Plotting an axial plane in a Stereonet is a fundamental skill in structural geology, crucial for understanding fold geometry and kinematics. The axial plane is an imaginary surface that bisects the angle between the fold limbs and contains the fold axis, defining the overall orientation of the fold.
What You'll Need
Before you begin, gather these essential tools:
- Stereonet: A transparent hemisphere projection, typically a Wulff or Schmidt net.
- Tracing Paper: A sheet of tracing paper that fits over the stereonet.
- Drawing Utensils: Pencils (different colors can be helpful), eraser, ruler.
- Protractor: For precise angle measurements if not using the stereonet's grid directly.
- Field Data: Measured strike and dip (or trend and plunge) of the fold limbs.
Understanding the Axial Plane in Stereonet Analysis
The axial plane represents the symmetry of a fold. Its orientation (strike and dip) is key to deciphering the deformation history of an area. In a stereonet, planes are represented by great circles, and lines (like the fold axis) are represented by points.
Step-by-Step Guide to Plotting an Axial Plane
The process involves plotting the fold limbs, finding the fold axis, and then geometrically constructing the axial plane.
1. Plotting the Fold Limbs
Begin by accurately plotting the orientation of the two limbs of the fold on your tracing paper.
- Mark North: Align the tracing paper over the stereonet and mark the North point on the primitive circle (the outer edge).
- Plot Great Circles: For each fold limb, rotate the tracing paper so that its strike aligns with the N/S line of the stereonet. Count inwards from the primitive circle along the E/W line by the dip angle and mark a point. Rotate the tracing paper back, then draw the great circle that passes through this point and aligns with the strike on the primitive circle. Label these great circles, for example, 'Limb 1' and 'Limb 2'.
2. Locating the Fold Axis
The fold axis is the line of intersection between the two fold limbs.
- Identify Intersection: The point where the great circles for Limb 1 and Limb 2 intersect is the fold axis. Mark this point clearly, perhaps with a different symbol or color.
3. Determining Limb Poles
Poles to planes are points 90 degrees away from their respective great circles, perpendicular to the plane.
- Plot Poles: For each limb, rotate its great circle to align with a N/S great circle on the stereonet. Count 90 degrees along the E/W line from the great circle and mark the pole. Repeat for both limbs. Label these as 'Pole to Limb 1' and 'Pole to Limb 2'.
4. Constructing the Plane Connecting Poles
Draw a great circle that connects the two limb poles you just plotted.
- Draw Great Circle: Rotate the tracing paper so the two limb poles lie on the same great circle of the stereonet. Draw this great circle. This plane represents the plane that contains the normals to the two fold limbs.
5. Drawing the Axial Plane
The axial plane has a specific geometric relationship to the fold axis and the limb poles.
- Define Axial Plane: The axial plane is the great circle that passes through the fold axis (from Step 2) and is perpendicular to the great circle connecting the limb poles (from Step 4).
- Find Perpendicular: The pole to the great circle connecting the limb poles will lie on the axial plane. Alternatively, if you have drawn the great circle connecting the limb poles, the axial plane's great circle will be 90 degrees away from it, passing through the fold axis.
Determining the Orientation of the Plotted Axial Plane
Once the great circle representing the axial plane is drawn on your tracing paper, you can determine its precise strike and dip using the following steps:
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Aligning the Axial Plane: Carefully rotate the tracing paper so that the great circle representing the axial plane lines up along with the N/S great circle of the underlying stereonet. This means the great circle on your tracing paper should perfectly overlap one of the N/S great circles on the stereonet.
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Measuring the Dip: While the axial plane's great circle is aligned with a N/S great circle, count the number of degrees from the primitive circle (the outermost circle) inwards along the E/W line to the point where your axial plane's great circle intersects this line. This count directly gives you the dip angle of the axial plane. Note the direction (East or West) of the dip.
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Reading the Strike: After measuring the dip, rotate the tracing paper back to its original position (where North on your tracing paper aligns with North on the stereonet). Read the bearing on the primitive circle where the great circle of the axial plane intersects it on the western side. This gives you the strike of the axial plane.
Example: Visualizing an Axial Plane
Imagine a fold with Limb 1 striking 045° and dipping 60° NW, and Limb 2 striking 225° and dipping 60° SE.
- Plotting these two limbs will show their great circles intersecting at a point. This point is your fold axis.
- Plot the poles to these limbs.
- Draw a great circle connecting these two poles.
- The axial plane will be the great circle that passes through the fold axis and is perpendicular to the great circle connecting the limb poles.
- Using the steps above, you can then measure its strike and dip, which in an ideal symmetrical fold like this, would typically strike 000°/180° and dip vertically (90°).
Practical Tips for Accuracy
- Use a Sharp Pencil: Precision is key in stereonet plotting.
- Label Clearly: Distinguish between limbs, poles, fold axis, and the axial plane.
- Double-Check: Re-plot if results seem inconsistent.
- Digital Tools: Consider using digital stereonet software (e.g., Stereonet by Rick Allmendinger) for complex analyses or verification, though manual plotting is fundamental.
Benefits of Stereonet Analysis for Axial Planes
- Geometric Interpretation: Visually understand the 3D orientation of folds.
- Kinematic Analysis: Infer stress fields and deformation mechanisms.
- Structural Context: Relate folds to other structural features like faults and cleavages.
- Predictive Mapping: Forecast the attitude of structures in unmapped areas.
By following these steps, you can accurately plot and determine the orientation of an axial plane, gaining valuable insights into the geometry of folded structures. For further information on stereonet fundamentals, refer to educational resources from reputable geological departments such as the University of Arizona.
| Component | Representation on Stereonet | Key Role |
|---|---|---|
| Fold Limb | Great Circle | Represents the attitude of a planar surface. |
| Fold Axis | Point (Line) | Intersection of fold limbs; axis of rotation. |
| Limb Pole | Point (Line) | Normal to a fold limb; used for geometric constructions. |
| Axial Plane | Great Circle | Bisects fold limbs; contains fold axis. |
