SUPERSONIC AND·HYPERSONIC SPEEDS AND AERIAL PHOTOGRAPHY 435

of attack, much greater Illminosity can occur.One assumption made throughout the

study is that the air is in thermodynamicequilibrium. It is distinctly possible that non­equilibrium thermodynamic effects can besignificant. This point requires further in­\"estiga tion.

REFEREi\CES

.\1ielsen, J. N., Goodwin, F. K.; Sacks, A. 1-1.,Rubesin, M. \V., Ragent, B., and Noble, C. E.:

"Effects of Supersonic and Hypersonic AircraftSpeed Upon Aerial Photography~Final Tech­nical Report, Phase I," Vidya ~eporl ~o. 17,March 1960. .

~ielsen, J. \"., Goodwin, F. K.; Sacks, A. H.;Rubesin, M. \\'., and Burnell, J. A.: "Effects ofSupersonic and Hypersonic Aircraft Speed UponAerial Photography~Final Report, Phase II,"Vidya Report 1'\0, 37, Jan. 1961.

Sine, Howard A. and \Vinovich, \\'arren: "LightDiffusion Through High-Speed TurbulentBoundary Layers," NACA RM A56B21, May25, 1956.

Geological Significance of Fracture Traces*

LAURENCE H. LATTMAN and RICHARD H. lVIATZKE,

Dept. of Geology, The Pennsylvania State Univ.,University Park~ Pa.

ABSTRACT: Fracture traces, as now mapped in a variety of tandscapes, areparallel or su.b-parallel to joints in areas of flat or l'ery gently folded rocks butare not parallel to the dominant joint sets in folded rocks. They apparentlyextend downward 10 depths of at least 3,000 feet at Bisbee, A rizona, where arepods, which are fracture-controlled, are parallel to surface fracture traces. A.probable wrench fault in Alaska separates two areas of difFerent fracture-traceorientations, but fracture-trace orientations are identical on both sides of athrust fault in central Pennsylvania. Extrusive (md intrusive igneous rocks inthe same area of A laska show di.tFereut fracture-trace orientation.

INTRODUCTION

F RACTURE traces (also known as micro­fractures and lineal'S) have been defined

(Lattman, 1958) as natural linear featuresthat have less than one mile of continuousexpression, as viewed on aerial photographs.These features are rarely visible except onaerial photographs, and hence are of particularinterest to the photogeologist. Recent in­vestigations have revealed systematic rela­tionships among fracture traces, joints, faults,folds and rock types. It is the purpose of thispaper to collate this scattered information,much of it as yet unpublished, so that photo­geologists may be made aware of currentprogress in understanding the geologic signifi­cance of fracture traces.

The authors are grateful to Dr. RichardJahns for his thoughtful critique of this paper.

FRACTURE TRACES

ORIGIN

Nearly all workers in this field (Blanchet,1957; Mallard, 1957; Lattman, 1958) haveconsidered that fracture traces are the surfaceexpression of joints or zones of joint concen­tration. No investigator has been able todemonstrate this hypothesis conclusively. Inmost areas a cross-section of a fracture tracecannot be found, owing chiefly to the absenceof bedrock exposures at the critical locality.But in the Powdel" River Basin of vVyoming,a mapped fracture trace passes across avertical sandstone cliff. Here a zone of jointconcentration can be seen to underlie thefracture trace (Figures 1 and 2). Strong cir­cumstantial evidence, described by the in­vestigators named above, supports the con-

* College of Mineral Industries: Contribution No. 60-94.

436 PHOTOGRAMMETRIC ENGINEERING

FIG. 1. Zone of joint concentration nnderlying a fracture trace in the Powder River Basin, \VyomingThe fracture trace extends directly away from the observer at the top center of the scarp. There is nooffset in the beds beneath the fracture trace, but a slight topographic sag which forms the surface frac­ture trace may be seen. Notice that the joints underlying the fracture trace are more closely spaced thanthose on either side. The width of cliff exposed in this view is about 50 feet.

cept that concentrations of joints are respon­sible for the occurrence of fracture traces.

RELATIONSHIP TO JOINTS

Photogeologic mapping of fracture traceshas been combined with ground mapping ofjoints in several recent studies. Lattman andNickelsen (1958) found a marked parallelism

between dominant fracture-trace and jointsets in flat to gently folded Carboniferousrocks of the Allegheny Plateau in centralPennsylvania. This relationship was con­firmed in a later study by Hough (1959).Subsequently Keim (1961), working in thefolded and faulted terrane around BisbeeArizona. found that the dominant fracture~

FIG. 2. Stereo pair showing fracture trace seen in cross section in Figure 1. The fracture trace is ex­pressed by drainage alignments in the western Powder River Basin, \Vyoming. The fracture trace isindicated by the two arrows.

GEOLOGICAL SIGNIFICANCE OF FRACTURE TRACES 437

trace sets and joi n t sets arc not parallel.Similarly ~Tatzkc, (1961), in a study of rela­tionships in the Folcled Appalachians nearState Colle~e, Pennsyh'ania, found that thedominant fracture-trace and joint sets differin trend by as much as 4S degrees.

On the basis of these limited investigations,it appears that fracture traces and joints areparallel to sub-parallel in areas underlain byHat to gently folded (dips less than 5 degrees)rocks, bu t tha t they are not parallel in areasof strongly folded rocks.

RELATIONSHIP TO LITHOLOGY

In a study of a published map of one of theAleutian Islands of Alaska, Lattman andSegovia (1961) found that fracture-traceorientations differ according to whether thearea is underlain by extrusive or intrusiveigneous rocks. This difference may be due tocontrasting responses of these t,,·o rock typesto the same deforming force or to contrastingstructural histories of the rock bodies. On theotl~er hand, Matzke (1961) found that frac­ture-trace orientation is constant across se\'­eral formations of limestone and dolomite incentral Pennsylvania.

