ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers 2

Question 1

What is the difference between ductile and brittle deformation in rocks, and what factors control which type occurs?

Accepted 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

Answer

Ductile deformation occurs only in metamorphic rocks; brittle deformation occurs only in sedimentary rocks, regardless of conditions

Answer

Ductile deformation only occurs under extensional tectonic regimes; brittle deformation only under compression

Answer

Ductile deformation involves cataclastic flow; brittle deformation involves crystal plastic mechanisms such as dislocation glide

Question 2

What does the Wilson Cycle describe in plate tectonics?

Accepted Answer

The complete life cycle of an ocean basin from continental rifting through seafloor spreading to subduction, collision, and suturing

Answer

The cyclical variation in polar wandering paths as continents drift over the geomagnetic poles

Answer

The episodic assembly and dispersal of supercontinents over billion-year timescales

Answer

The cycle of volcanic eruptions at a hotspot as a plate moves over it

Question 3

What is a mylonite, and in what crustal environment does it form?

Accepted 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

Answer

A mylonite is a fault breccia formed by cataclastic comminution in shallow crustal fault zones

Answer

A mylonite is an intrusive rock formed by emplacement of magma along a ductile shear zone

Answer

A mylonite is a sedimentary rock deposit formed in a fault-controlled lake basin

Question 4

In structural geology, what is the difference between a syncline and a synclinorium?

Accepted 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

Answer

A syncline has younger rocks at its hinge; a synclinorium has older rocks at its hinge

Answer

A syncline is an upward-closing fold; a synclinorium is a downward-closing fold

Answer

A syncline is a fold in sedimentary rocks; a synclinorium is the equivalent structure in metamorphic rocks

Question 5

What is the significance of ophiolite sequences in tectonic interpretation?

Accepted Answer

Ophiolites are fragments of ancient continental crust obducted onto the margin of a colliding continent, marking suture zones and former ocean basins

Answer

Ophiolites are remnants of ancient island arcs subducted and preserved in accretionary prisms

Answer

Ophiolites are sections of metamorphic core complexes exhumed by crustal extension

Answer

Ophiolites are massive sulfide ore deposits found along ancient divergent plate boundaries

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?

Accepted 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

Answer

P is the plunge of the fault plane; T is the trend of the fault plane as measured from the focal mechanism

Answer

P is the P-wave velocity; T is the S-wave velocity ratio used to determine fault type from seismic data

Answer

P is the principal strain axis; T is the tectonic transport direction of the fault as determined from kinematic indicators