Stress amp Strain Stress a force applied to an area Example tire pressure in psi Strain a change in original shape or volume produced by stress Elastic strain analogous to a steel spring or rubber band ID: 1021471
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1. GY111 Physical GeologyDeformation of the Earth’s Crust
2. Stress & StrainStress: a force applied to an area.Example: tire pressure in psi.Strain: a change in original shape or volume (produced by stress).Elastic strain: analogous to a steel spring or rubber band.Plastic strain: analogous to deforming mud or putty.
3. Types of StressLithostatic Stress: stress due to the burial and overlying overburden of rock.Lithostatic stress can only cause a change in volume referred to as dilation.Directed stress: stress is unequal in different directions.Directed stress is generated by plate tectonic motion and will cause a change in shape referred to as distortion.
4. Stress vs. Strain DiagramsIllustrate the mechanical behavior of rock materialsBrittle: rocks near the surface of the Earth behave as brittle materials- their behavior is mainly elasticStrain %StressElastic LimitDistortion below E.L. is 100%recoverableRuptureBrittle Deformation
5. Ductile DeformationDuctile deformation requires a significant component of plastic mechanical behaviorStrain %StressElastic LimitDistortion below E.L. is 100%recoverableRuptureelasticplasticPermanent strain
6. Mechanical Behavior of RocksNear-surface rocks that are under low T-P conditions behave as brittle material:Fault fracture (slippage).Joint fracture (no slippage).Deep rocks under elevated T-P conditions behave as ductile material:Folding.
7. Examples of Deformation ExperimentsLab equipment can reproduce all geological conditions except geologic timeLow T-P(brittle)UndeformedHigh T-P(ductile)
8. Mapping Geological StructuresOrientationPlanar: strike azimuth and dip angle with dip quadrant.Linear: trend azimuth and plunge angle.Azimuth: compass direction along the horizontal map surface.0-90: northeast quadrant.90-180: southeast quadrant.180-270: southwest quadrant.270-360: northwest quadrant.Strike is always read from a northern quadrant therefore it must always be 0-90 or 270-360.Dip: maximum angle of inclination in a geological plane (bedding, fault, joint fracture, etc.). The azimuth direction of the dip is always perpendicular to the strike.
9. Geologic Period AbbreviationsQuaternary (Q)Tertiary (T)Cretaceous (K)Jurassic (J)Triassic (Tr)Permian (P)Carboniferous (C)Devonian (D)Silurian (S)Ordovician (O)Cambrian (-C)Precambrian (p-C)These abbreviations are commonly used to indicate ages of beds on geologic maps.In North America the Carboniferous periodIs subdivided into the following 2 periods:Pennsylvanian (|P)Mississippian (M)
10. Examples of Planar StructuresBoth would be measured with a strike and dipBedding PlanesBedding & Fault Planes
11. Strike and Dip (Planar Structures)Strike is the azimuth direction of the horizontal line in a plane.By convention strikes are read from a north quadrant so the legal values are 0-90 or 270-360.Dip is the maximum angle of inclination in a planar structure. This angle will always be measured in a plane perpendicular to strike.The dip angle must be paired with a quadrant direction since there are 2 sides to any strike line.Example: 040 60NW (strike=040, dip angle = 60 in a 310 (NW) direction. Note that 310 is 90 degrees from 040).Maximum possible dip angle is 90. In this case there is no dip quadrant.A horizontal plane has no definable strike and 0 dip angle.
12. 09018027009018027009018027009018027009018027009018027009018027009018027009018027041(A)(B)(C)(D)(E)(F)(G)(H)(I)45386512258052Strike and Dip Symbols000, 52E000, 41W060, 38NW090, 65SN/A, 0315, 90300, 80NE330, 12NE OT030, 25SE
13. Dip Direction RelationshipsThe dip direction of bedding is in a direction toward younger strata- unless the strata is overturned (overturned folds are discussed later).Younger
14. Topography and Dip DirectionN502090“V” in dip direction is less pronounced with larger dip angleA vertical bed shows no “V”
15. Dip Direction SchematicNPTr JKTTr JKTTK50505050YoungerWhen beds are not overturned the dip directions points toward younger beds.
16. Overturned StrataNMDSO-C55-CODSO55555555OlderDip direction points toward older strata when overturned- note the special overturned bedding symbolIn this example the “V” of the contacts indicates the dip direction to the east
17. Trend and Plunge (Linear Structure)Trend: azimuth direction of a linear structure projected up to a horizontal plane.Plunge: incline angle of a linear structure.Note that the trend may have any azimuth value 0-360.Maximum possible plunge is 90.Linear structures with a plunge of 90 have no definable trend direction.
18. 090180270090180270090180270090180270090180270090180270090180270090180270090180270(A)(B)(C)(D)(E)(F)(G)(H)(I)15056540237255Trend and Plunge210, 15330, 05060, 65120, 40030, 00 N/A, 90240, 23300, 72150, 55
19. FaultingFaults are generated in brittle rock layers when the elastic limit is exceeded by deformation forces. Because brittle behavior is confined to the lithosphere faults do not extend into the asthenosphere.
