Heat vs Stretching Rate - Central Washington University D Huerta/gsa... · 2014-11-05 · Strain...
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Strain evolution and the relative role of lithospheric heterogeneities during continental rupture Audrey Huerta, Erik Rheams Dept. of Geological Sciences Central Washington University Ellensburg, WA 98926 CENTRAL WASHINGTON UNIVERSITY The evolution of strain patterns during continental rupture can be quite com- plex, and may be due to either changes in far-field stresses, and/or litho- spheric heterogeneities. The West Antarctic Rift system (WARS) is an ex- ample of a rupture system that experienced a distinct switch in deformation style. During the first stage extension was broadly distributed throughout most of the Ross Sea region. Later, the style of extension changed and was fo- cused primarily in the Terror Rift, near the boundary with the East Antarctic craton. Previous work (Huerta & Harry, 2007) indicates that the thermal structure of the lithosphere places a first-order control on the style of extension (wide vs narrow) and the location of rupture (central , or marginal). Here we explore the interplay between ther- mal structure and stretching rate on the rift- ing style and rupture of the lithosphere Again, we find that the thermal structure places first-order control on the style of ex- tension and the location of rupture. Stretch- ing rate places a second-order control on the style of rifting and location of rupture such that faster stretching can accomodate wider rifting, and faster stretching favors central ruptures. t=30 Ma East Antarcica WARS Victoria LandBasin- Terror Rift 0 2 4 a t e b l a t s u r c ) m k ( s s e n k c i h t beta crustal thickness East West t=40 Ma East Antarcica WARS Victoria Land Basin 0 2 4 a t e b l a t s u r c ) m k ( s s e n k c i h t beta crustal thickness East West t=60 Ma East Antarcica WARS 0 2 a t e b l a t s u r c ) m k ( s s e n k c i h t beta crustal thickness East West t=80 Ma East Antarcica WARS 0 2 a t e b l a t s u r c ) m k ( s s e n k c i h t beta crustal thickness East West t=100 Ma 7.5 km/my V.E.2:1 East Antarcica WARS l a t s u r c ) m k ( s s e n k c i h t crustal thickness East West Application to West Antarctic Rift System Model Results: Model results consistent with the geologic history of the WARS indicate that the transition from a prolonged period of broadly distributed extension to a later period of focused rifting did not require a change in the regional stress regime (changes in plate motion), or deep mantle thermal state (impingement of a plume). Instead, we attribute the transition from diffuse to focused extension to an early stage dominated by the initially weak accreted lithosphere of West Antarctica, and a later stage that shifted to a secondary weakness located at the boundary between the juvenile West Antarctica lithosphere and Precambrian East Antarctica craton. The modeled shift from the initially weak West Antarctica region to the secondary weakness at the West Antarctica-East Antarctica boundary was precipitated by strengthening of the West Antarctica lithosphere during syn-extensional thinning and cooling. This syn-extensional strengthening of the WARS lithosphere promoted a wide-rift mode of extension between 100 and 53 Ma. After 53 Ma, extension shifted to the Victoria Land Basin region. Localized extension in this area resulted in rapid mantle necking and syn-extensional weakening that promoted a narrow- rift mode of extension and the formation of the Terror Rift. Geodynamic models demonstrate that the transition from diffuse to focused extension occurs only under a limited set of initial and boundary conditions. The interplay between crustal heat production, heat conduction from the mantle, and lateral heat conduction are the key parameters that determine whether extension evolves from an early state of broadly distributed rifting to a later stage of strongly focused rifting. Initial, wide mode of rifting Late stage, narrow mode of rifting basement lower sediment package upper sediment package subvolcanic intrusions 0 100 km VE~9:1 Eastern Basin Central Basin Victoria Land Basin 20 mgal Free air gravity anomaly TAM WEST EAST MOHO 0 10 20 30 40 km