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Late Miocene and Early Pliocene Basaltic Rocks and Their Implications for Crustal Structure, Northeastern California and South-Central Oregon

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Late Miocene and Early Pliocene Basaltic Rocks and Their Implications for Crustal Structure, Northeastern California and South-Central Oregon Late Miocene and early Pliocene basaltic rocks and their implications for crustal structure, northeastern California and south-central Oregon WENDELL A.CDUFFIELD ) U S' Geo,°S'cal Survey, Menlo Park, California 94025 ROBERT J. STERN University of Texas at Dallas, Richardson, Texas 75085 ABSTRACT and (4) to compare these lavas with basaltic (Iocs. 11, 16, 29, Fig. 2), and future work lavas from various tectonic settings else- may identify other outliers, but the bulk of A dominantly basaltic late Miocene and where in the world. Such comparisons may the lava field appears to lie north of the Pit early Pliocene (about 5 to 10 m.y. old) lava help to interpret the tectonic setting of the River. A similar interpretation of the distri- field lies directly east of the Cascade Range study area, which occupies a zone that is bution of volcanic rocks in this region has on the northwesternmost edge of the Basin transitional between the Cascade Range to been presented by Luedke and Smith and Range province. More than 12,400 km2 the west and the Basin and Range province (1981a, 1981b); the outline of a 5- to of Modoc County, California, and Klamath to the east. lO-m.y.-old basalt unit on their maps de- and Lake Counties, Oregon, termed the fines a volcanic field nearly identical to the Devils Garden, is underlain by basalt with a AREA AND VOLUME OF BASALT Devils Garden lava field as described here. 3 total volume of about 850 km . OF DEVILS GARDEN The east margin of the field is well defined The basalt of Devils Garden is diktytax- by older, upfaulted, and tilted rocks of the itic olivine tholeiite, characterized by high Basalt lava flows form a plateau north- Warner Range and Abert Rim; the west A1 content; low K, Rb, and Cs content; and west of Alturas, California (Ford and margin, although less well defined, is mark- high K/Rb and K/Cs ratios. In these others, 1963, p. 57). This plateau and adja- ed by nearly conformable onlap of younger respects, it resembles mid-oceanic-ridge cent areas to the north in Oregon are volcanic rocks in the Medicine Lake and basalt (MORB), although it differs in other known locally as the Devils Garden (see Klamath Falls areas. The area of the basalt respects, such as high concentration of Ba U.S. Geological Survey Klamath Falls, of Devils Garden that is buried by these and Sr, low K/Ba, and higher 87Sr/86Sr Oregon and California, 2° sheet, and younger lavas is unknown, but we estimate 2 (0.7036-0.7039) ratios. Chemical character- Figs. 1, 2), a name which we adopt here as that a minimum of 12,400 km is now istics indicate that little or no contamina- an informal name for the basalt. The best exposed. tion by sialic crustal material has occurred. exposures of this basalt are along the south- Along its south margin, the lava field is a The basalts originated in the upper mantle eastern part of the plateau north of Altu- fairly constant 30 m thick. In the more and were erupted through crust thinned by ras, and one of the thickest sections is about mountainous part of the lava field near the tectonic extension behind the Cascade 50 km north of Alturas and 4 km south of state line and in Oregon, however, the total Range volcanic arc. the Oregon-California border. Here, about thickness varies but commonly exceeds 350 m of basalt forms a series of cliffs that 70 m and may be much greater. We esti- INTRODUCTION are the thickest known continuous expo- mate the volume of the basalt of Devils sures in the study area. Garden to be at least 850 km3. A poorly known sequence of late Mio- The south margin of the plateau under- cene and early Pliocene basaltic lavas lain by the basalt of Devils Garden is traced GEOLOGIC SETTING underlies part of northeastern California by the course of the Pit River near Alturas and south-central Oregon. Reconnaissance (Figs. 1,2). The lavas here are nearly flat- The Devils Garden lava field (Figs. 1, 2) field studies and K-Ar dating suggest that lying and are cut by north-northwest- lies between the Basin and Range and Cas- these basalt flows were emplaced during a trending normal faults with as much as a cade Range physiographic provinces and relatively brief period and thus constitute a few tens of metres of offset. Northward, at makes up much of a poorly defined region coherent age and compositional group of about the Oregon-California border and called the Modoc plateau (Macdonald, lavas. We report here the results of our beyond, the terrane is increasingly moun- 1966). The