RESEARCH Earliest Cretaceous Pacifi cward offset of the Klamath Mountains salient, NW California–SW Oregon W.G. Ernst DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES, STANFORD UNIVERSITY, STANFORD, CALIFORNIA 94305-2115, USA ABSTRACT Although Late Triassic igneous rocks are present, the Sierra Nevada–Klamath calc-alkaline arc began massive construction along the con- tinental margin at ca. 170 Ma during oblique underfl ow of paleo-Pacifi c oceanic lithosphere; intense activity continued throughout the volcanic-plutonic belt until at least ca. 140 Ma. This volcanic-plutonic arc supplied detritus to the Mariposa-Galice proximal clastic sequence starting by ca. 165–160 Ma. After onset of uppermost Jurassic Myrtle overlap sedimentation on the western fl ank of the Klamath Mountains, but before Hornbrook and Valanginian Great Valley Group overlap deposition on the eastern and southeastern sides, the Klamath Mountains salient was displaced ~200 km westward relative to the igneous arc. The orogen thus moved off the deep-seated magmagenic zone underly- ing the arc and did not participate in the massive Sierra Nevada igneous fl are-up between ca. 125 and ca. 85 Ma. I suggest that, beginning at ca. 140 Ma, underfl ow of a young, thin oceanic slab beneath the Klamath Mountains slid beneath the gently east-dipping stack of thrust sheets without disturbing their inclinations. Subduction and collision of much thicker oceanic lithosphere on both the north and south caused contraction, eastward relative displacement of the continental margin arc, and ductility-enhanced rotation of the superjacent stack of allochthons into near-vertical dips. After a magmatic lull, heightened igneous activity in the Sierra Nevada recommenced at ca. 125 Ma. The earliest Cretaceous oceanward plate junction rollback lay directly offshore the Klamath imbricate orogen, but to the south trapped the ca. 165 Ma Coast Range ophiolite on the North American side of the suture. After ca. 140 Ma, fi rst-cycle arc detritus began to accumulate on the mafi c igneous basement fl ooring the Great Valley forearc, and turbiditic clastic material also was carried oceanward across the forearc into the coeval Franciscan trench. LITHOSPHERE; v. 5; no. 1; p. 151–159 | Published online 14 November 2012 doi:10.1130/L247.1 INTRODUCTION types, structures, ages of the rock packages, timing of the offset and proposes a speculative the progressive oceanward assembly of succes- mechanism to explain it. Arcuate, shallowly east-dipping thrust sheets sively younger geologic units, and their times of in the Klamath Mountains consist mainly of deformation (Davis, 1969; Davis et al., 1980; TIME OF OUTBOARD RELATIVE OFFSET Paleozoic through earliest Late Jurassic oce- Wright and Fahan, 1988; Wright and Wyld, OF THE KLAMATH MOUNTAINS anic basement terranes and overlying superja- 1994; Irwin, 2003). However, the juxtaposed cent units that were stranded along the North Sierran Foothill terranes stand nearly vertically, Geologic constraints suggest an earliest Cre- American margin by chiefl y transpressive plate whereas, in contrast, Klamath thrust sheets root taceous westward displacement of the salient motions. Although high-pressure, low-temper- gently to the east. relative to the Sierran Foothills. Whether the ature phase assemblages attest to episodes of Figure 1 shows that the Klamath Moun- Klamath Mountains province moved westward Paleozoic to early Mesozoic subduction, the tains concave-to-the-east contractional assem- geographically, or the Sierra Nevada Range accretion of variably metamorphosed ophiol- bly lies well outboard of the trend of the Sierra moved eastward, or both were displaced, is itic terranes overlain by distal turbidites refl ects Nevada Range. Judging by the map relation- unknown. Only the differential offset is consid- chiefl y transform and transpressive lithospheric ships, this salient appears to be situated ~200 ered here. slip (Saleeby et al., 1992; Ernst et al., 2008). km west of the formerly contiguous Sier- (1) Late Jurassic deposition of the Galice Old, fault-bounded Klamath Mountain units on ran segment of the curvilinear arc (Fig. 2). Formation in the westernmost Klamath Moun- the east are structurally high in the accretionary North of the Klamath Mountains promontory, tains (MacDonald et al., 2006) and the correla- stack, whereas the ages of successively added a major eastward jog toward apparently cor- tive deposition of the Mariposa Formation in the lower allochthons decrease progressively toward relative lithologic units in the Blue Mountains westernmost Sierran Foothills (Snow and Ernst, the west (Irwin, 1972, 1994). The tectonized, (LaMaskin et al., 2011; Schwartz et al., 2011) 2008) exhibit the full ~200 km of apparent sinis- imbricated collage of west-vergent lithostrati- suggests the possibility of a much greater tral offset; hence, these proximal siliciclastic graphic terranes consists of basal ophiolitic oceanward offset of the Klamath Mountains units evidently were laid down unconformably units, chiefl y overlain by cherts and fi ne-grained relative