THEMED ISSUE: EarthScope IDOR project (Deformation and Magmatic Modification of a Steep Continental Margin, Western Idaho–Eastern Oregon) Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue B. Tikoff1, J. Vervoort2, J.A. Hole3, R. Russo4, R. Gaschnig5, and A. Fayon6 1DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN-MADISON, 1215 W DAYTON STREET, MADISON, WISCONSIN 53706, USA 2SCHOOL OF THE ENVIRONMENT, WASHINGTON STATE UNIVERSITY, PO BOX 642812, PULLMAN, WASHINGTON 99164, USA 3DEPARTMENT OF GEOSCIENCES, VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY, DERRING HALL, 1405 PERRY STREET, BLACKSBURG, VIRGINIA 24061, USA 4DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF FLORIDA, 241 WILLIAMSON HALL, GAINESVILLE, FLORIDA 32611, USA 5DEPARTMENT OF ENVIRONMENTAL, EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF MASSACHUSETTS LOWELL, 1 UNIVERSITY AVENUE, LOWELL, MASSACHUSETTS 01854, USA 6DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF MINNESOTA TWIN CITIES, 310 PILLSBURY DRIVE SE, MINNEAPOLIS, MINNESOTA 55455, USA ABSTRACT The EarthScope IDOR (Idaho-Oregon) project was designed to investigate the formation of the steep accretionary boundary associated with the Salmon River suture zone and western Idaho shear zone (WISZ), in western Idaho and eastern Oregon (western USA). The research was designed to test three hypotheses concerning the present geometry of the boundary: (1) a crustal detachment; (2) a lithospheric detach- ment; and (3) a curved original geometry for the WISZ. The data presented in this volume indicate that none of these original three models is correct. Rather, the WISZ continues vertically downward through the entire crust, its steep orientation consistent with a significant compo- nent of strike-slip and/or transpressional tectonism. Moreover, the western Idaho shear zone is extremely well preserved despite abundant subsequent magmatism (Idaho batholith, Challis event, Columbia River Basalt Group) and tectonism (Cretaceous contraction, Eocene exten- sion, Miocene–present extension). The ten research papers in this volume report on various aspects of this history. Four papers address the kinematics and tectonic history of rocks spatially associated with the WISZ, including its along-strike northward continuation. Two papers address the tectonic history of the accreted terranes, including geochemical arguments for different lithospheres sampled during magmatic episodes. Three papers document deformation, exhumation, and emplacement rates in the Idaho batholith. The final paper constrains the modern crustal architecture along a transect from the accreted terranes in the west to cratonic North America in the east. Together, these studies constrain the tectonic history of a subvertical tectonic boundary in the North American Cordillera. LITHOSPHERE; v. 9; no. 2; p. 151–156 | Published online 14 February 2017 doi:10.1130/L628.1 ACCRETED TERRANE-CRATON BOUNDARY IN WESTERN Following Giorgis et al. (2007), we refer to the Salmon River suture IDAHO: MOTIVATION FOR THE IDOR EARTHSCOPE PROJECT zone as the zone of western Idaho that records the collision of the accreted terranes of the Blue Mountains with North America. The western Idaho Understanding the growth and modification of the western North shear zone (WISZ), in contrast, is a younger shear zone within the over- American continental margin has been a long-standing goal in the geologi- all Salmon River suture zone. Although accretion occurred by the Late cal science community. The continental margin evolution in the western Jurassic–Early Cretaceous (Walker, 1986; Selverstone et al., 1992; Snee United States was dominated by active plate boundary processes, includ- et al., 1995; LaMaskin and Dorsey, 2016), the accretionary boundary ing subduction, terrane accretion and translation, distributed deformation, was significantly modified by the mid-Cretaceous subvertical WISZ (e.g., and voluminous magmatism. McClelland et al., 2000; Giorgis et al., 2005). Unusually sharp isotopic The Salmon River suture zone of western Idaho is an archetypal example gradients (Sr, Nd, O) representing the edge of continental lithosphere cor- of a steep terrane boundary of the North American Cordillera (Fig. 1). The respond to the surface location of the WISZ (Tikoff et al., 2001). Structural Salmon River suture zone juxtaposes accreted terranes (island arcs of the studies in the WISZ suggested dextral transpression with strong east-west Blue Mountains) and Precambrian North America. Hamilton (1963, 1969), shortening (Giorgis and Tikoff, 2004; Giorgis et al., 2005). working in the region around Riggins, Idaho, was the first to notice a spa- The EarthScope IDOR (Idaho-Oregon) project was