20. Fault ClassificationClassified by the nature of the slippage of one fault block past another block.Dip Slip: slippage is parallel to dip of fault.Normal: hanging wall down motionReverse: hanging wall up motionA special case of reverse where the fault dips < 45 degreesStrike Slip: slippage is parallel to strike of fault.Right lateral: a right-hand turn must be followed to find offset featuresLeft lateral: a left-hand turn must be followed to find offset featuresOblique Slip: has combined strike-slip and dip-slip motion.
21. Hanging Wall and Foot WallTo classify a dip-slip fault you must correctly identify the hanging wall and footwall blocksHanging WallFootwall
22. Dip-Slip Fault Motion ExamplesNote that normal faults accommodate tensional stress, whereas reverse faults accommodate compressional stress.
23. Fault OffsetsSome fault offsets are recognizable on the ground surface.Fault Scarp
24. Strike-Slip Fault Motion ExamplesMovement is parallel to strike of fault therefore offset is seen in a map view
25. Tectonic Associations of Fault TypesDivergent: tension tends to produce normal dip-slip faults.Convergent: compression tends to produce thrust (low-dip angle reverse dip-slip) faults.Transform: shear produces strike-slip faults.
26. FoldingFolding is produced by the compression generated at convergent plate boundaries.Folds require rocks to be under significant T and P so that the layers of rock can bend without breaking (i.e. ductile).
27. Fold GeometryAnticline: concave down (arch)Syncline: concave up (trough)
28. Fold Age RelationshipsAnticlines contain the oldest strata in the center of the structure. Bedding dips away from the center of the structure if the fold is not overturned.Synclines contain the youngest strata in the center of the structure. Bedding dips toward the center of the structure if the fold is not overturned.
29. Fold SymmetryBased on dip of axial plane
30. Plunging FoldAnticline: plunge of axis is in direction of arrow formed by beds on the mapSyncline: plunge of axis is opposed to the arrow formed by beds on the map
31. Surface Geologic MapNJTrPKTrJTKJTrNote the symmetrical patterns:P is symmetrically surrounded by younger bedsT is symmetrically surrounded by older beds.
32. Subsurface InterpretationNJTrPKTrJTKJTrP?TrJK?PTrAnticline axial trace symbolSyncline axial trace symbol
33. Plunging FoldsNQTaKplJoTrgPoKplJoKplTrgTaKplJoTrgQPoPoAnticlines: contacts point in plunge directionSynclines: contacts point opposite the plunge direction
34. Overturned FoldsNTrxP1JoKpkTaQaJoJoKpkTaTrxP1TrxJoKpkJoTrxP1JoKpkJoTaTrxOn the overturned limbs the Strike and dip symbol is overturned
35. Domes & BasinsDomes and Basins have circular contacts.Domes: oldest strata in the center of the structure. Bedding dips away from center of structure.Basins: youngest strata in the center of the structure. Bedding dips toward center of structure.
36. Structural DomeNp-C-CoOsSaDiDiMrMrDiSaOs-Cop-CSaDiOs-CoSa-CoOsp-CSaDiDiDiNote that bedding dips away from the center of the structure in a dome.
37. Dip-Slip FaultNOsp-CoOspSa-Co-CoOspSaSaOspp-CoUDFWHWFault Classification:____________________________Reverse dip-slipNote: slicken-side striations were found to be parallel to the dip of the fault plane.Note that “HW” always on the dip direction tic mark side of fault contact.Note the arrows indicating hanging-wall relative up dip-slip.Note that the upthrown block juxtaposes “old” against “young” strata.
38. Strike Slip FaultN-Ca-CpOxSjDoOx-Cp-CpFault Classification:____________________________Left-lateral strike slip-Ca353535353535DoSjOx-Cp-Ca7070HWFWNote that “HW” always on the dip direction tic mark side of fault contact.Note the arrows indicating left-lateral (sinistral) strike slip.Note: slicken-side striations were found to be horizontal in the fault zone.Sj+-
39. Dip-Slip FaultN-Ca-CpOxSjDoOx-Cp-CpFault Classification:____________________________Reverse dip-slip-Ca353535353535DoSjOx-Cp-CaSjNOTE: slickensides in fault zone were oriented parallel to dip line of fault.7070HFUDNote that “HW” always on the dip direction tic mark side of fault contact.Note the arrows indicating reverse relative dip-slip.Note that the slickensides constrain this fault to a dip-slip motion.Note that dip-slip juxtaposes old against young strata along fault in the up-thrown block.
40. Exam SummaryKnow definitions of stress and strainBe able to define brittle, ductile, elastic, elastic limit, plastic, lithostatic stress, directed stress.Know definitions of strike, dip, trend, and plunge.Know how to recognize anticlines, synclines, domes, and basins.Be able to recognize dip-slip, strike-slip, and oblique-slip faults.Be familiar with tectonic associations of different fault types.