Downloaded from geology.gsapubs.org on May 2, 2011 Geology Seismically imaged relict slab from the 55 Ma Siletzia accretion to the northwest United States Brandon Schmandt and Eugene Humphreys Geology 2011;39;175-178 doi: 10.1130/G31558.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2011 Geological Society of America Downloaded from geology.gsapubs.org on May 2, 2011 Seismically imaged relict slab from the 55 Ma Siletzia accretion to the northwest United States Brandon Schmandt and Eugene Humphreys Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA ABSTRACT depth from 30 km at 60–90 km depth, to 65 km Teleseismic tomography images a large high-velocity “curtain” extending vertically beneath at >820 km depth. The method was described the ca. 50 Ma Challis magmatic trend to maximum depths of 230–600 km. We interpret this in detail in Schmandt and Humphreys (2010b). structure as subducted Farallon ocean lithosphere that stalled with the ca. 55 Ma accretion of Resolution tests demonstrate excellent recovery the Siletzia microplate to North America within the Columbia Embayment, and we consider of structures with length scales ≥70 km, with the regional tectonic implications. The abrupt switch ca. 53 Ma from Laramide thrusting and typical peak amplitude recovery of 70%–80% magmatic quiescence to extension and vigorous magmatism in the northwestern United States for VP and 60%–70% for VS (Fig. DR1 in the 1 is evidence for foundering of the fl at-subducting Farallon slab. To account for the imaged GSA Data Repository ). VP resolution is slightly curtain, foundering apparently occurred by rollback after Siletzia accretion terminated sub- better than VS resolution in the Pacifi c North- duction within the Columbia Embayment. The magnitude of the high seismic velocity curtain west as a result of 150 additional short-period is approximately that expected for 20–50 m.y. old ocean lithosphere that stalled in the upper stations (Fig. 1), a greater number of high- mantle ca. 50 Ma. We suggest that the stalled slab became nearly neutrally buoyant as a result quality teleseismic P arrivals, and shorter wave- of removal of its basaltic crust, the melting of which likely contributed to the short-lived vigor length sampling of teleseismic P arrivals. of Challis magmatism. After Siletzia accretion, normal dip Cascadia subduction initiated west Figures 1 and 2 show three distinctive high- of Siletzia, evidenced by arc volcanism in Oregon and Washington beginning ca. 45–40 Ma, velocity structures beneath the Pacifi c North- while continued quiescence of the Sierra Nevada arc suggests persistence of fl at subduction to west. A prominent, relatively small and approx- the south. Kinematically, this requires a tear in the subducted Farallon slab near the latitude imately elliptical body is imaged below the of southern Oregon. We propose that north to south propagation of this torn slab edge propa- source area of the Columbia River fl ood basalts gated the ignimbrite fl are-up across what now is the northern Basin and Range and ended the in northeast Oregon (Fig. DR2). This structure Laramide orogeny. may owe its creation to basalt depletion or lithospheric delamination (Hales et al., 2005). INTRODUCTION the western United States by inverting 248,000 A second high-velocity feature is the slab- New western United States seismic images P and 84,000 S relative traveltime residuals of like structure beneath the Cascade Arc, which enabled by EarthScope’s USArray provide teleseismic earthquakes observed by USAr- must be the steeply dipping subducted Juan de for the fi rst time high-resolution information ray and more than 1700 additional stations Fuca slab. The subject of this paper is the high- on upper mantle structure over a large area of (Schmandt and Humphreys, 2010a). The inver- velocity curtain extending vertically from the continent. In this paper we discuss the Pacifi c sion uses recent advances in western U.S. crust base of North America lithosphere to depths Northwest portion of our recent tomographic models (Yang et al., 2008; Gilbert and Flesch, of ~230 km beneath eastern Washington and models (Schmandt and Humphreys, 2010a) to 2009) to better isolate the mantle component of 450–600 km beneath northern Idaho and west- focus