Lecture 17 and 18 - Deformation and Making Mountains PDF

Deformation and Making Mountains EA1110 LECTURES 17 AND 18: 30 TH SEPTEMBER AND 2 ND OCTOBER 2024 Learning Outcomes Explain and describe how the Earth functions as a dynamic system Explain and describe the physical and chemical evolution of the Earth Summarise deformation types and processes. Thi...

Deformation and Making Mountains EA1110 LECTURES 17 AND 18: 30 TH SEPTEMBER AND 2 ND OCTOBER 2024 Learning Outcomes Explain and describe how the Earth functions as a dynamic system Explain and describe the physical and chemical evolution of the Earth Summarise deformation types and processes. This includes: → What is the difference between stress and strain? → What is the difference between brittle and ductile deformation? → Determine strike and dip measurements → Describe the various types of faults → Describe the various geometries of folds → Explain how mountains are formed particularly in compressional settings 2 What Is This Lecture About? Deformation processes link Develop an understanding of closely with tectonic processes; how mountains are made; one good context to start from! side of the story of how landscapes are shaped 3 What is Orogenesis? Orogenesis is the natural process of mountain building → From Ancient Greek: oros = mountain, genesis = creation → Involves folding, faulting, magmatism, and metamorphism → Results in relatively narrow regions of deformed continental crust (e.g., Himalayas, Andes, Alps, etc.) Topographic map of the Himalayas Topographic map of the Andes 4 5 Orogenic Processes Orogenesis takes millions of years → Time for metamorphism → Time for deformation Orogenic (mountain) belts result from tectonic forces → Continental collisions → Subduction zones → Rifting → Transform faults The Matterhorn, Swiss Alps 6 Conditions for Orogenic Processes T < 200oC Rocks undergo changes in shape (geometry) and distribution only T > 200oC and < 1200oC Rocks undergo changes in geometry and distribution as well as mineralogy, composition and texture; at high T rocks may melt to form granites; realm of structural geology, metamorphic petrology and igneous petrology of granites T > 1200-1500oC Mantle melting, realm of igneous petrology of basalts 7  Orogenic Processes: Change in Chemistry Changes in the mineralogy, composition, and texture of rocks during orogenesis or similar falls withing the realm of metamorphic geology Metamorphic geology is the study of the way in which sedimentary and igneous rocks change due to Earth processes Images: Metamorphic minerals under a microscope 8 Orogenic Processes: Change in Geometry Changes in the shape (geometry) and distribution of rocks during orogenesis or similar falls withing the realm of structural geology Structural geology is the study of rock deformation → the way that rocks respond to deforming forces → the structures that form in the rocks (e.g., foliation, faults, folds) 9 Tectonic Structures Structure form in response to deformation, which ultimately links to forces generated by plate interactions (such as convergence, rifting etc.) → Include: joints, fractures, faults, folds, cleavage, foliations, lineations, shear zones, and many more... How did this happen? Harvey's Return, Kangaroo Island 10 What is Deformation? Deformation is a change in the shape, position, or orientation of a material by bending, breaking, or flowing. There are three main varieties: Displacement = movement of one part relative to another through faulting (a) Rotation = rotation of strata from their original orientation (b) Distortion = change in the shape/thickness of strata (c) 11 What Causes Deformation? Deformation occurs when a force is applied to a rock. The response depends on the magnitude of the force per unit of area, the direction of the force, and on the characteristics of the rock (pressure, temperature, composition) These forces can be described as: Compressional (b) Tensional (c) Shearing (d) Uniform (e) 12 Deformation: Stress and Strain Stress (geologically) is the push, pull, or shear that a material experiences when subjected to a force (often related to tectonic movement) Strain is the change in shape of an object in response to deformation (i.e., as a result of the application of a stress) Stretching (elongation) Shortening (contraction) Shearing 13 Elastic Deformation Elastic deformation is when the strain is reversable (like a rubber band – can go back to its original position / form) → The relationship between stress and strain is linear → When the stress is removed, the strain also goes away (strain is non-permanent) Example: Isostatic rebound → Extensive glaciers in the northern hemisphere weighed down the crust for thousands of years → Removal of these ice sheets after the ice age removes the downward stress → As a result, some continents are rebounding (returning to their pre-strain arrangement) Raised beaches Nunavut Northern Canada (isostatic rebound) 14 Permanent Deformation The extent of elastic (reversable) deformation is limited The yield point of a rock / system describes the limit of elastic deformation Once the elastic limit is reached, small additional stress results in a large increase in strain ‘Permanent’ (inelastic) deformation is when stress causes permanent strain that does not reverse when the stress is removed → Grouped into two categories: brittle and ductile 15 Ductile Deformation Ductile deformation is the bending and flowing of a material (without cracking and breaking) when subjected to stress beyond the yield point of the material → Usually involves an irreversible distortion (change in shape/thickness) → Also sometimes called plastic deformation 16 17 Brittle Deformation Brittle deformation is the cracking and fracturing of a material (without flowing) when subjected to stress beyond the yield point of the material → Usually involves an irreversible displacement (movement of one part relative to another through faulting) 18 19  Deformation of marble from low to Brittle versus Ductile high confining pressure (left to right) Brittle/ductile behavior depends on: 1. Confining pressure (depth) → Deeper deformation promotes ductile deformation 2. Temperature → High T promotes ductile deformation 3. Time (or strain rate, or rate of deformation) → Lower strain rate promotes ductile deformation 4. Rock composition (= rock strength) → Weaker rocks deform in a ductile way, stronger rocks fracture Can be ductile at first then brittle if stress exceeds what can be accommodated by ductile strain Iyare et al. 2021 20 Brittle versus Ductile (continued) The effects of P, T and strain rate predict that: → Fracturing (brittle deformation) is largely restricted to the upper crust → Flow (ductile deformation) dominates at greater depth Brittle-ductile transition zone is at around 10 to 15 km 21 Deformation Structures Deformation structures are the physical features that result from the combination of stress (the force applied to a rock) and strain (the way the rock reacts) → This means there are six basic structures, as seen in the table below → Uniform compression is common (confining / lithostatic pressure) but does not frequently result in deformation structures Stress Strain Compression Tension Shearing Crustal Shear zones, Ductile Folding thinning and mylonite deformation stretching zones Reverse Lateral Brittle Normal faults, thrust (strike-slip) deformation faults faults fault 22  Yellow arrows are dip Describing Structures surfaces we can measure! Many structural features result in planes or surfaces that we can measure using a compass with an inclinometer → Measure the cardinal direction the surface is facing → Measure the angle of incline → = Strike and dip Tilting AND faulting can be measured here Rocks exposed in the State Circle near Parliament House, Canberra 23 Strike and Dip Strike is the direction of a perfectly level horizontal line on the plane Dip is the angle of inclination of the plane (perpendicular to strike) The orientation of the rock in the diagram is written as 040o/30oE (or SE) 24 Identifying Strike and Dip Figuring out the correct strike direction can be tricky at first → Try a putting a drop of water on the surface and watch which way it flows = dip direction → Strike direction is always -90o from the dip direction The Right Hand Rule: put your right hand on the surface so that your arm follows the dip of the plane and is extended beside you = you are now facing the strike direction → Note there is a different version in Europe (other way) Demonstration of measuring the dip angle and strike (direction) Demonstration of the right-hand rule 25 Strike and Dip in Geological Mapping Strike and dip measurements are recorded on maps with a specific symbol → Sort of like a short, sideways T Apparent thickness is how thick a dipping unit may appear to be when intersected at any angle other than exactly perpendicular → Apparent thickness will always be thicker than true thickness Similar thing if dip isn’t viewed perpendicularly in cross section → Apparent dip will always be less than true dip 3D view Map view 26 BRITTLE DEFORMATION STRUCTURES Brittle Deformation