seismotectonics of intraplate thermal model strength ...thorne/eart118/... · shown by large...

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