- A seismologist observes that earthquakes in a particular region consistently show fault-plane solutions indicating that the axis of minimum stress (T-axis) is plunging steeply, while the axis of maximum stress (P-axis) is near-horizontal. What type of faulting dominates this region?
Answer: Reverse faulting
When analyzing earthquake fault-plane solutions, the key is understanding how stress orientations relate to different types of faulting. The P-axis represents maximum compressive stress, while the T-axis represents minimum stress (maximum tension).
In this scenario, you have a steeply plunging T-axis and a near-horizontal P-axis. This stress configuration indicates that maximum compression is acting horizontally while minimum stress (tension) is oriented nearly vertically. This arrangement drives reverse faulting, where rocks are compressed horizontally and the hanging wall moves up relative to the footwall. The steep T-axis reflects the vertical extension that accommodates this upward movement.
Looking at the wrong answers: (A) Normal faulting would show the opposite pattern—a steeply plunging P-axis and near-horizontal T-axis, because normal faults form under vertical compression and horizontal extension. (B) Left-lateral strike-slip and (C) right-lateral strike-slip faulting would both show P and T axes that are subhorizontal and oriented at roughly 45° to the fault trace, since strike-slip motion involves horizontal shearing with no significant vertical component. The lateral direction (left vs. right) doesn't change the stress orientation pattern.
The answer is (D) reverse faulting.
Remember this pattern: steep T-axis + horizontal P-axis = reverse faulting (compression), while steep P-axis + horizontal T-axis = normal faulting (extension). For strike-slip faulting, both axes remain subhorizontal but oriented obliquely to the fault.
- Imagine a block of rock containing a pre-existing, inactive fault that strikes north-south and dips 60 degrees to the west. If a new tectonic regime imposes a strong east-west compressional stress, what is the most likely outcome?
Answer: The fault will be reactivated as a reverse fault.
The imposed stress is east-west compression. This stress will push the eastern and western sides of the region together. For a fault that dips to the west, the block on the west side is the hanging wall. The east-west compression will push the hanging wall block up the fault plane relative to the footwall block. This type of motion defines a reverse fault. Pre-existing planes of weakness, like old faults, are often reactivated before new ones are created.
- In the diagram provided, a river has been offset by a fault. An observer standing on the west side of the fault notes that the river channel on the east side has moved to their left. What is the correct classification for this fault?
Answer: A left-lateral strike-slip fault caused by shear stress.
The description indicates horizontal movement parallel to the fault trace, which defines a strike-slip fault. To determine if it is left-lateral or right-lateral, imagine standing on one side and looking across the fault. From the west side, the east side has moved left. This defines a left-lateral strike-slip fault. All strike-slip faults are caused by a shear stress regime.
- In a particular normal fault, the fault plane dips at 45 degrees. A vertical borehole is drilled through the fault, and the same coal seam is found at a depth of 500m on the footwall side and 700m on the hanging wall side. What is the approximate horizontal extension (heave) across the fault at the level of the coal seam?
Answer: 200 meters
First, determine the vertical displacement, or throw. The coal seam is at 500m on one side and 700m on the other, so the throw is 700m - 500m = 200m. The throw, heave (horizontal extension), and fault dip are related by trigonometry. Specifically, tan(dip angle) = throw / heave. Since the dip angle is 45 degrees, and tan(45°) = 1, the throw must be equal to the heave. Therefore, the heave is also 200 meters.
- A mountain range is characterized by numerous folds and faults where older metamorphic rocks are found on top of younger sedimentary rocks. Seismic studies indicate the faults dip at low angles (less than 30°). Which combination of fault type and stress regime best explains these observations?
Answer: Thrust faults resulting from a compressional stress regime.
The presence of folds and the placement of older rocks over younger rocks are classic indicators of crustal shortening caused by compression. A fault that places older rocks over younger rocks is a reverse fault. When a reverse fault has a low dip angle (typically <45°, and especially <30°), it is specifically called a thrust fault. Therefore, the observations are best explained by thrust faulting under a compressional stress regime.
- Which statement accurately describes the relationship between stress and the formation of different fault types?
Answer: Tensional stress causes the crust to fail in shear, creating normal faults at an angle to the stress.
When you encounter questions about stress and fault formation, focus on understanding how different stress regimes cause rocks to fail and create specific fault geometries.
Tensional stress pulls the crust apart, causing it to fail along inclined planes rather than vertically. This creates normal faults where the hanging wall moves down relative to the footwall, typically at angles between 45-65° from horizontal. The key insight is that even though the stress is tensional (pulling), the actual failure occurs through shear along these angled planes because that's where the rock is weakest.
Let's examine why the other options miss the mark. Choice A incorrectly states that faults form parallel to maximum principal stress (σ1). In reality, faults typically form at angles to σ1 - usually around 30° for optimal shear failure. Choice B oversimplifies compressional faulting. While compression does create thrust faults, it commonly produces complex networks of multiple faults rather than a single large one, depending on factors like rock type and stress magnitude. Choice C contains a fundamental misconception about strike-slip faults. These faults actually result from horizontal compressive or shear stress, not "torsional stress" - they form when the maximum and minimum principal stresses are both horizontal.
