Structural Geology Flashcards

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

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

Cross-cutting relationships establish that any feature (fault, dike, fold axial planar cleavage) that cuts across pre-existing rocks or structures must be younger than those features, allowing construction of a relative chronology of geologic events.

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

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

Homoclinal dip is the consistent, uniform dip of strata in one direction (like a tilted table) over a broad area; a monocline is a local steepening — a one-limbed flexure connecting two areas of different elevation or dip without completing a fold closure.

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

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

S-C fabrics in mylonites consist of foliation (S-planes, oblique to shear zone) and shear bands (C-planes, parallel to shear zone walls); the angle between S and C and the sense of their obliquity (which C-plane end S deflects toward) indicate shear sense — a key kinematic indicator.

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

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

A doubly plunging anticline closes in map view to form an elliptical or oval dome structure with the oldest rocks at its center; this 3D closure traps buoyant hydrocarbons beneath an overlying seal rock, making it one of the most important types of structural oil and gas traps.

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

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

The principal stress tensor at a point in a rock has three mutually perpendicular principal stress axes: σ1 (maximum compressive stress), σ2 (intermediate), and σ3 (minimum compressive stress); their orientations determine fault type (normal if σ1 is vertical, thrust if σ3 is vertical, strike-slip if σ2 is vertical — Anderson's theory).

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

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)

Simple shear (non-coaxial) involves rotation of material lines and planes (gives asymmetric kinematic indicators); pure shear (coaxial) involves progressive flattening and stretching in fixed directions (gives symmetric fabrics). Natural shear zones typically involve a combination of both (general shear).