ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers 2 — Questions and Answers
Question 1: What is the difference between ductile and brittle deformation in rocks, and what factors control which type occurs?
- Ductile deformation occurs only in metamorphic rocks; brittle deformation occurs only in sedimentary rocks, regardless of conditions
- Ductile deformation involves continuous flow and folding without fracturing; brittle deformation involves fracturing and faulting. Temperature, pressure, strain rate, and rock type control which occurs — higher T and P, slower strain rates, and weaker minerals favor ductile behavior (Correct answer)
- Ductile deformation only occurs under extensional tectonic regimes; brittle deformation only under compression
- Ductile deformation involves cataclastic flow; brittle deformation involves crystal plastic mechanisms such as dislocation glide
Correct answer: Ductile deformation involves continuous flow and folding without fracturing; brittle deformation involves fracturing and faulting. Temperature, pressure, strain rate, and rock type control which occurs — higher T and P, slower strain rates, and weaker minerals favor ductile behavior
Ductile (plastic) deformation produces continuous strain (folds, foliations, mylonites) without fracturing; brittle deformation produces discontinuous fractures and faults. Higher temperature, higher confining pressure, slower strain rates, and mineral weakness all promote ductile behavior.
The brittle-ductile transition in the crust typically occurs at 10–15 km depth (300–400°C for quartz) and separates the seismogenic upper crust (brittle) from the aseismic lower crust (ductile). Factors promoting ductile behavior: high temperature (activates crystal plastic mechanisms — dislocation glide and creep), high confining pressure (suppresses fracturing), slow strain rate (allows creep), and mechanically weak minerals (halite, calcite, quartz at T > ~300°C). Brittle behavior is favored by low temperature, low confining pressure, high strain rates, and strong minerals. The same mineral can be brittle or ductile depending on these conditions.
Question 2: What does the Wilson Cycle describe in plate tectonics?
- The complete life cycle of an ocean basin from continental rifting through seafloor spreading to subduction, collision, and suturing (Correct answer)
- The cyclical variation in polar wandering paths as continents drift over the geomagnetic poles
- The episodic assembly and dispersal of supercontinents over billion-year timescales
- The cycle of volcanic eruptions at a hotspot as a plate moves over it
Correct answer: The complete life cycle of an ocean basin from continental rifting through seafloor spreading to subduction, collision, and suturing
The Wilson Cycle (named after J. Tuzo Wilson) describes the complete cycle of ocean basin evolution: continental rifting → young ocean (proto-oceanic gulf) → mature ocean → subduction (closing) → continental collision → suture zone (mountain belt), completing the cycle when the next rifting episode begins.
The Wilson Cycle (1966) formalizes the sequential stages of ocean basin evolution: (1) embryonic rift (East African Rift analog), (2) juvenile ocean (Red Sea/Gulf of Aden analog), (3) mature ocean (Atlantic analog), (4) declining ocean with subduction (Pacific analog), (5) terminal ocean with collision (Mediterranean analog), (6) suture zone and mountain building (Himalayas, Alps analog). The cycle repeats because suture zones are weak zones where the next rifting episode preferentially nucleates (the 'inside-out' Wilson Cycle). This framework explains the episodic nature of orogenic belts and the reassembly of supercontinents.
Question 3: What is a mylonite, and in what crustal environment does it form?
- A mylonite is a fault breccia formed by cataclastic comminution in shallow crustal fault zones
- A mylonite is a ductilely deformed rock with a strongly developed foliation and lineation, fine-grained matrix, and porphyroclasts formed by crystal plastic deformation in deep crustal shear zones (Correct answer)
- A mylonite is an intrusive rock formed by emplacement of magma along a ductile shear zone
- A mylonite is a sedimentary rock deposit formed in a fault-controlled lake basin
Correct answer: A mylonite is a ductilely deformed rock with a strongly developed foliation and lineation, fine-grained matrix, and porphyroclasts formed by crystal plastic deformation in deep crustal shear zones
Mylonites are highly deformed rocks produced by ductile shear along deep crustal or lithospheric shear zones, characterized by strong planar and linear fabric, fine-grained matrix from dynamic recrystallization, and augen/porphyroclasts of more resistant minerals.
Mylonites form in the ductile shear zones of the middle to lower crust (below the brittle-ductile transition) through intense ductile strain. During mylonitization, fine-grained minerals undergo dynamic recrystallization (subgrain rotation, grain boundary migration), reducing grain size and strengthening the foliation. Porphyroclasts (relict large grains of resistant minerals like feldspar, garnet) are surrounded by the fine-grained matrix. A classification from less to more deformed: protomylonite → mylonite → ultramylonite. Shear sense indicators in mylonites (S-C fabrics, σ and δ porphyroclasts, asymmetric boudins) allow determination of paleo-shear direction in ancient ductile shear zones.
