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State of Stress in the Conterminous United States University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 1980 State of Stress in the Conterminous United States Mary Lou Zoback U.S. Geological Survey, [email protected] Mark D. Zoback U.S. Geological Survey, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Zoback, Mary Lou and Zoback, Mark D., "State of Stress in the Conterminous United States" (1980). USGS Staff -- Published Research. 453. https://digitalcommons.unl.edu/usgsstaffpub/453 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. JOURNALOF GEOPHYSICALRESEARCH, VOL. 85, NO. BII, PAGES6113-6156, NOVEMBER 10, 1980 State of Stress in the Conterminous United States MARY LOU ZOBACK AND MARK ZOBACK U.S. GeologicalSurvey, Reston, Virginia 22092 Inferringprincipal stress directions from geologicdata, focal mechanisms, and in situstress measure- ments,we haveprepared a map of principalhorizontal stress orientations for the conterminousUnited States.Stress provinces with lineardimensions which range between 100 and 2000km weredefined on thebasis of thedirections and relative magnitude of principalstresses. Within a givenprovince, stress orientationsappear quite uniform (usually within the estimated range of accuracyof thedifferent meth- ods usedto determinestress). Available data on the transitionin stressdirection betweenthe different stressprovinces indicate that these transitions can be abrupt,occurring over <75 km in places.In the westernUnited States, a regionof activetectonism characterized by high levels of seismicityand gener- allyhigh heat flow, the stress pattern is complex,but numerous stress provinces can be well delineated. Despiterelative tectonic quiescence in the eastern and central United States, a major variation in princi- pal stressorientation is apparentbetween the Atlantic Coastand midcontinentareas. Most of the eastern UnitedStates is markedby predominantlycompressional tectonism (combined thrust and strike slip faulting),whereas much of the regionwest of the southernGreat Plains is characterizedby pre- dominantlyextensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreasfault and in partsof theSierra Nevada is nearly pure strike slip. Exceptions to this general pat- terninclude areas of compressionaltectonics in the westernUnited States (the PacificNorthwest, the ColoradoPlateau interior, and the Big Bend segment of theSan Andreas fault) and the normal growth faultingalong the Gulf CoastalPlain. Sources of stressare constrained not only by theorientation and relativemagnitude of the stresseswithin a givenprovince but alsoby the mannerof transitionof the stressfield from one province to another.Much of the modempattern of stressin the westernUnited Statescan be attributedto presenttransform motion and residual thermal and dynamic effects of Ter- tiarysubduction along the western edge of theNorth American plate. Abrupt stress transitions around activelyextending regions in the westernUnited Statesprobably reflect shallow sources of stressand anomalouslythin lithosphere.Large areas characterized by a uniformstress field in the centraland east- ernUnited States suggest broad scale plate tectonic forces. In themidcontinent region, both ridge push andasthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in thecold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute platemotion) correlate better with the data than do the ridge push directions. Asthenospheric counter- flowmodels do not apply in thisregion, as predicted stress orientations are about 90 ø off.A regionof compressionoriented approximately perpendicular to the continental margin and Appalachian fold belt is definedby thestress data along the Atlantic Coast. This NW-SE compression is in directcontrast with previousmodels predicting extension perpendicular topassive continental margins due to lateraldensity contrastsat the continental-oceanic crust interface. Ridge push forces, while capable of producinga com- ponentof compressionacross the coastalarea, do not explainthe observedorientation of the stress.Two speculativemechanisms are suggestedto explain the observedorientations: (1) rotationof stress(or strain)due to anisotropicAppalachian basement structure and (2) flexuraleffects associated with erosion and isostaticrebound of the Appalachians. INTRODUCTION Data are presentedin terms of the orientation and relative Detailedknowledge of the patternof intraplatestress pro- magnitude of maximum and minimum horizontal stressesand videsimportant constraints on modelsof globaltectonic proc- a