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

Shawn P. Willsey, Paul J. Umhoefer, George E. Hilley

Research output: Contribution to journalArticle

36 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)651-669
Number of pages19
JournalJournal of Structural Geology
Issue number4
StatePublished - Nov 7 2002



  • Extensional monocline
  • Normal fault
  • Upward propagation

ASJC Scopus subject areas

  • Geology

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