Early evolution of an extensional monocline by a propagating normal fault: 3D analysis from combined field study and numerical modeling

The Nopolo structure is located along the eastern margin of the Baja California peninsula and formed during the early stages (12–6 Ma) of development of the Gulf of California. The Nopolo structure is an ∼15-km-long series of two NW-striking, extensional monoclines produced by the upward propagation of normal faults. Where normal faults reach the surface, the footwall contains undisturbed, gently dipping strata, whereas the hanging wall is a highly deformed zone that contains fractured and faulted steep to subvertical strata. Long, narrow grabens with moderate to steep east-dipping strata are present in the hanging wall of the main normal faults. Initial monoclinal folding over blind normal faults produced a minimum of ∼300 m of structural relief. Once the faults propagated to the surface, they offset the monoclines ∼20–30 m before faulting ceased. We use an elastic dislocation model to invert fault geometry from bedding orientations around the Nopolo structure. The data are best matched by a listric normal fault that soles out at ∼5 km depth. The model suggests that the tip line of the fault was located ∼1 km below the surface prior to the breaching of the monocline. Because the offset along the main normal faults is minimal (∼20 m), the Nopolo structure is a unique example of an extensional faulted monocline and monocline system where faulting ended soon after the monoclines were breached and offset.