FOUR STAGES IN CORDILLERAN EVOLUTION

(1) Rifting of Rodinia started by 700 Ma, although complete separation to form the Laurentian margin evidently was not completed until ~520 Ma. Paleogeographic relationships between that margin and early Paleozoic and locally Neoproterozoic arc magmatic activity (in Yreka, Alexander terranes and arc detritus locally in southeastern British Columbia) are debated.

(2) In Middle Devonian time (~390 Ma) arc activity initiated along the Laurentian margin is recorded from Alaska to California. By the Carboniferous, successor arcs - in most Cordilleran terranes - were separated from the continental margin by an ocean basin (Slide Mountain and correlative terranes) that by the Permian possibly was 2000-3000 km wide but mostly was closed by the Middle Triassic. By Late Carboniferous time, Alexander terrane was linked to Wrangellia; furthermore, Permian faunal similarities between Wrangellia and Stikine terranes suggest those terranes were not widely separated and lay in eastern Panthalassa. The late Paleozoic tectonic setting probably resembled today’s western Pacific with its weakly coupled converging plates.

(3) In the early Mesozoic (~240-175 Ma), a west-facing Middle Triassic through Early Jurassic arc system (in Quesnel, Grindstone, Eastern Klamaths, northern Sierra and northwest Nevada terranes) formed close to, and in places on, the continental margin. The accompanying accretionary complex (Cache Creek and correlatives; offscrapings of Panthalassa’s floor) extends today from Yukon to California and delineates the western Pangean plate boundary. Except for Alexander terrane, early Mesozoic rocks best define older Cordilleran terranes; e.g. Wrangellia is distinguished from coeval arc terranes mostly by its distinctive Triassic plume-related basalt.

(4) By the Middle Jurassic, most arc terranes had been accreted - somewhere - to the margin of the newly formed North American Plate. Terranes (Alexander, Chilliwack, Stikinia, Wallowa, Wrangellia) now located oceanward of the Cache Creek were shuffled into position between ~175 Ma and 105 Ma during transpression which built the Cordillera. Mountain-building, equated for nearly a century (Taylor, 1910) with movement of North America towards the Pacific, results from strong coupling between converging North American and Pacific Ocean plates.