In an area of superposed folding with dome and basin pattern, late folds have $\text{E} - \text{W}$ vertical axial planes. Which of the following measurements on fold hinges from the area as given below is likely to belong to the early folds?
Plunge amount Plunge Direction
40 270
30 090
10 180
20 000

Question

In an area of superposed folding with dome and basin pattern, late folds have $\text{E} - \text{W}$ vertical axial planes. Which of the following measurements on fold hinges from the area as given below is likely to belong to the early folds?

Plunge amount	Plunge Direction
40	270
30	090
10	180
20	000

Prepp Team · Accepted Answer

Identifying Early Fold Measurements in Superposed Folding

This question concerns identifying measurements related to early folds within an area exhibiting superposed folding that results in a dome and basin pattern. We are given that the late folds have $\text{E} - \text{W}$ vertical axial planes.

Understanding Axial Planes and Fold Axes

An axial plane striking $\text{E} - \text{W}$ and being vertical implies that the fold axis associated with these late folds trends North-South (N-S).
In dome and basin structures, superposition of two fold sets with axes oriented at high angles is typical. If the late folds have N-S axes, the early folds would ideally have axes oriented perpendicular to this, i.e., East-West (E-W).
However, the provided fold hinge measurements (plunge and direction) must be analyzed directly.

Analyzing Fold Hinge Data

The provided measurements for fold hinges are:

Measurement 1: Plunge $40^\circ$, Direction $270^\circ$ (West)
Measurement 2: Plunge $30^\circ$, Direction $090^\circ$ (East)
Measurement 3: Plunge $10^\circ$, Direction $180^\circ$ (South)
Measurement 4: Plunge $20^\circ$, Direction $000^\circ$ (North)

The options group pairs of these measurements:

Option 1: ($40^\circ-270^\circ$; $10^\circ - 180^\circ$)
Option 2: ($30^\circ-090^\circ$; $20^\circ-000^\circ$)
Option 3: ($10^\circ-180^\circ$; $30^\circ - 090^\circ$)
Option 4: ($10^\circ-180^\circ$; $20^\circ-000^\circ$)

Determining Early Fold Characteristics

In the context of superposed folding, structures from the early phase are often characterized by less intense deformation or different orientations compared to later modifications. Observing the plunge values:

The measurements $10^\circ$ (Direction $180^\circ$) and $20^\circ$ (Direction $000^\circ$) represent the shallowest plunges among the given data.
These shallow plunges are often indicative of the earlier fold development. The later folding phases can modify these structures, potentially increasing plunge or changing orientation.
Option 4 combines these two measurements with the shallowest plunges: ($10^\circ-180^\circ$; $20^\circ-000^\circ$).

Therefore, the measurements likely belonging to the early folds are those with the shallowest plunges.