Deformation of the Earth’s Crust GY111 Physical Geology Deformation of the Earth’s Crust GY111 Physical Geology lectures on the deformation of the Earth’s crust.

Stress & 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. Plastic strain: analogous to deforming mud or putty. Stress & Strain: 1. Stress: a force applied to an area. Example: tire pressure in psi. 2. Strain: a change in original shape or volume (produced by stress). 3. Elastic strain: analogous to a steel spring or rubber band. 4. Plastic strain: analogous to deforming mud or putty.

Types of Stress Lithostatic 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. Types of Stress: 1. Lithostatic Stress: stress due to the burial and overlying overburden of rock. 2. Lithostatic stress can only cause a change in volume referred to as dilation. 3. Directed stress: stress is unequal in different directions. 4. Directed stress is generated by plate tectonic motion and will cause a change in shape referred to as distortion.

Stress vs. Strain Diagrams Illustrate the mechanical behavior of rock materials Brittle: rocks near the surface of the Earth behave as brittle materials- their behavior is mainly elastic Brittle Deformation Elastic Limit Rupture Stress Stress vs. Strain Diagram: X-axis = Strain %. Y-axis = Directed stress magnitude. Elastic Limit: point of rupture of elastic material. Distorsion below E.L. is 100% recoverable. Distortion below E.L. is 100% recoverable Strain %

Ductile Deformation Ductile deformation requires a significant component of plastic mechanical behavior Elastic Limit plastic Rupture Stress Ductile Deformation: Deformation past the E.L. is by plastic flow. Elastic component is still recoverable. Materials that are ductile will suffer permanent strain if E.L. is exceeded. elastic Distortion below E.L. is 100% recoverable Strain % Permanent strain

Mechanical Behavior of Rocks Near-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. Mechanical Behavior of Rocks: 1. Near-surface rocks that are under low T-P conditions behave as brittle material: Fault fracture (slippage). Joint fracture (no slippage). 2. Deep rocks under elevated T-P conditions behave as ductile material: Folding.

Examples of Deformation Experiments Lab equipment can reproduce all geological conditions except geologic time Deformation Expiremental Examples: Note brittle fractures at low T-P (near surface). Note ductile flow at high T-P (deep burial). Low T-P (brittle) High T-P (ductile) Undeformed

Mapping Geological Structures Orientation Planar: 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. Mapping Geological Structures: 1. Orientation Planar: strike azimuth and dip angle with dip quadrant. Linear: trend azimuth and plunge angle. 2. Azimuth: compass direction along the horizontal map surface. 0-90: northeast quadrant. 90-180: southeast quadrant. 180-270: southwest quadrant. 270-360: northwest quadrant. 3. Strike is always read from a northern quadrant therefore it must always be 0-90 or 270-360. 4. 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.

Geologic Period Abbreviations These abbreviations are commonly used to indicate ages of beds on geologic maps. Quaternary (Q) Tertiary (T) Cretaceous (K) Jurassic (J) Triassic (Tr) Permian (P) Carboniferous (C) Devonian (D) Silurian (S) Ordovician (O) Cambrian (-C) Precambrian (p-C) In North America the Carboniferous period Is subdivided into the following 2 periods: Pennsylvanian (|P) Mississippian (M) Geologic Time Scale: Phanerozoic and Precambrian (abbreviations in parentheses). Quaternary (Q) Tertiary (T) Cretaceous (K) Jurassic (J) Triassic (Tr) Permian (P) Carboniferous (C) Devonian (D) Silurian (S) Ordovician (O) Cambrian (-C) Precambrian (p-C)

Examples of Planar Structures Both would be measured with a strike and dip Bedding Planes Bedding & Fault Planes Planar Structures: all measured with strike and dip. Bedding planes. Fault planes Axial planes. Joint fractures Foliation

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. Strike and Dip: 1. Strike is the azimuth direction of the horizontal line in a plane. 2. By convention strikes are read from a north quadrant so the legal values are 0-90 or 270-360. 3. Dip is the maximum angle of inclination in a planar structure. This angle will always be measured in a plane perpendicular to strike. 4. The dip angle must be paired with a quadrant direction since there are 2 sides to any strike line. 5. Example: 040 60NW (strike=040, dip angle = 60 in a 310 (NW) direction. Note that 310 is 90 degrees from 040). 6. 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.

