EVOLUTION OF THE DEXTRAL TRANSPRESSIVE SALMON RIVER SUTURE, IDAHO: ISOTOPIC EVIDENCE FOR LATE-STAGE CONTINENTAL DELAMINATION IN THE SYRINGA EMBAYMENT

The Salmon River suture of west-central Idaho formed by dextral-oblique plate motion in the Cretaceous (~130-85 Ma). The suture is unusual because it (1) juxtaposes Mesozoic island-arc rocks directly against Proterozoic continental rocks; (2) is defined by abrupt changes in ⁸⁷Sr/⁸⁶Sr initial ratios, ages, and origins of nearby plutons and within stitching plutons; and (3) changes trend from north-northeast in Idaho to west in Washington forming the Syringa embayment. Northeast of the embayment, SHRIMP U-Pb dates on detrital zircon from paragneisses show that Cretaceous island-arc rocks are intercalated with Proterozoic continental rocks; U-Pb dates on inherited zircon and Sr and Nd isotopic analyses from orthogneisses indicate diverse plutonic sources; U-Pb intrusive ages constrain the timing of events.

Sample	Age (Inheritance)	87Sr/86Sri	eNdi
Gneiss of Elk City	Y (~1350-2500 Ma)
Gneiss of Syringa	Z (~900-1450 Ma; few X, W)
Orthogneiss of Apgar Creek	94 ± 1 Ma (X)	0.7083	-5.22
Orthogneiss of Andys Hump	73 ± 3 Ma (X, Y, J, K)	0.7102	-7.62
Gneiss of Swiftwater Creek	~90 Ma (91-300 Ma; few Z, Y)
Coolwater orthogneiss	~88 Ma (88-230 Ma)	0.7066	-2.91

Cretaceous island arc-derived Coolwater orthogneiss and gneiss of Swiftwater Creek are exposed in a window through the crustal-scale top-to-the-west Lowell thrust. The arc rocks were inserted into the continent as a wedge, delaminating Meso- and Neoproterozoic rocks (gneisses of Elk City and Syringa) and continent-derived orthogneiss of Apgar Creek from underlying basement. Orthogneiss of Andys Hump originally intruded along the top of the wedge. The entire package was folded into an antiformal culmination and exposed by normal faults. These detrital zircon data also delimit previously unknown extent of Neoproterozoic rift margin deposits around the Syringa embayment, suggesting that the embayment is an inherited segment that underwent local Cretaceous orthogonal contraction within overall dextral transpressive accretion.