See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/265996688 Extension and Subsidence of the Eastern Snake River Plain, Idaho Article CITATIONS READS 34 54 5 authors, including: David Rodgers Nadine McQuarrie Idaho State University University of Pittsburgh 92 PUBLICATIONS 1,401 CITATIONS 123 PUBLICATIONS 4,076 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Managing Idaho's Landscapes for Ecosystem Services (The MILES program) View project Water scarcity and governance across Social-Ecological Systems (PECS-WaterSES) View project All content following this page was uploaded by Nadine McQuarrie on 04 February 2015. The user has requested enhancement of the downloaded file. Extension and Subsidence of the Eastern Snake River Plain, Idaho David W. Rodgers,1 H. Thomas Ore,1 Robert T. Bobo,2 Nadine McQuarrie,3 and Nick Zentner4 ABSTRACT marginal normal faults. Northeast-striking marginal nor- mal faults only accommodated 2-5 percent extension as- The deformational history of the eastern Snake River sociated with crustal flexure, not downfaulting of east- Plain (SRP) is interpreted from rocks, structures, and land- ern SRP crust. Crustal flexure along the northern margin forms within and adjacent to it. Crustal extension is mani- of the eastern SRP is evidenced by Cretaceous fold hinges fested by west-dipping normal faults that define a half- whose southward plunges increase regularly toward the graben fault style along the north and south margins of plain. Isoplunge contours define a zone of flexure about the plain. Cumulative extension averages 15-20 percent 20 km wide that accommodated 4.5-8.5 km of subsid- and diminishes only slightly from the south margin to ence of eastern SRP rocks. Age-depth relations of east- the north, evidence of no lateral shear beneath the plain. ern SRP volcanic rocks indicate 1.7 km of rock subsid- Miocene-Quaternary volcanic and sedimentary rocks fill ence since 8.0 Ma, 3.1 km since 8.5 Ma, and the initia- the half grabens and provide evidence of two pulses of tion of subsidence before 10 Ma. extension that migrated northeast through time. One pulse Subsidence of the eastern SRP surface relative to the of minor extension began 16-11 Ma and ended 11-9 Ma, Basin and Range surface is manifested by axial drainage and a second pulse of major extension began 11-9 Ma that extends 50 km beyond the SRP margins. Streams and lasted a few million years in any one location as it that are tributary to the Snake River have deeply incised migrated eastward to the eastern edge of the Basin and Basin and Range valleys along the southern margin of Range, where it continues today. Because many Miocene the eastern SRP, a process attributed to SRP surface sub- faults project into the eastern SRP with no evidence of sidence and consequent base level lowering of the Snake diminishing offset, normal faults are interpreted to have River. The age and amount of surface subsidence are re- characterized the eastern SRP in middle and late Miocene corded by a strath terrace near Pocatello that cuts across time. Since then, upper crustal extension has been ac- tilted 7.3-Ma basin fill and is perched 900 m above the commodated primarily by mafic dike injection. modern plain. Drainage reversals at tributary stream head- Subsidence of eastern SRP rocks relative to Basin and waters provide evidence of ongoing Pleistocene subsid- Range rocks is manifested by the accumulation of a thick ence. volcanic succession, crustal flexure of its margins, and Space-time patterns of extension and subsidence are used to support a three-stage model of deformation. Stage Editors’ note: The manuscript was submitted in June 1998 and has 1 involved minor half-graben normal faulting and possi- been revised at the authors’ discretion. bly regional subsidence of the proto-eastern SRP. Stage 1Department of Geosciences, Idaho State University, Pocatello, ID 2 involved regional rock subsidence and major extension 83209-8072 via normal faulting. Extension was focused slightly east 2 595 Ford Road, McKenzie, TN 38201 of coeval silicic magmatism, and both migrated north- 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 east after 11 Ma. Deformation and coeval magmatism 4Department of Geology, Central Washington University, Ellensburg, were widely dispersed across the plain at about 10 Ma. WA, 98926 Stage 3 has involved ongoing rock subsidence, surface Rodgers, D.W., H.T. Ore, R.T. Bobo, Nadine McQuarrie, and Nick Zentner, 2002, Extension and subsidence of the eastern Snake River Plain, Idaho, in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 121-155. 