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