Strike and Dip Symbols 000, 52E 000, 41W 060, 38NW 090, 65S N/A, 0 (B) (C) 000, 52E 000, 41W 060, 38NW 090, 65S N/A, 0 315, 90 300, 80NE 330, 12NE OT 030, 25SE 38 270 52 90 270 41 90 270 90 180 180 180 45 (D) (E) (F) 270 90 270 90 270 90 65 180 180 180 (G) (H) (I) Strike and Dip examples. 80 12 270 90 270 90 270 90 25 180 180 180

Dip Direction Relationships The dip direction of bedding is in a direction toward younger strata- unless the strata is overturned (overturned folds are discussed later). Younger Younging Direction: 1. The dip direction of bedding is in a direction toward younger strata- unless the strata is overturned (overturned folds are discussed later).

Topography and Dip Direction 20 50 90 1. A contact line “V” in the dip direction across a stream valley is less pronounced with larger dip angle. 2. A vertical bed (dip=90) displays no “V” pattern across a stream valley. “V” in dip direction is less pronounced with larger dip angle A vertical bed shows no “V”

Dip Direction Schematic When beds are not overturned the dip directions points toward younger beds. N Tr J K T 50 50 50 T Younger K 50 T Dip Direction on Block Diagrams: 1. When beds are not overturned the dip directions points toward younger beds. Tr J K P

Overturned Strata Dip direction points toward older strata when overturned- note the special overturned bedding symbol In this example the “V” of the contacts indicates the dip direction to the east N 55 55 55 Older 55 -C D S O -C 55 Overturned Strata and Block Diagrams: 1. Dip direction points toward older strata when overturned- note the special overturned bedding symbol. 2. In this example the “V” of the contacts indicates the dip direction to the east. Note that the “V” is not pronounced because the dip angle is relatively large (>45). O M D S O

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 Structure Orientation: 1. Trend: azimuth direction of a linear structure projected up to a horizontal plane. 2. Plunge: incline angle of a linear structure. 3. Note that the trend may have any azimuth value 0-360. 4. Maximum possible plunge is 90. A lineation with a plunge of 90 has no definable trend.

Trend and Plunge (A) (B) (C) 210, 15 330, 05 060, 65 120, 40 030, 00 N/A, 90 240, 23 300, 72 150, 55 05 65 270 90 270 90 270 90 15 180 180 180 (D) (E) (F) 270 90 270 90 270 90 40 180 180 180 Examples of Linear Orientations: 210, 15 330, 05 060, 65 120, 40 030, 00 N/A, 90 240, 23 300, 72 150, 55 (G) (H) (I) 72 270 90 270 90 270 90 23 55 180 180 180

Faulting Faults 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. Faulting: 1. Faults are generated in brittle rock layers when the elastic limit is exceeded by deformation forces. 2. Because brittle behavior is confined to the lithosphere faults do not extend into the asthenosphere. 3. Earthquakes are generated along fault planes, therefore, earthquakes cannot be generated in ductile asthenosphere material.

Fault Classification Classified 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 motion Reverse: hanging wall up motion A special case of reverse where the fault dips < 45 degrees Strike Slip: slippage is parallel to strike of fault. Right lateral: a right-hand turn must be followed to find offset features Left lateral: a left-hand turn must be followed to find offset features Oblique Slip: has combined strike-slip and dip-slip motion. Fault Classification: Classified 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 motion. Reverse: hanging wall up motion. A special case of reverse where the fault dips < 45 degrees. Strike Slip: slippage is parallel to strike of fault. Right lateral: a right-hand turn must be followed to find offset features. Left lateral: a left-hand turn must be followed to find offset features. Oblique Slip: has combined strike-slip and dip-slip motion.

Hanging Wall and Foot Wall To classify a dip-slip fault you must correctly identify the hanging wall and footwall blocks Hanging Wall Footwall Hanging Wall and Foot Wall of a fault: HW overhangs the fault plane. FW is below the fault plane.

