The Peninsular Ranges orogeny occurred during the mid-Cretaceous at ∼100 Ma and affected rocks from southern Mexico to Alaska. The event resulted from the closing of an Early Cretaceous marine arc trough, named the Bisbee–Arperos seaway in Mexico and Arizona, and the Cinko Lake arc trough in the Sierra Nevada. The trough was an ocean that formed after the Late Jurassic – Early Cretaceous Nevadan orogeny and associated post-collisional magmatism. It was open for ∼40 million years and closed by westward subduction. Here, we focus initially on the most complete cross section, located in southwestern Mexico, where a west-facing Albian carbonate platform, with subjacent siliciclastic rocks built on the western margin of North America, was pulled down into a trench at 100 Ma, buried in hemipelagic mud and Cenomanian flysch, then overthrust from the west by rocks of the 140–100 Ma Santiago Peak – Alisitos arc and its substrate, the Guerrero Superterrane, which collectively document westerly subduction. This tectonically thickened collision zone was exhumed and intruded by 99–84 Ma distinctive post-collisional tonalite–granodiorite plutonic complexes, all with Sr/Y > 20, Sm/Yb > 2.5, Nb/Y > 0.4, and La/Yb > 10. These geochemical features are typical of slab failure, not arc magmas. The post-collisional plutons, previously considered to represent arc flare-ups, were derived from melting of the descending slab following arc-continent collision. Remnants of the arc, basin, related east-vergent 100 Ma thrusts, flexural foredeep, and 99–84 Ma slab failure plutons are traced from the Peninsular Ranges, through the Mojave Desert to the Sierra Nevada where similar rocks, relations, and ages occur. Along the western, back-arc, side of the orogen after collision and slab break-off, but during exhumation, east-dipping reverse faults with >10 km of east-side up movement shed 100–85 Ma plutonic and other debris westward from the hinterland into troughs such as the Valle and Great Valley. We extend our synthesis northward, from west-central Nevada to Alaska, in Part II.

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