Structural Geology Notes PDF

Topic 4: Structural Geology Topic 5: Structural Geology Earth is a dynamic planet. Tectonic forces rocks to deform produce our planet's spectacular mountain belts.  Rock Deformation Deformation refers to all changes in the original form and/or size of a rock body...

Topic 4: Structural Geology Topic 5: Structural Geology Earth is a dynamic planet. Tectonic forces rocks to deform produce our planet's spectacular mountain belts.  Rock Deformation Deformation refers to all changes in the original form and/or size of a rock body Deformation involves – Stress - force divided by area Types of stress – Compressional stress – shortens rocks – Tensional stress – extends rocks – Shear stress – makes the rocks to slide against each other – Strain – changes in the shape or size of a rock body from stress  Rock Deformation: How rocks deform? Rocks behave as elastic, ductile or brittle materials depending on: amount and rate of stress application, type of rock, temperature and, pressure Elastic deformation – Temporary change in shape or volume from which a material rebounds after the stress is removed (i.e. material return to original shape after stress removal) Rock Deformation: How rocks deform? Ductile deformation Permanent but gradual change in shape or volume of a material, caused by flowing or bending Brittle deformation Permanent change in shape or volume, in which a material breaks or cracks Rock Deformation: How rocks deform? Brittle deformation Permanent change in shape or volume, in which a material breaks or cracks v-22 ERSC  Mapping geologic structures Describing the orientation of a rock layer involves determining the strike and dip Strike The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane Dip (inclination) The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane When preparing a geological map, measurements called strike (trend) ad dip (inclination) are used to describe the orientation or attitude of a rock layer or fault surface. Strike is the compass direction of the line produced by the intersection of an inclined rock layer or fault, with a o16r-Nizovo-2n2 tal h Dip is the angle of inclination of the surface of a rock unit or16fN- aou2v-l2t surface measurEeRdSC2f1r01o, mFal2a022horizontal plane. Dip includes both an angle of inclination (angle of dip sh1o6-Nwovn-22I 30 degrees) … … and a direction toward which the rock is inclined. This di1r6-eNcvot-i2o2 n will always beERaStC2a1019, 0Fald20e22gree angle to the strike. To map a geological structure, geologist measure the st1r6iN- koe2v-a2 nd dip at as manERySCo21u01t, cFrllao20p2s as practical. Strike and dip of a rock layer Folds The two sides of a fold are called limbs. A line drawn along the points of maximum curvature is te1r6m-Noev-d22 the axis of the foERlSdC…2101, Fall2022 … and the axial plane is an imaginary surface that divides a 1f6oN- lod2-va2 s symmetricallyEaRSsC2p10o1,sFsailb20l2e2. The most common types of folds are anticlines… … and synclines.  core Antiform Synform Anticline Youngest Syncline ERSC21 01, Fa l l2 02 O ld e s t Antiform  Anticline Youngest 16O- Nlodve-  Antiform Shape Youngest Anticline Age 16O- Nlodve- Anticlines, which are commonly formed by upfolding or ar1c6-hNoivn-2g2 of rock layers… … are often found in association of toughs or synclines. Depending on their orientation, these basic folds are described as symmetrical when the limbs on either side of the axial plane diverge at the same angle. … and asymmetrical when they do not. An asymmetrical fold is said to be overturned if one limb is1t6-iNlotve-2d2 beyond the vertEiRcSaC2l1.01, Fall2022 If an overturn fold “lies on its side” so that the axial plane is1hN6- oorv-i2z2 ontal, it is calledERaSCr21e01c, uFam20llb2ent fold. Folds do not continue forever, rather, their ends die out much like wrinkles in a table cloth. These ends are said to pl1u6-nNog-ve2 because their axEiRsSCp21e0n1, eFatlr2l a02t2es into the View of plunging folds as they may appear after extensive er1o6- Although most folds are caused by compressional stresses, some are the consequences of vertical movements along faults. Monoclines are such structures. Domes and Basins structures Domes are structures in which the beds Basins are structures