4: Geological time and dating

Geological Time and Dating

In structural geology, deciphering the timing of deformation events is fundamental to reconstructing Earth's dynamic history. The vast scale of geological time, spanning billions of years, requires robust dating methods to position structural features—such as faults, folds, and foliations—within an accurate chronological framework. This temporal context transforms isolated observations into a coherent narrative of tectonic evolution.

Relative dating establishes the sequence of events without assigning numerical ages. It relies on foundational principles.

1. The law of superposition states that in an undisturbed stratigraphic sequence, younger layers overlie older ones.
2. Cross-cutting relationships dictate that any geological feature which cuts across another (e.g., a fault through a fold) is the younger of the two.
3. The principle of inclusions indicates that rock fragments contained within a host rock are older.
4. Unconformities, representing surfaces of erosion or non-deposition, signal significant temporal breaks.

Applying these principles allows structural geologists to determine the order of multiple deformation phases, such as establishing whether jointing occurred after folding.

Absolute dating provides quantitative age estimates, primarily through radiometric techniques that measure the decay of radioactive isotopes within minerals. Each method has specific applications based on the mineral's composition and closure temperature.

• Uranium-Lead (U-Pb) dating of zircon crystals is pivotal for dating ancient crustal rocks and Precambrian events.
• Potassium-Argon (K-Ar) and its variant Argon-Argon (Ar-Ar) are widely used for dating volcanic rocks and metamorphic minerals like micas.
• Rubidium-Strontium (Rb-Sr) and Samarium-Neodymium (Sm-Nd) systems are applied to whole rocks or minerals to date igneous crystallization and metamorphism.

These techniques calibrate the geological timescale and pin specific tectonic events to millions or billions of years.

In practice, structural geologists often target minerals that grew or recrystallized during deformation. Dating metamorphic minerals like garnet, muscovite, or monazite within a foliation can directly date the timing of fabric development. Similarly, analyzing syn-kinematic minerals from a shear zone provides an age for ductile shearing. Integrating relative sequencing with absolute ages builds detailed deformation chronologies, essential for understanding plate boundary evolution, orogeny, and the interplay between structural events.