Chapter 53 Relative Plate Motion Rifts and Triple Junctions

Year 3 · Geotectonics · G333-L2

Calculate relative plate vectors distinguish half-spreading and full spreading rates and test an idealized triple-junction model.

A schematic convergent plate boundary and subducting oceanic lithosphere. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Calculate relative velocities in a stated reference frame.
Separate spreading rate from half-spreading rate.
Explain why boundary geometry and motion must be compatible.

Motion needs a reference

Relative plate motion describes one plate compared with another. “Absolute” motion uses an explicitly chosen reference frame, such as a mantle-related model, and is not motion relative to a universal stationary point. Geodetic measurements describe present motion over observation intervals, whereas magnetic anomalies and dated geological offsets can yield longer-term averages. The two need not match exactly if motions changed.

On a small region treated as flat, velocity vectors can be resolved into east and north components. If vA and vB are velocities in the same frame, the velocity of B relative to A is vB/A = vB − vA. The vectors must use compatible units and reference directions. At global scale, rigid plate motion is represented by rotation about an Euler pole rather than uniform translation on a flat sheet.

Rates and boundary behaviour

A magnetic anomaly at a known distance from a spreading axis gives an average half-spreading rate if it formed at the ridge and moved away on that side. Full spreading is the relative separation of both sides. It equals twice the half-rate only for symmetric spreading. Ridge migration, asymmetric accretion and later deformation complicate the simple model.

Resolve relative velocity into components normal and parallel to a boundary. The normal component determines convergence or divergence; the parallel component describes strike-slip motion. Oblique convergence can be partitioned among several structures. A map of arrows without a boundary orientation cannot fully describe the interaction.

Evolving boundaries

Continental rifting can create faulted basins, thinning and magmatism. Some rifts proceed to oceanic spreading, while others fail; an aulacogen describes a failed rift arm in an appropriate tectonic interpretation. A sequence from rift to ocean to convergence is a useful long-term model, not a mandatory life cycle for every basin.

A triple junction is where three plate boundaries meet. The relative velocities must close as a vector circuit, but closure alone does not prove that a proposed junction geometry is stable through time. Each boundary must accommodate the relative motion assigned to it. Boundaries can migrate, reorganize and disappear, leaving older structures that no longer match present motions. Compare reconstruction ages rather than combining features from different times on one unlabelled diagram.

Keep vectors in one reference frame

Before subtracting plate velocities, express them in the same east–north frame or other stated system. A velocity quoted relative to plate A cannot be combined directly with one quoted relative to a different reference without transformation. In an illustrative local frame, vA = (10, 5) and vB = (−5, 15) mm/yr, so vB/A = (−15, 10) mm/yr with magnitude about 18.0 mm/yr. The signs indicate westward and northward components. On a globe, rigid rotations about Euler poles replace the simple translation model, especially over large distances.

Project motion onto the boundary

A relative velocity of 20 mm/yr directed 30° from a boundary’s normal has normal component 20 cos 30° ≈ 17.3 mm/yr and parallel component 20 sin 30° = 10 mm/yr. Geometry determines whether the normal component is convergence or divergence. In a real oblique boundary, separate structures may accommodate different components, and local block motions can depart from the rigid-plate approximation. A map arrow without its reference frame and boundary orientation does not tell a complete kinematic story.

Explain rifting as a sequence of possible outcomes

Continental extension may generate normal faults, tilted blocks, subsiding basins and magmatism. Continued extension can lead to crustal separation and oceanic spreading, but some rifts stop before that stage. An aulacogen is interpreted as a failed rift arm within an appropriate regional history. Identify its age, structures and relation to other arms rather than calling any long basin a failed rift. Later compression can reactivate older extensional structures, so a present reverse fault may occupy a surface with an earlier normal-fault history.

Test a changing junction

For plates A, B and C, the relative velocities must close: vB/A + vC/B + vA/C = 0. If one measured vector fails closure beyond uncertainty, investigate frames, units or the rigid-plate assumption. Closure alone does not ensure that three boundaries can retain the drawn junction geometry. A ridge, trench and transform each have different kinematic requirements, and a junction can migrate or reorganize. Compare maps reconstructed for the same time and track which boundary segments are active. A present arrangement should not be projected backward indefinitely simply because an older basin has a roughly matching trend. Geological age control is necessary to connect kinematics with the evolution of actual rock assemblages.

Worked activity

In a fictional local frame, A moves 20 mm/yr east and B moves 10 mm/yr north. B relative to A is therefore (−20, +10) mm/yr in east–north components. Speed is √(20² + 10²) ≈ 22.4 mm/yr, directed northwest.

A magnetic stripe lies 40 km from a ridge and is 2 million years old. The average half-rate is 20 mm/yr. Only if the other side spread symmetrically would full separation be 40 mm/yr. State both assumptions before using the calculation.

Self-check

How is B relative to A calculated?
Answer
Subtract A’s velocity from B’s in the same frame.
Does doubling one side always give full spreading?
Answer
No. That assumes symmetric spreading.
Does vector closure prove triple-junction stability?
Answer
No. Boundary geometry and allowable motions must also be compatible.

Chapter 53 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

In one east–north reference frame, A moves (30, 0) mm/yr and B moves (0, 40) mm/yr. What is B relative to A?
1. (−30, +40) mm/yr, with speed 50 mm/yr
2. (+30, −40) mm/yr, with speed 50 mm/yr
3. (+30, +40) mm/yr, with speed 70 mm/yr
4. (0, +40) mm/yr, with speed 40 mm/yr
Answer and explanation
A. (−30, +40) mm/yr, with speed 50 mm/yr Subtract A from B to obtain (−30, +40). Its magnitude is √(30² + 40²) = 50 mm/yr.
A 3 Ma magnetic stripe is 60 km from a ridge axis. What is the average half-spreading rate?
1. 10 mm/yr
2. 20 mm/yr
3. 40 mm/yr
4. 180 mm/yr
Answer and explanation
B. 20 mm/yr 60 km/3 million years = 20 km per million years = 20 mm/yr. This is one side’s half-rate.
Under what additional assumption can that 20 mm/yr half-rate be doubled to obtain full separation?
1. The ridge must coincide with a coastline
2. Both sides must have different magnetic ages
3. The opposite side has spread symmetrically at the same rate
4. The reference frame must be universally stationary
Answer and explanation
C. The opposite side has spread symmetrically at the same rate Full separation sums both sides. Doubling one side assumes symmetric spreading.
An oblique relative velocity crosses a plate boundary. Which decomposition identifies convergence and strike-slip components?
1. Components based only on the map’s paper edges
2. Components of rock colour and grain size
3. Components along arbitrary unrelated fossil zones
4. Components normal and parallel to the boundary
Answer and explanation
D. Components normal and parallel to the boundary Normal motion indicates convergence or divergence; parallel motion describes the strike-slip component.
Three relative velocity vectors close around a plate circuit. What remains to be tested for a proposed triple junction?
1. Whether the boundary geometry can accommodate the motions and persist through time
2. Whether vector subtraction should be replaced by scalar addition
3. Whether all three plates have the same crust type
4. Whether the junction lies exactly at sea level
Answer and explanation
A. Whether the boundary geometry can accommodate the motions and persist through time Vector closure is necessary but not a complete stability test. Geometry and allowed boundary motion must also agree.

All 84 chapters

Search This Blog

Geology Hub