Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and ⁴⁰Ar/³⁹Ar geochronology

Abstract

HAR•E• ET AL.' GE•ONOIX)O• OF THE JOSE•H•E O?H•oLrrE AND ITS EM•LACEMENTThe 4øAr/39Ar ages on igneous minerals will be similar to crystallization ages only if cooling was rapid (e.g., shallow level dikes). Many of the dated plutons are small and were intruded into rocks having ambient temperatures of <350øC. Such small intrusions would be expected to cool quickly below the -500øC closure temperature for hornblende. Many of these plutons, however, consist of two or more phases, so the temperature may have remained elevated by continual addition of heat from new intrusions, especially if substantial volumes of magma were transported through the plutonic complex.Zircon U/Pb age data are presented in Table 2, and the locations of the samples are given in Appendix A (available on microfiche). Notes on analytical procedures and assessment of uncertainties are given in the footnotes of Table 2 Table 2 with Table 1

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt

“…In Cordilleran-type ophiolites (Moores 1982), emplacement may involve underthrusting beneath the passive margin rather than overthrusting, as is the case for the Josephine ophiolite (Harper et al 1994). There, serpentinite mylonites occur at the base of the ophiolite, which is intruded by abundant syntectonic calcalkaline dikes and small plutons supplied by a hot island arc complex being underthrust beneath the ophiolite.…”

Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

“…If the resultant calcalkaline arc and apron of derived volcanogenic sediments initially formed as a continuous curvilinear belt, this time interval evidently attended a progressive $200 km outboard migration of the Klamath terrane amalgam relative to the Sierra Nevada . Initiation of volcanism-plutonism and the clastic detritus shed oceanward from the growing arc include Middle and Late Jurassic granitoids [Stern et al, 1981;Bateman, 1992;Dunne et al, 1998;Dickinson, 2008] and their outboard erosional products such as the western Klamath Galice Formation [Harper et al, 1994;Miller and Saleeby, 1995;Gray, 2006] and the western Sierran Foothills Mariposa Formation [Sharp, 1988;Edelman and Sharp, 1989], both of OxfordianKimmeridgian age. Dating the onset of construction of this Upper Jurassic sequence in terms of Mariposa sedimentation, based on macrofossils and microfossils reported by Imlay [1961], Clark [1964], and Graymer and Jones [1994], may well define the change from earlier, chiefly strike-slip plate motion to later transpression involving a major component of lithospheric convergence along the Californian sector of the North American margin.…”

Cretaceous sedimentary blanketing and tectonic rejuvenation in the Western Klamath moutains: Insights from thermochronology

“…Assembly of these terranes was completed in the late Jurassic Nevadan orogeny, during which the Western Klamath terrane ( Figure 1) was thrust beneath older rocks of the Klamath Mountains along the Orleans Fault, regionally metamorphosed to low-medium greenschist facies, and intruded by calcalkaline magmas [12][13][14]. Major thrusting in this orogenic event was completed by ∼150 Ma and plutons as old as ∼147 Ma overprint the Nevadan slaty cleavage, but the younger age limit of the Nevadan orogeny has been considered equivocal [15].…”

“…The timing and tectonic details of this event are somewhat controversial, but it is associated with a minor episode of plutonism cutting main-phase Nevadan structures in the Klamath Mountains and northern Sierra Nevada foothills between 143 and 138 Ma [13,15]. Ductile deformation features are well developed within and around these late-stage intrusions, and regional metamorphism may have continued until as recently as 135 Ma [15]. The younger age limit of the Separation Episode in the west-central Klamath Mountains is constrained by an extensive erosion surface on the crystalline basement.…”