to the late Mesozoic accretionary on the western edge of a continuous Sierra- terrigenous strata (e.g., Frost et al., 2006), all continental margin of eastern Oregon (Snoke Klamath arc prior to most of the differential invaded by Jurassic calc-alkaline arc plutons. and Barnes, 2006). The manner in which this slip (Ernst et al., 2008). Uppermost Jurassic to The accreted terrane assembly of the Klamath tectonic offset of the Klamath Mountains col- Lower Cretaceous Myrtle clastic strata overlie Mountains has long been correlated with the lage was accomplished remains obscure. This the Galice Formation in SW Oregon (Imlay et northern Sierran Foothills based on similar rock paper summarizes geologic evidence for the al., 1959; Dickinson, 2008, fi g. 3A) and so were LITHOSPHEREFor permission to| Volumecopy, contact 5 | Number [email protected] 1 | www.gsapubs.org | © 2012 Geological Society of America 151 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/1/151/3723189/151.pdf by guest on 01 October 2021 W.G. ERNST Myrtle Fm Hornbrook Fm lith (U-Pb zircon age 136 Ma) and the preex- isting eastern Klamath basement directly north 124°W 122°W 120°W Cascade subduction zone 42°N of the Cold Fork–Elder Creek fault zone at the Cascade 42°N Arc northern end of the Sacramento Valley (Jones and Irwin, 1971; Blake et al., 1988, 1999). 125–85 Ma granitic rocks (3) Jones and Irwin (1971) recognized and documented an upward nonmarine-marine tran- Modoc 170–140 Ma granitic rocks sition in Great Valley Group strata bordering Klamaths the SE Klamath Mountains. They reported that SB Plateau Jurassic metasedimentary this Valanginian paleoshoreline was offset more and metavolcanic rocks MF than 100 km eastward to the south of sinistral Franciscan Paleozoic-Triassic metased. OF-SS (?) “tear faults,” which are now considered as mem- Coast Complex and metavolcanic rocks Ranges bers of the Cold Fork–Elder Creek fault system Paleozoic and Mesozoic CF-EC ultramafic rocks (Blake et al., 1999). Thus, these offsets must Foothills Belt have occurred in post-Valanginian time. SAF Location of Fig. 3 (4) Locations of erosional remnants of silici- Great Valley Group clastic strata bordering the Klamath Mountains 39°N are shown schematically in Figures 1 and 2. Cenozoic sedimentary Based on the spatial distribution of erosional and volcanic rocks Sierra Nevada Batholith remnants of the Myrtle, Galice, and Mariposa Mainly Cretaceous Great Formations overlying Jurassic and older base- Valley Group (Lower K tan) ment rocks, outboard relative displacement Uppermost Cretaceous- of the Klamath salient began by ca. 140 Ma. Miocene coastal belt N? This apparent westward offset was well under Mainly Upper Cretaceous way by ca. 136 Ma, prior to Hornbrook + Great Franciscan mélange and Valley Group overlap deposition on the Shasta Nacimiento belts Bally Batholith and other eastern Klamath units. Mainly Lower to mid-Cretaceous SGF The seaward offset of the Klamath Mountains Franciscan eastern belt Salinian Block apparently occurred during a brief interval of chiefl y left-lateral slip along the western mar- gin of North America (Saleeby, 1992; Saleeby N et al., 1992; Harper et al., 1994) that terminated shortly after development of the Kimmeridgian– Tithonian cusp in the American apparent polar SAF W E wander paths (May and Butler, 1986; Schettino S and Scotese, 2005). (5) Geologic relationships among the Klam- 0 100 200 km ath Mountains, the Franciscan Complex, and the Great Valley Group in the vicinity of the so-called Yolla Bolly triple junction (Blake et 34°N al., 1999) are shown in Figure 3. In addition to 117°W the terrane-bounding, NW-trending South Fork Figure 1. Geology of northern and central California, emphasizing the Klamath-Sierran calc-alkaline and Coast Range faults, a family of transverse arc, Great Valley Group forearc basin, and Franciscan trench lithotectonic belts, simplifi ed after the breaks transects Great Valley Group strati- U.S. Geological Survey and California Division of Mines and Geology (1966) geologic map, the ter- rane map of Silberling et al. (1987), and coastal geologic maps of Dickinson et al. (2005). Granitoids graphic units in this somewhat more detailed are of Jurassic and Cretaceous emplacement ages, except for the dominantly Jurassic plutons map area. Especially signifi cant faults include, of the Klamath Mountains. Mariposa and Galice strata occupy the western parts of the Jurassic from north to south, the Oak Flat, Sulphur metasedimentary and metavolcanic rock units in the Sierran Foothills and the western Klamath Spring, Cold Fork, and Elder Creek structures. Mountains, respectively. Klamath-margin locations of the Myrtle and Hornbrook Formations are The Oak Flat–Sulphur Spring structures strike also indicated. SB—Shasta Bally Batholith. Fault zone abbreviations: CF-EC—Cold Fork and Elder ENE and are appropriately oriented to repre- Creek; OF-SS—Oak Flat and Sulphur Spring; N—on-land section of the Nacimiento, N?—its off- sent the inferred zone of earliest Cretaceous shore segment; SGF—San Gregorio–Hosgri; MF—Mendocino; and SAF—San Andreas.