Terror Rift Central Highland Fault Pre-Mesozoic Basement Cenozoic Igneous Pz-Mz 100 km + + + + + + + + + + + + + + + + S ° 2 7 S ° 4 7 S ° 6 7 160°E 170°E 170°E 180° Northern Basin Central Basin Victoria Land Basin Eastern Basin Ross Ice Shelf line of cross-section Central Highland TAM: Sediment-filled basins Highlands Ross Sea: East Antarctica 90°E v v v v v v v v v v v v v Inferred extent of the West Antarctic Rift System Marie Byrd Land 180° 90°E 90°W Ross Sea Region of detail Transantarctic Mts 1a) 1b) Two distinct stages of extension are recognized in the West Antarctic Rift system (WARS). During the first stage, beginning in the Late Cretaceous, extension was broadly distributed throughout much of West Antarctica. A second stage of extension in the Late Paleogene was focused primarily in the Victoria Land Basin, near the boundary with the East Antarctic craton. The transition to focused extension was roughly coeval with strike slip faulting and volcanic activity in the adjacent Transantarctic Mountains. This spatial and temporal correspondence suggests that the transition in extensional style could be the result of a change in plate motions or impingement of a plume. Initial set up and Boundary conditions Vertical exaggeration: 2:1 m k 0 5 1 1000 km 42 km 143 30 km v=10 km/my v=10 km/my T =1200°C T =0°C dT/dx=0 1300°C 2-> 20 km/my 2-> 20 km/my Table 1 Rheologic paramete rs Crust Mantle V iscous rheology a A,s - 1 Pa - n 5×10 - 18 4×10 - 25 Q, kJ mol - 1 219 498 n 2.4 4.5 ρ, kg m - 3 2850 3300 Plastic rheology b S, MPa 60 60 B , MPa km - 1 11 11 a σ =[ ε · / A] 1/ n e Q/ nRT [48] . b σ = S + Bz [47] . half-space spreading velocity (im/my) 2 5 10 15 20 0.50 0.75 1.0 1.25 1.75 Crustal Heat Production Rate (µW/m^3) Central Rupture Style Narrow<--------------------------------------------------------Wide Total Stretching: 260 km Total Stretching: 255 km Total Stretching: 240 km Total Stretching: 225 km Total Stretching: 228 km Total Stretching: 242 km Total Stretching: 255 km Total Stretching: 260 km Total Stretching: 270 km Total Stretching: 280 km Total Stretching: 340 km Total Stretching: 360 km Total Stretching: 340 km Total Stretching: 365 km Total Stretching: 202 km *(not to rupture) Total Stretching: 240 km *(not to rupture) Total Stretching: 540 km Total Stretching: 450 km Total Stretching: 420 km Total Stretching: 360 km *(not to rupture) Total Stretching: 295 km *(not to rupture) Total Stretching: 242 km *(not to rupture) Heat vs Stretching Rate Marginal Rupture Style Total Stretching: 192 km *(not to rupture) Total Stretching: 315 km *(not to rupture) Total Stretching: 350 km *(not to rupture) Temperature at top of the mantle: t=0 my: 450°C t=5 my: 460°C t=10 my: 470°C t=13 my: 465°C Extending Thinning of Lithosphere Warming of Upper Mantle Temperature (°C) 0 400 800 1200 ) m k ( h t p e D 0 50 100 150 t=0 my 0 400 800 1200 0 50 100 β=1.4 t=5 my 0 400 800 1200 0 50 β=2.8 t=10 my 0 400 800 1200 0 50 β= 5.8 t=13 my Low heat production: extension=>thinning of lithosphere thinning=> warming top of mantle warming=> weakening of lithosphere Weakening => narrow rift mode (e.g.Buck, 1991) Thinning + Warming= Weakening Integrated strengh: t=0 my: 14.9 TN/m t=5 my: 9.5 TN/m t=10 my: 2.9 TN/m t=13 my: 1.1 TN/m t=0 my β=1.4 t=5 my β=2.8 t=10 my β= 5.8 t=13 my Low Heat Production: Strength and Thermal Evolution Temperature at top of the mantle: t=0 my: 815°C t=5 my: 800°C t=10 my: 750°C t=15 my 680°C t=20 my: 635°C Extending Thinning of Crust Cooling of Upper Mantle t=0 my Temperature (°C) 0 400 800 1200 ) m k ( h t p e D 0 50 100 150 0 400 800 1200 0 50 100 0 400 800 1200 0 50 0 400 800 1200 0 50 0 400 800 1200 0 50 β= 1.6 t=5 my β=2.1 t=10 my β=2.4 t=15 my β= 2.5 t=20 my Integrated strengh: t=0 my: 2.0 TN/m t=20 my: 2.3 TN/m Extending Cooling t=0 my With Heat Production: Strength and Thermal Evolution Strengthening With heat production: extension=> thinning thinning of crust=> cooling top of mantle cooling=strengthening of lithosphere Strengthening => wide rift mode (e.g.Buck, 1991) Central Rupture Increasing Crustal Heat Production => Wider Rift Marginal Rupture Heat conducting in from un-extended region With low Nusselt # (advection/conduction): Strength and Thermal Evolution thinning of crust=> cooling top of mantle Extending and thinning region: With higher heat production: extension=> thinning cooling=strengthening of lithosphere Marginal region: Adjacent crust does not thin, remains warm Heat conducted to marginal, thinned, region Marginal areas are thin and warm => weakest region => extension accomodated at margins Extending Not as much Cooling Weakening Strengthening Extending Thinning of Crust Cooling of Upper Mantle