122 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province subsidence after about 7 Ma, and extension via dike in- the space-time pattern of eruptions. Our objective in this jection after about 4 Ma. Stage 3 deformation has been paper is to document the Neogene and Quaternary located west of coeval silicic magmatism. deformational history, the space-time pattern of defor- Key words: Yellowstone, Snake River Plain, Basin mation and magmatism, and the landscape evolution to and Range, extensional tectonics, lower crustal flow, develop an evolutionary model of eastern SRP deforma- crustal subsidence tion. Most of the data and interpretations in this paper were first presented in Master’s theses by Zentner (1989), Bobo (1991), and McQuarrie (1997) who worked under the su- INTRODUCTION pervision of D.W. Rodgers and H.T. Ore. Except for some abstracts and one paper (McQuarrie and Rodgers, 1998), The eastern Snake River Plain (SRP) is a bimodal the information was not widely disseminated or integrated, magmatic province with little surficial evidence of crustal a situation this paper attempts to rectify. deformation. A few basaltic rift zones, rare normal faults, and a relatively low regional elevation are the only indi- cation of crustal movement. In contrast, the adjacent Ba- STRUCTURAL SETTING sin and Range Province is characterized by a relatively high regional elevation and widespread normal faults that A complicated pattern of crustal extension has affected accommodate both crustal extension and vertical offset. the eastern SRP region (Figure 1). The eastern SRP itself Structural and geomorphic features clearly indicate that appears relatively unextended with just a few late Qua- different deformational processes are currently operat- ternary volcanic rift zones presumed to overlie mafic ing in the two provinces, but what causes the differences dikes. Neogene to early Quaternary structures are com- and what defines the transition remain difficult to dis- pletely obscured by younger basalt flows. Beyond the cern. The two provinces also experienced temporal eastern SRP are normal faults of the Basin and Range changes in deformation style and kinematics. The east- Province, some of which are active as indicated by arcu- ern SRP evolved during the past 16 Ma through a com- ate zones of high topography, high seismicity, and high plicated process of magmatism and deformation, but little to moderate Quaternary slip rates that are almost sym- is known about the Neogene deformational history dur- metrically distributed about the eastern SRP. Most work- ing the eruption of voluminous rhyolites and the early ers attribute the modern extensional pattern to the stages of basaltic magmatism. Neogene rocks and struc- Yellowstone magmatic system and its interaction with tures of the eastern SRP that probably record this earlier the southwest-drifting North American plate, though the history are concealed beneath Quaternary basalts. actual mechanics of this process are unclear. To better Fortunately, footwall uplifts of the adjacent Basin and understand the interaction between extension and Range contain rocks and structures that record several magmatism, a few workers have studied the older Neo- pulses of late Cenozoic deformation. Some pulses were gene history of normal faults near the eastern SRP. Anders restricted to the Basin and Range, whereas others were and others (1989, 1993) measured the ages and rates of more extensive and affected what is now the eastern SRP. faulting both north and south of the eastern SRP, and in- The late Cenozoic rocks have been studied by some work- terpreted them in terms of a migrating “seismic parabola” ers, notably Kirkham (1927, 1931), Trimble (1980), shaped like the modern one. Rodgers and others (1990) Trimble and Carr (1976), Allmendinger (1982), Kellogg compiled the space-time relations of SRP magmatism and (1992), Pierce and Morgan (1992), and especially Anders faulting south of the SRP and proposed a model of con- and his coworkers (Anders and others, 1989; Rodgers comitant tectonic activity, wherein the SRP experienced and Anders, 1990; Anders and others, 1993; Anders, 1994) crustal extension (via normal faulting, ductile attenua- who used the tectonic tilts of Neogene rocks to interpret tion, and magma injection) before, during, and after that the locus of Basin and Range faulting migrated east- magmatism at any one locality. Recent studies, in par- ward and away from the eastern SRP through time. ticular new dating and correlation techniques, have re- Similarly, we have worked in late Cenozoic rocks of vised data and interpretations concerning the space-time the Basin and Range, but we have focused along the east- pattern of extension. One goal of this paper is to present ern SRP margins where structures, basin fill, and geo- information that will explain the interaction of extension morphology record deformation that may have affected and magmatism on the eastern SRP. the eastern SRP. Results indicate that crustal extension Perhaps as unusual as the pattern of crustal extension and subsidence occurred in the eastern SRP throughout is the pattern of subsidence in the eastern SRP.