Strain localization processes at a magma-starved ridge ... · Strain localization processes at a...

Strainlocalizationprocessesatamagma-starvedridge:frommicro-scaletomacro-scaleBickert1,M.,Cannat1,M.,Tommasi2,A.,Jammes3,S.,Lavier4,L.

[modifiedfromCannatetal.,2008]

?

Detachment faults exhume mantle-derived rocks fromthebaseonthebrittlelithospheretotheseafloor.

§  What are the strain localizationmechanismsinthedeepaxiallithospherewhenthereisnomagma?

§  How to make detachments in a thicklithosphere(≥15km)?

1

2[Bickertetal.,tobesubmitted]

TheEasternSouthWestIndianRidge(SWIR)

40% of the samples are stronglydeformed,withplanarfine-grainedzones.

385samplesofultramaficrocks

99partiallyserpentinizedsamples

4059 Weakly

deformedStronglydeformed

Partiallyserpentinizedsamples(99)

Dredges realized on and off-axis recovered variablyserpentinizedperidotites,withminoramountsofgabbros(<4%)andbasalt(16%).

B C

ExtractingP-Tconditionsfromgrainsizereductionzones[Bickerteta

l.,to

besubm

itted]

Heterogeneousdeformationcombiningbrittleandductilemechanisms(Fig.A):

§  Pressuresinthespinelfieldi.e.atdepths>20km(Fig.B)§  Hightemperature(800-1000°C,Fig.B)fromorthopyroxenethermometry§  Highdeviatoricstresses(80-270MPa,Fig.C)fromolivinegrainsize

WeproposethatthisdeformationcharacterizestherootofSWIRaxialdetachments.A

3

[Sauter et al., 2013]

Isitpossibletoobtainflipflopdetachmentfaultswithsimilarlife-spamsasthoseobservedinnaturebyconsidering:§  Athickbrittlelithosphere(~20km)

§  Twostrainsofteningmechanismsobservedinthesamples:ü  Serpentinization(T≤350°C)ü  Grainsizereduction(800-1000°C)

Fromthemicroscaletothemacroscale:Platescaledeformationmodels

4059 Weakly

deformedStronglydeformed

4

T ~350°C

[Bickerteta

l.,to

besubm

itted]

5

Serpentinizationalone… doesnotproduceflipflopdetachments.

T≤350°C

εII.σII=108J

1) When conditions are appropriate:

2) Decrease of cohesion and friction:

3) Decrease of activation energy:

[Bickert et al., 2020]

6

Grainsizereductionalone…doesproduceflipflopdetachments.

T=800–1000°C

1) When conditions are appropriate:

2) Set up of new grain size:

3) New viscosity using diffusion creep law:

[VanderWaletal.,1993]

[Bickert et al., 2020]

Modelswithserpentinization+grainsizereduction…

Ø  … develop four faulting modes,including2detachmentmodes.

Ø  Serpentinization alone does not

produceflipflopdetachments.Ø  Grain size reduction makes

flipflopdetachmentspossible inathicklithosphere.

Ø  By combining the two, theresulting fault patterns approachthe flipflop detachment faultingpatternattheEasternSWIR.

7 [Bickert et al., 2020]

8

Bickert1,M.,Cannat1,M.,Tommasi2,A.,Jammes3,S.,Lavier4,L.

Questions?Comments?Feelfreetocontacttheauthors:

1manon.bickert@gmail.com/cannat@ipgp.frMarineGeosciences,InstitutdePhysiqueduGlobedeParis,UMR7154-CNRS,UniversitédeParis,France2andrea.tommasi@umontpellier.frGéosciencesMontpellier–CNRS&UniversitédeMontpellier,France3suzon.jammes@txstate.eduDepartmentofGeography,TexasStateUniversity,SanMarcos,Texas78666,USA4luc@jsg.utexas.eduDepartmentofGeologicalSciences,InstituteforGeophysics,JacksonSchoolofGeosciences,TheUniversityofTexasatAustin,UnitedStates