19 Tectonic Evolution and Competing Models for the Opening of the Gulf of California: From Subduction to Oblique Rifting

This symposium will explore major controversies in the tectonic evolution of the Gulf of California, highlighting the timing, kinematics, and driving mechanisms behind its transition from subduction to oblique continental rifting and transform faulting.

Thursday, 23 April 2026
8:00 AM - 12:00 PM
LMH, Fiesta Terrace Salon

Quaternary Emergence Recorded on Isla Carmen and Isla Monserrate, Bahía de Loreto, Baja California Sur, Mexico

Paper number: 19-11
Program: Technical Sessions
Time: 10:38 AM – 10:53 AM

Authors

Abstract

Islas Carmen and Monserrate are part of Parque Nacional Bahía de Loreto, in the Gulf of California, and record a multistage tectonic evolution involving (1) convergent tectonics and volcanism, (2) oblique rifting, subsidence, and marine deposition, and (3) subsequent emergence and fault-bounded block uplift. This study integrates geologic, structural, and geomorphic mapping on both islands; related results are presented in companion contributions. Fieldwork also included drone surveys that produced high-resolution orthomosaics and digital elevation models (DEMs).  Terrace surveys used dual-frequency GPS, post-processed to our new permanent GPS station in Loreto.

Basement rocks on both islands consist of intrusive and extrusive volcanic and volcaniclastic units of the Comondú Group. The transition from Comondú deposition associated with convergent tectonics to Miocene–Pliocene oblique extension is marked by a regional erosional surface. This surface ranges from a low-relief unconformity between Comondú rocks and Mio–Pliocene deposits of the Carmen Formation, as exposed in the Arroyo Blanco basin of Isla Carmen, to a high-relief ravinement surface across much of Isla Monserrate that separates Comondú rocks from overlying marine strata. Basal orange-colored sands are interpreted as lithic-rich erosional deposits accumulated in local marine basins of the proto–Gulf of California. These sands are overlain by biogenic deposits correlative with the Carmen Formation, including cemented carbonate caps preserved at the summit of Isla Monserrate.

The transition from subsidence and marine deposition (stage 2) to emergence and uplift (stage 3) is undated but may coincide with rift-margin uplift driven by unloading and isostatic response. Quaternary uplift is recorded on both islands by staircases of coastal terraces. Multiple terrace levels in the Arroyo Blanco and Bahía Marquer areas of Isla Carmen record progressive emergence, with the lowest terrace Uranium-series dated at 131.11 ±0.84 ka. Bedrock-cut terraces and wave-cut notches are also preserved near El Faro, on the southwestern coast of Carmen. On Isla Monserrate, terraces evolve upward from well-preserved coastal platforms with fossiliferous littoral deposits at low elevations to stripped and indurated carbonate caps, including broad platform areas at the crest of the island. Terraces on both islands are displaced by faults.  The modern topography of Islas Carmen and Monserrate reflects on-going faulting and differential uplift, with some fault-bounded blocks uplifting at rates surprising in this setting.

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

doi: 10.1130/abs/2026CD-411055

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