Structural Geology Stress and Strain

1. Structural GeologyStress and Strain • Structural Geology – the study of crustal deformation and basin/mountain development • Stress – force applied to a rock • Strain – change in shape and/or volume induced by stress (deformation)

2. Types of Stress • Compression – convergent plate boundaries • Crumpled, thickening vertically and shortening laterally • Creates folds, reverse and thrust faults • Himalayas, NW coast of N.A., Appalachians, western coast of S.A. • Tension – divergent plate boundaries • Extends crust, thins vertically and lengthens laterally • Creates basins, normal faults, grabens • Mid-Atlantic Ridge, East Pacific Rise (Gulf of California), Red Sea Rift • Shear – opposing forces along a plane • Forms parallel blocks, pull-apart basins, transform faults, folds and rotational structures • Gulf of California, San Andreas fault system, East Anatolian fault system, Enriquillo-Plantain Garden fault system (Caribbean and North America plates – Haiti)

3. Types of Deformation • Elastic – returns to original state • Temporary, not permanent • Yield point – point of deformation beyond which change is permanent • Plastic – irreversible change in shape or volume that occurs without the rock breaking • Usually under conditions of high temp and press • Usually a slow process giving atoms time to shift in response to force applied • Brittle – irreversible change that penetrates mineral bonding • Usually under conditions of low temp and press • Force applied suddenly not allowing atoms time to shift or move in response to force

4. Deformation Factors • Heat – Allows “fluid” behavior, > heat = > plastic deformation • Time – Great amount of time allows plastic deformation to occur if force is applied continually. Little time doesn’t allow atoms to adjust so brittle failure is the result • Composition – hard vs soft minerals and rocks (e.g. qtz vs micas or granite vs shale)

5. Folds • Monoclines – a single draped fold bending in only one direction (Fig. 10.18) • Synclines – Concave up, trough-like, youngest rocks at center (Fig. 10.9) • Anticlines – Convex up, arch-like, oldest rocks at center (Fig. 10.9) • Axial plane – divides fold into two “equal” parts along it’s crest or line of maximum curvature • Limbs – the sides of the fold • Symmetrical – near vertical axis, limbs are equal • Asymmetrical – rotated axial plane, limbs are not equal

6. Monoclinewith associated fault • Monoclinal folds form when deep-seated faults occur at depth and are not propagated through • overlying sedimentary rocks to the surface • More pliant sedimentary rocks are able to accommodate strain by folding instead of fracturing in a brittle manner

7. MonoclineA classic monocline near Mexican Hat, Utah is Comb Ridge. Mesozoic strata are bowed down along the fold. they are horizontal on the plateau at left and in the foreground but dip 45 degrees or more along the fold.

8. Monocline Comb Ridge, AZ-UT • Flat-irons & hog-backs • Tensional fracturing causes preferential erosion along fold axis • Differential erosion of units

9. MonoclineWaterpocket Foldsouthern Utah

10. Echo Cliffs MonoclineHwy. 89 near turnoff to Tuba City

11. SynclineEllesmere Island, Canadian Arctic (below left), Barstow syncline, a fold in Miocene shales and sandstones, Rainbow Basin, Mojave Desert, California (below top) Saint-Godard-de-Lejeune, Canada (below right)

12. AnticlineAnticlinal folds near : Calico ghost town Mojave Desert, CA (below top), Zagros Mountains, Iran/Iraq, Northcott Mouth, Cornwall (below left)

13. Anticlinal FoldBradshaw Mtns. west of I-17Granitic intrusion to right deformed overlying sedimentary and metamorphic rocks on left to form large anticlinal structure (only western limb of fold is visible in this image)

14. Folded Mtn. Belts

15. Appalachians Mountains • Complex folds broken by reverse faulting • Low-High grade metamorphic rx • Convergent plate boundaries create orogenic events • Taconic ~440mya lasting for >250mys

16. Himalayan Mountains • Suture Zone • Faulted, folded mtn. belt • Crustal thickening • Cont-cont collision closes Tethys Sea • Young mtn range • 40-50mya

17. Domes and Basins • Form in mid-continent/mid-plate locations • Result from vertical rather than lateral forces • Domes form when low-density or heated materials rise • Salt domes • Magma intrusions • Basins form when influx of material is enough weight to force subsidence

18. Type of Faults • Normal – dip-slip, more vertical than horizontal, hanging wall moves down relative to footwall (tensional) (Fig. 10.21) • Reverse – dip-slip, hanging wall moves upward relative to footwall (compressional) (Fig. 10.23) • Strike-slip – Horizontal slip of adjacent blocks, right and left (Fig. 10.27) • Thrust – special type of reverse, block moves at low angle (~45 degrees) (compressional)

19. FaultsHanging-wall/Foot-wall

20. Normal FaultA small normal fault cutting carbonaceous silt and mudstones

21. Reverse Faults Reverse faults in Entrada Fm., Grand Staircase-Escalante Nat. Mon., Utah (Note hanging walls move upward relative to foot walls)

22. Strike-slip fault Right-lateral SS along the Las Vegas Shear Zone, Nevada Part of a large-scale displacement feature trending NW-SE

23. Transform Fault • San Andreas fault and Pt. Reyes Peninsula, California • The San Andreas trends northwestward up the narrow Tomales Bay • Elongated transform fault between spreading centers in the Gulf of California (East Pacific Rise) and of the coast of NW U.S. (Juan De Fuca Ridge

24. Fault Scarps

25. Strike and Dip • Strike -Horizontal direction or orientation of a fault plane or dipping bed (azimuth) • Dip – the vertical plunge or angle of a fault plane or bed measured perpendicular to the strike

26. Strike & DipWhen a bed is not horizontal, it has dip and strike. Strike of a bed is the direction of the line of intersection of the bed with an imaginary horizontal plane. Dip of a bed is the angle a bed makes with a horizontal line in a vertical plane. The dip has two attributes - amount and direction.