Deformation & Structures

1. Deformation & Structures Associate Professor John Worden DEC University of Southern Qld

2. Deformation & Structures • Deformationis a continuous feature of the Earth. • Since Sea floor is < 200 Ma old (<5% of Earth History), Continents must be examined for history of Deformation. • Typical Mountain-building episode involves thick sequences of sediments deposited in extensive basins along ocean margins of Continental blocks (i.e. Accretionary Wedges). • During plate collisions, sediments are crumpled & deformed. • As sediment belt deforms sediments are: • Uplifted , Tilted , and Faulted , and • Distorted into Folds. • Continental Crust is thickened by: • Stacking multiple Thrust Sheets.

3. Deformation & Structures • Exotic displaced Terranes brought to collision margin & accreted/ welded onto a Continental block.. • Erosion continues to reduce the newly-formed mountain belt, & recycle the earlier sediments. • Evidence: • Continents marked by long, relatively narrow mountain chains of uniformly folded & faulted rocks; • Interpreted as remnants of old plate margin collision belts; • All involve compressive deformation; • Nearly all contain exotic terranes, and • Many episodes of intrusion & metamorphism.

4. Deformation & Structures • Rock Deformation: • Lab experiments reveal rock behaviour under compression. • Low confining P (270 A), applied strain causes 20% shortening of marble and extensive fracturing. Equates to shallow depths in Crust. High confining P (445 A), applied strain produces ‘ductile’ deformation corresponding to greater depths in Earth’s Crust. • Infer that marble would deform by ‘faulting’ at shallow depths, & ‘fold and flow’ at depths of > than a few kilometres. • If rock heated, & strain applied, rock deforms smoothly and continuously. • Predict rocks deform in a ductile manner if: • less applied stress over long time, & • rocks are wet & hot.

5. Deformation & Structures • Earthquakes: • At shallow depths in Crust & Mantle, rocks are ‘brittle’. • Accumulate strain until it exceeds elastic limit, when they rupture. • Earthquakes result from rock failure, & represent displacements of as much as 10-15 metres. • San Francisco (1906) earthquake due to 5 metres horizontal displacement along San Andreas Fault, and • Alaskan (1964) earthquake maximum of 13 metres vertical displacement. • Earthquake displacements are a major factor over time. • Earthquakes are sudden releases of ; • stored Elastic Energy; • accumulated during Plate Tectonics, and • heat, sound and vibrational waves are generated as rock slippage occurs along faults.

6. Deformation & Structures • Point where energy first released =‘ Earthquake Focus’. • Point vertically above ‘focus’ on the surface= ‘Epicentre’. • Earthquake sources quoted as ‘epicentre + depth below this location’. • Energy converted to vibrations termed ‘seismic waves’ that spread out spherically. • Seismic waves of two types- “Body and Surface waves”. • Body waves travel through solids & liquids. • Surface waves travel around Earth, but not through it (solids only). • Body wave mechanisms include: • Change in volume- compressional/expansion, and • Change in shape- shear waves. • Compressional (P) waves move at 6 km/sec, and • Denoted as “P” or Primary waves as first arrivals.

7. Deformation & Structures • ‘Shear’ (S) waves displace particles vertically as wave passes, and • Move at 3.5 km/sec. Therefore termed ‘Secondary waves’as second to arrive after an earthquake at a seismometer. • Note that S waves cannot travel through liquids , so do not traverse Earth’s core. • ‘Primary’(P) waves do transect the liquid outer core but with sharply attenuated velocity. • Both P & S waves are reflected and refracted at boundaries within Earth. • Surface waves travel around Earth’s surface, and • are last to arrive at recording station, • very long wavelengths (hundreds of kilometres), • Velocity of body waves depends on density of rocks.

8. Deformation & Structures • Orientation of Rock Strata: • Deformation is recognised by changes in shape & form of strata. • Sedimentary rocks and lava flows generally deposited horizontally. • Sedimentary rocks characterised by ‘bedding’ or bedding planes. • Deformation alters orientation of sedimentary layers from horizontal. • How is this determined? • Orientation is specified in terms of “strike and dip” directions. • Strike- magnetic bearing ( 0o-360o) of a horizontal line on the inclined bedding plane surface (i.e. a standing water level). • Dip- declination angle from the horizontal, measured perpendicular to the Strike bearing. • Dip quadrant direction is also specified. • Horizontal beds represented on maps by a cross .

9. Deformation & Structures • Folds and their geometry described by the following terms: • Hinge - the point of maximum curvature of a fold; • Hinge line - an imaginary line which joins all hinge points on a fold; • Limbs - relatively planar areas between hinges; • Axial surface - an imaginary plane that passes through all of the hinge lines of a fold; • Axial trace - intersection of axial surface with ground surface; • Anticline - fold with limbs dipping away from hinge and oldest rocks in the core of the fold; • Syncline - fold with limbs dipping towards hinge & • the youngest rocks in the core of the fold; • Crest - highest part of anticline, and • Trough - lowest part of syncline.

