The importance of diffusion, advection, and host-rock lithology on vein formation: A stable isotope study from the Paleozoic Ouachita orogenic belt, Arkansas and Oklahoma.
Ian J. Richards, Stable Isotope Laboratory, Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA; et al. Pages 1343-1355.
Keywords: Ouachita Mountains, quartz, stable isotopes, veins.
Oxygen is the most important element on Earth. Approximately fifty percent, by weight, of a rock is oxygen, and as such, oxygen isotopes can provide significant insight into geologic processes. In the Ouachita Mountains of western Arkansas and southeastern Oklahoma, there is a direct relationship between the oxygen isotopic composition of quartz veins and crystals and the rocks that host them. This relationship occurs at all scales and within an extremely wide variety of metasedimentary host-rock types, which indicates that the host rocks exert significant control over the fluids that assist in quartz vein formation. As fluids are the main agents of mass transport within Earth's crust and are responsible for economically important hydrocarbon reservoirs and ore deposits, the results of this study suggest that more confined and local flow of fluids within Earth's crust is a very important geologic process.
Late Cenozoic evolution and earthquake potential of an active listric thrust complex above the Hikurangi subduction zone, New Zealand
Philip M. Barnes, National Institute of Water and Atmospheric Research Ltd., P.O. Box 14901, Kilbirnie, Wellington, New Zealand; et al. Pages 1379-1405.
Keywords: earthquake potential, forearc, New Zealand, structure, subduction, tectonic deformation, thrust faulting.
This paper discusses the tectonic deformation of a large active thrust-fault complex beneath the eastern continental margin of New Zealand. The faulting occurs within the upper plate of the Hikurangi subduction zone, and results from convergence between the Pacific and Australian tectonic plates. The Lachlan Ridge thrust complex is one of the largest of such structures beneath the margin, at over 80 km in length, and with a slip rate of several millimeters per year. The displacement on the fault has resulted in uplift of Mahia Peninsula. Marine seismic reflection profiles, acquired by research organizations and industry, together with dated rock samples, have enabled a careful study of the evolution and structure of the thrust system. The data show that the active Lachlan fault is listric in geometry, originated as an extensional fault that developed on an older thrust system, and has since been reactivated and inverted. The offshore structure and uplift data from Mahia Peninsula enable estimates of potential earthquake size of up to Mw 7.6-8.0 and recurrence interval of about 600-2000 years for rupture events.
Active faulting and folding without topographic expression in an evaporite basin, Chile
T.E. Jordan, Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853-1504, USA; et al. Pages 1406-1421.
Keywords: active faults, Andes, geomorphology, halite, lakes, sequence stratigraphy.
Geologists commonly recognize zones of potential active faulting by identifying characteristic landforms, usually a linear scarp or an elongate hill. In a dry lake bed in northern Chile, however, more than 1000 m of rock salt accumulated during the last several million years, and there is no surface landform where a major fault reaches the land surface, despite having tens of meters of offset along the fault during the time of human occupation. The authors suggest that the rock salt is fed by ground water, and the salt-laden ground water is preferentially supplied to the down-thrown side of the fault, erasing any differential elevation that might have formed. Similar situations may exist in other parts of the world.
Rates and patterns of late Cenozoic denudation in the northernmost Atlantic Coastal Plain and Piedmont
Scott D. Stanford, New Jersey Geological Survey, P.O. Box 427, Trenton, New Jersey 08625, USA; et al. Pages 1422-1437.
Keywords: Atlantic Coastal Plain, denudation, eustasy, landform evolution, landscape evolution.
The form of Earth's landsurface in tectonically inactive areas is the result of erosion over hundreds of thousands to millions of years. The manner in which material is eroded from Earth's surface over long periods of time, and the landscape produced by the erosion, are the subject of active debate. To measure landscape change in detail, this study maps the paleotopography of a 2800 square-kilometer part of the Atlantic Coastal Plain at five times over the past 10 million years with a 60 meter spatial resolution. Relict river and hillslope deposits are the record of former landsurfaces and provide the data for the topographic reconstructions. The pattern and rates of erosion, calculated by subtracting successive landsurfaces, indicate that landforms in this coastal setting are created by river incision in response to long-term sea-level lowering, followed by retreat of hillslopes into uplands. Hillslope retreat is driven, in part, by groundwater seepage. As the volume of upland declines, seepage diminishes and slope retreat slows. In this way, small remnants of very old landsurfaces are preserved in the modern landscape.
Relationships between contact metamorphism and magnetite formation and destruction in a pluton's aureole, White-Inyo Range, eastern California
Agnes Kontny and Carlo Dietl, Department of Geology and Paleontology, Ruprecht-Karls-University Heidelberg, Im Neuenheimer Feld 234, D-69120 Heidelberg, Germany. Pages 1438-1451.
Keywords: Contact metamorphism, iron-magnesium silicate and iron-titanium oxide assemblage, magnetic properties, temperature-dependent susceptibility, oxygen fugacity.
Magnetic-petrologic studies on metasedimentary rocks from the contact metamorphic aureole of the Joshua Flat-Beer Creek pluton at the southern rim of the Inyo batholith, California, reveals the effect of contact metamorphism on magnetic mineralogy. Magnetic susceptibility, which is the response of a material to an external magnetic field, shows significant variations in the regional metamorphic biotite zone and three different contact mineral zones (andalusite-cordierite, transition, and cordierite-K-feldspar) in the aureole. This magnetic behavior is associated with a transformation of ilmenite + rutile + hematite into magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograde. Contact metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite-series plutons. This has to be considered when interpreting aeromagnetic data of granitoid rocks.
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