Remember this pattern: the type of fault depends on the orientation of principal stresses, but faults rarely form parallel to stress directions. Instead, they develop along planes of weakness where shear failure is most likely to occur, which is typically at specific angles to the stress field.
- A region's stress regime is characterized by a vertically oriented maximum principal stress (σ1) and a horizontally oriented minimum principal stress (σ3). Which type of faulting and tectonic setting would be most consistent with this stress orientation?
Answer: Normal faulting in a continental rift zone.
When the maximum principal stress (σ1) is vertical, the crust is being pushed down by gravity more forcefully than it is being squeezed horizontally. This leads to horizontal extension (tensional stress). Tensional stress regimes produce normal faults. Continental rift zones, like the East African Rift, are prime examples of such settings.
- Which of the following geologic settings is LEAST likely to be dominated by a compressional stress regime and its associated reverse and thrust faults?
Answer: The Mid-Atlantic Ridge, where new oceanic crust is being formed.
This question tests your understanding of stress regimes at different tectonic settings. When analyzing geological environments, you need to consider the forces acting on the crust and how they create characteristic fault patterns.
The Mid-Atlantic Ridge (D) is the correct answer because it's a divergent plate boundary where oceanic plates move apart. Here, the dominant stress regime is extensional (tensional), not compressional. As new oceanic crust forms through seafloor spreading, the lithosphere is being pulled apart, creating normal faults rather than reverse or thrust faults. This extensional environment is fundamentally different from compressional settings.
Option A is incorrect because the Himalayas represent a classic continental collision zone where compressional forces dominate. The ongoing collision between the Indian and Eurasian plates creates intense compression, resulting in extensive thrust faulting and mountain building.
Option B is wrong because accretionary wedges form at convergent margins where oceanic sediments are scraped off the subducting plate and compressed against the overriding plate. This process involves significant compression and produces abundant thrust faults.
Option C is incorrect because the Andes Mountains form at a subduction zone where oceanic crust descends beneath continental crust. The convergent motion creates compressional stress, leading to reverse and thrust faulting throughout the mountain range.
Remember this pattern: divergent boundaries = extension and normal faults; convergent boundaries = compression and reverse/thrust faults. When you see "spreading ridge" or "new crust formation," think extension, not compression.
- A major volcanic eruption is triggered by the intrusion of a dike, a vertical sheet of magma that forces the surrounding rock apart. What type of faulting and stress regime would you expect to find on the surface directly above and parallel to the propagating dike?
Answer: Normal faulting, due to localized extension as the crust is pushed apart by the dike.
When analyzing volcanic processes, you need to understand how magma intrusions create stress in the surrounding rock. A dike is a sheet-like intrusion that cuts vertically through existing rock layers, literally forcing the crust apart as it propagates upward.
As the dike intrudes, it creates a localized zone of extension directly above it. Think of it like inserting a wedge into a crack – the material on either side must move apart to accommodate the new volume. This extensional stress causes the overlying rock to stretch and thin, leading to normal faulting where one block drops down relative to another along inclined fault planes. The surface expression would show parallel normal faults running along the same orientation as the dike below.
Option A is incorrect because reverse faulting occurs under compression, not the extension created by dike intrusion. While magma does exert pressure, the dominant effect is pulling the crust apart, not squeezing it together.
Option B misunderstands the brittle nature of shallow crustal rocks. Even though deeper rocks may deform ductilely, the upper crust typically responds to dike intrusion with brittle fracturing and faulting.
Option C incorrectly suggests strike-slip motion. While the sides of the dike do separate, they move apart perpendicular to the dike walls, not parallel to them as in strike-slip faulting.
Remember this key principle: dike intrusions create extension perpendicular to their orientation, and extension in the brittle crust produces normal faults. Look for this pattern whenever volcanic processes involve vertical magma intrusions.
- A field geologist finds a fault plane with prominent linear grooves, called slickensides, that plunge almost directly down the 60-degree dip of the fault. This observation strongly suggests that the fault experienced...
Answer: primarily dip-slip motion under either a compressional or tensional regime.
Slickensides are scratches or grooves on a fault surface that indicate the direction of relative movement. If these grooves plunge directly down the dip of the fault, it means the movement was up or down the fault plane, which is defined as dip-slip motion. Dip-slip motion occurs in both normal faults (tensional stress) and reverse faults (compressional stress). Without knowing which block moved up or down, we cannot distinguish between the two, but we can confidently identify the motion as dip-slip.
- At a transform plate boundary, the primary motion is horizontal shear. However, if the boundary has a bend or 'step-over,' localized areas of compression or tension can develop. If a right-lateral transform fault has a left-stepping bend, what structures would be expected to form in the area between the fault segments?