Transcript of Heat vs Stretching Rate - Central Washington University D Huerta/gsa... · 2014-11-05 · Strain...
Strain evolution and the relative role of lithospheric heterogeneities during continental ruptureAudrey Huerta, Erik Rheams Dept. of Geological Sciences
Central Washington University Ellensburg, WA 98926
CENTRAL WASHINGTON UNIVERSITY
The evolution of strain patterns during continental rupture can be quite com-plex, and may be due to either changes in far-field stresses, and/or litho-spheric heterogeneities. The West Antarctic Rift system (WARS) is an ex-ample of a rupture system that experienced a distinct switch in deformation style. During the first stage extension was broadly distributed throughout most of the Ross Sea region. Later, the style of extension changed and was fo-cused primarily in the Terror Rift, near the boundary with the East Antarctic craton.
Previous work (Huerta & Harry, 2007) indicates that the thermal structure of the lithosphere places a first-order control on the style of extension (wide vs narrow) and the location of rupture (central , or marginal).
Here we explore the interplay between ther-mal structure and stretching rate on the rift-ing style and rupture of the lithosphere
Again, we find that the thermal structure places first-order control on the style of ex-tension and the location of rupture. Stretch-ing rate places a second-order control on the style of rifting and location of rupture such that faster stretching can accomodate wider rifting, and faster stretching favors central ruptures.
t=30 MaEast Antarcica WARS
Victoria LandBasin-Terror Rift
02
4
ateb
latsurc)
mk( ssenkciht
beta
crustal thicknessEastWest
t=40 Ma East Antarcica WARS
Victoria LandBasin
02
4
ateb
latsurc)
mk( ssenkciht
beta
crustal thicknessEastWest
t=60 MaEast Antarcica WARS
0
2 ateb
latsurc)
mk( ssenkciht
beta
crustal thicknessEastWest
t=80 MaEast Antarcica WARS
0
2 ateb
latsurc)
mk( ssenkciht
beta
crustal thicknessEastWest
t=100 Ma
7.5 km/my
V.E.2:1
East Antarcica WARS
latsurc)
mk( ssenkciht
crustal thicknessEastWest
Application toWest Antarctic Rift System
Model Results: Model results consistent with the geologic history of the WARS indicate that thetransition from a prolonged period of broadly distributed extension to a later period offocused rifting did not require a change in the regional stress regime (changes in platemotion), or deep mantle thermal state (impingement of a plume). Instead, we attribute thetransition from diffuse to focused extension to an early stage dominated by the initiallyweak accreted lithosphere of West Antarctica, and a later stage that shifted to a secondaryweakness located at the boundary between the juvenile West Antarctica lithosphere andPrecambrian East Antarctica craton. The modeled shift from the initially weak WestAntarctica region to the secondary weakness at the West Antarctica-East Antarcticaboundary was precipitated by strengthening of the West Antarctica lithosphere duringsyn-extensional thinning and cooling. This syn-extensional strengthening of the WARSlithosphere promoted a wide-rift mode of extension between 100 and 53 Ma. After 53Ma, extension shifted to the Victoria Land Basin region. Localized extension in this arearesulted in rapid mantle necking and syn-extensional weakening that promoted a narrow-rift mode of extension and the formation of the Terror Rift. Geodynamic modelsdemonstrate that the transition from diffuse to focused extension occurs only under alimited set of initial and boundary conditions. The interplay between crustal heatproduction, heat conduction from the mantle, and lateral heat conduction are the keyparameters that determine whether extension evolves from an early state of broadlydistributed rifting to a later stage of strongly focused rifting.
