Hinge lines from stereonets 2 - pole girdles.

Summary

This video explains how to analyze fold geometry using stereo nets, focusing on understanding fold hinges and profile planes. It builds on the previous video by examining poles to bedding, which offers more flexibility in analyzing various fold structures. The video demonstrates how to find the best-fit great circle through poles, identifying the hinge line and profile plane. It also explores non-cylindrical folds, where hinge lines curve or change trends, and suggests using multiple transects across the fold to analyze complex geometries. This technique is fundamental in regional fold belt analyses.

Takeaways

• 🔍 Stereo nets are used to understand fold geometry by analyzing limb geometries and hinge lines.
• 📐 Great circles are used to represent fold limbs, and their intersection helps find the hinge line of the fold.
• 🧭 Poles to bedding provide flexibility in analyzing different types of folds, particularly when dealing with curved or arched limbs.
• 🌍 The hinge line can be found by analyzing the poles to a folded layer and fitting a great circle to these poles.
• 📝 In a stereographic projection, poles to bedding are plotted, and a best-fit great circle helps identify the fold's profile plane and hinge line.
• 📊 The plunge and bearing of the hinge line can be measured directly from the stereonet based on the plotted great circles.
• 🔀 Fold limbs can be distinguished by analyzing clusters of poles on a stereonet, leading to a clearer understanding of fold geometry.
• 📏 The intersection of great circles from different limbs marks the hinge line, and the angle between the limbs (interlimb angle) can also be measured.
• 🔄 Cylindrical folds have a consistent hinge line orientation, but non-cylindrical folds exhibit varying hinge lines and profile planes.
• 🌀 Curvilinear folds can have changes in both hinge line plunge and trend, and different profiles across the fold can help analyze such variations systematically.

Q & A

What is the main focus of the video?

The video focuses on using stereo nets to understand fold geometry, particularly in the context of analyzing fold limbs and hinge lines through stereographic projections.

What is the role of great circles in fold analysis?

In fold analysis, great circles are used to represent the orientation of limbs. The intersection of these great circles helps identify the hinge line of the fold.

How does using poles to planes provide more flexibility in fold analysis?

Poles to planes offer more flexibility because they allow for the analysis of more complex folds where limbs gradually curve, unlike simple angular folds. This technique makes it easier to analyze broader hinge areas and varying fold geometries.

What is the significance of the hinge line in fold geometry?

The hinge line represents the line of maximum curvature within a fold. It plays a critical role in defining the fold's geometry, including the orientation and profile of the fold.

How do you identify the profile plane using poles?

To identify the profile plane, poles to bedding are plotted, and a best-fit great circle, or girdle, is found through these poles. The hinge line of the fold is then derived from the pole to this profile plane.

What is the relationship between the hinge line and profile plane?

The hinge line lies perpendicular to the profile plane. In stereographic projection, the hinge line can be found by determining the pole to the profile plane, which represents the intersection of the fold limbs.

How is a cylindrical fold represented in stereographic projection?

In a cylindrical fold, there is a single hinge line and a single profile plane. The hinge line appears as a fixed point, and the profile plane remains consistent across the fold, producing a clear, simple pattern on the stereogram.

What are the two end-member types of curvilinear fold hinges?

The two types are: 1) folds with varying plunge amounts but constant plunge directions, forming a saddle-shaped hinge line, and 2) folds where the hinge line changes trend but maintains a constant plunge, creating a fanned-out profile plane pattern.

What is the approach to analyzing non-cylindrical folds with variable hinge lines?

For non-cylindrical folds, transects are made across the fold, each providing a hinge line and profile plane. By systematically plotting each transect on a stereo net, the variation in fold geometry along the hinge line can be assessed.

Why are poles and girdles important tools in regional fold analysis?

Poles and girdles help visualize and quantify the variation in fold geometry. By plotting poles to bedding and finding the best-fit girdle, geologists can determine the fold's hinge line and profile plane, making these tools essential for understanding fold structures in regional analyses.