on a prominent but previously unresolved residual times and three-dimensional frequency- ernmost Montana (Figs. 1 and 2). Several recent “curtain” of high-velocity mantle extending dependent sensitivity kernels to map residuals tomography studies using USArray data found a vertically from beneath central Idaho to near (measured in multiple frequency bands) into large volume of vertically elongated high-veloc- the eastern edge of the Cascades in northern velocity structure. Horizontal node spacing is ity mantle in this region (e.g., Roth et al., 2008; Washington and extending down to depths of 40 km within the footprint of the USArray, and Sigloch et al., 2008; Burdick et al., 2009; Tian 230–600 km (Figs. 1 and 2). The only plausible vertical node spacing increases gradually with et al., 2009; Xue and Allen, 2010), although not explanation for this large volume of high-veloc- ity mantle is recently subducted ocean litho- 230 km 230 km sphere. We address here the seismic, tectonic, A B C and magmatic evidence that this structure is 48° subducted Farallon slab abandoned in the upper mantle during the ca. 55 Ma accretion of Silet- zia, which we defi ne as the fragment of Farallon lithosphere that fi lled the Columbia Embayment 44° in the Eocene. We then consider the seismic and geodynamic plausibility that ocean lithosphere VP VS & & subducted in the early Cenozoic did not lose 236° 240° 244° 248° - 2.7 0 2.7% - 4 0 4% its seismic contrast or sink into the Earth. We Figure 1. A: Regional topography and stations used in our study. Blue indicates stations then discuss the implications of this structure for used for P and S data; red indicates short-period stations used only for P data. Backarc early Cenozoic tectonic evolution of the western area underlain by Siletzia is well approximated by low-lying Columbia basin (see Fig. 2). United States. B: P-wave velocity variations. C: S-wave velocity variations. PACIFIC NORTHWEST TOMOGRAPHY 1GSA Data Repository item 2011073, supplemental fi gures, is available online at www.geosociety.org/ We created high-resolution P-wave (VP) and pubs/ft2011.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, S-wave (VS) images of the upper mantle beneath Boulder, CO 80301, USA. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2011; 2011 v. 39; no. 2; p. 175–178; doi: 10.1130/G31558.1; 2 fi gures; Data Repository item 2011073. 175 Downloaded from geology.gsapubs.org on May 2, 2011 as a feature distinct from the smaller anomaly ish Columbia ca. 48 Ma (Gehrels et al., 2009) Other factors contributing to the expected beneath northeast Oregon. Our better resolution and Challis magmatism waned 48–45 Ma seismic contrast of the slab with the astheno- in this area is a result of data from 30 stations (Gasch nig et al., 2009). Subsequently, magma- sphere include effects caused by dehydration in the Wallowa Mountains array. Based on the tism swept west across Oregon (the Clarno vol- upon formation of lithosphere at the spreading excellent recovery of input structure in synthetic canics, ca. 45–40 Ma; Retallack et al., 2000) and center and seismic anisotropy, although we have tests (Fig. DR1), we consider the high-velocity the Cascade arc initiated ca. 45–40 Ma (Fig. 2A; little knowledge about either of these properties. curtain and its separation from the Wallowa Christiansen and Yeats, 1992) as a response to For the effects of water, if we assume a 50 km Mountains anomaly to be robust features. The outward stepping of subduction required by thickness of dehydrated slab to be seismically maximum depth of the curtain being in the Siletzia accretion. faster by an amount equivalent to tempera- transition zone beneath northern Idaho is sup- This geologic record supports a history in ture decrease of 100 ºC (Karato, 2003), it can δ ported by receiver function results indicating which Farallon slab subducted fl at against North increase the expected imaged velocities by VP δ a spatially correlated upward defl ection of the America during the Laramide and fell away = 0.6% and VS = 1.0%. If ocean lithosphere 410 km discontinuity and no strong defl ection shortly after Siletzia accretion, at the time of anisotropy is preserved upon subduction and of the 660 km discontinuity (Eagar et al., 2010).