These results appear to indicate that in asingle area, orientation of fracture traces can\'ary significantly according to major changesin rock type, but that minor lithologic differ­ences may have no marked effect on orienta­tion.

RELATIO!\'SHIP TO GEOLOGIC STRUCTURE

Little detailed work has been done on therelationship of fracture traces to folding.Matzke (1961) found that the regional pat­tern of fracture-trace orientation is unaffectedby strongly compressed anticlines and syn­clines in the Folded Appalachians, wherejoint sets are parallel and perpendicular tothe strike of bedding. He also noted that,whereas the fracture traces are unrelated tolocal structures, they lie at a constan tangleof S2 degrees with respect to the regionalstructural trend. I n the area of study thistrend shifts from NSQoE to N62°E, and thepredominant fracture-trace direction shift,,;accordingly to maintain the angular relation­ship.

Keim (1961) and Lattman and Sego\'ia(1961) found that high-angle normal andlateral faults commonly separate areas ofdifferent fracture-trace orientation. Matzke(1961) noted that the Birmingham thrust, alow-angle fault in central Pennsylvania, hasno effect on fracture-trace orientation.

These results indicate that high-angle

faults may separate crustal blocks ".j thi n eachof "'hidl fracture-tra,e orientations are fairlyconstant.; ('a,h block, howe\'er, can be c1istin­guished hy a different orientation pattern.Low-angle faults may have no effect on frac­ture-trace patterns.

SUMMARY

The study of fracture traces is only be~in­

ning, and meaningful data are sparse and\"idely scattered. Later work may well con­travene some of the generalizations suggestedherein; the results summarized belo\\' are verytentative;

1. Fracture traces, which probably repre­sent zones of joint (and small fault) con­centration, are parallel to the trends ofmajor joint sets in areas of flat to gentlydipping rocks but are not parallel to thetrends of joint sets in areas of steep(greater than about 5 degrees) dip.

2. \Yithin a small area fracture trace ori­en ta tions are not the same on rock typethat are markedly different. :\To signifi­cant differences in orientation are foundon similar lithologic types \\'ithin a smallarea.

3. Steeply dipping faults may bound areasof diHeren t fracture- trace orientations.The orientations are, however, rela­tively constant within blocks boundedby such faults. One low-angle faultstudied did not separate areas of differ­en t fracture-trace orien tation.

4. Tn folded rocks the fracture trace ori­entations are not afi'ected by local foldsbut do maintain a constant angularrelation,,;hip to the regional structuraltrend.

CONCLUSIOto'S

From their constant angular relationshipto regional structural trend and apparent in­difference to local folds it is postulated that·fracture traces are related to regional tec­tonics rather than local structural deforma­tion.

Blocks separated by steeply dipping faultsmay each undergo a distinct tectonic historywhich gives rise to fracture trace directionswhich are constant in each block but perhapsdifferent from block to block.

To explain parallelism and non-parallelismof fracture traces and joints, the followingworking hypothesis is advanced. In areas oflittle or no structural deformation most of thejoints are parallel to fracture traces becausethey are the result of the same regional forces

438 PHOTOGRAMMETRIC ENGINEERING

which produced the zones of joint concentra­tion which give rise to fracture traces. Tnstrongly folded rocks the local structures giverise to joints not related to the regional pat­tern. Plots of orientation rosettes of joints insuch areas therefore show a non-parallelismbetween preferred directions of fracture traceand joint concentration.

By mapping fracture traces, which arevisible only on aerial photographs, the photo­geologist is in a position to make a uniquecontribution to our knowledge of the fracturepattern of the earth's surface. The greatestneed at the present is for detailed informationon the origin of fracture traces and possiblesignificance of different types of fracturetraces. The major effort now being expendedon these features is in careful mapping andsupporting field work. From the progressalready made, it appears that photogeologicfracture trace mapping is a promising part ofphoto interpretation.

REFERENCES CITED

Blanchet, P. H., 1957, "Development of Fracture

Analysis as Exploration Method," But!. Amer.Assoc. Petrol. Geol., Vol. 41, No.8, pp. 1748-1759. .

Hough, V. N. D., 1959, "Joint Orientations of theAppalachian Plateau in Southwestern Pennsyl­vania," Unpublished M.S. Thesis, The Penn­sylvania State University.

Keim, James, .1961, "Fracture Trace and JointPatterns at Bisbee, Arizona," UnpublishedM.S. Thesis, The Pennsylvania State Univer­sity.

Lattman, L. H., 1958, "Technique of MappingGeologic Fracture Traces and Lineaments onAerial Photographs," PHOTOGRAMMETRIC ENGI­NEERING, Vol. 24, pp. 568-576.

---, and Nickelsen, R. P., 1958, "PhotogeologicFracture Trace Mapping in AppalachianPlateau," Bull. Amer.- Assoc. Petrol. Ceol., Vol.42, No.9, pp. 2238-2245.

---, and Segovia, A. V., 1961, "Analysis ofFracture Trace Pattern of Adak and KagalaskaIsland, Alaska," Bull. Amer. Assoc. Petrol.Ceol., Vol. 45, No.2, pp. 249-251.

Matzke, R. H., 1961, "Fracture Trace and JointPatterns of Western Centre County, Pennsyl­vania," Unpublished M.S. Thesis, The Penn­sylvania State University.

Mollard, J. D., 1957, "A Study of Aerial Mosaicsin Southern Saskatchewan and Manitoba," Oilin Canada, Winnipeg, issue of Auglls~ 5, 1957.

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