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

Radiolarites from a lost ocean: tracking Paleo-Pacific plate fragments in Central America

Paper number: 37-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Radiolarites formed in deep-marine environments provide critical records of Paleo-Pacific plate evolution and are widely exposed within accreted terranes and exhumed complexes along the American Cordilleras. In Central America, Triassic–Jurassic radiolarites constitute the oldest exposed rocks of the Caribbean Plate and are commonly interpreted as allochthonous fragments of dismembered Pacific oceanic lithosphere. Yet, the absence of an identified igneous basement beneath these successions leaves unresolved whether they formed in situ on oceanic crust or were transported during later tectonic assembly.

 

In northwestern Costa Rica, radiolarites occur as disrupted fragments in tectono-magmatic contact with Cretaceous basalts and as concordant centimeter-scale radiolarian chert layers interbedded within basaltic sequences. Previous work has concentrated on the larger disrupted bodies, while the intrabasaltic cherts remain largely unstudied. These layers permit direct comparison between radiolarian sedimentation ages and geochronologic and paleomagnetic constraints from the host basalts.

 

We present new radiolarian biostratigraphic data from the San Juanillo site in Santa Cruz, Guanacaste, where tilted centimeter-thick chert layers are preserved within massive basaltic flows. Radiolarian assemblage is characterized by Theocampe salillum, Diacanthocapsa euganea, Diacanthocapsa fossilis, Gongylothorax verbeeki, Pseudodictyomitra lodogaensis, Pseudodictyomitra pentacolaensis, Hemicryptocapsa polyhedra, Holocryptocanium barbui, Dactyliosphaera silviae, and Praeconocaryomma universa. On the basis of the taxonomic identification, a concurrent acrozone is recognized, constraining the age of the studied rocks to the Middle–Late Albian. This age is in agreement with the 40Ar/39Ar basalt ages, previously registered in the area.  Correlating radiolarian assemblage ages with existing basalt geochronology and paleomagnetic data tests whether sedimentation and crust formation were broadly coeval, as expected for in situ ocean-floor deposition, or temporally offset, consistent with allochthonous emplacement.  

 

These results refine the radiolarian biochronology successions and constrain the timing of oceanic crust formation, sedimentation, and incorporation into the Caribbean margin. More broadly, this study illustrates how combined biostratigraphic, magmatic, and paleomagnetic datasets can identify fragments of lost Paleo-Pacific plates, evaluate terrane-accretion models, and improve paleogeographic reconstructions of the Central American Cordillera.

 

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

doi: 10.1130/abs/2026CD-410873

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