Ann M. Vlad¹, Paul J. Umhoefer¹

(1) Northern Arizona University, Flagstaff, AZ

ABSTRACT: Evolution of Normal-Faulted Monoclines in the Loreto Rift Segment, Baja California Sur, Mexico

The Baja California peninsula, Mexico, sits along an oblique-divergent plate boundary in the Gulf of California. The Escondido fault is located in the southernmost portion of the ~100-km-long Loreto rift segment on the peninsula. Along strike north of the Escondido fault is the Nopolo structure, a normal-faulted monocline.

The Escondido fault is a 16-km-long, NNW-striking down-to-the-east normal fault zone. Fault displacement is 1-2 km, and much of the hangingwall is buried beneath Quaternary alluvium. To the south, displacement decreases and terminates in an accommodation zone, while to the north, the fault dies into a monocline. The fault zone consists of secondary antithetic and synthetic normal faults within moderately east-dipping (20-55°) strata. The Escondido fault began as a propagating blind normal fault and produced an east-facing monocline. Once the Escondido fault approached the surface, the limbs of the monocline were steepened, and were eventually faulted. This stage of development is similar to the Nopolo structure. Evidence of Quaternary motion and a significant amount of offset, indicates that motion continued well after the monocline was breached. Thus, the Loreto area has evidence for the evolution from extensional monoclines to large normal faults.

The eastern Gulf of Suez, an Oligocene to Miocene rift, contains several faulted monoclines and is analogous to early rifting in the Gulf of California. In the Gulf of Suez, these faulted monoclines trap oil reserves. The Escondido fault is believed to have formed in a similar manner to structures in the Gulf of Suez and in experimental modeling.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado