In Late Devonian time (about 365 million years ago) the Antler Arc was still an island arc in the Pacific Ocean. The collision of this island arc with the North American continent at the present-day location of Nevada in Early Mississippian time (about 350 million years ago) marked the beginning of accretion of exotic terranes to the western margin. In Montana there were several effects of this far-away collision. The Central Montana Trough, an east-west feature across Montana, formed about 350 million years ago and there was movement on faults in the Williston Basin.

The Sonomian arc was to the west of the North American continent in Middle Pennsylvanian time (about 300 million years ago). It started docking onto the continental margin in Central California in Middle Permian time (about 260 million years ago). By Early Triassic time (about 245 million years ago) it had penetrated to western Nevada; and in Middle Triassic time (about 235 million years ago) it reached central Nevada as the thrust sheet above the Golconda thrust fault.

Blakey's map for the Latest Triassic (about 220 million years ago) labels small offshore areas as "Miscellaneous debris moving toward accretionary wedge." These schematically indicate the many small terranes being accreted to the western margin of the continent--a process that continued up to the Mid-Tertiary.

In Early Jurassic time (about 195 million years ago) a strike-slip fault (or transform fault) formed in Central Washington that extended north into British Columbia. There are a number of these faults that extend all the way to Alaska. As can be seen in the series of maps up to the present time, these faults have been continuously active. In Middle Jurassic time (about 175 million years ago) an oceanic plateau that would become the Wrangellia terrane was located by Baja California. By Late Jurassic time (about 150 million years ago) it had moved northward along the west coast to Washington. This terrane was sliced into pieces by strike-slip faults and (as shown by the large black dot pattern on the map in the Planet Earth text) the pieces of Wrangellia.were dispersed from Washington to Alaska. A closer view of the Alaska Panhandle area and adjacent British Columbia shows many accreted terranes (including Wrangellia) which have moved northward up the coast of North America and are now "sliver-shaped" terranes bounded by strike-slip faults. (The map shows Lithoprobe's SNORCLE study area.)

The strike-slip faults extending from Washington to Alaska are driven by oblique subduction in which the subducting oceanic plate moves not at right angles to the coast but rather at a smaller angle. Deformation of the accretionary wedge tends to be resolved into a thrusting component perpendicular to the coast and a strike-slip component parallel to the coast.

An indentation or jog to the east in the coast of North America along the Oregon/Washington border was apparent in the Cretaceous Period. This is called the Columbia Embayment and resulted from strike-slip faults moving accreted terranes up the coast from Washington towards Alaska. [See maps for the Early Cretaceous (about 120 million years ago) and Late Cretaceous (about 75 million years ago).]

Jim Sears refers to the Columbia Embayment as a "cusp" in the arc of volcanoes paralleling the coast. His animation of accretion of terranes from the Triassic to the Paleocene (220 - 60 million years ago) emphasize the importance of this cusp for Montana geology. The cusp penetrated eastward into the 1.5 billion year old Belt rift basin. The location of the Boulder batholith, a large body of granite between Helena and Butte, Montana, so far inland is nicely explained by this cusp. [Granite batholiths are considered to be part of arc volcanism, occurring some 100 or so kilometers inland from the coast and trench.] (The very large copper deposit at Butte is hosted by the Boulder batholith.)

The Sears animation also graphically displays the great amount of horizontal shortening caused by the subduction and thrusting at this strongly convergent plate margin. Terrane after terrane were added to the margin of the continent and all were pushed eastward along thrust faults. In the process the crust was thickened and great mountains formed.

Plate Tectonics of Montana (Page 9 of 14)