Structural Geology – Classifying and Recognizing Folds and Faults
I. INTRODUCTION & PURPOSE
Structural geology is the study of how geologic rock units are initially arranged and later deformed. Changing spatial relations between geologic units and the stress and strain that occur during deformation events are key aspects in understanding geologic structures. The purpose of this lab is to both learn and apply the concepts of structural geology to reading and interpreting geologic structures, including tilted beds, folds, and faults. The terms and concepts of geologic structures, the application of structural geology to mountain building events, and the techniques used to interpret geologic structures will be presented and discussed. The three types of graphic representations of geologic structures: 1) geologic maps, 2) geologic cross-sections, and 3) block diagrams will also be highlighted and discussed.
The purpose of this laboratory is to become successful at applying the principles of structural geology for both, interpreting surface and subsurface structural and geologic relations, stress and strain regimes, and solving structural problems, concerning geographic regions that expose a rock record of igneous, metamorphic, and sedimentary events, folding and faulting, and surface erosion.
II. MEASURING THE ATTITUDE OF ROCK UNITS
Attitude is the spatial orientation of planar rock structures. Two aspects of attitude are needed to constrain a rock unit or surface orientation in three- dimensional space: 1) Strike and 2) Dip. Strike is the compass bearing of a line formed by the intersection of a horizontal plane and the (inclined) plane of the layered rock feature. Strike can be expressed as either a quadrant or an azimuth bearing. Dip is the angle between the horizontal plane and the planar rock unit or feature. Dip direction is always down the inclined plane and is perpendicular to the strike. Strike and dip are drawn on geologic maps as a “T-like” symbol – the long segment is the strike; the short segment the dip. A number nest to the short segment represents the dip angle. Geologists measure attitude with a compass (strike) and an inclinometer (dip).
A. Geologic Map Symbols
Geologic symbols are used on geology maps to indicate one or more characteristics of the rock formation at the point on the map that they (the symbols) are placed. Some commonly used map symbols are found in the information pages on Canvas (you will refer to these symbols for interpreting and making geologic maps, cross-sections, and block diagrams). Map symbols indicate 1) attitude (e.g. strike and dip of either, bedding or foliation), 2) formation contacts, 3) fault lines (rock type, location, and planar orientation), 4) fold axes (type, location, and their limb orientations), and 5) rock formation information (type, name, and age). You will need to be able to recognize and interpret these symbols while working on geologic maps and diagrams.
B. Major Types of Geologic Structures
Mapable rock units are called formations. Locations where rock formations are exposed at the earth’s surface are called outcrops. Undisturbed rock formations such as sedimentary beds and lava flows are typically horizontal and planar in spatial orientation. However, shifting tectonic plates produce a variety of stresses in the crust that will, over time, cause crustal deformation such as uplift, tilting, erosion, faulting, and folding of formations. Faults and folds exposed at the earth’s surface in outcrops have unique structural characteristics that can be recorded, mapped, identified, categorized, and analyzed. Carefully study the major structural features listed and described in the information pages on Canvas. You will get to analyze these structures in Part II. Three-dimensional visualization of folds and faults are found on the web link at http://reynolds.asu.edu/blocks/menulist.htm. At this web site, you to be able to interactively manipulate the fold and fault blocks.
C. Structure Vocabulary – Define these structural terms below
- Outcrop –
- Formation –
- Strike –
- Dip –
- Contact –
- Anticlines –
- Synclines –
- (Fold) Plunge –
- Footwall –
- Hanging wall –
- Normal Fault –
- Reverse Fault –
- Thrust Fault –
- Strike-Slip Fault –
- Slickensides –
III. 3D GEOLOGIC BLOCK DIAGRAMS AND MAPS
Three-dimensional geologic block diagrams are scaled-down, abstract, simple representations, or models of Earth’s crustal rock structures, which include 1) formations, 2) unconformities, 3) faults, 4) folds, and 5) topography. Block diagrams are a 3-dimensional composite of both, a geologic map (horizontal map-view) and geologic cross-sections (vertical side-views). The key to successfully completing the block diagrams lies in visualizing the 2-D representations as 3-D structure.
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