physiography of the Modoc pla- efforts (1) to determine the areal extent and tainous and eroded. The basaltic lavas of teau is transitional between that of the to calculate the volume of these basaltic the entire region between Alturas and Basin and Range and the Cascade Range. lavas; (2) to establish the age of the lavas by Summer Lake in Oregon are similar in Part of the Modoc plateau is cut by north- K-Ar geochronology; (3) to determine the petrology and age and thus constitute a northwest-trending normal faults (Gay and petrology and origin of the lavas, using coherent lava field. Outliers of correlative Aune, 1958) that are roughly parallel to major and trace elements and Sr isotopes; basalt are known to the south of Alturas faults in a4jacent parts of the Basin and Geological Society of America Bulletin, v. 94, p. 292-304, 6 figs., 2 tables, February 1983. 292 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/2/292/3434543/i0016-7606-94-2-292.pdf by guest on 01 October 2021 BASALTIC ROCKS AND IMPLICATIONS FOR CRUSTAL STRUCTURE 293 44°- 43°- 42°-- Range (Lawrence, 1976). Figure 1 shows the positions of the major faults in southeastern Oregon, northeastern California, and northwestern Nevada. The basalt of Devils Garden unconfor- Figure 1. Northeastern California, mably overlies various tuffaceous continen- northwestern Nevada, aft'd southeast- tal and lacustrine sedimentary rocks that ern Oregon, showing major late crop out in several localities south of Cenozoic faults and Devils Garden Summer Lake, Oregon, and at the south lava field. Hachures on faults show margin of the volcanic field near Alturas, downdropped side. California. These sedimentary rocks range in age from early Miocene into earliest Plio- Range. Vertical offset is distributed across completely bury faults in older rocks. cene and may be progressively younger the Modoc plateau on many small faults Northwest-trending faults that may be prin- from north to south, based on scattered rather than being concentrated at the mar- cipally strike-slip (Gay, 1959) also cut the paleontological evidence. Near Summer gins of large horsts and grabens, as is typical Modoc plateau (Lawrence, 1976; Hannah, Lake, the sedimentary sequence is unnamed in the Basin and Range. This pattern of dis- 1977; Duffield and Fournier, 1974). These but appears to be lithologically similar to tributed fault offset extends westward from faults are parallel to the Brothers fault zone units of the John Day Formation of middle the Warner Range, without apparent inter- of central Oregon, which has been inter- Miocene to early Oligocene age in central ruption, to the east edge of the Cascade preted as a transcurrent structure that Oregon. Mammalian fossils from tufface- Range, where Quaternary lavas largely or bounds the northwest edge of the Basin and ous sedimentary rocks of the Alturas For- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/2/292/3434543/i0016-7606-94-2-292.pdf by guest on 01 October 2021 Figure 2. Northeastern California and 0 10 20 30 40 50 MILES south-central Oregon, showing sample localities, K-Ar ages, and outline of Devils Garden lava field. 0 10 20 30 40 50 60 70 80 KILOMETERS Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/2/292/3434543/i0016-7606-94-2-292.pdf by guest on 01 October 2021 BASALTIC ROCKS AND IMPLICATIONS FOR CRUSTAL STRUCTURE 295 mation of Dorf (1933) beneath the basalt of in the Medicine Lake Highlands area, but ally thinner than 10 m. Dikes that may have Devils Garden near Alturas are of early Pli- because there is no striking field evidence of fed some of the flows crop out locally; lava ocene (early Blancan) age (Repenning, a significant difference in age between these shields, interpreted to mark vent areas 1980). two groups of lavas, previous reconnais- (Duffield and Fournier, 1974), and cinder Basaltic to andesitic lava flows exposed in sance studies resulted in assignment of both cones occur unevenly throughout the field. fault scarps west of the Devils Garden lava groups to the Quaternary (Gay and Aune, Andesite flows, locally intercalated with field near Klamath Falls, Oregon, yield 1958). Radiometric dating done as part of basalt, account for perhaps 3% to 4% of the K-Ar ages of about 2 m.y. (Iocs. 31,32, Fig. this study, however, indicates that the basalt volume of the lava field. Late Miocene rhy- 2; Table I). These scarps are largely or of Devils Garden was emplaced during late olitic domes, flows, and associated pyro- wholly buried along strike by younger lavas Miocene and earliest Pliocene time (Table clastic deposits are fairly abundant in the associated with the Crater Lake volcanic 1) and is clearly older than the Quaternary northern part of the lava field but are nearly center to the north and the Medicine Lake lavas of the Medicine Lake area.
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