Tectonic Response to Siletzia Terrane Accretion Recorded in the Thermal and Displacement History of the Klamath Mountains Province

The Klamath Mountains province of northwestern California–southwestern Oregon is an anomalous element in the Cascadia margin; these mountains have the highest average topography, the oldest rocks, and the only identified example of late Cenozoic detachment faulting in the coastal mountains of the Cascadia forearc. Low-temperature thermochronology (apatite fission-track, apatite [U-Th]/He) analyses from the central and southern Klamath Mountains province record two distinct exhumation events—a Cretaceous–Paleocene regional cooling and a southward-migrating locus of rapid cooling/exhumation in the middle Tertiary. This younger event is localized within the geographic extent of the La Grange fault. We infer that this pattern reflects two distinct processes of exhumation: regional surface erosion (older) and migrating localized tectonic exhumation (younger). At the southern limit of this region of rapid cooling, slickenside striations on the exposed La Grange fault surface record southward displacement of the upper plate along a shallowly dipping (~20°) detachment surface. Thermochronologic data constrain average dip of the fault to a few degrees, upper-plate thickness to <~6–8 km, and fault slip rate to <2 mm/ yr for a duration of 30 m.y . (ca. 45 Ma to 15 Ma). The fault dip is unusually low compared to that of typical detachment faults; the duration of this extensional event is unusually long compared to other detachment faults; the north-south (margin-parallel) slip direction is roughly perpendicular to that of other Klamath Mountains province faults; and the Eocene to early Miocene timing of extensional faulting does not correlate with recognized tectonic events in northern California. Mid-Tertiary tectonic events in the Oregon Coast Ranges provide a context for understanding the unusual mid-Tertiary tectonism in the Klamath Mountains province. Immediately north of the Klamath Mountains province, early Eocene accretion of a large early Cenozoic igneous province, the Siletz terrane, initiated a westward jump of active subduction. Accretion was followed by late Eocene margin-parallel extension in the Oregon Coast Ranges, recorded by formation of a regional dike swarm. Both the timing of tectonic exhumation and the direction of extension on the La Grange detachment fault suggest that mid-Tertiary tectonism in the southern Klamath Mountains province was likely driven by plate tectonics associated with the accretion of Siletzia and the reestablishment of subduction outboard of the accreted terrane.

The Mesozoic and Cenozoic history of Western North America is characterized by terrane accretion, volcanism, and orogenesis. This history complicates the interpretation of paleogeography in northeastern Oregon and adjacent Idaho, where the age of mountainous topography in the Blue Mountains Province is important for validating hypothesized Miocene to present geodynamics. In this study, we analyze the distribution of Columbia River Basalt and low‐temperature apatite and zircon (U‐Th)/He thermochronometry collected from the Wallowa and Bald Mountain Batholiths to refine regional landscape history. These batholiths underly the Wallowa and Elkhorn Mountains respectively, two of the most prominent mountain ranges within the Blue Mountains Province. We find that low‐temperature thermochronometry data from the Bald Mountain and Wallowa Batholiths record distinct thermal histories associated with unroofing and magmatism from the Cretaceous to present. We propose that reheating during intrusion of the Chief Joseph dike swarm led to partial resetting of thermochronometers but did not completely overprint recorded Mesozoic to present thermal histories in the Wallowa Batholith. Using modeled thermal histories and the present‐day distribution of Columbia River Basalt, we conclude that the Wallowa and Elkhorn Mountains have distinct topographic histories. We propose that the Elkhorn Mountains began to form in the Eocene and that the Wallowa Mountains are geologically young, forming as a result of relief generation after the Miocene eruption of Columbia River Basalt.

The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a &amp;gt;10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features.

