geology of West Coast of North America

[Widdekind]

245-180 Mya

 

180-115 Mya

 

115-80 Mya

 

150-140 Mya

The following figures depict earth's tectonic plates, in the early Jurassic period. Although not explicitly depicted, the Insular Islands resided off the western coast of North America (above & right of the words 'Farallon Plate' in part B of figure 1). The Insular Islands formed c.325-300 Mya, in the late Carboniferous period. Retracing their journey, atop the Farallon plate, northeastwards across the ancient Pacific ocean, the Insular Islands apparently formed near earth's equator, at low latitudes, perhaps in the middle of the ancient Pacific ocean. Fossilized foraminifera, similar to fossils from China, Malaysia, and the ancient Tethys ocean, were eventually deposited, onto North America, when the Insular Islands collided with the coast, c.115 Mya.

 

Meanwhile, as North America obducted the ancient Farallon plate, beginning a quarter billion years ago, the coastal subduction zone apparently jumped out-board (sea-wards), with the accretion of coastal volcanic island arcs. For example, c.180 Mya, the Intermontane Islands accreted against the coast; and the Farallon-Insular plate, previously subducting under those islands, ruptured along the length of the Insular island chain. In-board, along the coast, the Insular micro-plate was created. Out-board, sea-wards, the Farallon plate began subducting under the island chain, activating volcanism.

 

Perhaps something similar had previously occurred, c.245 Mya, when the Farallon-Insular-Intermontane plate ruptured, into parallel subduction zones, with the in-board Intermontane micro-plate continuing to subduct North America, and the out-board Farallon-Insular oceanic plate starting to subduct the Intermontane, activating volcanism? Perhaps related to the Siberian traps flood basalt super-eruption, and the breakup of Pangea? Or, perhaps the Intermontane Islands had originally been a continental volcanic arc, overlying the coastal subduction zone, before being broken off from North America, by an opening Back-Arc Basin (which subsequently "failed" and began to be re-subducted, c.245 Mya)? Cp. Philippine plate.

 

 

 

References:

[Open University] Earth's Engine, ch.1-2

http://www.burkemuseum.org/static/geo_history_wa/Coast%20Range%20Episode.htm

http://www.sciencedirect.com/science/article/pii/S1342937X11003613

http://www.scotese.com/late.htm

http://www.scotese.com/newpage5.htm

Edited September 28, 2012 by Widdekind

[arc]

The Farallon Plate began its humble beginnings connected to a continent on its eastern edge, and likely fractured due to the same stresses that established the Pacific Plate's convergent boundaries along that plate's Western edge. It seems obvious that ocean plate's fracturing at particular widths is based on there structural limitations to excessive plate width. The question is was the forces involved compressive, expansive or both?