Structures “Brittle deformation is any permanent change in a solid material due to the growth of fractures and/or due to sliding on fractures once they have formed” Brittle deformation is favorable in low T / low P / fast strain rate conditions Includes: → Joints → Veins → Faults 28 Brittle Deformation: Joints Joins are fractures in rocks caused by tensional stress in brittle rocks → No lateral displacement occurs, just a crack Bottom: ‘Tessellated Pavement’, Tasman Peninsular 29 Brittle Deformation: Veins Veins are mineral-filled fractures → Form from fluid flow along pre-existing joints or from fault activity En echelon veins (brittle-ductile transition zone) 30 Brittle Deformation: Faults Faults are fractures with movement along the fracture → Result from brittle rock failures → Scale can be sub-millimeter (very small) to kilometer (very large) 31 Fault Terminology Orientation terms: Displacement terms: → Hanging-wall → Dip-slip: movement parallel to dip → Foot-wall → Strike-slip: movement parallel to strike → Trace → Oblique-slip: movement with both dip → Strike and strike slip components → Dip Fault planes can be moderately dipping, shallowly dipping, steeply dipping, vertical or sub-vertical 32 Fault Movement Described by the amount and direction of displacement → Heave: horizontal component → Throw: vertical component → Can have one or the other, or both! Movement direction can be shown by slickensides → Friction-related striations can form along the fault plane → Effectively small scratches that form in response to motion on a fault. slickensides 33 ‘Dip-Slip’ Faults There are three main varieties of dip-slip faults Normal fault: tensional stress, hanging wall moves down Reverse fault: compressional stress, hanging wall moves up Thrust fault: low angle reverse fault The are described by the relative movement of the hanging-wall (above the fault plane) and foot-wall (below the fault plane) Listric fault: usually a normal fault with a curved fault plane and includes hanging-wall block rotation 34 ‘Dip-Slip’ Fault Examples Hanging-wall Foot-wall Hanging-wall Hanging-wall Foot-wall Foot-wall 35 ‘Strike-Slip’ Faults Strike-slip (or lateral) faults are when the fault plane is roughly vertical (there isn’t a footwall or hanging wall) → Also sometimes called lateral faults Two varieties: → Right Lateral / Dextral Faults → Left Lateral / Sinistral Faults San Andreas Fault 36 Sinistral or Dextral? Pretend you’re standing on one side of the fault and looking at the rocks on the other side: → Moved left = left-lateral (sinistral) → Moved right = right-lateral (dextral) Sinistral fault sense Dextral fault sense 37 How to Recognise a Fault Faults can be recognized by: Displacement Fault breccia (cataclasite) Slickensides (scratches) Shear Zone (mylonite) 38 Brittle Deformation: Fault Rocks Fault breccia: broken rock along a fault plane Cataclasite: A fault rock that has been wholly or partly formed by the progressive fracturing and grinding of existing rocks, a process known as cataclasis Fault gouge: Rock that has been pulverized to a fine powder 39 DUCTILE DEFORMATION STRUCTURES Ductile Deformation Structures “Ductile deformation is any permanent change in a solid due to viscous flow” Ductile deformation is favorable in high T / high P / slow strain rate conditions Includes: → Fabrics → Folds @MaxLechte 41 Ductile Deformation: Mylonite Mylonite: A fine-grained metamorphic rock, typically banded, resulting from recrystallisation of minerals (ductile fabric) → Form in ductile shear zones Mylonite Granite 42 Ductile Deformation: Fabrics Fabric is a general term for the geometric arrangement of material (minerals, fossils, clasts, etc.) within a rock → A foliation is the general name for any kind of planar fabric in a rock → For example, folds and low grade (slaty) cleavage are often associated formation via pressure solution, rotation and/or new mineral growth Axial planar foliation 43 Ductile Deformation: Boudinage Boudinage is formed by extension (pulling apart), where a competent (rigid) tabular bed like a sandstone is stretched amidst less rigid beds like mudstone → The rigid bed begins to break up, forming sausage- shaped boudins → The less rigid beds simply deform without obviously breaking up 44 Ductile Deformation: Folds Anticline Folds represent curvature of a prior surface by ductile deformation An anticline is when the surfaces bend upwards (looks like an A) → Oldest rocks in the centre (if not overturned) → If ‘way up’ is not known = antiform Syncline A syncline is when the surfaces bend downwards (synclines ‘sag’) → Youngest rocks in the centre (it not overturned) → If ‘way up’ is not known = synform Both are structures in the rocks and do not always relate to topographic landforms (but they might!) 