seismotectonics of intraplate thermal model strength ...thorne/eart118/... · shown by large...
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Seismotectonics of intraplateoceanic regions
Thermal modelStrength envelopesPlate forcesSeismicity distributions
Cooling of oceanic lithosphere alsoincreases rock strength and
seismic velocity. Thus
elastic thickness of the lithosphereinferred from the deflection caused
by loads such as seamounts ,
maximum depth of intraplateearthquakes within the oceanic
lithosphere ,
& depth to the low velocity zonedetermined from surface wave
dispersion
all increase with age.
Stein and Stein, 1992
STRENGTH OF THE OCEANIC LITHOSPHERE
The strength of the lithosphere as a function of depth depends upon thedeformation mechanism.
At shallow depths rocks fail either by brittle fracture or frictional sliding onpreexisting faults. Both processes depend in a similar way on the normal stress,
with rock strength increasing with depth.
At greater depths the ductile flow strength of rocks is less than the brittle orfrictional strength, so the strength is given by the flow laws and decreases with
depth as the temperatures increase.
This temperature-dependent strength is the reason that thecold lithosphere forms the planet's strong outer layer.
To calculate the strength, a strain rate and a geothermal gradient givingtemperature as a function of depth are assumed
Strength increases with depth in the brittle region due to the increasing normalstress, and then decreases with depth in the ductile region due to increasing
temperature. Hence strength is highest at the brittle-ductile transition. Strengthdecreases rapidly below this transition, so the lithosphere should have little strength
at depths > ~25 km in the continents and 50 km in the oceans.
Strengthenvelope vsdepthdepends onmaterial, porepressure,geotherm,strain rate
Olivine isstronger inductile flowthan quartz,so oceaniclithosphere isstronger thancontinental
Brace & Kohlstedt, 1980
BRITTLE
DUCTILE
STRENGTH VERSUS AGE INCOOLING OCEANIC
LITHOSPHERE
As oceanic lithosphere ages andcools, the predicted strong regiondeepens. This seems plausible
since earthquake depths, seismicvelocities, and effective elastic
thicknesses imply the strong upperpart of the lithosphere thickens with
age
Wiens and Stein, 1983
STRENGTH VERSUS AGEIN COOLING OCEANIC
LITHOSPHERE
Strength envelopes areconsistent with theobservation that the maximumdepth of earthquakes inoceanic lithosphere isapproximately bounded by the650°C isotherm.
This makes sense, becausefor a given strain rate andrheology the exponentialdependence on temperaturewould make a limiting strengthfor seismicity approximate alimiting temperature.
Wiens and Stein, 1985
“RIDGE PUSH” - PLATE DRIVING FORCE DUE TO COOLING LITHOSPHERE
INTRAPLATESTRESS DUE TO
BALANCEBETWEEN:ridge push
drag at plate basestrength of ridge
FOCAL MECHANISM TYPE AS A FUNCTION OF LITHOSPHERIC AGE FOROCEANIC INTRAPLATE EARTHQUAKES
Older lithosphereis in compression
Youngerlithosphere hasboth extensional& compressionalmechanisms
Constrainsintraplate stressand plate drivingforces
Wiens and Stein,1984
INTRAPLATESTRESS DUE TO
BALANCEBETWEEN:ridge push
drag at plate basestrength of ridge
For 0 drag, ridgepush gives
compression at allages
If drag too high,get extension inold lithosphere,
which is notobserved
Wiens and Stein,1985
Age of transitionfrom ridge-normalextension tocompressionincreases withstrength of theridge
Wiens and Stein,1985
BASAL DRAG VALUE CONSTRAINSMANTLE VISCOSITYViscosity, the proportionality
constant between shear stressand the strain rate(or velocity gradient), controlshow mantle flows in responseto applied stress, and is thuscrucial for mantle convection
If drag on base of a plate due tomotion over the viscousmantle, compressiveearthquake mechanisms in oldlithosphere constrain viscosity
Data require low viscosity layerdecoupling plates from rest ofasthenosphere
Consistent with constraints fromgravity and glacial isostasy
McKenzie and Richter, 1978
Deformation in Central Indian Oceanshown by large earthquakes andwidespread basement folding inseismic reflection and gravity data
First attributed to intraplatedeformation of a single rigid Indo-Australian plate
Later model has distinct Indian andAustralian plates separated by diffuseplate boundary zone perhaps formedin response to Himalayan uplift.
Two-plate model fits focalmechanisms and magneticanomalies. Improved fit is statisticallysignificant, showing that two platescan be resolved.
Subsequent studies refined modeland show that India and Australiahave been distinct for at least 3 Myrand likely longer.
DISTINCT INDIAN AND AUSTRALIAN PLATES
Wiens et al., 1985
Intraplate stresspredicted by aplate driving forcemodel for theIndo-Australianplate
Location andorientation of thehighest stresses,such as thetransition betweencompressionand tension,are generallyconsistent withearthquakemechanisms in theregion nowregarded as adiffuse plateboundary
Cloetingh and Wortel, 1985
Supplemental Material on Ductile Flow
When rocks behave brittlely their behavior is not time-dependent; they either strain elastically or fail
In contrast, ductile rock deforms over time
A common model forthe time-dependentbehavior is a Maxwellviscoelastic material,which behaves like anelastic solid on shorttime scales and like aviscous fluid on longtime scales.
This model can describethe mantle, becauseseismic wavespropagate as though themantle were solid,whereas postglacialrebound and mantleconvection occur asthough the mantle werefluid.
Stress proportional tostrain
Stress proportional tostrain rate
MAXWELL VISCOELASTIC SUBSTANCE
Consider elastic materialas a spring, which exerts aforce proportional todistance. Thus stress andstrain are proportional atany instant, and there areno time-dependenteffects.
In contrast, a viscousmaterial is thought of as adashpot, a fluid damperthat exerts a forceproportional to velocity.Hence stress and strainrate are proportional, andthe material's responsevaries with time.
These effects are combined in a viscoelasticmaterial, which can be thought of as a spring
and dashpot in series
STRESS RESPONSETO STRAIN APPLIED
AT T=0 THATREMAINS
CONSTANT
Stress relaxes frominitial value
MAXWELL VISCOELASTIC SUBSTANCE
MAXWELL VISCOELASTIC SUBSTANCE
Stress decay
Response to a 150-km wide sediment load at apassive margin
If we model the mantle as viscoelastic,then a load applied on the surface hasan effect that varies with time.
Initially, the earth responds elastically,causing large flexural bending stresses.With time, the mantle flows, so thedeflection beneath the load deepensand stresses relax. In the time limit,stress goes to zero and the deflectionapproaches the isostatic solutionbecause isostasy amounts to assumingthe lithosphere has no strength.
Stress relaxation may explain whylarge earthquakes are rare atcontinental margins, except whereglacial loads have been recentlyremoved Although the large sedimentloads should produce stresses muchgreater than other sources of intraplatestress including the less dense iceloads, the stresses produced bysediment loading early in the margin'shistory may have relaxed.
Stein et al., 1989
Viscosity decrease with temperature is assumed to give rise to stronglithosphere overlying weaker asthenosphere, and the restriction of
earthquakes to shallow depths