Faults Folds and Joints Questions and Answers

Answer: A

Shortening the rock mass forces the layers to buckle, which is why fold belts mark former convergent margins. Tension produces normal faulting and jointing instead, so the structure records the stress regime that made it.

Answer: B

Correct answer: away from one another, oldest rocks in the core

An anticline is an upfold, so the beds arch over and the limbs dip away from the axis on either side. Because the sequence was deposited youngest on top, arching it upward and eroding the crest exposes progressively older rocks towards the centre. The syncline is the converse in both respects: limbs dipping towards the axis, youngest rocks in the core.

Answer: C

Correct answer: older toward the fold axis

Once erosion has planed off the crest, the age relationship rather than the topography identifies the fold. The oldest beds, originally deepest, are brought highest at the crest of the arch and so appear along the axis, with successively younger beds outcropping outward on both sides. A syncline shows the reverse. This is why a mapped anticline may occupy a valley and a syncline a ridge - the structure is read from the stratigraphy, not the landform.

Answer: D

Correct answer: containing the hinge lines of successive beds

Each folded bed has a hinge line where its curvature is greatest, and the surface joining those hinge lines through the whole folded sequence is the axial plane. Its orientation classifies the fold: vertical for a symmetrical fold, inclined for an asymmetrical or overturned one, horizontal for a recumbent one. It is not perpendicular to the axis - the axis lies within it.

Answer: A

Correct answer: vertical, with limbs dipping equally

A symmetrical fold has a vertical axial plane, and the two limbs are mirror images dipping at equal angles in opposite directions. It records compression applied more or less horizontally and equally from both sides. As the compression increases or becomes one-sided the axial plane rotates, giving first an asymmetrical fold, then an overturned one, and finally a recumbent fold with a horizontal axial plane.

Answer: B

Compression has pushed the fold over so far that one limb has rotated past the vertical, leaving both limbs inclined the same way. One limb is then inverted, so superposition no longer gives the true order of the beds.

Answer: C

Correct answer: horizontal or nearly so

In a recumbent fold the axial plane has been rotated until it lies flat, so one limb is inverted and lies upside down beneath the other. It represents the most intense compression in the sequence of fold styles, and it is characteristic of the cores of major mountain belts. Where such a fold is sheared through along its lower limb it becomes a nappe.

Answer: D

Correct answer: a single local step of steeper dip

A monocline is a one-limbed structure: the beds are horizontal, steepen briefly through a flexure, and resume horizontality at a different level. It has no opposed limbs and no true axial plane in the ordinary sense. Monoclines commonly form in a sedimentary cover draped over a fault in the basement beneath, the cover flexing where the rigid rock below has broken.

Answer: A

Correct answer: inclined rather than horizontal

A fold axis need not be horizontal; where it is tilted the fold is said to plunge, and the amount is the angle of that inclination below the horizontal. On a map a plunging anticline appears as a nose closing in the direction of plunge, with the outcrop pattern forming a V or a horseshoe rather than parallel bands. Recognising the closure is what tells the mapper the fold plunges and which way.

Answer: B

Correct answer: outward in all directions from a central point

A dome is a fold with no axis of elongation: the beds dip away in every direction from a central high, so the outcrop pattern is a set of concentric rings with the oldest in the middle. An anticline has a linear axis and its beds dip away in only two directions. The basin is the corresponding downfold, with the youngest rocks in the centre.