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The rock age of a normal fault can be determined by analyzing the age of the rocks on either side of the fault. Normal faults typically form in response to extensional forces, where older rocks are uplifted and younger rocks are deposited in the hanging wall. By dating the rocks on either side of the fault, geologists can determine the relative timing of fault movement.
A geologist can use relative dating techniques such as the principle of cross-cutting relationships to determine when a fault occurred. By observing which rock layers are cut by the fault and the sequence of events, they can infer the relative timing of the fault formation compared to the surrounding rocks. Additionally, studying the age of the rocks on either side of the fault can provide further constraints on the timing of faulting.
Given the law of superposition and assuming an undisturbed "pancake" stratigraphy each successive layer is younger than the the underlying one. Therefore, the fault is the 'youngest' feature in the system because the rocks need to form first in order for a fault to truncate them.
The relative age of a fault or igneous intrusion that cuts through an unconformity is younger than the unconformity but older than the rock it cuts through. This is because the fault or intrusion must have formed after the deposition of the rock layers below the unconformity but before the deposition of the rock layers above the unconformity.
Fossils can be used to determine the relative age of rock layers by comparing the types of fossils found in different layers. Fossils of organisms that existed for a short period of time can be used to date layers of rock containing them. Geologic features such as fault lines and unconformities can also help determine the relative age of rock layers by showing where layers have been disturbed or eroded.