# EMSC 3002 ## Structural Geology and Crustal deformation - **Louis Moresi** (convenor) - Chengxin Jiang (lecturer) - Romain Beucher (former lecturer) - Stephen Cox (curriculum advisor) Australian National University _**NB:** the course materials provided by the authors are open source under a creative commons licence. We acknowledge the contribution of the community in providing other materials and we endeavour to provide the correct attribution and citation. Please contact louis.moresi@anu.edu.au for updates and corrections._ <--o--> ## Resources 1. **Fossen, H, 2011.** *Structural Geology.* Cambridge University Press, 2nd Edition. 1. **McClay, K.R. 1991.** *The Mapping of Geological Structures.* John Wiley & Sons. 1. **Park, R.G., 1995.** *Foundations of Structural Geology.* Blackie & Sons Ltd. 1. **Davis, G.H. and Reynolds, S.J., 1996.** *Structural Geology of Rocks and Regions.* 2nd Edition, John Wiley & Sons. <!-- 1. **Hatcher, R.D., 1990.** *Structural Geology - Principles, Concepts, and Problems*, 2nd Edition, Prentice-Hall --> <!-- 1. **Ramsay, J.G. and Huber, M.I. 1983.** *Modern Structural Geology. Volume 1: Strain Analysis.* Academic Press. --> <!-- 1. **Ramsay, J.G. and Huber, M.I. 1987.** *Modern Structural Geology. Volume 2: Folds and Fractures.* Academic Press. --> <--o--> !<-- ## Intended learning outcomes --> ## Structural Geology and Crustal deformation **What is it about?** Structural geology covers deformation structures formed in the Earth's crust and in the underlying mantle in response to tectonic forces, in the cool upper part where the rocks have a tendency to fracture and in the hotter lower part where the deformation tends to be ductile. Deformations structures can form almost instantaneously as they do during earthquakes or over millions of years like mountains. <--o--> ## Structural Geology and Crustal deformation <div> <div style="width:50%; float:left; margin: 0; position: absolute; top: 50%; -ms-transform: translateY(-50%); transform: translateY(-50%)"> **What are we interested in?** - **Geometry** of structures; i.e. description of orientations and geometric relationships between structures. - **Styles of Deformation**, including quantitative estimates of amount of deformation. - **Processes of Deformation**, brittle versus ductile mechanisms. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="80%" --> </div> </div> <--o--> ## Structural Geology and Crustal deformation <div> <div style="width:50%; float:left"> Structural geology studies structures at the scale of hundreds of kilometers all the way down to micro- or atomic scales. - **Macroscopic**: regional, crustal scale observations. - **Mesoscopic**: outcrop observations. - **Microscopic**: hand specimen and smaller scale of observation. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## Structural Geology and Crustal deformation <div> <div style="width:50%; float:left; margin: 0; position: absolute; top: 50%; -ms-transform: translateY(-50%); transform: translateY(-50%)"> **Pumpelly's Rule**: *The geometry and style of small-scale structures usually reflect the geometry and styles of larger structures* (Raphael Pumpelly, USGS, 1894) </div> <div style="width:50%; float:right">  <!-- .element style="float: right" height="100%" --> </div> </div> <--v--> ## Structural Geology and Crustal deformation <div> <div style="width:50%; float:left; margin: 0; position: absolute; top: 50%; -ms-transform: translateY(-50%); transform: translateY(-50%)"> **Charles Lyell - Principles of Geology**: *the present is the key to the past* (Charles Lyell, Principles of Geology, published between 1830-1833) </div> <div style="width:50%; float:right">  <!-- .element style="float: right" height="100%" --> </div> </div> <--o--> ## Structural Geology, Tectonics, Geodynamics... Structural Geology, Tectonics and Geodynamics form a coherent and interdependent ensemble of sub-disciplines, the aim of which is the search for knowledge about how minerals, rocks and rock formations, and Earth systems (i.e., crust, lithosphere, asthenosphere ...) deform and via which processes. **Structural Geology** is about **structures** **Tectonics** aims at unraveling the geological context in which deformation occurs. It involves integration of structural geology information (maps, cross section, measurements etc.) and other disciplines (e.g. sedimentology, geochemistry, geophysics, geochronology etc.). It typically operates at scales ranging from 100m to 1000km. It focuses on processes such as **continental rifting**, **subduction**, **collision** etc. **Geodynamics** focuses on the forces that drives mantle convection, plate tectonics and the deformation of Earth's material. Geodynamics is interested in deep mantle and lithospheric processes and their link to plate motion and deformation. Geodynamics operates on scales greater than 100km. <--o--> ## Range of methods to approach structures <div> <div style="width:50%; float:left"> - **Field observations: final product of deformation.** - Laboratory experiments: useful to observe and understand progressive deformation. - Numerical modeling: useful to quantify and understand the role and interactions of forces. Each method has its own limitations but by combining them, the structural geologist is able to understand the formation of structures and their meaning. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## Range of methods to approach structures <div> <div style="width:50%; float:left"> - **Field observations: final product of deformation.** - Laboratory experiments: useful to observe and understand progressive deformation. - Numerical modeling: useful to quantify and understand the role and interactions of forces. Each method has its own limitations but by combining them, the structural geologist is able to understand the formation of structures and their meaning. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## Range of methods to approach structures <div> <div style="width:50%; float:left"> - Field observations: final product of deformation. - **Laboratory experiments: useful to observe and understand progressive deformation.** - Numerical modeling: useful to quantify and understand the role and interactions of forces. Each method has its own limitations but by combining them, the structural geologist is able to understand the formation of structures and their meaning. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="85%" --> </div> </div> <--o--> ## Range of methods to approach structures <div> <div style="width:50%; float:left"> - Field observations: final product of deformation. - Laboratory experiments: useful to observe and understand progressive deformation. - **Numerical modeling: useful to quantify and understand the role and interactions of forces.** Each method has its own limitations but by combining them, the structural geologist is able to understand the formation of structures and their meaning. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="100%" --> *Numerical Sandbox using UWGeodynamics, Romain Beucher* </div> </div> <--o--> ### Numerical Modeling  <!-- .element style="float: right" width="100%" --> <--o--> ## The Workflow of Tectonics and Structural Geology <div> <div style="width:40%; float:left"> **Description and analysis of 3D structures and microstructures (Structural Geology *sensu stricto*)** *Field Observations, mapping, cross-section, 3D block diagrams.* The aim is to understand **finite strain** (i.e. the end product of long, sometimes poly-phase deformation histories), and **incremental strain** (i.e., the small increment of deformation to unravel the stages of deformation.) </div> <div style="width:60%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## The Workflow of Tectonics and Structural Geology <div> <div style="width:40%; float:left"> We would like to understand **strain fields** by mapping features such as foliations, lineations etc. and determine the principal directions of shortening and lengthening. Structural geologists study the **kinematics** of faults and shear zones, and the magnitude of the displacement involved. This can be used to infer the direction of the maximum and minimum **stress directions** at a range of scales. </div> <div style="width:60%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## The Workflow of Tectonics and Structural Geology <div> <div style="width:40%; float:left"> **Design of tectonic models (Tectonics)** Propose a model that explain the deformation history and let to the observed 3D strain and stress fields. Synthesis a broad range of information from other disciplines. - Must obey **physical** laws. - Must be **testable**. - Must be **robust**, explain a wide range of facts. - Must be **simple**. Hypotheses should be kept at a minimum. </div> <div style="width:60%; float:right">  <!-- .element style="float: right" width="100%" --> source: Brune et al, 2017 </div> </div> <--v-->  <!-- .element style="float: right" width="50%" --> *Combining analog "sandbox" experiments with numerical geodynamic modeling we find that inherited rift structures explain the dramatic change in rift style from deep, narrow rift basins north and south of the Turkana area to wide, distributed deformation within the Turkana depression. The failed Cretaceous rift is also responsible for the eastward bend of the Ethiopian rift and the westward bend of the Kenyan rift when entering the Turkana depression, which generated the characteristic hook-shaped form of present-day Lake Turkana.* Brune, S., Corti, G., and Ranalli, G. Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analogue models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression: Tectonics, p. 2017TC004739, doi: 10.1002/2017TC004739. <--o--> ## Methods of Tectonics and Structural Geology ### Data Acquisition: Field Geology  <!-- .element style="float: right" width="50%" --> Reconstructing the 3D architecture requires lots of structural measurements. Structural geologists measure **strikes**, **dip**, **dip direction** of planar features (beeding, cleavage, fault, fold axial surface...), and **plunge**, **plunge direction** of linear features (fold axes, lineations...) When possible, structural geologist can determine the **magnitude** of **strain**, the **stress** orientations. They can also determine the **kinematics**, sense of displacement along faults and shear zones. **Cross-cutting relationships** are used determine the sequence of geological events. <--o--> ## Methods of Tectonics and Structural Geology ### Data Acquisition: Geophysics  <!-- .element style="float: right" width="50%" --> **Geophysics** data such as gravity or magnetic anomalies can reveal information about the buried structures. Spectral images at various wavelengths can be used to map the distribution of minerals etc. *Sixth edition Total Magnetic Intensity (TMI) Anomaly Grid of Australia* <--o--> ## Methods of Tectonics and Structural Geology ### Data Acquisition: Geophysics  <!-- .element style="float: right" width="40%" --> Information about structures at depth can be gathered *directly* via **drilling** and *indirectly* via geophysical method such as gravimetry annd seismic surveys. Gravimetry gives information about the distribution of **densities** whereas seismology reveal variation in **elastic properties**. Active seismic methods include seismic refraction and reflection and involve artificially induces elastic waves (explosive, air canon, vibration truck). Passive seimic uses earthquakes to image the structures and interfaces at depth (e.g., Moho). <--v-->  <!