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.