designed to unravel tially abrupt change in both age and rock composition along this zone. Since the tectonic history along this boundary. This project was submitted to that time, the region has also been noted for sharp isotopic gradients (Sr, Nd, the EarthScope program of the National Science Foundation (primary O) (e.g., Armstrong et al., 1977; Fleck and Criss, 1985, 2004; Manduca et investigators: Tikoff, Hole, Russo, and Vervoort). Specifically, the IDOR al., 1992) interpreted to represent the edge of continental mantle lithosphere. project investigated the tectonic history of western Idaho and eastern Ore- gon, including collision (e.g., Salmon River suture zone), syncollisional Basil Tikoff http://orcid.org /0000 -0001 -6022 -7002 to postcollisional transpressional deformation (e.g., WISZ), voluminous LITHOSPHERE© 2017 Geological | Volume Society 9 of| AmericaNumber 2| |For www.gsapubs.org permission to copy, contact [email protected] 151 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/151/1001391/151.pdf by guest on 23 September 2021 TIKOFF ET AL. 122° 118° 114° 110° Canada A Cenozoic, undivided 48° Cretaceous plutonic belts Washington B Wrangellia terrane and metamorphic Oregon BM rocks of the Washington Cascades Montana 44° Wyoming Cretaceous Ochoco and 44° Hornbrook basins WISZ Cretaceous subduction assemblages Idaho CoastCoast RangeRange OphioliteOphioli and Great VaValleylley (forearc basin)basin) Sequence 40° 40°40° Cordilleran Outboard exotic arc terranes Thrust Belt ColoradoColorado plplateauateau NorthNorth American fringing-arcfringi terranes Ne UtahUtah Pre-Cretaceous susubductionbd complexes vad Arrizonaizona 36° a 36°36° Paleozoic andand MesozoicMesoz eugeoclinal basinbasin assemblagesass LateLate ProterozoicProterozoic toto PermianP miogeoclinalmiogeoclinal rocksrocks 100 0 100 200 300 km California 32° 32°32° NorthNorth AmericanAmerican cratocraton 122° 118° 114° 110° 3 B 0 100 km Blue Mountains plutons 119° W 7 9 Volcanic, sedimentary 46° N Melange, schist Sedimentary, volcanic Volcanic, sedimentary SalmonRiver metamorphic N Blue Mountains Chief Joseph Dike Swarm 5 8 Province terranes Idaho batholith Z S Challis volcanics I Challis magmatic belt W Proterozoic platform sed Paleozoic platform sed 4 Cenozoic sediment 119° 1 Braudy et al. 2 2 Montz & Kruckenberg 6 10 3 Schmidt et al. 1 4 Giorgis et al. Idaho Batholith 5 Kurz et al. Atlanta Lobe 6 Gaschnig et al., 2017 WA MT 7 Byerly et al. OR 8 Fayon et al. ID Sr 0.706 9 Gaschnig et al., 2016 CA NV UT Oregon Idaho 43° N 10 Davenport et al. Figure 1. (A) Location of the study area in the western United States. WISZ—Western Idaho shear zone; BM—Blue Mountain terranes (diagram originally from Wyld et al., 2006). (B) Regional geological map of eastern Oregon and western Idaho (sed—sedimentary). The boxes indicate the areas of study for the publications within this themed issue. 152 www.gsapubs.org | Volume 9 | Number 2 | LITHOSPHERE Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/151/1001391/151.pdf by guest on 23 September 2021 Introduction: EarthScope IDOR project | THEMED ISSUE Cretaceous and Miocene magmatism (e.g., Idaho batholith, Columbia River basalt flows), and active extensional deformation. In particular, work A. OBSERVATIONS concentrated on the role of the steep accreted terrane-craton boundary in Idaho and Oregon as a major controlling feature for both subsequent OREGON IDAHO deformation (e.g., Tikoff et al., 2001; Giorgis et al., 2006, 2008; Lund et al., 2008) and magmatism (e.g., Carlson and Hart, 1987; Leeman et al., 1992). Miocene Cretaceous The following are the three overarching questions driving the EarthScope Sr 0.706 isopleth Sr 0.706 isopleth IDOR project. 1. How do fundamental lithospheric boundaries, such as the juxtaposi- WEbasalt granite tion of continental and oceanic lithosphere at the WISZ, guide subsequent deformation and magmatism? WISZ 2. How do magmatism and extensional deformation cause modifica- ? tion (destabilization) of the continental lithosphere, and at what litho- spheric levels? 3. What are the implications of this destabilization for magmatism, B-D. PROPOSED MODELS deformation, and evolving lithospheric strength? B. Crustal Detachment (Leeman et al., 1992) THREE REJECTED MODELS The study area was chosen because prior work had suggested that it con- WEWISZ tained an attribute that is unique in the North American Cordillera, i.e., an ID batholith offset vertical, lithospheric-scale boundary (Fig. 2). This vertical geometry detachment would allow investigation of the effect of different lithospheric layers on WISZ deformation and magmatism. Cretaceous intrusive rocks record a whole- Moho lithosphere vertical boundary beneath western Idaho