Geologic Maps & Structural Geology

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Slides

In this chapter the slides sets are more important than they usually are:

• (1) Motivation and Introduction to Faults: slides.

• (1.1) Large fault system example 1: The Basin and Range slides.

• Basin & Range: Volcanoes. An animated video about Basin and Range formation. Think back to your reading on volcanoes, melting processes, and chemical differentiation. How does that affect volcanic styles (explosive vs effusive)?

  • Deformation in the Basin and Range: IRIS Earthquake Sci on Twitter

    • Q: Where is most of the stretching, or strain, happening in the western US? A: In the Basin and Range Province, although there is only a very small amount of stretching currently happening. Scientists think that most of the stretching happened from 30 to 5 million years ago ... and that most of the current strain is occurring at the western and eastern edges of the Basin and Range Province. The greatest motion is on the western edge and appears to be shearing (side to side motion) more than stretching. How would we test this statement?

• (1.2) Large fault system example 2: The San Andreas Fault slides.

• (2) Orientation of geologic structures, folds, and geologic maps. slides.

• (2.1) Cross sections from geologic maps. slides.

Things to keep in mind

• Thinking three-dimensionally

• Fault types, and recognizing them. Here is a short "worksheet" you can use to think about fault types.

• Folds, anticlines, synclines. Relative ages of rocks exposed in a fold.

• Strike and dip symbols.

• Cross sections & geologic history

For fun - some historic aspects of maps

Maps are just neat, so enjoy this 1887 geologic map. Try to zoom in to see the rock units that are labeled and their estimated ages. An 1861 geologic map of Friuli, Italy. Also, and early geologic map in Ireland.

This is just a fun image to see how topographic maps were historically inked by hand: USGS on Twitter: "Just another day on the job (in 1952) - Inking contours on a topographic map

An informative history: how did the mountains form, anyway?

Please read this article, which gives an interesting historical account of ideas related to mountain building prior to Plate Tectonics. It's a fairly long read, but non-technical. Keep in mind the difference between classifying things vs. having a predictive model for how processes happen. An analogy might be classifying the wide variety of birds by color, wing shape, etc., vs. understanding the evolutionary processes that led to the observed diversity. What does plate tectonics predict about mountains (and other geologic processes)? More on this in CH 19.

Visualization: a classic "plunging" anticline

This is Sheep Mountain Anticline in Wyoming.

Note all the colors in the various sedimentary beds! Beautiful. An even more colorful photo (I don't know if the photographer has enhanced anything or not, but it does at least show the division between distinct sedimentary layers.)

Here is a gigpan of the Sheep Mountain Anticline (high resolution imagery you can scroll through). Very cool.

Here is a recent paper on the details of the Sheep Mountain anticline. Just take a look at Page 4, there are some neat pictures delineating some of the sedimentary formation names.

Finally, here you can read about the incredible variety of fossils found in this part of Wyoming.

A good question to ask yourself is "do I understand how a plunging anticline is formed?"

A video tour of the San Andreas Fault in the San Francisco Bay Area

Fault Images

What type of fault is this? (Normal, Reverse, Right-lateral strike-slip RLSS, Left-lateral strike-slip LLSS)

What type of fault is this? (Normal, Reverse, Right-lateral strike-slip RLSS, Left-lateral strike-slip LLSS)

The San Andreas Fault (SAF)

Extent of SAF

The San Andreas Fault is ~ 1,300 km long extending through most of California. In the south it meets a spreading center in the Gulf of California, and then meets the Pacific and Juan de Fuca plates just offshore in northern part of the state. The image below indicates the locations of various cities relative to the fault, the two largest known historic earthquakes, and some more recent events along the San Andrea Fault. Of course, earthquakes occur in other parts of the state, too.

SAF in the Carrizo Plain Area

The San Andreas Fault is perhaps best exposed near us, in the Carrizo Plain National Monument - only about a 90 minute drive from campus. Wallace Creek is probably the most famous site along the whole fault. Here are some resources:

• San Andreas Fault at Wallace Creek

• Wallace Creek Trail Guide

• Wallace Creek Interpretive Trail

SAF Lidar File

Load into Google Earth -- FUN!!! amazing detail (compare with/without) SAF Carrizo Plain area Lidar File

SAF GPS data in central California

I gave you this handout in class; here it is as an image:

and as a file: SAF GPS data in central California

Regional Fault Systems

The Los Angeles Basin has a complex set of faults (and this image is simplified!!!)

Subduction Zones are Faults

Subduction zones are reverse faults. Subduction zones represent the largest faults (think surface area of the fault plane surface) and therefore can host the largest earthquakes.

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You can now imagine that the Pacific Northwest region of the United States has a larger maximal earthquake magnitude potential (seismic hazard) than even the San Andreas Fault:

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