Exercises: Rock deformation

Distinguish between elastic, plastic and brittle deformation, and state which of the three is reversible when the stress ceases.

What is an anticline fold and what is a syncline? State where the oldest materials are found in each: in the core or on the limbs?

Explain the difference between a fault and a joint. In which of the two is there relative displacement of the blocks?

What is stress in geology? Name the three types (compressive, tensional and shear) and one typical structure produced by each.

Define the elements of a fold: hinge, limbs and axial plane. What does the orientation of the axial plane indicate about the symmetry of the fold?

What is a normal fault and under what type of stress does it occur? Give an example of a structure associated with crustal extension.

In a fault, an upthrown block and a downthrown block are distinguished. Explain how they are recognised in the field from the displacement of the strata.

What is the difference between a symmetrical fold and an asymmetrical one? What does the vergence of a fold indicate about the direction of compression?

In a cross-section of the terrain, two numbered tectonic structures are observed: structure 1 shows an upward-convex curvature and structure 2 shows an upward-concave curvature. Indicate which number corresponds to an anticline fold and which to a syncline, and explain the criterion you used to distinguish them.

In the same cross-section image, locate the two numbered faults and explain which is normal and which is reverse based on the relative displacement of their blocks. What type of stress, extensional or compressional, produced each one?

Subjected to the same compressional stress, slates fold while a quartzite fractures into joints. Explain what properties of each rock and what external conditions (temperature, pressure, rate of deformation) favour each behaviour.

Distinguish between a reverse fault and a thrust fault, and explain why both structures imply shortening of the crust. What inclination does the plane of a thrust fault usually have?

In a cross-section, the top of a limestone layer appears displaced vertically by $120\ \text{m}$ by a fault. Explain what the throw of a fault is and what additional information you would need to know the total displacement if the fault also had a strike-slip component.

Joints make block extraction easier in quarries, but reduce the quality of ornamental stone. Explain both statements and discuss why a heavily jointed rock is unsuitable for obtaining large pieces.

A block dropped between two normal faults forms a graben, and the raised block between them, a horst. Explain what stress regime generates them and give an example of each structure.

In the Appalachians, the cores of the anticlines form valleys and the synclines crown the summits. Explain this inversion of relief from the differential erosion acting on the fractured rocks of the anticline hinge.

In the preliminary report of a civil engineering project on the geological cross-section in the image, it is stated that: "The faults first originated in an extensional regime and, subsequently, a compressional episode folded both the strata and the preexisting faults."

1. a) Give a well-reasoned refutation of the report's hypothesis: analyze the geometry of the structures visible in the image and indicate which principles of relative dating this interpretation contradicts.
2. b) Reasonably reconstruct the chronological sequence of the processes that have shaped this area (sedimentation, tectonic deformations, and erosion).
3. c) Deduce and justify how the tectonic stress regime (or the physical conditions of deformation) changed between the time the folds were generated and the time the faults that cut across them acted.

During a geological survey for a tunnel alignment, mapping reveals that along a linear valley, the strata dip at $40^\circ$ in opposite directions away from the valley axis; fossils found on the valley floor belong to the Triassic, while those on the surrounding peaks belong to the Jurassic.

1. a) Determine the tectonic structure underlying the valley and justify why the current topography represents a case of inverted relief (relief inversion).
2. b) Explain the differential erosion mechanism that allowed the core of the structure, originally the structurally highest area, to be transformed into a depression (anticlinal valley or combe), indicating the role of fracturing along the hinge zone.
3. c) If a tunnel is planned to be bored perpendicular to the valley axis at a depth of $100\ \text{m}$ beneath its floor, predict two geomechanical or hydrogeological difficulties that the engineering team will encounter when crossing the hinge zone.

Geodetic studies using GPS and radar interferometry (InSAR) on a $30\ \text{km}$ section of the Alhama de Murcia fault indicate that the blocks on both sides are moving at a relative velocity of $1,5\ \text{mm/year}$. However, sensors located exactly on the surface trace have recorded zero velocity for more than 400 years, and there is scarcely any microseismicity.

1. a) Apply the elastic rebound theory to explain the mechanical state of this fault sector and justify why the prolonged absence of earthquakes does not imply safety, but rather an increase in seismic hazard.
2. b) Calculate the accumulated displacement that remains elastically locked after 400 years and compare the mechanical effect of its instantaneous release with that of a sector moving by continuous aseismic creep.
3. c) If in another segment of the same fault a continuous surface slip of $1,5\ \text{mm/year}$ is measured, accompanied by barely perceptible microearthquakes, deduce with reasoning which of the two segments poses a greater risk of generating a destructive earthquake in the short or medium term.

On a quarry face, a fault plane dipping $60^\circ$ to the west is exposed, bringing two blocks into contact (western block and eastern block). The following observations are recorded:

1. The eastern block contains a limestone marker bed at a depth of $10\ \text{m}$, whereas in the western block the same bed appears at $35\ \text{m}$.