Initial, wide mode of rifting
Late stage, narrow mode of rifting
basement
lower sediment package
upper sediment package
subvolcanic intrusions0 100 km
VE~9:1
EasternBasin
CentralBasinVictoria Land
Basin
20 mgal
Free airgravity
anomaly
TAMWEST EAST
MOHO
0
10
20
30
40km
TerrorRift
CentralHighland
Fault
Pre-Mesozoic Basement
Cenozoic Igneous
Pz-Mz
100 km
+++
+
++
+ + +
++++++
+ + + + +
+
+
S°27S°47
S°67
160°E
170°E 180°E 170°W
170°E 180°
Northern Basin
CentralBasin
VictoriaLandBasin
Eastern Basin
Ross Ice Shelf
line of cross-section
CentralHighland
TAM:
Sediment-filledbasins
Highlands
Ross Sea:
EastAntarctica
RossSea
90°E 90°W
180°
vvvvvvvvvvvvv
Inferred extent of theWest Antarctic Rift System
Marie Byrd Land
180°
90°E 90°W
RossSea
Region of detail
TransantarcticMts
1a) 1b)
Two distinct stages of extension are recognized in the West Antarctic Rift system(WARS). During the first stage, beginning in the Late Cretaceous, extension was broadlydistributed throughout much of West Antarctica. A second stage of extension in the LatePaleogene was focused primarily in the Victoria Land Basin, near the boundary with theEast Antarctic craton. The transition to focused extension was roughly coeval with strikeslip faulting and volcanic activity in the adjacent Transantarctic Mountains. This spatialand temporal correspondence suggests that the transition in extensional style could be theresult of a change in plate motions or impingement of a plume.
Initial set up and Boundary conditions
Vertical exaggeration: 2:1
143
mk 051
1000 km
400 km
280 km
42 km
143
30 km
v=10 km/my v=10 km/my
T=1200°C
T=0°C
dT/dx=0
1300°C
2-> 20 km/my 2-> 20 km/my
Table 1Rheologic parameters
Crust Mantle
Viscous rheologya
A, s−1 Pa− n 5×10−18 4×10−25
Q, kJ mol−1 219 498n 2.4 4.5ρ, kg m−3 2850 3300
Plastic rheologyb
S, MPa 60 60B , MPa km−1 11 11
a σ = [ ε· /A]1/neQ/nRT [48].b σ = S+ Bz [47].
