In Lab 7 you dealt with fault constructions that involved the separation of surfaces, but not with fault slip. This is because cut-offs for a single plane surface are inadequate to determine the displacement vector, or slip of a fault. In this lab you will analyse areas where piercing points can be located in both the footwall and hanging wall of a fault, allowing the true slip to be determined.
1.* Map 1 shows a simple map of a level landscape 500 m above sea level, in which a fault offsets a mafic dyke with a strike separation of 450 m. Slickenlines on the fault trend toward the south and have rake of 060°. Determine the slip of the fault.
a) *Plot the fault and the dyke as great circles on a stereographic projection. Also plot the slickenline lineation, and the intersection line, where the fault and dyke intersect. Use a small ‘×’ for the linear data.
b) *Now draw a fault-plane cross-section, viewed from the southwest. This section will show what you would see if you were able to remove the hanging wall block completely, and look directly at the fault face on the footwall. The land surface should be a horizontal line at the top of the section. The cutoff-line of the dyke in the footwall should be shown with its correct rake. Draw a few of the slickenlines, also with their correct rake.
c) *Now mark, using a dashed line, the cutoff of the dyke in the hanging wall (450 m across strike from its position in the footwall). Draw a line between the two cutoffs, parallel to the slickenline orientation. This is the net slip. Measure the distance of net slip and determine its trend and plunge from the stereographic projection. Say whether the dip-slip component is normal or reverse. Say whether the strike-slip component is sinistral or dextral.
2. Map 2 shows an area in which a thick sequence of conglomerate (circles) unconformably overlies mudstone (white) and coal (black). The area is cut by a major fault running NE-SW approximately. To the east of the fault the coal is not exposed, but three boreholes at A, B, and D encountered the coal at depth. Borehole C did not encounter coal. Your objective is to determine the subsurface structure, so as to show the extent of coal in the subsurface. In addition, you are to define the slip of the fault, using the subcrop of the coal below the unconformity.
|Borehole||Elevation of Ground||Depth to Unconformity||Depth to Coal|
a) Fault plane
i) Draw structure contours on the fault surface itself. Extrapolate contours down to sea level (0 m).
ii) Determine the strike and dip of the fault.
b) West of the fault the coal is exposed
i) Identify the unconformity by labelling it on the map.
ii) What is its orientation?
iii) Draw structure contours on the coal seam (assume that the coal seam is thin relative to the scale of the map, and therefore treat it as a single surface). Terminate the contours where they intersect the fault.
iv) What is the orientation of the coal?
v) Mark the subcrop line on the map where the unconformity surface cuts the coal seam.
c) East of the fault the coal is not visible
i) Identify the unconformity by labelling it on the map.
ii) What is the orientation of the unconformity?
iii) The borehole data allow the elevation of the coal seam to be calculated at three points (A, B & D). Use this information to construct structure contours on the coal seam.
iv) From your contours, determine the strike and dip of the coal east of the fault.
v) Mark the subcrop line where the unconformity surface cuts the coal seam.
d) Separation: Next, investigate the separation of the surfaces that are cut by the fault.
i) Using intersecting contours, mark and label the cutoff lines where the fault intersects the coal seam in the footwall and in the hanging wall. Between these two lines is a region where a drillhole would encounter no coal.
ii) Find the difference in elevation between the unconformity in the west and the same unconformity in the east. This is the vertical separation of the unconformity at the fault – the vertical distance between a surface and its projected counterpart from the other side of the fault. Because the surface is horizontal, it is also the throw (vertical distance between the hanging wall and footwall cutoffs, measured down the dip of the fault plane).
iii) Find the difference in elevation for the coal seam on either side of the fault (the vertical separation). (Note: Because the coal seam is gently dipping, its separation measurements – vertical separation, throw, and heave are all slightly different from the corresponding measurements made on the unconformity, which is horizontal.)