Dip-Slip Fault Motion Examples Note that normal faults accommodate tensional stress, whereas reverse faults accommodate compressional stress. Dip-Slip Fault Motion Examples: Note that normal faults accommodate tensional stress, whereas reverse faults accommodate compressional stress. Low dip angle reverse faults are “thrust” faults. Strike-slip faults are generated by horizontal shear forces like those associated with transform plate boundaries. Oblique-slip is a combination of strike- and dip-slip motion.

Fault Offsets Some fault offsets are recognizable on the ground surface. Fault Scarp Fault Offsets: Topographically expressed as fault escarpments (scarp). Offset ridges.

Strike-Slip Fault Motion Examples Movement is parallel to strike of fault therefore offset is seen in a map view Strike-Slip Faults: Motion is parallel to strike of fault. Motion is either right-handed or left-handed. Associated with transform plate boundary deformation.

Tectonic Associations of Fault Types Divergent: 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. Plate Tectonics and Faulting: Divergent – normal faults. Convergent – thrust faults (low-angle reverse). Transform – strike slip.

Folding Folding 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). Folding: Requires some level of ductile deformation to occur. Folding 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).

Fold Geometry Anticline: concave down (arch) Syncline: concave up (trough) Fold Geometry: Anticline – concave down shape with oldest strata in the core (center) of the structure. Syncline – concave up shape with youngest strata in the core (center) of the structure. Note that adjacent anticline-syncline pairs share a fold limb. A fold may have 0 plunge (no plunge), or it may plunge at up to 90 degrees if the fold axis is inclined.

Fold Age Relationships Anticlines 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. Fold Age Relationships: 1. Anticlines contain the oldest strata in the core of the structure. 2. Synclines contain the youngest strata in the core of the structure. 3. If the folds contain no overturned strata the dip direction will be away from the center of the anticline (“inclined opposite the center”) and toward the center of the syncline (“inclined toward the center”).

Fold Symmetry Based on dip of axial plane Fold Symmetry: Symmetrical fold axial plane dips 90 degrees. This causes the limbs of the folds to be of equal length. An Asymmetrical fold has an axial plane that dips at an angle not equal to 90 degrees. This causes the limbs of the folds to be of unequal length hence the term asymmetrical. Overturned folds are folds that contain overturned strata in one of the limbs of the fold. Overturned folds are always asymmetrical.

Plunging Fold Anticline: plunge of axis is in direction of arrow formed by beds on the map Syncline: plunge of axis is opposed to the arrow formed by beds on the map Plunging Folds: Fold axis has a plunge not equal to 0. Anticlines produce a “V” shape in map pattern that points in the direction of plunge. Synclines produce a “V” shape in map pattern that points opposite the direction of plunge. Note that an arrow is added to the axial trace symbol to indicate the plunge direction. Note that non-plunging folds have “straight” contacts whereas plunging folds have curved contacts.

Surface Geologic Map Note the symmetrical patterns: P is symmetrically surrounded by younger beds T is symmetrically surrounded by older beds. N J Tr P Tr J K T K J Tr Block diagram example of folded strata: Look for symmetrical age relationship patterns. For example “P” is the oldest exposed unit and “T” is the youngest. Both are symmetrically surrounded by younger and older strata respectively. “P” must therefore be the center of an anticline, “T” the center of a syncline. The fold is non-plunging because the contacts are straight.

Subsurface Interpretation Anticline axial trace symbol N Syncline axial trace symbol Tr J Tr P Tr J K T K J Tr P K Adding subsurface interpretation: Front face- anticline is concave down, syncline is concave up. Strata dips away from center of anticline, but towards the center of the syncline. Axial trace symbols for the anticline and syncline are added to the map view. The east side of the block diagram contains two interpreted contact that remain horizontal because there is no plunge to the folds. J ? Tr ? P

Plunging Folds Anticlines: contacts point in plunge direction Synclines: contacts point opposite the plunge direction Po Jo Trg N Kpl Po Ta Trg Jo Q Kpl Q Ta Plunging Folds: Anticlines and Synclines are determined by age relationships as in previous examples. The “V” shape of the contacts indicate that the folds are plunging. The Anticline “V” points south so south is the plunge direction. The Syncline “V” also indicated a south plunge. The east side face contains several contacts that are inclined in the plunge direction (south). Kpl Trg Kpl Jo Po

