17 Structural Inheritance in the Rocky Mountain Region

The Rocky Mountain region preserves a complex tectonic history shaped by multiple contractional and extensional events from the Proterozoic to present. This session examines the role of structural inheritance in guiding deformation across superposed orogens.

Monday, 18 May 2026
4:10 PM - 5:30 PM
Alvarado F

Examination of mid-Cretaceous porphyritic intrusions in the southeast Beartooth Mountains, Montana-Wyoming: Implications for magmatism, tectonism, and structural development in the northern Laramide foreland

Paper number: 17-4
Program: Technical Sessions
Time: 5:15 PM – 5:35 PM

Authors

Abstract

Detailed examination of low-volume porphyritic intrusions in the southeast Beartooth Mountains is being conducted in order to decipher their magmatic and tectonic history, and influence on Laramide structural development. Geologic mapping shows at least three units based on phenocryst assemblage that include Kpp (plagioclase), Kpkp (plagioclase+K- feldspar), and Kpq (plagioclase+quartz). Crosscutting relations amongst Kpp, Kpkp, and Kpq are difficult to discern due to periglacial cover, but Kpq appears to be youngest. Petrographic analysis shows embayed quartz, opacitic rims, and zoned plagioclase that suggest disequilibrium conditions in the magma system, and each intrusion type displays distinct abundances of plagioclase compositions. Major oxide geochemistry classifies the intrusions as trachydacite to trachyte and largely metaluminous and ferroan to magnesian. Trace elements exhibit LREE enrichment and slight HREE depletion relative to primitive mantle. Kpp and Kpq display distinct HREE trends, while Kpkp spans both types. A porphyritic sill in Cambrian shale at the Beartooth front displays unique, relatively enriched trace element contents. Zircon U-Pb dates span ca. 92–104 Ma and include significant Archean inheritance. Some samples display multiple mid-Cretaceous date populations that are interpreted to represent antecrystic behavior in the magma system. We leverage this data to describe the middle to upper crustal magma system during intrusion, and consider possible geodynamic mechanisms of flat-slab subduction, terrane accretion, or lithospheric flexure (although none may be mutually exclusive). Despite their low-volume nature, the intrusions raise questions about plate margin processes during mid-Cretaceous North American Cordilleran tectonism. At the local scale, structural development of the Beartooth Mountains may have been controlled by the intrusions, as these occur coincident with a pronounced bend in the trend of the range front from WNW to NNE and an apparent increased heave on the Beartooth thrust.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410581

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