ha
lf-s
pa
ce s
pre
ad
ing
ve
loci
ty (
im/m
y)
2
5
10
15
20
0.50 0.75 1.0 1.25 1.75
Crustal Heat Production Rate (µW/m^3)
Central Rupture StyleNarrow<--------------------------------------------------------Wide
Total Stretching: 260 km
Total Stretching: 255 km
Total Stretching: 240 km
Total Stretching: 225 km
Total Stretching: 228 km Total Stretching: 242 km
Total Stretching: 255 km
Total Stretching: 260 km
Total Stretching: 270 km
Total Stretching: 280 km Total Stretching: 340 km
Total Stretching: 360 km
Total Stretching: 340 km
Total Stretching: 365 km
Total Stretching: 202 km *(not to rupture)
Total Stretching: 240 km *(not to rupture)
Total Stretching: 540 km
Total Stretching: 450 km
Total Stretching: 420 kmTotal Stretching: 360 km
*(not to rupture)
Total Stretching: 295 km*(not to rupture)
Total Stretching: 242 km*(not to rupture)
Heat vs Stretching Rate
MarginalRupture Style
Total Stretching: 192 km *(not to rupture)
Total Stretching: 315 km*(not to rupture)
Total Stretching: 350 km*(not to rupture)
Integrated strengh:t=0 my: 2.0 TN/mt=5 my: t=10 my: 1.4 TN/mt=15 myt=20 my: 2.3 TN/m
Extending Cooling Strengthening
t=20 my
Integrated strengh:t=0 my: 2.0 TN/mt=5 my: t=10 my: 1.4 TN/mt=15 myt=20 my: 2.3 TN/m
Extending Cooling Strengthening
t=20 my
Temperature at top of the mantle: t=0 my: 450°C t=5 my: 460°C t=10 my: 470°C t=13 my: 465°C
Extending Thinning of Lithosphere Warming of Upper Mantle
Temperature (°C)0 400 800 1200
)mk(
htpe
D
0
50
100
150
t=0 my
0 400 800 12000
50
100
β=1.4t=5 my
0 400 800 12000
50
β=2.8t=10 my
0 400 800 12000
50β= 5.8t=13 my
Low heat production:extension=>thinning of lithospherethinning=> warming top of mantlewarming=> weakening of lithosphere
Weakening => narrow rift mode (e.g.Buck, 1991)
Thinning + Warming= Weakening
Integrated strengh: t=0 my: 14.9 TN/m t=5 my: 9.5 TN/m t=10 my: 2.9 TN/m t=13 my: 1.1 TN/m
t=0 my
β=1.4t=5 my
β=2.8t=10 my
β= 5.8t=13 my
Low Heat Production:Strength and ThermalEvolution
Integrated strengh:t=0 my: 2.0 TN/mt=5 my: t=10 my: 1.4 TN/mt=15 myt=20 my: 2.3 TN/m
Extending Cooling Strengthening
t=20 my
Integrated strengh:t=0 my: 2.0 TN/mt=5 my: t=10 my: 1.4 TN/mt=15 myt=20 my: 2.3 TN/m
Extending Coolinening
t=20 my
Temperature at top of the mantle: t=0 my: 815°C t=5 my: 800°C t=10 my: 750°C t=15 my 680°C t=20 my: 635°C
Extending Thinning of Crust Cooling of Upper Mantle
t=0 my
Temperature (°C)0 400 800 1200
)mk(
htpe
D
0
50
100
150
0 400 800 12000
50
100 0 400 800 12000
50
0 400 800 12000
50
0 400 800 12000
50
β= 1.6t=5 my
β=2.1t=10 my
β=2.4t=15 my
β= 2.5t=20 my
Integrated strengh: t=0 my: 2.0 TN/m t=20 my: 2.3 TN/m
Extending Cooling
t=0 my
With Heat Production:Strength and ThermalEvolution
Strengthening
With heat production:extension=> thinning
thinning of crust=> cooling top of mantlecooling=strengthening of lithosphere
Strengthening => wide rift mode (e.g.Buck, 1991)
Central RuptureIncreasing Crustal Heat Production => Wider Rift
Marginal RuptureHeat conducting in from
un-extended region
With low Nusselt # (advection/conduction):Strength and Thermal Evolution
thinning of crust=> cooling top of mantle
Extending and thinning region:With higher heat production:extension=> thinning
cooling=strengthening of lithosphere
Marginal region:Adjacent crust does not thin, remains warmHeat conducted to marginal, thinned, regionMarginal areas are thin and warm=> weakest region=> extension accomodated at margins
Extending Not as much Cooling
Weakening Strengthening
Extending Thinning of Crust
Cooling of Upper Mantle
