Geosphere, published online on 12 November 2014 as doi:10.1130/GES00990.1 Geosphere Synchronous late Pleistocene extensional faulting and basaltic volcanism at Four Craters Lava Field, central Oregon, USA Benjamin H. Mackey, Samuel R. Castonguay, Paul J. Wallace and Ray J. Weldon Geosphere published online 12 November 2014; doi: 10.1130/GES00990.1 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 Geosphere 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. 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Weldon2 1Department of Geological Sciences, University of Canterbury, PB 4800, Christchurch 8140, New Zealand 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA ABSTRACT tral Oregon (northwestern USA) has a complex In this paper we fi rst describe the regional set- network of extensional faults and lineaments ting, Four Craters Lava Field, and the adjacent Central Oregon (northwestern USA), (Pezzopane and Weldon, 1993) associated with Crack in the Ground fault. We then outline our where northern Basin and Range extension hundreds of cinder cones (Weldon et al., 2003). sampling strategy for cosmogenic 3He exposure diminishes in magnitude across the High Lava Although the faults and volcanic features have dating to determine the timing and nature of the Plains, exhibits widespread extensional fault- been studied independently, the absence of any interaction between the lava and fault. We pre- ing and Quaternary volcanism, yet the rela- historic eruptions means that the nature and sent results of cosmogenic 3He exposure dating tions between the processes are complex and timing of how these normal faults interact with and interpret these to unravel the interaction chronology is poorly constrained. Here we basaltic volcanism is largely unconstrained. between the Crack in the Ground fault and the use cosmogenic 3He exposure dating of basalt Beyond the implications for the tectonic evolu- Four Craters Lava Field. lava fl ows to quantify the timing of normal tion of central Oregon, and general interactions faulting and emplacement of a lava fi eld on between faulting and volcanism, there is a need FOUR CRATERS LAVA FIELD the margins of pluvial Fort Rock Lake. The to constrain the volcano-tectonic behavior for northwest-trending Christmas Valley fault hazard prediction. The propagation of Basin and Range exten- system displaces High Lava Plains volcanic Although central Oregon has many Quater- sion into the Oregon High Lava Plains (Fig. 1) rocks, forming an ~3-km-wide graben that nary volcanic features, few have absolute ages. generates a complex network of largely en eche- transects the eastern Fort Rock Basin. A Most geochronology has focused on Tertiary and lon normal faults (Walker and MacLeod, 1991; portion of the western edge of the graben is early Pleistocene basalt fl ows, rhyolite domes, Pezzo pane and Weldon, 1993; Crider, 2001). marked by a normal fault displaying fl exural and ash-fl ow tuffs of the High Lava Plains, This zone of extension is bounded by the Brothers shear folding with a prominent vertical hinge and how they may relate to Yellowstone plume fault zone to the north and the Walker Rim fault crack, called Crack in the Ground. Lava fl ows migration or the evolution of Newberry Volcano zone to the west, and coincides with pervasive of the Four Craters Lava Field fl owed into this (e.g., Jordan et al., 2004). Late Pleistocene and Neogene volcanism across the Oregon High Lava crack. Exposure dating of the Four Craters Holocene lava fl ows can be challenging to date Plains. Beyond stratigraphic and crosscutting Lava Field, on the eastern fl ank of the older (Sims et al., 2007) because they are generally too relations, or relative dating based on appearance Green Mountain shield volcano, indicates an young to have accumulated enough radiogenic (e.g., degree of weathering or vegetation), there emplacement age of 14 ± 1 ka. We dated Green 40Ar for 40Ar/39Ar dating. Furthermore, fi nding is limited information on the timing of faulting Mountain basalt exposed in the walls of the suitable charcoal under lava