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 recognized 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) favor each behavior.

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.

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).

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.