A growing body of evidence indicates that Middle Jurassic to Early Cretaceous plutons recorded changing sources during tectonic evolution of the Klamath Mountain province. The data set now includes U-Pb zircon ages and zircon trace element compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Thirteen rock samples were dated, and these data refine thermal ionization mass spectrometry (TIMS) data where inheritance was problematic, or provide new U-Pb ages. Individual plutonic suites, previously defined on the basis of crystallization age, isotope and elemental compositions, and petrogenetic style, show characteristic inherited zircon age ranges and zircon trace element patterns. Moreover, ages of inherited zircons in these suites are distinct and, in at least three suites, indicate the presence of cryptic (unexposed) source rocks. The zircon data complement oxygen, Nd, and Sr isotope whole-rock data that, when taken together, suggest a number of major changes in the crustal column with time. Middle Jurassic magmatism began with the oceanic(?) arc-related western Hayfork terrane comprising volcanic, volcaniclastic, and plutonic components. After regional thrusting on the ca. 170-Ma Wilson Point thrust, the Ironside Mountain batholith and Wooley Creek suite of plutons were emplaced. The former shows little evidence of interaction with the crust, but the latter contains Middle Jurassic inheritance and Sr, Nd, and oxygen isotope signatures suggestive of interaction with metasedimentary crustal rocks. Following Nevadan thrusting (ca. 153–150 Ma), emplacement of western Klamath suite plutons in the western Klamath Mountains province involved significant assimilation of Galice Formation metasedimentary rocks. This activity was followed by emplacement of tonalite-trondhjemite-granodiorite (ttg) plutons in the eastern Klamath Mountains province, which were derived by partial melting of metabasic rocks. Their zircon trace element signatures indicate diverse magma histories and, at least locally, multiple magma sources. Inherited zircons in ttg plutons suggest late Middle Jurassic to Late Jurassic sources, younger than the Josephine ophiolite. The youngest magmatism in the Klamath Mountains province consists of broadly granodioritic plutons, which, on the basis of limited data, show variable petrogenesis and zircon inheritance. At least one of these plutons (136-Ma Yellow Butte pluton) contains a ca. 150-Ma inheritance that indicates the presence of Late Jurassic crustal rocks beneath the eastern Klamath terrane.

Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along-strike heterogeneity of strain distribution, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We documented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben-filling sediments from sources outside of the Klamath Mountains, and initiation of mid-Miocene or younger right-lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 ± 2.1 Ma to 15.7 ± 2.1 Ma. Inverse thermal modeling of these data and published apatite fission-track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 ± 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 ± 5.1 Ma on the downthrown side and 22.7 ± 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaverville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south-central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these grabens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 ± 0.4 Ma and 16.7 ± 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault-controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to-bedrock contacts reported in well logs record E-side-up and right-lateral faulting at the eastern end of the Hayfork graben. A NE-striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Of fset of both the base of the Weaverville Formation and the cataclasite-capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right-lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation. Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or “subcretion” in the southern Cascadia subduction zone beneath the southern Klamath Mountains.

The southern Cascadia forearc undergoes a three-stage tectonic evolution, each stage involving different combinations of tectonic drivers, that produce differences in the upper-plate deformation style. These drivers include subduction, the northward migration of the Mendocino triple junction and associated thickening and thinning related to the Mendocino Crustal Conveyor (MCC) effect, and the NNW translation of the Sierra Nevada-Great Valley (SNGV) block. We combine geodetic data, plate reconstructions, seismic tomography and topographic observations to determine how the southern Cascadia upper plate is deforming in response to the combined effects of subduction and NNW-directed (MCC- and SNGV-related) tectonic processes. The location of the terrane boundaries between the relatively weak Franciscan complex and the stronger Klamath Mountain province (KMP) and SNGV block has been a key control on the style of upper-plate deformation in the southern Cascadia forearc since the mid-Miocene. At ∼15 Ma, present-day southern Cascadia was in central Cascadia and deformation there was principally controlled by subduction processes. Since ∼5 Ma, this region of the Cascadia upper plate, where the KMP lies inboard of the Franciscan complex, has been deforming in response to both subduction and MCC- and SNGV-related effects. GPS data show that the KMP is currently moving to the NNW at ∼8–12 mm/yr with little internal deformation, largely in response to the northward push of the SNGV block at its southern boundary. In contrast, the Franciscan complex is accommodating high NNW-directed and NE-directed shortening strain produced by MCC-related shortening and subduction coupling respectively. This composite tectonic regime can explain the style of faulting within and west of the KMP. Associated with this Mendocino Crustal Conveyor crustal thickening, seismic tomography imagery shows a region of low velocity material that we interpret to represent crustal flow and injection of Franciscan crust into the KMP at intracrustal levels. We suggest that this MCC-related crustal flow and injection of material into the KMP is a relatively young feature (post ∼5 Ma) and is driving a rejuvenated period of rock uplift within the KMP. This scenario provides a potential explanation for steep channels and high relief, suggestive of rapid erosion rates within the interior of the KMP.

