28-7 Pliocene-Quaternary Evolution of Faults and Topography in the Loreto Area, Baja California Sur – 1: Geology and Geochronology

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)

Presenting Author:

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Abstract:

The Gulf of California is a young, active oblique-transform rift that developed since ~12 Ma. Along its western margin, the Main Gulf Escarpment (MGE) represents a classic rift-margin topographic feature that runs discontinuously along the Baja California peninsula and exhibits up to ~2 km of relief. Near Loreto, Baja California Sur, the base of the MGE is separated from the main rift-bounding structure, the dextral-oblique normal Loreto fault, by a ~3–5-km wide pediment in the footwall. The timing and rate of escarpment retreat and footwall peneplanation are debated. Stratigraphic evidence from the cogenetic Loreto basin shows that the Loreto fault initiated ~3.5 Ma and experienced a rapid, short-lived subsidence history that effectively ended by ~2 Ma (Dorsey and Umhoefer, 2000), implying rapid escarpment retreat during the past ~2 Myr. In contrast, bedrock thermochronologic data and an ⁴⁰Ar/³⁹Ar date of 5.655 ± 0.152 Ma from a presumed extrusive "piedmont lava" at Cerro Papiñi near the western margin of the pediment have been interpreted to indicate an earlier history of faulting, footwall exhumation, and escarpment retreat between ~8 and 4 Ma (Mark et al., 2014). Here we synthesize new and published geologic mapping and geochronologic data from the footwall of the Loreto fault to reevaluate constraints on the evolution of footwall topography and erosional retreat of the MGE. Our 1:12,000-scale mapping at Cerro Papiñi demonstrates that the dated ~5.7-Ma rock is from an adakitic dike that intrudes the Lower Miocene Comondú Group, not a lava flow, requiring that escarpment retreat and footwall exhumation postdate 5.7 Ma. We further suggest that ~8–4 Ma apatite (U-Th)/He cooling ages reflect partial thermal resetting by Miocene rift-related heating, rather than prolonged footwall exhumation. Our synthesis supports a young, short slip history on the Loreto fault and indicates that the pediment formed in the past ~2 Myr. During this time, the steep erosional escarpment along the east flank of the Sierra La Giganta retreated westward ~3–5 km at rates of ~1.5–2.5 km/Myr (see also Dorsey et al., this volume). This rate overlaps with known rates of erosional escarpment retreat globally but is faster than expected for the regional arid climate, highlighting the need for new work to better understand surface processes at arid, rapidly evolving rift margins.

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

doi: 10.1130/abs/2026CD-13186

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