Sediments and tectonics of the Gorda-Juan de Fuca plate Public Deposited

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  • Cores taken from the ridge areas of the Gorda-Juan de Fuca plate have a sedimentation rate that is appropriate for the study of late Quaternary stratigraphy. An analysis of the clay and silt mineralogy of the cores using X-ray diffraction methods and by noting changes in the foraminiferan-radiolarian abundances in the cores were utilized in developing a stratigraphic sequence. The clay fractions of these sediments consists of chlorite, illite and smectite. Cores taken from bathymetric highs contain, on the average, less smectite than do the turbidites from the adjacent lowlands. The low smectite content suggests eolian enrichment of these sediments since dusts collected from the nearby continent also have low smectite concentrations. Changes in the relative abundances of radiolaria and foraminifera are used to put biostratigraphic constraints on the correlation of mineralogical datums. Two changes in the foraminiferan-radiolarian ratios, marked by sharp increases in the abundance of radiolaria, occurred at 12,500 years B.P. and 83,000 years B.P. as dated by carbon-14 and sedimentation rate extrapolations, respectively. Such faunal changes serve as an independent check of correlations of the mineralogical datums. In the 2 to 20 micron, silt fraction, quartz, chlorite, mica and feldspar are the predominant minerals. Intervals in which the relative abundance of quartz changes can be dated by carbon-l4 and sedimentation rates, and related to late Quaternary climatic events. The quartz-rich zones are synchronous with periods of high insolation, high stands of sea-level, and to a lesser degree with the catastrophic floods of the Columbia River. The correlation with high solar radiation reflects quartz enrichment of the sediment due to an increased eolian contribution. The coincident high sea level stands effectively decreased the sedimentation rate of quartz-poor continental detritus that otherwise dilutes the eolian component. The periodic floods of the Columbia River, caused by the failure of ice dams, swept quartz-rich loess from eastern Washington down the river and injected into the marine environment. Such sediment also increased the quartz abundance in the quartz-rich zones on the ridges. Thus, the late Quaternary stratigraphy of the cores can be related to global late Quaternary climatic variations as well as to events recorded on the adjacent continents. The structural development of the Gorda-Juan de Fuca plate over the last 10 million years can be explained by north-south shortening coupled with the normal tectonism associated with a spreading sea floor. This hypothesis for the development of the plate is based on the presently known magnetic anomaly pattern. A series of reconstructions of this pattern back through the past 10 million years shows that both the Gorda and Juan de Fuca portions of the plate have grown steadily smaller. The incorporation of sequentially shorter Gorda ridge anomalies into the Pacific plate appears to have led to the northwest-southeast orientation of the Blanco Fracture Zone, with consequent changes in the direction of spreading of the Juan de Fuca Ridge. On the Juan de Fuca portion of the plate, the shortening was accomplished by shear faulting in Cascadia Basin. Furthermore, this faulting resulted in the rapid subduction of this portion of the plate, which, in turn, produced a disconformity in the sediments of Cascadia Basin. The reconstruction strengthens the notion that right lateral strike slip motion between the Pacific and Gorda-Juan de Fuca plate does, indeed, exist.
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  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2012-04-04T15:35:11Z (GMT) No. of bitstreams: 1 PhippsJamesBenjamin1974.pdf: 1029660 bytes, checksum: b3dc5d767d69c4d8829bfd777996935b (MD5)
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