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Introduction: Techniques in Structural Geology
Ch. 1: p.4-‐
1. An Exercise in Perception: - Folds at Lulworth Cove, S. England. 2. An Exercise in Perception: - Wasatch fault near Salt Lake City, Utah. 3. An Exercise in Perception: - Pressure solution seam in limestone, Appalachian Mountains. 4. What was your Strategy?: In attempting to complete this exercise, what approach did you take?
Did you simply describe what you saw? Did you identify or name the type of feature? Did you attempt to interpret the features you observed? Did you infer something about the geologic history?
All of these approaches are possible and important when considering structural geology.
5. A Stepwise Approach: As we proceed with this course, you will learn how to adopt a stepwise approach to structural analysis, consisting of:
Nomenclature (DESCRIPTION) – based on field observations and remote sensing -‐ it is important to know the types of features, but this is not enough!
Processes (THEORY/EXPERIMENT) – based on theoretical and laboratory experiments -‐ we need to understand what processes control the different types of deformation we observe.
Models (INTERPRETATION) – based on analytical and numerical models -‐ ultimately, we need to place what we observe in the context of how we think things work.
6. Structural Geology vs. Tectonics: Structural geology and tectonics are closely intertwined. However, it is not possible to cover both topics in one course. We will only touch on the topic of tectonics where it pertains to understanding the structure.
Structural geology: Concerned with understanding the motions of/within the lithosphere in response to plate tectonics or other driving mechanisms, and the features that result from the breakage or flow of rocks during the process (i.e., deformation).
SUBMICROSCOPIC TO REGIONAL SCALE
Tectonics: Considers the underlying causes of plate tectonics and the regional patterns of deformation so-‐produced throughout Earth history.
REGIONAL TO GLOBAL SCALE
7. Structural Geology: As with most geologic processes, rock deformation is incredibly slow. Except in laboratory experiments, we rarely see deformation happening fast enough to see changes happening in our lifetimes (or even throughout all of recorded history). Notable exceptions include earthquakes and volcanic eruptions. 8. Earthquakes: We see rocks being deformed in response to these rapid events (e.g., Dixie Valley (Nevada, USA), 1954 – fault scarp from M6.8 earthquake). 9. Earthquakes: e.g., East African Rift Valley (Ethiopia), 2006 –earthquake scarp. 10. Volcanic Eruptions: Eruptions often occur through giant cracks (fissure eruptions) that appear during the eruptive period (e.g., Mauna Loa, Hawaii). 11. Volcanic Eruptions: e.g., Kilauea, Hawaii; Krafla, Iceland; Northern Volcanic Zone, Iceland. 12. Geodetic Measurement of Deformation: We can also measure deformation happening in real-‐time using geodetic techniques such as Global Positioning System (GPS) tools and Interferometric Satellite Aperture Radar (InSAR), both of which measure motions of the surface at the millimeter scale. [Fig. 1.5. GPS velocities in India and the Tibetan Plateau (relative to Europe) and resultant strain rates] 13. GPS Velocities: e.g., GPS velocities in the Snake River Plain and surrounding regions relative to stable North America (Payne et al., 2008). 14. InSAR: Measures changes in range from a satellite to a point on the surface using phase changes in reflected radar beams (e.g., InSAR pattern after the 1992 Landers earthquake, Mojave desert, California). 15. InSAR: e.g., Evidence of magma chamber inflation below Three Sisters, Oregon (USGS) from 1996-‐2000. The number of InSAR rings suggests 10 cm of central uplift due to an inflating magma chamber at a depth of 7 km ( mi). 16. Rates of Deformation: Although some deformation is measurable in real-‐time, the strain rates are still very small (mm/yr). Rocks can be pervasively deformed, so a great amount of time is needed to account for the deformation.
Still, significant deformation can occur over relatively short periods of geologic time. e.g., 2.5 mm/yr rate of uplift can make a 2 km (1.25 mi / 6562 ft) high mountain in only 1 million years (0.02% of all Earth history).
e.g., 3.5 cm/yr slip rate of the San Andreas fault amounts to ~1000 km of motion in its 29 m.y. history (this is the distance from Seattle to San Francisco!).
24. Remote Sensing: Satellite imagery can provide a regional context, but can also resolve details as small as a few meters across, which is particularly useful for observations on other planets.