The Josephine ophiolite and its sedimentary cover (Galice Formation) of the western Jurassic belt, Klamath Mountains, northern California, are the expression of a Late Jurassic rift basin which developed between an active Late Jurassic arc to the west and a remnant Middle Jurassic are to the east. The Devils Elbow ophiolite remnant (DEO) and its sedimentary and volcanic cover represent the southern continuation of the Josephine ophiolite and overlying Galice Formation. The DEO consists of a dike complex, volcanic sequence, and ophiolitic breccia. Overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks. The following relations suggest formation of the DEO in a marginal basin setting, in accordance with the setting inferred for the Josephine ophiolite basin: (1) dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts, by crystallization of clinopyroxene before plagioclase, and by trace-element geochemistry which is transitional between IAT and MORB; (2) overlying sedimentary rocks appear to have been derived from erosion of older Klamath Mountain terranes and were deposited into the DEO basin shortly after and/or during its formation; (3) the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source. The period of rifting which culminated in the formation of the Josephine ophiolite/DEO basin began by at least 164 Ma, the age of the DEO, but is unlikely to have begun much earlier, as the region was experiencing a major period of compression just prior to this time. Rifting began at least 5 m.y. prior to cessation of magmatism in the rifted Middle Jurassic arc. The overlap in rifting and arc magmatism can be explained if spreading in the Josephine ophiolite/DEO basin was asymmetric relative to the axis of Middle Jurassic are magmatism and/or if transform motion approximately parallel to the Middle Jurassic are was important in the spreading geometry of the basin. The western boundary fault of the western Jurassic belt, and Klamath Mountain province, is the South Fork fault (previously referred to as part of the Coast Range thrust). In the southern western Jurassic belt, a serpentinite matrix meelange (SMM) occurs along this fault. Blocks in the SMM consist largely of greenstones with the trace-element geochemistry of MORB. Other block types include gabbroic to dioritic intrusive rocks, diabase, harzburgite, amphibolite, chert, sandstone, and shale. Generally similar serpentinite matrix melanges have been described from other locations along the South Fork fault. This spatial association suggests that the South Fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix melange from a source at depth. Obvious sources for the SMM include parts of the Franciscan complex, the Josephine ophiolite/DEO, or possibly the Coast Range ophiolite. The geochemistry and relative abundances of different lithologies in the SMM, however, indicate that none of these could represent the source. In contrast, the SMM is nearly identical in block content and block geochemistry to serpentinite matrix melange of the Rattlesnake Creek terrane which structurally overlies the western Jurassic belt to the east. This suggests that a previously unrecognized part of the Rattlesnake Creek terrane may underlie the Josephine ophiolite/DEO at depth. As the Rattlesnake Creek terrane formed the western part of the basement for the rifted Middle Jurassic arc, the part of the Rattlesnake Creek terrane which underlies the Josephine ophiolite/DEO may represent a fragment of crust which was rifted oceanward with the opening of the Josephine ophiolite/DEO basin and then subsequently thrust beneath the basin during the Late Jurassic Nevadan Orogeny.