verticalstress. We haveassumed that oneprincipal stress is essesand the mechanismof plate motions.On a finer scale, vertical, and the horizontalcomponents of the in situ stress boththe patternof stressand variations in the patternmust be fieldrepresent principal stresses. Evidence supporting this as- known to understand intraplate volcanism and tectonism sumptionincludes observations of the nearlyvertical attitude properly.In addition,in regionsof relativetectonic quies- of dikesexposed over significant depth intervals at a givenlo- cence,knowledge of the in situstress field is requiredfor de- cality[R. B. Johnson,1961] as well asthe absenceof exposed lineationof potentialseismic hazards associated with pre- dikesthat appearnot to havebeen eraplaced vertically (D. existing zones of weaknessin the crust. Pollard,oral Communication,1980). In addition,measure- This studyrepresents an attemptto map the modernstress mentsof the completestress tensor in deepmines [McGarr field (_primarilyQuaternary in the westernUnited Statesand and Gay, 1978] and the near-horizontal orientations of the Tertiary and youngerin the East)in the conterminousUnited greatmajority of stressaxes inferred from earthquakefocal States. Principal stress orientations have been determined mechanismsalso indicate that regionally the principal stresses from geologicobservations, earthquake focal mechanisms, are horizontal and vertical. and in situ stressmeasurements. An attempthas also been The majorcontribution of the presentstudy is the inclusion madeto categorizebroad regions not only by the orientations of new geologicdata on the orientationof the stressfield, of the principalstresses but alsoby their relativemagnitudes, largelyin the westernUnited States.In addition,we have at- as inferredfrom currentlyactive tectonism. temptedto go backto the originalreferences and checkthe re- liability of previouslycompiled data, particularlyfor focal Thispaper is not subjectto U.S. copyright.Published in 1980by mechanisms.We haveexcluded poorly constrained points and the AmericanGeophysical Union. haverelied, whenever possible, on averagesof a numberof so- Paper number 80B 1093. 6113 6114 ZOBACKAND ZOBACK.'STATE OF STRESSIN CONTERMINOUSUNITED STATES lutions in a given locality. The quality criteria used are de- break can be identified, the maximum vertical and strike slip tailed below. offsets(along the same segmentof the break) can be used to determinethe net horizontalcomponents of slip. PRINCIPAL STRESS ORIENTATION INDICATORS Large-scalegrooves and parallel slickensideson an exposed Some discussionis warranted of the different techniques fault surface indicate the relative direction of motion of two used for determining principal stressorientations, especially crustal blocks. Unfortunately, well-preservedfault scarpsin the major assumptionsassociated with eachtechnique as well competentrocks are rarely exposed.In the Basin and Range as the inherent difficultiesand uncertainties.Incorporation of province, a region of abundant Quaternary faulting, most the geologicstress indicators is our main contributionto pre- young scarps occur in alluvium, basinward of the main vious compilationsof stressdata. ranges.However, surfaceexposures of fault scarpsare known and are sometimesexposed several kilometers (occasionally GeologicData tens of kilometers)along strike.Measurement of groovesand Geologicinformation on principal stressorientations can be slickensidesat a numberof sitesalong a fault providesa mean divided into two main categories:observations of fault slip horizontal direction of slip for a major crustal block. Con- and the alinements of young (<5 m.y.B.P., predominantly sistencyof the horizontalcomponent of slip despitechanges in Quaternary)volcanic feeders. For the westernUnited States, strike of as much as 90ø [M. L. Zoback, 1978; Pavlis and many of thesedata came from M. L. Zoback [1979], Thomp- Smith, 1980]indicates that thesegrooves and slickensidesac- son and Zoback [1979] and M. L. Zoback and M.D. Zoback curately record the major block motion. One sourceof error [1980]. In the east,geologic data on young faulting compiled in this kind of studyis the possibilityof a local downdipcom- by Prowell [1980] was especiallyuseful. ponent of motion of gravitationallyunstable blocks that slide Detailed information on fault slip can be usedto determine down into the valley. This sourceof error could result in mea- the net horizontal componentof motion on oblique slip nor- sured slip directionsthat are spuriouslyrotated toward the mal
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