This data is often rendered in a spatially-‐rectified mapping environment such as a GIS (Geographical Information System). Google Earth® is one example of a broad application of this technique. [Fig. 1.4. Satellite imagery (a and c) and aerial photograph (b) of the Canyonlands region, Utah]
25. Remote Sensing: Satellite imagery on Mars, taken by the Mars Reconnaissance Orbiter spacecraft, can resolve features as small as a compact car (e.g., Mars HiRISE image of Cerberus Fossae (25 cm/pix)). 26. Remote Sensing: Digital elevation models (DEMs) are produced through laser or radar reflection from a satellite or spacecraft. Although they only provide a digital rendering of surface topography, they can be used to make observations about structures (e.g., DEM of the Hat Creek fault, California. The fault can be mapped directly off the DEM). 27. Remote Sensing: e.g., DEMs of the Moon using data from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter spacecraft. The close-‐up of Mare Orientale can be used to identify ring faults around the impact crater. 28. Remote Sensing: e.g., DEM and fault interpretation at Alba Patera, Mars (courtesy of Ernst Hauber). 29. Remote Sensing: Light Detection and Ranging(LiDAR) data can be collected from an aircraft or from the ground and uses laser reflection technology to build a 3D rendering of any surface (e.g., an outcrop).
e.g., Earthscope (NSF) LiDAR images of active fault scarps. Top: the Wasatch fault near Nephi, Utah. Below: a fault cutting glacial moraines at the base of the Grand Tetons, Wyoming.
30. Remote Sensing: LiDAR data can be superposed on photographs to build a 3D model of an outcrop. [Fig. 1.3. LiDAR data from Svalbard, Norway, superposed on outcrop photos (should be viewed with 3D visualization tools)] 31. Geophysical Imaging: Observations of structures in the subsurface (vital for building a 3D picture of deformation) must rely on geophysical imaging techniques such as seismic reflection, seismic refraction, and ground penetrating radar.
Most seismic data has been collected in the pursuit of oil and gas reserves. Both 2D and 3D datasets exist. [Box 1.1. Offshore seismic reflection data acquisition]
32. Geophysical Imaging: [Fig. 1.6. Offshore Brazil 2D seismic reflection line clearly showing important structures like faults and salt diapirs. Vertical axis scale is in two-‐way travel time] 33. Geophysical Imaging: e.g., Below: faults interpreted from seismic reflection data in the Wytch Farm oilfield, English Channel. Right: time slice data shows fault locations. Caveat: cannot see offsets smaller than ~12 m.
34. Geophysical Imaging: Subsurface imaging can also be achieved using gravimetric data and magnetic data. These techniques rely on the different density and magnetic properties of different rock types. They are good for identifying structures buried below recent surface deposits. [Fig. 1.9. Gravimetric (left) and magnetic (right) data for the state of Minnesota, revealing bed rock structures covered by glacial deposits] 35. A Stepwise Approach: Let us return to how we began this lecture with our stepwise approach:
Nomenclature (DESCRIPTION) – based on field observations and remote sensing -‐ it is important to know the types of features, but this is not enough!
Processes (THEORY/EXPERIMENT) – based on theoretical and laboratory experiments -‐ we need to understand what processes control the different types of deformation we observe.
Models (INTERPRETATION) – based on analytical and numerical models -‐ ultimately, we need to place what we observe in the context of how we think things work.
36. Laboratory Experiments: Structural geologists have a long history of trying to replicate deformation using laboratory experiments. Although they help us understand processes, problems exist with scaling, strain rates (much higher than in nature), and deformation conditions (depth and temperature).
e.g., A. Sandbox models of extensional faulting (courtesy, NAGT). B. Sandbox models of strike-‐slip fault deformation (McClay and Bonora, 2001).
37. A Stepwise Approach: Our final technique we may utilize in structural geology is modeling:
Nomenclature (DESCRIPTION) – based on field observations and remote sensing -‐ it is important to know the types of features, but this is not enough!
Processes (THEORY/EXPERIMENT) – based on theoretical and laboratory experiments -‐ we need to understand what processes control the different types of deformation we observe.
Models (INTERPRETATION) – based on analytical and numerical models -‐ ultimately, we need to place what we observe in the context of how we think things work.
38. Numerical Modeling: Numerical modeling utilizes the underlying physical laws by which deformation proceeds to develop predictive models that may explain observed deformation.
e.g., A. 3D model of faults in the Los Angeles Basin for use in a numerical model of fault slip (courtesy, Michele Cooke). B. Numerical models of surface deformation caused by dike intrusion (left) and slip on a fault (right).
