The Rocky Mountains formed mainly through plate tectonics during the Laramide orogeny, roughly 80 to 55 million years ago, when the Farallon Plate (and related oceanic lithosphere) began subducting at a shallow angle beneath the western edge of the North American Plate. This shallow subduction caused extensive deformation far inland, stacking and thrusting crustal rocks to build the broad, high mountain belt we see today. Earlier, remnants of older, ancestral ranges (the Ancestral Rockies) were reworked and uplifted, and later uplift and erosion helped shape the current topography. Key nuances and regional variations:
- Subduction geometry: A shallow-angle (flat-slab) subduction of the Farallon plate pushed deformation deeper into the continent, promoting widespread crustal shortening and uplift rather than localized mountain-building near the trench.
- Pre-existing structures: The region collided with and accreted fragments of various offshore and microcontinental pieces, contributing to complex tilting and faulting that helped form the modern range.
- Canadian Rockies: In the far north, the Canadian Rockies show a slightly different but related story, with terrane accretion and thrusting over older basement rocks, producing a comparable high-elevation belt.
- Later modifications: After the Laramide orogeny, continued tectonic activity, magmatism, and erosion sculpted features like massive thrust faults, deep valleys, and asymmetrical ranges, shaping the present landscape.
If you’d like, I can tailor a concise timeline or a simple diagrammatic sequence illustrating the main tectonic events and how they produced the Rockies.
