37 Cordilleran Margin Evolution and Supercontinent Tectonics: From Pangea Assembly to North American Orogenesis (Posters)

This session integrates tectonics, petrology, geochronology, stratigraphy, and paleontology to illuminate how Paleo-Pacific plates drove subduction dynamics, crustal evolution, and paleogeographic reconstructions across the complex geological architecture of the American Cordilleras.

Friday, 24 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Subduction deformation recorded in High-Pressure blocks of the Puerto Nuevo Mélange complex (Baja California Sur, Mexico): Insights into Paleo-Pacific plate subduction

Paper number: 37-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Puerto Nuevo Mélange Complex (Baja California Peninsula, NW Mexico) constitutes the lowermost structural unit of the Triassic (221 Ma) North Vizcaíno suprasubducción ophiolite. This mélange comprises blocks of mafic-ultramafic and sedimentary protoliths enclosed within a serpentinite matrix and recording a wide range metamorphic degrees including greenschist, blue-schist, amphibolite, and eclogite facies assemblages related with a subduction processes. Nevertheless, the deformation mechanisms recorded by these blocks during their metamorphic history remain poorly constrained. To better understand the deformations mechanism active during the subduction of the downgoing plate, we present novel microstructural and geochronological data of two high-pressure (HP) metamorphic blocks (i.e., omphacitite and foliated eclogite) from the Puerto Nuevo Mélange Complex. This contribution focuses on the deformation mechanisms, with particular emphasis on omphacite microtextures, and in situ titanite U-Pb geochronology. Electron Backscatter Diffraction (EBSD) analysis reveal that the omphacitite exhibits a randomly oriented fine-grained omphacite aggregate crosscut by glaucophane and plagioclase bearing veins. The lack of a crystallographic preferred orientation (CPO) in omphacite grains suggests a grain size reduction under a brittle deformation regime likely during blueschist facies conditions. In contrast, foliated eclogite consists of omphacite + garnet + rutile overprinted by a retrograde amphibolite assemblage of hornblende + clinozoisite + titanite. Elongated omphacite crystals display subgrain development and LS-type (lineation and foliation-forming) CPO, implying deformation by dislocation creep under crystal-plastic deformation in the eclogite facies. Titanite U-Pb geochronology dating yielded a lower intercept age of 168.8 ± 12.6 Ma (Middle Jurassic) for the onset of metamorphic retrogression of the foliated eclogite rock. These results indicate contrasting deformation regimes within HP blocks of Puerto Nuevo Mélange, with brittle deformation experienced by the fractured and non-foliated omphacitite during blueschist facies, and crystal-plastic deformation recorded in the foliated eclogite during peak eclogite facies metamorphism, followed by amphibolite facies retrogression in the Middle Jurassic. Together, these microstructural and geochronological provide new insights into deformation processes operating in subduction zones and contribute to understand the tectonic evolution of western Paleo-Pacific margin of North America during Mesozoic.

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

doi: 10.1130/abs/2026CD-410990

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