Crustal structure beneath the Blue Mountains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic receiver functions

Abstract

We present new images of lithospheric structure obtained from P‐to‐S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho‐ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P‐to‐S receiver functions using the iterative time domain deconvolution method, and we used the H‐k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34 km, whereas the crust is 32–40 km thick beneath the Idaho batholith and the regions of extended crust of east‐central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, ~24 km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression. The presence of a strong midcrustal converting interface at ~18 km depth beneath the Idaho batholith extending ~20 km east of the WISZ indicates tectonic wedging in this region. A north striking ~7 km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.

Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project

“…South of the IDOR Passive deployment footprint, in Utah, the Farmington zone separates the Wyoming craton and the Archean Grouse Creek block (Figure 1;Foster et al, 2006;Mueller et al, 2011). The extension of this zone north into Idaho and SW Montana is consistent with exposures of similar aged rocks in SW Montana (∼2.4 Ga; Foster et al, 2006), and with a clear Moho offset within the IDOR footprint that is coincident with the presumed surface trace of the Farmington zone (Stanciu et al, 2016).…”

“…The Belt units are intracratonic rift deposits, and the Windermere rocks were deposited within rifts and on the rifted continental margin (e.g., Lund et al, 2003;Price & Sears, 2000;Ross, 1991;Stewart, 1972;Winston & Link, 1993). The Archean Grouse Creek block ( Figure 1; Foster et al, 2006; 10.1029/2018JB016350 Gaschnig et al, 2013) lies south of the Selway terrane and constitutes the majority of the Laurentia basement on which our seismic stations were deployed (Stanciu et al, 2016). Belt and Windermere units also partly cover the Grouse Creek block in the eastern portion of our study area.…”

“…To first order, the study area can be divided into two major geologic provinces lying east and west of the WISZ, a subvertical strike-slip fault zone that juxtaposes Precambrian North America, to the east, and Late Paleozoic to Jurassic island arc terranes and associated subduction complexes of the Blue Mountains Province (BMP) to the west (e.g., Davenport et al, 2017;Gaschnig et al, 2011;LaMaskin et al, 2015;Lund & Snee, 1988;Tikoff et al, 2001;Stanciu et al, 2016;Wyld & Wright, 2001; see Figure 1). The arc terranes of the BMP accreted to North America before Late Jurassic time (LaMaskin et al, 2015) and are today structurally imbricated with rocks of the Salmon River Belt (Gray & Oldow, 2005;LaMaskin & Dorsey, 2016;McKay et al, 2017) preserved at the surface to the north of the IDOR Passive seismic deployment.…”

“…We used a joint inversion of phase and group velocities because of their different depth sensitivities at a given period, which thus provides better constraints on structure than inverting either alone (Lin et al, 2008;Shapiro & Ritzwoller, 2002). The crustal velocities in the starting model were a network-averaged V s velocity model derived using receiver functions from the IDOR Passive data set; we also set the Moho depth to 34 km, the average crustal thickness derived from the receiver functions (Stanciu et al, 2016). We set the mantle velocity beneath the crust to the average P n velocity found by Buehler & Shearer (2010 for the IDOR network area.…”

“…We produced the final 1-D velocity model for each grid pixel from a joint inversion of each pixel's group and phase velocity dispersion curves (Herrmann, 2013). We used the 1-D initial V s model derived above, but at each pixel updated Moho depth from receiver functions from the IDOR data set (Stanciu et al, 2016), and updated mantle velocity from P n tomography results (Buehler & Shearer, 2010. Figure 9 shows the Moho depths and P n velocities across the IDOR network area used for these updates.…”