45 folds rarely form in isolation 46 Plunging Folds Plunging anticlines/synclines occur when the hinge is not horizontal → Layers ‘close out’ at one end → In antiforms, the end they close on is the direction the fold plunges; synforms are opposite Plunging syncline - Scotland 47 Fold Terminology Hinge: region of maximum curvature Limbs: relatively straight sides of the fold Axial Plane: surface that bisects the fold Fold Axis: intersection of axial plane and the folded surface Axial Plane Trace: intersection of the axial plane and the ground 48 Fold Geometry Fold geometry described by limb angle Gentle: 180° - 120° Open: 120° - 30° Tight: 30° - 0° Isoclinal: limbs are parallel Inclination of axial plane (and limbs) Vertical axial plane: upright Inclined axial plane: inclined Both limbs dip in the same direction: overturned fold Both limbs sub-horizontal: recumbent fold 49 Gentle Open Tight Isoclinal 50 Upright Inclined Overturned Recumbent 51  Isoclinal folds Chevron folds Recumbent folds 52  Monocline Complex Folds Monocline: a single flexure, like a step (often above a fault block) Dome: anticlines in all directions Think of the ‘Basin’: synclines in all directions Lapstone Monocline near Sydney Dome ‘Basin’ Ikara / Wilpena Pound ‘Basin’ (South Australia) 53 Exposed internal structure of the East Kaibab Monocline, Utah 54  Ripples Younging (way up) and fold names Younging direction is the direction that represents the order strata were deposited Up → The “way up” from when the rocks were originally made → The stem of the Y symbol points in the younging direction Way up indicators: features in the rock that tell you which way was up when the bed was deposited (e.g., graded bedding, ripples, flame structures, mud cracks, etc.) Cretaceous mudcracks in Utah: shape indicate up direction 55 Younging Direction and Fold Names The stem of the Y points in the younging direction If you don’t know younging direction of strata in a fold, use synform and antiform If you know the oldest rocks are on the bottom (right way up) use syncline and anticline If the rocks have been overturned, the oldest rocks are now on top 56 Foliation in Folds Axial planar foliation: Foliations form parallel to the axial plane in the direction of least stress → Direction of folding can be found on limbs, even when hinge zone not seen → If the orientation of the fold axis is known, the anticlinal and synclinal hinges can be determined 57 MAKING MOUNTAINS Landscape-Scale Forces Remember back to: Compression Tension Shearing We now know about deformation structures we can link these small- and large-scale processes Some tectonic forces are better for mountain-building than others 59 Landscape-Scale Shearing (Transform) Transform plate boundaries: where two tectonic plates slide past each other laterally → Examples = San Andreas fault and between mid ocean ridges → Not great for making large mountains Bends in fault lines can occur and these features either create basins (low points) where they pull apart or hills where they push together 60 San Andreas Fault 61 Landscape-Scale Extension Divergent plate boundaries: where plates move apart due to extensional forces → Examples = East African Rift, Basin and Range (USA), mid ocean ridges → Fairly good at making hills and mountains → Mostly uplifted highlands Horst and graben system in the Basin and Range region, Nevada USA 62  Extensional Deformation Structures Normal faults form due to extension (stretching) Rift systems are continent scale networks of normal faults related to extensional tectonics Symmetric rifts Asymmetric rifts * basal detachment 63 Mountains in Extensional Zones Rifting generates horst and graben system; the horsts are topographic highs Aerial view of the Great Rift Valley of Eastern Africa (Credit: Phillippe Bourseiller) 64 Mountains in Extensional Zones Rifting also results in uplifted rift flanks due to crustal thinning and heating which makes the crust near the boundary more buoyant than further away from the boundary → Applied to both continental and oceanic settings Highlands in Ethiopia related to uplifted rift flanks 65 Landscape-Scale Extension: Iceland Iceland is transacted by a continental scale rift system that results in much of the iconic landscape features from the mountains to the volcanos Pingvellir, Iceland 66 Landscape-Scale Compression Convergent plate boundaries: where plates move together due to compressional forces (mostly) → Subduction examples = Andes, ~New Zealand → Continental collision examples = Himalayas, Europeans Alps → Excellent at making mountains Subduction related orogenesis is both structural and igneous Continental collision related orogenesis is dominantly structural 67 Compressional Deformation Structures Reverse faults form due to contraction (shortening) → Called a thrust fault if the fault plane dip is < 30° → Hanging wall moves up relative to footwall Folds (most types of folds are compressional except monoclines) 68 Mountains in Compressional Zones: Subduction Mountains formed in subduction settings are often a combination of structural (faulting and folding) and igneous (volcanos and plutons) processes In front of Accretionary wedge forms a mélange the arc where material is scrape off the underlying plate and compressed up into a wedge Volcanic arcs are dominated by igneous processes that build mountains by adding extrusive and intrusive rock on top of and within the crust over a subduction zone Thrust belts can develop behind arcs that experience significant compression → Associated with retro-arc foreland basin Behind the arc systems (characterized by thrust faults) 69 Accretionary Wedge Mélange Mélange is a large-scale breccia, characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes in a fine-grained deformed matrix → Typically consists of a jumble of large blocks of varied lithologies → Commonly formed in the accretionary wedge above a subduction zone → Mélange broadly means “ a varied mixture” 70 The Andes 2nd highest mountain chain in the world Did not form through continent-continent collision! Nazca plate subducting under South American Plate Rare to form such high mountains during subduction All three subduction-related orogenesis settings present Accretionary wedge (relatively small) Volcanic arc (very tall and long mountain chain) Retro-arc thrust belt (widest around ~Bolivia) 71  New Zealand Alps 600 km long Moves 30m laterally every 1000 years (v. fast) Magnitude 8 earthquakes geologically frequently Transpressional tectonic setting → Subduction occurs to the north and south but the alps form along a special transform margin HTTPS://WWW.STUFF.CO.NZ/THE-PRESS/4253775/NASA-SATELLITE-CAPTURES-SOUTH-ISLAND 72 Mountains in Compressional Zones: Continental Collision Continental collision zones are characterized by intense and extensive orogenesis Leave lasting marks on the Earth where continents suture together Includes extensive brittle and ductile deformation Produces regional metamorphic rocks (heat + pressure) plus migmatites and igneous intrusions 73 Mountains in Compressional Zones: Continental Collision Orogenesis in these settings is dominated by structural and metamorphic processes although also includes igneous activity due to greater crustal thickness and increased geothermal gradient Intense folding (ductile) and thrust faulting (~brittle) Within a fold-and-thrust system, strata are compressed, faulted, folded, and crumpled An ocean disappears and a new mountain system lies in the interior of a major landmass 74 75 Geometry of the Himalayas The Himalayas are still growing! (~1cm per year) → What goes up, must come down → Upwards movement is also balanced with weathering and erosion Searle (2017) 76 Why do we care about mountains? The distribution and type of mountains on Earth significantly influence short and long-term climate Orogenic uplift results in rain on one side of a mountain range and a ‘rain shadow’ on the other Mountain building episodes in Earth history result in greater overall weathering which draws down CO2 from the atmosphere (next lecture!) 77 Topography and Climate Due to the position of the Himalayas relative to prevailing winds, northern India is very wet and the Tibetan Plateau is very dry 78 Australian Mountain Ranges: A Story of Landscape Evolution Great Dividing Range, Australia Highest part of Australia Hills, mountains, and plateau including the tallest mountain, Mount Kosciuszko (2.22 km high) To understand these mountains, we need to take a closer look at landscape evolution 79 Resources Types of fault (video – Liz Meredith, ~5 mins) https://www.youtube.com/watch?v=A_ZRtS3QGHw Types of fault (video – MooMooMath, ~3 mins) https://www.youtube.com/watch?v=qhSzpCpI38U Folding in rocks (video – rwm_gdf_uk, ~5 mins) https://www.youtube.com/watch?v=BtzMUXBc2Kg Folds, dip and strike (video – wvannorden, ~5 mins) https://www.youtube.com/watch?v=BtzMUXBc2Kg 80 Next Lecture: Landscape Evolution