Overturned Folds On the overturned limbs the Strike and dip symbol is overturned Ta Qa Ta N Kpk Ta Jo Kpk Trx Jo Kpk Jo Trx P1 Overturned Folds: Note the diagram is interpreted as before but the valley indicated all strata are dipping east. Each fold therefore must contain an overturned limb – note that change in the axial trace symbols indicating a overturned syncline/anticline pair. Where strata are overturned the “curl” must be added to the strike & dip symbol. Jo Jo Jo Kpk Trx P1 Jo Trx Trx P1

Domes & Basins Domes 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. Domes & Basins: Have circular contact patterns in map view. Domes have older strata in the core of the structure with all non-overturned strata dipping away from the center of the structure. Basins have younger strata in the core with all non-overturned strata dipping toward the center of the structure. For domes all cross-sections are concave down; for basins concave up.

Structural Dome Note that bedding dips away from the center of the structure in a dome. Di Os Sa Di Di -Co N Sa p-C Os -Co -Co p-C Di Os Mr Sa Di Mr Di Sa Sa Dome Block Diagram: Note that all beds dip away from the center. Oldest strata is located in the core. All cross-sections are concave down. General outcrop pattern is circular. Di Os -Co p-C

Dip-Slip Fault Reverse dip-slip Fault Classification:____________________________ Note: slicken-side striations were found to be parallel to the dip of the fault plane. HW FW N U Osp D Sa Sa Osp Osp Sa Dip-Slip Fault Block Diagram Interpretation: Sketch fault contact – assume strike is north and add dip tick pointed west. Determine relative motion from front diagram face- west block up. Add the “U” and “D” to map view. Determine the hanging and footwal blocks and add a “HW” and “FW” on the map view. Classify the fault – Reverse dip-slip because the HW moved up relative to the FW. Add strike and dip of strata symbols. Label strata ages on map view. -Co -Co Osp p-Co -Co 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.

Strike Slip Fault Left-lateral strike slip -Ca Fault Classification:____________________________ -Ca Note: slicken-side striations were found to be horizontal in the fault zone. -Cp HW FW 35 N 70 Ox -Ca 35 35 -Ca -Cp Sj 35 -Cp Ox 35 35 Do Sj 70 Ox Strike-Slip Fault Diagram: Note that slickenside striations indicate a strike-slip motion. If Ox is at the top of the west block on the front face the strike-slip motion must be west block toward the viewer, east block away from viewer. Add the arrows indicating the left-handed strike-slip motion. The contacts in the west block will be offset the exact same distance in a left-handed sense. We can estimate that the Ox/-Cp contact must be close to the front edge of the west block. Add strike and dip symbols for strata – all dip 35 south. Add dip tick mark for fault dip. Label HW and FW. Do + - -Cp Sj Note that “HW” always on the dip direction tic mark side of fault contact. Note the arrows indicating left-lateral (sinistral) strike slip.

Dip-Slip Fault Reverse dip-slip -Ca -Cp H F N Ox -Ca -Ca -Cp U D Sj Fault Classification:____________________________ NOTE: slickensides in fault zone were oriented parallel to dip line of fault. -Ca -Cp H F 35 N Ox 35 -Ca 35 -Ca -Cp U D Sj 35 70 -Cp Ox 35 35 Do Sj 70 Ox Dip-Slip Fault: Note this block diagram appears equivalent to the previous strike-slip diagram, however, the slickensides indicate dip slip motion. Remember that in the up-thrown block the strata contact are shifted in the dip direction (south in this case). Add the “U” and “D” to the west and east blocks respectively. Note that the –Ca/-Cp contact has been shifted south (dip direction) in the west fault block. The west block must be up-thrown to generate the above geometry. Because the HW went up relative to the FW this is a reverse fault. Do -Cp Sj Note 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.

Exam Summary Know definitions of stress and strain Be 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. Definitions of stress and strain. Geological examples of each. Definitions of brittle, ductile, elastic, elastic limit, plastic, lithostatic stress, directed stress. Geological examples of each. Definitions of strike, dip, trend, plunge. Recognition of anticlines, synclines, domes, basins, overturned strata, overturned folds from geological maps. Recognition of hanging wall block, footwall block, dip-slip, strike-slip, oblique-slip faults. Tectonic associations of structures.