Accreted Terrane
Terms & Phrases

The Accreted Terrane module was created by Digital Atlas staff members.

Look at an accreted terrane cross section

Most of the pre-Cretaceous rocks west of the Idaho Batholith in west central Idaho and east-central Oregon are oceanic or island arc assemblages. These rocks were formed offshore in island arcs and adjacent basins (Vallier, 1967, 1977; Brooks, 1979) and were accreted to the North American continent between Late-Triassic and mid-Cretaceous time. This means that before Jurassic time, the West Coast of North America was situated near Riggins, Idaho.

The Suture Line

Pre-Cenozoic rocks near the western boundary of Idaho fall into one of two settings. These two settings are separated by the strontium-isotope line. All the plutonic rocks west of the dashed line have low initial ratios (<O.7043); whereas, all rocks to the east of the line have high initial ratios (> 0.7055). This change in ratios is made in less than a distance of 6 miles. The strontium - isotope line therefore represents the suture line where the accreted island arc assemblages were welded to western North America.

Paleozoic sedimentary rocks overlying Precambrian rocks of the continent make up the miogeocline on the east side of the suture line. These sedimentary rocks have been intruded by the batholith. On the west side of the line is a complex assemblage of rock derived from oceanic crust and portions of an island arc. The ages of this accreted assemblage ranges from Devonian to Early Cretaceous. Granite plutons intruded the accreted terrane; and later, Late Cretaceous marine strata covered portions of the accreted terrane, which were depressed as a shallow basin.

Four Smaller Terranes

The oceanic and island arc terrane is divided into four smaller terranes: (1) the dismembered oceanic crust terrane or melange, (2) the Wallowa Mountains-Seven Devils Mountains volcanic arc terrane, (3) the Juniper Mountain-Cuddy Mountain volcanic arc terrane (may be a southern extension of the Wallowa-Seven Devils volcanic arc), and (4) Jurassic Flysch terrane of forearc basin marine sedimentary rocks. All four terranes are separated by major unconformities and faults and were intruded by plutons of Late Jurassic and Early Cretaceous age. These terranes were formed in the eastern Pacific Ocean, far from their present position, and were transported on lithospheric plates to be accreted on the edge of the continent.

Oceanic Crust Terrane

The dismembered oceanic crust terrane has undergone extreme deformation that is characteristic of tectonic melanges. This tectonic disruption probably happened in Late Triassic time. The oceanic crust terrane includes mafic rocks (ophiolite), metamorphosed chert, argillite, tuff, lava flows and limestone that ranges in age from Devonian to Middle Triassic. It includes the Canyon Mountain Complex, Elkhorn Ridge Argillite and Burnt River Schist of eastern Oregon and the lower part of the Riggins Group of western Idaho. Fossils and other evidence indicate that rocks derived from a deep ocean environment as well as from shallow water are mixed together.

Look at some accreted terrane photos:
* Hells Canyon 1
* Hells Canyon 2
* Hells Canyon 3

Volcanic Arc Terranes

The Wallowa Mountains-Seven Devils Mountains volcanic arc terrane includes the Lower Permian and Middle and Late Triassic volcanic rocks of the Seven Devils Group and the Clover Creek Greenstone. This terrane also includes overlying Late Triassic and Early Jurassic marine sedimentary rocks of the Martins Bridge, Hurwal and Coon Hollow Formations and the Lucille Slate.

The Juniper Mountain-Cuddy Mountain volcanic arc terrane includes assemblages of metamorphosed basalt, andesite, dacite and rhyolite flows that are interlayered with marine sedimentary rocks. The two volcanic arcs are separated by layers of Cenozoic age so their relationship is not known. The volcanism that created the two arcs ended in Late Triassic. Both arcs may represent different parts of the same arc or it is possible that the two terranes represent two different arcs with different origins. Based on structural and stratigraphic similarities it is probable that the volcanic terranes are different parts of the same arc.

The volcanic arc terranes are similar to the accreted island-arc terranes termed Wrangellia that lie between Alaska and Vancouver Island. However, based on the composition of volcanic and sedimentary rocks, the accreted terrane in Idaho is not related to the Wrangellia terrane.

The Jurassic Flysch

Flysch is defined as thin bedded, poorly sorted, deep water sandstone and mudstone rapidly deposited, usually during an orogenic pulse. The Jurassic Flysch accreted terrane includes siltstone, argillite, slates, phyIlites, volcanic wacke, arkosic wacke, limestone and conglomerate. Poor sorting, angular grains and rock fragments are common to these rocks. The Squaw Creek Schist, Fiddle Creek Schist, Lightening Creek Schist and the Berg Creek Amphibolite of the Riggins group are representative of the Jurassic Flysch.

The Flysch is situated between the volcanic arc and the oceanic crust terrane and is believed to have been compressed against the arc by the oceanic terrane. Deposition of the flysch probably ended in the Late Jurassic. The oceanic terrane and the volcanic arc terrane were sutured in Late Triassic and Early Jurassic time and the Flysch derived from the volcanism was deposited along the suture.

Arc Juncture

The continent-island arc juncture in west-central Idaho is narrow and well defined. Lund (1984) has recently described the geological character of this juncture or suture zone. On both sides of the suture, the metamorphic grade increases to amphibolite facies near the juncture. The suture zone lacks many of the features of a typical subduction zone. It is an abrupt, nearly vertical juncture between the continental metasedimentary rocks of Paleozoic to Middle Proterozoic age on the east side of the metamorphic rocks of the Permian - Triassic Seven Devils island arc and the overlying Riggins Group on the west. No transitional metasedimentary rocks with a marginal basin exists at the suture zone as is common for other known suture zones. According to Lund (1984), the suture was made by a convergent, right-lateral fault that sliced away the edge of the continent and then brought slabs of exotic oceanic (accreted) terrane in from the southwest.

Deformation and
Time of Accretion

The accreted terrane was deformed in the Late Triassic and again in the Late Jurassic. The Late Triassic deformation occurred following deposition of most of the volcanic rock units. The time of the accretion is estimated to have occurred 118 million years ago (Lund, 1984; Sutter and others, 1984). Deformation and metamorphism of the Riggins Group at the contact with continental rocks occurred at that time However, the accretion process probably occurred over a period of time ranging between Late Triassic and mid-Cretaceous. During this time and for a period afterwards, the Idaho Batholith was formed by magmas generated from subduction of the eastward moving plate.

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