geological significance of fracture traces
TRANSCRIPT
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 It is the purpose of thispaper to collate this scattered information,much of it as yet unpublished, so that photogeologists may be made aware of currentprogress in understanding the geologic significance of fracture traces.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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