2. On the slickenside, striations parallel to the dip line (line of maximum slope) and fault steps are observed whose rough protrusion to the touch is felt when sliding one's hand downward along the eastern block.

3. The strata of the western block curve upward as they approach the fracture.

1. a) Determine with justification which of the two blocks is the hanging wall and which is the footwall of the fault based on the dip of the plane.
2. b) Interpret the kinematic indicators (fault steps and bed curvature) and deduce the relative movement of the blocks.
3. c) A technician claims that the fault is normal «because the limestone bed is lower in the western block». Refute or qualify their conclusion by analyzing the consistency between the observable net displacement and the dynamic kinematic indicators (drag folds and striations).

In a geodynamics laboratory, identical cylindrical limestone specimens are subjected to three triaxial deformation tests:

• Test A: at $400\ ^\circ\text{C}$ and $400\ \text{MPa}$ (equivalent to about $15\ \text{km}$ depth), deformed at a very slow rate of $10^{-14}\ \text{s}^{-1}$ (typical tectonic rate).

• Test B: at identical temperature and confining pressure to Test A, but deformed abruptly at $10^{-3}\ \text{s}^{-1}$.

• Test C: at a slow rate ($10^{-14}\ \text{s}^{-1}$), but at $40\ ^\circ\text{C}$ and $25\ \text{MPa}$ (conditions at $1\ \text{km}$ depth).

1. a) Reasonably predict the type of deformation (brittle or ductile) that the specimen will undergo in each test, and sketch the expected stress-strain curves for Tests A and C.
2. b) Justify mechanically why a drastic increase in strain rate (Test B) alters the rock's response even though it is kept under deep conditions of high temperature and pressure.
3. c) In subduction zones, brittle earthquakes are recorded at depths greater than $60\ \text{km}$, where pressure and temperature should prevent fracturing. Formulate a justified hypothesis based on the role of pore fluids (pore pressure) to explain this anomaly.

When measuring a geological column, a stratigraphic repetition is observed: Marls (Cretaceous) $\rightarrow$ Sandstones (Paleocene) $\rightarrow$ Limestones (Eocene) $\rightarrow$ Marls (Cretaceous) $\rightarrow$ Sandstones (Paleocene) $\rightarrow$ Limestones (Eocene). The intermediate contact between the lower limestones and the upper marls is a sharp surface dipping $30^\circ$ that obliquely truncates the beds.

1. a) Propose two different geological hypotheses to account for this repetition: a structural one based on a tectonic feature and a stratigraphic one based on an eroded recumbent fold.
2. b) Design three field checks (sedimentary polarity criteria, microstructures on the contact surface, and relative dating of the contact with respect to the beds) to determine which of the two hypotheses is correct.
3. c) If the sedimentary polarity in both sequences is younging upwards (facing up) and a tectonic breccia with down-dip striations is found on the contact surface, conclude what specific type of geological structure has operated and deduce the responsible tectonic regime.

The 2011 Lorca earthquake had a moderate moment magnitude ($M_w\ 5,1$) but caused 9 fatalities and severe damage (intensity VIII), whereas the 2010 earthquake off the coast of Granada, of much greater magnitude ($M_w\ 6,3$), barely caused minor property damage.

1. a) Provide a physical justification for the disparity in damage between both events based on the relationship among magnitude, focal depth (hypocentral distance), and peak ground acceleration.
2. b) In Lorca, buildings located on Quaternary alluvial sediments suffered significantly greater damage than those founded on cohesive bedrock a short distance away. Explain this phenomenon by analyzing the propagation velocity, amplitude, and attenuation of seismic waves (site effect).
3. c) Based on these data, argue why a land-use planning scheme that classifies seismic risk based exclusively on the distance to active faults is insufficient, and indicate which complementary geological factors should be included in seismic microzonation maps.

In a $300\ \text{m}$ wide cortical deformation shear zone that cuts across a granitic massif, the following transition is observed:

• Toward the core of the zone, the original granite grades into a very fine-grained rock with continuous banding, dynamically stretched quartz grains, and rotated feldspars (mylonite).

• Crosscutting and crushing this mylonite, sharp fracture planes appear with angular breccias and cohesionless rock powder lacking crystalline preferred orientation (cataclasites).

1. a) Analyze the dominant mineral deformation mechanisms in the mylonite versus the cataclasite, and deduce the crustal regime (ductile or brittle) in which each was formed.
2. b) Justify what information the superposition of cataclasites over mylonites in the same outcrop provides about the uplift (exhumation) history and thermal evolution of that orogenic block.
3. c) If the kinematic analysis of the mylonite porphyroclasts indicates reverse (compressional) shear, but the striations on the cataclastic planes indicate normal (extensional) movement, develop a reasoned geodynamic reconstruction of the tectonic evolution of that fault zone.