Next article FreeA Summary of the Orogenic Epochs in the Geologic History of North AmericaEliot BlackwelderEliot Blackwelder Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 22, Number 7Oct. - Nov., 1914 Article DOIhttps://doi.org/10.1086/622180 Views: 389Total views on this site Citations: 17Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Arlo Brandon Weil, Adolph Yonkee The Laramide orogeny: Current understanding of the structural style, timing, and spatial distribution of the classic foreland thick-skinned tectonic system, (Jan 2023): 707–771.https://doi.org/10.1130/2022.1220(33)Tsilavo Raharimahefa, Bruno Lafrance, Douglas K. Tinkham, Fernando Corfu New structural, metamorphic, and U–Pb geochronological constraints on the Blezardian Orogeny and Yavapai Orogeny in the Southern Province, Sudbury, Canada, Canadian Journal of Earth Sciences 51, no.88 (Aug 2014): 750–774.https://doi.org/10.1139/cjes-2014-0025J. J. Schwartz, A. W. Snoke, F. Cordey, K. Johnson, C. D. Frost, C. G. Barnes, T. A. LaMaskin, J. L. Wooden Late Jurassic magmatism, metamorphism, and deformation in the Blue Mountains Province, northeast Oregon, Geological Society of America Bulletin 123, no.9-109-10 (Jun 2011): 2083–2111.https://doi.org/10.1130/B30327.1Arthur W. Snoke, Calvin G. Barnes The development of tectonic concepts for the Klamath Mountains province, California and Oregon, (Jan 2006): 1–29.https://doi.org/10.1130/2006.2410(01)Gary G. Gray Structural and tectonic evolution of the western Jurassic belt along the Klamath River corridor, Klamath Mountains, California, (Jan 2006): 141–151.https://doi.org/10.1130/2006.2410(07)Charlotte M. Allen, Calvin G. Barnes Ages and some cryptic sources of Mesozoic plutonic rocks in the Klamath Mountains, California and Oregon, (Jan 2006): 223–245.https://doi.org/10.1130/2006.2410(11)Kevin R. Chamberlain, Arthur W. Snoke, Calvin G. Barnes, Jonathan C. Bushey New U-Pb radiometric dates of the Bear Mountain intrusive complex, Klamath Mountains, California, (Jan 2006): 317–332.https://doi.org/10.1130/2006.2410(15)Bradley R. Hacker, Mary M. Donato, Calvin G. Barnes, M. O. McWilliams, W. G. Ernst Timescales of orogeny: Jurassic construction of the Klamath Mountains, Tectonics 14, no.33 (Jul 2010): 677–703.https://doi.org/10.1029/94TC02454Gregory D. Harper, Jason B. Saleeby, Matthew Heizler Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California-Oregon: U/Pb zircon and 40 Ar/ 39 Ar geochronology, Journal of Geophysical Research: Solid Earth 99, no.B3B3 (Sep 2012): 4293–4321.https://doi.org/10.1029/93JB02061Steven H. Edelman Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California, Tectonophysics 191, no.3-43-4 (Jun 1991): 223–236.https://doi.org/10.1016/0040-1951(91)90058-ZGregory D. Harper, James E. Wright Middle to Late Jurassic tectonic evolution of the Klamath Mountains, California-Oregon, Tectonics 3, no.77 (Jul 2010): 759–772.https://doi.org/10.1029/TC003i007p00759Jason B. Saleeby, Gregory D. Harper, Arthur W. Snoke, Warren D. Sharp Time relations and structural-stratigraphic patterns in ophiolite accretion, west central Klamath Mountains, California, Journal of Geophysical Research 87, no.B5B5 (Jan 1982): 3831.https://doi.org/10.1029/JB087iB05p03831Jason B. Saleeby, Cathy Busby-Spera, J. S. Oldow, G. C. Dunne, J. E. Wright, D. S. Cowan, N. W. Walker, R. W. Allmendinger Early Mesozoic tectonic evolution of the western U.S. Cordillera, (): 107–38.https://doi.org/10.1130/DNAG-GNA-G3.107W. B. Harland Interpretation of stratigraphical ages in orogenic belts, Geological Society, London, Special Publications 3, no.11 (Jun 2022): 115–135.https://doi.org/10.1144/GSL.SP.1969.003.01.07Hans Bürgl The orogenesis in the andean system of colombia, Tectonophysics 4, no.4-64-6 (Oct 1967): 429–443.https://doi.org/10.1016/0040-1951(67)90009-1R. H. Dott Mesozoic-cenozoic tectonic history of the southwestern Oregon coast in relation to cordilleran orogenesis, Journal of Geophysical Research 70, no.1818 (Dec 2012): 4687–4707.https://doi.org/10.1029/JZ070i018p04687Richard A. Sonder Die erdgeschichtlichen Diastrophismen im Lichte der Kontraktionslehre, Geologische Rundschau 13, no.33 (Nov 1922): 217–272.https://doi.org/10.1007/BF01799789

Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin.

Cryptic tectonic domains of the Klamath Mountains, California and Oregon

Research Article| June 01, 1988 An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California JAMES E. WRIGHT; JAMES E. WRIGHT 1Department of Geology, Stanford University, Stanford California 94305 Search for other works by this author on: GSW Google Scholar MARK R. FAHAN MARK R. FAHAN 2Holguin & Associates, Inc., 73 North Palm Street, Ventura, California 93001 Search for other works by this author on: GSW Google Scholar Author and Article Information JAMES E. WRIGHT 1Department of Geology, Stanford University, Stanford California 94305 MARK R. FAHAN 2Holguin & Associates, Inc., 73 North Palm Street, Ventura, California 93001 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (6): 859–876. https://doi.org/10.1130/0016-7606(1988)100<0859:AEVOJO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation JAMES E. WRIGHT, MARK R. FAHAN; An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California. GSA Bulletin 1988;; 100 (6): 859–876. doi: https://doi.org/10.1130/0016-7606(1988)100<0859:AEVOJO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. "Outboard" and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetic investigation of Cretaceous outliers and Tertiary sedimentary strata of the Klamath Mountains province, and of onlapping Cretaceous strata, has shown the rocks to be largely remagnetized. Samples studied are from the Upper Jurassic to Upper Cretaceous Great Valley sequence, Upper Cretaceous Hornbrook Formation, Eocene Montgomery Creek Formation, and Oligocene(?) Weaverville Formation. Cretaceous samples that survived the remagnetization have a mean remanence direction that is very close to the expected direction of the Cretaceous magnetic field at the locality of the Klamath Mountains. Data from both primary and remagnetized samples suggest the possibility of 11.5° ± 15.8° of post-Cretaceous clockwise rotation of the Klamath Mountains province. None of the data from either the primary or remagnetized samples shows evidence of the large amounts (∼ 70°) of clockwise rotation that other workers have measured for the lower Tertiary of the Oregon Coast Range. Our data indicate that the Oregon Coast Range and Klamath Mountains province did not behave as a single rigid block during the early Tertiary. They also suggest that any post-Oligocene rotation of the Klamath Mountains province is less than the approximately 30° post-Oligocene rotation recently proposed for a combined Oregon Coast Range–Klamath Mountains–Cascade Range block.

Previous article No AccessWestward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States: A ReplyTodd A. LaMaskin and Rebecca J. DorseyTodd A. LaMaskin1. Department of Geography and Geology, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403-5944, USA*Author for correspondence; e-mail: [email protected]. Search for more articles by this author and Rebecca J. Dorsey2. Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 124, Number 1January 2016 Article DOIhttps://doi.org/10.1086/684120 Views: 206Total views on this site Citations: 5Citations are reported from Crossref HistoryReceived September 04, 2015Accepted September 09, 2015Published online December 16, 2015 © 2015 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Todd A. LaMaskin, Jonathan A. Rivas, David L. Barbeau, Joshua J. Schwartz, John A. Russell, Alan D. Chapman A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America, GSA Bulletin 134, no.3-43-4 (Jul 2021): 965–988.https://doi.org/10.1130/B35981.1Paul M. Bremner, Mark P. Panning, R. M. Russo, Victor Mocanu, A. Christian Stanciu, Megan Torpey, Sutatcha Hongsresawat, John C. VanDecar, Todd A. LaMaskin, D. A. Foster Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project, Journal of Geophysical Research: Solid Earth 124, no.22 (Feb 2019): 1601–1625.https://doi.org/10.1029/2018JB016350B. Tikoff, J. Vervoort, J.A. Hole, R. Russo, R. Gaschnig, A. Fayon Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue, Lithosphere (Feb 2017): L628.1.https://doi.org/10.1130/L628.1N. Braudy, R.M. Gaschnig, D. Wilford, J.D. Vervoort, C.L. Nelson, C. Davidson, M.J. Kahn, B. Tikoff Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho, Lithosphere (Dec 2016): L519.1.https://doi.org/10.1130/L519.1Tracy L. Vallier, Keegan L. Schmidt, Todd A. LaMaskin Geology of the Wallowa terrane, Blue Mountains province, in the northern part of Hells Canyon, Idaho, Washington, and Oregon, (): 211–249.https://doi.org/10.1130/2016.0041(07)

