Academic literature on the topic 'Metamorphism (Geology) – Klamath Mountains (Calif. and Or.)'

Abstract The Eastern Belt of the Franciscan Complex in the northern California Coast Ranges consists of coherent thrust sheets predominately made up of ocean floor sediments subducted in the Early Cretaceous and then accreted to the overriding plate at depths of 25-40 km. Progressive packet accretion resulted in the juxtaposition of a series of thrust sheets of differing metamorphic grades. This study utilizes laser Raman analysis of carbonaceous material to determine peak metamorphic temperatures across the Eastern Belt and phengite barometry to determine peak metamorphic pressures. Locating faults that separate packets in the field is difficult, but they can be accurately located based on differences in peak metamorphic temperature revealed by Raman analysis. The Taliaferro Metamorphic Complex in the west reached 323-336°C at a minimum pressure of ~11 kbar; the surrounding Yolla Bolly Unit 215–290°C; the Valentine Springs Unit 282-288°C at 7.8±0.7 kbar; the South Fork Mountain Schist 314–349°C at 8.6–9.5 kbar, a thin slice in the eastern portion of the SFMS, identified here for the first time, was metamorphosed at ~365°C and 9.7±0.7 kbar; and a slice attributed to the Galice Formation of the Western Klamath Mountains at 281±13°C. Temperatures in the Yolla Bolly Unit and Galice slice were too low for the application of phengite barometry. Microfossil fragments in the South Fork Mountain Schist are smaller and less abundant than in the underlying Valentine Springs Unit, providing an additional method of identifying the boundary between the two units. Faults that record a temperature difference across them were active after peak metamorphism while faults that do not were active prior to peak metamorphism, allowing for the location of packet bounding faults at the time of accretion. The South Fork Mountain Schist consists of two accreted packets with thicknesses of 300 m and 3.5 km. The existence of imbricate thrust faults both with and without differences in peak metamorphic temperature across them provides evidence for synconvergent exhumation.

The Condrey Mountain Schist (CMS) occupies a window through Late Triassic amphibolite facies melange in the north central Klamath Mountains in northern California and southwest Oregon. The schists owe their present level of exposure to a large structural dome centered on the Condrey Mountain Window. Transitional blueschist-greenschist facies assemblages are widespread in mafic schists in the structurally lowest levels of the window; structurally higher CMS near the window margins contains medium- to high-pressure greenschist facies parageneses. An ⁴⁰Ar/³⁹Ar crossite age indicates a late Middle Jurassic age of metamorphism. All subunits of the CMS contain evidence of progressive, polyphase deformational and metamorphic histories. The styles and geometries of minor structures in the central part of the window suggest that early folding and transposition was the result of noncoaxial deformation, and that rotational strains were replaced by irrotational flattening strains with time. Rotational strains were accompanied by the development of epidote-crossite assemblages and the growth of deerite in meta-ironstones; irrotational flattening strains were accompanied and followed by the growth of albite, actinolite, spessartine, and the Ba-silicate, cymrite. Pressure-temperature estimates, the relative ages of mineral growth and deformation, and strain geometries are consistent with, but not restricted to, a subduction zone environment. High shear strains may reflect descent and burial, whereas flattening and late, static mineral growth occur during uplift. Pressure-temperature estimates for the overlying CMS greenschists suggest temperatures similar to those in the central part of the window, but at slightly lower pressures. Thrusting of the overlying amphibolites at 150-156 Ma occurred while the amphibolites were above about 500°C. Stretching lineations indicate a movement vector of about N45W. Comparisons of the sequence and timing of metamorphic and structural events, radiometric ages, and movement directions during thrusting indicate the CMS does not represent an inlier of Klamath Western Jurassic Belt flysch but is instead an older, isolated thrust plate. Similarities with the age of metamorphism and plutonism in the overlying amphibolites suggest the two plates may be remnants of the same Middle Jurassic paired metamorphic belt.<br>text

Indiana University-Purdue University Indianapolis (IUPUI)<br>The Antler and Sonoma orogenies occurred along the southwest-trending passive Pacific margin of North America during the Paleozoic concluding with the accretion of the McCloud Arc. A southeast-trending sinistral transform fault truncated the continental margin in the Permian, becoming a locus for initiation of an east-dipping subduction zone creating the Sierran magmatic arc. Constrained in age between two early Triassic tuff layers, the volcanic clasts in the breccia of Frog Lakes represent one of the earliest records of mafic magmatism in the eastern Sierra Nevada. Tholeiitic rock clasts found in the breccia of Frog Lakes in the Saddlebag Lake pendant in the east central Sierra Nevada range in composition from 48% to 63% SiO2. Boninites produced by early volcanism of subduction initiation by spontaneous nucleation at the Izu-Bonin-Mariana arc are more depleted in trace element concentrations than the clasts while andesites from the northern volcanic zone of the Andes produced on crust 50 km thick have similar levels of enrichment and provide a better geochemical modern analogue. Textural analysis of the breccia of Frog Lakes suggest a subaqueous environment of deposition from a mature magmatic arc built on continental crust > 50 km thick during the Triassic. The monzodiorites of Saddlebag and Odell Lakes are temporal intrusive equivalents of the breccia of Frog Lakes and zircon geochemistry indicates a magmatic arc petrogenesis.