-- .element style="float: right" width="100%" --> Faults and graben structure, offshore Tunisia (Riv Butler, Seismic Atlas https://see-atlas.leeds.ac.uk/) <--o--> ## Methods of Tectonics and Structural Geology ### Data Synthesis and Analysis <div> <div style="width:50%; float:left"> Structural data and analysis are summarized in various documents: - Geological Maps and Cross sections - Tectonic maps and strcutural profiles: Deformation related structures (faults, lineations, shear zones, fold axes etc.) - Structural Sketch maps: simplified geological map in which rock formations are grouped in packages that share the same deformation history. - Block Diagrams: 3D illustrations of structures. </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="90%" --> </div> </div> <--o--> ## What makes a good structural geologist / tectonicist ? </div> <div style="width:50%; float:left"> - The ability to think in 3D and to solve large scale 4D puzzles. - The ability to interact with a large range of geoscientists over a wide range of geological and environmental problems. - The ability to link field studies to computational modelling / theoretical modelling </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="100%" --> </div> </div> <--o--> ## Who needs Structural Geology? **Resource and Mineral exploration** - Structures control the migration, trapping and escape of deep fluids (e.g. hydrocarbons, hydrogen, helium) - Structures control the flow of groundwater, stored CO~2~ - Structural geology is the first stage to any regional geophysical and geochemical surveys aiming at identifying new mineralized provinces. It is also critical for the interpretation of geophysical, geochemical, and geochronological data. At the mine camp scale, structural geology guide the mining process. <--o--> ## Who needs Structural Geology? **Geotechnical Research and Applications** Structural geology / tectonics is at the core of geotechnical site assessment for bridges, dams, tunnels, nuclear reactors, waste disposals etc. Because of the obvious relationship between faults and earthquake, structural geology is that core of earthquake prevention and earthquake seismology. At the engineering-scale, structural geology guides the construction of underground structures and open excavations / slopes. Don't build a dam before understanding the local faults, geological units, and geomorphology. <--o--> ## Who needs Structural Geology? **Research** Structural geology is central to any study of past and present mountain belts and sedimentary basins. No geological, geochemical or geophysical study can be done without the input of structural geology. <--o--> ## Plate Boundaries and Morphology of the landscape  <!-- .element style="float:right; width:30%"--> The Theory of Plate Tectonics describe how Earth's crust is broken into tectonic plates that move about the surface. The interactions between those plates essentially occur at the boundaries. We typically distinguished between: - Convergent boundaries - Divergent boundaries - Transform boundaries Each of these different kind of plate boundaries produces unique features on the surface, including, faults, trenches, volcanoes, mountains, ridges and rift valleys. <--o--> ## Plate Boundaries and Morphology of the landscape **Large-scale structures associated with deformation induced changes to crustal thickness** Mountain Belts: predominantly in context of crustal shortening. <div> <div style="width:50%; float:left"> When two continental plates converge and neither of the buoyant plates is able to subduct beneath the other, this results in a collision zone. Stress translates into vertical and horizontal displacements forming mountain ranges. The Himalayas is an example of a geological feature formed by the collision of the Indian plate with Asia. </div> <div style="width:50%; float:right"> <!-- .element style="float: right" width="90%" --> </div> </div> <--o--> ## Plate Boundaries and Morphology of the landscape **Large-scale structures associated with deformation induced changes to crustal thickness** Sedimentary basins: crustal stretching. <div> <div style="width:50%; float:left"> When divergent boundaries occur in continental plates, a different geological feature, called a rift valley, is formed. These depressions slowly fill with water, forming lakes, as their level drops. Ultimately, they will form the floor of a new ocean. *An example of this type of geological feature is the East African Rift Zone.* </div> <div style="width:50%; float:right">  <!-- .element style="float: right" width="90%" --> </div> </div> <--o--> ## Plate Boundaries and Morphology of the landscape ### Transform boundaries  <!-- .element style="width:30%;float:right; margin-left:50px;" --> **Transform boundaries** are places where plates slip horizontally past one another. At transform boundaries lithosphere is neither created nor destroyed. Many transform boundaries are found on the sea floor, where they connect segments of diverging mid-ocean ridges. *Aerial view of the San Andreas Fault in the Carrizo Plain, Central California. Note the sudden change of course of the stream channel, consistent with 130 meters of right-lateral displacement. (source: Haakon Fossen)* <--o--> Structural Geology deals with structures created during rock deformation, not with primary structures formed by sedimentary or magmatic processes. However, being able to recognize tectonic deformation depends on our knowledge of primary structures... <--o--> Deformation structures depends on the material, its texture and structure. Different materials, such as sandstones, limestones, granites respond differently to deformation and produce different structures.