Rock contains &gt; 99% of Earth's reactive nitrogen (N), but questions remain over the direct importance of rock N weathering inputs to terrestrial biogeochemical cycling. Here we investigate the factors that regulate rock N abundance and develop a new model for quantifying rock N mobilization fluxes across desert to temperate rainforest ecosystems in California, USA. We analyzed the N content of 968 rock samples from 531 locations and compiled 178 cosmogenically derived denudation estimates from across the region to identify landscapes and ecosystems where rocks account for a significant fraction of terrestrial N inputs. Strong coherence between rock N content and geophysical factors, such as protolith, (i.e. parent rock), grain size, and thermal history, are observed. A spatial model that combines rock geochemistry with lithology and topography demonstrates that average rock N reservoirs range from 0.18 to 1.2 kg N m−3 (80 to 534 mg N kg−1) across the nine geomorphic provinces of California and estimates a rock N denudation flux of 20–92 Gg yr−1 across the entire study area (natural atmospheric inputs ~ 140 Gg yr−1). The model highlights regional differences in rock N mobilization and points to the Coast Ranges, Transverse Ranges, and the Klamath Mountains as regions where rock N could contribute meaningfully to ecosystem N cycling. Contrasting these data to global compilations suggests that our findings are broadly applicable beyond California and that the N abundance and variability in rock are well constrained across most of the Earth system.

A distinctive set of andesitic dikes crops out in the Klamath Mountains province south of ∼41°15′. These dikes are characterized by phenocrysts of Al-rich amphibole, Ca-rich almandine-pyrope garnet, plagioclase, and, commonly, quartz, set in a fine-grained groundmass of plagioclase, quartz, alkali feldspar, and amphibole. Conspicuous microphenocryst/accessory phases are allanite/epidote, zircon, and apatite. A variety of thermobarometric methods plus comparison with published experimental data indicates the phenocryst assemblage was stable at pressure >7 kb and at temperatures between 850 °C and the solidus. The best estimate of pressure of phenocryst equilibration is 8–9 kb and ∼800 °C, which corresponds to depths of 25–30 km. Emplacement pressure was probably 3–4 kb, and the lack of low-pressure equilibration features suggests that the dike magmas rose and cooled quickly. Rare earth element (REE) patterns for zircon are distinct from regional zircon compositions (samples in Klamath River sand) but are consistent with crystallization in equilibrium with garnet. Moreover, the zircons have strongly depleted light REE patterns, which is consistent with co-precipitation of allanite/epidote. A U-Pb (zircon) age of 160.5 ± 1.9 Ma was determined for one of the dike samples. This age is coeval with emplacement of the voluminous Wooley Creek plutonic suite in the central and northern parts of the province (north of latitude 41°15′). It is not clear why sparse dikes with high-pressure assemblages crop out in the southern Klamath Mountains province and voluminous coeval plutons are present in the central and northern parts. This distribution may reflect distinct tectonic regimes in the north (extension) versus the south (contraction), differences in melt productivity, or differences in the composition of deep crustal rocks.