No abstract

Exceptionally well exposed normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres to a few metres and are possibly the smallest visible faults studied to date. Displacement is greatest at or near the center of isolated faults and decreases toward the fault tips. Relay structures form between closely overlapping faults. The distribution of fault sizes in the study area follows a power-law (fractal) relation, and the maximum observed displacement scales linearly with fault length. The new fault data extend the global data set to more than eight orders of magnitude of fault length and indicate that there is no significant change in displacement geometry and the linear lengthdisplacement scaling relation between small and large faults.

Numerical and analogue models of fault-population evolution suggest that we should see a frequency-size transition from power law to exponential as brittle strain within a region increases. We now observe this transition in continental rift faults in the Afar region, Ethiopia and Djibouti. Furthermore, we observe that this strain transition is accompanied by a plateau in fault density and an increase in the displacement:length ratio of faults with increasing brittle strain. Once faults reach a critical density, stress shadows of nearby faults inhibit fault nucleation and restrict tip propagation. However, pinned faults continue to accommodate extension by accumulating more displacement, thus leading to higher displacement:length ratios in the higher strain regime. This strain regime transition is important to understanding the physics of fault growth and may also be important to rift localization and the development of plate boundaries.