Question 4: In structural geology, what is the difference between a syncline and a synclinorium?
- A syncline has younger rocks at its hinge; a synclinorium has older rocks at its hinge
- A syncline is a simple fold with younger rocks at the core; a synclinorium is a large-scale regional structure composed of multiple smaller folds but with an overall synclinal form with younger rocks in the center (Correct answer)
- A syncline is an upward-closing fold; a synclinorium is a downward-closing fold
- A syncline is a fold in sedimentary rocks; a synclinorium is the equivalent structure in metamorphic rocks
Correct answer: A syncline is a simple fold with younger rocks at the core; a synclinorium is a large-scale regional structure composed of multiple smaller folds but with an overall synclinal form with younger rocks in the center
A syncline is a simple fold in which younger rocks occur at the hinge (core); a synclinorium is a large-scale regional fold that has the overall form of a syncline but contains multiple smaller-scale fold closures within it.
In structural geology, fold orders are classified by scale. A syncline is a fundamental fold with the youngest strata in the core (hinge zone). A synclinorium (and its antithetical anticlinorium) is a first-order or regional fold structure whose limbs and hinge zone contain second- and third-order parasitic folds. The Appalachian Valley and Ridge province is a classic synclinorium/anticlinorium pair. On a map, a synclinorium shows a broad synclinal form with internally complex, smaller folds that young toward the center. Understanding fold orders is critical for subsurface extrapolation and resource exploration.
Question 5: What is the significance of ophiolite sequences in tectonic interpretation?
- Ophiolites are fragments of ancient continental crust obducted onto the margin of a colliding continent, marking suture zones and former ocean basins (Correct answer)
- Ophiolites are remnants of ancient island arcs subducted and preserved in accretionary prisms
- Ophiolites are sections of metamorphic core complexes exhumed by crustal extension
- Ophiolites are massive sulfide ore deposits found along ancient divergent plate boundaries
Correct answer: Ophiolites are fragments of ancient continental crust obducted onto the margin of a colliding continent, marking suture zones and former ocean basins
Ophiolites are slices of ancient oceanic lithosphere and upper mantle that have been obducted (thrust) onto continental margins, providing direct evidence of former ocean basins and the locations of ancient suture zones where oceans once existed.
Ophiolites are preserved slices of oceanic lithosphere (and sometimes overlying sedimentary cover) emplaced on land by obduction at convergent margins. The complete Penrose-conference ophiolite sequence (top to bottom) is: pelagic sediments → pillow basalts → sheeted dike complex → gabbros → ultramafic cumulates → depleted harzburgite mantle. This sequence directly mirrors the structure of oceanic crust and upper mantle drilled and sampled at mid-ocean ridges. Classic ophiolites include the Troodos Complex (Cyprus), the Semail Ophiolite (Oman), and the Bay of Islands Complex (Newfoundland). Their occurrence in orogenic belts marks the location of former oceans — Wilson Cycle suture zones.
Question 6: What do P and T axes in a focal mechanism (beach ball diagram) represent, and how do they relate to the regional stress field?
- P is the pressure axis (axis of maximum compressive stress); T is the tension axis (axis of minimum compressive stress). They are perpendicular and bisect the compressional and dilational quadrants of the focal mechanism, representing the principal stress axes driving the fault slip (Correct answer)
- P is the plunge of the fault plane; T is the trend of the fault plane as measured from the focal mechanism
- P is the P-wave velocity; T is the S-wave velocity ratio used to determine fault type from seismic data
- P is the principal strain axis; T is the tectonic transport direction of the fault as determined from kinematic indicators
Correct answer: P is the pressure axis (axis of maximum compressive stress); T is the tension axis (axis of minimum compressive stress). They are perpendicular and bisect the compressional and dilational quadrants of the focal mechanism, representing the principal stress axes driving the fault slip
In focal mechanisms, the P-axis (pressure axis) is the axis of maximum compressive stress bisecting the compressional quadrants, and the T-axis (tension axis) is the axis of minimum compressive stress bisecting the dilational quadrants, together approximating the principal stress orientations.
The P-axis and T-axis of a focal mechanism solution are derived from the orientation of the two nodal planes of the beach ball. The P-axis bisects the acute angle between the two nodal planes within the compressional (dark/black) quadrants and approximates the axis of maximum horizontal compressive stress (σ1). The T-axis bisects the dilational (white) quadrants and approximates the axis of minimum horizontal stress (σ3). These axes are perpendicular to each other. By inverting many focal mechanisms in a region, geologists can determine the regional stress tensor orientation, which governs fault reactivation, hydrocarbon migration, and earthquake hazard.
ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers 2
What is the difference between ductile and brittle deformation in rocks, and what factors control which type occurs?