ASBOG Structural Geology 2

Question 1

What is the principle of cross-cutting relationships in structural geology, and how is it applied to determine the relative ages of geologic events?

Accepted Answer

The principle states that any geologic feature (fault, dike, vein, unconformity) that cuts across another feature is younger than the feature it cuts; used to sequence deformation and igneous events relative to each other

Answer

The principle states that intrusive igneous bodies are always older than the rocks they intrude

Answer

The principle states that metamorphic grade increases consistently with depth, allowing ages to be correlated with grade

Answer

The principle states that sedimentary layers always maintain their original horizontal orientation unless deformed

Question 2

In structural geology, what is the difference between homoclinal dip and monocline?

Accepted Answer

Homoclinal dip refers to consistent dip direction and amount across a broad area; a monocline is a step-like fold where horizontal or gently dipping strata are abruptly bent to a steeper dip before resuming gentle dip, connecting two horizontal levels

Answer

Homoclinal dip refers to overturned bedding; a monocline is a symmetric fold with two limbs dipping equally

Answer

Homoclinal dip is measured only in vertical boreholes; a monocline is identified only from aerial photography

Answer

Homoclinal dip and monocline are synonymous terms for the same structural feature

Question 3

What is the significance of 'S-C mylonite fabric' in ductile shear zones, and what information does it provide?

Accepted Answer

S-C fabric consists of S-surfaces (foliation planes) at an angle to C-surfaces (shear band surfaces parallel to the shear zone boundary); the acute angle from C to S and the obliquity of S indicate the sense of shear and allow determination of kinematics of the shear zone

Answer

S-C fabric indicates two generations of folding in the same rock; the angle between S and C surfaces records the finite strain ratio

Answer

S-C fabric in mylonites indicates that the rock was deformed at two different metamorphic grades during two separate tectonic events

Answer

S-C fabric is found only in extensional core complexes and indicates normal-sense (extensional) shear

Question 4

What is the 'double plunging anticline' and why is it economically important in petroleum geology?

Accepted Answer

A double plunging anticline is a fold that plunges in opposite directions away from its highest point (doubly plunging closure), forming a closed dome-like structure that is an ideal trap for accumulation of oil and gas

Answer

A double plunging anticline is a fold that was deformed twice by two tectonic events, creating two overprinted fold axes

Answer

A double plunging anticline is an anticline with two axial surfaces, one for each limb of the fold

Answer

A double plunging anticline is a fold that formed only in deeply buried strata and was never exposed at the surface, making it detectable only by seismic reflection

Question 5

What is the stress tensor in structural geology, and what do the three principal stresses (σ1, σ2, σ3) represent?

Accepted Answer

The stress tensor describes the state of stress at a point in a rock mass; σ1 is the maximum principal compressive stress, σ2 is the intermediate, and σ3 is the minimum (least compressive) stress. Their orientations determine which faults can be activated and which type of faulting occurs

Answer

The stress tensor describes the distribution of rock strength with depth; σ1 is the highest strength, σ3 the lowest

Answer

The stress tensor describes the velocity field in a flowing rock mass; σ1 is the direction of fastest flow

Answer

The stress tensor describes the three crystallographic axes of deformed minerals used as paleostress indicators

Question 6

What is the difference between progressive simple shear and pure shear in structural geology, and what fabric elements distinguish them?

Accepted Answer

Simple shear involves rotation and distortion with no volume change along one direction; pure shear involves coaxial flattening and extension without rotation. Simple shear produces asymmetric fabrics (S-C fabric, rotated porphyroclasts); pure shear produces symmetric fabrics (symmetric boudinage, symmetric fold vergence)

Answer

Simple shear involves volume change and recrystallization; pure shear involves only rotation without any distortion

Answer

Simple shear produces folds only; pure shear produces faults only, regardless of rock type or temperature

Answer

Simple shear and pure shear are identical in terms of the finite strain state; they differ only in the kinematic pathway (strain path)