The deformation of rocks

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

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

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.

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.

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?

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 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 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.

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.

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

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.

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?

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 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.

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.