ASBOG Structural Geology 2 β Questions and Answers
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?
- The principle states that intrusive igneous bodies are always older than the rocks they intrude
- 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 (Correct answer)
- The principle states that metamorphic grade increases consistently with depth, allowing ages to be correlated with grade
- The principle states that sedimentary layers always maintain their original horizontal orientation unless deformed
Correct 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.
The principle of cross-cutting relationships is one of the fundamental principles of relative geochronology. It was formalized by James Hutton and Charles Lyell as part of the uniformitarian framework. In structural geology, it means: (1) a fault is younger than the rocks and structures it displaces; (2) a dike or sill is younger than the rocks it intrudes; (3) metamorphic foliation is younger than the bedding it overprints (if discordant); (4) a fold axial planar cleavage is younger than (or contemporaneous with) the fold if it is fan-shaped and parallel to the axial surface. Multiple episodes of deformation are unraveled by identifying all cross-cutting relationships in the field.
Question 2: In structural geology, what is the difference between homoclinal dip and monocline?
- 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 (Correct answer)
- Homoclinal dip refers to overturned bedding; a monocline is a symmetric fold with two limbs dipping equally
- Homoclinal dip is measured only in vertical boreholes; a monocline is identified only from aerial photography
- Homoclinal dip and monocline are synonymous terms for the same structural feature
Correct 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 β a one-limbed flexure connecting two areas of different elevation or dip without completing a fold closure.
Homoclinal structures have strata dipping consistently in one direction at a fairly uniform angle, like a tilted block. The Colorado Plateau is largely homoclinal. Monoclines are step-like folds with a single steep limb connecting two areas of different structural elevation; in map view they appear as a narrow band of closely spaced contours (steep dip) flanked by widely spaced contours (gentle dip). Monoclines are common on the Colorado Plateau where Laramide basement uplifts drape overlying sedimentary cover into flexures. The East Kaibab Monocline (100+ miles long) and the Waterpocket Fold in Utah are classic examples. In cross-section, monoclines are often associated with high-angle reverse faults or basement block uplifts at depth.
Question 3: What is the significance of 'S-C mylonite fabric' in ductile shear zones, and what information does it provide?
- S-C fabric indicates two generations of folding in the same rock; the angle between S and C surfaces records the finite strain ratio
- 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 (Correct answer)
- S-C fabric in mylonites indicates that the rock was deformed at two different metamorphic grades during two separate tectonic events
- S-C fabric is found only in extensional core complexes and indicates normal-sense (extensional) shear
Correct 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 β a key kinematic indicator.
In S-C (SchistositΓ©-Cisaillement) mylonite fabrics: S-surfaces are planar foliations oblique to the shear zone boundaries, produced by flattening and grain rotation; C-surfaces are discrete shear bands sub-parallel to the shear zone walls, representing localized zones of intense shear. In simple shear, the S-surface lies at an angle ΞΈ from C that decreases with increasing shear strain. The sense of obliquity of S relative to C indicates the shear sense: if the acute angle between S and C 'leans' in the transport direction (the S-planes are deflected as if swept in the shear direction), the shear sense can be determined as either dextral, sinistral, normal, or thrust sense. S-C fabrics are among the most reliable shear sense indicators in metamorphic terranes.
Question 4: What is the 'double plunging anticline' and why is it economically important in petroleum geology?
- 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 (Correct answer)
- A double plunging anticline is a fold that was deformed twice by two tectonic events, creating two overprinted fold axes
- A double plunging anticline is an anticline with two axial surfaces, one for each limb of the fold
- 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
Correct 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.
Anticlinal traps are the most commonly drilled structural targets in petroleum exploration. A doubly plunging anticline has its fold axis plunging at both ends, creating a closed structural contour pattern (domal structure) when viewed on a structure contour map. Hydrocarbons, being less dense than water, migrate upward through permeable carrier beds until they encounter the impermeable seal rock (cap rock) draping over the anticlinal crest. The closed structure prevents further lateral migration, trapping oil and gas beneath the seal. Classic anticlinal oil fields include the Kirkuk field in Iraq, the Zagros fold belt fields in Iran, and many Rocky Mountain fields. Seismic reflection data are used to map anticlinal closures before drilling.
Question 5: What is the stress tensor in structural geology, and what do the three principal stresses (Ο1, Ο2, Ο3) represent?
- The stress tensor describes the distribution of rock strength with depth; Ο1 is the highest strength, Ο3 the lowest
- The stress tensor describes the state of stress at a point in a rock mass; Ο1 is the maximum principal compressive stress, Ο2 is the intermediate, and Ο3 is the minimum (least compressive) stress. Their orientations determine which faults can be activated and which type of faulting occurs (Correct answer)
- The stress tensor describes the velocity field in a flowing rock mass; Ο1 is the direction of fastest flow
- The stress tensor describes the three crystallographic axes of deformed minerals used as paleostress indicators
Correct answer: The stress tensor describes the state of stress at a point in a rock mass; Ο1 is the maximum principal compressive stress, Ο2 is the intermediate, and Ο3 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: Ο1 (maximum compressive stress), Ο2 (intermediate), and Ο3 (minimum compressive stress); their orientations determine fault type (normal if Ο1 is vertical, thrust if Ο3 is vertical, strike-slip if Ο2 is vertical β Anderson's theory).
The stress tensor fully describes the state of stress at a point, requiring six independent components (a 3Γ3 symmetric matrix). The three principal stresses are the eigenvalues of this tensor, acting along mutually perpendicular eigenvectors (principal stress axes) where shear stress is zero. Anderson's (1951) theory of faulting relates the vertical principal stress (one principal stress is always vertical at the Earth's surface due to the free surface condition) to fault type: if Ο1 is vertical β normal faulting; if Ο3 is vertical β thrust faulting; if Ο2 is vertical β strike-slip faulting. The ratio (Ο1-Ο2)/(Ο1-Ο3) is the stress ratio R, which, combined with principal stress orientations, can be inverted from focal mechanisms or fault-slip data to determine the regional paleostress tensor.
Question 6: What is the difference between progressive simple shear and pure shear in structural geology, and what fabric elements distinguish them?
- 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) (Correct answer)
- Simple shear involves volume change and recrystallization; pure shear involves only rotation without any distortion
- Simple shear produces folds only; pure shear produces faults only, regardless of rock type or temperature
- Simple shear and pure shear are identical in terms of the finite strain state; they differ only in the kinematic pathway (strain path)
Correct 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).
Simple shear (non-coaxial flow) describes deformation where material is sheared parallel to a fixed reference surface β like a deck of playing cards. The instantaneous stretching axes (ISA) and finite strain axes (FSA) do not remain parallel during progressive deformation (non-coaxial). This rotation produces characteristic asymmetric kinematic indicators: S-C fabrics, Ξ΄ and Ο porphyroclasts, shear bands, asymmetric boudinage, and fold vergence in the shear direction. Pure shear (coaxial flow) is progressive flattening and extension along fixed orthogonal directions β like rolling out dough. The ISA and FSA remain parallel, producing symmetric fabrics: symmetric augen, symmetric boudinage, and conjugate shear bands. Most natural ductile shear zones involve general shear (transpression, transtension) combining components of both.
ASBOG Structural Geology 2
What is the principle of cross-cutting relationships in structural geology, and how is it applied to determine the relative ages of geologic events?