Answer: A compressional 'restraining bend' with uplift, reverse faults, and folds.
Imagine tracing the motion on a right-lateral fault. As you move along one segment and have to 'step' left to get to the next segment, the blocks on either side are being pushed into each other in the step-over region. This creates a zone of localized compression, known as a restraining bend or a transpressional zone. The crust in this zone will shorten and thicken, leading to uplift, folding, and reverse/thrust faulting.
- The formation of a volcanic caldera through the collapse of a magma chamber roof is often associated with a specific type of faulting around the caldera's perimeter. What type of fault and stress is most directly responsible for this collapse?
Answer: Ring faults, which are a form of normal fault, due to tensional stress as the roof loses support.
When a magma chamber empties, the overlying rock (the roof) loses its support. Gravity then becomes the dominant force, pulling the roof block downward. This creates a tensional stress regime in the rock surrounding the unsupported area. The crust fails along steeply dipping normal faults that form a circle or ring, allowing the central block to subside. These are called ring faults.
- An oblique-slip fault exhibits significant components of both dip-slip and strike-slip motion. This implies that the stress regime is NOT perfectly aligned with the fault's orientation. Which scenario would most likely produce an oblique-slip fault?
Answer: A transtensional regime, where overall shear is combined with a component of extension.
Oblique-slip motion requires a combination of shear stress (causing strike-slip) and either tensional or compressional stress (causing dip-slip). A transtensional regime combines transform (shear) motion with tensional (extensional) motion, resulting in both strike-slip and normal dip-slip components. Similarly, a transpressional regime combines shear and compression. The other options describe pure stress regimes that would produce pure normal, reverse, or strike-slip faults, not oblique-slip.
- Which statement correctly distinguishes between the hanging wall and the footwall of a non-vertical fault?
Answer: The hanging wall is the block of rock that is physically located above the fault plane, while the footwall is below.
The definitions of hanging wall and footwall are purely geometric and based on their position relative to the inclined fault plane, not on their movement. The hanging wall is the block above the fault plane (imagine a miner 'hanging' a lantern on it), and the footwall is the block below the fault plane (where the miner's 'feet' would be). This definition holds true for both normal and reverse faults.
- The Basin and Range Province of the western United States is characterized by a series of north-south trending mountain ranges separated by wide, flat valleys. This topography is a direct result of which combination of faulting and stress?
Answer: Widespread normal faulting due to east-west tensional (extensional) stress.
The 'basin and range' topography, with its alternating down-dropped blocks (grabens, which form valleys or basins) and uplifted blocks (horsts, which form mountain ranges), is the classic expression of large-scale crustal extension. This extension is accommodated by normal faulting. Since the ranges trend north-south, the tensional stress must be oriented perpendicular to them, in an east-west direction.
- A seismic hazard analysis is being conducted for a region dominated by a major east-west striking reverse fault. To model the earthquake potential, geophysicists need to define the principal stress orientations. What is the most likely configuration?
Answer: Maximum principal stress (σ1) is north-south; minimum principal stress (σ3) is vertical.
Reverse faults are caused by compressional stress, where the maximum principal stress (σ1) is horizontal and the minimum principal stress (σ3) is vertical. The compression acts perpendicular to the strike of the fault. Since the fault strikes east-west, the maximum compression (σ1) must be oriented north-south to cause the shortening.
- The San Andreas Fault is a right-lateral strike-slip fault. This implies that the dominant stress regime is shear. What is the orientation of the principal stress axes relative to the northwest-southeast trend of the fault?
Answer: The maximum compressive stress (σ1) is oriented roughly north-south, at an angle to the fault.
In a shear regime that produces strike-slip faulting, the maximum (σ1) and minimum (σ3) principal stresses are both horizontal, while the intermediate stress (σ2) is vertical. The faults typically form at an angle of about 30-45 degrees to the direction of maximum compression. For the San Andreas Fault system, the Pacific plate is moving northwest relative to the North American plate. This movement is driven by a maximum compressive stress (σ1) oriented roughly north-south, which is at an angle to the northwest-striking fault itself, creating the right-lateral shear.
- Which of the following provides the most direct evidence for identifying the specific type of dip-slip fault (normal vs. reverse) in a field outcrop where the rock layers are uniform and lack distinctive marker beds?
Answer: Small-scale Riedel shears or drag folds adjacent to the fault plane.
While strike and dip (A) define the fault's geometry and slickensides (B) show the direction of slip, neither can distinguish between normal and reverse motion on their own without stratigraphic context. Brecciation (D) indicates faulting but not type. However, small-scale structures like drag folds (where layers are bent or 'dragged' by the fault motion) or Riedel shears (subsidiary fractures) have a specific geometry that indicates the sense of shear. For example, drag folds adjacent to a fault will show beds on the hanging wall bent upwards in a reverse fault, indicating compression.
