Fault linkage and its controls on fault growth and basin evolution: Insights from analogue experiments

The evolution of fault-bounded basins and the concomitant migration of hydrocarbons and fluids are strongly influenced by fault activity and, in the case of an extensional tectonic setting, by the interaction of fault planes in relay zones. Fault linkage is a process that develops at relays between sufficiently closely spaced fault planes during their propagation. Fault interaction depends on several factors, such as the degree of under- or overlapping fault arrays, the similar or opposed polarity of fault planes and a separation that should not exceed a critical distance.

Motivated by observations at a km-scale fault relay of a major normal fault in the Magdalena Valley, Northern Andes of Colombia, we designed an analogous sandbox model, in which we simulated the linkage of rift zones separated at distances equivalent to two to four times the dimension of the height of a uniform sand layer. Fault nucleation took place at pre-designed seeds or at the velocity discontinuity of a moving sheet along the base of the sandbox and gave rise to two offset graben structures. Early fault linkage took place by means of two sub-vertical faults, which formed a shortcut between an inner and an adjacent outer border fault of the offset graben structures, enclosing a small horst in between.

The kinematic meaning of these short-cut faults became evident by the subsequent growth pattern of the faults opposite to the linked strands. On approaching the relay zone, these faults turned into an attitude almost perpendicular to their imposed trend. According to the displacement senses set up parallel to the axes of the offset graben structures, the displacement transfer on the two short-cut faults accommodated a strike-slip component. Particle analysis by means of the MATLAB’s PIVlab © tool and photogrammetric processes corroborated these findings. Displacement transfer on the short-cut faults set in at the very onset of the formation of the two graben structures. During successive deformation stages two distinct velocity fields parallel to the graben axes became established, each one pointing away from the structural high of the relay zone.

Although our boundary conditions are restricted to a uniform layer and orthogonal extension, this experimental scenario may form a starting point for testing new questions about the propagation of bounding faults at the termination of graben structures, such as those found at the East Africa Rift. Here, rifting evolved within a lithospheric high, impeding the accumulation of fine-grained or “soft” sedimentary sequences in precursor basins.