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

Geochronology, Geochemistry and Geodynamic Implications of Mafic Magmatism in the Sierra Madre Occidental Silicic Large Igneous Province

Paper number: 19-10
Program: Technical Sessions
Time: 10:23 AM – 10:38 AM

Authors

Abstract

Northwestern Mexico offers the rare opportunity to study the evolution of magmatism in the last stages of a long-lived subduction system and its transition to a transtensional rift. The Oligocene to middle Miocene magmatism of the Sierra Madre Occidental (SMO) silicic large igneous province has been traditionally associated to a supra-subduction arc, whereas rift-related volcanism would have started after the end of subduction at ca. 14–12.5 Ma. Nevertheless, the influence on magma generation processes of documented late Oligocene onset of extension in the SMO needs to be addressed. Most petrogenetic studies of the SMO have focused on the volumetrically dominant silicic ignimbrites and have scarcely addressed less voluminous but ubiquitous mafic rocks, which can provide new insights on the magmatic-tectonic evolution of this region. Here we present a geochronologic, geochemical and isotopic (Pb, Sr, Nd, Hf) study of mafic rocks emplaced in close association with grabens and found interspersed with or overlying the silicic ignimbrites in the central and southern SMO.

 

In this region, dominantly basaltic to andesitic-basaltic rocks are found as lava flows, sills, dikes, and rare shield volcanoes. Thirty-four new 40Ar-39Ar ages, and previously published ages, allow identifying two major magmatic pulses in the late Oligocene-early Miocene (~34-18 Ma) and middle to late Miocene (~15-9.5 Ma). Mafic rocks with geochemical features of intraplate magmas were emplaced in both pulses, with a trend to more enriched mantle sources with time. Magmas with geochemical signatures commonly ascribed to arc magmas, emplaced during the SMO ignimbrite flare-ups at ~30-28 Ma and ~24-18 Ma, have isotopic compositions that preclude a significant slab contribution and point to an upper continental crustal component, which decrease with time. Middle to late Miocene basaltic andesites have distinct composition, with higher Mg# at similar SiO2, high Gd/Yb and Sr/Y ratios and isotopic composition consistent with slab melting.

 

Intraplate lavas indicate decompression melting of asthenospheric mantle since the Oligocene, related to a growing slab window and piecemeal slab removal, with rapid ascent of magmas in an extending lithosphere. Slab-melt contributions to the middle-late Miocene magmas may reflect the final rupture of the slab. These findings confirm that the SMO is an example of the peculiar volcanism associated with a dying subduction system, rather than a classic supra-subduction arc.

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

doi: 10.1130/abs/2026CD-411032

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