Interpreting Map and Cross Sections In Normal Faulting Systems

Lab 7: Interpreting map and cross sections in normal faulting systems

For this lab, we will practice identifying features characteristic of normal faults in map view, 3D view, and in cross sections, and practice quantitatively relating normal fault geometries with the resulting folding that occurs due to displacement on these faults. For this exercise, you will accumulate all of your images in a Powerpoint file, which you will submit, along with this document, on D2L.

Part 1: Map patterns in normal faulting systems

First, we will explore map patterns of normal faults in the Central Oregon Fault Zone. In this region, Quaternary normal faulting has offset basalts, resulting in a beautiful expression of the fault geometries at the surface in the area (similar to the examples we looked at in class in Bishop California and in Iceland). We will focus on faults in the northern half of Lake County, Oregon (in the vicinity of the kml pins provided). Open these pins in Google Earth, and zoom in and rotate around to get a good feel for the normal faults expressed in this area.

Q1.1. Identify a hard-linkage between two previously-unconnected fault segments. Insert images into your Powerpoint in map-view, perspective cross section view, annotating what you seen in the images allowing you to identify this feature. Are the faults dipping toward the east or west? Put the correct map symbol on your normal fault accordingly. Make sure you include a scale bar and north arrow in your images.

Q1.2. Repeat this exercise with a soft-linkage between faults.

Part 2: Extensional fault-bend folds—construction methods

As we discussed in lecture, slip over non-planar faults requires folding of the overlying rocks. In extensional environments, this folding occurs as distributed shear, oriented parallel to the Coulomb failure orientation (or, ~65o dip relative to the earth’s surface); this kinematic modeling approach (after Xiao and Suppe, 1992) provides a predictive way of constructing a complete cross section from limited data that preserves cross sectional area before, during, and after deformation, and so is “area balanced,” and results in strain within the rocks that is sensible. These kinds of approaches are used extensively in published studies and in industry to create balanced cross sections and have been shown to be useful for approximating the first-order geometry of map-scale geologic structures.

In this method, the “active” axial surface (green dashed, below) is pinned to the bend in the fault, where rocks are progressively incorporated into the fold limb, and the “passive” axial surface (red dashed, below), represents the location in the rock mass that was originally positioned at the fault bend. The distance between the two, therefore, reflects how much slip has been on that segment of the fault.

Given a shear orientation (we’ll assume 65o here), it is possible to quantitatively predict the relationship between a change in fault dip orientation and the resulting dip of a fold limb that would result using trigonometry. This can be calculated, or, even easier—we can construct the solution using the “folding vector” concept, which is just manually constructing the requisite triangles.

Note that axial surfaces are parallel to each other in the pre-growth horizons, but note that the passive axial surface converges toward the active on in the growth, or syn-tectonic horizons, forming a “growth triangle” that represent the deposition of sediments while the structure is growing.

Q2.1. Construct a concave extensional fault bend fold on the slide in your Powerpoint using the concepts described above. Blue horizons represent “pre-growth” and orange horizons represent “growth”.

Q2.2. Complete the same exersie for a convex fault bend (the only difference will be that the axial surface will now dip 65 degrees, but “synthetic” to the main fault).

Q2.3. How much extension is accommodated on this model? Is the the same or different for each?

Part 3: Seismic reflection interpretation

Now we will use the concepts we learned during lecture and the methods we practiced in part 2 to interpret a seismic reflection profile from the North Aman Trough, Sumatra, Indonesia.

You can see that several stratigraphic horizons have been encountered in petroleum wells drilled in the area: the basement (bst), early Oligocene (O2), late Oligocene (O1), early Miocene (M3), middle Miocene (M2) and late Miocene (M1). (The black vertical line on the left side of the image represents a seam between two different seismic profiles)

Also notice how some portions of the seismic reflection data feature well-imaged, continuous reflectors, while other areas show discontinuous, irregular reflectivity (below).

Coherent, continuous reflectors indicative Discontinous, incoherent seismic character
of stratigraphic layers common of basement rocks

Q3A.1. In which direction do sedimentary layers thicken (toward the West/left, or toward the East/right)?

Q3A.2. Does the thickening sedimentary package stop abruptly at some point? If so, not the approximate location of this truncation for as many horizons as you can. What are some reasons that this is difficult to do precisely?

Q3A.3. Is this stratigraphic pattern suggestive of a particular type of fault structure? If so, what type (normal, thrust, strike slip, half-graben, horst-and-graben, etc.)?

Q3A.4. If the truncation you identified in 3.2 is a fault, do you find any evidence in the seismic reflection data that helps constrain where the fault might be deeper in the image? If so, point some arrows to it in an image you insert here:

The well in the center of the image lies within a dipping panel of rock, which is bounded by more flat-lying rocks to the east and to the west. Note the width of this panel of dipping rocks, and how its width varies as a function of height in the stratigraphic section.

Q3A.5. What is this upward-narrowing package of sedimentary rock units called? What does it help you understand about the fault?

Q3A.6. What was the growth history of the fault? What was occurring during the Oligocene? During the Early Miocene? During the Late Miocene? If the fault was moving during these times, was the basin that resulted filled, underfilled, or overfilled with sediments? Include images that point to the features that allow you to make these determinations.

Now we will construct a quantitative solution to use the parts of the seismic image that we can see to construct a balanced cross section and predict the geometry of the fault and fold where we can’t see so well. We’ll focus on the Oligocene and Basement portion of the stratigraphic column, and the central portion of the structure, to construct our solution.

Start with the features of an extensional fault-bend fold that we can see from the seismic—the direct fault plane reflector of the deep fault, the dip of the folded limb, and the width of the folded limb.

Q3B.1. For the width of the fold limb, should we use the width of O1, O2, or the basement? Explain why.

Recall that a dip panel contains layers all dipping approximately the same amount. Use this to your advantage, and expand out a line of this representative dip as far as you can to the east and west, until extending it further would cross seismic reflections (then stop! That is the location of your axial surface).

Note—there is of course some curvature to this axial surface and it is not razor-thin, making the choice of where to stop it somewhat subjective. The model you are constructing is a first-order solution, and a more accurate solution would require a few more subtle bends rather than one very angular, abrupt one, like the one you are constructing here. Nevertheless, your solution is correct to “first order” (and hand constructing the more detailed solution would take a very long time, so this is a task better suited to computer programs!).

Construct the geometry of you active and passive axial surfaces, and your dipping layers, figure out the folding vector, V. Move this vector, V, so that the top extent of it marks the location where the passive axial surface intersects your lower fault segment. The bottom extent of this vector, V, can be linked up with the location of the active axial surface; extend it past this point (parallel to this trajectory) to determine the orientation of the fault above the bend, and the length of this side of your “folding vector triangle” gives you the slip on the fault. Double check your solution by moving this vector up to see if the slip on the fault predicted in this way matches the cutoff of the basement along the fault. If it is not a good match, consider changing the dip of your horizons a little bit, or changing the width of your dip panel a little bit, to construct a solution that better honors your data constraints.

Q3B.2. Save the constructed solution for your extensional fault bend fold in your Powerpoint.

Q3B.3. How much slip is on the upper segment of the fault? On the lower segment?

Q3B.4. Now that you’ve constructed this first-order model, do you notice any second order features do you notice in the seismic data that don’t fit your model?

Submit your Powerpoint and this completed worksheet on D2L.