation 4o
triangle zone
backthrust
décollement level
syn-orogenic sand layers
38%
syntectonic erosion and exhumation 4o
No exhumation is observed. The wedge slope 8° (e) represents the criticalaccreting taper angle for the thrust wedges with high basal friction.
Exhumation of basal layers is observed in the dome-like antiformal stack at the rear of the wedge at the end of shortening (d). Thrust faults steeply plunge down the section at the frontal part of the growing anti-formal stack (c–d). The cover layers above décollement are completely detached from the basal layers and compressed in synformal klippe (c–e).
The independent system of thrusts develops above each décollement (red square). The basal layers below the lower décollement form antiformal stack growing through shortening (b–d) but they are not exhumed (d). The cover layers above the lower décollement are nearly completely eroded above the stack (d). Thrust faults steeply plunge down the section at the frontal part of the growing antiformal stack (d). The imposition of erosion taper with lower (6°) angle than the critical value (8°) in the models with décollements promoted formation of antiformal stack with high extent of exhumation.
Thrusts are frequently independent below and above the upper décollement (red square). The basal layers below the lower décollement form small-scale normal duplex slightly growing through shortening (b–d). Basal material is never exhumed (d). The cover layers form individual ramp-anticlines (c–d). Thrust faults are slightly inclined down the section at the frontal part of the growing basal duplex (d). Only cover layers above the upper décollement are eroded above the duplex (d). The 8° erosion profile (equal to critical slope) provided formation of individual ramp-anticlines in the upper wedge above the décolle-ment and small scale normal duplex below it with low amount of basal underplating.
Critical taper angle (α+β) composed of the dip angle of basal detachment β and the taper angle α determines the geometry of a thrust wedge, modified after Davis et al. (1983). Critical taper angle of our model wedges is equal to the imposed erosio-nal slope (α′) because basal detachment is horizontal.
(a) Proportions of initial (Sinit), eroded (Serod), and final (Sfinal) areas in model wedges used to calculate extent of erosional removal (Eeros). Serod=Sinit+Sinp−Sfinal. (b) Ratio of basal underplating (Rund) calculated as a ratio of the area of basal layers (Sund) with respect to final area (Sfinal) in model wedges. Sinit, Sfinal and Sund are measured from digitized photos of the model wedges. Sinp is calculated from experimental parameters.
Interactions between surface processes and wedge mechanics have important consequences on the structure and evolution of foreland thrust belts. Different accretion mechanisms are thus combined to account for wedge growth: frontal accretion, backthrusting, underthrusting and underplating due to décollement induced duplex formation at depth. These mechanisms may function simultaneously, being located at different parts across the wedge.
Our experiments clearly reflect these complex feedback mechanisms. Addition of erosion limits thrust wedge forward propagation and thickening. The higher rate of erosion leads to higher extent of exhumation of basal material. Variations in erosion slopes led in changes of fault kinematics and of the extent of basal material exhumation.
Time structure map of the Precambrian basement in the Quebec City area
General geology of the St. Lawrence Lowlands and the southern Quebec Appalachians
Geological map of the Quebec City area
modified after Globensky (1987)modified after Thériault et al. (2005)
Chambly-Fortie
rville Syn
cline
GRENVILLE BASEMENT
47oN
46o
73o 71oW
20 km
Montreal
Joliette
Trois Rivières
PortneufNeuville
Quebec City
Drummondville
Saint-Dominique
Chaud
ière N
appe Rich
ardso
n Nap
pe
N
Normal fault
Thrust fault
Grenville basement
Parautochthonousimbricate fault zone
+ + + +
+ ++
+ + +
+
+ +
Logan’s Line
Aston F
ault
M2001
78-824
M2002
BRNBN
BN
St. Lawrence LowlandsPlatform
Appalachian thrusts
M2001 Seismic line
Study area
Syntectonic sedimentation in the foreland of the fold-and-thrust belt changed the dynamics of the tectonic wedge and favoured the activation of weak décollement layers at the base or within the cover. This led to the forward propagation of detachments and induced the development of piggyback basins.
Syntectonic erosion retarded forward propagation of thewedge, enhanced material transport across the wedge, and focused internal deformation and exhumation close to the backstop.
Syntectonic erosion and sedimentation together affected the thrust wedge kinematics; a series of backthrusts and/or triangle zones developed at the rear of the wedge.
Deflection and rotation of frontal detachments was influenced by the presence of an oblique, steep basement escarpment in front of the growing wedge, by gradual changes of sand layer thickness across the ramp and by differential slip along the basal detachment. Deflection is produced by differential forward motion of the hanging wall along the fault strike and a counter-clockwise rotation about a vertical axis.
Re-interpreted seismic line across the Appalachian front showing location of the triangle zone (TZ).
h–height of rigid basement, HF–high basal friction, LF–low basal friction, DL–décollement level.
Variations in the initial thickness of sand layers are controlled by frontal and lateral rigid escarpments parallel or oblique to the shortening directions, which were introduced at the base of the models. Their geometry mimics the structure of the Grenville basement in the Quebec City area.
Experiments with syntectonic sedimentation and erosion Upper crustal structure across the Appalachian front
Note deflection in plan view of frontal thrust against the deep basement escarpment and development of backthrusts and triangle zone at the rear of the thrust wedge.
0.0
0.4
0.8
1.2
1.6
2.0
TWT
(s)
SE 155 205 255 305 355 405 455TRACE105 NW
1km 78-824
Aston FaultLogan’s Line
A168 A161
Neuville Fault
Neuville Fault
Backthrust
Ut-Tr
Pts
BmPC
LtUt-Tr
PC
CFSTZ
GB
Ni
Lb
Bt
LFBt
Lb LbNi
NiNi
BcPg Pg
Ni
155 205 255 305 355 405 455TRACE
0.0
0.4
0.8
1.2
1.6
2.0
TWT
(s)
SE NW105
1km 78-824
model cross-section
exhumation
50 km 0
20 km
10 km
E W
Eurasian Plate
2nd zone of underplating
1st zone of underplating
Volcanic arcFoothills
Central Range
~ 4 cm/yr~ 3 cm/yr~ 2 cm/yr
~ 9 cm/yr
Interpretive geological section of Taiwan orogenic wedge
Simplified structural section across Jumping Pound gas fields
2
4
0
SW NEMcConnell Thrust
Moose Mountain Thrust
1 2 3 4 5
5 km
M-D C
PRECAMBRIAN BASEMENT
kmM-D
M-DC
1, Tertiary; 2, Upper Cretaceous; 3, Lower Cretaceous and Jurassic; 4, Mississippian and Upper Devo-nian; and 5, Cambrian strata.
after Bruce, et al. (1995); Price and Monger (2000)
after Malavieille (2010)
TZ - triangle zone
sedimentationin the foreland basin
sedimentationin the foreland basin