10. Deformation & Structures • Folds are divided into Symmetrical & Asymmetrical: • Symmetrical folds have limbs of equal length, whereas • Asymmetrical folds have limbs of unequal length. • During the application of deformational forces, any initial tilting of the strata will result in one limb dipping more steeply than the other and an asymmetrical fold. • If deformation intense, fold may be overturnedwith upper limb of syncline and lower limb ofanticline tilted beyond vertical, and dipping inthe same direction. • In recumbent folds, axial planes are: • nearly horizontal

11. Deformation & Structures • Horizontal fold - the hinge line is horizontal. • Plunging fold - the hinge line is inclined. • Tightness of folding - measured by the angle between the limbs. Very tight folds are ‘Isoclinal’, when limbs are parallel. • Competency: • a relative property; • a competent formation is strong & can transmit compressive force much further than an incompetent formation; • crushing strength is some measure; • quartzite & marble stronger than sandstone & limestone, and all stronger than Shale. • Massiveness of a formation is also important. • Thick formations are more competent.

12. Deformation & Structures • Three types of Folding of sedimentary strata: • Flexure folding which involves bending & buckling of more competent layers under compressive force. The more incompetent layers flow into the space created & beds slide past one another. • Flow folding applies to incompetent layers. • Shear folding or slip folding results from minute displacements along closely spaced fractures/ micro-faults, producing a major fold with many minor folds. May be accompanied by ‘cleavage’. Beds are thinned, but never thickened. • Tight folds associated with flowage of mobile beds. • Open folds display no flowage. • All folds die out with depth.

13. Deformation & Structures • Outcrop patterns: • Rarely reveal folds of larger scale. • Their presence deduced from numerous observations of ”Strike & Dip”. • Topography may indicate folds if more resistant beds form ridges & less resistant horizons form valleys.

14. Deformation & Structures • Faults: • Form when rocks fail & rupture, and there is movement between opposite walls of a fault (move past each other). • Strike and Dip of fault plane measured in same manner as bedding. • Block above the inclined fault is termed the “ Hanging Wall”. • Block below is termed the “Footwall”. • Displacement may not be all in one plane,but distributed over many fractures in a“fault zone”. • Intersection with the Earth’s surface termed: • Fault line or Fault trace. • A common feature of mountain belts.

15. Deformation & Structures • Nature & Movement on Faults: • Movement may be translational or rotational; • Translational movement has no rotation of blocks relative to each other, • All faults normally have a certain amount of rotational movement. • Require ‘marker horizons’ to establish sense of movement on fault. • Term ‘slip’ used to indicate relative displacement. • Specify direction of slip as: • Strike slip - slip parallel to the strike of Fault, and • Dip slip - slip measured parallel to the dip of Fault.

16. Deformation & Structures • Classification of Faults: • Based on relative movements; • Dip-slip fault - movement of hanging wall either up or down relative to footwall • Normal Fault is when the hanging wall block moves down relative to footwall; • Reverse Fault when hanging wall moves up relative to footwall; • Normal faults are sometimes termed ‘Gravity Faults’; • Normal faults indicate lengthening of the Crust; • Reverse fault with shallow dip angle termed ‘Thrust’; • Thrust faults indicate shortening of the Crust • Strike-slip fault- displacement // to strike of fault. • Oblique-slip fault- combination of dip-slip and strike-slip movement.

17. Deformation & Structures • Sense of right-left movement on faults is determined by imagining that you are looking across the fault to the other side and: • If the opposite side is displaced to the LEFT, it is a left-lateral or Sinistral fault, • If the opposite side is displaced to the RIGHT, it is right-lateral or Dextral fault. • Criteria for recognition of faults: • Discontinuity of structures: • Strata suddenly end against different beds, • Do not confuse with unconformities and intrusive contacts. • Repetition or omission of strata: • Can also be due to unconformity. • Fault plane features: • Slickensides - polished & striated surfaces ; • Gouge - pulverised rock now clay-like; • Breccia -angular fragments in fine-crushed matrix.

18. Deformation & Structures • Silicification and Mineralisation: • Faults are often conduits for solutions. Replacement with fine-grained quartz referred to as Silicification. • Other mineralisation than quartz is common. • Sudden changes in sedimentary facies: • Sedimentary rock types usually interfinger and grade into each other over distance. Abrupt rock type changes suggest a fault. • Physiographic Data: • topographical features may indicate presence of fault. • Include scarps, springs, offset ridges, offset streams,etc.

19. Deformation & Structures • Joints: • Rocks are characteristically broken by smooth fractures known as “Joints”. • They are fractured surfaces along which there has beenno movement parallel to the surface. • There can be movement at right angles to the joint surface producing an open fracture. • Joints may have any orientation. • Joints are measured by strike and dip like bedding • Joints never occur alone, but in sets. • ‘Joint sets’ over a region make up a ‘joint system’.

20. Deformation & Structures • Joint classification: • Strike joints- strike parallel to bedding, schistosity or gneissosity. • Dip joints- strike parallel to dip of bedding, schistosity,& gneissosity. • Oblique or diagonal joints strike at an angle between the strike and dip of associated rocks. • Bedding joints- parallel to the sedimentary bedding. • Tension joints- form perpendicular to extensional forces. • Shear joints- occur when one part of rock slides past another. Usually form in pairs. • Columnar joints- three fractures at 120o • Form hexagonal columns in cooling Basalt. • Also occur in mud cracks, etc. • Sheetingis somewhat curved & // to topographic surface.