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A Tale of Two Walker Lane Pull-Apart Basins in the Ancestral Cascades Arc, Central Sierra Nevada, California GEOSPHERE; V

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A Tale of Two Walker Lane Pull-Apart Basins in the Ancestral Cascades Arc, Central Sierra Nevada, California GEOSPHERE; V Research Paper THEMED ISSUE: Origin and Evolution of the Sierra Nevada and Walker Lane GEOSPHERE A tale of two Walker Lane pull-apart basins in the ancestral Cascades arc, central Sierra Nevada, California GEOSPHERE; v. 14, no. 5 Cathy J. Busby1, K. Putirka2, Benjamin Melosh3, Paul R. Renne4,5, Jeanette C. Hagan6, Megan Gambs7, and Catherine Wesoloski1 1Department of Earth and Planetary Science, University of California, Davis, California 95616, USA https://doi.org/10.1130/GES01398.1 2Department of Earth and Environmental Sciences, California State University, Fresno, California 93720, USA 3U.S. Geological Survey, Menlo Park, California 94025, USA 15 figures; 1 table; 1 set of supplemental files; 4Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA 2 oversized figures 5Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA 6Exxon Mobil, Houston, Texas 77389, USA 7School of Oceanography, University of Washington, Seattle, Washington 98105, USA CORRESPONDENCE: cjbusby@ ucdavis .edu CITATION: Busby, C.J., Putirka, K., Melosh, B., Renne, P.R., Hagan, J.C., Gambs, M., and Wesoloski, ABSTRACT ter is dominated by Stanislaus Group basalt, trachybasaltic andesite, trachy- C., 2018, A tale of two Walker Lane pull-apart basins andesite, and andesite. Climactic eruptions at its southern end produced the in the ancestral Cascades arc, central Sierra Nevada, We integrate new geochronological, petrographic, and geochemical data Little Walker caldera and its Stanislaus Group trachydacite ignimbrites. In California: Geosphere, v. 14, no. 5, p. 2068–2117, https://doi.org/10.1130/GES01398.1. with previously published (Sierra Crest–Little Walker volcanic center) and new contrast, the Ebbetts Pass volcanic center erupted much less basalt, and an (Ebbetts Pass volcanic center) structural and stratigraphic data to describe two approximately equal mixture of basaltic andesite, andesite, dacite, and rhyo- Science Editor: Shanaka de Silva deeply dissected, spectacularly well-exposed Walker Lane pull-apart basins in lite. By virtue of its higher SiO2, on average, it shows geochemical evidence for Guest Associate Editor: David A. John the ancestral Cascades arc. The Miocene (ca. 12–5 Ma) Sierra Crest–Little Walker higher extents of crustal contamination. All of the volcanic rocks except the arc volcanic center and the Miocene–Pliocene (ca. 6–4.6 Ma) Ebbetts Pass arc Stanislaus Group are compositionally identical to Mesozoic plutonic rocks of Received 20 July 2016 volcanic center formed in pull-apart basins in the Walker Lane, a NNW-trending the Sierra Nevada Batholith, indicating a similar petrogenesis by shallow-level Revision received 25 January 2018 zone of dextral strike-slip and oblique normal faults at the western edge of the fractional crystallization with no evidence for deep-seated pyroxenite cumu- Accepted 15 May 2018 Published online 7 August 2018 Basin and Range Province. The Sierra Crest–Little Walker arc volcanic center is a lates. Only the high-K2O Stanislaus Group requires a significant degree of transtensional arc volcanic field that is as areally extensive (~4000 km2) and as fractionation of clinopyroxene and Fe-Ti oxides. The major-oxide trends and long-lived (~3 m.y.) as the Pliocene to Holocene Long Valley transtensional rift clinopyroxene-rich modes of the Stanislaus Group may be the volcanic expres- volcanic field, which is also in the Walker Lane. The Ebbetts Pass volcanic center sion of processes that produced pyroxenite cumulates. The basal and most is the next major volcanic center to the north within the ancestral Cascades voluminous unit of the Stanislaus Group, the Table Mountain Latite, is a likely arc. It formed within a smaller pull-apart basin. Its estimated original volume parent to succeeding units of the Stanislaus Group (Eureka Valley Tuff and (~270 km3) is comparable to the estimated volume for the Lassen volcanic cen- Dardanelles Formation), but this particular liquid line of descent is otherwise ter in the last 825 k.y. (200 km3), a major magmatic focus in the northernmost rare in the Mesozoic and Cenozoic history of the Sierra Nevada. Walker Lane pull-apart basin. Postvolcanic faulting and erosion provide spectac- Volcano-tectonic activity continued for a longer period (ca. 12–6 Ma) in the ular three-dimensional time-integrated views of the structure and stratigraphy northern half of the Sierra Crest–Little Walker volcanic center than it did in of the Miocene to Pliocene volcanic centers described herein. The volcanic stra- the southern half (ca. 12–9 Ma), overlapping for ~1 m.y. with the onset of vol- OLD G tigraphy of this region is divided into four unconformity-bounded sequences: cano-tectonic activity at the Ebbetts Pass volcanic center to the north. This (1) Oligocene silicic ignimbrites erupted in central Nevada (Valley Springs provides local-scale confirmation