INVENTORY OF STRUCTURES ACROSS A HIGH STRAIN ZONE IN WESTERN IDAHO

Long-lived accretionary orogens are commonly characterized by Barrovian metamorphism, syntectonic magmatism, and widespread ductile deformation associated with intraoceanic arc–continent boundary interactions. Sustained high-angle plate convergence involving crust of sufficient buoyancy gives rise to collisional orogenesis and accumulation of high-strain synmetamorphic structures across the orogen. In this context, the spatial and temporal characterization of regionally-developed tectonite fabric plays a key role in tracking the evolution of accretionary margins. Tracking fabric development over space and time requires mapping of lithotectonic units joined during the suturing process, with continuous coverage through the [1] accreted island arc assemblage, [2] autochthonous continental margin strata, and [3] intrusive bodies emplaced therein. In this process, the across-strike continuity and extent of penetrative deformation is established and external boundaries are defined for the high strain zone [Sander, 1930; Ramsay and Graham, 1970]. Boundaries may consist of diffuse brittle-ductile faults separating penetrative and non-penetrative domains, sharp intrusive contacts which truncate regional structural ± metamorphic trends, or strain gradients. Radiometric age constraints on pre- to postkinematic mineral growth provide the temporal context necessary for dating deformation over time. Across a 25–40 km-wide tectonite belt in west-central Idaho, a ca. 136–92 Ma history of progressive deformation is apparent based on integrated field mapping, polyphase fabric analysis, and mineral age dating. This history is derived from studies conducted along a 48-km transect extending from the northern Seven Devils Mountains east into the Salmon River canyon. Our unbroken transect crosses the easternmost exposures of marine Permian and Triassic metavolcanic rocks in western North America, intersects an arc–continent boundary at ~90°, and overlaps Precambrian continental rocks along the Laurentian margin. In this presentation, we inventory mesoscopic structures mapped between the Heavens Gate fault and undeformed Idaho batholith; i.e., external boundaries of the western Idaho suture zone, a long-lived and aptly abbreviated [WISZ] accretion-related contractional orogen.