iv) Find the distance measured along the strike of the fault, between equivalent structure contours on the coal seam on either side of the fault. For example you might find the point where the east side 300 contour hits the fault plane, and the point where the 300 contour on the west side hits the fault plane. This is the strike separation of the fault. It corresponds to the distance that would appear between the two halves of the coal seam if the land were eroded down to a horizontal surface.
v) Measure the heave of the fault at the coal seam, the width of the zone (measured perpendicular to the strike of the fault) between the two cutoff lines.
vi) Superimpose a sheet of tracing paper on the map; on it, shade the area where the coal seam is present below ground level. (Hint: To do this, the best argument is as follows. Originally the coal was present under the whole area. Since deposition, coal has been ‘removed’ in three ways. First, some coal was removed by erosion prior to deposition of the conglomerate, at the unconformity surface; second, coal is missing from the fault heave between the two cutoff lines; third, more coal has been removed by erosion to the present-day topography. Once you have removed all these areas, whatever is left is the area underlain by coal.)
e) Fault slip: So far, all the measurements you have made concern the separation of various surfaces. Although they would be very useful to an exploration project seeking the location of the coal seam, they do not tell us the actual movement on the fault, the net slip. To determine the net slip, it is necessary to find a unique line which has been offset. There is an identifiable line that has been severed by the fault. This is the line where the unconformity intersects the coal seam, the subcrop of the coal seam. This line has been split into two parts by the fault, creating piercing points on both walls. It can be used to pin down the true slip.
i) Set up a fault-plane cross-section.
- Use a piece of graph paper to make a fault plane cross-section, using the sea-level contour as a folding line. The trace of the fault on the map is somewhat longer than a normal letter-size sheet of tracing paper, but you will be able to show all the important intersections on a cross-section 25 cm (6.25 km) long, starting at the southern boundary of the map. First, find the point where the sea-level (0 m) contour on the fault plane intersects the south border of the map. Call this point X. Measure 25 cm NE along the contour and mark point Y. Now take a sheet of graph paper and draw a horizontal line 25 cm long, about 5 cm up from the base of the paper, and mark the ends X and Y.
- Next calculate the spacing of contours on the fault plane. If the fault has dip d, the contour spacing on the section will be: i’ = i / sin(d)
- ii) Add the footwall and hanging wall intersections.
- Place your graph paper with its top edge along the line XY on the map, and mark the places where the footwall and hanging wall cutoff lines have known elevation. Project these points to their correct elevations on the fault-plane section.
- Complete the cross-section by drawing the hanging wall and footwall traces of the unconformity and the coal. Use solid lines to represent the footwall cutoff and dashed lines to represent the hanging wall cutoffs.
iii) Determine the fault slip:
- Identify the piercing points where the subcrop line intersects the fault, in both the hanging wall and footwall. Join the two points with a dash-dot line representing the net slip.
- Measure and record the distance of net slip.
- The component of net slip parallel to the fault strike is the strike slip. Draw and label the strike slip on the fault-plane section, and record the distance of strike slip.
- The component down the dip of the fault is the dip slip. Draw the dip slip, and label it. Record the amount of dip slip.
- Characterize the fault in words overall, as mainly strike-slip, intermediate (oblique-slip), or dip-slip. Say whether the strike-slip component is sinistral or dextral. Say whether the dip-slip component is normal or reverse.
f) Check the results on a stereographic projection
i) Find the rake of the net slip in the fault plane, by measuring it with a protractor on the fault-plane section.
ii) Plot the fault plane, the orientation of the coal, the cutoff lines, and the net slip on a stereographic projection, and determine the plunge and trend of the net slip.
Answer table for fault measurements, Question 2
|Strike and dip of the fault:|
|Orientation of the unconformity west of the fault:|
|Orientation of the coal seam west of the fault:|
|Orientation of the unconformity east of the fault:|
|Orientation of the coal seam east of the fault:|
|Vertical separation of the unconformity at the fault:|
|Vertical separation of the coal seam at the fault:|
|Strike separation of the coal seam at the fault:|
|Heave of the fault at the coal seam:|
|Characterize the fault slip: |