of the regional-scale interpretation that a Formation), (2) Miocene arc volcanic and volcaniclastic rocks (Relief Peak For- transtensional rift tip propagated northward with time within the arc axis, in mation), (3) ca. 11.5–9 Ma high-K arc volcanic rocks (Stanislaus Group), and concert with northward migration of the Mendocino triple junction. Transten- OPEN ACCESS (4) Miocene–Pliocene arc volcanic rocks (Disaster Peak Formation). We present sional rift magmatism followed in its wake and continues today at various 19 new 40Ar/39Ar ages and integrate them with previous 40Ar/39Ar ages to erect points along the length of the Walker Lane. “Tectono-stratigraphic recycling” a detailed stratigraphic and intrusive chronology. For the first time, we present was a key geologic process throughout the development of these ancestral a systematic description of all of the volcanic and intrusive map units, using Cascades arc pull-apart basins, and it consisted of the transfer of megaslide outcrop photos, photomicrographs, modal analyses, and geochemical data. slabs, up to 2 km long and tens to hundreds of meters thick, from footwall to Geochemistry and modal analyses show that volcanism at the younger, hanging-wall blocks in the transtensional rift. A time-slice series of block dia- This paper is published under the terms of the smaller Ebbetts Pass volcanic center is more evolved than that of the Sierra grams is used to illustrate the structural controls on arc volcanism in the early CC-BY-NC license. Crest–Little Walker volcanic center. The Sierra Crest–Little Walker volcanic cen- stages of Walker Lane transtensional faulting (ca. 12–4.6 Ma). © 2018 The Authors GEOSPHERE | Volume 14 | Number 5 Busby et al. | Two Walker Lane pull-apart basins in the ancestral Cascades arc Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2068/4347053/2068.pdf 2068 by guest on 02 October 2021 Research Paper INTRODUCTION tionally, the importance of vertical-axis rotations along the western boundary of the central Walker Lane has been severely underevaluated (Carlson, 2017); The Sierra Nevada (California) forms an asymmetrical block-faulted moun- the stratigraphic and structural results described herein are ideal for future tain range with a gentle western slope and a steep fault-bounded range front studies on this. on the east. The steep eastern range front was previously interpreted to be the The Carson domain and Mina deflection represent very large right steps result of Basin and Range extension (Bateman and Wahraftig, 1966; Slemmons, in the central Walker Lane, ~60 km and 90 km, respectively, with vertical-axis 1953, 1966). However, it is now known to form the western boundary of the rotations described above. In this paper, we describe smaller right steps along Walker Lane, an ~100-km-wide, NNW-trending zone of dextral strike-slip and the western edge of the central Walker Lane (Fig. 1 and 2A): an ~15 km right oblique normal faults that lies along the western edge of the Basin and Range step, referred to herein as the Sierra Crest–Little Walker pull-apart, and an Province (e.g., Faulds and Henry, 2008; Jayko and Bursik, 2012; Busby, 2013). ~5 km zone of right-stepping faults, referred to herein as the Ebbetts Pass pull- The Walker Lane accommodates ~20%–30% of the right-lateral motion between apart. This paper describes Oligocene through Pliocene volcanic sections that the Pacific and North American plates (Argus and Gordon, 1991; Dixon et al., will be appropriate for much-needed paleomagnetic assessment of the impor- 2000; Oldow, 2003; Unruh et al., 2003; John et al., 2012). This region has been tance of vertical-axis rotations. In this paper, we demonstrate the controls of described as the northernmost extension of the Gulf of California transtensional these faults on two large volcanic centers that formed in the pull-apart basins. rift, where the process of continental rupture has not yet been completed, and The central Walker Lane (Fig. 1) has more abundant Cenozoic volcanic rift initiation can be studied on land (Faulds and Henry, 2008; Jayko and Bursik, rocks than the northern and southern Walker Lane, probably because it lies 2012; Busby, 2013). The northern and southern segments of the Walker Lane at a higher rift angle relative to the overall plate margin (Busby, 2013). The are dominated by translation along long NNW-trending dextral strike-slip faults abundant volcanic rocks make the central Walker Lane ideal for detailed re- (Faulds and Henry, 2008; Jayko and Bursik, 2012; Busby, 2013). In contrast, constructions of transtensional rifting and magmatism in a continental setting, the central Walker Lane (Fig. 1) shows evidence of distributed dextral shear because the volcanic strata are dateable by the 40Ar/39Ar method (e.g., Busby accommodated by oblique-normal faults, block rotations, and partitioning of et al., 2008a, 2008b, 2013a, 2013b, 2016; Hagan et al., 2009; Nagorsen-Rinke oblique deformation between subparallel normal and strike-slip faults (e.g., et al., 2013; Fleck et al., 2015). Our studies have focused on the western margin Wesnousky, 2005; Dong et al., 2014; Bormann et al., 2016).
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