fl ows for radiocar- or Pleistocene volcanic activity, and how faulting crack (the fault wall), which also yielded expo- bon dating can be diffi cult in poorly vegetated might relate to periods of volcanism. sure ages of 14 ± 1 ka. The similar ages sug- desert environments (Sherrod et al., 2012), and Fort Rock Basin is in a band of northwest- gest that substantial crack opening occurred frequently requires mechanical excavation (e.g., striking faults between Summer Lake and at about the same time the Four Craters lava Kuntz et al., 1986). Cosmogenic 3He has proven Newberry Volcano (Fig. 1). The closed basin was emplaced. These data indicate a period useful in dating young lava fl ows in the west- contained the ~60-m-deep pluvial Fort Rock of synchronous normal faulting and dike-fed ern U.S., particularly in the high deserts, where Lake during the last glacial maximum (Alli- cinder cone activity emanating from a fault- weathering and erosion of lava are minimal and son, 1979), with paleoshorelines preserved to parallel fi ssure ~2 km northeast of the crack cosmogenic nuclide production rates are high ~1350 m elevation. The basin hosts a range of ca. 14 ka, with minimal displacement since. (e.g., Cerling and Craig, 1994; Licciardi et al., phreatomagmatic Quaternary volcanic features 1999; Ely et al., 2012). (Heiken, 1971; Brand and Heiken, 2009). The INTRODUCTION Here we focus on the nature and timing of Christmas Valley fault system bisects the eastern interaction between the Four Craters Lava Field arm of the Fort Rock Basin, forming a north- Cinder cones and associated basalt fl ows are and the Crack in the Ground fault in central northwest–trending graben, creating a depres- common in extensional settings, and typically Oregon. Lava at the southwest margin of the sion in the older Green Mountain basaltic shield form along fi ssures (e.g., Tibaldi, 1995). Cen- lava fi eld fl owed into, and abuts, the Crack in and the playa delineated by the Crack in the the Ground fault, an extensional fi ssure in older Ground fault on the northwest and the View- *[email protected] †Present address: Geology Department, University basalt produced by normal faulting. The loca- point fault on the southeast (Fig. 2). 2 of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54702, tion is well suited to study the timing of lava The 19 km Four Craters Lava Field (Fig. 3) USA emplacement and extensional faulting. is within this graben, and covers a part of the Geosphere; December 2014; v. 10; no. 6; p. 1–8; doi:10.1130/GES00990.1; 5 fi gures; 2 tables. Received 10 October 2013 ♦ Revision received 11 July 2014 ♦ Accepted 8 October 2014 ♦ Published online XX Month 2014 For permission to copy, contact [email protected] 1 © 2014 Geological Society of America Geosphere, published online on 12 November 2014 as doi:10.1130/GES00990.1 Mackey et al. 121° W 120° W 119° W Br other Newberry s fault zone IDIDAHAHAHOO Volcano OREGON NENEVAVAVADADA Fort Rock basin High Lava Plains Extent of Fig. 2 43° N Summer Lake Lake Abert Elev (m) High : 2970 03060km Low : 870 Northern Basin and Range Figure 1. Location of study area in central Oregon (inset). Red—Quaternary faults from Weldon et al. (2003). Blue line is 1335 m contour line, showing the approximate extent of pluvial lakes in the late Pleistocene. Orange areas are the late Pleistocene basalt fi elds described in text (see Fig. 2). eastern fl ank of the older 25-km-diameter Green interpreted to indicate some postemplacement 1964). The fault averages 8 m of vertical separa- Mountain shield volcano (740 ± 59 ka; Jordan extension, with activity assumed to be Holocene tion (but can reach 13 m), although much of this et al., 2004). The Crack in the Ground fault, a in age (Pezzopane and Weldon, 1993). The East displacement is accommodated by the fl exural normal fault downthrown to the northeast, delin- lava fi eld, 6 km northwest of the Four Craters shearing forming a hanging-wall monocline eates a section of the southwest boundary of the Lava Field, has a similar morphologic appear- across the upper Green Mountain lava.