Geologic Time Line: Putting the Pieces Together - Literally!
About 365 million years ago, during the Devonian Period, a westward dipping subduction zone began west of the continent, creating a volcanic island arc chain. As the ocean crust between the arc and the continent subducted, the two landmasses began to collide. The effects were felt across present day Montana. This marked the beginning of tectonic activity along this boundary, and it resulted in the Antler orogeny. The arc accreted, and it can be seen today in the Klamath Mountains and northern Sierra Nevada Mountains of California and southern Oregon (Chernicoff, Stanley, et al. 2002). The Cordillera would continue to accumulate additions from the west.

The Sonomian arc started docking in Central California about 260 mya. By about 235 mya, the thrust sheet produced by the collision reached central Nevada.

Using maps and descriptions created by Dr. James Sears from the University of Montana, and used with his permission, we can get a visual idea of the sequence of tectonic events of the past 220 million years.

Click on each map to enlarge.

Late Triassic Hypothetical west-facing magmatic arc of same scale and curvature as modern Aleutian arc encloses a marginal sea floored by late Paleozoic-Triassic oceanic crust and marine sediments (Cache Creek, etc.). The arc collapses against North America beginning in late Triassic.
Map by James Sears©
Early Jurassic Westward subduction along the axis of the marginal sea consumes the east half beneath the west half. Volcanic terranes now comprising Sonomia and Stikinia grow on the overriding west half of the marginal sea, and approach the miogeocline. Collapse of the arc into a chord drives part of the marginal sea and distal miogeocline northward on a transform fault (white line).
Map by James Sears©
Middle Jurassic Complete closure of the marginal sea results in docking of the new volcanic terranes (Sonomia and Stikinia) against the miogeocline.
Map by James Sears©
Early Cretaceous Complete closure of the marginal sea results in docking of the new volcanic terranes (Sonomia and Stikinia) against the miogeocline.
Map by James Sears©
Middle Cretaceous Northward displacement of the distal miogeocline permits the main arc to dock directly against the Belt basin. The cusp in the main arc divides the Cordillera into two mechanical segments along the Lewis and Clark line sinistral shear boundary. The southern segment displaces the Sevier thrust belt, Colorado Plateau, and Wyoming province. The northern segment displaces the Canadian Rockies.
Map by James Sears©
Late Cretaceous The magmatic arc crosses into central Montana as the cusp deepens and the northern and southern domains shorten.
Map by James Sears©
Map by James Sears©




Late Paleocene Climax of compressional orogeny.





To see maps in full animation, click on map below:

Map by James Sears©

The terranes kept coming, moving eastward into the continent, northward along the great faults. In the process, crust thickened, great mountains formed.

The west coast is still forming, and terrane accretion continues around the planet.

This is an exciting and relatively new area of study! Better understanding of exotic terranes will contribute to a better understanding of tectonics of the past and the future. It will require the cooperation of many disciplines in geology and other sciences, and will most likely contribute greatly to each branch of science.

To learn more, browse your library, browse the World Wide Web,

and browse through the Reference Page to get started!

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Special thanks to George Head for encouragement and problemsolving!
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