in which the beds dip dip away from a central point toward a central point Faults Faults are fractures along which displacement has taken place. Faults can be grouped into the following based on the type of movement: dip-slip, strike-slip, or oblique-slip Before we describe these types, we need first to know some term16sN- aosv-s2o2 ciated with the moEvRSeCm12e10n, tFall2022 To describe the displacement along a dip-slip fault, we use nomenclature that arose from miners who excavated sh1a6-Nftovs-2a2 long fault zonesE.RSC2101, Fall2022 The rock surface above the fault is called the hanging wall …16-(NNovo2- te the lantern is hERuCSn2g101o, nlFlat20h2e hanging wall.) The rock surface above the fault is called the hanging wall …16-(NNovo2- te the lantern is hERuCSn2g101o, nlFlat20h2e hanging wall.) The rock surface above the fault is called the hanging wall …16-(NNovo2- te the lantern is hERuCSn2g101o, nlFlat20h2e hanging wall.) … and the rock surface below is the footwall. (Note the m1i6nN- eovr2- is standing on thEeRSfCo2o01t1w, Falall2l0.2)2 Normal Fault Dip-slip faults are classified as normal faults when the hanging 1 6- wall block moves down relative to the blN oov -22 footwall c k. Dip-slip faults are classified as normal faults when the hanging 1 6- wall block moves down relative to the blN oov -22 footwall c k. Dip-slip faults are classified as normal faults when the hanging 1 6- wall block moves down relative to the blN oov -22 footwall c k. Dip-slip faults are classified as normal faults when the hanging 1 6- wall block moves down relative to the blN oov -22 footwall c k. Dip-slip faults are classified as normal faults when the hanging 1 6- wall block moves down relative to the blN oov -22 footwall c k. Most normal faults are small, having displacements of on16l-yNova-22meter or so. Other examples of fault-block mountains are found in the Ba16s-Nionv-2a2 nd Range ProviEnRcSCe2.101, Fall2022 Normal faults indicate the existence of tensional stresses th1a6-tNoev-n2 d to pull the cruEsRtSaC2p10a1,rFta.2l022 Normal faults indicate the existence of tensional stresses th1a6-tNoev-n2 d to pull the cruEsRtSaC2p10a1,rFta.2l022 Normal faults indicate the existence of tensional stresses th1a6-tNoev-n2 d to pull the cruEsRtSaC2p10a1,rFta.2l022 Normal faulting may also resulted in a central block graben … ca1l6l- … which is bounded by uplifted structures called horsts. The Great Rift of the East Africa is made up of several la1r6g-Neov-g22rabens boundedEbRSyC2t1i0l1t, eFad2l0h22orsts. Reverse fault Reverse faults and thrust faults are dip-slip faults in which the hanging wall block moves up relative to the fo1o6-tNwov-a22ll Reverse faults and thrust faults are dip-slip faults in which the hanging wall block moves up relative to the fo1o6-tNwov-a22ll Reverse faults (dips greater than 45o) and thrust faults (d1i6pN-olve2- s2s than 45o) occurERiSnC21c01o, mFalp20r22essional environments. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. In mountain areas such as Alps, the Northern Rockies, and the Appalachians, thrust faults have displaced strata as16fN- aor2v-a2 s 50 kilometers oERvSCe2r10t1,hFe2lala0d22jacent rock units. The result of this large-scale movement is that older st1r6aN- toa2v-e2 nd up overlyingERySoC2u10n1,gFealr2l0r22ocks. Strike-slip Faults Strike-slip faults exhibit mainly horizontal displacement pa16rN- aolvl2-e2l to the strike uFarl2f0a22 ofEfRaSCu2l1t01s, ce. Strike-slip faults exhibit mainly horizontal displacement pa16rN- aolvl2-e2l to the strike uFarl2f0a22 ofEfRaSCu2l1t01s, ce. Strike-slip faults exhibit mainly horizontal displacement pa16rN- aolvl2-e2l to the strike uFarl2f0a22 ofEfRaSCu2l1t01s, ce. Strike-slip faults exhibit mainly horizontal displacement pa16rN- aolvl2-e2l to the strike uFarl2f0a22 ofEfRaSCu2l1t01s, ce. The San Andreas fault is a strike-slip (transform) fault th1a6-tNoivs-22located in land. Joints Fractures along which no appreciable displacement has oc1c6-uNorv-r22ed are called JoinERtSsC.2101, Fall2022 Columnar joints form when igneous rocks cool and that produce elongated, develop shrinkage fractures pill-ar-2l2ike Most joints are the results of brittle failure when rocks in th1e6-Noovu-2t2ermost crust areERdSeC2f1o01r, mlFae02ld2. End of Topic 10