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Phanerozoic History of North America. Tectonic Context of NA: Part 4. Last Topics. CM of Eastern North America CM of the Southern North America. ENACM-Tectonic Regimes. SNACM – Tectonic Regimes. Western North America.
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Phanerozoic History of North America Tectonic Context of NA: Part 4
Last Topics • CM of Eastern North America • CM of the Southern North America
Western North America • The western margin of the North American continent extends from the southern California to central part of Alaska and from the Pacific Coast to the craton below the Great Plains.
Tectonostructural Feature • Western North America Continental Margin consists of a series of margin parallel tectonic belts.
The most landward of these belts is composed of deformed Proterozoic North America. Seaward, the belts consist of a set of terranes, each of them being differentiated mainly by the age of attachment to North America. Generally, younger terranes are to be found seaward. Tectonostructural Feature
Belt Features • The total width of the belts is highly variable. Some belts seem to be continuous along the strike and extremely short on strike. • Some of them consist of a single terrane but in some others several discrete segments have been identified along the strike. • These terranes have undergone synattachment and postattachment deformations. • Moreover, the structural arrangement varies significantly on strike.
Belt Features • In California and British Columbia it includes the surfaces that accommodate the maximum rate of slip between the plates (San Andreas and Queen Charlotte faults respectively).
Pacific Basin Lithosphere • An oceanic basin has probably existed along the western margin of North America throughout the Phanerozoic. • Its evolution can be viewed in three phases • (i) end-Proterozoic breakup of Rodinia and sea-floor spreading; • (ii) Cambrian-Jurassic intrabasinal tectonic phase from which no plate survives; • (iii) a Cretaceous-Cenozoic phase for which plates exists for which their position and kinematics can be reasonably well restored.
Phase 1: End Proterozoic • Breakup of Rodinia caused the development of the Proto-Pacific oceanic basin and the growth of the Paleozoic oceanic lithosphere against the newly generated passive margin of the western North America. • By Permian times, the sea floor spreading produced a huge ocean and the Proto-Pacific was only a part of it. The name of the huge ocean that surrounded the Pangea supercontinent is Panthalassa.
Phase 2:Cambrian-Jurassic • Plate boundaries have been created within the Proto-Pacific lithosphere, dividing it into plates. • Moreover, convergent motions relative to the Proto-Pacific plates developed at various times and places along the western margin of North America. • The convergence may not have occurred completely along the western continental margin until the Middle Cretaceous times.
Phase 2: Characteristics • In this phase, the continent started to expand basinward (westward) by accretion. All the events are recorded in the rock and structural features of the terranes and by continental arcs. • Glimpses of the pre-Cretaceous plate tectonics within the Proto-Pacific Ocean come the tectonostratigraphic records of certain terranes transferred from the Proto-Pacific plates to North America at active and collisional margins.
Phase 2: Examples • Oceanic Spreading is documented by the age of crystallization of the ophiolites and oceanic basalt in the terranes. • Ex: Trinity Terrane of the Klamath Mts. (California) includes Lower Cambrian ophiolites that can document the early magmatism at the western edge of the newly formed North America. • Middle and and Late Paleozoic ophiolites of Sierra Nevada (Feather River & Keweah ultramafic rocks) indicate Paleozoic spreading in the Proto-Pacific Ocean.
Phase 2: Record Nature • For these events, we can infer the nature of the events but the real position and orientation of the plate boundaries at which they developed remain utterly unknown.
Phase 3: Cretaceous Cenozoic • There is a huge amount of data for the Cretaceous-Cenozoic tectonics (magnetic anomalies, fracture zones, hotspot tracks, paleomagnetic data, structure and fossil provinciality). • These data indicate the presence of three major oceanic plates: Pacific, Farallon and Kula with divergent boundaries among themselves that migrated towards and intersected the convergent boundaries of the North America plate at various times and places.
Collisional Margins • The first collision is dated Late Devonian and the last one in the Middle Cretaceous times. • Therefore, by the end of the Cretaceous the whole western margin of North America was an active one. • The main criterion to document an active margin is the existence of a continental magmatic arc. Collisional margins have no continental arc, although arc magmatic rocks can be thrust on them during collision.
Devonian Collision - Antler • The main effect of the Antler Orogeny (Uppermost Devonian-Mississippian) was the destruction of the Cordilleran passive margin that has been formed with the block up of Rodinia.
Devonian Collision - Antler Mississippian Pennsylvanian
Active Margins • At such margins there is a subduction of the pacific plates below the North American continent. Such margins have been classed as Andean, employing western South America as the global example.
