Geology topics

T.H. Dixon

Publications and source records attributed to T.H. Dixon.

4 recordsLinked to original sources

USGS2008

Space-based detection of wetlands' surface water level changes from L-band SAR interferometry

Interferometric processing of JERS-1 L-band Synthetic Aperture Radar (SAR) data acquired over south Florida during 1993-1996 reveals detectable surface changes in the Everglades wetlands. Although our study is limited to south Florida it has implication for other large-scale wetlands, because south Florida wetlands have diverse vegetation types and both managed and natural flow environments. Our analysis reveals that interferometric coherence level is sensitive to wetland vegetation type and to the interferogram time span. Interferograms with time spans less than six months maintain phase observations for all wetland types, allowing characterization of water level changes in different wetland environments. The most noticeable changes occur between the managed and the natural flow wetlands. In the managed wetlands, fringes are organized, follow patterns related to some of the managed water control structures and have high fringe-rate. In the natural flow areas, fringes are irregular and have a low fringe-rate. The high fringe rate in managed areas reflects dynamic water topography caused by high flow rate due to gate operation. Although this organized fringe pattern is not characteristic of most large-scale wetlands, the high level of water level change enables accurate estimation of the wetland InSAR technique, which lies in the range of 5-10??cm. The irregular and low rate fringe pattern in the natural flow area reflects uninterrupted flow that diffuses water efficiently and evenly. Most of the interferograms in the natural flow area show an elongated fringe located along the transitional zone between salt- and fresh-water wetlands, reflecting water level changes due to ocean tides. ?? 2007 Elsevier Inc. All rights reserved.

Remote Sensing of Environment↗

USGS1997

Continuous monitoring of surface deformation at Long Valley Caldera, California, with GPS

Continuous Global Positioning System (GPS) measurements at Long Valley Caldera, an active volcanic region in east central California, have been made on the south side of the resurgent dome since early 1993. A site on the north side of the dome was added in late 1994. Special adaptations for autonomous operation in remote regions and enhanced vertical precision were made. The data record ongoing volcanic deformation consistent with uplift and expansion of the surface above a shallow magma chamber. Measurement precisions (1 standard error) for “absolute” position coordinates, i.e., relative to a global reference frame, are 3–4 mm (north), 5–6 mm (east), and 10–12 mm (vertical) using 24 hour solutions. Corresponding velocity uncertainties for a 12 month period are about 2 mm/yr in the horizontal components and 3–4 mm/yr in the vertical component. High precision can also be achieved for relative position coordinates on short (less than 10 km) baselines using broadcast ephemerides and observing times as short as 3 hours, even when data are processed rapidly on site. Comparison of baseline length changes across the resurgent dome between the two GPS sites and corresponding two-color electronic distance measurements indicates similar extension rates within error (∼2 mm/yr) once we account for a random walk noise component in both systems that may reflect spurious monument motion. Both data sets suggest a pause in deformation for a 3.5 month period in mid-1995, when the extension rate across the dome decreased essentially to zero. Three dimensional positioning data from the two GPS stations suggest a depth (5.8±1.6 km) and location (west side of the resurgent dome) of a major inflation center, in agreement with other geodetic techniques, near the top of a magma chamber inferred from seismic data. GPS systems similar to those installed at Long Valley can provide a practical method for near real-time monitoring and hazard assessment on many active volcanoes.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Kinematics of the Eastern California shear zone: Evidence for slip transfer from Owens and Saline Valley fault zones to Fish Lake Valley fault zone

Late Quaternary slip rates and satellite-based geodetic data for the western Great Basin constrain regional fault-slip distribution and evolution. The geologic slip rate on the Fish Lake Valley fault zone (the northwest extension of the Furnace Creek fault zone) increases northward from about 3 to 5 mm/yr, in agreement with modeled geodetic data. The increase coincides with the intersections of the Deep Springs fault, connected to the Owens Valley fault zone, and of other faults connected to the Saline Valley fault. The combined geologic and geodetic data suggest that (1) the northwest-striking faults of the Eastern California shear zone north of the Garlock fault are connected by north- to northeast-striking normal faults that transfer slip in a series of right steps, and (2) the amount and distribution of slip among the many faults of this broad, complex plate boundary have changed through time.

Geology↗

USGS1984

Petrochemistry, age and isotopic composition of alkali basalts from Ponape Island, Western Pacific

Eleven analyzed lava samples from Ponape Island are alkali olivine basalt, basanite and basanitoid. Most lavas are aphyric or sparsely phyric (< 10% phenocrysts) and have phenocrysts of olivine (Fo77-80), clinopyroxene and titanomagnetite, and microphenocrysts of plagioclase (An53-68) in a fine-grained groundmass of olivine, clinopyroxene, plagioclase, opaques, potassic oligoclase, ?? nepheline and accessary phases. Oxygen isotope and Fe2O3 FeO data suggest that most samples are fresh, although H2O contents are high. Xenoliths of chromite-bearing harzburgites and dunites, both with cumulate textures occur in one locality. Major- and trace-element concentrations are similar to other oceanic volcanic islands. Most major elements and compatible trace elements vary systematically with respect to the Mg number [ 100Mg (Mg + Fe2+)]. In contrast, the incompatible trace elements do not correlate with the Mg number, but do covary with other incompatible elements. Simple closed-system shallow fractionation cannot be invoked to explain the observed chemical variation in the lavas. Derivation of the fractionated lavas (Mg number = 66-48) probably involved polybaric crystal fractionation from a high-Mg-number parental liquid. In addition, variable-source concentration of a trace-element-rich minor phase is postulated. However, the mantle was homogeneous with respect to the ratio of 87Sr 86Sr. New KAr age data are not consistent with the hypothesis that Ponape and the Caroline Ridge represent a simple "hot spot". ?? 1984.

Chemical Geology↗