28-8 Pliocene-Quaternary Evolution of Faults and Topography in the Loreto Area, Baja California Sur – 2: Stratigraphy and Geomorphology

Session: Recent Advances in the Gulf of California Oblique Rift: Offshore and Onshore Studies // Avances Recientes en el Rift Oblicuo del Golfo de California: Estudios Marinos y Terrestres (Part I)

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Abstract:

Pliocene-Quaternary faults, sediments, and geomorphology in the Loreto area of Baja California Sur, Mexico, provide a unique record of structural controls on landscape evolution in a tectonically active transtensional setting. The Loreto basin formed by westward tilting, asymmetric subsidence, and sedimentary filling between ~3.5 and 2 Ma in the hanging wall of the oblique dextral-normal Loreto fault (Dorsey and Umhoefer, 2000). Stratigraphic analysis shows that fault-controlled basin subsidence was initially slow (0.4 mm/yr) from ~3.5–2.5 Ma, accelerated to ~8 mm/yr for a short time (~2.46–2.36 Ma), and slowed again to the end of sediment accumulation at ~2.0 Ma. The contact between basal Sequence 1 (undated) and Sequence 2 (well-dated with interbedded tuffs) is a conformable marine flooding surface which indicates there is no gap in time that would permit an older age for the Loreto fault, as suggested by Mark et al. (2014) (see also Darin et al., this volume). A narrow belt of proximal alluvial-fan conglomerate requires steep topography in the immediate footwall during the lifetime of the fault. A 3–5 km wide pediment in the footwall of the Loreto fault has formed by erosional retreat of the Main Gulf Escarpment (MGE) at a spatially variable rate of ~2.0 ± 0.5 km/My since fault slip ended ~2 Ma. Interfluves are beveled by lateral channel migration. Thus, we confirm that the Loreto fault initiated ~3.5 Ma, it became inactive in the south at ~2.0 Ma, and slip continues very slowly in the north where basinal sediments remain in the subsurface. The retreat rate overlaps with known rates of erosional escarpment retreat globally but is faster than expected for the arid climate of Baja California. Channel profiles that cross the pediment reveal strong concavity in their upper reaches where they descend the MGE, and they are undeflected across the Loreto fault. The Escondido fault south of Loreto displays evidence for late Quaternary slip, providing a modern analog for the Loreto fault when it was active. Similarities and differences between channel profiles in footwalls of the two faults suggest a model in which footwall uplift initially sets steep topographic gradient and irregular channel concavity, which are maintained by positive feedbacks and smoothed by erosion during escarpment retreat after the fault becomes inactive.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

doi: 10.1130/abs/2026CD-12801

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